With the rapid proliferation of broadband wireless services, it is of paramount importance to understand how fast data can be sent through a wireless local area network (WLAN). Thanks to a large body of research following the seminal work of Bianchi, WLAN throughput under saturated traffic condition has been well understood. By contrast, prior investigations on throughput performance under unsaturated traffic condition was largely based on phenomenological observations, which lead to a common misconception that WLAN can support a traffic load as high as saturation throughput, if not higher, under nonsaturation condition. In this paper, we show through rigorous analysis that this misconception may result in unacceptable quality of service: mean packet delay and delay jitter may approach infinity even when the traffic load is far below the saturation throughput. Hence, saturation throughput is not a sound measure of WLAN capacity under nonsaturation condition. To bridge the gap, we define safe-bounded-mean-delay (SBMD) throughput and safe-bounded-delay-jitter (SBDJ) throughput that reflect the actual network capacity users can enjoy when they require finite mean delay and delay jitter, respectively. Our earlier work proved that in a WLAN with multi-packet reception (MPR) capability, saturation throughput scales superlinearly with the MPR capability of the network. This paper extends the investigation to the nonsaturation case and shows that superlinear scaling also holds for SBMD and SBDJ throughputs. Our results here complete the demonstration of MPR as a powerful capacity-enhancement technique for WLAN under both saturation and nonsaturation conditions.

Fast rate adaptation has long been recognized as an effective way to improve the PHY-layer data rate of wireless networks. However, in random access wireless networks such as IEEE 802.11 wireless LANs, MAC-layer throughput is dominated by stations with the lowest transmission rates, resulting in an underutilization of spectrum bandwidth. In this paper, we propose a novel two-level medium access framework, referred to as Two-Level MAC, to solve the aforementioned problem and to significantly improve system spectrum efficiency through the exploitation of multiuser diversity. The key idea of Two-Level MAC is to introduce a second level of deterministic channel access on top of the traditional IEEE 802.11 DCF protocol. By doing so, higher priority is granted to high-rate stations in a fully distributed manner. Meanwhile, collisions among potential contending stations are drastically reduced. Through analysis, we show how such Two-Level MAC can be optimized to achieve the maximum system throughput. The superiority of the proposed protocol is verified through analyses and extensive simulations.

This paper provides a queueing analysis for the slotted Aloha network. We assume the use of an exponential backoff protocol. Most prior work on slotted Aloha focuses on the analysis of its saturation throughput. Good saturation throughput, however, does not automatically translate to good delay performance for the end users. For example, it is well-known that the maximum possible throughput of slotted Aloha with a large number of nodes is e-1=0.3679 . Prior work showed that binary backoff factor of r = 2 can achieve a saturation throughput of 0.3466, which is very close to the e-1. However, this paper shows that if mean queuing delay is to be bounded, then the offered load must be below 0.2158, a drastic 41% drop from e-1. Fortunately, setting r= 1.3757 allows us to achieve bounded-mean-delay throughput of 0.3545, less than 4% lower than e-1. A general conclusion is that the backoff factor r may significantly affect the queuing delay performance. Our analysis provides a framework to set system parameters properly.

To date, most analysis of WLANs has been focused on their operation under saturation condition. This work is an attempt to understand the fundamental performance of WLANs under unsaturated condition. In particular, we are interested in the delay performance when collisions of packets are resolved by an exponential backoff mechanism. Using a multiple-vacation queueing model, we derive an explicit expression for packet delay distribution. It is found that under some circumstances, mean delay and delay jitter may approach infinity even when the traffic load is way below the saturation throughput. Saturation throughput is therefore not a sound measure of WLAN capacity when the underlying applications are delay sensitive. To bridge the gap, we define safe-bounded-mean-delay (SBMD) throughput and safe-bounded-delay-jitter (SBDJ) throughput that reflect the actual network capacity users can enjoy when they require bounded mean delay and delay jitter, respectively. The analytical model in this paper is general enough to cover both single-packet reception (SPR) and multi-packet reception (MPR) WLANs, as well as carrier-sensing and non-carrier- sensing networks. We show that the SBMD and SBDJ throughputs scale super-linearly with the MPR capability of a network. Together with our earlier work that proves super-linear throughput scaling under saturation condition, our results here complete the demonstration of MPR as a powerful capacity-enhancement technique for both delay-sensitive and delay-tolerant applications.

Today's IEEE 802.11 Wireless LANs (WLANs) usually support multiple transmission rates, where mobile stations adapt their data rates based on their link adaptation strategies. In this paper we analyze the throughput performance of multi-rate WLANs with link adaptation. Specifically, an analytical model is developed to derive the system throughput of both the basic access and RTS/CTS mechanisms. The proposed model is applicable to general multi-rate WLANs and independent of specific link adaptation strategies adopted. The accuracy of the model is validated by simulations in various network settings. Using the proposed model, we further derive the theoretical throughput limit of common multi-rate IEEE 802.11 WLANs. Interestingly, it is observed that for typical WLAN configurations, standard RTS/CTS mechanism performs closely to the throughput limit. Therefore, in multi-rate WLANs simply using standard RTS/CTS mechanism is preferred for its low complexity and close-to-optimal performance.

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 spectrum bandwidth. In this paper, we propose 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 actually access the channel. In this way, the R-DCF protocol not only effectively exploits multi-user diversity in a fully distributed manner but also drastically reduces the loss of throughput due to collisions. Through analysis, we derive a closed-form network throughput expression for R-DCF. Based on the analysis, we further derive the maximal throughput that can be achieved by R-DCF. For practical implementation, an offline adaptive backoff method is developed for R-DCF to achieve a close-to-optimal performance at low runtime complexity. The superiority of R-DCF is proven by extensive analyses and simulations.