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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_channel7/29/2019 Mobile Computing Report
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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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