Rake receiver only for CDMA

Hi,
I know this a very newbie question; but it seems to me that all RAKE
receiver implementation are for CDMA systems. Can it be used for
normal modulation schemes e.g. Coherent (BPSK etc) or non coherent
(FSK).

Thanks.

On Aug 23, 4:53 pm, Affan <int...@gmail.com> wrote:
> Hi,
> I know this a very newbie question; but it seems to me that all RAKE
> receiver implementation are for CDMA systems. Can it be used for
> normal modulation schemes e.g. Coherent (BPSK etc) or non coherent
> (FSK).
>
> Thanks.

Uh..... no.

It's intended for modulations with some "spreading" factor, i.e. the
width of the main lobe of the autocorrelation sequence is much
smaller than the symbol spacing.

BPSK and FSK has no spreading.

Julius

On Aug 23, 5:23 pm, julius <juli...@gmail.com> wrote:
> On Aug 23, 4:53 pm, Affan <int...@gmail.com> wrote:
>
> > Hi,
> > I know this a very newbie question; but it seems to me that all RAKE
> > receiver implementation are for CDMA systems. Can it be used for
> > normal modulation schemes e.g. Coherent (BPSK etc) or non coherent
> > (FSK).
>
> > Thanks.
>
> Uh..... no.
>
> It's intended for modulations with some "spreading" factor, i.e. the
> width of the main lobe of the autocorrelation sequence is much
> smaller than the symbol spacing.
>
> BPSK and FSK has no spreading.
>
> Julius

I may have spoken too quickly:  I assumed you were talking about
a multipath channel.  If there is no multipath, then a matched
filter basically works as a 1-tap rake receiver.  So it depends
on what you mean by "rake receiver".

Julius

>> I may have spoken too quickly
Maybe not... I would have answered the same, only in different words:

- A rake receiver works, because it can "block" a time-delayed part of the
signal by despreading. Different rake fingers can be assigned to different
time delays (multipath), and the outputs are then combined.

-For non-spread signals - for example standard QPSK/QAM/etc streams - a
time-delayed signal appears simply as a different symbol stream. A branch
of a rake receiver has no means to isolate a single delay path of the
channel profile.

Cheers

Markus