Do you know what to consider when specifying or designing digital radio networks for data transmission, as opposed to just voice? There are important coverage differences that you need to understand, to be sure you specify for the overall network performance you need. In this article, first featured in the latest issue of Connection Magazine, regular Connection contributor and Principal Engineer at Tait Communications Ian Graham sorts out some important differences when specifying digital voice and data.
DMR and P25 digital radio standards are specifically designed to provide voice coverage very similar to their analog narrowband FM predecessors. This means that migration from analog narrowband FM voice systems to more spectrally efficient, data-capable DMR or P25 networks does not require additional repeater sites. However, when specifying or designing digital radio networks for data transmission, there are important coverage differences that you need to understand, to be sure you specify for the overall network performance you need.
First, let’s define some typical performance metrics:
- Voice network performance is usually defined as a Delivered Audio Quality (DAQ);
- data performance is normally defined by Message Error Rate (MER).
For voice, a DAQ of 3.4 (speech understandable without repetition, although some noise or distortion present) is acceptable for most Public Safety applications. This equates to a Bit Error Rate (BER) of around 2% – two out of every 100 bits received are decoded incorrectly. The decoded audio is noticeably distorted, but is still perfectly understandable to the human ear.
Things are not so simple with data, unfortunately. For example, if a data message is 1000 bits long, then that same 2% BER will correspond to an average of 20 errors. Even with Forward Error Correction (FEC) techniques, a BER of 2% means few (if any) 1000-bit messages will be received correctly. That 2% BER equates to nearly 100% MER!
Let’s look at it the other way round. If, for example, we decide that MER of 1% is acceptable, that means only one 1000-bit message in 100 is received incorrectly – conceivably, only one bit in 100,000 has been received incorrectly. So our 1% MER equates to a BER of just 0.001%.
In terms of the acceptable received signal level, for a DMR voice network to achieve DAQ3.4, the signal must be at least 15.6dB above the noise and interference present, the carrier-to noise ratio (CNR) must be greater than 15.6dB. For a DMR data network, the minimum CNR will depend on message length; the longer the message, the greater the CNR required to achieve the desired 1% MER.
This reinforces an important difference: for DMR voice there is a single CNR threshold, whereas DMR data networks must be designed with the longest message length in mind, because the threshold varies with message length. Typically, the acceptable CNR for DMR data performance needs to be 10–15dB higher than it does for DMR voice.
Now let’s bring it back to what really matters—coverage. If we look at line-of-sight coverage according to free space path loss (FSPL), and the data network requires CNR 10dB higher than voice to achieve 1% MER, the resulting coverage radius will only be 30% of that for DAQ3.4 DMR voice (or 10% of the covered area). For longer message lengths – where the CNR requirement is 15dB higher than for DMR voice – the coverage radius will only be 17% of that for voice. That’s just 3% of the covered area.
WHAT DOES THIS MEAN IN REAL TERMS?
Luckily, the news isn’t quite as bad as these theoretical figures suggest. In practice, over real terrain with no line of sight, the covered area differences would be less, but would still be significant and will vary depending on the type of terrain. Similarly, there are technical solutions that can narrow the coverage gap further, e.g. involving technology selection system design, and at product level. That’s why it’s critical that you consult coverage experts when you are investing in a new digital radio network.
For more articles like this, check out the latest edition of Connection Magazine online, or subscribe today to receive new issues of Connection as they’re released, along with other informative content from Tait.