Adaptive DC Sub-carrier Handling in DFT-SOFDM Receivers
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Solution Overview
Problem
Direct conversion transmitters and receivers introduce significant DC distortion in DFT-SOFDM uplink transmissions, leading to performance degradation due to unsuppressed carrier feed-through and local oscillator leakage, which existing methods fail to adequately address, especially when discarding the DC sub-carrier causes 0.9 dB degradation for 16QAM modulation.
Innovation Solution
Adaptive DC bias suppression is implemented in the receiver, where the decision to suppress or not suppress DC distortion is made based on the distortion level, either by discarding sub-carriers in the frequency domain or subtracting DC distortion in the time domain, depending on the amount of interference, to mitigate the impact of DC distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If the DC sub-carrier is discarded (zeroed out) at the receiver, then the impact of DC distortion is reduced, but receiver performance degrades by 0.9 dB for 1 resource block allocations with 16QAM modulation
Solution Approach 1:
The patent implements adaptive DC sub-carrier handling where the receiver dynamically selects between discarding the DC sub-carrier and using it based on the measured DC distortion level. When DC distortion is below a threshold, the DC sub-carrier is utilized; when above, it is discarded. This dynamic adaptation resolves the contradiction by optimizing receiver performance across varying DC distortion conditions rather than using a fixed approach.
2Reliability
If the DC sub-carrier is not discarded, then receiver performance is maintained for low DC distortion values, but significant degradation occurs for high DC distortion values
Solution Approach 1:
The patent employs a feedback mechanism where the receiver measures the DC distortion level and uses this measurement to determine whether to discard the DC sub-carrier or not. This closed-loop control resolves the contradiction by adjusting the DC sub-carrier handling strategy based on actual DC distortion conditions, ensuring optimal performance across varying distortion levels.
Solution Approach 2:
The patent changes the operational parameter of DC sub-carrier utilization based on the DC distortion level. When DC distortion is low, the DC sub-carrier is active; when high, it is discarded. This parameter change approach resolves the contradiction by adapting the system behavior to the actual distortion conditions rather than maintaining a fixed state.
3Use of energy by moving object
If DFT-SOFDM is used instead of OFDM, then power de-rating properties are improved, but DC distortion from direct conversion transmitters and receivers causes increased CM and EVM degradation
Solution Approach 1:
The patent extracts and removes the harmful DC sub-carrier component when DC distortion is high, while preserving the beneficial DFT-SOFDM modulation scheme. By selectively removing only the distorted DC component rather than changing the entire modulation scheme, the patent maintains the improved power de-rating properties of DFT-SOFDM while eliminating the CM and EVM degradation caused by DC distortion.
Data Source
AI summary
Receiving units will switch between performing a DC bias suppression and not removing the DC distortion at the receiver depending on the amount of DC interference level observed/measured/estimated. Since the overall DC interference is from all uplink transmitters, potentially at different power levels, the amount of DC distortion can be measured based on the difference between the received power level and the expected power level on the DC sub-carrier. Additionally it can be estimated based on the number of active transmitters, their allocation bandwidth, power control target and/or a rough estimate of the DC distortion introduced by each active transmitter and also the distortion introduced by the receiver. Once this distortion level is estimated, a decision is then made whether or not to remove the DC distortion.