Broadband wireless access
One highly-elevated antenna serves a large geographic region:
Geographic region divided into cells:
Base stations/MTSO coordinate handoff and control functions.
When a mobile moves between two cells, its call must be handed off from source BS to destination BS.
First generation (1G): analog systems (25/30 kHz FM), mostly vehicular terminals:
Second generation (2G): digital systems (narrowband TDMA, TDMA/FDMA and CDMA), voice and low-speed data (e.g. SMS, MMS), circuit-switched, portable terminals:
Third generation (3G): digital systems (wideband TDMA and CDMA), voice/video and high-speed data, circuit and packet switching, portable terminals:
Born in the 70’s as “cable replacements” for conventional phone devices:
More sophisticated digital systems have similar functionalities to cellular phones (outdoor use, handoff, mobility support):
Strong competition with cellular phones limited their use mainly to home/indoor applications.
WLANs connect “local” computers (typical range < 100 m).
Breaks data into packets to improve reliability.
Channel access is shared (random access).
Backbone Internet provides best-effort service: poor performance in some applications (e.g., video).
IEEE 802.11b (1999)
IEEE 802.11a (1999)
IEEE 802.11g (2003)
The current trend is towards network interfaces supporting two or three standards.
Bidirectional wireless link between a fixed point and multiple fixed or low-mobility terminals:
LMDS and MMDS systems (USA), HiperACCESS (Europe).
WiMax (IEEE 802.16) is an emerging standard:
Broad coverage for providing short messaging services (SMS):
Optimized for one-way (downlink) transmission: answer-back not always supported.
Overtaken by cellular systems.
Cover very large areas.
Different orbit heights:
Optimized for one-way transmission:
Most two-way systems struggling or bankrupt (e.g., Iridium):
Low cost “cable replacement” RF technology:
Widely supported by telecommunications, PC, and consumer electronics companies (> 1300 companies).
Few applications beyond cable replacement: Smart home?
Low-rate low-cost WPAN (Wide Personal Area Network):
Star or peer-to-peer topology.
Lower power consumption than Bluetooth.
UWB is an impulse radio technique:
A carrier is not necessarily needed.
Uses a lot of bandwidth (GHz).
Low probability of detection.
Excellent ranging (localization) capabilities.
Multipath highly resolvable: good and bad:
Unique location and positioning properties
Low power CMOS transmitters
Very high data rates possible
7.5 GHz of “free spectrum” in USA
The wireless world encompasses many interesting systems and applications.
The main technical challenges are dealing with channel hostility at the lowest levels of the protocol stack and user/device mobility at the highest levels.
Existing and emerging systems provide excellent quality for certain applications but poor interoperability.
Standards and spectral allocation heavily impact the evolution of wireless technology.
3. Current and emerging wireless systems
9. Introduction to digital communications
A. Goldsmith. Wireless Communications. Cambridge University Press, 2005 (chap. 1 & app. D)
J.H. Schiller. Mobile Communications. Addison-Wesley, 2003 (chap. 1)
Supplementary material eventually available on the website