The invention is in the class of terrestrial cellular digital radio systems that provide demand assignment multiple access (DAMA) communications service to subscriber terminals equipped with narrowbeam radio antennas. Thus, the subject class of cellular radio systems has a set of terrestrial radio base stations distributed over the service area, where each base station is equipped with sector antennas, and each sector antenna has associated transmitter/receivers that communicate via frequency division multiplexed (FDM) radio wave with subscriber terminals in the vicinity of the base station. Communications is two-way with radio wave transmission in the direction from base station to terminal termed the downlink direction, and in the direction from terminal to base station termed the uplink direction. Each subscriber terminal, has a narrow beam antenna that is pointed into the aperture of one of the base station sector antennas assigned to the terminal. The terminal antenna has an associated transmitter and a receiver tunable to assigned sector FDM channels. The channel assignment originates in a service center on a DAMA basis.
The FCC allocates a block of radio spectrum for specified terrestrial cellular radio services. The radio frequency band allocated to a cellular radio system is a precious natural resource, and it is of utmost concern to national and international regulatory agencies that the allocated radio spectra be efficiently used. For the cellular radio service providers increased spectral utilization efficiency implies reduction of the density of base stations required to cover a given service area. The resultant reduction of infrastructure cost may then result in reduction of service cost to subscribers. Thus, the realization of increased spectral utilization efficiency of cellular radio systems, which is the objective of the invention has important economic and social benefits. Cellular radio systems employ frequency reuse of the block of spectra allocated to the cellular system in order to achieve high spectral efficiencies. Typically the allocated block is partitioned into two parts, one part for radio wave transmission from base stations to the subscriber terminals, hereafter defined to be the uplink direction, and the other part for radio wave transmission from terminals to base stations, hereafter defined to be the uplink direction. The uplink part is further partitioned into frequency-division multiplexed (FDM) channels, and the channels are partitioned into N groups of channels F1, F2, . . . , FN, where the number N is defined as the frequency reuse number. Each sector antenna of the cellular system is allocated one of the channel groups in such a way that for large numbers of sectors, the frequencies are reused. All sectors assigned the same channel group are termed co-channel sectors. A natural consequence of frequency reuse in cellular radio systems is that transmissions in the channel of one sector have the potential to interfere with transmissions in nearby co-channel sector and such interference is termed co-channel interference. Cellular systems utilize frequency allocation patterns that approximate idealized regular patterns known to have minimal co-channel interference levels for a given frequency reuse number N. The level of co-channel interference in a channel is defined in this disclosure as significant if it causes the channel to fail a defined channel quality criterion such as bit error rate.
In this disclosure spectral efficiency is defined as E=MP/N where M is the number of sectors per cell in a cellular pattern, and N is the number of channel groups. The efficiency thus equals the number of times the allocated frequencies are reused at each base station. The factor P is accounts for polarization usage and is generally equal to one. In the special case when both polarizations of a channel group are allocated to a sector, then P=2. For fixed base station complexity (M fixed) in general, reduction of the frequency reuse number N in an attempt to increase efficiency tends to generally increase the level of co-channel interference in the system so that there exists a fundamental tradeoff of spectral efficiency with the level of co-channel interference present in the network.
Conventional DAMA channel assignment algorithms associated with prior art typically treat all terminals uniformly without attempting to ascertain the co-channel interference characteristics of the terminal. A drawback to the conventional approach is that in order to guard against the random occurrence of unexpectedly large co-channel interference in assigned channels the use of spectrally inefficient cellular patterns with large reuse number N is needed, particularly when a guaranteed channel quality criterion is required by terminals.
Typically, design in prior art of cellular radio systems have been based on the requirements of mobile voice telephony service. However, the data transport requirements of an emerging new generation of broadband telecommunications services to be offered on cellular radio system are much more demanding in terms of the cellular system channel quality. Examples of such new generation services are interactive multimedia such as video conferencing, and video on demand. Two new data transport requirements that characterize the new generation services are (1) guaranteed channel quality and (2) DAMA channel assignment with short response times, typically measured in milliseconds or tens of milliseconds. Guaranteed channel quality means that channels that are assigned to terminals are guaranteed to meet some specified channel quality criterion such as bit error rate.
The allocation of ever higher frequencies of the radio spectrum by the FCC enables the use of relatively narrower terminal antennas of reasonable cost and size. For example, spectral allocation for local multipoint distribution systems (LMDS) has been recently allocated at 28 Ghz, and for that application low-cost terminals with beamwidths of a few degrees is feasible. Narrow beams have a known advantage in terms of mitigating multipath interference and reduction in the general level of co-channel interference. The objective of the present invention is to take advantage of the fact that the proportion of significant co-channel interferers in the system is typically very small so that special treatment in channel assignment for that population is possible. The objective of the present invention is to provide DAMA channel assignment under a guaranteed channel quality criterion and with much higher spectral efficiencies than prior art. This is achieved by measuring the co-channel interference characteristics of terminals. Knowledge of terminal interference characteristics is used by the DAMA channel assignment algorithm to control the co-channel interference in a systematic way so that all significant uplink co-channel interference is diverted into a relatively small number of unassigned channels. Similarly, the algorithm minimizes the number of downlink channel assignments that can induce significant downlink co-channel interference, and prohibits the assignment of those channels. The invention is applicable for example to LMDS systems.
The invention is a terrestrial cellular radio system comprised of a plurality of base stations distributed about a service area, and a plurality of subscriber terminals in the service area. Each base station has M sector antennas that service an area termed a cell that is centered at the base station. Each antenna defines a cell sector that has a channel FDM subgroup allocated for uplink communications and an FDM channel subgroup allocated for downlink communications in a cellular reuse pattern. Each terminal is assigned to one of the sectors according to a sector assignment algorithm, and the terminal""s antenna is accordingly pointed at the sector antenna to which it is assigned. Terminals request and receive assignment of channel resources from the channel group allocated to the sector from a DAMA channel assignment algorithm.
A unique feature of the invention is that its DAMA channel assignment procedure utilizes a database of prior information about the co-channel interference characteristics of terminals to assign channels in such a way that excessive co-channel interference gets harmlessly concentrated into a relatively small number of channels marked for interference and not assigned for communications. In this way the channel assignment algorithm is capable of offering guaranteed channel quality while avoiding the large frequency reuse numbers required in prior art to achieve such guarantees. In this way the invention maintains a high level of spectral utilization efficiency. The invention provides for automated means to generate the database of prior information about terminal characteristics, and the database is also used to ensure that adjacent channel interference is not excessive and received signal power is adequate to meet the guaranteed channel quality criterion.
The invention is based on two key observations about uplink co-channel interference when subscriber terminal antennas have narrow beamwidths: (1) typically only a small proportion of terminals are significant co-channel interferers, and (2) when a terminal is a significant co-channel interferer its transmissions typically only interfere with at most one nearby co-channel sector. In the invention, the DAMA uplink channel assignment procedure assigns channels to the relatively small number of subscribers known to be co-channel interferers in such a way that the co-channel interference always falls in one of a relatively small number of channels reserved only for such interference. The channels marked for interference are not assigned for communications in the sector in which the interference occurs so that significant co-channel interference is virtually eliminated from the channels assigned and used by terminals. Although the channels marked for interference represent wasted overhead, they are not only small in number relative to the totality of available channels, but also smaller in number than the population of terminals that are significant co-channel interferers. This is because under the DAMA assignment algorithm each unused channel marked for interference typically contains co-channel interference from more than one terminal. The invention works similarly for downlink assignments except that the location of interferer and interfered radio is reversed; i.e., certain downlink channels are marked as unassignable in certain sectors because they cause co-channel interference in terminal receivers in neighboring co-channel sectors.
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