Patent Description:
Transmission diversity is one of the techniques for easily improving transmission quality in a wireless communication system. The transmission diversity is a technique for improving transmission quality by a spatial diversity effect by transmitting signals based on the same information from a plurality of transmitting antennas to cause respective transmission signals to reach a reception device under the influence of different propagation paths.

Non Patent Literature <NUM> discloses a technique called cyclic delay diversity (CDD). The cyclic delay diversity is a technique for obtaining a diversity effect by providing delays different from each other to signals each transmitted from one of a plurality of antennas in block transmission to artificially create frequency selectivity. The cyclic delay diversity can also be said to be a technique for converting spatial diversity by a plurality of antennas into frequency diversity.

In addition, among modulation techniques used in a wireless communication system, frequency-shift keying (FSK) exhibits a constant envelope amplitude of a modulation signal and enables an input back-off value to be set small on a power amplifier, so that frequency shift keying is known to have higher power efficiency than phase shift keying (PSK), quadrature amplitude modulation (QAM), and the like. Also in a communication system using the frequency-shift keying, it is desirable to improve transmission quality by introducing a transmission diversity technique.

However, when the technique described in Non Patent Literature <NUM> is applied to a communication system using frequency-shift keying, a signal is intentionally distorted and thereby transmission quality is deteriorated in principle, which is a problem.

The present invention has been made in view of the above, and an object thereof is to obtain a wireless communication device capable of improving transmission quality while maintaining power efficiency.

In order to solve the above-described problems and achieve the object, the application provides a wireless communication device according to claim <NUM> and a wireless communication method according to claim <NUM>.

The wireless communication device according to the present invention achieves an effect that it is possible to improve transmission quality while maintaining power efficiency.

Hereinafter, a wireless communication device, a control circuit, a wireless communication method, and a storage medium according to embodiments of the present invention will be described in detail with reference to the drawings. The present invention is limited by the appended set of claims.

<FIG> is a diagram illustrating a functional configuration of a wireless transmission device <NUM> according to an embodiment of the present invention. The wireless transmission device <NUM> is a wireless communication device including a mapping unit <NUM>, a plurality of modulation units <NUM>, and a plurality of transmitting antennas <NUM>. The plurality of modulation units <NUM> are provided in one-to-one correspondence to the plurality of transmitting antennas <NUM>.

The mapping unit <NUM> is provided in a preceding stage of the plurality of modulation units <NUM>. The mapping unit <NUM> allocates carrier waves different from each other to the plurality of modulation units <NUM>. At that time, the mapping unit <NUM> allocates the carrier waves such that the transmission signals each transmitted from one of the plurality of transmitting antennas <NUM> are orthogonal to each other on a frequency axis. Therefore, the mapping unit <NUM> can also be referred to as a signal processing unit that performs an encoding process on a signal before modulation for transmission diversity.

Bit sequence b={b<NUM>, b<NUM>,. bm} is input to the mapping unit <NUM>. Here, m is the number of bits per modulation symbol, and when M is an FSK modulation level, m=log<NUM>M holds. Regarding the modulation units <NUM> each corresponding to one of the transmitting antennas <NUM>, the mapping unit <NUM> outputs, to each of the modulation units <NUM>, an FSK carrier number ki which is a number for identifying a carrier wave used by the modulation unit <NUM> depending on a value of the input bit sequence b. In a case where the number of transmitting antennas is denoted by N and an FSK modulation index is denoted by α, the FSK carrier number ki of the carrier wave used in an i-th (<NUM>≤i≤N) modulation unit <NUM> is expressed by using the following formula (<NUM>). [Formula <NUM>] <MAT>.

In formula (<NUM>), mod represents a modulo operation. In addition, K is a minimum carrier interval at which two carrier waves are orthogonal to each other, and is expressed by the following formula (<NUM>). By using formula (<NUM>), the mapping unit <NUM> defines a mapping rule which is an allocation rule of the carrier waves on the basis of the number N of antennas of the transmitting antennas <NUM>, and the FSK modulation level M and the FSK modulation index α of the modulation unit <NUM>. [Formula <NUM>] <MAT>.

W(b) is a mapping function that makes the bit sequence b and the FSK carrier number ki correspond to each other on a one-to-one basis, and in general, mapping based on gray labeling is used.

<FIG> is a diagram illustrating an example of a mapping rule used by the mapping unit <NUM> illustrated in <FIG> illustrates a case of N=<NUM>, M=<NUM>, and α=<NUM>. In the example illustrated in <FIG>, combinations of the FSK carrier numbers k<NUM> and k<NUM> are different for all values "<NUM>, <NUM>, <NUM>, and <NUM>" of an information bit sequence to be input. Therefore, the mapping unit <NUM> allocates the carrier waves such that the combinations of the carrier waves each allocated to one of the plurality of transmitting antennas <NUM> are different for all values of the information bit sequence to be input.

The modulation units <NUM> each generate a transmission signal to be transmitted from the transmitting antenna <NUM> by performing frequency-shift keying on a carrier wave on the basis of transmission data. The modulation units <NUM> each perform a frequency-shift keying process using the carrier wave allocated by the mapping unit <NUM>. Specifically, first, the modulation units <NUM> each determine an initial phase of an FSK modulation symbol of the transmitting antenna <NUM>. The initial phase of an i-th transmitting antenna <NUM> is expressed by the following formula (<NUM>). By determining the initial phase using formula (<NUM>), the modulation units <NUM> can use initial phases different from each other for the plurality of transmitting antennas <NUM>. [Formula <NUM>]<MAT>.

The modulation units <NUM> each generate an FSK modulation symbol on the basis of the FSK carrier number ki output by the mapping unit <NUM> and an initial phase ϕi determined above. An FSK modulation symbol Si(t) generated here is expressed by the following formula (<NUM>). [Formula <NUM>]<MAT>.

Here, fc is a center frequency, and 2Δf is a frequency interval between carrier waves. The modulation units <NUM> each transmit the generated FSK modulation symbol Si(t) from the transmitting antenna <NUM>.

<FIG> is a flowchart for explaining an operation of the wireless transmission device <NUM> illustrated in <FIG>. The mapping unit <NUM> first performs a spatial/frequency mapping process (step S101). Specifically, the mapping unit <NUM> allocates a carrier wave to each transmitting antenna <NUM>, and outputs the FSK carrier number ki for identifying the allocated carrier wave to each of the plurality of modulation units <NUM>.

The modulation units <NUM> each select an initial phase of the FSK modulation symbol (step S102). The modulation units <NUM> each perform an FSK modulation process using the initial phase selected in step S102 and the carrier wave of the FSK carrier number ki allocated by the mapping unit <NUM> (step S103). As a result, a signal including the FSK modulation symbol is generated. The modulation units <NUM> each transmit the generated signal from the transmitting antenna <NUM> (step S104).

<FIG> is a diagram illustrating a functional configuration of a wireless reception device <NUM> according to the embodiment of the present invention. The wireless reception device <NUM> is a wireless communication device including a receiving antenna <NUM>, a likelihood calculation unit <NUM>, and a demodulation unit <NUM>.

The likelihood calculation unit <NUM> calculates symbol likelihoods each corresponding to one of M number of FSK symbol candidates from frequency components of reception signals received by the receiving antenna <NUM>. The likelihood calculation unit <NUM> outputs the calculated symbol likelihoods to the demodulation unit <NUM>. Note that the likelihood calculation unit <NUM> may use power of each frequency component as the likelihood, or may use a Euclidean distance in a complex space as the likelihood on the basis of a complex value of each frequency component.

The demodulation unit <NUM> calculates bit sequence likelihoods corresponding to M number of information bit sequence candidates on the basis of M number of symbol likelihoods output by the likelihood calculation unit <NUM> and the mapping rule defined by formula (<NUM>). The demodulation unit <NUM> obtains an estimation value b (hat) of the information bit sequence by performing maximum likelihood estimation on the basis of the calculated bit sequence likelihoods. Note that, in a case where (hat) is described after a character, it means that ^ is added above the character.

<FIG> is a flowchart for explaining an operation of the wireless reception device <NUM> illustrated in <FIG>. The wireless reception device <NUM> receives signals at the receiving antenna <NUM> (step S201). The likelihood calculation unit <NUM> calculates symbol likelihoods each corresponding to one of the M number of FSK symbol candidates on the basis of frequency components of the reception signals received by the receiving antenna <NUM> (step S202). The likelihood calculation unit <NUM> outputs the plurality of symbol likelihoods to the demodulation unit <NUM>.

The demodulation unit <NUM> calculates bit sequence likelihoods corresponding to the M number of information bit sequence candidates on the basis of the plurality of symbol likelihoods and the mapping rule (step S203). The demodulation unit <NUM> obtains an estimation value b (hat) of the information bit sequence by performing a spatial/frequency demapping process (step S204). Specifically, the demodulation unit <NUM> obtains the estimation value b (hat) of the information bit sequence by performing maximum likelihood estimation based on the calculated bit sequence likelihoods.

Next, hardware configurations of the wireless transmission device <NUM> and the wireless reception device <NUM> according to the embodiment of the present invention will be described. The mapping unit <NUM>, the modulation units <NUM>, the likelihood calculation unit <NUM>, and the demodulation unit <NUM> are implemented by processing circuitry. The processing circuitry may be implemented by dedicated hardware, or may be a control circuit using a central processing unit (CPU).

In a case where the above-described processing circuitry is implemented by dedicated hardware, functions of the mapping unit <NUM>, the modulation units <NUM>, the likelihood calculation unit <NUM>, and the demodulation unit <NUM> are each implemented by processing circuitry <NUM> illustrated in <FIG> is a diagram illustrating dedicated hardware for implementing the functions of the wireless transmission device <NUM> and the wireless reception device <NUM> according to the embodiment of the present invention. The processing circuitry <NUM> is a single circuit, a composite circuit, a programmed processor, a parallel programmed processor, an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), or a combination thereof.

In a case where the above-described processing circuitry is implemented by a control circuit using a CPU, the functions of the mapping unit <NUM>, the modulation units <NUM>, the likelihood calculation unit <NUM>, and the demodulation unit <NUM> are implemented by, for example, a control circuit <NUM> having a configuration illustrated in <FIG> is a diagram illustrating a configuration of the control circuit <NUM> for implementing the functions of the wireless transmission device <NUM> and the wireless reception device <NUM> according to the embodiment of the present invention. As illustrated in <FIG>, the control circuit <NUM> includes a processor <NUM> and a memory <NUM>. The processor <NUM> is a CPU, and also referred to as a central processing device, a processing device, an arithmetic device, a microprocessor, a microcomputer, a digital signal processor (DSP), or the like. The memory <NUM> is, for example, a non-volatile or volatile semiconductor memory such as a random access memory (RAM), a read only memory (ROM), a flash memory, an erasable programmable ROM (EPROM), or an electrically EPROM (EEPROM (registered trademark)), a magnetic disk, a flexible disk, an optical disk, a compact disc, a mini disk, or a digital versatile disk (DVD).

In a case where the above-described processing circuitry is implemented by the control circuit <NUM>, the functions of the mapping unit <NUM>, the modulation units <NUM>, the likelihood calculation unit <NUM>, and the demodulation unit <NUM> are described as programs and stored in the memory <NUM>. The functions of the mapping unit <NUM>, the modulation units <NUM>, the likelihood calculation unit <NUM>, and the demodulation unit <NUM> are implemented by the processor <NUM> reading and executing a program stored in the memory <NUM>. The memory <NUM> is also used as a temporary memory in each process executed by the processor <NUM>. A part of the above functions may be implemented by dedicated hardware, and the rest thereof may be implemented by a program.

In the above, the wireless transmission device <NUM> and the wireless reception device <NUM> have been described as devices separate from each other, but a wireless communication device having the functions of the wireless transmission device <NUM> and the wireless reception device <NUM> may be provided.

As described above, the wireless communication device according to the embodiment of the present invention includes the mapping unit <NUM> provided in the preceding stage of the modulation units <NUM>. The mapping unit <NUM> allocates carrier waves different from each other to the plurality of transmitting antennas <NUM>. That is, a transmission diversity process by the mapping unit <NUM> is performed in the preceding stage of the modulation units <NUM>, and a signal obtained after frequency-shift keying is transmitted without being processed. Therefore, no phase discontinuity occurs in a case of the FSK modulation index α=<NUM>, and high power efficiency can be maintained.

Furthermore, the mapping unit <NUM> allocates the carrier waves such that the plurality of transmission signals each transmitted from one of the plurality of transmitting antennas <NUM> included in the wireless communication device are orthogonal to each other. With such a configuration, the signals each transmitted from one of the transmitting antennas <NUM> are orthogonal to each other on the frequency axis, and thus, full diversity can be acquired in a case of N<M in which there occurs one-to-one correspondence of combinations of respective values of the bit sequence b and the carrier waves. Therefore, it is possible to achieve high transmission quality compared with a case of using a technique of cyclic delay diversity that provides delays different from each other to signals each transmitted from one of the plurality of antennas in block transmission to artificially create frequency selectivity.

Furthermore, in the present embodiment, the orthogonality of the transmission signals from the transmitting antennas <NUM> is guaranteed in a closed form in one FSK modulation symbol. Therefore, it is possible to enhance resistance to a time variation of a communication path.

In the present embodiment, initial phases allocated to the transmitting antennas <NUM> are different from each other. With such a configuration, it is possible to increase a minimum distance between signal points in a signal point space formed by bit sequence likelihoods as compared with a case where the initial phases are the same. Therefore, transmission quality can be improved.

The present invention can also be used in combination with any secondary modulation technique such as orthogonal frequency-division multiplexing (OFDM) or direct sequence spread spectrum (DSSS), and high transmission quality which is an effect of the present invention can be achieved also in that case. For example, in a case where OFDM and the technology of the present invention are combined, improvement in spectral efficiency can be expected, and in a case where DSSS and the technology of the present invention are combined, improvement in interference immunity by a processing gain can be expected.

Claim 1:
A wireless communication device (<NUM>) comprising:
a plurality of transmitting antennas (<NUM>);
a plurality of modulation units (<NUM>) each provided corresponding to one of the plurality of transmitting antennas (<NUM>) to generate transmission signals transmitted from the transmitting antennas (<NUM>) corresponding thereto by performing frequency-shift keying on a carrier wave on a basis of transmission data; and
a mapping unit (<NUM>) provided in a preceding stage of the plurality of modulation units (<NUM>),
wherein the mapping unit (<NUM>) is configured to output, to each of the modulation units (<NUM>), a number for identifying a frequency shift on the carrier wave used by each of the modulation units (<NUM>) depending on a value of an input bit sequence (blb2) of the transmission data, wherein the numbers output from the mapping unit (<NUM>) based on the input bit sequence (blb2) to the plurality of the modulation units (<NUM>) are different from each other, such that the transmission signals each transmitted from one of the plurality of transmitting antennas are orthogonal to each other on a frequency axis,
wherein each of the modulation units (<NUM>) is configured to generate a transmission signal transmitted from each of the transmitting antennas (<NUM>) by performing frequency-shift keying on the carrier wave based on the number,
wherein the number for identifying a frequency shift on the carrier wave output to each of the modulation units is determined based on a minimum carrier interval at which two carrier waves are orthogonal to each other.