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The Physical Layer The Physical Layer
Curt Schurgers
Sources:
- Mani Srivastava, http://nesl.ee.ucla.edu/courses/ee206a/2002s/
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Electrical waveform
Electro-magnetic
waveform
Wireless Communication System Wireless Communication System
Source coding
Source coding
Multiple access
Modulation & baseband
Wireless channel
Channel coding
RF
Source decoding
Source decoding (^) Multiple
access
Demodulation & baseband
Channel decoding
RF
0 1 0 1 1 1 0 0 1 0 1 0
V, I
E H
r r ,
Multi- plexing
Demulti- plexing
Information
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Packet Structure Packet Structure
MPDU (MAC Protocol Data Unit): Link layer frames
PPDU (PLCP Protocol Data Unit) of PLCP (Physical Layer Convergence Protocol)
Correct packet reception involves many steps:
● Synchronization in frequency and time ● Packet detection ● Header reception: passing CRC check
Packet reception on the higher layers assumes all these
E.g. 802.11b DSSS
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Bits versus Symbols Bits versus Symbols
Modulation: information is grouped together into waveforms
Demodulation: inverse process (best effort)
● If M → ∝ the ‘performance’ goes up, but at a cost of complexity (Shannon limit)
(^01)
00 01 10 11
1 bit/symbol
2 bits/symbol
b bits/symbol = M possible waveforms
b = log 2 (M)
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QAM and PSKQAM and PSK
cos( ω t)
sin( ω t) (^) + Upconversion to fC
I input
Q input
I
Q
Based on two orthogonal waveforms: sin() and cos() at the same frequency
● Symbols are represented by various amplitude scalings of these basis waveforms: I (in-phase) and Q (quadrature) inputs for cos() and sin() respectively ● At the receiver, the I and Q parts can be separated again due to the orthogonality of the basis waveforms
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QAM and PSKQAM and PSK
4-QAM 16-QAM^ 64-QAM
4-PSK 8-PSK^ 16-PSK
QAM (Quadrature Amplitude Modulation)
PSK (Phase Shift Keying)
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Performance Evaluation Performance Evaluation
Q
I
ai + j·b i
r i
θ i
2 Ei ∝ r i
=
∝ =
M
i
S RMS ri M
E r 1
The average energy when each 2 1 2 symbol is transmitted with an equal probability
Energy in symbol i
(in-phase)
(quadrature)
● 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. Therefore the ‘closeness’ of symbols is an indication of the maximum tolerable noise. ● 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.
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Performance Metrics Performance Metrics
Signal to Noise Ratio (SNR):
● Signal energyE (^) S is per symbol
Energy per bit:
Symbol Error Rate (SER)
● Bit Error Rate (BER): if one symbol error corresponds to one bit error due to Gray coding
N W
E R
N
P
SNR R S S
0
log 2 ( M )
E
E b = S
S
b
M R
W
SNR
N
E
0 log^2 ( )
log 2 ( M )
SER
BER =
SER
N 0
E b
Note: Performance of a modulation scheme is expressed as function of E (^) b/N 0 rather than SNR
2
N 0
W^ frequency
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Example: Performance QAM Example: Performance QAM
log( )
log ( )
2
2 M
W
R M
W
R b S
b =
η = =
R S
T
W ≈ =
Data rate Error rate
log 2 ( )
0 0 0
M
N
E
N
E
N W
E R
N
P
SNR R^ S S = S = b ⋅
N 0
Eb
SER
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Comparison Comparison
SER = 10 -
Source: http://www.mhhe.com/e ngcs/electrical/proakis/s tudent/images.mhtml
FSK
W
Rb
η b =
N 0
Eb
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Other Issues Other Issues
Coherent versus non-coherent receiver
● Coherent: carrier phase is needed. E.g. QAM, PSK, … ● Non-coherent or envelope detection. E.g. DPSK, FSK (could also be coherent), …
Constant envelope or not
● If constant envelope, efficient amplifier can be used. E.g. PSK, FSK
Implementation complexity
Resilience against Interference
Out-of-band radiation
Effect of frequency offset, fading, time-variations, etc.
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Direct Sequence Spread Spectrum Direct Sequence Spread Spectrum
(DSSS)(DSSS)
1 1 -1 1 -1 -1 1 -
Receiver with correct code Receiver with incorrect code
T (^) c
8 -
1 -1 1
8 0 0 0
The input sequence is multiplied by a faster sequence, called the ‘chip’ sequence. Chip rateR (^) c = 1/T (^) c This chip sequence is PN (pseudo noise) The received sequence is multiplied by the same chip sequence and integrated over one symbol time.
T S
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CDMA versus FDMA CDMA versus FDMA
FDMA: frequency division multiple access ● Users have different frequency bands (possible use DSSS) CDMA: code division multiple access ● Users occupy the same frequency band, but use orthogonal codes
Time
Frequency
User k
Code
CDMA
User k…
Time
Frequency
FDMA
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Frequency Hopped Spread Spectrum Frequency Hopped Spread Spectrum
(FHSS)(FHSS)
Jump around between frequency bands in a pseudo random fashion.
Avoids being stuck in a bad frequency band.
As a multi-access technique, transmissions can collide, but occurrences are
infrequent.
Fast FHSS: jump multiple times during one symbol
Slow FHSS: multiple symbols per jump
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OFDM OFDM
FDM
● Frequency Division Multiplexing ● Frequency guard bands
OFDM
● Orthogonal Frequency Division Multiplexing ● Overlapping, but orthogonal bands (e.g. sinc functions) ● Much denser than FDM ● Multiplexing done in the digital domain using an FFT
frequency
frequency
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Frequency Response Frequency Response
time frequency
FFT
time
FFT
frequency
- Cyclic convolution results in multiplication of frequency data points with the
FFT of the channel response (= channel frequency response)
- Each data point is thus multiplied by a single factor
- As a result, equalization is easy in the frequency domain
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Ultra Wideband (UWB)Ultra Wideband (UWB)
Impulse radio is a form of ultra wideband radio transmission
● Narrow pulses in the time domain, nanoseconds or less ● Modulation: PPM, PAM, bi-phase ● Very broad spectrum ● Two definitions: BW > 500 MHz; BW > 0.2 * fcenter
Source: http://bwrc.eecs.berkeley.edu/Research/UWB/overview.htm
Conventional (narrowband)
Ultra Wideband (UWB)
Reference: [Siw01]
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UWB Properties UWB Properties
Operating conditions ● Limited power to reduce interference with existing systems: -41.3 dBm/MHz ● Limited range: few 10s of meters Benefits ● High data rate possible (up to Gbps) over short distances ● Simple radio design: mostly digital ● Reuse spectrum ● Inherent security: hard to detect ● Position determination: Aetherwire
Source: http://dessr2m.adm-eu.uvsq.fr/pdfsportesouvertes/Presentation_Ultra-Wideband.pdf
Research: ● Aetherwire, Time Domain, Intel, TI, XtremeSpectrum, etc. ● IEEE 802.15.3a http://www.ieee802.org/15/pub/TG3a.html ● IEEE 802.15.4a http://www.ieee802.org/15/pub/TG4a.html
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Smart AntennasSmart Antennas
Sectorized antennas ● Current cellular systems: 120º sectors with different frequencies Switched beam antennas ● M beams provide an M-fold gain ● Improve capacity by limiting interferers: space division multiplexing (SDMA)
Adaptive arrays ● Signals from the M antennas are weighted and combined
Reference: [Win98]
● Line-of-sight environment Steer antenna beam Can create M-1 nulls to cancel out M- interferers ● Multipath environment Consider signal space Cancel N interferers and provide (M-N) fold diversity gain
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MIMO MIMO
MIMO: multi-input multi-output system: 2 types
Space time diversity coding
● Provide diversity gain
Spatial multiplexing: e.g. BLAST
● Data is split in parallel streams ● The channel itself provides the decorrelation (orthogonalization) ● Capacity proportional to min(Tx-antennas, Rx-antennas)
Reference: [Wol98]
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Near- Near-FieldField IntrabodyIntrabody CommunicationCommunication
Near-field electrostatic coupling
Low carrier frequency (MHz)
Low power (few mW)
Prototype: 2.4 Kbps
Personal area network (PAN): e.g.
business card handshake
Reference: [Zim96]
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Smart DustSmart Dust
Sensor network project at UC Berkeley project
● Based on MEMS (Micro Electronic Mechanical Systems) ● Target volume: cube millimeter ● Target power: < 10 μW average ● Optical transmission: short wavelength fits in small package (as opposed to RF antennas)
Applications: defense, inventory control, virtual keyboard, etc.
Source: http://www- bsac.eecs.berkeley.edu/archive/use rs/warneke-brett/SmartDust/
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Smart Dust Optical Communication Smart Dust Optical Communication
● Active transmission: semiconductor laser and MEMS beam-steering mirror (mW so only short durations) ● Passive optical transmission: corner-cube reflector ● Spatial multiplexing: directed light and parallel read-out ● Transmission very directional, reception more omnidirectional
Reference: [Kah99]
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References References
[Pro] Proakis, “Digital Communications,” McGraw-Hill [Hay] Haykin, Moher, “Modern Wireless Communications,” Prentice Hall [IEEE] IEEE standards, http://standards.ieee.org/getieee802/portfolio.html
[Kai95] Kaiser, U., Steinhagen, W., “A Low-Power Transponder IC for High-Performance Identification Systems,” IEEE Journal of Solid-State Circuits, Vol.30, No.3, pp.306-310,
[Zho04] Zhou, F., Chen, C., Jin, D., Huang, C., Min, H., “Evaluating and optimizing power consumption of anti-collision protocols for applications in RFID systems,” ISLPED’04, Newport Beach, CA, pp.357-362, 2004. [Zim96] Zimmerman, T., “Personal Area Networks: Near-field intrabody communication,” IBM Systems Journal, Vol.35, No.3-4, pp.609-617, 1996. [Kah99] Kahn, J., Katz, R., Pister, K., “Next century challenges: mobile networking for smart dust”, Proc. Mobicom, pp. 483-492, 1999. [Siw01] Siwiak, K., "Ultra-Wide Band Radio: Introducing a New Technology," Proc. VTC, Vol. 2, pp. 1088-1093, 2001. [Win98] Winters, J., “Smart antennas for wireless systems,” IEEE Personal Communications, Vol.5, No.1, pp.23-27, 1998. [Wol98] Wolniansky, P., Foschini, G., Golden, G., Valenzuela, R., “V-BLAST: An Architecture for Realizing Very High Data Rates Over the Rich-Scattering Wireless Channel,” Proc. ISSSE- 98, Pisa, Italy, 1998.
