Performance of orthogonal stochastic code signals in the presence of delayed interference

Abstract

This article provides analytical expressions for performance evaluation of hybrid FH-DSSS system in delayed interference and fluctuation noise presence. Delayed interference is quite common in environment with multiple obstacles, occurring as consequence of signal reflection and multipath propagation (fig. 1). Introduction describes known methods of mitigating this interference many of whom include either use of direct sequence spreading or some variation on hybrid sequence spreading including pseudo random frequency hopping and periodical signal ensemble change. To improve on those methods considered dynamic program method for orthogonal stochastic spread spectrum code generation based on Viterby algorithm [6]. Section 1 of the paper describes FH-DSSS signal, it’s delayed interference, equation corresponding to their sum on the receiver input and noncoherent correlator response to it. By using Gaussian approximation, expressions derived for equivalent signal/noise level estimation and corresponding average probability of error at the output of the correlation receiver. By including actual signal statistic, like code length and cross-correlation value, final expressions (8)–(15) allow for accurate estimation of correlation receiver response to signal sequences with specific spread spectrum codes, as well as calculation for probability of error for M-ary spread spectrum communication system in delayed interference presence. Accuracy of the Gaussian approximation is examined in section 2 of the paper based on comparisons between analytical results and simulation modeling. Numerical results obtained, which showcase dependence of system performance on various signal characteristics, such as correlation properties, spreading code length, signal ensemble size and frequency hop delay time. Plotted graph on fig. 2 shows advantage from using M-ary orthogonal signals over binary in delayed interference presence for code sequences with length N = 32, 64, 128. Graph on fig. 3 shows gains from frequency hopping when interference delay equals to approximately half of signal information symbol. In the end performance gains from using hybrid FH-DSSS system under electronic suppression by delayed interference is also evaluated. As shown on fig. 4,a system performance significaly depend on value of delay between received signal and it’s delayed copy.