On the design of multidimensional signal sets for OFDM systems

Abstract

An orthogonal frequency division multiplexing (OFDM) system operating over a wireless communication channel effectively forms a number of parallel frequency-nonselective fading channels, thereby obviating the need for complex equalization and thus greatly simplifying equalization/decoding. However, the OFDM system also exhibits two weaknesses relative to its single-carrier counterparts: (1) the diversity achieved by the OFDM system can be less than a single-carrier system employing the same error control code in a signaling environment rich in diversity and (2) the baseband transmitted signal can exhibit significant amplitude fluctuation over time, thereby precluding efficient transmit amplifier operation. In this paper, nonstandard multidimensional signal sets matched to the OFDM framework are prescribed that address both of these issues. The proposed signal sets are chosen to maximize the diversity achieved by an uncoded system under a constraint to control the peak-to-mean envelope power ratio (PMEPR) of the baseband transmitted waveform. The cost of employing the proposed signal sets is an increase in decoding complexity, as essentially a small amount of controlled equalization has been added to the receiver; thus, the resulting system can be viewed as a hybrid between an OFDM system and a standard single-carrier system. Numerical results are presented which suggest that: (1) the system can provide an attractive alternative to a standard OFDM system in terms of required average transmitted SNR versus receiver complexity and (2) the system yields a modest reduction in PMEPR versus a standard OFDM system.