Physical Layer
รศ. ดร. อนันต์ ผลเพิ ม Assoc. Prof. Anan Phonphoem, Ph.D. [email protected] http://www.cpe.ku.ac.th/~anan Computer Engineering Department Kasetsart University, Bangkok, Thailand 1 Outline
• ModulationModulation TechniquesTechniques • Basic knowledge • Narrow Band • Spread Spectrum • Physical Layer Architecture • Physical Layer Operation
2 Modulation Techniques
• Physical Layer • Basic transformations • Conversion process from data in NIC to radio waves
3 Digital to Analog Encoding
Data in NIC Airwaves
4 Basic concepts
• Carrier signal (carrier frequency) • High frequency as a basis for information • Sender and receiver agree on the frequency • Digital data is modulated (shift keying) on the carrier by modifying carrier characteristics • 3 characteristics of carrier signal • Amplitude • Frequency • Phase
5/73 Digital/Analog Encoding
ASK FSK PSK
QAM Quadrature Amplitude Modulation
6 Amplitude Shift Keying (ASK)
7/73 Bandwidth for ASK
8/73 Frequency Shift Keying (FSK)
9/73 Bandwidth for FSK
10/73 Phase Shift Keying (PSK)
11/73 PSK Bandwidth
12/73 4-PSK
13/73 4-PSK Constellation
14/73 8-PSK Constellation
15/73 Quadrature Amplitude Modulation (QAM)
16/73 8-QAM Signal
17/73 Bit Rate and Baud Rate
18/73 Bit Rate and Baud Rate
19/73 Freq. Spectrum & Bandwidth
• Frequency Spectrum of a signal • Collection of all the component frequencies of the signal • Bandwidth of a signal • The width of the frequency spectrum
20/73 Bandwidth
21/73 Outline
• Modulation Techniques • Basic knowledge • Narrow Band • Spread Spectrum • Physical Layer Architecture • Physical Layer Operation
22 Modulation Technique
• Conversion process from data in NIC to radio waves • Narrow Band Modulation • Spread Spectrum
23 Narrow Band Modulation
• Concentrate all Tx power within a narrow band • Frequency Spacing • TV, AM/FM radio
24 Narrow Band Example: FM Radio
• Analog/Analog Encoding • Concentrate all Tx power within a narrow band
25 Spread Spectrum Modulation
• A signal’s power over a wider band of frequencies • Less susceptible to noise • Trade-off between BW and Signal-to-Noise Ratio (process gain)
26 Spread Spectrum Modulation Amplitude Narrow Band Signal Narrow Band Interference Spread Spectrum Signal
Frequency
27 Spread Spectrum Modulation
• Frequency Hopping Spread Spectrum (FHSS) • Direct Sequence Spread Spectrum (DSSS) • Orthogonal Frequency Division Multiplexing (OFDM)
28 FHSS
• A carrier signal • hops from frequency to frequency as a function of time • Hopping code: • frequency use and order to transmit • Federal Communications Corporation (FCC): Regulatory Agencies • >75 frequencies / transmission • Max dwell time is 400 msec
29 FHSS
https://www.okob.net/texts/mydocuments/80211physlayer/ 30 FHSS Time Hopping Pattern: E A C D B 5 B
4 D
3 C
2 A
1 E
2.40 2.41 2.42 2.43 2.44 2.45 Frequency (GHz)
31 FHSS & Noise Time Hopping Pattern: E A C D B 5 BB
4 Noise D
3 NoiseCrashC
2 A Noise Noise Noise 1 NoiseNoise EE
2.40 2.41 2.42 2.43 2.44 2.45 Frequency (GHz)
32 FHSS
• Achieve up to 2 Mbps • Faster data rate à high errors • Reduce the interference effect • Can operate radios with different hopping pattern in the same frequency band • Hopping code used is called Orthogonal
33 DSSS
• One of the most successful data transmission technique • Used in • cellular networks (CDMA systems) • Global Positioning Systems (GPS) • WLAN
34 DSSS
• Combines a data signal with higher data rate bit sequence • Each bit à multiple bits • Actually sends XOR of a sending bit and n- random bits • n-random bit à n-bit Chipping Code • Processing Gain
35 DSSS: 5-bit Chipping Code
1 Data Stream: 101 0
1 Random Seq: 0 01101 11000 10111
1 XOR (transmit): 0 10010 11000 01000
5-bit 5-bit 5-bit
36 DSSS
• Spread the signal (n-time) across the frequency band • Reduce power concentration over a frequency • Spreading ratio (Chipping code): • # of chip bits / data bit
37 Chipping Sequence
• Pseudo random binary sequence (PRN or PN) • PRN sequence properties: • Balanced sequence (difference between number of ones and zeros in the sequence is less or equal 1) • Autocorrelation function of the sequence should be close to 0 everywhere except at the shift 0. • è White noise properties
https://www.okob.net/texts/mydocuments/80211physlayer/ 38 DSSS
• Higher spreading ratio • Better interference resistance • Lower data throughput • Lower spreading ratio • Lower interference resistance • Better data throughput • FCC regulations: • Spreading ratio ³ 10 • IEEE 802.11 à 11
39 FHSS & DSSS
Power Spectral Density FHSS @ time T
Jamming Signal
DSSS
Frequency
40 FHSS & DSSS
• DSSS can achieve much higher than 2 Mbps • DSSS is harder to implement • More expensive than FHSS • FHSS is higher interference resistance • DSSS transmits farther than FHSS ?
41 OFDM
• Orthogonal Frequency-Division Multiplexing • Use many sub-carriers (in parallel) • Sub-carrier overlap • Sinc (sinx/x) Shape • Max amplitudes match the nulls of other sub-carriers • Each sub-carrier modulates at low rate (sum is high) • Use in 802.11 ac, WiMax, 4G, 5G Orthogonal è independent, no interference
42 sinc(x) Function
43 OFDM
• FDM principle • more bandwidth efficiency • Saving bandwidth by decreasing guard band • Decrease the Interference among channels (cross talk) • Decrease multipath fading channel effect
44 Regular FDM
Amplitude
Frequency
45 OFDM
Max amplitudes Amplitude
match the nulls of other sub-carriers
Frequency 46 OFDM Spectrum
https://www.nutaq.com/blog/introduction-orthogonal-frequency-division-multiplex-ofdm 47 FDMA & OFDM Bandwidth
Deepak Sharma, RF Engineer 48 Principles of orthogonal carriers
• Definition • The cross correlation integral between two orthogonal sine waves, over the symbol duration Ts, is zero:
∫ A1sin(2πf1t + φ1) · A2sin(2πf2t + φ2) = 0
Ts
www.dziwior.org/WirelessLAN/OFDM_Principles.html 49 OFDMA (Orthogonal Frequency Division Multiple Access)
• A digital modulation technique • Not a multiuser channel access technique • By transferring one-bit stream over one OFDM channel using one sequence of OFDM symbols
• Employs multiple closely spaced sub-carriers • The sub-carriers are divided into groups • Each group is named a sub-channel OFDMA • The sub-carriers that form a sub-channel need not be adjacent
50 Wi-Fi 6
Wi-Fi # Standard (Task group) Year Wi-Fi 1 IEEE 802.11b 1999 Wi-Fi 2 IEEE 802.11a 1999 Wi-Fi 3 IEEE 802.11g 2003 Wi-Fi 4 IEEE 802.11n 2009 Wi-Fi 5 IEEE 802.11ac 2014 Wi-Fi 6 IEEE 802.11ax 2019
Modified from: https://www.avtechguide.com/what-is-wi-fi-6-and-the-new-simple-name-wi-fi-alliance/ 51 802.11n/ac 20 MHz channel (OFDM subcarriers)
https://www.extremenetworks.com/extreme-networks-blog/ofdm-and-ofdma-subcarriers-what-are-the-differences/ 52 Subcarrier spacing
• 802.11ax introduces a longer OFDM symbol time of 12.8 μs • Subcarrier spacing is equal to the reciprocal of the symbol time
The narrow subcarrier spacing allows better equalization and therefore enhanced channel robustness.
https://www.extremenetworks.com/extreme-networks-blog/ofdm-and-ofdma-subcarriers-what-are-the-differences/ 53 OFDM & OFDMA
(IEEE 802.11 ax) Freq. (IEEE 802.11 a,g,n,ac) Freq. Bandwidth carrier channel - - Sub Sub Time Time User #1 User #2
User #3 54 OFDMA in Wi-Fi 6 (IEEE 802.11 ax)
https://www.juve-patent.com/sponsored/wi-fi-6-key-innovations-and-their-contributors-part-2/ 55 Structure of sub-carriers in OFDMA
56 OFDMA Transmitter
CP - Cyclic Prefix From Alcatel-Lucent Univ. By Dr. K.Sandrasegaran (2012) 57 (for multipath effect) Down Link VS. Up Link
Peak-to-average Ratio
By Dr. K.Sandrasegaran (2012) 58 OFDMA Vs. SC-FDMA
By Dr. K.Sandrasegaran (2012) 59 OFDMA Vs. SC-FDMA
By Dr. K.Sandrasegaran (2012) 60 Outline
• ModulationModulation TechniquesTechniques • Basic knowledge • Narrow Band • Spread Spectrum • Physical Layer Architecture • Physical Layer Operation
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