EECS 380: Wireless Communications CDMA

1
EECS 380 Wireless CommunicationsCDMA
Michael L. Honig Department of EECS Northwestern
University
May 2011
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The Multiple Access Problem
How can multiple mobiles access (communicate
with) the same base station?

• Use different frequencies (FDMA)
• Use different time slots (TDMA)
• Use different pulse shapes (CDMA)
• Use some combination of frequencies and time
  slots (OFDMA)

3
Two-User Example
1
0
bits
1
1
0
1
User 1
s1(t)
T
T/2
time
T
3T
2T
4T
5T
-1
chips
0
1
1
1
0
s2(t)
User 2
T
T
2T
3T
4T
5T
T/2
time
2
received signal r(t) s1(t)s2(t)
T
2T
4T
5T
3T
-2
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Orthogonality and Asynchronous Users
1
1
0
1
0
1
s1(t)
T
3T
2T
4T
5T
-1
1
0
1
1
0
s2(t)
T
2T
3T
4T
5T
time
Asynchronous users can start transmissions at
different times.

• Orthogonality among users requires
• Synchronous transmissions
• No multipath

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Correlator, or Matched Filter Receiver
delay
user 1's symbol multiple acess interference
(MAI) from user 2
s1(t) s2(t-?)
user 2's symbol multiple acess interference
(MAI) from user 1
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Processing Gain (PG)
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Processing Gain (PG)

• Fundamental tradeoff increasing the PG
• decreases the correlation between random
  signatures.
• decreases interference.
• increases the bandwidth of the signal.

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Correlation and Bandwidth
0
frequency
correlation between s1 and s2 ? multiple access
interference
s2
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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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Near-Far Problem
SO THEN THE THIRD TIME I CALLED CUSTOMER
SERVICE, I SAID
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Near-Far Problem
amplitude A1
User 1
amplitude A2
User 2
A1A2 (correlation of s1 and s2)
User 1s bits
A1 s1(t)A2 s2(t)
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!
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Closed-Loop Power Control
raise power
User 1
lower power
User 2
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Closed-Loop Power Control
raise power
User 1
lower power
User 2
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Closed-Loop Power Control Properties
raise power
User 1
lower power
User 2
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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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Bandwidth and Multipath Resolution
reflection (path 2)
direct path (path 1)
multipath components are resolvable
??(delay spread)
signal pulse
signal pulse
?
T lt ?
T gt ?
T
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CDMA and Path Diversity

• CDMA uses wideband signals (chips are very narrow
  pulses), which makes much of the multipath
  resolvable.
• A RAKE receiver collects (rakes up) the
  energy in the paths

?
power delay profile
received signal
received signal with combined multipath
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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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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

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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

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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

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CDMA Capacity Example

• For IS-95, want Eb/N0 7 dB
• For 3G, want Eb/N0 3 to 5 dB
• Suppose W1.25 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 N3
• K 8 (users/channel) ( of 200 kHz channels)
• 8 1.25 106 / (200 103 3) ? 16

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Increasing CDMA Capacity
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Increasing CDMA Capacity

• Must reduce interference
• Antenna sectorization
• Interference reduced by 1/3
• Trunking efficiency is not a majorissue (no
  channels/time slots).
• Voice inactivity automatically increasesthe
  capacity relative to TDMA with dedicatedtime
  slots.
• CDMA has a soft capacity each additional user
  marginally degrades performance for all users.

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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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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.

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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
  powerK ? 1/(11/2) K ? 108
• Multipath / fading
• Some multipath is combined by the Rake receiver,
  the rest is interference
• Power control inaccuracy

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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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Soft Handoff (CDMA) Make before break
DURING
AFTER
BEFORE
MSC
MSC
MSC
BSC
BSC
BSC
BSC
BSC
BSC
Hard Handoff (TDMA)
MSC
MSC
MSC
BSC
BSC
BSC
BSC
BSC
BSC
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Applications of Spread-Spectrum

• Military (preceded cellular applications)
• Cellular
• Wireless LANs (overlay)

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Military Spread Spectrum

• Can hide a signal by spreading it out in the
  frequency domain.
• Requires a very large PG (several 100 to 1000).
• Enemy must know spreading code (the key
  containing 100s of bits) to demodulate too
  complicated for simple search.
• Spread spectrum signals have the LPI/LPD
  property low probability of intercept / low
  probability of detect.

spread
noise level
frequency
0
0
frequency
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Applications of Spread-Spectrum

• Military (preceded cellular applications)
• Cellular
• Wireless LANs (overlay)

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CDMA vs. TDMA(early 1990s)
TDMA
CDMA
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2G CDMA IS-95 or cdmaOne

• Introduced by Qualcomm (San Diego)
• Direct-Sequence Spread Spectrum signaling
• FDD
• Wideband channels (1.25 MHz)
• Tight, closed-loop power control
• Sophisticated error control coding
• Multipath combining to exploit path diversity
• Noncoherent detection
• Soft handoff
• High capacity
• Air-interface only uses IS-41

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TDMA vs. CDMAPerformance Critera
Capacity Users per Hz per km2
Channel conditions System assumptions Perfect
power control? Modulation and coding?
Complexity
Power control (CDMA) Synchronization
(TDMA) Equalization Frequency assignment
Flexibility
Integrated services (voice/data) Multimedia Va
riable rate/QoS
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3G Air Interfaces

• Also referred to as multicarrier CDMA
• 1X Radio Transmission Technology (RTT) 1.25 MHz
  bandwidth (1 carrier)
• Supports 307 kbps instantaneous data rate in
  packet mode
• Expected throughput up to 144 kbps
• 1xEV (Evolutionary) High Data Rate standard
  introduced by Qualcomm
• 1xEV-DO data only, 1xEV-DV data and voice
• Radio channels assigned to single users (not
  CDMA!)
• 2.4 Mbps possible, expected throughputs are a few
  hundred kbps
• 1xEV-DV has twice as many voice channels as IS-95B

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Service Providers and Technologies
Verizon Cellular PCS (850 1900 MHz) CDMA 20001 x EV-DO LTE 8-128 Kbps up to 2.5 Mbps
ATT/Cingular Cellular (850 1900 MHz) GSM/GPRS/EDGE UMTS/HSPA up to 512 kbps
Sprint Clearwire PCS (1900 MHz) CDMA2000 1 x EV-DO WiMax 8-128 Kbps up to 2.5 Mbps
T-Mobile PCS (1900 MHz) GSM/GPRS/EDGE UMTS/HSPA 8-350 Kbps
NexTel Public service band (800 MHz) iDEN (TDMA) WiDEN4 25-64 kbps near 100 kpbs
U. S. Cellular Cellular PCS (850 1900 MHz) 1 x EV-DO up to 2.5 Mbps
4Wideband version of iDEN.
1Merged with Sprint. 2Limited LTE coverage.
3Limited WiMax coverage.
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Applications of Spread-Spectrum

• Military (preceded cellular applications)
• Cellular
• Wireless LANs (underlay)

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Spread Spectrum Underlay

• FCC requirements on spectrum sharing in the
  unlicensed (Industrial, Scientific, Medical
  (ISM)) bands
• Listen before talk
• Transmit power is proportional to the square root
  of the bandwidth.

telemetry
hospital monitor
spread spectrum signal
frequency
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Spread Spectrum Underlay

• FCC requirements on spectrum sharing in the
  unlicensed (Industrial, Scientific, Medical
  (ISM)) bands
• Listen before talk
• Transmit power is proportional to the square root
  of the bandwidth.
• Spread spectrum signaling is robust with respect
  to a narrowband interferer.
• To a narrowband signal, a spread spectrum signal
  appears as low-level background noise.

telemetry
hospital monitor
spread spectrum signal
frequency
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Variable-Rate CDMA
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Variable-Rate CDMA

• To increase the data rate we can
• Increase the number of signatures per user
• More signatures ? more power, more interference
• Reduce the number of chips per bit
• Decreases immunity to interference (must increase
  power)
• Increase the number of bits per symbol
• QPSK ? 8-PSK ? 16 QAM requires more power
• How is voice capacity affected by the presence of
  high-rate data users?

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Frequency-Hopped CDMA
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Hop Rate

• Can make synchronous users orthogonal by
  assigning hopping patterns that avoid collisions.
• Fast hopping generally means that the hopping
  period is less than a single symbol period.
• Slow hopping means the hopping period spans a
  few symbols.
• The hopping rate should be faster than the fade
  rate (why?).

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Hop Rate

• Can make synchronous users orthogonal by
  assigning hopping patterns that avoid collisions.
• Fast hopping generally means that the hopping
  period is less than a single symbol period.
• Slow hopping means the hopping period spans a
  few symbols.
• The hopping rate should be faster than the fade
  rate so that the channel is stationary within
  each hop.

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Properties of FH-CDMA
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Properties of FH-CDMA

• Exploits frequency diversity (can hop in/out of
  fades)
• Can avoid narrowband interference (hop around)
• No near-far problem (Can operate without power
  control)
• Low Probability of Detect/Intercept
• Spread spectrum technique can overlay
• Cost of frequency synthesizer increases with hop
  rate
• Must use error correction to compensate for
  erasures due to fading and collisions.
• Applications
• Military (army)
• Part of original 802.11 standard
• Enhancement to GSM
• Bluetooth

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1.
49
1. 2.
50

• 3.

1. 2.
51

• 3.
• 4.

1. 2.
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Inventor of Frequency-Hopping
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Bluetooth A Global Specification for Wireless
Connectivity

• Wireless Personal Area Network (WPAN).
• Provides wireless voice and data over short-range
  radio links via low-cost, low-power radios
  (wireless cable).
• Initiated by a consortium of companies (IBM,
  Ericsson, Nokia, Intel)
• Standard has been developed (IEEE 802.15.1 ).

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Bluetooth Specifications

• Allows small portable devices to communicate
  together in an ad-hoc piconet (up to eight
  connected devices).
• Frequency-hopped spread-spectrum in the 2.4 GHz
  UNII band.
• 1600 hops/sec over 79 channels (1 MHz channels)
• Range set at 10m.
• Gross data rate of 1 Mbps (TDD).
• 64 kbps voice channels
• Interferes with 802.11b/g
• Second generation (Bluetooth 2.0) supports rates
  up to 3 Mbps.Competes with Wireless USB.

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The Multiple Access Problem
How can multiple mobiles access (communicate
with) the same base station?

• Frequency-Division (AMPS)
• Time-Division (IS-136, GSM)
• Code-Division (IS-95, 3G)Direct
  Sequence/Frequency-Hopped
• Orthogonal Frequency Division Multiple Access
  (OFDMA) (WiMax, LTE)
• Random Access (Wireless Data)

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Orthogonal Frequency Division Multiplexing (OFDM)
substream 1
substream 2
source bits
substream M
OFDM Signal

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OFDM Spectrum
Total available bandwidth
Data spectrum for a single carrier
Power


f4
f1
f5
f6
f3
f2
frequency
? 0
subchannels
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OFDM vs OFDMA

• OFDM is a modulation technique for a particular
  user.
• OFDMA is a multiple access scheme (allows many
  users to access a single receiver).
• Can OFDM be combined other multiple access
  techniques?

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OFDM vs OFDMA

• OFDM is a modulation technique for a particular
  user.
• OFDMA is a multiple access scheme (allows many
  users to access a single receiver).
• Can OFDM be combined other multiple access
  techniques?
• Yes, e.g., FDMA and TDMA.
• OFDMA is different

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OFDM vs OFDMA
61
OFDM vs OFDMA
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Each color represents a different user, which is
assigned particular time slots.
OFDM/TDMA
subchannels
Different sub-carriers can be assigned to
different users.
time slot

• Each user can be assigned a time/frequency slice.
• Requires a time/frequency scheduler.

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WiMax OFDMA Frame Structure (TDD example)
(downlink)
(uplink)
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Adaptive Rate Control
channel gain
large channel gain ? higher data rate
small channel gain ? lower data rate
f1
f2
frequency
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Adaptive Rate Control
channel gain
large channel gain ? higher data rate
small channel gain ? lower data rate
f1
f2
frequency