1 prof. brandt-pearce lecture 3 transmitters, receivers, and modulation techniques optical wireless...
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Prof. Brandt-Pearce
Lecture 3Transmitters, Receivers, and
Modulation Techniques
Optical Wireless Communications
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Optical Transmitter LED Laser Lamp
Optical ReceiverDetection 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
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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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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 communicationsIndoor FSO systems
Optical Transmitters: LED
Illustration of the radiated optical power against driving current of an LED
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LED Types
Optical Transmitters: LED
Dome LED
Edge-Emitting LED
Planar LED
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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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Laser Types
Optical Transmitters: Laser
Fabry-Perot Laser
Distributed Feedback Laser Vertical-cavity surface-
emitting Laser (VCSEL)
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Optical Transmitters
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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:
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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.
Optical Receivers
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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:
• : the average value of the total output current • : the primary unmultiplied photocurrent
Typical gain values lie in the range 50–300 Excess noise factor:
• : 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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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)
• : mean of generated photo-current (A)
Noise in Optical Receivers
𝜎 𝑠2=2𝑞 ⟨𝑖 ⟩ 𝐵
𝜎 𝑠2=2𝑞 ⟨𝑖 ⟩ 𝐵𝐹 𝑀 2
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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 (Ω) and temperature (K)
White noise since the power spectral density (PSD) is
independent of frequency
Distributed as a zero mean Gaussian process
Thermal noise variance: (A2)
• K: Boltzmann Coefficient (m2 kg s-2)
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
σ2sig-sp: generated due to the interaction of ASE and main signal
σ2sp-sp: generated due to the interaction of ASE with itself
Noise in Optical Receivers
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Receiver performance Definition of SNR given received signal r(t):
, or
For an optical receiver without any optical amplifier, SNR can be calculated as:
SNR =Ip2 / (σ2
T + σ2s)
For an optical receiver containing a p-i-n diode preceded by an EDFA, SNR can be calculated as:
SNR =Ip2 / (σ2
T + σ2s+ σ2
sig-sp+ σ2sp-sp)
Signal to Noise Ratio in Optical Receivers
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.
Bit Error Rate and Bit Error Probability
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Received signal is a function of time corrupted by additive noise
Optimal detector assuming ideal channel and Gaussian noise is the
matched filter (MF)
Often use a low pass filter (LPF) or integrator and sample:
Detector for OOK
r(t)MF or LPF
XTs
ThresholdDecision statistic
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
μ1 x
p1(x)
σ12
μ0 x
p0(x)
σ02
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 σ0
2 (in most optical systems it is)
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
μ1
p (x)
σ12
μ0 x
σ02
Optimum Threshold
So the error probability is
We need to choose Threshold such that BER is minimized
When μ0=0, μ1=A and σ12 =σ0
2 =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)
Probability of Error for OOK
A
A/2
0
Threshold
Decide b=1
Decide b=0
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When μ0 ≠ 0, and/or σ12 ≠ σ0
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
01
1001
01
01factor Q
where)factor-Q(
QPe
Probability of Error for OOK
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Modulation Techniques
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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, : 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
Important Criteria in FSO
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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, :
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)
Important Criteria in FSO
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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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BER against the average photoelectron count per bit for OOK-FSO
in a Poisson atmospheric turbulence channel
Modulation Techniques: OOK
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Preferred Modulation Techniques in FSO Systems Pulse-Position Modulation (PPM)
• Orthogonal modulation technique
• The symbol time divided into 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
Modulation Techniques: PPM
Symbol
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 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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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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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 constellation symbols, ,
using BPSK, QPSK or M-QAM.
: Number of constellation symbols
N : Number of orthogonal subcarriers
Block diagram of an optical OFDM
Modulation Techniques: OFDM
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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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1.0E-06
1.0E-05
1.0E-04
1.0E-03
1.0E-02
1.0E-01
1.0E+00
0 5 10 15 20 25 30OSNR (dB)
BE
R
256 QAM
64QAM
32QAM16QAMDQPSK
128QAMDBPSK
Modulation Techniques: OFDM
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M-ary PAM M-ary PPM OOK
2 M 2 PAPR
log2 M log2M/M 1 Spectral Efficiency
Modulation Techniques
Optical power gain over OOK versus bandwidth efficiency (first spectral null) for conventional modulation schemes
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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 TechniquesError Control Coding
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