ESE680: Wireless Sensor Networks

1
ESE680 Wireless Sensor Networks Special Topics
in Embedded Systems Physical Layer - Part
II Lecture 6
Prof. Rahul Mangharam
2
Administrivia

• Lab 1
• Due on Today (before lecture)
• Lab 2
• Out on Today
• Due Thursday, Feb 19 (before lecture)

3
Outline of Previous Lecture

• Completion of Lecture on MAC Protocols
• Communication Schemes
• Propagation Modes
• Fading
• Encoding Schemes
• Spread Spectrum

4
Outline of Todays Lecture

• Physical Layer for Wireless Communications
• 802.11, Bluetooth and Zigbee
• Fundamentals of RF communications
• Basic concepts
• Modulation techniques
• IEEE 802.15 Standard
• Overview
• Physical Layer of IEEE 802.15.4

5
Network Communication Protocols

• Network communication protocols differ from net
  to net and determine who does what, when and how.
  
• Communication protocols are based on
  communication layers

6
OSI Model

• International Standards Organizations Reference
  Model for Open Systems Interconnection
• Determines the protocol interface, not the actual
  implementation
• Physical Layer
• The actual data is communicated via this layer
• Data arrives, interrupt is generated, data is
  accepted
• Data Link Layer
• Responsible for detecting and correcting
  transmission errors (EDC/ECC) and flow control
• e.g. synchronize the speeds of sending and
  receiving
• Network Layer
• Takes a message, breaks it into packets (for
  packet-switching networks)
• Takes care of congestion control (whether sender
  should slow down) and routing (which link to put
  the packet into)
• Each node must have at least these layers, so
  that they can function as a store and forward
  node (i.e. take a packet, and send it to the next
  hop towards the next destination)

7
OSI Model

• The bottom layers perform the communication of
  data
• Transport Layer
• Acts as the interface between the application
  programs and the underlying physical network
• Network independent software (device drives) are
  implemented at this layer
• May be used as a source of additional info
  (reliability, efficiency, etc.)
• Session Layer
• Establishes the connection with the remote host
• Responsible for sending and receiving the initial
  messages in order to establish that the channel
  is open and operational
• Presentation Layer
• After session establishment, messages are taken
  from the application and given to this layer,
  which performs transformations on the data
  to/from the application
• (e.g. MPEG compression/decompression, encryption
  for MIME messages, etc)

8
What does the Physical Layer do?

• Code, transmit, receive and decode frames
• Activate and deactivate radio transceiver
• Energy detection (ED) within current channel
• Link quality indicator (LQI) for received packets
  
• Channel selection
• Clear Channel Assessment (CCA) for CSMA-CA

9
Physical Layers of Wireless Communications

• WiFi (IEEE 802.11)
• Bluetooth (IEEE 802.15)
• Zigbee (IEEE 802.15.4)
• RFID

10
IEEE 802 - Wireless Standards
11
Wireless Network Applications
12
IEEE 802.11

• Designed for high data-rate networks
• Internet/LAN connectivity
• Entertainment systems
• Streaming video
• High power
• Device power device lifetime 23 hrs (laptops)
• Wired power supplied in most cases

13
IEEE 802.11

• Direct-sequence spread spectrum
• Operating in 2.4 GHz ISM band
• Data-rates of 1 and 2 Mbps
• Frequency-hopping spread spectrum
• Operating in 2.4 GHz ISM band
• Data rates of 1 and 2 Mbps
• Infrared
• 1 and 2 Mbps
• Wavelength between 850 and 950 nm

14
IEEE 802.11

• IEEE 802.11a
• 5-GHz band
• Provides rates of 6, 9, 12, 18, 24, 36, 48, 54
  Mbps
• Uses orthogonal frequency division multiplexing
  (OFDM)
• Sub-carrier modulated using BPSK, QPSK, 16-QAM or
  64-QAM
• IEEE 802.11b
• Provides data rates of 5.5 and 11 Mbps
• Complementary Code Keying (CCK) modulation scheme
  

15
IEEE 802.11

• IEEE 802.11a
• 40 mW
• 30 m range
• IEEE 802.11b
• 100 mW
• 100 m range

16
Bluetooth

• Designed for medium data-rate networks
• Computer cable replacement
• Gaming controllers
• Phone microphones
• Mobile device file transfer
• Fairly low power
• Device power lifetime is measured in weeks to
  months

17
Bluetooth

• Unlicensed 2.4 GHz radio band
• ISM band
• Used by microwave ovens, 802.11
• Fast Frequency Hopping
• 1600 hops per second
• 79 channels
• 1 MHz spacing
• 200 µs switching time

18
Bluetooth - Power Range

• Basic 10m range(0 dBm radio)
• Extended 100m range(20 dBm)
• Power classes
• Class 1
• Maximum output power 100mW (20dBm)
• Minimum output power 1mW (0dBm)
• Class 2
• Maximum output power 2.5mW (4dBm)
• Minimum output power 0.25mW (-6dBm)
• Class 3
• Maximum output power 1mW (0dBm)

19
Zigbee

• Designed for low data-rate systems
• Lighting
• Heating/Cooling
• Appliances
• Extremely Low Power
• Devices have lifetime measured in years

20
Zigbee

• 2.4 GHz Band
• 16-ary O-QPSK
• Sixteen 5 MHz channels
• Data-rate up to 250 Kbps
• 868/915 MHz Band
• BPSK
• 868 MHz European ISM band
• One 2 MHz channel
• 20 kbps
• 915 MHz North American ISM band
• Ten 2 MHz channels
• 40 kbps
• DSSS
• Chooses from 16 nearly orthogonal PN sequences

21
Zigbee - Power Range

• Scalable transmit power to meet range
  requirements
• Low power
• 1 mW transmit power
• 1020m range
• High power
• 60 mW transmit power
• 100m range

22
RFID

• Radio Frequency IDentification (RFID)
• A system that transmits the identity (in the form
  of a unique serial number) of an object
  wirelessly, using radio waves.
• RFID System
• RFID tag or transponder
• Antenna
• Wireless transducer
• Encapsulating material
• RFID reader or transceiver
• Antenna
• Transceiver
• Decoder
• Data processing subsystem

RFID Tag used in Walmart http//en.wikipedia.org/
wiki/RFID
23
RFID Frequency

• Frequency range
• Low-Frequency (LF)
• LF 125 -134.2 kHz
• LF 140 -148.5 kHz
• High-Frequency (HF)
• HF 13.56 MHz
• Ultra-High-Frequency (UHF)
• UHF 868 MHz -928 MHz

24
Signals

• A time-varying event
• Signals can be periodic or aperiodic
• A signal can be decomposed into a combination of
  pure tones, called sine waves, at different
  frequencies
• The different sine waves that compose a signal
  can be plotted as a function of frequency to
  produce a graph called the frequency spectrum of
  a signal.

25
Signal Bandwidth (1 of 3)

• Frequencies in the audio spectrum can be heard by
  the human ear. The ear hears by detecting very
  small changes in air pressure.
• The frequencies ranging from about 20 Hz to
  20,000 Hz are in the audio, or sound spectrum.
• Telephone speech cover the frequency range from
  about 300 Hz to 3000 Hz. The bandwidth is about
  3,000 Hz.

26
Signal Bandwidth (2 of 3)

• Bandwidth is defined as the frequency band around
  the carrier frequency containing 99 percent of
  the signal power
• Amplitude Modulation (AM) is the variation of the
  amplitude of a radio wave as to carry
  information. The bandwidth for an individual AM
  station is about 10,000 Hz
• Frequency Modulation (FM) is the variation of the
  frequency of a radio wave as to carry
  information. The bandwidth for an individual FM
  station is about 200,000 Hz
• The signal broadcast over the air by a television
  station has a bandwidth of about 6 MHz.

27
Signal Bandwidth (3 of 3)

• Analog systems
• Most signals occupy a range of frequencies.
• This frequency range is called the signal
  bandwidth
• Frequency range can be defined by the amount of
  power contained (say 50, gt 90 or even gt 99
  of the signal power
• Digital systems
• Rate at which symbols can be transmitted

28
Channel Bandwidth

• Amplitude Modulation (AM) is transmitted within a
  band of frequencies from 550 KHz to 1,600 KHz.
• Telephone carriers multiplex 12 telephone
  channels on a single cable. Each channel requires
  about 4 KHz bandwidth. The band of frequencies
  for telephone carriers is from 60 to 108 KHz
• The standard frequency assignments for television
  stations is as follows channels 2 through 13 are
  in the very high frequency (VHF) bands and
  channels 14 through 83 occupy the ultra high
  frequency (UHF) bands

29
Channel Capacity Noise-free

• In this case, the limitation on data rate is
  simply the bandwidth of the signal
• If bandwidth is B, then the highest binary signal
  rate that can be transmitted is 2B bps
• When multi-level signaling is used, the channel
  capacity becomes

Channel capacity is the tightest upper bound on
the amount of information that can be reliably
transmitted over a communications channel.
30
Shannons Capacity Theorem

• States the theoretical maximum rate at which an
  error-free bit can be transmitted over a noisy
  channel
• C the channel capacity in bits per second
• B the bandwidth in hertz
• SNR the ratio of signal power to noise power
• Channel capacity depends on channel bandwidth and
  system SNR

31
Shannons Theorem Example

• For SNR of 0, 10, 20, 30 dB, one can achieve C/B
  of 1, 3.46, 6.66, 9.97 bps/Hz, respectively
• Example
• Consider the operation of a modem on an ordinary
  telephone line. The SNR is usually about 1000.
  The bandwidth is 3.4 KHz. Therefore
• C 3400 X log2(1 1000)
• (3400)(9.97)
• 34 kbps

32
Bit Error Rate

• BER Errors / Total number of bits
• Error means the reception of a 1 when a 0 was
  transmitted or vice versa.
• Noise is the main factor of BER performance
  signal path loss, circuit noise,

33
Thermal Noise

• Thermal Noise
• white noise since it contains the same level of
  power at all frequencies
• kTB, where
• k is the Boltzmanns constant 1.381e-21 W / K /
  Hz,
• T is the absolute temperature in Kelvin, and
• B is the bandwidth.
• At room temperature, T 290K, the thermal noise
  power spectral density,
• kT 4.005e-21 W/Hz or
• 174 dBm/Hz

34
Receiver
Receive signal at RF frequencies and signal
processing at low frequencies

• Analog section
• signal conditioning, frequency conversion,
  analog-to-digital conversion
• Digital section
• synchronization, frequency correction, decoding

35
Receiver Sensitivity

• The minimum input signal power needed at receiver
  input to provide adequate SNR at receiver output
  to do data demodulation
• SNR depends on
• Received signal power
• Background thermal noise at antenna (Na)
• Noise added by the receiver (Nr)
• Pmin SNRmin (Na Nr)

36
Noise Figure

• Noise Figure (F) quantifies the increase in noise
  caused by the noise source in the receiver
  relative to input noise
• F SNRinput/SNRoutput (Na Nr)/Na
• Pmin SNRmin(Na Nr) SNRminF Na
• Example if SNRmin 10 dB, F 4 dB, BW 1 MHz
  Pmin 10 4 -174 10log(106) -100 dBm

37
Required Receiver Sensitivity

• Transmit power FCC regulation
• Path loss
• Receiver sensitivity govern by standards and
  applications
• Required SNR depends on BER requirement and
  modulation scheme
• Noise floor thermal noise or transmitter noise
  limited

38
802.15.4 Receiver Noise Figure Calculation

• Channel Noise bandwidth is 1.5 MHz
• Transmit Power is 1mW or 0 dBm
• Thermal noise floor is 174 dBm/Hz X 1.5 MHz
  112 dBm
• Total SNR budget is 0 dBm (112 dBm) 112 dBm
• To cover 100 ft. at 2.4 GHz results in a path
  loss of 40 dB
• i.e. Receiver sensitivity is 85 dBm
• Required SNR for QPSK is 12.5 dB
• 802.15.4 packet length is 1Kb
• Worst packet loss lt 1, (1 BER)1024 1 1, BER
  105
• Receiver noise figure requirement
• NF Transmit Power Path Loss Required SNR
  Noise floor
• 0 112 40 12.5 59.5 dB
• The design spec is very relaxed
• Low transmit power enables CMOS single chip
  solution Low cost and low power!

39
Modulation Techniques

• Purpose of Modulation?
• By modulating the signal onto a much higher
  carrier frequency, the communication systems can
  use a much smaller antenna.
• Also, more robust communication, multiplexing,
  etc.
• Modulation Types
• Analog modulation AM, FM, PM
• Digital modulation BPSK, QPSK, BFSK,MSK

40
Analog Modulation - Amplitude Modulation (AM)

• Amplitude of carrier is modulated by the baseband
  signal (XBB)

41
Analog Modulation FM

• Baseband signal is applied to control voltage of
  VCO

42
Analog Modulation -Pulse Modulation

• Derivative of baseband signal is applied to
  control voltage of VCO

43
Digital Modulation

• Advantages over analog modulation
• Better noise immunity
• Easier multiplexing of various forms of
  information (data, video..)
• Source coding, security encryption, error
  correction
• Performance Metrics
• Power efficiency
• A measure of how much signal power should be used
  to achieve a particular BER for a given
  modulation scheme
• Signal energy per bit / noise power spectral
  density Eb / N0
• Bandwidth efficiency
• Ability to accommodate data within a limited
  bandwidth
• Throughput data rate per hertz bps per Hz
• Wireless Sensor

44
Digital Modulation Techniques

• Amplitude Shift Keying (ASK) similar to AM
• On-off keying
• Frequency Shift Keying (FSK) similar to FM
• Binary frequency shift keying (BFSK)
• 4-level frequency shift keying (4-FSK)
• Phase Shift Keying (PSK) similar to PM
• Binary phase shift keying (BPSK)
• Quadrature phase shift keying (QPSK, OQPSK,
  p/4-QPSK)
• 8-level phase shift keying (8-PSK)
• 16-level phase shift keying (16-PSK)
• Quadrature Amplitude Modulation (QAM)
• 16-QAM
• 64-QAM

45
Amplitude Shift Keying (ASK)

• Advantages
• Very simple modulation and demodulation
• Disadvantages
• High sensitivity to noise
• Low bandwidth efficiency

46
Frequency Shift Keying (FSK)

• BFSK carrier frequency is shifted between f1 and
  f2
• 4-FSK carrier frequency is shifted on f1, f2, f3
  and f4.
• Better bandwidth efficiency than ASK
• Simple modulation and demodulation

47
Binary Phase Shift Keying (BPSK)

• Use alternative sine wave phase to encode bits
• Simple implement, low bandwidth efficiency
• Very robust, used extensively in satellite
  communications

48
Quadrature Phase Shift Keying

• Multi-level modulation technique 2 bits per
  symbol
• 2 x bandwidth efficiency of BPSK

49
QPSK Modulator
50
QPSK Time Domain Waveform
51
IEEE 802.15.4 Standard

• A standard for Low-Rate Wireless Personal Area
  Network (LR-WPAN).
• Provide a standard with ultra-low complexity,
  cost, and power for low-data-rate wireless
  connectivity among inexpensive fixed, portable,
  and moving devices.
• The standard only defines the Physical layer
  (PHY) and Medium Access Layer (MAC). The
  higher-layer protocols are left to industry and
  the individual applications.
• The Zigbee Alliance is an association of
  companies involved with building higher-layer
  standards based on IEEE 802.15.4.
• Network layer
• Application support layer
• Marketing

52
Approach for Low Power

• Reduce the amount of data transmitted
• Reduce the transceiver duty cycle and frequency
  of data transmissions
• Reduce the frame overhead
• Reduce complexity
• Reduce range
• Implement strict power management mechanisms
  (power-down and sleep modes)

53
Low-Rate WPAN Architecture
54
Wireless Sensor Network
Sensor Applications
WSN
55
802.15.4 Operating Band
56
802.15.4 - Modulation Scheme

• 2.4 GHz PHY
• 250 kb/s (4 bits/symbol, 62.5 kBaud)
• Data modulation is 16-ary orthogonal O-QPSK
• 16 symbols are orthogonal set of 32-chip PN
  codes
• 868 MHz/915 MHz PHY
• Symbol rate
• 868 MHz band 20 kbps (1bit/symbol, 20 Kbaud)
• 915 MHz band 40 kbps (1bit/symbol, 40 Kbaud)
• Spreading code is 15-chip
• Data modulation is BPSK
• 868 MHz 300 Kchips/s
• 915 MHz 600 Kchips/s

57
802.15.4 - Coding

• Symbol is transmitted as a series of chips
• Symbol (k bits) ?n chips ?waveform
• Ensure that there are clear edges, not too far
  apart so that receiver can stay synchronized with
  transmitter
• Differentiate valid symbol from noise

58
802.15.4 - PHY Communication Parameters

• Transmit power
• Capable of at least 0.5 mW
• Transmit center frequency tolerance
• 40 ppm
• Receiver sensitivity (packet error rate lt 1)
• -85 dBm _at_ 2.4 GHz band
• -92 dBm _at_ 868/915 MHz band
• Receiver Selectivity
• 2.4 GHz 5 MHz channel spacing, 0 dB adjacent
  channel requirement
• Channel Selectivity and Blocking
• 915 MHz and 2.4 GHz band 0 dB rejection of
  interference from adjacent channel
• 30 dB rejection of interference from alternate
  channel
• Rx Signal Strength Indication Measurements
• Packet strength indication
• Clear channel assessment
• Dynamic channel selection

59
802.15.4 - SNR Requirement

• For QPSK used in 802.15.4 to achieve a BER of
  10-5, a SNR of 12.5 dB is needed.
• For 64-QAM used in 802.11a to achieve the same
  BER, a SNR of 27 dB is needed!
• The hardware requirement for 802.15.4 is much
  more relaxed and therefore can operate at much
  lower power

60
Path Loss

• Relationship between received power and
  transmitted power

)
61
802.15.4 Measurement Data
62
802.15.4 Receiver Noise Figure Calculation

• Channel Noise bandwidth is 1.5 MHz
• Transmit Power is 1mW or 0 dBm
• Thermal noise floor is 174 dBm/Hz X 1.5 MHz
  112 dBm
• Total SNR budget is 0 dBm (112 dBm) 112 dBm
• To cover 100 ft. at 2.4 GHz results in a path
  loss of 40 dB
• i.e. Receiver sensitivity is 85 dBm
• Required SNR for QPSK is 12.5 dB
• 802.15.4 packet length is 1Kb
• Worst packet loss lt 1, (1 BER)1024 1 1, BER
  105
• Receiver noise figure requirement
• NF Transmit Power Path Loss Required SNR
  Noise floor
• 0 112 40 12.5 59.5 dB
• The design spec is very relaxed
• Low transmit power enables CMOS single chip
  solution Low cost and low power!

63
802.15.4 PHY Hardware Implementation
64
802.15.4 Packet Structure

• PHY Packet Fields
• Preamble (32 bits) synchronization
• All set to binary zero, allows sufficient number
  of bits to achieve synchronization
• Start of Packet Delimiter (8 bits)
• 0xe6, allows receiver to establish the
  beginning of the packet in the stream of bits
• PHY Header (8 bits)
• MSB reserved, remaining bits used to designate
  frame length information
• PSDU (0 to 1016 bits) data field

65
Summary of Todays Lecture

• Physical Layer for Wireless Communications
• 802.11, Bluetooth and Zigbee
• Fundamentals of RF communications (contd)
• Basic concepts
• Modulation techniques
• 802.15 Standard
• Overview
• Physical Layer of 802.15.4

66
Detailed Info
67
Duplexing - Principles

• FDD (Frequency Division Duplexing )
• Uses One Frequency for the DownLink, and a Second
  Frequency for the UpLink.
• TDD (Time Division Duplexing) Uses the same
  timeslot for the Downlink and the Uplink.
• In any configuration the access method is
  OFDMA/TDMA

68
OFDMA-TDMA Principles

• Using OFDMA/TDMA, Sub Channels are allocated in
  the Frequency Domain, and OFDM Symbols allocated
  in the Time Domain

69
Downlink OFDMA Symbol
70
PHY Services

• The PHY layer provides an interface between the
  physical radio channel and the MAC sublayer
  through the use of two services
• PHY data service
• Accessed by PHY layer data service access point
  (PD-SAP)
• PHY management service
• Accessed by PHY layer management entity service
  access point (PLME-SAP)

71
PHY Data Service (PD-DATA)
72
PHY Management Service Primitives
73
Receiver NF Calculation Example 802.11a

• FCC limits the PSD in 5 GHz to 2.5 mW/MHz
• Channel bandwidth is 16 MHz
• Transmit Power is 40 mW or 16 dBm
• Thermal noise floor is 174 dBm/Hz X 16 MHz
  102 dBm
• Total SNR budget is 16 dBm (102 dBm) 118 dBm
• To cover 300 ft. at 5 GHz results in a path loss
  of 86 dB
• i.e. Receiver sensitivity is 70 dBm (802.11a
  specification is 65 dBm)
• Required SNR for 64 QAM (54Mbps) is 27 dB
• 802.11a packet length is 8 kb
• Worst packet loss lt 10, (1 BER)8000 1 10,
  BER 105
• Receiver noise figure requirement
• NF Transmit Power Path Loss Required SNR
  Noise floor
• 16 102 86 27 5 dB