USRE40174E1 - Communication system - Google PatentsCommunication system Download PDF
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- USRE40174E1 USRE40174E1 US09/668,068 US66806800A USRE40174E US RE40174 E1 USRE40174 E1 US RE40174E1 US 66806800 A US66806800 A US 66806800A US RE40174 E USRE40174 E US RE40174E
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This application is a continuation-in-part of application Ser. No. 07/857,627, filed Mar. 25, 1992, pending.This is a reissue application of U.S. Pat. No. 5,600,672, issued Feb. 4, 1997, and a divisional application of reissue application No. 09/244,037, filed Feb. 4, 1999, which is also a reissue application of U.S. Pat. No. 5,600,672, issued Feb. 4, 1997 which is a Continuation-In-Part of application Ser. No. 07/857,627, filed Mar. 25, 1992 now abandoned. Further reissue divisional applications have been filed, all of which are reissues of Pat. No. 5,600,672. These further applications are: 09/677,421, filed Oct. 5, 2000; 09/678,014, filed Oct. 5, 2000; 09/677,420, filed Oct. 5, 2000; 09/680,177, filed Oct. 5, 2000; 09/680,176, filed Oct. 5, 2000; 09/686,467, filed Oct. 12, 2000; 09/686,463, filed Oct. 12, 2000; 09/686,466, filed Oct. 12, 2000; 09/688,028, filed Oct. 12, 2000; 09/686,464, filed Oct. 12, 2000; 09/686,465, filed Oct. 12, 2000; 09/666,012, filed Sep. 19, 2000; 09/667,525, filed Sep. 21, 2000; 09/667,438, filed Sep. 21, 2000; 09/668,068, filed Sep. 25, 2000; 09/669,916, filed Sep. 25, 2000; 09/672,948, filed Sep. 29, 2000; 09/672,946, filed Sep. 29, 2000; 09/672,947, filed Sep. 29, 2000; 10/133,347, filed Apr. 29, 2002; 10/133,364, filed Apr. 29, 2002; 10/692,469, filed Oct. 24, 2003; 10/693,526, filed Oct. 27, 2003; 10/635,468, filed Aug. 7, 2003; 10/690,297, filed Oct. 27, 2003; 10/860,666, filed Jun. 4, 2004; 10/782,411, filed Feb. 20, 2004; 10/783,588, filed Feb. 23, 2004; 10/773,811, filed Feb. 9, 2004; 10/882,126, filed Jun. 30, 2004; 10/885,572, filed Jul. 7, 2004; 10/911,680, filed Aug. 5, 2004; and 11/038,006, filed Jan. 19, 2006.
1. Field of the Invention
The present invention relates to a communication system for transmission and reception of a digital signal through modulation of its carrier wave and demodulation of the modulated signal.
2. Description of the Prior Art
Digital signal communication systems have been used in various fields. Particularly, digital video signal transmission techniques have been improved remarkably.
Among them is a digital TV signal transmission method. So far, such digital TV signal transmission systems are in particular use for transmission between TV stations. They will soon be utilized for terrestrial and/or satellite broadcast service in every country of the world.
The TV broadcast systems including HDTV, PCM music, FAX, and other information services are now demanded to increase desired data in quantity and quality for satisfying millions of sophisticated viewers. In particular, the data has to be increased in a given bandwidth of frequency allocated for TV broadcast service. The data to be transmitted is always abundant and provided as much as handled with up-to-date techniques of the time. It is ideal to modify or change the existing signal transmission system corresponding to an increase in the data amount with time.
However, the TV broadcast service is a public business and cannot go further without considering the interests and benefits of viewers. It is essential to have any new service compatible with existing TV receivers and displays. More particularly, the compatibility of a system is much desired for providing both old and new services simultaneously or one new service which can be intercepted by both the existing and advanced receivers.
It is understood that any new digital TV broadcast system to be introduced has to be arranged for data extension in order to respond to future demands and technological advantages and also, for compatibility to allow the existing receivers to receive transmissions.
The expansion capability and compatible performance of the prior art digital TV system will be explained.
A digital satellite TV system is known in which NTSC TV signals compressed to an about 6 Mbps are muitiplexed multiplexed by time division modulation of 4 PSK and transmitted on 4 to 20 channels while HDTV signals are carried on a signal channel. Another digital HDTV system is provided in which HDTV video data compressed to as small as 15 Mbps are transmitted on a 16 or 32 QAM signal through ground stations.
Such a known satellite system permits HDTV signals to be carried on the channel by a conventional manner, thus occupying a band of frequencies equivalent to the same channels of NTSC signals. This causes the corresponding NTSC channels to be unavailable during the transmission of the HDTV signal. Also, the compatibility between NTSC and HDTV receivers or displays is hardly concerned and data expansion capability needed for matching a future advanced mode is utterly disregarded.
Such a common terrestrial HDTV system offers an HDTV service on conventional 16 or 32 QAM signals without any modification. In any analogue TV broadcast service, there are developed a lot of signal attenuating or shadow regions within its service area due to structural obstacles, geographical inconveniences, or signal interference from a neighbor station. When the TV signal is an analogue from form, it can be intercepted more or less at such signal attenuating regions although its reproduced picture is low in quality. If the TV signal is a digital form, it can rarely be reproduced at an acceptable level within the regions. This disadvantage is critically hostile to the development of any digital TV system.
It is an object of the present invention, for solving the foregoing disadvantages, to provide a communication system arranged for compatible use for both the existing NTSC and newly introduced HDTV broadcast services, particularly via satellite and also, for minimizing signal attenuating or shadow region of its service area on the grounds ground.
A communication system according to the present invention intentionally varies signal points, which used to be disposed at uniform intervals, to perform the signal transmission and reception. For example, if applied to a QAM signal, the communication system comprises two major sections: a transmitter having a signal input circuit, a modulator circuit for producing m numbers of signal points, in a signal vector field through modulation of a plurality of out-of-phase carrier waves using an input signal supplied from the input circuit, and a transmitter circuit for transmitting a resultant modulated signal; and a receiver having an input circuit for receiving the modulated signal, a demodulator circuit for demodulating one-bit signal points of a QAM carrier wave, and an output circuit.
In operation, the input signal containing a first data stream of n values and a second data stream is fed to the modulator circuit of the transmitter where a modified m-bit QAM carrier wave is produced representing m signal points in a vector field. The m signal points are divided into n signal point groups to which the n values of the first data stream are assigned respectively. Also, data of the second data stream are assigned to m/n signal points or sub groups of each signal point group. Then, a resultant transmission signal is transmitted from the transmitter circuit. Similarly, a third data stream can be propagated.
At the p-bit demodulator circuit, p>m, of the receiver, the first data stream of the transmission signal if is first demodulated through dividing p signal points in a signal space diagram into n signal point groups. Then, the second data stream is demodulated through assigning p/n values to p/n signal points of each corresponding signal point group for reconstruction of both the first and second data streams. If the receiver is a P=n, the n signal point groups are reclaimed and assigned the n values for demodulation and reconstruction of the first data stream.
Upon receiving the same transmission signal from the transmitter, a receiver equipped with a large sized antenna and capable of large-data modulation can reproduce both the first and second data streams. A receiver equipped with a small sized antenna and capable of small-data modulation can reproduce the first data stream only. Accordingly, the compatibility of the signal transmission system will be ensured. When the first data stream is an NTSC TV signal or low frequency band component of an HDTV signal and the second data stream is a high frequency band component of the HDTV signal, the small-data modulation receiver can reconstruct the NTSC TV signal and the large-data modulation receiver can reconstruct the HDTV signal. As understood, a digital NTSC/HDTV simultaneous broadcast service will be feasible using the compatibility of the signal transmission system of the present invention.
More specifically, the communication system of the present invention comprises: a transmitter having a signal input circuit, a modulator circuit for producing m signal point, points in a signal vector field through modulation of a plurality of out-of-phase carrier waves using an input signal supplied from the input, and a transmitter circuit for transmitting a resultant modulated signal, in which the main procedure includes receiving an input signal containing a first data stream of n values and a second data stream, dividing the m signal points of the signal into n signal point groups, assigning the n values of the first data stream to the n signal point groups respectively, assigning data of the second data stream to signal points of each signal point group respectively, and transmitting the resultant modulated signal; and a receiver having an input circuit for receiving the modulated signal, a demodulator circuit for demodulating p signal points of a QAM carrier wave, and an output circuit, in which the main procedure includes dividing the p signal points into n signal point groups, demodulating the first data stream of which n values are assigned to the n signal point groups respectively, and demodulating the second data stream of which p/n values are assigned to p/n signal points of each signal point group respectively. For example, a transmitter produces a modified m-bit QAM signal of which first, second, and third data streams, each carrying n values, are assigned to relevant signal point groups with a modulator. The signal can be intercepted and the first data stream only reproduced by a first receiver, both the first and second data streams can be reproduced by a second receiver, and all the first, second, and third streams can be reproduced by a third receiver.
More particularly, a receiver capable of demodulation of n-bit data can reproduce n bits from a multiple-bit modulated carrier wave carrying m-bit data where m>n, thus allowing the communication system to have compatibility and capability of future extension. Also, a multi-level signal transmission will be possible by shifting the signal points of QAM so that a nearest signal point to the origin point of I-axis and Q-axis coordinates is spaced nf from the origin where f is the distance of the nearest point from each axis and n is more than 1.
Accordingly, a compatible digital satellite broadcast service for both the NTSC and HDTV systems will be feasible when the first data stream carries an NTSC signal and the second data stream carries a difference signal between NTSC and HDTV. Hence, the capability of corresponding to an increase in the data amount to be transmitted will be ensured. Also, on the ground, the service area will be increased will signal attenuating areas are decreased.
FIG. 25(a) and 25(b) are vector diagrams respectively showing an 8 and a 16 QAM signal of the first embodiment;
FIGS. 59(a) and 59(b) are diagrams showing assignment of signal points of the modified 4 ASK signal of the fifth embodiment of FIGS. 59(c) and 59(d) are diagrams respectively showing the slice levels of the modulated 4 ASK signal in subchannels 1 and 2;
FIG. 62(a) is a wave spectrum diagram of the ASK signal, i.e., a multi-value VSB signal before filtering, in the fifth embodiment of the invention and FIG. 62(b) is a wave spectrum diagram showing the characteristics of the filtered VSB signal;
FIG. 68(a) is an 8-level VSB constellation map in the fifth and sixth embodiments of the invention;
FIG. 68(b) is an 8-level VSB constellation map in the fifth and sixth embodiments of the invention;
FIG. 68(c) is an 8-level VSB signal-time waveform diagram in the fifth and sixth embodiments of the invention;
FIG. 74(a) is a block diagram of a video decoder of the fifth embodiment;
FIG. 74(b) is a diagram showing another time assignment of data components of the transmission signal according to the fifth embodiment;
FIG. 108(a) is a diagram showing a frequency distribution profile of a conventional TV signal;
FIG. 108(b) is a diagram showing a frequency distribution profile of a conventional two-layer TV signal;
FIG. 108(c) is a diagram showing threshold values of the third embodiment;
FIG. 108(d) is a diagram showing a frequency distribution profile of two-layer OFDM carriers of the ninth embodiment, and FIG. 108(e) is a diagram showing threshold values for three-layer OFDM of the ninth embodiment;
FIG. 119(a) is a diagram showing a time slot assignment of a conventional system;
FIG. 119(b) is a diagram showing a time slot assignment according to the eighth embodiment;
FIG. 120(a) is a diagram showing a time slot assignment of a conventional TDMA system;
FIG. 120(b) is a diagram showing a time slot assignment according to a TDMA system of the eighth embodiment;
FIG. 125(a) is a view showing a frequency assignment of a modulation signal of a conventional system;
FIG. 125(b) is a view showing a frequency assignment of a modulation signal according to the ninth embodiment;
FIG. 126(a) is a view showing a frequency assignment of an OFDM signal of the ninth embodiment, wherein no weighting is applied;
FIG. 126(b) is a view showing a frequency assignment of an OFDM signal of the ninth embodiment, wherein two channels of two-layer OFDM are weighted by transmission electric power;
FIG. 126(c) is a view showing a frequency assignment of an OFDM signal of the ninth embodiment, wherein carrier intervals are doubled by weighting;
FIG. 126(d) is a view showing a frequency assignment of an OFDM signal of the ninth embodiment, wherein carrier intervals are not weighted;
FIG. 128(a) is a block diagram of a trellis encoder (ratio ½) in embodiments 2, 4, and 5,
FIG. 128(b) is a block diagram of a trellis encoder (ratio ⅔) in embodiments 2, 4, and 5,
FIG. 128(c) is a block diagram of a trellis encoder (ratio ¾) in embodiments 2, 4, and 5,
FIG. 128(d) is a block diagram of a trellis decoder (ratio ½) in embodiments 2, 4, and 5,
FIG. 128(e) is a block diagram of a trellis decoder (ratio ⅔) in embodiments 2, 4, and 5,
FIG. 128(f) is a block diagram of a trellis decoder (ratio ¾) in embodiments 2, 4, and 5;
FIG. 145(a) is a diagram showing the waveform of the guard time and the symbol time in the multi-level OFDM according to the ninth embodiment, wherein multipath is short;
FIG. 145(b) is a diagram showing the waveform of the guard time and the symbol time in the multi-level OFDM according to the ninth embodiment, wherein multipath is long;
FIG. 149(a) is a diagram showing time slots of respective layers according to the ninth embodiment;
FIG. 149(b) is a diagram showing time distribution of guard times of respective layers according to the ninth embodiment;
FIG. 149(c) is a diagram showing time distribution of guard times of respective layers according to the ninth embodiment;
FIG. 159(a) is a signal point positioning diagram in 16-level VSB in the fifth embodiment of the invention;
FIG. 159(b) is a signal point positioning (8-level VSB) diagram in 16-level VSB in the fifth embodiment of the invention;
FIG. 159(c) is a signal point positioning (4-level VSB) diagram in 16-level VSB in the fifth embodiment of the invention;
FIG. 159(d) is a signal point positioning (16-level VSB) diagram in 16-level VSB in the fifth embodiment of the invention;
FIG. 160(a) is a block diagram of an ECC encoder in the fifth and sixth embodiments of the invention;
FIG. 160(b) is a block diagram of an ECC decoder in the fifth and sixth embodiments of the invention;
FIG. 165(a) is a block diagram of the Reed-Solomon encoder in the second, fourth, and fifth embodiments of the invention;
FIG. 165(b) is a block diagram of the Reed-Solomon decoder in the second, fourth, and fifth embodiments of the invention;
FIG. 168(a) is an interleave/deinterleave table for the second, third, fourth, and fifth embodiments of the invention;
FIG. 168(b) shows the interleave distance in the second, third, fourth, and fifth embodiments of the invention;
One embodiment of the present invention will be described referring to the relevant drawings.
In the preferred embodiment of the invention both the transmission apparatus, which comprises a transmitter for transmitting a digital HDTV signal or other digital signal and a receiver for receiving the transmitted signal, and the recording and reproducing apparatus, which records the digital HDTV signal or other digital signal on a magnetic tape or other recording medium and reproduces the recorded signal from said medium, are described.
It should be noted, however, that the configuration, operation, and principle of the digital modulator and demodulator, error correction encoder and decoder, and the encoder and decoder for image coding the HDTV signal are common to the transmission apparatus and the recording and reproducing apparatus, and apply essentially the same technologies. Therefore, to more concisely describe each embodiment, the block diagrams for either the transmission apparatus or the recording and reproducing apparatus are referenced in the description of each embodiment. In addition, the configuration of each embodiment of the invention can be achieved by means of any multi-value digital modulation method, e.g., QAM, ASK and PSK, positioning signal points in a constellation, and for brevity the embodiments of the present invention are described using only one modulation method.
The transmission signal is then sent down through three downlinks 21, 32 31, and 41 to a first 23, a second 33, and a third receiver 43 respectively. In the first receiver 23, the signal intercepted by an antenna 22 is fed through an input unit 24 to a demodulator 25 where its first data stream only is demodulated, while the second and third data streams are not recovered, before being transmitted further from an output unit 26.
Similarly, the second receiver 33 allows the first and second data stream of the signal intercepted by an antenna 32 and fed from an input unit 34 to be demodulated by a demodulator 35 and then, combined by a mixer 37 into a single data stream which is then transmitted further from an output unit 36.
The third receiver 43 allows all of the first, second, and third data streams of the signal intercepted by an antenna 42 and fed from an input unit 44 to be demodulated by a demodulator 45 and then, combined by a mixer 47 into a single data stream which is then transmitted further from an output unit 46.
As understood, the three discrete receivers 23, 33, and 43 have their respective demodulators of different characteristics such as that their outputs demodulated from the same frequency band signal of the transmitter 1 contain data of different sizes. More particularly, three different but compatible data can simultaneously be carried on a given frequency band signal to their respective receivers. For example, each of three, existing NTSC, HDTV, and super HDTV, digital signals is divided into low, high, and super high frequency band components which represent the first, the second, and the third data stream respectively. Accordingly, the three different TV signals can be transmitted on a one-channel frequency band carrier for simultaneous reproduction of medium, high, and super high resolution TV images respectively.
The NTSC TV signal is intercepted by a receiver accompanied by a small antenna for demodulation of small-sized data; the HDTV signal is intercepted by a receiver accompanied by a medium antenna for demodulation of medium-sized data, and the super HDTV signal is intercepted by a receiver, accompanied by a large antenna for demodulation of large-sized data. Also, as illustrated in
The first receiver 23 demodulates with its demodulator 25 the modulated digital signal supplied from the digital transmitter 51 into the original first data stream signal. Similarly, the same modulated digital signal can be intercepted and demodulated by the second receiver 33 or third receiver 43 into the first data stream or NTSC TV signal. In summary, the three discrete receivers 23, 33, and 43 all can intercept and process a digital signal of the existing TV system for reproduction.
The arrangement of the signal transmission system will be described in more detail.
Assuming that the input signal is a video signal, its low frequency band component is assigned to the first data stream, its high frequency band component to the second data stream, its super-high frequency band component to the third data stream. The three different frequency band signals are fed to a modulator input 61 of the modulator 4. Here, a signal point shifting circuit 67 shifts the positions of the signal points according to an externally given signal. The modulator 4 is arranged for amplitude modulation on two 90°-out-of phase carriers respectively which are then combined into a multiple QAM signal. More specifically, the signal from the modulator input 61 is fed to both a first AM modulator 64 62 and a second AM modulator 63. Also, a carrier wave of cos(2πfct) produced by a carrier generator 64 is directly fed to the first AM modulator 64 62 and also, to a π/2 phase shifter 66 where it is 90° shifted in phase to a sin(2πfct) form prior to being transmitted to the second AM modulator 63. The two amplitude modulated signals from the first and second AM modulators 64 62, 63 are combined by a summer 65 into a transmission signal which is then transferred to the transmitter unit 5 for output. The procedure is well known and will not be further explained.
The QAM signal will now be described in a common 4×4 or 16 state constellation referring to the first quadrant of a space diagram in FIG. 3. The output signal of the modulator 4 is expressed by a sum vector of two, Acos2πfct and Bcos Bsin 2πfct, vectors 81 and 82 which respectively represent the two 90°-out-of-phase carriers. When the distal point of a sum vector from the zero point represents a signal point, the 16 QAM signal has 16 signal points determined by a combination of four horizontal amplitude values a1, a2, a3, and a4 and four vertical amplitude values b1, b2, b3, and b4. The first quadrant in
c11 is a sum vector of a vector 0-a1 and a vector 0-b1 and thus, expressed as c11=a1cos2πfct−b1sin2πfct=Acos (2πfct+dπ/2).
It is now assumed that the distance between 0 and a1 in the orthogonal coordinates of
As shown in
When the distance between two adjacent signal points is great, it will be identified by the receiver with much ease. Hence, it is desirable to space the signal points at greater intervals. If two particular signal points are allocated near to each other, they are rarely distinguished and the error rate will be increased. Therefore, it is most preferable to have the signal points spaced at equal intervals as shown in
The transmitter 1 of the embodiment is arranged to divide an input digital signal into a first, second, and a third data or bit stream. The 16 signal points or groups of signal points are divided into four groups. Then, 4 two-bit patterns of the first data stream are assigned to the four signal point groups respectively, as shown in FIG. 6. More particularly, when the two-bit pattern of the first data stream is 11, one of four signal points of the first signal point group 91 in the first quadrant is selected depending on the content of the second data stream for transmission. Similarly, when 01, one signal point of the second signal point group 92 in the second quadrant is selected and transmitted. When 00, one signal point of the third signal point group 93 in the third quadrant is transmitted and when 10, one signal point of the fourth signal point group 94 in the fourth quadrant is transmitted. Also, 4 two-bit patterns in the second data stream of the 16 QAM signal, or e.g. 16 four-bit patterns in the second data stream of a 64-state QAM signal, are assigned to four signal points or sub signal point groups of each of the four signal point groups 91, 92, 93, and 94 respectively, as shown in FIG. 7. It should be understood that the assignment is symmetrical between any two quadrants. The assignment of the signal points to the four groups 91, 92, 93, and 94 is determined by priority to the two-bit data of the first data stream. As the result, two-bit data of the first data stream and two-bit data of the second data stream can be transmitted independently. Also, the first data stream will be demodulated by using a common 4 PSK receiver having a given antenna sensitivity. If the antenna sensitivity is higher, a modified type of the 16 QAM receiver of the present invention will intercept and demodulate both the first and second data stream streams with equal success.
When the low frequency band component of an HDTV video signal is assigned to the first data stream and the high frequency component to the second data stream, the 4 PSK receiver can produce an NTSC-level picture from the first data stream and the 16- or 64-state QAM receiver can produce an HDTV picture from a composite reproduction signal of the first and second data streams.
Since the signal points are allocated at equal intervals, there is developed in the 4 PSK receiver a threshold distance between the coordinate axes and the shaded area of the first quadrant, as shown in FIG. 9. If the threshold distance is ATO, a PSK signal having an amplitude of ATO will successfully be intercepted. However, the amplitude has to be increased to a three times greater value or 3ATO for transmission of a 16 QAM signal while the threshold distance ATO is maintained. More particularly, the energy needed for transmitting the 16 QAM signal is nine times greater than that for sending the 4 PSK signal. Also, when the 4 PSK signal is transmitted in a 16 QAM mode, energy waste will be high and reproduction of a carrier signal will be troublesome. Above all, the energy available for satellite transmitting is not abundant but strictly limited to minimum use. Hence, no large-energy-consuming signal transmitting system will be put into practice until more energy for satellite transmission is available. It is expected that a great number of the 4 PSK receivers will be introduced into the market as digital TV broadcasting is placed in service. After introduction to the market, the 4 PSK receivers will hardly be shifted to higher sensitivity models because a signal intercepting characteristic gap between the two, old and new, models is high. Therefore, the transmission of the 4 PSK signals must not be abandoned. In this respect, a new system is desperately needed for transmitting the signal point data of a quasi 4 PSK signal in the 16 QAM mode using less energy. Otherwise, the limited energy at a satellite station will degrade the entire transmission system.
The present invention resides in a multiple signal level arrangement in which the four signal point groups 91, 92, 93, and 94 are allocated at a greater distance from each other, as shown in
For clearing the relationship between the signal receiving sensitivity and the transmitting energy, the arrangement of the digital transmitter 51 and the first receiver 23 will be described in more detail referring to FIG. 1.
Both the digital transmitter 51 and the first receiver 2 3 23 are formed of known types for data transmission or video signal transmission e.g. in TV broadcasting service. As shown in
The resultant modulated signal is shown in the space diagram of FIG. 18.
It is known that the four signal points are allcated allocated at equal distances for achieving optimum energy utilization.
The first receiver 23 is arranged to receive at its small-diameter antenna 22 a desired or 4 PSK signal which is transmitted from the transmitter 1 or digital transmitter 51 respectively through the transponder 12 of the satellite 10 and demodulate it with the demodulator 24 25. In more detail, the first receiver 23 is substantially designed for interception of a digital TV or data communications signal of 4 PSK or 2 PSK mode.
The input signal to the first receiver 23 will now be explained in more detail referring to the vector diagram of FIG. 20. The 4 PSK signal received by the first receiver 23 from the digital transmitter 51 is expressed in an ideal form without transmission distortion and noise, using four signal points 151, 152, 153, and 154, as shown in FIG. 20.
In practice, the real four signal points appear in particular extended areas about the ideal signal positions 151, 152, 153, and 154 respectively due to noise, amplitude distortion, and phase error developed during transmission. If one signal point is unfavorably displaced from its original position, it will hardly be distinguished from its neighboring signal point and the error rate will thus be increased. As the error rate increases to a critical level, the reproduction of data becomes less accurate. For enabling the data reproduction at a maximum acceptable level of the error rate, the distance between any two signal points should be far enough to be distinguished from each other. If the distance is 1AR0, the signal point 151 of a 4 PSK signal close to a critical error level has to stay in a first discrimination area 155 denoted by the hatching of FIG. 20 and determined by |0-aR1|>AR0 and |0-bR1|>AR0. This allows the signal transmission system to reproduce carrier waves and thus, demodulate a wanted signal. When the minimum radius of the antenna 22 is set to r0, the transmission signal of more than a given level can be intercepted by any receiver of the system. The amplitude of a 4 PSK signal of the digital transmitter 51 shown in
Here, a case of receiving a QPSK signal will be considered. Similarly to the manner performed by the signal point setting circuit 67 in the transmitter shown in