FREQUENCY OFFSET ESTIMATION METHOD FOR OFDM-IM SYSTEM

Abstract

The invention provides a frequency offset estimation method for an OFDM-IM system. The method includes: S1. performing preliminary frequency offset compensation on a received signal subjected to non-uniform frequency offset by using a two-step method of: (1) resampling and down conversion; and (2) unified compensation for residual frequency offset , wherein in the step (2), a sum of energy of null sub-carriers is used as a cost function, an initial estimation value of is obtained by one-dimensional search, and the preliminary compensation is performed; S2. estimating positions of non-activated sub-carriers in the OFDM-IM system by using the signal subjected to the preliminary compensation; and S3. assigning certain weights to the estimated sub-carriers, adding energy of the estimated sub-carriers into the cost function according to different weights, obtaining a final estimation value of by the one-dimensional search performed on , and performing secondary compensation.

Claims

1. A frequency offset estimation method for an OFDM-IM system, the method comprising following steps: S1. performing preliminary frequency offset compensation on a received signal subjected to non-uniform frequency offset by using a two-step method of: (1) resampling and down conversion to obtain a discrete baseband received signal z=(()y; and (2) unified compensation for residual frequency offset, a sum of energy of null sub-carriers being used as a cost function in the step (2): $J_1\left(\right)=\underset{m{S}_N}{.Math.}.Math.{.Math.f_m^H.Math.{}^H\left(\right).Math.z.Math.}^2$ wherein y is the discrete baseband received signal when the frequency offset is 0, is a residual normalized frequency offset, S.sub.N is a null sub-carrier set, f.sub.m is an m.sup.th column of an inverse Fourier transformation matrix, () is a frequency offset matrix diag(1,e.sup.j2T.sup.c.sup., . . . , e.sup.j2T.sup.c.sup.(k1)), j={square root over (1)}, (.).sup.H sampling interval; and an initial estimation value {circumflex over ()}.sub.1=arg min.sub.J.sub.1() of is obtained by one-dimensional search performed on ; S2. using a signal z.sub.1=.sup.H({circumflex over ()}.sub.1)z subjected to the preliminarily estimated compensation of to estimate positions of non-activated sub-carriers in the OFDM-IM system based on the signal subjected to the compensation, with criteria that: {circle around (1)} a present power P.sub.null of the sub-carriers is lower; {circle around (2)} one of two adjacent sub-carriers with a higher power P.sub.side is larger; {circle around (3)} a specific value is P.sub.side/P.sub.null is large enough; and S3. sorting the estimated sub-carriers in a descending order of a size of P.sub.side, assigning a larger weight to energy of the sub-carrier with a larger side P.sub.side, and forming the weights of each sub-carrier into a diagonal matrix W; adding energy of the estimated sub-carriers into the cost function according to different weights, the cost function being modified as: $J_2\left(\right)=\underset{m{S}_N}{.Math.}.Math.{.Math.f_m^H.Math.{}^H\left(\right).Math.z.Math.}^2+\underset{m\widehat{S_N}}{.Math.}.Math.{.Math.{\mathrm{Wf}}_n^H.Math.{}^H\left(\right).Math.z.Math.}^2$ wherein is an estimated non-activated sub-carrier set; and obtaining a final estimation value {circumflex over ()}.sub.2=arg min.sub.J.sub.2() of by the one-dimensional search performed on , a signal subjected to final compensation being z.sub.2=.sup.H({circumflex over ()}.sub.2)z, wherein a number of sub-carriers of the OFDM-IM system is N, a MPSK modulation mode is used in the system, in addition to a pilot frequency, a guard interval and N.sub.null preset null sub-carriers, the system contains a plurality of OFDM sub-blocks, and each block has N.sub.total sub-carriers, wherein N.sub.active sub-carriers are activated sub-carriers, a position and a symbol of the pilot frequency are known, and positions of the preset null sub-carriers are known.

2. The frequency offset estimation method for the OFDM-IM system according to claim 1, wherein in the step S2, the positions of the non-activated sub-carriers are estimated through power detection by using the signal subjected to the preliminary compensation.

3. The frequency offset estimation method for the OFDM-IM system according to claim 1, wherein in the step S3, the energy of the sub-carriers estimated to be non-activated is added into a final cost function according to different weights.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a flow chart of a method according to the present invention.

[0023] FIG. 2 and FIG. 3 are respective trend charts of mean square errors of different {circumflex over ()}.sub.norm changing with a signal-to-noise ratio.

DESCRIPTION OF THE EMBODIMENTS

[0024] The present invention is further described in detail hereinafter with reference to the accompanying drawings and the specific embodiments, the implementation and protection of the present invention are not limited to the accompanying drawings and the specific embodiments. It should be noted that all the processes which are not specifically described hereinafter can be implemented or understood by those skilled in the art with reference to the prior art.

[0025] The present invention provides an OFDM-IM method based on multiple modes, wherein 128 sub-carriers are provided in an OFDM-IM system, and a QPSK modulation mode is used in the system. In addition to a pilot frequency, a guard interval and 4 preset null sub-carriers, the system contains 28 OFDM sub-blocks, and each block has 4 sub-carriers, wherein 3 of the 4 sub-carriers are activated sub-carriers. A position and a symbol of the pilot frequency are known, and positions of the preset null sub-carriers are known. The specific implementation includes the following steps.

[0026] In step S1, preliminary frequency offset compensation is performed on a received signal subjected to non-uniform frequency offset by using a two-step method of: 1. resampling and down conversion to obtain a discrete baseband received signal z=()y; and 2. unified compensation for residual frequency offset. A sum of energy of null sub-carriers is used as a cost function in the step 2:

[00004]$J_1\left(\right)=\underset{m{S}_N}{.Math.}.Math.{.Math.f_m^H.Math.{}^H\left(\right).Math.z.Math.}^2$

wherein y is the discrete baseband received signal when the frequency offset is 0, is a residual normalized frequency offset, S.sub.N is a null sub-carrier set, f.sub.m is an m.sup.th column of an inverse Fourier transformation matrix, () is a frequency offset matrix diag(1,e.sup.j2T.sup.c.sup., . . . , e.sup.j2T.sup.c.sup.(k1)), j={square root over (1)}, (.).sup.H is a conjugating operation, and T.sub.c is a sampling interval.

[0027] An initial estimation value {circumflex over ()}.sub.1=arg min.sub.J.sub.1() of is obtained by one-dimensional search performed on .

[0028] In step S2, positions of non-activated sub-carriers in the OFDM-IM system are estimated by using a signal z.sub.1=.sup.H({circumflex over ()}.sub.1)z subjected to the preliminarily estimated compensation, with criteria that: [0029] {circle around (1)} a power P.sub.null of the present sub-carriers is lower that an average value of a power of the non-activated sub-carriers; [0030] {circle around (2)} one of two adjacent sub-carriers with a higher power P.sub.side is larger than an average value of the power of the activated sub-carriers; [0031] {circle around (3)} a specific value is P.sub.side/P.sub.null>16; and [0032] at most 8 sub-carriers meeting the above criteria are used in the following steps for the system.

[0033] In step S3, the estimated sub-carriers are sorted in a descending order of a size of P.sub.side, a larger weight is assigned to energy of the sub-carrier with a larger P.sub.side, a sub-carrier with a serial number k is W.sub.k={square root over ((9k)/4)}, and the weights of each sub-carrier are formed into a diagonal matrix W.

[0034] Energy of the estimated sub-carriers is added into the cost function according to different weights, wherein the cost function is modified as:

[00005]$J_2\left(\right)=\underset{m{S}_N}{.Math.}.Math.{.Math.f_m^H.Math.{}^H\left(\right).Math.z.Math.}^2+\underset{m\widehat{S_N}}{.Math.}.Math.{.Math.{\mathrm{Wf}}_n^H.Math.{}^H\left(\right).Math.z.Math.}^2$

wherein is an estimated non-activated sub-carrier set.

[0035] A final estimation value {circumflex over ()}.sub.2=arg min.sub.J.sub.2() of is obtained by the one-dimensional search performed on . A signal subjected to final compensation is z.sub.2=.sup.H({circumflex over ()}.sub.2)z.

[0036] In step S6, Monte Carlo Simulation is performed on the above method. The following 3 situations are stimulated, as shown in Table 1.

TABLE-US-00001 TABLE 1 Number of null carriers Method Modulation mode used for estimation {circle around (1)} Method of Li OFDM QPSK 4 {circle around (2)} Method of Li OFDM-IM QPSK 4 {circle around (3)} Method of the OFDM-IM QPSK Initial: 4 present invention Supplemental: not more than 8

[0037] Performances of the above methods are measured in the stimulation through comparing mean square errors of an estimation value {circumflex over ()}.sub.norm={circumflex over ()}/. As shown in FIG. 2 and FIG. 3, graphic symbols from top to bottom in a stimulation result diagram are trend charts of mean square errors of {circumflex over ()}.sub.norm in methods {circle around (1)}, {circle around (2)}, and {circle around (3)} changing with a signal-to-noise ratio.

[0038] Stimulation results show that {circle around (2)} has a slightly better performance than {circle around (1)}, and {circle around (3)} has a better performance than {circle around (1)} and {circle around (2)}. Under a high signal-to-noise ratio, {circle around (3)} has a more obvious performance advantage. The stimulation results verify the effectiveness of the method according to the present invention.

[0039] The above specific embodiment is merely one implementation of the present invention, and is not used for limiting the scope of patent of the present invention. Any equivalent structure or equivalent process transformation made by using the spirit and principle of the present invention and the contents of the accompanying drawings should fall within the protection scope of patent of the present invention.