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LTE OFDM timing synchronization

Started by gobruins September 14, 2010
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
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
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 >
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 >
On Sep 14, 4:44&nbsp;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 Pre&#64257;x'. 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