On the sphere-decoding algorithm I. Expected complexity
IEEE Transactions on Signal Processing - Part I
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Recent research has shown that code-division multiplex orthogonal frequency-division multiplex (CDM-OFDM) significantly improves the robustness of conventional OFDM against subcarrier fading. In particular, the combination of CDM with multiple transmit antennas has been shown to be very effective in the context of IEEE 802.11n. In this paper another distinctive feature of the forthcoming WLAN standard, namely, the existence of a feedback channel, is exploited to implement a frequency diversity adaptive technique. In the proposed method the receiver estimates an effective diversity order of the underlying wireless channel and feeds it back to the transmitter, which then uses it to appropriately dimension the CDM component. At the same time, the receiver can switch between iterative or noniterative processing depending on the CDM dimension of the forthcoming packets. Results show that this adaptivity can bring along important reductions in complexity without compromising error performance.