On Dec 7, 7:30 am, Bob Bob wrote:
I think Sal has answered this well enough. Suffice to say that if you
start with the noise floor or amount of noise energy in the bandwidth
you are going to use, you have to have a "margin" above that for the
radio system to be able to transfer information. This even applies to
morse code and voice transmission and thus the human brain's ability to
do the filtering and demod! Morse code for example can actually be heard
below the noise floor (or if you like a negative margin) because you can
concentrate on the 500Hz odd tone rather than the wide band noise.
Repeating the message that is sent also lowers the margin as it is a
kind of forward error correction that might give you a few extra dB.
Even a voice you know vs one you dont know lowers the margin. A trained
brain is remarkably good and has a huge dynamic range as well.

Even something like a human shout of warning has "greater range" because
it is a very narrow bandwith data stream. ie the message sent is binary
(yes vs no) or very short (like "help" "fire" "911" or "000") rather
than something like "There is a fire down here in the trees"

The margin required is more or less linked to the specification of the
overall modulation method. I quoted QAM64. When you add FEC to QAM64 the
margin becomes less at the expense of less data bandwidth (as more bits
are sent) I saw an FEC calc for an amateur radio satellite telemetry of
6-7dB. I browsed sround and found 802.11a is around 12dB for a BER of 1
in 10E6. Data stuff tends to be layered. ie they have a basic radio
modulation method (eg QAM, QPSK etc) and the FEC on top of that (eg Reed
Solomon & Verterbi encoding) Above that you may also have resends at a
higher layer. If you are running TCP/IP for example, TCP ensures that a
packet is received and reassembled within a certain timeout period. If
not it request the data again.

Hope this helps. apologies for the excessive analogies..

Cheers Bob W5/VK2YQA

Thanks for the explanation. In your previous post you suggested that I
have to "add" the margin to the path loss. My current result doesn't
make sense because the power required to transmit at 250 kpbs for
Zigbee is less than the power required to reach the receiver at a
modest 300m away. Wouldn't adding to my path loss further deteriorate
my result? I'm trying to wrap my head around this.