Frederik Petré

Space-time block coding for single-carrier block transmission DS-CDMA downlink

The combination of space-time block coding (STBC) and direct-sequence code-division multiple access (DS-CDMA) has the potential to increase the performance of multiple users in a cellular network. However, if not carefully designed, the resulting transmission scheme suffers from increased multiuser interference (MUI), which dramatically deteriorates the performance. To tackle this MUI problem in the downlink, we combine two specific DS-CDMA and STBC techniques, namely single-carrier block transmission (SCBT) DS-CDMA and time-reversal STBC. The resulting transmission scheme allows for deterministic maximum-likelihood (ML) user separation through low-complexity code-matched filtering, as well as deterministic ML transmit stream separation through linear processing. Moreover, it can achieve maximum diversity gains of N/sub T/N/sub R/(L+1) for every user in the system, irrespective of the system load, where N/sub T/ is the number of transmit antennas, N/sub R/ the number of receive antennas, and L the order of the underlying multipath channels. In addition, it turns out that a low-complexity linear receiver based on frequency-domain equalization comes close to extracting the full diversity in reduced, as well as full load settings. In this perspective, we also develop two (recursive) least squares methods for direct equalizer design. Simulation results demonstrate the outstanding performance of the proposed transceiver compared to competing alternatives.

Interpolation-Based Multi-Mode Precoding for MIMO-OFDM Systems with Limited Feedback

Spatial multiplexing with multi-mode precoding provides a means to achieve both high capacity and high reliability in multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) systems. Multi-mode precoding uses linear transmit precoding that adapts the number of spatial multiplexing data streams or modes, according to the transmit channel state information (CSI). As such, it typically requires complete knowledge of the multi-mode precoding matrices for each subcarrier at the transmitter. In practical scenarios where the CSI is acquired at the receiver and fed back to the transmitter through a low-rate feedback link, this requirement may entail a prohibitive feedback overhead. In this paper, we propose to reduce the feedback requirement by combining codebook-based precoder quantization, to efficiently quantize and represent the optimal precoder on each subcarrier, and multi-mode precoder frequency down-sampling and interpolation, to efficiently reconstruct the precoding matrices on all subcarriers based on the feedback of the indexes of the quantized precoders only on a fraction of the subcarriers. To enable this efficient interpolation-based quantized multimode precoding solution, we introduce (1) a novel precoder codebook design that lends itself to precoder interpolation, across subcarriers, followed by mode selection, (2) a new precoder interpolator and, finally, (3) a clustered mode selection approach that significantly reduces the feedback overhead related to the mode information on each subcarrier. Monte-Carlo bit-error rate (BER) performance simulations demonstrate the effectiveness of the proposed quantized multi-mode precoding solution, at reasonable feedback overhead

Pilot-aided adaptive chip equalizer receiver for interference suppression in DS-CDMA forward link

In the forward link of DS-CDMA systems, the multipath propagation channel destroys the orthogonality of the spreading codes and therefore causes multi-user interference (MUI). Ideal non-adaptive zero-forcing (ZF) and MMSE chip equalizer receivers have been proposed to restore the orthogonality and suppress the MUI in DS-CDMA systems employing aperiodic scrambling codes. However, these receivers can only deal with slow fading multipath channels. Practical adaptive implementations of the chip equalizer receiver that can track fast fading multipath channels, are nor straightforward, since no continuous training chip sequence is available. In this paper we propose a new pilot-aided adaptive fractionally-spaced chip equalizer receiver; that is well-suited for fast fading multipath channels. By exploiting the presence of a continuous pilot signal in forthcoming third generation cellular and LEO satellite communication systems, it continuously updates at the symbol rate using a simple NLMS or a more advanced RLS adaptive scheme. The proposed receiver out-performs the RAKE receiver with perfect channel knowledge over a wide range of normalized Doppler spreads, with a complexity that is still independent of the number of users.

Downlink frequency-domain chip equalization for single-carrier block transmission DS-CDMA with known symbol padding

Single-carrier block transmission (SCBT) DS-CDMA, also known as chip-interleaved block-spread (CIBS) CDMA, is an interesting transmission technique for future broadband cellular systems because it inherits the benign properties of both SCBT and CDMA. By zero padding (ZP) each chip block, the orthogonality of the spreading codes is retained regardless of the underlying multipath channel which allows for deterministic maximum likelihood (ML) user separation employing low-complexity code-matched filtering. In this paper, we focus on downlink single-carrier block transmissions with known symbol padding (KSP) (as opposed to ZP) which pad each chip block with a postfix of known symbols that can be used for training purposes at the receiver. Specifically, we propose three methods for direct equalizer estimation that all exploit the presence of the known symbol postfix but differ in the amount of additional a-priori information they assume to determine the equalizer coefficients. Simulation results demonstrate the outstanding performance of the semi-blind joint CDMP/KSP-trained method, that additionally assumes knowledge of a code division multiplexed pilot (CDMP) and the multiuser code correlation matrix.

Semi-blind space-time chip equalizer receivers for WCDMA forward link with code-multiplexed pilot

In the forward link of WCDMA systems, the multipath propagation channel destroys the orthogonality of the spreading codes and therefore causes multi-user interference (MUI). We propose new training-based and semi-blind space-time chip equalizer receivers for the forward link of WCDMA systems with a continuous code-multiplexed pilot. Both least-squares (LS) algorithms for block processing and recursive least-squares (RLS) algorithms for adaptive processing are derived. The proposed receivers can track fast fading multipath channels and outperform the RAKE receiver with perfect channel knowledge.

Space-time chip equalization for space-time coded downlink CDMA

In downlink CDMA, frequency-selectivity destroys the orthogonality of the user signals and introduces multi-user interference (MUI). A space-time chip equalizer is an attractive tool to restore the orthogonality of the user signals and suppress MUI. Recently, efficient pilot-based methods have been developed to design such a space-time chip equalizer. In this paper, we show how these pilot-based methods can be generalized to space-time coded downlink CDMA. As space-time coded downlink CDMA transmission scheme, we consider the conventional single-antenna downlink CDMA transmission scheme followed by a space-time block code for single-carrier block transmissions that exploits the maximum achievable diversity in a frequency-selective fading channel. Simulation results show improved performance over a pilot-based space-time RAKE-type receiver applied to the space-time coded downlink CDMA transmission schemes that were proposed for the UMTS and IS-2000 W-CDMA standards.

From MIMO-OFDM algorithms to a real-time wireless prototype: a systematic Matlab-to-hardware design flow

To assess the performance of forthcoming 4th generation wireless local area networks, the algorithmic functionality is usually modelled using a high-level mathematical software package, for instance, Matlab. In order to validate the modelling assumptions against the real physical world, the high-level functional model needs to be translated into a prototype. A systematic system design methodology proves very valuable, since it avoids, or, at least reduces, numerous design iterations. In this paper, we propose a novel Matlab-to-hardware design flow, which allows to map the algorithmic functionality onto the target prototyping platform in a systematic and reproducible way. The proposed design flow is partly manual and partly tool assisted. It is shown that the proposed design flow allows to use the same testbench throughout the whole design flow and avoids time-consuming and error-prone intermediate translation steps.

Multicarrier block-spread CDMA for broadband cellular downlink

Effective suppression of multiuser interference (MUI) and mitigation of frequency-selective fading effects within the complexity constraints of the mobile constitute major challenges for broadband cellular downlink transceiver design. Existing wideband direct-sequence (DS) code division multiple access (CDMA) transceivers suppress MUI statistically by restoring the orthogonality among users at the receiver. However, they call for receive diversity and multichannel equalization to improve the fading effects caused by deep channel fades. Relying on redundant block spreading and linear precoding, we design a so-called multicarrier block-spread- (MCBS-)CDMA transceiver that preserves the orthogonality among users and guarantees symbol detection, regardless of the underlying frequency-selective fading channels. These properties allow for deterministic MUI elimination through low-complexity block despreading and enable full diversity gains, irrespective of the system load. Different options to perform equalization and decoding, either jointly or separately, strike the trade-off between performance and complexity. To improve the performance over multi-input multi-output (MIMO) multipath fading channels, our MCBS-CDMA transceiver combines well with space-time block-coding (STBC) techniques, to exploit both multiantenna and multipath diversity gains, irrespective of the system load. Simulation results demonstrate the superior performance of MCBS-CDMA compared to competing alternatives.

Quantized multi-mode precoding for spatial multiplexing MIMO-OFDM system

Spatial multiplexing with multi-mode precoding can achieve both high capacity and high reliability in multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) systems. Multimode precoding uses linear transmit precoding but adapts the number of transmit streams or modes according to the channel conditions. Multi-mode precoding typically requires complete knowledge of the transmit precoding matrices for each subcarrier at the transmitter. In this paper we propose to reduce the feedback requirements by sending back the quantized precoding matrices of a fraction of the subcarriers and obtaining the other precoders using interpolation. Two algorithms are proposed for the interpolation of unitary matrices. Bit error rates simulations demonstrate the performance improvements of the proposed algorithms as a function of the feedback rate.

Space-time chip equalization for maximum diversity space-time block coded DS-CDMA downlink transmission

In the downlink of DS-CDMA, frequency-selectivity destroys the orthogonality of the user signals and introduces multiuser interference (MUI). Space-time chip equalization is an efficient tool to restore the orthogonality of the user signals and suppress the MUI. Furthermore, multiple-input multiple-output (MIMO) communication techniques can result in a significant increase in capacity. This paper focuses on space-time block coding (STBC) techniques, and aims at combining STBC techniques with the original single-antenna DS-CDMA downlink scheme. This results into the so-called space-time block coded DS-CDMA downlink schemes, many of which have been presented in the past. We focus on a new scheme that enables both the maximum multiantenna diversity and the maximum multipath diversity. Although this maximum diversity can only be collected by maximum likelihood (ML) detection, we pursue suboptimal detection by means of space-time chip equalization, which lowers the computational complexity significantly. To design the space-time chip equalizers, we also propose efficient pilot-based methods. Simulation results show improved performance over the space-time RAKE receiver for the space-time block coded DS-CDMA downlink schemes that have been proposed for the UMTS and IS-2000 W-CDMA standards.