Reasons for Choosing Encoding Techniques. Signal Encoding Techniques. Reasons for Choosing Encoding Techniques. Signal Encoding Criteria

Size: px

Start display at page:

Download "Reasons for Choosing Encoding Techniques. Signal Encoding Techniques. Reasons for Choosing Encoding Techniques. Signal Encoding Criteria"

Geraldine Grant
6 years ago
Views:

1 Reaon for Chooing Enoding Tehnique Signal Enoding Tehnique Chapter 6 Digital data, digital ignal Equipment le omplex and expenive than digital-to-analog modulation equipment Analog data, digital ignal Permit ue of modern digital tranmiion and withing equipment Reaon for Chooing Enoding Tehnique Digital data, analog ignal Some tranmiion media will only propagate analog ignal E.g., optial fiber and unguided media Analog data, analog ignal Analog data in eletrial form an be tranmitted eaily and heaply Done with voie tranmiion over voie-grade line Signal Enoding Criteria What determine how ueful a reeiver will be in interpreting an inoming ignal? Signal-to-noie ratio Data rate Bandwidth An inreae in data rate inreae bit error rate An inreae in SNR dereae bit error rate An inreae in bandwidth allow an inreae in data rate Fator Ued to Compare Enoding Sheme Signal petrum With lak of high-frequeny omponent, le bandwidth required With no d omponent, a oupling via tranformer poible Tranfer funtion of a hannel i wore near band edge Cloking Eae of determining beginning and end of eah bit poition Fator Ued to Compare Enoding Sheme Signal interferene and noie immunity Performane in the preene of noie Cot and omplexity The higher the ignal rate to ahieve a given data rate, the greater the ot

Bai Enoding Tehnique Bai Enoding Tehnique Digital data to analog ignal Amplitude-hift keying (ASK) Amplitude differene of arrier frequeny Frequeny-hift keying (FSK) Frequeny differene near arrier

2 Bai Enoding Tehnique Bai Enoding Tehnique Digital data to analog ignal Amplitude-hift keying (ASK) Amplitude differene of arrier frequeny Frequeny-hift keying (FSK) Frequeny differene near arrier frequeny Phae-hift keying (PSK) Phae of arrier ignal hifted Amplitude-Shift Keying One binary digit repreented by preene of arrier, at ontant amplitude Other binary digit repreented by abene of arrier Ao () t = 0 ( t) binary 1 binary 0 Amplitude-Shift Keying Sueptible to udden gain hange Ineffiient modulation tehnique On voie-grade line, ued up to 1200 bp Ued to tranmit digital data over optial fiber where the arrier ignal i Ao( t) Binary Frequeny-Shift Keying (BFSK) Two binary digit repreented by two different frequenie near the arrier frequeny () t = ( 2 f ( t) Ao π 1 Ao 2 binary 1 binary 0 where f 1 and f 2 are offet from arrier frequeny f by equal but oppoite amount Binary Frequeny-Shift Keying (BFSK) Le ueptible to error than ASK On voie-grade line, ued up to 1200bp Ued for high-frequeny (3 to 30 MHz) radio tranmiion Can be ued at higher frequenie on LAN that ue oaxial able

3 Multiple Frequeny-Shift Keying (MFSK) More than two frequenie are ued More bandwidth effiient but more ueptible to error i () t A o 2π f t = 1 i M i Multiple Frequeny-Shift Keying (MFSK) To math data rate of input bit tream, eah output ignal element i held for: T =LT eond where T i the bit period (data rate = 1/T) So, one ignal element enode L bit f i = f + (2i 1 M)f d f = the arrier frequeny f d = the differene frequeny M = number of different ignal element = 2 L L = number of bit per ignal element Multiple Frequeny-Shift Keying (MFSK) Multiple Frequeny-Shift Keying (MFSK) Total bandwidth required 2Mf d Minimum frequeny eparation required 2f d =1/T Therefore, modulator require a bandwidth of W d =2 L /LT=M/T Phae-Shift Keying (PSK) Two-level PSK (BPSK) Ue two phae to repreent binary digit () t ( ) Ao t = A o ( 2 π f t +π ) ( ) ( ) Ao t = Ao t binary 1 binary 0 binary 1 binary 0 Phae-Shift Keying (PSK) Differential PSK (DPSK) Phae hift with referene to previou bit Binary 0 ignal burt of ame phae a previou ignal burt Binary 1 ignal burt of oppoite phae to previou ignal burt

Phae-Shift Keying (PSK) QPSK and OQPSK Four-level PSK (QPSK) Eah element repreent more than one bit () t = π Ao t + 11 4 3π Ao t + 01 4 3π Ao t 00 4 π A o t 10 4 Phae-Shift Keying (PSK) Multilevel

4 Phae-Shift Keying (PSK) QPSK and OQPSK Four-level PSK (QPSK) Eah element repreent more than one bit () t = π Ao t π Ao t π Ao t 00 4 π A o t 10 4 Phae-Shift Keying (PSK) Multilevel PSK Uing multiple phae angle with eah angle having more than one amplitude, multiple ignal element an be ahieved D = R L = R log 2 M D = modulation rate, baud R = data rate, bp M = number of different ignal element = 2 L L = number of bit per ignal element Performane Bandwidth of modulated ignal (B T ) ASK, PSK B T =(1+r)R FSK B T =2DF+(1+r)R R = bit rate 0 < r < 1; related to how ignal i filtered DF = f 2 -f =f -f 1 Performane Bandwidth of modulated ignal (B T ) MPSK MFSK B T B T 1+ r 1+ r R R L M = = log 2 ( 1+ r) M R M = log 2 L = number of bit enoded per ignal element M = number of different ignal element Performane

Performane Quadrature Amplitude Modulation QAM i a ombination of ASK and PSK Two different ignal ent imultaneouly on the ame arrier frequeny ( t) = d ( t) t d ( t) in t 1 o + 2 Quadrature Amplitude

5 Performane Quadrature Amplitude Modulation QAM i a ombination of ASK and PSK Two different ignal ent imultaneouly on the ame arrier frequeny ( t) = d ( t) t d ( t) in t 1 o + 2 Quadrature Amplitude Modulation Reaon for Analog Modulation Modulation of digital ignal When only analog tranmiion failitie are available, digital to analog onverion required Modulation of analog ignal A higher frequeny may be needed for effetive tranmiion Modulation permit frequeny diviion multiplexing Bai Enoding Tehnique Analog data to analog ignal Amplitude modulation (AM) Angle modulation Frequeny modulation (FM) Phae modulation (PM) Amplitude Modulation Amplitude Modulation ( t ) = [ 1 + n x( t )] o t o t = arrier x(t) = input ignal a n a = modulation index Ratio of amplitude of input ignal to arrier a.k.a double ideband tranmitted arrier (DSBTC)

6 Spetrum of AM ignal Amplitude Modulation Tranmitted power n = + a Pt P 1 2 P t = total tranmitted power in (t) P = tranmitted power in arrier 2 Single Sideband (SSB) Variant of AM i ingle ideband (SSB) Send only one ideband Eliminate other ideband and arrier Advantage Only half the bandwidth i required Le power i required Diadvantage Suppreed arrier an t be ued for ynhronization purpoe Angle Modulation Angle modulation ( t) = A o[ 2π f t + φ( t) ] Phae modulation Phae i proportional to modulating ignal φ n p = phae modulation index () t = n m() t p Angle Modulation Frequeny modulation Derivative of the phae i proportional to modulating ignal φ' () t = n m() t n f = frequeny modulation index f Angle Modulation Compared to AM, FM and PM reult in a ignal whoe bandwidth: i alo entered at f but ha a magnitude that i muh different Angle modulation inlude o( (t)) whih produe a wide range of frequenie Thu, FM and PM require greater bandwidth than AM

Angle Modulation Caron rule where B T = 2 ( β + 1)B np Am for PM β = F n f Am = for FM B 2πB The formula for FM beome B T = 2 F + 2B Bai Enoding Tehnique Analog data to digital ignal Pule ode

7 Angle Modulation Caron rule where B T = 2 ( β + 1)B np Am for PM β = F n f Am = for FM B 2πB The formula for FM beome B T = 2 F + 2B Bai Enoding Tehnique Analog data to digital ignal Pule ode modulation (PCM) Delta modulation (DM) Analog Data to Digital Signal One analog data have been onverted to digital ignal, the digital data: an be tranmitted uing NRZ-L an be enoded a a digital ignal uing a ode other than NRZ-L an be onverted to an analog ignal, uing previouly diued tehnique Pule Code Modulation Baed on the ampling theorem Eah analog ample i aigned a binary ode Analog ample are referred to a pule amplitude modulation (PAM) ample The digital ignal onit of blok of n bit, where eah n-bit number i the amplitude of a PCM pule Pule Code Modulation Pule Code Modulation By quantizing the PAM pule, original ignal i only approximated Lead to quantizing noie Signal-to-noie ratio for quantizing noie n SNR db = 20log db = 6.02n db Thu, eah additional bit inreae SNR by 6 db, or a fator of 4

8 Delta Modulation Delta Modulation Analog input i approximated by tairae funtion Move up or down by one quantization level (δ) at eah ampling interval The bit tream approximate derivative of analog ignal (rather than amplitude) 1 i generated if funtion goe up 0 otherwie Delta Modulation Two important parameter Size of tep aigned to eah binary digit (δ) Sampling rate Auray improved by inreaing ampling rate However, thi inreae the data rate Advantage of DM over PCM i the impliity of it implementation Reaon for Growth of Digital Tehnique Growth in popularity of digital tehnique for ending analog data Repeater are ued intead of amplifier No additive noie TDM i ued intead of FDM No intermodulation noie Converion to digital ignaling allow ue of more effiient digital withing tehnique

Basic Encoding Techniques

Basic Encoding Techniques Bai Enoding Tehnique Signal Enoding, Spread Spetrum Chapter 7 Digital data to analog ignal Amplitude-hift keying (ASK) Amplitude differene of arrier frequeny Frequeny-hift keying (FSK) Frequeny differene