User`s guide

196 Chapter 6
BERT Testing
RF Loopback BER–Option 300
PN synchronization is also a prerequisite to BER measurements. This is automatic but is
dependent on prior BCH or TCH synchronization.
The pattern to be recognized (PN9 or PN15) is determined by the corresponding transmit
timeslot pattern selection.
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 BER 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 6-1 shows the minimum requirements for the GSM received data frame structure