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Trends in Radio Technology
EE206A (Spring 2001): Lecture #3 Mani Srivastava UCLA - EE Department
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Readings for this Lecture
MANDATORY none 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 p Richley, R.A.; Butcher, L. Wireless communications using near field coupling. US Patent 5,437,057, Search at Zimmerman, T.G. Personal area networks: near-field intrabody communication. IBM Systems Journal, vol.35, (no.3-4), IBM, p
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Demodulator & Equalizer
Digital Radio Link antenna Source Coder Multiple Access Channel Coder Power Amplifier Source Multiplex Modulator Carrier fc transmitted symbol stream Radio Channel Radio Technology and Trends received (corrupted) symbol stream Source Decoder Multiple Access Channel Decoder Demodulator & Equalizer RF Filter Destination Demultiplex antenna Carrier fc
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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
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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
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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
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Constellation Diagrams
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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 BER and bandwidth efficiency R/B
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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) = W Eb/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 P and W W < log2(1+ W P )
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Receiver Performance
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Energy-Bandwidth Trade-off
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Energy Efficiency
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Energy Efficiency Examples
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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
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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
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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.
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Wireless EPL Objectives: Restrictions: 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/ )
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NCR’S Design Active transmitter for downlink Backscatter for uplink
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
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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 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.
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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
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Link Budget Power received by EPL P=PtGtLdgr Power received by CBS
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 = 35 dB of SNR
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Performance Data Rate Lifetime Error 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
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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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Wireless Communication Using Near Field Coupling
Patent in 1995 by Edward A. Richley and Lawrence Butcher of Xerox PARC a near field coupling radio for cell based wireless LANs Background: conventional cell-based systems Far field electric and magnetic fields drop with a rate 1/r => power rate 1/r2 Neighboring cells operating at different frequencies required complex switching mechanisms Too gradual drop for using the same frequency UHF frequencies used, small wavelengths and small antennas sensitive to reflections and multiple reflections cause resonance.
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Far Field Overlapping Cells
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Resonance Caused by Multiple Reflections
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Near Field Coupling Approach
The goal is to provide well-defined cell boundaries and good coverage within cells What is ‘near field’ ? Electric and magnetic components of an antenna field that do not propagate Analogous to reactive power in circuit theory For large distances far field components dominate, while short distances - near field components dominate Main idea is to operate within the near field radius Small loop antennas are ideal for this application
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Near Field Coupling for Cellular LANs
A LAN is assumed to have adequate bandwidth to accommodate several clients LAN consists of Base stations connected to wired infrastructure A set of portable clients Base stations use phase quadrature antennas overcomes directional sensitivity of the antenna Clients use a single antenna
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Basestation Architecture
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Portable Architecture
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Typical Values Carrier Frequency 5.3 MHz
(5 MHz - 15 MHz about 2 orders of magnitude lower than UHF) Longer wavelengths less sensitive to reflections Coverage feet (ideal for small office spaces) 250kbps
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Modification for Hallways
For more ‘rectangular’ areas coverage can be accommodated with antenna modifications. Long antennae can cause interference to neighboring cells Single loop antennas with twisted wires was found to work better
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Near Field Coupling Result
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Summary of Benefits Low power operation
Can accommodate higher densities Less interference Less sensitive to reflections Transmitted energy much lower than the 30m FCC limits
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Near-field Intrabody Communication
Personal Area Networks by T.G Zimmerman of IBM Idea: Use the human body as the communication medium communication using electric field modulation Many I/O devices are duplicated Watch,PDA,Cell-phone, all have displays Ubiquitous I/O instead of Ubiquitous Computing Network devices that are close to the body using capacity coupling though the body Such communication can be achieved in a power efficient manner
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Advantages Makes use of near field effects
Low frequencies, below 1MHz, typically MHz, Low power (few mW) Very short range Higher density More difficult to intercept Less Interference Well below FCC limits so no strict licensing limits Avoids duplication of personal devices
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Challenges Privacy/Security
Beacon signals can be used to keep track of people Personal information such as credit card numbers, telephone numbers and computer passwords can be stolen Communication becomes difficult or stops when touching good conductors
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A Closer Look Information transmitted on a modulating picoamp displacement current through the human bod
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Lumped Model of Communication Channel
PAN transmitter is modeled as an oscillator, and the receiver is modeled as a differential amplifier basic principle of a PAN communication channel is to break the impedance symmetry between the transmitter electrodes and receiver electrodes transmitter's and receiver's intraelectrode impedances are ignored since the former is a load on an ideal voltage source and the latter is modeled as an open circuit
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PAN Device Electric Field Model
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PAN Device Electric Field Model (contd.)
A small portion of the electric field G reaches the receiver R The transmitter T electrode closest to the body tb has a lower impedance to the body than the electrode facing toward the environment te. This enables the transmitter T to impose an oscillating potential on the body, relative to the earth ground, causing electric fields A, B, C, D, and E. The impedance asymmetry of the receiver electrodes (rb and re) to the body and environment allow the displacement current from electric fields F and G to be detected. Since the impedance between the receiver electrodes is nonzero, a small electric field H exists between them
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Electrical Model of PAN System
The transmitter T capacitively couples to receiver R through the body (modeled as a perfect conductor). The earth ground provides the return signal.
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Electrical Model of PAN System (contd.)
Body capacitance to the environment E degrades PAN communication by grounding the potential that the transmitter T is trying to impose on the body E.g standing barefoot reduced communication between wrist-mounted devices by 12 dB. Feet are the best location for PAN devices, providing large electrodes in close proximity to the body and environment, respectively Also suggest a novel power source: PAN devices embedded in shoe inserts that extract power from walking An adult dissipates several hundred milliwatts while walking. A piezoceramic pile charging a capacitor at an efficiency as low as 10 percent can provide enough power for a PAN device
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The PAN Prototype Using C = B log (1+S/N), -3dB BW is around 400KHz
max channel capacity 417kbps, assuming SNR =10 Actual transceiver prototype 2400bps Dimensions 8 x 5 x 1 cm Modulation schemes: on-off keying ( on = 1, off = 0 ), DSSS
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Electric Handshakes Transmitter placed inside shoe insets
can be charged during walking When 2 people are in close proximity they can exchange business card information. Business card information is continuously transmitted as a set of ASCII characters. When people shake hands the information can be downloaded to each others PDAs. Concept of power sneakers
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Radios for High Speed Wireless
Digital multimedia leading to increasing demand for broadband wireless over terrestrial links not a problem in satellite channel Orthogonal Frequency Division Multiplexing (OFDM) is a method that has emerged as the leading approach to transmit high data rates over extremely hostile channels at a comparable low complexity
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Data transmission over multipath channels
Multipath channel in terrestrial link transmitted signal arrives at the receiver in various paths leading to ISI Received symbol can theoretically be influenced by tmax/T previous symbols tmax is delay of the longest path with respect to the earliest path Hard to estimate and compensate
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Single Carrier Approach
ISI can be pretty high e.g. tmax/T 1600 in a DVB-T system Tremendous radio complexity
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Multi Carrier Approach
The original data stream of rate R is multiplexed into N parallel data streams of rate Rmc = 1/Tmc = R/N each of the data streams is modulated with a different frequency and the resulting signals are transmitted together in the same band Receiver consists of N parallel paths, each with ISI reduced by factor of N such little ISI can often be tolerated with no equalizer needed e.g. N = 8192 will reduce ISI in DVB-T to 0.2 But , receiver complexity tremendous for large N Leads to concept of OFDM
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FDM vs. OFDM FDM OFDM Frequency Division Multiplexing
Frequency guard bands OFDM Orthogonal FDM Overlapping, but orthogonal bands (e.g. sinc functions) Sample at appropriate points Much denser than FDM frequency frequency
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Digital Modulation/Multiplexing
Only values at sample points are important and contain data IDFT generates the time samples corresponding to these frequency points Implemented efficiently via IFFT IFFT generates an infinite periodic time sequence, but only the first N samples are sent over the channel (these samples contain exactly the same information as the frequency points) This windowing to N samples results in a convolution with a sinc function in the frequency domain (which has the orthogonality property) Only need a FFT at the receiver to reverse this operation frequency IFFT time
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Data Transmission Sequence
Data is grouped into blocks and each block is treated sequentially Each block i consists of N symbols which are transformed into N time samples using the IFFT time i = 1 frequency i = 2 frequency IFFT time i = 3 frequency i = 1 i = 2 i = 3
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Single Carrier versus Multicarrier
Single carrier and multicarrier both send N symbols in NT, or 1/T symbols/second have a total single sided bandwidth of about 1/T time frequency Single carrier frequency time Multicarrier
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Subcarrier Spectrum Overlap
Info can still be separated due to orthogonality by using IFFT the spacing of the subcarriers is implicitly chosen such that all other signals are zero at the frequency where we evaluate the received signal (arrows) Preserving orthogonality requires Receiver and transmitter must be perfectly synchronized. both must assume exactly the same modulation frequency and the same time-scale for transmission (which usually is not the case) requires sophisticated radio architectures The analog components must be of very high quality There should be no multipath channel yikes! … but solved by introducing a guard interval D > tmax via prolonging the OFDM symbols by periodically repeating the 'tail' of the symbol and preceding the symbol with it NT cyclic prefix
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Overall System View Typically QAM (quadrature amplitude modulation) is used to modulate the bits onto symbols, but any modulation is possible b = 2 bits/symbol b = 4 bits/symbol b = 6 bits/symbol 4-QAM 16-QAM 64-QAM Concentrator N-IFFT Cyclic prefix insertion Radio front-end Rb bits/s Modulator K bits/block b bits/symbol N symbols/block Rb/K blocks/s Rb/b symbols/s (N+C) symbols/block RS symbols/s freq time
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Without adaptive loading
Adaptive Bit Loading Not all subcarriers are equally good (different amounts of attenuation) Adaptive loading takes channel info into account at the sender Without adaptive loading With adaptive loading bk = f(k) freq bk = bav freq
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Adaptive Bit Loading (contd.)
Normalized channel response (dB) Subcarrier N = 256 bav = 4 bi bits/symbol Assign bi and Pi such that Ptot is minimized Send more information when channel is good Channel needs to be estimated (as for equalization) BER SNR (dB) N = 256 (uncorrelated) bav = 4 AWGN Loaded Unloaded Loading information needs to be fed back to the sender The channel must remain quasi-stationary between estimation updates (low Doppler rate)
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Things to Consider About OFDM
Dynamic range at output of IFFT is much larger than at input (or single carrier systems): large peak-to-average ratio (PAR) Very good frequency synchronization is crucial to maintain orthogonality (otherwise ISI) Example: use OFMA as multiple access technique ISI OFDMA downlink OFDMA uplink Sync problem !!! ISI
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Different Flavors of Multicarrier Systems
Discrete Multi Tone (DMT) Terminology used in xDSL systems In what is it different from OFDM? Nobody knows. DMT baseband OFDM passband / DMT loaded OFDM unloaded ?? Different Variations of OFDM (none of them use adaptive loading) W-OFDM or Wideband OFDM (WiLAN): Basis for a (54 Mbps) and new Data rate up to 30 Mbps at 70 mph Reed-Solomon coding with erasures large carrier spacing (‘wideband’) to simplify frequency synchronization V-OFDM or Vector OFDM (CISCO): MMDS microwave band, non-line-of-sight 2-fold receive antenna diversity (‘vector’), up to 44 Mbps Flash-OFDM (Flarion): Fast frequency hopping on OFDM (‘flash’) Data rate 384 kbps – 3 Mbps at highway speeds, aimed at 3G systems Others: Malibu Networks, Iospan Wireless etc.
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