Optical Wireless (Free Space Infrared) Communications

ADROIT GroupDept. of Electrical Engineering

Ryerson University

Acknowledgement

J. M. Kahn, Pahlavan, Tim O’ Farrell,Z. Ghassemlooy, N.M. Aldibbiat, A.

Hayes and C.K. SeeSheffield Hallam University, UK

Scenario for WLANusing Infrared Radiations:

Intensity Modulation/Direct Detection (IM/DD)

• Wireless infrared links are based on intensity modulation and direct detection of the optical carrier.

• Intensity modulation is performed by varying the current of the laser diode or LED.

• Direct detection is by photodiodes, producing electric current proportional to incident optical power.

Radiation Patterns

LambertianSource

More Directional Sources (LASER)

LED is more directional than a Lambertian source,worse than Laser

Multipath Propagation causes Temporal Dispersion.The Long tail lasts for 50 ns

This induces inter Symbol Interference in high bit rate communications

Inter Symbol Interference (ISI)

The broadening of transmitted pulse in time which results in the overlapping of neighboring pulses is called Inter Symbol Interference (ISI).

ISI happens because of the multipath propagation.The multipath propagation is reflected in the channel

impulse response hc(t).ISI makes it difficult for the receiver to make the

correct decision.ISI can be removed by equalization.

Block Diagram for Infrared WLANBinary

I/P Signal

OOK

Modulation

LASER

DIODE

WIRELESS

CHANNEL

Photo

DetectorOOK

DemodulatorEqualizer

Binary

O/P

Signal

Characterizing Infrared Channels:

• Average transmitted optical power that is required to achieve certain bit rate for a particular modulation scheme depends on the channel characteristics.

• This power requirement is characterized into two parameters.

1) Optical path loss.2) Delay spread.

Two Mathematical Models1) Exponential-Decay Model.

Because of multi path reflections, the impulse response is a sequence of delta functions whose amplitudes decay exponentially. This geometric series is represented by this mathematical model:

hc(t,τ) =1/τ exp(-t/ τ ) u(t).2) Ceiling Bounce Model.

Reflections from a single infinite-plane reflector make impulse response to develop Ceiling-bounce model:

hc(t, a) = 6a6 / (t + a)7 u(t).

An expression for the model is given as:

x(t) = Baseband information sequence (1,0)hc (t) = Impulse response of the wireless

channelhc(t,τ) =1/τ exp(-t/ τ ) u(t).y (t) = x (t) * h c (t) + n (t)

y (t) = Output of the channel at the receiver

n (t) = Noise added at the channel

Xx(t): Input Binary Signal Wireless Channel (hC(t))

X y(t) h equalizery2(t): Output Binary signal

Convolution

Convolution

An equalizer can compensate for wireless channel dispersion

Equalization:• In order to remove ISI, filter with transfer function

Hc (f)Heq(f) = Constant

The output after the detection is convolved with the transfer function of the Equalizer

The output of the equalizer gives back the information sequence as it is given at the input of the channel at the transmitter

Multiple Access Techniques

• When multiple users share a common channel, they should be coordinated

• TDMA – Time Division Multiple Access• FDMA – Frequency Division Multiple

Access• CDMA – Code Division Multiple Access• WDMA – Wavelength Div. Multi Access

Advantages of CDMA• Enables low power operation• Noise reduction by averaging the noise• Multipath (delayed) components get

removed• Multiuser Interference (other user signals)

also get rejected• No synchronization between users needed

But needs Much Higher Bandwidth.