Debdeep Chatterjee

On achievable rate regions for half-duplex causal cognitive radio channels

Coding for the causal cognitive radio channel, with the cognitive source subjected to a half-duplex constraint, is studied. A discrete memoryless channel model incorporating the half-duplex constraint is presented, and a new achievable rate region is derived for this channel. It is proved that this rate region contains the previously known causal achievable rate region of [1] for Gaussian channels.

Cooperative Transmission in a Wireless Relay Network Based on Flow Management

Cooperative transmission protocols using a flow optimization approach for a general multi-node half-duplex wireless relay network are presented. The proposed design involves solving a convex flow optimization problem on a graph that models the relay network. Two protocols are developed using the techniques of broadcasting (BC), multiple-access (MA), and time sharing (TS), and both are shown to achieve the optimal diversity-multiplexing tradeoff (DMT). Simulation results are used to quantify the performances of the proposed protocols in terms of outage probabilities, and to compare them against the max-flow-min-cut bound.

Cooperative Transmission in a Wireless Cluster Based on Flow Management

In this paper, a cooperative transmission design for a multi-node wireless cluster network is presented. It is assumed that the nodes operate in half-duplex mode and that channel information is available at the nodes. The proposed design involves solving a convex flow optimization problem on a fully connected graph that models the cluster network. A much simpler generalized routing protocol based on the above design is also presented. Both the proposed flow-optimized protocol and the generalized routing protocol are shown to achieve the optimal diversity-multiplexing tradeoff for the cluster network. Moreover, simulation results are presented to quantify the gap between the performance of the proposed protocols and that of a max-flow-min-cut type bound, in terms of outage probability.

Achievable rates in cognitive radio networks

In this paper, communication through an overlay cognitive radio network, wherein the cognitive ability of a secondary source needs to be acquired, i.e. the primary message may only be obtained by the cognitive source in a causal manner, is considered. A new achievable rate region for the discrete memoryless version of this network is derived using block Markov superposition coding (SPC), conditional rate-splitting, conditional Gelfand-Pinkser (GP) binning, and cooperative relaying. This rate region is evaluated for Gaussian channels, and numerical results are presented to demonstrate the contributions of the various coding strategies, used in the proposed coding scheme, towards enlarging the achievable rate region.

Resource allocation and cooperative behavior in fading multiple-access channels under uncertainty

The problems of resource allocation and the characterization of cooperative behavior in a two-user fading multiple-access channel (MAC) in the presence of uncertainty about the channel state information at the transmitters (CSIT) are considered. Game-theoretic techniques are used to characterize cooperative behavior and obtain the system operation point from an individualistic perspective. A two-user bargaining problem formulation, with the utility function being defined as the average rate each user can achieve, is used to obtain a fair and efficient operating point. It is shown that for the conventional bargaining problem formulation, the Nash Bargaining Solution (NBS) is very sensitive to the CSIT, and hence may result in considerable performance degradation. A modified bargaining problem formulation, that does not require an explicit modeling of the actual CSIT error, is proposed to address this issue, and shown to provide significant robustness to the system.

Active user cooperation in fading multiple-access channels

Consider a two-user fading cooperative multiple-access channel (CMAC), where each user, along with transmitting its own information to the destination, helps the other user by forwarding the latterpsilas information. It is assumed that the transceivers (nodes) are capable of half-duplex communication and that full channel state information (CSI) is available at all the nodes. Cooperative transmission strategies based on a flow-theoretic approach are proposed. The proposed strategies include a sub-optimal but simple orthogonal relaying strategy for the CMAC (OR-CMAC), and a flow-optimized strategy for the CMAC (FO-CMAC). The proposed protocols are evaluated in terms of achievable rate regions and outage performances, for different scenarios. The boundaries of the achievable rate regions for the proposed strategies are characterized by convex optimization formulations, and FO-CMAC is shown to provide improved achievable rate regions as compared to the conventional multiple-access channel (MAC), and a previously proposed strategy based on the decode-and-forward (DF) approach. Although sub-optimal in terms of the achievable rate region, OR-CMAC is shown to provide outage performance close to FO-CMAC for the scenarios considered. Moreover, by comparing to a lower bound on the outage probability, both the proposed strategies are shown to achieve the optimal diversity order of two for the required rate region of interest.

Performance Comparison of Optimal and Suboptimal Forward-Link Channel-Sharing Schemes

In this paper, we evaluate the performance of various channel-sharing schemes in the forward link from a base station (BS) to several mobile stations (MSs). Simultaneous transmission (broadcasting) to all mobile stations using the entire available bandwidth is known to yield rates dominating any other channel-sharing technique for Gaussian channels. However, broadcasting is very complicated from an implementation point of view. We analyze and compare different channel-sharing techniques on the basis of the asymptotic minimum signal-to-noise ratio (SNR) per MS necessary at the BS to transmit to a certain set of MSs in a single cell at a common data rate. From our results we conclude that suboptimal but practical methods like two- or three-level superposition coding when used in conjunction with frequency division multiplexing perform sufficiently close to the optimum and hence, are good alternatives to the optimal broadcasting scheme.