CDMA Technology Overview February, 2001 - Page 1-1
CDMA Technology CDMA Technology OverviewOverview
Lesson 1 – CDMA Basics
CDMA Technology Overview February, 2001 - Page 1-2
Course ObjectivesCourse Objectives
Upon completion of this course, you will have an understanding of the following concepts:
• CDMA and other access technologies
• CDMA coding, forward, and reverse channels
• Vocoding, multiplexing, and power control
• Components that comprise a CDMA system
• CDMA messaging and call flow
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Why CDMA?
• CDMA is the technology of choice for both 800 MHz Cellular and 1900 MHz PCS service providers
• CDMA satisfies CTIA Users’ Performance Requirements
• CDMA provides high capacity (many times the capacity of AMPS)
• CDMA provides privacy through its coding scheme
CDMA
CDMA
ode
ivision
ultiple
ccess
CDMA is extremely robust and provides excellent audio quality
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What is Multiple Access?
Since the beginning of telephony and radio, system operators have tried to squeeze the maximum amount of traffic over each circuit
Types of Media Twisted pair - copper Coaxial cable Fiber optic cable Air interface (radio signals)
Advantages of Multiple Access Increased capacity: serve more users Reduced capital requirements since fewer
media can carry the traffic Decreased per-user expense Easier to manage and administer
Transmission
Medium
Each pair of users enjoys a dedicated, private circuit through the transmission medium, unaware that the
other users exist.
Multiple Access: Simultaneous private use of a transmission medium by multiple, independent users.
CDMA Technology Overview February, 2001 - Page 1-5
Multiple Access Technologies
The physical transmission medium is a resource that can be subdivided into individual channels according to different criteria depending on the technology used:
Here’s how the three most popular technologies establish channels:
FDMA (Frequency Division Multiplex Access) each user on a different frequency a channel is a frequency
TDMA (Time Division Multiplex Access) each user on a different window period in time
(“time slot”) a channel is a specific time slot on a specific
frequency CDMA (Code Division Multiplex Access)
each user uses the same frequency all the time, but mixed with different distinguishing code patterns
a channel is a unique set of code patterns FrequencyTime
Power
FrequencyTime
Power
FrequencyTime
Power
FDMA
TDMA
CDMA
Channel: An individually-assigned, dedicated pathway through a transmission medium for one user’s information
CDMA Technology Overview February, 2001 - Page 1-6
CDMA System ComponentsCDMA System Components
• Mobile Telephone Exchange (MTX) provides call processing functions for AMPS/TDMA/CDPD/CDMA cellular systems
• Base Station Manager (BSM) provides a Graphical User Interface (GUI) for operations, administration and maintenance of the BSC, BTS and itself
• Base Station Controller (BSC) provides data routing, voice coding and some hand-off functions
• Base Station Transceiver Subsystem (BTS) provides the RF link to the subscriber
• MTX, BSC and BSM are identical for 800 and 1900 MHz products
• Mobile Telephone Exchange (MTX) provides call processing functions for AMPS/TDMA/CDPD/CDMA cellular systems
• Base Station Manager (BSM) provides a Graphical User Interface (GUI) for operations, administration and maintenance of the BSC, BTS and itself
• Base Station Controller (BSC) provides data routing, voice coding and some hand-off functions
• Base Station Transceiver Subsystem (BTS) provides the RF link to the subscriber
• MTX, BSC and BSM are identical for 800 and 1900 MHz products
DMS-MTX BTS
T1 or E1s
MTSO
T1s
BSM BSCMAP
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Defining Our Terms
CDMA Channel or CDMA Carrier or CDMA Frequency Duplex channel made of two 1.25 MHz-wide bands of electromagnetic
spectrum, one for Base Station to Mobile Station communication (called the FORWARD LINK or the DOWNLINK) and another for Mobile Station to Base Station communication (called the REVERSE LINK or the UPLINK)
In 800 Cellular these two simplex 1.25 MHz bands are 45 MHz apart In 1900 MHz PCS they are 80 MHz apart
CDMA Forward Channel
1.25 MHz Forward Link
CDMA Reverse Channel 1.25 MHz Reverse Link
CDMA Code Channel Each individual stream of 0’s and 1’s contained in either the CDMA
Forward Channel or in the CDMA Reverse Channel Code Channels are characterized (made unique) by mathematical codes Code channels in the forward link: Pilot, Sync, Paging and Forward Traffic
channels Code channels in the reverse link: Access and Reverse Traffic channels
45 or 80 MHz
CDMA CHANNELCDMA
ReverseChannel 1.25 MHz
CDMAForwardChannel 1.25 MHz
CDMA Technology Overview February, 2001 - Page 1-8
CDMA Is a Spread-Spectrum System
Traditional technologies try to squeeze the signal into the minimum required bandwidth
Direct-Sequence Spread spectrum systems mix their input data with a fast spreading sequence and transmit a wideband signal
The spreading sequence is independently regenerated at the receiver and mixed with the incoming wideband signal to recover the original data
The de-spreading gives substantial gain proportional to the bandwidth of the spreading signal
CDMA uses a larger bandwidth but then uses resulting processing gain to increase capacity
Spread Spectrum Payoff:Processing Gain
Spread SpectrumTRADITIONAL COMMUNICATIONS SYSTEM
SlowInformation
Sent
TX
SlowInformationRecovered
RX
NarrowbandSignal
SPREAD-SPECTRUM SYSTEM
FastSpreadingSequence
SlowInformation
Sent
TX
SlowInformationRecovered
RX
FastSpreadingSequence
Wideband Signal
CDMA Technology Overview February, 2001 - Page 1-9
Spread Spectrum Principles
1.25 MHz30 KHz
Power is “Spread” Over a Larger Bandwidth
MATHHAMMER
MATHHAMMER
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Spread Spectrum Principles
Many code channels are individually“spread” and then added together tocreate a “composite signal”
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Spread Spectrum Principles
UNWANTED POWERFROM OTHER SOURCES
Using the “right” mathematicalsequences any Code Channelcan be extracted from the receivedcomposite signal
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Anything We Can Do, We Can Undo
Any data bit stream can be combined with a spreading sequence The resulting signal can be de-spread and the data stream
recovered if the original spreading sequence is available and properly synchronized
After de-spreading, the original data stream is recovered intact
ORIGINATING SITE DESTINATION
SpreadingSequence
SpreadingSequence
InputData
(Base Band)
RecoveredData
(Base Band)
Spread Data Stream(Base Band + Spreading Sequence)
CDMA Technology Overview February, 2001 - Page 1-13
“Shipping and Receiving” via CDMA
Whether in shipping and receiving, or in CDMA, packaging is extremely important!
Cargo is placed inside “nested” containers for protection and to allow addressing
The shipper packs in a certain order, and the receiver unpacks in the reverse order
CDMA “containers” are spreading codes
Fed
Ex
Data Mailer
Fed
Ex
DataMailer
Shipping Receiving
CDMA Technology Overview February, 2001 - Page 1-14
CDMA’s Nested Spreading Sequences
CDMA combines three different spreading sequences to create unique, robust channels
The sequences are easy to generate on both sending and receiving ends of each link
The sequences are applied in succession at the sending end and then reapplied in opposite order to recover the original data stream at the receiving end
SpreadingSequence
A
SpreadingSequence
B
SpreadingSequence
C
SpreadingSequence
C
SpreadingSequence
B
SpreadingSequence
A
InputData
X
RecoveredData
X
X+A X+A+B X+A+B+C X+A+B X+ASpread-Spectrum Chip Streams
ORIGINATING SITE DESTINATION
CDMA Technology Overview February, 2001 - Page 1-15
Walsh CodesWalsh Codes
• 64 Sequences, each 64 chips longa chip is a binary digit (0 or 1)
• Each Walsh Code is Orthogonal to all other Walsh Codes
• It is possible to recognize, and therefore extract, a particular Walsh code from a mixture of other Walsh codes that are “filtered out” in the process
• Two same-length binary strings are orthogonal if the result of XORing them has the same number of 0s as 1s
WALSH CODES # ---------------------------------- 64-Chip Sequence ------------------------------------------ 0 0000000000000000000000000000000000000000000000000000000000000000 1 0101010101010101010101010101010101010101010101010101010101010101 2 0011001100110011001100110011001100110011001100110011001100110011 3 0110011001100110011001100110011001100110011001100110011001100110 4 0000111100001111000011110000111100001111000011110000111100001111 5 0101101001011010010110100101101001011010010110100101101001011010 6 0011110000111100001111000011110000111100001111000011110000111100 7 0110100101101001011010010110100101101001011010010110100101101001 8 0000000011111111000000001111111100000000111111110000000011111111 9 010101011010101001010101101010100101010110101010010101011010101010 001100111100110000110011110011000011001111001100001100111100110011 011001101001100101100110100110010110011010011001011001101001100112 000011111111000000001111111100000000111111110000000011111111000013 010110101010010101011010101001010101101010100101010110101010010114 001111001100001100111100110000110011110011000011001111001100001115 011010011001011001101001100101100110100110010110011010011001011016 000000000000000011111111111111110000000000000000111111111111111117 010101010101010110101010101010100101010101010101101010101010101018 001100110011001111001100110011000011001100110011110011001100110019 011001100110011010011001100110010110011001100110100110011001100120 000011110000111111110000111100000000111100001111111100001111000021 010110100101101010100101101001010101101001011010101001011010010122 001111000011110011000011110000110011110000111100110000111100001123 011010010110100110010110100101100110100101101001100101101001011024 000000001111111111111111000000000000000011111111111111110000000025 010101011010101010101010010101010101010110101010101010100101010126 001100111100110011001100001100110011001111001100110011000011001127 011001101001100110011001011001100110011010011001100110010110011028 000011111111000011110000000011110000111111110000111100000000111129 010110101010010110100101010110100101101010100101101001010101101030 001111001100001111000011001111000011110011000011110000110011110031 011010011001011010010110011010010110100110010110100101100110100132 000000000000000000000000000000001111111111111111111111111111111133 010101010101010101010101010101011010101010101010101010101010101034 001100110011001100110011001100111100110011001100110011001100110035 011001100110011001100110011001101001100110011001100110011001100136 000011110000111100001111000011111111000011110000111100001111000037 010110100101101001011010010110101010010110100101101001011010010138 001111000011110000111100001111001100001111000011110000111100001139 011010010110100101101001011010011001011010010110100101101001011040 000000001111111100000000111111111111111100000000111111110000000041 010101011010101001010101101010101010101001010101101010100101010142 001100111100110000110011110011001100110000110011110011000011001143 011001101001100101100110100110011001100101100110100110010110011044 000011111111000000001111111100001111000000001111111100000000111145 010110101010010101011010101001011010010101011010101001010101101046 001111001100001100111100110000111100001100111100110000110011110047 011010011001011001101001100101101001011001101001100101100110100148 000000000000000011111111111111111111111111111111000000000000000049 010101010101010110101010101010101010101010101010010101010101010150 001100110011001111001100110011001100110011001100001100110011001151 011001100110011010011001100110011001100110011001011001100110011052 000011110000111111110000111100001111000011110000000011110000111153 010110100101101010100101101001011010010110100101010110100101101054 001111000011110011000011110000111100001111000011001111000011110055 011010010110100110010110100101101001011010010110011010010110100156 000000001111111111111111000000001111111100000000000000001111111157 010101011010101010101010010101011010101001010101010101011010101058 001100111100110011001100001100111100110000110011001100111100110059 011001101001100110011001011001101001100101100110011001101001100160 000011111111000011110000000011111111000000001111000011111111000061 010110101010010110100101010110101010010101011010010110101010010162 001111001100001111000011001111001100001100111100001111001100001163 0110100110010110100101100110100110010110011010010110100110010110
EXAMPLE:
Correlation of Walsh Code #23 with Walsh Code #59
#23 0110100101101001100101101001011001101001011010011001011010010110#59 0110011010011001100110010110011010011001011001100110011010011001XOR 0000111111110000000011111111000011110000000011111111000000001111
Correlation Results: 32 1’s, 32 0’s: Orthogonal!!
CDMA Technology Overview February, 2001 - Page 1-16
Correlation and OrthogonalityCorrelation and Orthogonality
Correlation is a measure of the similarity between two binary strings
Code #23 0110100101101001100101101001011001101001011010011001011010010110
–(Code #23) 1001011010010110011010010110100110010110100101100110100101101001
Code #59 0110011010011001100110010110011010011001011001100110011010011001
PARALLELPARALLEL
XOR: all 0sXOR: all 0s
Correlation: 100%(100% match)
Correlation: 100%(100% match)
ORTHOGONALORTHOGONAL
XOR: half 0s, half 1sXOR: half 0s, half 1s
Correlation: 0%(50% match, 50% no-match)
Correlation: 0%(50% match, 50% no-match)
ANTI-PARALLELANTI-PARALLEL
XOR: all 1sXOR: all 1s
Correlation: –100%(100% no-match)
Correlation: –100%(100% no-match)
#23#23
–(#23)
#23
#23
#59
CDMA Technology Overview February, 2001 - Page 1-17
Creating the Walsh Code TableCreating the Walsh Code Table
0 0
0 1
CDMA Technology Overview February, 2001 - Page 1-18
Creating the Walsh Code TableCreating the Walsh Code Table
0 0
0 1
0 0
0 1
CDMA Technology Overview February, 2001 - Page 1-19
Creating the Walsh Code TableCreating the Walsh Code Table
0 0
0 1
0 0
0 1
0 0
0 1
CDMA Technology Overview February, 2001 - Page 1-20
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
Creating the Walsh Code TableCreating the Walsh Code Table
CDMA Technology Overview February, 2001 - Page 1-21
Creating the Walsh Code TableCreating the Walsh Code Table
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
1 1
1 0
1 1
1 0
1 1
1 0
0 0
0 1
0 0
0 1
0 0
0 1
0 0
0 1
1 1
1 0
CDMA Technology Overview February, 2001 - Page 1-22
The Short PN SequencesThe Short PN Sequences
The two Short PN Sequences, I and Q, are 32,768 chips long
• Together they can be considered a two-dimensional binary “vector” with distinct I and Q component sequences, each 32,768 chips long
• Each Short PN Sequence (and, as a matter of fact, any sequence) correlates with itself perfectly if compared at a timing offset of 0 chips
• Each Short PN Sequence is special: Orthogonal to a copy of itself that has been offset by any number of chips (other than 0)
IQ
32,768 chips long26 2/3 ms.
(75 repetitions in 2 sec.)
IQIQ
100% Correlation: All bits = 0
Short PN Sequence vs. Itself @ 0 Offset
IQIQ
Orthogonal: 16,384 1’s + 16,384 0’s
Short PN Sequence vs. Itself @ Any Offset
Unique Properties:
CDMA Technology Overview February, 2001 - Page 1-23
The Long PN Sequence The Long PN Sequence
• Each mobile station uses a unique User Long Code Sequence generated by applying a mask, based on its 32-bit ESN, to the 42-bit Long Code Generator which was synchronized with the CDMA system during the mobile station initialization
• Generated at 1.2288 Mcps, this sequence requires 41 days, 10 hours, 12 minutes and 19.4 seconds to complete
• Portions of the Users Long Codes generated by different mobile stations for the duration of a call are not exactly orthogonal but are sufficiently different to permit reliable decoding on the reverse link
Long Code Register (@ 1.2288 MCPS)
Public Long Code Mask (STATIC)
User Long CodeSequence
(@1.2288 MCPS)
1 1 0 0 0 1 1 0 0 0 PERMUTED ESNAND
=SUM
Modulo-2 Addition
CDMA Technology Overview February, 2001 - Page 1-24
How Many Spreading Codes Do We Need?(Discriminating Among Forward Code Channels)
A Mobile Station tuned to a particular CDMA frequency receives a Forward CDMA Channel from a sector in a Base Station.
This Forward CDMA Channel carries a composite signal made of up to 64 forward code channels
Some of these code channels are traffic channels while other are overhead channels needed by the CDMA system to operate properly.
A set of 64 mathematical codes is needed to differentiate the 64 possible forward code channels that can be contained in a Forward CDMA Channel.
The codes in this set are called “Walsh Codes”
SyncPilot
FW Traffic(for user #1)
Paging
FW Traffic(for user #2)
FW Traffic(for user #3)
CDMA Technology Overview February, 2001 - Page 1-25
How Many Spreading Codes Do We Need?(Discriminating Among Base Stations)
A mobile Station is surrounded by Base Stations, all of them transmitting on the same CDMA Frequency
Each Sector in each Base Station is transmitting a CDMA Forward Traffic Channel containing up to 64 distinct forward code channels
A Mobile Station must be able to discriminate between different Sectors of different Base Stations and listen to only one set of code channels
Two binary digit sequences called the I and Q Short PN Sequences (or Short PN Codes) are defined for the purpose of identifying sectors of different base stations
These Short PN Sequences can be used in 512 different ways in a CDMA system. Each one of them constitutes a mathematical code which can be used to identify a particular sector of a particular base station
A B
Up to 64Code Channels
Up to 64Code Channels
CDMA Technology Overview February, 2001 - Page 1-26
How Many Spreading Codes Do We Need?(Discriminating Among Reverse Code Channels)
The CDMA system must be able to uniquely identify each Mobile Station that may attempt to communicate with a Base Station
A very large number of Mobile Stations will be in the market
One binary digit sequence called the Long PN Sequence (or Long PN Code) is defined for the purpose of uniquely identifying each possible reverse code channel
This sequence is extremely long and can be used in trillions of different ways. Each one of them constitutes a mathematical code which can be used to identify a particular user (and is then called a User Long Code) or a particular access channel (explained later in this course)
RV Trafficfrom M.S.
#1837732008RV Trafficfrom M.S.
#8764349209
RV Trafficfrom M.S.
#223663748
System AccessAttempt by M.S.#4348769902
(on access channel #1)
CDMA Technology Overview February, 2001 - Page 1-27
Summary of Characteristics & FunctionsSummary of Characteristics & Functions
Cell
• Each CDMA spreading sequence is used for a specific purpose on the forward link and a different purpose on the reverse link
• The sequences are used to form “code channels” for users in both directions
Walsh Codes
Short PN Sequences
Long PNSequences
Type of Sequence
Mutually Orthogonal
Orthogonal with itself at any time shift value except 0
near-orthogonal
if shifted
Special Properties
64
2
1
How Many
64 chips1/19,200
sec.
32,768 chips
26-2/3 ms75x in 2
sec.
242 chips~41 days
Length
Orthogonal Modulation(information
carrier)
Quadrature Spreading
(Zero offset)
Distinguish users
Reverse Link
Function
User identity
within cell’s signal
Distinguish Cells & Sectors
Data Scrambling to avoid all 1’s or 0’s
Forward Link
Function
IQ
32,768 chips long26-2/3 ms.
(75 repetitions in 2 sec.)
64codes
64 chips long
AND
=SUM
Modulo-2 Addition
CDMA Technology Overview February, 2001 - Page 1-28
Lesson Review
1. If a signal is deliberately transmitted using more RF bandwidth than required, it is easier to detect at the receiver. This “waste” is formally defined as what?
Processing gain
2. What vocoder function stores a collection of arbitrary waveform segments?
Code book
3. Are all CDMA Walsh Codes orthogonal?
Yes
4. What sequence best describes this conversion relationship in CDMA:
chips symbols bits
5. List the four overhead (support) channels.
Paging, sync, access, pilot
CDMA Technology Overview February, 2001 - Page 1-29