Cellular Wireless Networks

Introduction• Cellular technology is the underlying technology for most mobile

wireless communications and data transfers• Main concept: Use of multiple low-power transceivers• An area is divided into cells, each of which are served by a base

station, consisting of transmitter, receiver, and a control unit• Adjacent cells are assigned different frequencies to avoid

interferences and crosstalk. Cells at sufficient distance from each other can use the same frequency bands

• The cells are hexagonal in shape• Each cell has a base transceiver – the transmission power is

carefully controlled to avoid interferences with neighboring cells• About 10-50 frequencies are assigned to each cell

Cellular Geometries

• Handout

Frequency Reuse

• How many cells must intervene between two cells using the same frequency?D: Minimum distance between centers of cells that use the same

band (co-channels)

R: Radius of a cell

d: distance between adjacent cells (d=3 R)

N: Number of cells in a repetitious pattern (reuse factor)

D/R = √(3N)

or D/d = √N

Frequency Reuse

• Handout

Increasing Capacity• Adding new channels:

– add unused channels• Frequency borrowing:

– borrowed from adjacent cells– Frequencies can be also assigned dynamically

• Cell Splitting: – Original cells (about 6.5-13km in size) can be split to smaller cells (1.5

km minimum)– Powers of transmitters are lowered– Handoff becomes more frequent

• Cell Sectoring: A cell is divided into number of wedge-shaped sectors– Each sector is assigned a separate subset of cells channel– Directional antennas at the base station are used to focus on each

sector

Operations

Base Station

Mobile Telecom Switching Office

(MTSO)

Base Station Base Station

Operations – cont.

• Base stations (BS) includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell

• Each BS is connected to an MTSO. – One MTSO serves a single or multiple BSs– MTSO assigns the voice channel to each call, performs handoff,

and monitors the call for billing information

• Channels between the mobile units and the base stations– Control channels: used to exchange information having to do

with setting up and maintaining calls– Traffic channels: carry a voice or data connections between

users

Typical Call - 1

• Mobile Unit Initialization:– The unit scans and selects the strongest set-up control channel– A handshake takes place between the mobile unit and the

MTSO controlling this cell, through the BS in this cell– The scanning process is repeated periodically

• Mobile-originated call:– The number of the called unit is sent on the preselected set-up

channel to the BS– The BS sends the request to the MTSO

• Paging:– The MTSO sends a paging message to certain BSs– Each BS transmit the paging signal on its own assigned set-up

channel

Typical Call - 2

• Call accepted:– The called mobile unit recognizes its number on the set-up

channel being monitored and responds to that BS, which send the response to the MTSO

– MTSO sets up a circuit between calling and called BSs– MTSO selects an available traffic channel within each BS’s cell

and notifies each BS, which in turn notifies its mobile unit.• Ongoing call:

– While the connection is maintained, the two mobile units exchange voice or data signals, going through their respective BSs and the MTSO

• Handoff:– If a mobile unit moves out of range of one cell and into the range

of another during a connection, the traffic channel has to change to one assigned to the BS in the new cell

Handoff

• Handoff Initiation:– Network-initiated or mobile assisted

• Performance Metrics:– Cell blocking probability– Call dropping probability– Call completion probability– Probability of unsuccessful handoff– Handoff blocking probability– Handoff probability– Rate of handoff– Interruption duration– Handoff delay

Handoff: When to?

• Handout

First Generation (Analog Systems)

• Advanced Mobile Phone System (AMPS) – developed by AT&T (early 1980’s)

• Uses FDD scheme• AMPS Parameters:

– Base station transmission band = 869-894 MHz– Mobile unit transmission band = 824-849 MHz– Spacing between forward and reverse channel = 45 MHz– Channel bandwidth = 30 kHz– Number of full-duplex voice channels = 790– Number of full duplex control channels = 42– Data transmission rate = 10 kbps– Cell size = 2-20km radius

AMPS - Operation

• Each AMPS-capable cellular telephone includes a numeric assignment module (NAM) in read-only memory

• NAM contains telephone number (provided by service provider) and the serial number of the phone

• When turned on, the phone transmits its serial number to MTSO

• MTSO maintains a database to take care of authentication and billing issues.

Sequence of Events

• The subscriber dials a number• The MTSO authorizes and issues a message to

the user’s cell phone indicating which traffic channel to use for sending and receiving

• The MTSO sends a ringing signal to the called party. When the party answers, the MTSO establishes a circuit between the two parties and initiates billing information

• When one party hangs up, the MTSO releases the circuit, frees the radio channels, and completes the billing information

Second Generation (TDMA/CDMA)

• Goal: higher quality signals, higher data rates, and greater capacity

• Advances:– Digital traffic channels: second generation systems provide

digital traffic channels, supports digital data; voice traffic is encoded in digital form before transmitting

– Encryption– Error detection and correction– Channel access: allows multiple users per channel using TDMA

and CDMA• Second Generation Cellular Telephone System:

– GSM– Interim Standard 136 (IS-136)– IS-95

Standards• GSM (Global System for Mobile Communications):

– Introduced in 1990, TDMA-based– Transmission range: BS=935-960 MHz, MS = 890-915 MHz– Channel bandwidth = 200 kHz, 8 users/channel– Deployed widely in Europe, Asia, Australia, South America, PCS

spectrum in US• IS-136: (Also known as North American Digital Cellular)

– Introduced in 1991, TDMA-based– Transmission range: BS = 869-894 MHz, MS = 824-849 MHz– Channel BW = 30 kHz, 3 users/channel– Deployed in North & South America, Australia

• IS-95: (Also known as cdmaOne)– Introduced in 1993, CDMA-based– BS and MS transmission bands = same as IS-136– Channel bandwidth = 1250 kHz, 64 users/channel– Deployed in North & South America, Korea, Japan, China, Australia

Global System for Mobile (GSM) Communications

• Services:– Teleservices: mobile telephony– Data services: packet-switched traffic

• User services:– Telephone services– Data services

• packet switched protocols and data rates from 300 bps to 9.6 kbps

– Supplementary ISDN services• Calling features, SMS, and cell broadcast

• On-the-air privacy:– The digital bit stream sent by a GSM transmitter is encrypted

according to a specific secret cryptographic key that is known only to the carrier. The key changes with time for each user.

GSM System Architecture

• Three subsytems:– Base System Subsystem (BSS)– Network and Switching Subsystem (NSS)– Operation Support Subsystem (OSS)

• Mobile station (MS) is considered a part of BSS

• Refer to the GSM Architecture (handout)

Mobile Station

• Mobile Equipment (ME)– Device including keypad, speaker, microphone, radio

transceiver, Digital signal processor

• Subscriber Identity Module (SIM):– A pluggable memory device that stores information

such as subscriber identification number, the network and countries where the subscriber is entitled to service, privacy keys, and other user-specific information

– Calls in GSM are directed to SIM

GSM Subsystems

• BSS: (also known as radio subsystem)– Provides and manages radio transmissions between MSs and

Mobile Switching Center (MSC)– Manages the radio interface between all subsystem, reserves

radio frequencies, manages handoff, controls paging– Each BSS consists of a Base Station Controllers (BSC) and

multiple Base Transceiver Stations (BTS)– Connects the MS to the NSS via the MSCs

• NSS:– Manages the switching functions of the system and allows the

MSCs to communicate with other networks such as PSTN and ISDN

• OSS: – Supports the operations and maintenance of GSM; monitor,

diagnose, troubleshoot all aspects of GSM system

Base Station Subsystem (BSS)

• The MSs communicate with the BSS over the radio interface

• BSS consists of one or more BSCs which connect to a single MSC

• Each BSC typically controls about several hundred Base Transceiver Stations (BTS)

• BTS may be colocated with the BSC or remotely distributed

• Mobile handovers between two BTS under the control of the same BSC are handled by the BSC – reduces switching burden on MSC

GSM Interfaces

• Um: radio interface• Abis: interface connecting a BTS to a BSC

– Carries traffic and maintenance data– Standardized for all manufacturers

• BSCs are physically connected to MSCs (A interface)– Uses an SS7 protocol called the signaling

correction control part (SCCP) to support communications between the MSC and BSSs

NSS

• NSS:– Handles the switching of GSM calls between

external networks and the BSCs– Responsible for managing and providing

external access to several customer databases

– MSC is the central unit in the NSS and controls traffic among all BSCs

Databases in NSS• Home Location Register (HLR)

– Contains subscriber information and location information for each user who resides in the same city as MSC

– Each subscriber in a particular GSM market is assigned a unique International Mobile Subscriber Identity (IMSI)

• Visitor Location Register (VLR)– Temporarily stores the IMSI and customer information for each roaming

subscriber who is visiting the coverage area of a particular MSC– Once a roaming mobile is logged in the VLR, the MSC sends the necessary

information to its HLR so that the calls can be appropriately routed • Authentication Center (AUC)

– Handles authentication and encryption keys for each of the subscriber in the HLR and VLR

• Equipment Identity Register (EIR)– Keeps the international mobile equipment identity (IMEI) that reveals

manufacturer, country of production, terminal type– Helps in managing faults and thefts– Implementation is optional to the service provider

OSS

• OSS supports one or several operation maintenance centers (OMC), which are used to monitor and maintain performance of each MS, BS, BSC, and MSC

• Functions:– Maintain all telecom hardware and network

operations– Maintain all charging and billing procedures– Manage all mobile equipments in the system

Second Generation CDMA (IS-95)

• Forward link: Up to 64 logical CDMA channel – each occupying 1228 kHz bandwidth

• Types of channels:– Pilot (channel 0): allows MS to acquire timing, phase

reference, and signal strength reference– Synchronization (channel 32): 1200 bps channel used

by MS to obtain identification information (system time, code state, protocol version, etc.)

– Paging (Channels 1-7)– Traffic channels (8-31, 33-63) up to 14.4 kbps

IS-95 (continued)

• Reverse Link: Up to 94 logical CDMA channels, each occupying 1228 kHz bandwidth– Supports up to 32 access channels and up to

64 traffic channels– The access channel is used by an MS to

initiate a call, respond to paging, and for location update

– Traffic channels is reverse links are mobile unique

Third Generation (3G) Wireless Networks

• Motivations:– Multi-megabit Internet access– Very high network capacity– VoIP– Integration of multiple technologies

• ITU formulated a plan to implement a global frequency band in the 2000 MHz range that would support a single, ubiquitous wireless communication standard for all countries – called International Mobile Telephone (IMT-2000)

• 3G evolution of CDMA – cdma2000• 3G evolution of GSM – wideband CDMA (W-CDMA),

also called Universal Mobile Telecommunications Service (UMTS)