Automatic power control apparatus

Abstract: An automatic power control apparatus according to the present invention comprises a level varying unit (1) such as a variable amplifier for amplifying a modulated wave output from a transmitter at an amplification factor specified by an external device, a level detecting unit (2) such as a level detector circuit for detecting a power level of the modulated wave output from the level varying unit, a computing unit (21) such as a high-speed computing unit for, during a period other than a time slot assigned to the transmitter, computing a power level of a non-modulated wave which is varied by a modulated wave obtained by linearly modulating a carrier wave in response to a modulation signal generated based on digital data to be transmitted in the next time slot, and a comparing unit (3) such as a comparator circuit for comparing a power level, which is obtained by adding a difference between the power level computed by the computing unit and an average power level in the time slot to the average power level, with the power level detected by the level detecting unit. In the automatic power control apparatus according to the present invention, an amplification factor of the level varying unit (1) is adjusted so that the power levels compared by the comparing unit (3) become equal to each other during a transmission period of the non-modulated wave, and the level varying unit (1) is then operated at the amplification factor during a transmission period of the modulated wave. Furthermore, a sending level of sending data is set based on the sending data to be transmitted during one time slot.

Description: The present invention relates to an automatic power control apparatus for stabilizing a level of output power in a radio communication system such as a cellular radio telephone system. In a radio communication system such as a cellular radio telephone system, generally, automatic power control (referred to as APC hereinafter) is performed to stabilize a level of output power while the system is operating. Fig. 4 is a block diagram showing the arrangement of a conventional APC apparatus. In Fig. 4, reference numeral 1 denotes a variable amplifier for amplifying a transmit signal output from a transmitter (not shown). The transmit signal is supplied to, for example, an antenna (not shown) via the variable amplifier 1. The transmit signal is also supplied from the variable amplifier 1 to a level detector 2. The level detector 2 outputs a detection signal having a voltage corresponding to a power level of the transmit signal. The detection signal is then input to a comparator circuit 3 comprising a differential amplifier 31, a variable resistor VR, and resistors R1, R2 and R3. A reference voltage VB is also applied to the comparator circuit 3 from a variable power source 4. The comparator circuit 3 compares the voltage of the detection signal output from the level detector 2 with the reference voltage VB and then supplies a difference between the voltages to a control voltage delivering circuit 5 as a comparison output voltage. The control voltage delivering circuit 5, which is constituted of a Darlington circuit, amplifies the comparison output voltage output from the comparator circuit 3 and supplies the amplified voltage to the variable amplifier 1 as a control voltage of the amplifier 1. The variable amplifier 1 amplifies the transmit signal at a gain corresponding to the control voltage supplied from the control voltage delivering circuit 5. Consequently, the transmit signal output from the variable amplifier 1 is always stabilized so as to have a fixed power level which depends upon the reference voltage VB preset in the variable power source 4. Digital modulation for modulating an analog carrier by a digital signal has recently been popularized in a radio communication system. For example, quadrative amplitude modulation (referred to as QAM hereinafter) system is generally known as the digital modulation. The use of such digital modulation enables so-called time-sharing transmission. For effective use of channels, a time division multiplexing access (referred to as TDMA hereinafter) system has been adopted. A transmitter of a radio communication system to which the QAM and TDMA are applied, will be outlined. In the TDMA, the transmitter performs a transmission operation only during the time slots assigned to the transmitter itself. When the transmission operation is performed, a predetermined period t0, during which a non-modulated wave is output, is set in the initial part of each of the time slots as shown in Fig. 5(b), and the ARC apparatus automatically controls power so that the non-modulated wave is adjusted to have a predetermined level P during the period t0. In the QAM, generally, an instantaneous level of a modulated wave varies with digital data to be transmitted. More specifically, when the QAM is performed using a digital pulse, the digital pulse is caused to pass through a low-pass filter to restrict the bandwidth of the digital pulse. Since the digital pulse is expanded with respect to time by the restriction of bandwidth, the waveform of the pulse is influenced by its preceding and succeeding pulses. This means that the level of a non-modulated wave varies with a modulation signal of a modulated wave subsequent to the non-modulated wave. Since the ratio of an average level of a signal transmitted during the time slot to that of a signal transmitted during the non-modulated wave output period changes with each of the time slots, an error ΔP occurs in the average level between the time slots as shown in Fig. 5(b). Fig. 5(a) is a view showing time slots in which TDMA signals are generated, and the hatched portions in Fig. 5(a) show time slots assigned to the transmitter. Fig. 5(b) is a view showing the waveforms of transmit signals output from the variable amplifier 1. As described above, when the conventional APC apparatus is applied to a radio communication system wherein linear modulation such as the QAM is used as a modulation system and the TDMA is used as a multiplexing access system, an error occurs in average power every time slot, and automatic power control cannot be correctly performed. Prior art document EP-A-0 460 280 (published after the priority of the present application) disclose a transmitting power control circuit which comprises a control unit. This control unit recognizes the period of transmitting a voice signal and issues an instruction to a reference signal generating unit to output a lower output reference value during this period. A reference value generating unit has, as one output reference value, an output reference value corresponding to a lower output level in the higher power level of the adjacent power levels and an output reference value corresponding to a higher output level in the lower power level. As a result of it, the reference value generating unit prepares a less number of output reference values and thereby outputs the output reference value during transmission of data signal in each power level and the output reference value during transmission of voice signal. It is an object of the present invention to provide an automatic power control apparatus capable of controlling the average power levels in time slots to have the same constant value when the apparatus is applied to a radio communication system in which linear modulation such as QAM is used as a modulation system and a TDMA is used as a multiplexing access system. To solve this object the present invention provides an automatic power control apparatus as specified in claim 1. An automatic power control apparatus comprises: a level varying unit such as a variable amplifier for amplifying a modulated wave output from a transmitter at an amplification factor specified by an external device; a level detecting unit such as a level detector circuit for detecting a power level of the modulated wave output from the level varying unit; a computing unit such as a high-speed computing unit for, during a period other than a time slot assigned to the transmitter, computing a power level of a non-modulated wave which is varied by a modulated wave obtained by linearly modulating a carrier wave in response to a modulation signal generated based on digital data to be transmitted in the next time slot; and a comparing unit such as a comparator circuit for comparing a power level, which is obtained by adding a difference between the power level computed by the computing unit and an average power level in the time slot to the average power level, with the power level detected by the level detecting unit. In the automatic power control apparatus, an amplification factor of the level varying unit is adjusted so that the power levels compared by the comparing unit become equal to each other during a transmission period of the non-modulated wave, and the level varying unit is then operated at the amplification factor during a transmission period of the modulated wave. With the above configuration, an output level of a non-modulated wave transmitted to the initial part of each time slot is computed during a period other than the time slot, that is, during a vacant time slot. A control level of the non-modulated wave is variably set on the basis of the computed output level, and the level varying unit is operated at the amplification factor set during the transmission period of the non-modulated wave. Power control is thus performed based on the average power level in each time slot. The power control is not influenced by an error in the average power level. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a block diagram showing a circuit arrangement of an automatic control apparatus in a transmitter, according to a first embodiment of the present invention; Fig. 2(a) is a view showing time slots of the automatic power control apparatus, in which TDMA signals are generated; Fig. 2(b) is a view showing waveforms indicative of variations in transmit signals output from a variable amplifier of the apparatus shown in Fig. 1; Fig. 3 is a flow chart showing a control routine program activated by a CPU 12 of the apparatus shown in Fig. 1; Fig. 4 is a block diagram showing a configuration of a conventional automatic control apparatus in a transmitter; Fig. 5(a) is a view showing time slots of the conventional automatic control apparatus, in which TDMA signals are generated; Fig. 5(b) is a view showing waveforms of transmit signals of the conventional apparatus; Fig. 6 is a block diagram showing an arrangement of an automatic power control apparatus according to a second embodiment of the present invention; Fig. 7 is a time chart showing a location of time slot S1; Fig. 8 is a view showing ramp time periods in a plurality of consecutive symbols; Fig. 9 is a view showing a waveform of signal output from modulator 63 after ramp time period 802; Fig. 10 is a level control rank table stored in a ROM; Fig. 11 is a flowchart showing a transmission operation of the automatic power control apparatus according to the second embodiment of the present invention; and Fig. 12 is a block diagram showing an arrangement of an automatic power control apparatus according to a third embodiment of the present invention. An automatic power control (APC) apparatus according to an embodiment of the present invention, which is applied to a radio communication system wherein linear modulation such as quadrative amplitude modulation (QAM) is used as a modulation system and a time division multiplexing access (TDMA) is used as a multiplexing access, will be described. Fig. 1 is a block diagram showing a circuit arrangement of the APC apparatus using in transmitter. The descriptions of the elements denoted by the same numerals as those in Fig. 4 showing a conventional APC apparatus, are omitted. In Fig. 1, an encoder 11 digitally encodes a signal (e.g. base band signal), which is to be transmitted, into sending data. The sending data is input to a CPU 12 via an input port. The CPU 12 activates a control routine program which is constituted of a plurality of steps described later and then performs the following control in accordance with the program. The CPU 12 converts the input sending data into burst modulating data suitable for the TDMA. The burst modulating data is supplied to a modulator 13 through an output port and then undergoes quadrative amplitude modulation (QAM). The burst modulating data is finally converted into a transmit signal. The above is the arrangement of a transmitter. In Fig. 1, reference numeral 20 indicates an APC apparatus. The APC apparatus 20 has the following function in addition to those of the conventional APC apparatus shown in Fig. 4. The burst modulating data, which is transmitted in the initial part of the next time slot, is received from the CPU 12, and a convoluting operation is performed on the basis of the burst modulating data. This operation allows a computation of the power level of a non-modulated signal transmitted in the initial part of the next time slot. The APC apparatus 20 comprises a high-speed computing unit 21 and a sample/hold unit 22, arranged between a comparator circuit 3 and control voltage delivering circuit 5, for receiving a signal indicative of the timing of a time slot from the CPU 12 and sampling an output of the comparator circuit 3 at timing corresponding to the timing of the time slot. An operation of the APC apparatus 20 having the above configuration will be described with reference to Figs. 2(a) and 2(b). Fig. 2(a) shows time slots in which TDMA signals of the APC apparatus are generated, and Fig. 2(b) shows waveforms representing variations in transmit signals output from the variable amplifier 1. A transmit signal is supplied to an antenna through the encoder 11, CPU 12, modulator 13, and APC apparatus 20. The APC apparatus then performs the following APC operations. In a vacant time slot before a time slot assigned to the sending station, the high-speed computing unit 21 receives sending data, which is to be transmitted in the next time slot, from the CPU 12. The sending data is modulated by the modulator 13 into a non-modulated wave. The unit 21 computes a power level of the non-modulated wave. The unit 21 then compares the average level of the computed power level with the level of the non-modulated wave output during the period t0 and sets a voltage corresponding to a power level obtained by adding a difference between the computed power level and the level of the non-modulated wave to a desired value, i.e., a control desired power level of the transmit signal, to the variable power source 4 as a reference voltage. More specifically, since the level of the non-modulated wave is lower than the control desired power level P in time slot S1 in Fig. 2(a), a voltage corresponding to power P1 obtained by subtracting a difference ΔP1 between the levels from the control desired power level P is set to the variable power source 4. Since the level of the non-modulated wave is higher than the control desired power level P in time slot S2 in Fig. 2(a), a voltage corresponding to power P2 obtained by adding a difference Δ between the levels to the control desired power level P is set to the variable power source 4. During the non-modulated wave output period T0 of the initial part of the time slot assigned to the sending station, the same APC operation as that of the conventional APC apparatus is performed by the variable amplifier 1, level detector 2, comparator circuit 3, and control voltage delivering circuit 5 on the basis of the reference voltage. Therefore, the power level of the non-modulated wave output from the variable amplifier 1 is set to P1 and P2 in the time slots S1 and S2, respectively. At a predetermined time tS when the APC operation is sufficiently operated and the power level of the non-modulated wave is optimized, during the period T0, the sample/hold unit 22 samples an output voltage of the comparator circuit 3 and holds the output voltage during the time slot. A sampling pulse is generated in the sample/hold unit 22 in response to a signal representing the timing of the time slot supplied from the CPU 12. As described above, after the non-modulated wave output period T0, in other words, while the modulated wave is outputting, the gain of the variable amplifier 1 is fixed to the gain controlled during the period T0, and the transmit signals whose powers P1, P2, ... are set as reference power levels are output from the variable amplifier 1. The average power level in each of the time slots is always kept to power P (i.e. constant value), and an error in the average power level does not occur between the time slots as is apparent from Figs. 2(a) and 2(b). The present invention is not limited to the above embodiment. In the above embodiment, the variable amplifier is used as a level varying means. An amplifier whose amplification factor is constant and a variable attenuator for varying the level of a signal input to the amplifier can be used as the level varying means to vary the amplification factor of the variable amplifier by changing the attenuation factor of the variable attenuator. In the above embodiment, the variable power source is used as a source from which a reference voltage is generated, and the reference voltage is varied by controlling the variable source. If the comparator circuit 3 has the conventional configuration as shown in Fig. 4, the resistor R3 can be used as a variable low resistor to control the variable power source. In the above embodiment, the APC apparatus is applied to a radio communication system wherein the QAM is used as a modulation system. The APC apparatus can be applied to a radio communication system wherein the linear modulation is used. Various changes and modifications can be made without departing from the spirit or scope of the present invention. Fig. 3 shows a flow chart of a control routine program which is activated by the CPU 12 shown in Fig. 1. The control routine program is executed by the following steps: (1) Step S1 of inputting sending data to a memory (i.e., RAM); (2) Step S2 of converting the input sending data into a burst modulation signal; (3) step S3 of determining whether or not the burst modulation signal is the initial signal of a time slot; (4) Step S4 of transmitting the burst modulation signal to a high-speed computing unit when it is determined that the signal is the initial signal of the time slot; and (5) Step S5 of transmitting the burst modulation signal to a modulator when it is determined that the signal is not the initial signal of the time slot. The automatic power control apparatus according to the present invention comprises: a level varying unit such as a variable amplifier for amplifying a modulated wave output from a transmitter at an amplification factor specified by an external device; a level detecting unit such as a level detector circuit for detecting a power level of the modulated wave output from the level varying unit; a computing unit such as a high-speed computing unit for, during a period other than a time slot assigned to the transmitter, computing an average power level of a modulated wave, which is obtained by linearly modulating digital data to be transmitted in the next time slot, based on the digital data; and a comparing unit such as a comparator circuit for comparing a power level, which is obtained by adding a difference between the average power level computed by the computing unit and a power level of the non-modulated wave to the average power level, with the power level detected by the level detecting unit. Further, the automatic power control apparatus according to the present invention is so constructed that an amplification factor of the level varying unit is adjusted so that the power levels compared by the comparing unit become equal to each other during a transmission period of the non-modulated wave, and the level varying unit is then operated at the amplification factor during a transmission period of the modulated wave. Even if the automatic power control apparatus is applied to a radio communication system wherein linear modulation such as QAM is used as a modulation system and TDMA is used as a multiplying access system, the average power levels of the time slots can be controlled so as to have the same value. Fig. 6 is a block diagram showing an arrangement of an automatic power control apparatus according to a second embodiment of the present invention. A transmit signal (baseband signal) is encoded by an encoder 61 and then input to a CPU 62. The CPU 62 stores the transmit signal in a memory region thereof, converts it into burst modulating data suitable for the TDMA, and inputs the burst modulating data to a modulator 63 at predetermined timing. The modulator 63 subjects a carrier wave to the quadrative amplitude modulation (QAM) using the input data and then supplies a carrier wave signal (modulated signal) to a power amplifier 64. The power amplifier 64 amplifies the input data in accordance with a control voltage applied from a control voltage delivering circuit 65. A signal output from the power amplifier 64 is detected by a detector circuit 66. A detection signal output from the detector circuit 66 is thus input to a comparator circuit 67. A ROM 68 stores level control data for correcting a level control signal, which is transmitted from a base station, in accordance with sending data. The level control data is converted from a digital signal to an analog signal by a D/A converter circuit 69 and then input to the comparator circuit 67 through an operational amplifier. The comparator circuit 67 compares the detection signal with the level control data and outputs a signal representing a result of the comparison to a sample/hold circuit 71. The sample/hold circuit 71 holds a signal output from the comparator circuit 67 under the control of CPU 62 and inputs the signal to the control voltage delivering circuit 65. The control voltage delivering circuit 65 converts an output of the sample/hold circuit 71 into a voltage signal and supplies it to the power amplifier 64. A transmission operation in the TDMA system will be described in order to explain the level control data stored in the ROM 68. A communication channel having a predetermined frequency is divided into, for example, three time slots, and a signal is transmitted from a moving communication device to a base station device during one of the time slots. A guard time period 801 for preventing collision of signals transmitted from different moving communication devices and a ramp time period 802 for starting the transmission of the signals are formed at the leading portion of each of the time slots. The ramp time period 802 corresponds to time required for transmitting, for example, a 6-bit signal (i.e., three symbols). It is desirable that a reference transmission output level (i.e., reference level of transmission output signal which does not rely upon the value of the sending data) for the time slot be determined at the end of the ramp time period thereof. In the second embodiment of the present invention, automatic power control is performed between symbols 5 and 6 shown in Fig. 8. In Fig. 6, when the burst modulating data is sent to the modulator 63, the CPU 62 activates the sample/hold circuit 71 only while data from the symbols 5 to 6 is being transmitted, and the control voltage is applied to the power amplifier 64 on the basis of the detection signal and level control data. Fig. 9 shows a waveform of signal output from the modulator 63 after the ramp time period 802. The output signal rises during the ramp time period and then varies with the sending data. Since the rise of the output signal during the ramp time period greatly depends upon values of the first two symbols in the time slot of the sending data, automatic power control between the symbols 5 and 6 is performed in view of the values of the first two symbols. For this reason, a level control data is stored in the ROM 68, as shown in Fig. 10. In Fig. 10, the addresses in the lateral direction correspond to level control information (e.g., eight levels of 000 to 111 if expressed in binary notation). The eight levels of the level control information are values of signals transmitted from the base station and extracted among signals received by a receiver 72 of the moving communication device. The addresses in the longitudinal direction correspond to sending data (constituted of four bits of 0000 to 1111) of the first two symbols of the time slot. The level control data is stored in regions where the addresses in the lateral direction cross those in the longitudinal direction. The level control data is acquired before-hand on the basis of simulation. Fig. 11 is a flowchart showing a transmission operation of the automatic power control apparatus according to the second embodiment. First it is determined whether it is a transmission slot assigned to the apparatus (step 101). When the transmission slot is available for the apparatus, the CPU 62 reads out level control data from the ROM 68 based on the values extracted by the receiver 72 and the values of the first two symbols (four bits) to be transmitted in the transmission slot (step 102). Since the CPU 62 is so constructed that sending data for at least one slot is stored and then the data are sequentially input to the modulator 63, the values of the first two symbols in the slot can be known before the symbols are transmitted. The level control data read out from the ROM 68 is input to the D/A converter circuit 69 and converted into an analog signal. The analog signal is then supplied to the comparator circuit 67 through the operational amplifier 70. Next it is determined whether guard time has elapsed or not (step 103). If the guard time has elapsed, data sending is started up (step 104). The CPU 62 closes a switch 73 of the sample/hold circuit 67 when the fifth symbol in the slot is transmitted (step 105). Therefore, the feedback of the power amplifier 64 is related to its amplification factor, and the amplification factor is set on the basis of level control information corrected by the sending data of the first two symbols which greatly affects the wave-form of a signal rising in a time slot. The CPU 62 opens the switch 73 (step 108) when the sixth symbol is transmitted (step 107). The amplification factor of the power amplifier 64 is thus kept constant during the rest period of the time slot. After that, the sending data is amplified at the constant amplification factor and transmitted during the time slot. In the next time slot, new automatic power control is performed in a ramp time period of the slot, so that an amplification factor of the power amplifier 64 is set in the slot. Similarly, an amplification factor of the power amplifier 64 is set for each time slot. Fig. 12 is a block diagram showing an arrangement of an automatic power control apparatus according to a third embodiment of the present invention. In Fig. 12, the same structural elements as those in Fig. 6 are denoted by the same reference numerals. In the apparatus shown in Fig. 12, a variable attenuator 71 is arranged before the power amplifier 64, and an output signal of the sample/hold circuit 67 varies an attenuation factor of the variable attenuator 71. Therefore, a signal input to the power amplifier 64 changes with an output and level control data of the detector circuit 66. According to the present invention, since a reference power of transmission output signal is set every time slot before sending data is transmitted in the time slot, the reference power is not varied with the value of the sending data. Furthermore, since the reference power is set in view of an influence of the value of the sending data upon a rise of the transmission output signal in the time slot, even though the rise of the transmission output signal is varied with the sending data, a variation in the rise is compensated and thus the reference power is not varied every time slot.

Claims: An automatic power control apparatus for use in a radio communication system for modulating a carrier wave using sending data to be transmitted to generate a modulated signal and performing burst transmission during a time slot assigned to the automatic power control apparatus itself, comprising: setting means (2-5, 21, 22; 65-70, 73) for setting a transmission output power level in the time slot based on a value of sending data to be transmitted during the time slot; and transmitting means (1; 64, 71) for transmitting the sending data at the transmission level. The automatic power control apparatus according to claim 1, characterized in that said setting means (2-5, 21, 22; 65-70, 73) comprises storing means (4, 21; 68) for storing level control data obtained from a control signal for controlling the transmission output power level and a first one symbol or first two symbols of the sending data in the time slot, detecting means (2; 66) for detecting an output signal of said transmitting means (1; 64, 71), comparing means (3; 67) for comparing the level control data stored in said storing means (4, 21; 68) and the output signal of said detecting means (2; 66), said transmission level being set based on a comparison of said comparing means (3; 67). The automatic power control apparatus according to claim 1, characterized in that said transmitting means (1; 64, 71) comprises amplifying means (1; 64) for amplifying the modulated signal, and said setting means (2-5, 21, 22; 65-70, 73) sets an amplification factor of said amplifying means (1; 64) based on the value of the sending data to be transmitted during the time slot. The automatic power control apparatus according to claim 1, characterized in that said transmitting means (64, 71) comprises amplifying means (64) for amplifying the modulated signal and attenuating means (71) connected to an input of said amplifying means (64) for adjusting the modulated signal input to said amplifying means (64) and said setting means (65-70, 73) sets an attenuation factor of said attenuating means (71) based on the value of the sending data to be transmitted during the time slot.

Inventors: TSUTSUMI SHYUITSU C O INTELLEC, YAMADA TATSUYA C O INTELLECTUA, TSUTSUMI, SHYUITSU, C/O INTELLECTUAL PROP. DIV., YAMADA, TATSUYA, C/O INTELLECTUAL PROP. DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H04B7
IPC codes: H03G3