Null Space Transmission for DC Offset Interference in OFDM
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Solution Overview
Problem
Direct conversion radio front-end architectures suffer from unknown constant DC-offset interference, leading to performance degradation and increased complexity in baseband processing, particularly in OFDM-based wireless communication systems, where residual DC offset affects spectral efficiency and requires separate estimation and compensation modules.
Innovation Solution
Transmitting data symbols in the null space of the desired frequency tone, which is orthogonal to the interference, allowing for interference-free recovery without estimation or compensation at the receiver, using a set of basis vectors that characterize the null space of the desired frequency tone.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If direct conversion front-end architecture is used, then power efficiency and cost effectiveness are improved, but DC-offset interference of unknown magnitude occurs
Solution Approach 1:
The patent transforms the harmful DC-offset interference into a useful component by treating it as part of the transmitted signal. Instead of attempting to remove or compensate for the DC offset, the system transmits data symbols in the time domain where the DC offset remains constant, allowing the receiver to recover data by subtracting the time-averaged signal (which contains the DC offset) from the received signal.
2Object-affected harmful factors
If DC offset compensation is implemented through baseband processing, then DC offset effects are reduced, but receiver complexity and cost increase
Solution Approach 1:
The patent extracts the DC offset component from the received signal by computing the time average of the received signal over a symbol period. This extracted DC offset component is then subtracted from the received signal to recover the transmitted data symbols, simplifying the receiver architecture compared to traditional estimation and compensation methods.
3Object-affected harmful factors
If DC subcarrier is left idle in OFDM systems, then DC offset issues are avoided, but spectral efficiency deteriorates
Solution Approach 1:
The patent inverts the conventional approach by actively using the DC subcarrier for data transmission instead of leaving it idle. By transmitting data symbols in the time domain and exploiting the constant nature of DC offset over a symbol period, the system enables the DC subcarrier to carry useful information while maintaining robustness against DC offset interference.
4Measurement precision
If suspended DC tone transmission period is extended for accurate DC offset estimation, then estimation accuracy is improved, but spectral efficiency is limited
Solution Approach 1:
The patent eliminates the need for suspended transmission periods by continuously transmitting data symbols on the DC subcarrier. The useful action of data transmission continues uninterrupted, while the DC offset is handled through time-averaging of the received signal, thereby maintaining both high estimation accuracy and high spectral efficiency.
Data Source
AI summary
Methods and systems are disclosed for transmitting data over a desired frequency tone in the presence of an interference that has an unknown magnitude that is substantially constant. In general, data symbols are transmitted in a null space of the desired frequency tone. The null space of the desired frequency tone is orthogonal to the desired frequency tone. As such, the data symbols transmitted in the null space of the desired frequency tone are not interfered with by the interference at the desired frequency tone regardless of the magnitude of the interference. The data symbols transmitted in the null space of the desired frequency tone can then be recovered by a receiver without estimation of or compensation for the interference at the desired frequency tone.