Diversity reception system in a portable radio apparatus

A portable radio used with this invention has two different types of antenna, such as a monopole antenna and a loop antenna. The diversity reception system continuously selects that one of the antenna which provides the better received signal. Antenna selection is performed such that the received signal level is compared with two or more preset threshold levels, and antennas are switched when and only when the received signal level fails to satisfy certain predetermined conditions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a diversity system in a portable radio for 
maintaining the high quality of received signals. A radio telephone 
apparatus or the like is a typical example of a portable radio of this 
type. 
2. Description of the Prior Art 
Many prior art portable radios have been provided with extendable monopole 
antennas. Monopoles, dipoles and other electric-field-type antennas are 
likely to be adversely affected by the human body; and if they are used in 
proximity to the human body the antenna's gain is lowered. Consequently, 
prior art portable radios of this type have the drawback that, when a 
radio is worn or placed in a pocket during use, the gain of the monopole 
antenna, even if extended, is lowered, thereby degrading reception. 
Some prior art portable radios have been provided instead with 
magnetic-field-type antennas such as loop antennas whose gain is improved 
by the influence of a human body, eventually avoiding the above stated 
drawback. These radios include pagers, portable radio telephones and the 
like. This type of portable radio has the drawback that when it is placed 
on a table or otherwise not worn by the user, the gain of the antenna will 
be reduced, thereby degrading reception. 
So-called mobile communications involve a complex propagation having 
multiple propagation paths generated by reflection of radio waves from 
buildings and other obstacles between a transmitter and a receiver by 
scattering and refraction from inhomogeneities in the atmosphere. A 
standing wave is likely to be produced at the point of reception. Movement 
over even a slight distance causes a variation of field strength 
("fading") which can be approximated by a Rayleigh distribution. This 
particular fading is generally called Rayleigh fading and is characterized 
by an occasional great decrease in field strength. Diversity is frequently 
used to achieve good communication in the presence of Rayleigh fading. In 
space diversity, for instance, two or more identical types of antennas 
spaced more than one quarter wavelength from each other receive signals 
which are then combined or selected to diminish the effects of the fading 
phenomenon. The space diversity system, of course, has been difficult to 
apply to portable radios because of the requirement for distance between 
antennas. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a diversity reception 
system in a portable radio capable of maintaining high quality 
communication against the effects of the human body. 
It is another object of the present invention to provide a diversity 
reception system in a portable radio capable of maintaining high quality 
communication against Rayleigh fading. 
It is another object of the present invention to provide a diversity 
reception system in a portable radio which may be effectuated by simple 
circuit construction. 
It is another object of the present invention to provide a diversity 
reception system in a portable radio which may be employed even in compact 
equipment. 
In order to achieve the above objects the invention uses two different 
antenna types such as a linear antenna (like a monopole or dipole antenna) 
and a loop antenna. The system selectively uses and continuously selects 
the antenna which provides the better received signal level. Antenna 
selection is performed so that the received signal level of the selected 
antenna is compared with two or more preset threshold levels, the selected 
antenna is continuously selected when the received signal level satisfies 
predetermined conditions, and the other antenna is selected when the 
received signal level (from the selected antenna) does not satisfy the 
predetermined conditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A portable radio telephone to which this invention may be applied is shown 
in FIG. 1 and includes an extendable monopole antenna 1, a loop antenna 2 
disposed within the radio housing, a microphone 3, a loud speaker 4, and a 
keypad 5 for dialing. As stated above, the gain of monopole antenna 1 
decreases, and that of loop antenna 2 increases, when the radio is placed 
very close to (within 1/4 wavelength of) a human body. The gain of loop 
antenna 2 is particularly increased when the plane of the loop is normal 
to the nearest surface of the human body, and therefore loop antenna 2 is 
located, as shown in FIG. 1, normal to the side of the housing where the 
perforations for microphone 3 and speaker 4 are formed. In particular, 
loop antenna 2 comprises a compact printed circuit board having a loop 
made of a conductor. The compact printed circuit board is attached to the 
housing perpendicular to the control panel. It should be noted that the 
monopole antenna can be collapsed into the housing. FIG. 2 is a graph 
showing changes in gain for monopole antenna 1 and loop antenna 2 with 
distance from the human body. The longitudinal axis of monopole antenna 1 
is arranged to be parallel to the longitudinal axis of the body of the 
user. 
The radio telephone is connected through a radio link to a base station 
(not shown), in the telephone subscription network, which in turn is 
connected to a landline and relays communications to another subscriber's 
telephone. Either antenna may be selected for reception of radio signals. 
For example, if, during reception, the radio is located on a table, 
monopole antenna 1 may be selected to receive the signals. Conversely, if 
the portable radio telephone is grasped by the user in the course of 
communication, the loop antenna will probably be selected to receive the 
signals. 
FIG. 3 is a block diagram showing one embodiment of the invention as 
applied to a portable radio telephone. Matching circuit 10 converts the 
impedance of monopole antenna 1 to the input impedance of a high-frequency 
amplifier 41, while matching circuit 20 converts the impedance of loop 
antenna 2 to the input impedance of amplifier 41. Switch 30 is used to 
select either monopole antenna 1 or loop antenna 2. High-frequency 
amplifier 41 performs well-known functions, such as amplifying 
high-frequency signals, and preventing local oscillator signals from 
frequency converter 42 from radiating. Converter 42 converts weak 
microwave signals to an intermediate frequency, enhancing reception 
sensitivity of the microwave signals. Intermediate frequency amplifier 43 
amplifies the intermediate frequency signal, demodulator 44 demodulates 
the amplified intermediate frequency signal, and audio frequency amplifier 
45 amplifies the demodulated signals to drive loudspeaker 4. 
Squelch circuit 40 comprises high pass filter 46, which extracts noise 
signals above the baseband, noise anplifier 47, and noise level detector 
48, which rectifies and filters the amplified noise. The result is the 
squelch signal, a DC voltage level representative of the received signal 
strength. 
The squelch signal is compared, in comparators 61 and 62, respectively, 
with two different reference voltage levels produced by reference voltage 
generator 51. The first reference voltage, input to comparator 61, 
corresponds to a received signal strength arbitrarily defined as "high." 
The second reference voltage, applied to comparator 62, corresponds to a 
lesser received signal strength--the minimum acceptable level for 
communication. Each comparator produces a logical "0" at its output when 
the reference voltage (first or second, as appropriate) applied to it is 
lower than the squelch signal, and a logical "1" otherwise. The operation 
of audio frequency amplifier 45 is controlled by the output of comparator 
62. 
An antenna switching operation of this embodiment will be described, with 
reference to FIG. 4, a flow chart of one mode of operation of control unit 
71. Assume that the monopole antenna 1 is connected through switch 30 to 
the high frequency amplifier 41, and that a "high" signal level is being 
received. In this condition, comparators 61 and 62 both produce logical 
"0" outputs. When the received signal is degraded by influences from a 
human body or for any other reason, changing the output of comparator 61 
to a "1," control unit 71 transmits a switching signal causing switch 30 
to connect the loop antenna 2 to the high frequency amplifier 41. Control 
unit 71 then determines whether the output of comparator 61 has returned 
to "0." If it has, switch 30 is not switched, and loop antenna 2 remains 
connected to high frequency amplifier 41. If the output of comparator 61 
is still "1," control unit 71 looks to the output of comparator 62. If 
this output is "1," control unit 71 transmits a switching signal to select 
the monopole antenna 1 again. However, if the output signal from 
comparator 62 is "0," control unit 71 waits for a predetermined length of 
time (T) and then detects the output from comparator 61 once again. If the 
output from comparator 61 is still "0," switch 30 is not switched, and the 
selection of loop antenna 2 is maintained. However, if comparator 61 is 
generating a "1," control unit 71 transmits a switching signal to select 
monopole antenna 1 again. If the squelch signal falls between the first 
and second reference levels, and if it does not exceed the first level 
within the predetermined time interval T, antennas 1 and 2 are switched. 
The predetermined time interval counted by control unit 71 preferably 
corresponds to the time required by a person carrying the radio to travel 
more than a quarter wavelength in the field of the standing wave, because 
there is a good probability of a decrease in the effects of fading after 
the lapse of this predetermined length of time (provided the user does not 
stand still). 
Audio frequency amplifier 45 receives the output from comparator 62. 
Amplifier 45 is allowed to operate normally when the output of comparator 
62 is "0," but operation is suspended by a hold circuit when the output is 
"1," so that the generation of audible sounds at speaker 4 is suppressed 
when the quality of the received signals deteriorates. Thus, generation of 
unfavorable noise due to poor quality of the received signals and from the 
antenna switching operation can be avoided. 
FIG. 5 is a timing chart for use in explaining antenna switching according 
to the embodiment of FIG. 3. Waveform a indicates the signal level 
received by loop antenna 2, while waveform b indicates the signal level 
received by monopole antenna 1. During good reception, as indicated by 
time interval I1, the level of at least one of the received signals (as 
indicated by the squelch signal) exceeds the first reference voltage 
level, and the selected one of the antennas continues to be used for 
reception. As shown in FIG. 5, during the time interval I1, loop antenna 2 
is selected, then monopole antenna 1 is selected, and then loop antenna 2 
is reselected. 
When the radio wave is influenced by a building, or a weak radio wave is 
received, or the portable radio telephone is located far away from the 
base station, the signals received by the antennas 1 and 2 are set at low 
level (i.e., below the first level). In this embodiment, the antenna 
having the reception signal level is selected in accordance with a second 
level as a reference. As shown in FIG. 5, when the levels of the signals 
received by the monopole antenna 1 and the loop antenna 2 fall within the 
range between the first level and the second level during a time interval 
I2, the monopole antenna 1 and the loop antenna 2 are alternately 
selected. When the level of the signal received by loop antenna 2 falls 
below the second level, monopole antenna 1 is continuously selected during 
the time interval T, and then loop antenna 2 is selected. However, when 
the level of the signal received by loop antenna 2 remains below the 
second level, the monopole antenna 1 is instantaneously selected. In this 
case, when the level of the signal received by the monopole antenna 1 
falls within the range between the first and second levels, monopole 
antenna 1 is selected and held again during the time interval T. 
Thereafter, when the level of the signal received by monopole antenna 1 
falls below the second level, and the level of the signal received by loop 
antenna 2 falls within the range between the first and second levels, loop 
antenna 2 is selected and held during the time interval T. In this manner, 
when the levels of the signals received by the antennas 1 and 2 become 
below the first level, the antenna for obtaining the reception signal 
level of not less than the second level can be selected in practice. 
Therefore, even if the radio wave strength is decreased, the good quality 
of reception signal can be guaranteed. 
Assume that the reception signal level is lowered due to fading or the like 
when a good reception state has been obtained and one of the antennas is 
continuously selected. As indicated by a time interval 13, the reception 
signal level is abruptly lowered. However, also assume that the portable 
radio telephone is in motion and that the influence of fading is 
eliminated within the time interval T. In this case, antenna switching 
during the time interval I3 is not performed. According to FIG. 5, the 
loop antenna 2 is selected and held during the time interval I3. In other 
words, even if the fading happens, unneccessary switching of antenna is 
not performed as long as the duration of fade is within the time interval 
T. 
According to this embodiment, therefore, the portable wireless telephone 
apparatus placed on a table or the like is not influenced by the human 
body, and monopole antenna 1 having a good antenna gain is selected. 
However, when the apparauts is used and the head of the user comes close 
to the apparatus for communication, loop antenna 2 is selected, thereby 
preventing a decrease in the reception signal level and hence preventing 
signal quality degradation. When the gain of monopole antenna 1 is not 
greatly decreased, a good quality reception signal can be obtained using 
monopole antenna 1. Furthermore, when the portable wireless telephone 
apparatus is located so as to correspond to the "valley" of the standing 
wave, the reception signal level is greatly lowered. In such a state 
wherein the intensity of the electric field is very low (valley of the 
electric field), the magnetic field component is great, so that loop 
antenna 2 often has a reception signal level higher than that of monopole 
antenna 1. As a result, the decrease in the reception signal level which 
corresponds to the valley of the standing wave can be prevented upon 
proper selection of the antenna. Therefore, the influence of fading can be 
reduced. 
In an alternate mode of operation, as shown in FIG. 6, control unit 71 may 
store the outputs of comparators 61 and 62 when the squelch signal falls 
in the range between the first and the second levels. Control unit 71 may 
then control the antenna switching operation to inhibit switching between 
the antennas if the last set of comparator outputs for the same antenna 
indicates that the squelch signal was in the range between the first and 
second reference voltage levels and the next preceding set of comparator 
outputs, for the same antenna, indicates that the squelch signal was not 
in that range. According to this embodiment, unlike the embodiment shown 
in FIG. 4, control unit 71 causes continuous selection of one of the 
antennas for a time interval of 2T, when the output signal level by 
selected antenna falls in the range between the first level and the second 
level. 
FIG. 7 shows a block diagram of another embodiment of the invention applied 
to a portable radio telephone. Identical components to those shown in FIG. 
3 are designated by the corresponding reference numerals from FIG. 3 and 
are excluded from any further description. The principal features of this 
embodiment are that either of high frequency amplifiers 411 and 412 may be 
selected, and that the voltage level from reference voltage generator 
circuit 52 is modified. 
High frequency amplifiers 411 and 412 together have a circuit arrangement 
as shown in FIG. 8. Grounded-emitter transistor 81, whose base is 
connected through a coupling capacitor 84 to the output terminal of 
matching circuit 10, and a similar grounded-emitter transistor 82, whose 
base is connected through a coupling capacitor 85 to the output terminal 
of another matching circuit 20, have their collector terminals commonly 
connected through a coupling capacitor 86 to frequency converter 42. To 
the common connection is also connected a parallel tuned circuit 83 so 
that the amplifier is able to amplify a desired frequency band. A bypass 
capacitor 89 provides a high frequency ground for stabilizing the tuned 
circuit 83. A switch circuit 90 comprises a conventional transistor 
circuit for applying operating power to either transistor 81 or transistor 
82. Reference numerals 91, 92, 93 and 94 designate bias resistors; 95, a 
resistor; and 87 and 88, bypass capacitors for high-frequency grounding 
the bases of the transistors 81 and 82. Assume that switch circuit 90 is 
switched as shown in FIG. 8, and a bias voltage is applied to the 
transistor 81, allowing output signals from matching circuit 10 to be 
amplified and introduced to frequency converter 42, whereas the base of 
transistor 82 is DC grounded, causing this transistor to be cut-off. As a 
result, the output impedance of transistor 82 increases. Thus, output 
signals from transistor 81 are not influenced by transistor 82. Because 
one of transistors 81 and 82 has a high output impedance, interference 
between output signals of the transistors 81 and 82 can be avoided. 
Reference voltage generator 52 has a circuit arrangement as shown in FIG. 
9, and comprises a Zener diode 521 for stabilizing a source voltage Vcc, a 
resistor 522, divider resistors 523, 524 and 525, analog switches 526 and 
527 and an inverter 528. The resistances of divider resistors 523, 524 and 
525 are preset such that analog switches 526 and 527 receive input 
voltages of the first and second reference voltage levels, respectively. 
Suppose that analog switches 526 and 527 are switched in a manner as shown 
in FIG. 9. The second preset level signal is supplied to comparator 60. 
The inverter 528 serves to control switch 526 such that switch 526 is off 
whenever switch 527 is on, and vice-versa. Thus, switch 527 is turned on 
when a control signal from control unit 72 is a logical "1," and switch 
526 is turned on when the control signal is logical "0." Comparator 60 
generates a logical "1" signal when the level of the first or second 
reference voltage is higher than the level of the squelch signal. However, 
when the level of the first or second reference voltage is lower than the 
level of the squelch signal, comparator 60 generates a logical "0." 
Referring to FIG. 10 of a flow chart of the operation of control unit 72, 
an antenna switching operation of still another embodiment of the 
invention will be described. Suppose that an output signal from high 
frequency amplifier 41 is supplied to frequency converter 42. Control unit 
72 supplies an "L" level control signal to reference voltage generator 52, 
causing the generator 52 to transmit the first preset level signal. 
Comparator 60 then compares the first preset signal with the output signal 
from squelch circuit 40. 
Control unit 72 supplies a selection signal (control signal) to switch 
circuit 90 in response to a signal of "L" level from comparator 60. In 
this case, switch circuit 90 selects monopole antenna 1. However, control 
unit 72 supplies another selection signal (control signal) to switch 
circuit 90 in response to a signal of "H" level from comparator 60. In 
this case, switch circuit 90 selects loop antenna 2. When loop antenna 2 
is selected due to influences from a nearby human body, control 72 detects 
an output signal level from comparator 60, and causes selection of loop 
antenna 2 to be maintained if it is at "L" level. If it is at "H" level, 
control unit 72 feeds a control signal of "H" level to reference voltage 
generator 52 to cause the latter to supply the second preset level signal. 
When the second preset level signal is thus supplied, control unit 72 
detects an output signal level from comparator 60. If the output signal is 
set at "L" level, control unit 72 supplies an "L" level control signal to 
reference voltage generator 52 when a predetermined time interval has 
elapsed. Control unit 72 supplies the first preset level signal. If the 
output signal from comparator 60 is set at "H" levels, control unit 72 
supplies the selection signal to switch circuit 90 which then selects 
monopole antenna 1. The level of the output signal from comparator 60 is 
then detected again. If the output signal level is at "L" level after 
selection of monopole antenna 1, control unit 72 supplies an "L" level 
control signal to reference voltage generator 52 after the elapse of a 
predetermined time interval, causing generator 52 to supply the first 
preset level signal. If the output signal from comparator 60 is set at "H" 
level, loop antenna 2 is selected again. The level of the output signal 
from comparator 60 is repeatedly detected. 
In short, when the output signal level from squelch circuit 40 falls within 
the range between the first and the second levels, selection between 
antennas is performed unless the output signal level is above the first 
level after the elapse of a determined time interval. 
The predetermined time interval counted by control unit 72 may be set in a 
similar way as in the previously described embodiment above. Control unit 
72 also controls the operation of audio frequency amplifier 45 to hold the 
latter for a determined short period of time so as to decrease switching 
noise upon switching between high-frequency amplifiers 411 and 412. Audio 
frequency amplifier 45 includes a holding circuit for performing this 
holding operation. When the output signal level falls within the range 
between the first and second levels, the control unit 71 may, as shown in 
FIG. 11, store the comparison results obtained by the comparator 60 and 
may control the antenna switching operation to inhibit switching between 
the antennas 1 and 2 if the last comparison result by same antenna 
indicates that the output signal level is in the range between the first 
level and the second level and the comparison result before last by same 
antenna indicates that the output signal level is not in the range between 
the first level and the second level. 
In this embodiment unlike the embodiment shown in FIG. 10, control unit 72 
causes continuous selection of one of the antennas for a time interval of 
2.times.T, when the output signal level by selected antenna falls in the 
range between the first level and the second level. 
Although comparators 60, 61 and 62 compare the output signals from squelch 
circuit 40 with the first and second level signals as they occur in analog 
form in each of the foregoing embodiments of the invention, the output 
signals from squelch circuit 40 can be converted to digital form by an A/D 
converter and can be compared with digital data corresponding to the first 
and second preset level signals stored in a memory means. The level of the 
output signal from squelch circuit 40 in each of the above-described 
embodiments is increased when the level of the reception signal is 
increased. However, the output signal from the squelch circuit may have a 
lower level when the reception signal has a higher level, and vice-versa. 
In the above embodiments, two preset levels are provided. However, three 
or four levels may be preset so as to use an antenna having a higher 
reception signal level. 
The switch diversity system has been conventionally known. According to 
this system, there is provided one preset level as a reference so as to 
select the antenna having the better reception state. It is assumed that 
the above-described loop and monopole antennas are applied to the switch 
diversity system. According to the diversity system of this type, when the 
reception level of one of the antennas exceeds the preset level, the 
diversity effect can be obtained (i.e., a good reception signal level can 
be obtained). However, if the reception signal levels of both these 
antennas are below the preset level, the antenna having a better reception 
state can not be selected. In order to prevent this drawback, the preset 
level is replaced with a lower level. However, the quality of the 
reception signal level higher than the lower level is degraded, resulting 
in inconvenience. More particularly, even if the reception signal from one 
antenna have poor quality than from another, the antenna for receiving 
this reception signal kept on being selected. As a result, an antenna 
having a better reception state cannot be selected. 
However, according to the present invention, at least two preset levels are 
provided to prevent the drawback described above. The antenna having the 
better reception state can always be selected regardless of given 
reception conditions.