Frequency Offset Tolerant Demodulators for UNB Communications

This work focuses on one of the prominent challenges in UNB communications which is Carrier Frequency Offset (CFO). CFO might be a consequence of the mismatch between oscillators in the transmitter and the receiver or the Doppler shift caused by the relative movement of the nodes. To overcome the CFO problem in UNB communication, offset tolerant demodulators are considered in this thesis while focusing on achieving scalable offset tolerance. In other words, the demodulator should obtain the same BER value for a certain Eb/N0 regardless of how large the frequency offset is. First, the performance of two offset tolerant demodulators for Frequency Shift Keying (FSK) and Phase Shift Keying (PSK) are investigated to find their limitations when a large CFO tolerance is needed. For FSK, a Discrete Fourier Transform based (DFT-based) demodulator is considered and for PSK an autocorrelation based demodulator for Double Differential PSK (DDPSK) is chosen. The limitation of the DFT-based demodulator is its complexity which stems from its window synchronization algorithm. Hence, a low complexity window synchronization algorithm is introduced with an efficient implementation. Furthermore, the performance of the PSK demodulator degrades when large CFO tolerance is needed. This is due to the increased noise bandwidth which is a consequence of increasing the bandwidth of the filter prior to the demodulator in presence of a large frequency offset. To tackle this problem, a demodulator based on shifted correlation is proposed for DDPSK modulated signals. In addition to the CFO, UNB communication systems are vulnerable to a time-varying fading channel. To combat distortion in a time-varying channel, time and frequency diversity techniques together with channel coding and interleaving can be utilized. To afford the redundancy required for these solutions, a higher bitrate is required. To increase bitrate while achieving a better power and bandwidth efficiency trade-off a hybrid Frequency/Phase modulation (FPSK) is a modulation is considered in this thesis. As a solution for CFO, an offset tolerant demodulator is proposed for FPSK modulation. Moreover, the performance of this modulation is considered in a time-varying channel while using a system designed for including time diversity.