Mobile Computing Cellular Concepts

Cellular Networks Wireless Transmission Cellular Concept Frequency Reuse Channel Allocation Call Setup Cell Handoffs Location Management

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

Wireless Transmission Communication Frequencies –Frequencies in the VHF range are used –Regulation bodies Antennas –Theoretically: equal radiation in all directions –Reality: directive effects, sectorized antennas

Wireless Transmission 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 Mobile hosts can change cells while communicating –Hand-off occurs when a mobile host changes base station Factors for determining cell size –No. of users to be supported –Multiplexing and transmission technologies

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 As demand increases (more channels needed) –Number of base stations is increased –Transmitter power is decreased correspondingly to avoid interference

Cellular Concept 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, using FDMA, TDMA, CDMA…

Cellular Concept Cell size: –100 m in cities to 35 km on the country side (GSM) –even less for higher frequencies

Cellular Concept 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 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

Cellular Concept Problems: –fixed network needed for the base stations –handover necessary –interference with 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

Geometric Representation Cells are commonly represented by hexagons. Why hexagon? How about circle? How about square, or triangle?

Hexagonal Cells

Channel Reuse The total number of channels are divided into K groups. – K is called reuse factor or cluster size. Each cell is assigned one of the groups. The same group can be reused by two different cells provided that they are sufficiently far apart.

Example: K = 7

Coordinate System Use (i,j) to denote a particular cell. Example: Cell A is represented by (2,1). A

R D Distance Formula Note: i and j are integers where Reuse factor

Cellular System Architecture MSC HLR VLR HLR VLR To other MSCs PSTN

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

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 and PSTN

Cellular System Architecture 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

Call Setup (contd) 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

Call Setup (contd) 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

Hand-Offs Mobile moves from 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

Hand-Offs (contd) 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

Hand-Offs Intersystem: (Roaming) –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 ( 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)

Cellular Implementations 2.5G: Packet switching extensions –Digital: GSM to GPRS –Analog: AMPS to CDPD 3G: –High speed, data and Internet services –IMT-2000