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On Radio Technology

On Radio Technology. EE206A (Spring 2002): Lecture #3. Mani Srivastava UCLA - EE Department mbs@ee.ucla.edu. Readings for this Lecture. MANDATORY

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On Radio Technology

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  1. On Radio Technology EE206A (Spring 2002): Lecture #3 Mani SrivastavaUCLA - EE Departmentmbs@ee.ucla.edu

  2. Readings for this Lecture • MANDATORY • Eugene Shih, Seong-Hwan Cho, Nathan Ickes, Rex Min, Amit Sinha, Alice Wang, and Anantha Chandrakasan, “Physical Layer Driven Algorithm and Protocol Design for Energy-Efficient Wireless Sensor Networks,” Proceedings of MOBICOM 2001, Rome, Italy, July 2001.http://www-mtl.mit.edu/research/icsystems/uamps/pubs/eugene_mobicom01.html • Curt Schurgers and Mani Srivastava, “Channel State Aware Protocol Framework for Loaded Wireless Multicarrier Systems,” Technical Report, UCLA, 2002.http://nesl.ee.ucla.edu/pw/ee206a/Schurgers02.pdf • RECOMMENED • Evans, J.G.; Shober, R.A.; Wilkus, S.A.; Wright, G.A. A low-cost radio for an electronic price label system. Bell Labs Technical Journal, vol.1, (no.2), Lucent Technologies, Autumn 1996. p.203-15.http://www.lucent.com/minds/techjournal/autumn_96/pdf/paper14.pdf • Richley, R.A.; Butcher, L. Wireless communications using near field coupling. US Patent 5,437,057, 1995. Search athttp://www.uspto.gov/patft/index.html • Zimmerman, T.G. Personal area networks: near-field intrabody communication. IBM Systems Journal, vol.35, (no.3-4), IBM, 1996. p.609-17.http://www.research.ibm.com/journal/sj/384/zimmerman.html

  3. SourceDecoder SourceCoder SourceDecoder SourceCoder Digital Radio Link antenna MultipleAccess ChannelCoder PowerAmplifier Multiplex Modulator Source Carrier fc transmitted symbol stream Radio Channel Radio Technologyand Trends received (corrupted)symbol stream MultipleAccess ChannelDecoder Demodulator& Equalizer RFFilter Demultiplex Destination antenna Carrier fc

  4. Things You Did Not Want to Know About Digital Communications!

  5. How is Information Communicated? 0 1 0 1 1 1 0 0 1 0 1 0 Information V, I Electrical waveform Electro-magnetic waveform

  6. Digital Modulation & Demodulation • Modulation: maps sequence of “digital symbols” (groups of n bits) to sequence of “analog symbols” (signal waveforms of length TS) • Demodulation: maps sequence of “corrupted analog symbols” to sequence “digital symbols” - e.g. maximum likelihood decision

  7. Grouping the Information Bits into Symbols • If M   the ‘performance’ goes up, but at a cost of complexity (Shannon limit) b bits/symbol = M possible waveforms 1 bit/symbol 0 1 11 10 01 00 2 bits/symbol

  8. Signal Space Representation • The basic idea is that we can transmit information in parallel over a set of orthogonal waveforms with respect to the symbol interval T. The inverse of this interval is called the symbol rate: Rs = 1/T.

  9. s1(t) Sample at t = T s2(t) Detection of the Symbols • Correlation or matched filter detector (basically equivalent)

  10. Commonly Used Modulation Techniques • Coherent or Synchronous Detection • process received signal with a local carrier of the same frequency and phase • e.g. phase shift keying, frequency shift keying, amplitude shift keying, continuous phase modulation • Noncoherent or Envelope Detection • requires no reference wave • e.g. FSK, differential PSK, CPM, ASK

  11. a2 . s1 a2 . s2 a1 . s1 a1 . s2 Amplitude Scaling • Instead of sending only s1, s2, s3 … sL etc. combine these with a set of possible scaling factors a1, a2, a3 … aK s1(t) Sample at t = T X s2(t) Y

  12. s2 s2 s1 s1 s2 s2 s1 s1 Information Mapping Examples M = 4 M = 2 Send s1, s2, both or none of them. Send either s1 or s2. M = 8 M = 4 Send any of these combinations. Send s1 or s2.

  13. f2 f1 Some Elementary Schemes FSK (Frequency Shift Keying) Baseband PAM (Pulse Amplitude Modulation) s1 Passband PAM (Pulse Amplitude Modulation) f1

  14. g(t) Q (ai, bi) I time T 0 Sinusoidal Waveforms Quadrature (Q) In-phase (I) s2 s1

  15. q(t) time T 0 cos(2.fc.t) ai ai . q(t) ai(t) = ai . g(t) si(t) D/A D/A g(t) bi bi(t) = bi . g(t) bi . q(t) -sin(2.fc.t) LPF LPF time T 0 Transmitter Structure

  16. Frequency Domain T 1/T time frequency Baseband BW (bandwidth) fc Passband BW (bandwidth)

  17. ni cos(2.fc.t) ai(t) si(t) bi(t) LPF LPF -sin(2.fc.t) Modulation and Demodulation Modulation Demodulation 2.cos(2.fc.t) a(t) ri(t) b(t) -2.sin(2.fc.t) In flat fading channel

  18. Q ai + j·bi ri i I Alternative Interpretation

  19. QAM and PSK QAM (Quadrature Amplitude Modulation) 64-QAM 16-QAM 4-QAM PSK (Phase Shift Keying) 8-PSK 16-PSK 4-PSK

  20. Example • QAM: Each symbol is represented by a tuple of amplitude and phase • FSK: Each symbol is represented by a frequency separated by twice the data bandwidth

  21. Constellation Diagrams

  22. Symbol Error SER 100 101 000 001 111 011 110 010 SNR • The demodulator chooses the symbol that is closest to the received one (maximum likelihood decoding) • If the noise (and distortions) is such that we are closer to another symbol than the correct one, a symbol error occurs. • Each symbol error results in a number of bit errors. By carefully choosing the mapping from bits to symbols (Gray encoding), one symbol error typically results in just one bit error.

  23. Selecting a Modulation Scheme • Provides low bit error rates (BER) at low signal-to-noise ratios (SNR) • Occupies minimal bandwidth • Performs well in multipath fading • Performs well in time varying channels (symbol timing jitter) • Low carrier-to-cochannel interference ratio • Low out of band radiation • Low cost and easy to implement • Constant or near-constant “envelope” • constant: only phase is modulated • may use efficient non-linear amplifiers • non-constant: phase and amplitude modulated • may need inefficient linear amplifiers No perfect modulation scheme - a matter of trade-offs!Two metrics: energy efficiency Eb/N0 for a certain BERand bandwidth efficiency R/B

  24. Parameters and Metrics to Evaluate Modulation Schemes • Bit rate: Rb = 1/Tb • Symbol rate: Rs = (Tb.log2M)-1 • Occupied bandwidth: W • E.g. 99% of signal energy lies within (-W,W) • Bandwidth Efficiency: W = Rb/W • Ratio of throughput data rate to bandwodth occupied by the modulated signal • SNR = P/N0W = P/N0Rb/W) = WEb/N0 • Energy Efficiency: P= Eb/N0 • Ratio of signal energy per bit to noise power spectral density required required at the receiver for a certain BER (e.g. 10-5) • Tradeoff between Pand W • W < log2(1+ W P )

  25. Receiver Performance

  26. Energy-Bandwidth Trade-off

  27. Energy Efficiency

  28. Energy Efficiency Examples

  29. Effect of Channel Coding (FEC) Moves curves to the leftby a “coding gain”

  30. Power allowed time time transmission time Control Knobs for Scaling the Performance-Energy Curve Modulation scaling fewer bits per symbol Code scaling more heavily coded Energy Energy transmission time transmission time

  31. Energy: the Deeper Story…. • Wireless communication subsystem consists of three components with substantially different characteristics • Their relative importance depends on the transmission range of the radio Tx: Sender Rx: Receiver Incoming information Outgoing information Channel Power amplifier Transmit electronics Receive electronics

  32. Examples Medusa Sensor Node (UCLA) Nokia C021 Wireless LAN GSM nJ/bit nJ/bit nJ/bit ~ 50 m ~ 10 m ~ 1 km • The RF energy increases with transmission range • The electronics energy for transmit and receive are typically comparable

  33. Energy Consumption of the Sender Tx: Sender • Parameter of interest: • energy consumption per bit Incoming information RFDominates Electronics Dominates Energy Energy Energy Transmission time Transmission time Transmission time

  34. Short-range Long-range Energy Medium-range Transmission time Effect of Transmission Range

  35. Power Breakdowns and Trends Radiated power 63 mW (18 dBm) Intersil PRISM II (Nokia C021 wireless LAN) Power amplifier 600 mW (~11% efficiency) Analog electronics 240 mW Digital electronics 170 mW • Trends: • Move functionality from the analog to the digital electronics • Digital electronics benefit most from technology improvements • Borderline between ‘long’ and ‘short’-range moves towards shorter transmit distances

  36. Another Issue: Start-up Time Shih et. al., Mobicom 2001

  37. Wasted Energy • Fixed cost of communication: startup time • High energy per bit for small packets Shih et. al., Mobicom 2001

  38. Communication View: Coding is Always good for Energy/Bit Shih et. al., Mobicom 2001

  39. Reality: Coding Not Always Good Due to Computation Energy • Encoding energy << Decoding energy • Computation energy dominates at higher target BER With Viterbi Decoder on a StrongARM With Viterbi Decoder on an ASIC(5X more efficient computation) Shih et. al., Mobicom 2001

  40. Interesting Radio Technologies

  41. Significant Radio Trends • Cheap low power / low rate / short range radios • e.g. for wake-up channel, personal area networking • Holy grail: single-chip radio, and radio-on-chip • High-speed radios • High bit rate! • Shift of Analog-Digital-Software boundary • Direct-DSP radios • sample RF band of interest at the antenna input, and then do all processing digitally • Holy grail: software-defined radios • Software for all digital processing • E.g. MIT’s Spectrumware Project, DoD’s JTRS

  42. Short Range Wireless • Conventional radios with low transmit power • Infrared • Focused: requires LOS • Diffused: high power 100s of mW • Passive radios as in RFID tags • powered by external RF energy • Backscatter radios • Inductive coupling • Near-field communication e.g. Electrostatic coupling • Modulated fluorescent lighting • Ultrasound

  43. Backscatter Radios • NCR’s Low-Cost Radio for an Electronic Price Label (EPL) System • for use in grocery stores to keep displayed prices in sync with the frequently updated prices in the main computer, flash sales etc.

  44. Wireless EPL • Objectives: • Ceiling-mounted basestations • networked to a pricing database residing in an “In Store Processor” (ISP) • Store, update, and display price • Acknowledge data received from modules • only then the ISP updates the price at the checkout counter • Restrictions: • Must be cheap! (under $1) • Must work for 5 years on a watch battery • Must have few errors (less than 1/1000000)

  45. NCR’S Design • Active transmitter for downlink • Generates its own wave • High power requirements • Cheap Modulation scheme can be used • Backscatter for uplink • Reflects the received wave back • Modulates the backwards reflection • Requires little power • Returns very little power • High SNR

  46. Downlink • Downlink (transmitting from basestation to an EPL): crystal radio! • Simple demodulation • Tuned antenna that is connected to a diode that rectifies the signal • after rectification, the signal envelope matches the modulated data stream • Manchester-encoded Amplitude Modulation chosen @ 1 kbps • can detect radio signals into the diode as low as –60 dBm • but need amplification without much power consumption! • special amp for 110 dB gain (10 uVp-p to 3V) with low noise, 33 uA peak (3.2 uA with cycling) • Manchester encoding: clock transmitted with the data • High Frequencies carrier chosen (2.4 Ghz ISM band) • small receive antenna size on the EPL • signal would scatter and reflect throughout the store so that LOS not needed • but multipath problems: movement ~ 1” will cause change in path loss • retransmission, spatial diversity (combine power from multiple ceiling basestations) • FCC allows 1W in 2.4 GHz ISM band, and receiver can detect –60 dBm, so that max path loss allowed would be around 90 dB.

  47. Uplink via Modulated Backscattering • Uplink is much more challenging • Active radio is out • Observation: antenna reflects as well as absorbs RF energy • Amount of power reflected from an antenna is: • a. No energy when connected to an open circuit • b. As much as it absorbs when connected to a matched impedance • c. Four times as much as in part (b.) if connected to a short circuit • Idea: modulate the energy reflected by the antenna by biasing the reflector diode used in the receiver! • modulated backscatter previously used in eavesdropping • Passive cavity transmitter found over the US Ambassador’s desk in Russia in 1952 • Metal cavity resonant at 330 MHz with an acoustic diaphragm and antenna so that the sound impinging on the diapragm modulated the RF reflection coefficient of the cavity so that when excited by an outside RF source, the device would send back a modulate backscatter signal carrying the ambassador’s voice: no battery, wires, or active components! • microprocessor in EPL modulates the diode by turning its bias on and off at the rate of 25 KHz • by forward biasing the diode, it acts as a short, thus reflecting much of the incoming wave • presence and absence of 25 KHz backscatter provides uplink communications • continuous wave from basestation is reflected back to the basestation with a 25 KHz sideband in the spectrum • sensitive receiver in the ceiling detects the reflected signal, much like a Doppler radar • but… severe path loss as the signal travels downlink and uplink • mitigated by having multiple receive units on the ceiling instead of simply colocating transmit and receive units on the ceiling

  48. Link Budget • Power received by EPL • P=PtGtLdgr • FCC resticts transmitted power to 1W (Pt) • Gt is restricted for max area coverage • Ld is downlink path loss l2/4pR2 • gr is receiver gain (0 dB for isotropic) • Power received by CBS • P = PincGgrLuGr • Pinc is the power received by the EPL • G is a form of the reflection coefficient • 0, 1, 4 depending on open, matched, or shorted antenna corresponding to no diode bias, 2.5 uA bias, and 2.5 mA forward bias respectively • gr is the EPL antenna gain • Lu the uplink path loss l2/4pR2 • Gr is the ceiling receive antenna gain • In order to reduce Lu and increase Gr, multiple receiver antennas may be added • P= PtGtLdgrGgrLuGr • Pt = 30 dBm, Gt = 3 dB, Gr = 2 dB, grGgr =-7 dB, LuLd=-161 dB • Total = -133 dBm • The thermal noise at this frequency is roughly -168 dBm • This gives us 168-133= 35 dB of SNR

  49. Performance • Data Rate • Interface provides 6 messages per 1.5 secs • (3000 / hour) • Lifetime • Approx 6 years • Error Rate • 1 / 10,000 false positives returned • 1% of sent transmissions fail to be received • Therefore 1/1,000,000 errors received

  50. Near Field vs Far Field • Far field (radio) • Isotropic radio radio transmitter propagates energy with a signal strength that decreases with distance squared • susceptible to eavesdropping and interference • transmission efficiency is maximized by matching the impedance of the transmitter to free space, typically by using a half- antenna • for small devices, would require carrier frequencies of several GHz • subject to regulations & licensing that vary from country to country • Near field (e.g. electrostatic coupling) • strength decreases with distance cubed • earth shunts the electric field, further attenuating the signal and making near-field communication more difficult to intercept • signal attenuation also reduces inter-PAN interference • near-field electrostatic coupling is proportional to electrode surface area • operate at very low frequencies (0.1 to 1 megahertz) that can be generated directly from inexpensive microcontrollers • 330 kilohertz (KHz) at 30 volts with a 10-picofarad electrode capacitance, consuming 1.5 milliwatts discharging the electrode capacitance • Near-field communication avoids regulatory complications • e.g. a prototype (about the size of a thick credit card) has a field strength of 350 picovolts per meter at 300 meters, 86 decibels (dB) below the field strength allowed by the Federal Communications Commission

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