cdma technology & terminology
TRANSCRIPT
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Basic CDMATechnology and Terminology
March 31, 2000
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What is CDMA?• CDMA (Code Division Multiple Access)
• Digital using DSSS (Direct Spread Spectrum System)
• N=1 Frequency Reuse
• Coherent Multiple and Reception
• Variable Speech Coding during conversation
• Uses PN (Pseudo Noise)
• Uses GPS as system clock (Base Station)
ft
ft
Traditional Narrowband System (e.g. AMPS)
Spread Spectrum System
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General CDMA Terminology
• Base Station to Mobile - Forward Link (Downlink)
• Mobile to Base Station - Reverse Link (Uplink)
• PN Sequence – Pseudo Noise (Periodic and Noise Like) generated by a shift register
• Chips and Chip Rate – Chip is a “Bit” (1 or 0) and the Chip Rate (Rc) for IS-95 is 1.2288Mbps
• Bit Rate (Rb) – digitize user data (e.g. voice). For 800Mhz IS-95 and IS-95-A maximum Rb is 9.6 kbps and 14.4kbps respectively
• Traffic Frame – data are processed in 20ms burst
More Terminology to Follow . . .
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CDMA PN Long Codes
• PN Long Code– 42 bits generated at 1.2288 Mbps
– Cycle every 41.4 days
• Long Codes are Used for – Uplink (MS to BTS)
• Spreading the signal
• Encryption
• Used to identify the user
– Downlink (BTS to MS)• Encryption
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CDMA PN Short Codes
• PN Short Code– 15 bits generated at 1.2288 Mbps
– Cycle every 26.67ms
• Short Codes are Used for – Uplink
• QSPK modulation
• Using only PN offset 0
– Downlink• Identification of the BTS (Sector)
• BTS/sectors have different PN offset
• There are currently 512 PN offset
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CDMA Walsh Codes
• PN Walsh Code– IS-95 CDMA uses 64 orthogonal (64 x 64) sequence
– Each Walsh Code is 64 bits long
• Walsh Codes are Used for – Uplink
• Identifies the user
• Loosely referred to as “CDMA Channels”
– Downlink• As modulation symbol
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CDMA Channel Types
• Base Station to Mobile: Forward Link (Downlink)– Identified by Walsh Codes (64 Bits)
• Pilot (Walsh Code 0: Pilot Channel to identify sites)
• Paging (Walsh Code 1 – 7: Used for control information)
• Sync (Walsh Code 32: Time Sync)
• Traffic (Wothers : Used for traffic and control during conversation and 20ms long)
• Mobile to Base Station: Reverse Link (Uplink)– Identified by Long Code (Unique Long Code per User )
• Traffic (Used for send voice and data)
• Access (Used for control information to access network)
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Forward Link (Downlink)• Forward Link Process
BTS Transmitter
MS Receiver
Variable Low BitRate Speech
Coding
Channel Coding1/2 Rate
VOCODERBit Interleaving
Long CodeScrambling and
Encryption
Walsh CodeModulation
Short CodeQuadratureSpreading
QSPKCarrier Modulation
Variable Low BitRate Speech
Decoding
Channel Decoding1/2 Rate
DECODERBit Deinterleaving
Long CodeDescrambling and
Decryption
Walsh CodeDemodulation
Short CodeQuadrature
Despreading
QSPKCarrier
Demodulation
RF
Downlink Speech / Channel Processing
Downlink Speech / Channel Processing
1
2 3 4
Overhead Data InsertedMM (Mixed Mode)
MM=1, Blank-and-Burst orDim-and-Burst
MM=0,
Gating Off Duplicated PowerControl Group to reduce selfinterference (Power ControlGroup = 6 Walsh Function /
Group)
RakeReceiver
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Forward Link (Downlink)• Forward Link Transmitter
1/2Rate
24x16
1/64 LongCode
Walsh CodeSequence
I SHORTCode
QSHORTCode
I CarrierModulation
Q CarrierModulation
20msBlocks
VOCODEDSPEECH
DATA
9.6kpbs
19.2kpbs
19.2kpbs
ConvolutionalEncoder
Interleaver
19.2kpbs
1.2288Mpbs
1.2288Mpbs
1.2288Mpbs
1.2288Mpbs
9.6kps*2(384bits)
42bit Mask
Take 1st Bit of 64and hold for 64 bits
1.2288Mpbs
19.2kpbs
Modulo2 Add
19.2kbps*64bitsI - Pilot PN Sequence +x PN Offset (there are
512)
I - Pilot PN Sequence +x PN Offset (there are
512)
1 2 34
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Reverse Link (Uplink)• Reverse Link Process
MS Transmitter
BTS Receiver
Variable Low BitRate Speech
Coding
Channel Coding1/3 Rate
VOCODERBit Interleaving
Walsh CodeSymbol
Modulation
Long CodeSpreading and
Encryption
Short CodeQuadratureSpreading
QSPKCarrier Modulation
Variable Low BitRate Speech
Decoding
Channel Decoding1/3 Rate
DECODERBit Deinterleaving
Walsh CodeSymbol
Demodulation
Long CodeDespreading and
Decryption
Short CodeQuadrature
Despreading
QSPKCarrier
Demodulation
RF
Uplink Speech / Channel Processing
Uplink Speech / Channel Processing
1
2 3 4
Overhead Data InsertedMM (Mixed Mode)
MM=1, Blank-and-Burst orDim-and-Burst
MM=0,
Insert Closed / Open LoopPower Control Bits at 800 bps16 control bits for every 20ms
frame (384/ / 16 = 24)19.2kbps / 24 = 800bps
RakeReceiver
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Reverse Link (Uplink)• Reverse Link Transmitter
1/64 LongCode
I SHORTCode
QSHORTCode
I CarrierModulation
Q CarrierModulation
20msBlocks
VOCODEDSPEECH
DATA
9.6kpbs
28.8kpbs
307.2kpbs
ConvolutionalEncoder1/3 Rate
Interleaver
1.2288Mpbs
1.2288Mpbs
1.2288Mpbs
28.8kpbs
64-aryModulator
(1-64 Walsh Code)
9.6kps*3(576bits)
see CDMAVOCODER
rate
28.8kbps/ 6bits blocksthen for every 6 bits
block is represented by64 bits walsh code
1/2 chipDelay
(406.9ns)
4PN Chips perWalsh bits.
4*307.2kbps
42Bit Mask - used toidentify the user
32x18
I - Pilot PN Sequence at0 offset
Compensate for the 180 degphase transition of Q
Q - Pilot PN Sequenceat 0 offset
4
3
2
1
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CDMA RAKE Receiver• Type of receiver used in CDMA to overcome multipath
(reflections, diffractions, scattering of radio signals)
• What is a RAKE Receiver– It is a Receiver wherein the Received Signal Level (RSL) will be
the vector sum of all the the multipath components
– Currently the CDMA RAKE receiver can vector sum only the three highest Coherent RSL.
• Rake Receiver may combine– Multipath Signal from a single transmitter
– Multipath Signal from multiple serving transmitter
Base Station (A)
Base Station (B)A1A2 B2 B1
A3A2
Time
h(t)
A1
A3B1
B2 A1
A2
B1
Received Signal Peaks
Magnitude of CoherentlyCombined Multipath
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CDMA Standards
• IS-95– Interim Standard for Mobile Station – Base Station Compatibility
Standard for Dual-Mode Wide Band Spread Spectrum Cellular
– Air Interface Specifications
– Maximum VOCODER rate of 9.6kbps
• IS-95-A– IS-95 with support for a much Higher VOCODER rate of 14.4kbps
• Other Specifications– IS-96-A : Voice Encoder Specifications
– IS-97 : Base Stations Performance Specifications
– IS-98 : Mobile Station Performance Specifications
– J-STD 8 : Defines RF Specifications for 1900MHz
– J-STD 18 : Recommends minimum performance for 1900MHz personal stations (MS)