mobile computing report

7/29/2019 Mobile Computing Report

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A REPORT ON

Multiple Access techniques

SUBMITTED IN PARTIAL FULFILLMENT FOR THE

DEGREE OF BACHELOR OF TECHNOLOGY IN

INFORMATION TECHNOLOGY

BY

VIDYA MHETRE(76)

KAJAL PATEL(77)

SNEHA PHALKE(79)

AKSHATA NAIK(87)

USHA MITTAL INSTITUTE OF TECHNOLOGY

S.N. D. T. WOMENS UNIVERSITY

MUMBAI400049

20122013.


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Abstract

Systems with single-carrier techniques show significant limits when high data speed

transmission is concerned. In order to increase data speed, meet QoS requirements and maximize

system capacity, multiple access techniques with efficient resource allocation are investigated.

Systems with multi-carriers and multiple spreading in time, frequency, code or space domain, as

well as radio link adaptation technologies, are favored due to huge capacity, robustness and

flexibility. Therefore, future research and design of mobile networks shall focus on multi-carrier

hybrid modulation systems with adaptive technology and multiple smart antennas. This paper

describes multiple access techniques suitable for mobile communication systems, stresses the

importance of resource allocation techniques, briefly touches on present mobile network

technology and finally gives an overview of multiple-access techniques for future mobile

networks


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Introduction

A limited amount of bandwidth is allocated for wireless services. A wireless system is required

to accommodate as many users as possible by effectively sharing the limited bandwidth.

Therefore, in the field of communications, the term multiple access could be defined as a means

of allowing multiple users to simultaneously share the finite bandwidth with least possible

degradation in the performance of the system. There are several techniques how multiple

accessing can be achieved.FDMA is one of the earliest multiple-access techniques for cellular systems

when continuous transmission is required for analog services. In this technique the bandwidth is divided

into a number of channels and distributed among users with a finite portion of bandwidth for permanent

use as illustrated in the figure. The vertical axis that represents the code is shown here just to make a clear

comparison with CDMA (discussed later in this chapter). The channels are assigned only when demanded

by the users. Therefore when a channel is not in use it becomes a wasted resource. In digital systems,

continuous transmission is not required because users do not use the allotted bandwidth all the time. In

such systems, TDMA is a complimentary access technique to FDMA. Global Systems for Mobile

communications (GSM) uses the TDMA technique. In TDMA, the entire bandwidth is available to the

user but only for a finite period of time. In most cases the available bandwidth is divided into fewer

channels compared to FDMA and the users are allotted time slots during which they have the entire

channel bandwidth at their disposal. This is illustrated in the figure. TDMA requires careful time

synchronization since users share the bandwidth in the frequency domain. Since the number of channels

are less, inter channel interference is almost negligible, hence the guard time between the channels is

considerably smaller. Guard time is a spacing in time between the TDMA bursts. In cellular

communications, when a user moves from one cell to another there is a chance that user could experience

a call loss if there are no free time slots available. CDMA systems utilize a spread spectrum

technique in which a spreading signal, which is uncorrelated to the signal and has a large

bandwidth, is used to spread the narrow band message signal. Direct Sequence Spread Spectrum

(DS-SS) is most commonly used for CDMA. In DS-SS, the message signal is multiplied by aPseudo Random Noise Code (PN code), which has noise-like properties. Each user has his own

code word which is orthogonal to the codes of other users. In order to detect the user, the

receiver is required to know the code word used by the transmitter. Unlike TDMA, CDMA does

not require time synchronization between the users.SDMA utilizes the spatial separation of the

users in order to optimize the use of the frequency spectrum. A primitive form of SDMA is when


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the same frequency is re-used in different cells in a cellular wireless network. However for

limited co-channel interference it is required that the cells be sufficiently separated. This limits

the number of cells a region can be divided into and hence limits the frequency re-use factor. A

more advanced approach can further increase the capacity of the network. This technique would

enable frequency re-use within the cell. It uses a Smart Antenna technique that employs antenna

arrays backed by some intelligent signal processing to steer the antenna pattern in the direction

of the desired user and places nulls in the direction of the interfering signals.Spread spectrum

systems is a class of (primarily) wireless digital communicationsystems specifically designed to

overcome a jamming situation, i.e., when anadversary intends to disrupt the communication. To

disrupt the communication, theadversary needs to do two things, (a) to detect that a transmission

is taking place and (b) to transmit a jamming signal which is designed to confuse the receiver. A

spread spectrum system is therefore designed to make these tasks as difficult as possible. Firstly,

the transmitted signal should be difficult to detect by an adversary, i.e., the signal should have a

low probability of intercept (LPI). Secondly, the signal should be difficult to disturb with a

jamming signal, i.e., the transmitted signal should possess an anti-jamming (AJ) property.


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FDMA

DESCREPTION:-

Frequency Division Multiple Access or FDMA is a channel access method used in multiple-

access protocols as a channelization protocol. FDMA gives users an individual allocation of one

or several frequency bands, or channels. It is particularly commonplace in satellite

communication. FDMA, like other Multiple Access systems, coordinates access between

multiple users. Alternatives include TDMA, CDMA, or SDMA. These protocols are utilized

differently. FDMA is distinct from frequency division duplexing (FDD). While FDMA allows

multiple users simultaneous access to a transmission system, FDD refers to how the radio

channel is shared between the uplink and downlink (for instance, the traffic going back and forth

between a mobile-phone and a mobile phone base station). Frequency-division

multiplexing (FDM) is also distinct from FDMA. FDM is a physical layer technique that

combines and transmits low-bandwidth channels through a high-bandwidth channel. FDMA, on

the other hand, is an access method in the data link layer. FDMA also supports demand

assignment in addition to fixed assignment.Demand assignmentallows all users apparently

continuous access of the radio spectrum by assigning carrier frequencies on a temporary basis

using a statistical assignment process. The first FDMA demand-assignmentsystem for satellite

was developed by COMSAT for use on theIntelsatseriesIVA and Vsatellites.

FEATURES:-

1. In FDMA all users share the satellite transponder orfrequency channel simultaneouslybut each user transmits at single frequency.

2. FDMA can be used with both analog and digital signal.3.

FDMA requires high-performing filters in the radio hardware, in contrastto TDMA and CDMA.

4. FDMA is not vulnerable to the timing problems that TDMA has. Since a predeterminedfrequency band is available for the entire period of communication, stream data (a

continuous flow of data that may not be packetized) can easily be used with FDMA.

5. Due to the frequency filtering, FDMA is not sensitive to near-far problem which ispronounced forCDMA.
http://en.wikipedia.org/wiki/Frequency_channelhttp://en.wikipedia.org/wiki/Time_division_multiple_accesshttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/Time_division_multiple_accesshttp://en.wikipedia.org/wiki/Near-far_problemhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/Near-far_problemhttp://en.wikipedia.org/wiki/Time_division_multiple_accesshttp://en.wikipedia.org/wiki/CDMAhttp://en.wikipedia.org/wiki/Time_division_multiple_accesshttp://en.wikipedia.org/wiki/Frequency_channel


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6. Each user transmits and receives at different frequencies as each user gets a uniquefrequency slots.

APPLICATIONS:-

1. FDMA is used in mobile communication.

2. It is used in 2G of mobile communication in the European Standard. GSM is one of these

technologies.

3. It is used in first generation cellular communication systems. (e.g. NMT, ETACS, AMPS, etc.)

4. This technique is also used in fiber optic communications systems.5. It is used to set the transmission and reception frequencies.

ADVANTAGES:-

1. Frequency band divided into small frequency channels and different channels are allocated to

different users.

2.Multiple users can transmit at the same time but on different frequency channels.3. FDMA is cheaper as compared to CDMA, TDMA and SDMA

4. Reducing the information bit rate and using efficient digital codes can obtain capacity

increases.

5. As FDMA systems use low bit rates (large symbol time) compared to average delay spread,

they reduce the cost, and there is low Inter Symbol Interference (ISI).

6. There is hardly any equalization required.

7. Technological advances required for implementation are simple. A system can be configured

so that improvements in terms of speech coder bit-rate reduction could be readily incorporated.

8. Since the transmission is continuous, less number of bits are needed for synchronization and

framing.


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DISADVANTAGES:-

1. It does not differ significantly from analog systems; capacity improvement depends on

reducing signal-to-interference ratio, or signal-to-noise ratio (SNR).

2. The maximum bit rate per channel is fixed and small.

3. The guard bands result in wastage of capacity.

4. Hardware involves narrow band filters, which cannot be realized in VLSI and thus increase

cost.

5. Impossible for receiver to receive the data from more than one station at a single point of

time

6. Maximum Data Rate for every channel is small and fixed.

7. Requires special filters to avoid interference between any narrow channels.


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TDMA

DESCRIPTION:-

Time division multiple access (TDMA) is a channel access method for shared medium networks.

It allows several users to share the same frequency channel by dividing the signal into different

time slots. The users transmit in rapid succession, one after the other, each using its own time

slot. This allows multiple stations to share the same transmission medium (e.g. radio frequency

channel) while using only a part of its channel capacity. TDMA is used in the digital 2G cellular

systems such as Global System for Mobile Communications (GSM), IS-136, Personal Digital

Cellular (PDC) and Iden, and in the Digital Enhanced Cordless Telecommunications (DECT)

standard for portable phones. It is also used extensively in satellite systems, combat-net radio

systems, and PON networks for upstream traffic from premises to the operator. For usage of

Dynamic TDMA packet mode communication, see below.

TDMA is a type of Time-division multiplexing, with the special point that instead of having

one transmitter connected to one receiver, there are multiple transmitters. In the case of

the uplinkfrom a mobile phone to a base station this becomes particularly difficult because the

mobile phone can move around and vary the timing advance required to make its transmission

match the gap in transmission from its peers.

FEATURES:-

1. Shares single carrier frequency with multiple users2. Non-continuous transmission makes handoff simpler3. Slots can be assigned on demand in dynamic TDMA4.

Less stringent power control than CDMA due to reduced intra cell interference

5. Higher synchronization overhead than CDMA6. Advanced equalization may be necessary for high data rates if the channel is frequency

selective and creates Inter symbol interference

7. Cell breathing (borrowing resources from adjacent cells) is more complicated than inCDMA


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8. Frequency/slot allocation complexity9. Pulsating power envelope: Interference with other devices

APPLICATIONS:-

1. TDMA (time division multiple access) is a technology used in digital cellular telephone

communication that divides each cellular channel into three time slots in order to increase the

amount of data that can be carried.

2. TDMA is used by Digital-American Mobile Phone Service.

3. TDMA is also used for Digital Enhanced Cordless Telecommunications (DECT).

4. TDMA is also used in GSM.

5. Time Division Multiple Access or TDMA is a method used to enable multiple earth stations

or VSAT terminals to transmit intermittently on the same frequency, but with the timing of their

transmissions so arranged that the bursts do not overlay when they arrive at the satellite but

arrive in sequence and thus are all successfully received by the teleport hub modem burst

demodulator.

ADVANTAGES:-

1. Permits flexible bit rates (i.e., multiple time

slots can be assigned to a user, e.g., if

each

time slot translates to 32Kbps, then a64Kbps user getsassigned 2 slots per frame).2. Can support bursts or variable bit rate traffic. Number of slots assigned to a user can be

changed frame by frame (e.g., 2 slots in frame 1, 3 slots in frame 2, 1 slot in frame 3, 0slots in

frame 4, etc.)

3. No guard bands required for wideband system.

4. No narrowband filters required for wideband system


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DISADVANTAGES:-

1. The high bit rates of wideband systems require complex equalization.

2. Because of burst mode of operation, a large number of overhead bits for synchronizationand

framing are required.

3. Guard time is required in each slot to accommodate time inaccuracies because of

clock instability.

4. Electronics operating at high bit rates increase power consumption.

5. Complex signal processing is required for synchronize within a short slot time.


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CDMA

DESCREPTION:-

Code Division Multiple Access system is very different from time and frequency division

multiplexing. In this system, a particular user has access to the entire bandwidth for the entire

time duration as shown in Figure 4 [22]. The basic principle of CDMA is that different codes are

used to distinguish between the different users. Typically used forms of modulation are Direct

Sequence spread spectrum (DS-CDMA), frequency hopping or Joint Detection CDMA (JD-

CDMA). Here a signal is generated that spreads out over a wide bandwidth. A code known as as

p reading code is used to perform this action. By using a group of codes, which are orthogonal to

each other, it is possible to pick out a signal with a given code in the presence of many

other signals with different orthogonal codes [24]. In fact many different baseband signals with

6 different spreading codes can be modulated onto the same carrier to enable many different

user store be supported. By using different orthogonal codes interference between the signals is

minimal. Conversely when signals are received from several mobile stations, the base station is

able to isolate each one as they have different orthogonal spreading codes.CDMA is an example

of multiple access, which is where several transmitters can send information simultaneously over

a single communication channel. This allows several users to share a band of frequencies

(see bandwidth). To permit this to be achieved without undue interference between the users

CDMA employs spread-spectrum technology and a special coding scheme (where each

transmitter is assigned a code).CDMA is used as the access method in many mobile phone

standards such as cdma One, CDMA2000 (the 3G evolution of cdma One), and WCDMA(the 3G

standard used by GSM carriers), which are often referred to as simply CDMA.

FEATURES:-

1. Soft capacity limit: system performance degrades for all users as number of users increase

2. Wide frequency spectrum reduces fading

3. Cell frequency re use 1: No frequency planning


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5. Soft Handover increases capacity

6. Utilization of voice activity is possible.

APPLICATIONS:-

1. CDMA is used in different stock exchanges.

2. It is also used in text communication

3. We can send photos over the air by using CDMA (e.g. MMS messages)

4. It is useful in position location services like navigation assistance

5. CDMA is used in games and entertainment.

ADVANTAGES:-

1. CDMA has a soft capacity. The more the number of codes, more the number of users.

However as more codes are used the S/I ratio will drop and the BER (Bit Error Rate) will go up

for all users.

2. CDMA requires tight power control as it suffers for far-near effect. In other words, a

user close to the base station transmitting with the same power as a user farther away will drown

the latters signal. All signals must have more or less equal power at the receiver.

3. Rake receivers can be used to improve signal reception. Time delayed versions (a chip or more

delayed) of the signal (multipath signals) can be collected and used to make bit level decisions.

4. Soft handoffs can be used. Mobiles can switch base stations without switching carriers. Twobase stations receive the mobile signal and the mobile is receiving from two base stations.

5. Burst transmissionreduces interference.


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DISADVANTAGES:-

1. The code length has to be carefully selected. A large code length can induce delay or even

cause interference.

2. Time synchronization is necessary.

3. Soft handoff increases use of radio resources and hence can reduce capacity.

4. As the sum of the power received at and transmitted from a base station has to constant, a tight

power control is needed. This can result in more handoffs.


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SDMA

DESCREPTION:-

Space-division multiple access (SDMA) is a channel access method based on creating parallel

spatial pipes next to higher capacity pipes through spatial multiplexing and/or diversity, by

which it is able to offer superior performance in radio multiple access communication systems.

In traditional mobile cellular network systems, the base station has no information on the

position of the mobile units within the cell and radiates the signal in all directions within the cell

in order to provide radio coverage. This results in wasting power on transmissions when there are

no mobile units to reach, in addition to causing interference for adjacent cells using the same

frequency, so called co-channel cells. Likewise, in reception, the antenna receives signals

coming from all directions including noise and interference signals. By using smart

antenna technology and differing spatial locations of mobile units within the cell, space-division

multiple access techniques offer attractive performance enhancements. The radiation pattern of

the base station, both in transmission and reception, is adapted to each user to obtain highest gain

in the direction of that user. This is often done using phased array techniques. In GSM cellular

networks, the base station is aware of the distance (but not direction) of a mobile phone by use of

a technique called timing advance (TA). The base transceiver station (BTS) can determine how

distant the mobile station (MS) is by interpreting the reported TA. This information, along with

other parameters, can then be used to power down the BTS or MS, if a power control feature is

implemented in the network. The power control in either BTS or MS is implemented in most

modern networks, especially on the MS, as this ensures a better battery life for the MS. This is

also why having a BTS close to the user results in less exposure to electromagnetic radiation.

This is why one may actually be safer to have a BTS close to them as their MS will be powered

down as much as possible. For example, there is more power being transmitted from the MS than

what one would receive from the BTS even if they were 6 meters away from a BTS mast.However, this estimation might not consider all the Mobile stations that a particular BTS is

supporting with EM radiation at any given time. In the same manner, 5th

generation mobile

networks will be focused in utilizing the given position of the MS in relation to BTS in order to

focus all MS Radio frequency power to the BTS direction and vice versa, thus enabling power

savings for the Mobile Operator, reducing MS SAR index, reducing the EM field around base


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stations since beam forming will concentrate rf power when it will be actually used rather than

spread uniformly around the BTS, reducing health and safety concerns, enhancing spectral

efficiency, and decreased MS battery consumption.

APPLICATIONS:-

1. Mobile Web applicationsrefer to applications for mobile devices that require only a Webbrowser to be installed on the device.

2. Enterprise risk management (ERM) applications support the establishment of an integrated,

thorough and strategic approach to the management of risk across the organization by helping

organizations to identify, measure and manage various risk classes, including market risks, credit

risks, liquidity risks and operational risks.

3. Location-aware applications use the geographical position of a mobile worker or an asset to

execute a task. Position is detected mainly through satellite technologies, such as a GPS, orthrough mobile location technologies in cellular networks and mobile devices. Examples include

fleet management applications with mapping, navigation and routing functionalities, government

inspections and integration with geographic information system application.

4. Content and applications service providers include providers focusing primarily on

information and media services, content, entertainment and applicationsservices. Examples

include, Yahoo, Google etc.

5. Model-driven packaged applications refer to enterprise applications that have explicit

metadata-driven models of the supported processes, data and relationships, and that generateruntime components through metadata models, either dynamically interpreted or compiled, rather

than hardcoded.

ADVANTAGES:-

1. SDMA increases the capacity of the system and transmission quality by focusing the signal

into narrow transmission beams. Through the use of smart antennas with beams pointed at the

direction of the mobile station, SDMA serves different users within the same region.

2. Mobile stations operating outside the bounds of these directed beams experience a near zerointerference from other mobile stations operating under the same base station with the same radio

frequency.

3. Since the beams are focused, the radio energy frequency can have increased base station

range. This attribute of SDMA allows base stations to have larger radio coverage with less


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radiated energy. This narrow beam width also allows greater gain and clarity.

4. Under traditional mobile phone network systems, the base station radiates radio signals in alldirections within the cell without knowledge of the location of the mobile station. SDMA

technology channels radio signals based on the location of the mobile station. Through this

method, the SDMA architecture saves on valuable network resources and prevents redundantsignal transmission in areas where mobile devices are currently inactive.

5. The main advantage of SDMA is frequency reuse. Provided the reuse distance is preserved in

the network architecture, interference can be near zero, even if mobile stations use the sameallocated frequencies.

DISADVANTAGES:-

1. As the sum of the power received at and transmitted from a base station has to constant, a tight

power control is needed. This can result in more handoffs.

2. Guard time is required in each slot to accommodate time inaccuracies because of

clock instability.

3. The guard bands result in wastage of capacity.

4. Electronics operating at high bit rates increase power consumption. Electronics operating at

high bit rates increase power consumption.

5. Hardware involves narrow band filters, which cannot be realized in VLSI and thus increase

cost.


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SPREAD SPECTRUM

DESCREPTION:-

In telecommunication and radio communication, spread-spectrum techniques are methods by

which a signal (e.g. an electrical, electromagnetic, or acoustic signal) generated with a

particular bandwidth is deliberately spread in the frequency domain, resulting in a signal with a

wider bandwidth. These techniques are used for a variety of reasons, including the establishment

of secure communications, increasing resistance to natural interference, noise and jamming, to

prevent detection, and to limit power flux density (e.g. in satellite downlinks). This is a technique

in which a (telecommunication) signal is transmitted on a bandwidth considerably larger than

the frequency content of the original information. Frequency hopping is a basic modulation technique

used in spread spectrum signal transmission. Spread-spectrum telecommunications is a signal structuring

technique that employs direct sequence, frequency hopping, or a hybrid of these, which can be used for

multiple access and/or multiple functions. This technique decreases the potential interference to other

receivers while achieving privacy. Spread spectrum generally makes use of a sequential noise-like signal

structure to spread the normally narrowband information signal over a relatively wideband (radio) band of

frequencies. The receiver correlates the received signals to retrieve the original information signal.

Originally there were two motivations: either to resist enemy efforts to jam the communications (anti-jam,

or AJ), or to hide the fact that communication was even taking place, sometimes called low probability of

intercept (LPI). Frequency-hopping spread spectrum (FHSS), direct-sequence spread

spectrum (DSSS), time-hopping spread spectrum (THSS), chirp spread spectrum (CSS), and

combinations of these techniques are forms of spread spectrum. Each of these techniques employs

pseudorandom number sequences created using pseudorandom number generators to

determine andcontrol the spreading pattern of the signal across the allocated bandwidth. Ultra-

wideband (UWB) is another modulation technique that accomplishes the same purpose, based on

transmitting short duration pulses. Wireless standard IEEE 802.11 uses either FHSS or DSSS in its radio

interface.

FEATURES:-

1. Ability to selectively address

2. Bandwidth sharing

3. Immunity to interference


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4. Security from eavesdropping

5. Ability of detection

APPLICATIONS:-

1. Cellular/PCS base station interconnect

2. Last-mile obstacle avoidance

3. Private networks

4. Railroads and transportation

5. Utilities like electricity, oil, gas and water

6. Banks, hospitals, universities and corporations

7. Disaster recovery and special event PSTN extensions

8. TELCO bypass

9. Rural telephony

10. Video conferencing

11. LAN/WAN/Internet connection

ADVANTAGES:-

1. Has ability to eliminate the effects of multipath interference

2. Can share the same frequency band with other users

3. Provides privacy due to unknown random codes

4. Involves low power spectral density since a signal is spread over a large

frequency band

DISADVANTAGES:-


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1. Bandwidth inefficient

2. Implementation is somewhat complex


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CONCLUSION

In this way we have seen that, FDMA is one of the earliest multiple-access techniques for

cellular systems when continuous transmission is required for analog services. TDMA is a

complimentary access technique to FDMA. CDMA systems utilize a spread spectrum technique

in which a spreading signal, which is uncorrelated to the signal and has a large bandwidth. A

primitive form of SDMA is when the same frequency is re-used in different cells in a cellular

wireless network. Frequency hopping is a basic modulation technique used in spread spectrum

signal transmission.


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References

[1]http://en.wikipedia.org/wiki/Frequency-division_multiple_access

[2]http://www.answers.com/topic/frequency-division-multiple-access

[3] http://old.optivera.ru/products/Wilan/whitepaper_mcdsss.pdf

[4]http://www.google.co.in/url?sa=t&rct=j&q=spread+spectrum+advantages+and+disadvantaspr

eadspectrum&ei=SxcAUvuTC82krQfIrIHgCA&usg=AFQjCNFJGmicW5q6Ub6vYMXBsq1urp

__WQ

[5] http://www.slideshare.net/varun_bansal90/spread-spectrum

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