Multiplex communications – Generalized orthogonal or special mathematical techniques – Quadrature carriers
Reexamination Certificate
2000-11-17
2004-06-08
Rao, Seema S. (Department: 2666)
Multiplex communications
Generalized orthogonal or special mathematical techniques
Quadrature carriers
C370S210000, C370S480000, C375S260000, C708S404000
Reexamination Certificate
active
06747946
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an apparatus for transmitting an orthogonal-multi-carrier signal (an orthogonal frequency division multiplexed signal or an OFDM signal). In addition, this invention relates to a method of transmitting an orthogonal-multi-carrier signal.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) employs multiple carriers which are orthogonal with respect to each other. The “orthogonal” multiple carriers mean that the spectrums of carriers neighboring one carrier are null at the frequency of the latter carrier. The multiple carriers are modulated in accordance with digital information pieces to be transmitted, respectively. For example, phase modulation (phase shift keying or PSK) or quadrature amplitude modulation (QAM) is used by the modulation of each carrier. The modulation-resultant multiple carriers are combined into an OFDM signal which has a form as a random signal. Since the multiple carriers are orthogonal, they do not interfere with each other. Accordingly, during transmission, the digital information pieces assigned to the respective multiple carriers are prevented from interfering with each other.
In general, OFDM employs a lot of carriers. Thus, the modulation rate for one carrier in OFDM can be lower than that in a transmission system using only one carrier. Therefore, an OFDM signal is less affected by multi-path signal distortions occurring in a spatial transmission line. Since the spectrum of an OFDM signal can be rectangular, the frequency band occupied thereby can be narrow. Thus, OFDM can provide efficient frequency use.
In OFDM-based signal transmission, a guard interval is added to every symbol interval related to an OFDM signal. During the guard interval, a redundant signal is transmitted. The guard interval is a buffer time for absorbing lags of delayed waves from a non-delayed wave and thereby reducing multi-path signal distortions. The transmitted redundant signal is remarkably smaller in bit rate than a main transmitted information signal. Therefore, the redundant signal causes only a small decrease in transmission efficiency.
Terrestrial digital broadcasting uses OFDM. A transmitter side of the terrestrial digital broadcasting implements IDFT (inverse discrete Fourier transform) for generating an OFDM signal. Information pieces to be transmitted are used as frequency-domain signals being phase and amplitude modulating signals with respect to multiple carries. The DFT converts the frequency-domain signals into time-domain signals composed of the multiple carries modulated with the information pieces. The time-domain signals are added and combined into a baseband OFDM signal. The baseband OFDM signal is up-converted into a radio-frequency (RF) OFDM signal. The RF OFDM signal is radiated by an antenna.
A receiver side of the terrestrial digital broadcasting down-converts an RF OFDM signal into a baseband OFDM signal. The receiver side implements DFT (discrete Fourier transform) for converting time-domain signals in the baseband OFDM signal into frequency-domain signals. Transmitted information pieces are recovered from the frequency-domain signals. In general, DFT is implemented by a signal processing circuit including a DSP (digital signal processor) or an LSI (large-scale integration circuit)
The terrestrial digital broadcasting uses an assigned frequency band divided into channels each having a width of 6 MHz and a predetermined number of carriers. The channels are separated by guard frequency bands. For example, the channels are distributed to a plurality of different users.
Data transmission based on OFDM is executed symbol by symbol. Each OFDM transmission symbol period is composed of a window interval and a guard interval. The window interval is assigned to IDFT. Multiple carriers are spaced at frequency intervals corresponding to the reciprocal of the symbol period. In general, all the multiple carriers are used for the transmission of information.
In the terrestrial digital broadcasting, multiple carriers are spaced at frequency intervals of about 1 kHz to 4 kHz. All the multiple carriers are used for the transmission of information.
OFDM is used in not only broadcasting but also communications and wireless LAN. Specifically, OFDM is widely used in radio communications because of its ability to reduce multi-path signal distortions.
In a prior-art OFDM signal transmission system, a signal processing circuit for implementing IDFT has a complicated structure. Also, a signal processing circuit for implementing DFT has a complicated structure.
SUMMARY OF THE INVENTION
It is a first object of this invention to provide a simple apparatus for transmitting an orthogonal-multi-carrier signal (an orthogonal frequency division multiplexed signal or an OFDM signal).
It is a second object of this invention to provide a simple method of transmitting an orthogonal-multi-carrier signal.
A first aspect of this invention provides an apparatus for transmitting an orthogonal-multi-carrier signal. The apparatus comprises first means for subjecting an information signal to first inverse discrete Fourier transform to convert the information signal into an orthogonal-multi-carrier signal; second means for outputting the orthogonal-multi-carrier signal generated by the first means to a transmission line; third means for receiving the orthogonal-multi-carrier signal from the transmission line; fourth means for subjecting the orthogonal-multi-carrier signal received by the third means to discrete Fourier transform to recover the information signal from the received orthogonal-multi-carrier signal; fifth means contained in the first means for subjecting the information signal to M-point inverse discrete Fourier transform and thereby implementing a first stage of the first inverse discrete Fourier transform, and for generating a transform result signal representative of results of the M-point inverse discrete Fourier transform, wherein M denotes a predetermined natural number greater than 2; and sixth means contained in the first means for subjecting the transform result signal generated by the fifth means to N-point inverse discrete Fourier transform and thereby implementing a second stage of the first inverse discrete Fourier transform to generate the orthogonal-multi-carrier signal, the second stage following the first stage, wherein N denotes a predetermined natural number greater than twice M.
A second aspect of this invention provides an apparatus for transmitting an orthogonal-multi-carrier signal. The apparatus comprises first means for subjecting a multi-channel information signal to first inverse discrete Fourier transform to convert the multi-channel information signal into an orthogonal-multi-carrier signal; second means for outputting the orthogonal-multi-carrier signal generated by the first means to a transmission line; third means for receiving the orthogonal-multi-carrier signal from the transmission line; fourth means for subjecting the orthogonal-multi-carrier signal received by the third means to discrete Fourier transform to recover the multi-channel information signal from the received orthogonal-multi-carrier signal; a plurality of fifth means contained in the first means for subjecting the multi-channel information signal to M-point inverse discrete Fourier transforms and thereby implementing a first stage of the first inverse discrete Fourier transform, and for generating transform result signals representative of results of the M-point inverse discrete Fourier transforms, wherein M denotes a predetermined natural number greater than 2; and sixth means contained in the first means for subjecting the transform result signals generated by the fifth means to N-point inverse discrete Fourier transform and thereby implementing a second stage of the first inverse discrete Fourier transform to generate the orthogonal-multi-carrier signal, the second stage following the first stage, wherein N denotes a predetermined natural number greater than twice M.
Kaneko Keiichi
Takaoka Katsumi
Moore, Jr. Michael J.
Rao Seema S.
Victor Company of Japan Ltd.
Woo Louis
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