5. Digitaaliset modulaatiomenetelmät

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1 5.1. Johdanto 5. Digitaaliset modulaatiomenetelmät modulaatiotavan valinnassa on yleensä kyse: käytettävissä oleva kaistanleveys vs. kapasiteettitarve tehonkulutus kohinavaikutus kustannukset digitaalimodulaation käytön etuja ovat suurempi siirtokapasiteetti yhteensopivuus datapalveluiden kanssa parempi tietoturva parempi siirron laatu nopeampi järjestelmien kehitysaika Huom: Baudi [Bd] on symbolinopeuden yksikkö 18-Mar-04 Siirtotekniikka / JPR 244 binaarisissa menetelmissä kantoaaltoa moduloidaan joko ykköseksi tai nollaksi Illustrative waveforms for the three basic forms of signaling binary information: (a) Amplitude-shift keying. (b) (c) Phase-shift keying. Frequency-shift keying with continuous phase. 18-Mar-04 Siirtotekniikka / JPR 245 1

2 Source: Global Wireless Education Consortium Amplitude Shift Keying Modulator on off on on off on off off 18-Mar-04 Siirtotekniikka / JPR 246 After analog to digital signal conversion is completed, the signal is modulated such that the zeros and ones are converted into on-off discrete energy pulses. This is known as on-off keying or amplitude shift keying (ASK). This method of on and off pulsing is not used much for today s radio transmission since multipath fading distorts the amplitude of the carrier. However, fiber optic and infrared transmission use on-off pulsing of the light signal for transmitting digital information. (For more information on multipath fading, refer to the GWEC module RT-RF Antennas and RT-RF Propagation.) 246

3 Source: Global Wireless Education Consortium Frequency Shift Keying Modulator Mar-04 Siirtotekniikka / JPR 247 Another way to modulate digital signals is to have two distinct frequencies that are offset from the carrier frequency. These two frequencies represent the zeros and ones from the original digital signal. This is called frequency shift keying (FSK). Frequency modulation and phase modulation are closely related. A static frequency shift of +1 Hz means that the phase is constantly advancing at the rate of 360 degrees per second (2 π rad/sec), relative to the phase of the unshifted signal. If you have two distinct frequencies, there are two states, 0 (on) and 1 (off). If you have four distinct frequencies, there are four states: 00, 01, 10, and 11. With eight frequencies there are eight states: 000, 001, 010, 100, 011, 101, 110, and 111. When a signal element changes states it is called a baud. With two states, there is one bit to one baud. With four states, there are two bits to one baud, and with eight states there are three bits to one baud. FSK is used in low transmission rates such as paging and for the control channel in some cellular systems. Some of the cordless systems include DECT (Digital Enhanced Cordless Telephone) and CT2 (Cordless Telephone 2). 247

4 Source: Global Wireless Education Consortium Phase Shift Keying Modulator Phase Shift Keying (PSK) Uses Different Phases to Indicate Bits 90 o 180 o o 270 o 18-Mar-04 Siirtotekniikka / JPR 248 Another way to modulate digital signals is phase shift keying (PSK) whereby the carrier frequency is modulated by a phase angle. The simplest example is binary PSK. Each 180- degree shift in phase represents a change from 0 to 1. Phase relationships can also be drawn without using sine waves by using a phase plan diagram. In a phase plan diagram, each vector represents a sine wave. In the example shown above, waves are 180 degrees out of phase with each other. Most digital radio signals use some form of PSK. 248

5 Multilevel Modulation Source: Global Wireless Education Consortium Modulator Amplitudes 4 Frequencies 4 Phases 18-Mar-04 Siirtotekniikka / JPR 249 Modulation can be explained in terms of degrees of freedom. Amplitude, frequency, and phase are the variables. Simple systems generally hold two of the three variables constant, varying only one of them. This makes decoding quite easy, but subject to error. More complex systems vary combinations of the variables, creating more reliable systems. Complex systems may also have more than the two-state binary levels to support higher data rates. Efficiency of the spectrum is defined by the number of bits that can be transmitted in a period of time (usually one second) with a defined bandwidth or channel. Since the width of the channel is in kilohertz (khz) or megahertz (MHz), spectrum efficiency is the number of bits per second per hertz. In other words, it is how many bits per hertz that can be stuffed into the radio channel. With ASK, FSK, or two-state PSK, each change of state represents one bit. Under ideal conditions, one bit per second per hertz of channel can be transmitted. With a 30 khz channel, like cellular, 30,000 bits per second (30 kb/s) can be transmitted. Using ASK, FSK, or two-state PSK is inefficient. If there were four states (four amplitudes, four frequencies, or four phases), the number of bits per second per hertz would double from one to two, and the occupied bandwidth would be cut in half. The modulation scheme would be more complex, but double the efficiency. On the other hand, eight levels allow for three bits. Now the efficiency of three bits per second per hertz could be achieved. By modulating at sixteen levels (four bits), the efficiency is four bits per second per hertz. 249

6 Kanavointimenetelmät Digitaaliset mod.menetelmät 18-Mar-04 Siirtotekniikka / JPR I/Q-modulaation käyttö amplitudia ja vaihetta voitaisiin moduloida yhtä aikaa ja erikseen, mutta se on käytännössä hankalaa sen sijaan käytännön järjestelmissä signaalit muodostetaan I- ja Q- komponenttien avulla (in-phase ja quadrature) komponentit ovat ortogonaalisia, eivätkä vaikuta toisiinsa 18-Mar-04 Siirtotekniikka / JPR 251 4

7 18-Mar-04 Siirtotekniikka / JPR 252 Signal characteristics that can be modified There are only three characteristics of a signal that can be changed over time: amplitude, phase, or frequency. However, phase and frequency are just different ways to view or measure the same signal change. In AM, the amplitude of a high-frequency carrier signal is varied in proportion to the instantaneous amplitude of the modulating message signal. Frequency Modulation (FM) is the most popular analog modulation technique used in mobile communications systems. In FM, the amplitude of the modulating carrier is kept constant while its frequency is varied by the modulating message signal. Amplitude and phase can be modulated simultaneously and separately, but this is difficult to generate, and especially difficult to detect. Instead, in practical systems the signal is separated into another set of independent components: I (Inphase) and Q (Quadrature). These components are orthogonal and do not interfere with each other. 252

8 Napakoordinaatistoesitys (polar diagram) amplitudi ja vaihe esitetään samanaikaisesti kantoaalto muodostaa referenssin, johon amplitudia ja vaihetta verrataan amplitudi voidaan esittää joko suhteellisena tai absoluuttisena arvona 18-Mar-04 Siirtotekniikka / JPR 253 Polar display magnitude and phase represented together A simple way to view amplitude and phase is with the polar diagram. The carrier becomes a frequency and phase reference and the signal is interpreted relative to the carrier. The signal can be expressed in polar form as a magnitude and a phase. The phase is relative to a reference signal, the carrier in most communication systems. The magnitude is either an absolute or relative value. Both are used in digital communication systems. Polar diagrams are the basis of many displays used in digital communications, although it is common to describe the signal vector by its rectangular coordinates of I (In-phase) and Q (Quadrature). Signal changes or modifications in polar form Figure 6 shows different forms of modulation in polar form. Magnitude is represented as the distance from the center and phase is represented as the angle. Amplitude modulation (AM) changes only the magnitude of the signal. Phase modulation (PM) changes only the phase of the signal. Amplitude and phase modulation can be used together. Frequency modulation (FM) looks similar to phase modulation, though frequency is the controlled parameter, rather than relative phase. One example of the difficulties in RF design can be illustrated with simple amplitude modulation. Generating AM with no associated angular modulation should result in a straight line on a polar display. This line should run from the origin to some peak radius or amplitude value. In practice, however, the line is not straight. The amplitude modulation itself often can cause a small amount of unwanted phase modulation. The result is a curved line. It could also be a loop if there is any hysteresis in the system transfer function. Some amount of this distortion is inevitable in any system where modulation causes amplitude changes. Therefore, the degree of effective amplitude modulation in a system will affect some distortion parameters. 253

9 I/Q-esitys polaariesitys suorakaidemuodossa 18-Mar-04 Siirtotekniikka / JPR 254 Lähetin- ja vastaanotinratkaisut I/Q-modulaation käyttö yksinkertaistaa toteutusta 18-Mar-04 Siirtotekniikka / JPR 255 6

10 Miksi käytetään I/Q-esitystä? Digitaalimodulaatiomenetelmät ovat helppoja toteuttaa I/Qmodulaattorien avulla Useimmissa digitaalisissa modulaatiomenetelmissä viesti kuvautuu rajalliseen määrään I/Q-tason pisteitä (ns. konstellaatiopisteet) Signaalin siirtyessä tilasta toiseen, aiheuttaa muutos tavallisesti sekä amplitudin että vaiheen muutoksen Em. toteuttaminen perinteisten amplitudi- ja vaihemodulaattorien avulla on vaikeaa ja toteutus tulee kalliiksi Sen sijaan yhtäaikainen AM and PM voidaan helposti toteuttaa I/Qmodulaattorilla 18-Mar-04 Siirtotekniikka / JPR 256 Whytouse I and Q? Digital modulation is easy to accomplish with I/Q modulators. Most digital modulation maps the data to a number of discrete points on the I/Q plane. These are known as constellation points. As the signal moves from one point to another, simultaneous amplitude and phase modulation usually results. To accomplish this with an amplitude modulator and a phase modulator is difficult and complex. It is also impossible with a conventional phase modulator. The signal may, in principle, circle the origin in one direction forever, necessitating infinite phase shifting capability. Alternatively, simultaneous AM and Phase Modulation is easy with an I/Q modulator. The I and Q control signals are bounded, but infinite phase wrap is possible by properly phasing the I and Q signals. 256

11 5.3. Tärkeimmät digitaaliset modulaatiomenetelmät Yleistä ao. taulukossa on menetelmien sovelluskohteita langattomassa tietoliikenteessä ja videotekniikassa 18-Mar-04 Siirtotekniikka / JPR 257 Digital modulation types variations The basic modulation types form the building blocks for many systems. There are three main variations on these basic building blocks that are used in communications systems: I/Q offset modulation, differential modulation, and constant envelope modulation. 257

12 PSK 18-Mar-04 Siirtotekniikka / JPR 258 Phase Shift Keying One of the simplest forms of digital modulation is binary or Bi-Phase Shift Keying (BPSK). One application where this is used is for deep space telemetry. The phase of a constant amplitude carrier signal moves between zero and 180 degrees. On an I and Q diagram, the I state has two different values. There are two possible locations in the state diagram, so a binary one or zero can be sent. The symbol rate is one bit per symbol. A more common type of phase modulation is Quadrature Phase Shift Keying (QPSK). It is used extensively in applications including CDMA (Code Division Multiple Access) cellular service, wireless local loop, Iridium (a voice/data satellite system) and DVB-S (Digital Video Broadcasting Satellite). Quadrature means that the signal shifts between phase states which are separated by 90 degrees. The signal shifts in increments of 90 degrees from 45 to 135, 45, or 135 degrees. These points are chosen as they can be easily implemented using an I/Q modulator. Only two I values and two Q values are needed and this gives two bits per symbol. There are four states because 2 2 = 4. It is therefore a more bandwidth-efficient type of modulation than BPSK, potentially twice as efficient. 258

13 QPSK Kaksi tyypillistä QPSK:n konstellaatiokuvaa; nuolet osoittavat QPSK-modulaattorin mahdolliset tilasiirtymät nelivaiheinen PSK (1 merkki välittää 2 bittiä) käyttää vain puolet BPSK:n kaistasta usein käytetään gray-koodattua bittiparia, jolloin vaihesiirtymät ovat (oik.puoleinen kuva) 10 => 45º (π/4) 00 => 135º (3π/4) 01 => 225º (5π/4) tai -135º (-3π/4) 11 => 315º (7π/4) tai -45º (-π/4) 18-Mar-04 Siirtotekniikka / JPR 259 s 2 x1 Q BPSK using sinewave carrier and transmitting the second bit QPSK Signal Space Diagram s 3 1x s 1 0x +135 o +45 o I s 4 x0 BPSK using cosine carrier and transmitting the first bit in two bit block -135 o -45 o Mar-04 Siirtotekniikka / JPR 261 The circles in the diagram above represent the target areas where the receiver looks for zeros and ones. It is not a single point, but is an area or search window defined by phase relationships. This adds reliability when wave forms are warped and reflected. HUOM! Tilasiirtymät ja vaiheet erilaiset kuin yo. kuvassa 259

14 Source: Global Wireless Education Consortium QPSK Modulator Block Diagram ODD Carrier X ~ + π/2 X EVEN 18-Mar-04 Siirtotekniikka / JPR 260 The diagram above and on the following page illustrate quadrature phase shift keying. Used by both cellular and PCS systems, quadrature phase shift keying (QPSK) increases the modulation efficiency from binary PSK. In binary PSK, one symbol phase (0 or 180 degrees) at the modulation stage represents one bit. In QPSK, one symbol (one of four phases) at the modulation stage represents two bits. The four phases used are 0, +90, -90, and 180 degrees. Half of the bit stream goes to the I (in-phase) multiplier, which has phase s 0 and 180 degrees, and the other half goes to the Q (quadrature or out-of-phase) multiplier, which has the phase s +90 and 90 degrees. QPSK can be thought of as two binary PSK modulators. While this is a relatively robust signal, it has a significant component of amplitude variation. The envelope of amplitude of the composite signal varies with modulation. Occasionally, the 180-degree phase shift can cause the envelope to go to zero instantly. Therefore, transmitter amplifiers should be linear. This type of modulation is typically used to transmit from the base station to the mobile (forward link). Since this form of modulation is the linear combination of two constant envelope modulation schemes, the result has a constant envelope as well. 260

15 (a) Input binary sequence. (b) Odd-numbered bits of input sequence and associated binary PSK wave. (c) Even-numbered bits of input sequence and associated binary PSK wave. (d) QPSK waveform defined as s(t) = s i1 φ 1 (t) + s i2 φ 2 (t). 18-Mar-04 Siirtotekniikka / JPR

16 Offset QPSK (OQPSK) I- ja Q-bitit eivät vaihdu yhtä aikaa => pienemmät amplitudivaihtelut ( db => 3 db), jolloin ei tarvita niin lineaarista RF-tehovahvistinta tällöin kerralla voi tapahtua vain ±90 vaihesiirto (vaihesiirtoja tulee kaksi kertaa tiheämmin kuin normaali QPSK:ssa) käytetään esim. CDMA:ssa Offset QPSK Signal Space o +45 o -135 o -45 o Mar-04 Siirtotekniikka / JPR 262 The circles in the diagram above represent the target areas in which the receiver looks for zeros and ones. It is not a single point due to the presence of noise, interference, or phase errors in the equipment, but is an area or search window defined by phase relationships. This adds reliability when wave forms are warped and reflected. I/Q offset modulation One example of offset modulation is Offset QPSK (OQPSK). This is used in the cellular CDMA (Code Division Multiple Access) system for the reverse (mobile to base) link. In QPSK, the I and Q bit streams are switched at the same time. The symbol clocks, or the I and Q digital signal clocks, are synchronized. In Offset QPSK (OQPSK), the I and Q bit streams are offset in their relative alignment by one bit period (one half of a symbol period). Since the transitions of I and Q are offset, at any given time only one of the two bit streams can change values. This creates a dramatically different constellation, even though there are still just two I/Q values. This has power efficiency advantages. In OQPSK the signal trajectories are modified by the symbol clock offset so that the carrier amplitude does not go through or near zero (the center of the constellation). The spectral efficiency is the same with two I states and two Q states. The reduced amplitude variations (perhaps 3 db for OQPSK, versus 30 to 40 db for QPSK) allow a more power-efficient, less linear RF power amplifier to be used. 262

17 Source: Global Wireless Education Consortium Offset QPSK Modulator Block Diagram ODD X ~ + π/2 X EVEN Τ/2 18-Mar-04 Siirtotekniikka / JPR 263 The diagram above and on the following page illustrate offset QPSK. Like QPSK, the unfiltered offset quadrature phase shift keying (OQPSK) signal has a constant envelope. However, the two streams do not change status at the same time, thus eliminating the 180-degree phase change. The envelope will not go to zero as it does with QPSK, meaning that nonlinear amplification can be used. Less linear but more efficient amplifiers can be used, which makes this modulation technique ideal for battery-powered transmitters. OQPSK is typically used for mobile to base station station (reverse link) modulation. Because the number of OQPSK bits per second is the same as in QPSK, OQPSK requires the same bandwidth. The slight delay avoids the 180-degree zero crossing shift. This allows cell phone manufacturers to use cheaper, less linear and more efficient amplifiers. This modulation technique is ideal for battery-powered transmitters. 263

18 π/4-dqpsk (käytetään myös π/4-qpsk) jokaisesta tilasta päästään neljään tilaan (ei siirrytä origon kautta) => tapahtuu aina tilan muutos Two commonly used signal constellations of QPSK; the arrows indicate the paths along which the QPSK modulator can change its state. Eight possible phase states for the π/4-shifted QPSK modulator. 18-Mar-04 Siirtotekniikka / JPR 264 Differential modulation One variation of differential modulation is differential QPSK (DQPSK). Differential means that the information is not carried by the absolute state, it is carried by the transition between states. In some cases there are also restrictions on allowable transitions. This occurs in π/4 DQPSK where the carrier trajectory does not go through the origin. 264

19 18-Mar-04 Siirtotekniikka / JPR 265 The reason for choosing π/4-dqpsk is based on having the choice of a powerefficient modulation or a spectral-efficient modulation. For a power-efficient modulation, the dynamic power range of the nonlinear amplifier is applied to increase power efficiency. In a power-efficient modulation, spectral regrowth is a problem that reduces spectral efficiency. For a modulation with spectral efficiency, a linear amplifier is used to reduce the spectral regrowth. 265

20 18-Mar-04 Siirtotekniikka / JPR 266 Source: Global Wireless Education Consortium π/4-differential QPSK ODD X ~ + π/4 π/2 X EVEN 18-Mar-04 Siirtotekniikka / JPR

21 18-Mar-04 Siirtotekniikka / JPR Mar-04 Siirtotekniikka / JPR