Post on 03-Apr-2018
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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 zhenyuzywei@ee.cuhk.edu.hk
March 27, 2006
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Contents
Introduction
Forward CDMA Channel
Reverse CDMA Channel
Specifications on CDMA Project
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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
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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
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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
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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).
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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
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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.
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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
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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
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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
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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
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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
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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
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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.
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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