Performance of FFH-GMSK signals with centre sampling single and double differential detection in frequency-selective Rayleigh channels with CCI

It is well known that classical (single) differential detection systems yield poor performance for channels that introduce into the carrier a random frequency shift, such as mobile radio (MR) channels. In such situations, one must resort to a form of differential detection (DD) that has the capability to remove the effect of frequency variations. The object of this paper is to study the performance of centre sampling double differential detection (CSDDD) system and compare its performance with the well-known centre sampling differential detection (CSDD) system. A fast frequency hopping (FFH) spread spectrum (SS) using Gaussian minimum shift keying (GMSK) signals transmitted through severely restricted bandwidth (BW) Rayleigh channels are used for data modulation. The performance of the proposed system is analyzed in a two-path Rayleigh fading channel taking into account frequency-selective fading (FSF), carrier random frequency shift, intersymbol interference (ISI), Doppler frequency shift, cochannel interference (CCI) and additive white Gaussian noise (AWGN), while the receiver (RX) rejects a specified amount of adjacent channel interference (ACI). It is shown that the CSDD scheme is sensitive to frequency offset while the CSDDD scheme has a potential to combat the effect of frequency shifts and its performance is better than the CSDD scheme when random frequency shifts are introduced into the carrier. Furthermore there is a significant improvement in the error rate performance by utilizing the frequency diversity inherent in the FFH system compared to other systems. Therefore, this system can accommodate more users than the others and a better spectrum efficiency can be achieved.