Achieving wireless voice and data in the licence-exempt spectrum

Voice is the perfect example of how networks of any type need to be able to support a high level of two-way communication. The fact that voice, in a digital topology, is ‘just data’, processed as packets is relevant, because the way those packets are handled is important. They contain information that has to be reassembled in the right order and as fast as possible, to minimise the inevitable and unwelcome delay that encoding and decoding introduces.

The first 5G networks and services were recently switched on in a number of locations around the world, after much anticipation. It is pertinent that there has been less focus on voice since 3G networks were introduced, and that supporting voice in cellular networks has been something of a burden since 2G. Consumers experience dropped calls or even no network access in heavily congested areas, such as city centres. Data is far more forgiving in this respect, and so it is perfectly suited to packet networks where capacity and congestion might otherwise impact voice traffic negatively. As a result, it is clear that broadband cellular networks are not always the best platform for voice traffic.

The narrowband alternative

Point-to-point and peer-to-peer communications, as opposed to using part of a large, shared network, has many advantages. Firstly, it is less susceptible to the fluctuations in capacity apparent with a commercial cellular network. The technology used also has benefits in terms of range and robustness. In most instances, Private Mobile Radio, or Land Mobile Radio as it’s also known, uses narrowband transmissions and operate in both the licensed and unlicensed parts of the RF spectrum, although the devices designed for use in licence-exempt frequencies are generally low-cost consumer devices.

Although it offers lower capacity, narrowband technology has the potential to carry both voice and data traffic with greater reliability and better security than cellular networks. The ‘shape’ of narrowband transmissions means they have other advantages, such as greater range for a given power. Despite being primarily used in mobile applications, narrowband voice and data systems can be very effective in fixed point-to-point systems, too.

Above: the SCT2400

When compared with other forms of wireless communication, narrowband technology can deliver significant power savings, however it is important to appreciate that this benefit can be eroded by the protocol used. By combining the best technologies, PMR systems can deliver exceptional range for voice and low bandwidth data services, with a high level of security. For many voice applications, only narrowband technology can deliver the required level of reliability, offering guaranteed coverage with a high degree of clarity and robust security.

PMR in the ISM

As most professional PMR systems require a licence to operate, their use comes with certain overheads that aren’t present when using cellular networks. However, the use of PMR in licence-exempt bandwidths isn’t prohibited, it’s just not very well supported by the underlying solutions. Until now, that is.

As mentioned earlier, operating PMR over ISM is largely limited to consumer devices, such as walkie-talkies. Professional users tend to opt for more secure and robust systems running over licensed bandwidth. However, the latest digital technology platform developed by CML Microsystems provides a complete digital voice and data transceiver that operates at 2.4GHz, making it licence-exempt in most regions in the world.

The SCT2400 integrates an RF transceiver with a digital baseband processor, so a complete PMR transceiver system can be realised by adding a low-cost microcontroller for management, an external codec, a speaker and a microphone. This narrowband solution uses spread spectrum modulation operating at 2.4GHz to deliver a high level of security against eavesdropping. It can achieve a range of up to 12km line of sight, or up to 2km in urban areas, yet only consumes around 64mA in transmit mode. This drops down to just 10mA in standby, so the SCT2400 can be used in smaller devices running from fewer primary cells or smaller rechargeable batteries.

The need for robust mobile voice systems isn’t going away, but the demand for high quality, long range voice and data solutions operating in the licence-exempt part of the RF spectrum is really just emerging. That’s thanks to solutions like the SCT2400, and it will continue to develop as manufacturers begin to supply the wide variety of communication devices the SCT2400 has the potential to enable.