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8/2/2019 CH4. is-95 System
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1Korea Aerospace University Mobile Communications Lab.
CH 4. Air Interface of the IS-95A CDMASystem
2Korea Aerospace University Mobile Communications Lab.
ContentsContents
Summary of IS-95A Physical Layer Parameters
Forward Link Structure
Pilot, Sync, Paging, and Traffic Channels
Channel Coding, Interleaving, Data Scrambling, and Modulation
Power Control Sub-channel, Spreading, and Pulse-Shaping
Reverse Link Structure
Access and Traffic Channels
Channel Coding, Interleaving, and Modulation
Burst Transmission, Direct and Quadrature Spreading
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3Korea Aerospace University Mobile Communications Lab.
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 and
13.35 / 6.25 / 2.75 / 1.05 kbps for rate set 2) and EVRC
Power Control Forward : Power AllocationReverse : Closed loop ( Rate : 800 Hz ) + Open loop + Outer loop
Diversity Forward : Path + Time + Space (Handover) diversityReverse : Path + Time + Space (Antenna, Handover) diversity
4Korea Aerospace University Mobile Communications Lab.
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. 4.1,
1 pilot channel 1 sync channel
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-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.
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5Korea Aerospace University Mobile Communications Lab.
Forward Link Structure (cont.)Forward Link Structure (cont.)
Fig. 4.1 An example of IS-95 forward link channel assignments.
6Korea Aerospace University Mobile Communications Lab.
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. 4.2 Pilot channel modulation.
0 0 0 0 0 0 0 .
symbolmapping pilot gain
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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 syncchannel 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
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.
8Korea Aerospace University Mobile Communications Lab.
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)
Symbol
Repetition
Block
InterleaverA
4.8 ksps2.4 ksps
Sync
Channel
Data 4.8 ksps
Walsh
32
Fig. 4.3 Sync channel super frame.
Fig. 4.4 Sync channel modulation.
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9Korea Aerospace University Mobile Communications Lab.
Forward Link Structure: Paging ChannelForward Link Structure: Paging Channel
The paging channel is used to transmit control information from the basestation 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 topages 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 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 will
receive control messages.
10Korea Aerospace University Mobile Communications Lab.
Forward Link Structure: Paging Channel (cont.)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 cell list, etc.
Fig. 4.5 Paging channel modulation.scrambling
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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) Rate Set 1: 9600, 4800, 2400, and 1200 bps
Rate Set 2: 14400, 7200, 3600, and 1800 bps
When signaling data is to be transmitted, data rate is always changed to
the full rate (9600 or 14400).
There are two options to transmit signaling data: Blank and burst, Dim andburst.
Blank and burst: The entire traffic channel frame is used to send only
signaling data.
Dim and burst: The traffic channel frame is used to send both primary
traffic and signaling data.
12Korea Aerospace University Mobile Communications Lab.
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_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. 4.6 Forward traffic channel modulation for RS1.
traffic Ch. gainsymbolmapping
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13Korea Aerospace University Mobile Communications Lab.
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 kbps1.4 kbps
ADD Tail
8 Bits
14.4 kbps
7.2 kbps
3.6 kbps1.8 kbps
Convolutional
Encoding
(r=1/2, k=9)
Symbol
RepetitionBlock
InterleaverB
28.8 ksps28.8 ksps
14.4 ksps
7.2 ksps3.6 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_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. 4.7 Forward traffic channel modulation for RS2.
traffic Ch. gainsymbolmapping
14Korea Aerospace University Mobile Communications Lab.
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. 4.8 Convolutional encoder.
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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 spanning26.6666 ms which involves 128 modulation symbols.
The 128 input symbols are written into a linear array with addresses viewedby 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.
16Korea Aerospace University Mobile Communications Lab.
Forward Link Block Interleaving (cont.)Forward Link Block Interleaving (cont.)
Table. 4.1 Write operation for 128 symbols with two time repetition.
Address 0
Address 127
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Forward Link Block Interleaving (cont.)Forward Link Block Interleaving (cont.)
Table. 4.2 Read operation for 128 symbols.
18Korea Aerospace University Mobile Communications Lab.
Forward Link Data Scrambling [2]Forward Link Data Scrambling [2]
Fig. 4.9 Data scrambling.
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Forward Link Power Control Sub-channel [2]Forward Link Power Control Sub-channel [2]
Fig. 4.10 Position of power control bits.
0 1
1011
20Korea Aerospace University Mobile Communications Lab.
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 zero
insertion.
The I and Q channel spreading is used to mitigate multipath interferenceand 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
= + + + + + +
= + + + + + + + +
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21Korea Aerospace University Mobile Communications Lab.
Forward Link Quadrature Spreading (cont.)Forward Link Quadrature Spreading (cont.)
Fig. 4.11 Forward channel signal constellation and phase transition.
22Korea Aerospace University Mobile Communications Lab.
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 (passband ripple), 2=40 dB, fp=590 kHz, fs=740 kHz.
Fig. 4.12 Frequency response specifications of a pulse-shaping filter.
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Forward Link Pulse-Shaping Filter (cont.)Forward Link Pulse-Shaping Filter (cont.)
Table 4.3 48 tap coefficients of the sample pulse-shaping filter with four
times over-sampling.
24Korea Aerospace University Mobile Communications Lab.
Forward Link Pulse-Shaping Filter (cont.)Forward Link Pulse-Shaping Filter (cont.)
Fig. 4.13 Comparison with a truncated ideal low-pass filter.
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Forward Link Pulse-Shaping Filter (cont.)Forward Link Pulse-Shaping Filter (cont.)
Fig. 4.14 Frequency response of the sample pulse-shaping filter.
26Korea Aerospace University Mobile Communications Lab.
Forward Link Pulse-Shaping Filter (cont.)Forward Link Pulse-Shaping Filter (cont.)
Fig. 4.15 Signal waveform after pulse-shaping.
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27Korea Aerospace University Mobile Communications Lab.
Reverse Link Structure [1],[2]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 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 constraint
length 9 is employed.
28Korea Aerospace University Mobile Communications Lab.
Reverse Link Structure (cont.) [1],[2]Reverse Link Structure (cont.) [1],[2]
Fig. 4.16 An example of IS-95 reverse link channels.
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29Korea Aerospace University Mobile Communications Lab.
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. 4.17.
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 framesand the minimum sizes are 1 and 3 frames, 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 previousone 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. 4.18.
30Korea Aerospace University Mobile Communications Lab.
Reverse Link Structure: Access Channel (cont.)Reverse Link Structure: Access Channel (cont.)
Fig. 4.17. Access probe structure.
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Reverse Link Structure: Access Channel (cont.)Reverse Link Structure: Access Channel (cont.)
Fig. 4.18. Access probe sequence (ALOHA).
32Korea Aerospace University Mobile Communications Lab.
Reverse Link Structure: Access Channel (cont.)Reverse Link Structure: Access Channel (cont.)
Fig. 4.19 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
Ch. gainsymbolmapping
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33Korea Aerospace University Mobile Communications Lab.
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
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 variablerate transmission, which is due to closed-loop power control in reverse
link.
When a signaling data is to be transmitted, the data rate is changed tothe full rate.
34Korea Aerospace University Mobile Communications Lab.
Reverse Link Structure: Traffic ChannelReverse Link Structure: Traffic Channel
Fig. 4.20 Reverse traffic channel modulation for RS1.
Short PN_I
Generator
FIR
cosct
-sin
ct
s(t)
Short PN_QGenerator
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. gainsymbolmapping
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35Korea Aerospace University Mobile Communications Lab.
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. 4.21 k=9, rate 1/3 convolutional encoder.
36Korea Aerospace University Mobile Communications Lab.
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
:bps1200At3228312730262925242023192218211716121511141013984736251
:bps2400At
3230312928262725242223212018191716141513121011986754231
:bps4800At
3231302928272625242322212019181716151413121110987654321
:bps9600At
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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 meansof 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 +++++=
32 322 64
32 32
N NNN N
H H H HH H
H H H H
= =
38Korea Aerospace University Mobile Communications Lab.
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. 4.22.
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 the data
burst randomizer.
For 4800 bps, transmission shall occur on power control groupsnumbered:
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 11
if 0, or 2+ if 1,
4 if 0, or 6+ if 1,8 if 0, or 10+ if 1,
12 if 0,
b b b b
b b b bb b b b
b b
= =
+ = =+ = =
+ = 7 11or 14+ if 1.b b =
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Reverse Link Burst Transmission (cont.)Reverse Link Burst Transmission (cont.)
Fig. 4.22 Reverse Traffic Channel variable rate transmission.
40Korea Aerospace University Mobile Communications Lab.
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.
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Reverse Link Quadrature Spreading [1],[2]Reverse Link Quadrature Spreading [1],[2]
Following the direct sequence spreading, the reverse traffic channel and
access 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 half a
PN chip and a resulting signal constellation is that of OQPSK, as shown inFig. 4.23.
( )
( )
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
= + + + + + +
= + + + + + + + +
42Korea Aerospace University Mobile Communications Lab.
Reverse Link Quadrature Spreading (cont.)Reverse Link Quadrature Spreading (cont.)
Fig. 4.23 Reverse CDMA channel signal constellation.
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ReferencesReferences
1. Samuel C. Yang, CDMA RF System Engineering, Artech House, 1998.
2. Qualcomm, CDMA System Training Handbook-vol. 1, 1993.