Rohde & Schwarz CMU-K20, Руководство по эксплуатации

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CMU-K20/K21/K22/K23/K24
Adaptive Multi-Rate (AMR) Speech Codec
1115.6088.12 4.203 E-12
Adaptive Multi-Rate (AMR) Speech Codec
(Option R&S CMU-K45)
If an AMR speech codec test is selected (see
Traffic Mode
softkey on p. 4.170 ) the
Network
tab
displays additional softkeys to configure the AMR codec and test the uplink and downlink codec
adaptation (inband signalling). The following settings are provided:
•
Selection of a subset of codec modes and switching thresholds
(AMR Rate Set)
•
Explicit setting of the codec mode at the CMU
(Codec Mode DL)
and the MS under test
(Codec
Mode UL).
•
BS signal level setting in the used and in the unused timeslots
(TCH Level).
AMR codec
The Adaptive Multi-Rate (AMR) codec is an integrated speech codec with six or eight fixed
user bit rates ranging from 4.75 kbit/s to 7.95 kbit/s (AMR Half Rate) or 12.2 kbit/s (AMR Full
Rate). The speech coder is capable of switching its user bit rate upon command.
Decreasing the bit-rate impairs the speech quality but leaves more bits for error protection.
This allows a dynamic trade-off between the speech quality and the stability of the connection
as the quality of the radio link varies.
Codec mode selection is done from a set of 1 to 4 active codec modes (ACS, Active Codec
Set) and an associated set of 1 to 3 switching thresholds for increasing and decreasing the
bit rate. The necessary signalling messages are included in the AMR speech frames (inband
signaling).
Test of inband
signalling
The MS conformance test specification 3GPP TS 51.010 describes the procedure and
conditions for inband signalling tests.
•
The purpose of the downlink adaptation test is to verify that the MS can monitor the
downlink quality of the dedicated traffic channel
(BS Signal)
and request a BS codec
mode according to the thresholds provided at call setup.
•
The purpose of the uplink adaptation test is to verify that the MS in the uplink
direction applies the codec mode indicated by the network, and that the MS
correctly signals the used codec mode to the network.
Both tests shall be performed with the codec mode and threshold settings quoted in Table 4-
11 below. The values are different for full rate and half rate AMR speech coders.
Table 4-11 AMR Rate Set according to the conformance test specification
AMR Full Rate
AMR Half Rate
3
Codec
Mode TCH data
rate Threshold
– Down Threshold
– Up TCH data rate Threshold
– Down Threshold
– Up
Mode 4 12.2 kbit/s 16.5 dB
+
∞ –
Mode 3 7.95 kbit/s 11.5 dB 18.5 dB 7.95 kbit/s 12.5 dB ∞
Mode 2 5.9 kbit/s 6.5 dB 13.5 dB 6.7 kbit/s 11.0 dB 15.0 dB
Mode 1 4.75 kbit/s
–
∞ 8.5 dB 5.15 kbit/s
–
∞ 13.0 dB
3 A test model with 4 codec modes is described in standard 3GPP TS 45.009.
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