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Transcript of Wcdma 1488
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WIDEBAND CODE DIVISION MULTIPLE
ACCESS & THE CAPACITY IN CODE
DIVISION MULTIPLE ACCESS
Presented by
Maheshwarnath BeharyAssisted by
Vishwanee Raghoonundun &
Y. Koteswara RaoMSc Computer Networks
Middlesex University
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Definition ofWCDMA
WCDMA: A high speed 3G mobile wireless technology with capacity to
offer higher data rate than CDMA.
WCDMA has the capacity to easily handle bandwidth-intensiveapplications such as video, data, and image transmission necessary for
mobile internet services.
WCDMA was adopted as a standard by the International Mobile
Telecommunications-2000 (IMT 2000) with the intention to create a global
standard for real time multimedia services and international roaming.
WCDMA is direct spread technology, which means that it will spread its
transmissions over a wide, 5MHz carrier.
WCDMA is the leading 3G wireless standard in the world today.
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BASICS OF THE WCDMA
WCDMA users share same radio resources whereas TDMA or FDMA users
have their own radio resources allocated.
User signals are differentiated based on codes.
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FEATURES OF THE WCDMA
144 kbps, 384 kbps, 2 MbpsData rates
FDDDuplexing
DL - QPSK
UL - BPSK
Data Modulation
DL- Gold sequences. (len-18)
UL- Gold/Kasami sequences (len-41)
Scrambling codes
Orthogonal variable spreading factor
(OVSF) SF: 4-256
Spreading codes
5, 10, 20 MHzBandwidth
(1) The actual data rate achieved is inversely proportional to the coverage and mobility ofthe user
(2) Larger bandwidth of 10 and 20 MHz is proposed for higher data rates.
(3) Spreading factor = tb / tc.(4) Scrambling codes permit the handling of multiple users in the network(using the same
frequency).
(5) Frequency Division Duplex used as there are two different frequency bands, one for theuplink and the second one for the downlink.
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ADVANTAGES OF USING CDMA SYSTEM
A number of advantages are:
Low power spectral density. As the signal is spread over a large
frequency-band, the Power Spectral Density is getting very small,
so other communications systems do not suffer from this kind of
communications. Interference limited operation. In all situations the whole
frequency-spectrum is used.
Privacy due to unknown random codes. The applied codes are
unknown to a hostile user. This means that it is hardly possible to
detect the message of another user. Random access possibilities. Users can start their transmission at
any arbitrary time.
Good anti-jam performance.
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CDMA CAPACITY
Blocking probability is used to measure theBlocking probability is used to measure the
capacity of any multiple access system.capacity of any multiple access system.
The capacity of CDMA systems mainlyThe capacity of CDMA systems mainly
depends on interference. Any reduction independs on interference. Any reduction in
interference directly results in the increase ininterference directly results in the increase in
the capacity of the system.the capacity of the system.
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SINGLE CELL SYSTEM
In a single cell system each user occupies entire frequency spectrum
employing direct sequence spread spectrum waveform.
Power control is the basic requirement and affects the capacity of the
system greatly.
Forward link: A pilot signal is included which is for initial power
control by the mobile to adjust the power inversely to the total power
its receives.
Reverse link: Here non coherent reception without any pilot signal is
used.
In reverse link no pilot signal is required because of
power efficiency considerations.
an independent pilot would be needed for each reverse signal.
Since the forward link combines the signals synchronously, its
performance is much superior to reverse link.
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MULTIPLE CELL SYSTEMIn multiple cell system power of each subscriber unit is
controlled by the base station of their own cell.
The maximum pilot power among cell sites decides which
cell site the subscriber will join.
The interference to a subscriber in another cell site is
proportional to the attenuation in the path to the subscribers
cell and inversely to the attenuation from the interfering
user to his own cell site.
Forward link:Power control is essential since signals in
adjacent cells fade independently and can cause interference
in the near cell boundaries.
Power control in the forward link is the allocation of power
to each subscriber according to each subscribers needs.
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MULTIPLE CELL SYSTEM(CTND)
REVERSE LINK:
The power control to a given mobile is exercised by the cell
whose pilot signal power is maximum to that mobile.
It follows that if the path loss is due to distance from cellsite, then the mobile would be power controlled by the
nearest cell site.
The path loss between the subscriber and the cell site is
proportional to r, the distance from the subscriber to cellsite.
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TECHNIQUES FOR IMPROVING
CHANNEL CAPACITY
SECTORIZATION
Use of directional antennas for transmission and reception.
The interference seen by directional antenna is one third of those seenby omni directional antennas.
This interference suppression improves the capacity of the system.
VOICE ACTIVITY MONITORING
The digital vocoders monitor the voice activity of each user.
Each user is active only 35%-40% of the time and the transmission
can be suppressed during the quiet periods. Voice activity monitoring reduces interference by a value
proportional to voice activity factor which is usually 3/8.
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ENHANCEMENTS IN WCDMA
Superior voice and data service quality.
Smooth evolution from GSM to WCDMA
Open standardisation process, global standard
Global markets and economies of scale
Service portability and roaming
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References
[1] fhttp://cas.et.tudelft.nl/~glas/ssc/techn/techniques.html
[2] www.ittc.ukans.edu
[3] http://www.umts-forum.org/servlet/dycon/ztumts/umts
[4]http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8
[5]http://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdm
[6] http://www.ittc.ukans.edu/~krishk/Documents/cdma.pdf
http://var/www/apps/conversion/current/tmp/scratch1690/A:/www.ittc.ukans.eduhttp://www.umts-forum.org/servlet/dycon/ztumts/umtshttp://www.umts-forum.org/servlet/dycon/ztumts/umtshttp://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cdma.htmlhttp://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cdma.htmlhttp://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cdma.htmlhttp://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cdma.htmlhttp://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.telecommagazine.com/default.asp?journalid=2&func=articles&page=0208i10&year=2002&month=8http://www.umts-forum.org/servlet/dycon/ztumts/umtshttp://www.umts-forum.org/servlet/dycon/ztumts/umtshttp://var/www/apps/conversion/current/tmp/scratch1690/A:/www.ittc.ukans.edu -
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END OF PRESENTATION
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