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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

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END OF PRESENTATION

THANK YOU FOR YOUR KINDATTENTION