ECGR 6120 Wireless Communications

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ECGR 6120 Wireless Communications
NetworkingLecture I

• Ivan Howitt

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Class 1 Outline

• Syllabus
• Q A Form
• Wireless Overview

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Communications A Systems Overview

• Analog Digital Systems
• Destination signal should approximate source
  signal to within a specified degree of fidelity
  (analog) or accuracy (digital)
• Source Destination may require transducers

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Communications A Systems Overview

• Distortion linear nonlinear. Linear can be
  thought of as passing the transmitted signal
  through linear filter
• Interference Other signals in the same
  frequency band
• Noise Thermal noise, i.e. white noise, caused
  by random motion in charged particles.

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Communications A Systems Overview

• Fundamental limitation of channel capacity
  measured in bits / second given by
• Where B is the channel band width, S is the
  average power of the transmitted signal and N is
  the average power in the noise

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Communications A Systems Overview

• Communication System Design Goals
• Received signal should be within desired
  tolerance of transmitted signal
• System design should be capable of tolerating
  anticipated levels of noise, interference
  distortion
• Minimize required channel bandwidth system cost

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Communications A Systems Overview

• Modulation Methods
• Involves two waveforms
• Modulating signal (i.e. information signal)
• Carrier waveform (selected based on application)
• Continuous - Wave Modulation
• AM - amplitude modulation
• FM - frequency modulation
• Pulse Modulation
• PAM - pulse amplitude modulation

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Communications A Systems Overview

• Why Modulate the Information Signal ?
• Match Transmission medium
• E.g. for wireless transmission antenna length
  needs to be gt 1/10 the signals wavelength
• For 100 Hz implies 300 km antenna, Impractical
  !!!
• For 100 MHz implies 0.3 m antenna, Practical
• Hardware considerations
• Minimize hardware complexity and cost,bandwidth
  / carrier frequency (0.01 to 0.1)
• Multiple Access

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Why Wireless ?

• Information Location
• Libraries vs. WWW
• Files vs. Databases
• Information Timeliness
• Industrial
• Mobile Workforce
• Efficiency Implementation
• Commercial
• Mobile Society
• Efficiency Implementation
• Untethered vs. Cost vs. Time

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Applications Telephony

• Terrestrial (Cellular/PCS/SMR)
• Expediential growth starting 30 years after
  introduction (1946)
• gt600 Million Subscribers Worldwide(2001)
• Satellite (Iridium/Global Star / etc.)
• 1998 Introduction (mobile services)
• Universal Coverage
• Multi - Mode
• Paging
• Low Cost vs. Flexibility
• Future ???

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Applications

• Wireless WAN/LAN/PAN
• Inventory Control
• Keyless Entry
• Electrical Power Distribution Automation
• Building Automation
• Manufacturing Automation
• Cordless Telephones
• Space Exploration/Underwater Communications

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Wireless Issues

• Information (Source Signal(s))
• Real time (audio / visual / fidelity)
• Data (impact due to latency / quantity /
  sensitivity to error)
• Location Range (Channel)
• Medium (air / water / vacuum )
• Environment ( buildings / hills / weather / dust
  / etc. )
• Distances (within meters / building / city /
  nation / planet / etc )
• Time Variations (mobile users / weather / new
  buildings / etc)
• Service Type
• Single User
• Multiple User
• One way or Two way (Simplex or Duplex)

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Focus Cellular Systems

• Applications
• Telephony
• Wireless Local Area Networks
• Wireless Local Loops
• Issues
• Signal - real time speech
• Quality - Error rates up to 1 to 5
• Coverage - Region (country / world)
• Expediential growth users
• Channel - many impairments

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Cellular Concept

• FCC licenses services in specific bands
• Cellular band - AMPS
• 25 MHz bandwidth
• 832 channels
• 30 kHz bandwidth channel
• Full Duplex
• 2 service providers per region
• 416 channels/provider

US Frequency Allocation for Commercial Use
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Cellular Concept

• Issue
• Large service regions
• Exponential user growth
• Finite (small ) voice channels
• Trunking
• Basestations have a pool of voice channels
• Channels assigned relinquished
• Frequency Reuse

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Signal Representation - Bandwidth
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Signal Representation - Digital Representation

• Quantization - represent amplitude with fixed
  bits
• Fidelity - 8 bits

• Time Sampling 2 x Highest Freq
• Speech 2 x 4kHz 8ksps

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Signal Representation - Digital

• Speech
• Sampling rate 8 ksamples/s
• Bits 8 bits/sample
• Required data rate8bps x 8 ksps 64kbps
• Bandwidth Data Rate
• Speech Digital Representation Bandwidth 64kbps
• Alternative Voice Encoders - VOCODER 8kbps
  (down to 4kbps)

• Remember AMPS
• Analog Standard
• Each channel 30 kHz
• 1 User/channel
• Using digital techniques
• 3 User/channel
• 3 fold increase in users
• US Digital StandardIS-136
• CDMA Cellular - IS-95

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Multiple Access Techniques
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Direct Sequence Spread Spectrum
bit
chip
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Wireless Channel Impairments

• Delay Spread
• Angle Spread

• Doppler Spread
• Cochannel Interference (CCI)

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Co-Channel Interference
Problem - Interference from user on same frequency
Possible solution directional antennas
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Multipath

• Multiple signal paths typical
• No line of site typical
• Use multipath to advantage, i.e, use signal
  processing to achieve reinforced signal

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Doppler Shift

• Shifts signal in frequency
• Relative motion
• Signal source (or reflected source) Receiver
• Motion in environment
• Multipath
• Different Doppler on each component

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Future

• Voice / Data / Video
• Integrated Services
• Universal handset
• Software Defined Radio
• Cognitive Radio
• FCC Spectral Reuse
• Security
• Ethics

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