ΕΠΛ 476: ΚΙΝΗΤΑ ΔΙΚΤΥΑ ΥΠΟΛΟΓΙΣΤΩΝ (MOBILE NETWORKS) Δρ. Χριστόφορος Χριστοφόρου Πανεπιστήμιο Κύπρου - Τμήμα Πληροφορικής Modulation Techniques (Τεχνικές Διαμόρφωσης)
Recall (Process and Elements of Radio Transmission) 1 Modulation Demodulation
Topics Discussed 2 What is modulation? Why modulate? Analog and Digital Signals Digital Modulation Bit rate, Baud rate Basic Digital Modulation Techniques (ASK, FSK, PSK, QAM) Constellation diagrams Factors that influence the choice of Digital Modulation Scheme Power Efficiency, Bandwidth Efficiency Spread Spectrum Techniques, FHSS, DSSS
Modulated Signals Modulation for Wireless 3 Message Signal (Data to be transmitted) Carrier signal with frequency f c Controlling the Amplitude of the carrier signal (ASK) Controlling the Frequency of the carrier signal (FSK) Controlling the Phase of the carrier signal (PSK)
Modulation for Wireless 4 Modulation is the process of encoding information (the data that we want to transmit) from a message source in a manner suitable for transmission through a medium (i.e., the air) (Είναι η διαδικασία κωδικοποίησης της πληροφορίας (του πηγαίου μηνύματος) που θέλουμε να στείλουμε, σε μια μορφή η οποία είναι κατάλληλη για μετάδοση μέσω κάποιου μέσου (π.χ., στον αέρα)) Modulation is used to modify the parameters of a high frequency carrier signal (i.e., Amplitude, Frequency, Phase), so as to include a message signal (which contains the information) onto the carrier (i.e., the channel).
Why Modulate? 5 Ease of radiation (Ευκολία εκπομπής) Reduce antenna size: The size of an antenna is proportional to the signal wavelength. By modulating the baseband signal (i.e., the message) into higher frequency carrier, the wavelength decreases, and thus the size of the antenna decreases. The size of antenna λ/4 = c/4f. E.g.: 3 khz 25 Km antenna 3 GHz 2.5 cm antenna
Why Modulate? 6 Allows the message data to be encoded into a form (i.e., into a Carrier signal with higher frequencies) suitable for Traveling Long Distances over the air: E.g., If we wish to throw a piece of paper, it cannot go too far by itself. But by wrapping it around a stone (carrier signal), it can be thrown over a longer distance!
Why Modulate? 7 Bandwidth Efficiency (Αποδοτική χρήση του διαθέσιμου εύρου ζώνης) By modulation we can use a narrow bandwidth (χρήση μικρού εύρος ζώνης) to send a large amount of data Higher order modulation schemes (8-PSK: 3 bits/symbol, 16-QAM: 4 bits/symbol, 64-QAM: 5 bits/symbol) can be used depending on the environmental conditions. Effective use (Αποτελεσματική χρήση) of limited frequency resources A carrier alone conveys no information A carrier signal by itself is like a train wagon without letters Modulation makes the signal carrier usefull. Includes the information that will be transmitted.
Why Modulate? 8 Allows simultaneous transmission of several signals (Multiplexing; Πολυπλεξία Πολλοί χρήστες μπορούν να στείλουν τα δεδομένα τους την ίδια ώρα χωρίς να προκαλούνται παρεμβολές) By modulating different signals into different carriers (with different frequencies) several users can transmit their signals at the same time (avoid interference). The Received parties will just tune (συντονίζονται) to the correct Frequency Carrier to receive the signal. Good Security features (Ασφάλεια δεδομένων) Greater security (encryption - κρυπτογράφηση) on the data can be applied during modulation..
Analog and Digital Signals Αναλογικά και Ψηφιακά Σήματα 9 Means by which data are propagated over a medium (Οι τρόποι με τους οποίους τα δεδομένα διαδίδονται μέσω κάποιου μέσου). An Analog Signal is a continuously varying electromagnetic wave (ένα συνεχόμενο εναλλασσόμενο ηλεκτρομαγνητικό κύμα) that may be propagated over a variety of media: Wire, fiber optic, coaxial, space (wireless) A digital signal is a sequence of discrete voltage pulses (είναι μια αλληλουχία διακριτών παλμών τάσης) that can be transmitted over a wire medium: E.g., a constant positive level of voltage to represent bit 1 and a constant negative level to represent bit 0.
Modulation for Wireless Digital Modulation 10 Digital modulation is the process by which an analog carrier is modulated to include a discrete (digital) signal. (Ψηφιακή διαμόρφωση είναι η διαδικασία κατά την οποία ένας μεταφορέας αναλογικού σήματος διαμορφώνεται έτσι ώστε να συμπεριλάβει ένα διακριτό (ψηφιακό) σήμα (π.χ., 1 or 0)) Digital modulation methods can be considered as Digital-to- Analog conversion, and the corresponding Demodulation (e.g., at the Receiver) as Analog-to-Digital conversion.
Modulation for Wireless Digital Modulation 11 The modulation that will be applied on the (analog) signal carrier to so as to include the data that will be carried (e.g., 1 or 0, etc.) are chosen from a finite number of M alternative symbols (or signal units or signal elements) based on the Digital Modulation Technique and the Modulation Alphabet that will be used. (Η διαμόρφωση που θα γίνει στον (αναλογικό) μεταφορέα σήματος για να συμπεριλάβουν την πληροφορία που θα μεταφερθεί (π.χ., 1 ή 0) επιλέγονται από ένα πεπερασμένο αριθμό από εναλλακτικά σύμβολα (σήματα) ανάλογα με την Τεχνική διαμόρφωσης και το Αλφάβητο Διαμόρφωσης που θα χρησιμοποιηθεί. Symbol Pattern 1 0 Symbol Pattern 2 1 This same Modulation Alphabet have to be used both from the Transmitter (for modulating the signal) and the Receiver (for demodulating the signal)
Modulation for Wireless Digital Modulation 12 The general form (pattern) of the modulated signal is (Η γενική μορφή ενός διαμορφωμένου σήματος): s(t) = A(t) sin(2π x (f c + f m (t)) t + (t)
Modulation for Wireless Digital Modulation 13 The three essential parameters that can be modulated (Οι τρείς βασικές παράμετροι που μπορούμε να διαμορφώσουμε) s(t ) = A sin(2π f t + ) Amplitude value (A) ASK (Amplitude Shift Keying) Frequency value (f) FSK (Frequency Shift Keying) Phase value (φ) PSK (Phase Shift Keying) Digital modulation: Amplitude (A), frequency (f) and Phase () are used to represent a digital state. (Στην Ψηφιακή διαμόρφωση το πλάτος, η συχνότητα, και η φάση του σήματος χρησιμοποιούνται για να αναπαραστήσουν μία ψηφιακή κατάσταση ή τιμή)
Basic Digital Modulation Techniques 14 Basic Digital Modulation Techniques work by varying the Amplitude, Frequency or Phase (or a combination of them) of a sinusoidal carrier depending on the information (data) that will be transmitted and the Modulation Alphabet that will be used. ASK: Amplitude Shift Keying s(t ) = A sin(2π f t + ) FSK: Frequency Shift Keying s(t ) = A sin(2π f t + ) PSK: Phase Shift Keying s(t ) = A sin(2π f t + ) QAM: Quadrature Amplitude Modulation or Amplitude Phase Shift Keying (APSK) s(t ) = A sin(2π f t + )
Basic Digital Modulation Techniques 15 Types of Digital to Analog Modulation
Basic Digital Modulation Techniques Illustration 16 A(t).sin[2.π.f c.t] sin[2.π.(f c +f m (t)).t] sin[2.π.f c.t + θ(t)] A(t).sin[2.π.f c.t + θ(t)]
Bit Rate and Baud Rate 17 Bit Rate is the number of bits (data) that can be carried per second. Baud Rate is the number of signal units (or symbols) per second used for carrying the bits (and achieve the Bit Rate). Baud Rate can be less than (in case each symbol can carry more than one bit) or equal (in case each symbol can carry only one bit) to the bit rate. Baud Rate is important in Bandwidth efficiency. Baud rate determines the bandwidth required to send the message signal (Καθορίζει το εύρος ζώνης που απαιτείται για να σταλεί μήνυμα) Baud Rate = Bit Rate / Number of Βits per Symbol Thus, the lower the Baud Rate the less the bandwidth required The number of bits that can be carried by one Symbol, depends on the Modulation Technique used. The Baud Rate depends on the type of Modulation used.
Bit Rate and Baud Rate Examples 18 Example 1: A modulated signal carries 4 bits in each signal unit. If 1000 signal units are sent per second, find the Baud Rate and the Bit Rate Baud Rate = 1000 baud/s Bit Rate = 1000 x 4 = 4000 bps Example 2: The bit rate of a modulated signal is 3000 bps. If each signal unit carries 6 bits, what is the baud rate? Baud Rate = 3000/6 = 500 (baud/s) Example 3: A modulated signal has a bit rate of 8000 bps and a baud rate of 1000 baud. How many bits are carried by each signal element? Bits/Baud = 8000/1000 = 8
Amplitude Shift Keying (ASK) 19 s(t) = The strength of the carrier signal (Amplitude) is varied to represent digital data (1 and 0) - Binary ASK. Frequency and Phase remains the same while the amplitude changes. A 1 sin(2π f t + ) 1 A 2 sin(2π f t + ) 0 One bit per symbol! Baud Rate = Bit Rate Carrier Modulated Signal Two states used
Amplitude Shift Keying (ASK) 20 ASK is susceptible to noise and interference (ευάλωτο στο θόρυβο) as noise can change the amplitude of a signal. This can cause errors on the Receiver during demodulation. Noise refers to unintentional voltages introduced onto a line by various phenomena such as heat or electromagnetic induction created by other sources. s(t) = On/Off Keying A sin(2π f t + ) 1 0 0
Amplitude Shift Keying (ASK) M-ary ASK 21 Binary ASK has only two possible states representing only 0 and 1 M-ary ASK has more than 2 possible states. For example, if M = 4, the ASK refers to four different Amplitudes in which the carrier is send. As 4 states are possible, two bits can be encoded per symbol. In general if number of possible states M > 2, each symbol can carry log2(m) bits. This scheme is therefore more bandwidth efficient. Modulation Alphabet with 4 possible states s(t) = A 1 sin(2π f t + φ) A 2 sin(2π f t + φ) A 3 sin(2π f t + φ) A 4 sin(2π f t + φ) A 1 = 0 : 00 A 2 = 1: 01 A 3 = 2: 10 A 4 = 3: 11 Example: M = 4 (4 different states) log 2 (4) = 2 2 bits/symbol Baud Rate= N, Bid Rate= 2N
Frequency Shift Keying (ASK) 22 The frequency of the carrier signal (f c ) is varied to represent digital data (1 and 0) - Binary FSK (2 frequencies: f 1 and f 2 ) Both peak amplitude and phase remain constant. In this case the carrier switches between 2 values (two states ; M = 2): A sin(2π (f 1 ) t + ) f 1 = f c + f m 1 s(t) = A sin(2π (f 2 ) t + ) f 2 = f c f m 0 Baud Rate = Bit Rate
Frequency Shift Keying (ASK) 23 Binary FSK has only two possible states representing only 0 and 1 M-ary FSK has more than 2 possible states. For example, if M = 4, the FSK refers to four different frequencies in which the carrier is modulated and send. As 4 states are possible, two bits can be encoded per symbol. Baud Rate = Bit Rate/2 Reduces the Required Bandwidth to half s(t) = A sin(2π (f 1 ) t + φ) A sin(2π (f 2 ) t + φ) A sin(2π (f 3 ) t + φ) A sin(2π (f 4 ) t + φ) Modulation Alphabet f 1 = fc + fm: 00 f 2 = fc - fm: 01 f 3 = fc + 2.fm: 10 f 4 = fc - 2.fm: 11
Frequency Shift Keying (ASK) 24 FSK is not susceptible to noise interference as it avoids noise interference by looking at frequencies (change of a signal) and ignoring amplitudes. Because the Receiving device is looking for specific frequency changes over a given number of periods, it can ignore voltage spikes. (Ο Receiver κοιτάζει μόνο για συγκεκριμένες αλλαγές στη συχνότητα του μεταφορέα σήματος σε συγκεκριμένες περιόδους (περίοδος ενός signal unit)) και αγνοεί αλλαγές στην τάση (πλάτος) του σήματος.)
Phase Shift Keying (PSK) 25 The phase of the carrier signal is varied to represent digital data (binary 0 or 1), i.e., Binary PSK (BPSK) Both peak amplitude and frequency remain constant as the phase changes. Phases are separated by 180 degrees. If we start with a phase of 0 o to represent bit 0, then we can change the phase to 180 o to send bit 1 (or inversely). The Constellation or phase-state Diagram shows the relationship by illustrating only the phases. Baud Rate = Bit Rate A sin(2π f t + 0 o ) 0 s(t) = A sin(2π f t + 180 o ) 1
Phase Shift Keying (PSK) 26 Phase Shifts Examples Phase Shifts Example
Quadrature Phase Shift Keying (QPSK) 27 QPSK refers to PSK with 4 states. The Q - Quadrature in QPSK refers to four phases in which a carrier is modulated and send in QPSK. Also, called 4-PSK. Because QPSK has 4 possible states, QPSK can encode two bits per symbol. Because 2 bits are allocated to each symbol, QPSK can achieve twice the Data Rate of a comparable BPSK scheme for a given bandwidth.
Quadrature Phase Shift Keying (QPSK) 28 Two Modulation Alphabets s(t) = A sin(2π f t + φ 1 ) A sin(2π f t + φ 2 ) A sin(2π f t + φ 3 ) A sin(2π f t + φ 4 ) φ 1 = 0 ο φ 2 = 90 ο φ 3 = 180 ο φ 4 = 270 ο OR φ 1 = 45 ο 00 φ 2 = 135 ο 01 φ 3 = 225 ο 10 φ 4 = 315 ο 11
Phase Shift Keying (PSK) 29 PSK is not susceptible to noise degradation that affects ASK, neither has the limitation of FSK that needs to repeatedly tune at different frequencies (i.e., more complicated demodulator needed filtering of the signal for different frequencies). Simple to implement, and is used extensively in satellite communication.
Constellation Diagrams Διαγράμματα Αστερισμού 30 A constellation diagram helps us to define the amplitude and phase of a signal that will be applied during the modulation (Το Διάγραμμα Αστερισμού μας βοηθά να καθορίσουμε το πλάτος και τη φάση του σήματος μας που θα εφαρμοστεί κατά τη διαμόρφωση του σήματος μας για να στείλουμε τα δεδομένα). It is a convenient way to represent the symbols of the Modulation Alphabet that will be used for modulating signal carrier and transmitting the signal. (Είναι ένας εύκολος τρόπος για να αναπαραστήσουμε τα σύμβολα του Αλφαβήτου Διαμόρφωσης που θα χρησιμοποιηθούν για τη διαμόρφωση του μεταφορέα σήματος για την αποστολή του σήματος) Examples:
Constellation Diagrams Circular Constellation Diagrams 31 Examples: 01 11 00 10 (a) Circular 4-QAM (b) Circular 8-QAM (c) Circular 16-QAM
Constellation Diagrams Rectangular Constellation Diagrams 32 Examples: +1 0010 0110 +3 1110 1010 0011 0111 +1 1111 1011-3 -1 0001 0101-1 +1 +3 1101 1001 I Rectangular 4-QAM Rectangular 8-QAM 0000 0100-3 1100 Rectangular 16-QAM: 1000
Higher Order Modulation: 8-PSK 33 s(t) = We can extend the Modulation Alphabet, by varying the signal by shifts of 45 ο (instead of 90 ο in QPSK). (Μπορούμε να επεκτείνουμε το Αλφάβητο Διαμόρφωσης με το μεταβάλλουμε το σήμα με μετατοπίσεις 45 ο παρά 90 o όπως το QPSK) With 8 (2 3 ) different phases, each phase (i.e., signal unit or symbol) can represent 3 bits. Baud Rate = Bit Rate/3 Reduces A sin(2π f t + φ 1 ) 000 A sin(2π f t + φ 2 ) 001 A sin(2π f t + φ 3 ) 010 A sin(2π f t + φ 4 ) 011 A sin(2π f t + φ 5 ) 100 A sin(2π f t + φ 6 ) 101 A sin(2π f t + φ 7 ) 110 A sin(2π f t + φ 8 ) 111 φ 1 = 0 ο φ 2 = 45 ο φ 3 = 90 ο φ 4 = 135 ο φ 5 = 180 ο φ 6 = 225 ο φ 7 = 270 ο φ 8 = 315 ο the Required Bandwidth to one third Using the Constellation Diagram we can easily produce the Modulation Alphabet
Higher Order Modulation: M-PSK 34 Obviously the bandwidth efficiency of a M-ary PSK scheme increases as M (the number of possible states) increases because more bits per symbol can be sent...however the distance between 2 points in the constellation is reduced and therefore the error rate gets larger. 01 11 011 010 110 0 1-1 +1 I 00 10 001 000 BPSK QPSK 8-PSK 16-PSK As M increases, the bandwidth efficiency increases but the waveform energy (i.e., the transmission power used to send the symbol) must be increased to keep the BER at a certain level. 100 101 111
Quadrature Amplitude Modulation (QAM) Phase and Amplitude Modulation 35 PSK is limited by the ability of the equipment to distinguish between small differences in phases. Limits the potential data rate. (Περιορίζει το πιθανό data rate) The principle of Quadrature Amplitude Modulation (QAM) of Amplitude Phase Shift Keying (APSK) is to have X variations in Phase (X διαφορετικές φάσεις) and Y variations of Amplitude (Υ διαφορετικά πλάτη). X Y possible variations More different states to modulate the signal carrier More bits can be carried per symbol Greater Data Rates QAM or APSK is an application of ASK to PSK (Εφαρμογή του ASK πάνω στο PSK)
Quadrature Amplitude Modulation (QAM) Phase and Amplitude Modulation 36 Example: 8-QAM example Two (2) different Amplitudes (A1 = 1; A2 = 2) Four (4) different Phases (0 o, 90 o, 180 o, 270 o ) Total of 8 QAM symbols 3 bits per symbol Baud Rate = Bit Rate/3 Modulation Alphabet A = 1, Phase = 0 o : 000 A = 2, Phase = 0 o : 001 A = 1, Phase = 90 o : 010 A = 2, Phase = 90 o : 011 A = 1, Phase = 180 o : 100 A = 2, Phase = 180 o : 101 A = 1, Phase = 270 o : 110 A = 2, Phase = 270 o : 111
Quadrature Amplitude Modulation (QAM) of APSK 37 We can have numerous variations (Διάφορες παραλλαγές) of Phase Shifts and Amplitude shifts Number of Phase Shifts > Number of Amplitude shifts. 16-QAM for example: There are sixteen QAM symbols 4 bits per symbol. A variety of constellations diagrams can be used
Quadrature Amplitude Modulation (QAM) 38
Quadrature Amplitude Modulation (QAM) 39 More higher order modulation 64-QAM (64-Quadrature Amplitude Modulation) Each symbol now carries 6 bits (log 2 (64)) Going higher: 256-QAM 8bits/symbol (log 2 (256)) 1024-QAM 10bits/symbol (log 2 (1024)).
Why Not Just Keep Going? 40 With Higher Order Modulation schemes Minor errors during modulation could create symbol errors in transmission Even a little noise in the transmission channel could create symbol errors Minor inaccuracies in the Receiver could create errors Signal-to-Noise requirements increases with higher order modulations (thus more power have to be used during transmission more interference caused in the cells)
Multipath Propagation Effect on Signal Demodulation at the Receiver 41 In the example illustrated below, we assume that the Receiver receives two copies of each signal, from two different paths (Direct Path, Reflected) each path having a different length --> Each signal copy will arrive to the Receiver in different times (delays) and with different amplitudes. Signal 1 Signal 2 Signal 3 Effect of Multipath Propagation
Multipath Propagation Effect on Signal Demodulation at the Receiver 42 The addition of the delayed contribution via the Reflected path has two consequences: The symbols have been extended and now have a tail due to the reflected contribution continuing to arrive for a short period after the direct symbol has ended. The Received Result has its effective Amplitude and Phase altered by an amount that depends on the time difference between the paths. Propagation Delay: Length of the path that the signal follows to reach the receiver (Z), divided by the Speed of Light (C)
Multipath Propagation Effect on Signal Demodulation at the Receiver 43 The two paths are of lengths we can call Z (direct) and Z (indirect). This allows us to define the time delay (dt) between their relative arrivals to be dt = (Z Z)/C C is the speed of light The above shows why it is useful to have a Guard Interval (T G ), in between the Useful Symbol Periods (T s ). For this case, if we arrange for T G > dt then the delayed version of each symbol will have finished arriving before the next symbol in the sequence starts to arrive. This prevents the pattern transmitted for a given symbol from affecting what we see during the next symbol period. ISI makes it harder for the receiver to demodulate the symbols correctly. The larger the T G The lower the possibility for ISI The less the number of symbols that can be transmitted per second The lower the data rates achieved.
Bid and Baud Rate Comparison 44 Assuming a Binary-FSK signal can send 1200 bps, it requires 1200 signal units (symbols) to send 1200 bits (each symbol represents one bit, Baud Rate = 1200 bauds/s) Assuming an 8-QAM (3 bits per symbol), using the same baud rate (1200 bauds/s), 3600 bits/s will be achieved (3 x Bit Rate).
Bid and Baud Rate Comparison Examples 45 A constellation diagram consists of eight equally spaced points on a circle. If the bit rate is 4800 bps, what is the baud rate? The constellation indicates an 8-PSK with the points 45 degrees apart. Since 2 3 = 8 (N = log 2 (8) N = 3) then 3 bits are transmitted with each signal unit (symbol). Therefore, the baud rate is 4800 / 3 = 1600 baud/s. What is the bit rate for a 1000 baud/s 16-QAM signal? A 16-QAM signal has 4 bits per signal unit (symbol) since log 2 16 = 4. Thus, (1000 x 4) = 4000 bps Compute the baud rate for a 72,000bps 64-QAM signal. A 64-QAM signal has 6 bits per signal unit (log 2 64 = 6). Therefore 72,000 / 6 = 12,000 baud/s
46 Factors that influence the choice of Digital Modulation Scheme Some Performance factors considered (depending on the environment): Achieve Low Bit Error Rate (BER) at low SNR or SINR (Να επιτυγχάνει χαμηλό ποσοστό σφάλματος δεδομένων με χαμηλή ενέργεια σήματος) Efficient use of battery power in Mobile Device (Να κάνει αποδοτική χρήση της μπαταρίας στις κινητές συσκευές) Resistance to Interference and Noise (Ανοχή στις παρεμβολές και το θόρυβο)
47 Factors that influence the choice of Digital Modulation Scheme Occupying a minimum amount of Bandwidth to send the data (Να δεσμεύει όσο το δυνατό πιο λίγο εύρος ζώνης για να στείλει τα δεδομένα) Easy and cheap to implement in a Mobile Device (Η τεχνική να είναι φτηνή και εύκολη να υλοποιηθεί στις κινητές συσκευές) No existing modulation scheme simultaneously satisfies all of these requirements well. Each one is better in some areas with tradeoffs of being worse in others.
48 Factors that influence the choice of Digital Modulation Scheme The performance of a modulation scheme is often measured in terms of its: Power Efficiency: Refers to the ability to preserve the fidelity (no errors) of a digital message at low power levels (i.e., low SNIR) (Περιγράφει την ικανότητα διατήρησης της ακεραιότητα ενός μηνύματος (no errors) σε χαμηλά επίπεδα ισχύος του σήματος) Bandwidth (or Link Spectral) Efficiency: Refers to ability to squeeze as much data into the least amount of bandwidth available (Περιγράφει την ικανότητα συμπίεσης όσον περισσότερων δεδομένων στο ελάχιστο διαθέσιμο εύρος ζώνης)
49 Factors that influence the choice of Digital Modulation Scheme Power Efficiency: In order to increase noise immunity, it is necessary to increase the signal power. (Για να αυξήσουμε την ανοσία ενός σήματος στο θόρυβο πρέπει να αυξήσουνε την ενέργεια (ισχύ) με την οποία θα το στείλουμε) The amount by which the signal power should be increased to maintain a certain BER depends on the modulation scheme. Higher Order Modulation Higher Signal Power. The Power Efficiency is expressed by the value of SNIR required at the Receiver to decode the signal correctly and guarantee a certain BER (The lower the SNIR required the higher the Power Efficiency).
50 Factors that influence the choice of Digital Modulation Scheme Bandwidth (Link Spectral) Efficiency: Is typically used to analyse how efficiently the allocated bandwidth is used by the modulation technique (Χρησιμοποιείται για να αναλύσουμε πόσο αποδοτικά χρησιμοποιείται το διαθέσιμο εύρος ζώνης από την τεχνική διαμόρφωσης). It is defined as the average number of bits per unit of time (bits per second) that can be transmitted per unit of bandwidth (per Hertz).
51 Factors that influence the choice of Digital Modulation Scheme Bandwidth (Link Spectral) Efficiency: Is the net bitrate (useful information rate excluding error-correcting codes) or maximum throughput* divided by the available Bandwidth (in hertz) of a communication channel. It is measured in bits per second per Herz (bps/hz) * Throughput or network throughput is the rate of successful message delivery over a communication channel. A typical method of performing a measurement is to transfer a 'large' file from one system to another system and measure the time required to complete the transfer or copy of the file. The throughput is then calculated by dividing the file size by the time required to complete the transfer. Then the throughput can be measured in megabits, kilobits, or bits per second.
Factors that influence the choice of Digital Modulation Scheme 52 Bandwidth Efficiency Examples: Example 1: What is the bandwidth efficiency of the modulation technique when 2 Kilohertz of bandwidth is required to transmit 1000 bps? 1000bps/2000Hz = 0.5 bps/hz Example 2: What is the bandwidth efficiency of the modulation technique when 3 Kilohertz of bandwidth is required to transmit 12000 bps? 12000bps/3000Hz = 4bps/Hz
Factors that influence the choice of Digital Modulation Scheme 53 Bandwidth Efficiency Examples: Example 3: What is the bandwidth efficiency of a modulation technique utilizing the frequency band between 107.4 MHz and 107.8 MHz to transmit 512,000 bps. Bandwidth Utilized: 107.8 107.4 = 400 KHz 512,000bps / 400,000Hz = 1.28 bps/hz
54 Factors that influence the choice of Digital Modulation Scheme Very often there is a tradeoff: M-ary schemes increase the Bandwidth Efficiency but require Higher Transmission Power (than Binary modulation schemes) to keep the same Bit Error Rate Lower Power Efficiency. (Κάνουν την χρήση του εύρους ζώνης πιο αποδοτική άλλα χρειάζεται περισσότερη ισχύς κατά την αποστολή των δεδομένων για να είναι το σήμα πιο δυνατό και να μπορεί ο Receiver να το αναγνωρίσει και να το αποκωδικοποιήσει σωστά διατηρώντας έτσι το BER στα επίπεδα που πρέπει)
55 Factors that influence the choice of Digital Modulation Scheme Very often there is a tradeoff: Power Efficiency can be increased by adding Error Control Coding in the packets transmitted but reduces the Bandwidth Efficiency as Redundancy is transmitted too. (Μπορούμε να αυξήσουμε το Power Efficiency με το να προσθέσουμε error control codes κατά την αποστολή των πακέτων αλλά αυτό μειώνει το Bandwidth Efficiency αφού στέλλουμε επιπλέον (μη χρήσιμη) πληροφορία) Error control (error detection and correction) are techniques that enable reliable delivery of digital data over unreliable communication channels. Many communication channels are subject to channel noise, and thus errors may be introduced during transmission from the source to a receiver. Error detection techniques allow detecting such errors, while error correction enables reconstruction of the original data in many cases.
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 56 Spread Spectrum techniques use a transmission bandwidth that is several orders of magnitude greater than the required bandwidth to spread the data (Χρησιμοποιούν ένα εύρος ζώνης πολύ μεγαλύτερο από αυτό που πραγματικά χρειάζεται για διασπείρουν τα δεδομένα).
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 57 Each bit of the data that we want to transmit is encoded using a sequence of digits (chips) known as a Spreading Code (Κάθε bit των δεδομένων που θα διαδοθούν κωδικοποιείται χρησιμοποιώντας μια ακολουθία ψηφίων (τα ψηφία αυτά ονομάζονται chips) η οποία είναι γνωστή ως ο Κώδικας Διασποράς). Each bit that will be transmitted by the transmitter in the specific channel is encoded using the same Spreading Code. During Spreading, data bit 0 is represented as -1 and data bit 1 is represented as +1.
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 58 Example: We want to transmit Data = (0, 1) using the Spreading Code = (1, 1, 1, -1, 1, -1, -1, -1) Data = (-1, +1) Bit 0 will be encoded and transmitted using the following chip sequence: (-1).(1, 1, 1, -1, 1, -1, -1, -1) = (-1, -1, -1, 1, -1, 1, 1, 1) Bit 1 will be encoded and transmitted using the following chip sequence: (+1). (1, 1, 1, -1, 1, -1, -1, -1) = (1, 1, 1, -1, 1, -1, -1, -1)
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 59 During Spreading, data bit 0 is represented as -1 and data bit 1 is represented as +1.
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 60 Example: Spreading and Despreading Step 0 1 Encode Sender (Spreading) Spreading Code (SC) = (1, 1, 1, -1, 1, -1, -1, -1), Data = (0, 1) Data (-1, +1) Encode (Spread) Data = ( (-1. SC), (+1. SC) ) = ((-1, -1, -1, 1, -1, 1, 1,1), (1, 1, 1, -1, 1, -1, -1,-1)) 2 Spread Data = (-1, -1, -1, 1, -1, 1, 1, 1, 1, 1, 1, -1, 1, -1, -1, -1)
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 61 The Receiver will use the same Spreading Code to Despread (Decode) the chip sequence received. Example: The Receiver receives the chip sequence (-1, -1, -1, 1, -1, 1, 1, 1) Decoding of the chip sequence (applying dot product) using the Spreading Code (1, 1, 1, -1, 1, -1, -1, -1): (-1, -1, -1, 1, -1, 1, 1, 1).(1, 1, 1, -1, 1, -1, -1, -1) = = (-1, -1, -1, -1, -1, -1, -1, -1) = = (-8) If decoded data < 0 Data bit 0 If decoded data > 0 Data bit 1
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 62 Example: Spreading and Despreading Step 0 1 Decode Sender (Despreading) Spreading Code (SC)= (1, 1, 1, -1, 1, -1, -1, -1) Received Spread Data (RSD) = (-1, -1, -1, 1, -1, 1, 1, 1, 1, 1, 1, -1, 1, -1, -1, -1) Decode = RSD. SC = ((-1, -1, -1, 1, -1, 1, 1, 1), (1, 1, -1, 1, -1, -1, -1)). (1, 1, 1, -1, 1, -1, -1, -1) = ((-1-1-1-1-1-1-1-1), (1, 1, 1, 1, 1, 1, 1,1)) 2 Decoded Data = ( -8, 8 ) Data (0, 1) If decoded data < 0 Data bit 0 If decoded data > 0 Data bit 1
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 63 As illustrated in the previous example, after despreading the amplitude of the signal increases by a factor of 8 (analogous to the length of the Spreading Code this is called the Spreading Factor (SF)) This effect is termed Processing Gain and is a fundamental aspect (είναι ένα θεμελιώδες στοιχείο) of all Spread Spectrum systems. Processing Gain = 10 log 10 (SF)
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 64 In the previous example the Processing Gain is 9dB (10 x log 10 (8)) This means that the signal energy can be increased by 9dB after despreading. Thus, assuming that the minimum SNIR required by the Receiver (Demodulator) for decoding the signal correctly is 5dB, the SNIR that the signal can have before despreading is therefore 5 db minus the Processing Gain (i.e., 5dB 9dB = 4 db). In other words, the signal power, can be 4 db under the interference or thermal noise power, and the Receiver (Demodulator) can still decode the signal correctly.
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 65 The number of chips that will be used (i.e., the length of the Spreading Code) to spread one bit of data is defined by the Spreading Factor. The Spreading Factor is given by: Spreading _ Factor Chip _ Rate Bit _ Rate
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 66 Using W-CDMA (Wideband-Code Division Multiple Access, which is used in 3G Networks) we have 5Mhz carrier bandwidth and a Chip Rate of 3.84 Mcps to Spread the data. Note: CDMA uses a carrier bandwidth of 1.25 MHz and a Chip Rate of 1.22Mcps. Thus, if we transmit a video clip with Bit Rate of 128Kbps the Spreading Factor will be: Spreading _ Factor 3,840,000chips / sec 128,000bits / sec 30 Each bit will be spread using a Spreading Code of length 30. Processing Gain = 10 x log 10 (30) = 14.77 db
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 67 Processing Gain allows the received signal power to be under the interference or thermal noise power (i.e., improves reception), and the Receiver can still detect the signal. Detection of a Spread signal is difficult without knowledge of the Spreading Code. Spread Spectrum systems originated in military applications as it is very difficult to interfere with (πολύ δύσκολα παρεμβάλλεται) and difficult to identify the signal (πολύ δύσκολα αναγνωρίζεται η πληροφορία που μεταφέρει το σήμα) without knowing the Spreading Code.
Spread Spectrum Techniques Spread Spectrum Advantages 68 Several advantages can be gained from this apparent waste of spectrum (από αυτή την προφανή σπατάλη του φάσματος) by this approach: The signals gains immunity from various kinds of noise and interference (Τα σήματα αποκτούν μεγαλύτερη ανοσία στο θόρυβο και στις παρεμβολές) Due to the Processing Gain that can be achieved The earliest applications of spread spectrum were military, where it was used for its immunity to jamming (ανοσία σε θόρυβο και παρεμβολές με σκοπό το μπλοκάρισμα των καναλιών).
Spread Spectrum Techniques Spread Spectrum Advantages 69 It can also be used for hiding and encrypting signals (Χρησιμοποιούνται για απόκρυψη και κρυπτογράφηση των σημάτων). Only a recipient who knows the spreading code can recover the encoded information. Several users can independently use the same bandwidth at the same time with very little interference. This property is used in cellular telephony applications (e.g., in UMTS Networks), with a technique known as Code Division Multiple Access (CDMA).
Spread Spectrum Techniques Τεχνικές Διασποράς Φάσματος 70 The first type of spread spectrum developed is known as Frequency Hopping (FH). A more recent type of spread spectrum is Direct Sequence (DS). Both of these techniques are used in various wireless communications standards and products.
Frequency Hopping Spread Spectrum (FHSS) 71 When using FHSS, the available Frequency Spectrum (bandwidth) is divided into channels (Το διαθέσιμο εύρος ζώνης διαιρείται σε κανάλια μικρότερου εύρους).
Frequency Hopping Spread Spectrum (FHSS) 72 Data are transmitted (spread) on these channels in a random pattern (με ένα τυχαίο μοτίβο) (this random pattern is defined by a Hopping sequence Table) generated by a pseudorandom algorithm known only to the Transmitter and Receiver. The Transmitter transmits data on one frequency channel for a certain time, then randomly jumping (hopping) to another, and transmitting again. The Receiver, hopping between frequencies in synchronization (σε συγχρονισμό) with the Transmitter, picks up the message.
Frequency Hopping Spread Spectrum (FHSS) 73 Because collocated FHSS networks follow different random patterns, multiple systems/networks can operate in close proximity with a minor possibility of interfering (Επειδή συνορεύοντα FHSS δίκτυα ακολουθούν διαφορετικά τυχαία μοτίβα αναπήδησης μπορούν να λειτουργήσουν σε κοντινές αποστάσεις με ελάχιστη πιθανότητα να προκαλούν παρεμβολές το ένα στο άλλο). If interference is present on one channel, data transmission is blocked (i.e., the data is lost). The Transmitter and the Receiver will hop to the next channel in the Hopping Sequence Table and the Transmitter will resend the data again.
Frequency Hopping Spread Spectrum (FHSS) 74 Advantages of FHSS: Allows a number of FHSS systems/networks to operate in close proximity in the same geographic area with a minor possibility of interfering. Eavesdroppers that do not know the hopping sequence will hear only unintelligible blips (ακατανόητους στιγμιαίους ήχους). Attempts to jam the signal on one frequency channel will affect only a few bits.
Frequency Hopping Spread Spectrum (FHSS) 75 Disadvantages of FHSS: It has a relatively low transfer limit, since only a specific amount of information can be sent over any given frequency channel (e.g., in the case of Bluetooth only 1 MHz of bandwidth can be used to spread the data at each hop) There is no built in redundancy or error checking, which means that once there is a certain critical limit of bad channels, FHSS becomes nearly unusable Too many bits come out corrupted.
Frequency Hopping Spread Spectrum (FHSS) 76 Due to the need for rapidly switching between different channels (hardware complexity), and the low data transfer limitation, FHSS is not widely implemented in today s WLAN or cellular systems, however Bluetooth (WPAN) does use FHSS. Bluetooth divides the available bandwidth into 79 channels of 1Mhz each. The first channel starts at 2,402 MHz and continues up to 2,480 MHz in 1 MHz steps. It performs 1600 hops per second ~0.625 ms per time slot
Direct Sequence Spread Spectrum (DSSS) 77 The DSSS encoder spreads the data across a broad range of frequencies (διασπείρει τα δεδομένα σε ένα ευρύ φάσμα συχνοτήτων; usually in the whole available bandwidth) using the Spreading Code. The higher the Bandwidth (and thus the higher the Chip Rate that can be used) to spread the data, the larger the Spreading Code Thus the higher the Processing Gain that can be achieved and thus lower power density is required during transmission! The Receiver uses the same Spreading Code to decode the data.
Direct Sequence Spread Spectrum (DSSS) 78 Compared to Narrowband transmissions, DSSS transmissions uses a much lower power density (power/frequency) to send the data.
Direct Sequence Spread Spectrum (DSSS) 79 At the Receiver, Narrowband interference appears as another narrowband transmission affecting a small part of the DSSS signal. Στο Receiver, μικρού εύρους ζώνης παρεμβολές παρουσιάζονται απλά σαν ένα άλλο μικρού εύρους ζώνης συχνοτήτων σήμα το οποίο επηρεάζει ένα μικρό κομμάτι του διασπαρμένου σε όλο το διαθέσιμο εύρος ζώνης (DSSS) σήματος.
Direct Sequence Spread Spectrum (DSSS) 80 When the DSSS signal is decoded back to its original narrowband state, the narrowband interference picked up during transmission is decoded to a lower power density signal thereby reducing its effects and thus ignored (filtered) by the Receiver (Due to Processing Gain) Όταν το DSSS σήμα αποκωδικοποιηθεί στην αρχική του κατάσταση, οι παρεμβολές που υπήρχαν στο μικρό εύρος συχνοτήτων αποκωδικοποιούνται σε ένα πολύ πιο αδύναμο σήμα και στο τέλος αγνοούνται από τον Receiver)
Direct Sequence Spread Spectrum (DSSS) 81 As the whole available bandwidth is used to spread the data, DSSS achieves better Processing Gain than FHSS (Spreading Codes with higher length can be used) Thus, better resistance to Interference and moreover improved reception at the Receiver. Better resistance to intended or unintended jamming (Καλύτερη αντοχή σε σκόπιμες ή μη παρεμβολές η θόρυβο για να μπλοκάρουν το σήμα) Lower levels of transmission power required to send a signal and thus: Less battery consumption in the Mobile Terminal Less Interference caused in the System
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