Elsevier

Computer Communications

Volume 32, Issue 5, 27 March 2009, Pages 943-953
Computer Communications

Slot allocation algorithms in centralized scheduling scheme for IEEE 802.16 based wireless mesh networks

https://doi.org/10.1016/j.comcom.2008.12.020Get rights and content

Abstract

In IEEE 802.16 based wireless mesh networks (WMNs), TDMA (Time Division Multiple Access) is employed as the channel access method and only TDD (Time Division Duplex) is supported and there are no clearly separate downlink and uplink subframes in the physical frame structure. As the uplink and downlink traffic has different characteristics in that the uplink traffic decentralizes in each MSS (Mesh Subscriber Station) and the downlink traffic centralizes in the MBS (Mesh Base Station), different scheduling methods should be taken in the uplink and downlink. This paper presents a uniform slot allocation algorithm which is suitable for both uplinks and downlinks. To achieve higher spatial reuse and greater throughput and to avoid switching frequently between receiving and transmitting within two adjacent time slots when a relay node forwards traffic, different link selection criteria are taken into account when allocating slots for uplinks and downlinks. A combined uplink and downlink slot allocation algorithm is proposed for further improving the spatial reuse and network throughput. The proposed algorithms are evaluated by extensive simulations and the results show that it has good performance in terms of spatial reuse and network throughput. To the best of the authors’ knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.

Introduction

Wireless mesh networks (WMNs), as the development trend of the next generation wireless networks have recently attracted much attention. Nodes in the WMN usually communicate with each other by multihop forwarding, and all nodes can automatically establish and maintain mesh links among themselves, which brings many advantages [1], [2], such as low up-front cost, easy network maintenance, robustness and reliable service coverage, etc. Also several IEEE standards establish special sub-groups to define the requirements to support mesh topology, such as IEEE 802.11s, IEEE 802.15.5, IEEE 802.16a, and IEEE 802.20, etc. More details can be found in [3].

WMN can support many special applications which cannot be supported by other wireless networks such as cellular networks, ad hoc networks, wireless sensor networks, standard IEEE 802.11, etc. Those special applications include: broadband home networking, community and neighbourhood networking, enterprise networking, metropolitan area networks, transportation systems, building automation, health and medical systems, security surveillance systems, etc [2]. The above applications are either community networks or broadband wireless access networks, and for IEEE 802.16 based WMN, which has high transmit rate and long transmit range, there is still another important application: wireless backhaul networks for WIFI Hotpoint or third generation cellular networks. Much previous work has been reported in this area [4], [5], [6], [7], [8], [9]. As we all know, in traditional cellular networks, point-to-point wired links such as T1/E1 or microwave links such as Local Multipoint Distribution Service (LMDS) are used as backhaul links. There are many disadvantages in the traditional backhaul networks [4], [6]: first, the construction and maintenance cost of wired T1/E1 and microwave links is very high; second, the point-to-point links are very easy result in single point failure; thirdly, the symmetric T1/E1 links and microwave links are not suited for the asymmetric data traffic. Fortunately, the IEEE 802.16 based WMN can resolve the above problems.

In IEEE 802.16 based WMN, TDMA is employed as the channel access method and only TDD is supported [10]. There are three schemes to allocate the wireless resources (time slots) for the nodes in the mesh networks: coordinate distributed scheduling, non-coordinate distributed scheduling and centralized scheduling. For backhaul applications, centralized scheduling is preferable because all the traffics are to or from the MBS. In TDMA based mesh networks, spatial reuse technology is usually taken to exploit the wireless resource more efficiently and to improve the network throughput. The spatial reuse in TDMA means multiple links or nodes in the mesh networks can transmit concurrently only if they do not interfere with each other.

In this work, a uniform slot allocation (USLA) algorithm which is suitable for both uplinks and downlinks is proposed first. Compared with the existing scheduling algorithms, the proposed algorithm can be used for both uplink and downlink slot allocation. To achieve higher spatial reuse and network throughput in uplink and downlink and to avoid the relay nodes switching frequently between receiving and transmitting within two adjacent time slots when forwarding traffics, different link selection criteria are taken in uplink and downlink slot allocation. To improve the spatial reuse and network throughput further, a combined uplink and downlink slot allocation(CSLA) algorithm is proposed, where uplinks are considered when allocating slots for downlinks. To the best of the author’s knowledge, this work is the first one that considers combined uplink and downlink slot allocation on the centralized scheduling scheme in IEEE 802.16 based WMNs.

The remainder of this paper is organized as follows. Section 2 introduces some related work. The basic centralized scheduling scheme in IEEE 802.16 mesh mode is described in Section 3. In Section 4 the uniform slot allocation algorithm and the combined uplink and downlink slot allocation algorithm are proposed. Section 5 presents extensive simulation results and Section 6 gives a conclusion.

Section snippets

Related work

As IEEE 802.16 based WMNs have many advantages such as high transmitting rate and long transmitting range, which makes it suitable for using as backhaul network for WIFI Hotpoint and the third generation cellular networks. Much work has been done in this area [4], [5], [6], [7], [8], [9]. Bu, Chan and Ramjee designed an IEEE 802.16 based wireless radio access network for third generation cellular network[4]. Wei and Ganguly also designed an IEEE 802.16 based radio access network to provide data

Basic centralized scheduling scheme in IEEE 802.16 mesh mode

In IEEE 802.16 mesh mode, traffic can be routed through other SSs and can occur directly between SSs, which is different from PMP (Point to Multi-Point) mode, in which traffic can only occur between BS and SSs. In a mesh network, the node has a direct connection to a backhaul services outside the mesh network is termed a mesh BS (referred to as the MBS), the other nodes are termed mesh SSs (referred to as the MSSs). Within mesh context, the uplink and downlink are defined as traffic in the

The proposed slot allocation algorithm

As mentioned in the last section, the proposed slot allocation algorithm will focus on how to determine the MSS’s actual transmitting time slot to achieve higher spatial reuse and network throughput assuming the routing tree and the uplink and downlink flow assignments are known. From the flow assignment, the link transmitting rate (which can be obtained from the link burst profile), the allocation cycle time, the slot cycle time, etc. the uplink and downlink slot demands of each MSS in an

Performance metrics

In this section, the performance of the proposed algorithm will be evaluated through extensive simulations. Both special reuse and network throughput are considered and the following performance metrics are used: scheduling length, link concurrency ratio and normalized network throughput, switching times.

The scheduling length is the total number of time slots needed to transfer all MSSs uplink traffic to the MBS or/and transfer all MSSs downlink traffic from the MBS to each MSS within the slot

Conclusion

In this work, a uniform slot allocation (USLA) algorithm which is fit for both uplinks and downlinks is proposed first, to achieve higher spatial reuse and network throughput both in uplink and downlink slot allocation and to avoid switching frequently between receiving and transmitting within two adjacent time slot when a relay node forwards traffic, a smallest hopcount first and hop by hop and largest hopcount first and hop by hop link selection criteria is taken in uplink and downlink slot

Acknowledgement

This work is supported by the Guangdong province and the Hong Kong government under Grant No. 20060104-2.

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