Date of Award

3-2002

Document Type

Thesis

Degree Name

Master of Science

Department

Department of Electrical and Computer Engineering

First Advisor

Rusty O. Baldwin, PhD

Abstract

The United States Air Force relies heavily on computer networks for every-day operations. The medium access control (MAC) protocol currently used by most local area (LAN) permits a single station to access the network at a time (e.g. CSMA/CD or Ethernet). This limits network throughput to, at most, the maximum transmission rate of a single node with overhead neglected. Significant delays are observed when a LAN is overloaded by multiple users attempting to access the common medium. In CSMA/CD, collisions are detected and the data sent by the nodes involved are delayed and transmitted at a later time. The retransmission time is determined with a binary exponential back-off-algorithm. Code Division Multiple Access (CDMA) is a technique that increases channel capacity by allowing multiple signals to occupy the same bandwidth simultaneously. Each signal is "spread" through multiplication with a unique pseudo-random code that distinguishes it from all other signals. Upon reception, the signal of interest is "despread" and separated from other incoming signals by multiplying it with the same exact code. With this technique, it is possible for multiple stations to transmit simultaneously with minimal ill effects. A simulation model is developed for a direct sequence spread spectrum CDMA (DS/CDMA) channel that incorporates the effects of multiple access interferers (MAI) having spreading codes from the same or different code families. The model introduces cross-correlation coefficients to calculate the signal-to-interference ratio and determine channel bit error performance. Transmission media attenuation and the near-far effects are accounted for in the model design. The model utility is demonstrated by determining the loss characteristics of a coaxial spread spectrum network. Due to the modular design, other transmission media characteristic can be easily incorporated. A bus network topology is simulated using 10Base2 coaxial cable. The model is compared and validated against a spread spectrum local area network hardware test bed.

AFIT Designator

AFIT-GE-ENG-02M-22

DTIC Accession Number

ADA401515

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