CCM 4300 Lecture 17Computer Networks, Wireless and Mobile

Communication Systems

1

Digital Communications for Wireless Systems

Dr S Rahman

Lesson objectives� To acquire a basic understanding of signals,

systems and modulation techniques used in

wireless communication systems.

� Basic of signals and noise, understand SNR

2

� Various Line coding methods and bandpass

modulation techniques

� Channel Capacity and Bandwidth Relation

Session Content

� Introduction to what signals are composed of

� Basic Communication system

�A few modulation techniques used in real systems

3

� Difference between baseband and bandpass

modulation schemes

� Latest OFDM technique

� Introduction to wireless channel design

Communication System? What is it?

InformationModulation/Coding

TransmitterWired/Wireless

ChannelCoax/UTP/Wireless

ReceiverWired/Wireless

Demodulation/Decoding

Data IN

Data OUT

Transfer of Information from one point to another

4

• Wired/Wireless communication system -> at physical layer on OSI• At Network/Data-link layer -> Frames/Packets • At physical layer -> real signals Digital/Analogue

We focus on Wireless Comm.'s Channel

Signals in Communication Systems

• Signal is the variation in time, voltage, light intensity, etc.

• Time Representation – Signal on an oscilloscope

• Frequency Representation – Signal on a Spectrum Analyzer

5

• Time/Frequency -> Two ways to view the same signal

• Signal is made up of different sinusoidal harmonics

Fact that signal is made up of frequency components

Frequency spectrum of a signal:1. Magnitude and Phase2. Real and Imaginary

Types of signals

Analog – Takes continuous values and is a function of time

Digital – Takes discrete values and is also a function of time

Ohms law: i(t) = v(t) /R p(t) =v(t)

2

RPower of a signal:

i(t), v(t) and p(t) are instantaneous current, voltage, power resp.

6

i(t), v(t) and p(t) are instantaneous current, voltage, power resp.

Average power of a periodic signal: P =1

Tg(t)

2dt

−T / 2

T / 2

∫T is the period of the signal

Periodic signal x(t) is called periodic in time if there exists a constant T0 such that x(t) = x(t+T0 ) for –INF < t < INF, t denotes time and T0 is called period of x(t), If T0 =0, it is a non-periodic signal

Sinusoidal Signal & its Representation

E.g.

s(t) = sin2πft• s(t), time domain

• f = frequency (Hz), 1Hz i.e. 1 cycle/every second

7

i.e. 1 cycle/every second

• Period T = 1/f = 1s

• T= 1/f or f= 1/T

• Frequency is reciprocal of the Period

s(t) = sin2*100* π * tSo when f = 100 Hz then

Other signals ….

Made up of different sinusoidal harmonics

What are harmonics?

8

Rectangular Signal

What are harmonics?

If a signal is periodic with a frequency f, the only frequencies composing the signal are integer multiples of fundamental frequency f, i.e., 2f, 3f, etc.

Other signals ….

A modem signal with 5Kbits/sec rate through a filter output, i.e., f=5000Hz, so T=1/f=1/5000=0.2ms

9

Fourier Series

� By summing sine signals of well chosen amplitude, frequency and phase -> a signal can be generated

� i.e. A signal is represented as a sum of sine waves

� Although, the sum of such signals is not always finite

10

� Although, the sum of such signals is not always finite

� Example -> 4

πsin2πt +

1

3sin6πt +

1

5sin10πt + ........+

1

ksin2kπt + ....

OR

4

π

1

ksin2kπt

k

∑

where, k is any odd number > 0

Noise

� Unwanted and beyond our control waves that disturb the transmission of a signal

� Atmosphere, interference by neighboring channels, galactic are external sources

11

� Shot, thermal (electronics) etc. are internal sources

� Thermal noise caused by rapid & random motion of electrons in a conductor. Gaussian distribution with zero mean

� Shot noise caused by discrete and non-continuous random motion of electrons, causing random current fluctuation (again Gaussian in nature)

Spectral DensityThe spectral density of a signal characterizes the distributionof the signal's energy or power in the frequency domain.Power Spectral Density (PSD)If x(t) is a periodic signal with period T0, it is classified as apower signal.The expression for the average power of a

Some Additional Definitions

12

power signal.The expression for the average power of aperiodic signal where the time average is taken over the signalperiod T0 is shown as follows:

White noise is a random signal (or process) with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency. (ref. wikipedia)

Terms used in real channel designs

� Additive White Gaussian Noise (AWGN) -> Communications channel fundamental

� What does AWGN channel mean?

White NoiseZero-mean Gaussian Dist.

13

Power spectral density (psd)Gn(f) is flat for all frequencies

Gn ( f ) = N0 /2 watts/Hz

Factor of 2 for two-sided psd

Gaussian distribution of amplitude

Ref.: Digital Comm., B

Skalar, p. 31

Signal to Noise Power Ratio (SNR)

SNR = Signal Power / Noise Power

Biggest constraint of communication channel -> how large is the signal compared to noise

14

Channel Capacity and Bandwidth

C = B log2(1+ SNR) C Channel Capacity, B Bandwidth,SNR (linear not dB)

Data Rate of a system (D) is always less than C

15

Range of frequencies occupied by the signal within Fmin as minimum and Fmax as maximum frequency, then

Bandwidth (B) = Fmax - Fmin

Data rate is the number of bits/sec sent. Data rate is reciprocalof bit period (T), i.e., D=1/T or T=1/D.If D = 1 bit/s, means each bit takes T=1sIf D=1000bits/s, means each bit takes T= 1/1000 = 0.001s

Digital Communication System

Formatting/Character Encoding

Pulse(baseband)

Modulation

Freq Spread and Multiple Access

Pulse(bandpass)

Modulation

CHANNEL

16

Pulse(baseband)

DEMOD

Freq De-spread and DE-MUX

Pulse(bandpass)DEMOD

De-Format/Char

acter Decoding

Pulse Code Modulation (PCM)Line Coding, M-level Signaling, NRZ, NRZI, RZ, etc.

Phase/Frequency/Amplitude Shift Keying, Gaussian Minimum (GMSK), QAM, QPSK, OFDM, etc.

PCM or Line Coding

Analog signal converted into digital

PCM is a baseband scheme that converts the analog signal intoits digital form/discrete-valued form.

Advantages:

17

Advantages:1. Digital signals are more immune to channel noise2. Retransmission of digital signal using repeaters3. All analog signals can be converted to a uniform format4. More like Analog to Digital conversion than modulation

Line Coding

18B. Sklar, Digital Communication – Fundamentals & Applications

Line Coding

� NonReturn-to-Zero (NRZ) -> L- Level, M- Mark, S- Space

� Return-to-Zero (RZ)

Change in Level from one to zero Mark(1) – change in level,

Space(0)- no changeAlso Differential Encoding

Complement of NRZ-M

19

� Return-to-Zero (RZ)• Unipolar –> 1 – half wide pulse, 0 – absence of pulse• Bipolar -> 1’s & 0’s by opposite-level pulses 1-half bit wide • Alternate Mark Inversion (AMI) -> 1’s by equal amplitude alternating pulses, 0’s by absence of pulses

� Delay Modulation -> (DM) or Miller Coding• 1 is represented by a transition at middle of bit interval.• 0 by no transition, unless followed by another 0.• Transition is placed at end of bit interval of first zero

Line Coding

� Dicode NRZ : • one-to-zero or zero-to-one data transition changes pulse polarity; otherwise a 0-level is sent.

� Dicode RZ : • one-to-zero or zero-to-one data transition produces a half-duration polarity change; otherwise a 0-level is sent.

20

duration polarity change; otherwise a 0-level is sent.

� Manchester or bi-Φ-L (bi-phase-level): • 1 is represented by a half-bit-wide pulse positioned during the first half of the bit interval• 0 is represented by a half-bit-wide pulse positioned during the second half of the bit interval

21

Pulse Amplitude Modulation (PAM)

• Simplest form of M-ary signalling -> Multi-levels

• Size of symbol, M, is M= 2k, where k is no. of bits in symbol• M-ary PAM - used to reduce the Tx b/w of the channel, as it transmits M-level pulses, each representing a k-bit symbol

One of M allowable levels is

22

One of M allowable levels isassigned to each of M possiblesymbol values

(a) -> PCM signalling(b) -> 8-level PAM

Eight and two level PCM

signalling (p. 93, Sklar)

Bandpass Modulation

• Baseband modulated signal modulate a carrier signal (radio)

• Carrier is converted into EM waves for propagation

• Size of Tx antenna depends on wavelength (λ) of e.m. wave• For cellular technology, antenna size is typically (λ/4• Imagine, sending a 3kHz baseband signal via that antenna

23

• Imagine, sending a 3kHz baseband signal via that antenna without a carrier wave.

λ =c

f

c= speed of light, 3x108 m/s, f=3kHz.Hence, (λ/4) would be = 2.5x104 m ~ 15miles

Other benefits include multiplexing for more signal Tx channels, less interference, etc.

Bandpass Modulation Schemes

Amplitude/Phase/Frequency of an RF carrier, or a combination of them is varied w.r.t the information to be transmitted.

General form of the carrier wave is: s(t) = A(t)cosθ (t)

A(t) is the time varying amplitude, Θ(t) is time-varying angle

θ(t) = ω + φ(t) binary s(t)

24

θ(t) = ω 0 + φ(t)

ω0 =carrier frequency in radians/sec, Φ(t) =phase, f = carrier frequency in Hz

ω = 2πf

binary stream

A(t)cos(2πft + φ(t))

s(t)

Coherent & Non-Coherent

Rx exploits knowledge of carrier’s phase to detect signals

Rx does not exploits knowledge of carrier’s phase to detect signals

Phase Shift Keying (PSK)

In binary PSK (simplest form BPSK), the binary symbol 1 is represented by setting the carrier phase Φ(t)=0 radians and likewise for binary symbol 0, it is Φ(t) = π radians or 180°.Correspondingly,

for symbol=1s(t) = A(t)cos(2πft)

25

φ(t) = 0o,90o,180o,270o

for symbol=1

s(t) = A(t)cos(2πft + π) for symbol=0

Quadrature PSK (QPSK) is obtained by adding two BPSK signals

s(t) = A(t)cos(2πft)

Four Phases

Frequency Shift Keying (FSK)

In FSK, two symbols 1 and 0 are distinguished from each otherby the transmission of one or two sinusoidal waves that differ infrequency by a fixed amount

si(t) =2E

Tcos(2πf it) 0 ≤ t ≤ Tb

26

T

= 0 otherwise

i=1,2,…M T is symbol (bit) duration, E is energy per bit

f i =nc + i

Tfrequency transmitted for a integer nc and i=1,2..M

Phase is continuously maintained everywhere

Carrier Wave Modulated Signals

27G. Corazza, Digital Satellite Communications

28

Gaussian Minimum Shift Keying (G-MSK)Widely used in GSM systems

• Form of PSK – Continuous phase scheme• Frequency changes occur at the carrier zero crossing points• Better spectral efficiency, noise resilience, distortion less• Drawback – consumes more power than QPSK

Input data is shaped into a Gaussian pulse shape using aGaussian Filter

29

Gaussian Filter

Orthogonal Frequency Division Multiplexing (OFDM) Modulation

• It’s a block Tx technique• Also known as Discrete Multi-tone Modulation (DMT)• Very high rate data stream is divided into multiple parallel lowdata rate streams• OFDM uses multiple sub-carriers – closely spaced without any

Used in DAB, WIFI, WIMAX, DVB, 4G

30

• OFDM uses multiple sub-carriers – closely spaced without anyguard bands. • Orthogonality – Peak of one sub-carrier coincides with the nullof adjacent sub-carrier. It allows simultaneous transmission on alot of sub-carriers in a tight frequency space without interferencefrom each other.• In OFDM the signal is first split into independent channels, Modulated by data and then re-multiplexed to create the OFDMcarrier. OFDM is a special case of FDM.

Orthogonal Frequency Division Multiplexing (OFDM) Modulation

• OFDM requires less bandwidth than FDM to carry same amountof information -> Hence, MORE SPECTRAL EFFICIENCY

• Effect of ISI is suppressed by the fact that there are longersymbol period based parallel OFDM sub-carriers

31

• OFDM only allows one user on the channel at a given time, Hence, additional multi-user technique is implemented over it.

• OFDMA distributes subcarriers among users so all users can Txand Rx at the same time within a single channel on what arecalled sub-channels (sub-carrier groups).Modulation_OFDM_etc\ofdm2.pdfModulation_OFDM_etc\armstrong_ofdm_good.pdf

OFDM Modulation Implementation

32

Summary

• Signals

• Communications Systems overview

• Modulation Techniques like ASK, PSK, FSK

• G-MSK used in GSM networks

• OFDM modulation implementation and benefits

33

• OFDM modulation implementation and benefits

• OFDM used in DAB, DVB, 4G and WIFI, WIMAX for higher

data rates

Further Reading

1. Digital Communications – Fundamentals & Applications,

B. Sklar, Prentice Hall

2. Digital Satellite Communications, G. Corazza, Springer

3. Advanced Wireless Network Technologies, S. Glisic,

Wiley

34

4. Communication Systems, B. Carlson, P. Crilly and J.

Rutledge, McGraw Hill

5. Gaussian Minimum Shift Keying GMSK, Brad Gaynor

6. OFDM Tutorial, www.complextoreal.com