Eric Jacobsen
Guest
|
 Posted:
Thu Dec 08, 2005 12:48 am Post subject:
Re: Timing Error Detection using Gardner and other methods |
|
|
On Wed, 07 Dec 2005 04:14:28 -0600, "Rajenish_jain"
<rajenish.jain@gmail.com> wrote:
| Quote: | Hi,
I simulated a QPSK Symbol Rrror Recovery mechanism using interpolation
menthod. I used a gardner TED which works only for the case where there is
a symbol trensition. I would like to know of any algorithm which works for
the case even without trnsition. (I didnt find Muller-Muller method very
good).
What if I want to simulate Symbol recovery using the same interpolation
method for 16-QAM. What algorithm would be suitable for TED of the QAM
system.
|
If there's no transition then the signal is essentially DC and there's
no way to determine the symbol boundaries. So no detector can
provide timing information when there's no transition, and this is one
reason it's important to have a scrambler in the system to keep the
entropy high enough that the detector always works.
Some of the "better" detectors know enough to generate exactly zero
when no transition is detected in order to minimize the noise input to
the timing loop.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org |
|
Steve Underwood
Guest
|
| Posted:
Thu Dec 08, 2005 8:08 am Post subject:
Re: Timing Error Detection using Gardner and other methods |
|
|
Hi,
Eric Jacobsen wrote:
| Quote: | On Wed, 07 Dec 2005 04:14:28 -0600, "Rajenish_jain"
rajenish.jain@gmail.com> wrote:
Hi,
I simulated a QPSK Symbol Rrror Recovery mechanism using interpolation
menthod. I used a gardner TED which works only for the case where there is
a symbol trensition. I would like to know of any algorithm which works for
the case even without trnsition. (I didnt find Muller-Muller method very
good).
What if I want to simulate Symbol recovery using the same interpolation
method for 16-QAM. What algorithm would be suitable for TED of the QAM
system.
If there's no transition then the signal is essentially DC and there's
no way to determine the symbol boundaries. So no detector can
provide timing information when there's no transition, and this is one
reason it's important to have a scrambler in the system to keep the
entropy high enough that the detector always works.
Some of the "better" detectors know enough to generate exactly zero
when no transition is detected in order to minimize the noise input to
the timing loop.
That answers Rajenish's first paragraph well, but misses his second. |
Gardner works OK for 16-QAM, or even much higher orders, with a couple
of caveats - you need more damping, and training sequences may have issues.
Because a scrambler is always used for real world QAM, the transition
density is kept high, and the transitions kept fairly random. A Gardner
test on any individual transition produces a rather variable result.
However, the average of a few results from a scrambled sequence forms a
cost function which homes the timing correctly. You need to use more
damping in the Gardner test, than you would with QPSK, to provide
appropriate averaging. Other than that, the same test will track the
symbols OK. The only thing to watch is that you don't make the damping
so great that you cannot track the greatest transmitter to receiver
symbol clock rate mismatch you might face.
Many QAM systems, such as telephone line modems, use an initial training
phase where the transmitter alternates between two states. This is the
section of the signal where you need to achieve your initial timing lock
on the symbols. In many QAM training sequences, especially the more
recent ones, the alternating symbols are of equal magnitude, and a
Gardner TED works fine. However, in some specs the states are not of
equal magnitude. The Gardner TED algorithm tends to lock to such a
signal with a fixed phase shift. It may be OK to just ignore this, and
let things settle down when the higher entropy sections of the signal
start. That really depends on how far out the timing is for the
particular training sequence. It might be possible to apply a fixed
sized kick to the symbol timing as the end of the alternating signal is
detected. It does, however, always need to be considered, to see the
design is unconditionally stable.
Regards,
Steve |
|
FAQ
Memberlist
Register
Profile
Log in to check your private messages
Log in