JP5745959B2 - OFDM transmitter and receiver for wireless microphone - Google Patents

OFDM transmitter and receiver for wireless microphone Download PDF

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JP5745959B2
JP5745959B2 JP2011152273A JP2011152273A JP5745959B2 JP 5745959 B2 JP5745959 B2 JP 5745959B2 JP 2011152273 A JP2011152273 A JP 2011152273A JP 2011152273 A JP2011152273 A JP 2011152273A JP 5745959 B2 JP5745959 B2 JP 5745959B2
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ofdm
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JP2013021465A (en
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啓之 濱住
啓之 濱住
直彦 居相
直彦 居相
円香 本田
円香 本田
正寛 岡野
正寛 岡野
俊二 中原
俊二 中原
池田 哲臣
哲臣 池田
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日本放送協会
一般財団法人Nhkエンジニアリングシステム
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Description

  The present invention relates to an OFDM transmitter and receiver for wireless microphones that transmit and receive digital audio signals by OFDM modulation.

  Conventionally, there are an analog method and a digital method as a transmission method of a wireless microphone. Non-Patent Document 1 describes the standards established for “radio equipment for land mobile stations of specific radio microphones”, and Non-Patent Document 2 describes “radio equipment for specific low-power radio stations radio microphones”. Standards written are written.

  An analog wireless microphone has a short delay time and is currently widely used, but has a problem that it is easily interrupted by an obstacle, has a short transmission distance, and easily interferes. Therefore, it is necessary to use a digital wireless microphone to provide high-quality sound outdoors or at a concert hall.

  For example, Patent Literature 1 discloses a wireless microphone system that compresses and encodes and transmits audio in a digital manner. FIG. 7 is a block diagram showing the configuration of such a conventional wireless microphone system. The wireless microphone transmission device 3 includes a microphone 31, an A / D conversion unit 32, a compression encoding unit 33, an interleave / error correction unit 34, a modulation unit 35, a D / A conversion unit 36, and a transmission frequency conversion. A unit 37 and a transmission antenna 38 are provided. The wireless microphone transmission device 3 converts an analog audio signal input from the microphone 31 into a digital signal by the A / D conversion unit 32, compresses and encodes the digital signal by the compression encoding unit 33, and interleaves and error correction unit 34. Interleave and error correction. Subsequently, in the wireless microphone transmission device 3, the modulation unit 35 modulates, for example, with the π / 4 shift DQPSK modulation method, the D / A conversion unit 36 converts the modulation signal into an analog signal, and the transmission frequency conversion unit 37 transmits the transmission frequency. And output to the transmitting antenna 38.

  The wireless microphone receiver 4 includes a reception antenna 41, a reception frequency conversion unit 42, an A / D conversion unit 43, a demodulation unit 44, a deinterleave / error correction unit 45, a decompression decoding unit 46, a D / A A conversion unit 47 and a speaker 48 are provided. The wireless microphone receiver 4 frequency-converts the signal input from the reception antenna 41 by the reception frequency conversion unit 42, converts it to a digital signal by the A / D conversion unit 43, and modulates the signal on the transmission side by the demodulation unit 44. The modulated signal is demodulated, and deinterleave and error correction unit 45 performs deinterleave and error correction. Subsequently, in the wireless microphone receiver 4, the decompression decoding unit 46 decompresses the signal compressed on the transmission side, the D / A conversion unit 47 converts the decompressed signal into an analog signal, and outputs the analog signal to the speaker 48.

  However, in the conventional digital wireless microphone system, in order to save the frequency band, the compression encoding unit 33 performs compression processing, and the expansion decoding unit 46 performs expansion processing of the signal. There is a delay time due to the process. In the conventional digital wireless microphone system shown in FIG. 7, a delay time of about 3 ms occurs in the wireless microphone transmission device 3 and the wireless microphone reception device 4 in total. Among them, the delay time due to the compression processing of the compression encoding unit 33 and the expansion processing of the expansion decoding unit 46 is said to be about 1 ms in total. In addition, when using a wireless microphone outdoors or while moving, fading due to multipath occurs and the quality deteriorates. Therefore, it is conceivable that a digital audio signal is modulated and transmitted by an OFDM (Orthogonal Frequency Division Multiplexing) modulation method.

JP-A-10-150692

"Radio equipment of land mobile stations with specific radio microphones", ARIB RCR STD-22, The Japan Radio Industry Association "Radio equipment for specified low-power radio stations radio microphones", ARIB RCR STD-15, Japan Radio Industry Association

  When transmitting a signal by the OFDM modulation method, a reference signal having a known phase and amplitude can be inserted as a pilot signal at a specific carrier symbol position. FIG. 6 is a diagram showing an arrangement of pilot signals in an ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) system and a DVB-T (Digital Video Broadcasting-Terrestrial) system, which are broadcasting systems for terrestrial digital television broadcasting. In the figure, S indicates a scattered pilot (SP) signal, and the others indicate data signals. Since the SP signal has known amplitude and phase at the time of signal generation, the transmission path characteristic can be estimated on the receiving side. When p is a non-negative integer, i is a symbol number, and k is a carrier symbol position where an SP signal is arranged, the SP signal is arranged at a carrier symbol position satisfying the following expression (1).

k = 3 × (mod4) + 12p (1)
Here, (imod4) indicates a remainder obtained by dividing the symbol number i by 4.

  That is, as shown in FIG. 6, when attention is paid to a signal in a certain symbol, the SP signal is arranged every 12 carriers in the carrier direction. Each time one symbol is advanced, the SP signal is shifted in the carrier direction by 3 carriers. In such an ISDB-T system, the SP signal insertion ratio is high, and the data transmission efficiency is 11/12 (91.7%). Actually, since a pilot signal other than the SP signal is also inserted, the transmission efficiency is further deteriorated. In addition, the environment to be used is different between the terrestrial digital television broadcast and the wireless microphone system, and each setting condition to be satisfied is different.

  In order to solve the above problems, an object of the present invention is to provide an OFDM transmitter and receiver for a wireless microphone that can optimize the insertion ratio of a pilot signal for a wireless microphone system and improve transmission efficiency. is there.

  In order to solve the above-described problem, an OFDM transmitter for a wireless microphone according to the present invention is an OFDM transmitter for a wireless microphone that transmits an OFDM signal obtained by modulating a digital audio signal by an OFDM modulation method. Mapping for the IQ plane according to a predetermined modulation scheme for each carrier, generating a carrier modulation signal, inserting a pilot signal into the carrier modulation signal, arranging the carrier, and an OFDM segment frame And an OFDM frame configuration unit that generates an insertion interval in the symbol direction of the pilot signal, and a value obtained by multiplying the insertion interval in the symbol direction of the pilot signal by the effective symbol length of the OFDM signal, 1/10 of the fading period, which is the reciprocal of the maximum Doppler frequency A value obtained by dividing the insertion interval in the carrier direction of the pilot signal by the effective symbol length of the OFDM signal divided by the maximum delay time of the reflected wave, and the insertion interval in the symbol direction of the pilot signal. It is characterized by setting as follows.

Further, in the OFDM transmitter for a wireless microphone according to the present invention, the OFDM frame configuration unit may transmit a TMCC signal and an SP signal to one carrier of each segment obtained by dividing the bandwidth of the OFDM signal into a plurality of segments for each channel. Are arranged as the first TMCC / SP signal, and the TMCC signal and the SP signal are arranged as the second TMCC / SP signal on the carrier having the highest frequency or the lowest frequency of each channel, The TMCC / SP signal and the second TMCC / SP signal have the same arrangement of the TMCC signal and the SP signal in the symbol direction.

  Furthermore, in the OFDM transmitter for a wireless microphone according to the present invention, the OFDM frame configuration unit sets the number of data carriers per segment to a value obtained by dividing the number of data carriers in the entire band by the number of segments. To do.

  Further, in the wireless microphone OFDM transmitter according to the present invention, an inner code encoder that inputs a digital audio signal in units of blocks and performs inner encoding for each block to generate an inner code, and the inner code encoder No is the number of bits of block-unit data input to the inner code, Ri is the coding rate of the inner code, M is the modulation multi-level number of the OFDM signal, and Nd is the number of data carriers of the OFDM signal. A transmission parameter setting unit that sets the No, the Ri, the M, and the Nd so that Ri × M × Nd, and the carrier modulation unit performs inner coding by the inner code coding unit The digital digital audio signal is mapped to an IQ plane according to a predetermined modulation method for each carrier to generate a carrier modulation signal.

In order to solve the above problem, an OFDM receiver for a wireless microphone according to the present invention receives an OFDM signal transmitted from the above-described OFDM transmitter for a wireless microphone and generates a digital audio signal. The receiving apparatus includes an OFDM demodulator that extracts a pilot signal, estimates a transmission path characteristic, and demodulates the OFDM signal .

  According to the present invention, the pilot signal insertion ratio can be optimized for the wireless microphone system, and the transmission efficiency can be improved.

It is a block diagram which shows the structure of the OFDM transmitter for wireless microphones of one Embodiment by this invention. It is a block diagram which shows the structure of the OFDM receiver for wireless microphones of one Embodiment by this invention. It is a figure which shows the example of the 1st signal arrangement | positioning in the OFDM transmitter for wireless microphones of one Embodiment by this invention. It is a figure which shows the example of the 2nd signal arrangement | positioning in the OFDM transmitter for wireless microphones of one Embodiment by this invention. It is a figure which shows the parameter example of the OFDM transmitter for wireless microphones of one Embodiment by this invention. It is a figure which shows arrangement | positioning of the pilot signal of the conventional terrestrial digital television broadcast. It is a block diagram which shows the structure of the conventional wireless microphone system of a digital system.

  Hereinafter, embodiments according to the present invention will be described in detail with reference to the drawings.

[OFDM transmitter for wireless microphone]
FIG. 1 is a block diagram showing a configuration of an OFDM transmitter for a wireless microphone according to the present invention. As shown in FIG. 1, an OFDM transmitter 1 for a wireless microphone includes a microphone 11, an A / D converter 12, an interleave / error corrector 13, an OFDM modulator 14, a D / A converter 15, A transmission frequency conversion unit 16, a transmission antenna 17, a transmission parameter setting unit 18, a crystal oscillator 19 (19-1 to 19 -n), and a clock supply unit 20 are provided. The interleave / error correction unit 13 includes an outer code encoding unit 131, an interleaving unit 132, and an inner code encoding unit 133. The OFDM modulation unit 14 includes an S / P conversion unit 141, a carrier modulation unit 142, and the like. OFDM frame configuration section 143, IFFT section 144, and GI addition section 145.

  The A / D conversion unit 12 converts an analog audio signal input from the microphone 11 into a digital signal and outputs the digital signal to the outer code encoding unit 131.

  Outer code encoding section 131 adds an RS code, BCH code, difference set cyclic code, or CRC, and outputs the result to interleaving section 132. This is because error correction is performed on the receiving side, or error detection is performed and concealment is applied to an erroneous block. In particular, the delay time can be reduced by using the BCH code. For example, in the case of the RS (204, 188) code, the number of bits included in the code 1 block is 204 (bytes) × 8 (bits / byte) = 1632 bits, and the delay time due to encoding and decoding is 1310 μs. . On the other hand, in the case of the BCH (144, 128) code, the number of bits included in one code block is 144 bits, and the delay time due to encoding and decoding is 115 μs. When a code having a code length No is generated from data having an information length Ko by block coding, this code is represented as a (No, Ko) code, and Ro = Ko / No is referred to as a coding rate. The code length No means the block length after outer coding. In order to distinguish from the coding rate of the inner code described later, the coding rate Ro of the outer code is referred to as the outer coding rate.

  Interleaving section 132 rearranges the order of outer codes input from outer code encoding section 131 and outputs the result to inner code encoding section 133 in order to increase the efficiency of error correction.

  The inner code encoding unit 133 performs inner encoding (for example, convolutional encoding) on the signal input from the interleaving unit 132 and outputs the signal to the S / P conversion unit 141. In general, when a code having a code length Ni is generated from data having an information length Ki by inner coding, Ri = Ki / Ni is referred to as a coding rate. In order to distinguish from the above-described outer coding rate (outer coding rate), the coding rate Ri of the inner code is referred to as an inner coding rate.

  The S / P converter 141 temporarily stores the inner code input from the inner code encoder 133 in a storage area such as an internal memory, and converts it into parallel data when a predetermined number of data is reached. And output to the carrier modulation unit 142. For example, when the number of carriers is Nd and the modulation level of the modulation scheme of each carrier is M, it is converted into Nd signals by M bits.

  The carrier modulation unit 142 performs mapping to the IQ plane according to a predetermined modulation method (modulation multilevel number M) for each carrier for a signal input in parallel from the S / P conversion unit 141 every M bits, A carrier modulation signal is generated and output to OFDM frame configuration section 143.

  The OFDM frame configuration unit 143 generates an OFDM segment frame by inserting and arranging a pilot signal with respect to the carrier modulation signal input from the carrier modulation unit 142 and outputs the OFDM segment frame to the IFFT unit 144. Here, the pilot signal includes an SP signal and a CP (Continual Pilot) signal that is a reference signal continuous in the symbol direction. Further, the pilot signal may include a TMCC (Transmission and Multiplexing Configuration Control) signal that is a signal for transmitting control information and an AC (Auxiliary Channel) signal that is a signal for transmitting additional information. An arrangement example of pilot signals will be described later.

  The IFFT unit 144 performs an IFFT (Inverse Fast Fourier Transform) process on the OFDM segment frame input from the OFDM frame configuration unit 143 to generate an effective symbol signal, which is output to the GI adding unit 145 To do.

  The GI adding unit 145 inserts a guard interval obtained by copying the latter half of the effective symbol signal at the head of the effective symbol signal input from the IFFT unit 144, and outputs the guard interval to the transmission rate adjustment buffer memory 145. The guard interval is inserted in order to reduce intersymbol interference when receiving an OFDM signal, and is set so that the delay time of the multipath delay wave does not exceed the guard interval length.

  The D / A converter 15 converts the digital signal input from the transmission rate adjustment buffer memory 145 into an analog signal. The transmission frequency conversion unit 16 modulates the analog signal input from the D / A conversion unit 15 to a transmission frequency, amplifies the power, and outputs the modulated signal to the reception antenna via the transmission antenna 17. Send.

  The transmission parameter setting unit 18 sets the information length Ko and the code length No for the outer code encoding unit 131 as parameters, sets the interleave parameter for the interleaving unit 132, and sets the interleave parameter for the inner code encoding unit 133. The inner coding rate Ri is set, the modulation multi-level number M and the data carrier number Nd are set for the S / P converter 141, and the modulation scheme type and the data carrier number Nd are set for the carrier modulation unit 142. The IFFT unit 144 sets the number of FFT points, the modulation multi-level number M, and the number of data carriers Nd, and the GI adding unit 145 sets the guard interval ratio. Of course, if the modulation multi-level number M is different for each type of modulation scheme, the modulation multi-level number M may be set in the carrier modulation unit 142.

  Here, if the code length of the outer code by the outer code encoder 131 is No, the data amount a per block output from the outer code encoder 131 is equal to No (the unit is bits). Further, if the inner coding rate by the inner code coding unit 133 is Ri, the modulation multi-level number of the modulation scheme in the carrier modulation unit 142 is M, and the number of carriers of the OFDM signal is Nd, the data amount b per OFDM signal symbol b Is represented by Ri × M × Nd (the unit is bits).

  In the case of a = b, a bit of the code length output every time processing is performed from the outer code encoding unit 131 and the interleaving unit 132 in units of blocks, and the inner code encoding unit 133 and the OFDM modulation unit 14 in units of symbols. Since the b bits of the amount of data to be processed are equal, the inner code encoding unit 133 and the OFDM modulation unit 14 buffer the a bit (= b bits) data output from the outer code encoding unit 131 and the interleaving unit 132. Then, the inner code encoding process and the OFDM modulation process can be immediately executed. Therefore, the transmission parameter setting unit 18 controls and sets parameters so as to satisfy the following expression (2).

  No = Ri * M * Nd (2)

  It is also possible to set the outer coding rate Ro = 1. In this case, the outer code coding unit 131 is unnecessary, and the inner code coding unit 133 inputs a digital audio signal in units of blocks. The inner code is generated by performing inner coding for each block. In this case, the parameter is set so as to satisfy Expression (2), where No is the bit length of the block unit data input to the inner code encoding unit.

  Thus, since the transmission parameter setting unit 18 can continuously generate OFDM signals by setting the parameters so that No = Ri × M × Nd, the buffer memory for adjusting the transmission rate is The delay of data output from the inner code encoding unit 133 and the OFDM modulation unit 14 with respect to data input to the outer code encoding unit 131 and the interleaving unit 132 can be reduced.

  The clock supply unit 20 selects the crystal oscillator 19 according to the symbol rate of the carrier of the OFDM signal, generates a clock, and supplies the clock to the interleave / error correction unit 13 and the OFDM modulation unit 14.

[OFDM receiver for wireless microphone]
Next, an OFDM receiver for a wireless microphone according to the present invention will be described. FIG. 2 is a block diagram showing a configuration of an OFDM receiver for a wireless microphone according to the present invention. As shown in FIG. 2, the wireless microphone OFDM receiving apparatus 2 includes a receiving antenna 21, a receiving frequency converting unit 22, an A / D converting unit 23, an OFDM demodulating unit 24, a deinterleaving / error correcting unit 25, , A D / A conversion unit 26, a speaker 27, a reception parameter setting unit 28, a crystal oscillator 29 (29-1 to 29-n), and a clock supply unit 30. The OFDM demodulation unit 24 includes a GI removal unit 241, an FFT unit 242, a carrier demodulation unit 243, and a P / S conversion unit 244. The deinterleave / error correction unit 25 includes an inner code decoding unit 251 and a deinterleave unit. Unit 252 and outer code decoding unit 253.

  The reception frequency conversion unit 22 amplifies the power of the audio signal received by the reception antenna 21, converts the frequency to intermediate frequency data, and outputs the data to the A / D conversion unit 23. The A / D conversion unit 23 converts the analog signal input from the reception frequency conversion unit 22 into a digital signal, outputs the parameter information to the reception parameter setting unit 28, and outputs the parameter information to the GI removal unit 241.

  The GI removal unit 241 removes the guard interval from the digital signal input from the A / D conversion unit 23, extracts a valid symbol, and outputs the effective symbol to the FFT unit 242.

  The FFT unit 242 performs FFT (Fast Fourier Transform) processing on the effective symbol input from the GI removal unit 241 and outputs the result to the carrier demodulation unit 243.

  The carrier demodulator 243 demodulates the signal input from the FFT unit 242 for each carrier and outputs the demodulated signal to the P / S converter 244. When demodulating, the SP signal is extracted and compared with a reference value (known amplitude and phase) to calculate the channel characteristic of the carrier in which the SP signal exists, and the calculated channel characteristic is calculated in the time direction and Interpolate in the frequency direction to calculate the estimated values of the transmission path characteristics of all OFDM carriers.

  The P / S converter 244 converts the signal input in parallel from the carrier demodulator 243 into a serial signal.

  The inner code decoding unit 251 performs inner code decoding on the inner code input from the P / S conversion unit 244 to generate an outer code, and outputs the outer code to the deinterleaving unit 252. In addition, when the transmission side performs the inner coding by the convolutional coding, the inner code decoding unit 251 performs Viterbi decoding to correct the error, and outputs the error to the deinterleaving unit 252.

  The deinterleaving unit 252 rearranges the data order with respect to the outer code input from the inner code decoding unit 251, and outputs the rearranged data to the reception rate adjustment buffer memory 254.

  Outer code decoding section 253 decodes the code encoded by outer code encoding section 131 of OFDM transmitter 1 for wireless microphone using an outer code such as a BCH code. If the wireless microphone OFDM transmitter 1 does not include the outer code encoder 131 with the outer code rate Ro = 1, the outer code decoder 253 is also unnecessary.

  The D / A conversion unit 26 converts the digital signal input from the outer code decoding unit 253 into an analog signal and outputs the analog signal to the speaker 27.

  The reception parameter setting unit 28 sets the same parameter as that set in the wireless microphone OFDM transmitter 1 in each block. For example, the guard interval ratio is set for the GI removal unit 241, the number of FFT points, the modulation multilevel number M, and the number of data carriers Nd are set for the FFT unit 242, and the modulation scheme is set for the carrier demodulation unit 243. Type (modulation multi-level number M) and data carrier number Nd are set, the inner coding rate Ri is set for the inner code decoding unit 251, and the interleave parameter is set for the deinterleaving unit 252. An information length Ko and a code length No are set for the outer code decoding unit 253. The parameter may be received from the wireless microphone OFDM transmitter 1, or the parameter information may be acquired from the TMCC signal.

  Here, since the parameter set by the reception parameter setting unit 28 satisfies the condition of No = Ri × M × Nd in Expression (2), the reception rate adjustment is performed in the same manner as the wireless microphone OFDM transmitter 1. No buffer memory is required.

  The clock supply unit 30 generates a clock by selecting the crystal oscillator 29 according to the symbol rate (parameter setting mode) of the carrier of the OFDM signal, and supplies the clock to the OFDM demodulation unit 24 and the deinterleave / error correction unit 25. Supply.

[Effective symbol length]
Next, regarding the symbol length of the OFDM signal in the wireless microphone system according to the present invention, the delay time between transmission and reception, that is, the voice input to the microphone 11 of the wireless microphone OFDM transmitter 1 is the speaker of the wireless microphone OFDM receiver 2. The optimum value is examined from the viewpoints of the delay time until output from 27 and the delay time of the reflected wave with respect to the fundamental wave due to multipath.

First, consider the delay time between transmission and reception. According to the subjective evaluation, it is said that when the delay time between transmission and reception is about 2 ms or less, it becomes difficult to detect the delay, and the delay becomes almost unnoticeable. Therefore, in this embodiment, the effective symbol length Tu is determined so that the delay time between transmission and reception is 2 ms or less. The delay time by the A / D converter 12 is about 400 μs, and the delay time by the D / A converter 26 is about 400 μs. The total delay time by the outer code encoding unit 131 and the outer code decoding unit 253 is about 115 μs when the BCH code is used. The total delay time by the interleaving unit 132 and the deinterleaving unit 252 is about 125 μs. The total delay time of the inner code encoder 133 and the inner code decoder 251 is about 270 μs. Then, in order to set the delay time in the entire transmission and reception to 2 ms or less, the total delay time T OFDM of the OFDM modulation unit 14 and the OFDM demodulation unit 24 needs to be set to 690 μs or less. Since the delay time of about 3 symbols is generated in the processing of the OFDM modulator 14 and the OFDM demodulator 24, the effective symbol length Tu needs to satisfy the condition of the following equation (3).

Tu ≦ T OFDM / 3 (3)

  Next, consider the maximum delay time due to multipath. When the maximum value of the propagation distance difference is L, the maximum delay time τ of the reflected wave is expressed by τ = L / c using the speed of light c. Under the use environment of the wireless microphone, even when an omnidirectional antenna is used, the maximum propagation distance difference L between the direct wave and the reflected wave is about 2000 m. In order to prevent fading due to multipath, the effective symbol length Tu needs to be about 10 times the delay dispersion or more. Therefore, the effective symbol length Tu needs to satisfy the condition of the following equation (4). Regarding the point that the effective symbol length Tu needs to be about 10 times the delay dispersion, see, for example, Takashi Shono, “Impress Standard Textbook Series WiMAX Textbook”, Impress R & D, July 16, 2008, P71. See description.

  Tu ≧ τ × 10 = 10 L / c (4)

If T OFDM ≦ 690 [μs] in Equation (3) and L = 2000 [m] in Equation (4), the effective symbol length Tu needs to be set in a range that satisfies the following Equation (5).

  66.6 [μs] ≦ Tu ≦ 230 [μs] (5)

[SP signal insertion interval]
Next, the insertion interval in the symbol direction (time direction) of the SP signal will be considered. When mobile reception is performed, interference between carriers occurs due to the Doppler effect due to movement. In order to compensate for the interference between the carriers, the OFDM frame configuration unit 143 inserts an SP signal in the time direction.

  In order to prevent the influence of transmission characteristic deterioration due to Doppler shift due to mobile reception, the effective symbol length Tu needs to be about 1/100 or less of the fading period Tf. The fading period Tf is the reciprocal of the maximum Doppler frequency fd, and the maximum Doppler frequency fd is expressed by the following equation (6) using the moving speed v, the carrier frequency fc, and the speed of light c. Regarding the point that the effective symbol length Tu needs to be about 1/100 or less of the fading period Tf, for example, Takashi Shono, “Impress Standard Textbook Series WiMAX Textbook”, Impress R & D, July 16, 2008 , Young-Cheol YU, M, OKADA and H. YAMAMOTO, “Dipole Array Antenna Assisted Doppler Spread Compensator with MRC Diversity for ISDB-T Receiver.”, Vol.E90-B, No.5, IEICE TRANS. COMMUN, May 2007 Please refer to the description.

  fd = v × fc / c (6)

  However, on the receiving side, the received OFDM signal is zero-order held in the symbol direction, so that it is possible to apparently reduce the insertion interval in the carrier direction and increase the number of SP signals used for the transmission path response. For example, in the case of the conventional ISDB-T system shown in FIG. 6, the apparent insertion interval of the SP signal in the carrier direction is set every three symbols by the 0th-order hold. Thus, in the case of zero-order hold on the receiving side, the symbol direction insertion interval It of the SP signal needs to satisfy the condition of the following equation (7). That is, the OFDM frame configuration section 143 is obtained by multiplying the insertion interval It in the symbol direction of the SP signal by the insertion interval It in the symbol direction of the SP signal and the effective symbol length Tu of the OFDM signal as a reciprocal of the maximum Doppler frequency fd. It is set to be 1/100 or less of a certain fading cycle Tf.

  It × Tu ≦ Tf / 100 (7)

  Next, the insertion interval of the SP signal in the carrier direction (frequency direction) will be considered. The insertion interval If in the carrier direction of the SP signal needs to be equal to or less than Tu / τ obtained by dividing the effective symbol length Tu by the maximum delay time τ of the reflected wave. However, as described above, the receiving side can hold the zeroth order in the symbol direction, and in this case, the carrier signal insertion interval If of the SP signal needs to satisfy the condition of the following equation (8). . That is, the OFDM frame configuration section 143 sets the insertion interval It in the SP signal symbol direction to the value obtained by dividing the insertion interval If in the carrier direction of the SP signal by the effective symbol length Tu of the OFDM signal by the maximum delay time τ of the reflected wave. Set to be less than or equal to the value multiplied by.

  If ≦ (Tu / τ) × It (8)

In a situation where sound is transmitted using a microphone, there is no need to assume an environment where the user is moving on a vehicle as in the case of reception by a mobile phone, and consideration is given to the case where an adult moves fast or lightly. Is enough. Therefore, specifically, the SP signal insertion interval required when v = 2.5 m / s is obtained. When the moving velocity v and 2.5 m / s, when the carrier frequency f c is 600MHz, fd = 2.5 × 600 × 10 6 / (3 × 10 8) = 5 [Hz], the carrier frequency fc is 1200MHz In this case, fd = 2.5 × 1200 × 10 6 / (3 × 10 8 ) = 10 [Hz]. Therefore, the fading cycle Tf = 1 / fd is 200 ms when the carrier frequency is 600 MHz, and 100 ms when the carrier frequency is 1200 MHz. Substituting 100 ms for Tf in Equation (7), 66.6 μs for Tu, substituting 66.6 μs for Tu in Equation (8), and 6.66 μs for τ, the symbol direction insertion interval It of the SP signal and the SP signal The carrier direction insertion interval If needs to be set in a range satisfying the following expression (9).

  It ≦ 15, If ≦ 10 × It (9)

  In this way, the OFDM frame configuration section 143 has a value obtained by multiplying the insertion interval It in the symbol direction of the SP signal by the insertion symbol It in the symbol direction of the SP signal and the effective symbol length Tu of the OFDM signal. It is set to be 1/100 or less of the reciprocal fading period Tf, and the insertion interval If in the carrier direction of the SP signal is divided by the effective symbol length Tu of the OFDM signal by the maximum delay time τ of the reflected wave, By setting the SP signal to be equal to or less than the value multiplied by the insertion interval It in the symbol direction, the SP signal insertion ratio can be optimized for the wireless microphone system and the transmission efficiency can be improved.

[Pilot signal arrangement example]
Next, an arrangement example of pilot signals in the wireless microphone system according to the present invention is shown. As in the ISDB-T system, the OFDM frame configuration unit 143 can insert an SP signal, a TMCC signal, and a CP signal as pilot signals. In this embodiment, SP signals are arranged every 20 carriers in the carrier direction. Further, the bandwidth of the OFDM signal allocated for the wireless microphone may be divided into a plurality of segments. In this embodiment, the number of carriers per segment is 20.

  FIG. 3 is a diagram illustrating an example of a first signal arrangement of each segment in the wireless microphone OFDM transmitter 1. In the example shown in FIG. 3A, the OFDM frame configuration unit 143 arranges SP signals at carrier symbol positions that satisfy the following equation (10). In the example shown in FIG. 3B, the OFDM frame configuration unit 143 SP signals are arranged at carrier symbol positions satisfying the following equation (11). That is, It = 5 and If = 20, and these values satisfy the condition of Expression (9).

k = 4 × (imod5) + 20p (10)
k = 2 × (imod10) + 20p (11)

  In addition, in order to enable control for each segment, the OFDM frame configuration unit 143 arranges a TMCC signal on one carrier of each segment (carrier number 11 in the figure). That is, the arrangement of TMCC signals is fixed in the symbol direction (the same carrier position for each symbol). When pilot signals are arranged in this way, the data transmission efficiency is 18/20 (90%).

  FIG. 3C shows the carriers of the entire band of each channel transmitted by the OFDM transmitter 1, and the CP signal is arranged on the carrier having the highest frequency. In addition, the spectrum of the segment may be inverted and the CP signal may be arranged on the carrier having the lowest frequency of each channel.

  FIG. 4 is a diagram showing an example of the second signal arrangement of each segment in the wireless microphone OFDM transmitter 1 according to the present invention. Similar to FIG. 3, in the example shown in FIG. 4A, the OFDM frame configuration unit 143 arranges the SP signal at the carrier symbol position that satisfies Equation (10), and in the example shown in FIG. The OFDM frame configuration unit 143 arranges SP signals at carrier symbol positions that satisfy Equation (11). In addition, the OFDM frame configuration unit 143 arranges the TMCC / SP signal on one carrier of each segment (carrier of carrier number 10 in the figure). Here, the TMCC / SP signal is a signal that is arranged on the same carrier number and includes a TMCC signal and an SP signal in the symbol direction.

FIG. 4C shows the carriers of the entire band of each channel transmitted by the OFDM transmitter 1, and the TMCC / SP signal is arranged on the carrier with the highest frequency. Note that the TMCC / SP signal may be arranged on the carrier having the lowest frequency of each channel. Here, the arrangement of the TMCC signal and the SP signal in the symbol direction of the TMCC / SP signal arranged in the carrier having the highest frequency (or the carrier having the lowest frequency) is the same as the TMCC / SP signal arranged in each segment. Arrange. For example, as shown in FIG. 4, an SP signal is arranged at a carrier symbol position where (imod5), which is a remainder obtained by dividing symbol number i by 5, is arranged, and a TMCC signal is arranged at other carrier symbol positions.

  As described above, the OFDM frame configuration unit 143 arranges the TMCC signal and the SP signal as the first TMCC / SP signal on one carrier of each segment, and furthermore, the highest frequency carrier or the highest frequency of each channel. The TMCC signal and the SP signal are arranged as the second TMCC / SP signal on the carrier having a low signal level, and the first TMCC / SP signal and the second TMCC / SP signal are arranged in the symbol direction. Since it is not necessary to provide a TMCC signal extraction circuit and a CP signal extraction circuit, it is only necessary to provide a common extraction circuit for TMCC / SP signals. Further, by adopting the TMCC / SP signal instead of the TMCC signal, the reception characteristics of the TMCC signal can be improved by the amount of the SP signal inserted.

[Parameter example]
Next, parameter examples used in the wireless microphone system according to the present invention will be shown. FIG. 5 is a diagram showing an example of parameters of the wireless microphone system according to the present invention. In this embodiment, the quantization bit length is 24 bits and the sampling frequency is 48 KHz. Therefore, the input information rate I is I = 24 × 48 = 1152 [kbps]. The wireless microphone OFDM transmitter 1 and the wireless microphone receiver 2 do not perform compression / decompression processing in order to reduce the amount of delay between transmission and reception, that is, the information compression rate is 1. Ro = 128/144 = 8/9 is set so that the outer coding rate Ro is constant. The rate Vo after the outer coding is Vo = I × 1 / Ro = 11152 × 9/8 = 1296 [kbps]. The inner coding rate Ri and the modulation multi-level number M are different values depending on the mode.

  In the case of mode 1, the inner coding rate Ri = 1/3 and the modulation multi-level number M = 2. Therefore, the symbol rate Vs of the entire OFDM signal is Vs = Vo × (1 / Ri) × (1 / M) = 1296 × 3/2 = 1944 [kHz].

  The outer code encoder 131 starts processing after waiting for the number of information bits to be transmitted in the number of bits transmitted in one block. Therefore, the delay time can be reduced as the number of information bits transmitted in one block is smaller. However, if the number of bits to be transmitted is not large to some extent, the coding efficiency is deteriorated, that is, the outer coding rate Ro is small unless the code length No is increased to some extent. Therefore, it is necessary to set a suitable value for the code length No in consideration of the delay time and the coding rate. In this embodiment, the code length No. of the outer code is 144 bits.

  In the case of mode 1, the number of data carriers Nd in the entire band is Nd = No × (1 / Ri) × (1 / M) = 144 × 3/2 = 216. Therefore, the symbol rate Vsc of one carrier is Vsc = Vs / Nd = 1944/216 = 9 [kbps]. At this time, the symbol length Ts is Ts = 1 / Vsc≈111.1 [μs]. This symbol length Ts satisfies the condition of the equation (5).

  Here, in order to prevent the generation of null carriers, the number of data carriers per segment Ndseg is set so as to satisfy the following equation (12) using the number of data carriers Nd and the number of segments S in the entire band. Must be set to

  Ndseg = Nd / S (12)

  That is, the OFDM frame configuration unit 143 preferably uses the number of data carriers per segment as a value obtained by dividing the number of data carriers in the entire band by the number of segments. In the case of mode 1, since Nd = 216 and S = 12, it is necessary to set Ndseg = 216/12 = 18 in order to satisfy Expression (12). Mode 8 and mode 11 show examples that do not satisfy equation (12). The number of null carriers Nnull is expressed as Nnull = S × Ndseg−Nd. In the case of mode 8, Nnull = 3 × 18−48 = 6, and in the case of mode 11, Nnull = 2 × 18−27 = 9.

  In the case of mode 1, since the number of segments S is 12, and the number of SP signals is 1 and the number of TMCC signals is 1 for each segment, the number of SP signals Nsp is 12 in all bands, and the number of TMCC signals Ntmcc Becomes 12. The number of CP signals Ncp is 1 in the entire band. Therefore, the total number of carriers Ncarr is Ncarr = 216 + 12 + 12 + 1 = 241.

  As described above, the OFDM frame configuration unit 143 prevents the occurrence of null carriers by setting the number of data carriers Ndseg per segment to a value obtained by dividing the number of data carriers Nd in all bands by the number of segments S. Can do.

  Although the above embodiments have been described as representative examples, it will be apparent to those skilled in the art that many changes and substitutions can be made within the spirit and scope of the invention. Therefore, the present invention should not be construed as being limited by the above-described embodiments, and various modifications and changes can be made without departing from the scope of the claims. For example, the wireless microphone OFDM transmitter 1 may not include the interleave unit 132, and the wireless microphone OFDM receiver 2 may not include the deinterleave unit 252.

  Thus, according to the present invention, the pilot signal insertion ratio can be optimized for a wireless microphone system, which is useful for any application for transmitting and receiving digital audio signals using the OFDM modulation scheme.

DESCRIPTION OF SYMBOLS 1 OFDM transmitter for wireless microphones 2 OFDM receiver for wireless microphones 11 Microphone 12 A / D conversion unit 13 Interleave / error correction unit 14 OFDM modulation unit 15 D / A conversion unit 16 Transmission frequency conversion unit 17 Transmission antenna 18 Transmission parameter setting Unit 19 crystal oscillator 20 clock supply unit 131 outer code encoding unit 132 interleave unit 133 inner code encoding unit 141 S / P conversion unit 142 carrier modulation unit 143 OFDM frame configuration unit 144 IFFT unit 145 GI addition unit 21 reception antenna 22 reception Frequency conversion unit 23 A / D conversion unit 24 OFDM demodulation unit 25 Deinterleave / error correction unit 26 D / A conversion unit 27 Speaker 28 Reception parameter setting unit 29 Crystal oscillator 30 Clock supply unit 241 GI removal unit 242 F T 243 in the carrier demodulator 244 P / S conversion unit 251 code decoding section 252 deinterleaver 253 outer code decoding section

Claims (5)

  1. An OFDM transmitter for a wireless microphone that transmits an OFDM signal obtained by modulating a digital audio signal by an OFDM modulation method,
    A carrier modulation unit for mapping a digital audio signal to an IQ plane according to a predetermined modulation method for each carrier and generating a carrier modulation signal;
    An OFDM frame configuration unit that inserts and arranges a pilot signal with respect to the carrier modulation signal and generates an OFDM segment frame, and
    The OFDM frame configuration unit is configured such that a value obtained by multiplying an insertion interval in a symbol direction of a pilot signal by an insertion interval in the symbol direction of the pilot signal and an effective symbol length of the OFDM signal is a reciprocal of a maximum Doppler frequency. Set to 100 or less,
    The pilot signal insertion interval in the carrier direction is set to be equal to or smaller than the value obtained by dividing the effective symbol length of the OFDM signal by the maximum delay time of the reflected wave and the pilot signal symbol interval insertion interval. An OFDM transmitter for a wireless microphone.
  2. The OFDM frame configuration unit arranges the TMCC signal and the SP signal as a first TMCC / SP signal on one carrier of each segment obtained by dividing the bandwidth of the OFDM signal into a plurality of segments for each channel , and The TMCC signal and the SP signal are arranged as the second TMCC / SP signal on the highest frequency carrier or the lowest frequency carrier of each channel,
    2. The OFDM transmitter for wireless microphone according to claim 1, wherein the first TMCC / SP signal and the second TMCC / SP signal have the same arrangement order of the TMCC signal and the SP signal. 3. .
  3.   3. The wireless microphone OFDM according to claim 1, wherein the OFDM frame configuration unit sets the number of data carriers per segment to a value obtained by dividing the number of data carriers in all bands by the number of segments. Transmitter device.
  4. An inner code encoder that inputs a digital audio signal in units of blocks and performs inner encoding for each block to generate an inner code;
    The number of bits of block unit data input to the inner code encoder is No, the coding rate of the inner code is Ri, the modulation multi-level number of the OFDM signal is M, and the number of data carriers of the OFDM signal is Nd. A transmission parameter setting unit that sets No, Ri, M, and Nd so that No = Ri × M × Nd.
    The carrier modulation unit maps the digital audio signal encoded by the inner code encoding unit to an IQ plane according to a predetermined modulation method for each carrier, and generates a carrier modulation signal. The OFDM transmitter for a wireless microphone according to any one of claims 1 to 3, wherein the device is an OFDM transmitter.
  5. An OFDM receiver for a wireless microphone that receives an OFDM signal transmitted from the OFDM transmitter for a wireless microphone according to any one of claims 1 to 4 and generates a digital audio signal .
    An OFDM demodulator that extracts the pilot signal, estimates transmission path characteristics, and demodulates the OFDM signal
    OFDM receiver for wireless microphone, characterized in that it comprises a.
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