METHOD AND APPARATUS FOR MEASURING THE STRENGTH OF A RADIO FREQUENCY SIGNAL

Abstract: A method and apparatus for measuring the strength of a radio frequency (RF) signal subject to Rayleigh fading. The strength of the RF signal is sampled two or more times during a predetermined time interval and the sampled signal strength having the largest magnitude is selected. The selected signal strength is a reasonably accurate measure of the signal strength since the true average strength of a Rayleigh fading signal is close to its peak signal strength. The inventive method and apparatus is particularly well adapted for use in a scanning receiver located in a base station radio (111, 121, 131) of a cellular radiotelephone communications system. The scanning receiver includes an antenna selector (102) for selecting one of a plurality of sector antennas, an RF signal receiver (104) tunable to a plurality of different RF signal frequencies, an analog-to-digital converter (106) for converting RF signal strength samples to a binary data signal and a microprocessor (114) together with peripheral devices (108, 110, 112, 116, 118, 120) for controlling the operation of the antenna selector (102), RF signal receiver (104) and analog-to-digital converter (106).

Description: METHOD AND APPARATUS FOR MEASURING THE STRENGTH OP A RADIO FREQUENCY SIGNAL Related Patent Applications The present invention is related to co-pending U.S. patent application, attorney's docket no. CM-54H, entitled "Scanning and Assigning Duplex Radio Channels to Mobile and Portable Radiotelephone in Cellular Radio- telephone Communications System", 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 O PI 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 Figure 1 is a block diagram of a radio communica¬ tions system that may advantageously utilize the signal strength measuring apparatus of the present invention. Figure 2 is a block diagram of a scanning receiver including signal strength measuring apparatus embodying the present invention. Figure 3 illustrates the envelope of an RF signal that is experiencing Rayleigh fading. Figure 4 is a flow chart illustrating the signal strength measuring method of the present invention. Detailed Description of the Preferred Embodiment Referring to Figure 1, there is illustrated a cellular radiotelephone communications system of the type described in U.S. patent numbers 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 "Motorola DYNATAC Cellular Radiotelephone Systems", published by Motorola, Inc., Schaumburg, Illinois, in 1982. Such cellular systems can provide telephone coverage to both OMPI - r >* IPO mobile and portable radiotelephones located throughout a large geographical area. Portable radiotelephones may be of the type described in U.S. patent numbers 3,962,553 and 3,906,166 and in U.S. patent application number 187,304, filed September 15, 1980, invented by Larry C. Puhl et al, entitled "Microprocessor Controlled Radio¬ telephone Transceiver", 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, Illinois, 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. patent numbers 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, Illinois, 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 OMPI 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's co-pending U.S. patent applications: serial number 22,757, filed on March 22, 1979, entitled "Instantaneously Acquiring Sector Anntena System" and invented by Frank J. Cerny, Jr. and James J. Mikulski; and serial number 268,613, filed on June 1, 1981, entitled "Large Dynamic Range Multiplier for a Maximal-Ratio Diversity Combiner", 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. patent numbers 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 strength 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 Figure 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 Figure 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 Figure 1. The scanning receiver in Figure 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. patent numbers 4,101,836 and 4,317,229. Receiver 104 may be a conventional synthesized re- ceiver (such as the scanning receiver in the aforemen¬ tioned 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 d.etector), 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 Figure 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 Figure 4 for measuring the signal strength of all active mobile and portable radiotelephones in its cell. Upon completion of the flowchart in Figure 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'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 radio¬ telephones, 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 Figure 2 and other processing circuitry in the base site controllers 112, 122 and 132 in Figure 1. Microprocessor 114, PIA'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 "Motorola Microprocessor Data Manual" , published by the MOS Integrated Circuits Group of the Microprocessor Division of Motorola, Inc., Austin, Texas in 1981. For example, microprocessor 114 may be a OMPI ■ IP Motorola type MC6800 microprocessor, PIA'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's, and data interface 120, a Motorola type MC6850 or MC6852 data adapter. Referring to Figure 4, a flow chart of the control program for microprocessor 114 in Figure 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 Figure 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 radio¬ telephone 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 Figure 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, "Programming the 6800 Microprocessor", by R. . Southern, published in 1977 and available from Motorola Semiconductor Products Inc., Literature Distribution Center, P.O. Box 20924, Phoenix, AZ 85036. TABLE I SCAN SUBROUTINE * 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 SE002 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 1 04 1 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 DO 00 LDA A PIA1 CLEAR INPUT FLAG 104E B6 DO 01 LOOP LDA A PIA1+1 WAIT FOR A/D CONVERSION COMPLETE 1051 2A FB BPL LOOP 1053 B6 DO 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 OMPI

Claims: Claims 1. A method for measuring the strength of a radio frequency (RF) signal that is subject to Rayleigh fading, said method comprising the steps of: a) sampling the strength of the RF signal N times over a predetermined time interval, where N is an integer number greater than one; and b) selecting the sampled signal strength having the largest magnitude. 2. The method according to claim 1 , further in¬ cluding the step of repeating said steps a) and b) a predetermined time after the end of said predetermined time interval. 3. The method according to claim 1, further including the step of repeating said steps a) and b) during consecutive predetermined time intervals. CMPI , 4. A method of determining which of a plurality of antennas is receiving the strongest radio frequency (RF) signal subject to Rayleigh fading and transmitted from a remote station having an unknown location in a geographical area, each of said antennas being tuned to the same RF signal frequency and receiving RF signals from a portion of the geographical area, said method comprising the steps of: a) sampling the strength of the RF signal com- ponents received by each antenna N times during a predetermined time interval , where N is an integer num¬ ber greater than one; b) selecting the antenna for which at least one of its N sampled signal strengths has a magnitude greater than the magnitude of any of the N sampled signal strengths for all other antennas. O OMMPPII KA- IPO 5. The method according to claim 4, further in¬ cluding the step of repeating said steps a) and b) a predetermined time after the end of said predetermined time interval. 6. The method according to claim 4, further including the step of repeating said steps a) and b) during consecutive predetermined time intervals. 7. Apparatus for measuring the strength of a radio frequency (RF) signal that is subject to Rayleigh fading, comprising: means for receiving the RF signal; means for sampling the strength of the RF signal N times over a predetermined time interval, where N is an integer number greater than one; and means for selecting the sampled signal strength having the largest magnitude. OMPI 8. The signal strength measuring apparatus according to claim 7, further including a plurality of antennas each coupled to said receiving means, said sampling means sampling the strength of the RF signal received by each antenna N times over a predetermined time interval, and said selecting means selecting the antenna for which at least one of its N sampled signal strengths has a magnitude greater than the magnitude of any of the N sampled signal strengths for all other antennas. 9. The signal strength measuring apparatus according to claim 7 or 8, wherein said sampling means repeatedly takes N samples of the RF signal during consecutive predetermined time intervals. CMPI O

Inventors: MCLAUGHLIN MICHAEL JOSEPH, MCLAUGHLIN, MICHAEL, JOSEPH
Applicants: MOTOROLA INC, MOTOROLA, INC.
Application year: 1983
Main classification: H04B7
IPC codes: H04B7, H04B17