cis 632 / eec 687 mobile computing
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CIS 632 / EEC 687
Mobile Computing
Mobile Communications (for Dummies)
Chansu Yu
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Contents
Modulation
Propagation
Spread spectrum
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3
1
0
t
digital signal
Digital Communication
Want to transform to since waves:
Why?
• Natural phenomena produce sine
waves
• When a microphone picks up an
audible tone, the output is a sine
• Electromagnetic radiation can be
represented as a sine wave
4
)2cos()2sin(2
1)(
11
nftbnftactgn
n
n
n
1
0
t
digital signal
Digital Communication - Fourier Transform
Want to send a variety of signals (FM, Wifi, Satellite,
Bluetooth, Remote control, etc.) concurrently.
This is why we need modulation!
1
0
t
decomposition
f
Frequency
domain100KHz
25KHz
50KHz 25k 100k
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Digital Communication - Modulation
f
Frequency
domain
25k 100k f2.4G 2.4G
+25k +100k
90M 90M
+5k +40k
FM WiFi
Modulation allows us to send a signal over a bandpass
frequency range. If every signal gets its own frequency range,
then we can transmit multiple signals simultaneously over a
single channel, all using different frequency ranges. Another
reason to modulate a signal is to allow the use of a smaller
antenna.* What 90M or 2.4G is called?
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Amplitude Shift Keying (ASK):
Frequency Shift Keying (FSK):
Phase Shift Keying (PSK):
1 0 1
t
1 0 1
t
1 0 1
t
Digital Communication – “Shift Keying”
: Digital 0/1Sine Waves
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4-ASK?
Binary Amplitude Shift Keying (ASK)
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4-FSK?
Frequency Shift Keying (FSK)
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9
0 180 0 0 180
(Phase changes are overlaid over the carrier signal)
Phase Shift Keying (PSK)
Constellation
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Quadrature PSK (QPSK)
Higher throughput: Encode 2 or more bits onto one signal element.
More fragile: Noise makes it more difficult to distinguish between, for example, 11 and 10.
11 10 00 01
A
t
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11
Two carriers
are 90 degree
shifted
Quadrature PSK (QPSK)
Constellation
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Quadrature PSK (QPSK)
QPSK
OffsetQPSK
Which is more robust to channel error?
8QPSK
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QAM (Quadrature Amplitude
Modulation)
In QAM, a finite number of at least two phases,
and at least two amplitudes are used.
In-phase signal (for example a cosine waveform)
Quadrature phase signal (for example a sine wave)
They are amplitude modulated with a finite
number of amplitudes, and summed.
The resulting signal is equivalent to a
combination of PSK and ASK.
Constellationfor 16 QAM
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802.11/a/b
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Bluetooth and Zigbee
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Contents
Modulation
Propagation
Spread spectrum
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Free-Space Propagation
In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)
Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²
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4
dGG
P
Ptr
t
r
Pr: received power
Pt: transmitted power
Gr, Gt: receiver and transmitter antenna gain
(=c/f): wave length
Sometime we write path loss in log scale:
Lp = 10 log(Pt) – 10log(Pr)
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Let’s Do the Math
Wifi radio
Transmit power (Pt) = 28 dBm = ??? Watt
Distance (d) = 1km (0.6 mile)
Pr = Pt*GtGrht2hr2/Ld4
Gt=Gr=1, ht=hr=1.5, L=1
Pr = 3.2 x 10-12 Watt
dB = 10 log (-------)P1
P2
dBm = 10 log (-------)P1
1mW
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1x10-12 Watt
2.5x10-12 Watt
2.5x10-12 Watt
4.0x10-12 Watt
What does
Pr = 3.2 x 10-12 Watt
mean?
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Real Antennas
Real antennas are not isotropic radiators
Some simple antennas: quarter wave /4 on car roofs or half wave dipole /2 size of antenna proportional to wavelength for better transmission/receiving
/4/2
Q: Assume frequency 1 Ghz, = ?
Real Measurements
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Receiving power additionally influenced by shadowing (e.g. through a wall or a door)
refraction depending on the density of a medium
reflection at large obstacles
scattering at small obstacles
diffraction at edges
reflection
scattering
diffraction
shadow fadingrefraction
Signal Propagation
Shadowing
Signal strength loss after passing through obstacles Some sample numbers
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Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction
Multipath
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Multiple Rates in
IEEE 802.11/a/b/g
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Trace from 2004 SIGCOMM
(CRAWDAD)
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802.11 PHY Packet Formats
synchronization SFD PLW PSF HEC Payload (MPDU)
PLCP preamble PLCP header
80 16 12 4 16 Variable (<4KB)
synchronization SFD signal service HEC Payload (MPDU)
PLCP preamble PLCP header
128 16 8 8 16
length
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DSSS
FHSS
* MPDU: MAC Protocol Data Units
Always 1Mbps with BPSK modulation
Always 1Mbps with GFSK modulation
Data rate for MPDU:
Up to 11Mbps (802.11b, DSSS) in steps of 100kbps
Up to 2Mbps (802.11, DSSS) in steps of 100kbps
Up to 2Mbps (802.11, FHSS) in steps of 0.5Mbps
Variable (<4KB)
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Multi-rate Control in WLANs
Access point
E.g., increase the rate upon consecutive successes; decrease the rate upon consecutive failures
ARF (Autorate Fallback)
• Relatively simple
• A client node has only
one communication
partner (access point)
• Each node can
individually optimize its
data rate for the link to
AP
• Already being used in
commercial products
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Auto-Rate Fallback (ARF)
The first multi-rate algorithm for 1&2Mbps WaveLAN
Many commercially available 802.11 cards support some sort of ARF for automatic rate selection
Use a higher rate upon 10 consecutive successful transmissionsWhen the rate is increased, the first transmission
must succeed or the rate is immediately decreased
Fall back to a lower rate after 2 consecutive transmission failures
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Receiver-Based Auto Rate (RBAR)
Upon receiving an RTS frame,
The receiver estimates the channel quality based on SINR of
the received RTS frame and then
Determines the best data rate that the transmitter must use
The estimated optimal rate is then sent back to the sender
piggybacking in the CTS packet
A major drawback: Requires
incompatible changes to the
IEEE 802.11 standard
RTS@1
CTS+Opt.rate
Data@5.5
ACK
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OAR & MAD
Opportunistic Auto Rate (OAR) protocol Built on a multi-rate algorithm such as ARF or RBAR
Although the channel condition fluctuates, it is consistent for the time duration, which is enough to transmit more than one packet
When a high-quality channel condition is observed, send multiple back-to-back data packets
Medium Access Diversity (MAD) Looks for the receiver whose channel condition is near its
peak based on channel probing
Group RTS (GRTS) for query (with list of receivers) and CTS’s for replies (with channel condition information)
The sender then chooses the receiver that can maximally utilize the channel
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Other Multi-rate Adaptation Methods
Sender-based Auto-Rate Fallback (ARF, 97)
Adaptive ARF (AARF, 04)
Estimated Rate Fallback (ERF, 05)
MADWIFI (04)
Adaptive Multi Rate Retry (AMRR, 05)
Collision-Aware Rate Adaptation (CARA, 06)
Receiver-based Receiver-Based Auto Rate (RBAR, 01)
Opportunistic Auto Rate (OAR, 02)
Medium Access Diversity (MAD, 05)
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Contents
Modulation
Propagation
Spread spectrum
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Why Spread Spectrum?
Initially adopted in military applications, for its resistance to jamming and difficulty of interception (seemingly a noise)
Adopted recently in CDMA, WiFi & Bluetooth & Zigbee
Power
Interference
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FHSS (Frequency Hopping SS)
User data
3 hops/bit,
in general, a very
short packet/hop or
even a bit/multiple hops
as in this example
0 1
tb
0 1 1 tf
t
f
f1
f2
f3
t
td
With a binary frequency-modulation scheme in which the carrier simply shifts
up or down in frequency by about 150 kHz to represent, respectively, a 1 or a 0
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OFDM (Orthogonal frequency
division multiplexing )
In OFDM technology, the bit string to be transmitted
is broken down into N (N>1) bit strings. The N bit
strings are then transmitted in parallel through N
orthogonal sub-channels.
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IEEE 802.11/a/b
Physical layer
802.11a/g
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Advanced Topics
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*
Physical Layer Watermarking
Adjusting phase difference in M-PSK
modulation
BPSK
modulation
8PSK
modulation: add
π/4 to embed
watermark bit 1
0 (0) π (1)
π/4 (?)
0 (0) π (1)
0 (0)
Watermarked DSSS (WDSSS)
Spreading
Correlation
Original PN sequence:
00010
Two watermark values
are embedded
Aware receiver
records the flipped
positions, interprets
them into watermark
values
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802.11 MAC and Collision
Alice Bob
AP
CSMA is not perfect…
Collision
Repeatedly collide… with some random jitter
“ZigZag Decoding: Combating Hidden Terminals in Wireless Networks”, S. Gollakota and D. Katabi (MIT), SIGCOMM 2008.
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ZigZag Decoding
AP
Alice Bob
Pa1 3 Pa1 3
Pb2 4Pb42∆1 ∆2
∆1- ∆2
1st collision 2nd collision
0
Can reconstruct both packets Pa and Pb!!
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