JPH118604A - Transmission system for orthogonal frequency division multiplex modulation signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a transmission code configuration for reducing a code error rate of a transmitted information code in an orthogonal frequency division multiplexing (OFDM) transmission apparatus. SOLUTION: A zero frequency carrier (a carrier giving a DC component) of a baseband signal to be multiplexed by the OFDM method.
Is configured to use the BPSK method as a modulation method or not use it for transmitting information codes. [Effect] The zero-frequency carrier of the baseband signal, which is likely to cause a code error due to the influence of the DC level adjustment accuracy of the receiving circuit and the temperature stability thereof, is not used for the transmission of the information code. The error rate can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an orthogonal frequency division multiplex modulation.
The present invention relates to a transmission code configuration for reducing a bit error rate of a transmitted information code in a transmission device of the g: OFDM system, and a transmission system using this code configuration.

[0002]

2. Description of the Related Art At present, an orthogonal frequency division multiplexing transmission system (hereinafter, referred to as an OFDM system) having characteristics of being resistant to multipath fading and ghosting is used as a multiplex transmission system for mobile or terrestrial digital radio communication. Is attracting attention. In this OFDM system, as shown in FIG. 3, a large number of tens or hundreds of carrier waves arranged at a predetermined frequency interval fs are digitally converted at a symbol frequency fs '(= 1 / Ts'). Modulated signal,
That is, an OFDM signal (orthogonal frequency division multiplex modulation signal)
Is a method of transmitting an information code using. Here, the time interval Ts' is a symbol period of the digital signal. As a digital modulation method for each carrier in the OFDM method, a QPSK method (four-phase phase shift keying method), a 16QAM method (16-level quadrature amplitude modulation method), and the like are being studied.

FIG. 4 shows a block circuit configuration of an OFDM transmission apparatus according to the prior art when each carrier is digitally modulated by the QPSK method. Here, the upper side of FIG. 4 represents the transmitting device, and the lower side represents the receiving device side. In the transmitting device, the information code to be transmitted is modulated by the QPSK modulation circuit 1 into a complex vector signal of the QPSK system (hereinafter, referred to as a QPSK signal). The QPSK signal obtained by the modulation is distributed to each carrier by the distribution circuit 2 and then inverse discrete Fourier transform (I
FFT). By this conversion process, the QPSK signal is
OFDM of baseband multiplexed by the orthogonal frequency division multiplexing modulation method composed of Ns carrier waves separated from each other at a frequency interval fs at a symbol period Ts' and orthogonal to each other.
Converted to a signal. Then, the OFDM signal is supplied to the mixer 4 and, for example, several hundred MHz by the transmitting side local signal of the frequency fr generated by the high frequency transmitting side local oscillator 5.
The signal is frequency-converted into a high-frequency signal of a band or a several GHz band, power-amplified, and transmitted from the transmitting antenna 6.

On the other hand, in the receiving device, a received signal received by the receiving antenna 7 is amplified and then input to the mixer 8. The received signal is frequency-converted by the receiving-side local oscillator signal of the frequency fr generated by the receiving-side local oscillator 9, and the multiplexed baseband OFDM signal is reproduced. This OFDM signal is further subjected to a discrete Fourier transform (FFT) by the FFT circuit 10 to be separated into a baseband complex vector signal Z (n) of each carrier. Here, n represents the number of the separated carrier. The complex vector signal Z (n) of each carrier separated in this manner is rearranged in the original time order in the coupling circuit 11 by the reverse procedure of the distribution circuit 2 on the transmission side, and the temporally continuous QPSK signal And demodulated by the QPSK demodulation circuit 12 and output as an information code.

[0005]

By the way, the baseband OFDM signal includes a carrier whose carrier frequency is a predetermined DC level corresponding to the information code to be transmitted during one symbol period (hereinafter referred to as carrier number 0). Note). On the other hand, in the FFT circuit 10 of the receiving device, as described above, the input signal is directly subjected to discrete Fourier transform. Therefore, if the adjustment of the DC level input to the FFT circuit 10 is shifted, the shifted DC level is also converted as a signal component and mixed as the carrier component of the number 0. As a result, the QPSK method and the 16
When demodulating by the QAM method, it appears as a large noise component and increases the bit error rate of the entire demodulated code. This problem can be avoided by adjusting the DC level of the signal input to the FFT circuit 10 to exactly the 0 level.
However, in an actual circuit, this adjustment is difficult, and a special circuit for suppressing a change in the operating point of the circuit due to temperature is required, which has a disadvantage that the circuit scale is increased and the transmission device is expensive. An object of the present invention is to provide a transmission code configuration for reducing a bit error rate in an OFDM signal demodulation process without increasing a circuit scale, and a transmission device using the transmission code configuration.

[0006]

According to the present invention, in order to achieve the above object, Ns carrier waves separated from each other by a frequency interval fs or an integer multiple thereof are represented by a time interval Ts' (= 1 / fs) as a symbol period. In the orthogonal frequency division multiplexing modulation signal transmission system for transmitting an information code by digital modulation, a predetermined direct current according to the information code to be transmitted is used for the Ns carrier waves which are orthogonal to each other in a baseband for one symbol period. This is a transmission method of an orthogonal frequency division multiplex modulation signal in which the above information code is transmitted using a carrier excluding a carrier that becomes a level.

Further, Ns carrier waves separated from each other by a frequency interval fs or an integer multiple thereof are divided into time intervals Ts' (= 1 /
fs) in the orthogonal frequency division multiplexing modulation signal transmission system in which the information code is transmitted by digitally modulating the symbol period as the symbol period, of the Ns carrier waves orthogonal to each other in the baseband, the carrier frequency is transmitted for one symbol period. This is a transmission method of an orthogonal frequency division multiplex modulation signal using a BPSK method (two-phase shift keying) as a modulation method of a carrier wave having a predetermined DC level according to a code. Furthermore, a carrier wave whose carrier frequency becomes a predetermined DC level corresponding to the information code to be transmitted for one symbol period is reproduced as a symbol synchronization signal or carrier wave frequency used for reproducing the synchronization of each symbol in the receiver. This is a transmission method of an orthogonal frequency division multiplex modulation signal used as a carrier for transmitting a synchronization signal for carrier recovery used in the above.

As described above, according to the present invention, when demodulating,
No information code is transmitted on the carrier of number 0 affected by the DC level. Alternatively, the information code is transmitted by modulating the carrier having the number 0 using the BPSK method which is strong against noise. Further, the carrier wave having the number 0 is used as a carrier wave for transmitting a symbol synchronization signal or a carrier wave reproduction synchronization signal used for reproducing a carrier wave frequency. for that reason,
In demodulation, a carrier code that frequently causes a code error due to a shift in the DC level does not cause an increase in the code error rate for the entire transmission code, and the code error rate of the transmitted code can be reduced.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the transmission apparatus according to the present invention will be described below with reference to FIG. Here, as a digital modulation method for each carrier, FIG.
2 shows a circuit configuration in the case of using the QPSK method, which is the same as the related art. As described above, almost all conventional circuits can be used as they are in the method of the present invention. The circuit shown in FIG. 1 is different from the FFT circuit of the receiver in that the distribution circuit 2 for distributing the QPSK signal modulated by the QPSK modulation circuit to each carrier is changed to a distribution circuit 2 'for distributing the carrier except for the carrier of number 0. A coupling circuit 11 for recombining the signals of the respective carriers obtained by the discrete Fourier transform at 10 and returning them to QPSK signals;
Circuit 1 except for the carrier signal of number 0
Only the point changed to 1 'is different from the conventional circuit of FIG. The operation of the transmission apparatus is performed in the same manner as the conventional circuit except that the information code is not transmitted on the carrier wave of number 0, and thus the description is omitted.

The number Ns of carriers multiplexed by the OFDM method is usually set to several hundred to several thousand. For example,
When the number of carrier waves is set to 1000, if 10% of the codes of the carrier wave of number 0 cause a code error,
Even if the code of the other carrier does not cause an error, the code error rate of the entire transmission code is 1/10 ⁴ . For this reason, even when the reception state is good and there is almost no noise, it is necessary to use an error correction code having a high error correction code capability and a long code length, which results in a reduction in the transmission rate.

On the other hand, in the present embodiment, since the carrier of number 0 having a high code error rate is not used, the code error rate of the entire transmitted code is obtained as the transmission code of the original code error rate determined by the reception state. be able to. In the code configuration of the present embodiment, the carrier of number 0 out of the 1000 carriers cannot be used for information code transmission, so that the transmission rate of the transmission device is slightly reduced. However, the reduction is only 1
/ 1000 only. In addition, the circuit scale is almost the same as the conventional one, and therefore, there is no increase in the price of the transmission device. As described above, in the transmission apparatus according to the present embodiment, since the carrier of number 0 affected by the DC level shift is not used for transmitting the information code, the carrier of the carrier that frequently causes a code error due to the DC level shift is used. The code does not cause an increase in the bit error rate for the entire transmission code.
Therefore, the code error rate of the transmitted code can be reduced, and the transmission device can be kept inexpensive without increasing the circuit scale to obtain this effect.

Next, a second embodiment of the present invention will be described with reference to FIG. In this embodiment, in addition to the QPSK modulation circuit 1, a BPSK modulation circuit 13 that performs noise-resistant BPSK modulation is provided in the transmission apparatus. Then, when modulating the carrier wave of number 0, the corresponding information code is modulated by the BPSK modulation circuit 13 and assigned to the carrier wave of number 0 by the distribution circuit 2 ″.

On the other hand, a QPSK demodulation circuit 1
2, a BPSK demodulation circuit 14 is provided. further,
Among the carrier signals obtained by the discrete Fourier transform by the FFT circuit 10, the carrier signals excluding the carrier signal of the number 0 are converted into the original time order by the reverse procedure of the distribution circuit 2 ″ on the transmission side. A coupling circuit 11 "is provided for rearranging and returning to a temporally continuous QPSK signal, and for returning the signal of the carrier wave of number 0 to a BPSK signal. And these FFT circuits 10,
The signals returned to the QPSK signal and the BPSK signal by the coupling circuit 11 ″ are respectively converted into a QPSK demodulation circuit 12,
The information code is demodulated by the SK demodulation circuit 14 and output.
In the present embodiment, the carrier of number 0, which is susceptible to a code error under the influence of the DC level shift, is modulated by the BPSK method which is strong against noise, so that the code error rate of the code of the carrier of number 0 is reduced. In addition, the code error rate of the entire transmitted code can be reduced. In this embodiment, since a new circuit is required, the circuit scale is slightly increased. However, the number 0
Since the information code is also transmitted by the carrier wave of (1), the reduction amount of the transmission rate can be reduced.

In the first embodiment, the description has been made on the assumption that the carrier of the number 0 is not used at all. However, in an OFDM transmission apparatus, a synchronization code for reproducing a symbol phase and a carrier frequency of a transmission signal necessary for demodulating a received signal is inserted for each predetermined symbol of the transmission signal. Therefore, the synchronization code may be transmitted using a carrier wave of number 0 which does not transmit the information code. By doing so, the information code can be transmitted using the symbols originally used for the transmission of the synchronization code, so that the amount of reduction in the transmission rate can be reduced.

[0015]

As described above, in the transmission code configuration according to the present invention, the information code is transmitted by modulating the carrier wave of number 0, which is affected by the shift of the DC level, by the noise-resistant BPSK method, or Since the information code is not transmitted by the carrier of the number 0, the code of the carrier that frequently causes a code error due to a shift in the DC level does not cause an increase in the code error rate for the entire transmission code. Therefore, the code error rate of the transmitted code can be reduced without increasing the circuit scale.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of an OFDM transmission apparatus according to the present invention.

FIG. 2 is a block diagram showing the configuration of a second embodiment of an OFDM transmission apparatus according to the present invention.

FIG. 3 is a diagram showing a frequency arrangement of carrier waves in a general OFDM system.

FIG. 4 is a block diagram showing a configuration of a conventional OFDM transmission apparatus.

[Explanation of symbols]

1: QPSK modulation circuit, 2, 2 ′, 2 ″: distribution circuit,
3: IFFT circuit, 4, 8: mixer, 5: transmitting local oscillator, 6: transmitting antenna, 7: receiving antenna, 9: receiving local oscillator, 10: FFT circuit, 11, 11 ', 1
1 "coupling circuit, 12: QPSK demodulation circuit, 13: BPS
K modulation circuit, 14: BPSK demodulation circuit.

Continuation of front page (72) Inventor Nobuo Tsukamoto 32, Miyukicho, Kodaira-shi, Tokyo Inside Koganei Plant of Hitachi Electronics Co., Ltd.

Claims (3)

[Claims] 1. Ns carrier waves separated from each other by a frequency interval fs or an integer multiple thereof are separated by a time interval Ts' (= 1 / fs).
In the orthogonal frequency division multiplex modulation signal transmission method of digitally modulating the symbol period and transmitting the information code,
A configuration in which the Ns carrier waves orthogonal to each other in the baseband are used as the Ns carrier waves except for a carrier wave whose carrier frequency is a predetermined DC level corresponding to the information code to be transmitted for one symbol period, and the information code is transmitted. A method of transmitting an orthogonal frequency division multiplex modulation signal, characterized in that: 2. Ns carrier waves separated from each other by a frequency interval fs or an integer multiple thereof are divided into time intervals Ts' (= 1 / fs).
In the orthogonal frequency division multiplex modulation signal transmission method of digitally modulating the symbol period and transmitting the information code,
Among the Ns carrier waves orthogonal to each other in the baseband, the BPSK method (two-phase shift keying) is used as a carrier modulation method in which the carrier frequency becomes a predetermined DC level corresponding to the information code to be transmitted for one symbol period. A transmission method of an orthogonal frequency division multiplex modulation signal. 3. Ns carrier waves separated from each other by a frequency interval fs or an integer multiple thereof are separated by a time interval Ts' (= 1 / fs).
In the orthogonal frequency division multiplex modulation signal transmission method of digitally modulating the symbol period and transmitting the information code,
Of the Ns carrier waves orthogonal to each other in the baseband, a carrier wave whose carrier wave frequency is a predetermined DC level corresponding to the information code to be transmitted for one symbol period is reproduced by a receiving device in synchronization with each symbol. A transmission method of an orthogonal frequency division multiplex modulation signal, which is used as a carrier for transmitting a symbol synchronization signal to be used or a carrier recovery synchronization signal used to recover a carrier frequency.