60% 4-4-44-44444在A4△ 50% 日日日日日日日日 510152025303 Number of stations Number of stations Figure 3. Network capacity comparison: a)saturation capacity; b) network capacity gain with respect to 802. 11g ooperation. Moreover, Coop MAC introduces a combining, not supported by any existing wire which is used by the helper to indicate its avail- generation wireless baseband chip. Given the ability after it receives the rtS message from constraint of using existing hardware, we have the source. If the destination hears the HTs developed a backward compatible mode of to resage, it issues a clear to send(CTS)message CoopMAC, which does not perform receiver serve channel time for a two-hop transmis combining and therefore only requires a driver sion. Otherwise, it still sends out the CTs, but or firmware upgrade nly reserves channel time for a direct transmis- Without diversity combining: If no combining capability is supported at the destination, the If both HTS and Cts are received at the packet should be transmitted on both the first source, the data packet should be transmitted to and second hop at the highest physical layer rate the relay first, and then forwarded to the desti- that the respective link can sustain nation by the relay. If the source does not receive With diversity combining: When receiver an HTS, it should then initiate a direct transmis- combining is enabled, the relay now can forward sion to the destination packets at a rate equal to or greater than the A normal ACK is used to acknowledge a one that it adopts in Coop MAC where combi orrect reception, regardless of whether the ing is not possible. More specifically, the trans- packet is forwarded by the relay, or is directly mission rate between the source and relay is transmitted from the source If ne cessa retransmission is attempted, again in a coopera- bility at the relay. Although the destination can- tive fashion not fully decode the packet after the first-hop It is crucial that each node obtains and con- transmission, this received signal will be stored stantly updates its information about the avail- If the relay can successfully receive the packet, it ability of potential relays. The CoopMac then forwards the packet to the destination. The protocol deals with this issue mainly through transmission rate on the second hop is the high maintaining a table called the Coop Table in its est one that meets a predetermined average management plane. Each entry in the CoopT- error rate at the destination, once the destin able corresponds to a potential relay, and con- tion combines the source and relay signals. tains such information as the ID(e. g, 48-bit The diversity combining capability allows MAC address) of the potential relay, the latest CoopMAC to leverage both the spatial diversity time at which a packet from that potential relay and the coding gain, thereby resulting in ever is overheard by the source, and the data rate better performance than the protocol without used for direct transmission between the poten- receiver combining. Using the coded coopera tial relay and destination, and between the cur- tion framework described above, the helper pro- rent node and the potential relay. A set of vides different coded bits than the source protocols have been defined in Coop MAC to leading to a better error performance than rep properly establish, manage, and update the table tition coding It is worthwhile to note that although the Due to the broadcast nature of the channel, protocol architecture and signaling mechanism the destination will receive the signals transmit- defined above are applicable both with and with ted by both the source and the relay. If the desti- out diversity combining at the receiver, the nation is capable of combining these two copies relay-selection scheme may not yield an optimal to decode the original information, then cooper- choice for CoopMAC with receiver combining ative diversity can be fully leveraged. Receiver any longer, because it does not take the possible IEEE Wireless Comm88 IEEE Wireless Communications • August 2006 cooperation. Moreover, CoopMAC introduces a new message called helper-ready to send (HTS), which is used by the helper to indicate its avail￾ability after it receives the RTS message from the source. If the destination hears the HTS message, it issues a clear to send (CTS) message to reserve channel time for a two-hop transmis￾sion. Otherwise, it still sends out the CTS, but only reserves channel time for a direct transmis￾sion. •If both HTS and CTS are received at the source, the data packet should be transmitted to the relay first, and then forwarded to the desti￾nation by the relay. If the source does not receive an HTS, it should then initiate a direct transmis￾sion to the destination. •A normal ACK is used to acknowledge a correct reception, regardless of whether the packet is forwarded by the relay, or is directly transmitted from the source. If necessary, retransmission is attempted, again in a coopera￾tive fashion. It is crucial that each node obtains and con￾stantly updates its information about the avail￾ability of potential relays. The CoopMAC protocol deals with this issue mainly through maintaining a table called the CoopTable in its management plane. Each entry in the CoopT￾able corresponds to a potential relay, and con￾tains such information as the ID (e.g., 48-bit MAC address) of the potential relay, the latest time at which a packet from that potential relay is overheard by the source, and the data rate used for direct transmission between the poten￾tial relay and destination, and between the cur￾rent node and the potential relay. A set of protocols have been defined in CoopMAC to properly establish, manage, and update the table in a timely manner. Due to the broadcast nature of the channel, the destination will receive the signals transmit￾ted by both the source and the relay. If the desti￾nation is capable of combining these two copies to decode the original information, then cooper￾ative diversity can be fully leveraged. Receiver combining, not supported by any existing wire￾less hardware, can be implemented in the next￾generation wireless baseband chip. Given the constraint of using existing hardware, we have developed a backward compatible mode of CoopMAC, which does not perform receiver combining and therefore only requires a driver or firmware upgrade. Without diversity combining: If no combining capability is supported at the destination, the packet should be transmitted on both the first and second hop at the highest physical layer rate that the respective link can sustain. With diversity combining: When receiver combining is enabled, the relay now can forward packets at a rate equal to or greater than the one that it adopts in CoopMAC where combin￾ing is not possible. More specifically, the trans￾mission rate between the source and relay is chosen so as to guarantee a desired error proba￾bility at the relay. Although the destination can￾not fully decode the packet after the first-hop transmission, this received signal will be stored. If the relay can successfully receive the packet, it then forwards the packet to the destination. The transmission rate on the second hop is the high￾est one that meets a predetermined average error rate at the destination, once the destina￾tion combines the source and relay signals. The diversity combining capability allows CoopMAC to leverage both the spatial diversity and the coding gain, thereby resulting in even better performance than the protocol without receiver combining. Using the coded coopera￾tion framework described above, the helper pro￾vides different coded bits than the source, leading to a better error performance than repe￾tition coding. It is worthwhile to note that although the protocol architecture and signaling mechanism defined above are applicable both with and with￾out diversity combining at the receiver, the relay-selection scheme may not yield an optimal choice for CoopMAC with receiver combining any longer, because it does not take the possible ■ Figure 3. Network capacity comparison: a) saturation capacity; b) network capacity gain with respect to 802.11g. Number of stations 0 5 7 8 Capacity (Mb/s) 9 10 11 12 13 14 10 15 20 (a) (b) 25 30 35 40 Number of stations 5 0% 10% Capacity gain (percentage) 20% 30% 40% 50% 60% 10 15 20 25 30 35 40 CoopMAC with receiver combining CoopMAC without receiver combining 802.11g CoopMAC without receiver combining CoopMAC with receiver combining ERKIP LAYOUT 8/3/06 1:24 PM Page 88