Subject: RE: [manet] Interference in ad hoc network

From: "[email-masked]" <[email-masked]>
Date: Sat, 13 Jul 2002 22:37:40 -0500

Hi,

[Manfredi, Albert E wrote:]
> I guess I'm saying that the ratio that matters here is S/(N+I), not just 
S/I?

Yes, I was just focusing on the "I" since (I thought) that was the topic
under discussion. If N<<I, then both S and N+I scale with the (global)
transmit power.

[Kaixin Xu wrote:]
> An interesting thing I have found is that interference range is
> usually 1.7 times the transmitter-receiver distance.

Is that an experimental or a simulation result? And how do you
define the interference range?

[Summeth Nagaraj wrote:]
> you are right but I I think it was in the context of  less number of nodes
> hearing the signal and thereby less interference from other nodes while
> contending for a channel. on the other hand if a large number of nodes are
> able to hear the signal then comes the problem of interference while all the
> nodes contend to access the channel.

Less nodes hearing the signal is again a noise issue, not an interference
issue. Are you thinking about a flooding scenario, assuming every nodes
tries to re-transmit what it receives? If yes, then the protocol could
make sure that only those that are far away (received signal strength
below a certain threshold) try to retransmit.
If the network is (at least locally) synchronized at the symbol level,
then collisions are actually beneficial when flooding, since the packets
from different transmitters add up constructively (a type of cooperative
diversity). Then re-transmission from a large number of nodes becomes merely
an energy issue, not an interference issue.

[ John P. Mullen wrote:]
> Although the ratio of signal to interference is constant, receivers work on
> the ratio of signal to noise + interference.
> If power reduction were not able to reduce interference, it would be
> impossible to reuse channels in cellular systems and even low power systems
> would interfere with each other.

I don't agree with that. Let's assume for a moment that there is no
noise, and let's focus on the SIR. Spatial reuse of the spectrum is 
still possible, since the SIR depends on the relative distances of
the receiver to the desired transmitter and to the interfering transmitters
(and the path loss exponent). So, if the interferers are far enough
away, the SIR will be high. Hence the fact that the SIR is independent of
the transmit power does not preclude channel reuse - it's a matter
of distance not of transmit power.
Including the noise does not change that, it just sets a lower bound
on the transmit power for successful transmission to the receiver.
(Whether the interference is low by itself or buried in the noise
does not matter here.)
 
[Jim Stevens wrote:]
> 2.  Martin Haenggi wrote that "if all nodes use the same power level, then
> the effect of the interference is completely independent of that power
> level, since the signal-to-interference ratio remains constant.  So, a
> (global) reduction of the power level does not help alleviate the
> interference problem."  Yes, but why should all nodes use the same power
> level instead of adaptively using different power levels depending upon
> which neighbor node (or set of neighbor nodes) they are trying to talk to.

Power control is certainly the strategy that leads to optimum performance,
but it may not be possible in all ad hoc nets, either because of resource
constraints, or because the locations of the nodes are not known or
cannot be kept track of.

> Also, by reducing transmit power across the network, frequency reuse goes
> up, which increases total network throughput.

If nodes have different transmit powers, and you just scale all of them
by the same factor, all the SIRs remain constant.

[Manfredi, Albert E wrote:]
>This seems like a non-sequitur to me. We just finished saying that many
>transmitters drop out of sight, so by defintion, we must have reduced
>interference. Ultimately, as power levels go to 0, there will be no
>interference at all.

True but there is also no signal. And the SIR is not determined.
(Luckily we have the noise to avoid this dilemma.)
Clearly the interference itself scales with the transmit power, but the
effect of that interference, measured by the SIR, does not depend on
the power.

Regards,
Martin

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From: Sumeeth Nagaraj <[email-masked]>
Date: Sun, 14 Jul 2002 21:11:21 -0600

Hi,

> [Sumeeth Nagaraj wrote:]
> issue.

If a node can reach a large number of nodes: the number of nodes contending
for
a channel would be more at a time. the number of RTS sent drastically
increases
and so does the collisions,  before one node gets a chance to transmit. This
also
increases the time for a packet to reach the destination.

>Are you thinking about a flooding scenario, assuming every nodes

> an energy issue, not an interference issue.

Here is another Question I have. Are you thinking about packets sent by 2
different
 transmitter to be the same? By all chances it could happen two different
sources
sending a RREQ(Broadcast) ? then I think it would surely be an interference
issue
 too and not just an energy issue.

Regards,
Sumeeth

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From: Sumeeth Nagaraj <[email-masked]>
Date: Sun, 14 Jul 2002 21:11:21 -0600

Hi,

Regards,
Sumeeth