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1587: COMMUNICATION SYSTEMS 1 Mobile Communications

1587: COMMUNICATION SYSTEMS 1 Mobile Communications. Dr. George Loukas. University of Greenwich, 2012-2013. Handheld mobile phones. 1983. 1973. 2000. 2008. Prior to cellular radio. mobile service was only provided by one high powered transmitter/receiver

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1587: COMMUNICATION SYSTEMS 1 Mobile Communications

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  1. 1587: COMMUNICATION SYSTEMS 1Mobile Communications Dr. George Loukas University of Greenwich, 2012-2013

  2. Handheld mobile phones 1983 1973 2000 2008

  3. Prior to cellular radio • mobile service was only provided by one high powered transmitter/receiver • typically supported about 25 channels • had a radius of about 80km

  4. 1984 1st Gen.: Cellular Networks 1-G 2-G 3-G 4-G

  5. Cellular Networks • Divide the area into cells using multiple low power transmitters in each cell • tiling pattern to provide full coverage • each with own antenna • each with own range of frequencies • served by a base station • consisting of transceiver (transmitter – receiver) and control unit • adjacent cells use different frequencies to avoid crosstalk • but cells sufficiently distant can use same frequency band 1-G 2-G 3-G 4-G

  6. Cellular Geometries Hexagons Circles Squares 1 1 1 1.4 1 1 All area is covered nicely, BUT antennas (at the centres of the squares) are not equidistant Equidistant BUT There are gaps (or overlaps) between the circles Equidistant No gaps 1-G 2-G 3-G 4-G

  7. Cellular Geometries For the same reasons, hexagons are also very common in board and computer games Hexagons 1 1 Equidistant No gaps 1-G 2-G 3-G 4-G

  8. Frequency Reuse • Power of Base Transceiver controlled • Allows communication within cell on given frequency • Limits power escaping to adjacent cells • Sharing cell frequencies with nearby (but not adjacent) cells without interfering with each other • Allows multiple simultaneous conversations • 10 to 50 frequencies per cell transceiver 1-G 2-G 3-G 4-G

  9. Frequency Reuse Patterns Typical parameters: • Reuse factor N = number of cells in a repetitious pattern (each cell in the pattern uses a unique band of frequencies) • D = minimum distance between centers of cells that use the same band of frequencies • R = radius of a cell D R 1-G 2-G 3-G 4-G

  10. Frequency Reuse Example Consider a geographical area A divided into (a) 32 hexagonal cells of 1.6 km radius or (b) 133 hexagonal cellsof 0.8 km radius. The reuse factor is 7 and there are 336channelsin total. Calculate: the number of channels per cell the maximum number of concurrent calls that can be handled in A the total area covered 336 / 7 = 48 channels per cell (a) ii) Total channel capacity (number of concurrent calls that can be handled) = 48 x 32 = 1,536 channels (b) ii) Total channel capacity (number of concurrent calls that can be handled) = 48 x 133 = 6,384 channels (b) iii) 2 2 Total area covered = 32 x 6.65 = 213 km2 Total area covered = 133 x 1.66 = 221 km2 1-G 2-G 3-G 4-G

  11. Increasing Capacity • add new channels • frequency borrowing • congested cells take frequencies from adjacent cells • assign frequencies dynamically • cell splitting • non-uniform topography and traffic distribution • use smaller cells in high use areas 1-G 2-G 3-G 4-G

  12. Increasing Capacity: Cell Splitting Cells can be divided to provide more capacity. To use a smaller cell, the power level must be reduced to keep the signal within the cell. As the mobile units move, they pass from cell to cell, which requires transferring of the call from one base transceiver to another. This process is called a handoff. The smaller the cells, the more frequent the handoffs. 1-G 2-G 3-G 4-G

  13. Increasing Capacity: Cell Sectoring Three directional antennas (120o sectoring) Single omni-directional antenna Six directional antennas (60o sectoring) Each sector is assigned a separate subset of the cell’s channels. This reduces transmission power and increases battery life 1-G 2-G 3-G 4-G

  14. Operation of Cellular System A base station (BS) at centre of cell. Each BS has one or more antennas, a controller (handling the call process) and a number of transceivers (for communicating on the channels) Each BS is connected to a Mobile Telecommunications Switching Office (MTSO) • Between the mobile unit and the base station: • Control channels exchange information for setting up and maintaining calls and establishing a relationship between a mobile unit and the nearest BS. • Traffic channels carry voice or data connection between users. 1-G 2-G 3-G 4-G

  15. Call Stages Monitor for strongest signal Request connection Paging Call accepted Ongoing Call Handoff MTSO 1-G 2-G 3-G 4-G

  16. Propagation Effects Types of transmission Impairment Attenuation Attenuation distortion Cross-talk noise Delay distortion Impulse noise Inter modulation noise Thermal noise transceiver signal strength • strength of signal between BS and mobile unit needs to be strong enough to maintain signal quality • but not too strong so as to create co-channel interference • and must handle variations in noise 1-G 2-G 3-G 4-G

  17. Propagation Effects: Fading Fast Fading. Rapid changes in strength over half wavelength distances. Slow Fading. Slower changes due to user passing different height buildings, gaps in buildings etc. Flat Fading. Affects all frequencies in same proportion simultaneously. Selective Fading. Affects different frequency components differently. transceiver Fading: Time variation of received signal caused by changes in transmission paths Even if signal strength is in effective range, signal propagation effects may disrupt the signal 1-G 2-G 3-G 4-G

  18. Propagation Effects: Multipath Propagation Reflection. A signal encounters a large surface. The reflected waves may interfere (positively or negatively). Diffraction. At the edge of a large impenetrable body. Helps receive signals even without line of sight. Scattering. If the size of the obstacle is on the order of the wavelength of the signal or less, it may scatter into several weaker signals. Lamp post R S D 1-G 2-G 3-G 4-G

  19. Error Compensation Mechanisms • Diversity (e.g. through spread spectrum) • by space, frequency or time Since different channels experience different fading: • send parts of a signal over different channels • Forward error correction (Typical ratio of total bits to data bits is 2-3:1) 1-G 2-G 3-G 4-G

  20. Design Factors When designing a mobile phone network, we need to take into account: • Geography - Propagation effects (difficult to predict. Often using Okumura/Hata model for path loss) • desired maximum transmit power level at BS and mobile units • typical height of mobile unit antennas • available height of the BS antenna Map of base stations around Greenwich from http://www.sitefinder.ofcom.org.uk 1-G 2-G 3-G 4-G

  21. 1991 2nd Gen.: Digital Networks 1-G 2-G 3-G 4-G

  22. 2nd Gen Vs. 1st Gen Digital channels • encryption • error detection and correction Better security Higher data rate Greater capacity Thanks to: Shared channel access • TDMA (Time division multiple access) • FDMA (Frequency division …) • CDMA (Code division …) 1-G 2-G 3-G 4-G

  23. Code Division Multiple Access (CDMA) • Different codes make sure that the receiver can recover individual transmissions from multiple transmissions • Frequency bandwidth is split in two • Half for reverse (mobile to BS) • Half for forward (BS to mobile) • Use direct-sequence spread spectrum (DSSS) 1-G 2-G 3-G 4-G

  24. Rake Receiver To counter multipath fading: We take into account only the dominant signal and ignore the rest as noise or Use a rake receiver, which takes into account all signals based on appropriate weights and time delays for each signal. R S D 1-G 2-G 3-G 4-G

  25. CDMA Advantages & Disadvantages • frequency diversity • noise bursts and fading have less effect, because of spread spectrum • multipath resistance • CDMA codes have low cross- and autocorrelation • privacy • inherent in use of spread-spectrum • graceful degradation • Users not fixed as in FDMA and TDMA • near-far problem • more remote transmissions might be more difficult to recover • self-jamming • some cross correlation between users 1-G 2-G 3-G 4-G

  26. Two types of 2G Uses CDMA Great capacity Very large cell sizes Even low signal is enough for good quality Dropped calls less likely But monopoly of a single company bars new entrants in market Few subscribers Uses FDMA, TDMA Many more subscribers. Covers the whole world, so roaming not an issue But more interference and cells limited to 120 km Introduced SMS messages 1-G 2-G 3-G 4-G

  27. Short Message Service Short Message Service Centre (SMSC): store-and-forward Introduced as part of the GSM standard First SMS was sent in the UK over the Vodafone GSM network (1992). Now, 200,000 SMS are sent every second Limited to ~160 characters Larger SMS messages can be sent, but need to be split and recombined when received Includes control information (e.g. destination number, timestamp, data coding scheme …) Best-effort delivery SMS sent to SMSC SMS forwarded if recipient reachable. Otherwise, retry or drop 1-G 2-G 3-G 4-G

  28. 2002 3rd Generation 1-G 2-G 3-G 4-G

  29. 3G Systems High-speed wireless communications to support multimedia, data, and video in addition to voice 3G capabilities: • voice quality comparable to PSTN • 144 / 384 kbps available to users (vehicles / pedestrians) • symmetrical and asymmetrical data rates • packet-switched and circuit-switched services • adaptive interface to Internet • support for variety of mobile equipment • allow introduction of new services and technologies 1-G 2-G 3-G 4-G

  30. Typical capacity demands 1-G 2-G 3-G 4-G

  31. CDMA Design Considerations – Multirate • provision of multiple fixed-data-rate channels to user • different data rates provided on different logical channels • logical channel traffic can be switched independently through wireless fixed networks to different destinations • flexibly support multiple simultaneous applications • efficiently use available capacity by only providing the capacity required for each service • use TDMA within single CDMA channel • use multiple CDMA codes 1-G 2-G 3-G 4-G

  32. 2010 4th Generation 1-G 2-G 3-G 4-G

  33. 4G Systems • rapid increase in data traffic on wireless networks • more terminals accessible to the Internet • permanent connections to e-mail • multimedia services • support for real time services 1-G 2-G 3-G 4-G

  34. 4G Development Two candidates Similar performance and both based on use of orthogonal frequency division multiple access (OFDMA) • Fading is frequency selective (does not affect the whole signal) • and easy to overcome with forward error correction • Overcomes intersymbol interference (ISI) in multipath environment and makes equalisers unnecessary 1-G 2-G 3-G 4-G

  35. Wireless Network Generations 1-G 2-G 3-G 4-G

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