Home >Documents >CDMA m1 Overview[1]

CDMA m1 Overview[1]

Date post:12-Nov-2014
Category:
View:14 times
Download:1 times
Share this document with a friend
Transcript:

Fundamentals of CDMALesson 1 - OverviewAlexander Sierra

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 1

Basics

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 2

Why CDMA?

C ode D ivision M ultiple A ccessFundamentals of CDMA - Luis Font

CDMA is extremely robust and provides excellent audio quality

Is the technology of choice for both 800 MHz Cellular and 1900 MHz PCS service providers Satisfies CTIA Users Performance Requirements Provides high capacity (many times the capacity of AMPS) Provides privacy through its coding scheme

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 3

What is Multiple Access?Multiple Access: Simultaneous private use of a transmission medium by multiple, independent users.

Since the beginning of telephony and radio, system operators have tried to squeeze the maximum amount of traffic over each circuit Types of Media -- Examples: 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 administerFundamentals of CDMA - Luis Font

Transmission

Medium

Each pair of users enjoys a dedicated, private circuit through the transmission medium, unaware that the other users exist.

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 4

Multiple Access TechnologiesChannel: An individually-assigned, dedicatedpathway through a transmission medium for one users information The physical transmission medium is a resource that can be subdivided into individual channels according to different criteria depending on the technology used: Heres how the three most popular technologies establish channels:

FDMAPower

FDMA Frequency Division Multiple Access

TDMAPower

each user on a different frequency a channel is a frequency TDMA Time Division Multiple 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 Multiple 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 pattern(s)Fundamentals of CDMA - Luis Font

CDMAPower

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 5

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 the 1.25 MHz Forward LinkCDMA CHANNELCDMA Reverse Channel1.25 MHz

CDMA Reverse Channel the 1.25 MHz Reverse Link

CDMA Forward Channel1.25 MHz

45 or 80 MHz

CDMA Code Channel each individual stream of 0s and 1s 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 channelsFundamentals of CDMA - Luis Font CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 6

Other Technologies: Avoiding Interference In conventional radio technologies, the desired signal must be strong enough to override any interference AMPS, TDMA and GSM depend on physical distance separation to keep interference at low levels Co-channel users are kept at a safe distance by careful frequency planning Nearby users and cells must use different frequencies to avoid interferenceFundamentals of CDMA - Luis Font

AMPS-TDMA-GSM1 4 7 6 1 4 2 3 6 1 5 1 7 3 5 1 4 2 3 6 5 1 2 7 1

Figure of Merit: C/I(carrier/interference ratio)

AMPS: +17 dB TDMA: +14 to 17 dB GSM: +12 to 14 dB

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 7

CDMA: Using a New Dimension All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators CDMA operates by using CODES to discriminate between users CDMA interference comes mainly from nearby users Each user is a small voice in a roaring crowd -- but with a uniquely recoverable code Transmit power on all users must be tightly controlled so their signals reach the base station at the same signal levelFundamentals of CDMA - Luis Font

Figure of Merit: Ec/Io, Eb/No(energy per chip [bit] / interference [noise] spectral density)

CDMA: Ec/Io -17 to -2 dB CDMA: Eb/No ~+6 dB

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 8

CDMA Is a Spread-Spectrum SystemTRADITIONAL COMMUNICATIONS SYSTEM Spread SpectrumSlow Information Sent TX Narrowband Signal Slow Information Recovered RX

SPREAD-SPECTRUM SYSTEMWideband SignalSlow Information Sent TX RX Slow Information Recovered

Fast Spreading Sequence

Fast Spreading Sequence

Spread Spectrum Payoff:

Processing Gain

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 capacityCDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 9

Fundamentals of CDMA - Luis Font

Spread Spectrum Principles

Power is Spread Over a Larger Bandwidth

30 KHz 1.25 MHz

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 10

Spread Spectrum Principles

Many code channels are individually spread and then added together to create a composite signal

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 11

Spread Spectrum Principles

Using the right mathematical sequences any Code Channel can be extracted from the received composite signal

UNWANTED POWER FROM OTHER SOURCES

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 12

Anything We Can Do, We Can Undo

ORIGINATING SITESpread Data Stream (Base Band + Spreading Sequence) Input Data (Base Band)

DESTINATIONRecovered Data (Base Band)

Spreading Sequence

Spreading Sequence

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 intactNote - The spread sequences actually shown are icons, not accurate or to scaleFundamentals of CDMA - Luis Font CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 13

Shipping and Receiving via CDMA Shipping FedEx FedEx Receiving

Data

Mailer

Mailer

Data

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

Fundamentals of CDMA - Luis Font

CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 14

CDMAs Nested Spreading Sequences

ORIGINATING SITEX+A

Spread-Spectrum Chip Streams X+A+B X+A+B+C X+A+B

DESTINATIONX+A

Input Data

Recovered Data

XSpreading Spreading Spreading Sequence Sequence Sequence Spreading Spreading Spreading Sequence Sequence Sequence

X

A

B

C

C

B

A

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 endFundamentals of CDMA - Luis Font CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 15

How Many Spreading Codes Do We Need? (Discriminating Among Forward Code Channels)Pilot FW Traffic (for user #1) Paging FW Traffic (for user #3)

Sync

FW Traffic (for user #2)

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 CodesFundamentals of CDMA - Luis Font CDMA Technical Market Support, CALA & EMEA - Nov. 9, 1998 - Page 16

How Many Spreading C

Click here to load reader

Embed Size (px)
Recommended