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UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICS UNESCO PROJECT Advanced Course on Networking Introduction to Cellular Wireless Networks. Outline. Introduction (Resource Management and Utilization). Compression and Multiplexing (Other related definitions).

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Outline

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  1. UNESCO/CISM SECOND ADVANCED SCHOOL OF INFORMATICSUNESCO PROJECT Advanced Course on Networking Introduction to Cellular Wireless Networks

  2. Outline • Introduction (Resource Management and Utilization). • Compression and Multiplexing (Other related definitions). • The Most Important Access Utilization Schemes (Techniques): • FDMA (Frequency Division Multiple Access) • TDMA (Time Division Multiple Access) • CDMA (Coded Division Multiple Access) • In which Systems the above Schemes are used?

  3. The idea behind the Resource Management in wireless communications was how to assign channels or frequencies to the radio cells in a way that the probability of interference is sufficiently low. The utilization of the capacity of a transmission medium can be improved through different methods that involved transmitting several connections simultaneously in a multiplex mode (Multiple Access Techniques). By using these techniques, the shared resource (spectrum) can be divided/shared among users, ensuring Quality of Service and the required amount of interference. Introduction

  4. Frequency Spectrum

  5. Frequency Spectrum in USA

  6. Definitions • Multiplexing: is sending multiple signals or streams of information on a carrier at the same time in the form of a single, complex signal and then recovering the separate signals at the receiving end. • Compression means transmitting/storing the same amount of information using less amount of resources

  7. Definitions Cont... • Both Compression and Multiplexing are attempts to make better use of resources: the Communication Channel. • Multiplexing attempts to make better use of a potentially underused resources. On the Other hand, Compression attempts to make better use of a potentially overused resources.

  8. Definitions Cont… • A signal: In electronics, is an electromagnetic field used to convey data from one place to another. In information technology is simply “That which is send or received”. • A signal can be analog or digital: • Analog technology refers to electronic transmission accomplished by adding signals of varying frequency or amplitude to carrier waves of a given frequency. • Digital: is a technology that generates, stores and process data in terms of two states: Positive and Non-positive ( 1 or 0 bits).

  9. Definitions Cont… • Frequency: for a periodic function, is the number of cycles or events per unit time. • Bandwidth: (The width of a band of electromagnetic frequencies) is used to mean: (1) how fast data flows on a given transmission path. (2) more technically, the width of the range of frequencies that an electronic signal occupies on a given transmission medium.

  10. Multiplexing Methods • FDM : Frequency Division Multiplexing • FDMA: Frequency Division Multiple Access • TDM : Time Division Multiplexing • FDMA: Time Division Multiple Access • CDM : Code Division Multiplexing • CDMA: Code Division Multiple Access

  11. FDMA • FDM divides the transmission frequency range (Bandwidth) into narrower bands called (subchannels). • The subchannels are smaller frequency bands and each band is capable of carrying a separate voice or data signals! • Guard bands are used to prevent interference on the receiving end of the signal (Accommodate the delay of the near-far-problem) • Disadvantage (FDMA): Full utilization of the available frequency band is not possible!! • Advantage: Multiple callers can share the frequency spectrum.

  12. FDM F6 Frequency Band F5 Guard Band F4 Frequency Band F3 Frequency Guard Band F2 Frequency Band F1 Guard Band F0 Frequency Band Time

  13. FDMA-Applications • FDMA is used in a variety of applications such as: telephone systems, radio systems, cable TV at homes. • The first generation of Mobile networks. FD,TD-MA. • GSM (Global System of Mobile Telecommunication) uses FDMA in combination with TDMA. • UMTS (Universal Mobile Telecommunication Systems-3G) in combination with other multiplexing techniques

  14. TDMA : Time Division • Why TDM ? • FDM sometime offer less frequency for the communication channel than the required amount • On the Other hand, in TDM the entire bandwidth of the radio channel is used but is divided into time slots that are periodically allocated to each station for the duration of the call • Pros and Cons : • It needs more synchronization between the sender and the receiver • It is more frequency-economic than FDM • This engagement of the transmission medium can cause using asynchronous time slots instead of the synchronous ones especially when the transmission pauses occurs

  15. TDM Frequency Slot 1 Slot 3 Slot 4 Slot 2 Guard Band Guard Band Guard Band Time

  16. FDMA + TDMA • A combination of FDMA/TDMA can be used. For example, in GSM systems. The traffic is burst onto the channel at a specific periods. • With this combination, more channels can be used with less interference. • How?

  17. FDMA+TDMA in GSM Burst Burst Burst Burst Burst Burst Burst Burst Burst 1 2 3 4 5 6 7 8 n ETC.. 200 kHz 200 kHz 200 kHz 1 2 3 4 5 6 7 8 1 Voice Channels Signalling Channels

  18. CDMA (Code Division Multiple Access) • Like TDMA, in CDMA the analog speech is coded into digital signals. • Unlike TDMA, in CDMA each conversation is assigned a unique code (a signature for each individual transmission). • The codes of different users are assigned to be different from each other (e.g orthogonal to each other). • The final signal at the receiver contains only the relevant conversation. Any other signals are picked up as a noise.

  19. Spread Spectrum (SS) in CDMA • The idea of the SS is to transform an information data signal into a transmission signal with much larger bandwidth. This is achieved by encoding the information data signal by a code signal called pseudo random or pseudo noise (PN) which is independent from the data signal and has a much larger bandwidth. • Example : Chip rate = 4MChip/s • Spreading Factor = 2  2 users with data rate = 2 Mbps • Spreading Factor = 4  4 users with data rate = 1 Mbps • Spreading Factor = 64  64 users with data rate = 64 kbps • Spreading Factor = 128  128 users with data rate = 32 kbps

  20. Spread Spectrum Principle 1 Spreading 1 De-spreading 2 1&2&3 1 2 2&3 3 3

  21. Basic SS Block diagram Information Sequence Recovered Information Sequence Channel encoder Modulation Channel Demodulation Channel Decoder PN Code Generator PN Code Generator

  22. Spreading Sequence Assignment (SSA) • The codes are said to be orthogonal when their normalized inner product is equal to 0. • Code Ci is said to be orthogonal to code Cj if Ci does not belong to the path from the root of the tree to Cj or to the sub-tree below code Cj . • In W-CDMA spreading codes have variable length: 4 to 512 chips in the downlink and 4 to 256 chips in the uplink.

  23. Code Tree C4,1=(1,1,1,1) C2,1 =(1,1) C4,1=(1,1,-1,-1) C1,1 =(1) C4,1=(1,-1,1,-1) C2,2 =(1,-1) C4,1=(1,-1,-1,1) SF=1 SF=2 SF=4

  24. How to obtain the code? • The first element in the tree is 1. • For each element, there are two possible sub-elements that we will term the top and bottom sub-element. • The top sub-element is generated by repeating the root of the sub-element twice. So that the top sub-element of 1 is (1,1). • The bottom sub-element can be constructed by concatenating the root of the sub-element with the inverse of itself. Thus, the bottom sub-element of 1 is (1,-1) and so on.

  25. CDMA Code User 3 User 2 Frequency User 1 Time

  26. CDMA • CDMA technique is used in UMTS (Universal Mobile Telecommunication Systems) (Called also W-CDMA) • CDMA provides better signal-to-noise ratio performance than the conventional TDMA and FDMA. Which means that the required high capacity can be approved!! • Advantage: It is easy to accommodate variable user capacity as long as the user does not increase the whole energy of the multi-user signal. • Disadvantage: The power control issue which limits the max. number of users in the cell. • There are different alternatives for CDMA in UMTS: • W-CDMA • TD-CDMA • FD-CDMA

  27. Multiple access in CDMA – An Example Same Code in Bi-polar form for demo Code assigned to each user Digital transmission from user before spreading Actual transmission on the air Receiver decodes the transmission of C sender from the total signal

  28. References? • Mobile Radio Networks, Networking and Protocols, Bernhard H. Walke. ISBN: 0-471-97595-8. WILEY. • UMTS, Mobile Communications for future, Flavio Muratore, ISBN: 0-471-498297. WILEY. • Mobile and Wireless Networks, Uyless Black, Prentice Hall Series in Advanced Communications Technologies. ISBN:0-13-440546-3. • Digital Communications, I. A. Glover, P. M. Grant, Prentice Hall. ISBN:0-13-565391-6. • Computer Networks, Andrew S. Tanenbaum, ISBN:0-13-394248-1. • CDMA, Principles of Spread Spectrum Communication, Andrew J. Viterbi. ISBN:0-201-63374-4. • Mobile IP, the internet Uplugged, James D. Solomon. ISBN:0-13-856246-6.

  29. But one of the best: Andrew Tanenbaum: Computer Networks, 4th Edition 2003, Prentice Hall

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