A Review of Key Wireless Physical Layer Concepts Raj Jain Professor of Computer Science and Engineering Washington University in Saint Louis Saint Louis MO 63130 Jain cse wustl edu Audio Video recordings of this lecture are available at http www cse wustl edu jain cse574 10 Washington University in St Louis CSE574s 3 1 2010 Raj Jain Overview Basic Concepts Coding Phase Shift Keying PSK QAM Decibels Channel Capacity Nyquist Theorem Shannon s Theorem Hamming Distance Error Correction Antenna Reflection Diffraction and Scattering Multipath Propagation Recent Development Spread Spectrum Code Division Multiple Access OFDM Turbo Codes Washington University in St Louis CSE574s 3 2 2010 Raj Jain Electromagnetic Spectrum Wireless Wireless communication uses 100 kHz to 60 GHz Washington University in St Louis CSE574s 3 3 2010 Raj Jain Frequency Period and Phase A Sin 2 ft A Amplitude f Frequency Phase Period T 1 f Frequency is measured in Cycles sec or Hertz Washington University in St Louis CSE574s 3 4 2010 Raj Jain Wavelength Amplitude Distance Distance occupied by one cycle Distance between two points of corresponding phase in two consecutive cycles Wavelength Assuming signal velocity v vT f v c 3 108 m s speed of light in free space 300 m s Washington University in St Louis CSE574s 3 5 2010 Raj Jain Time and Frequency Domains A Amplitude Frequency f Amplitude Amplitude A 3 3f A A 3 f Washington University in St Louis CSE574s 3 6 Frequency 3f Frequency 2010 Raj Jain Decibels Attenuation Log10 Pin Pout Bel Attenuation 10 Log10 Pin Pout Attenuation 20 Log10 Vin Vout Example 1 Pin 10 mW Pout 5 mW Attenuation 10 log 10 10 5 10 log 10 2 3 dB Example 2 Pin 100mW Pout 1 mW Attenuation 10 log 10 100 1 10 log 10 100 20 dB Washington University in St Louis CSE574s 3 7 decibel decibel 2010 Raj Jain Coding Terminology Pulse 5V 0 5V Bit 5V 0 5V Signal element Pulse of constant amplitude frequency phase Modulation Rate 1 Duration of the smallest element Baud rate Data Rate Bits per second Data Rate Fn Bandwidth signal noise ratio encoding Washington University in St Louis CSE574s 3 8 2010 Raj Jain Phase Shift Keying PSK Differential PSK 0 Same phase 1 Opposite phase 1 0 A cos 2 ft A cos 2 ft Quadrature PSK QPSK Two bits 11 A cos 2 ft 45 10 A cos 2 ft 135 10 11 00 A cos 2 ft 225 01 A cos 2 ft 315 Sum of two signals 90 apart in phase 00 01 In phase I Quadrature Q Up to 180 phase difference between successive intervals Washington University in St Louis CSE574s 3 9 2010 Raj Jain QAM Quadrature Amplitude and Phase Modulation QAM 4 QAM 16 QAM 64 QAM 256 Used in DSL and wireless networks Q 01 Q 11 I 0 1 Binary Washington University in St Louis Q I 00 I 10 QAM 4 QAM 16 CSE574s 3 10 2010 Raj Jain Antenna Transmitter converts electrical energy to electromagnetic waves Receiver converts electromagnetic waves to electrical energy Same antenna is used for transmission and reception Omni Directional Power radiated in all directions Directional Most power in the desired direction Isotropic antenna Radiates in all directions equally Antenna Gain Power at particular point Power with Isotropic Expressed in dBi Pr Pt Gt Gr 4 d 2 Omni Directional Washington University in St Louis Directional CSE574s 3 11 Isotropic 2010 Raj Jain Reflection Diffraction Scattering Eflection Phase shift cattering iffraction Washington University in St Louis CSE574s 3 12 2010 Raj Jain Reflection Diffraction and Scattering Reflection Surface large relative to wavelength of signal May have phase shift from original May cancel out original or increase it Diffraction Edge of impenetrable body that is large relative to May receive signal even if no line of sight LOS to transmitter Scattering Obstacle size on order of wavelength Lamp posts etc If LOS diffracted and scattered signals not significant Reflected signals may be If no LOS diffraction and scattering are primary means of reception Washington University in St Louis CSE574s 3 13 2010 Raj Jain Multipath Propagation Inter symbol Interference Delay Spread Time between first and last versions of signal Fading Fluctuation in amplitude phase or delay spread Multipath may add constructively or destructively Fast fading Washington University in St Louis CSE574s 3 14 2010 Raj Jain Doppler Shift If the transmitter or receiver or both are mobile the frequency of received signal changes Moving towards each other Frequency increases Moving away from each other Frequency decreases Frequency difference velocity Wavelength Example 2 4 GHz l 3x108 2 4x109 125m 120km hr 120x1000 3600 33 3 m s Freq diff 33 3 125 267 Hz Washington University in St Louis CSE574s 3 15 2010 Raj Jain Channel Capacity Capacity Maximum data rate for a channel Nyquist Theorem Bandwidth B Data rate 2 B Bi level Encoding Data rate 2 Bandwidth 5V 0 Multilevel Data rate 2 Bandwidth log 2 M Example M 4 Capacity 4 Bandwidth Washington University in St Louis CSE574s 3 16 2010 Raj Jain Shannon s Theorem Bandwidth B Hz Signal to noise ratio S N Maximum number of bits sec B log2 1 S N Example Phone wire bandwidth 3100 Hz S N 30 dB 10 Log 10 S N 30 Log 10 S N 3 S N 103 1000 Capacity 3100 log 2 1 1000 30 894 bps Washington University in St Louis CSE574s 3 17 2010 Raj Jain Hamming Distance Hamming Distance between two sequences Number of bits in which they disagree Example 011011 110001 Difference 101010 Distance 3 Washington University in St Louis CSE574s 3 18 2010 Raj Jain Error Correction Example 2 bit words transmitted as 5 bit word Data Codeword 00 00000 01 00111 10 11001 11 11110 Received 00100 Not one of the code words Error Distance 00100 00000 1 Distance 00100 00111 2 Distance 00100 11001 4 Distance 00100 11110 3 Most likely 00000 was sent Corrected data 00 b Received 01010 Distance 00000 2 Distance 11110 Error detected but cannot be corrected c Three bit errors will not be detected Sent 00000 Received 00111 Washington University in St Louis CSE574s 3 19 2010 Raj Jain Turbo Codes Data Input xi Systemic Output xi Upper Parity zi Encoder Parallel to Serial Coded Converter Output Interleaved Lower Interleave Interleaver Input x i Encoder Parity z i Normal FEC codes 3dB below the Shannon limit Turbo Codes 0 5dB below Shannon limit Developed by French coding theorists in 1993 Use two coders with an interleaver Interleaver rearranges bits in a prescribed but irregular manner 3rd Generation cellular networks use turbo codes Washington University in St Louis CSE574s 3 20 2010 Raj Jain Frequency Hopping Spread Spectrum Frequency 50 ms Time
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