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**Optical Wireless Communications**

Prof. Brandt-Pearce Lecture 3 Transmitters, Receivers, and Modulation Techniques

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**Transmitters/Receivers and Modulation in FSO Systems**

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

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**Optical Transmitters LED Laser Lamp 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 Provides higher power

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**Optical Transmitters: LED**

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 Illustration of the radiated optical power against driving current of an LED

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**Optical Transmitters: LED**

LED Types Planar LED Dome LED Edge-Emitting LED

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**Optical Transmitters: Laser**

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 Laser output power against drive current plot

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**Optical Transmitters: Laser**

Laser Types Fabry-Perot Laser Vertical-cavity surface-emitting Laser (VCSEL) Distributed Feedback Laser

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Optical Transmitters

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**Optical Transmitters: Lamp**

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

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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:

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**Optical Receivers 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.

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Photodetectors 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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**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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**Photodetectors Avalanche Photo-Diode (APD)**

provides an inherent current gain through the process called repeated electron This culminates in increased sensitivity since the photocurrent is now multiplied before encountering the thermal noise associated with the receiver circuit Multiplication (or gain) factor: 𝐼 𝑇 : the average value of the total output current 𝐼 𝑃 =𝑅 𝑃 𝑅 : the primary unmultiplied photocurrent Typical gain values lie in the range 50–300 Excess noise factor: 𝐹=𝜅𝑀+ 2− 1 𝑀 1−𝜅 𝜅: the ratio of the hole impact ionization rate to that of electrons

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Photodetectors APD vs p-i-n diode

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**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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**Noise in Optical Receivers**

Shot Noise Present in all photon detectors Is associated with the quantum nature of light The number of photons emitted by all optical sources, including coherent source in a given time is never constant For a constant power optical source, the mean number of photons generated per second is constant; yet the actual number of photons per second follows the Poisson distribution Shot noise in p-i-n: (A2 ) Shot noise in APD: (A2 ) q: Electron charge (coulombs) B: Receiver equivalent bandwidth (Hz) 𝑖 : mean of generated photo-current (A) 𝜎 𝑠 2 =2𝑞 𝑖 𝐵 𝜎 𝑠 2 =2𝑞 𝑖 𝐵𝐹 𝑀 2

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**Noise in Optical Receivers**

Thermal Noise Also known as Johnson noise Occurs in all conducting materials Caused by the thermal fluctuation of electrons in any receiver circuit of equivalent resistance 𝑅 𝐿 (Ω) and temperature 𝑇 𝑒 (K) White noise since the power spectral density (PSD) is independent of frequency Distributed as a zero mean Gaussian process Thermal noise variance: 𝜎 𝑇 2 = 4𝐾 𝑇 𝑒 𝐵 𝑅 𝐿 (A2) K: Boltzmann Coefficient (m2 kg s-2)

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**Noise in Optical Receivers**

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 σ2sig-sp: generated due to the interaction of ASE and main signal σ2sp-sp: generated due to the interaction of ASE with itself

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**Signal to Noise Ratio in Optical Receivers**

Receiver performance Definition of SNR given received signal r(t): SNR= 𝑟(𝑡) 𝑟(𝑡) 2 , or power of signal power of noise For an optical receiver without any optical amplifier, SNR can be calculated as: SNR =Ip2 / (σ2T + σ2s) For an optical receiver containing a p-i-n diode preceded by an EDFA, SNR can be calculated as: SNR =Ip2 / (σ2T + σ2s+ σ2sig-sp+ σ2sp-sp)

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**Bit Error Rate and Bit Error Probability**

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. It’s important to know what portion of the bits are in error so you can determine how much margin the system has before failure.

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**Detector for OOK r(t) X MF or LPF**

Received signal is a function of time corrupted by additive noise 𝑟 𝑡 =𝑠 𝑡 +𝑛 𝑡 Optimal detector assuming ideal channel and Gaussian noise is the matched filter (MF) Often use a low pass filter (LPF) or integrator and sample: Ts r(t) X MF or LPF Threshold Decision statistic

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**Probability of Error for OOK**

Assuming a Gaussian additive noise the probability of the received signal, x, conditioned on “0” and “1” are as follows p1(x) p0(x) σ12 σ02 μ1 μ0 x x μ1 : mean of x when bit “1” is transmitted μ0 : mean of x when bit “0” is transmitted σ12 : variance of x when bit “1” is transmitted σ02 : variance of x when bit “0” is transmitted σ12 can be different from σ02 (in most optical systems it is)

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**Probability of Error for OOK**

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 p (x) σ02 σ12 μ0 μ1 x Optimum Threshold So the error probability is We need to choose Threshold such that BER is minimized

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**Probability of Error for OOK**

When μ0=0, μ1=A and σ12 =σ02 =σ2 , the optimal threshold is A/2, and BER becomes Pe= Q(A/2σ) where Q(.) is Gaussian error function A2 is the energy received for bit “1” σ2 is the energy of the noise A2 /σ2 is called signal to noise ratio (SNR) and A/2σ is called Q-factor (Quality factor) A Decide b=1 A/2 Threshold Decide b=0

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**Probability of Error for OOK**

When μ0 ≠ 0, and/or σ12 ≠ σ02, the optimal threshold becomes Then the probability of error approximates as where Q(.) is Gaussian error function Same as for fiber systems!

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**Probability of Error for OOK**

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**Modulation Techniques**

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**Important Criteria in FSO**

Power Efficiency In portable battery-powered equipment, it is desirable to keep the electrical power consumption to a minimum, which also imposes limitations on the optical transmit power Power efficiency, 𝜂 𝑝 : the average power required to achieve a given BER at a given data rate Peak to Average Power Ratio (PAPR) The average optical power emitted by an optical wireless transceiver is limited due to the eye and skin safety regulations, and power utilization Optical Sources such as laser and LED have limited peak power PAPR = Peak Power Average Power

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**Important Criteria in FSO**

Spectral Efficiency (Bandwidth Efficiency) Although the optical carrier can be theoretically considered as having an ‘unlimited bandwidth’, the other constituents (optical source rise-time, photodetector area) in the system limit the amount of bandwidth that is practically available for a distortion-free communication system Also, the ensuing multipath propagation in diffuse link/nondirected LOS limits the available channel bandwidth Spectral efficiency, 𝜂 𝐵 : Acheivable Bit−Rate Bandwidth of the Transceiver or Channel Reliability A modulation technique should be able to offer a minimum acceptable error rate in adverse conditions as well as show resistance to the multipath-induced inter-symbol interference (ISI) (e.g., five 9s reliability) SNR is desired to be large and BER be smaller than some specification (after coding)

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**Modulation Techniques: OOK**

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

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**Modulation Techniques: OOK**

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

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**Modulation Techniques: PPM**

Preferred Modulation Techniques in FSO Systems Pulse-Position Modulation (PPM) Orthogonal modulation technique The symbol time divided into 𝑄 equal timeslots Only one of these time slots contains a pulse Low spectral efficiency: is used in FSO links where the requirement for the bandwidth is not of a major concern Does not require a threshold to make an optimal decision Transmitted energy per symbol decreases in peak power limited systems Symbol 𝑘

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**Probability of Error for PPM**

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

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**Binary PPM, No Turbulence**

For short-range FSO systems, the BER is

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

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**Modulation Techniques: PPM**

BER versus the scintillation index

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**Modulation Techniques: OFDM**

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

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**Modulation Techniques: OFDM**

Efficiently utilizes the available bandwidth Special version of subcarrier modulation where all the subcarrier frequencies are orthogonal Serial data streams are grouped and mapped into 𝑁 𝑑 constellation symbols, 𝑋 0 , 𝑋 1 , …, 𝑋[ 𝑁 𝑑 −1], using BPSK, QPSK or M-QAM. 𝑁 𝑑 : Number of constellation symbols N : Number of orthogonal subcarriers Block diagram of an optical OFDM

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**Modulation Techniques: OFDM**

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

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**Modulation Techniques: OFDM**

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**Modulation Techniques**

M-ary PAM M-ary PPM OOK 2 M PAPR log2 M log2M/M 1 Spectral Efficiency Optical power gain over OOK versus bandwidth efficiency (first spectral null) for conventional modulation schemes

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**Modulation Techniques**

Error Control Coding 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

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