Wireless Wide Area Network Primerfivedots.coe.psu.ac.th/~wsuntorn/ihpt2003/wireless... · IHPT...

Wireless Wide Area Network Primer

Suntorn Witosurapot

April 2003

InIn--House Practical Training House Practical Training ((IHPTIHPT)) Semester Semester 22//20032003

IHPT Semester 3/2002 Wireless WAN Primer - Part 1 2

Part 1

Digital Cellular Network


Outline

• Digital cellular networks

• Global system for mobile communication (GSM)

• General Packet Radio Service (GPRS)


Digital cellular networks

• ?Wireless mobile extension of the public switched

telephone network (PSTN)

– ?Used primarily for voice service today

– ?Data traffic is a small percentage of overall traffic,

but is growing rapidly

• ?Existing systems based on a voice service model

– ?Primarily circuit switched

– ?Typically low data rates, e.g. 9.6 or 19.2 Kbps


Topics in Digital Cellular Networks

• Cellular Concept

• Frequency Reuse

• Channel Allocation

• Call Setup

• Location Management

• Cell Handoffs

• Optimizations: Power control, Cell capacity

• Implementations: AMPS, GSM, GPRS, 3G…


Basic Cellular System• A single cellular system interconnects many small radio coverage

areas called (cells)– Base station (BS) is responsible for communication with

mobile hosts in its cell

– Mobile hosts can change cells while communicating• user equipment (UE) in 3rd generation (3G) system• mobile equipment (ME) or mobile station (MS) in 2G system

BSBS

MSC PSTN


Basic Cellular System (cont.)

• If a connected MS (MS in call phase) moves between two cells, the call is not dropped.

• Instead, the network performs a handover (US: hand-off).• Factors for determining cell size

– No. of users to be support

– Multiplexing and transmission technologies

– …

BSBS

MSC PSTN


Cellular Concept

• Limited number of frequencies => limited channels

• Single high power antenna=> limited coverage area =>serving limited number of users

• Smaller cells => frequency reuse possible => more number of users but need to

– increase the number of base stations

– reduce transmitter power accordingly to avoid interference


Cellular Concept (cont.)

• 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:– depends on environments, e.g. buildings, mountains,

weather conditions.– Hexagonal is useful for theoretical analysis– Practical footprint (radio coverage area) is amorphous


Cellular Concept (cont.)

• 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: Summary• Advantages (due to smaller cells):

– higher capacity, higher number of users (Why?)

– less transmission power needed (saving battery power for mobile life)

– more robust, decentralized

– base station deals locally with interference, transmission area etc.

• Problems:

– infrastructure needed to connect base stations

– handover necessary

– interference with other cells:co-channel, adjacent-channel

Bottom line: Attempt to maximize availability of channels in an area

Important Issues:• Cell sizing• Frequency reuse planning• Channel allocation strategies


Cellular System Architecture

• Each cell is served by a base station (BS or - ?µ??̧µ? )• Each BS is connected to a mobile switching center (MSC or

•»¤ - µ¥Ã? ¦ « ¡́ ? rÁ?¨ ºÉ° ? ? Ȩ́) through fixed links• Each MSC is connected to other MSCs and PSTN

MSC MSC

HLR

VLR

HLR

VLRTo otherMSCs PSTNPSTN


Cellular System Architecture: MSC

• 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 belonging to the MSC

• Visitor Location Register (VLR): database recording the cell of “visiting” mobiles

– Interfacing with• other MSCs • PSTN (traditional Public telephone network)


Cellular System Architecture: Channel Allocation

• One channel in each cell is set aside for signaling 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


Cellular System Architecture: 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


Cellular System Architecture: Call Setup (cont.)

• Incoming call setup:– Target MSC (covering current location of mobile) initiates a paging

message

– 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


Cellular System Architecture: Hand-Offs

• BS initiated handoff:– BS monitors the signal level of the mobile– Handoff occurs if signal level falls below threshold– Extra burden on BS:

• Monitor signal level of each mobile• Determine target BS for handoff

• Mobile assisted handoff :– Each BS periodically transmits beacon

– On hearing stronger beacon from a new BS, mobile sends a greeting in order to perform:• changing routing tables (i.e. make new BS becomes its default gateway)

• sends new BS identity of the old BS

– New BS acknowledges the greeting and begins to route mobile’s call


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, UMTS

Channel Multiplexing Technology


Frequency Division Multiplexing (FDM)

Advantages:• No dynamic coordination

Disadvantages:• Inflexible and inefficient

if channel load is dynamic and uneven

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Each channel gets a spectrum band


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Time Division Multiplexing (TDM)Each channel gets the entire spectrum for a certain amount of time

Advantage: Can assign more time to senders with heavier loads

Challenge: Requires precise synchronization


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Combining TDM and FDM

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A channel gets a certain frequency band for a certain amount of time• Example: GSM

Advantages:•More robust against frequency selective interference•Inherent tapping protection

Challenge:• Frequency hopping coordination


Code Division Multiplexing (CDM)• Each channel has a unique code• All channels use the same

spectrum at the same time - e.g. orthogonal codes

• Advantages:– bandwidth efficient – code

space is huge– no coordination or

synchronization necessary between different channels

– good protection against interference and tapping

• Challenge:– More complex signal

regeneration– Requires careful power control

• Spread spectrum technology

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Wireless Wide Area Network Primerfivedots.coe.psu.ac.th/~wsuntorn/ihpt2003/wireless... · IHPT...

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