Interference detection apparatus for use in digital mobile communications system
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 &#34;A new method of co-channel D/U measurement using squared-law envelope differential detection&#34;, 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 &#34;Digital Communications&#34;, 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 MIN  from the maximum-minimum differences. The detected PMD MIN  is provided to the interference detector 18. It should be noted that the PMD 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 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 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&gt;B), the comparator 311 outputs a control signal indicating A&gt;B to cause the data selector 312 to select the path metric A. Otherwise, the comparator 311 outputs a control signal indicating A&lt;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 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 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 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 MIN ) detector 61 per data symbol. PMD 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 H  (11111111). Thus, the detector 61 outputs the minimum path metric difference PMD MIN  for data of 52 bits. PMD 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 μPD74HC85s. Each of data selectors 312, 412 and 614 may be comprised of two μPD74HC257s. Each of buffers 313, 413 and 613 may be comprised of μPD74HC574. Each of adders 511 and 513 may be comprised of 74HC283. The inverter 512 may be made of μPD74HC240. 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) 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) FS  corresponding to the field strength (Eb/No) A  at step S3. 
     At step S4, the detector 18 receives PMD 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 MIN  at steps S5. The detector 18 calculates at step S6 a bit error ratio (BER) 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) SQ  and (BER) FS  at step S7 and if (BER) SQ  is greater than (BER) 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) SQ  may be compared with a predetermined set value instead of (BER) 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 MIN  of path metric comparators 16a and 16b are supplied to a signal quality (SQ) comparator 22. The comparator 22 compares two PMD 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.