The present embodiments relate to wireless mesh communication system and, more particularly, to a network with wideband beacon channels for frequency hopping systems.
A wireless mesh network is a type of wireless communication system where at least one wireless transceiver must not only receive and process its own data, but it must also serve as a relay for other wireless transceivers in the network. This may be accomplished by a wireless routing protocol where a data frame is propagated within the network by hopping from transceiver to transceiver to transmit the data frame from a source node to a destination node. A wireless node may be a wireless access point such as a wireless router, a mobile phone, or a computer capable of accessing the internet. In other applications, the wireless node may be an external security monitor, a room monitor, a fire or smoke detector, a weather station, or any number of other network applications for home or business environments.
FIG. 1 shows an exemplary wireless network of the prior art as disclosed in version 0v79 of the 2013 Wi-SUN Alliance Field Area Network Working Group, which is incorporated by reference herein in its entirety. The network includes an internet access circuit 150. The network also includes Personal Area Network (PAN) circuits A through C. Each of PAN communicates with circuit 150 through respective Master nodes MA 100, MB 120, and MC 130.
PAN A is an exemplary network that may be similar to PANs B and C. PAN A communicates with circuit 150 through Master node MA 100. MA 100 communicates directly with relay node (RN) 102 and with leaf node (LN) 114. Thus, MA 100 is a parent node of RN 102 and LN 114. RN 102 is a parent of RN 104 and communicates indirectly with LN 106 via RN 104. RN 102 also communicates directly with RN 108 and indirectly with RN 110 via RN 108. RN 108 also communicates directly with LN 112. RN 108 is a parent of both RN 110 and LN 112. Frequency Hopping Protocol (FHP) is often used within the network to reduce interference and provide frequency diversity.
Frequency hopping is used for many narrowband communication systems in the United States because the FCC regulations (15.247) allow higher transmit power for narrowband frequency hopping systems in the bands 902-928 MHz, 2400-2483.5 MHz, and 5725-5850 MHz. For the 902-928 MHz band if at least 50 hopping channels are used with 20 dB bandwidth less than 250 kHz, then 1 watt of transmit power can be used. For a 20 dB bandwidth between 250 kHz and 500 kHz then at least 25 hopping channels are needed. If frequency hopping is not used for these narrowband systems then they would fall under regulation 15.249, where the transmit power would be limited to −1.25 dBm or 0.75 mW. This is over 1000 times lower transmit power than the frequency hopping system. For wider bandwidth systems using digital modulation techniques, 1 watt of transmit power can be used if the 6 dB bandwidth is at least 500 kHz.
IEEE 802.15.4g defines frequency hopping systems for smart utility networks (SUN) using one of three physical layers: frequency shift keying (FSK), orthogonal frequency division multiplexing (OFDM), or direct sequence spread spectrum (DSSS). DSSS may also be referred to as offset quadrature phase shift keying (OQPSK). For the 902-928 MHz band there are 129 channels with a 200 kHz channel spacing or 64 channels with a 400 kHz channel spacing (FIG. 2). Both of these definitions meet the number of hopping channels required by 15.247, so 1 watt of transmit power can be used. One challenge with SUN frequency hopping systems is that the join procedure for a new node may take a long time.
When the network uses a star configuration where there is a central hub, then the central hub can transmit a beacon that other nodes can use to learn the hopping sequence so that they can join the network. However, often a mesh network is used where there is no central hub. In a mesh network each node can transmit to a neighboring node until the message reaches a data concentrator or network master which would be connected to a backbone to transmit data to the utility. For the reverse direction messages can hop from node to node to reach a leaf node. For a new node to join a mesh network it would need to either camp on one channel and wait until a neighboring node transmits a beacon message or it would need to be able to scan many channels sequentially to find a beacon.
This problem is illustrated with reference to FIG. 3. When powered up the slave nodes must perform an acquisition to find out where in the sequence of frequencies the master resides. A new acquisition may also have to be performed if synchronization is lost, for example, due to noise or being temporarily moved out of radio range. In the latter case the slave has an idea of which frequency the master is transmitting on, and this information is exploited to achieve faster acquisition. When performing acquisition, the slave must always listen for one full period at each frequency that is being examined to be sure of picking up the beacon. It starts at the frequency that it believes to be most likely and then moves on to the most nearby frequencies in the pseudo-random sequence. This is typically a random selection when the slave is powered up. At time 0 (first row), if the first master frequency guess is 0 (second row), the closest frequencies are 0, 1, −1, 2, −2, 3, −3, and so forth. However, since the master is also stepping through the frequencies, this sequence must be shifted by the sequence 0, 1, 2, 3, 4, 5, 6, etc. The actual frequencies examined, therefore, are 0, 2, 1, 5, 2, 8, 3 (third row). If all frequencies are examined without result, the acquisition sequence must start over again. Assuming the failure is due to the slave being out of radio range, the application may choose to have a delay between each acquisition to reduce power consumption.
For networks of the prior art, when the slave is powered up and does not know where the master is hopping, it chooses a random frequency and listens. It can take a long time until it finds a beacon from the master. For example, if there are 50 hopping frequencies, then the slave may need to listen through 50 beacon cycles to find a beacon if there is no fading or interference. If beacons are missed due to fading or interference, it can take much longer to find a beacon. A typical beacon packet format of the prior art is illustrated at FIG. 4 together with a data packet and acknowledge/negative acknowledge (ACK/NACK) packets. Each packet includes a preamble, packet length, and cyclic redundancy check (CRC) field. The beacon and ACK/NACK packets also include a source identification (ID) field. The beacon packet also includes control information which specifies the network frequency hopping protocol (FHP) and other network information to the slave.
Although network proposals of the prior art provide steady improvements in wireless network communications, the present inventors have recognized that still further improvements in mesh network protocol are possible. Accordingly, preferred embodiments described below are directed toward this and other improvements over the prior art.
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