Coded modulation for direct-sequence spread-spectrum communications.
B.D. Woerner · Deep Blue (University of Michigan) · 1991
In this thesis, we consider the application of coded modulation techniques to a direct-sequence spread-spectrum (DS/SS) communication system. Coded modulation or trellis coding combines the operations of error correction coding and signal modulation into a single step. We define a class of trellis codes, constructed over a biorthogonal signal constellation, which is suited to a DS/SS system. The biorthogonal signal constellation has constant amplitude and takes full advantage of the multidimensional signal space available in a DS/SS system. We present examples, and discuss several properties of these codes. We also present techniques for implementing these codes in a DS/SS system. We analyze the performance of trellis coded modulation in a DS/SS system with known deterministic sequences. We present an algorithm for computing the bit error probability for this system as a function of the number of interferers and the signal to background noise ratio. This technique is applicable to any coded system with soft-decision decoding. We verify the accuracy of our analysis with simulations. We use these analytic tools to demonstrate that the performance of several trellis coded systems compares favorably with the performance of uncoded systems and systems employing other coding techniques. We present an algorithm for computing the bit error probability of a trellis-coded DS/SS system using a model of randomly generated signature sequences. We also present a reduced complexity approximation. We derive an expression for the packet error probability of a trellis coded system. We also present a bound on the packet error probability of a DS/SS system which employs a conventional convolutional code and hard-decision decoding which is tighter than the best previously known bound. Finally, we consider a channel model which includes the effects of Rician fading. We analyze the performance of a coded system for three different cases. These cases include a system without interleaving, a system with interleaving, and an interleaved system which makes use of side information about the state of the channel. We present numerical results which indicate that performance is improved greatly by interleaving, but only marginally by the addition of channel state information.