Code Division Multiple Access- CDMA

CDMA – CODE DIVISION
MULTIPLE ACCESS
VIVEK PATEL
CWID- 10404232
17/30/2015 EE 583 Code Division Multiple Access

Outline:
 History And Introduction
 Multiple Access Method –
FDMA/TDMA/CDMA
 Comparison
 Spread Spectrum
 CDMA Introduction
 IS 95-CDMA
 General Specification
 How CDMA Works?
 Types Of CDMA
 Architecture
 CDMA Architecture
 Spreading Codes
 Walsh Codes
 PN Sequences And
Orthogonal
 CDMA Frame Structure
 CDMA Capacity
 Power control
 Rake receiver
 Handoffs
 Detection
 Third Generation 3G
 Commercial use
 Advantage And Disadvantage
Of CDMA
 Conclusion
 References

1st.Generation
(1980s)
Analog
NMT CT0
TACS CT1
AMPS
3rd.
Generation
(2000s)2nd.
Generation
(1990s)
Digital
GSM DECT
DCS1800 CT2
PDC PHS
IS-54
IS-95
IS-136
UP-PCS
IMT-2000
CDMA2000
W-CDMA
 Evolution of Cellular Systems :
7/30/2015 EE 583 Code Division Multiple Access 3

Time Time Time
Freq Freq Freq
PN Code
FDMA TDMA CDMA
 Access Technology :

 FDMA :
•In FDMA, the entire allocated cellular frequency
spectrum is divided into a number of 30-kHz
channels .
•The power transmitted by a cell is only large
enough to communicate with mobile stations
located near the edge of the cell’s coverage area.
•The radius of a cell might be one mile or less-
referred to as a small cell.

6
 FDMA
MS #1
MS #2
MS #n
BS
f1’
f2’
fn’
f1
f2
fn
…
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)

7
 FDMA: Channel Structure
1 2 3 … N
Frequency
Total Bandwidth W=NWc
Guard Band Wg
4
Sub Band Wc
Frequency
Protecting bandwidth
…
f1’ f2’ fn’
…
f1 f2 fn
Reverse channels Forward channels

 TDMA :
•TDMA is a digital wireless air interface .
•It divides each carrier frequency into a number of
time slots, each of which constitutes an independent
telephone circuit.
C B A C B A C B A C B A
C
A
B
Time
f0
Frequency

9
MS #1
MS #2
MS #n
BS
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)
t
Frequency f ’
#1
…
#1
…
Frame
Slot
…
#1
…
#1
Frame
…
t
Frequency f
Frame Frame
…
t
#2
…
#2
…
…
t
#n
… #n
…
…
#2
…
#2
…
t
…
#n
…
#n
…
t
 TDMA:

10
 TDMA: Channel Structure
… t
f
#1
#2
#n
#1
#2
#n
…
(a). Forward channel
…
#1
#2
#n
Frame FrameFrame
… t
f ’
#1
#2
#n
#1
#2
#n
…
(b). Reverse channel
…
#1
#2
#n
Frame FrameFrame

11
 TDMA: Frame Structure (Cont’d)
…
Time
Frequency
f = f ’
#1
#2
#n
#1
#2
#n
…
Forward
channel
Reverse
channel
…
#1
#2
#n
Forward
channel
Frame Frame
#1
#2
#n
…
Reverse
channel
Channels in Simplex Mode

12
 TDMA: Frame Structure (Cont’d)
…
Time
Frequency#1
#2
#n
#1
#2
#n
… …
#1
#2
#n
Frame FrameFrame
Head Data
Guard
time

 CDMA :
•A digital multiple access technique specified by the
Telecommunications Industry Association (TIA) as
"IS-95.“
•One of the unique aspect of CDMA is that while
there are certainly limits to the number of phone
calls that can be handled by a carrier, this is not a
fixed number .
•Code division multiple access (CDMA) is a digital
air interface standard, claiming eight to fifteen times
the capacity of analog.

14
 Code Division Multiple Access (CDMA)
MS #1
MS #2
MS #n
BS
C1’
C2’
Cn’
C1
C2
Cn
…
…
…
Reverse channels
(Uplink)
Forward channels
(Downlink)
Frequency f ’
Note: Ci’ x Cj’ = 0, i.e., Ci’ and Cj’ are orthogonal codes,
Ci x Cj = 0, i.e., Ci and Cj are orthogonal codes
Frequency f

15
 Comparisons of FDMA, TDMA, and CDMA
Operation FDMA TDMA CDMA
Allocated
Bandwidth
12.5 MHz 12.5 MHz 12.5 MHz
Frequency reuse 7 7 1
Required channel
BW
0.03 MHz 0.03 MHz 1.25 MHz
No. of RF channels 12.5/0.03=416 12.5/0.03=416 12.5/1.25=10
Channels/cell 416/7=59 416/7=59 12.5/1.25=10
Control
channels/cell
2 2 2
Calls/RF Channel 1 4*
40**
Voice channels/cell 57x1=57 57x4=228 8x40=320
Sectors/cell 3 3 3
Voice calls/sector 57/3=19 228/3=76 320
Capacity vs FDMA 1 4 16.8

 Spread Spectrum Principles :
7/30/2015 16EE 583 Code Division Multiple Access
•Does not attempt to allocate disjoint frequency or time
slot resources
•Instead, this approach allocates all resources to
simultaneous users, controlling the power
transmitted by each user to the minimum required to
maintain a given SNR
•Each user employs a noise-like wideband signal
occupying the entire frequency allocation
•Each user contributes to the background noise
affecting all users, but to the least extent possible.
•This restriction on interference limits capacity, but
because time and bandwidth resource allocations are
unrestricted, the resulting capacity is significantly
higher than the conventional system

•Suppose each user use a wideband Gaussian noise
carrier
•Suppose each user’s transmission is controlled so that
all signals received at the BS are of equal power
•Let Psbe the power of each user, and the background
noise be negligible.
•Then the total interference power, I, presented to each
user’s demodulator is
I = (K-1) Ps (1) where K is the number of users

•Let’s say demodulator of each user operates at bit-
energy-to-noise-density level of Eb/N0.
•So the noise density received by each user’s
demodulator is N0 = I/W (2), where W Hz is the
bandwidth of the wideband noise carriers.
•The received energy per bit is the received signal
power divided by the data rate R (bits/s), i.e., Eb= Ps/R
(3)

•Combining (1), (2) and (3) we get
K – 1 = I/Ps = (W/R) / (Eb/N0) (4)
•If W >> R then the capacity of the system can be large
i.e., transmission bandwidth should be much larger
than the message bandwidth
•If Eb/N0is small, then also the capacity can be large.
(since Eb/N0 SNR, this means SNR should be as smallα
as possible)

 Spread Spectrum (1)Spread Spectrum (1) ::
S(f)
ff0
Before spreading
Signal
S(f)
ff0
After spreading
Signal
S(f)
ff0
After despreading
signal
Interfering noise
f
S(f)
f0
Before despreading
Signal
Interfering noise

 Spread Spectrum(2)Spread Spectrum(2) ::
Radio
Channel
Channel Coding
(SS)
Carrier
Modulation
DS-PN
Source
Coding
Transmit
Antenna
Radio
Channel
Channel
Coding
(SS)
Carrier
Modulation
DS-PN
Source
Coding
Transmit
Channel
Decoding
Carrier
Demodulation
DS-PN
Source
Decoding
Receive
Channel
Decoding
Carrier
Demodulation
DS-PN
Source
Decoding
Receive
Antenna
A B

 Spreading Spectrum (3)Spreading Spectrum (3)
“Principle of Using Multiple Codes”“Principle of Using Multiple Codes”
Spreading
Sequence
A
Spreading
Sequence
B
Spreading
Sequence
C
Spreading
Sequence
C
Spreading
Sequence
B
Spreading
Sequence
A
Input
Data
X
Recovered
Data
X
X+A X+A+B X+A+B+C X+A+B X+A
Spread-Spectrum Chip Streams
ORIGINATING SITE DESTINATION

 Spread Spectrum (4)Spread Spectrum (4)

 Advantages of Spread SpectrumAdvantages of Spread Spectrum::
1. Avoid interference arising from jamming signal or
multi-path effects.
2. Covert operation : Difficult to detect
3. Achieve Privacy: Difficult to demodulate,
Noise like signal.
4. Impossible to Eavesdrops on the signal expect using
the same PN sequence

 CDMA:
•Classification of CDMA Systems :
CDMA
one
CDMA
2000
IS95 IS95B JSTD 008
Narrow Band
Wide Band
CDMA SYSTEMS

7/30/2015 26
CDMA Evolution PathsCDMA Evolution Paths
2Mbps
153.6kbps
CDMA
1xRTT
CDMA
1xEV-DV
CDMA
IS-95
CDMA
1xEV-DO
CDMA
3x 5x
2G 2.5G
3G
9.6kbps
EE 583 Code Division Multiple Access

CDMA-Its History & StatusCDMA-Its History & Status

CDMA-Its History & StatusCDMA-Its History & Status
1993, the first CDMA standard IS-95 was issued;
In 1995, CDMA technology was put into commercialization in
Hong Kong and America on large scale;
In April, 2001, China Unicom began to construct CDMA
networks—the largest in the world (about 70Million line now);
At present, CDMA commercial networks are established in
about 40 countries or area, almost 20% of all users in the
world.

CDMA Subscriber Growth History:CDMA Subscriber Growth History:
Sept.1997 through Sept.2003Sept.1997 through Sept.2003

 IS-95 CDMA:
•IS-95 (cdmaone) 2G digital cellular standard
•Motivation
–Intended as a new system (greenfield) or
replacement for AMPS (not an upgrade)
–Increase system capacity
–Add new features/services

 IS-95 CDMA:
•History:
–1990 Qualcomm proposed a code division multiple
access (CDMA) digital cellular system claimed to
increase capacity by factor 20 or more
–Started debate about how CDMA should be
implemented and the advantages vs. TDMA
(religious tones to debate)
–1992 TIA started study of spread spectrum cellular
•Several alternative CDMA proposals floated – large
debate in the CTIA
–came down to Interdigital vs. Qualcomm
–Qualcomm proposal won7/30/2015 EE 583 Code Division Multiple Access 31

 IS-95 CDMA:
• 1993 TIA IS-95 code division multiple access
• (CDMA) standards completed
–1995 IS-95A enhanced revision
–ANSI J-STD-008 (IS-95b) is standard up banded to
1900 MHz PCS band
–1996 Commercial deployment in US (Sprint PCS)
–Most popular system in U.S. and Korea
–1997 IS-95 name changed to cdmaone
• IS-95 evolves to cdma 2000 in 2.5 and 3G

 IS-95 System Features:
•Digital Voice
–QCELP fixed rate 14.4Kbps coder
–variable rate QCELP coder: 9.6, 4.8, 2.4, 1.2 Kbps
•Use of voice activation to reduce interference
•As data rate reduces, the transmitter can reduce the
power to achieve the same error rates
•Dual Mode (AMPS/CDMA), Dual Band (900, 1900
MHz bands)
•Low power handsets (sleep mode supported)
•Soft Handoff possible
•Digital Data services (text, fax, circuit switched data)
•Advanced Telephony Features (call waiting, voice
mail, etc.)

IS-95 System Features:
•Security: CDMA signal + CAVE encryption
•Air Interface Standard Only
•Code Division Multiple Access/FDMA/FDD
•Traffic Channel
–Pair of 1.25 MHz radio channels (up/downlink)
–Several users share a radio channel separated by a
code not a timeslot or frequency!
–Receiver performs a time correlation operation to
detect only desired code word
–All other code words appear as noise due to
decorrelation
–Receiver needs to know only codeword and
frequency used by transmitter7/30/2015 EE 583 Code Division Multiple Access 34

IS-95 System Features:
–Adjust power often to prevent near –far problem
•Universal frequency reuse (frequency reuse cluster
size K =1)
–Simple planning
–large capacity increase

 IS-95 Radio Aspects:

 Code Division Multiple Access – CDMA:
•Multiple users occupying the same band by having
different codes is known as CDMA - Code Division
Multiple Access system
Let
W - spread bandwidth in Hz
R = 1/Tb = Date Rate
S - received power of the desired signal in W
J - received power for undesired signals like multiple
access users, multipath, jammers etc in W
Eb - received energy per bit for the desired signal in W
N0 - equivalent noise spectral density in W/Hz

7/30/2015 38
• What is the tolerable interference over desired signal power?
( )min0max NE
RW
S
J
b
=





00
0
NE
RW
NE
WT
TE
WN
S
J
bb
b
bb
===
)()((db)marginJamming
min0max
db
N
E
db
R
W
S
J b






−==






•In conventional systems W/R ≈ 1 which means, for
satisfactory operation J/S < 1
•Example Let R = 9600; W = 1.2288 MHz
(Eb/N0)min = 6 dB (values taken from IS-
95)
Jamming margin (JM) = 10log10(1.2288*106
/9.6*103
)
- 6
= 15.1 dB ≡ 32
•This antijam margin or JM arises from Processing Gain
(PG) = W/R = 128
•If (Eb/N0)minis further decreased or PG is increased, JM
can be further increased

•JM can be used to accommodate multiple users in the
same band
•If (Eb/N0)min and PG is fixed, number of users is
maximized if perfect power control is employed.
•Capacity of a CDMA system is proportional to PG.

•Multiple Access in CDMA:
oEach user is assigned a unique PN code.
oEach user transmits its information by spreading
with unique code.
oDirect Sequence spread spectrum is used.
oUsers are separated by code not by time slot and
frequency slot.

•Concept of CDMA
oUsers share same
bandwidth
oUser axis shows
cumulative signal strength
of all users
Code 1
Code2
Code 3
Code 4
Freq
User

-5000 -4000 -3000 -2000 -1000 0 1000 2000 3000 4000 5000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
noise
massege
After SpredingChipDurationis0.4
ms.
Bit Durationis 6.4 ms.
ProcessingGainis 16
 How CDMA works?:
•Spectrum of the CDMA:

 Types of CDMA:
1. Frequency hopping
•Fixed sequence of frequency values & time is divided
into slots .
•In the first time slot, a given user transmit to the base
station using the first frequency in its frequency hopping
sequence.
•In the next time interval, it transmits using the second
frequency value in its frequency hopping sequence, and
so on.
•This way, the transmit frequency keeps changing in time.

 Types of CDMA:
2. Direct sequence
•Each user transmits its message to the base station
using the same frequency, at the same time.
•Here signals from different users interfere with each
other.
•But the user distinguishes its message by using a
special, unique code. This code serves as a special
language that only the transmitter and receiver
understand.

ME
SI
M
BT
S
BT
S
BSC
BSC
MSC
VLR
EIR
HLR
AuC
AUm
Abis
PSTN
ISDN
Mobile
station
Base Station
Subsystem
Network
Subsystem
MS BSS NS
 Architecture Of CDMA:

•Mobile Station (MS): Two Blocks
oMobile Equipment (ME)
oSubscribers Identity Module (SIM)
•Function of Mobile Station:
1. Personal Mobility
2. IMEI (International Mobile Equipment
Identity)
3. IMSI (International Mobile Subscriber Identity)

BSC
•Base Station Subsystem (BSS)
oBase Transceiver Station (BTS)
oBase Station Controller (BSC)

•BTS : Base Transceiver station
•1. It defines the cell .
•2. It handles the radio link protocol with the mobile
station
BSC: Base station Controller
•1. It manages radio resources for one or more BTS.
•2. Allocation and Deallocation of channels.
•3. Transmitter power control.
•4. Handoff control

BS
C MSC
VLR
EIR
HLR
AuC
PSTN
ISDN
•Network Subsystem
oMSC: Mobile Switching Center
oHLR: Home Location Register
oVLR: Visitor Location register
oAuC: Authentication Center
oEIR: Equipment Identity Register

•Mobile Switching center:(MSC)
1 Call set up/supervision/release
2 Call routing
3 Billing information collection
4 Mobility management
5 Paging, Echo cancellation
connection to BSC, other MSC and other local
exchange networks
6 Access to HLR and VLR

•Home Location Register (HLR)
1 One HLR per CDMA operator
2 Contains permanent M database of all the
subscribers in the network
3 Contains MSRN(mobile station routing no.)
4 It is referred for every incoming call

•Visitor Location Register(VLR)
1. Temporary visitors database
2. One VLR per MSC
•Authentication Center(AuC)
1. Provides security
2. Authentication and encryption
•Equipment Identity Register:
1. Contains IMEI

 Spreading Codes:
•A noise-like and random signal has to be generated at
the transmitter.
•The same signal must be generated at the receiver in
synchronization.
•We limit the complexity by specifying only one bit per
sample, i.E., A binary sequence.

 Desirable Randomness Properties Of SC:
•Relative frequencies of “0” and “1” should be ½
(balance property)
•Run lengths of zeros and ones should be (run
property):
•Half of all run lengths should be unity
•One - quarter should be of length two
•One - eighth should be of length three
•A fraction 1/2n
of all run lengths should be of length
n for all finite n

 Desirable Randomness Properties Of SC:
•If the random sequence is shifted by any nonzero
number of elements, the resulting sequence
should have an equal number of agreements and
disagreements with the original sequence
(Autocorrelation property)

 Walsh CodesWalsh Codes::
•64 sequences, each 64 chips long
•A chip is a binary digit (0 or 1)
•Each Walsh code is orthogonal to all other Walsh
codes
•This means that it is possible to recognize and
therefore extract a particular Walsh code from a
mixture of other Walsh codes which are “filtered
out” in the process
•Two same-length binary strings are orthogonal if
the result of xoring them has the same number of 0s
as 1s

 PN Sequences:
•A deterministically generated sequence that nearly
satisfies these properties is referred to as a
Pseudorandom Sequence (PN)
•Periodic binary sequences can be conveniently
generated using linear feedback shift registers (LFSR)
•If the number of stages in the LFSR is r, P ≤ 2r
- 1
where P is the period of the sequence

 PN Sequences:
•However, if the feedback connections satisfy a
specific property, P = 2r
- 1. Then the sequence is
called a Maximal Length Shift Register (MLSR) or a
PN sequence.
•Thus if r=15, P=32767.
•MLSR satisfies the randomness properties stated
before

 Randomness Properties of PN Sequences:
•Balance property - Of the 2r
- 1 terms, 2r-1
are one and
2r-1
–1 are zero. Thus the unbalance is 1/P. For r=50;
1/P≅10-15
•Run length property - Relative frequency of run
length n (zero or ones) is 1/ 2n
for n ≤ r-1 and 1/(2r
- 1)
for n = r
•One run length each of r-1 zeros and r ones occurs.
There are no run lengths for n > r
•Autocorrelation property - The number of
disagreements exceeds the number of agreements by
unity. Thus again the discrepancy is 1/p

 PN Sequences Specified in IS-95:
•A “long” PN sequence (r =42) is used to scramble the user
data with a different code shift for each user
•The 42-degree characteristic polynomial is given by:
•x42
+x41
+x40
+x39
+x37
+x36
+x35
+x32
+x26
+x25
+x24
+x23
+x21
+x20
+x17
+x16
+
x15
+x11
+x9
+x7
+1
•The period of the long code is 242
- 1 ≈ 4.4*102
chips and
lasts over 41 days

 PN Sequences Specified in IS-95:
•A short PN sequence (r = 15) is specific to a base
station and its period is (215
−1)Tc = 27ms.
•Two “short” PN sequences (r=15) are used to spread
the quadrature components of the forward and reverse
link waveforms

 CDMA Related Terms:
•CDMA Channel or CDMA Carrier or CDMA
Frequency:
Duplex channel made of two 1.25 MHz-wide bands of
electromagnetic spectrum, one for Base Station to
Mobile Station communication (called the
FORWARD LINK or the DOWNLINK) and another
for Mobile Station to Base Station communication
(called the REVERSE LINK or the UPLINK).
•In 800 Cellular these two simplex 1.25 MHz bands are
45 MHz apart,
•In 1900 MHz PCS they are 80 MHz apart

 CDMA Related Terms:
•CDMA Forward & Reverse Channel
1.25 MHz Forward / Reverse Link
CDMA Code Channel
Each individual stream of 0’s and 1’s contained in
either the CDMA Forward Channel or in the CDMA
Reverse Channel
•Code Channels are characterized (made unique) by
mathematical codes
Code channels in the forward link: Pilot, Sync, Paging
and Forward Traffic channels
Code channels in the reverse link: Access and Reverse
Traffic channels

 cdma2000 Uplink Frame Structure:
• Radio Configuration 3
7/30/2015 65
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

 cdma2000 Uplink Modulator:
7/30/2015 66
C
C
C
A
B
Σ
Σ
dβ
dβ
dβ
cβ
)cos( tcω
)sin( tcω
Ilong,c Qlong,c
+
+
+
+
Σ
+
_
sumsum
1Dw
Cw
2Sw
1Sw
Primary
Traffic
Secondary
Traffic 1
Secondary
Traffic 2
Control
Pilot
pulse
shape
pulse
shape

 cdma2000 Downlink Frame Structure:
7/30/2015 67
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

 Benefits of the cdma2000 1x StandardsBenefits of the cdma2000 1x Standards::
•Increased mobile standby battery life (via Quick
Paging Channel)
•Total backward compatibility to reuse switch and call
processing features
•2-3 dB better coverage
•High speed 153.6 kbps packet data capabilities
•cdma2000 1x = 1.25 MHz Radio Transmission
Technology

 cdma2000 1X DL Modulation Processing:
+
long
code
0  1
1  -1
channel
gain
fwd pwr
ctrl gain
powercontrol
symbolpuncture
DEMUX
decimate pwr ctrl
bit pos
I/Q scrambling
bit extract
power
control
bits
puncture
timing
800 Hz
W
YQ
YI
modulation
symbol
rate

 cdma2000 1X Downlink Modulation:
7/30/2015 70
I channel
pilot PN
Q channel
pilot PN
Walsh code
QOF code
IY
QY
Σ
Σ
Σ
Σ
Σ
pulse
shape
pulse
shape
+
+
+
-
)cos( tcω
)sin( tcω
other
channels
other
channels
phaserotateforQOF
QX
IX

 CDMA Capacity:
•CDMA Main Advantages
–resistant to narrow band
interference
–resistant to multipath fading
and ISI
–no hard limit on number of
users (soft capacity)
.1
.01
.001
.0001
10 20 30 40 50 60
users
BER of CDMA system with 128 cps.

 CDMA Capacity:
–As number of users on a frequency increase the
interference level increases and BER increases for all
users
–With proper limits all frequencies can be used in
every cell

 CDMA Capacity:
•CDMA is a interference limited system
–Must limit number of users on a frequency to limit
interference within a cell and between cells using same
frequency (All CDMA carriers can be assigned to each
sector in each cell!)
–Total Interference It=Ioc+ Io+ No
•Ioc = other cell interference, Io= own cell interference,
No= Noise
•uplink not downlink in CDMA systems considered
the constraining factor

 CDMA Capacity:
•Remember in direct sequence spread spectrum
Processing Gain = bandwidth of the spread signal to
the bandwidth of the data signal = W/R
–In IS-95 W/R = 10 log (1.23.MHz/9.6 KHz) = 21.1
dB for rate set 1, for rate set two (14.4 kbps) =>
19.3 db
•Number of traffic channels per carrier and cell
function of processing gain, interference, speech coder
tolerance for errors, error control coding, etc.

 CDMA Capacity:
•The effect of more users in a cell on frequency is to
degrade the channel for everyone – can be thought off
as decreasing the usable cell size

 Power Control in CDMA:
•CDMA goal is to maximize the number of
simultaneous users
•Capacity is maximized by maintaining the signal to
interference ratio at the minimum acceptable
•Power transmitted by mobile station must be therefore
controlled
•Transmit power enough to achieve target BER:
no less no more

 Power Control in CDMA:
•Controlling transmitted power affects the CIR
77
Pr
Pt
=
1







4πdf
c
α
Pt = Transmitted power
Pr = Received power in free space
d = Distance between receiver and transmitter
f = Frequency of transmission
c = Speed of light
a = Attenuation constant (2 to 4)

 Power Control: Open Loop vs. Closed
Loop
•Open loop:
–Base station transmits at a known power level (a
beacon) which mobile measures to estimate the path
loss
–Assumes path loss in both directions is the same
–Not very accurate as uplink and downlink are
separated in frequency
–Useful for coarse initial estimates at mobile used in
Access channel for signaling

 Power Control: Open Loop vs. Closed
Loop:
•Closed loop:
–Signal-to-Interference Ratio (SIR) measured at the
receiver and compared to a target value for SIR
–Receiver sends a power control command to
transmitter to reduce or increase the power level -
requires a bi-directional link
–Used in TCH – power control sub channel operates
at 800 bps by puncturing downlink data with
periodic bits – each power command adjusted MS
power in 1 dB increments

 Closed Loop Power Control: Inner Loop vs.
Outer Loop:
•Inner Loop (or fast power control)
–Measures received SIR, controls transmit power
–Commands sent several times per frame (hence fast
power control)

 Closed Loop Power Control: Inner Loop
vs. Outer Loop:
•Outer Loop (or slow power control)
–Measures packet error rate
–Changes target SIR for inner loop
–Directly modify transmit power based on FER
–Commands sent once per frame (hence slow power
control)

 Two factors important for power control:
•Propagation loss
•due to propagation loss, power variations up to
80 dB
•a high dynamic range of power control required
•Channel Fading
•average rate of fade is one fade per second per
mile hour of mobile speed
•power attenuated by more than 30 dB
•power control must track the fade

 Power Control in IS-95A:
•At 900 MHz and 120 km/hr mobile speed Doppler
shift =100Hz
•In IS 95-A closed loop power control is operated at
800 Hz update rate
•Power control bits are inserted (‘punctured’) into the
interleaved and encoded traffic data stream
•Power control step size is +/- 1 dB
•Power control bit errors do not affect performance
much

 Rake Receiver:
•Mobile station receives multiple attenuated and
delayed replicas of the original signal (multipath
diversity channels).
•Two multipath signals are resolvable only if their
relative delay exceeds the chip period Tc
•Amplitudes and phases of multipath components are
found by correlating the received waveform with
multiple delayed versions of the signal (delay = nTc).
•Searcher performs the above task for up to 3 different
multipath signals.
•3 parallel demodulators (RAKE fingers) isolate the
multipath components and the RAKE receiver
combines them.

d1
d2
t t t
d3
transmission
receiving
Racer combination
noise
 Rake Receiver:

Correlator 1
Correlator 2
Correlator 3
Search Correlator
Combiner
To De-Interleaver,
Viterbi Decoder
Multipath Delay
Components
( 150 µs > ∆t > 1µs)
Rake receiver can isolate multipath spaced > 1 chip
length.
 Rake Receiver:

•Handset uses combined outputs of the three traffic
correlators “rake fingers”
•Each finger can independently recover a particular PN
offset and Walsh code
•Fingers can targeted on delayed multipath reflections, or
even on different BTSs
•Searcher continuously checks pilots
 Rake Receiver:

7/30/2015 88
88
 Handoff in CDMA System:
•In GSM hard handoff occurs at the cell boundary
•Soft Handoff
•Mobile commences Communication with a new BS
without interrupting communication with old BS
•same frequency assignment between old and new BS
•provides different site selection diversity
•Softer Handoff
•Handoff between sectors in a cell
•CDMA to CDMA hard handoff
•Mobile transmits between two base stations with
different frequency assignment

Handoff is of
3 types
1. Intra BSC
2 Inter BSC
3. Inter MSC
 Handoff in CDMA System:
BSCBSC BSC
MSC MSC
GMS
C

 Soft Handoff- A unique feature of CDMA
Mobile:
Advantages
•Contact with new base station is made before the call
is switched
•Diversity combining is used between multiple cell
sites
•Diversity combining is the process of combining
information from multiple transmitted packets to
increase the effective SNR of received packets
• additional resistance to fading
•If the new cell is loaded to capacity, handoff can still
be performed for a small increase in BER
•Neither the mobile nor the base station is required to
change frequency

 Detection in the CDMA system:
•Multi-Users Detection (MUD)
•The capacity and the performance of the CDMA
system increase when MUD is implemented.
•The basic principle of MUD is the elimination of
the negative effect of each user on the other.
• MUD is also known as joint detection and
interference cancellation .
• Multi-user detection considers all users as signals
for each other.

•The ultimate technique for the MUD is the
optimum receiver.
•Optimum Receiver:
•Very complex. So, it is unpractical solution to
reduce the multiple access interference. Sup-
optimum detector will be implemented.

•Sup-optimum solution:
•Serial Interference cancellation (SIC):
•SIC is preferred in the absence of power
control.
• Serial canceling the interference generated
from the other users.
•Cancel the strongest signal first (most negative
effect )

•Parallel Interference Cancellation (PIC):
•PIC would be preferred when the amount of
interference from each user is similar.
•In the initial detection stage, all active users are
detected in parallel by a SUD technique.
•Then, the obtained interference is subtracted
from the received signal and data detection is
performed again with reduced MAI

Disadvantages:
1. Multiple Access Interference:
•Require multi-user detection (MUD) algorithms to
solve.
2. Near-far problem.
•Where stronger (near to the Base Station) user masks
the weaker user (far from the Base Station)
3. Requires wideband channel.

 What is Third Generation? :
•Flexible support of multiple services
•Voice
•Messaging – email, fax, etc.
•Medium-rate multimedia – Internet access,
educational
•High-rate multimedia – file transfer, video
•High-rate interactive multimedia – video
teleconferencing, telemedicine, etc.
•Mobility: quasi-stationary to high-speed platforms
•Global roaming: ubiquitous, seamless coverage
•Evolution from second generation systems

 W-CDMA Versus cdma2000 :
7/30/2015 97
97

 cdma2000 vs WCDMA:
•Chip rate
•Coherent Pilot Channels
•Transmit Diversity
•Underlying Network
•Single Carrier versus Multicarrier Spreading
•Cell Site Synchronization

Commercial usage of CDMA:
•CDMA was introduced in the 2nd
Generation (early of
the 1990s).
•E.g. IS-95 standard, also known as cdma One which
support up to 64 users that are orthogonally coded
and at the same time transmitted over 1.25 MHz
channel
•Used for Cellular Communication System
•(824-894 MHz in US Cellular)
• 3rd
Generation: cdma2000
•Allow high rate of packet transmitting in addition
of voice transmitting.

 CDMA AdvantagesCDMA Advantages::
•Spread Spectrum
•Soft & Softer Handoff
•Rake Receiver
•Variable Rate Vocoder
•High quality voice
•Power Control
•Coverage
•Simple Network Planning
•Green Handset
•Smooth migration to 3G and the operator’s benefit is
protected at the most

 DISADVANTAGES OF CDMA :
• Near far problem.
• CDMA is relatively low, and the network is not as mature
as GSM.
• CDMA can not offer international roaming, a large GSM
advantage.

 Conclusion:
•The CDMA will allow many signals to be transmitted
at the same channel at the same time. This done by
giving each user a Pseudo-Noise code which is a
binary sequence. This code should have low cross-
correlation between each other.
•Multiple access interference has bad effect on the
CDMA system so the multiple user detection is used
to reduce the MAI.
•CDMA cellular system is deemed superior to the
FDMA and TDMA
•cellular systems for the time being.

 Conclusion:
• Therefore, CDMA technique becomes more
important in radio
• communication systems.
•CDMA is based on the spread spectrum technique
which has been used at military field.

 References:
•http://www.tubex.com/category/cdma/gsm
•http://www.bsnl.co.in:cdma/architecture.html
•http://www.cellular.co.za/technologies/cdma/cdma_w_paper
•http://www.arcx.com/sites/CDMAvsTDMA.htm
•Lee JS and Miller LM, CDMA System Engineering
Handbook, Arttech Publishing House, 1998.
•Viterbi A, CDMA-Spread Spectrum Communication,
Addison Wesley 1995.
• R. L. Peterson, “Third Generation Personal
Communications: Physical Layer Status,” Presentation
at Clemson University, Feb. 1, 2001

 References:
• Manjit Singh and Manoneet Singh, “3G Wireless with Respect to
IMT-2000 and Beyond,” Telecom 99
• Harri Holma and Antti Toskala, WCDMA for UMTS: Radio Access
for Third Generation Mobile Communications, New York: Wiley,
2000
• “CDMA Evolution from IS-95, IS-2000, to 1XTREME,”
Technology Transfer Training Class, Motorola, Inc., July 2000

Code Division Multiple Access- CDMA

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