Cellular Revolution
• In 1990 mobile phone users populate 11 million. By
2004 the figure will become 1 billion
• Phones are most obvious sign of the success of
wireless technology. Handsets are getting smaller,
lighter, yet more powerful
• Service prices are dropping
• Service quality are being improved
• The applications have expanded from voice
application to Internet applications
Problems with Wireless Networks
• Operates in a less controlled environment, so is more
susceptible to interference, signal loss, noise, and
eavesdropping.
• Generally, wireless facilities have lower data rates
than guided facilities.
• Frequencies can be more easily reused with guided
media than with wireless media.
Mobile Telephony
• First Generation (AMPS)
– analog voice communication using frequency
modulation.
• Second Generation (GSM)
– digital techniques and time-division multiple access
(TDMA) or code-division multiple access (CDMA)
• Third Generation
– evolving from second-generation wireless systems
– will integrate services into one set of standards.
AMPS Components
• Mobile Units
– contains a modem that can switch between many
frequencies
– 3 identification numbers: electronic serial number, system
ID number, mobile ID number
• Base Transceiver
– full-duplex communication with the mobile
AMPS
• Spectral allocation in North America
– Two 25-MHz bands are allocated to AMPS: 869-894 MHz from the base station to the mobile unit, 824-849 MHz from the mobile unit to the base station
– The bandwidth has been split into two 12.5 MHz in each direction for two operators to compete each other.
– A 12.5 MHz channel allows 416 channels.
• Spatial allocation
– 10-50 frequencies are assigned to each cell
– Depends on the pattern of cells. Each cell may have N/n frequencies, where N = 395, and n = 7 is the smallest pattern
– Original cells are 6.5-13km in size. 1.5-km is the practical minimum size. Too small size will have more frequency change.
Frequency Reuse
A Seven-Cell Cluster
Global System
for Mobile Communication
• Developed to provide common 2nd-generation technology for Europe • 200 million customers worldwide, almost 5 million in the North America • GSM transmission is encrypted, using stream cipher A5 for transmissions
from subscriber to transceiver. A3 is used for authentication.
• It uses subscriber identity module (SIM) in the form of smart card.
• Supports both data and image services based on ISDN model, with rates up to 9.6 kbps
• Spectral allocation: 25 MHz for base transmission (935–960 MHz), 25 MHz for mobile transmission (890–915 MHz)
GSM Layout
Multiple Access
• Four ways to divide the spectrum among
active users
– frequency-division multiplexing (FDM)
– time-division multiplexing (TDM)
– code-division multiplexing (CDM)
– space-division multiplexing (SDM)
Choice of Access Methods
• A random access scheme using FDM, TDM, SDM or CDM to dynamically assign sub-channels to users is called random access method, e.g. FDMA, TDMA, CDMA, SDMA.
• FDM, used in 1st generation systems, wastes spectrum • Debate over TDMA vs CDMA for 2nd generation
– TDMA advocates argue there is more successful experience with TDMA.
– CDMA proponents argue that CDMA offers additional features as well, such as increased range.
– TDMA systems have achieved an early lead in actual implementations
Third Generation Systems
• IMT-2000 defined the 3rd-generation capacities:
– voice quality, 144kbps data rate for high speed mobile, 384 kbps data rate for low speed mobile, 2.048 Mbps office use, packet/circuit switching, Internet interface, more efficiency of spectrum use, more mobile equipment support, flexible for new services and technologies.
• Intended to provide high speed wireless communications for multimedia, data, and video
• Personal communications services (PCSs) and personal
communication networks (PCNs) are objectives for third-generation wireless.
• Planned technology is digital using TDMA or CDMA to provide efficient spectrum use and high capacity
• PCS handsets are designed to be low power, small and light
Wireless Application Protocol (WAP)
• A universal, open standard developed by WAP forum to provide services:
– wireless phone, pager, personal digital assistants, Internet, web, etc.
• It is designed to work with all wireless network technologies • It is based on Internet standards:
– IP, XML, HTML and http
• WAP specification includes:
– WWW Programming Model
– Wireless markup language (WML) – Specification of a small browser
– A lightweight communications protocol stack
Types of LEOs
• Little LEOs: Intended to work at
communication frequencies below1 GHz using
no more than 5 MHz of bandwidth and
supporting data rates up to 10 kbps
• Big LEOs: Work at frequencies above 1 GHz
and supporting data rates up to a few
Iridium: A 3
rd
Generation Satellite
System
• 66 small LEOs
• Services: voice, paging, wireless phone • Proposed in 1987
• Put in service 1999
• Named for the element iridium because 77 electrons match the number of satellites
• Transmissions between satellites • $5 billion to implement
• Motorola 9505 terminal for Iridium weighs about 13 oz. (370g) 2.4 hour talk time, 24 hours standby time
• Using L band (1600-1700 MHz) for ground communications and 18-30 GHz between satellites
