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 d ivision 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 envelo pe. Nyquist filtering of quadrature signal components yields quadratur e amplitude modulation which, of course, also contains pi/2 phase vari ations at the sampling instants. Demodulation is performed by a low-ov erhead 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. S ignal-to-noise ratio (SNR) and signal-to-interference ratio (SIR) perf ormance of the radio link was measured for a stationary channel and fo r a channel with several different fading rates. The results indicate that the link performance with interference is slightly better than wi th noise. In a Rayleigh fading environment, increasing the fading rate causes only a small performance degradation. The results also show th at selection diversity is effective against interference in a slow Ray leigh fading environment. Computer simulations show close agreement wi th the experimental results. A separate set of experiments were perfor med to measure the effects of delay-spread on the radio link. Both exp eriments and computer simulations showed that selection diversity is e ffective in reducing the word-error ratio (WER) floor caused by freque ncy-selective fading. As a result, relatively high data rates can be s upported by a multipath fading channel without employing adaptive equa lization. Thus, a portable radio communications system using low compl exity hardware design incorporating selection diversity can achieve go od performance.