Accurate data-aided frequency tracking circuit
A frequency compensation circuit for compensating for a frequency offset in a received signal, the received signal including a periodically repeated pilot sequence for phase locking. The circuit comprises a phase estimator for estimating a phase of the received signal; a phase compensator, associated with the phase estimator, for compensating for the phase; a frequency estimator, comprising a maximum likelihood estimator comprising a first modification for estimating a frequency offset which is small relative to a symbol time, from the pilot sequence, the frequency estimator being connected downstream of the phase compensator; and a frequency compensator for applying a compensation to the signal, thereby to compensate for the frequency offset. The compensator is suitable for the exacting conditions of the DVB-S2 standard.
1 . A frequency compensation circuit for compensating for a frequency offset in a received signal, the received signal including a periodically repeated pilot sequence for phase locking, the circuit comprising:
a phase estimator for estimating a phase offset of said received signal;
a phase compensator, associated with said phase estimator, for compensating for said phase offset;
a maximum likelihood frequency estimator, the frequency estimator being connected downstream of said phase compensator, the maximum likelihood estimator being configured to provide maximum likelihood estimation of a frequency offset in said received signal using said pilot sequence in a phase compensated version of said received signal; and
a frequency compensator, associated with said frequency estimator, for applying a compensation to said signal, thereby to compensate for said frequency offset.
2 . The frequency compensation circuit of claim 1 , wherein said maximum likelihood frequency estimator comprises a first modification for estimating the frequency offset under the assumption that the frequency offset is small relative to a symbol time.
3 . The frequency compensation circuit of claim 1 , further comprising a coarse frequency estimator connected upstream of said phase compensator and configured to obtain a course estimation of said frequency offset.
4 . The frequency compensation circuit of claim 1 , wherein said maximum likelihood estimator comprises a second modification for estimating a frequency offset of a phase compensated signal.
5 . The frequency compensation circuit of claim 4 , wherein said received signal comprises n complex symbols, and said compensation circuit comprises a separator for separating said n complex symbols into n real symbols Xn and n imaginary symbols Yn.
6 . The frequency compensation circuit of claim 5 , wherein said maximum likelihood estimator is configured to compute a frequency offset from:
f
^
=
1
π
∑
X
n
n
2
-
(
1
π
∑
X
n
n
2
)
2
-
2
π
2
(
∑
Y
n
n
)
(
∑
Y
n
n
3
)
2
·
∑
Y
n
n
3
7 . The apparatus of claim 6 , wherein said maximum likelihood estimator comprises a counter, a series of multipliers and accumulators, and a series of paths defined from real and imaginary outputs of said separator and from said counter, through said multipliers and accumulators, to accumulate summations for said maximum likelihood estimation, each path defining a different summation.
8 . The apparatus of claim 1 , wherein said frequency compensation comprises feedback compensation via a feedback loop, the feedback loop comprising a numerically controlled oscillator.
9 . The apparatus of claim 1 , further comprising an averager, adapted to perform averaging of frequency offsets over a series of pilot signals.
10 . A frequency compensation method for compensating for a frequency offset in a received signal, the received signal including a periodically repeated pilot sequence for phase locking, the method comprising:
estimating a phase of said received signal;
compensating for said phase;
estimating a maximum likelihood frequency offset of the received signal using the pilot sequence from within said phase compensated signal; and
applying a frequency compensation to said received signal, thereby to compensate for said frequency offset.
11 . The method of claim 10 , wherein said maximum likelihood frequency offset estimating comprises using a first modification of said maximum likelihood estimation wherein said first modification is based on an assumption that said frequency offset is small relative to a symbol time.
12 . The method of claim 10 , further comprising carrying out coarse frequency estimation of said frequency offset prior to said phase compensation.
13 . The method of claim 11 , wherein said maximum likelihood estimating further comprises a second modification for estimating a frequency offset of a phase compensated signal.
14 . The method of claim 13 , wherein said received signal comprises n complex symbols, and said method comprises separating said n complex symbols into n real symbols Xn and n imaginary symbols Yn prior to said maximum likelihood estimation.
15 . The method of claim 14 , wherein said maximum likelihood estimating comprises computing a frequency offset from:
f
^
=
1
π
∑
X
n
n
2
-
(
1
π
∑
X
n
n
2
)
2
-
2
π
2
(
∑
Y
n
n
)
(
∑
Y
n
n
3
)
2
·
∑
Y
n
n
3
16 . The method of claim 15 , wherein said calculating comprises accumulating summations for said maximum likelihood estimation via multiplications, each path defining a different summation.
17 . The method of claim 10 , wherein said frequency compensation comprises feedback compensation via a feedback loop.
18 . The method of claim 10 , further comprising averaging of frequency offsets over a series of pilot signals.
19 . A preparation circuit for providing summations to a maximum likelihood estimator, comprising:
a separator for separating a complex incoming signal into a real part at a real output and an imaginary part at an imaginary output,
a counter for providing an ongoing symbol count,
a plurality of multipliers,
a plurality of accumulators, and
a series of paths each connecting one of said outputs, said counter, at least one of said multipliers and one of said accumulators, such that each path of said series defines a different summation for said maximum likelihood estimation.