Mobile Computing & Modulation PDF

Physical layer Modulation Mobile Computing [email protected] TYPES OF SIGNALS (a) continuous time/discrete time (b) continuous values/discrete values  analog signal = continuous time, continuous values  digital signal = discrete time, discrete values Periodic signal - analog or digi...

Physical layer Modulation Mobile Computing [email protected] TYPES OF SIGNALS (a) continuous time/discrete time (b) continuous values/discrete values  analog signal = continuous time, continuous values  digital signal = discrete time, discrete values Periodic signal - analog or digital signal that repeats over time  s(t +T ) = s(t ) -< t < +  where T is the period of the signal signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift   sine wave as special periodic signal for a carrier: s(t) = At sin(2  ft t + t) SINE WAVE PARAMETERS  MODULATION Digital modulation  digital data is translated into an analog signal (baseband)  ASK, FSK, PSK - main focus in this chapter  differences in spectral efficiency, power efficiency, robustness Analog modulation  shifts center frequency of baseband signal up to the radio carrier Motivation  smaller antennas (e.g., /4)  Frequency Division Multiplexing  medium characteristics Basic schemes  Amplitude Modulation (AM)  Frequency Modulation (FM)  Phase Modulation (PM) MODULATION AND DEMODULATION analog baseband digital signal data digital analog 101101001 modulation modulation radio transmitter radio carrier analog baseband digital signal analog synchronization data demodulation decision 101101001 radio receiver radio carrier  DIGITAL MODULATION Modulation of digital signals known as Shift Keying 1 0 1 Amplitude Shift Keying (ASK):  very simple t  low bandwidth requirements  very susceptible to interference 1 0 1 Frequency Shift Keying (FSK):  needs larger bandwidth t 1 0 1 Phase Shift Keying (PSK):  more complex  robust against interference t EX. Draw the bit sequence “001101010” using the following types of digital modulation : 1. Amplitude Shift Keying (ASK) 2. Frequency Shift Keying (FSK) 3. Phase Shift Keying (PSK)  ASK 1 a “001101010” f 0  FSK 1 “001101010” 0  PSK 1 0  ADVANCED FREQUENCY SHIFT KEYING  A famous FSK scheme used in many wireless systems is minimum shift keying (MSK).  MSK is basically BFSK (Binary FSK) without abrupt phase changes, i.e., it belongs to CPM (Continuous Phase Modulation) schemes.  In a first step, data bits are separated into even and odd bits, the duration of each bit being doubled.  The scheme also uses two frequencies: f1, the lower frequency, and f2, the higher frequency, with f2 = 2f1. MSK According to the following scheme, the lower or higher frequency is chosen (either inverted or non-inverted) to generate the MSK signal: if the even and the odd bit are both 0, then the higher frequency f2 is inverted bit (i.e., f2 is used with a phase shift of 180°); even odd if the even bit is 1, the odd bit 0, then the lower frequency f1 is inverted. Signal value This is the case, e.g., in the fifth to seventh columns, if the even bit is 0 and the odd bit is 1, h: high frequency f1 is taken without changing the phase, n: low frequency +: original signal if both bits are 1 then the original f2 is taken. -: inverted signal EXAMPLE OF MSK 1 0 1 1 0 1 0 data even bits odd bits t EXAMPLE OF MSK 1 0 1 1 0 1 0 data bit even 0101 even bits odd 0011 odd bits signal hnnh value - - ++ low h: high frequency frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t EXAMPLE OF MSK 1 0 1 1 0 1 0 data bit even 0101 even bits odd 0011 odd bits signal hnnh value - - ++ low h: high frequency frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t EX: Draw the resultant signal for the given bit sequence “00110110” using MSK scheme? EX: Draw the resultant signal for the given bit sequence “00110110” using MSK scheme? 0 0 1 1 0 1 1 data bit even 0101 even bits odd 0011 odd bits signal hnnh value - - ++ low h: high frequency frequency n: low frequency +: original signal -: inverted signal high frequency MSK signal t ADVANCED PHASE SHIFT KEYING BPSK (Binary Phase Shift Keying):  bit value 0: sine wave  bit value 1: inverted sine wave  very simple PSK  low spectral efficiency  robust, used e.g. in satellite systems ADVANCED PHASE SHIFT KEYING 10 Q 11 QPSK (Quadrature Phase Shift Keying):  2 bits coded as one symbol  symbol determines shift of sine wave I  needs less bandwidth compared to BPSK  more complex 00 01 Often also transmission of relative, not absolute phase A shift: DQPSK - Differential QPSK (IS-136, PHS) t 11 10 00 01 EX. Draw the bit sequence “00110110” using QPSK (Quadrature Phase Shift Keying).  Q Draw the bit sequence “00110110” using QPSK 10 11 EX. (Quadrature Phase Shift Keying). I 00 01 Basic QPSK Modulator and Demodulator QUADRATURE AMPLITUDE MODULATION Q. Quadrature Amplitude Modulation (QAM) 0010 0001  combines amplitude and phase modulation  it is possible to code n bits using one symbol 0011 0000  2n discrete levels, n=2 identical to QPSK φ a I 1000 Bit error rate increases with n, but less errors compared to comparable PSK schemes  Example: 16-QAM (4 bits = 1 symbol)  Symbols 0011 and 0001 have the same phase φ, but different amplitude a.  0000 and 1000 have different phase, but same amplitude HIERARCHICAL MODULATION DVB-T modulates two separate data streams onto Q a single DVB-T stream High Priority (HP) embedded within a Low Priority (LP) stream Multi carrier system, about 2000 or 8000 carriers 10 QPSK, 16 QAM, 64QAM I Example: 64QAM  good reception: resolve the entire 64QAM constellation  poor reception, mobile reception: 00 resolve only QPSK portion 000010 010101  6 bit per QAM symbol, 2 most significant determine QPSK  HP service coded in QPSK (2 bit), LP uses remaining 4 bit FREQUENCY DOMAIN Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency Spectrum - range of frequencies that a signal contains Absolute bandwidth - width of the spectrum of a signal Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in BIT RATES, CHANNEL CAPACITY Impairments, such as noise, limit data rate that can be achieved For digital data, to what extent do impairments limit data rate? Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions SIGNAL-TO-NOISE RATIO Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission Typically measured at a receiver Signal-to-noise ratio (SNR, or S/N) signal power ( SNR ) dB  10 log 10 noise power A high SNR means a high-quality signal, low number of required intermediate repeaters SNR sets upper bound on achievable data rate SHANNON CAPACITY FORMULA Equation: C  B log 2 1  SNR Represents theoretical maximum that can be achieved In practice, only much lower rates achieved  Formula assumes white noise (thermal noise)  Impulse noise is not accounted for  Attenuation distortion or delay distortion not accounted for EXAMPLE OF NYQUIST AND SHANNON FORMULATIONS Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB B  4 MHz  3 MHz  1 MHz SNR dB  24 dB  10 log 10 SNR  SNR  251 Using Shannon’s formula C  B log 2 1  SNR C  10  log 2 1  251   10  8  8Mbps 6 6 SHANNON CAPACITY FORMULA – EXAMPLE 1 SHANNON CAPACITY FORMULA – EXAMPLE 1 EXAMPLE OF NYQUIST AND SHANNON FORMULATIONS How many signaling levels are required? C  2 B log 2 M 6   8 10  2  10  log 2 M 6 4  log 2 M M  16  FREQUENCIES FOR WIRELESS COMMUNICATION VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency Frequency and wave length   = c/f  wave length , speed of light c  3x108m/s, frequency f twisted coax cable optical transmission pair 1 Mm 10 km 100 m 1m 10 mm 100 m 1 m 300 Hz 30 kHz 3 MHz 300 MHz 30 GHz 3 THz 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible light UV Spread Spectrum A spread-spectrum system is one in which the transmitted signal is spread over a wide frequency band, much wider than the bandwidth required to transmit the message. Such a system would take a baseband voice signal with a bandwidth of a few kilohertz and spread it to a band of many megahertz. Spread Spectrum Concept Input fed into channel encoder  Produces narrow bandwidth analog signal around central frequency Signal modulated using sequence of digits  Spreading code/sequence  Typically generated by pseudonoise/pseudorandom number generator Increases bandwidth significantly  Spreads spectrum Receiver uses same sequence to demodulate signal Demodulated signal fed into channel decoder Spread Spectrum Two types of spread-spectrum systems are: Direct-sequence system: A digital code sequence with a bit rate higher than the message is used to obtain the modulated signal. Frequency-hopping system: The carrier frequency is shifted in discrete increments in a pattern dictated by a code sequence. We will not consider this here. DIRECT SEQUENCE SPREAD SPECTRUM (DSSS) Each bit represented by multiple bits using spreading code Spreading code spreads signal across wider frequency band  In proportion to number of bits used  10 bit spreading code spreads signal across 10 times bandwidth of 1 bit code One method:  Combine input with spreading code using XOR  Input bit 1 inverts spreading code bit  Input zero bit doesn’t alter spreading code bit  Data rate equal to original spreading code Performance similar to FHSS DIRECT SEQUENCE SPREAD SPECTRUM EXAMPLE DIRECT SEQUENCE SPREAD SPECTRUM TRANSMITTER DIRECT SEQUENCE SPREAD SPECTRUM TRANSMITTER DIRECT SEQUENCE SPREAD SPECTRUM USING BPSK EXAMPLE DIRECT SEQUENCE SPREAD SPECTRUM USING BPSK EXAMPLE APPROXIMATE SPECTRUM OF DSSS SIGNAL FREQUENCY HOPPING EXAMPLE GENERAL MODEL OF SPREAD SPECTRUM SYSTEM GAINS Immunity from various noise and multipath distortion  Including jamming Can hide/encrypt signals  Only receiver who knows spreading code can retrieve signal Several users can share same higher bandwidth with little interference  Cellular telephones  Code division multiplexing (CDM)  Code division multiple access (CDMA) PSEUDORANDOM NUMBERS Generated by algorithm using initial seed Deterministic algorithm  Not actually random  If algorithm good, results pass reasonable tests of randomness Need to know algorithm and seed to predict sequence FREQUENCY HOPPING SPREAD SPECTRUM (FHSS)  FHSS is a wireless technology that spreads its signal over rapidly hopping radio frequencies, it is highly resistant to interference and is difficult to intercept.  Interference at a specific frequency only affects the transmission during that extremely short interval, making FHSS inherently cybersecure. oSignal broadcast over seemingly random series of frequencies oReceiver hops between frequencies in sync with transmitter oEavesdroppers hear unintelligible blips oJamming on one frequency affects only a few bits BASIC OPERATION Typically 2k carriers frequencies forming 2k channels Channel spacing corresponds with bandwidth of input Each channel used for fixed interval  300 ms in IEEE 802.11  Some number of bits transmitted using some encoding scheme  May be fractions of bit  Sequence dictated by spreading code FREQUENCY HOPPING EXAMPLE FREQUENCY HOPPING SPREAD SPECTRUM SYSTEM (TRANSMITTER) FREQUENCY HOPPING SPREAD SPECTRUM SYSTEM (RECEIVER) SLOW AND FAST FHSS Frequency shifted every Tc seconds Duration of signal element is Ts seconds Slow FHSS has Tc  Ts Fast FHSS has Tc < Ts Generally fast FHSS gives improved performance in noise (or jamming) SLOW FREQUENCY HOP SPREAD SPECTRUM USING MFSK (M=4, K=2) FAST FREQUENCY HOP SPREAD SPECTRUM USING MFSK (M=4, K=2) FHSS AND WLAN ACCESS POINTS IEEE 802.11 FHSS WLAN specifies 78 hopping channels separated by 1 MHz in 3 groups (0,3,6,9,…, 75), (1,4,7,…, 76), (2,5,8,…,77) Allows installation of 3 AP’s in the same area.