03.503GPPTransmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS
The overall transmission performance of connections in alternate conversation mode can be considered as a summation of the effects of:
1) the audio part between the MRP/ERP and the UPCMI interface;
2) the speech transcoder part including the effects of radio transmission, and speech processing between the UPCMI and the POI;
3) the overall characteristics of the connection between POI and the other user.
There is not only a linear addition of these effects but there is also an influence from different parts of the connection on the performance of the speech transcoder and other speech processing devices.
Where possible, the transmission performance is specified between the MRP/ERP and the POI. Where this is not possible, the transmission aspects of the audio part mentioned above have been specified. The transmission aspects of the speech transcoder are specified in GSM 06 series specifications. In the following paragraphs, requirements are specified for the UPCMI, the Air Interface or the POI as appropriate.
The transmission requirements of the MS have been derived from the requirements of digital telephones stated in I‑ETS 300 245‑2 and 3.
MSs will have to work in a variety of environments ranging from quiet office locations to very noisy environments as found in moving cars. In noisy conditions, different values for the sending and receiving sensitivities relative to the nominal values can increase the performance of the terminal. Some guidance is given in annex E.
The overall transmission performance in full duplex conversation mode will also greatly depend on the performance of the echo control devices which may be included in the connection.
The handsfree requirements in the present document are designed to provide a basic level of performance and to avoid adverse interactions with other networks. Testing is carried out in one configuration in a vehicle, whilst this gives some confidence that the system can work in a typical environment, the testing is by no means complete. It is assumed that the manufacturer submits the handsfree system with the transducers fitted in reasonable locations within the vehicle (the transducers should not impede the normal operation of the vehicle or its fittings). Unusual installations should be agreed for suitability with the relevant Type Approval Authority before testing.
In a real vehicle installation, care should be taken to allow for the acoustic properties of that vehicle and the likely acoustic environment. It is important that the best possible coupling between the microphone and the MS user is achieved. Hence the microphone should be directional and mounted as close to the user’s mouth as practical. The loudspeakers should be mounted in such a way that the maximum receive signal is directed at the user, rather than dissipated by the various obstacles in that vehicle, such as the seats.
Proper consideration for the noise environment and the direct coupling between the microphone and loudspeakers is necessary. Excessive noise coupled into the microphone can mask the MS user’s send speech and potentially affect the operation of DTX. The vehicle noise environment can potentially mask the received speech, unless sufficient volume is provided. However, direct coupling between the transducers can cause annoying echo to be heard by the far end user.
Primary factors affecting the coupling between the loudspeaker(s) and microphone(s) include:
1) directionality of the microphone(s);
2) directionality of the loudspeaker(s);
3) location of the transducers in relation to each other and reflecting surfaces such as the windows and windscreen.