Subject: RE: [manet] tx power adjustment

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Mon, 1 Jul 2002 12:58:27 -0600

Hi David,

In general, as the distance is doubled, the received power is quartered, or
received S/N drops about 6 dB.  (This is actually overly optimistic in some
frequency bands).  One factor is how this 6 dB drop affects bandwidth.  The
Shannon limit would predict about a 50% decrease in available BW, but that
may not be critical.  Usually, BER and other errors do you in first.  Also,
absolute levels count.  If the original S/N is 20 dBm, there is probably not
a problem, but if it were 8 dBm, ...

Another factor is processing time.  If 80% of the time required to move a
packet from one node to the next is taken up in coding, decoding, etc., then
a two-hop path would be doubly slow (added processing time plus lower
transmission rate).

Of course, multiple transmissions tie up the channel longer than a single
one, but a "shout" ties up the channel over a greater geographic area.

The effect on battery life is probably worth considering, too.

Interesting problem.

John Mullen

-----Original Message-----
From: David Young [mailto:[email-masked]]
Sent: Monday, July 01, 2002 12:27 PM
To: [email-masked]
Subject: Re: [manet] tx power adjustment

In some research, a host transmits a beacon at each TX power level and
listens for beacon-replies to discover its neighbors. Then the host
labels each neighbor's route with the least TX power level necessary to
reach that neighbor.  Packets destined for a neighbor are transmitted
with the power level labeling the neighbor's route.

If a host discovers neighbors with beacons independently of discovering
routes with an ad hoc routing protocol, then sometimes a neighbor will
have two routes, one discovered by the beacon and one discovered by the
routing protocol. Then the host will have a choice between shouting at a
far away neighbor, or whispering to a nearer neighbor who will whisper
to the far away neighbor. A strategy for choosing between one "loud"
hop and several quiet ones could be quite complex, it seems. Suppose
you tried to estimate and minimize the product of a packet's on-air time
and the area silenced by its (re)transmission?

Dave

On Mon, Jul 01, 2002 at 09:20:41AM -0600, Mullen, John P. (Contractor)
wrote:
> Hi Bert,
>  
> It all turns on the density of your hosts and the patterns of
> communications.  If you have a very large number of hosts in a rather
small
> geographical area, you will have increased contention for the RF channel.
> By reducing power, you reduce the potential for interference among hosts
> attempting unrelated communications on the same channel, but in
> significantly different geographical locations.  If, on the other hand,
> those conversations are of interest to all nodes, then reducing power will
> increase the number of hops and channel utilization.  
>  
> For example, suppose your MANET consists of clusters of hosts that are
about
> 1 km apart.  However, within a cluster, each node is within a few meters
of
> at least one other node.  When communications is within a cluster, it
would
> be prudent, to limit transmit power to that level necessary to carry out
> communications within that cluster, but not high enough to cause
> interference between clusters.  On the other hand, if you need to
> communicate from one cluster to another, a higher power level would be
> needed.
>  
> Also, if we agree that the MANET hosts are likely to be portable
> battery-powered units, then managing transmitter power is also important
> from the perspective of conserving network resources.  
>  
> I would say that the ability to change transmitter power has potential for
> more efficient MANET management, but its success will turn on the ability
of
> the protocol to detect situations in which the different power levels are
> appropriate.
>  
> John Mullen
>  
> 
> -----Original Message-----
> From: Manfredi, Albert E [mailto:[email-masked]]
> Sent: Monday, July 01, 2002 8:18 AM
> To: '??'
> Cc: [email-masked]
> Subject: RE: [manet] RE: [manet] IP addresses in MANET
> 
> 
> Some miscellaneous rambling:
>  
> Decreasing power is useful to decrease the size of cells, to make more
> efficient use of RF spectrum. In a system with base stations or access
> points, the advantages seem obvious. The wired infrastructure will have to
> be upgraded for a greater traffic load, but the individual wireless cells
> will support the same number of hosts in a smaller area, resulting in
> greater density of mobile hosts.
>  
> In an ad-hoc network, the advantage doesn't seem so obvious (to me). Since
> ad-hoc networks depend on mobile hosts for all their intra-MANET routing,
> decreasing power means increasing the number of hops required, which means
> that there will be more choke points in the network and more hosts
involved
> in the average data path. I suppose that if one introduces faster mobile
> hosts while reducing the power, same as one had to upgrade the wired
> infrastructure in the cell-based network with base stations, then maybe I
> can understand how you'd expect better performance.
>  
> Bert
>  
> 
> -----Original Message-----
> From: [email-masked] [mailto:[email-masked]]
> Sent: Monday, July 01, 2002 3:52 AM
> To: [email-masked]
> Cc: [email-masked]; [email-masked]
> Subject: [manet] RE: [manet] IP addresses in MANET
> 
> 
> Hello Scott:
> As David Young says, Using power control to increase MANET performance is
an
> active research area in MANET. It sure will increase MANET?¡¥s throughput.
> However, firstly, because of practical reason, transmitting power can not
be
> decreasing without limit; secondly, when the transmitting power is low
> enough, it means that the talking area or clustering area is very small,
In
> such network, the mobility of nodes will become a big problem:) 
> Practically speaking, if MANET can serve more than 10000 nodes freely, I
> think it is enough for real implementation. I see no case which needs more
> nodes than that:)
> 
> Hai Wang
> 
> 

-- 
David Young             OJC Technologies
[email-masked]     Engineering from the Right Brain
                        Urbana, IL * (217) 278-3933

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---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 2 Jul 2002 07:10:01 -0600

Hi Miguel,

I follow your argument, but come to a different conclusion.

If we assume that only nodes on the path from sender to receiver "whisper,"
then a repeated whisper will affect only nodes in the vicinity of the
end-to-end path, whereas a loud shout will affect every node within the
radius of the distance from the originator to the destination.  

For example, if the MANET consists of 100 nodes arranged in a 10 by 10 array
with each at the limit of "whisper" range, a whisper path from the middle to
the right edge will affect, maybe 10 or so nodes, but a shout from the
center to one edge would involve about 80 nodes (pi r^2).

Also, reception power, though significant, is usually a couple of orders of
magnitude or more, less than transmission power and may actually be less
than idle power.  (A receiver searching for a signal may have its gain
cranked much higher than one actually locked on a signal).

I agree that circumstances could affect the analysis, which is why some sort
of adaptive control will probably be the answer. 

John Mullen

-----Original Message-----
From: Miguel Sanchez [mailto:[email-masked]]
Sent: Monday, July 01, 2002 4:21 PM
To: [email-masked]
Cc: [email-masked]
Subject: Re: [manet] tx power adjustment

Hi Dave,

You wrote:

>...  a choice between shouting at a
As a rule of thumb if you look at the tx power only, it is more energy 
efficient to whisper the packet twice than to transmit it loud once (due 
to the power law on the atenuation). However if you  take into account 
that receiving is not coming for free either, and if you assume that all 
neighbor nodes will receive a packet before they can check if it is 
addressed  to them or not (and therefore they will draw reception 
energy) then whispering twice could be a worse choice than one loud 
transmission. Of course this analysis may be interesting when you are 
worried about network survivability (i.e. low power sensor networks).

What it is more, the numbers of this observation will change a lot 
depending on the nodal density about these three involved nodes (i.e. 
sender, reciver and forwarder).

I just wanted to add this thought to the discussion.

Miguel Sanchez
Universidad Politecnica de Valencia, Spain

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 2 Jul 2002 08:49:24 -0600

Hi Miguel,

70% of transmit energy for receive!!  Wow!  I'm not sure if that means the
equipment has terribly efficient transmitters or terribly inefficient
receivers.

:-)

This boggles my mind.  Assuming omnidirectional transmission (logical in a
MANET), in a transmit operation, the receiver receives only a very small
fraction of the transmitted power.  Power is lost in the transmitter, the
final output stage, in mismatches between the output stage and antenna, in
the antenna, in all sorts of environmental nasties, and in the receive
antenna.  On top of this, you have the power law, which describes the effect
of dispersion with distance and attenuation by the medium.  

Receivers employ energy in a more-or-less closed system that involves
repeated amplification and transformation of the received signal.  There is
power lost between stages, but since the signal levels are so low, the
amount of power involved is very low.  Of course, there could be a
considerable amount of processing involved with decoding and deciphering.
Still ..

You are implying a very advanced technology which sounds very interesting.
I will sure check this out!

In my example, I did take into account the twice-reception rule.  Assuming a
path of five, including end points, four of the nodes will receive the
packet twice.  This is not all a loss, since the receipt of a packet
forwarded to the next node can function as an implicit ACK.  Of course, if
other nodes are in the vicinity, they too will get the double reception.
The power consumed in the receiver would also be a function of how much
processing is needed to determine what to do with the packet.

I think we agree, however, that conditions can speak in favor of either a
shout or whisper, so the protocol will have to be adaptive.  

John Mullen

-----Original Message-----
From: Miguel Sanchez [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 8:19 AM
To: Mullen, John P. (Contractor)
Cc: [email-masked]
Subject: Re: [manet] tx power adjustment

Hi John,

See my comments below ...

>I follow your argument, but come to a different conclusion.
That's the point of discussing things :-)

>radius of the distance from the originator to the destination.  
Hmm, I agree. But if you whisper twice some nodes will receive this 
packet twice too.  Depending on where  nodes are located this can be a 
large or small number of nodes ...

>center to one edge would involve about 80 nodes (pi r^2).
I agree.

>cranked much higher than one actually locked on a signal).
As other folks on the list, my thinking is tightly coupled to 802.11 
standards. Most of the available products require while receiving around 
70% of the energy used when transmitting (so transmit and receive energy 
consumption is, for this particular technology  now, withing the same 
magnitude order). Other technologies (or even the new generations of 
CMOS 802.11 chipsets could change this dramatically ...)

>of adaptive control will probably be the answer. 
Several authors (and I think Broadcom too) have proposed different 
adaptive schemes to do so. Cell phones , although not an ad-hoc network, 
have been routinely using transmission power control to achieve higher 
cell usage and/or to improve link quality.

Please note that I am not claiming that "whispering" twice is 
necessaryly better than one loud transmission, but I wanted to show that 
packet reception energy cost can be significant  and can favor one or 
the other choice, again strongly tied to the network topology and traffic.

Best regards,

Miguel Sanchez

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 2 Jul 2002 08:49:24 -0600

Hi Miguel,

:-)

John Mullen

-----Original Message-----
From: Miguel Sanchez [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 8:19 AM
To: Mullen, John P. (Contractor)
Cc: [email-masked]
Subject: Re: [manet] tx power adjustment

Hi John,

I agree.

Best regards,

Miguel Sanchez

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 2 Jul 2002 09:25:28 -0600

Hi Miguel,

You probably hit the key to the receiver power need when you mentioned
speed.  Decoding involves Fast Fourier Transforms, etc.  If the clock speed
is very high, so will be power consumption.

This RTS/CTS idea is intriguing.  Nodes outside the transmission range of
the originator will not know that some of the nodes are shut down, but even
so, if a distant node tried to transmit to one of the nodes that would be
shut down, there would be a data collision, anyhow.  Either way, the end
result is the same.  That could work.

Another idea might be a two part packet.  The first part is the packet's ID,
source, etc. and the second is the data.  They are sent as a single physical
packet, but encoded in a way that the first part can be decoded prior to
decoding the second.  A receiver, upon decoding the first part would realize
that the packet is not directed at it or a duplicate of a packet already
received and simply discard the rest of the packet without decoding it.
That could save on receiver power, too.

John Mullen

-----Original Message-----
From: Miguel Sánchez [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 9:01 AM
To: Mullen, John P. (Contractor)
Subject: Re: [manet] tx power adjustment

Hi John,

I think that most of the energy budget of an spread-spectrum radio has 
to do with the control logic. RF wideband amplfier will acount (I guess) 
a negligible amount of power, but correlator and decoding section (logic 
working at very high speeds) needs a considerable amount of enery.

However, if you are using an RTS/CTS control dialog between 
sender/receiver then an easy optimization can shutdown any neighboring 
node while the transmission is taking place (signaled by the RTS/CTS 
exchange), thus zeroing the unneeded receptions on neighbor nodes that 
are not the intended receiver (sorry I have no info about what 
manufacturers are actually doing this thing).

Best regards,

Miguel Sanchez

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 2 Jul 2002 09:25:28 -0600

Hi Miguel,

John Mullen

-----Original Message-----
From: Miguel Sánchez [mailto:[email-masked]]
Sent: Tuesday, July 02, 2002 9:01 AM
To: Mullen, John P. (Contractor)
Subject: Re: [manet] tx power adjustment

Hi John,

Best regards,

Miguel Sanchez

---

From: John Peter Tapsell <[email-masked]>
Date: Tue, 2 Jul 2002 16:29:51 +0100

---

From: John Peter Tapsell <[email-masked]>
Date: Tue, 2 Jul 2002 16:29:51 +0100

---

From: John Peter Tapsell <[email-masked]>
Date: Tue, 2 Jul 2002 16:32:41 +0100

70% boggles my mind too :)
<nl><nl>

Just to double check, this is per node?  So if you had 1 node
transmitting, and 10 listening, the listening would far outweigh
the transmitting by an order of magnitude..
<nl><nl>

---

From: John Peter Tapsell <[email-masked]>
Date: Tue, 2 Jul 2002 16:32:41 +0100