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Hi, Bin-Chul:
I think it is a good comment. The real valued sequences in frequency domain yield conjugate symmetric patterns in time domain.
But, the time synchronization using the symmetric structure has some problems. In addition, the frequency offset estimation cannot be done using the symmetric structure only.
Firstly, the computational complexity required in the algorithm using symmetric pattern is much higher than the algorithm using repetitive pattern.
The algorithm using the symmetric structure has to newly calculate the correlation value by correlating all the time samples at every sample timing. In the algorithm using the repetitive pattern, on the other hand, the correlation value can be obtained by only adding the newest one sample value and discarding the oldest one sample value at every sample timing.
Since the timing hypotheses should be collected at every sample timing for a frame duration at least, the synchronization using repetitive pattern can much lower the computational complexity than the symmetric based algorithm.
Secondly, the coarse frequency offset estimation cannot be achieved from
the symmetric structure only. Instead, you can easily obtain the frequency offset
estimation by using the repetitive pattern, where you only need to compare the
phase rotations between a pair of samples.
Regards,
Jiho Jang
Jiho Jang (Ph. D) NTP Development Team (System Lab. 2) Telecommunication System Division Telecommunication Network
SAMSUNG ELECTRONICS CO.,LTD
T. 82-31-279-3355 M. 016-9233-8541 |
Dear Anand Dabak
and all,
I think that initial symbol timing acquisition can be done using symmetric structure of preamble. Because the preamble is generally real-valued, its IFFT version in time domain has symmetric structure inherently.
For example, if Nfft=8 and preamble consists of only real, then samples in time domain within a symbol duration are symmetric as follows:
[ a b c d e d* c* b* ]
The samples around 'e' are conjugated symmetric and this property can be used for acquisition of symbol timing. The symmetric structure is more usefull to finding correlation peak than CP based method because the symmetric part has length of (Nfft/2-1) longer than CP.
And I think this inherent symmetric structure is more useful than repetitive structure. For example, let's consider a repetitive structure as following:
[ a b c d a b c d ]
The correlation at sample 'a' is |a|^2 + |b|^2+ |c|^2+|d|^2 and that at sample 'b' is |b|^2+ |c|^2+|d|^2 + ax*. This means that it is hard to discriminate the peak among correlations over several positions. However, in the symmetric structure, the correlation at sample 'e' is |b|^2+ |c|^2+|d|^2 and that at sample 'd*' is ec* + db* + cx. This means that it is easy to discriminate the peak among correlations over several positions.
In summary, it is enough for acquisition of symbol timing to use the inherent symmetric structure obtained without any manipulation to preamble structure in my opinion.
Best Regards
Bin-Chul Ihm LGE
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