Proceedings of IEEE 2008 6th National Conference on Telecommunication Technologies andIEEE 2008 2nd Malaysia Conference on Photonics, 26-27 August 2008, Putrajaya, Malaysia

Constructing Routing Tree Centralized Schedulingusing Multi-Channel System in 802.16

Ali Al-Hemyari, Chee Kyun Ng, Nor Kamariah Noordin, Alyani Ismail and Sabira KhatunDepartment of Computer & Communication Engineering, Faculty of Engineering, University Putra Malaysia.

Emails: alhemyari_agyahoo.com, {mpnck, nknordin, alyani, sabira}geng.upm.edu.my

Abstract- The IEEE 802.16 standard defines WiMAX(Worldwide Interoperability for Microwave Access) meshnetwork, using the base station (BS) as a coordinator forcentralized scheduling. This paper proposes a centralizedscheduling algorithm by constructing routing tree in WiMAXmesh network, which introduces the cross-layer conceptbetween the media access controller (MAC) and the networklayers. The Interference, Hop-Count, Spatial Reuse and Qualityof Services (QoS) guarantee are considered. In our scheme, eachnode has one transceiver and can be tuned between multiplechannels, intending to eliminate the secondary interference. Theresult of analysis shows this algorithm greatly improves thelength of scheduling, channel utilization ratio (CUR) and averagetransmission scheduling.

Keywords-802.16, Routing Tree, Scheduling, Multi-Channel.

I. INTRODUCTION

WiMAX (Worldwide Interoperability for MicrowaveAccess) [1] is an emerging wireless communication systemthat is expected to provide high data rate communications inmetropolitan area networks (MANs). In the past few years, theIEEE 802.16 working group has developed a number ofstandards for WiMAX [2]. The first standard was published in2001, which aims to support the communications in the 10-66GHZ frequency band. In 2003 IEEE 802.16a wasintroduced to provide additional physical layer specificationsfor 2-11 GHz frequency band. These two standards werefurther revised in 2004 (IEEE 802.16-2004) named 802.16dwhich does not support mobility.

Recently, IEEE 802.16e has also been approved as theofficial standard for mobile applications. According to theIEEE 802.16 standards, WiMAX technology supports twooperation modes: PMP (Point to Multipoint) and Mesh ornamed Relay Mode or MPMP (Multipoint Point toMultipoint). A WiMAX PMP network aims at providing last-mile access to broadband Internet Service Provider (ISP).WiMAX base stations can offer greater wireless coverageof about 5 miles, with LOS (Line of Sight) transmissionwithin bandwidth of up to 70 Mbps. In the Mesh Mode,several Mesh SS (Subscriber Station) can constitute a smallmultipoint to multipoint wireless connection, without specificuplink and downlink sub-frames. It may commendablysupport the NLOS (Non Line of Sight) transmission,adopting the Multi - Hop technology based on packetswitching to keep away from obstruction, interference andcongestion. In particular, the mesh modes of the WiMAX

standards direct communication between SS and hence reducedead zone while increasing the global throughput.

One of the major problems in the wireless communicationis dealing with interferences from transmission of theneighbouring nodes. Previous researches avoid the effectof the interference by constructing a routing tree and / orscheduling algorithms to or made a cross layerbetween the layers, Network and M\AC (Media AccessControl) Layers or between the M\AC and Physical Layers.Most of current researchers work is based on single channel.Wei [3] adopted interference-aware cross-layer to increase thethroughput but did not consider bandwidth request. Harish andVinod [4] produced algorithm for routing and centralizedscheduling and found that the shortest path routing tree providegood performance in reducing interference. A schedulingscheme using multi-path routing is presented in [5] which canimprove the system performance, but it is not well conformedto the WiMAX standard. M. Peng et al [6] designed crosslayers between Network, M\AC and Physical layers. They alsoproposed multi-channel scheduling algorithm but he did notfocus on routing tree [11]. Bo Han [7] designed schedulingalgorithm for 802.16 by considering the relay model usingsingle channel but did not consider bandwidth request [7]. Lu[8] took into account both interference and bandwidth requestfor each node during the construction of tree routing butconsidered only the interference at the receiver [8]. Du [9]constructed multi-channel system in the scheduling algorithmbut did not focus on routing tree. Jiao [10] designed centralizedscheduling, routing tree by constructing under multi-channelsystem but did not consider bandwidth request for each node.Sayenko [12] proposed a scheduling algorithm considering thebandwidth request and QoS but in PMP WiMAX.

The rest of this paper is organized as follows. In section II,we will present an overview of scheduling in WiMAX meshnetwork. In section III, we will describe the benefit of usingMulti-Channel system. In Section IV we will explain in detailthe construction of routing tree using multi-channel assignmentand centralized scheduling. Section V presents the expectedsimulation result and we conclude the paper in Section VI.

II. SCHEDULING IN WIMAX MESHNETWORKS

In the WiMAX mesh mode, there are two schedulingmethods: distributed scheduling and centralized scheduling.Distributed scheduling is also classified in two kinds:coordinated and uncoordinated distributed scheduling. Thecoordinated one used for coordinating their transmission

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extended two hop neighboring but the uncoordinatedscheduling used for temporary burst between a pair ofneighboring nodes like an ad-hoc network [12]. Thecentralized scheduling used to schedule the transmission fromthe BS to the SSs (downlink) or vice versa (uplink). In thecentralized scheduling, every SS send its bandwidth requestwith the message MSH-CSCH (Mesh CentralizedScheduling): request. After the BS receives this requestmessage, it will collect the resource request and send aresource allocation to each SS through the MSH-CSCH: grant.There is another message used for sending the routing tree andscheduling configuration to all SS, it is MSH-CSCF (MeshCentralized scheduling configuration).

We focus only on the centralized scheduling because ourtraffic is between the SSs to the BS and we will consider onlythe uplink traffic because it is more complicated than thedownlink traffic. In the downlink, the BS is the only one thattransmits. The packets are broadcasting to all SSs and only theSS picks up the packets that destined to it. On the contrary, inuplink traffic a lot of SSs transmit to BS and the BS allocatetime slot to each SS that will be transmit data [12]. The mainobjective of scheduling is transmitting the data to /or from theBS in a collision free transmission.

III. BENEFIT OF MULTI-CHANNEL SYSTEM

Wireless network uses a shared medium to communicate toeach other. In single channel TDD (Time Division Duplex)network any transmission must follows the principle that theremust be only one receiver among the neighborhood of atransmitter and there must be only one transmitter among theneighborhood of a receiver [13]. So that there is two type ofthe interference [14]:

* Primary interference: occurs when a station do morethan one thing (transmits or receives) at one time.

* Secondary interference: occurs when a station R whointends to receive from station T is within the rangeof another transmitter whose transmission is notintended for R.

Obviously the primary interference can not be eliminatedbecause we are using single transceiver, but the secondaryinterference, can be eliminate by using multi-channel singletransceiver in each time slot. Different SSs can work ondifferent channels at same time to eliminate the secondaryinterference, so that we will have a lot of SSs working at sametime. The benefit of multi-channel system assignmentalgorithm is presented in Figure 1. Figure l(a) shows therouting tree and mesh topology, the solid line represent therouting tree and dashed line represent the mesh topology whileFigure 1(b) shows the channels assignment on the routing tree,that linked SS1 to BS (link 1) and link SS2 to BS using thesame channel (denoted as link Number 1) because they haveprimary interferences and the same goes for for links SS3 toSS1, SS4 to SS1 (denoted as link Number 2). Link SS3 to SS1and link SS5 to SS2 use different channel because they havesecondary interference and similarly for links SS4 to SS1, SS6

to SS5. Links SS5 to SS2 and SS6 to SS5 use the samechannel because they are adjacent (denoted by link Number3).

(a) Routing tree and mesh topology

(b) Channels assignment

Figure 1. Multi-channel system assignment algorithm.

IV. CONSTRUCTING ROUTING TREE CENTRALIZEDSCHEDULING AND USING MULTI-CHANNEL SYSTEM

IN 802.16d.

This algorithm consists of three steps: Constructing routingtree, channel assignment strategy algorithm and centralizedscheduling algorithm.

A. Constructing Routing Tree

The primary focus of our algorithm is to provide scalablerouting in the presence of static node. The network topology isa tree rooted to the BS and the problem is to select the bestroot for the all SSs (source) to the BS (destination). One of themajor problems in the wireless communication is dealing withinterferences from transmission of the neighbouringnodes. In this procedure we need to take into account twoparameters, i.e the interference from the neighbouring node(number of neighbouring nodes) and number of packets foreach node along the root to the BS. In developing theprocedure we will also apply four rules as follows:

1. Always select the sponsoring nodes from the upperlevel (Minimum hop-count to the BS).

2. Number of blocked node for each node (number ofneighbouring nodes).

3. Number of packets for each node4. Blocking metric for each node: number of blocked

node multiply by number of packets.

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After that we calculate the blocking metric for each path bysumming the blocking metric for all the nodes along the pathto the BS. Finally the path with minimum blocking metric willbe selected. The routing tree construction algorithm can beillustrated as shown in Figure 2.

Figure 2(a) depicts the WiMAX mesh topology. In Figure2(b) we assume all the nodes have one packet, so that theconstruction of routing tree will depend only on theinterference from neighbouring node. In Figure 2(c) weassume random number of packets per each node (from 1 to 3packets).In Figure 2(b) and (c) the solid line represents therouting Tree while the dashed line represents the meshtopology.

(a) WiMAX mesh topology

(b) Consider only the interferences for each node.

(c) Consider the interference and number of packets.

Figure 2. Routing tree constructing.

B. Channel Assignment Algorithm

We will used the same techniques that we explained insection 3 in a multi channel system using single transceiver inwhich each node can support one channel in each time slot.However, we can tune it to another channel in another timeslot. The adjacent node should use the same channel so thatare connect to each other. By using our algorithm we caneliminate the secondary interference, hence reducing thelength of scheduling, CUR (Channel Utilization Ratio) and theaverage delay transmission because a lot of nodes transmit atsame time slot (using different channel). But we can notreduce the primary interference because we are assumingsingle transceiver.

The switching delay which is occurred when a transceivertuned between the channels is ignored because it is too small.We first should assign the channels on the edge set of therouting tree. Starting with the SS nearest to the BS followingthe node ID (Identifier) we can start the channel assignmentalgorithm with the ID equal 1 which the nearest to the BSbefore choosing the next ID. The nodes that have primaryinterference take the same channel but the nodes that havesecondary inferences used another channel.

C. Centralized SchedulingAlgorithm

In the 802.16, time slot allocation for each SS is controlledby centralized scheduling algorithm. We propose a centralizedscheduling algorithm considering the interference, multi-channel system, QoS and the relay model (each noderesponsible to transmit its packets and its child packets). Inorder to design according to IEEE 802.16, we make thefollowing assumptions:

* No node simultaneously transmits and receives data.* Nodes can not send or receive data in the signal

range of communicating nodes because ofinterference.

* The signal of a node can only cover the range of asingle-hop neighborhood.

* Non-interference links can communicateconcurrently.

* The traffic is always between the Mesh SS andMesh BS (uplink traffic).

* Each node can support only one channel in each timeslot.

* Node can transmit one packet in each time slot.* The buffer in each node work in FIFO [16] (First In

First Out) for sending the data from the node queue.

There are a lot of selection criteria to select the links to sendthe data: random, minimum interference, Nearest to BS (hopcount) and Farthest to BS. We used Nearest to BS algorithmbecause this common criteria use by other researchers.Furthermore since all the SSs send the data to the BS (relayedby the nodes that near the BS)there will be a lot of traffic nearthe BS so that using Nearest to BS algorithm give high priorityto the nodes near the BS to send the data. We will select the

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nodes in the same level (same hop-count to the BS) dependingon:

1. Maximum Number of packets.2. Maximum number of interference neighbors.3. Smallest Node ID.

First we select the node in the same level depend on linkNumber 1,~if all the nodes have same number of packets thenwe shift to link Number 2, if all the nodes have same thenumber of interfering neighbors then we shift to link Number3 Then we select the nodes with smallest Node ID number tosend the data before the other in same level. Note that we haveinitially set the counter equal to the number of packets foreach node; therefore when SS send data its counter is reducedby one and receiver node counter increased by one.

V. SIMULATION

A. Simulation Metrics

Three performance parameters are used for evaluating the

system performance:* Reduced length of scheduling the number of time

slots in the cycle).* Improved CUR (Channel Utilization Ratio): the ratio

between the number of occupied time slots (no. ofpacket * no. of hops to the BS for each node) and thetotal number of available time slots (the length ofscheduling multiplied by the number of nodes).

* Reduced average delay in transmission: the numberof time slots between the time slot when a packet istransmitted by the source Subscriber Station and thetime slot when the same packet arrives at thedestination.

B. Simulation SetupWe have proposed the construction of routing tree

centralized scheduling using multi-channel single transceiverin 802.16 considering the relay model in the schedulingalgorithm. We compare our result with the same algorithmusing single channel system. We use MatLab simulator toevaluate the performance metrics. We locate the BS at thecentre of the simulation area. We obtain the result in twoways: the first one all the SSs have equal traffic, the secondone the traffic for each SS selected randomly from 1 to 3packets. The number of nodes is increase from 10 to 120 withstep of 10.

130~

110

-ir

80

No. ofpackets= 1

-------------- -------------- -------------- ---------- --------------------------------- - --------------------------

-------------- -----------------------

- -T

-------------- -------------- ------------

-----------------------------------------------

------------ --- -------------- --------------- ---------------

No. of Nodes

(a) Number of packets 1 for each SS.

No. of packets =1 to 3

--- MulliO

330--~~~~~~~~~~~~--------L--24 0- -- -- --7

-- ----_- --_-- --_--

4060 60 70 60 90 100 110 120No. of Nodes

(b) Number of packets= (I to 3).Figure 3. The Length of Scheduling.

Figure 4 present the result about the average transmissiondelay, Figure 4(a) shows result when all the SSs have onepackets; Figure 4(b) presents the result when all the SSs haverandom number of packets. It is observed that we havereduced the average transmission delay. Notice that thetransmission ranges for the nodes are fixed.

30

25

165

1 0

C. Expected Simulations Result

This proposal is at preliminary stage, hence no simulated

results are available. However based on the analytical study

we anticipate the results as follows. Figure 3 gives the result

for the length of scheduling. Figure 3(a) gives the result when

all the SSs have one packets; Figure 3(b) gives the result when

all the SSs have random number of packets. The simulation

result shows that we have reduced the length of scheduling. In

No. of Packets =

soNo. of Nodes

(a) Number of packets for each SS.

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- - Multi-OHJ-- ---Single-COH----

--- multi-OH--in---gle--- --O H--

--------------------------

I

I--------------I-----

1-1

.o

No. of packets = to 3130 F F F

---Multi-CH~20 H --- -----Single-CH

11~~~~~~~~~~~-------~~~~~~~~~~~~; ---

100 --- --r -1- n--/ T----- ----- b- --r --

90n

8~~~~~~~~~~~N ofNje_

--

-- I-

,~~~~~~~~~~o of Nodes

(b) Number of packets= (I to 3).

Figure 4. Average Transmission Delay.

VI. CONCLUSION

We have proposed a routing tree centralized scheduling usingmulti-channel single transceiver for IEEE 802.16. We considerthe interference neighboring nodes and traffic load per eachnode in constructing the Routing Tree and considering theinterference neighboring nodes, traffic load, hop-count, nodeID and the relay model in the scheduling design. We compareour proposed scheme with a single-channel schedulingalgorithm. Our findings show that we have improved thesystem performance through reduced length of scheduling,reduced average delay in transmission scheduling andimproved CUR (Channel Utilization Ratio).

REFERENCES

[1] IEEE Standard 802.16-2004, "IEEE Standard for Local andMetropolitan Area Networks - Part 16: Air Interface for FixedBroadband Wireless Access System," 1 Oct. 2004.

[2] The WiMAX Forum, http://www.wimaxforum.org/[3] H. Y. Wei, S. Ganguly, R. Izmailov, Z. Haas, "Interference-aware IEEE

802.16 WiMax mesh networks," IEEE 61st Vehicular TechnologyConference (VTC), vol. 5, pp. 3102 - 3106, 29 May - 1 June 2005.

[4] H. Shetiya and V. Sharma, "Algorithm for Routing and Centralizedscheduling in IEEE 802.16 Mesh Networks," IEEE WirelessCommunications and Networking Conference (WCNC), vol. 1, pp. 147- 152, 2006.

[5] Y. Cao, Z.M. Liu and Y. Yang, "A Centralized Scheduling AlgorithmBased on Multi-path Routing in WiMAX Mesh Network," IEEEInternational Conference on Wireless Communications, Networking andMobile Computing (WiCOM), pp. 1 - 4, Sept. 2006.

[6] M. Peng, Y. Wang, and W. Wang, "Cross-layer design for tree-typerouting and level based centralized scheduling in IEEE 802.16basedwireless mesh network," IET Communications, vol. 1, pp. 999 - 1006,Oct. 2007.

[7] B. Han, W. J. Jia and L. D. Lin, "Performance evaluation of schedulingin IEEE 802.16 based wireless mesh networks," Elsevier Journal ofComputer Communications, vol. 30, pp. 782 - 792, 2007.

[8] L. Yanbin and Z. Guoqing, "Mainting Routing Tree in IEEE 802.16Centralized Scheduling Mesh Networks," IEEE ComputerCommunications and Networks conference (ICCCN), pp. 240 - 245,2007.

[9] P. Du, W. Jia, L. Huang and W. Lu, "Centralized Scheduling andChannel Assignment in Multi-Channel Single-Transceiver WiMax MeshNetwork," IEEE Wireless Communications and Networking Conference(WCNC), pp. 1734 - 173 9, 2007.

[10] W. Jiao, P. Jiang, R. Liu and M. Li, "Centralized Scheduling TreeConstructing under Multi-channel IEEE 802.16 Mesh Networks," IEEEGlobal Telecommunications Conference (GLOBECOM), pp. 4764 -4768, 2007.

[11] M. Peng, M. Li and W. Wang, "A Unified Architecture and the TreeLevel Based Scheduling Technique for Multi-radio Multi-channel inIEEE 802.16 Standard Based Wireless Mesh Networks," IEEECommunications and Networking in China (ChinaCom), pp.1 - 6, 2007.

[12] S. Alexander, A. Olli and H. Timo, "Scheduling Solution for the IEEE802.16 Base Stations," Elsevier Journal of Computer Communications,vol. 52, pp. 96-115, 2008.

[13] W. kitti and G. Aura, "Packet scheduling for QoS support in IEEE802.16 broadband wireless access systems," International Journal ofCommunication Systems, vol.16, pp. 81 - 96, 2003.

[14] J. Tao, F. Liu, Z. Zeng and Z. Lin, "Throughput Enhancement inWiMAX Mesh Networks Using Concurrent Transmission," IEEEWireless Communications, Networking and Mobile Computing,Conference, vol. 2, pp. 871 - 874, 2005.

[15] S. Ramanathan and E. L. Lloyd, "Scheduling Algorithms for MultihopRadio Networks," IEEE/ACM Transactions on Networking, vol. 1, pp.166-177, 1993.

[16] G Kamal, A. Mounir and G. Annie, "How Many Traffic Classes do weNeed in WiMAX," IEEE WCNC, 2007

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