TABLE OF CONTENTS If all the spreading sequences in a network of asynchronous CDMA systems have a common period equal to the data-symbol duration, then by the proper selection of the sequences and their relative phases, one can obtain a system performance better than that theoretically attainable with random sequences. However, the number of suitable sequences is too small for many applications, and long sequences that extend over many data symbols provide more system security. Furthermore, long sequences ensure that successive data symbols are covered by different sequences, thereby limiting the time duration of an unfavorable cross-correlation due to multiple-access interference. Even if short sequences are used, the random-sequence model gives fairly accurate performance predictions.
Consider the direct-sequence receiver of Figure 2.14 when the modulation is PSK and multiple-access interference is present. If the spreading sequence of the desired signal is modeled as a random binary sequence and the chip waveform confined to [0, T C), then the input V to the decision device is given by (2-84) and has mean value
The interference component is given by (6-5), (6-6), and (6-1). Since the data modulation d i( t) in an interference signal is modeled as a random binary sequence, it can be subsumed into q i( t) given by (6-3) with no loss of generality. Since q i( t) is determined by an independent, random spreading sequence, only time delays modulo- T c
