Transmission Techniques

8/6/2019 Transmission Techniques

1/13

1

Transmission Techniques

Geometric interpretation of modulated signals

Baseband transmission

Ultrawideband pulse transmission

Carrier modulated systems (BPSK, QPSK, Offset

QPSK and MSK)

Transmission in bandlimited channels

Fading channel performance Diversity

2

General Criteria for Modulation

Technique Selection

Detection efficiency

Bandwidth efficiency

Sensitivity to nonlinearities

Filtering and ISI

CCI and ACI performance

Sensitivity to frequency and phase uncertainties

Complexity

3

Transmission System Classification

Baseband systems: signal transmitted

without modulating with a carrier.

Systems with carriers: RF bandwidth

usually much smaller than carrier frequency

Ultrawideband systems: either no carrier

but very large bandwidth, or with carrier but

bandwidth a large percentage of carrier freq.

4

Baseband systems

Used in wired

systems, or ininfra-red (IR)

systems.

Employs line

coding and pulse

coding.


2/13

5

ULTRA-WIDEBAND

SYSTEMS

Although FCC defined UWB systems asthose which have bandwidths exceeding%25 of their center frequency or 1.5 GHz,whichever is less.

In industry, an UWB system is, which usesimpulses that have extremely fast rise and

fall times in sub-nanosecond range. As aresult their bandwidths are from near-DC toseveral GHz. There is no carrier frequencyin this system. 6

UWB PULSE EXAMPLE

[1] T. S. Rappaport et al, , Wireless communications: past events and a future

perspective. IEEE Commun. Mag., Vol. 40 Issue: 5 Part: Anniversary May 2002.

7

Modulation Techniques of Interest

M-ary PSK (for M=2,4,8 and perhaps 16)

M-ary FSK

Continuous Phase FSK (MSK,GMSK)

M-QAM

TCM

8

Geometric Representation of Signals

Suppose each waveforms represents 2 bits of information.


3/13

9

4 waveforms can be represented as points

in 3D using the following basis functions

10

s t E tT

t

i M

t T

i o1 2

1 2

0

( ) ( ) cos( )

, , ...,

= +

=

Amplitude Shift Keying (ASK)

11

Baseband filtered ASK

12

FSK Waveforms


4/13

13

s tE

Tt

i M

t T

i i( ) cos( )

, , ...,

= +

=

2

1 2

0

Frequency Shift Keying (FSK)

14

s tE

T

t i M

i M

t T

i o( ) cos( / )

, , ...,

= +

=

22

1 2

0

Phase Shift Keying (PSK)

15

Baseband filtered PSK

16

s tE t

Tt t

i M

t T

ii

o i( )( )

cos[ ( )]

, ,...,

= +

=

2

1 2

0

Amplitude & Phase Shift Keying (APK)


5/13

17

Performance of BPSK

Transmittedsignal

Received

signal

18

BER performance of BPSK

If we go through the analysis, we find

where,

erfc

energy per bit

noise spectral density

P QE

N

E N

E N

Q x x

E

N

bb

o

b o

b o

b

o

=F

HGI

KJ

=

2

4

1

2 2

exp( / )

/

( ) ( / )

:

:

19

2,4 and 8-PSK constellations

20

SER of coherent M-PSK


6/13

21

Why M-PSK ? (M>4)

The last figure clearly demonstrates that asM becomes larger than 4, there is a power

efficiency penalty. The question is why do

we pay this penalty. The answer is in the

next figure.

22

Bandwidth of M-PSK

23

What about QAM in wireless?

We now know that PSK is the most popular

modulation for many wireless systems. ButM-PSK for M>8 is not used in practice.

Clearly as M becomes large, putting the

points on a single circuit reduces the

distance for a given average power (or

energy) as shown next.

24

a

b

c d

4 different 8-QAM constellations


7/13

25

Is FSK used in cellular systems?

We know that FSK is a basic digitalmodulation format.

Is it frequently used in cellular systems?

If not, why not?

26

Signal separation in FSK

27

Spectrum definitions

(a) 3-dB, (b) noise equivalent, (c) null-to-null, (d) 99% power

28

Bandlimiting and ISI

When a signal is bandlimited in the frequency

domain, it is usually smeared in the timedomain. This smearing results in intersymbol

interference (ISI).

The only way to avoid ISI is to satisfy the 1st

Nyquist criterion.

For an impulse response this means at sampling

instants having only one nonzero sample.


8/13

29

Bandwidth requirements

For PSK or QAM for FSK

B rr

M B Mr

M r

M

B

r

r

M

sb

sb

s

b

= + =+

= =

<

( )( )

log log

:

:: ( )

:

11

0 1

2 2

: bandwidth in Hz

symbol rate in sps

bit rate in bpsroll off factor

number of points in the constellation30

State diagram of QPSK

31

Serial to parallel conversion in QPSK

32

Phase changes in QPSK


9/13

33

Envelope variations in QPSK

34

State diagram of filtered QPSK(square-root raised cosine with roll-off 0.5)

35

Offset QPSK

36

Serial to parallel conversion in OQPSK


10/13

37

Spectral regrowth in QPSK and OQPSK

38

/4 QPSK

39

GMSK Generation

40

Gaussian Filter

GMSK filter defined by bandwidth B

which is a function of symbol durationT.

Let = 1.177 / B, then impulse response is

and the transfer function is

h t t

H f f

G

G

( ) exp

( ) exp( )

= FHG

IKJ

=

2

2

2

2 2


11/13

41

Bandwidth as a function of BT

42

BER in AWGN and Rayleigh Fading Channels

43

BPSK in Rayleigh Fading

For coherent BPSK

PE N

e

b

1

4 0/

44

Objective of Diversity

If diversity is not employed, the resulting efficiency

would be very low, as it can be deduced from thecomparison of AWGN vs. Rayleigh channel BER.

Diversity refers to transmitting and/or receiving thesame information via different (preferablyindependent) ways.

Diversity combats fading and improves the BERperformance which

directly translates to power savings, increased system capacity.


12/13

45

Diversity Techniques Space Diversity

Receive

Transmit

Polarization Diversity

Angle Diversity

Frequency Diversity

Path Diversity

Time Diversity

46

Some relevant concepts

Explicit diversity (redundant transmission) Implicit diversity

Coherence distance

Coherence time

Coherence bandwidth

47

Diversity Combining Techniques

Selection Combining

Equal Gain Combining

Maximal Ratio Combining

48

Selection Combining

Logic

Select


13/13

49

Equal Gain Combining

+

Estimate

Phase

50

Maximal Ratio Combining

+

Estimate

Weights & Phase

Phase

Weights

51

Performance with Selection Diversity

Documents

Transmission Techniques