## 1 Introduction  
### Previous Work  
With the increasing demands for high-speed communications, full-duplex technologies have been taken into account as a promising solution for boosting the spectral efficiency in multiuser wireless communications systems [1]. However, the potential advantage of full-duplex systems may be limited by a new challenge--the inter-terminal interference--that does not appear in half-duplex systems. The problem of inter-terminal interference in full-duplex systems has recently been studied in the literature in terms of degrees of freedom (DoF) (known as the pre-log of the sum-rate capacity in the high signal-to-noise (SNR) regime) [2; 3]. In particular, if channels follow the ergodic phase fading model and full channel state information at the transmitter (CSIT) is available, then it was shown in [2] that the DoF of full-duplex systems can be ideally twice as large as that of half-duplex systems. Several inter-terminal interference cancellation schemes for a three-terminal full-duplex system were presented in [3]. In addition, the DoF of multi-antenna full-duplex systems was recently studied in [4; 5]. However, there are some practical challenges as follows. First, the computational burden of such schemes will increase steeply as the system dimensions increase. Second, the node cooperation and a massive number of CSI feedback bits are required. On the other hand, in multiuser wireless communications systems, opportunistic transmission techniques that exploit the usefulness of fading have been widely studied in the literature, where one can obtain a multiuser diversity gain as the number of users is sufficiently large. Specifically, opportunistic scheduling [6], opportunistic beamforming [7], and random beamforming [8] were introduced in single-cell broadcast channels. In particular, it was pointed that the same sum-rate scaling law as the optimal dirty-paper coding can be achieved for such broadcast channels via random beamforming with far less CSI feedback [8]. Moreover, scenarios exploiting the multiuser diversity gain were studied in cooperative networks by applying an opportunistic two-hop relaying protocol [9], a parallel opportunistic routing protocol [10], and an opportunistic network decoupling protocol [11] as well as in cognitive radio networks with opportunistic scheduling [12; 13; 14]. Using opportunistic communications, a certain user scaling law for achieving one DoF per user was also examined for \((n,K)\)-interference channels [15]. In addition, such opportunism was utilized in multi-cell broadcast channels (or, equivalently, interfering broadcast channels) by using multi-cell random beamforming [16; 17] and opportunistic interference alignment [18]. As a more challenging problem than the downlink case, the optimal DoF in multi-cell multiple access channels (or, equivalently, interfering multiple access channels) was analyzed by presenting opportunistic interference alignment strategies [19; 20; 21; 22] and distributed scheduling protocols [23; 24]. In [16; 18; 19; 20; 21], the minimum number of users required to achieve the optimal DoF was investigated (i.e., the user scaling law). It is worth noting that, for achieving these DoFs, the transmitters do not require the knowledge of the instantaneous channel realizations. Recently, in a full-duplex system composed of a \(2M\)-antenna full-duplex base stations (BSs) and a large number of single-antenna half-duplex mobile stations (MSs), opportunistic beamforming and scheduling methods were proposed in [25; 26]. In [25], a joint uplink-downlink opportunistic beamforming method was employed so that uplink and downlink sum capacities can be achieved under a certain user scaling condition. Unlike the beamforming method in [25], the scheme in [26] took advantage of the zero-forcing (ZF) receiver for uplink to achieve the full DoF since ZF filtering at the BS is sufficient to guarantee \(M\) DoF for uplink, which results in infinitely large sum-rates with increasing SNR.