NXG04-1: Interference Analysis in MQAM-OFDM Microcellular Mobile Communication Systems Considering Imperfect Inter-cell Synchronization

This paper presents a theoretical performance analysis of downlink (base-to-mobile) OFDM (Orthogonal frequency-division multiplexing) micro-cellular radio communication systems. The model developed in this analysis determines accurate interference analysis which takes into consideration frequency and timing synchronization errors between cochannel cells in a Nakagami/Rayleigh fading scenario. This leads to new, accurate and computationally efficient expressions for average bit error rates and cutoff rates of MQAM-OFDM in a micro-cellular environment. Results obtained are applicable in the investigation of the effect of synchronization errors on the spectral efficiency of OFDM based micro-cellular systems. Whilst the system is more robust against timing jitters resulting from separation distances from the victim receiver, frequency synchronization errors above 10^-1 have the potential to create a loss in spectral efficiency of up to 7.5% for 16 QAM.

I. Introduction

Orthogonal Frequency Division Multiplexing (OFDM) is a promising signaling technique that achieves high spectral efficiency over wideband wireless channels, and is therefore a strong candidate for next generation mobile communication systems. An OFDM system is a type of multicarrier modulation where each transmission band divides into subchannels that are modulated into different subcarriers. System performance improves because subcarriers are orthogonal to one another, thus, minimizing the threat of interference. However, performance of OFDM tends to suffer from degradation because of possible episodes of imperfect time and frequency synchronizations since a loss in orthogonality occurs between subcarriers at the OFDM receiver. This gives rise to interchannel interference. For example, in terms of cellular analysis, if subcarriers transmitted in a given cell are not aligned in time and/or frequency with a receiver located in a reference cell, then, there is a loss in orthogonality amongst the subcarriers resulting in intercellular interference from subcarriers associated with other cells.