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Microcell Antenna Heigth

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  • #49711 Reply
    Abdel
    Guest

    Hi everybody,

    I am a beginner in GSM RF optimization and planning. I would like to ask you guys I have planned a to split some area where traffic is very congested into several microcells. the area has so some buildings. As you know microcells antennas tends to be located below the roof level. For security reasons we have placed the antenna on the top of a building which is 12 meters high. Therefore how can I limit the coverage area of this cell since it has the same height of our macrocells. Do you think a great mechanical or electrical tilt may help or some kind of power output reduction made from the OMCR. I am waiting 4 ur answers. Thanks

    Abdel

    #49712 Reply
    pix
    Guest

    you forgot to tell us what is the vertical aperture (vertical HPBW) of the antenna !! 6, 8, 12 ??

    assuming the V-HPBW = 8°, a total downtilt of 12 degrees should help do the trick (the antenna coverage with 12 degrees tilt at 12m high is about 100m).
    With 10 degrees, you’ll have 120m theoretical coverage, but because reflections on buildings, it might go beyond 150m/200m.

    But don’t reduce the power unless you feel it is necessary. Micro BTS already has a low EIRP, and reducing it further more might impact the indoor coverage.

    Regards,

    #49713 Reply
    Abdel
    Guest

    Hi pix,

    Thanks for your quick answer. Well we are going to use low gain antennas with a Vertical of 55 and 28 degrees. Also what I am thiking of is a great tilt may cause a wider cell which may increase the overlap between the other neighbouring cells. By the way I am using MacroBTSs with a PA output power of 40W for microcells so I think reducing the power might be a good idea. What do you think pix?

    #49714 Reply
    pix
    Guest

    “low gain antennas with a Vertical of 55 and 28 degrees”

    that’s a bad choice to put this kind of antennas at 12m above ground.
    55° = you must put at least 30° tilt
    28° = you must put at least 17° tilt

    With less tilt than that, there is going to be overshooting and cell resurgence !

    i recommend you choose another antenna to install up there. The gain doesn’t really matter : you can always decrease the power of the BTS to obtain (in the end) a total EIRP of about 40dBm or 45dBm.

    “Also what I am thiking of is a great tilt may cause a wider cell which may increase the overlap between the other neighbouring cells.”

    I don’t understand this sentence ! If you increase the tilt, the antenna will point more towards the ground. Therefore increasedowntilt = reduce the cell range = reduce the overlap with neighbors.
    However, the cell you’re putting is a micro cell, it means that it will be totally overlaid by an umbrella cell.
    The overlap with the umbrella cell is 100%, whichever is the downtilt you choose for the microcell.

    Last thing : reduce the power will not work as good as increasing the downtilt. The priority is to put a high downtilt, to compensate for the high height.

    Pix

    #49715 Reply
    Abdel
    Guest

    Hi pix

    Well I have understood you very well and I have used the Kathrein scala calculator to see the coverage this caculator takes the input of the antenna height,V-beamwidth,and tilt and it will give you the range of the main beam, higher beam and lower beam. Yes the priority must be given to tilt to compensate the height of the tower. There are two kinds of tilt Mechanical and electrical. Great mechanical tilts may cause an overlap of neighbor cells of the left side and right side. because the strong signal will hit the ground and then reflects a strong signal to both the left side and right side increasing the overlap of both sides. While Electrical tilts may be better to use to avoid side overlaps. I tried to look for other kinds of antennas with a low gain and low Vertical beam width but all the low gain antennas have high vertical beamwidth. It seems that vertical beamwidth and gain are indirectly propotional to each other. reducing the gain antenna might increase the V-bmw.
    “The overlap with the umbrella cell is 100%, whichever is the downtilt you choose for the microcell.”
    but we have a lot of microcells close to each other about 100 meter so inorder to make sharp boundaries with minimul overlap so you may use a specific downtilt.

    Abdel

    #49716 Reply
    pix
    Guest

    Hello Abdel,

    You came to the right conclusions, but you got some facts a little bit wrong. I hope you don’t mind if I explain the “why” and “how” of your previous post ? It might of some help for other readers…

    The mechanical tilt doesn’t provok “bouncing” of the waves on the ground to the right and left : when you downtilt an antenna, you actually downtilt the front lobe.
    The problem comes from the side lobes and back lobe.

    You can imagine an antenna as a torchlight that sends light
    – from the front a lot
    – from the back a little
    – from the left and right sides a little

    When you tilt the torchlight’s front, there is an uptilt of the backlobe and there is no tilt of the side lobes.

    Backlobe is bad, because it will transmit your frequencies in the opposite direction, creating interferences with other BTS using this frequency.

    Sidelobe is “usually” less critical, but in your case… it is critical. The side coverage might go too far.

    So yes, I agree with you : choose an electrical tilt if available, but there are strong chances that this kind of big aperture antenna do not have electrical tilt.
    Anyway, electrical tilt can’t go further than 14°, as far as i know.

    Second point :
    an antenna is designed in such a way that
    1. high-gain antennas have narrow vertical aperture
    2. low-gain antennas have wide vertical aperture

    But this is fixed ! you cannot change the gain of an antenna by yourself. I you reduce the BTS power, you don’t reduce the gain of the antenna. The antenna gain will still be xx dBi, and the aperture will still be 55°.

    In my opinion, drop the installation of this site : you’re going to implement a microcell with the coverage of a macrocell… it’s going to bug your optimization for years 🙂

    Can’t you buy a small outdoor antenna you could install on a vertical wall, at 3 meters high ?

    -Pix

    #49717 Reply
    Abdel
    Guest

    Hi pix,

    Reducing the power output of MacroBTSs may involve a lot of changing parameters like Handover margin and Power Budget. Which may cause instability if not changed carefull so what is your recommendation. There are two options:
    1) Take low gain antennas with high aperature.
    2)Take high gain antennas with low aperature then decrease the power output.

    “But this is fixed ! you cannot change the gain of an antenna by yourself. I you reduce the BTS power, you don’t reduce the gain of the antenna. The antenna gain will still be xx dBi, and the aperture will still be 55°.”
    but if this is fixed why do antenna vendors still manufacture low gain GSM antennas. Sorry I have some doubts about it.

    #49718 Reply
    pix
    Guest

    abdel,

    Don’t worry, you can trust me about this, i really know what i’m talking about . I hope i’m not sounding too pretentious but that’s my job to know about this 🙂 anyway if i don’t know something then i would not pretend i do..

    the gain and aperture of an antenna are fixed characteristics.

    between your two solutions, i’d choose none of them ! i’d rather find another microcell site.

    but if there is no other possibility, i’d say choice 2 is better. Choose an antenna with around 9° vertical aperture and maximum amount of fixed electrical tilt (12° or 14°). This kind of antenna gives you a gain of about 15dBi and a size of about 1.0/1.5m.

    Install the antenna.

    Next step : measure the coverage. If the cell range is very much too large, then increase the downtilt by 2°.
    If the cell range is a little bit too large, then decrease the power.
    If the cell range is too short, decrease the downtilt by 2°.

    When you decrease the power of the cell, do NOT change the HO MARGIN, the power budget, or any other parameter : every radio algorithm uses a compensation in order to take into account the attenuation of the BTS TX POWER.

    no need to change any parameter if you put attenuation.

    if you don’t mind telling me, which country are you working in ?

    -pix

    #49719 Reply
    TNS
    Guest

    Hi friends
    I’m sorry to disturb your dialog… :-).
    I’ll express my self this way:
    the perfect micro cell – a DCS, micro BTS (4 or 8 W), integrated antenna, 4 m high, a hot spot area, a micro cell parameter set;
    the most terrible micro cell – a tower 40 m in the middle of the desert, GSM macro BTS 40 W…
    So what we got:
    DCS or GSM – I think DCS is better but doesnt mean GSM doesnt work;
    12 m high – tilt compensation;
    macro BTS – reduce the power to 8 W;
    antenna – my opinion is to choose a closer to the integrated antenna characteristics;
    a hot spot – you got it;
    a micro cell parameter set – you should got it;
    I’m agree with Pix, the tilt will be tuned impericaly… but at the beginning it should be = arctg(h/l) ( h=12,l=100 ) something like 6 deg … i think.
    Finaly I realy dont understand why you want to limit the overlapping…
    Anyway you will decrease the load of the macro cells which is you general objective.
    All overwritten is my personal opinion.
    I hope I was useful to you.

    Good look.

    B.R.

    #49720 Reply
    pix
    Guest

    Hello TNS,

    Thanks for your input, i hope to read more of your opinions here 🙂 It’s always great to mix different perspectives.

    Just one thing though, in your geometric formula, you forgot to take into account the vertical half beam power width of the antenna.

    “-3dB” point = arctg(h/l) + VHPBW/2

    with ( h=12,l=100 and VHBW = 28/2 or 55/2 … )

    #49721 Reply
    abdel
    Guest

    Hi guys

    Thanks for your answers and your advises. As I have mentioned before I am not an expert in RNO OR RNP either. Recently we asked from a aircom for a network consultancy and they have sent us a guy which I am not sure about his experience and knowledge. At least he was good on using Exel and some Kathrein antenna calculators plus the Asset3G tool for planning. He insisted that low gain antennas must be used and the one he mentioned was an antenna with a vertical HPBW of 55 degree and a gain of 8dbi. So there might be a possiblity that this guy was good software user rather than a RF engineer who have good ideas about RF.
    Anyway I am quite conviced with your ideas of microcellular antennas. Also I want to tell you that our Dual band network only uses two types of antennas:
    1)742 215 1800MHz
    2) 739 686 900MHz

    This antennas have high gain around 18dbi with a vertical HPBW of 7 degree.
    At the moment we don’t have any microBTSs which have integrated antennas. Yes DCS is much better for microcells since their coverage is less than the 900MHz.
    So this is what I have:
    I have the hotspot;
    I have a antenna height 12 m;
    I have the MacroBTS;
    I have the two types of antennas I mentioned above;
    But I don’t have the microcell params;
    From here what you can understand is how to tranform a macroBTS with the height of 12 m into a microBTS which have a coverage of 150 meters.
    Last but not the least do you think this antennas can work with a excellent performance on microcells?
    Hey I forgot to ask you too if you don’t mind where are working? me I am working in Somalia.

    Abdel

    #49722 Reply
    pix
    Guest

    Hello Abdel,

    Regarding the microcell parameters, it’s another problem. We can talk about this later.

    The “742 215 1800MHz” is a standard 18dBi, H 65° and V 7° antenna, right ? Isn’t it too tall to install on your current site ? Check if it is possible.

    Can you use it for a microcell ? yes, i guess, but that’s a wild shot.
    So let’s see.

    – Does it have electrical tilt ?
    If no –> forget about it

    – What are the average height of buildings surrounding the site (are they above the antenna height, or below ? From the top of the site, can you have a panoramic view, or do you have obstacles ?)

    If you can have a elec tilt of 12 or 14°, then do it. The EIRP of the sector would be :
    46.5 + 18 – 4 (losses) = 60.5dBm
    A typical microcell is 7W = 38.45dBm + antenna of 5dBm, so EIRP = 43.45dBm.

    Because you’re very high, you can initially decrease the EIRP to 38.5dBm ( = BTS TX attenuation of -22dB, it’s a parameter in OMC-R)

    Then adjust this parameter in order to take more or less traffic.
    Note : Maximum value for attenuation should be around 30dB (depends on your vendor)

    I’ve heard so many complaints about the “ability” of the consultants from AIRCOM, it’s a wonder they are still a reference when it comes to telecom consultancy.

    I live in france, working for alcatel.

    Pix

    #49723 Reply
    Abdel
    Guest

    Hi Pix,

    This antenna “742 215 1800MHz” has a adjustable tilt of 0-10 degrees. and we are using monopoles of 6 and small selfsupport towers of a height of 8 meters. At the moment we can install these antenna on the 6 meter monopole but I have some doubts about the weight of how much the pole can resist and the local technician who made this have no idea about physics.Anyway I will try to install this antenna on that, but for future expansion if the 1800MHz BTS is not enough we have to add an additional 900MHz BTS. At the moment if we try to put these two antennas 900 and 1800 under each other there will be no space left and for the 900 and 1800 vertical seperation which must be greater than 0.5 meter. and lower 900 antenna will reach the ground or the floor causing partial block of the signal.The 900 and the 1800 MHZ antennas have a height of 2.9 m and 1.57 m respectively. In that case we have to get short antenna with a height of 50cm-100cm. These size of antennas tends to be low gain antennas which have a big HPBW which is not recommended for microcells. so what do you think pix in this situation. As far as I know There are no short antennas with low HPBW.

    Abdel

    #49724 Reply
    pix
    Guest

    you’re right, there is no short antenna with a narrow vertical aperture.

    a microcell should not be dual band… it is a bit paradoxal, don’t you think ?
    you must (re)define a strategy for the microcells in your network. it should cover an area small enough so that only 2 TRX can carry the traffic of this area.

    Needing more than 2 TRX (for outdoor) means that your site is not really microcell… and you’re installing a macrocell, dual band : 1800 for capacity and 900 for coverage.

    the area should already be covered with an umbrella. Why do you need so much capacity ? You should rather reduce the coverage of this cell, and add other microcells around.

    Regards,
    Pix

    #49725 Reply
    Abdel
    Guest

    For me it is quite complex this kind of situation because the area I am trying to cover is the market which has a very high density, the width of this area is 700m and 500m which is a very small area so I have put some microcells within this area which was previously covered by two Macrocells. so this sites will be an underlay for that Macrocels(overlay/umbrella).If have Google Earth on your PC I can send the file to you and open it with Google earth so you can see the where the macros and micros are within the polygon which covers the market. If you don’t mind please send me your email to abdi.muse@yahoo.com so I can send you the google earth file. Thanks

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