Communication Beyond the Reach of Cellular Networks

By David Brooke

Data Product Manager

CML Microsystems

May 10, 2021


Communication Beyond the Reach of Cellular Networks

With the rollout of 5G and the broader availability of 5G-ready handsets, high-speed mobile data is here.

There is a whole generation who cannot believe there was a life without a smartphone. Many of their parents were just using the cell phone to call up a friend or send the occasional text message. And, before that?

The networks that support these smartphones are enormous, and with every new generation, their cost increases. As you would expect, network operators focus their attention on ensuring that they provide strong coverage in highly populated areas, which makes sense as this is where they get the highest return on investment. However, in some extreme cases, cellular coverage in sparsely populated areas may not be as reliable. Here, most individuals can cope with the occasional dropped call, slow data, or a busy network, but commercial and industrial applications cannot.

Why PMR is Essential

Quality of service and uptime are essential for public safety networks, such as the police, fire department, and ambulance service. Prior to cellular telephony, private mobile radio (PMR) met the need for reliable voice and data connectivity. In essence, it is a small, privately managed and maintained network that remains operational in dense and urban environments as well as industrial or isolated locations. However, despite its benefits, the use of PMR remains relatively niche, but that does not mean it is redundant.

PMR operates at a much lower frequency than cellular, in the region of hundreds of Megahertz rather than Gigahertz frequencies. At the same time, two-way radios, for example, need to remain operational for as long as possible, especially where there are no other forms of communication. However, meeting this level of low power operation is challenging.

Within the transceiver, the RF synthesizer is a critical element in radio systems. For those targeting PMR, it needs to be designed and optimized to operate in both the 136 - 174MHz and 403 - 527MHz frequency range. It also needs to deliver the best performance at the lowest power and smallest form factor to enable smaller PMR products to operate for longer on a single charge.

A key parameter that impacts the performance of RF synthesizers is phase noise. In conventional RF systems designers tend to tackle the phase noise problem by pumping more power to the sensitive analog components, which is widely accepted by the market but does not meet the low power requirements of PMR products.

A solution to the power-performance problem is to integrate the voltage-controlled oscillator (VCO) into the RF synthesizer, which saves on system power and lowers total cost. The CMX940 from CML, for example, features dual Phase Locked Loops (PLLs) and a fully integrated VCO. The PLL comprises an integral N PLL and a fractional PLL, enabling developers to tweak how the two PLLs operate to lower the phase noise figures and, therefore, save power. The dual PLL architecture also ensures low phase noise and spurious performance over the wide frequency bands that need to be covered by PMR terminals. The device only requires an external loop filter and clock reference, providing a complete and compact RF synthesizer solution.

PMR still has a vital role in communications, not only for voice but data too. Being able to operate where no other network can assure IoT applications connect even in the remotest of places and secures the future of PMR.

David has nearly 40 years of experience working in the electronics industry. As CML’s Wireless/Wireline Voice and Data Product Manager, specialising in baseband solutions, he works across all CML departments managing the products under his care.  When he gets some time to himself, he can often be found cooking at home with a glass of red wine in hand