3.4 Echo

3GPP43.050Release 17Transmission planning aspects of the speech service in the GSM Public Land Mobile Network (PLMN) systemTS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

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 1/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 shall be as specified in the 3GPP Network in [49], 3GPP TS 26.131.

For a dual mode GSM/3GPP telephone as well as a single mode GSM MS compliance shall be checked according to the procedures described in [50] , 3GPP TS 26.132.

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.168, but with an end delay of 60 ms. This refers to td in subclause 1.3.10 of ITU‑T Recommendation G.168. 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 in the PLMN shall be as specified in the 3GPP Network in [49], 3GPP TS 26.131.

For a dual mode GSM/3GPP telephone as well as a single mode GSM MS compliance shall be checked according to the procedures described in [50] , 3GPP TS 26.132.

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).