Proc. of Int. Conf. on Advances in Communication, Network, and Computing, CNC
Fuzzy Rule based Channel Allocation for Efficient Spectrum Management in WiMAX Networks Abdul Quaiyum Ansari1, Premchand Saxena2, and Koyel Datta Gupta3 1
Jamia Millia Islamia/Department of Electrical Engg, New Delhi, India Email:
[email protected] 2 Jawaharlal Nehru University/School of Computer & Systems Sciences, New Delhi, India Email:
[email protected] 3 Research Scholar, Jamia Millia Islamia/Department of Electrical Engg, New Delhi, India Email:
[email protected]
Abstract— The WiMAX (IEEE 802.16) technology offers a mechanism to create a multi-hop mesh network and provides a low cost alternative to the cabled access networks. This paper proposes a fuzzy rule based channel allocation technique for efficient spectrum management in multi-channel IEEE 802.16 network (WiMAX). The Subscriber Stations are categorized on the basis of their average traffic pattern. The frequency use by the network is optimized through re-allocation of sub-carrier frequency without degrading the QoS. Simulation using Matlab have been used to demonstrate the effectiveness of the proposed method in terms of Channel Utilization Ratio and Channel Acquisition Delay. Index Terms— Base station, Subscriber Station, Channel Acquisition Delay, Channel Utilization Ratio
I. INTRODUCTION With the exponential rise in the demand for broadband services, it is essential to provide cheap and reliable connections for urban as well as rural users. The Broadband Wireless Access (BWA) is considered effective since it does not require point to point connection and the maintenance cost is also low. Worldwide Interoperability for Microwave Access (WiMAX) is the new emerging technology which provides high data rate with secure wireless communication for long and short distances. In mobile WiMAX (IEEE 802.16e) we have both, fixed and mobile users. So the inter-cell movement of users also plays a vital role in the performance of the IEEE802.16e based wireless networks. In previous wireless cellular networks no support for wireless broadband technology was provided, hence permanent channels allocation was not required. Now for permanent allocation of channels in the network, Fixed Channel allocation scheme can be used. But Fixed Channel allocation may face blocking (no connection available) for a non-uniform traffic. As cell size supported by a WiMAX cell may be up to a radius of about 50 Km, so number of channels to be allocated to a specific cell may be more than even 200 channels keeping Co-Channel interference at its minimum. So for irregular or high traffic a combination of both Fixed Channel Allocation and Dynamic Channel Allocation schemes (Hybrid Channel Allocation) works well. The demands for wireless broadband technology have increased rapidly. WiMAX is evolving as the best © Elsevier, 2014
solution to provide BWA [1]. Besides BWA it also provides support for cellular mobile phone users. In this paper [1] a layered critical path algorithm is proposed which achieves better scheduling length for high density network. In [2-4], the authors employed different spatial reuse techniques to make non-interference links work concurrently. In [5], the authors proposed a centralized scheduling algorithm to provide high qualified wireless multimedia services. The algorithm emphasized on the delay function of the mesh nodes in transmission trees. Ref. [6], considered QoS, and proposes a centralized QoS, which guarantees throughput enhanced scheduling scheme. The authors [7], proposed a bidirectional concurrent transmission model, which simultaneously considered transmissions of uplinks as well as downlinks. Authors in [8], designed uniform slot allocation algorithms which exploit combined uplink and downlink slot allocation based on the centralized scheduling. In [9], the work presents a centralized scheduling and routing scheme for flows with different QoS requirements. The authors [10] proposed a routing and packet scheduling for a linear chain IEEE 802.16 mesh network to maximize the throughput considering traffic conditions and interference present of the network. They have proposed an integer linear programming formulation for the problem. Ref. [11] concluded the work by proving that the single, double and multiple channel allocation proposed by them can shorten the length of scheduling and increase the utilization of bandwidth. But the effect of neighbouring interference was not included. To reduce the channel acquisition delay and improve the channel utilization ratio, we have proposed a fuzzy rule based channel allocation scheme which also considers the neighboring interference. The rest of the paper is organized as follows. Section 2 presents the preliminary definitions. Section 3 presents our system model and channel allocation algorithm. Section 4 gives the simulation results. Section 5 concludes this paper. II. PRELIMINARY DEFINITIONS A. Interferring Neighbour The interfering neighbor of given node i in the network is defined as set of node j, j ∈ all the nodes in the network, which are within the transmission range of node i. B. Primary interference Primary interference [12-13] occurs when a node i has been scheduled to transmit and receive in the same time slot. C. Secondary interference Secondary interference [12-13] occurs when a receiver is within the transmission range of more than one transmitter transmitting during the same time slot. D. Adjacent channel interference Contraint Adjacent channel interference constraint states that channels assigned to interfering nodes must maintain a minimum separation of few channels, such that no two adjacent frequency spectrums are functional within the same transmission range. E. Fixed Channel Allocation (FCA) In a general wireless communication network Fixed Channel Allocation means to allocate a fixed set of channels to a specific cell irrespective of the actual demand. All cells in the network have their own set of disjoint channels [14-15]. F. Dynamic Channel Allocation (DCA) The concept of Dynamic Channel Allocation [14] is to allocate channels dynamically to the mobile users in the cellular network as when a request is raised, without allocating fixed sets of channels to the cells. The FCA may fail when the traffic load of cell is non-uniform and rises more than the fixed resource allotted to it. DCA may be either centralized or distributed in nature. H. Hybrid Channel Allocation (HCA) HCA is the combination of FCA and DCA. Total channels in the network are divided into two sets of disjoint channels. One set of channels is allocated using FCA to all the cells in the network. When the fixed channels are already in use, the second set is allotted dynamically on the basis of request generated for the new call in that cell [15]. 352
I. Orthogonal Frequency Division Multiple Access(OFDMA) Orthogonal Frequency Division Multiplexing (OFDM) is a technique which transmits large data over the radio wave by splitting the signal into multiple smaller sub-signals that are transmitted simultaneously over different frequencies (orthogonal to each other) to the destination. The concept of OFDM is extended in OFDMA by assigning subsets of subcarriers to individual users in a multi-user shared channel network. J. Time Division Multiple Access (TDMA) The Time Division Multiple Access technique is used for allowing several users to share the same frequency carrier by dividing the signal into different time slots. The users can transmit data through the shared channel in their respective time slot. III. PROBLEM FORMULATION In this section, we formally describe in details the system model and the formulation of the channel allocation algorithm. In a typical wireless scenario a user (stationary or mobile) may require initiating a transmission and in doing so requires a channel for the same. To use the available frequency spectrum efficiently (i.e. without wastage of frequency carrier and minimizing the request blocking) the spectrum is divided into fixed and dynamic sets. The objective is to provide carriers to all users based on their demand. A. System Model Our system model is IEEE 802.16 mesh network (multi-hop) with two BS (Base Station) and multiple SSs (Subscriber Stations) (shown in Fig. 1). We are using OFDMA and TDD techniques, where OFDMA is used to divide the channel into sub-channels and then TDD is deployed to divide each sub-channel into slots. Each SS is equipped with a single transceiver that restricts them from transmitting and receiving packets simultaneously in a single time slot. The number of mutually orthogonal channels available is 10.The number of sub-channels is 16.The number of slots per sub-channel is 128. The SSs can be stationary or mobile in our system model. The transmission range of all nodes is considered to be identical. During the transmission of packets between a sender-receiver pair, both should operate in the same channel. In a wireless mesh network, transmissions may collide in two ways—these are typically referred to as primary and secondary interference [12-13]. For a multi-channel system as our model, secondary interference can only be eliminated if the other interfering transmissions are carried through channels not in use by the operating receiver. Hence, to avoid any secondary interference, the interfering stations of any certain SS (acting as a receiver) cannot have more than one sender operating in the same carrier spectrum as that of the receiver. There are 2 BSs and 20 SSs in this model .The SS numbered 3 and 9 are mobile; all the other SS are stationary. A link between two SSs indicates they are within the transmission range of each other. The channels are divided into fixed and dynamic set (in the ratio of 1:4). The dynamic channels are allotted in a distributive manner by the base station once no fixed channel is free or available.
Figure 1. System Model with 2 BSs and 20 SSs
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Assumptions: 1. The network uses several mutually orthogonal channels, which do not interfere with one another avoiding any adjacent channel interference. 2. The SSs cannot receive and transmit concurrently so primary interference cannot take place. 3. The speed of movement of the mobile SSs (3 and 9) are set at 5 m/s. 4. Propagation Delay of the Wireless Link 1ms. B. The Algorithm The traffic load of the Subscriber Stations is evaluated and then they are categorized as heavily loaded or medium loaded or sparsely loaded based on average traffic pattern. The interfering neighbors for each SS are listed. The available frequency spectrum is divided into two disjoint sets as fixed and dynamic. Each channel is divided into orthogonal sub-channels (under OFDMA). Each sub-channel is divided into slots (using TDD). A slot is reserved for high priority requests. The SSs generates a request for a new demand/ handoff demand (for mobile SSs 3 and 9) and transmits the request to the BS. The call priority is set based on the combined effect of the nature of the request (new call/handoff) and density of traffic of the SS. The BS searches for a fixed channel, if available, allot the carrier to the requesting SS; otherwise looks for a noninterfering free dynamic channel by re-allocation. If no dynamic channel is available, but the request is categorized as high priority, the reserved slot is allotted. Algorithm-Channel Allocation Input: Geographical locations of the BSs and SSs (total n in number) Output: Channel Allocation: Maximum number of channels required Begin /*Initialization*/ 1. a. Sort all SS according to traffic as High, Medium and Sparse. b. The interfering neighbours (INij) for each SSi are identified through broadcasting. i,j∈ {1,n}& i≠j c. Channels are divided as fixed and dynamic. Sub-channels and slots are also derived. 2. a. Li←Ø for i ∈ {1,n} /*List of sub-channels/slots allotted to each node is null*/ b. Initialize list of sub-channels/slots allotted to interfering neighbours. 3. Each sub-channel (under OFDMA) is divided into slots (TDD).A slot is reserved for high priority calls. /* Channel Allocation*/ 4. An SSi sends a request (REQ) to the BS for sending packets to SSk; count_searchi=0 5. The BS searches a free slot sl. count_searchi++// no. of search incremented /*If the free slot/sub-channel is used by the interfering neighbours of the sender or by interfering neighbours of the receiver */ If sl≠Ø// free slot is available If sl ∈ INijL || sl ∈ INLkij /*the slot is in use by INij or by INkj */ Decision=0 /* Next statement executed when channel reallocation is used*/ Else if sl ∈ Adjacent{INLij} /*If adjacent sub-channel is in use by INij*/ Decision=f(Adjacent{INLij},sl) Else Decision =1 Endif 354
6. If Decision =0 /* Priority of the request is checked*/ If Proc_priority_REQ() = High Li← Li ∪ reserved slot Else /*3 unsuccessful search permitted*/ If count_searchi0 &
