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An Advanced course
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Table of Contents
Part I: Fundamentals of CDMA System
I.1 Introduction to CDMA System
I.2 CDMA system overview
I.3 spreading and scrambling
I.4 Multi-user downlink overview
I.5 Interference and processing gain
I.6 Power control
I.7 Quadrature spreading and modulation
I.8 Long codes, short codes and Walsh functions
Part II: UniversalMobile Telecommunication System (UMTS)
II.1 The Third Generation Frequency
The UMTS standard.
3G /UMTS 4 Zone concept data rates and applications.
II.2 UMTS Evolution
II.3 UMTS service concept.
II.4 The UMTS Network.
II.5 Identities
II.6 UMTS Radio Access: Basic Principles.
II.7 UTRA Aspects
Power control.
RAKE receivers.
Handover.
II.8 Radio Interface: Transport Concept
I.9 CDMA logical channels
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Part I:Fundamentals of CDMA System
I.1 Introduction to CDMA system
CDMA is a spread spectrum technology that uses digital code
division, notfrequency or time division, to realize multiple
access. The Telecommunications
Industries Association (has defined a standard that uses CDMA
for cellular It iscalled Standard (IS) 95.
Older wireless systems, like AMPS, relied on frequency division
to divide thespectrum among users. Right now, these systems are
being pushed to theircapacity.
new wireless system that could:
Offer much higher capacity along with improved service
quality.
Support future wireless services inc luding voice, data, and
video.
Be the basis for fur ther growth in the industry.
CDMA also of fers substantially improved:
Voice quality Coverage characteristics Privacy
CDMA carriers can be used in
Cellular RF spectrum (about 850 MHz).
Personal communication system (pcs) spectrum (about 1.9
GHZ).
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Multiple Access
Multiple access means that many users can access the system at
the same time.
Different mul tiple access methods use different strategies
to:
Divide the radio spectrum into channels. Al locate those
channels among users. Identify dif ferent users on an RF
carrier.
Multiple Acess TechniqeIt is used because the limitation of
transmission resources
comparing with the number of users
FDMA
- Large frequency range is divided into smaller slices called
channels.
-all Advanced Mobile Phone Service (AMPS) analog cellular
systems use FDMA.
-Only one user at a time is ever assigned to an FDMA channel.
Other users cannotaccess th is channel unt il the caller hangs up
or is handed-off to another channel.
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TDMA
TDMA systems use digital technology to divide the radio spectrum
by time so thateach RF carrier is shared by several users. This
multipl ies system capacity.
The RF carrier is divided into units of time called frames. Each
frame is further
divided into time slots. Each user is assigned a time slot (or
slots) in every timeframe.
CDMA
It supports many users on each RF carrier with relatively wide
bandwidth, anddistinguishes users by using digital codes. The same
process that encodes speechsignals also spreads them over a much
wider bandwidth than other multiple access
systems.
CDMA carriers1.23 MHz. Spread signals have different properties
from thenarrowband signals.CDMA transmissions appear more like
noise than likeinformation signals.
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For decades. The objective has always been to determine the
narrowest possiblebandwidth within a given frequency spectrum to
make more users channels.
CDMA system Introduction.
Spread spectrum signal is generated through a process called
Direct Sequence (DS)Spreading. The incoming digital speech signal
is multiplied by a digital pseudo-noise(PN) code through a process
called bit stream multiplication.
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Pseudo-Noise
-The PN codes used in spreading is unique to each user.
-spreading not only widens the user's signal, it also dig itally
encodes it.
-spread spectrum signals f rom all users are combined&
transmitted on a single RF
-often reception, dispreading process, which also requires the
subscriber's PN code
each mobile identifies the signal meant for it& return that
signal to i ts narrow band state.
Spread spectrum & privacy.
- Difficul t to jam, interfere with or identify.- Conventional
radio receives can't detect or decode CDMA message.
The Key to CDMA
Uses unique, digital PN codes to distinguish subscribers on the
samecarrier.
produces spread spectrum, noise-like, transmissions
Strength of CDMA
- Increase capacity, frequency reuse =1 & cell sites can be
larger.- Enhanced privacy due to digital coding.- Enhanced call
quality. Better and more consistent sound.- More reliable.- Reduce
interference from other sources.- Lower transmit RF power level,
longer battery l ife.
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I.2 CDMA System Overview
3 Major processes
-Spreading - transmission & reception - recovery
Downlink: Transmission from base station to mobile unite
(forward link).
Uplink: Transmissions from mobile unite to base station (reverse
link).
Downlink Overview
Now let's take a conceptual look at the downlink process from
beginning to end.
o Downlink Overview - The Input Signal
Downlink signal processing begins at the MSC where speech coders
convertincoming voice from 64 kbps to13 kbps or 8 kbps. This low
bit rate digitizedspeech is then transmitted to the base station
via
Downlink Overview - Signal Spreading
On the downlink, direct sequence (DS) spreading occurs at the
base station.Spreading combines the incoming digital speech signal
with the digital PN codethrough a process called bit stream
multiplication.
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o Downlink Overview - Transmission and Reception
Transmission is accomplished through processes called quadrature
spreading andquadrature modulation. Reception involves processes
called quadraturedespreading and quadrature demodulation.
o Downlink Overview - Signal Recovery
On the downlink, signal recovery occurs at the mobile through
the despreadingprocess. The output of downlink signal recovery is
analog voice.
Uplink Overview
On the uplink, signal spreading and transmission occur in the
mobile.
The spread signal from each mobile is transmitted at a minimum
needed power levelto the base station where signal recovery
occurs.
Uplink vs. Downlink
Although simi lar in theory, upl ink and downl ink processing
are qui te dif ferent inpractice. You wil l see that signal
processing in the mobile unit is quite different thanat the base
station. These differences are due the fact that downl ink
reception iscoherent whereas uplink reception is noncoherent.
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I.3 spreading and scrambling
There are three ways to spread the bandwidth o f the signal:
1-Direct sequence.
2-Frequency hopping.
3-Time hopping.
1- Direct Sequence Spread Spectrum
Bit Stream Multiplication Concepts
Bit stream multiplication is the process where an input bit
stream of informationcalled b(t) is multiplied by a pseudo-noise
(PN) code called c(t) to produce a newcomposite output s ignal
called y(t).
When the inputs to the multiplier b(t) and c(t) have the same
bit rate, theoutput y(t) will also have the same bit rate. In this
case the information signalb(t) is scrambled by the mul tipl ier
and the data rate remains the same.
When the input b(t) has a lower bit rate than the PN code c(t),
the output ofthe multiplier y(t) will have the higher bit rate
equal to the rate of c(t). In thiscase, the information signal b(t)
is spread by the multiplier and the data rateincreases.
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In the figure above, a spreading code c(t) is shown coming from
a pseudo-noise (PN)generator. The PN code is generated vs. being
stored in memory. This code will becalled the long code (privacy
code) or the short code depending upon its purpose(e.g., if used
for encryption it is called the long code or privacy code).
Multiplication is used to spread or scramble an input signal. As
this figure shows,mult iplication is also used to recover the
original signal at the receiver.
During generation, the input digital speech signal b(t) is
multiplied by a PN code to
create a composite product signal y(t). The product is RF
modulated, transmitted,and demodulated at the receiver. In the
receiver, the composite signal y(t) ismultiplied again by the same
PN code c(t) in order to recover the digital speechsignal b(t).
The generator and receiver must use the exact same PN code in
order to recover thedigital speech s ignal. Synchronization of the
received signal y(t) and the PN code c(t)at the receiver is also
required for signal recovery.
Mathematically, the product of b(t) and c(t) is sometimes shown
as "bc" , and afterrecovery may be shown as "bcc" . The following
formulas hold true for bit st ream
multiplication:
b(t) c(t) = y(t)
b(t) c(t) c(t) = b(t)
y(t) c(t) = b(t)
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In order for the multipl ication principles of CDMA to hold
true, bipolar coding is usedwith the assumed physical values of +1
and -1.
With bipolar coding, logical "1" bits are represented by
positive "+1" physicalsignals, and logical " 0" bi ts are
represented by negative "-1" physical signals.
Once the logical bit values are represented by physical +1 and
-1 bits, themultiplication formula b(t) c(t) c(t) = b(t) holds
true. That is because c(t) c(t)always equals +1. Look at the fol
lowing examples:
b c c = b
1 1 1 = 1
1 -1 -1 = 1
-1 1 1 = -1
-1 -1 -1 = -1
The same c(t) signal must be used in both multiplication
functions in order torecover the original signal b(t).
Spreading and Despreading
Remember that spreading occurs when a lower bit rate input
signal b(t) is multiplied
by a higher chip rate spreading code c(t) as shown.
When the chip rate of the product y(t) is greater than the bit
rate of the inputinformation signal b(t), this causes spreading of
the input signal in the frequencydomain. The input signal is said
to have its spectrum spread.
With spreading, the product y(t) will always have a greater
bandwidth than the inputsignal b(t). The product y(t) will always
have the same bandwidth as the spreadingcode c(t).
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Two bits of a digital speech signal, b(t), across the top. The
bit interval or bitduration is Tb,said "T sub B." (The bit rate is
1/Tb.)
The PN code, c(t), is shown in the middle. Its chip interval is
Tc, said "T sub C."
In this example, the spreading code c(t) and the product y(t)
have a bandwidthwhich is 5 times greater than the input information
signal b(t). Therefore, thebandwidth or spectrum of the product
y(t) is increased by a factor of 5 to 1.
The term chip rate is used to describe the bit rate of the
spreading code c(t).Thesmallest time duration of any bit in the PN
code c(t), and subsequently in the producty(t),is called a time
chip (Tc).
Now, let's look at the process of despreading.
The product of c(t) times c(t) is always equal to +1 (once the
local c(t) and theincoming signal are synchronized).
This is why b(t) c(t) c(t) = b(t).
Therefore, despreading takes y(t) and multiplies again by the
same spreading PNcode c(t) to recover the digi tal information
signal b(t).
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Let's look at this graphically.
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Spreading and Power Spectral Density
Now, consider what happens to the power spectral density of a
signal when it isspread. This figure shows the power spectral
density for b(t), c(t), and y(t).
The digital information signal b(t) has a high amplitude and
narrow bandwidth asshown. The bandwidth of b(t) is expressed as
fb.
The spreading PN code and the output spread signal both show a
low amplitude andwide bandwidth. Notice that the bandwidth of c(t)
is much greater as expressed by fc.Therefore, spreading a signal
has the effect of lowering the amplitude and wideningthe
bandwidth.
Although the shapes of power spectral densi ty are very di
fferent, the total power ofthe signal (total area) is equal. The
total power of y(t) is the same as b(t), however ithas been spread
over a greater bandwidth.
Spreading does not change total power. Spreading changes how the
power isdistributed over frequency.
The following formulas hold t rue:
Fb=1/Tb(the bit rate of the input signal)
Fc=1/Tc(the chip rate of the spreading code)
G (processing gain) = Fc/Fb
When a signal is spread, the bandwidth of the signal's spectral
density (power/Hz) isincreased by the factor G, called processing
gain, and its amplitude is reduced bythe same factor G.
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Processing gain (G):
is the ratio of the bandwidth of the narrowband information
signal to the bandwidthof the final transmit ted signal.
Calculate G by dividing the bit in terval (Tb) by the chip
interval (Tc).
Since the bandwidth is the reciprocal of the interval, G can
also be calculated bydividing the bandwidth of the spreading signal
(Fc= 1/Tc ) by the bandwidth of theinput signal (Fb= 1/Tb).
The following formula holds true.
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Direct Sequence Spread
Spectrum Transmitter
Direct Sequence Spread
Spectrum Receiver
Direct Sequence Spread
Spectrum Using BPSK Example
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Frequency-hopping spread spectrum (FHSS)
Frequency-hopping spread spectrum (FHSS) is a method of
transmitting radiosignals by rapidly switching a carrier among man
frequency channels, using apseudorandom sequence known to both
transmitter and receiver.
Frequency Hopping Spread
Spectrum System (Transmitter)
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Frequency Hopping Spread
Spectrum System (Receiver)
Slow and Fast FHSS
Frequency shi fted every Tc seconds Duration of signal element
is Ts seconds
Slow FHSS has Tc Ts Fast FHSS has Tc < Ts Generally fast FHSS
gives improved performance in noise (or jamming)
Time-hopping spread spectrum (THSS)
Time-Hopping in communications, a type of spread spectrum
technology in whichthe carrier is turned on and off by the
pseudorandom code sequence.
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PN Codes
Now let's take a closer look at pseudo-noise (PN) codes. You've
seen that it isspreading and scrambling using PN codes that gives
CDMA signals their noise-likeappearance.
You'll recall that although PN codes do eventually repeat, they
do so in a way that isvery hard to predict. PN codes are not stored
in memory. Instead, they aregenerated. This is because of their
size. As you will see, IS-95 defines two PN codesknown as the short
code and the long code. The PN short code repeats only after 215-1
or over 65 thousand bits, and the long code repeats only after 2
42-1 or almost 4.4trillion bits. That would require lots of
memory.
Although PN codes appear noise-like, they do contain specific bi
t patterns that al lowfor signal correlation, which gives a measure
of how much two signals are alike.
The PN code generator is a digital device that uses a shift
register that has m stages,that is, it takes m shifts to move an
input bit to the output of the register. The periodof a PN code, or
the time it takes to repeat, is a function both of the number of
stagesin the PN code generator and the duration of the time chip.
Consider this formula fora typical digital design:
L = 2m-1 where: "L" = is the number of chips that make up
theperiod and"m" = the number of stages in the generator.
Because 24-1 = 15, it is true that the 15-chip PN code below was
created by a 4-stage
generator.
The period of the PN code, or sequence, is calculated using the
formula below.
LTc= (2m
- 1)Tc
where: "L" = is the number of chips that make up theperiod
"m" = the number of stages in the generator.
"Tc" = the duration of 1 time chip
Therefore, LTcis the period of the PN code -- the time it takes
to repeat itself.
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Code Generation Circuit ry
The PN code stream is generated in an m-stage linear recursive
generator like theone shown below. Each stage is a digital
flip-flop with the output of one stagefurnishing the input to the
next stage. When several flip-flops are similarlyconnected, it is
called a register.
The flip-flop is a basic digital storage circuit which has an
input, an output, and aclock signal. Whenever the clock signal is
in a constant state, the output does notchange (it always remains
either 1 or 0). However, when the clock changes, theoutput becomes
whatever the input was.
Notice that the input to the first flip-flop stage is generated
by modulo-2 addition ofthe outputs from different stages. Depending
upon the specific stages which aremodulo-2 added to generate the
input to the first stage, the output sequence canhave different
properties. If the output repeats with maximum possible number
of
bits, it is called a maximal length sequence. Note that some of
the maximal lengthsequences also have the important pseudo-noise
properties of a PN code.
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Walsh CodesWalsh Codes
64 Sequences, each 64 chips long
A chip is a binary dig it (0 or 1)
Each Walsh Code is Orthogonal to all other WalshCodes
This means that it is possible to recognize andtherefore extract
a particular Walsh code from amixture of other Walsh codes which
are filteredout in the process
Two same-length binary strings are orthogonal ifthe result of
XORing them has the same numberof 0s as 1s
Coding Process on CDMACoding Process on CDMA
Forward ChannelsForward Channels
WALSH
19
BTSPilot Walsh 0
Walsh 19
Pagi Walsh 1
Walsh 6
Walsh 11
Walsh 20
Sync Walsh 32
Walsh 42
Walsh 37
Walsh 41
Walsh 56
Walsh 60
Walsh 55
PN OFFSET 116BTS
PN OFFSET 226BTS
PN OFFSET 510BTS
SPN
372
x
x
x
PN OFFSET
ANALOG
SUM/MUXPN OFFSET 372
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I.4 Multi-user downlink overview
Multi-User Downlink Overview
You have seen this diagram of overall CDMA signal
processing.
1. Spreading
2. RF Modulation3. Transmission4. Reception5. RF Demodulation6.
Despreading
At the base station, the signal meant for each mobile is fi rs t
spread. These signalsare then combined and transmitted. The same
signal is received at each mobile butthrough a different path and
with different propagation properties. Each mobile thenuses its own
PN code to despread the combined signal. It then detects the
particularsignal meant for it.
In the drawing below, the subscript "1" is used with all
variables associated withMobile A. For example, c1(t) is Mobile A's
PN code and y1(t) is its signal afterspreading. Similarly,
subscript "2" is used for Mobile B and subscript "3" is used
forMobile C.
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Transmit Receive
At the base station, there is a data stream, or bi t st ream,
for each subscr iber. Theinput is represented here for each
user:
b1(t) for mobile A
b2(t) for mobile B
b3(t) for mobi le C
Next, the base station uses each subscr iber's PN code to spread
the data stream:
y1(t) = b1(t)c1(t)
y2(t) = b2(t)c2(t)
y3(t) = b3(t)c3(t)
Now, because there are multiple users, the system adds together
the coded datastreams for all the users.
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Frequency ReuseFrequency Reuse
CDMA (IS-95)
frequency reuse = 1TDMA (IS-136)
frequency reuse = 7
Green HandsetGreen Handset
SystemsMean
transmission
power
Maxtransmission
power
GSM 125 mW 2W
CDMA 2 mW 200mW
Low transmission power:
Accurate power control, handoff
control, voice activation
Low transmission power:
Accurate power control, handoff
control, voice activation
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CDMA Techniques
CDMA 2000 System
Cdma2000 has already been implemented to several
networks as an evolut ionary step from cdmaOne as
cdma2000 provides full backward compatibility with
IS-95B. Cdma2000 is not constrained to only the
IMT-2000 band, but operators can also overlay a
cdma2000 1x system, which supports 144 kbps now
and data rates up to 307 kbps in the future.
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CDMA SystemCdma2000 Technical summary
Frequency band: Any exist ing band.Minimum frequency band
required:
1x: 2x1.25MHz, 3x: 2x3.75Chip rate:
1x: 1.2288, 3x: 3.6864 McpsMaximum user data rate:
1x: 144 kbps now, 307 kbps in the future
1xEV-DO: max 384 kbps - 2.4 Mbps,
1xEV-DV: 4.8 Mbps.
Frame length: 5ms, 10ms or 20msSpreading factors: 4 ... 256
UL
digital public land mobile communication
network(PLMN)
BSC MSC/VLR
HLR/AUC EIR MSC/VLR
OMC
SC
PLMN
PSTN
BTS
Um
Abis
A
BTS
No.7 BSSAP
No.7 MAP
No.7 MAP TUP
No.7 TUP
X.25/ No.7
X.25
Signaling voice
BSS
MSS
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Concept
BSC:Base Station Controller
BTS:Base transceiver station
MSC:Mobile Switching Center
HLR:Home Location Register
VLR:Visitor Location Register
AUC:Authentication Center
SC:Short Message Center
OMC:Operation and Maintenance Center
1.Main components of CDMA
MSS(mobile switching sub-system)
BSS(base station sub-system)
MS(mobile station)
OMM(operation and maintenance sub-system)
CDMA service area
PLMN Service area
MSCService area
cell
Location area
Wireless cover area structure
wireless Area partition
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I.6 Power control
Near/Far Problem
Consider the CDMA system uplink. Some mobiles may be very close
to the basestation while others are far away. If all the mobiles
are using the same carrierfrequency, then they are all creating
interference for each other.
If all mobile units are transmitting with the same power, then
the signal received atthe base station will be strongest for the
mobile unit nearest to the base station andweakest for the mobile
farthest away.
CDMA systems use technique called power control (controls the
transmitted powerby monitoring both the received signal Eb&
noise N0).
The goal of Power control is to achieve optimum power levels in
order to serve the
maximum number of subscribers.
o Each direction (up link & downl ink) uses different power
contro l procedure.
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I.9 CDMA logical channels
Overview of Logical Channels
Each physical channel supports approximately 15 users with
current technology.
Each physical channel is divided into several logical channels
by the Walsh
functions (downlink) or the unique long code (uplink).
Theoretically, on the downlink each physical channel can be
divided into 64 logicalchannels using every Walsh funct ion.
However, interference begins to degrade userssignals when more that
15 users share one IS-95 carrier
The logical channels can be broken down into two groups:
Control Channels used for transmission of control and system
setupinformation such as timing or synchronization and to pass
control messagesbetween the base station and mobile unit when the
mobile unit is not on atraffic channel.
Traffic Channelsused for transmission of user data (speech) once
the mobileunit is in an active talking state. The traffic channel
also passes associatedsignaling over the uplink and downlink once
it is setup for a call.
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Hand over
Types of CDMA HandoffSoft Handoffs:-Soft handoffs occur when
the
mobi le is involved in a call. CDMA uses the mobile
to assist the network in the handoff. The term sof thandoff is
used to describe the make-before-break process which takes place
during the
handoff.
Soft handoffs occur between cells, sectors in a
cell, or combination of cells and
sectors.
Types of CDMA Handoff
Hard Handoffs
Hard handoffs occur when the mobile is involved
in a call. During a hard handoff a CDMA phone is
not able to assist the network in the handoff. Theterm hard
handoff is used to describe the
break-before-make process that occurs during
the handoff hand over .
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Multi-path fading
Rake receiver
A rake receiveris a radio receiver designed tocounter the
effects of multipath fading. It does this byusing several
"sub-receivers" called fingers, that is,several correlators each
assigned to a differentmultipath component. Each finger
independentlydecodes a single multipath component; at a later
stage
the contribution of all fingers are combined in order tomake the
most use of the different transmissioncharacteristics of each
transmission path. This couldvery well result in higher
signal-to-noise ratio
(or Eb/N0) in a multipath environment than in a
"clean" environment.
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Multi-path fading rejection
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CDMA advantage
In this section the following topics wil l be discussed:
CoverageCapacity
Clarity
Cost
Compatibility
Customer Satisfaction
CDMA advantage
Coverage
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CDMA advantage
Capacity
CDMA advantageClarity
CDMA advantage
Cost
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CDMA advantage
With my best wishesAhmed
[email protected]
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