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Thread: Limit to densification

  1. #1
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    Limit to densification

    Bear with me, folks. This is a long one.

    So we all see posts where some markets are heavily densified (New York, Chicago, for instance). This provides excellent data experiences in those areas, and the principle is simple: Even though VZW does not necessarily have more spectrum in those areas, the densification forces a smaller number of people to latch on to an individual sector. This reduces air interface congestion. The spectrum is the same, but you're sharing it with fewer people.

    However, there have to be limits. Since each small cell (or even macro) is using identical spectrum, they can only stomp all over each other so much before you're creating more interference than it's worth. So what are those limits?

    I would assume a small cell on literally every corner of a downtown area blasting out band 4 is going to create a ton of problems. There would be large areas in the middle of every block where there is a ton of co-channel interference. In the real world, how is this handled? Do bands get split in half and spectrum alternates between each small cell? Is there a particular limit to small cell placement? Do they simply have to alternate between bands 2 and 4?

    Thanks for any help you can offer.

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    There definitely are limits, but you can Tx power and angle differently and use many other strategies to reduce interference like they do on a larger macro grid but on a smaller scale. Here are two possible techniques if interference is of concern.


    Galaxy S7 - Carrier Aggregation Band 30 10 MHz PCC + Band 4 10 MHz SCC1 + Band 12 10 MHz SCC2
    iPhone 7 - Carrier Aggregation Band 4 10 MHz PCC + Band 2 10 MHz SCC1 + Band 12 5 MHz SCC2

    -

    Intel iPhone 7 Plus- AT&T Unlimited Plus

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    Right, so that image seems to confirm the problem I thought. Everyone wants fast, fast, fast, so we've got all of band 4 deployed in a market. That means it's really not possible to deploy a small cell on a different frequency. We've already used it all on the big macro up the street. So the only choice is to interfere. However, your point is well taken. If we beam tilt and cut power, we can greatly reduce interference...albeit at the cost of also greatly reducing the geographical coverage of the small cell.

    Maybe that's exactly what all small cells have to do. I just don't have much experience with them.

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    There are seemingly an infinite amount of variables involved in answering this question, I certainly don't have any answers. However the 3GPP LTE spec(s) deal with this via ICIC (Inter Cell Interference Coordination), eICIC (enhanced) and FeICIC (Further Enhanced). If you have the time there's lots of info to be had by googling "LTE icic" etc. Unfortunately there are lots of "scholarly" articles that get very technical very quickly but may be worth your time.

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    Thank you. I will definitely do a search and see if I can wade through it.

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    Quote Originally Posted by weskeene View Post
    Do bands get split in half and spectrum alternates between each small cell?
    This was done with 2G/GSM because it couldn't handle co-channel interference. Each nearby cell had a different ARFCN (or different channel), which meant different parts of the spectrum was used. Both UMTS and LTE minimized this issue, because now you will see the same UARFCN and EARFCN on adjacent site/neighboring cells. I remember, that 2G/GSM systems used BCCH or broadcast/pilot/paging channel but I don't remember if that changed when you moved between cells.

    In the old 2G/GSM days, even traveling 1-3 miles will cause my phones to switch to a different ARFCN which was clue you changed cells.
    Today, going that same distance doesn't change the UARFCN and EARFCN.

    Is there a particular limit to small cell placement?
    I don't think there's a limit to small cell placement, when it comes to spectrum usage. More like physical and CAPEX limits.

    I say this, because I have been in underground metro stations and international airports where there's full LTE/HSPA coverage and these are stations/airports are quite a walk. How about the installation of small cells across an subway line?

    Do they simply have to alternate between bands 2 and 4?
    With both UMTS and LTE there's no need to alternate between B2 and B4 or assigned different UARFCN/EARFCN for adjacent site/neighboring cells. However, wireless operators never needed to alternate bands when covering large areas, whether small or wide. The only thing they did was assign different ARFCNs to avoid co-channel interference in 2G/GSM systems.

    Post #4 pretty much explains how UMTS/LTE systems minimize co-channel interference when you use the same UARFCN/EARFCN for the licenses you hold.
    Last edited by i0wnj00; 10-11-2017 at 10:51 AM.

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    Based on some research I was doing, it would appear that Almost Blank Subframe may be the most popular technique right now for minimizing co-channel interference. Do you have any input on that?

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    Quote Originally Posted by weskeene View Post
    Based on some research I was doing, it would appear that Almost Blank Subframe may be the most popular technique right now for minimizing co-channel interference. Do you have any input on that?
    It's been a few months since I did my searching/researching, I actually jotted down a few notes because the concepts were a little too large for my brain.

    Anyway sure does look like ABS and RP-ABS (Reduced Power) seem to be the winners for interference control on data channels. I believe data channels occupy the majority of an LTE "signal" so that's where they've looked first. I noted there were possibly other techniques for control channels but I don't know what those are.

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