Efficient Use of Partially Overlapped Channels in 2.4 GHz WLAN Backhaul Links

Shazia Abbasi Institute of Information and Communication Technology

University of Sindh Jamshoro, Pakistan

abbasi.shazia@gmail.com

Quratulain Kalhoro Research Scholar, CCSR

University of Surrey United Kingdom

q.kalhoro@surrey.ac.uk

Abstract—In 2.4 GHz WLANs, channel assignment is critical issue due to minimum availability of orthogonal channels (1, 6 and 11). WLANs which supports Multiple Access Points (APs), either require maximum number of orthogonal channels for individual links to reduce channel interference or some new strategies adopt to improve the utilization of orthogonal channels. This paper is deeming to find an efficient way to utilize the partially overlapped channels of 2.4 GHz band as the orthogonal channels for WLAN backhaul links, which has achieved high throughput due to minimum interference between static links of backhaul with directional antennas. In proposed work an equation is derived to find sets of orthogonal channels of 2.4 GHz band, directed graph applied on sets of orthogonal channels in two-step approach (direct links and directional links) of each node. This study has produced throughput results approximately double as compared to traditional channel assignment with utilization of Partially Overlapped Channels (POS). Future lines of this work would be to analyze proposed channel assignment over backhaul using 5 GHz band to reduce adjacent channel interference, same concept can be useful to increase the system performance with MIMO and wireless mesh networks.

Keywords- IEEE 802.11; Mesh Networks; Backhaul Links; Access Point; WLAN Channel.

I. INTRODUCTION The systems are moving to reshape the fundamental

communication service similar to that of provided by wired networks over standard LAN protocols such as Ethernet [1,2] to offer high quality communication, reliable delivery of data with high throughput. Maintaining high quality communication in WLAN is comparatively difficult to wired networks. WLAN backhaul comes up with a promise to provide the desired quality communication service [3], by keeping careful monitoring and attention to the conditions of backhaul links the desired quality communication can be achieved [4]. In multiple Access Point (AP) WLAN infrastructure, links on backhaul AP’s remain active simultaneously on different channels this may cause increase/decrease in interference depends on channel selection criteria for individual link [5]. In multi-hop wireless network, throughput decreases linearly with an increase in hop count due to generated interference of neighboring AP’s. Interference either primary (co-channel) or

secondary (adjacent channel) can be controlled by proper channel assignment scheme over links. Generally two approaches are being considered for channel assignment: (1) dynamic channel assignment scheme where channels are dynamically configured at each interface [8], making individual node capable enough to perform fast switching for channel selection, this property works efficiently for mobile nodes. (2) static channel assignment where fixed topology is set and channels are being assigned statically to the interfaces for required time period or can be set permanent, it is widely used for static nodes [9]. For this study we consider infrastructure based 2.4 GHz WLAN, where we focused on channel assignment to static links of backhaul with directional antennas. Each node of backhaul is equipped with three radios two are on 2.4 GHz for backhaul communication with directional antennas and one for Access Point functionality on 5 GHz. Interference level of static nodes with directional antennas can not only depended on the assignment of orthogonal channels to direct links of antenna, it depends on next to direct link which we call directional links Figure: 1.

Figure 1. Direct and directional links of wireless mesh network

It is because signals would be in specific direction and propagates up to longer distances, so it may cause an increase in interference when non-orthogonal (partially overlapped) channels would be assigned for the node being used in the directional antenna. The motivation of this work is to provide solution for improved system throughput, keeping effect of interference at its minimum while utilizing all channels of the

Masood Ahmed Kalhoro Research and Development (R & D)

TechnoTronix Pvt Ltd Karachi, Pakistan

Director@technotronix.net

2011 International Conference on Innovations in Information Technology

978-1-4577-0314-0/11/$26.00 ©2011 IEEE 7

spectrum including both non-overlapped and partially overlapped channels. This paper is contributing in particular to:

• Equation has been derived to find set of orthogonal channels for 2.4 GHz band which has Orthogonal and partially overlapped channels.

• Applied directed conflict graph in two-step approach, first on links for first tier (direct links) and then on 2nd tier (directional links) and present a system model for the backhaul links of 2.4 GHz WLAN.

• Simulation results (OPNET Modeler 14.0) proved that the proposed strategy provides double throughput as compared to traditional channel utilization scheme.

II. REVIEW The channel assignment problem is usually solved as a

coloring graphs, where colors represents the channels assigned to each node, the objective of coloring graph is to assign different colors to neighbor nodes. Traditional coloring graph (un-weighted coloring graph) is used to increase special re-use of channels when minimum colors are used to color the graph [10,11], consider each node as vertex in a graph and assign channels to the vertex in iterative process with greedy algorithm. Some work in [12] modeled channel assignment of non-overlapping Channels as Least Congested Channel Search (LCCS) where links continuously check the traffic rate on its channel and moved to LCC if traffic crosses the threshold level, while LCCS could not model the interference in some conditions where traffic increases rapidly. Scalable and fault tolerant distributed algorithms are presented in [13], they worked on link channel assignment with the intention to reduce interference between APs, they spread out the Least Congested Channel Search (LCCS) to a weighted graph coloring problem to reduce the sum of weights on all conflict edges thus reduced the number of conflict edges.

Their algorithm uses local information and is scalable in nature which could not model interference properly in terms of weight due to the rapid fluctuation of mobile devices on different links. Above all work focused only non-overlapped channels of 2.4 GHz band. In our work we applied the conflict graph on the static links of backhaul with directional antennas, where it work in two step to minimize channel interference of direct and directional links, with which all partially overlapped channels can be utilized efficiently. Alternatively partially overlapped channels can be used in WLAN with the concept of different energy masks at some loss of signal energy; energy loss is proportional to overlapped area of channels [14,15,16]. Work done in [17,18] converse some problems with the implementation of directional antennas in multi hop networks: Authors measure the interference between two directional antennas in [17], which have a 90 degree difference in orientation, but no information about the spatial separation of these antennas is given. The study of [18] observes the network performance related to its configuration of multi-radio 802.11g node through quantity study, their work confirmed that the performance of network is affected by the placement (orientation and distance) of directional antennas in multi-hop network [19] Proposed an algorithm, Directionality As Needed (DAN), which satisfy two objectives: Minimum network design cost and minimum interference between different links. Their work was limited to the interference level of direct links

while we try to minimize the channel interference on direct and directional links with orthogonal channel assignment.

III. SET OF ORTHOGONAL CHANNELS FOR BACKHAUL LINKS

WLAN systems working in Industrial Scientific and Medical (ISM) frequency band i.e. 2.4 GHz and 5.3/ 5.8 GHz (License Free Bands), both have multiple orthogonal and partially overlapped channels [20, 21]. The channel fundamentally represents the center frequency that the transceiver within the radio and access point uses (2.412 GHz for channel 1 and 2.417 GHz for channel 2) [22,23]. 2.4 GHz band is chosen by us to work as it has minimum number of orthogonal channels which has helped us to prove the benefit of proposed orthogonal channel assignment scheme for backhaul links of WLAN. 2.4 GHz is 83.5 MHz wide band, having 14 channels number of channels varies with the location Country to country in the world. Channels 1 to 11 used in America, channels 1 to 13 used in Europe, and all 14 channels are used in Japan. We are using 11 channels for our system model. There is 5 MHz separation between the center frequencies, and 2.4 GHz Signal occupies approximately 30 MHz of total frequency spectrum. The signal falls nearby 15 MHz on each side of the center frequency. As a result, one channel overlaps with several adjacent channel frequencies [24] this leaves the system model with only three channels (channels 1, 6, and 11) all are orthogonal while 8 are partially overlapped (for the U.S) [25]. For our work we need sets of channels which are orthogonal in nature.

A. Strategy forFinding Non-Overlapping Channels Non overlapping channels of 2.4 GHz band could be find

by set theory, sample space contain two sets:

{ , }S Orthogonal Non Orthogonal= − (1)

The elements of sample space are all channels of 2.4 GHz band (Ch1 to Ch11) Orthogonal set represented by On and contain all orthogonal channels of Chn, Non-orthogonal set represented by NOn and contain Non-orthogonal channels of Chn.

{ },n nS O NO= (2)

( )1

4N

nm

O n m=

= −∑ Where N=11 (3)

{ }n nNO S O= − (4)

Table: 1 contain sets of orthogonal channels for every channel.

B. Theorems With the help of above strategy Orthogonal (On) and Non orthogonal (NOn) sets pursue following two theorems. Theorem 1 (Union): Union of two sets must equal to sample space. Proof: { } { }n nS O U NO= (5) Theorem 2 (Mutually Exclusive Events): Two sets must be mutually exclusive sets.

Proof: { } { },n nO NO = (6)

8

TABLE I. SET OF ORTHOGONAL AND NON-ORTHOGONAL

NO. ( )

1

4N

nm

O n m=

= −∑ { }n nNO S O= −

1 {6,7,8,9,10,11} {1,2,3,4,5} 2 {7,8,9,10,11} {1,2,3,4,5,6} 3 {8,9,10,11} {1,2,3,4,5,6,7} 4 {9,10,11} {1,2,3,4,5,6,7,8} 5 {10,11} {1,2,3,4,5,6,7,8,9} 6 {1,11} {2,3,4,5,6,7,8,9,10} 7 {1,2} {3,4,5,6,7,8,9,10,11} 8 {1,2,3} {4,5,6,7,8,9,10,11} 9 {1,2,3,4} {5,6,7,8,9,10,11} 10 {1,2,3,4,5} {6,7,8,9,10,11} 11 {1,2,3,4,5,6} {7,8,9,10,11}

IV. SYSTEM MODEL

A. Proposed Network Model And Channel Assignment The network is modeled using direct conflict graph. In this work, we assume that there are ‘N’ network nodes and ‘L’ direct network links deployed in a network. Each wireless node is equipped with three radio interfaces. The complete network graph is then assumed as directed graph G= (N, L), where N= {n1,…,nn} is the number of nodes and L={l1,…,ln} is the number of direct links. Each node ni has a transmission range ri with directional antenna, where the condition for a node nj to receive proper signal from ni is || ni-nj || ≤ ri, where || ni-nj || is the distance between ni and nj. Notice that with the directional antennas || ni-nj || ≤ ri is not the actual condition for (ni,nj) ε L.

Figure 2. Network model and channel assignment

Some other links belong to G as signal range enhanced in the propagation direction of directional antenna. Each node ni has an interference range rint such that nj is interfered by the signal from ni if is || ni-nj || ≤ rint, the interference range rint in such cases would be much higher then the transmission range ri and nj can receive signals from ni beyond the range ri, in such systems rint of ni is up to next node of nj and suppose as nj+1. For efficient channel assignment orthogonal set of channels is assigned in rint ≤ || ni –(nj + nj+1) || of every node to reduce the interference This is the main distinction of our model.

B. Simulation Non-Orthogonal and Orthogonal With the band of 2.4 GHz,

each scenario consists of 8 AP’s each with 3 interfaces 2 used for backhaul connectivity with other AP’s and 1 interface is reserved for Access Point (AP) functionality for subscribers, each AP should serve multiple subscribers, one FTP Server which serves its connected clients uniformly. System parameters are shown in Table: II and default APs parameters are given in Table: III. The first scenario based on channel assignment without caring the orthogonal concept for backhaul links. In this scenario static channels have been assigned on

TABLE II. INFORMATION ABOUT SYSTEM

Scenario

Subscribers

Data rate

Supported to purposed

Non- Orthogonal 2.4G

26 11 Mbps NOT

Orthogonal 2.4G 26 11 Mbps Yes

Figure 3. Scenario 1 (Non Orthogonal Backhaul on 2.4 G)

backhauls as in Figure: 3, AP_0 is connected to two neighboring APs, it is connected to AP_1 on channel 1 and AP_2 on channel 3, it is mentioned above that channel 1and channel 3 are partially overlapped channels. Connectivity of nodes is shown in table: IV. The 2nd scenario based on our proposed channel assignment scheme with orthogonal concept for backhaul links. In this scenario static channels have been assigned on backhauls links as in Figure: 4. AP_0 is connected to neighbor AP_1 on channel 1 and AP_2 on channel 7. This show the orthogonality of channels as channel 1 and 7 are orthogonal to each other. Connectivity of nodes is shown in table: V.

Figure 4. Scenario 2 (Orthogonal Backhaul on 2.4 G)

TABLE III. DEFAULT AP’S PARAMETERS

Name of Parameter

Orthogonal & Non Orthogonal

(2.4 G) Physical Characteristics

Direct Sequence

Transmitted Power 0.005 Watt

Received power threshold -95

Short Retry Limit 7

Long Retry Limit 4

AP Beacon Level 0.02 sec

Max Life Time 0.5 sec

9

TABLE IV. PORT CONNECTIVITY AND CHANNEL ASSIGNMENT FOR NON-ORTHOGONAL

Name

Port 0

Port 1

Port 2

AP_0 AP Enable Ch 36 AP_1 with Ch1 AP_3 with Ch3

AP_1 AP Enable Ch 52 AP_1 with Ch1 AP_3 with Ch2

AP_2 AP Enable Ch 40 AP_1 with Ch3 AP_3 with Ch4

AP_3 AP Enable Ch 44 AP_1 with Ch4 AP_3 with Ch5

AP_4 AP Enable Ch 60 AP_1 with Ch5 AP_3 with Ch7

AP_5 AP Enable Ch 56 AP_1 with Ch6 AP_3 with Ch6

AP_6 AP Enable Ch 66 AP_1 with Ch7 AP_3 with Ch8

AP_7 AP Enable Ch 48 AP_1 with Ch8 AP_3 with Ch7

TABLE V. PORT CONNECTIVITY AND CHANNEL ASSIGNMENT FOR ORTHOGONAL

Name

Port 0

Port 1

Port 2

AP_0 AP Enable Ch 36 AP_1 with Ch1 AP_3 with Ch7

AP_1 AP Enable Ch 52 AP_1 with Ch1 AP_3 with Ch8

AP_2 AP Enable Ch 40 AP_1 with Ch8 AP_3 with Ch3

AP_3 AP Enable Ch 44 AP_1 with Ch7 AP_3 with Ch2

AP_4 AP Enable Ch 60 AP_1 with Ch3 AP_3 with Ch11

AP_5 AP Enable Ch 56 AP_1 with Ch2 AP_3 with Ch9

AP_6 AP Enable Ch 66 AP_1 with Ch9 AP_3 with Ch4

AP_7 AP Enable Ch 48 AP_1 with Ch4 AP_3 with Ch11

V. RESULTS AND DISCUSSIONS This section will provide some comparative results in terms of time line throughput, average throughput, time line media access delay and average media access delay of orthogonal and non-orthogonal channel assignment. Initially both systems are silent subscription so there is no transmission occurred that’s why the both system’s outcomes is zero during 0 to 100 sec it is configured in simulation tool that there will be no events occurred up to 100 sec.

Figure 5. Time line WLAN throughput of both scenarios

Figure: 5 shows time line throughput of orthogonal channel assignment along with a comparison to non-orthogonal channel

assignment, where throughput of orthogonal channel assignment is almost stable for the time when events occur, while throughput of non-orthogonal assignment is 10 times smaller then orthogonal time line throughput, in the middle of simulation non-orthogonal throughput touches to the orthogonal time line throughput. Figures 6 and 7 proofs that we can improve the performance of FTP Server by assigning proposed orthogonal channel in 2.4 G channels,

Figure: 6 shows the media access delay is extremely high in non-orthogonal channel assignment due to channel overlapping at backhaul which increases interference, while Figure: 7 shows the comparison Average throughput of both scenarios, it further proves that with non-orthogonal channels, system performance decreases in terms of throughput.

Figure 6. Media access delay in both scenarios

Here it could be seen that initially system is in silent state when clients resident in all BSSs starts communicating with FTP sever then throughput rapidly increases in our purposed orthogonal scheme its clearly observed that non-orthogonal assignment never crossed the orthogonal average line, so we could achieve more than 50% throughput by assigning the proper channel assignment on backhaul links.

Figure 7. Average of over all system throughput of both scenarios

VI. JUSTIFICATION It’s reasonably proved that proper orthogonal channel

assigning gets dramatically high throughput for further Justification see in figure: 8 and 9, when we are increasing the number of users in overall System the performance of system enhanced with orthogonal channel assignment.

10

Figure 8. Average WLAN delay vs number of users

Figure 9. Average WLAN throughput vs number of users

CONCLUSION An efficient way of utilizing orthogonal and partially

overlapped channels of 2.4 GHz band has been introduced for the wide deployment of 802.11 (WLAN) to achieve higher throughput. Proposed channel assignment is applied to backhaul of WLAN with 2.4 GHz band having limited number of orthogonal channels, concept was based on assigning sets of orthogonal channels to links in the interference area of every node, where nodes are static and working with directional antenna. By maintaining such orthogonality of channels on links in the interference area of every node, our study has found decrease in channel interference and increase in throughput, simulated results clearly shows that orthogonal concept achieves approximately double throughput as compared to non orthogonal arrangement.

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