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RTX Telecom solves 2.4 GHz band cordless phone interference problems

by RTX Telecom 
(Part of RTX February Newsletter - Click here to Join)

Wireless LAN-compatible high-performance digital cordless technology

Interference issue
More and more devices in our day-to-day lives are becoming wireless. We are surrounded by them, and have become more and more dependent on them. They include cordless phones, wireless LAN (802.11b/g) and an increasing variety of different Bluetooth-enabled devices. They all share the same resource – the radio spectrum. Unlicensed devices have to operate in limited frequency bands called ISM (Industrial, Science and Medical) bands. One of these is the 2.4 GHz band, which has become very popular. All devices using these bands have to fulfil certain requirements and are constructed to operate in the presence of other devices working within the same band. But with more and more devices, this band has become so crowded that performance and reliability of communication are in question. New products have subsequently emerged that combine cordless and WLAN functionality. The Bluetooth community has already drawn attention to problems that are likely to arise and is now proposing adaptive frequency hopping schemes in the next generation of the standard. This approach and others, such as implementing additional filtering, may improve the situation, but they do not solve this coexistence problem completely. In addition, these approaches may introduce extra production costs.

The RTX Telecom 2.4 GHz cordless technology has already built in adaptive frequency hopping algorithms and other algorithms to avoid interference from other radio technologies in the same frequency band. These algorithms are in fact so efficient that a WLAN device in close proximity scarcely affects the cordless phone range or audio quality. However, a WLAN (802.11b/g) operating in the same 2.4 GHz band may be severely affected, resulting in very poor throughput.

This issue is particularly important for new products combining WLAN and cordless phone functionality, and the problem must be solved.

Solutions
The solutions to this problem are either to use a common air interface protocol or to use separate frequency bands. Due to the nature of already existing devices using the ISM band, the first solution is not really applicable, leaving usage of separate frequency bands as the only real solution.

In Europe, DECT cordless telephones operate in a reserved frequency band at 1.9 GHz, which does not cause any interference with 2.4 GHz devices. In the US region, other ISM bands are located at 900 MHz and 5.8 GHz.

For a number of years, 900 MHz cordless phones have been widely used, but these have been mainly low-end analogue models.

New digital cordless phone models using 5.8 GHz are now reaching the market. Currently, these products are based on technology that is not yet mature, and typically provide:
    §         lower performance
 §         higher component costs
    §         reduced range due to the law of nature – higher frequencies generally introduce greater path loss when radio waves propagate through air and obstacles.

The best technical solution is to use the lowest possible frequency with better propagation properties and range. However, this contradicts some preconceptions, both in the market and among consumers. 

The RTX Telecom solution
The FCC requirements for the use of ISM bands have recently been changed and now allow the use of a digital modulation method that perfectly fits RTX Telecom’s cordless technology. RTX has implemented a high-performance 900 MHz cordless technology with:
 §         superior performance
    §         low cost
    §         low power consumption
    §         long range.

Implementation is comparable with our 2.4 GHz technology in terms of cost, component count and power consumption.

This new technology also includes all the interference avoidance techniques known from our existing 2.4 GHz technology. It provides the same high-quality audio and security as known from DECT, including support for multiple handset, multiple cell PBX systems, repeaters and integration of voice/low-speed data.

Please contact the RTX Telecom sales department for more information about your next cordless solution.

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