4G/LTE - MDT  

 

 

 

MDT (Minimization of Drive Test)

MDT stands for "Minimization of Drive Test".  Even if you have not experienced of performing any 'Drive Test', you may intuitively understand what it mean.

As you might have guessed, 'Drive Test' is a kind of testing in which you drive a testing vehicle around a certain route measuring various network performance (e.g, Cell Power, Interference) or UE performance (e.g, Call Drop, Throughput, Handover performance, Cell Reselection Performance etc). Usually during this period, you would collect a lot of protocol logs from UE and analyze by yourself or send them to developers when they have serious problems.

As the word MDT itself implies, MDT is a kind of special mechanism that may help minimize this kind of Drive Test.

Why do you want to minimize it ? Answer is simple.. 'to save time and money'.

The idea is .. in stead of dispatching test truck and engineers, let any UE around the interested area perform the all those measurement and store the result in the device and then report it later.

Now you may have following three questions and the answer to these questions would give you all the details of MDT mechanism.

How Network tell UE to perform measurement logging ?

As any other instruction, Network send special RRC message called 'loggedMeasurementConfiguration' to UE to trigger the measurement and logging on UE. For each of the information elements, you may refer to 36.331, but just looking at the name of the IEs, you may guess pretty accurate meaning.

What kind of measurement UE has to perform ?

The items to be measured and logged are a little difference depending on which call status UE are in (IDLE or Connected or MBSFN reception etc). Roughly some of following informations is supposted to be measured and logged.

  • Both RSRP and RSRQ for EUTRA
  • Both RSCP and Ec/No for UTRA FDD
  • P-CCPCH RSCP for UTRA 1.28 Mcps TDD
  • Rxlev for GERAN
  • Pilot Pn Phase and Pilot Strength for CDMA2000 if the serving cell is EUTRAN cell
  • MBSFN RSRP, MBSFN RSRQ, BLER for signalling
  • BLER for data per MCH

How Network can retrieve the log from UE ?

Last question is "How network can retrieve the measurement result logged in UE ?" or "How UE can transfer (report) the measurement result to Network ?"

Roughly this is done in three steps.

Step 1 : When I has any Logged measurement, it is supposed to report to network "Hey, I have a measurement result logged in me" via rrcConnectionSetupComplete message or UEInformationResponse message.

Step 2 : If network wants to retrieve the logged message indicated by the UE, Network can request it by sending 'ueInformationRequest' as shown below.

Step 3 : UE send the logged information via ueInformationResponse. (The UE can report max 520 measurement results using this single message. If it cannot report all the log in a single message, it can set LogMeasAvailable to be 'true' and NW can send the request message again if it wants.

Reference