Low Rate Stations Research Articles

Achieving VoIP Guarantee in Wireless Local Area Networks

Recently, voice over Internet Protocol (VoIP) has gained much attention in wireless local area networks (WLANs). Although IEEE 802.11e enhanced distributed channel access (EDCA) has been standardized to provide voice traffic with quality of service (QoS) support, the voice quality could still be severely degraded by the traffic transmitted from low-priority stations (LP-STAs). Like 802.11b, the 802.11e EDCA also has the performance anomaly problem. Therefore, the voice quality of the high-rate stations (HR-STAs) could be severely degraded by the traffic transmitted from low-rate stations (LR-STAs). In fact, the LP-STAs and LR-STAs can be present simultaneously in practical WLAN environments. In this paper, we consider the effects of LP-STAs and/or LR-STAs on voice traffic and propose an extended EDCA scheme called voice differentiation-EDCA (VD-EDCA) to protect voice traffic from LP-STAs and/or LR-STAs. With VD-EDCA, the voice AC in a HR-STA has an absolute priority over LP-STAs and LR-STAs when they are notified that the wireless delay of a voice frame at the QoS access point (QAP) is larger than or equal to the predefined maximal delay threshold. We evaluate the performance of the proposed VD-EDCA scheme through extensive simulations. The simulation results show that, with appropriate maximal delay threshold settings, the proposed scheme well protects all of the VoIP calls transmitted by HR-STAs from LP-STAs and/or LR-STAs

Relay-volunteered multi-rate cooperative MAC protocol for IEEE 802.11 WLANs

In IEEE 802.11, the rate of a station (STA) is dynamically determined by link adaptation. Low-rate STAs tend to hog more channel time than high-rate STAs due to fair characteristics of carrier sense multiple access/collision avoidance, leading to overall throughput degradation. It can be improved by limiting the transmission opportunities of low-rate STAs by backoff parameters. This, however, may cause unfair transmission opportunities to low-rate STAs. In an attempt to increase overall throughput by volunteer high-rate relay STAs while maintaining fairness, we propose a new cooperative medium access control (MAC) protocol, relay-volunteered multi-rate cooperative MAC (RM-CMAC) based on ready to send/clear to send in multi-rate IEEE 802.11. In the RM-CMAC protocol, we show that the effect of hogging channel time by low-rate STAs can be remedied by controlling the initial backoff window size of low-rate STAs and the reduced transmission opportunity of low-rate STAs can be compensated by the help of volunteer high-rate relay STAs. We analyze the performance of RM-CMAC, i.e., throughput and MAC delay, by a multi-rate embedded Markov chain model. We demonstrate that our analysis is accurate and the RM-CMAC protocol enhances the network throughput and MAC delay while maintaining the fairness of low-rate STAs.

Cooperative MAC protocol using active relays for multi-rate WLANs

Cooperative communications using relays in wireless networks have similar effects of multiple-input and multiple-output without the need of multiple antennas at each node. To implement cooperation into a system, efficient protocols are desired. In IEEE 802.11 families such as a/b/g, mobile stations can automatically adjust transmission rates according to channel conditions. However throughput performance degradation is observed by low-rate stations in multi-rate circumstances resulting in so-called performance anomaly. In this paper, we propose active relay-based cooperative medium access control (AR-CMAC) protocol, in which active relays desiring to transmit their own data for cooperation participate in relaying, and it is designed to increase throughput as a solution to performance anomaly. We have analyzed the performance of the simplified AR-CMAC using an embedded Markov chain model to demonstrate the gain of AR-CMAC and to verify it with our simulations. Simulations in an infrastructure network with an IEEE 802.11b/g access point show noticeable improvement than the legacy schemes.

PR-MAC: A practical approach for exploiting relay transmissions in multi-rate WLANs

In this paper, we propose a new MAC protocol, called proxy relay-enabled MAC (PR-MAC), to improve the capacity of multi-rate WLANs. PR-MAC introduces a new entity called proxy relay point (PRP), which serves as a relay between the AP and stations. The cooperation of the AP with PRP replaces direct transmissions for low-rate stations with fast two-hop transmissions while the stations think that they communicate directly with the AP. Our experiment and simulation results showed that PR-MAC can significantly improve the throughput of legacy stations without any modification on the MAC operation of the stations.

CRBAR: Cooperative relay-based auto rate MAC for multirate wireless networks

Cooperative communication is becoming a promising technology for wireless networks by exploiting two key characteristics of the wireless medium: broadcast nature and spatial diversity. To integrate cooperation into practical systems, efficient protocols are needed across the entire protocol stack. This paper presents a novel cooperative relay-based auto rate medium access control (MAC) protocol (CRBAR) for multirate wireless networks, in which low-rate stations can be adaptively assisted by high-rate stations. Different from state-of-the-art table-based proactive relay selection schemes, a reactive relay selection scheme is developed to adapt to dynamical channel variation and network topology in which the relay candidates adaptively select themselves as the relay nodes and determine the relay scheme and transmission rates based on the instantaneous channel measurements. In terms of relay schemes, cooperative relaying can be employed whenever possible in addition to simple relaying. The receiver can improve its capability to decode the original frame by combining the information from both the sender and the relay. We have developed analytical models of different schemes for a simple scenario and carried out further simulations under a realistic fading channel and mobility model. Both analysis and simulation show that CRBAR significantly outperforms the table-based proactive relay selection scheme and direct rate adaptation scheme.

Multirate Throughput Optimization With Fairness Constraints in Wireless Local Area Networks

In 802.11-based wireless local area networks (WLANs), it is difficult to simultaneously attain both high throughput and fairness for multirate traffic. There is a performance anomaly when there are stations whose data rates are much lower than the other stations, in which the aggregate throughput of the high-rate stations drastically degrades. The problem of maximizing the total throughput while maintaining time fairness among the competing stations was studied previously by the same authors. However, our previous solution sacrificed the throughput of low-rate stations. In this paper, we extend our previous work by solving the same optimization problem while maintaining both time fairness and throughput fairness. The optimization problem is formulated as a mixed-integer nonlinear programming problem. The two fairness constraints are maintained by means of changing the channel access probability and transmission time among the competing stations, which can be realized by adjusting their minimum contention window sizes and medium access control (MAC) frame sizes, respectively. A penalty function accompanied with a gradient-based approach is used to solve the problem, and its effectiveness is verified by computational experiments. The proposed solution is also compared with our previous solution in terms of convergence speed and total throughput.