Radio communication apparatus having a received signal strength measuring function

A dual mode cellular radio communication apparatus having a received signal strength measuring function is disclosed. The measured strength of the signal may be corrected when the apparatus operates in the digital mode. The value of corrected strength may be used for indicating the received signal strength to a user of the apparatus and for reporting the received signal strength to a base station. Similarly, the measured strength of the signal may be corrected for indicating the received signal strength to a user of the apparatus when the apparatus operates in the analog mode.

This application is related by subject matter to copending U.S. application 
Ser. No. 07/800,426 entitled DUAL MODE CELLULAR RADIO COMMUNICATION 
APATUS HAVING AN ECHO CANCELLER EMPLOYED IN BOTH ANALOG AND DIGITAL 
MODES filed on Nov. 27, 1991 and incorporated herein by reference thereto. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the field of radio communication apparatus 
such as mobile telephones, portable telephones, cordless telephones, and 
the like. More specifically, the present invention relates to a radio 
mobile unit used in a radio communication system in which the strength of 
a received signal is measured by the mobile unit. 
2. Description of the Related Art 
As the number of subscribers in cellular radio systems increase, it is 
desirable to incorporate digital signal transmission methods into the 
analog cellular system that are presently available in order to transmit 
speech signals at a more efficient transmission rate. A system of this 
type is called a Dual Mode Cellular System. In such a system, speech 
signals may be selectively transmitted between a base station and a mobile 
unit over speech radio links in either analog or digital mode. In the 
analog mode transmission, the speech signals are modulated by an analog 
modulation method, for example, by frequency modulation (FM). 
In the digital mode transmission, the speech signals are encoded into 
digital signals and transmitted at a more efficient transmission rate. 
Still, in the digital mode, speech communication links may be established 
by the time division multiple access (TDMA) method between a mobile unit 
and a base unit. In the TDMA, signals to be transmitted to a particular 
mobile unit are transmitted in designated time slots of a radio channel. 
The technical standard for the dual mode cellular system is specified in 
"Dual-Mode Mobile Station-Base Station Compatibility Standard" published 
by the EIA (Electronic Industries Association) as IS-54. 
In a dual mode mobile unit served by such a dual mode cellular system, a 
received signal strength indicator may be employed as has been employed in 
a conventional analog-only mobile unit. If a received signal strength 
measuring circuit device which is common to both of the digital and analog 
mode transmission is used, the way in which the device measures the signal 
strength in the digital mode is the same as that in the analog mode. 
A deficiency of such a dual mode mobile unit is in that the signal strength 
measured by the unit varies depending on whether the unit operates in the 
analog mode or in the digital mode even if the unit is located in a 
position where it is a certain distance away from a base station. This is 
because the signals are received continuously in the analog mode while the 
signals are received intermittently in the digital mode owing to the TDMA 
method adopted in the digital mode. Therefore, even if the strength of 
received signals in the analog mode were substantially the same as that in 
the digital mode, an average value of the strength of signals measured in 
the digital mode is lower than that measured in the analog mode. 
Another problem is that more accurate received signal strength information 
is required to be transmitted to a base station serving the mobile unit, 
in accordance with the technical standard. Specifically, the received 
signal strength information has to be included in the MAHO (Mobile 
Assisted Handoff) control signal which is defined in the technical 
standard. The MAHO control signal is transmitted to the base station in 
the digital mode operation. However, the received signal strength 
indicator employed in the conventional analog-only mobile unit, which 
consists of an integrated circuit device, is not capable of accurately 
measuring the received signal strength. This is because the integrated 
circuit device individually has measurement characteristics of its own 
when the device measures the received signal strength. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the present invention to provide a radio 
communication apparatus wherein received signal strength may be accurately 
corrected. 
It is another object of the present invention to provide a cellular 
communication apparatus in which an accurate indication of the signal 
strength of signals received by the apparatus may be provided. 
It is still another object of the present invention to provide a dual mode 
radio cellular communication apparatus capable of appropriately measuring 
the signal strength of signals received by the apparatus regardless of its 
operational mode. 
It is still another object of the present invention to provide a dual mode 
cellular communication apparatus in which an appropriate value of the 
signal strength of signals received by the apparatus may be reported to a 
base station. 
To achieve one or more of the objects, as embodied and described herein, 
the radio communication apparatus according to one aspect of the present 
invention, which is used in a dual mode cellular radio system wherein 
speech signals are selectively transmitted in an analog mode or a digital 
mode over a radio link established between a base station and the radio 
communication apparatus, comprises a receiver for receiving radio 
frequency signals transmitted over the radio link, a measurement device, 
coupled to the receiver, for measuring the strength of the radio frequency 
signals received by the receiver and outputting an output signal 
corresponding to the measured strength of the radio frequency signals, and 
a controller for performing a correction operation on the output signal 
when the apparatus operates in the digital mode. 
The measurement device may output a digital signal corresponding to the 
measured strength of the radio frequency signals and the controller may 
comprise one or more memories for storing conversion data and a 
microprocessor for calculating a corrected input level on the basis of the 
digital signal and the conversion data. 
The apparatus may further comprise a display unit for displaying the 
received signal strength on the basis of a corrected output signal which 
is output from the controller when the apparatus operates in the digital 
mode, and on the basis of the output signal which is output from the 
measurement device when the apparatus operates in the analog mode. 
The apparatus may further comprise a transmitter coupled to the controller 
for transmitting a control signal, including a corrected output signal 
which is output from the controller, when the apparatus operates in the 
digital mode. 
The radio communication apparatus according to another aspect of the 
present invention, which is used in a dual mode cellular radio system 
wherein speech signals are selectively transmitted in an analog mode or a 
digital mode over a radio link established between a base station and the 
radio communication apparatus, comprises a receiver for receiving radio 
frequency signals transmitted over the radio link, a measurement device 
coupled to the receiver for measuring the strength of the radio frequency 
signals received by the receiver and outputting an output signal 
corresponding to the measured strength of the radio frequency signals, a 
first memory for storing first conversion data for digital mode signals, a 
second memory for storing second conversion data for analog mode signals, 
and a microprocessor coupled to the measurement device and selectively 
coupled to either of the first memory or the second memory for calculating 
a corrected received signal strength on the basis of the output signal and 
the first conversion data when the apparatus operates in the digital mode, 
and for calculating a corrected received signal strength on the basis of 
the output signal and the second conversion data when the apparatus 
operates in the analog mode. 
The radio communication apparatus according to still another aspect of the 
present invention, which is used in a cellular radio system wherein speech 
signals are transmitted over a radio link established between a base 
station and the radio communication apparatus, comprises a receiver for 
receiving radio frequency signals transmitted over the radio link, a 
measurement device having individual measurement characteristics, and 
coupled to the receiver, for measuring the strength of the radio frequency 
signals received by the receiver and outputting an output signal 
corresponding to the measured strength of the radio frequency signals, a 
memory for storing conversion data which are stored in accordance with the 
individual measurement characteristics of the measurement device, and a 
microprocessor, coupled to the measurement device and the memory, for 
calculating a corrected received signal strength on the basis of the 
output signal and the conversion data.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic diagram showing the arrangement of a dual mode type 
radio telephone system in which a dual mode cellular mobile unit according 
to one embodiment of the present invention operates. The dual mode type 
radio telephone system comprises a control station CS connected to a wired 
telephone network NW, a plurality of base stations BSl-BSn connected to 
the control station CS via respective cable lines CLl-CLn, and a plurality 
of mobile units PSl-PSm. The base stations BSl-BSn are located in radio 
zones El-En, each of which is called a cell. Each of the mobile units 
PSl-PSm may be connected via radio links to a base station in the radio 
zone where it is located. The base station is further connected to the 
wired telephone network NW via the control station CS from the base 
station. Thus, the mobile units may communicate with telephones connected 
to the wired telephone network. When a mobile unit moves to an adjoining 
radio zone in the event that a speech communication link has been 
established between the mobile unit and a base station, the control 
station CS performs control to switch the radio channels to those of the 
base station in the adjoining radio zone so that a user may keep talking 
over the switched radio link. The control operation is called a handoff. 
FIG. 2 is a block diagram of the mobile unit according to an embodiment of 
the present invention. The dual mode cellular telephone has two 
operational modes. One mode is an analog mode, and the other mode is a 
digital mode. 
Referring to FIG. 2, radio frequency (RF) signals are received at an 
antenna 1 and applied to a receiving circuit (RX) 3 through a duplexer 
(DUP) 2. In the RX 3, the RF signals which are within a frequency range of 
800-900 MHz are frequency demodulated into intermediate frequency (IF) 
signals by local oscillation signals from a synthesizer (SYN)4. A received 
signal strength indicator (RSSI) 6 measures the signal strength of 
received signals. This measurement may be implemented by a plurality of 
diodes which are connected to each output of plural stage of amplifiers as 
shown and described in U.S. Pat. No. 4,996,715 (incorporated herein by 
reference thereto). A magnitude of voltage corresponding to the measured 
signal strength is sent to a controller (CONT) 20 through an 
analog/digital (A/D) converter 18. 
In the digital mode, radio frequency signals received at antenna 1 are 
applied to RX 3 through DUP 2. In RX 3, the radio frequency signals are 
converted into intermediate frequency (IF) signals by local oscillation 
signals from a synthesizer 4. The IF signals are demodulated after frame 
and bit synchronization signals are acquired in digital 
modulator/demodulator (MOD) 7. The acquired synchronization signals are 
sent to CONT 20. MOD 7 extracts digital control signals out of the 
demodulated signals and sends the digital control signals to CONT 20. 
Digital speech signals included in the demodulated signals are applied to 
an equalizer (EQL)8. Signal equalization of the demodulated signals is 
performed therein. The output of EQL 8 is coupled to an error correction 
coder/decoder (CH-COD) 9. 
CD-COD 9 deinterleaves the applied signals, performs error detection and 
correction on the deinterleaved signals by using a Cyclic Redundancy Cheek 
(CRC) technique and a convolutional coding technique. The output of CH-COD 
9 is coupled to a speech coder/decoder (SP-COD) 10. In SP-COD 10, the 
applied digital speech signals are decoded using a prescribed decoding 
technique and converted into analog speech signals. The analog speech 
signals are converted into acoustic signals and output from speaker 13. 
Speech signals from microphone 14 are reversely processed by SP-COD 10, 
CH-COD 9, and MOD 7 and transmitted by a transmitter (TX) 5 through DUP 2 
and antenna 1. 
In the analog mode, an analog audio circuit (A-AUD) 15 converts the 
demodulated signals output from RX 3 into analog speech signals and 
applies the analog speech signals to speaker 13. Also, A-AUD 15 converts 
speech signals applied from microphone 14 into IF signals and inputs the 
speech signals to TX 5. 
Operational panel (OP) 21 having a display unit 210 is provided for 
entering a telephone dial number, a call origination command, etc. Power 
supply 31 consists of a battery 30 to supply each section of the telephone 
with electric power. 
Then, a switch 11 and a switch 12 are provided to switch over the 
operational mode between two modes: an analog mode and a digital mode 
under the control of CONT 20. In the digital mode, switch 11 connects the 
output of SP-COD 10 to speaker 13 and switch 12 connects microphone 14 to 
this input of SP-COD 10. In the analog mode, switch 11 connects the output 
of an analog audio circuit (A-AUD) 15 to a speaker 13, and switch 12 
connects microphone 14 to the input of A-AUD 15. 
FIG. 3 is a detailed block diagram of the RX 3, RSSI 6, MOD 7, and CONT 20 
shown in FIG. 2. 
The RF signals are amplified in an amplifier 31. The amplified RF signals 
which are within a frequency range of 800-900 MHz are mixed with a first 
local oscillation signal from synthesizer 4 in a first mixer 32 and pass 
through a first band pass filter 33. The output signals of the first band 
pass filter 33 are signals of frequency range of 84 MHz (first IF 
signals). Similarly, the first IF signals are mixed with a second local 
oscillation signal from synthesizer 4 in a second mixer 34 and pass 
through a second filter 35. The output signals of the second band pass 
filter 35 are signals of frequency range of 450 KHz (second IF signals). 
In either of the analog mode or the digital mode, the received signals are 
demodulated into the second IF signals in the same way. 
In the digital mode, the second IF signals are applied to an amplifier 36 
in which the second IF signals are variably amplified. In other words, the 
gain of amplifier 36 is automatically controlled by a feedback loop. The 
second IF signals are mixed with a third local oscillation signal from 
synthesizer 4 in a third mixer 37 and converted into signals of frequency 
range of 60 KHz. The output of third mixer 37 is coupled to an 
analog/digital (A/D) converter 71 in MOD 7. The A/D converter 71 converts 
the third IF signals into the digital signals. The digital signals are 
subject to bit synchronization and frame synchronization processing in a 
MODEM portion 72 of MOD 7 as described above. The output signals of MODEM 
portion 72 are coupled to EQL 8 as described above in reference to FIG. 2. 
In the analog mode, the second IF signals are transmitted to a limiter 38. 
Limiter 38 may pass digital control signals and analog speech signals but 
not digital speech signals. The output signals of limiter 38 are applied 
to a frequency modulation demodulator (FM-DEM) 16 in A-AUD 15 shown in 
FIG. 2. FM-DEM 16 demodulates the applied signals. The output signals of 
FM-DEM 16 are amplified by a low frequency amplifier 17, and applied to 
speaker 13 through switch 11. 
The second IF signals are also applied to RSSI 6. RSSI 6 outputs an 
amplitude of voltage corresponding to the second IF signals. The amplitude 
of voltage is converted into a digital signal in an analog/digital (A/D) 
converter 18. The output of A/D converter 18 sends the digital signal 
corresponding to the received signal strength to CONT 20. 
CONT 20 comprises a microprocessor 201, an I/O interface 202, a ROM 23, and 
a RAM 24. CONT 20 controls various operations of the apparatus such as 
establishment of radio links. Microprocessor 201 determines the mode of 
the apparatus, either the analog mode or the digital mode, on the basis of 
a mode designation signal included in the control signals which are 
transmitted from a base station and which also designate the speech 
channel. When the mobile unit receives a mode designation signal over a 
control channel, a speech radio link is established for transmission of 
the speech signals, and microprocessor 201 performs certain correction 
operations against the digital signals representative of the received 
signal strength which are applied from A/D converter 18. 
FIG. 4 is a flow chart for explaining a received signal strength indicator 
operation of the apparatus in accordance with an embodiment of the present 
invention. 
First, on the basis of the mode designation signal included in the control 
signals, the apparatus determines whether an analog mode or a digital mode 
is designated (STEP 401). 
When the apparatus receives an analog mode designation signal, the 
apparatus tunes to an analog speech channel designated by a speech channel 
designation signal which is transmitted with the analog mode designation 
signal (step 402). The strength of the signals which are received through 
the analog speech channel is measured by RSSI 6 (step 403). The output 
signal of RSSI 6 is converted into a digital signal indicative of the 
signal strength. The digital signal is applied to microprocessor 201 in 
CONT 20 via I/O interface 202. 
Microprocessor 201 stores fifty samples of the digital signal indicative of 
the signal strength. Then, microprocessor 201 calculates an average value: 
RD-av of the fifty samples of digital signal (step 404), and calculates a 
corrected input level: L-av on the basis of the average value: RD-av and 
conversion data which are prestored in ROM 23 and a first portion of RAM 
24 (step 405). Details of the calculation will be described hereinafter. 
Upon completion of the calculation, microprocessor 201 controls display 
unit 210 so that the signal strength indication corresponding to the 
corrected input level: L-av is made (step 406). 
When the apparatus receives a digital mode designation signal, the 
apparatus tunes to a digital speech channel designated by a speech channel 
designation signal which is transmitted with the digital mode designation 
signal (step 407). The strength of the signals which are received through 
the digital speech channel is measured by RSSI 6 (step 408). The output 
signal of RSSI 6 is converted into a digital signal indicative of the 
signal strength. The digital signal is applied to microprocessor 201 in 
CONT 20 via I/O interface 202. 
Microprocessor 201 stores fifty samples of the digital signal indicative of 
the signal strength which are obtained from fifty TDMA slots assigned to 
the apparatus. Then, microprocessor 201 calculates an average value: RD-av 
of the fifty samples of digital signal (step 409) and calculates a 
corrected input level: L-av on the basis of the average value: RD-av and 
conversion data which are prestored in ROM 23 and a first portion of RAM 
24 (step 410). Details of the calculation will be described hereinafter. 
Upon completion of the calculation, microprocessor 201 controls display 
unit 210 so that the signal strength indication corresponding to the 
corrected input level: L-av is made (step 406). 
FIG. 5 shows the conversion data prestored in ROM 23. When the apparatus is 
assembled, the ROM 23 storing the conversion data is incorporated into the 
apparatus. In ROM 23, values of nine input levels: L1-L9 corresponding to 
nine points: P1-P9 are stored. In this instance, the input level means a 
signal level (strength) of signals which are substantially applied to RX 
3. 
FIG. 6 shows the conversion data prestored in RAM 24 which is preferably an 
EEPROM. After the apparatus is assembled in a factory, the conversion data 
are entered into RAM 24 in its data entry mode operation. In the 
operation, nine input levels: L1-L9, which are defined in ROM 23, of radio 
frequency signals are applied to RX 3, in either of the digital mode and 
analog mode transmission, by using a simulator device. Resultant outputs 
of A/D converter 18 are stored into the first portion for the digital mode 
signals and the second portion for analog mode signal in RAM 24 as RSSI 
data: RD1-9. Thus, ROM 23 and RAM 24 storing the conversion data 
constitute a conversion table for converting the actually measured signal 
strength to an appropriate input level. Accordingly, the conversion data 
of the conversion table is in accordance with the individual measurement 
characteristics of the RSSI 6. 
FIG. 7 shows the relationship between the input levels: L1-L9 and the 
resultant signal strength measured by RSSI 6 (which is output of RSSI 6). 
As appreciated from FIG. 7, the relationship is not linear due to the 
measurement characteristic of an integrated circuit device constituting 
RSSI 6. That is why correction operation on the actually measured signal 
strength, which is one aspect of the present invention, is necessary. 
Returning to step 405 and 410 in FIG. 4, L-av may be calculated as follows. 
FIG. 8 is a diagram for explaining the calculation of L-av. On the basis 
of calculation result of RD-av, microprocessor 201 determines RDm and RDn 
between which the RD-av is positioned by referring to the table stored in 
the first portion or the second portion of RAM 24. Next, on the basis of 
the points Pm and Pn corresponding to the RDm and RDn, microprocessor 201 
determines Lm and Ln by referring to the table stored in ROM 23. Then, 
microprocessor 201 calculates L-av in accordance with the following 
equation. 
EQU L-av=Ln+(Lm-Ln)*(RD-av-RDm)/(RDm-RDn) 
FIG. 9 shows a visual appearance of display unit 210 provided in 
operational panel 21. Responsive to the application of the corrected input 
level: L-av, display unit 210 determines which one of the four levels (as 
shown 211 in FIG. 9) the L-av corresponds to and drives LCD (Liquid 
Crystal Display) therein in accordance with the determination. 
Also, in the digital mode operation, microprocessor 201 provides the 
corrected input level: L-av to MOD 7 via I/O interface 202 so that MAHO 
control signal including the L-av may be transmitted to a base station 
serving the mobile unit. 
It should be noted that since the received signal strength information is 
not required to be transmitted to a base station in the analog mode 
operation according to the technical standard, the above mentioned 
correction operation on the signal strength measured by RSSI 6 may be 
omitted in the analog mode operation. In this event, the operational steps 
404 and 405 in FIG. 4 is skipped. 
It should be obvious from the above discussed embodiment that non-linear 
measurement characteristics of RSSI 6 may be compensated by referring to 
the conversion table in ROM 23 and RAM 24. Also, since the conversion data 
are provided for either of the digital mode signals and the analog mode 
signals in the first and second portion of RAM 24, accurate signal 
strength measurement may be achieved. 
Although the embodiment of the present to a dual mode cellular mobile unit 
used in a dual mode cellular system have been described above, the present 
invention may be applied to any kind of radio communication apparatus 
having a received signal strength measuring function, for example, an 
analog-only cellular radio telephone, a digital-only cellular radio 
telephone, or a cordless telephone. 
It should be obvious from the above-discussed apparatus embodiment that 
numerous other variation and modification of the apparatus of this 
invention are possible, and such will readily occur to those skilled in 
the art. Accordingly, the scope of this invention is not to be limited to 
the embodiment disclosed, but is to include any such embodiments as may be 
encompassed within the scope of the claims appended hereto.