Impact of Cyclic Prefix length on OFDM system Capacity

This paper is a study on the impact of the Cyclic Prefix (CP) length on the downlink Capacity in a base-band synchronized SISO-OFDM context. To measure this impact, the capacity, measured in bits per second per hertz, is chosen as quality parameter. The study shows how the lengthening of the CP affects Spectral Efficiency Loss (SEL) and Signal to Interference and Noise Ratio (SINR). SINR can be mapped into Capacity using Shannons formula. The optimum CP length is defined as the one that maximises the Capacity for a given set of system parameters. The parameters studied in this paper are: 1) the useful OFDM symbol duration, 2) the Signal to Noise Ratio (SNR) at the receiver and 3) the channel Power Delay Profile (PDP). Depending on the values of these parameters different optimum CP lengths are obtained. For a system using only one value of CP length we suggest an optimum value to be 4us for an OFDM symbol length of 40us and 6us for an OFDM symbol length of 80us.