Cdma Overview

Fundamentals of CDMA

Why CDMA?

C ode D ivision M ultiple A ccess

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

What is MultipleAccess?
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 administer

Transmission

Medium

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

Multiple Access Technologies
Channel: An individually-assigned, dedicatedpathway through a transmission medium for one user’s information 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
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• FDMA Frequency Division Multiple Access
− each user on adifferent 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)TDMA
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CDMA
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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 theREVERSE 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 Link CDMA Reverse Channel • the 1.25 MHz Reverse Link
CDMA CHANNEL
CDMA Reverse Channel
1.25 MHz

CDMA Forward Channel
1.25 MHz

45 or 80 MHz

CDMA Code Channel • each individual stream of 0’s and 1’scontained 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

Other Technologies: Avoiding Interference In conventional radio technologies, the desired signalmust 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 interference AMPS-TDMA-GSM
1 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: 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...