How is an IFER Measurement Made?
The test set requires that four codecs be defined in the Active Codec Set (ACS) before an IFER measurement can initiate. If the IFER measurement is initiated but the ACS does not contain four codecs, the measurement does not start and the following integrity indicator is displayed on the front panel:
Unsupported Configuration
. The codec that is used on the downlink (or the uplink) can change at a minimum of 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 used on the uplink is called the Codec Mode Command (CMC). The message sent by the mobile station to request the use of a new codec on the downlink is called the Codec Mode Request (CMR).
The test set requires that the mobile station supports loopback type I in order for the downlink CMI and CMC bits to be returned on the uplink. If the mobile is not capable of performing a loopback type I, synchronization between the uplink and the downlink will not occur and meaningful results will not be generated. When loopback type I is activated, no changes to the RATSCCH parameters are permitted. The RATSCCH parameters include Current Codec, Active Codec Set, Threshold and Hysteresis values. The test set allows the mobile station enough time to close loopback type I with the speech frame loopback delay control parameter. If the speech frame loopback delay parameter is set to AUTO, and the Operating Mode is not Active Cell, the test set tries to determine the speech frame delay every time the measurement begins. If the Operating Mode is in Active Cell, the test set only determines the speech frame delay the first time the measurement is made, unless the call drops or the speech frame delay control changes. If the speech frame loopback delay control is set to Manual, the test set uses the specified speech frame delay.
The IFER measurement measures the reference sensitivity of an AMR logical speech channel by forcing the CMI and CMC bits to cycle through the codec selection pattern 1,2,3,4,4,3,2,1 at a rate specified in the test set. When the IFER measurement is activated, it overrides the settings for the current codec. When IFER is deactivated, the setting for the current codec is set on the downlink back to what is was before the IFER measurement was activated. Changes to the current codec, codec set and/or thresholds that are made during the IFER measurement are ignored until the next call is initiated.
The 3GPP TS 51.010 recommends a minimum of 150000 speech frames to be tested to have accurate measurement results. This measurement can take over 50 minutes to complete.
The results of an AMR IFER measurement can be displayed in two ways; the number of errors counted or the ratio of bad bits to the total number of bits. If you are using the test set's front panel, select either
%
or
Count
from the
Measurement Units
field. If you are using the test set via the GPIB commands, the results are available using the fetch command, see
FETCh:IFERror:RATio?
or
FETCh:IFERror:COUNt?
. Alternatively, the command
FETCh:IFERror[:ALL]?
can also be used to obtain these results.
Impact on other Measurement
Only one of the following measurements can be made at a time: GSM Bit Error, GSM Fast Bit Error or AMR In-Band Frame Error. If one of these measurements is initiated while another of the remaining two are running, the measurement that is initiated last continues to run and the other is closed.
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