13-04-2010, 12:02 PM
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CDMA Technology Overview
CDMA Basics
Course 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
Why CDMA?
CDMA is the technology of choice for both 800 MHz Cellular and 1900 MHz PCS service providers
CDMA satisfies CTIA Usersâ„¢ Performance Requirements
CDMA provides high capacity (many times the capacity of AMPS)
CDMA provides privacy through its coding scheme
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
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:
Hereâ„¢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
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 0â„¢s and 1â„¢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
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 Principles
Spread Spectrum Principles
Spread Spectrum Principles
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
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
CDMAâ„¢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
Walsh Codes
64 Sequences, each 64 chips long
a 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
Correlation and Orthogonality
The 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)
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 page link
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
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
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)
Summary of Characteristics & Functions
Lesson Review
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
What vocoder function stores a collection of arbitrary waveform segments?
Code book
Are all CDMA Walsh Codes orthogonal?
Yes
What sequence best describes this conversion relationship in CDMA:
chips à symbols à bits
List the four overhead (support) channels.
Paging, sync, access, pilot
CDMA Technology Overview
CDMA Basics
Course 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
Why CDMA?
CDMA is the technology of choice for both 800 MHz Cellular and 1900 MHz PCS service providers
CDMA satisfies CTIA Usersâ„¢ Performance Requirements
CDMA provides high capacity (many times the capacity of AMPS)
CDMA provides privacy through its coding scheme
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
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:
Hereâ„¢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
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 0â„¢s and 1â„¢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
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 Principles
Spread Spectrum Principles
Spread Spectrum Principles
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
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
CDMAâ„¢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
Walsh Codes
64 Sequences, each 64 chips long
a 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
Correlation and Orthogonality
The 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)
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 page link
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
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
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)
Summary of Characteristics & Functions
Lesson Review
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
What vocoder function stores a collection of arbitrary waveform segments?
Code book
Are all CDMA Walsh Codes orthogonal?
Yes
What sequence best describes this conversion relationship in CDMA:
chips à symbols à bits
List the four overhead (support) channels.
Paging, sync, access, pilot
