KR100236039B1 - A coarse time acquisition for ofdm - Google Patents

Abstract

The present invention relates to a simplified time acquisition tracking circuit for acquiring symbol synchronization in a time domain of an orthogonal division band reception system. According to the present invention, the correlation between the effective period and the guard period, which is the OFDM symbol, is performed by using a guard period in which partial samples of the OFDM symbols are copied as it is and inserted in front of the OFDM symbol in order to remove inter-symbol interference. It is used to estimate the starting point of a symbol. The present invention includes a phase calculator 80 for calculating a phase of a complex sample; A rotation phase angle calculator 81 that calculates a moving angle between the one sample delayed previous sample and the current sample; A rotation phase difference calculator 82 for obtaining a difference value between the absolute value of the current rotation phase angle and the absolute value of the previous rotation phase angle delayed by N _u samples, and calculating and outputting an absolute value of the difference value; An estimation value calculation unit (83) which receives an absolute value provided from the rotation phase difference calculation unit (82) and accumulates it by a window size to obtain an estimated value; The detection unit 84 is configured to output a symbol start point acquisition signal by comparing the estimated value provided from the estimated value calculation unit 83 with a predetermined threshold value. There is an effect that can be implemented.

Description

A coarse time acquisition circuit for Orthogonal Frequency Division Multiplexing systems

The present invention relates to an Orthogonal Frequency Division Multiplexing (OFDM) reception system, and more particularly, an OFDM received in a time domain that is a step before performing a Fast Fourier Transform (FFT). A coarse time acquisition tracking circuit for acquiring symbol synchronization of a signal is provided.

In general, inter-symbol interference (ISI) is generated in a signal received by multipath fading in a wireless communication channel and a transmission channel of a digital high definition television (hereinafter referred to as HDTV). In particular, when high-speed data such as HDTV system is transmitted, the intersymbol interference is intensified, which causes a serious error in the data recovery process of the receiver. As a solution to this problem, recently, in Europe, an OFDM scheme capable of robustly operating in multipath fading has been proposed as a transmission method of digital audio broadcasting (DAB) and digital terrestrial television broadcasting (DTTB). There is a bar.

The OFDM method converts a symbol string input in serial form into parallel data by N symbols, and then multiplexes the parallelized symbols with different subcarrier frequencies, and adds and transmits each of the multiplexed data. Here, the N symbols parallelized are regarded as one unit block, and each subcarrier of the block is orthogonal to each other so that there is no influence between subcarrier channels (subchannels). Therefore, compared with the conventional single carrier transmission scheme, since the symbol period can be increased by the number of subchannels N while maintaining the same symbol rate, inter-symbol interference due to multipath fading can be reduced. In particular, when a guard interval (GI) is inserted between the transmitted symbols, the inter-symbol interference can be further reduced, thereby simplifying the structure of the channel equalizer. In addition, unlike the conventional frequency division multiplexing (FDM) method, the OFDM method has high bandwidth efficiency because the spectrum of each subchannel overlaps each other, and thus the power is uniformly distributed in each frequency band due to the square wave shape. There is also a strong advantage over co-channel interference signals. In general, modulation techniques frequently coupled to OFDM include pulse amplitude modulation (PAM), frequency shift keying (FSK), phase shift keying (PSK), and quadrature amplitude modulation (QAM).

1 is a block diagram illustrating a modulation principle of an orthogonal division band (OFDM) scheme and is a block diagram of an OFDM modulator using QAM as a basic modulation technique.

에 의해 곱해진 다음 수학식 1과 같이 합산된다.Referring to FIG. 1, a subcarrier signal perpendicular to each other after each complex symbol a _i inputted in series during QAM modulation is parallelized to N stages.

It is multiplied by and then summed as

[Equation 1]

들이 서로 수직이기 위해서는

(

는 한 심볼 주기)의 조건을 만족해야 하고, 샘플링 주기 T_A를

으로 정하면, 상기 수학식 1은 다음 수학식 2 와 같다.Here, T _A is a sampling period of a complex carrier. Each subcarrier signal

To be perpendicular to each other

(

Should satisfy the condition of a symbol period), and the sampling period T _A

In Equation 1, Equation 1 is represented by Equation 2 below.

[Equation 2]

Looking at Equation 2, it can be seen that the equation is the same as the N point Inverse Discrete Fourier Transform (IDFT). In other words, an OFDM modulated signal can be obtained using an IDFT and an inverse fast fourier transform (IFFT) structure, and the OFDM modulated signal can be demodulated into a DFT and FFT structure at a receiving side.

Accordingly, the OFDM scheme has a problem in that hardware complexity increases as the number of parallel subchannels increases. However, if the system is digitized, hardware can be simply implemented because the FFT structure can be implemented.

FIG. 2 illustrates a time domain change of the OFDM signal of FIG. 1 and shows a change in time domain of the QAM serial complex symbols a _i and the sum signal C (t) after OFDM is applied. If the N QAM complex symbols each have a symbol period T, the N complex symbols input in series are converted in parallel, multiplied by N vertical subcarriers, and then summed and transmitted. Then, the transmission time of the sum signal C (t) is equal to the time taken when the N complex symbols are transmitted in series, that is, the total symbol time (NT = Ts). Therefore, the period of the parallelized complex symbol is Ts, which means that the transmission time of each symbol is increased by N times the number of parallel channels.

FIG. 3 is a diagram illustrating a frequency-domain change of the OFDM signal of FIG. 1. Since the QAM serial complex symbol a _i has a T period, the frequency band is 1 / T, and the complex symbol a _i of the parallelized signal after OFDM is applied. Since NT has an NT period, the frequency band is approximated to 1 / NT. Since the sum signal C (t) is multiplied by N orthogonal signals and then added, the band between different subchannels is 1 / NT, so the band of sum signal C (t) is 1 / T, which is the original serial input. It has the same bandwidth as the signal. In other words, even when OFDM is applied, the bandwidth of the entire signal does not change.

On the other hand, it is assumed that the OFDM transmitter performs N-point IFFT conversion by performing parallel processing on one block of N QAM complex symbols and then serially converts and transmits it through the channel. Here, the N-point IFFT transformed signal is called an OFDM symbol. There are N samples modulated by different N vertical subcarriers in one OFDM symbol output from an IFFT converter, and the OFDM symbols are serially converted and transmitted in OFDM sample units. Now, the receiving side can receive the OFDM samples inputted through the serial stream, and on the transmitting side, the same block subjected to IFFT processing must be detected and FFT converted to restore the original data. That is, the receiving side must accurately detect the reference point at which the OFDM symbol starts in the OFDM sample serial streams.

Accordingly, the present invention has been devised to satisfy the above requirements, and utilizes the characteristics of the guard interval inserted between symbols to remove intersymbol interference between OFDM symbols. An object of the present invention is to provide a simple time acquisition tracking circuit of an OFDM system that estimates a start position and obtains symbol synchronization.

In the OFDM system according to the present invention for achieving the above object, an OFDM transmission that inserts and transmits a guard interval consisting of N _g samples between valid periods, an OFDM symbol consisting of N _u OFDM samples, is transmitted. A system comprising: a phase calculator for receiving a complex sample received in a time domain and calculating and outputting an argument of the complex sample; A rotation phase angle calculator for obtaining a difference value between the phase of the current complex sample provided by the phase calculator and the previous phase delayed by one sample, and calculating and outputting an absolute value of the difference value; A rotation phase difference calculation unit obtaining a difference value between the absolute value of the current rotation phase angle provided from the rotation phase angle calculation unit and the absolute value of the previous rotation phase angle delayed by N _u samples, and calculating and outputting an absolute value of the difference value; An estimation value calculator which receives an absolute value provided from the rotation phase difference calculator and accumulates the window size to obtain an estimated value; And a detector for outputting a symbol start point acquisition signal by comparing the estimated value provided from the estimated value calculator with a predetermined threshold value.

The present invention estimates the starting position of an actual valid OFDM symbol by investigating the correlation (rotation phase angle) of the two intervals by using a characteristic of copying a part of the OFDM complex samples of the actual valid interval and using the sample as the guard interval. It can be implemented with simple hardware.

1 is a block diagram illustrating a modulation principle of an orthogonal frequency division multiplexing (OFDM) scheme;

2 is a view showing a time domain change of the signal to which the OFDM of FIG.

3 is a view showing a frequency domain change of the signal to which the OFDM of FIG.

4 is a block diagram of an OFDM system for inserting and transmitting a typical fast Fourier transform (FFT) and a guard interval,

5 is a diagram illustrating a time-frequency domain of an OFDM signal with a guard interval inserted in the OFDM system of FIG. 4;

6 is a format diagram for a transmission symbol using a guard interval in the OFDM system of FIG. 4;

7 is a flowchart illustrating a simple time acquisition tracking algorithm using a correlation between an effective section and a guard section according to the present invention;

8 is a block diagram of a simplified time acquisition tracking circuit according to the present invention.

* Explanation of symbols for main parts of the drawings

80: phase calculation unit 81: rotation phase angle calculation unit

81-1: One-sample delay register 81-2: First subtraction and absolute value calculation unit

82: rotation phase difference calculation unit 82-1: N _u sample delay register

82-2: second subtraction and absolute value calculator 83: estimated value calculator

84: detector

Hereinafter, with reference to the accompanying drawings will be described in detail with respect to the present invention.

First, an OFDM system using a typical FFT and guard interval will be described with reference to FIG. 4 to help understand the present invention.

As shown in FIG. 4, the transmitter of the OFDM system includes a serial / parallel converter 40, a signal mapper 41, an IFFT chip 42, a parallel / serial converter 43, a guard interval inserter 44, and D. / A converter 45, up converter (Up Converter: 46), the receiving unit of the OFDM system down converter (Down Converter: 50), the A / D converter 51, the guard section eliminator 52, The serial / parallel converter 53, the FFT chip 54, the equalizer 55, the signal mapper 56, and the parallel / serial converter 57 are comprised.

The serial data input to the serial / parallel converter 40 on the transmitting side is converted into a parallel data form, grouped by n bits, and output as complex symbols ai through the signal mapper 41. Here, n is the number of bits determined according to the signal constellation. For example, n = 4 bits when the signal constellation of the signal mapper is 16QAM, and n = 5 bits when 32QAM. N complex symbols output in parallel from the signal mapper 41 are inverse Fourier transformed through the IFFT chip 42, and are converted into serial form again for transmission. The guard interval inserter 44 sets and inserts a guard interval to avoid inter-symbol interference (ISI) due to multipath fading. The discrete symbol output from the guard interval inserter 44 is converted into an analog signal, modulated to an uplink frequency, and transmitted through a channel. The receiver performs processing that is reversed from the transmitter. Here, the equalizer 55 serves to compensate for channel distortion due to non-ideal characteristics of the channel, that is, various noises, interference with adjacent channels, and multipath.

FIG. 5 is a diagram illustrating a time-frequency domain of an OFDM signal with a guard interval inserted in the OFDM system of FIG. 4. Two-dimensional representation of the signal transformed into the time-frequency domain by FFT transform. In the time domain, a symbol is transmitted Ts, a guard interval is transmitted Tg, and each subchannel band is 1 / Ts in the frequency domain. In the frequency domain, the symbols overlap each other, and in the time domain, the symbols are separated from each other by a guard interval.

6 is a format diagram for a transmission symbol using a guard interval in the OFDM system of FIG. Samples transmitted from the sender are composed of a useful part and a guard interval (GI). In the valid section, valid OFDM samples are transmitted, and a guard section is inserted at the front of the valid section to distinguish the OFDM symbol. In the guard interval, some samples located below the valid interval are copied. That is, since the same sample exists in the guard period and the valid period, the correlation between the valid period and the guard period, which is an OFDM symbol, can be related. This correlation can be used to estimate the starting point of the symbol.

However, in obtaining correlations, samples are subject to various distortions, frequency offsets, fading, and additive white gaussian noise (AWGN) while being transmitted from sender to receiver. Consideration should be given.

Therefore, in the present specification, a simple time acquisition tracking circuit using the algorithm is proposed after presenting an algorithm that can implement a correlator for obtaining a starting point of a symbol with simpler hardware, and reacts strongly to the distortion included in the symbol. We will implement it.

7 is a flowchart illustrating a simple time acquisition tracking algorithm using a correlation between an effective section and a guard section according to the present invention. The algorithm is applied to OFDM complex samples in the time domain before performing the FFT process on the receiving side.

First, if variables are defined, N _u is the total number of valid samples in the validity interval and N _g is the total number of samples in the guard interval. X _i is a received complex sample, and X _i ^d is a complex sample delayed by the number of N _u samples, meaning X _i ^d = X _i-Nu . k is an index for counting N _g samples (window size), i is an index indicating the first sample position of the set window, and ε _i is an estimated value calculated in the i-th window.

Steps S2 to S7 of FIG. 7 are obtained by estimating the estimated value ε _i for the i-th window.

[Equation 3]

In Equation 3, arg (X _{i + k} ) is a phase angle with respect to a complex sample in the time domain. Now, considering the range in which the estimated value calculated according to Equation 3 exists, there are two cases.

① When (X _{i + k} , X _{i + k-1} ) exists in the guard period (GI)

If (X _{i + k} , X _{i + k-1} ) is in the guard interval (GI), then (X _{i + k} , X _{i + k-1} ) samples are delayed by N _u samples (X _{i + k} ^d , X _{i + k-1} ^d ) is the same sample. (Copy N _g lower samples in the valid section and use it as a guard interval) That is, the same received sample (X _{i + k} , X _{i + k-1} ) and (X _{i + k} ^d , X _{i + k-1} ^d Are added with the same channel distortion. In particular, since the frequency offset causes a constant rotational movement between successive samples (one sample delay), the absolute value (| A | = | arg ( X _{i + k} ) -arg (X _{i + k-1} ) |) and the absolute value of the rotational angle of rotation between the N _u delayed samples (| B | = | arg (X _{i + k} ^d ) -arg (X _{i It can be seen that + k-1} ^d ) |) is the same.

Therefore, the rotational movement angle due to the frequency offset or the like in the two sections means that it can be canceled by subtraction (| A | = | B |). As a result, in an ideal environment with no damage, the value || A |-| B || becomes '0', so the estimated value ε _i accumulated in the window from k = 0 to k = N _g -1 converges to the value of '0'. do.

② (X _{i + k} , X _{i + k-1} ) does not exist in the guard period (GI)

' 값을 갖으며, ｜｜A｜-｜B｜｜은 '

' 값이다. 결국, k=0 ∼ k=N_g-1 까지 윈도우내에 누적된 추정값 ε_i는 '

' 평균값으로 수렴한다.If (X _{i + k} , X _{i + k-1} ) is not within the guard interval, the (X _{i + k} , X _{i + k-1} ) samples are samples delayed by N _u samples (X _{i + k} ^d It can be seen that the sample is completely independent of, X _{i + k-1} ^d ). Therefore, the phase angles of the respective samples are random values, and the absolute values | A | and | B | for the rotational movement angles between neighboring samples (one sample delayed) are not equal. (| A | ≠ | B |) The absolute values | A | and | B |

'Value, ｜｜ A ｜-｜ B ｜｜

'Value. As a result, the estimated value ε _i accumulated in the window from k = 0 to k = N _g −1 is

'Converge to average value.

'을 유지할 것이고, 슬라이딩 (sliding)되면서 윈도우가 보호구간으로 접근할 수록 상기 추정값은 점차 작아지기 시작하고, 윈도우와 보호구간이 동일하게 설정됐을 때 추정값은 '0'으로 수렴할 것이다. 따라서, 시간 영역상에서 복소 심볼의 위치를 추정할 수 있는 기 설정된 임계값과 상기 S2∼S7 단계를 통해 계산된 추정값 ε_i을 비교하여(S8) 심볼의 시작위치를 결정한다. 만약, i번째 윈도우 추정값이 임계값 보다 클 경우에는 슬라이딩하여 i+1 번째 윈도우 추정값을 다시 계산한다.(S9) 만약, 추정값이 임계값 보다 작을 경우에는 OFDM 심볼의 시작점을 획득하게 된다.(S10) 이와 같은 알고리즘을 통해서 보호구간이 갖는 특성을 이용하면 OFDM 복소 심볼의 시작 위치를 획득할 수 있다.As shown in the above ① and ② cases, when the window exists within the valid period of the OFDM symbol, the estimated value ε _i is'

As the window approaches the guard zone as it slides, the estimate starts to get smaller, and when the window and guard zone are set equal, the estimate will converge to '0'. Accordingly, the start position of the symbol is determined by comparing the predetermined threshold value for estimating the position of the complex symbol in the time domain with the estimated value ε _i calculated through the steps S2 to S7 (S8). If the i-th window estimate is larger than the threshold, the i-th window estimate is slid and recalculated (S9). If the estimate is less than the threshold, the start point of the OFDM symbol is obtained. By using the characteristics of the guard interval through this algorithm, the start position of the OFDM complex symbol can be obtained.

8 is a block diagram of a simple time acquisition tracking circuit according to the present invention. The present invention provides a phase calculation unit 80, a rotation phase angle calculation unit 81, a rotation phase difference calculation unit 82, and an estimated value calculation unit ( 83) and a detector 84. The rotation phase angle calculator 81 includes a one-sample delay register 81-1, a first subtraction and absolute value calculator 81-2, and the rotation phase difference calculator 82 is N. _u sample delay register 82-1 and second subtraction and absolute value calculator 82-2.

The phase calculator 80 receives a complex sample and calculates and outputs a phase angle arg (X _{i + k} ). The phase calculator 80 stores the phase angle corresponding to the complex sample in ROM and receives the complex sample as an address. It is implemented in the form of a look-up table that outputs phase angles.

The rotation phase angle calculator 81 calculates a difference between the phase of the current complex sample provided from the phase calculator 80 and the one-sample delayed previous phase output from the one-sample delay register 81-1. _{i + k} ) -arg (X _{i + k-1} )) is calculated and the absolute value (| A |) of the difference value is calculated and output.

The rotation phase calculator 82 has the rotational phase current rotational phase of each absolute value provided by the angle calculating unit (81) (| A |) and the N _u samples delay register the output from the N delayed sample _u 82-2 The difference value (| B |) of the absolute value of the previous rotation phase angle is obtained, and the absolute value (|| A |-| B ||) for the difference value is calculated and output. Where N _u is the total number of samples in the validity interval.

The estimated value calculator 83 receives an absolute value provided from the rotation phase difference calculator 82 and accumulates the estimated value ε _i by accumulating as much as a window size. Here, the window size is N _g , and N _g is the total number of samples of the guard interval.

The detector 84 compares the estimated value provided from the estimated value calculator 83 with a predetermined threshold value and outputs a symbol start point acquisition signal. Here, the symbol start point acquisition signal is generated when the estimated value is smaller than a predetermined threshold.

According to the basic specifications of the European digital terrestrial broadcasting system, the effective section size (FFT size 2K mode or 8K mode) and the protective section size (1/4, 1/8, 1/16 or 1/32 of the FFT size) Is an option, but for convenience, the present invention will be described based on 2K mode (FFT size: 2048). That is, in the case of the 2K mode, the effective period sample number N _u is 2K (= 2048), and the guard period sample number N _g is 512 (for 1/4 of 2K). The guard interval is a copy of the 1536 th sample to the 2047 th sample (a total of 512 samples), which is the lower portion of the valid section, and the guard section is inserted and transmitted at the front of the valid section.

The complex sample Xa, b is the b th sample of the a th symbol. In the 2K mode, samples in the guard period of the 0th symbol are X0,0 to X0,511, and samples in the valid period are X0,512 to X0,2559.

The receiving side receives the OFDM modulated signal transmitted through the channel, converts it to a downlink frequency, and converts the lowpass and analog signal into a digital signal to obtain a complex sample serial stream. Here, in the digital signal complex sample serial stream, the guard period is deleted and only the valid period is processed in parallel and input to the FFT chip. Therefore, the characteristic of the guard interval was used to obtain the position (that is, the first sample X0,512) at which the validity interval starts before performing the FFT. (Protection period samples X0,0 to X0,511 are the same as validity period samples X0,2048 to X0,2559.)

)을 구한다.The rotation phase angle calculator 81 calculates an absolute value of the phase difference between the current sample and the sample delayed by one sample in order to calculate the rotation phase in which the sample is moved due to the frequency offset. The rotation phase difference calculation unit 82 subtracts the absolute value of the rotation phase difference (| A |) of the current sample and the absolute value of the rotation phase difference (| B |) delayed by 2048 samples and subtracts the absolute value of the resultant value (| A |-). | B || The estimated value calculator 83 accumulates the absolute value || A |-| B || for the window size 512 to estimate the estimated value (

)

'으로 수렴된다. (물론, 추정값은 채널 환경에 따라 달라질 수 있으며 상기 제시된 값은 이상적인 환경으로 설정했을 경우이다.)Here, since the rotational movement angles generated by the influence of the frequency offset have the same value in the same sample, if the window is set to be completely coincident with the protection period, ｜ A | = | B | The estimate converges to '0'. As the window moves away from the guard interval, the estimate increases and when the window is completely out of the guard interval, the estimate is'

Converges to (Of course, the estimates may vary depending on the channel environment, and the values presented above are for ideal environments.)

The detection unit 84 determines a predetermined threshold value according to the channel environment, and compares the estimated value with the threshold value. When the estimated value is greater than or equal to the threshold value, the slicing and recalculation of the estimated value in the reset window is performed. When the estimated value is less than the threshold value, a symbol start point acquisition signal indicating the first sample position of the valid section is output. In this way, the estimation value is calculated by using the window sliding method to obtain a symbol start point acquisition signal. The acquisition signal is used to delete the guard interval at the next connected block and to perform FFT processing only the valid interval.

As described above, the simplified time acquisition tracking circuit of the present invention can obtain the symbol start point by using the characteristic that the rotational movement angles generated between the same samples in the protection period and the effective period are identical in the time domain, and the frequency offset influence. It shows a relatively strong characteristic and can be implemented with simple hardware.

Claims (5)

In an OFDM system in which a guard interval consisting of N _g samples is inserted and transmitted between valid periods, an OFDM symbol consisting of N _u OFDM samples, a complex sample received in a time domain is inputted, and the phase of the complex sample ( a phase calculator 80 for calculating and outputting argument); A rotation phase angle calculator (81) for obtaining a difference value between the phase of the current complex sample provided by the phase calculator (80) and the previous phase delayed by one sample, and calculating and outputting an absolute value of the difference value; The rotation phase difference calculator 82 obtains a difference value between the absolute value of the current phase angle provided by the rotation phase angle calculation unit 81 and the absolute value of the previous phase angle delayed by N _u samples, and calculates and outputs an absolute value of the difference value. ; An estimation value calculation unit (83) which receives an absolute value provided from the rotation phase difference calculation unit (82) and accumulates it by a window size to obtain an estimated value; And a detection unit (84) for outputting a symbol start point acquisition signal by comparing the estimation value provided by the estimation value calculation unit (83) with a predetermined threshold value. The quadrature division band system of claim 1, wherein the phase calculator 80 is a lookup table that stores a phase angle corresponding to a complex sample in advance and receives a complex sample as an address and outputs the phase angle. Simple time acquisition tracking circuit. 2. The apparatus of claim 1, wherein the rotation phase angle calculator (81) comprises: a one sample delay unit (81-1) for delaying and outputting one sample of the phase value of the complex sample provided from the phase calculator (80); A first subtracted absolute value calculating unit which receives the phase value provided from the phase calculating unit 80 and the phase value output from the first sample delay unit 81-1, calculates and outputs an absolute value of a difference value between the two phases ( 81-2), the simplified time acquisition tracking circuit in an orthogonal divided band system characterized in that it comprises a. The method of claim 1, wherein the rotational phase difference calculation unit 82 _u N sample delay unit (82-1) to which the output _u N sample delay receives the absolute phase value from the phase rotation angle calculating section 81 and; A phase absolute value received from the rotation phase angle calculation unit 81 and a phase absolute value output from the N _u sample delay unit 82-1, and calculating and outputting an absolute value of a difference value between the two absolute values; 2. A simplified time acquisition tracking circuit in an orthogonal divided band system, characterized in that it comprises two subtracted absolute value calculation units (82-2). The orthogonal partitioning according to claim 1, wherein the estimation value calculating unit 83 sets a window by window slicing to obtain an estimation value, wherein the window size is N _g equal to the number of samples of the guard interval. Simple time acquisition tracking circuit in band system.

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