PERFORMANCE OF A TDM/TDMA PORTABLE RADIO LINK FOR INTERFERENCE, NOISE, AND DELAY SPREAD IMPAIRMENTS

This paper presents the measured and simulated performance of a time division multiplexing/time division multiple access (TDM/TDMA) portable radio link for noise, interference and delay spread impairments. The radio link transmits short TDMA bursts of only 82 symbols at 450 Kbits/sec using 4-quadrature amplitude modulation (QAM). This modulation is also referred to 4-psk, but phase shift modulation is constant envelope. Nyquist filtering of quadrature signal components yields quadrature amplitude modulation which, of course, also contains pi/2 phase variations at the sampling instants. Demodulation is performed by a low-overhead digital coherent demodulator with 2-branch selection diversity. The port or base has two receivers for diversity, but the portable or handset uses only a single receiver to perform selection diversity. Signal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) performance of the radio link was measured for a stationary channel and for a channel with several different fading rates. The results indicate that the link performance with interference is slightly better than with noise. In a Rayleigh fading environment, increasing the fading rate causes only a small performance degradation. The results also show that selection diversity is effective against interference in a slow Rayleigh fading environment. Computer simulations show close agreement with the experimental results. A separate set of experiments were performed to measure the effects of delay-spread on the radio link. Both experiments and computer simulations showed that selection diversity is effective in reducing the word-error ratio (WER) floor caused by frequency-selective fading. As a result, relatively high data rates can be supported by a multipath fading channel without employing adaptive equalization. Thus, a portable radio communications system using low complexity hardware design incorporating selection diversity can achieve good performance.