CH 5. Air Interface of the IS-95A CDMA
System
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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
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� 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
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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
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� 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.)
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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.
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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
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� 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.
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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.
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� 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.
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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
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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
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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.
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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.
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Forward Link Block Interleaving (cont.)
Table. 5.1 Write operation for 128 symbols with two time repetition.
Address 0
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Address 127
Forward Link Block Interleaving (cont.)
Table. 5.2 Read operation for 128 symbols.
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Forward Link Data Scrambling [2]
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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
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� 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.)
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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.
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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.
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Forward Link Pulse-Shaping Filter (cont.)
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Fig. 5.11 Frequency response of the sample pulse-shaping filter.
Forward Link Pulse-Shaping Filter (cont.)
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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.
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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]
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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
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� 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.)
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Fig. 5.14. Access probe structure.
Reverse Link Structure: Access Channel (cont.)
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Fig. 5.15. Access probe sequence (ALOHA).
Reverse Link Structure: Access Channel (cont.)
Short PN_I
Generator cosωct
Long Code
GeneratorPublic Long
Code Mask
1.2288 Mcps1.2288 Mcps
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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
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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
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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).
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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 +++++=
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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.
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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.
� The characteristic polynomials of the PN sequences are
( ) 15 13 9 8 7 5 1P x x x x x x x= + + + + + +
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� 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.)
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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.
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