Performance of CRC-Aided Erasure Demodulation for M-ary Chirp Spread Spectrum Signal

In this paper, erasure demodulation for an M-ary chirp spread spectrum signal is proposed. This demodulation scheme is assumed to be implemented in IoT devices to receive a deactivation command. In the erasure demodulation scheme a demodulator regards bit elements in a coded bit sequence as erasure if the corresponding correlator output is larger than a threshold and they are different from bit elements in a coded bit sequence for the maximum correlator output. Those erasure bits are treated as both "0" and "1" and cyclic redundancy check (CRC) decoding following the erasure demodulation checks which output is correct. As one of those outputs must be the same as the transmit coded bit, block error rate (BLER) performance improves. Even though the erasure demodulation increases the probability of false alarm, it can reduce the miss probability of deactivation. On the other hand, as the number of erasure bits increases, the number of CRC decoding operations grows exponentially and the probability of CRC miss detection also increases. Thus, the threshold level has to be decided according to the computational capability of each IoT device, the target BLER, and the probability of CRC miss detection. The performance of CRC miss detection and the BLER on a Rayleigh fading channel, a Rician fading channel, and a Vehicular-B channel are evaluated through computer simulation. Furthermore, cumulative distribution function curves for the number of erasure bits are also presented for different channel models.