Download - CDMA & IS-95
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A MaheswaranPulsetone Industries
CDMA Basics & IS-95 System
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CDMA Basics
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What is CDMA?
CDMA is Code Division Multiple Access Also called Direct Sequence Spread Spectrum
(DS-SS) Belongs to broader communication systems
called Spread Spectrum This is a wideband system having many
advantages Immune to narrow band interferences Exploits multipath propagation Better handover due to soft handover Increased capacity (users / sq km / MHz)
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Understanding Spread Spectrum
Normally RF bandwidth is conserved In spread spectrum communication bandwidth is
deliberately increased Evolved out of military communication
• In avoiding detection and jamming
• For preventing eavesdropping There are 2 types of Spread Spectrum
Freq. Hopped SS (FH-SS) – Carrier frequency is changed periodically Bandwidth remains the same
Direct Sequence Spread Spectrum (DS-SS) – Suitable for data transmission & original data spread manifold (say 8 to 1024 times) - This will be our focus
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Direct Sequence Spreading
Here, the binary message bit sequence is multiplied (Ex-Ored) with a bipolar (binary) code (chip) sequence
If L chips multiply every bit, then the BW of the message seq. increases L times
Usually we choose L of the form L = 2k
The Coding (or Spreading) Gain of the system is 3k dB
Tc
Tb
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DS-SS Transmitter
Spread Spectrum Transmitter
RR
Information
Transmitted Signal
freq.WW
Processing Gain = W/R >> 1Processing Gain = W/R >> 1
Spreading Code
freq.
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DS-SS Receiver
Information
freq.
Spread Spectrum Receiver
RR
Received Signal
Despreading Code
freq.WW
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Interference Rejection In SS
Frequency Frequency
SpectralDensity
SpectralDensity Interfering
signal
Desiredsignal
Desiredsignal
Interferingsignal
a) At the SS receiver input b) Correlator output after despreading
Proc.Gain
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DS-SS techniques useful for multi access purpose Same wide band used by different users Spread sequences (or codes) of users to be different
and mutually orthogonal This is called Code Division Multi Access
• Orthogonal codes ensure signals do not interfere with each other
Code used for despreading at the receiver has to be exactly the same
• Otherwise decorrelation occurs• Orthogonal codes need to generated by simple
process
Using DS-SS For Multiple Access
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Spreading gain allows weaker signals to be
received without errors
Being a wide band signal, this is immune to
multipath fading and narrow band interference
Many orthogonal codes are available
Walsh Codes
PN Sequence – Long codes
PN Sequence – Short codes
Orthogonal codes allow efficient means of
sharing a given RF spectrum by different users
Using DS-SS For Multiple Access
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Properties Of Orthogonal Codes
Orthogonal property Two codes C1 & C2 having a periodicity over T are
orthogonal, if and only if
C1 * C2 = 0 (over the period T) Walsh codes derived from Hadamard matrixes have good
properties for use as orthogonal codes Hadamard matrices are square matrices with n x n binary
elements All rows of this matrixes are mutually orthogonal, (if we
consider an agreement as having a weight of +1 and disagreement as having a weight of –1)
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Hadamard Matrices
0 0
0 1H2 =0H1 =
A 2n x 2n Hadamard Matrix can be generated by following the recursive procedure
H4 =
0 0 0 0
0 1 0 1
0 0 1 1
0 1 1 0
H2N =HN
HN
HN
HN
All 2N rows of matrix are mutually orthogonal
H2N
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Walsh Codes
The rows of Hadamard matrix are used as code words and these are called Walsh codes
Walsh codes are extensively used in CDMA systems both for spreading and modulation In IS-95 system, 64 x 64 Walsh codes are used for
spreading on forward link and for modulation on reverse link
• When it is used for spreading, the spreading factor is 64
• When it is used for modulation, the spreading factor is (64 / 6) since 6 bits will be represented
by 64 chips
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Using Walsh Code For Spreading
Walsh Code Generator (Wi )
1.2288 Mcps
+
Traffic data of ith user (data rate of 19.2 kbps)
Output after spreading
In every symbol time (1 bit), we have 64 Walsh chips Wi
or inversion of Wi being sent depending upon whether the bit is 0 or 1
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Using Walsh Code For Spreading (Contd.)
Though Walsh codes have excellent properties by way of orthogonality, the transmitter and receiver need to have a perfectly synchronised copies – otherwise orthogonality is not guaranteed
If {0 0 1 1} is used as code word, then this can result in {0 1 1 0}. Hence Walsh code is used for spreading on forward link where perfect synchronisation can be assured
0 0 0 0
0 1 0 1
0 0 1 1
0 1 1 0
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Walsh Code
Chip # 1
Chip # 2
Chip # 4
Chip # 5
Chip # 6
W01 1 1 1 1 1 1 1
W11 -1 1 -1 1 -1 1 -1
W21 1 -1 -1 1 1 -1 -1
W31 -1 -1 1 1 -1 -1 1
W41 1 1 1 -1 -1 -1 -1
W51 -1 1 -1 -1 1 -1 1
W61 1 -1 -1 -1 -1 1 1
W71 -1 -1 1 -1 1 1 -1
Chip # 3
Chip # 7
Chip # 8
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Using Walsh Code For Spreading & Multiaccess
For demonstrating CDMA (using DS-SS techniques for multi access purpose) we will do the following:
Spreading (User1)
Spreading (User2)
Despreading (User2)
Despreading (User1)
User1 Data (0101)
User2 Data (0011)
W1
W5
W1
W5
Decoded User1 Data
(0101)
Decoded User2 Data
(0011)Code Division Multiplexed
Output
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User1 Data 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1
W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Output1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1
User2 Data 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1
Output2 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1
Sum 2 -2 2 -2 0 0 0 0 0 0 0 0 -2 2 -2 2
W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Product 2 2 2 2 0 0 0 0 0 0 0 0 -2 -2 -2 -2
Total = +8 = -8
User1 Data (+8/8 = +1 >0) 0 (-8/8 = -1 <0) 1
W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1
Product 2 2 2 2 0 0 0 0 0 0 0 0 2 2 2 2
Total = +8 = +8
User2 Data (+8/8 = +1 >0) 0 (+8/8 = +1 >0) 0
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User1 Data 1 1 1 1 1 1 1 1 -1 -1 -1 -1 -1 -1 -1 -1
W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Output1 1 -1 1 -1 1 -1 1 -1 -1 1 -1 1 -1 1 -1 1
User2 Data -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1 -1
W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1
Output2 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1
Sum 0 0 0 0 2 -2 2 -2 -2 2 -2 2 0 0 0 0
W1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1 1 -1
Product 0 0 0 0 2 2 2 2 -2 -2 -2 -2 0 0 0 0
Total = +8 = -8
User1 Data (+8/8 = +1 >0) 0 (-8/8 = -1 <0) 1
W5 1 -1 1 -1 -1 1 -1 1 1 -1 1 -1 -1 1 -1 1
Product 0 0 0 0 -2 -2 -2 -2 -2 -2 -2 -2 0 0 0 0
Total = -8 = -8
User2 Data (-8/8 = -1 <0) 1 (-8/8 = -1 <0) 1
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Using Walsh Code For Modulation
Walsh code is used in reverse traffic channel of IS-95 for modulating user traffic data
In everys symbol time (6 bits), we send chips of a particular Walsh code depending upon the 6 bit combination
Modulator using
Walsh Code
Traffic data of ith user (data rate of 28.8 kbps)
Output after Modulation
(307.2 kcps)
SymbolGenerator
4.8 ksps (6 bits / symbol)
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Using Walsh Code For Modulation (Contd.)
This improves reception at the base station – detecting forward link is more difficult due to Near-Far problem and non-coherent detection
Modulation using orthogonal Walsh codes enhances the decision making algorithm at the receiver and is computationally efficient
We can view this Walsh modulation as a form of block error correcting code with (n,k) = (64,6)
with dmin = 32 (in fact the distance between any
code word is 32)
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Using Walsh Code For Modulation
Data rate k bits per sec
Rate k/3 symbols per sec (3 bits / Symbol)
ModulationUsing Walsh
Codes
Serial to Parallel
Converter
Modulated output (k*8/3 chips per sec)
{W0, W1, W2, W3,W4, W5 , W6, W7}
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Modulation Scheme
Symbol Bits Walsh code chosen for modulation
D2 D1 D0
0 0 0 W0
0 0 1 W1
0 1 0 W2
0 1 1 W3
1 0 0 W4
1 0 1 W5
1 1 0 W6
1 1 1 W7
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Illustrating Modulation Using Walsh Code
Bit Sequence to be transmitted (101011001110) Converting to symbols with 3 bits / symbol
[101 011 001 110] Modulating with Walsh codes as per table yields
{W5 W3 W1 W6}
Corresponding chips are • {1 -1 1 -1 -1 1 -1 1
1 -1 -1 1 1 -1 -1 1 1 -1 1 -1 1 -1 1 -1 1 1 -1 -1 -1 -1 1 1}
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Illustrating Modulation Using Walsh Code (Contd.)
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
CorrelationReceiver
Threshold Decision
Maker
W0
W2
W4
W6
W7
W1
W3
W5
Received Chips
Decoded Symbol
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Illustrating Modulation Using Walsh Code (Contd.)
Demodulation is done by correlating the received chips with various Walsh chips and finding the match with maximum likelihood decoding
Taking the chips in the first symbol {1-11-1-11-11} and doing this for W0
1 -1 1 -1 -1 1 -1 1
1 1 1 1 1 1 1 1
1 -1 1 -1 -1 1 -1 1
= 0 (Minimum matching)
• For W11 -1 1 -1 -1 1 -1 1
1 -1 1 -1 1 -1 1 -1
1 1 1 1 -1 -1 -1 -1
= 0 (Minimum matching)
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Illustrating Modulation Using Walsh Code (Contd.)
For W2 1 -1 1 -1 -1 1 -1 1
1 1 -1 -1 1 1 -1 -1
1 -1 -1 1 -1 1 1 -1
= 0 (Minimum matching)
1 -1 1 -1 -1 1 -1 1
1 -1 -1 1 1 -1 -1 1
1 1 -1 -1 -1 -1 1 1
= 0 (Minimum matching)
1 -1 1 -1 -1 1 -1 1
1 1 1 1 -1 -1 -1 -1
1 -1 1 -1 1 -1 1 -1
= 0 (Minimum matching)
•For W3
•For W4
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Illustrating Modulation Using Walsh Code (Contd.)
For W5 1 -1 1 -1 -1 1 -1 1
1 -1 1 -1 -1 1 -1 1
1 1 1 1 1 1 1 1
= 8 (Maximum matching)
1 -1 1 -1 -1 1 -1 1
1 1 -1 -1 -1 -1 1 1
1 -1 -1 1 1 -1 -1 1
= 0 (Minimum matching)
1 -1 1 -1 -1 1 -1 1
1 -1 -1 1 -1 1 1 -1
1 1 -1 -1 1 1 -1 -1
= 0 (Minimum matching)
•For W6
•For W7
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Other Spreading Codes
CDMA systems use multiple spreading, each spreading serving a different purpose
So we need many classes of spreading codes User specific codes: A large number of mobiles
need to be allotted spreading codes and these have to be administered & synchronised
Station Specific codes: On the forward link we need to spread the combined signal to have a station specific spreading to provide isolation between transmissions of different base stations
Scrambling codes: Not all Walsh codes generate wide band signals (like W0) we need to scramble the data so that the resulting signal is truly wideband
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Properties Of Spreading Code Desired Randomness Properties
P1: Balance Property• Relative frequencies of occurrence of 1’s and
0’s should be 1/2
P2: Run Length Property• Run lengths of 1’s and 0’s are as expected in
a coin-flipping experiment• 1/2 of all run lengths are unity,• 1/4 of the run lengths are 2, • 1/8 of the run lengths are 3, and so on
P3: Delay and Add Property• Equal number of agreements and
disagreements between a sequence and its shifted version
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PN Sequences
Psuedo-random Noise (PN) Sequences A deterministically generated sequence
that `nearly’ satisfies properties P1 to P3 , within extremely small discrepancies
Maximum Length Shift Register (MLSR) generated sequences Are PN sequences which nearly satisfy P1 to
P3 Also called as m-sequence, where m is the
number of shift registers used to generate the sequence
Period of an m-sequence P, is given by
12 mP
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MLSR Sequence Generator
Generating Function, G(D), is given by[G(D) is the generated sequence]
Characteristic polynomial, f(D), is given by[ƒ(D) gives the tap connections of Seq Gen]
D: Delay
operator
r
i
iiDcDf
1
1)(
0
)(n
nnDaDG
r
iinin aca
1
1c 2c rc
D D D rna 2na1na
No connection
Connection
,1
,0ic
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3-stage MLSR Sequence Generator
+
D D D
Sequence No X0 X1 X2
0 0 0 11 1 0 02 0 1 03 1 0 14 1 1 05 1 1 16 0 1 17 0 0 1
PN Sequence Output
Clock
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Generating PN Sequence With Offset
Initial State Output Sequence
0 0 1 1 0 0 1 0 1 1
1 0 0 0 0 1 0 1 1 1
0 1 0 0 1 0 1 1 1 0
1 0 1 1 0 1 1 1 0 0
1 1 0 0 1 1 1 0 0 1
1 1 1 1 1 1 0 0 1 0
0 1 1 1 1 0 0 1 0 1
PN Sequences with known offset can be generated by starting with an Initial State (seed)PN codes are generated unique to mobiles by using ESN as seed at both MS and BS
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Autocorrelation Function Of PN Sequence
0 0 1 1 1 1 0 0 1 1 1 0 1 0 0 1 1 1 0 1 0 0 10
1 PN Period = N Tc
BinaryPN Seq.
Tc
111
1/N
Tc Tc
ACF
cT
For large N, 01
N
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Near-Far Problem
CDMA base station receives signals from all mobile stations All of them transmit on the same wideband
channel with only channelisation codes / PN long codes to discriminate the signals
If we assume that all of them transmit with same power
• The signal from far off mobile will be comparatively weaker than the signal from a near by mobile
• This has to be detected in the presence of a stronger signal from a nearby mobile
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Near-Far Problem (Contd.)
This problem is called “Near-Far” Problem and this problem gets further compounded by fading and other short term variations that take place due to the channel
Solution is to tightly control the power transmitted by all mobiles so that they are seen with equal power at the base station
• This is done by closed loop power control whereby the mobiles are asked to vary power continuously to meet the target
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Rake Receiver Principles
Multipath creates ISI (inter symbol interference) problem in conventional data transmission However, in the case of CDMA the chip rate
being much is handled differently by constructively combining the multipath signals to improve signal to noise ratio
This is done in RAKE receiver where a separate correlator (called RAKE finger) is assigned for each multipath signal
Typically MS has 4 RAKE fingers and out of these 3 are used for combining signals and 1 is used as a searcher
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Rake Receiver Principles (Contd.)
DEMOD
Code Generator
Input Data
MOD 2
3
1 a1
a2
a`3
a3
a`2
a´1
+
+
+
+ Output data
c(t-1)
c(t-3)
c(t-2)
RAKE receiver
Multipath channel
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Rake Receiver Principles (Contd.) RAKE fingers are used in the Mobile Rx for
combining multipath components 3 fingers for tracking and demodulating
upto 3 different multipath signals of Forward channel
1 searcher for searching and estimating signal strength on different pilots, coarse timing of different multipaths• to select desired (strongest) base station in
idle mode• to provide hypothesis testing and coarse
timing estimation• to generate pilot strength information
messages during traffic mode to enable Handoff
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Power Control On Reverse Link
Transmitter
Receiver
Gain
Duplexer
Open Loop Power Control
Transmitter
Receiver
Gain
Duplexer
Transmitter
Receiver
Duplexer
Base StationMobile Station
Closed Loop Power Control
Control path for closed loop power control
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Power Control
To combat the effect of fading, shadowing and distance losses
Transmit only the minimum required power to achieve a target link performance (e.g., FER) Minimizes interference Increases battery life
Forward Link Power Control To send enough power to reach users at cell
edge Reverse Link Power Control is very critical
To overcome “near-far” problem in DS-CDMA
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Handoffs In IS-95 CDMA
Types of Handoff 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
• Handoffs between sectors in a cell CDMA-to-CDMA Hard Handoff
• Mobile transmits between two base stations with different frequency assignment
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Micro Vs Macro Diversity
Diversity principles are used to improve RF signal quality This is based on the fact that statistical properties
of two or more paths will be uncorrelated • When one path is experiencing deep fade, the
other one is unlikely to experience deep fade In space diversity we may receive through 2
antennas separated in space and combine the signals to improve the performance
We call this Micro Diversity • RAKE receiver combining mulipaths from the
same BS can also be called some form of micro diversity
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Micro Vs Macro Diversity (Contd.)
What we do during soft hand off is Macro Diversity, where 2 or more BSs are beaming the same signal toward the same MS
The received signals are combined in RAKE receiver
Resulting signal will be much better than any individual one
Transmission by MS is received by all BSs and the best signal received is selected at the BSC / MSC
Macro diversity is a boon in CDMA system
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Handoffs In IS-95 CDMA (Contd.)
Basis for Handoff MS does measurements on Pilot channels on
serving base station + other base stations• Search finger in RAKE receiver helps Ms• Information about neighbouring PN offsets
provided by BS in System_Parameter helps MS classifies the Pilots into 4 categories
• Active Set• Candidate Set • Neighbour Set• Remaining Set
These sets are dynamically updated based upon the measurements
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Handoffs In IS-95 CDMA (Contd.)
Pilot Sets Active Set
• Pilots associated with Forward Link traffic channels assigned to the mobile in soft handoff
Candidate Set• Pilots that are not in Active Set but are
received by the mobile with sufficient strength
Neighbour Set• Pilots not in Active or Candidate Set but are
likely candidates for handoff Remaining Set
• Set in the current system on current freq assignment, excluding the above 3 sets
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Soft Handoff Architecture
Mobile
BSC BSC
BTS BTSBTS
New LinkOld Link
MSCTo other switch
Energy measurements are made at the mobile
Frame Selection:MSC selects the bit stream with lower error rate
BTS
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Micro Vs Macro Diversity (Contd.)
Time or Distance
EC/It
Pilot 1Pilot 2
Active Set Total EC/It
Dynamic Soft Handoff Region
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Handoff Example
Time
PilotStrength
(1)
T_ADD
T_DROP
(2) (3) (4) (5) (6) (7)
NeighbourSet
CandidateSet
Active Set
T_TDROP
NeighbourSet
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Handoff Example (Contd.)
(1) Pilot strength exceeds T_ADD. Mobile sends a Pilot Strength Measurement Message (PSMM) to base station and transfers pilot to the Candidate Set
(2) Base station sends a Handoff Direction Message (HDM)
(3) Mobile transfers pilot to Active Set and sends Handoff Completion Message (HCM)
(4) Pilot strength drops below T_DROP. Mobile starts handoff drop timer
(5) Handoff drop timer expires. Mobile sends a PSMM
(6) Base station sends a HDM(7) Mobile moves pilot from Active Set to Neighbour
Set and sends a HCM
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Recap Of CDMA
Being a wideband system, this is immune to multipath propagation
RAKE receiver exploits multipath to improve reception
No frequency planning is required since the same frequency can be used in nearby cell also
Offers advantage of soft capacity and soft handoff
Suffers form “Near-Far” problem
Tight closed loop power control is required to overcome this problem
Offers more capacity than FDMA or TDMA systems
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Advantages of CDMA
In mobile environment multi path propagation is a serious issue resulting in ISI Multi path is not resolvable in narrow band
systems where symbol time is comparable to multi path delay• GSM has symbol time of 3.69 sec vis-à-vis
delay spreads of 3-8 sec CDMA is a wideband system (IS-95 symbol time is
about 0.8 sec, much smaller than multi path delay) Multi path can be resolved and constructively
combined using RAKE receivers Thus we are able to exploit the multi path
propagation for improving the signal to noise ratio
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IS-95 System
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Overview Of IS-95 System
IS-95 is a proprietary design by Qualcomm Developed to overcome capacity issues in AMPS
Network architecture greatly influenced by GSM
Meant for supporting circuit mode voice services Has been enhanced for supporting data services
DS-SS on both Forward and Reverse links
Universal frequency reuse requiring no frequency
planning
64 Walsh Codes used supporting a theoretical
maximum of 64 active users
Channel of 1.25 MHz wide (1.2288 Mcps chip rate)
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Overview Of IS-95 System (Contd.)
Fast power control to combat Near-Far problem
RAKE receiver to take advantage of multipath
Soft handoff making use of macro diversity
Qualcomm 9600 bps Code Excited Linear Predictive
(QCELP) speech coder with a variable data rate from
9600 bps to 1200 bps is used
CDMA system is interference limited with soft capacity Interference reduction is a dominant theme
Variable rate coding for reducing the duty cycle
Closed loop power control
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Is-95 Network Architecture
Um
MS
BTS BSCAbis MSCA
TR-45 / 46 Reference Model
HLR VLR
ACEIR
C B
H
F
DMSC
E
OtherVLR
G
PSPDN
PSTN
ISDN
PLMNMi
Di
Ai
Pi
IWFAUXOS
LXO
WPT2
TAP
WPT1
WPT0
TE2
TE2
TE1
Rm
Sm
Sm
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Logical Channels In Is-95 System
Control Channels
Forward Reverse
Pilot Sync Paging Access
Dim & Burst
Traffic Channels
Speech or Data Associated Signaling
Full Rate 1/2 Rate 1/4 Rate 1/8 RateBlank & Burst
Power Control(Forward)
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Forward And Reverse Channels
Forward Channels (making up a total of max 64) 1 Pilot Channel 1 Synchronisation Channel Upto 7 Paging Channels Traffic Channels
Reverse Channels Access Channels called Random Access
Channels Traffic Channels
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Forward Link Channel Arrangement
...... ... .
Forward CDMA Link (1.2288 MHz channel transmitted by base station
Pilot
Sync
W0 W7W8
W63W32 W1
PCH#1 PCH#7
Code#1 Code#N
Code#P
Code#55
Code#M
FTCH FTCH with multiple code channel
FundamentalCode Channel Mobile Power
Control Subchannel
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General Transmission Scheme - Forward
Data rate ranges from 1200 to 9600 bps
This rate is made upto 19200 bps by rate ½ coding
and repetition of 8 to 1
This data is spread to a channel chip rate of 1.2288
Mcps using combination of techniques
Each IS-95 channel occupies 1.25 MHz of spectrum
Channel chip rate is 1.2288 Mchips/s
64 orthogonal Walsh functions are used in forward
channel for spreading purpose
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Pilot Channel
Pilot channel is the first channel mobile looks for Its power level is kept 4 to 6 dB above traffic
channels Provides phase reference for coherent
demodulation Allows pilot strength measurement for handoff
This is carried on Code Channel 0 (uses W0) Results in transmitting the station PN sequence
with a length of 32,768 {(215-1) PN Sequence plus a ‘0’}• Corresponds to a period of 26.667 msec
All base stations use the same PN sequence, with unique offsets in increments of 64 chips
Base stations are identified by their unique offset
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Modulation Structure Of Pilot Channel
1.2288 Mcps
I-Chl Pilot PN Seq
Q-Chl Pilot PN Seq
BalancedQPSK
Pilot takes 15-20% of total Tx power
Walsh Function 0(all zeros)
all zeros
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Station Specific PN Short Codes & Offsets
32,768 Chips
Short code 0
Short code i
Short code I+1
Short code k Short
code 0
64 Chips
64 Chips
Short code 511
64 Chips
…………………………………………………………………….
1 Chip
Short code 1
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Synchronisation Channel
This always operates at a fixed data rate of 1200 bps
After rate ½ convolutional coding and repetition (of 2) the date rate becomes 4800 bps
This gets spread by Code Channel 32 (W32) operating
at 1.2288 Mcps
Sync Channel message has the following information
System Identification (SID), Network Identification (NID), Pilot short PN sequence offset index (PILOT_PN), Long Code State, System time, Paging channel data rate (4.8 or 9.6 kbps), etc.
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Modulation Structure Of Synchronisation Channel
Convolutional Encoder
and Repetitionr =1/2, K = 9
Sync.Data
Block Inter-leave
1.2288 Mcps4.8 Kbps
Walsh Code 32
I-Chl Pilot PN Seq
Q-Chl Pilot PN Seq
1.2Kbps BalancedQPSK
4.8 Kbps
Sync. Channel typically has about 10% of the Tx power of the Pilot
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Paging Channel
Paging channel is used to transmit control information at either 4.8 or 9.6 kbps The format is similar to that of Sync channel
message 8 bit length indicator, 2-1146 bits long data, 30
bit CRC Paging message can use synchronised capsules
that end on a half frame boundary or unsynchronised capsules that can end anywhere• Synchronised capsules use padding bits to
reach the nearest half frame boundary• Synchronised paging messages carry
multiple of 48 bits (4.8 kbps) and 96 bits (9.6 kbps)
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Paging Channel (Contd.)
Eight paging channel half-frames are combined to forma a paging channel slot of length 80 msec (384 bits at 4800 bps and 768 bits at 9600 bps)
Type of messages carried by paging channel include System parameter message Access parameter message Neighbour list message CDMA carrier list message Slotted page / Page message Order messages Channel assignment messages Authentication challenge message
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Modulation Structure Of Paging Channel
Convolutional Encoder
(and Repetitionfor 4.8 kbps)r =1/2, K = 9
PagingData
Block Inter-leave
1.2288 Mcps19.2Kbps
Walsh Code p (1-7)
I-Chl Pilot PN Seq
Q-Chl Pilot PN Seq
9.6Kbps4.8Kbps Balanced
QPSK
19.2 Kbps
Totally, the Paging Channels have about 75% of the Tx power of the Pilot
Long Code generator Decimator
Long code mask for pth paging
channel
1.2288 Mcps
ScramblingCode -- 19.2Kcps
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Slotted Operation Of Paging Channel
I=0, T=2 =1; PGSLOT = Slot number 6 out of every 16 slots
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 162047 0
1 Slot Cycle 1.28 Seconds
80 ms
Mobile in Nonactive State
Mobile in Nonactive State
Paging Channel Slot
Reacquisition Time
8 Paging Channel Half- Frame
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Forward Traffic Channel
This channel is used for carrying user traffic Data rates are 9600, 4800, 2400 and 1200 with
lower data rates being associated with low voice activity
Convolutional coding, symbol repetition and block interleaving make the data rate 19.2 kbps
Scrambling of user data is achieved using long code generated with mobile’s ESN as long code mask and used for scrambling user data after a decimator block
Power control subchannel @ 800 bps is added by stealing the scrambled bits
Walsh code Wi (for the ith channel) spreads this rate to 1.2288 Mcps
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Modulation Structure Of Forward Traffic Channel
Convolutional Encoder
and Repetitionr =1/2, K = 9
User data(Taffic)
Block Interleaver
(24x16)
Long Code generator Decimator Decimator
MUX
Long code for ith user
1.2288 Mcps
1.2288 Mcps19.2 kbps
Power Control
Bit
800 Hz
Walsh Code i
(use 1/64x24)Scrambling(use 1/64)
I-Chl Pilot PN Seq
Q-Chl Pilot PN Seq
9600 bps4800 bps2400 bps1200 bps
BalancedQPSK
(Short Code of BS;215-1=26.67msecs)
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Access Channel
Access channel is used by the mobile to transmit control information like call origination and response to paging
Data rate is 4800 bps Each access channel is identified by a distinct long
code sequence having an access number, a paging channel number associated with the access channel and other system data
Types of access channel messages are Registration message, Order message, Origination
messages, Page response message, Authentication challenge response message and so on
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Modulation Structure of Access Channel
Convolutional Encoder
and Repetitionr =1/3, K = 9
Block Interleaver
32x18
Long Code generator
Long code Mask of Access Channel
1.2288 Mcps
Pilot PN Seq I Chl
4800 bps
28.8 Kbps
64-aryOrthogonalModulator
Pilot PN Seq Q ChlData
burstrandomizer
Code symbol
Walshchip
307.2 Kcps{ 28.8 (64/6) } D
1/2 chip delay
Modulator
OffsetQPSK
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Reverse Traffic Channel
This channel is used for carrying user traffic Data rates are 9600, 4800, 2400 and 1200 with
lower data rates being associated with low voice activity
Convolutional coding (rate 1/3), symbol repetition and block interleaving make the data rate 28.8 kbps
Interleaved data at 28.8 kbps is modulated using Walsh code with 6 bits being used as symbol to select one of 64 Walsh codes to get modulated data at 307.2 ksps
PN long code generated using user’s ESN as seed is used for taking out the repeated data and for spreading this 4-fold to 1.2288 msps
Finally the Pilot PN code is used for scrambling
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Modulation Structure of Reverse Traffic Channel
Convolutional Encoder
and Repetitionr =1/3, K = 9
Block Interleaver
32x18
Long Code generator
Long code Mask for user i
1.2288 Mcps
Pilot PN Seq I Chl
9600 bps4800 bps2400 bps1200 bps
28.8 Kbps
64-aryOrthogonalModulator
Pilot PN Seq Q ChlData
burstrandomizer
Code symbol
Walshchip
307.2 Kcps{ 28.8 (64/6) } D
1/2 chip delay
Modulator
OffsetQPSK
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Power Control
Types of Power Control Open Loop Power Control Closed Loop Power Control – based on feedback
Open Loop Power Control (on forward link) Channel state on the forward link is estimated by
mobile Reverse link transmit power made proportional to
forward link channel loss Works well if forward link and reverse link are
highly correlated• which is generally true for slowly varying
distance and shadow losses• but not true with fast multipath Rayleigh
fading
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Closed Loop Power Control in Reverse Link Reverse link subjected to
Inner Loop Power control – for overcoming Near-Far problem
• Control bits are punctured into the traffic data stream
• Closed loop power control step size is +/- 1 dB
• Errors on account of power control bits recovered by error decoding
Outer Loop Power control – for maintaining performance of individual links
• Done by means of messages
• Takes somewhat longer time to effect changes Both open (outer) and closed (inner) loops drive the
transmit power to ensure a target FER of 1-2 %
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Base station measures the received Eb/Io Compares it with the `Target Eb/Io’ and generates
power UP/DOWN command Sends UP/DOWN command to mobile asking it to
increase or decrease the transmit power PC rate must be fast enough (approx 10 times the
max Doppler BW) to track multipath fading At 900 MHz Carrier frequency and 120 km/h
mobile speed, Doppler = 100 Hz In IS-95A, closed loop power control is operated
at 800 Hz update rate Propagation and processing delays are critical to
loop performance
Closed Loop Power Control in Reverse Link (Contd.)
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Position Of Power Control Bits
20 21 22 23 0 1 2 3 4 5 6 7 8 9 10 1112 131415 16 17 18 19 2021 22 23 0 1 2 3
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1 Frame (20 ms) consists of 16 Power Control Groups
1.25 msec
1.25 msec = 24 scrambled traffic data bits
Power Control Bit repeated twice
20212223
0 0 1 1
The values of these 4 bits determine the location of power control bit in the subsequent frame (1100 equals 12 and hence this starts
at bit position 12)
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Generating Long Code
110001111(9 bits)
Access Channel Number
Paging Channel Number (3 bits)
Base Station Identification (16 bits)
Pilot Offset of Forward Channel
(9 bits)
(5 bits) (16 bits) (9 bits)
Access Channel
Reverse Traffic Channel
1100011000(10 bits)
Permuted ESN (32 bits)
Permuted ESN = (E0, E31, E22, E13, E4, E26, E17, E8, E30, E21, E12, E3, E25, E16, E7, E29, E20, E11, E2, E24, E15, E6, E28, E19, E10, E1, E23, E14, E5, E27, E18, E9)
(3 bits)
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Operation Of Data Burst Randomizer
Data Burst Randomiser
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1514
Operating at 2.4 kbps
PCG Bits transmitted
1.25 msec (1 PCG)
20 msec (16 PCGs)
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Forward And Reverse – Key Differences
Forward Reverse
Synchronous CDM – Walsh codes provide channelisation
Asynchronous CDMA – Long codes provide channelisation
Short code provides scrambling code, helps in identifying a BS and also provides pilot channel for timing recovery
Short codes provide scrambling code and helps in identifying BS
Data rate made up to 19.2 kbps by repeating
Data rate made up to 28.8 kbps by repeating and the extra bits are removed by randomiser
Walsh codes provide orthogonal spreading – unique for each channel
Walsh codes enablle 64-ary orthogonal modulation
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Forward And Reverse– Key Differences (Contd.)
Forward Reverse
Rate ½ Convolutional coding is used for error control
More robust rate 1/3 rate Convolutional coding is used for error control
Simple open loop power control used
Open loop power control as well as closed inner loop and closed outer loop power controls are used
QPSK modulation is used Balanced OQPSK is used to get a tighter control of spectrum
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Forward / Reverse Traffic Channel Payload
171 bits0 F=12 T=8
80 bits F=8 T=8
40 bits
T=8
T=8
16 bits
48 bits (20 msec)
96 bits (20 msec)
24 bits (20 msec)
192 bits (20 msec)
F – Frame Quality Indicator bits (CRC bits)
T – Tail bits (all zeros)
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Blank And Burst & Dim And Burst
Signaling / Secondary Traffic Bits (168)1
Primary traffic bits (80)
Primary trafficBits (40)
Primary traffic (16 bits)
172 bits0/1
1
1 0/1
0/1
11
00
01
1 0/1 10
Signaling / SecondaryTraffic Bits (88)
Signaling / Secondary Traffic Bits (128)
Signaling / Secondary Traffic Bits (152)
- MM bit (1) - TT bit (1) - TM bits (2)BB – Blank & Burst Format
DB – Dim & Burst Formats
MM – Mixed Mode TT – Traffic Type TM – Traffic Mode
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Transmission Formats (Full Rate)
MM Bit
TT Bit
TM Bits Voice Bits (Primary)
Data Bits (Secondary)
Signaling Bits
0 - - - 171 - -
1 0 1 1 - - 168
1 1 1 1 - 168 -
1 0 0 0 80 - 88
1 1 0 0 80 88 -
1 0 0 1 40 - 128
1 1 0 1 40 128 -
1 0 1 0 16 - 152
1 1 1 0 16 152 -
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General Transmission Scheme - Reverse Data rate ranges from 1200 to 9600 bps
This rate is made upto 19200 bps by rate 1/3 coding and repetition of 8 to 1
64 orthogonal Walsh codes are used in modulation
This data is spread to a channel chip rate of 1.2288 Mcps using user specific long code PN-sequence and (BS specific) Pilot PN short code sequence
Each IS-95 channel occupies 1.25 MHz of spectrum Channel chip rate is 1.2288 Mchips/s Traffic channels power control is as per base
station command
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Reverse Link Channel Arrangement
...... .. .
Reverse CDMA Link (1.23 MHz channel received by base station
. .AccessChannel(PCH1)
AccessChannel(PCH1)
AccessChannel(PCHN)
TrafficChannel# 1
TrafficChannel# T
Addressed by Long Code PN
FundamentalCode Channel
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IS-95 – Key Facts
CDMA/FDD based technology No need for frequency planning Walsh codes used in forward link for channelisation
and in reverse for modulation Supporting voice coding at 9.6 (4.8, 2.4, 1.2) kbps Channel spacing of 1.25 MHz Per user gross rate of 19.2 kbps Power control in reverse and forward link for interfere
reduction Soft handoff improves handoff efficiency
Hard handoff (make before break) not supported Offered at least three fold spectral efficiency
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IS-95 Evolution To IS-95B
Voice quality in IS-95 was not adequate Data rates in IS-95A was enhanced to 14.4, 7.2, 3.6 &
1.8 kbps (called Rate Set 1 or RS1) in addition to IS-95 rates of 9.6, 4.8, 2.4 & 1.2 kbps (Rate Set 2 or RS2) Main idea was to support QCELP-13
IS-95B defined forward and reverse traffic channels having 1 fundamental code channel and up to 7 supplementary channels Data rates of up to 76.8 kbps in RS1 (8 x14.4) or
115.2 kbps in RS2 (8 x 14.4) can be supported Data Inter Working Function (IWF) was defined for
supporting packet data using these traffic channels Inter frequency hard handoff supported in addition to
soft handoff leading to better interworking
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Forward Traffic Channel Arrangement In IS-95B
...... ... .
Forward CDMA Link (1.2288 MHz channel transmitted by base station
Pilot
Sync
W0 W7W8
W63W32 W1
PCH#1 PCH#7
Code#1 Code#N
Code#P
Code#55
Code#M
FTCH FTCH with multiple code channel
FundamentalCode Channel Mobile Power
Control Subchannel
SupplementaryCode Channel
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Reverse Traffic Channel Arrangement In IS-95B
...... .. .
Reverse CDMA Link (1.23 MHz channel received by base station
. .AccessChannel(PCH1)
AccessChannel(PCH1)
AccessChannel(PCHN)
TrafficChannel# 1
TrafficChannel# T
Addressed by Long Code PN
FundamentalCode Channel
SupplementaryCode Channel
SupplementaryCode Channel
.. ..