A robust cross coding scheme for OFDM systems

In wireless OFDM-based systems, coding jointly over all the sub-carriers simultaneously performs better than coding separately per sub-carrier. However, the joint coding is not always optimal because its achievable channel capacity (i.e. the maximum data rate) is inversely proportional to the dynamic range of the channel. In this paper, we propose a novel cross coding scheme to increase the maximum data rate (i.e. the noise floor) achieved in frequency selective channels. The proposed OEC-II is based on fountain codes. The key element in OEC-II is to exchange the modulation order with the discarded subcarriers. In such case, the system can increase the noise floor by only taking care of the sub-carriers with high energy. By transmitting a fountain-encoded packet over a single sub-carrier, the packets can be discarded if they have encountered a low-energy channel. Fountain codes can recover the source data by only using the surviving packets. With the same effective transmission data rate (i.e. 10.8 Mbits/s), OEC-II with QAM-16 has a SNR gain of around 7 dB over the FEC layer from the IEEE 802.11a standard, around 8 dB over the FEC layer from the IEEE 802.11n standard, around 7 dB over OEC-II with QPSK and around 3 dB over OEC-II with QAM-64.