Worldwide Interoperability for Microwave Access (Wimax)

Introduction

The need for cheaper, efficient and convenient ways to communicate digitally has become increasingly important. Worldwide Interoperability for Microwave Access (WiMAX) is the first fourth generation (4G) wireless technology standard that enables the delivery of last mile wireless broadband access as an alternative to cable and Digital Subscriber Line (DSL) (WiMAX Forum 2006). It uses microwave frequency band to transmit data wirelessly. WiMAX is standardized by the Institute of Electrical and Electronics Engineers (IEEE), belonging to IEEE 802.16 family of wireless networks which defines the wireless metropolitan area network (WMAN) formed in 1998. It was first deployed in 2006 by Korea Telecom, in the Seoul metropolitan area working on 2.3GHz version of the mobile WiMAX service (Roh, and Yanover, 2009).

Function

WiMAX delivers wireless high speed broadband service to both fixed and mobile users over a large geographic area. The initial standard (IEEE 802.16) operates in 10GHz to 66GHz range which was later amended (IEEE 802.16a) to work in the 2GHz to 11GHz range, easing regulatory issues and eliminating the need of line of sight. The standard defines both the physical layer and the medium access control layer (Omerovic n.d.). With the new revised standard, WiMAX coverage extends up to 30 miles radius and can be used by unlicensed users. In was further amended in 2005 as IEEE 802.16e (mobile WiMAX) to include mobility support. “It uses licensed bands at 3.5GHz and 10.5GHz internationally and 2.5GHz to 2.7GHz in the United States; and unlicensed 2.4GHz and 5.725 – 5.825GHz” (Andrews, Ghosh, and Muhamed 2007: 156). The 2GHz to 11GHz bands were assigned for mobility and 2GHz to 6GHz bands for mobile applications (Andrews, Ghosh, and Muhamed 2007).

Capacity

WiMAX throughput capability is based on the channel bandwidth which is selectable from a defined channel bandwidth (1.25MHz to 20MHz), this in turn provides a very flexible deployment. WiMAX provides a very high speed broadband access to the internet with speeds up to 70Mbps when using 20MHz channel bandwidth, with the use of multiple antennas providing a better signal. Subscribers further from the base station experience lower speeds as compared to those nearer to the base station and the speed is also affected by the number of subscribers using a particular base station. It provides a ratio of 3:1 uplink and downlink at 10MHz using time division duplex (TDD) scheme (Andrews, Ghosh, and Muhamed 2007). The amended standard (IEEE 802.16a) operating at 2-11GHz spectrum provides an idle coverage area of up to 30miles radius.

Access

WiMAX system is set up like a conventional cellular system employing frequency reuse that enables it to work efficiently without the need of line of sight, based on point to multipoint access. The WiMAX network consists of two basic parts, the base station (WiMAX tower) which provides internet service within a large coverage area with the help of a higher broadcasting power antenna and the WiMAX receiver (Customer Premise Equipment or CPE) which is at the subscriber end.  The CPE is connected to either a Wi-Fi system or directly to the computer using an access card.  A WiMAX backhaul is used in connecting between base stations and to a backbone, which enable users to roam between base stations. WiMAX network allocates upload and download bandwidth based on the requirements set by the subscriber. Current WiMax specification is based on a Time Division Duplex (TDD) method of multi-user access, which is best suited for mobile internet services (Roh, and Yanover, 2009).

Routing

The IEEE 802.16 defines the standard for physical and MAC layer but the routing protocol is not defined by the standard (Rasheed et al 2010). WiMAX network can be deployed in various modes; point to multipoint mode or as a mesh mode. The point to multipoint mode works in a situation where a base station with a large coverage area provides service to multiple subscriber stations within the coverage area. The mesh mode is optionally deployed when “traffic demands are aggregated at a set of subscriber nodes, the traffic demands at the subscriber nodes are delivered to a set of base stations nodes which functions in the point to multipoint mode” (He n.d.: 2). There other types of routing deployed in network like Dynamic Source Routing (DSR), Ad hoc On-demand Distance Vector routing (AODV) and Destination-Sequence Distance-Vector routing (DSDV) with DSDV providing the best performance when compared to the aforementioned (Rasheed et al 2010). As there is no defined standard, it is left to the provider to use the best routing scheme. 

Switching

WiMAX systems employ packet switching techniques to transport data.  A packet switch is a device in a data transmission network that receives and forwards packets of data. The packet switch receives the packet of data, reads its address, searches in its database for its forwarding address, and sends the packet toward its next destination.   This is in contrast to 3G networks such as W-CDMA where circuit switching is employed.  Circuit switching makes continuous path connections based on a signal’s time of arrival (TDM) port of arrival (cross-connect) or frequency of arrival. In WiMAX, each transmission is packetized and individually addressed to enable fast routing through the network. This leads to low latency in transmission making multimedia broadcasting possible. The switching utilized by WiMAX is called Adaptive MIMO (Multiple Input Multiple Output) Switching. “Adaptive MIMO switching (AMS) is a scheme to switch between multiple MIMO modes to maximize spectral efficiency with no reduction in coverage area” (Roh, and Yanover 2009)

Multiplexing

WiMAX systems use orthogonal frequency division multiplexing (OFDM) (Ibanez et al 2008). OFDM provide an efficient way to overcome the challenges of non line of sight (NLOS) multi path channel transmission. It also provides an easy transceiver structure with far superior performance, allowing available spectrum resources to be utilized efficiently by time and frequency subchannelization (Roh, and Yanover, 2009). The OFDM implementation in WiMAX networks “makes it easier to exploit frequency diversity and multiuser diversity to improve capacity” (Andrews, Ghosh, and Muhamed 2007: 449). There are slight difference in the implementation of OFDM physical layer between the mobile version and the fixed version. The fixed version uses 256 Fast Fourier Transform (FFT) based physical layer while the mobile version uses between 128 bits to 2.048 kilobits of FFT sizes on a scalable OFDM based physical layer. This concept enables the network to achieve higher performance with easy and flexible implementation.

REFERENCE

Roh, w., and Yanover, V.  (2009) WiMAX Evolution: Emerging Technologies and Applications Haboken:  John Wiley & Sons, Ltd

WiMAX Forum (2006) 5^th plugfest. Whitepaper. USA: Bechtel Labs [online] available from <www.wimaxforum.org/technology/downloads/Mobile_WiMAX_Plugfest_WhitePaper.pdf > [23 November 2011]

Omerovic, s. (n.d.) WiMAX Overview. Unpublished booklet. Slovenia: University of Ljubljana [online] available from <http://multiinfocom.ru/ru/artpdf/s_omerovic.pdf > [24 March 2011]

Andrews, G., Ghosh, A., and Muhamed, R. (2007) Fundamentals of WiMAX: Understanding Broadband Wirelss Networking Upper Saddle River: Pearson Education Inc

Rasheed, M., Khan, M., Naseem, M., and Hussain I. M. (2010) ‘Performance of Routing in WiMAX Networks’. IACSIT International Journal of Engineering and Technology 2 (5), 1793-8236

He, J., Fu, X., Xiang, J., Zhang, Y., and Tang, Z. (n.d.) Routing and Scheduling for WiMAX Mesh Networks Unpublished booklet. Goettingen: University of Goetingen [online] available from <http://www.net.informatik.uni-goettingen.de/publications/1558/WiMaxMeshNetRoutingScheduling09.pdf> [23 November 2011]

Ibanez, S., Santos, R., Licea, V., Block, A. E., and Ruiz, M. A. G. (2008) ‘Hybrid WiFi-WiMAX Network Routing Protocol’. [online] IACSIT International Journal of Engineering and Technology 4 (5), 193-236 available from < http://prof