cdma basic
TRANSCRIPT
1
CDMA Concepts andCDMA Concepts andApplications in Wireless PCSApplications in Wireless PCS
NetworksNetworks
WFI Technical Training Series:
Developed By: Kamran Etemad
2
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
InformationInformationSourceSource
Source Encoder
Source Encoder
ChannelEncoder
ChannelEncoder
DigitalModulator
DigitalModulator
Information Information DestinationDestination
Source Decoder
Source Decoder
ChannelDecoder
ChannelDecoder
DigitalDemodulator
DigitalDemodulator
AnalogWaveformChannel
Review of a Basic Communication SystemReview of a Basic Communication System
l The function of source coding is datacompression.
l Removing redundancies of thesignals, in its original form, andrepresenting it with minimumnumber of bits.
l Signal compression may be lossy orlossless.
l Question:» How do we compress analog signals such
as voice and music?» Which Applications require lossless
compression?
Source CodingSource Coding
Information Information BitsBits
Channel Channel BitsBits
CompressedInformation
Add Redundancyto Protect Info. bits.
Channel CodingChannel Coding
l Channel Coding: adds redundant bits to information bitsuch that Protects Information Bits against Channel Noiseand Interference by increasing the distance between validcodes.
(Eb/Io)minCapacity
l Using more powerful channelcoding and modulation schemesincrease the tolerance againstnoise and interference.
l This means for a given Bit ErrorRate (BER) coding reduces therequired (Eb/N0).
BER
(Eb/Io)
Without Coding
With Coding
CodingGain
Coding GainCoding Gain
A B C D E F G C E G A F B D Interleaver
A B C D E F G C E G A F B D De-Interleaver
ErrorsErrorsErrorsErrors
InterleavingInterleavingl Conventional FEC schemes work best when the errors are
randomly distributed in time as opposed to being clusteredin bursts.
l In mobile radio channels, however, errors tend to occur inbursts due to fading effects.
l The function of interleaver is randomization of errors intime.
l The bits' order of transmission is altered, so that uponundoing this altering at the receiver, the errors appear tohave random rather than correlated locations
8
Q
IX X
X X
QPSK
X
ωω1
X
X
ωω3
ωω2FSK
Q
IX X X X
ASK
l A Digital Modulator maps a block of L bits toone of 2L Waveforms suitable for transmissionover a physical channel.
l Examples:» ASK (Amplitude Shift Keying)» FSK (Frequency Shift Keying)» PSK (Phase Shift Keying)» QAM (Quadrature Amplitude Modulation)
is a hybrid modulation
Q
IX
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
16QAM
Examples of Digital ModulationsExamples of Digital Modulations
Insecure, Unreliable Digital Fading Channel
Insecure, Unreliable DigitalMemoryless Channel
Information Destination
Source Decoder
Channel Decoder
DemodulatorInsecure AnalogFading Channel
Deinterleaver
Secure, Reliable, DigitalMemoryless Channel
Decryption
Review of FunctionalitiesReview of Functionalities
10
Spread Spectrum IdeaSpread Spectrum Idea
l Based on Shannon’s Capacity equation:» C=W x log(1+S/R)» A spread spectrum communication is designed so that
the system can operate at much lower signal to noiseratio using a much larger bandwidth.
l Starting from a typically narrowband information Signall The energy of the signal is spread over a much larger
bandwidth using:» Direct Sequence Spreading:
– Modulating each information bit by a high rate sequence DirectSequence Spreading
» Frequency Hopping:– Randomly hopping the sub-carrier frequency within a wide
spectrum.
11
SourceEncoder
ChannelDecoder
Modulator
ChannelEncoder
SourceDecoder
Demodulator
PNSource
PNSource
Wideband Wireless Channel
Identical &SynchronizedPseudo-Noise
High RateSignals
TRANSMITTER RECEIVER
Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum
DS-Spread Spectrum FeaturesDS-Spread Spectrum Features
l Interference Rejectionl Anti-jamming Communicationl Frequency Diversity Against Multipath Fadingl Low Probability of Interceptsl Secrecy and Securityl Code Division Multiple Access (CDMA)
Capabilityl Provides high capacity and spectral efficiency in a
cellular network environment.l Provides no advantage over a pure additive
Gaussian Noise Channel.
13
ApplicationsApplications
l Military Based Applicationsl Second Generation Cellular and PCS
Systems (IS95)l Wireless Local loop Systemsl Third Generation/IMT2000 Systems
(CDMA2000, WCDMA,..)l Mobile-Satellite Systems (Global-Star)l ….
BTS
BTS
BTS
BSC
BTS
BTS
BTS
BSC
ISDN
PSTN
AUC
OMC
HLR VLR
MSC
DataNetworks
MS
MS
Base Station Subsystem
Network SwitchingSubsystem
PublicNetworks
Cellular Network ArchitectureCellular Network Architecture
15
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
16
Binary and Bipolar SequencesBinary and Bipolar Sequences
l Example of a binary sequence» 1,1,0,0,1,1,0,0
l Each binary sequence can be represented in abipolar form by mapping the ‘1’s to ’+1’s and ‘0’sto ‘-1’s. Example» 1,1,0,0,1,1,0,0 +1,+1,-1,-1,+1,+1,-1,-1
l For convenience, in our correlation analysis we usebipolar representation of binary sequences
[ 1 -1 -1 1 -1 1 ]
[ 1 1 -1 1 1 -1 ]
0
[ 1 -1 -1 1 -1 1 ]
1
[ 1 -1 -1 1 -1 1 ]
C1 & C2 are Orthogonal
Autocorrelation
Cross-correlationC1C1
C1C1
C2C2
C1C1<C1,C1>=1<C1,C1>=1
<C1,C2>=0<C1,C2>=0
C1 is Normalized
.dt∫
.dt∫
Correlation Between Two SequencesCorrelation Between Two Sequences
[ -1 1 1 -1 1 -1 1 -1-1 1]
Information Bits
Spreading Sequence
After Spreading
“1” “0”
-1
+1
Information Bits
SpreadingSequence
Spreading is achieved whenmultiplying the signal by thespreading sequence.
Spread Spectrum Signal
Spreading WaveformsSpreading Waveforms
Information SignalBefore Spreading
Information SignalAfter Spreading
R
W
Same Power i.e.Same Area
Frequency
2 Spreading Increases the rate of alternations andtherefore Bandwidth.
Effects of Spreading in SpectrumEffects of Spreading in Spectrum
20f
R
W
Processing GainProcessing Gain
l …Therefore, Spreading involves dividing each bit time intoL equal chip times and modulating the bit interval by asequence p(n) of length L.
l As a result of this multiplication/modulation thebandwidth of the transmitted signal increases by about afactor of L, thus the term spread spectrum.
l The ratio of bandwidth after spreading (W) to informationbit rate (R) is called processing gain (GP),» Thus : GP = W/R=L.
l Despreading is accomplished by correlating the receivedwaveform with the the same sequence p(n).
21
[ 1 -1 -1 1 -1 1 ]
[ 1 -1 -1 1 -1 1 ]
C1C1
C1C1
The original low rate “Information bit”
[ 1 -1 -1 1 -1 1 ]
[ 1 1 -1 1 1 -1 ]
C1C1
C2C2
Another high rate signal
freq.
freq.
freq.
freq.
Using theMatchingCode
Using theWrongCode
Despreading: Time/Frequency ViewsDespreading: Time/Frequency Views
1
0
.dt∫
.dt∫
Narrowband Signal
S.S. Interference
l Within the bandwidth of a narrowband system theSS signal looks like a white Gaussian noise.
SS Interference on NB SystemsSS Interference on NB Systems
Narrowband Interference
Narrowband Interference
Desired SS signal
Desired Signal
Before Despreading
After Despreading
Effect of Despreading NB InterferenceEffect of Despreading NB Interference
Despreading (review)Despreading (review)
l As a result of correlating with a user specific code at thereceiver:» The signal from the intended user gets despread.» The additive white gaussian noise remains the same.» The narrowband interference gets spread and appears as AWGN.» Also all unintended S.S. signals, that have been spread using
different codes, remain spread and therefore appear as AWGN.
l This is the basis for a multi-user secure communicationsystem based on spread spectrum idea.
25
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues» Link Budget for CDMA Systems» CDMA and 3rd Generation Wireless PCS Systems
Basic CDMA ConceptsBasic CDMA Concepts
l CDMA: assigns one distinct spreading code toeach user
l As long as the codes are orthogonal or almostorthogonal all users can send and receive theirsignal through the same wide band channel.
l Other users’ signals appear like noise.
Frequency
time
f
t
f
t
f
t
f
t
Code
User1: C1
User2: C2
User3: C3
User4: C4Spread Spectrum Channel
Time
27
CDMA and Universal Channel ReuseCDMA and Universal Channel Reusel A CDMA system allows multiple access using
a single CDMA channel.l The same channel can be used in adjacent
cells. Thus CDMA allows a universal reusepattern, or reuse of one.
l The universal reuse implies:» A significant improvement of spectral efficiency
because of increased spectrum available per cell.» A tremendous amount of Co-Channel Interference.
– Because of spread spectrum nature of signals allco-channel interference appear like noise tointended user.
– Since different base stations or users, usedifferent codes with almost zero correlation, thereceivers can reject co-channel interference aspart of despreading.
F1F1
F1
F1F1
F1
F1
Almost Orthogonal Code Sequences
Frequency
FF11 FF22 FF33 FF44
C D M
A
F D M A
PN1
PN2
PN3
PNn-1
PNn
Example: Multiple Access in IS95Example: Multiple Access in IS95
l In IS95, Physical Channels are formed based on acombination of Code Division Multiple Access (CDMA)and Frequency Division Multiple Access (FDMA).
C1
C2
C3
Cn-1
Cn
Uplink Downlink
C’1
C’2
C’3
C’n-1
C’n
Example: IS95 CDMA ChannelsExample: IS95 CDMA Channels
l Each CDMA Frequency Assignment (FA) consists of a pairof 1.23MHz channels for downlink and uplink.
l Within each CDMA RF channel, or FA, signals to and fromvarious users are distinguished using different codes.
l The spreading codes used in forward and reverse link aredifferent.
30
Forward and Reverse Link CDMAForward and Reverse Link CDMA
l In the forward link there are two levels ofspreading:» Each base station uses a different code, so that the
interference from adjacent cells can be rejected at themobile’s receiver..
» Within each cell, the base station uses a set oforthogonal codes for channelization, to separatedifferent users information signals.
– Different users signal are first spread by a distinct code,– Then all spread spectrum signals for all users are added– and the composite signal is spread by the BS stations specific
code.
l In the reverse link each user uses a differentspreading code.
31
A Two Receiver ScenarioA Two Receiver Scenario
l A CDMA base station (BS) intends to send a “1” to user 1.l The BS spread the information bit by code C1.
» User 1 uses C1 for despreading» The other user, User 2, uses a different code C2, which is orthogonal
to C1.
User 1
User 2
[ ]C1 1 1 1 1= + + − −, , ,
[ ]C2 1 1 1 1= + − + −, , ,
[ ] [ ]{ }< >= × + + − − + − + −
= × − + − =
C C1 2 1 4 1 1 1 1 1 1 1 1
1 4 1 1 1 1 0
, / , , , , , , ,
/ { }
Note That C1 andC2 are Orthogonal.
32
[ ]~ , , ,C C n n n n n1 1 1 1 1 11 2 3 4= + = + + − −
Channel Noise
RX= <C1, C1+N> = <C1,C1> + <C1,N> = 1 + ε’
After correlating with the same code
Correlating With the Same CodeCorrelating With the Same Code
[ ]C1 1 1 1 1= + + − −, , ,
[ ]C1 1 1 1 1= + + − −, , ,
33
Channel Noise
RX= <C2, C1+N> = <C2,C1> + <C2,N> = 0 + ε’
After correlating with a different code
Correlating With a Different CodeCorrelating With a Different Code
[ ]C1 1 1 1 1= + + − −, , ,
[ ]C2 1 1 1 1= + − + −, , ,
[ ]~ , , ,C C n n n n n1 1 1 1 1 11 2 3 4= + = + + − −
34
A More Realistic Downlink ScenarioA More Realistic Downlink Scenario
T a C T a C
T a C
T T T T
TX T D
N N N
N
A
1 1 1 2 2 2
1 2
== ====
== ++ ++ ++== ⊗⊗
. , .
. . . .
. . .
R X L TX L T D
R L R X T D
T T D D T
r T C a C a C a C C
r a C C a C a C C
r a a
A
A
A A
N N
N N
N
== ×× == ×× ⊗⊗== ×× ≈≈ ⊗⊗
=<=< ⊗⊗ >=>==<=< >=<>=< ++ ++ ++ >>=<=< >> ++ << ++ ++ >>== ++ ++ ++ == ++
( )
( / *)
( ),
, . . . . . . ,
. , . . . . . ,
( . . . )
1
1 1 1 1 2 2 1
1 1 1 1 2 2 1
1 1 21 1 1
)
εε εε εε
TX:At the BaseStation A
RX:At the MobileStation 1
Spreading with BS’s Code
Same CellChannelization Codes
2nd level despreading
Match Filtering
Despreading withBS’s Code
35
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
36
Spreading CodesSpreading Codes
l To maintain all signal power, after spreading anddespreading, the spreading sequences» Have to be Mutually Orthogonal to each other or» Have noise like characteristics with very small cross correlation.
l Example of such codes are Walsh codes & Pseudo-Noise(PN) codes.
l Walsh Codes are perfectly orthogonal to each other. Theycan be obtained from different rows of Haddamardmatrices.
l The PN codes» have Noise-like characteristics, e.g. Sharp Autocorrelation» Are easily implementable using shift registers» Are Periodic, Long and» Difficult to reconstruct from a short segment
H
H
HH H
H H
HH H
H Hn
n n
n n
1
2
42 2
2 2
2
2 1 2 1
2 1 2 1
0
0 0
0 1
0 0
0 1
0 0
0 10 0
0 1
1 1
1 0
=
=
=
=
=
− −
− −
[ ]
. . . . .
Hadamard Matrices
Rows areOrthogonalto each other.<Ci,Cj>=0
Rows areOrthogonalto each other.<Ci,Cj>=0<Ci,Cj>=0
C0C1C2C3
l In IS95 Walsh Codescorresponding to rows of H64are used
l There are only N orthogonalsequences of length N.
Orthogonal Sequences: Walsh CodesOrthogonal Sequences: Walsh Codes
38
Walsh Codes are Mutually Orthogonal:Walsh Codes are Mutually Orthogonal:
HH H
H H
C C
84 4
4 4
0 0
0 1
0 0
0 10 0
0 1
1 1
1 0
0 0
0 1
0 0
0 10 0
0 1
1 1
1 00 0
0 1
0 0
0 10 0
0 1
1 1
1 0
1 1
1 0
1 1
1 01 1
1 0
0 0
0 1
2 518
1 1 1 1 1 1 1 1
1 1 1 1 1 1 1 1
18
1 1 1 1
==
==
<< >=>= ×× << −− −− ++ ++ −− −− ++ ++
−− ++ −− ++ ++ −− ++ −− >>
== ×× ++ −− −− ++
, ( , , , , , , , ),
( , , , , , , , )
( −− ++ ++ −− ==1 1 1 1 0)
C2
C5
Convert to Bipolar
Pseudo-Noise CodesPseudo-Noise Codes
l The PN codes are pseudo-random sequences» They are deterministic codes which mimic randomness properties
l Randomness or Noise-like characteristics include» Sharp Autocorrelation» “1”s and “0”s appear randomly and independently in a sequence» The number of “1”s and “0” are (almost) the same in any long
segment of the sequence.» Difficult to reconstruct from a short segment
l Additional desirable properties for PN codes are» Easy to Implement» Periodic & Long
l Examples are :» m-Sequences,» Gold and Kasami codes
40
Pseudo-Noise CodesPseudo-Noise Codesl PN codes have very sharp autocorrelation,l It implies that the time-shifted code versions of the same
PN sequence have very small correlation with each other.l For a long periodic PN sequence of length N this
correlation is very small, i.e close to 1/N, so that differenttime-shifted version or offsets of the same pseudo-randomsequence are almost orthogonal to each other.
-1/N
0
RS (t,t+δδ)
1 PN Chip
Period of the Code
Generating PN SequencesGenerating PN Sequences
l Maximal Length Shift Register Sequences, also called m-sequences, aregenerated by an m stage shift register with appropriate linear feedbackconnections defined by prime polynomials with modulo 2 arithmetic.
l The specific feedback configurations, used, ensure than the sequence hasits maximum period, i.e. 2m-1.
l By loading different initial value into the shift register, one cangenerate different offsets of the same sequence.
+
1 2 3 4 . . . . . m-1 mOutput
Feedback
Initial Value
Example: Spreading Codes in IS95Example: Spreading Codes in IS95
l Walsh Codes» 64 Orthogonal Codes (W0-W63)» Each of Length 64 Chips
l Short Codes» A PseudoNoise M-Sequence» Generated by a Maximal Length Shift Register» of Length 215 and period 215 -1 Chips
l Long Codes» A PseudoNoise M-Sequence» Generated by a Maximal Length Shift Register» of Length 242 and period 242 -1 Chips
43
l Orthogonal Walsh Codes:» Walsh codes of length 64 are used in IS95 forward link» There are 64 Walsh codes used to isolate forward link
channels within one cell.» Examples:
l W0: 0000.......................000l W32: 0000....0001111....111
64bits
32bits 32bits
Spreading Sequences in IS95Spreading Sequences in IS95
….010...1110.......10010010011
(215-1) Chips
BS1
BS2 ….10.......10010010011 010...11PN Offset
(i x 64chips)
Same SequenceDifferent Offsets
PN Offset of Short CodesPN Offset of Short Codes
l The Short Code is an “m-sequence” of period 215-1 chips!!l Different BS’s use different offsets of the “short code”.l Each station (or sector) uses only one PN offset.l There are 512 possible offsets, of 64 chips apart, to be
assigned to base stations.
45
Usage of CodesUsage of Codes
Station A
Short Codes: Sa and SbLong Codes:L1 ,L2 and L3Walsh Codes: W1-W63
Sa.W23Sa.W12
Station B Sb.W23
L1 L2
L3
l Short codes are used for spreading as BS’s ID in the forward link.l Long Codes are used for scrambling and spreading as MS’s ID in the
reverse linkl Walsh codes are used for forward link channelization.
Need for SynchronizationNeed for Synchronization
l Spread Spectrum Signals are typically high ratesignals with very sharp autocorrelations
l Correlation based receivers rely heavily on almostperfect synchronization.
l Therefore, maintaining the synchronization has adirect effect on identifying the desired fromundesired signals.
Synch. Out of Synch.
TX
RXRX RXRX
TX
TX
RXRX
l Coarse Synchronization is performed for Code Acquisition.
l Fine Synchronization is performed during code tracking.
Time
Time
Course and Fine SynchronizationCourse and Fine Synchronization
Chip Time
δδδδ
Code Acquisition MethodsCode Acquisition Methods
l Code Acquisition Circuits can be implemented using» a parallel bank of correlators» or a sliding correlator with a feedback
u1
u2
Correlator with p (t-(2Nc-1) Tc) v 2Nc-1
InputSS Signal
Correlator with p (t-2Tc)
Correlator with p (t-nTc)
...
PN Generator
+
-
Adjust n
Correlator with p (t-Tc)
Sliding CorrelatorParallel Bank of Correlator
x
x
x bandpassfilter
envelopedetector
envelopedetector
bandpassfilter
loopfilter
clockVCO
PN generator
-
+
P(t+τ) to Data Demodulator
P(t+Tc/2+τ)
P(t-Tc/2+τ)
Delay Locked Loop
Code Tracking MethodsCode Tracking Methods
l After Code Phase Acquisitionwe need to adapt to timevariations and maintainlocking condition.
l Code tracking circuits operateusing some sort of a feedbackloop. For example» Delay Locked Loop» Tau Dither Loop
50
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues» Link Budget for CDMA Systems» CDMA and 3rd Generation Wireless PCS Systems
51
Multipath Effects on NB signalsMultipath Effects on NB signals
l Because of multipath effects, for each transmitted symbol, the receiver,receives a combination of the main symbol and and its echoes.
l In a Narrowband System (e.g. most TDMA based systems) the symbolis relatively wide in time so the main symbol and its echoes overlap intime. This overlap, called InterSymbol Interference (ISI), is not desiredand causes erroneous detection.
l Therefore most NB systems use adaptive equalizers to cancel ISI.l An equalizer in a NB system tries to estimate multipath components
and cancel them.
Transmitted Symbols
Received Symbols
52
Multipath Effects on WB signalsMultipath Effects on WB signals
l Because of multipath effects, for each transmitted symbol, the receiver,receives a combination of the main symbol and and its echoes.
l In a wideband system (e.g. most CDMA based systems) the symbols arerelatively narrow in time so a symbol and its echoes do not overlap intime and therefore they are resolvable.
l Most WB systems use a Rake Receiver to estimate and combine thesignal coming from multipath.
Transmitted Symbols
Received SymbolsMultipaths are Resolvable
Rake Receiver and MultipathRake Receiver and Multipath
T T+ t1 T+ t2
Delay Line &Correlators
Adaptive Combiner
ττ22
r t dtT
( )(. )∫
ττ11
r t dtT
( )(. )∫
ττ33
r t dtT
( )(. )∫
Input Data
MultiPath Components
BS1
Rake Receiver utilizes the spatial diversity.
Station A
Station B
Station A & B
Soft Hand-offSoft Hand-offl The mobile station continuously
scans for pilot signals transmittedby different stations/sectors andestablishes, both uplink and downlink, communication with up to 3stations whose pilot power exceedsa certain threshold.
l This results in a make before breakprocedure for Hand-off, where duringthe transition from one cell toanother the call is served by multiplecells.
l These simultaneous links to multiplebase stations is a form of spatialdiversity which provides a morerobust and smooth Hand-off andimproves capacity and coverageperformance of the system.
Soft HandoffSoft Handoff
Time Margin
Station AStation A
Station BStation B
TADD
TDROP
Signal Margin
Stations A & BStations A & B
Soft Handoff Region
Time/Space
Ec/
I o
Rake Rec. in Soft Handoff (Downlink)Rake Rec. in Soft Handoff (Downlink)
T T+ t1 T+ t2
Delay Line &Correlators
Adaptive Combiner
ττ22
r t dtT
( )(. )∫
ττ11
r t dtT
( )(. )∫
ττ33
r t dtT
( )(. )∫
Input Data
BS1 BS2
SelectionCombining
MSC
Soft Handoff and Spatial DiversitySoft Handoff and Spatial Diversityl There is a diversity gain associated with soft handoff in
both reverse and forward link.l The major gain is in the reverse link due to combiners at
each base station and the selective combining at the MSC.l Rake receivers in both forward and reverse link
contribute to this spatial diversity gain.
RAKE Rec. 1RAKE Rec. 1
RAKE Rec. 2RAKE Rec. 2
DiversityCombiner
RAKE Rec. 1RAKE Rec. 1
RAKE Rec. 2RAKE Rec. 2
DiversityCombiner
Selection DiversityCombining
Vocoder
2 Finger
2 Finger
2 Finger
2 Finger
BS1
BS2
MTSO
To PSTN
Selection Diversity in SHO (Uplink)Selection Diversity in SHO (Uplink)
59
SHO, Power Control and InterferenceSHO, Power Control and Interference
l SHO reduces the average transmit power of mobiles in thehandoff area» A mobile in soft handoff powers up only if all BS’s involved in soft
handoff ask for more power and» it powers down as soon as one of BS’s ask him to power down.
l Therefore statistically mobile’s transmitted power isreduced and so it contributes less to interference level in thesystem.
BS1 BS2
UP only if UP1 and UP2Down if Down1 or Down2
MSCMSCBS1
BS2
BS3
Adv. and Disadv. of Soft Hand-offAdv. and Disadv. of Soft Hand-off
l + Improvements in RF interface» Reduction in Interference» Improvement in Coverage» Increase in Capacity
l + Improvement in Voice Qualityl - Additional overhead to Allocate
» Channel Element» PowerFor users in soft Hand-off.
l In LBA to account for the Soft Hand-offDiversity Gain, for %30-%50 of users in softHand-off region, 2-3 dB is considered. Effectively,this gain is due to a reduction in the fade marginfor the combined signal.
61
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues» Link Budget for CDMA Systems» CDMA and 3rd Generation Wireless PCS Systems
Reverse Link InterferenceReverse Link Interference
Same Cell Interference: Isc Other Cell Interference: Ioc
Forward Link InterferenceForward Link Interference
Same Cell Interference: Isc Other Cell Interference: Ioc
Energy per BitEnergy per Bit
S = Received Signal PowerR= Bit Rate=#Bits/SecondEb= Energy per Bit = Signal Power/ (# Bits per Second)
EEbb= S/R= S/R
Assuming Perfect Uplink Power ControlAssuming Perfect Uplink Power Control
P1
P3
P2
R2R1
R3
P1 > P2 > P3P1 > P2 > P3 R1=R2=R3=SR1=R2=R3=SSuch Such ThatThat
Assuming Perfect Power Control
Interference Power Spectral DensityInterference Power Spectral Density
Interference Spectrum
Thermal/BackgroundNoise
Other Users’Interference
NNoo
IISCSC
IItt
No= Noise Spectral DensityIsc = Same Cell Interference Spectral DensityIt =Total Interference Spectral Density
IIscsc = Total Interference/BandwidthNN00= Noise Power/BandWidthTotal Interference Power =IItt.W=(N.W=(N00+I+ISCSC).W).W
Freq.
Total Interference (Single Cell)Total Interference (Single Cell)
PN-1
SSS
S S
(N-1)SP2
PN
P1
Total Interference= Other (N-1) users Interference + Thermal Noise
It W= IscW+NoW = (N-1)S +NoW
IIt t = (N-1)S/W + N= (N-1)S/W + N00
Assuming Perfect Power Control
EEbb/I/Itt
EI
S RN S W N
S RN S W
W RN
b
t
=− +
≈−
=−
/
[( ) ] /
/
[( ) ] /
/
1 1 10
Ignoring thebackground noiseE
IS R
I WE I E N
W RN
E N
NW R
E N
Capacity NW R
E NW R
E N
b
t totalb t b
b
b
b b
= > =
−>
< +
= = + ≈
/
/( / ) ( / )
/( / )
/
( / )
/( / )
/( / )
min min
min
min
maxmin min
0
0
0
0 0
1
1
1
Common Terminology
Coding and Processing GainCoding and Processing Gain
BER
(Eb/Io)
Without Coding
With Coding
CodingGain
(Eb/Io)min
Coding
Capacity
☺.
l Using more powerful channelcoding and modulation schemesincrease the tolerance againstnoise and interference.
l This means for a given Bit ErrorRate (BER) coding reduces therequired (Eb/N0).
l Also note the direct and expliciteffect of Processing Gain on thecapacity.Capacity
☺.
W/R
Speech ActivitySpeech Activity
60%Inactive(Silence)
40%Active
(Speech)
Human’ Speech signal has a duty cycle of about νν=40%
Effect of Speech Activity FactorEffect of Speech Activity Factor
ν νii
N
N
S N S N Seffective
= × − ≈ −=
−
∑1
1
1 0 4 1( ) . ( )1 24 34
ννi=1 with probability 0.4
ννi=1 with probability 0.6
ννi=1 with probability 0.4
ννi=1 with probability 0.6
N-1 mobiles,Only 0.4(N-1) active
RX=Neff. xS
Effect of Voice Activity on CapacityEffect of Voice Activity on Capacity
N NW R
E N
CapacityW R
E N
effb
b
. maxmin
min
/
( / )
/
( / ).
= ≈
=
ν
ν
0
0
1
Voice Activity Gain
> 1
l The effective capacity increases because of voice activity.l The increase in capacity is achieved without additional
overhead signaling and protocol considerations.
Effect of SectorizationEffect of Sectorization
l For 120o sectored cites, comparedto an Omni-Cite» Almost 1/3rd Interference received in
the uplink» Causes almost 1/3rd Interference in
the uplink
l Reduction in interference resultsin higher capacity in both links.
I I
G
CapacityW R
E NG
Sector Omni
S
bS
≈
= ≈
=
1 3
2 55 3
0
/
.
/( / )
.min
SectorizationGain
Sectorization GainSectorization Gain
l Interference reduction due to directional antenna patternsresults in increase in capacity.
l For three sectors the sectorization gain is close to 3.l There is no loss of trunking efficiency because all sectors of
a cite use a common pool of channels.
Reuse Efficiency FactorReuse Efficiency Factor
Ioc: Other Cells InterferenceIsc: Same Cell Interference = f= f
Total Interference= Isc + Ioc = (1+f ) Isc
EI
S RI I W N
S Rf I W N
EI
S Rf N S W N
W Rf N
b
t sc oc sc
b
t
=+ +
=+ +
=+ − +
≈+ −
/
( ) /
/
( ) /
/
( )[( ) ] /
/
( )( )
0 0
0
1
1 1 1 1ν νReuse Efficiency >1Effect of Other Cells
Ignoring thebackground noise
Ignoring thebackground noise
Effect of Effect of f f on Capacityon Capacity
EI
W Rf N
W Rf N
CapacityW R
E N f
b
t
b
≈≈++ −−
≈≈++
==++
/( )( )
/( )
/( / )
.( )min
1 1 1
110
νν νν
νν
Reduction in Capacitydue to other cellInterference
77
Loading FactorLoading Factor
l The simple capacity equation which ignores the effect ofnoise, called the pole capacity, is theoretical limit toCDMA cell capacity.
l To achieve this limit mobile has to transmit at infinitepower and the system becomes unstable.
l For stable operation of the system a loading factor of %50to %80 is usually considered.
CapacityW R
E N fG L
bS=
+/
( / ).( )
. . .min0
11
1
ν
Pole Capacity
Loading Factor
Big Picture (Reverse Link Cell capacity)Big Picture (Reverse Link Cell capacity)
CapacityW R
E N fG L
bS=
+/
( / ).( )
. . .min0
11
1
ν
Processing Gain
Speech AcivityGain
SectorizationGain
Minimum TechnologyRequirement Frequency
Reuse Efficiency
LoadingFactor
79
ExerciseExercise
l Using this capacity equation and the following practicalassumptions compute the cell capacity for CDMA system.
l Eb/No=7dB=5l Reuse Efficiency Factor f=0.55l Bandwidth W=1.23MHzl Data Rate R=9.6kbpsl Voice activity v=0.4l Sectorization Gain Gs= 2.65l Loading L=60%
.LPC. GSHO
>1<1
CapacityW R
E N fG L
bS=
+/
( / ).( )
. . .min0
11
1
ν
Some Implicit Effects on CapacitySome Implicit Effects on Capacity
l Soft handoff improves Capacity» Because of spatial diversity gains, users in soft handoff region
demand less power from BS and transmit less power therefore theycontribute less to the interference. Reduced Interference meansimprovement in capacity.
l Power Control errors reduces the capacity .» In all of calculations we assumed perfect power control. The effect
of errors in power control is modeled as a factor which is adecreasing function of the error variance.
Soft CapacitySoft Capacity
l In a CDMA system the capacity islimited by a threshold on acontinuous variable i.e. signalquality.
l It is always possible to allow oneextra user by sacrificing some ofthis quality, for all users.
l In FDMA/TDMA systems we havehard capacity, because capacity ishard limited by the number of RFcarriers and number of time slots.
# Active Users
Qu
alit
y In
dex
(E
b/N
o)
Desired Quality
NominalCapacity
More Users &less Quality
82
How About Forward Link Capacity?How About Forward Link Capacity?
l While the reverse link capacity is limited by aggregateinterference effects the forward link is power limited.
l The forward link capacity is defined as the maximumnumber of users, for whom the base station can provide» distinct code channels» enough power
l The code channel limitation practically never dominates.l The power limitation depends on user locations
» the worse case is when all users are far at the cell periphery, inwhich the base can support only few of them.
» The average case, where users are uniformly distributed, in whichcase the forward link capacity estimated using simulation and it isusually higher than reverse link.
l Therefore the CDMA cell capacity is usually determined bythe reverse link.
83
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
1.23MHz
41 x 30KHz AMPS Channels.
. . .
3dB
Downlink or Forward Channel
Uplink orReverse Channel
fN+ 45MHz
Physical ChannelsPhysical Channels
l Each CDMA channel occupies 1.23MHz of spectrum whichis equivalent of 41 AMPS channels.
l All CDMA channels within the Cellular band are organizedbased on AMPS channels.
[825+0.03N] MHz [870+0.03N] MHz for N=1,2,..,799[825+0.03(N-1023)] MHz [870+0.03(N-1023)] MHz for N=990,991,...,1023
Uplink
Downlink
A Band B’A’’ B Band A’
991
1023
1 333
334
666
667
716
717
799
Channel Numbering for Cellular BandChannel Numbering for Cellular Band
BlockDesignator
BandwidthAllocated
(MHz) Uplink Downlink
A (MTA) 30 (15/15) 1850-1865 1930-1945
D (BTA) 10 (5/5) 1865-1870 1945-1950
B (MTA) 30 (15/15) 1870-1885 1950-1965
E (BTA) 10 (5/5) 1885-1890 1965-1970
F (BTA) 10 (5/5) 1890-1895 1970-1975
C (BTA) 30 (15/15) 1895-1910 1975-1990
PCS BlocksPCS Blocks
D CA B E F
[1850+0.05N] MHz N=0,1,..,1199
Uplink Downlink
0 1199 0 1199
[1930+0.05N] MHz N=0,1,..,1199
D CA B E F20MHz
80MHz
1850MHz
CDMA PCS BLOCKSCDMA PCS BLOCKS
l PCS blocks (A, B and C) are 15MHz wide pairs, whereas blocks (D, Eand F) are 5MHz wide pairs.
l PCS spectrum allows up to 1200 center frequencies (and thereforeCDMA channel numbers) of 50KHz separation.
BlockDesignator
Preferred Set of CDMA Channel Numbers
A 25, 50, 75, 100, 125, 150, 175, 200. 225, 250, 275
D 325, 350, 375
B 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675
E 725, 750, 775
F 825, 850, 875
C 925, 950, 975, 1000, 1025, 1050, 1075, 1100, 1125, 1175
Preferred Channels for PCS Preferred Channels for PCS
l CDMA carriers are arranged in the middle of preselected channels toallow for sufficient guard bands, the set of these channels is called thePreferred set.
l To access the CDMA system in block A, the mobile station scans thePreferred set of black A until it finds a pilot channel. If no service isfound it may search for Preferred set of another block say block B.
System Information & Paging Information
Access Request
Voice Information & Signaling
Voice Information & Signaling
Control & Voice ChannelsControl & Voice Channels
l Pilot Channell Synch Channell Paging Channell Traffic Channel
» User Traffic Data» Blank & Burst
Signaling» Dim & Burst Signaling» Power Control
l Access Channel
Signaling &Control
IS95 Logical ChannelsIS95 Logical Channels
PilotSynchPaging
TrafficChannels
Forward Link ChannelsForward Link Channels
Pilot ChannelPilot Channel
l Pilot Channel is an unmodulated DSS signal continuouslytransmitted by each CDMA base station used to uniquelyidentify the base station.
l It transmits Walsh-0 (W0) sequence.l It serves as a phase reference for timing, bit
synchronization and coherent demodulation in the downlink
l Also since Pilot is not subject to dynamic power control itprovides a reference for comparing the signal strength ofdifferent base stations.
l Therefore pilot channel plays the major role in determiningbest server and servers in soft hand-off.
Synch ChannelSynch Channel
l Synch channel is demodulated by the mobile right tuning tostrongest pilot.
l It carries some of system ID parameters» System Identification number» Network Identification number
l And some information about timing» Pilot sequence offset index PILOT_PN» Long Code State» System Time» Offset of Local Time» Leap Seconds
l Paging Channel Data Rate.l The data rate of the Synch channel is 1.2kbps.
Paging ChannelPaging Channel
l Paging channel is continuously monitored by the mobileafter reading the information on the Synch channel.
l Some of typical Paging Channel messages are:» System parameters» Access parameters» Page or Slotted Page» Order Messages» SSD update» Data Burst» Authentication» CDMA Channel List» Channel assignment
Forward Traffic ChannelForward Traffic Channel
l Traffic channel carry variable rate voice/data.l In addition to user data/voice, the traffic channels is IS95
also carry some signaling information.» These signaling subchannels are associated to and are
time multiplexed with users data on the traffic channel.» In the Forward Traffic channel the following messages
are sent to the mobile– Order messages– Data Burst– Hand off Direction– In-Traffic System parameters– SSD Update– Power Control Parameters– Neighbor-list Update– MS Registered Message
Pag
ing
Cha
n. 1
Tra
ffic
Cha
n. 1
Pag
ing
Cha
n. 2
Pag
ing
Cha
n. 3
Pag
ing
Cha
n. 4
Pag
ing
Cha
n. 5
Pag
ing
Cha
n. 7
Pag
ing
Cha
n. 6
Tra
ffic
Cha
n. 2
Tra
ffic
Cha
n. 3
Tra
ffic
Cha
n. 2
4
Tra
ffic
Cha
n. 5
4
Tra
ffic
Cha
n. 5
5
Syn
c C
han.
Pilo
t C
han.
W0 W32 W1 W2 W3 W4 W5 W6 W7 W8 W9 W10 .......... W31W33 .................. W62 W63
Tra
ffic
Cha
n. 2
5
Tra
ffic
Cha
n. N
Traffic DataMobile PowerControl Sub-channel
1.23 MHz BandwidthTransmitted by Base Station
1 Pilot1 Sync7 Paging55 Traffic
Total of 64 Walsh CodesTotal of 64 Walsh Codes
CDMA Forward ChannelCDMA Forward Channel
AccessChannel
Mobile’s:OriginationResponds to OrdersPeriodic Reports
Random Access ChannelRandom Access Channel
l Access Channel Signaling consists of messages related to» Access or Call Origination» Respond to a Page» Authentication» Registration» User Generated Data Bursts for the base station.» Other order messages
l It operates based on a variation of Slotted ALOHAProtocol
l Some of the signaling information transmittedover uplink traffic channel are» Authentication Challenge Response» Power Measurement Report» Pilot Strength Measurement» Hand-off Completion» Dual Tone Multi-Frequency (DTMF) Signaling» Order Messages
– Long Code Transition Request and Response, SSD UpdateConfirmation/Rejection, Parameter Update Confirmation,Service Option Control, Base Station Challenge, Mobile StationAcknowledgment, Release (normal and with power-downindication), Local Control, Mobile Station Reject (with andwithout a reason).
Uplink Traffic Channel SignalingUplink Traffic Channel Signaling
..................................
Acc
ess
Cha
n. 1
Tra
ffic
Cha
n. 1
Acc
ess
Cha
n. 2
Acc
ess
Cha
n. n
Tra
ffic
Cha
n. 2
Tra
ffic
Cha
n. m
Addressed by Long PN CodesAddressed by Long PN Codes
1.23 MHz BandwidthReceived by Base Station
CDMA Reverse ChannelCDMA Reverse Channel
100
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
Variable RateSource EncodingVariable Rate
Source Encoding
Channel CodingChannel Coding
Bit InterleavingBit Interleaving
Long Code ScramblingLong Code Scrambling
Walsh and QuadratureSpreading
Walsh and QuadratureSpreading
Quadrature CarrierModulation
Quadrature CarrierModulation
Variable RateSource DecodingVariable Rate
Source Decoding
Channel DecodingChannel Decoding
Bit De-interleavingBit De-interleaving
Long Code De-scramblingLong Code De-scrambling
Walsh and QuadratureDe-spreading
Walsh and QuadratureDe-spreading
Quadrature CarrierDemodulation
Quadrature CarrierDemodulation
TX RX
WirelessWireless Channel Channel
BasebandProcessing
DownLink ProcessesDownLink Processes
OPERATION CHANNEL RATE
kbps
8 9.6
4 4.8
2 2.4
1 1.2
kbps
Variable Rate
Vocoder
Variable Rate VocoderVariable Rate Vocoder
l There are two rate sets corresponding to» 8 kbps Speech coders (Rate Set I)» 13 kbps Speech coders (Rate Set II)
l At each rate set:» There are 4 possible rates used based on the speech activity.» At lower rates, lower average power is transmitted
LPC filter Coef.
Pitch Parameters (Gain and Lag)
Excitation Parameters (Index and Gain)
MUX
Speech Analysis
Channel Coder
LPC Filter
Excitation
Pitch
Imitation of Vocal Cords
Imitation of Vocal Tract
Speech Waveform
Speech Generation Model
High RateSampled Speech
Low Rate OutputModel parameters
Code Excited Linear Predictive CoderCode Excited Linear Predictive Coder
LPC Filter Coef.Pitch
Codebook Index
LPC Filter Coef.Pitch
Codebook Index
LPC Filter Coef.Pitch
Codebook Index
LPC Filter Coef.Pitch
Codebook Index
40 bits10bits 10 bits 10 bits 10 bits
10b 10b 10b 10b 10b 10b 10b 10b
20 bits
10 bits
10 bits
10bits
10bits
10bits
0 bits
10bits 10bits
10bits 10bits 10bits 10bits
6bits
Rate 1
Rate 1/2
Rate 1/4
Rate 1/8
20 msec frame
171 bits
80 bits
40 bits
16 bits
Encoded Packet
Subframes
Speech Coder Rates (Rate Set 1)Speech Coder Rates (Rate Set 1)
TrafficBlock
Generator
SpeechCoder
CRC
ConvolutionalEncoder++Speech
Blocks
Signaling
Traffic Blocks
ChannelEncoder
Interleaver
Traffic Frames
Mixed Mode Bits
8 Tail Bits
Channel CodingChannel Coding
1/2 Rate Conv. Encoder &
Repeater ( for input rates < 9.6 kps )
1/2 Rate 1/2 Rate Conv. Encoder &Conv. Encoder &
RepeaterRepeater ( for input rates < 9.6 kps )( for input rates < 9.6 kps )
20ms20ms 20ms20ms
Traffic FramesTraffic Frames Coded FramesCoded Frames
Variable RateVariable Rate Fixed Rate Fixed Rate 19.2 kbps19.2 kbps
Convolutional CodingConvolutional Coding
l Traffic data frames are coded using a 1/2 rate convolutional encoder.l The convolutional encoder has a constraint length of 9 and uses an 8
bit shift register that for every input bit generates 2 output bits.l For input rates smaller than 9.6kbps, output bits are repeated to
provide a fixed number of output bits per frame.l So for all input rates there are 384 bits per 20 msec frame which gives
an output rate of 19.2kbps.
Convolutional Encoder&
Repetition Code
BlockInterleaver
Convolutional Encoder&
Repetition Code
BlockInterleaver
Convolutional Encoder&
Repetition Code
BlockInterleaver
Sync ChannelData
Paging ChannelData
Traffic ChannelData
•1.2kbps
•2.4kbps•4.8kbps•9.6kbps
•1.2kbps•2.4kbps•4.8kbps•9.6kbps
4.8kbps
19.2kbps
19.2kbps
Pilot Channel No Data
Forward Link Channel CodingForward Link Channel Coding
Long Code Generator
42 bitLong CodeMask
1/64 Long Code Generator
Scrambling
InterleavedBits 19.2kbpsPaging or Traffic
Encrypted Data 19.2 kbps
Walsh Code Spreading
Long Code ScramblingLong Code Scrambling
l Based on a user specific mask the long code generator, which uses 42 bitshift register, generates a PN sequence of length 242-1 chips of rate1.2288Mbps.
l 1/64 long code generator selects the first chip of every 64 long code chips,and holds it for the duration of 64 chips. This provides a 19.2 kbpssequence.
l The 1/64 long code, used as a MS key, is combined with the input bitstream through XOR operation to encrypts the data.
l The same 1/64 long code is generated at the receiver to undo thescrambling process.
1/64 Long Code Generator
DownlinkPower Control
Algorithm
Power Control Bit position
Power Control Bit
Bit Puncturer
Scrambled TrafficFrames
Walsh SequenceWi
19.2kbps 1.2288Mbps
Power Control Bit PuncturingPower Control Bit Puncturing
l Bit Puncturer replaces 2 consecutive input bits by one power control bitevery 1.25msec.
l The value of this bit is determined by power control algorithms and BSmeasurements from mobile.
l And the position of this bit is determined by long code.
1dB1dB
1.25msecPower Control Group Period
0 0 0 1 0 1 0 1 1 1 1 1 0 0 0 1 0 1 0 1 1 1 1 1
Power Control Bit00
11
timetime
MS PowerMS Power
Change inPower level
+1dB+1dB
-1dB-1dB
Power Control BitsPower Control Bits
Coded & Interleaved
Coded Interleaved
and Scrambled
CodedInterleaved
and Scrambled
W0
+W32
+Wp
+Wi
+
1.22MHz1.22MHz
1.22MHz1.22MHz
1.22MHz1.22MHz
Pilot Channel0kbps
Sync ChannelData
4.8kbps
Paging ChannelData
19.2kbps
Traffic ChannelData
19.2kbps
1.22MHz1.22MHz
Walsh SpreadingWalsh Spreading
Pilot ChannelNo Data, “All 0’s”
Sync ChannelData
4.8kbps
Paging ChannelData
19.2kbps
Traffic ChannelData
19.2kbps
Long CodeGenerator
+
Long CodeGenerator
+
W0
+W32
+Wp
+Wi
+MUX
Power Control Bit
++
++
++
++
Q-Channel Pilot PN Seq.
I-Channel Pilot PN Seq.
Paging Channel Long Code Mask
User i Long Code Mask
1.22MHz1.22MHz
1.22MHz1.22MHz
1.22MHz1.22MHz
Symbol Scrambling
Forward Link WaveformForward Link Waveform
Variable RateSource Encoding
Variable RateSource Encoding
Channel CodingChannel Coding
Bit InterleavingBit Interleaving
Long Code ScramblingLong Code Scrambling
QuadratureSpreading
QuadratureSpreading
Quadrature CarrierModulation
Quadrature CarrierModulation
Variable RateSource Decoding
Variable RateSource Decoding
Channel DecodingChannel Decoding
Bit De-interleavingBit De-interleaving
Long Code De-scramblingLong Code De-scrambling
QuadratureDe-spreadingQuadrature
De-spreading
Quadrature CarrierDemodulation
Quadrature CarrierDemodulation
TX RX
WirelessWireless Channel Channel
BasebandProcessing
UpLink ProcessesUpLink Processes
SpeechCoder
ChannelEncoder
Interleaver
TrafficBlock
Generator
CRC
++Speech Blocks
Signaling
Traffic Blocks
Traffic Frames
Mixed Mode Bits
8 Tail Bits
1/3 Rate Conv. Encoder &
Repeater ( for input rates < 9.6 kps )
1/3 Rate 1/3 Rate Conv. Encoder &Conv. Encoder &
RepeaterRepeater ( for input rates < 9.6 kps )( for input rates < 9.6 kps )
20ms20ms
Coded FramesCoded Frames
Fixed Rate Fixed Rate 28.8kbps28.8kbps
Uses a Different Interleaving Matrix than uplink
Same as downlink
Different Conv. Encoder
Uplink Channel CodingUplink Channel Coding
64ary Walsh Modulator
Coded & Coded & Interleaved bitsInterleaved bits
28.8kbps28.8kbpsTo EncryptorTo Encryptor307.2 kbps307.2 kbps
6 input bits
Each selects one of 64 Walsh Waveformsof 64 bit length.
26=64 combinations
Walsh ModulationWalsh Modulation
l Every block of 6 bits is mapped to one of 64 Walsh sequences of length 64.l This orthogonal modulation improves the error performance of the system.l Note that although Walsh Modulation increases the data rate, it and not
the same as Walsh spreading.
Long CodeLong CodeGeneratorGenerator
307.2kbps307.2kbps 1.2288Mcps1.2288Mcps
Voice PrivacyVoice PrivacyMask GeneratorMask Generator
Long Code Mask42 bits
Walsh ModulatedBits
To QuadratureShort Code
Spreading andModulation
Long Code Spreading/EncryptionLong Code Spreading/Encryption
l The data and signaling on the traffic channel are encrypted/spread witha long code based on user specific long code masks.
l The data on the access channel is not encrypted because the accesschannel long code mask is not private.
Reverse Link WaveformReverse Link Waveform
l No pilot Noncoherent receiversl 1/3 rate convolutional encoder.l 64ary orthogonal Walsh code modulation (not spreading in
down link)l Uplink channels are identified by long PN codes.l Interleaving matrix is different than down link.l Modulation is Offset QPSK.l Message encryption using Long Code Private Mask
IS95 Physical Layer (Rate Set I)IS95 Physical Layer (Rate Set I)
l Multi-access : CDMA combined with FDMAl Bandwidth 1.23MHz per carrierl Voice Circuits up to 55 per carrierl Modulation: Down link :QPSK, Uplink: OQPSKl Speech Coding Variable Rate, QCELPl Channel Coding CRC + Conv. Code + Interleavingl Coding Rate Down link: 1/2, Uplink:1/3l Bit or Chip Rate 1.2288Mcpsl Traffic Rates 9.6, 4.8, 2.4, 1.2 kbps.l Channel Rates Down link: 19.2, Uplink: 28.8
Convulotional Coding Rates:
•Downlink: 3/4
•Uplink: 1/2
Rate Set IIRate Set II
Rate Speech CoderRate
Traffic BlockRate
Channel
Rate1 13.35 14.4 19.2
1/2 6.25 7.2 9.6
1/4 2.75 3.6 4.8
1/8 1.05 1.8 2.4
l Rate Set II is an option that allocates more bits to voice coder and lessto convolutional coder.
l In this set speech and traffic rates are higher but convolutional codingrates are also higher, which adds up to unchanged channel rates.
l For Rate set II» Paging and Access channels are unchanged.» Frame duration, modulation and power control are unchanged.
120
Course OutlineCourse Outline
l Introductionl Part 1: CDMA Concepts
» Spreading/Despreading in Time and Frequency Domains» Concept of Multiple Access Using Codes» Spreading Codes (Walsh and Pseudo-Noise Codes)» Rake Receivers and Soft Handoff» CDMA Cell and System Capacity
l Part 2: Applications: IS95 and 3G-CDMA» Physical and Logical Channel Structure in IS95» Forward and Reverse Link Waveforms» Call Processing and Power Control Issues
Call Processing OverviewCall Processing Overview
l We will discuss the following call processingissues» Mobile Initialization and Access» Registration and Paging» Call Set up and Release (Mobile originated or
terminated)» (Soft) Handoff related Signaling» Power Control
InitializationInitialization
Power UpPower Up
Idle ModeIdle Mode
System AccessSystem Access
Conversation ModeConversation Mode(Traffic/Signaling)(Traffic/Signaling)
?
Mobile Station StatesMobile Station States
Pilot ChannelAcquisition
Synch ChannelAcquisition
Synchronization
System DeterminationAnalog or CDMA
Analog Initialization
To Idle StateTo Idle State
Identify and lock on to the strongest CDMAbase station.
Decodes Synch Channel Messages
Synchronizes it internal timing with the BS.
Analog
CDMA
?
Initialization StateInitialization State
Acquire Primary Paging Channel
Call Origination
Page Response
Registration
Authentication
Idle Hand-off
Update OverheadInformation
Page or Call Origination
No
Access StateAccess State
Idle StateIdle State
System AccessSystem Access
l System Access Mode consists of active or reactive messagetransfers in uplink through access channel.
l These message transactions take place before conversationstage.
l During access mobile BS tells MS about its assigned trafficchannel.
l It consists of the following substates:» MS origination attempt» MS Page Response» Order or message response» Registration Access» Update overhead/configuration information» MS Data Burst message transmission
Mobile Registration TypesMobile Registration Types
l There are various occasion during which MS send aregistration message to BS.» After power on» Before power off» Upon Registration Order» Time based and periodically» Distance based» Zone based» Implicitly during any access» implicitly during conversation on the traffic channel
Page Response
Traf. Ch. Initialization
Waiting for BS’s Order
Waiting for MS’s Answer
Conversation
Call Release
MS verifies the FTC & Transmits on the RTC MS Receives
Release Order
MS Receives Release Order
MS Receives anAlert with Information
MS Answers the Call
MS Receives Release Order or initiates Disconnect
MS Control on the Traffic ChannelMS Control on the Traffic Channel
MSC receivesOriginationfrom PSTN
Null Data (FTC)
Page Message (PC)
Page Response (AC)
Traffic Channel Assignment (PC)
Acknowledgement (FTC)
Traffic Channel Preamble (RTC)
Null Data (RTC)
Service Option Response (FTC)
Alert with Information (FTC)
Connect Order (FTC)
Conversation, Speech Frames (TC)
FTC set up
RTC set up Rec. 2 consecutive valid frames
User Answers, Ring Stops
Starts Ringing
Call flow for Mobile TerminationCall flow for Mobile Termination
Soft Hand-off
Softer Hand-off Soft Softer
Hand-off3-way Hand-off
A1 A2 A3
B1 B2 B3
C1 C2 C3
D1 D2 D3
{D2,C1}
{D1,D3}
{B1,D2,D3}
{A2,B1,D3}
Hand-Off’s in IS95Hand-Off’s in IS95
l Hard Hand-offl Soft Hand-offl Softer Hand-offl Soft Softer Hand-offl Three Way Hand-off
Simultaneous Softer & Soft Hand-off
Channel ElementChannel Element Channel ElementChannel Element
MSC
Signals from the two sectors arecombined locally at the BTS.Only the combined frame is sent toMSC.
Frame selection is performed at MSCbased on the signals received fromthe two sites.
Soft Softer Hand-offSoft Softer Hand-off
EI
PP N W
C
i
i
jj
0 0
==
++∑∑αα
ActiveSet
CandidateSet
Neighbor Set
Remaining Set
Pilot SetsPilot Sets
l MS evaluates each pilot strength based on its powerrelative to the total power received in the forward link.
l Based on their signal strength, the pilots identified by theMS are categorized in Four different sets:
Mobile Station Power ClassesMobile Station Power Classes
MobileStation
EIRP at Maximum Output
Class Minimum Maximum
I -2 dBW (630 mW) 3 dBW (2 W)
II -7 dBW (200 mW) 0 dBW (1 W)
III -12 dBW (63 mW) -3 dBW (0.5 W)
IV -17 dBW (20 mW) -6 dBW (0.25 W)
V -22 dBW (6.3 mW) -9 dBW (0.13 W)
P
S
K.S
P
Interference Spectrum
NNoo
IIoo
IIttEquivalent to K users
Near-Far ProblemNear-Far Problem
Excessive Interference
Poor Signal Quality
Just Enough PowerJust Enough Power
Power ControlPower Control
l The fundamental purpose of power control is to» maintain a satisfactory voice quality subject to» maximizing system capacity and» minimizing power consumption.
l Power Control is applied to:» Mobile Power on initial access» Mobile Power while on the traffic channel» Base Station Power
PilotSynchPaging
TrafficChannels
Within the traffic channel thepower is dynamically allocatedto different users according toto their path loss to maintainthe same voice quality or FERfor all users.
Forward Link: Power AllocationForward Link: Power Allocation
0
5
10
15
20
25
30
35
User1 User2 User3 User4
User1
User4User3
User2
Traffic Channel Power Allocation
Forward Loop Power ControlForward Loop Power Control
∆∆D
∆∆U
PMR shows High FER
N Frames
Forward Link Power Control ProcessForward Link Power Control Process
l MS measures Frame Error Rate (FER), every N frames, on the forwardlink and reports the measurement to the BS.
l The Power Message Report (PMR) contains the number of frames inerror and the total number of frames received during the report timeperiod. The ratio between these two numbers is FER.
l Whenever PMR shows high FER base station powers up by ∆∆Uotherwise it powers down by ∆∆D
Probe 1
1st Attempt 2nd Attempt 3rd Attempt 15th Attempt
RandomTime
Probe 2
Probe 16
Waitingfor ACK
Random Time Access Preamble
1-16 FramesMessage Capsule
3-10 Frames
Time
One Access Channel Slot
Power Increment
Initial Power
Reverse Link Access Power ControlReverse Link Access Power Controll During an access attempt mobile’s power has to be controlled.l Each access attempt consists of the entire process of sending one
message and receiving or failing to receive its acknowledgment.
P
Open Loop: Power control based on mobile measurement of pilot signal strength
Closed Loop: Power control based on BS commandsaccording to its uplink measurements.
Reverse Link Traffic Power ControlReverse Link Traffic Power Control
l Power control in the uplink is done both open loop andclosed loop..
l Open loop PC takes care of slow fading due to shadowingeffects.
l Closed loop PC tries to compensate for multipath fadingeffects.
EI
PP N W
C
t i
i
jj
=
+∑α
0
Open Loop Power ControlOpen Loop Power Control
l The mobile measures the pilot power level from its primarycell along with the total signal received.
l These two measurements, which are also used for Hand-offdecisions, are the basis for open loop power controldecisions.
l The mobile powers up if it receives low Ec/It and powersdown otherwise.
l To avoid too many unnecessary changes in the power dueto fast fading effects on the received signal, the open looppower control has a relatively large response time.
Until Frame Error Exceeds thethreshold.
The set-point value is reducedby a small amount for everyconsecutive frame....
∆∆U∆∆D
Ceiling(10dB)
Floor (10dB)
Time
Eb/No Target or set-point Value
Inner Loop Process
Closed (Outer) Loop Power ControlClosed (Outer) Loop Power Control
dBm Eb
20 msec Frame
1.25 msec
No
1dB
set-point Value From Outer Loop Process
From Outer Loop
Closed (Inner) Loop Power ControlClosed (Inner) Loop Power Control
l BS sends power control bits to the MS to ask it power up or down asneeded to reach the target Eb/No set point determined in the outer loopprocess.
l The power control bits are sent 16 times per 20msec frame.l Each “0” (”1”) bit changes the power level by +1 ( -1) dB.l When set-point is reached the power control bit alternates and
therefore signal level changes +/-1 dB around the set-point.
∆P= ∆POpen + ∆PClosed
Autonomous Slow Large-Scalechanges with largetime constant τ=30msec
Directed Fast changes+/-1dB per 1.25 msec.Dynamic Range 48 dB over 3 frames.
Reverse Link Power Control ( Recap)Reverse Link Power Control ( Recap)
l The faster response time of the closed loop control enablesit to overwrite the open loop commands when it isnecessary.
l The two power control mechanisms are independent andtogether can provide at least 80dB of dynamic range.
Power Control SummaryPower Control Summary
l Objective: Operate BS and MS at optimum power level to» Achieve the minimum FER (e.g. 0.01) to ensure voice quality.» Reduce interference to its minimum and thereby maximize the
operational capacity.» Maximize the battery life of the mobile.
l Process consists of» Mobile Access Power control» Dynamic Allocation of Power among Traffic channels at the BS in
Down Link.» Reverse Link Power Control on the Mobile
– The Autonomous Open Loop control at the MS based on its powermeasurement on the down link.
– The directed or closed loop control based on BS’s Eb/No set points andits power control commands on the traffic channel.
145
Course ReviewCourse Review
4 Introduction4 Part 1: CDMA Concepts
4 Spreading/Despreading in Time and Frequency Domains4 Concept of Multiple Access Using Codes4 Spreading Codes (Walsh and Pseudo-Noise Codes)4 Rake Receivers and Soft Handoff4 CDMA Cell and System Capacity
4 Part 2: Applications: IS95 and 3G-CDMA4 Physical and Logical Channel Structure in IS954 Forward and Reverse Link Waveforms4 Call Processing and Power Control Issues
146
Hope that you enjoyed this course.Hope that you enjoyed this course.
Thank You for Your Participation
147
Useful ReferencesUseful References
l “Applications of CDMA in Wireless/Personal Communications”, VijayK. Garg, Kenneth Smolik and Joseph E. Wilkes, Prentice Hall 1997.
l “CDMA”, Andrew j. Viterbi, Addison-Wesley, 1995l “Wireless Communications, Principles and Practice”, Theodore
Rappaport, Prentice Hall 1996.l “CDMA System Engineering Handbook”, Jhong S. Lee and Leonard E.
Miller, Artech House Publishers, 1998.l “Wideband CDMA for Third Generation Mobile Communications”,
Tero Ojanpera and Ramjee Prasad, Artech House Publishers, 1998.l Magazines:
» IEEE Communications Magazine (Recent Issues)» IEEE Personal Communication Magazines