Co channel Interference

Since frequency reuse in the GSM is unavoidable, co-channel interference will beeasily caused when the reuse distance between two cells using the same frequencyis too small compared with the cell radius. The frequency reuse in many cases,according to the experience, should be avoided

For A~D BTSs shown in the above figure, suppose that frequency N is assigned for cell A-3, then frequency N cannot be assigned to A1, A2, B1, B2, B3, C1, C2, C3, D1,D2, and D3; frequency N±1 cannot be

assigned to A1, A2, A3, B1, C2, D1, and D2(without frequency

hopping).The case of co-channel interference is scarce, there are only one earlier case

ando n e c a s e i n l a b o r a t o r y f o r r e f e r e n c e b e s i d e s t h e

a b o v e c a s e o f c o - c h a n n e l interference that the antenna is connected inversely.

Fault phenomenon:

The co-channel interference of Huawei early 2.0 BTS (O2) incertain area leads to high call drop rate and poor voice quality. Serious interruptionwith occasional strong noise (whizz in general) occurs.

It is after the BTS’s

normalrunning for a certain time that the call drop occurs. Located in a little town

(Du city)o n t h e b o r d e r o f t h e c i t y , t h e B T S i s s u r r o u n d e d b y t h e B T S s o f t h e o t h e r manufacturer.

Fault location process 1)

The frequencies assigned for the BTS are 64, and 92 (64 is of BCCH frequency).

2)

I n t h e o p t i m i z i n g t e s t , t h e r e c e i v i n g

q u a l i t y ( q u a l i t y l e v e l i s l e s s t h a n 3 ) i s continuously good as the downlink signal level is -95dBm in the direction awayfrom Huanggang and Du City. In the direction from Du City to Huanggang, thereceiving quality is also good when the receiving level is more than -70dBm.

T h e n m o v e f o r w a r d u n t i l t o t h e p l a c e w h e r e TA= 5 , t h e r e c e i v i n g q u a l i t y i s s o m e t i m e s g o o d , a n d s o m e t i m e s m o r e t h a n 5 i n a b o u t 1 m i n u t e w h e n t h e receiving level is about

-75dBm. And network-drop occurs frequently when Idle-mode test is made at this place. It is suspected that downlink interference mayexist on BCCH frequency.

3)

Carry out continuous conversation test with one test MS and scan test for 64#frequency with another MS. The test carried out again in the section from Du Cityto Huanggang reveals that the signal strength of 64#

frequency is already lessthan -100dBm near the Huanggang, and call drop already had occured. But thesignal strength of 64# frequency rises up to -65dBm and disappears after aduration of 100 seconds when entering the downtown area of Huanggang. So itcan be judged that the co-channel interference may be from the TCH frequencyof cell nearby.

4)

Carry out scan test for this frequency after arriving at the hotel, the signal levelstill remains high, but the conversation is not implemented on this frequency.

Thenext day, carry out designated scan test at the place with the strongest signal of this frequency in the street of Huanggang, and test in Idle mode with another testM S . F r o m t h e s y s t e m m e s s a g e , i t c a n b e c o n f i r m e d t h a t 6 4 # f r e q u e n c y

i s a s s i g n e d t o t h e T C H i n H G 0 8 c e l l w i t h B C C H f r e q u e n c y 4 5 , a n d t h e conversation is actually established once on 64# frequency in multiple times of conversation tests.

5)

The interference disappears after the application is made to the customer tomodify the frequency. At the same time, the customer should adjust the adjacentcell relation of peer equipment.

6)

Carry out test again at the place where the original co-channel interferenceexists after the frequency is modified, the call drop and network drop disappear,and conversation quality Rx_Qual<3. It indicates that the problem of co-channelinterference (downlink) is solved.The GSM is a duplex system, so the interface may occur on both the uplink anddownlink frequencies. Current Drive Test tools are unable to measure the co-channelinterference directly, the equipment such as SAFCO, and ANT with so-called C/I testfunction actually test the C/A. And in addition, the Drive Test equipment can onlymeasure the downlink frequency and nothing for the uplink frequency.The co-channel interference in downlink frequency can be confirmed by using currentDrive Test equipment to measure it indirectly. First, lock the test MS in the cell of theservice area to carry out Drive Test of 2~3 hours in continuous conversation mode.

If t h e r e c e i v i n g s i g n a l

i n s o m e a r e a s i s a r e l a t i v e l y h i g h ( e . g . , - 8 5 d B m ) w h i l e t h e receiving quality is very low (e.g., RxQual>4), it is quite possible that co-channelinterference exists in this channel.The interference of uplink can be judged with the help of interference band statisticsdata in traffic measurementCase 0008119

Fault phenomenon:

It is found in the training equipment room some day: it is hardfor the MS to access the network in BTS30 under BSC2. Even it accesses then e t w o r k n a r r o w l y , i t w i l l d r o p s o o n . T e s t M S s h o w s t h a t t h e s i g n a l e x i s t s intermittently and the receiving level is very high (about -50dBm).

Fault location process

1)

The correctness of the data configured by the trainees cannot be ensured in thetraining equipment room. So load standard data (which is tested to ensure thatcall can be connected) for BSC2, the fault still exists, so the data problem can beexcluded.

2)

For the fault phenomenon, it is quite possible that fault occurs in the carrier board. The fault still exists after replacing the TRX.

3)

C h e c k e a c h p r o c e s s r e l a t e d t o t h e v o i c e f o r t h e B T S : T M U , D I P s w i t c h

backplane cable, etc. No problem is found, so the BTS is all right.

4)

Then focus is turned to the BSC. a BTS20 instead of BTS30 and change thedata into BTS20 (in this way, overall check of BSC can be performed).And thecall is set up easily , so the BSC is all right.

5)

At this time, it seems that there is no way out, but restore the BTS30, however,the MS can call again after power-on loading.

6)

It seems that the problem is solved but the cause is still not located. Carefulconsideration from the beginning to end reveals that all data of BSC2 and BTS30are not modified, but BTS20 is switched off during BTS30 rollback. Think aboutthe phenomenon of the test MS: the signal exists intermittently and the receivinglevel is very high (about -50dBm), it is possible that BTS20 works interference onBTS30?

7)

Since BTS20 is in BSC1, check the data of BSC1. It is found that the

frequencyof BTS20 is set to be the same with that of BTS30. And the BSC2 data (BTS30),configured by the trainee, is loaded in BSC1 after it is changed into BTS20,leading to the same frequency of BTS20 and BTS30. The BTS30 is connectedwith the attenuator, and the signal is far weaker than that of BTS20, so the MScannot access the BTS30, while can

access the BTS20 . In this way, you tend tothink that it is the fault of BTS30, but the truth is hidden herein.

8)

The problem is solved after the frequency of BTS30 is modified.