# A Fair Cooperative MAC Protocol in IEEE 802.11 WLAN
## 2 Related Works  
The MAC protocols of the IEEE 802.11 standard have exploited cooperative techniques to provide improvements in spectrum efficiency and reliability efficiency. These techniques present two categories of cooperative MAC protocols in IEEE 802.11 networks. In the first category, the original cooperative MAC protocols in the IEEE 802.11 standard improve the network throughput by exploiting the optimum relay node providing the lowest delay, whereas in the second category, the cooperation mechanism improves energy efficiency. In this section, we present an overview of cooperative MAC protocols fitting these two categories. CoopMAC [6] and rDCF [7] are the earliest protocols presented in the first category. In the CoopMAC protocol, each source node collects information on the transmission time by overhearing the control and data packet exchanged between its neighbors. This information is stored in a database called CoopTable. When the source node has a packet to send to an intended destination, the optimal relay node is selected from the CoopTable. Similarly, the rDCF protocol employs the minimum transmission time as the metric. However, the main difference between the CoopMAC and rDCF protocols is that CoopMAC obtains the decision information completely from overhearing whereas rDCF uses overhearing to obtain the primary information and then broadcasts this information to neighbors. The next protocol of this category is ADC-MAC [8], which employs the minimum transmission rate as a metric. In this protocol, a pure two-hop transmission is used when there is no available direct transmission and information on the lowest transmission time is broadcasted after the primary overhearing. Some protocols, such as RAMA [9] and EMR [10], exploit the metric of lowest transmission time with some modifications. In RAMA, the highest data rate is the metric for relay selection and the final relay is chosen through a random contention mechanism. The EMR selects the highest effective throughput as a metric and the winning relay is selected through a priority-based contention between relay candidates. The other protocols fitting this category can improve the performance of an IEEE 802.11 MAC protocol with some special techniques and features, such as frame aggregation in CoopMAC [11] and RRS-CMAC [12], concurrent transmissions in CCMAC [13], network coding in CODE [14] and EAP-CMAC [15], a cross-layer approach in MACR-CCT [16], interference reduction in CLS-MAC [17] and contention-based relay selection in RCD-MAC [18] and CRP-CMAC [19]. In the second category of cooperative MAC protocols, the cooperation condition and cooperation metric are dependent on power parameters, such as SNR and SINR. As an example, in opportunistic relaying (OR) [20], each relay node estimates the \"instantaneous channel conditions\" from overhearing control packets. Then, the relay which is able to provide the highest SNR will transmit a flag to the corresponding source and destination nodes. Another example of this category is a cooperative proposal referred to in the UTD-MAC protocol [21], in which power channel nformation among source, destination, and potential relays is evaluated by each source node. The relay node providing the maximum power ratio is the winning node. The operation of the UTD-MAC protocol is improved by other protocols with special features, such as coding schemes in CMAC [22] and CD-MAC [23], Automatic Repeat reQuest (ARQ)based cooperative MAC schemes in NCSW [24] and PRCSMA [25], and network coding in the NCCARQ-MAC protocol [26]. A summary of cooperative MAC protocols, their categories, and their particular techniques which provide performance improvement are presented in Table 1. The interested reader can refer to [1; 2; 3; 4] for more information about the existing cooperative MAC protocols. Nevertheless, the existing protocols select the relay based on the maximum available rates only and this leads to reuse of the same relay again and again.