On Sep 14, 4:44 pm, "gobruins" <chunmei.kang@n_o_s_p_a_m.gmail.com>
wrote:

> we are working on the OFDM synchronization for 3GPP LTE,
> So far we have implemented the timing and fractional frequency

Typically the timing sync in ofdm is not that important as the phase
shift due to timing offset is going to be the same for all sub-
carriers. When you estimate the channel, the phase term is going to be
part of your channel. Thus, as long as you do not offset a lot(delta<
cp length - delay spread of the channel), you are fine.

> synchronization using the Maximum Likelihood method proposed in 'On
> Synchronization in OFDM Systems Using the Cyclic Preﬁx'. The ML estimates
> of timing and frequency offset are then processed by the loop filter. It
> works pretty well. After the equalizer clearly I can tell the symbols in
> the constellation.
>
> However, one problem we found is that, the timing offset estimate is not
> static, it fluctuates under noisy environment. The fluctuation of the
> timing offset will cause subcarrier dependent phase offset and result in
> the symbols smear around on the constellation plot. How do we compensate
> for that? Is there any way we can compensate this using the channel
> estimation?
>
> Thanks

Reply by gobruins●September 15, 20102010-09-15

I already tuned the loop filter so i have a very narrow bandwidth, i can
see the timing offset estimate is within around +-3 samples. Even the
timing offset curve is already very smooth, the little fluctuation will
smears the symbols on the constellation. Can this effect be compensated?
Is there any channel estimation algorithm to compensate for this?
I found one 'Robust MMSE Channel Estimation in OFDM
Systems with Practical Timing Synchronization' talk about this topic, but
do not quite understand how it is done.

>On 09/14/2010 01:44 PM, gobruins wrote:
>> we are working on the OFDM synchronization for 3GPP LTE,
>> So far we have implemented the timing and fractional frequency
>> synchronization using the Maximum Likelihood method proposed in 'On
>> Synchronization in OFDM Systems Using the Cyclic Preﬁx'. The ML

estimates

>> of timing and frequency offset are then processed by the loop filter.

It

>> works pretty well. After the equalizer clearly I can tell the symbols

in

>> the constellation.
>>
>> However, one problem we found is that, the timing offset estimate is

not

>> static, it fluctuates under noisy environment. The fluctuation of the
>> timing offset will cause subcarrier dependent phase offset and result

in

>> the symbols smear around on the constellation plot. How do we

compensate

>> for that? Is there any way we can compensate this using the channel
>> estimation?
>
>Turn down the bandwidth in your loop, to track through the fluctuation?
>
>--
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>

Reply by gobruins●September 14, 20102010-09-14

I already tuned the loop filter so i have a very narrow bandwidth, i can
see the timing offset estimate is within around +-3 samples. Even the
timing offset curve is already very smooth, the little fluctuation will
smears the symbols on the constellation. Can this effect be compensated?
Is there any channel estimation algorithm to compensate for this?
I found one 'Robust MMSE Channel Estimation in OFDM
Systems with Practical Timing Synchronization' talk about this topic, but
do not quite understand how it is done.

>On 09/14/2010 01:44 PM, gobruins wrote:
>> we are working on the OFDM synchronization for 3GPP LTE,
>> So far we have implemented the timing and fractional frequency
>> synchronization using the Maximum Likelihood method proposed in 'On
>> Synchronization in OFDM Systems Using the Cyclic Preﬁx'. The ML

estimates

>> of timing and frequency offset are then processed by the loop filter.

It

>> works pretty well. After the equalizer clearly I can tell the symbols

in

>> the constellation.
>>
>> However, one problem we found is that, the timing offset estimate is

not

>> static, it fluctuates under noisy environment. The fluctuation of the
>> timing offset will cause subcarrier dependent phase offset and result

in

>> the symbols smear around on the constellation plot. How do we

compensate

>> for that? Is there any way we can compensate this using the channel
>> estimation?
>
>Turn down the bandwidth in your loop, to track through the fluctuation?
>
>--
>
>Tim Wescott
>Wescott Design Services
>http://www.wescottdesign.com
>
>Do you need to implement control loops in software?
>"Applied Control Theory for Embedded Systems" was written for you.
>See details at http://www.wescottdesign.com/actfes/actfes.html
>

Reply by Tim Wescott●September 14, 20102010-09-14

On 09/14/2010 01:44 PM, gobruins wrote:

> we are working on the OFDM synchronization for 3GPP LTE,
> So far we have implemented the timing and fractional frequency
> synchronization using the Maximum Likelihood method proposed in 'On
> Synchronization in OFDM Systems Using the Cyclic Preﬁx'. The ML estimates
> of timing and frequency offset are then processed by the loop filter. It
> works pretty well. After the equalizer clearly I can tell the symbols in
> the constellation.
>
> However, one problem we found is that, the timing offset estimate is not
> static, it fluctuates under noisy environment. The fluctuation of the
> timing offset will cause subcarrier dependent phase offset and result in
> the symbols smear around on the constellation plot. How do we compensate
> for that? Is there any way we can compensate this using the channel
> estimation?

Turn down the bandwidth in your loop, to track through the fluctuation?
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" was written for you.
See details at http://www.wescottdesign.com/actfes/actfes.html

Reply by gobruins●September 14, 20102010-09-14

we are working on the OFDM synchronization for 3GPP LTE,
So far we have implemented the timing and fractional frequency
synchronization using the Maximum Likelihood method proposed in 'On
Synchronization in OFDM Systems Using the Cyclic Preﬁx'. The ML estimates
of timing and frequency offset are then processed by the loop filter. It
works pretty well. After the equalizer clearly I can tell the symbols in
the constellation.
However, one problem we found is that, the timing offset estimate is not
static, it fluctuates under noisy environment. The fluctuation of the
timing offset will cause subcarrier dependent phase offset and result in
the symbols smear around on the constellation plot. How do we compensate
for that? Is there any way we can compensate this using the channel
estimation?
Thanks