KR102013682B1 - Method and apparatus for compensating frequency offset in mobile communication system - Google Patents

Method and apparatus for compensating frequency offset in mobile communication system Download PDF

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KR102013682B1
KR102013682B1 KR1020160005587A KR20160005587A KR102013682B1 KR 102013682 B1 KR102013682 B1 KR 102013682B1 KR 1020160005587 A KR1020160005587 A KR 1020160005587A KR 20160005587 A KR20160005587 A KR 20160005587A KR 102013682 B1 KR102013682 B1 KR 102013682B1
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frequency offset
fft
signal
received signal
estimated
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KR1020160005587A
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KR20160091260A (en
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최승남
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한국전자통신연구원
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2649—Demodulators
    • H04L27/265—Fourier transform demodulators, e.g. fast Fourier transform [FFT] or discrete Fourier transform [DFT] demodulators

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Discrete Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Mathematical Physics (AREA)
  • Mobile Radio Communication Systems (AREA)

Abstract

Provided are a method and an apparatus for compensating for frequency offset in a mobile communication system. In an environment in which the UE moves at a high speed, the base station processes a fast fourier transform (FFT) on a received signal converted into a digital signal, and estimates a frequency offset using a pilot signal obtained from the FFT processed signal. The estimated frequency offset is then used to compensate for the frequency offset of the received signal before or after the FFT process.

Description

Method and apparatus for compensating frequency offset in mobile communication system TECHNICAL FIELD

The present invention relates to a method and apparatus for compensating for a frequency offset in a mobile communication system.

In an environment in which the terminal moves at high speed, for example, an environment in which the terminal is mounted on a high speed train and moves at high speed, communication performance is greatly degraded due to Doppler shift. Specifically, the terminal receives a signal having a frequency shifted by a frequency offset due to the Doppler shift phenomenon compared to the transmission frequency of the base station. Thereafter, even when the terminal transmits a signal, the base station receives a signal having a frequency shifted by a frequency offset from the transmission frequency of the terminal. As such, the communication performance is degraded by the frequency offset generated for the signals transmitted and received by the terminal and the base station.

Therefore, the frequency offset must be compensated for to prevent degradation of communication performance.

An object of the present invention is to provide a method and apparatus for compensating for an uplink frequency offset of a moving body moving at high speed in a high speed moving environment.

In the frequency offset compensation method according to an aspect of the present invention for the above object, in a method in which the base station compensates the frequency offset in an environment in which the mobile station moves at a high speed, the received signal, which has been converted into a digital signal, is converted into a FFT (Fast Fourier Transform). ) Processing; Estimating a frequency offset using a pilot signal obtained from the FFT processed signal; And using the estimated frequency offset, compensating for the frequency offset of the digitally received signal before the FFT processing, or compensating for the frequency offset of the received signal after the FFT processing.

According to an embodiment of the present invention, an uplink frequency offset of a moving object moving at a high speed in a fast moving environment may be digitally compensated at the base station receiving end. In particular, the base station of the fast mobile backhaul may compensate the frequency offset present in the uplink received signal in the baseband digitally.

1 is an exemplary view showing a high speed mobile environment according to an embodiment of the present invention.
2 is an exemplary diagram illustrating a frequency offset generated by a Doppler shift.
3 is a diagram illustrating a frequency offset compensation apparatus according to a first embodiment of the present invention, and FIG. 4 is a diagram illustrating a frequency offset compensation method according to a first embodiment of the present invention.
5 is a view showing a frequency offset compensation apparatus according to a second embodiment of the present invention, Figure 6 is a view showing a frequency offset compensation method according to a second embodiment of the present invention.
7 illustrates a frequency offset compensation apparatus according to a third embodiment of the present invention, and FIG. 8 illustrates a frequency offset compensation method according to a third embodiment of the present invention.
9 illustrates a frequency offset compensation apparatus according to a fourth embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may easily implement the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. In the drawings, parts irrelevant to the description are omitted in order to clearly describe the present invention, and like reference numerals designate like parts throughout the specification.

Throughout the specification, when a part is said to "include" a certain component, it means that it can further include other components, except to exclude other components unless specifically stated otherwise.

Throughout the specification, a terminal may be a mobile terminal (MT), a mobile station (MS), an advanced mobile station (AMS), a high reliability mobile station (HR-MS). May also refer to a subscriber station (SS), a portable subscriber station (PSS), an access terminal (AT), a user equipment (UE), and the like. It may also include all or part of the functionality of the HR-MS, SS, PSS, AT, UE and the like.

In addition, a base station (BS) may be an advanced base station (ABS), a high reliability base station (HR-BS), a node B (node B), an advanced node B (evolved node B, eNodeB), access point (AP), radio access station (RAS), base transceiver station (BTS), mobile multihop relay (MMR) -BS, relay serving as a base station station (RS), relay node (RN) serving as base station, advanced relay station (ARS) serving as base station, high reliability relay station (HR) serving as base station -RS), small base stations (femoto BS), home node B (HNB), home eNodeB (HeNB), pico base station (pico BS), metro base station (metro BS), micro base station (micro BS) ), Etc.), and all, such as ABS, Node B, eNodeB, AP, RAS, BTS, MMR-BS, RS, RN, ARS, HR-RS, small base station, or the like. It may include a negative feature.

Hereinafter, a frequency offset compensation method and apparatus therefor in a mobile communication system according to an exemplary embodiment of the present invention will be described with reference to the accompanying drawings.

1 is an exemplary view showing a high speed mobile environment according to an embodiment of the present invention.

As illustrated in FIG. 1, the terminal may transmit and receive signals to and from a base station in an environment in which a terminal is mounted on a moving object moving at a high speed such as a high speed train. In the high-speed mobile backhaul environment, a frequency offset occurs due to a Doppler shift when transmitting and receiving a signal between a terminal and a base station.

2 is an exemplary diagram illustrating a frequency offset generated by a Doppler shift.

When the terminal receives the first signal from the base station, as shown in Figure 2, the frequency offset f o by the Doppler shift compared to the transmission frequency f1 of the first signal transmitted from the base station The first signal having the frequency f2 shifted by is received. Thereafter, the terminal estimates the frequency offset f o and uses the frequency f2 shifted by f o as the transmission frequency of the terminal.

Thereafter, the terminal transmits a second signal having a transmission frequency f2 to the base station, where the base station has a frequency offset f o due to the Doppler shift compared to the transmission frequency f2 of the second signal transmitted from the terminal. The second signal having the frequency f3 shifted by is received. Therefore, the interval between the transmission frequency f1 of the base station and the reception frequency f3 of the base station is 2f o. Increases by. Therefore, for uplink signals, 2f o Since the Doppler frequency shift occurs, the base station must compensate for the frequency offset at a high speed, for example, at a speed of 300 km / h or more, to prevent degradation of transmission performance.

In a typical cellular communication environment, since there are many terminals connected to a base station, it is very difficult to compensate the frequency offset individually for each terminal, and if the base station estimates and compensates for the frequency offset for each terminal, the hardware complexity increases greatly and thus the base station cost is increased. There is no choice but to increase. However, in the scenario as shown in FIG. 1, since one or two terminals (here, fast moving bodies) are simultaneously connected to the base station, it is possible to compensate the uplink Doppler frequency offset.

In addition, the higher the speed of the moving body, the greater the influence on the transmission performance due to the increase in the Doppler frequency. Therefore, the Doppler frequency problem must be solved to provide stable wireless backhaul link quality to the moving body. In addition, in the environment as shown in FIG. 1, the base station provides the wireless backhaul link to the mobile, and since there are a large number of users in the mobile, the wireless backhaul link quality affects the communication quality of many users at the same time.

In an embodiment of the present invention, the base station compensates the frequency offset generated by the Doppler shift as described above from the received signal as follows.

In the case of a downlink received signal, the terminal generally estimates a frequency offset from the received signal and compensates the frequency offset by controlling a clock of the oscillator based on the estimation result. However, in the case of the uplink reception signal, the base station cannot adjust the clock of the oscillator to compensate for the frequency offset of the uplink reception signal, because the clock of the oscillator is a reference clock. Therefore, in order to compensate for the frequency offset in the base station, a method of directly compensating by estimating the frequency offset of the received signal in the baseband. Since a few terminals exist for the base station of the fast mobile backhaul, the above method can be easily applied.

In uplink transmission using orthogonal frequency division modulation (OFDM), the frequency offset of the receiver is

Figure 112016004885613-pat00001
Is estimated from
Figure 112016004885613-pat00002
Denotes an estimated channel.

Firstly

Figure 112016004885613-pat00003
Inverse Discrete Fourier Transform (IDFT) to Impulse Response
Figure 112016004885613-pat00004
Finding
Figure 112016004885613-pat00005
Of the channel impulse response from
Figure 112016004885613-pat00006
Is calculated as follows.

Figure 112016004885613-pat00007

here,

Figure 112016004885613-pat00008
Denotes the number of pilot symbols.

Figure 112016004885613-pat00009
Power of two channel impulse responses
Figure 112016004885613-pat00010
Sorts in descending order from the largest value, and index corresponding to the sorted value.
Figure 112016004885613-pat00011
New counterpart
Figure 112016004885613-pat00012
To
Figure 112016004885613-pat00013
Obtained from That is, channel impulse response
Figure 112016004885613-pat00014
In order of power
Figure 112016004885613-pat00015
to be.

Figure 112016004885613-pat00016
And channel impulse response
Figure 112016004885613-pat00017
May be calculated as in Equation 2 below.

Figure 112016004885613-pat00018

Here, (*) represents a conjugate complex number.

And

Figure 112016004885613-pat00019
Phase shift using the function, as in Equation 3
Figure 112016004885613-pat00020
Obtain

Figure 112016004885613-pat00021

Phase transition

Figure 112016004885613-pat00022
Based on the frequency offset as follows
Figure 112016004885613-pat00023
Calculate

Figure 112016004885613-pat00024

here,

Figure 112016004885613-pat00025
to be.
Figure 112016004885613-pat00026
Represents the ratio of the length of the cyclic prefix (CP) to the length of the OFDM symbol,
Figure 112016004885613-pat00027
Denotes the number of OFDM symbols between two pilot signals,
Figure 112016004885613-pat00028
Denotes a subcarrier spacing.

A method of estimating and compensating for a frequency offset calculated as described above is described.

3 is a diagram illustrating a frequency offset compensation apparatus according to a first embodiment of the present invention, and FIG. 4 is a diagram illustrating a frequency offset compensation method according to a first embodiment of the present invention.

As shown in FIG. 3, the frequency offset compensator 10 according to the first embodiment of the present invention includes an FFT unit 11 for outputting a received signal, which is processed from a reception antenna, by processing FFT (Fast Fourier Transform). The frequency offset estimator 12 estimates the frequency offset based on the signal output from the FFT unit 11, the compensation processor 13 compensates the frequency offset of the received signal based on the estimated frequency offset, and the frequency offset is compensated. And a demodulator 14 for demodulating the received signal.

The first embodiment of the present invention is based on an OFDM-based mobile communication system, and a transmitter, that is, a terminal, transmits a pilot signal for channel estimation together with transmission data.

The base station receives such a transmission signal, and the frequency offset compensating apparatus 10 processes a received signal received through a receiving antenna (not shown) and converted into a digital signal by FFT (Fast Fourier Transform) as shown in FIG. 4 ( S100 and S110, a frequency offset f 0 is estimated using a pilot signal obtained from the FFT processed signal (S120). The frequency offset of the received signal digitally converted at the front end of the FFT is compensated using the estimated frequency offset f 0 (S130). That is, the compensation processor 13 located at the input terminal of the FFT unit 11 processes the digital signal and multiplies the received signal with the estimated frequency offset to compensate for the frequency offset of the received signal.

The method according to the first embodiment of the present invention is possible when one terminal is connected to the base station.

5 is a view showing a frequency offset compensation apparatus according to a second embodiment of the present invention, Figure 6 is a view showing a frequency offset compensation method according to a second embodiment of the present invention.

As shown in FIG. 5, the frequency offset compensator 10 according to the second embodiment of the present invention is the FFT unit 11, the frequency offset estimator 12, and the compensation processor as in the first embodiment. 13 ', and a demodulator 14, unlike the first embodiment, a compensation processing unit 13' is located at the rear end of the FFT unit 11.

The frequency offset compensator 10 performs FFT processing on the received signal converted into a digital signal as shown in FIG. 6 (S300 and S310), and estimates the frequency offset f 0 using a pilot signal obtained from the FFT processed signal. (S320). The frequency offset of the received signal is compensated at the rear end of the FFT by using the estimated frequency offset f 0 (S330). That is, the compensation processor 13 located at the output terminal of the FFT unit 11 multiplies the received signal output from the FFT unit 11 by the estimated frequency offset to compensate for the frequency offset of the received signal.

7 illustrates a frequency offset compensation apparatus according to a third embodiment of the present invention, and FIG. 8 illustrates a frequency offset compensation method according to a third embodiment of the present invention.

The frequency offset compensation device according to the third embodiment of the present invention has a form combining the above first and second embodiments. Specifically, as shown in FIG. 7, the frequency offset compensator 10 according to the third embodiment of the present invention includes an FFT unit 11, a frequency offset estimator 12, and a demodulator 14. In addition, unlike the first and second embodiments, the first compensation processor 13a and the second compensation processor 13b are included. The first compensation processor 13a is positioned at the front end of the FFT unit 11 to compensate for the frequency offset of the received signal based on the frequency offset f 0 . The second compensation processor 13b is located at the rear end of the FFT unit 11 and compensates the frequency offset of the received signal again based on the frequency offset f 1 estimated based on the pilot signal of the received signal whose frequency offset is compensated.

The frequency offset compensator 10 performs FFT processing on the received signal converted into a digital signal as shown in FIG. 8 (S500 and S510), and estimates the frequency offset f 0 using a pilot signal obtained from the FFT processed signal. (S520). The frequency offset of the received signal is compensated at the front end of the FFT by using the estimated frequency offset f 0 (S530). In addition, the received signal having the frequency offset compensated is estimated by using the FFT process and the pilot signal obtained therefrom to estimate the frequency offset f 1 (S540 and S550). Next, the frequency offset f 1 is additionally used to compensate for the frequency offset later in the FFT (S560). At this time, based on the frequency offset f 1 , the remaining offset is compensated for by the frequency offset f 0 .

As in the first to third embodiments described above, the frequency offset of the received signal can be estimated and compensated at the baseband, and the digital design facilitates the hardware design and control.

9 illustrates a frequency offset compensation apparatus according to a fourth embodiment of the present invention.

In the fourth embodiment of the present invention, when two or more terminals are connected to the base station, the frequency offset is compensated.

As shown in FIG. 9, the frequency offset compensation apparatus 10 according to the fourth embodiment of the present invention may include an FFT unit 110, a first offset compensation processor 120, and a second offset compensation processor 130. Include.

The first offset compensation processor 120 may include a frequency offset estimator 121, a compensation processor 122 that compensates for the frequency offset of the received signal based on the estimated frequency offset, and a demodulator that demodulates the received signal with the frequency offset compensated ( 123).

The second offset compensation processor 130 may include a frequency offset estimator 131, a compensation processor 132 that compensates for the frequency offset of the received signal based on the estimated frequency offset, and a demodulator that demodulates the received signal with the frequency offset compensated ( 133).

The first and second offset compensation processing units 120 and 130 perform frequency offset compensation at the rear end of the FFT as in the second embodiment. For example, the first offset compensation processing unit 120 is received from the first terminal. The frequency offset compensation is performed on the signal, and the second offset compensation processor 130 performs the frequency offset compensation on the signal received from the second terminal.

The frequency offset compensator 10 having such a structure FFT processes a received signal converted into a digital signal as in the second embodiment, and estimates a frequency offset using a pilot signal obtained from the FFT processed signal. do. In this case, the first offset compensation processor 120 and the second offset compensation processor 130 use the pilot signal assigned to each user from the FFT-processed received signal to adjust the frequency offsets f 0_user1 and f 0_ user2 for each user. Estimate.

The frequency offset of the received signal is compensated for each user at the rear end of the FFT using the estimated frequency offsets f 0_ user1 and f 0_ user2, respectively.

An embodiment of the present invention is not implemented only through the above-described apparatus and / or method, but may be implemented through a program for realizing a function corresponding to the configuration of the embodiment of the present invention, a recording medium on which the program is recorded, and the like. Such implementations may be readily implemented by those skilled in the art from the description of the above-described embodiments.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of rights.

Claims (7)

In an environment where the terminal moves at a high speed, in the apparatus for the base station to compensate for the frequency offset,
An FFT unit configured to process a fast fourier transform (FFT) on a received signal converted into a digital signal;
A frequency offset estimator for estimating a frequency offset using a pilot signal obtained from the signal output from the FFT unit;
A first compensation processor positioned at a front end of the FFT unit to compensate for a frequency offset of the received signal based on a first frequency offset output from the frequency offset estimator;
A second compensation processor configured to compensate for a frequency offset of a received signal having a frequency offset output from the FFT unit based on a second frequency offset output from the frequency offset estimator located at a rear end of the FFT unit;
Including;
The first frequency offset is estimated using a pilot signal obtained by the received signal from the FFT processed signal, and the second frequency offset is obtained by FFT processing the received signal whose frequency offset is compensated based on the first frequency offset. And an estimated using a pilot signal obtained in the frequency offset compensation apparatus. The method of claim 1,
The frequency offset estimating unit estimates the frequency offset for each terminal from the FFT-processed signal using a pilot signal allocated for each terminal. delete delete In an environment in which a terminal moves at a high speed, in a method in which a base station compensates for a frequency offset,
Fast Fourier Transform (FFT) processing of the received signal converted into a digital signal;
Estimating a frequency offset using a pilot signal obtained from the FFT processed signal; And
Compensating for the frequency offset of the received signal using the estimated frequency offset
Including,
Compensating the frequency offset,
Compensating for the frequency offset of the received signal by multiplying the received first signal by processing the digital signal and the estimated first frequency offset before FFT processing; And
After the received signal whose frequency offset is compensated for is subjected to FFT processing, multiplying the FFT processed signal by a second frequency offset to compensate for the frequency offset of the received signal,
The estimated first frequency offset is estimated using a pilot signal obtained from the FFT processed signal, and the estimated second frequency offset is obtained by FFT processing a received signal whose frequency offset is compensated based on the first frequency offset. And then estimated using a pilot signal obtained. The method of claim 5,
Estimating the frequency offset,
A frequency offset compensation method for estimating a frequency offset for each terminal from the FFT processed signal using a pilot signal allocated to each terminal. delete
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WO2003028270A1 (en) * 2001-09-24 2003-04-03 Atheros Communications, Inc. Post-fft scaling to reduce multiple effects and fine frequency offset estimation
US20040120412A1 (en) * 2002-08-02 2004-06-24 Raja Banerjea Carrier frequency offset estimation in a wireless communication system

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Publication number Priority date Publication date Assignee Title
WO2003028270A1 (en) * 2001-09-24 2003-04-03 Atheros Communications, Inc. Post-fft scaling to reduce multiple effects and fine frequency offset estimation
US20040120412A1 (en) * 2002-08-02 2004-06-24 Raja Banerjea Carrier frequency offset estimation in a wireless communication system

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