Cross-layer Optimization for Uplink Transmission in OFDMA Cellular Networks with Fixed Relays

In this paper we consider cross-layer design for uplink transmission in an OFDMA-based cellular network with fixed relay stations (RSs) aimed to enhance performance. Since mobile stations (MSs) spend most of the power in the uplink to transmission, power efficiency resource allocation becomes very important to MSs. We develop a cross-layer optimization framework for two types of uplink flows (inelastic and elastic flows) which have different QoS requirements. For inelastic flows with fixed rate requirement we formulate the cross-layer optimization problem as the minimization of the sum transmission power of MSs under the constraints of flow conservation law, subcarrier assignment, relaying path selection and power allocation. For elastic flows with flexible service rate requirement, we consider the cross-layer tradeoff between uplink service rate and power consumption of MSs and pose the optimization problem as the maximization of a linear combination of utility (of service rates) and power consumption (of MSs). Different tradeoff can be achieved by varying the weighting parameters. Dual decomposition and subgradient method are used to solve the problems optimally with reduced computational complexity. Simulation results show that through the proposed cross-layer resource optimization framework and algorithms, significant benefits of deployment of multiple fixed relays in an OFDMA cellular network can be fully obtained in the sense of reducing power consumption, increasing service rate and saving energy in the uplink transmission of MSs.