Report

Modulation Techniques for Mobile Radio 1 Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission. It generally involves translating a base band signal (or source) to a band pass signal, centered at a high carrier frequency. Demodulation is the process of extracting the base band message from the carrier. Modulation Techniques Modulation Analog Modulation (First Generation (1G) Mobile Radio) 2 Digital Modulation (2G, 3G and 4G systems) Review of Analog Modulation Techniques Amplitude Modulation (AM) • Message Signal -• Carrier Signal -• AM Signal 3 m(t) Ac cos(2πfc t) -- SAM (t) = Ac [1+m(t)]cos(2fc t) AM Spectrum S AM (f) = 0.5A [ (f - f ) + M(f - f ) + (f + f ) + M(f + f )] c Carrier 4 c Sidebands c c c AM Parameters • Modulation Index -- k= A / A 1 m c • Bandwidth -- BAM = 2 fm • Total Power in AM Signal PAM = 0.5A 2 [ 1 + 2 <m(t)> + <m2 (t)> ] c • Power in the carrier 5 -- Pc = Ac 2 / 2 Single Singleband AM Signal Lower Sideband S SSB A c [m(t)cos(2fc t) m(t)sin(2fc t)] Upper Sideband Where the Hilbert transform is defined as: m(t) m(t) h(t) 6 ; h(t) 1/ t SSB Generation Filter Method Baseband Filter m(t) A cos(2f t) c c SSSB (t) Baseband filter passes upper or lower sidebands 7 Balanced Modulator Carrier fc m(t) 8 -90o phase shift ∑ 90o SSSB (t) Properties of SSB 9 Bandwidth of SSB is very efficient = fm .However, Doppler spreading and Rayleigh fading can shift the signal spectrum, causing distortion. Frequency of the receiver oscillator must be exactly the same as that of the transmitted carrier fc. If not, this results in a frequency shift fc f, causing distortion. Pilot Tone SSB 10 Transmit a low level pilot tone along with the SSB signal. The pilot tone has information on the frequency and amplitude of the carrier. The pilot tone can be tracked using signal processing FFSR - Feed Forward Signal Regeneration. TTIB (Transparent Tone In-Band) System a ~ b m(t) 11 a ~ c f1 ~ d e ~ f2 f a b c f1 d e f 12 f2 BW f 2 f 1 frequency Properties of TTIB system 13 Base band signal is split into two equal width segments. Small portion of audio spectrum is removed and a low-level pilot tone is inserted in its place. This procedure maintains the low bandwidth of the SSB signal. Provides good adjacent channel protection. Demodulation of AM signals • Coherent Modulation • Non-coherent demodulation • Envelope Detectors 14 Demodulation of AM signals Coherent Modulation SAM R(t)cos(2fc t r ) A o cos(2fc t o ) 15 LPF VOUT (t) 0.5A oR(t)cos(r o ) Frequency Modulation • Message Signal – m(t) • FM Signal – S (t) A cos[2f t 2k FM C c • Power in FM Signal – PFM A c / 2 2 t f m(t)dt] • Frequency modulation index – k A / W f / W f f M 16 W = Highest frequency component in message signal AM = Peak value of modulating signal Phase Modulation • PM Signal SPM (t) = Ac cos[2 fc t +k m(t)] 17 • Phase Modulation Index k A M • Power in PM signal PPM A c 2 / 2 • Bandwidth BT 2f FM methods FM Modulation Direct Method – VCO Indirect Method – Armstrong FM Detection Slope Detection Zero Crossing Detection PLL Detection Quadrature Detection 18 Comparison between AM and FM FM AM •FM signals are less noisy, •AM signals are more noisy, because amplitude of signal amplitude cannot be limited is constant •The modulation index can be varied to obtain greater SNR(6dB for each doubling in bandwidth) •FM signals occupy more bandwidth (good for audio) 19 •Modulation index cannot be changed automatically. •AM signals occupy lesser bandwidth (good for video) Digital Modulation 20 VLSI and DSP promoted the advent of Digital Modulation Low noise Easier multiplexing of information (voice, data, video) Can accommodate digital transmission errors, source coding, encryption and equalization. DSP can implement digital modulators, demodulators completely in software. Basics of digital communications 21 In digital communication systems, the message) is represented as a time sequence of symbols or pulses. Each symbol has m finite states Number of bits required for m states: n = log2m bits/symbol Shannon’s bandwidth theorem 22 Bandwidth efficiency B = R / B bps/Hz R=Data rate in bits/second B=Bandwidth of modulated RF signal Shannon's formula: Bmax = C/B = channel capacity (bits/s) RF bandwidth = log2(1 + S/N) S/N = Signal to Noise ratio Practical digital systems For US digital cellular standard, R = 48.6 kbps RF bandwidth = 30 KHz For SNR 20 dB => 100 C = 30000 * log2(1 + S/N) 23 = 30000 * log2(1 + 100) = 199.75 kbps For GSM standard, R = 270.833 kbps C = 1.99 Mbps for S/N = 30 dB Line Coding 24 Line codes are used to provide particular spectral characteristics of a pulse train. Line codes provide the pulses to represent 0s and 1s. Line codes can be: o Return-to-zero (RZ) o Non-return-to-zero (NRZ) Line codes are Unipolar (0,V) or Bipolar (-V, V ) 1 0 Unipolar NRZ 1 V 0 Unipolar RZ V Bipolar NRZ -V 25 1 0 Pulse Shaping Techniques Bandlimited Channel 26 ISI – Inter Symbol Interference errors in transmission of symbols Pulse shaping techniques reduce the intersymbol effects Pulse shaping filters Raised cosine filter hRC (t) 27 sin(t/Ts ) cos(at/Ts) [ ] 2 (t/Ts ) (1 4at/2Ts) As the value of a (roll-off factor) increases, the bandwidth of the filter also increases As the value of a (roll-off factor) increases, the time sidelobe levels decrease. Implementation of raised-cosine filter Use identical [HRC (f)]1/2 filters at transmitter and receiver Symbol rate possible through raised cosine filter Rs 1/ Ts 2B/(1 a) where B is the filter bandwidth 28 Types of Digital Modulation Linear Non-Linear Amplitude of Amplitude of transmitted signal carrier is constant varies linearly with message signal m(t) 29 Spread Spectrum Transmission bandwidth >> signal bandwidth Low bandwidthallows more users High bandwidth – More usersLow noise high bandwidth Example systems: BPSK, QPSK FSK, GMSK W-CDMA, cdma 2000 Linear digital modulation 30 PSK or Phase Shift Keying of carrier: SPSK = A cos(wt + fk) fk = 0, (BPSK) fk = 0, /2, , 3/2 (QPSK) fk = 0,/4,/2,3/4,,5/4,3/2,7/4 (0PSK) Constellation Diagram Q(Quadrature) I (In Phase) 31 Properties of BPSK and QPSK BPSK BW = 2 RB = 2 / TB Pe,QPSK = Q[√(2 EB / N0)] RB – Bit rate, TB – Bit period EB – Energy/bit, N0 – Noise spectral density QPSK BW = RB = 1 / TB Pe,QPSK = Q[√(2 EB / N0)] 32 Nonlinear or envelope modulation Frequency shift keying The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0) SFSK Vh (t) (2Eb / Tb ) cos[2fc 2f]t,0 T Tb 1/ 2 SFSK Vi (t) (2Eb / Tb ) cos[2fc 2f]t,0 T Tb 1/ 2 33 Properties of FSK Transmission Bandwidth BT = 2f + 2B B = Bandwidth of digital base-band signal If a raised cosine pulse-shaping filter is used BT = 2f + (1 + a)R Probability of error 1/2 Pe,FSK = Q[(EB / N0) ] 34 Spread Spectrum Modulation techniques 35 Spread spectrum techniques employ a transmission bandwidth >> signal bandwidth The system is inefficient for a single user, but is efficient for many users Many users use the same bandwidth without significantly interfering with one another Principle of spread spectrum 36 Spread spectrum signals are pseudo random, and spreading waveform is controlled by a PN (pseudo – noise) sequence or code. Spread spectrum signals are demodulated at the receiver by cross correlation (matching) with the correct PN sequence. Advantages of spread spectrum techniques 37 PN codes are approximately orthogonal, and the receiver can separate each user based on their codes. Resistance to multi-path fading, because of large bandwidths and narrow time widths. PN Sequences 38 Pseudo Noise or Pseudo random sequence is a binary sequence of 1s and -1s PN sequences are generated by using sequential logic circuits Very low cross correlation exists between any two PN sequences High cross correlation exists between identical PN sequences Frequency Hopped Spread spectrum (FHSS) 39 A frequency hopping signal periodically changes the carrier frequency in a pseudorandom fashion. The set of possible carrier frequencies is called a hopset. Bandwidth of channel used in hopset Instantaneous bandwidth B Bandwidth of spectrum over which the hopping occurs total hopping bandwidth Wss Methodology of FHSS 40 Time duration between hops hopping period Ts Data is sent by hopping the transmitter carrier to seemingly random channels Small bursts of data are sent using conventional narrow band modulation before T/R hops again Hit -> Two users using the same frequency band at the same time Frequency Hopping Modulator Modulator DATA Oscillator Code Block 41 Frequency Hopping Signal Frequency Synchronizer PN Code Generator Frequency hopping demodulator Wideband Filter Frequency Hopping Signal 42 BP Filter Demodulation DATA Frequency Synthesizer PN Code Generator Synchronization System Properties of FHSS 43 Fast frequency hopping More than one frequency hop during each transmitted symbol -> Hopping rate ≥ symbol rate Slow frequency hopping Hopping rate < symbol rate Parameters of FH-SS 44 Probability of error for BPSK Spread Spectrum Pe = 0.5 x e -Eb/ 2N0 x (1 – ph ) + 0.5 ph ph = probability of hit = 1 – (1 – 1/M)K-1 M = number of hopping channels K = Total number of users Processing gain = Wss / B Direct Sequence Spread Spectrum (DSSS) code C1 C2 CN time 45 frequency Properties of Message Signal 46 m(t) is a time sequence of non-overlapping pulses of duration T, each of which has an amplitude (+/-) 1. The PN waveform consists of N pulses or chips for message symbol period T. NTC = T where TC is the chip period. Example: N=4 1 -1 1 -1 47 PN Wave for N =4 DSSS Transmitter S1(t) m1 (t) PN1(t) cos(2fc t 1 ) k mk(t) 48 1 PNK (t) cos(2fc t k ) r(t) Principles of transmitter operation The narrowband message signal mi(t) is multiplied by a pseudo noise code sequence that has a chip rate >> data rate of message. All users use the same carrier frequency and may transmit simultaneously. The kth transmitted signal is given by: Sk (t) (2Es / Ts )1/ 2mk (t)pk (t)cos(2fc t k ) 49 CDMA Receiver k Zi (t) (.)dt r(t) PNK (t) 50 cos(2fc t k ) mk (t) Principles of receiver operation At the receiver, the received signal is correlated with the appropriate PN sequence to produce desired variable. Z i1 (t) iT 1 (i 1)T 1 51 r(t)p1 (t 1 )cos[2fc (t 1 ) 1 ]dt Correlator output for ith user Z i1 (t) iT 1 r(t)p1 (t 1 )cos[2fc (t 1 ) 1 ]dt (i 1)T 1 •The multiplied signal will be p2(t) = 1 for the correct signal and will yield the dispersed signal and can be demodulated to yield the message signal mi(t). S1(t) (2Es / Ts ) m1(t)p1(t)cos(2fc t 1) 1/ 2 52 Parameters of DSSS Probability of bit error Pe = Q {1/ [(K –1)/3N + (N0/2Eb)]1/2} K = Number of users N = Number of chips/ symbol When Eb/No Pe = Q{[3N/(K-1)]1/2 } 53 Important Advantages of CDMA 54 Many users of CDMA use the same frequency. Either TDD or FDD may be used. Multipath fading may be substantially reduced because of large signal bandwidth. There is no absolute limit on the number of users in CDMA. Rather the system performance gradually degrades for all users as the number of users is increased. Drawbacks of CDMA 55 Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN sequences are not exactly orthogonal. The near- far problem occurs at a CDMA receiver if an undesired user has high detected power as compared to the desired user. Modulation performance in fading channels Fading Channel 56 s(t) r(t) r(t) = a(t) e-j(t) s(t) + n(t) a(t) = gain of the channel (t) = phase shift of the channel n(t) = additive gaussian noise Average signal to noise ratio at receiver = (EB / N0) a2 Probability of error in fading channels Probability of error Pe Pe (X)p(X)dX 0 Pe (X)= Probability of error for parent modulation, such as BPSK, FSK p(X) = pdf of x due to fading channel = (1/ )exp( x / ),x 0 57 Pe (X) and Pe for different systems Coherent binary PSK Pe (X) Q[(2EB /N0 ) ] 1/ 2 Pe 0.5[1 /(1 )] Coherent binary FSK Pe (X) Q[(EB /N0 ) ] 1/ 2 Pe 0.5[1 /(2 )] 58 Differential Binary PSK Pe (X) 0.5exp[(EB /N0 )] Pe [0.5/(1 )] Non-coherent orthogonal binary FSK Pe (X) 0.5exp[(EB / 2N0 )] Pe [0.5/(2 )] 59 Coherent GMSK Pe (X) Q[(2EB )] Pe 0.5{1 [ /( 1)] 1/ 2 1/ 4 =0.68, BT = 0.25, = 0.68 =0.85, BT = , = 0.85 BT = Bandwidth – bit duration product for GMSK 60