EE 3414Multimedia Communication Systems-Part I

Spring 2003 Lecture 8

Overview of Wireless Communications

February 20, 2003Elza Erkip

Outline

• Overview of wireless communication systems• Digital modulation

– Simple representation– Error analysis– Channel coding

• Wireless channel– Properties– Effect on errors– Diversity

What is communication?• Transmit information over unreliable channels

– Wired• Telephone• Cable TV• Fiber optic

– Wireless• Mobile phone• Wireless local area network• Satellite

– Storage• Hard drive• Compact disc

Wireless Multimedia Networks

Backbone Network

Wireless LAN

Wireless Cellular

Wired LAN

Wireless LAN

Wireless Cellular

Wired LAN

Ad-hoc Network

Communication channel model

Channel Filter +

Noise + Interference

Input Output

• Channel filter causes distortions• Noise in receiver equipment• Interference from the environment• Limited bandwidth and power

Goal: Design signals to effectively send information over communication channels.

Digital Communications

• You have already seen analog modulation• Digital modulation

– Digital signals: both amplitude and time discrete• Analog to digital: Sampling and quantization

Amplitude

Time

Digital Modulation• Digital information: Sequence of 0’s and 1’s:

001101…..• One bit every T seconds. During 0 < t < T

– To send a 0, send – To send a 1, send

• Input signal

• Modulated signal

)2cos()(0 tfAts cπ=)2cos()(1 tfAts cπ−=

0 0 1 1 0 1

Simple Representation of Digital Modulation Signals

• In and the term is common– Equivalently s0(t) = A, 0<t<T

s1(t) = -A, 0<t<T

– Or

)(0 ts )(1 ts )2cos( tfcπ

t

s0(t)A

T

s1(t)

-A

Tt

0 A-Ax x1 0

Vector Representation

• The modulation scheme discussed is called Binary Pulse Amplitude Modulation (PAM) or Binary Phase Shift Keying (BPSK).

• M-ary PAM (M=4, send two bits at a time)

• Labeling is done in such a way that adjacent points only differ in one bit (called Gray mapping)

A-Ax x xx

-3A 3A

11 10 00 01

Two Dimensional Modulation• Modulated signal

– 4-QAM (Quadrature Amplitude Modulation)

– 8-PSK (Phase Shift Keying)

)2sin()2cos()( tfAtfAts cscci ππ +=

x

x x

x10 00

0111

xx

xx

x x

xx

A

AA

Parameters of Modulation

• Three important parameters of a modulation scheme:– Minimum distance dmin: The smallest distance

among points in vector representation– Average energy Eav

– Number of bits/symbol= log2(M)• Example:

– 4-PAM: dmin=2A, Eav=2(A2+9A2)/4=5A2, log2(M)=2– 4-QAM: dmin=2A, Eav=2A2, log2(M)=2

Effect of noise

• Simple channel: Additive White Gaussian

• Noise n(t) is Gaussian

Modulated Signal +

Noise n(t)

Received Signal

Mean

Variance 2σ

Optimum Receiver for AWGN

• Received signal r

• Decide 0 was sent if r>0, 1 was sent if r<0.• How often do we make errors?

A-Ax x

0

Error Probability of BPSK

• To have few errors we want A large compared to• Signal to noise ratio= SNR =

• Probability of error – Q-function related to Gaussian noise

2σ2σ

A

)( SNRQPe =

Pe

SNR

Channel coding• To decrease probability of error, add redundancy:

– Instead of sending one 0, send three 0’s.– If we receive more 0’s than 1’s: Decide 000 was sent– Error probability=

• Smaller than Pe

• This is called channel coding.• Above example is one way of doing channel coding: Repetition

coding• But the information rate is decreased

– Instead of R=1bit/sec, we have R=1/3 bits/sec• What are some channel coding techniques that are smarter

than repetition coding?

23 )1(3 eee PPP −+

Wireless Channel

• Reflection from buildings, cars etc: Multipath• Scattering: Fading

Multipath Fading ChannelTransmitted signal s Received signal r

t t

1 A1

A2 A3

T1 T2 T3

• Amplitudes Ai and delays Ti randomly vary in time

Delay T1

Delay T2s

A2

nA1

r

Flat Fading

• Only one path– Received signal r=As+n

• BPSK modulation– What is the effect in error performance?

• Remember: SNR in dB=10log10(SNR)– 3 dB=Factor of 2– 10 dB=Factor of 10– 30 dB=Factor of 1000

)( 2SNRAQEP Ae =

Effect of Fading

0 5 10 15 20 25 30 3510-4

10-3

10-2

10-1

100

SNR (dB)

PROBABILITY OF ERROR

FADING GAUSSIAN

Diversity • Diversity to combat channel variations

• Receive L independently fading signals representing source.– Even if one signal is weak, the others may be stronger

Source Destination

Copy 1 of signal

Copy 2 of signal

Diversity Mechanisms• Time diversity: Repetition coding, interleaving• Frequency diversity: Frequency hopping• Space diversity: Multiple antennas, multihop (cooperation)

• Multipath diversity: Process all the received paths (Rake receiver)

• The receiver can do– Switching: If current signal is not good, try another– Selection: Choose the strongest signal– Equal gain: Add received signals– Maximal ratio: Add received signals but not equally. Give more

weight to the ones that are more reliable (optimum)

Effect of Diversity on Performance

0 5 10 15 20 25 3010

−6

10−5

10−4

10−3

10−2

10−1

100

SNR per bit (dB)

BE

R

AWGN div 1div 2div 3div 4

Interference in Wireless

NoiseT1

T2

T3

Base station receiver

Time Division Multiple Access (TDMA)

Time

Frequency

User 1 User 2 User 3 User 4

• Each user transmits at a separate time slot

T sec

W Hz

Frequency Division Multiple Access (FDMA)

Time

FrequencyW Hz

T secUser 1

User 2

User 3User 4

• Each user transmits at a separate frequency slot

Code Division Multiple Access

Code 2

Code 4

Code 3

Code 1

Transmitted bit

• Each user transmits all the time over all the frequency band, but has a different “ spreading code.”• Code i belongs to user i.• The base station differentiates users based on their codes.

Spreading Codes

Transmitted bit

S(t)

bit

chip

b=1 b=-1

t

t

Transmitted signal

• User transmits signal bS(t) where

=−=

=0 bit if ,11bit if ,1

b

Orthogonal Codes

• Spreading codes belonging to different users are orthogonal

S1(t)

t1

t1

S2(t)

-1

• Note S1=(1,1), S2=(1,-1) and <S1,S2> = 0• The base station can easily identify users• Problem: Mobile users asynchronous, and hence they transmit at different times. So it is hard for s1(t) and s2(t) to be orthogonal.

What you should know

• What is digital modulation?• One and two dimensional modulation

schemes• Parameters of digital modulation schemes• How is wireless channel different?• What is diversity and what are methods of

obtaining diversity?• How do multiple users share the wireless

channel? TDMA, FDMA, CDMA