Issue link: https://read.uberflip.com/i/1428395
LoRaWAN ® L2 1.0.4 Specification © 2020 LoRa Alliance ® Page 71 of 90 The authors reserve the right to change specifications without notice. 2151 Note: T is GPS time and, unlike Unix time, it increases strictly 2152 monotonically and is not influenced by leap seconds. 2153 2154 2155 T BeaconDelay is 1.5 ms. It allows a slight transmit delay (1.5 ms) of the gateways required by 2156 their radio system to switch from receive to transmit mode. 2157 The variable T Accuracy denotes the timing precision guaranteed by the gateway. T Accuracy 2158 SHALL be provided by the gateway manufacturer with the set of operational conditions 2159 required to guarantee it. T Accuracy SHALL be less than or equal to the timing precision 2160 indicated by the Prec field of the beacon T Accuracy ≤ 10 −6+ Prec s. 2161 All gateways participating in the broadcast of the Class B beacon must be synchronized. 2162 Depending on the timing precision that can be guaranteed by the gateway, two possible 2163 modes of the Class B beacon transmission are possible. 2164 T Accuracy ≤ 1 µs: gateways tightly synchronized to GPS time. 2165 If the gateway transmissions can be synchronized to the GPS clock with an accuracy of better 2166 than 1 µs, the gateway MAY transmit the beacon every 128 s. The Prec field of the beacon 2167 is set to 0, indicating a timing accuracy of better than 1 µs for the listening end-devices. 2168 T Accuracy > 1 µs: gateways loosely synchronized to GPS time. 2169 If gateway transmissions can be synchronized to the GPS clock with an accuracy of better 2170 than 1 ms, but an accuracy of 1 µs cannot be guaranteed, the transmission of a Class B 2171 beacon SHALL be randomized. For each beacon, the gateway SHALL draw a random number 2172 P with a uniform distribution between 0 and 1. The gateway transmits the beacon if P < 2173 P Beacon . If P ≥ P Beacon , the gateway remains silent and does not transmit the beacon. 2174 The parameter P Beacon exists on the gateway and MAY be remotely set by the Network 2175 Server. The value of P Beacon SHALL be ≤ 0.5 (50%). Different gateways may use different 2176 values of the P Beacon parameter. 2177 The parameter P may be a pseudo-random number but, in this case, each gateway in the 2178 network SHALL use a different seed, resulting in a unique series of P values. 2179 If the Class B beacons of two or more loosely synchronized gateways reach the antenna of 2180 an end-device with equivalent power, the end-device may not be able to demodulate the 2181 beacon. The timing difference between the colliding beacons may be too great to be 2182 compensated correctly by the end-device's demodulator. To avoid systematic beacon collision 2183 at the antenna of a stationary end-device, randomization must take place. Each loosely 2184 synchronized gateway randomly transmits the beacon no more than half of the time. 2185 Therefore, although beacon collisions do happen at the end-device's antenna, they are not 2186 systematic, and the end-device can still demodulate the Class B beacon with sufficient 2187 probability to operate in Class B mode. The P Beacon parameter should be optimized by the 2188 network infrastructure based on the average number of gateways that local end-devices can 2189 receive. The denser the gateway population becomes, the higher the probability of beacon 2190 collision, so the lower the parameter should be. The optimal parameter value depends on too 2191 many network-driven factors and is beyond the scope of this specification. 2192