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Loopback Bit Error Ratio (BER) Measurement Description
Last updated: January 16, 2009
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).
3GPP TS 34.121 Figure F.6.1.9
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
).
UE Control During FDD Test Mode Operation
The
FDD Test Operating Mode
does not provide radio bearer setup control of the UE for test mode operation. You must configure the UE in loopback mode 1 or 2 and synchronize it to the test set's downlink signal in order to make this measurement in FDD test operating mode.
The uplink transport format must include both the DTCH and the DCCH; however, only the DTCH information is used for the loopback bit error ratio measurement.
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
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