routing and scheduling for wimax mesh networks · 2 chapter 1. routing and scheduling for wimax...

Chapter 1

Routing and Scheduling for WiMAX

Mesh Networks

Jianhua He†, Xiaoming Fu‡, Jie Xiang§, Yan Zhang§, Zuoyin Tang†

† Institute of Advanced Telecommunications, Swansea University, UK.

Email: [email protected].

‡ University of Goettingen, Germany.

§ Simula Research Laboratory, Norway.

1.1 Introduction

In today’s telecommunications networking and services are changing in a rapid way to sup-

port next generation Internet (NGI) user environment. Wireless networks will play an im-

portant role in NGI. Wireless broadband networks are being increasingly deployed and used

in the last mile for extending or enhancing Internet connectivity for fixed and/or mobile

clients located on the edge of the wired network [1].

With high data rate, large network coverage, strong QoS capabilities and cheap network

1

2 CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKS

deployment and maintenance costs, WiMAX is regarded as a disruptive wireless technology

and has many potential applications [1] [2]. It is expected to support business applications,

for which QoS support will be a necessity. Depending on the applications and network invest-

ment, WiMAX network can be configured to work in different modes, point-to-multipoint

(PMP) or Mesh mode. For example, the network can have a simple base station (BS) work-

ing in PMP mode and serving multiple subscriber stations (SS) if the potential SSs can be

covered by the BS. Mesh topology is an optional configuration for WiMAX networks. In

the Mesh mode, traffic demands are aggregated at a set of Subscriber Station (SS) nodes

which are equipped with 802.16 interfaces. Subsequently, the traffic demands at SS nodes

are delivered to a set of Base Stations (BS) nodes which functions in the PMP mode. These

BS stations can be connected by a backhaul and connected to Internet Access Point (IAP)

nodes. An amendment to the IEEE 802.16 specifications (where WiMAX is based) is IEEE

802.16j, Multihop Relay Specification. IEEE 802.16j is being developed by IEEE 802.16’s

Relay Task Group. It is expected to extend reach/coverage through relaying [3]. In this

chapter, we will focus on the Mesh mode.

Wireless mesh network offers increased reliability, coverage and reduced network costs [4]

[5]. There are extensive research, standardization and commercial development activities

on mesh networks [4] [5] [6] [7]. For example, several IEEE special task groups have been

established to define the requirements for mesh networking in wireless personal area networks

(WPANs), wireless local area networks (WLANs) and wireless metropolitan area networks

(WMANs). A brief description of the standardization activities can be found in [4]. Wireless

mesh networks can be a prospective solution for broadband wireless Internet access in a

flexible and cost-effective manner. However wireless mesh networks also raises a number of

research challenges, e.g., network routing, scheduling, QoS support, network management,

etc [4] [8]. Those challenges are faced by WiMAX mesh networks without exception. WiMAX

Mesh mode is defined with OFDM for frequency between 2 and 11 GHz and time division

multiple access (TDMA) is used in the MAC layer to support multiple users. Unlike the

single hop wireless networks, routing algorithms are required to determine routes for the

connections between a subscriber station (SS) and a base station (BS). As WiMAX networks

operate synchronously in a time slotted mode, it is also necessary to allocate time slots

without collision over the network to achieve assigned bandwidth for each connection. More

1.1. INTRODUCTION 3

LaptopUNIVERSITY

3G Base Station Hand held computerRS CS TR RD TD CDTALK / DATA

TALK

AccessPointRS CS TR RD TD CD

TALK / DATATALKWiMAX Mesh BSCloudInternet

(a) WiMAX PMP network

LaptopUNIVERSITY

3G Base Station Hand held computerRS CS TR RD TD CDTALK / DATA

TALK

AccessPointRS CS TR RD TD CD

TALK / DATATALKWiMAX Mesh BS

RS CS TR RD TD CDTALK / DATA

TALKWiMAX Mesh SS RS CS TR RD TD CDTALK / DATA

TALKWiMAX Mesh SSRS CS TR RD TD CD

TALK / DATATALKWiMAX Mesh SS

CloudInternet

(b) WiMAX mesh network

Figure 1.1: WiMAX PMP network and mesh network architectures.

challenging is that the routing and scheduling for WiMAX networks are tightly coupled. The

routing and scheduling problem for WiMAX networks is different from 802.11 based mesh

networks. In the 802.11 based mesh networks, the MAC layer is contention based; routing

algorithms and MAC layer protocols can be designed and operated separately.

Although IEEE 802.16 standards specify several QoS schemes and related message for-

mats, the problems of scheduling algorithms for both PMP and Mesh mode are left unsolved.

Routing algorithms for WiMAX networks are outside the scope of the standard work as well.

In this chapter, we will investigate the issues of routing and scheduling in WiMAX mesh

networks. Both distributed and centralized routing algorithms will be studied, and their

effectiveness on alleviating potential network congestion will be compared. The scheduling


problem will also be mathematically modelled by taking into account the interference con-

straints. Solutions are developed to maximize the utilization of network capacity subject to

fairness constraints on allocation of scarce wireless resource among the subscriber stations.

The Chapter is organized as follows. The WiMAX mechanisms defined for Mesh mode

is overviewed in Section I. Existing research on scheduling and routing for WiMAX mesh

networks is presented in Section II. Both distributed and centralized routing algorithms will

be presented in Section III. A scheduling problem for WiMAX mesh networks is mathemat-

ically modeled and solved in Section IV. Typical numerical results are presented in Section

V. Open research issues are discussed in Section VI. Finally we conclude the Chapter.

1.2 Overview of 802.16 Mechanisms for Mesh mode

Unlike the PMP mode that only allows communication between the BS and SS, each station

is able to create direct communication links to a number of other stations in the network

instead of communicating only with a BS. However, in typical network deployments, there

will still be certain nodes that provide the BS function of connecting the Mesh network

to the backbone networks. When using Mesh centralized scheduling to be describe below,

these BS nodes perform much of the same basic functions as the BSs do in PMP mode.

Communication in all these links in the network are controlled by a centralized algorithm

(either by the BS or decentralized by all nodes periodically), scheduled in a distributed

manner within each node’s extended neighborhood, or scheduled using a combination of

these. The stations that have direct links are called neighbors and forms a neighborhood.

A node’ s neighbors are considered to be one hop away from the node. A two-hop extended

neighborhood contains, additionally, all the neighbors of the neighborhood.

In this section, we will briefly introduce the frame structure, network entry procedures,

bandwidth request and grant mechanisms defined for WiMAX Mesh mode, which will be the

base requirement for the design of routing and scheduling algorithms to be presented later.

1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 5

1.2.1 Identifications

There are several types of identifications that an SS will use for different purposes.

Each SS has a 48-bit universal MAC address, which uniquely defines the SS from other SSs.

The MAC address is used during the network entry process and as part of the authorization

process by which the candidate SS and the network verify the identity of each other.

After authorized to the network, a candidate SS will receive a 16-bit node identifier (Node

ID) upon a request to the Mesh BS. Node ID is the basis for identifying SSs during normal

operation. The Node ID is transferred in the Mesh subheader, which follows the generic

MAC header, in both unicast and broadcast messages [2].

To facilitate communications with local neighboring SSs, an SS will use a 8-bit link iden-

tifiers (Link IDs). Each SS shall assign an ID for each link it has established to its neighbors.

The Link IDs are communicated during the Link Establishment process as neighboring SSs

establish new links. The Link ID is transmitted as part of the Connection ID in the generic

MAC header in unicast messages. The Link IDs are used in distributed scheduling to iden-

tify resource requests and grants. Since these messages are broadcast, the receiver nodes can

determine the schedule using the transmitter Node ID in the Mesh subheader, and the Link

ID in the payload of the MSH-DSCH (Mesh mode Schedule with Distributed Scheduling)

message. The MSH-DSCH message will be introduce later.

1.2.2 Frame structure

Unlike in PMP mode, there are no clearly separate downlink and uplink subframes in Mesh

mode. A Mesh frame consists of a control and a data subframe [2]. There are two types

of control subframes, which serves different functions, which will be introduced below. All

transmissions in the control subframe are sent using QPSK-1/2 with the mandatory coding

scheme. The data subframe is divided into minislots [2]. A scheduled allocation consists of

one or more minislots.

6 CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKSFrame n-1 Frame n Frame n+1 Frame n+2timeNetworkEntry NetworkConfigure NetworkConfigure PHY tr. Burstfrom SS#j PHY tr. Burstfrom SS#kNetwork Control subframe Data subframeNetworkEntryLongPreamble MAC PDU w/MSH-NENT GuardSymbol GuardSymbol GuardSymbolNetworkConfigureLongPreamble MAC PDU w/ MSH-NCFG GuardSymbolFigure 1.2: WiMAX Mesh Network Control subframe.

Network Control Subframe

The first type of control subframe is termed Network Control subframe, used to create

and maintain cohesion between the different systems. The Network Control subframe is

illustrated in Fig.1.2. Frames with a Network Control subframe occur periodically. The

period of occurrence is indicated in the Network Descriptor. The length of the control

subframe is fixed and of length OFDM symbols, which is also indicated in the Network

Descriptor. During a network control subframe, the first seven symbols are allocated for

network entry, followed by sets of seven symbols for network configuration with MSH-NCFG

messages.

MSH-NCFG messages provide a basic level of communication between nodes in different

nearby networks. All the nodes (BS and SS) in the Mesh network will transmit MSH-

NCFG [2]. The MSH-NCFG message format is shown in Fig.1.3. Through the MSH-NCFG

message, a BS or an SS will report a number of its neighbors. The number of neighbors

reported on may be a fraction of the whole set of neighbors known to this SS. A node

will also report the Mesh BSs that its neighbors report and report the distances in hops to

the BSs. The Embedded Packet Flag is used to indicate if an embedded data information

element (IE) is included in the message. Network Descriptor is one of the 5 defined embedded

data IE that can be included in MSH-NCFG. Transmit Antenna indicate the logical antenna

1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 7Management Message Type=39 (8 bits)NumNbrEntries (5 bits)NumBSEntries (2 bits)Embedded Packet Flag (1 bits)Transmit Power (4 bits)Transmit Antenna (3 bits)NetEntry MAC Address Flag (1 bit)Network base channel (4 bits)reserved (4 bits)NetworkConfig Count (4 bits)TimestampeFrame Number (12 bits)NetControl Slot Num in Frame (4 bits)Synchronization Hop Count (8 bits)

If (NetEntry MAC Address Flag)NetEntry MAC Address (48 bits)for (i=0; i<NumBSEntries; i++){ BS Node ID (16 bits)Number of hops (3 bits)Xmt energy/bit (5 bits) }NetConfig schedule infoNext Xmt Mx (3 bits)Xmt Holdoff Exponent (5 bits)

for (i=0; i<NumNbrEntries; i++){ Nbr Node ID (16 bits)MSH-Nbr_Physical_IE() (16 bits)if (Logical Link Info Present Flag)MSH-Nbr_Logical_IE() (16 bits) }If (Embedded Packet Flag)MSH-NCFG_embedded_data (variable)Figure 1.3: WiMAX Mesh Network Configure message format.

used for transmission of this message. Up to eight antenna directions can be supported.

NetworkConfig Count is the counter of MSH-NCFG messages transmitted by this node,

which is used by neighbors to detect missed transmissions. Xmt Holdoff Exponent and Next

Xmt Mx are the variables used by this node to indicate its next MSH-NCFG eligibility

interval. The Xmt Holdoff Exponent is used to calculate Xmt Holdoff Time, which is the

number of MSH-NCFG transmit opportunities after Next Xmt Time that this node is not

eligible to transmit MSH-NCFG packets. There are MSH-CTRL-LENC1 opportunities per

network control subframe, as indicated in Network Descriptor. Next Xmt Time is the next

MSH-NCFG eligibility interval for this node, and is calculated by Xmt Holdoff Exponent

and Next Xmt Mx.

Network Descriptor is an important embedded data IE that can be included in MSH-

NCFG. The parameters included in a Network Descriptor is shown in Fig.1.6. Similar to the

downlink and uplink network descriptor in PMP mode, the Network Descriptor in Mesh mode

defines the parameters associated to burst profiles, i.e., FEC code type, Mandatory Exit

Threshold and Mandatory Entry Threshold. It also defines the important parameters such as

the length of control subframe (MSH-CTRL-LEN), and the number of DSCH opportunities

in schedule control subframe (MSH-DSCH-NUM). The parameter Scheduling Frames defines

how frequent a schedule control subframe will appear.

8 CHAPTER 1. ROUTING AND SCHEDULING FOR WIMAX MESH NETWORKSFrame Length Code (4 bits)MSH-CTRL-LEN (4 bits)MSH-DSCH-NUM (4 bits)MSH-CSCH-DATA-FRACTION (4 bits)Scheduling Frames (4 bits)Num_Burst_Profiles (4 bits)Operator ID (16 bits)XmtEnergyUnitsExponent (4 bits)Channels (4 bits)MinCSForwardingDelay (7 bits)ExtendedNeighborhoodType (1 bits)If (chnnels)MSH-NCFG_Channel_IE() (variable)for (i=0; i<Num_Burst_Profiles; i++){ FEC Code Type (8 bits)Mandatory Exit Threshold (8 bits)Mandatory Entry Threshold (8 bits) }(a) Mesh Network Descriptor

Management Message Type=40 (8 bits)Type (3 bits)Xmt counter for this type (3 bits)reserved (2 bits)Sponsor Node ID (16 bits)Xmt Power (4 bits)Xmt Antenna (3 bits)reserved (1 bits)If (Type==0x2)MSH-NENT_Request_IE() (variable)

MSH-NENT message format

MSH-NENT Request IEMAC Address (48 bits)OpConfInfo (64 bits)Operator Authentication Value (32 bits)Node serial Number (32 bits)(b) Mesh Network Entry message

Figure 1.4: WiMAX Mesh Network Discriptor and Network Entry message format.

Schedule Control Subframe

The second type of control subframe is termed Schedule Control, which is used to coordinate

scheduling of data-transfers between systems. The Schedule Control subframe is shown in

Fig.1.5 [2]. The first symbols are allocated to transmission bursts containing Mesh central-

ized scheduling message (MSH-CSCH) and Mesh centralized configuration message (MSH-

CSCF), and the remainder of the Schedule Control subframe is allocated to transmission

bursts containing Mesh distributed scheduling messages (MSH-DSCH) [2].

• MSH-CSCH message: A MSH-CSCH message is created by a Mesh BS for the purpose

of centralized scheduling, including collecting bandwidth request from the SSs and de-

livering the transmission schedule to the SSs. To deliver a transmission schedule to the

SSs, a MSH-CSCH message is generated and broadcasted by the BS to all its neighbors.

Then all the nodes with hop count lower than a threshold, which is configured by the

BS, will forward the MSH-CSCH message to their neighbors that have a higher hop

1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 9Frame n-1 Frame n Frame n+1 Frame n+2timeCentralSchedule CentralConfigure DistributedSchedule PHY tr. Burstfrom SS#j PHY tr. Burstfrom SS#kSchedule Control subframe Data subframeCentralScheduleLongPreamble MAC PDU w/ MSH-CSCH GuardSymbolCentralConfigureLongPreamble MAC PDU w/ MSH-CSCF GuardSymbolDistributedScheduleLongPreamble MAC PDU w/ MSH-DSCH GuardSymbolFigure 1.5: WiMAX Mesh Schedule Control subframe.

count. For the forwarding direction starting from the BS, the Grant/Request Flag in

the MSH-CSCH message is set to 0. In addition, SSs can use MSH-CSCH messages

to request bandwidth from the Mesh BS, by setting the Grant/Request Flag in the

MSH-CSCH message to 1. Each SS reports the individual traffic demand requests of

each child node in its subtree from the BS. The SSs in the subtree are those in the

current scheduling tree to and from the Mesh BS. The scheduling tree is known to all

nodes in the network and is ordered by node ID. The parameter Flow Scale Exponent

is used to determines scale of the granted bandwidth. Its value typically depends on

the number of nodes in the network, the achievable PHY bit rate, the traffic demand,

and the provided service. For the downlink, Flow Scale Exponent gives the absolute

values of flow granted, so the total minislot range allowed for centralized scheduling

need not be used if not needed, with the remainder set aside for distributed scheduling.

For the uplink, the lowest exponent possibility is used at each hop, with quantization

of forwarded requests rounded up to avoid reducing any requests to zero.

• MSH-CSCF message: A MSH-CSCF message is also used in Mesh mode for the purpose

of centralized scheduling, and specifically for configuration. The Mesh BS generates and

broadcasts the MSH-CSCF message to all its neighbors, and all nodes forward (rebroad-


cast) the message according to its index number specified in the message. With each

new configuration message, the number of Configuration sequence number in the mes-

sage is incremented by 1. The parameter NumberOfChannels in the message indicates

the number of channels available for centralized scheduling. And the parameter Num-

berOfNodes determine the number of nodes in scheduling tree. For each node in the

scheduling tree, the node ID of its one-hop neighbors will be given as well as the the

Uplink/Downlink Burst Profile used for the link from/to the neighbors.

• MSH-DSCH message: During a Schedule Control subframe, MSH-DSCH-NUM Dis-

tributed Scheduling messages will occur. The number MSH-DSCH-NUM is indicated

in the Network Descriptor included in a MSH-NCFG message. Distributed Scheduling

messages may also occur in the data subframe if not in conflict with the scheduling dic-

tated in the control subframe. In the MSH-DSCH message, the parameter Coordination

Flag indicates the type of the distributed scheduling, being 0 for Coordinated and 1 for

Uncoordinated. Both type of distributed scheduling will require a threeway handshake

(Request, Grant, and Grant confirmation) to establish a valid schedule. The parameter

Grant/Request Flag indicate the type of the message, being 0 for Request message and

1 for Grant message. A number of Request IEs, Availability IEs, and Grant IEs can be

included in the message.

1.2.3 Mesh Network Entry Mechanism

The standard specifies network entry mechanism to find sponsor nodes and establish links

with neighbors [2]. In this Chapter, the network entry mechanism is used as one of the

alternatives to establish traffic routes for WiMAX mesh networks. Upon entering the Mesh

network, a new SS searches for MSH-NCFG to acquire coarse synchronization with the

network. Once the physical layer has achieved synchronization, the MAC layer can acquire

network parameters for the MSH-NCFG message. Meanwhile, the SS can builds a physical

neighbor list, from which the SS can select a potential sponsor node out of the eligible sponsor

nodes. How to select the sponsor node is not specified in the standard. The selection method

will be discussed in more details later in this Chapter. The SS then synchronizes its time

1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 11Management Message Type=42 (8 bits)Configuration sequence num (3 bits)Grant/Request Flag (1 bits)Frame schedule Flag (1 bits)Configuration Flag (1 bits)reserved (2 bits)NumFlowEntries (8 bits)Flow Scale Exponent (4 bits)

MSH-CSCH message format

Padding Nibble (4 bits)For (i=0;i<NoFlowEntries; ++i){ UplinkFlow (4 bits)if (Grant/Request Flag==0)DowlinkFlow (4 bits) }If (Grant/Request Flag==0) {No_link_updates (4 bits)for (i=0; i<No_availabilities; ++i) {Node Index self (8 bits)Node Index parent (8 bits)Uplink Burst Profile (4 bits)Downlink Burst Profile (4 bits) }} else { Sponor Node (8 bits)Downlink Burst Profile (4 bits)Uplink Burst Profile (4 bits) } }(a) Coordinated Scheduling mes-sage

Management Message Type=43 (8 bits)Configuration sequence number (4 bits)NumberOfChannels (4 bits)For (i=0; i<NumberofChannels; ++i)Channel index (4 bits)Padding Nibble (0 or 4 bits)NumberOfNodes (8 bits)For (i=0;i<NumberOfNodes; ++i) {NodeID (16 bits)NumOfChildren (8 bits)

MSH-CSCF message format

For (j=0; j<NumOfChildren; ++j) {Child Index (8 bits)Uplink Burst Profile (4 bits)Downlink Burst Profile (4 bits)(b) Coordinated Configurationmessage

Management Message Type=41 (8 bits)Coordination Flat (1 bits)Grant/Request Flag (1 bits)Sequence counter (6 bits)No. Requests (4 bits)No. Availabilities (4 bits)No. Grants (6 bits)reserved (2 bits)If (Coordination Flag==0)MSH-DSCH_Scheduling_IE() (variable)

MSH-DSCH message format

For (i=0;i<No_Requests; ++i)MSH-DSCH_Request_IE() (16 bits)For (i=0;i<No_availabilities; ++i)MSH-DSCH_Availabilities_IE() (32 b)For (i=0;i<No_Grants; ++i)MSH-DSCH_Grant_IE() (40 bits)(c) Distributed Scheduling mes-sage

Figure 1.6: WiMAX PMP network and mesh network architectures.

to the potential sponsor node and sends a network entry request message to the potential

sponsor node. If the candidate sponsor node accepts the request and opens a sponsor channel,

the channel is ready for use to register with the BSs. After the SS is authorized to enter the

network by the BS, it can request bandwidth from the BS via the sponsor node and can also

establish links with the SSs other than the sponsor node [2].

1.2.4 Bandwidth allocation and grant mechanisms

In the 802.16 standard, flexible bandwidth allocation and grant mechanisms have been de-

fined for PMP mode to guarantee the QoS of various service flows. Those mechanisms

include periodically polling, real-time polling, non-real-time polling, contention based band-

width request scheme, poll-me bit, bandwidth stealing and piggyback [2]. However, the

bandwidth request and grant mechanisms can not be used in Mesh mode due to the multi-

hop networking in Mesh mode. In WiMAX Mesh mode, all the communications in the links


in the network are controlled by three ways, i.e., using a centralized scheduling algorithm,

using a distributed scheduling algorithm within each node’s extended neighborhood, or us-

ing a combination of these two types of algorithms. All the scheduling algorithms will be

implemented based on the scheduling messages presented in Subsection 1.2.2.

In the coordinated distributed scheduling algorithm, all stations (BS and SSs) coordi-

nate their transmissions in their extended two-hop neighborhood. Coordinated distributed

scheduling does not rely on the operation of a BS and transmissions are not necessarily di-

rected to or from the BS. Within the constraints of the coordinated schedules, uncoordinated

distributed scheduling can be used for fast and ad-hoc setup of schedules on a link-by-link

basis with directed requests. Grants of the uncoordinated schedules need to ensure that

the resulting data transmissions do not cause collisions with the data and control traffic

scheduled by the coordinated scheduling algorithms.

Although distributed scheduling algorithms are more scalable, they are inefficient in QoS

guarantee. On the other hand, centralized scheduling ensures collision free scheduling over

the links in the network, typically in a more optimal manner than the distributed scheduling

method. Therefore better QoS support and network bandwidth utilization can be achieved.

In this Chapter centralized scheduling will be the only scheduling algorithm studied.

Centralized scheduling

With the centralized scheduling algorithm, transmission schedule for the SSs is defined by

the BS. The BS determines the flow assignments from the resource requests from the SSs.

Subsequently, the SSs validate the MSH-CSCH schedule and determine the actual schedule

from these flow assignments. The assignments determined by the BS extends to those SSs

not directly connected to the BS. Intermediate SSs are responsible for forwarding bandwidth

requests for SSs listed in the routing tree that are further from the BS (i.e., more hops

from the BS) and the MSH-CSCH message from the BS to their neighbors as required.

The SS resource requests and the BS assignments are both transmitted during the Schedule

Control subframe. Centralized scheduling ensures that transmissions are coordinated to

ensure collision-free scheduling over the links in the routing tree to and from the BS. The

1.2. OVERVIEW OF 802.16 MECHANISMS FOR MESH MODE 13

centralized scheduling will persist over a duration that is greater than the cycle time to relay

the new resource requests and distribute the updated schedule.

Determination of the flow assignment and routing tree is outside the scope of the standard,

and will be addressed in later of this Chapter.

Distributed scheduling

Coordinated distributed scheduling ensures that transmissions are scheduled in a manner

that does not rely on the operation of a BS, and that is not necessarily directed to or from

the BS. In the coordinated distributed scheduling mode, all the stations (BS and SSs) need

coordinate their transmissions in their extended two-hop neighborhood. The coordinated

distributed scheduling mode uses some or the entire control portion of each frame to regularly

transmit its own schedule and proposed schedule changes on a PMP basis to all its neighbors.

Within a given channel all neighbor stations receive the same schedule transmissions. All

the stations in a network have to use the same channel to transmit schedule information in

a format of specific resource requests and grants.

Within the constraints of the coordinated schedules (distributed or centralized), uncoordi-

nated distributed scheduling can be used for fast, ad-hoc setup of schedules on a link-by-link

basis. Uncoordinated distributed schedules are established by directed requests and grants

between two nodes, and shall be scheduled to ensure that the resulting data transmissions

(and the request and grant packets themselves) do not cause collisions with the data and

control traffic scheduled by the coordinated distributed nor the centralized scheduling meth-

ods.

The major differences between coordinated and uncoordinated distributed scheduling are

in the portions where the MSH-DSCH messages are transmitted. In the coordinated case,

the MSH-DSCH messages are scheduled in the control subframe in a collision free manner.

In the uncoordinated case, MSH-DSCH messages are transmitted in data subframes and

may collide.


1.3 Literature Survey

An in-depth survey on the general wireless mesh networks has been presented in [4] [5],

which covers the research activities and open issues from physical layer to applications, and

from mobility management to power management, from designs with single channel single

radio device to those with multiple channel multiple radios devices. However, most of the

existing researches about wireless mesh networks are based on IEEE 802.11 standard. The

routing and MAC layer protocols designed for 802.11 standard based wireless mesh network

can not be used directly or efficiently for WiMAX mesh networks. And not much work has

been done on WiMAX mesh networks. In this section, we will introduce the routing and

scheduling related work on WiMAX mesh networks.

1.3.1 Coordinated Distributed Scheduling

The main idea of the coordinated distributed scheduling is to coordinate the transmission of

MSH-DSCH messages over transmission opportunities in a collision-free manner. Through

the exchanges of collision-free MSH-DSCH over control subframes, collision-free data slot

reservations in the data subframes can be achieved.

To achieve the goal of collision-free MSH-DSCH transmission, nodes will exchange 2-hop

or 3-hop neighborhood scheduling information with each other. Since nodes shall run the

scheduling algorithm independently, a common algorithm has been specified in the standard

for each node in the neighborhood to calculate the same schedule. The algorithm is random

and predictable by dynamically constructing the seeds of a random number generator for

each node according to a common rule. In particular, the seed for a given node is constructed

based on its unique node ID and the index of the candidate transmission opportunity. The

most important parameters that have significant impacts on the performance of coordinated

distributed scheduling are XmtHoldoffExponet (3 bits) and NextXmtXm (5 bits), which

have been introduced in Subsection 1.2.2. They can be used to control the contention on

the transmission opportunities and improve bandwidth utilization.

1.3. LITERATURE SURVEY 15

Due to importance of the coordinated distributed scheduling on the network performances,

an analytical framework is needed to assess the performance of the scheduling scheme. Cao et

al analytically investigated how the channel contention is correlated with the total node num-

ber, exponent value and network topology [9] [10]. With the assumption that the transmit

time sequences of all the nodes in the control subframe form statistically independent renewal

processes, they developed methods for estimating the distributions of the node transmission

interval and connection setup delay. The analytical method will be helpful for evaluating

upper layer performance like throughput and delay. They implemented the coordinated dis-

tributed scheduling module in NS-2 and showed that their analytical model is quite accurate

under various scenarios, including both single hop and multihop networks.

Based on Cao’s analytical model, Bayer et al presented an enhancement of the model [11].

In particularly, they evaluated the scalability of the coordinated distributed scheduling. A

scalability problem was observed that leads to poor performance in dense networks and ag-

gravates QoS provisioning. The problem may result from the election based transmission

timing mechanism for scheduling the transmission of MSH-DSCH messages. They proposes

a dynamic adaptation mechanism to counteract the scalability problem, in which the pa-

rameter XmtHoldoffExponet is dynamically and locally adjusted according to the network

contention and the status of a node. The NextXmtXm is used as a contention indicator. If

NextXmtXm used by the node or its neighbors exceeds a specified threshold, the XmtHoldof-

fExponet is increased. The status of a node is defined according to its transmission activity,

if it is BS or if it is a sponsor node. Significant UDP throughput increase is observed with the

application of the adaptation mechanism for both single and multiple hop network scenarios.

In the 802.16 standard and the above analytical work, it is assumed that the control mes-

sages can be transmitted without collision in the extended neighborhood (2-hop or 3-hop).

However such kind of interference model may not hold in practice. Zhu and Lu investigate the

performance of coordinated distributed scheduling under a realistic interference model [12].

Extensive simulations were conducted to evaluate the reception collision performance of the

scheduling mechanism. It was reported that the collision ratio of control messages can be as

high as 20% for 2-hop extended neighborhood. They studied how to deal with the collision

problem by appropriate configuration of parameters such as XmtHoldoffExponent.


1.3.2 Coordinated Centralized Scheduling

An early work on multihop scheduling for WiMAX mesh networks is presented by Kim and

Ganz in [13]. They proposed a fair multihop centralized scheduling algorithm. The algorithm

consists of two phases, namely node ordering and link allocation, which is introduced as

below.

In the beginning of the algorithm, each node sends bandwidth request messages to the

BS. The BS uses a node ordering algorithm to determines an Order List for the nodes and

broadcast the ordering to the nodes. The nodes will forward the broadcast message according

to the ordering. In the node ordering algorithm, a measure satisfaction index is defined as

the ratio of average bandwidth allocated in a given number of frames to a node’s weight

(Wi). The weight can reflect the priority of the node. The total weight value Wi,total is

defined as the weight of itself plus the sum of all the weights of its children nodes [13]:

Wi,total = Wi +∑j∈Ci

Wj, (1.1)

where Ci denotes the set of child nodes of node i. The satisfaction index Si(f) of each node

i in a frame f , is defined as:

Si(f) =

f−1∑fp=f−T

Bi(fp)

TWi,total

, (1.2)

where Bi(f) denotes the link bandwidth allocated to node i in frame f . After calculate the

satisfaction index of all the nodes, the BS sorts bandwidth requests in increasing order of

the satisfaction index of each node on a per-hop basis. The nodes closer to the BS have

higher orders.

After the node ordering phase, each node computes its own schedule based on the MSC-

CSCF. The computation uses two types of matrices: Schedule Matrix and Collision Matrix.

Each node takes the Order List broadcast by the BS, bandwidth requirement of each node,

scheduled time slots and network topology as the input. Its schedule is determined from the

time slots in which the node is not identified in the Collision Matrix.


The proposed scheduling algorithm is stated to achieve maximum throughput in a heavily

loaded wireless network while also guarantee fairness of channel access among different nodes.

However, there are still three major problems in the scheduling algorithm. First, each node is

required to compute its own schedule based on the Order List in the link allocation phase. In

the link allocation algorithm, each node will require the information of scheduled time slots

by other nodes as input. However, such information is not easy to obtain in a distributed

way. Second, there is not an efficient to control the time slots that a node can request

and/or reserved in a frame. Third, the forwarding tree is not constructed. The traffic from

the children nodes of a node is also not taken into account in the link allocation algorithm.

A part of the problems in the scheduling algorithm proposed in [13] are addressed by Han

et al in [14]. They proposed a collision-free centralized scheduling algorithm for WiMAX

mesh networks. The traffic from children nodes is taken into account in the algorithm by the

means of designing a relay strategy. The scheduling algorithm is based on a simple routing

tree. The routing tree is constructed by selecting the nearest neighbor with the minimum hop

count to the BS as the sponsor node of this node. In the scheduling algorithm, service token

is used to determine the eligibility of a node to be scheduled in a time slot. Initially service

token of a link is assigned based on its traffic demand, with the purpose of guaranteeing

fairness. A link can be scheduled only if the service token number of the transmitter of

the link is nonzero. Each time after a link is assigned a time slot, the service token of

the transmitter is decreased by one and that of the receiver is increased by one. Through

this method of service token adjustment, the hop-by-hop relay model is integrated into the

scheduling algorithm. During the scheduling process, there can be more than one links

having nonzero service token. A link selection algorithm is further designed for the BS to

determine the order of the links to schedule. The selection can be based on the four criteria:

random, min interference, nearest to BS and farthest to BS. The scheduling algorithm is

evaluated in terms of scheduling length, channel utilization, and transmission delay, with

comparison on the four selection criteria. However, optimization by the joint routing and

scheduling is not investigated in their work.


1.3.3 Routing and Centralized Scheduling

As the problems of routing and scheduling are tightly coupled for WiMAX mesh networks,

joint routing and scheduling design has been an interesting research topic.

An early investigation on disjoint routing and scheduling for WiMAX mesh networks

is reported by Z. Haas et al in [15]. They developed an interference-aware framework for

WiMAX mesh networks with the goal of achieving high utilization. The proposed framework

includes an interference-aware route construction algorithm and an enhanced centralized

scheduling scheme. Both traffic load demand and interference conditions are taken into

account in the design. The interference-aware routing algorithm is designed by considering

interference conditions in the network. A blocking metric B(k) is defined for a given route

from the Mesh BS toward an SS node k, which is used to model the interference level of

routes in the networks. Let blocking value b(j) of a node j, as the number of interfered nodes

when node j is transmitting. Then blocking metric B(k) of a route to the node k will be the

summation of the blocking values of nodes that transmit or forward packets along the route.

Let S(k) denote the set of nodes on the considered route from BS to node k.

B(k) =∑

j∈S(k)

b(j) (1.3)

According to a predefined sequence of nodes joining the mesh network, each time the node

is processed to select a sponsor node with minimum blocking metric. A routing tree can be

constructed consequently.

With the interference-aware routing tree, an interference-aware scheduling is proposed

which exploit concurrent transmission opportunity to achieve high spectral utilization. The

order of the links selected to reserve a time slot at each allocation iteration is based on the

traffic demand. For each selected link, after it reserves a time slot at an allocation iteration,

the traffic demand for this link decreases by one. At the same time, the scheduling algorithm

find the maximum number of concurrent transmissions that are allowed in this slot. The

iterative allocation continues until there is no unallocated traffic demand.


The above interference-aware routing and scheduling algorithms are simulated and com-

pared to a basic scheduling algorithm and an optimal solution. In the basic scheduling

algorithm, only one transmission is scheduled at each time slot, therefore spectral reuse is

not utilized. The optimal solution is optimized with objective function of maximal network

throughput. The proposed routing and scheduling schemes are reported to achieve near-

optimal performance and significantly outperforms the basic algorithm. However, only small

size networks are investigated and full fairness is ensured in the optimal solution.

In [16] the neighbor degree metric is used to construct routing tree. The neighbor degree

metric of a node is set as a function of its transmission power and the neighbor number of this

node. A cross-layer design architecture is proposed, in which transmit power control, routing

construction and centralized scheduling are taken into account. However, it is observed there

is only a minor (3%-7%) improvement on network throughput by employing the cross-layer

design.

T. Tsai and C. Wang investigated the routing and admission control in WiMAX mesh

networks [17]. They proposed a new routing method with Shortest-Widest Efficient Band-

width (SWEB) as metric. The SWEB for a path is defined as follows. Let pi,j and Ci,j

denoted the packet error rate and capacity over a link (i,j), respectively. Let h denote the

hop account of the path. Then the SWEB metric for a path P can be calculated by [17]:

SWEB =min{Ci,j(1− pi,j)|(i, j) ∈ P )}

2h. (1.4)

A node will select the path from itself to BS with the largest path SWEB. They also proposed

a token bucket based admission control method. However, centralized scheduling is not

exploited. Another problem on the routing algorithm is that a node may be required to have

several sponsor nodes due to the path selections of its children nodes.

Cao proposed a joint routing and scheduling algorithm for WiMAX network under a new

fairness constraints [21]. Shetiya investigated QoS support in 802.16 mesh networks with

centralized scheduling and shortest path routing algorithm [22]. However, only centralized

routing algorithms are considered and spectral reuse is not allowed.


Due to the capabilities of direction antenna in increasing network throughput, applications

of direction antenna to the general wireless mesh networks have been widely studied. Cao et

al investigated the cross-layer design for 802.16 multi-hop wireless backhaul networks [18].

They considered the joint optimal design of routing, MAC scheduling and physical layer

resource allocation. Beamforming antenna arrays are equipped at the physical layer. They

introduced the notion of transmission set (TS) to separate the physical layer operations

from those at the upper layers. TS is defined as a set of links in the network that can

be simultaneously active at any given time. Due to the computation complexity of finding

an optimal solution, a column generation approach is employed to identify TSs. Near-

optimal performance in term of scheduling cost is reported. However, the cross-layer design

is observed to be complex and may require a long time to converge.

1.4 Routing Algorithms

In this and the next section, we will investigate routing and scheduling algorithms for

WiMAX mesh networks, with the target application of using WiMAX mesh networks to

network cellular BSs and gateways to the Internet. The gateways of general cellular net-

works can be mobile switching center (MSC) in GSM networks or MSC and Serving GPRS

Support Node (SGSN) in WCDMA networks. In this network architecture, the gateways act

as 802.16 mesh BSs in the 802.16 backhaul network. They manage the backhaul network

and allocate bandwidth to the SSs. The cellular BSs act as 802.16 SSs in the backhaul

network. In the left of the Chapter, the cellular BSs will be called SSs for simplicity. The

SSs forward aggregated traffic from the mobile users to Internet in the uplink direction and

deliver traffic to the mobile users from the Internet in the downlink direction via the 802.16

BSs. For simplicity, only uplink traffic with QoS requirement is considered in this Chapter.

However the approaches of routing and scheduling can be applied to bi-direction traffic. It

is assumed that the backhaul topology and traffic demands from the SSs will not change

frequently. Therefore, the frequency of updating traffic routes and scheduling will be low. It

is feasible and beneficial to use high performance algorithms such as the optimal centralize

scheduling and routing algorithms.

1.4. ROUTING ALGORITHMS 21

1.4.1 Classification

As IEEE 802.16 standards specify the MAC and PHY layer protocols, routing protocols are

outside the scope of the standard work. We can identify the following ways of solving routing

and scheduling (bandwidth allocation) problems for WiMAX mesh networks:

• Centralized routing and centralized scheduling (CRCS): all the traffic, position informa-

tion of the stations are sent to the BSs. The BSs jointly solve the routes and schedule

problems, and send the routes/schedule information back to the SSs.

• Distributed routing and centralized scheduling (DRCS): SSs distributedly find their

routes to the BSs. After the routes are found, the SSs send their information to the

BSs. The BSs determine collision-free transmission schedule.

• Distributed routing and distributed scheduling (DRDS): Stations distributively find

their routes to the BSs. After SSs find their routes to BSs, they work with their next-

hop stations toward the BSs to determine collision-free transmission schedules in a

distributed way.

• Hybrid routing and scheduling (HRS): Both routing and scheduling algorithms can be

implemented in either a distributed or a centralized way.

In WiMAX mesh networks, distributed scheduling can be used by SSs with their neighbor

SSs to reserve time slots. However the success of reserving time slots will depend on the

availability of free time slots not reserved by the centralized scheduling and other neighbor

stations. As centralized scheduling has priority over distributed scheduling in WiMAX, it

will be easier to provide time slots reservation for end-to-end QoS support. Therefore we

will consider only centralized scheduling in this Chapter.

Centralized scheduling algorithm will work together with either centralized or distributed

routing algorithms. For the centralized routing algorithm, routing, bandwidth allocation and

scheduling problem can be jointly investigated. We will formulate the joint routing, allocation

and scheduling problem with an optimal mathematical model, which will be described in the

next section. The input to the optimal model from the routing point of view will be network


connectivity matrix CM , CMij = 1 if and only if station i is in the communication range

of station j. After the optimal problem is solved, the route from an SS to a BS will be

determined.

For the centralized scheduling and distributed routing algorithms, routes from SSs to

BSs will be determined first. Then we can also derive network connectivity matrix CM for

distributed routing. However in this case, the station connectivity matrix CM will not only

be determined by the communication ranges but also the traffic routes. We have CMij = 1

if and only if the link eij between station i and station j is in any route of SSs to BSs.

Then the connectivity matric CM can be inputted to the optimization model for centralized

scheduling. In the next subsection, we will introduce several distributed routing algorithms.

1.4.2 Distributed Routing

In the WiMAX mesh networks, the BSs will periodically broadcast network configuration

messages, which are further forwarded by the neighbor stations. The forwarding process

continues toward network edge until the pre-defined maximum number of hops is reached.

For each forwarding neighbor stations, it will add the information of hops from itself to the

BSs. Although routing algorithm is not specified in the standards, the distance information

together with other local information can be utilized by a new SS to find a route to a BS.

For a new SS to join the WiMAX mesh network, it is required to listen to the network con-

figuration/synchronization from its neighbor stations. After it hears network configuration

message at least twice from a station, it can send request to join the network through this

station. With the available local information, such as the signal strength, distance between,

the new station can select which station to be its sponsor station to a BS. Therefore the

distributed routing algorithm can be reduced to find a sponsor station for a route toward a

BS. We simply give four methods.

• Random Selection (RS): In this method, a new station will choose a neighbor station

as its sponsor station randomly among the candidate stations with the same minimum

number of hops to a BS.

1.5. OPTIMAL BANDWIDTH ALLOCATION AND SCHEDULING ALGORITHM 23

• Minimum node ID (MID): Among all the neighbor stations with the same minimum

number of hops to any BSs, the station with minimum node ID will be selected as the

sponsor station.

• Maximum Signal Strength (MSS): The station with maximum average signal strength

will be selected as the sponsor station among the candidate stations, which can achieve

higher transmission data rate.

• Minimum aggregate traffic (MAT): To balance traffic routed over the neighbor stations,

a new station can also choose the station with minimum aggregated traffic load among

the candidate stations.

1.5 Optimal Bandwidth Allocation and Scheduling Algorithm

In the previous section, we described the distributed routing algorithms. Both central-

ized and distributed routing algorithms can work with bandwidth allocation and scheduling

scheme to provide optimal network performance. The required input from both centralized

and distributed routing algorithms are the connectivity matrix defined in the previous sec-

tion. The traffic routing and scheduling problem can be solved separately. Next we will

formulate the bandwidth allocation model to allocate bandwidth to wireless links. Then

time slots will be scheduled for the wireless links to achieve interference-free transmissions.

In the currently considered problem, we assume that all stations operate in one operation

frequency band with one omni-antenna. In practice, directional antennas and multiple op-

eration frequency bands can be employed to maximize network capacity.

In the network, all of the WiMAX stations are connected by wireless links to the stations

in their communications ranges. And BSs have wireline links to fixed networks providing

Internet access. The wireline links have finite capacities. The problems are then how to

efficiently and economically route the traffic aggregated in the individual SSs to the fixed

networks and allocate required bandwidth in each station on the traffic routes.

As network performance can be optimized in different ways, we choose two kinds of


performance metrics for optimization. The first is the maximum delivered traffic to Internet,

denoted by Ts. Ts is the sum of the delivered traffic from individual SSs Ti, Ts =∑

Ti.

As maximizing delivered traffic may starve the traffic from the SSs located far away from

the BSs, we also consider the second performance metric λ to ensure fairness. In order

to provide fair delivery of traffic to fixed network, we request the ratios of successfully

delivered traffic from the SSs to fixed networks to their traffic demands are equal to λ. Then

the optimization objective is to maximize proportion λ to achieve delivery fairness. If the

constraints on the capacities of wireline links to fixed networks is released, the bottleneck

will be inside the WiMAX backhaul. We set the optimization objective to maximize the

overall traffic delivered to fixed networks. The variables used in the optimization model are

described in Table.1.1.

Table 1.1: Variables description.

V The set of all the WiMAX stations.N Number of WiMAX stations including SSs and BSs.E The set of all the wireless links.Rij Capacity of wireless link from station i to station j.Qi Traffic demands from station i.Ui Capacity of wireline link from station i to the fixed networks.

CMij Station connectivity matrix,1 if a station i is in transmission range of station j, otherwise 0.

Iij The set of interference links to the link eij,iij ∈ Iij if iij interferes with eij.

Xij Traffic routed from station i to station j.Fi Traffic routed from BS i to fixed networks.

With constrained wireline link capacity, the delivered traffic will not only be limited by

the capacity of wireless networks, but also these of wireline links. For the wireless links, to

achieve interference-free transmissions, if a station (say SS t) is transmitting to a neighbor

station (say SS r) in one slot, there should be no other transmissions on the interfering links.

An interfering link euv of a link eij means that either station u or v is in the interference

range of station i or j. We denote the set of interfering links of a link eij by Iij. Combining

the constraints on the capacities wireline and wireless link and the interference-free wireless

transmissions, we can formulate the optimization problem in (1.5).

1.5. OPTIMAL BANDWIDTH ALLOCATION AND SCHEDULING ALGORITHM 25

Maximize: λ

Subject to:

CMij=1∑j∈V

(Xji −Xij) + λQi = 0, ∀i ∈ V, Ui = 0;

CMij=1∑j∈V

(Xji −Xij) + λQi − Fi = 0, ∀i ∈ V, Ui > 0;

Xij

Rij+

ekl∈Iij∑k,l∈V

Xkl

Rkl≤ 1, ∀i, j ∈ V, CMij = 1;

Xij ≤ Rij, ∀i, j ∈ V ;

Xii = 0, ∀i ∈ V ;

Fi ≤ Ui, ∀i ∈ V.

(1.5)

If we assume that the wireline link capacity is not constrained compared to the wireless

link data rate, the bottleneck will be inside the wireless networks. We can consider the

optimization objective to be the delivered traffic without fairness consideration. Similar to

the optimization model under fairness constraint, we can formulate the problem in (1.6).

Maximize:∑i∈V

Ti

Subject to:

CMi,j=1∑j∈V

(Xji −Xij) + Ti = 0, ∀i ∈ V, Ui = 0;

CMi,j=1∑j∈V

(Xji −Xij) + Ti − Fi = 0, ∀i ∈ V, Ui > 0;

Xij

Rij+

ekl∈Iij∑k,l∈V

Xkl

Rkl≤ 1, ∀i, j ∈ V,CMij = 1;

0 ≤ Xij ≤ Rij, ∀i, j ∈ V ;

Xii = 0, ∀i ∈ V ;

Fi ≥ 0, ∀i ∈ V.

(1.6)

As we have imposed the interference constraint in the optimization model, it is easy to

derive collision free scheduling to achieve the allocated bandwidth. A simple algorithm has


been presented in [20].

1.6 Simulation Results

In this section we will present some typical simulation and analytical results. The WiMAX

mesh network is configured with 20 and 40 stations. For both network size, we have 2 base

station configurations, 1 base station (station 1) and 2 base stations (station 1 and 2), and

the left stations are set as SSs. The network area is square with size of 3000m×3000m for 20

stations and 5000m× 5000m for stations respectively. All the stations are randomly located

in the network area. They have the same transmission range of 1500m and interference

range of 3000m. The network topology used in the simulations with station IDs and station

connectivities are shown in Fig.1.7 and Fig.1.11. The data rates of all the wireless links

are distance dependent. The relationship of distance d and link data rate R used in the

simulation is shown in Table.1.2.

Table 1.2: Station distance and link data rate.

d (km) <0.25 <0.5 <0.75 <1 <1.25 <1.5R (Mbps) 75 60 45 30 15 10

Typical simulation and analytical results for network with 20 stations are shown in Fig.1.8

and Fig.1.9. In the figures, the routing algorithms represented by algorithm 1 to 5 are

explained in Table.1.3. Each node has random traffic in the range of [0,22] Mbps to be

delivered to the Internet through BSs. Fig.1.8 presents the normalized maximal λ versus

different routing algorithms. The normalized maximal λ are obtained by dividing maximal λ

from optimization model (1) for the different routing algorithms by the maximal λ value. It

can be observed that for both base station configurations, centralized routing algorithm can

achieve more than 30% performance improvement. The four distributed routing algorithms

achieves similar performance while the MSS algorithm is a little better. As an example,

Fig.1.10 shows the final traffic routes from the SSs to the BS (station 1) with centralized

routing algorithm.

1.6. SIMULATION RESULTS 27

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

3000

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19 20

Figure 1.7: Network topology with 20 stations.

Table 1.3: Routing Algorithms.

Algorithm 1 Centralized routing algorithm.Algorithm 2 random routing.Algorithm 3 minimal station ID (MID) algorithm.Algorithm 4 maximal signal strength (MSS) algorithm.Algorithm 5 minimal aggregate traffic (MAT) algorithm.

Fig.1.9 presents the results of maximal successfully delivered traffic versus different routing

algorithms for 20 stations network, which are obtained through optimization model (2). It

is observed that joint centralized routing and scheduling does not achieve any performance

advantages. This is because that the network bottleneck is in the wireless links. Only SSs

close to BSs can successfully deliver their traffic to Internet, while SSs far away from BSs

will be starved. It is noted that no fairness constraints are put in the optimization model

(2). As a result, the maximal λ obtained from (2) is zero for all the investigated routing

algorithms.

Similarly, the results on the maximal λ and the maximal successfully delivered traffic

for network size of 40 stations are presented in Fig.1.12 and Fig.1.13. Similar performance

improvements can also be observed for both base station configurations.


1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Routing algorithm

Max

imal

λ

(a) One base station (station 1)

1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Routing algorithm

Max

imal

λ

(b) Two base stations (station 1 and 2)

Figure 1.8: Normalized maximal λ versus different routing and allocation algorithms withnetwork size of 20 stations.

1 2 3 4 50

10

20

30

40

50

60

70

80

Routing algorithm

Max

imal

suc

cess

fully

del

iver

ed tr

affic

(M

bps)


1 2 3 4 50

10

20

30

40

50

60

70

Routing algorithm

Max

imal

suc

cess

fully

del

iver

ed tr

affic

(M

bps)


Figure 1.9: Maximal successfully delivered traffic versus different routing and allocationalgorithms with network size of 20 stations.

1.7 Conclusion

Routing and bandwidth allocation are critical for QoS support over WiMAX networks.

In this Chapter, we investigate different routing and bandwidth allocation algorithms in

WiMAX mesh networks. We proposed several distributed routing algorithms, which are

compared with a centralized routing algorithm. The connection matrix of the stations are

used to solve a bandwidth allocation problem, The problem is formulated by a mathematical

optimization model. The solution developed from the optimization model can be used to

schedule the time slots for stations transmission without collision. Simulation and analytical

1.7. CONCLUSION 29

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

3000

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19 20

1

2

3

4 5

6

7

8

9

10

11

12

13

14

15

16

17

18

19 20

Figure 1.10: Traffic routes determined by centralized routing algorithm with 2 base stations.

0 500 1000 1500 2000 2500 3000 3500 4000 45000

500

1000

1500

2000

2500

3000

3500

4000

4500

1

2

3

4

5

6 7

89

10

11

12

13

14

1516

1718

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

34

35

36

37

38

3940

Figure 1.11: Network topology with 40 stations.

results with medium size networks show that joint centralized routing and scheduling can

achieve the best performance with over 30% improvement. The improvement is achieved

at the cost of increased network management complexity. In the future work, the overhead

brought by centralized routing algorithm will be evaulated, and the benefit of the perfor-

mance improvement will be justified. In addition, it is interesting to consider the problem


1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Routing algorithm

Max

imal

λ


1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Routing algorithm

Max

imal

λ


Figure 1.12: Normalized maximal λ versus different routing and allocation algorithms withnetwork size of 40 stations.

1 2 3 4 50

10

20

30

40

50

60

Routing algorithm

Max

imal

suc

cess

fully

del

iver

ed tr

affic

(M

bps)


1 2 3 4 50

10

20

30

40

50

60

Routing algorithm

Max

imal

suc

cess

fully

del

iver

ed tr

affic

(M

bps)


Figure 1.13: Maximal successfully delivered traffic versus different routing and allocationalgorithms with network size of 40 stations.

of increasing network capacity by using multiple radio interface over multiple frequency

channels in WiMAX mesh networks.

Acknowledgment

The work reported in this Chapter has been partially funded by EPSRC through the HIP-

Net project, United Kingdom, and by the European Union through the Welsh Assembly

Government, whose funding and support are gratefully acknowledged.

1.7. CONCLUSION 31

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