Robust and Energy Efficient MAC/PHY Layer Strategies of Wi-Fi

Abstract

Over the last quarter century, wireless local area network (WLAN) technology has become an essential and indispensable part of our daily lives. Recently, a tremendously growing number of portable devices, such as smartphones, tablets and laptops, are being equipped with Wi-Fi, the hallmark of the IEEE 802.11 WLAN, in order to meet ever-increasing traffic demands at extremely low cost. Encouraged by this remarkable success, Wi-Fi is facing two trends. First, the state-of-the-art IEEE 802.11 specifications, e.g., IEEE 802.11n and 802.11ac, have focused on improving physical layer (PHY) rate by enabling multiple antennas, called multiple-input multiple-output (MIMO), and bandwidth widening, known as chan- nel bonding [1, 2]. Second, to achieve high throughput and transmission efficiency at medium access control (MAC) layer, IEEE 802.11n/ac standards have defined two types of frame aggregation techniques: MAC service data unit (MSDU) aggregation and MAC protocol data unit (MPDU) aggregation which amortize PHY/MAC proto- col overhead e.g., binary random backoff, physical layer convergence protocol (PLCP) preamble, and acknowledgement (ACK), over multiple frames by packing several MS- DUs and MPDUs into a single aggregate MSDU (A-MSDU) and aggregate MPDU (A-MPDU), respectively. Apparently, there is no doubt that these state-of-the-art features meet and fulfill i the requirements of Wi-Fi equipped device users by offering several hundred-fold in- creases in PHY rate and ubiquitous access. However, with respect to the demands of the battery-powered portable device users, the belief is easily broken from two per- spectives. First, since using higher PHY rate and longer A-MPDU are more vulnerable to channel errors especially for mobile users, the significant throughput performance degradation can be observed. Second, the emerging Wi-Fi chipsets, based on IEEE 802.11n/ac, consume much more energy than its legacy IEEE 802.11a/b/g counterparts due to the usage of MIMO and channel bonding. Nowadays, as the battery-powered portable device users place increasingly complex demands on the functionality of their devices which have strict power limitation, satisfaction with robust communication and battery life time is becoming increasingly important. In this dissertation, to address these challenges, we propose robust and energy efficient MAC/PHY layer strategies of Wi-Fi. First of all, to confirm those changes, we have conducted extensive experiments using state-of-the-art commercial IEEE 802.11n/ac-equipped devices and Microsofts Software Radio (Sora) platform. Our experiment results have revealed strong evidence that the use of long A-MPDU frames seriously deteriorates the Wi-Fi performance, i.e., throughput, especially for the pedestrian mobile users. Besides, we have found that the use of channel bonding remarkably consumes more energy, thus making Wi- Fi a primary energy consumer in the battery-powered portable devices. Especially, the energy cost is dominated by excessive and unnecessary listening and receiving operations. We begin an intra-frame rate control algorithm (Intra-RCA) design, called SNR- aware Intra-frame Rate Adaption (SIRA), which enhances the system performance of Wi-Fi in fast time-varying environments [3]. Widely used inter-frame rate control algorithms (Inter-RCAs), which select the PHY rate of each frame based on the time- ii averaged frame loss rate and the signal strength statistics, perform poorly for a long A-MPDU due to the channel variation in mobile environments. Unlike the previous ap- proaches, SIRA adapts the PHY rate on intra-frame basis, i.e., the PHY rate is updated in the middle of a frame according to user mobility. The performance of the proposed scheme is also evaluated by a trace-driven link level simulator employing the collected channel traces from real measurements. The simulation results show that SIRA outper- forms a standalone Inter-RCA in all tested traces. Despite its enhanced performance and considerable frame error reduction, the performance degradation caused by the impact of user mobility still remains due to the inherent limitation of IEEE 802.11 PHY design. Therefore, we conclude that this challenge should be solved with the assistance of PHY modification, and propose Channel-Aware Symbol Error Reduction (ChASER), a new practical channel estimation and tracking scheme for Wi-Fi receivers [4]. ChASER utilizes re-encoding and re-modulation of the received data symbol to keep up with the wireless channel dynamics at the granularity of orthogonal frequency division multi- plexing (OFDM) symbols. In addition, its low-complexity and feasibility of standard compliance is demonstrated by Microsofts Sora prototype implementation and experi- mentation. To our knowledge, ChASER is the first IEEE 802.11n-compatible channel tracking algorithm since other approaches addressing the time-varying channel con- ditions over a single (aggregated) frame duration require costly modifications of the IEEE 802.11 standard. Even though the above proposed approaches enhance the Wi- Fis throughput performance and robustness over conventional technique, the rest re- quirement of the portable device user, i.e., energy efficient Wi-Fi system design, should be addressed as ever. Accordingly, we propose a new power save operation as well as the corresponding protocol, called WiFi in Zizz (WiZizz), which judiciously exploits the characteristic iii of the channel bonding defined in IEEE 802.11ac and efficiently handles the channel bandwidth in an on-demand manner to minimize the traumatic energy spent by IEEE 802.11ac devices [5]. Our extensive measurement and simulation show significant per- formance improvement (as high as 73% energy saving) over a wide range of commu- nication scenarios. In addition, the feasibility of easy implementation is demonstrated by a prototype with a commercial 802.11ac device. To the best of our knowledge, WiZizz is the first IEEE 802.11ac-congenial energy efficient bandwidth management while other existing approaches require costly modifications of the IEEE 802.11ac specification. In summary, we propose a number of compelling algorithms and protocols to im- prove the robustness and energy efficiency in accordance with the paradigm shift of Wi-Fi. Moreover, our evaluation results show that the proposed schemes in this disser- tation are effective and yield considerable performance gain based on both the trace- driven link level simulation and the network level simulation which well reflects the wireless channel characteristics of the real world and the operation of IEEE 802.11 WLAN, respectively. We demonstrate the feasibility of our approaches by implement- ing prototypes in off-the-self IEEE 802.11n/ac devices and software-defined radio.