Measured performance of 4-QAM with 2-branch selection diversity in frequency-selective fading using only one receiver

Abstract

T h s paper presents the measured performance of a 500 kbitisec. 4-QAM (Quadrature Amplitude Modulation) radio link with 2-branch selection diversity using only one receiver subjected to hardware simulated slow frequency-selective Rayleigh fading. The single coherent receiver input is switched between two uncorrelated Rayleigh-fading signals and the one with higher power is selected. This performance is compared to results obtained from computer simulation.The experimental radio link uses TDM (Time Division Multiplexing) with a coherent receiver. Since the down-link transmitter is on all the time (it may send idle markers or data to other users) the received signal measurements and selection can be performed just before the reception of the user's data burst.The measured performance compares favorably with computer simulation. The effect of delay between measurement/selection and the actual data burst demodulation is also presented. It is found from both experiment and computer simulation that selection diversity is effective in reducing the BER floor caused by frequency-selective fading. As a result, a higher data rate can be supported by a multipath fading channel without employing adaptive equalization. Thus, a low complexity hardware design incorporating selection diversity is feasible for a portable radio communications system.

A radio system proposal for widespread low-power tetherless communications

“…TDMA radio links have several advantages as discussed in [2, p. 459]. Briefly, a) fewer radio transceivers are required for a given number of user circuits at a radio port, reducing port complexity and cost, b) different transmission rates can be provided on demand to different users in increments of some minimum rate increment, c) nonactive portions of the TDMA frame can be used for portable sets to assess the quality of other frequency channels and to do measurements for microscopic diversity selection [47], [48], d) the diplexer function in a portable set can be a simple solid-state switch, e) frequency stability requirements are reduced, f) fewer frequency channels need to be synthesized, and g) stepping between frequency channels can be done faster. However, in the past, conventional receiver implementations have required significant bit overhead in each TDMA time slot for time slot synchronization, symbol timing and carrier recovery.…”

“…In conventional diversity receiver implementation s. the realization of effective diversity has required two receivers, thus increasing set compleY.ity . However, in the slowly varying multipath environment of portable radio, a TDMA radio link architecture pennits the reaJization of fully effective selection diversity in a portable set using only one receiver [47) , (48). [60] .…”

“…the portable set can switch between antennas during that preceding time slot, measure which antenna is receiving the best signal and use the best signal antenna for receiving its data t. ime slot. Laboratory implementations and measurements demonstrating the effectiveness of this inexpensive diversity approach are reported in (47]…”