Degrees of freedom of cellular networks: Gain from full-duplex operation at a base station

In this paper, we study the degrees of freedom (DoF) of cellular networks in which a base station with full-duplex operation simultaneously communicates to multiple half-duplex mobile stations. The half-duplex mobile stations are assumed to either communicate with the base station in the uplink or downlink. For such networks, inter-terminal interference needs to be taken into account to achieve the potential gain that doubles the throughput of conventional half-duplex cellular networks. We provide an achievable total DoF for such networks and show that having full-duplex operation at the base station only can indeed double the DoF when the number of mobile stations is sufficiently greater than the number of antennas at the base station. The key idea is to utilize the full DoF for the uplink transmission, while minimizing the inter-terminal interference via interference alignment. The downlink communication rate is then carefully chosen to occupy the remaining DoF at the receiving mobile stations.

I. Introduction

Current cellular communication systems operate in half-duplex (HD) mode by transmitting and receiving either at different times or over different frequency bands. The system is designed such that downlink and uplink traffic are structurally separated by time division duplexing (TDD) or frequency division duplexing (FDD). The main reason for such design principle is to avoid the high-powered self-interference that is generated during simultaneous transmission and reception. Recent results [1]–[3], however, have demonstrated the feasibility of full-duplex (FD) wireless communication by cancelling self-interference in the RF and baseband level. By enabling simultaneous transmission and reception, FD radio is expected to double the spectral efficiency of current HD systems [4] and is considered as one of the key technologies for next generation communication systems. Evidently, in situations where the base station and mobile station simultaneously transmit bidirectionally as shown in Figure 1(a), enabling full duplex doubles the overall throughput. This point-to-point bidirectional communication example however, is just one instance of how a FD cellular system will function. In Figure 1(b), the base station operates in FD mode while two terminals, terminal 1 and terminal 2 only transmits and receives, respectively. In this example, the base station is receiving from terminal 1, and transmitting to terminal 2. In FD cellular systems, this type of traffic will frequently appear since there may not always be simultaneous bidirectional traffic between two nodes. Furthermore, the system may have to support HD mobile stations which do not have FD radio due to extra hardware burden on mobile devices. The main advantage of such configuration is that, since the base station is simultaneously transmitting and receiving, there is still potential to double the system throughput with conventional HD mobile terminals. However, the configuration shown in Figure 1(b) induces a new source of interference that does not arise in HD networks. Since terminal 1 is transmitting to the base station and terminal 2 is receiving from the base station, the transmission from terminal 1 causes interference to terminal 2. In cases where the interference is strong, the gain of having FD radios can be severely limited. While this simple comparison between the two FD operation scenarios for cellular networks is to highlight the new source of interference between mobile stations, inter-terminal interference becomes more problematic when we consider multi-user MIMO (MU-MIMO) transmission with multiple antennas at the base station. Due to simultaneous MU-MIMO uplink and downlink transmission, even the bidirectional communication case suffers from inter-node interference. Fig. 1.

Full-duplex configurations.