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

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

_______________________________________________
manet mailing list
[email-masked]
https://www1.ietf.org/mailman/listinfo/manet

---

From: Miguel Sanchez <[email-masked]>
Date: Sun, 11 Aug 2002 19:20:47 +0200

Hi Martin,

Thanks for the explanation.

I have a few comments below:

>  this also leads to a higher transport capcity.
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).

>  increase by a factor d2/d.
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.

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

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).

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.

Best Regards,

Miguel Sanchez
Universidad Politécnica de Valencia, Spain

---

From: Miguel Sanchez <[email-masked]>
Date: Sun, 11 Aug 2002 19:20:47 +0200

Hi Martin,

Best Regards,

Miguel Sanchez