Block Error Measurement

Loopback Measurement Method

The test set generates one or more downlink PDTCHs at a known level with data using the payload pattern you specify. (See Payload Patterns .) The default payload pattern is a pseudo-random binary sequence, PRBS-15, which is the most appropriate for this measurement. The mobile station receives the data, loops it back to its transmitter, and returns the data to the test set in the available uplink PDTCHs. You can specify the first downlink burst to be looped back in the first uplink burst using the command CALL:(PDTCH|PDTChannel):MSLot[:FIRSt]:DOWNlink:LOOPback[:BURSt] . The test set compares data sent to data received. A block that is received which has one or more bit errors within it is marked as having a block error.

Each mobile station may have a different time delay between receiving a block of data and re-sending it on the uplink. By default, the loopback delay control mode is set to automatic, and the amount of delay needed is determined when the test set has, for four consecutive blocks, correctly received 80% of the downlink bits back on the uplink. The test set can be queried for the block delay value using FETCh:BLERror:DELay? .

If necessary, you can manually set the delay. First, change loopback delay control to Manual using the command SETup:BLERror:LDControl:AUTO , and then set the delay using SETup:BLERror:LDControl:AUTO .

The BLER measurement result is calculated irrespective of the Layer 1 reported demodulation status. The BLER measurement result a will vary depending on the setting for the BLER Bad Block Handling setting (see SETup:BLERror:BBLocks for more information on the BLER Bad Block Handling parameter).

• If a corrupted burst error, missing burst error or radio block frame check sequence error is received, the burst is still included in the BLER measurement result and contributes to the Block Error count if the BLER Bad Block Handling parameter is set to Include .
• Only valid radio blocks contribute to the BLER measurment result if the BLER Bad Block Handling parameter is set to Exclude . Radio blocks that fail the CRC (cyclic redundancy check or frame check sequence) are considered invalid an are excluded. See FETCh:BLERror:CRC? for more information on the CRC Error Count parameter.

Polling Measurement Method

The test set periodically polls the mobile station, deriving the BLER measurement result from the Radio Link Control (RLC) layer's Packet ACK/NACK (Acknowledged/Not Acknowledged) message. Polling the mobile station for this message provides an indication of which blocks have been successfully received, and which blocks have been received with an error.

The number of blocks indicated to be in error and the total number of blocks received are counted. The polling interval can be set using CALL:FUNCtion:DATA:BLER:POLLing:INTerval .

ETSI B Acknowledged Measurement Method Mode

For the ETSI B Ack connection type, the BLER measurement can be performed in using either the Loopback or Polling measurement method. You can select the method to be used by the measurement with the following command: SETup:BLERror:ETSIB:MODE .

Operating Considerations

The BLER and the GBER measurements are mutually exclusive, that is which ever of these measurements is activated last forces the other to become inactive.

BLER measurements cannot be made when the data connection type is set to ETSI Type A in Active Cell operating mode, nor in BCH test mode. The test set's operating mode can be set using CALL:OPERating:MODE . The data connection type can be set using CALL:FUNCtion:CONNection:TYPE .

You may want to collect BLER results for different GPRS coding schemes. Use the command CALL:(PDTCH|PDTChannel):CSCHeme:UPLink to change the GPRS coding scheme.

You may want to collect BLER results with different modulation and coding schemes for EGPRS. Use the command CALL:(PDTCH|PDTChannel):MCSCheme to set the EGPRS modulation coding scheme.

If you make a change to any of the following parameters while the BLER measurement is on, it will abort and restart.

• Operating Mode
• Connection Type
• Amplitude Offset
• Multislot Configuration
• Coding Scheme
• Modulation Coding Scheme

The multislot configuration used affects the speed of the BLER measurement. Using multiple uplink timeslots increases the rate at which blocks are tested.

You can send SETup subsystem commands to the test set which specify the number of blocks to test, block delay, trigger arm, and measurement timeout values.

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".

Types of Signal BLER can Measure

BLER measurements can be made on these types of input signals:

• One or more uplink and downlink PDTCH pairs with the mobile station in active cell mode and the data connection type set to ETSI Type B, ETSI Type B Acknowledged, or BLER. If you want to optimize measurement speed, it is important that the mobile station is set up for the same number of uplink and downlink PDTCHs.
• One or more uplink and downlink PDTCH pairs with the mobile station in either GPRS BCH+PDTCH test mode or EGPRS BCH+PDTCH test mode (see CALL:OPERating:MODE for details on setting the cell operating mode). Note that you have to manually set your mobile station to synchronize its internal frequency and timing reference with the test set. You also have to command the mobile station to loop back the PDTCH data.

BLER Measurement Results by Timeslot

When using the Polling measurement method, you can also retrieve the following measurement results for individual timeslots using FETCh:BLERror:BURSt<[1]|2|3|4|5>[:ALL]? .

• Integrity Indicator for the burst
• Blocks Tested
• Block Error Ratio
• Block Error Count