Binary Phase Shift Keying (BPSK)
Lecture Notes 6: Basic Modulation Schemes
The first modulation considered is binary phase shift keying. In this scheme during every bit
duration, denoted by T , one of two phases of the carrier is transmitted. These two phases are
180 degrees apart. This makes these two waveforms antipodal. Any binary modulation where
the two signals are antipodal gives the minimum error probability (for fixed energy) over any
other set of binary signals. The error probability can only be made smaller (for fixed energy
per bit) by allowing more than two waveforms for transmitting information.
In this lecture we examine a number of different simple modulation schemes. We examine the
implementation of the optimum receiver, the error probability and the bandwidth occupancy.
We would like the simplest possible receiver, with the lowest error probability and smallest
bandwidth for a given data rate.
VI-1
st
rt
and height
1.
bl pT t
lT
bl
"!
bt
2P cos 2 f ct
bt
BPSK Modulator
VI-2
nt
Modulator
The transmitted signal then is given by
Figure 33: Modulator for BPSK
otherwise.
lT
2P cos 2 f ct
bl cos 2 f ct pT t
where t is the phase waveform. The signal power is P. The energy of each transmitted bit
is E PT .
2P b t cos 2 f ct
pT t
To mathematically described the transmitted signal we define appulse
t function p T t as
T
2P
st
The phase of a BPSK signal can take on one of two values as shown in Figure VI-3.
T
Let b t denote the data waveform consisting of an infinite sequence of pulses of duration T
VI-3
VI-4
bt
iT
rt
5T
4T
0 dec bi
0 dec bi
1
1
1
1
X iT
3T
2T
LPF
2 T cos 2 f ct
-1
Figure 35: Demodulator for BPSK
t
The optimum receiver for BPSK in the presence of additive white Gaussian noise is shown in
Figure VI-3. The low pass filter (LPF) is a filter matched to the baseband signal being
transmitted. For BPSK this is just a rectangular pulse of duration T . The impulse response is
pT t The output of the low pass filter is
ht
"
0
3T
4T
5T
r d
2 T cos 2 f c h t
"
X t
2T
Figure 34: Data and Phase waveforms for BPSK
VI-5
VI-6
The sampled version of the output is given by
2 T cos 2 f c
2P b cos 2 f c
i 1T
r d
"
iT
2 T cos 2 f c pT iT
X iT
P
e,-1
n d
iT
2 P T bi
i 1T
2 f c cos 2 f c d
1 cos
e,+1
2n for some
i is Gaussian random variable, mean 0 variance N0 2. Assuming 2 f c T
integer n (or that f c T
1)
E bi
"
PT bi
X iT
X (iT )
E
Figure 36: Probability Density of Decision Statistic for Binary Phase Shift
Keying
VI-7
VI-8
Pe b
2E
N0
2Eb
N0
u2 2
Qx
1
e
2
10
-1
10
-2
10
-3
10
-4
10
-5
10
-6
10
-7
10
-8
10
-9
10
-10
where
P e,b
Error Probability of BPSK
Bit Error Probability of BPSK
du
For binary signals this is the smallest bit error probability, i.e. BPSK are optimal signals and
the receiver shown above is optimum (in additive white Gaussian noise). For binary signals
the energy transmitted per information bit Eb is equal to the energy per signal E. For
Pe b 10 5 we need a bit-energy, Eb to noise density N0 ratio of Eb N0 9 6dB. Note: Q x
is a decreasing function which is 1/2 at x 0. There are efficient algorithms (based on Taylor
2
e x 2 2 the error probability can be
series expansions) to calculate Q x . Since Q x
upper bounded by
1
Pe b
e Eb N0
2
which decreases exponentially with signal-to-noise ratio.
"
10
12
14
16
Eb/N 0 (dB)
Figure 37: Error Probability of BPSK.
VI-9
Bandwidth of BPSK
VI-10
The power spectral density is a measure of the distribution of power with respect to frequency.
The power spectral density for BPSK has the form
sinc2 f
fc T
fc T
PT
sinc2 f
2
S f
where
"
sin x
x
sinc x
The power spectrum has zeros or nulls at f f c i T except for i 0; that is there is a null at
1 T called the first null; a null at f f c
2 T called the second null; etc. The
f fc
bandwidth between the first nulls is called the null-to-null bandwidth. For BPSK the
null-to-null bandwidth is 2 T . Notice that the spectrum falls off as f f c 2 as f moves away
from f c . (The spectrum of MSK falls off as the fourth power, versus the second power for
BPSK).
"
S f df
1 2T
f fc
1 2T . The drawbacks are that the signal loses its constant envelope
property (useful for nonlinear amplifiers) and the sensitivity to timing errors is greatly
increased. The timing sensitivity problem can be greatly alleviated by filtering to a slightly
larger bandwidth 1 2T
f fc
1 2T .
Notice that
It is possible to reduce the bandwidth of a BPSK signal by filtering. If the filtering is done
properly the (absolute) bandwidth of the signal can be reduced to 1 T without causing any
intersymbol interference; that is all the power is concentrated in the frequency range
VI-11
VI-12
S(f) (dB)
S(f)
0.40
-20
0.30
-40
0.50
-60
0.20
-80
0.10
-100
-5
0.00
-4
-3
-2
-1
-4
-3
-2
-1
(f-f c )T
(f-fc)T
Figure 38: Spectrum of BPSK
Figure 39: Spectrum of BPSK
VI-13
Example
VI-14
Given:
21
180 dBm/Hz =10
10
13
Watts
Pr
S(f) (dB)
Watts/Hz.
Noise power spectral density of N0 2
-20
10 7 .
Desired Pe
-40
Find: The data rate that can be used and the bandwidth that is needed.
-80
An alternative view of BPSK is that of two antipodal signals; that is
-60
Solution: Need Q 2Eb N0
10 7 or Eb N0 11 3dB or Eb N0 13 52. But
Eb N0 Pr T N0 13 52. Thus the data bit must be at least T 9 0 10 8 seconds long, i.e.
the data rate 1 T must be less than 11 Mbits/second. Clearly we also need a (null-to-null)
bandwidth of 22 MHz.
"
"
"
"
E t
s0 t
and
-100
-8
-6
-4
-2
s1 t
10
E t
-10
(f-fc)T
where t
2 T cos 2 f ct 0 t T is a unit energy waveform. The above describes
the signals transmitted only during the interval 0 T . Obviously this is repeated for other
Figure 40: Spectrum of BPSK
VI-15
VI-16
Effect of Filtering and Nonlinear Amplification
on a BPSK waveform
intervals. The receiver correlates with t over the interval 0 T and compares with a
threshold (usually 0) to make a decision. The correlation receiver is shown below.
In this section we illustrate one main drawback to BPSK. The fact that the signal amplitude
has discontinuities causes the spectrum to have fairly large sidelobes. For a system that has a
constraint on the bandwidth this can be a problem. A possible solution is to filter the signal. A
bandpas filter centered at the carrier frequency which removes the sidbands can be inserted
after mixing to the carrier frequency. Alternatly we can filter the data signal at baseband
before mixing to the carrier frequency.
rt
T
0
dec s0
dec s1
Below we simulate this type of system to illustrate the effect of filtering and nonlinear
amplification. The data waveform b t is mixed onto a carrier. This modulated waveform is
denoted by
t
This is called the Correlation Receiver. Note that synchronization to the symbol timing and
oscillator phase are required.
2P cos 2 f ct
s1 t
The signal s1 t is filtered by a fourth order bandpass Butterworth filter with passband from
fc 4Rb to f c 4Rb The filtered signal is denoted by s2 t . The signal s2 t is then amplified.
VI-17
VI-18
Data waveform
1
0.5
0
100 tanh 2s1 t
s3 t
b(t)
The input-output characteristics of the amplifier are
0.5
This amplifier is fairly close to a hard limiter in which every input greater than zero is mapped
to 100 and every input less than zero is mapped to -100.
1
0
0.2
0.4
Simulation Parameters
0.6
time
0.8
1.2
5
x 10
Signal waveform
Sampling Frequency= 50MHz
Sampling Time =20nseconds
Center Frequency= 12.5MHz
Data Rate=390.125kbps
Simulation Time= 1.31072 m s
s(t)
1
0
1
2
0
0.4
0.6
time
0.8
1.2
5
x 10
VI-19
0.2
VI-20
Signal spectrum
100
40
80
60
60
40
80
20
S(f)
Output
100
120
20
40
140
60
160
80
180
0
0.5
1.5
frequency
100
2
2.5
7
1.5
0.5
x 10
0
Input
0.5
1.5
VI-21
VI-22
Filtered signal spectrum
Data waveform
1
45
0.5
b(t)
40
50
55
0
0.5
S2(f)
60
1
0
0.2
0.4
65
0.6
time
0.8
1.2
5
x 10
Filtered signal waveform
2
70
1
s2(t)
75
80
85
90
0
0.5
1.5
frequency
2
0
2.5
7
x 10
0.4
0.6
time
0.8
1.2
5
x 10
VI-23
0.2
VI-24
Data waveform
Amplified and filtered signal spectrum
20
0.5
b(t)
30
0
0.5
40
S2(f)
1
0
0.2
50
0.4
0.6
time
0.8
1.2
5
x 10
Amplified and filtered signal waveform
100
60
s3(t)
50
0
70
50
80
0
0.5
1.5
frequency
100
0
2.5
0.2
0.4
0.6
time
x 10
0.8
1.2
5
x 10
VI-25
VI-26
P cos 2 f ct
Quaternary Phase Shift Keying (QPSK)
bc t
The next modulation technique we consider is QPSK. In this modulation technique one of
four phases of the carrier is transmitted in a symbol duration denoted by Ts . Since one of four
waveforms is transmitted there are two bits of information transmitted during each symbol
duration. An alternative way of describing QPSK is that of two carriers offset in phase by 90
degrees. Each of these carriers is then modulated using BPSK. These two carriers are called
the inphase and quadrature carriers. Because the carriers are 90 degrees offset, at the output of
the correlation receiver they do not interfer with each other (assuming perfect phase
synchronization). The advantage of QPSK over BPSK is that the the data rate is twice as high
for the same bandwidth. Alternatively single-sideband BPSK would have the same rate in bits
per second per hertz but would have a more difficult job of recovering the carrier frequency
and phase.
st
bs t
P sin 2 f ct
Figure 41: Modulator for QPSK
VI-27
VI-28
bc l pTs t
bs t
bc l
bs l pTs t
lTs
bs l
lTs
! !
bc t
bc l
bs l
+1
+1
-1
+1
3 4
-1
-1
5 4
+1
-1
7 4
2P cos 2 f ct
bs t sin 2 f ct
P bc t cos 2 f ct
st
The transmitted power is still P. The symbol duration is Ts seconds. The data rate is
Rb 2 Ts bits seconds.
The phase t , of the transmitted signal is related to the data waveform as follows.
lTs
4 3 4 5 4 7 4
l pTs t
The relation between l and bc l bs l is shown in the following table
VI-29
VI-30
bc t
1
t
Ts
2Ts
3Ts
4Ts
5Ts
-1
The constellation of QPSK is shown below. The phase of the overall carrier can be on of four
values. Transitions between any of the four values may occur at any symbol transition.
Because of this, it is possible that the transition is to the 180 degree opposite phase. When this
happens the amplitude of the signal goes through zero. In theory this is an instantaneous
transition. In practice, when the signal has been filtered to remove out-of-band components
this transition is slowed down. During this transition the amplitude of the carrier goes throguh
zero. This can be undesireable for various reasons. One reason is that nonlinear amplifiers
with a non constant envelope signal will regenerate the out-of-band spectral components.
Another reason is that at the receiver, certain synchronization circuits need constant envelope
to maintain their tracking capability.
bs t
1
2Ts
3Ts
4Ts
5Ts
2Ts
3Ts
4Ts
5Ts
Ts
-1
7 4
5 4
3 4
4
Ts
Figure 42: Timing and Phase of QPSK
VI-31
VI-32
Quadrature-phase
Channel
The bandwidth of QPSK is given by
(+1,+1)
fc Ts
f c Tb
sinc2 2 f
In-phase
Channel
sinc2 f
f c Tb
fc Ts
PTb sinc2 2 f
PTs 2 sinc2 f
S f
(-1,+1)
since Ts Tb 2. Thus while the spectrum is compressed by a factor of 2 relative to BPSK
with the same bit rate, the center lobe is also 3dB higher, that is the peak power density is
higher for QPSK than BPSK. The null-to-null bandwidth is 2 Ts Rb .
(-1,-1)
(+1,-1)
Figure 43: Constellation of QPSK
VI-33
S(f)
S(f) dB
1.00
VI-34
BPSK
QPSK
-20
0.75
QPSK
-40
0.50
-60
BPSK
0.25
-80
0.00
-4
-3
-2
-1
-100
-5
-4
-3
-2
-1
(f-f c )T
(f-f c )T
Figure 44: Spectrum of QPSK
Figure 45: Spectrum of QPSK
VI-35
VI-36
2 Ts cos 2 f ct
QPSK
BPSK
LPF
Xc iTs
0 dec bc i
0 dec bc i
0 dec bs i
0 dec bs i
1
1
-20
iTs
S(f) dB
rt
-40
LPF
-80
Xs iTs
1
1
-60
iTs
2 Ts sin 2 f ct
-100
-10
-8
-6
-4
-2
10
(f-fc)T
Figure 47: QPSK Demodulator
Figure 46: Spectrum of QPSK
VI-37
VI-38
c i
E b bc i
Given:
c i
14
110 dBm/Hz =10
Watts/Hz.
Noise power spectral density of N0 2
Eb bs i
s i
Pr
where Eb PTs 2 is the energy per transmitted bit. Also c i and s i are Gaussian random
variables, with mean 0 and variance N0 2.
10
Watts
s i
PTs 2 bs i
Xs iTs
Example
PTs 2 bc i
Xc iTs
2n or 2 f c Ts
Assuming 2 f c Ts
Desired Pe
10 7 .
Find: The data rate that can be used and the bandwidth that is needed for QPSK.
Bit Error Probability of QPSK
Ts
N0
Pr T N0
13 52
Pe2 b
2Pe b
13 52. But
Pr
2
11 3dB or Eb N0
Pe b
or Eb N0
"
"
"
Eb N0
The probability that a symbol error is made is
Pe s
"
10
2Eb N0
Pe b
2Eb
N0
Solution: Need Q
since Ts 2T . Thus the data bit must be at least T 9 0 10 8 seconds long, i.e. the data
rate 1 T must be less than 11 Mbits/second. Clearly we also need a (null-to-null) bandwidth
of 11 MHz.
Thus for the same data rate, transmitted power, and bit error rate (probability of error), QPSK
has half the (null-to-null) bandwidth of BPSK.
VI-39
VI-40
P cos 2 f ct
Ts 2
bc t
Offset Quaternary Phase Shift Keying (OQPSK)
st
The disadvantages of QPSK can be fixed by offsetting one of the data streams by a fraction
(usually 1/2) of a symbol duration. By doing this we only allow one data bit to change at a
time. When this is done the possible phase transitions are 90 deg. In this way the transitions
through the origin are illiminated. Offset QPSK then gives the same performance as QPSK
but will have less distorition when there is filtering and nonlinearities.
bs t
P sin 2 f ct
Figure 48: Modulator for OQPSK
VI-41
VI-42
bc l pTs t
lTs
bc l
lTs
bs l
bc t
Quadrature-phase
Channel
bs l pTs t
st
(-1,+1)
(+1,+1)
2P cos 2 f ct
bs t sin 2 f ct
Ts 2 cos 2 f ct
P bc t
st
bs t
In-phase
Channel
The transmitted power is still P. The symbols duration is Ts seconds. The data rate is
Rb 2 Ts bits seconds. The bandwidth (null-to-null) is 2 Ts Rb . This modification of
QPSK removes the possibility of both data bits changing simultaneously. However, one of the
data bits may change every Ts 2 seconds but 180 degree changes are not allowed. The
bandwidth of OQPKS is the same as QPSK. OQPSK has advantage over QPSK when passed
through nonlinearities (such as in a satellite) in that the out of band interference generated by
first bandlimiting and then hard limiting is less with OQPSK than QPSK.
(-1,-1)
(+1,-1)
Figure 49: Constellation of QPSK
VI-43
VI-44
Ts 2
bc t
2 Ts cos 2 f ct
1
5Ts
0 dec bs i
0 dec bs i
1
1
Ts 2
Xc iTs
0 dec bc i
0 dec bc i
bs t
LPF
4Ts
3Ts
2Ts
Ts
-1
Ts 2
iTs
rt
1
t
2Ts
3Ts
4Ts
5Ts
1
1
Xs iTs
7 4
LPF
-1
iTs
Ts
2 Ts sin 2 f ct
5 4
3 4
Figure 51: Demodulator for OQPSK
Ts
2Ts
3Ts
4Ts
5Ts
Figure 50: Data and Phase Waveforms for OQPSK
VI-45
VI-46
Minimum Shift Keying (MSK)
c i
Eb bc i
c i
Xs iTs
PTs 2 bs i
s i
Eb bs i
s i
PTs 2 bc i
Ts 2
Xc iTs
Minimum shift keying can be viewed in several different ways and has a number of significant
advantages over the previously considered modulation schemes. MSK can be thought of as a
variant of OQPSK where the data pulse waveforms are shaped to allow smooth transition
between phases. It can also be thought of a a form of frequency shift keying where the two
frequencies are separated by the minimum amount to maintain orthogonality and have
continuous phase when switching from one frequency to another (hence the name minimum
shift keying). The advantages of MSK include a better spectral efficiency in most cases. In
fact the spectrum of MSK falls off at a faster rate than BPSK, QPSK and OQPSK. In addition
there is an easier implementation than OQPSK (called serial MSK) that avoids the problem of
having a precisely controlled time offset between the two data streams. An additional
advantage is that MSK can be demodulator noncoherently as well as coherently. So for
applications requiring a low cost receiver MSK may be a good choice.
where Eb PTs 2 is the energy per transmitted bit. Also c i and s i are Gaussian random
variables, with mean 0 variance N0 2.
Bit Error Probability of OQPSK
2Eb
N0
Pe b
The probability that a symbol error is made is
2
2Pe b
Pe2 b
Pe b
Pe s
2n or 2 f c Ts
Assuming 2 f c Ts
This is the same as QPSK.
VI-47
VI-48
bc l pTs t
lTs
bs l
ct
Ts 2
bs l pTs t
2 cos t Ts
2 sin t Ts
ct
Ts 2
bc l
l
lTs
bs t
bc t
P cos 2 f ct
Ts 2
ct
bc t
st
bs t c t sin 2 f ct
Ts 2 cos 2 f ct
Ts 2 c t
P bc t
st
bs t
2P
Ts 2 cos t Ts
sin 2 f ct
bs t sin t Ts
P sin 2 f ct
ct
cos 2 f ct
bc t
!
!
st
2P cos 2 f ct
Figure 52: Modulator for MSK
where
Ts 2 cos t Ts
bc t
cos t
VI-49
VI-50
t
+1
-1
-1
+1
-1
-1
t
Ts
t
Ts
t
Ts
t
Ts
+1
+1
In the above table, because of the delay of the bit stream corresponding to the cosine branch,
only one bit is allowed to change at a time. During each time interval of duration Ts 2 during
which the data bits remain constant there is a phase shift of 2. Because the phase changes
linearly with time MSK can also be viewed as frequency shift keying. The two different
1
frequencies are f c 2T1 s and f c 2T1 s . The change in frequency is f T1s
2T where
bs t sin t Ts
bc t Ts 2 cos t Ts
tan
bs t sin t Ts
sin t
bs t
Ts 2
bc t
Tb 1 2 Ts is the data bit rate. The transmitted power is still P. The symbols duration is Ts
seconds. The data rate is Rb 2 Ts bits seconds. The signal has constant envelope which is
useful for nonlinear amplifiers. The bandwidth is different because of the pulse shaping
waveforms.
VI-51
VI-52
Ts 2
bc t
1
bc 0
bc 3
3
(t)/
7Ts t
Ts
-1
2Ts
3Ts
4Ts
bc 1 bc 2
5Ts
6Ts
bc 4
bs t
bs 1
bs 3
0
7Ts t
Ts
2Ts
-1b
s0
bs 2
4Ts
5Ts
6Ts
bs 4
2
0
2
3Ts
7Ts t
2Ts
Ts
3Ts
4Ts
5Ts
6Ts
4
Figure 53: Data and phase waveforms for MSK
8
time/Tb
10
12
14
Figure 54: Phase of MSK signals
VI-53
(-1,+1)
Quadrature-phase
Channel
VI-54
The spectrum of MSK is given by
(+1,+1)
cos2 2Tb f fc
1 4Tb f fc 2
cos2 2Tb f fc
1 4Tb f fc 2
8PTb
2
S f
The nulls in the spectrum are at f f c Tb = 0.75, 1.25, 1.75,.... Because we force the signal
to be continuous in phase MSK has significantly faster decay of the power spectrum as the
frequency from the carrier becomes larger. MSK decays as 1 f 4 while QPSK, OQPSK, and
BPSK decay as 1 f 2 as the frequency differs more and more from the center frequency.
In-phase
Channel
(-1,-1)
(+1,-1)
Figure 55: Constellation of MSK
VI-55
VI-56
Spectrum of MSK
Spectrum of MSK
0
S(f) (dB)
1.00
M SK
QPSK
B PSK
Q PSK
-10
M SK
-20
0.75
-30
-40
-50
0.50
-60
-70
0.25
-80
BPSK
-90
-100
0.00
-4
-3
-2
-1
-5
-4
-3
-2
-1
(f-f c )T
(f-f c )T
Figure 56: Spectrum of MSK
Figure 57: Spectrum of MSK
VI-57
VI-58
ct
2 Ts cos 2 fc t
Ts 2
S(f)
-25
Ts 2
iTs
iTs
0 dec bs i
0 dec bs i
1
1
Ts 2
Xc iTs
0 dec bc i
0 dec bc i
LPF
rt
-50
1
1
2 Ts sin 2 fc t
Xs iTs
ct
-100
LPF
-75
-6
-4
-2
-8
(f-fc)T
Figure 59: Coherent Demodulator for MSK
Figure 58: Spectrum of MSK
VI-59
VI-60
Bit Error Probability of MSK with Coherent Demodulation
Xs iTs
PTs 2 bs i
s i
Eb bs i
s i
Since the signals are still antipodal
Pe b
2Eb
N0
c i
Eb bc i
c i
PTs 2 bc i
Ts 2
Xc iTs
2n or 2 f c Ts
Assuming 2 f c Ts
where Eb PTs 2 is the energy per transmitted bit. Also c i and s i are Gaussian random
variables, with mean 0 variance N0 2.
The probability that a symbol error is made is
Pe b
2Pe b
Pe2 b
Pe s
VI-61
VI-62
1.5
4
1
2
phi(t)
y(t)
0.5
-0.5
-2
-1
-4
-1.5
-2
10
time/Tb
12
14
16
18
-6
20
Figure 60: Waveform for Minimum Shift Keying
10
time/Tb
12
14
16
18
20
Figure 61: Phase Waveform for Minimum Shift Keying
VI-63
VI-64
0.5
40
20
-0.5
-1
10
15
20
25
-20
30
-40
-60
0.5
0
-80
-0.5
-100
-1
60
10
15
20
25
-120
30
Figure 62: Quadrature Waveforms for Minimum Shift Keying
10
Figure 63: Spectrum for Minimum Shift Keying
VI-65
VI-66
Noncoherent Demodulation of MSK
Because MSK can be viewed as a form of Frequency Shift Keying it can also be demodulated
noncoherently. For the same sequence of data bits the frequency is f c 1 2Ts if
bc t Ts 2
bs t and is f c 1 2Ts if bc t Ts 2
bs t .
For the example phase waveform shown previously we have that
at
2Ts 5Ts 2
bs 1
bc 0
bc 0
3Ts 2 2Ts
bs 0
bs 0
Detected Data
bs 1
bc 1
bc 1
bs 2
So detecting the frequency can also be used to detect the data.
Consider determining bc i 1 at time iTs . Assume we have already determined bs i 1 at time
i 1 2 Ts . If we estimate which of two frequencies is sent over the interval i 1 2 Ts iTs
the decision rule is to decide that bc i 1 bs i 1 if the frequency detected is f c 1 2Ts and
to decide that bc i 1
bs i 1 if the frequency detected is f c 1 2Ts .
bc
Frequency
Previous Data
Ts 3Ts 2
Ts 2 Ts
0 Ts 2
Time Interval
Consider determining bs i 1 at time i 1 2 Ts . Assume we have already determined bc i
time i 1 Ts . If we estimate which of two frequencies is sent over the interval
i 1 Ts i 1 2 Ts the decision rule is to decide that bs i 1 bc i 2 if the frequency
bc i 2 if the frequency detected is
detected is f c 1 2Ts and to decide that bs i 1
fc 1 2Ts .
The method to detect which of the two frequencies is transmitted is identical to that of
Frequency Shift Keying which will be considered later.
VI-67
VI-68
Serial Modulation and Demodulation
The implementation of MSK as parallel branches suffers from significant sensitivity to precise
timing of the data (exact shift by T for the inphase component) and the exact balance between
the inphase and quadriphase carrier signals. An alternative implementation of MSK that is
less complex and does not have these draw backs is known as serial MSK. Serial MSK does
2n 1 4T which may be important when f c is about
have an additional restriction that f c
the same as 1 T but for f c 1 T it is not important. The block diagram for serial MSK
modulator and demodulator is shown below.
2 sin 2 f 1t pT t
gt
1 4T for some
st
bt
1
1
where f 1 fc 4T
and f 2 fc 4T
. (For serial MSK we require f c
2n
integer n. Otherwise the implementation does not give constant envelope).
G f
2P cos 2 f 1t
The filter G f is given by filter
j f
f1 T
f1 T e
T sinc f
f1 T
j f
f1 T e
T sinc f
G f
VI-69
iT
rt
Demodulator
VI-70
The low pass filter (LPF) removes double frequency components. Serial MSK is can also be
viewed as a filtered form of BPSK where the BPSK signal center frequency is f 1 but the filter
is not symmetric with respect to f 1 . The receiver is a filter matched to the transmitted signal
(and hence optimal). The output is then mixed down to baseband where it is filtered (to
remove the double frequency terms) and sampled.
LPF
H f
X iT
2 T cos 2 f 1t
The filter H f is given by
f1 T
f1 T
0 25
e
0 25 2
j2 f
f1 T
"
"
4T cos 2 f
1 16 f
H f
VI-71
VI-72
Continuous Phase Modulation
MSK is a special case of a more general form of modulation known as continuous phase
modulation where the phase is continuous. The general form of CPM is given by
1 2 and
For example if CPM has h
2P cos 2 f ct
st
t 2
qt
"
where the phase waveform has the form
0 kT
1T
iT d
0 i 0
2h bi q t
0 kT
iT
1 2
bi g
2h
t k
then the modulation is the same as MSK. Continuous Phase Modulation Techniques have
constant envelope which make them useful for systems involving nonlinear amplifiers which
also must have very narrow spectral widths.
1T
i 0
The function g is the (instantaneous) frequency function, h is called the modulation index
t
and bi is the data. The function q t
0 g d is the phase waveform. The function
is the frequence waveform.
dg t
dt
gt
VI-73
VI-74
Example
Given:
14
110 dBm/Hz =10
Watts/Hz.
Noise power spectral density of N0 2
Gaussian Minimum Shift Keying
10
Watts
Pr
Desired Pe
10 7 .
Gaussian minimum shift keying is a special case of continuous phase modulation discussed in
the previous section. For GMSK the pulse waveforms are given by
Q
Find: The data rate that can be used for MSK.
gt
Bandwidth available=26MHz (at the 902-928MHz band). The peak power outside must
be 20dB below the peak power inside the band.
10 7 or Eb N0 11 3dB or Eb N0 13 52. But
Solution: Need Q 2Eb N0
Eb N0 Pr T N0 13 52. Thus the data bit must be at least T 9 0 10 8 seconds long, i.e.
the data rate 1 T must be less than 11 Mbits/second.
"
"
"
"
VI-75
VI-76
1.8
1.6
1.4
1.2
h(t)
phi(t)
10
0.8
0.6
0.4
0.2
10
2
8
time/Tb
10
12
0
0
14
Figure 64: Phase Waveform for Gaussian Minimum Shift Keying (BT=0.3)
10
time/Tb
12
14
16
18
20
Figure 65: Data Waveform for Gaussian Minimum Shift Keying (BT=0.3)
VI-77
VI-78
1.5
-10
1
-20
0.5
b(t)
|H(f)|
-30
-40
-50
-0.5
-60
-1
-70
-80
5
f
-1.5
10
Figure 66: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
10
time/Tb
12
14
16
18
20
Figure 67: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
VI-79
VI-80
20
1.5
10
1
0
y(t)
|X(f)|
0.5
-10
-0.5
-20
-1
-30
-1.5
-40
5
f
-2
10
Figure 68: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
10
time/Tb
12
14
16
18
20
Figure 69: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
VI-81
VI-82
3
4
2
2
phi(t)
phi(t)
-1
-2
-2
-4
-3
-6
10
time/Tb
12
14
16
18
-4
20
Figure 70: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
10
15
time/Tb
20
25
30
Figure 71: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
VI-83
VI-84
60
0.5
40
20
-0.5
-1
10
15
20
25
-20
30
-40
1
-60
0.5
-80
-100
-0.5
-1
-120
0
10
15
20
25
30
10
Figure 73: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
Figure 72: Waveform for Gaussian Minimum Shift Keying (BT=0.3)
VI-85
VI-86
Data Waveform
2
Real(x(t))
4 QPSK
0
1
2
0
As mentioned earlier the effect of filtering and nonlinearly amplifying a QPSK waveform
causes distortion when the signal amplitude fluctuates significantly. Another modulation
scheme that has less fluctuation that QPSK is 4 QPSK. In this modulation scheme every
other symbol is sent using a rotated (by 45 degrees) constellation. Thus the transitions from
one phase to the next are still instantaneous (without any filtering) but the signal never makes
a transition through the origin. Only 45 and 135 degree transitions are possible. This is
shown in the constellation below where a little bit of filtering was done.
10
15
20
time
25
30
35
40
25
30
35
40
Data Waveform
2
Imag(x(t))
1
0
1
2
0
10
15
20
time
Figure 74: Data Waveforms for 4 QPSK
VI-87
VI-88
2
2
1.5
1
0
1
2
0.5
3
0
0.5
1.5
2.5
3.5
4
0
0.5
2
1
0
1
1.5
2
3
0
0.5
1.5
2.5
3.5
2
2
1.5
0.5
0.5
1.5
Figure 76: Constellation for 4 QPSK.
Figure 75: Eye Diagram for 4 QPSK
VI-89
General Modulator
Lecture 6b: Other Modulation Techniques
VI-90
Orthogonal Signals
M
1 are said to be orthogonal (over the interval
T 0
A set of signals i t : 0
0 T ) if
b1 t
b2 t
"
i t j t dt
2P cos 2 f c
f0 t
Many signal sets can be described as linear combinations of orthonormal signal sets as we
will show later. Below we describe a number of different orthonormal signal sets. The signal
sets will all be described at some intermediate frequency f 0 but are typically modulated up to
the carrier frequency f c .
bk t
"
i t j t dt
st
ut
b3 t
In most cases the signals will have the same energy and it is convenient to normalize the
signals to unit energy. A set of signals i t : 0 t T 1 i M are said to be
orthonormal (over the interval 0 T ) if
Select
one of
M 2k
unit energy
signals
bl pT t
lT i
12
""
bi t
VI-91
VI-92
General Coherent Demodulator
t
0 T
lT
X1 l T
il t
lT
1 T
X2 l T
b1 t
b2 t
bk
b1 t
b2 t
bk
bk t
rt
1 bk t
bk t
Choose
Largest
f0 t
2 T cos 2 fc
t
lT
XM l T
VI-93
The symbol error probability can be upper bounded as
m T t is the impulse response of the m-th matched filter. The output of these filters
(assuming that the il -th orthogonal signal is transmitted is) given by
VI-94
log2 M
ln 2
ln 2
Eb
N0
Eb
N0
4 ln 2
4 ln 2
ln 2
0 if
Eb
N0
, Pe
where exp2 x denotes 2x . Note that as M
ln 2 = -1.59dB.
VI-95
ln 2
Eb
2N0
log2 M
"
dx
Eb
N0
exp2
Eb
N0
4 ln M
x2 2
exp2
Pe s
"
du
where u is the distribution function of a zero mean, variance 1, Gaussian random variable
given by
1
2
u2 2
E
N0
ln M
"
ue
E
2N0
4 ln M
exp
!
2E M
N0
The symbol error probability of M orthogonal signals with coherent demodulation is given by
1
2
E
N0
ln M
""
To determine the probability of error we need to determine the probability that the filter output
corresponding to the signal present is smaller than one of the other filter outputs.
ln M
ln M
Normally a communication engineer is more concerned with the energy transmitted per bit
rather than the energy transmitted per signal, E. If we let E b be the energy transmitted per bit
then these are related as follows
E
Eb
log2 M
Thus the bound on the symbol error probability can be expressed in terms of the energy
transmitted per bit as
where m m 0 1 2
M 1 is a sequence of independent, identically distributed
Gaussian random variables with mean zero and variance N0 2.
Pe s
exp
E
N0
il
Pe s
il
!
!
Xm l T
E
N0
VI-96
bk
b2 t
b1 t
il
1T
" " "
" " "
where for l
lT
ut
Pe , s
1 0- 1
1 0- 2
1 0- 3
M=2
1 0- 4
So far we have examined the symbol error probability for orthogonal signals. Usually the
number of such signals is a power of 2, e.g. 4, 8, 16, 32, .... If so then each transmission of a
signal is carrying k log2 M bits of information. In this case a communication engineer is
usually interested in the bit error probability as opposed to the symbol error probability. These
can be related for any equidistant, equienergy signal set (such as orthogonal or simplex signal
sets) by
1 0- 5
8
16
1 0- 7
Shannon
Limit
1 0- 8
M=32
1 0- 9
M=1024
"
M
Pe s
2M 1
1 0- 1 0
2k 1
Pe s
2k 1
Pe b
1 0- 6
-4
-2
10
12
14
16
Eb/N0 (dB)
Figure 77: Symbol Error Probability for Coherent Demodulation of Orthogonal
Signals
VI-97
1 0- 1
1 0- 2
lT
2
Xc 1 lT
1 T
General Noncoherent Demodulator
Pe,b
VI-98
1 0- 3
Z1 lT
1 0- 4
f0 t
2 T cos 2 fc
M=2
Xs 1 lT
lT
1 T
1 0- 5
1 0- 6
Shannon
Limit
Choose
Largest
16
rt
32
1 0- 9
1 0- 8
1 0- 7
M=1024
10
12
14
16
Xc M lT
f0 t
Eb/N0 (dB)
2 T sin 2 fc
-2
lT
-4
M T
1 0- 1 0
ZM lT
Xs M lT
M T
Figure 78: Bit Error Probability for Coherent Demodulation of Orthogonal
Signals
lT
VI-99
VI-100
1 T l T then
If signal 1 is transmitted during the interval l
c 1
E cos
Xc m l T
c m
s 1
The symbol error probability for noncoherently detection of orthogonal signals is
E sin
Xs m l T
s m
Eb m log2 MN0
m 2
M
e
m
As with coherent demodulation the relation between bit error probability and symbol error
probability for noncoherent demodulation of orthogonal signals is
2s 1
2k 1
Pe s
2k 1
M
Pe s
2M 1
"
Pe b
2s M
2c M
ZM l T
2s 3
Z3 l T
2c 1
2s 2
2c 3
2c 2
Z2 l T
s 1 sin
2 E c 1 cos
The decision statistic then (if signal 1 is transmitted) has the form
Z1 l T
Eb log2 MN0
1
e
M
Pe s
VI-101
P e,s
-1
10
-2
10
-3
10
-4
M =2
10
-5
10
-6
10
-7
10
-8
10
-9
M =8
M =3 2
M =4
M =1 6
10
12
14
16
10
10
10
10
16
10
10
10
10
10
Eb/N 0 (dB )
32
-1 0
10
10
Pb
10
10
S y m bol Error P robability forN oncoherent D etection
of O rthogonal S ignals
VI-102
10
10
15
Eb N0 (dB)
Figure 79: Symbol Error Probability for Nonocherent Detection of Orthogonal
Signals.
VI-103
Figure 80: Bit error probability of M-ary orthogonal modulation in an additive
white Gaussian noise channel with noncoherent demodulation
VI-104
A. Time-orthogonal (Pulse position modulation PPM)
B. Time-orthogonal quadrature-phase
1T M
1T M
2M
T
2iT
M
2i
T
M
M
2
01
M
2T
elsewhere
M even
f0
M
2T
"
cos 2 f 0t
f1
2i
elsewhere
M
2T
2i
2M
sin 2 f 1t dt
T
f0
2iT
M
2M
sin 2 f 0t
T
sin 2 f 0t
f0
M
n
2T
2M
T
"
"
i 1T M
2i t
elsewhere
01
iT M
"
"
iT M
sin 2 f 0t
2M
T
i t
VI-105
VI-106
D. Frequency-orthogonal quadrature-phase
C. Frequency-orthogonal (Frequency Shift Keying FSK)
i
t
T
2E
cos 2 f 0
T
nM
2T
f0
i
t
T
2i
"
2E
sin 2 f 0
T
2i t
i
t
2T
nM
f0
2T
"
01
""
i t
2E
sin 2 f 0
T
VI-107
VI-108
4):
Example (M
E. Hadamard-Walsh Construction
The last construction of orthogonal signals is done via the Hadamard Matrix. The Hadamard
matrix is an N by N matrix with components either +1 or -1 such that every pair of distinct
rows are orthogonal. We show how to construct a Hadamard when the number of signals is a
power of 2 (which is often the case).
H2
H2
H4
H2
H2
H2 l
H2 l
H2 l
H2 l
H2
"
Begin with a two by two matrix
"
Example (M
8):
Then use the recursion
H4
H4
H8
H4
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2
H2l
H4
"
Now it is easy to check that distinct rows in these matrices are orthogonal. The i-th modulated
signal is then obtained by using a single (arbitrary) waveform N times in nonoverlapping time
intervals and multiplying by the j th repetition of the waveform by the jth component of the
i-th row of the matrix.
VI-109
VI-110
1 t
T 8 T 4 3T 8 T 2 5T 8 3T 4 7T 8 T t
2 t
"
3 t
T 8
5T 8
7T 8
3T 8
8 t
3T 4
T 4
Figure 81: Hadamard-Walsh Orthogonal Signals
VI-111
VI-112
W2
Yi
W8
W4
X3
X4
X5
X6
X7
X8
Y2
X2
Y2
X2
Y3
X3
Y3
X3
Y4
Y5
Y6
Y7
Y8
X4
X5
X6
X7
X8
Y4
Y5
Y6
Y7
Y8
X4
X5
X6
X7
X8
Y4
Y5
Y6
Y7
Y8
X4
X5
X6
X7
X8
Y4
Y5
Y6
Y7
Y8
X1
Y1
Y2
X2
Y2
Y3
X3
Y3
2 T sin 2 f ct
X1
Y1
W7
LPF
W3
X2
W6
X1
Y1
iT M
W5
W2
Process
Choose
Largest
Y1
W4
rt
X1
Xi
W1
W3
LPF
iT M
W1
2 T cos 2 f ct
Noncoherent Reception of
Hadamard Generated Orthogonal Signals
Figure 82: Noncoherent Demodulator
VI-113
VI-114
Noncoherent Reception of
Hadamard Generated Orthogonal Signals
X5
X6
Y2
X7
X8
X2
Y5
Y6
Y7
Y8
X4
X5
X6
X7
X8
Y5
Y3
X4
X5
X6
Y4
Y5
Y6
Y8
X7
X8
Y7
Y8
X6
X7
X8
Y2
Y7
Y1
Y6
X3
Y4
X5
2
2
X2
Y3
X1
Y2
X4
Y4
X3
X8
Y8
X7
Y7
Y6
X6
X3
Y5
X5
X2
Y3
Y4
X4
Y2
Y3
X3
X2
X1
Y1
W8
X4
X1
Y1
W7
X3
Y1
W6
X2
X1
W5
X1
2
WX 1
WX 5
WX 2
WX 6
WX 3
WX 7
WX 4
WX 8
Figure 83: Fast Processing for Hadamard Signals
VI-115
VI-116
Biorthogonal Signal Set
A biorthogonal signal set can be described as
E0 t
s1 t
E1 t
s0 t
EM
2 1
2 1
sM
sM
E0 t
EM
If we define bandwidth of M signals as minimum frequency separation between two such
signal sets such that any signal from one signal set is orthogonal to every signal from a
frequency adjacent signal set are orthogonal then for all of these examples of M signals the
bandwidth is
M
W
M 2W T !
2T
Thus there are 2W T orthogonal signals in bandwidth W and time duration T .
2 1
sM
That is a biorthogonal signal set is the same as orthogonal signal set except that the negative
of each orthonormal signal is also allowed.. Thus there are 2N signals in N dimensions. We
have doubled the number of signals without changing the minimum Euclidean distance of the
VI-117
VI-118
Let H j be the hypothesis that signal s j was sent for j
is (given signal s0 sent)
1. The probability of correct
""
fs r0 Fn r0
r0 0
Fn
r0
r0 H0
M 2 1
2 1
rM
r0
0 r1
P r0
dr0
where f s x is the denisty function of r0 when H0 is true and Fn x is the distribution of r1
when H0 is true.
!
""
Pc 0
B8
"
Symbol Error Probability
signal set. For example:
Let H j be the hypothesis that signal s j was sent for j 0
M 1. The optimal receiver does
a correlation of the received signal with each of the M 2 orthonormal signals. Let r j be the
correlation of r t with j t . The decision rule is to choose hypothesis H j if r j is largest in
absolute value and is of the appropriate sign. That is, if r j is larger than ri and is the same
sign as the coefficient in the representation of s j t .
Fs x
fn x
Fn x
1
x
22
1
x
22
VI-119
1
exp
2
x
E
1
exp
2
x
!
!
fs x
VI-120
Bit Error Probability
The bit error probabiltiy for birothogonal signals can be determined for the usual mapping of
bits to symbols. The mapping is given as
000000
000
000000
001
s0 t
011111
111
111111
111
111111
110
100000
000 sM
N0 2. The error probability is then
where 2
M 2 1
dr0
s1 t
sM
r0 0
r0
Fn
fs r0 Fn r0
Pe 0
2 1
sM
s0 t
s1 t
sM
2 1
z2
dz
2
1
exp
2
2 2
1M
2E
2 z
N0
2 1
"
Pe s
sM
Using an integration by parts argument we can write this as
The mapping is such that signals with furthest distance have largest number of bit errors. An
error of the first kind is defined to be an error to an orthogonal signal, while an error of the
second kind is an error to the antipodal signal. The probability of error of the first kind is the
probability that H j is chosen given that s0 is transmitted ( j M 2) and is given by
rM
2 1
rj
0 H0
rj
r1
""
r0 r j
P rj
Pe 1
VI-121
fn r j d r j
M 2 2
rj
Fn
Fn r j
rj
Fs
Fs r j
VI-122
It should be obvious that this is also the error probability to H j for j M 2. The probability
of error of the second kind is the probability that HM 2 is chosen given that s0 is transmitted
and is given by
r0
rM
2 1
r0 H0
r0 r2
""
0 r1
P r0
Pe 2
2 z
1M
z2
dz
2
1
exp
2
2 Pe 1
Notice that the symbol error probability is Pe s
2 2
2E
N0
f n r0 d r 0
M 2 2
2E
N0
r0
0
Fn
r0 Fn r0
Fs
dr0
M 2 1
r0
Fn
fs r0 Fn r0
Pe 2 .
The bit error probability is determined by realizing that of the M 2 possible errors (all
equally likely) of the first kind, M 2 2 of them result in a particular bit in error while an
error of the second kind causes all the bits to be in error. Thus
M 2
Pe 1 Pe 2
Pe b
2
M 2
Fs u Fs u Fn u Fn u M 2 2 fn u d u
2
0
VI-123
VI-124
P e,b
1 0- 1
Pe , s
1 0- 1
1 0- 2
1 0- 2
1 0- 3
1 0- 3
1 0- 4
1 0- 4
M=4
1 0- 5
1 0- 5
1 0- 6
1 0- 6
M=2,4
1 0- 7
1 0- 7
8
1 0- 8
1 0- 8
32
M=128
1 0- 9
M=128
1 0- 9
32
1 0- 1 0
-4
-2
10
12
14
1 0- 1 0
16
Eb/N0(dB)
-4
-2
10
12
14
16
Eb/N0(dB)
Figure 84: Symbol Error Probability for Coherent Demodulation of Biorthogonal Signals
Figure 85: Bit Error Probability for Coherent Demodulation of Biorthogonal
Signals
VI-125
VI-126
For example
1
Simplex Signal Set
01
""
i
S8
si t
si t
When the orthogonal set is constructed via a Hadamard matrix this amounts to deleting the
first component in the matrix since the other components sum to zero.
1 M 1
si t
M i0
Same as orthogonal except subtract from each of the signals the average signal of the set, i.e.
These are slightly more efficient than orthogonal signals.
VI-127
VI-128
Pe , s
Pe,b
1 0- 1
1 0- 1
1 0- 2
1 0- 2
1 0- 3
1 0- 3
1 0- 4
1 0- 4
1 0- 5
1 0- 5
1 0- 6
1 0- 6
1 0- 7
1 0- 7
1 0- 8
1 0- 8
M=2
16
1 0- 9
32
16
1 0- 1 0
10
12
14
Simplex.data
16
M=2
32
1 0- 9
1 0- 1 0
10
12
14
Eb/ N0 (dB)
16
Eb/ N0 (dB)
Figure 86: Symbol Error Probability for Simplex Signalling
Figure 87: Bit Error Probability for Simplex Signalling
VI-129
VI-130
Multiphase Shift Keying (MPSK)
For this modulation scheme we should use Gray coding to map bits into signals.
2
i
0
M
Ac i cos 2 f ct As i sin 2 f ct
A cos 2f0t
si t
"
"
2i
M
2i
A sin
M
Ac i
E N0
QPSK
2E
cos
N0
2
4E
cos e cos 1
N0
(QPSK and BPSK are special cases of this modulation).
Pe s
As i
A cos
This type of modulation has the properties that all signals have the same power thus the use of
nonlinear amplifiers (class C amplifiers) affects each signal in the same manner. Furthermore
if we are restricted to two dimensions and every signal must have the same power than this
signal set minimizes the error probability of all such signal sets.
1,
01
where for i
BPSK
VI-131
VI-132
B it Erro r P ro b ab ility
S y m b o l Erro r P ro b ab ility
1 0 -1
1 0 -2
M=2
1 0 -3
10 0
1 0 -1
1 0 -2
M = 32
M = 16
M=8
M=4
B it Error R ate for M P S K
P erform ance of M P S K M odulation
10 0
1 0 -3
M = 2 ,4
1 0 -4
M=8
1 0 -4
1 0 -5
0
12
16
M = 16
M = 32
1 0 -5
20
24
E b /N 0 (d B )
Figure 88: Symbol Error Probability for MPSK Signalling
12
16
20
24
Eb /N 0 (d B )
Figure 89: Bit Error Probability for MPSK Signalling
VI-133
VI-134
M-ary Pulse Amplitude Modulation (PAM)
Pe,s
10-1
10-2
10-3
Ai s t
si t
10-4
where
i
10-5
10-6
""
Ei
1 M 1
Ei
M i0
A2i
A2 M 1
2i
M i0
01
M A
2i
Ai
10-7
10-8
M2 1 2
A
3
2M 1
M
6E
1 N0
10-10
Pe s
10-9
10
15
20
25
30
M2
Eb/N0 (dB)
Figure 90: Symbol Error Probability for MPAM Signalling
VI-135
VI-136
1
10-1
Pe,b
10-2
10-3
Quadrature Amplitude Modulation
10-4
M=8
""
M=4
M=16
Bi sin 2 f ct
Ai cos 2 f ct
si t
10-6
For i
10-5
10-7
2
for PAM with
M signals
Pe 1
Since this is two PAM systems in quadrature. Pe 2
10-8
10-9
10-10
0
10
15
20
25
Eb/N0 (dB)
Figure 91: Bit Error Probability for MPAM Signalling
VI-137
where u t is called the lowpass signal. For general CPM the modulation is nonlinear so that
the below does not apply. Also
Bandwidth of Digital Signals:
VI-138
In practice a set of signals is not used once but in a periodic fashion. If a source produces
symbols every T seconds from the alphabet A 0 1
M 1 with be representing the l th
letter l then the digital data signal has the form
"
"
Note that while u t is a (non stationary) random process u t
0 T is stationary.
nT
sbn t
where is uniform r.v. on
nT
l
In g t
where In is possibly complex and g t is an arbitrary pulse shape.
st
ut
!
Note: 1) si t need not be time limited to 0 T . In fact we may design si t iM 0 1 so that si t
is not time limited to 0 T . If si t is not time limited to 0 T then we may have intersymbol
interference in the demodulaton. The reason for introducing intersymbol interference is to
shape the spectral characteristic of the signal (e.g. if ther are nonlinear amplifiers or other
nonlinearities in the communication system).
!
where
2) The random variables bn need not be a sequence of i.i.d. random variables. In fact if we are
using error-correcting codes there will be some redundancy in b 2 so that it is not a sequence of
i.i.d. r.v.
E In In
j 2 f mT
F gt
gt e
j2 f t
dt
1 (i.i.d).
Example: BPSK In
m
G f
I f
In many of the modulation schemes (the linear ones) considered we can equivalently write the
signal as
st
Re u t e jct
u t
1
I f G f 2
T
F Eu t
u f
VI-139
VI-140
T
2
u f
A2 T
sinc2
4
sinc2
I f
fc
where P is total power and S f c is value of spectrum f
S f df
5. Gabor bandwidth
T
2
fc 2 S f d f
f df
WG
P S fc
WN
m 0
4. Noise bandwidth
E In In
A cos ct pT t
gt
6. Absolute bandwidth
1
2%
1
2
7. half null-to-null
for BPSK).
null-to-null.
bandwith such that
2. 99% containment bandwidth
lies below lower level.
2
T
bandwidth (in Hz) of main lobe (
1. Null-to-Null
min W : S f
WA
Definition of Bandwidth
lies above upper bandlimit 12 %
3. x dB bandwidth Wx bandwidth such that spectrum is x dB below spectrum at center of
band (e.g. 3dB bandwidth).
3 35dB
5 and 6
3 3dB
BPSK
2.0
20.56
35.12
1.00
0.88
QPSK
1.0
10.28
17.56
0.50
0.44
MSK
1.5
VI-141
VI-142
2Eb
N0
BPSK has Pe s
Comparison of Modulation Techniques
R
W
"
10
9 6dB
Eb N0
1
T
1
T
QPSK for same date rate T bits/sec
"
R
W
20
QPSK has same Pe but has
"
2
Ts
1
T
or
VI-143
1
2T
1
Ts
VI-144
E b /N 0 (d B )
M-ary PSK has same bandwidth as BPSK but transmits log2 M bits/channel use (T sec).
M-ary PSK
A chie v ab le
R e g io n
24
18
12
30
log2 M
T
R
log2 M
W
1
T
36
Capacity (Shannon Limits)
0
1 0 -3
1 0 -2
1 0 -1
101
102
R W
Eb N0
U nachie v ab le
R e g io n
-2
2R W
-1
or
log2 1
R W
R Eb
W N0
R /W (b its/se c/H z)
1 there is a
Figure 92: Capacity of Additive White Gaussian Noise Channel.
We can come close to capacity (at fixed R W ) by use of coding (At R W
possible 9 6 dB coding gain)
"
VI-145
VI-146
