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1 Prof. Brandt-Pearce Lecture 3 Transmitters, Receivers, and Modulation Techniques Optical Wireless Communications

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Optical Transmitter LED Laser Lamp Optical Receiver Detection Techniques: Direct Detection Coherent Detection Photodetectors p-i-n Avalanche Photo Diode (APD) Photo Multiplier Tube (PMT) Modulation Techniques Transmitters/Receivers and Modulation in FSO Systems 2

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3 LED Semiconductor device Medium modulation speed Incoherent output light Mainly used for short range FSO systems (shorter than 1 km) Laser Highly directional beam profile Used for long range FSO systems High modulation speed Coherent output light Lamp Lower efficiency compared to LED and laser Lower cost Low modulation speed Incoherent output light Provides higher power Optical Transmitters

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4 A semiconductor p–n junction device that gives off spontaneous optical radiation when subjected to electronic excitation The electro-optic conversion process is fairly efficient, thus resulting in very little heat compared to incandescent lights Mainly used for short-range FSO systems (shorter than 1 km) Ultraviolet communications Indoor FSO systems Optical Transmitters: LED Illustration of the radiated optical power against driving current of an LED

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5 LED Types Optical Transmitters: LED Dome LED Edge-Emitting LED Planar LED

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6 Laser: light amplification by stimulated emitted radiation Has highly directional beam profile Is used for long range FSO systems Has narrow spectral width compared to LED Optical Transmitters: Laser Laser output power against drive current plot

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7 Laser Types Optical Transmitters: Laser Fabry-Perot Laser Distributed Feedback Laser Vertical-cavity surface- emitting Laser (VCSEL)

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Optical Transmitters 8

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9 Can be used in FSO communications, not in fiber optics Wideband and continuous spectrum Have very high power, but undirected The electro-optic process is inefficient, and huge amount of energy is dissipated as heat (causes high temperature in lamps) Has very low modulation bandwidth Divided as follows Carbon button lamp Halogen lamps Globar Nernst lamp Optical Transmitters: Lamp

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Optical Receivers The purpose of the receiver is: To convert the optical signal to electrical domain Recover data Direct-Detection Receiver: 10

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Coherent-Detection Receiver For detecting weak signal, coherent detection scheme is applied where the signal is mixed with a single-frequency strong local oscillator signal. The mixing process converts the weak signal to an intermediate frequency (IF) in the RF for improved detection and processing. 11 Optical Receivers

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Photodetectors 12 A square-law optoelectronic transducer that generates an electrical signal proportional to the square of the instantaneous optical field incident on its surface The ratio of the number of electron–hole (e–h) pairs generated by a photodetector to the incident photons in a given time is termed the quantum efficiency, η Dark current: the current through the photodiode in the absence of light Noise-equivalent power (NEP): the minimum input optical power to generate photocurrent equal to the root mean square (RMS) noise current in a 1 Hz bandwidth Responsivity: photocurrent generated per unit incident optical power (W/A)

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13 Photodetectors p-i-n photodetector Consists of p- and n-type semiconductor materials separated by a very lightly n-doped intrinsic region In normal operating conditions, a sufficiently large reverse bias voltage is applied across the device The reverse bias ensures that the intrinsic region is depleted of any charge carriers

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14 Photodetectors

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15 Photodetectors APD vs p-i-n diode

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16 Photodetectors Photo Multiplier Tube (PMT) Multiplies the current produced by incident light by as much as 100 million times (i.e., 160 dB), in multiple dynode stages Enables individual photons to be detected when the incident flux of light is very low Superior in response speed and sensitivity (low light-level detection) Has low quantum efficiency and high dark current

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17 Noise in Optical Receivers

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18 Noise in Optical Receivers

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Amplified Spontaneous Emission (ASE) Noise Produced by spontaneous emission that has been optically amplified by the process of stimulated emission in a gain medium Inherent in lasers and optical amplifiers ASE usually limiting noise source for high power levels ASE is added to the optical signal when it is amplified In a nonlinear medium interacts with signal and generates a random output σ 2 sig-sp : generated due to the interaction of ASE and main signal σ 2 sp-sp : generated due to the interaction of ASE with itself 19 Noise in Optical Receivers

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20 Signal to Noise Ratio in Optical Receivers

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Bit Error Rate (BER) is defined as the ratio of the number of wrong bits over the number of total bits. Probability of error is the theoretically predicted expected BER. The more the signal is affected, the more bits are incorrect. The BER is the fundamental specification of the performance requirement of a digital communication system It is an important concept to understand in any digital transmission system since it is a major indicator of the health of the system. Its important to know what portion of the bits are in error so you can determine how much margin the system has before failure. Bit Error Rate and Bit Error Probability

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22 Detector for OOK r(t) MF or LPF X TsTs Threshold Decision statistic

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Assuming a Gaussian additive noise the probability of the received signal, x, conditioned on 0 and 1 are as follows Probability of Error for OOK μ1μ1 x p1(x)p1(x) σ12σ12 μ0μ0 x p0(x)p0(x) σ02σ02 μ 1 : mean of x when bit 1 is transmitted μ 0 : mean of x when bit 0 is transmitted σ 1 2 : variance of x when bit 1 is transmitted σ 0 2 : variance of x when bit 0 is transmitted σ 1 2 can be different from σ 0 2 (in most optical systems it is)

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We need a threshold to decide between bit 0 and bit 1 The rule is: If x > Threshold, then decide bit 1 was sent If x < Threshold, then decide bit 0 was sent Probability of Error for OOK μ1μ1 p (x)p (x) σ12σ12 μ0μ0 x σ02σ02 Optimum Threshold So the error probability is We need to choose Threshold such that BER is minimized

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When μ 0 =0, μ 1 =A and σ 1 2 =σ 0 2 =σ 2, the optimal threshold is A/2, and BER becomes P e = Q(A/2σ) where Q(.) is Gaussian error function A 2 is the energy received for bit 1 σ 2 is the energy of the noise A 2 /σ 2 is called signal to noise ratio (SNR) and A/2σ is called Q-factor (Quality factor) Probability of Error for OOK A A/2 0 Threshold Decide b=1 Decide b=0

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26 When μ 0 0, and/or σ 1 2 σ 0 2, the optimal threshold becomes Then the probability of error approximates as where Q(.) is Gaussian error function Same as for fiber systems! Probability of Error for OOK

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28 Modulation Techniques

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29 Important Criteria in FSO

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30 Important Criteria in FSO

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31 Preferred Modulation Techniques in FSO Systems On-Off Keying (OOK) Most common technique for intensity-modulation/direct-detection (IM/DD) Simple to implement, easy detection Requires a threshold to make an optimal decision: a problem due to time-varying fading Return-to-Zero (RZ): the pulse occupies only the partial duration of bit Non-Return-to-Zero (NRZ): a pulse with duration equal to the bit duration is transmitted to represent 1 Transmitted waveforms for OOK: (a) NRZ and (b) RZ Modulation Techniques: OOK

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32 BER against the average photoelectron count per bit for OOK-FSO in a Poisson atmospheric turbulence channel Modulation Techniques: OOK

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33 Modulation Techniques: PPM

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For PPM we integrate over all chip times and then choose the maximum Probability of Error for PPM The error probability can be written as Lets denote sampled value in time chip i by x i, then This is called union bound

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35 Binary PPM, No Turbulence For short-range FSO systems, the BER is

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36 Binary PPM, Turbulence In the presence of turbulence, the BER is bounded by

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37 Modulation Techniques: PPM BER versus the scintillation index

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38 Preferred Modulation Techniques in FSO Systems Orthogonal Frequency Division Multiplexing (OFDM) Harmonically related narrowband sub-carriers Sub-carriers spaced by 1/Ts The peak of each sub-carrier coincides with trough of other sub- carriers Splitting a high-speed data stream into a number of low-speed streams Different sub-carrier transmitted simultaneously Guard intervals (CP) are added to reduce ISI effect Modulation Techniques: OFDM

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39 Modulation Techniques: OFDM

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40 Challenges and problems with FSO systems Nonlinearity of optical devices cause distortion The main drawback of OFDM with IM/DD is its poor optical average power efficiency This is because the OFDM electrical signal has both positive and negative values and must take on both values A DC offset must be added As the number of subcarrier signals increase, the minimum value of the OFDM signal decreases, becoming more negative Consequently the required DC bias increases, thus resulting in further deterioration of the optical power efficiency Regarding the restrictions on the average transmitted optical power in FSO system, the number of subcarriers is limited Modulation Techniques: OFDM

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41 Modulation Techniques: OFDM

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42 M-ary PAMM-ary PPMOOK 2M2PAPR log 2 Mlog 2 M/M1Spectral Efficiency Modulation Techniques Optical power gain over OOK versus bandwidth efficiency (first spectral null) for conventional modulation schemes

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43 Error control coding (ECC) is required in communication systems to improve error rate. Extra parity bits are added at the transmitter, so improved performance at the expense of reduced spectral efficiency At the decoder, errors can be corrected using the redundant bits Reed-Solomon and convolutional codes are conventional forward error correction (FEC) schemes in optical links. New: LDPC codes Modulation Techniques Error Control Coding

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