Upload
bassel-imad-al-aawar
View
228
Download
0
Embed Size (px)
Citation preview
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.
8/6/2019 Transmission Techniques
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.
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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)
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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.
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
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.
8/6/2019 Transmission Techniques
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
8/6/2019 Transmission Techniques
13/13
49
Equal Gain Combining
+
Estimate
Phase
50
Maximal Ratio Combining
+
Estimate
Weights & Phase
Phase
Weights
51
Performance with Selection Diversity