Inter-unit digital signal transmitting method, digital signal transmitter and receiver equipment, digital signal transmitter, and digital signal receiver

Digital signal transmitter/receiver equipment includes a first transmitter/receiver unit that handles digital signals and a second transmitter/receiver unit connected to the first transmitter/receiver unit via a transmission line to subject a digital signal to both a modulation/demodulation process and a frequency conversion process. The object is to suppress the signal attenuation due to a cable connecting the first transmitter/receiver unit to the second transmitter/receiver unit. Each of the first and second transmitter/receiver units includes a first frequency-shift modulation unit that sets the frequency of the digital signal transmitted between the first and second transmitter/receiver units to a value lower than an intermediate frequency at a frequency conversion processed by the second transmitter/receiver unit, and a second frequency-shift demodulation unit for demodulating the digital signal subjected to a frequency-shift modulation.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates to an inter-unit digital signal transmitting 
method, a digital signal transmitter and receiver equipment, a digital 
signal transmitter, and a digital signal receiver. 
2) Description of the Related Art 
In recent years, with an increasing number of mobile telecommunications 
base stations, inter-cellular telecommunications between exchanges have 
been frequently utilized. In the radio telecommunications such as mobile 
telecommunications, as shown in FIG. 14, radio terminals are linked to the 
exchange 24 (EX) via the radio base station (BS) 23 covering a radio 
terminal area. Optical fiber cable telecommunications or radio 
telecommunications have been chiefly utilized to link the radio base 
station (BS) 23 to the exchange 24. 
However, an increasing number of radio base stations 23 causes expensive 
work which requires laying optical fiber cables to link to new exchanges 
24. Particularly, it is costly to network new optical fiber cables in 
urban areas. For that reason, it has been increasingly worthwhile to use 
wireless (radio) communications, instead of optical fiber cables, for 
telecommunications between the radio base station 23 and the exchange 24, 
because of the advantage of installation easiness and economy. 
On the other hand, in order to realize a telecommunication between the 
radio base station 23 and the exchange 24, they each must include the 
transmitter unit and the receiver unit to exchange radio data. In this 
case, the transmitter in each radio base station modulates an electrical 
signal in the baseband to a signal in an intermediate frequency (IF) band, 
subjects the IF band signal to a RF band frequency conversion process, and 
then transmits the outcome. 
Various limitations sometimes require that the radio base station 23 or 
exchange 24 be divided into a first unit for handling digital signals and 
a second unit for performing a frequency conversion process, each being 
installed at a different place. It has long been desired to perform an 
efficient data transmission with no signal distortion between the 
separated units. 
In response to such a desire, digital signal transmitter/receiver equipment 
has been used as shown in FIGS. 10 to 12. The digital signal 
transmitter/receiver equipment is constituted of the first 
transmitter/receiver unit acting as the first unit and the second 
transmitter/receiver unit acting as the second unit. FIG. 10 is a 
schematic diagram showing a device that performs a baseband transmission 
between first and second units. FIG. 11 is a schematic diagram showing a 
device that performs an IF band transmission using two local sources. FIG. 
12 is a schematic diagram showing a device that performs an IF band 
transmission using one local source. 
Explanation will be made below for the devices described above. 
First, the digital signal transmitter/receiver equipment shown in FIG. 10 
will be explained. Numeral 1 represents the first transmitter/receiver 
unit that handles digital signals and 2 represents the second 
transmitter/receiver unit that subjects a digital signal to a 
modulation/demodulation process and a frequency conversion process. 
The first transmitter/receiver unit 1 is connected to the second 
transmitter/receiver unit 2 via three cables: the communication lines 8a 
and 8b and the power source cable 8c. The digital signal is subjected to 
the baseband transmission via the transmission lines 8a and 8b. 
The first transmitter/receiver unit 1 is chiefly installed indoors. The 
first transmitter/receiver unit 1 is constituted of the bipolar/unipolar 
converting means (hereinafter referred to B/U converting means) 5, the 
unipolar/bipolar converting means (hereinafter referred to U/B converting 
means) 9, the CMI encoding means (CMI COD) 6A, and the CMI decoding means 
(CMI DCOD) 7B. 
The second transmitter/receiver unit 2 is chiefly installed outdoors. The 
second transmitter/receiver unit 2 is constituted of the transmitting unit 
including the CMI decoding means 7A, the main modulation unit (MOD) 10, 
the up-converter 11, the high-power amplifier 16 and the bandpass filter 
17; the receiving unit including the bandpass filter 17, the low-noise 
amplifier 18, the down-converter 13, the main demodulation unit (DEM) 14, 
and the CMI encoding means 6B; and the circulator 12; the antenna unit 15; 
and the local oscillator 19. 
At the signal transmission time, when the transmitter/receiver unit 1 
receives digital signal data in the baseband, the B/U converting means 5 
converts the digital signal data from a bipolar signal to an unipolar 
signal while the CMI encoding means 6A subjects the unipolar signal to a 
code mark inversion (CMI) encoding process. 
The second transmitter/receiver unit 2 receives the encoded signal at the 
baseband frequency via the sending transmission line 8a. In the second 
transmitter/receiver unit 2, the CMI decoding means 7A decodes first the 
input signal. The clock extracting unit 7a arranged in the CMI decoding 
means 7A synchronizes with the CMI coded signal from the CMI encoding 
means 6A to time the CMI decoding and the modulation in the main 
modulation unit 10. 
Thereafter, the main modulation unit (MOD) 10 inputs the data signal to 
modulate it to a signal in the intermediate frequency (IF) signal band. 
The up-converter 11 receives the local signal from the local oscillator 19 
to convert the IF frequency band data signal into a RF frequency band 
signal. The resultant signal is amplified by the high-power amplifier 16 
and then transmitted from the antenna unit 15 by way of the bandpass 
filter 17 and the circulator 12. 
At the signal receiving time, the signal is transmitted along the reverse 
path to that at the transmission time. That is, the signal received with 
the antenna unit 15 is input to the down-converter 13 by way of the 
circulator 12, the bandpath 5, filter 17 and the low-noise amplifier 18. 
Next, the down-converter 13 converts the data signal from the RF frequency 
band to the IF frequency band. Then, the main demodulation unit (DEM) 14 
demodulates the IF frequency band signal into the baseband signal and then 
the CMI encoding means 6B subjects the resultant signal to the CMI 
encoding process. 
The resultant signal in a baseband frequency band is input to the first 
transmitter/receiver unit 1 via the receiving transmission line 8b. In the 
first transmitter/receiver unit 1, the CMI decoding means 7B decodes the 
signal and the U/B to converting means 9 converts the resultant signal 
into a bipolar signal. The clock extracting unit 7b is similar to the 
clock unit 7a arranged together with the CMI decoding means 7A. 
Next the digital signal transmitter/receiver unit 2 shown in FIG. 11 will 
be explained briefly. Like the configuration shown in FIG. 10, numeral 1 
represents the first transmitter/receiver unit that handles digital 
signals and 2 represents the second transmitter/receiver unit that 
subjects the digital signal to a modulation/demodulation process as well 
as a frequency conversion process. 
The first transmitter/receiver unit 1 is connected to the second 
transmitter/receiver unit 2 via a pair of cables, or the transmission 
lines 8a and 8b, to transmit the digital signal in the IF frequency band 
via the same lines 8a and 8b. 
That is, at a signal transmission time, the main modulation unit 10 in the 
first transmitter/receiver unit 1 modulates the digital signal data of the 
baseband to a signal in the IF frequency band. The resultant signal is 
transmitted to the second transmitter/receiver unit 2 via the transmission 
line 8a. The first transmitter/receiver unit 1 also includes the 
capacitors 20A-1, 20A-2, and coils 21A-1, 21A-2, 21A-1, and the second 
transmitter/receiver unit 2 also includes the capacitors 20B-1, 20B-2 and 
coils 21B-1, 21B-2, A DC power source is connected to one ends of the 
coils 21A- 1, 21A-2, 21B- 1, and 21B-2. 
In the second transmitter/receiver 2, the amplifier 25A amplifies an input 
signal in the IF frequency band. Then, in response to the local signal 
from the local oscillator 19A, the up-converter 11 converts the data 
signal from the IF frequency band to the RF frequency band. The resultant 
signal is transmitted from the antenna unit 15 via the bandpass filter 17 
and the circulator 12. 
The signal received by the antenna unit 15 is inputted to the 
down-converter 13 via the circulator 12, the bandpass filter 17, and the 
low-noise amplifier 18. 
In response to a local signal from the local oscillator 19B of which the 
oscillation frequency is different from that of the local oscillator 19A, 
the down-converter 13 frequency-converts the data signal from the RF 
signal band to the IF frequency band and then the amplifier 25B amplifies 
the converted signal. The resultant signal is transmitted to the main 
demodulation unit 14 via the transmission line 8b to modulate the 
resultant signal into a baseband signal. 
In the configuration shown in FIG. 1, since the main modulation unit 10 and 
the main demodulation unit 14 are arranged on the side of the first 
transmitter/receiver unit 1, the attenuation of digital data due to a 
cable arranged between the transmitter/receiver units 1 and 2 can be 
relatively reduced by setting the IF frequency to a small value. However, 
two cables are needed between the transmitter/receiver units 1 and 2. 
Next, the digital signal transmitter/receiver equipment will be explained 
with reference to FIG. 12. In this configuration, a signal cable 
(transmission line 8) is connected between the first transmitter/receiver 
unit 1 and the second transmitter/receiver unit 2. The digital signal is 
subjected to an IF transmission via the transmission line 8 shared for 
transmission and reception. 
That is, in the configuration shown in FIG. 12, at the signal transmission 
time, the main modulation unit (MOD) 10 receives the digital signal data 
in the baseband input to the first transmitter/receiver unit 1 to modulate 
it to an IF frequency band signal. The modulated signal is transmitted to 
the second transmitter/receiver unit 2 via the hybrid circuit (composite 
and branch filter) 22A, 22B and the transmission line 8. 
The transmission signal inputted to the second transmitter/receiver unit 2 
is branched by the hybrid circuit 22B and then amplified by the amplifier 
25A. Moreover, in response to a local signal from the local oscillator 19, 
the up-converter 11 converts the data signal in the IF frequency band into 
that in the RF frequency band. 
The converted signal is amplified by the high-power amplifier 16 and the 
resultant signal is then transmitted to the antenna 15 via the bandpass 
filter 17 and the circulator 12. 
At the signal receiving time, the signal is transmitted along the reverse 
path to that in the transmission time. That is, the signal received by the 
antenna unit 15 is inputted to the down-converter 13 via the circulator 
12, the bandpass filter 17 and the low-noise amplifier 18. 
The down-converter 13 converts the data signal in the RF frequency signal 
into a signal in the IF frequency band in accordance with the local signal 
from the local oscillator 19 shared as one for the transmitter. Then, the 
amplifier 25B amplifies the converted signal. Then, the hybrid circuit 22B 
combines the signal from the amplifier 25B with the signal from the 
amplifier 25A and then transmits the appropriate digital signal to the 
first transmitter/receiver unit 1 via the transmission line 8. In the 
first transmitter/receiver unit 1, the hybrid circuit 22A branches the 
received signal and the main demodulation unit (DEM) 14 demodulates the 
resultant signal in the IF frequency band into a signal in the baseband. 
The configuration including the local oscillator 19 shared for transmission 
and reception allows the single cable (transmission line 8) to connect the 
first transmitter/receiver unit 1 to the second transmitter/receiver unit 
2. However, as shown in FIG. 13, the IF frequency of the transmission 
system or the receiving system is fairly high. This causes a relatively 
large amount of signal attenuation due to cable losses. 
As shown in FIG. 15, the radio base stations 23 are sometimes installed on 
general buildings (for example, office buildings) in an urban area. If a 
suitable space cannot be found, the indoor equipment (or the first unit) 
may be installed in a basement of the building and the outdoor equipment 
(or the second unit) may be installed on the roof thereof, as shown in 
FIG. 15. In this case, it may be required to lay very long cables between 
the indoor equipment and the outdoor equipment. 
In the consideration of the installation easiness and the cable laying 
cost, it is most desirable to select an arrangement in which a signal 
cable is laid between the indoor equipment and the outdoor equipment, as 
shown in FIG. 12. 
However, the problem arises that this configuration, where a single coaxial 
cable connects one equipment to another equipment, limits the coaxial 
cable laying to the length to which the device within the equipment can 
compensate for cable attenuation. 
In other words, in order to lay a longer cable between the indoor equipment 
and the outdoor equipment without increased cable attenuation, the 
configuration requires using a cable with a thick inner conductor (with 
less signal attenuation) or an internal device with a large compensation 
characteristic for attenuation. 
However, thickening an internal conductor in the cable results in a 
degraded flexibility of cabling as well as an equipment connection 
difficulty. Moreover, increasing the compensation characteristic of the 
internal device causes an increased manufacturing cost and bulky 
equipment. 
SUMMARY OF THE INVENTION 
The present invention is made to overcome the above mentioned problems. An 
object of the present invention is to provide an inter-unit digital signal 
transmitting method that can suppress the attenuation of a signal in a 
cable connecting the first transmitter/receiver unit to the second 
transmitter/receiver unit. 
Another object of the present invention is to provide digital signal 
transmitter/receiver equipment that can suppress the attenuation of a 
signal in a cable connecting the first transmitter/receiver unit to the 
second transmitter/receiver unit. 
Still another object of the present invention is to provide a digital 
signal transmitter that can suppress the attenuation of a signal in a 
cable connecting the first transmitter/receiver unit to the second 
transmitter/receiver unit. 
Further still, another object of the present invention is to provide a 
digital signal receiver that can suppress the attenuation of a signal in a 
cable connecting the first transmitter/receiver unit to the second 
transmitter/receiver unit. 
In order to achieve the above objects, according to the present invention, 
the inter-unit digital signal transmitting method which performs a digital 
signal modulation/demodulation process and a digital signal frequency 
conversion process between a first unit which handles digital signals and 
a second unit which is connected to the first unit via a transmission line 
is characterized by the step of performing a frequency shift keying or 
frequency-shift modulation of a digital signal transmitted between the 
first and second units so as to set the frequency of the digital signal 
transmitted between the first and second units to a value lower than an 
intermediate frequency at a frequency conversion processed by the second 
unit. 
According to the present invention, in the digital signal 
transmitter/receiver equipment including a first transmitter/receiver unit 
that handles digital signals and a second transmitter/receiver unit 
connected to the first transmitter/receiver unit via a transmission line 
to perform a digital signal modulation/demodulation process and a digital 
signal frequency process, each of the first and second 
transmitter/receiver units is characterized by a frequency shift keying or 
frequency-shift modulation unit and a frequency shift keying or 
frequency-shift demodulation unit. The frequency-shift modulation unit 
performs a frequency-shift modulation of a digital signal transmitted 
between the first and second transmitter/receiver units so as to set the 
digital signal transmitted between the first and second 
transmitter/receiver units to a value lower than an intermediate frequency 
at a frequency conversion processed by the second transmitter/receiver 
unit. The frequency-shift demodulation unit demodulates a digital signal 
subjected to a frequency-shift modulation by a frequency-shift modulation 
unit confronting the second frequency-shift demodulation unit. 
According to the digital signal transmitter/receiver equipment of the 
present invention, the transmission line between the first and second 
transmitter/receiver units is formed of a transmission line shared for 
transmission and reception. 
According to the digital signal transmitter/receiver equipment according to 
the present invention, the transmission line between the first and second 
transmitter/receiver units has a separate sending transmission line and a 
receiving transmission line. 
According to the present invention, the digital signal transmitter 
including a first transmitter that handles digital signals and a second 
transmitter that is connected to the first digital transmitter via a 
transmission line to perform a digital signal modulation process and a 
digital frequency conversion, is characterized by a frequency-shift 
modulation unit arranged in the first transmitter, for subjecting a 
digital signal transmitted between the first and second transmitters to a 
frequency-shift modulation to set the frequency of a digital signal 
transmitted between the first and second transmitters to a value lower 
than the intermediate frequency at a frequency conversion process 
performed by the second transmitter. The digital signal transmitter also 
includes a frequency-shift demodulation unit arranged in the second 
transmitter for subjecting a digital signal to a demodulation process, the 
digital signal being subjected to the frequency-shift modulation by the 
frequency-shift modulation unit arranged in the first transmitter. 
According to the digital signal transmitter of the present invention, the 
first transmitter includes a bipolar/unipolar converting means and a CMI 
encoding means that are connected to the front stage of the 
frequency-shift modulation unit. The bipolar/unipolar converting means 
converts a bipolar signal into a unipolar signal, and the CMI coding means 
subjects the unipolar signal converted by the bipolar/unipolar converting 
means to a CMI encoding process. The second transmitter includes a CMI 
decoding means connected to the rear stage of the frequency-shift 
demodulation unit, for decoding the signal encoded by the CMI decoding 
means. 
According to the present invention, the digital signal receiver includes a 
first receiver for subjecting a received digital signal to a demodulation 
process and a frequency conversion process and a second receiver connected 
to the first receiver via a transmission line for handling digital 
signals. A frequency-shift modulation unit is arranged in the first 
receiver for subjecting a digital signal transmitted between the first and 
second receivers to a frequency-shift modulation to set the frequency of a 
digital signal transmitted between the first and second receivers to a 
value lower than the intermediate frequency at a frequency conversion 
process performed by the first receiver. A frequency-shift demodulation 
unit is arranged in the second receiver for subjecting a digital signal to 
a demodulation process, the digital signal being one subjected to the 
frequency-shift modulation by the frequency-shift modulation unit arranged 
in the first receiver. 
According to the digital signal receiver of the present invention, the 
first receiver includes a CMI decoding means connected to the front stage 
of the frequency-shift modulation unit for subjecting the demodulated 
unipolar signal to a CMI decoding process; and the second receiver 
includes a CMI encoding means and a bipolar/unipolar converting means each 
connected to the rear stage of the frequency-shift demodulation unit. The 
CMI decoding means decodes the unipolar signal encoded by the CMI encoding 
means; and the unipolar/bipolar converting means converts a unipolar 
signal decoded by the CMI decoding means into a bipolar signal. 
According to the inter-unit digital signal transmitting method of the 
present invention, digital signals can be transmitted between the first 
and second units with almost no signal attenuation. The first and second 
units can be connected to each other at a lower cost without using an 
expensive cable with less signal attenuation. Since the first and second 
units can be largely separated from each other, there are no limitations 
to the installation places for the first and second units, whereby the 
flexibility for installation can be largely increased. 
According to the digital signal transmitter/receiver equipment of the 
present invention, digital signals can be transmitted between the first 
and second transmitter/receiver units with almost no signal attenuation. 
The first and second transmitter/receiver units can be connected to each 
other at a lower cost without using an expensive cable with less signal 
attenuation. Since the first and second transmitter/receiver units can be 
largely separated from each other, there are no limitations to the 
installation places for the first and second transmitter/receiver units, 
whereby the flexibility for installation can be largely increased. 
The equipment according to the present invention can be realized at a lower 
manufacturing cost by sharing the transmission line between the first and 
second transmitter/receiver units as a transmission line for transmission 
and reception. 
The configuration of each of the first and second transmitter/receiver 
units can be simplified by separating the transmission lines between the 
first and second transmitter/receiver units into a sending transmission 
line and a receiving transmission line. 
According to the digital signal transmitter of the present invention, 
digital signals can be transmitted between the first and second 
transmitters with low signal attenuation. This feature allows the first 
and second transmitters to be connected to each other at a lower cost 
without using an expensive cable with less signal attenuation. Since the 
first and second transmitters can be largely separated from each other, 
there are no limitations to the installation places for the first and 
second transmitters, whereby the flexibility for installation can be 
largely increased. 
Even when the modulation/demodulation processing function is performed in 
the second transmitter arranged separately from the first transmitter, the 
system can reliably maintain synchronous timing, thus providing great 
value in practical use. 
The digital signal receiver of the present invention enables a digital 
signal transmission between the first receiver and the second receiver 
with nearly no signal attenuation. This feature allows the first and 
second receivers to be connected to each other at low cost without using 
an expensive transmission line with less signal attenuation. 
The feature that the first and second receivers can be installed separately 
from each other at a large distance eliminates the limitations for the 
installation places, thus improving the flexibility of installation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the attached drawings, explanation will be made of a preferred 
embodiment according to the present invention. 
(a) Description of the the Present Invention: 
FIG. 1 is a block diagram showing an aspect of the digital signal 
transmitter/receiver equipment according to the present invention. 
Referring to FIG. 1, numeral 1 represents a first transmitter/receiver 
unit and 2 represents a second transmitter/receiver unit. The transmission 
line 8 connects the first transmitter/receiver unit 1 to the second 
transmitter/receiver unit 2. 
The first transmitter/receiver 1 includes a frequency shift keying or 
frequency-shift modulation unit (FSK MOD) 3A and a frequency shift keying 
or frequency-shift demodulation unit (FSK DEM) 4B. The second 
transmitter/receiver unit 2 includes a frequency-shift demodulation unit 
(FSK DEM) 4A, a main modulation unit (MOD) 10, frequency conversion units 
11 and 13, a main demodulation unit (DEM) 14, a frequency shift keying or 
frequency-shift modulation unit (FSK MOD) 3B, a circulator 12, and an 
antenna 15. The frequency-shift modulation unit 3A in the first 
transmitter/receiver unit 1 subjects a digital signal transmitted between 
the units to a frequency shift keying or frequency-shift modulation to set 
the frequency of the digital signal to a value lower than the intermediate 
frequency at which the second transmitter/receiver unit 2 performs a 
frequency conversion. The frequency-shift demodulation unit 4A in the 
second transmitter/receiver unit 2 demodulates the digital signal which 
has been subjected to a frequency shift keying or frequency-shift 
modulation by the frequency-shift modulation unit 3A in the first 
transmitter/receiver unit 1. 
The main modulation unit 10 modulates the digital signal demodulated by the 
frequency-shift demodulation unit 4A. The frequency conversion unit 11 
subjects the modulated signal to a frequency conversion (up-conversion). 
The frequency conversion unit 13 subjects the receive data input from the 
antenna 15 to a frequency conversion (down-conversion). The main 
demodulation unit 14 demodulates the signal frequency-converted by the 
frequency conversion unit 13. 
The frequency-shift modulation unit 3B in the second transmitter/receiver 
unit 2 subjects a digital signal transmitted between the units 1 and 2 to 
a frequency-shift modulation to set the frequency of the digital signal to 
a value lower than the intermediate frequency at which the first 
transmitter/receiver unit 1 performs a frequency conversion process. The 
frequency-shift demodulation unit 4B in the first transmitter/receiver 
unit 1 demodulates the digital signal subjected to a frequency-shift 
modulation by the frequency-shift modulation unit 3B in the second 
transmitter/receiver unit 2. 
The transmission line 8 between the first and second transmitter/receiver 
units 1 and 2 may be a transmission line shared for transmission and 
reception. The transmission lines 8 between the first and second 
transmitter/receiver units 1 and 2 may be separated into a sending 
transmission line and a receiving transmission line, respectively. 
FIG. 2 is a block diagram showing an aspect of the digital signal 
transmitter according to the present invention. Referring to FIG. 2, 
numeral 101 represents a first transmitter unit and 102 represents a 
second transmitter unit connected to the first transmitter unit 101 via 
the transmission line 8. 
The first transmitter unit 101 includes the frequency-shift modulation unit 
(FSK MOD) 3A. The transmitter 102 includes the frequency-shift 
demodulation unit (FSK DEM) 4A, the main modulation unit (MOD) 10, the 
frequency conversion unit 11, and the antenna unit 15. 
Both a bipolar/unipolar converting means that converts a bipolar signal 
into a unipolar signal and a CMI encoding means that subjects the unipolar 
signal converted by the bipolar/unipolar converting means to a CMI 
encoding may be arranged at the front stage of the frequency-shift 
modulation unit 3A in the first transmitter 101. A CMI demodulating means 
that demodulates the signal encoded by the CMI encoding means may be 
arranged at the rear stage of the frequency-shift demodulation unit 4A in 
the second transmitter 102. 
FIG. 3 is a block diagram showing an aspect of the digital signal receiver 
according to the present invention. Referring to FIG. 2, numeral 201 
represents a second receiver unit and 202 represents a first receiver 
unit. The units 201 and 202 are connected via the transmission line 8. 
The first receiver unit 202 is constituted of the antenna unit 15, the 
frequency conversion unit 13, the main demodulation unit 14, and the 
frequency-shift modulation unit (FSK MOD) 3B. The second receiver unit 201 
includes the frequency-shift demodulation unit (FSK DEM) 4B. 
A CMI encoding means that subjects a demodulated unipolar signal to a CMI 
encoding is arranged to the front stage of the frequency-shift modulation 
unit 3B in the first receiver unit 202. Both a CMI decoding means that 
decodes a unipolar signal encoded by the CMI encoding means and a 
unipolar/bipolar converting means that converts a unipolar signal decoded 
by the CMI decoding means into a bipolar signal are arranged at the rear 
stage of frequency-shift demodulation unit 4B in the second receiver 201. 
According to the present invention, in order to perform a digital signal 
transmission between the first and second receiver units, the second 
receiver unit subjects a digital signal to both a modulation/demodulation 
process and a frequency conversion process. 
By subjecting a digital signal between the first and second receiver units 
to a frequency-shift modulation, the frequency of the digital signal is 
converted to a frequency lower than the intermediate signal at which the 
second receiver unit performs a frequency conversion process. Thus, the 
attenuation of the digital signal in the transmission line connecting the 
first and second receiver units is greatly reduced. 
According to the present invention, in the second transmitter/receiver unit 
2 connected to the first transmitter/receiver unit 1 via the transmission 
line 8, shown in FIG. 1, each of the main modulation unit 10 and the main 
demodulation unit 14 subjects a digital signal to a 
modulation/demodulation process and each of the frequency conversion units 
11 and 13 perform a frequency-shift modulation process. 
In the frequency-shift modulation unit 3A in the first transmitter/receiver 
unit 1 and the frequency-shift modulation unit 3B in the second 
transmitter/receiver unit 2, the digital signal transmitted between the 
transmitter/receiver units 1 and 2 is subjected to a frequency-shift 
modulation to set the frequency of the digital signal to a value lower 
than the intermediate frequency at which the second transmitter/receiver 
unit 2 performs a frequency conversion process. In the frequency-shift 
demodulation unit 4A in the first transmitter/receiver unit 1 and the 
frequency-shift demodulation unit 4B in the second transmitter/receiver 
unit 2, the digital signal subjected to a frequency-shift modulation by 
each of the confronting frequency-shift modulation units 3A and 3B is 
demodulated. 
Thus, the attenuation of digital signals in the transmission line 8 
connecting the first and second transmitter/receiver units 1 and 2 can be 
reduced. 
Where the transmission line 8 is formed as a shared transmission line for 
transmission and reception, it can transmit the transmission signal and 
the receive signal between the transmitter/receiver units 1 and 2. 
Where the transmission lines are formed separately of a sending 
transmission line and a receiving transmission line, the transmission 
signal is transmitted between the first and second transmitter/receiver 
units 1 and 2 via the sending transmission line while the receive signal 
is transmitted between the first and second transmitter/receiver units 1 
and 2 via the receiving transmission line. 
According to the present invention as shown in FIG. 2, the frequency-shift 
modulation unit 3A in the first transmitter unit 101 subjects a digital 
signal transmitted between the transmitter units 1 to a frequency-shift 
modulation to set the frequency of the digital signal lower than the 
intermediate frequency at which the second transmitter 102 performs a 
frequency conversion process while the frequency-shift demodulation unit 
4A in the first transmitter unit 102 demodulates the digital signal which 
has been subjected to the frequency-shift modulation by the 
frequency-shift modulation unit 3A in the first transmitter 101. 
Thus, the attenuation of a digital signal in the transmission line 8 
connecting the first and second receivers can be largely eliminated. 
In the second transmitter 102 connected to the first transmitter unit 101 
via the transmission line 8, the main modulation unit 101 modulates the 
digital signal while the frequency conversion unit 11 performs a frequency 
conversion. 
Both the bipolar/unipolar converting means 5 and the CMI encoding means 6A 
are arranged at the front stage of the frequency-shift modulation unit 3A 
in the first transmitter unit 101. The CMI decoding means 7A is arranged 
at the rear stage of the frequency-shift modulation unit 3A. In this case, 
the bipolar/unipolar converting means 5 converts a bipolar signal into a 
unipolar signal. The CMI converting means 6A subjects the unipolar signal 
converted by the bipolar/unipolar converting means to a CMI encoding 
process. Then, the CMI decoding means 7A decodes the signal encoded by the 
CMI encoding means. 
According to the present invention shown in FIG. 3, in the first receiver 
unit 202, the frequency conversion unit 13 frequency-converts the digital 
signal received and the main demodulation unit 14 subjects the resultant 
signal to a demodulation process. 
The frequency-shift modulation unit 3B in the first receiver unit 202 
subjects the digital signal transmitted between the first and the second 
receiver units 201 and 202 to a frequency-shift modulation to set the 
frequency of the digital signal to a value lower than the intermediate 
frequency used for the frequency conversion process in the first receiver 
unit 202. 
Thus, the attenuation of a digital signal in the transmission line 8 
connecting the first and the second receiver units 202 and 201 can be 
reduced. 
The frequency-shift demodulation unit 4B in the first receiver unit 201 
demodulates the digital signal subjected to the frequency-shift modulation 
by the frequency-shift modulation 3B in the first receiver unit 202. 
The CMI encoding means 6B is arranged at the front stage of the 
frequency-shift modulation unit 3B in the first receiver unit 202 while 
the CMI decoding means 7B and the unipolar/bipolar converting means 9 are 
arranged at the rear stage of the unit 3B. In this case, the CMI encoding 
means 6B subjects the signal demodulated by the main demodulation unit 14 
to a CMI encoding process. 
The CMI decoding means 7B decodes the signal encoded by the CMI encoding 
means 6B. The unipolar/bipolar converting means 9 converts the unipolar 
signal decoded by the CMI decoding means 7B into a bipolar signal. 
As described above, according to the present invention, the digital signal 
can be transmitted between the first and second units with almost no 
attenuation. This feature allows the first unit and the second unit to be 
connected to each other at a lower cost without using expensive 
transmission lines, and with less attenuation. Furthermore, since the 
first and second units can be largely separated from each other at a large 
distance, no limitation of the installation place for them greatly 
increases flexibility. 
According to the present invention, the digital signal transmitter/receiver 
equipment includes a first transmitter/receiver unit 1 that handles 
digital signals and a second transmitter/receiver unit 2 connected to the 
first transmitter/receiver unit 1 via the transmission line 8 to perform a 
digital signal modulation/demodulation process and a digital signal 
frequency conversion process. Each of the first and second 
transmitter/receiver units 1 and 2 includes frequency-shift modulation 
units 3A, 3B and a frequency-shift demodulation unit 4A, 4RB. The 
frequency-shift modulation units 3A, 3B perform a frequency-shift 
modulation of a digital signal transmitted between the first and second 
transmitter/receiver units 1 and 2 so as to set the digital signal 
transmitted between the first and second transmitter/receiver units to a 
value lower than an intermediate frequency at a frequency conversion 
processed by the second transmitter/receiver unit. The frequency-shift 
demodulation units 4A, 4B demodulate a digital signal subjected to a 
frequency-shift modulation by a frequency-shift modulation unit 
confronting the second frequency-shift demodulation unit. Hence, the 
digital signal can be transmitted between the first transmitter/receiver 
units 1 and 2 with nearly no signal attenuation. This feature allows the 
first transmitter/receiver unit 1 and the second transmitter/receiver unit 
2 to be connected to each other at a lower cost without using the 
expensive transmission line with less attenuation. Furthermore, since the 
first and second transmitter/receiver units 1 and 2 can be largely 
separated from each other at a large distance, no limitation of the 
installation place for them allows the largely increased flexibility for 
installation. 
Since the transmission line 8 between the first and second 
transmitter/receiver units 1 and 2 is constituted of a transmission line 
shared for transmission and reception, the present equipment can be 
realized at a low manufacturing cost. 
Since the transmission line 8 between the first and second 
transmitter/receiver units 1 and 2 is constituted separately of a sending 
transmission line and a receiving transmission line, the configuration of 
each of the first and second transmission/receiver units can be 
simplified. 
According to the present invention, the digital signal transmitter, 
including a first transmitter 101 that handles digital signals and a 
second transmitter 102 that is connected to the first digital transmitter 
101 via a transmission line 8 to perform a digital signal modulation 
process and a digital frequency conversion, is constituted of a 
frequency-shift modulation unit 3A arranged in the first transmitter 101, 
for subjecting a digital signal transmitted between the first and second 
transmitters to a frequency-shift modulation to set the frequency of a 
digital signal transmitted between the first and second transmitters to a 
value lower than the intermediate frequency at a frequency conversion 
process performed by the second transmitter 102. Also included is a 
frequency-shift demodulation unit 4A arranged in the second transmitter 
102, for subjecting a digital signal to a demodulation process, the 
digital signal being one subjected to the frequency-shift modulation by 
the frequency-shift modulation unit 3A arranged in the first transmitter 
101. Hence, the digital signal can be transmitted between the first 
transmitter/receiver units 1 and 2 without significant signal attenuation. 
This feature allows the first transmitter unit 1 and the second 
transmitter unit 2 to be connected to each other at a lower cost without 
using the expensive transmission line with less attenuation. Furthermore, 
since the first and second transmitter units 1 and 2 can be largely 
separated from each other at a large distance, no limitation of the 
installation place for them allows largely increased flexibility for 
installation. 
The first transmitter 101 includes a bipolar/unipolar converting means and 
a CMI encoding means that are connected to the front stage of the 
frequency-shift modulation unit 3A, the bipolar/unipolar converting means 
converting a bipolar signal into a unipolar signal, the CMI coding means 
subjecting the unipolar signal converted by the bipolar/unipolar 
converting means to a CMI encoding process; and the second transmitter 102 
includes a CMI decoding means connected to the rear stage of the 
frequency-shift demodulation unit 4A, for decoding the signal encoded by 
the CMI decoding means. Hence, even when the modulation/demodulation 
function is prepared within the second transmitter 102 arranged separately 
to the first transmitter 101, a synchronous timing can be reliably 
extracted so that the value in practical use is high. 
According to the present invention, the digital signal receiver including a 
first receiver 202 for subjecting a received digital signal to a 
demodulation process and a frequency conversion process and a second 
receiver 201 connected to the first receiver 202 via a transmission line 8 
for handling digital signals, is constituted of a frequency-shift 
modulation unit 3B arranged in the first receiver 202, for subjecting a 
digital signal transmitted between the first and second receivers to a 
frequency-shift modulation to set the frequency of a digital signal 
transmitted between the first and second receivers to a value lower than 
the intermediate frequency at a frequency conversion process performed by 
the first receiver 202. Also included is a frequency-shift demodulation 
unit 4B arranged in the second receiver 201, for subjecting a digital 
signal to a demodulation process, the digital signal being one subjected 
to the frequency-shift modulation by the frequency-shift modulation unit 
arranged in the first receiver 202. Hence, the digital signal can be 
transmitted between the first receiver units 202 and 201 with almost no 
signal attenuation. This feature allows the first receiver unit 202 and 
the second receiver unit 201 to be connected to each other at a lower cost 
without using the expensive transmission line with less attenuation. 
Furthermore, since the first and second receiver units 202 and 201 can be 
separated from each other at a large distance, no limitation of the 
installation place for them allows greatly increased flexibility for 
installation. 
The first receiver 202 includes a CMI encoding means connected to the front 
stage of said frequency-shift modulation unit 3B for subjecting the 
demodulated unipolar signal to a CMI decoding process. The second receiver 
201 includes a CMI encoding means and a bipolar/unipolar converting means 
that are connected to the rear stage of the frequency-shift demodulation 
unit 4B, the CMI decoding means decoding the unipolar signal encoded by 
the CMI encoding means. The unipolar/bipolar converting means converts a 
unipolar signal decoded by the CMI decoding means into a bipolar signal. 
Hence, even when the modulation/demodulation function is prepared within 
the second transmitter arranged separately to the first transmitter, a 
synchronous timing can be reliably extracted so that the value in 
practical use is high. 
Description of the First Embodiment of the Present Invention: 
Explanation will be made below of the first embodiment according to the 
present invention. 
FIG. 4 is a block diagram showing the configuration of the digital signal 
transmitter/receiver equipment according to the first embodiment of the 
present invention. FIG. 5 is a block diagram showing a modification of the 
first embodiment. 
The digital signal transmitter/receiver equipment according to the present 
embodiment is applied to radio base stations and exchanges (numerals 23 
and 24 in FIG. 14). Referring to FIG. 4, the digital transmitter/receiver 
equipment is constituted of an indoor equipment 1 acting as the first 
transmitter/receiver unit and an outdoor equipment 2 acting as the second 
transmitter/receiver unit. The indoor equipment 1 is linked to the outdoor 
equipment 2 via a single transmission line 8. 
The indoor equipment 1 that processes previously generated digital signals 
is installed, for example, in the basement of a building. The outdoor 
equipment 2 that has an antenna 15 is installed, for example, on the roof 
of a building. 
In the indoor equipment 1, the bipolar/unipolar (B/U) converting means 5, 
the CMI encoding means (CMI COD) 6A and the frequency-shift modulation 
unit (FSK MOD) 3A are arranged along the digital signal transmission path. 
The frequency-shift demodulation unit (FSK DEM) 4B, the CMI decoding means 
7B and the unipolar/bipolar (U/B) converting means 9 are arranged along 
the digital signal receiving path. The indoor equipment 1 also includes 
the hybrid circuit 22A that performs a branching and combining operation 
of a transmission signal and a receive signal. 
The B/U converting means 5 converts a digital signal (or a bipolar signal) 
processed in the indoor equipment 1 into a unipolar signal and then 
transmits the resultant signal. The CMI encoding means 6 subjects the 
unipolar signal to a code mark inversion (CMI) encoding. 
The frequency-shift modulation unit 3A subjects an input data signal to a 
FSK modulation (frequency-shift modulation) and then outputs a digital 
signal transmitted between the indoor equipment 1 and the outdoor 
equipment 2, the digital signal having a value lower than the intermediate 
frequency (IF) at the frequency conversion process handled in the indoor 
equipment 2. 
The hybrid circuit 22A inputs the digital signal from the frequency-shift 
modulation unit 3A and then combines it with the received signal. The 
combined signal is transmitted to the outdoor equipment 2 via the shared 
sending/receiving transmission line (cable) 8. 
In the combined signal, the transmission data with a frequency lower than 
the intermediate frequency is transmitted to the outdoor equipment 2, 
nearly without attenuating signals in the transmission line 8. 
The hybrid circuit 22B inputs again data transmitted to the outdoor 
equipment 2 and branches into a transmission signal and a receive signal. 
The transmission data is transmitted from the antenna unit 15 via the 
frequency-shift demodulation unit (FSK DEM) 4A, the CMI decoding means 
(CMI DCOD) 7A, the main modulation unit (MOD) 10, the frequency converting 
unit (up-converter) 11, the high-power amplifier 16, and the bandpass 
filter 17 in the outdoor equipment 2. 
The frequency-shift demodulation unit 4A demodulates the digital signal 
subjected to the FSK modulation in the frequency-shift modulation unit 3A 
within the indoor equipment 1. The CMI decoding means 7A decodes the 
signal encoded by the CMI encoding means 6A in the indoor equipment 1. The 
clock extracting unit 7a in the CMI decoding means 7A synchronizes timely 
the CMI decoding means 7A and the main modulation unit 10 in accordance 
with the CMI encoded signal from the CMI encoding means 6A in the indoor 
equipment 1. 
The main modulation unit 10 modulates the input baseband signal into an IF 
(intermediate) frequency band signal. The modulated signal from the main 
modulation unit 10 is amplified by the amplifier 25A arranged integrally 
to the main modulation unit 10. 
The frequency conversion unit 11 frequency-converts the digital signal in 
the IF frequency band into a signal in the RF frequency band, in response 
to the local signal from the local oscillator 19. The converted signal is 
amplified by the high-power amplifier 16 and then the resultant signal is 
transmitted from the antenna unit 15 via the bandpass filter 17 and the 
circulator 12. 
On the other hand, the data received by the antenna unit 15 is transmitted 
to the indoor equipment 1 via the bandpass filter 17, the low-noise 
amplifier 18, the frequency converting unit (down-converter) 13, the main 
demodulation unit (DEM) 11, the CMI encoding means 6B, and the 
frequency-shift modulation unit (FSK MOD) 3B in the outdoor equipment 2. 
That is, the received data is input via the bandpass filter 17 and the 
circulator 12 and then amplified by the low-noise amplifier 18. 
The bandpass filter 17 is a filter passing only signals in a specific 
frequency band. The signal from the low-noise amplifier 18 is input to the 
frequency-shift conversion unit 13. In the reverse way to that of the 
frequency conversion unit 11, the frequency conversion unit 13 converts 
the receive digital signal in the RF frequency band into the signal in the 
IF frequency band, in response to the local signal from the local 
oscillator 19. 
The amplifier 25B arranged integrally to the main demodulation unit 14 
amplifies the receive data and then the main demodulation unit 14 
demodulates the resultant data into the signal in the baseband. The 
demodulated signal is subjected to a CMI encoding process by the CMI 
encoding means 6B. 
Moreover, the received CMI encoded data is subjected to a FSK modulation by 
the frequency-shift modulation unit 3B. Thus, the receive data is 
modulated to a signal of a lower frequency than the intermediate 
frequency. Thereafter, the hybrid circuit 22B combines the transmission 
data with the receive data to input the resultant receive data to the 
indoor equipment 1 via the transmission line 8. 
The receive data modulated to a frequency lower than the intermediate 
frequency can be transmitted to the indoor equipment 1, nearly without 
being attenuated in the transmission line 8. 
The hybrid circuit 22A in the indoor equipment 1 branches the receive data 
into different signals. The branched signal is subjected to a digital 
process via the frequency-shift demodulation unit 4B, the CMI decoding 
means 7B, and the U/B converting means 9. 
In the reverse way of the B/U converting means 5, the U/B converting means 
9 converts the input digital signal from a unipolar signal to a bipolar 
signal. Hence, the U/B converting means 9 converts the unipolar signal 
from the CMI decoding means 7B into a bipolar signal. Thereafter, the 
receive data is subjected to various digital processes. 
The frequency-shift demodulation unit 4B and the CMI decoding means 7B in 
the indoor equipment 1 corresponds to the frequency-shift demodulation 
unit 4A and the CMI decoding means 7A, respectively. Hence, the duplicate 
explanation will be omitted here. 
Numerals 20A and 20B represent capacitors and 21A and 21B represent coils 
each one end of which is connected to a DC power source. The signal 
transmission between the indoor equipment 1 and the outdoor equipment 2 is 
carried out with the transmission line 8 on which the DC power source is 
applied. 
According to the configuration of the first embodiment of the present 
invention, each of the hybrid circuits 22A and 22B subjects the 
transmission data and the receive data transmitted between the indoor 
equipment 1 and the outdoor equipment 2 to a combining and branching 
process and a FSK modulating process. Hence, there is an advantage in that 
the signal attenuation due to an extended transmission line can be 
suppressed so that the signal distortion can be prevented. Moreover, 
laying a signal transmission line 8 between the indoor equipment 1 and the 
outdoor equipment 2 results in a reduced cabling cost. 
Hence, even when a radio base station (or an exchange) is installed in a 
general building, the installation places for the indoor equipment 1 and 
the outdoor equipment 2 do not restrict laying a transmission line, 
whereby the freedom of installation can be largely increased. This feature 
allows radio base stations to be installed in most buildings. 
In the first embodiment, as shown in FIG. 5, the two-local system using the 
local oscillators 19A and 19B may be used to connect the indoor equipment 
to the outdoor equipment. 
In the brief explanation of the modification of the digital signal 
transmitter/receiver equipment shown in FIG. 5, the indoor equipment 1 is 
connected to the outdoor equipment via both the sending transmission line 
8a and the receiving transmission line 8b. 
In the indoor equipment 1, the digital signal from the frequency-shift 
modulation unit 3A is transmitted to the outdoor equipment 2 via the 
sending transmission line 8a. The transmission data which has a frequency 
lower than the intermediate frequency can be transmitted to the outdoor 
equipment 2 substantially with no signal attenuation in the transmission 
line 8a. 
In the outdoor equipment 2, the receive data is transmitted via the antenna 
15 via the frequency-shift demodulation unit 4A, the CMI decoding means 
7A, the main modulation unit 10, the frequency conversion unit 11 with the 
local oscillator 19A, the high-power amplifier 16, and the bandpass filter 
17. 
On the other hand, in the outdoor equipment 2, data received by the antenna 
15 is transmitted to the indoor equipment 1 via the bandpass filter 17, 
the low-noise amplifier 18, the frequency conversion unit 13 with the 
local oscillator 19B having an oscillation frequency different from that 
of the local oscillator 19A, the main demodulation unit 14, the CMI 
encoding means 6B, and the frequency-shift modulation unit 3B. 
In other words, the frequency-shift modulation unit 3B subjects the receive 
data to a FSK modulation to transmit the outcome to the indoor equipment 1 
via the receiving transmission line 8b. Since the receive data has a lower 
frequency than the intermediate frequency, the transmission line 8b 
transmits it with nearly no signal attenuation. 
As described above, in the two-local system, the digital signal between the 
indoor equipment 1 and the outdoor equipment 2 subjected to a FSK 
modulation can eliminate the signal attenuation due to the transmission 
line 8a and 8b. Unlike the two-local system shown in FIG. 11, the present 
two-local system does not require the main modulation unit 10 in the 
indoor equipment 1 and the frequency conversion unit 11 in the outdoor 
equipment 2. In addition, the present two-local system can install the 
main modulation unit 10 in the outdoor equipment 2, together with the 
frequency conversion unit 11, thus realizing easy fabrication as a unit of 
each of the indoor equipment 1 and the outdoor equipment 2, 
easy-maintenance and reduced manufacturing cost. 
(c) Description of the Second Embodiment of the Present Invention: 
Next, the second embodiment according to the present invention will be 
explained. 
FIG. 6 is a block diagram showing the configuration of the digital signal 
transmitter/receiver equipment of the second embodiment according to the 
present invention. FIG. 7 is a block diagram showing a modification of the 
second embodiment. 
Unlike the first embodiment, the indoor equipment 1 includes in the second 
embodiment 2 plural transmission systems and receiving systems. 
As shown in FIG. 6, the indoor equipment 1 includes the two-system digital 
signal output (data transmitting) path and the two-system digital signal 
input (data receiving) path. 
The bipolar/unipolar converting unit 5-1, the CMI encoding means 6A-1 and 
the frequency-shift modulation unit (FSK MOD) 3A- 1 as well as the 
bipolar/unipolar converting unit 5-2, the CMI encoding means 6A-2, the 
frequency-shift modulation unit (FSK MOD) 3A-2 are arranged along the 
digital signal transmitting paths in the indoor equipment 1, respectively. 
The frequency-shift demodulation unit (FSK DEM) 4B-1, the CMI decoding 
means 7B-1 and the unipolar/bipolar converting means (U/B converting 
means) 9-1 as well as the frequency-shift demodulation unit (FSK DEM) 
4B-2, the CMI decoding means 7B-2 and the unipolar/bipolar converting 
means (U/B converting means) 9-2 are arranged along the digital signal 
receiving paths, respectively. 
The indoor equipment 1 also includes the hybrid circuit 22A that performs a 
branching and combining operation of transmission signals and receive 
signals. 
In each digital signal transmission path, each of the B/U converting means 
5-1 and 5-2 converts a bipolar signal into a unipolar signal. Each of the 
CMI encoding means 6A-1 and 6A-2 encodes the unipolar signal. 
In each of the frequency-shift modulation units 3A-1 and 3A-2, the digital 
signal is modulated into a signal of a frequency lower than the 
intermediate (IF) frequency at a frequency conversion processed in the 
outdoor equipment 2. Each of the frequency-shift modulating units 3A-1 and 
3A-2 performs a conversion process at a different frequency in each 
transmission path. 
The hybrid circuit 22A combines the digital signal from each of the 
frequency-shift modulating units 3A-1 and 3A-2 with the signal from the 
receiving side to transmit the combined signal to the outdoor equipment 2 
via the sending and receiving transmission line (cable) 8. 
The transmission data that is transmitted at a lower frequency than the 
intermediate frequency along each transmission path can be transmitted to 
the outdoor equipment 2 nearly with no signal attenuation in the 
transmission line 8. 
The hybrid circuit 22B inputs the data transmitted to the outdoor equipment 
2 and branches it corresponding to the signal system in the indoor 
equipment 1. In this case, the transmission path is divided into two 
branches. 
The transmission data is transmitted from the antenna unit 15 via the 
frequency-shift demodulation units 4A-1 and 4A-2, the CMI decoding means 
7A-1 and 7A-2, the multiplexer (MUX) 26, the main modulation unit 10, the 
frequency conversion unit 11, the high-power amplifier 16, and the 
bandpass filter 17 arranged in the outdoor equipment 2. 
In other words, the frequency-shift demodulation unit 4A-1 demodulates a 
piece of transmission data from the hybrid circuit 22B in the outdoor 
equipment 2 into the digital signal FSK-modulated by the frequency-shift 
modulation unit 3A-1 in the indoor equipment 1. The frequency-shift 
demodulation unit 4A-2 demodulates a piece of transmission data from the 
hybrid circuit 22B in the outdoor equipment 2 into the digital signal 
FSK-modulated by the frequency-shift modulation unit 3A-2 in the indoor 
equipment 1. 
The CMI decoding means 7A-1 demodulates the signal encoded by the CMI 
encoding means 6A-1. The CMI decoding means 7A-2 demodulates the signal 
encoded by the CMI encoding means 6A-2. The CMI decoding means 7 includes 
the clock extracting units 7a-1 and 7a-2. The clock extracting unit 7a-1 
and 7a-2 synchronize the CMI decoding means 7A-1 and 7A-2 and the main 
modulation unit 10 with the signals CMI-encoded by the CMI encoding means 
6A-1 and 6A-2 in the indoor equipment 1, respectively. 
The multiplexer 26 arranged in the rear stage of each CMI decoding means 7 
multiplexes the two-system data to output unified data to the main 
modulation unit 10. The multiplexer 26 multiplexes the plural transmission 
systems into a single transmission system. 
The main modulation unit 10 inputs the multiplex transmission data to 
modulate it into an IF (intermediate) frequency signal. The modulated 
signal is amplified by the amplifier 25A integrally mounted to the main 
modulation unit 10. 
The frequency conversion unit 11 frequency-converts the digital signal from 
the IF frequency band to the RF frequency band, in response to the local 
signal from the local oscillator 19. The high-power amplifier 16 amplifies 
the converted signal. Then, the resultant signal is transmitted from the 
antenna unit 15 via the bandpass filter 17 and the circulator 12. 
On the other hand, the data received by the antenna unit 15 is transmitted 
via the bandpass filter 17, the low-noise amplifier 18, the frequency 
conversion unit 13, and the main demodulation unit 14 arranged in the 
outdoor equipment 2. Then the demultiplexer separates the data into two 
systems. One piece of the data is transmitted to the indoor equipment 1 
via the CMI encoding means 6B-1 and the frequency-shift modulation unit 
3B-1. The other piece of the data is transmitted to the indoor equipment 1 
via the CMI encoding means 6B-2 and the frequency-shift modulation unit 
3B-2. 
In other words, the low-noise amplifier 18 inputs the receive data via the 
bandpass filter 17 and the circulator 12 and amplifies it. 
The frequency conversion unit 13 frequency-converts the digital signal in 
the RF frequency band from the low-noise amplifier 18 into a signal in the 
IF frequency band, in response to the local signal from the local 
oscillator 19 with the same specification as that in the transmission 
system. 
After the amplifier 25 arranged integrally to the main demodulation unit 14 
amplifies the receive data, the main demodulation unit 14 modulates the 
resultant data to data in the baseband. Then the demultiplexer (DMUX) 27 
separates the receive data transmission path into plural transmission 
paths (two-systems). 
The CMI encoding means 6B-1 and 6B-2 are arranged along the separated 
receive data paths, respectively, to subject receive data to a CMI 
encoding process. 
Moreover, the CMI encoded receive data are input to the frequency-shift 
modulation unit 3B-1 and 3B-2 arranged along the transmission paths, 
respectively, to perform the FSK modulation at a different frequency. Each 
of the receive data is modulated to a frequency lower than the 
intermediate frequency. Thereafter, the hybrid circuit 22B combines two 
pieces of the modulated data and then transmits the outcome to the indoor 
equipment 1 via the transmission line 8. 
As described above, since the receive data is modulated to a frequency 
lower than the intermediate frequency, it can be transmitted to the indoor 
equipment 1 substantially with no signal attenuation in the transmission 
line 8. 
The hybrid circuit 22A in the indoor equipment 1 is branched corresponding 
to the signal transmission systems for the receive data in the outdoor 
equipment 2. In this case, the receiving transmission path is divided into 
two branches. 
Thereafter, two pieces of the branched receive data are subjected 
respectively to a digital signal process via the frequency-shift 
demodulation unit 4B-1, the CMI decoding means 7B-1, and the U/B 
converting means 9-1 and via the frequency-shift demodulation unit 4B-2, 
the CMI decoding means 7B-2, and the U/B converting means 9-2. Then, the 
receive data are further subjected to various digital processes. 
Numerals 20A and 20B represent capacitors and 21A and 21B represent coils 
of which one ends are connected to a DC power source. The signal 
transmission is carried out between the indoor equipment 1 and the outdoor 
equipment 2 with the transmission line 8 supplied with the DC power 
source. 
According to the second embodiment having the configuration described 
above, like the first embodiment, the FSK signal transmission between the 
indoor equipment 1 and the outdoor equipment 2 is carried out prior to the 
main modulation and demodulation processes while the transmission data and 
the receive data combined and branched by the hybrid circuits 22A and 22B 
are exchanged between the indoor equipment and the outdoor equipment. 
Therefore, there is an advantage in that the signal attenuation can be 
suppressed even in an extended transmission line so that the signal 
distortion can be prevented. Moreover, the single transmission line 8 can 
link the indoor equipment 1 to the outdoor equipment 2, thus reducing the 
cable laying cost. Hence, where radio base stations (or exchanges) are 
installed in general buildings, the limitation to the installation places 
for the indoor equipment 1 and the outdoor equipment 2 due to the length 
of the transmission line can be removed so that the freedom for 
installation can be improved. This feature allows radio base stations to 
be installed in most buildings. 
Arranging the multiplexer 26 and the demultiplexer 27 in the second 
transmitter/receiver equipment enables arranging a plurality of data 
transmission systems and data receiving systems in the first 
transmitter/receiver equipment. 
In the modification of the second embodiment, as shown in FIG. 7, the 
two-local system in which the local oscillators 19A and 19B are employed 
can be used to connect the indoor equipment 1 to the outdoor equipment 2. 
In other words, plural independent input/output systems can be connected 
using the transmission line 8a and 8b. 
The above configuration can further simplify the entire structure of the 
indoor equipment 1 and the outdoor equipment 2 by omitting the hybrid 
circuits 22A and 22B. 
Moreover, the configuration can reduce the signal attenuation in the 
transmission lines 8a and 8b because of the use of the FSK-modulated 
digital signal between the indoor equipment 1 and the outdoor equipment 2. 
(d) Description of the Third Embodiment of the Present Invention: 
Next, the third embodiment according to the present invention will be 
explained below. FIG. 8 is a block diagram showing the digital signal 
transmitter/receiver equipment according to the third embodiment of the 
present invention. FIG. 9 is a block diagram showing a modification of the 
third embodiment. 
In the third embodiment, the indoor equipment having plural input systems 
and output systems is shown, like the second embodiment. 
As shown in FIG. 8, the indoor equipment 1 includes a two-digital signal 
output (data sending) path system and a two-digital signal input (data 
receiving) path system. In the third embodiment, the bipolar/unipolar 
(B/U) converting means 5 is arranged along each digital signal 
transmission path in the indoor equipment 1. The multiplexer 26 is 
arranged in the rear stage of the B/U converting means 5. The multiplexer 
26 multiplexes data in two systems and then transmits data unified to one 
system to the main modulation unit 10. 
The CMI encoding means 6A encodes the digital signal unified by the 
multiplexer 26. Then, the frequency-shift modulation unit 3A modulates the 
converted digital signal to the signal of a frequency lower than the 
intermediate (IF) frequency band. 
The indoor equipment 1 includes the hybrid circuit 22A that branches and 
combines the transmission signal and the receive signal. The hybrid 
circuit 22A combines the digital signal from the frequency-shift 
modulation unit 3A with the signal on the receiving side and then 
transmits the combined signal to the outdoor equipment 2 via the 
transmission line (cable) 8 shared for transmission and reception. 
The transmission data of a frequency lower than the intermediate frequency 
can be transmitted to the outdoor equipment without undergoing significant 
signal attenuation in the transmission line 8. 
The hybrid circuit 22B inputs data transmitted to the outdoor equipment 2 
to subject a transmission signal and a receive signal to a branching 
process. In the outdoor equipment 2, the frequency-shift demodulation unit 
4A demodulates one transmission data output from the hybrid circuit 22B. 
As for one transmission data output from the hybrid circuit 22B in the 
indoor equipment 1, the frequency-shift modulation unit 4A demodulates the 
digital signal FSK modulated by the frequency-shift modulation unit 3A in 
the indoor equipment 1. 
The CMI decoding means 7A decodes a signal decoded by the CMI encoding 
means 6A. The CMI decoding means 7A with the clock extracting unit 7a 
synchronizes timely the CMI decoding means 7A and the main modulation unit 
10 in accordance with the signal CMI-encoded by the CMI encoding means 6A 
in the indoor equipment 1. 
The main modulation unit 10 modulates the transmission data to a signal in 
the IF (intermediate) frequency band. Then the modulated signal is 
amplified by the amplifier 25A arranged integrally with the main 
modulation unit 10. Thereafter, the frequency conversion unit 11 receives 
the transmission data and frequency-converts the digital signal from the 
IF frequency band to the RF frequency band, in accordance with the local 
signal from the local oscillator 19. 
The signal is amplified by the high-power amplifier 16 and then the outcome 
is transmitted from the antenna unit 15 via the bandpass filter 17 and the 
circulator 12. 
On the other hand, the data received with the antenna unit 15 is 
transmitted to the indoor equipment 1 via the bandpass filter 17, the 
low-noise amplifier 18, the frequency conversion unit 13, and the main 
demodulation unit 14, the CMI encoding means 6B, and the frequency-shift 
modulation unit 3B. 
That is, the low-noise amplifier 18 amplifies the receive data via the 
bandpass filter 17 and the circulator 12. 
The frequency conversion unit 13 frequency-converts the receive signal from 
the low-noise amplifier 18 from the RF frequency band to the IF frequency 
band in response to local frequency signal from the local oscillator 19 
with the same specification as that for transmission. 
The amplifier 25B arranged integrally with the main demodulation unit 14 
amplifies the receive data and then the main demodulation unit 14 
modulates the amplified signal into a signal in the baseband. The CMI 
encoding means 6B subjects the receive data to a CMI encoding process. 
The frequency-shift modulation unit 3B further subjects the CMI-encoded 
receive data to a FSK modulation, the data having a frequency lower than 
the intermediate frequency. Thereafter, the hybrid circuit 22B combines 
the transmission data with the receive data and then transmits the outcome 
to the indoor equipment 1 via the transmission line 8. 
The receive data modulated to a frequency lower than the intermediate 
frequency can be transmitted to the indoor equipment 1 nearly with no 
signal attenuation in the transmission line 8. In the indoor equipment 1, 
the hybrid circuit 22A separates the receive data from the transmission 
data. Then, the digital signal is demodulated via the frequency-shift 
demodulation unit 4B, the CMI decoding means 7B, and the U/B converting 
means 9 while it is subjected to the CMI decoding process. 
As shown in FIG. 8, the demultiplexer 27 that separates a one-signal 
receiving system into a two-signal receiving system is arranged at the 
rear stage of the CMI decoding means 7 to separate receive data into 
plural pieces of data. 
The U/B converting means 9 in each receiving system converts a unipolar 
signal into a bipolar signal. 
Then, the receive data is subjected to various digital processes. 
Numerals 20A and 20B represent capacitors and 21A and 21B represent coils 
each of which one end is connected to the DC power source. 
The configuration of the third embodiment according to the present 
invention can provide the same effect and advantage as the first 
embodiment. In the third embodiment, since the indoor equipment 1 includes 
the multiplexer 26 and the demultiplexer 27, the number of each of the CMI 
encoding means 6 and the CMI decoding means 7 can be halved to a 
simplified configuration, as compared to the second embodiment. 
In the modification of the third embodiment, as shown in FIG. 9, the 
two-local system using local oscillators 19A and 19B can be employed to 
connect the indoor equipment 1 to the outdoor equipment 2. That is, plural 
input/output systems can be connected to the independent transmission 
lines 8a and 8b, respectively. 
As described above, since the configuration can eliminate the hybrid 
circuits 22A and 22B, it can simplify structurally the indoor equipment 1 
and the outdoor equipment 2. Moreover, the configuration subjects the 
digital signal between the indoor equipment 1 and the outdoor equipment 2 
to a FSK modulation, thus eliminating most signal attenuation due to the 
transmission lines 8a and 8b.