IS95 CDMA Technology

Brief overview of CDMA tech..!!

1. CDMA Technology & IS-95

2. What is CDMA „
Both an access method and air-interface
„
Rest of the network is very similar
„
Radio resource management, mobility
management, security are similar
„
Power control and handoffs are different
„
Uses DSSS and ECC
„
Frequency reuse factor is 1
„
3 systems
„
IS-95 2G, W-CDMA, and CDMA2000

3. Advantages of CDMA Cellular „
Higher capacity
„
Improves voice quality (new coder)
„
Soft-handoffs
„
Less power consumption (6-7 mW)
„
Choice for 3G systems

4. Advantages of CDMA Cellular „
Frequency diversity –frequency-dependent
transmission impairments have less effect on
signal
„
Multipath resistance –chipping codes used for
CDMA exhibit low cross correlation and low
autocorrelation
„
Privacy –privacy is inherent since spread
spectrum is obtained by use of noise-like signals
„
Graceful degradation –system only gradually
degrades as more users access the system

5. Drawbacks of CDMA Cellular „
Self-jamming –arriving transmissions from
multiple users not aligned on chip boundaries
unless users are perfectly synchronized
„
Near-far problem –signals closer to the receiver
are received with less attenuation than signals
farther away
„
Soft handoff –requires that the mobile acquires
the new cell before it relin quishes the old; this is
more complex than hard handoff used in FDMA
and TDMA schemes

6. Drawbacks of CDMA Cellular „
Air-interface is the most complex
„
Not symmetrical (unlike TDMA)
„
Forward and reverse channels are different
„
Forward channel (1ÆMany) synchronized
„
Forward channel uses orthogonal spreading codes
„
Reverse channel transmissions are not
synchronized
„
Orthogonal codes are used for orthogonal
waveform coding

7. Mobile Wireless CDMA Design
Considerations „
RAKE receiver –when multiple versions of a
signal arrive more than one chip interval apart,
RAKE receiver attempts to recover signals from
multiple paths and combine them
„
This method achieves better performance than simply
recovering dominant signal and treating remaining
signals as noise
„
Soft Handoff –mobile station temporarily
connected to more than one base station
simultaneously

8. Principle of RAKE Receiver

9. IS-95 CDMA Forward Channel „
The forward link uses the same frequency spectrum as AMPS
(824-849 Mhz)
„
Each carrier 1.25MHz
„
4 types of logical channel: A pilot, a synchronization, 7 paging,
and 55 traffic channels
„
Channels are separated using different spreading codes
„
QPSK is the modulation scheme
„
Orthogonal Walsh codes are used (64 total)
„
After orthogonal codes, they are further spread by short PN
spreading codes
„
Short PN spreading codes are M sequences generated by LFSRs
of length 15 with a period of 32768 chips.

10. Forward channel-2 „
Why we have two spreading codes?
„
The orthogonal codes are used to differentiate
between the transmissions within a cell
„
The PN spreading codes are used to isolate different
cells (BSs) that are using the same frequencies.
„
The same PN sequence is used in all BSs.
„
The offset for each BS is different. Of course, this
requires synchronization
„
Synchronization is achieved by GPS.

11. One Forward CDMA Link, 1.25 MHz in the 824 –
849 MHz bands
PCH
1
PCH
7
Code
1
Code
N
Code
P
Code
S
Code
55
Pilot
Synch
W
0
W
32
W
1
W
7
W
8
Funda
ment
a
l

Code Channel
Data
Mobile Pow
e
r
Control
Subchannel
Funda
ment
a
l

Code Channel
Data
Supple
m
ent
a
ry
Code Channel
Data
W
63
Mobile Pow
e
r
Control
Subchannel
Figure 8.4: IS-95 Forward Channel

12. 1.2288
Mcps
I Pilot PN at 1.288 Mcps
Walsh Code
Ba
seband Filter
Channel
Dependent
Symbols
Ba
seband Filter
Q Pilot PN at
1.288 Mcps
Figure 8.5: Basic Spreading Procedure on the
Forward Channel in IS-95

13. The pilot channel „
Provide a reference signal for all MSsthat
provides the phase reference for COHERENT
demodulation
„
4-6 dB stronger than all other channels
„
Used to lock onto other channels
„
Obtained using all zero Walsh code; i.e.,
contains no information except the RF carrier
„
Spread using the PN spreading code to
identify the BS. (512 different BS*64 offsets)
„
No power control in the pilot channel

14. To QPSK Modulator
Ba
seband Filter
Ba
seband Filter
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Walsh Code W
0
All 0s
BBF BBF
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Walsh Code W
32
Convolutional
En
cod
e
r
Sy
nch
C
hann
e
l
Me
ssage
1
.
2 k
s
ps
2.
4
ks
p
s
4.
8
ks
p
s
C
ode
Symbol
(a)
(b)
4.
8
ks
p
s
Symbol
Repetition
Bloc
k
Interleaver
Modu
la
ted
Symbol
Rat
e
1/
2
Figure 8.6: (a) Pilot and (b) Sync Channel Processing in IS -95

15. Sync channel „
Used to acquire initial time synchronization
„
Synch message includes system ID (SID),
network ID (NID), the offset of the PN short
code, the state of the PN-long code, and the
paging channel data rate (4.8/9.6 Kbps)
„
Uses W32 for spreading
„
Operates at 1200 bps

16. To QPSK Modulator
Ba
seband Filter
Ba
seband Filter
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Walsh Code W
0
All 0s
BBF BBF
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Walsh Code W
32
Convolutional
En
cod
e
r
Sy
nch
C
hann
e
l
Me
ssage
1
.
2 k
s
ps
2.
4
ks
p
s
4.
8
ks
p
s
C
ode
Symbol
(a)
(b)
4.
8
ks
p
s
Symbol
Repetition
Bloc
k
Interleaver
Modu
la
ted
Symbol
Rat
e
1/
2
Figure 8.6: (a) Pilot and (b) Sync Channel Processing in IS -95

17. Paging channels „
Used to page the MS in case of an incoming
call, or to carry the control messages for call
set up
„
Uses W1-W7
„
There is no power control
„
Additionally scrambled by PN long code,
which is generated by LFSR of length 42
„
The rate 4.8 Kbps or 9.6Kbps

18. I Pilot PN at 1.2288 Mcps
Walsh Code W
1-
7
Convolutional
En
cod
e
r
Pag
ing
C
hann
e
l
Me
ssage
4.
8
or 9
.
6
ks
p
s
Rat
e
1/
2
9
.
6 o
r
19.2
ks
p
s
19.2 ks
p
s
Long Code De
cimator
Long Code Generator
C
ode
Symbol
Modu
la
ted
Symbol
64
:1
1
.
228
8
Mcp
s
Long Code
Ma
s
k
For
P
a
ging Ch
an
nel
1.2288
Mcps
BBF
19.2 ks
p
s
Symbol
Repetition
Bloc
k
Interleaver
BBF
19.2 ks
p
s
Q Pilot PN at 1.2288 Mcps
Figure 8.7: Paging Channel Processing in IS -95

19. The traffic channels „
Carry user information
„
Two possible date rates
„
RS1={9.6, 4.8, 2.4, 1.2 Kbps}
„
RS2={14.4, 7.2, 3.6, 1.8 Kbps}
„
RS1 is mandatory for IS-95, but support
for RS2 is optional
„
Also carry power control bits for the
reverse channel

20. Convolutional
En
cod
e
r
Vo
ic
e
Traffic
Rat
e
1/
2
BBF BBF
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Walsh Code W
i
MUX
19.2 ks
p
s
800 bps
Po
we
r
C
ontro
l
Bits
800 bps
L
ong
Code
Decima
to
r
L
ong
Code
Generato
r
64
:1
1
.
228
8
Mcp
s
Long Code
Ma
s
k
19.2 ks
p
s
L
ong
Code
Decima
to
r
Bloc
k
Interleaver
Symbol
Repetition
19.2 ks
p
s
24
:1
Figure 8.8: Forward Traffic Channel Processing in IS –95 (Rate Set 1)

21. Convolutional
En
cod
e
r
Vo
ic
e
Traffic
Rat
e
1/
2
Symbol
Repetition
I Pilot PN at 1.2288 Mcps
1.2288
Mcps
BBF BBF
Q Pilot PN at 1.2288 Mcps
Walsh Code W
i
MUX
L
ong
Code
Decima
to
r
L
ong
Code
Generato
r
64
:1
1
.
228
8
Mcp
s
Long Code
Ma
s
k
19.2 ks
p
s
L
ong
Code
Decima
to
r
Bloc
k
Interleaver
19.2 ks
p
s
Po
we
r
C
ontro
l
Bits
800 bps
P
u
nc
ture
2
of
Ever
y 6 inputs
19.2 ks
p
s
800 bps
24
:1
Figure 8.9: Forward Traffic Channel Processing in IS –95 (Rate Set 2)

22. IS-95 CDMA Reverse Channel „
Fundamentally different from the forward channels
„
Uses OQPSK for power efficiency
„
QPSK demodulation is easy
„
869-894 MHz range.
„
No spreading of the data using orthogonal codes
„
Same orthogonal codes are used for WAVEFORM
encoding
„
Two types of logical channels: The access channels
and the reverse traffic channels

23. One Reverse CDMA Link, 1.25 MHz in the
869 –
894 MHz
Access Channel
PCH1
Funda
ment
a
l

Code Channel
Data
Supple
m
ent
a
ry
Code Channel
Data
Supple
m
ent
a
ry
Code Channel
Data
Supple
m
ent
a
ry
Code Channel
Data
Supple
m
ent
a
ry
Code Channel
Data
Figure 8.10: IS-95 Reverse Channel
Access Channel
1
Access Channel
PHCP
Access Channel
PHCP
Traffic
Channel
1
Traffic
Channel
T

24. 000
001
100
010
110
101
111
011
W
4
W
0
W
1
W
5
W
2
W
6
W
3
W
7
Figure 8.11: Mapping data bits to Walsh encoded symbols

25. BBF BBF
I Pilot PN at 1.2288 Mcps Q Pilot PN at 1.2288 Mcps
1.2288
Mcps
Long Code Generator
Long Code
Ma
s
k
Bloc
k
Interleaver
Symbol
Repetition
Convolutional
En
cod
e
r
4
.
8 k
s
ps
14.4 ks
p
s
Access
Me
ssage
28.8 ks
p
s
64-ary
Orthogonal Modul
a
tor
Rat
e
1/
3
Figure 8.12: Access Channel Processing in IS-95