Digital shapes the future for land mobile radio
Digital technology affects the functions, features and maintenance techniques for land mobile radio equipment, especially specialized mobile radio (SMR). Technicians are spending more time optimizing entire radio systems.
Land mobile radio continues to borrow features and technology from the cellular industry. As with cellular, radios in land mobile service use a growing number of digital functions that defy traditional trouble analysis and feature a growing list of maintenance aids.
Trends in land mobile radio testing and new radio techniques are bringing about changes in the test equipment market. The rush to adopt various digital radio proposals will usher in a new wave of powerful and reliable self-test features and self-optimizing networks.
Land mobile radio_it’s not cellular Considering the growing dominance of trunked radio techniques used in the land mobile industry, especially SMR (specialized mobile radio) systems that increasingly dominate land mobile services, it might seem that cellular radio techniques and systems eventually will replace even the most modern SMR systems. Although there may be some slight truth to this assumption, fundamental differences between cellular and SMR systems, particularly in the way they are used, will continue to drive the need for each market. Basic variations between the two technologies include: 1. PSTN access: A cellular system’s primary goal is to provide a mobile link to the PSTN (public switched telephone network). Although SMRs also can make mobile connections to the PSTN, it is not their primary job. 2. Open channel: An SMR user or customer prefers and expects open channel working through the universal PTT (push-to-talk) switch. Access times of less than one-tenth of a second are common in modern SMRs. 3. Back-to-back: SMR terminals most often are used in back-to-back working situations through a repeater. This practice is unusual in cellular systems. 4. User partitioning: The familiar channel selector knob on the typical SMR radio is the primary instrument through which users are partitioned among themselves on the system. 5. User statistics: SMR users access their systems more often than cellular phone users access their cellular systems. The cellular radio connection is full- duplex_lasting several minutes_and the frequency of the connections established during the day is typical of that with normal desk phone use. The SMR user gains access to the system hundreds of times each day, and the connections last only a few seconds. 6. Dispatch: SMR systems can be configured so that any terminal or dispatch console can simultaneously establish communications with many radios. 7. Coverage: One goal in typical SMR system design is to maximize system coverage, usually from a repeater site in some ideal location. Cellular systems have an opposite goal, which is to maximize the system’s capacity or the number of phones that can be connected to the PSTN at the same time. 8. Terminals: SMR users, particularly in demanding public safety applications, generally require highly specialized terminals within a wide range of hostile operating conditions. 9. Data: Cellular traffic is dominated completely by full-duplex voice. SMR systems carry a much broader mix of voice and data than cellular systems. Examples 2-7 can be simulated in modern digital cellular systems through the use of packet radio techniques, but usually at the expense of the cellular system’s capacity. The more closely the SMR feature set is simulated in a host cellular system, the more the system’s capacity for its PSTN connection traffic suffers.
Digital radio and modulation Digital cellular systems are popular among the mobile phone network operators because they can multiply the traffic that a physical channel, such as a 30kHz channel, can carry. They perform the multiplication through a variety of radio access techniques. The enabling technology for all of the access techniques is digital modulation, which reduces the information in the radio channel to symbols. These 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 trans-mitter’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 variances is in the preferred modulation types. Because land mobile systems tend to have many operation modes and sometimes need to interwork with analog systems, they tend to avoid some of the complex in-phase and quadrature (I/Q) modulation types used in certain digital cellular radios. This linear modulation 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, such as access time, rather than to optimize system capacity. For public safety service users, this is significant because of their preference of system performance characteristics over capacity.
Testing digital radios Aside from the numerous technical advantages inherent in digital modulation, such as resisting noise and enabling new radio access techniques, digital modulation removes the possibility of analog signals anywhere in the radio except at the microphone and speaker. This greatly simplifies radio testing and maintenance. The greatest similarity among the service entities_cellular and land mobile_is seen in the service shop, where fully digital radios are serviced with digital testing technologies.
Digital radios have two analog interfaces: the audio interface (speaker and microphone); and the RF (radio frequency) interface at the antenna (transmitter output and receiver input). These analog elements notwithstanding, everything between the audio and the antenna is digital. * Digital transmitters: A digital modulator imparts information to an assigned carrier by adjusting the carrier’s power, phase or frequency among a small dictionary of possibilities. Because there is no amount involved with digital modulation, no adjustments are generally required in the modulation path. Instead, it is usually sufficient to examine the quality of the modulation, or how good the modulator is at making its adjustments to the carrier. This parameter is measured in a percentage, or some other quality score against a goal. When the goal is not met, one looks for a defective component that is causing the problem. An important example is the transmitter path, which ranges from the RF to the power amplifier, including the antenna. Anything beyond the digital modulator can add its own distortions to the modulation. Some of the devices may be linear, such as feedlines and antennas, and some, such as amplifiers, may not. Informed substitution, tempered by knowledge of how a device could distort a digital modulator’s output, is enough to clear low-quality (high-error) readings of the modulation somewhere in the transmit path. A radio test set with some kind of modulation domain display applicable to the modulation under consideration is helpful in transmit path analysis. * Digital receivers: Except for their decision circuit, digital receivers are remarkably similar to their analog cousins. Their differences occur in the manner in which they are tested. In the case of analog, the sinad (signal-to-noise and distortion) procedure is efficient because it tests the whole receiver path in one pass. As part of the sinad procedure, a low-level carrier is inserted, modulated in a manner appropriate to the demodulator under test, at a known rate, such as FM at 1kHz.
The audio output of the receiver is examined to separate the original test tone (1kHz) from any distortion and noise that the receiver added to the original test signal. The ratio of the two separated signals can be equated to some test threshold. The receiver’s circuitry can be manipulated to optimize the ratio so that the distortion and noise portion are minimized.
The procedure applied to a digital receiver is functionally the same. However, the absence of a 1kHz tone has to be accounted for to take advantage of the presence of symbols on the test signal. The RF test source is modulated in a manner appropriate to the demodulator under test, such as 4-level FM: frequency shift keying between four frequencies instead of two (4FSK). The symbols emerging from the receiver’s decision circuit are compared with the symbol stream used to modulate the test source.
Depending on how channel coding is performed, generated and recovered symbols can be compared on a per-symbol basis (BER), or comparisons can be made on blocks or frames of symbols (FER). The BER (bit error rate) expresses a ratio of the total number of decision opportunities presented to the receiver (from the test generator), relative to the corresponding number of wrong decisions made by the receiver. A BER rating of 0.20 is an indication of a better receiver than a BER rating of 1.20.
Channel coding, a baseband process used in most digital radio systems, deliberately adds carefully contrived redundancy to a symbol stream. The receiver uses these extra symbols, or bits, to repair damaged data streams from the transmitter. As a result, radios that protect all of the user traffic with redundancy bits, which are also encoded into symbols, lose the ability to detect individual bit errors. Instead, the frames or blocks of bits (symbols) that cannot be repaired are marked by the receiver as having suffered an error. An FER of 0.10 is worse than an FER of 0.00. The second result indicates no frames were received with uncorrected errors.
Baseband processes In place of various audio processing circuits found in analog radios, digital radios perform numerous bit manipulations that do not require testing or adjustments of any kind. One reason digital radio has become so popular is because of the reduced maintenance represented. The digital domain processes, collectively called baseband processes, include: * Channel coding: Gain can be added to a digital radio system by encoding and scrambling traffic data (voice data or computer files) in some clever way known to the receiver. The receiver can “figure out” what the original data or symbol stream actually was, even in the presence of a high BER (a high proportion of wrong decisions). Many types of channel coding schemes exist, some more powerful than others. The process is similar to sending a message and then adding all the consonants at the end. (For example, OVER THE WALL! + OVRTHW!) The cost to the system is a considerable number of extra bits in the channel. * Voice coding: Voice coding, common to all types of digital radio systems, is the process in which an analog voice waveform is digitized and then coded to remove redundancy. Numerous ways to encode voice have been identified. A general tradeoff exists between the perceived quality of the voice recovered in the receiver and the number of bits needed to encode the voice. Because the process of recovering the original voice waveform requires a detailed knowledge of how the encoding was performed, both the receiver side and transmitter side functions are performed in the same chip or software module. As a narrowband service, land mobile radio confines itself to so-called low-rate voice coding processes, which favor low data rates with sacrifice to the perceived quality from the receiver. Digital FM broadcast services are exactly the opposite. They employ the highest rates possible to get excellent music * Equalization: An equalizer, found in most digital receivers, removes the ISI (intersymbol interference) that the radio channel imparted to the signal from the transmitter. 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.
Equalizers in digital radios are much more complex than their simple analog cousins. Low-cost advances in DSPs (digital signal processing) and analog-to-digital converters have made digital radio affordable.
Looking ahead The road to reliable digital radios is well-marked into the future. The field of land mobile radio will witness an increased effort to use spectrally efficient modulation through digital means. The radio networks will become more intelligent and efficient in their ability to automatically test all parts of the radio system. As a result, technicians will spend less time testing and repairing radios, and spend more time optimizing whole radio systems.
