Digital AMPS and code division multiple access

A digital cellular system based on CDMA which promises increased capacity and has been standardized as Interim Standard 95 (IS-95) by the U.S Telecommunications Industry Association (TIA).

IS-95 allows each user within a cell to use the same radio channel, and users in adjacent cells also use the same radio channel, since this is a direct sequence spread spectrum CDMA system .

The IS-95 system is designed to be compatible with the existing U.S. analog cellular system (AMPS) frequency band, hence mobiles and base stations can be economically produced for dual mode operation.

To facilitate graceful transition from AMPS to CDMA, each IS-95 channel occupies 1.25 MHz of spectrum on each one-way link.

1. GSM
PDC
cdmaOne
TDMA
(IS-136)
GPRS
EDGE
cdma2000
3G
Today 2G
19.2kbps
2000 evolved 2G
64--115kbps
3G
115--384kbps | 0.384--2Mbps
IMT-2000
Capable Systems
New SpectrumExisting Spectrum
TDMA/
GPRS
TDMA/
EDGE
Evolution to 3G

2. AMPS is an analog cellular phone
system using FDMA.
Note

3. Cellular bands for AMPS

4. AMPS reverse communication band

5. Second-generation cellular phone systems

6. •
Digital Advanced Mobile Phone System (D-AMPS)is a digital version of Advanced
Mobile Phone Systems (AMPS), the original analog standard for cellular phones.
•
D-AMPS combines time division multiple access (TDMA) and frequency division
multiple access (FDMA).
•
It adds TDMA to get three channels per AMPS channel, thus tripling the number of
calls on a channel.
D-AMPS - Digital Advanced Mobile Phone System

7. •
They are standardised by IS-54 and IS-136.
•
As in AMPS, D-AMPS also uses frequencies from 800 to 900MHz for
transmission. Half of the spectrum is used for sending signals and the
other half for receiving signals.
•
The frequency band is divided into 30KHz sub-bands, called channels, by
FDMA.
•
The channels for uplink are called forward channels and the channels for
downlink are called reverse channels.
•
TDMA is applied to each channel thus tripling the number of available
channels.
•
Presently, all D-AMPS channels are replaced by GSM or CDMA
technology.
Features:

8. D-AMPS
D-AMPS, or IS-136, is a digital cellular
phone system using TDMA and FDMA.

9. Rules for Efficient Multiple
Access 
Three laws

Know the channel

Minimize interference to others

Mitigate interference received from others

Requirements of wireless multiple access

Channel measurement

Channel control and modification

Multiple user channel isolation

11. Introduction:
Code division multiple access (CDMA) is a fixed assignment access technique
that uses spread spectrum and a special kind of coding scheme to allow the
transmitters to share the media at the same time.
Why it is considered as a form of spread spectrum technique?
It is considered as a form of the spread-spectrum techniques, because the
modulated coded signal has a higher bandwidth than the data being
transmitted.

12. Introduction (Cont.):
How does it support more than one group in the same channel?
It allows multiple groups to share the same channel at the same time by
assigning each group to a different code.
CDMA Modulation Steps:
1.
Generate a local pseudo-random code with a higher rate than the data to
be transmitted.
2.
XOR the data which need to be transmitted with the generated
code.

13. T
b
Tc
Data Signal
Pseudo-random
Code
Transmitted Signal
Time
Introduction (Cont.):

14. Channel
Encoder
Modulator Channel Demodulator
Channel
Decoder
Pseudo-noise GeneratorPseudo-noise Generator
General Model of Spread Spectrum Digital Communication System
Input
data
Output
data
Introduction (Cont.):

15. Introduction (Cont.):
What can we gain from the spreading of the spectrum?

The immunity from the various types of noise.

The immunity from the multipath distortion.

It can be used as an encrypting technique.

Several users can use the same channel with little interference.

16. Introduction (Cont.):
Generating Pseudo-Random Codes:
The base station generates a unique pseudo-random code for every connection
and these codes should have the following properties:

It should be deterministic.

Itmustappear randomto a listenerwhodoesnot
have prior knowledge of the code.

The cross-correlation between any two codes must be small.

The code should have a long period.

17. Introduction (Cont.):
Code Correlation:

Each user uses different code to modulate his signal.

The best performance will be when the users’ signals are separated
as much as possible.

The separation can be done by correlating the received signal with the local
generated code.

If the correlation was high, then we exclude this code and we search for
another code with less correlation.

18. Introduction (Cont.):
Correlation Functions:
1.
Cross-Correlation: The correlation of two different codes. “Used to
separate the transmitters’ signals from each other”.
2.
Auto-Correlation: The correlation of a code with a time-delayed version of
itself. “Used to reject multipath interference”.

19. Spread Spectrum Methods:
Direct Sequence Spread Spectrum (DSSS):

Eachbit from the data whichneedto be sentis
representedby multiple bits in the transmitted data, by using the
spreading code.

The originaldata is spread out across a widerfrequency band by
using the spreading code.

The spreading proportion depends on the number of bits used in the code.

20. T
b
Tc
Original
Data
Pseudo- random
Bit
Transmitted Data
Time
1 1 1
0 0
01101011001111011010
01100100001100100101
Example of Direct Sequence Spread Spectrum

21. Spread Spectrum Methods
(Cont.):
Frequency Hopping Spread Spectrum (FHSS):

At the transmitter, the original signal is broadcasted over a series of radio
frequencies, hopping from different frequencies in a fixed pattern.

The receiver should use the same hopping pattern simultaneously with the
transmitter in order to receive the data correctly.

The spreading code specifies the sequence of channels and the receiver
should use the same code to tune into the sequence of channels that are used
by the sender.

22. f1
f2
f3 f4
f5
f6 f7
f8
(a) Channel assignment
f8
f7
f6
f5
f4
f3
f2
f1
1
2
3
4
5
6
7
8
Example of Frequency Hoping Spread Spectrum
(b) Channel use
Frequency
Frequency
Time
Energ
y

23. Spread Spectrum Methods
(Cont.):
Chirp Spread Spectrum (CSS):

Itis thefirstandsimplest applicationof
MultiDimensionalMultiple Access (MDMA) technology.

It encodes the information by using wideband linear frequency modulated
chirp.

It uses the entire allocated bandwidth to broadcast the signal, which helps
making it robust to channel noise.

Also, it is immunized from the multi-path fading, because it utilizes the
spectrum broadband.

It finds the noise by relying on the linear nature of the chirp pulse.

24. Spread Spectrum Methods
(Cont.):
Time Hopped Spread Spectrum (THSS):

The period and duty cycle of a pulsed radio frequency carrier are varied
pseudo-randomly under the control of coded sequence similar to FHSS.

It consumes low power due to the very low duty cycles.

In THSS, the pseudo random number determines the sequence of
how the burst will be transmitted.

25. Time-Hopping Spread Spectrum
Time
f
T f 2T 3T f
f
4T0
Transmitted time slots (k bits)
Frame
t

26. IS-95 (CDMA ONE) forward transmission
IS-95 is a digital cellular phone system
using CDMA/DSSS and FDMA.

27. IS-95 (CDMA ONE) reverse transmission

28. cdma2000 Uplink Frame Structure
CRC
encoder
tail bits
Convolutional
or Turbo Coder
symbol
repetition
symbol
puncture
block
interleaver
modulation
symbol
C
channel
bits
Bits/
Frame
16
40
80
172
350
744
1512
3048
6120
CRC bits
6
6
8
12
16
16
16
16
16
tail
bits
8
8
8
8
8
8
8
8
8
Data Rate
kbps
1.5
2.7
4.8
9.6
19.2
38.4
76.8
153.6
307.2
Code
Rate
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/2
Repeats
16
8
4
2
1
1
1
1
1
Delete
1 of 5
1 of 9
none
none
none
none
none
none
none
Symbols
1536
1536
1536
1536
1536
3072
6144
12288
12288
Radio Configuration 3
C.S.0002-A-1 Fig 2.1.3.1.1.1-8

29. cdma2000 Uplink Modulator
C
C
C
A
B


d

d

d
c

)cos(t
c

)sin(t
c
Ilong,
c
Qlong,
c
+
+
+
+

+
_
s
u
m
s
u
m
1
Dw
Cw
2

S
w
1
Sw
Primary
Traffic
Secondary
Traffic 1
Secondary
Traffic 2
Control
Pilot
pulse
shape
pulse
shape

30. cdma2000 Downlink Frame Structure
Other similar tables in specification.
Bits/
Frame
21
55
125
267
552
1128
2280
4584
9192
20712
CRC bits
6
8
10
12
16
16
16
16
16
16
tail
bits
8
8
8
8
8
8
8
8
8
8
Data Rate
kbps
1.8
3.6
7.2
14.4
28.8
57.6
115.2
230.4
460.8
1036.8
Code
Rate
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
1/2
Repeats
8
4
2
1
1
1
1
1
1
1
Delete
none
none
none
none
none
none
none
none
none
2 of 18
Symbols
576
576
576
576
1152
2304
4608
9216
18432
36864
Radio Configuration 9
CRC
encoder
tail bits
Convolutional
or Turbo Coder
symbol
repetition
symbol
puncture
block
interleaver
modulation
symbol
channel
bits
W

31. cdma2000 1X DL Modulation
Processing
+
long
code
long
code
mask
0  1
1  -1
channel
gain
fwd pwr
ctrl gain
p
o
w
e
r

c
o
n
t
r
o
l
s
y
m
b
o
l

p
u
n
c
t
u
r
e
D
E
M
U
X
decimate
pwr ctrl
bit pos
I/Q scrambling
bit extract
power
control
bits
puncture
timing
800 Hz
W
YQ
YI
C.S.0002-A-1 Fig 3.1.3.1.1.1-18
modulation
symbol
rate

32. cdma2000 1X Downlink Modulation
I channel
pilot PN
Q channel
pilot PN
Walsh code
QOF code
I
Y
Q
Y





pulse
shape
pulse
shape
+
+
+
-
)cos(t
c
)sin(t
c

other
channels
other
channels
p
h
a
s
e


r
o
t
a
t
e

f
o
r

Q
O
F
QX
I
X

33. cdma2000 3X DL Modulation
Processing
+
long
code
long
code
mask
0  1
1  -1
channel
gain
fwd pwr
ctrl gain
p
o
w
e
r

c
o
n
t
r
o
l
s
y
m
b
o
l

p
u
n
c
t
u
r
e
D
E
M
U
X
decimate
pwr ctrl
bit pos
I/Q scrambling
bit extract
power
control
bits
puncture
timing
800 Hz
W
YQ1
YI1
modulation
symbol
rate
YQ2
YI2
YQ3
YI3

34. cdma2000 3X Downlink Modulation
same as below
same as above
output
carrier 1
output
carrier 2
output
carrier 3
YQ1
YI1
YQ2
YI2
YQ3
YI3

35. W-CDMA Versus cdma2000
Parameter W-CDMA cdma2000
Carrier spacing 5 MHz 3.75 MHz
Chip rate 4.096 MHz 3.6864 MHz
Data modulation BPSK FW – QPSK; RV - BPSK
Spreading Complex (OQPSK) Complex (OQPSK)
Power control frequency 1500 Hz 800 Hz
Variable data rate implement.Variable SF; multicode Repet., puncturing, multicode
Frame duration 10 ms 20 ms (also 5, 30, 40)
Coding Turbo and convolutionalTurbo and convolutional
Base stations synchronized?Asynchronous Synchronous
Base station acquisition/detect3 step; slot, frame, codeTime shifted PN correlation
Forward link pilot TDM dedicated pilot CDM common pilot
Antenna beam forming TDM dedicated pilot Auxiliary pilot

36. cdma2000 vs WCDMA

Chip rate

Coherent Pilot Channels

Transmit Diversity

Underlying Network

Single Carrier versus Multicarrier
Spreading

Cell Site Synchronization