There are two sources of echo in a TETRA system.
Echo is only of concern for duplex calls. In general the echo coming from the PSTN is characterised by a short reverberation time and a low dispersion. The echo from the terminal is more complex due to the time variant nature of acoustic echo which can be severe in a mobile environment.
[G.131] discusses the effects of echo at some length and presents curves showing user tolerance to echo against round trip delays. GSM [GSM 03.50] has a requirement for echo cancellation of 46dB which is, if anything, lower than the figure suggested by [G.131]. This figure applies to both the terminal and PSTN interfaces.
The large bulk delays within the TETRA system will result in difficulties with conference calls in duplex mode as the TETRA users will not want to hear their own delayed speech being relayed back to them. Avoiding this will complicate the processing within the infrastructure and may limit conference calls to non-duplex only.
The TETRA terminal shall have echo cancellation sufficient to ensure that the Echo Loss (EL) shall not be less than 46dB. This may be achieved by handset and headset design or by the provision of an effective echo cancellation circuit.
Compliance is tested using artificial speech on an end to end call as described in [GSM 03.50, Annex C.9.1]. Because of the complete isolation between paths in the TETRA network an equivalent test can be performed using the Linear PCM Reference point within the terminal.
The Echo Loss for a handsfree terminal shall be 40dB at the nominal volume setting in quiet background conditions and 33dB at the maximum volume setting. The echo cancellation algorithm shall have to be capable of handling the expected dispersion and reverberation in the mobile environment, the reverberation and dispersion may be time variant.
Compliance is tested using artificial speech on an end to end call as described in [GSM 03.50, Annex C.9.2]. Because of the complete isolation between paths in the TETRA network an equivalent test can be performed using the Linear PCM Reference point within the terminal.
The echo control device at the PSTN interface should meet the requirements of [G.165], but with an end delay of 60mS (Td in paragraph 3.2). This is in line with the requirements in [GSM 03.50].
Copyright ©2002 Andy Fletcher andy@x31.com