Modulation Techniques for Mobile Radio l Modulation is
Modulation Techniques for Mobile Radio l Modulation is the process of encoding the baseband or source information (voice, video, text) in a manner suitable for transmission. l 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 l 1
Modulation Techniques Modulation Analog Modulation (First Generation (1 G) Mobile Radio) 2 Digital Modulation (2 G, 3 G and 4 G systems)
Review of Analog Modulation Techniques Amplitude Modulation (AM) 3 • Message Signal -- • Carrier Signal -- • AM Signal --
AM Spectrum Carrier 4 Sidebands
AM Parameters 5 • Modulation Index -- • Bandwidth -- • Total Power in AM Signal • Power in the carrier --
Singleband AM Signal Lower Sideband Upper Sideband Where the Hilbert transform is defined as: ; 6
SSB Generation Filter Method Baseband Filter Baseband filter passes upper or lower sidebands 7
Balanced Modulator Carrier fc -90 o phase shift 8 ∑ 90 o
Properties of SSB l l l 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 l l l 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 ~ e ~ ~ 11 ~
12
Properties of TTIB system l l 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
signals Demodulation of AM Coherent Modulation 15
Frequency Modulation 16 • Message Signal – • FM Signal – • Power in FM Signal – • Frequency modulation index – W = Highest frequency component in message signal AM = Peak value of modulating signal
Phase Modulation • PM Signal SPM (t) = Ac cos[2 p fc t +k m(t)] 17 • Phase Modulation Index • Power in PM signal • Bandwidth
FM methods FM Modulation l Direct Method – VCO l Indirect Method – Armstrong FM Detection l Slope Detection l Zero Crossing Detection l PLL Detection l Quadrature Detection 18
Comparison between AM and FM 19 FM • FM signals are less noisy, because amplitude of signal is constant • The modulation index can be varied to obtain greater SNR(6 d. B for each doubling in bandwidth) • FM signals occupy more bandwidth (good for audio) AM • AM signals are more noisy, amplitude cannot be limited • Modulation index cannot be changed automatically. • AM signals occupy lesser bandwidth (good for video)
Digital Modulation l l l 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 l In digital communication systems, the message) is represented as a time sequence of symbols or pulses. l Each symbol has m finite states l Number of bits required for m states: n = log 2 m bits/symbol
Shannon’s bandwidth theorem l l 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 = log 2(1 + S/N) S/N = Signal to Noise ratio
l Practical digital systems For US digital cellular standard, R = 48. 6 kbps RF bandwidth = 30 KHz For SNR 20 d. B => 100 C = 30000 * log 2(1 + S/N) l 23 = 30000 * log 2(1 + 100) = 199. 75 kbps For GSM standard, R = 270. 833 kbps C = 1. 99 Mbps for S/N = 30 d. B
Line Coding l l 24 Line codes are used to provide particular spectral characteristics of a pulse train. Line codes provide the pulses to represent 0 s and 1 s. 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 )
Unipolar NRZ 1 0 1 1 V 0 V -V 25 Unipolar RZ Bipolar NRZ 0
Pulse Shaping Techniques Bandlimited Channel l l 26 ISI – Inter Symbol Interference errors in transmission of symbols Pulse shaping techniques reduce the inter-symbol effects
Pulse shaping filters l l l 27 Raised cosine filter As the value of (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 l l Use identical [HRC (f)]1/2 filters at transmitter and receiver Symbol rate possible through raised cosine filter where B is the filter bandwidth 28
Types of Digital Modulation Linear 29 Amplitude of transmitted signal varies linearly with message signal m(t) Low bandwidthallows more users Example systems: Non-Linear Spread Spectrum Amplitude of carrier is constant Transmissio n bandwidth >> signal bandwidth High bandwidth – Low noise FSK, GMSK More usershigh bandwidth W-CDMA, cdma 2000
Linear digital modulation l l 30 PSK or Phase Shift Keying of carrier: SPSK = A cos(wt + fk) fk = 0, p (BPSK) fk = 0, p/2, p, 3 p/2 (QPSK) fk = 0, p/4, p/2, 3 p/4, p, 5 p/4, 3 p/2, 7 p/4 (0 PSK)
Constellation Diagram Q(Quadrature) I (In Phase) 31
Properties of BPSK and QPSK l 32 BPSK BW = 2 RB = 2 / TB Pe, BPSK = Q[√(2 EB / N 0)] RB – Bit rate, TB – Bit period EB – Energy/bit, N 0 – Noise spectral density l QPSK BW = RB = 1 / TB Pe, QPSK = Q[√(2 EB / N 0)]
Nonlinear or envelope modulation Frequency shift keying l 33 The frequency of a constant amplitude carrier signal is switched between 2 values ( 1 and 0)
Properties of FSK l Transmission Bandwidth BT = 2 f + 2 B B = Bandwidth of digital base-band signal l If a raised cosine pulse-shaping filter is used BT = 2 f + (1 + )R l Probability of error 1/2 Pe, FSK = Q[(EB / N 0) ] 34
l 35 Spread Spectrum Modulation techniques Spread spectrum techniques employ a transmission bandwidth >> signal bandwidth l The system is inefficient for a single user, but is efficient for many users l Many users use the same bandwidth without significantly interfering with one another
Principle of Spread Spectrum l l l 36 Spread spectrum signals are PN (pseudo – noise) sequence or code. Spread spectrum signals are demodulated at the receiver by cross correlation with the correct PN sequence. PN codes are approximately orthogonal, and the receiver can separate each user based on their codes.
Advantages of spread spectrum techniques 37 l Spread spectrum communications (3 G) offer high bandwidth compared to 1 G and 2 G systems. l Resistance to multi-path fading, because of large bandwidths and narrow time widths.
PN Sequences l l 38 Pseudo Noise sequence is a binary sequence of 1 s and -1 s 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) l l 39 A frequency hopping signal periodically changes the carrier frequency by using PN control 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 l l 40 Time duration between hops hopping period Ts Data is sent by hopping the transmitter carrier over the hopset generated by PN codes Small bursts of data are sent before T/R hops again Hit => Two users using the same frequency band at the same time
Frequency Hopping Modulator DATA Frequency Oscillator Synchronizer Code Block 41 Frequency Hopping Signal 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 l Fast frequency hopping More than one frequency hop during each transmitted symbol -> Hopping rate ≥ symbol rate l Slow frequency hopping Hopping rate < symbol rate
Parameters of FH-SS l l 44 Probability of error for BPSK Spread Spectrum Pe = 0. 5 x e -Eb/ 2 N 0 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 (PG) = Wss / B
Direct Sequence Spread Spectrum (DSSS) code C 1 C 2 CN time 45 frequency
Properties of DSSS signal 46 l Message signal is a time sequence of non-overlapping pulses of duration T, each of which has an amplitude (+/-) 1. l 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 47 PN Wave for N =4
DSSS Transmitter 1 mk(t) 48 k
Principles of transmitter operation l The narrowband message signal mi(t) is multiplied by a pseudo noise code sequence that has a chip rate >> data rate of message. l 49 All users use the same carrier frequency and may transmit simultaneously. The kth transmitted signal is given by:
CDMA Receiver (. )dt 50
Principles of receiver operation 51 l At the receiver, the received signal is correlated with the appropriate PN sequence l The multiplied signal will be p 2(t) = 1 for the correct signal and can be demodulated to yield the message signal mi(t)
Parameters of DSSS l Probability of bit error (BER) Pe = Q {1/ [(K – 1)/3 N + (N 0/2 Eb)]1/2} K = Number of users N = Number of chips/ symbol l 52 When Eb/No Pe = Q{[3 N/(K-1)]1/2 }
Important Advantages of CDMA l l 53 Many users of CDMA use the same frequency Multipath fading may be substantially reduced because of large signal bandwidth. There is no absolute limit on the number of users in CDMA System performance gradually degrades for all users as the number of users is increased.
Drawbacks of CDMA 54 l Self-jamming is a problem in a CDMA system. Self-jamming occurs because the PN sequences are not exactly orthogonal. l 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 s(t) r(t) Fading Channel r(t) = (t) e-j (t) s(t) + n(t) = gain of the channel 55 (t) = phase shift of the channel n(t) = additive Gaussian noise
l 56 Probability of error in fading channels Probability of error (BER) = BER for parent modulation (BPSK, FSK) p(X) = pdf of X due to fading channel = = (EB / N 0) 2
Pe (X) and Pe for different systems Coherent binary PSK Coherent binary FSK 57
Differential Binary PSK Non-coherent orthogonal binary FSK 58
Coherent GMSK =0. 68, BT = 0. 25, = 0. 68 =0. 85, BT = , = 0. 85 BT = Bandwidth – bit duration product for GMSK 59
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