Hongji Xu

Lifetime Maximization via a New Cooperative MAC Protocol in Wireless Sensor Networks

Lifetime extension is a key design issue for wireless sensor networks (WSN) with battery-operated nodes. Compared with direct transmission, transmission power can be significantly reduced by cooperative communication, because it can effectively mitigate multi-path fading by introducing space diversity. In this paper, BER analysis for M-PSK and M-QAM modulation in non-cooperative and cooperative situations is firstly presented. Then power is optimally allocated to source and relay nodes with the objective of minimizing the total transmission power under the average BER constraint. Based on these works, a new distributed cooperative MAC protocol is proposed to improve the lifetime of WSN. In this protocol, both channel state information (CSI) and residual energy information (REI) of sensor nodes are considered to choose the cooperative nodes. Simulation results show that when the access point (AP) is above a certain height, the proposed cooperative protocol can significantly prolong network lifetime compared with non-cooperative MAC protocols.

Maximise lifetime of wireless sensor networks via a distributed cooperative routing algorithm

Combining cooperative diversity, truncated automatic repeat request scheme and distributed energy-aware routing strategy, a novel cooperative routing algorithm adopting decode-and-forward fashion is proposed to maximise the lifetime of wireless sensor networks from the cross-layer design perspective. In this algorithm, the transmission power is optimally allocated while satisfying the per-link symbol error rate or the end-to-end throughput requirement. The average total consumed power weighted by the normalised remaining energy of every participating node is used as the link cost to avoid the overuse of certain nodes with extremely little energy. With the use of the traditional distributed shortest path algorithm, the best route that includes a cascade of single-relay building blocks is constructed with polynomial complexity. In contrast to the non-cooperative routing schemes, this cooperative routing algorithm can significantly prolong the network lifetime and improve the energy efficiency by reducing the total network residual energy. Copyright

Performance of opportunistic relaying with truncated ARQ over Nakagami-m fading channels

On the basis of the decode-and-forward protocol, the opportunistic relaying scheme selects the best relay out of the N available relays with the correct data copy of the source. The best relay forwards the data of source to the destination in the second time slot. While in the truncated automatic repeat request scheme, the relay will cooperatively do the data transmission only when the original signal is erroneously received by the destination. Through combining these two conceptions, the system throughput can be improved without sacrificing the space diversity. The performance of traditional noncooperative and such cooperative schemes with M-ary phase-shift keying and M-ary quadrature amplitude modulation symbols over Nakagami-m fading channels is analysed in terms of packet error rate, symbol error rate and throughput. In addition, we suppose that the destination either combines the previous and retransmitted packets using maximal ratio combining technique or simply (SIM) discards the previous wrong packet and uses the current packet alone (SIM). Monte Carlo simulations are included to verify the accuracy of analytical results. Copyright

Receive antenna selection using convex optimization for MIMO systems

In this paper, a receive antenna selection algorithm based on the theory of convex optimization is proposed to improve the system performance over Rayleigh fading multiple-input multiple-output (MIMO) channels. The algorithm is based on the aim of minimizing error rate, and by relaxing the antenna selection variables from discrete to continuous, we arrive at a standard semidefinite convex problem that can be solved very efficiently. The Monte-Carlo simulations show that the algorithm proposed can provide the performance very close to that of the optimal selection based on exhaustive search.

A suboptimal user selection algorithm for wireless broadcast channels

In downlink broadcast channels, the multiple-antenna base station (BS) often communicates with several users simultaneously. However, the co-channel interference increases as the number of users increases, therefore, BS can not afford too many users. So user selection is necessary. In this paper, we propose a suboptimal user selection algorithm based on signal-to-leakage-noise ratio (SLNR) for the wireless broadcast systems. The property of the proposed user selection algorithm is studied compared with some other selection algorithms in conjunction with the SLNR precoding. Simulation results show that the performance of the proposed algorithm is close to that of the optimal one. For fairness, the impact of scheduling algorithm is also considered.

Multi-Hop Collaborative Relay Beamforming

In this paper, we consider a three-hop multi-relay network with one transmitter, one receiver and two clusters of relay nodes. With the aid of perfect channel state information (CSI), two different design approaches are devised. In the first approach, we aim to maximize the received signal to noise ratio (SNR) under different transmit power constraint at the relay nodes by performing optimization on the beamforming vectors of the two clusters jointly, whilst in the second design, the total transmit power of the relay nodes is minimized under the QoS constraint at the receiver. It is shown that, in both approaches, upon the coefficients of either one cluster being determined, the other one could be optimized, and therefore the overall optimization problem can be resorted to an efficient iteration process. Simulation results show that our proposed approaches outperform the existing solutions for improved energy efficiency and increased receiving SNR.

Optimization for Outage Probability Constrained Robust Downlink Collaborative Beamforming

In this paper, we design an outage probability constrained robust collaborative beamforming approach for the distributed multi-relay network in the downlink, where the channel state information (CSI) available is imperfect. We aim to minimize the total transmit power of the relay nodes whilst keeping the outage probability at the destination node below the predefined threshold. Assuming that the CSI mismatches follow Gaussian distribution, the equivalent counterpart for the outage probability constraint on the required signal-to-noise-ratio (SNR) is given explicitly. We show that though the original optimization problem is non-convex thus very intractable, it could be optimally solved by using the well-known interior-point method together with an efficient one-dimension search. Simulation results reveal that our proposed approaches can guarantee the quality-of-service (QoS) in term of outage probability in statistical sense while the non-robust scheme fails to do so.

Decode-and-forward two-path successive relaying with wireless energy harvesting

We consider an energy harvesting (EH) based cooperative relaying network, where two half-duplex relays can alternatively forward source data to the destination. The power-splitting technique is implemented at each relay. That is, part of the received source signal and inter-relay interference is allocated for the EH, while the remaining part is used for the information decoding (ID). The inter-relay interference is harmful for the ID, but it is beneficial for the EH. The transmit powers of the two relays are properly set by letting the average energy consumption equal the average amount of EH. The optimal power splitting ratio between EH and ID is numerically determined to maximize the system throughput. Numerical results show that our proposed two-path successive relaying scheme can greatly outperform the single-relay system in terms of average throughput by enabling much higher energy and spectral efficiency.

Joint receiver-and-collaborative beamforming for the multi-user relay network in the uplink

In this paper, the problem of receiver-and-collaborative beamforming is considered for a network which consists of K single-antenna transmitters, R single-antenna relay nodes, and a destination base station equipped with N antennas. For such network, we propose two different design approaches. In the first approach, we jointly design the two beamformers aiming to minimize the total transmit power of the relay nodes, while maintaining the quality of service (QoS) at a predefined level. We apply the genetic algorithm (GA) to obtain the receiver equalizer and relay weight. In the second approach, we design the beamformer by maximizing the smallest SINRk at the receiver subject to the total relay power constraint. The GA is used iteratively to optimize the relay weight and equalizer matrix at the relay nodes and the receiver equalizer. Simulation results demonstrate that significant performance improvements are achieved by our proposed schemes as compared with the existing solutions.

Outage probability constrained robust downlink collaborative beamforming

We consider a downlink collaborative relay network where communication between the base station and the terminal node is assisted by a cluster of relay nodes with imperfect channel state information (CSI). The equivalent condition for the probability constraint on the signal-to-noise-ratio (SNR) is derived, by modeling the CSI uncertainties as Gaussian. Though the original problem is quite intractable, it could be effectively solved by working out an semidefinite programming (SDP) iteratively. Subsequently, computer simulations are carried out to validate our approach compared with the non-robust design techniques. Simulation results show that, the quality-of-service (QoS) can be guaranteed in statistical sense for the proposed approach while violated by the non-robust solution.