power llimited and band limited channels

Hello ;
Can anyone explain me what exactly is power limited channels and band
limited channels??
Can a same signal be transmitted through power limited as well as band
limited channel??what could be limitations?
And how one can decide that a particular signal is suitable for power
limited or band limited channels??

On Oct 22, 6:45&#4294967295;am, "parwaz" <manish_mb...@yahoo.co.in> wrote:
> Hello ;
> Can anyone explain me what exactly is power limited channels and band
> limited channels??
> Can a same signal be transmitted through power limited as well as band
> limited channel??what could be limitations?
> And how one can decide that a particular signal is suitable for power
> limited or band limited channels??


Remembering that the shannon capacity formula for gaussian channels
C = W log (SNR +1)
=> C = W log(P/(N0*W)+1).

Here C is the capacity (in bits/s), W is bandwidth, P is signal power,
N0 is noise power spectral density.

Now consider two cases
CASE1 (PLR):
In case SNR is very low (SNR<<1), we can use power series
approximation of log, and capacity is given by
C =~ W* P/(N0*W) =~ P/N0
So in this case capacity is virtually independent of bandwidth and
directly proportional to signal power. So this channel is called power
limited regime. The most common examples are usually the space
communication and wireless communication channels.

CASE2 (BLR):
In case SNR is high (SNR>>1), we assume that log(SNR+1) ~ log(SNR) and
hence approximately
C =~ W*log(P/N0).
So in this case capacity is directly proportional to bandwidth and but
proportianal only to the logarithm of power . So increase in bandwidth
becomes much more important than increase in power. Hence it is called
bandwidth limited regime. Common examples are wireline channels, data-
modems, DSL etc.

RULE OF THUMB:

Defining nominal spectral efficiency SE (in bits/s/hertz) as
SE = C/W = log (P/(N0*W)).
We usually use Rho <= 2 in power limited regme. And use Rho > 2 in
bandwidth limited regine. Like most rules of thumb, that is not always
true for modern systems, but still quite close to the truth. So we
usually use QPSK (rho=2) or 16-QAM(rho=4) in wireless. In wireline rho
can go even beyond 10 (1024 QAM!!).

Hope this helps.

Regards
Piyush