9/3/13 Short-term and Long-term Fading
q
Power of received signal varies over time if transmitter
and/or receiver are mobile
Ø
Ø
Short-term fading – quick changes
Long-term fading – caused by varying distance
power
long term
fading
t
short term fading
19 MULTIPLEXING
Space, Frequency, Time, Code
20 1 9/3/13 Multiplexing
q
Multiplexing:
Ø
q
Multiplexing in 4 dimensions:
Ø
q
Goal: multiple use of a shared medium
Space (s), Frequency (f), Time (t), Code (c)
Task:
Ø
Ø
Assign space, time, frequency, code to each
communication channel
Minimize interference
§  Using “guard spaces”
Ø
Maximize medium utilization
21 Space Division Multiplex
q
A separate sender for each
communication channel
Ø
channels ki
k1
With wide enough distance
between senders
Used by FM radio stations
Ø
q
k3
k5
t
k6
c
t
s1
f
Many stations around the
world use same frequency
without interference
What if several radio
stations want to broadcast
in same city?
k4
c
§  Guard spaces
q
k2
s2
f
c
t
s3
f
22 2 9/3/13 Frequency Division Multiplex
q
Subdivide the frequency dimension into several nonoverlapping frequency bands
Ø
Senders can use their assigned band continuously
q
Used for radio stations within same region
q
Pros:
Ø
q
No dynamic coordination
Cons:
Ø
Ø
Ø
k1
k2
k3
k4
k5
k6
c
Waste of bandwidth if
usage is non-uniform
Limits # of senders
Inflexible
assignment
f
t
23 Time Division Multiplex
q
q
A sender gets the whole bandwidth for a certain
amount of time
Advantages
Ø
Ø
q
Only one carrier in the medium at any time
Throughput high even for many users
Disadvantages
Ø
Precise synchronization
necessary
k1
k2
k3
k4
k5
k6
c
f
t
24 3 9/3/13 Frequency and Time Multiplex
q
Combination:
Ø
Ø
q
Pros:
Ø
Ø
q
A sender can use a frequency band for a certain amount
of time
Used by GSM
(weak) protection against tapping
protection against frequencyk1
selective interference
k3
k4
k5
k6
c
Cons:
Ø
k2
f
Precise coordination
required
t
25 Code Division Multiplex
q
Each channel has a unique code
Ø
Ø
q
k2
k3
All channels use same spectrum at same time
“guard spaces” realized by using
orthogonal code
k4
k5
k6
c
Pros:
Ø
Ø
q
k1
Good protection against interference
and tapping
Bandwidth efficient
f
Cons:
Ø
Ø
Ø
Relatively complex receiver
Precise synchronization required
Precise power control required
t
§  All signals should reach receiver with nearly equal strength
26 4 9/3/13 DIGITAL MODULATION
Amplitude, Frequency, Phase
27 Modulation
digital
data
101101001
digital
modulation
analog
baseband
signal
analog
modulation
Radio Transmitter
radio
carrier
Radio Receiver
analog
demodulation
analog
baseband
signal
synchronization
decision
digital
data
101101001
radio
carrier
q
Digital modulation:
Ø
q
Digital data is translated into an analog (baseband) signal
Analog modulation:
Ø
Ø
Shifts center frequency up to that of radio carrier
Small antennas, FDM, frequency-dependent medium
characteristics
28 5 9/3/13 Digital Modulation
q
Modulation of digital signals known as Shift Keying
q
Issues:
Ø
Spectral efficiency:
§  How efficiently is the frequency spectrum utilized
Ø
Power efficiency:
§  How much power is needed to transfer bits
§  Very important for portable devices
Ø
Robustness to multi-path propagation, noise, interference
29 Digital Modulation Methods
q
Amplitude Shift Keying
Ø
Ø
Ø
Binary values are represented by
different amplitudes
Low bandwidth requirements
Very susceptible to interference
§  Used only in directed infra-red beams
q
0
1
t
1
0
1
Frequency Shift Keying
Ø
Ø
q
1
Needs larger bandwidth
Less susceptible to errors
Phase Shift Keying
Ø
Uses shifts in the signal phase
t
1
0
1
§  180o, if change in data
Ø
Ø
Ø
Synchronization required (freq, phase)
Most resistant to interference
Complex receiver/transmitter
t
30 6 9/3/13 Advanced Frequency Shift Keying
Minimum Shift Keying (MSK): avoids abrupt phase changes
Ø  Bits durations doubled and divided into even, odd bits
q
§  Depending on the bit values (even, odd) the higher or lower
frequency, original or inverted is chosen
Ø
Higher frequency is twice the lower frequency
1
0
1
1
0
1
0
bit
even
odd
0101
0011
odd bits
signal
value
hnnh
- - ++
low
frequency
h: high frequency
n: low frequency
+: original signal
-: inverted signal
data
even bits
high
frequency
MSK
signal
t
31 Advanced Phase Shift Keying
q
Ø
q
Q
Binary PSK: phase shift of 180o
Very simple, but low efficiency
1
Quadrature PSK: phase shift of 45o
Ø
0
I
Q
10
11
Codes two bits into one phase shift
§  Relative to phase of a reference signal
I
–  Requires frequent synchronization
§  Relative to phase of previous two bits
00
01
–  More efficient and less complex
Ø
q
Achieves high bit-rates for same bandwidth
Can be extended to:
Ø
Ø
Ø
More angles
Combine with ASK
Leads to more receiver complexity
A
t
11
10
00
01
32 7 9/3/13 Multi-carrier modulation
q
Splits high bit-rate stream into many with lower bit-rates
Ø
q
Each is sent using an independent carrier frequency
Pros:
Ø
Ø
Ø
Independent frequencies do not interfere with each other
Frequency-selective fading doesn’t influence whole signal
Guard spaces used between symbol (groups)
§  Helps handle multi-path propagation, ISI mitigation
Ø
q
Computationally efficient based on FFT
DAB uses 192-1536 sub-carriers
33 8