2_cellular_network of mobile computing explained
- 1. Cellular Network • Govt. regulatory agencies could not make spectrum allocations in proportion to the increasing demand for mobile services • Restructuring of radio telephone system became imperative – To achieve high capacity – With limited radio spectrum – Covering very large areas
- 2. The cellular concept • Replace a single high power transmitter (large cell) with many low power transmitter (small cells) • Each cell provides coverage to only a small portion of service area • Cellular concept was a major breakthrough in solving – Spectral congestion – User capacity
- 4. terminology • Base Station (BS) – A fixed station in a mobile radio system – Used for radio communication with MS – Located at the centre or on the edge of a coverage region – Consists of • Radio channels • Transmitter • Receiver • antennas
- 5. terminology • Mobile Switching Centre (MSC) – Co-ordinates routing of calls in a large service area – Connects BS and MSs to the PSTN – Also called mobile telephone switching office (MTSO) • Transceiver – Device capable of simultaneously transmitting and receiving radio signals
- 6. terminology • Mobile Station (MS) – Contains • Transceiver • Antenna • Control circuitry – Mounted in a vehicle – Used as a portable hand-held unit • Forward Voice Channel (FVC) – Channels used for voice transmission from BS to MS • Reverse Voice Channel (RVC) – Channels used for voice transmission from MS to BS
- 7. terminology • FCC/ RCC – Two channels responsible for initiating mobile calls – Involved in setting up a call – Often called set-up channels – FCC serve as beacons – Continuously broadcast all of the traffic requests for all MSs in the system – 5% of total channels available are control/ setup channels • A typical MSC handles – 100,000 cellular subscribers – 5,000 simultaneous conversation at a time – billing and system maintenance functions
- 8. How a call is made ? • When a cell phone is turned on – scans the group of FCC to determine the one with the strongest signal – MS monitors the FCC until the signal drops below usable level • Call initiated by MS – A call initiation request is sent on RCC – The servicing BS receives this data – Received data is sent to the MSC – The MSC validates the request – Makes connection to the called party through PSTN – Instructs the BS and MS to move to an unused FVC and RVC
- 9. • Call initiated by land phone – MSC dispatches the request to all BSs – Request message is broadcast as a paging message over all of the FCC – The MS for which MIN is broadcast responds by identifying itself over RCC – Servicing BS relays the acknowledgement to MSC – MSC instructs the BS to move the call to an unused voice channel – BS signals the MS to change frequencies to an unused FVC/ RVC pair – An alert message is also sent by FCC to instruct MS to ring
- 10. • Role of MSC during Call in progress – apply special control signal to control MS by BS and MSC – adjusts the transmitted power of MS – maintain call quality by changing the channel of MS
- 11. Cell Shape • Actual radio coverage of a cell is known as footprint determined from – field measurements – propagation prediction models • Real footprint is amorphous in nature • Regular cell shape is essential for – systematic system design – future growth
- 12. Cell Shape • Regular shapes – rectangle – circle – hexagon • For a given distance between the centre of a polygon and its farthest perimeter points – hexagon has the largest area of the three – hexagon closely approximates a circular radiation pattern matches with • omni-directional BS antenna • free-space propagation
- 13. Frequency reuse • Place BSs systematically • Distribute channel groups throughout the coverage zone • Channels are reused as many times as necessary – Interference between co-channel cells should be below acceptable level
- 14. Co-channel cells • Cells labeled with ‘A’ use the same group of channels • No. of cells per cluster/ compact pattern N N = i2 + ij + j2 i=3, j=2; N=19 • Each cell has 6 equidistant neighbour Fig. 2 : Calculation of co-channel cells
- 15. Nearest co-channel neighbours • Find the nearest co-channel cells – move i cells along any chain of hexagons – turn 600 counter-clockwise – move j cells • Co-channel interference – Interference between signals of a cell and its co-channel cells is called co-channel interference
- 16. Cell Design • Let a cellular system has – total of S duplex channels – each cell is allocated a group of K channels – N cells are in a cluster S = KN • A cluster is replicated M times • Total no. of duplex channels C (capacity) = MKN = MS
- 17. Cell Design • Capacity is directly proportional to the no. of times a cluster is replicated • A large cluster size indicates – Ratio between the cell radius and the distance between co-channel cells D/R is large • A small cluster size indicates – Co-channel cells are located much closer – Co-channel interference is significant
- 18. Cell Design • Design criteria (over a given coverage) – maximise capacity • smallest possible value of N – no interference • biggest possible value of N – tradeoff between capacity and interference/QOS
- 19. Co-channel interference and system capacity co-channel interference Q = D/R (see Fig) Q is too large – Improves the transmission quality Q is too small – Capacity increases Trade-off must be made between quality and capacity Reduce co-channel interference – Separate co-channel cells by a minimum distance – Provide sufficient isolation due to propagation
- 21. Hand Off strategies • The process of transferring a mobile station from one channel or base station to another – Identify a new BS – Voice and control channels be allocated to channels associated with new BS • Hand off must be performed – Successfully – As infrequently as possible – Imperceptible to the users
- 22. • In order to meet these requirements – System designer must specify an optimal signal level to initiate a hand off – Specify a particular signal level as minimum usable signal for acceptable voice quality at BS receiver – A slightly stronger signal level is used as threshold – This margin is given by delta = Prhandoff – Prminimum usable
- 23. • Delta is too large – Unnecessary handoffs which burden the MSC may occur – Delta is too small insufficient time to complete a handoff before a call is lost due to weak signal
- 24. Fig. 3 : Illustration of a handoff scenario at cell boundary
- 25. • Call may be dropped for the following reasons • Excessive delays may occur during high traffic condition for following reasons – computational loading at MSC – no channels are available on any of the nearby BSs – forces MSC to wait until a channel in a nearby cell becomes free
- 26. • Ensure the following before handoff – The drop in the measured signal level is not due to momentary fading – MS is actually moving away from the serving BS • In order to ensure this – BS monitors the signal level for a certain period of time before a handoff is initiated • Duration of time depends on speed of MS • If the slope of the short-term average received signal level is steep, hand off should be made quickly
- 27. • 1st generation hand off – BS measures signal strength (ss) – MSC supervises – Each BS constantly monitors ss of RVCs – Determine relative location of each MS • 2nd generation hand off – Hand off decisions are MS assisted (MAHO) – MS measures the received power from neighboring BSs – Continually reports the measurements to the serving BS
- 28. – A hand off is initiated • Power received from the BS of a neighbouring cell > power received by the current BS by a certain level or for a certain period of time • Merits of 2nd gen hand off – Hand off is made by each MS – MAHO hand over a call much faster – MSC no longer constantly monitors ss
- 29. Roaming • Roaming is a mechanism by means of which intersystem hand off takes place • When roaming ? – Ss received by MS becomes weak – MSC can’t find another cell within its system to transfer the control of MS • What are the issues of roaming ? – MS moves out of its home system – Local call becomes long distance call – Compatibility between the two MSCs must be determined before implementing an intersystem hand off
- 30. Prioritizing hand off User’s view – A call abruptly terminated while in the middle of a conversation is more annoying than being blocked occasionally on a new call attempt – Hand off prioritization over new call attempt is desirable Solution ? – Guard channel concept • A fraction of channels is reserved for hand off – Queuing of hand off requests
- 31. Hand off – some of the practical constraints Problem 1 • Accommodating a wide range of mobile velocities during design Solution ? – Use different antenna heights – Use different power levels – Provide ‘large’ and ‘small’ cells co-located – Known as umbrella cell approach (see Fig) – Large/small area coverage to high/low speed users
- 32. Fig. 4 : The umbrella cell approach
- 33. Problem 2 • Cell dragging – Hand off is not made even when essential – results from pedestrian users providing very strong signal to the BS – occurs in an urban environment – LOS radio path exists between MS and BS
- 34. GSM • GSM (Global System for Mobile) is a 2nd generation cellular system standard • World’s first cellular system to specify digital modulation and network level architectures and services • Before GSM, European countries used different cellular stds throughout the continent – Not possible for a customer to use a single subscriber unit throughout Europe
- 35. GSM contd. • GSM services and features – Classified as either teleservices or dataservices • Tele services – Calling – Fascimile • Data services – Data rates from 300 bps to 9.6 kbps – SMS while simultaneously carrying normal voice traffic
- 36. GSM contd. Remarkable features from the user’s point of view • SIM is a memory device that stores following information – subscriber’s identification module – Networks – Country – Privacy key – Other user specific information
- 37. GSM contd. • Without SIM installed all GSM mobiles are identical and nonoperational • Subscriber may plug their SIM into any suitable terminal
- 38. GSM contd. • GSM system architecture consists of three major interconnected subsystem – Base Station Subsystem (BSS) – Network and Switching Subsystem (NSS) – Operating Support Subsystem (OSS)
- 39. Fig. 5 GSM system architecture
- 40. GSM contd. • BSS provides – Radio transmission paths between the MS and MSCs – Manages the radio interface between the MSs and all other subsystems of GSM – BSCs connect the MS to the NSS via MSCs • NSS manages – Switching functions of the system – Allows the MSCs to communicate with other networks (PSTN, ISDN)
- 41. GSM contd. • OSS supports – Operation and maintenance of GSM – Allows system engineers to monitor, diagnose and troubleshoot all aspects of the GSM – Solely for the staff of the GSM operating company • Mobile handoffs between two BTSs under the control of the same BSC are handled by the BSC, and not the MSC
- 42. GPRS GPRS (General Packet Radio Service) • Multicast packet switched technology • Enhanced 2nd generation cellular system with faster data service • Particularly suited for sending and receiving small bursts of data such as email and web browsing
- 43. GPRS contd. • Runs at speed up to 115 kbps compared with 9.6 kbps – Promises to support data transmission typically at 20 to 30 kbps (max 50 kbps) – Theoretically max up to 171.2 kbps • Pay only for the amount of information you download rather than duration of the connection