WiMAX Overview _Updated

Wireless Data Communication

The prospects for wireless data communication appeared as early as 1985 when the

U.S. Federal Communications Commission (FCC) released its reserved industrial, scientific

and medical (ISM) radio spectrum bands for unlicensed use. However, the idea was not fully

explored until June 1997 when the Institute of Electrical and Electronics Engineers (IEEE)

formally directed its LAN/MAN Standards Committee (IEEE 802) to study the feasibility of

having an "over-the-air interface between wireless clients and base stations". Subsequently,

the findings of the committee was compiled and later came to be known as the 802.11

Standard – a document that specified guidelines, protocols, interoperability among wireless

devices and other technical data related to the technology. Wireless Fidelity or WiFi was

born.

Upon its inception, WiFi experienced some deployment issues such as low data

transfer rates, interference from other wireless streams, etc. but these and more were

quickly and systematically addressed via a series of architectural revisions such as 802.11a,

802.11b, etc. From the end user’s perspective, the cabling mess had been drastically

reduced and restricted only to mission critical systems such as servers, backups, etc.

Workstations were no longer in the confines of a cabled network and could be moved about

anywhere within the range of the transmitter (~100m) and theoretically, up to 254 wireless

devices could link up to a single transmitter. By early 2003, the technology had proven its

worth as was reflected by market response. Many business outlets began providing free

access to the net via WiFi (a.k.a HotSpots) as a part of their own value added service.

In the four year span between 1997 and 2000, internet usage had increased by

about 500% and that’s about 80million new users linking up every year. The cause of this

ballooning of the Global Internet Community can be largely attributed to the unabated

online advent of activities such as banking, shopping, education, meetings, etc. that were

traditionally terrestrial. This coupled to the fact that more and more people are

continuously moving about between places and need access to various online services

without comprising on mobility. Thus the feature of being connected anytime-anywhere

was fast becoming a need. The goal here was to provide broadband web access beyond the

confines of an enclosure (room, office) or the limited range of a ‘HotSpot’. This presented an

imminent challenge to the IEEE 802 Standards Committee, which in 2001 released the IEEE

802.16 Standard that addressed these requirements.

Worldwide Interoperability for Microwave Access (WiMAX) is a wireless architecture

that went live around mid-2004, based on the IEEE 802.16 Standard (2001). Apart from its

obvious range advantage over WiFi, WiMAX is more tolerant of interference, more

bandwidth efficient, provides greater range (upto 50km) and better global support due to

ease of deployment and viable economic model

As mentioned earlier, the migration from static internet to mobile internet is not an

easy task simply because current mobile technology caters mainly to voice and small data

exchanges such as short messaging service (SMS) and with recent hardware improvements

some audio-video as well. The underlying problem here is that transmission issues arise at

high vehicular speeds and that’s where the latest revision of WiMAX, known as 802.16m or

Mobile WiMAX 2.0 has a clear advantage. Although expected only around early 2012, but it

already promises impressive figures such as data access rates of upto 1 Gbps from within a

vehicle that’s moving at up to 350 km/h.

With this impressive specification sheet and more, WiMAX is fast becoming the

choice medium for cost-effective delivery of high speed broadband access to remote and

emerging regions of the world.

WiMAX Technology

The technology itself is not an entirely new concept. It is an adaptation of WiBRO

(Wireless Broadband), an air modulation technique that was already being used in South

Korea with much success. With the formation of WiMAX Forum; a international non-profit

organisation, in June 2001, WiBRO and other related technologies merged and WiMAX was

established. The forum directly administers the standards for WiMAX deployment

worldwide by certifying broadband wireless products for compatibility and inter-operability.

For certification, these products must pass through any one of the six WiMAX Forum

Designated Certification Laboratories (WFDCLs) worldwide; AT4 Wireless (Spain & U.S),

Telecommunications Technology Association (TTA - S.Korea), China Academy of

Telecommunications Research (CATR - China), Advanced Data Technology Corporation

(ADTC - Taiwan) and SIRIM QAS, Malaysia. After successful testing, only then can such

devices carry the "WiMAX Forum Certified" mark, else they can only display “WiMAX Ready”

or “WiMAX Compatible”.

WiMAX was initially deployed as a fixed Non-Line-Of-Sight (NLOS) service. Unlike

traditional broadband, it did not require a pre-existing land line telephony service to carry

data signals between ISP and end user. As with cellular networks, the set up consists of

strategically located transmission base stations and as long as the WiMAX receiver was

within the transmission zone, connectivity was possible. From a technical perspective, fixed

WiMAX can be considered as a WiFi on steroids. Both are microwaves, their receiver-

transmitter systems share similar architecture except for the former being more powerful in

every sense.

In cellular networks, the receiver is not confined to one zone and is able to move

between zones as one base station transfers coverage to another base station as the

receiver travels between zones. Unlike cellular systems, fixed WiMAX architecture did not

showcase this feature and as the demands grew for a truly mobile internet, the WiMAX

Forum then established the 802.16e architecture or Mobile WiMAX. This is the current

standard and will be around until its replacement, Mobile WiMAX 2.0 arrives around early

2012.

The following table summarises the pertinent differences between the 3 types of

WiMAX architectures. Following that, each type is briefly outlined.

Fixed WiMAX

o For fixed and nomadic applications (outdoor and indoor). Caters for day-to-day movement seen in homes/offices e.g. 1st floor to 2nd floor, hall to garden, etc.

o PHY technology – OFDM 256

o 64QAM, 16QAM, QPSK, and BPSK modulation schemes

o 1.25MHz to 20MHz channel bandwidths

o WiMAX profiles for 2.5GHz, 3.5GHz and 5.8GHz

o TDD/FDD/HD-FDD

o Supports both point-to-point and point-to-multipoint access

o Quality of Service (QoS) levels – best effort, non-real-time polling service, real-time polling service, unsolicited grant service

Mobile WiMAX

o For portability and mobility (including handoff and roaming) as seen in web-enabled devices such as notebooks, mobiles, etc. Caters for on-the-move pedestrians and vehicles.

o PHY technology – scalable OFDMA 128, 512, 1024, 2048

o Improves NLOS coverage by utilizing advanced antenna diversity schemes, and Hybrid-Automatic Retransmission Request (HARQ)

o Increases system gain by use of denser sub-channelization, thereby improving indoor penetration

o Introduces high-performance coding techniques such as Turbo Coding and Low-Density Parity Check (LDPC), enhancing security and NLOS performance

o Introduces downlink sub-channelization, allowing administrators to trade coverage for capacity or vice versa

o Improves coverage by introducing Adaptive Antenna Systems (AAS) and Multiple Input Multiple Output (MIMO) technology

o Eliminates channel bandwidth dependencies on sub-carrier spacing, allowing for equal performance under any RF channel spacing (1.2MHz to 14MHz)

o Resistance to multipath interference can be enhanced by employing Enhanced Fast Fourier Transform or FFT algorithm, which showcases greater tolerance for delay spreads.

The Market for WiMAX

Mid-2008 saw Malaysia’s first deployment of WiMAX. The service was provided by

Packet Green Bhd under the label P1 WiMAX. Since then, a number of companies have

begun providing WiMAX service, namely REDtone International Bhd, YTL e-Solutions Bhd

and Asiaspace Dotcom Sdn Bhd. The Malaysian Communication and Multimedia

Commission (MCMC) administers the use of the airspace and has allocated the 2.3GHz to

2.4GHz spectrum with a 15MHz bandwidth to these providers. The many features of WiMAX

mentioned earlier (deployment, range, data rates and mobility) helps it to position itself as a

worthwhile alternative to other wireless solutions currently in the market such as public

hotspots, 3G, etc.

Mobile WiMAX however, faces a different playing field altogether with the launch of

TD-LTE (Time-Division Long-Term Evolution) or 4G for short as both are head-to-head in

terms of performance and furthermore, 4G is heavily patronised by Mobile Telco providers,

as its easier to upgrade to 4G support compared to switching over in the case of WiMAX.

Likewise for cellular manufacturers who have to invest more in R&D to incorporate WiMAX

technology in their handhelds. One feature that is in favour of Mobile WiMAX is VoIP (Voice

over Internet Protocol) i.e. the ability to utilise the Internet Protocol to make voice calls to

land lines or handhelds.

Microstrip

A microstrip has been considered for use in the design of this device. It is a planar

transmission line or simply stated; an electrical transmission line that carries microwave

grade frequency signals through circuit board interconnections and is easily fabricated by

photolithographic processes or more

commonly by using printed circuit board

technology.

In its simplest form, it consists of a

conducting strip which is separated from a

ground plane by a dielectric layer known as the

substrate. See figure opposite:

Microwave components such as

antennas, couplers, filters, power dividers etc. can be formed from microstrips. It all

depends on the pattern of metallization on the substrate. One occurrence that is particularly

important in this design context is that of ‘microstrip losses’. Losses here mean the loss in

energy when resistivity is encountered in a material placed in a varying electric field.

Three possible types of losses can occur in a microstrip line: dielectric substrate loss,

conductor loss and radiation loss.

Dielectric Substrate Loss: is defined as a loss of energy which eventually produces a

rise in temperature of a dielectric placed in an alternating electrical field. Losses can be

minimised by; (a) using thicker substrates with lower dielectric constants (εr) e.g. alumina,

quartz, or sapphire as these will produce wider and thus lower loss transmission lines.

However this will cause increased radiation at higher frequencies, generation of higher

order modes in the lines and discontinuities; (b) Substrates with smoother edges.

Conductor Loss: is a result of several factors related to the metallic material

composing the ground plane and walls, among which are conductivity, skin effects and

surface roughness. With finite conductivity, there is a non-uniform current density starting

at the surface and exponentially decaying into the bulk of conductive metal. This is the

alleged skin effect and its effects can be visualized by an approximation consisting of a

uniform current density flowing in a layer near the surface of the metallic elements to a

uniform skin depth, δ. To minimize conductor loss while simultaneously minimizing the

amount of metallic material flanking the dielectric, the conductor thickness should be

greater than approximately three to five times the skin depth. The fabrication process of

microstrip devices creates scratches and bumps on the metal surfaces. The inside surfaces

of the strip conductor and the ground plane facing the substrate repeat the shape of the

substrate. The current, concentrated in the metal surface next to the substrate, follows the

uneven surface of the substrate and encounters a greater resistance compared to the case

of a smooth substrate. As the roughness of the surface increases, the lengths of the current

path increases and cause the losses to increase.

Radiation Loss: increased radiation such as that which results from the use of low εr

dielectrics contributes to this form of loss. Radiation losses depend on the dielectric

constant, the substrate thickness and the circuit geometry and can be minimised by the use

of high dielectric constant substrate materials as most of the EM field is concentrated in the

electric field between the conductive strip and the ground plane.

Since microstrip losses are multi-factorial, a handsome trade-off must be considered

between the aforementioned factors to achieve a microstrip with low net loss.

Solid State Devices

Diodes, Transistors, Chips, etc. are referred to as solid state devices due to their solid

design and construction characteristics which are very unlike glass tubes of the vacuum tube

era that were susceptible to burn-outs, breaks, etc. These aforementioned devices however,

account for just a small part of the pantheon of solid state electronic devices in the market.

Bipolar transistor

This bipolar transistor was jointly developed by Shockley, Bardeen and

Brittain in 1948. It is widely used in high frequency applications such as in

microwaves. For example, the Si bipolar junction transistor (BJT) is useful for

frequencies ranging from Ultra High Frequencies (UHF) (i.e. hundreds of

megahertz) to the X band (8-12GHz) while AlGaAs/GaAs heterojunction

bipolar transistor is useful to over 200GHz.

The majority of bipolar transistors are either fabricated from Si or GaAs based

epitaxial material and sport a more or less general structural design

consisting

of three

separately

doped

regions and

two

junctions

that are

close

enough for

interactions to occur between them. The doped regions are known as the

emitter, base and collector and based on the doping style, can be either pnp

or npn. For high frequency applications, npn is preferred because the

operation of the device is dependent upon the ability of minority carriers to

diffuse across the base region for which electrons are best suited as they

have superior transport characteristics.

Appearance wise, the base is very thin, and its doping is moderate unlike

other elements. The base provides the base current and thus is the control

element of the entire device and without it there will be no current flow in

the circuit. As a current-controlled device, its current gain β (hfe) is a function

Ic/Ib. In linear bias condition, the emitter-base junction is forward biased and

the collector-base junction is reverse biased. Under these conditions, the

collector current is approximately 95% to 99% of the emitter current. The

device is considered to be a conventional amplifier.

Silicon bipolar NPN devices have an upper cut-off frequency at about 25GHz.

However, at higher frequencies field-effect transistor (FET) is more

preferable. The primary limitations at higher frequencies that exist in bipolar

transistor are base and emitter resistance, capacitance and transit time.

Heterojunction bipolar transistors have been designed with much higher

maximum frequencies. The improvements are by scaling down the size of the

device and narrowing widths of the elements within the transistor. Further

improvement is changing the conventional construction geometry in the way

to control

widths and

other

problem

associated

with high

frequency

work. One of

popular

geometric is the inter-digital construction which yields thin wide-area low-

resistance base regions that increase the operating frequencies. The pitch, or

emitter-to-emitter centreline spacing, controls the high performance aspects

of transistor. Finer pitches result in more gain and a lower noise figure at

higher frequencies. The number of emitter fingers controls the current-

handling ability of the device and is a measure of output power capability.

Devices with larger numbers of fingers are suitable for power applications

such as transmitter stages while devices with small numbers of fingers

operate at lower biases and are often the choice of battery-operated

applications [1].

Field Effect transistor (FET)

This type of transistor relies on an electric field to control the shape of a

channel. The shape determines the conductivity of a channel of one type of

charge carrier in a semiconductor material. FET devices are majority-charge-

carriers and consist of an active channel through which majority charge

carriers, electrons or holes, flow from the source to the drain. Source and

drain terminal conductors are connected to semiconductor through Ohmic

contacts. The conductivity of the channel is a function of potential applied to

the gate.

An FET has three terminals, Source (S) through which the majority carriers

enter the

channel.

Conventional

current entering

the channel at S

is designated by

IGS; Drain (D)

through which

the majority

carriers leave

the channel. Conventional current entering the channel at D is designated by

IDS. Drain to Source voltage is VDS; Gate (G) the terminal that modulates the

channel conductivity. By applying voltage VGS to G, one can control IDS.

To contrast their single-carrier-type operation with the dual-carrier-type

operation of bipolar (junction) transistors (BJTs), FETs are sometimes called

unipolar transistors. It is interesting to note that the concept of the FET

predates the BJT, though it was not physically implemented until after BJTs

due to the limitations of semiconductor materials and the relative ease of

manufacturing BJTs compared to FETs at that time.

There are a number of FET variants currently available in the market. The

most popular of these in the context of microwave applications, are

mentioned below:

o Junction FET (JFET)

The JFET is the most common type of field-effect transistor in use

today and has a rather simplified design as well. It can be made to

function as an electronic switch or resistor by controlling the voltage

at its terminals. The

electrical principle behind

its function is simple; The

space between "source"

and "drain"

terminals

acts as a

semiconducting channel for

electric charge and when a

bias voltage is set at the

"gate" terminal, the channel

‘narrows’ so as to impede or totally stop current flow as shown.

Further explanation follows. The JFET works on the depletion region.

The region in this matter is an N-type material, while the gate is of P-

type material. The gate lead connects to the P-type material. Reverse

bias is normally applied to the gate-source junction. The applied

electric field extends into the depletion region and controls the level

of current reaching the drain.

o Metal–Oxide–Semiconductor FET (MOSFET)

The MOSFET is another common FET based transistor used primarily

in amplifying or switching circuits. It consists of an oxide-insulated

gate electrode which induces a conducting channel between the

“source” and “drain” contacts when a voltage is applied to it. The

channel can be of n-type or p-type and is accordingly called an

nMOSFET or a pMOSFET (also commonly nMOS, pMOS). The figures

below illustrate this function in an nMOS device.

In an nMOS device, the source and drain are 'n+' regions and the body

is a 'p' region. When sufficient gate voltage is reached, holes in the

body are driven away from the oxide gate, forming an inversion layer

or n-channel at the interface between the p region and the oxide. This

conducting channel extends between the source and the drain, and

current is conducted through it when a voltage is applied between

source and drain contacts. Increasing the voltage on the gate leads to

a higher electron density in the inversion layer and therefore

increases the current flow between the source and drain.

o Metal Semiconductor FET (MESFET)

Design-wise, MESFETs resemble JFETs. Instead of using a p-n junction

gate, a Schottky (metal-semiconductor) junction is used. A useful

feature of MESFETs is that they can operate in microwave frequency

regions of up to 45 GHz, and therefore are commonly used for

communications and radar. The introduction of GaAs has further

enhanced the high frequency operability of MESFETs, by having

higher-mobility carriers and smaller sized transistor which in turn has

led to low-noise characteristics and improved temperature stability

during high power operations. The most crucial design aspect of the

MESFETs is the gate width and length. These considerations decide

their performance characteristics in high frequency applications.

Generally, shorter gate lengths result in superior performance. Gate

width, on the other hand determines the devices electrical properties

such as trans-conductance (gm), saturated drain current (Idss), Power

producing capability, S-parameters and optimum operating

frequency. Each type of gate dimensions gives rise to a particular

mask type, and its common practice to mix and match mask types to

different processes to extract desired performance characteristics.

o High Electron Mobility Transistor (HEMT)

There are two high performance FETs available nowadays which are

the high electron mobility transistor (HEMT) and the pseudomorphic

high electron mobility transistor (pHEMT). The HEMT has high power

gain and capable to operate at frequency up to 100GHz with low

noise levels. The device is built using ion implantation, molecular

beam epitaxy (MBE), or metal organic chemical vapor deposition

(MOCVD). The pHEMT uses the MBE material to create a GaAs-

AlGaAs-InAlGaAs structure that results in superior mobility to

standard HEMT devices. This process is optimized for the lowest noise

figure for critical receiver applications. Nowadays, the pHEMT has

been one of popular choice for most radio applications utilizing

frequencies in microwave frequency region [1].