For propose of enhancing the reliability and the robustness of the WLAN operation, a cooperative communication MAC [7] is proposed.
In this protocol, the partner (relay node) retransmits the MAC frame received from the source when the frame is received in error at the destination. That is to say, if an acknowledgement from the destination is not received in aSIFSTime after completion of receiving the data frame, the partner(s) that received the data frame correctly from the source shall transmit this frame.
This protocol may utilize more than one partner and thus the transmission by partner is governed by a random backoff process to resolve potential collision between partners. The protocol is shown in Fig.2-1.
The partners shall always choose their backoff time within [0, CWp] for transmitting other nodes’ frames regardless of the retransmission result. The value of CWp is announced by the AP in its beacon based on the number of associated stations. In ad hoc wireless networks, the nodes choose CWp
= CW
min .Each node requires two MAC queues with the first queue being the data queue for its own outgoing data and the second queue, called partner queue that keeps the copy of the
currently transmitted frame that has not been acknowledged by the destination.
Figure 2-1 Basic CMAC protocol
The paper [8] proposed a cooperative MAC protocol with a rate sensitive relay selection scheme. Several rate adaptation algorithms, which are used to choose the optimal rate, have been proposed in 802.11 systems. These rate adaptive algorithms like Auto Rate Fallback (ARF) [9] and Receiver Based Auto Rate (RBAR) [10] may choose different modulation schemes according to the performance of the transmission quality or the communication state information. In this protocol, the physical modulation scheme is not changed between nodes during the transmission period. But packets are transmitted at different rates depending on the
distance between the source and the destination.
Figure 2-2 Cooperative regions for CoopMAC
In this protocol, the source will decide the dedicated helper (relay node) by observing the rate used for transmission between neighbor nodes. Each station maintains a table, called the CoopTable, of potential helpers that can be used for relaying data during transmission. The CoopTable contains the following fields: (1) the ID field, which stores the MAC address of the potential helpers. (2) The Time field, which stores the time of the last frame transmission heard from this node. (3) The Rhd field, which stores the data rate from the helper to the destination. (4) The Rsh field, which stores the data rate from the source to the helper. (5) The
NumOfFailures field, which records the sequential failures associated with the helper.
With the
R
hd and Rsh, the needed time to accomplish the two hop transmission is able to be calculated. In the RTS/CTS mode, the condition for a cooperative transmission can be expressed asWhere is the data rate for a direct transmission from the source to the destination and , , and are the additional time with a helper-aided transmission for the overhead. The HTS is a new message used to inform the source and the destination that the
direct
R
T
PLCPT
HTST
SIFShelper is available during the cooperative transmission time.
The source decides the helper by means of calculating the total transmission time by (1).
The helper with the minimum transmission time will be selected.
Figure 2-3 Control frame and Data frame exchange in CoopMAC
The paper [11] proposed a scheme that selects the best relay between the source and destination based on instantaneous channel measurements. This communication scheme exploits the wireless channel at its best, via distributed cooperative relays, is called
opportunistic relaying.
Figure 2-4 The “best” relay among M candidates is selected to relay information
A single relay among a set of M relay nodes is selected, depending on which relay node provides for the “best” end-to-end path between the source and destination. As Fig. 2-4 indicates, the wireless channel asi between the source and each relay i, as well as the channel
a
id between relay i and destination affect performance. The channel estimates asi, aid at each relay, describe the quality of the wireless path between source-relay-destination, for each relay i.Since the two hops are both important for end-to-end performance, each relay should quantify its appropriateness as a relay, using two functions that involve the link quality of both hops. The two functions are: under policy I, the minimum of the two is selected (equation (2)), while under policy II, the harmonic mean of the two is used (equation (3)).
Policy I selects the “bottleneck” of the two paths while Policy II balances the two link strengths and it’s a smoother version of the first one.
}
The relay
i that maximizes function h
i is the one with the “best” end-to-end path between the source and destination.Figure 2-5 Control frame and Data frame exchange in opportunistic relaying
As Fig. 2-5 indicates, the relay nodes overhear a single transmission of a Ready-to-Send (RTS) packet and a Clear-to-Send (CTS) packet from the destination. The transmission of RTS from the source allows for the estimation of the instantaneous wireless channel asi
between the source and the relay i. Similarly, the transmission of CTS from the destination, allows for the estimation of the instantaneous channel aid between relay i and the destination.
As soon as each relay receives the CTS packet, it starts a timer from a parameter hi based on the instantaneous channel measurements asi, aid. Each relay i will start its own timer with an initial value Ti, inversely proportional to the channel quality hi, according to the following equation:
There’re other relay selection schemes based on location information with respect to source and destination. The idea was suggested by [12]. But such schemes require knowledge or estimation of distances between all relays and destination.
In CMAC, it proposed a method that all the source and partners randomly backoff to contend for relaying the data. The relay selection scheme is out of its concern. Although the relay is randomly decided, the concept of all source and relays contending for relaying is inspiring and relay only when the first transmission fails to reduce the retransmission times is reasonable. The scheme that relay after failed in the first transmission is adopted in our cooperation MAC scheme.
Similar with CMAC, the “opportunistic relaying” which is introduced also lets the relay