To approach the potential multiple-input multiple-output (MIMO) capacity while optimizing the system bit-error rate (BER) performance, the joint transmit and receive minimum mean squared error (joint Tx/Rx MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams p as well as fixed modulation and coding across these spatial streams. However, the number of spatial streams has been arbitrarily chosen and fixed, which may lead to an inefficient power allocation strategy and a poor BER performance. In this paper, we relax the constraint of fixed number of streams p and optimize this value for the current channel realization, under the constraints of fixed average total transmit power P/sub T/ and fixed rate R, what we refer to as mode selection . Based on the observation of the existence of a dominant optimal number of streams value for the considered Rayleigh flat-fading MIMO channel model, we further propose an "average" mode selection that avoids the per-channel adaptation through using the latter dominant value for all channel realizations. Finally, we exhibit the significant BER improvement provided by our mode selection over the conventional joint Tx/Rx MMSE design. Such significant improvement is due to the better exploitation of the MIMO spatial diversity and the more efficient power allocation enabled by our mode selection.

To approach the potential MIMO capacity while optimizing the system bit error rate (BER) performance, the joint transmit and receive minimum mean squared error (MMSE) design has been proposed. It is the optimal linear scheme for spatial multiplexing MIMO systems, assuming a fixed number of spatial streams as well as a fixed modulation and coding across these spatial streams. However, state-of-the-art designs arbitrarily choose and fix the value of the number of spatial streams , which may lead to an inefficient power allocation strategy and a poor BER performance. We have previously proposed to relax the constraint of fixed number of streams and to optimize this value under the constraints of fixed average total transmit power and fixed spectral efficiency, which we referred to as spatial-mode selection. Our previous selection criterion was the minimization of the system sum MMSE. In the present contribution, we introduce a new and better spatial-mode selection criterion that targets the minimization of the system BER. We also provide a detailed performance analysis, over flat-fading channels, that confirms that our proposed spatial-mode selection significantly outperforms state-of-the-art joint Tx/Rx MMSE designs for both uncoded and coded systems, thanks to its better exploitation of the MIMO spatial diversity and more efficient power allocation.

The two major obstacles towards high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth. A combined OFDM/SDMA approach efficiently tackles both obstacles and paves the way for low cost, high-capacity wireless indoor networks (see Vandenameele, P. et al., VTC Spring, p.1712-16, 1999; IEEE J. Selected Areas in Commun., vol.18, no.11, p.2312-21, 2000). Channel distortion due to multipath propagation is efficiently mitigated by orthogonal frequency division multiplexing (OFDM) while the bandwidth efficiency is increased by the use of spatial diversity multiple access (SDMA). To keep the WLAN cost low, simple SDMA processors with good performance are necessary. Hence, we propose a new, low-complexity multiuser SDMA detector, which is designed for constant modulus signals. This linear constrained least squares (CLS) receiver, which explicitly exploits the constant modulus nature of subcarrier modulation to achieve better separation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receivers. Simulations show that the CLS detector significantly outperforms the ZF detector and comes close to the performance of the MMSE detector for QPSK. For higher order MPSK, the CLS detector outperforms the MMSE detector substantially.

To approach the potential MIMO capacity while optimizing the system performance, a joint Tx/Rx MMSE design has been proposed. It is an optimal linear scheme for spatial multiplexing MIMO systems assuming a fixed number of parallel data streams as well as fixed modulation and coding across these streams. The use of OFDM enables a low-complexity implementation of this design for frequency-selective MIMO channels. In such a MIMO/OFDM set-up, the state-of-the-art joint Tx/Rx MMSE approach would arbitrarily choose and fix the number of space-frequency data streams p to be transmitted, which may lead to an inefficient power allocation strategy and a non-minimum bit-error-rate (BER). Therefore, in this paper, we propose to relax the fixed number of streams p constraint and address the issue of optimizing p for this design under fixed average total transmit power P/sub T/ and fixed global rate R constraints. In typical frequency-selective indoor channels, the resulting space-frequency optimized joint Tx/Rx MMSE design is shown to lead to a 10 dB SNR gain over the full spatial multiplexing conventional joint Tx/Rx MMSE design for a (2,2) MIMO/OFDM set-up at BER = 10/sup -3/ and an average spectral efficiency of 4 bits/carrier. Such a dramatic improvement is due to the better exploitation of the system's frequency and space diversity enabled by the proposed optimization.

The combination of orthogonal frequency division multiplexing (OFDM) with linear processing-based spatial diversity multiple access (SDMA) enables high-capacity wireless local networks at reasonable implementation complexity. We investigate adaptive loading of the carriers as a means for lowering the relatively high symbol error rate (SER) of such linear OFDM/SDMA networks and propose efficient closed-form expressions for the optimal bit distributions. We analyze the performance and complexity of the proposed algorithms and show that substantial gains in SER can be achieved. Furthermore, simplified algorithms are proposed which have a small performance penalty but which possess a lower computational and signaling cost.

We propose a new stochastic model for the time-variance of channels for fixed wireless communications. In the classical Jakes' Doppler spectrum, the receiver (or the transmitter) is assumed to move at a certain speed. However, in fixed wireless communication systems, both the transmitter and the receiver are stationary and time-variations are actually due to moving scatterers. A novel stochastic model for this sort of time-varying channels is introduced specifically for indoor environments, which can be employed to more accurately simulate the performance of fixed wireless communications.

An exact knowledge of the problem, is often the most crucial step towards finding a solution. Therefore, when making plans for getting wireless OFDM systems to work, it is sound practice to start with an assessment of the wireless transmission channel. This chapter introduced the basic propagation effects to take into account. The focus is mainly on the indoor environment. The propagation is shown to be subject to rich multipath reflection, and slow time variation.

The average bit-error rate of transmit antenna selection combined with receive maximum-ratio combining is computed as a function of the transmit antenna update rate when using binary phase-shift keying in flat Rayleigh fading channels. This scheme achieves an order of diversity equal to the product of the number of transmit and receive antennas. Therefore, it can gain significant diversity benefits over traditional receive diversity schemes by distributing the antennas over the transmit and receive side.

The two major obstacles towards high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth which necessitates a high spectral efficiency. The combination of single carrier with cyclic prefix (SC-CP) modulation and spatial division multiple access (SDMA) tackles both obstacles and paves the way for low cost, high-capacity wireless indoor networks (see Vandenameele, P. et al., ICASSP, p.3714-17, 2000). The channel distortion due to multipath propagation is elegantly mitigated with SC-CP while the bandwidth efficiency is increased with the use of SDMA. However, to keep the WLAN cost low, simple SDMA processors with good performance are necessary. Hence, we propose a new low-complexity multiuser SDMA detector, which is designed for constant modulus signals. This linear constrained least squares (CLS) receiver, which explicitly exploits the constant modulus nature of the modulation, is compared in terms of performance and complexity with the zero forcing (ZF) and the minimum mean square error (MMSE) receiver. Simulations show that the CLS detector significantly outperforms the ZF detector and comes very close to the performance of the MMSE detector. Furthermore, the estimation complexity for the CLS detector is substantially lower than for the MMSE detector, which additionally requires estimation of the noise power

A combined OFDM/SDMA approach substantially increases the capacity of wireless local networks while maintaining a feasible implementation complexity. In the downlink of OFDM/SDMA-based networks however, the standard channel inversion approach significantly boosts the transmit power. We introduce a low-complexity downlink OFDM/SDMA strategy, based on the combination of channel inversion and Tomlinson-Harashima (1971, 1972) pre-coding, which lowers the required transmit power by 4 dB. Additionally, adaptive loading of the subcarriers as a means for further lowering the required transmit power in the downlink of OFDM/SDMA networks utilizing the proposed pre-compensating strategy is studied. We derive an efficient closed-form expression for the optimal bit distribution and show that an additional 8.5 dB gain in transmit power can be achieved. Finally, a simplified loading algorithm is proposed which possesses a lower computational and signaling cost in return for a small performance penalty.

A combined OFDM/SDMA approach substantially increases the capacity of wireless local networks while maintaining a feasible implementation complexity. We introduce a new precompensating downlink OFDM/SDMA strategy which achieves diversity A-U+1 for all U users, with A the size of the basestation's antenna array. We compare this scheme with a pre-compensating strategy we have proposed previously. Furthermore, adaptive loading as a means for increasing the performance in the downlink of such OFDM/SDMA networks is studied. We derive an efficient closed-form expression for the optimal bit distribution and show that substantial gains in symbol error rate (SER) can be achieved at low additional complexity. Finally, a simplified loading algorithm is proposed which possesses a lower computational and signaling cost in return for a small performance penalty

Adaptive loading is able to significantly reduce the average Symbol Error Rate (SER) of OFDM-based wireless local networks by exploiting the channel frequency diversity. In time-varying channels however these gains disappear quickly when the channel state feedback rate is limited. We show in this paper that by combining ML channel estimation with robust MMSE prediction filters. we can predict the optimal constellation sizes and regain most of the achievable gain of adaptive loading. With only 4 channel measurements spaced one MAC frame apart, the loss compared to the performance of adaptive loading with perfect channel knowledge is limited to 0.7 dB and a gain of 10 dB is achieved over non-adaptive OFDM for a SER = 10(-6).

Emerging standards for broadband wireless LANs (WLANs) such as IEEE802.11a, Hiperlan-II and MMAC require orthogonal frequency division multiplex (OFDM) modulation for the physical layer (PHY) interface. OFDM modems were previously realized in the context of very high speed digital subscriber lines (VDSL) and digital audio broadcast (DAB). However, the WLAN application puts different constraints on the OFDM transceiver presented here because the terminals are portable. Therefore, an adaptive frequency domain equalizer is integrated to mitigate the variations of the indoor wireless channel. Fast programmable on-chip acquisition hardware supports burst mode communications.

Wireless LANs (WLANs) still offer low capacity compared to wired LANs. The two major obstacles towards increasing the capacity are (a) distortion due to the indoor channel and (b) limitations on the bandwidth-power budget. In this paper we propose an OFDM/SDMA approach that combines orthogonal frequency division multiplexing (OFDM), for multipath channel distortion mitigation, with SDMA, for bandwidth efficiency. In addition, our approach concentrates all SDMA functionality in the basestation, which reduces the overall system cost. We give algorithms for both the uplink and downlink, determine their performance and analyse their implementation complexity. It is shown that OFDM/SDMA enables high-capacity WLANs at reasonable implementation complexity