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 M-ary PSK modulation is provided. With an objective to maximize the minimum device lifetime under a constraint on bit-error-rate 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.

In this paper, cooperative sensor protocol and relay deployment in wireless sensor networks (WSNs) is proposed to improve the network lifetime. First, cooperative transmission is employed among sensor nodes. With an objective of maximizing the network lifetime under a constraint on bit-error-rate performance, we determine which sensors should cooperate and how much power to allocate for cooperation. A closed form solution is provided for a two-sensor WSN. Based on the obtained two-sensor solution, a fast suboptimal algorithm is developed for multiple sensor case. Moreover, the network lifetime is further improved by deploying cooperative relays in the network to help sensors transmit their information. The network lifetime is maximized by choosing the optimum location for each relay and optimally allocating the power that the relay helps each sensor. A suboptimal algorithm is proposed for the WSN with multiple relays. Simulation results show that the network lifetime of the proposed WSN with cooperative sensor employment improves 2.3 times compared with the non-cooperative WSN. In addition, deploying a cooperative relay in the proper location improves network lifetime up to 3 times of that for the non-cooperative WSN.

In this paper, we propose a multi-node differential amplify-and-forward scheme for cooperative communications. The proposed scheme efficiently combines signals from the direct and multiple relay links to improve communication reliability. Bit-error-rate (BER) analysis for M-ary differential phase shift keying is provided as performance measure of the proposed scheme, and optimum power allocation is investigated. While the exact BER formulation of the proposed scheme is not available currently, we provide as a performance benchmark a tight BER formulation based on optimum combining weights. A simple BER upper bound and a tight BER approximation show that the proposed scheme can achieve the full diversity which equals to the number of cooperating nodes. We further provide simple BER approximation in order to provide analytical result on power allocation scheme. A closed-form optimum power allocation based on the tight simple BER approximation is obtained for single-relay scenario. An approximate optimum power allocation scheme is provided for multi-relay systems. The provided BER formulations are shown to closely match to the simulation results. Moreover, simulation results show that the optimum power allocation scheme achieves up to 2 dB performance gain over the equal power allocation scheme.

This paper proposes a threshold-based differential decode-and-forward cooperative scheme that efficiently exploits the cooperative relay channels via the use of a pre-determined decision threshold. In the proposed scheme, the source information is forwarded by the relay only if it is correctly decoded. The properly-designed threshold enables the destination to decide whether the received signal from the relay contains information such that the received signals from the source and the relay can be efficiently combined and jointly decoded. The bit error rate (BER) analysis of the proposed scheme is analyzed in case of differential M-ary phase shift keying signals. A tight BER approximation is established, and BER upper bound and lower bound are determined. Based on the tight BER approximation, joint optimum decision threshold and power allocation is numerically evaluated. Both analytical and simulation results reveal that the decision threshold and the power allocation depend on channel link qualities. Interestingly, when the link quality between the relay and the destination is very good, the effect of the threshold dominates the effect of the power allocation at high signal-to-noise ratio. Extensive simulation results are provided to validate the merit of the proposed scheme and confirm the theoretical analysis

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.

This paper proposes a threshold-selection relaying scheme, in which each relay decides whether to forward the source information by comparing the received signal power with a decision threshold. The challenging problem is to design the threshold such that the relay is able to forward only correctly decoded information. In this work, bit-error-rate (BER) performance analysis is provided for the proposed scheme with BPSK signals. We develop a BER upper bound which is tight over the entire range of signal-to-noise ratio values. Based on the established BER formulation, we jointly determine optimum decision threshold and power allocation. An interesting result is that the effect of optimum threshold dominates that of optimum power allocation, especially when the relay is close to the destination. For example, in case of equal power allocation, the proposed scheme with optimum threshold yields 10 dB performance improvement over the fixed relaying scheme without Q threshold at a BER of 10-2. If both the power allocation and the threshold are jointly optimized, then the performance of the proposed scheme can be further improved by 2 dB.

In this paper, we propose a differential encoding and decoding scheme for multiband UWB systems. The proposed scheme incorporates frequency-domain differential en/decoding with the hopping multiband OFDM modulation. To capture the effect of multipath-rich clustering property of UWB channels, we characterize the pairwise error probability performance of the proposed scheme in terms of cluster and ray arrival rates. It turns out that the diversity advantage does not strongly depend on the random-clustering of UWB channels, and we can achieve the same diversity gain in different channel environments. However, the system performance relies on the clustering behavior through the coding gain. Simulation results show that the proposed differential scheme achieves good performance in the short-range line-of-sight scenarios. In addition, the jointly encoded differential multiband UWB scheme is able to yield superior performance to the uncoded coherent multiband UWB system at high SNR

Cooperative transmission has been shown to be able to greatly improve the system performance by exploring the broadcasting nature of wireless channels and cooperation among users. While most existing works concentrate on improving the peer-to-peer link quality, we focus, in this paper, on resource allocation among users such that the system performance can be improved. In this work, two important questions are answered: who should help whom among the distributively located users; and how the users should cooperate to improve the performance. To quantify the questions, a power management problem is formulated over a multiuser OFDM network to minimize the overall system transmit power under the constraint of each user's desired transmission rate. Then, we develop an algorithm to find solutions for a two user case. From the simulation results, the proposed scheme achieves up to 50% overall power saving for the two-user system.

In this paper, we propose a differential encoding and decoding scheme for MIMO-OFDM systems under frequency-selective fading channels. We differentially encode the signal within each OFDM symbol period. This scheme not only reduces encoding and decoding delay, but also relaxes the restriction on channel assumptions. The successful differential decoding of the proposed scheme depends on the assumption that fading channels keep constant over two OFDM symbol periods rather than multiple periods as required in previous schemes. We provide the pairwise error probability formulation, and quantify the performance criteria in terms of diversity and coding advantages. Our design criteria reveal that the existing diagonal cyclic codes can be applied to achieve full diversity with high coding gain. Performance simulations in various channel conditions show that our proposed scheme yields superior performance to the previously proposed differential schemes.

We consider the design of matrix rotation based space-time signals based on the design criterion of minimizing the union bound on block error probability. We further propose to design the signal parameters via noninteger searching to get better signals. Superior performance of our improved design over the previous design are demonstrated through numerical calculations and performance simulations. With our proposed design for two transmit antennas and one or two receive antennas, we achieve the coding gain of about 1 dB over that of the previous design.

We propose a differential amplify-and-forward (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 bit-error-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 optimum-combining 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.

In this paper, we consider the design of matrix rotation based (MRB) space-time signals based on the design criterion of minimizing the union bound on pairwise block error probability (PBEP). We further propose to design the signal parameters via noninteger searching to get better signals. Superiority of our improved design over the previous design are demonstrated through numerical calculations and performance simulations. With our proposed design for two transmit antennas and one or two receive antennas, we achieve the coding gain of about 1 dB over that of the previous design.