Analog Costas Loop Carrier Recovery for m-QAM Receivers
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Solution Overview
Problem
High-performance communication systems, such as optical and millimeter wave systems, face challenges in performing carrier recovery in the digital domain due to the high cost of analog-to-digital converters, leading to the use of simpler modulation schemes like QPSK, whereas more complex amplitude and phase modulated schemes like m-QAM are desirable for increased channel throughput.
Innovation Solution
An apparatus and method for analog-domain carrier recovery using a receiver with a mixer, voltage-controlled oscillator, equalizers, analog-to-digital converters, and a Costas loop, which includes amplifiers, an adder, and a feedback circuit to determine the difference between amplified signals and adjust the local oscillator signal, enabling carrier recovery without the need for digital domain processing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If digital-domain carrier recovery is used, then carrier recovery performance is improved, but system cost increases due to expensive ADCs
Solution Approach 1:
The patent introduces an analog-domain Costas loop as an intermediary mechanism that performs carrier recovery functions before digital conversion. The Costas loop includes analog components (mixers, VCO, amplifiers, filters) that work together to recover the carrier signal in the analog domain, eliminating the need for expensive high-performance ADCs while maintaining carrier recovery performance.
Solution Approach 2:
The patent replaces the digital-domain carrier recovery system (which requires expensive ADCs) with an analog-domain system using electronic circuit components. The analog Costas loop uses mixers, voltage-controlled oscillators, and analog filters to perform carrier recovery, substituting the need for high-performance digital conversion hardware.
2Device complexity
If constant amplitude modulation is used, then carrier recovery is simplified, but channel throughput is limited
Solution Approach 1:
The patent enables dynamic modulation schemes (m-QAM with varying amplitude and phase) by providing robust analog-domain carrier recovery. The Costas loop dynamically tracks carrier frequency and phase while allowing the modulation scheme to vary amplitude levels, thus enabling higher throughput without sacrificing carrier recovery capability.
Solution Approach 2:
The patent changes the modulation parameters from constant amplitude to variable amplitude and phase (m-QAM). The analog Costas loop is designed to handle these parameter changes by continuously tracking the carrier signal, allowing the system to use higher-order modulation schemes that increase channel throughput while maintaining reliable carrier recovery.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enables carrier recovery for amplitude and phase modulated communications in analog domain, facilitating the use of more complex modulation schemes like m-QAM, thereby increasing channel throughput without the overhead of digital domain processing and reducing costs associated with high-performance converters.
Implementation Method 1
a mixer that is coupled to the input circuit
Implementation Method 2
a voltage controlled oscillator (VCO) that is coupled to the mixer
Implementation Method 3
a first equalizer that is coupled to the mixer; a second equalizer that is coupled to the mixer
Implementation Method 4
a Costas loop having: a first amplifier that is coupled to the first equalizer; a second amplifier that is coupled to the first equalizer
Implementation Method 5
a feedback circuit that is coupled between the adder and the VCO
Data Source
AI summary
A method is provided. A multi-amplitude signal is received and downconverted so as to generate I and Q signals using a local oscillator signal. The I and Q signals are equalized, and the equalized I and Q signals are digitized. First and second gains are adjusted with the second and first digital signals, respectively, and applied to the equalized I and Q signals, respectively. The difference between the first and second amplified signals is determined, and an error signal is generated from the difference between the first and second amplified signals. The local oscillator signal is then adjusted with the error signal.