The PSM mechanism is enhanced in WLAN energy saving by distinguishing the delay-sensitive data traffic, delay tolerance data traffic [11
], and priority-based data traffic [12
]. Many researchers have investigated the active/SLP mode scheduling that ensures the lower delay requirements [13
]. Several techniques for increasing the power performance of WLANs have been suggested. Vukadinovic et al. [17
] proposed a traffic alert approach for updating the standard PSM approach of ad hoc WLANs: when a data frame is transmitted over several hops, only the next hop STA is informed of the pending frame; STAs on other hops remain in doze mode and thus cause a long end-to-end (E2E) delay. Their proposed scheme requires each STA along the routing route to forward a traffic alert to its downstream neighbor. It results in the transmission of the data frames in a single beacon cycle over several hops, and the E2E delay in the multi-hop transfer is significantly minimized. Radwan et al. [18
] proposed a solution where STAs are required to work together to take advantage of the strong channel capacity of short-range (SR) connections to minimize transmission time and preserve energy. The neighboring STAs form a cluster, and a head of the cluster is chosen to relay data traffic. Instead of transmitting data directly through the long-range (LR) communication protocol to AP, the STAs send their data to the cluster head using SR networking. The cluster head will then, on behalf of other STAs, relay the traffic to the AP using the LR communication. Tang et al. [19
] provided a power-saving protocol for reduced power consumption of APs. This strategy helps AP to reach SLP state while there is no transmission traffic for a while. Equipped wake-up transceivers will relay wake-up signals to AP. This strategy reduces APs’ power consumption by reducing the amount of time spent in IDL state. However, their suggested strategy includes installing new radios with STAs, and additional methods are often required to handle the operation of the radios. Lin et al. [20
] points out that WLAN STAs also waste their power on IDL state in communication mode, as the STAs can constantly feel the channel and overhear the continuing transmissions of the other STAs. Additionally, the collisions between the STAs which wake up for data recovery at the same time may cause power loss. The authors proposed a DeepSleep scheme for energy-harvesting systems to improve the WLAN PSM, where STAs short of power will reach long-term IDL state and only access the channel with a higher priority. In [21
], He et al. present a TDMA-based MAC protocol to decrease contention among WLAN STAs. An AP divides a BI into many equal-time slices and allocates the slices to single or groups of STAs. Therefore, each STA wakes up in its allocated time slot for data retrieval instead of contending for channel access. By eliminating channel contention, this approach effectively decreases the energy consumption of PSM devices. However, if a PSM system does not wake up in its time slot, it will waste the allocated channel. Moreover, because all time slots have the same length without considering frame length or traffic load, in the case of short frames or light traffic, the allocated time slots can be used ineffectively. Eun-Sun et al. [22
] proposed an improved PSM (IPSM) to change the size of the ATIM window accordingly. During the predefined ATIM window, when a certain number of IDL channels are sensed, STAs may terminate the ATIM window and start transmitting data frames. Otherwise, if the current ATIM window length is too short, STAs will dynamically expand the window size for a given scale. While this protocol can efficiently enhance the WLAN PSM performance, a hidden terminal problem is missing. Lei et al. [23
] suggest a reservation scheme for back-off counters (BC), which is paired with a neighboring polling solution. The authors propose a BC reservation system for STAs to minimize the risk of selecting the same BC at random. Based on the proposal, devices that have successfully transmitted a control frame, ATIM frame, will reserve a BC by that frame, and use the reserved BC to continue with the following data transmissions. Furthermore, the authors present a neighboring polling scheme to minimize the hidden STA problems. As the wireless transmission has a broadcast nature, the STAs located at the transmission range of the transmitter will overhear an ongoing transmission colliding with a transmission from a hidden STA. It helps one of the neighbors to poll the transmitter again, using continuing transmission from the neighbor. However, a BC reservation scheme has similar results to a fixed channel access mechanism, such as TDMA.
Hence, from the above-related research discussion, we see that the PSM-based MAC protocols can provide higher performance with low energy consumption. However, this performance enhancement is greatly affected by the increase of the number of STAs in the WLAN. It increases the contention among the STAs, which increases the channel sensing time of the STAs.