Digital Advantages of TETRA
The characteristics of voice quality are clarity, distortion, noise and end to end transmission delay. Because PMR is a narrow band wireless technology, low bit rate voice coder/decoders (Codecs) operating around 4 kbits/s are used to convert voice signals into a digital code for transmission and then convert the digital code at the receiving end into a representation of the original voice signal. Depending on the type of codec used, the individual characteristics of voice quality can vary.
As a general rule, all codecs will provide constant good quality voice communications throughout the coverage area independent of RF signal strength, simply because digital either works or it doesn’t work. As soon as a digital signal is corrupted, such as that caused by severe RF fading conditions, voice quality drops rapidly. However, analogue will provide high quality voice communication in high signal strength areas but this will gradually degrade down to poor voice quality in low RF signal strength areas.
The extent of RF coverage is mainly determined by transmitted RF power and receiver sensitivity, combined with the propagation characteristics of the radio frequency being used. Assuming these determining factors are the same between analogue and digital the difference in RF coverage performance should be minimal. However, the way in which receiver sensitivity is specified in analogue is different than that with digital. For example, the accepted method of specifying analogue receiver sensitivity is the RF signal level required to produce a 20 dB signal to noise ratio, whereas with digital it is the RF signal level at which a particular Bit Error Rate (BER) is exhibited. The relationship between BER and minimum acceptable voice quality (the receiver sensitivity) is a function of the forward error correction and detection algorithm used to protect the integrity of the voice codec.
Another factor that affects voice quality is bearer circuits. In a large nation-wide system several hundred km of bearer circuits could be used to connect base stations to switches as well as switches to switches. In some countries analogue bearer circuits are still used which means that communications between users located a large distance apart could experience poor voice quality caused by added noise, variations in signal amplitude and frequency response distortion.
In a digital system all bearer circuits are digital, which means users communicating with each other from opposite ends of the system will experience the same perceived voice quality as users communicating with each other on the same local base station site.
The transmission of digital information in analogue systems is normally carried out using sub-carrier modulation. For example, in MPT1327 Fast Frequency Shift Keying (FFSK) is used to provide a gross data rate of 1200 bits/s resulting in a net data rate of around 600 bits/s after error correction and detection. A typical digital system has a gross data rate of around 8,000 bits/s and a net user rate of around 4,800 bits/s after forward error correction and detection.
Overall, for the same occupied channel bandwidth, digital systems offer a higher data throughput than analogue systems that are primarily designed to carry voice communications. This is mainly because digital systems are designed only for the transmission of digital information, which could be voice and/or data, with no differentiation.
The best form of voice security against eavesdropping is that provided by using digitally encoded voice encryption algorithms, which by nature of being digital make them difficult to employ in analogue systems. As digital systems are designed only for the transmission of digital information, the voice information elements of these transmissions can be digitally encrypted more easily.
The component cost to build an analogue radio and a digital radio are approximately the same. The main differences affecting cost, which are not technology related, are:
- Economies of Scale
- Technology Maturity
- Life Cycle Cost
Economies of Scale:
The main factors affecting economies of scale are the size of the market and how well that market is harmonised in both technology and frequency bands. For example, if there was a very large market for one type of technology operating in one globally harmonised frequency band, the economies of scale would be very high. If however, the market was still very large but there were several technology options and several frequency bands to be supported, the economies of scale would be relatively low.
In the case of analogue PMR the market is declining in size but is still relatively large. However, the number of analogue technology options available and the number of frequency bands to be supported are numerous resulting in lower economies of scale compared with digital PMR which is experiencing rapid market growth towards an equally large market but with fewer technology choices and frequency bands.
This means that the size of the digital market will soon exceed that of the analogue market. Also, a large digital market served by only a few different technologies operating in the same harmonised frequency bands will provide greater economies of scale than those being experienced in the analogue market.
The larger the market the more suppliers you have competing for business, which results in lower costs to the user. In the case of digital, there are many competing manufacturers looking for business, which has already resulted in a significant drop in terminal prices over the last few years, a trend that is likely to continue as the market grows.
In the case of analogue, particularly technologies such as MPT1327, the number of competing manufacturers is getting less as the market starts to decline. Another factor is that several manufacturers are no longer developing new analogue products because development resources are required for digital technologies.
As a consequence the number of manufacturers serving the analogue market will decrease resulting in price increases until only one remains, at which point the last manufacturer has a small captive market and can demand what price they like for products until the analogue market virtually disappears.
As mentioned previously, most manufacturers are no longer developing new analogue products and the sales revenues they have already obtained from existing analogue products has already covered their development investment. This means that analogue product sales no longer need to consider investment recovery and can therefore be offered at lower prices and still make an acceptable profit.
In the case of digital, product developments are still continuing and are likely to continue for at least another few years until the full portfolio of facilities and services are available. This means that product sales prices need to include a significant portion for investment cost recovery, even though the total investment cost is usually amortised over the projected sales volume for products. As a result, digital products are usually more expensive than analogue products.
However, it is important to note that this only holds true if the sales volumes are the same for both analogue and digital. As sales volumes decline the actual manufacturing cost per product will increase because the fixed manufacturing overhead cost will need to be apportioned against actual product volumes.
Life Cycle Cost:
Typically, a PMR network is expected to have a life cycle of around 15 to 20 years before replacement. Therefore, investing in a technology that has good longevity will minimise the cost of product replacement and/or expansion.
Because voice signals in digital systems are translated into a digitally coded signal that best represent the voice sample in the codec’s reference table, background noises with no recognisable voice characteristics are not usually encoded. This voice codec characteristic means that digital transmissions using low bit rate voice codecs are often immune to background noise. In some cases this can be advantageous such as when operating high noise environments. However, in some operational scenarios the ability for a receiving radio user to hear background sounds that are not voice is considered advantageous.
As this is a general codec characteristic the actual background noise rejection will vary depending on the type of codec used. Also, the actual acoustic design of radio terminals, as well as accessories, will cause differences in performance independent of the codec type employed.
Source: TETRA Association