On Energy Efficient Relay Selection For Ofdma Wireless Multi-Hop Relay Networks

The fourth generation of mobile communication has higher requirements on spectrum efficiency and transmission rate. Putting relays into traditional single-hop networks has been considered as one of the most possible schemes to increase throughput and extand coverage simultaneously. Compared with the traditional cellular network, wireless multihop relay networks have many advantages in the aspects of coverage, throughput, energy saving, operating cost, and etc. Othgonal Frequency Division Multiplexing (OFDM) technology which can overcome multipath fading and inter-symble interference efficiently is used as a key physical layer technology. Othgonal Frequency Division Multiple Access (OFDMA) technology is based on OFDM technology. It allocates different subchannels to different users to achieve multi-channel and muti-user diversity gains. Because of the combination of relay and OFDMA technology, the wireless multihop relay network can improve system performance significantly.However, when relay stations are used in networks, the network structure becomes complicated. Users can not only access to base station, but also access to relay stations. As a result, relay selection is the first problem that needs to be solved before user access to the network. With the increase of the energy consumption of communication industry, much attention is paid to energy-saving technologies in relay networks. Considering user’s quality of service (QoS), this thesis focuses on energy efficient relay seletion schemes in wireless multi-hop relay networks.In this thesis, basic research methods for relay selection schemes are summarized firstly. Accoding to the research purpose, some metrics are defined to evaluate the performance of relay selection schemes. A simulation platform is also built for wireless multi-hop relay cellular networks. Then, five typical relay selection schemes based on different decision metrics are introduced. In order to modify schemes desighed for energy-saving, two modified schemes are proposed. Firstly, direct links are added to the traditional overall power minimization scheme and a new mathematic model is built. Secondly, a power cost minimization scheme via pricing is presented on the basis of power minimization scheme. By using this scheme, power consumption between the base station and relay stations can be balanced. All these traditional schemes and modified schemes are simulated and compared with each other. The results show that these two modified schemes save much energy when power resource is very tight. The performance of power cost minimization scheme is slightly worse than the modified overall power minimization scheme, but its complexity is much less than the latter.Previous relay selection schemes always focus on equal time allocation. However it is proved theoretically that asymmetric time allocation enhances the freedom of transmission and improves system performance. Due to the above reasons, an energy efficient relay selection scheme with adaptive time allocation for relay zone and access zone is proposed, which uses adaptive modulation and coding technology, By taking channel statement information, rate requirements of all users and the new user into consideration, each relay adaptively adjusts the transmission time of relay zone and access zone to minimize the total energy consumption in a time slot when a new user applies for access. The minimum energy consumption mode for a single relay is established, and an ergodic method is used to find the optimal time allocation. Then, the relay with the smallest energy increase for attached users is selected as the best relay. Simulation results show that the proposed scheme can reduce energy consumption efficiently.