Polarization diversity system suitable for radio communication in indoor space

A polarization diversity radio communication system suitable for radio communication in indoor space is provided for transmitting from a transmitter a series of data sequentially from one of two or more transmission antennas of the transmitter, and for reproducing the data received at a receiver through a single reception antenna of the receiver when the series data are received from the transmitter. Thus, the transmitted data from any part of the indoor space can be received efficiently, and a high S/N ratio can be assured on the reception side.

TECHNICAL BACKGROUND OF THE INVENTION 
This invention relates to a polarization diversity radio communication 
system including a plurality of transmission antennas of different main 
polarization components and a reception antenna for receiving transmitted 
waves from the respective transmission antennas. 
DISCLOSURE OF PRIOR ART 
In recent years, radio communication systems employing electric waves as 
communicating media have been increasingly utilized in indoor spaces. When 
the electric waves propagate through free space, radiation field intensity 
of the antenna shows a monotonous decrease as the distance increases. In 
the indoor space, however, the electric wave is readily caused to reflect 
in a complicated manner thus producing many propagation paths. also, 
multipath fading results due to phase difference of the electric wave at 
the respective propagation paths. 
In view of the above problem, there has been suggested in, for example, 
Japanese Patent Application Laid-Open Publication No. 54-118117 of Y. 
Nakano a space diversity radio communication system, in which two or more 
reception antennas are provided as mutually spaced within an indoor space, 
and the system is so arranged as to obtain a relatively large reception 
signal from the electric wave received at one of the reception antennas at 
a required field intensity and through a change-over operation at a 
reception circuit connected to the reception antennas. With this 
arrangement, the reception can be carried out without suffering from any 
multipath fading due to the phase difference of the electric wave passing 
the respective propagation paths, and the reception of the electric wave 
coming from any part of the space can be attained. 
In the space diversity system, it is necessary to have the two or more 
reception antennas disposed mutually spaced by such a predetermined 
distance as 0.4 wave length, for example, 40 cm with respect to the 
electric wave of 300 MHz, so that a receiver including the reception 
antennas has to be enlarged in its dimension, and there arises a problem 
in respect of occupation space as the receiver of the radio communication 
system to be employed in the indoor space. Further, it is another problem 
that the change-over operation in the reception circuit, with respect to 
the two or more reception antennas, is likely to be accompanied by a 
larger loss, and no sufficient S/N ratio can be attained on the reception 
side. 
It has been further suggested in, for example, Japanese Patent Application 
Laid-Open Publication No. 56-98036 by Y. Ogata et al to employ a 
polarization diversity radio communication system provided with two or 
more reception antennas of different main polarization components, 
according to which the reception object is the different main polarization 
components of the transmitted electric wave so that no interval between 
the antennas installed is needed. In this configuration the reception 
antennas are able to be disposed closer and a minimization in size of the 
receiver can be realized. However, the loss at the reception circuit upon 
signal composition at the reception circuit becomes higher in this case, 
too, and there remains the problem that the S/N ratio on the reception 
side still cannot be made sufficiently high. 
TECHNICAL FIELD 
A primary object of the present invention is, therefore, to provide a 
polarization diversity radio communication system which allows the 
transmission from any part of an indoor space to be excellently received 
without suffering from any multipath fading due to the phase difference of 
the electric wave at every path within the indoor space where the system 
is employed, and its receiver to be sufficiently minimized in size and to 
be likewise lowered in manufacturing cost while assuring a high S/N ratio. 
According to the present invention, the above object can be realized by a 
polarization diversity radio communication system for transmission of 
radio signals, which comprises a transmitter including a plurality of 
antennas of different main polarization components, a transmission 
circuit, and an antenna change-over circuit connecting the transmission 
circuit to select one of the transmission antennas for sequentially 
selecting the antennas and transmitting a series of data from the 
respective transmission antennas; and a receiver separated properly from 
the transmitter and including a single reception antenna for reproducing 
the series of data from the transmitter upon reception of the series data. 
Other objects and advantages of the present invention shall be made clear 
in following description of the invention detailed with reference to 
embodiments shown in accompanying drawings.

DISCLOSURE OF PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, there are shown in the block diagrams the 
transmitter and receiver in the polarization diversity radio communication 
system according to the present invention. In the transmitter 10 shown in 
FIG. 1, a pair of transmission antennas llH and llV are included, and an 
antenna change-over circuit 12 which selects one of the antennas llH and 
llV as well as a transmission circuit 13 which generates a transmission 
output are connected to the antennas llH and llV. The transmission circuit 
13 includes an oscillation circuit 14 which comprises a crystal oscillator 
that oscillates at a constant frequency. This oscillation circuit 14 
oscillates as an output of a control circuit 15 is received to provide an 
oscillation output through a step up circuit 16 to a modulation circuit 17 
as raised to a desired frequency at the step up circuit 16. A digital 
signal is being provided from the control circuit 15 to the modulation 
circuit 17 through an encoder circuit 18 so that the oscillation output 
will be thereby modulated at the circuit 17 and then power-amplified at a 
transmission output circuit 19, and the power-amplified output can be 
transmitted out of one of the transmission antennas 11H and llV which is 
selected by the antenna change-over circuit 12. In this case, the control 
circuit 15 is connected to a power source E, while a switch SW or the like 
for the purpose of, for example, an emergency alarm can be connected to 
the control circuit 15 so that, when the switch SW is operated, the 
control circuit 15 will be driven. Further, a control output is provided 
from the control circuit 15 to the antenna change-over circuit 12 so that 
the antenna change-over operation will be carried out in conformity to the 
oscillation and modulation. It may be possible to connect, alternatively, 
to the control circuit 15 such disaster-preventive sensor as a fire 
detector, a crime-preventive sensor or the like. 
The transmission antennas llH and llV are so set that the main polarization 
components of their output electric waves mutually intersect at right 
angles, so that signals of different polarization components can be 
transmitted at different times from the both antennas 11H and llV with 
their change-over operation carried out by the antenna change-over circuit 
12. 
The receiver 20 includes, on the other hand, a single reception antenna 21 
to which a front-end circuit 22 which amplifies a reception signal, a 
demodulation circuit 23 which demodulates the modulated signal, and a 
decoder circuit 24 arc connected, so that the transmitted signal 
originated from the control circuit 15 and encoded at the encoder circuit 
18 will be reproduced and provided to a signal processing circuit 25 as 
its input. An output circuit 26 is connected to this signal processing 
circuit 25 so that an indication or alarm will be executed depending on 
contents of the reception signal. 
Referring in detail to the operation of the transmitter 10 and receiver 20, 
the transmitter 10 prepares first the data to be transmitted such that a 
series of data are made as a unit and two sets of data of the same 
contents are prepared for every unit. That is, as the switch SW is 
operated, two sets of the data of the same contents are formed at the 
control circuit 15 into first and second frames F1 and F2 as shown in FIG. 
3(a). In this case, the contents of the first and second frames F1 and F2 
arc constituted by digital signals as shown in FIG. 3(b), and the series 
of data in the first frame F1 are transmitted in a state where one of the 
two transmission antennas, for example, the antenna llH is selected by the 
antenna change over circuit 12 while the series of data in the second 
frame F2 are transmitted in a state where the other transmission antenna 
llV is selected. In this manner, the data sets of the same contents are to 
be transmitted at different time with different polarization components 
contained, so that a data transmission of the polarization diversity radio 
communication system can thereby be realized. 
When the transmission antenna llH is selected and the main polarization 
component of the transmitted set of the data is made horizontal, the field 
intensity distribution will be rendered to be as shown in FIG. 4 due to 
multipath fading, in which the maximum and minimum values of the field 
intensity involve a difference of 32 dB. When the other transmission 
antenna llV is selected and the main polarization component is made 
vertical, then the field intensity distribution will be as shown in FIG. 5 
due to the multipath phasing, where the maximum and minimum field 
intensity values involve a difference of 26 dB. Therefore, so long as the 
horizontal and vertical polarization components of which the maximum and 
minimum field intensity values are relatively large are utilized, the 
difference between the maximum and minimum values of the field intensity 
will be 13 dB as shown in FIG. 6, and the field intensity distribution can 
be made more uniform than in the case where only one type of the 
transmission antennas llH and llV is employed, as will be readily 
appreciated. 
In the receiver 20, on the other hand, signal processing is carried out at 
the signal processing circuit 26 on the basis of the polarization 
component of a value above a threshold value of the field intensity which 
can be received so long as only one of the horizontal and vertical 
polarization components is of the value above the threshold value. Where 
both polarization components are concurrently above the threshold value, 
the data of the first frame Fl are processed at the signal processing 
circuit 26. While the processing is not carried out at the signal 
processing circuit 26 when both polarization components are of a value 
below the threshold value, the arrangement according to the present 
invention is to select one of the polarization components which is larger 
in field intensity as has been referred to so that the field intensity 
distribution can be made even, and the probability of non-processing state 
of the signal processing circuit 26 is made extremely low as will be clear 
from a following table: 
______________________________________ 
Processing at 
First Frame F1 
Second Frame F2 
Circuit 26 
______________________________________ 
Below the threshold 
Above the threshold 
Carried out 
value value 
Above the threshold 
Below the threshold 
Carried out 
value value 
Above the threshold 
Above the threshold 
Carried out 
value value (for first frame) 
Below the threshold 
Below the threshold 
Not carried out 
value value 
______________________________________ 
With the receiver 20 disposed within a transmission range of the 
transmitter 10, the transmitted signals from any part can be received by 
the receiver 20 and an excellent diversity radio communication system can 
be realized. 
The transmission antennas shall be detailed next. In FIG. 7, there is shown 
a circuit diagram of the transmission antenna llH or llV with which the 
antenna change-over circuit 12 is made integral. In the present instance, 
as will be clear when a circuit of FIG. 8 and its equivalent circuit of 
FIG. 9 are referred to in conjunction, the transmission circuit includes 
an annular conductor 31 opened at a portion thereof and a tuning capacitor 
C1 is inserted between both ends at the open portion of the annular 
conductor 31. Further connected in series to this capacitor C1 is a 
bypassing capacitor C2 which is set to show a sufficiently low impedance 
with respect to high frequency signals provided out of the transmission 
circuit 13, and there are provided at two points of a loop including the 
annular conductor 31 and the two capacitors C1 and C2 a power-supplying 
point S1 and a grounding point G1 so that the both points S1 and G1 will 
be at both ends of the capacitor C2 while the grounding point G1 will be 
positioned between the both capacitors C1 and C2. An output terminal of 
the transmission circuit 13 is connected through another capacitor C3 to 
the power supply point S1, and a diode D1 connected in parallel with the 
capacitor C1 is connected at its cathode to the grounding point G1, while 
the diode D1 is connected at its anode and through the annular conductor 
31 to a change-over signal line L1 for supplying a bypass current, the 
line L1 including a bias resistor R1 and being connected to a change-over 
signal input terminal SS. In this arrangement of the transmission antenna, 
the bypassing capacitor C2 functions to break in series relationship 
between the grounding point G1 and the change-over signal line L1 which 
supplies the bypass current to the diode D1, and the bias resistor R1 is 
inserted so as to set the bypass current to the diode D1 and is made to 
have an impedance high enough for rendering any high frequency signal 
leaking through the current-over signal L1 to be of a negligible level. 
When in the foregoing transmission antenna the change-over signal input 
terminal SS is made open or to be at zero potential, the bias current does 
not flow to the diode D1, and the impedance of the diode D1 shows a 
capacitive value of about several pF which is low in the loss. 
Accordingly, it can be regarded that a capacitor due to an equivalent 
capacity of the diode D1 is connected in parallel with the tuning 
capacitor C1, and, with a composite capacity considered to be CO, an 
equivalent circuit considered in view of the high frequency signal will be 
as shown in FIG. 9. When the antenna is so set as to achieve the tuning at 
the value of the composite capacity CO, the antenna can function as a 
small loop antenna. 
When a predetermined voltage is applied to the change-over signal input 
terminal SS, on the other hand, the bias current is caused to flow to the 
diode D1, the impedance of the diode D1 becomes equivalent to a small 
resistance and a small inductive reactance. Accordingly, the composite 
capacity CO deviates from the capacitive value at which the tuning is 
attained and a state in which a resistor of a small value is connected in 
parallel with the capacitor C1, so that the selectivity Q of the loop 
including the annular conductor 31 is lowered to increase the loss and the 
operation as the transmission antenna is stopped. 
Such antenna operation of the transmission antenna 11H or llV is to be made 
on and off by means of the presence and absence of a voltage application 
to the change-over signal input terminal SS. In the case of ON, the loss 
at the diode D1 upon the biasing off is small and the diode D1 can be 
regarded substantially as a capacitor so that the loss can be made 
extremely small. In the event of OFF, the resonance point is deviated in 
the loop including the annular conductor 31 and the selectivity Q of this 
loop is lowered as has been referred to, whereby the ON/OFF ratio can be 
taken large. 
Referring again to FIG. 7, the transmission antenna of the foregoing 
arrangement is provided in a pair of antennas llH and llV which are 
connected in parallel relationship to each other, to the transmission 
circuit 13, to the change-over signal input terminal SS, and to the 
transmitter 10 according to the present invention as shown in FIG. 1. The 
antenna change-over input signal is applied to the terminal SS to be 
provided, more concretely, through a logic circuit comprising inverters Il 
and I2 to the respective transmission antennas llH and llV. The logic 
circuit is so formed as to selectively actuate the two transmission 
antennas with respect to a theoretical value of each of the antenna 
change-over signal which comprises binary signals, and the antenna 
change-over circuit 12 is to be formed by the logic circuit and diode D1. 
In FIG. 7, the transmission antennas llH and llV are formed identical to 
each other except that they are arranged to show mutually different 
polarization components, and respective constituents of the antenna 11V 
are denoted by the same reference numbers as those assigned to the 
foregoing constituents of the antenna 11H but respectively with an 
addition of 10. 
More concretely, the transmission antennas llH and 11V are mounted to a 
proper mounting surface, as shown in FIG. 10, so that their antenna loops 
respectively including the annular conductor 31 or 41 will intersect each 
other at right angles and thus their main polarization components also 
will intersect each other perpendicularly. Referring now to FIG. 11, there 
is shown another working aspect of the transmission antenna employed in 
the present invention, in which the transmission antenna llHa or llVa 
comprises an annular conductor 31a including a series circuit of a 
capacitor C1a and diode D1a connected across both ends at an open portion 
of the conductor, and a change-over signal line L1a for providing the bias 
current to the diode D1a is connected through a biasing resistor R1a to 
anode side of the diode D1a. A predetermined-voltage application to a 
change over signal input terminal SSa of this transmission antenna causes 
the impedance of the diode D1a to become equivalent to a small resistance 
and a slight inductive reactance as will be clear when an equivalent 
circuit with respect to the high frequency signal as shown in FIG. 12 is 
also referred to. In the drawing, the inductive reactance component is not 
shown as being regarded to be contained in the inductance of the loop 
including the annular conductor 31a. In the loop, further, a small 
transmission antenna having a certain extent of loss resistance is formed, 
but such loss can be made almost negligible by means of the diode D1a 
which shows excellent ON characteristics. Therefore, so long as tuning for 
the capacitor C1a is performed upon feeding of the bias current, the 
antenna can be effectively employed as a small transmission antenna. 
In this case, too, the zero potential or the opening of the change-over 
signal input terminal SSa renders the impedance of the diode D1a to show 
the capacitive reactance which is of such capacity value of less than 
several pF that causes the resonance point of the loop including the 
annular conductor to be deviated, whereby the loop is made to be of a high 
impedance with respect to circulating current of the loop and stops 
substantially the operation as the antenna. Accordingly, the transmission 
antenna of the present instance also carries out the ON and OFF operation 
in response to the presence or absence of the voltage applied to the 
change-over signal input terminal SSa and the antenna operation is to be 
made ON and OFF by varying the resonating state with the ON and OFF of the 
bias current to the diode D1a so that, when the antenna is regarded as a 
switch, it will be possible to render the ON/OFF ratio to be large and to 
reduce any loss at ON time with an optimum selection of the diode D1a. 
When the transmission antennas llHa and llVa of the foregoing arrangement 
are connected to be parallel to each other with respect to the 
transmission circuit 13a and change-over signal input terminal SSa as 
shown in FIG. 13, it is possible to realize the same transmitter as in the 
embodiment of FIG. 7 and to achieve the same function. Further, as 
required, it is possible to substitute such transmission antennas llHb, 
llVb or llHc, llVc as shown in FIG. 14 or 15 for the transmission antennas 
of FIG. 11, in the former antennas of which the connection of the diode 
D1b or D1c and the change-over signal input terminal SSb or SSc is 
altered. 
Referring now to FIG. 16 showing another working aspect of the transmission 
antenna employed in the present invention, the transmission antennas llHd 
and llVd comprise respectively a strip-shaped annular conductor 31d opened 
at one portion, and a tuning capacitor C1d is inserted between both ends 
of the conductor 31d at the open portion. In this annular conductor 31d, 
there are provided respectively at different positions the power-supplying 
point S1d and grounding point G1d, an air-core coil 32d is connected at 
its one end to another position of the conductor 31d than the 
power-supplying and grounding points S1d and G1d, and at the other end to 
a conductive wire 33d and loading 34d. In this case, the length of the 
conductive wire 31d can be made shorter by sequentially providing thereto 
the loading 34d. For this loading 34d, it may be possible to employ one of 
various shapes, while in the aspect of FIG. 16 or FIG. 17(a), there is 
employed one formed by dividing an end of the conductive wire 33d into two 
in T-shape and extending both divided ends backward along the wire 33d 
while turning toward and away from the wire 33d in zigzag shape on both 
sides of the wire 33d. 
For the loading 34d, further, it may be possible to employ such simply 
T-shaped one as in FIG. 17(b), such substantially T-shaped one having both 
divided ends in zigzag form as in FIG. 17(c), such substantially L-shaped 
one having straight or zigzag end as in FIG. 17(d), 17(e) or 17(f), or 
such annular crown-shaped one as in FIG. 17(g). If the provision of only 
the conductive wire 33d suffices the purpose, the loading 34d may of 
course be omitted to have the wire 33d terminated straight as in FIG. 
17(h). 
In respect of the transmission antenna llHd or llVd of FIG. 16, such an 
equivalent circuit as in FIG. 18 may be considered, in which event a first 
resonance circuit RESld is formed by the annular conductor 31d and 
capacitor Cld so that the circuit will have a resonance frequency 
determined by the inductance of the annular conductor and the capacity of 
the capacitor, and the power-supplying point Sld is set to be at a 
position where impedance matching with any circuit connected thereto can 
be attained. Therefore, when the power is supplied from the transmission 
circuit 13d, the circulating current is fed to the first resonance circuit 
RESld upon occurrence of resonance and supplied energy from the 
transmission circuit 13d is consumed at loss resistance RLld and radiation 
resistance RRld. AT this time, a loss component due to the radiation 
resistance RRld is to be radiated into the space, that is, a contracted 
loop antennas is to be formed by means of the annular conductor 31d and 
capacitor Cld. 
On the other hand, there is formed a second resonance circuit RES2d by 
means of an inductance of the air-core coil 32d and conductive wire 33d as 
well as a capacity component between the annular conductor 31d and the 
conductive wire 33d. If the foregoing contracted loop antenna is in the 
resonance state at this time, the maximum potential in the first resonance 
circuit RES1d is caused to occur at a point on opposite side of the 
grounding point G1d with respect to the capacitor C1d, l and the air-core 
coil 32d is connected at an end to this maximum potential point. 
Accordingly, an energy from the first resonance circuit RES1d is supplied 
to the second resonance circuit RES2d, and the air-core coil 32d and 
conductive wire 33d function as a monopole antenna in an event where the 
air core coil 32d and conductive wire 33d are so set as to have the both 
resonance frequencies of the first and second resonance circuits RES1d and 
RES2d coincided with each other. 
That is, when the power supply is carried out from the first resonance 
circuit RES1d to the second resonance circuit RES2d, the power is consumed 
at the loss resistance RL2d and radiation resistance RR2d, and the loss 
component at the radiation resistance RR2d is radiated to the space, 
whereby the present transmission antennas llHd and llVd are made to be 
equivalent to a case where the contracted loop antenna and contracted 
monopole antenna are concurrently employed. Accompanying the operation of 
the second resonance circuit RES2d, here, a current flows to a part of the 
first resonance circuit RES1d and, when the both resonance circuits are 
made to be of the same resonance frequency, the first resonance circuit 
RES1d is to be provided with a constant different from that when the same 
is employed alone. 
Referring to the operational characteristics of the transmission antennas 
llHd and llVd of the present working aspect, it should be assumed here 
that the first resonance circuit RES1d is a microloop antenna, the second 
resonance circuit RES2d is a microdipole antenna (dipole antenna is a 
modification of the monopole antenna), the characteristics of the 
transmission antennas llHd and llVd are approximated to composite 
characteristics of the microloop and microdipole antennas, and the 
radiation fields from the microloop and microdipole antennas are in 
equiphase. Referring here to FIG. 19, the microdipole antenna 36d is 
disposed to be in parallel including the microloop antenna 35d so as to 
correspond to the foregoing positional relationship between the 
transmission antennas llHd and llVd, then the both antennas are to show 
such directivities as shown in FIGS. 20(a) to 20(c) and FIGS. 21(a) to 
21(c). When it is so arranged that the polarization components present in 
the same direction are to be added to each other but those present in 
directions mutually intersecting at right angles are kept independent of 
each other, then the composite directivity of the both antennas will be as 
shown in FIGS. 22(a) to 22(c). The composite directivity represented by 
electric power pattern with a unit of dB will be as in FIGS. 23(a) to 
23(c) so that the maximum/minimum ratio will be about 4 dB, and it will be 
possible to render the transmission antennas llHd and llVd to be 
substantially isotropic in the directivity. In FIGS. 20 through 22, 
further, solid lines denote that the polarization components which are 
vertical to the plane of the drawings while dotted lines denote the 
components parallel to the plane. Further, the transmission antennas llHd 
and llVd of the present embodiment can be formed by means of conductive 
pattern of printed circuit board. 
Now, the transmission antennas llHd and llVd of the foregoing arrangement 
are so disposed that, as shown in FIG. 24, the planes including the 
annular conductors 31d and 31dl will intersect each other at right angles, 
and the respective polarization components of the microloop antennas 35d, 
35dl and the microdipole antennas 36d, 36dl in the both transmission 
antennas llHd and llVd are thereby made to mutually intersect at right 
angles. In this event, the transmission antennas llHd and llVd are 
connected through the antenna change-over circuit 12 to the transmission 
circuit 13 so that, as will be clear when FIGS. 25 and 26(a) to 26(c) and 
FIGS. 27 and 28(a) to 28(c) are jointly referred to, the transmission 
signals in two sets having the polarization components different by 90 
degrees from each other can be transmitted out of the transmission 
antennas llHd and llVd and the polarization diversity radio communication 
system can be realized. In the present invention, further, it is possible 
to employ various design modification. For example, while the polarization 
diversity radio communication system is realized in the foregoing 
embodiments by changing over the two transmission antennas llH and llV the 
main polarization components of which mutually intersect at right angles, 
the system can also be realized by using more than three transmission 
antennas in combination, and their main polarization components of the 
respective transmission antennas may not be always intersecting one 
another at right angles.