A Novel Four Dimensional CDMA Modem for Wireless Multimedia Communications

Abstract

A description and analysis of a four dimensional (4D) Direct Sequence Spread Spectrum (DSSS) modem architecture, comprising a transmitter and a receiver, are presented. The DSSS modulator employs Complex Spreading Sequences (CSS). Two integrated synchronization loops are incorporated in the DSSS receiver, namely a four dimensional Decicion-Directed Costas Carrier Recovery Loop (DD-CCRL) combined with a 4D Decision-Directed Complex Delay-Locked-Loop (CDLL). These loops are analysed and the system performance in Additive White Gaussian Noise (AWGN) verified by means of computer simulation. It is shown that the new DSSS modulation method does not suffer any losses compared to a conventional BPSK DSSS modem, whereas the multidimensional spreading technique achieves four times the data throughput rate of a BPSK DSSS system and two times the throughput of a QPSK modulation method in the same bandwidth and with identical spreading sequence lengths. The multidimensional spreading technique therefore offers extra flexibility in terms of the choice of data rate and DSSS processing gain achievable for a given DSSS transmission bandwidth without loss of performance compared to BPSK and QPSK in AWGN. Similar results are achieved in fading channels. A prototype of the new multidimensional DSSS modulation method is presently implemented with forefront Field Programmable Gate Array (FPGA) technology with a view of realising a multimedia wireless communication system for rural areas.