S. Thoen

imec Belgium, Louvain, Flanders, Belgium

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Publications (21)8.83 Total impact

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    N. Khaled, S. Thoen, L. Deneire
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    ABSTRACT: 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</sub> 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.
    IEEE Transactions on Communications 05/2005; · 1.75 Impact Factor
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    ABSTRACT: To approach the potential MIMO capacity while optimizing the system performance, the 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 high bit-error rate (BER). In this paper, we propose to relax the constraint of fixed number of streams p and optimize this value under the constraints of fixed average total transmit power P<sub>T</sub> and fixed spectral efficiency R. The resulting spatially optimized joint Tx/Rx MMSE design is shown to lead to a 5.7 dB SNR gain over its full spatial multiplexing counterpart for a (3, 3) MIMO set-up at BER = 10<sup>-2</sup> and R = 12 bits/s/Hz over flat-fading channels. Such a significant improvement is due to the better exploitation of the MIMO spatial diversity and the more efficient power allocation enabled by the proposed spatial optimization.
    Signal Processing Advances in Wireless Communications, 2003. SPAWC 2003. 4th IEEE Workshop on; 07/2003
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    ABSTRACT: 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</sub> 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</sup> 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.
    Vehicular Technology Conference, 2003. VTC 2003-Spring. The 57th IEEE Semiannual; 05/2003
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    ABSTRACT: The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited bandwidth which necessitates a high spectral efficiency. A combined orthogonal frequency division multiplexing (OFDM)/spatial division multiple access (SDMA) approach can efficiently tackle both obstacles and paves the way for cheap, high-capacity wireless indoor networks. The channel distortion due to multipath propagation is efficiently mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. However, to keep the cost of an indoor wireless network comparable to its wired counterpart's cost, low-complexity SDMA processors with good performance are of special interest. In this paper, we propose a new multiuser SDMA detector which is designed for constant modulus signals. This constrained least squares (CLS) receiver, which deterministically exploits the constant modulus nature of the 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) receiver. Additionally, since the CLS detector relies on reliable channel knowledge at the receiver, we propose a strategy for estimating the multiple input multiple output (MIMO) channels. Simulations for a Hiperlan II-based case-study 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 M-PSK, the CLS detector outperforms the MMSF detector. Furthermore, the estimation complexity for the CLS detector is substantially lower than that for the MMSE detector which additionally requires estimation of the noise power.
    IEEE Transactions on Wireless Communications 02/2003; · 2.42 Impact Factor
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    ABSTRACT: The two major obstacles toward high-capacity indoor wireless networks are distortion due to the indoor channel and the limited availability of bandwidth which necessitates a high spectral efficiency. A combined orthogonal-frequency division multiplexing/spatial-diversity multiple access (OFDM/SDMA) approach can effectively tackle both obstacles. The channel distortion due to multipath propagation is easily mitigated with OFDM while the bandwidth efficiency can be increased with the use of SDMA. In order to keep the network's cost acceptable, simplified SDMA processors are preferred over the exponentially complex optimal maximum-likelihood processors. However, these simplified processors perform significantly worse than the optimal ones in terms of average bit error rate (BER). In this paper, we show that by adapting the constellation sizes applied on the individual subcarriers to the channel conditions, the performance of OFDM/SDMA processors can be significantly enhanced. In the uplink, we derive simple closed-form equations for the optimal constellation sizes for both a simple linear minimum mean-square error (MMSE) detector and for a nonlinear MMSE decision feedback equalizer (DFE) detector. Furthermore, for each detector, we introduce a simplified loading algorithm which lowers the computational and signaling complexity substantially at a small performance penalty. In the downlink, we study the dual precoders of the uplink detectors, respectively, the linear MMSE precoder and the nonlinear Tomlinson-Harashima (TH) MMSE-based precoder. For both precoders, we derive expressions for the optimal and the simplified constellation sizes. Additionally, we show that in time-division duplexing systems, the constellation distribution of a set of dual detectors/precoders is identical for up- and downlink, which effectively halves the computational complexity of adaptive loading. In the fully loaded uplink, the proposed adaptive loading algorithm results in a gain of 9 dB for a BER=10<sup>-3</sup> for the linear MMSE detector and a gain of 4.5 dB for the nonlinear MMSE-DFE detector. In the fully loaded downlink, a gain of 6.3 dB is achieved for the MMSE precoder and 5.5 dB for the TH-MMSE precoder.
    IEEE Transactions on Communications 12/2002; · 1.75 Impact Factor
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    ABSTRACT: Future wireless applications target multimedia and high-speed internet access, all requiring techniques to improve the link capacity and robustness. For wireless Local Area Network (LAN) the standards have chosen orthogonal frequency division multiplex (OFDM) as modulation scheme. An extension beyond the standards to improve the link capacity is adaptive loading. This paper presents the implementation aspects of an OFDM based wireless LAN modem together with adaptive loading on field programmable gate array (FPGA). The FPGA implementation of the wireless LAN modem enhances rapid prototyping and allows flexible integration of extensions to improve the quality of service. Implementation results are given and a comparison is made with an ASIC implementation of the modem. The wireless LAN system will be demonstrated on a generic platform that is developed for prototyping and demonstration of high speed communication systems.
    Solid-State Circuits Conference, 2002. ESSCIRC 2002. Proceedings of the 28th European; 10/2002
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    ABSTRACT: 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.
    IEEE Communications Letters 09/2002; · 1.16 Impact Factor
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    ABSTRACT: 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
    IEEE Transactions on Communications 02/2001; · 1.75 Impact Factor
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    ABSTRACT: 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
    Global Telecommunications Conference, 2001. GLOBECOM '01. IEEE; 02/2001
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    ABSTRACT: 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
    Vehicular Technology Conference, 2001. VTC 2001 Spring. IEEE VTS 53rd; 02/2001
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    ABSTRACT: 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
    Vehicular Technology Conference, 2001. VTC 2001 Fall. IEEE VTS 54th; 02/2001
  • IEEE Transactions on Communications. 01/2001; 49:5-8.
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    ABSTRACT: This paper reports on the activities at IMEC that will lead to a 100Mb/s indoor communication system based on multi-carrier modulation. The goal is to build a wireless local area network, which can compete with today's wired solutions in terms of cost and performance. It is shown how a dedicated ASIC architecture implements an OFDM modem with 256 carriers that allows transmission of complex symbols (QPSK, 16 QAM) at 50Mbaud/s. It is also demonstrated how adaptive turbo-decoding allows operation close to the Shannon limit and how it can be used to provide quality of service. Adaptive loading of the carriers provides a further capacity enhancement. A gain of 6.5dB in the required signal to noise ratio can be observed compared to standard non-adaptive schemes. Finally, an SDMA scheme with full channel characterization is proposed in order to increase the channel capacity even further. It turns out that even with short training sequences a substantial capacity improvement can be observed.
    10/2000;
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    ABSTRACT: 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
    Vehicular Technology Conference Proceedings, 2000. VTC 2000-Spring Tokyo. 2000 IEEE 51st; 02/2000
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    ABSTRACT: 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
    Solid-State Circuits Conference, 2000. Digest of Technical Papers. ISSCC. 2000 IEEE International; 02/2000
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    ABSTRACT: 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
    Global Telecommunications Conference, 2000. GLOBECOM '00. IEEE; 02/2000
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    ABSTRACT: 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 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 an SER=10<sup>-6</sup>
    Vehicular Technology Conference, 2000. IEEE VTS-Fall VTC 2000. 52nd; 02/2000
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    ABSTRACT: 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
    Vehicular Technology Conference, 1999 IEEE 49th; 08/1999
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    ABSTRACT: Orthogonal frequency division multiplexing (OFDM) shows clear benefits as a modulation technique for wireless local area networks (WLAN), and is hence being validated in recent standardization efforts. This paper presents some results of a high-speed OFDM transceiver ASIC recently developed at IMEC. This transceiver provides a large degree of flexibility through programmable parameters with respect to both channel and user requirements. The integration of a high-speed OFDM baseband transceiver on a single ASIC is an important step towards high-capacity WLAN at competitive prices
    Vehicular Technology Conference, 1999. VTC 1999 - Fall. IEEE VTS 50th; 02/1999
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    ABSTRACT: Wireless LANs based on OFDM are limited in their performance by the dips in the frequency spectrum of the indoor multi-path channel. To simulate performance, we first derive a simplified channel model from a ray-tracing analysis of the indoor channel. The performance of OFDM-based WLANs can be augmented by adaptive loading of the subcarriers, which has shown to be very valuable for DMT modems on wired media (for example in ADSL). In this paper, we investigate the application of adaptive loading to a 100 Mbit/s OFDM-based wireless LAN. From the different adaptive loading algorithms, the Fischer and Huber (1996) algorithm is selected because it offers computational advantages for similar performance. This algorithm is then integrated in a complete system by implementing a channel and noise power estimation. Simulations show that the proposed adaptive loading strategy improves the system performance considerably (6 dB gain for a BER=10<sup>-2</sup>). Moreover, the simulations confirm that the simplified channel model is a reliable description of the indoor channel
    Global Telecommunications Conference, 1998. GLOBECOM 98. The Bridge to Global Integration. IEEE; 02/1998