MULTIPLE-ACCESS CAPABILITY OF FREQUENCY-HOPPED SPREAD-SPECTRUM REVISITED - AN ANALYSIS OF THE EFFECT OF UNEQUAL POWER LEVELS

In this paper we present a new method for the evaluation of the probability of error of uncoded asynchronous frequency-hopped spread-spectrum multiple-access (FH / SSMA) communications. For systems with binary FSK modulation our method provides an accurate approximation and a tight upper bound to the bit error probability; for systems with M-ary FSK modulation, it provides tight upper bounds to the symbol error probability. The method enables the computationally efficient averaging of the error probability with respect to the delays, phase angles, and data streams of the different users. It relies on the integration of the product of the characteristic function of the envelope of the branch of the BFSK demodulator, which carries the desired signal, and of the derivative of the characteristic function of the envelope of the other branch. For sufficient frequency separation between the BFSK tones, the new method can achieve any desirable accuracy. Moreover, the computational effort required for its evaluation grows linearly with the number of interfering users. In the M-ary case, tight upper bounds based on the union bound and the results of the binary case are derived. The new method allows us to quantify accurately the effect of unequal power levels on other-user interference in FH / SSMA systems for the first time. Comparison of the multiple-access capability of FH / SS systems without error-control can support (at a given error rate) considerably many more simultaneous users than previously thought when the relative received powers of the users are not significantly different. This trend is amplified further for systems with error control. Our results indicate that the FH / SSMA systems also suffer from the near-far problem, although less seriously than direct-sequence SSMA systems. © 1990 IEEE