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Downlink Interference

 

I want to talk a little bit about Interference between multi cells (Inter Cell Interference). In this page, I will show you some of the measurement result but don't pay too much attention to the exact measurement values. You would get different value in different situations or the value shown here would be different from the value measured by UE in live network. But general trend and characteristics of the interence would apply to most of the cases. You should be well aware of this kind of properties especially if you are working on test cases related to various mobility issues (Cell Selection, Reselection, Measurement Report, Handover, Redirection etc).

 

Followings are the general characteristics (tendencies) in intercell interference.

  • Interfence among Cells with the same frequency (Intrafrequency Interference) is much higher (more serious) than the interference among cells with different frequency. (Compare the result between Interfrequency and Intrafrequency measurement result shown below).
  • Within Intra frequency cells, the interference between cells with same reference signal location tend to be higher than the interference between cells with different reference signal locations.

 

< When the Interference come into play  ? >

 

Now the question is When we have to take this interference seriously ?.

The general answer is 'Always'. We have to take precautions anytime and as much as possible. I think The most crucial situation is at cell detection/selection/reselection stage because it would be the worst case if you power on the mobile phone and it fail to detect any cell due to the interference. To understand on possibile mode of interference, you need to know the detailed procedure/factors for this process. Following is what's happenning when you power on UE.

    i) Frequency Aquisition

    ii) Primary Sync Signal Aquisition (Slot Timing Aquired, Secondary Sync Signal Scrambling Code Aquired)

    iii) Secondary Sync Signal Aquisition (Frame timing Aquired, Cell Group ID sequence aquired)

    iv) with PSS and SSS, Cell ID can be calculated

    v) with Cell ID, Reference Signal Location is detected

    vi) With the help of Reference Signal, PBCH (MIB) can be detected

    vii) From MIB, SFN and System BW can be detected

    viii) Decode PCFICH and detect how many symbols are allocated for PDCCH.

    ix) Decode DCI for SIB1 from PDCCH

    x) Decode SIB1 and get the scheduling information for other SIBs

    xi) Decode SIBs (other than SIB1)

Now let's think of each of the steps and what might be the issues with those steps in terms of interference.

    i) Frequency Aquisition

    • Most of the frequency Aquisition is based on RSSI Scan for each subcarriers within all the bands that the UE is configured to support. Because of this mechanism, UE may have difficulties to find correct frequency if the frequency of two or more cells are in very close neighbouring location or the frequencies are partially overlapping.

    ii) Primary Sync Signal Aquisition (Slot Timing Aquired, Secondary Sync Signal Scrambling Code Aquired)

    iii) Secondary Sync Signal Aquisition (Frame timing Aquired, Cell Group ID sequence aquired)

    iv) with PSS and SSS, Cell ID can be calculated

    • There wouldn't be much problem if the frequency of cells are different (Interfrequency), but UE may suffer from inteference if the frequency of the neighbouring cells are same.
    • This inteference would be more serious if the reception of Sync signal from multiple cells are better aligned in time domain. (Even though they tried to design the sync signal interfering as little as possible, but still the sync signal is not 100% orthogonal).

    v) with Cell ID, Reference Signal Location is detected

    • This inferference would be especially serious among intra frequency cells.
    • The location of Reference Signal is primarily determined by Physical Cell ID and the Scrambling Code is also influenced by Physical Cell ID(PCI). So if the Physical Cell ID are different, there should be no interference for this reference signal. But in reality there would be a certain degree of interference whatever PCIs you allocate for the neighbouring cells.
    • Location of Reference signal gets same in some case even though PCI is different. (Refer to Downlink Reference Signal page if you are interested in details on this). So you need to very carefully assign PCI to each of the cells in such a way to minimize the interference.

    vi) With the help of Reference Signal, PBCH (MIB) can be detected

    vii) From MIB, SFN and System BW can be detected

    viii) Decode PCFICH and detect how many symbols are allocated for PDCCH.

    • This interference would be especially serious among intra frequency cells.
    • Location of PCFICH is mainly determined by System Bandwidth and Physical Cell ID (PCI). In most intra frequency settings, the neighbouring cells are using the same system bandwidth. So even though PCI is different, it is highly likely that there are certain level of overlapping of PCFICH location between cells.
    • Since PCFICH is located only at the first symbol of each subframe, this PCFICH interference would get less serious if the timing sync of the signal coming from neighbouring cell is not well synchronized.

    ix) Decode DCI for SIB1 from PDCCH

    • This interference would be especially serious among intra frequency cells.
    • If the location of DCI in PDCCH region is same or overlapping among multiple cells, UE would have more difficulties of decoding the DCI and as a result may fail to decode SIB1.
    • If the multiple cells are transmitting SIB1 in the same RB (Resource Blocks), the possibility of interferences would get higher.

    x) Decode SIB1 and get the scheduling information for other SIBs

    xi) Decode SIBs (other than SIB1)

    • This interference would be especially serious among intra frequency cells.
    • If the SIB transmission scheduling is same between neighbouring cells, the interference would get higher
    • If the multiple cells are transmitting SIBs in the same RB (Resource Blocks), the possibility of interferences would get higher

 

 

< Inter Frequency Interference between LTE and LTE with Varying Channel Power >

 

In this example, I setup two cells with different band (Inter frequency, Inter band) as follows. I used BTS1 as a serving cell and BTS2 as a neighbouring interfering cell. and I used a Vector signal analyzer as a kind of DUT and measured EVM (Error Vector Magnitude) detected by the serving cell.

  • BTS 1 = Band 4
  • BTS 2 = Band 17
  • Test Variable : Cell of BTS2 changes
  • Measurement : BTS1

As you see in the result shown below, there is almost no difference in the measured EVM at BTS1 regardless of the power of interfering cell. (In this example, the frequency of two cell is very far away from each other, so interference from the other cell is negligible, if the fequency of neighbouring cell is closer to each other, you would see stronger interference than in this example).

 

 

 

< Intra Frequency Interference between LTE and LTE with Varying Channel Power >

 

In this example, I setup two cells with same frequency (Intra frequency) as follows. I used BTS1 as a serving cell and BTS2 as a neighbouring interfering cell. and I used a Vector signal analyzer as a kind of DUT and measured EVM (Error Vector Magnitude) detected by the serving cell.  I set the BTS1 power to be fixed and increased the power of BTS2 step by step and checked how the measurement EVM varies.

  • BTS 1 = Band 4
  • BTS 2 = Band 4
  • Test Variable : Cell of BTS2 changes
  • Measurement : BTS1

Just by looking at the constellation, you can intuitively notice that EVM gets larger (constellation gets worse) as the interferering cell power gets higher.

The EVM result shown here is a little extreme case. According to this result, the DUT (Vector signal Analyzer) fail to decode the signal if the cell power  difference between two cell is less than 20. But in live network, the situation would be much better than this. Isolation between the two cell would be much better than this test environment and real UE (mobile phone) can decode signal much which is much worse than this since the chipset has channel estimation and use various error correction. However the point is that UE would experience pretty serious interference when it seems multiple cells with the same frequency around the UE.

You can see obvious difference (outstanding difference) if you compare this result with the previous case (Inter frequency case).

 

 

 

< Intra Frequency Interference between LTE and LTE with Varying Physical Cell ID (PCI) >

 

In this example, I setup two cells with same frequency (Intra frequency) as follows. I used BTS1 as a serving cell and BTS2 as a neighbouring interfering cell. and I used a Vector signal analyzer as a kind of DUT and measured EVM (Error Vector Magnitude) detected by the serving cell.

  • BTS 1 = Band 4
  • BTS 2 = Band 4
  • Test Variable : Cell of BTS2 changes
  • Measurement : BTS1

 

I set the PCI of BTS1 to be fixed and changed  PCI of BTS2 to various different values to how the measured EVM changes.  This is to check how the location of reference signal of the serving cell and neigbhouring cell can influence on the interference (If you are not sure about how PCI is related to Reference Signal location, refer to Downlink Reference Signal page)

 

As shown in the following result, when there is almost no inteference between two cells when the cell power of the two different cell is very large.

 

 

But the cell power difference between two cells is relatively small (actually this difference in this example is still very big difference in live network), you would see the DUT (Signal Analyzer) fails to measure signal when the PCI is configured in such a way that the reference signal of the two cells are same.