for FBWA systems that can support multimedia services [2]. In 802.16 protocol stack, the medium access control layer. (MAC) supports both point-to-multipoint ...
Proceedings of the 2009 IEEE 9th Malaysia International Conference on Communications 15 -17 December 2009 Kuala Lumpur Malaysia
Cross-Layer Routing and Scheduling for IEEE 802.16 Mesh Network Yaaqob. A. Qassem, A. Al-Hemyari, Chee Kyun Ng and N. K. Noordin Department of Computer and Communication Systems Engineering, Faculty of Engineering, University Putra Malaysia, UPM Serdang, 43400 Selangor, Malaysia. {y_alrefaei, alizuhair10}@yahoo.com, {mpnck, nknordin}@eng.upm.edu.my Abstract— In the last few years, demand for high-speed internet access and multimedia service has increased greatly. The IEEE 802.16 standard defines the wireless broadband access technology called WiMAX (Worldwide Interoperability Microwave Access) aims to provide broadband wireless last-mile access, easy deployment, high speed data rate for large spanning area. In this paper, we propose an Energy/bit Minimization routing and centralized scheduling (EbM-CS) based algorithm to multi-transceiver in WiMax mesh network (WMN), which introduces the cross-layer concept between the MAC and network layers. The results show that our algorithm has improved the system performance in the aspect of system throughput. Keywords—WiMAX, EbM routing, centralized scheduling, throughput, multi-transceiver, cross layer.
I. INTRODUCTION The rapid growth of high-speed multimedia services for residential and small business customers has created an increasing demand for last mile broadband access. Traditional broadband access is offered through digital subscriber line (xDSL), cable or T1 networks. While cable and DSL are already being deployed on a large scale, Fix Broadband Wireless Access (FBWA) systems are gaining extensive acceptance for wireless multimedia services with several advantages. These include rapid deployment, lower maintenance and upgrade costs, and granular investment to match market growth [1]. Recently, study group 802.16 was formed under IEEE Project 802 to recommend an air interface for FBWA systems that can support multimedia services [2]. In 802.16 protocol stack, the medium access control layer (MAC) supports both point-to-multipoint (PMP) and mesh topologies. The main difference between the PMP and mesh modes is that in the PMP mode, traffic only occurs between the base station (BS) and subscribers station (SSs), while in the mesh mode, traffic can be routed through other SSs and can occur directly between SSs. The algorithm for scheduling the transmission between the BS and SSs, and among the SSs may be done in a distributed manner, in a manner centralized by the BS, or as a combination of both. The MAC scheme used is TDMA (Time division multiple access) and the resource allocation is in terms of time slots within a frame. Both uplink and downlink can operate in different frequencies using Frequency Division Duplexing (FDD) technique or
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share the same frequencies using Time Division Duplexing (TDD). The standard does not specify an algorithm for scheduling of the time slots to different SSs; neither does it specify any routing algorithm. In IEEE 802.16 mesh mode, routing and scheduling will have significant impact on the performance of the system and will largely decide the end to end quality of service (QoS) to different users. A scheduling is a sequence of fixed-length time slots, where each possible transmission is assigned a time slot in such a way that the transmissions assigned to the same time slot do not collide. Generally, there are two kinds of scheduling broadcast and link. In a broadcast scheduling, the entities scheduled are the nodes themselves. The transmission of a node is intended for, and must be received collision-free by all of its neighbors, while in a link scheduling; the links between the nodes are scheduled. The transmission of a node is intended for a particular neighbor, and it is required that there be no collision at this receiver [3]. Authoritative information on IEEE 802.16 mesh networks can be found in the official IEEE specification [2]. Algorithms for wireless mesh networks have been proposed in [4]. While this result is not specifically within 802.16 frame work, the insights they provide are helpful nevertheless. In [5], authors have presented the routing and centralized scheduling depending on different traffic models (i.e., CBR, VBR). The authors consider that the routing tree is fixed as a shortest path routing, and the routing tree is more effective in deciding the overall performance of the network. In [6], authors have presented a routing, channel and link scheduling (RCL) algorithm. The spatial reuse concept was brought forward to make the non-interference links concurrent transmission in [7]. In [8], authors have presented a cross-layer design for tree type routing, level-based centralized scheduling and distributed power control to improve the network throughput. Wang in [9] used Breadth first search (BFS) algorithm to construct the rooting tree which first choose the BS as the new routing tree’s root node and then chose the neighboring node by selecting the node which has small ID number. In this paper we propose an Energy/bit Minimization routing and centralized scheduling (EbM-CS) based to multitransceiver in WiMAX mesh network, which introduce the cross-layer concept between the MAC and network layers. The results show that our algorithm has improved the system performance in the aspect of system throughput.
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The rest of the paper is organized as follows. Section 2 introduces the routing tree construction. Inn section 3, multitransceiver scheduling algorithm has desscribed. Section 4 presents the simulation results. Finally, section s 5 draws a conclusion for the paper. II. ROUTING TREE CONSTRU UCTION The performance of centralized scheduuling benefits from well structured routing tree. To reduce links’ l interference, balance traffic load, shorten the period of request and grant, the structure of routing tree plays a key rolle. Optimization of routing tree contributes to the overall throoughput. We apply the EbM routing tree construction algorithhm for centralized scheduling in IEEE 802.16d mesh networkss. A. Connectivity Graph Construction For centralized scheduling, the routing tree is rooted at a MBS and constructed on the connectivityy [10]. We assume every Mesh Subscriber Station (MSS) nodee would transmit at the maximum power. The signal travvelling from the transmitter to the receiver will sufferr from path loss attenuation, which is a function of distannce d between two nodes. Suppose a node wants to transmitt to node j. The transmission is successful if (1) where SNRij denotes the signal-to-noise ratiio at the node j for signal received from node i, and SNRthressh can be obtained from the Table 1 [2], which with differennt modulation and coding schemes with targeted Bit-Error- Rate R (BER) of less than 10-6. We calculate SNRij at the receeiver of every link according to the following [10]: –
– 10 – 10 log 0
If SNR is below the thresholld of QPSK 1/2, the two nodes are disconnected and the capaacity of the link is set to 0. Following the method above, we w get connectivity graph G (V, E) with links marked with its capacity. c The routing tree will be built based on this graph. Figure F 1 shows an example of connectivity graph for 18 MSS nodes. MSSs are randomly w the MBS is located on the distributed in a cell of 3.2 km while top of the cell. Other topology plans are also possible, for example, MBS can be in other positions of the network, or MBS only supports one sector inn a multi-sector cell. B. Routing Tree Construction Routing tree will be consttructed after the connectivity graph is obtained. Beginning with the mesh base station (MBS), the MSS nodes are addeed into the tree one by one. We construct a routing tree based on minimum energy/bit. This algorithm is looking for a shortt path from current node to BS, the optimal path achieved whenn the whole path has the lowest Energy/bit Minimization (EbM M). We define the neighbor which is within the node’s transsmission range set of node u as Neighbors[u]. The node PN is defined d as the sponsoring node which relays MAC messages too and from the BS for node u. While the set Sons[u] contains all the neighbor nodes whose sponsoring node (parent node) is node u. The strategy how to transfer traffic from a node too the BS is called the routing strategy which has a substanttial impact on the number of retransmissions. Due to the unique path charracteristic of the tree topology, for candidate subscriber nodes CSN with N candidate parent n yet in the routing tree but nodes (the nodes, which are not have neighbors already in thee tree), there are N potential routes toward the MBS, each of o which can be represented as , . The folloowing routing tree construction algorithms is used to find the paarent node .
(2)
In which, w, 0 = -144 / = Equipartition Law = Receiver noise figure, = Mean power at the antenna port, = Occupied bandwidth, = Antenna gain for Tx, = Antenna gain for Rx, = path loss at distance d,
C. EbM Algorithm , is defined as the The energy value energy value consumed for onne byte data while node n is transmitting to its parent nodde . We introduce the energy metric of a givenn route from node i to MBS to evaluate the total energy spent in transmitting one byte along the path (3) ∑
TABLE 1. RECEIVER SNR ASSUMPTTIONS
Modulation BPSK QPSK 16-QAM 64-QAM
coding
Receiiver
rate
SNR R (dB)
1/2 1/2 3/4 1/2 3/4 2/3 3/4
6.4 9.4 11.2 16.4 18.2 22.7 24.4
Fig. 1. An example of connectivity grapph (18 nodes within a radius of 3.2 km).
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We first choose the BS as the new routing tree’s root node then we use the breadth-first search (BFS) algorithm to construct the rooting tree. For each CSN n, we choose its parent node (hence the route to MBS) by selecting the one with the minimal energy , (4)
throughput. This aim can be realized by maximize simultaneous transmissions without introducing exceeding interference for other transmissions. The following is the centralized scheduling algorithm that was used to transmit the data using the routing tree created. 1. A MSS is assigned service token based on its traffic demand. We use service token to allocate time slots to each link proportionally according to the traffic demand Figure 2 shows an example of the routing tree constructed of the link’s transmitter, thus, the fairness is guaranteed. by EbM algorithm on the connectivity graph shown in Fig. 1. 2. A link can be scheduled only if the service token number From Fig. 2, we can see that this algorithm typically leads to of its transmitter is nonzero, this link is marked as the use of short links using very high orders of modulation, available, and otherwise, it is marked as idle. but tends to result in a fairly high hop-count to reach the MBS. 3. An available link satisfied nearest to the MBS (the link whose transmitter has the minimal hop count to the BS is scheduled.) is scheduled in the current time slot. 4. The selected link is marked as scheduled and all the conflicting neighboring links of it are marked as interfered. 5. 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. 6. The same procedure is repeated until the service tokens of all these SSs are decreased to 0. Fig. 2. An example of routing tree (18 nodes within a radius of 3.2 km). Thus, using the change of service token, we can easily integrate the hop-by-hop relay model of WMN into our III. MULTI-TRANSCEIVER SCHEDULING algorithm. ALGORITHM IV. RESULTS AND DISCUSSION The network model that we consider here occurs with network with multiple channels and multiple transceivers The results are show that the algorithm developed has available for transmission. It is believed that interference is a improved the system performance in the aspect of system major factor that limits the system throughput and scalability throughput. of WiMax mesh networks. In general, multiple channel with In the simulation, a given number of SSs were uniformly multiple transceiver applied, the better spectrum utilization and randomly distributed. Each SS has a fixed transmission can be achieved while limiting the interference and improving range r. The SS's movement is not considered. Thus, two SSs robustness. Thus the network throughput and scalability will are neighbors when their distance is smaller than their be ameliorated. transmission range r. We assume the service token of each node are one and distributed randomly from one to three. The A. Assumptions results are set as follows: The node's transmission range is 30 In order to design according to the IEEE 802.16 the units, the number of nodes in the network ranges from 5 to following assumptions were made: 100 with increment step of 5. 1. A node is assumed to be interference free with other There are two schemes employed in the evaluations of nodes that is two-hop away. system performances. The first scheme, constructs of the 2. A node is assumed to be interference free with the nodes EbM-CS routing tree by considering the multi-transceiver that use different channel. multi-channel, interference and fairness. The second scheme 3. Nodes with multiple transceiver transceivers can is proposed in [3] which based to a single-transceiver and did communicate, interference-free, simultaneously with not consider the routing tree algorithm. more than one neighbor at the same time using different Fig. 3 gives the simulation results about the throughput of channels. the BS which each node has one service token to send. System 4. The signal of a node can only cover the range of a throughput is the amount of data that receives by BS in a time single-hop neighborhood. slot is to be improved. Since all packets should be routed 5. Non-interference links can communicate concurrently. through the BS, the throughput of BS equals the throughput of 6. The topology doesn’t make any change during the the system. The throughput is measured in data packets per scheduling period. time slot. According to the assumption of one transceiver can 7. The control and scheduling sub-frame are long enough. transmit at most one token at a time slot, we can deduce that the up bound throughput of the BS is defined by the number B. Centralized scheduling algorithm of transceivers it carries. So the upper limit is 1 for single The aim of proposed scheduling is to utilize concurrent transceiver and 2 for double transceivers. From Fig. 3, we transmission opportunity to achieve a higher system
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almost double the throughput when we equip the first nearest hop nodes with two transceivers when the node's transmission range is fixed. As a number of nodes add to the system the throughput will decrease. Figure 4 shows that the throughput goes down as a number of service tokens and number of nodes adds to the system as similar to Fig. 3. However, it is applying one to three packets to send compared to Fig. 3 which only sending one packet. The spectrum efficiency is said to be increased when three packets are sent in a time slot.
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[5]
[6]
[7] [8] Fig. 3. Throughput which each node has one service token (in token). [9] [10]
Fig. 4. Throughput which each node has random service tokens from 1 to 3 (in token).
V. CONCLUSIONS In this paper, we propose a multi-transceiver and multichannel centralized scheduling algorithm based on Energy/bit Minimization routing EbM-CS in WiMax mesh network, which introduces the cross-layer concept between the MAC and network layers. The system throughput is synthetically considered. Analyzing EbM-CS with random network structure and traffic flow, we have compared the algorithm with the one in [3]. The analysis shows that this algorithm will improve the network throughput and the efficiency of the centralized scheduling while ensuring collision free and fairness among the SSs in the WiMax mesh networks, especially when the number of hops is large. There are many issues related to this work that requires further research such as consider the length of time slot, the different traffic services and consider the mobility for each node. .
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