COMP9336/4336 Mobile Data Networking …cs9336/basic_concepts.pdf · 2014-08-04 · Spectrum regulation and licensing Many users use the same airspace – Recipe for collision –

© 2013 Mahbub Hassan, UNSW 1

COMP9336/4336 Mobile Data Networking www.cse.unsw.edu.au/~cs9336 or ~cs4336

Basic Concepts


Lecture overview

Mobile networking is different from conventional wired (fixed) networking in many ways. This lecture will cover some of the basic concepts and terminologies of mobile data networking.


Topics to be covered (1)

  Wireless communication medium –  Spectrum, regulation, frequencies –  Power, battery, range –  Interference –  Line-of-sight, propagation, received signal strength –  Free-space propagation model –  Impact of obstacles –  Antenna – omni vs directional



  Wireless interfaces in mobile devices –  Bluetooth –  WiFi –  NFC –  GSM, GPRS, 3G, LTE –  GPS



  Cellular concept –  Cells, towers and cellular coverage –  Frequency reuse –  Cell size and capacity –  Handoff



  Connectivity models –  Device-infrastructure –  Device-to-device (single-hop, multihop)

  Service provider models –  Cellular provider –  Third-party provider


Wireless Communication Medium


Spectrum

  Wireless transmissions use the airwaves –  Airwaves are radio frequencies

  Useful frequencies constitute the Spectrum   Spectrum is ‘virtual’

–  We cannot touch and feel   A gift from nature (the force field)

–  Has been there ever since earth was created   A (limited) natural resource


Spectrum regulation and licensing

  Many users use the same airspace –  Recipe for collision –  No one would get anything useful done

  Spectrum use is highly regulated –  By govt. authorities (eg FCC in the USA)

  Spectrum is often licensed –  By big companies, eg Telstra –  Gives exclusive rights to certain freq. bands –  Interference avoidance by regulation


Spectrum allocation

  Bulk of it reserved for government use –  Scientific exploration –  Public safety –  Military

  Some for commercial services –  TV broadcast –  Mobile phone

  Some for free-to-use –  High-speed wireless local area network (WiFi) –  Cordless phone handsets at home –  Can you name a few more?


Key principles of spectrum allocation

  Maximise spectrum utilisation   Spectrum made available to new

technologies and services –  Adapt to new market needs

  Fair licensing   Promote competition   Ensure spectrum availability for public

safety, health, defence, scientific experiments…


Spectrum use by TV and cellular services

Broadcast TV (54 –806 MHz)

Cellular 1G (806-902MHz)

Cellular 2G (1.8-1.9 GHz)

Cellular 3G (746-764MHz, 776-794 MHz, 1.7-1.8 GHz, 2.5-2.6 GHz)

50MHz

3GHz

1GHz

2GHz

Cellular 4G (700MHz, 1.8GHz, 2.1GHz, 2.3GHz, 2.6GHz)



Free (unregulated) spectrum

  Not subject to license   Has rules for products (eg power limitation)   More frequencies released as license-free   Some current license-free frequencies

–  900 MHz –  2.4 GHz ISM band (WiFi, Microwave etc.) –  5.2/5.3/5.8 GHz (WiFi, Cordless phone etc.)


Power, battery, range

  Higher the transmit power –  Longer the range of communication –  Greater energy consumption (quicker battery

depletionlarger battery needed) –  Higher interference (less system capacity)


Range as a function of transmit power

T R1

R2 R3

P1

P2

P3

Transmitter T has 3 power levels to choose from: P3 > P2 > P1

R1 is closed to transmitter T

R3 is farthest from transmitter T

There are 3 receivers in the vicinity of transmitter T


Example

  Consider the scenario of previous slide   Q1 how many receivers can the transmitter

reach if it transmits at power P3 ?   Q2 what would be the minimum power the

transmitter could use if it intended to reach the majority of the receivers?

  Q3 at what power level the battery of the transmitter will run out the soonest


Solution

  A1 – three   A2 – P2

–  R3 will not hear the transmitter! –  But 2 out of 3 (majority) receivers will

  A3 – P3


Interference

  Signals transmitted by different sources may interfere with each other at the receiver –  If frequencies are close to each other

  Interference may corrupt data –  Reduces achievable data rates


Interference is experienced at the receiver

R T1 T2

Signals from T1 and T2 interferes at R


Line-of-sight, propagation, signal strength

  Transmitter and receiver have line-of-sight (LOS) when they have clear view of each other –  Best reception when there is LOS

  Received signal power is likely to be less than transmitted power

  A propagation (path loss) model is used to estimate received signal power


Free-space propagation model

  Rx power = Tx power x path loss factor   Rx power (Pr) falls off as the square of the tx-rx distance d   The higher the frequency, the more path loss (e.g. FM radio has

a shorter coverage than AM)

= Pr (d) Pt λ2

(4π)2 d2

Pt : transmit power, λ : wavelength of transmitted frequency

π = 3.14……..

for unit antenna gain and system loss


Frequency-wavelength conversion

λ = c/f

c: speed of light 3x108 m/s

f: operating frequency


Example wavelengths

  Higher the frequency, smaller the wavelength   900 MHz has a wavelength of 33.33 cm   2.4 GHz has a wavelength of 12.5 cm   60 Ghz has a wavelength of only 5 mm (this

technology is called millimeter wave)


Power in decibel   Power for practical mobile systems vary by many

orders of magnitude –  100kW or kilowatt (FM radio station) –  500mW or milliwatt (cellular phone tx power) –  2.5mW (Bluetooth with ~10m range) –  100pW or picowatt (typical WiFi rx threshold) –  Femto watt? (nanosensor communication) –  1 W = 103 mW = 106 µW = 109 nW = 1012 pW

  Decibel is a more convenient (logarithmic scale) unit to compare these powers with each other


Conversion to dBm

  dBm is in reference to 1 milliwatt   First, express power in milliwatt   Then apply following formula to obtain dBm

Power in dBm = 10 log (power in milliwatt)


Conversion to dBW

  dBW is in reference to 1 watt   First express power in watt   Then apply following formula to obtain dBW

Power in dBW = 10 log (power in watt)


Relationship between dBm & dBW

  Note that 1 W = 1000 mW   This gives us following relationship

–  Note log(axb) = log(a)+log(b)

dBm = dBW + 30

If you’ve calculated a power in dBW, you can simply derive the equivalent dBm by adding 30 to dBW, or vice versa.


Example (1)

  Express 1 mW power in units of dBm

10 log (1) = 10x0 = 0 dBm

So, zero dBm does not mean there is no power !


Example (2)

  Express 50 W in –  (a) dBW and (b) dBm

P(dBW) = 10 log (50) = 17 dBW

P(dBm) = 10 log (50x1000) dBm

= 10 log (50) + 10 log (1000) dBm

= 17 + 30 = 47 dBm


Example (3)

  If 50W tx power is applied to a 900 MHz frequency, find the rx power in dBm at a distance of 100 meter from tx. Assume free space with unit gains.

Pr (100) = Pt λ2

(4π)2 1002 = 3.5 x 10-3 mW

Pr(dBm) = 10 log (3.5 x 10-3) = -24.5 dBm


Example (4)

  Consider the same setting of the previous question, i.e., 100 meter between Tx and Rx. If you use 60 GHz instead, but still want to receive the same power at the Rx, i.e., -24.5 dBm, what power you need at the transmitter?


Impact of obstacles

  Many different types of obstacles –  Trees, building, hills, metallic body of vehicles

  Different obstacles affect signal differently   Free-space path loss model explained in the previous slides will

not provide accurate results when obstacles exist   Different path-loss models exist to estimate signal strength at

a given location based on the geographic profile of the location (not covered in this course) –  e.g, log-distance, Longley-Rice, Okumura


Antenna

  Transmits or receives radio waves   Some antennas are visible, some are hidden

within the device or not obtrusive


Relationship between Antenna size and frequency

  Antennas are designed to transmit or receive a specific frequency band –  Cannot use a TV antenna for wireless router, or vice-versa

(why?)

  End-to-end antenna length = ½ wavelength –  So that electrons can travel back and forth the

antenna in one cycle   If dipole (two rods), each rod is ¼ wavelength


Antenna – omni vs directional

  Antenna shape and orientation affect the way wireless signals propagate

  Omni-directional antenna –  Signal travels all directions from transmitter –  Signal received from all directions at receiver –  Small in size, low-cost –  Communication range is limited (power is divided in all

directions)   Directional antenna

–  Signal travels in a particular direction of choice –  Bulky, expensive –  Range can be extended with a directional antenna (total

power is focused in one direction)


Wireless Interfaces


Bluetooth

  Short-range communication (few meters)   Device-to-device communication

–  Exchange data between mobile phones –  Connect two handheld game consoles –  Connect microphone to wireless headset (a.k.a. bluetooth

headset)   Reasonable data rates (up to 1Mbps)   Operates at 2.4GHz ISM band

–  ISM – industrial scientific medical –  Can hop among 79 channels each 1MHz channel


WiFi   IEEE 802.11a/g standard   Range: 20-200 meters

–  Power, antenna, and other factors influence range   Bit rate: 2-54 Mbps

–  b: 11 Mbps (2Mbps at poor signal strength) –  g: 54 Mbps

  Mainly for device-to-infrastructure   Usually for data communication

–  But voice is possible (VOIP)   Operates at 2.4GHz or 5GHz (Dual Band)

–  2.4GHz would interfere with microwave, bluetooth, and others ISM band devices

  2.4GHz allows up to 12 (overlapping) channels, each 22 MHz, but 5GHz offers significantly more channels


WiFi-Direct

  A device-to-device technology, which does not require any AP/router

  A different standard from WiFi, which needs an AP

  2.4GHz frequency band (same as WiFi)


Near Field Communication (NFC)

  Builds on RFID (tag) technology (unlicensed 13.56 MHz)

  Device-to-device with a range of 2-3 cm (S-beam in Galaxy S3)

  Maximum data rate of 424 kbps   Previously used in credit cards for

contactless payments (e.g pay pass)   Now available in mobile devices enabling

mobile payment, ticketing, etc. using mobile phone

  Also used to bootstrap WiFi/Bluetooth for ‘heavy’ communication (eliminates WiFi/Bluetooth configuration delay)


GSM

  Global system for mobile communication   Long range: upto 10Km   Mainly for circuit-switched digital voice   Data possible at low rate – about 9Kbps   Short message service (SMS) over GSM

–  Uses control/signaling channels –  Can be used for some sort of data communication

  Operates at 1.8 GHz   Device-to-device not possible

–  Device must connect to tower/infrastructure


GPRS

  General packet radio service   Extension of GSM

–  GSM chipset has GPRS as well –  Same interface has both GSM and GPRS capabilities

  Designed for data communication   Operates at 1.8 GHz   Bit rates: 9-21 Kbps

–  Depending on coding schemes used –  Range reduces with higher data rates


3G   Designed for data communication   Data rates from a few hundred Kbps to a few Mbps

depending on distance to tower and obstacles   Draws more power than 2G   This interface cannot be turned off manually (by the users)

in the mobile devices   But cellular networks employ power management protocols to

dynamically switch this interface in different power modes


4G   Designed for data communication   Data rates from a few Mbps to tens of Mbps depending on

distance to tower and obstacles   Draws more power than 3G   This interface cannot be turned off manually (by the users)

in the mobile devices   But cellular networks employ power management protocols to

dynamically switch this interface in different power modes   Some older smartphones have 3G, but they do not have 4G   Newer phones all come with this interface


GPS

  Global positioning systems   Interface to communicate with GPS satellites   Operates at 1.5 GHz   Used for detecting

–  Position –  Speed –  Direction of movement –  Universal clock

  High power consumption (why?)   Turned on only when needed


The Cellular Concept


Cells

  Divide a large geographical region (service area) into many ‘small’ wireless coverage cells

  No matter where you are within the service area, you are always in a cell

  A radio tower is erected for each cell to provide wireless coverage to users in the cell

  All radio towers are connected to cellular service providers ‘main system’


Frequency reuse

  Adjacent towers cannot use same frequencies because of interference

  But ‘distant’ towers can! –  Towers separated by one or more cells

  Allows reuse of same set of frequencies –  Over many towers throughout the service area –  Can serve more customers per frequency


Example of frequency reuse   Assume 3 frequencies to be reused

–  Red, green, and blue   Adjacent cells cannot use same frequencies


Handoff

  Change of cell, change of tower   If user moves from one cell to another during an

ongoing service, a transparent tower reassignment is required

  User is handed off from one tower to another   (frequency) Handoff may be necessary if user

needs to be switched to a new frequency –  User may stay in the same cell


Connectivity Models


Device-to-infrastructure

  Device always connect to a ‘tower’ –  Tower: access point, base station, satellite, …

  Device requires high power if tower is far –  Satellite phones require lots of power!

  Single wireless hop to connect to network   1G,2G,3G and 4G must operate in this mode


Device-to-Device

  Also known as adhoc, mobile-to-mobile, or peer-to-peer

  Two devices can talk to each other without needing access to infrastructure –  example: Nintendo DS multiplayer games (using

bluetooth)   Multihop is possible

–  A mobile may get to tower via another mobile!   1-3G do not support this mode (4G/LTE does)


Service Provider Model


Cellular provider

  Owns cellular infrastructure   Mobile telephony --- the primary service   But can provide a host of new services

–  SMS –  Internet –  Navigation


Third-party provider

  No need to own cellular infrastructure   No need to offer mobile telephony   Builds on top of cellular service provided by

someone else   Can deliver a host of value added services

–  Tracking (dogs, cats, childs, vehicles, …) –  Advanced navigation based on current traffic

conditions –  Location-based services

COMP9336/4336 Mobile Data Networking …cs9336/basic_concepts.pdf · 2014-08-04 · Spectrum regulation and licensing Many users use the same airspace – Recipe for collision –

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