Curso De Introducción Cdma

CDMA
101
Scott Baxter

CDMA 101 Outline
❖ CDMA

Basics

N Multiple

Access Technology Survey
N CDMA coding principles
N Spread Spectrum principles
N Forward and Reverse Channel Structure
❖ Nortel
N MTX

❖ CDMA

CDMA System Architecture
BSC BSM BTS

Details and Operation

N Power

Control
N Handoff mechanics
N Optimization concepts

CDMA
Basics

Multiple Access
Technologies

FDMA
Power

❖

FDMA(example: AMPS)
Frequency Division Multiple Access
N each user has a private frequency

❖

TDMA (examples: IS-54/136, GSM)
Time Division Multiple Access
N each user has a private time on a private
frequency

❖

Tim

CDMA (IS-95, J-Std. 008)
Code Division Multiple Access
N users co-mingle in time and frequency but
each user has a private code

e

Fr

e

e
qu

nc

y

TDMA
Power
Tim

e

Fre

e
qunc

y

CDMA
Power
Tim

e

Fr

e

e
qu

nc

y

Other Technologies:
Other
Avoiding 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
1

4
7
6

2
7

3
5

1

6

4
2

4
7

36
1

5

1

1
2

3
5
1

Figure of Merit: C/I

(carrier/interference ratio)

AMPS: +17 dB
TDMA: +14 to 17 dB
GSM: +7 to 9 dB.

CDMA: Using A New Dimension
CDMA:
❖ All

CDMA users occupy the
same frequency at the same
time! Time and frequency are
not used as discriminators
❖ CDMA interference comes
mainly from nearby users
❖ CDMA operates by using
CODING to discriminate
between users
❖ Each user isa small voice in
a roaring crowd -- but with a
uniquely recoverable code

CDMA

Figure of Merit: C/I

(carrier/interference ratio)

AMPS: +17 dB
TDMA: +14 to +17 dB
GSM: +7 to 9 dB.
CDMA: -10 to -17 dB.
CDMA: Eb/No ~+6 dB.

CDMA Uses Code Channels
CDMA
❖A

CDMA signal uses many chips
to convey just one bit of
information
❖ Each user has a unique chip
pattern, in effect a code channel
❖ To recovera bit, integrate a large
number of chips interpreted by
the user’s known code pattern
❖ Other users’ code patterns
appear random and integrate
toward low values, hence don’t
disturb the bit decoding decision

Building a
CDMA Signal
Bits

from User’s Vocoder
Forward Error
Correction

Symbols
Coding and
Spreading

Chips

CDMA is a Spread-Spectrum System
❖ Traditional

technologies try to
squeezesignal
into minimum
required
bandwidth
❖ CDMA uses larger
bandwidth but
uses resulting
processing gain to
increase capacity

TRADITIONAL COMMUNICATIONS SYSTEM
Spread Spectrum
Narrowband
Signal

Slow
Information
Sent
TX

Slow
Information
Recovered

RX

SPREAD-SPECTRUM SYSTEM
Wideband
Signal

Slow
Information
Sent
TX
Fast
Spreading
Sequence

Slow
Information
Recovered
RX

Fast
Spreading
SequenceSpread Spectrum Payoff:
Processing Gain

Spreading: What we do, we can undo
ORIGINATING SITE

DESTINATION
Spread Data Stream

Input
Data

Recovered
Data

Spreading
Sequence

❖ Sender

Spreading
Sequence

combines data with a fast spreading
sequence, transmits spread data stream
❖ Receiver intercepts the stream, uses same
spreading sequence to extract original data

“Shipping and Receiving” viaCDMA

Mailer

❖ Whether

FedEx

Data

Receiving
FedEx

Shipping

Mailer

Data

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

CDMA’s Nested Spreading SequencesORIGINATING SITE
X+A

DESTINATION

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

X+A

Input
Data

Recovered
Data

X

X

Spreading Spreading Spreading
Sequence Sequence Sequence

A

❖ CDMA

B

C

Spreading Spreading Spreading
Sequence Sequence Sequence

C

B

A

combines three different spreading
sequences to create unique, robust channels
❖ The sequences are easy to generate on both
sending and...