The present invention relates to an OFDM modulation technique for improving frequency use efficiency.
Recently, for spread of an information processing technique and rapid development of IT (Information Technology) society, demand for and expansion of information and communication is very remarkable. It is demanded that a communication infrastructure that connects an individual with a society in addition to among societies, is capable of high-speed and wireless communication. This increasing demand for mobile communication dries up rich frequency resources.
Now, so-called space-division multiplexing communication is studied as a technique, such as MIMO (Multi Input Multi Output), for improving frequency efficiency by transmitting modulated signals through a plurality of antennas. That is, by using a plurality of channels formed between transmission antennas and reception antennas, the individuality between modulated signals is secured as much as possible, so that frequency use efficiency is improved.
However, because space-division multiplexing communication such as this utilizes a channel environment which changes over time, it is necessary not only at a base station but also at a terminal apparatus of an end user to perform a great amount of signal processing. It naturally follows that that output voltage is increased, an apparatus becomes heavy and large and a cost goes up as a result.
For example, by using vertical polarization and horizontal polarization, it is possible to send various information at the same frequency. Therefore, by using QPSK on various information, theoretically it is possible to achieve 4 bit/sec/Hz at maximum. However, signal processing for optimally utilizing the orthogonality (individuality) of vertical polarization and horizontal polarization on the receiving side from reflected waves, in the mobile environment, requires twice as many apparatuses as before. In addition, signal processing takes on heavy burden for extracting parameters for following changes in the environment over time.
It would be very difficult to realize N times faster transmission rate by using N antennas, because not only N times quantity and signal processing are required but also N radio channels are required.
Therefore, instead of taking the advantage of a channel environment which changes over time, basically, there is a priority to improve modulation efficiency in the baseband radiating in free space.
Current modulation schemes in mobile communication are based on quadrature phase modulation, commonly referred to as “digital communication”, and obtains the highest frequency use efficiency at present. Major ones include quadrature amplitude modulation (QAM) and orthogonal frequency division multiplexing communication scheme using quadrature amplitude modulation for first modulation. The frequency use efficiency for an OFDM scheme utilizing QPSK, which is a base of orthogonal multiplexing and which does not apply multiple values to amplitude, is 2 bits/sec/Hz. That is, the current maximum value of a frequency use efficiency technique in the baseband is 2 bit/sec/Hz.
The spectrum assumes only positive values shown in
OFDM modulation performs multiplexing by placing the center of another spectrum at the position of ω0. Accordingly, when the number of subcarriers is sufficiently large, the average frequency density becomes ω0 per symbol.
Input signal S1 which is the target of first modulation (digital quadrature modulation) is inputted to encoding section 3. Input signal S1 has I axis signal 1 and Q axis signal 2. Encoding section 3 encodes input signal S1 to add error robustness and converts encoded input signal S1 to N parallel signals corresponding to the number of OFDM subcarriers. N parallel signals in both the I domain and Q domain modulated and converted by encoding section 3 are provided to inverse Fourier transformer 4. Inverse Fourier transformer 4 forms digital signals in the I domain and digital signals of the Q domain constituting N subcarriers.
These digital signals are converted to digital signals by digital-to-analogue (D/A) converters 5 and 6. After unnecessary frequency components are canceled by filters 7 and 8, these analogue signals are inputted to quadrature modulation section 20.
Quadrature modulation section 20 multiplies a cosine wave supplied from frequency source 11 providing a central frequency of OFDM with an I axis signal at modulator 9 and, multiplies a sine wave shifted by π/2 of the phase by phase shifter 12, to a cosine wave of frequency source 11 with a Q axis signal at modulator 10, thereby performing quadrature modulation of a cosine wave and a sine wave. After modulated outputs are added and then are canceled unnecessary frequency components by third filter 13, modulated signal 14 of OFDM is obtained.
After unnecessary frequency components are canceled by filters 27 and 28, detected outputs outputted from quadrature detection section 40 are provided respectively to analogue-to-digital (A/D) converters 29 and 30. Digitized signals are provided to Fourier transformer 31 from A/Ds 29 and 30. Fourier transformer 31 performs OFDM demodulation by Fourier transforming input signals. OFDM demodulated outputs transformed from a frequency domain signal to a time domain signal by Fourier transformer 31 is decoded and converted to serial signals by decoder 32. As a result, demodulated I axis signal 33 and demodulated Q axis signal 34 are outputted from decoder 32. Thus, demodulated I axis signal 33 and demodulated Q axis signal 34 corresponding respectively to I axis input 1 and Q axis input 2 shown in
In an OFDM scheme, subcarriers can be arranged such that subcarriers are overlapped by ½, so that it is possible to improve frequency use efficiency. However, an OFDM scheme uses bare, unformatted pulse sequences as input signals, and so each individual carrier (that is, subcarrier) forming an OFDM signal requires twice as much bandwidth as the Nyquist frequency twice the transmission rate. It is desirable to improve much better frequency use efficiency by performing limitation of bandwidth on a pulse wave.
It is an object of the present invention to provide an OFDM modulating apparatus, OFDM demodulating apparatus, OFDM modulating method, and OFDM demodulating method that make it possible generating an OFDM signal with improved frequency use efficiency.
According to an embodiment of the present invention, OFDM modulating apparatus employs a configuration including: a Nyquist formation section that performs Nyquist formation of a first pulse signal and a second pulse signal; an inverse Fourier transform section that performs inverse Fourier transform on the first pulse signal and the second pulse signal after the Nyquist formation and obtains a first orthogonal frequency division multiplexing signal and a second orthogonal frequency division multiplexing signal; a delay section that gives a delay of half of a symbol period of an orthogonal frequency division multiplexing symbol between the first orthogonal frequency division multiplexing signal and the second orthogonal frequency division multiplexing signal; and a synthesizing section that switches and selects between the first orthogonal frequency division multiplexing signal and the second orthogonal frequency division multiplexing signal with the delay of half of the symbol period of the orthogonal frequency division multiplexing symbol at every half of the symbol period of the orthogonal frequency division multiplexing symbol and synthesizes the selected orthogonal frequency division multiplexing signal.
According to this configuration, by forming an OFDM signal after Nyquist formation, one frequency channel can be accommodated in approximately ½ of the bandwidth of a conventional OFDM wave, and, when modulation is performed using a carrier, a null can be provided at every ½ time of the symbol period. In addition, it is possible to reduce voltage at both ends remarkably, so that cutoff of both ends does not cause remarkably decreasing symbol error. Two such OFDM signals (first OFDM signal and second OFDM signal) are generated using a Nyquist formation means, a first inverse Fourier transform means and a second Fourier transform means, and, moreover, differential delay of ½ of the symbol period is provided between the OFDM signals by a delayer and then the OFDM signals are switched and selected at every ½ of the symbol period by a synthesizing means, so that symbol error due to cutoff is prevented and two OFDM signals can be accommodated in the conventional, same bandwidth. As a result, two OFDM signals can be accommodated in good condition in the bandwidth required that was conventionally required to transmit one OFDM signal, so that it is possible to transmit twice as much information as before in the same, conventional bandwidth.
According to an embodiment of the present invention, OFDM modulating apparatus employs a configuration wherein the synthesizing section keeps a portion of an orthogonal frequency division multiplexing signal before and after a switching time and synthesizes the first orthogonal frequency division multiplexing signal and the second orthogonal frequency division multiplexing signal such that the first orthogonal frequency division multiplexing signal and the second orthogonal frequency division multiplexing signal partially overlap.
According to this configuration, more original OFDM signals can be kept in proportion to the overlap, so that it is possible to further reduce symbol error.
According to an embodiment of the present invention, OFDM demodulating apparatus employs a configuration including: a first Fourier transform section and a second Fourier transform section; and a switching section for selectively switching a received orthogonal frequency division multiplexing modulated signal at half of a symbol period of an orthogonal frequency division multiplexing symbol to the first Fourier transform section or the second Fourier transform section.
According to this configuration, it is possible to demodulate OFDM signals generated by an OFDM modulating apparatus of the present invention in good condition.
According to the present invention, it is possible to form an OFDM signal with improved frequency use efficiency.
Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings.
OFDM modulating apparatus 100 has two systems each transmitting the same amount of information as the conventional OFDM apparatus shown in
Input signal S10 for the first system is inputted directly to encoding section 103. By contrast with this, input signal S20 for the second system is given a delay of approximately ½ of the symbol period T by delayers (DL) 123 and 124 and then inputted to coding section 125. Each encoding section 103 and 125 adds error robustness to input signals S10 and S20 by performing encoding, and converts the encoded signals to N parallel signals corresponding to the number of OFDM subcarriers.
N parallel signals in the I domain and Q domain obtained respectively from encoding sections 103 and 125 are inputted to Nyquist filters 104 and 126. Further, for simplifying the drawing,
Switching section 130 switches and selects the signal inputted from inverse Fourier transform section 105 and the signal inputted from inverse Fourier transform section 127, at a cycle of ½ of the symbol period T, and outputs the selected signal. For example, switching section 130 selects and outputs the signals inputted from D/As 106 and 107 during the time period of 0 to T/2, and selects and outputs the signals inputted from D/As 128 and 129 during the time period of T/2 to T.
After unnecessary components are canceled by filters 131 and 132, the I axis signal and Q axis signal outputted from switching section 130 are inputted to quadrature modulation section 140. Quadrature modulation section 140 multiplies a cosine wave supplied from frequency source 135 providing a central frequency of an OFDM signal with an I axis signal at multiplier 133 and multiples a sine wave shifted by ½ of the phase by phase shifter 136, to a cosine wave of frequency source 135 with a Q axis signal at multiplier 134, thereby performing quadrature modulation of a cosine wave and a sine wave. After modulated outputs are added and then are canceled unnecessary frequency components by third filter 137, OFDM modulated signal 138 is obtained.
Quadrature demodulation section 230 inputs the signal after the filtering to quadrature detection sections 203 and 204. In quadrature detection section 203, a cosine wave from detection frequency source 205 is multiplied. In quadrature detection section 204, a sine wave shifted by π/2 of the phase by phase shifter 206 is multiplied with a cosine wave of detection frequency source 205. The outputs of these quadrature detectors 203 and 204 are inputted to switching section 209 after unnecessary components are canceled by filter 207 and filter 208.
Switching section 209 divides the outputs of analogue-to-digital converters 211 and 212 into the I axis signal and Q axis signal for the first system and the I axis signal and Q axis signal for the second system, by dividing the period of the symbol period T by two into T/2. Switching section 209 sends out the divided I axis signal and Q axis signal for the first system to Fourier transformer 213 through analogue-to-digital (A/D) converters 211 and 212, and sends out the divided I axis signal and Q axis signal for the second system to Fourier transformer 223 through analogue-to-digital (A/D) converters 221 and 222.
Frequency domain information of the I axis signal and Q axis signal for the first system is changed to time domain information by Fourier transformer 213, and frequency domain information of the I axis signal and Q axis signal for the second system is changed to time domain information by Fourier transformer 223. Thus, the signals for the first system and second system are OFDM-modulated by Fourier transformers 213 and 223.
Fourier transformer 213 and Fourier transformer 223 performs Fourier transform which sets the integration period at ½ of the symbol period of the OFDM symbol. In addition, the integration period is shifted by ½ of the symbol period between Fourier transformer 213 and Fourier transformer 223. Thus, it is possible to transform the signals outputted alternately switching at ½ of the symbol period from switching section 209 to the signals before inverse Fourier transform, by Fourier transformers 213 and 223.
The OFDM demodulated outputs obtained by Fourier transformers 213 and 223 are sent out to decoders 214 and 224, respectively. Decoders 214 and 224 decode the inputted OFDM demodulated outputs and convert the results to serial signals. By this means, signal S30 formed with I axis signal 215 and Q axis signal 216 for the first system is outputted from decoder 214 and signal S40 formed with I axis signal 225 and Q axis signal 226 for the second system is outputted from decoder 224. That is, demodulated output signal S30 corresponding to input signal S10 for the first system in
Next, operation of the present embodiment is described.
Then, the frequency property So(ω) of the signal after Nyquist formation can be represented as follows using the roll-off factor α as a parameter:
Further,
Further, in OFDM, where the synchronized state of symbols are secured and individual carrier frequencies differ by integer times, a plurality of waveforms shown in
Meanwhile, as shown in
That is, the signals for the second system become very small in amplitude near time 0 and time T, so that it is possible to insert zero by cutting off this null portion, similar to signals for the first system.
According to the present embodiment, Nyquist filters 104 and 126 for Nyquist formation of each signal for two systems (the first system and the second system), delayers 123 and 124 for delaying signals of one of the two systems by ½ of the symbol period T, inverse Fourier transformers 105 and 127 for performing OFDM processing on each symbol after Nyquist formation and switching section 130 for synthesizing signals for two systems by switching at an interval of ½ of the symbol period T and selectively outputting signals for two systems subjected to OFDM processing are provided, so that it is possible to divide and multiply two OFDM signals without interference from one another. As a result, it is possible to realize OFDM modulating apparatus 100 capable of achieving twice as much frequency use efficiency (that is, transmission of twice as much information as before in the conventional, same frequency bandwidth) as a conventional OFDM signal.
In Embodiment 1, two OFDM signals are multiplexed by switching and alternately selecting the first system and the second system. However, this results in cutting off power of information in the respective systems, and leads to deterioration of the error rate to some degree.
Therefore, in the present embodiment, when OFDM signals for the first system and OFDM signals for the second system are synthesized, one of OFDM signals is not cut off at all. Instead, a method is proposed of keeping the original OFDM signals as much as possible by partially keeping OFDM signals before and after switching time and permitting partial overlaps of OFDM signals.
That is, in OFDM modulating apparatus 300, a digital signal is inputted to switching section 301, and, by performing digital processing in switching section 301, OFDM signals for the first system and OFDM signals for the second system are synthesized by making these signals partially overlapped. That is, it is difficult to perform partial overlapping processing of two signals as described above by using analogue processing described in Embodiment 1. In the present embodiment, this processing is realized by configuring switching section 301 with a digital processing configuration.
That is, in OFDM demodulating apparatus 400, digital signals are inputted to switching section 403 and are divided to the I axis signal and the Q axis signal for the first system and the I axis signal and the Q axis signal for the second system by switching section 403. The I axis signal and Q axis signal for the first system are sent out to Fourier transformer 213, and the I axis signal and the Q axis signal for the second system are sent out to Fourier transformer 223. Switching section 403 performs digital processing and divides signals for the first system and signals for the second system which are partially overlapped during signal input by keeping the overlapped portion.
Thus, compared to Embodiment 1, decoders 214 and 224 perform decoding processing by sparingly using signals of the overlapped portion, so that the error rate characteristics of decoded data S30 and S40 is further improved than Embodiment 1.
Therefore, according to the present embodiment, in addition to Embodiment 1, a portion before and after switching time is kept and OFDM signals for the first system and OFDM signals for the second system are synthesized by making these signals partially overlapped, so that it is possible to realize OFDM communication with much better error rate characteristics than Embodiment 1.
Although cases have been described with above Embodiments 1 and 2 where delayers 123 and 124 are provided at a stage prior to encoding section 125, a section that adds delay is not limited to this, differential delay of ½ of the symbol period of the OFDM symbol may be given between first OFDM signals (OFDM signals for the first system) and second OFDM signals (OFDM signals for the second system), which are the targets of synthesis.
The present application is based on Japanese Patent Application No. 2005-015835, filed on Jan. 24, 2005, the entire content of which is expressly incorporated by reference herein.
The present invention provides an advantage of improving frequency use efficiency in OFDM communication and is suitable for use in a radio system, such as wireless LAN, a cellular system and broadcasting system.
Number | Date | Country | Kind |
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2005-015835 | Jan 2005 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2006/300976 | 1/23/2006 | WO | 00 | 7/23/2007 |