256-Point FFT Tone Design for Reduced Preamble Overhead
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Solution Overview
Problem
Current multiple antenna wireless communication systems face inefficiencies in preamble training, particularly when transitioning from 20 MHz to 40 MHz bandwidth, where increasing subcarriers to 256 tones necessitates a training mechanism that covers all subcarriers without increasing symbol time, and existing solutions do not adequately address the need for reduced guard interval overhead.
Innovation Solution
Implementing a 256 point FFT in 40 MHz with a tone design that reduces pilot tones, increases populated tones, and employs central tones, along with a unified training and signaling scheme that allows for shorter preambles and reduced cyclic prefix overhead, enabling efficient channel estimation and increased system efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the number of subcarriers is increased to 256 in 40 MHz, then the system throughput increases, but the preamble training overhead increases
Solution Approach 1:
The 256-subcarrier channel is divided into multiple 128-subcarrier segments. The training preamble uses only 128 tones instead of all 256, training one segment at a time. This segmentation reduces the training overhead while maintaining the ability to serve the full 256-subcarrier bandwidth through multiple training phases or selective activation.
Solution Approach 2:
Instead of training all 256 subcarriers, the system performs partial training on only 128 essential tones. This partial action is sufficient to establish the necessary channel estimation for effective communication, avoiding the excessive overhead of complete 256-tone training while still supporting the extended bandwidth capability.
2Productivity
If the FFT size is increased to 256 points, then the system efficiency increases by reducing cyclic prefix overhead, but the device complexity increases
Solution Approach 1:
The 256-point FFT processing is segmented into multiple 128-point FFT operations. By processing the extended bandwidth in smaller manageable segments, the device complexity is reduced while still achieving the benefits of the larger FFT size for improved efficiency and reduced cyclic prefix overhead.
Solution Approach 2:
The patent employs nested FFT structures where 128-point FFT operations are nested within the overall 256-point FFT framework. This allows the system to achieve 256-point FFT efficiency benefits while using computationally simpler 128-point FFT building blocks, thereby reducing device complexity.
3Productivity
If the guard interval is reduced to increase efficiency, then the system efficiency improves, but the resistance to multipath effect deteriorates
Solution Approach 1:
The patent addresses the guard interval limitation by moving to another dimension - extending the FFT size and symbol duration in the frequency-time domain rather than simply increasing the guard interval in the time domain. The extended 6.4 μs symbol duration with 256 subcarriers provides better multipath resistance through increased time spreading without requiring a proportionally larger guard interval.
Data Source
AI summary
Methods and apparatus are provided for improved efficiency in an extended multiple antenna communication system. A multiplier is employed on the number of points in the FFT that is greater than the multiplier on the frequency (bandwidth) of the legacy 802.11 a/g system. In one exemplary implementation, a 256 point FFT is employed in 40 MHz (with a 4N multiplier on the number of possible tones and a 2N multiplier on the frequency). While the efficiency for the OFDM symbol is improved, additional overhead is required in the preamble training (the length of the preamble is proportional to the number of tones in the FFT). Thus, a number of preamble constructs are provided that couple the improved efficiency with shorter preambles. In addition, an improved tone design provides additional efficiency gains.