OFDM Pre-coding with Frequency Offset for Diversity Gain
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Solution Overview
Problem
Current Orthogonal Frequency Division Multiplexing (OFDM) systems fail to achieve diversity gain, which is essential for stable signal transmission, especially in mobile communication systems where users move at high speeds and experience frequency selective fading.
Innovation Solution
A pre-coding method is introduced that applies a frequency offset using a unitary pre-coding matrix based on Discrete Fourier Transform (DFT) to improve diversity gain, specifically designed for Quadrature Amplitude Modulation (QAM) signals, ensuring that any row of the pre-coding matrix is not spanned by differential signals of the constellation, thereby preventing basis vectors from being spanned by BPSK and QPSK signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If OFDM system uses multiple sub-carriers for high-speed data transmission, then data transmission rate is improved, but diversity gain cannot be achieved
Solution Approach 1:
The transmission signal is divided into multiple parallel sub-carrier signals through Serial-to-Parallel conversion. Each sub-carrier carries a portion of the data stream, enabling high-speed transmission while the segmented structure allows independent processing of diversity coding across different frequency channels
Solution Approach 2:
Diversity coding is applied in advance to the parallel sub-carrier signals before modulation and transmission. The pre-coder generates multiple coded versions of each data symbol and assigns them to different sub-carriers, ensuring diversity protection is established before the signal encounters frequency-selective fading channels
2Reliability
If frequency offset is applied in pre-coding, then diversity gain is improved, but signal reconstruction complexity increases
Solution Approach 1:
A frequency offset parameter is introduced into the pre-coding process, where each sub-carrier is assigned a specific frequency shift. This parameter change enables the system to achieve diversity gain by spreading transmitted symbols across frequency-shifted carriers, while the receiver can reverse the process using the known frequency offset values to reconstruct the original signal
3Stability of the object's composition
If pre-coding is applied to spread error symbols, then transmission stability is improved, but processing time increases
Solution Approach 1:
The diversity coding function is extracted as a separate pre-coding stage that operates independently on parallel sub-carrier signals. By taking out this function into a dedicated processing block, the system can apply diversity spreading without significantly increasing overall processing time, as the operation occurs in the frequency domain where parallel processing is efficient
Data Source
AI summary
A pre-coding method for improving diversity gain in an Orthogonal Frequency Division Multiplexing (OFDM) system and a transmission apparatus and method are disclosed. The transmission method includes converting an input transmission signal into parallel signals, pre-coding the parallel signals with a predetermined frequency offset, performing Inverse Discrete Fourier Transform (IDFT) on the pre-coded signals, and up-converting the IDFT signals and outputting the up-converted signals to a wireless network. By performing pre-coding on a transmission signal with a frequency offset in an OFDM system, diversity gain can be improved.