KR20040028226A - TDS-OFDM transmission system add pilot signal in frequence domain and a method proessing OFDM signal thereof - Google Patents

Abstract

The TDS-OFDM transmission system includes: an FEC unit for coding an OFDM signal in a frequency domain for detecting and correcting an error at a receiving end, a mapping unit for mapping an OFDM signal in a coded frequency domain by a predetermined mapping method, and a mapping; A pilot inserter which adds a pilot signal having a relatively small size compared to the average size of the OFDM signal in the frequency domain to the OFDM signal in the frequency domain, and inverse discrete Fourier transform the OFDM signal in the frequency domain into which the pilot signal is inserted The inverse Discrete Fourier Transform unit for modulating the OFDM signal in the domain, the guard interval insertion section for inserting the guard interval into the OFDM signal in the time domain, and the synchronization of the transmitting side and the receiver side to the OFDM signal in the time domain in which the guard interval is inserted And a shaping filter section for shaping and filtering the inserted synchronous signal of the OFDM signal in the time domain. Therefore, the reception performance can be improved by inserting a low power pilot signal having the same time period as the OFDM signal in the frequency domain.

Description

TDS-OFDM transmission system for adding a pilot signal to the frequency domain and a signal processing method according to the TDS-OFDM transmission system add pilot signal in frequence domain and a method proessing OFDM signal

The present invention relates to a digital broadcasting system, and more particularly, to a digital broadcasting system employing a Time Domain Synchronous-Orthogonal Frequence Division Multiplexing (TDS-OFDM) scheme for improving reception performance.

Orthogonal Frequency Division Multiplexing (OFDM) is a type of multicarrier modulation that has excellent performance in multipath and mobile reception environments.

The OFDM method improves frequency utilization efficiency by using a plurality of carriers having mutual orthogonality, and is a method suitable for high-speed data transmission by using a multi-carrier in a wired or wireless channel. In the case of a high-speed data transmission with a short symbol period in a wireless communication channel having multipath fading, when the single carrier method is used, the inter-symbol interference becomes more severe, whereas the complexity of the receiver is greatly increased. Since the symbol period in each subcarrier can be extended by the number of subcarriers while maintaining the data rate, a simple equalizer with one tap can cope with severe frequency selective fading channels by multipath.

In the OFDM method, the frequency utilization efficiency is increased by using a plurality of carriers having mutual orthogonality, and the process of modulating and demodulating the plurality of carriers at the transmitting and receiving end is the same as performing the IDFT and the DFT, respectively. Can be implemented.

One of the OFDM schemes, TDS-OFDM (Time Domain Synchronous-Orthogonal Frequency Division Multiplexing) scheme, is characterized by inserting a PN (Pseodo Noise) sequence, which is synchronization information for obtaining synchronization between a transmitter and a receiver in the time domain. That's the way it is.

FIG. 1 is a schematic block diagram of a digital broadcasting system employing the TDS-OFDM scheme.

The TDS-OFDM transmission system includes a forward error correction (FEC) unit 10 that performs encoding for detecting and correcting an error at a receiving end, and a mapping unit 20 which maps coded data to QPSK, 16QAM, 64QAM, or the like. And an Invertes Discrete Fourier Transform (IDFT) unit 30 which modulates the OFDM signal in the frequency domain into the OFDM signal in the time domain, and the end of the OFDM signal modulated to prevent inter syambol interference (ISI) in a multipath environment. A guard section inserting section 40 for inserting a section at the front of the OFDM signal with the section as a guard section, a sync information inserting section 50 for inserting a sync signal in a time domain characterized by the TDS-OFDM scheme, and the inserted sync information Shaping filter 60 for filtering for pulse shaping, and RF section 70 carrying a signal in a frequency band to which an OFDM signal is to be sent.

As described above, in the transmission system using the TDS-OFDM scheme, the synchronization signal is inserted only in the time domain without inserting the pilot signal which is the synchronization signal in the frequency domain.

FIG. 2 is a diagram illustrating the configuration of a mapped OFDM signal in a TDS-OFDM transmission system. As shown in FIG. 2, a synchronization signal does not exist in an OFDM signal in a mapped frequency domain. Therefore, in the transmission system to which the TDS-OFDM method is applied, the synchronization and acquisition of the time domain and the frequency domain are performed using only the sync segment of the time domain, thereby limiting the performance and limiting performance of the reception system. You have a problem.

SUMMARY OF THE INVENTION An object of the present invention is to provide a TDS-OFDM transmission system capable of improving reception performance by inserting a pilot signal in a frequency domain while being compatible with a conventional TDS-OFDM transmitter. will be.

1 is a schematic block diagram of a typical TDS-OFDM transmission system;

2 is a block diagram of a mapped OFDM signal according to FIG. 1;

3 is a schematic block diagram of a DMB-T transmission system applying the TDS-OFDM scheme according to the present invention;

4 is a configuration diagram of a signal mapped by FIG. 2;

5 is a conceptual diagram of adding a pilot signal and a mapped signal inserted in the pilot inserting unit 220 of FIG. 2, and

6 is a flowchart illustrating a signal processing method according to the DMB-T transmission system of FIG. 2.

Explanation of symbols on the main parts of the drawings

100 channel encoding unit 110 scrambler

120: FEC unit 200: DMB-T modulation unit

210: mapping unit 220: pilot insertion unit

230: IDFT unit 240: protective section insertion unit

250: synchronization information insertion unit 260: molding filter unit

270: RF section

In order to achieve the object of the present invention, a TDS-OFDM transmission system includes an FEC unit for coding an OFDM signal in a frequency domain for detecting and correcting an error at a receiving side, and a predetermined mapping scheme for the coded OFDM signal in the frequency domain. And a pilot insertion unit for adding a pilot signal having a size relatively smaller than the average size of the mapped OFDM signal in the frequency domain to the OFDM signal in the frequency domain, and the pilot signal inserted therein. An inverse discrete Fourier transform unit that modulates an OFDM signal in a frequency domain by inverse discrete Fourier transform and modulates the OFDM signal in a time domain, a guard interval insertion unit for inserting a guard interval into the OFDM signal in the time domain, and the guard interval inserted A synchronization information insertion unit for inserting synchronization information for synchronization between a transmitting side and a receiving side in the OFDM signal in the time domain, and inserted copper of the OFDM signal in the time domain. And a shaping filter unit for shaping the old signal.

Here, the OFDM signal in the frequency domain consists of an I signal and a Q signal, and the pilot signal has an I pilot signal added to the I signal and a Q pilot signal added to the Q signal.

In addition, the magnitude of the I pilot signal is set to a value in which a predetermined number of I pilot signals are accumulated by an arbitrary method to be larger than an average magnitude value of the I signal, and the magnitude of the Q pilot signal is a predetermined number of Q. The accumulated value of the pilot signal by any method is set to be larger than the average magnitude value of the Q signal. The time period of the I pilot signal and the Q pilot signal is equal to the time period of the I signal and the Q signal.

On the other hand, the signal processing method of the TDS-OFDM transmission system according to the present invention comprises the steps of: coding the OFDM signal in the frequency domain to detect and correct the error at the receiving side; Mapping the coded OFDM signal in the frequency domain by a predetermined mapping method; Adding a pilot signal having a size relatively smaller than the average size of the mapped OFDM signal in the frequency domain to the OFDM signal in the frequency domain; An inverse discrete Fourier transform step of inverse discrete Fourier transforming the OFDM signal in the frequency domain into which the pilot signal is inserted and modulating the OFDM signal in the time domain; Inserting a guard interval into the OFDM signal in the time domain; Inserting synchronization information for synchronization between a transmitter and a receiver into the OFDM signal of the time domain in which the guard interval is inserted; And shaping filtering the inserted synchronization information of the OFDM signal in the time domain.

Therefore, by inserting a low power pilot signal (Hidden Piolt) having the same time period as the OFDM signal in the frequency domain, it is possible to improve reception performance than a conventional TDS-OFDM transmission system.

Hereinafter, a TDS-OFDM transmission system will be described in which a pilot signal, which is an additional synchronization signal, is inserted into a frequency domain in a transmission system applying the TDS-OFDM scheme according to the present invention to improve reception performance.

Recently, China has proposed a new terrestrial digital TV transmission standard, DMB-T, which can be applied in China in order to improve the speed of the terrestrial digital TV transmission system. DMB-T transmission system is a DVB transmission plan developed by Tsinghua University according to service requirements, transmission conditions and channel characteristics of terrestrial multimedia TV broadcasting. Is applying.

3 is a schematic block diagram of a DMB-T transmission system for a Chinese system using a TDS-OFDM scheme, and a preferred embodiment of the present invention will be described in detail with reference to this.

The DMB-T transmission system may be divided into a channel encoding unit 100 and an OFDM modulation unit 200 according to its operation. The channel encoding unit 100 includes a scrambler 110 and a forward error correction (FEC) unit 120, and the OFDM modulation unit 200 includes a mapping unit 210, a pilot inserter 220, and an Inveres discrete fourier transform. ) 230, a protection section insertion unit 240, a synchronization information insertion unit 250, a shaping filter unit 260, and an RF unit 270.

The channel encoding unit 100 includes a scrambler 1100 that randomizes transmitted data to prevent data loss during synchronous data transmission, and a forward error correction (FEC) unit that performs coding for detecting and correcting an error at a receiving end. Has 1200.

The channel encoding, that is, the encoding of the forward error correction unit 120 is divided into a TV mode and a multimedia mode, and is applied differently.

First, FEC (Forward error correction) for the TV transmission mode is as follows.

Stepwise coding consisting of 2/3 trellis code, convolutional code, and reed-solomon (RS) code is used as forward error correction (FEC) of a TV broadcast program.

A convolutional code is inserted between the inner code and the outer code of the DMB-T transmission system to eliminate the effects of consecutive error codes caused by burst impulse interference.

Subsequently, the forward error correction (FEC) for the multimedia transmission mode is as follows.

Multi-level block product code (BPC) is employed for forward error correction (FEC) for multimedia integrated data traffic services. Multi-level BPS is a system code composed of block codes, and is a code composed of portions of two Dimensional Product Codes (DPS). And, the decoder of the multi-level BPC can adopt a high performance turbo algorithm. Multi-level BPS is divided into three levels. Different levels are mapped onto different bits of the 64QAM symbol constellation according to the set point set to give different anti-interference adaptability. Different protection priorities can be assigned to the multimedia data stream of the DMB-T transmission system according to the requirements.

Regarding the channel characteristics of terrestrial radio broadcasting, interleaved encoding is performed on data in time domain and frequency domain. Interleaved encoding in the time domain is performed among a plurality of frames, and has four operation modes according to a symbol constellation based on a convolutional interleaved encoder. Interleaved encoding in the frequency domain is performed in one frame according to the map table. For example, an input symbol vector consisting of 3780 symbols is mapped to a new output vector by an interleaving encoder in the frequency domain.

The OFDM modulator 200 applies the TDS-OFDM scheme.

The mapping unit 210 maps the error coded OFDM data to symbol constellations such as QPSK, 16QAM, and 64QAM. The constellation of a typical DMB-T transmission system uses 64QAM.

The symbol constellation is also applied differently for each mode by the channel encoding method applied differently according to the TV transmission mode and the multimedia transmission mode. That is, in the DMB-T transmission system using forward error correction (FEC) of the TV transmission mode, the coordinates of the projection of I and Q are (-7, -5, -3, -1,1,3,5,7). It has a regularly distributed symbol constellation. In addition, the DMB-T transmission system using forward error correction (FEC) of the multimedia integrated data traffic service has the coordinates of the projections of I and Q (-9, -7, -4, -2,2,4,7,9). Have irregularly distributed symbol constellations.

As illustrated in FIG. 4, the pilot inserting unit 220 includes an I pilot signal P _I and a Q pilot signal P _Q , which are pilot signals Hidden_Pilot of a very small size, in addition to the I and Q signals, which are OFDM signals. Is added to the I signal and the Q signal in the same period as the time period of the I signal and the Q signal (1 symbol time). At this time, the size of the I pilot signal P _I is set so that the value accumulated by the predetermined number of pilot signals P _I is larger than the average size value of the I signal. As a method of accumulating the power of the I pilot signal P _I , a correlation method or any method may be used. The size of the Q pilot signal P _Q is also set in the same way as the size of the I pilot signal P _I.

When the pilot signal Hidden_Pilot (P _I , P _Q ) is added to the I signal and the Q signal, which are OFDM signals, the signal is constructed as shown in FIG. 5. That is, mapped data (mapped data) and a pilot signal (Hidden_Pilot: P _I , P _Q ) are carried on each of N subcarriers of IDFT.

As described above, the reception side performs synchronization of time and frequency, channel equalization, and the like using the pilot signals Hidden_Pilot (P _I , P _Q ) inserted in the I and Q signals. In addition, extremely small amounts of pilot signals: by adding (Hidden_Pilot P _I, P _Q) may have a conventional system with compatibility.

The Inveres discrete fourier transform (IDFT) unit 230 converts an OFDM signal in the frequency domain into an OFDM signal in the time domain. That is, an OFDM symbol composed of a plurality of sample data in a time domain is output by assigning and modulating an OFDM signal in a frequency domain composed of a plurality of parallel data to a plurality of subcarriers.

The guard interval (GI) unit 240 inserts a guard interval (GI) in front of an OFDM symbol in units of OFDM symbols output by an Inverse Discrete Fourier Transform (IDFT). That is, the guard interval GI copies sample data of a portion of an end of an OFDM symbol and inserts it in front of the OFDM symbol in order to prevent inter symbol interference (ISI) in a multipath environment.

The synchronization information insertion unit 250 inserts a timing synchronization signal and a PN sequence for channel prediction in front of the guard period GI.

The shaping filter 260 filters PN-covered OFDM symbols into predetermined waveforms and transmits them to the wireless channel through the RF unit 270.

As described above, the pilot signal in the frequency domain according to the format of the signal mapped in order to have compatibility with the existing signal: inserts (Hidden_Pilot P _I, P _Q). At this time, the pilot signals (P _I, P _Q) and mapped to the OFDM signal (mapped data) will have the same time (1 symbol time). The format of the pilot signal Hidden_Pilot (P _I , P _Q ) may be a PN (Pseodo Noise) sequence or may be any value. Here, the pilot signal Hidden_Pilot (P _I , P _Q ) is a signal known between the transmitter and the receiver.

The pilot signal (Hidden_Pilot: P _I , P _Q ) sent from the DMB-T transmission system is used in the receiver for time, frequency synchronization, channel equalization, etc., thereby improving reception performance.

6 is a flowchart illustrating a process of processing a signal by a DMB-T transmission system using the TDS-OFDM scheme according to the present invention.

The FEC unit 120 for detecting and correcting an error in the receiving apparatus encodes the corresponding mode according to each mode, that is, the TV transmission mode and the multimedia transmission mode (S10).

The mapping unit 210 maps the OFDM signal encoded according to each mode to symbol constellations such as QPSK, 16QAM, and 64QAM (S20).

The pilot inserting unit 220 converts the I pilot signal P _I and the Q pilot signal P _Q , which are pilot signals Hidden_Pilot of very small size, to the I and Q signals that are mapped OFDM signals. Each is added in the same time period (1 symbol time) as shown in FIG. 3 (S30). Here, the format of the pilot signal Hidden_Pilot (P _I , P _Q ) may be a PN sequence or an arbitrary value. Of course, the pilot signal (Hidden_Pilot: P _I , P _Q ) is a signal known between the transmitter and the handset. .

As a result, reception performance is improved, and a small pilot signal (Hidden_Pilot: P _I , P _Q ) is inserted, thereby making it compatible with existing systems.

The IDFT (Inveres discrete fourier transform) unit 230 converts an OFDM signal in the frequency domain into an OFDM signal in the time domain (S40).

Thereafter, the guard interval (GI) unit 240 inserts a guard interval (GI) at the front end of the OFDM symbol in units of OFDM symbols output by the inverse fast Fourier transform (IFFT) (S50). In other words, the sample data of the terminal corresponding to 1/6, 1/9, 1/12, 1/20, 1/30 of all OFDM symbols is a guard period.

Next, the synchronization information inserting unit 250 inserts a time synchronization signal and a PN sequence for channel prediction in front of the guard period GI into an OFDM symbol in which the guard period GI is inserted (S60).

The shaping filter 260 filters the OFDM symbol covered with the PN sequence into a predetermined waveform (S70), and transmits the radio frequency through the RF unit 270 (S80).

In the above, a method of inserting a pilot signal (Hidden_Pilot: P _I , P _Q ) in the frequency domain has been described as a Chinese-based DMB-T transmission system using the TDS-OFDM method as an example. Naturally, it can be widely applied to the system.

In this way, by adding a low power pilot signal having the same time period as the OFDM signal to the frequency domain, a low power pilot signal, which is synchronization information other than the PN sequence in the time domain, is assigned to obtain time, frequency synchronization, and channel equalization in the receiver. Receiving performance can be improved by performing

Therefore, compared with a receiver that performs time, frequency synchronization acquisition, and channel equalization using only the PN sequence in the time domain, it is possible to overcome the limitations of the system and also to solve problems such as performance degradation.

According to the present invention, reception performance can be improved by inserting a pilot signal in the frequency domain, compared to a conventional TDS-OFDM transmission system.

In addition, by setting the power of the pilot signal inserted in the frequency domain very low, it is possible to be compatible with the existing system.

Although the preferred embodiments of the present invention have been illustrated and described above, the present invention is not limited to the specific embodiments described above, and the present invention is not limited to the specific embodiments of the present invention without departing from the spirit of the present invention as claimed in the claims. Anyone skilled in the art can make various modifications, as well as such modifications are within the scope of the claims.

Claims (10)

An FEC unit for coding an OFDM signal in a frequency domain to detect and correct an error at a receiving side; A mapping unit for mapping the coded OFDM signal in the frequency domain by a predetermined mapping method; A pilot inserter for adding a pilot signal having a size relatively smaller than the average size of the mapped OFDM signal in the frequency domain to the OFDM signal in the frequency domain; An inverse discrete Fourier transform unit for inverse discrete Fourier transform the OFDM signal in the frequency domain into which the pilot signal is inserted and modulate the OFDM signal in the time domain; A guard interval insertion unit inserting a guard interval into the OFDM signal in the time domain; A synchronization information insertion unit for inserting synchronization information for synchronization and channel equalization of a transmitting side and a receiving side into the OFDM signal of the time domain in which the guard interval is inserted; And And a shaping filter unit for shaping the inserted synchronous signal of the OFDM signal in the time domain. The method of claim 1, The OFDM signal in the frequency domain has an I signal and a Q signal, And the pilot signal has an I pilot signal and a Q pilot signal added to the I signal and the Q signal, respectively. The method of claim 2, The magnitude of the I pilot signal is set such that a value accumulated by a predetermined number of the I pilot signal of a predetermined number is larger than an average size of the I signal. And the Q pilot signal has a predetermined number of Q pilot signals which are accumulated by an arbitrary method to be larger than an average size of the Q signals. The method of claim 2, And the time periods of the I pilot signal and the Q pilot signal added to the I signal and the Q signal, respectively, are the same as the time periods of the I signal and the Q signal. The method of claim 1, And the pilot signal is the synchronization information. Coding an OFDM signal in a frequency domain to detect and correct an error at a receiving side; Mapping the coded OFDM signal in the frequency domain by a predetermined mapping method; Adding a pilot signal having a size relatively smaller than the average size of the mapped OFDM signal in the frequency domain to the OFDM signal in the frequency domain; An inverse discrete Fourier transform step of inverse discrete Fourier transforming the OFDM signal in the frequency domain into which the pilot signal is inserted and modulating the OFDM signal in the time domain; Inserting a guard interval into the OFDM signal in the time domain; Inserting synchronization information for synchronization and channel equalization of a transmitting side and a receiving side into the OFDM signal of the time domain in which the guard interval is inserted; And Shaping and filtering the inserted synchronization information of the OFDM signal of the time domain. The method of claim 6, The OFDM signal in the frequency domain has an I signal and a Q signal, And the pilot signal has an I pilot signal and a Q pilot signal added to the I signal and the Q signal, respectively. The method of claim 7, wherein The magnitude of the I pilot signal is set such that a value accumulated by a predetermined number of the I pilot signal of a predetermined number is larger than an average size of the I signal. And the Q pilot signal has a predetermined number of Q pilot signals set by a predetermined method to be larger than an average size of the Q signals. The method of claim 7, wherein And a time period of the I pilot signal and the Q pilot signal added to the I signal and the Q signal, respectively, is equal to the time period of the I signal and the Q signal. The method of claim 6, And the pilot signal is the synchronization information.