PETER PAZMANY CATHOLIC UNIVERSITY Consortium members SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER

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PETER PAZMANY CATHOLIC UNIVERSITY SEMMELWEIS UNIVERSITY Development of Complex

framework** Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY Consortium members

the support of the European Union and has been co-financed by the European Social

fejlesztése konzorciumi keretben ***A projekt az Európai Unió támogatásával, az

1 PETER PAZMANY CATHOLIC UNIVERSITY SEMMELWEIS UNIVERSITY Development of Complex Curricula for Molecular Bionics and Infobionics Programs within a consortial* framework** Consortium leader PETER PAZMANY CATHOLIC UNIVERSITY Consortium members SEMMELWEIS UNIVERSITY, DIALOG CAMPUS PUBLISHER The Project has been realised with the support of the European Union and has been co-financed by the European Social Fund *** **Molekuláris bionika és Infobionika Szakok tananyagának komplex fejlesztése konzorciumi keretben ***A projekt az Európai Unió támogatásával, az Európai Szociális Alap társfinanszírozásával valósul meg TÁMOP /2/A/KMR

2 Peter Pazmany Catholic University Faculty of Information Technology Ad hoc Sensor Networks Érzékelő mobilhálózatok Digital modulation Digitális moduláció Dr. Oláh András TÁMOP /2/A/KMR

3 Lecture 3 review Signal propagation overview Path loss models Log Normal Shadowing Narrowband Fading Model Wideband Multipath Channels TÁMOP /2/A/KMR

4 Outline Advantage of digital modulation Bandwidth of signals ISI-free system requirements IQ modulator Constellation and eye diagrams Tradeoff between spectral efficiency and power efficiency Linear and constant envelope modulation scheme Spread Spectrum Modulation TÁMOP /2/A/KMR

5 Structure of a wireless communications link TÁMOP /2/A/KMR

6 Modulation It is defined as a technique of mapping the information signal to a transmission signal (modulated signal) which is better suited for the operating medium (i.e. the wireless channel). The transmitted radio signal can be described as st = Atcos 2π ft+θ t By letting the transmitted information change the amplitude, the frequency, or the phase to carry the information we get the three basic types of digital modulation techniques: ASK (Amplitude Shift Keying) FSK (Frequency Shift Keying) PSK (Phase Shift Keying) ( ) ( ) ( ) ( ) Amplitude Frequency Phase Constant amplitude TÁMOP /2/A/KMR

7 Modulation (cont ) ( ) ( ) = ( ) cos 2π +Θ( ) st At ft t TÁMOP /2/A/KMR

8 Advantage of digital communications It allows information to be packetized It can compress information in time and efficiently send as packets through network. In contrast, analog modulation requires circuit-switched connections that are continuously available. Inefficient use of radio channel if there is dead time in information flow. It allows error correction to be achieved Less sensitivity to radio channel imperfections. It enables compression of information. More efficient use of channel. It supports a wide variety of information content. Voice, text and messages, video can all be represented as digital bit streams TÁMOP /2/A/KMR

9 Digital modulation Better performance and more cost effective than analog modulation methods (AM, FM, etc.) Performance advantages: the digital transceivers are much cheaper, faster and more power-efficient than analog transceivers; higher data rates are achieved compared to analog with the same signal bandwidth; powerful error correction techniques make the signal much less susceptible to noise and fading, and equalization can be used to mitigate ISI [ see later]; more efficient multiple acces strategies (spread spectrum techniques applied to digital modulation can remove or combine multipath, resist interference, and detect multiple users simultaneously); better security and privacy for digital systems; combination of multiple information types (voice, data, & video) in a single transmission channel; implementation of modulation/demodulation functions using DSP software (instead of hardware circuits) TÁMOP /2/A/KMR

10 Digital modulation (cont ) Choice of digital modulation scheme Many types of digital modulation methods small differences Performance factors to consider (corresponding metrics) low Bit Error Rate (BER) at low S/N (BER performance) resistance to interference and multipath fading high data rate high spectral efficiency (SE [bps/hz]) easy and cheap implementation of mobile units (Receiver complexity) transmission power amplifier linearity requirements (linearity) efficient use of battery power in mobile unit (Power efficiency, PE) No existing modulation scheme can simultaneously satisfy all of these requirements. Each one is better in some areas with trade-offs of being worse in others TÁMOP /2/A/KMR

11 Bandwidth of a signal: the concept Many definitions depending on application. Recall from DSP course FCC definition (99%) TÁMOP /2/A/KMR

12 Frequency ranges of a some natural signals Biological Signals Seicmic signals Electromagnetic signals Type of Signal Electroretinogram 0-20 Pneumogram 0-40 Electrocardiogram (ECG) Electroenchephalogram (EEG) Electromyogram Sphygmomanogram Speech Seismic exploration signals Eartquake and nuclear explosion signals Radio bradcast 3x10 4-3x10 6 Frequency Range [Hz] Common-carrier comm. 3x10 8-3x10 10 Infrared 3x x10 14 Visible light 3.7x x TÁMOP /2/A/KMR

13 A simplified communication modell Recall from ICT course PROBLEM: Bandlimited channel! TÁMOP /2/A/KMR

14 Digital signal transmission over analog channel Recall from ICT course PROBLEM: 1. Nyquist pulse are noncausal and of infinite duration. 2. We cannot implement the ideal lowpass filter in practice. 3. It decays very slowly (~1/t) TÁMOP /2/A/KMR

ISI-free system requirements Recall from ICT course Nyquist criterion for ISI-free communication: g l = δl0 = 1 l = 0 0 otherwise FT () G( f ) g t (Not to mention

15 ISI-free system requirements Recall from ICT course Nyquist criterion for ISI-free communication: g l = δl0 = 1 l = 0 0 otherwise FT () G( f ) g t (Not to mention the ISI caused by the coherence bandwidth of the wireless channel.) FT 1 n g( nt) GS ( f ) = G f + = 1 T n T f 1 2T TÁMOP /2/A/KMR

For the minimum bandwidth the impulse response is Nyquist pulse.

16 ISI-free system requirements (cont ) Nyquist criterion: FT 1 n g( nt) GS ( f ) = G f + = 1 T n T Recall from ICT course Observations: To satisfy the Nyquist criterion, the channel bandwidth B must be at least 1/(2T) For the minimum bandwidth the impulse response is Nyquist pulse. The pulse shape g(t) fulfills the Nyquist criterion if it is centersymmetric for 1/2T: (basis pulse shapping) TÁMOP /2/A/KMR

tt 1 ( 2α tt) RC 2 B=1/T B ~1/T B=(1+α)1/T

17 ISI-free system requirements (cont ) Nyquist Pulse Raised-Cosine Pulse Recall from ICT course g N () t = sin ( π f T ) π f T α=0 g () t sin = π ( π tt) cos( απ tt) tt 1 ( 2α tt) RC 2 B=1/T B ~1/T B=(1+α)1/T TÁMOP /2/A/KMR

( ) = ( ) 2 G f G f G f Uncorrelated η k : N0 k = l R( k) = E{ ηk lηk} = 0 k l R 2 (

18 Nyquist criterion with matched filtering Recall from ICT course 1 n n G T f + G R f + = 1 T T T n min G R ( f ) B B R ( ) 2 G f df G R (f) = G T (f) * matched filter T ( ) ( ) = ( ) 2 G f G f G f Uncorrelated η k : N0 k = l R( k) = E{ ηk lηk} = 0 k l R 2 ( ) ( ) ( ) Sη f = N G f = NG f 0 R TÁMOP /2/A/KMR

19 Error probability Recall from ICT course { ˆ } { ˆ } ( ) { ˆ } ( ) BER= Pr y y = Pr y= 1 y= 1 p y= 1 + Pr y= 1 y= 1 p y= 1 = { ( η) } ( ) ( η) BERBPSK =Φ { } ( ) = Pr sgn y+ = 1 y= 1 p y= 1 + Pr sgn y+ = 1 y= 1 p y= 1 = = Pr{ η 1} Pr{ η < 1} 0.5 = 1 Φ +Φ =Φ 2 N 0 N 0 N 0 ( SNR ) TÁMOP /2/A/KMR

20 Receiver sensitivity or power efficiency (PE) It describes the ability of a modulation technique to preserve the quality of digital messages at low power levels (low SNR): required PE = E b / N 0 for a certain BER (e.g ) where E b : energy/bit and N 0 : noise power/bit Tradeoff between signal power and fidelity: as E b / N 0,thanBER It depends on the particular type of modulation employed TÁMOP /2/A/KMR

Bandwidth efficiency or spectral efficiency (SE) Ability of a modulation technique to accommodate data in a limited BW SE = R / B, where R is the data rate, B is the

21 Bandwidth efficiency or spectral efficiency (SE) Ability of a modulation technique to accommodate data in a limited BW SE = R / B, where R is the data rate, B is the system bandwidth. Trade of between R data rate and B bandwidth: as R, than B For a digital signal 1 R B so as R, Ts and B T s TÁMOP /2/A/KMR

22 M-ary Keying each pulse or symbol having m finite states represents n = log 2 M bits/symbol e.g. M =0or1(2states) n = 1 bit/symbol (binary) e.g. M =0,1,2,3,or4(4states) n = 2 bits/symbol E.g.: when a system is changed from binary to 4-ary: In the case of binary: "0" = - 1V and "1" = 1 V In the case of 4-ary: "0" = - 1V, "1" = V, "2" = 0.33 V, "3" = 1 V What would be the new data rate compared to the old data rate if the symbol periods were kept constant? TÁMOP /2/A/KMR

23 Maximum SE: Shannon s theorem (1948) Most famous result in communication theory. SE C P E R = = log 1 log 1 B + P = + N B S b max 2 2 N 0 B : bandwidth C : channel capacity (bps) of real data (not retransmissions or errors) To produce error-free transmission, some of the bit rate will be taken up using retransmissions or extra bits for error control purposes. Lower bit error rates from higher power results more real data As noise power P N increases, the bit rate would still be the same, but SE max decreases. SE max is fundamental limit that cannot be achieved in practice. Recall from ICT course Claude Elwood Shannon ( ) TÁMOP /2/A/KMR

24 Fundamental trade-off between SE and PE If SE improves then PE deteriorates (or vice versa) One may need to waste more power to get a better data rate. One may need to use less power (to save on battery life) at the expense of a lower data rate. SE vs. PE is not the only consideration, we use other factors to evaluate, e.g.: resistance to interference and multipath fading; easy and cheap implementation in mobile unit; etc TÁMOP /2/A/KMR

Ad hoc Sensor Networks: Wireless channel characterization and models The canonical form of a band pass transmitted radio signal is () ( ) ω ( ) Convex envelope

The signal s(t) can be written as st = Atcos Θ t cos ωt Atsin Θ t sin ωt () ( ) ( ) ( ) ( ) ( ) ( ( )) ( ) Recall from Chapter 3 We will define the following

25 Ad hoc Sensor Networks: Wireless channel characterization and models The canonical form of a band pass transmitted radio signal is () ( ) ω ( ) Convex envelope theory ( ) ( ) j2πft jθ( t) { } st = Atcos t+θ t = Re Ate e where e j2πft is the carrier factor. The signal s(t) can be written as st = Atcos Θ t cos ωt Atsin Θ t sin ωt () ( ) ( ) ( ) ( ) ( ) ( ( )) ( ) Recall from Chapter 3 We will define the following quantities s t = A t cos Θ t Q I () ( ) ( ) () = () sin Θ() The complex envelope of s(t) is now written as s t = s + js and ( ) ( ) s t A t t () I Q j2π { } ( ) = st ( ) st Re e ft TÁMOP /2/A/KMR

distinguish between different symbols Assign each I/Q symbol to a

26 Constellation diagrams Plot I/Q samples on x-y axis The constellation diagram provides a sense of how easy it is to distinguish between different symbols Assign each I/Q symbol to a set of digital bits (eg. Gray code) TÁMOP /2/A/KMR

Constellation diagrams Noise corrupts sampled I/Q values The points in the constellation diagram no longer consist of single dots for each symbol

27 Constellation diagrams Noise corrupts sampled I/Q values The points in the constellation diagram no longer consist of single dots for each symbol What is the best way to match received I/Q samples with their corresponding symbols? (Detection) TÁMOP /2/A/KMR

28 Constellation diagrams properties Distance between signals is related to differences in modulation waveforms Large distance easy to discriminate good BER at low SNR Power Efficient related to density Occupied BW as number of signal states If we can represent more bits per symbol, then we need less BW for a given data rate. Small separation dense more signal states/symbol more information/hz!! Bandwidth Efficient TÁMOP /2/A/KMR

Eye diagrams Key idea: wrap signal back onto itself in periodic time intervals and

symbols eventually reveals all possible symbol transition trajectories It shows the

Eye diagram allows visual inspection of the impact of sample time and decision

29 Eye diagrams Key idea: wrap signal back onto itself in periodic time intervals and retain all traces Similar to the action of an oscilloscope Increasing the number of symbols eventually reveals all possible symbol transition trajectories It shows the ISI present as well as timig jitter present. Eye diagram allows visual inspection of the impact of sample time and decision boundary choices Large eye opening implies less vulnerability to symbol errors TÁMOP /2/A/KMR

30 Modulation schemes TÁMOP /2/A/KMR

31 Binary Phase Shift Keying (BPSK) Phase transitions force carrier amplitude to change from + to. Amplitude varies in time TÁMOP /2/A/KMR

32 Quaternary Phase Shift Keying (QPSK) Four different phase states in one symbol period Two bits of information in each symbol double the SE of BPSK or twice the data rate in same signal BW same PE (same BER at specified E b /N 0 ) TÁMOP /2/A/KMR

33 Transmit power amplifier When a modulation signal encounters a nonlinearity the signal becomes distorted and its occupied frequency bandwidth increases (spectrum re-growth). The most significant source of nonlinearity comes from the transmission PA TÁMOP /2/A/KMR

34 Offset QPSK QPSK OQPSK TÁMOP /2/A/KMR

amplifier, helps eliminate spectrum regrowth after

35 Quaternary Phase Shift Keying (QPSK) OQPSK ensures there are fewer baseband signal transitions applied to the RF amplifier, helps eliminate spectrum regrowth after amplification TÁMOP /2/A/KMR

36 Frequency Shift Keying (FSK) Constant Envelope as compared to AM Linear: Amplitude of the signal varies according to the message signal. Constant Envelope: The amplitude of the carrier is constant, regardless of the variation in the message signal. It is the phase that changes TÁMOP /2/A/KMR

37 M-ary Phase Shift Keying (MPSK) The SE with M The PE with M TÁMOP /2/A/KMR

bandwidth Need much more power to keep constellation points far enough

38 M-ary QAM Basic trade-off: Better bandwidth efficiency at the expense of power efficiency More bits per symbol time better use of constrained bandwidth Need much more power to keep constellation points far enough apart for acceptable bit error rates TÁMOP /2/A/KMR

39 M-ary FSK Frequencies are chosen in a special way so that they are easily separated at the demodulator (orthogonality principle). M-ary FSK transmitted signals: 2Es π si( t) = cos ( nc i) t T + T 0 t T i = 0,1,..., M f c = n c /2T for some integer n c The M transmitted signals are of equal energy and equal duration The SE of an M-ary FSK signal with M The PE with M Since M signals are orthogonal, there is no crowding in the signal space TÁMOP /2/A/KMR

40 Modulation scheme comparisons Given a modulation scheme and a targeted BER then the communication system designer can determine the SE (spectral efficiency) and the PE (E b /N 0 required to maintain the average BER target) TÁMOP /2/A/KMR

41 Spread Spectrum Modulation (SSM) The transmitter expands (spreads) signal B s bandwidth many times with a p(t) spreading code and the signal is then collapsed (despread) in receiver side with the same code. Other signals created with other codes just appear at the receiver as random noise. Processing Gain (PG)= B s /B T TÁMOP /2/A/KMR

42 Spread Spectrum Modulation (SSM) advantage Resistant to narrowband interference. It allows multiple users with different codes to share same the wireless channel no frequency reuse needed rejects interference from other users It combats multipath fading if a multipath signal is received with enough delay (more than one chip duration), it also appears like noise. As number of simultaneous users the SE TÁMOP /2/A/KMR

43 Spreading codes Signal spreading is done by multiplying the data signal by a pseudo-noise (PN) code or sequence the pseudo-noise signal looks like noise to all observers except those who know how to recreate the sequence TÁMOP /2/A/KMR

44 Spreading codes: PN codes Binary sequence with random properties noise-like (called "pseudo-noise" because they are not noise technically) equal # s of 1 s and 0 s Very low correlation between time-shifted versions of same sequence Very low cross-correlation between different codes each user assigned unique code that is approximately orthogonal to all other codes the other users signals appear like random noise! TÁMOP /2/A/KMR

45 Direct Sequence (DS) Ad hoc Sensor Networks: Digital modulation Type of spread spectrum modulation Multiply baseband data by PN code (same as above) Spread the baseband spectrum over a wide range. The Rx spread spectrum signal 2Es si() t = m() t p()cos t ( 2 π fct+ θ) Ts where m(t) : the data sequence and p(t) the PN sequence Frequency Hopping (FH) Randomly change f c with time In effect, this signal stays narrowband but moves around a lot to use a wide band of frequencies over time. Hopset: the set of possible carrier frequencies Hop duration: the time during between hops Classified as fast FH or slow FH fast FH: more than one frequency hop during each Tx symbol slow FH : one or more symbol are Tx in the time interval between frequency hops TÁMOP /2/A/KMR

46 Spread spectrum modulation and the multiple access With Spread Spectrum Modulation, users are able to share a common band of frequencies yielding a multiple access technique TDMA: Users share a band of frequencies, but use a different time slot FDMA: Users share a band of frequencies, but use a different slice of frequency SSM enables CDMA (Code Division Multiple Access): Users share a band of frequencies and a number of time-slots, but each use a different spreading code TÁMOP /2/A/KMR

47 Summary Nyquist criterion for ISI-free communication. No existing modulation scheme simultaneously satisfies all of these requirements well. Given a modulation scheme and a targeted BER then the communication system designer can determine the SE (spectral efficiency) and the PE (E b /N 0 required to maintain the average BER target). With Spread Spectrum Modulation, users are able to share a common band of frequencies a multiple access technique (CDMA) Next lecture: Detection and channel equalization TÁMOP /2/A/KMR

Lecture 9: Spread Spectrum Modulation Techniques

Lecture 9: Spread Spectrum Modulation Techniques Spread spectrum (SS) modulation techniques employ a transmission bandwidth which is several orders of magnitude greater than the minimum required bandwidth