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Claims  |
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What is claimed is:
1. An interference detection apparatus comprising:
receiver means for receiving and demodulating a radio signal including
convolutionally coded data to produce demodulated data;
decoder means for decoding said demodulated data and producing path metrics
for said convolutionally coded data;
comparator means responsive to said path metrics for selecting a minimum
difference from differences between maximum ones of said path metrics and
minimum ones of said path metrics;
detector means responsive to said minimum difference for determining a
signal quality of said demodulated data, said detector means comprising
means responsive to said minimum difference for determining a first bit
error ratio corresponding to said minimum difference;
means for detecting an electromagnetic field strength of said radio signal
to produce an EMFS signal;
means responsive to said EMFS signal for determining a second bit error
ratio corresponding to said electromagnetic field strength; and
means for comparing said first and second bit error ratios and producing an
interference detect pulse if said first bit error ratio is greater than
said second bit error ratio.
2. An apparatus as claimed in claim 1, wherein said comparator means
comprises:
maximum detector means for detecting said maximum path metric from said
path metrics for each data symbol;
minimum detector means for detecting said minimum path metric from said
path metrics for each data symbol;
subtracter means for subtracting said minimum path metric from said maximum
path metric for each data symbol; and
minimum difference detector means for detecting said minimum difference
from the output of said subtracter means.
3. An apparatus as claimed in claim 2, wherein said maximum detector means
comprises:
first magnitude comparator means for comparing each of said path metrics
with a first buffer output for one data symbol to produce a first select
signal;
first data selector means responsive to said first select signal for
selecting the bigger one of each of said path metrics and said first
buffer output to produce an output as the output of said maximum detector
means; and
first buffer means for storing the output of said first data selector means
and providing the stored output to said first magnitude comparator as said
first buffer output, said first buffer means being reset at the end of
each data symbol,
and wherein said minimum detector means comprises:
second magnitude comparator means for comparing each of said path metrics
with a second buffer output for one data symbol to produce a second select
signal;
second data selector means responsive to said second select signal for
selecting bigger one of each of said path metrics and said second buffer
output to produce an output as the output of said minimum detector means;
and
second buffer means for storing the output of said second data selector
means and providing the stored output to said second magnitude comparator
as said second buffer output, said second buffer means being reset at the
end of each data symbol.
4. An apparatus as claimed in claim 3, wherein said subtracter means
comprises:
complimentary number generator means responsive to the output of said
second data selector means for generating a complimentary number of said
minimum path metric;
adder means for adding said maximum path metric and said complimentary
number to produce a subtracted number; and
latch means for latching said substracted number at the end of each data
symbol to produce an output as the output of said subtracter means.
5. An apparatus as claimed in claim 2, wherein said minimum difference
detector means comprises:
magnitude comparator means for comparing each of the outputs of said
subtractor means with a buffer output to produce a select signal;
data selector means responsive to said select signal for selecting the
bigger one of each of the outputs of said subtracter means and said buffer
output to produce an output as the output of said minimum difference
detector means; and
buffer means for storing the output of said data selector means and
providing the stored output to said magnitude comparator means as said
buffer output, said buffer means being reset at the beginning of said
demodulated data.
6. An apparatus as claimed in claim 1, wherein said decoder means comprises
a Viterbi decoder.
7. An apparatus comprising:
receiver means for receiving and demodulating a radio signal including
convolutionally coded data to produce baseband data;
decoder means for decoding said baseband data and decoding the received
data to produce decoded data and providing path metrics;
comparator means responsive to said path metrics for determining a signal
quality of said decoded data;
detector means responsive to said quality for producing a detect signal
when said signal quality falls below a predetermined value;
field strength detector means for detecting an electromagnetic field
strength of said radio signal to produce an EMFS signal; and
means for changing said predetermined value in accordance with said EMFS
signal.
8. An apparatus as claimed in claim 7, further comprising means responsive
to said detect signal for changing a radio channel of said receiver means.
9. An apparatus as claimed in claim 7, wherein said comparator means
comprises:
first detector means for selecting a miximum one from said path metrics;
second detector means for selecting a minimum one from said path metrics;
subtractor means for subtracting said minimum path metric from said maximum
path metric to produce a subtracted path metric; and
detecting means responsive to said subtracted path metric for detecting
said signal quality.
10. An apparatus as claimed in claim 9, wherein said detecting means
comprises:
means for storing in advance a table indicating a signal quality vs.
subtracted path metric characteristic; and
means for reading out of said table a signal quality corresponding to said
subtracted path metric to produce an output as the output of said
detecting means.
11. An apparatus as claimed in claim 7, wherein said decoder means
comprises a Viterbi decoder.
12. An interference detection apparatus installed in a mobile subscriber
station which is to be connected to a mobile base station through a radio
channel, said apparatus comprises:
receiver means for receiving and demodulating a radio signal including
convolutionally coded data to produce baseband data;
decoder means for receiving said baseband data and decoding the received
data to produce decoded data and providing path metrics;
comparator means responsive to said path metrics for determining a signal
quality of said decoded data;
detector means responsive to said signal quality for producing in
interference detect signal when said signal quality falls below a
predetermined value;
transmitter means for transmitting said interference detect pulse to said
mobile base station;
field strength detector for detecting an electromagnetic field strength of
said radio signal to produce an EMFS signal; and
means for changing said predetermined value in accordance with said EMFS
signal.
13. An apparatus as claimed in claim 12, wherein said decoder means
comprises a Viterbi decoder.
14. An interference detection apparatus installed in a mobile base station
which is connected to a public switched telephone network and is to be
connected to a mobile subscriber station through a radio channel, said
apparatus comprises:
receiver means for receiving and demodulating a radio signal including
convolutionally coded data to produce baseband data;
decoder means for receiving said baseband data and decoding the received
data to produce data and providing path metrics;
comparator means responsive to said path metrics for determining a signal
quality of said decoded data;
detector means responsive to said signal quality for producing an
interference detect signal when said signal quality falls below a
predetermined value;
radio control means responsive to said interference detect signal for
changing a radio channel of said receiver means;
field strength detector means for detecting an electromagnetic field
strength of said radio signal to produce an EMFS signal; and
means for changing said predetermined value in accordance with said EMFS
signal.
15. An apparatus as claimed in claim 14, wherein said decoder means
comprises a Viterbi decoder.
16. An apparatus comprising:
first receiver means for receiving and demodulating a first radio signal
including convolutionally coded data to produce first baseband data
containing more than two bits;
first header detector means for detecting said first header out of said
first radio signal to produce a first header detect signal;
first Viterbi decoder means for receiving said first baseband data and
decoding the received data to produce a first decoded data and providing a
first set of path metrics;
first comparator means responsive to said first set of path metrics for
determining a first quality of said first decoded data, said first
comparator means being reset by said first header detect signal;
second receiver means for receiving and demodulating a second radio signal
including a second header followed by convolutionally coded data to
produce second baseband data containing more than two bits;
second header detector means for detecting said second header out of said
second radio signal to produce a second header detect signal;
second Viterbi decoder means for receiving said second baseband data and
decoding the received data to produce second decoded data and providing a
second set of path metrics;
second comparator means responsive to said second set of path metrics for
determining a second signal quality of said second decoded data, said
second comparator means being reset by said second header detect signal;
third comparator means for comparing said first and second signal qualities
to produce a first switch signal when said first signal quality is better
than said second signal quality and a second switch signal when said
second signal quality is better than said first signal quality; and
switch means for selecting first decoded data in response to said first
switch signal and selecting said second decoded data in response to said
second switch signal.
17. An apparatus as claimed in claim 16, wherein each of said first and
second comparator means comprises:
maximum detector means for detecting a maximum path metric from said path
metrics for each data symbol;
minimum detector means for detecting a minimum path metric from said path
metrics for each data symbol;
subtractor means for subtracting said minimum path metric from said maximum
path metric for each data symbol; and
minimum difference detector means for detecting a minimum path metric
difference from the output of said subtracter means.
18. An apparatus comprising:
first and second antennas each picking up first and second radio signals;
first and second receivers respectively connected to said first and second
antennas for respectively receiving and demodulating said first and second
radio signals to produce first and second demodulated baseband signals
respectively including first and second headers respectively followed by
first and second convolutionally coded data each of which contains more
than two bits;
first and second header detectors for detecting said first and second
headers to produce first and second header detect pulses, respectively;
first and second Viterbi decoders for respectively decoding said first and
second demodulated baseband signals to produce first and second decoded
data and respectively providing first and second sets of path metrics;
first and second path metric comparators responsive to said first and
second sets of path metrics for respectively determining first and second
signal qualities of said first and second demodulated baseband signals,
said first and second path metric comparators being reset by said first
and second header detect pulses, respectively; and
control means responsive to said first and second signal qualities for
selecting one of the outputs of said first and second Viterbi decoders.
19. An apparatus as claimed in claim 18, wherein said control means
comprises:
a signal quality comparator for comparing said first and second signal
qualities to produce a first switch signal when said first signal quality
is better than said second signal quality and a second switch signal when
said second signal quality is better than said first signal quality; and
a switch circuit for selecting the output of said first Viterbi decoder in
response to said first switch signal and selecting the output of said
second Viterbi decoder in response to said second switch signal.
20. A method of detecting a signal quality of a received radio signal,
comprising the following steps of:
receiving and demodulating a radio signal including convolutionally coded
data to produce baseband data;
decoding said baseband data and producing path metrics for said baseband
data;
responsive to said path metrics, determining a signal quality of said radio
signal;
producing a detect signal when said signal quality falls below a
predetermined value;
detecting the electromagnetic field strength of said radio signal to
produce an EMFS signal; and
changing said predetermined value in accordance with said EMFS signal.
21. A method as claimed in claim 20, wherein said decoding step comprises
the step of decoding said baseband data in accordance with the Viterbi
algorithm.
22. A method of detecting interference with a radio signal on a radio
channel between a mobile base station and a mobile subscriber station,
said method comprising the following steps of:
receiving said radio signal to produce a received radio signal;
decoding said received radio signal and producing path metrics;
detecting an electromagnetic field strength associated with said radio
signal; and
responsive to said path metrics and to said field strength, detecting
interference with said radio signal to produce a detect signal.
23. A method as claimed in claim 22, further comprising the step of,
responsive to said detect signal, changing said radio channel to another
radio channel.
24. A method as claimed in claim 22, wherein said decoding step comprises
the step of decoding said received radio signal in accordance with the
Viterbi algorithm.
25. A method of determining a signal quality of a radio signal including
convolutionally coded data, said method comprising the steps of:
receiving and demodulating said radio signal to produce demodulated data;
decoding said demodulated data to produce decoded data using the Viterbi
algorithm;
providing path metrics by means of said Viterbi algorithm;
responsive to said path metrics, selecting a minimum difference from
differences between maximum ones of said path metrics and minimum ones of
said path metrics for one data symbol of said decoded data;
responsive to said minimum difference, determining a signal quality of said
radio signal, said determining step comprising the step of determining a
first bit error ratio corresponding to said minimum difference;
detecting an electromagnetic field strength of said radio signal;
determining a second bit error corresponding to said electromagnetic field
strength; and
comparing said first and second bit error ratios to produce an interference
detect signal when said first bit error ratio is greater than said second
bit error ratio.
26. A method of selecting one of first and second radio signals picked up
by first and second antennas, respectively, said method comprising the
following steps of:
respectively demodulating said first and second radio signals to produce
first and second baseband data respectively including first and second
headers respectively followed by first and second convolutionally coded
data each of which contains more than two bits;
detecting said first and second headers to produce first and second header
detect pulses, respectively;
respectively decoding said first and second baseband data through the
Viterbi algorithm to produce first and second decoded data;
respectively producing from said first and second baseband data first and
second sets of path metrics through said Viterbi algorithm, the production
of said first and second sets of path metrics being reset by said first
and second header detect pulses, respectively;
responsive to said first and second sets of path metrics, respectively
determining first and second signal qualities respectively associated with
said first and second decoded data;
comparing said first and second signal qualities to produce a switch
control signal indicating which signal quality is better than the other;
and
responsive to said switch control signal, selectively passing one of said
first and second decoded data.
27. A method as claimed in claim 26, wherein each of said determining steps
comprises the steps of:
detecting a maximum path metric from said path metrics for each data
symbol;
detecting a minimum path metric from said path metrics for each data
symbol;
subtracting said minimum path metric from said maximum path metric for each
data symbol to produce subtracted path metrics for said decoded data;
responsive to said subtracted path metrics, detecting a minimum path metric
difference; and
responsive to said minimum path metric difference, calculating said signal
quality.
28. A method as claimed in claim 27, wherein said calculating step
comprises the steps of:
storing beforehand a table indicating a minimum path metric difference vs.
the number of bit errors characteristic;
reading out of said table bit errors respectively corresponding to said
minimum path metric differences; and
responsive to the read-out bit errors, determining said signal qualities. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to an interference detection apparatus and,
more particularly, to an interference detection apparatus for use in a
digital mobile communications system.
To detect co-channel interference, a conventional apparatus uses a beat
phenomenon caused by an interference wave. A reference is made to a paper
entitled "A new method of co-channel D/U measurement using squared-law
envelope differential detection", vol. 83, No. 3, IEICE Technical Report,
by Kozono et al 1983. By detecting and processing a received signal
envelope, the apparatus obtains the average power of the received signal
and power of the beat component. Based on the obtained powers, the
apparatus calculates a desired signal level-to-undesired signal level
(D/U) ratio. If the D/U ratio falls below a predetermined level, the
apparatus determines that interference exists.
Quality of such an interference detection depends on the linearity of the
envelope detector. Thus, if manufacturing errors in the envelope detector
occur, they deteriorate the interference detection quality. In addition,
the conventional apparatus needs a relatively long time to detect
interference, because obtaining the D/U ratio involves a complicated
calculation. This detection delay adversely affects communications.
SUMMARY OF THE INVENTION
An object of the present invention is, therefore, to provide a generally
improved interference detection apparatus which eliminates the
above-mentioned problems.
Another object of the present invention is to provide an interference
detection apparatus having a high detection quality.
Yet another object of the present invention is to provide an interference
detection apparatus capable of detecting interference for a relatively
short period.
A further object of the present invention is to provide an interference
detection apparatus suitable for a digital mobile communications system.
Yet a further object of the present invention is to provide an interference
detection apparatus applicable to an antenna diversity system.
According to the present invention, there is provided an interference
detection apparatus comprising a receive section which receives and
demodulates a radio signal including data of convolutional codes. A
Viterbi decoder decodes the demodulated data in accordance with the
Viterbi algorithm and provides path metrics for the convolutional codes.
In response to the path metrics, a path metric comparator selects a
minimum difference among differences between maximum ones of the path
metrics and minimum ones of the path metrics. By using the minimum
difference, an interference detector determines a signal quality of the
radio signal. The signal quality may be used for detecting interference
existing in the radio signal. Alternatively, the signal quality may be
used for an antenna diversity system.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other object, features and advantages of the present
invention will become more apparent from the following description with
reference to the accompanying drawings, in which:
FIG. 1 is a schematic diagram showing a mobile communications system to
which the present invention is applicable;
FIG. 2 is a block diagram showing an interference detection apparatus
embodying the present invention;
FIG. 3 is a block diagram showing a header detector of the FIG. 2
apparatus;
FIGS. 4A to 4C show received data, decoded data and a header detect pulse,
respectively;
FIG. 5 is a block diagram showing a path metric comparator of the FIG. 2
apparatus;
FIG. 6 is a flow chart demonstrating the operation of an interference
detector in FIG. 2;
FIG. 7 is a flow chart demonstrating the operation of a signal controller
17 in FIG. 2;
FIG. 8 is a graph showing a path metric vs. bit errors characteristic;
FIG. 9 is a graph showing an electromagnetic field strength vs. bit error
ratio characteristic; and
FIG. 10 is a block diagram showing an antenna diversity appararus embodying
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In FIG. 1, a mobile communications system includes a mobile control center
1 connected to a public switched telephone network (PSTN) and to mobile
base stations 2a to 2c through wired lines. The base stations 2a to 2c
have coverage areas 3a to 3c, respectively, and are to be connected to a
mobile subscriber station (MSS) 4 through radio channels. Only one MSS is
shown but a plurality of MSSs may be included in the system.
An interference detection apparatus according to the present invention may
be installed in either MSS 4 or the mobile base station 2a, 2b, 2c. If an
interference detection apparatus which is installed in the base station 2b
detects interference, the base station 2b changes a communications channel
used for communicating with MSS 4 to another communications channel. If an
interference detection apparatus which is installed in MSS 4 detects
interference, the apparatus informs the base station 2b of the
interference detection. In response, the base station 2b changes its
communications channel used for communicating with MSS 4 to another
channel and transmits a channel designate signal to MSS 4 to cause MSS to
change its channel to the another channel. It is to be noted that
interference may be interference waves illegally produced from
unidentified signal sources.
In FIG. 2, an interference detection apparatus 10 includes an antenna 11
picking up a radio signal which may contain convolutionally coded data.
The antenna 11 provides the radio signal to a receive section 13 through a
band-pass filter (BPF) 12. The receive section 13 demodulates the radio
signal to produce baseband data and provide the data to a header detecter
14. The receive section 13 includes a conventional clock recovery circuit
13 which provides necessary timings to various parts of the apparatus 10.
The receive section 13 also includes a conventional function to change it
radio channel in response to a channel change signal provided from a radio
control circuit 19. The receive section 13 provides an intermediate
frequency (IF) signal to an electromagnetic field strength (EMFS) detector
21.
EMFS detector 21 may be composed of a voltage comparator to detect the
field strength of the received radio signal and provides the result to an
interference detector 18. The header detector 14, which will be described
in detail later, detects a header or start code contained in the baseband
data to produce a header detect pulse. The header detect pulse activates a
Viterbi decoder 15 and the interference detector 18 and is provided as a
reset pulse to a path metric comparator 16.
Upon the header detect pulse, the Viterbi decoder 15 decodes the baseband
data through the Viterbi algorithm and provides the decoded data to a
signal controller 17. In the decoding algorithm, the Viterbi decoder 15
provides path metrics as well known in the art and provides them to the
path metric comparator 16. The Viterbi algorithm is disclosed in "Digital
Communications", pp. 295-298, written by John G. Proakis and published by
McGraw Hill, Inc. 1983. The path metric comparator 16, which will be
described in detail later, compares the path metrics with each other to
detect the maximum and minimum path metrics among them for each data
symbol. The comparator 16 then subtracts the minimum path metric from the
maximum path metric to produce a maximum-minimum difference therebetween
for each data symbol. The comparator 16 detects the minimum path metric
difference PMD.sub.MIN from the maximum-minimum differences. The detected
PMD.sub.MIN is provided to the interference detector 18. It should be
noted that the PMD.sub.MIN has a very close relationship with the number
of errors in the received data, i.e., with a signal quality of the
received data, as shown in FIG. 8.
The interference detector 18, whose operation will be discussed in detail
later, has two tables indicating a PMD.sub.MIN vs. number of bit errors
characteristic shown in FIG. 8 and an electromagnetic field strength
(Eb/No) vs. bit error ratio characteristic shown in FIG. 9. The FIG. 8
table has been obtained through a computer simulation. The FIG. 9 table
has been obtained through a field test. Based on the minimum path metric
difference PMD.sub.MIN and the field strength respectively provided from
the path metric comparator 16 and EMFS detector 21, the interference
detector 18 determines bit error ratios (BERs) from the FIG. 8 and 9
tables to see if there is interference.
More specifically, if BER determined from the FIG. 8 table, i.e., from the
signal quality is larger than that from the FIG. 9 table, the interference
detector 18 determines that interference exists. Otherwise, the detector
18 determines that no interference exists. The determined result is
provided to the signal controller 17.
If the determined result indicates that no interference exists, the signal
controller 17 transfers the decoded data from the Viterbi decoder 15 to
the radio control circuit 19. If, however, the determined result indicates
that interference exists, the controller 17 informs the control circuit 19
of this interference existence and transfers the decoded data to the
circuit 19.
When the interference detection apparatus 10 is installed in a mobile base
station, the radio control circuit 19 provides a channel change signal to
the receive section 13 and a transmitter 20 to change their communications
channel to another channel in response to the interference existence. The
transmitter 20 may be a conventional one having a function to change its
channel in response to the channel change signal. Regardless of
interference existence, the control circuit 19 provides the decoded data
to a fixed telephone (not shown) through the PSTN.
When the apparatus 10 is installed in a mobile subscriber station, the
radio control circuit 19 informs a mobile base station of the interference
existence through the transmitter 20 in response the interference
existence from the signal controller 17. In response, the mobile base
station provides the mobile subscriber station with a channel designate
signal indicating an unoccupied radio channel to be used for
communications. By receiving the channel designate signal, the apparatus
10 in the mobile subscriber station controls the receive section 13 and
transmitter 20 to tune them to the designated channel. Regardless of
interference existence, the control circuit 19 provides the decoded data
to a speaker (not shown), or the like.
In FIG. 3, the header detector 14 is comprised of a serial-to-parallel
(S-P) converter 141 and a read-only memory (ROM) 142. The S-P converter
141 is made of a sixteen-stage shift register which receives demodulated
baseband data from the receive section 13 and a clock from the clock
recovery circuit 131. The outputs of each stages of shift register 141 are
respectively applied to the address terminals of ROM 142. ROM 142 stores
in advance data which is read therefrom in response to either the start
code of 16 bits (see FIG. 4A) or a one-error containing code that is
similar to the start code but different from the start code by one bit. If
the received data includes either the start or one-error containing code,
ROM 142 produces a header detect pulse (FIG. 4C) and provides the pulse to
the Viterbi decoder 15, path metric comparator 16 and interference
detector 18.
FIG. 4A shows a received data which is convolutionally coded with code rate
1/2 and constraint length 5 and has a preamble of 16 bits, a start or
header code of 16 bits and data of 104 bits. Since the code rate is 1/2, a
decoded data has 52 bits, as shown in FIG. 4B. In FIG. 4C, the header
detect pulse provided by the header detector 14 (FIG. 3) appears at the
end of the start code. The Viterbi decoder 15 decodes the FIG. 4A data to
produce the FIG. 4B data and during the Viterbi algorithm produces 16 path
metrics. Reference should be made to the above-mentioned Proakis book.
In FIG. 5, the path metric comparator 16 comprises a maximum path metric
detector 31, a minimum path metric detector 41, a subtractor 51 and a
minimum path metric difference detector 61. The maximum path metric
detector 31 is composed of an 8-bit magnitude comparator 311, a data
selector 312 and a buffer 313. The comparator 311 compares the path metric
A from the Viterbi decoder 15 and the output B of buffer 313. If the path
metric A is greater than the output B (A>B), the comparator 311 outputs a
control signal indicating A>B to cause the data selector 312 to select the
path metric A. Otherwise, the comparator 311 outputs a control signal
indicating A<B to cause the data selector 312 to select the buffer output
B. The selected output is latched by the buffer 313. The buffer 313 is
reset by a reset pulse provided from the clock recovery circuit 131 per
data symbol to be loaded with the minimum value 00.sub.H (00000000). Thus,
the maximum path metric detector 31 detects the maximum path metric for
each data symbol.
Similarly, the minimum path metric detector 41 is composed of an 8-bit
magnitude comparator 411, a data selector 412 and a buffer 413. The data
selector 412 selects the smaller one of data A and B in response to the
output of magnitude comparator 411. The buffer 413 is reset by the reset
pulse to be loaded with the maximum value FF.sub.H (11111111). Thus, the
detector 41 detects the minimum path metric for each data symbol.
The subtractor 51 comprises an adder 511, a latch 514 and a complementary
number generator which includes an inverter 512 and an adder 513. The
subtractor 51 subtracts the minimum path metric from the maximum path
metric. Suppose that the maximum and minimum path metrics 5 and 3 are
outputted from the maximum and minimum path metric detectors 31 and 41,
respectively. The inverter 512 inverts 3 (00000011) into (11111100). The
adder 513 adds the inverted data (11111100) and 01.sub.H (00000001) to
produce the complementary number (11111101) of 3. The adder 511 adds the
complementary number (11111101) and 5 (00000101) to produce an added
output (100000010). Since the adder 511 has an 8-bit output, the most
significant bit (MSB) of the added output is discarded. Thus, the adder
511 produces the subtracted number 2 (00000010).
The output of addder 511 is provided to the latch 514 which in response to
the reset pulse, latches that output at the end of each data symbol. Thus,
the subtracted output is provided to the minimum path metric difference
(PMD.sub.MIN) detector 61 per data symbol. PMD.sub.MIN detector 61
comprises an 8-bit magnitude comparator 611, a data selector 612 and a
buffer 613. Like the minimum path metric detector 41, the detector 61
detects the minimum difference between the maximum and minimum path metric
respectively detected by the maximum and minimum path metric detectors 31
and 41. The buffer 613 is reset by the header detect pulse to be loaded
with the maximum value FF.sub.H (11111111). Thus, the detector 61 outputs
the minimum path metric difference PMD.sub.MIN for data of 52 bits.
PMD.sub.MIN is provided to the interference detector 18 (FIG. 2).
Each of 8-bit magnitude comparators 311, 411 and 611 may be comprised of
two .mu.PD74HC85s. Each of data selectors 312, 412 and 614 may be
comprised of two .mu.PD74HC257s. Each of buffers 313, 413 and 613 may be
comprised of .mu.PD74HC574. Each of adders 511 and 513 may be comprised of
74HC283. The inverter 512 may be made of .mu.PD74HC240. All the
above-mentioned integrated circuits (ICs) are manufactured and marketed by
NEC.
Next, the operation of interference detector 18 will be described referring
to FIG. 6. If the interface detector 18 receives the header detect pulse
at step S1, the detector 18 moves on to step S2 at which it calculates an
average electromagnetic field strength (Eb/No).sub.A for data period of 51
bits based on the output of EMFS detector 21. The detector 18 reads out of
the FIG. 9 table a bit error ratio (BER).sub.FS corresponding to the field
strength (Eb/No).sub.A at step S3.
At step S4, the detector 18 receives PMD.sub.MIN from the detector 61 (FIG.
5). The detector 18 reads out of the FIG. 8 table the number of bit errors
which corresponds to the received PMD.sub.MIN at steps S5. The detector 18
calculates at step S6 a bit error ratio (BER).sub.SQ indicating the signal
quality of the received data by dividing the number of bits error by the
number of data bits 104. The detector 18 compares (BER).sub.SQ and
(BER).sub.FS at step S7 and if (BER).sub.SQ is greater than (BER).sub.FS,
the detector 18 goes on to step S8. Otherwise, the operation ends. At step
S8, the detector 18 outputs an interference detect pulse indicating that
interference exists. The interference detect pulse is provided to the
signal controller 17. It is to be noted that (BER).sub.SQ may be compared
with a predetermined set value instead of (BER).sub.FS at step S7. In this
case, EMFS detector 21 need not be provided in the apparatus 10 (FIG. 2).
The operation of signal controller 17 will now be described referring to
FIG. 7. At step S11, the controller 17 check if there is decoded data from
the Viterbi decoder 15. If yes, the controller 17 sees at step S12 if the
interference detect pulse is provided from the interference detector 18.
If no, the controller 17 transfers the decoded data to the radio control
circuit 19. Otherwise, the controller 17 transfers both the decoded data
and the interference detect pulse to the radio control circuit 19. As
mentioned earlier, if the control circuit 19 receives the interference
detect pulse, it performs the channel change operation.
In FIG. 10, the present invention is applied to an antenna diversity
apparatus 100. The apparatus 100 includes antennas 11a and 11b, BPFs 12a
and 12b, receive sections 13a and 13b, header detectors 14a and 14b,
Viterbi decoders 15a and 15b and path metric comparators 16a and 16b.
These elements have the same structures as their counterparts in FIG. 2,
respectively, and thus their detailed description should not be provided
in this specification. Both the outputs PMD.sub.MIN of path metric
comparators 16a and 16b are supplied to a signal quality (SQ) comparator
22. The comparator 22 compares two PMD.sub.MIN to select the better one of
them. In response to the output of SQ comparator 22, a switch circuit 23
selects one of decoded data from the Viterbi decoders 15a and 15b.
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