Spatial diversity in a SIMO-OFDM hybrid powerline-wireless system

Abstract

Spatial diversity is an effective diversity technique used in multiple antenna wireless systems. It requires that antenna elements be sufficiently distributed by at least ten times the signal wavelength (10λ). Sufficient spatial distribution of antenna elements renders performance improvements through diversity gain. In consumer electronics, antenna elements are often located in close proximity, < λ, due to form size constraints. Therefore performance improvement through spatial diversity is limited even though multiple antenna systems are employed. The work in this dissertation addresses the use of power line communication as a method to spatially distribute antenna elements of a multiple antenna wireless system. Addressing this problem requires the cooperative use of both wireless and powerline channels to form a hybrid channel. A hybrid powerline-wireless channel is defined as the concatenation of a powerline channel and a wireless channel to establish a unified channel. A sequence-based channel sounding approach using maximum length sequences and software-defined radio was designed, implemented andapplied to attain channel transfer characteristics of the hybrid channel. The measurement campaign was carried out for an indoor low-voltage level powerline network. The attained results include the channel impulse responses, frequency responses and power delay profiles. Coherence bandwidth, average channel attenuation and power delay profile timing delays are channel parameters that were extracted from the measurements. The extracted channel parameters reveal that the hybrid channel is a challenging medium for data transfer and communication applications. Based on the attained hybrid channel characterisation parameters, a physical layer structure for a 1 × 2 single-input multiple-output orthogonal frequency division multiplexing (SIMOOFDM) hybrid powerline-wireless system was designed and implemented. An investigation of the bit-error-rate performance of single-carrier and multi-carrier modulation for a multipath powerline channel was carried out in simulation. Using OFDM as a suitable candidate for multi-carrier modulation, it was found to outperform single-carrier modulation. OFDM was used as the channel access method for the hybrid channel. Source encoding and decoding of the physical layer structure was designed to be robust against inherent interferences of the hybrid channel. Selection combining, equal gain combining and maximal ratio combining methods were designed and implemented. Estimation methods for error rate, data transfer rate and SNR of the SIMO-OFDM system were designed and implemented. Capacitive signal coupling was used to interface the powerline channel to the wireless propagation environment, hence establishing the hybrid channel. This method also allowed powerline transmission to be performed at a much lower frequency than wireless transmission. An experiment was conducted to investigate the effect of spatial distribution on the 1 × 2 SIMO-OFDM hybrid powerline-wireless system for indoor environments. The results of the experiment were error rate curves produced for different spatial distribution lengths through a powerline channel for each signal combining method. Error rate performance improvements in the SIMO-OFDM system were achieved with an increase of antenna element spatial distribution due to lowered signal envelope correlation. Small but yet notable diversity gains were observed by the increase in the slope of the produced error rate and signal-to-noise ratio curves for each signal combining method. Experimental parameters and apparatus placed a limitation on the achievable spatial distribution of diversity branches, hence the achievable diversity gains. This was a result of overwhelming inherent interferences of the hybrid channel