Sunday, 4 October 2015

FDD-eNodeB-FL-LTE FDD uplink throughput troubleshooting

 Problem Cause Analysis
The general troubleshooting strategy is described below and the covered reasons for bad throughput are shown in the figure below. 
Figure 1. Low Throughput causes in the Uplink for LTE networks.
Step 1: Identify cell with low UL (uplink) throughput
a) The first thing is to identify those cells with low throughput. What is considered as low throughput is a threshold defined by your network policies and best 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 Uplink interference
a) Run a report for RSSI in the uplink. Most equipment suppliers provide with counters and or tools to assess the RSSI in a span of days. Cells with uplink interference are those whose RSSI values are high (higher than -90dBm, for instance).
b) Typical scenarios where these values are high are indoor environments (i.e. airports, etc.)
c) Common sources of interference in the 800 MHz band are: high values of traffic in the uplink, external source of interference, high values of P0-nominalPUCCH and P0-nominalPUSCH .
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 uplink interference, bad coverage (holes in the network, etc.)
Step 4: Low Demand
a) Run a report using the counters provided by the supplier 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 5: Scheduler Type
a) Find the scheduler types your supplier 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. Usually you may switch the scheduler in your network but recommend one in particular.
c) The wrong scheduler may be the reason for bad throughput.
Step 6: Power Headroom
a) Run a report to find out the average power headroom that UEs have in your network.
b) A low value of power headroom means that UEs do not have available power to transmit in the uplink and hence, the throughput is low.
c) Low values of power headroom are 5 dB or smaller.
d) Typical causes of low power headroom are uplink interference and/or incorrect power control parameter settings, to mention a few.
Step 7: Other
a) High values of VSWR result in low throughput due to losses.
b) Check your backhaul capacity. Often times, the backhaul links are shared among multiple RATs. Make sure your backhaul is properly dimensioned.
 Summary and Notes
At the end of this methodology, you will be able to determine if the reasons for low throughput in your cells is one of the following or a combination, thereof:
- BLER (bad coverage)
- Uplink Interference (high RSSI)
- Low Power headroom
- Scheduling algorithm
- Low Demand
- Other (VSWR, Backhaul capacity)

Budi Prasetyo

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