Re: Effective Range/Throughput - Comparisons and review - (see postings 31-01-06)
- From: Jeff Liebermann <jeffl@xxxxxxxxxxxxxxxxxxxxxx>
- Date: Fri, 17 Feb 2006 23:34:19 GMT
On Fri, 17 Feb 2006 10:49:32 -0800, Jeff Liebermann
<jeffl@xxxxxxxxxxxxxxxxxxxxxx> wrote:
Let's play with the numbers and see what worst case produces instead
of best case. Going to the link calcs at:
http://en.wikibooks.org/wiki/FAQ_for_alt.internet.wireless/Wi-Fi#Link_Calculations
and plugging in some numbers. A WRT54G dribbles about +15dBm tx
power. The antenna is about 2dBi gain, but the coax and connectors
eat about -1dB in loss. The laptop uses an marginal receiver with
about -88dBm sensitivity at 6Mbits/sec connection speed, the slowest
OFDM speed. The ceramic backed antennas in the laptop lid are good
for about -2dBi gain and another -2dB in coax and connector losses.
I'll aim for 20dB fade margin (just to be consistent).
TX power = +15dBm
TX coax loss = 1dB
TX ant gain = 2dBi
Distance = unknown
RX ant gain = -2dBi
RX coax loss = -2dB
RX sens = -88dBm (at 6Mbits/sec)
Fade margin = 20dB
Plugging into:
http://www.terabeam.com/support/calculations/som.php
and trying various distances until I get about 20dB fade margin, I get
a range of 0.06 miles or 316ft. That's quite good at 6Mbits/sec.
Maybe I'm being too optimisitic somewhere.
Now, running the same numbers with a receive sensitivity of -68dBm at
54Mbits/sec, I get 0.006 miles or 32ft. So, your range is going to
be between those numbers (for OFDM).
I suspected that was too good to be true. The range is about twice
what I would expect from worst case calcs. Let's try again with
better numbers.
The RX coax loss and RX sensitivity on the laptop end are the problem.
I just ripped apart a Toshiblah A75-S213:
http://802.11junk.com/jeffl/pics/A75-S213/
and noticed that the tiny coax cables to the antennas are about 2ft
long. I only allowed for -2dB coax and connector loss. The
connectors are Hirose u-FL or iPex MHF connectors. The coax appears
to be Sumitomo 0.4DS-PBA-H with an attentuation of about 7dB/meter
(extrapolated from unreadable graph), or about -5dB loss for the coax
and about 0.5dB for the connector.
I dug out some tests I made with an Intel 2200BG card at various
speeds. The numbers from my chart came from a DLink DI-624 and tend to
be somewhat better than those found in the typical MiniPCI client
radio. I found that the RX sensitivity at 6Mbits/sec (slowest ODFM
speed) was about -84dB at the connector (including compensation for
test leads). Therefore, the numbers should look like:
TX power = +15dBm
TX coax loss = 1dB
TX ant gain = 2dBi
Distance = unknown
RX ant gain = -2dBi
RX coax loss = -5.5dB (including connectors)
RX sens = -84dBm (at 6Mbits/sec)
Fade margin = 20dB
http://www.terabeam.com/support/calculations/som.php
I now get 0.025 miles or 132ft. That's about right.
For 54Mbits/sec, the sensitivity is about -63dBm from my 2200BG
tinkering, which yields a range of 0.0022 miles or 11.6ft. Yeah,
that's a bit shorter than I expect but in the ballpark.
--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@xxxxxxxxxxxxxxxxxxxxxx
# http://802.11junk.com jeffl@xxxxxxxxxx
# http://www.LearnByDestroying.com AE6KS
.
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