Adaptive Multi-Rate (AMR) Feature Descriptions

Adaptive Multi-Rate (AMR) Overview

The test set has two options for AMR logical speech channels: Adaptive Full-Rate Speech (AFS) and Adaptive Half-Rate Speech (AHS). During an AMR speech call setup, the test set assigns the mobile station a set of up to four codecs for use during the call. These four codecs comprise what is called the Active Codec Set (ACS) and can be defined for each AMR logical speech channel.

The test set uses one codec from the ACS at a time. The codec being used from the ACS is called the current codec. The current codec switches to other codecs in the ACS as the signal quality changes. If the signal quality decreases, then a more robust codec is used if one exists in the ACS. Likewise, if the signal quality increases, a less robust codec is used if one exists in the ACS. The point at which the current codec should change to another codec in the ACS is determined by the Threshold and Hysteresis values that can be specified in the test set. The test set allows parameters such as the AMR Logical Speech Channel, codecs that define the ACS, current codec, Threshold and Hysteresis values to be changed whether a call exists or not. A protocol log captured with the Wireless Protocol Advisor (WPA) can be used to verify that the mobile station responded properly to changes in AMR parameters. For more information see Protocol Logging .

Adaptive Multi-Rate (AMR) Channel Mode Configuration

The AMR specific logical speech channels that the channel mode can be configured to are Adaptive Full-Rate Speech (AFS) and Adaptive Half-Rate Speech (AHS). For instructions on setting the channel mode configuration on the front panel of the test set, see steps one through three of General AMR Parameter Setup . The GPIB command to configure the Logical Speech Channel is CALL:TCHannel:CMODe:LSPeech:CHANnel .

Adaptive Multi-Rate (AMR) Codec Setup

The speech quality of a codec in the test set uses to code/decode a signal is inversely proportional to the amount of error correction and protection used. For example, the Adaptive Half-Rate Speech (AHS) codec with the rate of 4.75 kbps is the lowest speech quality codec available for the AHS logical speech channel, but is the most robust because it provides the most error correction and protection. Furthermore, the AHS codec with a speech rate of 7.95 kbps is the highest speech quality codec the AHS logical speech channel offers, but is the least robust because it contains the least amount of error correction and protection.

• Active Codec Set

  An Active Codec Set (ACS) for AMR is a group of up to four codecs (codecs 1 through 4 on the front panel) the test set can command the mobile station to use. The following are general guidelines for defining the ACS:

  • There can be up to four codecs in an Active Codec Set (ACS).

  • Each codec in the ACS must have a different rate.

  • The ACS must be defined in sequential order such that the first codec is the most robust codec and the last codec is the least robust codec. If codecs one through four are not in ascending rate order the following error occurs: Illegal parameter value; Invalid AMR codec set specified.

  • If less than four codecs are desired in the codec set, the codecs must be eliminated consecutively from codec 4 to codec 2 by selecting the Unused option for each codec that is unwanted. For example, if only two codecs are desired in the ACS, codec 4 and codec 3 must be eliminated. The Unused option is not applicable to codec 1 in the ACS, as the test set requires there to be at least one codec in the ACS.

  The codecs available for the ACS depends on which AMR specific logical speech channel is selected as the channel mode. If AHS (Adaptive Half) is chosen as the AMR Logical Speech Channel, there are seven options to choose from when defining the ACS: AHS 4.75, AHS 5.15, AHS 5.9, AHS 6.7, AHS 7.4, AHS 7.95, and Unused . If AFS (Adaptive Full) is chosen as the AMR Logical Speech Channel, there are nine different options to choose from when defining the ACS: AFS 4.75, AFS 5.15, AFS 5.9, AFS 6.7, AFS 7.4, AFS 7.95, AFS 10.2 , AFS 12.2 , and Unused.

  The units used for the codec options differ between the front panel and GPIB commands. The unit of measurement used on the front panel is kbps and is used in the Table 1: AMR Configurable Parameters and Default Values , whereas the unit of measurement used for the GPIB commands is bps. The same codecs are available via the GPIB commands as on the front panel. For Example, the codecs available to choose from when AHS (Adaptive Half) Logical Speech Channel is selected in the Channel Mode Configuration window must be specified in the GPIB commands in the following form: AHS4750, AHS5150, AHS5900, AHS6700, AHS7400, and AHS7950 . Also, the codecs available when AFS (Adaptive Full) is selected as the Logical Speech Channel in the Channel Mode Configuration menu must be specified in the GPIB commands in the following form: AFS4750, AFS5150, AFS5900, AFS6700, AFS7400, AFS7950, AFS10200 , and AFS12200 .

     

  NOTE

  In order to activate the changes to the Active Codec Set on the front panel Apply Changes must be pressed, otherwise any changes will be discarded if the menu closes.

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  Table 1: AMR Configurable Parameters and Default Values below shows the configurable parameters in the codec set (once an AMR specific Logical Speech Channel, AHS (Adaptive Half) and AFS (Adaptive Full) has been chosen) along with their corresponding GPIB commands.

Table 1: AMR Configurable Parameters and Default Values

• Current Codec

  The current codec is the codec within the Active Codec Set that is in use by the mobile station. The codec that is used on the downlink (or the uplink) can at most change every two speech frames. To inform the receiver of the codec that has been used for a group of two speech frames, a control message is transmitted in-band (with the speech data) every two speech frames, on specific multiframe positions. This is known as the Codec Mode Indication (CMI). The message sent by the test set to the mobile station to change the codec mode used on the uplink is called the Codec Mode Command (CMC). According to the 3GPP 51.010, Section 26.16.2, a mobile station must respond to a CMC within three speech frames. The message sent by the mobile station to request the use of a new codec on the downlink is known as the Codec Mode Request (CMR).

  In a GSM/GPRS/EGPRS system, changes in the codec mode used on the downlink should occur automatically in response to codec mode requests from the mobile station. In the test set, when an Logical Speech Channel is changed to another Logical Speech Channel, the initial codec used after the change is what the current codec was last set to while in that AMR Logical Speech Channel. Otherwise, the default current codec is the codec with the most error correction and protection for the AMR Logical Speech Channel. Codec changes can be verified by capturing a protocol log using the Wireless Protocol Advisor (WPA), see Logging Adaptive Multi-Rate (AMR) ". When the Operating Mode is Active Cell , and the Serving Cell is GSM, GPRS, or EGPRS, there are two additional choices for the current codec. These are Stress and MS Request .

  • Stress

    When the current codec is set to Stress , a Codec Mode Command (CMC) message is sent to force the mobile station to change to a different codec mode every second uplink speech frame. See Stress Testing with Frequent Changes of Codec Mode .

  • MS Request

    When the current codec is set to MS Request , the test set responds to Codec Mode Request (CMR) messages from the mobile station. CMRs are generated in response to when changes in the C/I ratio measured at the mobile station are large enough to exceed the threshold and hysteresis values. See Forcing Codec Mode Requests .

     

  NOTE

  It is only possible to choose Stress or MS Request when the Operating Mode is Active Cell. The Serving Cell can be GSM, GPRS, or EGPRS.

  The following error will occur if the current codec is outside the Active Codec Set: Illegal Parameter value; Invalid AMR initial codec specified.

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• Threshold and Hysteresis Values

  The codec's threshold is the Carrier to Interference (C/I) ratio at which the mobile station will request a codec mode change. Hysteresis settings create a buffer that prevents the codec mode from continually changing when the C/I ratio varies repeatedly above or below the threshold value.

  The figure below shows the how the behavior of codec changes depends on both the threshold and hysteresis values. Notice the codec change will occur at different C/I values depending on whether the change is to a more robust codec or less robust codec. For example, if the test set has commanded the mobile station to use Codec 2 and the C/I ratio increases, Codec 2 will not change to a less robust codec, Codec 3, until the Codec 2 to 3 Threshold plus Codec 2 to 3 Hysteresis point is reached. However, if the test set initially commands the mobile station to use Codec 3 and the C/I ratio is decreasing, it will not command the mobile station to switch to a more robust codec until the Codec 1 to 2 Threshold point is reached.

  

  Since threshold and hysteresis values determine when to switch between codec modes the number of threshold and hysteresis values that need to be chosen is one less than the total number of codecs defined in the Active Codec Set (ACS). Also, as can be seen in the figure above, the threshold value between Codec 1 and Codec 2 must be less than or equal to the threshold value between Codec 2 and Codec 3 and so on. Similarly, the sum of the threshold and hysteresis values between Codec 1 and Codec 2 must be less than or equal to the sum of the threshold and hysteresis value between Codec 2 and Codec 3 and so on. See Table 1: AMR Configurable Parameters and Default Values for the range and default settings for the threshold and hysteresis values.