>>The use of an oversampled signal when correlating especially
>>confuses me
Hello,
ideally, if the signals are perfectly aligned and filtered, it does not
matter at what rate I do the correlation. This is the case in a typical
link level simulation, unless one intentionally uses blocks that delay the
signal by a fractional number of samples. Transmitted and received signal
are always sampled at the same phase.
In an actual link, the sampling phase is not the same, and any time offset
deteriorates reception (lower numbers from correlation).
One solution is to use a higher a sampling rate, and pick the one that
aligns the signals best, as explained in previous posts.
BTW:
The system response is raisedcosine (two RRCs at Tx/Rx), and together
they meet the Nyquist pulse shaping criterion.
It means I can sample back down to rate 1 without losing information, *if*
the timing (=sampling phase) is accurate.
Cheers
Markus
Reply by raghs●August 28, 200720070828
>In article <rfudnUFUnZP3iHHZnZ2dnUVZ_tKdnZ2d@giganews.com>,
> "HSDPAboy" <fearsome.green@gmail.com> wrote:
>
> I see. Since you mention increased accuracy, are you assuming RRC
> filtering in the digital domain?
>
>Yes.
>
>
> Could you point me to some literature that discusses these matters?
It
> would be interesting to see how the oversampling factor affects
> performance. I would very much like to know which oversampling factor
is
> likely to be used in practice.
>
>24X is typical; higher levels of oversampling increase cost (MIPS) vs
>benefit. If you do a Google search for rake finger oversampling, you'll
>have plenty of papers to read.
>
>  Tim Olson
>
Hi
gain of the rake fingers is reduced by miss alignment of sampling points
and nulls/maximum of the filter. This is reduced if sampling rate is
increased. For example gain reduction due to misalignment is 0.91 dB at
4X than 3.9 dB at 2X. Correlation in fingers run at chip rate only. You
will find this clearly explained in section 3.4.3 of the book WCDMA
requirements and practical design edited by R. Tanner and J. Woodard.
Raghs
Reply by Tim Olson●August 23, 200620060823
In article <rfudnUFUnZP3iHHZnZ2dnUVZ_tKdnZ2d@giganews.com>,
"HSDPAboy" <fearsome.green@gmail.com> wrote:
 I see. Since you mention increased accuracy, are you assuming RRC
 filtering in the digital domain?
Yes.
 Could you point me to some literature that discusses these matters? It
 would be interesting to see how the oversampling factor affects
 performance. I would very much like to know which oversampling factor is
 likely to be used in practice.
24X is typical; higher levels of oversampling increase cost (MIPS) vs
benefit. If you do a Google search for rake finger oversampling, you'll
have plenty of papers to read.
 Tim Olson
Reply by HSDPAboy●August 23, 200620060823
>
>You are correct  multipaths which are separated by less than half a
>chip interval in time will not be individually resolvable. However,
>oversampling by greater than 2X still can make sense  it increases the
>accuracy of the matched filter on transmit/receive, and can increase the
>resolution of peak locations for individual multipaths or combined
>multipaths which are being tracked by the rake elements.
>
>  Tim Olson
>
I see. Since you mention increased accuracy, are you assuming RRC
filtering in the digital domain?
Could you point me to some literature that discusses these matters? It
would be interesting to see how the oversampling factor affects
performance. I would very much like to know which oversampling factor is
likely to be used in practice.
/Fred
Reply by Tim Olson●August 22, 200620060822
In article <Z8GdnXa8NuBaP3fZnZ2dnUVZ_v2dnZ2d@giganews.com>,
"HSDPAboy" <fearsome.green@gmail.com> wrote:
 According to theory the time resolution at the receiver is approximately
 one over the system bandwidth. Since the bandwidth of the RRC filter is
 actually a bit larger than one over the symbol (chip) duration I can see
 why an oversampling of 2x would make sense. But is useful to oversample
 even more? I mean, two multipaths that are separated by less than half of
 the chip duration would appear as a single multipath after RRC filtering.
 Or am I wrong in my reasoning?
You are correct  multipaths which are separated by less than half a
chip interval in time will not be individually resolvable. However,
oversampling by greater than 2X still can make sense  it increases the
accuracy of the matched filter on transmit/receive, and can increase the
resolution of peak locations for individual multipaths or combined
multipaths which are being tracked by the rake elements.
 Tim Olson
Reply by HSDPAboy●August 22, 200620060822
>In article <Bo6dnbodSYaPwHnZnZ2dnUVZ_o6dnZ2d@giganews.com>,
> "HSDPAboy" <fearsome.green@gmail.com> wrote:
>
> Hi,
>
> I have a question about oversampling when doing RRC filtering and
> correlations in WCDMA receiver. Both these operations are done at
base
> band in the digital domain. In the literature I've read that the
signal is
> oversampled say 4 or 6 times when doing both the RRC filtering and
> subsequent correlation. What is the motivation for oversampling in
these
> two cases?
>
>Multipath delays aren't quantized to the chip rate, so it is beneficial
>to oversample at the receiver to better center the individual rake
>delays to match peak correlation for a given mulitpath. For example, if
>a multipath delay is exactly a multiple of 1/2 the chip rate, then
>oversampling by 2x would give a 30% gain to the detected peak.
>
>To help minimize intersymbol interference when doing this, the
>transmitted data is oversampled by the same amount, and filtered
>through a rootraised cosine filter at both ends.
>
>  Tim Olson
>
Thanks for the reply, Tim.
According to theory the time resolution at the receiver is approximately
one over the system bandwidth. Since the bandwidth of the RRC filter is
actually a bit larger than one over the symbol (chip) duration I can see
why an oversampling of 2x would make sense. But is useful to oversample
even more? I mean, two multipaths that are separated by less than half of
the chip duration would appear as a single multipath after RRC filtering.
Or am I wrong in my reasoning?
Looking forward to your thoughts.
/Fred
Reply by Tim Olson●August 17, 200620060817
In article <Bo6dnbodSYaPwHnZnZ2dnUVZ_o6dnZ2d@giganews.com>,
"HSDPAboy" <fearsome.green@gmail.com> wrote:
 Hi,

 I have a question about oversampling when doing RRC filtering and
 correlations in WCDMA receiver. Both these operations are done at base
 band in the digital domain. In the literature I've read that the signal is
 oversampled say 4 or 6 times when doing both the RRC filtering and
 subsequent correlation. What is the motivation for oversampling in these
 two cases?
Multipath delays aren't quantized to the chip rate, so it is beneficial
to oversample at the receiver to better center the individual rake
delays to match peak correlation for a given mulitpath. For example, if
a multipath delay is exactly a multiple of 1/2 the chip rate, then
oversampling by 2x would give a 30% gain to the detected peak.
To help minimize intersymbol interference when doing this, the
transmitted data is oversampled by the same amount, and filtered
through a rootraised cosine filter at both ends.
 Tim Olson
Reply by HSDPAboy●August 17, 200620060817
Hi,
I have a question about oversampling when doing RRC filtering and
correlations in WCDMA receiver. Both these operations are done at base
band in the digital domain. In the literature I've read that the signal is
oversampled say 4 or 6 times when doing both the RRC filtering and
subsequent correlation. What is the motivation for oversampling in these
two cases? The use of an oversampled signal when correlating especially
confuses me. I've always been under the impression that correlation is
done with one sample per chip. Hope you guys can help me understand
this...