Patrick Vandenameele

A combined OFDM/SDMA approach

Two major technical challenges in the design of future broadband wireless networks are the impairments of the propagation channel and the need for spectral efficiency. To mitigate the channel impairments, orthogonal frequency division multiplexing (OFDM) can be used, which transforms a frequency-selective channel in a set of frequency-flat channels. On the other hand, to achieve higher spectral efficiency, space division multiple access (SDMA) can be used, which reuses bandwidth by multiplexing signals based on their spatial signature. In this paper, we present a combined OFDM/SDMA approach that couples the capabilities of the two techniques to tackle both challenges at once. We propose four algorithms, ranging from a low-complexity linear minimum mean squared error (MMSE) solution to the optimal maximum likelihood (ML) detector. By applying per-carrier successive interference cancellation (pcSIC), initially proposed for DS-CDMA, and introducing selective state insertion (SI), we achieve a good tradeoff between performance and complexity. A case study demonstrates that, compared to the MMSE approach, our pcSIC-SI-OFDM/SDMA algorithm obtains a performance gain of 10 dB for a BER of 10/sup -3/, while it is only three times more complex. On the other hand, it is two orders of magnitude less complex than the ML approach, for a performance penalty of only 2 dB.

A low-complexity ML channel estimator for OFDM

Orthogonal frequency-division multiplexing with cyclic prefix enables low-cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum-likelihood channel estimator. In addition to being optimal (up to the modeling error), this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

Adaptive loading strategy for a high speed OFDM-based WLAN

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/). Moreover, the simulations confirm that the simplified channel model is a reliable description of the indoor channel.

A combined OFDM/SDMA approach for WLAN

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.

A low complexity ML channel estimator for OFDM

Orthogonal frequency-division multiplexing (OFDM) with cyclic prefix enables low cost frequency-domain mitigation of multipath distortion. However, to determine the equalizer coefficients, knowledge of the channel frequency response is required. While a straightforward approach is to measure the response to a known pilot symbol sequence, existing literature reports a significant performance gain when exploiting the frequency correlation properties of the channel. Expressing this correlation by the finite delay spread, we build a deterministic model parametrized by the channel impulse response and, based on this model, derive the maximum likelihood (ML) channel estimator. In addition to being optimal, this estimator receives an elegant time-frequency interpretation. As a result, it has a significantly lower complexity than previously published methods.

A novel class of uplink OFDM/SDMA algorithms for WLAN

Wireless LANs (WLANs) still offer low capacity compared to conventional wired LANs. The design of high-capacity WLANs is impeded mainly by the impairments of the indoor channel and by bandwidth and transmit-power restrictions. To mitigate these, Vandenameele et al. (1999) proposed an OFDM/SDMA approach that combines the advantages of orthogonal frequency division multiplexing (OFDM) with those of space division multiple access (SDMA). However, the comparison of two uplink OFDM/SDMA algorithms, respectively based on the minimum-mean-squared-error and maximum-likelihood criterion, revealed a serious performance and complexity gap between them. In this paper, we propose a novel class of uplink OFDM/SDMA algorithms that bridge this gap. To this end, we introduce per-carrier successive interference cancellation and interference-dependent state insertion. Compared to the minimum-mean-squared-error approach, a performance gain of 10 dB is obtained for a BER of 10/sup -3/ This improvement comes at less than three times the initial implementation cost.

A novel class of uplink of OFDM/SDMA algorithms: a statistical performance analysis

The design of high-capacity wireless LANs (WLANs) is impeded mainly by indoor channel distortions and bandwidth limitations. To mitigate these, we previously proposed an OFDM/SDMA approach that combines the advantages of orthogonal frequency division multiplexing (OFDM) with those of space division multiple access (SDMA). We also introduced a novel class of implementation-efficient uplink algorithms. In this paper, based on results in multivariate statistical analysis, we present the analysis of the full statistics of the performance of OFDM/SDMA. In addition to offering theoretical evidence for the simulated BER results, this analysis allows the direct calculation of the packet error rate (PER), features scalability in terms of the number of antennas and/or users and calculates the effect of imperfect power control.

A digital 80 Mb/s OFDM transceiver IC for wireless LAN in the 5 GHz band

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.

An SDMA algorithm for high-speed WLAN. Performance and complexity

Wireless LANs (WLANs) still offer low capacity compared to conventional wired LANs. To increase the capacity of a bandwidth-limited wireless system, its spatial efficiency can be improved. Pico-cellular WLAN architectures have been proposed to this aim. However, we showed in a previous study that pico-cellularisation has serious disadvantages. Instead, space division multiple access (SDMA) enables spatial selectivity within one cell. We present a blind SDMA algorithm suitable for a 100 Mbps WLAN. Its design is based on rigorous indoor channel modeling and is targeted towards implementation. The performance of the proposed algorithm in typical office environments is assessed using a semi-analytic simulation approach. Furthermore, the complexity required for a hardware realization is determined. We conclude that a frequency re-use factor of four is achieved by a four antenna receiver using 2000 snapshots for channel estimation.

A High IIP2 SAW-Less Superheterodyne Receiver With Multistage Harmonic Rejection

In this paper, we propose and demonstrate the first fully integrated surface acoustic wave (SAW)-less superheterodyne receiver (RX) for 4G cellular applications. The RX operates in discrete-time domain and introduces various innovations to simultaneously improve noise and linearity performance while reducing power consumption: a highly linear wideband noise-canceling low-noise transconductance amplifier (LNTA), a blocker-resilient octal charge-sharing bandpass filter, and a cascaded harmonic rejection circuitry. The RX is implemented in 28-nm CMOS and it does not require any calibration. It features NF of 2.1-2.6 dB, an immeasurably high input second intercept point for closely-spaced or modulated interferers, and input third intercept point of 8-14 dBm, while drawing only 22-40 mW in various operating modes.