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 1. sensitivity gain of -80 dBm  1. Sensitivity gain of -80 dBm

802.11a is a protocol based on the 5.8GHz UnniBand (instead of the 2.4GHz IsmBand), this protocol can reach theoretical speeds of 52Mbs.

In typical outdoor usage it will have a shorter range (though it may be effected by water less) however it's use of OfdmModulation means it's better at dealing with MultiPathFading issues which are especially prevelant in warehouse / metropolitan area deployments due to reflections.

See also: ["802.11b"], ["802.11g"], AccessPointReviews

A recent posting by JimThompson to the PersonalTelco MailingList has more information:

{{{SamChurchill writes: > I thought 5 Ghz could deliver backbone services to multiple users 10 miles > away. Am I mistaken? > > A Multi-Point 5.8 Ghz tower on a hill reaching 10-15 miles might make a good > backbone for the Satellite Truck. But upon looking at the FCC part 15 rules > now I'm not so sure. The 5.8 Ghz band can go 20 or miles but only point to > point. Antenna gain limits EIRP for multipoint connections. Perhaps too much > to be useful. Am I wrong?}}}

  • Depends if you're looking at the U-NII or ISM rules. In 5.8 under U-NII (part 15.407) rules, (which you quote) in the 5.725-5.825GHz range, we're limited to 1W + 6dBi of gain, unless we're running point-to-point, when we get 23dBi. If we're running under 15.247 (ISM), and point-to-point, then:
    • (ii) Systems operating in the 5725-5850 MHz band that are used exclusively for fixed, point-to-point operations may employ transmitting antennas with directional gain greater than 6 dBi without any corresponding reduction in transmitter peak output power.
    Otherwise, with p2mp (or omni) we're limited to 1W and 6dBi of gain, just like U-NII (and 2.4!). However, the Atheros AR5000 chipset (without the LNA) has a RX sensitivity of -71dBm for 72Mbps @10% PER (Packet Error Rate) and -85dBm for 6 Mbps @10 % PER. With the LNA, things get better (by about 3dBm), but I only know of two vendors shipping with the high-quality LNA. Since 6Mbps isn't that interesting, lets guess that the equation is mostly linear, and figure -80dBm @ 36Mbps. So, using your figures:
  • A 10-mile link using 5.8-GHz radios would experience a total path loss of approximately 132 dB.
  • Deduct another 4-5 dB for 50 feet of cable and connectors or a total loss around 136 dBm.
  • Sensitivity gain of -80 dBm
  • With two, 6 dBi gain antennas, total gain is 12dBm.
  • A 1 watt radio is equivalent to 30 dBm.

    Total gain would be (80+12+30) or +122 dBm gain BR Minus total loss ~ (-136 dBm) BR Net result equals (-14 dBm) BR So, 10 miles is 'too far' at 36Mbps. 2 miles yeilds 118dBm of free space path loss @ 5.8GHz. Stretching this link to 5 miles will get you a 6Mbps data rate. Suggestions:

  • I'm fairly sure you can run 23dBm of gain on the "point to" (non-tower) end. This changes things somewhat. 139dBm total gain, or something near to your 10 mile goal (which has a 132dBm free space loss
  • EIRP is measured sans connector loss.
  • Phased array antennas (which deliver incredible gain) 'count' as PTP. I've seen the FCC letter.
  • 24dBm of headroom is a bit much (IMO). Remember that OFDM is a bit better at dealing with fading, esp multipath fading.
  • Mesh networking (eg. AdhocRouting)


802.11a (last edited 2007-11-23 18:01:23 by localhost)