Loopback Bit Error Ratio (BER) Measurement Description

How Is a Loopback Bit Error Ratio Measurement Made?

3GPP TS 34.121, F.6.1.1 defines bit error ratio (BER) as follows: "The Bit Error Ratio is defined as the ratio of the bits wrongly received to all data bits sent. The bits are the information bits above the convolutional/turbo decoder."

When the test set measures loopback bit error ratio, it sends a known data pattern (see DL DTCH Data ) on the downlink dedicated traffic channel (DTCH) to a UE that is configured in loopback mode 1 or 2. (Although the measurement will operate with UE Loopback Type set to 2 , all TS 34.121 tests utilizing the BER measurement require loopback mode 1). The UE decodes the data and re-transmits it on the uplink DTCH. The test set analyzes the uplink data to see how closely it matches the data bits originally sent on the downlink. The test set compares the downlink and uplink data one transport block at a time and reports the Bit Error Ratio .

To decrease test time, you may set Confidence State to On and specify the BER Requirement , which allows the test set to stop testing when an early pass or early fail event occurs, and to report a Confidence Test Result (see 3GPP TS 34.121 F.6 and 3GPP TS 34.901).

Because the test set must compare the UE's uplink data with the corresponding downlink data, the measurement must be able to synchronize to the UE's signal (align received uplink transport blocks with their corresponding downlink transport blocks. In doing so, the test set determines the UE's Loopback Delay ). See Input Signal Requirements .

When testing the UE's receiver performance, it is assumed that the uplink is error free. Any errors occurring on the uplink invalidate the receiver test. The loopback BER measurement returns information about uplink CRC Errors to assist in ensuring that the uplink is error free (to perform the loopback BER measurement, you must set Uplink DTCH RMC CRC Presence to Present so that an uplink CRC is generated).

To perform the loopback BER measurement while in Active Cell Operating Mode , you must be on an RB test mode call, with Channel Type set to a symmetrical RMC. Loopback BER can not be measured for Asymmetrical RMCs .

The trigger source for this measurement is always protocol-based (see Trigger Source Description ).

Loopback BER Measurement Parameters

• Number of Bits to Test

  When Confidence State is Off , you may set the number of data bits to test during the measurement using the Number of Bits to Test setting. However, the actual number of bits tested is always the largest multiple of the number of bits needed to fill a DTCH transport block for the transmitted format. For example, for the 12.2 kbps reference measurement channel (RMC) the transport block is 244 bits. If you set the number of bits to test to 1000, the actual number of bits tested will be 1220 (1000 bits/244 bits per block = 4.1 blocks, so 5 blocks are required to test at least 1000 bits. 5 blocks x 244 bits per block = 1220 bits).

  When Confidence State is On , you may set the maximum number of bits to test during the measurement using the Number of Bits to Test setting. However, the actual number of bits tested will be less than this number if an early pass or early fail event occurs (see Confidence Test Result ). If a Confidence Test Result of Max Bits Tested is returned, then the actual number of bits tested is the largest multiple of the number of bits needed to fill a DTCH transport block for the transmitted format, as explained above.

  GPIB command: SETup:WBERror:COUNt

• Trigger Arm - see Trigger Arm (Single or Continuous) Description .

• Measurement Timeout - see Measurement Timeouts .

• Confidence State

  To decrease test time, you may set Confidence State to On and specify BER Requirement , which allows the test set to stop testing when an early pass or early fail event occurs, and to report a Confidence Test Result (see 3GPP TS 34.121 F.6 and 3GPP TS 34.901).

  From the front panel, Confidence State can be set by pressing the ON or OFF key while Confidence Level is selected.

  GPIB commands: SETup:WBERror:CONFidence[:SLEVel] , SETup:WBERror:CONFidence:STATe

• Confidence Level

  3GPP TS 34.121 F.6 defines:
  F = probability of a wrong decision
  (1 - F) = probability of a correct decision
  The probability (risk) to fail a good DUT shall be </= F according to the following definition: A DUT is failed, accepting a probability of </= F that the DUT is still better than the specified error ratio (Test requirement (see BER Requirement )).
  The probability to pass a bad DUT shall be </= F according to the following definition: A DUT is passed, accepting a probability of </= F that the DUT is still worse than M times the specified error ratio (see BER Requirement ). (M>1 is the Bad DUT Factor ).

  The Confidence Level parameter is equivalent to (1 - F). 3GPP TS 34.121 table F.6.1.8 specifies an F of 0.2% ( Confidence Level of 99.8% ) for all Receiver and Performance Characteristics tests in 3GPP TS 34.121 sections 6 and 7 (except for the fail limit for 6.5 Blocking Characteristics).

  Confidence Level is thus fixed to 99.8%. Similarly, Bad DUT Factor is fixed to 1.5.

  GPIB commands: SETup:WBERror:CONFidence[:SLEVel] , SETup:WBERror:CONFidence:LEVel

• BER Requirement

  3GPP TS 34.121 F.6 defines TR = Test Requirement. As testing progresses, the bit error ratio trajectory converges to a value (see 3GPP TS 34.121 Figure F.6.1.9 ). The BER value that the UE is converging to shall not exceed the Test Requirement. The BER Requirement parameter is equivalent to Test Requirement expressed as a percentage. 3GPP TS 34.121 table F.6.1.8 specifies a Test Requirement of between 0.001 and 0.1 (0.1% and 10% BER Requirement ) for the Receiver and Performance Characteristics tests in 3GPP TS 34.121 sections 6 and 7.

  This setting is only available when Confidence State is On .

  GPIB command: SETup:WBERror[:RATio]:REQuirement

• Bad DUT Factor

  3GPP TS 34.121 F.6 defines M = bad DUT factor. A Bad DUT Factor of greater than 1 shifts the early pass limit curve up (the early pass limit curve is defined by 3GPP TS 34.121 F.6.1.5 equation (2). See also 3GPP TS 34.121 Figure F.6.1.9 .) This allows an early pass event to occur after fewer bits have been tested for a given number of errors, which results in a higher bit error ratio (see the early pass equation for NL( ne ) in 3GPP TS 34.121 F.6.1.9). As M is increased, a greater number of bad DUTs are passed.

  3GPP TS 34.121 table F.6.1.8 specifies a Bad DUT Factor of 1.5 for all Receiver and Performance Characteristics tests in 3GPP TS 34.121 sections 6 and 7. Thus, Bad DUT Factor is fixed to 1.5.

  This setting is only available when Confidence State is On .

  GPIB command: SETup:WBERror:CONFidence:DUTFactor

• Uplink DTCH RMC CRC Presence
• UE Loopback Type

Loopback BER Measurement Results

• Integrity Indicator
• Bit Error Ratio

  The ratio of incorrectly received bits to the Bits Tested , expressed as a percentage (%).

  GPIB command: FETCh:WBERror<[:ALL]|:INTegrity|:RATio|:COUNt|:BITS>?

• Confidence Test Result

  The Confidence Test Result is only available when Confidence State is On .

  • Pass : Few enough bit errors were detected for a large enough number of Bits Tested that the test set is able to predict (with the specified Confidence Level ) that the bit error ratio trajectory is converging to a value below the BER Requirement . An early pass event is determined according to 3GPP TS 34.121 5.6.1.5 equation (2).
  • Fail : Enough bit errors were detected for the number of Bits Tested that the test set is able to predict (with the specified Confidence Level ) that the bit error ratio trajectory is converging to a value above the BER Requirement . An early fail event is determined according to 3GPP TS 34.121 5.6.1.5 equation (1).
  • Max Bits Tested : After testing the number of bits specified by Number of Bits to Test , the test set is unable to predict (with the specified Confidence Level ) whether the bit error ratio trajectory is converging to a value above or below the BER Requirement .

  GPIB command: FETCh:WBERror:RATio:FAIL?

• Bit Error Count

  The number of incorrectly received bits.

  GPIB command: FETCh:WBERror<[:ALL]|:INTegrity|:RATio|:COUNt|:BITS>?

• Bits Tested

  Number of bits tested to determine the Bit Error Count and Bit Error Ratio results.

  GPIB command: FETCh:WBERror<[:ALL]|:INTegrity|:RATio|:COUNt|:BITS>?

• Missing Blocks

  The UE sends a TFCI for each uplink DTCH transport block. When operating in Active Cell Operating Mode , the test set monitors the uplink TFCI, and can report uplink missing blocks. If the TFCI indicates that there is no data block sent for that TTI, the test set reports a missing block. When a missing block occurs, Bit Error Count and the number of Bits Tested are not incremented. The test set returns a Questionable MS-to-Cell Data Integrity Indicator .

  When in FDD Test Operating Mode , the test set does not monitor the uplink TFCI; rather, it assumes that a data block is sent in every TTI. However, when a missing block occurs, the test set detects a CRC error. Thus, the data within the block is not analyzed, and the number of Bits Tested is not incremented.

  GPIB command: FETCh:WBERror:MBLocks?

• CRC Errors

  To determine if an uplink CRC error has occurred, the test set calculates the CRC for the data sent in the uplink DTCH transport block, and compares the result to the CRC sent by the UE for that transport block. When a CRC error occurs, the data within the block is not analyzed, and the number of Bits Tested is not incremented. If a CRC error occurs, the test set returns a Questionable MS-to-Cell Data Integrity Indicator .

  GPIB command: FETCh:WBERror:CRCerrors?

• Loopback Delay

  3GPP TS 25.211 section 7.6.3 specifies that the uplink frame transmission take place 1024 chips after reception of the corresponding downlink frame. In reality, there is often a larger delay before the UE is able to transmit the uplink frame. 3GPP TS 34.109, 5.3.2.9 specifies this delay between received downlink radio frames and their corresponding uplink radio frames (produced from the received data) as the loopback delay. Loopback delay is measured at the UE antenna connector and is specified in units of radio frames. While the UE test loop is closed and the radio bearer configuration is not changed, the UE is required to maintain a fixed loopback delay, which must not exceed the number of radio frames corresponding to 10 times the TTI (Transmission Timing Interval) of the actual transport channel configuration. For example, if TTI is 10 ms, the UE is required to maintain a fixed loopback delay not to exceed 10 radio frames (the test set can actually tolerate a loopback delay of up to 50 TTI).

  To calculate loopback delay, the test set correlates the data in the uplink radio frame to the data sent in the downlink radio frame. Thus, the data sent by the test set must be different for each radio frame ( DL DTCH Data must not be set to All Zeros or All Ones ).

  GPIB command: FETCh:WBERror:LDELay?

Input Signal Requirements

• The expected power into the test set must be set in order to maintain the link with the UE. This is done using the UE Target Power setting or by using manual receiver power control and specifying the expected power level. See Receiver Control .

• Frequency: 800 to 1000 MHz, 1700 to 1990 MHz
• Input level: -61 dBm to +28 dBm
• The test set can synchronize to the UE's signal only if the bit error ratio is less than 10% on a block-by-block basis for three consecutive blocks, and the loopback delay is less than 50 transmission time intervals of the DTCH. See Synchronization requirements .

Related Topics

Manual Operation: How Do I Make a Loopback Bit Error Ratio (BER) Measurement?

Programming a Loopback Bit Error Ratio Measurement

Loopback Bit Error Ratio Troubleshooting

Radio Bearer Test Mode and Channel Type

Block Error Ratio (BLER) Measurement Description

What 3GPP W-CDMA/HSPA Conformance Tests Are Supported?