CA1266705A - Digital communication device - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

A digital communication device includes first and second systems in a transmitter of a base station and first and second systems in a receiver of the base station. Inserted in the first system of the transmitter and the first system of the receiver are delay circuits for ensuring a delay time difference between D and U
waves, which difference is employed to obtain an improved bit error rate when utilizing a communication system such as a DSK system, a BPSK-RZ system or a QPSK-RZ system.

Description

7~

The presen,t invention relates to a digita].
communication device. More specificall.y, it re].ates to a digital communication device for exchanging digita].
signals between a base stat.ion and a mob.ile station.
This application i.s ~ di.vision of Canadi,an patent application serial number 501,201 filed on February 5, 1986.
According to one aspect of the present invention a digital communication system using signals for changing 10 phase of a carrier wave corresponding to each of the symbols of digita]. information transmitted at every 1 time slot is characterized in that each of the signa].s has the amplitude and phase determined corresponding to the symbol in an arbitrary T/2 interva]. of 1 time slot and the 15 amplitude becomes effective].y zero in the remai,ni.ng intervals.
The embodiments of the invention will now be described with reference to the accompanying drawings in which:
Figures 1 and 2 illustrate a conventional DSK
system;
Figure 3 shows structure of a demodulator in the DSK system;
Figure 4 illustrates two signa].s with delay time 25 difference in the DSK system;
Figures 5 and 6 are characteristic diagrams showing relation between the delay time di.fference and the bit error rate in the DSK system;
Figures 7~ to 7C illustrate a BPSK-~Z system;
Figure 8 shows structure of a demodulator i.n the BPSK~~Z sys tem;
Figure 9 illustrates two signa].s with delay time difference in the BPSK-RZ system;
Figures 10 and 11 are characteristic diagrams 35 showing relation between the de],ay time di.fference and the bit error rate in the BPSK~Z system;
Figure 12 is a characteristic comparison di.agram with respect to the BPSK-RZ system and the QPSK system;

7~
la Figures 13 and 14 are characterist.ic compari.son diagrams in case oE applying band restri.cti.on through a base band Eilter;
Figure 15 is a b].ock diagram showing an 5 embodiment of the present invention;
Figure 16 is a block diagram showing another example of a delay system in a transmission area according to the present invention;
Figure 17 is a block di.agram showing still 10 another example of the delay system i.n the receiving area according to the present invention; and Figure 18 is a block diagram showing another embodiment of the present invention.
In mobile radio communicati.on for exchanging 15 signals between a base station and a mobile station, for example, 7~5 circuit design has been considered to be e~ceedingly difficult since multiple waves arrive through di~ferent paths, causiny deterioration of bit error rate.
In order to solve such a problem, a double-phase shift Xeying system ~hereinafter referred to as DSK
system~ and a binary-phase shift ~eying re-turn-to-zero system (hereinafter referred to as BPSK-RZ system) have beer. proposed.
Figures 1 and 2 illustrate a conventional DSK system and Figure 3 shows structure of a demodulator in the DSK
system, while F~gure ~ illustrates two signals which differ in delay time by r and are to be received by the DSK system and Figures 5 and 6 are characteristic diagrams showing the relationship between the delay time difference and the bit error rate in the DSK system.
With reference to Figures 1 to 6, description is now made of the DSK system. ~s shown in Figure 1, the DSK
system is adapted to shift carrier wave phases with respect to binary information symbols "0" and ~ twice by ~t2 per 1/2 time slot. For e~ample, the carrier wave phases are shifted twice by -tT~/2 with respect to the binary information symbol "1" and twice by -~/2 with re~pect to "0" respectively.
Figure 2 shows relation of phase-shlfted DSK signals with respect to binary lnformation symbols "1, 0, 1" in the DSK system performing the aforementioned phase shift operation.
A demodulator employed in the DSK system has structure ~uch as that shown in Figure 3. Signals received at an input terminal 1 are divided into those delayed by T/2 (T: -time slo-t length) through a T/2 delay circuit 3a and non-delayed signals, which are respectively multiplied by a multiplier 2 and passed through a low-pas3 filter (LPF) ~, whereby signals e(t) corresponding to the original modulated ~ignals are obtained at an output terminal 5.
Tn propagation paths of a mobile radio communication system, signals from a transmission point arrive at a 7~

receivi11g point through reflection by various ma-tters.
Cons~deration is,now made o~ two signals shown as D and U
waves in Figure 4, which are transmitted fro~ the 3ame point and differ by I from each other, in time, as received at the recelving point. It has been theoretically confirmed that a signal composed of such two signals shows such error characteristics as shown in Figure 5 when demodulated by the demodulator as shown in Figure 3.
In Figure 5, the ordinate shows the bit error rate and the abscissa shows ~/T (T: time slot width, l: time difference between D and U waves), while symbol Eb lndicates signal energy per bit, symbol No noise power per hert~, symbol Pd/Pu average power ratio of the D wave to the U wave and symbol f~ the maximum Doppler frequency.
As obvious from the characteris-tics as shown in Figure 5, the bit error rate is remar~ably improved within the range of 0.1 to 0.35 of l/T.
The above description has been made on a ~/2 DSK
system for shiftiny the carrier wave phases with respect to the binary information symbols "1" and "0" twice by ~/2 per 1/~ time slot, while it has been conEirmed that this descriptlon al30 applies -to a ~/2~DSK system for shifting the carrier wave phases by ~ 0/2 (0<~ 0<~) per l/2 time slot.
Fiyure 6 shows the bit error rate of a ~/4 DSK s~s-tem in which the value ~/2 is replaced by ~/4, and a~ obvious from the characteristics shown in Figure 6, the bit error rate is also improved wlthin the range of 0.1 to 0.3 of ~/T in the ~/4 DSK system.
The above description has been made with respect to the case where the carrier wave phases are shifted ~tepwi~e in first and second halves of the time slots as shown in Fi~ures 1 an~ 2, while this description also applies to the case where the carrier wave phases are smoothly shifted, e.g. to rise ln raise~ cosine curves.
As hereinabove described, the bit error rate is ~mproved wlthin the range of 0.1 to 0.35 or 0.1 to 0.3 of ~T in -the DSK system, and hence correct communlcation i3 enabled throu~h, use of such a range. However, in a general mobile communication system having a data transfer rate (small~r than several thousand bauds) employing an audio range, the delay time difference I is so small that the value I~T is considerably smaller than 0.1, whereby no application is effected in regions with improved bit error rate to effect the characteris-tics of the DSK system.
Description is now made of a BPSK-RZ system with reference to Figures 7~ to 7C. BPSK-~Z signals are obtained by multiplying normal BPSK signals as shown in Figure 7~ by ON-OFF signals as shown in Figure 7B, which become "1" in arbitrary T/2 intervals of time slots T. In other words, the BPSK-RZ signals are identical in ampl~tude and phase to the normal BPSK signals in first or second T/2 intervals of the respective time slots T, while the same are effectively zero in carrier wave amplitude in the remaining T~2 intervals.
Figure ~ shows structure of a demodulator in the 20 BPSK-~Z system. In Figure 8, signals received in an inpu-t terminal 1 are delayed by a delay clrcuit 3 which has a delay amount of a unit time slot T, and the delayed signal~ and non-delayed rece~ved signals are multiplied by a multi.plier 2 and supplied to an output terminal 5 2S through a low-pass filter ~.
The BP5~-RZ signals recelved in the input ter-minal 1 are multiple waves formed by overlapping fir~t BPSK-RZ
signal waves thereinafter referred to a~ D waves) generated by the same digital in~ormation and second BPSK-RZ signal waves ~hereinafter referred to as U waves)arriviny at a delay ~ with respect to the D wave~.
Figure 9 illustrates the time relation betwe~en -the D
ancl U wave3. In E'igure ~, symbol ~ indicates thc length of a time slot for transmitting one digital symbol of digital information. Symbol _ indicates an interval from rise of the D waves to rise of the U waves and symbol b .indicates an interval from the rise of the U waves to a lapse of T/2 of the D waves while ~ymbol c indicates an ~2~

interval from the lapse of T/2 of the D waves to a lap~e of T~2 of the ~ waves and symbol d indlcates an interval from the lapse of T/2 of the U w~ves to a lapse of T of the D waves.
Signals e(t) obtaincd at the output terminal 5 ~n the respective intervals are as follows:
W th res~t to 0<~/T<0.5:
Interval a:
2e(t) ~ = 1 symbol: "1"
~ = -1 symbol: "O"
Interval b:
2e(t) ( = 1l-p2-l2pcos~ symbol: "1"
( = -(l~p2l-2pco~ symbol: "0"
Interval c:
2ett) ( =p 2 symbol: "1"
( = _ pa symbol: "0"
Interval d:
2e(t) = 0 W-th res ect to 1>~/T~0.5:
Interval a:
( = l-pa symbol: "0" -~ "1"
~ pa-12pcos~ symbol: "1" -~ "1"
2e(t) ( = -(1-p2) symbol: "1" -> "0"
( = ~(11-pa-2pcosç5) symbol: "O" -~ "1"
Interval b:
2e(t) ( = 1 symbol: "1"
( = --1 symbol: "0"
:[nterval c e(t) =
Interval d:
2e(t) ( = pa symbol: "1"
( _ a symbol: "0"
35 where p repre~ents the relative amplltude ratio of the U
wave3 to the D wave~ and ~ represents carrier wave phase difEerence between the D and U wavcs.

~ s will be obvious from tlle above calculatlon, effec-tive outputs can be always obtained in the intervals a and c in the case of Ocl~T<0.5 and in the intervals b and d in the case of l>-~/T>0.5, regardless of phase difference between the D and U waves, and hence there no problems arlse such as annihilation of the eye of the so-called eye pa-ttern and increase in lateral fluctuation, which have been caused in the conventional BPSK modulation sy3tem.
Figures 10 and 11 are characteristic diagrams showing tlle relation between the delay time d:ifference and the bit error ra-te in the BPSK-RZ system. In further detail, Figure 10 shows changes in the bit error rate with varied signal-to-noise ra-tios and Figure 11 shows changes in the bit error rate with re3pect to changes in delay time difference between the D and U waves at a constant signal-to-noise ratio.
The respective items are as follows: The D and U
waves are chanyed in Rayleigh distribu-tion, symbols Pd and Pu indicate average power levels of the D and U waves respectively and symbol Eb indicate3 signal energy per bit while symbol No indicates nol~e power per unit frequency and symbol fD indicate3 the ma~cimum Doppler frequency for fading.
The broken line BPSK in Figure 10 shows characteristics of the normal BPSK sy3tem for re~erence.
As ~hown in E'igure 10, the BPSK--RZ system is further improved in bi-t error rate in comparison with the normal BPSK system.
~0 FuIther, as ~hown in Fi~ure 11, the bi-t error rate is remarl~ably lmproved withln t~le ran~e of 0.1 to 0.~5 of ~/T. Although the description has been c~iven above with reference to the BPSK ~ystem ~two-pha~e modulation), similar improvements is obtcained in a QPSK system (four-phase modulation) in which two-bit binary information symbols "00", "01", "11" and "10" are respectively as3igrled to, e.y. phase changes 0, rc/2, rc and -r~/2 by employin~ a Q~ K-RZ (quadri-phase shift keying return-to-~Z~ 5`

~ero) modulation system in which the second or first half of one time slot'is zero in amplitude.
Figure 12 is a characteristic comparison diagram of the BP~K-~Z system and the QPSK--RZ system, while Figure3 13 and 1~ are characteristic diagrams showing the case of applying band restriction through a base band filter.
It will be clearly apparent from Figure 12 that the improved characteristics of -the QPS~-RZ modulation system are similar to those of the ~PSK-RZ system.
Although -the characteristics are adversely affected when ON-OFF signals are band-restricted through a base band filter (bandwidth ~d), it has been confirmed that the effect on the characteristics is remar~ably small, as shown in Figures 13 and 14. Thus, the QPSK-RZ modulation system is sufficiently practical even if the band restriction i5 applied. Such a technical idea is applicable not only to the binary information symbols but also extends -to multi--phase modulation corresponding to multilevel information symbols.
In the DSK, BPSK-RZ and QPSIC-RZ ~ystems, thus, the bit error rate is improved in specific regions of l/T (0.1 to 0.35 in the DSK system and 0.1 to 0.75 in the BPSK-RZ
and QPSY~-RZ systems).
~owever, in a general mobile radio communication ~ystem utili~ing audio ran~es at a data transfer rate smaller than several thousand bauds, the delay -time differencc I is so small that l/T is considerably smaller than 0.1, and hence the characteristics of the DSK and BPSK-RZ systems cannot be affected in the r~gions wi-th the improved blt error rate.
For e~ample, assuming that the transfer rate is 6000 bauds in low--speed data transmi~sion by mobile radio communlcation and the propagation time difference between D and U waves is 10 ~sec., the time slot T is as follows:
T = 1/6000 (sec.) = 167 ~sec.
Conse~uently, ~/T = 10 ~sec.~167 ~sec. = 0.06 7~S

whereby the value ~/T ls smaller than 0.1.
Accordingly,. it i9 an object of the present invention to provide a digital communlcation device which improves the bit error rate characteristics in a communication system such as the DSK, BPSK-RZ or QPSK-RZ system.
BrieEly stated, the present invention prepares a transmitter and a receiver, respectively, of a base station, in two systems to insert delay circuits for ensuring a delay time difference between D and U waves required for performing optimum operation through a DSK
system, a BPSK-RZ system or a QP5K-RZ system in one of the transmission systems and one of the receiving systems, respectively, or either one of the transmission systems or one of the receiving systems.
A~cording to the pre~ent invention, therefore, the delay circuits for ensuring the delay time difference required for performing prescribed bit error rate improvement operation through the DSK or BPSK-RZ system etc. are inserted between respective ones of the DSK or BPSK-R2 signals transmitted through two transmission paths having small correlation in transmission characteristics.
Therefore, even if the delay time of each space propagation path i-tself is smaller than the delay time difference required for performin~ the pre3cribed error improvlng operation through the D5K or B~SK-RZ system, the prescribed delay time difference can be provided by -the delay circuits, thereby to perform the operation in the optimum operation point of the ~SK or BP5K-~Z 3ystem.
In thc preferred embodiment of the present invention, paths throug}l two an-tennas differin0 ln position from each other or paths throucJh two antennas differing in antenna direc-tivity from each other are employed as the transmission paths having small correla-tion.
Fur-ther, an embocliment of -the present invention provides re~pective modula-tors in a rou-te from the base station to the mobile station and vice versa to connect the delay circuits in a cascade manner to one of the two signal transmission paths with small correla-tion provided x per route, thereby to at-tain a prescribed delay time di~ference between t-he signals through the two signal transmission paths of each route required for bit error rate lmprovement operation of a prescribed modula-tion system, to compose signals received through the two signal -trallsmi3sion paths of each route and to demodulate the signals in modulation by differential detection.
The present invention will become more apparent from the following detailed description of embodiment3 thereof when ta~en in con~unction with Figures l5-le of the accompanying drawinys.
Figure 15 i3 a bloc~ diagram showing an embodiment of thc present invention. With reference to Figure 15, description i3 now made on a digital communication device employing the DSK system. In Figure 15, a base station 10 is formed by a transmit-ter 110, a receiver 120 and an antenna part 13. The transmitter 110 is formed by two systems of transmission circuit~. In other words, signals supplied to a data input terminal 111 are divided in-to two systems. ~ first system transmission circuit i5 formed by a delay circuit (DL) 112, a modulator (MOD) 113 for DSK
modulation and a power amplifier (PA) 115, while the second system transmission circuit i3 formed by a modulator 114 for DSK modulatlon and a power amplifier 116.
On the other hand, the receivcr 120 in the base station 10 is formed by two systems of receiving circuits.
The fir3t system receivlng circuit is formed by a high-frequency amplifier (PF~) 121, a mi~cer (MIX) 123, an intermediate frequency amplifier (IF~) 125 and a delay circuit 12~. The second sy3tcm receiving circuit is ~ormed by a high-frequency amplifier 122, a mixer 124 and an intermediate frequency amplifier 126. Further, the receiver 120 includes a demodula-tor (DEM) 128 for demodulating composite signals ob-tained by composing the output signalG from the two receiving circuit systems through a T/2 delay circuit and an output terminal 129.

7~ 5 The antenna part 13 o~ the base station 10 is formed by duplexers (DUP) 131 and 132 a horizontal polarization antenna 133 and a vertical polarization antenna 13~.
A mobile station 20 exchanges digital signals wi-th the base station 10. The mobile station 20 is formed by a first antenna 21 a second antenna 22 a hybrid circuit 23 a receiver 24 a transmitter 25 a receiving output terminal 6 and a -transmission input terminal 27. The first and second antennas 22 are formed by horizontal and vertical polarization antennas so that radio waves thereErom cross each o-ther.
Description i5 now made of the operation of the above-described digital communication device. In the transmitter 110 of the base station 10 a signal received in the data input terminal 111 i9 divided in-to ~wo signals so that one of the signals is delayed by a prescribed period throllgh -the delay circuit 112 and modulated by the modulator 113 and then amplified by the power amplifier 115. The si~nal thus amplified by the power amplifier 115 is transmitted through the duplexer 131 and the horizontal polarization antenna 133.
The other signal i5 no-t delayed but directly supplied to the modulator 114 to be modulated and amplified by the power ampllfier 116 and transmitted through the duple~er 132 and the vertical polarization antenna 13-~. Thus the mobile station 10 receives the signal delayed by the delay circuit 112 of the transmitter 110 and the non-delayed signal.
When th~ transmission rate is small the delay time diEferencc in a spacc pro~agation path of the mobile radio communication system is remar~ably small in comparison with the timc slot width, antl hence the delay amount la of the delay circllit 112 is set in such a manner that the delay time dif~erence reqllired to obtain the minimum bit error rate in the DSK system is attained through the delay circuit 112.
Assumin~ that for example, the transmission rate is 6000 bauds and the delay time difference ~ between the D

w ~

and U waves i5 10 ,usec. as hereinabove describecl, the preferable value 0.1 to 0.3 of (~ a)~T is obtained as follows: Since the value T is equal to 167 ~lsec. as hereinabove described.
(~ a)/167 ~sec. = 0.1 to 0.3 ~ a - 16.7 to 50.1 (~sec.) where -c = 10, and hence:
la = 6.7 to 40.1 (,usec.) Although the delay time difference I between the D
and U waves is 10 ,usec. in the above description, the same may become -ero and hence the value la is preferably larger than 16.7 ~sec. so tha-t the value (~ a)/T is larger than 0.1 even if the value -c is equal to zero.
The mobile station 20 receives/detects the signals transmitted from the transmitter 110 of the base station 10 in the aforementioned manner, whereby a remarkably good bit error rate i5 obtained.
On the other hand, the mobile station 20 transmits the sigrlals crom the transmitter 25 through the first and second antennas 21 and 22 with crossed planes of polarization, 50 as to be received in the antenna part 13 of the base station 10. The antenna part 13 of the base station 10 receives the signals from the transmitter 25 oE
the mobile station 20 through the horizontal and vertical polarization antennas 133 ancl 134 of the two-sy~tem receiver 120 corresponding to the horizontally and vertically polarized signals. In other words, the signals picl~ed-up by the horizontal polariza-tion antenna 133 are supplied to the high-freq-lency amplifier 121 through the duple}~er 131 and amplified a-t a high frequency, frequency-mlxed by the mixer 123, converted to intermediate frequ~ncy signals by the in-ter~Decliate frequency amplifier 1~5 and clelayed by the delay circuit 127, and then supplied to the demodulator 123.
On the other hand, the signals pic~ed-up by -the vertical polarization antenna 13~ are supplied to the high-frequcncy amplifier 122 through the duplexer 132 and amplified at a high frequency, frequency-mixed by the --1~
mixer 124, converted -to intermediate fre~uency siynals by the intermediate.frequency amplifier 126 and supplied to the demodulator 128. The delay circuit 127 introduces the delay amount re~uired to obtain the optimum bit error rate of the DSK sy~tem in a manner similar to that described above, and hence the bit error rate is remar~ably improved in the receiver 110 of the base station 10.
As hereinabove described, the demodulator 128 differentially detects the two D~K signals such as the horizon-tally and vertically polarized signals in the mobile radio communication system transmitted through propagation paths with no correlation (or e~c-tremely small correlation), whlch have a delay time difference set at a prescribed value, i.e. a delay time which can improve the bit error rate characteristics of the DSK system.
Therefore, the location of the delay circuit 127 is not restricted to that in the aforementioned embodiment but DSK si~nals may be directly sent into two transmission paths with no correlation so tha~ one of the sign~ls received throuyh the first and second transmission pa-ths is delayed by a prescribed period and then composed with the other signal, which is differentially detected. Yet other modlfications are as -follows:
Figure 16 is a bloc~ diagram showing another e~ample of a delay system in the transmission area accordiny to the present inven-tion, ~hile Fiyure 17 is a bloc~ diayram showing still another e~cample of the delay system in the receiving area accordiny to the present invention.
In the e7cample as shown in Figure 16, a transmitter 130 is formed by a modulator 103, a delay circuit 112 and power ampliflers 115 and 116. A siynal received in a data input terminal 111 is DSK--modulated by -the modulator 103 to be divicled into two slgnals, so that one of the same is delayed by the delay circuit 112 to be supplied to th~
power amplifier 1~5 and the other signal is no-t delayed but directly supplied to the power amplificr 116 to be amplified.

Although the dela~ circuit 12~ as shown in Fiyure 15 is inserted between the inter~nediate frequency amplifier 125 and the demodulator 12~, a delay circuit 127 of a receiver 140 as shown in Figure 17 is inserted between a duplexer 131 and a high-frequency ampllfier 121.
Further, although the delay circu~ts 112 and 12~ are provided only in the base s-tation 10 in the examples as shown in Figures 16 and 1~, the same may be inserted only in the mobile station 20, or separately inserted in the base station 10 and the mobile station 20.
Although the above description has been made on the case of employing the horizontally and vertically polarized signals as the transrnission paths with no correlation, the presen-t invention is also appli~able to a system employing the so-called space diversity idea u-tilizing -two antennas deviating in position from each other and to a system employing the so-called antenna pattern diversity idea utilizing two antennas diEferent in antenna pattern from each other. Further, the present invention can be applied to a system employing two antennas in two or three combinations of the polarization, the deviation in position and the difference in antenna pattern, i.e., two or three combinations of the polarization, the space diversity and the antenna pattern diversity.
Further, although the above description ha~ been made on the case where both of the base and mobile stations 10 and 20 are providecl with the horizontal and vertical polarization arltennas, the planes of polarization are e~tremely dis-turbed by multipath reflection in the propagation paths of mobile radio communication, while correla-tion between horizontal and vertical polariza-tion components thereof is e~-tremely reduced. Namely, even :if either the horizontal or vertical polarization component is lowered, the other polarisa-tion component is not lowerecl and hence the an-tennas 21 and 22 of the mobile s-tation 20 may have only -the horizontal or vertical plane of polarization. The radio waves transmitted from the mobile station 20 are detected by the horizontal and vertical polarization antennas 133 and 134 of the base station lo as the signals having horizontal and vertical polarization components to be separated into the signals in the pre~cribed delay time through the two-~ystem receiving circuits and then composed with each other, whereby the optimum operation characteristics are obtained in 'che D5K system. The respective signals can be transmitted with circular polari~ation to obtain a similar effect.
~lthough the aforementioned embodiment is applied to a digital communication device employing the DSK system in the above description, the present invention may also be applied to sy~tems for lmproving the bit error rate wi-th increased delay time difference, e.g., the BPSK-RZ system and the QPSK-RZ system.
Figure 1~ is a bloc~ diagram showing another embodiment of the present inven-tion. In the embodiment as shown in Figure le, the present invention is applied to a digital communication device of the ~PSK-RZ sys-tem. The device as shown in Figure 18 ls identical in structure to that shown in Figure 15, e,~cep-t for the following points:
Although the modulators 113 and 114 as shown in Figure 15 are adap-ted to perform DSY~ modulation, modulator6 113' and 114' as shown in Fiyure 13 are adapted to perform BPSK-RZ
modulation. Fur-ther, while the modulator 128 a3 shown in Figure 15 has the T/2 delay circuit, a demodulator 128' as shown in E'igure 18 is implemented by a delay demodulator having a T delay circult. In addition, while the delay clrcuits 112 and 127 as shown in E'igure 15 are selccted to be in the delay time requ:irecl to obtain the minimum bit err-or rate in the DSK system, delay circuits 112' and 127' as shown in E'igure 18 are selected to be in delay time required to obtaln the bes~ bi-t error rate in the BPSK-RZ
3ystem.
The embodiment as shown in Figure 18 is identical in operation to -that shown in Figure 15, and dcscription thereof is omitted.

AlthouJh the above description has been made on -the case of applying the modulation system wi-th -the improving effect 3uch as the DSK or BPSK-RZ system to botil of the route from the transmit-ter 110 of the base 3tation 10 to the mobile station 20 and the route from the mobile station ~0 to the receiver 120 of the base station 10, the system of the present invention can be applied to only one of the routes while employing another improving sys-tem for the other route.
In the digital communication device according to the presen-t invention, the delay circuits are cascade-connected partially in the two signal transmission paths in order to ensure delay time difference required for improving the bit error rate of the DSK or BPSK-~Z signals tran3mitt~d ~hrough two transmission paths with no correlation or small correlation in transmission characteristic~ throuc3h use of the DSK or BPSK-RZ sys-tem as hereinabove described. Thus, even if the space propagation paths -them3elves have a delay time smaller than the delay time difference required for the prescrlbe~
improving operation by the DSK or BPSK-RZ system, prescribed delay time difference can be provided by the delay circuits to operate the communication device in the optimum operation point of the DSK or BPSK-RZ system.
Although embodiments of the present invention have been described and illustrated in detail, it is clearly understoocl that the 3ame are given by way of lllu3tration and e~ample only and are not to be ta~en by way of limitation, the spirit and scopc of the present invention being limitecl only by the terms of the appended clalms.

Claims (3)

.
THE EMBODIMENTS OF THE INVENTION IN WHICH AN
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS
FOLLOWS:

1. A digital communication system using signals for changing phase of a carrier wave corresponding to each of the symbols of digital information transmitted at every 1 time slot, characterized in that each of said signals has the amplitude and phase determined corresponding to the symbol in an arbitrary T/2 interval of 1 time slot and the amplitude becomes effectively zero in the remaining intervals.

2. A digital communication system in accordance with claim 1, wherein said modulation system is binary PSK (Binary Phase Shift Keying) system.

3. A digital communication system in accordance with claim 1, wherein said modulation system is quadruple PSK
(Quadruple Phase Shift Keying) system.