Polar modulation is analogous to
quadrature modulation
Quadrature may refer to:
In signal processing:
*Quadrature amplitude modulation (QAM), a modulation method of using both an (in-phase) carrier wave and a 'quadrature' carrier wave that is 90° out of phase with the main, or in-phase, carrier
*Qua ...
in the same way that
polar coordinates
In mathematics, the polar coordinate system is a two-dimensional coordinate system in which each point on a plane is determined by a distance from a reference point and an angle from a reference direction. The reference point (analogous to the or ...
are analogous to
Cartesian coordinates
A Cartesian coordinate system (, ) in a plane is a coordinate system that specifies each point uniquely by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular oriented lines, measured in t ...
. Quadrature modulation makes use of Cartesian coordinates, ''x'' and ''y''. When considering quadrature modulation, the ''x'' axis is called the ''I'' (in-phase) axis, and the ''y'' axis is called the ''Q'' (quadrature) axis. Polar modulation makes use of polar coordinates, ''r'' (amplitude) and ''Θ'' (phase).
The quadrature modulator approach to
digital radio
Digital radio is the use of digital technology to transmit or receive across the radio spectrum. Digital transmission by radio waves includes digital broadcasting, and especially digital audio radio services.
Types
In digital broadcasting syst ...
transmission requires a
linear
Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
RF power amplifier
A radio-frequency power amplifier (RF power amplifier) is a type of electronic amplifier that converts a low-power radio-frequency signal (electrical engineering), signal into a higher-power signal. Typically, RF power amplifiers drive the anten ...
which creates a design conflict between improving power efficiency or maintaining amplifier linearity. Compromising linearity causes degraded signal quality, usually by adjacent channel degradation, which can be a fundamental factor in limiting network performance and capacity. Additional problems with linear RF power amplifiers, including device parametric restrictions, temperature instability, power control accuracy, wideband noise and production yields are also common. On the other hand, compromising power efficiency increases power consumption (which reduces battery life in handheld devices) and generates more heat.
The issue of linearity in a power amplifier can theoretically be mitigated by requiring that the input signal of the power amplifier be "
constant envelope
Constant envelope is achieved when a sinusoidal waveform reaches equilibrium in a specific system. This happens when negative feedback in a control system, such as in radio automatic gain control or when an amplifier reaches steady state. Steady ...
", i.e. contain no amplitude variations. In a polar modulation system, the power amplifier input signal may vary only in phase. Amplitude modulation is then accomplished by directly controlling the gain of the power amplifier through changing or modulating its supply voltage. Thus a polar modulation system allows the use of highly
non-linear power amplifier architectures such as
Class E and
Class F.
In order to create the polar signal, the phase transfer of the amplifier must be known over at least a 17 dB amplitude range. As the phase transitions from one to another, there will be an amplitude perturbation that can be calculated during the transition as,
:
where n is the number of samples of I and Q and should be sufficiently large to allow an accurate tracing of the signal. One hundred samples per symbol would be about the lowest number that is workable.
Now that the amplitude change of the signal is known, the phase error introduced by the amplifier at each amplitude change can be used to pre-distort the signal. One simply subtracts the phase error at each amplitude from the modulating I and Q signals.
History
Polar modulation was originally developed by
Thomas Edison
Thomas Alva Edison (February 11, 1847October 18, 1931) was an American inventor and businessman. He developed many devices in fields such as electric power generation, mass communication, sound recording, and motion pictures. These inventio ...
in his 1874
quadruplex telegraph
The Quadruplex telegraph is a type of electrical telegraph which allows a total of four separate signals to be transmitted and received on a single wire at the same time (two signals in each direction). Quadruplex telegraphy thus implements a fo ...
– this allowed 4 signals to be sent along a pair of lines, 2 in each direction. Sending a signal in each direction had already been accomplished earlier, and Edison found that by combining amplitude and phase modulation (i.e., by polar modulation), he could double this to 4 signals – hence, quadruplex.
See also
*
Angle modulation
Angle modulation is a class of carrier modulation that is used in telecommunications transmission systems. The class comprises frequency modulation (FM) and phase modulation (PM), and is based on altering the frequency or the phase, respectively, ...
*
Phase modulation
Phase modulation (PM) is a modulation pattern for conditioning communication signals for transmission. It encodes a message signal as variations in the instantaneous phase of a carrier wave. Phase modulation is one of the two principal forms ...
*
Phase-shift keying
Phase-shift keying (PSK) is a digital modulation process which conveys data by changing (modulating) the phase of a constant frequency reference signal (the carrier wave). The modulation is accomplished by varying the sine and cosine inputs at a ...
(PSK)
External links
Fundamentals of Digital Quadrature ModulationMatsushita (formerly Tropian)RF Micro DevicesSkyworksAnadigics, IncPolar Modulation Ups Efficiency in Mobile PA Designs - CommsDesign
Telecommunication theory
Radio modulation modes
Physical layer protocols
Data transmission