Frequency Offset in LTE

Technical Field

The present invention relates to communication technology, and especially to estimating and correcting of frequency offset in LTE (Long Term Evolution).

Background

With the increased demand for higher data rate provided by mobile communication systems, multi-carrier with orthogonal frequencies is introduced into the next generation industry standard, such as OFDM for downlink or SC-FDMA for uplink in LTE. LTE is the name given to a project within the Third Generation Partnership Project to improve the UMTS mobile phone standard to cope with future technology evolutions. One of the goals of LTE is improving spectral efficiency. Since the orthogonal frequency provides much more flexibility to allocate the radio resource than the current cellular standards, thus improves the spectral efficiency.

But one inherent disadvantage with orthogonal frequency is sensitivity to frequency error. Frequency error can destroy the orthogonality among the sub-carriers, thus can introduce performance degradation into communication system. It is vital to decrease frequency error. Both fixed frequency offset within a certain time and the frequency spread could contribute to frequency errors.

There are many different ways to obtain frequency offset estimation. In which estimating of frequency offset in OFDM systems is generally performed by looking at the repeated segments in time domain over a period, for example, the Cyclic Prefix (CP), or some specially designed repetitive OFDM symbols. In an OFDM system, the Cyclic Prefix is a copy of the last portion of the data symbol appended to the front of the symbol during the guard interval. The repetition of the Cyclic Prefix over the time gives hints on a certain frequency offset. Accumulation on the repeated part is proportional to the frequency offset. But this method can normally work for a low frequency offset and it could not tolerate too much noise. i Disclosure of Invention

An object of the present invention is to provide a method for estimating frequency offset in LTE, which can work for relatively high frequency offset and can tolerate relatively more noise. The method comprises: (a) at transmitter side, applying Zadoff-Chu sequence to signal to be transmitted; (b) at receiver side, scanning the received signal with the Zadoff-Chu sequence with all different cyclic shifts to get an auto-correlation function of the Zadoff-Chu sequence; (c) detecting first two largest peaks in the auto-correlation function and/or a power of the autocorrelation function to obtain a power difference between the first two largest peaks; (d) approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; and (e) estimating the frequency offset between the transmitter and the receiver with the relationship between the frequency offset and the power difference of the first two largest peaks.

Another object of the present invention is to provide a method for estimating frequency offset in LTE, which can work for relatively high frequency offset and can tolerate relatively more noise. The method comprises: (a) at transmitter side, applying the Zadoff-Chu sequence to signal to be transmitted; (b) at receiver side, scanning the received signal with the Zadoff-Chu sequence with all different cyclic shifts to get an auto-correlation function of the Zadoff-Chu sequence; (c) detecting first two largest peaks in the auto-correlation function and/or a power of the auto -correlation function to obtain a power difference between the first two largest peaks; (d) approximating a relationship between the frequency offset and the power difference between the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; (e) estimating signal power and noise power, and obtaining the signal to noise ratio estimated; and (f) estimating the frequency offset between the transmitter and the receiver with the relationship between the frequency offset and the power difference of the first two largest peaks. A further object of the present invention is to provide an apparatus for estimating frequency offset in LTE, which can work for relatively high frequency offset and can tolerate relatively more noise. The apparatus comprises: a correlator for obtaining an inner product of a received signal and a conjugate of the zadoff-Chu sequence with a certain cyclic shift, in which the zadoff-Chu sequence is applied to the signal to be transmitted; a detecting and computing means for detecting first two largest peaks in the output from the correlator and/or the power of the output and then computing the power difference between the first two largest peaks; a computing means for approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; a calculator for calculating an estimated value of the frequency offset using the relationship between the frequency offsets and the power difference of the first two largest peaks.

A further object of the present invention is to provide an apparatus for estimating frequency offset in LTE, which can work for relatively high frequency offset and can tolerate relatively more noise. The apparatus comprises: a correlator for obtaining an inner product of a received signal and a conjugate of the zadoff-Chu sequence with a certain cyclic shift, in which the zadoff-Chu sequence is applied to the signal to be transmitted; an estimator for estimating power of noise; an estimator for estimating power of the signal received; a computing means for obtaining signal to noise ratio; a detecting and computing means for detecting first two largest peaks in the output from the correlator and/or the power of the output and then computing the power difference between the first two largest peaks; a computing means for approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; and a calculator for calculating an estimated value of the frequency offset using the relationship between the frequency offsets and the power difference of the first two largest peaks.

A further object of the present invention is to provide a method for correcting frequency offset in LTE. The method comprises: (a) at transmitter side, applying Zadoff-Chu sequence to signal to be transmitted; (b) at receiver side, scanning the received signal with the Zadoff-Chu sequence with all different cyclic shifts to get an auto-correlation function of the Zadoff-Chu sequence; (c) detecting first two largest peaks in the auto-correlation function and/or a power of the auto -correlation function to obtain a power difference between the first two largest peaks; (d) approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; (e) estimating the frequency offset between the transmitter and the receiver with the relationship between the frequency offset and the power difference of the first two largest peaks; and (f) correcting the frequency offset based on the estimated frequency offset.

A further object of the present invention is to provide a method for correcting frequency offset in LTE. The method comprises: (a) at transmitter side, applying the Zadoff-Chu sequence to signal to be transmitted; (b) at receiver side, scanning the received signal with the Zadoff-Chu sequence with all different cyclic shifts to get an auto-correlation function of the Zadoff-Chu sequence; (c) detecting first two largest peaks in the auto-correlation function and/or a power of the auto -correlation function to obtain a power difference between the first two largest peaks; (d) approximating a relationship between the frequency offset and the power difference between the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; (e) estimating signal power and noise power, and obtaining the signal to noise ratio estimated; (f) estimating the frequency offset between the transmitter and the receiver with the relationship between the frequency offset and the power difference of the first two largest peaks; and (g) correcting the frequency offset based on the estimated frequency offset.

A further object of the present invention is to provide an apparatus for correcting frequency offset in LTE. The apparatus comprises: a correlator for obtaining an inner product of a received signal and a conjugate of zadoff-Chu sequence with a certain cyclic shift, in which the zadoff- Chu sequence is applied to signal to be transmitted; a detecting and computing means for detecting first two largest peaks in the output from the correlator and/or the power of the output and then computing the power difference between the first two largest peaks; a computing means for approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; a calculator for calculating an estimated value of the frequency offset using the relationship between the frequency offsets and the power difference of the first two largest peaks; and a corrector for correcting the frequency of the received signal using the frequency offset calculated.

A further object of the present invention is to provide an apparatus for correcting frequency offset in LTE. The apparatus comprises: a correlator for obtaining the inner product of the received signal and the conjugate of zadoff-Chu sequence with a certain cyclic shift, in which the zadoff-Chu sequence is applied to signal to be transmitted; an estimator for estimating power of noise; an estimator for estimating power of the signal received; a computing means for obtaining signal to noise ratio; a detecting and computing means for detecting first two largest peaks in the output from the correlator and/or the power of the output and then computing the power difference between the first two largest peaks; a computing means for approximating a relationship between frequency offsets and the power difference of the first two largest peaks by using a set of given frequency offsets and a set of corresponding power differences of the first two largest peaks, in which the corresponding power differences are calculated according to the relationship between power profile values and the frequency offsets; a calculator for calculating an estimated value of the frequency offset using the relationship between the frequency offsets and the power difference of the first two largest peaks; and a corrector for correcting the frequency of the received signal using the frequency offset calculated.

Brief Description of the Drawings

Figure 1 shows the flowchart of frequency offset estimating with noise according to one embodiment of the invention; and

Figure 2 shows the real values of frequency offset and the estimated values of frequency offset according to one embodiment of the invention. Detailed Description of the Invention

In LTE, a kind of special polyphase sequence-Zadoff-Chu sequence is introduced, which provides a possibility of estimating frequency offset in a completely different way. Zadoff-Chu sequence has perfect auto-correlation characteristics. The auto -correlation is either non-zero when aligned or zero when shifted if there is no frequency offset. With the presence of frequency offset, there will be two largest peaks in the auto-correlation function. The inventors note that the distance between these two largest peaks indicates the root number of the Zadoff- Chu sequence, and the amplitude difference between the two peaks is proportional to the amount of frequency offset. In other words, the frequency offset is directly related to the power difference of the two largest peaks.

The basic idea of the invention is that if the relationship between the frequency offset and the power difference of the two largest peaks is known, then this relationship could be utilized to estimate the frequency offset by measuring the power difference of the two peaks. And of course, the noise level should be taken into account as a feasible solution. ^λ

One detailed solution is described in the following by applying to preamble sequences as one example, and it could be similarly applicable to any other situation in which Zadoff-Chu sequences are defined, for example, the dedicated pilots.

In the situation that the noise is ignored, the presence of a certain frequency offset leads to a phase shift to each received sample shown as the following:

f{n) = e ^]hωn r{n) (1) where f(ή) is the received signal, n is a time label which corresponding to one sample of the

received signal, Δω is the normalized frequency offset over the sampling rateΔ<» = — , r(n) is the received sample without frequency offset, in which Δ/ is the frequency offset, and f _s is sampling frequency.

The output of a preamble detection correlator is the inner product of the received vector and the conjugate of the concerned Zadoff-Chu sequence with a certain cyclic shift k: ΛL.-1 πun(n+\) m(n+k)(n+k+\) J^-k)

N. c _v (k) = Jy^ J-

ΛL. ΛL. (l - _g ^ ^Δfl) )

(2)

1 - e ^

Where u is the root number of the Zadoff-Chu sequence, it is a pre-defined parameter known by both transmitter and receiver. And N _zc is the length of Zadoff-Chu sequence.

Then the power of C _v (k) is obtained as:

Different sequence has different peaks distance, for example, when u=150 and the frequency offset is 500Hz, and in test the largest peak is defined at 0, then the second peak is located at 330. And power ratio between the largest peak and the second largest peak is around 4dB.

The difference between the two largest peaks varies with the frequency offset, and the next important step for this invention is to find a good approximation to the relationship between the power peak difference and the frequency offset.

To get the relationship between the power peak difference and the frequency offset, the power profile values in relation to the frequency offset could be derived theoretically according to the above equations (2) and (3) at first. Then the power peak difference could be calculated numerically for a given frequency offset. In this way, for a set of given frequency offsets, a set of corresponding power peak differences can be calculated. And with the set of given frequency offsets and the set of corresponding power peak differences calculated, the relationship between them could be approximated by using one of certain mathematics approximating methods, and the relationship could be represented by one kind of mathematics model, that is, one kind of mathematics equation.

Then in the process of estimating and correcting the frequency offset, after the power peak differences are obtained at the receiver side, the corresponding frequency offsets could be calculated by using the relationship approximated and the power peak difference obtained.

Alternatively, any kind of approximating methods known in the art can be also used for obtaining the relationship between the power peak difference and the frequency offset, and different equation forms can be also obtained for indicating the relationship between power peak difference and the frequency offset. Then different coefficients and their values in the equation approximated are obtained for estimating and correcting the frequency offset.

In one embodiment of the invention, in one approximating scheme, we have found that this relationship could take on the form of a second-order polynomial, thus the frequency offset could be approximated by the second-order polynomial, where Af is the frequency offset:

D(Af) = A (Af) ² + B (Af) ₊ C (4)

With our method of approximation, we have found that when A= -1.4818, B= -179 and C = 7.164e5, we can get the best approximating effect.

If the noise is considered, then the normalized peak power difference is revised as:

λ = ^^ = p - D(Af) (5) σ where Pi is the power of the largest peak, P ₂ is the power of the second largest peak, σ is the power of noise estimated, and p is the signal-to-noise ratio.

So we can get D(Af) =- , then the second-order polynomial is revised as:

P

In this way, the frequency offset could be estimated according to the above revised second- order polynomial as:

According to the equations (5) and (7), the frequency offset could also be estimated as:

According to the above description, we can estimate the frequency offset by measuring the power difference between the first two peaks in the auto-correction function, the power of noise, and signal to noise ratio, if there is noise.

In one embodiment, when there is noise, the process of estimating frequency offset according to the invention is illustrated in figure 1. In step 101, at a receiver side, it is to scan the signal received with the Zadoff-Chu sequence with all different cyclic shift to get the autocorrelation function of the Zadoff-Chu sequence. In step 102, the largest peak of the power of the auto-correlation function is obtained. In step 103, the second largest peak of the power of the auto -correlation function is obtained. Alternatively, the first two largest peaks of the autocorrelation function can also be obtained first, and then the power of the first two largest peaks is calculated. Then in step 104, it is to calculate the difference between the power of the first two largest peaks obtained. In step 106, at the receiver side, it is to estimate the power of the signal received. And in step 107, it is to estimate the power of the noise which is generated during the transmitting process. Next in step 108, it is to calculate signal to noise ratio using the results estimated in step 106 and 107. With the power of the noise and the signal to noise ratio obtained, we continue to step 105, where the frequency offset can be calculated as:

Then at step 109, the process is ended.

In this embodiment, the step 106 and the step 107 could be performed at any time before step 108, and the step 108 could be performed at any time before step 105.

From our experiment results, we list the real values of frequency offset and the estimated values of frequency offset according to this embodiment of the invention in Figure 2. For this figure, it can be seen that the estimated values obtained according to the present invention match the real values very well.

According to another aspect of the invention, a method for correcting frequency offset in LTE is provided. If the frequency offset has been estimated, then frequency of the received signal can be corrected at the receiver side. So a method for correcting frequency offset in LTE can be accordingly realized. According to the invention the method for correcting frequency offset comprises a step of correcting frequency offset based on the estimated frequency offset in accordance with the method for estimating frequency offset of the present invention.

According to another aspect of the invention, when the noise can not be ignored, then the apparatus for estimating frequency offset comprises: a correlator for obtaining the inner product of received signal and the conjugate of zadoff- Chu sequence with a certain cyclic shift, in which the zadoff-Chu sequence is applied to the signal to be transmitted; an estimator for estimating power of noise; an estimator for estimating power of the signal received and a computing means for obtaining signal to noise ratio; a detecting and computing means for detecting the first two largest peaks in the output from the correlator and/or the power of the output and then computing the power difference between the first two largest peaks; and a calculator for calculating frequency offset by

According to another aspect of the invention, an apparatus for correcting frequency offset in LTE is provided. If the frequency offset has been estimated, then frequency of the received signal can be corrected at the receiver side. So an apparatus for correcting frequency offset in LTE can be accordingly realized. According to the invention, the apparatus for correcting frequency offset comprises the apparatus for estimating frequency offset of the present invention and a corrector for correcting frequency offset based on the estimated frequency offset.

In other embodiments, other approximating methods can be used, and different equation forms can be obtained for indicating the relationship between power peak difference and the frequency offset, and then different coefficients and their values are obtained. Accordingly, the method and apparatus for estimating and correcting frequency offset in LTE will be modified according to the approximating methods used. The embodiments above are described in relation to preamble sequences, however the equations obtained could be similarly applicable to any other situation in which Zadoff-Chu sequences are defined, for example, the dedicated pilots.

From the description above, the embodiments according to the invention can estimate higher frequency offset, and can tolerate more noise.

While embodiment of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes and modifications may be made without departing from the spirit and scope of the invention.