Partial DS-CDMA for mobile satellite data networks.
In recent years, random access protocols have received considerable attention as useful multiple access techniques for systems with high peak to average traffic ratios (bursty traffic). In most conventional random access protocols, when a single packet is transmitted in a given time period, it is received correctly, while if two or more transmissions occur, then all the packets are destroyed. If direct sequence code division multiple access (DS-CDMA) is used, this need not to be the case, because the DS-CDMA system allows many packets to coexist in a given slot. Thus packets are correctly received with certain probability which depends on the simultaneous number of packets in the slot. This suggests the possibility of using the DS-CDMA in a random access mode of operation. In addition, it is well known that CDMA transmission can provide other benefits, such as greater immunity to interference and interception; and its inherent resistance to multipath fading. In this thesis we propose to use the DS-CDMA in a random access mode of operation for the mobile satellite data packet networks. First, we analyze the DS-CDMA over a multipath fading channel and develop new and accurate approximations of the probability of bit error and probability of packet success. In these approximations an improved Gaussian process is used to model the multiple access interference (MAI) and the bit-to-bit dependence caused by the MAI and fading is taken into account. These approximations are used to investigate the performance of the DS-CDMA packet networks. The second stage of our work is to propose a new DS-CDMA system referred to as partial DS-CDMA in which a limited number of spread spectrum codes is required. It is well known that a major problem in the design of a DS-CDMA packet networks is the required large number of spreading codes when the network is aimed to serve a large population of users. The proposed partial DS-CDMA copes with this problem and suggests the use of only a limited number of spreading codes at the price of relatively small degradation on the performance. The last stage of this work is to propose a collision resolution algorithm (CRA) as random access protocol to be used in conjunction with the proposed partial DS-CDMA system. The combined protocol is analyzed under Poisson traffic and the delay-throughput performance is analyzed for different system parameters and under different fading conditions. By combining the partial DS-CDMA with the proposed CRA we obtain a new protocol which is stable, implementable and has many advantages as we prove throughout the thesis.
Engineering, Electronics and Electrical.
Source: Dissertation Abstracts International, Volume: 56-11, Section: B, page: 6311.
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