SESSION: Wireless Communication Principles

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SESSION Wireless Communication Principles

• Wireless Network Classification
• Transmitters/Receivers
• Antennas
• Frequency Allocation
• Propagation Modes
• Noise characteristics
• Signal Encoding
• Error Detection and Correction

Ahmed Sameh
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(No Transcript)
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Factors in Designing Wireless Networks
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Wireless Frequency Allocation

• Radio frequencies range from 9KHz to 400GHZ (ITU)
• Microwave frequency range
• 1 GHz to 40 GHz
• Directional beams possible
• Suitable for point-to-point transmission
• Used for satellite communications
• Radio frequency range
• 30 MHz to 1 GHz
• Suitable for omnidirectional applications
• Infrared frequency range
• Roughly, 3x1011 to 2x1014 Hz
• Useful in local point-to-point multipoint
  applications within confined areas

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Wireless Radio Spectrum Frequency Allocation
Wavelength
Frequency
Gamma-rays
X-rays
3000 GHz
Infrared
0.1 m
300 GHz
1 mm
THF - terribly high frequency
30GHz
10 mm
EHF - extra high frequency
Micro Waves
3GHz
100 mm
SHF - super high frequency
300 MHz
1m
UHF - ultra high frequency
30 MHz
10 m
VHF - very high frequency
Radio Waves
3MHz
100 m
HF - high frequency
300KHz
1 Km
MF - medium frequency
30Khz
10 Km
LF - low frequency
3KHz
100 Km
VLF - very low frequency
Source Bekkers, R. and Smits, J., Mobile
Telecommunications, Artech, 2000.
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Frequency Regulations

• Frequencies from 9KHz to 300 MHZ in high demand
  (especially VHF 30-300MHZ)
• In wireless, lower frequencies (omnidirectional)
• Two unlicensed bands in the US (counterparts
  elsewhere)
• Industrial, Science, and Medicine (ISM) 2.4 GHz
• Unlicensed National Information Infrastructure
  (UNII) 5.2 GHz
• Regional, national, and international issues
• Procedures for military, emergency, air traffic
  control, etc
• Different agencies license and regulate
• www.fcc.gov - US
• www.open.gov.uk/radiocom -- for UK
• Others (e.g., ETSI, five agencies in Japan)
• Interferences across national borders handled
  through Radio Communications Bureaus

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ITU (International Telecom Union)

• Headquartered in Geneva (next to UN)
• Several sectors
• ITU-R(radiocommunications)- several study groups
  and World Radio Conferences (WCRs)
• ITU-T (standards) - subsummed formerly CCITT
• ITU-D (development) - developing countries

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National Telecommunications and Information
Administration (NTIA - www.ntia.gov)

• NTIA is part of The United States Commerce
  Department
• Maintain a spectrum chart, dated March 1996, that
  depicts the radio frequency spectrum allocations
  to radio services operated within the United
  States.
• Graphically partitions the radio frequency
  spectrum, extending from 9 kHz to 300 GHz, into
  over 450 frequency bands
• Copies of this chart can be viewed on line at
  http//www.ntia.doc.gov/osmhome/allochrt.html
  and printed copies of this chart are available
  from the U.S. Government Printing Office (ph 202
  512 1800 stock 003-000-00652-2 cost is 6.00

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Location Based Services (LBSs)

• Techniques
• Cell-id based
• GPS assisted
• Angle of arrival
• Others

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Wireless Transmission
Antenna
Antenna
Transmitter
Receiver

• Wireless Communication systems consist of
• transmitters
• Antennas radiates electromagnetic energy into
  air
• Receivers
• In some cases, transmitters and receivers are on
  same device, called transceivers (e.g., cellular
  phones)

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Transmitters
Antenna
Amplifier
Mixer
Filter
Amplifier
Transmiter
Oscilator

• Suppose you want to generate a signal that is
  sent at 900 MHz and
• the original source generates a signal at 300
  MHZ.
• Amplifier - strengthens the initial signal
• Oscilator - creates a carrier wave of 600 MHz
• Mixer - combines original signal with oscilator
  and produces 900 MHz
• (does modulation, etc)
• Filter - selects correct frequency (required by
  FCC)
• Amplifier - Strengthens the signal before sending
  it (higher f in some cases)
• Receivers perform similar operations but in
  reverse direction

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Antennas

• An antenna is an electrical conductor or system
  of conductors to send/receive RF signals
• Transmission - radiates electromagnetic energy
  into space
• Reception - collects electromagnetic energy from
  space
• In two-way communication, the same antenna can be
  used for transmission and reception

Directional Antenna (higher frequency)
Omnidirectional Antenna (lower frequency)
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Radiation Patterns

• Radiation pattern
• Graphical representation of radiation properties
  of an antenna
• Depicted as two-dimensional cross section
• Reception pattern
• Receiving antennas equivalent to radiation
  pattern
• Antenna Types
• Isotropic antenna (idealized)
• Radiates power equally in all directions
• Dipole antennas
• Half-wave dipole antenna (or Hertz antenna)
• Quarter-wave vertical antenna (or Marconi
  antenna)
• Parabolic Reflective Antenna (highly focussed,
  directional)

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Smart Antennas

• Basic idea propagate signals to follow objects
  as they move around and minimize noise.
• Mixture of
• Switched beam systems a number of fixed beams at
  an antenna site the beam with least
  interference and best signal strength is chosen.
• Adaptive antennas array of antennas that can
  adjust patterns based on noise, interference, and
  location of objects
• Great deal of activity
• Liberti, J. and Rappaport, T., Smart Antennas
  for Wireless Communications, Prentice Hall

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Smart Antennas
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Terrestrial Microwave (1GHz to 40GHz)

• Description of common microwave antenna
• Most common Parabolic "dish", 3 m in diameter
• Fixed rigidly and focuses a narrow beam
• Achieves line-of-sight transmission to receiving
  antenna (relays used in between)
• Located at substantial heights above ground level
  
• Applications
• Long haul telecommunications service (instead of
  fiber, coax) -- requires less repeaters but line
  of sight
• Short point-to-point links between buildings
  (e.g, closed circuit TV, LANs, bypass local
  telephone companies)
• Most common BW 4GHZ (can give up to 200 Mbps)
• Loss proportional to log (d/w)

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Satellite Microwave (1GHz to 20 GHz, typically)

• Description of communication satellite
• Microwave relay station
• Used to link two or more ground-based microwave
  transmitter/receivers
• Receives transmissions on one frequency band
  (uplink), amplifies or repeats the signal, and
  transmits it on another frequency (downlink)
• Applications
• Television distribution (e.g., PBS uses
  sattelites exclusively)
• Long-distance telephone transmission between
  telephone exchange offices
• Private business networks (lease channels,
  expensive)

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Broadcast Radio (30 MHz to 1GHz)

• Description of broadcast radio antennas
• Omnidirectional (main differentiator from
  microwave)
• Antennas not required to be dish-shaped
• Antennas need not be rigidly mounted to a precise
  alignment
• Applications
• Broadcast radio
• VHF and part of the UHF band 30 MHZ to 1GHz
• Covers FM radio and UHF and VHF television
• Due to new apps, the frequency range is expanded
  frequently
• Infrared
• does not penetrate walls
• used in remote control devices

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Propagation Modes
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LOS Wireless Transmission Impairments

• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
• Thermal noise

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Attenuation

• Strength of signal falls off with distance over
  transmission medium
• Attenuation factors for unguided media
• Received signal must have sufficient strength so
  that circuitry in the receiver can interpret the
  signal
• Signal must maintain a level sufficiently higher
  than noise to be received without error
• Attenuation is greater at higher frequencies,
  causing distortion
• Approach amplifiers that strengthen higher
  frequencies

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Categories of Noise

• Thermal Noise
• Intermodulation noise
• Crosstalk
• Impulse Noise

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Multipath Propagation
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Types of Fading

• Fast fading
• Slow fading
• Flat fading
• Selective fading
• Rayleigh fading
• Rician fading

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Signal Encoding (Modulation)

• Modulation of digital signals
• When only analog transmission facilities are
  available, digital to analog conversion required
• Modulation of analog signals
• A higher frequency may be needed for effective
  transmission
• Modulation permits frequency division
  multiplexing
• PCM and variants used frequently

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Pulse Code Modulation

• Based on the sampling theorem (sample rate should
  be higher than twice highest frequency)
• Each analog sample is assigned a binary code
• Analog samples are referred to as pulse amplitude
  modulation (PAM) samples
• The digital signal consists of block of n bits,
  where each n-bit number is the amplitude of a PCM
  pulse
• 8000 samples per second, 8 bits for levels (256)

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Pulse Code Modulation
Amplitude
Samples
This shows 12 samples, each sample represents the
amplitude of the wave. These samples as sent as
digital data and then reconstructed into the
original signal on the receiving side.
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Delta Modulation

• Analog input is approximated by staircase
  function
• Moves up or down by one quantization level (?) at
  each sampling interval
• The bit stream approximates derivative of analog
  signal (rather than amplitude)
• 1 is generated if function goes up
• 0 otherwise

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Delta Modulation
Analog Signal
Signal Amplitude
Staircase Function
Time
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Multiple Access Techniques
Session4
Session2
Session3
Session3
Session1
Session4
Frequency
Session2
Frequency
Session1
Time
Time
Time Division Multiple Access (TDMA) PCM, PSK
(6 ms frames) Used by ATT wireless, Bellsouth,
Ericson
Frequency Division Multiple Access (FDMA)
All sessions based on a code
Rarely used at present
Frequency
Time
Spread spectrum, Direct Used by Sprint PCS, 3G
systems
Code Division Multiple Access (CDMA)
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FDMA and TDMA

• FDMA
• FM radio divides the spectrum into 30 Khz
  channels.
• FDMA divides 30 Khz channels into 3 (10 KHz each)
• Base station cost is high and very limited
  capacity
• TDMA
• available since 1992
• each subscriber transmits at different times
• 6 millisecond frames, each divided into 1 ms time
  slots
• each time slot has a header and data
• errors may corrupt headers and cause time slots
  and in some cases the whole frame is lost
• TIA standard IS-54 defines the TDMA interface
  between a mobile station and cell-site radio
  (uses PCM for speech encoding, DQPSK for
  modulation)
• Call quality is similar to FDMA but can handle
  more calls (ATT)
• Several extensions of TDMA (can support 15 users
  per voice channel)

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CDMA

• Based on spread spectrum - direct sequencing is
  more prevalent (TIA IS-95)
• Groups of bits from digitized speech are tagged
  with a unique code that is associated with a
  cellular call.
• Several cellular calls are combined and
  transmitted over 1.25MHz and then reassembled on
  the receiver side
• Receiver detects a signal by tuning to correct
  phase position between incoming and locally
  generated signals from code
• Speech coder operates at a variable rate (fully
  when user is talking)
• Adjusts for near-far power adjustments (nearer
  stations generate less powerful signals)
• When powered on, the mobile system knows the CDMA
  frequency, so it tunes to that frequency and
  searches for a pilot signal (pilot signals
  represent base stations)
• Mobile station will pick the strongest pilot and
  register
• When moving from cell to cell, new pilot is
  picked up

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TDMA versus CDMA Controversy

• TDMA and CDMA are accepted TIA (telecom Industry
  Association) standards (IS-54, IS-95)
• Hardware vendors are lobbying hard
• Many, many variants in industry
• Performance reports are conflicting and confusing
  in terms of
• Call clarity CDMA appears to be better but
  questioned
• Network capacity CDMA may be more efficient than
  TDMA
• Privacy CDMA codes provide more privacy
• Economy TDMA allows same equipment for multiple
  users
• Maturity TDMA is very mature (in use since
  1992)
• More features TDMA offers more but CDMA can do
  it also

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Spread Spectrum

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Spread Spectrum

• Input is fed into a channel encoder
• Produces analog signal with narrow bandwidth
• Signal is further modulated using sequence of
  digits
• Spreading code or spreading sequence
• Generated by pseudonoise, or pseudo-random number
  generator
• Effect of modulation is to increase bandwidth of
  signal to be transmitted
• On receiving end, digit sequence is used to
  demodulate the spread spectrum signal
• Signal is fed into a channel decoder to recover
  data

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Frequency Hopping Spread Spectrum (FHSS)

• Signal is broadcast over seemingly random series
  of radio frequencies
• Signal hops from frequency to frequency at fixed
  intervals
• Channel sequence dictated by spreading code
• Receiver, hopping between frequencies in
  synchronization with transmitter, picks up
  message
• Advantages
• Eavesdroppers hear only unintelligible blips
• Attempts to jam signal on one frequency succeed
  only at knocking out a few bits

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Frequency Hopping Spread Spectrum (FHSS)
Energy
Data Bits
4 7 5 1 6 8 3 2
Frequency
f1 f2 f3 f4 f5 f6 f7 f8
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FHSS Using MFSK

• MFSK signal is translated to a new frequency
  every Tc seconds by modulating the MFSK signal
  with the FHSS carrier signal
• Large number of frequencies used
• Results in a system that is quite resistant to
  jamming
• Jammer must jam all frequencies
• With fixed power, this reduces the jamming power
  in any one frequency band

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Direct Sequence Spread Spectrum (DSSS)

• Each bit in original signal is represented by
  multiple bits in the transmitted signal
• Spreading code spreads signal across a wider
  frequency band
• Spread is in direct proportion to number of bits
  used
• One technique combines digital information stream
  with the spreading code bit stream using
  exclusive-OR (Figure 7.6)

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Code-Division Multiple Access (CDMA)

• Basic Principles of CDMA
• D rate of data signal
• Break each bit into k chips
• Chips are a user-specific fixed pattern
• Chip data rate of new channel kD

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CDMA Example

• If k6 and code is a sequence of 1s and -1s
• For a 1 bit, A sends code as chip pattern
• ltc1, c2, c3, c4, c5, c6gt
• For a 0 bit, A sends complement of code
• lt-c1, -c2, -c3, -c4, -c5, -c6gt
• Receiver knows senders code and performs
  electronic decode function
• ltd1, d2, d3, d4, d5, d6gt received chip pattern
• ltc1, c2, c3, c4, c5, c6gt senders code

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CDMA Example

• User A code lt1, 1, 1, 1, 1, 1gt
• To send a 1 bit lt1, 1, 1, 1, 1, 1gt
• To send a 0 bit lt1, 1, 1, 1, 1, 1gt
• User B code lt1, 1, 1, 1, 1, 1gt
• To send a 1 bit lt1, 1, 1, 1, 1, 1gt
• Receiver receiving with As code
• (As code) x (received chip pattern)
• User A 1 bit 6 -gt 1
• User A 0 bit -6 -gt 0
• User B 1 bit 0 -gt unwanted signal ignored

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Coding and Error Control
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Error Control

• Mechanisms to detect and correct transmission
  errors
• Types of errors
• Lost PDU a PDU fails to arrive
• Damaged PDU PDU arrives with errors
• Error detection
• Receiver detects errors and discards PDUs
• Positive acknowledgement
• Destination returns acknowledgment of received,
  error-free PDUs
• Retransmission after timeout
• Source retransmits unacknowledged PDU
• Negative acknowledgement and retransmission
• Destination returns negative acknowledgment to
  PDUs in error

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Coping with Data Transmission Errors

• Error detection codes
• Detects the presence of an error
• Automatic repeat request (ARQ) protocols
• Block of data with error is discarded
• Transmitter retransmits that block of data
• Error correction codes, or forward correction
  codes (FEC)
• Designed to detect and correct errors

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Error Detection Process

• Transmitter
• For a given frame, an error-detecting code (check
  bits) is calculated from data bits
• Check bits are appended to data bits
• Receiver
• Separates incoming frame into data bits and check
  bits
• Calculates check bits from received data bits
• Compares calculated check bits against received
  check bits
• Detected error occurs if mismatch

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Wireless Transmission Errors

• Error detection requires retransmission
• Detection inadequate for wireless applications
• Error rate on wireless link can be high, results
  in a large number of retransmissions
• Long propagation delay compared to transmission
  time
• Best to correct errors by using
• Block Error Correction
• Turbo Codes

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Block Code (Error Correction)

• Hamming distance for 2 n-bit binary sequences,
  the number of different bits
• E.g., v1011011 v2110001 d(v1, v2)3
• For each data block, create a codeword
• Send the codeword
• If the code is invalid, look for data with
  shortest hamming distance (possibly correct code)
  
• Datablock (k2) Codeword (n5)
• 00 00000
• 01 00111
• 10 11001
• 11 11110
• Suppose you receive codeword 00100 (error)
• Closest is 00000 (only one bit different)

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Turbo Codes

• Good block error correction requires large
  codewords
• Large codewords are complex to process and waste
  bandwidth
• Turbo codes break the codewords into two
• Two encoders on transmitters
• Two decoders on receivers
• Shown to be very efficient
• Main limitation decoding is complex and
  introduces delays
• Many applications in deep space communications
• Very active area of work

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Summary

• Wireless Network Classification
• Transmitters/Receivers
• Antennas
• Frequency Allocation
• Propagation Modes
• Noise characteristics
• Signal Encoding
• Error Detection and Correction