Basics CDMA.ppt

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How Did We Reach This Point?How Did We Reach This Point?

TelegraphyTelegraphy

TelephonyTelephony

Radio MilestonesRadio Milestones

“Hello”

“Shalom” “Guten Tag”

“Time Division” “Frequency Division!”

“CHAOS”

“Buenos Dias”

“Bonjour”

The SYMPHONY!The SYMPHONY!

GSM Vs. CDMAGSM Vs. CDMA

7 cell re-use patternf7

f7

f2

f2

f6

f6

f1

f5f3

f4

f1

f5f3

f4

Frequency Reuse- GSM

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

f1

FREQUENCY REUSE IN CDMAFREQUENCY REUSE IN CDMA

f1f1

GSM Vs CDMA

FREQUENCY REUSE IN CDMA & TDMA

TYPICAL TDMA SYSTEMEACH CELL USES DIFFERENT FREQUENCY

THE PATTERN IS REPEATED FOR THE NEXT SET OF CELL SITES

TYPICAL CDMA SYSTEMEACH CELL USES SAME FREQUENCY

F 1 F 1

F 1

F 1

F 1

F 1

F 1

F 1

F 2

F 5

F 4F 6

F 7 F 3

CODE DIVISION MULTIPLE ACCESS (CDMA)

CDMA - Code Division Multiple AccessCDMA - Code Division Multiple Access

CDMA is a "spread spectrum" technology– Spreads the information contained in a

particular signal of interest over a much greater bandwidth than the original signal.

Goal of spread spectrum – Interference mitigation

Spread spectrum helps mitigate the harmful effects of interference• Deliberate - Military Jammer (Use first by

DOD)• Inadvertent - Co-channel users

Shanon – Hartley TheoremShanon – Hartley Theorem To consider the technology used in CDMA, consider the Shannon-Hartley theorem,

which is given as follows: C = W log2 (1 + S/N)Where, C = capacity W = Bandwidth S/N = Signal to Noise RatioAccording to the theorem, an increase in the bandwidth causes a decrease in the signal

to noise ratio, thus requiring a trade-off between the two parameters. However, CDMA operates by maximizing the bandwidth, which brings about a corresponding reduction in the SNR This reduction is countered by the utilization of an efficient error correction code, which ensures optimum performance even in low SNR conditions.

Spread Spectrum Principles• Shannon’s work relates capacity to both bandwidth and signal tonoise ratio. It allows some simple conclusions to be drawn. In a bandwidth limited system the only way to increase capacity (ordata rate) is to do one or both of the following:• Increase the Signal Power• Decrease the Noise Power

Forward & Reverse LinksForward & Reverse Links

824MHz

869MHz

849MHz

894MHz

Forward linkReverse link

Uplink Downlink

Physical ChannelPhysical Channel

The 3 dB bandwidth of a channel is the frequency range where the signal at the edges is 3 dB

lower than the peak value at the center frequency, fc

CDMA Technology BenefitsCDMA Technology Benefits Capacity increase Improved call quality Simplified system planning

– use of the same frequency in every sector of every cell Enhanced privacy Improved coverage characteristics

– possibility of fewer cell sites– better performance in fading/interference prone

environment Increased talk time for portables Bandwidth on demand

Capacity in CDMACapacity in CDMA

Capacity of the system depends on interference (Total power received in the Receiving Antenna) in the system

Power Control Techniques are employed to reduce the total interference in the system.

By controlling power, the capacity can be dynamically adjusted (Capacity in CDMA systems is soft)

CDMA is altering the face of TelecommCDMA is altering the face of Telecomm Dramatically improving the telephone traffic (Erlang) capacity Dramatically improving the voice quality and eliminating the

audible effects of Multipath fading Reducing the incidence of dropped calls due to handoff

failures Providing reliable transport mechanism for data

communications, such as facsimile and internet traffic Reducing the number of sites needed to support any given

amount of traffic Simplifying site selection Reducing deployment and operating costs because fewer

cell sites are needed Reducing average transmitted power Reducing interference to other electronic devices Reducing potential health risks

All CDMA users occupy the same frequency at the same time! Frequency and time are not used as discriminators

CDMA operates by using CODING 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

CDMA: Using A New Dimension

Frequency-Hopping Each user’s narrowband signal hops

among discrete frequencies, and the receiver follows in sequence

Spectrum (FHSS) CDMA is NOT currently used in wireless systems, although used by the military

Direct Sequence• Narrowband input from a user is coded (“spread”) by a user-unique broadband code, then transmitted.• Broadband signal is received; receiver knows, applies user’s code, recovers users’ data

TYPES of CDMA Technology

At Originating Site: Input A: User’s Data @

19,200bps Input B: Walsh Code #23 @

1.2288 Mcps Output: Spread Signal

Spectrum

At Receiver Site: Input A: Received spread

spectrum signal Input B: Walsh Code #23 @

1.2288 Mcps Output: User’s Data

@19,200bps just as originally sent

DSSS Spreading: Time-Domain View

Spreading from a Frequency-Domain View

CDMA uses larger bandwidth but uses resulting processing gain to increase capacity

Spreading: What we do, we can undo

Sender 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” via CDMA

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

CDMA’s Nested Spreading Sequences

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.

U1 = 0110010101001000

C1 ( 100110….10110010)*

=U1C1 ( 100110………………………0000)

U1C1 ( 100110………………………00000)

U1 = 0110010101001000

C1 ( 100110….10110010)*

=

UnCn

U4C4

U3C3

U2C2

UnCn*C1 = 0, UnCn*Cn = Un

U4C4*C1 = 0, U4C4*C4 = U4

U3C3*C1 = 0, U3C3*C3 = U3

U2C2*C1 = 0, U2*C2*C2 = U2

C1*C1 = 1, C2*C2 = 1…. Cn*Cn = 1 BUT C1*C2 = 0…C1*Cn = 0

DSSS Spreading/ DespreadingDSSS Spreading/ Despreading

End to end overviewEnd to end overview

Types of CodeTypes of Code

Forward & Reverse LinkForward & Reverse Link

The Three CDMA Spreading TechniquesThe Three CDMA Spreading Techniques

Orthogonal SequencesOrthogonal SequencesOrthogonal SequencesOrthogonal Sequences• Definition:Orthogonal functions have zero correlation. Two binary sequences are orthogonal if the process of “XORing” them results in an equal number of 1’s and 0’s. Example:Example:

00000000((XOR) 01010101

------------01010101

0 0

0 1- Repeat: right & below

- Invert: diagonally

0 0

0 1

0 0

0 1

0 0

0 1

1 1

1 0

Walsh Codes 64 “Magic” Sequences, each 64 chips long Each Walsh Code is precisely Orthogonal with respect

to all other Walsh Codes

• it’s simple to generate the codes, or

• they’re small enough to use from ROM

Short codeShort code

Usage of codeUsage of code

Need for SynchronisationNeed for Synchronisation

Functions of the CDMA Forward Channels

WALSH CODE WALSH CODE contd. contd.

WALSH CODES ARE USED TO SEPARATE INDIVIDUAL USERS WHILE THEY SIMULTANEOUSLY OCCUPY THE SAME RF BANDWIDTH

THE SEQUENCE ARE ORTHOGONAL TO EACH OTHER AND ARE GENERATED USING THE HADAMARD MATRIX

WALSH-0 IS NOT USED TO TRANSMIT ANY BASEBAND DATA

PN CODEPN CODE

PN SEQUENCES

USED TO SPREAD THE BANDWIDTH OF THE MODULATED SIGNAL TO LARGER TRANSMISSION BANDWIDTHS

DISTINGUISH BETWEEN DIFFERENT USER SIGNALS

MULTIPLICATION BY A SHORT PN SEQUENCE IS DONE TO PROVIDE ANOTHER LAYER OF ISOLATION ON THE FORWARD LINK

WE CAN HAVE A MAXIMUM OF 512 DIFFERENT PN SEQUENCES EACH WITH A SEPARATION OF 64 CHIPS FROM EACH OTHER

SecuritySecurity

CHANNELSCHANNELS

CDMA IS-2000 HIGH LEVEL ARCHITECTURE

• FORWARD CHANNELS

PILOT CHANNEL (1)

SYNC CHANNEL (32)

FORWARD TRAFFIC

PAGING CHANNELS ( 1-7)

• REVERSE CHANNELS

REVERSE TRAFFIC CHANNELS

ACCESS CHANNELS

SummarySummary

Forward Reverse

Short PN Different offset- Each Sector

Zero offset-Used by MS

Walsh code 64 Channel 64 Ary modulation

Long code Use to scramble TCH

Identify TCH for each MS

Information about the long code is broadcast to the mobile station by the Sync Channel (or Control Channel) to help the mobile lock onto the base station, and helps provide separation from other base stations.

One of the codes used in conjunction with the Walsh Code is the PN (pseudo-random noise) short code. The PN short code on the forward link is used to provide the base station with a unique identification that the mobile station uses to identify the serving base station.

The user signal (or control channel) is multiplied by the Walsh code. The Walsh code provides each user or channel with a unique identifier and, in DS spreading, may spread the frame across the bandwidth.

Main process:– y(t) = b(t) c(t)• Scrambling:– If b(t) and c(t) have the same rate then y(t) has the same rate,and the spectrum of the signal is unchanged b(t) is said to be encrypted or scrambled• Spreading:– If c(t) has a higher rate than b(t), y(t) has the faster rate and itscorrespondingly wider spectrum .In addition to being scrambled, b(t) is said to have had its spectrum spread• CDMA codes are used to perform scrambling andspreading.

Rayleigh Fading In addition to delay spread, the same multipath environment causes severe local variations in signal strength as thesemultipath signals are added constructively and destructively at the receiving antenna. This type of variation is called Rayleigh fadingThis can cause large blocks of information to be lost.If the set of reflected signals have one dominant component, such as a line-of-sight signal, the fading is more appropriately modeled using the Rician model. Note that if the mobile speed is zero, there is no fading, except if signals are reflected frommoving objects.For slow fading, CDMA uses power control to adjust the transmitted power in order to overcome the fades. Power control is too slow for the fast Rayleigh fading; instead, FEC encoding and bit interleaving are used.

Channel Contd.Channel Contd.

Forward channelForward channel

PILOT CHANNELPILOT CHANNEL

• PILOT SIGNALS ARE TRANSMITTED BY EACH CELL SITE TO ASSIST MOBILE RADIO IN ACQUIRING AND TRACKING THE CELL SITE DOWNLINK SIGNAL

• PILOT CHANNEL IS ASSIGNED CODE CHANNEL NUMBER ZERO

• THE SIGNAL STRENGTH = Ec/Io• Ec/Io IS THE ENERGY PER CHIP PER INTERFERENCE

DENSITY MEASURED ON THE PILOT CHANNEL• Ec/Io EFFECTIVELY DETERMINES THE FORWARD

COVERAGE AREA OF A CELL OR A SECTOR

SYNC CHANNELSYNC CHANNEL

• SYNC CHANNEL IS GIVEN THE CODE CHANNEL NUMBER 32; FIXED DATA RATE 1200 KBPS

• ALLOWS RECIEVER TO OBTAIN FRAME SYNCHRONIZATION ON SIGNAL

• MESSAGES SENT ON SYNCH CHANNEL ARESYSTEM TIMECHARACTERISTICS OF THE SYSTEM

FORWARD TRAFFIC CHANNELSFORWARD TRAFFIC CHANNELS

• PAGING CHANNELS ARE GIVEN THE CODE CHANNEL NUMBER 1 THRU 7

• FORWARD TRAFFIC CHANNELS GROUPED INTO RATE SET 1( 9.6, 4.8, 2.4 or 1.2 KBPS) AND RATE SET 2 (14.4, 7.2, 3.6 or 1.8 KBPS)

• RATE SET 1 IS REQUIRED FOR IS-95 WHEREAS RATE SET 2 IS OPTIONAL

• SPEECH IS ENCODED WITH VARIABLE RATE VOCODER TO GENERATE FORWARD TRAFFIC CHANNEL DATA DEPENDING ON VOICE ACTIVITY

Reverse ChannelReverse Channel

REVERSE TRAFFIC CHANNELSREVERSE TRAFFIC CHANNELS

• IDENTIFIED BY LONG USER CODE OFFSET

• DATA TRANSMITTED ON REVERSE CHANNEL IS CONVOLUTIONALLY ENCODED, BLOCK INTERLEAVED, MODULATED BY MEANS OF 64-ary ORTHOGONAL MODULATION, AND DIRECT SEQUENCE SPREAD PRIOR TO TRANSMISSION

• DATA RATE IS 9.6, 4.8, 2.4 OR 1.2 KBPS

ACCESS CHANNELSACCESS CHANNELS

• ENABLES THE MOBILE TO COMMUNICATE NONTRAFFIC INFORMATION

• DATA RATE IS FIXED AT 4.8 KBPS

• IDENTIFIED BY A DISTINCT ACCESS CHANNEL LONG-CODE SEQUENCE OFFSET

• A PAGING CHANNEL NUMBER IS ASSOCIATED WITH ACCESS CHANNEL

Forward LinkForward Link

Putting it All Together: CDMA Channels

How a BTS Builds the Forward Code Channels

Code Channels in the Reverse Direction

Functions of the CDMA Reverse Channels

Variable rate vocoderVariable rate vocoder

Vocoders compress speech, reduce bit rate, greatly increasing capacity

CDMA uses a superior Variable Rate Vocoder

• full rate during speech

• low rates in speech pauses

• increased capacity

• more natural sound Voice, signaling, and user secondary data may be

mixed in CDMA frames.

Variable Rate Vocoding & Multiplexing

In CDMA, most call processing events are driven by messages

Some CDMA channels exist for the sole purpose of carrying messages; they never carry user’s voice traffic

• Sync Channel (a forward channel)

• Paging Channel (a forward channel)

• Access Channel (a reverse channel)

Some CDMA channels exist just to carry user traffic

• Forward Traffic Channel

• Reverse Traffic Channel

• On these channels, most of the time is filled with traffic and messages are sent only when there is something to do

Messages in CDMA

Rake ReceiverRake Receiver

What’s In a Handset? How does it work?

The Rake Receiver Handset uses combined outputs of the three traffic correlates (“rake fingers”). Each finger can independently recover a particular PN offset and Walsh code. Fingers can be targeted on delayed Multipath Reflections, or on different

BTSs. Searcher continuously checks pilots.

The Pilot Searching ProcessThe Pilot Searching Process

Handoff SignalingHandoff Signaling

Near-far Problem Path Loss Fading Performance Objectives

Power Control Is Required ?

Power Control in CDMAPower Control in CDMA

Open Loop power control– Purely mobile unit function– Done during initial stage when mobile is

turned onClosed Loop power control

– Involves both base station and mobile unit

Forward Power Control

The BTS continually reduces the strength of each user’s forward base band chip stream

When a particular handset sees errors on the forward link, it requests more energy

The complainer’s chip stream gets a quick boost; afterward ,continues to diminish

Reverse Power Control

• Reverse Open Loop: Handset adjusts power up or down based on received BTS signal (AGC).

• Reverse Closed Loop: Is handset too strong? BTS tells up or down 1 dB 800 times/second.

• Reverse Outer Loop: BSC has FER trouble hearing handset? BSC adjusts BTS set point.

Types of HandoffTypes of Handoff

SOFT HANOFF

1. Soft handoff - 2 BTS are involved

2. Soft Soft - 3 BTS are involved

3. Softer - Two Sector of same BTS

HARD HANDOFF

- Between different frequency.

Hard & Soft handoverHard & Soft handover

CDMA Soft Handoff Mechanics CDMA soft handoff is driven by the handset Handset continuously checks available pilots Handset tells system pilots it currently sees System assigns sectors (up to 6 max.), tells handset Handset assigns its fingers accordingly

The Handset considers pilots in set ACTIVE SET: Pilots of sectors in use CANDIDATE SET: Pilots mobile

requested, but not yet set up NEIGHBOUR SET: Pilots told to mobile

by system, as nearby sectors to check REMAINING SET: Any pilots used by

system but not already in the other sets Handset sends Pilot Strength

Measurement Message to the system whenever:

• It notices a pilot in neighbor or remaining set exceeds T_ADD

• An active set pilot drops below T_DROP for T_TDROP time

• A candidate pilot exceeds an active by T_COMP

The Complete Rules of Soft Handoff

Softer Handoff Handset will ask for whatever pilots it wants If multiple sectors of one BTS simultaneously serve a

handset, this is called Softer Handoff Handset can’t tell the difference, but softer handoff

occurs in BTS in a single channel element Handset can even use combination soft-softer handoff

on multiple BTS & sectors

Soft H/O ADV. & DIS. ADV.Soft H/O ADV. & DIS. ADV.

Reverse linkReverse link

Forward linkForward link

Processing GainProcessing Gain

Spreading gain or processing gain is achieved when noise components, or noise-like

components, remain spread when the original signal (user 1 in the figure) is despread. The

original signal appears to have gained energy relative the noise. It can also be seen as if the noise

has been suppressed. By filtering out most of the wideband noise energy the original signal

can be extracted, provided sufficient bit energy over noise ratio, Eb/NT. It can be seen that the

“signal to noise” ratio after despreading will favor user 1 by a factor of G = BW/bw (or Fc/Fb or

Tb/Tc). G is then called spreading gain or processing gain. Processing gain can also be seen

as the number of chips per bit.

Processing GainProcessing Gain

Processing GainProcessing Gain

Energy per BitEnergy per Bit

•Eb/NT is traffic channel bit energy over noise.•Ec/I0 is pilot channel chip (bit) energy over interference.

Capacity EnhancementCapacity Enhancement

Capacity FactorCapacity Factor

Idle Mode Handoff

An idle mobile always demodulates the best available signal In idle mode, it isn’t possible to do soft handoff and listen to

multiple sectors or base stations at the same time -- the paging channel information stream is different on each sector, not synchronous -- just like ABC, NBC, CBS, and CNN TV news programs aren’t in word-sync for simultaneous viewing

Since a mobile can’t combine signals, the mobile must switch quickly, always enjoying the best available signal

The mobile’s pilot searcher is constantly checking neighbor pilots If the searcher notices a better signal, the mobile continues on the

current paging channel until the end of the current superframe, then instantly switches to the paging channel of the new signal

The system doesn’t know the mobile did this! (Does NBC’s Tom Brokaw know you just switched your TV to CNN?)

On the new paging channel, if the mobile learns that registration is required, it re-registers on the new sector

Signal processingSignal processing

•Speech encoding. This step is only used if speech information is transmitted. Data transmission omits this step.•Quality indicator•Forward Error Correction (FEC) encoding•Interleaving•Scrambling•Spreading•Digital modulation•RF modulation•Amplification of RF signal.

Speech encodingSpeech encodingIn order to transmit speech over a digital system, it must be digitized and encoded using a vocoder. Normal speech is received as an analog signal. The analog signal is converted into a digital signal using a process called Nyquist sampling, in which the analog input is typically sampled 8,000 times per second. The product of Nyquist sampling is a digital waveform called PCM (pulse code modulation).The PCM output is transferred to a vocoder (voice coder), which compresses the digitized voice signal into either Rate Set 1 (RS1) with an output of 8 kbps, or Rate Set 2 (RS2) with an output of 13 kbps, depending on the type of vocoder. In CDMA, variable rate vocoders are used.

Variable Rate VocoderThe variable rate vocoder employs a codec (coder/decoder) that compresses digitized speech from the analog-to-digital (A/D) converter and produces an output that complies with the data rate to be transmitted. During these “lulls” in theconversation, the vocoder can reduce its bandwidth requirements, before the FEC encoder, from full rate (9600 bps for EVRC) to 1/2 rate, 1/4 rate, or 1/8 rate (1200 bps for EVRC). Since the transmitter only transmits the lowest bit rate required, the required transmit power is minimized, and the channel interference is reduced.

FrameInformation bits are grouped into frames. A frame is the basic

timing interval in the system. Thelength of a frame depends on what channel on which it is

transmitted (e.g., Sync Channel,Traffic Channel), what type of information transmitted in the frame

(e.g., overhead messages,traffic information), and what air-interface standard is used

Error MeasurementsThere are a number of error measurements available in CDMA

transmission: Bit Error Rate(BER), Frame Error Rate (FER), and Packet Error Rate (PER).The frame quality indicator is a Cyclic Redundancy Code (CRC).

End of presentationEnd of presentation