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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)