EP2087686A1 - Hierarchical modulation apparatus and method using pilot signal, and apparatus and method for receiving hierarchically modulated signals - Google Patents

Hierarchical modulation apparatus and method using pilot signal, and apparatus and method for receiving hierarchically modulated signals

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Publication number
EP2087686A1
EP2087686A1 EP07833781A EP07833781A EP2087686A1 EP 2087686 A1 EP2087686 A1 EP 2087686A1 EP 07833781 A EP07833781 A EP 07833781A EP 07833781 A EP07833781 A EP 07833781A EP 2087686 A1 EP2087686 A1 EP 2087686A1
Authority
EP
European Patent Office
Prior art keywords
sub
signal
carrier signals
pilot signal
respect
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP07833781A
Other languages
German (de)
French (fr)
Other versions
EP2087686A4 (en
Inventor
Hyoungsoo Lim
So-Ra Park
Sung-Hoon Kim
Jae-Hwui Bae
Jong-Soo Lim
Soo-In Lee
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Electronics and Telecommunications Research Institute ETRI
Original Assignee
Electronics and Telecommunications Research Institute ETRI
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Electronics and Telecommunications Research Institute ETRI filed Critical Electronics and Telecommunications Research Institute ETRI
Publication of EP2087686A1 publication Critical patent/EP2087686A1/en
Publication of EP2087686A4 publication Critical patent/EP2087686A4/en
Withdrawn legal-status Critical Current

Links

Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2602—Signal structure
    • H04L27/2604—Multiresolution systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/36—Modulator circuits; Transmitter circuits
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/022—Channel estimation of frequency response
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L25/00—Baseband systems
    • H04L25/02—Details ; arrangements for supplying electrical power along data transmission lines
    • H04L25/0202—Channel estimation
    • H04L25/0224—Channel estimation using sounding signals
    • H04L25/0228—Channel estimation using sounding signals with direct estimation from sounding signals
    • H04L25/023—Channel estimation using sounding signals with direct estimation from sounding signals with extension to other symbols
    • H04L25/0236—Channel estimation using sounding signals with direct estimation from sounding signals with extension to other symbols using estimation of the other symbols
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
    • H04L27/20—Modulator circuits; Transmitter circuits
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems

Definitions

  • the present invention relates to an orthogonal frequency division multiplexing
  • OFDM orthogonal frequency division multiplexing
  • Hierarchical modulation is a method of synchronously modulating two or more signals modulated differently through different modulating methods, combining the synchronously modulated signals and transmitting the combined signals.
  • a base layer signal is a signal transmitted in a conventional system and an enhancement layer signal is a signal for providing services added to the base layer signal.
  • a receiver requires an in-phase demodulation method in order to receive and demodulate the hierarchically modulated signals. For this, a corresponding transmitter is required to transmit an additional pilot signal. Disclosure of Invention Technical Problem
  • the present invention provides a method and apparatus for inserting a pilot signal into an enhancement layer signal, modulating the enhancement layer signal and demodulating the modulated signal in order to ensure in-phase demodulation performance for hierarchically modulated signals.
  • a hierarchical modulation apparatus comprising: a first encoder encoding a first layer signal sequence; a second encoder encoding a second layer signal sequence; a pilot inserting unit inserting a pilot signal into the signal sequence encoded by the second encoder; and a hierarchical modulator modulating the signals output from the first encoder and the pilot inserting unit and combining the modulated signals.
  • a hierarchical modulation method comprising: encoding a first layer signal sequence; encoding a second layer signal sequence; inserting a pilot signal into the encoded second layer signal sequence; and modulating signal sequences output from the encoding a first layer signal sequence and the inserting a pilot signal and combining the modulated signal sequences.
  • a receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated comprising: a channel estimator estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; an equalizer outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; a decision unit deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; a channel estimation updating unit re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results and updating the estimated channel values using the re-estimated sub-carrier channel values; and a demodulator equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
  • a method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; updating the estimated channel values using the re-estimated sub-carrier channel values; and equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
  • a method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; replacing the estimated channel values with respect to carrier signals corresponding to the sub-carrier signals of the pilot signal with the re-estimated sub-carrier channel values; giving correlation to channel estimation values with respect to neighboring sub-carrier signals using the replaced sub-carrier channel values to update the estimated channel values of the sub- carrier signals; equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and de
  • a receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated comprising: a layer separator separating the first layer signal sequence and the second layer signal sequence from the received signal; a first layer demodulator demodulating the first layer signal sequence; and a second layer demodulator estimating sub-carrier signal channels with respect to the pilot signal, equalizing sub-carrier signal channels other than the sub-carrier signal channels with respect to the pilot signal using the estimated channel values and demodulating the second layer signal sequence.
  • OFDM based communication or broadcasting systems employing hierarchical modulation can additionally transmit a pilot signal while maintaining compatibility with the existing systems to improve receiving performance.
  • FIG. 1 is a block diagram of a hierarchical modulation apparatus according to an embodiment of the present invention.
  • FIG. 2 illustrates exemplified constellation of a sub-carrier signal of an enhancement layer signal sequence and a sub-carrier signal of a pilot signal inserted into the enhancement layer signal sequence in a hierarchically modulated ⁇ /4-DQPSK signal;
  • FIG. 3 illustrates exemplified periodic insertion of a pilot signal into a enhancement layer signal sequence
  • FIG. 4 is a block diagram of a receiver for receiving hierarchically modulated signals according to an embodiment of the present invention
  • FIG. 5 is a flow chart of a process of demodulating hierarchically modulated signals according to an embodiment of the present invention.
  • FIG. 6 is a flow chart of an operation 54 illustrated in FIG. 5 according to another embodiment of the present invention.
  • FIG. 7 is a block diagram of a receiver for receiving hierarchically modulated signals according to another embodiment of the present invention.
  • Mode for Invention
  • FIG. 1 is a block diagram of a hierarchical modulation apparatus according to an embodiment of the present invention.
  • the hierarchical modulation apparatus includes a first encoder 10, a second encoder 11, a pilot inserting unit 12 and a hierarchical modulator 13.
  • the first encoder 10 encodes a base layer signal sequence and the second encoder 11 encodes an enhancement layer signal sequence.
  • an encoding method convolution encoding, trellis encoding, turbo encoding, low density parity check (LDPC) encoding or concatenated encoding that concatenates two or more of these encoding methods can be employed.
  • LDPC low density parity check
  • the pilot inserting unit 12 inserts a pilot signal into the encoded enhancement layer signal sequence.
  • the pilot signal will be described later.
  • the hierarchical modulator 13 modulates the signals output from the first encoder 10 and the pilot inserting unit 12 and maps the modulated signals to corresponding signal spaces.
  • the base layer signal may be mapped to a quadrature phase-shift keying (QPSK) signal space and the enhancement layer signal may be mapped to a binary phase-shift keying (BPSK) signal space.
  • QPSK quadrature phase-shift keying
  • BPSK binary phase-shift keying
  • FIG. 2 illustrates exemplified constellation of a sub-carrier of an enhancement layer signal sequence and a sub-carrier of a pilot signal inserted into the enhancement layer signal sequence when hierarchical modulation is applied to ⁇
  • /4-DQPSK Differential Quadrature Phase-Shift Keying
  • DAB digital audio broadcasting
  • DMB digital multimedia broadcasting
  • squares represent each sub-carrier signal of odd-numbered OFDM symbols beginning from a phase reference symbol (PRS), which is transmitted earlier than any other symbol of the DAB transmission frames.
  • Circles represent each sub-carrier signal of even-numbered OFDM symbols beginning from the PRS. Hatched squares or circles represent pilot signals.
  • a pilot signal is required for channel estimation and equalization for in-phase demodulation in terms of characteristic of hierarchical modulation. In the hierarchical modulation can only an enhancement layer signal be changed without changing a ⁇
  • the pilot signal in order to ensure compatibility with existing systems, and thus the pilot signal can be inserted into only the enhancement layer signal.
  • the pilot signal has a modulated form of random data that is not previously agreed between a transmitting side and a receiving side instead of a form used in most communication or broadcasting systems.
  • the pilot signal has power lower than that of a base layer signal. Accordingly, in order to obtain appropriate channel estimation performance using the pilot signal at a receiving side, a decision directed channel estimation manner is required, which detects a base layer signal and uses it for channel estimation.
  • the pilot signal to be transmitted through an enhancement layer signal has correlation with a base layer signal having the same sub- carrier signal as the pilot signal.
  • the most preferable case is illustrated in FIG. 2. That is, if a base layer signal corresponding to a sub-carrier signal through which the pilot signal is transmitted among even-numbered OFDM symbol periods from the PRS is placed in the second quarter plane, transmission of the signal corresponding to the constellation point 21 as a pilot signal has an identical effect with increase of the transmission power of the base layer signal and results in lowering decision errors of the base layer signal.
  • Another example of selecting a pilot signal is to increase the transmission power of the base layer signal corresponding to a sub-carrier signal through which the pilot signal is transmitted by a predetermined value or a predetermined ratio.
  • the enhancement layer signal may not be transmitted.
  • the enhancement layer signal functions as noise due to its relatively low power compared with the power of the base layer signal.
  • channel estimation performance can be improved by increasing the transmission power of the base layer signal and not transmitting the enhancement layer signal.
  • the increased value or increased ratio of the transmission power may not coincide with one of the constellation points of the sub-carrier signals through which the pilot signal is not transmitted as shown in FIG.2. This also occurs when the base layer signal is transmitted with the original transmission power and the enhancement layer signal is not transmitted.
  • Still another example of selecting a pilot signal is to use a base layer signal having power lowered by a predetermined value or having an amplitude lowered by a predetermined ratio as the pilot signal.
  • the pilot sub-carrier signal having the above-described constellation can be pe- riodically inserted into an enhancement layer signal and transmitted, as illustrated in FIG. 3.
  • the pilot signal is periodically inserted for every ten sub- carriers.
  • the pilot signal can also be non-periodically inserted. For example, when the length of data to be actually transmitted in an enhancement layer signal sequence is shorter than a maximum allowed length, the pilot signal can be inserted into an empty region after data.
  • FIG. 4 is a block diagram of a receiver for receiving hierarchically modulated signals according to an embodiment of the present invention.
  • the receiver includes a channel estimator 40, an equalizer 41, a decision unit 42, a channel estimation updating unit 43, and a hierarchical demodulator 44.
  • FIG. 5 is a flow chart of a process of demodulating hierarchically modulated signals according to an embodiment of the present invention. The operation of the receiver illustrated in FIG. 4 will now be explained with reference to FIG. 5.
  • the channel estimator 40 estimates channels for all sub-carriers using PRS of a DAB frame, which is first transmitted such as a preamble included in a received signal in operation 50.
  • the PRS is a signal of a fixed transmission pattern, and thus channel estimation values can be easily obtained from the PRS signal.
  • the channel can be estimated using any one of conventional channel estimation methods including the least square estimation and the maximum likelihood estimation.
  • the equalizer 41 compensates for channel distortion with respect to sub-carrier signals corresponding to pilot signals in currently received OFDM symbols using the channel values estimated by the channel estimator 40 in operation 51.
  • the decision unit 42 decides constellation points corresponding to the sub-carrier signals of the pilot signals on the basis of the constellation as illustrated in FIG. 2 in operation 52.
  • the channel estimation updating unit 43 re-estimates the channels of the pilot sub-carrier signals based on the decision results and reflects the re-estimated channel values to update the estimated channel values of all the sub-carrier signals, which are estimated in operation 50, through an interpolation etc.
  • the linear signal process can employ a moving average method and the nonlinear signal process can employ a median filtering method.
  • the hierarchical demodulator 44 demodulates in-phase data sub-carrier signals of
  • OFDM symbols other than the pilot sub-carrier signals using the channel values updated by the channel estimation updating unit 43 in operation 55.
  • FIG. 6 is a flow chart of the operation 54 illustrated in FIG. 5 according to another embodiment of the present invention.
  • operation 60 channel estimation values with respect to carrier signals corresponding to the pilot sub-carrier signals are replaced with decision-directed channel estimation values.
  • the channel estimation noise can be attenuated by processing the replaced values linearly or nonlinearly as described above and then channel estimation with respect to sub-carrier signals can be updated by giving correlation to channel estimation values of neighboring sub-carrier signals in operation 61.
  • FIG. 7 is a block diagram of a receiver for receiving hierarchically modulated signals according to another embodiment of the present invention.
  • the receiver includes a layer separator 71, a base layer demodulator 72, a channel estimator 73, an equalizer 74, a decision unit 75, a channel estimation updating unit 76, and an enhancement layer demodulator 77.
  • the layer separator 71 separates a base layer signal sequence and an enhancement layer signal sequence from a received signal.
  • the base layer demodulator 72 demodulates the base layer signal sequence according to a demodulation method corresponding to a modulation method of a transmitting side.
  • the enhancement layer signal sequence is demodulated as follows.
  • the channel estimator 73 estimates channels for all sub-carrier signals using PRS included in the received signal.
  • the PRS is a signal having a fixed transmission pattern, and thus channel estimation values can be easily obtained from the PRS signal.
  • the equalizer 74 compensates for channel distortion with respect to sub-carrier signals corresponding to pilot signals in currently received OFDM symbols using the channel values estimated by the channel estimator 73.
  • the decision unit 75 decides constellation points corresponding to the sub-carrier signals of the pilot signals on the basis of the constellation as illustrated in FIG. 2.
  • the channel estimation updating unit 76 re-estimates the channels of the pilot sub- carrier signals based on the decision results and and reflects the re-estimated channel values to update the estimated channel values of all the sub-carrier signals through an interpolation etc.
  • the linear signal process can employ a moving average method and the nonlinear signal process can employ a median filtering method.
  • the enhancement layer signal demodulator 77 demodulates in-phase data sub- carriers signals of OFDM symbols other than the pilot sub-carrier signals using the channel values updated by the channel estimation updating unit 76.
  • the present invention can also be embodied as computer readable codes on a computer readable recording medium.
  • the computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet).
  • ROM read-only memory
  • RAM random-access memory
  • CD-ROMs compact discs
  • magnetic tapes magnetic tapes
  • floppy disks optical data storage devices
  • carrier waves such as data transmission through the Internet

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Power Engineering (AREA)
  • Digital Transmission Methods That Use Modulated Carrier Waves (AREA)
  • Circuits Of Receivers In General (AREA)

Abstract

Provided are hierarchical modulation apparatus and method using a pilot signal and an apparatus and method for receiving a hierarchically modulated signal. The hierarchical modulation apparatus comprises a first encoder encoding a first layer signal sequence, a second encoder encoding a second layer signal sequence, a pilot inserting unit inserting a pilot signal into the signal sequence encoded by the second encoder, and a hierarchical modulator modulating the signals output from the first encoder and the pilot inserting unit and combining the modulated signals.

Description

Description
HIERARCHICAL MODULATION APPARATUS AND METHOD USING PILOT SIGNAL, AND APPARATUS AND METHOD FOR
RECEIVING HIERARCHICALLY MODULATED SIGNALS
Technical Field
[1] The present invention relates to an orthogonal frequency division multiplexing
(OFDM) transmission system, and more particularly, to an apparatus and method for hierarchical modulation or demodulation using a pilot signal in an OFDM system. Background Art
[2] Hierarchical modulation is a method of synchronously modulating two or more signals modulated differently through different modulating methods, combining the synchronously modulated signals and transmitting the combined signals. In the hierarchical modulation, a base layer signal is a signal transmitted in a conventional system and an enhancement layer signal is a signal for providing services added to the base layer signal. A receiver requires an in-phase demodulation method in order to receive and demodulate the hierarchically modulated signals. For this, a corresponding transmitter is required to transmit an additional pilot signal. Disclosure of Invention Technical Problem
[3] The present invention provides a method and apparatus for inserting a pilot signal into an enhancement layer signal, modulating the enhancement layer signal and demodulating the modulated signal in order to ensure in-phase demodulation performance for hierarchically modulated signals. Technical Solution
[4] According to an aspect of the present invention, there is provided a hierarchical modulation apparatus comprising: a first encoder encoding a first layer signal sequence; a second encoder encoding a second layer signal sequence; a pilot inserting unit inserting a pilot signal into the signal sequence encoded by the second encoder; and a hierarchical modulator modulating the signals output from the first encoder and the pilot inserting unit and combining the modulated signals.
[5] According to another aspect of the present invention, there is provided a hierarchical modulation method comprising: encoding a first layer signal sequence; encoding a second layer signal sequence; inserting a pilot signal into the encoded second layer signal sequence; and modulating signal sequences output from the encoding a first layer signal sequence and the inserting a pilot signal and combining the modulated signal sequences. [6] According to still another aspect of the present invention, there is provided a receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: a channel estimator estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; an equalizer outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; a decision unit deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; a channel estimation updating unit re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results and updating the estimated channel values using the re-estimated sub-carrier channel values; and a demodulator equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[7] According to still another aspect of the present invention, there is provided a method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; updating the estimated channel values using the re-estimated sub-carrier channel values; and equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[8] According to still another aspect of the present invention, there is provided a method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; replacing the estimated channel values with respect to carrier signals corresponding to the sub-carrier signals of the pilot signal with the re-estimated sub-carrier channel values; giving correlation to channel estimation values with respect to neighboring sub-carrier signals using the replaced sub-carrier channel values to update the estimated channel values of the sub- carrier signals; equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[9] According to still another aspect of the present invention, there is provided a receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: a layer separator separating the first layer signal sequence and the second layer signal sequence from the received signal; a first layer demodulator demodulating the first layer signal sequence; and a second layer demodulator estimating sub-carrier signal channels with respect to the pilot signal, equalizing sub-carrier signal channels other than the sub-carrier signal channels with respect to the pilot signal using the estimated channel values and demodulating the second layer signal sequence. Advantageous Effects
[10] According to the present invention, OFDM based communication or broadcasting systems employing hierarchical modulation can additionally transmit a pilot signal while maintaining compatibility with the existing systems to improve receiving performance. Description of Drawings
[11] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:
[12] FIG. 1 is a block diagram of a hierarchical modulation apparatus according to an embodiment of the present invention;
[13] FIG. 2 illustrates exemplified constellation of a sub-carrier signal of an enhancement layer signal sequence and a sub-carrier signal of a pilot signal inserted into the enhancement layer signal sequence in a hierarchically modulated π /4-DQPSK signal;
[14] FIG. 3 illustrates exemplified periodic insertion of a pilot signal into a enhancement layer signal sequence;
[15] FIG. 4 is a block diagram of a receiver for receiving hierarchically modulated signals according to an embodiment of the present invention;
[16] FIG. 5 is a flow chart of a process of demodulating hierarchically modulated signals according to an embodiment of the present invention;
[17] FIG. 6 is a flow chart of an operation 54 illustrated in FIG. 5 according to another embodiment of the present invention; and
[18] FIG. 7 is a block diagram of a receiver for receiving hierarchically modulated signals according to another embodiment of the present invention. Mode for Invention
[19] The present invention will now be described more fully with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the invention to those skilled in the art. Throughout the drawings, like reference numerals refer to like elements.
[20] FIG. 1 is a block diagram of a hierarchical modulation apparatus according to an embodiment of the present invention. Referring to FIG. 1, the hierarchical modulation apparatus includes a first encoder 10, a second encoder 11, a pilot inserting unit 12 and a hierarchical modulator 13.
[21] The first encoder 10 encodes a base layer signal sequence and the second encoder 11 encodes an enhancement layer signal sequence. As an encoding method, convolution encoding, trellis encoding, turbo encoding, low density parity check (LDPC) encoding or concatenated encoding that concatenates two or more of these encoding methods can be employed.
[22] The pilot inserting unit 12 inserts a pilot signal into the encoded enhancement layer signal sequence. The pilot signal will be described later.
[23] The hierarchical modulator 13 modulates the signals output from the first encoder 10 and the pilot inserting unit 12 and maps the modulated signals to corresponding signal spaces. For example, the base layer signal may be mapped to a quadrature phase-shift keying (QPSK) signal space and the enhancement layer signal may be mapped to a binary phase-shift keying (BPSK) signal space.
[24] FIG. 2 illustrates exemplified constellation of a sub-carrier of an enhancement layer signal sequence and a sub-carrier of a pilot signal inserted into the enhancement layer signal sequence when hierarchical modulation is applied to π
/4-DQPSK (Differential Quadrature Phase-Shift Keying) terrestrial digital audio broadcasting (DAB) or digital multimedia broadcasting (DMB) system. In FIG. 2, squares represent each sub-carrier signal of odd-numbered OFDM symbols beginning from a phase reference symbol (PRS), which is transmitted earlier than any other symbol of the DAB transmission frames. Circles represent each sub-carrier signal of even-numbered OFDM symbols beginning from the PRS. Hatched squares or circles represent pilot signals. [25] A pilot signal is required for channel estimation and equalization for in-phase demodulation in terms of characteristic of hierarchical modulation. In the hierarchical modulation can only an enhancement layer signal be changed without changing a π
/4-DQPSK signal in order to ensure compatibility with existing systems, and thus the pilot signal can be inserted into only the enhancement layer signal. In this case, the pilot signal has a modulated form of random data that is not previously agreed between a transmitting side and a receiving side instead of a form used in most communication or broadcasting systems. In addition, the pilot signal has power lower than that of a base layer signal. Accordingly, in order to obtain appropriate channel estimation performance using the pilot signal at a receiving side, a decision directed channel estimation manner is required, which detects a base layer signal and uses it for channel estimation.
[26] Therefore, it is preferable that the pilot signal to be transmitted through an enhancement layer signal has correlation with a base layer signal having the same sub- carrier signal as the pilot signal. The most preferable case is illustrated in FIG. 2. That is, if a base layer signal corresponding to a sub-carrier signal through which the pilot signal is transmitted among even-numbered OFDM symbol periods from the PRS is placed in the second quarter plane, transmission of the signal corresponding to the constellation point 21 as a pilot signal has an identical effect with increase of the transmission power of the base layer signal and results in lowering decision errors of the base layer signal.
[27] Another example of selecting a pilot signal is to increase the transmission power of the base layer signal corresponding to a sub-carrier signal through which the pilot signal is transmitted by a predetermined value or a predetermined ratio. In this case, the enhancement layer signal may not be transmitted. More specifically, the enhancement layer signal functions as noise due to its relatively low power compared with the power of the base layer signal. Thus, channel estimation performance can be improved by increasing the transmission power of the base layer signal and not transmitting the enhancement layer signal. In this case, the increased value or increased ratio of the transmission power may not coincide with one of the constellation points of the sub-carrier signals through which the pilot signal is not transmitted as shown in FIG.2. This also occurs when the base layer signal is transmitted with the original transmission power and the enhancement layer signal is not transmitted.
[28] Still another example of selecting a pilot signal is to use a base layer signal having power lowered by a predetermined value or having an amplitude lowered by a predetermined ratio as the pilot signal.
[29] The pilot sub-carrier signal having the above-described constellation can be pe- riodically inserted into an enhancement layer signal and transmitted, as illustrated in FIG. 3. Referring to FIG. 3, the pilot signal is periodically inserted for every ten sub- carriers.
[30] The pilot signal can also be non-periodically inserted. For example, when the length of data to be actually transmitted in an enhancement layer signal sequence is shorter than a maximum allowed length, the pilot signal can be inserted into an empty region after data.
[31] FIG. 4 is a block diagram of a receiver for receiving hierarchically modulated signals according to an embodiment of the present invention. Referring to FIG. 4, the receiver includes a channel estimator 40, an equalizer 41, a decision unit 42, a channel estimation updating unit 43, and a hierarchical demodulator 44.
[32] FIG. 5 is a flow chart of a process of demodulating hierarchically modulated signals according to an embodiment of the present invention. The operation of the receiver illustrated in FIG. 4 will now be explained with reference to FIG. 5.
[33] The channel estimator 40 estimates channels for all sub-carriers using PRS of a DAB frame, which is first transmitted such as a preamble included in a received signal in operation 50. The PRS is a signal of a fixed transmission pattern, and thus channel estimation values can be easily obtained from the PRS signal. The channel can be estimated using any one of conventional channel estimation methods including the least square estimation and the maximum likelihood estimation.
[34] The equalizer 41 compensates for channel distortion with respect to sub-carrier signals corresponding to pilot signals in currently received OFDM symbols using the channel values estimated by the channel estimator 40 in operation 51.
[35] The decision unit 42 decides constellation points corresponding to the sub-carrier signals of the pilot signals on the basis of the constellation as illustrated in FIG. 2 in operation 52.
[36] In operation 54, the channel estimation updating unit 43 re-estimates the channels of the pilot sub-carrier signals based on the decision results and reflects the re-estimated channel values to update the estimated channel values of all the sub-carrier signals, which are estimated in operation 50, through an interpolation etc.
[37] Here, it is possible to reduce channel estimation noise by carrying out a linear or nonlinear signal process on the re-estimated results. The linear signal process can employ a moving average method and the nonlinear signal process can employ a median filtering method.
[38] The hierarchical demodulator 44 demodulates in-phase data sub-carrier signals of
OFDM symbols other than the pilot sub-carrier signals using the channel values updated by the channel estimation updating unit 43 in operation 55.
[39] FIG. 6 is a flow chart of the operation 54 illustrated in FIG. 5 according to another embodiment of the present invention. In operation 60, channel estimation values with respect to carrier signals corresponding to the pilot sub-carrier signals are replaced with decision-directed channel estimation values. The channel estimation noise can be attenuated by processing the replaced values linearly or nonlinearly as described above and then channel estimation with respect to sub-carrier signals can be updated by giving correlation to channel estimation values of neighboring sub-carrier signals in operation 61.
[40] FIG. 7 is a block diagram of a receiver for receiving hierarchically modulated signals according to another embodiment of the present invention. Referring to FIG. 7, the receiver includes a layer separator 71, a base layer demodulator 72, a channel estimator 73, an equalizer 74, a decision unit 75, a channel estimation updating unit 76, and an enhancement layer demodulator 77.
[41] The layer separator 71 separates a base layer signal sequence and an enhancement layer signal sequence from a received signal. The base layer demodulator 72 demodulates the base layer signal sequence according to a demodulation method corresponding to a modulation method of a transmitting side.
[42] The enhancement layer signal sequence is demodulated as follows.
[43] The channel estimator 73 estimates channels for all sub-carrier signals using PRS included in the received signal. The PRS is a signal having a fixed transmission pattern, and thus channel estimation values can be easily obtained from the PRS signal.
[44] The equalizer 74 compensates for channel distortion with respect to sub-carrier signals corresponding to pilot signals in currently received OFDM symbols using the channel values estimated by the channel estimator 73.
[45] The decision unit 75 decides constellation points corresponding to the sub-carrier signals of the pilot signals on the basis of the constellation as illustrated in FIG. 2.
[46] The channel estimation updating unit 76 re-estimates the channels of the pilot sub- carrier signals based on the decision results and and reflects the re-estimated channel values to update the estimated channel values of all the sub-carrier signals through an interpolation etc.
[47] Here, it is possible to reduce channel estimation noise by carrying out a linear or nonlinear signal process on the re-estimated results. The linear signal process can employ a moving average method and the nonlinear signal process can employ a median filtering method.
[48] The enhancement layer signal demodulator 77 demodulates in-phase data sub- carriers signals of OFDM symbols other than the pilot sub-carrier signals using the channel values updated by the channel estimation updating unit 76.
[49] The present invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.
[50] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

Claims
[1] A hierarchical modulation apparatus comprising: a first encoder encoding a first layer signal sequence; a second encoder encoding a second layer signal sequence; a pilot inserting unit inserting a pilot signal into a signal sequence encoded by the second encoder; and a hierarchical modulator modulating signals respectively output from the first encoder and the pilot inserting unit and combining the modulated signals.
[2] The hierarchical modulation apparatus of claim 1, wherein the pilot inserting unit inserts the pilot signal into the encoded second layer signal sequence at a predetermined interval or inserts the pilot signal after the encoded second layer signal sequence.
[3] The hierarchical modulation apparatus of claim 1, wherein the pilot signal has correlation with the first layer signal sequence.
[4] The hierarchical modulation apparatus of one of claims 1, 2 and 3, wherein the pilot signal is the first layer signal sequence with amplified power or a lowered amplitude.
[5] A hierarchical modulation method comprising: encoding a first layer signal sequence; encoding a second layer signal sequence; inserting a pilot signal into the encoded second layer signal sequence; and modulating signal sequences output from the encoding a first layer signal sequence and the inserting a pilot signal and combining the modulated signal sequences.
[6] The hierarchical modulation method of claim 5, wherein the pilot signal is inserted into the encoded second layer signal sequence at a predetermined interval or inserted after the encoded second layer signal sequence.
[7] The hierarchical modulation method of claim 5, wherein the pilot signal has correlation with the first layer signal sequence.
[8] The hierarchical modulation method of one of claims 5, 6 and 7, wherein the pilot signal is the first layer signal sequence with amplified power or a lowered amplitude.
[9] A receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: a channel estimator estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; an equalizer outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; a decision unit deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; a channel estimation updating unit re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results and updating the estimated channel values using the re-estimated sub-carrier channel values; and a demodulator equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[10] The receiver of claim 9, wherein the re-estimated sub-carrier signals with respect to the pilot signal is linearly or nonlinearly processed to reduce channel estimation noise.
[11] A method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; updating the estimated channel values using the re-estimated sub-carrier channel values; and equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[12] The method of claim 11, wherein the re-estimated sub-carrier signals with respect to the pilot signal is linearly or nonlinearly processed to reduce channel estimation noise.
[13] A method of receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: estimating channels of sub-carrier signals with respect to the received signal using a reference symbol included in the received signal; outputting sub-carrier signals with respect to the pilot signal using the estimated channel values; deciding which constellation point in a constellation each of the sub-carrier signals output from the equalizer corresponds to; re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results; replacing the estimated channel values with respect to carrier signals corresponding to the sub-carrier signals of the pilot signal with the re-estimated sub- carrier channel values; giving correlation to channel estimation values with respect to neighboring sub- carrier signals using the replaced sub-carrier channel values to update the estimated channel values of the sub-carrier signals; equalizing sub-carrier signals other than the sub-carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the hierarchically modulated first and second layer signal sequences.
[14] The method of claim 13, wherein the re-estimated sub-carrier signals with respect to the pilot signal is linearly or nonlinearly processed to reduce channel estimation noise.
[15] A receiver for receiving a signal into which a first layer signal sequence and a second layer signal sequence having an inserted pilot signal are hierarchically modulated, comprising: a layer separator separating the first layer signal sequence and the second layer signal sequence from the received signal; a first layer demodulator demodulating the first layer signal sequence; and a second layer demodulator estimating sub-carrier signal channels with respect to the pilot signal, equalizing sub-carrier signal channels other than the sub-carrier signal channels with respect to the pilot signal using the estimated channel values and demodulating the second layer signal sequence.
[16] The receiver of claim 15, wherein the second layer demodulator comprises: a channel estimator estimating channels of sub-carrier signals with respect to the second layer signal sequence using a reference symbol included in the second layer signal sequence; an equalizer outputting the sub-carrier signals with respect to the pilot signal using the estimated channel values; a decision unit deciding which constellation point the sub-carrier signals output from the equalizer correspond to; a channel estimation updating unit re-estimating the channels of the sub-carrier signals with respect to the pilot signal using the decision results and updating the estimated channel values using the re-estimated sub-carrier channel values; and a second layer demodulator equalizing sub-carrier signals other than the sub- carrier signals with respect to the pilot signal using the updated channel estimation values and demodulating the second layer signal sequence.
EP07833781A 2006-12-01 2007-10-31 HIERARCHICAL MODULATION APPARATUS AND METHOD USING A PILOT SIGNAL, AND APPARATUS AND METHOD FOR RECEIVING HIERARCHICALLY MODULATED SIGNALS Withdrawn EP2087686A4 (en)

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