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