Re: CDMA receiver sensitivity

On Fri, 12 Jun 2009 09:02:56 -0700, ashu wrote:
(top posting fixed)
On Jun 12, 5:13 pm, Tim Wescott <t...@xxxxxxxxxxxxxxxx> wrote:
On Fri, 12 Jun 2009 06:44:39 -0700, ashu wrote:
Hi All,

This is a theoritical type question.

We know that, under all conditions there is an ambient thermal noise
present and its value is  -174+10log(B) dBm. Where B is bandwidth in
Hz. For any signal to be received the input power of signal should be
higher than this limit.

Now if you consider the case of wideband radios for example CDMA-SS
or UWB signals, each symbol is coded over multiple chips(pulses).
Hence for each chip to be correctly detected at the receiver, the
received power of each chip should be greater than -174+10log(Bc)
(where Bc is the bandwidth of chip).

Is my understanding correct ?

Now since Bc is much higher number than B of narrowband radios, is it
safe to assume that sensitivity of Wideband receivers is much lower
than that of narrow band receivers ?

For a given pathloss does this then mean CDMA transmitters trasmit at
much high power than GSM transmitters?

No, your understanding is not correct.

The important way that it is not correct is in assuming that each chip
has to be correctly received for CDMA (or any other spread-spectrum
technique) to work.  The CDMA receive process works like a bandpass
filter in that many small bits of input are assembled together into a
slowly-varying signal -- it's just that what's assembled together
doesn't happen to be narrow band.  To use the noise power in your
calculations you have to take the bandwidth of the signal _after_
despreading.

The unimportant (at least for this application) way that your
understanding is incorrect is in taking the -178dBm/Hz number as
gospel. Yes, that's the correct number when all the antenna sees is
black-body radiation at 30 degrees centigrade.  No, that's not the
correct number if you have a narrow beam antenna pointed at the sun,
the moon, or at a dark patch of the sky.  So for cell phone
applications you can take the effective temperature of the surroundings
to be 30C, and your assumption is correct.  But it's not universally
correct.

--http://www.wescottdesign.com

Hi Tim,
thanks, not considering the temprature issue to be important. (lets say
its 30C)

I think we still have to receieve all the chips correctly, only then
orthogonal PN sequence will despread the signal properly else the
correlation will give another noise like signal.. isnt it ? that is the
way we maintain multi user possibility in CDMA.

Another thing is for each individual chip to be received correctly, the
chip power after BPF must be greater than -174+10log(Bc) where Bc is the
bandwidth of the chip. (which is very large).

Essentially what I want to say is that the chip power must be greater
than the noise power in the chip bandwidth(or the bandwidth after band
pass filter) for the chip to be received correctly.

Nope nope nope and nope. If you don't believe that, consider that I'm
working on a CDMA application _right now_ that works just fine, and which
has the chips buried deep in the noise even for the "strong" signals.

The whole _point_ of spread spectrum is that your desired signal is below
the noise floor until it's despread, at which point you just low pass
filter and enjoy.

You don't decode the chips, so there's no need for them to be out in the
clear. You take a whole bunch of chips and despread them, so the desired
signal is narrow band, then you average the heck out of the despread
signal. Wideband noise stays wideband, so you can filter it out. Narrow
band noise gets spread into wideband, so you can filter it out. Only the
desired signal gets narrowed up, so it makes it through your filter.

--
www.wescottdesign.com
.

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