Ericsson LTE Throughput Troubleshooting Techniques.ppt

LTE L11 Throughput Troubleshooting TechniquesSlide title In CAPITALS 44 pt Slide subtitle 20 pt
LTE L11 Throughput Troubleshooting Techniques
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Slide title In CAPITALS 44 pt Slide subtitle 20 pt
Introduction
Why learn about Throughput Troubleshooting
LTE provides data, lots of data
Throughput is shared in time and frequency
Users notice throughput problems
Learn to isolate the domain causing throughput degradation
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Data throughput is the main driver for 3G and 4G network.
It is more important than ever to know the factors affecting throughput degradation
This module will present an analysis of various domains which cause throughput degradation
At the end of the module the course participant will have the key knowledge to enable them to isolate problem causing domains and perform a complete LTE RAN and end-to-end analysis of throughput problems.
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Scope and objectives
Pinpoint causes of throughput degradation clearly within domains through theory, traces and practical examples
Objectives
Scope
> Overview
Agenda
Overview
LTE RBS User plane Overview
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LTE RBS User Plane Overview
User plane visualisation
User Plane Domains
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This course aims to explore the throughput troubleshooting possibilities for the LTE RAN.
End user throughput degradation is very visible to end customers and operators alike. Hence these problems are sensitive in nature.
Throughput investigations involve many different nodes and protocols.
A coordinated approach to troubleshooting is then beneficial to isolating the problem.
Of note is that we subdivide the throughput analysis into several smaller domains:
Radio Domain - Specifically concerned with the radio interface and the L1/L2 protocols.
Transport Domain - This domain is concerned with the northbound IP network that connects the LTE RBS to the Core Network.
e2e Domain - This domain is concerned with the end-2-end aspects of throughput. It focus’ on what the user experiences.
The user protocols are presented here for clarity and reference.
Note: the core interfaces are not explored in this course.
Data Flow over Air (RBS/UE)
CRC
Payload
Payload
Payload
CRC
Header
Header
Header
Payload
Payload
Payload
Header
Header
Header
PDCP
Header
PDCP
Header
PDCP
Header
PDCP
RLC
MAC
RLC
Header
RLC
Header
RLC
Header
MAC
Header
MAC
Header
Uses sequence numbers.
Support ROHC (not in L10) - shown in diagram with reduction of header size.
Direct mapping to an RLC SDU.
RLC:
Segments and Concatenates.
Uses HARQ for fast retransmission (RLC handles overall payload reliability)
Multiplexes multiple RLC PDUs (SRB and/or DRB)
Mapped to Transport Block (1-2 TB per TTI depending on the CWs supported)
The combination of hybrid-ARQ and RLC attains a good combination of small roundtrip time and a modest feedback overhead where the two components complement each other. Thus, the two-level structure combines the best of two worlds – fast retransmissions due to the hybrid-ARQ mechanism and reliable packet delivery due to the RLC.
network assumptions
Node alarms verified
MO status
Cell availability
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Release Limitations
L11A contains some limitations that directly affect end user throughput
One SE per TTI in UL and DL (in L11A GA)
Each cell is treated individually, so there could be up to 3 users simultaneously in an eNB
SIB is scheduled the same as user data, so nothing can be scheduled at the same time as SIB
DUL user plane capacity limited to 150 Mbps (20MHz)
100PRBs UL, 150 PRBs DL.
16QAM UL (up to MCS24)
MCS28 disabled in DL by default (requires CFI=1 also)
Fixed CFI (number of OFDM symbols for PDCCH)
Default is CFI=2 for 5MHz and less.
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Initial Checks
Initial checks
Network changes & Basic troubleshooting
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Before we investigate any throughput issues, it is best to rule out the most obvious and basic issues that might affect end user throughput.
This checklist aims to provide some items that should be checked/ruled out prior to detailed radio or transport network investigations.
NW Changes and Basic Troubleshooting
Network/node changes can affect network throughput
Some common examples include:
RBS parameter changes (all MOs under ENodeBFunction, system constants, EricssonOnly hidden parameters, e.g. DataRadioBearer)
IP address plan changes
DNS updates
Basic troubleshooting checks include:
MO health status
Planned network changes can cause throughput issues if implemented incorrectly (or untested).
A history (verbal) of network changes should always be requested prior to beginning investigations
The listed Moshell commands are considered important inputs prior to commencing more detailed investigations.
PC/Server Settings
Confirm the end user PC settings:
Laptop specification can impact throughput (processors, memory, USB bus, HDD speed, plugged into AC power, etc)
MTU settings in PC (1360 optimal for eNB in L11A to prevent fragmentation)
Throughput monitors (e.g. Netpersec, only good for downlink UDP measurements, uplink must be measured at receiving side for UDP)
TCP enhancements in Vista (experimental), Vista should “auto-tune”.
Confirm server settings:
FTP server configuration
Linux TCP setting/guide
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Laptop specs:
Some laptops may not provide the necessary power to run high throughput
The OS and hardware specifics can reduce the achievable throughput in these cases
If possible, test throughput with other PCs to benchmark the UE/PC performance
MTU of 1360 avoids IP fragmentation/reassembly (process that requires many resources and causes delay)
Use appropriate Application
UE Categories
5 UE Categories are defined in 3GPP TS 36.306
The UE-Cat is sent in the UE Capability Transfer procedure (RRC UECapabilityInformation)
The COLI ue command provides detailed capability info ( KO ) for connected UEs
DL
UL
UE Category
Maximum number of DL-SCH transport block bits received within a TTI
Maximum number of bits of a DL-SCH transport block received within a TTI
Total number of soft channel bits
Maximum number of supported layers for spatial multiplexing in DL
Category 1
UE Category
Maximum number of bits of an UL-SCH transport block transmitted within a TTI
Support for 64QAM in UL
Category 1
Highlight UL limitation in L10A (16QAM max)
Highlight Rx div assumed
General Comment on Charts:
Column 1 defines total bits received. Simply multiply these by 1000ms/sec for throughput on L1.
For DL chart:
Soft channel bits: Defines the total number of soft channel bits available for HARQ processing.
COLI command for UE cap.
UE Subscriber profile
End User (EPS User) subscription data is stored in the HSS
The EPS User Profile data is identified by its IMSI number
The profile consists of:
RAT frequency selection priority
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ODB - The type of ODB applied to the EPS User.
APN Replacement - Domain name to replace the APN OI when constructing the Packet Data Network (PDN) Gateway (GW) Fully Qualified Domain Name (FQDN), upon which to perform a Domain Name System (DNS) resolution.
Charging - Charging Characteristics associated to the overall EPS User Profile, according to 3GPP TS 32.299.
AMBR - Aggregate maximum bandwidth of the overall Internet Protocol (IP) flow associated to the EPS User Profile.
RAT - The Subscriber Profile Identity for RAT/frequency priority.
APN - Information on all the APNs associated to the EPS User Profile.
RBS Parameters RN
RN MO parameters:
(nrOfSymbolsPdcch) (Control Region Size) NOTE: currently controlled by SC38 in L11A
noOfUsedTxAntennas controls whether OLSM MIMO is used (2) or not.
partOfRadioPower NOTE: this is the % part of RU capability independent of SectorEquipmentFunction::confOutputPower settings
pZeroNominalPucch some UEs need this to be increased or ACK/NACKs are not received successfully on PUCCH.
pZeroNominalPusch some UEs need this to be increased from default or lots of errors seen on PUSCH
SectorEquipmentFunction=Sx
DataRadioBearer
Various parameters for RLC status reporting and retransmission. Should be set to recommended values.
MACConfiguration
xxMaxHARQTx – enable (>1) or disable (1) HARQ. Recommended to use 4 HARQTx.
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ChannelBandwidth tied to license - > BW = more RBs and higher payload
PDCCH ranges from 1-3 OFDM symbols. Less provides more symbols for PDSCH. Currently controlled by system constant number 38 in L11A
Rx/Tx Antenna - Multiple for Rx and Tx diversity and Spatial Multiplexing.
pZeroNominalPucch – Some UEs need this to be increased to get successfully ACK/NACK reception in eNB. However it could cause neighbour cell interference.
pZeroNominalPusch – Some UEs need this to be increased for eNB to successfully decode PUSCH transmission.
CyclicPrefix - can limit the number of OFDM symbols. Sizes cater for different cell sizes. Smaller = more OFDM symbols.
Sector
conf
fqBand
RBS PARAMTERS TN
TN MO parameters:
GigabitEthernet=1
actualSpeedDuplex – if you see half-duplex, it could be a problem with auto-negotiation
dscpPbitMap (QoS mapping from L3 to L2)
IpInterface=2 (rec. MO id for Signalling and Payload)
vLan/vid (true/false and vlan id)
IpAccessHostEt=1
IpSyncRef (if NTP synchronisation is used)
syncStatus should be OK
nodeSystemClock should be in LOCKED_MODE.
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Enabled Features
The following features directly impact end user throughput
Downlink/Uplink Baseband Capacity
64-QAM DL / 16-QAM UL
To quickly check active licenses (including states):
moshell> inv
Channel Bandwidths:
64-QAM DL / 16-QAM UL:
These features provide higher capacity modulation types for the DL and UL respectively. More bits/symbols are supported, hence greater transport block sizes (throughput rates).
Dual Antenna DL Performance Package:
Provides two radio transmission modes: Transmit Diversity and Open Loop Spatial Multiplexing (OLSM).
OLSM is an antenna technology that can effectively double the available throughput in the air interface.
Expected Throughput (Simplified)
This reference shows the expected L1 downlink throughput for different antenna/radio configurations.
Knowing what L1 throughput to expect in ideal conditions for different radio configurations is extremely important.
Some inputs to radio setup are thus defined as:
Cyclic Prefix in use
Scheduling Blocks (= 2 RBs)
Configured Control Region Size
Frequency Bandwidths
L1 throughput will include overheads, so expected user throughput is less than this.
DL SB to Bit calculation
DL Scheduling Block (SB) -> Bit calculation (Normal CyclicPrefix)
Tx Diversity
2x2 MIMO
Resource Elements (RE) per Resource Block (7 OFDM symbols x 12 SubCarriers)
84
168
168
336
16
32
1
2
3
1
2
3
RE per CRS (OFDM*12 - 4 RS Tx) (OFDM*12 - 8 RS MIMO)
8
20
32
16
40
64
144
132
120
288
264
240
288
264
240
576
528
480
576
528
480
1152
1056
960
864
792
720
1728
1584
1440
20 MHz => 100 RB (64 QAM)
86.4
79.2
72
172.8
158.4
144
64.8
59.4
54
129.6
118.8
108
43.2
39.6
36
86.4
79.2
72
21.6
19.8
18
43.2
39.6
36
76
64
52
152
128
104
Identify the domain
Further analysis required:
Analysis steps to perform:
Single UE call scenario
Optionally use a radio monitor (e.g. TEMS)
Decide - Radio or Transport analysis:
Radio issues provide more control for LTE RAN analysis
Transport issues blend/carry-on towards core elements
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Steps:
Avoid multi-use and mobility cases. Selective tracing in baseband is currently limited.
Take logs from a controllable location e.g. on S1 using Wireshark or internal tools.
Aside from the node logs, we should obtain as many logs as possible (e.g. TEMS, Routers, Switches, etc).
Iperf is a flexible client/server tool. It runs on multiple operating systems and can change roles (between server/client)
UDP allows us to test the pipes and avoids TCPs congestion control algorithms (which complicate analysis)
TCP will be covered later for completeness
Decide
Radio:
Transport:
?What of cell throughput? - i.e. why only 1 UE
?Is iperf the best tool for the job? Why not Ixia or <vendor>?
Radio Analysis
Radio Analysis
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Radio Analysis
Ericsson’s LTE Baseband provides a detailed mechanism for tracing the complete L1 and L2 interaction, including MAC scheduling decisions and L1 decoding results.
Using this information we can further isolate the cause of the problem and pinpoint either:
UE problem
Scheduling abnormality
eNB northbound problem
S1 user plane
RADIO ANALYSIS
To perform targeted radio analysis, it’s useful to know radio aspects specific to the following traffic scenarios:
Downlink
Uplink
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We’ll start by presenting an overview of the Uplink and Downlink Radio Analysis Areas
Following that, we’ll deep dive into downlink analysis followed by uplink analysis
The analysis will consist of theory relevant to the area combined with signals and traces to view the behaviour in a live system
Finally a few slides show post-processing tools available for traces to simplify analysis.
Radio Analysis - Downlink
Areas of analysis for Downlink:
CQI (Channel Quality Index) and RI (Rank Indicator) reported from UE.
Transmission Mode: MIMO (tm3) vs. TxD (tm2) vs. SIMO (tm1)
MCS vs. number of assigned PRBs vs. assignable bits in scheduler
UE Scheduling percentage of TTIs (how often is the UE scheduled)
CFI (number of OFDM symbols for PDCCH) vs. MCS vs. % scheduling
HARQ
These are the fundamental areas of analysis for downlink:
CQI and RI provides us the SINR/antenna layer reception reports from the UE point of view
Transmission modes 1 (SIMO),2 (Transmit Diversity) and 3 (Open Loop Spatial Multiplexing) are supported by the eNB in L11A.
Understanding the relationship between chosen MCS, assigned PRBs and assignable bits in the scheduler are important for sorting core network issues/UE issues from air interface issues
Scheduling percentage means the amount of TTIs (typically measured per second) that the UE was scheduled. This is also related to the resources allocated for PDCCH (control channels)
PDCCH CFI impacts the % TTI scheduling but also reduces the maximum MCS achievable.
HARQ will be presented using tracing. Check also the Uplink HARQ for more detailed theoretical explanation.
RLC retransmissions will be touched on briefly, including how to trace RLC status messages and some parameters involved.
Radio Analysis – Uplink
Uplink scheduling overview
PHR (Power Headroom Report) – is the UE at maximum power?
Cell bandwidth vs. maximum allowable PRBs
Link Adaptation
HARQ (less important, because we can measure SINR)
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These are the fundamental areas of analysis for uplink:
We’ll begin this section with an overview of uplink scheduling
BSR is the mechanism the UE uses to inform the eNB about the amount of data waiting in its RLC buffers
PHR is the mechanism the UE uses to inform the eNB about remaining power at the transmitter (or power limitations)
The number of PRBs available for uplink scheduling has some 3GPP specified limitations which different from downlink. This means that, for example, the maximum number of PRBs (for a single UE) able to be scheduled in 5MHz is 20 and not 23 (with 2 reserved for PUCCH).
Link adaptation inputs for uplink vary from downlink. One difference is the CQI (estimate of SINR) is not needed because the eNB can itself measure SINR of PUSCH transmissions.
PDCCH collisions can occur with SIB/downlink transmissions as downlink and uplink grants are both scheduled using the same PDCCH resources.
The theory of HARQ in uplink is presented.
Radio Analysis DL – CQI/RI and TM
The eNB needs knowledge of the SINR conditions of downlink transmission to a UE in order to select the most efficient MCS/PRB combination for a selected UE at any point in time.
Channel Quality Index (CQI):
Informs eNB of current channel conditions as seen at UE
Directly maps to 3GPP defined modulation/code rate (TS36.213 Table 7.2.3-1)
Defined as the highest coding rate the UE could decode at 10% BLER on HARQ rv=0 transmission
CQI 1-6 map to QPSK
CQI 7-9 map to 16QAM
CQI 10-15 map to 64QAM
Rank Indicator (RI)
Is a feedback mechanism from UE to eNB
Informs eNB whether UE can successfully decode RS from 1 or 2 (or more) antennas.
eNB scheduler uses this feedback to transmit with either:
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Radio analysis DL – CQI/RI and TM
The UE measures DL channel quality and reports to eNodeB in the form of Channel Quality Information (CQI)
The average CQI (periodic-CQI reporting) for the whole band (wide-band CQI) is reported periodically on PUCCH (or on PUSCH if user data is scheduled in that TTI) with configured periodicity.
Sub-band CQI (aperiodic-CQI reporting) is reported when requested by the eNB. This report is for the PDSCH. Report sent on PUSCH.
CQI polling is triggered on demand by eNB based on DL traffic activity.
When 2 antennas are configured, Rank Indicator is also reported. Precoding Matrix Indicator (PMI) also reported in case of transmission mode 4 (not in L11A).
CQI
Periodic CQI reports are WCQI only.
Sent on PUCCH (unless PUSCH data is scheduled, in which case it’s multiplexed onto PUSCH).
Aperiodic CQI reports (eNB must specifically request aperiodic CQI report in the uplink grant) include RI+WCQI+SCQI
(NOTE: may be requested periodically by the eNB)
In order to transmit with MIMO (OLSM) we should check the following:
eNB cell is configured with two working transmit antennas.
Check EUtranCellFDD::noOfUsedTxAntennas > 1
L11A GA (default) system constant SC125:3 means that tm3 is used in case 2 TX antennas are defined.
If only one TX antenna is configured, then tm1 is used
In order to force Transmit Diversity (i.e. prevent OLSM), SC125:2 must be set
UE CQI/RI report from UE shows RI > 1
Rank 1: TxDiversity (transmission mode 2, tm2)
Rank 2: MIMO (Open Loop Spatial Multiplexing in L11A) (transmission mode 3, tm3)
mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir OUTGOING
This signal (from L1 to MAC scheduler) shows the reported CQI and RI
(also shows HARQ ACK/NACK for downlink data transmission)
(also shows rxPowerReport and timingAdvanceError)
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EUtranCellFDD::noOfTxAntennas may be configured as 0 or 2 to support MIMO (0 means automatically detect number of transmit antennas available).
Default system parameters in L11A are for open-loop spatial multiplexing (MIMO), otherwise known as tm3. If only one transmit antenna is configured, the system will automatically switch to SIMO.
Therefore it’s possible to switch between SIMO and MIMO using MOM parameters.
It’s not possible to configure transmit diversity without using system constants (SC125:2) in L11A.
Rank Indicator is the only way that eNB switches between:
RI=2 OLSM (tm3) and
RI=1 Transmit Diversity (tm2)
cfrPusch { cfrInfo { ri = 2, cfrLength = 22, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [61440, 0, 0, 0] as hex: [f0 00 00 00 00 00 00 00] }
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND UpUlMacPeCiUlL1Meas2DlIndS {
cfrPucch { cfrInfo { ri = 0, cfrLength = 4, cfrFormat = 0, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [0, 0] as hex: [00 00 00 00] }
cfrFormat=0 is a WCQI report only (ignore RI)
Valid report if cfrValid=1,cfrExpected=1,cfrCrcFlag=1
mtd peek -ta ulMacPeBl -signal LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND -dir OUTGOING
cfrFormat=4 is a SCQI + RI report
WCQI is first half octet (f => 15). Octets thereafter are subband CQI reports for each RBG.
A number of subband CQIs follow (see next slide)
cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] }
Rank Indicator = 2 (indicates UE can decode both antenna streams)
WCQI = 11. 5MHz bandwidth means 4PRBs subbands.
SCQI = F49C = 11 11 01 00 10 01 11 00
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We only show the relevant parts of the signal content here for presentation purposes.
The top example is WCQI on PUCCH (periodic CQI report)
Middle example is aperiodic CQI report on PUSCH including RI+WCQI+SCQI where all reported subbands have same value as WCQI (15). This is the best possible channel quality report available, CQI15 is maximum.
Bottom example is aperiodic CQI report on PUSCH including RI+WCQI+SCQI with variable SCQI reports as indicated. The subbands only indicate an offset from the WCQI value (this case 11). So, for example, subband PRBs 0-3 are CQI 10 or less and so on.
Radio Analysis DL – SCQI Visualisation
From the previous slide, SCQI is visualised here..
For 5MHz, each RBG is 4 PRBs wide (except for SCQI group 7)
SCQI is given relative to WCQI which was 11 in this example
f
0 1 2 3 4 5 6 7 8 9 10
11 12 13 14 15 16 17 18 19 20
21 22 23 24
5 MHz
SCQI PRBs: 0-3 -1, 4-7 -1, 8-11 +1, 12-15 0, 16-19 +2, 20-23 +1, 24 -1
Sub-band 1 2 3 4 5 6 7
CQI value ( 10 10 12 11 13 12 10 )
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cfrFormat = 4 consists of:
4 bit Wideband CQI (i.e. CQI across whole bandwidth)
Up to 13 subband CQI differentials (depends on bandwidth of cell)
Subband CQI (3GPP TS36.211 Ch 7.2.1)
RBG width depends on bandwidth:
3 & 5MHz – subband width 4 PRBs
10MHz – subband width 6 PRBs
15 & 20MHz – subband width 8 PRBs
Subband Differential mapping, see table below:
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7 possible cfrFormats defined in L11A.
Typically see reports cfrFormat 0 and 4 as described previously
Note that PMI is not yet used (requires tm4)
cfrFormat
Transmission Mode and MCS can be traced out with the following:
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0} noOfSBs={4294443008 0} rv={0 1} ndi={0 0} rmGbits={21600 0}"
MCS for each codeword. In this case, tm2 so only one MCS listed.
lhsh gcpu01024 te e trace4 UpcDlMacCeFt_DL_SCHEDULER
LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND (330) UpDlMacPeCiDlUeAllocIndS {
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
commonTb { newDataFlag = 1, tbSizeInBytes = 999, l1Tb { rvIndex = 0, modType = 2 (UPDLMACPEMode64Qam), nrOfRateMatchedBits = 21600, rmSoftBits = 1237248 } }
PRB list in RBGs, for 5MHz RBG size is 2. fff8 corresponds to 25 PRBs (last PRB is 1 less).
MCS is a combination of tbSize and modType.
999 bytes = 7992 bits then put into TS36.213 Table 7.1.7.2.1-1 for NPRB=25. That gives ITBS of 16.
Convert ITBS to MCS using Table 7.1.7.1-1.
mtd peek -ta dlMacPeBl -signal LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND -dir INCOMING
-filter {(U16SIG)8,NEQ,(U16)0x00}
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There are two possible traces for viewing MCS+TM+PRBs+HARQ info. They both show the same information in a different format:
This trace show everything on one line. Transmission mode is not explicitly stated.
mtd peek -ta dlMacPeBl -signal LPP_UP_DLMACPE_CI_DL_UE_ALLOC_IND -dir INCOMING -filter {(U16SIG)8,NEQ,(U16)0x00}
This is the signal between MAC and L1 informing all the HARQ information.
prbResourceIndicatorType will be explained in the next slide
RBG for Resource Allocation Type 0
Defined in 3GPP TS36.213 Ch 7.1.6.1
One bit used to represent a certain number of consecutive PRBs
1.4MHz is RBG size 1
3 & 5MHZ is RBG size 2
10MHz is RBG size 3
15 & 20MHz is RBG size 4
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Resource Allocation Types vary between system bandwidths to reduce the required message size for larger bandwidths.
Larger bandwidths use larger RBG sizes.
Example of switching transmission modes based upon RI
(bfn:3352, sfn:280, sf:5.47, bf:128) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=1 tbs={7992 0} mcs={18 0} noOfSBs={4294443008 0} rv={0 1} ndi={0 0} rmGbits={21600 0}"
TM=2 transmission with MCS 18
cfrPusch { cfrInfo { ri = 2, cfrLength = 18, cfrFormat = 4, cfrValid = 1, cfrExpected = 1, cfrCrcFlag = 1 }, cfr[] = [48969, 49152, 0, 0] as hex: [bf 49 c0 00 00 00 00 00] }
Rank Indicator = 2 received from UE. eNB will now switch to tm3 (OLSM MIMO) transmission
WCQI 11 + SCQI.
bfn:3352, sfn:280, sf:6.47, bf:131) ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL2 cellId=12 : Selected SE and HARQ: rnti=61 bbUeRef=201327456 HARQ idx=0 tbs={5736 5736} mcs={13 13} noOfSBs={4294443008 0} rv={0 0} ndi={0 1} rmGbits={14400 14400}"
25 PRBs as according to previous example
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This example shows a switch between tm2 and tm3 based upon RI=2 aperiodic CQI report reception.
Radio Analysis DL – Assignable Bits
If UE is sending with high CQI (in the range 10-15) and RI=2 but throughput is still very low, then the next check should be assignable bits.
Assignable bits means the amount of data in the downlink buffer available for the scheduler to schedule for this UE.
A classic symptom of low assignable bits is that the UE is scheduled with a high MCS but a low number of PRBs.
The scheduler always attempts to send with the highest possible MCS and least number of PRBs so that left-over PRBs could be assigned to another UE.
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Possible causes for low assignable bits:
RLC STATUS messages are not being received fast enough and RLC buffers are full.
Until RLC STATUS ACK messages are received, already transmitted RLC SDUs are kept in memory in UE and/or eNB
Check for RLC DISCARDs but low (or 0) assignable bits
Data received from core network is not enough to fill the RLC buffers in eNB.
Check that non-TCP based traffic is not being sent with too large packet size. For iperf based traffic, recommended size 1360 bytes (default is 1470).
Set MTU of 1360 in UE (or UE laptop).
RLC DISCARDs will trigger TCP congestion control and lower thpt.
In L11A the default RLC buffer size per RB is 750 IP packets
Trace discards with lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD
Discards on UDP traffic will not affect throughput
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ULMA3/UpDlPdcpPeFt_DISCARD TRAFFIC_ABNORMAL Discarding DL PDCP PDU due to exceeding limits. maxBufferedPacketsInRlc=751 totalNumNonAckedDrbPackets=751 cellId=12 bbUeRef=201327456 bbBearerRef=201327458 receiveFromTeid=3779046158 payloadLength=1506 bytes incl GTP-U header. hoState=0"
750 is default PDCP/RLC buffer per UE in eNB (L11A)
TRAFFIC_ABNORMAL corresponds to trace1. Traffic discards for UDP are normal, but for TCP traffic it will cause severe throughput degradation
ULMA4/UpcDlMacCeFt_DL_SCHEDULER LEVEL3 cellId=12 : Selected SE and PQ: rnti=61 bbUeRef=201327456 PQ lcid=1 assignableBits=0 minPduSize=56 selectedHarq=0"
LCID 3 is for the default bearer. LCID 1 and 2 for SRB
About 1MByte of data available for scheduling. Check for low value of assignable bits which indicates e2e problems affecting data available to schedule on air for eNB. Low assignable bits for UDP traffic may indicate MTU problems.
lhsh gcpu00768 te e all UpDlPdcpPeFt_DISCARD
*
Radio Analysis DL – CFI and Scheduling
Another cause of low (or lower than expected) throughput is that the UE is not being scheduled in every TTI.
This may be caused by:
Limitations in current scheduler implementation
3GPP defined compromises between control channel efficiency and scheduling efficiency (especially for lower number of users)
L11A software has some limitations to be aware of:
Only one SE per TTI is supported in L11A
SIBs are scheduled in the same was a user data (i.e. they are sent to the scheduler).
When a SIB is transmitted, no user data can be transmitted in the DL at the same time (using default parameters).
It is possible to use (System Constant) SC43 to enable 2SE/TTI in DL
*
System Constant SC43 controls the number of SE/TTI in the downlink. The default value of SC43 is 1 in L11A GA, however it is possible for lab testing to set SC43:2 and have 2 SE/TTI in the downlink.
System constant definition for L11A GA is found at http://cdmweb.ericsson.se/WEBLINK/ViewDocs?DocumentName=1%2F19059-HRB105500&Revision=PE2-10
Radio Analysis DL – CFI and Scheduling
SIBs require PDCCH resources
Typically SIBs consume 4 or 8 CCEs of PDCCH resources.
If a UE is in good SINR conditions, the scheduler may allocate only one CCE for that UE.
In that case, because of limited positions in PDCCH, it is quite likely that a PDCCH collision occurs (especially in low system bandwidths)
If a UE is in bad SINR conditions, the scheduler may allocate a large number of CCEs for that UE (2 or 4 or 8 CCEs)
Depending on the configured CFI there may only be common search space available or it may still collide with other PDCCH users.
*
When the UE is in good SINR, only one CCE is used for PDCCH transmission
In this case, a CCE index is calculated per subframe and may be at the beginning of a SIB (8 CCEs). In this case, up to 6 consecutive slots will be attempted. All of these will overlap with SIBs and then PDCCH collision has occurred for all possible search spaces.
When the UE is in bad SINR, many CCEs may be allocated
Large CCEs have limited possible search spaces and a collision may be unavoidable when SIB is scheduled or when other downlink users are scheduled.
Radio Analysis Dl – HARQ
Each transport block transmission is represented as a HARQ process.
Each HARQ process data is held in memory until NDI is toggled (i.e. New data is to be sent).
This allows fast retransmission of erronerously received data.
The schedulers representation of an HARQ process is as follows:
Feedback status
MCS – modulation and coding scheme
RV – redundancy version. HARQ has 4 redundancy versions, rv0, rv2, rv3, rv1.
NDI – New Data Indicator (physical layer bit toggled for new data).
Do not confuse with newDataFlag which is scheduler internal flag where 1 means new data and 0 means retransmission.
Number of transmission attempts (max 4 transmissions in L11A default paramters)
In case of rank 2 spatial multiplexing there are 16 HARQ process per UE instead of 8, but there are two processes that share the same ID
*
4 redundancy versions exist for HARQ and they are used in the following order the order RV0, RV2, RV3, RV1.
In case > 4 transmissions are configured, the cycle repeats, e.g. RV0, RV2, RV3, RV1, RV0, RV2, RV3, RV1, etc
Default is for 4 HARQ transmissions on L11A
Increasing the default number of transmissions means that RLC parameters also need to be modified and will require larger RLC buffers
In case one CW is ACKed and one CW is NACKed then the eNB retransmits BOTH CWs (even though one was received successfully).
L11A does not send new data on one branch and retransmission on another.
Another solution could be to send the NACKed CW using transmit diversity (but L11A does not do this).
Radio Analysis Dl – HARQ Example
The following slides will show an example of tracing out downlink HARQ
Initial downlink grant is sent with rv=0 (MIMO, 2 codewords)
SFN 280/subframe 8
SFN 281/subframe 2 (DL Grant + 4TTI)
First retransmission sent with rv=2
SFN 281/subframe 6 (8 TTI past initial transmission is earliest occasion)
HARQ ACK received on both code words
SFN 282/subframe 0 (DL Grant ReTx + 4TTI)
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Radio Analysis Dl – HARQ DL Grant
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
swapFlag = 0
macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 }
rlcTb { nrOfBearer = 1, bearerAlloc[0] { bbBearerRef = 201327458, lcid = 3, rbScheduledSizeInBytes = 717 } }
}
...
SFN/subframe where DL PDSCH will occur. PDCCH DL Grant sent at same sfn/subframe.
RNTI, TM, used PRBs (same for both code words)
If re-transmission, this indicates if CW0 and CW1 swapped layers
newDataFlag indicates if it is new data or not
HARQ redundancy version. rv0 used for initial transmission, rv2, rv3, rv1 used for re-transmission.
HARQ process number. 8 HARQ processes exist in FDD LTE L11A.
CW1 defined here.
Radio Analysis Dl – HARQ FEEDBACK (NACK/NACK)
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS {
timingAdvanceError { timingAdvanceError = 1 }
}
}
}
SFN/subframe +4 from DL grant (i.e. where the HARQ ACK/NACK is received from UE).
HARQ NACK received for DL HARQ Process 0 on both code words.
*
Radio Analysis Dl – HARQ ReTX
prbList[] = [4294443008, 0, 12, 0]dec
[ff f8 00 00 00 00 00 00 00 00 00 0c 00 00 00 00]hex
swapFlag = 0
macTb { dlHarqProcessId = 0, nrOfMacCtrlElem = 0 }
rlcTb { nrOfBearer = 0 }
SFN/subframe where DL PDSCH will occur. PDCCH DL Grant sent at same sfn/subframe.
RNTI, TM, used PRBs (same for both code words)
Same as previous transmission
HARQ redundancy version. rv2 is used for first retransmission
HARQ process number (same as before)
CW1 defined here.
Radio Analysis Dl – HARQ FEEDBACK (ACK/ACK)
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_DL_IND (431) UpUlMacPeCiUlL1Meas2DlIndS {
timingAdvanceError { timingAdvanceError = 0 }
}
}
}
SFN/subframe +4 from DL grant (i.e. where the HARQ ACK/NACK is received from UE).
HARQ ACK/ACK received for DL HARQ Process 0 on both code words.
*
Radio Analysis DL – RLC
RLC retransmissions are triggered:
When HARQ fails to transmit a transport block within the maximum number of configured retransmissions
Default number of HARQ transmissions is 4 in L11A
If RLC STATUS messages are not received within the time frames configured
RLC STATUS messages are sent between peer nodes (eNB and UE) to inform about lost RLC packets. They can be traced out using
mtd peek -ta dlRlcPeBl -si UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND
Check:
ACK_SN should be increasing, otherwise RLC buffers are not released
NACK_SN indicates RLC retransmissions (occasionally is OK)
DataRadioBearer::tStatusProhibit governs how often RLC STATUS messages may be generated, default is 25ms in L11A.
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Radio Analysis DL – RLC
RLC PDU {
ACK_SN = 743
}
}
Indicates the SN (Sequence Number) of the last successfully received RLC packet
NACK_SN indicates RLC retransmissions (HARQ failures)
0xd4205d4f=(sfn:324, sf:5.33, bf:212): UP_DLRLCPE_FI_STATUS_FOR_DL_TRAFFIC_IND (343) UpDlRlcPeRlcStatusForDlTrafficIndS {
RLC PDU {
ACK_SN = 787
}
}
Check that ACK_SN is increasing or RLC buffers not released
*
Radio Analysis – Uplink
Uplink scheduling overview
Link Adaptation
*
Radio Analysis UL – UPlink Scheduling
UL
eNodeB
Channel sounding
UL grant
Uplink Scheduling consists of the following components:
(if a UE has no PUSCH resources allocated) SR is sent by UE to eNB in order to request UL grant for BSR transmission
UL grant sent by eNB to UE. Initial buffer size in UL grant is set to size of BSR report + size of small ping/VoIP packet (to improve latency).
UE transmits BSR + data (in case data is small enough to fit in the initial UL grant buffer size allocated)
UL grant based upon BSR is allocated to UE (according to existing UL demands and link adaptation parameters)
UE transmits data on PUSCH according to UL grant
Channel sounding DMRS and SRS (not used in L11A) are used so that eNB can understand channel response across PUSCH and schedule UE accordingly.
macCtrlElementList[0] {
}
Radio Analysis Ul – BSR
Buffer Status Report (BSR) is used to inform the eNB of the current data waiting for transmission in the UE (3GPP TS36.213 Ch. 6.1.3.1)
Values ranges from 0 up to >15000 bytes using 64 index values.
e.g. index 0 for BS=0, index 1 for 0 < BS <= 10 and so forth.
Can be traced out through LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND. Expect to see high values for maximum UL throughput. Low values indicate UE/laptop problem.
Type of MAC report, this case short BSR (6)
LSB 6 bits are the BSR index (this case >150000 bytes)
MSB 2 bits is the LCID
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Radio Analysis UL – PHR
Power Headroom Report (PHR) is used to inform the eNB of the remaining transmit power available at the UE. (3GPP TS36.321 Ch. 6.1.3.6)
Defined as difference between configured maximum UE output power and estimated power used for PUSCH transmission
Reports a index value similar to BSR with values between -23 up to 40 dB
PH values are close to (or less than) 0 means the UE is power limited
Ideally we look for positive values somewhat greater than 0
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Radio Analysis UL – PHR
}
Type of MAC report, this case PHR (3)
PHR value of 55 which corresponds to 32 <= PH < 33. In this case there is no power limitation on the UE side.
PH Index values <= 23 indicates the UE has reached maximum transmission power
Negative values indicate the UE was power limited
See 3GPP TS36.133 Ch 9.1.8.4 for index mapping
*
Radio Analysis UL – PUCCH and PUSCH
PUCCH takes a minimum 1 PRB on each side of the uplink band for uplink control signalling, reducing the size of PUSCH
E.g. 5MHz bandwidth, 25 PRBs available. Minimum 2 PRBs for PUCCH.
23 PRBs available for PUSCH
0
1
2
3
4
5
6
7
8
9
PUSCH – Used for UE data scheduling and UL RA msgs
PUCCH – Semi-static allocation of CQI, SR, ACK/NAK
PUCCH – Semi-static allocation of CQI, SR, ACK/NAK
PUCCH
PUCCH
PUSCH
Radio Analysis UL – PRB Limitations
Due to 3GPP specified design limitations in the UL it is not always possible to utilise all free PRBs for UL transmissions
3GPP TS36.211 Ch 5.3.3 defines the following formula for the number of PRBs on PUSCH for a single transmission:
Where a, b and c are integers.
For 5MHz:
23 PRBs are available for PUSCH (2 allocated to PUCCH)
Max number of PRBs for a single PUSCH transmission is 20 PRBs.
This corresponds to a=2, b=0 and c=1 (i.e. 3 PRBs are unavailable to be used).
In L11A, 3 PRBs would be unused (only one SE/TTI possible).
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Radio Analysis UL – Link Adaptation
Goal: Select MCS for a certain allocation size to maintain the target BLER (10%) for the first transmission
Inputs to Uplink Link Adaptation are:
UL interference power:
LPP_UP_ULMACPE_CI_UL_L1_MEAS2_UL_IND outgoing from ulMacPeBl
LPP_UP_ULMACPE_CI_UL_MAC_CTRL_INFO_IND outgoing from ulMacPeBl
cellStatusReportInd(interferencePower (-125 .. -80dB)
Input to Link Adaptation
LPP_UP_ULCELLPE_CI_CELL_STATUS_REPORT_IND UpUlCellPeCiCellStatusReportIndS {
sfn = 456
subFrameNo = 3
interferencePower = -1170
High values here (>-104)
rxPwr = -95.6dBm over those PRBs (pZeroNominalPusch= -96dBm)
= ~22.9dB
L11A supports up to MCS 24 in the uplink by default
MCS21-24 are defined as 64QAM
However, according to 3GPP TS36.213 Ch 8.6.1 if a UE does not support 64QAM then 16QAM can be used for MCS21-24.
Check that MCS24 is selected. If not, check link adaptation inputs for problems
In UL, the eNB itself can directly measure SINR of the received signal
Therefore CQI is not necessary for UL transmission
*
Check for:
Could there be some external interferer?
Are the values of pZeroNominalPusch in neighbour cells too high?
rxPower too low
PHR shows UE at maximum Tx power
Is EUtranCellFDD::pZeroNominalPusch too high causing UE to exceed maximum transmit power?
Closed-loop power control TPC ignored by UE?
Low values of SINR
Is EUtranCellFDD::pZeroNominalPusch too low?
*
Radio Analysis UL – PDCCH
Time (ms)
Radio Frame
PDCCH carries both the UL (PUSCH) assignment and DL (PDSCH) assignment.
In case many PDCCH CCEs are used for DL transmission (e.g. SIB with 8 CCEs) it may be that UL grant is not possible to be scheduled in this TTI for a single UE!
PUSCH
DL subframe (current)
PDSCH
PDCCH
*
Downlink (PDSCH) assignments
Uplink (PUSCH) grants
In case of a downlink SIB transmission, 8 CCEs of PDCCH may be used for downlink grant.
To reduce processing load when decoding PDCCH, 3GPP defines particular search spaces within PDCCH depending on:
Number of CCEs for grant
Number of CCEs for PDCCH
RNTI of the UE
Depending on these parameters, it may not be possible to allocate a PDCCH uplink grant resource and therefore the UE may not be able to be scheduled every TTI even if there are unused PUSCH resources.
See 3GPP TS36.213 Ch 9.1.1
*
Search space for 1 CCE completely overlaps 8 CCE search space.
*
Radio Analysis UL – HARQ
LTE defines uplink with synchronous HARQ to reduce PDCCH signaling load and simplify the uplink HARQ processing
Example 1, successfully received PUSCH data:
Subframe n: UL grant sent to UE
Subframe n+4: PUSCH data received (rv=0)
Subframe n+8: ACK sent, UL grant with New Data Indicator toggled
Subframe n+12: new PUSCH data received (new HARQ process)
Example 2, HARQ retx:
Subframe n+4: PUSCH data received (rv=0)
Subframe n+8: NACK sent, NO UL grant is signaled on PDCCH
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Radio Analysis UL – HARQ ReTx
Postponed reTx
PUCCH
PUCCH
PRACH
Radio Analysis UL – HARQ
Because of the synchronous nature of Uplink HARQ, the following scheduling priority is used:
Random Access Message 3 (RRC Connection Request). Scheduled 6 subframes before, special case.
Non-adaptive HARQ retransmission
Adaptive HARQ retransmission
New Data transmission
Non-adaptive means no UL grant is explicitly scheduled for the retransmission
Adaptive means that scheduling collision occurred (e.g. collision with PRACH) and an explicit UL grant was signalled to:
Move the allocated PRBs to another part of the UL spectrum
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Radio Analysis – Traffic Abnormal
TRACE1 in baseband is defined as TRAFFIC_ABNORMAL. It should be used to trace out abnormal conditions in baseband processing.
Normally the output gives a good description of the problem encountered
Some useful TRAFFIC_ABNORMAL traces:
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Radio Analysis – Post-Processing Tools
3GPP has specified L1 messages in order to reduce the bits required for transmission on the air interface.
These formats can be difficult to read
For this reason, many values in the traces are presented in formats which require conversion to human readable formats, for example:
PRBs allocated in DL/UL grant messages
PHR values
BSR values
MIMO HARQ feedback, etc..
Tools exist to perform these conversions and compact the data presentation to the end user
One such tool is bbfilter or scheduling_filter.pl
Check the flowfox web page for details
*
Radio Analysis – bbfilter Downlink
$ cat decoded_dl_log.log | ./bbfilterv2.2 -bw 5 –dl
sfn|sf|mode|dlModul|mcs1|mcs2|prb|Ndf|Tbs1|Tbs2|AssBits|Harq|dlBler|cqi|ri|
280| 4|TxDi| 64QAM | 16 | 0 |25 | Y|7736| 0|8771784| | | 11| 2|
280| 5| | | | | | | | | |A | 0% | | |
280| 6|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8764088|A | 0% | | |
280| 7|TxDi| 64QAM | 18 | 0 |25 | Y|7992| 0|8756144|A | 0% | | |
280| 8|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8748192|A | 0% | | |
280| 9|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8736760| | | | |
281| 0|Mimo| 16QAM | 12 | 12 |25 |Y Y|4968|4968|8737384|A | 0% | | |
281| 1|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8763568|N | 0% | | |
281| 2|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8776208|N N | 2% | | |
281| 3|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8800856|N N | 4% | | |
281| 4|Mimo| 16QAM | 13 | 13 |25 |Y Y|5736|5736|8825504|N N | 6% | | |
281| 5|TxDi| 16QAM | 30 | 0 |25 | N|7992| 0|8862160|N N | 8% | | |
281| 6|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|N N | 10% | | |
281| 7|Mimo| 16QAM | 30 | 30 |25 |N N|5736|5736|8862200|A A | 10% | | |
HARQ ACK/NACK refers to the transmission 4 subframes earlier!
NOTE: Format modified to fit on slide, only example!
*
Here we present an example of trace parses.
This example is a modified output of bbfilter –dl which summaries a number of traces onto one line.
Note how RI=2 is reported then transmission changes from TxDiversity to MIMO.
BLER rates are approximate values.
Radio Analysis – bbfilter Uplink
$ cat decoded_ul_log.log | ./bbfilterv2.2 -bw 5 –ul
sfn|sf|rxPwrPus|prb|ulTpc|sinr|ulModul|mcs|ndf|ul bsr |phr |ul tbs| ul crc |har|ulBler|
266| 6| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 2% |
266| 7| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 2% |
266| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| N | | | 25456| ERR 3182| N | 5% |
266| 9| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 24496| | A | 5% |
267| 0| -95.7 | 48| 0:1 | 22 | 16QAM | 24| Y |>150000 | | 24496| | A | 5% |
267| 1| -95.8 | 40| 0:1 | 22 | 16QAM | 24| Y | | | 21384| | A | 5% |
267| 2| -95.6 | 48| | 23 | 16QAM | 24| Y | | | 25456| | A | 5% |
267| 3| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | | 25456| | A | 4% |
267| 4| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y | | | 25456| | A | 4% |
267| 5| -95.6 | 48| 0:1 | 22 | 16QAM | 23| Y |>150000 | | 25456| | A | 4% |
267| 6| -95.6 | 48| 0:1 | 23 | 16QAM | 24| N |>150000 | | 25456| | A | 4% |
267| 7| -95.6 | 48| 0:1 | 23 | 16QAM | 24| Y | | | 25456| | A | 4% |
267| 8| -95.6 | 48| 0:1 | 22 | 16QAM | 24| Y | | 32 | 24496| | A | 4% |
UL BSR and PHR values decoded
NOTE: Format modified to fit on slide, only example!
*
Here we present an example of trace parses.
This example is a modified output of bbfilter –ul which summaries a number of traces onto one line.
BSR and PHR are decoded into human readable formats along with PRBs.
BLER rates are approximate values.
Radio Analysis - Summary
CQI / RI (Rank Indicator) reported from UE.
Transmission Mode (MIMO, TxD, SIMO)
MCS vs. number of assigned PRBs vs. assignable bits in scheduler
UE Scheduling percentage of TTIs (how often is the UE scheduled)
PDCCH CFI and scheduling impacts
HARQ
BSR (Buffer Status Report)
Link Adaptation
*
These are the fundamental areas of analysis for downlink:
CQI and RI provides us the SINR/antenna layer reception reports from the UE point of view
Transmission modes 1 (SIMO),2 (Transmit Diversity) and 3 (Open Loop Spatial Multiplexing) are supported by the eNB in L11A.
Understanding the relationship between chosen MCS, assigned PRBs and assignable bits in the scheduler are important for sorting core network issues/UE issues from air interface issues
Scheduling percentage means the amount of TTIs (typically measured per second) that the UE was scheduled. This is also related to the resources allocated for PDCCH (control channels)
PDCCH CFI impacts the % TTI scheduling but also reduces the maximum MCS achievable.
HARQ will be presented using tracing. Check also the Uplink HARQ for more detailed theoretical explanation.
RLC retransmissions will be touched on briefly, including how to trace RLC status messages and some parameters involved.
Transport Analysis
Transport Analysis
Several Transport Network topologies (L2/L3) provide great flexibility in design
Several router redundancy methods are supported
Transport network dimensioning provides insights into the peak provisioning on the S1 link
The LTE RBS is a QoS enabler, providing end user and transport network QoS differentiation
*
Transport Topology
*
This diagram shows the different transport network topologies supported in the LTE RAN.
RBS A is connected directly to a Layer 2 network (Net A) which also provides direct connectivity to the SGW and MME. Connectivity to the OSS-RC O&M router R3 is achieved via router R7.
RBS B is connected to a Layer 2 network (Net C) and is routed to the core network and OSS-RC via routers R2 and R4.
RBS C is connected to a small on-site Layer 2 network and has its first hop router R6 at the RBS site.
RBS A, B and C have only Layer 3 connectivity to each other. Other RBSs not shown in the figure may possibly have Layer 2 connectivity to each other.
The network must be designed so that only one IP address exists for the next hop router.
This is achieved by placing a router between the RBS and the MME and SGW pools.
In the case of multiple next hop routers between the RBS and the MME and SGW pools, Router Path Supervision (RPS) can be used or the routers can share one IP address using the Virtual Router Redundancy Protocol (VRRP), for example.
The same setup is required for OSS-RC, if COMINF is not placed at the same site as the SGW and MME.
Transport Topology
No strict requirements on using a L2 switched or L3 routed LTE RAN transport network
No specified topology requirement
A router is required in the network, but LTE RAN transport network does not have to be L3
Network design is important (number of hops for L3 vs. size of broadcast domain for L2)
This topology flexibility could complicate troubleshooting efforts depending on the nodes involved (say 3PP support is required)
*
The RBS network does not need to follow a topology method like full meshed, partial mesh or point to point.
This ensures that the network is flexible and allows for expansions using any topology methodology.
When using a Layer 2 Transport Network (such as Carrier Ethernet), a router is still required at the CN sites.
The transport dimensioning then plays an important role in ensuring end user data rates.
QoS is also important considering the different types of traffic supported in the network.
Transport configuration
A 2 VLAN configuration is recommended (separating O&M and Transport):
O&M VLAN
The DU supports the following Internet Protocol (IP) logical interface configurations:
One IPv4 interface for S1, X2, SoIP and O&M IP traffic
Two IPv4 interfaces (on the same GE port): one IPv4 interface for S1, X2, and SoIP traffic, and one IPv4 interface for O&M traffic.
VLAN configuration is required when deploying more than one IP logical interface on the same GE transport network port.
When applicable, each IP interface must be configured on a separate VLAN.

Router Path Supervision (RPS)
*
RPS is a CPP function (it can be used in products that have CPP as a platform).
The optional Router Path Supervision function supervises the IP (layer 3) connections towards a number of configured routers and decides which one should currently be used as the default router. I.e. it provides a default router redundancy function.
The router redundancy may be handled by the routers themselves using a router redundancy protocol, e.g. Virtual Router Redundancy Protocol
(VRRP). In case the routers are using some redundancy protocol, the RPS function may have to be turned off in order to avoid interference.
If RPS is turned off, router 0 will be used as default router.
Up to three default routers per IP interface can be supervised. This is done by periodically sending ICMP Echo Requests (with TTL = 1) and a unique IP id field of 0xFFFF to all the routers and awaiting an ICMP Echo Reply in return.
The default prioritization of the routers, in descending order, is:
• Router 0 (first preferred default router)
• Router 1 (second preferred default router)
• Router 2 (third preferred default router)
The time between echo requests, as well as the maximum time until the corresponding echo reply should be received, is configurable. An echo request not being replied to within the stipulated time is regarded as a failure, which results in measures as described below.
If the first default router is considered lost by the RBS, it will automatically switch to the next router (priority in order of assignment via attributes).
Note that this allows for redundancy only (not a hot standby solution).
Router recovery time is determined by when RBS notices the router/link is down to when another router is selected -> can be in the order of seconds.
The RPS function is fully configurable via the IpInterface MO (num of failed pings, max wait for ping reply, num pings before ok, etc).
virtual router redundancy protocol (VRRP)
LTE RBS supports VRRP (a router redundancy protocol)
VRRP uses an election method to assign responsibility for a virtual router to one of the VRRP routers on a LAN
The Master VRRP router controls the IP address(es) associated with a virtual router and forwards packets sent to these IP addresses
If the Master fails, one backup VRRP router will act as the virtual router
LTE RBS is transparent to the process, it does not directly participate in VRRP
Master
Backup
eNB
eNB
eNB
*
Either RPS or VRRP can be used, but not both router redundancy features at the same time.
Hence RPS must be turned off in order to use VRRP.

Transport Dimensioning
Dimensioning of the northbound transport network will impact achievable end user throughput rate
LTE RBS transport network dimensioning process (mobile backhaul):
Dimensioning is based on payload only!
Determine bandwidth needed for last mile
Determine cell thpt in a loaded network and avg. cell thpt during busy hour
Calculate agg. bandwidth required in mobile backhaul
*
As assumed, the Radio network inputs are required when dimensioning the transport network.
The dimensioning process includes:
1. Determining the bandwidth required for the last mile to the eNodeB by using selected cell peak rate and the transport overhead to calculate the required bandwidth.
2. Determining the values of Average cell throughput during busy hour and Cell throughput in a loaded network. The values can be based either on input from the radio network dimensioning or from simulations.
3. Calculating the aggregate bandwidth required in the mobile backhaul, using bandwidth requirements for the last mile and variants of cell throughput.
Signalling for S1 and X2, together with operation and maintenance data, generate a relatively small amount of data and are not considered when dimensioning the backhaul.
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Ericsson LTE Throughput Troubleshooting Techniques.ppt

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