Energy-Efficient Resource Allocation in Downlink OFDM Wireless Systems With Proportional Rate Constraints

Orthogonal frequency-division multiplexing (OFDM), with its own advantages of spectral efficiency (SE) enhancement and flexible resource allocation, is a promising basic technique for fulfilling the ever increasing demand for high-data-rate transmission and various service type support for mobile multimedia communications. In the meantime, energy efficiency (EE) has now become a critical metric for green system design. In this paper, energy-efficient and fair resource allocation is investigated in a downlink OFDM-based mobile communication system. Given a subcarrier assignment, the bisection-based optimal power allocation (BOPA) is proposed at first, which achieves the maximum EE and guarantees proportional data rates for users. Then, a two-step subcarrier assignment is designed to avoid unaffordable computational complexity of exhaustive search. In the first step, the estimated energy-efficient transmit power is found via the assumptions on flat fading and subcarrier sharing. In the second step, the traditional spectral-efficient subcarrier assignment (SESA) is introduced to complete the bandwidth resource allocation among users. Although the two-step subcarrier assignment is suboptimal due to the fact that the optimization is done independently in two separate steps, numerical results demonstrate that its performance is very close to the optimum. This paper also studies the difference between the energy-efficient solution and the traditional spectral-efficient policy and observes that they are similar with each other in the low channel-gain-to-noise ratio (CNR) regime. This observation is helpful and could enable that the energy-efficient design can be turned into the relatively simpler spectral-efficient policy when the CNR is low.