Maxime Guillaud

A practical method for wireless channel reciprocity exploitation through relative calibration

We present a relative calibration method for a wireless TDD link, which, after a calibration phase involving feedback, lets the transmitter acquire knowledge of the downlink channel state from the uplink channel estimates, through proper modeling and estimation of the RF circuitry impulse responses. Contrarily to previous methods, relative calibration does not require specific calibration hardware. Experimental results confirm the validity of the proposed linear reciprocity model, and of the calibration approach.

Cellular Interference Alignment with Imperfect Channel Knowledge

Interference alignment is evaluated as a technique to mitigate inter-cell interference in the downlink of a cellular network using OFDMA. The sum mutual information achieved by interference alignment together with a zero-forcing receiver is considered, and upper and lower bounds are derived for the case of imperfect channel knowledge. The sum mutual information achieved by interference alignment when the base stations share their information about the channels is shown to compare favorably to the achievable sum-rate of methods where the base stations do not cooperate, even under moderately accurate knowledge of the channel state.

Dual-polarized wireless communications: from propagation models to system performance evaluation

In this paper, we address the potential benefits of dual-polarized arrays in multi-antenna wireless systems. After an extensive literature overview of experimental data, we present a new and simple analytical framework to model dual-polarized Rayleigh and Ricean fading channels for arbitrary array sizes. The model relies on a limited number of physical parameters, such as the channel spatial correlations, the channel co-polar and the cross-polar ratios and the antenna cross-polar discrimination. Then, we investigate the multiplexing advantage of dual-polarized transmissions through the evaluation of the ergodic mutual information, for both TITO and MIMO systems. Finally, the performance of two space-time coding schemes (Alamouti OSTBC and uncoded spatial multiplexing) is evaluated via a detailed analysis of the pairwise error probability.

On the achievability of interference alignment in the K-user constant MIMO interference channel

Interference alignment in the K-user MIMO interference channel with constant channel coefficients is considered. A novel constructive method for finding the interference alignment solution is proposed for the case where the number of transmit antennas equals the number of receive antennas (NT = NR = N), the number of transmitter-receiver pairs equals K = N + 1, and all interference alignment multiplexing gains are one. The core of the method consists of solving an eigenvalue problem that incorporates the channel matrices of all interfering links. This procedure provides insight into the feasibility of signal vector spaces alignment schemes in finite dimensional MIMO interference channels.

Clustered interference alignment in large cellular networks

Spatial interference alignment among a cluster of base stations is considered as a technique to mitigate inter-cell interference in the downlink of a large cellular network using OFDMA. We derive an expression for the mutual information achieved by clustered base station cooperation together with a zero-forcing receiver. Numerical performance results with throughput gains of up to 30% with respect to the non-cooperating base stations case are demonstrated for users located in the central area of the cells. Those results provide insight into the applicability of interference alignment methods in upcoming cellular networks.

Performance of interference alignment in clustered wireless ad hoc networks

Spatial interference alignment among a finite number of users is investigated as a technique to increase the probability of successful transmission in an interference limited clustered wireless ad hoc network. Using techniques from stochastic geometry, we build on the work of Ganti and Haenggi dealing with Poisson cluster processes with a fixed number of cluster points and provide a numerically integrable expression for the outage probability using an intra-cluster interference alignment strategy with multiplexing gain one. For a special network setting we derive a closed-form upper bound. We demonstrate significant performance gains compared to single-antenna systems without local cooperation.

Limited feedback for interference alignment in the K-user MIMO Interference Channel

A simple limited feedback scheme is proposed for interference alignment on the K-user Multiple-Input-Multiple-Output Interference Channel (MIMO-IC). The scaling of the number of feedback bits with the transmit power required to preserve the multiplexing gain that can be achieved using perfect channel state information (CSI) is derived. This result is obtained through a reformulation of the interference alignment problem in order to exploit the benefits of quantization on the Grassmann manifold, which is well investigated in the single-user MIMO channel. Furthermore, through simulations we show that the proposed scheme outperforms the naive feedback scheme consisting in independently quantizing the channel matrices, in the sense that it yields a better sum rate performance for the same number of feedback bits.

Channel modeling and associated inter-carrier interference equalization for OFDM systems with high Doppler spread

We address the problem of OFDM transmission over a time-varying, frequency-selective channel with high Doppler spread. This creates situations where the channel significantly evolves over the time span of one OFDM symbol. We analyze the CP-OFDM transmission mechanism, and the impairments due to channel variations, using a decomposition of these variations over a base of sinusoid functions sampling the Doppler spectrum at subcarrier frequencies. On the one hand, this leads to a fairly parsimonious parameterization of the time-varying channel impulse response. On the other hand we show that, considering a whole CP-OFDM symbol, this leads to a duality between equalization of the delay spread of a time-varying channel in the time domain, and the equalization of the Doppler spread of a frequency selective channel in the frequency domain. Using this duality, we show that equalization in the frequency domain can benefit from all known time domain equalization methods.

Interference Alignment in Partially Connected Interfering Multiple-Access and Broadcast Channels

We consider interference alignment (IA) in the L-interfering multiple access channels (MACs) network, a particular case of the partially connected interfering MACs network whereby the number of interference links per MAC is bounded regardless of the total number of MACs in the network. Conversely to the fully-connected case, we show that interference alignment can be achievable in a network of arbitrary size, while the per-user signaling dimension remains bounded. We provide necessary conditions for the feasibility of IA, discuss their sufficiency, and introduce an algorithm capable of providing practical solutions. These results also apply to the dual case of partially connected interfering broadcast channels (IBCs), and have practical applications to both the uplink and downlink of large cellular networks.

Interference Alignment With Quantized Grassmannian Feedback in the K-User Constant MIMO Interference Channel

A simple channel state information (CSI) feedback scheme is proposed for interference alignment (IA) over the K-user constant multiple-input-multiple-output interference channel (MIMO IC). The proposed technique relies on the identification of invariants in the IA equations, which enables the reformulation of the CSI quantization problem as a single quantization on the Grassmann manifold at each receiver. The scaling of the number of feedback bits with the transmit power sufficient to preserve the multiplexing gain that can be achieved under perfect CSI is established. We show that the CSI feedback requirements of the proposed technique are better (lower) than what is required when using previously published methods (including state-of-the-art codebook-based as well as analog feedback techniques), for system dimensions (number of users and antennas) of practical interest. Furthermore, we show through simulations that this advantage persists at low SNR, in the sense that the proposed technique yields a higher sum-rate performance for a given number of feedback bits. Finally, to complement our analysis, we introduce a statistical model that faithfully captures the properties of the quantization error obtained for random vector quantization (RVQ) on the Grassmann manifold for large codebooks; this enables the numerical (Monte Carlo) analysis of general Grassmannian RVQ schemes for codebook sizes that would be impractically large to simulate.