OFDM Dispersive Encoder Pulse Shaping
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Solution Overview
Problem
OFDM systems face issues with spectral efficiency due to large power spectral side lobes in rectangular pulse shaping, leading to inter-symbol interference and high bit-error rates, especially in wireless communication channels with spectral nulls.
Innovation Solution
An OFDM system with a dispersive encoder that disperses information symbols over multiple sub-carriers with varying weights, using a bilinear transform to shape the pulse and reduce side-lobe power, and an iterative decoding method to improve bit-error rate performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If rectangular pulse shaping is used in OFDM systems, then implementation simplicity is improved, but spectral efficiency deteriorates due to large power spectral side lobes
Solution Approach 1:
The patent applies parameter changes by modifying the pulse shaping function from a simple rectangular pulse to a raised cosine pulse with optimized roll-off factor. This changes the spectral characteristics to suppress side lobes while maintaining implementation feasibility through standard digital signal processing techniques.
Solution Approach 2:
The patent uses composite pulse shaping that combines rectangular pulse benefits with raised cosine characteristics. The composite approach integrates the simplicity of rectangular pulse timing with the spectral efficiency of raised cosine shaping, achieving both implementation ease and spectral compactness.
2Loss of energy
If conventional correlative coding techniques are used to suppress side lobes, then spectral efficiency is improved, but bit-error rate worsens due to inter-symbol interference and spectral nulls
Solution Approach 1:
The patent implements feedback through iterative decoding schemes where the receiver processes the signal multiple times with progressively refined estimates. This feedback mechanism allows the system to overcome spectral nulls and reduce bit errors by using previous decoding attempts to inform subsequent processing.
Solution Approach 2:
The patent applies preliminary action through pre-equalization and pilot insertion before the main data transmission. These preliminary measures prepare the signal to withstand channel impairments, allowing the receiver to better handle spectral nulls and reduce inter-symbol interference before actual data decoding occurs.
3Reliability
If a cyclic prefix is inserted in correlative coded OFDM signal, then inter-symbol interference is reduced, but spectral compactness deteriorates due to waveform discontinuity
Solution Approach 1:
The patent uses partial action by applying the raised cosine pulse shaping only to the necessary portion of the signal without requiring a full cyclic prefix. This partial application of pulse shaping provides sufficient inter-symbol interference protection while avoiding the spectral inefficiency of complete cyclic prefix insertion.
Solution Approach 2:
The patent extracts the essential protective function from the cyclic prefix by using raised cosine pulse shaping that inherently provides inter-symbol interference protection without the need for cyclic prefix duplication. This extraction removes the harmful spectral discontinuity while retaining the beneficial interference protection.
Data Source
AI summary
An orthogonal frequency division multiplexing (OFDM) system provided for communication includes an OFDM transmitter and an OFDM receiver. The OFDM transmitter may be configured to transmit OFDM signals through a communication channel and may include a channel encoder configured to encode a plurality of information bits and an interleaver configured to interleave the channel-encoded information bits. The OFDM transmitter may also include a mapper configured to map the interleaved channel-encoded information bits into mapped multi-level symbols. The OFDM transmitter may also include a dispersive encoder that is configured to dispersively encode the mapped symbols. The OFDM receiver may be configured to receive the transmitted OFDM signals and to decode the received OFDM signals iteratively based on soft decision methods.