Friday, 16 October 2015

LTE Drive Test Compare Simulation

Incident Description (Incident Phenomena)
How to compare the difference between FDD-LTE simulation result and actual network?

 Problem Cause Analysis
How much is the difference between FDD-LTE simulation result and actual network is always concerned by the Mobile Communications Operator. The Operator care about the predictive accuracy, and the budgetary of the equipment investment obtained by the simulation. After all the operators concerned about the accuracy of the simulation, namely whether the coverage quality of the network simulation results can match the actual basic needs of the network coverage. Therefore, the simulation and drive test Compare project comes out.  

Compare indicators should choose RSRP

The Compare analysis is only Compare RSRP index, don't Compare the SINR. Because there are currentlly interference between unicom and telecom network and the different network load between the simulation and the actual network, the simulation is set at the 50% load condition. When the Compare is SINR index, the deviation is very large, because of the Atoll interference calculation is different from actual network, Atoll interference calculation is via calculating the number of RB conversion interference energy sent by neighboring cell, and actually can't get the RB number send by adjacent cell each time instantly used for modeling when driving test is performing, which cause the deviation between Atoll interference calculation and the actual network.
In LTE Network, RSRP is a basic parameter of RS coverage, this parameter can reflect the status of network coverage, It is not affected by the site opened or not and the influence of load level in the test area around, this Compare will use the indicator to analysis the similarity between the simulation and drive tests.

Comparing theoretical basis

Comparing actually is a calculation and analysis process of comparing the difference between simulation link model and drive test link model. Simulation link model simulate the actual scene, there is deviation with the actual scene. Drive test can accurately feedback actual scenario covering quality, however, it can only represent the instantaneous indicator of specific kind of terminal in car.
To obtain accurate results without deviation, first of all, with a focus on ensuring that the drive test network engineering parameters and simulation network engineering parameters is completely consistent, but the drive test network engineering parameters and simulation network engineering parameters is not always consistent, this needs careful engineering parameters check to maximize these errors, thereby, eventually complete Compare verification test. In fact, Engineering parameters check is the process of recheck each link engineering parameters of wireless signal from the main equipment to terminal.

Engineering Parameters Check

Engineering parameter Used by simulation and actual execution may be inconsistent, we need through original engineering parameter verification, site survey etc. to maximum reduce error from the this part. Engineering parameters including the cell opening situation, the cell antenna RS reference signal power setting, antenna location, antenna height, azimuth, mechanical downtilt, electronic downtilt, antenna gain,  feeder loss, loss of combiner, etc. Get the actual installation and opening engineering parameter results is the key of the comparison work, this piece of work need to occupy more than half or more amount of the simulation and drive test comparation work. The content of need to be carefully verify are:

(1)           Due to certain reasons, some sites or sectors has not opened when drive test is performing, so need to get the real-time network cell opening situation from the network management unit.
(2)           Because some cells may have power adjustment, some sectors reference signal power setting data are likely have been changed. we need get the real time reference signal setting.
(3)            When using high precision three-dimensional simulation, the antenna location accuracy become more important. Due to GPS has own existence error, as well as Operator provided latitude and longitude are usually the center of the entire site, the different sector antenna of the station is located in different places of the building, so we need to obtain the real antenna position and adjust it in the simulation tool.
(4)            Antenna height is also a very important basic data, antenna height, and whether the location of the antenna in the building can match with the electronic map building is also important. When site survey is performing, height tester is recommended to give a more accurate antenna height.
(5)           Antenna azimuth requires accurate, error should be not more than 5 degrees, especially to avoid antenna misplace construction problem, etc.
(6)           Antenna mechanical downtilt should to be accurately measured by slope instrument, not more than 1 degree error is allowed.
(7)           Antenna electronic downtilt need precise measurement, error requires no more than 1 degree.
(8)            Antenna gain and field strength in different azimuth is also very important, it is best to obtain the actual antenna file.
(9)           For estimating the relatively accurate feeder loss, need to get the feeder type and length  from base station equipment to the antenna port.
(10)       Some antenna is combined with 2/3G antenna, thus increased the combiner, introduced the combiner loss, these data also need to be counted.

CW M       CW  Model calibration advice

Erection of the CW antenna and precise propagation model calibration can be drastically reducing the deviation of simulation results compared with drive test results. Because, the CW model calibration itself is use drive test results to calibrate the simulation model. It is better that the scene of simulation and drive tests for comparing is the CW model calibration scene, this can also greatly reducing the gap between drive test results compared with the simulation results.

Drive        Drive test data acquisition and processing

Some matters need to be attention in the drive test data acquisition
Avoid repeat route.
Data acquisition shall be suspended when parking.
For receiver antenna is inside or outside, set corresponding receiver loss.
Fast fading and the signal fluctuation will lead the test result has a certain randomness, thus, when drive test data export, in order to eliminate the effects of fast fading, CXA using geography average features to process  measured sampling points data and deal with the following ways:
Remove blank value sampling points.
Remove invalid values sampling points (RSRP >= - 141dbm)
 Remove sample point outside the area.
Remove sample point indoor for GPS error.
 Summary and Notes
From the above content, the simulation and drive test compare analysis is a continuously process of eliminate the deviation which is introduced by all aspects of basic data. It need accurate consistency check of comprehensive data and key data.
Ensure the accuracy of the engineering parameters, the simulation result and drive test result can achieve basic match, can draw the conclusion of using simulation forecast network coverage has strong guiding significance.

Budi Prasetyo

About Budi Prasetyo

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