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8960 GSM/GPRS/EGPRS Online User's Guide
E1968A, E6701F, E6704A
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Bit Error Measurement

Last updated: May 19, 2009

There are three bit error measurements available in the test set:

GSM Bit Error Measurement

Bit Error Measurements versus Fast Bit Error Measurements

There are four commonly used types of bit error measurements in GSM:

  • "BER with Frame Erasure" or "Residual BER" when the mobile station has been configured to loopback A.
  • "BER without Frame Erasure" or "Non-residual BER" when the mobile station has been configured to loopback B.
  • BER using burst-by-burst loopback when the mobile station has been configured to loopback C.
  • "BER with Frame Erasure and Unreliable Frame Rate Signaling" or "Residual (D) BER" when mobile station has been configured to loopback D.

The test set allows you to select between loopback A, B or D for Bit Error Measurement. Loopback C can only be used in the fast bit error measurement. Refer also to Fast Bit Error Measurement Description .

   
NOTE
If the test set has codeware version A.02.00 or above, unnecessary loopback commands and delays can be eliminated by taking advantage of enhancements available.

Previous versions of the test set required you to set the loopback type, and did not have a feature that allowed time for the loop to close.

   

How is a Bit Error (BER) Measurement Made?

During BER measurements, the test set generates a downlink TCH with pseudo-random binary sequence, PRBS-15, data at a known power 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 BER is calculated.

SETup subsystem commands are sent to the test set to specify the time taken to close its loopback path, whether to open or close a loop during downlink signaling operations (for example, channel assignment), the number of bits to test, measurement type, loopback delay, measurement units, trigger arm, and measurement timeout values.

When a call is established on a TCH, the loopback type, corresponding to one of the BER measurement types, must be sent to the mobile station. The test set closes the loopback automatically and re-opens it when the measurement is closed (that is, when INITiate:BERRor is OFF).

You must set the measurement type from one of the available measurement types, (see SETup:BERRor[:TYPE] ). If you query a residual result when a non-residual measurement is initiated, the test set returns 9.91 E+37 (NAN). The Measurement type must be set before initiating a BER measurement. See BER Measurement Types

The loop must be closed before a BER test can start. Using the close loop signalling delay time feature allows time for the loop to close. See SETup:BERRor:CLSDelay[:STIMe] for more details.

Each mobile station may have a different time delay between receiving a frame and re-sending it on the uplink. By default, the test set is configured to automatically determine the amount of delay needed. This 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 loopback delay value.

If necessary, you can manually set the loopback delay (see SETup:BERRor:LDControl:AUTO ).

   
NOTE
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 appears to "hang".
   

The BER measurement trigger source is always set to immediate. The BER measurement does not offer multi-measurement results. See Statistical Measurement Results

BER, FBER, and DAUDIO (uplink speech level) measurements are mutually exclusive measurements. Whichever of these measurements is activated last forces the others to stop.

BER Measurement Types

Type IA, Type IB, and Type II for both residual and non-residual refer to the different class types describing GSM speech coding bits.

Residual:

  • Residual Type IA (50 bits per speech frame)
  • Residual Type IB (132 bits per speech frame)
  • Residual Type II (78 bits per speech frame)
  • Residual Data

When one of these residual measurement types is selected, the test set sends the mobile station a message instructing it to close loopback A. When the mobile station is in loopback A, it erases frames received with CRC (cyclic redundancy check) errors and replaces all the bits in those frames with zeros. The BER measurement is then able to return the frame erasure count or ratio results as well as the results of the bitwise comparison of the un-erased frames looped back from the mobile station.

Non-residual:

  • Type IA (50 bits per speech frame)
  • Type IB (132 bits per speech frame)
  • Type II (78 bits per speech frame)
  • Data

When you select one of these non-residual measurement types, the test set sends the mobile station a message instructing it to close loopback B. A BER measurement with CRC's (cyclic redundancy check) returns the CRC count or ratio results. The mobile station does not indicate if any frames in the downlink were erased.

Residual (D):

  • Residual Type IA (D) (50 bits per speech frame)
  • Residual Type IB (D) (132 bits per speech frame)
  • Residual Type II (D) (78 bits per speech frame)
  • Residual Data (D)

When one of these residual (D) measurement types is selected, the test set sends the mobile station a message instructing it to close loopback D. The test set works almost the same as the correspondent Residual measurement type with one exception: the FER display during the measurement is actually Frame Error Rate plus Unreliable Frames Rate.

BER Measurement Results

The results of a BER 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 manually, select either Count or % from the Measurement Units field. If you are using the test set remotely, these results are available using the FETCh command, see FETCh:BERRor:COUNt[:BITS]? or FETCh:BERRor:RATio[:BITS]? . Alternatively the FETCh:BERRor[:ALL]? or FETCh:BERRor:FULL? can also be used to return the results.

Type A Residual Measurement Results

  • Integrity Indicator
  • Bit Error Ratio
  • Bits Tested
  • Bit Error Count
  • Frame Error Rate
  • Frame Erasure Count

Type B Non-Residual Measurement Results

  • Integrity Indicator
  • Bit Error Ratio
  • Bits Tested
  • Bit Error Count
  • CRC Ratio
  • CRC Count

Type D Residual Measurement Results

  • Integrity Indicator
  • Bit Error Ratio
  • Bits Tested
  • Bit Error Count
  • Frame Error Rate (which is actually Frame Error Rate plus Unreliable Frames Rate)
  • Frame Erasure Count

Using the GSM Bit Error Measurement for Circuit Switched Data (CSD) Bit Error Testing

The GSM bit error measurement is also useful for CSD bit error testing. For more information on GSM CSD, see GSM Circuit Switched Data (CSD) .

You must start a CSD call before initiating the GSM BER measurement.

Measurement Types for CSD Bit Error

Since all the data in the CSD data payload is equally important and convolutionally encoded with same protection, the test set allows you to choose two new measurement types to ensure appropriate results are returned: Residual Data and Data.

In addition, the frame-by-frame Fast Bit Error measurement is also applicable to CSD bit error testing.

Operating Considerations

GSM Bit Error Measurement is available when the operating mode is set to Active Cell, GSM BCH+TCH, GPRS BCH+PDTCH+TCH and EGPRS BCH+TCH+PDTCH.

GPRS Bit Error Measurement

How is a GPRS Bit Error (GBER) Measurement Made?

During GBER measurements, 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. The test set compares data sent to data received, and BER is calculated.

If the test set's operating mode is set to Active Cell, the connection type must be set to ETSI Type B (Unacknowledged) or ETSI Type B (Acknowledged) in order to make BER measurements. This can be done using CALL:FUNCtion:CONNection:TYPE . The measurement does not return any results if the connection type is set to SRB Loopback.

Alternatively, you can make BER measurements when the test set's operating mode is set to GPRS BCH+PDTCH or EGPRS BCH+PDTCH test modes (see CALL:OPERating:MODE ).

SETup subsystem commands are sent to the test set to specify the loopback delay control mode, number of bits to test, block delay, measurements units, trigger arm, measurement timeout values, and bad block handling setting.

The bad block handling parameter determines how bad blocks are interpreted by the GBER measurement. If you want more details on this parameter, see SETup:GBERror:BBLocks .

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:GBERror:DELay? .

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] .

If necessary, you can manually set the delay using SETup:GBERror:LDControl:AUTO .

   
NOTE
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 GBER measurement trigger source is always set to immediate. The GBER measurement does not offer multi-measurement results. If you require more details on multi-measurement results, see Statistical Measurement Results .

Operating Considerations

The GBER measurement aborts and restarts if the Operating Mode, Multislot Configuration, or Coding Scheme is changed while the GBER measurement is running.

Any change to the Multislot Configuration or the Coding Scheme affects the rate at which bits are tested. More uplink timeslots per frame will increase the number of bits tested per second as well as coding scheme used. For example, if the coding scheme CS-4 is selected on the test set, the GBER measurement can test more bits per second than if the coding scheme CS-1 is selected. Therefore, the recommended channel coding scheme for GPRS BER measurements is CS-4. Use the command CALL:(PDTCH|PDTChannel):CSCHeme:UPLink to set the GPRS coding scheme.

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

GSM Bit Error Measurement is available when the operating mode is set to Active Cell, GSM BCH+TCH, GPRS BCH+PDTCH+TCH and EGPRS BCH+TCH+PDTCH.

Types of Signal GBER can Measure

GBER 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 Unacknowledged or ETSI Type B Acknowledged. 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 GPRS BCH+PDTCH or EGPRS BCH+PDTCH test modes (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.

GBER Measurement Results

The results of a GBER measurement can be displayed in two ways; the number of errors counted or the ratio of bad bits (errors) to total bits counted. If you are using the test set manually, both Count and Bit Error Ratio are displayed on the Bit Error measurement screen. If you are using the test set remotely, these results are available using the FETCh command, see FETCh:GBERror:COUNt? or FETCh:GBERror:RATio? . Alternatively the command FETCh:GBERror[:ALL]? can be used to return all of the following results:

  • Integrity Indicator
  • Bit Error Ratio
  • Bits Tested
  • Bit Error Count

In addition, the Intermediate Count of Bits Tested result can be queried using FETCh:GBERror:ICOunt? , the Block Delay can be queried using FETCh:GBERror:DELay? , and the CRC error count can be queried using FETCh:GBERror:CRC? .

EGPRS SRB Bit Error Measurement

How is a EGPRS SRB Bit Error (SBER) Measurement Made?

During the SBER measurement, 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, with is the most appropriate for this measurement. To accurately measure bit error, the measurement must reflect the bit error rate before error correction is applied. For EGPRS, this is provided by implementing the ETSI defined SRB Loopback test mode in the mobile station. In this test mode, the mobile station loops back the received downlink PDTCH after demodulation but before decoding, thus bypassing the error correction stage. 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 BER is calculated.

The SBER measurement is made on the raw data after coding and before decoding. This means not all the bits returned by the mobile station are included in the comparison. Only the encrypted bits which are not "stealing bits" are included in the calculation. Therefore only the following bits are used in the SBER measurement:

  • GMSK Normal Burst - bit numbers 3-59 and 88-144
  • 8PSK Normal Burst - bit numbers 9-182 and 263-434.

Asymmetric SRB Loopback Mode

Asymmetric SRB (ASRB) loopback mode is defined under the circumstance that the MS is unable to support 8PSK modulation capability on the uplink. In this scenario the bits on 8PSK downlink PDTCH burst are looped back over three successive GMSK uplink bursts. The radio block sent on a downlink timeslot using 8PSK modulation should be followed by two radio blocks on the same timeslot where GSM dummy bursts are transmitted, and MS shall not retransmit the last 16 encrypted bits received in an 8PSK radio block when retransmitting it using GMSK modulation.

In order to let MS go to ASRB loopback mode, the corresponding mode flag in the layer 3 message should be set and sent to MS at the beginning of starting a data connection, otherwise MS will not go to ASRB mode by any change. For example, changing downlink coding scheme to 8PSK and uplink to GMSK in transferring will not get the right results.

Operating Considerations

If the test set's operating mode is set to Active Cell, the serving cell must be set to EGPRS and the data connection type must be set to SRB Loopback in order to make SBER measurements. This can be done using CALL:FUNCtion:CONNection:TYPE .

Alternatively, you can make SBER measurements when the test set's operating mode is set to EGPRS BCH+PDTCH test mode (see CALL:OPERating:MODE ).The SBER measurement will work correctly in this mode if the modulation format used on a downlink slot has the same modulation format as the corresponding uplink slot. If the SRB Loopback mode is supported, the SBER measurement will also function when an 8PSK downlink modulation format is looped into a GMSK uplink modulation format.

For ASRB loopback mode BER measurement, the result could either synchronize incorrectly or return high BER when the 8PSK uplink capable MS is used, As there is no phone can support both SRB and ASRB, do not change the PDTCH coding scheme type from SRB mode to ASRB mode, or opposite, during the data connection, otherwise the SBER result could be incorrect

Measurement Setup Parameters

SETup subsystem commands are sent to the test set to specify the loopback delay control mode, number of bits to test, block delay, measurements units, trigger arm, and measurement timeout values. See SETup:SBERror .

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:SBERror:DELay? .

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] .

If necessary, you can manually set the delay using SETup:SBERror:MANual:DELay .

   
NOTE
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 SBER measurement trigger source is always set to immediate. The SBER measurement does not offer multi-measurement results. If you require more details on multi-measurement results, see Statistical Measurement Results .

   
NOTE
When in SRB or ASRB loopback mode BER measurement
  • The downlink and uplink Modulation Coding Schemes are chosen to the same modulation format (GMSK or 8PSK) for normal SRB BER measurement
  • The downlink Modulation Coding Scheme is set to 8PSK and the uplink is set to GMSK for ASRB BER measurement.

   

Types of Signal SBER can Measure

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

  • One uplink and downlink PDTCH pair with the mobile station in active cell mode, serving cell EGPRS, and the data connection type set to SRB Loopback.
  • One uplink and downlink PDTCH pair with the mobile station in EGPRS BCH+PDTCH test mode (see CALL:OPERating:MODE for details on setting the cell operating mode).

       
    NOTE
    When operating the test set in EGPRS BCH+PDTCH operating mode, you must
    • Manually set your mobile station to synchronize its internal frequency and timing reference with the test set, and configure the mobile station for SRB Loopback test mode
         

SER Measurement Results

The results of an SBER measurement can be displayed in two ways; the number of errors counted or the ratio of bad bits (errors) to total bits counted. If you are using the test set manually, both Count and Bit Error Ratio are displayed on the EGPRS SRB Bit Error measurement screen. If you are using the test set remotely, these results are available using the FETCh command, see FETCh:SBERror:COUNt? or FETCh:SBERror:RATio? . Alternatively the command FETCh:SBERror[:ALL]? can be used to return all of the following results:

  • Integrity Indicator
  • Bits Tested
  • Bit Error Ratio
  • Bit Error Count

In addition, the Intermediate Count of Bits Tested result can be queried using FETCh:SBERror:ICOunt? and the Block Delay can be queried using FETCh:SBERror:DELay? .

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