Lecture 13 Evolution of Cellular Systems

1
Lecture 13 Evolution of Cellular Systems

• Introduction
• 1st Generation cellular systems
• 2nd Generation cellular systems
• 3rd Generation cellular systems
• Ben Slimane
• slimane_at_kth.se

2
Multiple Access/Multiplexing Methods

• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Frequency-Hop Code Division Multiple Access
  (FH-CDMA)
• Direct Sequence-Code Division Multiple Access
  (DS-CDMA)

3
Cellular System Evolution
4
1G Cellular Systems

• Appeared in late 1970s and deployed in early
  1980s
• All based on analog techniques
• All used FDMA and FM modulation
• Date rate 8-10 kbps
• Low system capacity (reuse N7)
• Large cells with omni-directional base station
  antennas

5
1G NMT

• 1981 Nordic Mobile Telephone
• First generation analog technology
• NMT450 and NMT900
• Free standard ready 1973, 1977
• Network open 1981 in Sweden and Norway
• Based on FDMA
• Channel bandwidth 25/12.5 kHz
• Total number of channels 1999
• Analog traffic channel, digital control channel

6
1G AMPS

• Advanced Mobile Phone System (AMPS)
• Appeared late 1970s,
• First deployed in 1983, US, South America, China,
  and Australia
• Based on FDMA
• Channel bandwidth 30 kHz
• Total number of channels 832 channels

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1G AMPS
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1G versus 2G

• Digital traffic channels first-generation
  systems are almost purely analog
  second-generation systems are digital
• Encryption all second generation systems
  provide encryption to prevent eavesdropping
• Error detection and correction
  second-generation digital traffic allows for
  detection and correction, giving clear voice
  reception
• Channel access second-generation systems allow
  channels to be dynamically shared by a number of
  users

9
2G Cellular Systems

• Deployed in mid 1990s
• 2G cellular systems all use digital voice coding
  and digital modulation
• Can provide advanced call capabilities and a
  better system capacity
• More users per unit bandwidth
• Designed before the widespread of the Internet
• Voice services and limited data services (SMS,
  FAX)
• Data rate on the order 10 kbps

10
2G cellular Systems
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2G GSM

• Global Systems for Mobile Communications (GSM)
• Based on TDMA
• Channel bandwidth
  200 kHz
• Traffic channels (slots) per RF channel 8
• Maximum cell radius (R) 35 km
• Frequency
  900/1800 MHz
• Maximum vehicle speed (Vm) 250
  km/hr
• Maximum coding delay
  approx. 20 ms
• Maximum delay spread (?m) 10 ?s

12
Steps in Design of TDMA Timeslot
13
GSM Frame Format

• Transmission bit rate 156.25/0.577 270.833
  kbps

14
Time Slot Fields

• Trail bits allow synchronization of
  transmissions from mobile units
• Encrypted bits encrypted data (ciphertext bits)
• Training sequence
• A known bit pattern
• used to estimate the multi-path radio channel
• Stealing bit block contains data or stolen for
  control
• Guard bits
• used to avoid overlapping with other bursts
• Speech information
• The actual information data

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GSM Signal Processing
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GSM Network Architecture
17
Mobile Station

• Mobile station communicates across Um interface
  (air interface) with base station transceiver in
  same cell as mobile unit
• Mobile equipment (ME) physical terminal, such
  as a telephone or PCS
• ME includes radio transceiver, digital signal
  processors and subscriber identity module (SIM)
• GSM subscriber units are generic until SIM is
  inserted
• SIMs roam, not necessarily the subscriber devices

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Base Station Subsystem (BSS)

• BSS consists of a base station controller and one
  or more base transceiver stations (BTS)
• Each BTS defines a single cell
• Includes radio antenna, radio transceiver and a
  link to a base station controller (BSC)
• BSC reserves radio frequencies, manages handoff
  of mobile unit from one cell to another within
  BSS, and controls paging

19
Network Subsystem (NS)

• NS provides link between cellular network and
  public switched telecommunications networks
• Controls handoffs between cells in different BSSs
• Authenticates users and validates accounts
• Enables worldwide roaming of mobile users
• The Mobile Switching Center (MSC) is the central
  element of the NS
• The MSC controls four databases

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The MSC Databases

• Home location register (HLR) database stores
  information about each subscriber that belongs to
  it
• Visitor location register (VLR) database
  maintains information about subscribers currently
  physically in the region
• Authentication center database (AuC) used for
  authentication activities, holds encryption keys
• Equipment identity register database (EIR)
  keeps track of the type of equipment that exists
  at the mobile station

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2.5G

• GPRS General Packet Radio Service
• Bitrates from 9.05 to 171.2 kbit/s
• Multiple Time slots allocated to user
• Link adaptations.
• EDGE Enhanced data rates for GSM evolution
• Data rates up to 384 kbit/s
• Two modulation schemes (GMSK, 8PSK)
• Link adaptations

22
Cellular CDMA

• Frequency diversity resolve multi-paths by
  means of the RAKE receiver
• 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

23
Cellular CDMA

• Self-jamming non-orthogonal codes create
  interference between users
• Near-far problem weak users jammed by strong
  users
• Soft handoff smooth handoff from one cell to
  the next
• more complex than hard handoff
• Frequency reuse of 1
• No frequency planning needed (N1)

24
Cellular CDMA

• The RAKE receiver
• Resolves multi-path components and combine them
  coherently
• A diversity gain with order equals to the number
  of resolved paths is obtained
• Soft Handoff
• Mobile station temporarily connected to more
  than one base station simultaneously
• Require more radio resources

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The RAKE Receiver

• Spreading codes with low correlation properties
  allow the separation of the different radio paths
• The RAKE receiver uses this property and locks on
  the different paths

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The RAKE Receiver
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Soft handoff in CDMA

• When a mobile unit is in soft handover
• Two codes are needed on the downlink
• Only one code is needed on the uplink

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Spreading in Cellular CDMA
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2G IS-95
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2G IS-95

• Downlink
• Pilot (0) cell detection, channel estimation
• Synchronization (32) identification information
• Paging (1-7) messages to mobiles
• Traffic (8-31, 33-63) 55 traffic channels with
  data rate of 9600 bps
• A unique channel for each user
• Uplink
• Access channels
• Traffic channels

31
ITUs View of 3G

• Voice quality comparable to the public switched
  telephone network
• 144 kbps data rate available to users in
  high-speed motor vehicles over large areas
• 384 kbps available to pedestrians standing or
  moving slowly over small areas
• Support for 2.048 Mbps for office use
• Symmetrical / asymmetrical data transmission
  rates
• Support for both packet switched and circuit
  switched data services

32
ITUs View of 3G

• An adaptive interface to the Internet to reflect
  efficiently the common asymmetry between inbound
  and outbound traffic
• More efficient use of the available spectrum in
  general
• Support for a wide variety of mobile equipment
• Flexibility to allow the introduction of new
  services and technologies

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Alternative Interfaces
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CDMA Design Considerations

• Bandwidth limit channel usage to 5 MHz
• Chip rate depends on desired data rate, need
  for error control, and bandwidth limitations 3
  Mcps or more is reasonable
• Multi-rate advantage is that the system can
  flexibly support multiple simultaneous
  applications from a given user and can
  efficiently use available capacity by only
  providing the capacity required for each service

35
UMTS

• Wideband CDMA
• Uplink 1920-1980 MHz
• Downlink 2110-2170 MHz
• Bandwidth 4,4-5 MHz
• HSDPA High Speed Downlink Packet Access
• Data rates 1,8, 3,6, 7,2 and 14,4 Mbit/s

36
W-CDMA
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LTE

• Long Term Evolution (LTE)
• Advanced OFDM for downlink
• Single carrier FDMA for uplink
• Data rates exceeding 100 Mbps in the downlink
  with full mobility
• Scalable bandwidth (1.25 to 20 MHz)
• Frequency-reuse 1
• Multiple transmit and receive antennas