DMT Optimal Cooperative Protocols with Destination-Based Selection of the Best Relay

We design a cooperative protocol in the context of wireless mesh networks in order to increase the reliability of wireless links. Destination terminals ask for cooperation when they fail in decoding data frames transmitted by source terminals. In that case, each destination terminal D calls a specific relay terminal B with a signaling frame to help its transmission with source terminal S. To select appropriate relays, destination terminals maintain tables of relay terminals, one for each possible source address. These tables are constituted by passively overhearing ongoing transmissions. Hence, when cooperation is needed between S and D, and when a relay B is found by terminal D in the relay table associated with terminal S, the destination terminal sends a negative acknowledgment frame that contains the address of B. When the best relay B has successfully decoded the source message, it sends a copy of the data frame to D using a selective decode-andforward transmission scheme. The on-demand approach allows maximization of the spatial multiplexing gain and the cooperation of the best relay allows maximization of the spatial diversity order. Hence, the proposed protocol achieves optimal diversitymultiplexing trade-off performance. Moreover, this performance is achieved through a collision-free selection process

Random Matrix Theory applied to the Estimation of Collision Multiplicities

This paper presents two techniques in order to estimate the collision multiplicity, i.e., the number of users involved in a collision [1]. This estimation step is a key task in multi-packet reception approaches and in collision resolution techniques. The two techniques are proposed for IEEE 802.11 networks but they can be used in any OFDM-based system. The techniques are based on recent advances in random matrix theory and rely on eigenvalue statistics. Provided that the eigenvalues of the covariance matrix of the observations are above a given threshold, signal eigenvalues can be separated from noise eigenvalues since their respective probability density functions are converging toward two different laws: a Gaussian law for the signal eigenvalues and a Tracy-Widom law for the noise eigenvalues. The first technique has been designed for the white noise case, and the second technique has been designed for the colored noise case. The proposed techniques outperform current estimation techniques in terms of mean square error. Moreover, this paper reveals that, contrary to what is generally assumed in current multi-packet reception techniques, a single observation of the colliding signals is far from being sufficient to perform a reliable estimation of the collision multiplicities

Model Order Selection for Collision Multiplicity Estimation

The collision multiplicity (CM) is the number of users involved in a collision. The CM estimation is an essential step in multi-packet reception (MPR) techniques and in collision resolution (CR) methods. We propose two techniques to estimate collision multiplicities in the context of IEEE 802.11 networks. These two techniques have been initially designed in the context of source separation. The first estimation technique is based on eigenvalue statistics. The second technique is based on the exponentially embedded family (EEF). These two techniques outperform current estimation techniques in terms of underestimation rate (UNDER). The reason for this is twofold. First, current techniques are based on a uniform distribution of signal samples whereas the proposed methods rely on a Gaussian distribution. Second, current techniques use a small number of observations whereas the proposed methods use a number of observations much greater than the number of signals to be separated. This is in accordance with typical source separation techniques

Optimal Cooperative MAC Protocol with Efficient Selection of Relay Terminals

A new cooperative protocol is proposed in the context of wireless mesh networks. The protocol implements ondemand cooperation, i.e. cooperation between a source terminal and a destination terminal is activated only when needed. In that case, only the best relay among a set of available terminals is re-transmitting the source message to the destination terminal. This typical approach is improved using three additional features. First, a splitting algorithm is implemented to select the best relay. This ensures a fast selection process. Moreover, the duration of the selection process is now completely characterized. Second, only terminals that improve the outage probability of the direct link are allowed to participate to the relay selection. By this means, inefficient cooperation is now avoided. Finally, the destination terminal discards the source message when it fails to decode it. This saves processing time since the destination terminal does not need to combine the replicas of the source message: the one from the source terminal and the one from the best relay. We prove that the proposed protocol achieves an optimal performance in terms of Diversity-Multiplexing Tradeoff (DMT)

Estimation of Collision Multiplicities in IEEE 802.11-based WLANs

Abstract—Estimating the collision multiplicity (CM), i.e. the number of users involved in a collision, is a key task in multipacket reception (MPR) approaches and in collision resolution (CR) techniques. A new technique is proposed for IEEE 802.11 networks. The technique is based on recent advances in random matrix theory and rely on eigenvalue statistics. Provided that the eigenvalues of the covariance matrix of the observations are above a given threshold, signal eigenvalues can be separated from noise eigenvalues since their respective probability density functions are converging toward two different laws: a Gaussian law for the signal eigenvalues and a Tracy-Widom law for the noise eigenvalues. The proposed technique outperforms current estimation techniques in terms of underestimation rate. Moreover, this paper reveals that, contrary to what is generally assumed in current MPR techniques, a single observation of the colliding signals is far from being sufficient to perform a reliable CM estimation

On-Demand Cooperation MAC Protocols with Optimal Diversity-Multiplexing Tradeoff

This paper presents access protocols with optimal Diversity-Multiplexing Tradeoff (DMT) performance in the context of IEEE 802.11-based mesh networks. The protocols are characterized by two main features: on-demand cooperation and selection of the best relay terminal. The on-demand characteristic refers to the ability of a destination terminal to ask for cooperation when it fails in decoding the message transmitted by a source terminal. This approach allows maximization of the spatial multiplexing gain. The selection of the best relay terminal allows maximization of the diversity order. Hence, the optimal DMT curve is achieved with these protocols

Adapting DVB-SH system parameters to mobile environments

A performance analysis of the digital video broadcasting - satellite to handheld (DVB-SH) system in presence of ground mobile terminals (GMTs) is presented. The paper focuses on the Doppler spread issue. Indeed, the mobility of GMTs induces a Doppler spread in the orthogonal frequency division multiplexing (OFDM) signal that destroys the orthogonality of subcarriers. The loss of orthogonality produces inter-carrier interference (ICI) and hence a degradation of the system performance in terms of symbol error probability. The paper presents the conditions in which this degradation can be compensated for by an increase in the signal to noise ratio (SNR) at the receiver side. The result depends on both the modulation scheme and the speed of GMTs. Inversely, having a maximum allowable margin on the received SNR allows us to determine an upper bound on the mobile station velocity

Finite element formulation to study thermal stresses in nanoencapsulated phase change materials for energy storage

Nanoencapsulated phase change materials (nePCMs) – which are composed of a core with a phase change material and of a shell that envelopes the core – are currently under research for heat storage applications. Mechanically, one problem encountered in the synthesis of nePCMs is the failure of the shell due to thermal stresses during heating/cooling cycles. Thus, a compromise between shell and core volumes must be found to guarantee both mechanical reliability and heat storage capacity. At present, this compromise is commonly achieved by trial and error experiments or by using simple analytical solutions. On this ground, the current work presents a thermodynamically consistent and three-dimensional finite element (FE) formulation considering both solid and liquid phases to study thermal stresses in nePCMs. Despite the fact that there are several phase change FE formulations in the literature, the main novelty of the present work is its monolithic coupling – no staggered approaches are required – between thermal and mechanical fields. Then, the FE formulation is implemented in a computational code and it is validated against one-dimensional analytical solutions. Finally, the FE model is used to perform a thermal stress analysis for different nePCM geometries and materials to predict their mechanical failure by using Rankine’s criterion

DMT Optimal On-Demand Relaying for Mesh Networks

This paper presents a new cooperative MAC (Medium Access Control) protocol called BRIAF (Best Relay based Incremental Amplify-and-Forward). The proposed protocol presents two features: on-demand relaying and selection of the best relay terminal. “On-demand relaying” means that a cooperative transmission is implemented between a source terminal and a destination terminal only when the destination terminal fails in decoding the data transmitted by the source terminal. This feature maximizes the spatial multiplexing gain r of the transmission. “Selection of the best relay terminal” means that a selection of the best relay among a set of (m-1) relay candidates is implemented when a cooperative transmission is needed. This feature maximizes the diversity order d(r) of the transmission. Hence, an optimal DMT (Diversity Multiplexing Tradeoff) curve is achieved with a diversity order d(r) = m(1-r) for 0 ≤ r ≤ 1