5.3 Poor Performance of Data Transfer
5.3.4 Analyzing Poor Performance of Data Transfer by HSDPA at RAN Side
The HSDPA network schedules power and code resources by code division or time division between multiple subscribers. When there is only one HSDPA subscriber in a cell, the following factors affect the rate for data transfer:
HSDPA available power
Number of HS-PDSCH codes in cell (when there is only one subscriber, a HS-SCCH is necessary)
Category of UE (maximum number of codes supported by UE and whether to support 16QAM)
Radio signals near UE
In addition, the following factors affect the reachable maximum rate:
Subscribed rate
Bandwidth at Iub interface
Maximum rate supported by RNC, NodeB, GGSN, and SGSN.
When there are multiple subscribers, besides previous factors, the scheduling algorithm used by NodeB and number of HS-SCCH configured to cell affects the rate of data transfer.
An HSDPA subscriber works as below:
The UE reports CQI on HS-DPCCH. The NodeB obtains the CQI of UE's location.
The scheduling module inside NodeB evaluates different subscribers by channel conditions, the amount of data in cache for each subscriber, the last serving time. It then determines the HS-DSCH parameters.
The NodeB sends HS-DSCH parameters on HS-SCCH, and after two slots it sends data on HS-DSCH.
The UE monitors HS-SCCH for information sent to it. If there is any schedule
information, it starts receiving HS-DSCH data and buffers them.
According to HS-SCCH data, the UE judges whether to combine the received HS-DSCH data and data in soft buffer.
The UE demodulates the received HS-DSCH data, and send the ACK/NACK message on uplink HS-DPCCH according to CRC result.
If the NodeB receives the NACK message, it resends the data until it receives the ACK message or reaches the maximum retransmission times.
In the DT tool Probe, out of consideration for multiple subscriber scheduling and retransmission at MAC-HS layer, there are three rates at MAC-HS layer:Scheduled Rate，Served Rate，MAC Layer Rate.
Served Rate = Scheduled Rate * HS-SCCH Success Rate MAC Layer Rate = Served Rate * (1- SBLER)
Scheduled rate
Schedule rate = total bits of all TBs received in statistics period/total time with TB scheduled in statistics period
The total bits of all TBs received in statistics period include all the bits of received correct and wrong TBs.
The total time with TB scheduled in statistics period includes the time with data received and excludes the time without data received.
Served rate
Served rate = total bits of all TBs received in statistics period/statistics period The total bits of all TBs received in statistics period include the bits of received correct and wrong TBs.
The statistics period includes the time with and without data received.
MAC layer rate
MAC Layer Rate = total bits of correct TBs received in statistics period/statistics period The total bits of correct TBs received in statistics period include the bits of correct TBs and exclude bits of wrong TBs.
The statistics period includes the time with and without data received.
HS-SCCH success rate is the success rate for receiving HS-SCCH data by UE
SLBER = wrong TBs received at MAC-HS layer/(received correct and wrong TBs)
ACK->NACK/DTX is the ratio that NodeB judges the ACK message as NACK/DTX message.
Figure 1.1 shows the flow for analyzing poor performance of data transfer on HSDPA at RAN side.
Figure 1.1 Flow for analyzing poor performance of data transfer on HSDPA at RAN side
NE Alarms
When the performance of data transfer for PS services is poor, analyze the NodeB and RNC alarms.
The clock alarms, alarms on transport code error, and transmission interruption may lead to fluctuation of PS data. For querying NodeB and RNC alarms, see W-Equipment Room Operations Guide.
Whether the Service Is Set Up on HSDPA Channel
Check the IE serving HSDSCH RL indicator of the message RB SETUP on RNC. If the IE is True, and the SF of downlink channel code is 256, the service must be carried by HSDPA channel, as shown in Figure 1.2.
Figure 1.2 Confirming in the RNC message that PS service is set up on HSDPA channel
You can also check the information like reported CQI in the WCDMA HSDPA Link Statistics window in the DT software Probe. If no information is in the window, the service must be carried on DCH, as shown in Figure 1.3.
Figure 1.3 Confirming in Probe that service is set up on HSDPA channel
If the service is not set up on HSDPA channel, it will automatically be set up on DCH. Now the service rate is the rate of R99 service, usually equal to or smaller than 384 kbps.
If it is confirmed that the service is not set up on HSDPA channel, analyze it from the following aspects.
HSDPA cell is not set up
Check at RNC side whether the HSDPA cell is activated by executing the command LST CELLHSDPA.
Check at NodeB side whether the local cell supports HSDPA. Check by executing the command LST LOCELL whether the value of the local cell is TRUE or FALSE.
If the HSDPA cell at RNC side is not activated, activate it by executing the command MOD LOCELL: LOCELL=0, HSDPA=TRUE.
In addition, during modifying the HSDPA cell configuration on RNC, if HSDPA codes are statically assigned, and if there are excessive R99 subscribers connected to the cell so the code assigned to HSDPA is inadequate, the RNC still displays that the modifying HSDPA cell configuration succeeds. However, actually the HSDPA cell is not
successfully set up. Check whether the codes assigned to HSDPA cell are successful by selecting Realtime Performance Monitoring > Cell Performance Monitoring > Code Tree Tracing on RNC.
Incorrect type of HSDPA AAL2PATH or No Configuration
Set the type of HSPDA AAL2PATH to HSDPA_RT or HSDPA_NRT. Otherwise the cell can support R99 services only, but not HSDPA services. It is recommended that one HSDPA AAL2PATH is configured to one NodeB. If multiple HSDPA AAL2PATHs are configured, the data packets are easily dropped in the current version. Query it at RNC or NodeB side by executing the command
LST AAL2PATH.
If the HSDPA AAL2PATH is set to RT or NRT, the downlink subscription rate of UE is 2 Mbps. When the UE accesses the network, setting subscriber plane for HSDPA service fails, and the RNC will automatically set up the subscriber plane of PS 384kbps service.
According to signaling of the RB Setup message, the service is set up on R99, and SF is 8.
HSDPA subscriber's admission failure
The HSDPA subscriber's admission failure leads to that the RNC reconfigures HSDPA service to be carried by PS384K channel of R99 service. If the service cannot be set up, the UE continues to access the network after lowering the rate of R99 service. If the rate of connected HSDPA subscriber is as low as 384 kbps, 128 kbps, or 64 kbps of R99 services according to test, confirm whether the service is set up on HSDPA channel and whether the admission fails.
Check whether the following aspects are rational:
− Uplink and downlink load of R99 services
− Downlink code resource
− Iub transmission resource
− Number of HSDPA subscribers
− Threshold of HSDPA cell rate
− Guaranteed rate threshold of streaming service
− Guaranteed power threshold
Over high HSDPA threshold for downlink BE service
The HSDPA threshold for downlink BE service defines the rate judgment threshold for background or interactive services carried on HS-DSCH in PS domain. If the request rate is great than or equal to the threshold, the PS service is carried on HS-DSCH; otherwise, the PS service is carried on DCH.
Set HSDPA threshold for downlink BE service by executing the command SET FRC:
DlBeTraffThsOnHsdpa=D384 on RNC.
Low Scheduled Rate
The TB size of NodeB scheduling depends on CQI, HSDPA codes, available power for HSDPA, and so on. TB size/2ms is scheduled rate.
Normally, there is mapping relationship (depending on mapping table of NodeB CQI in actual use) between the schedule rate and CQI reported by UE. The NodeB will filter and adjust the CQI reported by UE, so the scheduled rate and CQI scheduled by NodeB have mapping relationship, not completely having mapping relationship with the CQI reported by UE.
Table 3.1 lists the relationship between CQI and TB size according to the protocol 3GPP 25.306. It is only for reference, the product realization does not completely consist with protocol.
Table 3.1 Relationship between CQI and TB size when the UE is in category 11–12 CQI
CQI
value Transport
Block Size Number of
HS-PDSCH Modulati
Table 3.2 Relationship between CQI and TB size when the UE is at the level 1–6 CQI
value Transport
Block Size Number of
HS-PDSCH Modulatio
CQI
value Transport
Block Size Number of
HS-PDSCH Modulatio
The following factors affect scheduled rate:
CQI
If the downlink rate of UE is low, check whether the CQI reported by UE is over low, and check the PCPICH RSCP and Ec/Io of the serving cell from the following aspects:
− The coverage is weak, and the CQI reported by UE is low.
− The interference is strong, and there is pilot pollution, and the CQI reported by UE is low.
− When the HSDPA serving cell is frequently updated, the HSDPA subscribers cannot change accordingly due to punishment, so the CQI reported by UE is low.
If the coverage is weak, improve the CQI reported by UE by RF optimization and constructing sites.
If the interference is strong, adjust the azimuth and down tilt in RF optimization. This forms a primary cell.
If the HSDPA serving cell is frequently updated, avoid frequent handover by adjusting antenna azimuth and down tilt or constructing sites in RF optimization.
Available power of HSDPA cell
If the available power of HSDPA cell is over low, the TB size of NodeB scheduling will be affected.
HSDPA power configuration includes dynamic and static configuration.
The RNC MML is MOD CELLHSDPA: HSDPAPOWER=430. The unit of HSDPA power is 0.1 dB. The total power of all HS-PDSCHs and HS-SCCHs must not exceed the HSDPAPOWER.
When HSDPAPOWER in previous formula is higher than or equal to total power of cell, the HSDPA power configuration is dynamic configuration. The available power of HSDPA cell = total power of cell * (1 – power margin) – power used by R99 TCH and CCH.
When HSDPAPOWER in previous formula is lower than total power of cell, the HSDPA power configuration is static configuration. Namely, the available power of HSDPA cell is the HSDPAPOWER. However, the maximum available power = total power of cell * (1 – power margin) – CCH power.
In static power distribution, the R99 services may occupy the power of HSDPA cell, so the actual power used by HSDPA cell is not the configured power.
Analyze the factors affecting available power of HSDPA cell from the following aspects:
− Query power margin by executing the command LST MACHSPARA on NodeB. The default power margin is 10%, namely, the total downlink load of cell can use 90% of total power of cell.
− On RNC LMT, select Realtime Performance Monitoring > Cell Performance Monitoring > Tx Carrier Power. Observe the transmit carrier power and power used by non-HSDPA subscribers. The available power of HSDPA = transmit carrier power - power used by non-HSDPA subscribers. If the power used by non-HSDPA subscribers is over high, the available power of HSDPA cell becomes low, so the scheduled rate is affected.
Available codes of HSDPA cell
If inadequate codes are assigned to HSDPA subscribers, the TB size of NodeB scheduling will be affected..
HSDPA UE CATEGORY
The 3GPP protocol 25.306 defines 12 types of UE category. In a TTI, the UE of a type obtains different maximum TB size, so the maximum scheduled rate obtained by UE is different.
The UE reports its capability in the IE hsdsch physical layer category of the RRC Connection Setup Complete message..
Amount of data to be transmitted being smaller than the maximum TB size
The TB size scheduled by NodeB depends on the available power and codes of the subscriber, as well as the amount of data transferred by the subscriber. If the amount of data sent is smaller than the maximum scheduled TB size, the rate at physical layer is lower than the expectation.
This problem occurs when there is data in NodeB buffer but the amount of data is
inadequate for a scheduled maximum TB size.
Low Served Rate
According to the previous formula Served Rate = Scheduled Rate * HS-SCCH Success Rate, if the scheduled rate is normal, over low HS-SCCH success rate leads to over low served rate. If there is only one subscriber in normal conditions, and the HS-SCCH power and traffic are not restricted, the success rate of HS-SCCH is shall be highly approach to 100%.
The success rate of HS-SCCH is relevant to HS-SCCH power, number of HS-SCCHs, number of subscribers, scheduling algorithm, and transported traffic. The following paragraphs describe them respectively.
HS-SCCH power distribution
The HS-SCCH is a downlink CCH, shared by all subscribers. The UE keeps monitoring UE ID on HS-SCCH, and judge whether the UE ID is for itself. If the UE ID is for itself, it demodulates HS-PDSCH data. Therefore, correct demodulation of HS-SCCH goes before data transfer.
There are three types of HS-SCCH power, transit ，SET MACHSPARA， in NodeB , 0 shows that HS-SCCH power control is based on CQI . 1 shows HS-SCCH power changeless; 2 shows use a power control mode which go with DCH and keep a fixed power deflection value. Default is 0. (Attention: the edition before
NodeB3812EV100R007C03B040 can’t be set to type 0, need use type 1 .) The HS-SCCH power is in static configuration or dynamic configuration.
The default configuration is static configuration. Set the HS-SCCH power to a fixed ratio of maximum transmit power of cell as below:
− Set the ratio to 3% in indoor environment.
− Set the ratio to 5% in outdoor environment.
Set the HS-SCCH power on NodeB LMT by executing the command below:
SET MACHSPARA: PWRFLG=FIXED, PWR=5;
HS-SCCH power can be configured as dynamic power control, which is achieved by setting a power offset to the pilot bit of DL-ADPCH. The power offset is relevant to spreading factor of downlink DPCH and whether the UE is in SHO state. When this method is used, the HS-SCCH power offset is listed as in Table 3.3.
The MML command is as below:
SET MACHSPARA: PWRFLG=DYNAMIC;
Table 3.3 HS-SCCH power offset Spreading
128 +4.25 +8.25
256 +7.25 +11.25
0 shows that HS-SCCH power control is based on CQI , which works like this:
First set HS-SCCH initialization TX power
Then according to CQI change , adjust HS-SCCH power, like DCH inner-loop power control.
At last , according to the ACK/NACK/DTX information from HS-DPCCH’s feedback ,adjust HS-SCCH power , like DCH outer-loop power control.
The parameter of the power control which base on CQI’s HS-SCCH : HS-SCCH’s initial power , Default is 28(-3 dBm), relative to pilot power ; HS-SCCH power control’s aim FER , Default is 10%(1%)
Number of HSDPA subscribers and number of HS-SCCHs
The success rate of HS-SCCH is relevant to number of subscribers.
− If there is only one HSDPA subscriber in a cell, the traffic is not restricted and HS-SCCH power is adequate, the success rate of HS-HS-SCCH for the subscriber approaches 100%.
− If there are multiple HSDPA subscribers in the cell, the success rate of HS-SCCH for each subscriber is relevant to scheduling algorithm and number of HS-SCCHs.
Usually set the HS-SCCH according to available power of HS-PDSCH, code resource, and traffic of service source. For example, if UEs used in the cell are all category 12 UE, set number of HS-PDSCH codes and number of HS-SCCHs as below:
− If you set 5 codes to HS-PDSCH, it is recommended to set 2 HS-SCCHs.
− If you set 10 codes to HS-PDSCH, it is recommended to set 3 HS-SCCHs.
− If you set 14 codes to HS-PDSCH, it is recommended to set 4 HS-SCCHs.
Scheduling algorithm
Using different scheduling algorithm for multiple subscribers enables each subscriber to be scheduled at different probability. For example, after Max C/I scheduling algorithm is used, the subscribers far from the cell center will hardly or even never be scheduled due to low CQI.
The scheduling algorithm is one function of new function entity of HSDPA, the MAC-hs function entity. Four factors are involved as below:
− CQI
CQI is the quality of signals received by UE at the location.
− Wait_Inter_TTI
It indicates the length of time that the UE must wait for service.
− Queue priority
− Queue length
The following scheduling algorithms are typical:
− Max C/I (only considering CQI value)
− RR (only considering wait_Inter_TTI)
− Classic PF (proportional fair, considering previous factors)
− EPF，Enhanced Proportional Fair，，V17 edition
Parameters are not configured for current scheduling algorithm. Select one of previous three algorithms by executing the command below:
SET MACHSPARA: LOCELL=10131, SM=PF;//The previous algorithm corresponds to the PF scheduling algorithm.
Traffic
After previous configuration and checks, there is no problem and CQI reported by UE is high, but the rate of subscribers fluctuates. Check downlink traffic in Connection Performance Monitoring window on RNC LMT, and see whether there is enough traffic for scheduling. Or check downlink traffic in HSDPA User Flow Control Performance Periodic Report window on NodeB LMT.
The cause of this problem is unstable source rate, single thread used upon downloading, and small TCP window.
In the HSDPA User Flow Control Performance Periodic Report window, there are following selections:
− Queue Priority
− Queue Buffer Used Ratio
− RLC User Buffer Size
− Input Data Size
− Output Data Size
Select Queue Buffer Used Ratio to draw picture on LMT, and check the occupation of NodeB queue.
Select RLC User Buffer Size to check RLC buffer.
Select Input Data Size and Output Data Size to check the sending and receiving queue data. The data involved in Output Data Size is the data with ACK indicator received.
Restricted Rate at UE side
The request service type, uplink and downlink maximum rate are sent to UE by AT commands. The UE sends the information to CN in the following Active PDP context request message. When the subscribed rate is higher than or equal to the requested maximum rate, the CN sends the RAN Assignment request message at the requested maximum rate. If the resource is not restricted at RNC side, the final output rate is the
The request service type, uplink and downlink maximum rate are sent to UE by AT commands. The UE sends the information to CN in the following Active PDP context request message. When the subscribed rate is higher than or equal to the requested maximum rate, the CN sends the RAN Assignment request message at the requested maximum rate. If the resource is not restricted at RNC side, the final output rate is the