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Material Type: Notes; Class: Wireless Communications; Subject: Electrical & Computer Engr; University: Villanova University; Term: Unknown 1989;
Typology: Study notes
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Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Yimin Zhang, Ph.D.
Department of Electrical & Computer Engineering
Villanova University
http://yiminzhang.com/ECE
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Outlines
Spread Spectrum Communications
Frequency Hopping
Direct Sequence
Spreading Codes
RAKE Diversity
Power Control
Soft Handoff
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Frequency Hopping
Typical frequency-hopping waveform pattern
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
DSSS Using BPSK
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Direct Sequence Transmitter side of system
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Direct Sequence Receiver side of system
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
0
5
1 0^
1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^20) - 2^0
5
1 0^
1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^20) - 2^0
5
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1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^20) - 2^0
5
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1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^20) - 2^0
5
1 0^
1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^50) - 5^0
5
1 0^
1 5^
2 0^
2 5^
3 0^
3 5^
4 0^
4 5^
5 0
(^50) - 5^0
5
1 0^
1 5^
2 0^
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5 0
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Spreading Codes
Two requirements for codes:(1) Easy to distinguish a signal from its delayed version.
other users.- very important in CDMA
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
PN Sequence
PN sequence provides low and constant autocorrelation fornon-zero lags. It well satisfies the first condition, but is notgood for the second condition.
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Walsh Function
Different Walsh functions are orthogonal. Walsh function wellsatisfies the second condition, but is not good for the firstcondition.
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Walsh Function in Forward Link
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
CDMA: Features
Greatly reduces fading effect
less fading margin Ï
reduces transmission power
Ï
Single frequency reusing
no need of frequency assignment to cells Ï
improves frequency efficiency
Ï
Gradually quality degradation
variable capacity
Ï
Easy error correction
less burst error
Ï
RAKE diversity
path diversity Ï
ISI problem not significant
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
Code Division Multiple Access - CDMA
Multiple users occupying the same band by having different codesis known as a CDMA - Code Division Multiple Access system Let
W - spread bandwidth in HzR = 1/T
= Date Rate (data signal bandwidth in Hz)b
S - received power of the desired signal in WJ - received power for undesired signals like multiple access
users, multipath, jammers etc in W E
W/R is called processing gain.
min 0
max
b
Yimin Zhang, Villanova University
ECE 8708 Wireless Communications :Spread Spectrum Communications and CDMA
CDMA (contd…)
In conventional systems W/R
1 which means, for satisfactory
operation J/S < 1.
-^
Example
Let R = 9600; W = 1.2288 MHz(E
/Nb
min
= 6 dB (values taken from IS-95)
Jamming margin (JM) = 10log
10
3 )-6 = 15.1 dB
This antijam margin or JM arises from Processing Gain
If (E
/Nb
min
is further decreased or PG is increased, JM can be further
increased.
-^
JM can be used to accommodate multiple users in the same band
-^
If (Eb/N0)min and PG is fixed, number of users is maximized if perfectpower control is employed.
-^
Capacity of a CDMA system is proportional to PG.
