Subject: Feasibility of busy tones

From: Ken Tang <[email-masked]>
Date: Mon, 25 Oct 1999 17:52:22 -0700

Hi,

I was wondering if any of you know anything about the feasibility of
using busy tones as described in Tobagi/Kleinrock [1] and Hass [2] in
real implementations or is busy tones primarily an academic/research
interest?

[1]. Fouad A. Tobagi and Leonard Kleinrock, “Packet Switching in Radio
Channels: Part II – The Hidden Terminal Problem in Carrier Sense
Multiple Access and the Busy Tone Solution”, IEEE Trans. Comm, COM-23,
1975.

[2]. Jing Deng and Zygmunt J. Haas, “Dual Busy Tone Multiple Access
(DBTMA): A New Medium Access Control for Packet Radio Networks,” IEEE
ICUPC’98, Florence, Italy, October 5-9, 1998.

Ken

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Mon, 25 Oct 1999 22:39:35 -0400 (EDT)

Hi Ken,

Could you be more specific in what do you mean by "feasibility"?

Zygmunt Haas.
==-=-=-=-=-=-=-=

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Prof. Zygmunt J. Haas                    tel: +1-607-255-3454
Wireless Networks Laboratory		                  
School of Electrical Engineering         fax: +1-607-255-9072 
Cornell University
323 Frank Rhodes Hall			 e-mail: [email-masked]
Ithaca, NY 14853			 
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From: "Ken Tang" <[email-masked]>
Date: Mon, 25 Oct 1999 21:37:41 -0700

Hi Prof. Haas,

What I meant by "feasibility" is that can current radios support busy tones?
From what I read about busy tones, the busy tone solution seems to require
additional transceivers to support it.  However, most radios only have one
transceiver.  From what I understand, having more than one transceiver is
extremely expensive and difficult to implement.  I guess another way of
wording
my question is "Is busy tones practical?"

Ken

----- Original Message -----
From: Prof. Zygmunt J. Haas <[email-masked]>
To: Ken Tang <[email-masked]>
Cc: Zygmunt Haas <[email-masked]>; manet <[email-masked]>
Sent: Monday, October 25, 1999 7:39 PM
Subject: Re: Feasibility of busy tones

Hi Ken,

Zygmunt Haas.
==-=-=-=-=-=-=-=

On Mon, 25 Oct 1999, Ken Tang wrote:

> [1]. Fouad A. Tobagi and Leonard Kleinrock, "Packet Switching in Radio
> Channels: Part II - The Hidden Terminal Problem in Carrier Sense
> Multiple Access and the Busy Tone Solution", IEEE Trans. Comm, COM-23,
> 1975.
>
> [2]. Jing Deng and Zygmunt J. Haas, "Dual Busy Tone Multiple Access
> (DBTMA): A New Medium Access Control for Packet Radio Networks," IEEE
> ICUPC'98, Florence, Italy, October 5-9, 1998.
>
>
> Ken
>
>

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Tue, 26 Oct 1999 10:21:05 -0400 (EDT)

Hi Ken,

OK, I understand now your concern.

Although, to the best of my knowledge, you are right in claiming that
"busy tones" are not used in current MAC layer (anyone, please correct me
if I am wrong), practical use of busy tones is rather simple. As a point
of reference, today's cellular air protocols use similar features to busy
tones. For instance, the Supervisory Tone (SAT) in AMPS is nothing more
than a tone in the band within the respective channel. Other air protocols
use similar technology. There is no need for any extra transmitters or 
receivers at the nodes, as the tones can be easily mixed with the actual
transmission at IF and extracted by a notch filter at the receiver.

Best,

Zygmunt Haas.
==-=-=-=-==--=

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Wed, 27 Oct 1999 09:50:19 -0400 (EDT)

Hi Ajay,

> base-station. The ST (supervisory tone) is used for signalling
> and the carrier for data. Both ST and data are transmitted by node
> A and received by Node B. That way node A is only transmitting and 
> node B is receiving. The node B now has to demultiplex the data 
> and the signalling message. This is similar to frequency division
> multiplexing which has been done for a while.
>  

In my e-mail, I was refering to SAT *not* to ST in AMPS. SAT is a narrowband
signal (a tone, at one of three frequencies) and can be notched out by a
simple filter, as it is done in AMPS. There is no need to demultiplex the 
data to get the busy tones.

> operates in full duplex mode. As far as I know, full duplex 
> wireless transceivers (that transmit and receive 
> in the same channel) have not been built. The main problem
> is isolating the transmit and receive paths in the RF front-end.
> A big fraction of the transmitted signal will leak into
> the receive path with the current Rf isolators. 

As stated, the busy tones are signals that should be notched out at the
receiver. Furthermore, there is no need to *continuously* monitor the
busy tones - rather looking at discrete times is sufficient. Thus, a 
simple transmitter/receiver design is possible in which the transmitter
does not receive during its transmission. As such, no problem of signal 
isolation exists. 

Whether such a receiver was built or not is not an issue here.

Best,

ZYgmunt Haas.
==--=-==--=-=

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From: Ajay Gummalla <[email-masked]>
Date: Wed, 27 Oct 1999 09:11:14 -0400

Dear Ken: 
The implementation of a single channel radio transceiver for busy tone
protocols is not as simple as Dr. Haas had explained. There is one
significant difference from the operation of the ST in AMPS and
the busy tone. 

  Consider an example: Node A wants to send a packet to Node B
In the AMPS case, Node A is the cell phone and node B the 
base-station. The ST (supervisory tone) is used for signalling
and the carrier for data. Both ST and data are transmitted by node
A and received by Node B. That way node A is only transmitting and 
node B is receiving. The node B now has to demultiplex the data 
and the signalling message. This is similar to frequency division
multiplexing which has been done for a while.
 
In the case of busy-tone protocols, the node A should 
transmit data  and receive the busy-tone at the same time.
The node B has to transmit busy-tone and receive data 
at the same time. This amounts to the RF front end that 
operates in full duplex mode. As far as I know, full duplex 
wireless transceivers (that transmit and receive 
in the same channel) have not been built. The main problem
is isolating the transmit and receive paths in the RF front-end.
A big fraction of the transmitted signal will leak into
the receive path with the current Rf isolators. 
This is why two different transceivers are required to 
implement busy-tone protocols. 

To our advantage, the busy-tone channel is a narrowband channel 
compared to the data channel. Taking advantage of this, we beleive
a single transceiver radio can be built with the careful
engineering.  Please refer to our work:
http://www.cc.gatech.edu/~ajay/papers/vtc99.ps.gz
 
Ajay
--
Ajay Chandra
[email-masked]

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From: Ajay Gummalla <[email-masked]>
Date: Wed, 27 Oct 1999 11:02:39 -0400

Dr. Haas:
   I think we are trying to discuss whether the transmitter can be
built or not. I think SAT is one of the supervisory tone.  However, my
point was that the solution is not as simple as notching the SAT in AMPS.

The key difference is, in AMPS the data carrier
and the SAT are coming into the receiver at relatively the same 
signal strength.  As an example consider
the transmitted signal is 0dB and the received signal is -60dB. 
In AMPS both the data and SAT tone are received at -60dB. Therefore
the SAT can be notched out at the IF. 

Even though the busy tone can be sampled at specific instants, the busy
tone is always 'ON' at the destination node. Current day isolators
can give about -20dB isolation. Therefore RF front end LNA will see,
the busy-tone coming in at -20dB and the data coming
in at -60dB. Therefore the notch filter should provide -40 dB attenuation to 
notch the tone at the RF frequency or the front-end LNA will saturate. 
Now if the centre frequency of the carrier is 1 GHz, and the 
bandwidth of the data channel say is 1MHz.  The filter is required 
to notch the busy-tone and pass the data signal unchanged. 
Say we allow 10 KHz for the notch  bandwidth. So we are 
trying to design a analog notch filter, 10 KHz wide
at 1 GHz and needs to provide attenuation of atleast -40 dB. 
I think RF designers will agree that it is a non trival design.

As a final comment, there are advantages to *continously* monitoring 
the busy-tone like power control. Either way, the discrete sampleing
only make solves the problem at the transmitter not the receiver.

Ajay

--
Ajay Chandra
 [email-masked]

Nearly all men can stand adversity, but if you want to test a man's
character, give him power.
                -- Abraham Lincoln

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Wed, 27 Oct 1999 16:51:44 -0400 (EDT)

First, let me say that I believe that the busy tone should be transmitted
at much higher power than the data signal itself, so you have some of the
attenuation requirement of the notch filter reduced this way, already.

Second, although I am not a filter designer, I have seen amazing values of
the Q-factor in some technologies. Maybe some filter designer on the group
can comment on the severity of these requirements.

Third, everything is a question of trade-off, if you enlarge your
bandwidth requirements, your filter requirements are eased as well.

Finally, using busy tones for *continuous* signal sampling for power
control will not work, as at any point in time there may be many nodes
that transmit the busy tone. It would be very difficult (impossible?) to
associate a specific measurement with a particular node.

Zygmunt Haas.
==-=-=-=-==-=-=

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From: "Ajay Gummalla" <[email-masked]>
Date: Wed, 27 Oct 1999 19:58:10 -0400

> attenuation requirement of the notch filter reduced this way, already.

If the busy tone is actually transmitted at a higher power than the
data signal, the attenuation requirements at the receiver are now much
more stringent. Though at the tranmitter they would be reduced.

Ex data at 0dB and BT at 10dB. The data power coming into the receiver
is -60dB and the BT leaking into the receive path -10dB. Attenuation
required -50dB. Therefore, if we assume that we have only
one notch filter we would have to design for this case.

> associate a specific measurement with a particular node.

Here again i would not agree. As an example consider,
receiver initiated busy tone multiple access (RI-BTMA).
A sends data for B. If B receives the data and knows it is the
destination, it asserts the busy tone.  In this protocol, if one
assumes reciprocal links, then node A can receive the BT only
from B.  In real world, A might receive some power from other BTs but
they will be very insignficant compared to the power from B. (otherwise
the BT is not serving its purpose ?)
Maybe it is not all that difficult to use it for power control.

Ajay

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Wed, 27 Oct 1999 21:20:21 -0400 (EDT)

As I said in my previous e-mail, you may want to get an opinion of a
filter designer.

Best,

ZYgmunt Haas.
=-=-=-==--=-=-=

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From: [email-masked]
Date: Thu, 28 Oct 1999 10:51:37 +0200

Dear Ajay,

You will probably need a physical separation of the transceiver antennas.
Anyhow it is difficult to obtain two frequencies from regulatory
authorities far enough to allow full duplex.

As I said in a previous mail, a discontinuous busy tone transmission on
same base frequency could be an affordable solution. 

Philippe

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From: [email-masked]
Date: Thu, 28 Oct 1999 15:38:22 +0200

>Anyhow it is difficult to obtain two frequencies from regulatory
authorities >far enough to allow full duplex.

>As I said in a previous mail, a discontinuous busy tone transmission on
same >base frequency could be an affordable solution. 

Dear Ajay,

Nobody is perfect: it was part of a mail sent to Ken. I give a copy below:

Philippe

>As far practical issue is concerned you must consider that all your access
>signals must be done on the same frequency domain as the data and you
should >forget about transmitting while receiving. 

>One practical implementation of busy tone could be that long packets are
>interrupted by regularly spaced gaps, where receiver(s) can insert short
busy >tone bursts. This may have several advantages:
>1. short bursts inform hidden nodes that a transmission occurs and they
have >to make silence during the intergap period. 
>2. short bursts can be used as intermediate ACK between transmitter and
>receivers.

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From: "Prof. Zygmunt J. Haas" <[email-masked]>
Date: Thu, 28 Oct 1999 17:17:25 -0400 (EDT)

One more remark on this matter and I am "off the air" (:-).

The busy tones do not need to be implemented in the signal band. For
instance, (and this is what we have proposed in our Dual Busy Tone
Multiple Access Scheme (DBTMA)), the tones are on the two sides of the
data transmission band. This way, if the tones are sufficently far from 
the transmission band, getting enough filter attenuation can be a much 
simpler problem.

Of course, there will be some penalty in bandwidth (there is no free
lunch), but if the data rate is high (large data bandwidth), the overhead
should be reasonable.

Now, back to normal reception (:-) ...

Zygmunt Haas.
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