Cellular Architecture and Basics

1
Cellular Architecture and Basics

• Session 2
• Nilesh Jha

2
Mobile Communications Space

• Subscribers
• Terminal Price, Size/Wt., Service Price and Model
  (Minutes/Free), Range of Services (call forward,
  email, etc), Roaming charges
• Performance dropped calls, Voice quality,
  Outage areas and coverage, Battery life,
  Availability sp. during busy hours
• Operating companies
• Performance and Cost Infrastructure technology
  and cost, capacity, technology spectral
  efficiency cell location and sizes, deployment
  schedules, technology and deployment evolution,
  Impact of data/IP
• Business bottom line ARPU (revenues/user),
  penetration and market share, Quality of service
  (total), churn (lost customers -- can be
  30/year, costs 400 to recover), Financing

3
Mobile Communications Space

• Equipment and Technology Providers
• Technology, Selling existing infrastructure and
  terminals, owning the interface to the
  terrestrial PSTN switches, Developing and Selling
  new infrastructure and terminals, Financing,
  Impact of data, Impact of IP, New innovative
  competitors

4
Cellular Network Organization

• Use multiple transmitters
• Areas divided into cells
• Each served by its own antenna
• Served by base station consisting of antennas,
  transmitters, receivers, and control unit
• Band of frequencies allocated to carrier, carrier
  allocates portions to each base station

5
Cellular System Overview
6
Wireless Infrastructure
From iec.org
7
Cellular Systems Elements

• Base Station (BS) includes an antenna, a
  controller, and a number of receivers
• Made up of Base Transceiver Station (BTS) and
  Base Station Controller (BSC) --- often one BSC
  controls multiple BTSs
• Mobile telecommunications switching office (MTSO)
  connects calls between mobile units --Also
  called Mobile Switching Center (MSC)
• Two types of channels available between mobile
  unit and BS
• Control channels used to exchange information
  having to do with setting up and maintaining
  calls
• Traffic channels carry voice or data connection
  between users

8
Cellular Architecture and Mobility

• Cellular Architectures Include
• Mobiles
• Base Stations
• In Adjacent Cells
• Switching Centers MSCs
• Interfaces to the PSTN
• With Networked MSCs
• A Key Factor is Mobility
• Requires Handoff as User Moves from Cell to Cell
• Requires Databases to Track User Mobility -- Used
  by MSCs for Mobility Management

Other MSCs
Figure adapted from IEC cellular and wireless
tutorials. Iec.org
9
Steps in an MSC Controlled Call between Mobile
Users

• Mobile unit initialization (power on)
• Mobile signals Base Station its there, Base
  Station and MSCs update location registers
• Mobile-originated call (push send button)
• Base Station and MSC accept route call
• Paging -- to called user via his Base Station
• Or call routed to PSTN phone
• Call accepted by called user
• Ongoing call (traffic channels with controls)
• Handoff as mobile unit moves to adjacent cell

10
System Operations

• Information Transport
• Air Interface Defines the Protocol -- Frequency
  Band(s), Uplink/Downlink, Multiple Access Scheme,
  Modulation/ Demodulation and Coding/Decoding,
  Bandwidths and Data Rates
• Call Management
• Call setup, release, maintenance
• Uses control channel signaling, backend I/F to
  PSTN
• Mobility Management
• Registers/tracks user location, handoff, handles
  roaming
• Authentication/Encryption
• Authentication in network, encryption options
• Radio Resource Management
• Manages power, assigns channels, supports other
  functions

11
How can a wireless system (e.g. cellular) have
users in the same geographic area not interfere
with one another?
Multiple Access
Answer Multiple Access Technique
Wireless System
U
U
U
U
U
U
U Users Throughout Area
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Multiple Access

• N channels, shared by M users
• Multiple Access technique defines the N channels
  and assigns them to M users
• One example is frequency channels, each assigned
  to a TV station (nearly) permanently -- the M
  users are the M TV stations and a license gives
  each a channel
• Here NM --- in fact some UHF channels can
  remain unlicensed, unassigned
• In cellular the M users are usually many more
  than the N channels -- users ask for channels and
  later complete phone calls so the assignments are
  dynamically changing

3
.
N
2
1
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Multiple Access Techniques

• Multiple Access is the scheme used to allow
  multiple users to operate transceivers without
  interfering with each other
• It defines physical channel separations based on
  some physical signal characteristic
• Common schemes
• FDMA -- based on frequency separation
• TDMA -- based on time separation
• CDMA -- based on code separation
• SDMA -- based on spatial separation

14
Frequency Division Multiple Access --- FDMA
15
Time Division Multiple Access -- TDMA
16
Multiple Access Schemes
From Goodman
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Multiple Access Efficiency

• The M.A. technologies driven by critical
  features efficiencies
• FDMA is oldest, used most with analog modulations
• Used in analog AMPS cellular systems, power
  hungry
• In AMPS provides 2.25 calls/MHz/cell or 56
  calls/cell
• TDMA was next, used in 2G cellular/PCS, digital
  phones
• Digital modulations and technology, less power
  hungry
• In US TDMA does 7 calls/MHz/cell or 210
  calls/cell
• CDMA introduced for 2G cellular/PCS, also digital
• Spread spectrum modulations and codes, also less
  power hungry
• IP/chipsets initially Qualcomm controlled
• Does about 15-30 calls/MHz/user (soft limit), or
  450-900 calls/cell
• Broadband version is 3G, coming slowly, first in
  Europe, Japan
• Also used in WLAN (802.11), no Qualcomm IP

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Channels

• Physical Channels
• The physical resource unit assigned to carry
  information
• eg, a wire, a frequency and bandwidth (for FDMA,
  like for radio and TV channels), a time slot (for
  TDMA), a code (for CDMA) ---- defined at layer 1
  of the ISO stack
• Logical Channels
• Defined by the nature of the information they
  carry and the format ---- defined at layer 2 of
  the ISO stack
• eg, paging channel, or sync channel, or pilot
  channel, or traffic channel
• Circuits often used interchangeably with
  channels or links

19
Air Interface Layers
From Goodman
20
Links

• Multiple use term
• Refers to point to point electromagnetic path
  between transmitter and receiver
• Also refers to the physical connection
  established between transmitter and receiver
• Forward or Downlink refers to Base Station to
  user link
• Reverse or Uplink refers to user to Base Station
  link

21
Channel Types

• Logical channels are mapped to physical channels
  by assigning specific physical parameters for
  transmission
• Channels can be traffic channels (carries user
  data) or control channels (controls the
  communications link)
• Traffic channels typically want be be as high a
  data rate as possible so the link is used
  effectively to transfer the user data
• Control channels typically want to be as low a
  data rate as possible so this overhead does not
  waste channel capacity
• In cellular control channels are used to make
  calls, synchronize the different radios and
  clocks for transmissions, handoff as you move to
  another cell, and hang-up
• Traffic channels carry the voice

22
Mobile Radio Propagation Effects -- The
Challenge of Wireless

• Signal strength
• Must be strong enough between base station and
  mobile unit to maintain signal quality at the
  receiver
• Must not be so strong as to create too much
  cochannel interference with channels in another
  cell using the same frequency band
• Distance based signal attenuation -- often not
  line of sight (ie, worse than free space)
• Fading and ISI
• Signal propagation effects such as multipath
  cause fading of signal strength and some
  distortion
• Fast and slow fading
• Slow fading by interposed physical structures as
  the user moves in and out of sight ---- requires
  a margin for shadowing to be used in planning
  needed power
• Fast fading due to interference from a number of
  multi-path signals adding and subtracting
  together.
• Also causes signal shape distortion and (ISI)
  inter-symbol interference
• Requires significant signal processing to
  mitigate effect

23
Traffic Engineering

• Ideally, available channels would equal number of
  subscribers active at one time
• In practice, not feasible to have capacity to
  handle all possible loads
• For N simultaneous user capacity and L
  subscribers
• L lt N non-blocking system
• L gt N blocking system

24
Blocking System Performance Questions

• Probability that call request is blocked?
• What capacity is needed to achieve a certain
  upper bound on probability of blocking?
• What is the average delay?
• What capacity is needed to achieve a certain
  average delay?

25
Traffic Intensity in Erlangs

• Load presented to a system, defined in Erlangs
• ? mean rate of calls attempted per unit time
• h mean holding time per successful call, call
  duration
• A average number of calls arriving during
  average holding period
• Example 1 One call lasting 10 minutes made every
  hour is a traffic load of (1/hour)(one sixth of
  an hour)1/6 Erlangs
• Example 2 a channel used for twenty minutes
  during an hour is passing .33 Erlangs of traffic

26
Factors that Determine the Nature of the Traffic
Model

• Manner in which blocked calls are handled
• Lost calls delayed (LCD) blocked calls put in a
  queue awaiting a free channel
• Blocked calls rejected and dropped
• Lost calls cleared (LCC) user waits before
  another attempt
• Lost calls held (LCH) user repeatedly attempts
  calling
• Number of traffic sources
• Whether number of users is assumed to be finite
  or infinite

27
Probability of Blocking
For Erlang type B, LCC and infinite sources Note
this is what is normally used in traffic analysis
C number of channels needed Atotal traffic
offered in Erlangs
28
Source Rappaport p.49
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Blocking Example

• Traffic Intensity per user
• 2 calls/hour, 3 minutes each
• Au .1 Erlangs (traffic intensity per user)
• If 20 channels available, how many users can be
  supported with blocking probability of .5
• AUxAu so UA/Au -- A is total Traffic Intensity
• From figure, A11
• So U110

30
Issues in Wireless -- 1

• Operating Companies --- Critical
• ARPURevenues/user gtgtgt services
• Quality of Service (Total) voice quality/BER,
  dropped calls, calls blocked
• Spectrum Owned or Trying to Get/Buy
• Deployment schedules and coverage
• Churn of customers lost --- expensive

31
Issues in Wireless -- 2

• Operating Companies --- Other
• Infrastructure Technology -- Multiple Access
  Technique, Cost
• Evolution from One to the Next One
• 2G to 2.5G to 3G
• Capacity and Coverage
• Spectral Efficiency -- Spectrum is Expensive
• Calls/MHz/sq.km or Calls/MHz/cell

32
Issues in Wireless -- 3

• Users
• Terminal size/weight, price, features, looks
• Battery life
• Service price and model (flat, weekend, long
  distance, roaming)
• Quality of Service (as in Issues 1) and Coverage
• Anytime, anywhere --- Including busy hour calls,
  busy intersections, out in boonies

33
Prevalent Cellular/PCS Protocols

• Protocols require (inter)national standardization
• Air Interface and Backbone or Core Network
• AMPS --- analog, 50 MHz in 824-895 MHz in US, FM,
  FDMA, 30 KHz channels
• European band is 50 MHz in 890-960 MHz, used for
  GSM and AMPS
• US TDMA (IS-54/136) --- digital, AMPS bandsPCS
  band, 30 KHz carries 3 TDMA channels, DQPSK,
  multiple 30 KHz carriers (so, called FDMA/TDMA
  also)
• CDMA (IS-95) --- digital, AMPS bandsPCS band,
  1.25 MHz carrier uses CDMA channelization, PSK
• PCS/DCS bands --- 1850-1910 and 1930-1990 MHz
  (PCS) in US, 1710-1785 and 1805-1885 MHz (DCS) in
  Europe
• GSM --- digital, in 890-960 MHz band DCS band,
  TDMA, GMSK, 200 KHz carries 8 TDMA channels,
  multiple 200 KHz carriers (so, called FDMA/TDMA
  also)
• 3G --- Two prevalent forms, WCDMA and CDMA2000,
  both CDMA but first also TDMA, 5 MHz carriers
  uses CDMA for channelization in WCDMA and CDMA200
  3x, CDMA2000 1X is like CDMA above. 3-4 other
  standards incl. TDMA versions.
• Backend networks fall into two types
  GSM/European, and IS-41 in US