混合OFDM-WDM可见光通信系统实验研究

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"混合OFDM-WDM可见光通信技术是一种结合了正交频分复用(OFDM)和波分复用(WDM)的新型可见光通信系统。该技术利用直接检测替代复杂的数字信号处理，如信道估计、信道均衡和前向错误校正，以实现高效、简单的通信方案。实验中，通用的红、绿、蓝发光二极管(LED)被用于高速WDM为基础的三通道VLC，而白光LED则用于室内照明。每个通道采用直流光学OFDM来提高频率利用率。已实现在1米传输距离上的室内应用中，数据速率达到了288Mbps。关键词：可见光通信，OFDM，波分复用" 本文是一篇研究论文，探讨了混合OFDM-WDM可见光通信系统的设计与应用。在可见光通信（VLC）领域，这种混合系统具有显著的优势。OFDM技术因其对多径衰落和频率选择性衰落的抵抗力强，以及易于实现高速数据传输而被广泛应用。WDM则通过在不同波长上同时传输多个信号，极大地增加了光通信系统的容量。在本研究中，作者选择了直接检测方法，这种方法比基于数字信号处理的方法更为简单，减少了硬件复杂性和功耗。采用红、绿、蓝LED作为光源，构建了三个独立的通信通道，每个通道利用直流光学OFDM技术，这有助于提高频谱效率，减少带宽浪费。直流光学OFDM是将直流偏置添加到OFDM信号中，以克服LED的非线性特性，同时保持高效的频率使用。实验结果显示，该混合系统在室内1米的范围内实现了288Mbps的数据传输速率，这对于室内环境，如办公室、家庭和公共场所的无线通信来说，是一个非常有前景的速度。这表明混合OFDM-WDM VLC系统在提供高带宽无线接入和照明服务的同时，具有巨大的潜力。这项研究为可见光通信的发展提供了新的思路，特别是在室内短距离通信场景下，能够满足高速数据传输的需求，并且简化了系统的实现。未来的研究可能将探索如何进一步提高数据速率，扩大传输距离，以及优化系统的稳定性和可靠性，以推动可见光通信在物联网、5G网络等领域的实际应用。

A hybrid OFDM-WDM Visible Light

Communications

(Invited paper)

Zabib Ghassemlooy

, Zhen Zhan

, Min Zhang

, Dahai Han

, and H. Le-Minh

Optical Communications Research Group, NCRLab, Northumbria University, UK, Email: z.ghassemlooy@northumbria.ac.uk

State Key Lab of Info.Photon. & Opt. Comm., BUPT, Beijing 100876, China, Email: zhenzhan@bupt.edu.cn

Abstract—The paper presents a hybrid orthogonal frequency

division multiplexing (OFDM) – wavelength division

multiplexing (WDM) visible light communication (VLC). Direct

detection is adopted in place of complex digital signal processing

such as channel estimation and channel equalization, or even

forward error corrections. Generic red, green and blue light

emitting diodes (LED) are used for high-speed WDM based 3-

channel VLC and white LED for indoor illumination. In each

channel, we have used direct current optical OFDM to improve

the frequency utilization efficiency. We report an experimental

data rate of 288 Mb/s over a transmission span of 1m for indoor

applications.

Keywords—Visible light communications, OFDM, Wavelength

division multiplexing

I. INTRODUCTION

Due to the exponentially growing demand for bandwidth by

the end-users, we are continuously looking for new ways of

exploiting further the bandwidth efficiency potential of current

radio frequency (RF) based wireless technologies. However,

the spectrum congestion has become a real issue in RF

technologies demanding radical action to address the future

demand for the bandwidth. One altenaive complementary

technology that could be exploited in number applications both

for indoor and outdoor environments is the emerging optical

wireless technologies due to their broad bandwidths, license

free spectra and inherent security. In recent years we have seen

growing research and development activities at a global level in

visible light communications (VLC) with the wavelength

between ~ 400nm (750THz) and ~ 700nm (428THz) as a

complementary technology to WiFi based systems in a number

of applications requiring high data rates connectivity including

homes, offices, hospitals, transportation, underwater, etc. [1,2].

This growing interest is mostly driven by the availability of

white light emitting diodes (LED) for illumination, which are

highly energy efficient and green, harmless for human body,

eye safety, low cost, and have longer life-span. The unique

feature of ceiling mount LEDs is their triple functionalities of

indoor illumination, data communications and indoor

localization [2-4].

There mainly two innovative light-generating process using

LEDs. Traditionally, a blue-emitting chip is coated along with

a yellow phosphor coating that converts the narrow wavelength

light into something the human eye perceives as white. With

the appropriate mix of phosphor materials, one can set the tone

of the light from cool to warm, depending on the application

they have in mind.!Warmer hues is achieved by stretching the

phosphor to longer wavelengths, but at the cost of reduced

emitted photons energy and ultimately reduced efficiency. To

overcome a combined red and blue LEDs are coating a

phosphor, which acts as a diffuser, thus ensuring a uniform

white light emission in all direction. The bandwidth of the

commercial phosphor-based LEDs is thus limited to few MHz

(typically 2-3 MHz [1]), because of the slow response of the

phosphors. In order to overcome this limitation, a blue filter at

the receiver can be used to suppress the slow phosphorescent

components; this increases the bandwidth up to 20 MHz [5].

One of the current white LED technologies is based on

combining red, green and blue (RGB) chip devices. The RGB

based LEDs clearly offers at least two significant advantages

for data communications including higher capacity due to the

bandwidth of the individual components in the triplet and the

potential for parallel transmission using wavelength division

multiplexing (WDM). But the downside with RGB LEDs is the

colour balancing, since the individual LEDs are switched on at

arbitrary and uncorrelated intervals [6]. The single chip LED is

the most widely used because of the reduced complexity of the

driving circuitry without the need for the colour balancing in

general lighting industry [7]. However, this is a trade-off

between reduced complexity and the slow transient response of

the phosphor. The latter reduces the device modulation

bandwidth by almost an order of magnitude to typically 3-4

MHz [2]. The phosphor effect may be mitigated by using a

blue optical filter at the receiver, but at the cost of substantial

power penalties where up to 90% of the received optical power

is filtered. In [8] OOK based VLC scheme employing a blue

filter was reported with a data rates of > 100 Mbps.

To increase the bandwidth a number of techniques have been

adopted including equalization, which has increased the

bandwidth by more than an order of magnitude over a very

short transmission distance [6,7,9]. Spectrally efficient

modulation schemes such orthogonal frequency division

multiplexing (OFDM) or discrete multi-tone (DMT) [10],

which also take advantage of the high signal-to-noise ratio

(SNR) available in VLC systems have been used to increase

the data rate [11, 12]. A number of high-speed VLC solutions

at hundreds of Mb/s and even at Gbps level have been reported

[13-18]. For example, a 1.1 Gbps off-line VLC system

employing 32 quadrature amplitude modulation (QAM) with

four space division multiplexing channels and 4 special LEDs

with a 3-dB bandwidth of 20 MHz and a fish eye lens over a

link span of 1 m has been reported [17]. A 750 Mbps off-line

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