CH 5. Air Interface of the IS-95A CDMA System

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

System

1Korea Aerospace University Mobile Communications Lab.

System

Contents

� Summary of IS-95A Physical Layer Parameters

� Forward Link Structure

� Pilot, Sync, Paging, and Traffic Channels

� Channel Coding, Interleaving, Data Scrambling, and Modulation

� Spreading and Pulse-Shaping

� Reverse Link Structure

� Access and Traffic Channels


� Access and Traffic Channels

� Channel Coding and Modulation

� Direct and Quadrature Spreading

Summary 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 / QPSK Reverse : 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 and 13.35 / 6.25 / 2.75 / 1.05 kbps for rate set 2) and EVRC

Power Control Forward : Power Allocation Reverse : Closed loop ( Rate : 800 Hz ) + Open loop + Outer loop

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

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,

� 1 pilot channel

� 1 sync channel

� up to 7 paging channels


� up to 7 paging channels

� 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-cell

interference.

� In IS-95 forward link, BPSK data modulation is employed.

� In IS-95 forward link, a convolutional code with rate ½ 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.)


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

Forward 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 the total transmit power is assigned to the pilot channel.


Fig. 5.2 Pilot channel modulation.

0 0 0 0 0 0 0… .

symbol

mapping

0�1, 1�-1

pilot gain

Forward 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/Q

short 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.

� The sync channel message is transmitted at a rate of 1200 bps.

� The sync channel message contains


� The sync channel message contains

� System time

� System and network identification

� Pilot PN offset of the base station

� State of the long code shift register

� Paging channel data rate, etc.

Forward Link Structure: Sync Channel (cont.)

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

96 bits

80 ms

Fig. 5.3 Sync channel super frame.


Fig. 5.4 Sync channel modulation.

Forward 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 particular

paging channel.

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


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

Forward Link Structure: Paging Channel (cont.)

� The paging channel message contains

� Page messages

� System parameters: PN offset, system ID, network ID, base station ID,

search windows, handoff parameters, etc.

� Access parameters: Number of access channels, number of access

probes, authentication data, etc.

� Neighbor cells list, etc.


Fig. 5.5 Paging channel modulation.

Data scrambling

Forward Link Structure: Traffic Channel

� The forward traffic channel is mainly used to transfer voice and data from

the base station to the mobile station.

� Traffic Channel Data Rates (Variable Data Rates)

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

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


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

W

cosωct

Long Code

Generator

Long Code

mask for

user m

Decimator Decimator

Data scrambling


FIR

Σ

W64,n

-sinωct

s(t)

Short PN_Q

Generator

FIR

MUXB

1.2288 Mcps

800 bps

user m

19.2 Ksps

Power

control bit

800 Hz

1.2

288 M

cps

1.2288 Mcps

1.2288 Mcps

Fig. 5.6 Forward traffic channel modulation for RS1.

traffic Ch. gainsymbol

mapping

Forward Link Channel Coding [2]

� The sync, paging, and forward traffic channels shall be convolutionally

encoded 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. 5.7 Convolutional encoder.

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.

� 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.

� For reading, the mapping of addresses shall be performed as c0=>a6,

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


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.)

Table. 5.2 Read operation for 128 symbols.


Forward Link Data Scrambling [2]


Fig. 5.8 Data scrambling.

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 = 26.666…ms) due to zero

insertion.

� The I and Q channel spreading is used to mitigate the multipath

interference and other-cell interference.

� The characteristic polynomials of the PN sequences are



( )

( )

15 13 9 8 7 5

15 12 11 10 6 5 4 3

1

1

I

Q

P x x x x x x x

P x x x x x x x x x

= + + + + + +

= + + + + + + + +

Forward Link Quadrature Spreading (cont.)


Fig. 5.9 Forward channel signal constellation and phase transition.

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 δδδδ1=1.5 dB (pass band ripple), δδδδ2=40 dB, fp=590 kHz, fs=740 kHz.


Fig. 5.10 Frequency response specifications of a pulse-shaping filter.

Forward Link Pulse-Shaping Filter (cont.)

Table 5.3 48 tap coefficients of the sample pulse-shaping filter with four

times over-sampling.




Fig. 5.11 Frequency response of the sample pulse-shaping filter.



Fig. 5.12 Signal waveform after pulse-shaping.

Reverse Link Structure [1],[2]

� The IS-95 reverse link is composed of access channels and reverse traffic

channels.

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

period of 242----1 Tc.

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

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

� In IS-95 reverse link, the quadrature spreading by I/Q short PN codes is

employed, along with the direct spreading by the long PN code.


employed, along with the direct spreading by the 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, the noncoherent 64-ary orthogonal modulation

scheme is employed.

� In IS-95 reverse link, a convolutional code with rate 1/3 and constraint

length 9 is employed.

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


Fig. 5.13 An example of IS-95 reverse link channels.

Reverse 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 and

message capsule as shown in Fig. 5.14.

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

mobile.

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


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

and the minimum sizes are 1 and 3 frames, respectively.

� After transmitting an access probe, the mobile waits a specified period for an

acknowledgement 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. 5.15.

Reverse Link Structure: Access Channel (cont.)


Fig. 5.14. Access probe structure.



Fig. 5.15. Access probe sequence (ALOHA).


Short PN_I

Generator cosωct

Long Code

GeneratorPublic Long

Code Mask

1.2288 Mcps1.2288 Mcps


Fig. 5.16 Access channel modulation.

FIR

Σ

-sinωct

s(t)

Short PN_Q

Generator

FIR

A

GeneratorCode Mask 1.2288 Mcps

1.2288 Mcps

D

1/2 Tc

Ch. gainsymbol

mapping

Reverse Link Structure: Traffic Channel

� Transmits user information such as voice and data.

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

� Reverse Traffic Channel Data Rate

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

� 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 the closed-loop power control in


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

reverse link.

Reverse Link Structure: Traffic Channel

Short PN_I

Generator cosωct

Long CodeUser Long Code 1.2288 Mcps


Fig. 5.17 Reverse traffic channel modulation for RS1.

FIR

Σ

-sinωct

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

Ch. gainsymbol

mapping

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 g1

equals 663(octal), and g2 equals 711(octal).


Fig. 5.18 k=9, rate 1/3 convolutional encoder.

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 c5represents the latest (or most recent) and c

0the first (or oldest)

543210 3216842Index Symbol Modulation cccccc +++++=


where c5represents the latest (or most recent) and c

0the 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 µµµµs).

32 32

2 64

32 32

N N

N

N N

H H H HH H

H H H H

= ⇒ =

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

� Prior to transmission, the reverse traffic channel and the access channel

shall be spread by either a private user long code or a public long code for

channel identification.

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


Reverse Link Quadrature Spreading [1],[2]

� Following the direct sequence spreading, the reverse traffic channel and

access channel are spread in quadrature to mitigate the other user

interference and other cell interference.

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

the forward link channel.


( ) 15 13 9 8 7 5 1P x x x x x x x= + + + + + +


� The data spread by the Q channel PN sequence shall be delayed by half

PN chip time and a resulting signal constellation is that of OQPSK, as

shown in Fig. 5.19.

( )

( )

15 13 9 8 7 5

15 12 11 10 6 5 4 3

1

1

I

Q

P x x x x x x x

P x x x x x x x x x

= + + + + + +

= + + + + + + + +

Reverse Link Quadrature Spreading (cont.)


Fig. 5.19 Reverse CDMA channel signal constellation and phase transition.

References

1. Samuel C. Yang, CDMA RF System Engineering, Artech House, 1998.

2. Qualcomm, CDMA System Training Handbook-vol. 1, 1993.


Documents

CH 5. Air Interface of the IS-95A CDMA System