Physical Layer

รศ. ดร. อนันต์ ผลเพิม 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 • • Physical Layer Architecture • Physical Layer Operation

2 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 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

RU

(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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