Conventionally, receiving devices, which receive radio waves including desired waves and interfering waves via antennas, carry out analog-to-digital conversion, extract desired waves by use of received signal strength indicators (RSSI), carry out digital demodulation, and carry out automatic gain control (AGC) to adjust the input levels of demodulators, have been developed and applied to base-station receiving devices in cellular and mobile communication systems, base-station receiving devices in fixed-line telecommunication networks, grand-station receiving devices in satellite communication systems, and broadcasting systems.
Patent Literature Document 1 discloses an automatic gain control method in a mobile orthogonal frequency division multiple access (OFDMA) network, which is characterized in that the power level of an analog baseband signal is adjusted based on the average power of cyclic prefixes which are calculated based on the received signal intensity. Patent Literature Document 2 discloses a receiving device which is able to substantially expand the dynamic range with the input and the output of an RSSI circuit, receive a wide range of signal levels, and transmit accurate reception levels. Patent Literature Document 3 discloses a receiving device preventing intermodulation, which reliably suppress intermodulation waves included in reception channels so as to effectively prevent a reduction of reception quality due to intermodulation. Patent Literature Document 4 discloses a broadcasting receiving device which effectively attenuates signal levels of interfering stations by use of AGC and RSSI so as to suppress received desired frequency signals from being reduced in levels. Patent Literature Document 5 discloses a receiving device which determines interfering waves based on RSSI of desired waves so as to change the linearity of a low-noise amplifier (LNA), thus reducing influences of interfering waves. Patent Literature Document 6 discloses an AGC circuit which applies an optimum gain to the power of desired waves irrespective of interfering waves being input to a receiver. Patent Literature Document 7 discloses a wireless cellular communication terminal which prevents wireless characteristics from being degraded by changing the capacitance of a capacitor connected to a detection and rectifier circuit depending on the existence or nonexistence of interfering waves or the magnitude of interfering waves. Patent Literature Document 8 discloses a receiver which carries out AGC and digital demodulation processes. Patent Literature Document 9 discloses a receiving device which carries out a software demodulation process without using AGC irrespective of fluctuations in reception levels. Patent Literature Document 10 discloses a receiver having an AGC circuit adapted to a digital CATV tuner, which is characterized by gaining optimum AGC characteristics irrespective of deviations of tuner's gains or deviations of gains between channels. Patent Literature Document 11 discloses a wireless receiving device which is able to reliably amplify desired signals while suppressing increasing power consumption at an RF frontend, which is characterized by using a DC-DC converter which controls power consumption of a low noise amplifier (LNA) based on the received signal strengths before and after a low pass filter. Patent Literature Document 12 discloses a receiving device employing a direct conversion system or a Low-IF system. Patent Literature Document 13 discloses a receiving module of a terrestrial digital television receiver which is able to suppress interfering waves due to unwanted waves of adjacent channels by applying AGC to an RF amplifier circuit based on the signal level after IF tuning. Patent Literature Document 14 discloses a receiving device having a high resistance against interfering waves, which determines the existence or nonexistence of interfering waves based on differences between RF input levels and IF input levels, thus improving a reception efficiency.
FIG. 4 shows a block diagram of a conventionally-known receiving device (see Patent Literature Document 2). The receiving device includes an antenna 1, a mixer 3, a local oscillator 4, an A/D converter 5, an analog variable gain function part 6, a channel selecting filter 8, a received signal strength indicator (RSSI) 9, a comparison controller 11, a demodulator 12, and a low noise amplifier (LNA) 13 with a variable gain function. The receiving device prevents saturation and secures backoff (i.e. a process needed to transmit modulation signals having peak components without distortions) in the analog-to-digital (A/D) converter 5 based on the direct conversion system, the single conversion system, or the multiple conversion system.
For this reason, the comparison controller 11 determines the level detected with the RSSI 9 (i.e. a circuit used to monitor the desired-wave level at the output side of the channel selecting filter 8) which is arranged at the output side of the A/D converter (ADC) 5, wherein the analog variable gain function part 6, arranged at the input part of the A/D converter 5, controls a gain reduction on desired waves exceeding the allowable threshold.
FIG. 5 is used to explain problems of the conventionally-known receiving device, wherein FIG. 5(a) is used to explain the level diagram and the AGC behavior in the case of the receiving device solely inputting desired waves, while FIG. 5(b) is used to explain the level diagram and the AGC behavior in the case of the receiving device inputting both low desired waves (close to the minimum sensitivity) and strong interfering waves.
When the level of a desired wave is increased from the minimum sensitivity in FIG. 5(a), the comparison controller 11 determines the level detected with the RSSI 9 in the AGC loop based on the level of a desired wave by use of a threshold, and therefore the analog variable gain function part 6 controls a gain reduction (−a [dB], −b [dB]) on a desired wave exceeding the threshold level in order to aim to prevent saturation and secure backoff in the A/D converter 5.
Since the level of a desired wave input to the demodulator 12 becomes higher than the minimum definition level in FIG. 5(a), it is possible to demodulate the desired wave while maintaining the carrier-to-noise (C/N) ratio to the noise level, which is needed to demodulate the desired wave. When a desired wave is solely input to the receiving device, it is possible to reduce the dynamic range which is required as the input level of the demodulator 12.
However, the following problem is raised when the receiving device receives a strong interfering wave and a low desired wave (close to the minimum sensitivity) as shown in FIG. 5(b). That is, when an interfering wave instead of a desired wave becomes dominant due to the increasing level of an interfering wave, the comparison controller 11 determines the level of an interfering wave detected with the RSSI 9 in the AGC loop, and therefore the analog variable gain function part 6 controls a gain reduction (−a [dB], −b [dB]) on the interfering wave whose level exceeds the threshold in order to aim to prevent saturation due to the interfering wave and to secure backoff in the A/D converter 5. In this case, the analog variable gain function part 6 controls a gain reduction (−a [dB], −b [dB]) on a low desired wave close to the minimum sensitivity similar to an interfering wave, and therefore the level of a desired wave will be decreased in the circuit portion subsequent to the A/D converter 5.
Additionally, due to a desired wave and an interfering wave passing through the channel selecting filter 8 following the analog-to-digital (A/D) converter 5, the level of an interfering wave is extremely attenuated and suppressed to be lower than the level of a desired wave, wherein the level of a desired wave input to the demodulator 12 remains lower than the minimum definition level as shown in FIG. 5(b), and therefore the original information is lost to cause extreme degradation of demodulation or disability of demodulation because the information of a desired wave below the minimum definition level of the demodulator 12 is cut out. the channel selecting filter 8 following the A/D converter 5, the level of an interfering wave is extremely attenuated and suppressed to be lower than the level of a desired wave, wherein the level of a desired wave input to the demodulator 12 remains lower than the minimum definition level as shown in FIG. 5(b), and therefore the original information is lost to cause extreme degradation of demodulation or disability of demodulation because the information of a desired wave below the minimum definition level of the demodulator 12 is cut out.
When the conventionally-known receiving device is used to receive and demodulate a low desired wave close to the minimum sensitivity along with a strong interfering wave, it is necessary to extremely increase the dynamic range necessary for the input level of the demodulator while increasing the number of bits necessary for the demodulator, thus increasing the circuit scale of the receiving device while increasing power consumption.
FIG. 6 shows a block diagram of another conventionally-known receiving device (see Patent Literature Document 1). The receiving device includes an analog block 22, an antenna 24, and a digital baseband part 32. The analog block 22 includes a band-pass filter (BPF) 26, a low noise amplifier (LNA) 28, a local oscillator (LO) 30, and amplifiers (VGA) 34, 36. The digital baseband part 32 includes a received signal strength indicator (RSSI) 38, a control logic part 40, and A/D converters 46, 48. Reference sign 42 shows enable pulses.
Patent Literature Document 4 discloses the receiving device which monitors and compares the levels before and after a channel selecting filter, which is arranged to suppress interfering waves, so as to compare the level difference and the allowable threshold, thus determining the existence or nonexistence of an interfering wave. Specifically, it is possible to determine the existence of an interfering wave due to a large difference between the levels before and after the channel selecting filter but to determine the nonexistence of an interfering wave due to a small difference between the levels.
For example, Patent Literature Document 4 discloses the broadcasting receiving device which determines that a small amount of radio waves (i.e. interfering waves) are being received via channels other than the currently serving channel due to a small difference between the levels before and after IF-BPF, and therefore it performs AGC via the current channel by controlling the gain of the variable-gain RF-AMP, preceding MIXER, based on the RSSI level before IF-BPF. On the other hand, the broadcasting receiving device determines that a large amount of radio waves are being received via channels other than the currently serving channel due to a large difference between the levels before and after IF-BPF, and therefore it performs AGC collectively via the other channels and the current channel by controlling the gain of the variable-gain RF-AMP, preceding MIXER, based on the level representing a plurality of reception channels which is calculated by subtracting the RSSI level after IF-BPF from the RSSI level before IF-BPF. Herein, it is possible to achieve an AGC function by applying a variable gain to RF-AMP in connection with desired waves which are detected with respect to the current channel alone or a plurality of channels including the current channel.
The present invention, which will be discussed later, detects the existence of a strong interfering wave based on a level difference of RSSI before the channel selecting filter while reducing the gain of the analog variable gain function part (i.e. a first AGC-controlled subject) preceding the A/D converter such that the output of the A/D converter, serving as an AGC-controlled subject in the minimum sensitivity reception and preceding channel selection, will not be distorted due to a strong interfering wave. In this case, the present invention increases the gain of the digital variable gain function part, serving as a second AGC-controlled subject, so as to correct the low level of a desired wave with the demodulation-enabled level.
Patent Literature Document 8 imports an idea into the AGC circuit including RSSI to receive desired waves with a wide range of levels even when high-level interfering waves are intermixed with desired waves. That is, it is possible to appropriately control the AGC gain entirely over the circuitry by activating an AGC operation by use of the processing result of the broadband RSSI1 based on the output of the A/D converter (ADC) reflecting desired waves and interfering waves and the processing result of the narrowband RSSI2 solely reflecting desired waves after being subjected to the band limitation of BPF following ADC. In this connection, Patent Literature Document 9 and Patent Literature Document 10 disclose prior art which aim at improvement of Patent Literature Document 8.
Patent Literature Document 8 discloses a method of extracting data by use of ADC at two points in different gain stages arranged in the reception IF system depending on the input level in consideration of the necessity of two systems for ADC.
Patent Literature Document 10 discloses a method of independently varying and controlling the AGC gain of RF (radio frequency) and the AGC gain of IF (intermediate frequency) in consideration of the gain control reference solely reflecting desired waves precluding interfering waves.
The first embodiment of Patent Literature Document 8 refers to the circuitry in which the broadband RSSI1 based on the ADC output reflecting desired waves and interfering waves is not interlocked with the narrowband RSSI2 based on the BPF output after ADC solely reflecting desired waves. First, due to inputting of desired waves and interfering waves having significantly high levels, it is necessary to set a source voltage or a gain of LNA without exceeding the full scale of ADC by way of the determination of RSSI1. Due to inputting of further high-level signals, it is necessary to control and reduce the gain of the preceding digital IF amplifier by way of the determination of the level of desired waves with RSSI2.
For the above reason, it is necessary to perform the RSSI1 operation without causing saturation of ADC. However, it is necessary to apply a variable gain to the digital IF amplifier interposed between the BPF and the demodulator in the RSSI2 operation in order to keep desired waves before the demodulator within the predetermined level without determining the existence or nonexistence of interfering waves (cf. Patent Literature Document 10 in which RSSI solely operates). This results from an expectation in which that the determination of RSSI2 will be performed with the level of a desired wave on the assumption that the level of an interfering wave has been adequately reduced by way of the band-limiting BPF. The present invention needs to compare differences between RSSI1 and RSSI2 so as to estimate the existence or nonexistence of interfering waves, thus controlling the digital AGC gain.
The second embodiment of Patent Literature Document 8 refers to an AGC control method using both of RSSI1 and RSSI2. The second embodiment is designed to control and greatly reduce the gain of LNA to prevent AGC saturation with respect to the very low level of a desired wave and the very high level of an interfering wave, whereas it may raise the fear of degrading the reception sensitivity due to an unnecessary reduction in the level of a desired wave. To prevent this drawback, Patent Literature Document 8 discloses a correction method of preventing a rapid reduction in a control voltage applied to LNA in order to alleviate a significant reduction in the gain of LNA which is controlled via the processing result of RSSI1.
Patent Literature Document 11 discloses the technology similar to the technology of Patent Literature Document 5. It is possible to detect the existence of interfering waves and the levels of interfering waves by comparing differences between the level of RSSI1 (interfering waves+desired waves), preceding the analog baseband part including. MIXER through ADC or the band-limiting LPF arranged in the IF system and the level of a desired wave detected with RSSI2 following the LPF. It is necessary to control the voltage of a DC-DC converter used for LNA in order to prevent saturation of LNA, occurrence of distortion, and suppression of sensitivity (i.e. a phenomenon in which the gain or the desired-wave gain is decreased below the appropriate value due to gain compression caused by interfering waves) while improving the linearity of LNA when the strength of an interfering wave is being varied over time during reception of an interfering wave and a desired wave which are intermixed together.
Upon determining a high level of an interfering wave, it is possible to improve the saturation power of LNA (i.e. to expand the backoff) by boosting the control voltage of LNA. In contrast, upon determining a low level of an interfering wave, it is possible to prevent an increase of power consumption, which is necessary to constantly cope with interfering waves, by restoring the original control voltage of LNA.
Patent Literature Document 11 aims at improvement of Patent Literature Document 12. Patent Literature Document 12 aims to prevent an increase of power consumption which is necessary to constantly cope with interfering waves, wherein it determines the existence or nonexistence of interfering waves by detecting a difference of RSSI at two points, and it copes with high-level interfering waves by changing the paths regarding the LNA and the analog baseband part in the direct conversion system.
It is possible to determine the existence or nonexistence of interfering waves based on a difference between RSSI1 representing the RF output of the LNA reflecting interfering waves and desired waves and RSSI2 regarding the output of the digital baseband part solely reflecting desired waves, transmitted through the channel filter, after ADC. In this connection, it is possible to secure a net gain by arranging the digital AGC amplifier before ADC irrespective of a gain correction when the LNA is bypassed upon detecting interfering waves above the allowable value.