Illumination LEDs, but also infrared LEDs have limited bandwidth. To achieve high throughput, one needs to modulate the LED significantly above its 3 dB bandwidth. Orthogonal Frequency Division Multiplexing (OFDM) is a popular modulation technique to cope with the frequency selectivity of the LED channel. In this article, we challenge whether its large Peak-to-Average-Power Ratio (PAPR) and resulting large DC bias are justified. We compare systems using the same power and derive how PAM and OFDM variants reach their optimum throughput at different bandwidths and differently shaped spectral densities, thus at very different Signal to Noise Ratio (SNR) profiles but nonetheless the same transmit power.When corrected for the path loss and normalized to the noise power in the 3 dB bandwidth of the LED, we call this the Normalized Power Budget (NPB). OFDM can exploit the low-pass LED response using a waterfilling approach. This is attractive if the NPB exceeds 60 dB. OFDM will then have to spread its signal over more than ten times the LED bandwidth and requires a DC bias of more than 4 times the rms modulation depth. Second-order distortion and LED droop may then become a limitation, if not compensated. At lower power (NPB between 30 and 60 dB), DCO-OFDM outperforms PAM, provided that it significantly reduces its bias and only if it uses an appropriate adaptive bit and power loading. Without adaptive bit loading, thus using a frequency–constant modulation order, for instance made feasible by a pre-emphasis, OFDM always shows lower performance than PAM; about 2.5 dB at a NPB around 60 dB. Below 30 dB of NPB, even waterfilling cannot outweigh the need for a larger bias in OFDM, and PAM should be preferred. We argue that a mobile system that has to operate seamlessly in wide coverage and short–range high–throughput regimes, needs to adapt not only its bandwidth and its bit–loading profile, but also its DCO-OFDM modulation depth, and preferably falls back from OFDM to PAM.

中文翻译：

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用于光无线通信的M-PAM和光OFDM调制的优化和比较

照明LED，还有红外LED的带宽有限。为了实现高吞吐量，需要将LED调制到明显高于其3 dB带宽。正交频分复用（OFDM）是一种流行的调制技术，用于应对LED通道的频率选择性。在本文中，我们质疑其大的峰均功率比（PAPR）和由此产生的大DC偏置是否合理。我们比较使用相同功率的系统，并得出PAM和OFDM变体如何在不同的带宽和不同形状的频谱密度，因此在非常不同的信噪比（SNR）曲线，但相同的发射功率下达到最佳吞吐量的情况。损耗和归一化到LED的3 dB带宽内的噪声功率，我们称之为归一化功率预算（NPB）。OFDM可使用注水方法来利用低通LED响应。如果NPB超过60 dB，这将很有吸引力。然后，OFDM将不得不将其信号扩展到LED带宽的十倍以上，并且需要大于有效值调制深度4倍的DC偏置。如果不进行补偿，则二阶失真和LED下降会成为限制。在较低功率（NPB在30至60 dB之间）的情况下，DCO-OFDM的性能要优于PAM，前提是它可以显着降低其偏置并且仅在使用适当的自适应位和功率负载的情况下。没有自适应比特加载，因此使用频率恒定的调制阶数（例如通过预加重使之可行），OFDM总是表现出比PAM更低的性能。NPB约为60 dB时约为2.5 dB。NPB低于30 dB时，即使充水也不能超过OFDM中更大偏置的需求，和PAM应该是首选。我们认为，必须在宽覆盖范围和短距离高通量模式下无缝运行的移动系统，不仅需要适应其带宽和比特加载特性，还需要适应其DCO-OFDM调制深度，并且最好回退从OFDM到PAM。