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Full Paper Proc. of Int. Conf. on Advances in Electrical & Electronics 2012

Study and Analysis of Throughput Improvement in Wi-MAX Networks (802.16j) using Multi-Hop Relay Stations K.Vinoth Babu 1, Dr.G.Ramachandra Reddy 2, Boniface A A3 and Sunit Gupta 4 1

VIT University, Vellore, India Email: vinothbab@gmail.com 2 VIT University, Vellore, India Email: {reddygr007, boniface.aa, sunit2gupta}@gmail.com decoded, but source codes the received signal and is retransmitted. It is also called hybrid relaying [2].

Based on the ability to manage resources, relays are classified into two categories, Type-I (non transparent) and Type-II (transparent) relays [1]. These two types are shown in Fig. 1. Type-I relays are deployed when the MS is located far away from BS i.e. out of cell coverage. This kind of relay acts like a separate BS and appears like a separate cell. It generates its own control messages. They are mainly used for coverage extension. The second types of relays are deployed within the coverage area of BS. Even MS can directly communicate with BS. They are mainly used for improvement in signal quality. Type-II relay deployments suffer by the path selection problems. In this paper we consider Type-II relays for analysis.

Abstract— The IEEE 802.16j task group has developed a novel frame work for IEEE 802.16, i.e. the multihop relay (MR) network for enhancing throughput and cell coverage extension. However, deploying several relay nodes causes inefficient time-slot. So selecting the optimum number of relays and relay placements are essential to optimize the system capacity. Furthermore, cell sectoring and spatial reuse can be employed to minimize the noise and improve the throughput. In this paper we have discussed path selection, cell sectoring and optimum number of relays for throughput enhancement.

Index Terms— Wi-MAX, Multihop Relay Base Station (MRBS), Mobile Station (MS), Relay Station (RS), Modulation and Coding scheme (MCS), Signal to Noise Ratio (SNR), Radio Resource Utilization Index (RRUI)

I. INTRODUCTION Wi-MAX is a wireless standard designed to provide the data rate of 30 to 40 Mbps for mobile nodes and 1Gbps for fixed nodes. It is mainly used for wireless broadband access and a best alternate to cable and Digital Subscriber Line (DSL).

IEEE 802.16j is exceptional for supporting Quality of Service (QoS), Broadband Wireless Access (BWA) and mobility for networks with multimedia applications. However, a Mobile Station (MS) within the coverage region of a Base Station (BS) may fail if the relative signal strength at the MS is too weak or due to Non-line of Sight (NLOS). Wi-MAX operators are expected to increase the density of BS to address these issues. But cost involved for BS deployment is too high. To counter these problems, IEEE 802.16j adds a new type of station to the Wi-MAX called Relay Station (RS). Relaying technology has numerous advantages, such as cost reduction, coverage extension, shadowing combat, network capacity enhancement [4].

Radio links between MR-BS—RS, RS—RS are relay links and between MR-BS—MS or RS—MS is called access links.

Depending on how the received signal is processed, the relay is classified as Amplify and Forward (AF), Decode and Forward (DF) or Estimate and Forward (EF). In AF, the signal is just amplified and retransmitted along with the noise. In DF, the signal is demodulated and decoded before retransmission. This does not contain any additional noise or degradation, but only symbol errors. In EF, the input is not © 2012 ACEEE DOI: 02.AETAEE.2012.3. 47

Figure 1. Type-I and Type –II Relays

(1) where s,r,p denotes the percentage gain on the throughput of using the path r + p instead of the path s. Ws is a weight associated with the BS-MS link, Wp a weight associated with the RS-MS link and Wr a weight associated with the BS-RS link. Each of these weights reflects in link efficiency.

A typical three node relay network is shown Fig. 2. The link s and p is called access link, which is directly associated with the MS and link r is called relay link, which is the link between a RS-BS or RS-RS. Relay positioning and the number of relays also play an important role in throughput enhancement and coverage extension [4]. The system performance is sensitive to the number of relays deployed and their location. The relay 76

Full Paper Proc. of Int. Conf. on Advances in Electrical & Electronics 2012 schemes like QAM and QPSK with different code rates. Fig. 3. Indicates how a cell is divided into three zones based on SNR. RS is installed in a zone where it can provide maximum link quality. Assuming that most of the time user will be stationary and confined to a single cell, RS can be easily positioned. RS position is varied assuming MR-BS and MS are fixed and it is placed in the position where maximum throughput is achieved.

III. NETWORK MODEL The network model consists of one MR-BS, RSs at the vertices of the hexagonal cell with MR-BS positioned at the centre. There are several MSs deployed randomly, which is having multiple downlink paths, namely MR-BS—MS direct path or MR-BS—RS—BS multihop path. Here a Type II, transparent relay is considered, so the MS has to be in the coverage area of MR-BS for ranging. The path selection is done by the MR-BS. In order to calculate the path cost, the MR—BS requires the necessary information of each link, such as number of relays in the path, list of burst profiles for the access link and relay links.

The OFDMA PHY of IEEE 802.16 supports different forward error correction (FEC) encoding schemes, of which convolutional coding (CC) with tail biting is mandatory and other techniques which optionally supported are convolutional turbo coding (CTC), block turbo coding (BTC), low density parity check coding (LDPCC). In Table II. the burst profile represents the type of modulation used, FEC encoding technique, code rate and repetition rate along with their corresponding RRUI value is given respectively.

Figure 2. Path selection in a simple Relay based network

placement problem has been studied in different multi-hop wireless networks such as wireless sensor networks. It is demonstrated in some papers that, if above a certain number of relays are deployed no additional gain is achieved.

The paper is organized in the following way. Section II discusses about relay placements. Section III discusses about the simple path selection algorithm and path selection using RRUI metric and section V about simulation and results.

II. RELAY PLACEMENTS The positioning of relays between MR-BS and MS is one of the major issues in Wi-MAX based wireless networks [3].

The cell is divided into zones with different modulation schemes based on received SNR. The SNR of the receiver node is calculated as follows.

The received signal strength Pr is calculated using standard approach.

A. Simple path selection algorithm The path selection is a key factor in a relay based mobile networks. In some cases MS may get better quality signal in direct path (MR-BS—MS) itself than the indirect path (MRBS—RS—MS). In some cases, direct path may use fewer resources than indirect path. In these situations, a good decision has to be made in selecting the path. There are many path selection algorithms available like in literature [5, 6].

Here we use a simple path selection algorithm to test throughput performance. The proposed scheme uses weights of links in each path, i.e. BS-MS and MR-BS—RS—MS to calculate the total weight and select the path that gives maximum throughput. The weight gives the cost of OFDMA symbols associated with specific MCS to transmit a bit through the link.

(2) where Pt is the transmitted power in dBm , Gt and Gr is the antenna gain at the transmitter and receiver, PL is the path loss.

SNR in the receiver is calculated using (3) where B is the effective channel bandwidth in Hz, Nf the noise figure in dB and No the thermal noise level in dBm. B represents the channel bandwidth specifically used for information transmission in an OFDMA system. This can be determined by the following formula (4) where Fs is the sampling frequency, Nfft is the FFT size and Nused is the active subcarriers. The calculated received signal strength is the main parameter in deciding MCS between any two nodes. Based on SNR at the receiver node, transmission parameters are mapped to one of the thresholds specified in 802.16e-2005 standard which is given in Table I. In Wi-MAX standards mainly they use two modulation © 2012 ACEEE DOI: 02.AETAEE.2012.3.47

Figure 3. Different cell zones based on modulation

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Full Paper Proc. of Int. Conf. on Advances in Electrical & Electronics 2012 TABLE II. BURST PROFILE

The following steps are executed in sequence for path selection 1. Calculate the weight associated with the direct path.

2. Calculate the weight associated with the indirect path.

3. Find the percentage gain in throughput using (1).

If percentage gain is more than zero, select the path via relay. Otherwise transmit through direct link.

TABLE I. PATH DECISION PARAMETERS

The cost function for a particular path can be calculated as, In a simple relay based network shown in Fig. 2, it is assumed that MS is available in QPSK ¾ zone from the BS. Relay is placed in 16 QAM ½ zone from BS. MS is distanced from RS by 16 QAM ½ zone. By calculation we can find out the weight of the link between BS—MS Ws as 3/2 and that of the links via the relay BS—RS—MS as Wr =1/2 and Wp =1/ 2.Therefore, the percentage gain on throughput is calculated as in (1) and is 50 %.Since the path via relay is giving 50% gain on throughput, the path (r, s) via relay will be selected.

(5) where rj is the RRUI of jth hop in a DL path.

C. Example In Fig. 4. the network is deployed as per the model described earlier. For simplicity, we have deployed 9 MS in the MR-BS coverage area and 6 RS at the vertex of hexagonal cell with MR-BS at the centre. Consider MS9, having 3 different paths, i.e. direct MS-BS path, through relay RS3 to MR-BS and through RS6 to MR-BS. To determine which path is better, the path cost is calculated.

B. RRUI path selection metric Here we device a metric called RRUI, Radio Resource Utilization Index, to calculate the cost of the link. It is the normalized measurement of the number of subcarriers required to transmit 30 bytes in specific sub-carrier utilization index (SUCI) [6].

The path can be calculated as the summation of the weighted vales of RRUI of each link in the path. For MS having multiple paths for DL (download link), the path with lowest cost is selected.

B. Path cost calculation algorithm  Determine the number of MS i (N), i=1,2,...,N  Determine the number paths for each MSi (Pi,k), k=1,2..,P paths  For a MSi , determine the number of hops (h) for each path Pi,k =1,2,….j  Transmit all the link information to MR-BS © 2012 ACEEE DOI: 02.AETAEE.2012.3.47

Figure 4. A simple network scenario

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Full Paper Proc. of Int. Conf. on Advances in Electrical & Electronics 2012 In Fig. 5. the detailed view of the paths of MS9 in the above scenario is shown. From Table II. the cost of each path can be calculated. For the direct path the cost is same as the RRUI value of QPSK3/4 which is 5. The cost of the path via RS6 can be calculated as 10/9 +10/9 =20/9 (64QAM3/4) and that of path via RS3 is 15/8 +15/8=15/4 (16QAM1/2,1 ) from (5). So it can be seen that the path cost via RS6 is the minimum and is 20/9. In Fig. 6, it can be seen that the lowest cost path selected is through RS6. Similarly for all other MSs the same procedure is done and the path with minimum overall cost is selected. Fig 6. showing the complete path of the MSs in the network using RRUI path selection algorithm.

In these experiments, the Friss free space path loss model is used. It is also assumed that signal does not suffer from any shadowing or fading effect. Different simulation scenarios are taken like varying number of relays, varying the positions of relays to test the throughput improvement. One of the simulation scenarios is given in Fig. 7.

Figure 7. Snap shot of the simulation scenario in Qualnet Tool

Figure 5. Example showing the paths of MS9 in the network

IV. SIMULATION AND RESULTS For simulating this Relay based Wi-MAX networks we have used Qualnet 5.0 tool which supports advanced wireless library which includes 802.16e and the RRUI path selection algorithm and is implemented in matlab. For testing the path selection algorithm, relay placement and resource reuse, we take single cell system with LOS/LOS environments and omni directional antennas. A traffic load with Constant Bit Rate (CBR) traffic is assumed.

Figure 8. Increase in Throughput vs. Number of RS

Fig. 8. is plotted between increase in throughput and number of RS. The system throughput gain flattens or stabilizes when four or more relays are deployed. For this scenario, the BS cell is covered by the optimum number of relays found out earlier and the additional relay nodes don’t have any significant impact on the throughput gain or system capacity. Fig. 9, is plotted between Throughput gain vs.BSRS distance. The position of RS between BS and MS is varied and every time throughput is calculated. When the RS is nearer to BS or far from BS, the gain in throughput is less.

Fig. 10, gives comparison between Throughput gain and number of relays with and without spatial reuse. Spatial reuse allows RSs to operate on the same channel to transmit using the same frequency simultaneously without interference. This appreciably enhances the cell capacity by reusing the

Figure 6. RRUI metric based path selection

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Full Paper Proc. of Int. Conf. on Advances in Electrical & Electronics 2012 frequency within the cell. The key challenge is to maximize the number of concurrent transmissions ensuring successful transmissions. The relay is sectored into three and the resources are reused. From the plot it is clear that spatial reuse provides more throughput gain than the conventional one.

CONCLUSIONS The results prove that the RS between BS and MS increases the throughput than the conventional one without relays. This paper also discusses the optimum number of relays to be selected between BS and MS. After a certain number, throughput will not increase. These results avoid unnecessary deployments of more number of relays and the exact positioning of relay where we can achieve maximum throughput. This paper also discusses various path selection algorithms for transparent relays to reduce the usage of resources. It is also proved that spatial reuse along with relay makes better gain in throughput. The basic results pave path to use advanced path selection algorithms and relay positioning algorithms as the future work.

REFERENCES [1] Genc, V. Murphy, S. Murphy, J. “Performance Analysis of Transparent Relays in 802.16j MMR Networks”, Modeling and Optimization in Mobile, Ad Hoc, and Wireless Networks and Workshops, 2008. WiOPT 2008. 6th International Symposium, Issue. 1-3 April 2008, pp. 273-281.

[2] IEEE Standard, “Local and metropolitan area networks Part 16: Air Interface for Broadband Wireless Access Systems Amendment 1: Multihop Relay Specification” IEEE Std 802.16 June 2009, pp. 1-290.

[3] Bakaimis, B.A.; “Opportunistic Use of Mobile Relays for Mobile Positioning for Next Generation Networks”, EUROCON 2009, IEEE, pp. 1748-1754.

[4] Lei Xiao; Fuja, T.E.; Costello, D.J.; “Mobile Relaying: Coverage Extension and Throughput Enhancement”, Communications, IEEE Transactions on, vol.58, Issue.9, September 2010, pp. 2709-2717.

[5] Sojeong Ann Kyung Geun Lee Hyung Seok Kim “A Path Selection Method in IEEE 802.16j Mobile Multi-hop Relay Networks”, Sensor Technologies and Applications, 2008.

SENSORCOMM ’08. Second International Conference, 2531 Aug. 2008, pp. 808-812.

[6] Wang S-S, Yin H-C, Tsai Y-H, Sheu S-T “An effective path selection metric for IEEE 802.16-based multi-hop relay networks”, In Proceedings of the IEEEsymposium on computers and communications; 2007. p. 1051–1056.

Figure 9. Throughput Gain vs. BS-RS distance

Figure 10. Throughput gain vs. Number of RS with and without spatial reuse

© 2012 ACEEE DOI: 02.AETAEE.2012.3.47

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