Drive Test

Drive Test is done to:-
  •  Troubleshoot the customer complaint
  • Diagnose other problems impacting customer.
  • Bench mark the other network parameters.
  • Reduce interference in the network.
  • Optimise the network.

Drive Test Tool
                                                                                             
Output of Drive Test tells you about:-
  • Unsuccessful calls.
  • Dropped calls.
  • Coverage
  • Poor received quality
  • Hand over indication
  • Hand over failure.
  • Failure signaling message


 RF Parameters

    1.  RX  Level  -10 to –120 dbm
      • -35 to -75
< -65 is Good
We can receive signal on road as well as in house.
·         -75 to -85
 Satisfactory (So-So).
We can receive signal only on road and not in house.
·         -85
Bad
The signal quality is very poor.                                                                                                                

    1. RQuality 0 to 7.
·         0 to 7
0 is the best. 
Rx quality depends upon Bit Error Rate BER.
BER                Rx Q
0 – 0.2             0
0.2 – 0.4          1
0.4 – 0.8          2
 0.8 – 1.6         3
1.6 – 3.2          4
3.2 – 6.4          5
6.4 – 12.8        6
12.8 – 25.6      7

Call without Hoping Rx Q should be less then 4.
Call with Hoping RxQ should be less then 5.

    1.  Speech Quality Index (SQI) -20 to 30.
·         -20 to 30
 Maximum value is good.
Speech Quality is dependent on Rx Quality and C/I ratio.
            
                                                     
    1. Speech Quality
·         1 to 5
 Maximum value is good for speech quality.


  Index                      Quality Scale
1                                                            Unsatisfactory (Speech not understandable)
2                                                            Poor
3                                                            Fair
4                                                            Good
5                                                            Excellent


    1.  
     Carrier to Interference (C/I) or Co-channel interference ratio.
·         -5 to 25 db
> 15 Good  
>  9 So-So.

    1. Carrier to Adjacent Interference Ratio (C/A)
>  23.


    1. Frame Error Rate
               It should be 0.

    1. Timing Advance TA
1 TA = 554m


´ T (bit) = (2d) ¤  c
Where T= Timing Advance
 C = vel.of light 3´10^5 m /ms
T(bit) = 1 ¤  270.833

Now d = T ( ( T (bit) ´ c )  ¤  2 )

                        =  T ( ( 1  ¤  270.833 ) ´ 3 ´ 10^5 )  ¤  2 )
           
Now after calc. d =  T ´ 554 m

Total TA is from 0 to 63.


Drive test before network is available for general public. The signal strength from all BTS is measured at each geographical locations.


SD DROP
1) Bad Radio link quality (any sort of interference, if highly destructive)
2) Non availability of TCH timeslots.
3) could b a problem at NSS end.
4) hardware problem.
etc. ..

1*1 & 1*3 Solution

In 1*1 hopping all sectors uses Same MA list and 1*3 hopping all 3 sectors uses three different MA list

1*3 is also called discrete hopping
in which no of trx = no of MAL LIST

Handover failures other than Co-Bcch and Neighbour

1. Weak cell boundaries,
2. Congestion on target cell,
3. BCCH and TCH interference,
4. Too many neighboring cell, and
5. incorrect handover parameters

 

difference of RX full and Sub

simple definition is

RX Lev Full :- DTX is OFF
RX Lev Sub :- DTX is On

detailed

RX Lev Full:
is nothing but the Mobile transmit the measurement report(SACCH multiframe) for every 480ms. this multiframe containes 104 TDMA frames, in 104 TDMA frames 4 TDMA frames for Decode the BSIC and remaining 100 TDMA frames for Average measurement of serving cell and neighboring cell

This average measurement of 100 TDMA frames are RX Lev Full

RX Lev Sub:

DTX is a discontinuous transmission, When the mobile conversation 40% of the time either Transmitter or Receive is idle. When DTX is ON, DTX will switch off the Transmitter or Receiver when they is no speech Pulses. only few TDMA frames will trasmit, the average of this TDMA frames is called RX Lev Sub, give you proper measurment of RX level

Separation b/w Antenna should be minimum 3Meters.


C1 & C2

C1 is for cell selection and C2 is for celll reslection.


(when mobile is switched on, it calculates C1 first. Then after recieving neighbour cell information, it calculates C2 based on the strongest serving cell.)



c1 is used for cell selection & parameter C1 to determine
whether a cell is suitable to camp on.
C1 depends on 4 parameters:
Received signal level (suitably averaged)
The parameter rxLevAccessMin, which is broadcast on the
BCCH, and is related to the minimum signal that the operator
wants the network to receive when being initially accessed by
an MS
The parameter msTxPwrMaxCCH, which is also broadcast on
the BCCH, and is the maximum power that an MS may use when
initially accessing the network
The maximum power of the MS.
c2 is used for reselection & encouraging MSs to select some suitable
cells in preference to others
Cell re-selection is needed if:
Path Loss criterion C1 < 0 for cell camped on, for more than 5 sec
There is DL signaling failure
The cell camped on has been barred
The is a better cell in terms of C2 criterion

HSN – MAIO:
HSN (values 0-63) is basically an Algorithm that assigns frequency to the cell from block/list of frequencies... Assignment of frequency from the list whatever HSN value is totally random/algorithm dependent (HSN=0 being cyclic)... normally HSN assigns frequency after each TDMA frame (4.615msec)... this hopping rate is changable...

MAIO is used as an offset from the frquency, assigned by HSN, to avoid co/adjacnt frquency clash on the samecell/cosite cell... In b/m exmple as well we have atleast a difference of 2 in MAIOs to avoid frquency clash...

Taking 1x1 example...

We hve a three sectored site having 1x1 implemented A,B,C... Each hving 4 TRXs, TRXs A1,B1,C1 being BCCH TRXs

Lets say we have b/m list of frequencies in 1x1 hopping pool...
f1,f2,f3,f4,f5,f6,f7,f8,f9,f10,f11,f12,13,f14,f15,f16,f17,f18

Lets suppose HSN=17 for three cells of the site

TDMA______1____2___3
frame

freq assgned
by
HSN_______f2___f11__f9 (suppositions)

TRX_MAIO
A2___0____f2___f11__f9

A3___6____f8___f17__f15
__________ ^ f8 is assigned which
is 6 blocks ahead of f2 because offset
(MAIO) is 6... so on...

A4___12___f14___f5__f3

B2___2____f4___f13__f11

B3___8____f10___f1__f17

B4___14___f16___f7__f5

C2___4____f6___f15__f13

C3___10___f12___f3__f1

C4___16___ f18___f9__f7

in above mentioned example hopping TRXs have hopped on to 3 frquencies during 3 TDMA frames without any freq clash...

In 1x1 HSN is same for cells of same site... Sites in close vicinity are given different HSN values to avoid assignment of same frequencies...

Multiple coverage and Call muting


1) Increase the HO Margin from serving cell to target cells side only
2) Reduce the power of cell to whom serving cell is attempting to HO
3) Increase the RXP for Nokia System(For Example.-95 to -110)
4) Delete the unnecessary HOs those are not required
5) Reduce the power of Overshoots.
6) Try to make single dominant.

Call Mute:

At the time of Hand over failure and Reversion, Call can be in the position of muting or one sided.



TILTS:
tilts are of two different types...
electrical and mechanical
!). Mechanical tilt:- in this you bend the antenna mechanically without any change in the internal ckt.
The major lobe will become heart shaped in case of larger tilts and lead to unwanted distribution of signal in uncontrollable manner.

2).Electrical tilt:-
this is done with the help of phase shifters.
The phase of the feed(voltage) to the dipoles is changed further leading to change in the radiation pattern in a better manner.

Effects:-
Mechanical Tilt Causes:
Beam Peak to Tilt Below Horizon
Back Lobe to Tilt Above Horizon
At ± 90° No Tilt

Electrical Tilt Causes:
Beam peak to tilt below horizon
Back lobe to tilt below horizon
At ± 90° to tilt below horizon
All the pattern tilts


dB - Decibel
The Decibel is a unit of comparison, in which the ratio of two power values are expressed using a logarithmic scale usually to the base 10. Although the dB is a unit of comparison it is sometimes useful to have an agreed reference point. A common reference is 1mW, which is expressed as 0dBm. Consequently 2W, the typical maximum power of a GSM handset, is rated as 33dBm.


dBc


A ratio in Decibels of the sideband power of a signal, measured in a given bandwidth at a given frequency offset from the centre frequency of the same signal, to the total in band power of the signal

dBm

A measure of power expressed in terms of its ratio (in Decibels) to one milliwatt.

dBW


A measure of power expressed in terms of its ratio (in Decibels) to one Watt.