TANG AND WANG:MAC FOR EFFICIENT COEXISTENCE BETWEEN FULL AND HALF-DUPLEX COMMUNICATIONS 5873 II.RELATED WORK A.Capture effect A当A mitted SFS ACK 1 destroy all the frames.If the receiving power of one of the arge eough than that other,the others NAVIRTS of receiving a frame from colliding frames is called (a) effect.The physical layer model for capture effect has been Delay ti ng de comes ear r has AP CTS DATA to node B ACK synchronized with the first frame.However.if a weaker frame B SIFS ACK 1 since the ver n be wit other be much larger than the weak one so that the receiver can re. 16].Since the receiver ta scenario:the transmis mak even e [121 the experimental results show that even two frame n time of the Howeve a pream ne th the SIRe and channel estimation which is consistent with [12].If packet alignment-based capture ofar the work in[]provides the best model for capture eftect is leveraged,then dB SIR is sufficient [12].For other des (ie. te),the pnbtknfhMaCp phor net ork It ador B.Full Duplex MAC noise(PN)sequence-based signatures toidentify the concurren links on-the-fly and explore the cha So far there exist a few mac protocols that supp full ance of exposed transmi duplex communications [6]-[8).In [6],both symmetric and ed.The ghput c are for by two no ng sig in Fig.1.A set of asymmetric dual links are esta IIL MAC PROTOCOL DESIGN ablished by three nodes in a two-hop setup.such as the links (BAP →C) In this section e the Ap full duplex C)in Fig. 11. full du ilit ork links need to be established.As explained before,to explore capture effect in an efficient way,the packet from the AP to downlin C)at the Client C:b)the A Howe since a lent is in th In 7.a reservation based MAC protocol was developed for the channel busy and cannot start a second link with the ap a network where all nodes have full du lex radios.All nodes are To resolve this issue,an RTS/CTS mechanism is added into the allocated wit tim ots by the Al In this M. pro to star dual links v re the AP-to-client lin dual lin -AP-C n00 an RTS if they are not hidden nodes,since the interference by Client Three cases are considered below.When a client gets the A is small at Client C.In this sche channel via the RTS frame,two cases need to be AP a pac ever packet-aigment based capture effect are not considered third case is that the Ap acquires the channel first.Note that TANG AND WANG: MAC FOR EFFICIENT COEXISTENCE BETWEEN FULL- AND HALF-DUPLEX COMMUNICATIONS 5873 II. RELATED WORK A. Capture effect In random access, if more than one frames are transmitted concurrently, collision occurs. However, it does not always destroy all the frames. If the receiving power of one of the colliding frames is larger enough than that of the other, then the receiver can decode this frame correctly. Such a process of receiving a frame from colliding frames is called capture effect. The physical layer model for capture effect has been explored by various experiments in [10]–[12], [15]. Usually if the stronger frame comes earlier than other frames, it is easier to decode this frame since the receiver has already been synchronized with the first frame. However, if a weaker frame comes first, MIM is needed to decode the stronger frame. In this case, since the receiver has been synchronized with the weaker frame, the signal strength of the stronger frame must be much larger than the weak one so that the receiver can re￾synchronize with the stronger one to utilize capture effect [12], [13], [16]. Since the receiver takes a certain amount of time to achieve synchronization, even if the stronger frame arrives earlier, it is necessary to make sure the lead time is sufficient. In [12], the experimental results show that even two frames come at the same time, capture effect may still succeed in 802.11a networks. However, if the first frame is earlier by a preamble time, the success probability of capture effect can be improved, because a clean preamble is helpful to conduct synchronization and channel estimation. So far the work in [12] provides the best model for capture effect in 802.11a networks. It provides the measurement-based SIR capture thresholds for all the 802.11a bit rates. B. Full Duplex MAC So far there exist a few MAC protocols that support full duplex communications [6]–[8]. In [6], both symmetric and asymmetric dual links are considered. A set of symmetric dual links are formed by two nodes transmitting signal to each other simultaneously. An example is the link (A → AP → A) shown in Fig. 1. A set of asymmetric dual links are established by three nodes in a two-hop setup, such as the links (B → AP → C) shown in Fig. 1. In [6], the MAC protocol mainly explores symmetric dual links to improve network throughput, and in the asymmetric case (e.g., links (B → AP → C) in Fig. 1), dual links can be established only under the following conditions: a) the uplink transmission (B → AP) does not collide with the downlink transmission (AP → C) at the Client C; b) the AP transmits to Client C later. In [7], a reservation based MAC protocol was developed for a network where all nodes have full duplex radios. All nodes are allocated with some time slots by the AP. In this MAC protocol, asymmetric dual links are allowed even for non-hidden nodes. For example, dual links A-AP-C in Fig. 1 are allowed even if they are not hidden nodes, since the interference by Client A is small at Client C. In this scheme, AP first collects the interference information of clients and then schedules the trans￾mission order according to the interference relationship. How￾ever, packet-alignment based capture effect are not considered Fig. 3. Dual link establishment. (a) The “AP-shorter” scenario: the transmis￾sion time of the packet from the AP to B is shorter than the packet transmission time of the other link plus a preamble time. (b) The “AP-longer” scenario: the transmission time of the packet from the AP to B is longer than the packet transmission time of the other link plus a preamble time. in this scheme. Thus, the performance of capture effect is limited. For example, in the basic operation mode (i.e., lowest rate with robust modulation/coding), the SIR needs to be 10 dB, which is consistent with [12]. If packet alignment-based capture effect is leveraged, then 1 dB SIR is sufficient [12]. For other modes (i.e., higher rate), the situation is similar. In [8], the MAC protocol addresses the exposed terminal problem in a full duplex network. It adopts the pseudo-random noise (PN) sequence-based signatures to identify the concurrent links on-the-fly and explore the chance of exposed transmis￾sions. As a result, the throughput of a full duplex network can be improved. The protocol also assumes all nodes have full duplex capability. III. MAC PROTOCOL DESIGN In this section, a MAC protocol called A-Duplex is designed for a wireless LAN where the AP has full duplex capability, but all clients can only work in the half duplex mode. To utilize full duplex capability in this wireless LAN, asymmetrical dual links need to be established. As explained before, to explore capture effect in an efficient way, the packet from the AP to a client needs to start first. However, since a client is in the half duplex mode, once it detects such a transmission, it considers the channel busy and cannot start a second link with the AP. To resolve this issue, an RTS/CTS mechanism is added into the MAC protocol to start dual links where the AP-to-client link can be started first. As shown in Fig. 3, the procedure to set up a full duplex link always starts with an RTS frame by a client. Three cases are considered below. When a client gets the channel via the RTS frame, two cases need to be considered: 1) the AP can send a packet to another client to establish dual links; 2) the AP does not have a packet for another client. The third case is that the AP acquires the channel first. Note that