Subject: [manet] Feasibility of simultaneous TX and RX at an MANET node?

From: Leon Martin <[email-masked]>
Date: Wed, 16 Nov 2005 10:26:19 -0800 (PST)

Hi all,

I've seen a lot of papers talking about that it is
feasible for a MANET node to transmit and receive at
the same time using two radios.

However, I have serious doubt on this. To see this,
let's say the transmit power is Pt, and the power of
the received signal is Pr. If the signal source is far
away, then Pt could be easily greater than Pr by
several orders, say 10000 times. We know that the
radios are not ideal, which causes channel leakage. So
even a tiny fraction of the energy of the transmit
radio leaks into the receiving channel, the signal
being received will be destroyed. 

Can anyone tell me how feasible is it to do TX and RX
at the same time?

Thanks!

- Leon 

		
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From: Jim Thompson <[email-masked]>
Date: Wed, 16 Nov 2005 08:33:56 -1000

Leon Martin wrote:

>Can anyone tell me how feasible is it to do TX and RX at the same time?
>  
>
On the same channel?   Thats a 100dB problem, my friend.    Not in our 
lifetime.

On different channels?  Probably, with sufficient separation (frequency 
and/or space) and/or filtering.

In different bands?  Definitely (but this is just a degenerate case of 
"different channels").

Jim

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From: Joe Carey <[email-masked]>
Date: Thu, 17 Nov 2005 08:15:41 -0700

Leon,

To elaborate a little on what Jim Thompson has already said,

Most MANET research has been conducted on radios using 802.11 cards and 
many people assume 802.11 when we talk about MANET.  I agree with your 
assesment for radios in the 2.4 GHz ISM band using 802.11 PHY - there 
simply isn't enough frequency separtion between channels to build a 
practical filter for most applications.    I have heard of some 
experiments that used very large, heavy, exceptionally narrow high Q 
cavity filters to pull this off.  Without knowing the details of the 
particular radio chain, it is difficult to estimate what the filter 
specs need to be.  In the best case, the problem becomes one of figuring 
out how much the final power amplifier raises the noise floor on nearby 
channels (this is sometimes called "splatter") and filtering the signal 
by a bit more than that.

Transmitting and receiving on the same channel it is essentially 
impossible.  Reflections from nearby objects will make the required 
isolation impossible to achieve.

However, if you didn't need to maintain 802.11 PHY layer, then it would 
be possible.  For example, CDMA handsets always transmit and receive 
simultaneously in essentially the same band (although they separate it 
into a TX region and an RX region).  The filter they use is called a 
duplexer and it is one of the most critical parts of the radio.  
Cellular is of course a point-to-multipoint topology; I'm not sure this 
split-band radio architecture can be extended to ad-hoc topologies.

joe

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From: "Paul A.S. Ward" <[email-masked]>
Date: Thu, 17 Nov 2005 11:00:59 -0500

We do it in out lab.  The channels have to be sufficiently separated, 
but there is enough separation among a channels to find non-cliflicting 
pairs.  It also varies by hardware.