Brains beat brawn in radio linking

By Paul Daigneault, Chief Executive Officer, MiMOMax.

A common misconception among radio professionals is that a large bandwidth is needed when linking analog and digital PMR base-station sites. However, this isn’t necessarily the case.

Network cablesWith the development and adoption of more spectrally-efficient radio technologies, it can be argued that there’s no overall shortage of radio spectrum. However, the unique propagation properties particularly associated with radio waves in 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 into smaller chunks or channels to accommodate more users and make the best use of this finite and valuable resource.

In the past, radio infrastructure was typically linked utilizing UHF linking frequencies. This not only provided propagation advantages, but also predictability in terms of latency, jitter, availability and reliability. Recently, however, it is common to assume that microwave links are required to accommodate the growing bandwidth requirements of digital PMR networks.

The truth is that “megahertz bandwidths” are not needed to optimize low-latency PMR network linking. Instead, cleverly designed, ultra spectrally-efficient narrowband will achieve the desired quality of service (QoS)—a good fit for linking PMR sites. In fact, if the QoS issues are overlooked, then systems selected on the basis of “megahertz bandwidths” alone may still lead to excess delays and data loss. Behind all this is the underlying fabric of the internet: the Internet Protocol. The increasing popularity of IP has shifted the paradigm from “IP over everything” to “everything over IP”, including linking of PMR sites. IP was designed to provide best-effort service for data packet delivery and to run across virtually any network transmission medium 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 combining 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.

How is this achievable?
MiMOMax uses multiple-input, multiple-output technology and space–time diversity, in combination with high orders of modulation and M-DAP (MiMOMax Data Acceleration Protocols), to provide very high capacity in narrow bandwidths.

A high-performance narrowband radio with optimized compression algorithm, combined with QoS management and forward error-correction processing architecture, enhances the reliability. Finally, fast training algorithms to equalize in fading channel conditions ensure high availability. The M-DAP assigns high priority to critical data packets.

M-CAM (MiMOMax Cognizant Adaptive Modulation) is an optional adaptive modulation feature that can further enhance system reliability by enabling the constellation to be varied from higher order quadrature amplitude modulation to a lower order modulation, based on the equalizer’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 tables above give the minimum number of P25 trunked or conventional channels that can be carried, using a particular modulation rate in a typical 2×2 MIMO link. The residual bandwidth is available for PMR link management. It is possible to either carry more channels (depending on the Ethernet’s non-VoIP data rate requirements) or additional traffic. As a result of bandwidth misconceptions—and despite the RF propagation, implementation and cost advantages—narrowband radio may be unjustifiably discounted as a backhaul solution for PMR and other higher bandwidth applications, when, in fact, it may be the best solution.

In conclusion, PMR linking solutions needs 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. Optimum linking solutions support comprehensive FEC (Forward Error Correction) schemes but not at the cost of excessive latency. Using a combination of intelligent radio design and smart software features, it is possible to reliably support a large number of PMR channels in narrow bandwidth while still maintaining very low latency and jitter. This will provide users with an efficient, economical and reliable customized communications solution.

What can seriously smart technology do?

25 kHz P25 DMR
  Minimum Trunked Channels Residual Ethernet BW Minimum Trunked Channels Residual Ethernet BW
QAM256 10 channels 123 Kbps 13 Channels 126 Kbps
QAM64 8 channels 79 Kbps 11 Channels 79 Kbps
QAM16 5 channels 47 Kbps 7 Channels 49 Kbps
QPSK 2 channels 13 Kbps 3 Channels 18 Kbps
12.5 kHz P25 DMR
  Minimum Trunked Channels Residual Ethernet BW Minimum Trunked Channels Residual Ethernet BW
QAM256 5 channels 46 Kbps 7 Channels 53 Kbps
QAM64 4 channels 24 Kbps 5 Channels 38 Kbps
QAM16 2 channels 13 Kbps 3 Channels 23 Kbps
QPSK N/A N/A 1 Channel 8 Kbps

Tait Connection MagazineThis article is taken from Connection Magazine, Edition 3. Connection is a collection of educational and thought-leading articles focusing on critical communications, wireless and radio technology.

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