Polar modulation
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Polar modulation is analogous to quadrature modulation in the same way that polar coordinates are analogous to Cartesian coordinates. 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 transmission requires a linear RF power amplifier 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 include 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", i.e. contain no amplitude variations. In a polar modulation system, the power amplifier input signal varies only in phase. Amplitude modulation is then accomplished by directly controlling the gain of the power amplifier. Thus a polar modulation system allows the use of highly non-linear power amplifier architectures such as Class E and Class F.
[edit] External links
- Fundamentals of Digital Quadrature Modulation
- Matsushita (formerly Tropian)
- Sequoia Communications
- RF Micro Devices
- Skyworks
