Brice Djeumou

Practical quantize-and-forward schemes for the frequency division relay channel

We consider relay channels in which the source-destination and relay-destination signals are assumed to be orthogonal and thus have to be recombined at the destination. Assuming memoryless signals at the destination and relay, we propose a low-complexity quantize-and-forward (QF) relaying scheme, which exploits the knowledge of the SNRs of the source-relay and relay-destination channels. Both in static and quasistatic channels, the quantization noise introduced by the relay is shown to be significant in certain scenarios. We therefore propose a maximum likelihood (ML) combiner at the destination, which is shown to compensate for these degradations and to provide significant performance gains. The proposed association, which comprises the QF protocol and ML detector, can be seen, in particular, as a solution for implementing a simple relaying protocol in a digital relay in contrast with the amplify-and-forward protocol which is an analog solution.

Combining Decoded-and-Forwarded Signals in Gaussian Cooperative Channels

The goal of this paper is to investigate the different ways of combining signals that have been decoded-and-forwarded by a bunch of relays. We more deeply look at the case where the relays are in bad reception conditions and the cooperation powers are sufficiently high. In this situation using a conventional MRC severely degrades the receiver performance especially when the number or relays increases. On the other hand the MMSE- and ML-based combiners can almost always extract from their partners some performance improvements

Resource allocation games in interference relay channels

In this paper we study a distributed network comprising an interference channel in parallel with an interference relay channel. Therefore each source node can use two frequency bands and has to implement a certain power allocation policy. An example of application of such a model is the case where the performance of terminals operating in unlicensed bands would be enhanced by being allowed to exploit an additional frequency band in which a relay is available. In this network model, each user is selfish and wants to maximize its Shannon transmission rate. We analyze two cases. In the first case, the relaying node is assumed to implement an amplify-and-forward (AF) protocol while in the second case it implements the decode-and-forward (DF) protocol introduced by Cover and El Gamal. For both cases we analyze the existence and uniqueness issues of the equilibrium of the aforementioned power allocation games. Several interesting and new results are provided. In particular: 1. The existence of a Nash equilibrium is shown to be always guaranteed in the case of the AF protocol; 2. The performance of a user or the network does not necessarily increase with the transmit power available at the relay; 3. We show that there is naturally a game in interference relay channels (even if the power allocation policy is fixed) when the DF protocol is used; this game is induced by the decentralized choice of the cooperation degree between each source node and the relay node.

Power allocation games in interference relay channels: existence analysis of nash equilibria

We consider a network composed of two interfering point-to-point links where the two transmitters can exploit one common relay node to improve their individual transmission rate. Communications are assumed to be multiband, and transmitters are assumed to selfishly allocate their resources to optimize their individual transmission rate. The main objective of this paper is to show that this conflicting situation (modeled by a non-cooperative game) has some stable outcomes, namely, Nash equilibria. This result is proved for three different types of relaying protocols: decode-and-forward, estimate-and-forward, and amplify-and-forward. We provide additional results on the problems of uniqueness, efficiency of the equilibrium, and convergence of a best-response-based dynamics to the equilibrium. These issues are analyzed in a special case of the amplify-and-forward protocol and illustrated by simulations in general.

Gaussian broadcast channels with an orthogonal and bidirectional cooperation link

This paper considers a system where one transmitter broadcasts a single common message to two receivers linked by a bidirectional cooperation channel, which is assumed to be orthogonal to the downlink channel. Assuming a simplified setup where, in particular, scalar relaying protocols are used and channel coding is not exploited, we want to provide elements of response to several questions of practical interest. Here are the main underlying issues: (1) the way of recombining the signals at the receivers; (2) the optimal number of cooperation rounds; (3) the way of cooperating (symmetrically or asymmetrically, which receiver should start cooperating in the latter case); and (4) the influence of spectral resources. These issues are considered by studying the performance of the assumed system through analytical results when they are derivable and through simulation results. For the particular choices we made, the results sometimes do not coincide with those available for the discrete counterpart of the studied channel.

Performance analysis for the AF-based frequency division cooperative broadcast channel

This paper considers a system where one transmitter broadcasts a single common message to two receivers. These receivers can cooperate through a bidirectional channel that is assumed to be orthogonal to the downlink channel. For the case where the assumed cooperation protocol is amplify-and-forward we calculated the final equivalent SNR in the MRC output at each receiver for an arbitrary number of cooperation exchanges. The corresponding analytical expressions can then be used for evaluating different performance criteria in order to discuss issues such as: Which receiver should start cooperating first? Is there an optimum number of cooperation exchanges? What is the difference between asymmetric and symmetric cooperations?

A cheap relaying protocol for orthogonal relay channels

The proposed relaying scheme, which is an optimized scalar quantize-and-forward (QF) protocol, has at least three attractive features: 1. it is simple; 2. it exploits the signal-to-noise ratios (SNR) of the source-relay and relay-destination channels; 3. it can be seen as a digital alternative of the conventional (analog) amplify-and-forward (AF) in a digital relay transceiver. The presented QF protocol is optimized in terms of end-to-end distortion, extending the idea of joint source-channel coding. Using this cooperation protocol over orthogonal relay channels, it is shown that the quantization noise introduced by the relay can significantly degrade the receiver performance if the latter uses a maximum ratio combiner (MRC) to combine the two signals from the source and relay. In order for the receiver to compensate for this effect, we propose a maximum likelihood detector (MLD), which is optimum for the QF protocol.

Combining coded signals with arbitrary modulations in orthogonal relay channels

We consider a relay channel for which the following assumptions are made. (1) The source-destination and relay-destination channels are orthogonal (frequency division relay channel). (2) The relay implements the decode-and-forward protocol. (3) The source and relay implement the same channel encoder, namely, a convolutional encoder. (4) They can use arbitrary and possibly different modulations. In this framework, we derive the best combiner in the sense of the maximum likelihood (ML) at the destination and the branch metrics of the trellis associated with its channel decoder for the ML combiner and also for the maximum ratio combiner (MRC), cooperative-MRC (C-MRC), and the minimum mean-square error (MMSE) combiner.