Cellular Networks Wireless Transmission Cellular Concept Frequency Reuse Channel Allocation Call Setup Location Management Cell Handoffs Optimizations: Power control, Cell capacity Implementations: AMPS, GSM, GPRS, 3G…

Basic Idea Single hop wireless connectivity to the wired world Space divided into cells A base station is responsible to communicate with hosts in its cell Mobile hosts can change cells while communicating Hand-off occurs when a mobile host starts communicating via a new base station Factors for determining cell size No. of users to be support Multiplexing and transmission technologies …

Wireless Transmission Communication Frequencies Frequencies in the VHF – SHF range are used Regulation bodies Antennas Theoretically: equal radiation in all directions Reality: directive effects, sectorized antennas Signal Propagation Classification: Analog/Digital, Periodic/Aperiodic Parameters: Amplitude, Frequency and Phase shift Modulation Techniques Amplitude, Frequency, Phase Multiplexing Mechanisms Space (SDM), Frequency (FDM), Time (TDM), Code (CDM)

Cellular Concept Limited number of frequencies => limited channels Single high power antenna => limited number of users Smaller cells => frequency reuse possible => more number of users Base stations (BS): implement space division multiplex Each BS covers a certain transmission area (cell) Each BS is allocated a portion of the total number of channels available Cluster: group of nearby BSs that together use all available channels Mobile stations communicate only via the base station FDMA, TDMA, CDMA may be used within a cell As demand increases (more channels are needed) Number of base stations is increased Transmitter power is decreased correspondingly to avoid interference

Cellular Concept Cell size: Cell shape: BS placement: 100 m in cities to 35 km on the country side (GSM) even less for higher frequencies Umbrella cell: large cell that includes several smaller cells Avoid frequent handoffs for fast moving traffic Cell shape: Hexagonal is useful for theoretical analysis Practical footprint (radio coverage area) is amorphous BS placement: Center-excited cell: BS near center of cell omni-directional antenna Edge-excited cell: BSs on three of the six cell vertices sectored directional antennas

Cellular Concept Advantages: Problems: Important Issues: higher capacity, higher number of users less transmission power needed more robust, decentralized base station deals with interference, transmission area etc. locally Problems: fixed network needed for the base stations handover (changing from one cell to another) necessary interference with other cells: co-channel, adjacent-channel Important Issues: Cell sizing Frequency reuse planning Channel allocation strategies Bottom line: Attempt to maximize availability of channels in an area

Cellular System Architecture Each cell is served by a base station (BS) Each BS is connected to a mobile switching center (MSC) through fixed links Each MSC is connected to other MSCs and PSTN MSC HLR VLR To other MSCs PSTN

Cellular System Architecture Each MSC is a local switching exchange that handles Switching of mobile user from one base station to another Locating the current cell of a mobile user Home Location Register (HLR): database recording the current location of each mobile that belongs to the MSC Visitor Location Register (VLR): database recording the cell of “visiting” mobiles Interfacing with other MSCs Interfacing with PSTN (traditional telephone network) One channel in each cell is set aside for signalling information between BS and mobiles Mobile-to-BS: location, call setup for outgoing, response to incoming BS-to-Mobile: cell identity, call setup for incoming, location updating

Call Setup Outgoing call setup: Network activity: Incoming call setup: User keys in the number and presses send (no dial tone) Mobile transmits access request on uplink signaling channel If network can process the call, BS sends a channel allocation message Network proceeds to setup the connection Network activity: MSC determines current location of target mobile using HLR, VLR and by communicating with other MSCs Source MSC initiates a call setup message to MSC covering target area Incoming call setup: Target MSC (covering current location of mobile) initiates a paging msg BSs forward the paging message on downlink channel in coverage area If mobile is on (monitoring the signaling channel), it responds to BS BS sends a channel allocation message and informs MSC Network completes the two halves of the connection

Hand-Offs Hand-off necessary when mobile moves from area of one BS into another BS initiated: BS monitors the signal level of the mobile Handoff occurs if signal level falls below threshold Increases load on BS Monitor signal level of each mobile Determine target BS for handoff Mobile assisted: Each BS periodically transmits beacon Mobile, on hearing stronger beacon from a new BS, sends it a greeting changes routing tables to make new BS its default gateway sends new BS identity of the old BS New BS acknowledges the greeting and begins to route mobile’s call Intersystem: Mobile moves across areas controlled by different MSC’s Handled similar to mobile assisted case with additional HLR/VLR effort Local call may become long-distance

Cellular Implementations First-generation: Analog cellular systems (450-900 MHz) Frequency shift keying for signaling FDMA for spectrum sharing NMT (Europe), AMPS (US) Second-generation: Digital cellular systems (900, 1800 MHz) TDMA/CDMA for spectrum sharing Circuit switching GSM (Europe), IS-136 (US), PDC (Japan) 2.5G: Packet switching extensions Digital: GSM to GPRS Analog: AMPS to CDPD 3G: High speed, data and Internet services IMT-2000