Fast rate adaptation has been established as an effective way to improve the PHY-layer raw date rate of wireless networks. However, within the current IEEE 802.11 legacy, MAC-layer throughput is dominated by users with the lowest data rates, resulting in underutilization of bandwidth. In this paper, we propose and analyze a novel distributed MAC strategy, referred to as Rate-aware DCF (R-DCF), to leverage the potential of rate adaptation in IEEE 802.11 WLANs. The key feature of R-DCF is that by introducing different mini slots according to the instantaneous channel conditions, only contending stations with the highest data rate can access the channel. In this way, the R-DCF protocol not only exploits multi-user diversity in a fully distributed manner but also reduces the loss of throughput due to collisions. Through analysis, we develop an analytical model to derive the throughput of R-DCF in general multi-rate WLANs. Using the analytical model we investigate the performance of R-DCF protocol in various network settings with different rate adaptation strategies and channel variations. Based on the analysis, we further derive the maximal throughput achievable by R-DCF. For practical implementation, an offline adaptive backoff method is developed to achieve a close-to-optimal performance at low runtime complexity. The superiority of R-DCF is proven via extensive analyses and simulations.
Deep Dive into Distributed MAC Strategy for Exploiting Multi-user Diversity in Multi-rate IEEE 802.11 Wireless LANs.
Fast rate adaptation has been established as an effective way to improve the PHY-layer raw date rate of wireless networks. However, within the current IEEE 802.11 legacy, MAC-layer throughput is dominated by users with the lowest data rates, resulting in underutilization of bandwidth. In this paper, we propose and analyze a novel distributed MAC strategy, referred to as Rate-aware DCF (R-DCF), to leverage the potential of rate adaptation in IEEE 802.11 WLANs. The key feature of R-DCF is that by introducing different mini slots according to the instantaneous channel conditions, only contending stations with the highest data rate can access the channel. In this way, the R-DCF protocol not only exploits multi-user diversity in a fully distributed manner but also reduces the loss of throughput due to collisions. Through analysis, we develop an analytical model to derive the throughput of R-DCF in general multi-rate WLANs. Using the analytical model we investigate the performance of R-DCF p
IDELY adopted at home, offices, and hot spots, IEEE 802.11 WLANs (wireless local area networks) are expected to provide services parallel to its wired counterpart in near future [1]. To address this requirement, fast rate adaptation, which is capable of drastically enhancing the PHY-layer raw data rate, holds significant promise [2]. Various rate adaptation algorithms have been proposed for WLANs in recent years, targeting at selecting the most suitable transmission rates according to stations' time-varying channel conditions [3], [4], [5], [6], [7]. With rate adaptation, it is common in today's WLANs (e.g. IEEE 802.11a/b/g) that multiple data rates coexist in the network. Unfosion rates, leading to unexpected performance degradation of high-rate stations. This phenomenon, referred to as performance anomaly [8], is due to the fact that current MAC protocol implicitly guarantees throughput fairness to all the users regardless of their transmission rates. Consequently, low rate links occupy much more airtime than high rate links, if the packet sizes are the same. Besides, whenever collision happens, the airtime wasted is typically determined by the lowest transmission rate involved in the collision. It is therefore essential to redesign the MAC strategy for multi-rate WLANs to leverage the advantage of fast rate adaptation and achieve an overall high network throughput.
Various schemes have been proposed in recent years to mitigate the performance anomaly and increase the total system throughput of multi-rate WLANs [9], [11], [15], [16], [17]. Bruno et al [9] apply the dynamic backoff method [11] to approach the throughput limit of multirate WLANs. However, due to the existence of the lowrate transmissions, the throughput limit of a multi-rate WLAN is often much lower than its high PHY-layer available transmission rates. Therefore, directly applying dynamic backoff does not effectively improve the system throughput. In [11], the authors present an Opportunistic Auto Rate (OAR) protocol, where a station transmits multiple packets in proportion to its instantaneous data rate. Similar concept is also introduced in IEEE 802.11e [12] through the use of transmission opportunity (TXOP), where equal airtime is allocated to stations with different data rates and multiple frames can be transmitted within the TXOP interval. Accordingly, airtime fairness instead of throughput fairness is achieved. In all the above schemes, however, only the time-domain channel variation (time diversity) is considered in the protocol design. Low-rate stations have the same chance of grabbing the channel as high rate stations.
In multi-rate WLANs, the channel spectrum is more efficiently utilized when high-rate stations transmit. It is therefore natural to opportunistically grant high-rate stations better chances of accessing the channel by differentiating users at the MAC layer. The significant spectrumefficiency enhancement therefore achieved, often referred to as multiuser diversity [13], [14], has recently attracted extensive research interest. For example, assuming that stations use different but fixed rates, the authors in [15] propose a remedy scheme to differentiate stations’ long-W xxxx-xxxx/0x/$xx.00 © 200x IEEE term channel access opportunities by using different backoff parameters according to their data rates. However, due to the rapid fluctuating nature of the wireless channel, it is very common for a mobile station to frequently adapt its transmission rate during one session. Consequently, the long-term backoff-based remedy scheme becomes ineffective when fast rate adaptation is taken into account. In [16] the authors develop an Opportunistic Scheduling and Auto-Rate (OSAR) protocol, where the transmitter probes the channels of multiple intended receivers through RTS/CTS exchange and then choose the best one. Nevertheless, in many applications data frames in a station’s buffer are generally targeted to a fixed destination due to service burst, which implies that there is usually only one potential candidate receiver. Hence, OSAR can not effectively gain multi-user diversity in general situations. In [17], the authors propose a Useraware Rate Adaptive Control (UARAC) scheme, where the stations reply CTS packets with a probability that is a function of the channel conditions estimated from RTS packets. The computational complexity, however, is prohibitively high. In addition, in all the above schemes, though low-rate stations no longer occupy excessive air time, the collision cost is still dominated by the lowest data rate, resulting in an unnecessary waste of bandwidth.
In this paper we propose and analyze a novel MAC strategy, referred to as R-DCF (Rate-aware Distributed Coordination Function), to exploit multi-user diversity as well as time diversity in a fully distributed manner in IEEE 802.11 DCF-based WLANs. By dynamically adjusting a station’s channel access priority through the use of different mini slots, the R-DCF
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