LTE: Issue on UE and NW side

Issue-1->In MMU, BW change related Issue Observed which was assign to L$$$$M team

Issue-2 ->DISCA Scell TP was not constant in RDU.

Actually on MAC perspective there is no any NACK DTX observed on MAC, but I see BO coming from RLC being fluctuated

-> From RLC log, We see that message id 0xd804 is received from Scell MAC to Scell RLC side and forwarded to GTP.

Mostly we need to verify if GTP received this message or if there are any drop or discard.

MAC->RLC->GTP->

as per RLC and MAC analysis Scell TP was 0 for long time around 5s to 10 sec, MAC observed that there is a lot of Scell deactivation happening for which reason is ISCA_BACKHAUL delay_calc is lost many times when sent from Scell. It doesn't reach Pcell which cause lot of deactivation and activation causing TP degradation observed.

Conclusion:

RLC has shared one debug patch for missing count to check GTP to MAC and MAC to GTP msg are sent.

Issue-3->  During TDD-TDD CA while SSF state is not similar between 2 cell(Pcell(2-5) and Scell(2-7) of same RRH then CRC346 is increasing.

Issue-4-> Reconfig of Victim UE was failing during scheduling with ABS(1,1,1,1).

Issue-5->  Random Access Fail in good Radio condition.

•            UE camped to LTE n/w.
•            RA Procedure is triggered for rrcconnectionrequest, 
•            UE sent RA Preamble and UE Receive RAR 
•            UE sent msg3ts but not received any response from network due to which backoff               happened.
•           It repeat multiple time random access procedure and RA procedure fail.

Issue-6-> DUT detect RLF due to max no of Retransmission.

Issue-7-> RLF Occurred due to RLC MAX retransmission.

• UE was attached to LTE n/w  
• Scell CC2/CC3/CC4 is added/activated Successfully
• UE receive RRCConnectionreconfiguration for 256QAM modulation
• UE sent rrcconnectionreconfigcomplete at index 0 but we observed that at RLC Ack where not received for RB33.
• Observed NACK during the process causing RLC Occurred
• RLF RB 33 vta 3   vts 10///vta also zero on RLC Layer

Issue-8->  Reason for Low DL Throughput

• BLER (Low coverage)
• Downlink Interface(Bad CQI)
• MIMO Parameter
• Scheduling Algorithm(RR, PFS, Qos)
• Low Demand
• CQI Reporting
• Other(VSWR, Backhaul Capacity
• 
  
• 1 - Poor coverage (BLER)
• 2 - Avaibility issue
• 3 - Uplink Interference
• 4 - A poor RACH deconding SR
• 5 - High error on S1 link.
• 6 - Delay on S1 link towards MME & SGW.
• 7 - Accessiblity issue.
• 8 - Handover failure.
• 9 - Lack of PRBs.
• 10 - Problemtic UE.
• 11 - Downlink Interference (Bad CQI)
• 12 - MIMO Parameters
• 13 - High VSWR
• 14 - High radio errors or instability issue.
• 15 - Maximum number of RRC connections active per cell
• 16 - Maximum number of users per TTI supported per cell.
• 17 - Core network, MME/SGW, etc
• 18 - Transmission insability & bottleneck.
• 19 - Incorrect parameter setting.
• 20 - Badly tuned handover parameters.
• 21 - High traffic on Cell limit THp
• Radio Analysis - Downlink
• 22 - CQI (Channel Quality Index) and RI (Rank Indicator) reported from UE.
• 23 - Transmission Mode: MIMO (tm3) vs. TxD (tm2) vs. SIMO (tm1)
• 24 - MCS vs. number of assigned PRBs vs. assignable bits in scheduler
• 25 - UE Scheduling percentage of TTIs (how often is the UE scheduled)
• 26 - CFI (number of OFDM symbols for PDCCH) vs. MCS vs. % scheduling
• 27 - HARQ
• 28 - RLC retransmissions
• 29 - Another cause of low (or lower than expected) throughput is that the UE is not being scheduled in every TTI. Packet loss can lead to (retransmissions, dropped in RBS, etc)
• Radio Analysis – Uplink
• 30 - Uplink scheduling overview
• 31 - BSR (Buffer Status Report)
• 32 - PHR (Power Headroom Report) – is the UE at maximum power?
• 33 - Cell bandwidth vs. maximum allowable PRBs
• 34 - Link Adaptation
• 35 - MCS available and 16QAM
• 36 - PDCCH SIB scheduling colliding with UL grant
• 37 - HARQ (less important, because we can measure SINR)
• 38 - QPSK cases in UL has increased from ~30% to ~40%. If more QPSK cases then there are more UEs in poor radio conditions

In this write up we will learn briefly general troubleshooting guidelines for downlink throughput in LTE networks with MIMO 2x2

 

The general troubleshooting strategy is described below and the covered reasons for bad throughput are shown in the figure below. Low Throughput causes in the Downlink for LTE networks.

Step 1: Identify cell with low DL (downlink) throughput
a) The first thing is to identify those cells with low throughput. This threshold is defined by your network policies and practices (it also depends on your design parameters). Reports should be run for a significant number of days so that data is statistically valid.

 

Step 2: Identify Downlink interference
a) Cells with downlink interference are those whose CQI values are low (an exception to this rule is when most traffic is at the cell edge –bad cell location-).

 

Analyze the CQI values reported by the UE for

          1.    Transmit Diversity

2.    MIMO one layer

3.    MIMO two layers

 

Typical values for transmit diversity oscillate between 7 and 8.

Typical values for MIMO one and two layers oscillate between 10 and 12.

b) If low CQI values are found after a CQI report is obtained, then downlink interference might be the cause of low throughput.

c) Common sources of interference in the 700 MHz band (LTE deployment in the USA) are: inter-modulation interference, cell jammers and wireless microphones

Step 3: BLER Values

a) Run a report for BLER in the cells identified. The BLER should be smaller or equal than 10%. If the value is larger, then, there is an indication of bad RF environment.

b) Typical causes of bad BLER are downlink interference, bad coverage (holes in the network, etc.)

Step 4: MIMO Parameters

a) Identify the transmission mode of your network. There are seven transmission modes as shown in the table below

b) Adjust the SINR thresholds for transition of transmission modes as recommended by the OEM. Request the Link Level simulations they used to set these thresholds and see if the conditions under which the values were calculated apply to your network. Otherwise, update them if the parameters are settable and not restricted.

Step 5: Low Demand

a) Run a report using the counters provided by the OEM to find

1.    Maximum number of RRC connections supported per cell (parameter or feature)

2.    Maximum number of RRC connections active per cell

3.    Average number of RRC connections active per cell

4.    Maximum number of users per TTI supported per cell (parameter or feature)

5.    Maximum number of users scheduled per TTI in the cell(s) of interest

6.    Average number users scheduled per TTI in the cell(s) of interest

b) If the maximum number of RRC connections active per cell is close or equal to the maximum number of RRC connections supported, then. The cause for low throughput is load.

c) A high number of scheduled users per TTI does not necessarily mean that demand is the cause for low throughput.

Step 6: Scheduler Type

a) Find the scheduler types your OEM supports

b) Select the one that is more convenient for the type of cell you are investigating. Examples of schedulers are: round robin, proportional fairness, maximum C/I, equal opportunity, etc. OEMs allow you to switch the scheduler in your network but recommend one in particular.

c) The wrong scheduler may be the reason for bad throughput.

Step 7: CQI reporting parameters

a) Check if your network is using periodic or aperiodic CQI reporting (or both).

b) Verify the frequency in which the CQI reporting is carried out for periodic reporting as well as the maximum number of users supported per second.

c) If the value is too small compared with the maximum number of RRC active connections, then, increase the values of the parameters CQIConfigIndex as well as RIConfigIndex (deal with in future blog).

d) If your network is not using aperiodic CQI reporting, then enable it.

e) Slow frequencies of CQI reporting might yield bad channel estimations that prevent the eNodeB from scheduling the right amount of data and Modulation and Coding Schemes to UE.

Step 8: Other

a) Run a VSWR report or ask your OEM to run it for you.

b) High values of VSWR result in low throughput due to losses.

c) Check your backhaul capacity. Often times, the backhaul links are shared among multiple RATs. Make sure your backhaul is properly dimensioned.

 At the end of this methodology, you will be able to determine is the reasons for low throughput in your cells  one of the following or a combination, thereof:

- BLER (bad coverage)

- Downlink Interference (Bad CQI)

- MIMO Parameters

- Scheduling algorithm

- Low Demand

- CQI reporting frequency

- Other (VSWR, Backhaul capacity) 

Issue-9-> Reason for Low UL Throughput

•  BLER
•  PHR Low
•  Scheduling
•  BSR

Issue-10-> CSFB Call is originated from DUT and after releasing the CS call, DUT failed to come back to LTE network and DUE stay in UMTS/2G n/w.

SIB 19.

Issue-12-> meas-id mapping wrong when eICIC and power control(OLPC) is enable and also Interfreq neighbour was configured.

Issue-13 ->

CDUT UDP UL -21% Vs Ref -18% Vs DU

Analysis:

---------

 

Test 1/Iteration 1:

 

CDUT-7360:

 

-- DUT camped to earfcn/pci:   1725/286:

-- radio conditions were moderate since it’s a far cell conditions rsrp/rsrq/snr( -108.0/-14.75/  2.0;)

-- UL MCS allocated was 8.7

-- PTM to FW level there is not drop in throughput

-- PTM bundle discards seen in DUT which is expected

-- no EPDCP discards seen

-- UL scheduling was around 58.9%

-- BSR reposted was 62

-- UL BLER was around 9% at MCS but high BLER seen at higher MCS

-- UL PRB allocated was 20.31

-- DUT was transmitting at max power all the time resulting in -VE PHR

-- MTU size was set to 1428

 

DUT-7480

 

-- UL MCS allocated was 10

-- UL scheduling ratio was 73

-- UL PRB was 20

-- UL BLER was 9%

-- camped to earfcn/pci:   1725/286

 

 

REF:

 

-- DUT camped to earfcn/pci:   1725/286:

-- radio conditions were similar to DUT

-- UL MCS allocated was 12

-- UL Scheduling ratio was 91.7%

-- UL PRB was around 18

-- REF was also transmitting at MAX power

 

 

 

Test 1/Iterations 3:

 

CDUT-7360:

 

-- DUT camped to earfcn/pci:   1725/286

-- UL MCS allocated was 5.8

-- PTM to FW level there is not drop in throughput

-- PTM bundle discards seen in DUT which is expected

-- no EPDCP discards seen

-- UL scheduling was around 97%

-- BSR reposted was 62

-- UL BLER was around 10% at MCS but high BLER seen at higher MCS

-- UL PRB allocated was 20

-- DUT was transmitting at max power all the time resulting in -VE PHR

-- MTU size was set to 1428

 

DUT-7480:

 

-- camped to earfcn/pci:   1725/286

-- UL MCS allocated was 7

-- UL scheduling was around 99

-- UL PRB was around 20

-- UL BLER was 9%

 

REF:

 

-- DUT camped to earfcn/pci:   1725/286:

-- radio conditions were similar to DUT

-- UL MCS allocated was 8

-- UL Scheduling ratio was 91%

-- UL PRB was around 18.7

-- REF was also transmitting at MAX power

 

 

Test 2/Iteration 1:

 

CDUT-7360:

 

-- DUT camped to earfcn/pci:   1725/286

-- UL MCS allocated was 5.8

-- PTM to FW level there is not drop in throughput

-- PTM bundle discards seen in DUT which is expected

-- no EPDCP discards seen

-- UL scheduling was around 97%

-- BSR reposted was 62

-- UL BLER was around 10% at MCS but high BLER seen at higher MCS

-- UL PRB allocated was 20

-- DUT was transmitting at max power all the time resulting in -VE PHR

-- MTU size was set to 1428

 

DUT-7480:

 

-- camped to earfcn/pci:   1725/286

-- UL MCS allocated was 6.32

-- UL scheduling was around 73%

-- UL PRB was around 20

-- UL BLER was 10%

 

 

REF:

 

-- DUT camped to earfcn/pci:   1725/286:

-- radio conditions were similar to DUT

-- UL MCS allocated was 7

-- UL Scheduling ratio was 90%

-- UL PRB was around 15.6

-- REF was also transmitting at MAX power

 

Conclusion:

 

UL MCS was better in REF and DUT as compared to CDUT

PTM UL Bundle discards seen in DUT

No discards seen at EPDCP,BSR reported good in DUT

UL MCS allocation was low resulting in low scheduling

High BLER seen at higher MCS

as part of EPHY_EMAC_ACTIVATE_IND    UL grant all HRQ process was very less(2-3 digit) though pending data  at RLC was good (4digit)

 

Though radio conditions and CQI reporting was similar in all 3 devices, UL MCS allocation and scheduled throughput was low in CDUT-7360 resulting in low throughput

 

Note : Discussed with LTE FW (Srikar)

 

Next Step:

----------

LTE SE to check why UL MCS allocation and scheduled throughput was low under similar radio conditions affecting the overall throughput