Major contributions of this dissertation are listed as follows:
∙ An adaptively direct communication approach for SSs in an IEEE 802.16 OFDM/TDD-based PMP network (Chapter 2)
∙ A predictive motion and interference-based scheduling algorithm for MSs with direct communication (Chapter 3)
∙ A flexible listening window adjustment approach for IEEE 802.16m sleep mode operation (Chapter 4)
∙ A statistical sleep window control approach for IEEE 802.16m sleep mode operation (Chapter 5)
A brief overview of each chapter are given in subsequent paragraphs.
Chapter 2 provides a flexible and contention-free approach to support direct commu-nication between SSs in an IEEE 802.16 OFDM/TDD-based PMP network. The BS is co-ordinating and arranging specific time periods for the two SSs that are actively involved in packet transmission. According to the channel conditions among the BS and SSs, the packet transmission operation is switched between the direct link and indirect links in the proposed approach, which improves the bandwidth utilization and consequently increases the user throughput. A comprehensive architecture design associated with the extended frame structures for the proposed approach are described. Moreover, an analytical model is also conducted to justify the correctness and effectiveness of the proposed scheme.
In Chapter 3, a scheduling algorithm for each pair of MSs that is expected to conduct direct communication is presented. Both the interference region and feasible region for the pair
of MSs to perform direct communication are studied and calculated. Based on these two types of information, the algorithm properly arranges the MSs to conduct direct communication in either DL subframe or UL subframe, or to communicate with the conventional communication manner, i.e., via the BS. Furthermore, since MSs may move around, a motion prediction mechanism is proposed and is considered within the scheduling algorithm, which effectively reduces the control overheads regarding the updates of MSs positions.
A flexible listening window adjustment approach for an MS in the sleep mode with real-time traffic is proposed in Chapter 4. Instead of window-based operation specified in the IEEE Std. 802.16e-2005, a cycle-based mechanism is considered in the proposed approach.
With the consideration of tolerable delay constraint, the approach dynamically adjusts the ratio of listening window to sleep window for each sleep cycle based on the number of both buffered and retransmitted packets. Therefore, the performance of both energy efficiency and packet loss rate are improved in the proposed scheme. It is worthwhile to mention that the concept of flexible listening window adjustment approach has been proposed by the authors and is adopted in the IEEE 802.16m standard draft.
For an MS in the sleep mode with non-real-time downlink traffic, a statistical sleep window control approach, based on the concepts of IEEE 802.16m sleep mode operation, is provided in Chapter 5. The proposed approach constructs a discrete-time Markov-modulated Poisson process (dMMPP) for representing the states of non-real-time traffic. Furthermore, a partially observable Markov decision process (POMDP) is exploited to conjecture the present traffic state. Based on the estimated traffic state and the considerations of tolerable delay and/or queue size, two suboptimal policies for maximizing energy conservation are proposed within the approach.
Each of the following four chapters are began with an introduction and some preliminaries to describe the subject problem. The proposed approach, analytical model, and performance evaluation are presented in the subsequent sections. Finally, the concluding remarks are given in the end of each chapter.
Chapter 2
Adaptively Direct Communication Approach for Subscriber Stations in Broadband Wireless Networks
2.1 Introduction
The IEEE 802.16 standards for wireless metropolitan area networks (WMANs) are designed to satisfy various demands for high capacity, high data rate, and advanced multimedia services [9]. The medium access control (MAC) layer of IEEE 802.16 networks supports both point-to-multipoint (PMP) and mesh modes for packet transmission [6]. Based on application requirements, it is suggested in the standard that only one of the modes can be exploited by the network components within the considered time period, and the PMP mode is considered the well-adopted one. In the PMP mode, packet transmission is coordinated by the base station (BS), which is responsible for controlling the communication with multiple subscriber stations (SSs) in both downlink (DL) and uplink (UL) directions. All the traffic within an IEEE 802.16 PMP network can be categorized into two types, including inter-cell traffic and intra-cell traffic. For the inter-cell traffic, the source/destination pair of each traffic flow are located in different cells. On the other hand, the intra-cell traffic is defined if they are
situated within the same cell. The inefficiency within the PMP mode occurs while two SSs are intended to conduct packet transmission, i.e., the intra-cell traffic between the SSs. It is required for data packets between the SSs to be forwarded by the BS even though the SSs are adjacent with each others. Due to the packet rerouting process, the communication bandwidth is wasted which consequently increases the packet-rerouting delay.
In order to alleviate the drawbacks resulted from the indirect transmission, a directly communicable mechanism between SSs should be considered in IEEE 802.16 networks. Several direct communication approaches have been proposed for different types of networks. The direct-link setup (DLS) protocol is standardized in the IEEE 802.11z draft standard to support direct communication between two SSs in wireless local networks [10]. However, the DSL protocol is designed as a contention-based mechanism, which does not guarantee the access of direct link setup and data exchanges between two SSs. The dynamic slot assignment (DSA) scheme for Bluetooth networks is proposed in [11, 12], which is primarily implemented based on the characteristics of the Bluetooth standard [13]. Since the frame structures and the medium access mechanisms are different among these wireless communication technologies, both the DLS protocol and DSA scheme cannot be directly applied to IEEE 802.16 networks.
For the IEEE 802.16 PMP networks, the virtual direct link access (VDLA) mechanism is proposed in [14], which partially overlaps the DL and UL subframes within a single frame.
The source SS and destination SS are scheduled in the overlapped time intervals in order to accomplish the direct transmission. However, since the channel conditions among the BS and the two SSs can be different, it will not always attainable to assume that both the source and destination SSs process the common burst profile in the VDLA scheme.
In this work, an adaptive point-to-point communication (APC) approach is proposed to support direct communication for SSs. The APC approach is designed as a flexible and contention-free scheme especially for the time division duplexing-based IEEE 802.16 PMP networks. The complete data structures and procedures for implementing direct communi-cation are proposed. According to the relative locommuni-cations and channel conditions among the BS and SSs, the packet transmission operation is switched between the direct link and
in-direct links in the APC approach, which results in enhanced network throughput. While the direct link approach is selected, the required bandwidth, communication overhead, and packet latency can be greatly reduced. The effectiveness of the APC approach is evaluated and validated via both the numerical analysis and extensive simulation studies. It can be shown that the proposed APC approach outperforms the conventional IEEE 802.16 transmis-sion mechanism in terms of user throughput, communication overhead, and packet latency.
In summary, the contributions of this work are listed as follows: (a) the proposal of a com-prehensive architectural design associated with the extended frame structures for conducting direct communication; (b) an adaptive communication approach that can dynamically select the most efficient packet transmission scheme between the direct communication and indirect communication; (c) an approach that is fully compatible and can be directly integrated with the existing protocols defined in the IEEE 802.16 standard; and (d) numerical analysis and extensive simulations to justify the effectiveness of the proposed scheme.
The remainder of this chapter is organized as follows. Section 2.2 briefly reviews the MAC frame structure and packet transmission mechanism in IEEE 802.16 PMP networks. The proposed APC approach, consisting of management structures, admission control scheme, and direct communication procedures, is described in Section 2.3; while the numerical analysis is carried in Section 2.4. Both the performance evaluation and validation of the APC approach are conducted in Section 2.5. Section 2.6 draws the concluding remarks.