The effect of phase noise on the capacities of coherently demodulated hybrid frequency-division multiple access/code division multiple-access (FDMA/CDMA) systems operating over a multipath Rayleigh fading channel is investigated. Using an approximate upper bound on the bit-error rate performance, which has been derived and presented in a previous paper, the capacities of the FDMA/CDMA systems are estimated for several combinations of channel and system parameters. Simulation results are also included to show the effect of the bounding error.

Based on the observation of Kim, Kwon, Hong and Whang (see ibid., vol.46, no.9, p.1109-10, 1998), we discovered that in our derivation (see ibid., vol.44, p.1117-29, 1996), a factor of one-half in the exponent of one of three exponentials was inadvertently dropped and this, in turn, affected two other equations.

The bit-error rate (BER) performance of a direct sequence spread spectrum (DS-SS) signal, operating over a multipath Rayleigh fading channel, is investigated when corrupted by phase noise as well as additive white Gaussian noise (AWGN). The phase noise arises from phase locked loop (PLL) dynamics and results in imperfect receiver phase estimates whereby the phase errors assume Tikhonov densities. The phase estimates are used by a multipath-combining RAKE receiver for demodulation. Approximate upper-bounds on the bit error probability are obtained and evaluated for different combinations of channel parameters and for various values of the average loop signal-to-noise ratio (SNR). Results indicate that for a PLL with loop SNR 10 dB above the system E/sub b///spl eta//sub 0/, the degradation is less than 3 dB, and for a loop SNR of 20 dB above E/sub b///spl eta//sub 0/, the degradation is less than 1 dB.

Several methods of diversity combining for a Rayleigh-faded channel are evaluated and compared. The methods considered are, for coherent reception, maximal ratio combining (MRC), selection combining (SC), and a generalization of SC, whereby the two (three) signals with the two (three) largest amplitudes are coherently combined. We will call this method second (third) order SC, and denote it SC2 (SC3). Similar techniques are also investigated for noncoherent reception, with equal gain combining (EGC) replacing MRC, and noncoherent versions of SC2 and SC3. Numerical results indicate that SC2 and SC3 significantly enhances the bit-error rate (BER) performance relative to that achievable with SC, and under certain conditions approaches the performance achieved by MRC or EGG. The performance enhancement of SC2 and SC3 is especially noticable for noncoherent reception, where EGC is seen to provide the best performance only for low BER values. In fact, when the BER is 10/sup -3/ or greater, SC2 and SC3 performed comparably to EGG, and in some cases performed better than EGC.

The coherent reception of direct sequence-code division multiple access (DS-CDMA) signals in a multipath fading channel is considered. The channel model assumes independent paths with Nakagami fading statistics; this model includes the Rayleigh channel os a special case and is a reasonable model for a frequency-selective fading channel. The bit error rate (BER) performance of a RAKE receiver under various multipath fading conditions is derived and evaluated. The results indicate that over 50 users may access the channel (assuming 127 chips per bit) simultaneously under typical fading conditions, and can achieve an uncoded BER of 0.01 with an average received signal-to-noise ratio per bit of 10 dB.>

A hybrid FDMA/CDMA scheme operating over a frequency selective Rayleigh fading channel is described and analyzed. The performance of the hybrid system is then compared with that of a wideband CDMA system occupying the same total bandwidth. Both coherent and noncoherent modulation formats are investigated; it is found that, for coherent modulation with a RAKE receiver, wideband CDMA has greater capacity than does the hybrid. However, for the noncoherent modulation formats (either DPSK or square-law detected orthogonal signalling), a hybrid system can have a greater capacity if a high channel error rate can be tolerated. Otherwise, a wideband noncoherent system remains optimal.>