1587: COMMUNICATION SYSTEMS 1 Mobile Communications

1. 1587: COMMUNICATION SYSTEMS 1Mobile Communications Dr. George Loukas University of Greenwich, 2011-2012

2. WANs quick revision: Topology Wide Area Network WAN Use local and long-distance telecommunications Usually very high speed with low error rates Usually follow a mesh topology A mesh is a network where all nodes can send, receive and relay data

3. pkt 1 pkt 1 pkt 1 pkt 2 pkt 2 pkt 2 queuing delay pkt 3 pkt 3 pkt 3 WANs quick revision: Sub-network types Intermediate node 1 source Intermediate node 2 destination transmission delay Message-switched Store-and-forward the whole message Circuit-switched A dedicated circuit (physical path) is established between sender and receiver and all data go through this circuit. Packet-switched Messages transmitted in suitably-sized packets. No dedicated physical path. • datagrams (packets routed independently) • virtual circuit (emulating a physical circuit) propagation delay source Intermediate node 2 Intermediate node 1 destination

4. WANs quick revision: Types of traffic and congestion Network Congestion: Queues overflow and packets are lost due to: • excessive TRAFFIC • link FAILURES • Stream traffic - lengthy and continuous • Bursty traffic - short sporadic transmissions Maria Lin Maria: Good morning Lin. Good morning Lin.

5. WANs quick revision: Routing Dijkstra’s least-cost algorithm Flooding B (7, A) 7 2 A 3 F (8, C) 5 C (3, A) 7 7 1 2 D (5, C) G (9,F) E (8, D) 3 Routing tables can be kept centrally or at each node separately 3

6. WANs quick revision: Examples X.25 Frame Relay ATM PPP ISDN DSL / ADSL Uses asynchronous time division multiplexing Takes congestion very seriously Uses admission control QoS and error control Originally designed for voice, but used by cash machine and credit card verification networks Designed for speed rather than reliability Simple and cheap Uses packet switching Digital Subscriber Line (DSL) Uses multiplexing ADSL: Asymmetric DSL Standard home broadband is usually ADSL Digital phone circuit Can transmit voice and data from 64 Kbps to 1.544 Mbps Point to point protocol for dial-up

7. Mobile communications 1983 1973 2000 2008

8. 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

9. 1984 1st Gen.: Cellular Networks

10. Cellular Networks Developed to increase capacity for mobile radio telephone service • multiple low power transmitters • area divided into cells • 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 • cells sufficiently distant can use same frequency band

11. 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

12. Cellular Geometries For the same reasons, hexagons are also very common in board and computer games Hexagons 1 1 Equidistant No gaps

13. 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

14. 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 (called cochannels) • R = radius of a cell D R

15. Frequency Reuse Example 336 channels. Reuse Factor N = 7 336/7 = 48 channels per cell Total channel capacity (number of concurrent calls that can be handled) = 48 x 128 = 6,144 channels 128 cells, each with radius R = 0.8 km and area covered: Total channel capacity (number of concurrent calls that can be handled) = 48 x 32 = 1,536 channels 32 cells, each with radius R = 1.6 km and area covered: Total area covered 32 x 6.65 = 213 km2 Total area covered 128 x 1.66 = 213 km2

16. 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

17. 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.

18. Increasing Capacity: Cell Sectoring Replace a single omni-directional antenna with 3 directional antennas (120o sectoring) or with 6 directional antennas (60o sectoring) Each cell is divided into 3 (or 6) wedge-shaped sectors. Each sector uses a directional antenna at the Base station and is assigned one third (or sixth) of the channels in the cell This reduces transmission power and increases battery life

19. 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.

20. Call Stages Monitor for strongest signal Request connection Paging Call accepted Ongoing Call Handoff MTSO

21. Other Functions • call blocking • after repeated attempts, if all traffic channels are busy, a busy tone is returned • call termination • when a user hangs up, channels at the BS are released • call drop • when BS cannot maintain required signal strength • calls to/from fixed and remote mobile subscriber • MTSO connects to the PSTN

22. 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

23. 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

24. 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

25. Error Compensation Mechanisms • Diversity (e.g. spread spectrum) • by space, frequency or time • individual channels experience independent fading events • use multiple logical channels between transmitter and receiver • send part of signal over each channel • Forward error correction • for digital transmissions • Typical ratio of total bits to data bits is 2-3:1 • Adaptive equalisation • applied to transmissions that carry analog or digital information • used to combat intersymbol interference • involves gathering the dispersed symbol energy back together into its original time interval

26. 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

28. 1991 2nd Gen.: Digital Networks

29. 2nd Gen Vs. 1st Gen • better signal quality • higher data rates for support of digital services • greater overall capacity key differences: • digital traffic channels • encryption • error detection and correction • shared channel access • time division multiple access (TDMA) • frequency division multiple access (FDMA) • code division multiple access (CDMA)

30. Two types of 2G Greater capacity Very large cell sizes Even low signal is enough for good quality Dropped calls less likely Uses CDMA But monopoly of a single company bars new entrants in market Fewer subscribers More mature technology 3.1 KHz voice, 9.6 Kbps data SMS messages Uses FDMA, TDMA Many more subscribers Covers the whole world, so roaming not an issue But more interference and cells limited to 120 km

31. 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 SMS are sent every second Limited to ~160 characters Large (concatenated) 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, etc.) Best-effort delivery SMS sent to SMSC SMS forwarded if recipient reachable. Otherwise, retry or drop

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

33. Rake Receiver • To counter multipath fading: • We take into account only the dominant signal and ignore the rest as noise • or • In CDMA, we use a rake receiver, which takes into account all signals appropriately: • Original binary signal is spread with a code. • Spread sequence is modulated for wireless transmission • Because of multipath, multiple copies of signal arrive with different delay • The Rake receiver • demodulates combined signal and feeds it into “correlators”, each with different delay • combines signals using weighting factors estimated from the channel R S D

34. 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 • more users means more noise and more errors until signal degradation is unacceptable • near-far problem • signals closer to receiver are received with less attenuation than signals farther away • transmissions more remote might be difficult to recover • self-jamming • some cross correlation between users

35. 2002 3rd Generation

36. 3G Systems • high-speed wireless communications to support multimedia, data, and video in addition to voice • 3G capabilities: • voice quality comparable to PSTN • 144 kbps available to users over large areas • 384 kbps available to pedestrians over small areas • symmetrical and asymmetrical data rates • packet-switched and circuit-switched services • adaptive interface to Internet • more efficient use of available spectrum • support for variety of mobile equipment • allow introduction of new services and technologies • greater security than predecessors

37. Typical capacity demands

38. 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

39. CDMA Multirate Time and Code Multiplexing

40. 2010 4th Generation

41. 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

42. 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

43. Wireless Network Generations