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An Overview of Indoor OFDM/DMT Optical Wireless Communication Systems

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### An Overview of Indoor OFDM/DMT Optical Wireless Communication Systems

Speaker: Shao-CiJheng

Advisor: Dr. Ho-Ting Wu

2013/12/23

Outline

- Background
- Introduction
- Optical Wireless OFDM Techniques
- Half-wave symmetry OFDM techniques
- DC-biased OFDM techniques
- Results
- Conclusions
- References

Background-VLC

- 全名為Visablevisiblelight communication (VLC)
- 由Harald Haas取名為Li-Fi
- 是一種無線通訊技術
- 使用可見光做為傳輸媒介，波長介於400 THz(780 nm; 1 THz = 1000 GHz)至800 THz (375 nm)之間
- 以人肉眼無法察覺的速度快速閃爍燈泡來傳輸資料

Background-VLC 特點

- 通常使用LED或是發光二極體，成本比Wi-Fi更便宜
- 光無法穿透牆面
- 能在電磁敏感的領域中使用，如：核電廠、飛機上
- 可見光的頻譜比無線電波大10,000倍
- 傳輸距離可以到達 1-2 公里

Background-VLC 發展

- 在2003年，由日本慶應大學的實驗室第一次被實作出來
- 在2011年11月，在 TED Global talk，Li-Fi第一次被公開展示

其作者Professor HaraldHaas被公認為是Li-Fi之父

在此次展示中，Li-Fi傳輸速率可達10Mb∕秒

- 在2013年11月，英國研究學者以紅、藍、綠三原色，每種燈光3.3Gb/s的速度傳輸資料，合起來傳輸速率接近10Gb/s
- 目前推動這個技術的主要組織是visible light communication consortium(VLCC)、 IEEE Wireless Personal Area Networks working group (802.15)

Introduction

- Next generation wireless communication systems (5G) will be based on several complementary access technologies and OW(Optional Wireless) is expected to be essential in the 5G vision.

Introduction

- OW technology can be considered motivated by the several benefits as compared to RF(Radio frequency) systems:
- unregulated huge (THz) bandwidth
- license-free operation
- low-cost front-ends
- no interference with RF which makes it a preferred solution for RF-sensitive operating environments
- free of any health concerns as long as eye and skin safety regulations are fulfilled

Introduction

- OW basic system types fall intodiffuse or line of sight (LOS) systems.
- LOS systems:

high data rates in the order of Gbit/s can be achieved

but the system is vulnerable to blockage/shadowing becauseof its directionality.

- Diffuse OW system:

several paths from source to receiver exist, which makes the system robust to blockage/shadowing.

However, the path losses are high and multi-paths create inter-symbol interference (ISI) which limits the achievable data rate.

Introduction

- A promising solution to combat multipath distortion and boost the data rate without any bandwidth or power expansion is by using OFDM technique.
- In general, the output of the OFDM modulator is complex and bipolar. In IM(intensity modulation) optical systems, quadrature(法文:正交) modulation is not possible.
- This paper provides an overview of existing OFDM modulationtechniques suitable for OW communication.

Optical Wireless OFDM Techniques

q(k) : raw data bits are to be transmitted over the optical channel

data vector x (k) : modulation using QAM, PAM, or phase shift keying(PSK) maps

the bits to data symbols and arrange

N : number of available subcarriers

B: the channel bandwidth

Optical Wireless OFDM Techniques

- Half-wave symmetry OFDM techniques
- the subcarriers are assigned to produce a half-wave symmetrytime signal.
- DC-biased OFDM techniques
- assigns data to all available subcarriers.
- DC-biased OFDM techniques increasing the data rate as compared to the Half-wave symmetry OFDM techniques but the output signal is no longer symmetrical.

Half-wave symmetry OFDM techniques

- The half-wave symmetry of s(n) means that the same information in the first N/2 samples is repeated in the second half of the OFDM symbol.
- Two techniques are reported in literature where a half-wave symmetry OFDM time signal can be achieved
- ACO-OFDM(asymmetrically clipped optical OFDM)
- PAM-DMT(pulse-amplitude-modulated discrete multi-tone)

Half-wave symmetry OFDM techniques

x *(N -k) is the complex conjugate transpose of the input data vector x (k)

Ng : number ofguard subcarriers

m = log(M)

M : thesize of the considered constellation diagram

Half-wave symmetry OFDM techniques

- Half-wave symmetry OFDM systems have several advantages among of which are the following:
- A DC biased is avoided
- A larger amplitude of the signal can be considered which covers the full dynamic range of the LED.
- It is orthogonal to the transmitted data on the odd subcarriers and has no significant impact on the performance.

Half-wave symmetry OFDM techniques

- Conversely, these systems suffer from several drawbacks as follows,
- techniques sacrifice a significant portion of the available signaling bandwidth (typically up to 30 MHz).
- For VLC (visible light communication) systems, a DC bias is needed for the lighting purposes which makes the clipping at the zero level redundant.
- For PAM-DMT systems, the signal constellation is limited to real values only which suffers from larger probability of error as compared to QAM and PSK constellations.

DC-biased OFDM techniques

- DC-OFDM
- OPAM-DMT

DC-biased OFDM techniques

The data rate of DC-OFDM system can be calculated using the same equation given in (3).

DC-biased OFDM techniques

- OPAM-DMT

DC-biased OFDM techniques

- In summary, DC-biased OFDM systems achieve higher data rates as compared to half-wave symmetry OFDM systems, but they are subject to several issues:
- The DC bias results in an additional power consumption which reduces the power efficiency of these systems.
- The LED nonlinearity and clipping effects are more significant and highly depends on the considered bias point.
- The bias point reduces the LED dynamic range as compared to half-wave symmetry OFDM transmission approaches. This limits the transmitted optical power.

Conclusions

- The first group of OW OFDM techniques produces half-wave symmetry time signals and allow clipping at the zero level. Thus, reducing DC power consumption by avoiding the need for a DC bias.
- These advantages are achieved at the expense of a major reduction in data rate as compared to DC-biased OFDM systems.
- It is shown that high modulation orders are impractical for large signal power. It is also shown that AWGN noise dominates at low SNR values and clipping distortion dominates at large SNR values.

References

- RaedMesleh , JanyElgala , Harald Haas , ” An Overview of Indoor OFDM/DMT Optical Wireless Communication Systems”, 2010 7th International Symposium on Communication Systems Networks and Digital Signal Processing (CSNDSP), 21-23 July 2010,pp.566 – 570
- MyOOPS開放式課程,

URL:http://www.myoops.org/main.php?act=course&id=2363

- WiKi,

URL: http://en.wikipedia.org/wiki/Orthogonal_frequency-division_multiplexing

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