Spectrum selection
 


The current WiMAX license bands in operation reside in the 2.3GHz, 2.5GHz and 3.5GHz ranges, with still more occupation in the upper 5GHz unlicensed region. It’s these frequency allocations that we'll be discussing in more detail.

WiMAX CPE customer provided equipment built to 802.16de IEEE standards such as laptops, cellular phones and portable hotspot devices are designed to operate on these frequencies the same way WiFi Technology products are build to 2.4GHz and 5GHz product specifications.

WiMAX License Bands

2.3GHz
The 4G WiMAX spectrum allocation for this RF range is divided between two bands holding WiMAX license to 2305 - 2320MHz, and 2345 - 2360MHz, and are known as the Wireless Communication Services bands.

This RF range is shared with Digital Audio Radio Service which operates on neighboring bands bordering its frequency; a questionable concern for radio interferences exists here.

2.5GHz
This region of the WiMAX license bands in the range of 2500 - 2690MHz contains many MMDS Multichannel Multipoint Distribution Service providers including some of those in the television service arena.

This band shows great promise in future deployment and expansion in upcoming years as WiMAX solutions are pushing the limits to increase coverage in service areas that provide connectivity for newer 4G WiMAX tri band products that offer true WiMAX Interoperability.

3.5GHz (3.65GHz United States)
The 300MHz of bandwidth from 3.3GHz to 3.6GHz is a licensed WiMAX spectrum that has been allocated for broadband wireless access in numerous countries around the world with the exception of the United States which has allocated this portion of the electromagnetic spectrum mainly for military use.

With this 3.65GHz WiMAX spectrum opportunity for broadband wireless access rolling out across North America it is very clear that 4G WiMAX wireless solutions will take a firm position in the future of high speed wireless service for years to come.



Spectrum allocation
There is no uniform global licensed spectrum for WiMAX, however the WiMAX Forum has published three licensed spectrum profiles: 2.3 GHz, 2.5 GHz and 3.5 GHz, in an effort to drive standardisation and decrease cost.

In the USA, the biggest segment available is around 2.5 GHz, and is already assigned, primarily to Sprint Nextel and Clearwire. Elsewhere in the world, the most-likely bands used will be the Forum approved ones, with 2.3 GHz probably being most important in Asia. Some countries in Asia like India and Indonesia will use a mix of 2.5 GHz, 3.3 GHz and other frequencies. Pakistan's Wateen Telecom uses 3.5 GHz.

Analog TV bands (700 MHz) may become available for WiMAX usage, but await the complete roll out of digital TV, and there will be other uses suggested for that spectrum. In the USA the FCC auction for this spectrum began in January 2008 and, as a result, the biggest share of the spectrum went to Verizon Wireless and the next biggest to AT&T. Both of these companies have stated their intention of supporting LTE, a technology which competes directly with WiMAX. EU commissioner Viviane Reding has suggested re-allocation of 500–800 MHz spectrum for wireless communication, including WiMAX.

WiMAX profiles define channel size, TDD/FDD and other necessary attributes in order to have inter-operating products. The current fixed profiles are defined for both TDD and FDD profiles. At this point, all of the mobile profiles are TDD only. The fixed profiles have channel sizes of 3.5 MHz, 5 MHz, 7 MHz and 10 MHz. The mobile profiles are 5 MHz, 8.75 MHz and 10 MHz. (Note: the 802.16 standard allows a far wider variety of channels, but only the above subsets are supported as WiMAX profiles.)

Since October 2007, the Radio communication Sector of the International Telecommunication Union (ITU-R) has decided to include WiMAX technology in the IMT-2000 set of standards. This enables spectrum owners (specifically in the 2.5-2.69 GHz band at this stage) to use WiMAX equipment in any country that recognizes the IMT-2000.


Spectral efficiency
One of the significant advantages of advanced wireless systems such as WiMAX is spectral efficiency. For example, 802.16-2004 (fixed) has a spectral efficiency of 3.7 (bit/s)/Hertz, and other 3.5–4G wireless systems offer spectral efficiencies that are similar to within a few tenths of a percent. The notable advantage of WiMAX comes from combining SOFDMA with smart antenna technologies. This multiplies the effective spectral efficiency through multiple reuse and smart network deployment topologies. The direct use of frequency domain organization simplifies designs using MIMO-AAS compared to CDMA/WCDMA methods, resulting in more effective systems.
Inherent limitations

WiMAX cannot deliver 70 Mbit/s over 50 kilometers (31 mi). Like all wireless technologies, WiMAX can operate at higher bitrates or over longer distances but not both. Operating at the maximum range of 50 km (31 mi) increases bit error rate and thus results in a much lower bitrate. Conversely, reducing the range (to under 1 km) allows a device to operate at higher bitrates.

A city-wide deployment of WiMAX in Perth, Australia demonstrated that customers at the cell-edge with an indoor Customer-premises equipment (CPE) typically obtain speeds of around 1–4 Mbit/s, with users closer to the cell site obtaining speeds of up to 30 Mbit/s.

Like all wireless systems, available bandwidth is shared between users in a given radio sector, so performance could deteriorate in the case of many active users in a single sector. However, with adequate capacity planning and the use of WiMAX's Quality of Service, a minimum guaranteed throughput for each subscriber can be put in place. In practice, most users will have a range of 4-8 Mbit/s services and additional radio cards will be added to the base station to increase the number of users that may be served as required.

 


Selection of various characteristics for comparison depends upon many factors, notably:
 
A.  Licensing
Feasibility of applicability of a certain frequency spectrum  range depends upon its availability and licensing and regulatory requirements.  Between 2 to 6 GHz, there are
some license free bands.  These unlicensed bands are freely available for any experimental or enterprise application, as opposed to the licensed bands which are currently owned by carriers and users who have paid for the use of these bands.  3.5 GHz band is licensed spectrum, available for Broad band Wireless Access (BWA) and is used in many European and Asian countries, but not in the US.  It is the most heavily allocated band, representing the largest global BWA market. Covering 300 MHZ of   bandwidth, from 3.3 GHz to 3.6 GHz, this band offers great flexibility for large pipeline back hauling to WAN services.  With this license spectrum, major carriers could be able to offer competitive subscriber fees through the economy of scale and the lower equipment costs that is achieved through

WIMAX certification.
The 5 GHz Unlicensed National Information Infrastructure (UNII) bands are in three major frequency groups:
• Low and middle U-NII bands in the 5150 MHz-5350 MHz frequency range, included in IEEE 802.11a
• Newly adapted World Radio Conference (WRC) in the 5470 MHz – 5725 MHz frequency range
• Upper U-NII / Industrial, Scientific and Medical (ISM) band  in the 5725 MHz – 5850 MHz frequency range.

Most WIMAX services are in the upper U-NII band because there are fewer competing services and hence less interferences.  There are many other operational frequency bands that are being considered for a variety of domain specific applications.  The two Wireless Communications Services (WCS) bands are twin 15 MHz slots, 2305 MHz to 2320 MHz and 2345 to 2360 MHz. 2.4 GHz band is dedicated to ISM band.  It is also an unlicensed band and offers roughly 80 MHz of Bandwidth for BWA deployment. Another option, the Multichannel Multipoint Distribution Services (MMDS) Spectrum include 31 channels of 6MHz is also considered as viable solution within the 2-10 GHz range. Spacing in the 2500 MHz to 2690 MHz range and include the Instructional Television Fixed Service (ITFS).   IEEE 802.15.3a, Ultra Wideband (UWB), also uses 3.1 GHz to 10 GHz spectrum. However, It is well established that there are two main working Spectrums for WIMAX which are 2.5 GHz and 3.5 GHz.  
 
B.  RF Interference
An interfering RF source disrupts a transmission and decreases performance by making it difficult for a receiving station to interpret a signal. Problems in RF communication which frequently encounter are interference and attenuation.  When same signal receives
from various paths then multi-path errors are encountered. Bluetooth offers short-range radio frequency (RF) technology that operates at 2.4 GHz and is capable of transmitting voice and data.  Some times at 2.4 GHz, Bluetooth networks also create Interference problem in Wireless MAN, working in this particular range.
 
C.  Attenuation
Attenuation occurs when an RF signal passes through a solid object, such as a tree, the strength of signal reduces and subsequently its range.  It is critical to identify inherent interference sources in the spectrum range under consideration.    
 

COMPARISON CRITERIA
 The two main working spectrums of WIMAX are compared on the basis of various important parameter

A.  Comparison With Respect To Losses (f1) The 1st comparable factor between the 2.5 GHz & 3.5GHz spectrums is the effective Range or Coverage area.  For large coverage, it is necessary that the losses are small.  .  

B.  Comparison With Respect To Interference Factors Since an interfering RF source disrupts transmission and decreases performance by making it difficult for a receiving station to interpret a signal, this factor has a great significance in developing a omparison
model. Forms of RF interference frequently encountered are Multipath interference and attenuation. Multipath interference is caused when signals are reflected off objects resulting in reception distortion. Attenuation occurs when an RF signal passes through a solid object, such as a tree, the strength of the signal reduces and subsequently its range.

C.  Comparison With Respect To Power And Coverage Area                                
The important factors for the comparison with respect to Power and Coverage area

D.  Comparison With Respect to CAPEX and OPEX Area The critical factors for the comparison with respect to CAPEX and OPEX

E.  Comparison With Respect To Cell Radius & Range
Most mobile applications are best adapted in the < 3GHz bands range.  However, for fixed applications typically 3.5 GHz spectrum is mostly adopted.  Thus IEEE 802.16-2004 mainly focuses on 3.5 GHz spectrum while IEEE 802.16e standard works with 2.5 GHz spectrum.

F.  Comparison with respect to Mobility
For mobile networks and mobile applications including MANET based systems it is necessary to remain connected at the least above the Transport layer.  For this purpose the 802.16e is the standard which supports mobility and its main focus is on the 2.5 GHz frequency spectrum.


G.  Comparison With Respect to Data Rates
 The important factors for the comparison with respect to Data Rates


The most suitable and preferable spectrum for WIMAX implementations appears to be in
the 2.5 GHz range.  The 2.5 GHz range promises to provide most if not all the features which are required to compete with most current wireless technologies and has a prospective to go a long way in the future. This is particularly true in the cases where mobile applications or MANET installations have to be augmented by some sort of gateway based solutions between fixed and ad hoc infrastructures. However, since WIMAX is still in its infancy, it is important to consider the following factors before a full blown commitment is made:
 
• Compatibility issues between different (2.5 GHz, 3.5 GHz) bands equipment
• Interference issues throughout the 2-11 GHz spectrum.
• Issues of frequencies allocations  
• Wireless Local loop still using the frequencies in the 3.5 GHz frequency band