Channel Spacing for Maximum Performance of 5 GHz WLAN Backhaul Links

Shazia Abbasi Institute of Information & Communication Technology

University of Sindh, Jamshoro, Pakistan abbasi.shazia@gmail.com

Quratulain Kalhoro Department of Computer Science ISRA University

Hyderabad, Pakistan Kallhoro.quratulain@gmail.com

Abstract—this study deals to quantify the impact of Adjacent Channel Interference (ACI) in multi-radio 802.11a based wireless nodes designed for WLAN. The nodes are equipped with three 802.11a wireless interfaces, two for backhaul connectivity with directional antennas and one for Access Point (AP) functionality. Adjacent channels in 802.11a are considered to be non-overlapping and expected ACI would be almost negligible. Observed results are, 802.11a channels produces significant interference when adjacent channels are assigned in consecutive order to nodes or to the nodes which are placed in the beam of directional antennas, they are being known as directed nodes. Our work focuses mainly on backhaul of 802.11a, we propose a channel assignment technique where adjacent nodes and directed nodes are placed with spacing of one channel, Conflict graph and greedy algorithms for channel assignment to backhaul links have been used. We evaluate our technique by OPNET 14 simulations, where we quantify the effect of ACI in terms of throughput, observed results for our proposed technique have shown that the throughput of the network is doubled as compared to system where channels are assign to adjacent nodes with consecutive channels without spacing of channel.

Keywords- ACI; 802.11a; AP; Multi-radio; WLAN;Throughpu;,Backhaul

I. INTRODUCTION

A. Background and Motivation Wireless networks are coming with advanced features

and techniques for efficient use of spectrum, such as multi APs WLAN to utilize available channels of spectrum for concurrent communication; Interference has been a primary issue in such networks due to broadcast nature of the wireless medium. There is possibility that one channel assign to multiple nodes in the effective area of node which create interference of concurrent transmissions known as co-channel interference, or by the leakage of near by channels assign to adjacent nodes known as adjacent channel interference[1]. Interference, particularly adjacent channel interference, has significant cause of reduction in performance of network, hence should be carefully managed in the design of network [2]. Interference either co-channel or adjacent channel, controlled by proper channel assignment of the links, further ACI could be reduced by increasing channel separation and using directional antennas [3].

B. Contributions We have considered WLAN based backhaul infrastructure, Backhaul links are static in nature and working on 5 GHz, Each node of backhaul is equipped with three ‘3’ radios two for backhaul communication with directional Antenna and one for Access Point functionality, This paper contributes to alleviate the effect of ACI in 802.11a WLAN. We have taken a case where each of the nodes in a multi-hop system is equipped with multiple radio interfaces and discuss the adjacent channel interference (ACI) issue that prevents the total throughput from scaling up with the number of radio interfaces. . This paper is contributing in particular to:

• Conflict graph model applied in two step approach (main links of node and directional link of node) to model ACI.

• Greedy channel assignment algorithm is used to fully exploiting the spectrum resource.

• Findings through simulator OPNET 14 Modeler produces improved results of our proposed technique that has given system Throughput approximately doubled as compared system uses nodes without channel spaces.

The road map of paper is as follows. We summarize literature review in Section II. Section, III presents the overview of channels of 5GHz. In Section IV, discussion on the system design is presented. The performance evaluation is shown in Section V. Findings of our study is defined in Section VI.

C. Related Work In the literature review we found two ways to improve the performance of multi-channel wireless networks:[4] use of multiple omni-directional antennas at each node working on separate channels, [5] or replace omni directional antennas with directional antennas which allowing a node to communicate with neighboring nodes on separate beams on same channels[6], this leads good spatial reuse while prior technique is not efficient in terms of spatial reuse. [7], [8] worked on ACI problem and found that interference can be managed by separating the affected radios by distance. In [9] the authors carry out some experiments on 802.11a testbed to compute the measurements of Adjacent Channel Interference (ACI) on a dual-radio multihop network using spectrum analyzer, they use omni directional antennas for in-lab testbed and field measurements. In [10] uses SINR to

Masood Ahmed Kalhoro IQRA University Karachi, Pakistan

director@technotronix.net

2009 Second International Conference on Computer and Electrical Engineering

978-0-7695-3925-6/09 $26.00 © 2009 IEEE

DOI 10.1109/ICCEE.2009.213

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2009 Second International Conference on Computer and Electrical Engineering

978-0-7695-3925-6/09 $26.00 © 2009 IEEE

DOI 10.1109/ICCEE.2009.213

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measure ACI of partially overlapped channels along with a theoretical model. [11] presents some analytical studies, that adjacent channels of 802.11a have such a power that make considerable interference, their study have covered spectral properties (inter-channel spectral distance, channel width and spectral mask) of channels and radio channel separation between nodes, they have identified that ACI effect can be observe on links with low path loss. In [12] authors presents the model based on Signal to Interference plus Noise (SINR) principle to calculate the effect of ACI in 802.11a/g they have observed that ACI effect enhanced when directional antennas placed closely.

D. 5 GHz spectrum 5 GHz is 300 MHz wide band spectrum having 12 non overlapped channels (8-channel in the 5.3 GHz and 4-channels in 5.8 GHz). Bandwidth of each channel is 20 MHz [13]. The 5 GHz (Us based) Unlicensed National Information Infrastructure (U-NII) band consists of three sub-bands: UNII1 (5.15-5.25 GHz) having 4 channels for indoor use, UNII2 (5.25-5.35 GHz) having 4 channels for indoor/ outdoor use and UNII3 (5.725-5.825 GHz) having 4 channels for outdoor use [14]. Table 1 lists the channels supported by 802.11a wireless networks [15].

II. SIMULATION DESIGN In the design phase, two scenarios have been designed

with the 5 GHz band • Backhaul nodes without channel spacing • Backhaul nodes with channel space

Both scenarios have 24 nodes served by a FTP server. The static topology has been designed with 8 AP’s; each access point has at least three ‘3’ interfaces every AP is averagely served to 3 users. Each AP has three ports one is reserved for access point functionality and others are used for backhaul connectivity. Initially system is silent up to 100ms, and then Traffic for FTP clients is generated in uniformly distributed manner. Parameters of system design are described in table 1

TABLE I. DEFAULT PARAMTERS OF WLAN SYSTEM

S NO Default Parameters Values Set

1 Data Rate bits/sec 11Mbps

2 Transmits Power (W) 0.005 W

3 Packet Received Power -95

4 Threshold CTS to Self Option Enable

5 Short Relay 7

6 Long Relay 4

7 AP Beacon Interval 0.02

8 Max Received Life-Time 0.5

9 Buffer size 256K

10 Large packet processing Drop

A. Backhaul nodes without channel spacing In first scenario, channels are assigned to backhaul links without keeping channel space which creates ACI particularly in directed links. Static channels have been assigned on backhauls as in figure: 1, AP_0 is connected to adjacent AP_1 on channel 36 and further connected to direct AP_2 on channel 40, while it has been mentioned that channel 36 and 40 are adjacent channels without any guard band which causes ACI in the system

Figure 1. Backhaul links without channel space

B. Backhaul nodes with channel space In second scenario, channels are assigned to backhaul links with keeping at least one channel Spacing either to adjacent links or to directed links, this leads guard band between channels and can reduce ACI in the system. Static channels have been assigned on backhauls as in figure:2. AP_0 is connected to two adjacent APs; link to AP_1 is on channel 36 and to AP_3 is on channel 44, while directed link of AP_0 to AP_2 is on channel 48

Figure 2. Backhaul links with channel space

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With keeping this channel spacing we have achieved high performance of system in terms of throughput with Minimum delay.

III. RESULTS AND DISCUSSIONS The This section represents desired simulated results

using OPNET 14 Modeler based on above mentioned two scenarios, we have quantified the effect of ACI in terms throughput and delay. The throughput in terms of bits per seconds, Figure.3 plots Time line Throughput and Figure. 4 shows average throughput of both scenarios

Figure 3. Timeline Throughput of both Scenarios

Produced results are much clear here to develop an understanding about the increase in Throughput second scenario is much higher in comparison to 1st scenario. Initially throughput is zero because system is in ideal conduction up to 100 sec as clients from BSS_0 starts transferring files from ftp server by using uniform distribution Figure. 3 shows that up to 150 seconds throughput is almost same for both scenarios but as

Figure 4. Average Throughputs of both Scenarios

System starts dealing with events from various BSSs then deference in Throughput have been observed shown in figure 3 and 4 that channel spacing average graph linearly increase up to 1Mbps but no channel spacing peek level is 0.4 Mbps

Figure 5. Average Delay of both scenarios

Delay analysis graph is presented in figure. 5 channel spacing delay’s peek value reaches up to 4ms but no channel spacing has 4.5ms its due to channel interference.

A. Justification For further justification we have simulated the deferent

experiments scenarios which give more authentic confidence approach to our channel spacing concept. In Figure.7 Average system delay increase linearly by increasing number of users in both approaches with channel spacing and without channel spacing, noticeable

Figure 6. Average wireless delay vs No. of users in per cell of both

scenarios

Point is that at every stage our proposed channel spacing concept has minimum average delay. Initially we have observed less difference but as number of users increases the

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deference gets clear that by giving channel spacing in 5G we can minimize the delay. Similarly when we are increasing the number of users per cell the maximum throughput could be achieved via giving channel spacing shows in Figure. 7 the log average throughput compared with number of users per cell in both scenarios as we have seen in figure 6 that delay was minimum when the number of users was minimum at initial stage here in Figure 7 simulated results shown maximum throughput achieved at first few points and in last when number of users are dense then over systems throughput is relatively low because the wireless LAN delay increases due to non-spaced channels.

Figure 7. Log average throughput via No. of users per cell in both scenarios

From above simulated results and discussion we have found that the even though in 5G has all orthogonal channels but still spacing between channels can gives better systems performance by increasing throughput and minimizing WLAN delay

IV. CONCLUSIONS By keeping channel spacing in backhaul links in 5G

WLAN systems have achieved higher system performance and relatively lower system delay. This concept would produce much better performance in dense systems we have seen in figure. 6 and 7 where delay and throughput and there relationships proved that performance of system is doubled in comparison to non-spaced channels in backhaul links.

REFERENCES

[1] Lili Qiu, Yin Zhang, Feng Wang, Mi Kyung Han, Ratul Mahajan. A General Model of Wireless Interference, in Proceedings of ACM MOBICOM, September 2007.

[2] Chen-Mou Cheng, Pai-Hsiang Hsiao, H.T. Kung and Dario Vlah, "Parallel Use of Multiple Channels in Multi-hop 802.11 Wireless Networks," Military Conference on Computer Communications and Networks (ICCCN 2006), October 2006.

[3] Cheng, C.-M., Hsiao, P.-H., Kung, H. T., and Vlah, D.: Adjacent Channel Interference in Dual-radio 802.11 Nodes and Its Impact on Multi-hop Networking, IEEE GLOBECOM 2006, San Francisco, CA, November 2006.

[4] Alicherry, M., Bhatia, R., and Li, L., “Joint Channel Assignment and Routing for Throughput Optimization in Multiradio Wireless Mesh Networks,” ACM MobiCom 2005, August 2005.

[5] Ko, Y. B., Shankarkumar, V., and Vaidya, N., “Medium Access Control Protocols using Directional Antennas in Ad Hoc Networks,” IEEE INFOCOM, March 2000.

[6] Yi, S., Pei, Y., and Kalyanaraman, S.,“On the Capacity Improvement of Ad Hoc Wireless Networks Using Directional Antennas,” ACM MobiHoc 2003, June 2003.

[7] Robinson, J., Papagiannaki, K., Diot, C., Guo, X. and Krishnamurthy, L, “Experimenting with a Multi-Radio Mesh Networking Testbed,” 1st workshop on Wireless Network Measurements (WiNMee 2005), Trento, Italy, April 2005.

[8] Adya, A., Bahl, P., Padhye, J., Wolman, A. and Zhou, L., “A Multi-Radio Unification Protocol for IEEE 802.11 Wireless Networks,” First International Conference on Broadband Networks (BROADNETS’04), 2004.

[9] Cheng, C.-M., Hsiao, P.-H., Kung, H. T., and Vlah, D., “Adjacent Channel Interference in Dual-radio 802.11 Nodes and Its Impact on Multi-hop Networking,” IEEE GLOBECOM 2006, San Francisco, CA, November 2006.

[10] A. Mishra, et. al. , “Partially Overlapped Channels Not Considered Harmful”, SIGMetrics/Performance’06, Saint Malo, France, June 26–30, 2006.

[11] V. Angelakis, S. Papadakis, V. Siris and A. Traganitis, “Adjacent Channel Interference in 802.11a: Modeling and Testbed validation”, 2008 IEEE Radio and Wireless Symposium, Orlando, FL, USA, Jan. 2008.

[12] V. Angelakis, A. Traganitis, V. Siris, "Adjacent channel interference in a multi-radio wireless mesh node with 802.11a/g Interfaces", IEEE INFOCOM 2007, Anchorage, Alaska, USA, May 2007.

[13] Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, IEEE 802.11 Working Group Std., September 1999 - 2005.

[14] James C. Chen, Jeffrey Gilbert, Atheros Communications, Inc. Measured Performance of 5-GHz 802.11a Wireless LAN Systems

[15] Channels and Maximum Power Settings for Cisco Aironet AutonomousAccessories: http://www.cisco.com/en/US/docs/wireless/access_point/channels/ios/reference/guide/1300_chp.html

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