Patent Publication Number: US-4549311-A

Title: Method and apparatus for measuring the strength of a radio signal frequency

Description:
This is a continuation of application Ser. No. 405,123, filed Aug. 3, 1982 now abandoned. 
    
    
     RELATED PATENT APPLICATIONS 
     The present invention is related to co-pending U.S. patent application, U.S. patent application Ser. No. 576,805 (corresponding to U.S. Pat. No. 4,485,486) entitled &#34;Method and Apparatus for Assigning Duplex Radio Channels and Scanning Duplex Radio Channels Assigned to Mobile and Portable Radiotelephones in a Cellular Radiotelephone System&#34;, invented by Larry C. Puhl and Ronald J. Webb, and assigned to same assignee and filed the same date as the instant invention. By reference thereto, the foregoing related U.S. patent application is incorporated in its entirety into the written description of the present invention. 
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to radio frequency (RF) signal strength measuring apparatus, and more particularly to an improved method and apparatus for measuring the strength of an RF signal subject to rapid and deep fading, such as Rayleigh fading. 
     In the prior art, the strength of RF signals has been determined by averaging a number of samples taken either by an analog meter or an analog-to-digital converter. The resulting signal strength is reasonably accurate as long as the RF signal is not subject to rapid and deep fading. However, if an RF signal is subject to rapid and deep fading and only a few signal strength samples are taken, a signal strength sample taken during a deep fade will cause a large error in the computed average signal strength. The effects of a sample taken during a deep fade can be alleviated somewhat by taking a large number of samples over a relatively long time interval. However, in radio systems where it is necessary to quickly measure the signal strength of many different RF signals subject to Rayleigh fading, the average of a small number of signal strength samples will not accurately reflect the true average signal strength due to the errors introduced by samples taken during deep fades. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide an improved method and apparatus for accurately measuring in a relatively short time period the strength of an RF signal subject to rapid and deep fading. 
     It is another object of the present invention to provide an improved method and apparatus for accurately measuring the signal strength of a plurality of RF signals that are being received by a plurality of antennas. 
     Briefly described, the present invention encompasses a method for measuring the signal strength of an RF signal that is subject to Rayleigh fading. The inventive method comprises the steps of sampling the strength of an RF signal N times over a predetermined time interval, where N is an integer number greater than one, and selecting the sampled signal strength having the largest magnitude. The sample having the largest magnitude is a good estimate of the signal strength, since the true average signal strength is reasonably close to the peak signal strength and is relatively unaffected by the Rayleigh fading. Improved apparatus for measuring the strength of an RF signal comprises a receiver for receiving an RF signal, circuitry for sampling the received RF signal N times over a predetermined time interval, where N is an integer number greater than one, and circuitry for selecting the sampled signal strength having the largest magnitude. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a radio communications system that may advantageously utilize the signal strength measuring apparatus of the present invention. 
     FIG. 2 is a block diagram of a scanning receiver including signal strength measuring apparatus embodying the present invention. 
     FIG. 3 illustrates the envelope of an RF signal that is experiencing Rayleigh fading. 
     FIG. 4 is a flow chart illustrating the signal strength measuring method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is illustrated a cellular radiotelephone communications system of the type described in U.S. Pat. Nos. 3,663,762, 3,906,166; in an experimental cellular radiotelephone system application filed under Docket No. 18262 with the Federal Communications Commission by Motorola and American Radio-Telephone Service, Inc. in February 1977; and more recently in a system description entitled &#34;Motorola DYNATAC Cellular Radiotelephone Systems&#34;, published by Motorola, Inc., Schaumburg, Ill., in 1982. Such cellular systems can provide telephone coverage to both mobile and portable radiotelephones located throughout a large geographical area. Portable radiotelephones may be of the type described in U.S. Pat. No. 3,962,553 and 3,906,166 and in U.S. patent application Ser. No. 187,304 (now U.S. Pat. No. 4,486,624, filed Sept. 15, 1980, invented by Larry C. Puhl et al, entitled &#34;Microprocessor Controlled Radiotelephone Transceiver&#34;, and assigned to the instant assignee; and mobile radiotelephones may be of the type described in Motorola instruction manual number 68P81039E25, published by Motorola Service Publications, Schaumburg, Ill., 1979. The geographical area may be subdivided into cells 10, 20 and 30, which each may include a base station radio 111, 121 and 131 and an associated base site controller 112, 122 and 132. Base site controllers 112, 122 and 132 are each coupled by data and voice links to a radiotelephone control terminal 140, which may be similar to the terminals described in U.S. Pat. Nos. 3,663,762, 3,764,915, 3,819,872, 3,906,166 and 4,268,722. These data and voice links may be provided by dedicated wire lines, pulse-code-modulated carrier lines, microwave radio channels, or other suitable communication links. Control terminal 140 is in turn coupled to a conventional telephone central office 150 for completing telephone calls between mobile and portable radiotelephones and landline telephones. 
     Each base station radio 111, 121 and 131 in FIG. 1 includes a plurality of transmitters and receivers for operating on at least one duplex signalling channel and a plurality of duplex voice channels. Base station 111, 121 and 131 may be of the type described in Motorola instruction manual number 68P81060E30, published by Motorola Service Publications, Motorola, Inc., Schaumburg, Ill., in 1982. Base station radios 111, 121 and 131 are located substantially at the center of each of the corresponding cells 10, 20 and 30. The base station transmitters may be combined onto one omni-directional antenna, while the base station receivers may be intercoupled to two or more directional or omni-directional sector antennas. In FIG. 1, the base station radios 111, 121 and 131 each include six 60° sector antennas. Each sector antenna primarily covers a portion of a cell, such as portion 21 of cell 20, and typically has a coverage area that overlaps the coverage area of adjacent sector antennas. Since the signalling channel requires an omni-directional receiving pattern, the signals received by the six sector antennas may be combined by a maximal ratio predetection diversity combiner, as illustrated and described in the instant assignee&#39;s co-pending U.S. patent applications: Ser. No. 22,757, (now U.S. Pat. No. 4,369,520) filed on Mar. 22, 1979, entitled &#34;Instantaneously Acquiring Sector Anntena System&#34; and invented by Frank J. Cerny, Jr. and James J. Mikulski; and Ser. No. 268,613, filed on June 1, 1981, now abandoned in favor of Ser. No. 493,802, filed on May 12, 1983, which is also now abandoned in favor of Ser. No. 580,775, filed on Feb. 21, 1984, entitled &#34;Large Dynamic Range Multiplier for a Maximal-Ratio Diversity Combiner&#34;, and invented by Frank J. Cerny, Jr. Furthermore, coverage of a portion of a cell may be provided by combining the signals received by two or more of the sector antennas. The sector antennas and associated receiving apparatus may be of the type described in U.S. Pat. Nos. 4,101,836 and 4,317,229. 
     In order to determine whether or not a mobile or portable radiotelephone leaves one cell, e.g. 20, and enters another, e.g. 10 or 30, the signal strength of the mobile or portable radiotelephone must be monitored by the base site controller 122. When the signal stength of a mobile or portable radiotelephone becomes too weak, the mobile or portable radiotelephone can be handed off to a base site controller 112 or 132 in another cell. Hand off involves transferring a particular mobile or portable radiotelephone from a duplex voice channel in one cell to a duplex voice channel in another cell. By utilizing the present invention, the strength of an RF signal can be quickly and accurately measured for each operating mobile and portable radiotelephone so that a particular mobile or portable radiotelephone can be reliably handed off before communications with it are degraded or interrupted due to weak signal conditions. 
     Since the RF signal frequencies utilized in the cellular system in FIG. 1 are typically above 800 mHz, the RF signals are subject to random, deep and rapid fading, commonly referred to as Rayleigh fading. Thus, the problem of accurately determining the strength of RF signals from mobile and portable radiotelephones is complicated not only by the fact that there may be a large number of mobile and portable radiotelephones to monitor, but also by the fact that the RF signal from the mobile and portable radiotelephones is subject to Rayleigh fading. In other words, the base site controllers 112, 122 and 132 must be capable of rapidly and accurately measuring the signal strength from a large number of mobile and portable radiotelephones to prevent communications with them from being unacceptably degraded or interrupted. By utilizing the present invention, a reasonably accurate measure of the signal strength from operating mobile and portable radiotelephones can be obtained by sampling the strength of the RF signal from each mobile and portable radiotelephone at least two times during a predetermined time interval and selecting the sampled signal strength for each that has the largest magnitude. As illustrated by the RF signal envelope in FIG. 3, the peak signal strength for each mobile and portable radiotelephone is a reasonably accurate measure of the signal strength since the peak signal strength is relatively close to the true average signal strength. Moreover, the peak signal strength is closer to the true average signal strength than an average of a number of signal strength samples that includes a sample taken in a null. Thus, according to the present invention, a scanning receiver can be coupled to the sector antennas of each base station radio 111, 121, and 131 for periodically measuring the peak signal strength of each operating mobile and portable radiotelephone in each cell 10, 20 and 30. 
     Referring to FIG. 2, there is illustrated a scanning receiver embodying the present invention. The scanning receiver may be part of base station radios 111, 121 and 131 in FIG. 1. The scanning receiver in FIG. 2 includes an antenna selector 102 that is coupled to each of six sector antennas of the base station radio. Binary input signals applied to antenna selector 102 by peripheral interface adapter (PIA) 108 cause antenna selector 102 to couple a selected one of the six sector antennas to receiver 104. The sector antennas and antenna selector 102 may be any conventional apparatus, such as that described in the aforementioned U.S. Pat. Nos. 4,101,836 and 4,317,229. 
     Receiver 104 may be a conventional synthesized receiver (such as the scanning receiver in the aforementioned Motorola instruction manual no. 68P81060E30), whose frequency of reception may be determined by an eleven-bit data signal applied to receiver 104 by PIA 108. The eleven-bit data signal is strobed into a register (not shown) of receiver 104 by a strobe signal from PIA 108. An output from receiver 104, which is proportional to the instantaneous strength of the RF signal being received (eg., such as the output of an envelope detector), is coupled to analog-to-digital converter (ADC) 106, which converts the magnitude of the receiver output to an eight-bit data signal. ADC 106 is controlled by PIA 110, which applies a start signal to ADC 106 for initiating the analog-to-digital conversion process. ADC 106 is also coupled to 600 kHz oscillator 112, and completes a conversion in approximately 120 microseconds. When the analog-to-digital conversion is complete, a complete signal is applied to PIA 110 by ADC 106. ADC 106 may be any suitable conventional analog-to-digital converter, such as, for example, an ADC0803 manufactured by National Semiconductor Corp. 
     The operation of the scanning receiver in FIG. 2 is controlled by microprocessor 114. Microprocessor 114 may be any sutiable conventional microprocessor, such as, for example, a Motorola type MC6800 microprocessor. At predetermined time intervals, microprocessor 114 executes the flow chart in FIG. 4 for measuring the signal strength of all active mobile and portable radiotelephones in its cell. Upon completion of the flowchart in FIG. 4, microprocessor 114 has measured and stored the signal strength for each active mobile and portable radiotelephone and has determined which of the six sector antennas is receiving the strongest RF signal from each active mobile and portable radiotelephone. 
     Microprocessor 114 is intercoupled to PIA&#39;s 108 and 110, read only memory (ROM) 116, random access memory (RAM) 118 and data interface 120 by way of data bus 124, address bus 126 and timing signals 128. ROM 116 stores a control program including subroutines for measuring the signal strength of active mobile and portable radiotelephones, such as the subroutine in the Appendix hereinbelow. RAM 118 is a scratch pad memory used during execution of the control program amd subroutines. Data interface 120 provides a conventional data link between the scanning receiver in FIG. 2 and other processing circuitry in the base site controllers 112, 122 and 132 in FIG. 1. Microprocessor 114, PIA&#39;s 108 and 110, ROM 116, RAM 118, data interface 120 can each be provided by conventional integrated circuit devices, such as those described in the &#34;Motorola Microprocessor Data Manual&#34;, published by the MOS Integrated Circuits Group of the Microprocessor Division of Motorola, Inc., Austin, Tex. in 1981. For example, microprocessor 114 may be a Motorola type MC6800 microprocessor, PIA&#39;s 108 and 110 a Motorola type MC6821 peripheral interface adapter, ROM 116 a Motorola type MCM68A316 2KX8 ROM, RAM 118 two Motorola type MCM2114 1KX4 RAM&#39;s, and data interface 120, a Motorola type MC6850 or MC6852 data adapter. 
     Referring to FIG. 4, a flow chart of the control program for microprocessor 114 in FIG. 2 is illustrated. The flow chart may be executed periodically with all of the transmitting frequencies of active mobile and portable radiotelephones. Entering at START block 402, and proceeding to block 404, one of the active frequencies is loaded into receiver 104 in FIG. 2. Next, at block 406, the SCAN subroutine in the Appendix hereinbelow is executed. The SCAN subroutine samples each sector antenna a number of times determined by the variable COUNT and stores the largest sample for each sector antenna sequentially beginning at address IDAT. The scanning is repeated twenty-five times to provide some time diversity between signal strength measurements for each sector antenna. Then, at block 408, the sector antenna having the largest sample is identified by comparing the six samples stored at addresses IDAT, IDAT+1, IDAT+2, IDAT+3, IDAT+4 and IDAT+5. The selected sector antenna and its sampled signal strength may be stored in a location associated with the mobile or portable radiotelephone operating on the particular frequency. Next, at decision block 410, a check is made to see if all operating frequencies have been scanned. If all operating frequencies have been scanned, YES branch is taken to RETURN block 412. Otherwise, NO branch is taken to block 404 for scanning the next operating frequency. 
     In summary, an improved method and apparatus for accurately measuring in a relatively short period of time the strength of an RF signal subject to Rayleigh fading has been described. The inventive method and apparatus is particularly well adapted for use in cellular radiotelephone systems, where it necessary to quickly and accurately measure the signal strength of mobile and portable radiotelephones so that communications with them can be maintained as they move from cell to cell. 
     APPENDIX 
     Table I hereinbelow illustrates a suitable program for the SCAN subroutine referred to in block 406 in the flow chart in FIG. 4. The program is coded in mnemonic instructions for the Motorola type MC6800 microprocessor, which mnemonic instructions can be assembled into machine code instructions by a suitable assembler. The mnemonic instructions and operation of the Motorola type MC6800 microprocessor are described in further detail in a publication entitled, &#34;Programming the 6800 Microprocessor&#34;, by R. W. Southern, published in 1977 and available from Motorola Semiconductor Products Inc., Literature Distribution Center, P.O. Box 20924, Phoenix, AZ 85036. 
     
                                           TABLE I                                 
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SCAN SUBROUTINE                                                           
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*    THIS SUBROUTINE TAKES SIGNAL STRENGTH MEASURE-                       
*    MENTS FROM SIX SECTOR ANTENNAS AND STORES THE                        
*    LARGEST VALUE FOR EACH SECTOR. TWENTY FIVE                           
*    SCANS THROUGH THE SIX SECTORS ARE TAKEN                              
0100             ORG  $100                                                
D000       PIA1  EQU  $D000 SIGNAL STRENGTH                               
                            MEASUREMENT                                   
E002       PIA2  EQU  $E002 SECTOR SELECTION                              
0100       IDAT  RMB  6     TABLE OF SIGNAL                               
                            STRENGTH MAXIMA                               
0106       COUNT RMB  2     NO. OF SAMPLES                                
                            PER SECTOR                                    
0108       TABAD RMB  2     DATA ADDRESS IN                               
                            TABLE                                         
010A       SECTOR                                                         
                 RMB  1     SECTOR SELECTED                               
010B                                                                      
   00 50   DEND  FDB  $50   WAIT INTERVAL                                 
010D       CNT   RMB  1     SCAN NUMBER                                   
*                                                                         
*    SELECT SECTOR AND KEEP TRACK OF THE NUMBER                           
*    OF SCANS THROUGH SIX SECTORS                                         
*                                                                         
1000       ORG   $1000                                                    
1000                                                                      
   CE 01 00                                                               
           SCAN  LDX  #IDAT BEGINNING                                     
                            ADDRESS                                       
                            OF DATA                                       
1003                                                                      
   FF 01 08      STX  TABAD                                               
1006                                                                      
   6F 00         CLR  X     CLEAR TABLE                                   
                            OF MAXIMA                                     
1008                                                                      
   6F 01         CLR  1,X                                                 
100A                                                                      
   6F 02         CLR  2,X                                                 
100C                                                                      
   6F 03         CLR  3,X                                                 
100E                                                                      
   6F 04         CLR  4,X                                                 
1010                                                                      
   6F 05         CLR  5,X                                                 
1012                                                                      
   7F 01 0D      CLR  CNT                                                 
1015                                                                      
   C6 01         LDA B                                                    
                      #1    GENERATE FIRST                                
                            SECTOR NUMBER                                 
1017                                                                      
   20 1B         BRA  N2                                                  
1019                                                                      
   F6 01 0A                                                               
           M1    LDA B                                                    
                      SECTOR                                              
                            CALCULATE NEXT                                
                            SECTOR ADDRESS                                
101C                                                                      
   5C            INC B                                                    
101D                                                                      
   C1 06   M2    CMP B                                                    
                      #6    HAVE ALL SIX                                  
                            SECTORS                                       
                            BEEN SCANNED                                  
101F                                                                      
   23 13         BLS  N2                                                  
1021                                                                      
   CE 01 00      LDX  #IDAT REINITIALIZE                                  
                            POINTER TO TABLE                              
1024                                                                      
   FF 01 08      STX  TABAD                                               
1027                                                                      
   7C 01 0D      INC  CNT   HAVE 25 SCANS                                 
                            THROUGH THE                                   
                            6 SECTORS BEEN                                
                            COMPLETED?                                    
102A                                                                      
   B6 01 0D      LDA A                                                    
                      CNT                                                 
102D                                                                      
   81 19         CMP A                                                    
                      #25   NO                                            
102F                                                                      
   2D 01         BLT  M6                                                  
1031                                                                      
   39            RTS        YES                                           
1032                                                                      
   C6 01   M6    LDA B                                                    
                      #1                                                  
1034                                                                      
   F7 01 0A                                                               
           N2    STA B                                                    
                      SECTOR                                              
1037                                                                      
   F7 E0 02      STA B                                                    
                      PIA2  SELECT SECTOR                                 
103A                                                                      
   CE 00 00      LDX  #0                                                  
103D                                                                      
   08      WT    INX        WAIT FOR COMPLE-                              
                            TION OF SECTOR                                
                            SWITCHING                                     
103E                                                                      
   BC 01 0B      CPX  DEND                                                
1041                                                                      
   26 FA         BNE  WT                                                  
*                                                                         
*    TAKE SIGNAL STRENGTH SAMPLES ON SELECTED                             
*    SECTOR AND RETAIN LARGEST VALUE                                      
*                                                                         
1043                                                                      
   FE 01 08      LDX  TABAD GET MAXIMUM FROM                              
                            PREVIOUS SCAN                                 
1046                                                                      
   E6 00         LDA B                                                    
                      X                                                   
1048                                                                      
   FE 01 06      LDX  COUNT                                               
104B                                                                      
   B6 D0 00      LDA A                                                    
                      PIA1  CLEAR INPUT FLAG                              
104E                                                                      
   B6 D0 01                                                               
           LOOP  LDA A                                                    
                      PIA1+1                                              
                            WAIT FOR A/D                                  
                            CONVERSION                                    
                            COMPLETE                                      
1051                                                                      
   2A FB         BPL  LOOP                                                
1053                                                                      
   B6 D0 00      LDA A                                                    
                      PIA1  READ SAMPLE                                   
                            VALUE                                         
1056                                                                      
   11            CBA        IS NEW SAMPLE                                 
                            LARGEST?                                      
1057                                                                      
   23 01         BLS  N3    NO                                            
1059                                                                      
   16            TAB        YES                                           
105A                                                                      
   09      N3    DEX        HAVE ALL SAMPLES                              
                            BEEN TAKEN?                                   
105B                                                                      
   26 F1         BNE  LOOP                                                
105D                                                                      
   FE 01 08      LDX  TABAD STORE LARGEST                                 
                            SIGNAL STRENGTH                               
                            SAMPLE VALUE IN                               
                            THE TABLE                                     
1060                                                                      
   E7 00         STA B                                                    
                      X                                                   
1062                                                                      
   08            INX                                                      
1063                                                                      
   FF 01 08      STX  TABAD                                               
1066                                                                      
   7E 10 19      JMP  M1                                                  
1069             END                                                      
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