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

From: Martin Haenggi <[email-masked]>
Date: Sun, 11 Aug 2002 16:41:30 -0500 (EST)

Hi Miguel

>>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.
>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).

If the interferers raise their power by the same factor as the
desired transmitter (or less), then the transport capacity increases.
So, the only scenario where this is not the case is when the 
interferers increase their power over-proportionally.

>>- 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 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.

Sure, this only makes sense if there is another node approximately
in the right direction. And it does not have to be the same SNR; if
the new destination node is closer (but still further away than the old one),
there is a gain in transport capacity.

>>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).
>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).

Yes, to be precise, we could (should) define the distance part in the
transport capacity as the distance projected onto the end-to-end axis.

>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.

I agree. For some applications, the question may be whether it is
worthwile spending resources on power control.

Best,
Martin

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---

From: Sumeeth Nagaraj <[email-masked]>
Date: Mon, 12 Aug 2002 10:28:01 -0600

----- Original Message -----
From: "Martin Haenggi" <[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Sent: Sunday, August 11, 2002 3:41 PM
Subject: Re: [manet] Interference in ad hoc network

> interferers increase their power over-proportionally.

I think, all the nodes use the same power unless you have an algorithm
running to control the power in the network to different nodes. if this is
the case, does the power control algorithms take care of Uni directional
links, if so what would be the complexity?.

> there is a gain in transport capacity.

In my opinion, when you increase the transmitting power, you tend to address
more nodes in the vicinity. thereby more nodes tend to receive the packets
and decode the packet before dropping them since it *may* not belong to
them. This would certainly increase the medium access delay for each node,
and hence the packets could be easily dropped because the retries might
exceed max_retry_limit.

> worthwile spending resources on power control.

Best Regards,
Sumeeth

---

From: Sumeeth Nagaraj <[email-masked]>
Date: Mon, 12 Aug 2002 10:28:01 -0600

----- Original Message -----
From: "Martin Haenggi" <[email-masked]>
To: <[email-masked]>
Cc: <[email-masked]>
Sent: Sunday, August 11, 2002 3:41 PM
Subject: Re: [manet] Interference in ad hoc network

Best Regards,
Sumeeth