Wireless LANs 2014 - Kasetsart University

1

Physical Layer

รศ. ดร. อนันต์ ผลเพิม่

Assoc. Prof. Anan Phonphoem, [email protected]

http://www.cpe.ku.ac.th/~anan

Computer Engineering Department

Kasetsart University, Bangkok, Thailand

Wireless LANs2014

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Outline

• Modulation Techniques

•Basic knowledge

•Narrow Band

• Spread Spectrum

• Physical Layer Architecture

• Physical Layer Operation

Modulation Techniques

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Modulation Techniques

• Physical Layer

• Basic transformations

•Conversion process from data in NIC to radio waves

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Digital to Analog Encoding

Data in NIC Airwaves

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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

Digital/Analog

Encoding

FSK PSKASK

QAMQuadrature Amplitude Modulation

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Amplitude Shift Keying (ASK)

8/73

Bandwidth for ASK

9/73

Frequency Shift Keying (FSK)

10/73

Bandwidth for FSK

11/73

Phase Shift Keying (PSK)

12/73

PSK Bandwidth

13/73

4-PSK

14/73

4-PSK Constellation

15/73

8-PSK Constellation

16/73

Quadrature Amplitude Modulation(QAM)

17/73

8-QAM Signal

18/73

Bit Rate and Baud Rate

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Bit Rate and Baud Rate

20/73

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

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Bandwidth

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Modulation Technique

• Conversion process from data in NIC to radio waves• Narrow Band Modulation

• Spread Spectrum

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Narrow Band Modulation

• Concentrate all Tx power within a narrow band

• Frequency Spacing

• TV, AM/FM radio

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Narrow Band Example: FM Radio

• Analog/Analog Encoding• Concentrate all Tx power within a narrow band

25/73

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)

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Narrow Band Signal


Frequency

Amplitude

Narrow Band Interference

Spread Spectrum Signal

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• Frequency Hopping Spread Spectrum (FHSS)

• Direct Sequence Spread Spectrum (DSSS)

• Orthogonal Frequency Division Multiplexing (OFDM)

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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

FHSS

29https://www.okob.net/texts/mydocuments/80211physlayer/

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FHSS

Frequency(GHz)

Time

E

A

C

D

B

2.40 2.41 2.42 2.43 2.44 2.45

1

2

3

4

5

Hopping Pattern: E A C D B

E

A

C

D

B

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FHSS & Noise

Frequency(GHz)

Time

B

2.40 2.41 2.42 2.43 2.44 2.45

1

2

3

4

5

ENoise

A Noise Noise

Noise

CNoise

Noise

DNoise

Noise

Noise

CNoise

Noise

Crash

E

A

C

D

B

Noise

Noise

Crash

Hopping Pattern: E A C D B

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FHSS

• Achieve up to 2 Mbps

• Faster data rate high errors

• Reduce the interference effect

• Can operate radios with different hopping pattern in the same frequency band

•Hopping code used is called Orthogonal

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DSSS

• One of the most successful data transmission technique

• Used in

• cellular networks (CDMA systems)

•Global Positioning Systems (GPS)

•WLAN

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DSSS

• Combines a data signal with higher data rate bit sequence

•Each bit multiple bits

•Actually sends XOR of a sending bit and n-random bits

• n-random bit n-bit Chipping Code

•Processing Gain

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5-bit5-bit5-bit

DSSS: 5-bit Chipping Code

1

0Data Stream: 101

1

0

Random Seq: 01101 11000 10111

1

0

XOR (transmit): 10010 11000 01000

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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

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.

• White noise properties


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DSSS

• Higher spreading ratio

• Better interference resistance

• Lower data throughput

• Lower spreading ratio

• Lower interference resistance

• Better data throughput

• FCC regulations:

• Spreading ratio 10

• IEEE 802.11 11

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FHSS & DSSS

DSSS

Jamming Signal

FHSS @ time T

Frequency

Power Spectral Density

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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 ?

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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)

sinc(x) Function

42

43/73

OFDM

• FDM principle

•more bandwidth efficiency

• Saving bandwidth by decreasing guard band

• Decrease the Interference among channels (cross talk)

44/73

Regular FDM

Frequency

Amplitude

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OFDM

Amplitude

match the nulls of other sub-carriers

Max amplitudes

Frequency

Ts

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

46www.dziwior.org/WirelessLAN/OFDM_Principles.html

http://www.dziwior.org/WirelessLAN/OFDM_Principles.html

Principles of orthogonal carriers

• The amplitude (An) and phase (φn) of each carrier can have any value, but must remain constant during the duration of the symbol (Ts). These characteristics are used to encode the data.

• It turns out that f1 − f2 = n · (1 / Ts), where n is an integer. The important fact about this orthogonal relationship is that a demodulator "looking at" f1 over the period Ts will be completely unaffected by f2 – without requiring an RF filter.

• You can have a set of orthogonal sine waves, such that fn = f0+ n · (1 / Ts), i.e. a set of carriers at regularly spaced frequency intervals, 1 / Ts apart. A demodulator "looking at" one carrier will be unaffected by the other carriers – provided it integrates over the period Ts.

47www.dziwior.org/WirelessLAN/OFDM_Principles.html

http://www.dziwior.org/WirelessLAN/OFDM_Principles.html

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Outline


•Basic knowledge

•Narrow Band

• Spread Spectrum


• Physical Layer Operation

Physical Layer Architecture

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Physical Layer

Physical Layer Architecture

PMD Sublayer

PMD SAP

PLCP Sublayer

PHY SAP

MAC Layer

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PLCP Sublayer

• Physical Layer Convergence Procedure

• Communicate to MAC via primitives through Physical Layer Service Access Point (SAP)

• Prepare PLCP protocol data unit (PPDU) (append fields to MPDU)

• PPDU provides for asynchronous transfer of MPDU between stations

51

PMD Sublayer

• Physical Medium Dependent

• Provide actual transmission and reception of Physical Layer entities via wireless medium

• Interface directly to the medium

• Provides modulation and demodulation of the transmission frame

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Outline


•Basic knowledge

•Narrow Band

• Spread Spectrum


• Physical Layer OperationPhysical Layer Operation

53

Physical Layer Operations

• 3 State machines

•Carrier Senses: determine the state of the medium

•Transmit: send the data frame

•Receive: receive the data frame

54

Physical Layer state

ตรวจสอบสื่อ

รับข้อมลูสง่ข้อมลู

55

Physical Layer Service Primitives

MAC PLCP PLCP MAC Description

PHY-TXSTART.request PHY-TXSTART.confirm Start TX

PHY-DATA.request PHY-DATA.confirm Transfer Data

PHY-TXEND.request PHY-TXEND.confirm End TX

PHY-CCARESET.request PHY-CCARESET.confirm Reset Clear Channel Assessment state machine

PHY-CCA.indication (busy/idle : send every channel changes state)

PHY-DATA.indication Transfer Data

PHY-RXSTART.indication

PHY-RXEND.indication

Received a valid start frame/PLCP header

MAC PMDPLCP

Carrier Sense Function

• Station is not in Tx or Rx mode

• Clear channel assessment

• Medium Idle

• Clear channel assessment

• Medium Busy

• Check preamblemonitor header

• Try to synchronize

MAC PLCP PMD

Transmit Function

•Switch to TX mode

… …

•Sending preamble & header to antenna @ 1 Mbps•Transmit data @ specified rate

•Switch to RX mode

…

MAC PMDPLCP

Receive Function

• Clear channel assessment foundmedia busy

• Check preamblemonitor header

• Final Octet

…

• Check Power level > 85 dBm

• Check CRC

• Set octet counter

……

59

Multiple Antenna Diversities

• Receive function will operate with• Single Antenna

• Multiple Antennas

• Signal Degradation Factors• Distance

• Atmosphere

• Barrier

• Multiple-path propagation• Decrease the signal strength

• Use multiple antennas (diversity) to improve the received signal

Multiple Antennas

60

Wireless Transmitter

Wireless Receiver

SISO


Wireless Receiver

::

SIMO


Wireless Receiver

:: MISO


Wireless Receiver

::

:: MIMO

Multiple Antennas

61http://techon.nikkeibp.co.jp/english/img2/nea_0211comnet-1fig1.gif

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IEEE 802.11 PHY Layer

• FHSS Physical Layer

• DSSS Physical Layer

• Infrared (IR) Physical Layer

63

FHSS Physical Layer

• Low cost

• Low power consumption

• Most tolerant to noise

• Low potential data rate

• Medium range (< DSSS)

64

FHSS Architecture

• FHSS PLCP Sublayer

• FHSS PMD Sublayer

• Primitives

65

FHSS PLCP frame

Start FrameDelimiter

PLW PSFHeaderErrorCheck

Whitened PSDU

80 bits 16 bits 12 bits 4 bits 16 bits 0-4095 Octets

SYNC

0 & 1 alternating : synchronization purpose

PSDU Length word

0000110010111101: define the beginning of a frame

16-bit CRC

Payload (MPDU)Reduce DC bias, scramble

PLCP Signaling Field: data rate (1- 4.5 Mbps)

PLCP Preamble PLCP Header

PSDU=PLCP Service Data Unit

66

FHSS HW

พีแอลซีพี

Data Whitenerการเปล่ียน

สญัญลกัษณ์การกรองแบบ

Guassianมอดดเูลชัน่แบบ GFSK



Data Dewhitener

การกูคื้น ช่วงการกระโดด

ฝ่ังสง่

ฝ่ังรับ

ปรับปรุงจาก 802.11 Wireless Networks, Gast, O’reilly

67

FHSS PMD

• Perform actual Tx/Rx of PPDU by hopping between channel (hopping sequence)

• Provides FHSS modulation/demodulation

68

FHSS PMD Service Primitives

PLCP PMD PMD PLCP Description

PMD_TXRX.request Set Tx/Rx mode

PMD_PA_RAMP.request Set Ramp up/down Tx power

PMD_ANTSEL.request Select antenna (1..N)

PMD_PWRMGMT.request Put radio in sleep mode

PMD_TXPWRLVL.request

PMD_FREQ.request

Select power level

Tx Freq (channel ID)

PMD_DATA.request PMD_DATA.indicate Transfer Data

PMD_RSSI.indication Signal Strength (0-15)

69

DSSS Physical Layer

• High cost

• High power consumption

• High potential data rate

• More range

70

DSSS Architecture

• DSSS PLCP Sublayer

• DSSS PMD Sublayer

• Primitives

71

DSSS PLCP frame

0 & 1 alternating : synchronization purpose

Modulation type: data rate


#microsec. To transmit the MPDU

16-bit CRC

Reserved



Signal ServiceFrameCheck

SequenceMPDU

128 bits 16 bits 8 bits 8 bits 16 bits

SYNC Length

8 bits

DSSS

• Preamble and PLCP header are transmitted at 1Mbps

• Regardless of data transmission speed

• Payload prepared by the MAC layer is sent at the rate specified in the services field

• The transmitter uses DBPSK (1Mbps) and DQPSK(2Mbps) modulation

72

https://www.okob.net/texts/mydocuments/80211physlayer/images/dsss_dbpsk.gif

https://www.okob.net/texts/mydocuments/80211physlayer/images/dsss_dqpsk.gif

73

DSSS HW

ปรับปรุงจาก 802.11 Wireless Networks, Gast, O’reilly

พีแอลซีพี ตวักระจายตวักรอง

Transmit Markมอดดเูลชัน่แบบDBPSK/DQPSK

Descrambler

Correlator

ดีมอดดเูลชัน่แบบ DBPSK/DQPSK

การกูคื้น ช่วงเวลา

ฝ่ังสง่

ฝ่ังรับ

ล าดบัชิปปิง้


Complementary Code Keying (CCK)

• 1998

• Proposed by Lucent Technologies and Harris Semiconductor (later owned by Intersil)

• Achieve 5.5Mbps and 11Mbps Tx rates


CCK

• IEEE adopted the CCK and released the 802.11b in 1999

• new option to transmit PLCP header with a short (56 bits) preamble

• only the Synchronization and Start Frame Delimiter fields are transmitted at 1Mbps

• The rest of the PLCP header is transmitted at 2Mbps (using DSSS DQPSK)

• data payload at either the same 2Mbps, or using CCK at 5.5Mbps or 11Mbps.

75

https://www.okob.net/texts/mydocuments/80211physlayer/images/dsss_dqpsk.gif

CCK

• based on polyphase complementary codes

• not binary complex codes

• real component placed in the vertical plane

• complex component in the horizontal plane

76

CCK

• The modulator uses

• 6 bits of each byte to pick one of 64 unique orthogonal eight chips long polyphase complementary codes

• two bits of the byte are used to rotate the whole code word (0, 90, 180 or 270 degrees)

• real and complex parts of the resulted code go to the I(in-phase) and Q(quadrature) channels of the IQ modulator

77

78

More on Preamble Frame

http://searchmobilecomputing.techtarget.com/searchMobileComputing/downloads/CWAP_ch8.pdf

79

Signal Field (802.11b)

Data Rate (Mbps)

Signal Field Value

1 00001010

2 00010100

5.5 00110111

11 01101110

80

DSSS PMD

• Perform actual Tx/Rx of PPDU

• Provides DSSS modulation/demodulation

81

DSSS PMD Service Primitives

PLCP PMD PMD PLCP Description

PMD_TXSTART.request

PMD_TXEND.request PMD_TXPWRLVL.request

Start Tx

End Tx

Select power level

PMD_ANTSEL.request PMD_ANTSEL.indicate Select antenna (1..N)

PMD_RATE.request PMD_RATE.indicate Select data rate

PMD_ED.request PMD_ED.indicate Energy > Threshold

PMD_DATA.request PMD_DATA.indicate Transfer Data

PMD_RSSI.indication

PMD_SQ.indicate

PMD_CS.indicate

PMD_CAA.indicate

Signal Strength (0-15)

Signal Quality (PN code)

Valid 802.11 DS

Detect RF as CCA algo.

82

IR Physical Layer

• Lowest cost

• Highest tolerant to RF noise

• Lowest range

• Need ceiling

• More secure

• No frequency regulating

• No product ?

• IrDA: Infrared Data Association Standard

83

IR Architecture

• IR PLCP Sublayer

• IR PMD Sublayer

84

IR PLCP frame

Pulse alternating : synchronization purpose

Data rate


#microsec. To transmit the MPDU

16-bit CRC

Specified for 1 and 2 Mbps



DataRate

DC LevelAdjust.

FrameCheck

SequenceMPDU

57-73 slots 4 slots 3 slots 32 slots 16 slots

SYNC Length

16 slots 0-2500 octets

85

IR PMD

• Mostly use diffused infrared

• Perform actual Tx/Rx of PPDU, translate binary to infrared light

• Provides IR modulation/demodulation

86

IR PMD

• Noise affects amplitude (not phase)

Pulse position reduces interference

• Pulse position modulation :PPM

•Vary position of pulse

• For 1 Mbps 16 PPM

• For 2 Mbps 4 PPM

87

Pulse Position Modulation

Data bits 16-PPM signal

0000 0000 0000 0000 0001

0001 0000 0000 0000 0010

… …

1000 1000 0000 0000 0000

Data bits 4-PPM signal

00 0001

01 0010

10 0100

11 1000

88

OFDM PLCP Frame

Rate Resv. MAC Data

12 สญัญลกัษณ์

SYNC

พรีแอมเบิล้ พีแอลซีพี เฮดเดอร์

Length

ขนาดเปลีย่นแปลงได ้4 บติ 1 บติ 12 บติ

พารติี้

1 บติ

Tail

6 บติ

Service

16 บติ

DSSS-OFDM PPDU

89http://wireless.agilent.com/wireless/helpfiles/n7617b/dsss-ofdm_ppdu_format_short.jpg

90

Transmission Rate

บติข้อมูล ความเร็วที่ได้ (Mbps) ช่วงความเร็ว

1101 6 1(BPSK)1111 9

0101 12 2(QPSK)0111 18

1001 24 3(16-QAM)1011 36

0001 48 4(64-QAM)0011 54

IEEE 802.11g OFDM


Wireless LANs 2014 - Kasetsart University

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