On the Capacity Analysis of A DS/FH CDMA System for the PCS

Abstract

<p>The main objective of this thesis is to design a PCS system with high capacity. We find that the hybrid DS/FH CDMA method is a promising technology to achieve this goal. Designing such a system and analyzing its capacity are the major contributions of this thesis.</p> <p>CDMA technology potentially offers higher capacity than the other multiple access methods. However, there are serious problems in implementing DS-CDMA to PCS. These are the long acquisition and "near-far" problem. Both these two problems reduce the system performance, and require complicated hardware solutions.</p> <p>The idea of applying hybrid DS/FH CDMA technique to PCS system is initiated for trying to solve the long acquisition problem. We also find that by taking advantage of FH, we can solve the "near-far" problem without increasing system complexity.</p> <p>The capacity of CDMA systems is limited by mutual interference of users, and the interference level is determined by two major factors: the precision of synchronization and the code property. The most important property of the codes is the cross-correlation. The lower the cross correlation among the codes, the higher the capacity will be. Based on this criterion we explore the performance of different codes. In addition, we proposed to use codes generated from the same maximum-length sequence with a unique phase, which offer the lowest cross correlation in the family of non-orthogonal codes.</p> <p>Another detriment to the performance of the system is the imperfection of synchronization due to random access and propagation delays. We propose to implement time slots and deterministic coordinated hopping patterns. Such a measure not only helps to save acquisition time and power control, but also increases the capacity dramatically, by way of establishing synchronous access and avoiding frequency collisions. We show that quasi-synchronous access can be achieved for uplink and completely synchronous access can be achieved for downlink. Our theoretical analysis as well as simulations showed that, by ignoring the adjacent cell interference, system capacity in a single cell can approach its hard limit defined as the ratio of total bandwidth to the message bandwidth for a reasonable bit error rate.</p> <p>A major concern in applying CDMA to PCS is the interference from adjacent cells, since the whole spectrum is reused in all the adjacent cells. Mathematical models for calculating such interference power have been developed, in terms of propagation characteristics, code properties, etc. The results reveal significant impairment on the capacity due to this interference. Voice activity cycle plays an important role now and can triple the capacity in one cell to approach the hard limit N. Without it, the capacity can only be about N/3 in one cell, for an acceptable BER (10ˉ³) and reasonable Eb/N₀. The capacity of this hybrid system has the potential of being much larger than that of TDMA system.</p>