In order to solve Problem1, an adaptive reservation-assisted collision resolution (ARCR) protocol modified from conventional DCF is proposed in Chapter2to both mitigate packet collision and reduce the backoff delays from the random access scheme for the WLANs.
With its adaptable reservation period, the contention-based channel access can be adap-tively transformed into a reservation-based system if there are pending packets required to be transmitted between the WSs and the AP. Furthermore, in order to support QoS requirements for multiple prioritized traffic, the enhanced-ARCR (E-ARCR) protocol is further proposed in Chapter3 as the enhanced version of the ARCR scheme. Analytical models are derived for both proposed schemes to evaluate their throughput performance.
It can be observed from both analytical and simulation results that the proposed ARCR and E-ARCR protocols outperform existing schemes with enhanced channel utilization and network throughput.
Furthermore, for the IEEE 802.11n standard, frame aggregation is considered one of the major factors to improve the system performance of WLANs from the MAC perspective.
In order to achieve both reliable data transmission and high system throughput (Problem 2), two MAC-defined ARQ protocols are proposed in Chapter4to consider the effect from frame aggregation for the enhancement of network performance. An aggregated selective repeat ARQ (ASR-ARQ) scheme is proposed which incorporates the selective repeat ARQ scheme with the consideration of frame aggregation. On the other hand, for worse chan-nel quality, the aggregated hybrid ARQ (AH-ARQ) mechanism is proposed to further enhance the throughput performance by adopting the Reed-Solomon (RS) block code as forward error correction scheme. Novel analytical models for both the ASR-ARQ and AH-ARQ protocols are established to evaluate the cumulative distribution function (CDF) of packet service time. Simulations are conducted to validate and compare the proposed ARQ mechanisms based on the service time and system throughput. Numerical evalua-tions show that the proposed AH-ARQ protocol can outperform the other schemes under worse channel condition; while the ASR-ARQ scheme is superior to the other mechanisms under better channel condition.
On the other hand, how to choose a feasible femtocell access strategy is considered an important issue for LTE-A HetNet system. It is concluded in [13] that open access mode can alleviate the interference and improve throughput for the entire HetNet compared to closed access mode. Moreover, from the economic viewpoint, open access is suggested
by telecommunication operators if the operators can charge from the users that connect to the femtocell. However, according to [12], subscribers or customers prefer femtocells with closed access than with open access since it is undesirable that their femtocells are accessed by the other users. In order to acquire better tradeoff between closed and open access modes, it is necessary to propose a new design, hybrid access mode, to allow some nonsubscribers to possess limited connections to the fBS. In order to evaluate the perfor-mance of closed, open, and hybrid access modes for Problem3, subscribers’ throughput, system throughput and operator’s revenue will be compared in Chapter5. The cell selec-tion schemes for nonsubscribers to access the fBS are formulated based on theoretical gam-ing models to describe selfish behavior of individual UE. The existences of pure strategy Nash Equilibria (NEs) will be illustrated and proven for the defined cell selection games.
By the observation at NEs, it is suggested to adopt hybrid access mode in order to provide higher flexibility for the performance enhancement of all subscribers, entire HetNet system and operator. Finally, conclusions are drawn in the Chapter6for this dissertation.
Reservation-Assisted Protocol for IEEE 802.11 WLANs
- Adaptive reservation-assisted collision resolution (ARCR) protocol is proposed in this chapter to solve Problem1for IEEE 802.11 wireless local area networks. By adoption of adaptive reserva-tion periods within the contenreserva-tion-based channel access networks, proposed ARCR algorithm can significantly improve packet collision and reduce the backoff delay compared to the other existing works.1
2.1 Introduction
In recent years, the techniques for wireless local area networks (WLANs) have been prevailing exploited for both indoor and mobile communications. The applications for WLANs include wireless home gateways, hotspots for commercial usages, and ad-hoc networking for inter-vehicular communications. Among different techniques, IEEE 802.11 standard is considered the well-adopted suite due to its remarkable success in both design and deployment. Various amendments are contained in the IEEE 802.11 standard suite, mainly including IEEE 802.11a/b/g [1–3] and IEEE 802.11e [4] for quality-of-service (QoS)
1The chapter is based on
- Jia-Shi Lin and Kai-Ten Feng, “QoS-based Adaptive Contention/Reservation Medium Access Control Protocols for Wireless Local Area Networks,” IEEE Trans. Mobile Computing, vol. 10, no. 12, pp. 1785-1803, Dec. 2011.
- Jia-Shi Lin and Kai-Ten Feng, “Design and Performance Analysis on Adaptive Reservation-Assisted Collision Resolution Protocol for WLANs,” ACM/Springer Journal on Wireless Networks (WINET), vol.
17, no. 4, pp. 973-986, May 2011.
support. The medium access control (MAC) protocol within the IEEE 802.11 standard supports the distributed coordination function (DCF) to regulate the random and complex medium accessing behaviors among the wireless stations (WSs) within the same WLAN.
How to alleviate the probability of packet collision has been considered a crucial issue for the enhancement of network throughput.
Different types of schemes have been proposed in order to resolve the packet colli-sion problem within the WLAN. The adjustment of contention window (CW) size has been considered an effective scheme in most of the existing research work [1,14–21]. The binary exponential backoff scheme [1] as described in the IEEE 802.11 MAC protocol con-trols the waiting time duration for channel contention. The CW size will be increased or decremented with failed or successful transmission respectively. In general, the probabil-ity of packet collision can be decreased with augmented value of CW size, especially with a larger number of WSs in the network. However, excessive idle time resulted from the enlarged CW size will consequently degrade the channel utilization. In order to enhance the throughput performance for the conventional IEEE 802.11 protocol, the algorithm pro-posed in [14] increases the transition rate between the backoff stages associated with de-creased value of minimum CW and incremented value of maximum CW size. The hybrid algorithm proposed in [15] combines both the exponential and the linear backoff for the purpose of decreasing packet collision; while the slow CW decrease (SD) scheme in [16]
either doubles or halves the CW size according to the successfulness of packet transmis-sion. The early backoff announcement (EBA) protocol [17] proposed a WS to record its next backoff number into the MAC head while transmitting data packets. All the other WSs will select their corresponding backoff numbers excluding this value in order to avoid po-tential packet collisions. Similar design concept was also presented in [18]; while [19] pro-posed a MAC protocol with multiple-step distributed in-band channel reservation. The gentle DCF (GDCF) protocol as proposed in [20, 21] maintains a larger value of the CW size compared to the conventional backoff scheme in order to decrease the probability of packet collision. The work in [22] proposed a handshake-based channel aware (HCA) MAC protocol which selects the WSs to access the channel according to its corresponding channel condition. Nevertheless, all the existing contention-based protocols suffer from the tradeoff between packet collision and transmission delay. Moreover, the throughput performance by adopting these algorithms is greatly influenced by the total number of WSs within the WLAN.
Compared to the DCF-based random access schemes, there are also polling-based
al-gorithms proposed for WLAN in order to provide feasible performance to fulfill time-constrained requirements. Various centralized polling protocols and scheduling algo-rithms (e.g., [23]) have been proposed to increase the channel utilization for the IEEE 802.11 point coordination function (PCF) [1]. The operation time period for each WS is divided into cycles of contention period (CP) and contention-free period (CFP), where CFP is utilized by either the PCF for real-time packet delivery. A reservation-based MAC protocol was employed in [24] to provide support for real-time traffic. The work in [25,26]
proposed piggyback schemes by adjusting the transmission rate of WS for throughput en-hancement. However, the requirement to specifically assign the designated CFP for the implementation of polling-based algorithms will lead to excessive overhead if the WSs have no packet to be delivered to the access point (AP). Moreover, these AP-initiated polling protocols can not completely fulfill the throughput requirement while the WSs are intending to conduct uplink data transmissions. Moreover, the channel reservation MAC protocol was presented in [27] to provide automatic scheduling of channel usage between the WSs. However, this scheme requires both a busy tone channel and multple antennas in order to resolve the hidden terminal problem.
It is noted that there are tradeoffs between the centralized-based and contention-based schemes under different network environments. It will be beneficial to provide a channel access mechanism that can adaptively switch between these two types of schemes. There-fore, an adaptive reservation-assisted collision resolution (ARCR) protocol is proposed in this chapter in order to alleviate packet collisions within the random access scheme. The main feature of proposed ARCR scheme is that the original contention-based channel ac-cess will be adaptively transformed into a reservation-based system in the case that there are pending requests for packet transmission from the WSs. With the adaptable reser-vation period by exploiting the ARCR algorithm, packet collision resulting from chan-nel contention can be effectively reduced which consequently leads to enhanced network throughput. Analytical model for throughput analysis is developed in this chapter to provide feasible observations on the behaviors of proposed ARCR protocol. Numerical results are conducted via simulations both to provide validation on the analytical models and to evaluate the effectiveness of proposed scheme. It can be observed that network throughput can be enhanced by adopting the ARCR algorithm compared with other ex-isting protocols.
The remainder of this chapter is organized as follows. Section2.2briefly summarizes the IEEE 802.11, the GDCF, and the EBA protocols. The proposed ARCR scheme is
de-scribed in Section 2.3 associated with its throughput analysis presented in Section 2.4.
Section2.5illustrates the performance validation and evaluation for the proposed ARCR protocol; while conclusions are drawn in Section2.6.