Apparatus and methods for estimating and compensating sampling clock offset
View Patent ↗An apparatus for sampling clock recovery (SCO) and methods for estimating and compensating SCO are provided. The apparatus comprises a symbol timing adjustment module for shifting forward or backward symbol timing of the transmitted OFDM symbols; a discrete Fourier transform (DFT) processor for performing DFT to an output from the symbol timing adjustment module; a channel estimator for undertaking a channel frequency response estimation based on a channel estimation sequence; a SCO phase rotator for receiving and performing phase shift on the transmitted OFDM symbols of a frame header and a frame payload; an SCO estimation stage for undertaking an SCO estimation based on a pilot-subcarrier-related output of the SCO phase rotator and the CFR estimation; and an SCO compensation distributor for dividing the SCO estimation into integer and fractional portions and then distributing them into the symbol timing adjustment module and the SCO phase rotator, respectively.
1. A method for estimating and compensating sampling clock offset (SCO) for a multi-band orthogonal frequency division multiplexing (OFDM)-based ultra-wideband (UWB) system on a plurality of transmitted OFDM symbols; the transmitted OFDM symbols being divided into groups, indexed with m; each group having K OFDM symbols, indexed with i, i=0, 1, . . . , K−1; each OFDM symbol having R pilot subcarriers, ascendingly indexed with {p(0), p(1), . . . , p(R−1)}; a method for obtaining an accumulated normalized SCO (ANSCO) estimation, {circumflex over (η)} m+1 (i) , on the ith OFDM symbol in the (m+1)th group, using an ANSCO estimation {circumflex over (η)} m (i) , on the ith OFDM symbol in the mth group, comprising:
grouping R pilot subcarriers into Q pilot pairs; each pilot pair, {p(l 1 ), p(l 2 )}, having a separation factor, β(l 1 ,l 2 );
obtaining a channel frequency response (CFR) estimation, h r , on an rth sub-band;
obtaining a pilot-subcarrier related input of an N-point inverse discrete Fourier transform (IDFT) processor and a pilot-subcarrier related output of an N-point discrete Fourier transform (DFT) processor;
obtaining a residual SCO estimation related value, b m (i) (l 1 ,l 2 ), by using a pilot-pair related input of the IDFT processor, a pilot-pair related output of the DFT processor and a pilot-pair related CFR estimation, where the pilot-pair indicates one pilot pair {p(l 1 ), p(l 2 )} in the R pilot subcarriers of the ith OFDM symbol in the mth group;
summing up Q residual SCO estimation related values, respectively obtained from the Q pilot pairs of the ith OFDM symbol in the mth group to yield a symbol-level residual SCO estimation related value, γ m (i) ;
obtaining a combined residual SCO estimation, Δ{circumflex over (η)} m , by using K symbol-level residual SCO estimation related values, γ m (0) , γ m (1) , . . . , γ m (K−1) , respectively obtained from the K OFDM symbols in the mth group;
obtaining the ANSCO estimation, {circumflex over (η)} m+1 (i) , based on the combined residual SCO estimation, Δ{circumflex over (η)} m and the ANSCO estimation {circumflex over (η)} m (i) , on the ith OFDM symbol in the mth group,
dividing the ANSCO estimation into integer and fractional portions;
shifting sample timing at an interval of a sampling period in the time-domain by an amount of the modulus of the integer portion of the ANSCO estimation; and
correcting a phase shift in the frequency-domain with a rotating factor corresponding to the fractional portion of the ANSCO estimation.
2. The method as claimed in claim 1 , wherein during the step of grouping R pilot subcarriers into Q pilot pairs, the Q pilot pairs are obtained as {p(l 1 ), p(l 2 )}, l 1 εZ 0 R/2−1 and l 2 εZ R/2 R/2+l 1 , and each pilot pair's separation factor is given by β(l 1 ,l 2 )=p(l 2 )−N−p(l 1 ), l 1 εZ 0 R/2−1 and l 2 εZ R/2 R/2+l 1 , where Z P 1 P 2 denotes the finite integer set {P 1 , P 1 +1, . . . , P 2 }.
3. The method as claimed in claim 1 , wherein the step of obtaining a residual SCO estimation related value, b m (i) (l 1 ,l 2 ), comprises: obtaining the pilot-pair related input, {s m (i) (p(l 1 )), s m (i) (p(l 2 ))} of the IDFT processor, the pilot-pair related output, {ŷ m (i) (p(l 1 )), ŷ m (i) (p(l 2 ))} of the DFT processor and the pilot-pair related CFR estimations, {ĥ r (p(l 1 )), ĥ r (p(l 2 ))}; computing b m (i) (l 1 ,l 2 )=ℑ[(ŷ m (i) (p(l 1 ))ĥ r (p(l 2 )))*ŷ m (i) (p(l 2 ))ĥ r (p(l 1 ))s m (i) (p(l 1 ))(s m (i) (p(l 2 )))*], where (x)* means to perform conjugation on x, and ℑ[x] means to take the imaginary part of x; and dividing b m (i) (l 1 ,l 2 ) by β(l 1 ,l 2 ) to yield the residual SCO estimation related value, b m (i) (l 1 ,l 2 ).
4. The method as claimed in claim 1 , wherein the step of summing up Q residual SCO estimation related values respectively obtained from the Q pilot pairs of the ith OFDM symbol in the mth group to yield a symbol-level residual SCO estimation related value, γ m (i) , is performed as:
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5. The method as claimed in claim 1 , wherein the step of obtaining a combined residual SCO estimation, Δ{circumflex over (η)} m , by using K symbol-level residual SCO estimation related values, γ m (0) , γ m (1) , . . . , γ m (K−1) , respectively obtained from the K OFDM symbols in the mth group, comprises: finding two values, N m p , the number of the symbol-level residual SCO estimation related values, γ m (0) , γ m (1) , . . . , γ m (K−1) , which are greater than zero, and, N m n , the number of the symbol-level residual SCO estimation related values, γ m (0) , γ m (1) , . . . , γ m (K−1) , which are less than zero; and obtaining a sign of the combined residual SCO estimation, Δ{circumflex over (η)} m , by comparing N m p and N m n .
6. The method as claimed in claim 5 , wherein the step of obtaining a combined residual SCO estimation, Δ{circumflex over (η)} m , by using K symbol-level residual SCO estimation related values γ m (0) , γ m (1) , . . . , γ m (K−1) , respectively obtained from the K OFDM symbols in the mth group, is implemented as:
Δ{circumflex over (η)} m =sgn( N m p −N m n ) (| N m p −N m n |)μ
where sgn(N m p −N m n ) indicates that the sign is positive if N m p is greater than N m n , and negative if N m p is less than N m n , otherwise, the combined residual SCO estimation, Δ{circumflex over (η)} m , is zero; μ is a system-design defined parameter indicating a micro-shift of SCO; and (l), l=0, 1, . . . , K, is the lth element of a system-design defined non-negative integer vector, , which satisfies (l+1)≧ (l).
7. The method as claimed in claim 1 , wherein the step of obtaining the ANSCO estimation, {circumflex over (η)} m+1 (i) , based on the combined residual SCO estimation, Δ{circumflex over (η)} m and the ANSCO estimation {circumflex over (η)} m (i) , on the ith OFDM symbol in the mth group, is performed as:
{circumflex over (η)} m+1 (i) ={circumflex over (η)} m (K−1) +( i+ 1)Δ{circumflex over (η)} m +{circumflex over (η)} m (i) /m
where {circumflex over (η)} m (K−1) corresponds to the ANSCO estimation on the last OFDM symbol in the mth group.
8. The method as claimed in claim 1 , wherein during the step of shifting sample timing at an interval of sampling period in time-domain by an amount of the modulus of the integer portion, the sample timing is shifted forward if the integer portion of the ANSCO estimation is less than zero and backward if the integer portion of the ANSCO estimation is greater than zero.
9. The method as claimed in claim 1 , wherein during the step of correcting phase shift in frequency-domain with a rotating factor corresponding to the fractional portion of the ANSCO estimation, the rotating factor on subcarrier k is obtained as e −jφ m (i) (k) with the phase shift, φ m (i) (k), on subcarrier k being obtained from the fractional portion of the ANSCO estimation on the ith OFDM symbol in the mth OFDM symbol group as
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where the transmitted OFDM symbols are divided into a plurality of OFDM symbol groups, indexed with m, each of which has a certain number of OFDM symbols, indexed with i, N indicates the number of subcarriers employed by one OFDM symbol, k indicates subcarriers index, Z P 1 P 2 denotes the finite integer set {P 1 , P 1 +1, . . . , P 2 }, {circumflex over (η)} m (i) is the ANSCO estimation on the ith OFDM symbol in the mth group, and F({circumflex over (η)} m (i) ) is the fractional portion of the ANSCO estimation, {circumflex over (η)} m (i) ; and, the phase shift is corrected in an SCO phase rotator in frequency-domain as:
ŷ m (i) ( k )= y m (i) ( k ) e −jφ m (i) (k) ,kεZ 0 N−1
where y m (i) (k) and ŷ m (i) (k) are an input and an output of the SCO phase rotator.