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In the last decade, various techniques have been developed to
calculate the data bit error probability caused by
multiple-access interference (MAI) for direct-sequence /
spread-spectrum multiple-access (SS/SSMA) communication systems
with random signature sequence. However, it is computationally
difficult in the exact calculation, so emphasis has been on
approximations and bounds. First, we inquire the model of
DS/SSMA communication systems and derive the expression of the
MAI. Random variables characterizing various aspects of the MAI
are in terms of the random signature sequences, the data bits,
the correlation functions of the chip waveform, and the
propagation delays. Second, improved Gaussian approximation is
developed to find bit error probability for DS/SSMA
communication systems in conjunction with aperiodic
autocorrelation function of the first signature sequence by
using the central limit theorem. The accuracy of the
improved Gauusian approximation with no noise for data
bit error probability is greater than that offered by a
widely used Gaussian method which we call the " standard
Gaussian approximation " . This method can also be applied in
a straightforward manner to derive the relation between
the bit error probability and signal-to-noise ratio for DS/
SSMA communication systems with additive white Gaussian
noise. The calculation for the improved Gaussian
approximation is very complicated, so it takes long time to
calculate the data bit error probability and the
accuracy for the simple difference approximation is not
very good. We derive a simple and accurate approximation for DS
/ SSMA communication systems by using the difference
approximation and compare the results with the improved
Gaussian approximation and simple difference approximation.
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