Optical wireless transmission system, transmission device, and optical wireless transmission method

Abstract

Provided are: an optical wireless transmission system which transmits a high-frequency wireless signal to a remote unit directly through an optical fiber and which enables stable operation even in the case of a commercially-available optical module for general-purpose digital communication; an optical wireless transmission system; a transmission device; and an optical wireless transmission method.　An optical wireless transmission system (100) comprises: a transmission device (110) that includes a digital baseband processing unit (111) which generates a plurality of orthogonal signals, a pulse modulator (112) which performs pulse modulation on the orthogonal signals, and a dither modulator (113) which applies a dither signal to the pulse-modulated signal; an optical fiber module (120) that converts into an optical signal the signal to which the dither signal has been applied, transmits the optical signal through an optical fiber, and converts the optical signal into a digital electrical signal; and a remote unit (130) that transmits, as a wireless signal, the digital electrical signal which has been transmitted by the optical fiber module (120).

Description

Technical Field

[0001] The present invention relates to an optical wireless transmission system transmission device and an optical wireless transmission method.

Background Art

[0002] As one of optical wireless transmission systems, there is an optical fiber wireless system that directly transmits a high-frequency wireless signal generated by a transmission device to a slave unit via an optical fiber. In this optical fiber wireless system, since a DAC (Digital to Analog Converter) is not required in the slave unit, cost reduction, high efficiency, and improved ease of installation of the slave unit can be expected. For example, Patent Document 1 describes a transmission system in which a radio control station up-converts to a radio frequency band and converts it into an optical intensity modulation signal, transmits the signal, and a radio base station converts the optical intensity modulation signal into an electrical signal and synthesizes each signal.

[0003] On the other hand, in order to use general-purpose optical components (optical modules) for digital communication for cost reduction, it is required that the transmission waveform is rectangular and does not have an extremely large or small mark ratio (80% or more, or 20% or less, etc.).

[0004] In a general pulse modulation signal that modulates a wireless signal into a rectangular wave, since the amplitude is represented by the pulse width, when the amplitude of the wireless signal becomes small, the mark ratio becomes small. Note that the mark ratio is the ratio of the time occupied by "high" within a certain time.

[0005] On the other hand, in wireless signals used in recent mobile communications such as OFDM (Orthogonal Frequency Division Multiplexing), the amplitude changes dynamically and greatly, so there are moments when the mark ratio becomes extremely small.

[0006] The marking rate of modulation signals such as OFDM exceeds the range of the marking rate of optical modules for general-purpose digital communication, so optical modules for general-purpose digital communication cannot currently be used for mobile signals. Therefore, in an optical wireless transmission system using a modulation signal such as OFDM, it is necessary to use a dedicated optical module. This leads to an increase in the cost of the optical module.

Prior Art Documents

Patent Documents

[0007]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0008] As described above, in an optical fiber wireless system that directly transmits a high-frequency wireless signal to a slave unit via an optical fiber, there is a problem that it is difficult to use a commercially available optical module for general-purpose digital communication.

Means for Solving the Problems

[0009] An optical wireless transmission system according to an embodiment includes a digital baseband processing unit that generates a plurality of orthogonal signals, a pulse modulator that pulse-modulates the orthogonal signals, and a dither modulator that adds a dither signal to the pulse-modulated signal. An optical fiber module that converts the signal with the dither signal added into an optical signal, transmits it via an optical fiber, and converts the optical signal into a digital electrical signal. A slave unit that transmits the digital electrical signal transmitted by the optical fiber module as a wireless signal.

[0010] A transmission device according to an embodiment includes a digital baseband processing unit that generates a plurality of orthogonal signals, a pulse modulator that pulse-modulates the orthogonal signals, and a dither modulator that adds a dither signal to the pulse-modulated signal.

[0011] In one embodiment of the optical wireless transmission method, in a transmission device, a plurality of orthogonal signals are generated, the orthogonal signals are pulse-modulated, and a dither signal is added to the pulse-modulated signal. In an optical fiber module, the signal added with the dither signal is converted into an optical signal, transmitted through an optical fiber, and the optical signal is converted into a digital electrical signal. In a slave unit, the digital electrical signal transmitted by the optical fiber module is transmitted as a wireless signal.

Advantages of the Invention

[0012] According to the optical wireless transmission system, transmission device, and optical wireless transmission method of the present invention, in an optical wireless transmission system that directly transmits a high-frequency wireless signal to a slave unit through an optical fiber, a commercially available general-purpose optical module for digital communication can also operate stably.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

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Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Mode for Carrying Out the Invention

[0014] (Embodiment 1) Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing an example of an optical wireless transmission system according to Embodiment 1. In FIG. 1, the optical wireless transmission system 100 includes a transmission device 110 and a slave unit 120.

[0015] The transmission device 110 includes a DBB (Digital Base Band) 111, a pulse modulator 112, and a dither modulator 113. The dither modulator 113 includes a dither signal generator 114 and an adder 115. The slave unit 120 includes a BPF (Band Path Filter) 121, a power amplifier 122, and an antenna 123. Further, the transmission device 110 and the slave unit 120 are connected via an E / O 131, an optical fiber cable 132, and an O / E 133. The E / O 131, the optical fiber cable 132, and the O / E 133 constitute an optical fiber module 130.

[0016] The DBB 111 generates two wireless orthogonal signals I and Q. Then, the wireless orthogonal signals are output to the pulse modulator 112.

[0017] The pulse modulator 112 pulse-width modulates the wireless orthogonal signals to generate pulse wireless signals. Then, the pulse modulator 112 outputs the pulse wireless signals to the adder 115.

[0018] The dither signal generator 114 generates a dither signal. Then, the dither signal generator 114 outputs the dither signal to the adder 115.

[0019] The adder 115 adds the dither signal to the pulse radio signal to generate a dither - contained pulse radio signal. Then, the adder 115 outputs the dither - contained pulse radio signal to the E / O 131.

[0020] The E / O converter 131 converts the dither - contained pulse radio signal, which is an electrical signal, into an optical signal. Then, the E / O converter 131 transmits the optical signal to the O / E converter 133 via the optical fiber cable 132.

[0021] The O / E converter 133 converts the optical signal transmitted from the E / O converter 131 into an electrical signal. Then, the O / E converter 133 outputs the converted electrical signal to the BPF 121 of the slave unit 120.

[0022] The BPF 121 extracts a desired frequency component from the electrical signal. Then, the BPF 121 outputs the electrical signal to the power amplifier 122.

[0023] The power amplifier 122 amplifies the signal extracted by the BPF 121. Then, the power amplifier 122 outputs the amplified signal to the antenna 123. The antenna 123 radiates the output of the power amplifier 122 into the air as radio waves.

[0024] With the above configuration, optical wireless transmission is performed. Next, the operation of the optical wireless transmission system 100 will be described.

[0025] First, in the transmission device 110, in the DBB 111, wireless orthogonal signals I(t), Q(t) are generated.

[0026] Here, the wireless orthogonal signals I(t), Q(t) can be described by the following equations (1) and (2) using the amplitude signal A(t) and the phase signal θ(t). I(t)=A(t)cosθ(t) …(1) Q(t)=A(t) sinθ(t) …(2) Note that the following equations (3) and (4) also hold. TIFF0007708223000001.tif11153…(3) TIFF0007708223000002.tif11153…(4)

[0027] Here, it is assumed that A(t) is normalized including I and Q so that it becomes at most 1. That is, 0 ≦ A ≦ 1. And if the carrier frequency is fc and the angular frequency obtained by multiplying fc by 2π is ωc, the radio signal RF(t) can be described by the following equation (5). RF(t)=A(t)cos(ω C ·t - θ(t)) …(5)

[0028] Here, Rect(a,b) is defined as a function that outputs either 1 or -1 depending on the values of a and b as shown in the following equations (6) and (7). Rect(a,b)=1 …(6): -b + 2n·π ≦ a ≦ b + 2n·π (n is an arbitrary integer) Rect(a,b)= -1 …(7): otherwise

[0029] For example, when a = (2π / T)·t, Rect(a,b) is a rectangular wave with a period T and a duty cycle ratio (hereinafter referred to as DCR) of b / π. In particular, when b = π / 2, the DCR is 1 / 2, that is, 50%.

[0030] The time waveform of Rect(a,b) is shown in FIG. 2. FIG. 2 is a graph showing the time variation of Rect(a,b). In FIG. 2, the vertical axis represents the value of Rect(a,b), and the horizontal axis represents a.

[0031] And the pulse modulator 112 outputs a pulse signal Spwm(t) shown in the following equations (8), (9), and (10) with I(t) and Q(t) as inputs. Spwm(t)= Rect(phase(t),θedge (t)) …(8) phase(t) = ω C ·t - θ(t) …(9) θ edge (t) = arcsin{A(t)} …(10)

[0032] Assuming A(t) and θ(t) are approximately constant, from this equation, the Fourier component of cos(ω C ·t - θ(t)), which is the fundamental wave of Spwm(t), can be seen to be proportional to A(t) by taking the product of Spwm(t) and cos(ω C ·t - θ(t)) and then integrating over one period (= 2π / ωc).

[0033] Therefore, Spwm(t) can be expressed as the sum of RF(t) and its harmonics as shown in the following equation (11). Spwm(t) = k·A(t)·cos(ωc·t - θ(t)) + Hm(t) = k·RF(t) + Hm(t) …(11) k is a constant. Also, Hm(t) is the harmonic component of RF(t). Also, DCR is θ edge (t) / π.

[0034] As shown in equation (11), by using a filter to remove the harmonic components, only the fundamental wave, i.e., the desired wave component, can be extracted.

[0035] Figure 3 shows the time waveform of Spwm(t). Figure 3 is a graph showing the time waveform of Spwm(t) and the IQ plane. In Figure 3, the vertical axis represents Spwm(t). Also, the horizontal axis represents time t.

[0036] In the pulse waveform of Spwm(t), the pulse width becomes larger at high output levels where A(t) is large. The mark ratio is maximized when A(t) = 1, at which time θ edge (t) = π / 2 and the mark ratio becomes 50%.

[0037] Conversely, at low output where A(t) becomes small, the mark rate becomes small. At the minimum, when A(t) = 0, θ edge = 0, and the mark rate becomes 0. That is, the mark rate of Spwm(t) transitions between 0 and 50%.

[0038] An example with a dither signal inserted will be described. FIG. 4 is a graph showing the time waveform of Spwm(t) with a dither signal inserted and the IQ plane. In FIG. 4, the vertical axis represents Spwm(t). Also, the horizontal axis represents time t.

[0039] The dither modulator 113 generates Spwm_dither by inserting the dither signal generated by the dither signal generator 114 into the Spwm signal.

[0040] In the Spwm_dither at low output shown in FIG. 4, when Spwm is at low output, replacement is performed with a dither signal in which high and low alternate. In Spwm(t), for example, when the mark rate is 50%, Spwm_dither becomes 75%. Similarly, when it is 18.75%, it becomes 59.375%. Also, when it is 0%, it all becomes a dither signal and the mark rate becomes 50%.

[0041] Also, in this dither signal, Spwm_dither in FIG. 4 is a pulse signal in which high and low alternate, but the pulse width of this pulse signal is smaller than the pulse width of Spwm(t). As a result, the band occupied by this dither signal becomes higher than the desired signal band, and interference with the desired signal can be avoided. Also, as long as it is a signal that can avoid interference as a dither signal, any signal can be used. For example, high and low may change irregularly. However, the pulse width of the dither signal at that time needs to be smaller than the pulse width of Spwm(t).

[0042] As an example in which another dither signal is inserted, Spwm_dither2 is shown in Fig. 5. Fig. 5 is a graph showing the time waveform and IQ plane of Spwm(t) with the dither signal inserted. In Fig. 5, the vertical axis represents Spwm(t), and the horizontal axis represents time t. In Spwm_dither2, the waveform at maximum output is the same as that of Spwm. As the output decreases, the ratio of high in Spwm(t) decreases, but a part of the low signal is replaced with the dither signal to complement the decrease in high. In this Spwm_dither2, the dither signal is inserted so that the mark rate becomes 50% at any output.

[0043] As described above, when the mark rate of Spwm(t) transitions between 0 and 50%, in Spwm_dither, it transitions between 50% - 75%, and in Spwm_dither2, the mark rate is constant at 50%. By using this method, it becomes possible to use a general-purpose optical module for digital communication (operating stably at approximately a mark rate of 20 - 80%), and the cost of the optical fiber wireless system can be reduced.

[0044] Thus, according to the optical wireless transmission system of Embodiment 1, by inserting a dither signal into the pulse width modulation signal, stable operation can also be achieved with a general-purpose optical module for digital communication.

[0045] (Embodiment 2) In Embodiment 2, an example in which the slave unit outputs an intermediate signal will be described. Fig. 6 is a block diagram showing an example of the optical wireless transmission system according to Embodiment 2. In Fig. 6, the same components as those in Fig. 1 are given the same numbers, and the description thereof is omitted. In Fig. 6, the optical wireless transmission system 200 includes a transmission device 110 and a slave unit 220. The slave unit 220 includes a BPF 121, an LO signal generator 221, a mixer 222, a power amplifier 122, and an antenna 123.

[0046] BPF121 extracts a desired frequency component from an electrical signal. Then, BPF121 outputs the electrical signal to mixer 222.

[0047] LO signal generator 221 generates a local oscillation signal having a frequency that is the difference between the desired frequency and the frequency of the signal extracted from BPF121. Then, LO signal generator 221 outputs the local oscillation signal to mixer 222.

[0048] Mixer 222 mixes the local oscillation signal with the signal extracted from BPF121 and outputs a signal having the desired frequency to power amplifier 122.

[0049] Power amplifier 122 amplifies the signal having the desired frequency. Then, power amplifier 122 outputs the amplified signal to antenna 123.

[0050] Thus, according to the optical wireless transmission system of Embodiment 2, the signals transmitted by fiber and the signals radiated from the antenna can have different frequencies.

[0051] (Embodiment 3) In Embodiment 3, an example using multi-valued pulse modulation will be described. FIG. 7 is a block diagram showing an example of an optical wireless transmission system according to Embodiment 3. In FIG. 7, the same components as those in FIG. 1 are denoted by the same reference numerals, and the description thereof is omitted. In FIG. 7, optical wireless transmission system 300 includes a transmission device 310 and a slave unit 320.

[0052] Transmission device 310 includes DBB111, multi-valued pulse modulator 312, and dither modulators 113-1 and 113-2. Slave unit 320 includes adder 321, BPF121, power amplifier 122, and antenna 123. Also, transmission device 310 and slave unit 320 are connected via E / O 131-1, 131-2, optical fiber cables 132-1, 132-2, and O / E 133-1, 133-2. E / O 131-1, 131-2, optical fiber cables 132-1, 132-2, and O / E 133-1, 133-2 constitute optical fiber module 130.

[0053] DBB111 generates two quadrature signals I and Q, and outputs the quadrature signals to a multi-valued pulse modulator 312.

[0054] The multi-valued pulse modulator 312 pulse-width modulates the quadrature signals to generate multi-valued pulse radio signals, and outputs the multi-valued pulse radio signals to dither modulators 113-1 and 113-2. Note that multi-valued pulse modulation is called so because the sum of bundled signals results in multiple values. As an example of multi-valued pulse modulation, in the case of three values described later, two binary pulse modulation signals (Spwm1(t) and Spwm2(t)) that output 1 or -1 are used, and the sum (Sdes(t)) takes on three values: 2, 0, and -2. The processing of multi-valued pulse radio signals will be described later.

[0055] The dither modulators 113-1 and 113-2 have the same configuration as the dither modulator 113 described in Embodiment 1. The dither modulator 113-1 adds a dither signal to the multi-valued pulse radio signal Spwm1 generated by the multi-valued pulse modulator 312, and outputs the dither-embedded pulse radio signal to E / O131-1.

[0056] Similarly, the dither modulator 113-2 adds a dither signal to the multi-valued pulse radio signal Spwm2 generated by the multi-valued pulse modulator 312, and outputs the dither-embedded pulse radio signal to E / O131-2.

[0057] E / O131-1 and 131-2 have the same configuration as the E / O131 described in Embodiment 1. The optical fiber cables 132-1 and 132-2 have the same configuration as the optical fiber cable 132 described in Embodiment 1. O / E133-1 and 133-2 have the same configuration as the O / E133 described in Embodiment 1.

[0058] O / E133-1 and O / E133-2 output the electrical signals obtained by converting optical signals to the adder 321.

[0059] The adder 321 adds the electrical signals from O / E133-1 and O / E133-2. Then, the adder 321 outputs the added electrical signal to the BPF121.

[0060] With the above configuration, optical wireless transmission is performed. Next, the operation of the optical wireless transmission system 300 will be described.

[0061] The case of a three-valued multi-valued pulse radio signal is shown in FIG. 8. FIG. 8 shows the multi-valued pulse radio signal and the IQ plane in Embodiment 3. In FIG. 8, the vertical axis represents Spwm1(t) and Spwm2(t). Also, the horizontal axis represents time t. The amplitude signal A(t) is decomposed into two amplitude signals A1(t) and A2(t) as shown below according to the value. When 0 <= A(t) <= 0.5, A1(t) is represented by Equation (12). Also, A2(t) is represented by Equation (13). A1(t)=2·A(t) …(12) A2(t)=0 …(13) When 0.5 < A(t) <= 1, A1(t) is represented by Equation (14). Also, A2(t) is represented by Equation (15). A1(t)=1 …(14) A2(t)=2·A(t)-1 …(15)

[0062] Note that since A(t) takes values from 0 to 1, as is clear from the above Equations (12) to (15), A1(t) and A2(t) also take values from 0 to 1 in the same way. Also, the following Equation (16) also holds. A(t)=0.5·{A1(t)+A2(t)} …(16)

[0063] As shown in the following Equations (17) to (22), the multi-valued pulse radio signal generates Spwm1(t) and Spwm2(t) as the multi-valued pulse radio signal from A1(t) and A2(t). Spwm1(t)=Rect(phase(t),θ edge1 (t)) …(17) phase(t)=ω C ·t - θ(t) …(18) θ edge1 (t)=arcsin{A1(t)} …(19) Spwm2(t)=Rect(phase(t),θ edge2 (t)) …(20) phase(t)=ω C ·t - θ(t) …(21) θ edge2 (t)=arcsin{A2(t)} …(22)

[0064] As shown in Fig. 8, in the time waveforms of Spwm1(t) and Spwm2(t), both take on two values of 1 and -1. Also, assuming that A(t) and θ(t) are approximately constant, the Fourier components of cos(ω C ·t - θ(t)), which are the fundamental waves of Spwm1(t) and Spwm2(t), are proportional to A1(t) and A2(t), respectively. Therefore, Spwm1(t) and Spwm2(t) are expressed by the following equations (23) and (24). Spwm1(t)=k·A1(t)·cos(ω C ·t - θ(t)) + Hm1(t) …(23) Spwm2(t)=k·A2(t)·cos(ω C ·t - θ(t)) + Hm2(t) …(24) k is a constant, and Hm1(t) and Hm2(t) are the harmonic components of A1(t)·cos(ω C ·t - θ(t)) and A2(t)·cos(ω C ·t - θ(t)), respectively.

[0065] If the sum of Spwm1(t) and Spwm2(t) is defined as Sdes(t), the following equation (25) is obtained. Sdes(t)=2k·RF(t)+Hm1(t)+Hm2(t) …(25)

[0066] Therefore, Sdes(t) includes the desired signal RF(t). As shown in Equation (25), by using a filter to remove harmonic components, only the desired wave component can be extracted.

[0067] Also, since Spwm1(t) and Spwm2(t) are binary values of 1 or -1, Sdes(t), which is their sum, takes on three values: 2, 0, and -2. Therefore, Sdes(t) is a three-valued pulse signal.

[0068] Also, the DCR of Spwm1(t) and Spwm2(t) are each θ edge1 (t) / π, θ edge2 (t) / π respectively.

[0069] In the pulse waveform of Spwm1(t), the pulse width becomes larger when the output is high with a large A1(t). When A1(t) = 1, θ edge1 (t) = π / 2, and the mark ratio reaches its maximum of 50%.

[0070] Conversely, when the output is low with a small A1(t), the mark ratio becomes smaller. When A1(t) = 0, θ edge1 = 0, and the mark ratio reaches its minimum of 0. That is, the mark ratio of Spwm1(t) transitions between 0 and 50%.

[0071] Similarly, the mark ratio of Spwm2(t) also transitions between 0 and 50%.

[0072] The two dither modulators 113-1 and 113-2 shown in Figure 7 each generate Spwm1_dither and Spwm2_dither by inserting the dither signal generated by a dither signal generator into Spwm1 and Spwm2 respectively.

[0073] As described in the explanation of Spwm_dither in FIG. 2, by using a dither modulator, due to a similar effect, the mark ratios of Spwm1_dither and Spwm2_dither transition between 50% and 75%. Also, if the dither modulator described in the explanation of Spwm_dither2 in FIG. 2 is used, the mark ratio is constant at 50%.

[0074] Thus, according to the optical wireless transmission system of Embodiment 3, a general-purpose optical module for digital communication (operating stably at approximately a mark ratio of 20 - 80%) can be used, and the cost of the optical fiber wireless system can be reduced. And according to the optical wireless transmission system of Embodiment 3, by multiplexing the pulse signal, the quantization noise of the desired signal can be reduced, and a signal with a high SNR can be transmitted.

[0075] Note that although the fiber for transmitting a pulse modulation signal including a plurality of dithers has been described as being configured with a plurality for each signal, it may be configured with one using wavelength division multiplexing.

[0076] (Embodiment 4) In Embodiment 4, an example of decomposing and modulating A(t) of Embodiment 3 into an n-bit signal will be described. FIG. 9 is a block diagram showing an example of the optical wireless transmission system according to Embodiment 4. In FIG. 9, the same components as in FIG. 1 are denoted by the same reference numerals, and the description thereof will be omitted. In FIG. 9, the optical wireless transmission system 400 includes a transmission device 410 and a slave unit 420.

[0077] The transmission device 410 includes a DBB 111, an n-bit pulse modulator 412, and dither modulators 113-1 to 113-n. The slave device 420 includes amplifiers 421-1 to 421-n, an adder 422, a BPF 121, a power amplifier 122, and an antenna 123. Also, the transmission device 410 and the slave device 420 are connected via E / O 131-1 to 131-n, optical fiber cables 132-1 to 132-n, and O / E 133-1 to 133-n. E / O 131-1 to 131-2, optical fiber cables 132-1 to 132-2, and O / E 133-1 to 133-2 constitute an optical fiber module 130.

[0078] The DBB 111 generates two wireless orthogonal signals I and Q. Then, the wireless orthogonal signals are output to the n-bit pulse modulator 412.

[0079] The n-bit pulse modulator 412 pulse-width modulates the wireless orthogonal signals to generate n-bit pulse wireless signals. Then, the n-bit pulse modulator 412 outputs the n-bit pulse wireless signals to the dither modulators 113-1 to 113-n. Note that n-bit pulse modulation is pulse modulation represented using n bits. Examples of n-bit pulse wireless signals are Spwm_bit1(t), Spwm_bit2(t), ···, Spwm_bitn(t), which will be described later.

[0080] The dither modulators 113-1 to 113-n have the same configuration as the dither modulator 113 described in Embodiment 1. The dither modulator 113-1 adds a dither signal to the multi-value pulse wireless signal Spwm_bit 1 generated by the multi-value pulse modulator 312. Then, the dither modulator 113-1 outputs the dither-included pulse wireless signal Spwm_bit1_dither to the E / O 131-1.

[0081] Similarly, the dither modulator 113-n adds a dither signal to the n-bit pulse radio signal Spwm_bitn generated by the multi-value pulse modulator 312. Then, the dither modulator 113-n outputs the dither-embedded pulse radio signal Spwm_bitn_dither to E / O131-n.

[0082] E / O131-1 to 131-n have the same configuration as E / O131 described in the first embodiment. The optical fiber cables 132-1 to 132-n have the same configuration as the optical fiber cable 132 described in the first embodiment. O / E133-1 to 133-n have the same configuration as O / E133 described in the first embodiment.

[0083] O / E133-1 outputs the electrical signal obtained by converting the optical signal to the amplifier 421-1. Similarly, O / E133-n outputs the electrical signal obtained by converting the optical signal to the amplifier 421-n.

[0084] The amplifiers 421-1 to 421- amplify the electrical signals at a magnification corresponding to the positions of the respective bit signals, and output the amplified electrical signals to the adder 422. For example, when the electrical signal amplified by the amplifier 421-1 corresponds to the MSB (Most Significant Bit) and the electrical signal amplified by the amplifier 421-N corresponds to the LSB (Least Significant Bit), the amplification ratio of the amplifier 421-1: the amplification ratio of the amplifier 421-2: the amplification ratio of the amplifier 421-n is 2 -1 :2 -2 :2 -n becomes.

[0085] The adder 422 adds the electrical signals from the amplifiers 421-1 to 421-n. Then, the adder 422 outputs the added electrical signal to the BPF121.

[0086] With the above configuration, optical wireless transmission is performed. Next, the operation of the optical wireless transmission system 400 will be described.

[0087] When the number of bits is n, A(t) is decomposed as shown in the following equation (26). Here, A_bit1(t) is the MSB and A_bitn(t) is the LSB. A(t)=2 -1 A_bit1(t)+2 -2 ·A_bit2(t)+2 -3 ·A_bit3(t) +····+2 -n ·A_bitn(t) …(26)

[0088] Here, let the signals obtained by pulse-modulating each of A_bit1(t), A_bit2(t), ···, A_bitn(t) be Spwm_bit1(t), Spwm_bit2(t), ···, Spwm_bitn(t). These are pulse signals taking two values of 1 and -1. Pulse signals Spwm_bit1_dither and Spwm_bitn_dither obtained by adding a dither signal to Spwm_bit1(t) to Spwm_bitn(t) are fiber-transmitted, transmitted at the slave unit, and synthesized by multiplying by a predetermined coefficient, whereby a desired signal can be generated.

[0089] Thus, according to the optical wireless transmission system of Embodiment 4, quantization noise can be further reduced and a higher SN ratio can be realized.

[0090] (Embodiment 5) In Embodiment 5, an example in which the multilevel pulse modulator of Embodiment 2 is replaced with a hybrid modulator will be described. FIG. 10 is a block diagram showing an example of an optical wireless transmission system according to Embodiment 5. In FIG. 10, the same components as those in FIG. 1 or FIG. 7 are denoted by the same reference numerals and the description thereof is omitted. In FIG. 10, the optical wireless transmission system 500 includes a transmission device 510 and a slave unit 320. The transmission device 510 includes a DBB 111, a Hybrid modulator 512, and dither modulators 113-1 and 113-2.

[0091] The DBB 111 generates two wireless orthogonal signals I and Q. Then, the wireless orthogonal signals are output to the Hybrid modulator 512.

[0092] When the input signal is greater than a predetermined value, the Hybrid modulator 512 represents the input signal in an outphasing manner. When the input signal is less than or equal to the predetermined value, the Hybrid modulator 512 pulse-modulates the input signal. Then, the processed signal is output to the dither modulators 113-1 and 113-2.

[0093] The dither modulators 113-1 and 113-2 have the same configuration as the dither modulator 113 described in Embodiment 1. The dither modulator 113-1 adds a dither signal to the multi-valued pulse radio signal Shyb1 processed by the Hybrid modulator 512. Then, the dither modulator 113-1 outputs the dither-embedded pulse radio signal to E / O131-1.

[0094] Similarly, the dither modulator 113-2 adds a dither signal to the multi-valued pulse radio signal Shyb2 processed by the Hybrid modulator 512. Then, the dither modulator 113-2 outputs the dither-embedded pulse radio signal to E / O131-2.

[0095] With the above configuration, optical wireless transmission is performed. Next, the operation of the optical wireless transmission system 500 will be described.

[0096] The Hybrid modulator 512 switches between the outphasing method and the pulse modulation method according to the magnitude of the amplitude signal A(t). Hereinafter, the magnitude of the amplitude at the switching point is denoted as Amid. When Amid < A(t) <= 1, the outphasing method When 0 <= A(t) <= Amid, the pulse modulator

[0097] Note that the outphasing signal represents a radio signal with a fluctuating amplitude as the sum of a pair of two amplitude-constant vector signals. These vector signals OP1 and OP2 are described by the following equations (27) and (28). OP1(t) = cos(ω C ·t - θ(t) - θamp(t)) …(27) OP2(t) = cos(ωC ·t - θ(t) + θamp(t)) …(28) However, θamp(t) = Arccos(A(t)) ω C = 2π·f C is as follows.

[0098] Note that considering the desired signal RF(t) is A(t)cos(ω C ·t - θ(t)), the following equation (29) holds. OP1(t) + OP2(t) = 2·RF(t) …(29) Therefore, the sum of OP1(t) and OP2(t) includes RF(t).

[0099] Next, the time waveforms of the Hybrid modulator 512 are shown. FIG. 11 is a graph showing an example of the time waveform of Hybrid modulation and the IQ plane in the optical wireless transmission system of Embodiment 5. In FIG. 11, the two signals output from the Hybrid modulator 512 are denoted as Shyb1(t) and Shyb2(t). When Amid < A(t) <= 1, Shyb1 and Shyb2 are signals obtained by rectifying the outphasing signal. Shyb1(t) = k·OP1(t) + Hmh1(t) …(30) Shyb2(t) = k·OP2(t) + Hmh2(t) …(31) In the above equations (30) and (31), Hmh1(t) and Hmh2(t) are harmonic signals generated when rectifying the outphasing signal respectively. k is a constant. Their sum includes the desired signal RF(t) as shown in the following equation (32). Shyb1(t) + Shyb2(t) = 2k·RF(t) + Hmh1(t) + Hmh2(t) …(32)

[0100] When A(t) = Amid, since θamp(t) = Arccos(Amid), the following equations (33) and (34) are obtained. Shyb1(t) = k·cos(ω C·t - θ(t) - arccos(Amid)) + Hmh1(t) …(33) Shyb2(t) = k·cos(ω C ·t - θ(t) + arccos(Amid)) + Hmh2(t) …(34) The sum of the two contains the desired signal RF(t) in the same way as when Amid < A(t) <= 1.

[0101] When 0 <= A(t) < Amid, it becomes pulse modulation and the following equations (35) to (39) are obtained. Shyb1(t) = Rect(phase_h1(t), θ edge_h (t)) …(35) Shyb2(t) = Rect(phase_h2(t), θ edge_h (t)) …(36) phase_h1(t) = ω C ·t - θ(t) - arccos(Amid) …(37) phase_h2(t) = ω C ·t - θ(t) + arccos(Amid) …(38)

[0102] θ edge_h (t) = arcsin{A(t) / Amid} …(39)

[0103] Similar to Embodiment 3, the fundamental waves of Shyb1(t) and Syb2(t) are represented by cos(ωc·t - θ(t) - arccos(Amid)) and cos(ωc·t - θ(t) + arccos(Amid)) respectively, and the Fourier components of these fundamental waves contain A(t) / Amid as shown in the following equations (40) and (41). Shyb1(t) = k·(A(t) / Amid)·cos(ωc·t - θ(t) - arccos(Amid)) + Hmh1(t) …(40) Shyb2(t) = k·(A(t) / Amid)·cos(ωc·t - θ(t) + arccos(Amid)) + Hmh2(t) …(41)

[0104] In this formula, the sum of the two includes the desired signal RF(t) as shown in the following formula (42) when A(t) >= Amid, which is the same as the previous case. Shyb1(t) + Shyb2(t) = 2k·RF(t) + Hmh1(t) + Hmh2(t) …(42) As described above, the time waveform of the pulse is shown in FIG. 11.

[0105] When A(t) is greater than or equal to Amid, the mark rate is 50%. However, when A(t) is less than Amid, the mark rate deviates from 50% (in FIG. 11, the mark rate decreases. It can also be considered in terms of the inverted signal, in which case it increases). Therefore, when A(t) is less than Amid, by adding a dither signal, it is possible to avoid extremely large or small mark rates. As a result, it becomes possible to use a commercially available optical module for digital communication.

[0106] Thus, according to the optical wireless transmission system of Embodiment 5, when A(t) is greater than or equal to Amid, it is not necessary to add a dither signal. Therefore, the intensity of the dither signal in the signal synthesized by the slave unit becomes small, and the cutoff characteristics of the filter for removing the unnecessary wave components of the dither signal can be relaxed. As a result, the cost can be reduced.

[0107] Note that the present invention is not limited to the above embodiments and can be appropriately modified without departing from the spirit. For example, Embodiments 1 to 5 may be arbitrarily combined.

[0108] Also, in the above embodiment, the transmission device 110 is configured such that the pulse modulator and the dither modulator are connected in series. However, any configuration that can generate the signal waveforms described in FIG. 2 is applicable. Therefore, the present invention can include any configuration that can generate the signal waveforms of FIG. 2. For example, a configuration including a processor that directly derives and outputs Spwm_dither by calculation using IQ as an input signal may be used.

[0109] Each element described in the drawings as a functional block that performs various processes can be composed of a CPU, memory, and other circuits in terms of hardware, and can be realized by a program loaded into the memory or the like in terms of software. Therefore, it is understood by those skilled in the art that these functional blocks can be realized in various forms by only hardware, only software, or a combination thereof, and are not limited to any one of them.

[0110] In addition, the above-described program can be stored using various types of non-transitory computer-readable media and supplied to a computer. Non-transitory computer-readable media include various types of tangible recording media. Examples of non-transitory computer-readable media include magnetic recording media (e.g., flexible disks, magnetic tapes, hard disk drives), magneto-optical recording media (e.g., magneto-optical disks), CD-ROM (Read Only Memory), CD-R, CD-R / W, semiconductor memories (e.g., mask ROM, PROM (Programmable ROM), EPROM (Erasable PROM), flash ROM, RAM (Random Access Memory)). Also, the program may be supplied to the computer by various types of temporary computer-readable media. Examples of temporary computer-readable media include electrical signals, optical signals, and electromagnetic waves. The temporary computer-readable media can supply the program to the computer via wired communication paths such as electric wires and optical fibers, or wireless communication paths.

Explanation of Reference Numerals

[0111] 100, 200, 300, 400, 500 Optical wireless transmission system 110, 310, 410, 510 Transmission device 111 DBB 112 Pulse modulator 113, 113-1 to 113-n Dither modulator 114 Dither signal generator 115, 321, 422 Adder 120, 220, 320, 420 Sub-units 121 BPF 122 Power Amplifier 123 Antenna 130 Optical Fiber Module 131, 131-1 to 131-n E / O Converters 132, 132-1 to 132-n Optical Fiber Cables 133, 133-1 to 133-n O / E Converters 221 LO Signal Generator 222 Mixer 312 Multilevel Pulse Modulator 412 Pulse Modulator 421 Amplifier 512 Hybrid Modulator

Claims

1. A transmission device comprising a digital baseband processing unit that generates a plurality of orthogonal signals, a pulse modulator that pulse-modulates the orthogonal signals, and a dither modulator that adds a dither signal to the pulse-modulated signal, an optical fiber module that converts the signal with the dither signal added into an optical signal, transmits it via an optical fiber, and converts the optical signal into a digital electrical signal, and a slave unit that transmits the digital electrical signal transmitted by the optical fiber module as a wireless signal. An optical wireless transmission system comprising the same.

2. The optical wireless transmission system according to claim 1, wherein the dither modulator includes a dither signal generator that generates a dither signal and an adder that adds the dither signal to the pulse-modulated signal.

3. The optical wireless transmission system according to claim 1 or 2, wherein the dither modulator creates a signal by bundling a plurality of binary pulse modulations for an input of three or more values.

4. The optical wireless transmission system according to any one of claims 1 to 3, wherein the dither modulator is a Hybrid modulator that represents an input signal in an outphasing manner when the input signal is greater than a predetermined value and pulse-modulates the input signal when the input signal is less than or equal to the predetermined value.

5. The optical wireless transmission system according to any one of claims 1 to 4, wherein the slave unit includes an LO signal generator that generates a local oscillation signal and a mixer that mixes the local oscillation signal with the digital electrical signal.

6. A transmission device comprising a digital baseband processing unit that generates a plurality of orthogonal signals, a pulse modulator that pulse-modulates the orthogonal signals, and a dither modulator that adds a dither signal to the pulse-modulated signal.

7. In a transmission device, a plurality of orthogonal signals are generated, the orthogonal signals are pulse-modulated, and a dither signal is added to the pulse-modulated signal. In an optical fiber module, the signal with the dither signal added is converted into an optical signal, transmitted via an optical fiber, and the optical signal is converted into a digital electrical signal. In a slave unit, the digital electrical signal transmitted by the optical fiber module is transmitted as a wireless signal. An optical wireless transmission method.

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