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Wireless Communication

Modulation Techniques for Mobile Radio 12 2558

o ? n

n

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o Last few weeks: o Properties of cellular radio systems n Reuse by using cells n Clustering and system capacity n Handoff strategies n Co-Channel Interference (CCI) n Trunking and grade of service (GOS)

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o Electromagnetic propagation properties and hindrances n Free space path loss n Large-scale path loss - Reflections, diffraction,

scattering n Multipath propagation

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o Now what we will study n We will look at modulation and demodulation. n Then study error control coding and diversity.

o Then the remainder of the course will consider the ways whole systems are put together (bandwidth sharing, modulation, coding, etc.) n IS-95 (first CDMA by Qualcomm. ) n GSM n 802.11

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Introduction

o Modulation: Encoding information in a baseband signal and then translating (shifting) signal to much higher frequency prior to transmission

n Message signal is detected by observing baseband to the amplitude, frequency, or phase of the signal.

n Our focus is modulation for mobile radio. n The primary goal is to transport information

through the MRC (maximum ratio combining) with the best quality (low BER), lowest power & least amount of frequency spectrum o Must make tradeoffs between these objectives.

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o Must overcome difficult impairments introduced by MRC: n Fading/multipath n Doppler Spread

o Challenging problem of ongoing work that will likely be ongoing for a long time. n Since every improvement in modulation methods

increases the efficiency in the usage of highly scarce spectrum.

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I. Analog Amplitude and Frequency Modulation

o A. Amplitude Modulation

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Spectrum of AM wave

Spectrum of baseband signal. Spectrum of AM wave.

( ) [ ( ) ( )] [ ( ) ( )]2 2c a c

c c c cA k AS f f f f f M f f M f f = + + + + +

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o B. Frequency Modulation n Most widely used form of Angle modulation for

mobile radio applications o AMPS o Police/Fire/Ambulance Radios

n Generally one form of "angle modulation" o Creates changes in the time varying phase (angle) of

the signal. n Many unique characteristics

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o Unlike AM, the amplitude of the FM carrier is kept constant (constant envelope) & the carrier frequency is varied proportional to the modulating signal m(t) :

n fc plus a deviation of kf m(t) n kf : frequency deviation constant (in Hz/V) - defines

amount magnitude of allowable frequency change

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(a) Carrier wave.

(b) Sinusoidal modulating signal.

(c) Amplitude-modulated signal.

(d) Frequency-modulated signal.

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o So

o FM signal spectrum carrier + Message signal frequency # of sidebands

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II. Digital Modulation

o Better performance and more cost effective than analog modulation methods (AM, FM, etc.)

o Used in modern cellular systems o Advancements in VLSI, DSP, etc. have made

digital solutions practical and affordable

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o Performance advantages: 1) Resistant to noise, fading, & interference 2) Can combine multiple information types (voice,

data, & video) in a single transmission channel 3) Improved security (e.g., encryption) deters phone

cloning + eavesdropping 4) Error coding is used to detect/correct transmission

errors 5) Signal conditioning can be used to combat hostile

MRC environment 6) Can implement mod/dem functions using DSP

software (instead of hardware circuits).

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o Choice of digital modulation scheme n Many types of digital modulation methods subtle

differences n Performance factors to consider

1) low Bit Error Rate (BER) at low S/N 2) resistance to interference (ACI & CCI) & multipath

fading 3) occupying a minimum amount of BW 4) easy and cheap to implement in mobile unit 5) efficient use of battery power in mobile unit

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n No existing modulation scheme simultaneously satisfies all of these requirements well.

n Each one is better in some areas with tradeoffs of being worse in others.

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o Power Efficiency : ability of a modulation technique to preserve the quality of digital messages at low power levels (low SNR) n Specified as Eb / No @ some BER (e.g. 10-5) where Eb : energy/

bit and No : noise power/bit n Tradeoff between fidelity and signal power

BER as Eb / No ()

p

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o Bandwidth Efficiency : ability of a modulation technique to accommodate data in a limited BW

n R : data rate B: RF BW n Tradeoff between data rate and occupied BW

as R , then BW () n For a digital signal :

o

bps/HzBRB

=

B

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o each pulse or symbol having m finite states represents n = log2 m bits/symbol n e.g. m = 0 or 1 (2 states) 1 bit/symbol (binary) n e.g. m = 0, 1, 2, 3, 4, 5, 6, or 7 (8 states) 3 bits/

symbol

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o Implementation example: A system is changed from binary to 2-ary. n Before: "0" = - 1 Volt, "1" = 1 Volt n Now "0" = - 1 Volt, "1" = - 0.33 volts, "2" = 0.33 Volts, "3" = 1 Volt n What would be the new data rate compared to the old data

rate if the symbol period where kept constant?

o In general, called M-ary keying

o Multiple frequency-shift keying (MFSK) is a variation of frequency-shift keying (FSK) that uses more than two frequencies.

o MFSK is a form of M-ary orthogonal modulation, where each symbol consists of one element from an alphabet of orthogonal waveforms. M, the size of the alphabet, is usually a power of two so that each symbol represents log2M bits.

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o Maximum BW efficiency Shannons Theorem n Most famous result in communication theory.

n where

o B : RF BW o C : channel capacity (bps) of real data (not retransmissions

or errors) o To produce error-free transmission, some of the bit rate

will be taken up using retransmissions or extra bits for error control purposes.

o As noise power N increases, the bit rate would still be the same, but max decreases.

maxB

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maxB

n So

n note that C B (expected) but also C S / N o an increase in signal power translates to an increase in

channel capacity o lower bit error rates from higher power more real

data o large S / N easier to differentiate between multiple

signal states (m) in one symbol n n max is fundamental limit that cannot be

achieved in practice

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n People try to find schemes that correct for errors.

n People are starting to refer to certain types of codes as capacity approaching codes, since they say they are getting close to obtaining Cmax. o More on this in the chapter on error control.

Channel Capacity o Channel Capacity (Data rate) Shannons

Theorem 2G (USDC)

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Bo Fundamental tradeoff between and (in general) n If improves then deteriorates (or vice versa) o May need to waste more power to get a better data rate. o May need to use less power (to save on battery life) at the

expense of a lower data rate. n vs. is not the only consideration. o Use other factors to evaluate complexity, resistance to

MRC impairments, etc.

p

B p

Bp

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o Bandwidth Specifications n Many definitions depending on application all use

Power Spectral Density (PSD) of modulated bandpass signal

n Many signals (like square pulses) have some power at all frequencies.

2( )( ) lim TW T

W fS f

T

=

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o B : half-power (-3 dB) BW o B : null-to-null BW o B : absolute BW

range where PSD > 0 n FCC definition of occupied BW BW contains 99%

of signal power n FCC stands for Federal Communications Commission.

It is an agency of the U.S

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III. Geometric Representation of Modulation Signal

o Geometric Representation of Modulation Signals - Constellation Diagrams

n Graphical representation of complex ( A & ) digital modulation types o Provide insight into modulation performance

n Modulation set, S, with M possible signals

o Binary modulation M = 2 each signal = 1 bit of information ( 1 2 )

o M-ary modulation M > 2 each signal > 1 bit of information (M > 2 > 1 )

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o Example: Binary Phase Shift Keying (BPSK)

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n Phase change between bits Phase shifts of 180 for each bit.

n Note that this can also be viewed as AM with +/- amplitude changes

n

n Dimension of the vector space is the # of basis signals required to represent S.

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n Plot amplitude & phase of S in vector space :

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o Constellation diagram properties : 1) Distance between signals is related to differences in

modulation waveforms o Large distance sparse easy to discriminate

good BER @ low SNR (Eb / No )

Above example is Power Efficient (related to density with respect to # states/N)

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2) Occupied BW as # signal states n If we can represent more bits per symbol, then we

need less BW for a given data rate. ( BW )

n Small separation dense more signal states/symbol more information/Hz !!

Bandwidth Efficient

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IV. Linear Modulation Methods

o In linear modulation techniques, the amplitude of the transmitted signal varies linearly with the modulating digital signal.

o Performance is evaluated with respect to Eb / No

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BPSK

o BPSK Binary Phase Shift Keying

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o Phase transitions force carrier amplitude to change from + to . n Amplitude varies in time

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BPSK RF signal BW

n Null-to-null RF BW = 2 Rb = 2 / Tb n 90% BW = 1.6 Rb for rectangular pulses

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o Probability of Bit Error is proportional to the distance between the closest points in the constellation. n A simple upper bound can be found using the

assumption that noise is additive, white, and Gaussian.

n d is distance between nearest constellation points. n No is noise

BPSK constellation

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Gaussian distribution

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White noise

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n Q(x) is the Q-function, the area under a normalized Gaussian function (also called a Normal curve or a bell curve)

n Here

dyezQ yz

2/2

21)(

=

Q function

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o Demodulation in Rx n Requires reference of Tx signal in order to properly

determine phase o carrier must be transmitted along with signal

n Called Synchronous or Coherent detection o complex & costly Rx circuitry o good BER performance for low SNR power efficient

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QPSK

o QPSK Quadrature Phase Shift Keying

n Four different phase states in one symbol period n Two bits of information in each symbol

Phase: 0 /2 3/2 possible phase values Symbol: 00 01 11 10

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o Note that we choose binary representations so an error between two adjacent points in the constellation only results in a single bit error

n For example, decoding a phase to be instead of /2 will result in a "11" when it should have been "01", only one bit in error.

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o Constant amplitude with four different phases o remembering the trig. identity

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n Now we have two basis functions n Es = 2 Eb since 2 bits are transmitted per symbol n I = in-phase component from sI(t). n Q = quadrature component that is sQ(t).

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QPSK RF Signal BW

o null-to-null RF BW = Rb = 2RS (2 bits / one symbol time) = 2 / Ts o double the BW efficiency of BPSK or twice the data rate in same

signal BW

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o BER is once again related to the distance between constellation points.

n d is distance between nearest constellation points.

n

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o How does BER performance compare to BPSK?

n Why? same # of states per number of basis

functions for both BPSK and QPSK (2 states per one function or 4 states per 2 functions)

n same power efficiency (same BER at specified Eb / No)

n twice the bandwidth efficiency (sending 2 bits instead of 1)

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o QPSK Transmission and Detection Techniques

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OQPSK

o Offset QPSK n The occasional phase shift of radians can cause the

signal envelope to pass through zero for just in instant.

n Any kind of hard limiting or nonlinear amplification of the zero-crossings brings back the filtered sidelobes o since the fidelity of the signal at small voltage levels is

lost in transmission. n OQPSK ensures there are fewer baseband signal

transitions applied to the RF amplifier, o helps eliminate spectrum regrowth after amplification.

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o Example above: First symbol (00) at 0, and the next symbol (11) is at 180. Notice the signal going through zero at 2 microseconds. n This causes problems.

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o Using an offset approach: First symbol (00) at 0, then an intermediate symbol at (10) at 90, then the next full symbol (11) at 180. n The intermediate symbol is used halfway through

the symbol period. n It corresponds to allowing the first bit of the symbol

to change halfway through the symbol period. n The figure below does have phase changes more

often, but no extra transitions through zero. n IS-95 uses OQPSK, so it is one of the major

modulation schemes used.

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o In QPSK signaling, the bit transitions of the even and odd bit streams occur at the same time instants.

o but in OQPSK signaling, the even and odd bit Streams, mI(t) and mQ(t), are offset in their relative alignment by one bit period (half-symbol period)

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n the maximum phase shift of the transmitted signal at any given time is limited to 90o

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o The spectrum of an OQPSK signal is identical to that of a QPSK signal, hence both signals occupy the same bandwidth

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/4 QPSK

o /4 QPSK n The /4 shifted QPSK modulation is a quadrature

phase shift keying technique o offers a compromise between OQPSK and QPSK in

terms of the allowed maximum phase transitions. n It may be demodulated in a coherent or noncoherent

fashion. o greatly simplifies receiver design.

n In /4 QPSK, the maximum phase change is limited to 135o

n in the presence of multipath spread and fading, /4 QPSK performs better than OQPSK

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