2. Using I/Q modulation to convey information.
2.1 Transmitting information
To transmit a signal over the air, there are three main steps:
1. A pure carrier is generated at the transmitter.
2. The carrier is modulated with the information to be transmitted. Any reliably detectable change in signal characteristics can carry information.
3. At the receiver the signal modifications or changes are detected and demodulated.
Figure 3. Transmitting Information... (Analog or Digital)
2.2 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.
Figure 4. Signal Characteristics to Modify
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 ommunications 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 (In-phase) and Q (Quadrature). These components are orthogonal and do not interfere with each other.
2.3 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).
Figure 5. Polar Display - Magnitude and Phase Represented Together
This figure shows different forms of modulation in polar form. Magnitude is represented as the distance from the center and phase is represented as the angle.
Figure 6. Signal Changes or Modifications
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.
2.5 I/Q formats
In digital communications, modulation is often expressed in terms of I and Q. This is a rectangular representation of the polar diagram. On a polar diagram, the I axis lies on the zero degree phase reference, and the Q axis is rotated by 90 degrees. The signal vector’s projection onto the I axis is its “I” component and the projection onto the Q axis is its “Q” component.
Figure 7. “I-Q” Format
2.6 I and Q in a radio transmitter
I/Q diagrams are particularly useful because they mirror the way most digital communications signals are created using an I/Q modulator. In the transmitter, I and Q signals are mixed with the same local oscillator (LO).
A 90 degree phase shifter is placed in one of the LO paths. Signals that are separated by 90 degrees are also known as being orthogonal to each other or in quadrature. Signals that are in quadrature do not interfere with each other. They are two independent components of the signal. When recombined, they are summed to a composite output signal. There are two independent signals in I and Q that can be sent and received with simple circuits. This simplifies the design of digital radios. The main advantage of I/Q modulation is the symmetric ease of combining independent signal components into a single composite signal and later splitting such a composite signal into its independent component parts.
Figure 8. I and Q in a Practical Radio Transmitter
2.7 I and Q in a radio receiver
The composite signal with magnitude and phase (or I and Q) information arrives at the receiver input. The input signal is mixed with the local oscillator signal at the carrier frequency in two forms. One is at an arbitrary zero phase. The other has a 90 degree phase shift. The composite input signal (in terms of magnitude and phase) is thus broken into an in-phase, I, and a quadrature, Q, component. These two components of the signal are independent and orthogonal. One can be changed without affecting the other.
Normally, information cannot be plotted in a polar format and reinterpreted as rectangular values without doing a polar-to-rectangular conversion.
This conversion is exactly what is done by the in-phase and quadrature mixing processes in a digital radio. A local oscillator, phase shifter, and two mixers can perform the conversion accurately and efficiently.
Figure 9. I and Q in a Radio Receiver
2.8 Why use 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 principal, 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.
Lihat juga
DIGITAL MODULATION; INTRODUCTION
DIGITAL MODULATION ; WHY DIGITAL MODULATION
DIGITAL MODULATION ; USING I/Q MODULATION TO CONVEY INFORMATION
DIGITAL MODULATION ; DIGITAL MODULATION TYPES AND RELATIVE EFFICIENCIES
DIGITAL MODULATION ; FILTERING
DIGITAL MODULATION ; DIFFERENT WAYS OF LOOKING AT A DIGITAL MODULATED SIGNAL TIME AND FREQUENCT DOMAIN VIEW
DIGITAL MODULATION ; SHARING THE CHANNEL
DIGITAL MODULATION ; HOW DIGITAL TRANSMITTER AND RECEIVER WORK
DIGITAL MODULATION ; MEASUREMENT ON DIGITAL RF COMMINICATION SYSTEMS
DIGITAL MODULATION ; SUMMARY
DIGITAL MODULATION ; OVERVIEW OF COMMUNICATION SYSTEM
DIGITAL MODULATION ; GLOSSARY OF TERM
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