Method and apparatus for orthogonal frequency division multiplexing communication

A method and apparatus for transmitting and receiving a digital signal using an orthogonal frequency division multiplexing (OFDM) communication system are provided. In this method, subcarriers are split into a plurality of subcarrier groups according to available frequency bandwidths and are respectively transmitted in a transmitting portion and the subcarrier groups are combined and are restored to the original signal in a receiving portion. Therefore, wireless resources may be used efficiently by combining with cognitive radio technology.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application claims priority from Korean Patent Application No. 10-2006-0111232, filed on Nov. 10, 2006, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Methods and apparatuses consistent with the present invention relate to wireless communication, and more particularly, to transmitting and receiving a digital signal using an orthogonal frequency division multiplexing (OFDM) communication system.

2. Description of the Related Art

Recently, active research has been carried out on cognitive radio technology and the use of frequency resources for efficient wireless communication.FIGS. 1A and 1Bare diagrams for describing cognitive radio technology. Referring toFIG. 1A, generally, frequency resources available for wireless communication are allocated in a manner so as not to overlap with a variety of wireless communication standards. Therefore, communication with a device, which communicates according to a certain standard, may not be possible if all the channels allocated to the corresponding standard are being used, although channels allocated to other communication standards are not being used.

Cognitive radio technology searches available wireless channels according to regions and time in order to use available channels. As illustrated inFIG. 1B, the available channels can be searched for and used regardless of time and frequency bands.

FIG. 2Ais a block diagram of an OFDM transmitting apparatus. Referring toFIG. 2A, when a digital signal is input, a serial-to-parallel (S/P) converter21splits the digital signal into a plurality of signals so as to input the signals to an inverse fast Fourier transformation (IFFT) device22. InFIG. 2A, it is assumed that three-point IFFT is used. The IFFT device22performs IFFT on the input signals. When the IFFT is completed, digital signals corresponding to a plurality of subcarriers are generated. Since the IFFT and a FFT are well known and are disclosed in a variety of documents, detailed descriptions thereof will be omitted.

A parallel-to-serial (P/S) converter23combines the digital signals output from the IFFT device22and then converts the signals into a serial signal. A digital-to-analog (D/A) converter24converts a digital signal output from the P/S converter23into an analog signal. A mixer25performs frequency up-conversion using carriers which have radio frequencies (RFs).

FIGS. 2B and 2Care frequency domain graphs illustrating signals output from {circle around (1)} and {circle around (2)} of the OFDM transmitting apparatus illustrated inFIG. 2A. As illustrated inFIG. 2B, the subcarriers at baseband generated by the IFFT are modulated into RF signals at a frequency band of the carriers by the mixer25ofFIG. 2Aand then the modulated signals are transmitted externally.

In the above described OFDM system, although available frequency resources are searched for using cognitive radio technology, the frequency resources cannot be enabled if an available frequency bandwidth is less than the bandwidth of subcarriers.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for using frequency resources efficiently by splitting subcarriers and transmitting the subcarriers, respectively, in an OFDM system.

According to an aspect of the present invention, there is provided a method of transmitting a digital signal, the method including splitting the digital signal at baseband into a plurality of signals according to available frequency bands; performing inverse fast Fourier transformation (IFFT) on each of the split signals to generate subcarriers; and transmitting the subcarriers through the available frequency bands.

The method of transmitting a digital signal may further include searching for the available frequency bands using cognitive radio technology.

The signals split according to the available frequency bands may include a plurality of subcarrier groups, and each of the subcarrier groups may include at least one subcarrier. In this case, the IFFT is performed on each of the subcarrier groups. Further, the transmitting may include low-pass-filtering each of the subcarrier groups corresponding to the split signals; modulating each of the low-pass-filtered subcarrier groups independently using different RF signals; and transmitting the modulated subcarrier groups.

The low-pass-filtering may be performed variably according to a bandwidth of each the subcarrier groups.

The RF signals which have different frequencies from each other may be generated by a signal output from a phase-locked loop (PLL) circuit using at least one of a frequency divider and a frequency multiplier.

According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing the method of transmitting a digital signal.

According to another aspect of the present invention, there is provided an apparatus for transmitting a digital signal, the apparatus including a splitter which splits the digital signal at baseband into a plurality of signals according to available frequency bands; a plurality of IFFT units which performs IFFT on each of the split signals to generate subcarriers; and a transmitter which transmits the subcarriers generated through the available frequency bands.

According to another aspect of the present invention, there is provided a method of receiving a digital signal, the method including receiving subcarrier groups modulated using RF signals which have different frequencies from each other; demodulating each of the subcarrier groups so that all subcarriers of each of the subcarrier groups are arranged adjacent to each other in a predetermined order at baseband; performing fast Fourier transformation (FFT) on each of the demodulated subcarrier groups independently; and combining the subcarrier groups on which the FFT is performed.

The demodulating may include frequency down-converting each of the subcarrier groups independently using the RF signals which have different frequencies from each other; and low-pass-filtering each of the frequency down-converted subcarrier groups variably according to a bandwidths of each of the subcarrier groups.

According to another aspect of the present invention, there is provided a computer readable recording medium having recorded thereon a computer program for executing the method of receiving a digital signal.

According to another aspect of the present invention, there is provided an apparatus for receiving a digital signal, the apparatus including an RF receiver which receives subcarrier groups modulated using RF signals which have different frequencies from each other; a demodulator which demodulates each of the subcarrier groups so that all subcarriers of each of the subcarrier groups are arranged adjacent to each other in a predetermined order at baseband; a plurality of FFT units which performs FFT on each of the demodulated subcarrier groups independently; and a combiner which combines the subcarrier groups on which the FFT is performed.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Hereinafter, the present invention will be described in detail by explaining exemplary embodiments of the invention with reference to the attached drawings.

FIGS. 3A and 3Billustrate an OFDM communication system according to an exemplary embodiment of the present invention.

FIG. 3Ais a block diagram illustrating a configuration of a digital signal transmitting apparatus according to an exemplary embodiment of the present invention. Referring toFIG. 3A, a subcarrier splitter splits a digital signal into a plurality of subcarrier groups. A bandwidth of each of the subcarrier groups is determined by controlling the number of subcarriers according to available frequency resources, i.e., bandwidths.

An inverse fast Fourier transformation (IFFT) is performed with respect to the signals split by the subcarrier splitter and then the signals are converted into analog signals by digital-to-analog converters (DACs). The analog signals are low-pass-filtered and then are respectively modulated to RF signals by using different carrier frequencies, according to currently available frequency bands. The currently available frequency bands may be searched for by cognitive radio technology.

FIG. 3Bis a block diagram illustrating a configuration of a digital signal receiving apparatus according to an exemplary embodiment of the present invention. Operation of the digital signal receiving apparatus is the inverse of the operation of the digital signal transmitting apparatus ofFIG. 3A.

That is, when subcarrier groups transmitted through wireless channels are received, the subcarrier groups are respectively demodulated into signals adjacent to each other at baseband using carriers which have different frequencies and then are low-pass-filtered. The filtered signals are converted into digital signals by analog-to-digital converters (ADCs).

An FFT is performed with respect to the digital signals. The signals on which FFT is performed are combined by a subcarrier combiner and then are restored to the original signals.

FIG. 4is a flow chart of a method of transmitting digital signals according to an exemplary embodiment of the present invention.

In operation405, bandwidths of available wireless channels are searched for using cognitive radio technology.

In operation410, a digital signal at baseband is split into a plurality of signals. Each of the split signals forms an independent subcarrier group. Bandwidths of subcarrier groups are determined by the bandwidths of the wireless channels searched for in operation405. In other words, the reason why the digital signal at baseband is split in operation410is because the bandwidths of the subcarrier groups have to be controlled appropriately in order to fit the bandwidths of the searched for wireless channels.

In operation420, an IFFT is performed on each of the split signals.

In operation430, the signals on which the IFFT is performed are converted into analog signals.

In operation440, the subcarrier groups are respectively low-pass-filtered. That is, one low-pass filter is used with respect to one subcarrier group. Since the bandwidths of the subcarrier groups can change constantly over time, the low-pass filters used in operation440have to be able to modify filtering sections according to the bandwidths of the input subcarrier groups.

In operation450, the subcarrier groups are respectively frequency up-converted using RF signals which have different frequencies from each other. Here, the frequencies of the RF signals are determined by the frequencies of the wireless channels searched for in operation405. The RF signals may be generated from one signal source in order to maintain orthogonality between the subcarriers. A plurality of phase-locked loops (PLLs) can be generated using a crystal oscillator, however, PLL hardware cannot be implemented easily and frequency errors can be generated due to the nonlinear PLLs. Preferably, but not necessarily, the required RF signals may be generated by a signal output from one PLL using a frequency divider or a frequency multiplier.

In operation460, the frequency up-converted RF signals are transmitted.

FIG. 5is a flow chart of a method of receiving digital signals according to an exemplary embodiment of the present invention.

In operation510, signals corresponding to subcarrier groups are received.

In operation520, the subcarrier groups are respectively frequency down-converted so that all subcarriers of the subcarrier groups are arranged adjacent to each other at baseband. Here, different RF signals are used with respect to each of the subcarrier groups. Preferably, but not necessarily, the required RF signals may be generated by a signal output from one PLL using a frequency divider or a frequency multiplier as in the previous exemplary embodiment of the present invention.

In operation530, the frequency down-converted signals are respectively low-pass-filtered. Accordingly, the same number of low-pass filters is used as the number of subcarrier groups. Since bandwidths of the subcarrier groups can change constantly over time, the low-pass filters have to be able to modify filtering sections according to the bandwidths of the input subcarrier groups.

In operation540, the low-pass-filtered signals are converted into digital signals.

In operation550, an FFT is performed on each of the digital signals corresponding to the subcarrier groups.

In operation560, the signals on which the FFT is performed are combined together and then are restored to the original signal.

FIG. 6is a block diagram illustrating a configuration of a digital signal transmitting apparatus600according to an exemplary embodiment of the present invention.

Referring toFIG. 6, the digital signal transmitting apparatus600includes a splitter610, a cognitive radio unit620, IFFT units630, a transmitter640, a PLL circuit650, a frequency divider660, and a frequency multiplier670.

The cognitive radio unit620searches for available wireless channels using cognitive radio technology.

The splitter610splits a digital signal at baseband into a plurality of signals with reference to the result of the search by the cognitive radio unit620. Each of the split signals forms a subcarrier group. The transmitter640distributes a plurality of subcarrier groups among the available wireless channels and transmits the subcarrier groups. The transmitter640includes low-pass filters641, mixers643and RF transmitters642.

The low-pass filters641low-pass-filter signals output from the IFFT units630. The signals output from the IFFT units630are digital signals, and the signals are converted into analog signals and then are processed. However, a description of digital-to-analog conversion will be omitted for the sake of brevity. Hereinafter, a description of digital-to-analog conversion or analog-to-digital conversion will also be omitted. Since bandwidths of the subcarrier groups can change constantly over time, the low-pass filters641may modify filtering sections according to the bandwidths of the input subcarrier groups.

The mixers643modulate the low-pass-filtered signals to RF signals, respectively. The RF transmitters642transmit the RF signals externally. The mixers643frequency up-convert each of the subcarrier groups in order to transmit them through the available wireless channels. Accordingly, the RF signals have different frequencies from each other. The RF signals are generated by a signal output from one PLL circuit650using the frequency divider660and/or the frequency multiplier670. InFIG. 6, both the frequency divider660and the frequency multiplier670are illustrated. However, only one of the frequency divider660and the frequency multiplier670can be used depending on circumstances.

FIG. 7is a block diagram illustrating a configuration of a digital signal receiving apparatus700according to an exemplary embodiment of the present invention.

Referring toFIG. 7, the digital signal receiving apparatus700includes a cognitive radio unit705, an RF receiver710, a demodulator720, FFT units740, a PLL circuit750, a frequency divider760, a frequency multiplier770, and a combiner780.

The cognitive radio unit705searches for frequency bandwidths in which data to be received is transmitted. The RF receiver710receives subcarrier groups through wireless channels and the demodulator720demodulates the received subcarrier groups, respectively, so that the subcarrier groups are arranged adjacent to each other at baseband. The demodulate720includes mixers721and low-pass filters722. The mixers721frequency down-convert the signals received by the RF receiver710, respectively, and the low-pass filters722low-pass-filter the frequency down-converted signals, respectively. Since bandwidths of the subcarrier groups can change constantly over time, the low-pass filters722may modify filtering sections according to the bandwidths of the input subcarrier groups.

The RF signals used for frequency down-conversion of the subcarrier groups have different frequencies from each other. The RF signals are generated by a signal output from one PLL circuit750using the frequency divider760and/or the frequency multiplier770.

The FFT units740perform a FFT with respect to each of the signals output from the low-pass filters722, i.e., the subcarrier groups.

The combiner780combines the signals output from the FFT units740and then outputs signals corresponding to the subcarrier groups arranged adjacent to each other at baseband.

FIG. 8is a diagram of a method of transmitting and receiving digital signals according to an exemplary embodiment of the present invention.

Although not shown inFIG. 8, a digital signal at baseband is separated into 10 signals, i.e., 10 subcarriers, by a serial-to-parallel converter. Then, the subcarriers are split into 3 subcarrier groups by a splitter. For convenience of description, the subcarriers are denoted as first through tenth subcarriers in numerical order. A first subcarrier group includes the first through third subcarriers, a second subcarrier group includes the fourth through seventh subcarriers, and a third subcarrier group includes the eighth through tenth subcarriers. Since three subcarrier groups are generated according to the current exemplary embodiment, three IFFT units and three mixers will be used in a transmitting portion, and three mixers and three FFT units will be used in a receiving portion. In the current exemplary embodiment, 4-point IFFT is assumed.

The three subcarrier groups are frequency up-converted by carriers which have frequencies of fa, fband fc, respectively, and then are transmitted. Here, fa, fband fcare searched for by cognitive radio technology in order to transmit the subcarrier groups through available frequency bands.

In the receiving portion, the three subcarrier groups are received and then are respectively frequency down-converted. Here, RF signals of f1, f2and f3are used with respect to the subcarrier groups. f1, f2and f3are set so as to arrange the subcarriers of the three subcarrier groups after frequency down-conversion adjacent to each other at baseband in the same order as the subcarriers are arranged before the frequency down-conversion. For example, in order to arrange the subcarriers in the first subcarrier group having the frequency f1in the same order as the original signal, the subcarriers have to be frequency down-converted for 3Δf more than fa(f1=fa+3Δf). Here, Δf is sub-channel spacing, that is, a bandwidth of one subcarrier.

Likewise, f2is the same as fb(f2=fb), and f3is calculated by subtracting 4Δf from fc(f3=fc−4Δf). As described above, the frequency down-converted signals are converted into frequency domain signals in the FFT units and then are combined by a combiner so as to be restored to the original signal.

Exemplary embodiments of the present invention can be written as computer programs and can be implemented in general-use digital computers that execute the programs using non-transitory a computer readable recording medium. Examples of the computer readable recording medium include magnetic storage media (e.g., ROM, floppy disks, hard disks, etc.), optical recording media (e.g., CD-ROMs, or DVDs), and other storage media.

According to an exemplary embodiment of the present invention, data may be transmitted and received by splitting the data among desired bandwidths using orthogonality between subcarriers in an OFDM system. Therefore, wireless resources may be used efficiently by combining with cognitive radio technology.