Nelson R. Sollenberger

Architecture and implementation of an efficient and robust TDMA frame structure for digital portable communications

The architecture and implementation of a time-division multiple-access digital portable communication system using short bursts are considered. An efficient and robust TDMA frame structure that requires very low overhead for burst demodulation and synchronization while being robust against fading and carrier frequency offset is described. Based on this frame structure, an experimental radio link was implemented by using a hardware architecture that is suitable for digital signal processing and very large-scale integration implementation. Implementation of demodulation, synchronization, error detection, and fading recovery using combined software and off-the-shelf hardware is described, followed by a summary of the performance results. >

Performance of a TDM/TDMA portable radio link for interference, noise, and delay spread impairments

Presents the measured/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 82 symbols at 450 kbits/sec using 4QAM. This modulation is also referred to 4PSK, but phase shift modulation is constant envelope. Nyquist filtering of quadrature signal components yields quadrature amplitude modulation which, of course, also contains /spl 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 and signal-to-interference ratio performance of the link was measured for a stationary channel and for a channel with different fading rates. 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. A separate set of experiments were performed to measure the effects of delay-spread on the link. Selection diversity is effective in reducing the word-error ratio floor caused by frequency-selective fading. As a result, relatively high data rates can be supported by a multipath fading channel without using adaptive equalization. Thus, a portable radio communications system using low complexity hardware design incorporating selection diversity can achieve good performance. >