A New Receiver Structure for DFT Spread OFDM (DFT-SOFDM) in Time-Selective Fading Channel

The transmission of a Discrete Fourier Transform Spread Orthogonal Frequency Division Multiplexing (DFT-SOFDM) signal through a time-selective fading channel suffers from severe performance degradation due to the Doppler shifting of the transmitted spectrum. In this paper, we propose a multiple-input multiple-output (MIMO) DFT-SOFDM scheme and a new technique to overcome the adverse effects caused by Doppler shift. By appropriately interleaving the transmitted signals in space and time, our scheme allows us to use linear equalization techniques such as the zero-forcing (ZF) and minimum mean-squared error (MMSE) equalization. In addition, the use of sphere decoding algorithm (SDA) provides superior BER performance compared to the ZF or MMSE solution. Simulation results show that at a BER of 10 3, SDA provides a 13.5 dB gain over ZF solution, and a 13 dB gain over MMSE solution. The system with SDA shows negligible performance degradation when the system load in the uplink is increased: at E
_b
/N
_o
= 21 dB, a BER of 1.88 times 10
^-5
is achieved when there are 2 users in the system, but degrades only to 2.19 times 10
^-5
for a 8 user system. In addition, SDA used with our proposed signal model offers significantly lower worst case complexity as compared to the conventional DFT-SOFDM with SDA.