A common misconception among radio professionals is that a large bandwidth is needed when linking analogue and digital PMR base-station sites. However, as Paul Daigneault concludes, this isn’t necessarily the case.
With the development and adoption of new higher spectrally efficient radio technologies, it can be argued that there’s no overall shortage of radio spectrum. However, the set of unique propagation properties particularly associated with radio waves in the UHF bands, make this the most sought after and crowded band in the usable spectrum. As a consequence, the spectrum in these “golden” bands has been regularly divided up into smaller chunks or channels to accommodate more users and make the best utility out of this finite and valuable resource.
In the past, linking of radio infrastructure had typically been implemented utilising UHF linking frequencies where possible, which not only provided propagation advantages, but also predictability in terms of latency, jitter, availability and reliability. More recently, however, it has become common to assume that microwave links are required to accommodate the growing bandwidth requirements of digital PMR networks.
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 ultra-spectrally 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.
At the same time, the increasing popularity of IP has shifted the paradigm from “IP over everything” to “everything over IP, including linking of PMR sites”. Behind all this success is the underlying fabric of the internet: the Internet Protocol. IP was designed to provide best-effort service for delivery of data packets and to run across virtually any network transmission media and system platform. As a consequence, issues such as delay (latency), variation in delay (jitter), packet loss, late packet arrival, availability and bandwidth requirements all become very significant.
By being aware of these issues and using a combination of intelligent radio design and smart software features, it’s possible to support a large number of PMR channels in narrow bandwidth with minimal packet loss and late packet arrival, while still maintaining very low latency and jitter, even across challenging terrains.
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