JP2003264522A - Multicarrier modulation device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To avoid the generation of tertiary distortion in a synchronous symbol of an OFDM (orthogonal frequency division multiplexing) signal by an amplifier in a receiver side of the OFDM modulation signal. <P>SOLUTION: When a synchronous symbol is added to an OFDM signal in each frame of an information symbol for the synchronization of FFT (fast Fourier transform) processing on a receiver side, the amplitude of the synchronous symbol is made to be almost equal to the maximum amplitude of the OFDM signal. This makes the maximum power of the OFDM signal almost equal to mean power, thereby preventing the generation of tertiary distortion in the synchronous symbol in an amplifier of a receiving set. <P>COPYRIGHT: (C)2003,JPO

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM (Orthogonal Frequency Divisio) used for performing stable and high-speed wireless communication under a multipath fading environment.
The present invention relates to a multi-carrier modulation device that generates a modulation signal. 2. Description of the Related Art In mobile radio communication, the environment around a mobile unit changes with time as the mobile unit on the receiving side moves. In addition to radio waves directly arriving from the base station, radio waves (delayed waves) via various paths reflected by surrounding buildings and the like also arrive at this mobile object, and receive so-called multipath radio waves. Will be. When such radio waves are received, interference occurs between them. However, as described above, if the surrounding environment changes with the movement of the moving object, the path through which the radio waves pass also changes, and as a result, these interferences change. Multipath fading occurs. [0003] By the way, when an information signal is transmitted digitally, it is transmitted in a symbol unit. Such a digital signal also causes multipath fading. And
When performing digital wireless communication with a high transmission rate, the symbol period is shortened. However, when the symbol period is shortened in this way, a delayed wave symbol interferes with a symbol of the direct wave that is different from the corresponding symbol. As a result, the influence of the delayed wave increases, and the quality of the received signal is degraded. As means for compensating for such deterioration in quality,
Adaptive equalizers are known. This is, in principle, to add a received signal and a plurality of delayed signals obtained by delaying the received signal by a symbol unit at a predetermined ratio, respectively, and to control these ratios according to an error to be detected. . However, if the arrival time (delay time) of the delayed wave becomes longer, the influence of the delayed wave appears after the longer delay time, so that more delay means to be used are required, and the processing amount increases exponentially. For this reason, it is realistic to set the delay amount that can be equalized to about several symbols, and
Even an adaptive equalizer cannot sufficiently suppress multipath fading. [0005] As a method of further compensating for the influence of the delayed wave, an OFDM modulation system for collectively modulating a plurality of mutually orthogonal subcarriers (multicarriers) using an inverse Fourier transform circuit is typically used. FIG. 6 is a block diagram showing an example of a conventional multi-carrier modulation apparatus, wherein 1 is an SP (serial-parallel) conversion circuit, and 2 is an IFFT (Inverse Fast Fourier).
er Transform: inverse fast Fourier transform) circuit, 3 is a changeover switch, 4 is a D / A (digital / analog conversion) circuit,
5 is a modulation circuit, 6 is a BPF (band pass filter), 7
Is a mixer, 8 'is a synchronization symbol generation circuit, and 9 is an oscillation circuit. In FIG. 1, a serial digital information signal is converted into a parallel digital information signal by an SP circuit 1, inversely Fourier-transformed by an IFFT circuit 11, and further subjected to OFDM modulation with a card interval added. (OFDM signal) is obtained. This OFDM signal is supplied to the changeover switch 3, and a synchronization symbol generated by the synchronization symbol generation circuit 8 'is added to each frame formed by a plurality of information symbols. The synchronization symbol is composed of a null section having an average power of 0 and a symbol section for defining a specific time on the time axis (hereinafter, this symbol section is referred to as a synchronization symbol).
As the synchronization symbol, a sweep signal or PN (Pseu
do Noise (pseudo noise) code. The OFDM signal to which the synchronization symbol is added is D / A
After being converted into an analog OFDM signal by the conversion circuit 4,
The signal is quadrature-modulated by the modulation circuit 5, mixed with the output signal of the oscillation circuit by the mixer 7, becomes an RF band signal, and transmitted from the antenna. On the receiving side, a reference signal is extracted from the synchronization symbol by performing correlation detection, and the received signal is synchronized with this reference signal by an FFT (Fast Fourier Transform).
rm: Fast Fourier Transform) Generally, Fourier transform is performed by a circuit. By the way, in the above-mentioned conventional multi-carrier modulation apparatus, the D / A conversion circuit 4 uses the amplitude value (digital value) of the synchronization symbol generated by the synchronization symbol generation circuit 8 ′. Bit adjustment is performed so that the maximum amplitude is output. Therefore, when the OFDM signal is random, a difference in amplitude occurs between the OFDM signal and the synchronization symbol. FIG. 7 is a waveform diagram showing a synchronization symbol portion of the output signal of the D / A conversion circuit 4 in FIG. 6. In the conventional multi-carrier modulation device, the amplitude of the synchronization symbol with respect to the OFDM signal is reduced. It turns out that it is quite large. According to actual measurement, when converted to power, OF
A power about 6 dB larger than the average power of the DM signal is instantaneously generated. On the receiving side, when the OFDM signal is received, it is first amplified by an amplifier. The dynamic range of the amplifier is set according to the amplitude of the synchronization symbol of the received signal. For this reason, when the received signal of the above-mentioned power distribution is supplied to such an amplifier, the amplifier operates in the full dynamic range with the synchronization symbol, so that a third-order distortion is generated in the synchronization symbol, and from the synchronization symbol, A reference signal for synchronizing the above-mentioned FFT processing cannot be accurately extracted, so that the reception performance deteriorates. In order to avoid this, it is conceivable to use an amplifier having a wider dynamic range, but the power consumption increases accordingly. FIG. 8 is a graph showing the measured frequency spectrum of a received signal output from such an amplifier in the synchronization symbol period, and is approximately 170 MHz centered at 190 MHz.
The band of up to 210 MHz is the band of the synchronization symbol, and the whisker-like frequency component extending above this band is the third-order distortion. An object of the present invention is to provide a multi-carrier modulation apparatus which solves such a problem and makes it possible to prevent deterioration of reception performance by using a synchronization symbol. In order to achieve the above-mentioned object, the present invention provides a method of making an amplitude of a synchronization symbol added for each frame of an OFDM signal of an information symbol substantially equal to the amplitude of the OFDM signal. To set. Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a multi-carrier modulation apparatus according to the present invention. Reference numeral 8 denotes a synchronization symbol generation circuit, and portions corresponding to those in FIG. Omitted. In this figure, the configuration of this embodiment is almost the same as that of the conventional multicarrier apparatus shown in FIG. 6, but the synchronization symbol generation circuit 8 is different from the synchronization symbol generation circuit 8 'in FIG. . That is, the synchronization symbol generating circuit 8 is capable of adjusting the amplitude of the generated synchronization symbol, and has adjusted the amplitude to be substantially equal to the maximum amplitude of the OFDM signal. FIG. 2 is a block diagram showing a specific example of the synchronization symbol generation circuit 8 in FIG. 1. Here, the synchronization symbol is a PN (Pseudo Noise: pseudo noise) code. An IFFT circuit, 3 is a changeover switch in FIG. 1, 8a is a PN signal generation circuit, 8b is a cutout circuit, 8c is a band limiting circuit, and 10 is a carrier modulation circuit. In FIG. 1, a PN signal generating circuit 8a includes:
A PN signal in which 127 samples are repeated at a sample rate equal to the FF sample rate in the IFFT circuit 2 is generated. The extracting circuit 8b outputs a PN signal for each frame of the OFDM signal.
Synchronizing with the synchronization symbol addition position in this frame,
80 samples out of 127 samples of the PN signal are cut out. The waveform at this time is shown in FIG. The output of the extracting circuit 8b is supplied to a band limiting circuit 8c, and a bandwidth equal to the information symbol of the OFDM signal, that is, a bandwidth equal to the bandwidth = {l / 2 FFT sample frequency × the number of subcarriers} / the number of FFT samples. Limited. The output signal of the band limiting circuit 8c is supplied to the changeover switch 3 as a synchronization symbol.
It is added to the OFDM signal from the IFFT circuit 2. After the output signal of the changeover switch 3 is up-sampled by the carrier modulation circuit 10, the D / A conversion circuit 4 of FIG.
Supplied to FIG. 3B is a waveform diagram of a synchronization symbol output from the band limiting circuit 8c, and FIG. 3C is a diagram showing a frequency spectrum of the synchronization symbol. FIG. 4 is a waveform diagram showing a synchronizing symbol portion of the output signal of the D / A conversion circuit 4 in FIG. 1. As shown, the amplitude of the synchronizing symbol is substantially equal to the maximum amplitude of the OFDM signal. That is, when converted into power, the maximum power of the power of the synchronization symbol becomes the average power of the OFDM signal. FIG. 5 is a diagram showing an actually measured frequency spectrum of a synchronizing symbol of an output signal of an amplifier in a receiver having received a signal having a waveform as shown in FIG. The receiver used at this time is the same as the receiver used for measuring the frequency spectrum shown in FIG. As is apparent from the frequency spectrum shown in FIG. 5, in the case of this embodiment, the third-order distortion as shown in the frequency spectrum shown in FIG. 8 when a transmission signal from a conventional multicarrier apparatus is received is: Not observed. Therefore, in this embodiment, the amplifier having the same characteristics as the conventional one on the receiving side does not cause third-order distortion in the synchronization symbol, the FFT processing is accurately synchronized, and the high-output amplifier is used. Therefore, the reception quality can be improved without increasing power consumption. As described above, according to the present invention,
Occurrence of third-order distortion in a synchronization symbol on the receiving side of an OFDM signal can be suppressed while avoiding an increase in power consumption, thereby improving reception quality.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing an embodiment of a multicarrier modulation device according to the present invention. FIG. 2 is a configuration diagram illustrating a specific example of a synchronization symbol generation circuit in FIG. 1; FIG. 3 is a diagram showing signal waveforms and frequency spectra of synchronization symbols of respective parts of the cutout circuit in FIG. 2; FIG. 4 is a waveform diagram showing a measurement result of a synchronization symbol portion of an output signal of the D / A conversion circuit in FIG. 1; 5 is a diagram showing an actual measurement result of a frequency spectrum of an output of the amplifier on the receiver side based on the waveform shown in FIG. 4; FIG. 6 is a block diagram showing a conventional example of a multicarrier modulation device. 7 is a waveform chart showing an actual measurement result of a synchronization symbol portion of an output signal of the D / A conversion circuit in FIG. 8 is a diagram showing an actual measurement result of the frequency spectrum of the output of the amplifier on the receiver side based on the waveform shown in FIG. 7; [Description of Signs] 1 SP conversion circuit 2 IFFT circuit 3 Changeover switch 4 D / A conversion circuit 5 Modulation circuit 6 BPF 7 Mixer 8 Synchronous symbol generation circuit 8a PN signal generation circuit 8b Cutout circuit 8c Band limiting circuit 9 Oscillation circuit 10 Carrier modulation circuit

   ────────────────────────────────────────────────── ─── Continuation of front page    (72) Inventor Kiyoshi Hamaguchi             4-2-1 Nukikitamachi, Koganei-shi, Tokyo Independent             Inside the Communications Research Laboratory (72) Inventor Hiroyo Ogawa             4-2-1 Nukikitamachi, Koganei-shi, Tokyo Independent             Inside the Communications Research Laboratory F term (reference) 5K022 DD01 DD13 DD17 DD19 DD23                       DD33                 5K047 BB01 GG34 HH43

Claims (1)

Claims 1. An information signal is converted from serial to parallel.
Further, the information symbol O obtained by the inverse Fourier transform
FDM (Orthogonal Frequency Division Multiplexin)
g) A multicarrier modulation apparatus for forming a transmission signal by adding a synchronization symbol to each frame of a signal, wherein the amplitude of the synchronization symbol is set substantially equal to the amplitude of the OFDM signal.