Optical Wireless
Communications
Prof. Brandt-Pearce
Lecture 3
Transmitters, Receivers, and
Modulation Techniques
1
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
2
Optical Transmitters

LED





Laser





Semiconductor device
Medium modulation speed
Incoherent output light
Mainly used for short range FSO systems (shorter than 1 km)
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
3
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
4
Optical Transmitters: LED

LED Types


Planar LED
Dome LED

Edge-Emitting LED
5
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
6
Optical Transmitters: Laser

Laser Types


Vertical-cavity surfaceemitting Laser (VCSEL)

Fabry-Perot Laser
Distributed Feedback Laser
7
Optical Transmitters
8
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
9
Optical Receivers
The purpose of the receiver is:


To convert the optical signal to electrical domain
Recover data
Direct-Detection Receiver:
10
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.
11
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)
12
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
13
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:
• 𝐼𝑇 : the average value of the total output current
• 𝐼𝑃 = 𝑅𝑃𝑅 : the primary unmultiplied photocurrent


Typical gain values lie in the range 50–300
Excess noise factor:
1
1−𝜅
𝑀
• 𝜅: the ratio of the hole impact ionization
rate to that of electrons
𝐹 = 𝜅𝑀 + 2 −
14
Photodetectors

APD vs p-i-n diode
15
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
16
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: 𝜎𝑠2 = 2𝑞 𝑖 𝐵 (A2 )
 Shot noise in APD: 𝜎𝑠2 = 2𝑞 𝑖 𝐵𝐹𝑀2 (A2 )
• q: Electron charge (coulombs)
• B: Receiver equivalent bandwidth (Hz)
• 𝑖 : mean of generated photo-current (A)
17
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 𝑅𝐿 (Ω) and temperature 𝑇𝑒 (K)
 White noise since the power spectral density (PSD) is
independent of frequency
 Distributed as a zero mean Gaussian process
 Thermal noise variance: 𝜎𝑇2 =
4𝐾 𝑇𝑒 𝐵
𝑅𝐿
(A2)
• K: Boltzmann Coefficient (m2 kg s-2)
18
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
19
Signal to Noise Ratio in Optical Receivers
 Receiver performance
 Definition of SNR given received signal r(t):
SNR =
𝑟(𝑡)
𝑟(𝑡)
2
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)
20
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.
Detector for OOK
 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:
r(t)
Ts
MF or LPF
X
Threshold
Decision statistic
22
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
p0(x)
p1(x)
σ02
σ12
μ1
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)
μ0
x
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
μ0
σ12
μ1
Optimum Threshold
 So the error probability is
 We need to choose Threshold such that BER is minimized
x
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)
A
where Q(.) is Gaussian error function
Decide b=1
A/2
Threshold
Decide b=0
A2 is the energy received for bit “1”
0
 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
 When μ0 ≠ 0, and/or σ12 ≠ σ02, the optimal threshold becomes

 1 0   0 1 

1   0
Then the probability of error approximates as
Pe  Q(Q - factor) where
1   0
Q factor 
1   0
where Q(.) is Gaussian error function
Same as for fiber systems!
26
Probability of Error for OOK
Modulation Techniques
28
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, 𝜂𝑝 : 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
29
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, 𝜂𝐵 :
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
30
coding)
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
31
Modulation Techniques: OOK

BER against the average photoelectron count per bit for OOK-FSO
in a Poisson atmospheric turbulence channel
32
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
Symbol 𝑘
• Transmitted energy per symbol decreases in peak power limited systems
33
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
Binary PPM, No Turbulence
For short-range FSO systems, the BER is
35
Binary PPM, Turbulence
In the presence of turbulence, the BER is bounded by
36
Modulation Techniques: PPM
BER versus the scintillation index
37
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 subcarriers

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
38
Modulation Techniques: OFDM
 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 𝑁𝑑 constellation symbols,
𝑋 0 , 𝑋 1 , … , 𝑋[𝑁𝑑 − 1], using BPSK, QPSK or M-QAM.
 𝑁𝑑 : Number of constellation symbols
 N : Number of orthogonal subcarriers
 Block diagram of an optical OFDM
39
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
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
poor optical
40
Modulation Techniques: OFDM
1.0E+00
1.0E-01
BER
1.0E-02
1.0E-03
1.0E-04
256 QAM
16QAM 32QAM
DQPSK
1.0E-05
64QAM
DBPSK
1.0E-06
0
5
10
15
OSNR (dB)
128QAM
20
25
30
41
Modulation Techniques
OOK
M-ary PPM
M-ary PAM
PAPR
2
M
2
Spectral Efficiency
1
log2M/M
log2 M
Optical power gain over
OOK versus bandwidth
efficiency (first spectral
null) for conventional
modulation schemes
42
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
43