Method of estimating carrier frequency offset in an orthogonal frequency division multiplexing system

[0002] (a) Field of the Invention

[0003] The present invention relates to a method of estimating carrier frequency offset in an OFDM (orthogonal frequency division multiplexing) system. More particularly, the present invention relates to a method of estimating carrier frequency offset in an OFDM system in which a repetitive signal structure inside of a 1 OFDM symbol is used to enlarge the range and increase the accuracy of offset estimation.

[0004] (b) Description of the Related Art

[0005] OFDM systems are used for high-speed data transmission. In the OFDM system, input bits are converted to data symbols (Xk), and the data symbols are modulated to OFDM symbols through an OFDM modulator using IFFT (inverse fast Fourier transform), and then transmitted to a receiver. At this time, the OFDM symbols are comprised of an N samples and a cyclic prefix, which is a copy of the last part of the OFDM symbol. The cyclic prefix is introduced to maintain orthogonality between sub-carriers, and is, in general, assumed to be longer than the impulse response of a time-dispersive channel.

[0006] In the receiver, after compensating for the carrier frequency offset of the received signals and removing the cyclic prefix, a FFT is executed and the data symbols are demodulated.

[0007] An offset estimation method of carrier frequency proposed by Schmidl has been widely used among those. In this method, two identical signal intervals are made in a 1 OFDM reference symbols within a time domain, and the carrier frequency offset is estimated by using these signals.

[0008] The conventional method for estimating carrier frequency offset will be described hereinafter.

[0009] In an OFDM system, when there is carrier frequency offset, a phase of a received signal (y_n) is changed with time by a carrier frequency (ε) in a time domain as shown in Equation 1 below.

[0010] y_h=x_ne^jπεn/N  [Equation 1]

[0011] where x_nis a transmitted time-domain signal of reference OFDM symbol designed to estimate carrier frequency offset and ε is a carrier frequency offset normalized to a sub-carrier spacing. N denotes the number of samples, that is, FFT size.

[0012] In the case where two identical signals are transmitted, each signal having N/2 number of samples, Equation 2 below can be obtained by correlating them. $\begin{array}{cc}\begin{array}{c}\sum _{n=0}^{N/2-1}{y}_n^{*}{y}_{N/2+n}=\sum _{N=0}^{N/2-1}{\left(x_n{e}^{j2\pi ϵn/N}\right)}^{+}\left(x_{N/2+n}{e}^{j2\mathrm{\pi ϵ}\left(N/2+n\right)/N}\right)\\ =e^{j\pi ϵ}\sum _{n=0}^{N/2-1}{\left|x_n\right|}^2\end{array}& \left[\mathrm{Equation}2\right]\end{array}$

[0013] where x_nand x_N/2+n(n=0, 1, 2, 3, . . . , N/2-1) are repeated identical signals in one OFDM symbol. Here * denotes a complex conjugate.

[0014] In Equation 2, as |x_n|²has a real number value, the carrier frequency offset (ε) can be obtained by the following Equation 3. $\begin{array}{cc}ϵ=\frac{1}{\pi }\arg \left(\sum _{n=0}^{N/2-1}{y}_n^{*}{y}_{\frac{N}{2}+\pi }\right)& \left[\mathrm{Equation}3\right]\end{array}$

[0015] where arg is an argument function having a value from −π to π. Therefore, according to the conventional method for estimating carrier frequency offset, a possible estimation range of carrier frequency offset is limited to |ε|<1.

[0016] FIG. 1 shows a block diagram of a conventional method for estimating carrier frequency offset. As shown in the drawing, in the conventional method, signals y_nare received in a time domain, and a value for the signals is placed in Equation 3 to estimate carrier frequency offset.

[0017] However, it is common that the carrier frequency offset caused by oscillator instability, thermal noise in a base station and a mobile station exceeds the limited estimation range, |ε|<1. As a result, the estimation of carrier frequency offset can be failed for a large value.

[0018] The present invention has been made in an effort to solve the above problem.

[0019] It is an object of the present invention to provide a method of estimating carrier frequency offset in an OFDM system in which a range of carrier frequency offset estimation is enlarged and accuracy of the same is maintained by using only time-domain received signal.

[0020] To achieve the above object, the present invention provides a method of estimating carrier frequency offset in an OFDM system. The method includes the steps of forming a 1 OFDM symbol using an N samples, which has M repeated identical sub-blocks, form a 2ⁱnumber of identical ith sub-blocks using an M/2ⁱnumber of the M number of base sub-blocks as a unit; and estimating an ith carrier frequency offset ε_iusing a correlation between two adjacent ith sub-blocks in the 2ⁱnumber of ith sub-blocks.

[0021] According to a feature of the present invention, the offset ε_iof the ith carrier frequency is estimated using the following equation: ${\widehat{ϵ}}_i=\frac{2^{\prime }}{2\pi }\arg \left(\sum _{n=0}^{N/2^{\prime }-1}{y}_n^{*}{y}_{\frac{N}{2^{\prime }}+\pi }\right)$

[0022] where * is a complex conjugate and y_nis a received signal.

[0023] According to another feature of the present invention, M satisfies the condition of 2^k(k=a positive integer).

[0024] According to yet another feature of the present invention, the method further includes the step of obtaining an enlarged ith carrier frequency offset estimate values {tilde over (ε)}_i(q_i) from the estimated ith carrier frequency offset {circumflex over (ε)}_l, the offset estimate values {tilde over (ε)}_i(q_i) satisfying the condition of {tilde over (ε)}_i(q_i)=2ⁱq_i+{circumflex over (ε)}_i(i=1,2,3,4 . . . ,k), where −2^k·(i+1)≦q_i<2^k·(i+1)when {circumflex over (ε)}_i≧0, and {tilde over (ε)}_i(q_i)=−2^k·(i+1)+1 ≦q_i<2^k·(i+1)+1 when ε_i<0.

[0025] According to still yet another feature of the present invention, the method further includes the steps of selecting values closest to {tilde over (ε)}_p(q_p) values among {tilde over (ε)}₁(q₁) values, where p<k; and obtaining carrier frequency offset from the selected {tilde over (ε)}₁(q₁) values.

[0026] According to still yet another feature of the present invention, the method further includes the steps of selecting values closest to a {tilde over (ε)}₂(q₂) value among {tilde over (ε)}₁(q₁) value; selecting values closest to {tilde over (ε)}₃(q₃) values among the selected {tilde over (ε)}₁(q₁) values; selecting {tilde over (ε)}₁(q₁) value closest to {tilde over (ε)}_p(q_p) values by repeating the above step, where p≦k; and obtaining carrier frequency offset from the selected {tilde over (ε)}₁(q₁) value.

[0027] According to still yet another feature of the present invention, p=k, and a last remaining value among the {tilde over (ε)}₁(q₁) values is a carrier frequency offset.

[0028] According to still yet another feature of the present invention, the 1 OFDM symbol is obtained by using a data symbol Xk of a frequency scope having a random symbol that is not 0 in a sub-channel corresponding to a constant multiple of M, and having a 0 value in remaining portions.

[0029] According to still yet another feature of the present invention, the data symbol Xk of a frequency scope satisfies the condition of X_k={square root over (M)}X_m^Dwhen k =mM (m=0, 1, 2, . . . , N/M−1), where X_kis a transmitted symbol to a kth sub-carrier, and X_m^Drefers to a random symbol other than 0, and k is 0 with other values.

[0030] Further objects and other advantages of the present invention will become apparent from the following description in conjunction with the attached drawings, in which:

[0031] FIG. 1 is a block diagram of a conventional method for estimating carrier frequency offset;

[0032] FIG. 2 is a schematic drawing of a time domain 1 OFDM symbol used in a first preferred embodiment of the present invention;

[0033] FIG. 3 is a schematic drawing of a time domain 1 OFDM symbol realized through 8 sub-blocks used in the first preferred embodiment of the present invention;

[0034] FIG. 4 is a flow chart of a method for establishing carrier frequency offset according to the second preferred embodiment of the present invention;

[0035] FIG. 5 is a block diagram of a method for estimating carrier frequency offset according to the second preferred embodiment of the present invention; and

[0036] FIG. 6 is a chart used to describe an example for estimating carrier frequency offset according to the second preferred embodiment of the present invention.

[0037] Preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

[0038] Referring first to FIG. 2, in a method for estimating carrier frequency offset according to a first preferred embodiment of the present invention, a time domain 1 OFDM symbol (composed of N samples) is comprised of signals repeated M number of times (M=2^k, where k is a positive integer). At this time, a signal portion that is repeated in minimum(i.e., A0, A1, . . . , A_M-1) is defined as a base sub-block.

[0039] In such a symbol structure, an M/2 number of base sub-blocks are bundled to obtain a new primary sub-block such that two primary sub-blocks of an N/2 samples come to exist in the 1 OFDM symbol. Further, an M/4 number of base sub-blocks are bundled to obtain a new second sub-block such that four secondary sub-blocks come to exist in the 1 OFDM symbol. In the same manner, M/2ⁱof base sub-blocks are bundled to obtain a 2ⁱnumber of ith sub-blocks. A final kth sub-block is identical to the base sub-block. An example of a time domain 1 OFDM symbol structure when M=8 is illustrated in FIG. 3.

[0040] In the first preferred embodiment of the present invention, a correlation between two adjacent its sub-blocks is used to estimate carrier frequency offset.

[0041] First, assuming that a signal interval between two identical symbols is Δ (i.e. X_n=X_n+Δ), Equation 2 can be changed into Equation 4 below. $\begin{array}{cc}\begin{array}{c}\sum _{n=1}^{\Delta -1}{y}_n^{*}{y}_{n+\Delta }=\sum _{n=0}^{\Delta -1}\left(x_n{e}^{j\pi ϵ/N}\right)*\left(x_{n+\Delta }{e}^{j\pi ϵ\left(n-\Delta \right)/N}\right)\\ =e^{j\pi \mathrm{ϵ\Delta }/N}\sum _{n=0}^{\Delta -1}{\left|x_n\right|}^2\end{array}& \left[\mathrm{Equation}4\right]\end{array}$

[0042] Since, Δ=N/2ⁱin the case of the ith sub-block, ε_ican be obtained by Equation 5 below derived from Equation 4. $\begin{array}{cc}{\widehat{ϵ}}_i=\frac{2^{\prime }}{2\pi }\arg \left(\sum _{n=0}^{N/2^{\prime }-1}{y}_n^{*}{y}_{\frac{N}{2^{\prime }}+\pi }\right)& \left[\mathrm{Equation}5\right]\end{array}$

[0043] where {circumflex over (ε)}_irepresents carrier frequency offset when using an ith sub-block. At this time, an estimation range of carrier frequency offset is defined as |ε_i|<2^|˜|.

[0044] Accordingly, if the carrier frequency offset is obtained using a high degree sub-block, the range of estimation is increased. As a result, it is possible to estimate the large carrier frequency offset which is caused by oscillator instability and thermal noise in a base station and mobile station.

[0045] However, in the method of estimating carrier frequency offset according to the first preferred embodiment of the present invention, the number of samples becomes N/2ⁱin the case of the ith sub-block. That is, because the number of samples deceases exponentially with the increase of the degree of sub-block used for carrier frequency offset estimation, the accuracy of the carrier frequency offset estimation is reduced.

[0046] In a method of estimating carrier frequency offset in an OFDM system according to a second preferred embodiment of the present invention, the range of estimation is increased by utilizing the above description, and a high level of accuracy in an estimation value is ensured as in the Schmidl's method only by processing the received signal in time-domain. That is, in the method according to the second preferred embodiment of the present invention, a multistage estimation algorithm (MEA) is combined with the method of the first preferred embodiment to achieve increased range.

[0047] In the MEA according to the second preferred embodiment, characteristics of the value of {circumflex over (ε)}_land the argument function of Equation 5 are used.

[0048] Using the properties of the argument function, the value of {circumflex over (ε)}_lestimated using Equation 5 can be used to obtain all possible values of carrier frequency offset {tilde over (ε)}_i(q_i) which can be obtained using ith sub-block.

[0049] {tilde over (ε)}_i(q_i)=2ⁱq_i+{circumflex over (ε)}_i, i=1,2, . . . ,k  [Equation 6]

[0050] where {tilde over (ε)}_l(q_q) represents an estimated carrier frequency offset value at ith step, and q_iis an integer satisfying the following conditions of Equation 7.

[0051] −2^k·(i+1)≦q_i<2^k-(i+1), ε_l≧0  [Equation 7]

[0052] −2^k·(i+1)+1≦q_i<2^k-(i+1)+1, ε_i<0

[0053] As shown by Equations 6 and 7, in each step the estimated offset value ε_i(q_i) has various values according to q_i. Accordingly, the number of offset values is reduced by ½ according to the increase in step i, because the intervals (2ⁱ) between adjacent {tilde over (ε)}_i(q_i) values at each step increase by two times.

[0054] The method of estimating carrier frequency offset according to the second preferred embodiment of the present invention will be described hereinbelow with reference to FIGS. 4 and 5.

[0055] First, {circumflex over (ε)}_lis obtained from 1 OFDM using Equation 5. Next, all {tilde over (ε)}₁(q₁) values existing within the range of estimation ({tilde over (ε)}₁(q₁)<2^i-1) for carrier frequency offset are obtained. This range refers to the estimation range of carrier frequency offset solely obtained using a kth sub-block.

[0056] Subsequently, {tilde over (ε)}₂(q₂) values within the range of estimation are obtained. Since an interval (2²) between the {tilde over (ε)}₂(q₂) values according to q₂is twice that (2¹) between the {tilde over (ε)}₁(q₁) values, the number of {tilde over (ε)}₂(q₂) values is ½ the number of {tilde over (ε)}₁(q₁) values within the range, solely obtained using an kth sub-block. If the {tilde over (ε)}₁(q₁) values closest to the {tilde over (ε)}₂(q₂) values are selected, the number of {tilde over (ε)}₁(q₁) values is reduced by ½, thereby becoming identical to the number of {tilde over (ε)}₂(q₂) values. If this process is continued to {tilde over (ε)}_k(q_k) values, only a single {tilde over (ε)}₁(q₁) value is left remaining. This single {tilde over (ε)}₁(q₁) value is the estimated offset value of carrier frequencies obtained through MEA.

[0057] At this time, since the value estimated through the MEA is one of the {tilde over (ε)}₁(q₁) values, the accuracy of estimation is equal to that when a primary sub-block is used to obtain the estimation value (i.e. when Schmidl's method is used).

[0058] FIG. 6 is a chart used to describe an example for estimating carrier frequency offset according to a second preferred embodiment of the present invention. The example is that of estimating ε=2.8 with respect to the symbol structure shown in FIG. 2.

[0059] An OFDM system using an N number of orthogonal sub-carriers is modulated by IFFT (inverse fast Fourier transform). Accordingly, in the second embodiment of the present invention, an OFDM symbol with M repeated signal block is obtained using Equation 8 below, with a data symbol X_khaving the following conditions.

[0060] X_k={square root over (M)}X_m^D, k=mM (m=0,1,2 . . . , N/M-1)  [Equation 8]

[0061] 0, otherwise

[0062] where X_kis a transmitted symbol to kth sub-carrier, and X_m^Drefers to a random symbol.

[0063] In the second preferred embodiment of the present invention, a random symbol is inserted in a sub-channel corresponding to a constant multiple of M, and 0 is inserted in remaining portions. Further, a power of a symbol is increased by as much as {square root over (M)} and transmitted to prevent the reduction in power caused by the insertion of 0s.

[0064] In the second preferred embodiment, it was described all sub-blocks to a maximal degree are used to estimate carrier frequency offset, but in the case when carrier frequency offset is not large as a result of good channel conditions, it is possible to use a sub-block of a degree lower than the maximal degree sub-block to estimate the carrier frequency offset.

[0065] Further, if the offset value of carrier frequencies, estimated using high-degree sub-blocks, is accurate, it is possible to directly selected the ε₁(q₁) values from the estimated carrier frequency offset using the high-degree sub-blocks, without requiring the use of all the consecutive lower-degree sub-blocks. Utilizing this method, the amount of calculation needed is minimized.

[0066] In the method of estimating carrier frequency offset in an OFDM system of the present invention, the accuracy of estimation is same as that of the conventional method while the range of estimation is increased by using only the received signals in time-domain.

[0067] While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.