CH5_IS 95 CDMA

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1Korea Aerospace University Mobile Communications Lab.

CH 5. Air Interface of the IS-95A CDMASystem


ContentsContents

Forward Link Structure

z Pilot, Sync, Paging, and Traffic Channels

z Channel Coding, Interleaving, Data Scrambling, and Modulation

z Power Control Sub-channel, Spreading, and Pulse Shaping

CDMA System Time

Reverse Link Structure

z Access and Traffic Channels

z Channel Coding, Interleaving, and Modulation

z Burst Transmission, Direct and Quadrature Spreading

Summary of IS-95A Physical Layer Parameters

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Forward Link Structure [1],[2]Forward Link Structure [1],[2]

The IS-95 forward link (base station-to-mobile station direction) consists of

pilot, sync, paging, and traffic channels.

Among these, pilot and sync channels are called the broadcasting channel.

IS-95 base stations may support up to 64 forward link channels per each

sector for 1.23MHz band, as shown in Fig. 5.1,

z 1 pilot channel

z 1 sync channel

z up to 7 paging channels

z up to 55 traffic channels

In IS-95 forward link, 64 Walsh codes are used to isolate each channel, along

with I/Q short PN codes to reduce the multipath interference and other-cellinterference.

In IS-95 forward link, BPSK data modulation is employed. In IS-95 forward link, a convolutional coding with rate and constraint

length 9 is employed.

All forward link channels are summed at base band prior to transmission.

All forward link channels should be aligned within 1/8 PN chip errors.


Forward Link Structure (cont.)Forward Link Structure (cont.)

Fig. 5.1 An example of IS-95 forward link channel assignments.

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Forward Link Structure: Pilot ChannelForward Link Structure: Pilot Channel

No information data (all zero data): Only I/Q short PN codes

Used for code and carrier synchronization

Used for multi-path searching for rake combining

Used for channel estimation for coherent demodulation

Used for power measurements for handover, etc.

10% ~ 20% of total transmit power is assigned to the pilot channel.

Fig. 5.2 Pilot channel modulation.

0 0 0 0 0 0 0 .


Forward Link Structure: Sync ChannelForward Link Structure: Sync Channel

Used to transmit the system time obtained from GPS satellites.

A sync channel frame is 26.666 ms in length equivalent to the period of I/Qshort PN codes and is aligned with the PN codes.

A sync channel super frame is 80 ms in length consisting of three sync

channel frame.

Messages to be transmitted on the sync channel shall begin only at the start

of a sync channel super frame.

The sync channel messageis transmitted at a rate of 1200 bps.

The sync channel messagecontains

z System time

z System and network identification

z Pilot PN offset of the base station

z State of the long code shift register

z Paging channel data rate, etc.

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Forward Link Structure: Sync Channel (cont.)Forward Link Structure: Sync Channel (cont.)

Sync Frame #2Sync Frame #1 Sync Frame #3

96 bits

80 ms

1.2 kbps

Convolutional

Encoding

(r=1/2, k=9)

SymbolRepetition

BlockInterleaver A

4.8 ksps2.4 ksps

Sync

Channel

Data 4.8 ksps

Walsh

32

Fig. 5.3 Sync channel super frame.

Fig. 5.4 Sync channel modulation.


Forward Link Structure: Paging ChannelForward Link Structure: Paging Channel

The paging channel is used to transmit control information from the base

station to the mobile station for call setup.

Up to 7 paging channels can be associated with a single FA (frequency

assignment, 1.23MHz).

The mobile station always monitors a paging channel and responds to

pages through one of access channels associated with that particularpaging channel.

The paging channel data is transmitted at 4800 or 9600 bps.

The paging channel is normally operated in slot mode, where control

messages for a particular mobile is sent in a pre-defined time slot.

During registration, the mobile is assigned a time slot in which it willreceive control messages.

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Forward Link Structure: Paging Channel (cont.)Forward Link Structure: Paging Channel (cont.)

The paging channel message contains

z Page messages

z System parameters: PN offset, system ID, network ID, base station ID,search windows, handoff parameters, etc.

z Access parameters: Number of access channels, number of accessprobes, authentication data, etc.

z Neighbor cell list, etc.

Fig. 5.5 Paging channel modulation.


Forward Link Structure: Traffic ChannelForward Link Structure: Traffic Channel

The forward traffic channel is mainly used to transfer voice and datafrom

the base station to the mobile station.

The forward traffic channel is also used for transmission of signaling data.

Traffic Channel Data Rates (Variable Data Rates)

z Rate Set 1: 9600, 4800, 2400, and 1200 bps

z Rate Set 2: 14400, 7200, 3600, and 1800 bps

When signaling data is to be transmitted, data rate is always changed tothe full rate (9600 or 14400).

There are two options to transmit signaling data: Blank and burst, Dim and

burst.

z Blank and burst: The entire traffic channel frame is used to send only

signaling data.

z Dim and burst: The traffic channel frame is used to send both primary

traffic and signaling data.

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Forward Link Structure: Traffic Channel (cont.)Forward Link Structure: Traffic Channel (cont.)

ADD

CRC

8.6 kbps

4.0 kbps

2.0 kbps

0.8 kbps

9.2 kbps

4.4 kbps

2.0 kbps

0.8 kbps

ADD Tail

8 Bits

9.6 kbps

4.8 kbps

2.4 kbps

1.2 kbps

Convolutional

Encoding

(r=1/2, k=9)

Symbol

Repetition

Block

InterleaverB

19.2 ksps19.2 ksps

9.6 ksps

4.8 ksps

2.4 ksps

Traffic Channel

Information

Data

19.2 ksps

Short PN_I

Generator

FIR

W64,n

cosct

-sinct

s(t)

Short PN_Q

Generator

FIR

MUXB

1.2288 Mcps

800 bps

Long Code

Generator

Long Code

mask for

user m

Decimator Decimator

19.2 Ksps

Power

control bit

800 Hz

1.2

288Mcps

1.2288 Mcps

1.2288 Mcps

Fig. 5.6 Forward traffic channel modulation for RS1.


Forward Link Structure: Traffic Channel (cont.)Forward Link Structure: Traffic Channel (cont.)

ADD

CRC

13.4 kbps

6.3 kbps

2.8 kbps

1.1 kbps

14.0 kbps

6.8 kbps

3.2 kbps

1.4 kbps

ADD Tail

8 Bits

14.4 kbps

7.2 kbps

3.6 kbps

1.8 kbps

Convolutional

Encoding

(r=1/2, k=9)

Symbol

RepetitionBlock

InterleaverB

28.8 ksps28.8 ksps

14.4 ksps

7.2 ksps

3.6 ksps

Traffic Channel

Information

Data

19.2 ksps

Puncturing

19.2 ksps

Short PN_I

Generator

FIR

W64,n

cosct

-sinct

s(t)

Short PN_QGenerator

FIR

MUXB

1.2288 Mcps

800 bps

Long Code

Generator

Long Code

mask for

user m

Decimator Decimator

19.2 Ksps

Power

control bit

800 Hz

1.2

288Mcps

1.2288 Mcps

1.2288 Mcps

Fig. 5.7 Forward traffic channel modulation for RS2.

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Forward Link Channel Coding [2]Forward Link Channel Coding [2]

The sync, paging, and forward traffic channels shall be convolutionallyencoded prior to transmission.

The convolutional code used in the forward link shall be of rate 1/2 with a

constraint length of 9. The generator functions of the code shall be g0 and g1 that equal 753(octal)

and 561(octal), respectively.

Fig. 5.8 Convolutional encoder.


Forward Link Block Interleaving [2]Forward Link Block Interleaving [2]

All symbols after repetition are block interleaved by using a bit reversal

method or modified bit reversal method.

For example, the sync channel shall use a block interleaver spanning

26.6666 ms which involves 128 modulation symbols.

z The 128 input symbols are written into a linear array with addresses viewed

by 7-bit binary number a6 a5 a4 a3 a2 a1 a0.z For reading, the mapping of addresses shall be performed as c0=>a6,

c1=>a5, c2=>a4, c3=>a3, c4=>a2, c5=>a1, c6=>a0.

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Forward Link Block Interleaving (cont.)Forward Link Block Interleaving (cont.)

Table. 5.1 Write operation for 128 symbols with two time repetition.

Address 0

Address 127


Forward Link Block Interleaving (cont.)Forward Link Block Interleaving (cont.)

Table. 5.2 Read operation for 128 symbols.

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Forward Link Data Scrambling [2]Forward Link Data Scrambling [2]

Fig. 5.9 Data scrambling.


Forward Link Power Control Sub-channel [2]Forward Link Power Control Sub-channel [2]

Fig. 5.10 Position of power control bits.

0 1

1011

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Forward Link Quadrature Spreading [1],[2]Forward Link Quadrature Spreading [1],[2]

Following Walsh orthogonal spreading, each channel is spread in

quadrature.

The I and Q channel spreading sequences (also called short PN codes)

have a length of 215 chips (i.e., 32768 chips = 26.666ms) due to zeroinsertion.

The I and Q channel spreading is used to mitigate multipath interference

and other-cell interference.

The characteristic polynomials of the PN sequences are

( )

( ) 1

1

345610111215

57891315

++++++++=

++++++=

xxxxxxxxxP

xxxxxxxP

Q

I


Forward Link Quadrature Spreading (cont.)Forward Link Quadrature Spreading (cont.)

Fig. 5.11 Forward channel signal constellation and phase transition.

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Forward Link Pulse-Shaping Filter [2]Forward Link Pulse-Shaping Filter [2]

Following the I/Q spreading operation, I and Q impulses are applied to

pulse-shaping filters to limit the spectrum of a transmitted signal.

The pulse-shaping filter should satisfy the condition that 1=1.5 dB (pass

band ripple), 2=40 dB, fp=590 kHz, fs=740 kHz.

Fig. 5.12 Frequency response specifications of a pulse-shaping filter.


Forward Link Pulse-Shaping Filter (cont.)Forward Link Pulse-Shaping Filter (cont.)

Table 5.3 48 tap coefficients of the sample pulse-shaping filter with fourtimes over-sampling.

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Fig. 5.15 Comparison with a truncated Sinc filter.



Fig. 5.16 Signal waveform after pulse-shaping.

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CDMA System Time [2]CDMA System Time [2]

Fig. 5.17 CDMA system time-line.

midpoint


Reverse Link Structure [1],[2]Reverse Link Structure [1],[2]

The IS-95 reverse link is composed of access channels and reverse trafficchannels.

Each channel in the reverse link is identified by the long PN code with the

period of 242 1 Tc.

z Each traffic channel is identified by a private user long code.

z Each access channel is identified by a public long code.

In IS-95 reverse link, the quadrature spreading by I/Q short PN codes isemployed, along with the direct spreading by long PN code.

The I/Q short PN codes are the same as those used in the forward link.

The Q channel PN sequence is delayed by half a PN chip to reduce the

signal fluctuation due to zero crossing (OQPSK).

In IS-95 reverse link, noncoherent 64-ary orthogonal modulation schemeis employed.

In IS-95 reverse link, a convolutional coding with rate 1/3 and constraintlength 9 is employed.

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Reverse Link Structure (cont.) [1],[2]Reverse Link Structure (cont.) [1],[2]

Fig. 5.18 An example of IS-95 reverse link channels.


Reverse Link Structure: Access ChannelReverse Link Structure: Access Channel

Used for call origination by a mobile, response to paging, and registration.

Up to 32 access channels are associated with a single paging channel.

The data rate on the access channel is 4800 bps.

Each access probe (or access slot ) consists of an access preamble andmessage capsule as shown in Fig. 5.19.

The access preamble is used for a base station to obtain a synchronization to amobile.

The maximum sizes of access preamble and message capsule are all 16 frames

and the minimum sizes of access preamble and message capsule are 1 and 3frames, respectively.

After transmitting an access probe, the mobile waits a specified period for anacknowledgement from the base station.

If an acknowledgement is received, the access attempt is completed. Otherwise,the next access probe is transmitted at a power level higher than the previous

one after a pseudo-randomly generated delay.

The entire process to send an access probe and receive an acknowledgement is

called an access attempt, which is depicted in Fig. 5.20.

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Reverse Link Structure: Access Channel (cont.)Reverse Link Structure: Access Channel (cont.)

Fig. 5.19. Access probe structure.



Fig. 5.20. Access probe sequence.

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Fig. 5.21 Access channel modulation.

Short PN_I

Generator

FIR

cosct

-sinct

s(t)

Short PN_Q

Generator

FIR

A

Long Code

GeneratorPublic Long

Code Mask

1.2288 Mcps1.2288 Mcps

1.2288 Mcps

D

1/2 Tc


Reverse Link Structure: Traffic ChannelReverse Link Structure: Traffic Channel

Transmits user information such as voice and data.

Transmits also signaling data.

Each traffic channel is identified by a private user long code.

Reverse Traffic Channel Data Rate

z Rate Set 1: 9600, 4800, 2400, and 1200 bps

z Rate Set 2: 14400, 7200, 3600, and 1800 bps

The reverse traffic channel data is transmitted in burst mode for variable

rate transmission, which is due to closed-loop power control in reverselink.

When a signaling data is to be transmitted, the data rate is changed to

the full rate.

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Reverse Link Structure: Traffic ChannelReverse Link Structure: Traffic Channel

Fig. 5.22 Reverse traffic channel modulation for RS1.

Short PN_I

Generator

FIR

cosct

-sinct

s(t)

Short PN_Q

Generator

FIR

A

Long Code

Generator

Data Burst

Randomizer

User Long Code

Mask

1.2288 Mcps1.2288 Mcps

1.2288 McpsFrame Data

Rate

D

1/2 Tc


Reverse Link Channel Coding [1],[2]Reverse Link Channel Coding [1],[2]

The access channel and reverse traffic channel shall be convolutionally

encoded prior to transmission.

The convolutional encoder shall be of rate 1/3 with a constraint length of 9.

The generator functions of the code shall be g0 equals 557(octal) and g1equals 663(octal), and g2 equals 711(octal).

Fig. 5.23 k=9, rate 1/3 convolutional encoder.

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Reverse Link Block Interleaving [2]Reverse Link Block Interleaving [2]

The mobile station shall interleave all coded symbols on the reverse traffic

channel and access channel prior to modulation and transmission.

The interleaver shall be an array with 32 rows and 18 columns (576 cells),

spanning 20 ms.

Coded symbols shall be written into the interleaver by columns filling the

complete 32 x 18 matrix.

Reverse Traffic channel coded symbols shall be output from the interleaver

by rows in the following order.

3224312330222921282027192618251716815714613512411310291

:bps1200At

3228312730262925242023192218211716121511141013984736251

:bps2400At

3230312928262725242223212018191716141513121011986754231

:bps4800At

3231302928272625242322212019181716151413121110987654321

:bps9600At


Reverse Link Modulation [1],[2]Reverse Link Modulation [1],[2]

Modulation for the reverse link channel shall be 64-ary orthogonal

modulation.

After interleaving, every six consecutive symbols are grouped to form a

Walsh symbol, which is then mapped to one of Walsh functions.

The modulation symbols shall be selected according to the following rule:

where c5

represents the latest (or most recent) and c0

the first (or oldest)

binary valued code symbol of each Walsh symbol.

The 64 by 64 Walsh matrix is used to generate Walsh functions by means

of the following recursive procedure:

The period of a Walsh symbol shall be 64 Walsh chips, which correspond

to 256 PN chips (208.333 s).

543210 3216842IndexSymbolModulation cccccc +++++=

32 32

2 64

32 32

N N

N

N N

H H H HH H

H H H H

= =

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Reverse Link Burst Transmission [2]Reverse Link Burst Transmission [2]

Prior to transmission, the Walsh symbol stream is gated with an ON-OFF

filter that allows transmission of certain power control groups and deletion

of others, as shown in Fig. 5.24.

The gated-on and gated-off groups are determined by the data rate of theframe and by a block of 14 bits taken from the long PN code in data burst

randomizer.

For 4800 bps, transmission shall occur on power control groups

numbered:

For 2400 bps, transmission shall occur on power control groups

numbered:

76543210 14,12,10,8,6,4,2, bbbbbbbb +++++++

0 8 1 8

2 9 3 9

4 10 5 10

6

if 0, or 2+ if 1,

4 if 0, or 6+ if 1,

8 if 0, or 10+ if 1,

12 i

b b b b

b b b b

b b b b

b

= =+ = =

+ = =

+ 11 7 11f 0, or 14+ if 1.b b b= =


Reverse Link Burst Transmission (cont.)Reverse Link Burst Transmission (cont.)

Fig. 5.24 Reverse Traffic Channel variable rate transmission.

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Reverse Link Direct Sequence Spreading [1],[2]Reverse Link Direct Sequence Spreading [1],[2]

Prior to transmission, the reverse traffic channel and the access channelshall be spread by either a private user long code or a public long code.

The long code shall be periodic with period 242-1.


Reverse Link Quadrature Spreading [1],[2]Reverse Link Quadrature Spreading [1],[2]

Following the direct sequence spreading, the reverse traffic channel andaccess channel are spread in quadrature.

The sequences used for this spreading shall be the same as those used on

the forward link channel.

The characteristic polynomials of the PN sequences are

The data spread by the Q channel PN sequence shall be delayed by halfPN chip time and a resulting signal constellation is that of OQPSK, as

shown in Fig. 5.25.

( )

( ) 1

1

345610111215

57891315

++++++++=

++++++=

xxxxxxxxxP

xxxxxxxP

Q

I

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Reverse Link Quadrature Spreading (cont.)Reverse Link Quadrature Spreading (cont.)

Fig. 5.25 Reverse CDMA channel signal constellation.


Summary of IS-95A Physical Layer ParametersSummary of IS-95A Physical Layer Parameters

Chip Rate 1.2288 Mcps

BW / Carrier Spacing 1.23 MHz / 1.25 MHz

Spreading Codes

Forward : I/Q short PN codes(215 = 32768 chips : 26.666 ms)

Reverse : I/Q short PN codes(215 = 32768 chips : 26.666 ms)and Long PN code(242-1)

Frame Length 20 ms, 26.666ms (Sync Ch.)

Forward Orthogonal Code Walsh code

Modulation / Spreading Forward : BPSK / QPSKReverse : 64-ary orthogonal / OQPSK

Channel Coding Forward : Convolutional code (r=1/2, k=9)Reverse : Convolutional code (r=1/3, k=9)

Voice Coding Variable rate QCELP (8.6 / 4.0 / 2.0 / 0.8 kbps for rate set 1 and13.35 / 6.25 / 2.75 / 1.05 kbps for rate set 2) and EVRC

Power ControlForward : Power AllocationReverse : Closed loop ( Rate : 800 Hz ) + Open loop + Outer loop

Diversity Forward : Path + Time + Space (Handover) diversityReverse : Path + Time + Space (Antenna, Handover) diversity

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