Apparatus and method for estimating carrier frequency offset

Abstract

An apparatus for estimating carrier frequency offset includes an M.sup.th-power circuit, a spectrum generating circuit, a spectrum adjusting circuit, a peak frequency determining circuit and a frequency offset determining circuit. The M.sup.th-power circuit performs an M.sup.th-power calculation on an input signal to generate an M.sup.th-power calculation result. The spectrum generating circuit generates a spectrum according to the M.sup.th-power calculation result. The spectrum adjusting circuit identifies a partial energy peak value in a partial frequency range from the spectrum, and increases the partial energy peak value to be higher than any other energy in the spectrum to generate an adjusted spectrum. The peak frequency determining circuit identifies a peak frequency having a maximum energy peak value from the adjusted spectrum. The frequency offset determining circuit determines an estimated carrier frequency offset result according to the peak frequency.

Claims

1. An apparatus for estimating carrier frequency offset (CFO), comprising: an M.sup.th-power calculation circuit for performing an M.sup.th-power calculation on an input signal to generate an M.sup.th-power calculation result, where M is an integer greater than 1 and is associated with a modulation scheme of the input signal; a spectrum generating circuit for generating a spectrum according to the M.sup.th-power calculation result; a spectrum adjusting circuit for increasing a partial energy peak value in a partial frequency range in the spectrum to generate an adjusted spectrum, wherein the partial frequency range is T to T and T represents a symbol duration of the input signal; a peak frequency determining circuit for identifying a peak frequency from the adjusted spectrum, wherein the peak frequency has a maximum peak energy in the adjusted spectrum; and a frequency offset determining circuit for generating an estimated CFO result.

2. The apparatus according to claim 1, wherein the spectrum adjusting circuit comprises: a search circuit for identifying the partial energy peak value in the partial frequency range from the spectrum, identifying a corresponding partial peak frequency according to the partial energy peak value, and identifying energies corresponding to a plurality of frequencies that are spaced by integral multiples of 1/T from the partial peak frequency as a plurality of energy increments; an addition circuit for adding the energy increments and the partial energy peak value to generate an adjusted energy; and an adjusting circuit, adjusting the spectrum according to the partial peak frequency and the adjusted energy to generate the adjusted spectrum, wherein the partial peak frequency corresponds to the adjusted energy in the adjusted spectrum.

3. The apparatus according to claim 1, wherein the spectrum adjusting circuit comprises: a search circuit for identifying the partial energy peak value in the partial frequency range from the spectrum, and identifying a corresponding partial peak frequency according to the partial energy peak value; an addition circuit for adding a predetermined energy increment and the partial energy peak value to generate an adjusted energy; and an adjusting circuit for adjusting the spectrum according to the partial peak frequency and the adjusted energy to generate the adjusted spectrum, wherein the partial peak frequency corresponds to the adjusted energy in the adjusted spectrum.

4. A method for estimating carrier frequency offset (CFO), comprising: performing an M.sup.th-power calculation on an input signal to generate an M.sup.th-power calculation result, where M is an integer greater than 1 and is associated with a modulation scheme of the input signal; generating a spectrum according to the M.sup.th-power calculation result; identifying a partial energy peak value in a partial frequency range from the spectrum, wherein the partial frequency range is T to T and T represents a symbol duration of the input signal; increasing the partial energy peak value to generate an adjusted spectrum; identifying a peak frequency from the adjusted spectrum, wherein the peak frequency has a maximum energy peak value in the adjusted spectrum; and determining an estimated CFO result according to the peak frequency.

5. The method according to claim 4, wherein the step of increasing the partial energy peak value to generate the adjusted spectrum comprises: identifying a corresponding partial peak frequency according to the partial energy peak value; identifying energies corresponding to a plurality of frequencies that are spaced by integral multiples of 1/T from the partial peak frequency as a plurality of energy increments; adding the energy increments and the partial energy peak value to generate an adjusted energy; and adjusting the spectrum according to the partial peak frequency and the adjusted energy to generate the adjusted spectrum, wherein the partial peak frequency corresponds to the adjusted energy in the adjusted spectrum.

6. The method according to claim 4, wherein the step of adjusting the partial energy peak value to generate the adjusted spectrum comprises: identifying a corresponding partial peak frequency according to the partial energy peak value; adding a predetermined energy increment and the partial energy peak value to generate an adjusted energy; and adjusting the spectrum according to the partial peak frequency and the adjusted energy to generate the adjusted spectrum, wherein the partial peak frequency corresponds to the adjusted energy in the adjusted spectrum.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(A) is a block diagram of carrier frequency offset (CFO) estimating apparatus applicable to a quadrature phase shift keying (QPSK) signal;

(2) FIG. 1(B) and FIG. 1(C) are examples of spectra of signals transmitted through a single path and a multipath, respectively;

(3) FIG. 2 is a block diagram of an apparatus for estimating carrier frequency offset according to an embodiment of the present invention;

(4) FIG. 3 is a block diagram of a spectrum adjusting circuit according to an embodiment of the present invention;

(5) FIG. 4 (A) is an example of a spectrum; FIG. 4(B) is an example of an adjusted spectrum generated by a spectrum adjusting circuit according to an embodiment of the present invention;

(6) FIG. 5 is a block diagram of a spectrum adjusting circuit according to another embodiment of the present invention; and

(7) FIG. 6 is a flowchart of a method for estimating carrier frequency offset according to an embodiment of the present invention.

(8) It should be noted that, the drawings of the present invention include functional block diagrams of multiple functional modules related to one another. These drawings are not detailed circuit diagrams, and connection lines therein are for indicating signal flows only. The interactions between the functional elements/or processes are not necessarily achieved through direct electrical connections. Further, functions of the individual elements are not necessarily distributed as depicted in the drawings, and separate blocks are not necessarily implemented by separate electronic elements.

DETAILED DESCRIPTION OF THE INVENTION

(9) An apparatus and method for estimating carrier frequency offset (CFO) of the present invention may coordinate or be integrated in receivers in various types of communication systems needing to estimate carrier frequency offset, e.g., Digital Video BroadcastingSatellite (DVB-S) receivers and Digital Video BroadcastingCable (DVB-C) receivers.

(10) FIG. 2 shows a block diagram of an apparatus for estimating CFO according to an embodiment of the present invention. As shown in FIG. 2, a CFO estimating apparatus 200 includes a 4.sup.th-power circuit 21, a spectrum generating circuit 22, a spectrum adjusting circuit 25, a peak frequency determining circuit 23, and a frequency offset determining circuit 24. An input signal y(t) of the 4.sup.th-power circuit 21 is a quadrature phase shift keying (QPSK) baseband signal. In practice, the baseband signal may be, for example but not limited to, a corresponding baseband signal generated by processing a radio-frequency (RF) signal, which is received by a receiver coordinating with the CFO estimating apparatus 200, by circuits such a low-noise amplifying (LNA) circuit, a down-converting circuit, an analog-to-digital converter (ADC) and a low-pass filter (LPF). The 4.sup.th-power circuit 21 performs a 4.sup.th-power calculation on the input signal y(t) to generate a 4.sup.th-power calculation result y.sup.4(t). Implementation details of the 4.sup.th-power calculation circuit 21 are generally known to one person skilled in the art, and shall be omitted herein.

(11) Next, the spectrum generating circuit 22 generates a spectrum Z(f) according to the 4.sup.th-power calculation result y.sup.4(t). In practice, the spectrum generating circuit 22 may generated the spectrum by, for example but not limited to, fast Fourier transform (FFT). It should be noted that, details for generating the spectrum are generally known to one person skilled in the art, and shall be omitted herein.

(12) One task of the spectrum adjusting circuit 25 is identifying a partial energy peak value P.sub.SEL from the spectrum Z(f). The partial energy peak value P.sub.SEL is, in the partial frequency range T to T, a peak value having the maximum energy value, where T represents a symbol duration of the input signal y(t), and may be learned from parsing an input signal y(t) by other circuit in the receiver coordinating with the CFO estimating apparatus 200 or may be a constant value agreed by both of the transmitter and the receiver. The spectrum adjusting circuit 25 then identifies a partial peak frequency f.sub.SEL according to the partial energy peak value P.sub.SEL, wherein the partial peak frequency f.sub.SEL is the frequency the partial energy peak value P.sub.SEL corresponds to. Based on the rule of thumb, the carrier frequency offset f is not a large value, and the four-fold carrier frequency offset 4f usually falls within the above partial frequency range T to T. Thus, the frequency (i.e., the partial peak frequency f.sub.SEL) corresponding to the partial energy peak value P.sub.SEL the spectrum adjusting circuit 25 identifies is usually the four-fold carrier frequency offset 4f. Next, the spectrum adjusting circuit 25 increases the energy corresponding to the partial peak frequency f.sub.SEL to generate an adjusted spectrum C(f). FIG. 3 and FIG. 5 depict two embodiments for illustrating how the spectrum adjusting circuit 25 adjusts the energy corresponding to the partial peak frequency f.sub.SEL.

(13) In the embodiment in FIG. 3, the spectrum adjusting circuit 25 includes a search circuit 25A, an addition circuit 25B and an adjusting circuit 25C. The search circuit 25A identifies the partial energy peak value P.sub.SEL from the spectrum Z(f), and identifies the corresponding partial peak frequency f.sub.SEL according to the partial energy peak value P.sub.SEL. The search circuit 25A further identifies a plurality of energy increments P.sub.ADD, which are, in the spectrum Z(f), corresponding energies of a plurality of frequencies associated with the partial peak frequency f.sub.SEL and the symbol duration T of the input signal. Based on observations on characteristics of QPSK signals, relative energy peaks also exist at frequencies that are spaced by integral multiples of 1/T from the partial peak frequency f.sub.SEL. That is to say, relative energy peaks exist at positions of frequencies (f.sub.SEL1/T), (f.sub.SEL2/T), (f.sub.SEL3/T) . . . in the spectrum Z(f). Further, the sampling frequency F.sub.S of the input signal y(t) limits the frequency range of the CFO estimating apparatus 200. For example, the frequency range the spectrum Z(f) covers may be limited between F.sub.S/2 and F.sub.S/2. Thus, in one embodiment, the search circuit 25A identifies energies corresponding to a plurality of frequencies (F.sub.SEL+n/T) spaced by integral multiples of 1/T from the partial peak frequency f.sub.SEL as the energy increments P.sub.ADD, where n is an integral index value in a predetermined range such that the plurality of frequencies (F.sub.SEL+n/T) fall in the frequency range F.sub.S/2 and F.sub.S/2.

(14) FIG. 4(A) shows an example of a spectrum of a signal transmitted through a multipath. In FIG. 4(A), in the frequency range F.sub.S/2 and F.sub.S/2, there are seven frequencies (F.sub.SEL+n/T) that are spaced by integral multiples of 1/T from the partial peak frequency f.sub.SEL. The search circuit 25A may select the energies (P.sub.4 to P.sub.1 and P.sub.+1 to P.sub.+3) corresponding to these frequencies as the energy increments P.sub.ADD.

(15) The addition circuit 25B adds up the energy increments P.sub.ADD and the partial energy peak value P.sub.SEL to generate an adjusted energy P.sub.SUM. Next, the adjusting circuit 25C adjusts the spectrum Z(f) according to the partial peak frequency f.sub.SEL and the adjusted energy P.sub.SUM to generate an adjusted spectrum C(f). In the adjusted spectrum C(f), the energy corresponding to the partial peak frequency f.sub.SEL is equal to the adjusted energy P.sub.SUM. In one embodiment, the adjusting circuit 25C changes only the energy corresponding to the partial peak frequency f.sub.SEL but not the energies corresponding to the other frequencies in the spectrum Z(f); that is, the energies corresponding to the other frequencies in the adjusted spectrum C(f) and in the spectrum Z(f) are the same. Taking FIG. 4(A) for example, the adjusted spectrum C(f) in FIG. 4(B) is an example of the spectrum Z(f) that is adjusted by the adjusting circuit 25C. As seen from FIG. 4(B), in the adjusted spectrum C(f), an adjusted energy P.sub.SUM corresponding to the partial peak frequency f.sub.SEL is a sum of eight energies P.sub.4 to P.sub.+3 in the spectrum Z(f), and the other parts in the adjusted spectrum C(f) are identical to those in the spectrum Z(f).

(16) In practice, the energies corresponding to the frequencies in the spectrum Z(f) may be respectively stored in multiple registers by the spectrum generating circuit 22. The adjusting circuit 25C can adjust the spectrum by setting contents of the registers corresponding to the frequencies to be adjusted.

(17) After the spectrum adjusting circuit 25 generates the adjusted spectrum C(f), the peak frequency determining circuit 23 identifies a peak frequency from the adjusted spectrum C(f). Observing the adjusted spectrum C(f) in FIG. 4B, it is seen that the peak frequency determining circuit 23 selects the adjusted peak energy P.sub.SUM as the maximum peak value after the spectrum Z(f) is adjusted by the spectrum adjusting circuit 25, and determines the partial peak frequency f.sub.SEL corresponding to the adjusted energy P.sub.SUM as the peak frequency . Next, the frequency offset determining circuit 24 determines an estimated CFO result f.sub.E according to the peak frequency . More specifically, the frequency offset determining circuit 24 may divide the peak frequency by 4 to generate the estimated CFO result f.sub.E.

(18) As previously stated, the partial peak frequency f.sub.SEL that the frequency adjusting circuit 25 identifies is usually the four-fold carrier frequency offset 4f. Through adjusting the energy corresponding to the partial peak frequency f.sub.SEL to the sum of the plurality of energy increments P.sub.ADD and the partial energy peak P.sub.SEL by the spectrum adjusting circuit 25, in the adjusted spectrum C(f), the energy corresponding to the partial peak frequency f.sub.SEL becomes higher than the energies corresponding to other frequencies. Thus, even in a situation where the energies corresponding to the other frequencies in the original spectrum Z(f) are caused to be higher than the energy corresponding to the partial peak frequency f.sub.SEL due to the effects of echo signals (e.g., the energy P.sub.+1 in FIG. 4(A) is higher than the energy P.sub.0), the peak frequency determining circuit 23 is still capable of identifying the peak frequency corresponding to the actual four-fold carrier frequency offset 4f according to the adjusted spectrum C(f), such that the frequency offset determining circuit 24 may calculate the estimated CFO result f.sub.E.

(19) FIG. 5 shows the spectrum adjusting circuit 25 according to another embodiment. In this embodiment, the spectrum adjusting circuit 25 includes a search circuit 25D, an addition circuit 25E and an adjusting circuit 25F. Similar to the search circuit 25A, the search circuit 25D identifies the partial energy peak value P.sub.SEL from the spectrum Z(f), and identifies the partial peak frequency f.sub.SEL according to the partial energy peak value P.sub.SEL. Different from the search circuit 25A, the search circuit 25D does not need to identify the energies corresponding to the frequencies spaced by integral multiples of 1/T from the partial peak frequency f.sub.SEL. The addition circuit 25E adds a predetermined energy increment P.sub.DFT and the partial peak frequency f.sub.SEL identified by the search circuit 25D to generate an adjusted energy P.sub.SUM. In practice, the predetermined energy increment P.sub.DFT may be designed according to actual communication environments in a way that the adjusted energy P.sub.SUM is usually higher than other energies in the spectrum Z(f).

(20) Similar to the adjusting circuit 25C, the adjusting circuit 25F adjusts the spectrum Z(f) according to the partial peak frequency f.sub.SEL and the adjusted energy P.sub.SUM to generate an adjusted spectrum C(f). In the adjusted spectrum C(f), the energy corresponding to the partial peak frequency f.sub.SEL is equal to the adjusted energy P.sub.SUM.

(21) It should be noted that, in other embodiments of the present invention, the 4.sup.th-power circuit 21 may be replaced by an M.sup.th-power circuit, where M is an integer greater than 1. In one embodiment, the integer M may be associated with the modulation scheme of the input signal y(t). For example, when the modulation scheme that the transmitter performs on the input signal y(t) is QPSK, the integer M may be designed to equal to 4 or an integral multiple of 4. Similarly, when the modulation scheme that the transmitter performs on the input signal y(t) is 8 phase shift keying (8PSK), the integer M may be designed to equal to 8 or an integral multiple of 8. However, for the M.sup.th-power circuit, in an ideal situation, the peak frequency the peak frequency determining circuit 23 identifies corresponds to an M multiple of the carrier frequency offset f. Thus, the frequency offset determining circuit 24 may generate the estimated CFO result f.sub.E according to the peak frequency and the value M.

(22) In practice, the peak frequency determining circuit 23, the frequency offset determining circuit 24 and the spectrum adjusting circuit 25 may be realized by various types of control and processing platforms, including fixed and programmable logic circuits, e.g., programmable logic gate arrays, application-specific integrated circuits (ASIC), microcontrollers, microprocessors and digital signal processors (DSP). Further, the peak frequency determining circuit 23, the frequency offset determining circuit 24 and the spectrum adjusting circuit 25 may be designed to complete multiple tasks through executing commands stored in a memory (not shown) by one or more processors. One person skilled in the art can understand that, there are many circuit configurations and elements capable of realizing the concept of the present invention without departing from the spirit of the present invention.

(23) A carrier frequency offset (CFO) estimating method is provided according to another embodiment of the present invention. FIG. 6 shows a flowchart of the method, which includes following steps. In step S61, an M.sup.th-power calculation is performed on an input signal to generate an M.sup.th-power calculation result, where M is an integer greater than 1 and is associated with a modulation scheme of the input signal. In step S62, a spectrum is generated according to the M.sup.th-power calculation result. In step S63, a partial energy peak value in a partial frequency range is identified from the spectrum, wherein the partial frequency range is approximately T to T and T represents a symbol duration of the input signal. In step S64, the partial energy peak value is increased to be higher than any other energy in the spectrum to accordingly generate an adjusted spectrum. In step S65, a peak frequency corresponding to a maximum energy peak value is identified from the adjusted spectrum. In step S66, an estimated CFO result is determined according to the peak frequency.

(24) One person skilled in the art can understand that, operation variations (e.g., means for adjusting the partial energy peak value) in the description associated with the CFO estimating apparatus 200 are applicable to the CFO estimating method in FIG. 6, and are thus omitted herein.

(25) While the invention has been described by way of example and in terms of the embodiments, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.