Cellular Wireless Networks

1
Cellular Wireless Networks

• ICS 620
• Fall 2003
• Week 9

2
Importance of Wireless

• Freedom of movement
• No loss of connectivity
• Increase in productivity

3
Cellular Network Organization

• Use multiple low-power transmitters (100 W or
  less)
• Areas divided into cells
• Each served by its own antenna
• Served by base station consisting of transmitter,
  receiver, and control unit
• Band of frequencies allocated
• Cells set up such that antennas of all neighbors
  are equidistant (hexagonal pattern)

4
Cellular Spectrum
Phone Transmit
825
835
845
824
846.5
849
A band 10 MHz 333 channels 30khz
B band 10 MHz 333 channels 30khz
A band
A band
B band
1 MHz 33 chan
1.5 MHz 50 chan
2.5 MHz 83 chan
20 MHz Guard
Base Transmit
870
880
890
869
891.5
894
A band 10 MHz 333 channels 30khz
B band 10 MHz 333 channels 30khz
A band
A band
B band
1.5 MHz 50 chan
2.5 MHz 83 chan
1 MHz 33 chan
5
Frequency Reuse

• Adjacent cells assigned different frequencies to
  avoid interference or crosstalk
• Objective is to reuse frequency in nearby cells
• 10 to 50 frequencies assigned to each cell
• Transmission power controlled to limit power at
  that frequency escaping to adjacent cells
• The issue is to determine how many cells must
  intervene between two cells using the same
  frequency

6
Approaches to Cope with Increasing Capacity

• Adding new channels
• Frequency borrowing frequencies are taken from
  adjacent cells by congested cells
• Cell splitting cells in areas of high usage can
  be split into smaller cells
• Cell sectoring cells are divided into a number
  of wedge-shaped sectors, each with their own set
  of channels
• Microcells antennas move to buildings, hills,
  and lamp posts

7
Cellular System Overview
8
Cellular Systems Terms

• Base Station (BS) includes an antenna, a
  controller, and a number of receivers
• Mobile telecommunications switching office (MTSO)
  connects calls between mobile units
• 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

9
Steps in an MTSO Controlled Call between Mobile
Users

• Mobile unit initialization
• Mobile-originated call
• Paging
• Call accepted
• Ongoing call
• Handoff

10
Additional Functions in an MTSO Controlled Call

• Call blocking
• Call termination
• Call drop
• Calls to/from fixed and remote mobile subscriber

11
Mobile Radio Propagation Effects

• 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
  co-channel interference with channels in another
  cell using the same frequency band
• Fading
• Signal propagation effects may disrupt the signal
  and cause errors

12
Handoff Performance Metrics

• Cell blocking probability probability of a new
  call being blocked
• Call dropping probability probability that a
  call is terminated due to a handoff
• Call completion probability probability that an
  admitted call is not dropped before it terminates
• Probability of unsuccessful handoff probability
  that a handoff is executed while the reception
  conditions are inadequate

13
Handoff Performance Metrics

• Handoff blocking probability probability that a
  handoff cannot be successfully completed
• Handoff probability probability that a handoff
  occurs before call termination
• Rate of handoff number of handoffs per unit
  time
• Interruption duration duration of time during a
  handoff in which a mobile is not connected to
  either base station
• Handoff delay distance the mobile moves from
  the point at which the handoff should occur to
  the point at which it does occur

14
Handoff Strategies Used to Determine Instant of
Handoff

• Relative signal strength
• Relative signal strength with threshold
• Relative signal strength with hysteresis
• Relative signal strength with hysteresis and
  threshold
• Prediction techniques

15
Power Control

• Design issues making it desirable to include
  dynamic power control in a cellular system
• Received power must be sufficiently above the
  background noise for effective communication
• Desirable to minimize power in the transmitted
  signal from the mobile
• Reduce co-channel interference, alleviate health
  concerns, save battery power
• In SS systems using CDMA, its desirable to
  equalize the received power level from all mobile
  units at the BS

16
Types of Power Control

• Open-loop power control
• Depends solely on mobile unit
• No feedback from BS
• Not as accurate as closed-loop, but can react
  quicker to fluctuations in signal strength
• Closed-loop power control
• Adjusts signal strength in reverse channel based
  on metric of performance
• BS makes power adjustment decision and
  communicates to mobile on control channel

17
Traffic Engineering

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

18
First-Generation Analog

• Advanced Mobile Phone Service (AMPS)
• In North America, two 25-MHz bands allocated to
  AMPS
• One for transmission from base to mobile unit
• One for transmission from mobile unit to base
• Each band split in two to encourage competition
• Frequency reuse exploited

19
Frequency Division Multiple Access

• Definition - FDMA is a multiple access method in
  which users are assigned specific frequency
  bands. The user has sole right of using the
  frequency band for the entire call duration.
  (Qualcomm, 1997)

20
FDMA

• Frequency Division Multiple Access

Chan D
Chan C
Frequency
Chan B
Chan A
Time
21
AMPS Operation

• Subscriber initiates call by keying in phone
  number and presses send key
• MTSO verifies number and authorizes user
• MTSO issues message to users cell phone
  indicating send and receive traffic channels
• MTSO sends ringing signal to called party
• Party answers MTSO establishes circuit and
  initiates billing information
• Either party hangs up MTSO releases circuit,
  frees channels, completes billing

22
Differences Between First and Second Generation
Systems

• 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

23
Time Division Multiple Access

• Definition - TDMA is an assigned frequency band
  shared among a few users. However, each user is
  allowed to transmit in predetermined time slots.
  Hence, channelization of users in the same band
  is achieved through separation in time.
  (Qualcomm, 1997)

24
TDMA

• Time Division Multiple Access

Chan B
Frequency
Chan A
Time
25
Mobile Wireless TDMA Design Considerations

• Number of logical channels (number of time slots
  in TDMA frame) 8
• Maximum cell radius (R) 35 km
• Frequency region around 900 MHz
• Maximum vehicle speed (Vm)250 km/hr
• Maximum coding delay approx. 20 ms
• Maximum delay spread (?m) 10 ?s
• Bandwidth Not to exceed 200 kHz (25 kHz per
  channel)

26
GSM Network Architecture
27
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

28
Base Station Subsystem (BSS)

• BSS consists of 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

29
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
• Central element of NS is the mobile switching
  center (MSC)

30
Mobile Switching Center (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

31
TDMA Format Time Slot Fields

• Trail bits allow synchronization of
  transmissions from mobile units
• Encrypted bits encrypted data
• Stealing bit - indicates whether block contains
  data or is "stolen"
• Training sequence used to adapt parameters of
  receiver to the current path propagation
  characteristics
• Strongest signal selected in case of multipath
  propagation
• Guard bits used to avoid overlapping with other
  bursts

32
GSM Speech Signal Processing
33
GSM Signaling Protocol Architecture
34
Functions Provided by Protocols

• Protocols above the link layer of the GSM
  signaling protocol architecture provide specific
  functions
• Radio resource management
• Mobility management
• Connection management
• Mobile application part (MAP)
• BTS management

35
Code Division Multiple Access

• Definition - CDMA is a method in which users
  occupy the same time and frequency allocations,
  and are channelized by unique assigned codes. The
  signals are separated at the receiver by using a
  correlator that accepts only signal energy from
  the desired channel. Undesired signals contribute
  only to noise. (Qualcomm, 1997)

36
CDMA

• Code Division Multiple Access

Code
Frequency
Time
37
Capacity

• CDMA has the ability to deliver 10 to 20 times
  the capacity as FDMA for the same bandwidth. CDMA
  also has a capacity advantage over TDMA by 5 to 7
  times.

38
Advantages of CDMA 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

39
Drawbacks of CDMA Cellular

• 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

40
Mobile Wireless CDMA Design Considerations

• RAKE receiver when multiple versions of a
  signal arrive more than one chip interval apart,
  RAKE receiver attempts to recover signals from
  multiple paths and combine them
• This method achieves better performance than
  simply recovering dominant signal and treating
  remaining signals as noise
• Soft Handoff mobile station temporarily
  connected to more than one base station
  simultaneously

41
Principle of RAKE Receiver
42
Types of Channels Supported by Forward Link

• Pilot (channel 0) - allows the mobile unit to
  acquire timing information, provides phase
  reference and provides means for signal strength
  comparison
• Synchronization (channel 32) - used by mobile
  station to obtain identification information
  about cellular system
• Paging (channels 1 to 7) - contain messages for
  one or more mobile stations
• Traffic (channels 8 to 31 and 33 to 63) the
  forward channel supports 55 traffic channels

43
Forward Traffic Channel Processing Steps

• Speech is encoded at a rate of 8550 bps
• Additional bits added for error detection
• Data transmitted in 2-ms blocks with forward
  error correction provided by a convolutional
  encoder
• Data interleaved in blocks to reduce effects of
  errors
• Data bits are scrambled, serving as a privacy mask

44
Forward Traffic Channel Processing Steps (cont.)

• Power control information inserted into traffic
  channel
• DS-SS function spreads the 19.2 kbps to a rate of
  1.2288 Mbps using one row of 64 x 64 Walsh matrix
• Digital bit stream modulated onto the carrier
  using QPSK modulation scheme

45
ITUs View of Third-Generation Capabilities

• 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

46
ITUs View of Third-Generation Capabilities

• 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

47
Alternative Interfaces
48
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
• Multirate 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

49
Paging SMS

• Evolution of Paging
• Tone Boy, early 1960s
• Tone-Voice, late 1960s
• Digital Pagers, 1970s
• Numeric Paging Systems
• Alpha/Numeric Paging Systems

50
Paging

• Larger coverage area in each site
• Signal, Numeric, Alpha-numeric
• Marketed by coverage area.
• Features--Web messaging, modem messaging

51
Paging

• Current Applications
• Fax Forwarding
• E-Mail Forwarding
• Voice Mail Notification
• Automated Problem Notification
• Two-way Paging

52
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53
Wireless Local Loop

• Wired technologies responding to need for
  reliable, high-speed access by residential,
  business, and government subscribers
• ISDN, xDSL, cable modems
• Increasing interest shown in competing wireless
  technologies for subscriber access
• Wireless local loop (WLL)
• Narrowband offers a replacement for existing
  telephony services
• Broadband provides high-speed two-way voice and
  data service

54
WLL Configuration
55
Advantages of WLL over Wired Approach

• Cost wireless systems are less expensive due to
  cost of cable installation thats avoided
• Installation time WLL systems can be installed
  in a small fraction of the time required for a
  new wired system
• Selective installation radio units installed
  for subscribers who want service at a given time
• With a wired system, cable is laid out in
  anticipation of serving every subscriber in a
  given area

56
Propagation Considerations for WLL

• Most high-speed WLL schemes use millimeter wave
  frequencies (10 GHz to about 300 GHz)
• There are wide unused frequency bands available
  above 25 GHz
• At these high frequencies, wide channel
  bandwidths can be used, providing high data rates
• Small size transceivers and adaptive antenna
  arrays can be used

57
Propagation Considerations for WLL

• Millimeter wave systems have some undesirable
  propagation characteristics
• Free space loss increases with the square of the
  frequency losses are much higher in millimeter
  wave range
• Above 10 GHz, attenuation effects due to rainfall
  and atmospheric or gaseous absorption are large
• Multipath losses can be quite high

58
Fresnel Zone

• How much space around direct path between
  transmitter and receiver should be clear of
  obstacles?
• Objects within a series of concentric circles
  around the line of sight between transceivers
  have constructive/destructive effects on
  communication
• For point along the direct path, radius of first
  Fresnel zone
• S distance from transmitter
• D distance from receiver

59
Atmospheric Absorption

• Radio waves at frequencies above 10 GHz are
  subject to molecular absorption
• Peak of water vapor absorption at 22 GHz
• Peak of oxygen absorption near 60 GHz
• Favorable windows for communication
• From 28 GHz to 42 GHz
• From 75 GHz to 95 GHz

60
Effect of Rain

• Attenuation due to rain
• Presence of raindrops can severely degrade the
  reliability and performance of communication
  links
• The effect of rain depends on drop shape, drop
  size, rain rate, and frequency
• Estimated attenuation due to rain
• A attenuation (dB/km)
• R rain rate (mm/hr)
• a and b depend on drop sizes and frequency

61
Effects of Vegetation

• Trees near subscriber sites can lead to multipath
  fading
• Multipath effects from the tree canopy are
  diffraction and scattering
• Measurements in orchards found considerable
  attenuation values when the foliage is within 60
  of the first Fresnel zone
• Multipath effects highly variable due to wind

62
Multipoint Distribution Service (MDS)

• Multichannel multipoint distribution service
  (MMDS)
• Also referred to as wireless cable
• Used mainly by residential subscribers and small
  businesses
• Local multipoint distribution service (LMDS)
• Appeals to larger companies with greater
  bandwidth demands

63
Advantages of MMDS

• MMDS signals have larger wavelengths and can
  travel farther without losing significant power
• Equipment at lower frequencies is less expensive
• MMDS signals don't get blocked as easily by
  objects and are less susceptible to rain
  absorption

64
Advantages of LMDS

• Relatively high data rates
• Capable of providing video, telephony, and data
• Relatively low cost in comparison with cable
  alternatives

65
802.16 Standards Development

• Use wireless links with microwave or millimeter
  wave radios
• Use licensed spectrum
• Are metropolitan in scale
• Provide public network service to fee-paying
  customers
• Use point-to-multipoint architecture with
  stationary rooftop or tower-mounted antennas

66
802.16 Standards Development

• Provide efficient transport of heterogeneous
  traffic supporting quality of service (QoS)
• Use wireless links with microwave or millimeter
  wave radios
• Are capable of broadband transmissions (2 Mbps)

67
Protocol Architecture

• Physical and transmission layer functions
• Encoding/decoding of signals
• Preamble generation/removal
• Bit transmission/reception
• Medium access control layer functions
• On transmission, assemble data into a frame with
  address and error detection fields
• On reception, disassemble frame, and perform
  address recognition and error detection
• Govern access to the wireless transmission medium

68
Protocol Architecture

• Convergence layer functions
• Encapsulate PDU framing of upper layers into
  native 802.16 MAC/PHY frames
• Map upper layers addresses into 802.16 addresses
• Translate upper layer QoS parameters into native
  802.16 MAC format
• Adapt time dependencies of upper layer traffic
  into equivalent MAC service

69
IEEE 802.16.1 Services

• Digital audio/video multicast
• Digital telephony
• ATM
• Internet protocol
• Bridged LAN
• Back-haul
• Frame relay

70
IEEE 802.16.3 Services

• Voice transport
• Data transport
• Bridged LAN

71
IEEE 802.16.1 Frame Format
72
IEEE 802.16.1 Frame Format

• Header - protocol control information
• Downlink header used by the base station
• Uplink header used by the subscriber to convey
  bandwidth management needs to base station
• Bandwidth request header used by subscriber to
  request additional bandwidth
• Payload either higher-level data or a MAC
  control message
• CRC error-detecting code

73
MAC Management Messages

• Uplink and downlink channel descriptor
• Uplink and downlink access definition
• Ranging request and response
• Registration request, response and acknowledge
• Privacy key management request and response
• Dynamic service addition request, response and
  acknowledge

74
MAC Management Messages

• Dynamic service change request, response, and
  acknowledge
• Dynamic service deletion request and response
• Multicast polling assignment request and response
• Downlink data grant type request
• ARQ acknowledgment

75
Physical Layer Upstream Transmission

• Uses a DAMA-TDMA technique
• Error correction uses Reed-Solomon code
• Modulation scheme based on QPSK

76
Physical Layer Downstream Transmission

• Continuous downstream mode
• For continuous transmission stream (audio, video)
• Simple TDM scheme is used for channel access
• Duplexing technique is frequency division duplex
  (FDD)
• Burst downstream mode
• Targets burst transmission stream (IP-based
  traffic)
• DAMA-TDMA scheme is used for channel access
• Duplexing techniques are FDD with adaptive
  modulation, frequency shift division duplexing
  (FSDD), time division duplexing (TDD)

77
Wireless LAN Technology
78
Wireless LAN Applications

• LAN Extension
• Cross-building interconnect
• Nomadic Access
• Ad hoc networking

79
LAN Extension

• Wireless LAN linked into a wired LAN on same
  premises
• Wired LAN
• Backbone
• Support servers and stationary workstations
• Wireless LAN
• Stations in large open areas
• Manufacturing plants, stock exchange trading
  floors, and warehouses

80
Multiple-cell Wireless LAN
81
Cross-Building Interconnect

• Connect LANs in nearby buildings
• Wired or wireless LANs
• Point-to-point wireless link is used
• Devices connected are typically bridges or routers

82
Nomadic Access

• Wireless link between LAN hub and mobile data
  terminal equipped with antenna
• Laptop computer or notepad computer
• Uses
• Transfer data from portable computer to office
  server
• Extended environment such as campus

83
Ad Hoc Networking

• Temporary peer-to-peer network set up to meet
  immediate need
• Example
• Group of employees with laptops convene for a
  meeting employees link computers in a temporary
  network for duration of meeting

84
Wireless LAN Requirements

• Throughput
• Number of nodes
• Connection to backbone LAN
• Service area
• Battery power consumption
• Transmission robustness and security
• Collocated network operation
• License-free operation
• Handoff/roaming
• Dynamic configuration

85
Wireless LAN Categories

• Infrared (IR) LANs
• Spread spectrum LANs
• Narrowband microwave

86
Strengths of Infrared Over Microwave Radio

• Spectrum for infrared virtually unlimited
• Possibility of high data rates
• Infrared spectrum unregulated
• Equipment inexpensive and simple
• Reflected by light-colored objects
• Ceiling reflection for entire room coverage
• Doesnt penetrate walls
• More easily secured against eavesdropping
• Less interference between different rooms

87
Drawbacks of Infrared Medium

• Indoor environments experience infrared
  background radiation
• Sunlight and indoor lighting
• Ambient radiation appears as noise in an infrared
  receiver
• Transmitters of higher power required
• Limited by concerns of eye safety and excessive
  power consumption
• Limits range

88
IR Data Transmission Techniques

• Directed Beam Infrared
• Ominidirectional
• Diffused

89
Directed Beam Infrared

• Used to create point-to-point links
• Range depends on emitted power and degree of
  focusing
• Focused IR data link can have range of kilometers
• Cross-building interconnect between bridges or
  routers

90
Ominidirectional

• Single base station within line of sight of all
  other stations on LAN
• Station typically mounted on ceiling
• Base station acts as a multiport repeater
• Ceiling transmitter broadcasts signal received by
  IR transceivers
• IR transceivers transmit with directional beam
  aimed at ceiling base unit

91
Diffused

• All IR transmitters focused and aimed at a point
  on diffusely reflecting ceiling
• IR radiation strikes ceiling
• Reradiated omnidirectionally
• Picked up by all receivers

92
Spread Spectrum LAN Configuration

• Multiple-cell arrangement (Figure 13.2)
• Within a cell, either peer-to-peer or hub
• Peer-to-peer topology
• No hub
• Access controlled with MAC algorithm
• CSMA
• Appropriate for ad hoc LANs

93
Spread Spectrum LAN Configuration

• Hub topology
• Mounted on the ceiling and connected to backbone
• May control access
• May act as multiport repeater
• Automatic handoff of mobile stations
• Stations in cell either
• Transmit to / receive from hub only
• Broadcast using omnidirectional antenna

94
Narrowband Microwave LANs

• Use of a microwave radio frequency band for
  signal transmission
• Relatively narrow bandwidth
• Licensed
• Unlicensed

95
Licensed Narrowband RF

• Licensed within specific geographic areas to
  avoid potential interference
• Motorola - 600 licenses in 18-GHz range
• Covers all metropolitan areas
• Can assure that independent LANs in nearby
  locations dont interfere
• Encrypted transmissions prevent eavesdropping

96
Unlicensed Narrowband RF

• RadioLAN introduced narrowband wireless LAN in
  1995
• Uses unlicensed ISM spectrum
• Used at low power (0.5 watts or less)
• Operates at 10 Mbps in the 5.8-GHz band
• Range 50 m to 100 m