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

From: Martin Haenggi <[email-masked]>
Date: Thu, 8 Aug 2002 16:37:08 -0500 (EST)

Bert:

>Martin, I agree that there's a difference between noise and interference. And 
that in a Rayleigh propagation channel, interference is usually the biggest 
problem. But this Rayleigh channel you mention is typically a problem in short 
range situations, where the echo paths equal the strength of the direct signal 
path.

>At longer range between hosts, two things happen. First, echo energy is reduced 
compared with the main signal path, because those longer-traveled echoes paths 
will dissipate more quickly than the direct signal path. And second, the main 
signal at the frequencies we're talking about here (at or above UHF frequencies) 
will soon start going off into space. So signal or carrier energy of the 
interfering and the wanted transmitters are reduced dramatically, and noise 
becomes the only concern when dealing with C/(N+I) reception thesholds. The 
propagation path goes to Ricean (echo energy less than direct path) and then 
Gaussian (direct signal and white noise only) as range between hosts increases.

You assume that there is a direct path, which may not be the case.

> At these longer ranges, C/(N+I) --> C/N rather than C/I.

I don't quite see why the interference should become smaller over
longer distances. Interferers may be close to the intended receiver,
irrespective of the transmission range.

>About saving battery life, I have not seen a good answer to this. Reducing 
transmitter output theoretically increases battery life, right? Not necessarily. 
If everyone reduces transmitter power to the lowest possible, the MANET routing 
protocol will involve many more hops. So more MANET hosts are going to be 
providing relay services for other communications than their own, and their 
battery will deplete unexpectedly. Just another one of those "it depends" MANET 
things.

Sure. Also, amplifier nonlinearities have to be considered. A reduction
of the transmit power does not lead to a proportional reduction of the
energy consumption.

>As to increasing the MANET aggregate throughput, seems to me that reducing the 
transmitter power should certainly increase aggregate throughout on average, as 
we already discussed.

If you consider the total amount of packets transmitted over the lifetime
of the battery, I agree. If energy consumption (or lifetime) is not an issue,
I don't. Then boosting power will increase the throughput.
To be more precise, maybe we should rather speak of transport capacity
of the network (bit-meters per second). This quantity certainly increases
with increased transmit power.

Martin

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From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 8 Aug 2002 18:23:17 -0400

> You assume that there is a direct path, which may not be the case.

Agreed. If there isn't a direct path, and there are lots of loud echo paths (Rayleigh propagation), you're probably close to the transmitter(s).

> irrespective of the transmission range.

Well, perhaps you should specify the distribution of MANET hosts, then. Let's assume for the sake of argument that the distribution is even. Approximately the same density throughout the MANET. Now make the area large enough that hosts toward opposite edges of the coverage area are just out of direct communication range, even with high power transmitters. In that sort of MANET, if you reduce power, you can cause more and more hosts to be out of *direct* range of each other. Which allows clusters of hosts within this MANET that are far apart to use the same spectrum simultaneously. This happens because for a good many of the MANET hosts, the C and the I in the C/(N+I) ratio are both low with respect to N, when hosts are far apart as described. They will drop off the horizon quickly, as you reduce power.

Remember that it's not just the inverse square law that works for you here, but also the fact that signals at these freqs will be propagating into space.

You say above, "Interferers may be close to the intended receiver, irrespective of the transmission range." I assume you mean that a host is attempting to receive a signal from far away directly, and that there are close-by hosts causing interference.

Now, what happens if you reduce everyone's power? The far-away host will not be capable of reaching you directly. Those packets will be relayed. Meanwhile, the hosts closer to you will be communicating on that same frequency. *On average*, more packets will be transferred at any given time, in the aggregate. 

> with increased transmit power.

I was assuming infinite battery life. If you add in battery life, the problem becomes difficult. Low power means more hosts have to be involved in relaying packets hop by hop, which means more batteries will die without their owners even being aware they are being used. High power means shorter battery life for those actually transmitting packets, but at least there's less relaying going on because fewer hops would be involved in data paths. The article I had mentioned went into this, using more and more elaborate constraint sets:

IEEE Communications Magazine, June 2001, "Maximum Battery Life Routing to Support Ubiquitous Mobile Computing in Wireless Ad Hoc Networks," C.K. Toh, Georgia Tech, pp. 138-147.

Bert

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From: Martin Haenggi <[email-masked]>
Date: Fri, 9 Aug 2002 19:08:05 -0500 (EST)

Bert:

>Well, perhaps you should specify the distribution of MANET hosts, then. Let's 
assume for the sake of argument that the distribution is even. Approximately the 
same density throughout the MANET. Now make the area large enough that hosts 
toward opposite edges of the coverage area are just out of direct communication 
range, even with high power transmitters. In that sort of MANET, if you reduce 
power, you can cause more and more hosts to be out of *direct* range of each 
other. Which allows clusters of hosts within this MANET that are far apart to 
use the same spectrum simultaneously. This happens because for a good many of 
the MANET hosts, the C and the I in the C/(N+I) ratio are both low with respect 
to N, when hosts are far apart as described. They will drop off the horizon 
quickly, as you reduce power.

I cannot quite follow your explanation. If the C is low compared to N (or N+I),
then the packets will be lost anyway (negative SINR[dB]), and the
throughput will be zero.
And if clusters of hosts that are far apart communicate with high power,
spectral reuse does not suffer since both C and I are scaled. On the
contrary, the SINR will increase slightly since N is constant, hence
the throughput increases.

>I was assuming infinite battery life. If you add in battery life, the problem 
becomes difficult. Low power means more hosts have to be involved in relaying 
packets hop by hop, which means more batteries will die without their owners 
even being aware they are being used.

At higher power, this gets even worse: although less nodes are used as
relays, their increased transmit power will more than compensate for that.
Let's consider a "low-power case" with unit distances for all hops,
where all nodes along the path are needed as relays, transmitting at power P.
Now if we increase the power to 2^alpha*P (where alpha is the path
loss exponent), we only need every second node as a relay. So, on
average, the intermediate nodes are used only 50% of the time, but
they have to transmit at 8*P or even 16*P, which depletes their
batteries much faster.
So, the benefit of higher-power transmission is not longer battery lifetime,
but a lower delay.

> High power means shorter battery life for those actually transmitting packets, 
> but at least there's less relaying going on because fewer hops would be
> involved in data paths.

As pointed out above, I'd say higher power means shorter battery lifetime
for everybody, whether they generate packets on their own or just
volunteer as a relay.

Martin

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From: Martin Haenggi <[email-masked]>
Date: Fri, 9 Aug 2002 19:08:05 -0500 (EST)

Martin

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From: Martin Haenggi <[email-masked]>
Date: Fri, 9 Aug 2002 19:29:10 -0500 (EST)

Hi Miguel

>I think I agree most of the time with your comments but, could you 
>please elaborate a bit more on your last point (transport capacity bps/m 
>increases with transmit power)? I do not understand why is this.

Assume you transmit at a power level P over a link of distance d.
The SNR at the receiver translates into a "reception probability" p_r
(the probability that a packet is received correctly).
Given a certain bandwidth and this probability p_r, the throughput
can be determined.
If the transmit power is increased, we may consider two cases:
- We keep the same receiver node. Over the same distance d, the average SNR
  and thus p_r increase, resulting in a higher throughput. Multiplied by d,
  this also leads to a higher transport capcity.
- We choose a different receiver at distance d2>d such that the SNR at
  this new receiver is the same as the previous one. Then p_r and 
  the throughput remain the same, but the transport capacity will
  increase by a factor d2/d.
In practice, we will mostly opt for the second case, i.e., choosing
a more distant receiver to reduce the number of hops. If the SNR
(and, accordingly, p_r) at transmit power P are already reasonably high,
then the benefit in the first case is negligible. For example, if we increase
the transmit power to 10*P, we will gain only a few percent in the first case,
whereas the second case leads to an improvement (of the transport capacity)
of about 100% (depending on the large-scale path loss exponent).

Hope that helps.

Martin

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From: "Manfredi, Albert E" <[email-masked]>
Date: Sun, 11 Aug 2002 17:29:15 -0400

>   increase by a factor d2/d.

I don't understand your last comment, unless we don't agree on what "transport capacity" means. I would say it should be the aggregate throughout of the MANET, rather than the capacity between two hosts.

Now add a third case. Reduce power to the point that the original two hosts have such a low SNR (or C/(N+I)) that they can longer communicate directly. But of course, hosts that are closer together than those original two, i.e. d3<d, still can communicate.

> of about 100% (depending on the large-scale path loss exponent).

I agree when you say that if the original SNR was "good enough" increasing power won't help much at those distances. But I don't understand what you mean by "transport capacity."

Look at the third case. The hosts in the two examples you mentioned can no longer communicate directly. But if there are hosts closer together, they see what appears to be a free RF medium, so they can initiate sessions on that medium. If they don't need to communicate with the original hosts you mentioned, they can initiate sessions on the MANET's RF channel without being aware that a cluster of hosts relatively far away is doing the same thing. Therefore, you can have several clusters all using the same RF channel at the same time.

To me, this says that "transport capacity" has increased.

I think the effect is going to be dynamic over time, depending which pairs of hosts are trying to communicate. There will be times when the bandwidth can be reused more than other times.

Bert

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From: "Manfredi, Albert E" <[email-masked]>
Date: Sun, 11 Aug 2002 17:29:15 -0400

Bert

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From: Martin Haenggi <[email-masked]>
Date: Sun, 11 Aug 2002 16:41:30 -0500 (EST)

Hi Miguel

>I think this is ok for the case of a single transmitter 
>(noise-limitted). However, I do not see this to be the case in an ad-hoc 
>network environment where multiple transmissions can take place 
>simultaneously(interference-limitted). The reason is that in such a 
>scenario, by raising the transmission power, the "reception probabiliy", 
>p_r, can actually be lower due to a higher interfence level. Network 
>topology (transmitters and receivers location and path-loss matrix) 
>information is required to study what happens here (I think).

If the interferers raise their power by the same factor as the
desired transmitter (or less), then the transport capacity increases.
So, the only scenario where this is not the case is when the 
interferers increase their power over-proportionally.

>In the ad-hoc network we want the "receiver" (next-hop) to be one in the 
>route to the destination node. We can find that a node fulfilling this 
>equal SNR could be in the opossite direction of the destination, then a 
>succesful reception would render useless.

Sure, this only makes sense if there is another node approximately
in the right direction. And it does not have to be the same SNR; if
the new destination node is closer (but still further away than the old one),
there is a gain in transport capacity.

>I think this is correct, but I reckon we want a "useful" carrying 
>capacity (or throughput). So we need to take into account if, while 
>carrying data, the network is actually delivering to the destination. If 
>not, the network may reach a high carrying capacity but delivering 
>nothing to the users. (Which will bring the network to saturation in a 
>short time frame).

Yes, to be precise, we could (should) define the distance part in the
transport capacity as the distance projected onto the end-to-end axis.

>Cellular networks (while not an ad-hoc network example) have multiple 
>simultaneous transmitters and they carefully carry transmission power 
>control to maximize carrying capacity. I think that though a different 
>scenario, ad-hoc networks capacity can be increased by using 
>transmission power control. Some algorithms have been proposed to do 
>this, like COMPOW or PARO.

I agree. For some applications, the question may be whether it is
worthwile spending resources on power control.

Best,
Martin

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From: "Martin Layley" <[email-masked]>
Date: Mon, 12 Aug 2002 17:54:54 +0100

I a real network, where power adjustment is available (e.g. cellular, bluetooth), the power adjustment will occur at the phy or link-layer level and will be different for each device and for each path.

	Martin Layley

-----Original Message-----
From: Sumeeth Nagaraj [mailto:[email-masked]]
Sent: 12 August 2002 17:28
To: Martin Haenggi; [email-masked]
Cc: [email-masked]
Subject: Re: [manet] Interference in ad hoc network

----- Original Message -----
From: "Martin Haenggi" <[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Sent: Sunday, August 11, 2002 3:41 PM
Subject: Re: [manet] Interference in ad hoc network

> interferers increase their power over-proportionally.

I think, all the nodes use the same power unless you have an algorithm
running to control the power in the network to different nodes. if this is
the case, does the power control algorithms take care of Uni directional
links, if so what would be the complexity?.

> there is a gain in transport capacity.

In my opinion, when you increase the transmitting power, you tend to address
more nodes in the vicinity. thereby more nodes tend to receive the packets
and decode the packet before dropping them since it *may* not belong to
them. This would certainly increase the medium access delay for each node,
and hence the packets could be easily dropped because the retries might
exceed max_retry_limit.

> worthwile spending resources on power control.

Best Regards,
Sumeeth

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From: "Martin Layley" <[email-masked]>
Date: Mon, 12 Aug 2002 17:54:54 +0100

Martin Layley

-----Original Message-----
From: Sumeeth Nagaraj [mailto:[email-masked]]
Sent: 12 August 2002 17:28
To: Martin Haenggi; [email-masked]
Cc: [email-masked]
Subject: Re: [manet] Interference in ad hoc network

----- Original Message -----
From: "Martin Haenggi" <[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Sent: Sunday, August 11, 2002 3:41 PM
Subject: Re: [manet] Interference in ad hoc network

Best Regards,
Sumeeth