Subject: [manet] Interference in ad hoc network

From: "Jack Wang" <[email-masked]>
Date: Tue, 9 Jul 2002 17:27:47 -0400 (EDT)

How to prevent interference in ad hoc network? If there are several node within a node's transmission range, they may interfere with each other. In traditional cellular network, this is done by channel allocation. And in bluetooth, frequency hopping is used and master node of a piconet decides the frequency.How about ad hoc network? Are there any published papers about it? Thanks. Regards, Jack

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From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 9 Jul 2002 16:10:02 -0600

Hi Jack,

In a MANET, usually all nodes share the same channel.  There are exceptions
to this, but you can also think of these exceptions as collections of MANETs
that are interfaced with one another.

The problem is that in order for a node to receive a packet, it must be
tuned to the channel of the sender.  If a MANET consists of just three
nodes, each broadcasting on its own channel, each receiver would have to be
tuned to both of the other node's transmit channels to be sure it doesn't
miss anything.  That means, each node will need two receivers.  In general,
if there are n nodes, each would need n-1 receivers.  This quickly gets out
of hand.

Frequency hopping is just a scrambled version of the same situation.  If
there is just one hop pattern, the transmitter and receiver can be
synchronized, but if there is more than one hop pattern, then you will need
a separate receiver to keep track of each pattern, or one receiver per
frequency to monitor all streams and a sequencer for each active hop
pattern.

There is a cheat on this.  If one uses time division multiplexing, then each
node can have its own time slices to use.  For example, if there are three
nodes, the first uses slices 1, 4, 7, and so on, while the second uses 2, 5,
8, ... and the third 3, 6, 9,...  In this way, each node just needs one
receiver and, if all goes well, there are no data collisions.  However,
given the narrow bandwidth of wireless channels, this means each node in a
n-node network only gets one n-th of the band to work with.  If the need to
communicate is evenly spread out, this can work, but in most cases, the
offered load is too "lumpy" and, as a result, some nodes have far too little
bandwidth while others have far too much.  You can do the same with
frequency division multiplexing and face pretty much the same problem.

Nets with centralized control get around this because the controllers can
dynamically allocate frequencies and time slices, notifying each node of
what is going on.  Because these controllers are in direct contact with each
node, these changes can be sent quickly and reliably as needed.  However,
MANETs do not have centralized control and nodes must keep in touch with
each other, often by way of relay hops, to know what they have to do to.  

Hope this helps,
John Mullen

-----Original Message-----
From: Jack Wang [mailto:[email-masked]]
Sent: Tuesday, July 09, 2002 3:28 PM
To: [email-masked]
Subject: [manet] Interference in ad hoc network
Importance: High

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Content-Type: text/plain; charset="us-ascii"
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 How to prevent interference in ad hoc network? If there are several node
within a node's transmission range, they may interfere with each other. In
traditional cellular network, this is done by channel allocation. And in
bluetooth, frequency hopping is used and master node of a piconet decides
the frequency.How about ad hoc network? Are there any published papers about
it? Thanks. Regards, Jack

--EXCITEBOUNDARY_000__8cdb11f6d0157d59e6d70e8083fae3c7
Content-Type: text/html; charset="us-ascii"
Content-Transfer-Encoding: 7bit

 <table cellpadding=10 cellspacing=0 border=0 width=100% bgcolor=white><tr
height=200><td width=100%><font size=2 color=black><DIV>How to prevent
interference in ad hoc network? If there are several node within a node's
transmission range, they may interfere with each other. In traditional
cellular network, this is done by channel allocation. And in bluetooth,
frequency hopping is used and master node of a piconet decides the
frequency.</DIV><DIV>How about ad hoc network? Are there any published
papers about it? Thanks.</DIV><DIV> </DIV><DIV>Regards,</DIV><DIV>
</DIV><DIV>Jack<BR><BR><BR><BR></DIV></font></td></tr></table><p><hr><b>Join
Excite! - <a href="http://www.excite.com/?PG=Email&SEC=Signature"
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From: "Manfredi, Albert E" <[email-masked]>
Date: Tue, 9 Jul 2002 18:01:49 -0400

Within a single MANET, the nodes need to contend for channel access, and perhaps even reduce their transmitter power level (this was discussed recently) so as not to interfere with each other. The basic scheme used in IEEE 802.11 is similar to Ethernet CSMA. Since MANET depends on each node acting as a relay station potentially, all nodes must be capable of hearing one another, if within range. This is essential. You do not rely on base stations for coverage, with MANET.

But there's no reason why multiple MANETs can't be created in a given geographical area, is there? If you use IEEE 802.11b for the RF solution, for example, you could assign a different DSSS code to different MANETs, so the nodes of different MANETs will not interfere with one another. This is analogous to your Bluetooth example, where different frequency hop sequences are used.

Or you could use a simple frequency divided radio scheme for this too. Just assign a different RF frequency band to each MANET in the area, and let these nodes broadcast to all nodes within reach, within their own MANET.

If the packets are explicitly routed, this should work fine.

If there are clusters of nodes in a given MANET that are very far apart, you could have cases where intra-cluster traffic of these far-apart clusters occurs simultaneously, without causing interference. That would happen naturally, as the clusters move away from one another, and don't need to worry about hearing other traffic from the remote cluster any longer.

Bert

-----Original Message-----
From: Jack Wang [mailto:[email-masked]]
Sent: Tuesday, July 09, 2002 5:28 PM
To: [email-masked]
Subject: [manet] Interference in ad hoc network
Importance: High

How to prevent interference in ad hoc network? If there are several node within a node's transmission range, they may interfere with each other. In traditional cellular network, this is done by channel allocation. And in bluetooth, frequency hopping is used and master node of a piconet decides the frequency.
How about ad hoc network? Are there any published papers about it? Thanks.
Regards,
Jack

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Tue, 9 Jul 2002 16:10:02 -0600

Hi Jack,

Hope this helps,
John Mullen

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Wed, 10 Jul 2002 07:53:57 -0600

Hi Bert,

Good points.  However, just a couple of wrinkles.

There is certainly no reason you cannot have multiple MANETs in the same
geographical region, but some mechanism is needed to assure channels are
allocated in a way that will avoid interference.  Cellular systems use a
supercel of seven channels which is repeated as needed to cover the area.
The result is much like a honeycomb.  The reused channels are separated by
two cell diameters so interference is unlikely.  Cellular antennas do not
move, so, once established, this pattern reliably prevents interference.
However, the hosts in MANETs do move, so there is the additional problem of
making sure clusters using the same channel stay far enough apart from each
other.  The alternative is to give each of the MANETs its own channel, but
we need to remember there is only so much BW available.

If the different MANETs have occasional need to communicate with each other,
some hosts will have to be gateways.  These will either need two
transceivers each or have to drop out of one MANET to communicate with the
other.  The first solution would be the best to avoid the need to rejoin
after every gateway operation.  Then, there is the problem of which hosts
are to be gateways.  If you cannot be sure that the right host will be in
the right place, you may need to have two transceivers in every host.  There
is also the problem that if each MANET has its own channel, the gateways
will have to determine which other MANETs are in the area and which one(s)
to join.  (Of course, if a gateway joins k other MANETS, it will need k
additional transceivers).  A way around this is to have a single channel
solely for communication between MANETs.  Then, each gateway only needs to
monitor two channels.  Of course, there can be contention on the inter-MANET
channel and, using the additional channel reduces BW available for the
individual MANETs.

Looking more deeply into the matter of available bandwidth, if the band is
sliced up among different users, then each user will have less bandwidth
available.  The most efficient use of a shared resource is usually a pooled
approach so that whoever needs higher capacity can get it, without
limitations caused by subdividing the band.  Of course, there could be
security reasons or technical limitations for subdividing.  For example, one
MANET could be for police, one for fire fighters, one for EMT, etc.
However, a fixed allocation will limit the BW available for each.  A shared
channel allows each to use what they need (up to channel limits).  

Perhaps, a solution could be a dynamic allocation, in which each MANET has a
minimal assignment, but there is a pool of BW which any MANET can use,
providing there is a need.

Sincerely,

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Tuesday, July 09, 2002 4:02 PM
To: [email-masked]'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

Within a single MANET, the nodes need to contend for channel access, and
perhaps even reduce their transmitter power level (this was discussed
recently) so as not to interfere with each other. The basic scheme used in
IEEE 802.11 is similar to Ethernet CSMA. Since MANET depends on each node
acting as a relay station potentially, all nodes must be capable of hearing
one another, if within range. This is essential. You do not rely on base
stations for coverage, with MANET.

But there's no reason why multiple MANETs can't be created in a given
geographical area, is there? If you use IEEE 802.11b for the RF solution,
for example, you could assign a different DSSS code to different MANETs, so
the nodes of different MANETs will not interfere with one another. This is
analogous to your Bluetooth example, where different frequency hop sequences
are used.

Or you could use a simple frequency divided radio scheme for this too. Just
assign a different RF frequency band to each MANET in the area, and let
these nodes broadcast to all nodes within reach, within their own MANET.

If there are clusters of nodes in a given MANET that are very far apart, you
could have cases where intra-cluster traffic of these far-apart clusters
occurs simultaneously, without causing interference. That would happen
naturally, as the clusters move away from one another, and don't need to
worry about hearing other traffic from the remote cluster any longer.

Bert

-----Original Message-----
From: Jack Wang [mailto:[email-masked]]
Sent: Tuesday, July 09, 2002 5:28 PM
To: [email-masked]
Subject: [manet] Interference in ad hoc network
Importance: High

the frequency.

---

From: Jorge Nuevo <[email-masked]>
Date: Wed, 10 Jul 2002 11:25:04 -0400 (EDT)

Hello Jack:

As already said, in a MANET all nodes share the same channel where the 
choice of medium access scheme is difficult due to the time-varying network
topology and the lack of centralized control. Use of TDMA or dynamic
assignment of frequency bands is complex since there is no centralized
control as in cellular networks, FDMA is inefficient in dense networks,
CDMA is difficult to implement due to node mobility and the consequent
need to keep track of the frequency hopping patterns and/or spreading
codes for nodes in the time-varying neighborhood, so random
access appears to be the current favorite.

Applicability of the random access schemes are limited by two 
interference problems: the hidden terminal problem and the 
exposed terminal problem. In general, the hidden terminal problem 
reduces the capacity of a network due to the increasing  number of 
collisions, while the exposed terminal problem reduces the network
capacity due to the unnecessarily deferring nodes
from transmitting. Several attempts have been made in the
literature to reduce the effect of these two problems. Protocols
based on a RTS/CTS dialogue such as MACA, MACAW and FAMA deal with 
these problems. Information of these can be found at

"MACA - A new channel access method for packet radio", P. Karn,
"MACAW: A media access protocol for wireless LAN's", V. Bharghavan and 
        others
"Floor acquisition multiple access (FAMA) for packet radio networks",
       C.L. Fullmer and J.J. Garcia-Luna Aceves
       
Hope it helps

Jorge Nuevo
[email-masked]

---

From: Jorge Nuevo <[email-masked]>
Date: Wed, 10 Jul 2002 11:25:04 -0400 (EDT)

Hello Jack:

       
Hope it helps

Jorge Nuevo

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 11:47:36 -0400

Nikhil Jain [mailto:[email-masked]] wrote:

> I would recommend that we think about using CDMA. In CDMA we 
> use a reuse of 
> 1. Which means that the same channel is used in all the 
> cells. Ofcourse the 
> moving users present new issues that have to be solved.

I must be missing something in this thread.

IEEE 802.11b uses DSSS: direct sequence spread spectrum. Is this not the foundation of CDMA? I know 802.11b doesn't need all the bells and whistles that cellular CDMA uses to set up channels, but the basics are the same, yes?

For a single ad-hoc network, be it just IEEE 802.11 link layer ad-hoc, or MANET, all mobile hosts in the ad-hoc net have to share the same channel. That is, the same code of DSSS is used. (This is obviously different from cellular CDMA, where each RF link is merely a point-to-point link between one mobile host and the base station.) In ad-hoc nets, mobile hosts are used as hops. They must share the channel. So in 802.11b, you don't have to worry about mobile hosts being told in real time what code to use, or to adjust their power levels, or all the other intricacies of cellular telephone CDMA. In effect, you have a bunch of mobile hosts all sharing one CDMA channel.

If we used frequency hopping (FHSS), the same basic concept applies. In this case, it would be the same sequence of frequency hops, rather than the same code in a DSSS example, used by any given ad-hoc network.

If we used plain old frequency division, the same concept applies. The hosts of one ad-hoc net would have to share one frequency band.

In all of the above, different ad-hoc networks could coexist in the same geographical area simply be using a different channel.

The issues with moving users must be managed at higher layers than the physical layer, because they are fundamental to the routing of ad-hoc network data frames.

I guess what I'm saying is that interference between different ad-hoc nets is managed at the physical layer, by doing whatever the RF technique requires to separate channels, but within an ad-hoc net, you _want_ the hosts to hear each other.

How great the coverage of each host is creates interesting tradeoffs of number of hops required between A and B, vs. number of potential collisions experienced by MANET hosts when trying to transmit something. Not clear to me that the optimal solution is obvious. Low power = lots of hops = lots of contentions for the channel, but fewer hosts contending each time.

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 11:47:36 -0400

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 10:22:32 -0600

Hi Bert,

Well, if you are missing something, I don't see it.

Just one thing, though.  As you point out, the choice of many low-power hops
vs. a few high-power hops is not obvious.  I would like to add that the
preferred strategy might differ as one goes from region to region within a
single MANET.  That is, the final answer might be a protocol that can decide
which option to employ locally based on local conditions, but able to
operate seamlessly with other parts of the MANET, even if they are using
different strategies.

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 9:48 AM
To: 'Nikhil Jain'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

IEEE 802.11b uses DSSS: direct sequence spread spectrum. Is this not the
foundation of CDMA? I know 802.11b doesn't need all the bells and whistles
that cellular CDMA uses to set up channels, but the basics are the same,
yes?

For a single ad-hoc network, be it just IEEE 802.11 link layer ad-hoc, or
MANET, all mobile hosts in the ad-hoc net have to share the same channel.
That is, the same code of DSSS is used. (This is obviously different from
cellular CDMA, where each RF link is merely a point-to-point link between
one mobile host and the base station.) In ad-hoc nets, mobile hosts are used
as hops. They must share the channel. So in 802.11b, you don't have to worry
about mobile hosts being told in real time what code to use, or to adjust
their power levels, or all the other intricacies of cellular telephone CDMA.
In effect, you have a bunch of mobile hosts all sharing one CDMA channel.

If we used frequency hopping (FHSS), the same basic concept applies. In this
case, it would be the same sequence of frequency hops, rather than the same
code in a DSSS example, used by any given ad-hoc network.

If we used plain old frequency division, the same concept applies. The hosts
of one ad-hoc net would have to share one frequency band.

In all of the above, different ad-hoc networks could coexist in the same
geographical area simply be using a different channel.

The issues with moving users must be managed at higher layers than the
physical layer, because they are fundamental to the routing of ad-hoc
network data frames.

I guess what I'm saying is that interference between different ad-hoc nets
is managed at the physical layer, by doing whatever the RF technique
requires to separate channels, but within an ad-hoc net, you _want_ the
hosts to hear each other.

How great the coverage of each host is creates interesting tradeoffs of
number of hops required between A and B, vs. number of potential collisions
experienced by MANET hosts when trying to transmit something. Not clear to
me that the optimal solution is obvious. Low power = lots of hops = lots of
contentions for the channel, but fewer hosts contending each time.

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 10:22:32 -0600

Hi Bert,

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 9:48 AM
To: 'Nikhil Jain'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 12:20:08 -0400

Good point about asymmetry not required or desirable in an ad-hoc net, Phil. I forgot about that. With base stations, asymmetry helps.

I have a suggestion on a transmitter power rule of thumb. If the typical path of MANET data packets covers a large area of the MANET, then high power is good because it reduces the number of hops required for each packet, reducing the busyness of every host.

If the typical data path is short, mostly between adjacent hosts, then the transmitter power is best kept low, because this permits different clusters to transmit packets among their own hosts simultaneous to other clusters doing the same. More aggregate throughput.

Bert

> From: Phil Neumiller [mailto:[email-masked]]
> Sent: Thursday, July 11, 2002 12:09 PM
> To: Mullen, John P. (Contractor); Manfredi, Albert E; [email-masked];
> Nikhil Jain
> Subject: Re: [manet] Interference in ad hoc network
> 
> 
> MeshNetworks uses CDMA, but not the Qualcomm type.  Rather, a 
> version that
> is suitable for ad hoc networks.  The Qualcomm type (3G type) 
> has asymetric
> channel structure, i.e. the reverse and forward links are 
> structured differently at
> the phy and link layers.  This poses problems for a 
> peer-to-peer network like a
> MANET.  There is also a lot of intelligence in the IS-94A/B/C 
> (3G) BTS that
> you would not want to replicate in an inexpensive ad hoc 
> radio.  Our MAC
> is called QDMA.  It supports aspects of CDMA, TDMA, FDMA, and CSMA
> all simultaneously.

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 12:20:08 -0400

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 10:54:49 -0600

Hi Bert,

I agree on the short path, but on long paths, it is not so clear.  If there
are many hosts within the broadcast range that are not involved in the
interchange, they cannot use the channel  for other interchanges during the
transmission.  If low power is used, fewer hosts are affected at each hop
and some may not be affected at all.  There could be several low-power hops,
each involving different messages, at the same time without interference, as
long as they are far enough from each other.

The preferred strategy would depend on how many uninvolved hosts are
impacted by each transmission, as well as the topology of the situation.
For example, if the hosts are clustered near the sender and the receiver is
a great distance away, the high power hop would be better since a
transmission from any of the clustered hosts powerful enough to reach the
destination will affect all the other clustered hosts anyhow.  

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 10:20 AM
To: 'Phil Neumiller'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

Good point about asymmetry not required or desirable in an ad-hoc net, Phil.
I forgot about that. With base stations, asymmetry helps.

I have a suggestion on a transmitter power rule of thumb. If the typical
path of MANET data packets covers a large area of the MANET, then high power
is good because it reduces the number of hops required for each packet,
reducing the busyness of every host.

If the typical data path is short, mostly between adjacent hosts, then the
transmitter power is best kept low, because this permits different clusters
to transmit packets among their own hosts simultaneous to other clusters
doing the same. More aggregate throughput.

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 10:54:49 -0600

Hi Bert,

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 10:20 AM
To: 'Phil Neumiller'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

Bert

---

From: Ken Peirce <[email-masked]>
Date: Thu, 11 Jul 2002 10:48:10 -0700

Phil,
Let me be indelicate and ask if this version of CDMA is encumbered by patents?
Cheers,
Ken

-----Original Message-----
From: Phil Neumiller [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 9:09 AM
To: Mullen, John P. (Contractor); Manfredi, Albert E; [email-masked];
Nikhil Jain

MeshNetworks uses CDMA, but not the Qualcomm type.  Rather, a version that
is suitable for ad hoc networks.  The Qualcomm type (3G type) has asymetric
channel structure, i.e. the reverse and forward links are structured differently at
the phy and link layers.  This poses problems for a peer-to-peer network like a
MANET.  There is also a lot of intelligence in the IS-94A/B/C (3G) BTS that
you would not want to replicate in an inexpensive ad hoc radio.  Our MAC
is called QDMA.  It supports aspects of CDMA, TDMA, FDMA, and CSMA
all simultaneously.

----- Original Message -----
From: "Nikhil Jain" <[email-masked]>
To: "Mullen, John P. (Contractor)" <[email-masked]>; "Manfredi, Albert E" <[email-masked]>;
<[email-masked]>
Sent: Thursday, July 11, 2002 10:57 AM
Subject: RE: [manet] Interference in ad hoc network

> I would recommend that we think about using CDMA. In CDMA we use a reuse of
> 1. Which means that the same channel is used in all the cells. Ofcourse the

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 17:46:57 -0400

Martin Haenggi wrote:

> > Within a single MANET, the nodes need to contend for channel access,
> 
> > and perhaps even reduce their transmitter power level (this 
> was discussed recently)
> 
> > so as not to interfere with each other. 
> 
> 
> One comment on 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.

Hi Martin,

Hmmm. If the distance between hosts remains the same, receiver sensitivity remains the same, and the power level is reduced for all hosts in the MANET, then each host will detect fewer neighbors. This causes whatever routing algorithm is being used to have to create more hops between any random A and B host in the MANET, on average.

I guess I'm saying that the ratio that matters here is S/(N+I), not just S/I?

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 18:40:19 -0400

Kaixin Xu wrote:

> Just want to say a complement point. I remember that Prof. 
> Kumar at UIUC has
> one paper saying that reducing the transmission power will reduce the
> interference among nodes. However, to maintain the 
> connectivity, there is a
> minimal power level. Thus, from my point of view, reducing 
> transmission
> power is equivalent to reducing the node density.

I agree. And this is because receivers can only receive signals above a given C/N, or C/(N+I), or C/(N+I+echo) margin. As power levels go down, transmitters that were once receivable drop out of sight. All else being equal, this effectively reduces node density.

> So I agree 
> with Haenggi
> that global reduction of the power level cannot help reducing 
> interference.

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.

> It's just a optimization between the node density and 
> transmission range.

Yes, I agree here too. It's an optimization problem with no universal optimal solution. Depends on traffic characteristics in the MANET. Perhaps I just don't understand how the term "interference" is being used here.

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Thu, 11 Jul 2002 18:40:19 -0400

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 16:57:39 -0600

Hi Kaixin,

Although the ratio of signal to interference is constant, receivers work on
the ratio of signal to noise + interference.  As Sumeeth points out in
another email, each node will be able to receive from (and be interfered
with) by fewer nodes.  This serves to localize the impact of the
interference.  It also reduces the number of nodes that can receive the
signal, but that is why you need more hops.

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.  The management of output power is a key
element of reducing interference in broadcast AM radio.  Once the signal
gets down into the grass, it is essentially background noise to the receiver
and ceases to be interference.

That 1.7 factor is interesting.  It sounds about right, but might be band
sensitive, since the inverse square law is overly optimistic for some bands.

John Mullen

-----Original Message-----
From: Kaixin Xu [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 4:25 PM
To: Manfredi, Albert E; 'Martin Haenggi'
Cc: [email-masked]
Subject: Re: [manet] Interference in ad hoc network

Just want to say a complement point. I remember that Prof. Kumar at UIUC has
one paper saying that reducing the transmission power will reduce the
interference among nodes. However, to maintain the connectivity, there is a
minimal power level. Thus, from my point of view, reducing transmission
power is equivalent to reducing the node density. So I agree with Haenggi
that global reduction of the power level cannot help reducing interference.
It's just a optimization between the node density and transmission range.
What really matter are how fast the signal decays and the
transmitter-receiver distance. An interesting thing I have found is that
interference range is usually 1.7 times the transmitter-receiver distance.
Large interference range is harmful since RTS-CTS can only handle
interference nodes within transmission range.

Kaixin

----- Original Message -----
From: "Manfredi, Albert E" <[email-masked]>
To: "'Martin Haenggi'" <[email-masked]>
Cc: <[email-masked]>
Sent: Thursday, July 11, 2002 2:46 PM
Subject: RE: [manet] Interference in ad hoc network

> Hmmm. If the distance between hosts remains the same, receiver sensitivity
remains the same, and the power level is reduced for all hosts in the MANET,
then each host will detect fewer neighbors. This causes whatever routing
algorithm is being used to have to create more hops between any random A and
B host in the MANET, on average.

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Thu, 11 Jul 2002 16:57:39 -0600

Hi Kaixin,

John Mullen

-----Original Message-----
From: Kaixin Xu [mailto:[email-masked]]
Sent: Thursday, July 11, 2002 4:25 PM
To: Manfredi, Albert E; 'Martin Haenggi'
Cc: [email-masked]
Subject: Re: [manet] Interference in ad hoc network

Kaixin

---

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:]
> the ratio of signal to noise + interference.
> 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

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Sun, 14 Jul 2002 17:48:25 -0400

[email-masked] (Martin) wrote:

> I had written:
>
> > I guess I'm saying that the ratio that matters here is 
> S/(N+I), not just 
> S/I?
> 

I think the general topic of discussion was that the reduction of transmitter power causes a reduction of inter-host interference in a MANET, and the extent to which this helps (or hurts?) the MANET problem. And some interesting discussions were spawned, I think. (I don't think one can ignore the effect of background noise in this discussion, since that would create an unrealistic and fairly trivial special case.)

There are clearly many variables to consider, from comments made in all the discussions.

Trying to sum this up, I'll use a special initial condition, and then generalize from there.

If virtually all sessions in the MANET were between very distant hosts, hosts on periphery of the MANET, then high transmit power would reduce the number of hops required for each session, but would also reduce the number of users simultaneously capable of gaining access to the medium.

Keeping the same initial condition (sessions mostly between hosts on the periphery), low transmit power would mandate many hops, with consequently many medium access events, and with hosts in the core of the MANET having to relay packets of multiple sessions. So this requires computationally faster hosts. The multiple medium access evolutions would not necessarily create more latency, though, because each one would encounter less congestion than the high power case. So if you have such fast hosts in the MANET, then even this special initial condition is better met with low transmit power (just enough to reach a neighbor). Because the aggregate throughput of the MANET would go up.

Since, in the more general case, the vast majority of sessions would _not_ be between hosts on the periphery, but would be distributed throughout, I think one can conclude that in the more general case, low power would be beneficial (as long as one or a few neighbors can be reached) _and_ that very fast hosts would not necessarily be required, because all sessions would not require so many hops after all.

Does pretty much cover what everyone said?

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Sun, 14 Jul 2002 17:48:25 -0400

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Mon, 15 Jul 2002 07:25:25 -0600

Hi Bert,

Looks fine to me, except that although I'm sure there are situations in
which low power would be beneficial, I'm not entirely sure that low power
would necessarily be better in the general case.  

The trouble is that in the case of MANETs, the "general case" covers a lot
of territory.

:-)

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Sunday, July 14, 2002 3:48 PM
To: [email-masked]'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

> under discussion. If N<<I, then both S and N+I scale with the (global)

I think the general topic of discussion was that the reduction of
transmitter power causes a reduction of inter-host interference in a MANET,
and the extent to which this helps (or hurts?) the MANET problem. And some
interesting discussions were spawned, I think. (I don't think one can ignore
the effect of background noise in this discussion, since that would create
an unrealistic and fairly trivial special case.)

There are clearly many variables to consider, from comments made in all the
discussions.

Trying to sum this up, I'll use a special initial condition, and then
generalize from there.

If virtually all sessions in the MANET were between very distant hosts,
hosts on periphery of the MANET, then high transmit power would reduce the
number of hops required for each session, but would also reduce the number
of users simultaneously capable of gaining access to the medium.

Keeping the same initial condition (sessions mostly between hosts on the
periphery), low transmit power would mandate many hops, with consequently
many medium access events, and with hosts in the core of the MANET having to
relay packets of multiple sessions. So this requires computationally faster
hosts. The multiple medium access evolutions would not necessarily create
more latency, though, because each one would encounter less congestion than
the high power case. So if you have such fast hosts in the MANET, then even
this special initial condition is better met with low transmit power (just
enough to reach a neighbor). Because the aggregate throughput of the MANET
would go up.

Since, in the more general case, the vast majority of sessions would _not_
be between hosts on the periphery, but would be distributed throughout, I
think one can conclude that in the more general case, low power would be
beneficial (as long as one or a few neighbors can be reached) _and_ that
very fast hosts would not necessarily be required, because all sessions
would not require so many hops after all.

Bert

---

From: "Mullen, John P. (Contractor)" <[email-masked]>
Date: Mon, 15 Jul 2002 07:25:25 -0600

Hi Bert,

:-)

John Mullen

-----Original Message-----
From: Manfredi, Albert E [mailto:[email-masked]]
Sent: Sunday, July 14, 2002 3:48 PM
To: [email-masked]'; [email-masked]
Subject: RE: [manet] Interference in ad hoc network

Bert

---

From: "[email-masked]" <[email-masked]>
Date: Wed, 31 Jul 2002 11:20:12 -0500

Hi John,

sorry for the delayed response. This belongs to a thread that
is two weeks old.

>Well, This might work in Ethernet, but I doubt it will in radio nets.
>
>In theory you can ignore noise, but it is quite evident in wireless
>communications and any discussion that ignores its effect is a bit too
>theoretical for me.

I agree, noise can't be ignored. But you can look at noise issues
and interference issues separately. If you use a Rayleigh fading
model for the physical channel, the probability that the packet gets
through is a product of two probabilities, one that only depends on
the noise, and one that only depends on the interference. This permits
the separate analysis of the effects caused by noise and by interference.
So, to determine the interference part, the noise is put to zero,
and vice versa.

Best,
Martin

---

From: "[email-masked]" <[email-masked]>
Date: Wed, 31 Jul 2002 11:20:12 -0500

Hi John,

Best,
Martin

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Wed, 31 Jul 2002 13:37:59 -0400

> So, to determine the interference part, the noise is put to zero,

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

So unless you worry only about short range effects, I think reducing transmitter power will have a significant effect in reducing interference between MANET hosts that are, say, several miles apart anyway. But not close together hosts, I agree.

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.

There's an article that covers this battery power problem in the June 2001 IEEE Communications Magazine, Maximum Battery Life Routing to Support Ubiquitous Mobile Computing in Wireless Ad Hoc Networks, by C.K. Toh. This article talks among other things about using a "sleep" mode in MANET hosts, to conserve battery power. But, depending on traffic patterns, hosts would have less chance of going into "sleep" mode if virtually every host has to provide relay services. The article introduces something they call "minimum battery cost routing," then a min-max battery cost routing, then a conditional min-max battery cost routing, where the path cost in a Dijkstra shortest path algorithm includes a battery power metric for reception and transmission. Cool. They keep elaborating the problem statement, with things like not asking too much of any one node, and considering the battery capacity of each node, rather than just the instantaneous power 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.

Bert

---

From: "Manfredi, Albert E" <[email-masked]>
Date: Wed, 31 Jul 2002 13:37:59 -0400

Bert