0. Trends in Radio Technology
EE206A (Spring 2001): Lecture #3
Mani Srivastava UCLA - EE Department

1. 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

2. 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

3. 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

4. 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

5. 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

6. Constellation Diagrams

7. 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

8. 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 )

9. Receiver Performance

10. Energy-Bandwidth Trade-off

11. Energy Efficiency

12. Energy Efficiency Examples

13. 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

14. 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

15. 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.

16. 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/ )

17. 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

18. 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.

19. 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

20. 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

21. 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

22. 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

23. 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.

24. Far Field Overlapping Cells

25. Resonance Caused by Multiple Reflections

26. 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

27. 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

28. Basestation Architecture

29. Portable Architecture

30. 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

31. 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

32. Near Field Coupling Result

33. 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

34. 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

35. 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

36. 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

37. A Closer Look
Information transmitted on a modulating picoamp displacement current through the human bod

38. 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

39. PAN Device Electric Field Model

40. 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

41. 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.

42. 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

43. 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

44. 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

45. 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

46. 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

47. Single Carrier Approach
ISI can be pretty high
e.g. tmax/T  1600 in a DVB-T system
Tremendous radio complexity

48. 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

49. 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

50. 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

51. 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

52. 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

53. 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

54. 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

55. 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

56. 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)

57. 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

58. 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.