Pitfalls on the way to high speed paging from the service provider’s perspective
By Selwyn E. Hill
Most of us in the paging industry are familiar with the issues of simulcasting on a single frequency and the stringent requirements placed on the transmitters and network in order to achieve these tight specifications. The following discussion will not attempt to analyze these issues in great depth since there are other sources that explain this in great detail. I will concentrate mainly on the real world experiences that we have lived through in transitioning to high-speed paging and question some of the original ideas and expectations that were put forward when the FLEX protocol was first introduced. Not having any experience with ERMES, I will not draw any conclusions about its robustness or advantages in the RF world but there are certain features in the FLEX protocol, which make it extremely attractive in the transitioning process. I will focus mainly on RF related issues but may also touch on network issues in the context of our system architecture that play a major role in supporting higher speed protocols. Some of the “growing pains” are relative to specific types of equipment deployed in the network, but will probably serve to highlight where some of the pitfalls can occur, even though different equipment may be involved.
PageMart’s satellite based system architecture
FIGURE 1: PageMart coverage - NAFTA wide
The company currently has over 2 million subscribers, which has been achieved through internal growth, and no acquisitions. It is the fastest growing company in the industry and has added more than 100,000 units in service for the past twelve consecutive quarters. PageMart is ranked fifth largest in the industry and is ranked among the “big four” paging companies in the United States in terms of the amount of nationwide spectrum owned (which includes narrowband PCS channels). These statistics are presented to emphasize the desperate need for high speed paging to keep system capacities ahead of growth projections. This will allow PageMart to continue providing a service, which has minimal delays in the delivery of messages to the end user. These are typically less than a minute.
PageMart operates 24 regional terminals across the nation including Honolulu. Figure 2 illustrates an example of the consolidation of messages to a regional terminal in the Midwest. To send a message to a subscriber in Kansas City, a local call is placed, which connects directly to the central office of PageMart’s long distance carrier. The message is then routed to a large regional terminal in Chicago instead of a terminal in Kansas City. This architecture minimizes the number of terminals and simplifies maintenance, while only slightly increasing long distance costs. Each of the terminals across the nation provides a similar function.
FIGURE 2: Regional Terminals consolidate messages
Figure 3 depicts how consolidated traffic at the regional terminals is sent over high capacity lines to the satellite uplink in order to minimize long distance costs. This network is being converted to a frame relay system, which allows higher speed and many alternative routes for a single connection. This is obviously desirable for back-up purposes when outages occur. An additional back-haul network has been implemented in the network for increased redundancy through a separate satellite link, which can route TNPP traffic from Dallas to Chicago.
FIGURE 3: Long distance lines connect terminals to satellite uplink
The Direct Broadcast Satellite (DBS) architecture is the most valuable element of the network because it allows messages to be delivered anywhere in the nation. This point to multi-point distribution is shown in Figure 4.
The satellite uplink transmits all messages to the satellite, which relays the signal to all transmitters across the country. These are currently located at more than 1700 sites in the USA. The Galaxy 4 satellite provides the backbone for this DBS architecture. FM (cubed) modulation is the protocol used to communicate with the majority of the transmitters although SCPC protocol is also used for the more distant sites such as Hawaii. Each transmitter “hears” all the traffic being sent from the uplink, but addressing in the satellite receivers will filter out only the traffic being sent to groups of transmitters on what is referred to as a “space segment.” The transmitter group that receives the message is determined by a code, which is derived from the coverage selected by the user. The transmitters combined into a “space segment” vary from a single city to a number of different markets around the country. This grouping is determined by geography as well as the volume of traffic on a channel and not by the number of transmitters in a group.
FIGURE 4: DBS architecture provides point to multi-point Distribution
The available satellite bandwidth is divided into channels with bandwidths, which are typically 9600 bps or 19200 bps “pipes.” Using different technologies, these “pipes” can be as large as 76.8 kBd. Both the Motorola developed satellite protocol C-NET and the C2000 protocol from Glenayre are used in the network.
A diagram depicting the overall connectivity of the network is shown in Figure 5.
Since a major concern about this network approach is “what happens if the satellite falls from the sky?” PageMart has made provision at a number of key sites around the country to have the satellite antenna mounted in such a way that it can be easily redirected to an alternative satellite which is simultaneously being fed with redundant data.
FIGURE 5: Overall diagram showing PageMart’s Direct Broadcast Satellite System.
Initial design strategies
The following discussion presents some of the key recommendations originally submitted by the designers of high-speed protocols such as FLEX.
Transmitter power: In the early days of 512 bps POCSAG, paging transmitters were laid out with little regard to issues such as delay spread and as transmitters were developed to output more and more power, existing transmitters were replaced with higher powered equipment to compensate for the reduced sensitivity of pagers as they were redesigned to operate at 1200 bps and then 2400 bps. This reduction in sensitivity as measured in a laboratory environment and calculated based on the reduced energy per bit, was in the order of 2 to 3 dB for each increase in baud rate. Hence, when transmitters went from 150 watts to 250 watts and then to 500 watts, the service providers could essentially upgrade their equipment and save increasing the number of sites to maintain the same coverage footprint.
Further reason for increasing transmitter power was to provide in-building coverage and “brute force” was the approach taken by most providers to penetrate heavily structured buildings and those constructed with reflective glass. These “quality” type problems resulted in transmitter sites being installed randomly within a city based only on customer demands with little regard to the overall system design. There are numerous examples of 3 or more sites all within a radius of a few miles in densely populated areas. Sites that are installed to deal with these quality issues are generally equipped with high power transmitters to penetrate buildings.
The ultimate justification for increasing the power of base stations occurred in preparation for 6400 bps paging. The cellular type concept proposed, required high powered transmitters with unity gain antennas, to provide strong signals in smaller “cells” and hence, ensure capture of the paging receiver at all times from the nearest transmitter. This approach would necessitate more sites to maintain the existing coverage at 6400 bps.
Coverage footprint: As discussed in the previous paragraph, the effect of increasing the baud rate would result in a corresponding reduction in the coverage footprint by up to 6 dB or more when systems are “up-speeded” from 512 bps to 6400 bps. In the ideal case of flat earth and 2 transmitters spaced perfectly apart with the minimum of overlap, we would also expect to see a “hole” materialize between the transmitters when the system is “up-speeded.” In order to compensate for these losses, it would be necessary to raise the transmitter powers accordingly.
High sites: Up-speeding to 6400 bps poses a completely new challenge when mountainous terrain is involved. Since, delay spread is a major factor to contend with at 6400 bps, these systems need to be redesigned as the high mountain sites are removed. In order to maintain the identical coverage provided by these high sites, alternative sites need to be found which are low in elevation, have low antenna heights, and make use of low gain antennas. This will obviously mean that many more sites will be required.
Delay spread: A delay-spread model that has gained support in the paging industry is proposed by Hess (Ref 1). For high-speed data systems he considers multiple interferers and views the received signal as a single transmission undergoing multipath delay spread. It is found that for delays limited to a fraction of the symbol time, the amount of signal degradation depends not on the actual delay profile, but on the rms value of the delays, weighted by their respective power levels. This offers an attractive way of handling multisource simulcast because it reduces the multiple delays and signal powers to a single parameter called multipath spread, which is equal to twice the rms of the delay profile. This expression is shown in Figure 6 where Tm refers to the multipath spread.
FIGURE 6: Delay Spread expression
The multipath spread for N simulcasting signals is given by:
where Pi and di is the power and delay of the i-th signal, respectively.
A rule of thumb requirement that has been accepted in the paging industry for some time is that the delay spread needs to be limited to an amount less than one quarter the symbol time for accurate decoding to take place. At 1200 bps this is 208 microseconds, at 2400 bps it is 104 microseconds and at 3200 b/s symbol time (6400 bps) this number is 78 microseconds. In terms of system design, this means we need to avoid any overlapping conditions where signal levels are within about 6 dB of each other and where one or more of the distant transmitters is greater than 15 miles away. Figure 7 illustrates in simplistic terms how shadowing of a signal from a nearby site can cause almost equal overlapping signals between 2 sites with a resultant delay that can be potentially damaging.
FIGURE 7: RF shadowing causes delay spread
The diagram in Figure 8 shows areas of potential capture and where delay spread can occur.
FIGURE 8: Potential areas of capture and delay spread
The challenge is to find these areas of delay spread and this can be achieved by either measurement or by computer modeling. Delay spread is the main reason why the designers of high-speed RF systems are proposing to move off high sites and use low sites with low gain antennas. Since many existing systems, can have transmitters on high sites spaced 20 miles or more apart, meeting the design criteria of no more than 15 miles of delta overlap will necessitate a redesign of the system. Large urban areas such as Los Angeles are difficult to redesign because of the need to have high sites to cover populations on either side of mountain ranges that run in different directions. Many more sites will be required to match the existing footprint. In a city such as Los Angeles, finding sites that meet our design criteria is difficult and lease rates for those sites that are available are extremely expensive.
Frequency offsets: In previous POCSAG systems, frequency offsets have always been recommended due to the fact that overlapping signals of almost equal signal strength beat against each other creating nulls when 2 or more signals cancel each other. In a stationary position this can last for a significant time and hence corrupt the received data if the signal falls below receiver sensitivity.
A frequency offset plan for POCSAG is recommended to have offsets in a range ± 500 Hz of the carrier frequency between simulcasting transmitters. Most pagers are designed for 4500 Hz deviation, and hence an offset of an additional 500 Hz would not exceed the band limit of 5 kHz and a deviation of 4000 Hz could easily be accommodated by the paging receiver. Before the advent of FLEX, transmitters were not required to have the frequency stability needed for 4-level modulation and hence most transmitters had an inherent offset by default. We may not have seen frequency variations as much as 500 Hz but there was probably enough difference between transmitters to essentially do the job without intervention. Paging reliability at lower baud rates was good enough so as not to be overly concerned about offsets and were hence, not implemented in the PageMart network. When high stability transmitters were introduced into the network, a slight drop in performance at 2400 bps was noticed but not significant enough to be a concern.
The recommendation for FLEX transmission is that offsets are to be avoided at all costs. This is mainly due to concerns about the decoding of the inner symbols at ± 1600 Hz, which are required to have tolerances better than ± 10 Hz between transmitters.
Implications and moves away from the initial proposals
High power: This may not be a major concern in the UK and European markets since the level of transmitter ERP is tightly controlled and the issues we face in the States may not occur to the same extent in other countries.
Since most paging companies did not design their site locations with any kind of “finesse” based on accurate RF models and ideal locations, sites were more or less selected based on what towers or buildings were available. Hence, in urban areas, most systems were over-designed to start with. Increasing the baud rate from 512 to 2400 bps and ultimately to 6400 bps (3200 b/s symbol rate), would probably not have affected the overall coverage footprint, even if the transmitter powers had not changed from the original 150 watt base stations.
PageMart has never been a proponent of high transmitter powers and has limited the maximum power of transmitters to 300 watts (which are normally turned down to a little over 250 watts) and has preferred to make use of high gain antennas. The extra expense involved in increasing transmitter power to 500 watts, which is the maximum, offered for paging transmitters, does not seem to be warranted for the extra 2 dB of gain. A 500-watt transmitter will usually require a higher voltage source (in the US) than the 110 volts typically provided for at most sites and the power supplies add weight to the equipment. In addition, the antenna systems need special consideration since the types of connectors and filters used are generally rated below 500 watts.
More important than this, however, is that the use of high power transmitters together with low gain antennas has indirect consequences, which has resulted in a huge cost to the industry. Signal levels within a half-mile radius of a high-power transmitter and unity gain antenna, attain levels between -20 to -30 dBm. Pager receivers have sensitivities better than -100 dBm. and signal levels of this magnitude create inter-modulation (IM) interference in the receiver that was never considered a few years ago. In general, IM rejection numbers for pagers at low signal levels are in the order of 55 dB. At high levels of signal, a pager receiver will probably achieve less than 10 dB of rejection. This means that a pager receiver can be rendered useless even if the desired signal is at -40 dBm (which is considered to be a strong signal), but happens to be near a high-powered transmitter creating “intermod.” This problem has resulted in pager manufacturers having to redesign their receivers. Introducing AGC can improve IM rejection to better than 40 dB in high signal environments. Even this cannot always resolve the problem when the signal level from the offending transmitter is very high. Also, this does not offer a solution to the thousands of pagers on the street that do not have this protection. To deal with this situation transmitters need to be added where they would normally not be required. The result is additional cost, maintenance, and more transmitters, which in turn may create IM problems for some other service provider. This issue becomes a spiraling “no-win” situation for all in the industry. The increased number of transmitters also impacts the system layout and design as it effects reliability at 6400 bps, which I will deal with later. “Intermod” problems have incurred a tremendous cost for everyone involved (pager manufacturers and carriers) and I believe this has been aggravated by the need to have higher-powered transmitters.
Coverage footprint: As the transition occurred from 1200 bps to 2400 bps POCSAG, PageMart conducted propagation studies and field tests to determine the impact on our advertised footprint. The design criteria used for our coverage maps is reasonably conservative and tries to achieve better than 95% of the area covered in all types of environments from heavy structures to residences and in vehicles. Land use and land cover data (LULC) for the whole of the United States is utilized and loss factors up to 24 dB are assigned to different categories of land cover. In addition to these losses, allowance is made for up to 20 dB of Rayleigh type fading. Results of field trials from a single transmitter in a flat area with relatively no foliage, showed no major reduction of performance at 2400 bps within the boundaries of our 95% reliability contour. At the fringes of coverage, signal variations due to Rayleigh type fading are so significant (swings of ± 10 dB and more) that the reduced sensitivity of the pager by 2 to 3 dB at 2400 bps relative to 1200 bps is hardly a significant factor. The pager is just as likely to receive the page at either of these baud rates, but at a reliability level which is much below that which is acceptable for the industry. What happens beyond our published boundaries cannot be guaranteed and slightly reduced performance beyond these boundaries was not sufficient justification to increase transmitter powers or add more sites.
As mentioned previously, coverage between transmitters could be a problem if the reduced sensitivity of the pager creates a “hole” in the middle of a published map. Our studies showed that only in rare instances did the reduced signal sensitivity at 2400 bps create the need for new sites, because most systems were probably over designed to begin with. With the advent of 6400 bps FLEX, similar studies were conducted around the country. A number of different factors have to be considered when 4-level modulation is introduced and initially, these were not fully understood. I will deal with the reduced sensitivity issue first. The comparison in this case is between 3200 bps 4-level (1600 bit/sec symbol rate) and 6400 bps 4-level (3200 b/s symbol rate). Figures 9 through 11 show the results of drive tests superimposed on the coverage footprint of the Dallas system. These tests were conducted in the fringe areas of coverage. Only a single dominant transmitter was used to transmit the test pages. Signal levels along this route are predominantly from this one site and are shown in Figure 9.
FIGURE 9: RSSI plot in fringe area
Signal levels or RSSI are shown in color where strong signals are indicated in red (> -50 dBm) and change in steps of 10 dB. Signal levels shown in dark blue are < -90 dBm and are considered marginal for reliable coverage. Since these levels were recorded with an external antenna on the roof of a vehicle, we see slightly better levels than what the pager would “see.” Coverage within the boundaries shown is where we would expect to see better than 95% reliability. The Bit Error Rate (BER) plots in Figures 10 and 11 show respectively the results of test pages transmitted at 3200 bps and 6400 bps over the same route. The color scale ranges from green (corresponding to good reliable alpha paging) through blue, yellow, and red (which corresponds to poor numeric paging) and is calibrated to actual pager performance shown between marker numbers in the tables. In both cases, we achieved better than 95% reliability within our predicted coverage footprint. The range at 3200 bps extends a few miles further than at 6400 bps.
FIGURE 10: Single site BER plot at 3200 bps in fringe area
FIGURE 11: Single site BER plot at 6400 bps in fringe area
It should be mentioned at this point that these tests are conducted for a worst case situation where a test page is a 40 to 80 character alpha message and the criteria for measurement is a perfect page or nothing. In other words, a single error in one of the characters being transmitted or a totally missed page is assumed to have the same weight in the reliability calculation. Another factor needs to be considered in carrying out these tests, and that is the sensitivity of a FLEX pager depends on the phase of the received page. The FLEX protocol requires interleaving of the data bits and the bits associated with the “b” and “d” phases always correspond to the inner bit of each symbol as shown in Figure 12.
FIGURE 12: The four phases of FLEX at 6400 bps
The decision processes in the decoding algorithm have a much better probability of correctly determining “a” and “c” phases since the threshold for this decision is whether the recovered symbol is above or below the zero crossing line only. The other 2 phases require a more critical determination around the inner level of modulation and has therefore more room for error. The net result is that “b” and “d” phase pagers have a reduced sensitivity by a factor of 2 to 3 dB relative to the other 2 phases. Hence, all our field testing is done for worst case with the test page always being sent in one of these 2 phases. Other protocols such as ERMES may not have phases as defined for FLEX but the resultant effect in decoding the inner-symbols will still be the same and hence reduced sensitivity will be apparent on all pages sent.
The previous test considers the simplistic situation when only one transmitter is being considered. In this case it is apparent that from a sensitivity aspect alone, that we do not see a marked reduction in coverage. Even at 6400 bps the justification for increased transmitter power is not valid. A more realistic test, of course, is what happens in a simulcast environment? This will be discussed in the section under delay spread.
High sites: From a service provider’s perspective, this is one of the more controversial and difficult issues to contend with. The implications of coming down from the high sites are obvious. The expense of replacing a single site could amount to 2 or 3 sites to many more depending on the situation. There is no question that at 6400 bps, high sites are a problem in that they affect delay spread. If one chooses to keep the existing high sites, managing the power from these sites becomes quite a challenge.
One approach, which was attempted in the Los Angeles (LA) area, was to add a few more sites in the urban areas where our propagation studies showed weak signal levels. The idea would be to raise the general signal powers in the low lying areas to a point where only near-by sites would be “seen” by the receiver and would hence, capture the receiver over the distant high sites. Results from this exercise were not too encouraging. Pages were still being missed in strong signal areas and in some cases within a mile from the nearest transmitter. Plots of BER were studied and it became clear that high bit error rates were occurring in areas of overlap between low elevation sites within the LA basin. These areas correlated closely with where we noticed poorer pager performance.
In an urban environment, the clutter causes a tremendous amount of RF scatter to occur. As a result, the receiver is subjected to a combination of Rayleigh fades and beating of signals from nearby transmitters. Fades of 25 to 30 dB can occur for short durations particularly in simulcast overlap areas. The cancellation of the strong signals from near transmitters will provide the opportunity for distant mountain sites to become a significant factor once more. The use of low gain antennas and high powered base stations will not help since signal beating and fading will still occur in the overlap zones. Total capture of the receiver will only take place in very close proximity of less than 1 mile from a transmitter site.
Similarly, the use of low gain antennas on high sites will not help much in reducing the delay spread contribution from these sites. Consider the case where a 5000 ft elevation site has a clear shot at a receiver 20 miles away at sea level. The path loss can be considered to be close to free space. This will present an angle of a little less than 3 degrees below the horizon from the transmitter, which is well within the main beam of a typical gain antenna having a beam width of about 8 degrees. Changing the antenna at the high site to a unity gain will only reduce the contribution of this transmitter by approximately 8 dB. Experience has shown that this amount of signal reduction is not sufficient to reduce delay spread distortion. To really be successful at eliminating high sites from the equation, we need to reduce their signal contribution by more than 20 db in some cases.
A reasonable compromise in dealing with the high site issue is to continue to make use of these sites, but to carefully control the energy radiated by making use of appropriate antennas. The use of high gain antennas with down tilt can reduce the signal on the horizon by up to 15 dB. In certain cases it may be necessary to use cardioid shaped patterns to get signals down to levels previously mentioned. This approach has the added benefit of concentrating most of the transmitted energy close to the mountain site where it is needed and is far more efficient than a unity gain antenna, which will radiate more than half of the transmitted energy above the horizon.
In the same way that high mountain sites create delay spread at a distance, the inverse of having a receiver located at a height, such as in a high-rise building, results in the same problem. In relatively flat terrain and in a city such as Dallas, a paging receiver in a high-rise building above 100 ft will be subject to signals arriving from many distant sites. This high-rise phenomenon can be a major problem and the use of low gain antennas will not provide sufficient power reduction on the horizon as was previously described.
The only real effective way of controlling the signal radiated by sites more than 15 miles away is with the use of specially designed high gain antennas with the appropriate amount of down tilt.
Delay Spread: As alluded to in previous sections, the affect of delay spread is probably one of the most significant factors in the overall performance of systems at 6400 bps. We have conducted extensive tests at ground level and in high-rise buildings to determine what the appropriate values of delay spread should be for use in our models and the results are not all that encouraging. First, I want to briefly review how we can recognize the effects of delay spread by looking at the signal and then I will discuss three different scenarios where delay spread has a detrimental effect at high speed.
When we look at the recovered audio signal from a paging receiver or service monitor, we expect to see a reasonable looking square wave. This may be somewhat smoothed at the transition edges due to the required filtering to limit the generation of frequencies beyond the specified bandwidth of transmission. In a simulcast environment where 2 or more signals are presented to the receiver with different times of arrival and when the signal level differences are less than 6 dB, we will notice spikes of overshoot occurring at the transition edges which could be positive or negative. These spikes are generated when phase cancellations occur in the discriminator as one signal which is undergoing a change in deviation level sweeps across another signal which has because of a time of arrival difference, not yet started its change in deviation. The multiple cancellations manifest as a single spike due to the filtering characteristics in the receiver. The size and width of the spike will depend on the depth of nulls generated and the amount of delay present. This is shown diagrammatically in Figure 13.
FIGURE 13: Offset nulls and data modulation
Spikes are also generated because of signal cancellations due to null beating and will be dealt with in the next section. However, these spikes can be discerned from simulcast spikes in that they can occur anywhere in the signal and are not confined the edge transitions (see Figure 20).
Simulcast spikes are present whether we are transmitting 512 bps POCSAG or 6400 bps FLEX but “ringing” or “overshoot” is not a significant factor at lower speeds since we have ample time to sample the bit after the spike has occurred. This is not the case with 4-level modulation at 6400 bps. Distortion of the inner symbols makes the correct determination of the symbol a lot more difficult to achieve. This results in a significant reduction in performance when comparing 3200 bps 2-level with 6400 bps 4-level, which is at the same symbol rate
The implication of this is that we can expect to see degraded performance, even in systems optimized for 3200 bps 2-level when 4-level modulation is introduced. Service providers need to be aware of this fact when implementing 4-level modulation schemes such as ERMES and FLEX.
We have carried out many measurements to determine at what level of delay spread we start to see degraded performance at 6400 bps. It is important to conduct these tests with “b” or “d” phase pagers since optimization of systems need to accommodate for the worst case. Worst case is taking into consideration the decoding of the inner symbols in conditions as described above. Unfortunately, the minimum delay spread that we are able to tolerate before degradation in performance is about 40 microseconds. This is about half the number we had previously assumed and has serious implications in how we plan and design our systems for 6400 bps.
The 3 scenarios of where and when to expect simulcast delay spread are presented below:
(a) Overlap between sites within the coverage footprint:
Even though sporadic occurrences of delay spread will affect reliability on a random basis, it is possible to predict areas where delay spread is going to be consistently bad in overlap areas. By using delay spread numbers ranging from 40 to 80 microseconds and by taking the beating of the 2 strongest signals into account, we have been very successful in identifying areas of reduced paging performance by using computer models. Figure 14 shows a BER plot at 6400 bps in the heart of the Dallas coverage area superimposed on a delay-spread plot, which is predicted by our software model. The BER results correlate very closely to results of actual pager performance, and verify our predictions. Delay spread is depicted with varying shades of gray where lighter shaded areas indicate 40 microsecond and darker areas 80 microseconds.
FIGURE 14: BER plot at 6400 bps superimposed on delay-spread plot in center of the DFW market
(b) Low signal level simulcast.
As previously discussed, reduced sensitivity at 6400 bps can account for a slightly reduced footprint when compared to 3200 bps and this was demonstrated with pager performance from a single transmitter. However, this is not the case in a 6400 bps simulcast environment where the fringe areas of coverage are subject to signals arriving from the many transmitters within a large metropolitan area. Pager performance at 6400 bps is dramatically worse than at 3200 bps beyond the coverage boundary and is mainly due to the effects of delay spread and simulcast “spikes.” This dramatic degradation of BER beyond the coverage zone is depicted in Figures 15 and 16 which compare performance at 3200 bps and 6400 bps over the same route as shown previously for a single site transmission. Neither improved pager sensitivity or increased transmitter power is likely to improve performance in this scenario.
FIGURE 15: Simulcast BER plot at 3200 bps in fringe area
FIGURE 16: Simulcast BER plot at 6400 bps in fringe area
Again, tests with actual pagers are always carried out for worst case, and pagers are programmed for capcodes on the “b” or “d” phase. Test messages are approximately 80 characters of text. As is the case for reduced sensitivity of the ‘b’ and ‘d’ phase pagers, where noise makes it more difficult for good detection of the inner symbols, simulcast spikes create much more distortion of the inner symbols than the outer symbols. This again explains why we have consistently worse performance of pagers on ‘b’ and ‘d’ phase compared to those on ‘a’ and ‘c’ phases.
(c) Strong signal simulcast in high-rise buildings and on the ground.
These tests are carried out with pagers distributed around the room. What this tells us, is that capture as we know it in a laboratory environment seldom exists in the real world. Multiple signals reflecting off the walls and structures within the room and from outside the building result in an uneven pattern of nulls and peaks around the room with a Rayleigh like distribution. Some nulls may be more than 30 dB below average signal level. Signals from each transmitter result in their own independent pattern and each has its own slow time variant component, which can also be rapid depending on people movement within the room. A three dimensional depiction of this is shown in Figure 17.
FIGURE 17: Multipath nulls in 3 dimensions
This complex signal environment explains why capture is not possible except in a few isolated locations within the room and also explains the pager performance previously described. The other component, which makes this environment even more complicated, is the presence of low frequency beats, which occur between simulcasting transmitters that are held to very tight tolerances. This time variant component will result in nulls of various depths occurring anywhere in the room. In a strong signal situation as I have described, total signal cancellation will not occur very often, but any amount of cancellation between 2 strong signals has the potential for delay spread to occur in the nulls. These simulcast-induced spikes are shown diagrammatically in Figure 18, which illustrate how the spikes occur in the nulls at the beat frequency between 2 signals. In this case delay spread spikes are as a result of 2 transmitters only and no other signals are involved.
FIGURE 18 Simulcast spikes due to delay spread
Shown in Figure 19 are actual over-the-air-snapshots, which indicate total annihilation of some bits even at a 3200 bps symbol rate. In this picture, which captures a complete frame at 6400 bps, it is clear how “spikes” occur at the offset frequency of 16 Hz between the 2 transmitters involved. In this case, the delta signal level between the 2 transmitters is greater than 10 dB on average, but the receiver happens to be in a “multipath null” as previously described.
It is also clear from these pictures that the bursts of “spikes” that occur in the nulls, last for longer than 20 milliseconds and is greater than the fade protection margin that the FLEX protocol can handle. This is partly due to the fact that independent beats occur for each deviation level and hence the nulls do not all occur at the same point in time. Keep in mind that not all the “spikes” are real damaging and error correction code will help tremendously in this situation.
FIGURE 19: 6400 bps frames over the air indicating spikes in nulls at the offset frequency between 2 TX
As we add more and more transmitters into the mix, the null patterns become more complex and random and delay spread as a result of more distant sites can now become a factor within the nulls of the stronger signals from nearby sites. In this scenario, offset frequencies can be helpful if only 2 or 3 sites are involved. When many sites are involved, the benefit of offset frequencies is probably not significant. This is the very reason why a high density of transmitters in an urban area can be very damaging to high speed FLEX or ERMES.
The high rise phenomenon is particularly susceptible to many sites being “seen” and sites with low gain antennas will be just as much a factor as those with high gain antennas. However, with appropriately designed high-gain antennas, we have the ability to control the level of signal on the horizon and can reduce the amount of interference from distant sites significantly.
The scenario described above for high-rise buildings can also be found on the ground in the presence of very high signal levels where the receiver happens to be in the overlap between sites fairly close to each other (5 to 10 miles). This situation was previously described where we experienced high BER in the overlap between low elevation sites in the city of Los Angeles. The effect of beating and delay spread within the nulls is the same effect as in the high-rise. This coupled with Rayleigh fading in a moving vehicle explains the lack of capture and missed or corrupted pages even close to transmitters.
Frequency offset and beat effects: As mentioned earlier, the recommendation for high speed FLEX systems was not to implement frequency offsets due to the tight tolerances required for recovery of the inner symbols. However, the downside of having simulcasting transmitters held to within 3 Hz of each other, which is typical for the transmitters we currently have installed, is that in an overlap situation where signal deltas are less than 6 dB, beating will occur between these signals at a very slow rate. Instantaneous peak-to-null differences can vary from 10 dB to greater than 60 dB if the receiver is in a stationary position. Because of the slow beat frequency, nulls can occur for longer than the 10 ms fade protection that is provided for in the FLEX interleaving code.
It is also apparent from actual over the air measurements, that because of slight differences in deviation levels between transmitters, the beat associated with each deviation level is unique and affects each deviation level independently. This spreads the nulls over more data bits and causes more corruption than if the nulls occurred on all deviation levels at the same time.
The result of these nulls on the recovered data is shown in Fig 20 which is an actual over-the-air snapshot illustrating that noise “spikes” can occur anywhere within the data signal. This can be extremely damaging in low signal simulcast overlap areas where nulling causes the resultant signal to drop below receiver threshold. Where there are strong signals present such as in high-rises, total signal cancellation does not happen often. Far more damaging, is when cancellation of the dominant signals due to beating, creates the opportunity for delay spread “spikes” to occur in the nulls. This of course also happens in low signal situations.
FIGURE 20: Simulcast spikes associated with nulls from over-the-air snapshot
Reference to the previous Figure 19 also illustrates graphically the effect of “noise spikes” on the recovered data.
The presence of spikes on the transition to the inner symbol of deviation and during the inner symbol is the worst case situation and explains why we see significantly worse performance of pagers programmed to receive ‘b’ and ‘d’ phases. The length of time a symbol remains at a particular level is dependent on the combinations of phases that are being transmitted simultaneously. For example, ‘b’ phase only will result in worse results than ‘a’, ‘b’, ‘c’ and ‘d’ together since the number of transitions to the inner symbols is greater and there are more symbols of 312 microsecond duration than in the latter case. An example of what the FLEX 6400 bps signal will look like for various phase combinations is shown in Figure 21. This example represents a string of “A” characters being transmitted. The net result is that a fully loaded channel will tend to have slightly better results than if only one of the ‘b’ or ‘d’ phases is being transmitted.
FIGURE 21: 6400 bps FLEX signal for different phase combinations
Laboratory measurements have shown that by implementing even small amounts of frequency offset between transmitters such as 8 Hz, we will significantly improve the performance of FLEX paging at 6400 bps. Figure 22 shows pager reliability as a function of frequency offsets between two Nucleus transmitters.
FIGURE 22 Reliability measurements for different frequency offsets between 2 Nucleus TX
The charts shown in Figure 23 give comparative results between two transmitters from different manufacturers for ‘a’ and ‘b’ phases. We notice a roll off starting to occur at 64 Hz for the ‘b’ phase pagers and a significant “dip” in reliability at 100 Hz and 200 Hz which is expected due to the interleaving rate of 200 Hz or 5 ms and multiples of this frequency. The expected lower reliability at specific frequencies below 100 Hz did not occur. It is highly unlikely that these “traps” will ever be noticed in the real world because of the inherent “jitter” of the nulls that we see in a multi-path environment.
FIGURE 23: Pager reliability at different frequency offsets for two different transmitter types
PageMart has been implementing offsets in systems as we transition to 6400 bps and we have been receiving favorable reports from the field. Special programs have been developed and incorporated in our modeling tools to calculate the appropriate offsets for each system. Minimum and maximum criteria are specified for acceptable limits between adjacent transmitters. The benefits of offset frequencies are mainly evident where only 2 or 3 transmitters are involved in the overlap and the error correcting code in the FLEX protocol has an opportunity to correct corrupted data in the null periods (or fades) that occur for less than 10 ms. When more transmitters are added to the mix in an overlap zone, less improvement can be expected from frequency offsets but our experience indicates that we still do better with than without.
Other network related issues needing consideration in up-speeding systems
System synchronization: From the material already presented, it is clear that the one issue, which has the largest impact on performance at 6400 bps, is the effect of delay spread. Fundamental to “power-delay” management is the assumption that all transmitters are perfectly synchronized to begin with. This is probably the biggest hurdle to overcome on the path to successful high-speed implementation. It is imperative that the service provider understands the issues involved in synchronizing their systems from the start. Many providers lean heavily on automated systems for synchronizing transmitters such as the use of monitor receivers and GPS receivers. Both these methods have problems and PageMart has moved away from relying on these techniques alone. Monitor receivers work reasonably well provided they are located in an “RF quiet” environment and are not subject to interference. This is obviously not the case in many urban areas. GPS systems are much better but we have also found instances around the country where interference or other local factors create problems in getting satisfactory GPS readings at a site. This has resulted in incorrect delay numbers being installed in the transmitter.
Since the PageMart network architecture is solely satellite based, it is easy to compensate for time delay differences that occur on the paths from the satellite to each transmitter in a system. We have developed software tools to calculate these numbers exactly. Compensating for path delay differences is one thing, however, it is not always possible to have a completely homogeneous equipment mix in a network. Hence, measuring and knowing to within a few microseconds the differences in delay propagation through transmitters from different vendors, of different vintages, and different versions of software is essential. The method of synchronization through the use of monitor receivers will usually compensate for these differences but for reasons mentioned above, their readings are not always reliable. Use of GPS receivers also requires knowledge of the above-mentioned variables. Knowing the delay times through the Exciter and Power Amplifier (PA) of the transmitters is not sufficient and it will also be necessary to know the propagation times through the complete control and interface configuration. Component changes in newer versions of control equipment and upgrades to software have to be continually monitored and re-measured so that adjustments can be made to the delay numbers at the sites.
During initial transitions from low speed to high-speed transmitters, it may be necessary to make adjustments to the rise-times of the modulating waveforms. The recommended number for FLEX 6400 is 88 microseconds whereas earlier POCSAG systems were 120 microseconds and greater. In certain transmitters, adjusting the rise-times would also affect bulk delay through the exciter. Since these adjustments were independent for POCSAG and FLEX transmission, if not done correctly, the monitor receivers would show systems to be perfectly synchronized whereas in reality, transmitters could be 100’s of microseconds out of synch.
In systems where it is required to dynamically transmit FLEX and POCSAG over the same frequency, there may be different deviation limits set for each protocol. Some types of transmitters may not be able to dynamically alter deviation levels according to which protocol is being transmitted and it is essential that at least for the high-speed 4-level protocols these levels of deviation are held to exact tolerances between different types of transmitters.
One of the major benefits of the FLEX protocol which has “saved our skin” in the transition to 6400 bps, is the ability to step up from 1600 bps to 3200 2-level and 4-level transmission modes. Early transitions to 3200 2-level were made because not all transmitters had been modified to provide 4-level modulation. Even though the symbol rate at 3200 bps 2-level is the same as at 6400 bps, we now know that simulcast spikes degrade performance when we introduce an inner symbol of modulation. Having the ability to debug system synchronization issues at 3200 bps where effects of delay spread are not that dramatic, is a tremendous help in the transition to 6400 bps. Obviously, as soon as a point is reached when all transmitters in a system have 4-level modulation capability, it makes no sense to run 3200 bps 2-level, and the system should immediately be switched to 3200 bps 4-level since delay spread effects at 3200 bps 4-level are almost non-existent. The inability of the ERMES protocol to provide this intermediary transition phase would be a major concern.
Network related issues: This topic does not really fall within the scope of this discussion but does warrant some mention. Obviously when contemplating up speeding to 6400 bps, the network as a whole needs to be considered. In the PageMart configuration, one aspect of the network, which is critical, is the bandwidth and capacity of the satellite link to the transmitters. Various protocols are available in the United States and there can be large differences in the efficiencies of these protocols. In the beginning of our transition to 6400 bps, PageMart was using one type of protocol almost exclusively. Early expectations were that we would be able to transmit 2 channels of 6400 bps traffic on a single 19.2 kbps “pipe.” However, inefficiencies in the protocol caused dropped packets of data on adjacent channels if the bandwidth constraints were not being met. This resulted in extremely inefficient usage of the available bandwidth and was an unexpected expense when more “segments” had to be purchased in order to accommodate the traffic load.
Another network related issue deserves some comment and is not necessarily a 6400 bps problem but more a symptom of the FLEX or synchronous protocols. In a satellite based configuration, there will be times when systems are adjacent to each other and are operating on different space segments. Paging receivers that operate on the same frequency will at times be able to receive signals from either segment. There is hence, a strong likelihood of a pager locking on to the wrong segment. These adjacent segments will need to be synchronized so that the FLEX frames are synchronous with each other. In order to achieve this, it is necessary to have a GPS receiver interface to each segment. Unfortunately, differences in satellite link protocols and even differences in baud rates between different channels may cause a time delay to occur between adjacent segments resulting in pagers “locking” on to the wrong system.
RF modeling and measurement tools
Without the necessary tools, contemplating 6400 bps speeds in urban areas where there is a significant build-out of transmitters can be a very risky proposition. RF models have traditionally only provided an approximation of expected signal levels. However, to adequately predict pager performance at high speeds, we need to include an additional feature of “delay spread” prediction and algorithms, which allow for system optimization. This can be implemented by making adjustments to transmit powers, antennas, and link delays. Delay spread predictions should take into account the effects of null beating and Rayleigh fading to get close approximations to reality.
The PageMart model is custom designed and uses the TIREM (Terrain Integrated Rough Earth Model) algorithm for RF propagation prediction. The basic RF signal level and Delay Spread prediction model should at a minimum be able to accept the following inputs to make predictions as accurate as possible:
The model should have the following capabilities:
Many other features have been added to our model to allow for quick access to available data and the ability to be interactively responsive to changes in power, antennas, and site selections.
Examples of drive test results superimposed on coverage and delay spread plots have been shown elsewhere in this paper.
Obviously the most reliable piece of test equipment to measure pager performance is the pager itself. Given the variety of pagers on the market with just as large a variation in performance specifications, it is critical that a standard set of criteria be met. PageMart has a very stringent pager quality test program and only the best performing pagers are allowed access on the network. A standard test pager is selected for pager reliability testing and typically a minimum of 5 pagers will be used simultaneously to acquire sufficient statistical data. Test pages are transmitted at least once a minute. Gathering all this data, however, is not very useful unless it corresponds to location information so as to compare performance to predicted coverage. We do this test together with equipment, which records location co-ordinates from a GPS receiver. Manually entered marker numbers allow us to keep track of average reliability over short distances.
The device we normally use for drive tests will have up to 4 receivers and decoders that can simultaneously record different frequencies, protocols and baud rates. RSSI samples are averaged and recorded at selected intervals but the sample rate is at least once every 800 microseconds. POCSAG and FLEX capcodes and associated data are recorded on a laptop computer. BER is calculated after every batch for POCSAG and after every frame for FLEX. The average BER is calculated between GPS recordings which is typically sampled every 5 seconds. The data files that are recorded are then processed and calibrated against actual pager performance before plotting and comparing to predicted coverage.
Besides giving us the ability to view the data graphically and compare to actual pager performance, this test equipment is also invaluable in debugging system problems without which the transition to high speed paging systems would be, if not impossible, at least very difficult. Use of the traditional “service monitors” provides very limited information and has almost no value in diagnosing RF systems designed to transmit high-speed data.
Recommended design strategies and transitionary phases on the path to high-speed paging on existing networks
In this section I will essentially summarize the basic steps a service provider should take in the transition to high-speed paging using an existing infrastructure.
FIGURE 24: RSSI plot superimposed on coverage for Denver market
FIGURE 25: BER plot superimposed on coverage for Denver
FIGURE 26: BER plot in Denver superimposed on Delay spread before optimization.
FIGURE 27: BER plot in Denver after optimization.
However well we design and maintain the network infrastructure, in the end, the overall quality of service we provide the end user, is dictated by the quality of the paging receiver products. For this reason, PageMart sets high standards for acceptance and all pagers are thoroughly tested before introducing them into service. Pagers are tested in a screen room and placed in a TEM (Transverse Electromagnetic Mode) cell, which is essentially a section of rectangular transmission line, which provides a uniform field at the desired frequency. The basic parameters evaluated are:
Sensitivity is usually tested against benchmark pagers that are known to have good performance. Pagers are evaluated in worst-case situations only. Therefore, tests are conducted at 6400 bps on ‘b’ or ‘d’ phase capcodes only. Sensitivity measurements are done in 8 different positions in the horizontal and vertical planes. The best position is used for the final measurement and is the RF level, which provides better than 90% reliability. If a pager does not meet a minimum range of acceptable performance, it is rejected at this point. Normally a sample set of at least 5 pagers will be tested.
If the pager passes the sensitivity test and is a relatively unknown quantity, we will normally carry out further tests in the field. These tests are conducted in areas known for low signal simulcast as well as in locations where high signal level intermodulation exists. It is not uncommon to find paging receivers that have excellent sensitivity but extremely poor IM protection and simulcast performance. There is often a trade-off of sensitivity against these two qualities. Since most high-end alpha-numeric paging products will more than likely be used in large urban environments, we are often prepared to sacrifice a few dB of sensitivity for improved IM and simulcast performance.
Not all pager manufacturers are aware of the issues at 6400 bps in a simulcast environment and it is critical that they focus more attention to delay spread and how it affects the decoding of the inner symbols. PageMart has worked very closely with major vendors in improving performance in this area and we have seen significant progress over the last year. Some of the latest products are able to tolerate delay spread much higher than 40 microseconds and the goal should be set at 100 microseconds. This is achievable with the use of DSP technology and we are seeing close to this performance level with pagers designed for two-way which have the same simulcast issues to contend with.
The ultimate high-speed network using 2-way transmission
In July 1994 PageMart was awarded narrowband spectrum licenses for frequencies in the 940 MHz and 901 MHz bands. These frequencies are designed for advanced messaging systems called Narrowband Personal Communications Services (NPCS). NPCS frequencies will permit two-way paging as well as voice messaging and will move paging into applications that are currently served by one-way technology.
Currently there are over 42 million pager users in the U.S. and NPCS will provide the bandwidth to support continued customer growth and enhanced wireless message delivery to laptops and personal digital assistants (PDAs). Increased capacity will encourage two-way messaging and integration of paging with computers for message creation.
In addition to providing a faster forward channel (940 MHz), the NPCS reverse channel (901 MHz) enables the system to pinpoint the location of a subscriber within a geographic area. The message can then be broadcast within a pre-defined region rather than across a whole network. This location technology allows for “roaming” and facilitates frequency reuse, which can increase network capacity by many times.
Various types of acknowledgment services will be provided.
With two-way capability, the doors open to many more applications and extend to telemetry type services (either mobile or fixed). The future of this technology promises to be extremely exciting. PageMart plans to implement all these new services in the future. However, in the near term, the one major aspect of two-way, which is most attractive, is that the technology will provide the capacity to meet projected usage and be able to deliver high speed reliable alpha text messaging. In the U.S., alpha messaging is forecast to be the next most rapid growth market. This is the next logical step after the astronomical growth of numeric paging which has gained widespread acceptance in every market sector. Use of Internet and e-mail services allows easy origination of alpha messages from PCs and has resulted in an accelerated use of alpha text paging. In order to plan for this expected demand, PageMart is building out a nationwide network of transmitters and receivers, which will, for the most part, overlay on existing sites. This two-way network is the only way to support the capacity needs of the future and provide for reliable delivery of alpha messaging as we move into the next century.
Figure 28 illustrates in a top-level diagram how one-way and two-way services will be integrated over a single network. The network will continue to make use of satellite links only as the backbone for both outbound and inbound transmissions.
FIGURE 28: Network configuration for 1-way and 2-way Paging combined.
A number of design strategies were initially proposed to prepare systems for high-speed paging. These all require significant investment from the service provider’s perspective in that large scale changes to the network are needed. These changes involve increasing transmitter powers, changing antennas to unity gain, removing high sites, and adding new sites to compensate for the high sites that are removed. Since these design changes result in reduced coverage from each site, creating an almost “cellular” like layout, we need to add more sites to provide the same coverage footprint. Recent proposals recommend designs, which make use of directional panel type antennas with lower height above average terrain. Again, this will probably require some re-arrangement and addition of sites. This approach will work well if a system is being designed from scratch.
I have presented a number of reasons, which show that the former approach to the design of high-speed systems is expensive and flawed in that a high density of sites transmitting at high power causes many problems in a simulcast environment. These range from “intermod” issues to signal fades even in strong signal areas, which allow destructive delay spread “spikes” to occur almost anywhere in the coverage footprint. The expectation that the “capture” effect of FM will eliminate delay spread distortion from more distant sites did not work out and I have given reasons to explain this.
The single most important factor to consider in the design of high-speed systems is delay spread. The results of drive tests were shown which demonstrate the effect of delay spread on pager performance at 6400 bps in fringe areas of coverage and well within the coverage footprint. Three different scenarios were presented where delay spread can affect paging.
The PageMart approach to dealing with delay spread and the migration to 6400 bps is somewhat different from the strategies proposed above. No antenna changes, site moves or additions are considered until deemed absolutely necessary. Fundamental to this strategy is a heavy reliance on accurate computer simulation and having the tools to verify computer predictions. Systems are analyzed carefully for areas of delay spread before up speeding to 6400 bps. In the majority of cases, particularly in the more rural areas where simulcast is not a major concern, no changes other than frequency offsets are implemented.
When the computer model does predict areas of delay spread in major centers, we attempt to “fine tune” the system for optimal performance in critical areas based on population densities. This optimization is carried out by making slight changes to link delays, which we call “simulcast offsets.” After this step has been implemented, the system is again thoroughly tested to verify the computer model.
The final step in this optimization process is to change out antennas at high sites or those identified to be the cause of delay spread. Additional sites in “RF holes” and in the fringes of coverage may be required. This 4 step process of frequency offsets, simulcast offsets, some antenna changes and a few site additions, has proven to be very successful for PageMart and provides the quickest and most economically viable method of implementing a high speed paging network.
A special thanks to our team of engineers and technicians in the RF Department at PageMart who have helped compile the data presented and for their many hours in the field acquiring this data.
Also, a special thanks to the engineers and managers at Motorola in the Advanced Messaging Systems Division in Fort Worth and the Paging Subscriber Division in Boynton Beach for allowing me to share their data and diagrams as well as their support and help in trying to improve system and product performance at 6400 bps.
1. Hess, Garry C., Land-Mobile Radio System Engineering, 1993.
FLEX, ReFLEX, FLEXsuite, and InFLEXion, are trademarks or registered trademarks of Motorola, Inc.
If you would like to have a "pdf" file of this document, click here.
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International Path Delay Service
The path delay service uses the output of numerical weather models and
computes slant path and zenith path delays in the atmosphere at radio,
infra-red or optic ranges, including dry and wet components. Handling
the output of numerical weather models is not an easy task: their size
exceed 1 Tb/year. The service solves for logistic problem of downloading
voluminous datasets, their preprocessing, performing CPU-intensive computations
at a dedicated server, and presenting the results in a form that can be used
for analysis of GNSS, SLR, DORIS, VLBI, radioastronomy, InSAR, air-born and
- The goal of the Path Delay Service is
to ingest outputs of numerical models, compute path delay through
the neutral atmosphere, and to disseminate the results for the entire
geodetic community on a regular basis, continuously, and with low
latencies. The service is provided free of charge, and
no registration is required.
- Slant path delay through the neutral atmosphere in a form of 3D
time series is provided for 459
geodesy sites. The dimensions runs over elevation, azimuth, and time.
In total, there are 124351
files to download from this site. To facilitate downloading files with slant
path delay, the list of URLs for downloading can be generating by filling a web form:
Downloading pre-computed time series
of slant path delay for 459
space geodesy sites. This form will generate a list URLs of individual
files or a control file for wget.
The series are updated within 1 hour of availability of new output of
numerical weather models.
Software SPD for computing computing slanted path delay through the neutral
atmosphere using the output of numerical weather models and for the end-user who
needs only interpolation of slant path delay for data reduction can be
This work was supported by NASA
Earth Surface & Interior program, grant NNX12AQ29G.
This page was prepared by Leonid Petrov
Last update: 2016.02.26_20:11:10


My general idea was to use one or more mac mini’s for the ‘heart’ of the installation, the main advantage being their compact size. Although not high-powered machines, they would likely contain enough processing power for my application (i think!).
I have been doing a little research into using multiple monitors with them, which through some brief searching, isn’t as easy as hoped. I currently have an older generation (2006) mac mini loaned from University, which only has 1 DVI output. The newer generation (2009>) mac mini’s do, unfortunately for me, support 2 monitor outputs.
Seemingly the only way to connect another monitor to the older generation of devices is through the use of a VGA splitter device, as detailed here. More info here and here. The main disadvantage of this looks to be a max resolution of 1024x768 per screen, which shouldn’t be too much of an issue, as I’d prefer to use as smaller monitors as possible, to constrain the potentially large size of the structure.
Tonight I have quickly experimented with using a second monitor with a (mac OS hacked) Dell netbook, which seems to work well. This will be considered through future development as a possible combination or alternative to the mac mini.
Another advantage to using the mac mini however, was the inbuilt bluetooth. I have tested the processing app on the netbook with a bluetooth USB dongle though and it seems to work fine - I had read about issues when using dongles with the bluetoothDesktop processing library, but it seems these don’t affect my setup.


Surge Arrester: An appliance used to protect electrical equipment from high transient overvoltage hazards during lightning strikes, and to limit freewheeling time, and often limits freewheeling. It is also called Lightning arresters sometimes,or called overvoltage protectors, overvoltage limiters
Surge Arrester are usually connected between the grid conductors and the ground wire, but are sometimes connected next to the electrical windings or between the conductors.
The AC metal oxide arrester is used to protect the insulation of the AC power transmission and transformation equipment from lightning overvoltage and operating overvoltage. Suitable for overvoltage protection of transformers, transmission lines, power distribution panels, switch cabinets, power metering boxes, vacuum switches, shunt compensation capacitors, rotating electrical machines and semiconductor devices.
There are many types of folding arresters, such as metal oxide surge arresters, line metal oxide surge arresters, gapless line metal oxide surge arresters, fully insulated composite jacket metal oxide surge arresters, and detachable arresters.
According to shape, the main types of surge arresters are tubular surge arresters, valve surge arresters and zinc oxide surge arresters. The main working principle of each type of arrester is different, but their work is essentially the same, in order to protect the communication cable and communication equipment from damage.
If the above Surge Arrester specifications do not meet your requirements,our creative design engineer can help customer focused on full implementation insulator designing services of all sizes according to customer’s request.
We offer a full range of design services to meet different needs from different customers.


India has acquired a significant milestone in India’s moon mission. The Indian Space Research Organisation (ISRO) will fire Chnadrayaan-2’s liquid engine on Tuesday. “It’s tomorrow morning (tentatively between 8:30 am and 9:30). It’s challenging,” Chairman of Indian Space Research Organisation K Sivan said on Monday.
ISRO has also mentioned that the spacecraft will enter its final orbit passing over the lunar poles at a distance of about 100 km from Moon’s surface. Accordingly, the Vikram lander will be separated from the orbit on September 2 as per Bengaluru -headquartered space agency. On the onset of the initiation of powered descent to make a soft landing on the lunar surface on September 7, two-orbit maneuvers will be performed on the lander.
The spacecraft has been continuously monitored on a regular basis from the Missile Operation Complex (MOX) at ISRO Telemetry, Tracking and Command Network (ISTRAC) antennas at Byalalu, near Bengaluru.
The space agency has said- “This mission will help us gain a better understanding of the origin and evolution of the Moon by conducting detailed topographical studies, comprehensive mineralogical analyses, and a host of other experiments on the lunar surface.”
Furthermore, this mission will also discover the presence of water molecules on the Moon and new rock types with unique chemical composition as told by the agency.
To get more such updates delivered first to you, like our Facebook Page. Also dont forget to follow us onTwitter, Instagram and Google News. To send guest post or press release, mail us at email@example.com


After a 205-day journey through space that covered 300 million miles, NASA’s three-legged InSight probe descended safely after a final, intricate seven-minute maneuver on the red planet Mars to whoops of joy at NASA’s Jet Propulsion Laboratory in California and enthusiasts around the world following live, edge-of-the-seat action on social media platforms. NASA spacecraft ‘InSight’ is designed to burrow beneath the surface of Mars and crack open crucial secrets held deep inside the planet’s rose-hued atmosphere. With Monday’s achievement, Mars has just received its newest robotic resident. It’s been a long wait for NASA’s scientists tracking InSight’s every move since it launched from Vandenberg Air Force Base in California on 5 May. The lander touched down near Mars’ equator on the western side of a flat, smooth expanse of lava called Elysium Planitia – a plain almost as flat as a parking lot – just moments before 3 pm EST.
The InSight team has chosen a “boring” spot for the landing because they want the probe’s two primary instruments, a sensitive seismometer, and an underground temperature probe to be undisturbed – to measure the tiniest fluctuations in the planet’s interior. The mission is expected to last about two Earth years. The stationary 360-kilogram lander will use its 6 feet robotic arm to place a mechanical mole and seismometer on the ground. The self-hammering mole will burrow 16 feet down to measure the planet’s internal heat, while the seismometer listens for possible quakes. No lander has dug deeper on Mars than several inches, and no seismometer has ever worked on the planet.
“Today, we successfully landed on Mars for the eighth time in human history,” said NASA Administrator Jim Bridenstine. “InSight will study the interior of Mars and will teach us valuable science as we prepare to send astronauts to the Moon and later to Mars. This accomplishment represents the ingenuity of America and our international partners, and it serves as a testament to the dedication and perseverance of our team. The best of NASA is yet to come, and it is coming soon.”
Now InSight’s camera has sent back some clearer pictures of the Red Planet, showing a much lighter Mars than you would have imagined. The photo also shows a dusty, rocky surface without any major craters in sight. The new robotic resident sent signals to earth about 12.30pm AEST indicating its solar panels were open and it was collecting sunlight on the Martian surface. The image was relayed from InSight to Earth via NASA’s Odyssey spacecraft, currently orbiting Mars.
NASA last landed on Mars in 2012 with the Curiosity rover. “Landing on Mars is one of the hardest single jobs that people have to do in planetary exploration,” InSight’s lead scientist, Bruce Banerdt, said before Monday’s success. “It’s such a difficult thing, it’s such a dangerous thing that there’s always a fairly uncomfortably large chance that something could go wrong.”
Mars has been the graveyard for a multitude of space missions. Up to now, the success rate at the red planet was only 40 percent, counting every attempted flyby, orbital flight and landing by the U.S., Russia and other countries since 1960.
The U.S., however, has pulled off seven successful Mars landings in the past four decades, not counting InSight, with only one failed touchdown. No other country has managed to set and operate a spacecraft on the dusty surface.


Fraunhofer Institute for Manufacturing Engineering and Automation IPA
The Fraunhofer Institute for Manufacturing Engineering and Automation IPA in Stuttgart is one of the largest institutes in the Fraunhofer-Gesellschaft and employs almost 1,200 people. The “Robot and Assistive Systems” department involved in Sim4Dexterity designs, with over 70 scientists, automation solutions for industrial and commercial applications, the service sector and everyday support for private individuals. The department is characterized by its many years of competence in all subject areas that are required for the development of robot systems and their key technologies. In industrial robotics, handling technologies are further developed with the help of 3D image processing and machine learning. Fraunhofer IPA is also known for developing innovative handling software, such as the bin picking software bp3™, which is already successfully employed in production systems for component separation for over 12 years. The Sim4Dexterity research project is lead by Fraunhofer IPA.
The website of the Robot and assistive systems department can be found here (https://www.ipa.fraunhofer.de/robotsystems).


ST robots can be fun too!
— CEO of ST Robotics, serial inventor David Sands
PRINCETON, NJ, USA, May 18, 2023/EINPresswire.com/ — Increasingly robots are finding applications in the entertainment industry. Often that is coupled with vending functions, so the robot entertains you then sells you something.
Here are some examples
Exciting vending application with an R12 on a linear track
>> ST Robotics supplied an R12 on a linear track to Zone 28 in Pittsburg PA.
ST Robotics supplied the robot and the track together with pneumatics for vacuum pickup.
The whole setup was developed and programmed by Gregg Jackson at Zone 28 including programming the touch screens. A customer chooses the item he/she wants on the touch screen and pays using token cards. The robot then fetches his/her selection from shelves behind the robot and drops it down a chute for the customer to retrieve.
For flat packs like beef jerky or candy arranged in piles the robot searches down to find the top of the pile. A change in vacuum pressure tells the robot it has reached a pack then to stop downward motion and head back up again holding the pack. All this is easily programmed in the embedded robot software called RoboForth. Control of the robot is all from the embedded software and the PCs just need to send ASCII commands.
ST provided additional software to handle the search function and the vacuum generator and sensors. Gregg Jackson did the rest.
You can see the whole system in action on Youtube:
https://youtu.be/soWvq-h6vA8 (jump to about 12:15 to see the robot).
Airbrush body art
ST Robotics has been collaborating with Itatu in providing R17 robot arms for airbrush body art aka tattoos. No, the robot is not wielding an actual tattoo pen!
This is a whole new field for robotics. The developer, Chris Martin has been running trials for some time and the concept is proven and safe. It is part of a 23B$ growing market for wearables.
In the art world Pindar Van Arman is using an R17 robot arm to paint portraits, using AI and machine learning. You can see the robot in action on Youtube:
ST robots are in films too, such as Space Precinct, This Means War and The Last Mimzy
About ST Robotics:
ST Robotics has been in business making robots for 30 years. Its R12 robot arm was the first truly industrial arm in such a small size. ST have taken a modular approach to options to keep costs to a minimum with options such as a small electric grippers, miniature 6th axis module and a unique miniature vacuum pickup. ST also provides the larger R17 robot arm including a very high speed version (that is 3 times as fast as the nearest competition), the R19 cylindrical format arm and the R18 delta format arm as well as dozens of accessories.
About Zone 28
Based in Pittsburgh PA , Zone 28 is a big family fun center with 33 bowling lanes, over 85 arcade games. They are at 2525 Freeport Rd, Pittsburgh, PA 15328 if you want to visit.
Based in Myrtle Beach, SC. They are at 509 9th Ave N, Myrtle Beach, SC 29577
Originally published at https://www.einpresswire.com/article/634284312/robots-in-entertainment-art-and-film


The multi-zone evohome controller controls the time and temperature of up to 12 heating zones plus a domestic hot water supply if required. Each zone has independent time and temperature control.
It has a backlit colour LCD screen. The unit can either be wall mounted with an external power supply or table mounted on the table stand. This is powered by a 13amp power cable.
The evohome controller can be removed from the stand for easy programming for up to 2 hours before returning to the stand for charging.
The colour of the surround can be changed to either black or silver. A choice of idle screens, analogue or digital clock displays and a choice of day or night mode increases the aesthetic properties of the most visible part of the evohome system. When controlling the heating system the evohome controller has a number of features that make operation easy.
The evohome controller can act as a room thermostat on its own. Zone names can be input to match the actual property and the zones can be controlled as multi-room or as single-room zones. Zones can be configured to give individual local override of a room or rooms within the zone.
The programming in the unit is split between in-use and installation sections with a wide variety of control choices for the property owner. There is a guided scheduling wizard for the property owner to help them set up time and temperature profiles in each zone and a guided configuration facility for installers.
The screen will show energy efficiency messages to the homeowner as each zone’s temperature will be colour coded to show potential over or under set point temperatures.
The Heating Off button will turn all the zones off unless the frost protection temperature level is reached
The evohome controller provides access for the installer to fully control the heating system.
It is suitable for radiators, hydronic
under floor and a mixture of both types of heating. Cycle rates, minimum on times and fail safe parameters can be set.
OpenTherm® capability is built-in to work with OpenTherm® boilers (not hot water systems). Boilers can be controlled by either a boiler relay or an OpenTherm® bridge. You can programme a temperature offset for the evohome controller screen when used as a temperature sensor in its own right.
Hot water cylinder overrun and temperature offset levels can be programmed as can the hot water temperature itself. You can carry out a wireless Radio Frequency (RF) check using the evohome controller screen. Wireless binding of the system components is carried out via the touchscreen and additional sensors can be easily bound in post application if the zoning requirements change.
In-built optimisation features such as ‘delayed start’ and ‘optimum stop’ allow evohome to intelligently optimise start and stop times. A learning TPI algorithm ensures each zone adapts to the weather and the seasons.
Where there is a requirement to use a wireless room thermostat as a temperature sensor, both Honeywell’s DT92 Digital Room Thermostat and the Y87 Single Zone Room Thermostat can be included into the evohome system.
Easy to set up with two way wireless communication (30m range) and a signal strength indicator. Simple to operate with up and down buttons (DT92) or by a turn of the Dial (Y87). Individual zone temperature override.
5 to 35°C set point range offers a wide range of comfort levels. The DT92 digital thermostat can be wall mounted or free standing to ensure best possible positioning. Y87 Single zone thermostat is wall mounted
The remote relay is wired into either a motorised zone valve or the boiler and provides the wireless interface between these devices and the evohome system. The relay box requires a permanent 230V~ supply.
Control domestic hot water via the evohome controller screen by installing an externally- mounted wireless cylinder thermostat onto the hot water cylinder; Sensors are provided for vented and unvented cylinders.
Honeywell’s externally-mounted wireless cylinder thermostat is a combination of a temperature sensor that is mounted against the metal of the hot water cylinder and a wireless battery powered transceiver that communicates the water temperature to the evotouch controller.
By controlling the hydronic under floor heating via the Honeywell under floor heating controller, the time and temperature of the under floor heating can be achieved via the evohome controller screen.
The under floor heating controller provides for up to 5 zones of under floor heating and the optional extension controls a further
3 zones. It operates on the same 868 MHz frequency as the rest of the evohome components and has an internal antenna.
To enable the evohome system to be controlled remotely by either a wifi-enabled tablet or smartphone, a remote access gateway needs to be connected to the internet network within the property.
LED lights are provided on the gateway for feedback during binding and operation. This is achieved by connecting the supplied ethernet cable into the remote access gateway and a spare ethernet port on the home broadband router.
The remote access gateway is powered by a supplied 13 amp power supply and there is one button binding of the gateway to the evohome system via the evohome controller.
Once bound, the homeowner is then able to visit the total connect website to register their evohome system, uniquely identify their evohome remote access gateway (MAC ID & CRC code) and receive instructions on how to download the smartphone or tablet app that suits their Apple or Android system.
The smartphone or tablet app that enables remote control of the evohome system will be available for download from the iTunes Store and the Google Play store.
(Click to open Note: Not mobile compatible)


When a solenoid needs to be actuated based on a signal input condition, a major concern is how to obtain the best performance from the solenoid while at the same time minimizing heat dissipation. Solenoids are inductive loads, so when a voltage is first applied no current flows through the solenoid coil, and the voltage across the coil sits at the peak driving voltage. Then the voltage across the coil drops off exponentially while the current rises exponentially.
If the same voltage is applied during this time, an extremely high current would eventually flow which would cause unnecessary heat build-up. Initially, the voltage or current source driving a solenoid should be at the maximum to ensure that the solenoid’s armature is actuated. Once the armature is pulled in on the solenoid, voltage or current from the driving source should be dropped to a lower value that is just sufficient to hold the armature in place.
There are many ways that this can be accomplished by analog circuitry, where the output of the voltage source or current source is controlled by feedback from the solenoid coil or just designed to drop off as a function of time. This article discusses an alternative scheme, where the driving source voltage is either full “on” or full “off”, with no in between levels. The driver circuit output is digital instead of analog, and the voltage driving the solenoid is in a pulse-width modulated (PWM) format.
This technique is used in the DRV solenoid driver series from Texas Instruments (TI) - the DRV101, in particular, is discussed here. When a signal is sent the DRV101 to actuate a solenoid (pin 1), full power is applied to the solenoid initially. This full power signal is programmed to last only for a set time which is controlled by a timing capacitor, CD, connected to pin 2. This applied pulse should be long enough to make sure the solenoid is activated. After this pulse, the signal changes to a PWM signal with a programmed duty cycle that is set by a resistor, RPWM, connected to pin 3.
It is possible to connect pin 3 to a voltage or current source rather than a resistor. This source can be controlled by a feedback loop to maintain an optimum duty cycle time to keep the solenoid on while minimizing power requirements. The duty cycle is low enough so the current through the solenoid doesn’t reach a high value during the part of the cycle when full voltage is applied. Increasing the resistor value or voltage connected to pin 3 always results in a decreased duty cycle (100% duty cycle is full “on”, 0% is full “off”).
With the set duty cycle, the average voltage and average current output are set to some controlled value lower than the maximum, which reduces the power dissipation (P = VI) in the solenoid. The power dissipated in the DRV101 itself is also reduced to a very low level by using this PWM scheme, whereas in analog drivers, power dissipation is a problem. In the “off” state of the duty cycle, the current through the output driver is zero while its voltage is at the full supply voltage. In the “on” state, the driver current becomes high, but its voltage drop is almost zero. Therefore, over the full cycle, the power dissipated in the device is very small (according to P = VI). Because of the relatively low power dissipation in the DRV101, a heat sink is not usually required - but a heat sink should be considered anyway to increase reliability and to make sure that the maximum operating junction temperature (125°C) is not exceeded.
A unique feature of the DRV101 is that it can drive solenoids with a much higher pull-in current requirement than hold requirement. A constant DC voltage is applied initially for a set time period (100% duty cycle), and then switched to PWM mode with a set duty cycle (10% to 100%) to save power. The period (cycle time) of the PWM mode in DRV101 is constant, and is set by an internal 24kHz oscillator. This frequency is not externally adjustable.
The voltage supply and ground connections are made to pins 5 and 4 respectively on the DRV101. Pin 5 is the output that actually drives the solenoid. There is one pin left (pin 7), and this is used as a status flag, and provides a fault indication for over-current, under-current, and thermal shutdown conditions. An over-current fault condition occurs when the output current is greater than about 2.3 A. An under-current fault is when the output current is below the under-scale current threshold (about 23mA). These two fault indications would detect both open and short circuits in the drive circuit. A thermal fault occurs when the device reaches a temperature of about 165°C. The device is shut off during a thermal or over-current fault.
The DRV101 is suitable not only for driving electromechanical loads such as relays, actuators, and solenoids, but it can also work as a linear driver for valves, positioners, heaters, and lamps. The DRV101 comes in a 7-lead staggered TO-220 package and a 7-lead surface-mount DDPAK plastic power package. It has an output drive capability of 2.3 A, and a voltage supply range of 9V to 60V. It is rated to operate over the extended industrial temperature range of –40°C to +85°C. Other solenoid drivers in this family include the DRV102, DRV103, and DRV104. The DRV102 has a 2.7 A output current drive capability, while the DRV103 and DRV104 have adjustable internal oscillators and a lower 8V to 32V voltage supply range.
Daniel is a recent graduate of the City University in London. His Masters thesis looked at how companies use social media for marketing purposes. In his first degree, he focused on electric engineering.


A paint industry in Oman for its operation, a central load controller with a second/any higher order model is to be designed. The control systems should be equipped with a motor constant and a suitable amplifier gain to balance the operation. The controller should be designed to operate with unity feedback H(s)=1. The system is expected to perform on unit step/ramp input. Obtain the response for such system if the amplifier gain is varying within the rage of 20 to 55, using mathematical evaluation.
The system is also expected to be dealt in noisy environments. In such a case, if a disturbance is acting on the system with a unit step phase, determine the steady state error component for the system for the same amplifier gains. Also critically analyze the impact of disturbance on the system to ensure the stability and obtain the responses in mathematical evaluation.


Electric Vehicles or EVs have brought in a revolution in the automobile industry. However, the innovative infrastructure that comes with EVs makes them expensive for the masses. But Hybrid Electric Vehicles act as the bridge between the traditional cars and the electric cars.
What is a HEV?
An Hybrid Electric Vehicle uses the combination of an Internal Combustion (IC) engine and an electric propulsion system. The electric powertrain enhances the fuel efficiency and performance of the vehicle. It can also independently propel the vehicle on just the electric power, depending on the kind of hybrid system you choose.
An HEV has a conventional fuel engine as well as an electric powertrain, as the electric motor assists the engine to extract more performance and better fuel economy.
The Working Mechanism of HEV
An HEV is powered by an IC engine and electric motor. The electric motor utilizes the electrical energy stored in the battery pack, which is charged through regenerative braking that is run by the internal combustion engine. An HEV does not need to be plugged into a power source to charge the battery.
The additional power from the electric motor assists the engine, and it enhances the performance and fuel economy. The battery pack can also power other electrical components like lights. The powertrain saves fuel with the engine start/stop technology. In an HEV, the engine automatically shuts off when idle and starts automatically when the throttle pedal is pressed.
The main components of the HEV are the internal combustion engine and the electric motor that are responsible for propelling the vehicle and maintaining its performance and fuel economy. It also consists of a battery pack, generator, transmitters and a fuel tank, much like a conventional vehicle.
There are three types of Hybrid Electric Vehicles are series hybrid, parallel hybrid and series-parallel hybrid. The division is based on the power delivery and distribution.
Hybrid Electric Cars stand out for their fuel efficiency, and with the help of electric powertrain, they function more efficiently than their conventional counterparts. The most common HEVs in India are Toyota Camry, Lexus ES 300h Exquisite, Maruti Suzuki Brezza, and Maruti Suzuki Ertiga.
The automobile industry is continuously innovating and so are we! With Kuwy, the best finance deal for your dream car is just a few clicks away. We are India’s first automotive fintech platform that connects dealers, lenders, OEMs and customers digitally. We simplify auto loans by facilitating end-to-end digital loan approval, processing and disbursal. Download the Kuwy app to know more about the ease of car finance!


Keep an eye on your home or business - even when you’re away using your Smartphone or Tablet.
A Bespoke Security CCTV (Closed Circuit Television) is a video surveillance system, purposefully designed for monitoring a variety of locations,
areas and activities. For retail, commercial or residential, have that edge with your security with multi-camera viewing of critical locations
such as entrances, Point of Sale areas, stockrooms and warehouses. The CCTV images can now be viewed live using your iPhone or Android
Smartphone while they are recorded locally or remotely on to a NVR (Network Video Recorder) for viewing later.
With completely customisable recording hard drives, we can offer you footage recording ranging back from a few weeks to months*.
CCTV was originally developed as a means of security for banks and casinos however; today it has been developed to the point where it is simple
and inexpensive enough to be used with any business and home security systems for everyday surveillance. Individuals and businesses today
install CCTV as a major deterrent against theft, robbery and fraud. There are many CCTV solutions in the market today, including the older
analogue technology and the modern Digital IP systems.
Bespoke Security & Screens offers state of the art and the very latest in technology in there camera systems to give reliability and superior
image quality. Camera types such as full body cameras, bullet camera’s, dome cameras, variable zoom cameras and fixed lens, to fully motorised
and controllable PTZ cameras.
Bespoke Security Brochure (34788 KB)
Bespoke CCTV Brochure (1121 KB)
CCTV Cameras Gallery


Recently, there will always be news on the Internet or on TV that rains have fallen in a certain area. Residents need to go boating. Extraordinary heavy rains not only cause a lot of inconvenience to people’s travel, but also may cause some LED display manufacturers’ warehouses. Water inflow causes water in the display screen body. In response to this phenomenon, what measures should LED display manufacturers for MRV 300 take in the face of water in the screen body? Let’s explain the solution of water in the LED display screen.


Ningbo Honghzou Electronics Development Co., Ltd was founded in 1985, a Chinese famous company in maritime technology sphere, specializing in the design, manufacture , marketing and service of marine electronics, navigation and radio equipment. Ningbo Hongzhou Electronics Development Co., Ltd has been established good relationship with many leading radio electronics company in Europe, America, Japan, Korea etc.
The distribution in China to: Jotron AS (SART, EPIRB,TWO-WAYS, AIS, )
Pole Star Electronics UK ( SSAS )
Ningbo Honghzou Electronics Development Co., Ltd has been set up wide radio service network for providing convenient and quickly service at main port in China and the company was qualified bu CCS in Jan ,2002 and also obtained the qualification certificates of radio inspection by ABS, BV, NK, LR, KR, DNV, CR, GL, RINA , etc .
Ningbo Chiyang Electronics Co., Ltd was belongs to Ningbo Hongzhou Electronics Development Co., Ltds, which involving in the field of the manufacturing and researching in marine products, the following marine products are launched and all type tested and approved CCS :
Lithium Battery package for all types of SART , EPIRB ,TWO-WAYs.
Hydrostatic Release Unit (HRU) for all types EPIRB
Together with all member of Ningbo Hongzhou Electronics Development Co., Ltd. President Mr. Lin Lijun warmly and heartily greets all friend home and abord, welcomes all of you contact with us for business and cooperation .