Tuesday, 22 September 2015

Planning & Optimization Solution for TDD-LTE-Solving LTE Network Congestion

Incident Description (Incident Phenomena)
During the football match occasion on Feb. 26th, the background monitoring personnel located that heavy data congestion occurred within the stadium. To be specific, the main indicators are listed below:
You can infer from the congestion indicators shown in the previous table that the main traffic congestion occurred in cell 1, cell 4, and cell 5 of the indoor distribution network site, in which, most severe traffic congestion happed in cell 4.
 Networking Environment
This stadium is covered by section 2 and the point at the west gate of the stadium, where two distribution network sites are located. The congestion points gathered at the grandstands. 
The cell planning at the grandstands are shown in the figure below:
As shown in the figure above, section 9 and section 10 represent the rostrum area, whose network was covered by one cell. The coverage cells for all sections are describe below:
For the other areas, the commissioning was not implemented due to engineering construction.
 Problem Cause Analysis
After collecting the data of the congestion cell on the very night, the number of UEs that are activated at their maximum was huge during the congestion period. That is, at most 70 UEs were activated to their maximum in a single cell. An increasing number of UEs was the main reason to the congestion.
 When the test was executed on site, you would find that the on-site network was also faulty:
1/Unclear division of the primary coverage cell and the secondary coverage cell: As the grandstand of the stadium was open and that the radio signal propagation environment was severe, and additionally, the regular directional plate-type antenna was used for network coverage, the antenna lobe was relatively large and the side lobe suppression was relatively poor, causing an unanimous signal strength in the entire area of the stadium. According to the test results, sections 1, 4, and 5 of Stadium West Gate WE were the primary cell, causing the fact that most UEs of the stadium would access to the three sections as an advantage. Thus, a huge number of UEs caused the traffic congestion.
2/Sever intra-frequency interference: As the grandstand of the stadium was open, the signal strength difference between cells was minor. The isolation between cells was poor. As the field strength difference between the main coverage area and its neighboring cells was only 5 dB, interference would be severe when intrafrequency networking was applied, causing relatively low SINR values and thus affecting the network performance.

3/Single-stream networking caused poor network performance. The stadium used the R8972M192023 (RRU type), which is a dual-mode (TDS/TDL) RRU. This type of RRU can support only the LTE single-channel networking. For the antenna system, only a single-route system was provided. The single-stream LTE network performance decreased by around 1/2, compared with the dual-stream LTE network. 4/The coverage was insufficient. The stadium was only half covered. To be specific, a total of 12 cells were designed initially but the actual number of cells is 7. The area opposed to the rostrum was not covered. Therefore, the UEs at the grandstand across from the rostrum need to access to the network through other cells, causing overloading of other cells. Meanwhile, as the strength of signals from the other cells was identical, intra-frequency interference was severe, badly affecting the network performance.
As the hardware expansion relevant negotiation work is tough and that the negotiation cannot be completed before March 8th, the hardware expansion cannot be executed for the moment.
1/Improve the cell coverage area so as to equip each area with different main coverage cells. As the antennas cannot be adjusted, adjust only the power relevant parameters to control the cell coverage range. The power relevant parameters are adjusted below:

2/For adjacent cells, interfrequency networking is applied so as to reduce the intrafrequency interference and mode-3 interference. When the frequencies are modified, inter-frequency networking is achieved between adjacent cells. Meanwhile, mode-3 interference between cells can be prevented.

3/Enable load balancing between cells. Arrange as many cells as possible within the stadium so as to balance the load and to prevent the traffic congestion.
4/ Improve the coverage through addition of adjacent 3G cells. According to the 3G cell distribution on the 3G network side, add neighboring cells at the 3G network side.
5/For a long-term scheme, the coordination with the property management company is highly recommended in order to finish the commissioning of the planned cells.  
 Summary and Notes
 1/For the problem of high UE density, you need to check whether the cell coverage is consistent with the previous network planning through the refining and optimizing of the engineering and radio parameters. Prevent possible traffic congestion within the cell whose coverage range is excessive. 
2/ For an open scenario such as the grandstand, as the radio environment is relatively tough, defining the cell edge is also difficult. The UEs can be migrated to the other cells through the function of load balancing;
3/ During the sports events, the timeslot configuration can be accordingly adjusted so as to improve the UL capacity if the UL rate is demanding.

Budi Prasetyo

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