Pulse Shaping in OFDM Based Single Frequency Networks

In systems using Orthogonal Frequency Division Multiplexed (OFDM) all transmitters in a radio broadcasting network may transmit identical signals in the same frequency block, forming a so called Single Frequency Network (SFN). The simultaneous transmission creates a severe artificial multipath propagation causing interference at the receiver. This interference consists not only by intersymbol interference (ISI), but also interchannel interference (ICI) due to orthogonality losses between OFDM scheme's subcarriers. A method that is often used to counter the self-interference is to insert a guard interval between consecutive OFDM frames combined with an increased symbol duration. The receiver in an SFN is normally experiencing a diversity gain of receiving several signals. Doubts concerning the performance has been raised due to the OFDM scheme's sensitivity to frequency errors. If the received signals are exposed to different carrier offsets it will lead to ICI, resulting into reduced coverage. In this paper we apply pulse shaping to reduce the effects of ISI and ICI. Pulse shaping of the transmitted OFDM signal is shown to give significant reductions in self-interference. It is hereby possible to achieve the same coverage with a reduced amount of transmitters. The system also becomes less sensitive to time and frequency synchronisation errors. A method to analyse the performance for time limited pulse shapes is presented using weight functions describing the amount of useful and interfering powers. In the receiver analysis, generalised expressions are given using Fourier transforms and series.