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) 5th 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
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