Bit-to-bit error dependence in direct-sequence spread spectrum multiple-access packet radio systems
Abstract
Slotted direct-sequence spread-spectrum multiple-access (DS/SSMA) packet broadcasting systems with random signature sequences are analyzed within the framework of the lower three layers of the International Standards Organization Reference Model of Open Systems Interconnection. At the physical layer, we show that a widely-used Gaussian approximation (which we call the Standard Gaussian Approximation) for the probability of data bit error in a chip and phase asynchronous system is accurate only when there are a large number of simultaneous users on the channel; otherwise, this approximation can be optimistic by several orders of magnitude. For interfering signals with fixed delays and phases relative to the desired signal, however, the Standard Gaussian Approximation is quite accurate for any number of simultaneous users. To obtain a closer approximation to the probability of data bit error for an asynchronous system, we introduce the Improved Gaussian Approximation, which involves finding the distribution of the multiple-access interference variance over all possible delay and phase values and then taking a Gaussian approximation over the support of the distribution and averaging the results. To accurately analyze packet performance at the data link layer, we first use the theory of moment spaces to gain insight on the effect of bit-to-bit error dependence caused by the constant relative delays and (possibly) phases of the interfering signals over the duration of a desired packet. If no error control is used, we find that this error dependence increases the average probability of packet success. When error control is employed and the channel is lightly loaded, then packet performance diminishes when bit error dependencies exist, but performance improves when the channel is heavily loaded and the multiple-access interference is high. Numerical results for the probability of packet success are obtained through the Improved Gaussian Approximation. At the network layer, provided packet losses occur only from data bit errors due to multiple-access interference, we show that a DS/SSMA packet radio system using the slotted ALOHA protocol possesses a significant throughput advantage over that of an equivalent narrow-band slotted ALOHA system. Furthermore, if error control is used to correct some of the data bit errors in the packet, then the maximum throughput per unit bandwidth of the DS/SSMA system is also higher.
Degree
Ph.D.
Advisors
Lehnert, Purdue University.
Subject Area
Electrical engineering
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