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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

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Cellular Wireless Networks

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  • 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
frequency reuse
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

increasing capacity
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

Mobile Telecom

Switching Office


Base Station

Base Station

Base Station

operations cont
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
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
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 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
first generation analog systems
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
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
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
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
  • 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
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
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 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
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
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
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:
    • 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
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 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
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
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
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)