Two-Stage Gain Control for Frequency-Hopped OFDM Subbands
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Solution Overview
Problem
Ultrawideband (UWB) communication systems face challenges in performing automatic gain control (AGC) due to bursty transmissions and frequency hopping, which limits the time available for AGC operations and complicates signal processing, leading to signal distortion.
Innovation Solution
A two-stage automatic gain control system is implemented in a receiver, comprising a coarse AGC stage with a low noise amplifier and mixer, and a fine AGC stage with a programmable gain amplifier (PGA), where gains are initialized to maximum values and adjusted based on received signal power and digitized output counts to maintain optimal signal strength and prevent clipping.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-stage AGC system is used, then the device complexity is reduced, but the signal processing accuracy deteriorates due to inability to handle bursty UWB transmissions with frequency hopping
Solution Approach 1:
The AGC system is divided into two independent stages: a first stage AGC that processes signals in the analog domain before ADC, and a second stage AGC that processes digital signals after ADC. Each stage has its own control mechanism and operates independently to handle different aspects of gain control, thereby improving overall signal processing accuracy without excessive complexity increase.
Solution Approach 2:
The patent introduces an intermediary RSSI measurement mechanism that bridges the two AGC stages. The RSSI value is measured from the analog signal and used to control the first stage AGC, while digital signal levels are monitored to control the second stage AGC. This intermediary measurement system enables coordinated control between stages, improving precision while managing complexity.
2Adaptability or versatility
If gain adjustment is performed rapidly to handle bursty transmissions, then the adaptability improves, but the signal distortion increases due to insufficient settling time
Solution Approach 1:
The gain control process is segmented into two stages with different response characteristics. The first stage AGC operates on analog signals with faster response to capture bursty transmission characteristics, while the second stage AGC operates on digital signals with slower, more stable adjustment. This segmentation allows the system to adapt rapidly to transmission bursts while preventing excessive gain changes that would cause signal distortion.
Solution Approach 2:
The first stage AGC performs preliminary gain control on the analog signal before it enters the ADC. By pre-adjusting the signal level in the analog domain, the system prepares the signal for optimal digital processing, reducing the burden on the second stage AGC and minimizing the risk of distortion during rapid gain transitions.
3Reliability
If the gain is increased to improve signal strength, then the sensitivity improves, but the clipping of ADC outputs increases
Solution Approach 1:
The patent segments the gain control into two independent stages to separately manage signal strength and clipping prevention. The first stage AGC amplifies the analog signal to improve sensitivity and signal strength, while the second stage AGC monitors digital signal levels and applies attenuation if necessary to prevent ADC clipping. This segmentation allows simultaneous optimization of both signal strength and output quality.
Solution Approach 2:
The second stage AGC implements a feedback mechanism that monitors the levels of digitized outputs from the ADC. When clipping is detected or signal levels exceed optimal ranges, the feedback control adjusts the second stage gain downward to prevent distortion. This feedback loop ensures that signal strength is maintained while preventing ADC output clipping, thereby preserving signal quality.
Data Source
AI summary
An automatic gain control method and system for use in signal processing of OFDM symbols at a receiver. Two stages of coarse and fine automatic gain control are implemented that adjust different gains in an analog RF processing stage of the receiver. Gain of a low noise amplifier and a mixer are adjusted during a first and coarse automatic gain control stage based on feedback from a digital baseband stage. During a subsequent fine gain control period, the gain of a programmable gain amplifier is adjusted separately for each frequency band used by the OFDM symbols based on a histogram bin that counts the number of output samples of an analog to digital converter whose magnitude falls within certain ranges. Coarse and fine gains are updated after each OFDM symbol.