# A Joint Strategy for Fair and Efficient Energy Usage in WLANs in the Presence of Capture Effect
## 1 Introduction  
With the increase in the popularity of smart phones, laptops, PDAs (Personal Digital Assistant), and other hand-held devices, wireless local area networks (WLANs) are rapidly growing and are widely deployed. These handheld devices are battery operated and need to conserve energy. It is stated that the Internet today accounts for about 2% of the total energy consumption, with further shifting of offline services online, this percentage may grow in coming years [1]. A major fraction of the energy consumption in these handheld devices comes from the WLAN interface that accounts for more than half of the total device power budget under typical use [2] and can quickly drain the phone's battery when transmitting at high peak rates. This enormous energy consumption by the wireless devices not only puts a burden on the user but also affects the environments in the form of global warming. In order to conserve energy in WLANs, IEEE 802.11 devises a power save mode (PSM) [3]. In PSM, a node periodically wakes up to listen to a beacon frame transmitted by access point (AP) that announces the buffered frames. If there is any frame waiting for the node, then it sends a PS-POLL (Power Save Polling) to retrieve the buffered frame. Otherwise it goes back to sleep mode. In contrast to PSM, IEEE 802.11 also operates in continuous active mode or constantly awake mode (CAM). Nodes operating in CAM use a carrier sense multiple access with collision avoidance(CSMA/CA)-based distributed coordinated function (DCF) to send frames without going into sleep mode. Similarly, nodes remain awake all the times and can receive frames from the AP anytime. All clients initially choose randomly a backoff counter from the minimum contention window (CW) and starts counting down after sensing the channel idle for distributed inter-frame space (DIFS) time. A client makes a transmission attempt when its backoff counter reaches zero and all other clients hearing the transmission freeze their counters and set their network allocation vector (NAV) for the time of transmission. The transmission is successful when no other client's backoff counter reaches zero simultaneously. In this case the sender receives ACK frame after inter-frame space (IFS) duration from the destination otherwise collision happens. In case of collision, all clients involved in collision wait for the extended inter-frame space (EIFS) duration, increase their CW according to the binary exponential backoff algorithm and the clients again start contending for the channel access by decreasing the backoff counter. The same process is repeated unless the transmission is successful or the number of retries reaches predefined retry limit in which case the frame is discarded and the upper layers are notified of the transmission failure. Obviously, PSM which is specifically devised for power saving are more energy efficient than CAM as it goes to the sleep mode in case it does not have data to send or receive. However, the PSM is not suitable for delay sensitive applications, as the client that has a frame buffered at the AP has to wait for a beacon interval to retrieve it [4]. In addition, to reduce delay, some devices change its operation mode from PSM to CAM as the traffic load exceeds a certain level [5]. In this paper, we analyze the energy consumption in CAM in the presence of some real network conditions such as capture effect, unsaturated network, and erroneous channel conditions. In particular, we put emphasis on the effect of PHY capture on the energy efficiency of network. PHY capture is the capability of wireless devices of successfully receiving one frame when two or more than two frames simultaneously arrive [6] at the network interface of receiver. For example, there are two nodes, \(S_{1}\) and \(S_{2}\) in a network and are \(d_{1}\) and \(d_{2}\) distance apart from AP as shown in Figure 1. Both \(S_{1}\) and \(S_{2}\) transmit to AP.