TIME AND CARRIER SYNCHRONIZATION IN MULTIMODE CPM-DSSS SCHEME FOR POINT-TO-POINT COMMUNICATION

Abstract

Point-to-Point (P2P) communication is employed in several fields, such as military systems, 5G mobile communication, to improve the spectral efficiency and off shed load in cellular systems. The general requirements of P2P communication are high level of reliability, throughput, spectral efficiency, and infallible security. In P2P communication, the devices frequently enter or leave the network; therefore, the security requirements of such links are of paramount importance, where the possible solutions are either narrowband or wideband communication. Wideband communication is often used to achieve the high data rate requirements of P2P communication. Direct Sequence Spread Spectrum (DSSS) is a viable solution to provide security at high-data rates, but it suffers inefficient spectral utilization. Continuous Phase Modulation (CPM), on the other hand, is a constant envelop signaling scheme known for high spectral efficiency. Thus, Continuous Phase Modulation–Direct Sequence Spread Spectrum (CPM-DSSS) can be used for modern P2P communications. Synchronization is considered a vital activity in a wireless communication system. It involves estimation and compensation of some reference parameters for reliable detection of the incoming signals. Examples of reference parameters include timing offset, carrier frequency offset and carrier iii phase offset. The acquisition of these parameters is considered as a challenging task because most of the wireless communication systems operate under harsh channel conditions such as strong fading, low Signal-to-Noise Ratio (SNR), and high interference. Like other communication systems for reliable detection of data, synchronization of CPM-DSSS based systems is also required. The major objective of this work is to provide reliable high throughput for P2P link supporting a variety of wireless communication applications using multiple modes of operation for CPM-DSSS scheme. The objective is achieved by developing a joint timing and phase offset estimation algorithm and devising a joint carrier frequency and phase offset estimation algorithm for CPM-DSSS scheme. This research is divided in two phases. First phase of this work presents the three modes of operation for CPM-DSSS, i.e., low data rate reliable mode, moderate data rate reliable mode and high data rate mode. The low data rate mode of CPM-DSSS has a higher throughput as compared to the standard BPSK-DSSS based scheme. These modes are suitable to address the diverse data rate requirement of modern P2P communication. In second phase of this work, we focus on the time and carrier synchronization of CPM-DSSS based P2P communication, where transmission is considered in the form of bursts. Each burst comprises two parts, the training sequence, and the actual data. Firstly, the entire burst is spread using DSSS and then modulated with CPM to form a CPM-DSSS burst which is later upconverted and passed on to the channel. The channel adds various effects on the transmitted signal, for example, Additive White Gaussian Noise (AWGN) and unknown timing, phase, and frequency offsets. After down-conversion, the proposed algorithms estimate the reference paramiv eters from the received spread training sequence at the receiver. We have proposed two algorithms for the estimation of the reference parameters. The first is the joint timing and phase offset estimator, and the second is the joint carrier frequency and phase offset estimator. Performance comparison of the proposed algorithms is performed with the Modified Cramer-Rao Bound (MCRB). For both the algorithms, timing, frequency, and phase offset estimators’ performance is approaching the MCRB, which is authenticating the validity of both the algorithms. Simulations are also carried out to demonstrate the effectiveness of the proposed algorithms in multipath fading channels and comparison of its performance is performed with the AWGN channel. Both the algorithms are tested for more detailed packet structures by varying symbol and training sequence lengths. Simulations were carried out on both the algorithms for higher order modulation schemes as well. The results suggest that in the case of all estimators, the performance of both the algorithms for different modulation schemes and for different packet lengths approaches the AWGN channel at almost all SNRs.