Karine Gosse

Transmit diversity and spatial multiplexing in four-transmit-antenna OFDM

This paper studies a practical design for achieving both spatial diversity and spatial multiplexing in an OFDM system with four transmit antennas. In a certain range of spectral efficiency, the proposed hybrid space-time block coding/multiple-input multiple-output (STBC/MIMO) design is found to be a good design choice based on performance and receiver complexity, when compared with MIMO-only or diversity-only STBC designs. For the hybrid design, we propose a simple receive algorithm that requires no matrix inversion. Outage capacity evaluated with a Monte Carlo approach is used to provide insight into the simulation-based comparison of the different designs. The robustness to channels that lack spatial separability is also studied in the simulations.

The road to IMT-advanced communication systems: State-of-the- art and innovation areas addressed by the WINNER + project

Phases I and II of the WINNER project contributed to the development, integration, and assessment of new mobile network techniques from 2004 to 2007. Some of these techniques are now in the 3GPP LTE and IEEE 802.16 (WiMAX) standards, while others are under consideration for LTE-Advanced and 802.16m. The WINNER+ project continues this forwardlooking work for IMT-advanced technologies and their evolution, with a particular focus on 3GPP LTE-advanced. This article provides an overview of the WINNER system concept and several of its key innovative components.

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.

Perfect reconstruction versus MMSE filter banks in source coding

Classically, the filter banks (FBs) used in source coding schemes have been chosen to possess the perfect reconstruction (PR) property or to be maximally selective quadrature mirror filters (QMFs). This paper puts this choice back into question and solves the problem of minimizing the reconstruction distortion, which, in the most general case, is the sum of two terms: a first one due to the non-PR property of the FB and the other being due to signal quantization in the subbands. The resulting filter banks are called minimum mean square error (MMSE) filter banks. Several quantization noise models are considered. First, under the classical white noise assumption, the optimal positive bit rate allocation in any filter bank (possibly nonorthogonal) is expressed analytically, and an efficient optimization method of the MMSE filter banks is derived. Then, it is shown that while in a PR FB, the improvement brought by an accurate noise model over the classical white noise one is noticeable, it is not the case for the MMSE FB. The optimization of the synthesis filters is also performed for two measures of the bit rate: the classical one, which is defined for uniform scalar quantization, and the order-one entropy measure. Finally, the comparison of rate-distortion curves (where the distortion is minimized for a given bit rate budget) enables us to quantify the SNR improvement brought by MMSE solutions.

Orthogonal full diversity space-time block coding based on transmit channel state information for 4 Tx antennas

In this paper we consider the design of space-time block codes to benefit from multiple antennas at the emitter and possibly at the receiver. Theory of orthogonal designs has shown that such codes can not simultaneously be orthogonal, of rate 1, with full diversity order, for more than 2 transmit antennas. We propose in this paper to exploit channel state information available at the emitter to overcome this limitation and we describe a new rate 1 coding scheme with full diversity for 3 or 4 transmit antennas. This scheme is an orthogonal one thanks to transmit weights, which allow to use a simple linear receiver having optimal performance. The performance of the code is assessed by simulations in the 5 GHz OFDM-based WLAN environment (HIPERLAN/2). The practical interest of such approach is finally discussed.

A MIMO-OFDM testbed for wireless local area networks

We describe the design steps and final implementation of a MIMO OFDM prototype platform developed to enhance the performance of wireless LAN standards such as HiperLAN/2 and 802.11, using multiple transmit and multiple receive antennas. We first describe the channel measurement campaign used to characterize the indoor operational propagation environment, and analyze the influence of the channel on code design through a ray-tracing channel simulator. We also comment on some antenna and RF issues which are of importance for the final realization of the testbed. Multiple coding, decoding, and channel estimation strategies are discussed and their respective performance-complexity trade-offs are evaluated over the realistic channel obtained from the propagation studies. Finally, we present the design methodology, including cross-validation of the Matlab, C++, and VHDL components, and the final demonstrator architecture. We highlight the increased measured performance of the MIMO testbed over the single-antenna system.

Filter bank design for minimum distortion in presence of subband quantization

This paper presents source coding schemes, based on both parallel and tree-structured filter banks, in which synthesis filters and quantizers are jointly optimized under bit-rate constraint. The design of reconstruction filter banks thus takes into account the amount of subband quantization noise, and minimizes the output mean square error (MSE). All other constraints on the filters, such as perfect reconstruction or maximum selectivity, are relaxed, and noticeable improvements over perfect reconstruction schemes with optimal bit-rate allocation are shown by means of rate-distortion curves, for both synthetic and audio signals.

Architectures for IP-based network-assisted mobility management across heterogeneous networks

In this article new architectures for network- assisted management of vertical handovers are proposed, relying on IP-based protocols, aiming at achieving the most relevant intersystem mobility decision with respect to user satisfaction and operator added value. The key design principle used is ease of deployment, with flexible hierarchical handover decision entities allowing the distribution of the decision process across local and global levels. The decision itself relies on a broad set of parameters at the radio, networking, application, user preferences, and operator policies levels. The advantage of the approach is highlighted in the specific cellular/WLAN scenario, and a proof-of-concept testbed has enabled to validate it. Standardization in this area is currently ongoing in 3GPP, IEEE, and IETF.

Impact of RF front-end impairments and mobility on channel reciprocity for closed-loop multiple antenna techniques

We focus on multiple antennas TDD OFDM-based WLANs. In this context, we assume that channel state information (CSI) is available at the transmitter (closed-loop techniques). In TDD systems, it is commonly assumed that the propagation channel is reciprocal, i.e. the same on the up link (UL) and the down link (DL). This implies that the channel estimated on the UL might be used as CSI at the transmitter for DL transmission. However, the RF front-ends contribute to the channel experienced, and their imperfections might be inconsistent with the reciprocity assumption. Based on observed characteristics of actual components, a model for these imperfections is introduced in order to quantify their effect on the performance of closed-loop techniques based on channel reciprocity. Simulation results for the TxAA and transmit selection diversity (TSD) schemes are provided in the context of a multiple transmit multiple receive (MTMR) IEEE 802.11a WLAN system with 4 transmit and one receive antennas. The impact of the delay between UL and DL is also considered in a mobility scenario.

Trading rate versus diversity in space-time-frequency block coding schemes

This paper presents space-time-frequency (STF) designs applied to a MIMO-OFDM system. The basic idea is to make use of frequency diversity in combination with spatial diversity for reducing the number of antennas. The problem is that the use of additional diversity (given that we stick to complex orthogonal codes) has an impact on the spectral efficiency. This impact can be overcome by an increase of the modulation order, and the increase of diversity must compensate for the drawbacks coming from this higher order. The situation is first explained through a simple combination of Alamouti schemes, resulting in a global rate of 1/2. This scheme provides the same diversity as the 4-antenna Tarokh code of same rate. We then show how the scheme can be modified in order to better see the impact of the diversity in a coded transmission. Finally, we illustrate that such schemes can be useful in a two transmit antenna setting, since, in the lower rate mode of IEEE 802.11a standard, they provide better performance than the Alamouti scheme, while the Tarokh scheme would require 4 antennas. Other consequences are highlighted : (i) such schemes are not useful for higher order modulations, (ii) frequency does not always bring as much diversity as space in practical situations, depending on the channel model.