3.4 Echo

03.503GPPTransmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS

3.4.1 General

There are two main sources of echo:

1) acoustic echo caused by the acoustic path between receive and transmit transducers;

2) electrical echo caused by coupling between the transmit and receive directions of transmission. The primary source of this form of echo is a two‑to‑four wire converter.

Electrical echo can be eliminated by the use of end‑to‑end four‑wire transmission. Acoustic echo will be generated in all telephone instruments with the exception of carefully designed headsets.

In general, electrical echo is characterized by a short reverberation time and low dispersion while acoustic echo is likely to have a longer reverberation time and greater dispersion. The case of the acoustic echo may be further complicated by the time variant nature of acoustic echo which may be more severe in the mobile environment.

Curves showing the tolerance to echo, taking account of the relationship between the delay and the level of the echo, are given in ITU‑T Recommendation G.131 figure 2/G.131. In practice, it has been found that for any connection with a delay of greater than 25 ms, some form of echo control will be required to reduce the level of the echo (ITU‑T Recommendation G.131 Rule M).

With the expected maximum one‑way delay in the PLMN of 90 ms, acoustic echo control will be required in the MS to reduce the echo returned to the distant end and electrical echo control will be required at the POI to reduce the echo returned to the PLMN user from the PSTN. The design of these echo control devices should be such as to provide operation in full duplex mode (as opposed to alternate mode).

The echo loss (EL) presented by the PLMN at the POI should be at least 46 dB during single talk. This value takes into account the fact that a MS is likely to be used in a wide range of noise environments. This requirement should be met for handset and handsfree MSs. Compliance shall be checked by the test described in annex C, clause C.9.

3.4.2 Electrical echo control in the PLMN (Reference configurations A)

The electrical echo control device at the interface with the PSTN should meet the requirements given in ITU‑T Recommendation G.165, but with an end delay of 60 ms. This refers to td in clause 3.2 of ITU‑T Recommendation G.165. The 60 ms is calculated as follows. ITU‑T Recommendation G.131 states that the maximum length of connection which need not have echo control has a mean one‑way propagation time of 25 ms. However, this figure is the sum of the delays of the international connection and the maximum national delays at each end of the connection. Since the interconnection of the PLMN to the PSTN is unlikely to be at a point where the PSTN delay is > 22 ms, and the dispersion may be up to 8 ms, the maximum expected end delay which the echo canceller in the MSC should expect is:

(22 + 8) x 2 = 60 ms (see figure 7).

Certain countries on the geographical limits of a continent may need to increase this limit as there may be a proportion of connections which do not comply with ITU‑T Recommendation G.131 having a mean one‑way delay of greater than 25 ms and yet are not provided with echo control.

3.4.3 Acoustic echo control in the PLMN

Acoustic echo control provided in the MS should provide a TCLw of 46 dB at the POI (see clause 3.4.1) over the likely range of acoustic end delays. If acoustic echo control is provided by voice switching, comfort noise should be injected. This comfort noise shall operate in the same way to that used in Discontinuous Transmission system (DTX). Effectively, the acoustic echo loss is provided by MS as the GSM network is zero loss from the air interface to the POI and hence the 46 dB requirement should be applied to the MS.

3.4.3.1 Acoustic echo control in a handsfree MS

The TCLw for the handsfree MS shall be 40 dB at the nominal setting of the volume control in quiet background conditions and 33 dB at the maximum user selectable volume control setting. If acoustic echo control is provided using some form of echo cancellation technique, the cancellation algorithm should be designed to cope with the expected reverberation and dispersion. In the case of the handsfree MS, this reverberation and dispersion may be time variant.

3.4.3.2 Acoustic echo control in a handset MS

The TCLw for the handset MS shall be 46 dB. Careful acoustic design of the handset body and selection of the mouth and ear piece transducers may facilitate the required acoustic echo loss without the need for active echo control techniques. However, should echo cancellation be employed the echo canceller should be capable of dealing with the variations in handset positions when in normal use. The implications of this are under study.

3.4.3.3 Acoustic echo control in a headset MS

The TCLw for a headset MS shall be 46 dB. Due to the obstacle effect of the head in this type of terminal, careful design might mean that no active echo control is necessary.

3.4.4 Interaction between tandem echo control devices (reference configurations B & C)

On long international routes or routes containing a satellite path, network echo control devices will be present in accordance with ITU‑T Recommendation G.131 Rule M. These devices will be echo suppressers or echo cancellers generally with centre clippers. The tandem connection of such devices can lead to increased clipping and, if echo suppressers are used, additional loss. It is recommended that signalling or routeing means be used to avoid the tandem connections of echo control devices whenever possible (see figure 7).