2.1 BLOCK ERROR RATE (BLER) MEASUREMENTS
2.1.2 DOWNLINK BLOCK ERROR RATE (DL BLER)
Compared to uplink block error rate DL BLER does not need to be computed by any performance measurement equipment based on transport block counters. Neither it is meaningful to compute it on lub, because transmission errors will appear on Uu – after data has been sent to the cell via Iub in downlink.
The job to compute and report DL BLER is assigned to UE using the RRC measurement control message (message example 2.2).
The measurement can be reported periodically or if a predefined threshold of CRC errors on downlink transport channels is exceeded. For a threshold event triggered reporting using event 5A will be monitored. From the performance monitoring point of view the disadvan- tage of event-triggered reporting is that one can no longer follow a certain measurement over a long period of time. Instead only single peak values (exceptions) are reported while all normal values are hidden to lower load on SRNC caused by measurement tasks. In the case of DL BLER measurements presented in Figure 2.2 only the extreme values shown on the right side of the histogram have a chance of being reported.
Message example 2.2RRC measurement control to set up periodical DL BLER reporting
| ID Name | Comment or Value |
| TS 25.331 DCCH-DL - V5.9.0 (RRC_DCCH_DL) measurementControl (ẳmeasurementControl) |
| dL-DCCH-Message |
| 2 message |
| 2.1 measurementControl |
| 2.1.1 r3 |
| 2.1.1.1 measurementControl-r3 |
| 2.1.1.1.1 rrc-TransactionIdentifier |0 |
| 2.1.1.1.2 measurementIdentity |16 |
| 2.1.1.1.3 measurementCommand |
|2.1.1.1.3.1 setup |
| 2.1.1.1.3.1.1 qualityMeasurement |
|2.1.1.1.3.1.1.1 qualityReportingQuantity |
|2.1.1.1.3.1.1.1.1 dl-TransChBLER |true |
| 2.1.1.1.3.1.1.2.1.1 reportingAmount |ra-Infinity |
| 2.1.1.1.3.1.1.2.1.2 reportingInterval |ril1 |
| 2.1.1.1.4 measurementReportingMode |
| 2.1.1.1.4.1 measurementReportTransferMode |acknowledgedModeRLC |
| 2.1.1.1.4.2periodicalOrEventTrigger |periodical |
Figure 2.2 Distribution of DL BLER measurement reports monitored during a single call
DL BLER values for each transport channel of a connection are reported by the UE using the RRC measurement report message as shown in message example 2.3.
In this example the UE computed the transport channel BLER for downlink transport channels. DL BLER is only measured on those channels for which a CRC procedure has been defined during channel establishment. Whether a CRC for a certain transport channel applies can be ascertained from transport format settings found in the NBAP Radio Link Setup Request or NBAP Radio Link Reconfiguration Preparation Request message.
Report values are sent in bin format. Conversion from percentage ratio into bins follows the mapping table found in 3GPP 25.133 (see Table 2.1).
There is no unit in this measurement, but it could be expressed in %. Bin value 62 (BLER_LOG_62), for example, represents a BLER in the range from 74% to 86%. If the Message example 2.3RRC Measurement report containing DL BLER report for a speech call
| ID Name | Comment or Value |
| TS 25.331 DCCH-UL - V5.9.0 (RRC_DCCH_UL) measurementReport (ẳmeasurementReport) |
| uL-DCCH-Message |
| 2 message |
| 2.1 measurementReport |
| 2.1.1 measurementIdentity | 16 |
| 2.1.2 measuredResults |
| 2.1.2.1 qualityMeasuredResults |
| 2.1.2.1.1 blerMeasurementResultsList |
| 2.1.2.1.1.1 bLER-MeasurementResults |
| 2.1.2.1.1.1.1transportChannelIdentity |32 |
| 2.1.2.1.1.1.2dl-TransportChannelBLER |0 |
| 2.1.2.1.1.2 bLER-MeasurementResults |
| 2.1.2.1.1.2.1transportChannelIdentity |1 |
| 2.1.2.1.1.2.2dl-TransportChannelBLER |0 |
| 2.1.2.1.1.3 bLER-MeasurementResults |
| 2.1.2.1.1.3.1transportChannelIdentity |2 |
| 2.1.2.1.1.4 bLER-MeasurementResults |
| 2.1.2.1.1.4.1transportChannelIdentity |3 |
| 2.1.2.1.2 modeSpecificInfo |
| 2.1.2.1.2.1 fdd | 0 |
Table 2.1 Bin mapping table for DL transport channel BLER reports Reported value Measured quantity value
BLER_LOG _00 Transport channel BLERẳ0
BLER_LOG _01 1<Log10(transport channel BLER)<4.03 BLER_LOG _02 4.03Log10(transport channel BLER)<3.965 BLER_LOG _03 3.965Log10(transport channel BLER)<3.9
. . . . . .
BLER_LOG _61 0.195Log10(transport channel BLER)<0.13 BLER_LOG _62 0.13Log10(transport channel BLER)<0.065 BLER_LOG _63 0.065Log10(transport channel BLER)0
reported bin value is displayed as a percentage value, it is necessary to specify which value to show when the UE sends reports with DL BLERẳ62. Will the lowest assumed value of 74%, the highest possible value of 86% or the value in the middle of the range (80%) be displayed and used for further calculations? Whatever is decided will have a significant impact on calculated average values and graphical presentation.
Note: it is not possible to derive exact DL BLER measurement results in % from values found in the RRC measurement report.
2.1.2.1 DL BLER per Call or Service
It is not easy to compute DL BLER for a certain call or call type. The most important reason is the bin format used in the RRC measurement report. In the case of a voice call there are four or two (if no CRC is activated for transport channels that carry AMR B and C bits) different bins reported by the UE to the SRNC, which makes it difficult to build an average on bin values. This is explained in the following example.
Imagine a conversion function for ratio/bins based on a logarithmic function as described in the imaginary mapping table shown in Table 2.2.
There are two ratios measured and converted into bins:
Measure #1ẳ102)bin 20 Measure #2ẳ103)bin 30 The correct averageof these measurement results is:
ð102ỵ103ị
2 ẳ0:0055ẳ102:25936 )bin 22
This calculation proves that to calculate the average of the bin values in the case of the above measurement leads to awrong result, because:
ð20ỵ30ị
2 ẳ25
And the average measurement result is not represented by bin 25!
It must also be known which bearer service applies to which transport channel. For a voice call there are four transport channels: one for the signalling radio bearer (RRC signalling) and three for AMR speech. While monitoring the speech service BLER it must be known on which channel AMR A-class bits are transmitted. The problem is that especially in the case
Table 2.2 Imaginary bin mapping table to demonstrate calculation of bin average
Ratio value Bin value
102to 102.1 20
102.1to 102.2 21
. . . . . .
103to 103.1 30
of multi-RAB calls it cannot be distinguished from transport channel ID on which channel A-class bits are transmitted, because CS and PS services of other calls may use transport channels with the same ID. Hence, the individual transport format settings for each transport channel have to be analysed based on NBAP signalling information and stored as call-specific context as long as the call is active. Looking at this context is could be evaluated by perform- ance measurement software at any time on which transport channel specific speech information (e.g. AMR A-class bits) or IP data is transmitted. Although it would not be impossible to implement this in performance measurement software the problem is that in a live network environment with hundreds or thousands of calls to be analysed simultaneously, there is a lot of information that the software needs ‘to keep in mind’, which leads to a significant impact on system performance. For this reason this feature might not be found in existing applications.