Wireless Transmission Fundamentals (Physical Layer)

Professor Honggang Wang

Email: hwang1@umassd.edu

Electromagnetic Spectrum Wireless communication uses 100 kHz to 60

GHz

The Layered Reference Model

3

Application

Transport

Network

Data Link

Physical

Medium

Data Link

Physical

Application

Transport

Network

Data Link

Physical

Data Link

Physical

Network Network

Radio

Often we need to implement a function across multiple layers.

Outline RF introduction

Antennas and signal propagation How do antennas work Propagation properties of RF signals

Modulation and channel capacity

What is Antenna

Conductor that carries an electrical signal and radiates an RF signal. The RF signal “is a copy of” the electrical

signal in the conductor

Also the inverse process: RF signals are “captured” by the antenna and create an electrical signal in the conductor. This signal can be interpreted (i.e. decoded)

Efficiency of the antenna depends on its size, relative to the wavelength of the signal. e.g. half a wavelength

Types of Antennas Antenna is a point source that

radiates with the same power level in all directions – omni-directional or isotropic An antenna that transmits equally in all

directions (isotropic) Shape of the conductor tends to create

a specific radiation pattern Common shape is a straight

conductor Shaper antennas can be used to

direct the energy in a certain direction Well-know case: a parabolic antenna

A parabolic antenna

Signal Propagation Ranges Transmission range

communication possible low error rate

Detection range detection of the signal

possible no communication

possible Interference range

signal may not be detected

signal adds to the background noise

distance

sender

transmission

detection

interference

Signal propagation

Propagation in free space always like light (straight line) Receiving power proportional to 1/d² in vacuum – much more in real

environments(d = distance between sender and receiver)

Receiving power additionally influenced by fading (frequency dependent) Shadowing Reflection at large obstacles Refraction depending on the density of a medium Scattering at small obstacles Diffraction at edges

reflection scattering diffractionshadowing refraction

Propagation Degrades RF Signal

Attenuation in free space Signal gets weaker as it travels over

longer distance Free space loss- Signal spreads out Refraction and absorption in the

atmosphere Obstacle can weaken signal

through absorption or reflection. Part of the signal is re-directed.

Multiple path effects Multiple copies of the signal interfere

with each other Mobility

Moving receiver causes another form of self interference

Node moves ½ wavelength cause big change in signal strength

path loss

log (distance)

Received Signal

Power

(dB)

location

Decibels Attenuation = 10 Log10 (Pin/Pout) decibel Attenuation = 20 Log10 (Vin/Vout) decibel

Example 1: Pin = 10 mW, Pout=5 mW Attenuation = 10 log 10 (10/5) = 10 log 10 2 = 3

dB

Example 2: Pin = 100mW, Pout=1 mW Attenuation = 10 log 10 (100/1) = 10 log 10 100 =

20 dB

Shadowing

11

Signal strength loss after passing through obstacles

Some sample numbers

i.e. reduces to ¼ of signal10 log(1/4) = -6.02

Multipath Signal can take many different paths between

sender and receiver due to reflection, scattering, diffraction

Multipath propagation Signal can take many different paths between sender

and receiver due to reflection, scattering, diffraction

Time dispersion: signal is dispersed over time interference with “neighbor” symbols, Inter Symbol

Interference (ISI) The signal reaches a receiver directly and phase shifted

distorted signal depending on the phases of the different parts

signal at sendersignal at receiver

LOS pulsesmultipathpulses

Multipath Effects

Receiver receives multiple copies of the signal, each following a different path

Copies can either strengthen or weaken each other Depends on whether they are in our out of phase

Small changes in location can result in big changes in signal strength

Larger difference in path length can cause intersymbol interference (ISI) More significant for higher bit rates (shorter bit

times)

Free-Space Isotropic Signal Propagation

15

In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)

Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²

Loss depends on the frequency: Higher loss with higher frequency

Loss increase quickly with distance (d^2)

2

4

dGG

P

Ptr

t

r

Pr: received power Pt: transmitted power Gr, Gt: receiver and

transmitter antenna gain (=c/f): wave length

Sometime we write path loss in log scale: Lp = 10 log(Pt) – 10log(Pr)

Outline RF introduction

Antennas and signal propagation How do antennas work Propagation properties of RF signals

Modulation and channel capacity

Signals Physical representation of data

Function of time and location

Signal parameters: parameters representing the value of data classification continuous time/discrete time continuous values/discrete values analog signal = continuous time and continuous values digital signal = discrete time and discrete values

Signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift sine wave as special periodic signal for a carrier:

s(t) = At sin(2 ft t + t)

Signals Different representations of signals

amplitude (amplitude domain) frequency spectrum (frequency domain) phase state diagram (amplitude M and phase in polar coordinates)

Composed signals transferred into frequency domain using Fourier transformation

Digital signals need infinite frequencies for perfect transmission modulation with a carrier frequency for transmission (analog signal!)

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]

Multiplexing Multiplexing in 4 dimensions

space (si) time (t) frequency (f) code (c)

Goal: multiple use of a shared medium

Important: guard spaces needed!

s2

s3

s1f

t

ck2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

Frequency multiplex Separation of the whole spectrum into smaller

frequency bands A channel gets a certain band of the spectrum for

the whole time Advantages

no dynamic coordination necessary

works also for analog signals

Disadvantages waste of bandwidth

if the traffic is distributed unevenly

inflexible

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

k2 k3 k4 k5 k6k1

Time multiplex A channel gets the whole spectrum for a certain

amount of time

Advantages only one carrier in the

medium at any time throughput high even

for many users

Disadvantages precise

synchronization necessary

f

Time and frequency multiplex

Combination of both methods A channel gets a certain frequency band for a

certain amount of time Example: GSM Advantages

protection against frequency selective interference

but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Code multiplex

Each channel has a unique code

All channels use the same spectrum at the same time

Advantages bandwidth efficient no coordination and synchronization

necessary good protection against interference

Disadvantages varying user data rates more complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c

Modulation Digital modulation

digital data is translated into an analog signal (baseband) ASK, FSK, PSK differences in spectral efficiency, power efficiency, robustness

Analog modulation shifts center frequency of baseband signal up to the radio carrier

Basic schemes Amplitude Modulation (AM) Frequency Modulation (FM) Phase Modulation (PM)

Modulation and Demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter

Digital modulation

Modulation of digital signals known as Shift Keying

Amplitude Shift Keying (ASK): very simple low bandwidth requirements very susceptible to interference

Frequency Shift Keying (FSK): needs larger bandwidth

Phase Shift Keying (PSK): more complex robust against interference

1 0 1

t

1 0 1

t

1 0 1

t

Frequency Shift Keying (FSK):

Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying): 0 = Same phase, 1=Opposite

phase A cos(2πft), A cos(2πft+π) low spectral efficiency robust, used e.g. in satellite

systems QPSK (Quadrature Phase Shift

Keying): 2 bits coded as one symbol symbol determines shift of

sine wave needs less bandwidth

compared to BPSK 11=A cos(2πft+45°), 10=A cos(2πft+135°), 00=A cos(2πft+225°), 01=A cos(2πft+315°)

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t

Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM)

combines amplitude and phase modulation it is possible to code n bits using one symbol 2n discrete levels, n=2 identical to QPSK

Bit error rate increases with n, but less errors compared to comparable PSK schemes Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have

the same phase φ, but differentamplitude

0000 and 1000 havedifferent phase, but same amplitude.

0000

0001

0011

1000

Q

I

0010

φ

a

Channel Capacity Capacity = Maximum data rate for a channel Nyquist Theorem: Bandwidth = B

Data rate < 2 B Bi-level Encoding: Data rate = 2 × Bandwidth

Multilevel: Data rate = 2 × Bandwidth × log 2 M

Example: M=4, Capacity = 4 × Bandwidth

Shannon’s Theorem Bandwidth = B Hz Signal-to-noise ratio = S/N Maximum number of bits/sec = B log2 (1+S/N) Example: Phone wire bandwidth = 3100 Hz

S/N = 30 dB10 Log 10 S/N = 30

Log 10 S/N = 3S/N = 1000

Capacity = 3100 log 2 (1+1000)= 30,894 bps

Q/A

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