Robust fine frequency and time estimation in mobile receivers

Abstract

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.

Description

φ_μ=E[Δφ_k]=Δf  (2)

FIG. 3 illustrates the structure of scattered OFDM pilots considered in order to obtain an estimate of a straight line in a MediaFLO™ system receiver design according to an embodiment herein. As illustrated, the phase difference across OFDM pilots are taken every other symbols; i.e., either odd symbols 310 or even symbols 320. An estimate of the timing offset φ_Δ and the carrier frequency offset φ_μ may then be obtained as follows:

$\begin{array}{cc}{\phi }_{\Delta }=\frac{4}{L^2}\stackrel{\frac{L}{2}-1}{\sum _{k=0}}\left({\mathrm{\Delta \phi }}_{\frac{L}{2}+k}-{\mathrm{\Delta \phi }}_k\right),& \left(2\right)\\ {\phi }_{\mu }=\frac{1}{L}\stackrel{L-1}{\sum _{k=0}}{\mathrm{\Delta \phi }}_k& \end{array}$
where L is the total number pilots involved in the estimation within one OFDM symbol.

In the case of channels with high Doppler frequency, the straight line 210 in FIG. 2 will not be clean and can be very noisy which leads to the wrap up of some values of Δφ_k. Wrapping up happens for angle values that exceed 2π. This is due to the fact that exp(φ+2π) is equal to exp(φ). Thus, values for angle values that exceed 2π may not be distinguishable.

FIGS. 4A and 4B illustrate examples of Doppler fading effect on timing and carrier offset estimation. In the example of FIG. 4A, the Doppler fading effect shown is of a TU6 channel with Doppler 150 Hz, timing offset 100 ppm, and delta angles even index 24. In the example of FIG. 4B, the Doppler fading effect shown is of a ideal channel with timing offset 100 ppm, and delta angles even index 34. The line 410 of FIG. 4A and the line 420 of FIG. 4B are not clean and may be very noisy, which leads to the wrap up of some values of Δφ_k. Thus, it is desirable to use moving averaging based techniques to smooth out the noisy effect caused by fast Doppler channel changes. In an embodiment, a moving averaging technique such as a leaky integrator may be used to smooth out the noisy effect.

As the differential phase are obtained between odd symbols 310 or even symbols 320 in FIG. 3, the relationship between the differential phase Δφ_k, the timing offset δ, and the carrier offset Δf for the MediaFLO™ system application may be then given by the following equation:

$\begin{array}{cc}{\mathrm{\Delta \phi }}_k=4\pi \left(\mathrm{\Delta f}+\frac{\delta }{T_u}·k\right)& \left(4\right)\end{array}$

From equation (4) it may be observed that, in case the timing offset δ=0 ppm, (i.e., no timing offset) in an embodiment, Δf takes the maximum value when Δφ_k=±π, according to equation (4), Δf_max=0.25 (bins), which translates to approximately 340 Hz. In case the timing offset δ≠0 ppm in another embodiment, it may be observed that Δf_max<0.25 (bins).

As it is desired to further increase the carrier and timing offset carrier ranges, accordingly in accordance with an embodiment, the pilots are interpolated on the odd symbols 310 or the even symbols 320 of FIG. 3. FIG. 5 illustrates the structure of interpolated scattered pilots for a MediaFLO™ system receiver design according to an embodiment herein. The interpolated pilots are then used to determine the phase difference between two successive symbols instead of using the raw pilots over every other symbol. The corresponding differential phase may be then given by:

$\begin{array}{cc}{\mathrm{\Delta \phi }}_k=2\pi \left(\mathrm{\Delta f}+\frac{\delta }{T_u}·k\right)& \left(5\right)\end{array}$

From equation (5), it may be observed that, in case δ=0 ppm, in an embodiment, Δf takes the maximum value when Δφ_k=±π, i.e., Δf_max=0.5 (bins), which translates to approximately 680 Hz. Thus, the estimation ranger has been doubled using this technique.

FIG. 6A illustrates a simulation result of the straight line 610 obtained using a technique based on raw pilots. As illustrated in the example of FIG. 6A, the carrier offset tolerance is about 0.10 bin with 50 ppm timing offset. Further, the straight line 610 tends to wrap with an increase of the carrier offset tolerance from 0.10 bin. FIG. 6B illustrates a simulation result of the straight line 620 obtained using a technique based on interpolated pilots. In the example of FIG. 6B, it can be seen that with 50 ppm timing offset, the carrier offset tolerance is about 0.25 bin. The straight line 620 tends to wrap with further increase of the carrier offset tolerance from 0.25 bin. Thus, the technique using interpolated pilots doubles the estimation range of the carrier offset frequency to +/−0.5 OFDM sub carrier spacing, which corresponds to about +/−680 Hz at almost no complexity increment.

FIG. 7 is a block diagram illustrating an embodiment of a system 700 for estimating a slope and a mean of a straight line obtained using a technique based on interpolated pilots. The system 700 comprises angle estimators 710, 720 to estimate angles of the interpolated pilots. The angle estimator 710 estimates an angle of current pilots and the angle estimator 720 estimates an angle of the previous pilots. A calculator 730 estimates the phase difference Δφ between the current pilot and the previous pilot. The resulting output of the calculator 730 is then provided to a slope estimator 740 and a mean estimator 750. As mentioned, graphically Δφ is a straight line. The slope estimator 740 estimates the slope and the mean estimator 750 estimates the mean intercept of the straight line of Δφ. To smooth out the noisy effect caused by fast Doppler channel changes, the resulting output of the slope estimator 740 is provided to a line gradient leaky integrator 760 and the resulting output of the mean estimator 750 is provided to a line mean leaky integrator 770. The resulting output of the line gradient leaky integrator 760 provides the slope of the straight line of Δφ, wherein the slope equals the timing offset φ_Δ and the resulting output of the line mean leaky integrator 770 provides the mean intercept of the straight line of Δφ, wherein the mean intercept equals the frequency offset φ_μ.

FIG. 8, with reference to FIGS. 1 through 7, illustrates a flow diagram illustrating a method of estimating a carrier frequency offset and a timing offset in a MediaFLO™ (Forward Link Only) system according to an embodiment herein, wherein the method comprises receiving (801) Orthogonal Frequency Division Multiplexing (OFDM) symbols; interpolating (803) pilots on odd or even symbols of the received OFDM symbols; determining (805) a phase difference between two successive symbols using the interpolated pilots; obtaining (807) an estimate of the carrier frequency offset and the timing offset from the determined phase difference between two successive symbols; and correcting (809) a sampling frequency in accordance with the estimated carrier frequency offset and timing offset.

Preferably, determining (805) the phase difference occurs using relation:

${\mathrm{\Delta \phi }}_k=2\pi \left(\mathrm{\Delta f}+\frac{\delta }{T_u}·k\right),$
wherein Δφ_k is a differential phase between two successive symbols of sub-carrier index k in rad/symbol, Δf is the carrier offset between a receiver and a transmitter in said MediaFLO™ (Forward Link Only) receiver system in terms of sub-carrier bin duration,

$\delta =\frac{T-T^{\prime }}{T^{\prime }},$
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.

Moreover, the method may further comprise determining Δφ_k for multiple sub-carrier index k using said relation; and representing the resulting values of Δφ_k graphically. Additionally, obtaining (807) of the estimate of the carrier frequency offset may be derived as the mean of intercept of the graphically represented values of Δφ_k and the timing offset may be derived as the slope of the graphically represented values of Δφ_k.

Preferably, an estimate of the timing offset φ_Δ and the carrier frequency offset φ_μ is obtained using:

$\begin{array}{c}{\phi }_{\Delta }=\frac{4}{L^2}\stackrel{\frac{L}{2}-1}{\sum _{k=0}}\left({\mathrm{\Delta \phi }}_{\frac{L}{2}+k}-{\mathrm{\Delta \phi }}_k\right),\\ {\phi }_{\mu }=\frac{1}{L}\stackrel{L-1}{\sum _{k=0}}{\mathrm{\Delta \phi }}_k\end{array}$
wherein L is a total number pilots involved in the estimation within one OFDM symbol.

Further, a relationship between the phase difference Δφ_k, the timing offset δ, and the carrier offset Δf is given by:

${\mathrm{\Delta \phi }}_k=4\pi \left(\mathrm{\Delta f}+\frac{\delta }{T_u}·k\right),$
wherein when there is no timing offset, Δf takes a maximum value when Δφ_k=±π.

The techniques provided by the embodiments herein may be implemented on an integrated circuit chip (not shown). The chip design is created in a graphical computer programming language, and stored in a computer storage medium (such as a disk, tape, physical hard drive, or virtual hard drive such as in a storage access network). If the designer does not fabricate chips or the photolithographic masks used to fabricate chips, the designer transmits the resulting design by physical means (e.g., by providing a copy of the storage medium storing the design) or electronically (e.g., through the Internet) to such entities, directly or indirectly. The stored design is then converted into the appropriate format (e.g., GDSII) for the fabrication of photolithographic masks, which typically include multiple copies of the chip design in question that are to be formed on a wafer. The photolithographic masks are utilized to define areas of the wafer (and/or the layers thereon) to be etched or otherwise processed.

The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

The embodiments herein can take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment including both hardware and software elements. The embodiments that are implemented in software include but are not limited to, firmware, resident software, microcode, etc.

Furthermore, the embodiments herein can take the form of a computer program product accessible from a computer-usable or computer-readable medium providing program code for use by or in connection with a computer or any instruction execution system. For the purposes of this description, a computer-usable or computer readable medium can be any apparatus that can comprise, store, communicate, propagate, or transport the program for use by or in connection with the instruction execution system, apparatus, or device.

The medium can be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device) or a propagation medium. Examples of a computer-readable medium include a semiconductor or solid state memory, magnetic tape, a removable computer diskette, a random access memory (RAM), a read-only memory (ROM), a rigid magnetic disk and an optical disk. Current examples of optical disks include compact disk-read only memory (CD-ROM), compact disk-read/write (CD-R/W) and DVD.

A data processing system suitable for storing and/or executing program code will include at least one processor coupled directly or indirectly to memory elements through a system bus. The memory elements can include local memory employed during actual execution of the program code, bulk storage, and cache memories which provide temporary storage of at least some program code in order to reduce the number of times code must be retrieved from bulk storage during execution.

Input/output (I/O) devices (including but not limited to keyboards, displays, pointing devices, etc.) can be coupled to the system either directly or through intervening I/O controllers. Network adapters may also be coupled to the system to enable the data processing system to become coupled to other data processing systems or remote printers or storage devices through intervening private or public networks. Modems, cable modem and Ethernet cards are just a few of the currently available types of network adapters.

A representative hardware environment for practicing the embodiments herein is depicted in FIG. 9. This schematic drawing illustrates a hardware configuration of an information handling/computer system 900 in accordance with the embodiments herein. The system 900 comprises at least one processor or central processing unit (CPU) 910. The CPUs 910 are interconnected via system bus 912 to various devices such as a random access memory (RAM) 914, read-only memory (ROM) 916, and an input/output (I/O) adapter 918. The I/O adapter 918 can connect to peripheral devices, such as disk units 911 and tape drives 913, or other program storage devices that are readable by the system 900. The system 900 can read the inventive instructions on the program storage devices and follow these instructions to execute the methodology of the embodiments herein. The system 900 further includes a user interface adapter 919 that connects a keyboard 915, mouse 917, speaker 924, microphone 922, and/or other user interface devices such as a touch screen device (not shown) to the bus 912 to gather user input. Additionally, a communication adapter 920 connects the bus 912 to a data processing network 925, and a display adapter 921 connects the bus 912 to a display device 923 which may be embodied as an output device such as a monitor, printer, or transmitter, for example.

The sampling time of the receiver is not commensurate with that of the transmitter, and a carrier and time offset exists between the transmitter and the receiver. To ensure efficient communication between the transmitter and the receiver, the carrier and time offset need to be estimated and then corrected to ensure reliable quality communication. Accordingly, the embodiments herein provide a manner of achieving this.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.

Claims

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:

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:

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:

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:

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:

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:

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:

15. The apparatus of claim 14,

wherein said

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:

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:

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.