Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W52/00—Power management, e.g. TPC [Transmission Power Control], power saving or power classes
  • H04W52/04—TPC
  • H04W52/52—TPC using AGC [Automatic Gain Control] circuits or amplifiers
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/3052—Automatic control in amplifiers having semiconductor devices in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver
• • H—ELECTRICITY
  • H03—ELECTRONIC CIRCUITRY
  • H03G—CONTROL OF AMPLIFICATION
  • H03G3/00—Gain control in amplifiers or frequency changers
  • H03G3/20—Automatic control
  • H03G3/30—Automatic control in amplifiers having semiconductor devices
  • H03G3/3052—Automatic control in amplifiers having semiconductor devices in bandpass amplifiers (H.F. or I.F.) or in frequency-changers used in a (super)heterodyne receiver
  • H03G3/3078—Circuits generating control signals for digitally modulated signals
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04B—TRANSMISSION
  • H04B1/00—Details of transmission systems, not covered by a single one of groups H04B3/00 - H04B13/00; Details of transmission systems not characterised by the medium used for transmission
  • H04B1/06—Receivers
  • H04B1/16—Circuits
  • H04B1/30—Circuits for homodyne or synchrodyne receivers
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L27/00—Modulated-carrier systems
  • H04L27/18—Phase-modulated carrier systems, i.e. using phase-shift keying
  • H04L27/22—Demodulator circuits; Receiver circuits

Definitions

• This invention is concerned with digital automatic gain control. More particularly, this invention is concerned with Automatic Gain Control (AGC) for discontinuous signals in a receiver having limited dynamic range.
• AGC Automatic Gain Control
• TDMA Time-division-multiplexed
• the time slots are arranged into periodically repeating frames.
• a radio communication of interest may be periodically discontinuous - interleaved with unrelated signals transmitted in other time slots.
• the unrelated signals (of widely varying strength) must not influence the gain control of the signals of interest. A daunting challenge then is to provide Automatic Gain Control of these periodically discontinuous TDMA signals.
• the challenge is further enhanced by attempting to provide digital AGC in inexpensive receivers - those having limited dynamic range. Since these signals may vary by as much as 100dB in the land-mobile environment, but modest 8-bit Analog-to-DigitaJ converters (A/D) for digital signal processing are limited to 48dB dynamic range, techniques must be developed for controlling the gain of the signal to keep it within the limited dynamic range of the receiver. The challenge then is to handle a 100dB discontinuous signal with a 48dB device; otherwise, prohibitively expensive A/Ds with greater dynamic range must be utilized.
• A/D Analog-to-DigitaJ converters
• GMSK Gaussian Minimum Shift Keying
• This invention takes as its object to overcome these challenges and realize certain advantages, presented below.
• a mechanism for Automatic Gain Control in a receiver comprises: determining, within a certain dynamic range, the difference in power between the desired signal and a signal received, and providing open loop gain control for the signal in response to the differential so determined, scaled by the receiver's gain characteristics, such that the signal is positioned within dynamic range so as to reduce saturation and noise.
• AGC digital Automatic Gain Control
• the method comprises detecting the level of a received and AGC'd discontinuous signal, comparing the level of the AGC'd signal relative to the dynamic range of the receiver, and adjusting the AGC to establish a desired relationship between the AGC'd signal and the dynamic range limitation.
• a method of handoff in a TDMA cellular-type transmission system utilizing this method of AGC control.
• the method for Automatic Gain Control (AGC) of discontinuous signals in a receiver having limited dynamic range is further characterized by: digitizing a received and AGC'd discontinuous signal and converting the digitized samples to a power sample to sense the power of and detect the level of the signal, comparing the level of the AGC'd signal relative to the dynamic range of the receiver; and coarsely-adjusting by either progressively attenuating the signal until it falls within the dynamic range of the receiver or by progressively gain-amplifying the signal until it falls within the limited dynamic range of the receiver and finely-adjusting the AGC of the received signal until optimum use of the full (albeit limited) dynamic range of the signal processing stages is about 6-12 dB below the maximum to establish a desired relationship between the AGC'd signal and the dynamic range limitation.
• Figure 1 is a simplified block diagram of the invention.
• Figure 2 is a functional block diagram of the preferred embodiment of the invention.
• FIG. 3 is a diagram of the AGC control process according to the invention.
• Figure 4 is a simplified diagram of an alternate embodiment of the invention.
• Figure 5 is a diagram of a preferred embodiment of the AGC control process according to the invention.
• Figure 6 illustrates five overlapping regions of the useful A D range (30dB) spanning the expected signal range of -20dB to -110dB.
• Figure 1 is a simplified block diagram of the invention; it illustrates gain control in a digital quadrature receiver. It illustrates, in series, an RF receiver section (IF), a quadrature demodulator (l/Q) having In-phase and Quadrature phases, Analog-to-Digital converters (A/D), a Digital Signal Processor (DSP), and a Digital-to-Analog converter (D/A) providing Automatic Gain Control (AGC) to the receiver section (RF/IF).
• IF RF receiver section
• l/Q quadrature demodulator
• A/D Analog-to-Digital converters
• DSP Digital Signal Processor
• D/A Digital-to-Analog converter
• AGC Automatic Gain Control
• the signal is received, converted to an intermediate frequency and gain amplified in the receiver section (RF/IF); quadrature demodulated (l/Q) into In-phase and Quadrature components; digitized in Analog-to-Digital converters of limited dynamic range (A D); and converted to a power sample in the Digital Signal Processors (DSP) to detect the level of the signal.
• DSP Digital Signal Processors
• the signal level is compared relative to the dynamic range of the receiver, and the AGC is adjusted in the Digital-to-Analog converter (D/A) to establish a desired relationship between the AGC'd signal and the dynamic range limitation of the receiver.
• the power level of the AGC'd signal is compared relative to a desired power level in the stage having the dynamic range limitation.
• Figure 2 is a functional block diagram of the preferred embodiment of the invention.
• Figure 2 illustrates, in line, an RF receiver section (RF/IF); a quadrature demodulator (l/Q) having In-phase (I) and Quadrature (Q) phase mixers whose outputs are Low Pass Filtered (LPF); and, under Direct Memory Access control (DMA), 8-bit Analog-to-Digital converters (A/D), tri-state gates, Random Access Memories (RAM), and a 56001 Digital Signal Processor (56001 DSP); and a latching Digital-to-Analog converter (D/A) providing Automatic Gain Control (AGC) to the receiver section (IF).
• DMA Direct Memory Access control
• A/D 8-bit Analog-to-Digital converters
• RAM Random Access Memories
• 56001 DSP 56001 Digital Signal Processor
• D/A Digital-to-Analog converter
• AGC Automatic Gain Control
• This GMSK receiver is comprised of a conventional RF stage, mixing and filtering that feeds a 10.7 MHz IF signal to a conventional AGC-type IF amplifier (IF), such as a Motorola MC1350.
• the IF amplifier feeds a conventional l/Q demodulator comprised of a 10.7 MHz local oscillator, a 90 degree phase shifter, a pair of mixers and a pair of low pass filters (LPF).
• the 8-bit flash A/Ds such as RCA CA331 ⁇ CE's, provide 48dB of dynamic range and are, in large part, responsible for the dynamic range limitation of the receiver.
• a Motorola 56001 Digital Signal Processor (56001 DSP) is used for signal acquisition, signal level detection, and AGC control.
• the 56001 DSP is supported by conventional clock and timing circuitry (not shown) and ROMs for programmed control (not shown).
• An Analog Devices 7528LN is suitable as the latching Digital-to-Analog converter (D/A) that provides Automatic Gain Control (AGC) to the receiver section (IF).
• D/A Digital-to-Analog converter
• AGC Automatic Gain Control
• the receiver operates in a TDMA system having 8 time slots in each 4.8 millisecond frame; 135 kilobits/second are transmitted in each quadrature phase.
• a retained previous AGC setting is fetched (DMA) from memory (RAM) through the Digital Signal Processor (56001 DSP) and applied to the Digital-to-Analog converter (D/A), providing Automatic Gain Control (AGC) to the receiver section (RF/IF).
• DMA Digital-to-Analog converter
• AGC Automatic Gain Control
• the received signal after being gain-controlled and quadrature-demodulated is digitized by the Analog-to-Digital converters (A/D) to provide multiple pairs of samples per bit interval, which are stored in memory (RAM) under Direct Memory Access control (DMA) of the tri-state gates.
• A/D Analog-to-Digital converters
• the samples are retrieved from memory (RAM) and converted in the 56001 DSP to a power sample by summing N pairs (32 to 128 pairs in the preferred embodiment) to obtain a Q value and an I value, and taking the square root of the sum of the squares of the Q and I values.
• the square root is proportional to the average power of the received signal (an instantaneous power sample from a single pair cannot be reliably obtained due to the variability in the received signal strength).
• a preferred alternative measure for the power sample may be obtained by simply summing the squares of the Q and I values.
• the power level of the AGC'd signal is compared relative to a desired power level in the stage having the dynamic range limitation.
• the AGC wants to establish and maintain the level of the AGC'd signal at a nominal level of about 6-12dB (9dB in the preferred embodiment) below the maximum output of the A/D.
• FIG. 3 is a diagram of the AGC control process according to the invention.
• the basic control process is to:
• AGC Error AGC Error
• the signal is progressively gain-amplified (or gain-attenuated) until the signal falls within the dynamic range of the A/Ds and is further amplified (or attenuated) until optimum use (with appropriate margin) of the full (albeit limited) dynamic range of the A/Ds is obtained.
• the result of these various approximations for a plurality of TDM time slots may then be retained in memory (RAM) for resuming AGC control when the respective signals are expected to resume.
• these various gain calculation results are representative of the actually received signal strength (with appropriate compensation for overall loop gain characteristics)
• these gain determinations can be reported to the transmitting station for purposes of establishing transmission gain levels that optimally utilize the dynamic range of the receiver, thereby increasing spectral efficiency and frequency reuse in th ⁇ system -- particularly cellular systems.
• the signal strength (gain determination) may be reported to the transmitting station by the receiver and hand off the transmission when the AGC adjustment crosses a certain threshold.
• the signal strength (AGC level) of an adjacent cell (time slot) can be determined and evaluated to facilitate hand-off.
• Figure 4 is a simplified diagram of an alternate embodiment of the invention. It illustrates an analog implementation of Automatic Gain Control that utilizes a power averaging circuit and comparator to implement the control process of Figure 3, described above.
• the power averaging circuit is well known by those ordinarily skilled in the relevant art and can readily be adapted to conform to the control process described above.
• the method comprises detecting the level of a received and AGC'd discontinuous signal, comparing the level of the AGC'd signal relative to the dynamic range of the receiver, and adjusting the AGC to establish a desired relationship between the
• the method for Automatic Gain Control (AGC) of discontinuous signals in a receiver having limited dynamic range has further been characterized by: digitizing a received and AGC'd discontinuous signal and converting the digitized samples to a power sample to sense the power of and detect the level of the signal, comparing the level of the AGC'd signal relative to the dynamic range of the receiver; and coarsely-adjusting by either progressively attenuating the signal until it falls within the dynamic range of the receiver or by progressively gain-amplifying the signal until it falls within the limited dynamic range of the receiver and finely-adjusting the AGC of the received signal until optimum use of the full (albeit limited) dynamic range of the signal processing stages is about 6-12 dB below maximum sensitivity to establish a desired relationship between the AGC'd signal and the dynamic range limitation.
• Figure 5 is a diagram of a preferred embodiment of the AGC control process according to the invention. It illustrates the control process for an open loop improvement to the digital AGC presented thus far.
• the foregoing embodiment iteratively settled on the proper AGC through progressive, closed loop control.
• This preferred embodiment utilizes a lookup table (incorporating all the receiver characteristics and non- linearities, including the A/D non-linearities) with the calculated difference in power between that desired to obtain maximum utilization of the A/D dynamic range and the current actual power received at the A/Ds as an index into the table to obtain the next AGC setting required to settle at the desired power level.
• the table is derived in a laboratory setting where the AGC (power) level is established at the desired level while the power generated from a signal generator coupled to the antenna input necessary to establish certain power differentials at the A/Ds is noted. In this fashion, the AGC level required for any given power differential can be extrapolated.
• the power seen at the A/Ds is calculated (501) as the sum over sixty-four samples of the squares of the demodulated (l/Q) signal samples.
• the difference in power ( ⁇ dB) between that seen at the A/Ds (PA/D) and that desired (Pd) is determined (502).
• the power (PdBm) during the timeslot is determined (503) from the cur red D/A setting and the power differential ( ⁇ dB) and then indexing into the lookup table with the to find the power for that timeslot.
• the lookup table is a function of the receiver's gain control characteristics. This PdBm setting from many timeslots (comprising the discontinuous communication) is averaged in an FIR filter (504) to form a better power estimate of the faded signal that is reported to the transmitter for handoff determinations (506).
• the power differential ( ⁇ dB) itself is also averaged over several timeslots (since the AGC cannot track through signal fades) in an FIR filter (505) to determine the average power difference (AVG ⁇ dB) from the desired (Pd) to determine when AGC settling has occurred (507). If this short-term average error (AVG ⁇ dB) is not greater than, say, half-scale (6dB) off of A/D saturation (508) and not less than (510) the noise quantization level (-30dB), then an IIR filter, or "leaky integrator,” (513) determines the speed of the AGC response to correct the present D/A setting by table lookup (512). This updated D/A value is then stored (514) for use with subsequent timeslots.
• the useful range of the A/Ds is windowed up through gain reduction (509) when the signal is clipped and the A Ds are saturated and windowed down through increased gain (511) when noise quantization occurs and the signal is insufficiently strong (see Figure 6).
• the AGC gain is scaled (and the A/Ds window adjusted) by the receiver's gain control characteristics, incorporated in the lookup table (509 & 511 ).
• FIG. 5 illustrated the AGC control process for tracking timeslots comprising one communication that is discontinuously transmitted.
• the receiver may, while idle, be monitoring up to 32 other carriers with strength varying across the entire -20dB to -110dB range (see Figure 6).
• This same basic process is employed for this adjacent cell monitoring.
• each carrier is sampled just three times during the multi-frame and the filter coefficients (504, 505 & 513) must be adjusted for this slower AGC control (for example, the IIR filter 513 averages over eight samples for monitoring, rather than the thirty-two for tracking, so that it becomes more responsive for monitoring).
• the saturation headroom (508) is increased from the tracking value of 6dB to 15dB because there is less certainty that the signal may be at the previously observed power level.
• a mechanism for Automatic Gain Control in a receiver comprises: determining, within a certain dynamic range, the difference in power between the desired signal and a signal received, and providing open loop gain control for the signal in response to the differential so determined, scaled by the receiver's gain characteristics, such that the signal is positioned within dynamic range so as to reduce saturation and noise.
• this invention need not be limited to TDMA land-mobile systems, but is adaptable to AGC of digital and analog signals, including AM, FM or TV signals.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Control Of Amplification And Gain Control (AREA)
• Circuits Of Receivers In General (AREA)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

Country Status (8)

Families Citing this family (14)

Citations (3)

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• 1991
  • 1991-04-09 EP EP19910907940 patent/EP0484476A4/en not_active Withdrawn
  • 1991-04-09 WO PCT/US1991/002409 patent/WO1991017606A1/en active IP Right Grant
  • 1991-04-09 KR KR1019910701981A patent/KR950009559B1/ko not_active IP Right Cessation
  • 1991-04-09 CA CA002062776A patent/CA2062776C/en not_active Expired - Fee Related
  • 1991-04-09 JP JP3507876A patent/JP2586214B2/ja not_active Expired - Fee Related
  • 1991-04-09 BR BR919105733A patent/BR9105733A/pt not_active Application Discontinuation
  • 1991-04-09 AU AU76895/91A patent/AU635134B2/en not_active Ceased
  • 1991-12-27 FI FI916141A patent/FI107089B/fi active

Patent Citations (3)

Non-Patent Citations (3)

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