KR20010048447A - Method and apparatus for implementing the guard interval using cyclic suffix in ofdm systems - Google Patents

Abstract

PURPOSE: A method and apparatus for implementing guard interval using cyclic suffix in OFDM system is provided to reduce the chip size of hardware by copying front portion of valid data row and inserting them to immediate next of the valid data row. CONSTITUTION: An apparatus for implementing guard interval is for removing effect of intersymbol interference and interchannel interference in communication system. A delay device(62-1-62-G) is supplied with the first to nth samples of valid data row(N samples) IFFT(Inverse Fast Fourier Transform) modulated at transmitter and delays the samples by length of the guard interval to output CS(cyclic suffix) data row. A selection device(65) selects one of the valid data row and the delayed CS data row and outputs the delayed CS data row immediately after the valid data row.

Description

TECHNICAL AND APPARATUS FOR IMPLEMENTING THE GUARD INTERVAL USING CYCLIC SUFFIX IN OFDM SYSTEMS}

TECHNICAL FIELD The present invention relates to Orthogonal Frequency Division Multiplex (OFDM) technology, and in particular, a guard interval for reducing intersymbol interference (ISI) and interchannel interference (ICI). And a method and apparatus for processing GI).

In general, the OFDM method multiplexes the parallelized N transmission data with different subcarrier frequencies, and transmits each of the multiplexed data. In this case, if the parallelized N data are regarded as one symbol, each of the N subcarriers in the unit symbol has orthogonality 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 symbols by extending the transmitted symbol period, the inter-symbol interference can be further reduced, so that the structure of the channel equalizer is very simple. In addition, the insertion of the guard interval increases the band efficiency and the power is uniformly distributed in each frequency band in the form of a square wave in the form of spectrum, which is also strong in the co-channel interference signal.

Due to the advantages of OFDM, high-speed data transmission systems such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Digital Terrestrial Television Broadcasting (DTTB), Wireless Local Area Network (WLAN), and Wireless ATM (Wireless ATM) OFDM wireless communication systems such as Asynchronous Transfer Mode are being developed. Of course, the technique of inserting the guard interval in these OFDM systems is a necessary requirement.

In addition, a technology for inserting a guard interval is essential in a digital wired communication system employing a Discrete MultiTone (DMT) method such as an Asymmetric Digital Subscriber Line (ADSL) and a Very-high Bit Rate Digital Subscriber Line (VDSL).

In the present specification, a technique for inserting and removing a guard interval will be described based on the OFDM scheme, but it is applied to the DMT scheme and the like, but can be variously modified without being limited thereto.

FIG. 1 is a schematic block diagram of a transmitting and receiving end of a typical OFDM system, and FIG. 2 is a structural diagram of an OFDM symbol implementing a guard interval using a conventional cyclic prefix (CP) scheme.

Referring to FIG. 1, the OFDM transmitter 110 includes a serial / parallel conversion unit 111, an IFFT chip 112, a parallel / serial conversion unit 113, a guard interval insertion unit 114, and a D / A conversion unit ( 115, the OFDM receiver 130 includes an A / D converter 131, a guard interval eliminator 132, a serial / parallel converter 133, an FFT chip 134, synchronization and channel estimation ( 135), the equalizer 136, the bottle / serial converter 137.

The modulated serial data to be transmitted at the transmitting end 110 passes through N parallel data ( ), The IFFT chip 112 and the parallel / serial converter 113 are IFFT converted and then output as IFFT serial data.

For the IFFT serial data, a predetermined guard interval is inserted through the guard interval inserter 114 to generate an OFDM symbol. ) Is converted into an analog signal through the D / A converter 115 and transmitted. Here, the signal passes through the wireless communication channel H (k) and noise ( ) Is added.

The received analog signal including the noise at the receiver is passed through the A / D converter 131 to the digital signal ( ) And deletes the guard interval inserted through the guard interval removal unit 132, thereby valid OFDM data ( Will be printed).

Valid OFDM data ( ) Is parallel data (FFT converted through the serial / parallel conversion unit 133 and the FFT chip 134) ). Next, the FFT transformed parallel data ( Is a channel equalized signal through the equalizer 136. ) Is output to the demodulator (not shown) via the bottle / serial converter 137.

The synchronization and channel estimator 135 performs symbol synchronization and frame synchronization and performs channel estimation for tap setting of the equalizer 136.

The conventional guard interval insertion method in the guard interval insertion unit 114 removes the influence of interference between adjacent symbols and interference between adjacent channels by inserting a CP longer than an impulse response of a channel between adjacent OFDM symbols in the guard period. That is, as shown in Fig. 2, in order to maintain the orthogonality of the subchannels, the guard interval 23 is placed immediately before the N valid data 21 in the time domain, and the guard interval 23 is used to store the valid data. The CP is constructed by copying the guard interval length (ie, N _G samples 22) at the rear part.

Therefore, the size of the OFDM symbol is the sum of N _G samples for CP and N samples for valid data.

As such, the IFFT data in the OFDM system is output in a serial form and input to the guard interval insertion unit 114. In this case, considering that the guard interval is to be inserted, the input data format includes IFFT valid data (N samples) in a serial form, followed by a blank signal corresponding to the guard interval (N _G samples) size. Come.

The reason for doing this is that, as can be seen in the time domain of FIG. 2, since the data 22 to be inserted comes after the position of the protection section 23 to be inserted, the valid data 21 to be inputted before the data to be inserted is inputted. Because you need to save).

Therefore, when implementing the protection period of the conventional CP method, since a delay element (buffer) or a memory or a memory address generator equal to FFT / IFFT size N is required, it occupies a considerable chip area and an initial delay as much as N clocks. There is a problem that occurs.

Accordingly, the present invention has been made to solve the above problems, by extending the period of the symbol by inserting a plurality of pieces of data at the front of the valid data sequence immediately preceding the valid data sequence, thereby reducing the hardware chip area The purpose is to provide a method and apparatus for implementing a protection section using CYCLIC SUFFIX.

In order to achieve the above object, a method for implementing a guard interval according to the present invention removes the influence of interference between adjacent channels and interference between adjacent symbols in a digital wired / wireless communication system using an orthogonal frequency division multiplex (OFDM) or discrete multitone (DMT) scheme. In the implementation method of the guard interval, the guard interval is set by extending the guard interval from the end of the valid data interval consisting of N samples IFFT at the transmitting end, by copying the N _G samples of the front portion of the valid data interval the guard interval Inserting a CS into a cyclic suffix, and generating and transmitting a sample of the valid data and a sample of the inserted CS as an OFDM symbol (N + N _G ).

The guard interval realization apparatus of the present invention for achieving the above another object is to remove the effects of interference between adjacent channels and interference between adjacent symbols in a digital wired / wireless communication system of Orthogonal Frequency Division Multiplex (OFDM) or Discrete Multitone (DMT). An apparatus for implementing a guard interval, wherein the transmitter receives an input from the first sample of an inverse fast fourier transform (IFFT) -modulated valid data sequence (N samples) and transmits a predetermined guard interval length (N _G samples). Delay means for delaying and outputting a CS (cyclic suffix) data string; And selecting means for selecting one of the valid data string and the delayed CS data string and outputting the delayed CS data string (N _G samples) immediately after the valid data string.

The present invention for achieving the above another object is to extend the protection from the end of the valid data interval consisting of N samples in the transmission symbol is inserted into the protection interval for removing the effects of inter-channel interference and inter-symbol interference The interval is set, and the N _G samples of the front part of the valid data interval are copied and inserted into the protection interval as CS (cyclic suffix) to form a unit transmission symbol.

1 is a schematic block diagram of a transmitting and receiving end of a typical Orthogonal Frequency Division Multiplex (OFDM) system

2 is a structural diagram of an OFDM symbol implementing a guard interval using a conventional cyclic prefix (CP) method

3 is a structural diagram of an OFDM symbol that implements a guard interval using a Cyclic Suffix (CS) method according to the present invention

4 is a view illustrating a form in which corrupted data is removed from an OFDM symbol of the CS scheme of FIG.

5A and 5B are block diagrams illustrating one embodiment of a protection section insertion circuit of the CS system of FIG.

<Description of the symbols for the main parts of the drawings>

62-1 to 62-G, 73-1 to 73-G: delay elements 65, 74, 76: multiplexer

64: AND gate 72: demultiplexer

CS: Cyclic Suffix CP: Cyclic Prefix

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

FIG. 3 is a structural diagram of an OFDM symbol in which a guard interval is implemented by a Cyclic Suffix (CS) method according to the present invention, and FIG. 4 is a diagram illustrating a form in which corrupted data is removed from an OFDM symbol of the CS method of FIG.

Referring to FIG. 3, in the CS method, a guard interval 43 is provided by the length of N _G samples from the end of the section of the IFFT-signed valid data string 41 of N samples displayed from 0 to (N-1) on the axis in the time domain. Extend to set one symbol period.

Then, the data 42 of the preceding portion marked from 0 to N _G of the valid data string 41 is copied and inserted into the protection section 43. The OFDM symbol size of the CS scheme thus formed is the sum of the length of valid data and the length of the CS protection interval.

That is, the OFDM symbol to be transmitted is copied into the CS in the protection section immediately after the valid data by copying some data 42 in front of the valid data string coming in a serial form in the time domain and transmitted through the wired / wireless communication channel.

Now, as shown in (a) of FIG. 4, ISI and ICI are generated in an OFDM symbol of a CS scheme received through a wired / wireless communication channel. To solve this problem, as shown in (b) of FIG. 4. damage in an OFDM symbol of the received CS-that is, the damaged front data in the valid data columns (N _G samples) is removed.

Then, even if the damaged valid data portion is removed at the receiving side, the removed valid data portion remains in the CS protection section area, so that it is possible to restore the original data and effectively remove the ISI and ICI.

The characteristics of the OFDM symbol of the CS scheme proposed above will be described in detail through the following mathematical signal analysis. After analyzing the transmission signal and the reception signal of the conventional CP method and the corresponding transmission and reception signal of the CS method of the present invention will be compared and prove the efficiency of the CS method.

First, the l- th OFDM symbol transmitted with the guard interval of the conventional CS scheme inserted is as follows.

here, Is the data symbol, N is the FFT / IFFT size, Represents the number of samples used in the guard interval. In addition, "-" means the OFDM signal after applying CP.

The signal received through the channel has a frequency offset due to phase jitter, a Doppler shift, and a timing offset in the sampler. .

here, Is a frequency offset, Is the subchannel spacing, Is the normalized frequency offset, Denotes the normalized timing offset, H _l (k) denotes the channel frequency response of the l- th symbol.

Inter-symbol interference (ISI) is generated by adjacent symbols while passing through the channel, so that damaged CPs are removed from the received signal, and N valid data that are not affected by the interference between adjacent symbols are extracted and demodulated by FFT. same.

In the above Equations 3 and 4, the frequency offset is not taken into account for the convenience of the expansion of the equation, but the process is the same in the general case in which the frequency offset exists.

Where frequency offset , Timing offset δ, channel Estimates using a training signal or a received signal during a training interval in the modem initialization process, and constructs a frequency domain equalizer using the estimated channel value.

In contrast, when the CS scheme of the present invention is used, the l- th OFDM symbol transmitted through the channel is as follows.

Where ^ represents an OFDM signal after the CS scheme is applied.

In addition, the signal received at the receiver is expressed as follows, including a frequency offset and a timing offset as in the CP.

The area where data is damaged by the previous symbol is the CP area of the protection section in the CP method, but the valid data area is damaged in the CS method of the present invention.

However, when the transmitter inserts the guard interval, the same data is copied to the guard interval at the back, so the copied CS region is not affected by the previous symbol even after passing through the channel.

Therefore, when the guard section is removed from the receiver, the damaged valid data area (0 to N _G ) must be removed from the OFDM symbol having the damaged portion as shown in (a) of FIG. 4, and the signal is removed from FIG. As shown in FIG.

OFDM data having an intact valid data area and a guard interval CS are represented as follows.

As in Equation 7, in the case of the CS method, the signal in which the guard interval is removed is different from the CP method in that the time domain index starts from N _G , the size of the guard interval, and the original n = 0 data exists in the guard interval. . As a result, in the case of the CS method, the effective data is expressed in an annular shifted state by N _G as compared with the CP method.

That is, the annular shift in the time domain is a phase shift in the signal demodulated by the FFT due to the property of the DFT represented by the linear phase shift in the frequency domain as follows.

Again, frequency offset is not taken into account for ease of expression expansion.

As can be seen from Equation 8, when the CS method is used as compared to the demodulated signal of the CP method represented by Equation 4, As much as phase shift occurs, the relationship between the two methods is given by Equation (9).

Phase shift θ (k) by CS is expressed as a function of subchannel index k . As the subchannel index increases, the phase shift increases linearly.

However, the shift in phase generated by using the CS method is an undesirable result and therefore requires compensation. However, no additional equalizer is needed for this phase shift compensation.

This is because phase shift can be compensated only by the frequency domain equalizer used in the existing system. That is, when estimating a channel using a training symbol, since the phase shift by the channel and the CS are estimated together in the estimation value, there is no need for an extra phase shift estimation.

Therefore, if the coefficient is set using the estimated channel value, phase shift is compensated together. A brief summary of this process is as follows.

1. [Training period]

The training symbol and the received symbol are used to estimate the channel. At this time, the transmitter inserts N _G CSs in the call section, and the receiver removes the N _G valid data at the front.

Where Xp (k) is the training symbol transmitted in the frequency domain Indicates a reception symbol. At this time, the coefficient of the frequency domain equalizer is obtained by taking the inverse of the estimated channel.

2. [Data Transfer Period]

Since the θ (k) term is included in the estimated channel value, it is used to compensate for phase shift by CS. Ignoring the effects of noise, it is assumed that the channel experienced by the training symbol and the l- th symbol is the same.

That is, the transmission signal is estimated by multiplying the estimated equalizer coefficient by the received signal after the FFT.

The CS guard interval inserter receives CS from the first sample of the IFFT-modulated valid data sequence (N samples) and delays the defined guard interval length (N _G samples) by delaying CS (cyclic suffix) data. And a selector for selecting one of the valid data string and the delayed CS data string, and outputting the delayed CS data string (N _G samples) immediately after the valid data string.

5A and 5B are block diagrams illustrating one embodiment of a protection section insertion circuit of the CS system of FIG. 3.

Referring to FIG. 5A, the CS protection zone inserting device includes N _G buffers 62-1 to 62 _-G connected in series, an AND gate 64, and a multiplexer 65.

The N _G buffers 62-1 to 62-G connected in series receive IFFT serial data strings and shift them by one clock to output them to the multiplexer input terminal. The AND gate 64 performs an AND operation on the enable signal and the clock signal to provide the clock signals of the buffers 62-1 to 62-G. At this time, the enable signal has a logic 'high' value while the first portion of the valid data string (N _G samples) is input.

The multiplexer 65 outputs the incoming IFFT serial valid data string 61 according to the selection signal and then outputs the delayed guard interval data string 63 from the buffers 62-1 to 62-G. In this case, the OFDM symbol of the CS scheme is generated by inserting the preceding valid data string into the guard interval at the rear of the valid data string.

Referring to FIG. 5B, the CS protection zone insertion device includes a first demultiplexer 72, N _G parallel-connected buffers 73-1 to 73 -G, a first multiplexer 74, and a multiplexer 76.

The first demultiplexer 72 sequentially selects valid data strings coming in the serial form according to the first selection signal SEL1 and stores them in buffers 73-1 to 73-G connected to respective output terminals, respectively. The first multiplexer 72 sequentially selects the N _G buffers 73-1 to 73 _-G connected in parallel according to the second selection signal SEL2, and receives and outputs the stored data in order.

In this case, the first and second selection signals SEL1 and SEL2 are respectively output terminals and input terminals 0, 0 of the first demultiplexer 72 and the first multiplexer 74 while the first 0 to N _G samples of the valid data string are input. Switch in order of 1,2, ...

The second multiplexer 76 outputs the IFFT serial valid data string 71 coming in according to the third selection signal SEL3 and then outputs the guard interval data string 75 delayed from the first multiplexer 74. .

In this way, the output of the second multiplexer 76 generates an OFDM symbol by inserting the preceding valid data sequence into the protection section immediately after the valid data sequence.

That is, as shown in FIGS. 5A and 5B, the input data is stored in the buffer only as much as the size N _G of the protection interval, and the data stored in the buffer is output after the last data of the OFDM valid data is output. Configure.

Since the hardware complexity is proportional to the memory device, the CS method increases in proportion to the size N _G of the protection interval, and no initial delay occurs.

Table 1 is a table comparing the hardware components required in the hardware implementation of the protection period using the conventional CP method and the CS method according to the present invention.

suggestion Transmitter Hardware configuration (n-bit data bus) Initial clock Cyclic Prefix Method # 1 N buffers and demultiplexers NN _G Method # 2 2 memory of size N N Cyclic suffix Method # 1 AND gate, multiplexer and N _G D flip-flops none Method # 2 Demultiplexer, multiplexer, multiplexer2 and N _G D flip-flops none

As shown in Table 1, the conventional CP method requires a memory (delay element) as much as the effective data size (N samples), but the CS method of the present invention uses the memory (as much as N _G samples) as the guard interval size (N _G samples). Delay element), and no initial delay occurs, enabling efficient hardware.

In addition, in the case of the CS scheme of the present invention, the position of removing the guard interval at the receiver is the same as that of the conventional CP scheme, and the phase shift generated by using the CS scheme is compensated only by the frequency domain equalizer used in the conventional OFDM system. This is possible. Since the structure of the CS receiver is also the same as that of the CP scheme, it can be seen that the compatibility between the modem using the conventional CP scheme and the proposed CS scheme modem is maintained.

The present invention is not limited to the embodiments described above, and various modifications and changes can be made by those skilled in the art, which are included in the spirit and scope of the present invention as defined in the appended claims.

As described above, in the conventional OFDM system or the DMT system, N _G CPs are inserted into the guard intervals between the adjacent symbols between the adjacent symbols, but the present invention inserts and transmits the N _G CSs into the guard intervals of the back. By removing the N _G valid data at the front, the inter-symbol interference and the inter-channel interference can be eliminated. In addition, the phase shift that is increased according to the subchannel index when the effective data of the CS method is removed is estimated at the same time as the channel distortion and the phase shift by the CS are estimated at the time of channel estimation by the training symbol. If the tap coefficient of the frequency domain equalizer is set using the estimated channel value, phase shift can be compensated with channel distortion.

Therefore, in the OFDM system having the protection interval using the CS scheme of the present invention, the same protection scheme as the conventional CP scheme is used, but only the data of the size of the protection interval N _G need to be stored. The insertion apparatus can be implemented, and there is no initial delay, and an additional compensator is not required for the phase shift generated during data symbol transmission.

Claims (12)

In a method of implementing a protection interval for removing the effects of interference between adjacent channels and interference between adjacent symbols in a digital wired / wireless communication system using an orthogonal frequency division multiplex (OFDM) or discrete multitone (DMT) method, The guard interval is set by extending the guard interval from the end of the valid data interval composed of N samples modulated by the transmitter, copying N _G samples from the front of the valid data interval, and inserting into the guard interval as a CS (cyclic suffix), And generating and transmitting the sample of the valid data and the sample of the inserted CS as an OFDM symbol (N + N _G ). 2. The method of claim 1, wherein the interval to be removed at the receiving end is set to N _G samples in front of the transmitted OFDM symbol, and the remaining (NN _G ) samples are removed after removing the first N _G samples of valid data of the received OFDM symbol. And demodulating using valid data and N _G CSs inserted by the transmitting end. The method of claim 2, further comprising simultaneously compensating for the linear phase shift generated during the demodulation and the channel distortion generated during the transmission using a frequency domain equalizer. The method of claim 3, wherein the frequency domain equalizer coefficient in the compensation step is to be obtained using a training symbol, CYCLIC SUFFIX is characterized by dividing the received OFDM symbol signal by the transmitted training symbol, estimating the channel value including phase shift, taking the inverse of the estimated value, and setting it as the frequency domain equalizer coefficient. How to implement the protection intervals used. The transmission symbol of the CS method according to claim 3, wherein N symbols of valid data modulated using the IFFT and N _G samples of CS are transmitted. Is (Formula 1), and the transmission symbol of the received CS scheme If is given by (Equation 2), Transmission symbol of the received CS scheme After removing corrupted data part Is a method of implementing a guard interval using CYCLIC SUFFIX, characterized in that (3) is annularly shifted by the guard interval length (N _G samples) in the time domain. (Equation 1): (Equation 2): (Equation 3): here, Is the frequency offset, Timing offset, Channel function, H _l (k) is the channel frequency response of the l th symbol, Is noise. The signal of claim 5, wherein the annular shifted signal Demodulated by FFT Equation 4 generates linear phase shift in the frequency domain, and the linear phase shift is a method of implementing a guard interval using CYCLIC SUFFIX, characterized in that the phase shift increases linearly as the subchannel index k increases. . (Equation 4): here, Is the lth transmit symbol, Timing offset, Channel function, H _l (k) is the channel frequency response of the lth symbol, N _G is the guard interval length, Is noise. An apparatus for implementing a guard interval for removing the effects of interference between adjacent channels and interference between adjacent symbols in a digital wired / wireless communication system using an orthogonal frequency division multiplex (OFDM) or discrete multitone (DMT) method, The transmitter receives the first sample of the inverse fast fourier transform (IFFT) modulated valid data sequence (N samples) and delays the predetermined guard interval length (N _G samples) to decode the cyclic suffix (CS) data sequence. Delay means for outputting; And And selecting means for selecting one of the valid data string and the delayed CS data string, and outputting the delayed CS data string (N _G samples) immediately after the valid data string using CYCLIC SUFFIX. Implementation device of protection section. The method of claim 7, wherein the delay means is implemented in the enable period, a serial form from the incoming protection with CYCLIC SUFFIX, characterized in that configured in series, N _G buffers connected to putting the valid data string by one clock shifted a period during Device. 8. The apparatus of claim 7, wherein the delay unit comprises: a demultiplexer for selecting and outputting the valid data strings by one sample in order of input according to a first selection signal; N _G buffers connected in parallel to store a sample of the output from the demultiplexer temporarily; And a multiplexer for selecting the N _G buffers connected in parallel in order of output according to the second selection signal and outputting the selected buffers to the selection means. In a transmission symbol having a guard interval inserted to remove the effects of interference between adjacent channels and interference between adjacent symbols, The guard interval is set by extending from an end of the valid data interval consisting of N samples, copying N _G samples of the front portion of the valid data interval, and inserting a unit transmission symbol by inserting a CS (cyclic suffix) into the guard interval. CS transmission symbol characterized in that it is formed. 12. The transmission symbol of claim 10, wherein the CS is inserted with a length longer than an impulse response of a channel. The transmission symbol of claim 10, wherein the transmission symbol of the CS scheme is transmitted in an extended symbol period by adding N samples of an IFFT-validated valid data sequence and N _G samples of an inserted CS. The received CS transmission symbol is demodulated by the FFT using the remaining valid data (NN _G ) and the inserted CS after removing the corrupted N _G samples of the valid data.