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Cellular system
1.
2. 1G: First generation wireless cellular: Early 1980s
Analog transmission, primarily speech: AMPS (Advanced
Mobile Phone Systems) and others
2G: Second generation wireless cellular: Late 1980s
Digital transmission
Primarily speech and low bit-rate data (9.6 Kbps)
High-tier: GSM, IS-95 (CDMA), etc.
Low-tier (PCS): Low-cost, low-power, low-mobility e.g. PACS
2.5G: 2G evolved to medium rate (< 100kbps) data
3G: future Broadband multimedia
144 kbps - 384 kbps for high-mobility, high coverage
2 Mbps for low-mobility and low coverage
Beyond 3G: research in 4G
3. Handoffs (typically 30 milliseconds):
1. At any time, mobile station (MS) is in one cell and under the control of a BS
2. When a MS leaves a cell, BS notices weak signal
3. BS asks surrounding BSs if they are getting a stronger signal
4. BS transfers ownership to one with strongest signal
5. MTSO assigns new channel to the MS and notifies MS of new boss
Mobile
Telephone
Switching
Center
(MTSC)
Cell 1
Cell 2
HLR VLR
Public
Switched
Telephone
Network
(PSTN)
4. Mobile radio telephones were used for military
communications in early 20th century
Car-based telephones first introduced in mid 1940s
Single large transmitter on top of a tall building
Single channel used for sending and receiving
To talk, user pushed a button, enabled transmission and disabled reception
Became known as “push-to-talk” in 1950s
CB-radio, taxis, police cars use this technology
IMTS (Improved Mobile Telephone System) introduced in
1960s
Used two channels (one for sending, one for receiving)
No need for push-to-talk
Used 23 channels from 150 MHz to 450 MHz
5. Advanced Mobile Phone Service (AMPS) invented at Bell
Labs and first installed in 1982
Used in England (called TACS) and Japan (called MCS-L1)
Key ideas:
Exclusively analog
Geographical area divided into cells (typically 10-25km)
Cells are small: Frequency reuse exploited in nearby (not adjacent)
cells
As compared to IMTS, could use 5 to 10 times more users in same area
by using frequency re-use (divide area into cells)
Smaller cells also required less powerful, cheaper, smaller devices
6. Based on digital transmission
Different approaches in US and Europe
US: divergence
Only one player (AMPS) in 1G
Became several players in 2G due to competition
Survivors
IS-54 and IS-135: backward compatible with AMPS frequency
allocation (dual mode - analog and digital)
IS-95: uses spread spectrum
Europe: Convergence
5 incompatible 1G systems (no clear winner)
European PTT development of GSM (uses new frequency
and completely digital communication)
7. Voice, data and fax can be integrated into a
single system
Better compression can lead to better channel
utilization
Error correction codes can be used for better
quality
Sophisticated encryption can be used
8. Circuit mode data
Transparent mode
Non-transparent mode using radio link protocol
Data rate up to 9.6kb/s
Short message service
Limited to 160 characters
Packet mode data: Plans for GSM Phase 2+
Architecture specification very detailed (500
pages)
Defines several interfaces for multiple suppliers
9. IS-95 is the best known example of 2G with CDMA
Advantages of CDMA for Cellular
Frequency diversity – frequency-dependent
transmission impairments have less effect on signal
Multipath resistance – chipping codes used for
CDMA exhibit low cross correlation and low
autocorrelation
Privacy – privacy is inherent since spread spectrum
is obtained by use of noise-like signals
Graceful degradation – system only gradually
degrades as more users access the system
10. Self-jamming – arriving transmissions from
multiple users not aligned on chip boundaries
unless users are perfectly synchronized
Near-far problem – signals closer to the receiver
are received with less attenuation than signals
farther away
Soft handoff – requires that the mobile acquires
the new cell before it relinquishes the old; this is
more complex than hard handoff used in
FDMA and TDMA schemes
11. Major technical undertaking with many organizational and
marketing overtones.
Questions about the need for the additional investment for 3G
(happy with 2.5G)
Wireless LAN in public places such as shopping malls and airports
offer options
Other high-speed wireless-data solutions compete with 3G
Mobiles low data rates (nominally 8 Kbps), it uses a narrowband
(2.5KHz) as compared to 30 KHz (GSM) and 5 MHz (3G).
Ricochet: 40 -128 kbps data rates. Bankruptcy
Flash-OFDM: 1.5 Mbps (up to 3 Mbps)
12. Wireless networks with cellular data rates of 20 Mbits/second
and beyond.
AT&T has began a two-phase upgrade of its wireless
network on the way to 4G Access.
Nortel developing features for Internet protocol-based 4G
networks
Alcatel, Ericsson, Nokia and Siemens found a new Wireless
World Research Forum (WWRF) for research on wireless
communications beyond 3G.
Many new technologies and techniques (multiplexing,
intelligent antennas, digital signal processing)
Industry response is mixed (some very critical)