Multi-access properties of transform domain spread spectrum systems

Summary form only given, as follows. This paper examines the multi-access properties of transform domain spread spectrum systems for use in tactical battlefield scenarios. Transform domain spread spectrum (XDSS) systems evolved as an attractive solution for low probability of detection/interception (LPD/I) systems that operate in interference and multipath environments. However, the same properties that make XDSS systems robust to interference and multipath, when combined with multi-access features, make XDSS systems attractive for tactical battlefields as well. We illustrate the advantages of transform domain spread spectrum over more traditional direct sequence spread spectrum methods by examining three multi-access techniques: (1) orthogonally coded direct sequence (OCDS), (2) cyclic code shift keying (CCSK), and (3) XDSS. All of these systems can be either coherent or noncoherent; however, we concentrate on non-coherent M-ary waveform modulation methods. We first show that while OCDS systems mitigate multi-user interference problems when the received user waveforms are synchronized, they lose this property when any time offsets are introduced by multipath and propagation delay. In addition, OCDS systems do not naturally resolve the time shifted versions caused by multipath, so RAKE processing for the M-ary signal set can be expensive. On the other hand, a CCSK system can readily use fast convolution methods to naturally resolve circularly delayed versions of the signal. Provided the processing gain is sufficiently large, a CCSK system can easily implement a RAKE combiner. The CCSK system, however, only provides isolation from other users by taking advantage of the low cross correlation properties of the spreading code. This make CCSK systems subject to near/far interference problems. By showing that XDSS systems have orthogonal CCSK signal sets, we illustrate how XDSS systems can provide robust, multi-access communications in tactical battlefield scenarios where ECCM and LPD/I properties are important