Spread spectrum and CDMA
Welcome to the World of CDMA
Processing gain=
bandwidth expansion factor
Capacity
Signal –noise ratio
Capacity
Example: WCDMA: THE SPREADING PROCESS
WCDMA uses Direct Sequence spreading, where spreading process is
done by directly combining the baseband information to high chip rate
binary code. The Spreading Factor is the ratio of the chips (UMTS =
3.84Mchips/s) to baseband information rate. Spreading factors vary from
4 to 512 in FDD UMTS. Spreading process gain can in expressed in dBs
(Spreading factor 128 = 21dB gain).
•
WCDMA Spreading
TDD WCDMA uses spreading factors 4 - 512 to spread the base band data over ~5MHz band. Spreading factor in dBs
indicates the process gain. Spreading factor 128 = 21 dB process gain). Interference margin is calculated from that:
Interference Margin = Process Gain - (Required SNR + System Losses)
Required Signal to Noise Ration is typically about 5 dB
•System losses are defined as losses in receiver path. System losses are typically 4 - 6
dBs
Advantages of CDMA
• Improving the voice quality & eliminating the
audible effects of multi-path fading.
• Enhancing privacy and security through the
spreading of voice signals.
• Reducing average transmitted power
• Also reducing interference to other electronic
devices.
Multiple access using spread spectrum
Orthogonal code properties
•Correlation Property
•Balance Property
• Run Property
Orthogonal code properties
Codes for CDMA
Codes in CDMA
Orthogonal codes PN codes
Walsh code M-Sequence Gold code
Orthogonal Codes
Walsh Codes
nn
nn
nHH
HHH2
0 0
0 1 0
0 0 0 0
0 0
0 0
0 0
1
1 1
1
1 1
Etc.
Pseudo-Random Noise Codes:
Feedback Shift Register:
Logic
1 2 m
Clock
O/P
Flip-Flop
Maximal Length Sequences
(M-Sequences)
Feed-
Back
symbol
1 0 0 0 0 O/P
Symbol
0 0 1 0 0 0 0
1 1 0 1 0 0 0
0 0 1 0 1 0 0
1 1 0 1 0 1 0
1 1 1 0 1 0 1
1 1 1 1 0 1 0
0 0 1 1 1 0 1
0 1 0 1 1 1 0
0 1 1 0 1 1 1
Gold Codes:
1) Barker Codes
Examples of Barker codes are:
Barker 11: [1,1,1,1,-1,
-1,1,1,-1,1,-1]
Barker 13: [1,1,1,1,1,-1,
-1,1,1,-1,1,-1,1]
0 5 10 15 20 25-2
0
2
4
6
8
10
12
14auto-correlation of B13
2) Maximal Length Sequences
(M-Sequences)
Clock
Output
sequence
S3 S0 S1 S2
2 3 1
N=2n-1
Choosing a maximum length sequence
t register
ngth, m feedback taps#m-
sequences
2* [2,1] 2
3* [3,1] 2
4 [4,1] 2
5* [5,2],[5,4,3,2],[5,4,2,1] 6
6 [6,1],[6,5,2,1],[6,5,3,2] 6
7* [7,1],[7,3],[7,3,2,1],[7,4,3,2],[7,6,4,2],[7,6,3,1],
[7,6,5,2],[7,6,5,4,2,1],[7,5,4,3,2,1]
18
8 [8,4,3,2],[8,6,5,3],[8,6,5,2],[8,5,3,1],[8,6,5,1],
[8,7,6,1],[8,7,6,5,2,1],[8,6,4,3,2,1]
16
0 2 4 6 8 10 12 14 16 18-2
-1
0
1
2
3
4
5
6
7m-sequence auto-correlation
3) Gold Codes Families
clk Gold-sequence (k)
M-sequence 2
(T = k .Tc)
+
M-sequence 1
(T = 0)
Example
[5,3]
+
+ + +
1 2 3 4 5
1 2 3 4 5
[5,4,3,2]
Gold-
sequence
0 10 20 30 40 50 60 70-10
-5
0
5
10Crosscorrelation of m-sequence 1&2
5 10 15 20 25 30
0
0.5
1
Gold code of (3,5),(2,3,4,5)
5 10 15 20 25 30
0
0.5
1
Gold code of (3,5),(2,3,4,5)
0 10 20 30 40 50 60 70-20
0
20
40Autocorrelation of g
0 10 20 30 40 50 60 70-20
0
20
40Autocorrelation of G
0 10 20 30 40 50 60 70-10
-5
0
5
10Crosscorrelation of g,G
0 20 40 60 80 100 120 140-60
-40
-20
0
20
40
60
80auto-correlation of Walsh #10 of length 64 bit
0 20 40 60 80 100 120 140-15
-10
-5
0
5
10
15cross-correlation of Walsh #30 & Walsh #50 of length 64 bit
Orthogonal Gold Codes:
0 10 20 30 40 50 60 70-15
-10
-5
0
5
10
CDMA is altering the face of cellular and PCS communication by:
• Dramatically improving the telephone traffic capacity
• Dramatically improving the voice quality and eliminating the audible
effects of multipath fading
• Reducing the incidence of dropped calls due to handoff failures
• Providing reliable transport mechanism for data communications, such as
facsimile and internet traffic
• Reducing the number of sites needed to support any given amount of
traffic
• Simplifying site selection
• Reducing deployment and operating costs because fewer cell sites are
needed
• Reducing average transmitted power
• Reducing interference to other electronic devices
• Reducing potential health risks
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