Truncated IIR Filter 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, while FIR filters are more hardware-efficient but less effective in filtering.
Innovation Solution
Implementing a truncated and decimated IIR filter that operates in a regular mode during the preamble and 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 eliminating ISI.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If IIR filters are used for OFDM baseband processing, then hardware efficiency is improved and fewer storage registers are needed, but inter-symbol interference increases due to longer impulse response
Solution Approach 1:
The patent divides the continuous data stream into discrete symbol periods and applies the IIR filter separately to each symbol. By resetting the filter state at the beginning of each symbol period, the long impulse response of the IIR filter is segmented into isolated segments that do not interfere with adjacent symbols, thus eliminating ISI while maintaining hardware efficiency
Solution Approach 2:
The patent implements periodic resetting of the IIR filter registers at the start of each OFDM symbol period. This periodic action clears the filter state regularly, preventing the accumulation of interference from previous symbols while allowing the IIR filter to maintain its computationally efficient structure throughout the transmission
2Object-generated harmful factors
If FIR filters are used for OFDM baseband processing, then inter-symbol interference is reduced, but hardware efficiency decreases and more storage registers are needed
Solution Approach 1:
The patent changes the operational parameters of the IIR filter by periodically resetting its state registers at the beginning of each symbol period. This parameter change (resetting the filter coefficients to initial values) effectively transforms the behavior of the IIR filter to match the ISI-reducing characteristics of FIR filters, while maintaining the IIR filter's superior hardware efficiency
3Manufacturing precision
If the IIR filter impulse response is extended to cover the cyclic prefix, then filtering accuracy is improved, but inter-symbol interference increases due to multipath delay spread
Solution Approach 1:
The patent applies the IIR filter to the cyclic prefix portion of each symbol before the main data portion. By pre-filtering the cyclic prefix and resetting the filter state afterward, the system achieves accurate filtering of the guard interval while preventing filtered samples from leaking into subsequent symbols, thus maintaining filtering accuracy without introducing ISI
Data Source
AI summary
A filter for receiver and operative on a stream of OFDM symbols has a symbol timing identifier which indicates the time interval for each symbol and also indicates a non-truncation interval and a truncation interval of the stream of symbols. The stream of OFDM symbols is applied to an infinite impulse response (IIR) filter with a reset input for resetting internal registers such that during the non-truncation interval, the reset input is not asserted, and during the truncation interval of the stream of OFDM symbols, the reset input is asserted during the intervals between symbols, as identified by the symbol timing identifier.A transmit filter for a stream of OFDM symbols, each symbol being separated into a first Tg interval, a second Tg interval, a symbol interval, and a final Tg interval, the filter has a stream modifier which discards the first Tg interval, accepts said second Tg interval, accepts the symbol interval and discards said final Tg interval, presenting to an infinite impulse response filter, in sequence, the second Tg interval, the symbol interval and the second Tg interval.