Application specific, low power, wireless telemetry

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Article

Application specific, low power, wireless telemetry

Land Mobile, Telemetry Feature, June 2004

Wireless telemetry can be defined as machine-to-machine communications via a radio link. This is often the transfer of data from sensors for remote monitoring or data collation purposes. Typical applications include:

Building alarms

Vehicle and boat alarms

Remote meter reading

Environmental applications, such as weather stations and earthquake status monitoring

Border access control

Monitoring of water, gas or oil pipelines/facilities

It is often tempting to consider addressing a telemetry requirement with a standards-based solution such as GSM/GPRS or 3G. Whilst these established standards do offer well defined hardware and software specifications and have an existing network infrastructure, they have not been optimised for the specific telemetry application under consideration. The power consumption, terminal equipment cost, data transport cost and/or data rate are unlikely to all be optimal for any given application. Other standards-based, wireless data transfer solutions such as Bluetooth, DECT or 802.11b may offer more attractive cost models but may fall short in other ways such as achievable range or battery life.

Key requirements of many of the telemetry applications mentioned above include:

Low unit cost

Low data transport costs

Long battery life (low operating power)

Adequate range

Low/moderate data rates

A custom-designed, application-specific telemetry system should result in the optimum solution for a given application because the correct trade-offs between the conflicting requirements can be made during the design process. Plextek has extensive experience in the design of wireless telemetry products and systems and has successfully implemented novel design approaches that allow enhanced sensitivity, low transmit power, low cost and long battery life. Two examples are:

An automatic remote utility meter reading system, which was designed for a maximum density of 4000 terminals/sq km, battery life of 10 years and range of 1km.

A stolen vehicle recovery system, which achieves an average range of 20km and 90% UK area coverage with just 60 base sites.

In the case of the remote utility meter reading system, a low cost radio transmitter is fitted to or integrated with existing metering equipment. An interface to that equipment allows local data storage. shows the remote customer premises equipment for both an electricity meter and a gas meter. Data is transmitted from the remote terminal to a basestation with a subsequent back-haul link to a central server. The rate of transmission from the meter is programmable from several times per hour to daily or even weekly.

Figure 1: Gas and electric meter versions of a wireless utility meter reader

The adoption of a custom telemetry solution allows the system architecture and radio interface technology to be tailored to minimise the cost, complexity and power consumption of the remote unit at the meter. Although it is essential to keep the cost of the remote unit very low, it is also necessary to ensure that the communications range of the radio link is adequate in order to keep the infrastructure deployment costs down. The key to achieving these goals is the development of a sophisticated, custom radio communications protocol and the use of digital signal processing in the basestation receiver. The resulting system supports packet transmissions of nominally 2kbps with payloads typically in the range 32-64 bytes. The remote unit is a narrow band, phase-modulated transmitter operating on a randomised time and frequency basis. The basestation receiver utilises an advanced, wideband multichannel DSP architecture able to decode up to forty simultaneously received signals. A single basestation is able to support several thousand remote terminals.

The stolen vehicle recovery system was developed in collaboration with TRACKER Networks (UK) Ltd. The system uses VHF radio to communicate between the vehicle and base site (and on to TRACKER headquarters) allowing tracking and location of stolen vehicles by the police. The recovery procedure commences when TRACKER headquarters receives notification that a vehicle has been stolen. Following confirmation of theft with the owner, a telemetry link is used to activate the vehicular unit, which will then repeatedly transmit its ID. Police cars and helicopters fitted with the tracking equipment can then find the stolen vehicle when it is in proximity.

The police tracking equipment is a four antenna receiver which uses time of arrival information from the different antennas to determine the bearing of the stolen vehicle and then track and locate it. Variants of the equipment that interface with GPS modules have also been developed by Plextek. In this case the stolen vehicle also transmits its precise location (as determined by the GPS receiver) along with its ID.

The use of a custom-designed telemetry link was necessary in this application to allow the in-car unit to be hidden discreetly in one of several dozen places around the vehicle and to still be able to form a robust communications link with a base site.

Contact details:

Plextek Ltd, London Road, Great Chesterford, Essex CB10 1NY. Tel: +44 (0)1799
533200. Fax: +44 (0)1799 533201. E-mail: lmd@plextek.co.uk Website: www.plextek.com

Author profile

Liam Devlin is currently director of RF integration with Plextek. He joined the company in 1996 to develop the RF and microwave IC design capability and since then he has led the design and development of over 30 custom ICs on a range of GaAs and Si processes. He has also led programmes to develop microwave and mm-wave equipment at frequencies up to 60GHz and Low Temperature Co-fired Ceramic (LTCC) modules. He was previously chief designer with Marconi Caswell where he designed GaAs ICs for both the commercial product line and for customer specific applications. Prior to this, Liam was employed by Philips Research Laboratories. He graduated from Leeds University in 1988 with a first class honours degree in electrical and electronic engineering.

Liam was the chairman of the 2001 European Conference on Wireless Technologies and was previously chairman the IETprofessional group E12 (Microwave Devices and Techniques). He has published 26 technical papers.