The test set allows you to select between Loopback Type A or B, and the Fast Bit Error Measurement, which uses Loopback Type C. Refer also to Bit Error Measurement .
During FBER measurements, the test set generates a downlink TCH with (Pseudo Random Binary Sequence) PRBS-15 data at a known low level. The mobile station receives the data, loops it back to its transmitter, and returns the data to the test set. The test set compares data sent to data received, and FBER is calculated. see CALL:TCHannel
SETup subsystem commands are sent to the test set to specify close loop delay, signal loopback control, the number of bits to test, TDMA frames delay, measurement unit, trigger arm, and measurement timeout values.
When a call is established on the TCH, the loopback type is sent to the mobile station if the signal loopback control is on, see SETup:FBERror:SLControl[:STATe] . If you set signal loopback control to off, the loopback type is controlled using CALL:TCHannel:LOOPback .
FBER measurements use mobile station burst-by-burst loopback, referred to as loopback type C. In loopback type C the comparison is made between the 114 bits of data sent from the test set to the mobile station, then looped back and received by the test set.
The loop must be closed before a FBER test can start, using the close loop signaling delay time feature allows time for the loop to close. See SETup:FBERror:CLSDelay[:STIMe] for more details.
Each mobile station may have a different delay between receiving a TDMA frame and re-sending it on the uplink. By default, the test set is configured to LDControl:AUTO:ON, and the amount of delay needed is determined automatically when the test set has, for two frames, correctly received 80% of the downlink bits back on the uplink. The test set can be queried for the TDMA frames delay value.
In case the test set is not able to correlate the data it transmits on the downlink with the data it receives on the uplink, a Measurement Timeout value should be set. If a timeout is not set and the test set is unable to correlate, the measurement will appear to "hang".
The FBER, BERR and the DAUDIO (uplink speech level) measurements are mutually exclusive, that is which ever of these measurements is activated last forces the other to become inactive. see Decoded Audio Measurement Description
FBER measurement trigger source is always set to immediate. The FBER measurement does not offer multi-measurement results. see Statistical Measurement Results
The results of a FBER measurement can be displayed in two ways, (number of errors counted) or (the ratio bad bits (errors) to total bits counted). If you are using the test set remotely these results are available by using the FETCh command, see FETCh:FBERror:COUNt? or FETCh:FBERror:RATio? . If you are using the test set manually select either Count or % from the Measurement Units field.