New expressions for the soft decision bit log-likelihood ratio (LLR) of a MIMO system using quadrature amplitude modulation (QAM) taking into account channel estimation error (CEE). The bit LLR for the maximum likelihood (ML) and the linear minimum mean-squared error (MMSE) receivers are derived, showing in both receivers explicit scaling of the LLR that is a function of the QAM symbol and the CEE variance. These new expressions for the LLRs are used to show that only modest improvements in the link performance are achieved relative to the LLRs that do not take into account the CEE. This indicates that separating the detector design from channel estimation does not significantly impact the system performance, which leads to simplifications in the overall receiver implementation.

A log-max generalized bit log-likelihood ratio (GLLR) was derived in an earlier work of Wanget al.for soft decision decoding of quadrature amplitude modulation that takes into account channel estimation error. The results in the above paper show that the GLLR metric outperforms the conventional LLR metric that does not account for channel estimation error. In trying to reproduce the results of the above paper, our simulations indicate that their results were obtained without proper scaling of the transmit constellation. Conversely, through simulations, we demonstrate that by properly scaling the constellation to have unity average energy, the GLLR metric does not result in a noticeable advantage in the bit error rate over the conventional LLR metric for practically meaningful values of the channel estimation error.

This paper describes a frame work for evaluating the coverage performance of an orthogonal frequency division multiplexing (OFDM) system as defined in the IEEE 802.16e/WiMAX standard. Both the downlink and the uplink coverage are presented under a system with multiple cells and sectors. The spatial area coverage for a single frequency reuse and reuse of 3 are evaluated. Our results show that a WiMAX system can be uplink coverage limited for base station receivers with only two antennas. It is shown that there is a tradeoff between the uplink sector throughput and coverage performance. On the downlink, it is shown that a combination of cyclic delay diversity (CDD) with segmented frequency reuse of 3 significantly improves coverage performance. Also, shown is a comparison of downlink coverage performance between maximum ratio combining (MRC) and minimum mean square (MMSE) receivers at the subscriber station. It is shown that for a single frequency reuse an MMSE receiver with CDD at the transmitter are needed for adequate downlink coverage performance.