What TX power does distortion start to occur

Discussion in 'Cisco/Linksys Wireless Routers' started by crawdaddy, Feb 21, 2006.

  1. crawdaddy

    crawdaddy Network Guru Member

    the topic says most of it. at what TX power does the router start to induce noise and distortion into the signal? I am running a 15db outdoor antenna on it, and I'm currently running at 80mw, but I'm looking at maybe kicking it up higher with some additional cooling. I would think that distortion levels are relative to the antenna on it, so I would like to know where it starts to distort with the stock antenna on. Thanks
  2. jchuit

    jchuit Network Guru Member

    Peak to average power ratio. (PAPR)

    This isn't a simple question:

    The wrt54g has two modulation techniques: this is DSSS for the 802.11b and OFDM for the 802.11g. The wrt54g amplifier is designed to give a max of 1 watt (30dbm).

    The OFDM transmitter need a lot of headroom for the radio signal, this because it is a parallel signal, which consists of 64 subchannels.

    64 subchannels which are fully correlated will give a PAPR (peak to average power ratio) from 18 db. This would mean a max of (30 - 12) 12dbm outputpower with no distortion.

    To get a better performance the OFDM decorrelates the signals, this by altering the angle of the radio-waves. This technique decorrelates the half of the signals fully. The PAPR value will lower to 15 db. The max power with no distortion will be around (30 -15) 15 dbm.

    This is all worst case, in reality it is better, because the 64 subchannels are not fully correlated, the broadcom radio should take care of the decorrelation.

    The expected value for the PAPR, (RMS PAPR) will be the half, this means around 8 to 10 db.

    The max OFDM output power will be around (30dbm- 9db=) 21 dbm, above 100mW to 200 mW is the power where some distortion can occur.

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  3. vincentfox

    vincentfox Network Guru Member

    Why would you want to increase it any case?

    Do you have a distant client also with high-gain antenna, with similarly adjustable transmit power?

    Much like turning up the stereo to "11" in your house, I think there is also a consideration with making the receiver go deaf, for very nearby clients.
  4. t4thfavor

    t4thfavor Network Guru Member

    this is not true, although scientifically accurate the antennas that are on the router will not handle much more than 50mw without increasing the swr (standing wave ratio) to an unsafe level and then burning out the radio
  5. vincentfox

    vincentfox Network Guru Member

    Hey favor, can you explain that again? And which point where you responding to, one of mine, or one of jchuit?
  6. t4thfavor

    t4thfavor Network Guru Member

    Re: Peak to average power ratio. (PAPR)

    this one is accurate scientifically but the increased power creates reflected signal that can harm the Xmitter also known as SWR

    the SWR decreases as the quality of the antenna increases
  7. crawdaddy

    crawdaddy Network Guru Member

    well, I have a huge outdoor 15db omni attached to it with a max input power of 100watts, so I'm not worried. I actually just recieved a 1 watt bidirectional amp today, so bumping up my local power is not neccesary :)
  8. -mb-

    -mb- Network Guru Member

    This is not true either, the standing wave ratio of the antenna circuit is constant, and does not change with output power level. With a non-resonant antenna system, reflected power is usually dissipated as heat in the feedline between the transmitter output and radiator (antenna). In theory, a mismatch could cause the power amplifier semi-conductor to fail. At the frequencies used by these devices, it is difficult to create low loss feedline and I'd bet that any reflected power would turn to heat energy in the feedline. More times than not at these frequencies any gain advantage of a remote antenna is offset by feedline loss.
  9. jchuit

    jchuit Network Guru Member

    lineairity and OFDM.

    The transmission of the OFDM modulation, needs a lineair transmission system. This means: when the input power is doubled the output will double too => strong signals behave the same as weak signals.

    This lineairity also applies to the antenna's, vSWR will stay the same for the given frequency.

    The amplifier is non-lineair at high power, a part of the wave is chopped of, this part will give distortion on a higher frequency. The antenna will reflect (not radiate) this (small) part because the antenna is not tuned for use of this frequency.

    Cables and connectors: a good cable is Aircom plus, 10 meters will give 2.15 db loss, the Aircom connectors are also very low loss.

  10. xrattiracer

    xrattiracer Network Guru Member

    I am not sure how it relates to the linksys, but at the wisp I work at we use 200mw radios with 12-24db directional antennas. this is of course for long distance links of upwards of 12 miles or so, but i dont see why the linksys radio would be any less capable. i also question the need for cooling, as i dont think they dissapate much heat (its still very low power consuption after all, and the heat has to come from somewhere). the radios we use are in pcmcia cards and never even get warm that ive noticed.
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