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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