channels in cdma

7/29/2019 Channels in Cdma

1/17

An Introduction to CDMA Air Interface:

TIA/EIA/IS-95A

Visual Signal Processing & Communication LabSHB Rm301

Department of Electronic EngineeringThe Chinese University of Hong Kong, Shatin, N.T., Hong Kong

Tutor: wei [email protected]

March 27, 2006


2/17

Contents

Introduction

Forward CDMA Channel

Reverse CDMA Channel

Specifications on CDMA Project


3/17

Multiple Access

FDMA

Advanced Mobile Phone Service (AMPS) and Total AccessCommunications System (TACS)

AMPS systems use 30 kHz "slices" of spectrum for each channel

Narrowband AMPS (NAMPS) requires only 10 kHz per channel

TACS channels are 25 kHz wide

TDMA

IS-54 systems, designed to coexist with AMPS systems, divide 30 kHz ofspectrum into three channels

PDC divides 25 kHz slices of spectrum into three channels

GSM systems create 8 time-division channels in 200 kHz wide carriers


4/17

Multiple Access

CDMA

unique digital codes are used to differentiate subscribers codes are shared by both MS and BS

all users share the same range of radio spectrum

Benefits of CDMA:

Capacity increases: 8 to 10 times (AMPS); 4 to 5 times (GSM)

Improved call quality

Simplified system planning

Enhanced privacy Improved coverage characteristics

Increased talk time for portables

Bandwidth on demand


5/17

CDMA

There are two CDMA common air interface standards:

Cellular (824-894 MHz) - TIA/EIA/IS-95A PCS (1850-1990 MHz) - ANSI J-STD-008

They are very similar in their features, with exceptions of thefrequency plan, mobile identities, and related message fields.

IS-95A 45 MHz spacing for forward & reverse channel

Permissible frequency assignments are on 30 kHz increments

W/R = 10 log (1.2288 MHz/9600Hz) = 21 dB for the 9600 bps rate set

ANSI J-STD-008 80 MHz spacing for forward & reverse channel

Permissible frequency assignments are on 50 kHz increments


6/17

Forward CDMA Channel of IS-95A

From BSS to MS

It carries traffic, a pilot signal, and overhead information.

Pilot is a spread but unmodulatedDSSS signal.

Pilot and overhead channels establish the system timing and

station identity. Pilot channel is also used in the mobile-assisted handoff

(MAHO) process as a signal strength reference.

FEC code rate is 1/2 and the PN rate is 1.2288 MHz (1.2288MHz = 128*9600 bps).


7/17

Forward Link Channel Parameters

Channel Sync Paging Traffic

Data rate 1200 4800 9600 1200 2400 4800 9600 bps

Code repetition 2 2 1 8 4 2 1

Modulationsymbol rate 4800 19,200 19,200 19,200 19,200 19,200 19,200 sps

PN chips/modulation symbol 256 64 64 64 64 64 64

PN chips/bit 1024 256 128 1024 512 256 128


8/17

Signal Structure

The forward link consists of up to 64 logical channels (code

channels). Code channels is one of a set of 64 so-called Walsh functions.

Only whole periods of the Walsh functions occur in each code

symbol, the Walsh makes the channels completely separable inthe receiver.

Each forward code channel is spread by the Short Code, whichhas I- and Q-components.

The two coded, covered, and spread streams are vector-modulated on the RF carrier. The spreading modulation is thusQPSK, superimposed on a BPSK code symbol stream.


9/17

Overhead Channels

There are three types of overhead channel in the forward link:

pilot, is required in every station sync

paging

Pilot channel

pure short code with no additional cover or information content

always code channel zero

a demodulation reference for the mobile receivers and for handoff levelmeasurements

carries no information

all stations use the same short code, distinguished by the phase

period of the short code, 215= 26.667 ms at the 1.2288 MHz chip rate


10/17

Overhead Channels

Pilot channel (Conti.)

pilot phases always be assigned to stations in multiples of 64 chips,giving a total of 215-6 = 512 possible assignments

9-bit number that identifies the pilot phase assignment is called thePilotOffset

Sync Channel sync channel carries timing and system configuration information

data rate is always 1200 bps

interleaver period is also 80/3 = 26.667 ms, simplifies finding frame

boundaries, once the mobile has located the pilot

code period ambiguity is then resolved by the long code state and systemtime fields


11/17

Overhead Channels

Paging channel

used to communicate with MSs when they are not assigned to a trafficchannel

successful accesses are normally followed by an assignment to adedicated traffic channel

paging channel may run at either 4800 or 9600 bps each BS must have at least one paging channel per sector, on at least one

of the frequencies in use


12/17

Traffic Channel

Traffic channels

assigned dynamically, in response to MS accesses, to specific MS always carries data in 20 ms frames

carry variable rate traffic frames, either 1, 1/2, 1/4, or 1/8 of9600 bps

rate variation is accomplished by 1, 2, 4, or 8-way repetition of code

symbols, but the energy per bit approximately constant rate is independently variable in each 20 ms frame

the 800 bps reverse link power control subchannel is carried on the trafficchannel by puncturing 2 from every 24 symbols transmitted.

Timing all base stations must be synchronized within a few microseconds


13/17

Reverse CDMA Channel of IS-95

From MS to BSS

It carries traffic and signaling information. FEC code rate is 1/3, the code symbol rate = 28,800 symbols/sec,

6 code symbols/modulation symbol, and the PN rate is 1.2288

MHz modulation is 64-ary orthogonal Walsh functions, each period of

the Walsh function is repeated for four chips of the PN code

Walsh symbol rate is 1.2288 MHz/(4 chips per Walsh chip)/(64

Walsh chips per Walsh symbol) = 4,800 modulation symbols/second


14/17

Reverse CDMA Channel Parameters, Rate Set 1

Channel Access Traffic

Data rate 4,800 1,200 2,400 4,800 9,600 bps

Code Rate 1/3 1/3 1/3 1/3 1/3

Symbol Ratebefore Repetition 14,400 3,600 7,200 14,400 28,800 sps

Symbol Repetition 2 8 4 2 1

Symbol Rate after Repetition 28,800 28,800 28,800 28,800 28,800 sps

Transmit Duty Cycle 1 1/8 1/4 1/2 1

Code Symbols/Modulation Symbol 6 6 6 6 6PN Chips/Modulation Symbol 256 256 256 256 256

PN chips transmitted/bit 256 128 128 128 128


15/17

Signal Structure

Reverse CDMA Channel consists of 242-1 logical channels

One of the logical channel is permanently and uniquely associatedwith each MS. The channel does not change upon handoff.

Reverse link addressing is accomplished through manipulation of

period 2

42-1

Long Code, which is part of the spreading process. The reverse CDMA Channel does not use strict orthogonality inany sense to separate logical channels. Rather, it uses a very longperiod spreading code, in distinct phases. The correlations

between stations are not zero, but they are acceptably small.


16/17


17/17

Main blocks

IS-95A CRC Generator

IS-95A Reverse Channel Convolutional Encoder IS-95A Reverse Channel Interleaver/Deinterleaver

IS-95A Reverse Channel Repeater/Derepeater

IS-95A Reverse Channel Walsh Modulation and Spreading(inside Spreading and Modulation subsystem)

IS-95A Short Code Generator

channels in cdma

Documents