BER Modified Decode-and-Forward Protocol for OFDM-Based Linear Multihop Networks

In orthogonal frequency division multiplexing （OFDM） based multihop communications, the conventional decodeand-forward （DF） relay scheme severely suffers from the error propagation problem. This drawback is serious in multihop networks as errors made by any relay node may fail the decoder at the destination in great chance. In this paper, we propose a bit error rate （BER） modified DF protocol （BMDF） which can be applied to systems where error correction channel coding and M-ary modulation are used. By modeling all links except the last one as a binary symmetric channel （BSC）, we derive a log likelihood ratio （LLR） modification function relying only on the accumulated BER of all previous links to be applied to the output of the soft demapper. Furthermore, to reduce the computational complexity and signaling overhead, the modification function is simplified from its original exponential expression and less BERs are delivered between nodes by making successive subcarriers share the same BER. In addition, for situations where the channel state information （CSI） of forward link is available, the proposed BMDF can be further enhanced by combining with subcarrier pairing （SP） and power allocation （PA）, where a sorted-channel gain SP scheme and a greedy PA algorithm are proposed. The simulation results verify thesignificant performance improvement to the conventional DF.

Energy efficiency based multi-relay selection and power allocation in OFDM cooperation networks

Getting along with the improvement of green communications, the energy efficiency （EE） of wireless network becomes more and more important. However, in nmlti-relay-assisted systems, the related work obviously depends on the end-to-end performance, whereas EE of relays has not attracted enough attention. The authors propose an orthogonal frequency division multiplexing （OFDM） multi-relay system based on amplify and forward （AF） mode. Taking incorporating EE and remaining energy as factor, decision criteria of attending cooperation is designed. The EE based asynchronous power iteration method is presented, and the existence and convergence of nash equilibrium （NE） is proven. Furthermore, a joint algorithm in subcarrier pairing, relay selection and power allocation is given. Genetic algorithm （GA） and iteration method are integrated to improve the convergence speed additionally. Simulation demonstrates the obvious improvement of EE, decrease of power consumption and increase of lifetime in relays of this algorithm with the constraint of minimum data transmission rate.

A novel resource optimization scheme for multi-cell OFDMA relay network

In cellular networks, users communicate with each other through their respective base stations（BSs）.Conventionally, users are assumed to be in different cells. BSs serve as decode-and-forward（DF） relay nodes to users.In addition to this type of conventional user, we recognize that there are scenarios users who want to communicate with each other are located in the same cell. This gives rise to the scenario of intra-cell communication. In this case, a BS can behave as a two-way relay to achieve information exchange instead of using conventional DF relay.We consider a multi-cell orthogonal frequency division multiple access（OFDMA） network that comprises these two types of users. We are interested in resource allocation between them. Specifically, we jointly optimize subcarrier assignment, subcarrier pairing, and power allocation to maximize the weighted sum rate. We consider the resource allocation problem at BSs when the end users＇ power is fixed. We solve the problem approximately through Lagrange dual decomposition. Simulation results show that the proposed schemes outperform other existing schemes.

Joint Resource Allocation for Lifetime-aware Cooperative Multi-carrier DF System

In this paper,we study the problem of optimal resource allocation for lifetime maximization in an orthogonal-frequencydivision multiplexing(OFDM)system with decode-and-forward relay.The goal is to minimize total energy cost of the system by jointly optimizing power allocation,subcarrier pairing and relay selection.We present a heuristic solution that is composed of two parts.The first part is an optimal power allocation approach to allocate power to a subcarrier pair of the source and the relay.The second part is a modified Hungarian algorithm to make subcarrier pairing and relay selection.Evaluations show that the presented scheme outperforms other schemes in the total transmitted data and the network lifetime.