Wireless transmission method, wireless transmission system, and transmission apparatus and reception apparatus of wireless transmission system

In a wireless transmission system that transmits and receives a modulated signal between a transmitter and a receiver that are coupled through a wireless transmission path, the transmitter includes a spectrum division filter bank dividing the modulated signal and generating a plurality of sub-spectrum signals each of which is arranged at a predetermined frequency position, and subjects the plurality of sub-spectrum signals arranged in spectra to a direct spectrum division transmission, and the receiver includes a spectrum combination filter bank extracting the plurality of sub-spectrum signals from the received signals arranged in spectra and subjected to the direct spectrum division transmission to combine the sub-spectrum signals into an original modulated signal.

CROSS REFERENCE TO RELATED APPLICATION

This application is a U.S. National Stage application claiming the benefit of prior filed International Application Number PCT/JP2010/002356, filed on Mar. 31, 2010, in which the International Application claims priority from Japanese Patent Application Number 2009-088857, filed on Apr. 1, 2009, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a wireless transmission method, a wireless transmission system, and a transmitter and a receiver of the wireless transmission system by which a plurality of users efficiently use a limited frequency band to perform wireless communication.

BACKGROUND ART

FIG. 25shows a first exemplary configuration of a conventional multicarrier transmission system (Patent Document 1).

InFIG. 25, a transmitter of the conventional multicarrier transmission circuit includes modulation circuits1001to100Nfor each user, a Tx filter bank101and a transmitting circuit102. A receiver includes a receiving circuit103, an Rx filter bank104and demodulation circuits1051to105Nfor each user.

The modulation circuits1001to100Nin the transmitter modulate (map) data1to N for each user, respectively. The Tx filter bank101converts respective modulated signals to respective predetermined carrier frequencies, which are in turn combined and transmitted by the transmitting circuit102. The Rx filter bank104in the receiver filters multicarrier signals received at the receiving circuit103for each carrier frequency, and the demodulation circuits1051to105Ndemodulate data1to N for each user, respectively.

FIG. 26shows a second exemplary configuration of a conventional multicarrier transmission system. Here, an example is shown in which, in the conventional multicarrier transmission system shown inFIG. 25, a user A uses an unused frequency band to transmit a signal when other users8, C and D have already occupied frequency bands for communication.

A serial-parallel converter110in the transmitter serial-to-parallel converts data for the user A, modulation circuits1111and1112modulate serial-parallel converted data, respectively. A Tx filter bank112converts each of modulated signals A1and A2for the user A to a predetermined carrier frequency, so as to be allocated to an unused frequency band and transmitted by a transmission circuit113. Meanwhile, an Rx filter bank115in the receiver filters multicarrier signals received at a receiving circuit114for each carrier frequency for frequency conversion, and demodulation circuits1161to1162demodulate modulated signals A1and A2for the user A, respectively. The demodulated modulated signals A1and A2are parallel-serial converted by a parallel-serial converter117and restored to data for the user A.

FIG. 27shows an exemplary configuration of a conventional orthogonal frequency division multiplexing (OFDM) transmission system.

InFIG. 27(a), a conventional OFDM transmission system includes an OFDM modulation circuit120on the transmission side, and an OFDM demodulation circuit121on the reception side. The OFDM modulation circuit120includes a serial-parallel converter122, modulation circuits1231to123Nand an inverse fast Fourier transform (IFFT) circuit124. The OFDM demodulation circuit121includes a fast Fourier transform (FFT) circuit125, demodulation circuits1261to126Nand a parallel-serial converter127.

Usually, with the orthogonal frequency division multiplexing-time division multiple access (OFDM-TDMA) scheme, which divides users' signals into time slots to distinguish the users according to time, the users' signals that are divided into time slots are serial-to-parallel converted by the serial-parallel converter122, and each of the parallel-output signals is modulated by each of the modulation circuits1231to123N, independently. Subsequently, the parallel-output modulated signals are converted to time domain by the IFFT circuit124, and transmitted as multicarrier signals.

Meanwhile, in the OFDM demodulation circuit121, after establishing OFDM frame synchronization, the signals are converted to frequency domain by the FFT circuit125, and demodulated by the demodulation circuits1261to126Nfor each sub carrier. The demodulated signals are input into the parallel-serial converter127, and restored from per-sub carrier signals to the original one-system signals.

RELATED ART DOCUMENT

Patent Document

DISCLOSURE

Problems to be Solved

The conventional multicarrier transmission system shown inFIG. 26requires as many modulation circuits and demodulation circuits as the number of carriers into which a user signal is divided. In addition, a serial-parallel converter and a parallel-serial converter require suitable amount of memory for the number of carriers into which a user signal is divided, leading to a problem that a circuit size becomes large.

Meanwhile, in an OFDM transmission system shown inFIG. 27, a plurality of modulation circuits and demodulation circuits are required, and an OFDM signal presents an SINC function response in a frequency domain so that sub carriers become orthogonal. Accordingly, as shown inFIG. 27(b), there is the problem that, when a signal for a user A is divided into spectra, a side lobe becomes superposed with an adjacent, another user signal, causing interferences. If a sufficient guard band is provided between the OFDM signal and the signal of the other user in order to avoid this problem, a problem occurs that frequency utilization efficiency is degraded.

A proposition of the present invention is to provide a wireless transmission method, a wireless transmission system, and a transmitter and a receiver of the wireless transmission system by which direct spectrum division transmission of one modulated signal is achieved at a small circuit size, allowing unused frequency bands of a transmission path to be used efficiently.

Means for Solving the Problems

According to a first invention, in a wireless transmission system that transmits and receives a modulated signal between a transmitter and a receiver that are coupled through a wireless transmission path, the transmitter includes a spectrum division filter bank dividing the modulated signal and generating a plurality of sub-spectrum signals each of which is arranged at a predetermined frequency position, and subjects the plurality of sub-spectrum signals arranged in spectra to a direct spectrum division transmission, and the receiver includes a spectrum combination filter bank extracting the plurality of sub-spectrum signals from the received signals arranged in spectra and subjected to the direct spectrum division transmission to combine the sub-spectrum signals into an original modulated signal.

The spectrum division filter bank D1in the wireless transmission system according to the first invention includes a Fourier transform unit converting the modulated signal to frequency domain; a plurality of spectrum division units dividing an output from the Fourier transform unit and outputting the plurality of sub-spectrum signals; a plurality of frequency shift units shifting each of the plurality of sub-spectrum signals output from the plurality of spectrum division units to the predetermined frequency position; an addition unit performing an addition of outputs from the plurality of frequency shift units, and arranging each of the outputs at the predetermined frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

The spectrum division filter bank D2in the wireless transmission system according to the first invention includes a Fourier transform unit converting the modulated signal to frequency domain; a plurality of frequency shift units each shifting an output from the Fourier transform unit to the predetermined frequency position; a plurality of spectrum division units dividing each output from the plurality of frequency shift units and outputting the plurality of sub-spectrum signals; an addition unit performing an addition of outputs from the plurality of spectrum division units, and arranging each of the outputs at the predetermined frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

The spectrum combination filter bank C1in the wireless transmission system according to the first invention includes a Fourier transform unit converting the received signals to frequency domain; a plurality of spectrum extraction units dividing an output from the Fourier transform unit and extracting the plurality of sub-spectrum signals; a plurality of frequency shift units shifting each of the plurality of sub-spectrum signals output from the plurality of spectrum extraction units to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position; an addition unit performing an addition of outputs from the plurality of frequency shift units, and combining the outputs at the original frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

The spectrum combination filter bank C2in the wireless transmission system according to the first invention includes a Fourier transform unit converting the received signals to frequency domain; a plurality of frequency shift units each shifting an output from the Fourier transform unit from the predetermined frequency position to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position; a plurality of spectrum extraction units dividing each output from the plurality of frequency shift units and extracting the plurality of sub-spectrum signals; an addition unit performing an addition of outputs from the plurality of spectrum extraction units, and combining the outputs at the original frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

According to a second invention, the transmitter and the receiver in the wireless transmission system of the first invention transmit the plurality of sub-spectrum signals through respective wireless transmission paths.

The spectrum division filter bank D1′ in the wireless transmission system according to the second invention includes a Fourier transform unit converting the modulated signal to frequency domain; a plurality of spectrum division units dividing an output from the Fourier transform unit and outputting the plurality of sub-spectrum signals; a plurality of frequency shift units shifting each of the plurality of sub-spectrum signals output from the plurality of spectrum division units to the predetermined frequency position, and output each of the plurality of sub-spectrum signals arranged at the predetermined frequency position; and a plurality of inverse Fourier transform units that convert each output from the plurality of frequency shift units to time domain.

The spectrum division filter bank D2′ in the wireless transmission system according to the second invention includes a Fourier transform unit converting the modulated signal to frequency domain; a plurality of frequency shift units each shifting an output from the Fourier transform unit to the predetermined frequency position; a plurality of spectrum division units dividing each output from the plurality of frequency shift units and outputting each of the plurality of sub-spectrum signals arranged at the predetermined frequency position; and a plurality of inverse Fourier transform units converting each output from the plurality of spectrum division units to time domain.

The spectrum combination filter bank C1′ in the wireless transmission system according to the second invention includes a plurality of Fourier transform units converting each of received signals transmitted through the plurality of wireless transmission paths to frequency domain; a plurality of spectrum extraction units dividing each output from the plurality of Fourier transform units and extracting the plurality of sub-spectrum signals; a plurality of frequency shift units shifting each of the plurality of sub-spectrum signals output from the plurality of spectrum extraction units to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position; an addition unit performing an addition of outputs from the plurality of frequency shift units, and combining the outputs at the original frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

The spectrum combination filter bank C2′ in the wireless transmission system according to the second invention includes a plurality of Fourier transform units converting each of received signals transmitted through the plurality of wireless transmission paths to frequency domain; a plurality of frequency shift units shifting each output from the plurality of Fourier transform units from the predetermined frequency position to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position; a plurality of spectrum extraction units dividing each output from the plurality of frequency shift units and extracting the plurality of sub-spectrum signals; an addition unit performing an addition of outputs from the plurality of spectrum extraction units, and combining the outputs at the original frequency position; and an inverse Fourier transform unit converting an output from the addition unit to time domain.

The spectrum division units in each of the spectrum division filter banks D1, D1′, D2and D2′ in the wireless transmission system according to the first invention or second invention each multiplies the modulated signal by more than one spectrum division weighting function BDk(ω) to generate N sub-spectrum signals Sbk(ω). The spectrum extraction units in each of the spectrum combination filter banks C1, C1′, C2and C2′ in the wireless transmission system according to the first invention or second invention each multiplies N sub-spectrum signals Sbk(ω) contained in the received signals by a spectrum combination weighting function BCk(ω) corresponding to a transfer function G(ω) between the transmitter and the receiver and the spectrum division weighting function BDk(ω), where k represents a natural number from 1 to N, N represents the number of divided spectra and ω represents a frequency.

In addition, preferably, an overall transfer function BTk(ω) that is the product of the spectrum division weighting function BDk(ω) and the spectrum combination weighting function BCk(ω) in an occupied spectrum of the modulated signal is represented as follows:
Σ|BTk(ω)G(ω+ωk)|=A
where A represents a constant and ωkrepresents a value determined by the frequency allocation of the sub-spectrum signal.

In addition, preferably, the spectrum division weighting function BDk(ω) and the spectrum combination weighting function BCk(ω) making up a pair are both the same root roll-off function.

Further, preferably, the product of a mean frequency spectrum F(ω) of the modulated signal and the spectrum division weighting function BDk(ω) satisfies
|F(ω)BDk(ω)G(ω+ωk)|=|BCk(ω)|
and the spectrum combination weighting function BCk(ω) is a root roll-off function.

According to a third invention, the transmitter in the wireless transmission system of the first invention includes spectrum division filter banks D1and D2.

According to a fourth invention, the transmitter in the wireless transmission system of the second invention includes spectrum division filter banks D1′ and D2′.

According to a fifth invention, the receiver in the wireless transmission system of the first invention includes spectrum combination filter banks C1and C2.

According to a sixth invention, the receiver in the wireless transmission system of the second invention includes spectrum combination filter banks C1′ and C2′.

According to the seventh invention, in a wireless transmission method that transmits and receives a modulated signal between a transmitter and a receiver that are coupled through a wireless transmission path, the transmitter uses a spectrum division filter bank to divide the modulated signal, generates transmitted signals from a plurality of sub-spectrum signals each of which is arranged at a predetermined frequency position, and subjects the plurality of sub-spectrum signals arranged in spectra to a direct spectrum division transmission, and the receiver uses a spectrum combination filter bank to extract the plurality of sub-spectrum signals from received signals arranged in spectra and subjected to the direct spectrum division transmission to combine the sub-spectrum signals into an original modulated signal, which is in turn subjected to demodulation processing.

The spectrum division filter bank in the wireless transmission method according to the seventh invention converts the modulated signal to frequency domain by a Fourier transform unit; divides an output from the Fourier transform unit and outputs the plurality of sub-spectrum signals by a plurality of spectrum division units; shifts each of the plurality of sub-spectrum signals output from the plurality of spectrum division units to the predetermined frequency position by a plurality of frequency shift units; performs an addition of outputs from the plurality of frequency shift units, and arranges each of the outputs at the predetermined frequency position by an addition unit; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

The spectrum division filter bank in the wireless transmission method according to the seventh invention converts the modulated signal to frequency domain by a Fourier transform unit; shifts an output from the Fourier transform unit to the predetermined frequency position by each of a plurality of frequency shift units; divides each output from the plurality of frequency shift units and outputs the plurality of sub-spectrum signals by a plurality of spectrum division units; performs an addition of outputs from the plurality of spectrum division units, and arranges each of the outputs at the predetermined frequency position by an addition units; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

The spectrum combination filter bank in the wireless transmission method according to the seventh invention converts the received signals to frequency domain by a Fourier transform unit; divides an output from the Fourier transform unit and extracts the plurality of sub-spectrum signals by a plurality of spectrum extraction units; shifts each of the plurality of sub-spectrum signals output from the plurality of spectrum extraction units to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position by a plurality of frequency shift units; performs an addition of outputs from the plurality of frequency shift units, and combines the outputs at the original frequency position by an addition unit; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

The spectrum combination filter bank in the wireless transmission method according to the seventh invention converts the received signals to frequency domain by a Fourier transform unit; shifts an output from the Fourier transform unit from the predetermined frequency position to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position by each of a plurality of frequency shift units; divides each output from the plurality of frequency shift units and extracts the plurality of sub-spectrum signals by a plurality of spectrum extraction units; performs an addition of outputs from the plurality of spectrum extraction units, and combines the outputs at the original frequency position by an addition unit; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

According to an eighth invention, the transmitter and the receiver in the wireless transmission system of the seventh invention transmit a plurality of sub-spectrum signals through respective wireless transmission paths.

The spectrum division filter bank in the wireless transmission method according to the eighth invention converts the modulated signal to frequency domain by a Fourier transform unit; divides an output from the Fourier transform unit and outputs the plurality of sub-spectrum signals by a plurality of spectrum division units; shifts each of the plurality of sub-spectrum signals to the predetermined frequency position, and outputs each of the plurality of sub-spectrum signals arranged at the predetermined frequency position by a plurality of frequency shift units; and converts each output from the plurality of frequency shift units to time domain by a plurality of inverse Fourier transform units.

The spectrum division filter bank in the wireless transmission method according to the eighth invention converts the modulated signal to frequency domain by a Fourier transform unit; shifts an output from the Fourier transform unit to the predetermined frequency position by each of a plurality of frequency shift units; divides each output from the plurality of frequency shift units and outputs each of the plurality of sub-spectrum signals arranged at the predetermined frequency position by a plurality of spectrum division units; and converts each output from the plurality of spectrum division units to time domain by a plurality of inverse Fourier transform units.

The spectrum combination filter bank in the wireless transmission method according to the eighth invention converts each of received signals transmitted through the plurality of wireless transmission paths to frequency domain by a plurality of Fourier transform units; divides an output from the Fourier transform unit and extracts the plurality of sub-spectrum signals by a plurality of spectrum extraction units; shifts each of the plurality of sub-spectrum signals output from the plurality of spectrum extraction units to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position by a plurality of frequency shift units; performs an addition of outputs from the plurality of frequency shift units, and combines the outputs at the original frequency position by an addition unit; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

The spectrum combination filter bank in the wireless transmission method according to the eighth invention converts each of received signals transmitted through the plurality of wireless transmission paths to frequency domain by a plurality of Fourier transform units; shifts each output from the plurality of Fourier transform units from a predetermined frequency position to an original frequency position where each of the sub-spectrum signals is at before arranged to the predetermined frequency position by a plurality of frequency shift units; divides each output from the plurality of frequency shift units and extracts the plurality of sub-spectrum signals by a plurality of spectrum extraction units; performs an addition of outputs from the plurality of spectrum extraction units, and combines the outputs at the original frequency position by an addition unit; and converts an output from the addition unit to time domain by an inverse Fourier transform unit.

According to the present invention, since one modulated signal is divided to generate a plurality of sub-spectrum signals which are in turn subjected to direct spectrum division transmission with the plurality of sub-spectrum signals arranged in spectra, a direct spectrum division transmission effectively using an unused frequency band of a transmission path occupied by another user can be achieved. In addition, since a plurality of sub-spectrum signals can be handled by one modulation circuit and one demodulation circuit, a modulation circuit or a demodulation circuit for each sub-spectrum signal is not required, enabling direct spectrum division transmission with a reduced circuit size for the wireless transmission system.

Further, since one modulated signal is divided into spectra, allowing the peak average power ratio (PAPR) to be smaller compared to conventional multicarrier transmission, the size of the amplifier in the RF circuits of the transmitter and the receiver can be reduced.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1shows a first embodiment of a wireless transmission system according to the present invention.

InFIG. 1, the wireless transmission system according to the present embodiment has a configuration in which a transmitter and a receiver are coupled with each other through a wireless transmission path. The transmitter includes a modulation circuit10, a spectrum division filter bank11and a transmitting circuit12, and transmits a plurality of sub-spectrum signals which result from the spectrum division of a modulated signal, and each of which is arranged at a predetermined frequency position. The receiver includes a receiving circuit13, a spectrum combination filter bank14and a demodulation circuit15, and extracts a plurality of sub-spectrum signals from received signals subjected to direct spectrum division transmission, and combines the sub-spectrum signals into an original modulated signal for demodulation.

FIG. 2shows a first exemplary configuration of a spectrum division filter bank11. Here, an exemplary configuration is shown, in which spectrum division is performed to generate two sub-spectrum signals.

InFIG. 2, the spectrum division filter bank11includes: an FFT circuit11athat converts an input modulated signal to frequency domain; a spectrum division circuit11bthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the FFT circuit11aby a spectrum division weighting function1; a spectrum division circuit11cthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the FFT circuit11aby a spectrum division weighting function2; a frequency shift circuit11dthat shifts the sub-spectrum signal output from the spectrum division circuit11bby a frequency shift1; a frequency shift circuit11ethat shifts the sub-spectrum signal output from the spectrum division circuit11cby a frequency shift2; an addition circuit11fthat performs an addition of the outputs from the frequency shift circuits11dand11e; and an IFFT circuit11gthat converts the output from the addition circuit11fto time domain.

FIG. 3show a flow of the signal processing of the spectrum division filter bank11of the first exemplary configuration.

InFIGS. 3(a) and (b), a modulated signal input into the spectrum division filter bank11is subjected to a fast Fourier transform processing by the FFT circuit11a, and converted from time domain to frequency domain to obtain a modulated signal A. The spectrum division circuit11bmultiplies the modulated signal A output from the FFT circuit11aby a spectrum division weighting function1, and outputs a sub-spectrum signal resulting from the spectrum division in the frequency domain of the modulated signal A. The spectrum division circuit11cmultiplies the modulated signal A output from the FFT circuit11aby a spectrum division weighting function2to output a sub-spectrum signal resulting from the spectrum division in the frequency domain of the modulated signal A. The frequency shift circuit11dshifts the sub-spectrum signal output from the spectrum division circuit11bby a frequency shift1to generate a sub-spectrum signal A1equivalently frequency-converted. The frequency shift circuit11eshifts the sub-spectrum signal output from the spectrum division circuit11cby a frequency shift2to generate a sub-spectrum signal A2equivalently frequency-converted.

The addition circuit11fperforms an addition of the outputs from the frequency shift circuits14dand14ein the frequency domain, arranges each of the sub-spectrum signals A1and A2resulting from spectrum division and frequency conversion at a predetermined frequency position, and outputs the signals to the IFFT circuit11g. The IFFT circuit11gperforms an inverse fast Fourier conversion processing to convert the modulated signal from frequency domain to time domain.

The modulated signal is converted to a radio signal and transmitted from the transmitting circuit12shown inFIG. 1. At that time, as shown inFIG. 3(c), when modulated signals B, C and D for other users occupy respective frequency bands on a wireless transmission path, the sub-spectrum signals A1and A2are inserted into unused frequency bands. The frequency bands and frequency positions of these sub-spectrum signals A1and A2are set by spectrum division weighting functions1and2and frequency shifts1and2depending on respective unused frequency bands.

FIG. 4shows a second exemplary configuration of a spectrum division filter bank11. Here, an exemplary configuration is shown, in which spectrum division is performed to generate two sub-spectrum signals.

InFIG. 4, the spectrum division filter bank11includes: an FFT circuit11athat converts an input modulated signal to frequency domain; a frequency shift circuit11dthat shifts the output from the FFT circuit11aby a frequency shift1; a frequency shift circuit11ethat shifts the output from the FFT circuit11aby a frequency shift2; a spectrum division circuit11bthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the frequency shift circuit11dby a spectrum division weighting function1; a spectrum division circuit11cthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the frequency shift circuit11eby a spectrum division weighting function2; an addition circuit11fthat performs an addition of the outputs from the frequency shift circuits11band11c; and an IFFT circuit11gthat converts the output from the addition circuit11fto time domain.

FIG. 5show a flow of the signal processing of the spectrum division filter bank11of the second exemplary configuration.

InFIGS. 5(a) and (b), a modulated signal input into the spectrum division filter bank11is subjected to a fast Fourier transform processing by the FFT circuit11a, and converted from time domain to frequency domain to obtain a modulated signal A. The frequency shift circuit11dshifts the modulated signal A output from the FFT circuit11aby a frequency shift1so as to be equivalently frequency-converted. The frequency shift circuit11eshifts the modulated signal A output from the FFT circuit11aby a frequency shift2so as to be equivalently frequency-converted. The spectrum division circuit11bmultiplies output from the frequency shift circuit11dby the spectrum division weighting function1, and outputs a sub-spectrum signal A1resulting from spectrum division in the frequency domain of the modulated signal A. The spectrum division circuit11cmultiplies output from the frequency shift circuit11eby the spectrum division weighting function2, and outputs a sub-spectrum signal A2resulting from the spectrum division in the frequency domain of the modulated signal A.

The addition circuit11fperforms an addition of the outputs from the spectrum division circuits14band14cin the frequency domain, arranges each of the sub-spectrum signals A1and A2resulting from frequency conversion and spectrum division at a predetermined frequency position, and outputs the signals to the IFFT circuit11g. The IFFT circuit11gperforms an inverse fast Fourier conversion processing to convert the modulated signal from frequency domain to time domain.

The modulated signal is converted to a radio signal and transmitted from the transmitting circuit12shown inFIG. 1. At that time, as shown inFIG. 5(c), when modulated signals B, C and D for other users occupy respective frequency bands on a wireless transmission path, the sub-spectrum signals A1and A2are inserted into unused frequency bands. The frequency bands and frequency positions of these sub-spectrum signals A1and A2are set by spectrum division weighting functions1and2and frequency shifts1and2depending on respective unused frequency bands.

As described above, in the past, if sequential unused frequency bands could not be acquired, no frequency band could be assigned to the modulated signal A. In addition, dispersed unused frequency bands could not be used effectively. On the contrary, in the wireless communication system according to the present invention, the spectrum division filter bank11shown inFIG. 2orFIG. 4is used to perform spectrum division and arrangement of the modulated signal A over the dispersed unused frequency bands, so that direct spectrum division transmission of the modulated signal A becomes possible even if sequential unused frequency band is not acquired, allowing frequency utilization efficiency to be improved as the whole system.

FIG. 6shows a first exemplary configuration of the spectrum combination filter bank14. Here, an exemplary configuration is shown, in which spectrum combination of two sub-spectrum signals is performed.

InFIG. 6, the spectrum combination filter bank14includes: an FFT circuit14athat converts an input modulated signal to frequency domain; a spectrum extraction circuit14bthat extracts a sub-spectrum signal by multiplying the output from the FFT circuit14aby a spectrum combination weighting function1; a spectrum extraction circuit14cthat extracts a sub-spectrum signal by multiplying the output from the FFT circuit14aby a spectrum combination weighting function2; a frequency shift circuit14dthat shifts the sub-spectrum signal output from the spectrum extraction circuit14bby a frequency shift1; a frequency shift circuit14ethat shifts the sub-spectrum signal output from the spectrum extraction circuit14cby a frequency shift2; an addition circuit14fthat performs an addition of the outputs from the frequency shift circuits14dand14e; and an IFFT circuit14gthat converts the output from the addition circuit14fto time domain.

Note that, if needed, an Rx spectrum shaping filter14his inserted between the addition circuit14fand the IFFT circuit14g. The Rx spectrum shaping filter14hincludes a multiplication circuit14ithat multiplies the output from the addition circuit14fby a spectrum shaping filter function in the frequency domain so as to remove noise and signal components out of a predetermined band.

FIG. 7show a flow of the signal processing of the spectrum combination filter bank14of the first exemplary configuration.

InFIG. 7(a), a received signal input into the spectrum combination filter bank14is subjected to a fast Fourier transform processing by the FFT circuit14a, and converted from time domain to frequency domain. The sub-spectrum signals A1and A2are arranged at predetermined frequency positions on the received signal.

InFIGS. 7(b) and (c), the spectrum extraction circuit14bmultiplies the received signal output from the FFT circuit14aby the spectrum combination weighting function1, and extracts a sub-spectrum signal A1from the received signal in the frequency domain. The spectrum extraction circuit14cmultiplies the received signal output from the FFT circuit14aby the spectrum combination weighting function2, and extracts a sub-spectrum signal A2from the received signal in the frequency domain. That is, the spectrum extraction circuits14band14cperform equivalent filter processing in the frequency domain by multiplying the received signal and the spectrum combination weighting functions1and2to remove noise and signal components out of the pass band of the spectrum combination weighting functions1and2, and extract the sub-spectrum signals A1and A2.

InFIG. 7(d), the frequency shift circuit14dshifts the output from the spectrum extraction circuit14bby a frequency shift1so as to be equivalently frequency-converted. The frequency shift circuit14eshifts the output from the spectrum extraction circuit14cby a frequency shift2so as to be equivalently frequency-converted. The addition circuit14fperforms an addition of the signals each of which is frequency-converted to combine the sub-spectrum signals A1and A2at a frequency position where the sub-spectrum signals A1and A2are at before being arranged to the predetermined frequency positions and restore the original modulated signal A.

InFIG. 7(e), the Rx spectrum shaping filter14hremoves modulated signals B and D in neighboring bands contained in the output from the addition circuit14f, selects restored modulated signal A and outputs it to the IFFT circuit14g. The IFFT circuit14gperforms inverse fast Fourier conversion processing to convert the modulated signal from frequency domain to time domain, and outputs the modulated signal to the subsequent demodulation circuit.

Note that, the spectrum combination weighting functions1and2of the spectrum combination filter bank14of the first exemplary configuration shown inFIGS. 6 and 7are set to values corresponding to the spectrum division weighting functions1and2of the spectrum division filter bank11of the second exemplary configuration shown inFIGS. 4 and 5and the transfer function between the transmitter and the receiver. In addition, the frequency shifts1and2of the spectrum combination filter bank14of the first exemplary configuration shown inFIGS. 6 and 7are set to values complementary to the frequency shifts1and2of the spectrum division filter bank11of the second exemplary configuration shown inFIGS. 4 and 5.

FIG. 8shows the second exemplary configuration of the spectrum combination filter bank14. Here, an exemplary configuration is shown, in which spectrum combination of two sub-spectrum signals is performed.

InFIG. 8, the spectrum combination filter bank14includes: an FFT circuit14athat converts an input modulated signal to frequency domain; a frequency shift circuit14dthat shifts the output from the FFT circuit14aby a frequency shift1; a frequency shift circuit14ethat shifts the output from the FFT circuit14aby a frequency shift2; a spectrum extraction circuit14bthat extracts a sub-spectrum signal by multiplying the output from the frequency shift circuit14dby a spectrum combination weighting function1; a spectrum extraction circuit14cthat extracts a sub-spectrum signal by multiplying the output from the frequency shift circuit14eby a spectrum combination weighting function2; an addition circuit14fthat performs an addition of the outputs from the spectrum extraction circuits14band14c; and an IFFT circuit14gthat converts the output from the addition circuit14fto time domain.

Note that, if needed, the Rx spectrum shaping filter14his inserted between the addition circuit14fand the IFFT circuit14g. The Rx spectrum shaping filter14hincludes a multiplication circuit14ithat multiplies the output from the addition circuit14fby a spectrum shaping filter function in the frequency domain so as to remove noise and signal components out of a predetermined band.

FIG. 9show a flow of the signal processing of the spectrum combination filter bank14of the second exemplary configuration.

InFIG. 9(a), a received signal input into the spectrum combination filter bank14is subjected to a fast Fourier transform processing by the FFT circuit14a, and converted from time domain to frequency domain. The sub-spectrum signals A1and A2are arranged at predetermined frequency positions on the received signal.

InFIGS. 9(b) and (c), the frequency shift circuit14dshifts the output from the FFT circuit14aby a frequency shift1so as to be equivalently frequency-converted. The spectrum extraction circuit14bmultiplies the received signal output from the frequency shift circuit14dby the spectrum combination weighting function1, and extracts a sub-spectrum signal A1from the received signal in the frequency domain. The frequency shift circuit14eshifts the output from the FFT circuit14aby a frequency shift2so as to be equivalently frequency-converted. The spectrum extraction circuit14cmultiplies the received signal output from the frequency shift circuit14eby the spectrum combination weighting function2, and extracts a sub-spectrum signal A2from the received signal in the frequency domain. That is, the spectrum extraction circuits14band14cperform equivalent filter processing in the frequency domain by multiplying the frequency-converted received signal and the spectrum combination weighting functions1and2to remove noise and signal components out of the pass band of the spectrum combination weighting functions1and2, and extract the sub-spectrum signals A1and A2.

InFIG. 9(d), the addition circuit14fperforms an addition of the sub-spectrum signals A1and A2extracted from the received signal to combine the sub-spectrum signals A1and A2at a frequency position where the sub-spectrum signals A1and A2are at before being arranged to the predetermined frequency positions and restore the original modulated signal A.

InFIG. 9(e), the Rx spectrum shaping filter14hremoves modulated signals B and D in neighboring bands contained in the output from the addition circuit14f, selects restored modulated signal A and outputs it to the IFFT circuit14g. The IFFT circuit14gperforms inverse fast Fourier conversion processing to convert the modulated signal from frequency domain to time domain, and outputs the modulated signal to the subsequent demodulation circuit.

Note that, the spectrum combination weighting functions1and2of the spectrum combination filter bank14of the second exemplary configuration shown inFIGS. 8 and 9are set to values corresponding to the spectrum division weighting functions1and2of the spectrum division filter bank11of the first exemplary configuration shown inFIGS. 2 and 3and the transfer function between the transmitter and the receiver. In addition, the frequency shifts1and2of the spectrum combination filter bank14of the second exemplary configuration shown inFIGS. 8 and 9are set to values complementary to the frequency shifts1and2of the spectrum division filter bank11of the first exemplary configuration shown inFIGS. 2 and 3.

In addition, the spectrum division filter bank11and the spectrum combination filter bank14may adopt a well-known overlap and add method in which in order to process sequential signals, an input signal is partitioned at fixed intervals, and processed at each interval, and processed signals are added and output. Further, the spectrum division filter bank11and the spectrum combination filter bank14may adopt a well-known overlap and storage method in which an input signal is partitioned at partially overlapping fixed intervals, processed at each interval and after some of the overlapping portions are dropped from the processed signals, addition is carried out.

FIG. 10shows an exemplary configuration of the spectrum division filter bank11to which an overlap and add method is applied.

InFIG. 10, a modulated signal input from a modulation circuit10is branched into two lines, one of which is input into a first spectrum division filter bank11-1through a first time window21, and the other is delayed by a delay circuit22, and input into a second spectrum division filter bank11-2through a second time window23. Note that, the first time window21and the second time window23are time windows having complementary characteristics in the time domain. The first spectrum division filter bank11-1and the second spectrum division filter bank11-2have the same circuit configuration as that of the spectrum division filter bank11shown inFIG. 2orFIG. 4. The output from the first spectrum division filter bank11-1is delayed by a delay circuit24, and then input into an addition circuit25, added to the output from the second spectrum division filter bank11-2and output to the transmitting circuit12. By applying such an overlap and add method, FFT processing at a limited interval can be continuously performed on sequential modulated signals in the time domain.

FIG. 11shows an exemplary configuration of the spectrum combination filter bank14to which an overlap and add method is applied.

InFIG. 11, a received signal input from a receiving circuit13is branched into two lines, one of which is input into a first spectrum combination filter bank14-1through a first time window31, and the other is delayed by a delay circuit32, and input into a second spectrum combination filter bank14-2through a second time window33. Note that, the first time window31and the second time window33are time windows having complementary characteristics in the time domain. The first spectrum combination filter bank14-1and the second spectrum combination filter bank14-2have the same circuit configuration as that of the spectrum combination filter bank14shown inFIG. 6orFIG. 8. The output from the first spectrum combination filter bank14-1is delayed by a delay circuit34, and then input into an addition circuit35, added to the output from the second spectrum combination filter bank14-2and output to the demodulation circuit15. By applying such an overlap and add method, FFT processing at a limited interval can be continuously performed on sequential received signals in the time domain.

FIG. 12shows a second embodiment of a wireless transmission system according to the present invention.

InFIG. 12, the wireless transmission system according to the present embodiment has a configuration in which transmitters and receivers are coupled through a plurality of wireless transmission paths. Here, a plurality of wireless transmission paths include a multiplex transmission path such as polarization division multiplexing and space division multiplexing.

The transmitter includes a modulation circuit10, a spectrum division filter bank11′ and transmitting circuits12-1,12-2, . . . ,12-N corresponding to a plurality of wireless transmission paths, and transmits through respective corresponding transmitting circuits12-1,12-2, . . . ,12-N a plurality of sub-spectrum signals which result from the spectrum division of a modulated signal, and each of which is arranged at a predetermined frequency position. The receiver includes receiving circuits13-1,13-2, . . . ,13-N corresponding to a plurality of wireless transmission paths, a spectrum combination filter bank14′ and a demodulation circuit15, receives received signals that are subjected to direct spectrum division transmission through the receiving circuits13-1,13-2, . . . ,13-N, extracts a plurality of sub-spectrum signals from respective received signals, and combines them into the original modulated signal for demodulation.

FIG. 13shows a first exemplary configuration of a spectrum division filter bank11′. Here, an exemplary configuration is shown, in which spectrum division is performed to generate two sub-spectrum signals.

InFIG. 13, the spectrum division filter bank11includes: an FFT circuit11athat converts an input modulated signal to frequency domain; a spectrum division circuit11bthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the FFT circuit11aby a spectrum division weighting function1; a spectrum division circuit11cthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the FFT circuit11aby a spectrum division weighting function2; a frequency shift circuit11dthat shifts the sub-spectrum signal output from the spectrum division circuit11bby a frequency shift1; a frequency shift circuit11ethat shifts the sub-spectrum signal output from the spectrum division circuit11cby a frequency shift2; IFFT circuits11g-1and11g-2that convert each output from the frequency shift circuits11dand11eto time domain.

The difference from the first exemplary configuration of the spectrum division filter bank11shown inFIG. 2is that the sub-spectrum signals A1and A2output from the frequency shift circuits11dand11eare output to the transmitting circuits12-1and12-2shown inFIG. 12through the IFFT circuits11g-1and11g-2, respectively. Accordingly, the sub-spectrum signals A1and A2are transmitted to the receiver through wireless transmission paths that are independent from each other.

FIG. 14shows a second exemplary configuration of the spectrum division filter bank11′. Here, an exemplary configuration is shown, in which spectrum division is performed to generate two sub-spectrum signals.

InFIG. 14, the spectrum division filter bank11′ includes: an FFT circuit11athat converts an input modulated signal to frequency domain; a frequency shift circuit11dthat shifts the output from the FFT circuit11aby a frequency shift1; a frequency shift circuit11ethat shifts the output from the FFT circuit11aby a frequency shift2; a spectrum division circuit11bthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the frequency shift circuit11dby a spectrum division weighting function1; a spectrum division circuit11cthat outputs a sub-spectrum signal resulting from spectrum division by multiplying the output from the frequency shift circuit11eby a spectrum division weighting function2; and IFFT circuits11g-1and11g-2that convert each output from the spectrum division circuits11band11cto time domain.

The difference from the second exemplary configuration of the spectrum division filter bank11shown inFIG. 4is that the sub-spectrum signals A1and A2output from the spectrum division circuits11band11care output to the transmitting circuits12-1and12-2shown inFIG. 12through the IFFT circuits11g-1and11g-2, respectively. Accordingly, the sub-spectrum signals A1and A2are transmitted to the receiver through wireless transmission paths that are independent from each other.

FIG. 15shows a first exemplary configuration of the spectrum combination filter bank14′. Here, an exemplary configuration is shown, in which spectrum combination of two sub-spectrum signals is performed.

InFIG. 15, the spectrum combination filter bank14′ includes: FFT circuits14a-1and14a-2that convert a plurality of input modulated signals to frequency domain, respectively; a spectrum extraction circuit14bthat extracts a sub-spectrum signal by multiplying the output from the FFT circuit14a-1by a spectrum combination weighting function1; a spectrum extraction circuit14cthat extracts a sub-spectrum signal by multiplying the output from the FFT circuit14a-2by a spectrum combination weighting function2; a frequency shift circuit14dthat shifts the sub-spectrum signal output from the spectrum extraction circuit14bby a frequency shift1; a frequency shift circuit14ethat shifts the sub-spectrum signal output from the spectrum extraction circuit14cby a frequency shift2; an addition circuit14fthat performs an addition of the outputs from the frequency shift circuits14dand14e; and an IFFT circuit14gthat converts the output from the addition circuit14fto time domain. Note that, the Rx spectrum shaping filter14his arranged as described above if needed.

The difference from the first exemplary configuration of the spectrum combination filter bank14shown inFIG. 6is that a plurality of modulated signals input from the receiving circuits13-1and13-2shown inFIG. 12are input to the corresponding spectrum extraction circuits14band14cthrough the FFT circuits14a-1and14a-2, respectively. Accordingly, respective sub-spectrum signals A1and A2are extracted from the received signals transmitted through the wireless transmission paths independently from each other and combined.

FIG. 16shows the second exemplary configuration of the spectrum combination filter bank14′. Here, an exemplary configuration is shown, in which spectrum combination of two sub-spectrum signals is performed.

InFIG. 16, the spectrum combination filter bank14′ includes: FFT circuits14a-1and14a-2that convert input modulated signals to frequency domain, respectively; a frequency shift circuit14dthat shifts the output from the FFT circuit14aby a frequency shift1; a frequency shift circuit14ethat shifts the output from the FFT circuit14aby a frequency shift2; a spectrum extraction circuit14bthat extracts a sub-spectrum signal by multiplying the output from the frequency shift circuit14dby a spectrum combination weighting function1; a spectrum extraction circuit14cthat extracts a sub-spectrum signal by multiplying the output from the frequency shift circuit14eby a spectrum combination weighting function2; an addition circuit14fthat performs an addition of the outputs from the spectrum extraction circuits14band14c; and an IFFT circuit14gthat converts the output from the addition circuit14fto time domain. Note that, the Rx spectrum shaping filter14his arranged as described above if needed.

The difference from the second exemplary configuration of the spectrum combination filter bank14shown inFIG. 8is that a plurality of modulated signals input from the receiving circuits13-1and13-2shown inFIG. 12are input to the corresponding frequency shift circuits14dand14ethrough the FFT circuits14a-1and14a-2, respectively. Accordingly, respective sub-spectrum signals A1and A2are extracted from the received signals transmitted through the wireless transmission paths independently from each other and combined.

FIG. 17shows an exemplary configuration of the spectrum division filter bank11′ to which an overlap and add method is applied.

InFIG. 17, a modulated signal input from a modulation circuit10is branched into two lines, one of which is input into a first spectrum division filter bank11′-1through a first time window21, and the other is delayed by a delay circuit22, and input into a second spectrum division filter bank11′-2through a second time window23. Note that, the first time window21and the second time window23are time windows having complementary characteristics in the time domain. The first spectrum division filter bank11′-1and the second spectrum division filter bank11′-2have the same circuit configuration as that of the spectrum division filter bank11′ shown inFIG. 13orFIG. 14. The first output from the first spectrum division filter bank11′-1is delayed by a delay circuit24-1, and then input into an addition circuit25-1, added to the first output from the second spectrum division filter bank11′-2and output to the transmitting circuit12-1. In addition, the second output from the first spectrum division filter bank11′-1is delayed by a delay circuit24-2, and then input into an addition circuit25-2, added to the second output from the second spectrum division filter bank11′-2and output to the transmitting circuit12-2. By applying such an overlap and add method, FFT processing at a limited interval can be continuously performed on sequential modulated signals in the time domain.

FIG. 18shows an exemplary configuration of the spectrum combination filter bank14′ to which an overlap and add method is applied.

InFIG. 18, a received signal input from a receiving circuit13-1is branched into two lines, one of which is input into a first spectrum combination filter bank14′-1through a first time window31-1, and the other is delayed by a delay circuit32-1, and input into a second spectrum combination filter bank14′-2through a second time window33-1. In addition, a received signal input from a receiving circuit13-2is branched into two lines, one of which is input into a first spectrum combination filter bank14′-1through a first time window31-2, and the other is delayed by a delay circuit32-2, and input into a second spectrum combination filter bank14′-2through a second time window33-2. Note that, the first time window31-1and the second time window33-1, and the first time window31-2and the second time window33-2are time windows having complementary characteristics in the time domain, respectively.

The first spectrum combination filter bank14′-1and the second spectrum combination filter bank14′-2have the same circuit configuration as that of the spectrum combination filter bank14′ shown inFIG. 15orFIG. 16. The output from the first spectrum combination filter bank14′-1is delayed by the delay circuit34, and then input into the addition circuit35, added to the output from the second spectrum combination filter bank14′-2and output to the demodulation circuit15. By applying such an overlap and add method, FFT processing at a limited interval can be continuously performed on sequential received signals in the time domain.

Each circuit of the spectrum division filter banks11and11′ and spectrum combination filter banks14and14′ described above is not limited to a hardware circuit, and for example may be made up of software processing.

In the wireless transmission system and the wireless transmission method of the present invention, it is important that the spectrum division filter bank11divides the modulated signal A to generate the sub-spectrum signals A1and A2, which are in turn extracted by the spectrum combination filter bank14, and combined to restore the modulated signal A. A spectrum division weighting function and a spectrum combination weighting function will now be described in detail.

FIG. 19shows an example of a spectrum division weighting function and a spectrum combination weighting function.

When a modulated signal is filtered, convolution is performed in the time domain. Meanwhile, in the frequency domain where a Fourier transform is used, multiplication may be performed instead.

In the spectrum division filter bank11, let a modulated signal to be input be F(ω), a spectrum division weighting function1be H1(ω), a spectrum division weighting function2be H2(ω), a frequency shift1be ω1and a frequency shift2be ω2, a transmitted signal Tx(ω) obtained by adding sub-spectrum signals A1and A2in the frequency domain may be represented as follows:
Tx(ω)=F(ω−ω1)H1(ω−ω1)+F(ω−ω2)H2(ω−ω2)  (1)
provided that ω1and ω2are selected so that, after addition, the signal bands of the sub-spectrum signals A1and A2do not overlap in the frequency domain.

Next, let a transfer function G(ω) between the transmitter and the receiver be 1, the received signal Rx(ω) to be input into the spectrum combination filter bank14may be represented as follows:
Rx(ω)=G(ω)Tx(ω)=Tx(ω)  (2)

Meanwhile, let the same H1(ω) as the spectrum division weighting function1on the transmission side be the spectrum combination weighting function1, the same H2(ω) as the spectrum division weighting function1on the transmission side be the spectrum combination weighting function2, a frequency shift1be −ω1and a frequency shift2be −ω2, the output from the addition circuit14f, Rx1(ω) may be represented as follows:

Here, the spectrum combination weighting function BCk(ω) is a function corresponding to the spectrum division weighting function BDk(ω) and a transfer function G(ω) between the transmitter and the receiver where k represents a natural number from 1 to N, N represents the number of divided spectra and ω represents a frequency. An overall transfer function BTk(ω) that is the product of the spectrum division weighting function BDk(ω) and the spectrum combination weighting function BCk(ω) in an occupied spectrum of the modulated signal is represented as follows:

∑B⁢⁢Tk⁡(ω)⁢G⁡(ω+ωk)=∑B⁢⁢Dk⁡(ω)⁢B⁢⁢Ck⁡(ω)⁢G⁡(ω+ωk)=A(5)
where A represents a constant and ωkrepresents a value determined by the frequency allocation of the sub-spectrum signal. The spectrum division weighting function BDk(ω) and the spectrum combination weighting function BCk(ω) making up a pair are both the same root roll-off function.

Here, let G(ω)=1 as in Formula (2), Formula (5) may be represented as follows:

Let H1(ω)>0 and H2(ω)>0, Formula (4) may be represented as follows:

R⁢⁢x2⁡(ω)=F⁡(ω)⁢(H12⁡(ω)+H22⁡(ω))⁢Roll⁡(ω)=A⁢⁢F⁡(ω)⁢Roll⁡(ω)(7)
and the Rx spectrum shaping filter14hperforms filtering to extract a transmitted signal.

Meanwhile, when a delay time τ is assumed between the transmitter and the receiver, the transfer function G(ω) of an undistorted transmission path is represented as follows:
G(ω)=exp(−j(ωτ+θ0))
The received signal RX(ω) to be input into the receiver may be represented as follows:
Rx(ω)=G(ω)Tx(ω)=exp(−j(ωτ+θ0))Tx(ω)  (8)
Here, let the spectrum combination weighting function1on the reception side be H1(ω), and the spectrum combination weighting function2be H2(ω)exp(−j(ω1−ω2)τ), the output Rx1(ω) of the addition circuit14fmay be represented as follows:

That is, the modulated signal F(ω) is rotated by a phase exp(−j(ω1τ+θ0)) and delayed by time τ for demodulation.

Since the phase rotation and time delay can be adjusted by a carrier recovery circuit and a timing recovery circuit that are usually provided on the demodulation circuit15, an undistorted modulated signal F(ω) can be extracted by the demodulation circuit15.

The above description represents a case where the transfer function G(ω) is undistorted. On the contrary, if amplitude or the like of the transfer function G(ω) is not flat, the spectrum combination weighting function1and the spectrum combination weighting function2are selected or the spectrum division weighting function1and the spectrum division weighting function2are selected so that the amplitude becomes flat after combination, thus the distortion of the transmission path can be compensated.

In addition, a spectrum division weighting function and a spectrum combination weighting function that satisfy the present invention will be described in detail with reference toFIG. 19.

In order to satisfy the present invention, it suffices that, as described above, a pass band of the sum in the frequency domain of an overall transfer function1obtained by multiplying the spectrum division weighting function1and the spectrum combination weighting function1in the frequency domain, and an overall transfer function2obtained by multiplying the spectrum division weighting function2and the spectrum combination weighting function2in the frequency domain is flat with respect to the occupied spectrum of the modulated signal. For example, the characteristics resulting from frequency-shifting by ωha root roll-off filter with a roll-off factor α and a cut-off frequency ωhis represented by the following formula where ωx=αωh:
H1(ω)=1(|ω+ωh|<ωh−ωx)  (10-1)
H1(ω)=sin(π(ωx−|ω+ωh|+ωh)/4ωx)(ωh−ωx≦|ω+ωh|<ωh−ωx)  (10-2)
H1(ω)=0(|ω+ωh|≧2ωh−ωx)  (10-3)

Here, for the sake of simplification, it is assumed that the transfer function G(ω)=1, Formula (10) may be calculated with the spectrum division weighting function1and the spectrum combination weighting function1.

Here, for the sake of simplification, it is assumed that the transfer function G(ω)=1, Formula (11) may be calculated with the spectrum division weighting function2and the spectrum combination weighting function2.

From Formula (9), the gain of the filter characteristics H12(ω)+H22(ω) combining transmission and reception is 1 with |ω|<ωh(2+α) (pass band). Accordingly, for the modulated signal F(ω) having an occupied spectrum of |ω|<ωh(2+α), a signal transmission without waveform distortion is possible.

When the above-described spectrum division weighting function and spectrum combination weighting function are applied, a modulated signal can be divided in the frequency domain, and combined and demodulated on the reception side.

Note that, the above example is an example of spectrum division weighting function and spectrum combination weighting function, and is not limited to this filter function. That is, the above example is an example in which a modulated signal is equally divided into two signals, but the modulated signal may be divided into three or more signals, for example, seven signals, as shown inFIG. 20(a) depending on the situation of an unused frequency band, or may be divided into sub-spectrum signals with different bandwidths as shown inFIG. 20(b).

FIG. 21show an example of transmitted signals obtained by dividing a modulated signal into two sub-spectrum signals.FIG. 22show an example of a combined signal obtained by combining the two sub-spectrum signals of the received signals.FIG. 23shows the combined signal of the sub-spectrum signals and the roll-off characteristics of the modulated signal.

In this example, a broadband modulated signal F(ω) is divided and transmitted by two narrow-band filters, and each of
|F(ω)BDk(ω)G(ω+ωk)|
which is the absolute value of the product between the transmitted signal F(ω)BD(ω) and the propagation path characteristics G(ω+ωK), and |BCk(ω)|, which is the absolute value of the spectrum combination weighting function, is the same root roll-off-function.

The division and combination of a transmitted signal using the spectrum division weighting function BCk(ω) and the spectrum combination weighting function BDk(ω) that satisfy the characteristics will now be described.

On the transmission side, when a modulated signal F(ω) shown inFIG. 21(a) is multiplied by the spectrum division weighting functions BD1(ω) and BD2(ω) shown inFIG. 21(b), the modulated signal is divided to generate two sub-spectrum signals F(ω)BD1(ω) and F(ω)BD2(ω) as shown inFIG. 21(c). Subsequently, when the center frequencies of the sub-spectrum signals resulting from division are shifted to respective predetermined frequencies ω1and ω2, the transmitted signals shown inFIG. 21(d) are generated:
F(ω−ω1)BD1(ω−ω1)
F(ω−ω2)BD2(ω−ω2)

Meanwhile, on the reception side, a case is assumed where the transmitted signals shown inFIG. 21(d):
F(ω−ω1)BD1(ω−ω1),
F(ω−ω2)BD1(ω−ω2)
are affected by a transmission path G(ω), so as to become received signals shown inFIG. 22(a):
F(ω−ω1)BD1(ω−ω1)G(ω),
F(ω−ω2)BD2(ω−ω2)G(ω)
When these signals are frequency-converted, signals shown inFIG. 22(b) are obtained:
F(ω)BD1(ω)G(ω+ω1),
F(ω)BD2(ω)G(ω+ω2)

On the reception side, if a spectrum combination weighting function BCk(ω) that satisfies
|F(ω)BDk(ω)G(ω+ωk)|=|BCk(ω)|  (13)
is selected, and |BCk(ω)| becomes a root roll-off function with the same roll-off factor as that of the spectrum of each sub-spectrum signal, then, after spectrum combination filtering, a signal with a waveform shown inFIG. 22(c) is obtained. Here, inFIG. 22(c), since each of
F(ω)BD1(ω)G(ω+ω1)BC1(ω), and
F(ω)BD2(ω)G(ω+ω2)BC2(ω)
satisfies full roll-off characteristics, the sum of the levels of the transition regions where the two sub-spectrum signals overlap becomes equal to the level of the band pass. Accordingly, as shown inFIG. 22(d), the spectrum F′(ω) where the sub-spectrum signals are combined would also satisfy the full roll-off characteristics.

In addition, the relationship of the waveform F′(ω) ofFIG. 22(d) of the combined sub-spectrum signals, and the full roll-off characteristics F″(ω) of the transmitted signal inFIG. 21(a) is shown inFIG. 23.

Although F′(ω) and F″(ω) each satisfy the roll-off characteristics, the transition region of the roll-off function is steeper for F′(ω), as shown inFIG. 21(b), since, at the time of spectrum division, multiplication is performed by a spectrum division weighting function having a steeper transition region than that of the modulated signal. That is, as shown inFIG. 23, the spectrum has an equivalent shape with a smaller roll-off factor than that of the modulated signal F″(ω). On the demodulation side, regardless of the roll-off factor, Nyquist timing with no Intersymbol interference can be extracted as long as the full roll-off characteristics are satisfied, such that, a signal obtained by combining the sub-spectrum signals can be used to perform demodulation without degradation of the characteristics.

In this case, since the signal F′(ω) obtained by combining the sub-spectrum signals already satisfies the full roll-off characteristics, subsequent spectrum shaping filtering is not required, thus the Rx spectrum shaping filter14hin the spectrum combination filter bank14shown inFIGS. 6 and 8is not required.

FIG. 24show an example of a comparison between the band of a modulated signal F(ω) and the sum of the bands occupied by the sub-spectrum signals.

The band occupied by the sub-spectrum signal is the product of the spectrum division weighting function BDk(ω) and the modulated signal F(ω) as shown inFIG. 21(c). Accordingly, by appropriately selecting BDk(ω), the bandwidth of the sub-spectrum signal can be adjusted. For example, when a steep function having a narrower transition region than that of the pass band is selected as BDk(ω), each sub-spectrum signal is a steep function in which the pass band is broad and the transition region is narrow, as shown inFIG. 24(b). In this case, since the sub-spectrum signal becomes close to a rectangular wave, the sum of the signal bands of the sub-spectrum signals1and2can also be made narrower than the band of the modulated signal F(ω). Note thatFIG. 24(a) shows a case in which the sum of the occupied spectrum of the sub-spectrum signal is broader than the band of the modulated signal F(ω).

As described above, by selecting the spectrum division weighting function BDk(ω), the total band width required for the transmission may become be equal to or less than the occupied spectrum width of the modulated signal, thus allowing the frequency utilization efficiency to be improved.