Can you help me interpret this spectrum analysis noise plot?

Discussion in 'Wireless Internet' started by Danny D'Amico, Dec 18, 2013.

  1. Can you help me interpret this 2.4GHz WiFi spectrum analysis noise plot
    that I just ran from my Ubiquiti Rocket M2 rooftop antenna?

    I'm trying to debug why I have -88dBm of noise at my rooftop radio.

    My Rocket M2 rooftop radio is on channel 10.
    My home broadband router inside the house is on channel 1.

    Here is the "waterfall" plot:

    Here is the "channel" plot:

    Here's a site survey:

    And, here's a view of the rooftop radio signal to noise strength:

    Any observations with respect to the source of the -88dB noise?
    (I don't really know how to interpret this stuff.)
    Danny D'Amico, Dec 18, 2013
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  2. Danny D'Amico

    hh Guest

    only guy who can do this is gone, Jeff Liebermann

    --- news:// - complaints: ---
    hh, Dec 18, 2013
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  3. Jeff is great. He was in Santa Cruz, and frequented alt.internet.wireless
    before moving mostly on to

    What do you mean he's "gone"?
    Isn't he still around?
    Danny D'Amico, Dec 18, 2013
  4. Q: What's the problem.
    A: It's my understanding that every dB of noise reduces the dB of signal
    by that amount. The less signal, the slower the Internet speeds.

    Q: How much noise do you consider normal?
    A: I have no idea. That's why I'm asking! :) Googling, I find you "should"
    have about 20dB of headroom between noise & signal. My signal is about
    -52dBm and my noise is -88dBm, so I'm within that range, but, my
    signal to noise ratio is -52dBm - -88dBm = 36

    Q: What's the bandwidth & noise figure of your receiver?
    A: Googling for the "Rocket M2 bandwidth gain specifications", I find
    this datasheet for a "RM2" receiver & "2G-24" 24dBi dish reflector:
    Which says, on page 6:
    Rocket M2, Operating Frequency 2412-2462 MHz
    llg = 1-24 Mbps => -97 dBm min +/- 2 dB
    11g = 36 Mbps => -80 dBm +/- 2 dB
    11g = 48 Mbps => -77 dBm +/- 2 dB
    11g = 54 Mbps => -75 dBm +/- 2 dB
    11n = MCS0 => -96 dBm +/- 2 dB
    11n = MCS1 => -95 dBm +/- 2 dB
    11n = MCS2 => -92 dBm +/- 2 dB
    11n = MCS3 => -90 dBm +/- 2 dB
    11n = MCS4 => -86 dBm +/- 2 dB
    11n = MCS5 => -83 dBm +/- 2 dB
    11n = MCS6 => -77 dBm +/- 2 dB
    11n = MCS7 => -74 dBm +/- 2 dB
    11n = MCS8 => -95 dBm +/- 2 dB
    11n = MCS9 => -93 dBm +/- 2 dB
    11n = MCS10 => -90 dBm +/- 2 dB <=== this is my channel
    11n = MCS11 => -87 dBm +/- 2 dB
    11n = MCS12 => -84 dBm +/- 2 dB
    11n = MCS13 => -79 dBm +/- 2 dB
    11n = MCS14 => -78 dBm +/- 2 dB
    11n = MCS15 => -75 dBm +/- 2 dB

    Given that my Rocket M2 is 11n MIMO, and on channel 10, I'd say
    the receiver sensitivity is from -88 to -92 dBm.

    Hmmm... I just noticed, that this is the same (essentially) as
    my noise figure. But, I'm not sure what that tells me.
    Danny D'Amico, Dec 18, 2013
  5. Rumors of my demise might be a bit premature. I'm buried in projects,
    work, Christmas Chrisis', ladyfriend, and trying to untrash my house,
    office, and vehicle. Incidentally, this is what I've been doing for
    Jeff Liebermann, Dec 18, 2013
  6. Hi Jeff,

    Ooooh. Yours is bigger than mine!

    That looks like a neat project.

    Well, I'm here in the Santa Cruz mountains, just trying to figure
    out what my noise plot is trying to tell me (I'm on channel 10):
    Danny D'Amico, Dec 18, 2013
  7. Here's a better screenshot of the bandwidth numbers, without the
    text formatting that went on by my newsreader client:

    Since my Rocket M2 radio is on channel 10, it looks like the
    sensitivity is from -88dBm to -92dBm.

    The 24dBi antenna has specifications here:
    Which say:
    • Frequency Range: 2.3-2.7 GHz
    • Gain: 24 dBi
    • Hpol Beamwidth: 3.8 deg. (Rx Dish) / 6.6 deg. (Tx Dish)
    • Vpol Beamwidth: 3.8 deg. (Rx Dish) / 6.6 deg. (Tx Dish)
    • F/B Ratio: -50 dB (Rx Dish) / -65 dB (Tx Dish)
    • Max VSWR: 1.6:1
    • Polarization: Dual Linear
    • Cross-pol: Isolation 35 dB min

    The Rocketdish RD-2G24 datasheet is here:
    which shows these specifications:

    So, in summary, the 24dBi dish has a narrow bandwidth of 3.8° to 6.8°
    degrees, and the receiver has a sensitivity of -90dBm ±2dB.

    I don't know how to translate those facts into an interpretation of
    the spectrum analysis noise charts provided in the OP though.
    Danny D'Amico, Dec 18, 2013
  8. Danny D'Amico

    Tony Hwang Guest

    First do you understand what decibel means in the context of voltage,
    current or power? Can you calcualte receiver sensitivity of -92dbm
    comes out (?) volts? On what modulation mode are we talking about?
    Tony Hwang, Dec 18, 2013
  9. Googling, I can make a guess that my transient noise (blue line) is very
    high at channel 10 of -40dBm, but that my average noise (green line) is
    very very low at -90dBm.

    The receiver sensitivity that someone asked me to look up is at about
    my average noise level, at -90dBm ± 2dB for 802.11n signals.

    So, the question I ask, without having the experience to know what "good"
    transient noise and average noise levels are, is whether or not these
    numbers are "good" or "average" or "bad"?

    Also, I have no experience whether a peak instantaneous noise of -40dBm
    (which is admittedly high) has any detrimental effect on my radio
    performance (even as the average seems very low, at -90dBm).

    I just don't know. Do you?
    Danny D'Amico, Dec 18, 2013
  10. I (think I) do understand decibels, at least at a rudimentary level.
    Here's my simple summary of how I organize my thoughts around decibels:

    1) Every 3 decibels is a doubling (or halving) of power.
    2) A halving of power would be if it went from -87dBm to -90dBm.
    3) My radio is transmits at 27 decibels (compared to the mW reference).
    4) When I want to convert dBm to Watts, I google "dbm to watts".
    5) The first hit is always the best dbm-to-Watts converter.
    7) So, 27 dBm is 1/2 Watt (Note: 30 dBm would be 1 Watt, & so on).
    8) The receiver is sensitive to -90dBm at 802.11n channel 10.
    9) That means it can pick up a signal strength of 1 picowatt.
    10) The signal to noise headroom needs to be around 20dBm.
    11) That means I need signal to be 1/10 Watt greater than noise.
    12) The transmitter is claimed to be 28dBm±2dB at channel 10.
    13) So, the transmitter (without antenna) is 6/10ths of a Watt.
    14) However, the Rocketdish reflector & antenna add another 24dBi.
    15) A dBi is relative to a fictional spherical-radiation pattern.
    16) So, my effective isotropic radiated power is 28+24=52dBm!
    17) An EIRP of 52 dBm is a whopping 158 Watts!
    18) The FCC only allows me an effective power of 4 Watts
    19) Googling for "watts to dbm", the first link is the best.
    21) That 4 Watts is 36dBm
    22) Even though the equipment is capable of 52dBm, it's toned
    down to that legal limit of 36dBm.

    So, given all that, my average noise is rather low, on channel 10,
    of about -90dBm or 1 picoWatt (which is the green part of the bottom
    graph below).

    However, my instantaneous noise is rather high, at -40dBm or
    10 microWatts (which is the blue line in the bottom graph above).

    So, I guess my question is how much will my radio be adversely affected
    by 10 microWatts of instantaneous noise, when the average noise is only
    1 picoWatt?

    I have no experience with what noise levels are good, and which ones
    are bad - so I have no background to interpret the spectrum analysis.

    Transceiver datasheet (Ubiquiti Rocket M2):

    Antenna (Ubiquiti Rocketdish RD-2G-24):
    Danny D'Amico, Dec 18, 2013
  11. I never deal with volts when I'm working with the radio.
    I always deal in power. So, I don't know what the volts are,
    but, I do know this about -92dBm:

    0) First, I always google "dbm to watts".
    1) The first hit is always a great time-saving calculator.
    2) That's .
    3) So, -92dBm is about 631 femtoWatts (which is pretty small).
    4) I don't know how to convert that to volts though.
    I'm not sure if I understand the question, but, the radio operates
    in the 802.11n MIMO channel 10 (with vertical & horizontally polarized
    antennas) that have a 3.8° to 6.8° beamwidth (which is pretty narrow).

    NOTE: The narrow beamwidth is how the antenna gets all that gain in
    the first place. You can't create or destroy power, so, I'd have a
    wider beamwidth with a lower-gain antenna. This is a pretty high-gain
    antenna, so, the beam width is pretty narrow, but, since it's pointed
    at the WISP access point a few miles away, it doesn't have to be broad.
    Danny D'Amico, Dec 18, 2013
  12. I didn't explain that one all too well.

    Here's another try at my thought process:

    1. If I put the legal limit of 4 Watts into an antenna with a wide
    radiation pattern, it goes only so far.
    2. If I then change the antenna pattern to be more narrow, the
    radiated signal goes farther in the direction that it is pointed.

    So, my antenna & dish reflector, having a gain of 24dBi, is pretty
    narrow at around 5° beamwidth (in both horizontal & vertical planes).

    Breaking out my trig (SOH, CAH, TOA), I see that I can create a right
    triangle of half the 5° beamwidth, with the Adjacent being 3 miles.

    Since I have the angle and the Adjacent, and I want the Opposite,
    it looks like the tangent will tell me the how large of a circle is
    painted on the WISP antenna 3 miles away.

    1. TOA means Tangent is equal to the Opposite over the Adjacent.
    2. So the Adjacent times the tangent is the Opposite.
    3. 3 miles time the tangent of 2.5° is what I need to know.
    4. Googling for "tangent calculator", again I take the first hit.
    5. That's .
    6. The tangent of 2.5° = 0.04366094 .
    7. So 3 miles x 0.04 is about 633 feet.
    8. The 2.5° was half the beamwidth (to make a right triangle).
    9. So the beam paints a pattern twice that, at about 1200 feet.
    10. This tells me that a beamwidth of 5° isn't really all that narrow!

    Note: The 5° is defined, I think, by where the furthest lobe's power
    is cut in half (i.e., by 3dB).

    Here is a picture of that pattern from the Rocketdish RD-2G-24:

    Notice that this radio is pretty directional, but even so, I have
    a catcher's mitt about 1200 feet wide to hit my access point.

    Here are the Internet speeds I get by hitting that catcher's mitt:

    All I'm trying to do is *improve* on those Internet speeds, by
    understanding first, and then lowering my noise (or raising my
    signal-to-noise levels) within legal limits of 36 decibels EIRP.

    Danny D'Amico, Dec 18, 2013
  13. Danny D'Amico

    miso Guest

    First things first. I'm guessing your WISP picked channel 10 and you
    can't change that. Is that the case?

    A waterfall is just time dependent sniffing. If your WISP wasn't
    spraying you with wifi on channel 10 (again, my guess), you would
    examine the waterfall display and find the area with the least activity.
    Now the waterfall is useful if someone burps wifi at you, as in a
    telemetry application. If the band was crowded, you would pick the
    channel with the occasional belch of wifi rather than one that is busy
    all the time. [If you were doing SIGINT, you would look for patterns in
    the occasional wifi belch. This is knows as traffic analysis.]

    My next guess is you WISP provider has a customer on channel 2 at the
    same location that feeds you. [It could be another WISP from a different
    vendor.] The WISP provider has a beam antennas at the transmitter site.
    One beam for you on channel 10, another beam for somebody else on
    channel 2. That person may live near you since you are seeing the
    signal, but there are no red blobs in the waterfall, so the antenna
    isn't pointed directly at you. [And why would it be?] Red means a strong
    signal. The WISP on channel 2 is only 7 db less than your signal, but
    channel 2 and channel 10 have no common frequencies, so nothing to worry

    You have two neighbors on channel 3, so that would be a bad channel for
    you, as would any channel that overlaps channel 3. Probably the WISP
    installer already knew that from when the site survey was done.

    I'm not really sure how they determine the noise floor. At any one time,
    there is somebody on a wifi channel. It might be really low RF level,
    but not zero. Probably the receiver makes a determination that if it
    can't sniff a signal, it must be noise. Not a good assumption.

    At -50dBm over 3 miles, I'd call it a day and go looking for something
    else to fix.
    miso, Dec 18, 2013
  14. Danny D'Amico

    Tony Hwang Guest

    When you talk about noise there are many different kinda noise.
    Tony Hwang, Dec 18, 2013
  15. In the case here, it's all 2.4GHz signals which are not coming
    from the access point that I'm connecting to, yet, which the
    rooftop antenna sees.
    Danny D'Amico, Dec 18, 2013
  16. Yes. The WISP has two antennas that I'm aimed at (because they're on the same

    I'm pretty sure both humps in the graphs are the WISP's antennas, one of
    which I'm connected to.

    He's on Channel 10 and on Channel 2, and notice you see two humps, one at
    channel 10, and the other at channel 2.

    I can only connect to channel 10 (because Channel 2 is a different password).
    Danny D'Amico, Dec 18, 2013
  17. Wow. You're good! The WISP is on both channels, 10 and 2.

    10 is for me. I don't know who is on 2, but, it's his channel too.

    You got all that from the waterfall? You're good, because I only know
    that from other means.
    Danny D'Amico, Dec 18, 2013
  18. It's rare to see time on the Y axis, so I see how it's time-dependent
    sniffing. I guess it also looks like a waterfall, since it's columnar
    but in layers. Blue seems to be where I'd want to be, if I wasn't
    constrained to be on channel 10, which is my access point channel.
    You are correct. The same WISP is feeding two different neighborhoods with
    two antennas, both at the same mast, one on channel 2 and the other on
    channel 10.

    I can now see the waterfall is yellow'er on those two channels, as are the
    power levels bluer, and the real-time view greener.
    You figured out a lot from that waterfall graph that I hadn't mentioned
    (because I didn't realize it might be relevant). Yes, the WISP is on both
    channel 2 and 10, and both antennas are on the same tower; but only one
    (channel 10) is meant for me to connect to.
    The point is duly noted to stay away from channel 3.
    I think anything the antenna sees which is not a connection signal,
    is considered noise.
    You have a point that the -50dBm isn't bad for a distance of 3 miles.
    I was more worried about the -88dBm of noise, but, now, after looking
    further, I think that the noise level is just about at the receiver
    sensitivity of -90dBm ± 2dB.

    The main figure that worries me is the instantaneous noise of -40dBm.
    Do you know what effect this instantaneous noise might have on the radio?
    Danny D'Amico, Dec 18, 2013
  19. Danny D'Amico

    mike Guest

    Would wrapping the feed in tinfoil give a measure of internally
    generated noise?
    mike, Dec 18, 2013
  20. They do sell RF Armor for the Rocketdish, but, it's prohibitively expensive:

    I guess I could fabricate the RF Armor out of steel sheathing.

    That would make it a neat home-repair project!
    Danny D'Amico, Dec 18, 2013
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