CDMA 120 – cdmaOne and CDMA2000 Concepts and Terminology phần 2 pptx

CDMA 120 Section 2-9cdma university Multiple Access Methods FDMA: Frequency Division Multiple Access FDMA is a multiple access method in which users are assigned specific frequency bands

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CDMA 120 Section 2-7

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Standardization Progress

TR 45.5 is currently working to develop the next evolution in the cdmaOne family

The working title of the IS-95A revision is IS-95B, but the standard will be released as

TIA/EIA-95

Contents of TIA/EIA-95

This new revision will combine IS-95A, TSB-74, and ANSI J-STD-008 into a single document andeliminate much of the redundancy between the three documents Most of the analog informationwill be deleted and the standard will reference the existing analog standard

IS-553A when applicable Lastly, TIA/EIA-95 will add technical corrections and new capabilities.TIA/EIA-95 is Protocol Revision 4

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z Backward-compatible with IS-95 mobile stations

z Provides Third Generation (3G) wireless services– Voice

– Packet-switched data– Circuit-switched data

z CDMA2000 works in a wide range of environmentsincluding:

– Outdoor mega-cells/macro-cells– Indoor/Outdoor microcells– Indoor/Outdoor picocells

CDMA2000

The first revision of CDMA2000 was Revision 0, developed by the Telecommunications IndustryAssociation (TIA) standards body The TR45 Committee completed the revision in July 1999.Release A of CDMA2000 was developed by Third Generation Partnership Product 2 (3GPP2), aconsortium of five standards bodies:

z TIA in North America

z Telecommunications Technology Association (TTA) in Korea

z Association of Radio Industries and Businesses (ARIB) and Telecommunications

Technology Committee (TTC) in Japan

z China Wireless Telecommunication Standards Group (CWTS) in China

Release A was completed in March 2000 The discussion of CDMA2000 in this course assumesCDMA2000 revision A unless otherwise stated

Revision B of CDMA2000 is expected to be completed by 3GPP2 in 2002

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cdma university Multiple Access Methods

FDMA: Frequency Division Multiple Access

FDMA is a multiple access method in which users are assigned specific frequency bands The userhas sole right of using the frequency band for the entire call duration

TDMA: Time Division Multiple Access

TDMA is an assigned frequency band shared among a few users However, each user is allowed totransmit in predetermined time slots Hence, channelization of users in the same band is achievedthrough separation in time

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cdma university Multiple Access Methods (continued)

CDMA: Code Division Multiple Access

CDMA is a method in which users occupy the same time and frequency allocations, and are

channelized by unique assigned codes The signals are separated at the receiver by using a

correlator that accepts only signal energy from the desired channel Undesired signals contributeonly to the noise

In December of 1991, QUALCOMM presented to CTIA the results of some of the first CDMAfield trials Following these presentations, the CTIA Board of Directors unanimously adopted aresolution requesting that TIA, the Telecommunications Industry Association, prepare structurally

to accept contributions regarding wideband cellular systems

In March of 1992, a new subcommittee within the TR45 Committee was formed to develop spreadspectrum cellular standards That subcommittee published the first CDMA cellular standard, IS-95,

in July 1993 CDMA systems based on the IS-95 standard and related specifications are referred to

as cdmaOneTMsystems

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The CDMA “Cocktail Party”

The CDMA concept is analogous to the situation encountered at a party At the “CDMA CocktailParty,” all subscribers are talking in the same room together simultaneously This is analogous tothe technique used in CDMA Imagine that every conversation in the room is being carried out in adifferent language that you do not understand They would all sound like noise from your

perspective If you “knew the code,” the appropriate language, you could imagine filtering out theunwanted conversations and listening only to the conversation of interest to you A CDMA systemmust filter the traffic in a similar way

Even with knowledge of the appropriate language, the conversation of interest may not be

completely audible The listener can signal the speaker to speak more loudly and can also signalother people to speak more softly A CDMA system uses a similar power control process

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cdma university Example CDMA Network Architecture

10 00 00 11 00 0

1010010000110011001110100001000

10111 1 10 10

111

01010 00

T1 / E1

T1/ E1

T1 / E1 T1 / E1

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(with BTS)

Subscriber Units

This is the generic term for all types of telephones: mobile, portable, or Wireless Local Loop

(WLL) Subscriber units support FAX and data transmission as well as voice service

Base Stations

A Base Station is a fixed station used for communicating with mobile stations Unfortunately,

industry usage varies considerably Depending on the context the term “Base Station” may refer to

a cell, a sector within a cell, or part of the infrastructure equipment Common terms for

infrastructure elements are BTS and BSC:

z Base Station Transceiver Sub-system (BTS)

The BTS is the radio link between the subscriber units and the system A BTS is the systemelement that provides the CDMA signal generation and transmits the signal at the

appropriate frequency

z Base Station Controller (BSC)

The BSC controls several BTS elements The BSC may perform the switching function andprovide other services such as billing

Public Switched Telephone Network (PSTN)

Often ignored as a part of the wireless network, the PSTN is the link that connects the wirelesssystem to all international and domestic land line traffic

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cdma university The “Cellular” Concept and “Sectors”

Cell Cell

Cell

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The Conventional Cellular Concept

To support a very large subscriber base with a limited number of frequencies, wireless systemsmust re-use frequency assignments

To reduce co-channel interference, a cellular concept was adopted Rather than broadcasting a

signal over a very large area, RF propagation was restricted to a small segment of the coverage

area called a cell Directional antennas were employed at a single site to cover multiple cells The

same frequency is then used in another cell some distance away The same frequency cannot beused in neighboring cells Co-channel interference would be too severe

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cdma university The “Cellular” Concept and “Sectors” (cont.)

Sector Sector

advantage in CDMA systems to communicate with a mobile through multiple sectors

In CDMA systems, then, it is important to identify this special relationship Conventional systems

generally refer to each isolated region as a cell Rather than refer to each region as a cell, in CDMA

the group is collectively referred to as a cell Each of the sub-divisions of the cell is referred to as a

sector of the cell Typically they are referred to as the alpha, beta, and gamma sectors.

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cdma university cdmaOne Overview and Terminology

Information

Information Bits

Code Symbols

Spreading Code Generator

A/D

FEC

Spreader ChipsChips

mux

add check bits

PSK

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An Overview of cdmaOne Modulation

cdmaOne systems convert the analog voice signal into a digital signal for transmission There areseveral steps in the digital transmission process Many of these steps are common to digital

wireless schemes After each step in digital processing, the signal conveys a different meaning andseveral terms are used to refer to the signal at different stages in the process:

The “Bit”

A bit is the fundamental unit of information: a single binary digit Analog information is encoded

into a sequence of binary digits (A/D conversion) Both user data and error detection code digits

are considered bits The bit rate (bits per second) is a measure of the volume of information being

transmitted

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cdma university cdmaOne Overview and Terminology (cont.)

Information

Information Bits

Code Symbols

Spreading Code Generator

A/D

FEC

Spreader ChipsChips

mux

add check bits

PSK

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The Code Symbol

In cdmaOne systems, a code symbol is the output of the coding process (Forward Error Correction

[FEC]) Each bit produces several code symbols The symbol rate is a measure of the redundancyintroduced by the FEC scheme Each symbol is also a single binary digit

The Chip

The output digits of a spreading code generator are commonly termed chips A chip is also a single binary digit Several chips are used to spread a single code symbol The chip rate is a measure of

the amount of spreading performed

Bits, symbols, and chips all look the same: a single binary digit What distinguishes one from

another is their relationship to the information signal

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LPF

Voice Signal

Sample

Quantize

Binary Encode Digital

Signal Clock

Vocoder MMT98010125Ag.emf

Analog-to-Digital Conversion (A/D)

Elementary analog-to-digital conversion is referred to as waveform coding and generally requires

four processing steps:

z Low pass filtering of the analog signal in order to limit its spectrum.

z Sampling of the filtered signal at the Nyquist rate (The Nyquist Rate is equal or higher than

2 times the spectrum of the signal.)

z Quantizing the samples into 2nlevels, where n is the number of bits needed per sample toachieve the accuracy desired

z Binary encoding the resulting levels into a baseband digital signal.

Voice Coding

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cdma university Error Detection Codes

Block of data bits Additional check bits

CRC Bits

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Cyclic Redundancy Checking (CRC)

CRC provides a high level of error detection The price paid for this detection capability is

overhead bits These overhead bits are appended to the data stream prior to transmission

In cdmaOne systems, for example, a Traffic Channel data block (frame) is generated once every 20milliseconds When transmitted at a rate of 14,400 bps, each frame contains 288 bits; twelve ofthese bits are CRC bits

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cdma university Error Correction Codes (continued)

Voice is Real Time

Voice transmissions require “real time” transmission Delays are unacceptable to the human ear It

is impractical to attempt to retransmit a portion of a conversation that arrives at the receiver in

error For this reason, the signal must be transmitted reliably One method to increase reliability is

to simply transmit at higher power An alternative method is to use some form of coding that

enables the receiver to correct some detection errors

FEC Coding

Coding that enables the receiver to correct errors is referred to as Forward Error Correction

Coding A very unsophisticated form of coding is illustrated in the figure Redundancy is

introduced simply by repeating the digits The receiver can use a simple majority rule device tomake a detection

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cdma university Spread Spectrum Techniques

Spread spectrum technology is over 50 years old Applying spread spectrum principles to the

multiple access environment is a development occurring over the last decade

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cdma university Spread Spectrum Techniques (continued)

Direct Sequence Spread Spectrum

The information signal is inherently narrowband, on the order of less than 10 KHz The energyfrom this narrowband signal is spread over a much larger bandwidth by multiplying the

information signal by a wideband spreading code Direct sequence spread spectrum is the

technique used in cdmaOne systems How this spreading is accomplished is the topic of the nextsection

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cdma university Spread Spectrum Techniques (continued)

Data

M(t)

IF Modulated with m(t)

s(t) Local Oscillator

Hop Word Hop Clock

Modulator

Synthesizer

PN Generator

Spectrum s(t)

m(t) Spectrum

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Frequency Hopped Spread Spectrum

Spreading can also be achieved by hopping the narrowband information signal over a set of

frequencies This type of spreading can be classified as Fast or Slow depending on the rate of

hopping to the rate of information:

z Fast hopping — the hopping rate is larger than the bit rate

z Slow hopping — more than one bit is hopped from one frequency to another

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cdma university Binary Phase Shift Keying

-sinwct(logic 0)

sinwct(logic 1)

1

0

1

DigitalSignal

BPSK

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Binary Phase Shift Keying (BPSK)

Binary phase shift keying is a popular constant-amplitude digital modulation technique A BPSKsignal is an analog signal whose phase is switched between two phases (0, 180°) in accordancewith the binary information signal

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01 11

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Quadrature Phase Shift Keying (QPSK)

QPSK is another form of constant-amplitude digital modulation The QPSK carrier cycles throughfour output phases The figure shows a QPSK transmitter and a QPSK constellation where the

phases are: 45, 135, 225, and 315 degrees All transitions are allowed including those through theorigin (180°)

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I- ChannelInput Data

b0 b1 b2 b3 b4

Q- ChannelInput Data

a0 a1 a2 a3

tb

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Offset Quadrature Phase Shift Keying (OQPSK)

Offset QPSK (OQPSK) is a modified form of QPSK where the bit waveforms on the I and Q

channel are shifted from each other by one-half of a bit time Since the bit waveforms are offset, atany given time only one bit stream can change the value In OQPSK, phase shifts of 180°are

prohibited, therefore simplifying amplifier design

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cdma university Analog and RF Concepts

The Decibel (db)

In communication systems, the numbers representing power gains and power levels are often verylarge or very small To facilitate working with these numbers, a logarithmic scale called decibel isused The decibel may be thought of as a mapping from a linear scale to a logarithmic scale Oncenumbers are converted to decibels, the operations of multiplication and division in the linear

domain are substituted by the operations of addition and subtraction respectively

The ratio of two power levels or power gain is converted to decibel using:

W

P

1

mW P

1

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cdma university Analog and RF Concepts (continued)

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Watts 1,000,000 100.000 10,000 1,000 100 10 4 2 1 0.5 0.25 0.1 0.01 0.001 0.0001 0.00001 0.000001

dBw +60 +50 +40 +30 +20 +10 +6 +3 0 -3 -6 -10 -20 -30 -40 -50 -60

dBm +90 +80 +70 +60 +50 +40 +36 +33 +30 +27 +24 +20 +10 0 -10 -20 -30

Notes

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cdma university Radio Channel Characteristics

Signals may add

Small Scale Effects: Multipath Fading

The fluctuation in signal strength at a receiver is often due to interference caused by the reception

of the same signal over multiple paths The signals may arrive with different path delays and phase

shifts This is referred to as Multipath Fading, Fast Fading, or Small Scale Fading.

The figures show multipath components adding constructively or destructively Small-scale fadingdepends on several variables including speed of the mobile, condition of surrounding environment,and transmission bandwidth In the cellular band, the received power may vary

30 to 40 dB as the mobile moves a fraction of a wavelength

Large-Scale Effects: Shadowing

Large-scale fading is mainly due to the separation between the transmitter and receiver and theclutter in the channel At any particular radius from the cell, the received signal strength will varydue to varying obstructions in the channel Blockage of the signal by obstructions in the channel is

referred to as Shadowing or Slow Fading This is a large scale effect that does not change

substantially over small distances

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