TETRA Transmission planning

Introduction

There are several factors which influence the perceived audio quality of a radiocommunications system when it is connected to the Public Switched Telephone Network (PSTN). These include Frequency Response, Loudness Rating, TETRA CODEC reference levels. This document specifies a set of parameters which can be used to ensure that a particular TETRA system can interoperate with other systems and the PSTN.

A transmission plan is important because it defines the audio performance of a system. If it is got seriously wrong then it can can result in disturbing changes in volume or unintelligible speech. ETSI decided not to produce a transmission plan for TETRA unlike GSM, DECT, ISDN and analogue systems.

This document is the result of an analysis of the transmission plans of other systems and presents what is in the opinion of the author a minimum set of requirements which should allow a TETRA system to interoperate with the PSTN and other systems.

ETSI defined TETRA interoperability testing appears to concentrate on the air protocol and doesn't consider any audio sensitivity or related issues. Unless there is agreement on system audio transmission between the various vendors interoperating with different equipments could result in user problems in areas such as echo and loudness where one end of a call complains about excessive volume and the other user complains that the first is too quiet.

TETRA is similar to GSM in many respects. The main differences are an increase in bulk delay through the system and the lower bit rate of the CODEC. The CODEC makes testing TETRA systems harder than GSM because it has a much more severe impact on the quality of the signals which pass through it. This makes single frequency audio testing impossible for all but the most basic tests resulting in the need for simulated speech testing to be performed.

The bulk delays through a TETRA system are about twice that of a GSM system with round trips figures exceeding 300mS. It is possible for end to end calls with two TETRA systems across the PSTN to have round trip delays as high as 700mS. Users will have to be very disciplined if they are to communicate in a satisfactory manner under these circumstances and good echo cancellation is absolutely essential. This high delay is similar to that encountered in DECT systems where the specified echo losses are the same as GSM.

Another area where TETRA differs from GSM is that TETRA use a wider range of call types. These include half duplex and group calls. Under these situations portable handset users will treat their handset like a conventional radio with the loudspeaker volume turned up to a much higher level than would be normal for a duplex call. This higher loudspeaker volume can easily exceed the Acoustic Shock limits were the handset to be placed close to the ear as would be done for duplex calls and therefore the user has to be warned about this situation.

Both the TETRA and GSM systems differ from conventional telephone systems in that they are often use in areas of high ambient noise. Because of this the loudness ratings are adjusted so that the microphone is less sensitive and the loudspeaker is louder than is normal for telephones. In addition adaptive gain control is often used to improve the performance of the handset under noisy conditions. Reducing the gain of the microphone can help to ensure that the TETRA CODEC does not interpret background noise as speech.

It is assumed that the PSTN transmission imparements are negligible in comparison to those introduced by the TETRA CODEC.

The requirements within these pages are written with duplex operation in mind. Non-duplex calls shall, where relevant, conform to all these requirements. It should be noted that the operation of portable equipments is generally different in the case of non-duplex calls and it may be best to regard them as loudspeaking terminals in these modes.

These requirements apply to all TETRA terminal equipments including line and radio connected despatchers.