OFDM for 5G: A General Framework

A general framework is presented for orthogonal frequency-division modulation and multiple-access (OFDM/OFDMA) in 5G cellular wireless access networks. It 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. The recent UF-OFDM and f-OFDM proposals from Alcatel-Lucent and Huawei, respectively, are both special cases thereof. A unified signal model is developed for all the OFDM variants, which in addition accommodates both windowing-based and filtering-based receivers. Sufficient conditions on the channel impulse response and the windowing/filtering at the transmitter and receiver are given for the avoidance of inter-symbol and intra-symbol interference. Several performance results for all four OFDM variants are included, comparing the rates achievable with them on a three-user uplink channel. With each user occupying a 180 kHz subband, and SNRs in the 6-15 dB range, standard OFDM (without any windowing or filtering) is shown to suffer a rate loss of 10-30% due to asynchronous operation and/or unequal subcarrier spacings and symbol periods for the users. Filtered OFDM (with judiciously chosen parameters) can cut this loss down to 4-11%. The results further support the following conclusions regarding rate-versus-SNR performance: (a) CP-OFDM and ZP-OFDM are nearly indistinguishable, and filtering is preferable to windowing; (b) at SNRs of interest, filters with impulse response durations exceeding the guard period are beneficial (despite the increased inter-symbol interference); and (c) filtering is required at both ends of the link (transmitter and receiver) for the most effective suppression of inter-subband interference.