April 9, 1998

New standards and certified modules ease low-power-radio designs

Newell White, Contributing Technical Editor

Europe's new 868- to 870-MHz licence-free band encourages the trend to a "more professional" class of wireless-link design. Designers can now implement wireless links using precertified modules without mastering RF design or grappling with type-approval procedures.

Short-range radio devices, unlicenced low-power radios, wireless data links--whatever you call them, demand for such devices is booming. The potential market is as large as that for telephone handsets--but regulation of spectrum utilisation and appropriate access technology must occur before these devices achieve such market penetration.

The cost and performance of these devices range from vehicle-access key fobs, which cost a few pounds for 2 kbps at 10m, to wireless-LAN and telemetry interfaces, costing hundreds of pounds for 1 Mbps at 3 km. The ability to act at a distance--or get up-to-date information minute-by-minute from remote sites--is the key advantage these devices offer. Business benefits include improved safety in hazardous operations, better quality of service, lower inventories, and increased productivity of mobile plants and staff. Social benefits are shorter response times to emergencies and improved security for homes and public places.

But questions remain about how to develop a wireless link that satisfies various national regulatory authorities, even within a single market such as the European Union, and how to locate a good RF designer to join your team. You cannot pretend that these problems have been solved completely, but recent developments in spectrum allocation, standards, and technology have lowered some of the hurdles that equipment designers must clear when targeting European markets.

Available technology now lets an application team build a wireless-linked system without an RF designer. This development is welcome news, given the difficulty of recruiting RF specialists. However, without an in-house RF designer, you still need access to some RF knowledge for top-level system design.

Designers also need to keep close watch on the Pan-European regulatory environment: frequency allocation is the province of the Conference Europenne des Ad-ministrations des Postes et des Telecommunications (CEPT), whereas equipment standards are the concern of the European Telecommunications Standards Institute (ETSI). It is vital to understand which bits of the spectrum are available for your application in the national markets you seek to penetrate, but be aware that this quest for understanding is a moving target. Fortunately, new recommendations and standards are now being published in formats that clarify design requirements. What's more, you can download them directly from the Internet: For CEPT material, go to www.ero.dk, and for ETSI information, see www.etsi.fr.

National authorities, such as the United Kingdom's Radiocommunications Agency or Germany's Bundesamt für Post und Telekommunikation, are responsible for implementing these recommendations, and they may place restrictions on designing and operating wireless links within their boundaries. A list of contacts for these authorities is available from the Low Power Radio Association (LPRA) (see box "Contacting the Low Power Radio Association").

If you want to design wireless-link functions into your equipment, you have two main approaches. First, if you have in-house RF expertise, you can design your own interface using ICs; a single chip may provide a complete solution, but otherwise you should consider a partial or complete chip set for wireless-LAN or telephony applications. Second, you can buy a complete interface module from a specialist supplier. Obviously, your material cost increases significantly with the latter option, but there are several advantages to selecting this route:

• You may not need an RF designer on the product team--although for some development stages you may opt for a consultant.

• The module may already be type-approved for use in CEPT markets. However, the complete product still must achieve EMC compliance for CE marking.

• You may achieve faster time to market and better application focus.

Table 1 lists representative wireless-interface chips and chip sets--as well as guide prices. Output power is normally at chip output into a 50ohms load, and sensitivity is at chip input for a 3% bit-error rate (BER) on FSK signals.

Devices designed for the 434-MHz band typically suit digital data transfers to or from a µC. So, the chip user has to design any forward-error-correction (FEC) or cyclic-redundancy-check (CRC) coding scheme and interleaving process in software.

Devices serving the new 868- to 870-MHz band are just emerging, and designers will be able to choose between digital-I/O devices and those accepting the baseband signals a modem produces. Representative FSK and Gaussian-filtered minimum-shift keying (GMSK) modems appear in Table 1. Available modems produce packeted data compatible with commercial wireless-data-network formats, such as Ericsson's (www.ericsson.com) Mobitex or Motorola's RD-LAP.

Serving the 2.4- to 2.5-GHz band, Harris' chip set is designed for half-duplex wireless-LAN use. Full-duplex chips are also available--but at greater cost. The baseband processor converts between digital data (as much as 4 Mbps) and analog in-phase and quadrature (I,Q) signals using differential binary or quad phase-shift keying (PSK) for direct-sequence spread-spectrum signals. These signals are authorised under CEPT/ERC recommendation 70-03, which does not impose a channel structure on the 2.4- to 2.4835-GHz band. For a list of additional telecommunications-IC manufacturers offering chips for this frequency band, see box "For more information...".

If you opt for the packaged-interface-module route, you'll discover a far wider selection from which to choose. The cheapest modules provide basic transmitting or receiving functions and can be extremely compact. More sophisticated units offer RS-232C telemetry, video links (duplex for pan and zoom), or remote control as drop-in or bolt-on options.

Table 2, based on manufacturers' data sheets, provides a small sample of available module offerings; the table also offers guide prices. Note that maximum range in clear air may be at transmitter power above CEPT/ERC 70-03-recommended levels.

Step 1: system design

Primary issues system designers must address include communication mode, channel usage, operating range, peak data rate, and reliability. Such considerations determine the suitable frequency ranges available to your application. However, make sure to create a clear statement of who will send how much of what to whom--and when. Also, try to determine what quality of service users expect. If you lack definitive answers to these questions, you're not yet ready for the design process. And, remember that if you don't have ready access to RF expertise, you will likely benefit from a consultant. Consider the following:

• Communication modes are simplex (one-way traffic, such as vehicle keys and garage-door openers), half-duplex (traffic in alternate directions for polled telemetry and alarm systems, as well as command and acknowledge), or full-duplex (simultaneous two-way traffic, as in mobile telephones).

• How many channels will you use? Having the ability to select one of several channels adds cost and complexity, but traffic density or interfering signals may force this feature on you.

• How will you deal with collisions between multiple transmitters? Re-member that, although your base station must detect signals from any out station, there is no guarantee that an out station can hear all others. Obviously, a system in which out stations speak only when spoken to can get by with one channel and no collision problems.

• Operating range, transmission power, and antenna requirements are interlinked. Ballpark figures for transmitters in the 1- to 10-mW range span 30 to 500m outdoors and 10 to 100m within buildings.

By now you probably have identified a number of eligible frequency bands. If you're designing a system for a particular locality, a survey of the RF environment will help. If your system will be used in other countries, check the national limitations on frequency usage. Or, if material cost is critical, you may opt for the lowest eligible frequency. But if reliability of first-time communication or high data rates is most important, a move to higher frequency bands may be worthwhile.

According to Graham Sharples, managing director of RadioMetrix, the new European-harmonised 868- to 870-MHz band promises better reception than the existing 433- to 434-MHz band, because of lower density of interfering sources and higher permitted power levels. These features will allow increased range or higher data rates.

Sharples notes, however, that this advantage must be set against higher material cost. Tighter frequency tolerances may require crystal/PLL frequency sources, which together with higher carrier frequencies and IFs increase power consumption. The same factors apply to the 2.4-GHz band, only more so. Increased power consumption, GaAs devices, and spread-spectrum modulation all push up costs.

Although you have set goals for range, data rate, and BER for your system design, the equipment design determines at what cost and power consumption you can meet these requirements. And, this last factor is critical for handheld equipment.

Thus, a key part of your business plan is the decision to design your own wireless link or to buy ready-made modules; the economic break-even point is likely to be approximately 2000 to 3000 units per year. If your design involves much higher volumes, consult RF IC suppliers about ASICs--some of these chips can add a µC to a transceiver, realising a one-chip solution to cost-critical requirements.

Even if you decide to use buy-in modules, some study of design issues helps you choose from the wide selection available for most applications.

Alan Wood, managing director of Wood & Douglas, offers the following advice: "In most cases, receiver design is the critical factor determining how well a system performs in the field. It is important to avoid mixing unwanted signals into the first IF output." For example, there may be an interfering signal at the image frequency 2×IF away from the input signal. You can take this interference out of the RF filter's passband by choosing a large enough IF. The ratio of IF-to-passband width also determines how well local-oscillator leakage into the environment is suppressed (Figures 1a and 1b).

Other sources of risk are sidebands or wide skirts on the spectrum of the local oscillator, which can cause "reciprocal mixing"--where attenuated copies of adjacent channel signals are mixed into the IF signal (Figure 2). Alleviating these problems involves compromise, most crucially in power consumption.

You can improve BER using FEC schemes, CRC, or requests for retransmission. You can trade-off this improvement for benefits to range, data rate, or power consumption. (For more detailed information, see Reference 1.)

Matthew Philips, marketing manager with Consumer Microcircuits, notes that "FEC is the friendliest technique in systems with dense traffic. At most working signal-to-noise levels, an FEC scheme increasing message length by about 33% will improve BER by at least an order of magnitude."

In addition to various other considerations, remember that your equipment must have an integral or dedicated plug-in antenna to comply with ETS 300 220. You have a choice between designs providing 360° horizontal coverage and those providing significant directional gain--if the user can point the equipment in the right direction.

Note that you should also take into account antenna gain when testing radiated transmitter power (or spurious output from receivers) for compliance with ETS 300 220. The direction of maximum measured power performs limit checking, so the benefit of a directional transmitting antenna stems from power consumption, not absolute signal strength. For a receiver, there may be no benefit in the worst case of a single interfering signal.

New applications fuel growth

The biggest use of short-range radio has been access/alarm key-fob applications for motor vehicles. This reality is hardly surprising given the mass market for cars, the relatively small number of suppliers, and the rudimentary nature of the entry system: single-channel, one-way security-code transmission. If the message arrives garbled and nothing happens, you have just to press the button again.

Handheld data terminals are currently spreading this technology into new application areas. Helmut Meier of HM-Funktechnik reports that in Germany, no single application dominates, and end users are found in all business and state sectors. But most of the companies providing these users with radio-linked equipment are less than three years old, a sure sign of a market in exponential growth.

A typical handheld terminal has an LCD, keys, an input device such as a bar-code reader, and a wireless link to a local base station. The base station can provide access to a LAN. In roving applications, the data terminals may link directly to a third-party wireless data network. Tasks benefiting from instant information include parcel service, warehousing, and mobile-plant tracking; more esoteric uses include restaurant-wait tracking and real-time assessment in training and education environments.

The implementation of remote metering by electricity, gas, and water utilities will trigger the real gold rush. Different models for such applications are competing for designers' consideration. Should the base station be the domestic telephone, accessed by a no-ring call? Or should it be a dedicated lamp-post transceiver for each service, or a mobile unit cruising the streets in interrogation mode, investigating communications failure on the spot?

Whichever scheme (or schemes) wins acceptance, it is likely that each meter will eventually contain a transmitter or transceiver. Suppliers are excited by the sheer size of this prospective market but recognise that it may be some years away. If this reality allows time for a common specification to be adopted by several utilities--or across national boundaries--so much the better.

Reference

1. "Increasing Data Throughput in Radio Telemetry Systems," Consumer Microcircuits (www.cmlmicro.co.uk), November 1997.

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