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Giacinto Gelli » 3.Current and emerging wireless systems

Outline

Cellular systems
Cordless phones
Wireless LANs
Broadband wireless access
Satellite systems
Paging systems
Wireless PANs

Bluetooth, Zigbee, UWB

Cellular systems: the market

Cellular systems: the market (cont’d)

Pre-cellular (0G) mobile phone

One highly-elevated antenna serves a large geographic region:

small number of channels, high blocking probability
very low capacity (max 543 users in NY city in 1976!)
example: MTS/IMTS systems in USA (1962)

BS = Base Station MT = Mobile Terminal

The cellular concept: frequency reuse

Geographic region divided into cells:

Channels reused at spatially-separated locations
Co-channel interference between cells using the same channels (same colors)
First proposed by D.H. Ring at Bell Laboratories in 1947

Base stations/MTSO coordinate handoff and control functions.

Shrinking cell-size increases capacity, as well as networking burden

MTSO = Mobile Telephone Switching Office

Handoff

When a mobile moves between two cells, its call must be handed off from source BS to destination BS.

Handoffs are coordinated by MTSOs and must be transparent to the user
Handoff is initiated when the signal power of the mobile to the source BS decreases below a threshold
Smaller cells imply more frequent handoffs

Cellular network architecture

PSTN = Public Switched Telephone Network

Cellular generations

First generation (1G): analog systems (25/30 kHz FM), mostly vehicular terminals:

AMPS, TACS/ETACS, NMT (450/900), NTT, C-450, RC2000

Second generation (2G): digital systems (narrowband TDMA, TDMA/FDMA and CDMA), voice and low-speed data (e.g. SMS, MMS), circuit-switched, portable terminals:

GSM, IS-54/136, IS-95 (cdmaOne), PDC

Third generation (3G): digital systems (wideband TDMA and CDMA), voice/video and high-speed data, circuit and packet switching, portable terminals:

cdma2000, W-CDMA/UMTS

Cellular standards evolution

World cellular subscribers by technology (June 2006)

2.41 billion cellular customers worldwide GSM+ UMTS totals 82.4%

World cellular subscriber distribution (June 2006)

GSM growth: 1993 to June 2006

Cordless phones

Born in the 70’s as “cable replacements” for conventional phone devices:

voice quality comparable to wired handsets
CT-1 (1G, analog); CT-2 (2G, digital)

More sophisticated digital systems have similar functionalities to cellular phones (outdoor use, handoff, mobility support):

DECT (Europe)
PHS (Japan)

Strong competition with cellular phones limited their use mainly to home/indoor applications.

Wireless Local Area Networks

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).

WLAN main standards

IEEE 802.11b (1999)

2.4 GHz ISM band (83.5 MHz bandwidth)
Modulation: DSSS, IR
Max data-rate 11 Mb/s (raw), range 100 m

IEEE 802.11a (1999)

5 GHz ISM band (300 MHz bandwidth)
Modulation: OFDM
Max data-rate 54 Mb/s (raw), range 30 m
Similar to HiperLAN/2 in Europe

IEEE 802.11g (2003)

2.4 GHz (83.5 MHz bandwidth)
Modulation: OFDM
Max data-rate 54 Mb/s (raw), range 30 m

The current trend is towards network interfaces supporting two or three standards.

Broadband Wireless Access

Bidirectional wireless link between a fixed point and multiple fixed or low-mobility terminals:

interactive video, video on demand
high-speed Internet access
infrastructure-based (star) topology

LMDS and MMDS systems (USA), HiperACCESS (Europe).
WiMax (IEEE 802.16) is an emerging standard:

frequency 2-11 GHz (NLOS), 10-66 GHz (LOS)
data-rate 40 Mb/s (fixed), 15 Mb/s (mobile)
competitor to WLAN, 3G cellular systems, and wired systems (cable, ADSL)

Paging systems

Broad coverage for providing short messaging services (SMS):

Messages are broadcasted from all BS
Simple terminals

Optimized for one-way (downlink) transmission: answer-back not always supported.

Overtaken by cellular systems.

Satellite networks

Cover very large areas.
Different orbit heights:

GEO (40000 km) vs. MEO (9000 km) vs LEO (2000 km)

Optimized for one-way transmission:

Radio (DAB) and TV (DVB) broadcasting

Most two-way systems struggling or bankrupt (e.g., Iridium):

expensive alternative to terrestrial systems
limited market

Bluetooth

Low cost “cable replacement” RF technology:

short range (10 m, extensible to 100 m)
2.4 GHz unlicensed band (crowded)
1 data channel (723.3 kb/s) or 3 voice channels (64 kb/s)

Widely supported by telecommunications, PC, and consumer electronics companies (> 1300 companies).
Few applications beyond cable replacement: Smart home?

IEEE 802.15/Zigbee

Low-rate low-cost WPAN (Wide Personal Area Network):

20, 40, 250 kb/s
Range 30 m
ISM band

Star or peer-to-peer topology.
Lower power consumption than Bluetooth.
Applications:

sensor networks
inventory tags (RFID)

Ultrawideband Radio (UWB)

UWB is an impulse radio technique:

Sends pulses of tens of picoseconds (10^-12) to nanoseconds (10^-9)
Duty-cycle only a fraction of percent

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:

can use OFDM to solve multipath problem (e.g. WiMedia).

Advantages of UWB

Unique location and positioning properties

1 cm accuracy possible

Low power CMOS transmitters

100 times lower than Bluetooth for same range/data-rate

Very high data rates possible

500 Mb/s @ 3m under current regulations

7.5 GHz of “free spectrum” in USA

FCC recently legalized UWB for commercial use
spectral allocation overlays existing users, but its allowed power level is very low to minimize interference

Wireless networks: a comparison

Future wireless networks

Conclusions

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.