1

04/10/23

Basic CDMATechnology and Terminology

March 31, 2000

2

04/10/23

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

3

04/10/23

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 . . .

4

04/10/23

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

5

04/10/23

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

6

04/10/23

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

7

04/10/23

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)

8

04/10/23

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

9

04/10/23

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

10

04/10/23

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

11

04/10/23

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

12

04/10/23

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

13

04/10/23

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)