Multi-carrier transmission system

Multi-carrier transmission system comprises transmitter including acquisition unit configured to acquire 2m (m: a natural number) modulated signals including no-information signals which are failed to be used for information transmission and 2n (n: a natural number, n<m) signals, acquisition unit subjecting modulated signals to inverse discrete Fourier transform to obtain transformed signals, no-information signal included in Lth modulated signal of modulated signals being used as first no-information signal of no-information signals, every Kth modulated signal of modulated signals that is counted from first no-information signal being used as no-information signal (K: a natural number, L: an integer, K=2m−n, 0≦L≦K−1), and transmission unit configured to transmit transformed signals, and receiver including receiving unit configured to receive the transformed signals, and detection unit configured to detect synchronization timing based on at least one no-information signal included in the transformed signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2003-330170, filed Sep. 22, 2003, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital data transmission technique, and more particularly to a multi-carrier transmission system that utilizes an orthogonal frequency division multiplexing (OFDM) transmission technique using a multi-carrier.

2. Description of the Related Art

In OFDM as a type of multi-carrier transmission system, a transmitter multiplexes frequency-base signals into time-base signals using an inverse discrete Fourier transformer (IDFT), while a receiver extracts frequency-base signals from received time-base signals using a discrete Fourier transformer (DFT). No further particulars of OFDM will be explained since OFDM is a well-known technique.

When an OFDM receiver receives a transmission signal, a DFT performs block processing. Accordingly, it is necessary to accurately set the positions of blocks, i.e., to perform symbol synchronization. In general, to realize symbol synchronization, a transmitter adds a redundant symbol to a to-be-transmitted signal.

There is a method in which a guard symbol is inserted and synchronization is performed based on the symbol. Specifically, if, for example, there is eight IDFT outputs x0, x1, . . . , x7, the last four outputs x4, x5, x6and x7are copied, and the four copies are positioned before the original four outputs and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. When a receiver executes correlation computation on each pair of adjacent ones of the twelve outputs, it finds that the four points of the guard symbol and the four outputs positioned after the guard symbol show high correlation values, since the four guard symbol points are copies of the four outputs. From this, a symbol synchronization position can be specified. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-99486)

Further, Jpn. Pat. Appln. KOKAI Publication No. 2003-69546, for example, discloses a technique for transmitting, as a preamble, a known signal for synchronization, and making a receiver detect the known preamble as a symbol synchronization position.

A block signal as an IDFT output signal from a transmitter is used as a transmission symbol. The transmitter continuously transmits the transmission symbol. A receiver accurately detects the leading portion of the transmission symbol by detecting, for example, a preamble contained therein. Thus, the receiver performs symbol synchronization, and then inputs each symbol to a DFT to perform signal reproduction.

When the transmission channel has multipath characteristics, the receiver receives delayed waves as well as direct waves. Accordingly, when synchronization is established at the leading portions of direct waves, inter-symbol interference in which present and preceding symbols are mixed occurs. In the prior art, to eliminate such inter-symbol interference, a transmitter inserts, for example, a guard symbol in each symbol to be transmitted. Assuming, for example, that the output signals of an IDFT with eight input/output points are x0, x1, . . . , x7, the last four outputs X4, x5, x6and x7are copied, and the four copies are positioned before the original four outputs and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. In this case, if a multipath delay is within a time corresponding to four outputs, the time of inter-symbol interference is limited to the period of the guard symbol. Therefore, if x0, x1, . . . , x7are input to the DFT with eight input/output points, signal reproduction with suppressed inter-symbol interference can be performed. Further, if the multipath delay is longer than the above, the number of guard symbols may be increased (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 2002-374223).

A block signal as an IDFT output signal from a transmitter is used as a transmission symbol. The transmitter continuously transmits the transmission symbol. A receiver accurately detects the leading portion of the transmission symbol by detecting, for example, a preamble contained therein. Thus, the receiver performs symbol synchronization, and then inputs each symbol to a DFT to perform signal reproduction.

In OFDM transmission, the range of amplitude variation is large, therefore non-linear distortion may easily occur. Accordingly, a receiver for performing OFDM transmission needs to have an analog receiving circuit of high linear performance that can receive, without distortion, signals having significantly different amplitudes, or needs to perform control for suppressing the maximum amplitude of a transmitter output (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-46480).

When an OFDM receiver receives a transmission signal, accurate setting of the block position, i.e., symbol synchronization, is indispensable since a DFT employed therein performs block processing. In general, to realize symbol synchronization, a transmitter adds a redundant symbol to a signal to be transmitted.

For example, there is a method in which a guard symbol is inserted, and synchronization is performed using this symbol. Specifically, assuming, for example, that the output signals of an IDFT with eight input/output points are x0, x1, . . . , x7, the last four outputs x4, X5, x6and x7are copied, and the four copies are positioned before the original four outputs, and used as a guard symbol. The thus-obtained outputs, twelve outputs in total, are transmitted as one symbol. When a receiver executes correlation computation on each pair of adjacent ones of the twelve outputs, it finds that the four points of the guard symbol and the four outputs positioned after the guard symbol show high correlation values, since the four guard symbol points are copies of the four outputs. From this, a symbol synchronization position can be specified. (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 7-99486)

There is another method in which a known signal for synchronization is transmitted as a preamble, and a receiver detects the preamble to detect the symbol synchronization position (see, for example, Jpn. Pat. Appln. KOKAI Publication No. 2003-69546).

In the above-described multi-carrier transmission system, however, symbols for synchronization must be inserted to enable a receiver to perform symbol synchronization, which reduces the transmission efficiency.

Further, the above-described method for increasing the number of guard symbols is disadvantageous in that the transmission efficiency is inevitably reduced.

Concerning the analog receiving circuit of high linear performance, this circuit is expensive, therefore the use of the circuit inevitably increases the cost of the communication system. If the maximum amplitude of the transmitter output is suppressed, the feature of the OFDM transmission system cannot sufficiently be utilized.

BRIEF SUMMARY OF THE INVENTION

The present invention has been developed in light of the above-described techniques, and aims to provide a multi-carrier transmission system that realizes symbol synchronization without inserting synchronization symbols.

It is another object of the present invention to provide a multi-carrier transmission system that can reduce the degree of inter-symbol interference without reducing the transmission efficiency.

According to a first aspect of the invention, there is provided a multi-carrier transmission system comprising:

a transmitter including: an acquisition unit configured to acquire 2m(m: a natural number) modulated signals including a plurality of no-information signals which are failed to be used for information transmission and 2n(n: a natural number; n<m) signals, the acquisition unit subjecting the modulated signals to inverse discrete Fourier transformer to obtain a plurality of transformed signals, a no-information signal included in an Lthmodulated signal of the modulated signals being used as a first no-information signal of the no-information signals, every Kthmodulated signal of the modulated signals that is counted from the first no-information signal being used as a no-information signal (K: a natural number; L: an integer; K=2m−n; 0≦L≦K−1) and a transmission unit configured to transmit the transformed signals; and

a receiver including: a receiving unit configured to receive the transformed signals; and a detection unit configured to detect synchronization timing based on at least one no-information signal included in the transformed signals.

According to a second aspect of the invention, there is provided a multi-carrier transmission system comprising:

a transmitter including: an acquisition unit configured to acquire 2m(m: a natural number) modulated signals including a plurality of no-information signals which are failed to be used for information transmission, and 2n(n: a natural number; n<m) signals, the acquisition unit subjecting the 2mmodulated signals to inverse discrete Fourier transformer to obtain a plurality of transformed signals, a no-information signal included in an Lthmodulated signal of the modulated signals being used as a first no-information signal of the no-information signals, every Kthmodulated signal of the modulated signals that is counted from the first no-information signal being used as a no-information signal (K: a natural number; L: an integer; K=2m−n; 0≦L≦K−1); and a transmission unit configured to transmit 2mtransformed signals; and

a receiver including: a receiving unit configured to receive the 2mtransformed signals; a calculation unit configured to calculate, based on the 2mtransformed signals received, a constraint given by a relational expression established between the 2mreceived signals; and a correction unit configured to correct at least one of the transformed signals based on the constraint.

According to a third aspect of the invention, there is provided a multi-carrier transmission system comprising:

a transmitter including: an acquisition unit configured to acquire 2m(m: a natural number) modulated signals including a plurality of no-information signals which are failed to be used for information transmission, and 2n(n: a natural number; n<m) signals, the acquisition unit subjecting the 2mmodulated signals to inverse discrete Fourier transformer to obtain a plurality of transformed signals, a no-information signal included in an Lthmodulated signal of the modulated signals being used as a first no-information signal of the no-information signals, every Kthmodulated signal of the modulated signals that is counted from the first no-information signal being used as a no-information signal (K: a natural number; L: an integer; K=2m−n; 0≦L≦K−1); and a transmission unit configured to transmit 2mtransformed signals; and

a receiver including: a receiving unit configured to receive 2mtransmitted signals; a detection unit configured to detect 2mreceived signals which have distorted amplitudes; a correction unit configured to correct at least one of detected signals; a transforming unit configured to transform, if the correction unit fails to correct at least one of the detected signals, both the at least one detected signal corrected by the correction unit and the at least one detected signal which are failed to be corrected; and a setting unit configured to set, to no-information signals, the received signals which correspond to the no-information signals, to subject the no-information signals and a plurality of transformed signals to inverse discrete Fourier transformer, and to input, to the transforming unit, a plurality of inverse-discrete Fourier-transformed signals which correspond to a plurality of amplitude-distorted signals, as corresponding input signals for the transforming unit.

According to a fourth aspect of the invention, there is provided a multi-carrier transmission system comprising:

a transmitter including: an acquisition unit configured to acquire 2m(m: a natural number) modulated signals including a plurality of no-information signals which are failed to be used for information transmission, and 2n(n: a natural number; n<m) signals, the acquisition unit subjecting the 2mmodulated signals to inverse discrete. Fourier transformer to obtain a plurality of transformed signals, a no-information signal included in an Lthmodulated signal of the modulated signals being used as a first no-information signal of the no-information signals, every Kthmodulated signal of the modulated signals that is counted from the first no-information signal being used as a no-information signal (K: a natural number; L: an integer; K=2m−n; 0≦L≦K−1); and a transmission unit configured to transmit 2mtransformed signals of the transformed signals; and

a receiver including: a receiving unit configured to receive the transmitted 2mtransformed signals; and an estimation unit configured to estimate a value of L based on the received 2mtransformed signals.

DETAILED DESCRIPTION OF THE INVENTION

Multi-carrier transmission systems, receivers and transmitters according to embodiments of the invention will be described in detail with reference to the accompanying drawings.

In OFDM as a type of multi-carrier transmission system, a transmitter multiplexes frequency-base signals into time-base signals using an inverse discrete Fourier transformer (IDFT), while a receiver extracts frequency-base signals from received time-base signals using a discrete Fourier transformer (DFT). No further particulars of OFDM will be explained since OFDM is a well-known technique.

First Embodiment

Referring toFIG. 1, the configuration of a multi-carrier transmission system according to a first embodiment will be described.FIG. 1is a block diagram illustrating a multi-carrier transmission system according to first and second embodiments of the invention.

The multi-carrier transmission system of the embodiments at least comprises a multi-carrier transmitter10and multi-carrier receiver20.

The multi-carrier transmitter10at least includes an inverse discrete Fourier transformer (IDFT)11and transmission unit12. The multi-carrier receiver20at least includes a receiving unit21, synchronization circuit22and discrete Fourier transformer (DFT)23. In the embodiment, the IDFT11and DFT23each have eight inputs and outputs as shown inFIG. 1. However, the number of the inputs (outputs) of each of the IDFT11and DFT23is not limited to 8, but may be set to an arbitrary value. Concerning this point, a detailed description will be given later using, for example, equations [14].

The IDFT11receives eight modulated signals as input signals, subjects them to inverse discrete Fourier transform, and outputs the transformed modulated signals as output signals. If the input signals of the IDFT11are defined as X0, X1, . . . , X7, the output signals are defined as x0, x1, . . . , x7, and W=exp(−j2π/8), j2=−1, the relationships between the input and output signals are given by
xk=(⅛) (X0+W−kX1+W−2kX2+ . . . +W−7kX7)  [1]

where, for example, W−2k=(W)−2k. The IDFT11transforms the modulated signals into those determined by the equation [1].

The transmission unit12uses, as one transmission symbol, the eight output signals x0, x1, . . . , x7of the IDFT11. Thus, the IDFT11successively generates transmission symbols, and the transmission unit12transmits a sequence of transmission symbols.

In the embodiment, two of the input signals of the IDFT11, i.e., X0and X4, are set as follows:
X0=0, X4=0  [2]

If these values of X are substituted into the equation [1], constraints expressed by the following equations [3-1] and [3-2] are established:
x0+x2+x4+x6=0  [3-1]
x1+x3+x5+x7=0  [3-2]

The receiving unit21receives, as a signal sequence, a transmission symbol sequence having passed through a transmission channel30. The synchronization circuit22receives the transmission symbol sequence from the receiving unit21, extracts a series of eight signals from the transmission symbol sequence in the order of reception, and synchronizes the signal transmitted from the multi-carrier transmitter10, with the signal received by the multi-carrier receiver20.

Assume that a series of eight signals extracted by the synchronization circuit22at a certain point in time are y0, y1, . . . , y7. The DFT23subjects the signal sequence to inverse discrete Fourier transform, and outputs the resultant modulated signals as output signals. Assuming that the input and output signals of the DFT23are y0, y1, . . . , y7and Y0, Y1, . . . , Y7, respectively, the input and output signals have the following relationships:
Yk=y0+W1ky130W2ky2+ . . . +W7ky7[4]

Assume that an ideal transmission channel that is free from noise, multipath fading, etc. is used. In this case, if the output timing of eight signals from the IDFT11of the multi-carrier transmitter10is identical to the input timing of eight signals to the DFT23of the multi-carrier receiver20, i.e., if symbol synchronization is established, the following relationships are established in the time-base input signal of the DFT23:
y0+y2+y4+y6=0  [5-1]
y1+Y3+y5+y7=0  [5-2]

Further, the following relationships are established in the output frequency-base signal of the DFT23:
Y0=0, Y4=0  [6]

Since the input and output of the DFT23has a 1:1 relationship, if the equation concerning the input (or output) is established, the equation concerning the output (or input) is also established. On the other hand, if no symbol synchronization is established, none of the equations [5-1], [5-2] and [6] are established.

Referring now toFIG. 2, the synchronization operation of the synchronization circuit22will be described.FIG. 2is a view useful in explaining the process performed by the synchronization circuit22.

The synchronization circuit22extracts a sequence of received time-base signals in units of eight signals, while shifting the extraction position by one signal at a time. Specifically, as shown inFIG. 2, the circuit22firstly extracts a signal sequence sq1of x0, x1, . . . , x7as the (n−1)thoutput signal sequence of the IDFT11. Subsequently, the circuit22shifts the to-be-extracted signal sequence by one signal, and extracts x1, . . . , x7included in the (n−1)thoutput signal sequence of the IDFT11, and x0included in the nthoutput signal sequence of the IDFT11. In this way, the circuit22successively extracts signal sequences. It extracts, for example, a signal sequence sq6that is formed of x5, x6and x7included in the (n−1)thoutput signal sequence of the IDFT11, and x0, x1, x2, x3and x4included in the nthoutput signal sequence of the IDFT11.

After that, the synchronization circuit22determines, for each signal sequence formed of extracted eight signals, whether the equations [5-1] and [5-2] are established. Concerning, for example, the signal sequences shown inFIG. 2, the equations [5-1] and [5-2] are established between the signal sequence sq1, x0, x1, . . . , x7, as the (n−1)thoutput signal sequence of the IDFT11, and the corresponding received signal sequence, y0, y1, . . . , y7. Similarly, the equations [5-1] and [5-2] are established between a signal sequence sq2, signal sequence sp9and signal sequence sp10and the respective corresponding received signal sequences.

Thus, by virtue of the synchronization circuit22for determining a sequence of eight time-base signals that satisfy the above-mentioned equations, correct timing in synchrony with the output of a transmission signal from the multi-carrier transmitter10can be acquired.

In an actual transmission channel, however, the equations [5-1] and [5-2] are not established because of noise, multipath fading, etc. In light of this, signals are extracted which require power smaller than a certain value of v2, as shown in the following inequalities [7-1] and [7-2]:
(y0+y2+y4+y6)2<v2[7-1]
(y1+y3+y5+y7)2<v2[7-2]

Alternatively, y0, y1, . . . , y7that minimize the value of (y0+y2+y4+y6) and the value of (y1+y3+y5+y7) may be detected, thereby determining synchronizing timing based on the detected signals.

Although the above case utilizes the constraints on the output signals of the DFT23required when two of the input signals of the IDFT11are set to a level of 0, the embodiment is not limited to this. It is not essential to set two of the input signals of the IDFT11to a level of 0. There can be other cases. Some specific cases will be described referring toFIGS. 3A,3B,3C and3D. In the case ofFIG. 3A, all modulated signals input to the IDFT11ofFIG. 1are 4-PSK (4-Phase-Shift Keying) signals. The case ofFIG. 3Bis obtained by changing one of the modulated signals shown inFIG. 3Afrom the 4-PSK signal to a no-information signal. Similarly, the case ofFIG. 3Cis obtained by changing two of the modulated signals shown inFIG. 3Ato no-information signals. The case ofFIG. 3Dis obtained by changing four of the modulated signals shown inFIG. 3Ato no-information signals.FIG. 3Acorresponds to a prior art technique and illustrates a case where all input signals X0, X1, . . . , X7contain information, i.e., none of the signals are no-information signals.FIGS. 3B,3C and3D correspond to the present embodiment.

(Case 1) Where only one of the input signals of the IDFT11is set to a level of 0 (corresponding toFIG. 3B), i.e., where
X0=0  [8]

In this case, the constraint on the output signals of the IDFT11is given by
x0+x1+x2+x3+x4+x5+x6+x7=0  [9]

The following inequality, in which v1represents a certain power level, is used by the synchronization circuit22to detect a synchronization position:
(y0+y1+y2+y3+y4+y5+y6+y7)2<v1[10]

(Case 2) Where two of the input signals of the IDFT11is set to a level of 0 (corresponding toFIG. 3C). This case corresponds to the aforementioned case explained with reference toFIG. 1. Under the constraint expressed by the equation [2], the equations [3-1] and [3-2] are established. In this case, the synchronization circuit22detects a synchronization position using the inequalities [7-1] and [7-2].

(Case 3) Where four of the input signals of the IDFT11is set to a level of 0 (corresponding toFIG. 3D), i.e., where
X0=0, X2=0, X4=0, X6=0  [11]

In this case, the following constraints are required for the output signals of the IDFT11:
x0+x4=0  [12-1]
x1+x5=0  [12-2]
x2+x6=0  [12-3]
x3+x7=0  [12-4]

In this case, the synchronization circuit22detects a synchronization position using the following inequalities in which v4represents a certain power level:
(y0+y4)2<v4[13-1]
(y1+y5)2<v4[13-2]
(y2+y6)2<v4[13-3]
(y3+y7)2<v4[13-4]

Any one of the above equations enables symbol synchronization to be established between the multi-carrier transmitter10and multi-carrier receiver20. As is understood from the above, the larger the number of 0-level signals, the more constraints can be acquired.

In a multipath transmission channel, there may exist a symbol that arrives later than another symbol. If such a delay symbol exists, there may be a case where, for example, x0included in the nthoutput signal sequence of the IDFT11coexists with x7included in the (n−1)thoutput signal sequence of the IDFT11, resulting in inter-symbol interference. Depending upon the conditions for the transmission channel, even x1in the nthoutput signal sequence may interfere with x7included in the (n−1)thoutput signal sequence. Thus, a conditional expression for synchronization (relation expression established between output signals of the IDFT11) that includes x0, or x0and x1is easily influenced by inter-symbol interference, which makes it difficult to perform accurate synchronization. In particular, in the above case 1, there is only one conditional expression (i.e., only the equation [9]), which includes x0, or x0and x1. Therefore, it is difficult to eliminate the influence of the above-mentioned inter-symbol interference.

On the other hand, the case 2 has, as a conditional expression for synchronization, the equation [3-2] that does not include x0. Therefore, symbol synchronization can be established using the inequality [7-2] acquired from the equation [3-2]. Further, the case 3 has, as conditional expressions for synchronization, the equations [12-2] to [12-4] that do not include x0. Accordingly, symbol synchronization can be established using the inequalities [13-2] to [13-4] acquired from the equations [12-2] to [12-4]. Moreover, the case 3 has, as conditional expressions for synchronization, the equations [12-3] and [12-4] that do not include x0or x1. Accordingly, symbol synchronization can be established using the inequalities [13-3] and [13-4] even when x1is also involved in inter-symbol interference.

Transmission efficiency will now be described with reference toFIGS. 4A,4B and4C.FIG. 4Ais a view illustrating a case where modulated signals input to the IDFT11are all 4-PSK signals.FIG. 4Bis a view illustrating a case where two of the modulated signals shown inFIG. 4Aare no-information signals, and the other six modulated signals are all 4-PSK signals.FIG. 4Cis a view illustrating a case where two of the modulated signals shown inFIG. 4Aare no-information signals, and other two modulated signals are 16-QAM (Quadrature Amplitude Modulation) signals.

The transmission efficiency is lower by the transmission bits of input signals X0and X4in the case shown inFIG. 4Bwhere the two input signals X0and X4included in the IDFT input signals X0, X1, . . . , X7are signals with no information, than in the case shown inFIG. 4Awhere none of the IDFT input signals X0, X1, . . . , X7contain information.

In light of this, in the embodiment, if one input signal is made as a no-information signal, the modulation circuit13modulates, into a signal with a larger number of transmission bits, one of the IDFT input signals other than the no-information signal, as is shown inFIG. 4C. The modulation circuit13is a circuit for modulating an input signal into a modulated signal corresponding to a predetermined modulation scheme.

For instance, the modulation circuit13modulates a 4-PSK signal into a 16-QAM signal or 64-QAM signal, etc., which has a larger number of transmission bits than the former.

FIG. 4Cshows an example, where the number of transmission bits is identical to that in the example ofFIG. 4Awhere all input signals X0, X1, . . . , X7are 4-PSK signals. Since the number of transmission bits of a 16-QAM signal is double the number of transmission bits of a 4-PSK signal, two 4-PSK input signals X1, X5are replaced with respective 16-QAM signals in the example ofFIG. 4Cwhere two input signals X0, X4are no-information signals.

The case where only one of the input signals X0, X1, . . . , X7is a no-information signal is shown inFIG. 3B. Similarly, the case where four of the input signals X0, X1, . . . , X7are no-information signals is shown inFIG. 3D. These cases are applications of the case shown inFIG. 4C.

As above-mentioned, the embodiment is not limited to the use of the 16-QAM scheme as in the examples ofFIG. 4CandFIG. 3B and 3D. For example, to make the number of transmission bits identical to that in the example ofFIG. 4A, two 4-PSK signals included in X0, X1, . . . , X7may be replaced with respective 8-PSK signals. Alternatively, one 4-PSK signal included in X0, X1, . . . , X7may be replaced with a 64-QAM signal.

Further, if no-information signals are included in X0, X1, . . . , X7, and if the power is reduced by the number of the no-information signals, the resistance to errors is reduced. To prevent a reduction in resistance to errors, the embodiment employs a power-adjusting unit14for increasing the power of the modulated signals X1′ and X5′ of the 16-QAM scheme in order to make the total power of X0, X1′, . . . , X5′, . . . , X7shown inFIG. 4Cidentical to that of X0, X1, . . . , X7shown inFIG. 4A. If the former total power can be made identical to the latter, the resistance to errors can be made identical.

As described above, some of the IDFT input signals can be set to no-information signals without degrading the resistance to errors and without reducing the number of transmission bits per one symbol. In other words, the modulation scheme and power can be set on condition that the input signals of the IDFT11have the same number of bits and the same power.

However, if a reduction in the number of transmission bits by setting a certain 4-PSK input signal of the IDFT11to a level of 0 is allowed, it is not necessary to change the modulation scheme for another input signal to another multi-value modulation scheme. It is sufficient if the modulation scheme is kept at the 4-PSK scheme. Further, if a reduction in error ratio due to a change in modulation scheme for a certain input signal is allowed, no power adjustment is needed.

Even in the standard OFDM transmission system, the input signals of the IDFT11may include a no-information signal. Referring then toFIGS. 5A and 5B, a description will be given of a case where the multi-carrier transmission system of the embodiment is applied to the OFDM transmission system.FIG. 5Ais a view illustrating a case where those two of the modulated signals input to the IDFT11, which are positioned at both ends, are no-information signals.FIG. 5Bis a view illustrating a case where the positional relationship of the no-information signals shown inFIG. 5Ais changed.

In the standard OFDM transmission system, when an IDFT having 2048 input/output points is utilized, there is a case where no signals are input to several hundreds of input/output points positioned at each end of the IDFT, i.e., no-information signals are input to those input/output points.FIG. 5Aillustrates a typical case where the input signals X0and X7at both ends are no-information signals. If the present embodiment is applied to the input signal arrangement as shown inFIG. 5A, it is necessary to change the positional relationship of the no-information signals, as shown inFIG. 5B, so that the constraint on symbol synchronization (in this case, the constraint expressed by the equation [2]) can be satisfied. Since this change process is performed only by shifting the positions of no-information signals, the ratio of transmission bits to a symbol is unchanged, therefore the transmission efficiency is not reduced.

The number of no-information signals inserted can be varied in accordance with the state of the transmission channel. This will be described with reference toFIGS. 6A and 6B.FIG. 6Ais a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station50to a terminal40differs from that from the terminal40to the base station50.FIG. 6Bis a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station70to a terminal60is identical to that from the terminal60to the base station70.

The terminal40or the base station70detects the state of the transmission channel, and controls the modulation circuit contained in an OFDM transmitter52or73. For example, if the multipath delay time is long, the base station controls the modulation circuit contained in the OFDM transmitter52or73to increase the number of no-information signals to be inserted. On the other hand, if the multipath delay time is short, the base station controls the modulation circuit to reduce the number of no-information signals to be inserted. The base station detects the state of the transmission channel in the manner stated below.

FIG. 6Aillustrates frequency division duplex (FDD) communication in which up-link and down-link transmission channels are used between the base station50and terminal40. In this case, when OFDM transmission is performed from the base station50to the terminal40using the down-link transmission channel, the base station50instructs the terminal40to inform the base station of the transmission condition for the down-link transmission channel via the up-link transmission channel. Based on the transmission condition for the down-link transmission channel supplied from the terminal40, the base station50executes OFDM transmission.

More specifically, for instance, in the terminal40, a down-link transmission channel estimation unit42estimates the state of the down-link transmission channel based on a signal received by the OFDM receiver41. Subsequently, a transmitter43transmits, to the base station50, information concerning the state of the down-link transmission channel estimated by the estimation unit42. In the base station50, a receiver51receives the information concerning the state of the down-link transmission channel, and outputs the information to the OFDM transmitter52. The OFDM transmitter52transmits a signal to the terminal40, based on the input information concerning the state of the down-link transmission channel.

On the other hand,FIG. 6Billustrates time division duplex (TDD) communication in which only a single transmission channel is used as both an up-link transmission channel and down-link transmission channel between the base station70and terminal60. In this case, when OFDM transmission is performed from the base station70to the terminal60, the base station70detects a transmission condition for the down-link transmission channel, from the characteristics of a signal received. Based on the detected transmission condition for the down-link transmission channel, the base station70executes OFDM transmission.

More specifically, for instance, in the base station70, a down-link transmission-channel estimation unit72estimates the state of the down-link transmission channel from a signal received by a receiver71. Based on the estimated state, the OFDM transmitter73transmits a signal to the terminal60.

Although the above-described embodiment employs an IDFT and DFT having eight input/output points, it is a matter of course that the number of the input/output points is not limited to eight, but may be set to an arbitrary value. Specifically, in a transmitter, assuming that Xpk(p=0, 1, . . . , N−1, M=KN, N=2n) included in the input signals X0, X1, . . . , XM−1of an IDFT with M input/output points (M=2m) is set to a level of 0, the output signals x0, x1, . . . , xM−1satisfy the following equations:
xp+xp+N+ . . . +Xp+(K−1)N=0  [14]

Accordingly, symbol synchronization can be realized by detecting received signals y0, y1, . . . , yM−1that have passed through the transmission channel and satisfy the following inequalities:
(yp+Yp+N+ . . . +Yp+(K−1)N)2<v[15]

where v represents a small power value.

A method using a voltage instead of the power value v may be possible.

Further, for a DFT and IDFT having a large number of input/output points, algorithms based on fast Fourier transformer (FFT) and inverse fast Fourier transformer (IFFT) are utilized.

Second Embodiment

In the first embodiment, in the transmitter, every kthXpk(p=0, 1, . . . , N−1; M=KN; N=2n), which is included in the input signals X0, X1, . . . , XM−1of the IDFT with M input/output points (M=2m) and begins from X0, is set to a level of 0. In the second embodiment, the contents of the first embodiment are generalized, and every kthXi+pk, beginning not from X0but from Xi(i=0, 1, . . . , K−1), is set to a level of 0.

Assuming that the input signals of the IDFT are X0, X1, . . . , XM−1, the output signals of the IDFT are x0, x1, . . . , xM−1, WM=exp(−j2π/M), and j2=−1, the relationships between the input and output signals are given by
xk=(1/M) (X0+WM−kX1+WM−2kX2+ . . . +WM−(M−1)kXM−1)  [18]

(k represents an integer, and 0≦k≦M−1)

(p represents an integer, and 0≦p≦N−1)

If u0, u1, . . . , uN−1are input to a DFT with N input/output points, the output signal Uk(k represents an integer, and 0≦k≦M−1) of the DFT are given by
Uk=u0+WNku1+WN2ku2+ . . . +WN(N−1)kuN−1[20]
where WN=exp(−j2π/N)=WMK. Using the equations [19], the equations [20] can be modified in the following manner:
Uk=x0+WM(i+kK)x1+WM2(i+kK)x2+ . . . +WM(M−1)(i+kK)xM−1[21]

On the other hand, if x0, x1, . . . , xM−1are input to a DFT with M input/output points, the output signal Xk(k represents an integer, and 0≦k≦M−1)of the DFT are given by
Xk=x0+WMkx1+WM2kx2+ . . . +WM(M−1)kxM−1[22]

From the equations [21] and [22], the followings are acquired:
Xi+pK=Up[23]

This equations [24] are used as constraints on the output signals of the IDFT with the M input/output points when Xi+pk(i=0, 1, . . . , K−1, p=0, 1, . . . , N−1, M=KN, N=2n) are set to a level of 0.

These are constraints identical to those in the first embodiment. If, for example, M=8 and N=4, K is 2, and accordingly the equations [25] become:
xp+xp+4=0(p=0, 1, 2, 3)  [26]

Thus, the equations [26] are equivalent to the equations [12-1] to [12-4] derived in the first embodiment.

In general, symbol synchronization is performed by presetting, for M received signals y0, y1, . . . , yM−1, a small power value v that can be detected by the synchronization circuit, and detecting received signals that satisfy the following inequalities [27]:

Referring toFIGS. 7A,7B,7C and7D, a specific example will be described.FIG. 7Ais a view of a multi-carrier transmission system according to the second embodiment, illustrating a case where two no-information signals are input.FIG. 7Billustrates a case where the position of each no-information signal is shifted by one signal from the position shown inFIG. 7A.FIG. 7Cillustrates a case where the position of each no-information signal is shifted by two signals from the position shown inFIG. 7A.FIG. 7Dillustrates a case where the position of each no-information signal is shifted by three signals from the position shown inFIG. 7A.

FIGS. 7A,7B,7C and7D show the cases where M=8, N=2 and K=4, which correspond to i=0, 1, 2 and 3, respectively.FIG. 7Acorresponds to i=0, and the conditional expressions for synchronization are the equations [25], as described above. If 8, 2 and 4 are substituted for M, N and K, respectively, in the equations [25], the followings are acquired:
xp+xp+2+xp+4+xp+6=0(p=0, 1)  [28]

The equations [28] are equivalent to the equations [5-1] and [5-2] derived in the first embodiment.

The case where i=1 corresponds toFIG. 7B. From the equations [24], the constraints on synchronization are:

If 8, 2 and 4 are substituted for M, N and K, respectively, in the equations [29], the followings are acquired:

These equations are a conditional expression required for synchronization when i=1, M=8, N=2 and K=4.

The case where i=2 corresponds toFIG. 7C. From the equations [24], the conditional expressions for synchronization are:

If 8, 2 and 4 are substituted for M, N and K, respectively, in the equations [31], the followings are acquired:

These equations are conditional expressions required for synchronization when i=2, M=8, N=2 and K=4.

The case where i=3 corresponds toFIG. 7C. From the equations [24], the conditional expressions for synchronization are:

If 8, 2 and 4 are substituted for M, N and K, respectively, in the equations [33], the followings are acquired:

These equations are conditional expressions required for synchronization when i=3, M=8, N=2 and K=4.

As described above, in the second embodiment, the position of a no-information signal can be changed in a desired manner.

Third Embodiment

A third embodiment of the invention is acquired by combining the first embodiment with a synchronization detection method using a guard symbol. In the synchronization detection method using a guard symbol, the signals output from some latter output points of an IDFT are copied, and the copies are positioned before the signal output from the first output point, and are used as guard symbol points. Symbol synchronization is established using the correlation between the guard symbol points and original signals.

In the third embodiment, a detailed description will be given of a case, similar to the case of the first embodiment, where a transmitter has an IDFT11with eight input/output points, and a receiver has a DFT23with eight input/output points, referring toFIG. 8.FIG. 8is a block diagram illustrating a multi-carrier transmission system according to the third embodiment of the invention. In the transmitter, the input signals are defined as X0, X1, . . . , X7, and the output signals as x0, x1, . . . , x7. As in the first embodiment, input signals X0and X4are set to a level of 0. In this case, the output signals of the IDFT11satisfy the equations [3-1] and [3-2]. That is, the following equations are satisfied:
x0+x2+x4+x6=0
x1+x3+x5+x7=0

In the third embodiment, to set a guard symbol, x6and x7are copied and positioned before X0, as shown inFIG. 8. More specifically, the last two of x0, x1, . . . , x7, i.e., x6and x7, are positioned before x0and used as a guard symbol. As a result, the combination of x6, x7, x0, x1, x2, x3, x4, x5, x6and x7is used as a single transmission symbol. Thus, transmission symbols are sequentially generated and a resultant transmission symbol sequence is transmitted via the transmission channel30.

The receiver receives the transmission symbol sequence as an adjacent signal sequence, and extracts therefrom eight sequential signals at certain timing, and regards them as received signals y0, y1, . . . , y7. If this extraction is performed at correct timing where there is no noise or multipath fading, the followings are established:
y0+y2+y4+y6=0
y1+y3+y5+y7=0

While the position of extraction of eight signals is shifted, the timing at which the signals that satisfy the above equations are extracted is detected as synchronization timing. However, in actual transmission, in which noises, for example, are mixed, a synchronization circuit221detects, as synchronization timing, the detection timing of the signals that satisfy the above equations, the total power of which is minimum. Alternatively, synchronization timing may be extracted by extracting signals, the total power of which is lower than a certain power value as in the inequalities [7-1] and [7-2].

On the other hand, since the transmission symbol is the combination of x6, x7, x0, x1, x2, x3, x4, x5, x6and x7, the received symbol has y6and y7placed before the combination of y1, y1, . . . , y7. Accordingly, when eight signals are extracted at correct timing, the last two signals of the eight signals are identical to the two signals placed before the eight signals. In other words, the same combinations of signals Y6and y7exist with six signals y0, y1, y2, y3, y4and y5interposed therebetween.

Using this regularity, the synchronization circuit221extracts the combination of two signals while shifting the position of extraction, thereby detecting, as synchronization timing, the timing at which the correlation of such combinations of two signals is maximum, i.e., the timing at which the addition result of multiplied values is maximum.

In actual degraded transmission in which noise and multipath fading, etc. exist, the power of the total sum of the received signals in the above relational expressions employed in the first embodiment is increased, and the correlation value is reduced in the synchronization detection method using a guard symbol. Utilizing these features, degradation of the synchronization detection accuracy in a degraded transmission environment can be suppressed.

For instance, the synchronization circuit221executes synchronization detection using a guard symbol in a relatively satisfactory transmission environment, and executes both the method using a guard symbol and the synchronization detection method employed in the first embodiment, in a degraded transmission environment. In the latter case, the position which both the above two methods regard as a synchronization position is used as a synchronization position.

Fourth Embodiment

Referring toFIG. 9, the configuration of a multi-carrier transmission system according to a fourth embodiment will be described.FIG. 9shows the multi-carrier transmission system of the fourth embodiment.

As shown, a multi-carrier transmitter10at least includes an inverse discrete Fourier transformer (IDFT)11and transmission unit12. A multi-carrier receiver20at least includes a receiving unit21, switching units (22-1,2-3,22-3,22-4) and discrete Fourier transformer (DFT)23. In the fourth embodiment, the IDFT11and DFT23each have sixteen inputs and outputs as shown inFIG. 9. However, the number of the inputs (outputs) of each of the IDFT11and DFT23is not limited to 16, but may be set to an arbitrary value. Concerning this point, a detailed description will be given later using, for example, equations [A8-1], [A8-2], [A8-3], and [A8-4].

The IDFT11receives sixteen modulated signals as input signals, subjects them to inverse discrete Fourier transform, and outputs the transformed modulated signals as output signals. If the input signals of the IDFT11are defined as X0, X1, . . . , X15, the output signals are defined as x0, x1, . . . , x15, and W=exp(−j2π/16), j2=−1, the relationships between the input and output signals are given by

The transmission unit12uses, as one transmission symbol, the sixteen output signals x0, x1, . . . , x15of the IDFT11. Thus, the IDFT11successively generates transmission symbols, and the transmission unit12transmits a sequence of transmission symbols.

In the fourth embodiment, four of the input signals of the IDFT11, i.e., X0, X4, X8and X12, are set as follows:
X0=0, X4=0, X8=0, X12=0  [A2]

If these values of X are substituted into the equation [A1], constraints expressed by the following equations [A3-1] to [A3-4] are established, as are also expressed by equations [A35]:
x0+x4+x6+x12=0  [A3-1]
x1+x5+x9+x13=0  [A3-2]
x2+x6+x10+x14=0  [A3-3]
x3+x7+x11+x15=0  [A3-4]

The receiving unit21receives, as a signal sequence y0, y1, . . . , y15, a transmission symbol sequence having passed through a transmission channel30. The switching units (22-1,2-3,22-3,22-4) are connected to positions of a transmission symbol that are expected to be inter-symbol interference occurrence positions. In the example ofFIG. 9, y0, y1, y2and y3included in received signals y0, y1, . . . , Y15are input to the switching units. Each switching unit performs switching utilizing the constraints.

Assuming here that the transmission channel30is an ideal channel free from noise, multipath fading, etc., if a boundary between two symbols is detected at correct timing in the symbol sequence received by the multi-carrier receiver20, i.e., if accurate symbol synchronization is performed, the followings are established between the time-base signals:
xk=yk(k=0, 1, . . . , 15)  [A4-1]

Since, in general, each input and corresponding output of a DFT is in a one for one relationship, if the equation concerning the input or output is established, the other equation is also established. On the other hand, if signal transmission is out of synchrony with signal reception, i.e., if symbol synchronization is not established, the above equations [A4-1] or [A4-2] are not established.

Accordingly, if the transmission channel is an ideal one, constraints expressed by the following equations are established between the received signals y0, y1, . . . , Y15from the above equations [A3-1] to [A3-4], [A4-1] and [A4-2]:
y0+y4+y8+y12=0  [A5-1]
y1+y5+y9+y13=0  [A5-2]
y2+y6+y10+y14=0  [A5-3]
y3+y7+y11+y15=0  [A5-4]

Since it is estimated that inter-symbol interference occurs at y0, y1, y2and y3, these received signals y0, y1, y2and y3are not reliable signals. However, when signal transmission is performed under the constraint expressed by the equation [A1], the above equations [A6-1] to [A6-4] are established if the transmission channel is an ideal one. The received signals y4, y5, . . . , Y15are considered reliable since they are substantially free from inter-symbol interference. Therefore, if y0, y1, y2and y3are replaced with other signals using the equations [A6-1] to [A6-4], signals y0′, y1′, y2′ and y3′ corresponding to y0, y1, y2and y3and free from inter-symbol interference can be acquired.

The DFT23performs discrete Fourier transform on a signal sequence, and outputs the resultant signal sequence as an output signal sequence. Specifically, assuming that the input and output signals of the DFT23are y0′, y1′, y2′, Y3′, Y4, . . . , y7and Y0, Y1, Y2, Y3, Y4, . . . , Y7, respectively, the input and output signals have the following relationships:

Some examples in which the switching units (22-1,2-3,22-3,22-4) perform switching of received signals will be described. In the above-mentioned example, the received signals y0, y1, y2and y3are interfered by the preceding transmission symbol. Other types of inter-symbol interference may occur.

Example (1-1): Assume that, in an ideal transmission channel, the received signals y0and y1are interfered by the preceding transmission symbol, and the received signal y15is interfered by the next transmission symbol. In this case, the following inequalities and equation are established:
y0+y4+y8+y12≠0  [A8-1]
y1+y5+y9+y13≠0  [A8-2]
y2+y6+y10+y14=0  [A8-3]
y3+y7+y11+y15≠0  [A8-4]

This inter-symbol interference can be eliminated using the equations [A6-1] and [A6-2] and the following equation [A9] that is acquired from the equation [A5-4].
y15=−y3−y7−y11[A9]

Thus, inter-symbol interference can be eliminated without a guard symbol, if the received signals are switched appropriately using the constraints established therebetween.

The transmission channel30is assumed so far to be an ideal one. Actually, however, noise may well exist in the transmission channel30. Therefore, it is needed to determine whether the channel is an ideal one. To this end, some of the equations [A5-1] to [A5-4] as the constraints on noise determination are utilized. Specifically, since it is known, depending upon the transmission/reception system used, at which received signals inter-symbol interference occurs, noise determination is performed, using equations that express constraints concerning received signals free from inter-symbol interference.

Example (1-2): Assume that, the received signals y0and Y1are interfered by the preceding transmission symbol, the received signal Y15is interfered by the next transmission symbol, and noise exists in the transmission channel. In this case, the following inequalities and equation are established:
y0+y4+y8+y12≠0  [A10-1]
y1+y5+y9+Y13≠0  [A10-2]
y2+y6+y10+Y14=v≠0  [A10-3]
y3+y7+y11+y15≠0  [A10-4]

The inequalities [A10-1] and [A10-2] express cases in which no constraint is established because of the influence of inter-symbol interference and noise. The equation [A10-3] expresses a case where inter-symbol interference does not exist but noise exists. If the transmission channel is an ideal one in which no noise exists, the equation [A10-3] is identical to the equation [A5-3]. Therefore, the closer to 0 the left part of the equation [A10-3], the lower the noise. Conversely, the remoter from 0, the higher the noise. In light of this, the degree of influence of noise can be determined from a value of power at which any constraint, which is established between received signals that are detected in an ideal transmission channel and are free from inter-symbol interference, is not established. In the example (1-2), the influence of noise is determined from whether the value v of the equation [A10-3] is high or low.

For example, if v is less than a certain value, noise is considered to be low, thereby regarding the transmission channel30as ideal. After that, like the example (1-1), the received signals y0, y1and y15are replaced with other appropriate signals, using the equations [A6-1] and [A6-2] and the equation [A9] acquired from the equation [A5-4], thereby appropriately eliminating inter-symbol interference. On the other hand, if v is not less than the certain value, noise is considered to be high, thereby determining that the transmission channel30cannot be regarded as ideal. In this case, control is performed so as not to perform the elimination of inter-symbol interference based on the equations [A6-1] and [A6-2] and the equation [A9] acquired from the equation [A5-4]. This is because noise is too high and therefore a significant error may occur if it is assumed that the constraints are established. The value v is preset in accordance with, for example, the level of a signal transmitted from a transmitter, or the performance of a receiver.

In the above-described examples, four no-information signals are assigned to each transmission symbol. However, the number of no-information signals is not limited to 4. Variations will now be described.

Example (2-1): Where only one input signal input to the IDFT11is set to a no-information signal, as expressed by, for example, the following equation:
X0=0  [A11]

In this case, the constraint established between the output signals of the IDFT11is given by
x0+x1+x2+ . . . +x14+x1=0  [A12]

This constraint can be used where a single received signal is interfered. More specifically, the constraint can be used when only y0is interfered by the preceding transmission symbol, or only y15is interfered by the next transmission symbol. Further, when inter-symbol interference exists, the level of noise may be determined depending upon the constraint.

Example (2-2): Where two input signals input to the IDFT11are set to no-information signals, as given by, for example, the following equations:
X0=0, X8=0  [A13]

In this case, the constraints expressed by the following equations are established between the output signals of the IDFT11:
x0+x2+x4+ . . . +x12+x14=0  [A14-1]
x1+x3+x5+ . . . +x13+x15=0  [A14-2]

These constraints can be used where two received signals are interfered. More specifically, the constraints can be used when y0and y1are interfered by the preceding transmission symbol, or y14and y15are interfered by the next transmission symbol. Further, if a single received signal is interfered by the preceding or next transmission symbol where two constraints exist, the level of noise can be determined using the constraint equations irrelevant to the signal. For example, where it is known that only y0is interfered, it is checked how far the value of
y1+y3+y5+ . . . +y13+y15[A14-2-1]
corresponding to the left part of the equation [A14-2] is from 0. If it is determined that noise does not have a significant impact as stated above, it is sufficient if the following equation [A14-1-1]
y0=−y2−y4−y6− . . . −y12−y14[A14-1-2]
is extracted from the equation [A14-1], thereby correct y0. On the other hand, the value of [A14-2-1] is far from 0, which means that noise has a significant impact and hence unignorable, no correction for y0is performed.

Example (2-3): Where four input signals input to the IDFT11are set to no-information signals (X0=0, X4=0, X8=0, X12=0). This case has already been described in detail with reference to the equations [A2] et seq.

Further, if, for example, received signals y0, y1, y2and y3are interfered by the preceding transmission symbol, and received signals y14and y15are interfered by the next transmission symbol, these received signals can be corrected, using the following equations:
y0=−y8, y1=−y9, y2=−y10, y3=−y11, y14=−Y6, y15=−y7[A18]

In this case, two constraints included in the constraints expressed by equations [A16-1] to [A16-8] are not used for correcting inter-symbol interference. Therefore, these two constrains can be utilized for determining the influence of noise. Specifically, it is determined whether each of u and v in the following equations [A19] are not less than a given value.
y4+y12=u, y5+y13=v[A19]

If each of u and v is not less than the given value, it is determined that the noise level is high, and equations [A18] are not utilized. On the other hand, each of u and v is less than the given value, it is determined that the noise level is low, and equations [A18] are utilized to correct interfered signals. Further, if either u or v is less than the given value, control is performed in which, for example, the difference between u and v is measured, and only when this difference is relatively small, interfered received signals are corrected.

As described above, the larger the number of no-information signals, the larger the number of acquired constraints independent of each other, and the larger the number of interfered signals that can be corrected.

However, as the number of no-information signals is increased, the transmission efficiency is reduced. Referring now toFIGS. 10A,10B and10C, a method for preventing the transmission efficiency from reduction will be described. Although an IDFT15described below has eight input/output points, the number of input/output points is not limited to this. The method is applicable to an IDFT with an arbitrary number of input/output points.FIG. 10Ais a view illustrating a case where modulated signals input to the IDFT15with eight input/output points are all 4-PSK signals.FIG. 10Bis a view illustrating a case where two of the modulated signals shown inFIG. 10Aare no-information signals, and the other six modulated signals are all 4-PSK signals.FIG. 10Cis a view illustrating a case where two of the modulated signals shown inFIG. 10Aare no-information signals, and other two modulated signals are 16-QAM signals.

Compared to the case ofFIG. 10Awhere none of the input signals X0, X1, . . . , X7of the IDFT15are no-information signals, the transmission efficiency is reduced, in the case ofFIG. 10Bwhere X0and X4included in the input signals X0, X1, . . . , X7of the IDFT15, by the number of transmission bits of X0and X4.

In light of this, in the fourth embodiment, if one input signal is made as a no-information signal, the modulation circuit13modulates, into a signal with a larger number of transmission bits, one of the IDFT input signals other than the no-information signal. The modulation circuit13is a circuit for modulating an input signal into a modulated signal corresponding to a predetermined modulation scheme.

For instance, the modulation circuit13modulates a 4-PSK signal into a 16-QAM signal or 64-QAM signal, etc., which has a larger number of transmission bits than the former.

FIG. 10Cshows an example where the number of transmission bits is identical to that in the example ofFIG. 10Awhere all input signals X0, X1, . . . , X7are 4-PSK signals. Since the number of transmission bits of a 16-QAM signal is double the number of transmission bits of a 4-PSK signal, two 4-PSK input signals are replaced with respective 16-QAM signals in the example ofFIG. 10Cwhere two no-information signals are input.

The fourth embodiment is not limited to the use of the 16-QAM scheme as in the example ofFIG. 10C. For example, to make the number of transmission bits identical to that in the example ofFIG. 10A, two 4-PSK signals included in X0, X1, . . . , X7may be replaced with respective 8-PSK signals. Alternatively, one 4-PSK signal included in X0, X1, . . . , X7may be replaced with a 64-QAM signal.

Further, if no-information signals are included in X0, X1, . . . , X7, and if the power is reduced by the number of the no-information signals, the resistance to errors is reduced. To prevent a reduction in resistance to errors, the embodiment employs a power-adjusting unit14for increasing the power of the modulated signals X1′ and X5′ of the 16-QAM scheme in order to make the total power of X0, X1′, . . . , X5′, . . . , X7shown inFIG. 10Cidentical to that of X0, X1, . . . , X7shown inFIG. 10A. If the former total power can be made identical to the latter, the resistance to errors can be made identical.

As described above, some of the IDFT input signals can be set to no-information signals without degrading the resistance to errors and without reducing the number of transmission bits per one symbol. In other words, the modulation scheme and power can be set on condition that the input signals of the IDFT15have the same number of bits and the same power.

However, if a reduction in the number of transmission bits by setting a certain 4-PSK input signal of the IDFT11to a level of 0 is allowed, it is not necessary to change the modulation scheme for another input signal to another multi-value modulation scheme. It is sufficient if the modulation scheme is kept at the 4-PSK scheme. Further, if a reduction in error ratio due to a change in modulation scheme for a certain input signal is allowed, no power adjustment is needed.

The number of no-information signals input to the IDFT11can be varied in accordance with the state of the transmission channel. This will be described with reference toFIGS. 11A and 11B.FIG. 11Ais a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station50to a terminal40differs from that from the terminal40to the base station50.FIG. 11Bis a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station70to a terminal60is identical to that from the terminal60to the base station70.

The terminal40or the base station70detects the state of the transmission channel, and controls the modulation circuit contained in an OFDM transmitter52or73. For example, if the multipath delay time is long, the base station controls the modulation circuit contained in the OFDM transmitter52or73to increase the number of no-information signals to be inserted. On the other hand, if the multipath delay time is short, the base station controls the modulation circuit to reduce the number of no-information signals to be inserted. The base station detects the state of the transmission channel in the manner stated below.

FIG. 11Aillustrates frequency division duplex (FDD) communication in which up-link and down-link transmission channels are used between the base station50and terminal40. In this case, when OFDM transmission is performed from the base station50to the terminal40using the down-link transmission channel, the base station50instructs the terminal40to inform the base station of the transmission condition for the down-link transmission channel via the up-link transmission channel. Based on the transmission condition for the down transmission channel supplied from the terminal40, the base station50executes OFDM transmission.

More specifically, for instance, in the terminal40, a down-link transmission channel estimation unit42estimates the state of the down-link transmission channel based on a signal received by the OFDM receiver41. Subsequently, a transmitter43transmits, to the base station50, information concerning the state of the down-link transmission channel estimated by the estimation unit42. In the base station50, a receiver51receives the information concerning the state of the down-link transmission channel, and outputs the information to the OFDM transmitter52. The OFDM transmitter52transmits a signal to the terminal40, based on the input information concerning the state of the down-link transmission channel.

On the other hand,FIG. 11Billustrates time division duplex (TDD) communication in which only a single transmission channel is used as both an up-link transmission channel and down-link transmission channel between the base station70and terminal60. In this case, when OFDM transmission is performed from the base station70to the terminal60, the base station70detects a transmission condition for the down-link transmission channel, from the characteristics of a signal received. Based on the detected transmission condition for the down-link transmission channel, the base station70executes OFDM transmission.

More specifically, for instance, in the base station70, a down-link transmission-channel estimation unit72estimates the state of the down-link transmission channel from a signal received by a receiver71. Based on the estimated state, the OFDM transmitter73transmits a signal to the terminal60.

Although the above-described embodiment employs an IDFT and DFT having sixteen input/output points, it is a matter of course that the number of the input/output points is not limited to sixteen, but may be set to an arbitrary value. Specifically, in a transmitter, assuming that Xpk(p =0, 1, . . . , N−1, M=KN, N=2n) included in the input signals X0, X1, . . . , XM−1of an IDFT with M input/output points (M=2m) is set to a level of 0, the output signals x0, x1, . . . , xM−1satisfy the following equations:
xp+xp+N+ . . . +xp+(K−1)N=0  [A20]

Accordingly, assuming that the received signals having passed through the transmission channel are y0, y1, . . . , yM−1, v is fine power, and the noise level is low, maximum number N interfered received signals can be corrected, using the following equations:
yp+yp+N+ . . . +yp+(K−1)N≈0  [A22]

Further, DFTs and IDFTs with a large number of input/output points utilize algorithms of fast Fourier transform (FFT) and inverse FFT.

Fifth Embodiment

In the above-described fourth embodiment, in the transmitter, every kthXpk(p =0, 1, . . . , N−1, M=KN, N=2n), which is included in the input signals X0, X1, . . . , XM−1of the IDFT with M input/output points (M=2m) and begins from X0, is set to a level of 0. In the fifth embodiment, the contents of the fourth embodiment are generalized, and every kthXL+pk, beginning not from X0but from XL(L=0, 1, . . . , K−1), is set to a level of 0.FIG. 12is a block diagram illustrating a multi-carrier transmission system according to the fifth embodiment of the invention.FIG. 12shows a case where every fourth signal beginning from X2is set to a level of 0, i.e., where L=2 and K=4. Referring to the example ofFIG. 12, a description will be given of a case where L and K assume respective arbitrary values.

Assuming that the input signals of the IDFT are X0, X1, . . . , XM−1, the output signals of the IDFT are x0, x1, . . . , xM−1, WM=exp(−j2π/M), and j2=−1, the relationships between the input and output signals are given by

(k represents an integer, and 0≦k≦M−1)

Further, up is defined for the output signals x0, x1, . . . , xM−1of the IDFT, using the following equations:

(p represents an integer, and 0≦p≦N−1)

If u0, u1, . . . , uN−1is input to a DFT with N input/output points, the output signal Uk(k represents an integer, and 0≦k≦M−1)of the DFT are given by

Uk=u0+WNk⁢u1+WN2⁢k⁢u2+…+WN(N-1)⁢k⁢uN-1[A26]
where WN=exp(−j2π/N)=WMK. Using the equations [A25], the equations [A26] can be modified in the following manner:

On the other hand, if x0, x1, . . . , xM−1is input to a DFT with M input/output points, the output signal Xk(k represents an integer, and 0≦k≦M−1)of the DFT is given by

From the equations [A27] and [A28], the followings are acquired:
Xi+pK=Up[A29]

In the equations [A29], if Xi+pk=Up=0, the output signal up of an IDFT with N input/output points assumed when U0, U1, . . . , UN−1are input thereto is naturally up=0 (p=0, 1, . . . , N−1). Accordingly, from the equations [A25], the followings are acquired:

Each of the left and right parts of each of the equations [A30] is divided by WMpL. Further, if WMN=exp(−j2πN/M)=exp(−j2π/K)=WKis considered, then the following equations are acquired from the above equation [A30]:

The equations [A31] are used as constraints on the output signals of the IDFT with the M input/output points when XL+pk(L=0, 1, . . . , K−1, p=0, 1, . . . , N−1, M=KN, N=2n) is set to a level of 0.

These equations express the constraint employed in the fourth embodiment. If M=16 and N=4, K=4. Accordingly, the equations [A33] become:
xp+xp+4+xp+8+xp+12=0  [A34]

Thus, the equations [A33] are identical to the equations [A3-1], [A3-2], [A3-3] and [A3-4] extracted in the fourth embodiment.

From the equations [A31], the following equations are established for a series of received signals y0, Y1, . . . , YM−1:

The equations [A35] enable maximum number N interfered signals to be corrected. In the fifth embodiment, the degree of freedom in positioning a no-information signal at the transmit side is increased compared to the fourth embodiment.

In the example ofFIG. 12, M=16, L=2 and K=4, therefore N=2 and the following equations are established:
yp+W42yp+4+W44yp+8+W46yp+12=0  [A36]

The equations [A36] enable four interfered signals, at maximum, to be corrected.

Sixth Embodiment

The sixth embodiment is obtained by combining the fourth embodiment with an inter-symbol interference reduction method using a guard symbol. In the inter-symbol interference reduction method using a guard symbol, the last several ones of the output signals of an IDFT are copied before the first output signal and used as a guard symbol, thereby absorbing any inter-symbol interference that may occur on the guard symbol, to protect information generated by a transmitter. Even if the guard symbol copied before the original output signals is interfered by the preceding transmission symbol, the information contained in the original signals located after the guard symbol is protected from the interference.

Referring now toFIG. 13, a detailed description will be given of a multi-carrier transmission system according to the sixth embodiment in which a multi-carrier transmitter10has an IDFT11with sixteen input/output points and a multi-carrier receiver20has a DFT23with sixteen input/output points, as in the fourth embodiment.FIG. 13is a block diagram illustrating the multi-carrier transmission system of the sixth embodiment. In the transmitter10of the system, the input signals of the IDFT11are defined as X0, X1, . . . , X15, the output signals are defined as x0, x1, . . . , x15. Further, it is assumed that X0=0, X4=0, X8=0 and X12=0. In this case, the constraints expressed by the following equations are established between the output signals of the IDFT11as in the fourth embodiment:
x0+x4+x8+x12=0  [A3-1]
x1+x5+x9+x13=0  [A3-2]
x2+x6+x10+x14=0  [A3-3]
x3+x7+x11+x15=0  [A3-4]

The multi-carrier receiver20receives, as an adjacent signal sequence, the transmission symbol sequence supplied through the transmission channel, thereby extracting a sequence of sixteen signals at certain timing, and regarding it as a received-signal sequence of y0, y1, . . . , Y15. In other words, the multi-carrier receiver20extracts, as y0, y1, . . . , y15, received signals detected immediately after each guard symbol. As a result, even if the four signals included in each guard symbol are interfered ones, they are not extracted by the receiver20, which means that the influence of inter-symbol interference can be avoided.

The sixth embodiment is characterized in that if inter-symbol interference occurs at signals positioned after each guard symbol, the signals interfered by the inter-symbol interference are corrected. Specifically, as shown inFIG. 13, interfered signals y0, y1, y2, y3are corrected, utilizing the equations [A6-1] to [A6-4] employed in the fourth embodiment. Thus, the sixth embodiment is useful in a case where inter-symbol interference may well occur at signals positioned after a guard symbol. The manner of correction performed in this case is similar to that described in the fourth embodiment, therefore no detailed description is given thereof.

Seventh Embodiment

Referring toFIG. 14, the configuration of a multi-carrier transmission system according to a seventh embodiment will be described.FIG. 14is a block diagram illustrating the common configuration of multi-carrier transmission systems according to seventh and eighth embodiments of the invention.

The multi-carrier transmission system of the embodiment at least comprises a multi-carrier transmitter10and multi-carrier receiver20.

The multi-carrier transmitter10at least includes an inverse discrete Fourier transformer (IDFT)11and transmitting unit12. The multi-carrier receiver20at least includes a receiving unit21, amplitude detector22, determination unit23, switch unit24, discrete Fourier transformer (DFT)25, IDFT26, memory27and controller28. In the embodiment, the IDFTs11and26and DFT25each have eight inputs and outputs as shown inFIG. 14. However, the number of the inputs (outputs) of each of the IDFTs11and26and DFT25is not limited to 8, but may be set to an arbitrary value. Concerning this point, a detailed description will be given later using, for example, equations [B17].

The IDFT11receives eight modulated signals as input signals, subjects them to inverse discrete Fourier transform, and outputs the transformed modulated signals as output signals. If the input signals of the IDFT11are defined as X0, X1, . . . , X7, the output signals are defined as x0, x1, . . . , x7, and W8=exp(−j2π/8), j2=−1, the relationship between the input and output signals is given by
xk=(1/8)(X0+W8−kX1+W8−2kX2+ . . . +W8−7kX7)  [B1]

where, for example, W8−2k=(W8)−2k. The IDFT11transforms the modulated signals into those determined by the equation [B1].

The transmitting unit12uses, as one transmission symbol, the eight output signals x0, x1, . . . , x7of the IDFT11. Thus, the IDFT11successively generates transmission symbols, and the transmitting unit12transmits a sequence of transmission symbols.

In the seventh embodiment, two of the input signals of the IDFT11, i.e., X0and X4, are set as follows:
X0=0, X4=0  [B2]

If these values of X are substituted into the equations [B1], constraints expressed by the following equations [B3-1] and [B3-2] are established:
x0+x2+x4+x6=0  [B3-1]
x1+x3+x5+x7=0  [B3-2]

The receiving unit21receives, as an adjacent signal sequence, a transmission symbol sequence having passed through a transmission channel (not shown). In the seventh embodiment, the receiving unit21is formed of a receiving amplifier with a saturation input/output characteristic. Alternatively, an analog-to-digital (A/D) converter of a limited level is interposed between the IDFT11and the output of the receiving unit21. The amplitude detector22detects distorted signals included in the output signals of the receiving unit21. However, the amplitude detector22cannot detect the amplitude of a large-amplitude signal that is not distorted. When there exists such a receiving amplifier of a saturation input/output characteristic or level-limited AD converter as stated above, if the output signals of the IDFT11include a so-called large-amplitude signal having an amplitude larger than a value determined by the receiving amplifier or AD converter, the amplitude detector22cannot detect the accurate amplitude of the large-amplitude signal. This will be described with reference toFIGS. 15A,15B and15C.

Assuming that an ideal transmission channel that is free from noise, multipath fading, etc., if a boundary between the two symbols is detected at correct timing in the symbol sequence received by the multi-carrier receiver20, i.e., if accurate symbol synchronization is performed, the following is established concerning the time-based signal:
xk=yk(k=0, 1, . . . , 7)  [B4-1]

Since each input and corresponding output of a DFT is in a one for one relationship, if the equations concerning the input or output are established, the other equations are also established. On the other hand, if signal transmission is not in synchrony with signal reception, i.e., if symbol synchronization is not established, the above equations [B4-1] or [B4-2] are not established.

Accordingly, when an ideal transmission channel is used, constraints given by the following equations are established between the received signals y0, y1, . . . , y7from the equations [B3-1], [B3-2] and [B4-1]:
y0+y2+y4+y6=0  [B5-1]
y1+y3+y5+y7=0  [B5-2]

The determination unit23determines, in units of transmission symbols, which one of the eight signals received by the receiving unit21is a large-amplitude signal (i.e., which signal is distorted), and executes a predetermined process based on the determination result. If a large-amplitude signal is included in the received signals, the constraints given by the equations [B5-1] and [B5-2] are used to determine whether the received signal determined to be a large-amplitude signal can be replaced with a received signal determined not to be a large-amplitude signal. For example, if the amplitude detector22determines that y1and y4are large-amplitude signals as shown inFIG. 14, y1and y4can be replaced with other signals in the following manners, using the equations [B5-1] and [B5-2]:
y4=−y0−y2−y6[B6-1]
y1=−y3−y5−y7[B6-2]

Thus, y1and y4can be replaced with y0, y2, y3, y5, y6and y7determined not to be large-amplitude signals. Since it is considered that the signals determined not to be large-amplitude signals are correctly received ones, they are reliable signals. On the other hand, it is considered that the received signals y1and y4determined to be large-amplitude signals are distorted and different from the original. This means that they are not reliable signals. If the received signals y1and y4can be replaced with other reliable signals as indicated by the equations [B6-1] and [B6-2] acquired from the equations [B5-1] and [B5-2], it is considered that they are corrected. Thus, reliable signals can be acquired if any received signal determined to be a large-amplitude signal can be replaced with received signals determined not to be large-amplitude signals, using a constraint.

Further, assume that received signals determined to be large-amplitude signals cannot be replaced with received signals determined not to be large-amplitude signals, using constraints. In this case, the determination unit23performs the following. The number of unreliable received signals is minimized in a pre-process, using a constraint, whereby a particular one of the output signals of the DFT25is input to the IDFT26, and one of the output signals of the IDFT26that corresponds to each unreliable received signal is input to the DFT25. Particulars concerning this process will be described later with reference toFIGS. 16 and 17. If this process is repeated a predetermined number of times, any unreliable received signal can be replaced with a reliable one. However, unless the number of no-information signals set in one transmission symbol is larger than that of pre-processed signals (i.e., the number of unreliable received signals), it is not guaranteed whether all received signals can be replaced with respective reliable signals. This problem of guarantee will be described later with reference toFIG. 18.

The switch unit24uses constraints established between received signals to replace received signals determined to be large-amplitude signals with received signals determined not to be large-amplitude signals. For instance, in the case ofFIG. 14, the received signal y1is replaced with the right part of the equation [B6-2], while y4is replaced with the right part of the equation [B6-1]. The switch unit24receives, from the determination unit23, a result of determination as to whether which one(s) of the received signals is a large-amplitude signal, and replaces the large-amplitude received signal(s), using the constraints.

The DFT25subjects a signal sequence to discrete Fourier transform, and outputs the resultant modulated signals as output signals. Assuming that the input and output signals of the DFT25are y0, y1, . . . , y7and Y0, Y1, . . . , Y7, respectively, the input and output signals have the following relationship:
Yk=y0+W81ky1+W82ky2+ . . . +W87ky7[B7]

The IDFT26receives the output signals of the DFT25, subjects them to inverse discrete Fourier transform, and outputs those of the transformed signals that correspond to the received signals determined to be large-amplitude signals by the amplitude detector22. In the example ofFIG. 14, for example, the output signals Y0, Y1, . . . , Y7of the DFT25are input as input signals U0, U1, . . . , U7to the IDFT26. Since Y0=0 and Y4=0 from the equations [B2] and [B4-2], U0=0 and U4=0. Assuming that the output signals of the IDFT26corresponding to U0, U1, . . . , U7are u0, u1, . . . , u7, the input and output signals of the IDFT26have the following relationships:
uk=(⅛) (U0+W8−kU1+W8−2kU2+ . . . +W8−7kU7)  [B8]

The signals included in uk(k=0, 1, . . . , 7) and determined to be large-amplitude signals by the amplitude detector22are output to the DFT25. In the example ofFIG. 14, since the received signals y1and Y4determined to be large-amplitude signals are already replaced with reliable signals in the pre-process using constraints, it is not necessary to correct the received signals y1and y4using the IDFT26. In this case, the determination unit23supplies the controller28with a control signal indicating that the IDFT26should be turned off. The controller28, in turn, outputs a control signal to the IDFT26to turn off the IDFT26.

The memory27stores the levels (e.g. amplitudes) of all signals output from the IDFT26. The memory27is used to monitor any signal level that is input from the IDFT26to the DFT25and processed by the DFT25, and stores signal levels so that which one of the is received signals included in each transmission symbol is indicated by one of the signal levels. More specifically, concerning each transmission symbol, the memory27stores two signals, i.e., a signal that has been just processed by the IDFT26and output from the DFT25, and a signal output from the DFT24and having processed by the IDFT26a number of times (including 0 time) smaller by one time than the first-mentioned signal.

The controller28receives the two output signal levels and calculates the absolute value of the difference therebetween. If the absolute value of the difference is not higher than a predetermined value, the controller28supplies the IDFT26with a signal for turning off the IDFT26. The controller estimates the degree of correction performed by the IDFT26on received signals determined to be large-amplitude signals (i.e., determined to be distorted signals), thereby determining when the process of operating the IDFT26should be stopped.

Alternatively, the time when the process of operating the IDFT26is stopped may be determined from the number of occasions in which the IDFT26and/or DFT25is operated for each transmission symbol. The controller28supplies the DFT25and IDFT26with respective operation signals for causing them to operate. The controller28includes, for example, a counter for holding the number of occasions in which each operation signal is output, and where each operation signal is output. As a result, the controller28can monitor the number of occasions in which each operation signal is output for each transmission symbol.

The above-described structure can correct any large-amplitude received signal (i.e., any distorted received signal), thereby providing accurate received signals.

Referring now toFIGS. 15A,15B and15C, a description will be given of the mechanism of occurrence of a distorted received signal detected by the amplitude detector22.FIG. 15Ais a graph illustrating the relationship between the time before an input signal is input to the receiving unit21including a non-linear circuit, and the amplitude of the input signal.FIG. 15Bis a graph illustrating the input/output characteristic of a non-linear circuit having a saturation characteristic (clipping characteristic).FIG. 15Cis a graph illustrating the relationship between the time and the amplitude of a signal output from the receiving unit21including the non-linear circuit.

If the receiving unit21has a non-linear circuit of a clipping characteristic as shown inFIG. 15B, and if the input signal level of the non-linear circuit is higher than a value of a, the output signal level of the non-linear circuit always becomes a value of b. Further, if the input signal level of the non-linear circuit is higher than a value of −a, the output signal level of the non-linear circuit always becomes a value of −b. Accordingly, if the signal as shown inFIG. 15Ais input to a non-linear circuit of the clipping characteristic as shown inFIG. 15B, the signal as shown inFIG. 15Cis output from the non-linear circuit. When the amplitude detector22detects a signal having a non-smooth portion (more particularly, a non-differentiable portion) as shown inFIG. 15C, it determines that the signal is a distorted received signal.

On the other hand, if the input signal level of the non-linear circuit falls between −a and a, an output signal level proportional to the input signal level is output. For example, inFIG. 15A, the signal portion having a negative amplitude that falls between −a and a is output as a smooth signal portion that reproduces the original input signal, as is shown inFIG. 15C.

In the example described so far with reference toFIG. 14, each unreliable received signal can be corrected simply by a pre-process. A description will now be given of a case where signal correction is performed not only by the pre-process but also by the use of the IDFT26, referring toFIG. 16.FIG. 16is a block diagram illustrating a structural example used for performing the pre-process and correcting received signals utilizing the IDFT26. The block diagram ofFIG. 16differs from that ofFIG. 14, in that in the former, each transmission symbol processed by the multi-carrier transmitter10and multi-carrier receiver20includes sixteen signals. Accordingly, in the case ofFIG. 16, each of the IDFT11, transmitting unit12, receiving unit21, DFT25and IDFT26has sixteen input/output points.

In the example ofFIG. 16, the IDFT11receives sixteen modulated signals as input signals, subjects them to inverse discrete Fourier transform, and outputs the transformed modulated signals as output signals. If the input signals of the IDFT11are defined as X0, X1, . . . , X15, the output signals are defined as x0, x1, . . . , x15, and W16=exp(−j2π/16), j2=−1, the relationship between the input and output signals is given by
xk=( 1/16) (X0+W16−kX1+W16−2kX2+ . . . +W16−15kX15)  [B9]

In the example ofFIG. 16, four input signals included in input signals X0, X1, . . . , X15of the IDFT11are set to no-information signals as expressed by the following equations:
X0=0, X4=0, X8=0, X12=0  [B10]

If these values of X are substituted into the equations [B9], constraints expressed by the following equations [B11-1] to [B11-4] are established between the output signals of the IDFT11, as is also given by equations [B26] recited later:
x0+x4+x8+x12=0  [B11-1]
x1+x5+x9+x13=0  [B11-2]
x2+x6+x10+x14=0  [B11-3]
x3+x7+x11+x15=0  [B11-4]

Assuming that an ideal transmission channel that is free from noise, multipath fading, etc., as in the case ofFIG. 14, if a boundary between the two symbols is detected at correct timing in the symbol sequence received by the multi-carrier receiver20, i.e., if accurate symbol synchronization is performed, the following is established concerning the time-based signal:
xk=yk(k=0, 1, . . . , 15)  [B12-1]

Since each input and corresponding output of a DFT is in a one for one relationship, if the equations concerning the input or output are established, the other equations are also established. On the other hand, if signal transmission is not in synchrony with signal reception, i.e., if symbol synchronization is not established, the above equations [B12-1] or [B12-2] are not established.

Accordingly, if the transmission channel is an ideal one, constraints expressed by the following equations are established between the received signals y0, y1, . . . , y15from the above equations [B11-1] to [B11-4], [B12-1] and [B12-2]:
y0+y4+y8+y12=0  [B13-1]
y1+y5+y9+y13=0  [B13-2]
y2+y6+y10+y14=0  [B13-3]
y3+y7+y11+y15=0  [B13-4]

In the example ofFIG. 16, the amplitude detector22determines that the received signals y0, y5, y6and y10are large-amplitude signals. The received signals y0and y5can be appropriately corrected using the equations [B13-1] and [B13-2], as in the case ofFIG. 14. In other words, the received signals y0and y5can be replaced with reliable signals. However, each of the received signals y6and y10cannot be corrected since they are both included in the equation [B13-3]. Therefore, in this example, the IDFT26is used to correct the received signals Y6and y10.

To correct the received signals Y6and y10, the determination unit23operates the DFT25and IDFT26via the controller28. Assume that the frequency-based input signals of the IDFT26are U0, U1, . . . , U15, and the time-based output signals of the IDFT26are u0, u1, . . . , u15.

(Step 2) The received signals y6and y10are input to the DFT25after they are processed using the equations [B16]. Concerning the other received signals y0, y1, y2, y3, y4, y5, y7, y8, y9, y11, y12, y13, y14and y15, i.e., the pre-processed signals and the reliable signals that do not have to be pre-processed, the outputs of the receiving unit21are directly input to the DFT25. The DFT25performs DFT operations on those input signals.

By the repetition of the steps 1 and 2, the received signals Y0, Y4, Y8and Y12become closer to 0. This is equivalent to that the received signals Y6and Y10become closer to their respective levels assumed before the signals are distorted. Thus, repetition of the steps 1 and 2 enables acquisition of the levels of the signals assumed before they are distorted.

As shown inFIG. 17, DFT and IDFT operations are repeated to eliminate distortions, based on the time-based constraint that the received signals other than the received signals y6and y10are not distorted, i.e., the actual levels of the received signals other than the received signals y6and y10are known, and the frequency-based constraint that y6=0, y4=0, y8=0, Y12=0, and the actual levels of the received signals other than the received signals Y6and Y10are known.

FIG. 17is a view illustrating the operations for eliminating distortions in each distorted received signal.

This principal is described as a method for correcting a degraded image signal in, for example, the following documents 1 and 2:

These documents describe image signal correcting methods in which DFT and IDFT operations are repeated based on time-based and frequency-based constraints, thereby correcting degraded image signals.

From these documents, the following is understood: When the frequency-based input signals of the IDFT of a transmitter include N no-information signals, the constraint that the frequency-based output signals of the DFT of a receiver include N no-information signals occurs. If predetermined operations are executed using, as unknown signals, the maximum number N distorted signals included in the time-based input signals of the IDFT of the receiver, the unknown signals can be corrected. Therefore, actually, in the case of, for example,FIG. 14where N=2 and two distorted received signals exist, the distorted received signals can be corrected simply by executing the above-described steps, i.e., without pre-processing. Similarly, in the case ofFIG. 16, where N=4 and four distorted received signals exist, the distorted received signals can be corrected simply by executing the above-described steps, i.e., without pre-processing.

However, in the case of correcting a received signal by pre-processing, simple numerical value replacement is performed. Therefore, the required processing speed and throughput are significantly lower than in the case of executing the above-described steps. Even in the case of executing the above-described steps, the smaller the number of to-be-corrected received signals, the earlier the correction process is finished, i.e., the smaller the number of repetitions of the steps. Thus, it is preferable that the above steps be executed when necessary after pre-processing is performed as far as possible.

It has been assumed so far that the transmission channel is an ideal one. In actual transmission channels, however, noise may well exist. Therefore, it is necessary to determine whether the transmission channel is an ideal one. For noise determination, some of the equations [B13-1] to [B13-4] as constraints are utilized. In the case ofFIG. 16, since y0, y5, y6and y10are distorted received signals, the equations [B13-4] are relational expressions that are related only to received signals of reliable levels, i.e., non-distorted received signals. Accordingly, if the equation [B13-4] is not established, i.e., if y3+y7+y11+y15≠0, it is estimated that there is an influence of noise in the transmission channel. Since the equation [B13-4] is established in an ideal channel, it is considered that the closer to 0 the left part of the equation [B13-4], the lower the level of noise. Conversely, the remoter from 0, the higher the level of the noise. In light of this, the degree of influence of noise can be determined from a value of power at which any constraint, which is established between received signals that are detected in an ideal transmission channel, is not established. In this case, the influence of noise is determined from whether the value v of (y3+y7+y11+y15) is high or low.

For example, if v is less than a certain value, noise is considered to be low, and the transmission channel30is regarded as ideal. After that, the received signals y0, y1and y15are replaced, as pre-processing, with other appropriate signals, using the equations [B13-1] to [B13-4], thereby appropriately correcting the distorted received signals y0and y5. On the other hand, if v is not less than the certain value, noise is considered to be high, thereby determining that the transmission channel cannot be regarded as ideal, and performing no pre-processing that uses the equations [B13-1] to [B13-4]. In this case, noise is too high to estimate that the constraints are established. The value v is preset in accordance with, for example, the level of a signal transmitted from a transmitter, or the performance of a receiver.

If it is determined that the noise level in the transmission channel is high and hence pre-processing cannot be performed, the above-described steps 1 and 2 are repeated without executing pre-processing. Since pre-processing is not executed, replacement in the equations [B16] is not performed. Thus, any received signal detected as a large-amplitude signal (distorted signal) by the amplitude detector22is corrected.

In the above cases, the number of distorted received signals included in one transmission symbol is not more than the number of no-information signals included in one transmission symbol. Referring now toFIG. 18, a description will be given of a case where the number of distorted received signals included in one transmission symbol is more than the number of no-information signals included in one transmission symbol, for example, the former is 3 and the latter is 2. The example ofFIG. 18is similar to the example ofFIG. 14except that the received signals detected as large-amplitude signals (i.e., distorted signals) by the amplitude detector22are y1, y4and y5. In the description below, only the points differing from those in the example ofFIG. 14will be described in detail.

The receiving unit21of the multi-carrier receiver20receives, as received signals y0, y1, . . . , y7, a single transmission symbol transmitted from the multi-carrier transmitter10. The equations [B5-1] and [B5-2] are established between the received signals. The amplitude detector22detects that y1, y4and y5included in y0, y1, . . . , y7are large-amplitude signals. The determination unit23determines that y1, y4and y5are large-amplitude signals, and also determines whether each signal determined to be a large-amplitude one can be replaced with signals determined not to be large-amplitude signals. In the example ofFIG. 18, the received signal y4can be replaced with only reliable signals (signals determined not to be large-amplitude ones), as is evident from the equations [B5-1] and [B6-1]. On the other hand, both y1and y5are both included in the equation [B5-2], and therefore cannot be replaced with only reliable signals.

Thus, three large-amplitude signals can be reduced to two large-amplitude signals by executing pre-processing. In the example ofFIG. 18, one transmission symbol contains two no-information signals and it is considered that the number of large-amplitude signals (non-corrected signals) can be reduced to 2. Accordingly, the received signal y1and y5can be corrected by twice repeating the above-mentioned steps 1 and 2.

Referring toFIGS. 19A,19B and19C, the transmission efficiency will be described.FIG. 19Ais a view illustrating a case where modulated signals input to the IDFT appearing inFIG. 14are all 4-PSK signals.FIG. 19Bis a view illustrating a case where two of the modulated signals shown inFIG. 19Aare no-information signals, and the other six modulated signals are all 4-PSK signals.FIG. 19Cis a view illustrating a case where two of the modulated signals shown inFIG. 19Aare no-information signals, and other two modulated signals are 16-QAM signals.

In the case shown inFIG. 19Bwhere X0and X4included in the input signals X0, X1, . . . , X7of the IDFT11are no-information signals, the transmission efficiency is lower by the transmission bits of the no-information signals X0and X4than the case shown inFIG. 19Awhere none of the input signals X0, X1, . . . , X7are no-information signals.

In light of this, in the embodiment, when one input signal is set to a no-information signal, the modulation circuit13modulates, into a signal of a larger number of bits, one of the input signals of the IDFT11other than the no-information signal, as is shown inFIG. 19C. The modulation circuit13is a circuit for modulating an input signal into a modulated signal corresponding to a predetermined modulation scheme.

For instance, the modulation circuit13modulates a 4-PSK signal into a 16-QAM signal or 64-QAM signal, etc., which has a larger number of transmission bits than the former.

FIG. 19Cshows an example, where the number of transmission bits is identical to that in the example ofFIG. 19Awhere all input signals X0, X1, . . . , X7are 4-PSK signals. Since the number of transmission bits of a 16-QAM signal is double the number of transmission bits of a 4-PSK signal, two 4-PSK input signals are replaced with respective 16-QAM signals in the example ofFIG. 19Cwhere two input signals are no-information signals.

As above-mentioned, the embodiment is not limited to the use of the 16-QAM scheme as in the examples ofFIG. 19C. For example, to make the number of transmission bits identical to that in the example ofFIG. 19A, two 4-PSK signals included in X0, X1, . . . , X7may be replaced with respective 8-PSK signals. Alternatively, one 4-PSK signal included in X0, X1, . . . , X7may be replaced with a 64-QAM signal.

Further, if no-information signals are included in X0, X1, . . . , X7, and if the power is reduced by the number of the no-information signals, the resistance to errors is reduced. To prevent a reduction in resistance to errors, the embodiment employs a power-adjusting unit14for increasing the power of the modulated signals X1′ and X5′ of the 16-QAM scheme in order to make the total power of X0, X1′, . . . , X5′, . . . , X7shown inFIG. 19Cidentical to that of X0, X1, . . . , X7shown inFIG. 19A. If the former total power can be made identical to the latter, the resistance to errors can be made identical.

As described above, some of the IDFT input signals can be set to no-information signals without degrading the resistance to errors and without reducing the number of transmission bits per one symbol. In other words, the modulation scheme and power can be set on condition that the input signals of the IDFT11have the same number of bits and the same power.

However, if a reduction in the number of transmission bits by setting a certain 4-PSK input signal of the IDFT11to a level of 0 is allowed, it is not necessary to change the modulation scheme for another input signal to another multi-value modulation scheme. It is sufficient if the modulation scheme is kept at the 4-PSK scheme. Further, if a reduction in error ratio due to a change in modulation scheme for a certain input signal is allowed, no power adjustment is needed.

The number of no-information signals included in each transmission symbol input to the IDFT11can be changed in accordance with the state of the transmission channel. This will be explained referring toFIGS. 20A and 20B.FIG. 20Ais a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station50to a terminal40differs from that from the terminal40to the base station50.FIG. 20Bis a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station70to a terminal60is identical to that from the terminal60to the base station70.

The terminal40or the base station70detects the state of the transmission channel, and controls the modulation circuit contained in an OFDM transmitter52or73. For example, if the multipath delay time is long, the base station controls the modulation circuit contained in the OFDM transmitter52or73to increase the number of no-information signals to be inserted. On the other hand, if the multipath delay time is short, the base station controls the modulation circuit to reduce the number of no-information signals to be inserted. The base station detects the state of the transmission channel in the manner stated below.

FIG. 20Aillustrates frequency division duplex (FDD) communication in which up-link and down-link transmission channels are used between the base station50and terminal40. In this case, when OFDM transmission is performed from the base station50to the terminal40using the down-link transmission channel, the base station50instructs the terminal40to inform the base station of the transmission condition for the down-link transmission channel via the up-link transmission channel. Based on the transmission condition for the down-link transmission channel supplied from the terminal40, the base station50executes OFDM transmission.

More specifically, for instance, in the terminal40, a down-link transmission channel estimation unit42estimates the state of the down-link transmission channel based on a signal received by the OFDM receiver41. Subsequently, a transmitter43transmits, to the base station50, information concerning the state of the down-link transmission channel estimated by the estimation unit42. In the base station50, a receiver51receives the information concerning the state of the down-link transmission channel, and outputs the information to the OFDM transmitter52. The OFDM transmitter52transmits a signal to the terminal40, based on the input information concerning the state of the down-link transmission channel.

On the other hand,FIG. 20Billustrates time division duplex (TDD) communication in which only a single transmission channel is used as both an up-link transmission channel and down-link transmission channel between the base station70and terminal60. In this case, when OFDM transmission is performed from the base station70to the terminal60, the base station70detects a transmission condition for the down-link transmission channel, from the characteristics of a signal received. Based on the detected transmission condition for the down-link transmission channel, the base station70executes OFDM transmission.

More specifically, for instance, in the base station70, a down-link transmission-channel estimation unit72estimates the state of the down-link transmission channel from a signal received by a receiver71. Based on the estimated state, the OFDM transmitter73transmits a signal to the terminal60.

In the cases shown inFIGS. 14,16and18, reverse discrete Fourier converters having eight or sixteen input/output points are employed. A description will now be given of an IDFT having M input/output points (M=2m; m is a positive integer).

Assuming that the input signals of the IDFT are X0, X1, . . . , XM−1, the output signals of the IDFT are x0, x1, . . . , xM−1(M=8 in the cases ofFIGS. 14 and 18, and M=16 in the case ofFIG. 16), WM=exp(−j2π/M), and j2=−1, the relationship between the input and output signals is given by

If u0, u1, . . . , uN−1is input to a DFT with N input points, the output signal Uk(k represents an integer, and 0≦k≦M−1)of the DFT is given by

Uk=u0+WNk⁢u1+WN2⁢k⁢u2+…+WN(N-1)⁢k⁢uN-1[B19]
where WN=exp(−j2π/N)=WMK. Using the equations [B19], the equations [B20] can be modified as follows:

On the other hand, if x0, x1, . . . , xM−1is input to a DFT with M input points, the output signal Xk(k represents an integer, and 0≦k≦M−1) of the DFT is given by

From the equations [B20] and [B21], the followings are acquired:
Xi+pK=Up[B22]

Each of the left and right parts of each of the equations [B23] is divided by WMpL. Further, if WMN=exp(−j2πN/M)=exp(−j2π/K)=WKis considered, then the following equations are acquired from the above equations [B23]:

These equations [B24] are use as a constraint on the output signals of the IDFT with the M input/output points when XL+pk(L=0, 1, . . . , K−1; p=0, 1, . . . , N−1; M=KN, N=2n) is set to a level of 0.

These are constraints on the systems shown inFIGS. 14,16and18. If, for example, M=16 and N=4, K is 4, and accordingly the equations [B25] become:
xp+xp+4+xp+8+xp+12=0  [B26]

Thus, the equations [B26] are equivalent to the equations [B11-1] to [B11-4] derived in the seventh embodiment.

From the equations [B24], the relationships expressed by the following equations are established between M serial received signals y0, y1, . . . , yM−1:

Using the equations [B27], maximum number N large-amplitude received signals can be corrected.

Large-amplitude received signals are subjected to pre-processing that uses the equations [B27], and any received signal that cannot be corrected by pre-processing is subjected to the above-described steps.

As described above, even if a receiver having a saturation characteristic is used, received signals free from distortion can be acquired. Further, if the frequency-based input signals of the IDFT of a transmitter include N no-information signals, the constraint that N signals included in the frequency-based output signals of the DFT of a receiver have a level of 0 occurs. In this case, when maximum number N distorted signals included in the time-based input signals of the IDFT of the receiver are regarded as unknown signals and corrected by repeating a certain operation, the number of repeated operations can be reduced by beforehand replacing some of the unknown signals with known signals using the IDFT input constraint. As a result, the entire signal process can be performed quickly.

Even in a standard OFDM transmission system, the input signals of the IDFT11may include a no-information signal. Referring then toFIGS. 21A and 21B, a description will be given of a case where a multi-carrier transmission system according to an eighth embodiment is applied to the OFDM transmission system.FIG. 21Ais a view illustrating a case where those two of the modulated signals input to the IDFT11, which are positioned at both ends, are no-information signals.FIG. 21Bis a view illustrating a case where the positional relationship of the no-information signals shown inFIG. 21Ais changed.

In the standard OFDM transmission system, when an IDFT having 2048 input/output points is used, there is a case where no signals are input to several hundreds of input points positioned at each end of the IDFT, i.e., no-information signals are input to those input points.FIG. 21Aillustrates a typical case where input signals X0and X7at both ends are no-information signals. In the case ofFIG. 21A, the constraint that the input signals located at the opposite ends are no-information signals exists, therefore two distorted received signals, at maximum, can be corrected by repeating the steps 1 and 2 described in the seventh embodiment. However, in the arrangement shown inFIG. 21A, in which every Ath(A=2, 4, 8) signal is set to a no-information signal, the constraints on pre-processing described in the seventh embodiment are not satisfied. Accordingly, the speedup of the entire process as realized in the seventh embodiment is impossible.

Therefore, to establish the pre-processing constraints described in the seventh embodiment, the signal X4input to the IDFT11is set to a no-information signal as shown inFIG. 21B. In this case, if the transmission channel is an ideal one, the constraints expressed by the following equations are established between the input signals yk(k=0, 1, . . . , 7) of the DFT of the receiver as in the seventh embodiment:
y0+y2+y4+y6=0  [B5-1]
y1+y3+y5+y7=0  [B5-2]

By using these constraints as pre-processing before executing the steps 1 and 2, distorted received signals can be corrected.

Furthermore, fast Fourier transformers (FFT) and inverse fast Fourier transformers (IFFT) may be utilized instead of all DFTs and IDFTs employed in the seventh and eighth embodiments.

Ninth Embodiment

Referring toFIG. 22, the configuration of a multi-carrier transmission system according to a ninth embodiment will be described.FIG. 22is a block diagram illustrating the configuration of the multi-carrier transmission systems.

The multi-carrier transmission system of the embodiment at least comprises a multi-carrier transmitter10and multi-carrier receiver20.

The multi-carrier transmitter10at least includes an inverse discrete Fourier transformer (IDFT)11and transmitting unit12. The multi-carrier receiver20at least includes a receiving unit21, extracting unit22, four-point discrete Fourier transformer (4-DFT)23, estimation circuit24and DFT25. In the ninth embodiment, the IDFT11and DFT25each have eight inputs and outputs as shown inFIG. 22. However, the number of the inputs (outputs) of each of the IDFT11and DFT25is not limited to 8, but may be set to an arbitrary value. Concerning this point, a detailed description will be given later using, for example, equations [C8].

The IDFT11receives eight modulated signals as input signals, subjects them to inverse discrete Fourier transform, and outputs the transformed modulated signals as output signals. If the input signals of the IDFT11are defined as X0, X1, . . . , X7, the output signals are defined as x0, x1, . . . , x7, and W8=exp(−j2π/8), j2=1, the relationship between the input and output signals is given by
xk=(1/8) (X0+W8−kX1+W8−2kX2+ . . . +W8−7kX7) (k=0, 1, . . . , 7)   [C1]
where, for example, W8−2k=(W8)−2k. The IDFT11transforms the modulated signals into those determined by the equations [C1].

The transmitting unit12uses, as one transmission symbol, the eight output signals x0, x1, . . . , x7of the IDFT11. Thus, the IDFT11successively generates transmission symbols, and the transmitting unit12transmits a sequence of transmission symbols.

FIG. 22shows a case where L=0. From the equations [C1] and [C2], constraints given by the following relational expressions [C3-1] and [C3-2] are established between the output signals of the IDFT11on condition that W4=exp(−j2π/4), as indicated by equations [C15] used in a tenth embodiment, described later:
x0+W4Lx2+W42Lx4+W43Lx6=0  [C3-1]
x1+W4Lx3+W42Lx5+W43Lx7=0  [C3-2]

The receiving unit21receives, as an adjacent signal sequence of y0, y1, . . . , y7, a transmission symbol sequence having passed through a transmission channel30. The extraction unit22receives the transmission symbol sequence from the receiving unit21, extracts, therefrom, signal sequences of yp, yp+2, yp+4and yp+6(p: 0, 1), and outputs the extracted signal sequences to the 4-DFT23. Assuming here that the transmission channel30is an ideal one free from noise, multipath fading, etc., if a boundary between the two symbols is detected at correct timing in the symbol sequence received by the multi-carrier receiver20, i.e., if accurate symbol synchronization is performed, the following is established between the time-base signals:
xk=yk(k=0, 1, . . . , 7)  [C4-1]

Since, in general, each input and corresponding output of a DFT is in a one for one relationship, if the equations concerning the input or output are established, the other equations are also established. On the other hand, if signal transmission is out of synchrony with signal reception, i.e., if symbol synchronization is not established, the above equations [C4-1] or [C4-2] are not established.

The 4-DFT23has four input/output points and receives the signal sequence of yp, yp+2, yp+4and yp+6. The 4-DFT23performs DFT transform on the signals to calculate SP,Lusing the following equations:
Sp,L=yp+W4Lyp+2+W42Lyp+4+W43Lyp+6(p=0, 1)  [C5]

Thus, the 4-DFT23, receiving the signal sequence of yp, yp+2, yp+4and yp+6, performs discrete Fourier transform concerning the signals input to the 4 (=K) input/output points, and outputs a signal sequence of Sp,0, Sp,1, Sp,2and Sp,3. Since in the ninth embodiment, p=0, 1, the 4-DFT23performs four-point discrete Fourier transform twice (=N). The 4-DFT23outputs the calculated Sp,Lto the estimation circuit24. Sp,L=0 (p=0, 1) is a constraint on the equations [C3-1] and [C3-2]. The estimation circuit24sequentially receives Sp,L(p=0, 1; L=0, 1, 2, 3), thereby determining whether each Sp,Lis 0, and estimating the value of L. In other words, the estimation circuit24estimates the value of L that makes Sp,L0 when p=0, 1.

Accordingly, even if the multi-carrier receiver20cannot detect those two (=N) of the eight (=M) modulated signals input to the IDFT11of the multi-carrier transmitter10, which are set to a level of 0, i.e., even if the receiver cannot detect the value of the non-negative integer L not higher than N−1 and included in the equations [C2], the estimation circuit24can estimate the value of L. Specifically, the estimation circuit24examines which ones of the four output signals Sp,0, Sp,1, Sp,2and Sp,3(p=0, 1) output when the four signals yp, yp+2, yp+4and yp+6are input to the 4-DFT23with four (=K) input/output points are 0, thereby estimating the value of L included in Sp,Lwith the level of 0. Thus, the estimation circuit24can detect the two (=N) of the eight (=M) modulated signals input to the IDFT11, which are set to a level of 0. This will be described later in more detail with reference toFIGS. 23A,23B,23C and23D and24.

Referring now toFIGS. 23A,23B,23C and23D, a description will be given of the principle used by the estimation circuit24to estimate the value of L.FIG. 23Ais a view illustrating the relationship between no-information signals in IDFT11and Sp,L(p=0, 1; L=0, 1, 2, 3) when the value of L that makes Sp,L0 is 0.FIG. 23Bis a view illustrating the relationship between no-information signals in IDFT11and Sp,L(p=0, 1; L=0, 1, 2, 3) when the value of L that makes Sp,L0 is 1.FIG. 23Cis a view illustrating the relationship between no-information signals in IDFT11and Sp,L(p=0, 1; L=0, 1, 2, 3) when the value of L that makes Sp,L0 is 2.FIG. 23Dis a view illustrating the relationship between no-information signals in IDFT11and Sp,L(p=0, 1; L=0, 1, 2, 3) when the value of L that makes Sp,L0 is 3.

The estimation circuit24receives Sp,0, Sp,1, Sp,2and Sp,3from the 4-DFT23with the four input/output points. Since the 4-DFT23simultaneously outputs four signals Sp,o, Sp,1, Sp,2and Sp,3, the estimation circuit24simultaneously receives them. Accordingly, in the ninth embodiment, the estimation circuit24receives all signals Sp,L(p=0, 1; L=0, 1, 2, 3) required for determining the value of L, after the 4-DFT23performs discrete Fourier transform twice.

Conversely, if it can be determined whether the value of Sp,L(p=0, 1; L=0, 1, 2, 3) is 0, which ones of X0, X1, . . . , X7are set to a level of 0, i.e., the value of L, can be determined.

Referring then toFIG. 24, a description will be given of how the multi-carrier receiver20estimates the value of L included in received signals.FIG. 24is a view useful in explaining successive reception of transmission symbols by the multi-carrier receiver20, and determination of a value L for each transmission symbol by the estimation circuit24. In the case ofFIG. 24, the insertion positions of no-information signals are changed in units of transmission symbols. Specifically, in the ithtransmission symbol, no-information signals are inserted at the position L=0. In the (i+1)thtransmission symbol, no-information signals are inserted at the position L=2. Further, in the (i+2)thtransmission symbol, no-information signals are inserted at the position L=0.

Concerning received time-base signals y0, y1, y2, y3, y4, y5, y6and y7, the extraction unit22extracts a sequence of signals in units of eight signals, while shifting the extraction position by one signal at a time. For instance, as shown inFIG. 24, firstly, a signal sequence sq1of received signals y0, y1, y2, y3, y4, y5, y6and y7as the ithoutput signal sequence of the IDFT11is extracted. Subsequently, the extraction position is shifted by one signal, and the next signal sequence sq2is extracted, which is formed of the received signals y1, y2, y3, y4, y5and y6of the ithoutput signal sequence of the IDFT11, and the received signal y0of the (i+1)thoutput signal sequence of the IDFT11. The extraction unit22repeats the same extraction as the above. Thus, the extraction unit22extracts, for example, a signal sequence sq6that is formed of the received signals y5, y6and y7of the ithoutput signal sequence of the IDFT11, and the received signals y0, Y1, y2, y3and y4of the (i+1)thoutput signal sequence of the IDFT11.

The extraction unit22reorders each extracted sequence of eight signals into y0, y1, y2, y3, y4, y5, y6and y7, and extracts a signal sequence of yp, yp+2, yp+4and yp+6in which p is 0, and a signal sequence of yp, yp+2, yp+4and yp+6in which p is 1. The unit22outputs these signal sequences to the 4-DFT23. More specifically, in the case of the signal sequence sq6inFIG. 24, y5, y6and y7of the ithoutput signal sequence of the IDFT11, and y0, y1, y2, y3and y4of the (i+1)thoutput signal sequence of the IDFT11are reordered into y0, y1, y2, y3, y4, y5, y6and y7. That is, y5of the ithoutput signal sequence is replaced with y0, y6of the ithoutput signal sequence with y1, y7of the ithoutput signal sequence with y2, y0of the (i+1)thoutput signal sequence with y3, y1of the (i+1)thoutput signal sequence with y4, y2of the (i+1)thoutput signal sequence with y5, y3of the (i+1)thoutput signal sequence with y6, and y4of the (i+1)thoutput signal sequence with y7. After that, a signal sequence of yp, yp+2, yp+4and yp+6, in which p is 0, and a signal sequence of yp, yp+2, yp+4and yp+6, in which p is 1, are extracted from the signal sequence of y0, y1, y2, y3, y4, y5, y6and y7, and are output to the 4-DFT23.

Based on the signal sequence of yp, yp+2, yp+4and yp+6(p=1, 0), the 4-DFT23calculates SP,L(p=0, 1; L=0, 1, 2, 3), and outputs it to the estimation circuit24. The estimation circuit24, in turn, determines whether SP,Lis 0 (p=0, 1; L=0, 1, 2, 3), thereby acquiring the value of L that satisfies SP,L=0 (p=0, 1) on condition that the other values of L do not make SP,L0.

For example, in the case of the signal sequence sq1, Sp,0=0 (p=0, 1) and SP,L=0 (p=0, 1; L=1, 2, 3). Accordingly, the estimation circuit24estimates that L=0. Similarly, in the case of a signal sequence sq10, Sp,2=0 (p=0, 1) and SP,L=0 (p=0, 1; L=0, 1, 3). Accordingly, the estimation circuit24estimates that L=2. In the signal sequences sq1to sq10shown inFIG. 24, any other value of L does not satisfy SP,L=0 (p=0, 1).

Thus, the estimation circuit24can determine the value of L that satisfies SP,L=0 (p=0, 1) on condition that the other values of L do not make SP,L0. Accordingly, even if the multi-carrier receiver20cannot detect those two (=N) of the eight (=M) modulated signals input to the IDFT11of the multi-carrier transmitter10, which are set to a level of 0, i.e., even if the receiver cannot detect the value of the non-negative integer L not higher than N−1 and included in the equations [C2], the estimation circuit24can estimate the value of L.

In an actual transmission channel, the equations [C4-1] and [C4-2] are not satisfied because of the influence of noise, multipath fading, etc. Therefore, a certain value v2is preset, and if the power level is slower than v2, as is given by the following inequality, SP,Lis considered to be 0, thereby determining the value of L:
(SP,L)2<v2[C7]

There is another method for determining the value of L. In this method, in each signal sequence of yp, yp+2, yp+4and yp+6(p=0, 1) output from the extraction unit22, if the output signals S0,L(L=0, 1, 2, 3) and S1,L(L=0, 1, 2, 3) output from the 4-DFT23have respective minimum power levels, they are regarded as 0, thereby determining the value of L. More specifically, the one of the four signals Sp,L(p=0; L=0, 1, 2, 3) output from the 4-DFT23, which has the minimum power level, is regarded as 0, and the one of the next four signals Sp,L(p=1; L=0, 1, 2, 3) output from the 4-DFT23, which has the minimum power level, is also regarded as 0. If a value of L is found which satisfies SP,L=0 (p=0, 1) on condition that the other values of L do not make SP,L0, it is regarded as the target value of L.

Referring now toFIG. 25, a description will be given of an information example contained in a transmission symbol by setting a value L in the symbol.FIG. 25is a view illustrating an example case where unique values of L are assigned to respective base stations.

In the case ofFIG. 25, different base stations transmit signals using different values of L. In other words, in different base stations, the insertion position of each no-information signal differs. In this case, different values of L are assigned to different cells that indicate the service areas of the base stations. Each terminal prestores, in its memory, a table, for example, which stores base stations and their values of L so that it can detect, from each value of L, the base station from which it has received a transmission symbol. By virtue of this, mobile terminals can detect the base station they are now accessing, i.e., the cell they now belong to. In the example ofFIG. 25, the terminal is receiving a symbol of L=0, therefore detects that it belongs to the cell of a base station35. Further, if the terminal shifts to the cell of another base terminal and receives a transmission symbol therefrom, it can detect the cell to which it belongs.

Although in the example ofFIG. 25, the terminal recognizes each base station (cell) by changing the value of L between 0, 1, 2 and 3, the embodiment is not limited to this. For example, the terminal can detect the base station that has sent a transmission symbol thereto, from a pattern of values of L included in transmission symbols. Specifically, the terminal can detect to which cell it now belongs, from a pattern of, for example, three values of L included in three successive transmission symbols. For example, the base station35utilizes a pattern of values of L included in three successive transmission symbols, i.e., 0, 2 and 0. Similarly, the base station36utilizes a pattern of 2, 1 and 3, the base station37a pattern of 2, 2 and 2, and the base station38a pattern of 3, 0 and 2. Thus, if L has four values of 0, 1, 2 and 3, and three successive transmission symbols are utilized to discriminate each cell (base station), 64 (=4×4×4) cells (base stations) can be discriminated at maximum. The larger the range of the values of L, and the larger the number of successive transmission symbols, the larger the number of recognizable cells (base stations).

Other than the above, various types of information can be assigned to transmission symbols using the values of L. For example, when a plurality of transmission symbols are sent to a terminal, the values of L can indicate the boundaries between the two symbols of successively transmitted symbols. Specifically, when one hundred of transmission symbols are transmitted as one packet, to indicate boundaries of packets, the value of L included in the first and last transmission symbols of each packet are set to, for example, 0, and the value of L included in the other transmission symbols of each packet are set to, for example, 1. In this case, packet boundaries are detected between the transmission symbols with L of 0.

Further, a destination to which a transmission symbol is sent can also be designated using a value of L. If a base station transmits different types of information to users1and2, the values of L included in, for example, four successive transmission symbols to be sent to user1as one packet are set to 0, 1, 2 and 3, while the values of L included in, for example, four successive transmission symbols to be sent to user2as one packet are set to 1, 0, 3 and 2. By setting different patterns of values of L for different users, the base station can transmit information to individual users. In this case, the user terminals can detect the boundaries of packets. User1detects a boundary when the value of L shifts from 3 to 0, while user2detects a boundary when the value of L shifts from 2 to 1.

Furthermore, the modulation scheme for transmitting signals employed in a base station can be reported to a terminal. In this case, a value of L for a transmission symbol is preset in accordance with the modulation scheme utilized. For example, if L=0, L=1 and L=2 are set to indicate 4-PSK, 16-QAM and 64-QAM, respectively, a terminal as a receiver can detect the modulation scheme of symbols transmitted from a base station, by detecting the value of L.

Referring toFIGS. 26A,26B and26C, transmission efficiency will be described.FIG. 26Ais a view illustrating a case where modulated signals input to the IDFT appearing inFIG. 22are all 4-PSK signals.FIG. 26Bis a view illustrating a case where two of the modulated signals shown inFIG. 26Aare no-information signals, and the other six modulated signals are all 4-PSK signals.FIG. 26Cis a view illustrating a case where two of the modulated signals shown inFIG. 26Aare no-information signals, and other two modulated signals are 16-QAM signals.

In the case shown inFIG. 26Bwhere X0and X4included in the input signals X0, X1, . . . , X7of the IDFT11are no-information signals, the transmission efficiency is lower by the transmission bits of the no-information signals X0and X4than the case shown inFIG. 26Awhere none of the input signals X0, X1, . . . , X7are no-information signals.

In light of this, in the ninth embodiment, when one input signal is set to a no-information signal, the modulation circuit13modulates, into a signal of a larger number of bits, one of the input signals of the IDFT11other than the no-information signal, as is shown inFIG. 26C. The modulation circuit13is a circuit for modulating an input signal into a modulated signal corresponding to a predetermined modulation scheme.

For instance, the modulation circuit13modulates a 4-PSK signal into a 16-QAM signal or 64-QAM signal, etc., which has a larger number of transmission bits than the former.

FIG. 26Cshows an example, where the number of transmission bits is identical to that in the example ofFIG. 26Awhere all input signals X0, X1, . . . , X7are 4-PSK signals. Since the number of transmission bits of a 16-QAM signal is double the number of transmission bits of a 4-PSK signal, two 4-PSK input signals are replaced with respective 16-QAM signals in the example ofFIG. 26Cwhere two input signals are no-information signals.

The embodiment is not limited to the use of the 16-QAM scheme as in the examples ofFIG. 26C.

For example, to make the number of transmission bits identical to that in the example ofFIG. 26A, two 4-PSK signals included in X0, X1, . . . , X7may be replaced with respective 8-PSK signals. Alternatively, one 4-PSK signal included in X0, X1, . . . , X7may be replaced with a 64-QAM signal.

Further, if no-information signals are included in X0, X1, . . . , X7, and if the power is reduced by the number of the no-information signals, the resistance to errors is reduced. To prevent a reduction in resistance to errors, the ninth embodiment employs a power-adjusting unit14for increasing the power of the modulated signals X1′ and X5′ of the 16-QAM scheme in order to make the total power of X0, X1′, . . . , X5′, . . . , X7shown inFIG. 26Cidentical to that of X0, X1, . . . , X7shown inFIG. 26A. If the former total power can be made identical to the latter, the resistance to errors can be made identical.

As described above, some of the IDFT input signals can be set to no-information signals without degrading the resistance to errors and without reducing the number of transmission bits per one symbol. In other words, the modulation scheme and power can be set on condition that the input signals of the IDFT11have the same number of bits and the same power.

However, if a reduction in the number of transmission bits by setting a certain 4-PSK input signal of the IDFT11to a level of 0 is allowed, it is not necessary to change the modulation scheme for another input signal to another multi-value modulation scheme. It is sufficient if the modulation scheme is kept at the 4-PSK scheme. Further, if a reduction in error ratio due to a change in modulation scheme for a certain input signal is allowed, no power adjustment is needed.

The number of no-information signals included in each transmission symbol input to the IDFT11can be changed in accordance with the state of the transmission channel. This will be explained referring toFIGS. 27A and 27B.FIG. 27Ais a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station50to a terminal40differs from that from the terminal40to the base station50.FIG. 27Bis a block diagram illustrating a multi-carrier transmission system in which a transmission channel from a base station70to a terminal60is identical to that from the terminal60to the base station70.

The terminal40or the base station70detects the state of the transmission channel, and controls the modulation circuit contained in an OFDM transmitter52or73. For example, if the multipath delay time is long, the base station controls the modulation circuit contained in the OFDM transmitter52or73to increase the number of no-information signals to be inserted. On the other hand, if the multipath delay time is short, the base station controls the modulation circuit to reduce the number of no-information signals to be inserted. The base station detects the state of the transmission channel in the manner stated below.

FIG. 27Aillustrates frequency division duplex (FDD) communication in which up-link and down-link transmission channels are used between the base station50and terminal40. In this case, when OFDM transmission is performed from the base station50to the terminal40using the down transmission channel, the base station50instructs the terminal40to inform the base station of the transmission condition for the down-link transmission channel via the up-link transmission channel. Based on the transmission condition for the down-link transmission channel supplied from the terminal40, the base station50executes OFDM transmission.

More specifically, for instance, in the terminal40, a down-link transmission channel estimation unit42estimates the state of the down-link transmission channel based on a signal received by the OFDM receiver41. Subsequently, a transmitter43transmits, to the base station50, information concerning the state of the down-link transmission channel estimated by the estimation unit42. In the base station50, a receiver51receives the information concerning the state of the down-link transmission channel, and outputs the information to the OFDM transmitter52. The OFDM transmitter52transmits a signal to the terminal40, based on the input information concerning the state of the down-link transmission channel.

On the other hand,FIG. 27Billustrates time division duplex (TDD) communication in which only a single transmission channel is used as both an up-link transmission channel and down-link transmission channel between the base station70and terminal60. In this case, when OFDM transmission is performed from the base station70to the terminal60, the base station70detects a transmission condition for the down-link transmission channel, from the characteristics of a signal received. Based on the detected transmission condition for the down-link transmission channel, the base station70executes OFDM transmission.

More specifically, for instance, in the base station70, a down-transmission-channel estimation unit72estimates the state of the down-link transmission channel from a signal received by a receiver71. Based on the estimated state, the OFDM transmitter73transmits a signal to the terminal60.

Tenth Embodiment

Although the above-described ninth embodiment is directed to a reverse discrete Fourier transformer having eight (=M) input/output points, a tenth embodiment is directed to a generalized case where M is set to 2m(m is a positive integer). This case will be described with reference toFIG. 28.FIG. 28is a block diagram illustrating a multi-carrier transmission system according to the tenth embodiment.

Assuming that the input signals of the IDFT81are X0, X1, . . . , XM−1, the output signals of the IDFT are x0, x1, . . . , xM−1, WM=exp(−j2π/M), and j2=−1, the relationship between the input and output signals is given by

Further, up is defined for the output signals x0, x1, . . . , xM−1of the IDFT81, using the following equations (the combination of the output signals x0, x1, . . . , xM−1is a transmission symbol):

If u0, u1, . . . , uN−1is input to a DFT with N input/output points, the output signal Uk(k represents an integer, and 0≦k≦M−1) of the DFT is given by

Uk=u0+WNk⁢u1+WN2⁢k⁢u2+…+WN(N-1)⁢k⁢uN-1[C10]
where WN=exp(−j2π/N)=WMK. Using the equations [C9], the equations [C10] can be modified in the following manner:

On the other hand, if x0, x1, . . . , xM−1is input to a DFT with M input/output points, the output signal Xk(k represents an integer, and 0≦k≦M−1) of the DFT is given by

If each of the right and left parts of the equations [C14] are divided by WMpL, and if WMN=exp (−j2πN/M)=exp (−j2π/K)=WKis considered, then the following equations are acquired from the above equations [C14]:

The equations [C15] are used as a constraint on the output signals of the IDFT81with the M input/output points when XL+pk(L=0, 1, . . . , K−1; p=0, 1, . . . , N−1; M=KN; N=2n) is set to a level of 0.

For example, in the ninth embodiment where M=8, N=2 and K=4, the equation [C15] are modified as follows:
xp+W4Lxp+2+W42Lxp+4+W43Lxp+6=0 (p=0, 1)  [C16]

Thus, the equations [C16] are identical to the equations [C3-1] and [C3-2] extracted in the ninth embodiment.

A receiving unit91receives, as an adjacent signal sequence of y0, y1, . . . , yM−1, a transmission symbol sequence having passed through a transmission channel100. An extraction unit92receives the transmission symbol sequence from the receiving unit91, extracts, therefrom, signal sequences of yp, yp+N, yp+2N, . . . , yp+(K−1)N(p=0, 1), and outputs the extracted signal sequences to a K-points DFT93. Assuming here that the transmission channel100is an ideal one free from noise, multipath fading, etc., if a boundary is detected at correct timing in the symbol sequence received by the multi-carrier receiver20, i.e., if accurate symbol synchronization is performed, the following is established between the time-base signals:
xk=yk(k=0, 1, . . . , M−1)  [C17-1]

Since, in general, each input and corresponding output of a DFT is in a one for one relationship, if the equations concerning the input or output are established, the other equations are also established. On the other hand, if signal transmission is out of synchrony with signal reception, i.e., if symbol synchronization is not established, the above equations [C17-1] or [C17-2] are not established.

The K-points DFT93has M input/output points and receives the signal sequence of yp, yp+N, yp+2N, . . . , yp+(K−1)N. The K-points DFT93performs DFT transform on the signals to calculate SP,Lusing the following equations:

Accordingly, even if the multi-carrier receiver20cannot detect the N signals set to a level of 0, which are included in the M modulated signals input to the IDFT81of the multi-carrier transmitter80, i.e., even if the receiver cannot detect the value of the non-negative integer L not higher than N−1 and included in the equations XL+pK=0, the estimation circuit94can estimate the value of L. Specifically, the estimation circuit94examines which ones of the K output signals Sp,0, Sp,1, . . . , Sp,K−1(p=0, 1, . . . , N−1) output when the K signals yp, yp+N, yp+2N, . . . , yp+(K−1)Nare input to the K-points DFT93with the K input/output points are 0, thereby estimating the value of L included in Sp,Lwith the level of 0. Thus, the estimation circuit94can detect the N signals set to a level of 0, which are included in the M modulated signals input to the IDFT81.

The other principles are similar to those employed in the ninth embodiment.

A description will be given of the DFT transform operations performed. In the case of using the equations [C18], discrete Fourier transform is performed N times at each of the K points of the K-points DFT93. Assume here that a DFT with M input/output points is used, instead of the K-points DFT93, to directly receive the signal sequence of y0, y1, . . . , yM−1In this case, DFT operations are performed M times to acquire Sp,L. In both cases, the same calculation result is acquired. Considering that M=KN, the total number of operations in the case (1) where discrete Fourier transform is performed N times at each of the K points will now be compared with that in the case (2) where discrete Fourier transform is performed one time at each of the M points. The number of operations corresponds to the number of complex operations since the numbers included in the equations [C18] are complex numbers.

The number of complex operations performed by the DFT with the M input/output points is M2. Accordingly, the number of complex operations in the case (1) is K2N, while the number of complex operations in the case (1) is K2N2(=M2). Thus, the number of operations is smaller in the case (1) than in the case (2).

Further, fast Fourier transformers (FFTs) and inverse fast Fourier transformers (IFFTs) may be utilized instead of all DFTs and IDFTs employed in the tenth embodiment. The total number of operations in the case (3) where fast Fourier transform is performed N times at each of the K points will be compared with that in the case (4) where fast Fourier transform is performed one time at each of the M points. The number of complex operations in the case (3) is (K/2) (log2K−1)N, while the number of complex operations in the case (4) is (M/2) (log2M−1)=(KN/2) (log2K+log2N−1). Thus, the number of operations is smaller in the case (3) than in the case (4).

In addition, concerning the number of operations, there is a more efficient operation method. When a value of L is detected in each transmission symbol, M signals are extracted as y0, y1, . . . , yM−1from a transmission symbol sequence at certain timing, thereby executing DFT operations N times at each of the K points to acquire Sp,Lgiven by the equations [C19]. After that, while the extraction position is shifted by one signal, the same operations are executed. At this time, however, a greater part of the required operations can be omitted by utilizing the results of the preceding operations. That is, when M signals are extracted as y1, y2, . . . , yM, and the operations given by the following equations are executed, it is actually sufficient if one DFT operation is performed at each of the K points to acquire Sp,Lwhen p=N−1 and L=0, 1, . . . , K−1:

This is because Sp,Lobtained when p=0, 1, . . . , N−2 and L=0, 1, . . . , K−1 have already been calculated by the preceding DFT operations concerning the already extracted y0, y1, . . . , yM−1. Similarly, concerning the next M signals, too, it is sufficient if one DFT operation is performed at each of the K points.

The above-described embodiments are not limited to radio communication between a base station and terminal, but also applicable to wireless or wired broadcasting.

The present embodiment is not limited to the above-described embodiments, but may be modified in various ways without departing from its scope. Further, various embodiments can be realized by appropriately combining the structural elements disclosed in the embodiments. For example, some element may be deleted from the entire elements employed in the embodiments. Furthermore, elements employed in different embodiments may be appropriately combined.