msit 413: wireless technologies
TRANSCRIPT
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
MSIT 413: Wireless Technologies Week 6
Michael L. Honig Department of EECS
Northwestern University
November 2014
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Outline
• Finish diversity
• Multiple Access techniques – FDMA, TDMA – CDMA (3G, 802.11b) – OFDMA (4G, WiMax)
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Diversity • Idea: Obtain multiple independent copies of the received signal.
– Improves the chances that at least one is not faded.
• Macroscopic (space): copies of signal are received over distances spanning many wavelengths.
• Microscopic (space): copies of signal are received over distances spanning a fraction of a wavelength
• Different types…
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Macroscopic Diversity
Copies of signal are separated by many wavelengths.
MSO
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Microscopic Space Diversity
• Want signals s1 and s2 to experience independent fading (why?). – distance between antennas should be ≥ ½ wavelength. – Ex: 900 MHz, λ = c/f ≈ 1/3 meter
2 GHz, λ ≈ 0.15 meter
Antenna 1 Antenna 2
s1 s2
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Frequency Diversity
frequency
channel gain
Wideband signals exploit frequency diversity. Spreading power across many coherence bands reduces the chances of severe fading. Wideband signals are distorted by the channel fading (distortion causes intersymbol interference).
coherence bandwidth Bc
f1 f2
Frequencies far outside the coherence bandwidth are affected differently by multipath.
signal power (wideband)
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Time Diversity
• Transmit multiple copies of the signal in time. – Error control coding: add redundant bits
• Problem: slow fading – Combine with power control
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Path Diversity
• Called a “RAKE” receiver, since it “rakes” up (combines) the energy in the different paths. – Can substantially increase the S/I.
• An important component of CDMA receivers. – Each branch in the Rake is typically referred to as a “finger”.
Delay τ2 - τ1
adjust phase
adjust phase
+ τ1 τ2
received signal
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiuser Diversity
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiuser Diversity
d1
d2 > d1
Received power user 1
user 2
time transmit to user 1
transmit to user 2
transmit to user 2
transmit to user 1
The BST can choose to transmit to the user with the best channel. Exploits variations in signal strength across users.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Selection Diversity
Received power
antenna 2
time select ant. 1
select ant. 2
Choose the best signal (highest instantaneous SNR). Easy to implement (antenna switch).
Antenna 1 Antenna 2
s1 s2
antenna 1
select ant. 2
select ant. 1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Benefit of Selection Diversity (Example) • Suppose that the signal on each antenna experiences
independent Rayleigh fading.
• Determine the probability that the received signal is faded: Recall Rayleigh fading formula: Probability that the signal power is less than a x P0 (average received power) = 1 – e-a Hence the probability that the signals on both antennas are less than a x P0 = (1 – e-a)2 Without diversity, probability of a signal fade = 1 – e-1 = 0.63 With 2-branch diversity, probability of a signal fade = 0.632 = 0.39
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Benefit of Selection Diversity (cont.) • Suppose that there are N copies of the signal
(e.g., N antennas, paths, coherence bands, etc.)
Probability that the signal power is less than a x P0 (average received power) = 1 – e-a Hence the probability that all N signals are less than a x P0 = (1 – e-a)N Without diversity, probability of a signal fade = 1 – e-1 = 0.63 With 4-branch diversity, probability of a signal fade = 0.634 = 0.16 Without diversity, Prob(signal is faded by more than 10 dB) = 1 – e-0.1 ≈ 0.1 With diversity this probability is (1 – e-0.1)4 ≈ 0.0001 !
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Coherent Combining
• “Coherent” means that the phases of the two signals are estimated at the receiver and aligned.
• Performs better than selection combining (why?).
• Example: RAKE receiver
• Can weight the combined signals to maximize the received SNR. (How should the weights depend on the signal levels?)
adjust phase
adjust phase
+
S1 (ant. 1)
S2 (ant. 2)
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Probability of Error with Fading
add diversity
• Diversity can transform a fading channel back to a non-fading (additive noise) channel. • Essential for mobile wireless communications.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiple Antennas: Multi-Input/Multi-Output (MIMO) Channel
Multi-Channel Detector
Transmitted
Data (multiple data streams)
Estimated Data
• Multiple (M) antennas at receiver and transmitter • Channel has multiple inputs and multiple outputs.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Single Transmit Antenna
Multi-Channel Detector
Transmitted
Data (single stream)
Estimated Data
• Multiple receiver antennas provides spatial diversity • Lowers error rate
• “Single-Input/Multiple-Output (SIMO)” channel
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multi-Input/Single Output (MISO) Channel
Single-Channel Detector
Transmitted
Data (single or multiple streams)
Estimated Data
• Transmitting the same symbol from all transmitters provides transmit spatial diversity (e.g., select the best antenna, turn the others off).
• Practical for cellular downlink. 18
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Downlink Beamforming
Different beams can use the same frequency!
Narrow “beam” focused on one user
19
• M antennas at the base station (single or multiple antennas at mobiles)
• Can support up to M data streams. • Multi-user MIMO: multiple users on the same channel
– Introduced in LTE, 802.11ac
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
OFDM Signal
Orthogonal Frequency Division Multiplexing (OFDM)
Split into M substreams source
bits
Modulate Carrier f1
+
Modulate Carrier f2
Modulate Carrier fM
substream 1
substream 2
substream M …
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel
• Multiple (M) antennas at receiver and transmitter.
Multi-Channel Detector
Transmitted Data Estimated
Data
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel
• Multiple (M) antennas at receiver and transmitter. • Transmitted data is divided into M substreams, one for each antenna.
• Transmit antennas are used to multiplex multiple data streams.
Multi-Channel Detector
Substream 1
Estimated Data Substream M
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel
• Multiple (M) antennas at receiver and transmitter. • Transmitted data is divided into M substreams, one for each antenna.
• Transmit antennas are used to multiplex multiple data streams. • Multiple receiver antennas (plus signal processing) are used to
remove interference from the different antennas.
Multi-Channel Detector
Substream 1
Estimated Data Substream M
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Multiple Antennas: Multi-Input/Multi-Output (MIMO)Channel
• Multiple (M) antennas at receiver and transmitter. • Transmitted data is divided into M substreams, one for each antenna.
• Transmit antennas are used to multiplex multiple data streams. • Multiple receiver antennas (plus signal processing) are used to
remove interference from the different antennas. • Data rate (Shannon capacity) is proportional to M!
Multi-Channel Detector
Estimated Data
Substream 1
Substream M
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
WiFi Evolution: 802.11n
• Technology based on OFDM with multiple antennas at the transmitter and receivers
• Supports data rates up to 540 Mbps – 4 spatial streams, 40 MHz bandwidth – Can replace USB 2.0 connections.
• Also important part of 802.11ac (multi-user MIMO)
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
The Multiple Access Problem
• Frequency-Division (AMPS)
• Time-Division (GSM)
• Code-Division (3G, Bluetooth) Direct Sequence/Frequency-Hopped
• Orthogonal Frequency Division Multiple Access (OFDMA)
• Random Access (Wireless Data)
How can multiple mobiles “access” (communicate with) the same base station?
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Duplexing (Two-way calls)
Frequency-Division Duplex (FDD)
Time-Division Duplex (TDD)
Channel 1
Channel 2
Time slot (frame) 1
Time slot (frame) 2
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Combinations
• FDMA/FDD (AMPS) • TDMA/FDD (GSM) • TDMA/TDD (IS-136 or 2G in the U.S.) • CDMA/FDD (IS-95, CDMA2000) • CDMA/TDD (3G/UMTS) • Frequency-Hopped CDMA/TDD (Bluetooth) • OFDMA/TDD and FDD (WiMax, 4G)
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
The Multiple Access Problem
• Frequency-Division (AMPS)
• Time-Division (IS-136, GSM)
• Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped
• Orthogonal Frequency Division Multiple Access (OFDMA)
• Random Access (Wireless Data)
How can multiple mobiles access (communicate with) the same base station?
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Cellular Spectrum (50 MHz)
A* A B A* B*
uplink
downlink
824
869 870 880 890 891.5 894
825 835 845 846.5 849
AMPS (1G): 30 kHz Channels
416 FDD Channels (requires 12.5 MHz): • 395 FDD voice channels • 21 FDD control channels
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of FDMA • Can be analog or digital (AMPS is analog).
• Narrowband: channel contained within coherence
bandwidth – undergoes flat fading.
• Low capacity
• Best for circuit-switched (dedicated) connections.
• Requires guard channels for adjacent channel interference.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
The Multiple Access Problem
• Frequency-Division (AMPS)
• Time-Division (GSM)
• Code-Division (IS-95, 3G) Direct Sequence/Frequency-Hopped
• Orthogonal Frequency Division Multiple Access (OFDMA)
• Random Access (Wireless Data)
How can multiple mobiles access (communicate with) the same base station?
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Frame
H 1 2 N H . . .
N time slots H: Frame Header
Time Division Multiple Access
Time slots
H 1 2 N H . . .
H 1 2 N H . . .
Channel f1
Channel f2
Channel fK
. . .
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Frame
H 1 2 N H . . . N time slots H: Frame Header
Data to or from user K + control information
Time Slot
Preamble and synch
Guard time
Time Slot
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
H 1 2 H
TDMA/Time-Division Duplex
3 4 5 6
Uplink time slots Downlink time slots { {
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of TDMA • Data transmission occurs in bursts.
– Must ensure small delays for speech. – High peak to average power on reverse link.
• Can measure signal strength in idle time slots (e.g., for handoff).
• Can assign multiple time slots for higher data rates.
• Significant overhead/complexity for synchronization. – Guard times needed between time slots for delay spread.
• May require an equalizer to mitigate intersymbol interference.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Global System for Mobile Communications (GSM)
• Originated in Europe – Main objective: allow roaming across countries – Incompatible with 1G systems
• More than an air-interface standard: specifies wireline interfaces/functions
• TDMA/FDMA, FDD • Dynamic frequency assignment • 200 kHz channels • 270.833 kbps
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
GSM Frame Structure
• 200 kHz FDD channels divided into 8 time slots per frame • Total number of available channels = (12.5 MHz – 2 X Guard Band)/200 kHz
100 kHz guard bands è 62 channels • Total number of traffic channels = 8 X 62 = 496 • Channel data rate = 270.839 kbps • Without overhead, data rate/user = 24.7 kbps
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
4.615 ms
156.25 bits 576.92 µs
T0 T1 T2 T22 T23 T24 I/S T10 T11 T12 S T13 T14 T15 ... … 120 ms
Tn: nth TCH frame S: Slow Associated Control Channel frame I: Idle frame
Speech Multiframe = 26 TDMA frames
Frame
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
GSM Frame Structure
• 200 kHz FDD channels divided into 8 time slots per frame • Total number of available channels = (12.5 MHz – 2 X Guard Band)/200 kHz
100 kHz guard bands è 62 channels Total number of traffic channels = 8 X 62 = 496 Channel data rate = 270.839 kbps Without overhead, data rate/user = 24.7 kbps
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
4.615 ms
156.25 bits 576.92 µs
T0 T1 T2 T22 T23 T24 I/S T10 T11 T12 S T13 T14 T15 ... … 120 ms
Tn: nth TCH frame S: Slow Associated Control Channel frame I: Idle frame
Speech Multiframe = 26 TDMA frames
Frame
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
GSM Frame Structure
• 200 kHz FDD channels divided into 8 time slots per frame • Total number of available channels = (12.5 MHz – 2 X Guard Band)/200 kHz
100 kHz guard bands è 62 channels • Total number of traffic channels = 8 X 62 = 496 • Channel data rate = 270.839 kbps • Without overhead, data rate/user = 24.7 kbps
TS0
TS1
TS2
TS3
TS4
TS5
TS6
TS7
4.615 ms
156.25 bits 576.92 µs
T0 T1 T2 T22 T23 T24 I/S T10 T11 T12 S T13 T14 T15 ... … 120 ms
Tn: nth TCH frame S: Slow Associated Control Channel frame I: Idle frame
Speech Multiframe = 26 TDMA frames
Frame
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
GSM Time Slots
0 1 2 3 4 5 6 7
4.615 ms
120 ms
6.12 s
3 57 1 26 1 57 3 8.25
576.92 µs
“Normal Burst” Traffic Channel (TCH)
• 148 bits/time slot • 114 coded information bits • Frame efficiency 74% (total bits – overhead bits)/(total bits)
Superframe
Multiframe
Frame
Time slot
Hyperframe = 2048 superframes lasts ~3 hrs 28 min 54 sec
≈
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
GSM Capacity
• Total bandwidth = 12.5 MHz, 200 kHz channels è 62 channels – With cell cluster size N=3 (typical),
capacity is (62/3) x 8 ~ 165 users/cell
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
The Multiple Access Problem
• Use different frequencies (FDMA)
• Use different time slots (TDMA)
• Use different pulse shapes (CDMA)
How can multiple mobiles access (communicate with) the same base station?
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Code Division Multiple Access • Users transmit simultaneously over
the same frequency band • Performance limited by interference
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Two-User Example
time User 1:
T T/2
time T
T/2
User 2:
T 2T 3T 4T 5T
s2(t) 1 0 1 1 0
T 2T 3T 4T 5T s1(t)
1 1 0 1 0 bits:
T 2T 4T 5T
received signal r(t)= s1(t)+s2(t)
How to recover each users’ bits? 3T
2
-2
1
-1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Chip Sequence
time T
T/2
User 2:
T 2T 3T 4T 5T
s2(t) 1 0 1 1 0
chips
bits:
1 -1 1 1 -1 Transmitted chips: -1 1 -1 -1 1
chip duration Tc
symbol duration T=2Tc
User 2’s chip sequence (1, -1) is called a signature.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Chip Sequence
time T T/2
User 1:
chips
1 1 -1 1 -1 Transmitted chips: 1 1 -1 1 -1
chip duration Tc
symbol duration T=2Tc
T 2T 3T 4T 5T s1(t)
1 1 0 1 0 bits: 1
-1
User 1’s signature is (1, 1).
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Two-User Example
T 2T 3T 4T 5T
s2(t)
T 2T 3T 4T 5T s1(t)
T 2T 4T 5T
r(t)= s1(t)+s2(t)
3T
2
-2
1
-1
1 -1 1 1 -1 Transmitted chips: -1 1 -1 -1 1
2 0 0 2 -2 Received chips: 0 2 -2 0 0
1 1 -1 1 -1 Transmitted chips: 1 1 -1 1 -1
1 0 1 1 0
1 1 0 1 0
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlation
Given two sequences: a1, a2, a3, …, aN
b1, b2, b3, …, bN
The correlation between the sequences is defined as:
(a1 x b1) + (a2 x b2) + (a3 x b3) + … + (aN x bN)
Examples: 1 1 1 1 1 correlated with 1 1 1 1 1 = 5 1 1 1 1 1 correlated with 1 -1 1 -1 1 = 1 2 4 1 3 1 correlated with -2 1 8 2 0 = -4 + 4 + 8 +6 + 0 = 14
If the correlation between two sequences is zero, they are said to be orthogonal.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlator Receiver
r(t) Sample Chips
Correlate with desired user’s signature
Bit Decision < 0 à 0 > 0 à 1
estimated bits
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Why Does This Work?
Correlate with User 1’s signature
A1 s1 + A2 s2
signature (1,1) amplitude
Correlate with User 1’s signature
A2 s2 0
The user signatures are orthogonal.
Now observe that:
Correlate with User 1’s signature
A1 s1
2A1
2A1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlator, or “Matched Filter” Receiver
Correlate with User 1’s signature
A1s1+ A2s2 User 1’s bits
Correlate with User 2’s signature
User 2’s bits
The correlator is “matched” to user 1’s signature s1, and rejects s2 (and vice versa).
Bit Decision < 0 à 0 > 0 à 1
Bit Decision < 0 à 0 > 0 à 1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Observations • Users transmit simultaneously (not TDMA). • Users overlap in frequency (not FDMA).
frequency
Spectrum: User 1
0
signal bandwidth is roughly 1/T
frequency 0
signal bandwidth is roughly 1/Tc = 2/T Spectrum: User 2
Bandwidth expansion (factor of 2) è “spread spectrum” signaling.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Users and Bandwidth Expansion To guarantee orthogonal signatures (no interference), the length of the signatures must be ≥ the number of users.
Example (4 users):
time T/2 T/4 User 3: 3T/4
T time T/2 T/4 User 4: 3T/4
T
time T/2 T/4 User 2: 3T/4
T time T/2 T/4 User 1: 3T/4
T
The chip rate is 4 times the symbol rate, hence the bandwidth expansion is a factor of 4.
signature: 1 1 1 1 signature: 1 1 -1 -1
signature: 1 -1 -1 1 signature: 1 -1 1 -1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlator Receiver (4 users)
Correlate with User 1’s signature
s1+ s2 + s3 + s4 User 1’s bits
Correlate with User 2’s signature
User 2’s bits
Bit Decision < 0 à 0 > 0 à 1
Bit Decision < 0 à 0 > 0 à 1
Correlate with User 3’s signature
User 3’s bits Bit Decision < 0 à 0 > 0 à 1
Correlate with User 4’s signature
User 4’s bits Bit Decision < 0 à 0 > 0 à 1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Processing Gain
• The PG is essentially the bandwidth expansion factor, given by (1/Tc)/(1/T) = T/Tc (chips per symbol), which is the length of the signature.
• The signature (sequence of chip values) is also called a spreading code. The signature may be randomly generated, in which case it is called a pseudo-noise (PN) sequence.
• “Direct-Sequence” CDMA uses a spread spectrum signalling scheme in which the signal is spread by transmitting a sequence of chips at a rate faster than the symbol rate.
RateSymbolSignal Spectrum Spread of Bandwidth)PG( Gain Processing ≡
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
DS-CDMA Transmitter
Spreader (generate chips)
Modulator (e.g., QPSK)
Source bits chips RF signal
Ex: 100 source symbols, processing gain = 10 è 1000 chips
X
sin 2πfct
Baseband signal Passband (RF) signal
time
Nyquist chip shape
Tc
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Orthogonality and Asynchronous Users
• Orthogonality among users requires: – Synchronous transmissions – No multipath
T 2T 3T 4T 5T s1(t)
1
-1
1 1 0 1 0
T 2T 3T 4T 5T
s2(t) 1 0 1 1 0
time
Asynchronous users can start transmissions at different times.
Chips are misaligned è signatures are no longer orthogonal!
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlator, or “Matched Filter” Receiver
Correlate with User 1’s signature
s1(t) + s2(t-τ)
Correlate with User 2’s signature
The multiple access interference adds to the background noise and can cause errors. For this reason, CDMA is said to be interference-limited. Because CDMA users are typically asynchronous, and because of multipath, it is difficult to maintain orthogonal signatures at the receiver. Consequently, in practice, the signatures at the transmitter are randomly generated.
Bit Decision < 0 à 0 > 0 à 1
Bit Decision < 0 à 0 > 0 à 1
delay Signal 1 + multiple acess interference (MAI) From user 2
Signal 2 + multiple acess interference (MAI) From user 1
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlation and PG
Example: PG=4 s1: 1 -1 -1 1 s2: 1 1 1 -1 Correlation = -2
Energy in s1 (or s2) is 12 + (-1)2 + (-1)2 + 12 = 4 Normalized correlation = correlation/energy = -2/4 = -1/2
Example: PG=10
Conclusion: On average, the correlation between signatures decreases as the signature length (PG) increases.
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Correlation and Bandwidth
T 2T 3T 4T 5T
s2(t) 1 0 1 1 0
0 1
T 2T 3T 4T s2(t)
1 1 0
5T
frequency 0
frequency 0 Increasing the PG increases bandwidth, but decreases the correlation between user signatures.
Correlate with User 1’s signature
A2 s2
correlation between s1 and s2 à multiple access interference
Increasing the PG decreases multiple access interference. Bandwidth expansion therefore provides “immunity” to interference (all kinds: analog, multiple access, multipath, narrowband, etc).
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Example
• IS-95 (2G CDMA) – Total bandwidth = 1.25 MHz – Data rate = 9.6 kbps – PG ≈ 130
• 3G/CDMA2000
– Total bandwidth = 1.25 MHz – Data rate varies between 14.4. kbps (voice)
up to 2 Mbps (1X-DO) – PG varies from 1 to 130
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of CDMA • Robust with respect to interference
• No frequency assignments (eases frequency planning)
• Asynchronous
• High capacity with power control – Power control needed to solve near-far problem.
• Wideband: benefits from frequency/path diversity.
• Benefits from voice inactivity and sectorization. • No loss in trunking efficiency.
• Soft capacity: performance degrades gradually as more users are added.
• Soft handoff
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Near-Far Problem SO… THEN THE THIRD
TIME I CALLED CUSTOMER SERVICE, I SAID &%$#%^…
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Near-Far Problem
Correlate with User 1’s signature
A1 s1(t)+A2 s2(t) User 1’s bits Bit Decision < 0 à 0 > 0 à 1
User 1
User 2
A1+A2 × (correlation of s1 and s2)
amplitude A1
amplitude A2
Output of correlator receiver is signal + interference. As the interferer moves closer to the base station, the interference increases. In practice, power variations can be up to 80 dB!
Conclusion: User 1’s signal is overwhelmed by interference from user 2!
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Closed-Loop Power Control
User 1
User 2
• Base station gives explicit instructions to mobiles to raise/lower power. • Needed to solve near-far problem (equalizes received powers). • Introduced by Qualcomm in the late 80’s. • Requires “closed-loop” feedback.
• BST controls powers through feedback channel. • Why “closed-loop”?
raise power
lower power
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Closed-Loop Power Control
User 1
User 2
• Base station gives explicit instructions to mobiles to raise/lower power. • Needed to solve near-far problem (equalizes received powers). • Introduced by Qualcomm in the late 80’s. • Requires “closed-loop” feedback.
– “Open-loop” power control (no feedback) is inadequate due to frequency-selective fading.
raise power
lower power
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Closed-Loop Power Control: Properties
User 1
User 2
• Crucial part of CDMA cellular systems (IS-95, 3G). • Minimizes battery drain. • Complicated (increases cost) • Requires overhead: control bits in feedback channel to tell transmitter
to lower/raise power. • Cannot compensate for fast fading.
raise power
lower power
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of CDMA • Robust with respect to interference
• No frequency assignments (eases RF planning).
• Asynchronous
• High capacity with power control. – Power control needed to solve near-far problem.
• Wideband: benefits from frequency/path diversity.
• Benefits from voice inactivity and sectorization. – No loss in trunking efficiency.
• Soft capacity: performance degrades gradually as more users are added.
• Soft handoff
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Bandwidth and Multipath Resolution
direct path (path 1)
reflection (path 2)
signal pulse
Narrow bandwidth è low resolution Receiver cannot distinguish the two paths.
signal pulse
Wide bandwidth è high resolution Receiver can clearly distinguish two paths.
multipath components are resolvable
τ (delay spread)
τ
T
T < τ T > τ
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
CDMA and Path Diversity
• CDMA uses wideband signals (chips are very narrow pulses), so that multipath is resolvable.
• A “RAKE” receiver collects (“rakes up”) the energy in the paths:
power delay profile
τ
delay τ + adjust phase
received signal
received signal with combined multipath
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of CDMA • Robust with respect to interference
• No frequency assignments (eases RF planning).
• Asynchronous
• High capacity with power control. – Power control needed to solve near-far problem.
• Wideband: benefits from frequency/path diversity.
• Soft capacity: performance degrades gradually as more users are added.
• Benefits from voice inactivity and sectorization. – No loss in trunking efficiency.
• Soft handoff
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
CDMA Capacity Performance depends on
Let S= Transmitted power (per user), R= information rate (bits/sec), W= Bandwidth, K= Number of users Eb= S/R (energy per second / bits per second) N0= (Number of interferers x S)/W = ((K-1) x S)/W Therefore Eb/N0 = (W/R)/(K-1) = (Processing Gain)/(K-1) For a target Eb/N0, the number of users that can be supported is K = (Processing Gain)/(Eb/N0) + 1
bandwidthunit per power Noise ceInterferenbitper Energy
0 +NEb ≡
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
CDMA Capacity: Example
• For IS-95, want Eb/N0 ≥ 7 dB • For 3G, want Eb/N0 ≥ 3 to 5 dB • Suppose W=1.25 MHz (single-duplex), R= 14.4 kbps,
target Eb/N0 = 7 dB: K= 1 + [(1.25 × 106)/(14.4 × 103)]/5.01 ≈ 18
• Compare with GSM, cluster size N=3: K= 8 (users/channel) × (# of 200 kHz channels) = 8 × 1.25 × 106 / (200 × 103 × 3) ≈ 16
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Increasing CDMA Capacity • Must reduce interference
• Antenna sectorization
– Interference reduced by 1/3 – Trunking efficiency is not a major
issue (no channels/time slots).
• Voice inactivity automatically increases the capacity relative to TDMA with dedicated time slots.
• CDMA has a “soft” capacity: each additional user marginally degrades performance for all users.
other-cell interference
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of CDMA • Robust with respect to interference
• No frequency assignments (eases RF planning).
• Asynchronous
• High capacity with power control. – Power control needed to solve near-far problem.
• Wideband: benefits from frequency/path diversity.
• Soft capacity: performance degrades gradually as more users are added.
• Benefits from voice inactivity and sectorization. – No loss in trunking efficiency.
• Soft handoff
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Interference and CDMA Capacity If interference is reduced by a factor 1/g, then the number of interferers can be increased by g (N0 is replaced by g x N0): If W/R is large, then reducing interference by 1/g (approximately) increases the capacity by a factor of g.
)Ng)(E(RW+=Kb 0//1/1
Previous example: voice activity of 1/3 combined with 120o sectors increases capacity by a factor of 9!
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Refining the Capacity Estimate
• Capacity for previous example is 9 × 18 ≈ 162 • Have not accounted for:
– Other-cell interference • Approximately 1/3 to 1/2 of total interference power
K à 1/(1+1/2) × K ≈ 108
– Multipath / fading • Some multipath is combined by the Rake receiver, the rest is
interference
– Power control inaccuracy
Precise capacity predictions become difficult, best to rely on field trials…
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Properties of CDMA • Robust with respect to interference
• No frequency assignments (eases RF planning).
• Asynchronous
• High capacity with power control. – Power control needed to solve near-far problem.
• Wideband: benefits from frequency/path diversity.
• Benefits from voice inactivity and sectorization. – No loss in trunking efficiency.
• Soft capacity: performance degrades gradually as more users are
added.
• Soft handoff
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N O R T H W E S T E R N MSIT | Master of Science in Information Technology
U N I V E R S I T Y
Soft Handoff (CDMA) ”Make before break”
MSC
BSC BSC
MSC
BSC BSC
MSC
BSC BSC
Hard Handoff (TDMA)
MSC
BSC BSC
MSC
BSC BSC
MSC
BSC BSC
BEFORE DURING AFTER