CDMA System Design 2

8/12/2019 CDMA System Design 2

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Lecture 12

Spread Spectrum

Dr. Mohammed Usman



Summary of previous lecture

Capacity of single cell CDMA system is

Capacity of multi-cell CDMA system is

Assumptions made Continuous transmission by all users Omni-directional antennas

NOTE: CDMA system is interference limited reduceinterference to increase capacity

SIR R

R

M s

c 1

MAI I

N SINR

PG

M 011



CDMA performance improvement

factor Pf

Typical value of inter-cell interference factor σ = 1.6 (2dB

Typical voice conversation – user speaks for only 50%

of the time voice activity = 50% Exploiting the pauses in natural speech, voice activity

reduces to40% interference reduction to 40%

Voice activity factor gain G v = 2.5

Capacity can be improved by a factor of 2.5



CDMA performance improvement

factor Pf

Sectorization reduces interference

For a 3 sector cell site (120 degree directional antenna),interference reduction by a factor of 3

Antenna patterns are not ideal in practice Typical interference reduction for 120 degree sectors is

2.5 sectorization gain G A = 2.5

Capacity improvement by a factor of 2.5

Performance improvement factor Pf = G v G A /σ



Illustration

Consider an interference limited IS-95 cellular system with interference to noise ratio of 6 dB. Assumingcontinuously transmitting users compute the averagenumber of users allowed per cell if the inter-cellinterference factor is 0.55 and the required SINR at the

receiver is 8 dB. Compare by taking performanceimprovement factor into account

Given:

IMAI/N0 = 6 dB IMAI/N0 (ratio) = 3.98

N0/IMAI = 1/3.98 = 0.251 σ = 0.55

Required SINR = 8 dB SINR(ratio) = 6.3

Processing Gain (PG) = 128 (for IS-95 standard)



Illustration

No. of users

Here, it was assumed that users are continuouslytransmitting.

MAI I

N PG

M

SINR

011

1

MAI I

N SINR

PG M

011

users M 1047.10

251.013.655.01

128



Illustration

Taking performance improvement factor into account

σ already accounted for in capacity equation but practical value =1.6

Using voice activity detection and cell sectorization,capacity increased by a factor of 4 for the same processinggain and receiver SINR threshold but with higher inter-cellinterference factor

Av

MAI

GG

I

N SINR

PG M

011

users M 3904.395.25.2

251.013.66.11

128



Illustration

Now consider lowering the receiver threshold SINR to5 dB (from the original 8 dB), and all other settingsremaining same

Required SINR =5 dB SINR(ratio) = 3.16

By making the receiver capable of operating at lower

SINR, capacity is improved (doubled in this case)

users M 7783.775.25.2

251.0116.36.11

128

Av

MAI

GG

I

N SINR

PG M

011



IS-95 Forward Channels

Forward Channels

Downlink channels Four types of logical channels

Pilot Channel Reference channel for sync and tracking

Carries phase reference for other channels Sync Channel

Carries accurate timing information (sync with GPS)

Provides PN code offset, system time

Paging Channel Carries paging information for call set up/tear down

Traffic Channel Carries user data



IS-95 Forward Channels

Forward Channels

Downlink channels Defined using 64 Walsh codes

Forward channel contains

One pilot channel – defined by Walsh code 0

One sync channel – defined by Walsh code 32

Seven paging channels – Walsh codes 1-7

55 traffic channels (W 8 to W 31 & W 33 to W 63)

Forward traffic channels support two rate sets–

RS1and RS2



IS-95 Forward Traffic Channel parameters

for RS1

Parameter Value 1 Value 2 Value 3 Value 4User data rate(bps) 1200 2400 4800 9600

Code symbolrepetition period

8 4 2 1

Code rate ½ ½ ½ ½

Modulation symbolrate

19200 19200 19200 19200

PN chips permodulation symbol

64 64 64 64

PN chips per bit 1024 512 256 128PN chip rate(Mcps) 1.2288 1.2288 1.2288 1.2288



IS-95 Forward Traffic Channel parameters

for RS2

Parameter Value 1 Value 2 Value 3 Value 4User data rate(bps) 1800 3600 7200 14400


8 4 2 1

Code rate ½ ½ ½ ½

Puncturing rate 4/6 4/6 4/6 4/6

Effective code rate ¾ ¾ ¾ ¾


19200 19200 19200 19200

PN chips permodulation symbol

64 64 64 64

PN chips per bit 682.67 341.33 170.67 85.33

PN chip rate(Mcps) 1.2288 1.2288 1.2288 1.2288



IS-95 Paging Channel parameters

Parameter Value 1 Value 2

Data rate(bps) 4800 9600


2 1

Code rate ½ ½


19200 19200

PN chips per

modulation symbol

64 64

PN chips per bit 256 128

PN chip rate(Mcps) 1.2288 1.2288



IS-95 Sync Channel parameters

Parameter Value

Data rate(bps) 1200


2

Code rate ½


4800

PN chips per

modulation symbol

256

PN chips per bit 1024

PN chip rate(Mcps) 1.2288



Short Quiz

If PN sequences are not orthogonal, is CDMA stillpossible?

Why is perfect synchronization required in CDMA? What is the significance of processing gain?

Comment on the complexity of the air interface ofCDMA systems as compared to other cellular systems.



Documents

CDMA System Design 2