CDMA System Design 2
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Transcript of CDMA System Design 2
8/12/2019 CDMA System Design 2
http://slidepdf.com/reader/full/cdma-system-design-2 1/16
Lecture 12
Spread Spectrum
Dr. Mohammed Usman
8/12/2019 CDMA System Design 2
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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
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IS-95 Forward Traffic Channel parameters
for RS2
Parameter Value 1 Value 2 Value 3 Value 4User data rate(bps) 1800 3600 7200 14400
Code symbolrepetition period
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
Modulation symbolrate
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
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IS-95 Paging Channel parameters
Parameter Value 1 Value 2
Data rate(bps) 4800 9600
Code symbolrepetition period
2 1
Code rate ½ ½
Modulation symbolrate
19200 19200
PN chips per
modulation symbol
64 64
PN chips per bit 256 128
PN chip rate(Mcps) 1.2288 1.2288
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IS-95 Sync Channel parameters
Parameter Value
Data rate(bps) 1200
Code symbolrepetition period
2
Code rate ½
Modulation symbolrate
4800
PN chips per
modulation symbol
256
PN chips per bit 1024
PN chip rate(Mcps) 1.2288
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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.