03 Radio System Design 1

Bhaskar Banerjee, EERF 6330, Sp2013, UTD

RF System Design I

Modulation, Detection and Multiple Access Techniques

Prof. Bhaskar Banerjee

EERF 6330- RF IC Design


Outline

Whats a wireless system?

Modulation schemes

Multiple Access Techniques

Reading:

Chapters 3 and 4: RF Microelectronics by Razavi

2

Bhaskar Banerjee, EERF 6330, Sp2013, UTD 3

The wireless telegraph is not too difficult to understand. The ordinary telegraph is like a very long cat. You pull the tail in New York, and it meows in Los Angeles. The wireless is the same, only without the cat.

- Albert Einstein, 1938


A Typical Communication System

InformationSource ADC

Compression/Coding Modulator

DriverAmplifier

Channel

Low-NoiseAmplifier

Down-converterADCDemodulator

De-coding/De-compressor DAC

InformationSink

Receiver

Transmitter

Digital

Digital

voice, video, data, music

mixer,VCO/PLL, PA, Filters

mixer,VCO/PLL, VGA, Filters

speakers, displays


Why modulate?

In wired systems, coaxial lines exhibit superior shielding at higher frequencies

In wireless system, antenna size is a significant fraction of the wavelength

Spectrum is precious - most wireless communication occurs in allocated frequency bands

In many cases, modulation allows simpler detection at the receive end in the presence of non-idealities in the communication channel.

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Modem Modulation

converting a baseband signal to a passband coding an information onto a carrier varying certain parameters of a carrier according to the baseband signal

Demodulation (or Detection) the inverse of modulation with the goal of being to extract the original

baseband signal with minimum noise, distortion, ISI, etc. Modem

Modulation + Demodulation

Simple communication system

Passband Signal


Modem Choice of modem is based on

Signal quality Signal to Noise Ratio (SNR), Bit Error Rate (BER) affect talk time, communication distance at a given

conditions Spectral efficiency

a certain bandwidth occupied by the modulated signal should be considered for limited-bandwidth applications such

as wireless networks Power efficiency

a reference to the type of power amplifier that can be used in the transmitter


Modem To transmit and receive a signal over the air, there are three

main steps At the transmitter, a pure carrier is generated The carrier is modulated with the information to be transmitted.

Any reliably detectable change in signal characteristics can carry information

At the receiver, the signal modifications or changes are detected and demodulated


Modulation Carrier

Amplitude: Phase

Total phase: Excess phase:

Frequency Total frequency:

Excess frequency:


Analog Modulation Amplitude Modulation (AM)

Modulation index Baseband signal

AM in time domain AM in frequency domain

AM detector

xAM (t) = [1 +m xBB(t)] cos(!ct)


Analog Modulation Phase Modulation (PM)

excess phase is linearly proportional to baseband signal

Frequency Modulation (FM)

excess frequency is linearly proportional to baseband signal

xPM (t) = Accos[!ct+m xBB(t)]


Analog Modulation Phase/Frequency Modulation


Why Digital Modulation?

Digital modulation provides more information capacity Increased channel capacity Transmit and receive data with greater accuracy compatibility with digital data services Higher data security Better quality communications


Digital Modulation: ASK, PSK, FSK

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ASK

PSK


Basic Concepts for Digital Modulation Symbol rate and Bit rate

Four-level digital representation of a binary data stream

In this case, the Symbol rate is one-half the original bit rate


Digital Modulation: ISI

A signal cannot be both time-limited and band-limited Inter-symbol Interference: Corruption of every bit/symbol by

the tail of the previous bit/symbol16


Basic Concepts for Digital Modulation Bandwidth

Definition with a percentage Typically frequency range containing the signal power of 99%

Definition with the relative amount of power that appears in an adjacent channel Usually defined by ACLR (Adjacent Channel power Leakage

Ratio)


Basic Concepts for Digital Modulation Signal Constellation

For dimensions below 4, the modulated waveforms can be visualized in terms of the coefficients in their inner products.

Basis functions are ignored and all coefficients of the vector are plotted in cartesian coordinates

coefficients basis functions

Amplitude modulation Frequency modulation

x(t) = [12 ] [1(t)2(t) ]


Digital Modulation Overview

Amplitude modulation Amplitude Shift Keying (ASK)

Frequency modulation Frequency Shift Keying (FSK)

Phase modulation Phase Shift Keying (PSK)

Both amplitude and phase modulation Quadrature Amplitude Modulation

(QAM)


Digital Modulation Binary Frequency Shit Keying (BFSK)

Modulated signal

Modulator

Demodulator

Constellation


Digital Modulation Binary Phase Shit Keying (BPSK)

Modulated signal

Modulator

Demodulator

Constellation


Digital Modulation Quadrature Phase Shit Keying (QPSK)

The modulation of two bits at a time using orthogonal functions Because symbol rate is half the bit rate, the required bandwidth is half

of BPSK Quadrature signal permits the use of image reject mixer

Modulated signal

bm and bm+1 are consecutive bits


Digital Modulation Quadrature Phase Shit Keying (QPSK)

Modulator

Demodulator

Constellation

b1b2 b3


Digital Modulation

Further considerations on digital modulation schemes Problem of 180o changes in QPSK

Offset QPSK: data streams are offset in time by half the symbol period to avoid simultaneous transitions in the waveform

/4-QPSK: consists of two QPSK systems, one rotated by 45o with respect to the other

Problem of bandlimited systems Use raised-cosine filters to minimize the band limiting influence

Other forms of digital modulation Minimum Shift Keying (MSK): to make the phase change

smoother Gaussian Minimum Shift Keying (GMSK): a PSK system that

approaches FSK


Digital Modulation Summary


Basic Concepts for Mobile Communication Cellular system

Concept of frequency reuse is implemented in a cellular structure

It is a system where the geographical area is divided into physical cells

Mobile units in each cell are served by a base station, and all of the base stations are controlled by a Mobile Telephone Switching Office (MTSO)

simple cellular system


Basic Concepts for Mobile Communication Co-Channel Interference (CCI)

How much two cells that use the same frequency interfere with each other

dependent on the ratio of the distance between two co-channel cells to the cell radius

Independent of the transmitted power


Basic Concepts for Mobile Communication Handoff

What happen when a mobile unit roams from cell A to cell B like following figure?

The mobile must change its server to the base station in cell B

because the power level received from the base station in cell A is insufficient to maintain communication

the channel because adjacent cells do not use the same group of

frequencies


Basic Concepts for Mobile Communication Path loss and Multipath fading

Signals propagating through free space experience a power loss proportional to the square of the distance

In reality, when the signal travels through both a direct path and an indirect, reflective path, the loss increases with the fourth power of the distance

Since the two signals shown in the following figure generally experience different phase shifts, the net received signal may be very small at certain distances

multipath fading loss profile


Basic Concepts for Mobile Communication

Diversity To avoid the effect of fading

Space diversity (antenna diversity)Adding redundancy to the transmission or reception of the

signal by two or more antennas spaced apart by a significant fraction of the wavelength

Frequency diversityUsing multiple carrier frequencies with the idea that fading

is unlikely to occur simultaneously at two frequencies sufficiently far from each other

Time diversityA technique whereby the data is transmitted or received

more than once to overcome short-term fading


Basic Concepts for Mobile Communication Delay spreading

When two signals in a multipath environment experience roughly equal attenuation but different delays

Frequency dependent fading due to


Duplexing Techniques Time Division Duplexing (TDD)

Same frequency band is utilized for both Tx and Rx paths, but the system transmits for half of the time and receives for the other half

Low loss RF switch is required Merit

No interference between Tx and Rx Drawback

Desensitizing Rx due to strong signals transmitted by other mobile units


Duplexing Techniques Frequency Division Duplexing (FDD)

Employing two different frequency bands for the Tx and Rx paths

Duplexer filter is required Trade-off between loss and Q-factor of filter

Merit Rx is immune to strong signals transmitted by other mobile

units Drawback

Leakage from Tx to Rx Filter loss


Multiple Access Techniques Frequency Division Multiple Access (FDMA)

Separation in frequency Continuous transmission and reception Applications: AMPS (Advanced Mobile Phone Service)

Power

Frequency

Time

FDMA


Multiple Access Techniques Time Division Multiple Access (TDMA)

Separation in time Transmission and reception in bursts Antenna switch instead of duplexer is possible Applications: GSM, D-AMPS (IS-136), PHS (Personal Handy-

Phone Serv.)

Frequency

Power Time

TDMA


Multiple Access Techniques Code Division Multiple Access (CDMA)

Separation in code domain (each user has different code) All users in cell use same frequencies all the time Applications: IS-95, UMTS (Universal Mode Telecomm. Serv.)

Frequency

CDMA

Power

Time


Multiple Access Techniques Hybrids

Combination of two multiple access techniques Example: GSM

TDMA+FDMA 124 frequency channels Each channel divided into 8 different timeslots


Spectral Viewpoint of Wireless Communication

Cellular communication (0.9~1.9GHz) Digital audio broadcast (AM, FM, 2.4GHz) Wireless local area networks (2~5GHz, 50~60GHz) Wireless access to homes (1.9~5.8GHz) Satellite communications (12~18GHz, 26~40GHz) Local multipoint distribution (28~30GHz) Wireless Multimedia (57-65 GHz) Auto radar (77GHz), Vehicle wireless systems


Wireless Standards Advanced Mobile Phone Service (AMPS)

One of the earliest wireless standards Multiple access: FDMA Duplexing: FDD Modulation: analog FM Channel bandwidth: 30kHz


Wireless Standards North American Digital Cellular (NADC)

The first digital cellular system in U.S. Designed to be compatible with AMPS Multiple access: TDMA

Frame: 6 time slots, 40ms Tx and Rx time slot offset: 1.85ms

Duplexing: FDD Modulation: /4-DQPSK Channel bandwidth: 30kHz


Wireless Standards Global System for Mobile communication (GSM)

Originally developed as a unified wireless standard for Europe The most widely used cellular standard in the world Multiple access: TDMA

Frame: 8 time slots, 4.615ms Tx and Rx time slot offset: 3 time slots

Duplexing: FDD Modulation: GMSK Channel bandwidth: 200kHz


Wireless Standards IS-95 (Qualcomm CDMA)

Proposed by Qualcomm, Inc. and adopted for the North America

Multiple access: DS-CDMA Duplexing: FDD Modulation: OQPSK Channel bandwidth: 1.23MHz


Wireless Standards Digital European Cordless Telephone (DECT)

Originally adopted as a cordless phone framework in Europe Designed to allow connection to other systems such as GSM Multiple access: TDMA / FDMA

Frame: 24 time slots, 10ms Duplexing: TDD Modulation: GFSK Channel bandwidth: 1.73MHz

03 Radio System Design 1

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