Iterative detection for pretransformed OFDM by subcarrier reconstruction

In this paper, an iterative detection method for an uncoded pretransformed (PT) orthogonal frequency division multiplexing (OFDM) system where the channel is not known at the transmitter is proposed. The iterative detection starts with linear detection. The noiseless received signal at the weakest subcarrier (corresponding to the smallest channel amplitude) is estimated based on all the detected data symbols using a hard or soft decision. Then, the actual received signal at the weakest subcarrier is replaced by the estimated one. This process is referred to as reconstruction here. After reconstruction, linear detection is carried out again to generate the next set of symbol estimates. The whole process proceeds iteratively to reconstruct the received signal at the next-weakest subcarrier. The transform coefficients and the iterative process are designed to maximize the minimum signal-to-noise power ratio. Under the assumption that the previous detection is error free, it is shown analytically that the iterative method achieves a diversity advantage of i+1 in the ith iteration, thus providing an explanation of its superior performance. Due to the flexibility of the transform design, the analysis conducted is applicable for other common systems as well. Simulations in realistic channels are carried out, and the bit-error rate performance of the iterative detection is superior as compared to that of the conventional detectors for the PT-OFDM or OFDM system.