Plants and construction sites need a radio technology which is easy to install and maintain. If several square kilometres or parts of an operation in different locations do not need to be covered, then TETRA equipment seems quite expensive. This is where DMR (Digital Mobile Radio) comes into play. Originally developed by Motorola and branded MOTOTRBO, it has since become an ETSI standard and is being marketed by many suppliers at appropriately attractive prices.
DMR has come to fill the gap between commercial mobile phone technology (GSM), trunked radio (TETRA) and simple walkie-talkies. This technology is ideal for construction sites, small and medium-sized companies in need of short call setup times and group calls with a few users but on their own frequencies. Due to the call setup time, commercial mobile phones and cordless telephones are out of the question.
DMR offers several different operating modes. The common denominator is a system with two time slots of 30ms duration each, in which, 4,800 symbols per second can be sent in 4FSK modulation.
The channel bandwidth of 12.5KHz suits common European channel spacing below 1GHz, and thus there are applications on various licensed and free bands starting at 68MHz. The transmission power of the terminal devices is fixed and is typically one watt or less. With digital voice transmission and encryption on the air interface, listening in is usually made difficult.
This technical benchmark data forms the basis for different variants: the direct mode, communication via a repeater, andcommunication via a base station. In direct mode, the devices transmit and receive on a single frequency; in simplex mode, only one time slot is used in alternation; in duplex operation, both are used simultaneously. The timing on the radio interface is specified by the radio device currently transmitting. The channels are operated in TDD (Time Division Duplex), both time slots are 30ms long.
If the system reaches its limits even with a repeater, then it can be expanded to include base stations, which further expand the range of the radio communication. Here too, frequency duplex comes into play. Per channel pair, however, there are still only two time slots available on a carrier, and the number of simultaneous conversations remains limited. Communication between the base stations is not standardised; for small systems with few base stations, the customer will rely on a single supplier.
During development, in production and during the testing of the devices between missions, various types of transmitter and receiver measurements are performed. Parameters which are always important for the transmitter of mobile radio devices are transmit power, frequency, and modulation errors.
There is a particularity for the power-related measurements: in the magnitude error measurements, deviations from the average output power are broken down according to symbol values, that is, according to the frequency deviation at the point of maximum effect of the symbol. Therefore, it is possible to establish a relationship between the respective frequency deviation and a deviation of the transmission power.
Another criterion for the modulation quality is the symbol clock error. Here, a trend in the temporal deviation of the symbol midpoint from the nominal value is regarded. The symbol clock error is measured in Millihertz; A significant value may be caused by an imprecise modulation frequency (target: 4,800Hz). Up to 48MHz is tolerable, however.
A technical particularity of DMR is its support of two essentially different duplex processes. In direct mode, the radio device is dependent on its own frequency stability and may not exceed a particular frequency and symbol clock error. In repeater mode, however, the radio must synchronise up to the frequency and timing of the receiver and may even have to switch cyclically between transmission and reception frequencies. For the measurements, this means essentially that the transmission quality must be tested in two different operating modes, that is, in the worst case, all measurements must be carried out in duplicate. The manufacturer’s specifications help to reduce the test time, for who knows better in which operating mode the radio design is most critical or which measurements do not have to be repeated. Experience in testing with various modes in commercial mobile phones also shows that the switching time between the modes (e.g. GSM – WCDMA – HSPA) has great influence on the overall test time.
As for all digital communication systems, so for DMR, the quality of the receiver is determined using statistical measurements of the bit error rate. In direct mode (simplex operation), for example, the measuring device repeatedly sends a longer, defined bit sequence. For sensitivity measurements on the lower limit, the test set’s output level is set to a low value. The radio synchronises itself to the (known) bit sequence, counts the incorrectly received bits and outputs the bit error rate calculated from this. For this, the radio must be put into a test mode. In contrast to most standards, the DMR standard does not define loopback of the signal with measurement of the bit error rate in the measuring device. The reason for this is that there are radios which are designed only for simplex operation, so they cannot send the signal received directly back to the measuring device.
Its master’s voice
The DMR standard supports a special test mode for audio measurements for which a tone of 1031Hz is transmitted. Thus, only the audio reception part can be tested. Therefore, a radio test set should also have the capability of looping the audio signal received back to the terminal. In combination with the test mode for audio measurements, errors such as signal distortions can then not only be determined, but their cause can be localised.
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