OFDM Signal Analysis Using Spectrum Shifting and Low-Pass Filtering
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Solution Overview
Problem
Existing devices for analyzing wideband OFDM signals, such as those in wireless LAN systems, are expensive due to their requirement for wideband signal processing, and reducing the bandwidth leads to reduced accuracy in estimating synchronization parameters due to fewer usable pilot channels.
Innovation Solution
The method involves shifting the OFDM signal spectrum to fit within a reduced bandwidth, using low pass filtering to suppress mirror frequencies and limit input bandwidth, and compensating for reduced pilot channels by averaging synchronization parameters across OFDM symbols.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If wideband signal processing is used to analyze OFDM signals, then measurement precision is improved, but device cost increases
Solution Approach 1:
The wideband OFDM signal is divided into multiple narrowband segments that can be processed sequentially by a reduced-bandwidth analyzing device. The device analyzes one segment at a time, then moves to the next segment, effectively processing the entire wideband signal without requiring wideband hardware capability.
Solution Approach 2:
The analyzing device dynamically shifts its analysis window across different frequency segments of the OFDM signal. By moving the analysis bandwidth to different frequency positions, the device can measure different portions of the wideband signal using a single reduced-bandwidth receiver.
2Device complexity
If bandwidth is reduced to lower device cost, then device complexity is improved, but measurement precision deteriorates
Solution Approach 1:
Multiple measurements from different frequency segments are combined and averaged to produce a final synchronization parameter estimate. This merging of information from multiple segments compensates for the reduced precision in any single segment, achieving overall accuracy comparable to wideband processing.
Solution Approach 2:
The analyzing device performs continuous measurements across all frequency segments of the OFDM signal. By accumulating data from every segment and processing it continuously, the device maximizes the useful information extracted from the limited bandwidth, improving the reliability of synchronization parameter estimates.
3Device complexity
If low pass filtering is applied to limit input bandwidth, then device complexity is improved, but harmful factors increase due to mirror frequency suppression requirements
Solution Approach 1:
A low-pass filter is applied before the resampler to pre-limit the input bandwidth and suppress mirror frequencies that would otherwise alias into the signal band during downsampling. This preliminary filtering action prevents harmful artifacts from entering the processing chain.
Solution Approach 2:
The low-pass filter acts as an intermediary element between the wideband input signal and the reduced-bandwidth resampler. It mediates the bandwidth transition by selectively passing desired frequency components while blocking unwanted mirror frequencies, protecting the subsequent processing stages from interference.
Data Source
AI summary
The invention concerns a method for analysing an OFDM signal (rIF(t)) transporting a series of data symbols on several orthogonal carrier frequencies, each data symbol having a useful part separated by a guard period from neighboring data symbols, with an analysing device (40) having a signal section (20) with a bandwidth (BWA) smaller than the bandwidth (BWOFDM) of the OFDM signal. The method comprises the steps of low pass filtering of the OFDM signal with a low pass filter (24) and shifting (26) the spectrum of the OFDM signal in order to obtain a frequency shifted filtered OFDM signal (r(i)), whereby the length of the impulse response of the low pass filter (24) is shorter than the length of the guard periods of the data symbols.