Performance of turbo codes in overloaded MIMO-OFDM systems using joint decoding

In order to support high data rate communications, the third generation partnership project (3GPP) long term evolution (LTE) implements multiple-input multiple-output (MIMO) systems. In addition, to achieve excellent performance, LTE also employs turbo codes as the forward error correcting (FEC) standard. Although conventional turbo codes give satisfactory performance in the ideal MIMO, significant performance degradation occurs in an overloaded MIMO system when the number of transmit antennas is larger than the number of receive antennas. This paper proposes a joint decoding scheme for turbo codes in overloaded MIMO-OFDM systems. In joint turbo decoding, the calculations of soft information are based on the bit combinations from all of streams. In addition, a super-trellis diagram is used to combine the trellis diagrams. Numerical results obtained through computer simulation show that the proposed scheme achieves better performance by 2.0dB for 4 transmit antennas and 2 receive antennas in the BER of 10^-5. The proposed scheme also achieves throughput improvement with the ratio close to the theoretical overloaded MIMO capacity.