Around the globe, public safety communications agencies looked at the explosion of digital cellular communications technologies with great interest. Operating a myriad of analog systems, users realized that digital was the answer to several critical issues confronting the industry. Chief among these concerns were:
- The growing scarcity of available radio spectrum
- Better voice quality over greater areas
- The growing demand for the integration of new, bandwidth intensive, data functions
- Better communications security
The drive to develop a digital solution began in earnest, driven mainly by European and US-based standards bodies. Like its cellular brethren, competing camps developed all of the various access techniques, however, the techniques were unified by digital modulation.
Digital modulation reduces the information in the radio channel to symbols. The symbols are generated in the transmitter’s digital modulator in accordance with a set of instructions from the baseband part of the transmitter. The instructions are bit patterns that represent the information coded into the symbols. The symbols are recovered in a receiver that demodulates the transmitter’s manipulations of its carrier and passes the demodulated waveform on to a decision circuit that decides which of the symbols the transmitter sent at any instant.
SMR systems have followed cellular into the wide adoption of digital radio techniques, but with some minor, though significant, differences. One of the differences is in the preferred modulation types. Since land mobile systems tend to have many modes of operation and need to sometimes interwork with analog systems, they have tended to avoid some of the complex plane types of modulation used in some digital cellular radios. This simplifies amplifier and system design. Another difference is in the use of access techniques. Land mobile systems use various radio access techniques to enhance system performance, e.g. access time, rather then to optimize system capacity.
The digital domain processes are collectively called baseband processes. Three baseband processes are relevant for this discussion:
Channel Coding - We can add gain to our digital radio system by encoding and scrambling our traffic data (voice data or computer files) in some clever way known to the receiver such that the receiver can "figure out" out what the original data or symbol stream actually was even in the presence of a high BER (a high proportion of wrong decisions). There are many types of channel coding schemes, some much more powerful then others. The process is something like sending a message together with , e.g. all the consonants in the message, "OVER THE WALL! + OVRTHW!" The cost to the system is a considerable number of extra bits in the channel.
Voice Coding - is common to all types of digital radio systems, and is the process where an analog voice waveform is digitized and then coded to remove redundancy. There are many ways to encode voice. There is a general tradeoff between the perceived quality of the voice recovered in the receiver against the number of bits needed to encode the voice. Since the process of recovering the original voice waveform requires a detailed knowledge of how the encoding was performed in the first place, both the receiver side and transmitter side functions are performed in the same chip or software module. Being a narrow band service, land mobile radio confines itself to so-called low rate voice coding processes, which are those that favor relatively low data rates with some sacrifice to the perceived quality from the receiver. The digital FM broadcast services are exactly the opposite; they employ the highest rates possible in order to get excellent music reproduction.
Equalization - An equalizer is a device found in most digital receivers that removes ISI (InterSymbol Interference). The radio channel represents a linear, band limited process that can be looked upon as a filter that spreads a symbol’s influence into the previous and next symbols’ times. If left uncorrected, the receiver’s decision circuits would cease to function. As is the case with most digital baseband processes, there is a huge catalog of equalizer schemes to select from.