Linking PMR sites is not all about high bandwidths

Analog and digital PMR (professional mobile radio) base station sites can be reliably and efficiently linked using narrowband point-to-point wireless networks operating in the UHF band, generally at OSI layer-2, ie, the data link layer.

However, a common misconception about PMR linking solutions is that a large amount of bandwidth is needed. The fact is that “megahertz bandwidths” are not needed to provide an optimised low latency PMR network linking solution. Instead, there is a growing recognition of the value that ultraspectrally efficient narrowband systems that have been cleverly designed to achieve the desired QoS (quality of service) are a good fit for linking PMR sites. In fact, if the QoS issues are overlooked then the systems selected on the basis of ‘megahertz bandwidths’ alone may still lead to excess delays and data loss.

Low latency, jitter and high reliability are the most critical attributes for a network link carrying VoIP packet data. By being aware of these issues, it is possible to design narrowband products that are able to achieve latencies that are better than 10 milliseconds.

By using a combination of intelligent radio design and smart software features, it is possible to support a large number of PMR channels in narrow bandwidth while still maintaining very low latency and jitter.

MiMOMax uses multiple input multiple output technology and space time diversity, in combination with high orders of modulation and M-DAP (data acceleration protocol), to get the capacity and QoS management. M-CAM modulation, forward error correction processing architecture combined with a timely re-send feature are used to enhance the reliability.

Finally, training algorithms for the equalising in fading channel conditions ensure high availability. The M-DAP assigns high priority to critical data packets. M-CAM is an adaptive modulation feature that enables the constellation to be varied from higher order quadrature amplitude modulation to quadriphase shift keying based on the equaliser’s error performance. The total system bandwidth available to transport the required number of channels is dictated by the modulation rates available for a particular application.

The table below gives the absolute minimum number of P25 trunked or conventional channels that can be carried when using a particular modulation rate in a typical 2x2 MIMO link occupying 25 kHz bandwidth.

The residual bandwidth in the table above is available for PMR link management and it is possible to carry more channels depending on the ethernet’s non-VoIP data rate requirements.

In conclusion, PMR linking solutions need to be robust and have the ability to rapidly re-train in the presence of destructive interference so that the overall availability is as high as possible in any given environment. Moreover, very low latency and jitter is critical so that the linking solution does not add to the overall system latency and jitter performance. Optimum linking solutions are those that support FEC schemes but not at the cost of excessive latency.

MiMOMax Wireless senior designer Dr Kishore Mehrotra has contributed towards the research and development of products in the areas of power electronics, control systems, wireless protocols, power amplifier linearisation and MIMO wireless communications. He has over 30 years of experience in the electronics industry. He has a B.Tech (Electrical) degree from IIT Kanpur and a PhD from IISc Bangalore.