IIR Filter Reset for OFDM Inter-Symbol Interference
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Solution Overview
Problem
In OFDM signal processing systems, IIR filters introduce inter-symbol interference (ISI) due to their longer impulse response, degrading wireless link performance, especially in multipath reflections, despite offering hardware efficiency and fewer storage requirements compared to FIR filters.
Innovation Solution
Implementing a truncated and decimated IIR filter that operates in a regular mode during the preamble and switches to a per-symbol truncated mode during data transmission, with register resets before each cyclic prefix, and using a remapper for transmit interpolation to rearrange samples and reset IIR filters at the start of each symbol interval, thereby minimizing ISI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If IIR filters are used for decimation and interpolation in OFDM baseband processors, then hardware efficiency is improved and storage register requirements are reduced, but inter-symbol interference increases due to longer impulse response
Solution Approach 1:
The patent segments the IIR filter operation into distinct phases: a preamble processing mode where the filter operates continuously, and a data symbol mode where the filter is reset before each symbol. This segmentation allows the system to benefit from IIR filter hardware efficiency while eliminating ISI by clearing the filter state between symbols.
Solution Approach 2:
The patent implements periodic resetting of the IIR filter registers at the start of each data symbol after preamble processing. This periodic action removes the accumulated impulse response from previous symbols, preventing inter-symbol interference while maintaining the hardware efficiency of IIR filter structure during active processing.
2Productivity
If IIR filters with longer impulse response are used, then hardware efficiency is improved, but the filter tail extends into subsequent symbols causing inter-symbol interference
Solution Approach 1:
The patent applies preliminary resetting of the IIR filter registers before processing each data symbol. This preliminary action ensures that the filter starts with a clean state for each symbol, preventing the filter tail from contaminating subsequent symbols and thereby maintaining reliable wireless link performance.
Solution Approach 2:
The patent discards the filter state (by resetting registers) at the beginning of each data symbol processing after the preamble. This discarding of the accumulated impulse response prevents ISI, while the filter is recovered and continues operating efficiently during the symbol processing itself.
Data Source
AI summary
A receive filter for a stream of OFDM symbols has an infinite impulse response (IIR) filter which receives packets having a preamble part followed by a data part, the data part having a succession of cyclic prefixes followed by OFDM symbols. The packet is provided to the IIR filter, and the registers of the IIR filter are reset before or during each cyclic prefix. The IIR filter may be formed from registers which are coupled to return a value to a predecessor register, or as an array of registers, such as in a BIQUAD configuration, where the registers are reset by a shared reset signal.