Subject: [manet] Bandwidth vs Transmission Range

From: "Prasanna.Padmanabhan-1" <[email-masked]>
Date: Sun, 23 Nov 2003 15:14:43 -0600

Hello Everyone,

In one of the WLAN cards (Wireless LAN Mini PCI Type 3A card), I see that for higher bandwidth cards they offer, the transmission range decreases.

For example, a 1 Mbps WLAN card has a tranmission range of 550m,
a 2 Mbps WLAN card has a tranmission range of 400m,
a 11 Mbps WLAN card has a tranmission range of 160m.

Does that mean that bandwidth and transmission range are inversely proportional.
I understand transmission range is related to Power. But how is it related to Bandwidth?

Please let me know.
Thanks,
Prasanna

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From: John Stine <[email-masked]>
Date: Mon, 24 Nov 2003 13:20:02 -0500

Prassana,

    The modulation schemes that provide the 1, 2, 5.5 and 11 Mbps
data rates all use the same bandwidth of RF spectrum.  The result
is that the 1 Mbps data rate  has about a 10 dB processing gain as
compared to the 11 Mbps data rate.  So as a rough estimate in
802.11, given the same transmit power, the SNR advantage data rate
x has over data rate y is y/x.  That is, data rate y needs y/x
more signal power in order to have the same BER as data rate x. 
Since the maximum transmit power of 802.11 is regulated, the
transmission range of the data rates change.   Graphs in the
following reference will give you a better comparison of the
performance of the modulation schemes.

C. Heegard, et al, "High Performance Wireless Ethernet," IEEE
Communications Magazine, Nov 2001.  pp. 64 -73.

    An interesting problem that I have not found any information
on is how these modulation schemes affect each other.  MANET
channels are interference limited not noise limited.   The data
rate control schemes that I have read about assume communication
to and from base stations and so no interference.  I would imagine
in MANETS, using the different data rates with the data rate
selection schemes that come with the devices will exacerbate the
hidden terminal problems that result from interference having a
greater range than the RTS/CTS virtual sensing range.

    By the way, I imagine that most M&S environments fail to
capture these effects and the problems they cause.

John