Energy Efficient Physical-Layer Transmission Strategies In Relay-assisted Wireless Networks

With the exponential growth of the number of nodes and amount of data transmissions,the demand for wireless network resources will increase rapidly in the upcoming communication systems.Given that the resources is finite,green and energy efficient transmission schemes that guarantee various performance metrics,including outage probability,diversity order,throughput,fairness are required.The networks have to deliver more data with limited energy and time slots.Related investigations fulfilling the above requirements are significant for academic and theoretical researches as well as potential engineering applications.In the thesis,a series of theoretical researches are conducted to make relay-assisted wireless networks more energy efficient.Main work and innovative contributions are listed as below:1.Two-way relay(TWR)system that has no fixed energy supply but applies energy transferring is studied.The fairness-utility is maximized by optimizing the power splitting strategies and data transmission rates to satisfy various fairness levels,channel conditions and transmission durations.The optimization problem is solved by the proposed iteration algorithm.The impacts of fairness levels,relay locations and time proportions of different transmission stages on the throughput are analysed.Then the three-node TWR mode is extended to parallel TWR scenario that employs energy harvesting.Energy consumption/storage,relay matching and rates in consecutive transmissions are jointly optimized to maximize the energy efficiency(EE).A fast iteration algorithm is proposed based on the variable relaxation,fractional programming and Lagrangian dual decomposition.The performance comparisons with the policies in the current state of the art demonstrate that our policies can provide notable EE gains.Additionally,it is illustrated that high rate threshold can cost significant EE penalty;2.Energy efficient relay scheduling(RS)and power allocation(PA)for multi-user multirelay systems with the maximum diversity network coding(MDNC)is proposed.In the networks,the transmitters only have the knowledge of channel statistics.A fast decomposition and iteration algorithm is proposed to solve the maximizing EE problem under the outage probability constraints.With geometric and fractional programmings,the optimization problem is converted into its log-convex and mixed-integer nonlinear form.Further,a generalized outer approximation(GOA)algorithm is applied to decompose the optimization problem such that it can be solved in an iterative manner.Both theoretical and simulation results reveal that MDNC,RS and PA can provide the system EE gains.The impacts of relay locations on the EE are also illustrated;3.A multi-user multi-relay network with integrated energy harvesting,transferring(IEHT)and network coding technique is studied for uplink data transmissions.A simultaneous twolevel cooperation,i.e.,information-level and energy-level cooperation is conducted for uplink data transmissions.Specifically,ET is adopted to share the harvested energy among the users for the energy-level cooperation;meanwhile,network coding is employed at the relays to facilitate the information-level cooperation.The problem of energy efficiency(EE)maximization under constraints of a predefined outage probability threshold and the energy causality is solved.Close-to-optimal power allocation and energy cooperation policies across consecutive transmissions are found under both Rayleigh fading and Nakagami-m fading environment.The performance comparisons with the current state of solutions demonstrate that the proposed policies can manage the harvested energy more efficiently;4.A new network coding scheme,i.e.,cascaded network coding is proposed to further improve the EE.In this scheme,more source messages are combined into one network codeword at each individual relay,which will be forwarded to the destination at the lower cost of time slots and energy consumptions.The source messages are finally jointly recovered at the destinations by network decoding.Both theoretical and simulation results imply that the proposed scheme can bring EE gains without decreasing the diversity order in the high signal-to-noise ratio(SNR)region.