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S. K. Hashemi 1 , Z. Ghassemlooy 1 , L. Chao 2 , and D. Benhaddou 3

Orthogonal Frequency Division Multiplexing for Indoor Optical Wireless Communications using Visible Light LEDs. S. K. Hashemi 1 , Z. Ghassemlooy 1 , L. Chao 2 , and D. Benhaddou 3 1 Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK

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S. K. Hashemi 1 , Z. Ghassemlooy 1 , L. Chao 2 , and D. Benhaddou 3

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  1. Orthogonal Frequency Division Multiplexing for Indoor Optical Wireless Communications using Visible Light LEDs S. K. Hashemi1, Z. Ghassemlooy1, L. Chao2, and D. Benhaddou3 1Optical Communications Research Group, NCRLab, Northumbria University, Newcastle upon Tyne, UK Web site: http://soe.unn.ac.uk/ocr 2Department of Electronic and Information Engineering, Hong Kong Polytechnic University 3Engineering Technology Department, College of Technology, University of Houston, USA CSNDSP 2008

  2. Contents • Introduction • Optical Wireless Communication (OWC) Links • Combating Inter-symbol Interference • OFDM • Channel Estimation • Simulation results • Conclusions CSNDSP 2008

  3. Introduction • Invented more than 40 years ago. • Has been adopted for several technologies: • Asymmetric Digital Subscriber Line (ADSL) services. • IEEE 802.11a/g, IEEE 802.16a. • Digital Audio Broadcast (DAB). • Digital Terrestrial Television Broadcast: DVD in Europe, ISDB in Japan • 4G, IEEE 802.11n, IEEE 802.16, and IEEE 802.20. CSNDSP 2008

  4. Rx Tx OWC Links • Non-LOS • Multipath Propagation • Intersymbol interference (ISI) • Difficult to achieve high data date due to ISI • LOS • No multipath Propagation • Only noise is limiting factor • Possibility of blocking • Tracking necessary to maintain LOS link • LOS Rx Tx CSNDSP 2008

  5. Techniques to Overcome ISI • Guard slots - in digital modulation (PPM, PIM) • Spread spectrum - at the expense of reduced bandwidth efficiency • Angle diversity - based on multibeam-narrow FOV transceiver • Adaptive decision equalizer • USE OF multi-carriers transmission • In this paper we investigate OFDM system where multipath induced ISI and the effects of background noise are reduced by taking the advantage of narrowband frequency interference, which affects only one of the frequency sub-bands when compare to the single-carrier modulation. CSNDSP 2008

  6. OFDM • OFDM mainly used in RF based wireless communication schemes offering • high data rates capability • high bandwidth efficiency • and capable of dealing with the multipath induced ISI [9] • Has been proposed in optical wireless communication systems [10] • Uses a large number of closely-spaced orthogonal sub-carriers, each modulated at a low symbol rate CSNDSP 2008

  7. OFDM - Types Flash OFDM from Flarion www.flarion.com Vector OFDM (V-OFDM) of Cisco, Iospan,etc. www.iospan.com Wideband-OFDM (W-OFDM) of Wi-LAN www.wi-lan.com -- 2.4 GHz band -- 30-45Mbps in 40MHz -- large tone-width (for mobility, overlay) -- MIMO Technology -- non-LoS coverage, mainly for fixed access -- upto 20 Mbps in MMDS -- Freq. Hopping for CCI reduction, reuse -- 1.25 to 5.0MHz BW -- mobility support • Wi-LAN leads the OFDM Forum -- many proposals submitted to • IEEE 802.16 Wireless MAN • Cisco leads the Broadband Wireless Internet Forum (BWIF) CSNDSP 2008

  8. OFDM Signal Conventional multicarrier techniques Ch.1 Ch.2 Ch.3 Ch.4 Ch.5 Ch.6 Ch.7 Ch.8 Ch.9 Ch.10 Frequency Orthogonal multicarrier techniques Ch.2 Ch.4 Ch.6 Ch.8 Ch.10 Ch.1 Ch.3 Ch.5 Ch.7 Ch.9 50% bandwidth saving Frequency CSNDSP 2008

  9. The FFT (and its inverse, the IFFT) are used to create a multitude of orthogonal subcarriers using just a single TX. Subcarrier orthogonality must be preserved in the presence of timing jitter frequency offset fading. T0 OFDM - Spectrum Magnitude Frequency CSNDSP 2008

  10. OFDM System Architecture Pilot signals uniformly inserted into all symbols (sub-carriers) for channel estimation for diffuse optical links only. CSNDSP 2008

  11. OFDM • IFFT converts X(k) of length N into a complex time-domain OFDM signal. • In order for the IFFT/FFT to create an ISI-free channel, the channel must appear to provide a circular convolution. • To mitigate the effects of multipath induced ISI, a guard interval of G-sample (or cyclic prefix (CP)) is inserted between symbols. • The length of CP depends on the channel delay spread and is normally considered to be grater than or equal to the channel length (impulse response time) and less than symbol duration CSNDSP 2008

  12. Circular convolution allows DFT! Circular Convolution & DFT/IDFT • Circular convolution: • Detection of X (knowing H): • (note: ISI free! Just a scaling by H) CSNDSP 2008

  13. OFDM - Cyclic Prefix (CP) • OFDM signal with CP is x[n]L, and so y[n] = x[n] * h[n]. • First v samples of ycp interference from preceding OFDM symbol => discarded. • The last v samples disperse into the subsequent OFDM symbol => discarded. • This leaves exactly L samples at output y, to recover the L data symbols embedded in x. CSNDSP 2008

  14. OFDM • The received OFDM signal propagated through the channel h(n) is given by: • where w[n] is the additive white Gaussian noise, is the photodetector responsivity, and denotes the circular convolution. Channel models: • Recursive algorithm to take into account higher order reflections [11], • MIMO method for speedy calculation of the impulse response [12]. • Mont Carlo ray-tracing algorithm and its modified version uses both the Lambert’s diffuse and the Phong’s models to approximate surfaces with a strong specular component [13-14]. • The Sphere model in [15] • Fast iterative model capable of calculating high-order reflections [17]. CSNDSP 2008

  15. Time Carriers Time Carriers OFDM - Channel Estimation • With no knowledge of the channel, channel estimation is required for equalization and decoding provided • In one dimensional channel estimation schemes pilot insertion is done in Block-type - all sub-carriers is used as pilot in a specific period. It uses the least square (LS) or the minimum mean-square error in a slow fading channel [18]) Comb-type - part of the sub-carriers are always reserved as pilot for each symbol, and it uses LS with interpolation and the maximum likelihood in a rapidly changing channel [19]) - Adopted Block type Come type CSNDSP 2008

  16. OFDM – Interpolation Schemes • Pilot symbols uniformly inserted into symbols according to • Information on the exact locations of the pilot and data symbols as well as the values of the pilot symbols are already available at the receiver. • The estimated channel at pilot subcarriers for the LS estimation (no need of prior knowledge of noise variance) is given as: • Interpolation schemes • linear interpolation, • second-order interpolation, • spline cubic interpolation • time domain interpolation which are used in the one dimensional model to extract channel information on the data sub-carriers Hd [18]. In [21, 23] detailed information on the number of pilots used and how to located them as optimal pilot design techniques are given. CSNDSP 2008

  17. I/Q I/Q Channel coding / interleaving Symbol mapping (modulation) OFDM modulation (IFFT) Guard interval 0110 010101001 1 OFDM symbol N symbols Receiver FFT-part Decoding / deinter-leaving symbol de-mapping (detection) OFDM demod. (FFT) Guard interval removal time I/Q I/Q Channel impulse response: Channel est. Time sync. OFDM - Simulation Transmitter CSNDSP 2008

  18. Simulation Parameters ParametersValues No. of Les 4 Transmitted optical power Ps 20.0 mW Centre luminance intensity of LED 730 mcd Field of view (FOV) 60o Photodetector surface area Ar 1 cm2 Lambert mode 0.6461 Concentrator refractive index nr 1.5 Photodetector responsivity 0.53 (A/W) FFT size 256 Number of data subcarriers 128 Number of pilots 16, 32, 64 Cyclic prefix (CP) ratio 1/4 The radiation pattern is assumed to be Lambertian CSNDSP 2008

  19. Horizontal Illuminance Distribution CSNDSP 2008

  20. Results - Error Performance for LOS optical wireless OFDM link employing BPSK, QPSK and 16-QAM CSNDSP 2008

  21. Results – Error performance for diffuse OW OFDM link with QPSK & LS channel estimation for a number of pilot symbols CSNDSP 2008

  22. Conclusions • Visible light LED’s was proposed for both lightening and communication system • OFDM technique employed to combat ISI due to the multipath reflections • Due to the variation of the optical channel impulse response in OW links, pilot signals added and the channel is estimated. • We simulated the illumination to meet the ISO for lighting and found the number of LEDs for the simulation parameters. • Performance of the system for both LOS and Diffuse link for BPSK,QPSK and M-QAM was simulated. CSNDSP 2008

  23. Thank you! CSNDSP 2008

  24. Transmitted signal: Received Signals: Line-of-sight: Reflected: The symbols add up on the channel  Distortion! Inter-Symbol-Interference (ISI) due to Multi-Path Reflections Delays CSNDSP 2008

  25. OFDM Symbols • Group L data symbols into a block known as an OFDM symbol. • An OFDM symbol lasts for a duration of T seconds, where T = LTs. • Guard period > delay spread • OFDM transmissions allow ISI within an OFDM symbol, but by including a sufficiently large guard band, it is possible to guarantee that there is no interference between subsequent OFDM symbols. • The next task is to attempt to remove the ISI within each OFDM symbol CSNDSP 2008

  26. Received LOS Power Maximum_power = 1.2270 [dBm] Minimum_power = -5.2891 [dBm] CSNDSP 2008

  27. OFDM Systems CSNDSP 2008

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