300 Chapter 9
BERT
RF Loopback BER–Option 300
Erased Frame Detection
When an incorrect CRC in an uplink speech frame is detected by a BTS conforming to the GSM standards,
the BTS substitutes an all-zero speech frame.
In the loopback mode, the BTS transmitter then recodes this substituted zero speech frame before
transmission back to the test equipment receiver (VSA/ESG).
The ESG receiver detects any coded all-zeros speech frames in the returned downlink signal and increments
the (erased) frame event count.
Downlink Errors
The loopback method of measuring BTS receiver BERT quality requires a high quality downlink return path
which introduces no errors.
To cater for the situation where the downlink path is faulty, the VSA/ESG provides a measure of downlink
quality based on TCH payload errors detected by convolutional decoding of the return path. This records
errors introduced into the TCH between the downlink coding (BTS transmitter) and decoding (VSA/ESG)
processes and should normally be zero. For any payload errors detected during a measurement, the
measurement is extended by a speech frame for every downlink error detected.
If a downlink problem results in errors introduced into the downlink midamble, the measurement aborts,
providing further security against downlink problems.
Frame Structure
GSM Frame Structure
26-frame TCH multiframe structure.
Frames: Frame 12 (SACCH) and frame 25 (idle) empty.
Identical repetitive frames. Content as per ESG GSM capability.
GSM Received Data
The Table 9-1 shows the minimum requirements for the GSM received data frame structure during
synchronization to the BTS and during measurements.
Tabl e 9-1 G SM R eceiv ed Dat a
During BCH Synchronization
TS0 51-frame BCH Multiframe Structure
SCH in frames 1, 11, 21, 31, 41
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