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BY
CDMA OVERVIEW
D. V . JOSHI
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MULTIPLE ACCESS
SPREADING SPECTRUM
CODES IN CDMA
CDMA CHANNEL STRUCTURE
POWER CONTROL
DIVERSITY
HAND OFFS
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Copper Local Loop
FSU BTS
ExchangeSubscriber
Wireless Local Loop
ExchangeSubscriber
BSC
TWO METHODS FOR SU SCRI ER CONNECTION
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WHAT IS MULTIPLE ACCESS?
NUMBER OF USERS ACCESS AND SHARE
TRANSMISSION MEDIUM
BANDWIDTH AVAILABLE
FOR COMMUNICATION AT THE SAME TIME.
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MULTIPLE ACCESS
MULTIPLE SIMULTANEOUS TRANSMISSIONS.
SHARING FINITE SPECTRUM AMONG LARGE
NO. OF SIMULTANEOUS USERS.
NO PRE-ASSIGNED CIRCUIT TO ANY USER.
ANY USER CAN ACCESS ANY CIRCUIT AND MAY
ACCESS DIFFERENT CIRCUITS FOR DIFFERENT
CALLS.
DEMAND ASSIGNED CIRCUITS ON FIRST COME
FIRST SERVE BASIS.
PRIVACY.
TRUNKING EFFICIENCY; THEREBY
IMPROVEMENT IN SYSTEM CAPACITY.
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MULTIPLE ACCESS METHODS
Frequency Division Multiple Access (FDMA)
FDMA is a multiple access method in which users areassigned specific frequency bands. The user has sole
right of using the frequency band for the entire call
duration.
Time Division Multiple Access (TDMA)
In TDMA an assigned frequency band shared among
a few users. However, each user is allowed totransmit in predetermined time slots. Hence,
channelization of user is achieved through separation
in time.
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CODE DIVISION MULTIPLE ACCESS
(CDMA)
LARGE NUMBER OF TRANSMISSIONS ARE COMBINED
ON THE SAME RF CHANNEL AT THE SAME TIME BUT ARESEPERATED BY CODES.
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MULTIPLE ACCESS TECHNIQUES
F
R
E
Q
U
E
N
C
Y
TIME
FDMA
TDMA
CDMA
RK/TR/ALTTC 7 WILL/ overview
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CDMA: Code Division Multiple CDMA is a methodin which users occupy the same time and frequency
allocations, and are channelized by unique assigned
codes. The signals are separated at the receiver by using
a correlator that Access
accepts only signal energy from the desired channel.
Undesired signals contribute only to the noise.
A CDMA system uses effective power control process.
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Advantages:-
The main advantages of this technologyare:
1. Fast Network deployment.
2. Reduced service interruptions.
3. Low Maintenance & operational cost.
4. Better system coverage flexibility
5. Higher capacity
6. Easy transition to mobile services.
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Salient Features of CDMA
It is an advanced comm. Technology. It has Anti-jam and security features.
Large capacity as compared to other Technology.
like FDMA and TDMA. It uses spread spectrum technology.
Better use of the multipath.
Frequency Reuse.
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Frequency Reuse
In CDMA reuse patterns are not required.
Subscriber in every cell can use the same frequency
at the same time. Subscriber is discriminated fromanother by the assignment of a unique code to every
conversation.
In GSM freq. Reuse pattern of 7 is used.
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C
D
B
AC
F
GE B
A
E
G
B
D
CF E
G
F
C
E
B
D
C
FDB
A
Frequency Reuse pattern of 7 (FDMA)
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A
A
A
A
A
A
AA A
A
A
A
A
A
AA A
A
A
A
A
A
A
A
AAA
A
CDMA Frequency Reuse
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Frequency Bands
CDMA 824- 849 MHz
869- 894 MHz
PCS 1850- 1910 MHz
1930- 1990 MHz
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45 MHz
1.25 MHz 1.25 MHz
80 MHz
1.25 MHz 1.25 MHz
CDMA Channel in PCS band
CDMA channel is 1.25 MHz wide and there is a separation
of 45 MHz in the forward and the reverse channel in case
of cellular band & and 80 MHz in case of PCS band.
CDMA Channel in 800 MHz band
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SPREADING SPECTRUM
Shannons Equation
C= W Log (1+S/N)
Where C=Capacity (bps)
W=Bandwidth
S=Signal Power
N=Noise Power
Shannons Capacity Equation is basis for spread spectrum.
System with large band width can operate at very low SNR level
& can provide acceptable data rate per user.
Therefore in CDMA-All users uses same 1.25 MHz spectrum.
-Each user has unique Digital code identifier.
-Digital codes separate users to avoid interference.
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SPREAD SPECTRUM TECHNIQUES
1. Frequency Hopped Spread Spectrum:Spreading can also be achieved by hopping the narrowband
information signal over a set of frequencies. The type of
spreading can be classified as fast or slow depending upon
the rate of hopping to the rate of information.
2. Direct Sequence Spread Spectrum:
The information signal is inherently narrowband, on the
order of less than 10KHz. The energy from this narrowbandsignal is spread over a much larger bandwidth by
multiplying the information signal by a wideband spreading
code. DSS technique is used in the IS-95 CDMA cellular
system.
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Direct Sequence Spread using Walsh code
Consist of 64 orthogonal codes each 64 bits long
Spreads spectrum to 1.2288 M bps from 9.6 Kbps
Channel Capacity
C=W log (1+S/N)
Increasing BW improves Signal Transmission with lower S/N
Frequency
Power
Spectral
Density-------------------------------------------------------------------------------------- Noise Level
Narrow Band Waveform
Spread Waveform
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PROCESSING GAIN
One of the major advantages with an SS system is its
robustness to interference. The system processing
gain Gp quantifies the degree of interference
rejection. The system processing gainis the ratio of
RF bandwidth to the information rate:
Gp =W/R
=1.2288x 106 /9.6 x 103
=128
dBgain =10log10 128
=21
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Spread spectrum principle:
Originally spread spectrum radio technology wasdeveloped for military use to counter the
interference by hostile jamming. The broad
spectrum of the transmitted signal gives rise
tospread spectrum. A spectrum signal isgenerated by modulating the radio frequency (RF)
signal with a code consisting of different pseudo
random binary sequences, which is inherently
resistant to noisy signal environment.
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A number of spread spectrum RF signals thus
generated share the same frequency spectrum and
thus the entire bandwidth available in the band willbe used by each of the users using same frequency
at the same time.
On the receive side only the signal energy
with the selected binary sequence code is accepted
and information content is recovered. The other user
signals, whose codes do not match contribute only
the noise and are not de-spread back inbandwidth. This transmission and reception of
signals differentiated by codes using the same
frequency simultaneously by a number of users is
known as code Division Multiple Access (CDMA).
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Techniques as opposed to conventional method of
Frequency Division Multiple Access and Time
Division Multiple Access.
In the fig. It has been tried to explain that how
the base band signal of 9.6 kbps is spread using a
long pseudo-random Noise(PN) source to occupy
entire bandwidth of 1.25 Mhz. At the receiving endthis signal will have interference from signals of
other users of the same cell, user different cells and
interference from other noise sources. All these
signals get combined with the desired signal but
using a correlator and correct PN code, the original
data can be reproduced back.
ORTHOGONAL SPREADING:
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CellCell
TXRX
USER INPUT
1 0 0 1 1
10011001100110011001
CODE
USER OUTPUT
1 0 0 1 1
10011001100110011001
CODE
01101001100101100110
User Input 1 0 0 1 1
Spreading
Sequence1001 1001 1001 1001 1001
TX Data 0110 1001 1001 0110 0110
ORTHOGONAL SPREADING:-
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Decoding using correct Orthogonal Function
0110 1001 1001 0110 0110
1001 1001 1001 1001 1001
1111 0000 0000 1111 1111
Rx Data
Correct
Function
1 0 0 1 1
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Decoding using incorrect Orthogonal Function
0110 1001 1001 0110 0110
0101 0101 0101 0101 0101
0011 1100 1100 0011 0011
Rx Data
Incorrect
Function
? ? ? ? ?
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Spreading Codes
cdmaOne systems use two types of code sequences: Orthogonal sequences (Walsh codes).
Pseudorandom Noise (PN) sequences.
Long codes (242 =4400 Billion)
Short codes (215=32768)
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Walsh Codes:-
In CDMA the traffic channels are separated byUnique Walshcode. These are
(a) 64 codes of 64 Bit Length.
(b) Forwarded traffic channel Codes.(c) All codes are orthogonal to each other.
(d) These codes provide Isolation between
multiple signals transmitted by base stations
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The basic concept behind creation of the code is
as follows:
(a)Repeat the function right
(b)Repeat the function below
(c) Invert function (diagonally)
Fig: Seed 0 0 0 0 0 0 0
0 1 0 1 0 1
0 0 1 1
0 1 1 0
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By using this technique we create a set of 64
such codes of 64 bit length which is known as
Walsh codes. All such codes are orthogonal toeach other. The individual subscriber now can
start communication using one of these codes.
These codes are traffic channel codes and are
used for orthogonal spreading of the information inthe entire bandwidth. Orthogonality provides
nearly perfect isolation between the multiple
signals transmitted by the base station.
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PN Code Generation
-PN Codes are generated from prime polynomials using
modulo-2 arithmetic.
-State machines generating PN Codes consists of shift
registers & XOR gates.
-The length of the PN Code is equal to 2 -1 ( N= no. of shift
registers).
0out
0 1
Output will be a 7-digit sequence that
repeats continually 1001011
N
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0 0 1 1
0 1 1 1
1 1 1 1
1 0 0 0
0 1 0 0
1 0 1 11 1 0 0
Sequence = 1001011 1001011
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PN offset (Masking)
- Masking will cause the generator to produce the same
sequence but offset in time.
-Masking provides the shift in time for PN codes.
-Different masks corresponds to different time shifts.
-ESN are used as masks for users on the traffic channels.
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LONG CODE
(a) 242 Bits polynomials.
(b) Forwarded channel Data (traffic and paging chls)
scrambled.
(c) Provides channelizations for the reverse chls.
(d) This code is unique for every subscriber.(e) It is known as user address mask or user
identification.
(f) Subscriber are differentiated as no two same
codes are used.
(g) Repeats every 41 days (at a clock rate of 1.2288
Mcps)
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SHORT CODES
(a) This PN sequence is based on 215
characteristics polynomial.
(b) Differentiates cells and sectors.
(c) Identifies cells and sectors.
(d) Consist of codes for I & Q chls.
(e) Each cell uses different codes.
(f) Repeats every 26.67 msec (at a clockrate of 1.2288 Mcps)
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CDMA Channels
Forward Link Channels
Pilot Channel
Sync Channel
Paging Channels
Traffic Channels
Reverse Link Channels
Access Channels
Traffic Channels
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CDMA CHL ARCHITECTURE
Pilot channel (W0)
The pilot is used by the subs unit to obtain initialsystem synchronization and to distinguish cell
sites. Every sector of every cell site has a unique
pilot channel.
Transmitted constantly.
Allows the mobile to acquire the system.
Provides mobile with signal strength comparison.
Approximately 20% of the radiated power is inthe pilot.
Has unique PN Offset(215 ) for each cell or sector.
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Sync channel(W32)
Used during system Acquisition stage. Sync chl
provides the subs unit with network informationrelated to cell site identification, pilot transmit
power & cell site PN offset.
Used by mobile to synchronize with the system
Transmits sync message with
- Pilot PN offset - System time
- Long PN code - System ID
- Network ID - Paging chl data rate
Tx at 1200 bps
PAGING CHLS (W1-W7)
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PAGING CHLS (W1-W7)
On this chl base station can page the subs unit
and it can send call set-up and traffic chl
assignment information.
Means of communication between base to
mobile station.
Paging CHL data Rates can be 2.4,4.8 or 9.6
Kbps.
CDMA assignment has 7 paging CHLs.
Each paging CHL supports 180 pages per set.
Total pages/ CDMA RF chl = 1260
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Provides mobile with
- System Parameter message - Neighbour list
- Access Parameter list - CDMA Channel list
Used by base station to :
- Page mobile - Transmit overhead information
- Assign mobile to traffic channel
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Traffic Channels ( W8-W31 & W33-W63)The traffic chl carries the actual call. That is, the
voice and control information between the subs unit
& base station.
TX upto 9.6kbps on rate set 1 and upto 14.4kbps
on rate set 2.
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Access CHLS.
(a) Provides communication from Mobile to base
station when mobile is not using traffic Chl. Theaccess chl is used for call origination & for
response to pages, orders & registration
requests. It is paired with corresponding paging
chl.
(b) Each Access CHL use long PN code.
(c) Base station responds to transmission on a
particular Access CHL.(d)Mobile responds to base station message by
emitting on Access CHL.
(e) Tx at 4800bps
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cdmaOne Modulation
A/D Vocoder
FEC
Spreading code
generatorSpreader PSK
Information
Information Bits
Code symbols
Chips
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Forward Traffic Channel Generation
9600 bps
4800 bps
2400 bps
1200 bps
Rate set 1
I PN
ConvolutionalEncoder & Repetition
Blockinterleaver
Long
Code PN decimator decimator
UserAddress
Mask
(ESN)
O PN
Power
Control
bit
19.2 ksps
1.2288
19.2
ksps
R=1/2
Mcps800bps
Rate set 2
14400 bps
7200 bps
3600 bps
1800 bps
R=3/4
Wt
1.2288
Mcps
64:1 24:1
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Reverse Traffic Channel Generation
9600 bps
4800 bps
2400 bps
1200 bps
Rate set 1
I PN
Convolutional
Encoder & RepetitionBlock
interleaver
Long
Code PN
User
Address
Mask
(ESN)
O PN
28.8 ksps
1.2288
R=1/3
Mcps
Rate set 2
14400 bps
7200 bps
3600 bps
1800 bps
R=1/21.2288
Mcps
Data
Burst
Rand.
Orthogonal
Modulation
307.2KHz
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Rake receiver
CDMA mobiles use rake receivers. The rake
receiver essentially a set of four or more receivers(or fingers). One of the receivers constantly
searches for different multipaths and helps to direct
the other three fingers to lock onto strong multipath
signals.
Allows combined reception of up to three different
paths.
Provides searcher receiver to identify changes in
path characteristics/new cells.
Provides both path diversity and frequency diversity.
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Correlator 1
Correlator 2
Correlator 3
Searcher
Correlator
c
o
m
b
i
ne
r
CDMA mobile rake receiver
Rake receiver
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Rake receiver
T0 T1 T2 T3
W3W2W1W0
Summing
Circuit
OUTPUT
ANTENNA
DELAY
TAPS
TAP
WEIGHTS
VOCODER
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A/DSpeech
Encoder
CDMA
ModemRF
Mobile
RF
Channel
RFCDMA
Modem
Speech
DecoderD/A
S(t) S(n) P(n)
S(t) S(n) P(n)
Speaker
Microphone
Disadvantage of Digital Comm. System
Bandwidth expansion of digitally sampled speech
Solution :
Variable rate vocoder
S(t): Input analog Speech
S(n): Input digitized speech blocked into 20 msec frames
P(n): Encoded packets every 20 msec representing parameters of speech
such as spectral envelope, pitch, energy and phases
S(n): Reconstructed digital speech
S(t): Reconstructed analog speech
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Hand offs
Softer handoff
Multi sector hand off (Intra BTS)
Can have upto 3or 6 sectors involved (same cell)
Voice data is combined at cell and passed as one cell to BSC
Make before break
Soft handoff
Multi-cell Handoff (Inter BTS)
Can have upto 3 cells involved (same FA) Each cell provides voice data to BSC
Voice data is selected at SVC to vocoder in BSC
Make before break
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CDMA Capacity
W/R 1 1N= -------*-----*--------*n *gEb/Io d 1+f
Where
N= calls per sector
W= Spread spectrum Bandwidth (1.25 MHz)
R= data rate (9.6 kbps or 14.4 kbps)
Eb/Io= Bit energy/ other user interference density (7dB)
d= Voice activity factor (0.4)
f= other interference/ same interference (0.6)
n= loading factor (0.8)g= reduction for variable power (0.85)
N= 27 users per sector for R=9.6Kbps
18 users per sector for R=14.4Kbps
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Power control
CDMA will not work without an effective power control,
because of the near-far problem, fading & varying pathloss.
The system requires fast closed loop power control for
Raleigh fading.Requires wide dynamic range open loop power control to
handle variations in path losses in different locations.
Open loop power control:
An original estimate is made by the mobile. Mobile
adjusts its trans power according to changes in its received
power from the base station.
Cl d l t l
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Closed loop power control:
Base station provides rapid correction to the mobiles.
Compares with the threshold value.
Takes decision for increasing or decreasing the power.
Commands the mobile to adjust the output power
accordingly.
=== 0 ===
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Power control in CDMA
There are basically two methods of powercontrol in CDMA
Open loop power control: This is purely amobile unit function. It gives open estimation.This is done only during the initial stage assoon as the mobile is turned on.
Closed loop power control:Thisinvolves both,the Base station and the mobile unit and givesthe closed loop power correction.
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Open loop power control
When the mobile is turned on, it locks onto the pilot, paging and synch channel.
There is no traffic channel assigned to
the mobile and hence no closed loop.
The mobile Tx power will be inversely
proportional to the pilot strength received.
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Open loop power control
The first access probe is sent at a nominalpower given by:
(Pt)mobile=-(Prx)-73+ Nom_pwr +Init_pwr+correction
Init_pwr= its range is 16 to +15 db with 0db nominalvalue
Nom_pwr= correction required(range 8 to+7db with 0db nom value)
For example,if the nominal rec. level is 90dbm, thenthe mobile Tx power is +17dbm without anycorrection.
If BTS does not responds,then pwr_step=0-7db
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Closed loop power control
After the traffic channel is assigned, thepower control shifts to closed loop control
Reverse channel has got 16 power control
groups of 1.25ms in one 20ms frame. BTS receives the mobile receive power once
every 1.25ms and BTS send Power control bit
in the 2nd next 1.25 ms cycle to increase(0) ordecrease(1) the power by 1db.
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