Robust fine frequency and time estimation in mobile receivers
View Patent ↗A technique for estimating a carrier frequency offset and a timing offset in a MediaFLO™ (Forward Link Only) communication system, wherein the method comprises includes receiving Orthogonal Frequency Division Multiplexing (OFDM) symbols; interpolating pilots on odd or even symbols of the received OFDM symbols; determining a phase difference between two successive symbols using the interpolated pilots; obtaining an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and correcting a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.
1. A method of estimating a carrier frequency offset and a timing offset in a mobile multimedia multicast communication system, said method comprising:
receiving Orthogonal Frequency Division Multiplexing (OFDM) symbols in a receiver;
interpolating pilots on odd or even symbols of the received OFDM symbols;
determining a phase difference between two successive symbols using the interpolated pilots;
obtaining an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and
correcting a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.
2. The method of claim 1 , wherein determining the phase difference occurs using relation:
Δφ
k
=
2
π
(
Δf
+
δ
T
u
·
k
)
,
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol, Δf is the a carrier offset between a the receiver and a transmitter in said mobile multimedia multicast communication system in terms of sub-carrier bin duration,
δ
=
T
-
T
′
T
′
,
where T is a transmitter sampling period and T′ is a receiver sampling period, k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
3. The method of claim 2 , further comprising:
determining Δφ k for multiple sub-carrier index k using said relation; and
representing the resulting values of Δφ k graphically.
4. The method of claim 3 , wherein the obtaining of the estimate of the carrier frequency offset is derived as the mean of intercept of the graphically represented values of Δφ k and the timing offset is derived as the slope of the graphically represented values of Δφ k .
5. The method of claim 1 , wherein an estimate of the timing offset φ Δ and the carrier frequency offset φ μ is obtained using:
φ
Δ
=
4
L
2
∑
k
=
0
L
2
-
1
(
Δφ
L
2
+
k
-
Δφ
k
)
,
φ
μ
=
1
L
∑
L
-
1
k
=
0
Δφ
k
wherein L is a total number pilots involved in the estimation within one OFDM symbol,
wherein
Δφ
L
2
+
k
is a phase difference between a current pilot and a previous pilot defined by half of said total number of pilots plus a value of a sub-carrier index k in rad/symbol; and
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol.
6. The method of claim 1 , wherein a relationship between the phase difference Δφ k , the timing offset represented by δ, and the a carrier offset represented by Δf is given by:
Δφ
k
=
4
π
(
Δf
+
δ
T
u
·
k
)
,
wherein when there is no timing offset, Δf takes a maximum value when Δφ k =±π, and wherein k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
7. A non-transitory program storage device readable by computer, tangibly embodying a program of instructions executable by said computer to perform a method of estimating a carrier frequency offset and a timing offset in a mobile multimedia multicast communication system, said method comprising:
receiving Orthogonal Frequency Division Multiplexing (OFDM) symbols in a receiver;
interpolating pilots on odd or even symbols of the received OFDM symbols;
determining a phase difference between two successive symbols using the interpolated pilots;
obtaining an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and
correcting a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.
8. The program storage device of claim 7 , wherein determining the phase difference occurs using relation:
Δφ
k
=
2
π
(
Δf
+
δ
T
u
·
k
)
,
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol, Δf is the a carrier offset between a the receiver and a transmitter in said mobile multimedia multicast communication system in terms of sub-carrier bin duration,
δ
=
T
-
T
′
T
′
,
where T is a transmitter sampling period and T′ is a receiver sampling period, k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
9. The program storage device of claim 8 , wherein said method further comprises:
determining Δφ k for multiple sub-carrier index k using said relation; and
representing the resulting values of Δφ k graphically.
10. The program storage device of claim 9 , wherein the obtaining of the estimate of the carrier frequency offset is derived as the mean of intercept of the graphically represented values of Δφ k and the timing offset is derived as the slope of the graphically represented values of Δφ k .
11. The program storage device of claim 7 , wherein an estimate of the timing offset φ Δ and the carrier frequency offset φ μ is obtained using:
φ
Δ
=
4
L
2
∑
k
=
0
L
2
-
1
(
Δφ
L
2
+
k
-
Δφ
k
)
,
φ
μ
=
1
L
∑
L
-
1
k
=
0
Δφ
k
wherein L is a total number pilots involved in the estimation within one OFDM symbol,
wherein
Δφ
L
2
+
k
is a phase between a current pilot and a previous pilot defined by half of said total number of pilots plus a value of a sub-carrier index k in rad/symbol; and
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol.
12. The program storage device of claim 7 , wherein a relationship between the phase difference Δφ k , the timing offset represented by δ, and the a carrier offset represented by Δf is given by:
Δφ
k
=
4
π
(
Δf
+
δ
T
u
·
k
)
,
wherein when there is no timing offset, Δf takes a maximum value when Δφ k =±π, and wherein k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
13. An apparatus for estimating a carrier frequency offset and a timing offset in a mobile multimedia multicast communication system, said apparatus comprising:
a receiver adapted to receive Orthogonal Frequency Division Multiplexing (OFDM) symbols;
a processor adapted to interpolate pilots on odd or even symbols of the received OFDM symbols;
a calculator adapted to determine a phase difference between two successive symbols using the interpolated pilots;
an estimator adapted to obtain an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and
an integrator adapted to correct a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.
14. The apparatus of claim 13 , wherein determining the phase difference occurs using relation:
Δφ
k
=
2
π
(
Δf
+
δ
T
u
·
k
)
,
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol, Δf is the a carrier offset between said receiver and a transmitter in said mobile multimedia multicast communication system in terms of sub-carrier bin duration,
δ
=
T
-
T
′
T
′
,
where T is a transmitter sampling period and T′ is a receiver sampling period, k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
15. The apparatus of claim 14 ,
wherein said
Δφ
k
=
2
π
(
Δf
+
δ
T
u
·
k
)
is determined for multiple sub-carrier index k, and wherein the resulting values of Δφ k are represented graphically.
16. The apparatus of claim 15 , wherein the obtaining of the estimate of the carrier frequency offset is derived as the mean of intercept of the graphically represented values of Δφ k and the timing offset is derived as the slope of the graphically represented values of Δφ k .
17. The apparatus of claim 13 , wherein an estimate of the timing offset φ Δ and the carrier frequency offset φ μ is obtained using:
φ
Δ
=
4
L
2
∑
k
=
0
L
2
-
1
(
Δφ
L
2
+
k
-
Δφ
k
)
,
φ
μ
=
1
L
∑
L
-
1
k
=
0
Δφ
k
wherein L is a total number pilots involved in the estimation within one OFDM symbol,
wherein
Δφ
L
2
+
k
is a phase difference between a current pilot and a previous pilot defined by half of said total number of pilots plus a value of a sub-carrier index k in rad/symbol; and
wherein Δφ k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol.
18. The apparatus of claim 13 , wherein a relationship between the phase difference Δφ k , the timing offset represented by δ, and the a carrier offset represented by Δf is given by:
Δφ
k
=
4
π
(
Δf
+
δ
T
u
·
k
)
,
wherein when there is no timing offset, Δf takes a maximum value when Δφ k =±π, and wherein k is the sub-carrier index, and T u is an OFDM symbol duration excluding a guard interval.
19. The apparatus of claim 13 , further comprising a transmitter adapted to transmit said OFDM symbols.
20. The apparatus of claim 19 , further comprising a communication link between said receiver and said transmitter.
21. The method of claim 1 , wherein said communication system comprises a mobile television communication system.
22. The program storage device of claim 7 , wherein said communication system comprises a mobile television communication system.
23. The apparatus of claim 13 , wherein said communication system comprises a mobile television communication system.