In this paper, we consider the performance of WiMax systems in fast varying environments. While optimal in static channels, we first show that the conventional single tap OFDM equalizer is not satisfying even at moderate terminal speeds of 60km/h. We then study the performance of linear sliding window equalizers in the WiMax context before focusing on decision feedback equalizers (DFE). Based on our analysis, we propose a new DFE equalizer using sliding window that offers a good trade off between performance and complexity.

Mobile WiMAX systems are based on the IEEE 802.16e specifications, which include two mandatory MIMO profiles for the downlink. One of these is Alamouti's space-time code (STC) for transmit diversity, and the other is a 2times2 spatial multiplexing MIMO scheme. In this paper, we compare the two schemes assuming that the latter employs maximum-likelihood detection. The analysis shows that at the same spectral efficiency, Alamouti's STC combined with maximum-ratio combining (MRC) at the receiver significantly outperforms the 2times2 spatial multiplexing scheme at high values of the signal-to-noise ratio (SNR). Next, selection of a MIMO option is included in link adaptation to maximize network capacity, and operating SNR regions are determined for different modulation, coding and MIMO combinations.

This article extends turbo demodulation to the zero-padded OFDM (ZP-OFDM) system by accounting for the noise color and the symbols estimates correlation introduced by equalization. Resorting to realistic simulations, we show that turbo demodulation used with set partitioning labeling can significantly outperform noniterative decoding with Gray labeling. We also show that it increases the performance gap between ZP-OFDM and OFDM with cyclic prefix (CP-OFDM) relative to noniterative decoding, because it amplifies the performance gain due to the guaranteed symbol recovery of ZP-OFDM.

Zero padding (ZP) of multicarrier transmissions has been proposed as an appealing alternative to the traditional cyclic prefix (CP) orthogonal frequency-division multiplexing (OFDM) to ensure symbol recovery regardless of the channel zero locations. In this paper, both systems are studied to delineate their relative merits in wireless systems where channel knowledge is not available at the transmitter. Two novel equalizers are developed for ZP-OFDM to tradeoff performance with implementation complexity. Both CP-OFDM and ZP-OFDM are then compared in terms of transmitter nonlinearities and required power backoff. Next, both systems are tested in terms of channel estimation and tracking capabilities. Simulations tailored to the realistic context of the standard for wireless local area network HIPERLAN/2 illustrate the pertinent tradeoffs.

The widespread application of OFDM for local area mobile wireless broadband systems is strongly motivated by the simple equalization it affords. OFDM's bottleneck of channel-dependent performance was only previously addressed by replacing the cyclic prefix (CP) with trailing zeros (TZ). Such zero-padded OFDM transmissions enable FIR channel-irrespective symbol recovery which results in significant BER gains. The price paid is increased receiver complexity compared to the classical CP-OFDM. To reduce complexity, this paper proposes two novel equalizers for TZ-OFDM. One is based on an overlap and add approach and exhibits exactly the same performance and complexity as CP-OFDM. The second offers BER performance close to the TZ-OFDM MMSE equalizer and incurs a moderate complexity increase. An extension of an existing CP-OFDM pilot-based channel estimation method is also derived for the proposed equalization schemes. Simulations are conducted in a realistic HiperLAN/2 scenario comparing the various OFDM transceivers.

This paper proposes a simple equalizer for a multicarrier block transmission scheme which pads zeros (as opposed to a cyclic prefix) in each transmitted block. In the absence of high-power amplifier induced nonlinear effects, it is shown that the resulting scheme is simply the dual of the classical cyclic prefix OFDM transceiver. Comparison between the two systems that takes into account nonlinear distortions introduced by the clipper is also performed in the practical context of the wireless broadband 5 GHz HiperLAN/2 system. Both the classical pilot-based as well as novel (semi-) blind subspace algorithms are applied for channel estimation. The advantage of subspace methods in lowering the variation of channel tracking is corroborated with simulations which verify practically accurate channel estimates along the bursts.

Two new blind channel identification methods suited to multicarrier system (OFDM) exploiting the redundancy introduced by the adjunction of a cyclic prefix at the emitter and relying on the evaluation of the received signal autocorrelation matrix are presented. The proposed algorithms are able to identify any channel without any constraint on their zeroes location (including non-minimum phase channels) and are robust to the addition of white noise. Moreover a further enhancement of the estimation accuracy can easily be achieved by taking advantage of already present training symbols in current systems operating in a semi-blind context. Furthermore one of the two identification strategies has a very low arithmetical complexity which makes it particularly attractive in practice. Notice that these methods are not restricted to classical DFT-type modulators and still apply with any perfect reconstruction modulator.