1 / 81

Physical Layer

Physical Layer. Overview of physical layer Channel limitation Modulation/demodulation. Introduction. Physical layer. What You Need for Better Understanding. Representation of Information. Digital representation Information that occurs naturally in digital form data files or image files

elliot
Download Presentation

Physical Layer

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Physical Layer • Overview of physical layer • Channel limitation • Modulation/demodulation

  2. Introduction

  3. Physical layer

  4. What You Need for Better Understanding

  5. Representation of Information • Digital representation • Information that occurs naturally in digital form • data files or image files • Analog information: be digitized • Voice • Music • Video • Most communications networks are digital!

  6. Source Coding • Networks are handling streams of 0’s and 1’ • Source Encoding: compression, according to statistics of 0’s and 1’s, map blocks of bits to more regular “shorter” blocks! Get rid of redundancy • Source Decoding: inverse of source encoding

  7. Channel Coding • Channel Encoding: According to channel conditions, add redundancy for more efficient transmission, interleaving may be used too. • Channel decoding: the inverse • Observation: source encoding attempts to eliminate “useless information”, while channel encoding add “useful information”, both deal with redundancies!

  8. Modulation/Demodulation • Modulation: maps blocks of bits to well-defined waveforms or symbols (a set of signals for better transmission), then shifts transmission to the carrier frequency band (the band you have right to transmit) • Demodulation: the inverse of modulation • Demodulation vs. Detection: Detection is to recover the modulated signal from the “distorted noisy” received signals

  9. Physical Components • Transmitter • Receiver • Transmission media • Guided: cable, twisted pair, fiber • Unguided: wireless (radio, infrared)

  10. Signal Types • Basic form: A signal is a time function • Continuous signal: varying continuously with time, e.g., speech • Discrete signal: varying at discrete time instant or keeping constant value in certain time interval, e.g., Morse code, flash lights • Periodic signal: Pattern repeated over time • Aperiodic signal: Pattern not repeated over time, e.g., speech

  11. Continuous & Discrete Signals

  12. Periodic Signals

  13. Information Carriers • s(t) = A sin (2pft+ ) * Amplitude: A * Frequency: f --- f=1/T, T---period * Phase:  , angle (2pft+ )

  14. Varying Sine Waves

  15. Frequency Domain Concept • Signal is usually made up of many frequencies • Components are sine waves • Can be shown (Fourier analysis) that any signal is made up of component sine waves • Can plot frequency domain functions • Time domain representation is equivalent to frequency domain representation: they contain the same information! • Frequency domain representation is easier for design

  16. Fourier Representation

  17. Addition ofSignals

  18. Received Signals • Any receiver can only receive signals in certain frequency range (channel concept), corresponding to finite number of terms in the Fourier series approximation: • physically: finite number of harmonics • mathematically: finite number of terms • Transmitted signal design: allocate as many terms as possible in the intended receiver’s receiving range (most of power is limited in the intended receiving band)

  19. Spectrum & Bandwidth • Spectrum: the range of frequencies contained in a signal • Absolute bandwidth: width of spectrum • Effective bandwidth: just BW, Narrow band of frequencies containing most of the energy • 3 dB BW • Percentage BW: percentage power in the band • DC Component: Component of zero frequency

  20. Data Rate and Bandwidth • Any transmission system has a limited band of frequencies • This limits the data rate that can be carried • The greater the BW, the higher the data rate • Channel capacity (later)

  21. Analog vs Digital • Analog: Continuous values within some interval, the transmitted signal has actual meaning, e.g., AM and FM radio • Digital: Digital=DSP+Analog, raw digital bits are processed and mapped to well-known signal set for better transmission, the final transmitted signal is still analog! You could not “hear” though!

  22. Analog Transmission • Analog signal transmitted without regard to content • Attenuated over distance • Use amplifiers to boost signal, equalizers may be used to mitigate the noise • Also amplifies noise

  23. Digital Transmission • Concerned with content • Digital repeaters used: repeater receives signal, extracts bit pattern and retransmits the bit pattern! • Attenuation is overcome and distortion is not propagated!

  24. Advantages of Digital Transmission • Digital technology: low cost, can use low power • Long distance transmission: use digital repeaters • Capacity utilization: get rid of useless information and add useful redundancy for data protection • Security & privacy: encryption • Integration: treat analog and digital data similarly

  25. Channel Impairments • Attenuation and attenuation distortion: signal power attenuates with distance • Delay distortion: velocity of a signal through a guided medium varies with frequency, multipath in wireless environments • Thermal noise • Co-channel Interference: wireless • Impulse noise (powerline communications)

  26. Channel Capacity • Data rate is limited by channel bandwidth and channel environment (impairments) • Data rate, in bits per second, is the number of bits transmitted successfully per second! Should not count the redundancy added against channel impairments! • It represents how fast bits can be transmitted reliably over a given medium

  27. Factors Affecting Data Rate • Transmitted power (energy) • Distance between transmitter and receiver • Noise level (including interference level) • Bandwidth

  28. Nyquist Capacity • Nyquist Rate: 2B (baud), where B is the BW of a signal • Sampling Theorem: Any signal whose BW is B can be completely recovered by the sampled data at rate 2B samples per second • Nyquist Capacity Theorem: For a noiseless channel with BW B, if the M level signaling is used, the maximum transmission rate over the channel is C = 2B log2( M) • Digital Comm: symbol rate (baud) vs. bit rate

  29. Shannon Capacity • All channels are noisy! • 1948 paper by Claude Shannon: “A mathematical theory of communications” “The mathematical theory of communications” • Signal-to-noise ratio: SNR=signal power/noise power (watt)

  30. Shannon Capacity (cont) • Shannon Capacity Theorem: For a noisy channel of BW B with signal-to-noise ratio (SNR), the maximum transmission rate is C = B log2 (1+SNR) • Capacity increases as BW or signal power increases: Shout as you can! • Some exercise: B=3400Hz, SNR=40dB • C=45.2 kbps

  31. Shannon Capacity (cont) • Shannon Theorem does not give any way to reach that capacity • Current transmission schemes transmit much lower rate than Shannon capacity • Turbo codes: iterative coding schemes using feedback information for transmission and detection • Sailing towards Shannon capacity!

  32. Modulation/Demodulation • Line coding: representation of binary bits without carrier (baseband coding) • Modulation/demodulation: representation of digital bits with carrier (broadband coding) • Analog to Digital Coding

  33. Line Coding • Unipolar: all signal elements have same sign • Polar: one logic state represented by positive voltage the other by negative voltage • Data rate: rate of transmitted data (bps) • Bit period: time taken for transmitter to emit the bit, the duration or length of a bit • Modulation rate: rate at which the signal level changes, measured in baud (symbols per sec)

  34. Schemes • Non-return to Zero-Level (NRZ-L) • Non-return to Zero Inverted (NRZI) • Bipolar-AMI • Pseudo-ternary • Manchester • Differential Manchester

  35. Nonreturn to Zero-Level (NRZ-L) • Two different voltages for 0 and 1 bits • Voltage constant during bit interval • no transition, i.e. no return to zero voltage • e.g., Absence of voltage for zero, constant positive voltage for one (Unipolar NRZ) • More often, negative voltage for one value and positive for the other---NRZ-L (Polar NRZ)

  36. Nonreturn to Zero Inverted • Nonreturn to zero inverted on ones • Constant voltage pulse for duration of bit • Data encoded as presence or absence of signal transition at beginning of bit time • 1: Transition (low to high or high to low) • 0: No transition • An example of differential encoding

  37. NRZ

  38. Differential Encoding • Data represented by changes rather than levels • More reliable detection of transition rather than level • In complex transmission layouts it is easy to lose sense of polarity

  39. Multilevel Binary • Use more than two levels • Bipolar-AMI • 0: no line signal • 1: positive or negative pulse • pulses for 1’s alternate in polarity • No loss of sync if a long string of ones (zeros still a problem) • No net dc component • Lower bandwidth • Easy error detection

  40. Pseudo-ternary • 1: absence of line signal • 0: alternating positive and negative • No advantage or disadvantage over bipolar-AMI Change for 1’s No signal No signal Change for 0’s

  41. Biphase • Manchester • Transition in middle of each bit period • Transition serves as clock and data • 1: low to high, 0: high to low • Used by IEEE 802.3 (Ethernet) • Differential Manchester • Midbit transition is clocking only • 0: transition at start of a bit period • 1: no transition at start of a bit period • Used by IEEE 802.5 (Token Ring)

  42. Manchester Coding

  43. Spectra • Used for the selection of line codes in conjunction with the channel characteristics: design the system so that most power is concentrated in the allowed range Figure 3.26

  44. Modulation Schemes (Binary) • Public telephone system • 300Hz to 3400Hz • Use modem (modulator-demodulator) • Amplitude Shift Keying (ASK) • Frequency Shift Keying (FSK) • Phase Shift Keying (PSK)

  45. Binary ASK,FSK, PSK Bit-stream ASK FSK PSK

  46. Binary Keying Schemes

  47. Digital Modulation • Binary keying schemes are simple, but not efficient! • Digital modulation uses multiple symbols (waveforms) to improve the efficiency • Information bearers: - Amplitude - Frequency - Phase • Mapping: a block of bits to a waveform

  48. QPSK • Quadrature Phase Shift Keying

  49. 2-D signal Bk Ak 4 “levels”/ pulse 2 bits / pulse 2W bits per second Signal Constellation • QPSK and QAM Bk 2-D signal Ak 16 “levels”/ pulse 4 bits / pulse 4W bits per second Figure 3.33

  50. QAM • Quadrature Amplitude Modulation (QAM)

More Related