Cellular Networks Wireless Transmission Cellular Concept Frequency Reuse

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, 3 G…

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: – 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: – – 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 PSTN MSC To other MSCs HLR VLR 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: – – § 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 activity: – 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: – – – § Mobile assisted: – – – § 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 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. 5 G: Packet switching extensions – Digital: GSM to GPRS – Analog: AMPS to CDPD § 3 G: – High speed, data and Internet services – IMT-2000