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OVERVIEW OF CDMA

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Multiple Access?

Number of users access and share

• Transmission medium

• Bandwidth available

For communication at the same time.

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Multiple Access:

• Multiple simultaneous transmissions.

• Sharing finite spectrum among large no. of simultaneous users.

• No pre-assigned circuit to any user.

• Any user can access any circuit and may access different circuits for different calls.

• Demand assigned circuits on first come first serve basis.

• Privacy.

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Multiple Access MethodsFrequency Division Multiple Access (FDMA): FDMA is a multiple access method in which users are assigned specific frequency bands. The user has sole right of using the frequency band for the entire call duration.

Time Division Multiple Access (TDMA): In TDMA an assigned frequency band shared among a few users. However, each user is allowed to transmit in predetermined time slots. Hence, channelization of user is achieved through separation in time.

Code Division Multiple Access (CDMA): Large number of transmissions are combined on the same RF channel at the same time but are separated by “codes”.

Time Fre

quenc

y

Power

FDMA TDMA CDMA

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CDMA: Code Division Multiple Access:

Large number of transmissions are combined on the same RF channel at the same time but are separated by “codes”.

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 corelator that accepts only signal energy from the desired channel. Undesired signals contribute only to the noise.

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SPREADING SPECTRUMSPREADING SPECTRUMShannon’s EquationC= W Log (1+S/N)Where C=Capacity (bps), W=Bandwidth, S/N=Signal to Noise Ratio Shannon’s Equation is basis for spread spectrum. Basics principle of spread spectrum is that the narrowband information signal is spread over a much larger bandwidth by multiplying the information signal by a wideband spreading code. Hence system with large band-width can operate at very low SNR level and can provide acceptable data rate per user.

Therefore in CDMA - All users uses same 1.25 MHz spectrum.- Each user has unique Digital code identifier.- Digital codes separate users to avoid interference.

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There are two types of Spread Spectrum namely Frequency Hopped Spread Spectrum (FH-SS) and Direct Sequence Spread Spectrum (DS-SS). CDMA cellular systems are use DS-SS technique.

The base-band signal of 9.6 Kbps is spread using a Pseudo-random Noise (PN) source to occupy entire bandwidth of 1.25 MHz. At the receiving end this signal will have interference from signals of other users of the same cell, users of different cells and interference from other noise sources. All these signals get combined with the desired signal but using a correct PN code the original data can be reproduced back.

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Direct Sequence Spread using Walsh code

Consist of 64 orthogonal codes each 64 bits long

Spreads spectrum to 1.2288 Mbps from 9.6 Kbps

Channel Capacity

C=W log (1+S/N)

Increasing BW improves Signal Transmission with lower S/N

Frequency

Power

Spectral

Density-------------------------------------------------------------------------------------- Noise Level

Narrow Band Waveform

Spread Waveform

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Cell Cell

TX RX

USER INPUT

1 0 0 1 1

10011001100110011001

CODE

USER OUTPUT

1 0 0 1 1

10011001100110011001

CODE

01101001100101100110

User Input 1 0 0 1 1

SpreadingSequence

1001 1001 1001 1001 1001

TX Data 0110 1001 1001 0110 0110

ORTHOGONAL SPREADING:

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Decoding using correct Orthogonal Function

0110 1001 1001 0110 0110

1001 1001 1001 1001 1001

1111 0000 0000 1111 1111

Rx Data

CorrectFunction

1 0 0 1 1

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Decoding using incorrect Orthogonal Function

0110 1001 1001 0110 0110

0101 0101 0101 0101 0101

0011 1100 1100 0011 0011

Rx Data

IncorrectFunction

? ? ? ? ?

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TECHNICAL PATRAMETERS of CDMA 

CDMA Standard IS-95A/B, CDMA 2000 1x or 3x & EV-DO

Frequency 824-844 & 869-889 Mhz (In India)

824-849 & 869-894 Mhz (ITU Standard)

 Access Method CDMA 

Duplexing Method FDD

  Modulation QPSK/ OQPSK

   RF Channel Spacing 1.25 Mhz (IS-95 & CDMA 2000)

5 Mhz (W-CDMA/3G)

Voice Coding Algorithm Q-CELP/EVRC

Voice Coding Rate 8K/13 K

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4.0 DIFFERENT CODES USED IN CDMA

4.1. Walsh Codes (Orthogonal Codes)

4.2. Long PN Code

4.3. Short PN Codes

IPN & QPN

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4. DIFFERENT CODES

4.1. Walsh Code: In CDMA the forward traffic channels are separated by unique “Walsh” code. All such codes are orthogonal to each other. Orthogonality provides nearly perfect isolation between the multiple signals transmitted by the base station. The basic concept behind creation of the code is as follows:

(a) Repeat the function right

(b) Repeat the function below

(c) Invert the function diagonally

0 00 1

Seed 0 0 0 0 00 1 0 10 0 1 10 1 1 0

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4.2. Long Code:

The long pseudo random noise (PN) sequence is based on 242characteristic polynomial. Reverse traffic channels (Mobile to Base) are separated by this long code and the data in the forward direction (Base to Mobile) is scrambled. The PN codes are generated using linear shift registers. The long code is unique for the subscribers and is known as users address mask. It repeats every 41 days (at a clock rate of 1.2288 Mcps)

PN offset (Masking)

Masking will cause the generator to produce the same sequence but offset in time.

-         Masking provides the shift in time for PN codes.

-         Different masks correspond to different time shifts.

-         ESN are used as masks for users on the traffic channels.

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4.3. Short Code:

The short Pseudo random Noise (PN) sequence is based on 215 characteristic polynomial.

This short code differentiates the cells & the sectors in a cell. It also consists of codes for I & Q channel feeding the modulator.

Each cell uses different PN offsets.

It repeats every 26.67 msec (at a clock rate of 1.2288 Mcps)

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5.0 CDMA Channels5.1 Forward Link Channels

•Pilot Channel

•Sync Channel

•Paging Channels

•Traffic Channels5.2 Reverse Link Channels

•Access Channels

•Traffic Channels

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Pilot channel (W0) The pilot is used by the subs unit to obtain initial system

synchronization and to distinguish cell sites. Every sector of every cell site has a unique pilot channel.

•         Transmitted constantly.

•         Allows the mobile to acquire the system.

•         Provides mobile with signal strength comparison.

•         Approximately 20% of the radiated power is in the pilot.

•         Has unique PN Offset for each cell or sector.

 

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Sync channel (W32)

Used during system Acquisition stage. Sync channel provides the subscriber unit with network information related to cell site identification, pilot transmit power & cell site PN offset.

•         Used by mobile to synchronize with the system

•         Transmits sync message with

- Pilot PN offset - System time

- Long PN code - System ID

- Network ID - Paging channel data rate

•         Tx at 1200 bps

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PAGING CHLS (W1-W7)

On this channel base station can page the subs unit and it can send call set-up and traffic channel assignment information.

•         Means of communication between base to mobile station.

•         Paging CHL data Rates can be 2.4,4.8 or 9.6 Kbps.

•         CDMA assignment has 7 paging channel.

•         Each paging CHL supports 180 pages per second.

•         Total pages/ CDMA RF channel = 1260

•         Provides mobile with

•         - System Parameter message - Neighbour list

•         - Access Parameter list - CDMA Channel list

•         Used by base station to :

•         - Page mobile - Transmit overhead information

•         - Assign mobile to traffic channel

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Forward Traffic Channel Generation

9600 bps4800 bps2400 bps1200 bpsRate set 1

I PN w

Convolutional Encoder & Repetition

Blockinterleave

r

LongCode PN decimat

ordecimat

or

User AddressMask(ESN)

O PN

Power Controlbit

19.2 ksps

1.2288

19.2 ksps

R=1/2

Mcps800bps

Rate set 214400 bps7200 bps3600 bps1800 bps

R=3/4

Wt

1.2288 Mcps

64:1 24:1

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Reverse Traffic Channel Generation

9600 bps4800 bps2400 bps1200 bpsRate set 1

I PN

Convolutional Encoder & Repetition

Blockinterleav

er

LongCode PN

User AddressMask(ESN)

O PN

28.8 ksps

1.2288

19.2 ksps

R=1/3

Mcps

Rate set 214400 bps7200 bps3600 bps1800 bps

R=1/21.2288 Mcps

Data Burst Rand.

OrthogonalModulation

307.2 KHz

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CDMA Capacity

W/R 1 1

N= -------*-----*--------*n *g

Eb/Io d 1+f

N= calls per sector

W= Spread spectrum Bandwidth (1.25 MHz)

R= data rate (9.6 kbps or 14.4 kbps)

Eb/Io= Bit energy/ other user interference density (7dB)

d= Voice activity factor (0.4)

f= other interference/ same interference (0.6)

n= loading factor (0.8)

g= reduction for variable power (0.85)

N= 27 users per sector for R=9.6Kbps

18 users per sector for R=14.4Kbps

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Rake Receiver

CDMA mobiles use rake receivers. The rake receiver essentially a set of four or more receivers (or fingers). One of the receivers constantly searches for different multipaths and helps to direct the other three fingers to lock onto strong multipath signals.

•         Allows combined reception of up to three different paths.

•         Provides searcher receiver to identify changes in path characteristics/new cells.

•         Provides both path diversity and frequency diversity.

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Data only 2.4 Mbps RF backward compatible

Voice, 9.6k Data only 10-60k

GSM(Europe)

CDMA2000 1xIS-95A

GPRS

EDGE

WCDMA

CDMA2000 1xEV-DO

IS-95B

Evolution of CDMA & GSM Networks

Voice, 14.4K Voice, 64K

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Evolution of CDMA Networks

 

First deployment of CDMA in commercial cellular systems was in 1994-95 only with IS-95 A as air-interface standard and IS-41 in core network; the complete network known as cdmaOne.

Next evolutionary step was use of IS-95B air interface standard which supported maximum data rate up-to 64 kbps to a user. Further in CDMA 2000 1x version many of the limitations of earlier IS-95 standard were overcome and new features were

added. As a result CDMA 2000 1x has a higher voice capacity and better handling of packet data services.

 

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Salient Features of CDMA 2000 1x

 

•Backward Compatibility with IS-95A & IS-95B

•Support for High data rates on same 1x Carrier

•Support for Simple IP and Mobile IP functionality for seamless mobility for data services.

•Higher capacity for voice communication

•Increased battery life

•Faster forward Power control (relative to IS-95)

•New Radio Configuration to support high data rates and more voice capacity.

 

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Architecture of CDMA 2000 1x Network:

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Architecture of CDMA 2000 1x Network:

 CDMA 2000 1x Network Architecture is divided in to three parts.

•CS-CN (Circuit Switched Core Network)

•PS-CN (Packet Switched Core Network)

•RAN (Radio Access Network)

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Circuit Switched Core Network:

This section is dedicated for voice communication and also for wireless authentication. This section includes four parts

MSC (Mobile Switching Center)

HLR (Home Location Register)

VLR (Visitor Location Register)

AUC (Authentication Center)

 

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MSC (Mobile Switching Center): It is responsible for setting up, managing and clearing connections as well as routing the calls to the proper user & provides the network interfaces, the charging function and the function of processing the signaling. MSC get data for call handling from 3 databases: VLR/HLR/AUC.

HLR (Home Location Register): It is a static database. When a user applies for mobile service, all data about this subscriber will be stored in HLR. It have information of a subscriber like ESN, MDN, IMSI, MIN, service information and valid term.

It also stores the mobile subscriber location (MSC/VLR address), to set up the call.

VLR (Visitor Location Register): VLR is a dynamic database used by MSC for information index. It stores all related information of mobile subscribers that enter its coverage area, which enables MSC to set up incoming and outgoing calls. It stores the subscriber parameters which includes subscriber number, location area identity (LAI), user’s status, services which subscriber can use and so on. When the subscriber leaves this area, it should register in another VLR, and the previous VLR will delete all the data about this subscriber. VLR can be built together with the MSC or set separately.

AUC (Authentication Center): It is an entity to prevent illegal subscribers from accessing CDMA network. It can generate the parameter to confirm the subscriber’s identity. At the same time it can encrypt user’s data according to user’s request.

AUC can be built separately or together with HLR

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Packet Switched Core Network: To provide better connectivity to the internet a new core network i.e. PS-CN is introduced to the CDMA 2000 1x network. This section includes four parts

PDSN (Packet Data Serving Node)

AAA Server

Home Agent/ Foreign Agent Server

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PDSN (Packet Data Serving Node): Packet Data Serving Node (PDSN) provides the function of routing of data between Radio Access Network (RAN) and internet

AAA Server: PS-CN also has the responsibility to authenticate, authorise and account for the CDMA 2000 subscribers wishing to obtain packet data services & to fulfil these task PDSN requires support of AAA server.

Authenticate: verifying that the user is valid & allowed to use packet data services.

Authorization: subscription to the service being offered is valid.

Accounting: Accounting for the service used.

Home Agent/ Foreign Agent Server: HA & FA server is used when mobile IP services are supported by CDMA 2000 PDSN.

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RAN (Radio Access Network): As in IS-95 RAN is composed of number of BSCs & BTSs The CDMA 2000 1x RAN is enhanced to support a higher no. of users on air interface or in other words it has a better spectral efficiency relative to IS-95. It is also modified to support the new packet data services on same 1.25 Mhz channel. This is achieved by software up-gradation at BTS and BSC and addition of a new hardware unit called Packet Control Function (PCF) at BSC.

The CDMA 2000 1x air interface is very different from IS-95 but still maintains the backward compatibility with IS-95.

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ADVANTAGES :

5.1. Larger Capacity: Let us discuss this issue with the help of Shannon’s Theorem. It states that the channel capacity is related to product of available band width and S/N ratio.

C = W log2 (1+S/N)

  Where C = channel capacity

W = Band width available

S/N = Signal to noise ratio

 It is clear that even if we improve S/N to a great extent the advantage that we are expected to get in terms of channel capacity will not be proportionally increased. But instead if we increase the bandwidth (W), we can achieve more channel capacity even at a lower S/N. That forms the basis of CDMA approach, wherein increased channel capacity is obtained by increasing both W & S/N. The S/N can be increased by devising proper power control methods.

 

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5.1.1 Vocoder and variable data rates: As the telephone quality speech is band limited to 4 Khz, when it is digitized with PCM its bit rate rises to 64 Kb/s. Vocoding compress it to a lower bit rate to reduce bandwidth. The transmitting vocoder takes voice samples and generates an encoded speech/packet for transmission to the receiving vocoder. The receiving vocoder decodes the received speech packet into voice samples. One of the important features of the variable rate vocoder is the use of adaptive threshold to determine the required data rate. Vocoders are variable rate vocoders. By operating the vocoder at half rate on some of the frames the capacity of the system can be enhanced without noticeable degradation in the quality of the speech. This phenomenon helps to absorb the occasional heavy requirement of traffic apart from suppression of background noise. Thus the capacity advantage makes spread spectrum an ideal choice for use in areas where the frequency spectrum is congested.

 

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5.2. Less (Optimum) Power per cell:

Power Control Methods: In CDMA the entire bandwidth of 1.25 MHz, is used by all the subscribers served in that area. Hence they all will be transmitting on the same frequency using the entire bandwidth but separated by different codes. At the receiving end the noise contributed by all the subscribers is added up. To minimize the level of interfering signals in CDMA, very powerful power control methods have been devised and are listed below:

 

1. Reverse link open loop power control

2. Reverse link closed loop power control

3. Forward link power control

 

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5.3. Seamless Hand-off: CDMA provides soft hand-off feature for the mobile crossing from one cell to another cell by combining the signals from both the cells in the transition areas. This improves the performance of the network at the boundaries of the cells, virtually eliminating the dropped calls.

5.4. No Frequency Planning: A CDMA system requires no frequency planning as the adjacent cells use the same common frequency.

5.5. High Tolerance to Interference: The primary advantage of spread spectrum is its ability to tolerate a fair amount of interfering signals as compared to other conventional systems. This factor provides a considerable advantage from a system point of view.

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5.6. Multiple Diversity:

Diversity techniques are often employed to counter the effect of fading. The greater the number of diversity techniques employed, the better the performance of the system in a difficult propagation environment.

  CDMA has a vastly improved performance as it employs all the three diversity techniques in the form of the following:

A. Frequency Diversity: A wide band RF signal of 1.25 Mhz being used.

B. Space Diversity: Employed by way of multipath rake receiver.

C. Time Diversity: Employed by way of symbol interleaving, error detection and correction coding.

 

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First deployment of CDMA in commercial cellular systems was in 1994-95 only with IS-95 A as air-interface standard and IS-41 in core network; the complete network known as cdmaOne.

Next evolutionary step was use of IS-95B air interface standard which supported maximum data rate up-to 64 kbps to a user.

Further in CDMA 2000 1x version many of the limitations of earlier IS-95 standard were overcome and new features were added.

As a result CDMA 2000 1x has a higher voice capacity and better handling of packet data services.

 

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Salient Features of CDMA 2000 1x

•Backward Compatibility with IS-95A & IS-95B

•Support for High data rates on same 1x Carrier

•Support for Simple IP and Mobile IP functionality for seamless mobility for data services.

•Higher capacity for voice communication

•Increased battery life

•Faster forward Power control (relative to IS-95)

•New Radio Configuration to support high data rates and more voice capacity.

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CDMA 2000 1x EV-DO:Although IS-2000 is already capable of meeting the 3G data rate requirement of 2 Mbps (By using 3x option) Qualcomm proposed a new standard 1xEV-DO (1x Evolution for Data Optimized) in March of 2000 as another option that supports high-rate data services.

EVDO is optimized for delivering high speed IP wireless data to many mobile and stationary terminals running multiple applications. EVDO is designed for an always on user experience.

 In a classical CDMA 2000 system base station controls its power by using the power control algorithms to provide the mobile a constant data rate and a quality of service for voice applications

But in EV-DO networks the base station transmits at a fixed power at all the times and controls the rate of data transmission given a constant transmit power.

 

 

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Since EV-DO is specially designed for packet data services therefore EV-DO designs its air interface to takes advantage of the characteristics of some data services, which are

Data rates are mostly asymmetrical: Data rate requirements downstream (on the forward link) are usually higher than those upstream (on the reverse link).

Latency can be tolerated: Data services, unlike voice services, can withstand delays of up to seconds.

Transmissions are bursty in nature: A burst of data transmission is often followed by a period of inactivity.

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Salient features of EV-DO

 

• EV-DO uses both CDMA and TDMA.

• Uses its own dedicated 1.25 Mhz carrier.

• It can support a maximum data rate of 2.4 Mbps in forward link.

• It can support a maximum data rate of 153.6 Mbps in reverse link.

• No power control on forward link is required.

• RF system components may be shared with 1xRTT.