RU2459354C1 - Method to assess bearing frequency shift in up-link for wireless telecommunications systems - Google Patents

Abstract

FIELD: information technologies.

SUBSTANCE: after detection of the first OFDM symbol and detection of delay time, mutual correlation function is calculated with a reference sequence, then the second, third and fourth serial OFDM symbols are received, the first and second symbols contain the code X, the third and fourth symbols contain the code X+1. The mutual correlation function is calculated for each of them. Maxima of mutual correlation functions of all OFDM symbols are identified within the main peak, and the maximum of the mutual correlation function of the first OFDM symbol is summed with the complex conjugate maximum of the mutual correlation function of the second OFDM symbol. Similarly maxima of mutual correlation functions of the second and third OFDM symbols are summed, as well as of the third and fourth OFDM symbols. Difference of phases is calculated between the first and second, second and third, and between third and fourth OFDM symbols by division of an imaginary part of the sum of mutual correlation function of the first OFDM symbol with the complex conjugate maximum of the mutual correlation function of the second OFDM symbol into the valid part of this sum. The difference of phases is averaged between the first and second, second and third, third and fourth OFDM symbols. The produced averaged value is divided into 2π and duration of the OFDM symbol.

EFFECT: improved accuracy of assessment of bearing frequency shift.

2 dwg, 1 tbl

Description

The invention relates to radio engineering, in particular to methods for estimating the frequency in the uplink (uplink), and can be used in base station equipment based on the 802.16 standard, for example, in the mobile WiMAX system, as well as in other wireless telecommunication systems using OFDM signals.

A known method of synchronizing the reception of signals (patent US 5,8125,23 C1, 09/22/1998, H04J 11/00). In this method, frequency synchronization is performed by a pilot signal consisting of two repeating parts. To evaluate the time delay and frequency shift of the signal, a complex product of the complex conjugate received signal and the received signal delayed by half the pilot signal is calculated. Then, in a sliding window with a duration equal to half the pilot signal, the average value of the obtained products is calculated. Based on the temporary position of the maximum value of the module and the phase of the complex value of the product corresponding to this maximum module, the time delay and frequency shift are estimated, respectively.

The disadvantage of this technical solution is that both the initial and refined estimates of the frequency shift are determined by two pilot signals, which at low signal-to-noise ratios and frequency-selective fading leads to a deterioration in the characteristics of the estimation of signal parameters.

Closest to the claimed method is a method for estimating the frequency shift in the uplink (uplink), in wireless communication systems (patent US 7,949,034 C1, HB04 1/69, 05/24/2011). In this method, the uplink carrier frequency offset is estimated using two identical consecutive OFDM (Orthogonal frequency-division multiplexing) symbols. To estimate the frequency shift, the first symbol is received, the maximum of the correlation function of the first symbol is calculated, then the second symbol is received, and the maximum of the correlation function of the second symbol is also calculated, after which the frequency shift estimate is obtained by dividing the maximum of the correlation function of the second symbol by the maximum of the correlation function of the first symbol and the resulting ratio divided by 2π.

The disadvantage of this method is that estimates of the frequency shift are determined by two identical consecutive OFDM symbols (ranging code), which does not allow a sufficiently accurate estimate of the frequency shift at low signal-to-noise ratios and frequency selective fading.

The main technical problem, the solution of which the proposed technical solution is aimed at, is to increase the accuracy of estimating the shift of the carrier frequency.

The main technical problem is achieved in that in the method for estimating the carrier frequency shift in the uplink including receiving two identical OFDM symbols, calculating the maximum correlation function of each OFDM symbol, dividing the maximum correlation function of the second OFDM symbol by the maximum correlation function of the first OFDM symbol, and then dividing the received values by 2π, according to the proposed solution, two identical consecutive OFDM symbols are additionally introduced, the first and second OFDM symbols containing the code X, the third and the fourth OFDM symbols contain the code X + 1, after detecting the first OFDM symbol and determining the delay time, calculate the cross-correlation function with the reference sequence, then take the second, third and fourth OFDM symbols and calculate the cross-correlation function for each of them, then determine the maxima of the mutual correlation functions of the first, second, third, and fourth OFDM symbols, within the main peak, and sum the maximum of the mutual correlation functions of the first OFDM symbol with complex conjugate max a maximum of the mutual correlation function of the second OFDM symbol, similarly summarize the maxima of the mutual correlation functions of the second and third OFDM symbols, as well as the third and fourth OFDM symbols, then calculate the phase difference between the first and second OFDM symbols by dividing the imaginary part of the sum of the mutual correlation functions of the first OFDM symbol with the complex conjugate maximum of the mutual correlation function of the second OFDM symbol by the real part of this sum, then the phase difference between the second and third OFDM symbols is similarly determined olami and between the third and fourth OFDM symbols, and the averaged phase difference between the first and second, second and third, third and fourth OFDM symbols, the average value obtained is divided by 2π and the duration of the OFDM symbol.

The invention is illustrated by drawings, where Fig. 1 shows a functional diagram for implementing the proposed method, Fig. 2 shows the structure of OFDM symbols used to estimate the carrier frequency.

The functional diagram includes a block receiving a signal and estimating the delay time of a signal 1, a reference signal generator 2, blocks calculating the temporal cross-correlation function of the reference and received signals 3, 4, 5, 6, blocks searching for the maximum of the temporal cross-correlation function (VKF) VKF 7, 8 , 9, 10, VKF maximum maximum summing blocks 11, 12, 13, phase difference calculation blocks 14, 15, 16, frequency shift estimation blocks between symbols 17, 18, 19, carrier frequency shift estimation block 20.

The method is as follows.

The subscriber station (AC) generates the Initial ranging (IR) shown in FIG. 2 containing four consecutive OFDM symbols, the first two of which are the same and contain the code X (Code X) and the second of which are the same and contain the code X +1 (Code X + 1). An initial ranging signal is generated by converting the input data stream (CDMA code) into a BPSK code and then writing the BPSK code as spectral coefficients of the inverse discrete Fourier transform (ODPF)

C _k = I _k + j0.

After processing the prepared array using the OTF, samples of quadrature components are formed (complex signal):

A (n) = IFFT [C _k = I _k + j0] = I (n) + jQ (n),

where I (n) = I (-n) and Q (n) = - Q (-n).

The signal is sent to the base station (BS):

,

,

where ⊗ is the convolution sign;

h (n, l) is the impulse response of the RRV channel.

When modeling the method, we used the characteristics of the six-beam channel shown in the table.

, Ts-интервал ортогональности и случайная фаза (-π…π).where γ _i (n), m _i (n) are time-dependent complex proportionality coefficients and time-dependent delay of the rays, and γ _i (n) includes the frequency offset within

, Ts-interval of orthogonality and random phase (-π ... π).

Communication channel characteristics Beam number Channel b Relative Average power one 0 0 2 200 -0.9 3 800 -4.9 four 1200 -8.0 5 2300 -7.8 6 3700 -23.9

In the receiving portion of an OFDMA system, the useful portion of the received signal is described by the expression

where k is the number of rays;

- задержка в i-м луче;

- delay in the ith beam;

- нормированное смещение частоты.

- normalized frequency offset.

, получаем спектральное представление полезной части принятого сигнала:Applying the direct discrete Fourier transform (FFT) to

, we obtain the spectral representation of the useful part of the received signal:

The reference generator generates a BPSK code matching the BPSK code in the received signal for each received OFDM symbol. We multiply the BPSK code of the reference generator with the complex conjugate BPSK code of the received signal for the first, second, third, and fourth OFDM symbols. Applying the inverse discrete Fourier transform to the product of the complex conjugate BPSK code of the reference sequence generator and the spectrum with the same BPSK code received from the AS, we obtain the VKF R (n) samples that connect the received and reference codes to each other:

выбранного луча, значение ВКФ для него в точке приближенно представлено как:After detecting the OFDMA signal, measuring the time delay

of the selected ray, the VKF value for it at the point is approximately represented as:

,

,

,Where

,

,

,

φ _γ is the random phase,

- оценка времени задержки сигнала,

- estimate the delay time of the signal,

T _s is the duration of the OFDM symbol.

Then the estimate of the frequency offset is written as:

The frequency Δf estimate includes the random phase of the signal φ _γ ; therefore, it is impossible to obtain an unambiguous estimate of the frequency shift from one OFDM symbol of the IR signal. For an unambiguous estimation of the frequency, difference schemes are used, and the symmetry properties of the IR signal are used (Fig. 2).

.Figure 2 shows that the code of the first pair of symmetric OFDM symbols is different from the code of the second pair of symmetric OFDM symbols. Since OFDM symbols have a short duration, the multipath PPB channel, which acts on two pairs of OFDM symbols, remains unchanged and the random phase φ _γ introduced by this channel will be the same for all OFDM symbols. To exclude the random phase φ _γ , the VKF is determined for all received OFDM symbols with reference sequences, after which the maxima of two VKFs (one of which is complex conjugate) are summed up within the main peak, which is determined provided that the information on the signal time delay is known

.

After that, the carrier frequency is estimated by calculating the phase difference Δφ between adjacent OFDM symbols:

where Δφ ₁₂ , Δφ ₂₃ , Δφ ₃₄ is the phase difference between adjacent symbols;

Im and Re are the values of the imaginary and real parts of the multiplication of the VKF of one OFDM symbol with the complex conjugate VKF of the neighboring. The carrier frequency offset Δf is calculated:

where T _b is the duration of the OFDMA symbol.

The functional diagram depicted in figure 1, implements the proposed method described above.

Improving the accuracy of estimating the shift of the carrier frequency at low signal-to-noise ratios in the proposed method is achieved by using four consecutive symbols and is about 35% compared with the prototype method.

Claims (1)

The method of estimating the carrier frequency shift in the uplink, including receiving two identical consecutive OFDM symbols, calculating the maximum correlation function of each OFDM symbol, dividing the maximum correlation function of the second OFDM symbol by the maximum correlation function of the first OFDM symbol, and then dividing the obtained value by 2π, characterized in that two identical consecutive OFDM symbols are additionally introduced, the first and second OFDM symbols contain the code X, the third and fourth OFDM symbols contain the code X + 1, after OFDM detection of the first symbol and determination of the delay time compute the cross-correlation function with the reference sequence, then take the second, third and fourth OFDM symbols and calculate the cross-correlation function for each of them, then determine the maxima of the cross-correlation functions of the first, second, third and fourth OFDM symbols within the main peak and sum the maximum of the mutual correlation function of the first OFDM symbol with the complex conjugate maximum of the mutual correlation function of the second OFDM symbols, similarly summarize the maxima of the mutual correlation functions of the second and third OFDM symbols, as well as the third and fourth OFDM symbols, then calculate the phase difference between the first and second OFDM symbols by dividing the imaginary part of the sum of the mutual correlation functions of the first OFDM symbol with the complex conjugate maximum of the mutual correlation the functions of the second OFDM symbol on the real part of this sum, then similarly determine the phase difference between the second and third OFDM symbols and between the third and fourth OFDM symbols and averaged phase difference between the first and second, second and third, third and fourth OFDM symbols, the average value obtained is divided by 2π and the duration of the OFDM symbol.

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