WO2007052993A1

WO2007052993A1 - Apparatus for receiving an orthgonal frequency division multiplexing signal

Info

Publication number: WO2007052993A1
Application number: PCT/KR2006/004645
Authority: WO
Inventors: Jong Woong Shin
Original assignee: Lg Electronics Inc.
Priority date: 2005-11-07
Filing date: 2006-11-07
Publication date: 2007-05-10
Also published as: KR100739552B1; KR20070048874A; CN101917383A; CN101305608A; CN101305608B

Abstract

An apparatus for receiving an orthogonal frequency division multiplexing signal is disclosed. An apparatus for receiving an orthogonal frequency division multiplexing (OFDM) signal using a pseudo noise (PN) sequence as a training signal includes a frame synchronization unit for generating a frame synchronous signal of a received signal, a synchronous-signal remover for removing the frame- sync signal contained in a frame of the synchronous received signal and a Discrete Fourier Transform (DFT) unit for receiving the output signal of the synchronous-signal remover, and converting the received signal into a frequency-domain signal. Therefore, the apparatus prevents an OFDM signal from being distorted by a multipath channel, and increases a performance of channel equalization.

Description

APPARATUS FOR RECEIVING AN ORTHGONAL FREQUENCY DIVISION MULTIPLEXING SIGNAL

Technical Field

[1] The present invention relates to an apparatus for receiving orthogonal frequency division multiplexing (OFDM) signals, and more particularly to an apparatus for receiving OFDM signals to prevent the OFDM signals from being distorted by a multipath channel. Background Art

[2] Recently, the Tsinghua University in China has proposed a new communication standard for implementing Chinese terrestrial digital television (hereinafter referred to as a terrestrial DTV) broadcasting. The above-mentioned new communication standard proposed by the Tsinghua University relates to a broadcast specification called a terrestrial digital multimedia/television broadcasting (DMB-T). The DMB-T uses a new modulation scheme called a Time Domain Synchronous - OFDM (TDS-OFDM).

[3] An Inverse Discrete Fourier Transform (IDFT) scheme is applied to transmission data modulated by a transmitter of a TDS-OFDM system in the same manner as in a cyclic prefix OFDM (CP-OFDM) scheme.

[4] A pseudo noise (PN), instead of a cyclic prefix (CP), is inserted into a guard interval, such that it is used as a training signal. The above-mentioned scheme for use of the pseudo noise (PN) reduces an amount of overhead and increases channel-use efficiency during the transmission of broadcast signals, such that it can increase performances of a synchronizer and a channel estimator of a broadcast- signal receiver.

[5] However, unlike the cyclic prefix structure, the signal according to this scheme is configured by inserting a PN sequence into a guard interval . As a result, if the signal is transmitted to a destination over a multi-channel, a data interval of a frame body in the signal is unavoidably distorted by the PN sequence. Disclosure of Invention

Technical Problem

[6] Accordingly, the present invention is directed to an apparatus for receiving an orthogonal frequency division multiplexing (OFDM) signal that substantially obviates one or more problems due to limitations and disadvantages of the related art. An object of the present invention is to provide an apparatus for receiving an OFDM signal, such that it prevents an OFDM signal from being distorted by a multipath channel, and increases a performance of channel equalization. Technical Solution [7] To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, an apparatus for receiving an orthogonal frequency division multiplexing (OFDM) signal capable of using a pseudo noise (PN) sequence as a training signal, the apparatus include a frame synchronization unit for generating a frame synchronous signal (i.e., a frame-sync signal) of a received signal; a synchronous-signal remover for removing the frame-sync signal contained in a frame of the synchronous received signal; and a Discrete Fourier Transform (DFT) unit for receiving the output signal of the synchronous-signal remover, and converting the received signal into a frequency-domain signal.

[8] The synchronous-signal remover may include a PN-sequence generator for generating a frame-sync interval signal contained in the received signal; a channel estimator for calculating a channel characteristic value of the output signal of the PN- sequence generator; a channel-characteristic application unit for calculating the frame- sync interval signal received from the PN-sequence generator and the calculated channel characteristic value, and generating a channel characteristic of the frame-sync interval signal; and a frame-sync remover for generating a signal from which the frame-sync interval signal of the received signal is removed, and using the generated signal as a difference between the output signal of the frame synchronization unit and the output signal of the channel-characteristic application unit.

[9] The channel estimator can estimate a channel impulse response of the synchronous signal generated from the frame synchronization unit.

[10] The channel-characteristic application unit can perform convolution between the output signal of the channel estimator and the frame- sync interval signal generated from the PN-sequence generator, and outputs the convolution result.

[11] The frame-sync interval signal includes 255 bit sequences and cyclic-extended bit sequences of the 255 bit sequences.

[12] The frame-sync interval signal may include 420 bit sequences.

Advantageous Effects

[13] The apparatus for receiving an orthogonal frequency division multiplexing (OFDM) signal according to the present invention has the following effects. The apparatus for receiving an OFDM signal prevents an OFDM signal from being distorted by a multipath channel, and increases a performance of channel equalization. Brief Description of the Drawings

[14] FIG. 1 is a block diagram illustrating a DMB-T transmitter according to the present invention;

[15] FIG. 2 is a structural diagram illustrating an exemplary DMB-T signal frame having a guard interval of 1/9 length mode; [16] FIG. 3 is a block diagram illustrating an apparatus for receiver a TDS-OFDM signal an embodiment of the present invention; [17] FIGS. 4 to 5 are structural diagrams illustrating two OFDM broadcast signals acquired when the same transmission signal is received through a multipath channel; [18] FIG. 6 is a block diagram illustrating an apparatus for receiving a broadcast signal according to an embodiment of the present invention;

[19] FIG. 7 is a structural diagram illustrating an arithmetic structure of a channel- characteristic application unit of the OFDM signal receiver according to an embodiment of the present invention; and [20] FIGS. 8 to 9 are graphs illustrating simulation results for indicating performances of the OFDM signal receiver according to the present invention.

Best Mode for Carrying Out the Invention [21] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. [22] FIG. 1 is a block diagram illustrating a DMB-T transmitter. The DMB-T transmitter will hereinafter be described with reference to FIG. 1. [23] Referring to FIG. 1, a channel encoder 10 outputs a channel-encoded bitstream so as to detect an error in a receiver. [24] A modulator 20 receives the encoded bitstream, and modulates the received bitstream according to a 4-ary, 16-ary or 64-ary quadrature amplitude modulation

(QAM) scheme, etc.

[25] An Inverse Discrete Fourier Transform (IDFT) unit 30 modulates a frequency- domain OFDM modulation signal into a time-domain OFDM signal. Generally, the

DMB-T scheme converts a frequency-domain signal associated with 3780 points of transmission data into a time-domain signal. [26] A Pseudo Noise (PN) generator 40 generates a PN sequence to be used as a training signal of a broadcast signal to be transmitted. [27] A multiplexer 50 distributes of the OFDM signal received from the IDFT unit 30 and the generated PN sequence in a time domain, multiplexes the distributed signals, and outputs the multiplexed signals. [28] A filter unit 60 limits a bandwidth of the multiplexed DMB-T signal, and outputs the limited DMB-T signal. The filter unit 60 includes a filter, and a square root raised cosine (SRRC) filter may be used as the filter. A specific value of 0.05 may be used as a roll-off factor (α) for limiting a bandwidth of the filter. [29] A radio frequency (RF) transmitter 70 converts the output signal having the limited bandwidth into a RF transmission band of a frequency (f ), and transmits the converted broadcast signal. [30] FIG. 2 is a structural diagram illustrating an exemplary DMB-T signal frame having a guard interval of 1/9 length mode according to the present invention. A transmission frame structure having the guard interval 1/9 will hereinafter be described with reference to FIG. 2.

[31] Referring to FIG. 2, the frame includes a frame sync and a frame body.

[32] The frame body includes data to be transmitted, and acts as a DFT (Discrete Fourier

Transform) block. The DFT block generally includes 3780 stream data units.

[33] The frame sync includes a PN sequence. The PN sequence for use in the frame sync may use a sequence having an order value of 8 (i.e., m=8). If the order value (m) is set to 8, 255 sequences different from each other may occur. Each sequence is extended to a preamble and a postamble such that it can be used for a guard interval.

[34] The preamble and the postamble correspond to a PN-sequence repeating interval for a cyclic extension of the PN sequence.

[35] Initial 115 PNs (Pseudo Noises) of the PN sequence from among 255 PN sequences of the frame sync are added to the end of the 255 PN sequences as the postamble. The last 50 PNs of the PN sequence are added to the header of the 255 PN sequence as the preamble, and are then extended.

[36] A polynomial of the PN sequence is denoted by P(x) = x + x + x + x + 1. Thus, the phase generated as an initial status of the PN sequence is changed from 0 to 254.

[37] If the guard interval is set to 1/9 length mode, the preamble is added to a front part of the 255 PNs and the postamble is added to a rear part of the 255 PNs, such that the frame sync composed of 420 data units may be configured. In other words, the number of data units corresponding to 1/9 of 3780 data units of the DFT block is 420, such that 420 data units are used for the frame sync. A single OFDM frame includes a frame sync composed of 420 data units and a frame body composed of 3780 data units.

[38] The data frame structure may be changed according to a length mode of the guard interval, and the number of data units of each frame may be changed to another number as necessary.

[39] The length of guard interval is set to a specific lenght mode, such as 1/4 length mode, 1/9 length mode, and so on. If required, the guard- interval value may also be set to another value 1/6 length mode. Therefore, the guard interval length may be changed to another length according to the system specification.

[40] FIG. 3 is a block diagram illustrating an apparatus for receiving a TDS-OFDM signal according to the present invention. The apparatus for receiving the TDS-OFDM signal will hereinafter be described with reference to FIG. 3.

[41] Referring to FIG. 3, a tuner 110 of the apparatus for receiving the TDS-OFDM signal converts a RF-band signal into a baseband signal.

[42] An automatic gain controller (AGC) 120 normalizes power of the baseband signal received from the tuner 110, and outputs the power-normalized analog signal.

[43] An analog-to-digital (AD) converter 130 converts the power- normalized analog signal received from the AGC 120 into a digital signal.

[44] A phase splitter 140 separates an inphase signal (i.e., I signal) and a quadrature signal (i.e., Q signal) from the output signal of the AD converter 130, and outputs the separated I and Q signals.

[45] An automatic frequency control (AFC) unit 177 compensates for estimated frequency errors of the I and Q signals. A filter unit 160 acts as a filter capable of limiting a bandwidth of the received signal in the same manner as in the transmitter. For example, the SRRC filter may be used as the filter unit 160.

[46] A frame synchronization unit can be mainly divided into three parts, i.e., an AFC unit 177, a signal acquisition unit 172, and a signal tracking unit 174.

[47] The AFC unit 177 calculates a frequency error of the received signal, receives the product of the received signal and the frequency-error signal from a multiplier 145 and compensates for the frequency error of the received signal.

[48] The signal acquisition unit 172 synchronizes the PN sequence of a signal received from the transmitter.

[49] The signal tracking unit 174 compensates for a symbol error using the PN sequence synchronized by the signal acquisition unit 172.

[50] The frame synchronization unit of the received signal uses the correlation result of a

PN correlator 171.

[51] The output data of the frame synchronization unit is converted into a frequency- domain signal by a Fast Fourier Transform (FFT) of first and second DFT units 180 and 182, is channel-estimated by the equalizer 190, and is then outputted to a channel decoder (not shown).

[52] A frame structure of a signal having a guard interval of 1/9 length mode from among two OFDM broadcast signals received through a multipath channel is shown in FIGS. 4 to 5. For the convenience of description, it is assumed that a signal received through a second path from among the multipath is delayed by a specific frame length "L" on the basis of another signal received through a first path from among the multipath. It is assumed that an interval "L" from among the signal received through the first path is denoted by "A" interval.

[53] Under the aforementioned assumptions, FIG. 4 shows an exemplary case in which a signal received through the first path and a signal received through the second path include the frame sync intervals, respectively.

[54] Referring to FIG. 4, a symbol contained in the signal received through the second path is delayed by a predetermined length "L" on the basis of a symbol corresponding to the first path. Therefore, if the signals received through the first and second paths are DFT-processed at a beginning interval of the frame body of the signal received through the first path, and channel compensation is performed, the signal received through the second path includes the PN sequence of the frame sync, such that unexpected distortion occurs in the "A" interval of the signal received through the first path.

[55] Under the aforementioned assumptions, FIG. 5 shows an exemplary case in which the frame sync interval is removed from the broadcast signals received through the first and second paths.

[56] Referring to FIG. 5, although the second-path broadcast signal is delayed by the frame length "L", the frame sync interval is removed from the delayed second-path broadcast signal, such that the "A" interval of the first-path channel signal is not affected by the PN sequence of the second-path channel signal on the condition that the DFT process is performed on the above-mentioned resultant signal having no frame sync interval.

[57] Therefore, the embodiment of the present invention provides an apparatus for receiving an OFDM signal capable of removing a frame-sync interval from an OFDM broadcast signal using the PN sequence as a training signal.

[58] FIG. 6 is a block diagram illustrating an apparatus for receiving an OFDM signal according to an embodiment of the present invention.

[59] Referring to FIG. 6, an apparatus for receiving an OFDM signal according to an embodiment of the present invention may include a frame synchronization unit 170 and a synchronous-signal remover. The synchronous-signal remover may include a PN-sequence generator 210, a channel estimator 220, a channel-characteristic application unit 230, and a frame-sync remover 240.

[60] The frame synchronization unit 170 can search for a beginning point of the frame body by using PN correlation characteristics as for a transmission signal using the PN sequence as a training signal.

[61] The channel estimator 220 estimates a channel impulse response in a frame-sync interval from among the synchronous signal of the frame synchronization unit 170, sue h that it can calculate a channel-characteristic value.

[62] The PN-sequence generator 210 may generate a PN sequence contained in the guard interval of the output signal of the frame synchronization unit 170.

[63] The channel-characteristic application unit 230 applies the channel characteristic value estimated by the channel estimator 220 to the PN sequence received from the PN-sequence generator 210. In order to apply the channel-characteristic value to the PN sequence, The channel-characteristic application unit 230 may operates convolution between the channel impulse response of the channel estimator 220 and the PN sequence. [64] The convolution can be represented by the following Math Figure 1 :

[65] MathFigure 1

[66] In Math Figure 1, L is indicative of a channel order, p(n) is indicative of a PN sequence, h(n) is indicative of a channel impulse response, and y(n) is indicative of an output signal of the channe characteristic application unit 230.

[67] The frame-sync remover 240 outputs a signal from which a frame-sync-interval signal including a PN sequence of the broadcast signal received through a multipath channel is removed. The output signal of the frame-sync remover 240 is denoted by z(n) of FIG. 6.

[68] The frame-sync remover 240 subtracts the output signal y(n) of the channel- characteristic application unit 230 from the output signal of the frame synchronization unit 170. As a result, the frame-sync interval of the output signal of the frame synchronization unit 170 is set to 0, and the frame-sync remover 240 generates a signal having only the frame body interval, and outputs the generated signal to the DFT unit 180.

[69] If the guard interval of the OFDM signal is set to 1/9 length mode, calculation performed by the frame-sync remover 240 contained in the embodiment of the present invention can be represented by the following Math Figure 2:

[70] MathFigure 2

(where n < G/(420)+L)

here n ≥ G/(420)+L)

[71] In Math Figure 2, x(n) is indicative of the output signal of the frame synchronization unit 170, GI is indicative of a guard interval, and L is indicative of a delayed interval of the frame. The value of n is in the range from 0 to 4199 under the a guard interval of 1/9 length mode.

[72] FIG. 7 is a diagram illustrating an arithmetic structure of a channel-characteristic application unit 230 in the apparatus for an OFDM signal according to an embodiment of the present invention.

[73] Referring to FIG. 7, p(n) is indicative of the output signal of the PN-sequence generator 210, and y(n) is indicative of the output signal of the channel-characteristic application unit 230. And, h(n) is indicative of the output signal of the channel estimator, h (n) is indicative of an inphase signal, and h (n) is indicative of a quadrature signal. In this case, the output signal y(n) can be represented by the following Math Figure 3: [74] MathFigure 3 y{n) = _yi (ή) + jy_Q {n) = (H₁ + j h_Q ){\ + j) = (A₇ - h_Q ) + j{h_} + h_Q )

[75] The PN sequence p(n) is sequentially transmitted to at least one delay (D) shown in

FIG. 7. The output value of the delay (D) is multiplied by h (n) (where n = 0 ~ N-I, N = integer) in a multiplier, such that the resultant inphase signal

H₁ is acquired. The output value of the delay (D) is multiplied by h (n) (where n = 0 ~ N- 1, N = integer) in the multiplier, such that the resultant quadrature signal h_Q is acquired. If the value of

H₁ and the value of

are summed up, the value of y (n) can be acquired. Otherwise, if the value of

is subtracted from the value of

A₇

, the value of y (n) can be acquired. [76] FIGS. 8 to 9 are graphs illustrating simulation results for indicating performances of an apparatus for receiving an OFDM signal according to an embodiment of the present invention. [77] In more detail, bit error rate (BER) results acquired when the frame-sync interval signal including a PN sequence is removed from the OFDM signal received through two paths are represented by a signal-to-noise ratio (SNR) in FIGS. 8 to 9. It should be noted that a modulation method of the transmitter of FIGS. 8 to 9 is an uncoded

64-QAM (Quadrature Amplitude Modulation). [78] The BER result of FIG. 8 indicates that a first path has a specific value 0 dB and no symbol delay, and also indicates that a second path has a specific value 0 dB simultaneously with maintaining one symbol delay (i.e., symbol delay static).

[79] The BER result of FIG. 9 indicates that a first path has a specific value 0 dB and no symbol delay, and also indicates that a second path has a specific value 0 dB simultaneously with maintaining one hundred symbol delay (i.e., symbol delay static).

[80] In FIGS. 8 to9, the line including several circles represents the result acquired by a signal having no PN sequence, and another line including several triangles represents the result acquired by a signal from which the PN sequence is not removed. As can be seen from FIGS. 8 to 9, the BER result of the first case in which the PN sequence is removed is superior to that of the second case in which the PN sequence is not removed. Compared with the BER result of FIG. 8, the BER result of FIG. 9 is more prominent than the BER result of FIG. 8 in proportion to the length of a delay spread.

[81] The broadcasting signal according to TDS-OFDM scheme is not configured in the form of a cyclic prefix, such that the PN sequence must be removed from the signal to eliminate the influence of the PN sequence. If the PN sequence in the signal is removed, Os (zeros) are inserted in a guard interval of the signal, such that a performance of the channel equalizer can be improved. Mode for the Invention

[82] Various embodiments of the present invention can be described with the above- described Best Mode together. Industrial Applicability

[83] The above embodiments are disclosed for some examples, and it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention.

Claims

[1] An apparatus for receiving an orthogonal frequency division multiplexing

(OFDM) signal using a pseudo noise (PN) sequence as a training signal, the apparatus comprising: a frame synchronization unit generating a frame synchronous signal of a received signal; a synchronous-signal remover removing the frame-sync signal contained in a frame of the synchronous received signal; and a Discrete Fourier Transform (DFT) unit receiving the output signal of the synchronous-signal remover, and converting the received signal into a frequency-domain signal.

[2] The apparatus of claim 1, wherein the synchronous-signal remover comprises: a PN- sequence generator generating a frame- sync interval signal contained in the received signal; a channel estimator calculating a channel characteristic value of the output signal of the PN-sequence generator; a channel-characteristic application unit calculating the frame-sync interval signal received from the PN-sequence generator and the calculated channel characteristic value, and generating a channel characteristic of the frame-sync interval signal; and a frame-sync remover generating a difference between the output signal of the frame synchronization unit and the output signal of the channel-characteristic application unit, and generating a signal from which the frame-sync interval signal of the received signal is removed, and using the difference.

[3] The apparatus of claim 2, wherein the channel estimator estimates a channel impulse response of the synchronous signal generated from the frame synchronization unit.

[4] The apparatus of claim 2, wherein the channel-characteristic application unit performs convolution between the output signal of the channel estimator and the frame-sync interval signal generated from the PN-sequence generator, and outputs the convolution result.

[5] The apparatus of claim 1, wherein the frame-sync interval signal includes 255 data units and cyclic-extension data units of the 255 bits.

[6] The apparatus of claim 1, wherein the frame-sync interval signal includes 420 data units.