5s72 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS.VOL 14.NO.10.OCTOBER 2015 Preamble PACKET1 PACKET 2 AP to Client C will be longe Fig 1.A wireless LAN with a full duplex AP. AP ne packet om Chent A to the capture effect can help establish dual links but it cann no capability of full duplex communications.only asymr netric ensure throughput improvement.Asa result,to better utilize the capture effect n in Fig.I nds less LAN with a full duplex AP and hal AP the AP C).In [6] etric dual links (i.e.,AAP- c)do first:2)the received signal strength of the packet from the AP o a client is stronger than the interference from the packet sen cap from ano ther chent to th o fully leverage the capability of the full duplex AP.we develop that throug hput can be imp oved.To this end,the AP needs re effect at the client side to collect relative signal strength between nodes to build an be properly f full h clients can selecte n 1 is de ed to Thanks to the asymmetric dual links,our MAC pr ocol sur ports efficient co-existen ce between a full duplex AP and hal Ao be hidden to cachtke fom the AP to Client B.dug ex (A-Dup utilized to form full duplex communications at the AP.even though Clients A and B are not hidden from each other. improved by 48%and 188%as compared to DCF [14]wit RTS/CT 2线 proeL-align nt for hetter cantur A-Duplex is distinct with several features.First.it leverage Sant first.In either type.eket as shown in packet o fully uti lize the capa for the fir to ba cond packet (This e links example e first packet blist the based on this map asymm nks are e ba that of the sc environment.Third.a virtual deficit round robin algorithm is effect ma asymmetric dual links are established (eA ver update is peeded.To the best of our knowledge.this is the first MAC that holds these features the transmission rate from the AP to Client C is lower than nt sign of A-Dup links in A-Duplex are elaborated in Section IV.The fairness issue is studied in Section V.Simulation results are presented in5872 IEEE TRANSACTIONS ON WIRELESS COMMUNICATIONS, VOL. 14, NO. 10, OCTOBER 2015 Fig. 1. A wireless LAN with a full duplex AP. no capability of full duplex communications, only asymmetric dual links can be utilized to boost network throughput. For example, as shown in Fig. 1, while Client A sends a packet to the AP, the AP sends a packet to another client (e.g., Client C). In [6], asymmetric dual links (i.e., A → AP → C) do not allow collisions at both receivers, i.e., the signal from Client A does not interfere Client C. However, such a condition cannot be easily satisfied in a wireless LAN, so the capability of the full duplex AP cannot fully utilized. In order to fully leverage the capability of the full duplex AP, we develop a novel MAC protocol to explore capture effect at the client side to improve opportunity of full duplex communications in a wireless LAN. Capture effect [10]–[12] means, when a receiver receives two colliding packets, the stronger one can still be decoded correctly. For example, Client A and Client B may not be hidden to each other, but a packet from Client A to the AP may not corrupt the packet from the AP to Client B, due to capture effect at Client B. In this situation, capture effect is utilized to form full duplex communications at the AP, even though Clients A and B are not hidden from each other. The performance of capture effect can be significantly im￾proved through alignment of two colliding packets. To leverage packet-alignment for better capture effect,1 there are two op￾tions: 1) the desired packet is sent first; 2) the interfering packet is sent first. In either type, if the preamble of the first packet does not collide with the second packet as shown in Fig. 2, it is relatively easy for the first packet to be decoded correctly. However, it is rather difficult to decode the second packet (This case is also called message in message (MIM) in [13]). For example, under the basic rate in IEEE 802.11a, the first packet can be decoded when its signal strength is 0 dB stronger than that of the second packet [12]. If we want to decode the second packet correctly, the signal strength of the second packet need to be 11 dB stronger than that of the first packet [12]. However, the asymmetric dual links supported via capture effect may not always improve the network performance. When asymmetric dual links are established (e.g., A → AP → C), the transmission rate from the AP to Client C is lower than that in the half duplex case since Client C is interfered by Client A. Thus, the transmission time of the packet from the 1If successive interference cancellation (SIC) is taken into account, then packet alignment is not necessary. However, the client side consists of legacy nodes, so any change to the physical layer is infeasible. Thus, SIC is not considered in this paper. Fig. 2. Capture effect without collision in preamble time. AP to Client C will be longer than that in the half duplex case. The concurrent transmission in dual links can save some time; however, if the transmission time from the AP to Client C is much longer than that of the packet from Client A to the AP, the dual links may reduce the throughput. In other words, the capture effect can help establish dual links but it cannot ensure throughput improvement. As a result, to better utilize the capture effect in a wireless LAN with a full duplex AP and half duplex clients, the MAC protocol must take into account three requirements: 1) the packet from the AP needs to be transmitted first; 2) the received signal strength of the packet from the AP to a client is stronger than the interference from the packet sent from another client to the AP; 3) the AP needs to choose a client with a proper transmission rate to establish dual links so that throughput can be improved. To this end, the AP needs to collect relative signal strength between nodes to build an information map from which clients can be properly selected to establish dual links effectively. In this paper, a dynamic information update scheme is designed to establish such a map. Thanks to the asymmetric dual links, our MAC protocol sup￾ports efficient co-existence between a full duplex AP and half duplex clients, so it is called asymmetrical-Duplex (A-Duplex) in this paper. A-Duplex not only improves the throughput but also reduces packet delay. Simulation results show that the throughput is improved by 48% and 188% as compared to IEEE 802.11 DCF [14] with RTS/CTS and without RTS/CTS, respectively. The packet delay is reduced by 26% and 22%, respectively. A-Duplex is distinct with several features. First, it leverages packet-alignment based capture effect to establish asymmetric dual links so as to fully utilize the capability of the full duplex AP. As a result, throughput is improved effectively as compared to the case of only using the hidden nodes to establish dual links. Second, a map of signal-to-interference ratio (SIR) is built at the AP; based on this map asymmetric dual links are established to take advantage of capture effect. The SIR map can be updated dynamically to capture the variable network environment. Third, a virtual deficit round robin algorithm is developed for the AP to select a downlink in dual link setup, which improves fairness of the MAC protocol. Finally, no physical layer change is needed at legacy nodes; only MAC driver update is needed. To the best of our knowledge, this is the first MAC that holds these features. The rest of the paper is organized as follows. Related work is introduced in Section II. The protocol design of A-Duplex is de￾scribed in Section III. Detailed procedures of establishing dual links in A-Duplex are elaborated in Section IV. The fairness issue is studied in Section V. Simulation results are presented in Section VII. Compatibility and practicality issues are discussed in Section VIII. The paper is concluded in Section IX