LEDs, particularly those used for Visible Light Communications (VLC), have a limited bandwidth, while above their 3 dB bandwidth, the roll-off is relatively gentle. If the modulation bandwidth would be limited to the 3 dB LED bandwidth, the achievable rate would be unacceptably constrained. Hence, effective communication systems need to optimize the use of bandwidth significantly above this 3 dB point. Orthogonal Frequency Division Multiplexing (OFDM) is a popular method to fine-tune the amount of power and constellation as a function of the channel response over different frequencies. Various power and bit loading strategies have been proposed and simulated in literature, but their performance was not captured in expressions. This manuscript derives these for optimal waterfilling, uniform and pre-emphasized power loading for the LED channel, that severely attenuates high frequencies. We also investigate the influence of practical discrete constellations and verify our new results experimentally. Interestingly, simple uniform loading only falls less than 1~2% short of the throughput achieved by waterfilling, but when we restrict OFDM to discrete QAM constellation sizes, the penalty for uniform loading is 1.5 dB. Inspired by the good performance of uniform power loading, we propose an algorithm to find the best discrete bit loading for uniform power within an optimized band. As pre-emphasis is nonetheless attractive because a flattened channel does not need adaptive sub-carrier loading, we quantify its penalty. This can be modest provided that the system can adapt its transmit bandwidth, thereby adaptively switching upper sub-carriers to zero power.

中文翻译：

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DCO-OFDM LED 通信的子载波加载策略

LED，尤其是那些用于可见光通信 (VLC) 的 LED，具有有限的带宽，而在其 3 dB 带宽以上时，滚降相对温和。如果调制带宽被限制为 3 dB LED 带宽，则可实现的速率将受到无法接受的限制。因此，有效的通信系统需要优化显着高于该 3 dB 点的带宽使用。正交频分复用 (OFDM) 是一种流行的方法，可根据不同频率上的信道响应对功率和星座进行微调。文献中已经提出并模拟了各种功率和位加载策略，但它们的性能并未体现在表达式中。这份手稿为 LED 通道的最佳注水、均匀和预强调的功率负载推导了这些，严重衰减高频。我们还研究了实际离散星座的影响，并通过实验验证了我们的新结果。有趣的是，简单的均匀加载仅比充水实现的吞吐量低不到 1% 到 2%，但是当我们将 OFDM 限制为离散 QAM 星座尺寸时，均匀加载的代价是 1.5 dB。受到均匀功率加载的良好性能的启发，我们提出了一种算法，以在优化频带内找到均匀功率的最佳离散位加载。由于预加重仍然很有吸引力，因为平坦的信道不需要自适应子载波加载，我们量化了它的惩罚。如果系统可以调整其传输带宽，从而自适应地将上副载波切换到零功率，则这可以是适度的。我们还研究了实际离散星座的影响，并通过实验验证了我们的新结果。有趣的是，简单的均匀加载仅比充水实现的吞吐量低不到 1% 到 2%，但是当我们将 OFDM 限制为离散 QAM 星座尺寸时，均匀加载的代价是 1.5 dB。受到均匀功率加载的良好性能的启发，我们提出了一种算法，以在优化频带内找到均匀功率的最佳离散位加载。由于预加重仍然很有吸引力，因为平坦的信道不需要自适应子载波加载，我们量化了它的惩罚。如果系统可以调整其传输带宽，从而自适应地将上副载波切换到零功率，则这可以是适度的。我们还研究了实际离散星座的影响，并通过实验验证了我们的新结果。有趣的是，简单的均匀加载仅比充水实现的吞吐量低不到 1% 到 2%，但是当我们将 OFDM 限制为离散 QAM 星座尺寸时，均匀加载的代价是 1.5 dB。受到均匀功率加载的良好性能的启发，我们提出了一种算法，以在优化频带内找到均匀功率的最佳离散位加载。由于预加重仍然很有吸引力，因为平坦的信道不需要自适应子载波加载，我们量化了它的惩罚。如果系统可以调整其传输带宽，从而自适应地将上副载波切换到零功率，则这可以是适度的。