N I ] 2 7 A pr 2 01 5 Full-duplex MAC Protocol Design and Analysis

The idea of in-band full-duplex (FD) communications revives in recent years owing to the significant progre ss in the self-interference cancellation and hardware designtechniques, offering the potential to double spectral efficienc y. The adaptations in upper layers are highly demanded in the desig n of FD communication systems. In this letter, we propose a novel medium access control (MAC) using FD techniques that allows transmitters to monitor the channel usage while transmitting, and backoff as soon as collision happens. Analytic al saturation throughput of the FD-MAC protocol is derived wit h the consideration of imperfect sensing brought by residualselfinterference (RSI) in the PHY layer. Both analytical and simulation results indicate that the normalized saturation throughput of the proposed FD-MAC can significantly outperforms conventional CSMA/CA under various network conditions. I. I NTRODUCTION The idea of single-channel full-duplex (FD) communication, where a node can transmit and receive using the same time and frequency resources has been proposed for over a decade because of its potential of double capacity. These several years witness the revival of FD research thanks to th e development of self-interference cancellation and suppre ssion techniques [1]. In addition to the increasing interest and r apid development in the PHY layer realization, it also requires adaptations in protocol design for the upper layers like the medium access control (MAC) layer. Some full-duplex MAC protocols have been proposed during the last few years [1]–[4]. In [1] and [2], the centralize d FD-MAC protocols are proposed: [1] considers bidirectiona l transmission between a pair of nodes, and uses busytone to eliminate the hidden terminal problem, and [2] design the pr otocol with three new elements, namely, shared random backof f, header snooping and virtual backoffs. Decentralized FD-MA C protocols are proposed in [3], [4] based on CSMA/CA. The former mainly focuses on bidirectional transmission betwe n users, and the latter discusses simultaneous transmission s among two or three FD users. However, the way how to fully utilize FD techniques for distributed wireless networks wi th multiple contending users, and comprehensive analysis fro m PHY to MAC layers still requires further investigation. To this end, in this letter, we propose a new distributed FDMAC protocol for decentralized FD communication networks in which a number of FD transceiver pairs compete for transmission opportunities. Due to FD techniques, transmi tters are able to sense and monitor the channel usage state while they are transmitting. Thus, if collision happens, users ar e able to backoff in short time before finishing the whole data packet and waiting for the absence of the ACK packet. We derive the analytical throughput of the proposed FD-MAC protocol by taking imperfect sensing caused by residual sel finterference (RSI) into consideration [5]. Both analytica l and simulation evaluations show that the proposed protocol can significantly improve system throughput and achieve high normalized saturation throughput. Note that different from the CSMA/CD in Ethernet, the proposed FD-MAC operates in wireless environment with fading channels and RSI caused by FD techniques. Thus, the sensing performance is imperfect and the design and analysi s of the FD-MAC is non-trivial. II. FD-MAC PROTOCOL In this section, we elaborate the proposed FD-MAC protocol based on FD techniques and carrier sense multiple access wit h collision avoidance (CSMA/CA) concepts, such that each use r can sense the spectrum and determine whether other users are occupying it while transmitting its own data. This feature l eads to the possibility of a new MAC protocol design in which users are not assumed “blind” when transmitting. A. System Model We consider a system with M FD-enabled contending users operating under the saturated condition, i.e., each user al ways has a packet to transmit. All users sense the target channel continuously to determine whether other users are occupyin g it. FD techniques allow users to keep sensing the channel while transmitting. Thus, users can detect channel state ch ange swiftly, and change their own states accordingly. Note that the sensing in FD users can never be perfect due to the impact of RSI. In analysis of conventional CSMA, noise term is neglected, and sensing is commonly assumed perfect. For simplicity and comparison fairness, we also omit noise term in sensing, and only consider the impact of RSI [5]. Furthermore, we assume that when more than two users collide, the received signal from other users overwhelms the RSI, and the sensing is perfect. Thus, the effects of imperfect sensi ng including false alarm and miss detection 1 are considered only in the following two cases, respectively: (1) one single use r occupies the channel; (2) two users collide with each other. B. Protocol Design Fig. 1 shows the proposed protocol, which consists of the following several parts. Sensing:All users keep sensing the channel continuously regardless of its own activity, and make decisions of the channel usage at the end of each slot with duration τ , which 1The values of false alarm and miss detection are related to th e RSI and slot length in PHY [5]. Generally, they both increase with th e RSI, i.e., the sensing performance gets degraded with self-interference . Also, there exist requirements for minimum slot length to achieve reliable se nsing, which are related to sampling frequency in sensing, channel conditio ns, number of users, and the RSI level, etc.