Praise for Fundamentals of WiMAX

398 Chapter 11 • Link-Level Performance of WiMAXiterative MIMO receivers based on the MAP detection perform even better than MLD receiversdo. An iterative MAP receiver uses the log likelihood ratios (LLR) from the channeldecoder output of the previous iteration as an input to the MIMO receiver. Thus, with eachiteration, the reliability of the received symbols is improved.Several suboptimal but low-complexity variants of the maximum-likelihood receivers, suchas QRM-MLD [11] have been proposed. It has been shown that these suboptimal receivers performwithin a dB of the full MLD receiver, thus significantly outperforming MMSE and O-SICreceivers. Table 11.9 shows the link gain for various receiver structures over the baseline MMSEreceiver.Table 11.9 Advanced Receiver Gain at 10 –4 BER for PUSC in Ped B 2 × 2 MIMOChannelQPSK16QAMR 1/2 (dB) R 3/4 (dB) R 1/2 (dB) R 3/4 (dB)O-SIC Receiver 1.0 2.0 0.8 1.5MLD Receiver 4.5 6.0 3.5 5.511.6 Summary and ConclusionsThis chapter provided some estimates of the link-level performance of a WiMAX and its dependenceon various physical-layer parameters and receiver structures. Based on these results, wecan derive the following high-level conclusions on the behavior of a WiMAX system.• The capacity curve of a WiMAX link is within 3dB of the Shannon capacity curve at lowto moderate SNRs. At high SNRs, the capacity of a WiMAX link is limited by allowedmodulation constellations.• The optional turbo codes provide a significant performance advantage over the mandatoryconvolutional codes. The additional complexity of the decoder for turbo codes is well justifiedby their performance benefit over the convolutional codes.• In fading channels, the band AMC subcarrier permutation provides significant performancebenefit over the PUSC subcarrier permutation at low speeds (< 10kmph). However,at moderate to high speeds, PUSC subcarrier permutation outperforms band AMC.• Multiantenna techniques give WiMAX a significant performance advantage over otherbroadband wireless techniques, such as HSDPA and 1xEV-DO. Closed-loop multiantennatechniques provide > 5dB of gain at low speeds (< 10kmph).• Advanced MIMO receivers based on the principles of maximum-likelihood detections andtheir derivatives provide additional link gains in excess of 5dB compared to linear receivers,such as MMSE.