Ch1 Fundamental of CDMA

Course Outline

• Introduction

• Ch1: Fundamental Of CDMA

• Ch2: UMTS Evolution & Air Interface.

• Ch3: The UMTS Network.

• Ch4: UMTS Transmission Chain.

• Ch5: UMTS Protocols

• HSPA Introduction

Transmission

Medium

As AMPS and NMT

As GSM and IS-136

FDMA TDMA

Advantages

•low cost hardware technology

•No need for network timing

•No restriction regarding the type of base

band (voice or data) or type of modulation

•Permits a flexible bit rate

•Easy for mobile or base stations to

initiate and execute hands off

Disadvantages

•The presence of guard bands

•Requires Ideal RF filtering to

minimize adjacent channel interference

•Requires network-wide timing

synchronization

•Requires signal processing for

advancing

Band Saturation.

Capacity (more than ½ The population of the globe use mobile

phones).

More BW efficiency.

New service.

Why do we need a new technology?

What about SDMA?

Switched Beam System Adaptive Array

Antenna

Array

Desired

User

Interfering

User Antenna

Array

Active

Beam

Signal Spreading

Fast Frequency Hopping

Signal Spreading Preview

In DS Spreading, the incoming digital speech signal is

multiplied by a digital pseudo-noise (PN) code through a

process called bit stream multiplication.

The result of DS spreading is an encoded, spread spectrum, CDMA signal.

Spread Spectrum

Anti-Jamming System

S(f)

f f0

Before spreading

Signal

S(f)

f f0

After spreading

Signal

S(f)

f f0

After despreading

signal

Interfering noise

f

S(f)

f0

Before despreading

Signal

Interfering noise

CDMA Spread Spectrum

As Channel BW get smaller More Capacity NO More necessary

Sharing the frequency by using Different codes


Fast Frequency Hopping

This approach assumes that the two stations have:

A common knowledge of the frequency hopping plan.

Synchronized clocks

CDMA options

Direct sequence and freq hopping CDMA


Time Hopping

Time hopping assumes that the two stations in communication have:

A common knowledge of the time-hopping "plan."

Synchronized clocks

The Strengths of CDMA

Increased system capacity.

Cell sites can be larger.

Enhanced privacy due to digital coding of CDMA signals.

Enhanced call quality.

Lower transmit RF power levels, longer battery life, and increased

talk time for hand-held units.

Simplified System Planning.

Bandwidth on Demand

Bit Stream Multiplication Concepts

Bit stream multiplication is the process where an input bit

stream of information (usually a digital speech signal) called

b(t) is multiplied by a pseudo-noise (PN) code called c(t) to

produce a new composite output signal called y(t).


b(t) · c(t) = y(t)

b(t) · c(t) · c(t) = b(t)


Consider the difference between spreading and scrambling:

Spreading and Despreading

Remember that spreading occurs when a lower bit rate input

signal b(t) is multiplied by a higher chip rate spreading code

c(t) as shown.

Bit

Chip

Input Data +1 -1 +1

+1 -1 +1 Divide by

Code Length

Receiver and Transmitter use identical code at same time offset

+1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 PN code used

in Transmitter

x x x

+8 -8 +8

Integrate

Result

Integrate Integrate Integrate

+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1 Transmitted

Sequence

= = =

+1 +1 +1 +1 +1 +1 +1 +1 -1 –1 –1 –1 –1 –1 –1 -1 +1 +1 +1 +1 +1 +1 +1 +1

= = =

+1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 PN Code

Used in Receiver

x x x

Transmitter

Receiver


Coding Concept…

Transmitter

Input Data +1 -1 +1

+1 +1 +1 +1 +1 +1 +1 +1

+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1

Orthogonal code

in Transmitter

Transmitted

Sequence

Orthogonal

different Code

used in Receiver

0 0 0

Integrate

Result

0 Divide by

Code Length

x x x

Integrate Integrate

= = =

x x x

= = Receiver

Receiver and Transmitter use two uncorrelated codes at same time offset

+1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1 +1 –1 +1 +1 –1 -1 +1 -1

+1 –1 +1 +1 –1 -1 +1 -1 -1 +1 -1 -1 +1 +1 -1 +1 +1 –1 +1 +1 –1 -1 +1 -1

+1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1 +1

=

Integrate

0 0

Spreading and Power Spectral Density

Spreading does not change total power. Spreading changes

how the power is distributed over frequency.

The following formulas hold true:

Fb =1/Tb (the bit rate of the input signal)

Fc =1/Tc (the chip rate of the spreading code)

G (processing gain) = Fc/Fb =Tb/Tc

G = Spreading factor

Scrambling and Descrambling

Scrambling is often used for privacy. When scrambling is used

for encryption, then descrambling is used for decryption.

Repeated Spreading and Scrambling

CDMA systems use scrambling and spreading in several ways

for:

Encryption for privacy

Channel identification

Base station identification

PN Codes

A binary sequence with fixed length and has noise-like

randomness

Nearly equal number of zeros and ones

Low correlation between shifted versions of the sequence

Low correlation with other user signals (interference) and noise

Good autocorrelation properties with own signal in

synchronization

Code Generation Circuitry

Long code = 242-1

Short code = 2 15-1

Code Generation Circuitry

D

Flip Flop D

Flip Flop D

Flip Flop

Quick Review

RF Transmission Delay and its Effect

Radio signals typically travel at a rate of about five microseconds to

the mile. Assuming that a mobile is ten miles away from the base

station, a 50 microsecond (µs) delay would be the result.

Chip = .81 µsec

RF Transmission Delay and its Effect

Locking On to a Particular PN Sequence Cont.

Signal Correlation With Offsets Less Than 1 Time Chip

Multi-User Downlink Overview

Message detection

C3

Multi-User Uplink Overview

Interference limited system

Traditional Systems Performance is measured by signal-to-noise

ratio (S/N). The desired user's signal versus noise.

CDMA Systems Performance is measured by signal-to-

interference ratio (S/I). The desired user's signal versus

interference from other users signals.

Interference limited system

Multipath and Delay Spread

Non-RAKE CDMA Reception

RAKE Receiver Operation

Related Terms and Definitions

Term Definition Narrow Band Signal Signal occupies a relatively small bandwidth i.e. (GSM signal has 200KHz bandwidth) Wide Band Signal Signal occupies relatively wide bandwidth i.e. (WCDMA signal has 5 MHz bandwidth) Pseudo Noise Signal Signal has a noise like behaviour - actual noise never repeats - Spreading Converting a signal with low bit rate into another

signal with much higher bit rate Scrambling Converting a signal into another coded version of

it keeping the same bit rate

Related Terms and Definitions

Term Definition

Auto Correlation Measurement for how much a signal is related to another version of itself

Cross Correlation Measurement for how a signal is related to another different signal

Orthogonal Codes Codes has Auto Correlation = 1 and

Cross Correlation = 0

Pseudo Noise Codes Codes has Auto Correlation very close to 1

and Cross Correlation very close to 0

Ch1 Fundamental of CDMA

Education

Transcript of Ch1 Fundamental of CDMA