mobile computing cdma

7/30/2019 mobile computing cdma

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Oxford University Press 2007. All rights reserved. 1

Wireless Medium Access Control and

CDMA-based Communication

Lesson 01

Modulation Methods for Medium-access


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Number of signal sources access to

wireless medium simultaneously

Simultaneously transmitted Signals

(actually electromagnetic radiation) mayinterference with each other, when they

travel through a medium

Network has to receive signals from each

radio carrier distinctly


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Modulation with radio carrier frequency

(ies)

Voice-data or data signals propagate

through the medium after modulation

Wireless station accesses a medium by

modulation of radio carrier(s) with thesignal symbols


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Symbols

Digitized form of the analog signals

Symbolbit(s) prepared for transmission

after encoding of data bits and insertion

of the error control and other bits


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Instantaneous value of signal amplitude,

s(t) at an instant t

s(t) = S s0sin (2 c/c t + t0)

s(t) = S s0 sin (2 fc t + t0)

where S is the symbol to be transmitted, 1 or

0


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Amplitude shifted keying (ASK)

modulation

s0 = A0whenSymbol = 0 (1)

s0 = A1 when

Symbol = 1 (2)


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BFSK (binary frequency shift keying) of

fc when S = 1

s0(t) = s0/2 . sin (2 fc t + t0) when

S = 0

s1(t) = s0/2 . sin (2 (fc+ fm) t + t0)

when S = 1 Bandwidth > f c

Harmonics of (fc+ 2 fm), (fc+ 3 fm) , (fc+

4 fm) present


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GMSK (Gaussian Minimum shift keying)

DSP based Gaussian low pass filter

Bandwidth is 2 .fmplus guard band


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QPSK

One of the four possible distinct

sequences (00, 01, 10, or 11) transmittedusing a specific phase angle of the radio

carrier frequency

Symbols (00, 01, 10, or 11) represent a

sequence

Each symbol actually a sequence of 2bits


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OQPSK

Each alternate symbol is in the next

quadrature

90 are added to the phase angle, the

second symbol shifts to the nextquadrature during transmission


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OQPSK

An OQPSK receiver subtracts the phase

angle by 90 and, therefore, receives thesignal in the original quadrature and,

therefore, also the original second symbol,

and so on


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Advantage of OQPSK

In-phase and quadrature signals overlap,

because now they are in the same phasequadrant

Thus, the number of sharp transitions inthe signals reduces to half of its original

value


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OQPSK

Transmitted envelope smoother as

compared to one transmitted throughQPSK

Utilization-efficiency of the bandwidthallotted to a mobile service improves


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/4-QPSK

A form of QPSK modulation in which the

signal phase shifts by 45 after every twosymbols


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Advantage of/4-QPSK

There are no sharp transitions of in the

phase angle

The number of sharp transitions of the

signals halves


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/4-QPSK

Lesser sharp transitions imply a lower

number of significant higher harmonics,lesser bandwidth requirement per channel,

and increased utilization of the allotted

bandwidth to a wireless service provider

Bandwidth utilization-efficiency improves


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Example

Symbol sequence 10 00 11 01 to be

transmitted after QPSK modulation

After each successive time interval of T,

the phase angles of the transmitted signal

s(t), which are 3/4, 3/4, /4, +/4become 3/4, /2, 0, /2 in /4-QPSK


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Example

The /4-QPSK demodulator at the

receiving end subtracts /4 after eachsuccessive bit pair, the original QPSK

angles

3/ 4, 3 / 4, /4, + /4 found and thebits are recovered as 10 00 11 01


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Four-bit per symbol 16-QAM method

Each symbol actually a sequence of 4 bits

Four symbol sequences representing a 16bits grouped and transmitted by phaseshift keying

One of the 16 possible distinct sequencestransmits by a specific phase angle of theradio carrier frequency at a specific

amplitude (one of the three values ofamplitude s0)


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64-QAM

Two most significant bits for QPSK while

reserving the remaining 4 for the 16-QAMsignals

64-QAM thus transmits 6 symbols (bits) in

a sequence

When the bits are transmitted after 64-

QAM, the spectrum bandwidthrequirement reduces greatly


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64-QAM Example

For example, assume that a 64-QAM

modulated signal is generated andtransmitted at 19.2 ksymbol/s

One of the 64 possible distinct sequences

is transmitted at a specific phase angle,

frequency, and amplitude

Six symbols represent a sequence


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64-QAM

The bit transmission rate is 6 19.2

ksymbol/s = 115.2 kbps when 64-QAM istransmitted at 19.2 ksymbol/s

Each symbol actually a sequence of 6 bits

The bandwidth requirement, in this case,

is thus reduced by a factor of 1/6


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Summary

Modulation methods, ASK, FSK, GMSK

QPSK for each symbol representing a pairof bits

OQPSK

/4 QPSK

16 QAM for each symbol representing a

set of 4 bits 64 QAM 16 QAM for each symbolrepresenting a set of 4 bits


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End of Lesson 01

Modulation Methods for Medium-access

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