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Sec. 8–8 Cellular Telephone Systems 629
received data establish the parameters for the receiver speech synthesizer so that the VF
signal may be reproduced for the listener.
2G systems have achieved significant improvements over the 1G AMPS system in
terms of system capacity, service quality, information security, and other features such as
text messaging and camera phones. 2G systems were developed mainly for voice transmission,
not data transmission. Most 2G systems operate at relatively slow data rates of 8 kbs
or 13 kbs. Consequently, transmission of computer-system data is severely limited in these
systems. Higher data rates have been the main driving force in the development of 3 G systems.
Some of these systems can obtain data rates of 384 kbs for users traveling at speeds
up to 120 kmhr whereas data rates of up to 2 Mbs can be obtained for stationary users.
An interesting wireless system application is the OnStar subscription service provided
by General Motors (GM). This service provides GM vehicle drivers and their passengers
contact with an OnStar representative for emergency and concierge-like services. The communication
link is via the cellular telephone network and the vehicle-position information is
provided by an on-board GPS (Global Positioning System) receiver. Some of the services
provided are: stolen-vehicle location assistance, emergency services, vehicle-trouble remote
diagnosis, roadside assistance, accident assistance, remote door unlocking, driver directions,
and accident detection via automatic notification of air bag deployment.
Third Generation (3G)—Digital with Circuit
and Packet Switching
As indicated earlier, users are demanding data services for applications such as browsing Web
pages and e-mail, in addition to VF calls. 3G networks have evolved from 2G networks using
packet switching for data transmission and circuit switching for VF calls. Most of the 3G networks
use protocol extensions of 2G-type technology. At the end of 2007, 9% of worldwide
subscribers (295 million) were using 3G technology. Data rates vary widely for the different
types of 3G systems. For a HSDPA (High Speed Downlink Packet Access) type of 3G cellular
network, the typical downlink data rate is 2Mbs and the typical uplink data rate is 200kbs.
Fourth Generation (4G)—Digital with Packet Switching
4G networks differ from 3G networks by the complete elimination of circuit switching and
using packet switching with IP (internet protocol). Thus, the networks are optimized to transmit
and receive high-speed data. VF calls are handled using VoIP (Voice over Internet
Protocol) on the data network. Two competing technologies are used: LTE (Long Term
Evolution) and WiMax.
LTE uses OFDM on the downlink and FDMA on the uplink, with channel bandwidths
(presently) up to 20 MHz. The modulation on the OFDM and FDMA carriers is either
QPSK, 16QAM, or 64QAM. In the United States, Verizon, AT&T, and Metro PCS rolled
out LTE systems in 2011. On the Verizon 4G network, measured rates on the operating system
in Washington, DC., are 16Mbs on the downlink and 6.6 Mbs on the uplink
[Mossberg, 2011].
The WiMax technology is used by Sprint, in partnership with Clearwire, in the United
States. For a description of the WiMax technology, see Section 8–11.