TANG AND WANG:MAC FOR EFFICIENT COEXISTENCE BETWEEN FULL AND HALF-DUPLEX COMMUNICATIONS 5875 to set up a dual link Details of this functionre explained in the then they compute the SIR value with the two recorded values an A-Duplex may experience two types of collisions.In the first Client B)contends the channel successfully,it transr mits its type of collisions.more than on clients send an Rohe recorded SIR value via the RTS frame to the AP.Since the AP the last roun colision ha and do n the RTSfm ws th SIR In the second type of collisions,the AP starts a transmission given the interference from Client A.Thus,it updates such the SIR MAP.If the same client llican sim eive,it the AP stop cha value is igno and let the client's RTS frame pr channel successfully In this scenario all client just update ceed.so the client can still contend the channel successfullv thethe channe clents pdate the IV.ESTABLISHMENT OF ASYMMETRICAL DUAL LINKS the the The critical step of setting up dual links is to ensure capture STR value in the AP's SIR MAP By building up SIR MAP the effect to be properly utilized.Forexampe if du whe interfere other words,the AP needs to find out the signal-to-interference RTS frame from the interfering client.To achieve this goa ent ve use the transmit the RTS that othe an of SIR for all client what follow h is effec an information map is called the sir map wireless LAN the transmit power ofan RTS frame is usuall set the same as that of a regular data packet.Sec ond,an RT A.Establishment of SIR MAP rame uses the rate for tra n,so it The SIR MAP contains the SIR information of all clients a busy channel but can nnot receive an rTS frame it relies or Considering Clients A and B in a wireless LAN.SIR at Client wce from repre nted by sh le that ere was an RT nal str Cto A.A to B.Cto B.A to C.and B toC.because there are 6 interference signal strength en two clients all clients can find out whether 10 the a clie on happens via kI ge.M cally,if ing an RTS frame from a client,it knows collision has happened and thus skips updating its interference signal strength. ansmits sa packet toa client (B).th ue to channel f .an A)tre AP,the current client (i.e.Client B)can also get the inter- the value instead a moving average strategy is necessary one erence strength by o erhearing the pack et.Sinc we make n casy approach is to use the average of the most recent value change to the physic signal str the Al store to the Strength Indicator (RSSD when a client succeeds in receiving a needs to stores 5 SIR values for each client.In order to reduce packet in the MAC layer. the storage of AP.a simple moving average scheme is given as AC layer m follows d help buld SIR=SIROm x (1-0)+ (3 the channel.it first transmits an KTS frame to the AP All where 6 is a weight strength nter re ing the R 4,the the current the signal strength from AP according to the CTS frame,and scheme.the AP only needs to store one SIR value for each TANG AND WANG: MAC FOR EFFICIENT COEXISTENCE BETWEEN FULL- AND HALF-DUPLEX COMMUNICATIONS 5875 to set up a dual link. Details of this function are explained in the next section. Fourthly, ACK frames in dual links are protected via the duration in RTS/CTS frames (i.e., the NAV mechanism). A-Duplex may experience two types of collisions. In the first type of collisions, more than one clients send an RTS to the AP. Without correct reception of an RTS, AP does not reply a CTS frame. As a result, the competing clients know that collision happens and do not start data packet transmission. In the second type of collisions, the AP starts a transmission simultaneously with an RTS frame from a client. Since the AP can simultaneously transmit and receive, it can find out collisions while it is transmitting. In this case, the AP stops transmitting immediately and let the client’s RTS frame pro￾ceed, so the client can still contend the channel successfully. IV. ESTABLISHMENT OF ASYMMETRICAL DUAL LINKS The critical step of setting up dual links is to ensure capture effect to be properly utilized. For example, if dual links (from A to AP and from AP to C) are to be set up, the AP needs to make sure that the signal strength at Client C is stronger enough than interference from Client A according to the transmission rate. In other words, the AP needs to find out the signal-to-interference ratio (SIR) for Client C and decides the transmission rate based on the SIR value. To this end, it is necessary for the AP to build an information map of SIR for all clients. In what follows, such an information map is called the SIR MAP. A. Establishment of SIR MAP The SIR MAP contains the SIR information of all clients. Considering Clients A and B in a wireless LAN, SIR at Client A given the interference from B can be represented by SIR of B to A. Thus, if a wireless LAN has 3 clients: clients A, B, and C, then SIR map table must have 6 items such as SIR of B to A, C to A, A to B, C to B, A to C, and B to C, because there are 6 different scenarios of interference between two clients. To measure the SIR value experienced by a client, two values are needed: the signal strength from the AP and that from the interfering client. However, an AP cannot get such information by itself, so it has to rely on clients to collect such information. When the AP transmits a packet to a client (e.g., Client B), the client can get the signal strength of this on-going transmission. When another client (e.g., Client A) transmits a packet to the AP, the current client (i.e., Client B) can also get the inter￾ference strength by overhearing the packet. Since we make no change to the physical layer of legacy nodes, the signal strength that can be collected in the MAC layer is the Received Signal Strength Indicator (RSSI) when a client succeeds in receiving a packet in the MAC layer. MAC layer messages are used to support SIR measurements at clients and help build the SIR MAP at the AP. As explained in the previous section, when a client (e.g., Client A) accesses the channel, it first transmits an RTS frame to the AP. All other clients who receive this RTS frame record the interference strength. After receiving the RTS frame, the AP replies a CTS frame and records this client address. All other clients update the signal strength from AP according to the CTS frame, and then they compute the SIR value with the two recorded values: the signal strength from the AP and interference strength. In the next round of RTS/CTS/Data/ACK, if one client (e.g., Client B) contends the channel successfully, it transmits its recorded SIR value via the RTS frame to the AP. Since the AP records which client (i.e., Client A) started the last round of RTS/CTS/Data/ACK, it knows that the SIR value carried in the RTS frame is the SIR of a new client (i.e., Client B) given the interference from Client A. Thus, it updates such an SIR value (of A to B) in its SIR MAP. If the same client contends the channel successfully, the SIR value is ignored. As explained in the protocol design, the AP may contend the channel successfully. In this scenario, all clients just update the signal strength from the AP and recompute the SIR value. If the AP continuously gets the channel, all clients update the SIR value until a client contends the channel successfully. Since then, the above procedure is followed to record or update the SIR value in the AP’s SIR MAP. By building up SIR MAP, the AP can have a dynamic view of SIR for each client. In the MAC layer, RSSI information is only available when a MAC packet is received. Given a client, in order to detect signal strength from an interfering client, it needs to get the RTS frame from the interfering client. To achieve this goal, we use the basic rate to transmit the RTS frame so that other clients can rely on this frame to detect strength of an interfering signal. This approach is effective for two reasons. First, in a wireless LAN the transmit power of an RTS frame is usually set the same as that of a regular data packet. Second, an RTS frame uses the basic rate for transmission, so it can mostly cover the large area of a wireless LAN. In case a client can sense a busy channel but cannot receive an RTS frame, it relies on a CTS frame to get RSSI. More specifically, when the client receives a CTS frame, it can conclude that there was an RTS frame before. Thus, the client can use the minimum required signal strength for decoding an RTS frame to approximate the interference signal strength. As for the collision case, all clients can find out whether a collision happens via RTS/CTS exchange. More specifically, if a client does not receive a CTS frame from the AP after receiv￾ing an RTS frame from a client, it knows collision has happened and thus skips updating its interference signal strength. Due to channel fading, an instantaneous SIR value cannot always accurately reflect the channel quality very well. Thus, when the AP updates the MAP table, it cannot simply substitute the value. Instead, a moving average strategy is necessary. One easy approach is to use the average of the most recent values. However, this demands the AP to store too many SIR values. For example, if we want to get the average of 5 SIR values, AP needs to stores 5 SIR values for each client. In order to reduce the storage of AP, a simple moving average scheme is given as follows: SIRNew = SIROld × (1 − θ ) + SIRupdate × θ , (3) where θ is a weight factor from 0 to 1, the value SIROld is from the current SIR MAP, SIRUpdate is newly learned from frame exchange, and SIRNew is the new SIR in the MAP. With this scheme, the AP only needs to store one SIR value for each