25-08-2017, 09:32 PM
SYSTEM-LEVEL MODELLING OF IEEE 802.16E MOBILE WIMAX NETWORK: KEY ISSUES
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ABSTRACT
WiMAX has attracted a lot of attention recently in the telecommunication community including researchers, product developers, and service providers. the Application Working Group of the WiMAX Forum has developed a standard simulation methodology that describes the key features to be simulated, the method of simulating these features. This system-level methodology has been used in several public and commercial WiMAX simulation models. This article presents a summary of the simulation methodology, and discusses the key issues and common mistakes in simulating various features of network configuration, and the physical, MAC, and application layers.
INTRODUCTION
The WiMAX Forum is a consortium of over 400organizations interested in IEEE 802.16 based broadband wireless access. The members include service providers, equipment vendors, chip vendors, researchers, and users. The Forum develops interoperability specifications for equipment using IEEE 802.16 standards. WiMAX Forum certification ensures that the equipment purchased from different vendors will interoperate. Most networking and telecommunications technologies have similar interoperability organizations; for example, the WiFi Alliance for IEEE 802.11 wireless LAN standards. The Application Working Group of the WiMAX Forum is chartered to study the performance of various applications on Wi-MAX networks and recommend best practices for optimizing the applications performance. The tasks include developing standard application workload models.
The simulation methodology described in the WiMAX Forum document and summarized here is general, so it can be used with any modeling plat form such as Network Simulator 2 (NS2). The organization of this article is as follows The next section provides an overview of amobile WiMAX system and its key technology(OFDMA). We then highlight the system-leveland link-level approaches of modeling The following section addresses physical layer (PHY) modem abstraction for system simulation. We then coverthe medium access control (MAC) layer model ing issues, and discuss scheduler design andmodeling, which are key to the performance of aWiMAX system. The final section captures the issues in application traffic modeling.
OVERVIEW OF MOBILE WIMAX AND OFDMA
IEEE 802.16 supports a variety of PHY layers. To allow non-LOS(NLOS) communication, the IEEE 802.16 working group designed the orthogonal frequency division multiplexing (OFDM) PHY using spectrum below 11 GHz. Most of the current WiMAX products implement this PHY. In this PHY multiple subscribers use time-division multiple access (TDMA) to share the media. OFDM is multicarrier transmission in which thousands of subcarriers are transmitted, and each user is given complete control of all subcarriers. Formobile users, it is better to reduce the number of subcarriers so that the subscriber station can use higher transmission power per subcarrier and increase their signal-to-noise ratio. This combination of time- and frequency-division multiple access in conjunction with OFDM is called OFDMA.
SYSTEM LEVEL VS.LINK LEVEL MODEL
A link-level model is used to study the transmission between a base station (BS) and one or more users. The key is to study the point-to- point link and the effect of various link-leveldesign decisions on the performance. Link-level models generally concentrate on the PHY layer Higher layers, such as IP routing, TCP sessions or applications, may or may not be modeled A system-level model, on the other hand, consists of a network of BSs, and the emphasis is on the application layer performance as expressed by user-perceived quality of service parameters such as voice or video quality. The PHY layer is abstracted as much as possible. Figure 1 shows important issues in various layers. A link-level model concentrates on the PHY layer, while a system-level model concentrates on the other layers.
CENTER CELL CONFIGURATION
It is well known that the performance of a wireless transmission is determined by the signal-to noise ratio (SNR). The noise usually arises from stray transmissions and thermal causes. However, in WiMAX as well as other cellular networks, “interference” from nearby BSs may affect the signal much more than noise; therefore, it is important to model the signal-to-interference plus-noise ratio (SINR). To determine the interference,
we need an accurate representation of various sectors of a cell and the surrounding cells. System-level models traditionally use a center cell configuration consisting of 19 cells, as
shown in Fig.2. Basically the network consists of 19 hexagonal cells with six cells surrounding the center cell in the first tier. There are 12 cells surrounding the center cell in the second tier. Each cell has three sectors by default; thus, there are 57 totalsectors. A number of mobile subscriber stations(SSs) are randomly dropped over these 57 sectors.
MAC LAYER MODELING
The main functions at the MAC layer that havesignificant impact on the performance ofWi-MAX systems are: scheduling, automaticrepeat request (ARQ) and hybrid ARQ(HARQ), fragmentation and packing, and packetheader suppression (PHS). Among these, thescheduler is possibly the most significant andcomplex. If an inefficient scheduler is used, it ispossible to get meaningless results from simulationmodels. Also, the IEEE 802.16 standarddoes not specify any scheduling mechanism. Inthis section we discuss the simulation issues related to scheduling. The MAC scheduler must efficiently allocatethe available radio resources in response tobursty data traffic, time-varying channel conditions,and specified scheduler criteria, if any.The data packets are associated with serviceflows with well-definedQoS parameters in theMAC layer so that the scheduler can correctlydetermine the packet transmission ordering overthe air interface. The scheduling service is provided for bothdownlink and uplink traffic. In order for theMAC scheduler to make an efficient resourceallocation and provide the desired QoS in theuplink, the SS must feedback accurate and timely information as to the traffic conditions andQoS requirements. The downlink scheduler basically allocates
CONCLUSION
System-level modeling of IEEE 802.16e-based mobile WiMAX networks requires a careful consideration at every layer including the physical, MAC, and application layers. It is possible to come up with misleading results unless careful attention is paid to various key features at these layers. Even network configuration and user placement are import- ant. For this reason, the WiMAX Forum has developed a system-level simulation methodology. The summary guidelines presented in this article can be used by WiMAX service providers, equipment vendors, and users in their performance evaluation projects. These guidelines are general in that they can be used for any kind of simulation tool such as NS2, OPNET, OMNET, and Qualnet. In this article we discuss many of the WiMAX system modeling issues: physical layer modeling, MAC layer modeling, and applicationmodeling.