OFDM for 5G: Cyclic prefix versus zero postfix, and filtering versus windowing

Owing to its low spectral decay rate, conventional orthogonal frequency-division modulation (OFDM) is unsuitable for supporting asynchronous access, user-specific subcarrier spacing and symbol period, and spectrum aggregation, all of which are envisioned for fifth-generation (5G) cellular networks. This has spurred recent work on designing new multi-carrier waveforms for 5G. In this paper, a unified signal model for OFDM is presented, which encompasses variants employing either a cyclic prefix (CP) or a zero postfix (ZP) as the guard interval technique, and either windowing or filtering to increase the spectral decay rate of the transmitted signal; both windowing-based and filtering-based receivers are also accommodated. Sufficient conditions on the channel impulse response and any windowing/filtering are given for the avoidance of inter-symbol and intra-symbol interference. Numerical results are then presented to show that, in a multiuser system operating asynchronously and with user-specific subcarrier spacing and symbol period, filtering and windowing yield substantial and comparable rate gains over standard OFDM when implemented at both transmitter and receiver; these gains can be realized with no performance degradation relative to standard OFDM (at SNRs typically of interest) when the system operates synchronously and all users have identical subcarrier spacing and symbol period. Further, CP-OFDM and ZP-OFDM have near-identical rate-versus-SNR performance.