Sebastien Simoens

On the Gaussian MIMO Relay Channel With Full Channel State Information

This paper addresses the problem of source and relay transmit covariance optimization on the Gaussian MIMO relay channel with full channel state information (CSI), i.e., assuming perfect knowledge of all channels. For full-duplex relaying, we show that the cut-set bound on capacity can be computed as the solution of a convex problem, thus providing a tighter bound than previously published. For time division duplex (TDD) relaying, both upper and lower bounds on capacity are derived, and the transmit covariance matrices are optimized for decode-and-forward (DF) strategies with either partial or full decoding at the relay. A generic procedure is introduced to formulate these problems into a standard convex form, and to solve them efficiently. Suboptimum precoders are also proposed which have a specific matrix structure that either leads to a closed-form expression or at least reduces the dimension of the optimization problem. Practical aspects related to transmit power constraints and CSI availability are then discussed. Finally, simulations in a cellular downlink scenario show that the partial DF strategy can achieve a rate very close to capacity for realistic values of the source to relay SNR, and that the rate loss due to suboptimum precoder structures remains small for typical antenna configurations.

Compress-and-Forward Cooperative MIMO Relaying With Full Channel State Information

This paper addresses cooperative time division duplex (TDD) relaying in the multiple-antenna case with full channel state information (CSI), i.e., assuming perfect knowledge of all channels. The main focus of the paper is on the compress-and-forward (CF) strategy, for which an achievable rate on the Gaussian MIMO relay channel can be derived by applying distributed vector compression techniques. The processing at the CF relay consists in a conditional Karhunen-Loeve transform (CKLT) followed by a separate Wyner-Ziv (WZ) coding of each output stream at a different rate. The paper provides a simple analytical expression for the optimum WZ coding rates, and also proposes an iterative procedure to perform this optimization jointly with that of the transmit covariance matrices at the source and relay. The multiple access channel (MAC) formed by the source and relay transmitting simultaneously to the destination is considered, and it is shown that an optimal decoding order exists at least in the single-antenna case. We discuss the extension to MIMO-OFDM, as well as practical source coding implementation. The CF achievable rates are benchmarked with other upper and lower bounds on capacity. Simulation results show that CF can outperform decode-and-forward (DF) and approach capacity for realistic SNR values, which validates the performance of the proposed optimization procedure.

Cooperative Wireless Networking Beyond Store-and-Forward

In future wireless networks devices may cooperate to form logical links. Each of these links may consist of several independent physical channels which are shared by the cooperating partners. Even without multiple antennas this cooperation provides diversity in time and space. This so-called cooperation diversity increases the robustness of the link vs. fading and interference. After surveying approaches in cooperation diversity we focus on optimizing its performance by combining several cooperation schemes and by integrating cooperation into space-time coding. For multiple scenarios, we further discuss the factors and benefits introduced by user cooperation and how cooperation-aware resource allocation can be employed to further increase the performance of cooperative networks. When it comes to implementation, the question arises how cooperation can be integrated efficiently into existing wireless networks. A case study for 802.11-based WLANs reveals the issues that need to be solved in order to deploy cooperative techniques. We provide an overview of the state of the art in implementing cooperation approaches, analyze how appropriate these approaches solve the issues, and, where appropriate, point out their deficiencies. We conclude with a road map for future research necessary to tackle these deficiencies for the practical implementation of cooperation in next generation mesh, WLAN, WMAN, and cellular standards.

Error prediction for adaptive modulation and coding in multiple-antenna OFDM systems

In this paper, the problem of packet error rate (PER) prediction is addressed in the multiple-antenna broadband OFDM context, and its impact on adaptive modulation and coding (AMC) is quantified. The analysis is based on a physical layer comprising various modulation and coding schemes, ranging from robust space-time block coding (STBC) modes to high bit rate spatial division multiplexing (SDM) modes, and also hybrid SDM-STBC schemes. For each mode the expression of several link quality metrics (LQM) enabling PER prediction in the broadband OFDM channel, such as instantaneous signal-to-noise ratio (SNR), capacity, or exponential effective SNR metrics are provided. Their advantages and limitations are investigated. Finally, their performance is benchmarked in the IEEE 802.11a/g/n context. It is shown that the choice of the LQM has a significant impact on the throughput performance of the AMC algorithm.

The evolution of 5GHz WLAN toward higher throughputs

A standardization effort has started within the IEEE 802.11 working group to define the next generation of 802.11 wireless LANs. This article illustrates how throughput achieved above the MAC layer of 5 GHz WLANs can be increased from an existing 30 Mb/s maximum with 802.11a/g to rates exceeding 90 Mb/s. After a brief review of ongoing WLAN standardization activities, the support of a higher physical-layer bit rate by various standardized MAC protocols (802.11, 802.11e, and HIPERLAN/2) is discussed, showing the PHY and MAC layers must be considered jointly in order to achieve a significant throughput increase. Various physical layer techniques are compared in terms of performance and complexity. In particular, simulations show that by relying on MAC layers with good efficiency like 802.11e and HIPERLAN/2, a combination of space-time block coding with a possibility of channel bundling could bring a peak throughput increase from 30 to 90 Mb/s as well as a significant cell range increase.

A MMSE successive interference cancellation scheme for a new adjustable hybrid spread OFDM system

The effects of uniform spreading in OFDM-CDMA systems is the harmonization at the reception of the signal to noise ratio between the sub-bands which prevents the good performance of successive decoding algorithms. This paper proposes a new hybrid spread OFDM (SOFDM) transmission scheme in which the spreading of the information is adjustable and not uniform along the carrier (frequency selective). Moreover a MMSE version of the V-BLAST successive interference cancellation algorithm suited for this hybrid modulator is derived. The performance of the combination of SHOFDM and MMSE V-BLAST is shown to both outperform COFDM and conventional and classical iterative detection algorithms for SOFDM in the realistic scenario of the 5 GHz HiperLAN/2 system.

New I/Q imbalance modeling and compensation in OFDM systems with frequency offset

In this paper, a new model and compensation scheme of quadrature imbalance in analog I/Q OFDM transceivers is presented. Classically, the effect of I/Q imbalance is modeled by a crosstalk between pairs of symmetrical OFDM sub-carriers. We show that this model is not valid when the carrier frequency offset is compensated after the source of I/Q mismatch. Assuming a perfect digital compensation of the frequency offset in baseband, the I/Q imbalance generates interference between all sub-carriers and not only symmetrical ones. When analog components, multipath propagation and frequency offsets are taken into account, the complete matrix analysis has to make use of mathematical results such as the diagonalization of pseudo-circulant matrices. The existing models happen to be a sub-case of this new more general result. A new compensation scheme applying to every case is proposed as well as a receiver autocalibration procedure. It is shown that system-specific assumptions can simplify the implementation of the compensation. These assumptions are validated by simulation in the framework of a 5 GHz WLAN transceiver design with realistic production spread of the analog components.

Optimum performance of link adaptation in HIPERLAN/2 networks

In this paper link adaptation is investigated in the HIPERLAN/2 context. It is shown that updating the modulation and coding on a frame basis can bring a gain theoretically greater than 2.5 dB over the optimum long term approach. The influence of the small-scale fading properties in the various propagation environments and of the transmit and receive filters selectivity is studied. The throughput performance is computed both analytically and by network simulations, assuming constant interference and constant transmit power as well as perfect knowledge of the SNIR or PER. Finally, the throughput loss resulting from PER estimation errors is illustrated by simulations and a sub-optimum method better suited to implementation is presented.

Performance evaluation of some hybrid ARQ schemes in IEEE 802.11a networks

This paper investigates how a type II hybrid ARQ (HARQ) scheme combined with adaptive modulation and coding (AMC) can improve the throughput on top of the MAC layer, by comparing it to the existing type I HARQ + AMC mechanism in the IEEE 802.11a context. The studied type II HARQ+AMC strategy relies on incremental redundancy (IR) by use of rate compatible punctured codes as well as chase combining. The performance of the various strategies is mainly affected by two parameters. One of these is the inaccuracy of the link quality predictor, since the SNIR inherently fluctuates due to fading and interference; the other is the MAC overhead, which is especially important in IEEE 802.11a due to the CSMA/CA protocol. From the simulation results presented in this paper, it can be observed that even when MAC overhead is taken into account, the type II HARQ+AMC strategy still provides up to 1.5 dB gain with respect to current type I HARQ + AMC scheme.

Achievable Rates of Compress-and-Forward Cooperative Relaying on Gaussian Vector Channels

The compress-and-forward (C&F) cooperative relaying strategy is known to outperform decode-and-forward (D&F) when the relay is close to the destination. In this paper, we derive achievable rates on Gaussian vector channels with cooperative C&F relaying. In order to extend previous information-theoretic results from the scalar to the vector Gaussian channel, we exploit recent results in distributed source coding. Like in source coding with side information at the decoder, the relay applies a conditional Karhunen Loeve transform (CKLT) to its observed signal, followed by a separate Wyner-Ziv encoding of each output stream with a different rate and under a sum-rate constraint. However, these Wyner-Ziv coding rates are such that the total mutual information between the source and destination is maximized. This differs from the conventional source coding approach in which the rates are selected to minimize the total squared distortion, leading to the well-known reverse-waterfilling algorithm. We show that the maximization of the C&F mutual information is also a convex problem. The optimum Wyner-Ziv coding rates have a simple analytical expression, and can be obtained by a waterfilling algorithm. Finally, we illustrate these results by simulations of MIMO-OFDM relaying in a system similar to IEEE802.16.