Offset Tolerant Demodulator for Frequency/Phase Modulation in Time-Varying Channel

Carrier frequency offset (CFO) and time-varying fading channels are two problems that emerging ultra-narrowband solutions for Internet of Things and Wireless Sensor Networks need to tackle. Previously, an offset tolerant demodulator for Double Differential PSK (DDPSK) has been proposed to overcome CFO. To combat channel distortion, time or frequency diversity together with coding techniques can be utilized. However, these necessitate transmitting extra bits and leads to an increase in packet time. A longer packet time corresponds to longer on-time of the RF front-end and more power consumption. To avoid longer packet for the same symbol rate, a higher order modulation is required which considerably degrades BER performance. In this work, instead of higher order DDPSK, hybrid frequency/phase modulation is used. An offset tolerant demodulator for hybrid modulation is proposed which provides the same robustness against CFO as DDPSK. Besides, it provides higher order of modulation with less performance loss compared to higher order DDPSK. Simulation results show that a combination of BFSK and QPSK using the proposed demodulator can achieve almost 4 dB improvement at BER=0.001 in a Rayleigh channel compared to Double Differential 8PSK (DD8PSK) which can provide the same modulation order.