 operate in the same frequency band, each rejecting the interfer- ence produced by the others by a factor approximately equal to its energy gain. This asynchronous form of spectrum sharing is often called spread- spectrum multiple-access (SSMA) or code-division multiple-access (CDMA). As an illustration of SSMA operation, consider a transmitted signal XT(t), (1.63) and an interfering signal x,(t), (1.64) impinging on a receiver which is frequency synchronous with the transmit- ted signal and which computes the inner product of the received signal with a reference modulation mr(t). We assume without loss of generality that mr(t),m,(t), and mR(t) are unit power waveforms, i.e., the time-averaged and ensemble-averaged value of |m;(t)p is one for j = T,1, R. The output of the receiver correlator is the sum of two terms, corresponding to the desired inner product VPTVT and the interference inner product / P N . These terms for the k-th correlation interval ((k - 1)T,kT), normalized to unit input power, are (1.65) (1.66) where is the phase tracking error in the receiver (see Figure 1.3), and A A Spread-Spectrum Overview is the carrier frequency offset between the interference and the transmitted signal. The average signal-to-interference energy at the input to the corre- lator is simply (1.67) and the frequency-synchronous correlator's output signal-to-interference measurement ratio is (1.68) Here <. ) denotes discrete time averaging over the parameter k. The above calculation is based on the supposition that outputs from both the in-phase and quadrature channels of the correlator are necessary, i.e., both the real and imaginary parts of UT(k) are necessary in the reception process. If instead reception is phase-synchronous, then VT(k) is real, and on the average only half of the interference power contributes to the distur- bance of Re{v7(k)}. Hence, for phase-synchronous detection, (1.69) The key to further analysis is the evaluation of the interference level at the correlator output. (1.70) The variable t denotes an