WiMAX


Introduction
WiMAX is a new technology that promises to deliver high data rates in two separate frequency bands. The lower band is in the frequency range 2-11 GHz and the higher band is in the range 10-66 GHz. At the higher frequencies, line of sight is required as the frequencies tend to become directional at these frequencies. In the lower region, the signals will propagate without the requirement for line of sight. WiMAX is a point to multipoint technology that will allow for ad-hoc networks to be created.


Technological Overview
IEEE 802.16x
CPE Equipment
LOS vs. NLOS Installations
Multipath Interference
NLOS Base Stations
NLOS vs. Mesh
Network Deployment Considerations
Convergence of Technologies
Conclusion
References


Technological Overview
IEEE 802.16's Task Group a developed IEEE Standard 802.16a, an amendment to IEEE Standard 802.16 ("Air Interface for Fixed Broadband Wireless Access Systems"). The amendment covers "Medium Access Control Modifications and Additional Physical Layer Specifications for 2-11 GHz." Both licensed and license-exempt bands are included. Courtesy of http://www.ieee802.org/16/tga/

The IEEE 802.16a Air Interface Standard is intended for fixed wireless applications on a point to multipoint (PMP) network. This is similar to the current operation of an 802.11x wireless access point where a central node has an omnidirectional antenna to allow all-round coverage of its area of transmission. Long-range versions of WIMAX may include multi-sectored antennae similar to the antennae used in cellular telephony base stations. WIMAX is intended to provide a MAN solution to backhaul 802.11 hotspots and other WLAN networks to the Internet. It will also provide solutions for campus connectivity.



IEEE 802.16x
This is the part of the spectrum that lies in the 2-11 GHz range. This is non-directional and can provide up to 50 km (30 miles) for fixed stations, and 3 - 10 miles (5 - 15 km) for mobile stations. A data rate of 280 Mbps is envisaged as being the bandwidth available per base station. This will be able to provide high data rates to many businesses or individuals. WIMAX's ability to bridge the last mile will provide a cheap upgrade to the installed copper base in urban areas and offer broadband access to rural areas that had been previously limited to POTS and 56 kbps only. The cheapness of a wireless solution means that this technology will compete with dark fibre as a high bandwidth data solution to the subscriber.

WiMAX in the higher frequency band is line of sight (LOS) and is envisaged to provide the high data rate backhaul to a core network. WiMAX is a complimentary technology to 802.11x intended to bridge the distance between the 802.11x wireless access point and a core network, e.g. the Internet. WiMAX may also replace 802.11x in certain network topologies.
WiMAX has been refined to provide 2 standards, 802.16d for fixed applications, operating at a higher data rate and greater distance than the version for mobile devices 802.16e.


Customer Premises Equipment
, CPE
The hardware required for connecting to a WiMAX network will be available in several types.

1st Generation
For the long-distance, high frequency reception, external small dishes will be required. These will need to be placed on a wall or roof and will need to be aimed at the transmitting node as the transmissions are highly directional. 802.16d

2nd Generation
These devices will be indoor devices similar to cable or DSL modems and should reach the marketplace some time in 2006. Likely to be WiMAX and WiFi enabled. 802.16d

3rd Generation
These devices will be for laptops and other portable devices and are expected to reach the marketplace by 2007. 802.16e


LOS vs. NLOS Installations
The winning principle behind non line of sight (NLOS) installations is that the customer can buy their hardware, remove it from the box and be operational in a short time. This does not require the services of a technician to visit the customer premises.

For LOS applications, e.g. Sky TV, a technician needs to visit the customer premises. This is known as a 'truck roll' and the cost of sending a technician perhaps more than once to a customer's premises to install a transceiver is an expensive part of a company's operations and the cost of the visit may take 12 months or more to recoup from the customer's rental fee.
Another factor here is that the directional antenna will tend to be costly and this will reduce customer take-up. As such, the NLOS solution is preferred due to the cost-effectiveness of implementation.

LOS is only preferred when a high bandwidth backhaul is required. These tend to be p-to-p and do not run to the end hosts, instead these links will run between a company's infrastructure nodes or alternatively between a service provider and a customer who requires very high BW to distribute within their organisation. Once the LOS antenna has been set up on the customer's premises, the customer will then attach the antenna's output to their LAN for distribution.

Multipath Interference
Multipath Interference (sometimes called picket fencing or flutter) happens when wireless signals bounce around between obstructions that lie between the transmitter and receiver. There may be a direct path between the transmitter and receiver and a secondary path. This secondary path will be a longer path and therefore a percentage of the power of the original signal will arrive later than the signal that travelled via the direct path.



It is possible that there will be a few microseconds of delay between the signal arriving by direct path and the signal that has taken a longer path. This can limit data rate and also limits the area of coverage of any transmitter.


NLOS Base Stations
Instead of using an omnidirectional or multi-sectored antenna, the NLOS base station will be a smart device that uses DSP to control an array of radiating elements. Each element is fed the same signal, but will differ in its amplitude and phase and time delay. The signals that are transmitted are calculated to arrive at their destination at the same time, having taken into account the verious different paths that the signals will undergo on their journey to the receiver.

This is accomplished by the base station listening to the signal transmitted by the receiving node and calculating a complimentary signal that it must transmit to overcome the physical limitations of the local area.

For example, a base station may recieve a signal from the west and than 1 microsecond later the same signal, 6dB weaker from the south. The DSP hardware in the base station will then transmit a signal in 2 parts. It transmits a strong signal to the south and a microsecond later a 6dB weaker signal to the west. This overcomes the physical limitations of the area and the arriving signal at the customer premises arrives all at once and in phase, producing an amplified signal.

The task of monitoring the CPE is continuous as it may be items other than buildings that are responsible for the multiple paths e.g. cars.

NLOS vs. Mesh
 Another method of distribution of wireless connectivity is to rely on a meshed solution. In the past, mesh wireless networks relying on 802.11x have been trialled and failed owing to the large number of nodes that the service provider would need to set up, typically one per 50 metres (see the story of Ricochet).

The solution here is to allow each customer's router to send and receive not only its own data signals but those of other customers too. This cuts down on the vast number of repeaters that the service provider needs to install and maintain. Instead, the neighbourhood is divided into a number of AirHoods each of which is served by an Airhead.


Nokia are trialling the technology in the USA. A drawback of any meshed solution is latency due to the number of hops that data has to cross. The mesh means that more hops will be experienced by a particular packet of data on its journey around the network, but the shortness of these hops will keep signal power and hence data rates high. It is envisaged that real-time applications will suffer in a mesh network of this type, however QoS software upgrades to the participating end-user nodes are addressing the real-time latency issues with smart queuing techniques.

Unfortunately, the licensing of the 2.4 GHz band in Europe only allows for a transmission power of 100 mW as compared to the USA who have licenced the maximum power of the same band at 4W. Unless and until the maximum power allowable on this band changes upwards, it is unlikely that this solution will be seen in Europe.

Network Deployment Considerations
Whenever planning to design a wireless network, consideration must be given to regulations relating to the use of the wireless spectrum.
The following is an extract from http://www.networkcomputing.com/showitem.jhtml?articleID=172301706&pgno=3

Since the 1950s, the FCC has increased available spectrum several times, starting with rules for unlicensed devices in the 27-MHz band and above 70 MHz. From 1960 to 1985, additional Part 15 rules were created to accommodate everything from cordless phones to garage door openers. In 1985, the FCC created the unlicensed ISM (Industrial, Scientific and Medical) bands that spurred the development of wireless LANs, allocating spectrum in three separate ranges: 902 MHz to 928 MHz, 2.4 GHz to 2.4835 GHz, and 5.725 GHz to 5.850 GHz. Additional 5-GHz wireless spectrum (5.15 GHz to 5.35 GHz) was allocated in 1998, governed by the U-NII (Unlicensed National Information Infrastructure) rules.

In November 2003, the FCC allocated another 255 MHz of bandwidth between 5.47 GHz and 5.725 GHz for use by U-NII devices, consistent with global spectrum policy recommendations of the ITU. Then, in March 2005, the FCC enacted new regulations allocating 50 MHz of spectrum between 3.6 GHz and 3.65 GHz for wireless broadband services.

Note that the above articla id from USA where the regulator is the FCC. In the UK, the regulator is known as OFCOM and it is this body that supplies licensing in the UK.

The primary choice is whether to choose licensed or unlicensed portions of the spectrum.  If the licensed option is chosen, the regulating authority in force in that area  will require a payment to be made for use of that spectrum. A few years ago, the five licences in the UK  for  3G telephony were sold for £22.6 billion. This money will have to be recovered from the customers before any profit can be made.

If an unlicensed band is chosen, the licensing cost is zero. The major difference between the unlicenced portions of the spectrum is their propagation characteristics and the maximum allowable power at the transmitter.

These factors must be borne in mind whenever the decision to design or deploy a wireless solution  has been taken.

Convergence of Technologies
It is likely that for the short-term future, the 802.11x hotspot market will grow and will be serviced by p-to-p LOS WiMAX for the backhaul in regions where such a backhaul would have been prohibitively expensive using current technologies.  The WiMAX market will grow over time and wireless access points will eventually support WiMAX and 802.11x.
It is also highly likely that VoIP will penetrate all of the communication market and that cellular telephony will also be WiMAX enabled. It is easy to envisage access points that will be WiMAX, 802.11x and Cellular-enabled, however only time will tell as to which the winning technology combination is likely to be.

Conclusion
WiMAX is a technology in its infancy. It supplies solutions for backhaul connectivity for existing wireless hotspots and also offers end-user access in the unlicensed portion of the wireless spectrum. Local regulations may prevent WiMAX from reaching its full potential in the unlicensed spectrum.
Its LOS soluions lie in the licenced portion of the spectrum and these will be able to supply remote areas with Internet connectivity in a cost-effective manner providing that the licensing authorities do not overcharge providers for the licences.

References
WiMAX Forum
WiMAX Industry
Players in the NLOS field include equipment manufacturers like Nokia Corp. (Espoo, Finland) and Navini Networks Inc. (Richardson, Texas); companies like Iospan Wireless Inc. (San Jose, Calif.), which provide transmitter and receiver designs and chips; and Internet service providers (ISPs) like Vista Broadband Networks Inc. (Petaluma, Calif.) and T-Speed (Dallas), which sell wireless access service to customers. Courtesy of http://www.netstumbler.com/2002/06/06/wireless_broadband_in_a_box/
WiMAX hardware manufacturers: Airspan Networks, Axxcelera Broadband Wireless, Picochip, Aperto Networks, Redline Communications, Sequans Communications and Wavesat. Other participants were announced as Huawei Technologies, Proxim Wireless, WiNetworks and ZTE Corporation. Courtesy of http://www.netstumbler.com/2005/11/17/wimax_on_the_horizon/
Case Study of WiMAX in the UK (pdf)