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Ai Mod
- 11. Frequency Modulation for Mobile Radio
- 12. FM Signaling A Constant-Amplitude Carrier A Modulating Signal Center Frequency Above Center Frequency Below Center Frequency Center Frequency Center Frequency Frequency Modulation
- 14. Frequency Shift Keying F F-shift F+shift F= Frequency
- 20. Quadrature Phase Shift Keying Modular Circuit Carrier
- 21. Quadrature Phase Shift Keying Signal Space
- 22. Offset QPSK Modular Circuit
- 23. Offset QPSK Signal Space Diagram
- 25. Phase Shift Truth Table
- 26. /4-DQPSK Signal Space Diagram
- 28. Summary
Editor's Notes
- © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws. Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners.
- Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
- During this module, the following topics will be discussed: Modulation basics Frequency modulation for mobile cellular Shift keying Types and uses Constant envelope modulation
- After completing this module and all of its activities, you will be able to: Explain basic modulation concepts. Describe frequency modulation for mobile radio. Explain different types of shift keying and when each one is used. Describe constant envelope modulation.
- Modulation is a process by which an input signal is encoded in a particular format suitable for transmission. Common transmission mediums include optical fiber, coaxial cable (coax), wave guide, or over-the-air. The baseband is the band of frequencies that must be sent to a particular destination. In audio systems these frequencies are typically in the 20-200 kHz range. For this baseband to be transmitted unmodulated the antenna would have to handle a wavelength ratio higher than 200:1, which is highly impractical. Thus, these baseband signals are transmitted at a much higher frequency bandpass signal level. Commercial frequency modulated (FM) broadcast bands have one hundred 200 kHz bandwith channels centered at about 100 MHz, which gives a 20% wavelength ratio. Advanced mobile phone service (AMPS) uses 832-30 kHz channels in a 25 MHz band centered at about 860 MHz, which is less than a 3% wavelength ratio. A smaller antenna is practical at the shorter wavelength. Antennas must be at least one-fourth wavelength of the propagation frequency to be efficient. Radio reception degrades when the transmitted signals bounce off of objects and arrive at the receiving antenna at different times or out of phase. “Ghosts” on television receivers are an example of the problem. Modulation schemes can be used to counter the effects of multipath fading and time-delay spread in mobile radio communication. (For more information on this topic, refer to the GWEC module RT- RF Propagation .)
- AM means amplitude modulation or changing of the amplitude of a signal in response to a second modifying signal. The signal whose amplitude is modulated is the carrier signal ; the modifying signal is the information signal . The information signal is also known as the baseband (BB) signal or the modulating signal . AM modulation is used in wireless telecommunications and in broadcast AM radio. In amplitude modulation, the receiver can be very simple in design; the early crystal radio set is an example of this. The information signal modulates (modifies) the carrier signal by being multiplied with it. The resulting composite signal is the amplitude-modulated carrier that is transmitted , i. e., the AM signal. The envelope of the AM signal is constantly changing because the amplitude is constantly being adjusted proportionally to the input signal. AM is a variable envelope type of transmission. When an AM receiver decodes an AM signal, it strips the carrier frequency from the composite signal, leaving the information signal to be heard. When you tune into an AM radio, you are matching the carrier signal’s frequency. AM radio is still very susceptible to noise (lightning, ignition, etc.) and is not used due to this detrimental factor in spite of its inherent simplicity of hardware.
- FM means frequency modulation , or changing the frequency of a carrier signal based on the amplitude of an information (baseband) signal. Frequency modulation is a special case of angle modulation ( phase modulation is another special case) in which the angle of the carrier is varied proportionately to the amplitude of the baseband signal. In angle modulation, the amplitude of the carrier signal is kept constant; hence, angle modulation methods are constant envelope transmissions. The diagram above illustrates amplitude, phase, and frequency modulation by a sine wave. The remainder of this module will focus on FM for mobile radio.
- In FM signaling , the frequency of the carrier signal is varied in accordance with the information signal being transmitted. The amplitude of the modulated wave does not change, no matter how strong the modulating signal is. When the modulating signal is zero, the carrier signal (modulated wave) is at the center frequency (resting frequency). When the FM frequency is not modulated, it is at the value F. When it is modulated, it deviates from F-f to F+f , where f is the change representing the modulation information. In the FM detection process the noise or interference amplitude increases linearly with frequency variation from the carrier. Therefore, noise power at the detector output will vary proportionately with the square of the frequency. An advantage of this is that input noise components close to the carrier frequency are suppressed. This makes narrowband FM relatively “noise free” and it produces a much higher quality signal. An FM receiver captures the strongest FM signal in the receiver bandwidth. FM uses more bandwidth than an equivalent AM signal and is relatively insensitive to received signal strength. The increased bandwidth required by FM is exploited to make FM relatively insensitive to noise. One disadvantage of FM is that it suppresses weaker signals. For this reason, air traffic systems do not use FM signals. If they did, air traffic controllers would hear urgent message superimposed on the message being listened to.
- Sampled speech can be transmitted over a radio channel by amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) signals. When a signal is converted to energy pulses or waves, it is known as shift keying . In this section, we will look at four types of shift keying: Frequency shift keying Amplitude shift keying Phase shift keying Multilevel shift keying
- Frequency shift keying is represented in the diagram above.
- 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 .)
- 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). 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.
- 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.
- 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.
- Because multipath fading distorts the amplitude of the carrier in a mobile radio, the signal is sent by modulating the phase or frequency of the carrier, which has no impact on the amplitude. These modulations are called constant envelope modulations since no signal is modulated on the amplitude. Distortion of carrier amplitude by other factors such as fading or nonlinear amplification will not affect the signal, making it possible to use a nonlinear amplifier. Constant envelope modulation can be a linear or a nonlinear modulation in digital mobile systems. Linear modulation is where the carrier signal is linearly proportional to the input signal. It can vary based on a constant factor. Nonlinear modulation allows the carrier signal to be a more complex mathematical proportion of the input signal. Constant envelope modulation is used in digital mobile systems and for the nonlinear modulation in analog mobile systems. A number of different modulation schemes are used by today’s digital cellular and PCS systems.
- 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.
- 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.
- 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, l ess linear and more efficient amplifiers. This modulation technique is ideal for battery-powered transmitters.
- 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.
- Also used for cellular and PCS systems is /4-DQPSK (differential quadrature phase shift keying ). /4-DQPSK is a type of QPSK . This modulation architecture uses symbols represented as the relative changes in phase rather than the absolute phase. Two carriers in quadrature with each other generate the waveform. The information source drives two amplifiers: s I and s Q. When their outputs are 0 & 0 the -3 /4 phase code is transmitted .
- The angle k is the phase shift based on the input symbols s I and s Q, which form a pair. Gray code is used in mapping the two-bit symbol. All possible combinations are shown in the table above. The reason for choosing /4-DQPSK is based on having the choice of a power-efficient 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. For example, a 4 watt RF amplifier can operate only at a 1 watt maximum power (when operating in the linear range) due to a 6 db output back off. A nonlinear amplifier does not have this back off, so it has a gain of 6 db. 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.
- QPSK has an instantaneous 180 o phase shift, which leads to significant spectral regrowth. /4-DQPSK has an instantaneous phase transition of 135 o , and a OQPSK system has only 90 o instantaneous phase transitions, making it the lowest spectral regrowth and the highest spectral efficiency of all three. Demodulation of OQPSK is much more difficult than with /4-DQPSK. /4-DQPSK with a linear amplifier is used to achieve the spectral efficiency for the cellular and PCS systems because it can be easily demodulated and it has an easier hardware implementation.
- Gaussian filtered minimal shift keying (GMSK ) is used in PCS systems and European GSM cellular systems. It is a variant of binary FSK. GMSK is a type of constant envelope FSK whereby the frequency modulation is a carefully handled phase modulation. The constant amplitude of the GMSK signal makes it suitable for use with high-efficiency amplifiers. The carrier signal has two frequency versions: high frequency and low frequency. The signals also carry a “sense”, which can be either positive or negative. When the carrier frequency is unchanged, the sense is positive. When the sense is negative, the frequency is upside down. The resulting waveform has a relatively smooth phase transition from one frequency to the next which reduces the spectral regrowth. In the QPSK family of modulations, phase transitions are discontinuous. Therefore, the power spectrum density of OQPSK has a wider spectral re-growth than that of GMSK.
- Modulation enables analog audio voice information to be efficiently transferred over the air interface. Modulation is also used in data transmission in modems. Each modulation method has strengths and weaknesses, as discussed in the module. An understanding of modulation methods will help wireless professionals support current radio systems and adapt to third generation (3G) technology. (For more information on third generation technology, refer the GWEC module AI-3GES .)