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  1. 1. WIDEBAND CODE DIVISION MULTIPLE ACCESS & THE CAPACITY IN CODE DIVISION MULTIPLE ACCESS Presented by Maheshwarnath Behary Assisted by Vishwanee Raghoonundun & Y. Koteswara Rao MSc Computer Networks Middlesex University
  2. 2. Introduction <ul><li>Definition of WCDMA </li></ul><ul><li>Basics of the WCDMA </li></ul><ul><li>Features of the WCDMA </li></ul><ul><li>Advantages of using CDMA system </li></ul><ul><li>CDMA capacity </li></ul><ul><li>Single cell system </li></ul><ul><li>Multiple cell system </li></ul><ul><li>Techniques for improving channel capacity </li></ul><ul><li>Enhancements in WCDMA </li></ul><ul><li>Reference </li></ul>
  3. 3. Definition of WCDMA <ul><li>WCDMA: A high speed 3G mobile wireless technology with capacity to offer higher data rate than CDMA. </li></ul><ul><li>WCDMA has the capacity to easily handle bandwidth-intensive applications such as video, data, and image transmission necessary for mobile internet services . </li></ul><ul><li>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. </li></ul><ul><li>WCDMA is direct spread technology, which means that it will spread its transmissions over a wide, 5MHz carrier. </li></ul><ul><li>WCDMA is the leading 3G wireless standard in the world today. </li></ul>
  4. 4. BASICS OF THE WCDMA <ul><li>WCDMA users share same radio resources whereas TDMA or FDMA users have their own radio resources allocated. </li></ul><ul><li>User signals are differentiated based on codes. </li></ul>
  5. 5. FEATURES OF THE WCDMA (1) The actual data rate achieved is inversely proportional to the coverage and mobility of the user (2) Larger bandwidth of 10 and 20 MHz is proposed for higher data rates. (3) Spreading factor = t b / t c. (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 the uplink and the second one for the downlink. 144 kbps, 384 kbps, 2 Mbps Data rates FDD Duplexing 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 MHz Bandwidth
  6. 6. ADVANTAGES OF USING CDMA SYSTEM <ul><li>A number of advantages are: </li></ul><ul><ul><ul><ul><li>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. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Interference limited operation. In all situations the whole frequency-spectrum is used. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>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. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Random access possibilities. Users can start their transmission at any arbitrary time. </li></ul></ul></ul></ul><ul><ul><ul><ul><li>Good anti-jam performance. </li></ul></ul></ul></ul>
  7. 7. CDMA CAPACITY <ul><ul><ul><li>Blocking probability is used to measure the capacity of any multiple access system. </li></ul></ul></ul><ul><ul><ul><li>The capacity of CDMA systems mainly depends on interference. Any reduction in interference directly results in the increase in the capacity of the system. </li></ul></ul></ul>
  8. 8. SINGLE CELL SYSTEM <ul><li>In a single cell system each user occupies entire frequency spectrum employing direct sequence spread spectrum waveform. </li></ul><ul><li>Power control is the basic requirement and affects the capacity of the system greatly. </li></ul><ul><li>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. </li></ul><ul><li>Reverse link : Here non coherent reception without any pilot signal is used. </li></ul><ul><li>In reverse link no pilot signal is required because of </li></ul><ul><ul><li>power efficiency considerations. </li></ul></ul><ul><ul><li>an independent pilot would be needed for each reverse signal. </li></ul></ul><ul><li>Since the forward link combines the signals synchronously, its performance is much superior to reverse link. </li></ul>
  9. 9. MULTIPLE CELL SYSTEM <ul><li>In multiple cell system power of each subscriber unit is controlled by the base station of their own cell. </li></ul><ul><li>The maximum pilot power among cell sites decides which cell site the subscriber will join. </li></ul><ul><li>The interference to a subscriber in another cell site is proportional to the attenuation in the path to the subscriber’s cell and inversely to the attenuation from the interfering user to his own cell site. </li></ul><ul><li>Forward link : Power control is essential since signals in adjacent cells fade independently and can cause interference in the near cell boundaries. </li></ul><ul><li>Power control in the forward link is the allocation of power to each subscriber according to each subscriber’s needs. </li></ul>
  10. 10. MULTIPLE CELL SYSTEM(CTND) <ul><li>REVERSE LINK: </li></ul><ul><li>The power control to a given mobile is exercised by the cell whose pilot signal power is maximum to that mobile. </li></ul><ul><li>It follows that if the path loss is due to distance from cell site, then the mobile would be power controlled by the nearest cell site. </li></ul><ul><li>The path loss between the subscriber and the cell site is proportional to r, the distance from the subscriber to cell site. </li></ul>
  11. 11. TECHNIQUES FOR IMPROVING CHANNEL CAPACITY <ul><li>SECTORIZATION </li></ul><ul><li>Use of directional antennas for transmission and reception. </li></ul><ul><li>The interference seen by directional antenna is one third of those seen by omni directional antennas. </li></ul><ul><li>This interference suppression improves the capacity of the system. </li></ul><ul><li>VOICE ACTIVITY MONITORING </li></ul><ul><li>The digital vocoders monitor the voice activity of each user. </li></ul><ul><li>Each user is active only 35%-40% of the time and the transmission can be suppressed during the quiet periods. </li></ul><ul><li>Voice activity monitoring reduces interference by a value proportional to voice activity factor which is usually 3/8. </li></ul>
  12. 12. ENHANCEMENTS IN WCDMA <ul><li>Superior voice and data service quality. </li></ul><ul><li>Smooth evolution from GSM to WCDMA </li></ul><ul><li>Open standardisation process, global standard </li></ul><ul><li>Global markets and economies of scale </li></ul><ul><li>Service portability and roaming </li></ul>
  13. 13. References <ul><li>[1] f http://cas.et.tudelft.nl/~glas/ssc/techn/techniques.html </li></ul><ul><li>[2] www.ittc.ukans.edu </li></ul><ul><li>[3] http://www.umts-forum.org/servlet/dycon/ztumts/umts </li></ul><ul><li>[4] http:// www.telecommagazine.com/default.asp?journalid =2&func=articles&page=0208i10&year=2002&month=8 </li></ul><ul><li>[5] http://people.deas.harvard.edu/~jones/cscie129/nu_lectures/lecture7/cdma/cdma.html </li></ul><ul><li>[6] http://www.ittc.ukans.edu/~krishk/Documents/cdma.pdf </li></ul>