Modulating waveforms for OFDM

Orthogonal frequency division multiplexing (OFDM) is a popular transmission technique that is employed in applications such as Digital Audio Broadcasting, Asymmetric Digital Subscriber Line and wireless LAN. In this work we consider design of modulating waveforms for OFDM in the presence of the delay spread and system impairments such as frequency offset and timing mismatch. We give a complete parameterization of OFDM modulating waveforms. Increasing robustness of OFDM to frequency offsets requires using long modulating waveforms. To make the implementation of OFDM systems with long modulating waveforms feasible we propose fast implementation algorithms. Some preliminary modulating waveform design examples are presented. The presented waveforms demonstrate that the robustness of OFDM systems to impairments can be improved by allowing certain degradation of unnecessarily good performances of the state of the art OFDM systems in ideal operating conditions. 1. PRINCIPLES OF OFDM Consider a communication system which transmits a symbol stream a, with a time interval 0 using a modulating waveform '. Transmitted signal, s, is a linear combination of translates of ', s(t) = P m a[m]'(t m 0): In the presence of multipath propagation the received signal, sr , is a superposition of delayed and attenuated replicas of the transmitted signal, sr(t) = P i ris(t di): Note that the propagation parameters ri and di in general vary with time. If the intersymbol interval 0 is significantly shorter than the possible delay spread, d = maxi;j jdi djj, this multipath propagation causes severe intersymbol interference. An immediate way to cope with this problem would be to transmit symbols with time intervals larger than the delay spread, while using some form of parallelism, e.g. frequency multiplexing, to maintain the symbol rate in a given range. This requires dividing the symbol sequence into a number, N , of subsequences and as many multiplexing waveforms. With this multiplexing the transmitted signal takes the form