Cyclic prefix length analysis for 4G OFDM systems

Channel rays outside the cyclic prefix (CP) interval (also known as the "guard interval") in orthogonal frequency division modulation (OFDM) are encountered due to excessively long channels and possibly due to timing errors. This causes inter-carrier and inter-symbol interference (ICI/ISI) and hence a loss in per-subcarrier signal-to-interference and noise ratio (SINR). As observed in prior works, there is a trade-off between ICI/ISI power and capacity when lengthening the CP; for some channel profiles it is advantageous to allow some of the weaker channel rays to "escape" the CP. In this paper we examine how OFDM symbol length, CP length, and timing mismatch of a certain variance, impact the expected SINR and capacity based on recently measured channel profiles in a 20 MHz band at 3.7 GHz in suburban and urban environments. Capacity for the measured channels is evaluated for two representative symbol lengths of 10 μs and 40 μs. For the measured channels, capacity is maximized for a CP of 1-3 microseconds. However, a CP of 4-5 microseconds has only slightly less capacity and is much more robust to timing errors.