Transcript Code Division Multiple Access (CDMA) Transmission Technology

Code Division
Multiple Access
(CDMA)
Transmission
Technology
EE 578 Assignment #5
Mohammad Alkhodary
200806080
Outline
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Introduction
Direct Sequence Spread Spectrum (DSSS)
Frequency Hopping Spread Spectrum (FHSS)
Time Hopping Spread Spectrum (THSS)
Hybrid Systems Basic Principle of Spread
Spectrum Systems
Code Division Multiple Access
M-Sequence
Gold , Orthogonal Gold Sequence
Simulation Result
Introduction
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Multiple-access capability is primarily achieved by means
of coding.
Each user is assigned a unique code sequence that he
uses to encode his information signal.
The receiver. knowing the code sequences of the user,
decodes the received signal after reception and recovers
the original data.
Because the bandwidth of the code signal is chosen to be
much larger than the bandwidth of the information signal,
the encoding process enlarges (spreads) the spectrum
modulation. (SS)
The resulting encoded signal is also called an SS signal.
Multiple Access
 FDMA
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Frequency Division Multiple Access
 TDMA
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Time Division Multiple Access
 CDMA
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FDM
A
Code Division Multiple Access
time
TDMA
time
code
TDMA
freq
CDMA
freq
time
freq
Spread Spectrum Advantages?
 Hide
a signal below the noise floor
 Resistance to narrowband jamming and
interference
 Mitigate performance degradation due to
intersymbol and narrowband interference
 In conjunction with RAKE receiver, SS can provide
coherent combining of different multipath
components
 Allow multiple users to share the same signal
bandwidth
 Wide bandwidth of SS signals is useful for location
and timing acquisition
Spread Spectrum Properties
 Signal
occupies a bandwidth much larger
than is needed for the information signal
 Spread spectrum modulation is done
using a spreading code independent of
the data in the signal
 Despreading at the receiver is done by
correlating the received signal with a
synchronized copy of the spreading code
Spread Spectrum System: DSSS
Vs. FHSS
Direct Sequence Spread
Spectrum (DSSS)
General Block Diagram
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Input
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Binary data dt with symbol rate Rs=1/Ts (=Rb for BPSK)
Pseudo-noise code: pnt with chip rate Rc=1/Tc (an integer of
Rs)
Spreading
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Transmitted baseband signal: txb=dt * pnt
Spreading
Processing gain
Despreading
DSSS Example
C=AxB
A=CxB
Processing Gain
GP 
TS
f
 C
TC
fS
Multiple Access
CDMA for DSSS
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n users each using different orthogonal PN sequence
Modulate each users data stream
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Using qPSK
Multiply by spreading code of user
Seven Channel CDMA Encoding and
Decoding
CDMA Power Control
Frequency Hopping Spread Spectrum
(FHSS)
 Signal
broadcast over seemingly random
series of frequencies
 Receiver hops between frequencies in
sync with transmitter
 Eavesdroppers hear unintelligible blips
 Jamming on one frequency affects only a
few bits
Basic Operation
 Typically
2k carriers frequencies forming 2k
channels
 Channel spacing corresponds with
bandwidth of input
 Each channel used for fixed interval
FHSS System
Transmitter
Receiver
Slow and Fast Frequency
Hopping
 Frequency
shifted every Tc seconds
 Duration of signal element is Ts seconds
 Slow FHSS has Tc  Ts
 Fast FHSS has Tc < Ts
 Generally fast FHSS gives improved
performance in noise (or jamming)
Fast Hopping versus Slow Hopping
 Fast
Hopping
Fast Hopping versus Slow Hopping
 Slow
Hopping
Hybrid DS/FH Spread Spectrum
System
Maximal Length Sequence (msequence)
Gold Sequence
IMT-2000: Gold Sequence
 Mutually
Orthogonal Gold Codes (256)
 Generator Polynomials:
1 X 2  X 3  X 4  X 8
1 X 3  X 5  X 6  X 8
Simulation Result
CDMA M-Sequence performance in AWGN
Simulation Result
CDMA Gold Sequence performance
in AWGN
Simulation Result
CDMA Orthogonal Sequence Gold
performance in AWGN Chanel
Simulation Result
CDMA M-Sequence performance in Fading
Channel
Simulation Result
CDMA Gold Sequence performance in Fading
Channel
Simulation Result
CDMA Orthogonal Sequence Gold
performance in Fading Channel