2 0 M ar 2 01 5 Multi-Objective Optimization for Power Efficient Full-Duplex Wireless Communication Systems

In this paper, we investigate power efficient resource allocation algorithm design for multiuser wireless communication systems employing a full-duplex (FD) radio base station for serving multiple half-duplex (HD) downlink and uplink user s simultaneously. We propose a multi-objective optimization framework for achieving two conflicting yet desirable system design objectives, i.e., total downlink transmit power minimization and total uplink transmit power minimization, while guaran teeing the quality-of-service of all users. To this end, the weight ed Tchebycheff method is adopted to formulate a multi-objecti ve optimization problem (MOOP). Although the considered MOOP is non-convex, we solve it optimally by semidefinite program ing relaxation. Simulation results not only unveil the trade-off between the total downlink and the total uplink transmit power, but a lso confirm that the proposed FD system provides substantial pow er savings over traditional HD systems. I. I NTRODUCTION The exponential growth in high data rate communication has triggered a tremendous demand for radio resources such as bandwidth and energy. A relevant technique for reducing the energy and bandwidth consumption for given quality-ofservice (QoS) requirements is multiple-input multiple-ou tput (MIMO) as it offers extra degrees of freedom for more efficien t resource allocation. However, the MIMO gain may be difficult to achieve in practice due to the high computational complex ity of MIMO receivers. As an alternative, multiuser MIMO (MUMIMO) has been proposed as an effective technique for achieving the MIMO performance gain. In particular, in MUMIMO, a transmitter with multiple antennas (e.g. a base stat ion (BS)) serves multiple single-antenna users which shifts th e computational complexity from the receivers to the transmi tter [1]. Nevertheless, the spectral resource is still underuti lized even if MU-MIMO is employed due to the traditional halfduplex (HD) operation of the BS, where uplink and downlink communication are separated in either time or frequency whi ch leads to a significant loss in spectral efficiency. As a result, full-duplex (FD) wireless communications has recently received significant attention from both academia and industry due to its potential to double the spectral efficien y of the existing wireless communication systems [2]–[6]. In co trast to conventional HD transmission, FD enables simultaneous downlink and uplink transmission at the same frequency. However, in practice, a major challenge in FD communication is self-interference (SI) caused by the signal leakage from the downlink transmission to the uplink signal reception. Besi d , the resource allocation for FD systems is complicated by the cochannel interference (CCI) caused by the uplink transmissi on to the downlink transmission. Thus, different resource alloc tion designs for FD systems were proposed and studied to overcome these challenges. The authors of [3] investigated the end-t o-end Derrick Wing Kwan Ng and Robert Schober are also with the Univ ersity of British Columbia, Canada. outage probability of MIMO FD single-user relaying systems . In [4], a resource allocation algorithm was proposed for the maximization of the end-to-end system data rate of multicarrier MIMO FD relaying systems. In [5], massive MIMO was applied in FD relaying systems to facilitate SI suppress ion and to improve spectral efficiency. Simultaneous downlink a d uplink transmission in small cells was studied in [6], where a suboptimal downlink beamformer was designed to improve the system throughput. However, the power consumption in FD systems has not been thoroughly studied in the literature so far [3]–[6]. On the other hand, a large amount of work has been devoted to power efficient/green communications because of the pressure stemming from environmental concerns and the rapidly increasing cost of energy [7]–[10]. In [7], downlin k beamforming was proposed to minimize the total transmit power while guaranteeing secure communication for a single desired user. In [8], both uplink and downlink beamforming were studied for total transmit power minimization in MUMIMO systems. In [9], a power efficient scheduling scheme was designed to minimize the power consumption of multihop wireless links. In [10], the maximization of the energy effic iency of multi-carrier systems was studied. However, all of the ab ove works focus on HD systems and the obtained results may not be applicable to FD communication systems. In particula r, total downlink and total uplink transmit power minimizatio n are conflicting design objectives in FD systems. However, the intricate trade-off between downlink and uplink power consumption in FD systems has not been studied in the literat ure [7]–[10]. Such a trade-off does not exist in HD systems since downlink and uplink transmission are separated either in th e time domain or in the frequency domain. Besides, the singleobjective optimization frameworks in [7]–[10] are not suit able for studying the trade-off between downlink and uplink powe r consumption in FD systems. Motivated by the aforementioned observations, we propose a multi-objective optimization problem (MOOP) formulatio n which minimizes the total downlink and the total uplink tran smit powers subject to QoS constraints for MU-MIMO FD wireless communication systems. The proposed MOOP is formulated by adopting the weighted Tchebycheff method and solved optimally by semidefinite programming (SDP) relaxation.