LASER: A load-aware spectral-efficient routing metric for path selection in IEEE 802.16j multi-hop relay networks

Computers and Electrical Engineering 38 (2012) 953–962

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LASER: A load-aware spectral-efficient routing metric for path selection in IEEE 802.16j multi-hop relay networks q Sheng-Shih Wang a, Chan-Ying Lien b, Wen-Hwa Liao c, Kuei-Ping Shih b,⇑ a

Department of Information Management, Minghsin University of Science and Technology, Hsinchu 304, Taiwan Department of Computer Science and Information Engineering, Tamkang University, Tamshui 251, Taipei, Taiwan c Department of Information Management, Tatung University, Taipei 104, Taiwan b

a r t i c l e

i n f o

Article history: Received 26 August 2011 Received in revised form 6 February 2012 Accepted 7 February 2012 Available online 7 March 2012

a b s t r a c t For coverage extension and throughput enhancement, the IEEE 802.16j task group has developed a novel multi-hop relay network architecture to enable typical IEEE 802.16 networks to achieve data transmission between base stations and mobile stations via a multihop path with relay stations deployment. How to determine an effective path for throughput gain and overhead reduction is emerging and crucial in IEEE 802.16 multi-hop relay networks. This paper introduces a load-aware spectral-efficient routing metric, called LASER, to evaluate paths, and proposes an efficient scheme to determine a proper path. Based on the LASER metric, the proposed path selection scheme formulates the path cost as the summation of cost of each link, and the path with the minimum cost will be selected as the appropriate one. Simulation results show that the proposed LASER-based path selection scheme significantly outperforms existing path selection schemes in network throughput and map overhead. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction In recent years, the remarkable upsurge in demand for high-quality and high-speed applications has created a rapid development of broadband wireless access (BWA) technologies [1]. Among a variety of BWA technologies, IEEE 802.16 is a promising one to enable various kinds of services, such as data, audio/voice distribution, and real-time videoconferencing. A fundamental IEEE 802.16 network has two kinds of stations, base station (BS) and mobile station (MS). The two kinds of stations operate on the point-to-multipoint (PMP) mode [2], so that BSs can transmit data to MSs via a direct radio link, and vice versa. Relaying technology has been broadly used in diverse wireless networks in [3–6], as it derives manifold advantages, including coverage extension, shadowing combat, deployment cost reduction, network capacity enhancement, and system reliability improvement. Thus, the IEEE 802.16j task group adopts this technique in the legacy IEEE 802.16 network and designs a so-called multi-hop relay (MR) network [7]. The components in typical IEEE 802.16j MR networks are threefold: MR– BS (i.e., the traditional BS in IEEE 802.16 networks), MSs, and relay stations (RSs). Radio links originating or terminating at an MS are access links, while radio links between MR–BS and RSs, or between a pair of RSs are relay links. Typically, the MR–BS performs scheduling for radio resource and allocates the time frequency resource to RSs and MSs, and provides MSs to connect to the backhaul network [8]. The RS supports multi-hop communications between MR–BS and MSs. Due to the provision of multi-hop transmissions, downlink (DL) and uplink (UL) transmissions can be achieved via a direct or a multi-hop manner, and multiple paths may exist between MR–BS and an MS. Nevertheless, path selection is not mentioned at all in the IEEE

q

Reviews processed and proposed for publication to Editor-in-Chief by Associate Editor Dr. Sherali Zeadally.

⇑ Corresponding author. Tel.: +886 2 26215656x2748; fax: +886 2 26209749. E-mail address: [email protected] (K.-P. Shih). 0045-7906/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.compeleceng.2012.02.005

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802.16j Spec. If selecting an improper relay path, data transmission is likely to experience disappointing network throughput and render more map overhead. Path selection is one of the significant issues in a variety of wireless networks, such as mobile ad hoc networks, wireless sensor networks, vehicular ad hoc networks [9–15]. It is generally regarded as the routing issue and is commonly investigated at the network layer. Numerous routing metrics have been widely explored in much prior research [9,10,14], in that hop count (HOP) [14], per-hop round trip time (RTT) [9], and expected transmission count (ETX) [10] are well-known and widely used in the aforementioned networks. Unfortunately, these metrics are unable to be directly exploited in IEEE 802.16j MR networks, as they are largely used to assist in determining proper relay path in multi-hop wireless local area networks as well as cellular networks and the media access control (MAC) design of such networks is different from that of IEEE 802.16j MR networks. Much study concentrating on the design of routing metrics to selection of efficient paths in IEEE 802.16j MR networks has been investigated [16–21]. However, the schemes with the consideration of these metrics are not suitable for path selection in IEEE 802.16j MR networks because this kind of network is quite sensitive to radio resource utilization. The IEEE 802.16 standard specifies a variety of burst profiles to provide flexibility in data transmission [2]. Each burst profile stands for a digital modulation technique in combination with a specific coding scheme, and support different levels of radio resource utilization. The influence of radio resource utilization on path selection in IEEE 802.16j MR networks is first discussed in [22]. This study introduces a radio resource utilization index (RRUI) to assist in selecting a proper relay path. The RRUI primarily reflects the utilization of orthogonal frequency division multiple access (OFDMA) radio resource of a link, and is defined as the amount of data which can be transmitted on a link at a time unit. This RRUI-based scheme will determine the fixed path for data communication between MR–BS and a specific MS consequently. Nevertheless, if the MR–BS has heavy DL traffic for a designated MS in the MR cell, only considering the RRUI in path selection is more likely to cause the link overloading problem, which will be discussed in Section 4, and therefore degrades the system throughput. In general, the efficiency of a specific burst profile used on a link can be typically represented by link spectral efficiency [23]. Adaptive modulation has been proven as an effective technique to increase the link spectral efficiency for communications [24,25]. In addition, much previous research addresses traffic load as a crucial factor in path selection [26–29], because overload on a link significantly degrades the network throughput. Nevertheless, to our best knowledge, link spectral efficiency and link load are not taken into account in the existing path selection schemes. Thus, this paper proposes a novel metric, called LASER, which is associated with traffic load and spectral efficiency of links, to assist in selecting the effective path for DL transmission. The spectral efficiency generally manifests channel capacity of links with a given burst profile, and thus it can actually indicate the amount of radio resource utilized on a link. Apparently, the higher spectral efficiency a link possesses, the higher throughput this link gains. On the basis of LASER, this paper further introduces a link cost to evaluate the suitability of links in IEEE 802.16j MR networks. As the cost of each link of a candidate path is determined, the MR–BS calculates the path cost, which is formulated as the sum of each link cost, and then selects the path with the smallest path cost as an appropriate one. Simulation results confirm that using the proposed LASER-based path selection scheme outperforms the existing approaches in network throughput as well as map overhead, and achieves a significant improvement. The rest of this paper is organized as follows. Section 2 gives the background of IEEE 802.16j and introduces the path selection (PS) problem in IEEE 802.16j MR networks. Section 3 presents the network model. Section 4 presents the proposed load-aware spectral-efficient routing metric, LASER, and the LASER-based path selection scheme in detail. Simulation results are shown in Section 5, followed by the concluding remarks in Section 6. 2. Preliminaries As IEEE 802.16j is the amendment of IEEE 802.16, this section gives an overview of IEEE 802.16, followed by the description of the PS problem in IEEE 802.16j MR networks. 2.1. Overview of IEEE 802.16 IEEE 802.16 considers OFDMA as an efficient multiple access approaches to allow a user to share a subset of available and mutual orthogonal subcarriers for channel access. An OFDMA symbol is made up of subcarriers, and a logical collection of subcarriers forms a subchannel. The mapping of logical subchannels to physical subcarriers is called permutation [1]. As addressed in IEEE 802.16, a unit of resource, called OFDMA slot, is allocated to a specific user in the time and frequency domains. The size of OFDMA slots depends on the OFDMA symbol structure, which varies with transmission direction (UL or DL) and the subcarrier permutation scheme. In IEEE 802.16, the OFDMA PHY specifies five forward error correction (FEC) encoding techniques, among which convolutional coding (CC) with tail-biting method is mandatory, while block turbo coding (BTC), convolutional turbo codes (CTC), low-density parity check coding (LDPCC), and CC with zero-tailing are optionally supported. These codes have multiple rates of 1/3, 1/2, 2/3, 3/4, and 5/6 to protect data sent over wireless channels. In addition, modulations that the IEEE 802.16 standard supports include Gray-mapping Quadrature Phase Shift Keying (QPSK), and two types of Quadrature Amplitude Modulation, 16-QAM and 64-QAM. In order to increase margin over the modulation and FEC encoding schemes, the repetition codings of 2, 4, and 6 can be applied to QPSK modulation. Specifically, a combination of a specific modulation scheme, FEC encoding method with a certain coding rate, and repetition coding scheme is corresponding to a single burst profile.

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Due to a great diversity of encoding, modulation, and repetition techniques, multiple burst profiles are provided in IEEE 802.16, and thus stations are able to utilize the most robust burst profile for data transmission. As specified in [7], RSs fall into two categories: transparent RS (T-RS) and non-transparent RS (NT-RS), depending on the ability of transmitting DL frame-start preamble, FCH, downlink map (DL-MAP), uplink map (UL-MAP), downlink channel descriptor (DCD), and uplink channel descriptor (UCD). All the stations in IEEE 802.16j MR networks typically follow a time division duplex (TDD) OFDMA frame structure, which consists of a DL subframe period and an optional UL subframe period. In the MR–BS frame structure, the DL subframe includes one access zone and one optional transparent zone primarily for RS to transmit data to its subordinate RSs and MSs, and the UL subframe may include an UL access zone and an UL relay zone. In the RS frame structure, the DL subframe includes one access zone for RS to receive data from MR–BS and may include one transparent zone for RS to transmit data to its subordinate RSs and MSs, and the UL subframe may include one access zone and one relay zone for RS to transmit data to MR–BS and its superordinate RSs. 2.2. The path selection (PS) problem in IEEE 802.16j networks Basically, in IEEE 802.16j MR networks, MSs adhere to the IEEE 802.16 standard to perform the network entry procedure if they intend to join a network [2]. During the network entry procedure, the MS first scans for the DL channel, and then establishes synchronization with the MR–BS. When receiving UL parameters, MSs perform an initial ranging process to adjust the timing offset and power level, followed by numerous mandatory processes, including basic capabilities negotiation and registration. Consequently, the connection between MR–BS and MSs is set up. As mentioned in [7], RSs like MSs also perform the network entry procedure to join the network and perform the additional important processes to measure the neighbor station, select the access station, and create the path so as to support multi-hop communications. Although path creation is invoked during the network entry procedure, the IEEE 802.16j standard does not provide any strategy to path selection. As data transmission can be achieved via a direct or a multi-hop way, multiple paths may exist. IEEE 802.16j allows the MR–BS to make a centralized determination to discover paths for DL and UL directions. Obviously, selection of proper paths is subject to various constraints, such as network topology, available radio resource, radio link quality, and load condition of RSs. As mentioned before, various metrics for path selection in traditional wireless networks cannot be effectively exploited in IEEE 802.16j MR networks. For example, hop count is a representative metric to be considered in path discovery due to the inherence of low latency. However, small hop count implies that data transmission probably uses low modulation owing to long distance. Such low modulation significantly degrades network throughput, renders MAP overhead, consumes much energy, and incurs more latency. On the basis of above discussion, we summarize that the main challenge to the PS problem is to design an efficient metric to evaluate all the possible paths. 3. Network model For brevity, this paper uses ‘‘MR networks’’ to represent IEEE 802.16j MR networks. The MR network we consider comprises one MR–BS and a number of RSs and MSs, all of which operate in the time-division-duplex (TDD) mode. The MR cell is assumed to be served by a 3-sector MR–BS, which locates at the center of a hexagonal cell. RSs are placed at the vertices of the cell, and are assumed to have enough buffer to hold the data traffic being relaying for MR–BS, RSs, and MSs. All MSs are randomly deployed with a uniform distribution in the network. Assume that each MS has multiple DL paths, including direct or multi-hop relay paths. The minimum configuration for an in-band transparent relay frame structure is considered, and thus signal interference can be ignored via decentralized scheduling and spatial reuse. Moreover, we assume that MR–BS is capable of using all of radio resources, and each RS are permanently installed at a fixed location (i.e., fixed relay station (FRS)). We model an MR network as a graph, G ¼ ðV; EÞ, where V is the set of stations comprising MR–BS, RSs and MSs, and E is the set of wireless links including relay and access links. An edge between two stations means that the two stations can directly communicate with each other. Let N MS be the number of MSs in MR networks, and all the MSs in the network are denoted as MSðiÞ, where i ¼ 1; 2; . . . ; N MS . In this paper, we define the DL path as the concatenation of consecutive links from MR–BS to a designated MS. Let N p ðiÞ be the number of possible DL paths, and all the DL paths from MR–BS to MSðiÞ are represented as P j ðiÞ, where j ¼ 1; 2; . . . ; N p ðiÞ. Let Lj ðiÞ be the length of Pj ðiÞ, which is defined as the hop distance from MR–BS to MSðiÞ. This paper denotes Pj ðiÞ in the form of a sequence of stations from MR–BS to MSðiÞ. That is, Pj ðiÞ ¼ fSj0 ðiÞ; Sj1 ðiÞ; Sj2 ðiÞ; . . . ; SjLj ðiÞ1 ðiÞ; SjLj ðiÞ ðiÞg, where Sj0 ðiÞ indicates MR–BS, SjLj ðiÞ ðiÞ indicates MSðiÞ, and Sjk ðiÞ where k ¼ 1; 2; . . . ; Lj ðiÞ  1 indicate RSs in P j ðiÞ. Let ejk;kþ1 ðiÞ be the link between Sjk ðiÞ and Sjkþ1 ðiÞ. As a result, the set of all relay links of P j ðiÞ, denoted as RPj ðiÞ , can be represented as RPj ðiÞ ¼ fejk;kþ1 ðiÞ j 0 6 k 6 Lj ðiÞ  2g. On the other hand, the single access link in P j ðiÞ can be obtained as ejLj ðiÞ1;Lj ðiÞ ðiÞ. An example of network model discussed in this paper is illustrated in Fig. 1. 4. The proposed DL path selection scheme This section discusses the link spectral efficiency and link load in DL path selection, aiming to design our load-aware spectral-efficient routing metric (LASER). Then, we present the LASER-based DL path selection scheme for IEEE 802.16j MR networks.

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Fig. 1. Example of the network model. For brevity, MR–BS is denoted as Sx0 ðkÞ, where x and k indicate the path index and the identifier of the designated MS, respectively. In this example, when k ¼ i, x = 1 or 2, and when k ¼ j, x = 1, 2 or 3.

4.1. Design of LASER To consider the influence of resource utilization on link throughput, we use link spectral efficiency as a factor to determine link cost. This link spectral efficiency is defined as follows. Definition 1. Link spectral efficiency (LSE) is defined as the data bitrate which can be transmitted over a given bandwidth of a link. It is the result of the gross bitrate divided by the bandwidth in hertz of a link. h Let Bjk;kþ1 ðiÞ be the amount of data in byte on ejk;kþ1 ðiÞ. Let ejk;kþ1 ðiÞ and 1jk;kþ1 ðiÞ be the numbers of symbols and subcarriers required for transmitting Bjk;kþ1 ðiÞ-byte data, respectively. By Definition 1, the LSE of ejk;kþ1 ðiÞ, termed LSEjk;kþ1 ðiÞ, can be represented as

LSEjk;kþ1 ðiÞ ¼

e

Bjk;kþ1 ðiÞ : j j k;kþ1 ðiÞ  k;kþ1 ðiÞ

1

ð1Þ

In the IEEE 802.16 standard, an OFDMA burst profile is defined as a combination of a specific modulation, coding scheme, and repetition used in the underlying PHY layer. It supports a variety of burst profiles, and MR–BS, RS, or MS can use the robust one for data transmission. Let M n represent the modulation scheme in which n is the modulation factor (i.e., net bitrate), indicating the number of bits associated with a subcarrier, Rcode be the coding rate, and C rep be repetition code, respectively. We use BPðn; Rcode ; C rep Þ to denote the burst profile using n, Rcode , and C rep as parameters. For a link, ejk;kþ1 ðiÞ, using BPðn; Rcode ; C rep Þ, according to Eq. (1), its LSE can be rewritten as Eq. (2). As the convolutional code (CC) is mandatory in IEEE 802.16 standard, and the convolutional turbo code (CTC) is widely adopted in many telecommunication systems including WiMAX, W-CDMA and CDMA2000 due to its high-performance, we list the link spectral efficiency of each different burst profile with CC and CTC coding schemes only, as shown in Table 1.

LSEjk;kþ1 ðiÞ ¼

n  Rcode : C rep

ð2Þ

In general, if multiple paths share a common link, such link is much more likely to be overloaded. This probably causes failure in data transmission. As shown in Fig. 2, suppose MS(1) is within the coverage areas of MR–BS and RS(1), and MS(2) is within the coverage areas of RS(1) and RS(2). Assume that RS(1) is the common access station of both MS(1) and MS(2). If DL traffic to MS(1) and MS(2) reaches RS(1), RS(1) is responsible to relay the traffic towards MS(1) and MS(2). However, the link between MR–BS and RS(2) is unable to forward the total traffic in the same frame due to the constraint of channel capacity. The downlink transmissions for MS(1) and MS(2) will be dropped if the traffic is time-bounded (i.e., QoS requirement), or be postponed to following frames. We call this the link overloading problem. Notice that in Fig. 2 MS(2) is also within the coverage area of RS(2). If data transmission from MR–BS to MS(2) can be through RS(2) instead of RS(1), the link overloading problem will be diminished. This obviously increases the network throughput. In IEEE 802.16-based networks, as DL traffic comes from the backhaul network, MR–BS can be aware of the amount of DL traffic to be transmitted to each MS and on each link. 4.2. The LASER-based DL path selection scheme Similar to much research on path determination, this paper utilizes the path cost to select the most proper path. The path cost is the summation of the cost of all links in a path, in which link cost is defined as follows.

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S.-S. Wang et al. / Computers and Electrical Engineering 38 (2012) 953–962 Table 1 Summary of link spectral efficiency of each burst profile. Burst profile ID

Modulation ðnÞ

Coding scheme ðRcode Þ

Repetition ðC rep Þ

LSE

1 2 3 4 5 6 7 8 9 10 11

QPSK (2) QPSK (2) QPSK (2) QPSK (2) QPSK (2) 16-QAM (4) 16-QAM (4) 64-QAM (6) 64-QAM (6) 64-QAM (6) 64-QAM (6)

CC/CTC (1/2) CC/CTC (1/2) CC/CTC (1/2) CC/CTC (1/2) CC/CTC (3/4) CC/CTC (1/2) CC/CTC (3/4) CC/CTC (1/2) CC/CTC (2/3) CC/CTC (3/4) CTC (5/6)

6 4 2 1 1 1 1 1 1 1 1

1/6 1/4 1/2 1 3/2 2 3 3 4 9/2 5

Fig. 2. Link overloading problem.

Definition 2. Link cost is defined as the number of OFDMA slots required for data transmission in a link. It can be derived as the ratio of link load to link spectral efficiency of the link. h As an OFDMA slot is equivalent to 48 subcarriers [2], the amount of traffic in byte an OFDMA slot can transmit for any link ejk;kþ1 ðiÞ

is

48LSE

j ðiÞ k;kþ1

8

cjk;kþ1 ðiÞ ¼

¼ 6  LSEjk;kþ1 ðiÞ. Denote the cost of ejk;kþ1 ðiÞ as cjk;kþ1 ðiÞ. We have

Bjk;kþ1 ðiÞ 6  LSEjk;kþ1 ðiÞ

ð3Þ

:

j

Let bk;kþ1 ðiÞ, cjk;kþ1 ðiÞ and jjk;kþ1 ðiÞ respectively denote the modulation parameter, coding rate, and repetition code of Eq. (3) can be rewritten as

ejk;kþ1 ðiÞ.

cjk;kþ1 ðiÞ ¼

Bjk;kþ1 ðiÞ  jjk;kþ1 ðiÞ j

6  bk;kþ1 ðiÞ  cjk;kþ1 ðiÞ

:

ð4Þ

Let Cj ðiÞ denote the cost of P j ðiÞ. Thus, Cj ðiÞ can be formulated as

Cj ðiÞ ¼

LjX ðiÞ1

cjk;kþ1 ðiÞ:

ð5Þ

k¼0

As MR–BS serves as a central controller in the proposed LASER-based scheme, it is responsible to select the final DL path. In order for MR–BS to calculate the cost of each possible path, MR–BS requires all the information of links of relay paths, which is reported by each RS in a relay path. The information includes the number of its subordinate stations, the list of its subordinate stations, and the list of the burst profiles used on all the subordinate links, which are denoted as N sub , Lsub ,

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and LIDBP , respectively. This paper indicates this link information as a three-tuple with the format (N sub , Lsub , LIDBP ), and uses a new reporting message, namely LINK_INFO_REP to report these information. Algorithm 1 shows the algorithm to select the most proper DL path for each MS.

.Algorithm 1

Consider an example, in which Pj ðiÞ has two intermediate RSs, termed Sj1 ðiÞ and Sj2 ðiÞ, as shown in Fig. 3. Let the burst profile corresponding to ej0;1 ðiÞ, ej1;2 ðiÞ, and ej2;3 ðiÞ be BPj0;1 ðiÞ, BPj1;2 ðiÞ, and BPj2;3 ðiÞ, respectively. After MSðiÞ performs the network entry process, Sj2 ðiÞ obtains the information about the burst profile on ej2;3 ðiÞ (i.e., BPj2;3 ðiÞ), and meanwhile, Sj1 ðiÞ also obtains the information about the burst profile on ej1;2 ðiÞ (i.e., BPj1;2 ðiÞ). During the initial ranging process, Sj2 ðiÞ transmits the message with (1, hSj2 ðiÞi, hBPj2;3 ðiÞi) to Sj1 ðiÞ, and Sj1 ðiÞ then transmits the message with (2, hSj1 ðiÞ; Sj2 ðiÞi, hBPj1;2 ðiÞ, BPj2;3 ðiÞi) to its subordinate station (i.e., MR–BS). Certainly, MR–BS has the information of ej0;1 ðiÞ due to immediate link. As receiving the reporting message from Sj1 ðiÞ, MR–BS is able to obtain the burst profiles corresponding to all the links in the path, and therefore calculates the cost of each possible path. Once the appropriate path is determined, MR–BS requires to notify the immediate RSs in the path of the routing information, including the RSs in the selected relay path. The information is carried in an extended information element (IE), called the path information advertisement IE, instead of a new management message since we consider a transparent system which is under a centralized control. This IE with a new code for extended-2 downlink interval usage code (DIUC) is carried in the DL-MAP and R-MAP messages. Whenever receiving DL-MAP messages issued from MR–BS, the immediate RSs in the path are aware of their own superordinate stations. 5. Performance evaluations This section first presents the simulation environment and parameters, and then presents simulation results.

Fig. 3. Example of link information reporting. The dark, gray, and white stations indicate the MR–BS, RS, and MSðiÞ, respectively. The thick arrow is the LINK_INFO_REP message.

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5.1. Simulation setup This paper simulates the DL path selection scheme with LASER consideration on an MR cell of the typical urban environment, which is a two-hop transparent MR network, as illustrated in Fig. 4. The cell serves in the hexagon area with the radius of 500 m. MR–BS is situated at the center of the cell. Six RSs are deployed in a hexagonal lattice shape, and the distance from MR–BS to each RS is 300 m. One hundred MSs are randomly scattered within the coverage radius of MR–BS. In addition, each MS is assumed to be able to directly connect to at least one RS in the cell. MR–BS is configured with three sectors and one segment. The system model follows the minimum configuration for an in-band transparent relay frame structure, and assumes that signal interference can be reasonably ignored via the TDD approach. The burst profiles defined in IEEE 802.16 are assumed to be available. This study only focuses on the design of an effective metric for DL path selection instead of the scheduling issue, and thus the scheduling technique is not thoroughly considered. In the simulation, MR–BS solely adopts the simple but representative round-robin scheduling algorithm, which regards the OFDMA slot as the scheduling unit to serve each MS for max–min fairness achievement. The system-level parameters in our simulations refer to the WiMAX forum suggestion [30], and consider P802.16j evaluation guideline [31] and baseline document [32], as listed in Table 2. 5.2. Simulation results We use utilization, termed q, which represents the congestion level of traffic load in the queue as a primary factor to investigate the performance. This factor stands for the proportion of the system’s resources used for the traffic that arrives at such system, and is formulated as

k

q¼ ; l

ð6Þ

where k and l are the mean packet arrival rate and the mean service rate of a queue. The value of utilization ðqÞ in the simulations ranges from 0.1 to 1.0 with a step of 0.1. The mean packet length is 384 bytes. All simulation results are observed in 200 frames (i.e., 10 s) and averaged over 50 runs. Extensive simulations are conducted under error-free networks to compare the performance of the proposed LASERbased relaying scheme with IEEE 802.16 standard consideration, the IEEE 802.16j-based (two-hop) relaying scheme, the RRUI-based relaying scheme [22], which are denoted as No-relaying, Relaying, RRUI-relaying, respectively. The No-relaying scheme follows the IEEE 802.16 standard, DL data transmission between MR–BS and an MS does not rely on any RS. In the Relaying scheme, MR–BS achieves DL data transmission to MSs via the path with a single RS. In the RRUI-relaying scheme, DL data transmission between MR–BS and an MS is achieved via the path depending on the RRUI metric. Fig. 5 shows the network throughput of various schemes under the scenarios with different utilizations. The network throughput of the No-relaying, Relaying, RRUI-relaying, and LASER-relaying schemes are between 2.6 and 11.3 Mbps, between 2.2 and 8.7 Mbps, between 2.7 and 12 Mbps, and between 2.8 and 16 Mbps, respectively. Note that without the consideration of link load, the RRUI-relaying scheme outperforms the No-relaying and Relaying schemes. We reason that the RRUI-relaying scheme adaptively enables the source station to transmit data to its destined station via the relay path with higher spectral efficiency. It implies that data is transmitted via the higher transmission rate, and thus network throughput gain and cell capacity compensation are achieved. Additionally, the LASER-relaying scheme obviously has better performance in comparison with the RRUI-relaying scheme. The reason is that the LASER-relaying scheme not only considers the link spectral efficiency, but also adaptively distributes the traffic load among the possible relay paths. Significantly, the traffic load on the common link of multiple relay paths is probably diminished.

Fig. 4. Illustration of an MR cell with two-hop relay.

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S.-S. Wang et al. / Computers and Electrical Engineering 38 (2012) 953–962 Table 2 System-level parameters in the simulations. Parameter

Value

Operational frequency Frame length OFDMA symbol structure Channel bandwidth FFT size (NFFT) No. of sub-channels Sub-carrier frequency spacing Useful symbol time ðT b ¼ 1=f Þ Guard time ðT g ¼ T b =8Þ OFDMA symbol duration No. of OFDMA symbols Ratio of occupied OFDMA symbols Channel coding

2.5 GHz 5 ms PUSC 10 MHz 1024 30 (DL), 35 (UL) 10.94 KHz 91.4 ls 11.4 ls 102.9 ls 48 29(DL):18(UL) Convolutional turbo code (CTC)

Fig. 5. Network throughput of various schemes for different scenarios.

Fig. 6. Map overhead generated by various schemes for different scenarios.

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Fig. 7. Enhancement of the LASER-relaying scheme.

In Fig. 6, the RRUI relaying scheme generates less map overhead than those generated by the No-relaying and Relaying schemes, apparently. Notice that taking RRUI into account MR–BS prefers the relay path with the high modulation and coding schemes for data transmission. Using the high modulation technique, time consumed for data transmission is reduced. Therefore, the fewer number of the map messages is required. The map overhead generated by the LASER-relaying and RRUIrelaying schemes are between 3.6% and 5.8%, and between 5.1% and 8.3%, respectively. Substantially, if traffic load on the bottleneck link is efficiently dispersed to other links, data transmission incurs the less queuing delay and significantly data will be rapidly transmitted. As a result, the LASER-relaying scheme outperforms the RRUI scheme in map overhead. In order to explicitly outline the effectiveness of the LASER-relaying scheme, we give Fig. 7 to illustrate the improvement of the LASER-relaying scheme in both network throughput and map overhead compared with the RRUI-relaying scheme. Overall, the LASER-relaying scheme outperforms the RRUI-relaying scheme in network throughput to a maximum of 37%, and in map overhead to a maximum 36%. 6. Conclusions This paper has revealed that spectral efficiency and traffic load dominate the routing performance in IEEE 802.16j multihop relay networks. In this paper, we have proposed a load-aware spectral-efficient routing metric, called LASER, to evaluate a link to participate in downlink transmission. The key concepts of the proposed LASER-based path selection scheme are selecting a higher modulation link due to having a higher spectral efficiency and distributing the traffic load on links among different paths due to avoidance of the overloading problem. The proposed LASER is quantified as link cost, and the path cost is formulated as the sum of each link cost. When the cost of each candidate path is determined, MR–BS chooses the path with the smallest cost as a proper path for downlink data transmission. The performance evaluations focus on system throughput and map overhead of the proposed LASER-based path selection scheme, and simulation results have conducted under different link conditions and congestion levels of traffic load. As a conclusion, the proposed LASER-based path selection scheme, which takes link spectral efficiency and link load into account, can significantly outperforms the comparative schemes. Our future work includes investigating the influence of link error on routing performance and designing an appropriate metric based on LASER to determine the appropriate path for bi-directional transmissions. Future studies can also explore the impact of load balance and scheduling issues on symmetric and asymmetric path selection in IEEE 802.16j multi-hop relay networks. Acknowledgements This work is supported by the National Science Council of the Republic of China under Grants NSC 100-2221-E-032-028 and NSC 99-2218-E-119-001. References [1] Andrews JG, Ghosh A, Muhamed R. Fundamentals of WiMAX – understanding broadband wireless networking. Prentice Hall; 2007. [2] IEEE Standard 802.16 Working Group. Standard for local and metropolitan area networks, part 16: air interface for broadband wireless access systems; 2009.

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His current research interests include design of routing protocols and MAC protocols for wireless sensor networks, vehicular ad hoc networks, and WiMAX systems. He is a member of the IEEE. Chan-Ying Lien received the M.S. degree in computer science from Tamkang University, Tamshui, Taiwan, in 2009. He is currently a Ph.D. student in Tamkang University, Tamshui, Taiwan. His current research interests include IEEE 802.16 and wireless sensor networks. Wen-Hwa Liao received the Ph.D. degree in computer science and information engineering from National Central University, Taiwan, in 2002. Currently, he is an Associate Professor in the department of information management, Tatung University, Taiwan. He served as a member of Editorial Board of International Journal of Distributed Sensor Networks and an Associate Guest Editor of International Journal of Ad Hoc and Ubiquitous Computing. His current research interests include wireless sensor networks, mobile communications, and cloud computing. Kuei-Ping Shih received the Ph.D. degree in computer science and information engineering from the National Central University, Taiwan in 1998. Currently, he is a Professor in the Department of Computer Science and Information Engineering, Tamkang University, Taiwan. He served as an Executive Editor of Journal of Applied Science and Engineering. He also respectively served as a Guest Editor and a Program Committee member for several international journals and conferences. His current research interests include network protocol designs for wireless LANs, WiMAX, underwater acoustic networks, wireless sensor networks, and mobile ad hoc networks. He is a member of the IEEE.