2.1 BLOCK ERROR RATE (BLER) MEASUREMENTS
2.1.1 UPLINK BLOCK ERROR RATE (UL BLER)
There is no measurement report in UTRAN that contains uplink BLER values although UL BLER is an important criterion for the radio network controller (RNC) to make handover decisions based on uplink transmission quality. Due to the fact that UL BLER is only computed and used in the RNC internally it also is only available inside RNC software and as a rule is not shown in any performance measurement statistics. Therefore, it is a typical example for performance measurement based on protocol analysis. The UL BLER is especially a very critical parameter to measure user perceived quality of services using RLC transparent mode. While AMR voice calls can compensate an UL BLER of up to 1%
using AMR-specific error concealment algorithms the quality of CS videotelephony will be heavily impacted, because each block error will directly result in pixel errors in video and/or background noise in audio information. Hence, for these services UL BLER gives a correct impression of the user’s perceived quality of service.
2.1.1.1 Uplink Transport Channel BLER
Usually UL BLER as well as DL BLER are computed per transport channel. This means that, e.g. for a voice call, four transport channels are used simultaneously: one dedicated channel (DCH) for the dedicated control channel (DCCH) (radio resource control (RRC) signalling) and three DCHs for voice packets (one DCH for each adaptive multi rate (AMR) A, B and C bits). This specific filtering is not mentioned in the general formula, but in 3GPP 25.215: ‘The BLER estimation shall be based on evaluating the CRC of each transport block associated with the measured transport channel. . .’.
3GPP standards only describe BLER measurement on the downlink, but it must be assumed that the same rules apply for UL BLER computing, which is performed by the RNC internally and also computed by the performance measurement software. Therefore, for a voice call there will not be only one UL BLER, but four different UL BLER measurements.
However, as explained in the next paragraphs UL BLER may appear in many different versions, but it may also be noted from above that the quote from 3GPP 25.215 is incomplete. The remainder of the quoted sentence is as follows: ‘. . .after RL combination’.
This points to another important requirement: it is necessary to take macro-diversity combining into account if the call is in a soft handover situation. A typical measurement scenario may then look as shown in Figure 2.1.
The user equipment (UE) sends UL data on three different radio links. Since each radio link is provided by a cell belonging to a different Node B, the UE is in soft handover. The same transport blocks may be sent on three different radio links, and because of the three different Node Bs involved in this scenario, also on three different lub physical transport bearers (AAL2 SVCs). Due to high synchronisation in UTRAN all uplink transport blocks will arrive nearly simultaneously on the serving RNC (SRNC). The usual time difference between identical blocks is approximately 1 ms. Due to the fact that in a voice call transport channels carrying AMR speech information belong together, they are set up as coordinated dedicated transport channels (DCHs). Hence, transport blocks from all three AMR DCHs are found in the same UL frame protocol (FP) data frame. If the cells belong to three different Node Bs that are involved in soft handover scenario as shown in Figure 2.1, the same FP data frame is sent on each Iub interface that is involved. Following this three identical FP frames, as presented in message example 2.1, are received by SRNC.
The Iub FP data frames in uplink contain some radio-related measurement results known as the quality estimate (QE). This represents the estimated bit error rate (BER) measured by Node B on the uplink radio link of a single cell. Since three cells are involved on each Iub the reported QE may be different. Following the rule explained in Chapter 1 of this book that only the UL FP data frame with the best QE is accepted by SRNC, all other frames are discarded. Although this rule is introduced to eliminate as many blok errors as possible. One erroneous transport block (#1) in the example passes the SRNC to be forwarded via IuCS and possibly core network interfaces to the B-party of the call.
Note 1: if UE is located in cell border areas CRC errors occur more often than usual.
However, in cell border areas soft handover procedures are triggered, too. To measure UL BLER correctly in such situations it is first necessary to see the first uplink data frame transmitted on the new Iub interface as a trigger for the start of macro-diversity combining of uplink frames. It is also useful to correlate BLER measurements with the active set size of calls to find out which specific impact softer and soft handover situations have on radio transmission quality.
Note 2: some network equipment manufacturers do not use QE for the macro-diversity combining algorithm. Instead they select the best frame using another uplink radio quality para- meter associated with the receiving radio link, e.g. uplink signal-to-interference ratio (SIR).
The size and type of each transport block is indicated by the transport format index, a value that corresponds with the settings of the transport format set as seen in Node B application part (NBAP) radio link setup or radio link reconfiguration preparation proce- dures. If the transport block is erroneous or not is indicated by a so-called CRC indicator associated with each transport block. This CRC indicator is a parameter in an FP trailer, an appendix transmitted on Iub in the uplink direction only. It indicates the result of a CRC after
Figure 2.1 UL transport blocks for UL BLER calculation
transmission of the transport block on the radio interface. However, not every transport channel is protected by CRC. If CRC is activated or not and how many bits are used in the check sequence is indicated in NBAP Radio Link Setup or Radio Link Reconfiguration Request messages. In downlink there is no BLER measurement on channels that are not CRC protected, but for uplink data frames, the FP entity of Node B considers blocks as Message example 2.1UL FP data frame including transport blocks and CRC indicators
| TS 25.322 (RLC) / 25.321 (MAC) / 25.435, 25.427 (FP) - V3.13.0 (RLC/MAC) FP DATA DCH (ẳFP
Data Frame DCH) |
| FP Data Frame DCH |
| FP: VPI/VCI/CID | "188/65/234" |
|
| FP: Radio Mode | FDD (Frequency Division Duplex) |
| FP: Direction | Uplink |
| FP: Transport Channel Type | DCH (Dedicated Channel) |
| 1 FP: Header |
| 1.1 FP: Header CRC | ‘35’H |
| 1.2 FP: Frame Type | Data |
| 1.3 FP: Connection Frame Number | 220 |
| 1.4 FP: Spare | 0 |
| 1.5 FP: Transport Format Index | 2 |
| 1.6 FP: Spare | 0 |
| 1.7 FP: Transport Format Index | 1 |
| 1.8 FP: Spare | 0 |
| 1.9 FP: Transport Format Index | 1 |
| 2 Transport Block Set DCH |
| 2.1 FP: DCH Index | 0 |
| 2.2 FP: Transport Block |
| 2.2.1 MAC: Target Channel Type | DTCH (Dedicated Traffic Channel) |
| 2.2.2 MAC: RLC Mode | Transparent Mode |
| 2.2.3 RLC: Whole Data | ‘101000110101000100101101100111001’B|
| 3 Transport Block Set DCH |
| 3.1 FP: DCH Index | 1 |
| 3.2 FP: Transport Block |
| 3.2.1 MAC: Target Channel Type | DTCH (Dedicated Traffic Channel) |
| 3.2.2 MAC: RLC Mode | Transparent Mode |
| 3.2.3 RLC: Whole Data | ‘010100100000111011100111010101110’B |
| 4 Transport Block Set DCH |
| 4.1 FP: DCH Index | 2 |
| 4.2 FP: Transport Block |
| 4.2.1 MAC: Target Channel Type | DTCH (Dedicated Traffic Channel) |
| 4.2.2 MAC: RLC Mode | Transparent Mode |
| 4.2.3 RLC: Whole Data | ‘010001001101000000110010100110001’B |
| 5 FP: Trailer |
| 5.1 FP: Quality Estimate | 132 |
| 5.2 FP: CRC Indicator (Transport Block) | Not Correct |
| 5.3 FP: CRC Indicator (Transport Block) | Correct |
| 5.4 FP: CRC Indicator (Transport Block) | Correct |
| 5.5 FP: Padding | 00 |
transmitted correctly if no CRC is executed. If the target is to measure UL BLER with highest precision, the BLER measurement application must check if CRC is defined for each transport channel set up by NBAP. Otherwise CRC indicator values reported for this transport channel in frame protocol will be ignored for UL BLER measurement.
Note: UL transport channel BLER is usually not measured on the random access channel (RACH) although it is theoretically possible. This is to be aligned with 3GPP 25.215 that says: ‘The measurement Transport channel BLER does not apply to transport channels mapped on a P-CCPCH and a S-CCPCH’. Transport channels mapped on mentioned common physical channels are the broadcast channel (BCH) and forward access channels (FACHs). If BLER is not measured on FACHs (downlink) a corresponding measurement on RACHs (uplink) does not seem to make much sense.
2.1.1.2 UL BLER per Call
UL BLER can also be used to estimate the uplink transmission quality of a call. In this case it is not necessary to differentiate between transport channels that carry user plane and control plane information. Merely count transport blocks and their CRC indicators according to the standard formula given at the beginning of this chapter, no matter to which transport channel single transport blocks belong.
2.1.1.3 UL BLER per Call Type
Another possibility is to map UL BLER to the type of call: voice, packet switched (PS) data, video-telephony, multi-RAB, signalling. Depending on differentiation based on higher layer information more types of call may be defined according to end-user services. However, definitions should be considered carefully. Transmitting a short message in uplink direction only requires a few transport blocks. In addition it must be considered that quality of end- user services like speech can only be measured on user plane transport channels. Especially for voice calls it is further necessary to define filter functions or special algorithms if there is no CRC on transport channels that carry AMR B and C class bits.