Robust bit-loaded wireless OFDM

Bit-loaded OFDM systems have been widely studied in the literature. In these systems, a bit-loading algorithm assigns different power and number of bits to the sub-channels according to their quality, thus improving the performance of the system. To do this, channel state information (CSI) must he available at the transmitter. When the CSI available at the transmitter is not reliable, the performance gain achieved by bit-loading is degraded. In this paper we consider the effect of unreliable CSI on the performance of a bit-loaded OFDM system. In particular, we consider the case where CSI is outdated as a result of a delay between the channel sensing and transmission of the bit-loaded symbol. The combination of such delay and time variations of the channel results in outdated CSI. We then propose a robust bit-loaded system which uses frequency domain code division multiplexing (CDM). In our proposed system, the bit and power allocation is assigned to groups of sub-channels. Within each group, the data symbols are spread in frequency using orthogonal codes. Therefore, frequency diversity is achieved without any loss in the transmission rate. Simulation results show that the proposed system is considerably less sensitive to outdated CSI.