Testing is an essential element in the successful manufacture, commissioning and maintenance of TETRA mobiles and base stations. As with other radio systems, transmitters need to be tested for power level, frequency and distortion; receivers need to be tested for sensitivity. TETRA is a digital system, using Differential Quaternary Phase Shift Keying (DQPSK) modulation and Time Division Multiple Access (TDMA) transmission. Transmitter distortion is replaced by 'Error Vector Magnitude' (EVM), a measurement of the impairment of the modulated signal. Receiver SINAD is replaced by 'Bit Error Rate' (BER), a measurement of the effect of the receiver noise on the ability to receive digital data. Mobiles and base stations are normally tested using a Radio Test Set, such as the IFR 3901 from Aeroflex, which combines a signal analyzer for testing transmitter performance and a signal generator for testing receiver performance. Mobiles and base stations employ different techniques for activating and controlling their transmitters and receivers, with essentially the same measurements. For mobiles, some additional measurements are performed as a consequence of the discontinuous TDMA transmission.Testing TETRA base stationsTesting a TETRA Base Station transceiver normally requires that the base station is operated in a proprietary test mode, typically using a control program supplied by the manufacturer. Some base stations also require a synchronization pulse lead between the base station and the test set. When testing a base station transmitter, the control program is used to command the base station to transmit at a particular power level and frequency. The test set signal analyzer is tuned to the base station transmitter frequency, and measurements are made of power level and modulation accuracy (EVM). When testing a receiver, the test set needs to synchronize the timing of its digital signal generator to that expected by the base station, using either a synchronization p pulse generated by the base station, or the timing information contained in the digital signal generated by the base station transmitter. The IFR 3901 supports both methods. The base station may perform its own BER measurement on the test set digital signal, or the base station may re-transmit (loopback) the received signal, allowing the test set to perform receiver sensitivity (BER) measurement directly.

Essential network parameters...

Essential network parametersTETRA mobiles are individually programmed with a configuration specific to the TETRA network. The test set must be configured with the correct TETRA network parameters, the Channel Plan, Mobile Country Code (MCC) and Mobile Network Code (MNC), otherwise the mobile is likely to ignore the test set and not perform registration or call placement. The MCC and MNC specify the network for which the mobile has been configured. Following registration and call set-up, measurements can be made on the mobile's transmitter. The IFR 3901 provides diagnostic displays to track down any poor frequency error and modulation (EVM) performances, which may be due to unstable transmit frequency.Power controlTETRA mobiles use Open Loop Power Control (OLPC) to calculate the transmit power level, based on the strength of the signal received from the base station (or test set). OLPC can be used to test the mobile's transmitter performance at different power levels by varying the test set RF Generator Level.

Testing receiver parameters at...

Testing receiver parameters at different frequenciesTETRA receivers are tested with an RF signal containing known digital data, usually a Pseudo Random Binary Sequence (PRBS). Comparison with the known data determines how well the mobile has received the RF signal. Normally the RF signal is generated at a low level, typically the reference sensitivity level, to determine adequate receiver sensitivity. Each time a call is set up, the mobile's transmitter and/or receiver performance at different frequencies can be tested. Some mobiles also support 'hand off' to a different traffic channel frequency during a call.Audio assessment and configurationAudio performance can be assessed subjectively using the 'talk-back' facility. Alternatively, a TETRA encoded 1kHz test tone can be generated for testing the mobile's audio receive performance in isolation. Functional behaviour and configuration is automatically assessed when the mobile registers, providing details of talk groups that it wants to 'attach' to. A time-stamped log with provides details of protocol configuration operations. The IFR 3901 can also check the mobile's ability to send and receive status messages and Short Data Service (SDS) messages.Automatic testing of TETRA mobilesThe Auto Test System is useful for an Auto Test script to include Test Mode registration, which obtains three parameters from the mobile for test automation. The TETRA Equipment Identity (TEI) is the mobile's electronic serial number that uniquely identifies the hardware under test. The Power Class informs the test script of the mobile's power rating whilst the Receiver Class informs the test script of the appropriate test limits to apply when performing the receiver BER tests using TT Loopback. To assist in some cases where it may not be possible to register a mobile and place calls, mobiles can be tested in a special ETSI-defined test mode. 'T1 test mode' is a one way control mode, requiring no registration or call set-up, i in which the test set generates a 'T1 test signal' to stimulate the mobile's receiver and control the mobile's transmitter.ConclusionThe IFR 3901 from Aeroflex is an invaluable tool for commissioning TETRA base stations and maintaining TETRA mobiles, but testing requirements need to be considered during the procurement phase. In order to test base stations and mobiles, it is normally necessary to refer to the manufacturer or supplier to obtain configuration information and access to test modes, and should form part of the procurement process. Allowances for purchasing test equipment or third party testing and maintenance services should be considered when setting procurement budgets.