The essential elements of WiMAX technology, and the component choices available to the WiMAX device designer

by Cecilia Montloin, FAE, Future Electronics (France)

READ THIS TO FIND OUT ABOUT: Where WiMAX stands amongst the many communication technologies on the market today.

New communications technologies are prone to hype, and WiMAX has been no exception. It promised to deliver greater range and higher data rates than the widelydeployed 802.11 (WiFi) standards. However, the reality is that WiMAX has suffered from a kind of paralysis in the industry as many manufacturers wanted to deploy, but no one seemed to know where. Cecilia Montloin, FAE, Future Electronics (France) explains.

Some of the grander predictions for WiMAX are unlikely to come true, at least in the short term. WiMAX, in its current form, is poorly suited to complex networking applications, and is not ready to supplant 3G, LTE standards for mobile telecommunication where demands include protocols, billing procedures, monitoring.

It is now becoming clear, however, that WiMAX is going to be used for last-mile broadband access, providing DSL-like performance in areas where the cost of installing cable cannot be justified, such as the developing world. It is also likely to serve as the backhaul channel for WiFi hot-spots.

A definite market for WiMAX equipment, then, is beginning to emerge, creating an opportunity for manufacturers and developers of WiMAX equipment, such as base stations and receivers. So what are the essential elements of WiMAX technology that designers of such hardware have to know? And to what extent have component vendors developed products optimised for WiMAX applications?

 

 

WiMAX standards and radio specifications

Unlike WiFi (the various flavours of IEEE 802.11), which is ideal for pockets of connectivity up to 300m outdoors and 100m indoors, WiMAX and mobile phone technologies such as 3G are suited for long-distance wireless expanses. Each long-distance technology is important and distinct for different reasons and will be deployed accordingly with a few overlapping edges.

Unlike UMTS, mobile phone, technology, WiMAX is built purely for data services, not for voice. Voice transmission over WiMAX can only be accomplished through a Voice over Internet Protocol (VoIP) application. UMTS technology, on the other hand, offers voice and multimedia services of guaranteed quality even when users are moving at high speed.

The main standard for WiMAX is 802.16d, which is intended for DSL-like applications, whilst the 802.16e standard is suitable for mobile applications. Privacy and encryption features are included in these 802.16 standards to support secure transmissions, authentication and data encryption.

The basic architecture of a WiMAX deployment is essentially a base station supporting hundreds of fixed subscriber stations, which could be WiFi hot-spots or firewalled enterprise networks. The base station itself is connected to the high-speed public network or backhaul.

In order to allow roaming by a WiMAX subscriber from one base station cell to another, it is theoretically possible to connect several base stations via high-speed backhaul microwave links. To provide interoperable networks, and to allocate uplink and downlink bandwidth to subscribers according to their needs in real time, base stations can use the common interface of the Media Access Control (MAC) layer defined in the WiMAX standard.

The 802.16 standard also defines profiles for the Physical (PHY) layer. The MAC layer packs and unpacks raw data, whereas the PHY layer handles the air interface and modulation schemes based on subscribers’ needs and the quality of the RF link.

Two frequency bands are used for these transmissions: Frequencies between 2GHz and 11GHz are referred to as the centimeter bands, whilst frequencies between 10GHz and 66GHz are referred to as the millimeter bands.

The centimeter bands are best for multi-point, Near-Line-Of-Sight (NLOS) and last-mile distribution. The millimeter bands have much wider channel bandwidths suitable for high-data-rate, Line-Of-Sight (LOS) backhauling applications. The LOS access service employs a dish antenna that points straight at the WiMAX tower from a rooftop or pole, with a coverage area of 9,300km². This LOS transmission is stronger than NLOS transmissions as it has a lower error rate, and is more stable.

The 802.16a extension to the standard, however, uses lower-frequency 2GHz -11GHz NLOS connections, in order to connect more customers to a single tower and reduce installation/equipment costs. These lowerfrequency transmissions are better able to diffract around obstacles, but only offer a coverage area of up to 65km². This is lower than with LOS and comparable to a GSM cell.

 

WiMAX signalling technologies

WiMAX uses Orthogonal Frequency Division Multiplexing (OFDM), a multicarrier technique that allows broadband transmission in a mobile environment with fewer multi-path effects than a single signal with broad bandwidth modulation. OFDM is an excellent method for highspeed bi-directional wireless data communication. It is good at combating both multi-path fading and narrowband interference.

OFDM effectively squeezes multiple modulated carriers tightly together, reducing the required bandwidth but keeping the modulated signals orthogonal so that they do not interfere with each other. OFDM is similar to FDM, but achieves better spectral efficiency by spacing the sub-channels much more closely together, until they are actually overlapping.

modulation schemes: Quadrature Amplitude Modulation (QAM), in which both the phase and amplitude are changed, and phase shift keying, in which only the phase is changed. The modulation scheme employed depends on the channel conditions and data throughput requirements, making WiMAX an adaptative modulation protocol.

For example, while QAM-mode WiMAX can send more bits per symbol to improve throughputs and spectral efficiency, it also requires a high signal-to-noise ratio to overcome the effects of interference and to maintain an adequate bit error rate.

One of the principal promises of WiMAX is that it provides a standards-based technology that can operate in both licensed and licence-exempt frequency bands. The first deployments are expected to use the unlicensed 5.8GHz band, and the licensed 2.5GHz (North America) and 3.5GHz (Europe, Asia) bands.

 

 

Key components in WiMAX equipment

The roadmap for 802.16 implementation requires the major IC vendors to produce specialised chipsets for the subscriber station and base station.

A subscriber station is composed of three main elements: PHY (which includes the baseband); MAC; and analogue front end.

Base stations have particular component requirements. Typically, they will be built on an architecture of microprocessors and RF components. Tight control of power levels when transmitting and receiving is crucial if the system is to be efficient. The main method used to match power inputs to the dynamic requirements of the transmitters and receivers is the implementation of power-control algorithms. This places a considerable processing overhead on the CPU.

There are also specific challenges in the design of the RF portion of a base station. Power amplifiers in base stations have to meet very tight specifications in terms of linearity, efficiency and operating frequencies.

Support for WiMAX can be seen in the offerings of the world’s leading IC vendors. National Semiconductor, for example, offers two 12-bit, 170Msample/s ADCs and a 14-bit, 155Msample/s ADC, each with a full-power bandwidth of 1.1GHz. The 12-bit ADC12C170 is available with parallel CMOS outputs, and has dual data-rate, parallel Low-Voltage Differential Signalling (LVDS) outputs. The 14-bit ADC14V155 has dual data-rate, parallel LVDS outputs.

In high Intermediate Frequency (IF) sampling receiver applications, the 1.1GHz bandwidth of these ADCs provides a high spurious free dynamic range and a high signal-to-noise ratio that extends well beyond 250MHz input frequencies. This enables the system designer to digitise the signal at the first IF, thereby eliminating a downconversion stage, which reduces component count and system power in WiMAX applications.

Avago also offers components for WiMAX equipment. The MGA 645T6 is a low-cost, highly-linear, easy-to-use GaAS Monolithic Microwave Integrated Circuit (MMIC) low-noise amplifier with bypass/shut-down mode, ideal for receiver applications.

Normally, the power amplifier in a WiMAX base station requires three or four stages of amplification delivered by discrete RF power transistors.

However Freescale has introduced a family of RF devices that integrates two stages of gain in an innovative physical design, housed in a single over-moulded plastic package. The MW7IC2725NB and MW7IC2750NB RFICs operate from 2.3GHz to 2.7GHz, and the MW7IC3825NB operates from 3.4GHz to 3.8GHz. All three devices use Freescale’s 7th-generation High-Voltage (HV7) Laterally Diffused Metal Oxide Semiconductor (LDMOS) process technology.

The devices promise to reduce cost, part count and board footprint compared to amplifiers employing discrete RF power transistors, and should also offer better linearity and efficiency. They operate from a 28V DC-32V DC supply and are extremely rugged, with the ability to handle a Voltage Standing Wave Ratio (VSWR) of 10:1 at 32V, while delivering their rated continuous wave output power. VSWR is an indicator of reflected waves bouncing back and forth within the transmission line. An increase in SWR reflects an increase in power in the line beyond the actual transmitted power. This power dissipation increases RF losses.

With the use of innovative over-moulded plastic packaging, Freescale’s RFICs can also achieve tight mechanical tolerances, enabling designers to maintain the high manufacturing yields required at WiMAX frequencies. This would have been difficult to accomplish using traditional discrete devices.

 

Conclusion

Refinement of the WiMAX standards has coincided with the potential commercial applications of wire-free broadband internet access technology, to produce a surge in demand for WiMAX subscriberstation and base-station equipment. This may have been slow in coming, but after the recent flurry of WiMAX product introductions, it is now commercially and technically feasible for equipment vendors to start venturing into the WiMAX market.