Abstract-In this letter, we propose a differential amplify-andforward (AF) transmission scheme for a two-user cooperative communications system. By efficiently combining signals from both direct and relay links, the proposed scheme provides superior performance compared to those of direct transmissions with either differential detection or coherent detection. While the exact biterror-rate (BER) formulation of the proposed scheme is not available currently, we provide, as a performance benchmark, an exact BER formulation and its simple bounds for a case of optimumcombining cooperation system with differential M-ary phase-shift keying (DMPSK) signals. The optimum power allocation is also determined based on the provided BER formulations. We show that the proposed differential cooperative transmission scheme together with the optimum power allocation yields comparable performance to the optimum-combining scheme. Simulation results show that the proposed differential scheme with optimum power allocation yields significant performance improvement over that with an equal power allocation scheme.

Abstract-Extending lifetime of battery-operated devices is a key design issue that allows uninterrupted information exchange among distributed nodes in wireless networks. Cooperative communications has recently emerged as a new communication paradigm that enables and leverages effective resource sharing among cooperative nodes. In this paper, a general framework for lifetime extension of battery-operated devices by exploiting cooperative diversity is proposed. The framework efficiently takes advantage of different locations and energy levels among distributed nodes. First, a lifetime maximization problem via cooperative nodes is considered and performance analysis for Mary PSK modulation is provided. With an objective to maximize the minimum device lifetime under a constraint on bit-errorrate performance, the optimization problem determines which nodes should cooperate and how much power should be allocated for cooperation. Since the formulated problem is NP hard, a closed-form solution for a two-node network is derived to obtain some insights. Based on the two-node solution, a fast suboptimal algorithm is developed for multi-node scenarios. Moreover, the device lifetime is further improved by a deployment of cooperative relays in order to help forward information of the distributed nodes in the network. Optimum location and power allocation for each cooperative relay are determined with an aim to maximize the minimum device lifetime. A suboptimal algorithm is developed to solve the problem with multiple cooperative relays and cooperative nodes. Simulation results show that the minimum device lifetime of the network with cooperative nodes improves 2 times longer than the lifetime of the non-cooperative network. In addition, deploying a cooperative relay in a proper location leads up to 12 times longer lifetime than that of the non-cooperative network.Index Terms-Cooperative diversity, wireless networks, decode-and-forward protocol, lifetime maximization.

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Abstract-Cooperative transmissions have been shown to be able to greatly improve system performance by exploring the broadcasting nature of wireless channels and cooperation among users. We focus, in this paper, on leveraging cooperation for resource allocation among users such that the network performance can be improved. Two important questions are answered-who should help whom among the distributively located users, and how many resources the users should use for cooperation to improve the performance. To answer these questions, a poweroptimization, subcarrier-allocation, and relay-selection problem is formulated over a multiuser orthogonal frequency-division multiplexing (OFDM) network, which is applicable to systems such as wireless local area networks (WLANs). In the multiuser OFDM network, cooperation among different users is conducted by assigning the subcarriers of the helping users to relay a certain part of the helped users' data, while maintaining the desired rates of both helping users and helped users by means of power control and rate adaption. This way, the bandwidth efficiency of the multiuser OFDM system with cooperation is the same as that of the noncooperative OFDM system. The formulated optimization problem is an assignment problem for subcarrier usage and corresponding bit loading as well as power control. We provide an approximate closed-form solution for a two-user two-subcarrier case. Then, a suboptimal heuristic algorithm for a multiple-user multiple-subcarrier case is proposed and implemented in the base station to solve the formulated NP-hard problem. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system and 19%-54% overall power saving for the multiuser case with random locations, compared with the current multiuser OFDM system without cooperative diversity. The proposed scheme is also compared to a much more complicated orthogonal frequency-division multiple access (OFDMA) system.

In this paper, we study the impact of cooperative routing for maximizing the network lifetime in sensor network applications. We assume nodes in the network are equipped with a single omnidirectional antenna and they perform cooperative transmission to achieve transmit diversity. We propose a joint cooperative transmission and energy aware routing algorithm to prolong the network lifetime. In contrast to the previous works, our approach uses the maximum lifetime power allocation, instead of minimum power allocation. Using the maximum lifetime power allocation, the cooperative nodes allocate their transmit power according to the channel condition and the residual energy in the nodes. Our proposed scheme combines the maximum lifetime power allocation and the energy aware routing to maximize the network lifetime. We study the performance of the cooperative routing in terms of network lifetime (defined as the time until the first node dies) and the total delivered packets before the first node dies. We demonstrate that the proposed solution achieves 1 ∼ 3.5 and 1 ∼ 2 times longer network lifetime and more total delivered packets compared to noncooperative routing, when it is used with MTE and FA algorithms, respectively. Furthermore, the maximum lifetime power allocation achieves 1 ∼ 2 times longer lifetime, compared to minimum power allocation in MTE and FA routing schemes. We also provide distributed implementation of the proposed algorithm. I. INTRODUCTIONAdvances in low power integrated circuit devices and communications technologies have enabled the deployment of lowcost, low power sensors that can be integrated to form sensor networks. This sensor network has vast important applications and has been identified as one of the most important technologies nowadays. The deployment of the low cost and energy limited sensors implies that the energy efficient communication protocol is imperative to extend the lifetime of the network. The problem of energy efficient protocol can be approached from different communication layers; from physical layer, data-link layer, MAC layer, network layer to the application layer. Moreover, the cross layer approach has been shown to be an effective energy saving method in the energy constrained communication [1], [2]. In ad hoc networking environment, most of the energy consumption is due to the packet transmission [3]. Motivated by this fact, we focus on the cross layer approach by jointly design the energy efficient routing algorithm in network layer and the energy efficient signal combining in physical layer.The energy efficient routing and transmit diversity have been studied separately in the literatures. The transmit diversity, pioneered by Alamouti's paper [4] shows the significant performance gain can be achieved in the multiple-input-multiple-output (MIMO) systems. However, multiple antennas in a sensor node may be impractical due to the cost. To overcome this problem, the cooperative communication concept has been recently proposed [5]. This cooperative communication explore...