Basic understanding of Linearity & Nonlinearity in Amplifiers, DPD, Harmonic and Intermodulation Distortion

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Linearity in Amplifiers

In practical communication systems, maintaining linearity in amplifiers is crucial to avoid distorting the transmitted or received signals. If the amplifier is perfectly linear, the output signal will always be a scaled version of the input signal, regardless of the input amplitude.

Example of Linearity:

• Application in Communication: Consider a base station in a 5G network where multiple signals are being transmitted at once (like voice, video, and data). If the power amplifier in the base station is linear, all signals can be amplified simultaneously without mixing or distorting the signals. Each waveform is preserved as-is, ensuring clear and accurate communication.
• Modulated Signals: When amplifying modulated signals such as QAM (Quadrature Amplitude Modulation), the amplifier must preserve the integrity of both amplitude and phase. A linear amplifier ensures that the constellation points of a QAM signal are transmitted accurately, preventing errors during demodulation.

Nonlinearity in Amplifiers

Nonlinear amplifiers exhibit behaviours where the output is no longer proportional to the input. This introduces various forms of distortion that can severely degrade the performance of communication systems.

Types of Nonlinearity:

• Clipping: clipping occurs when the amplifier cannot supply enough output power for large input signals, causing the signal peaks to be truncated. This leads to severe distortion and the generation of harmonics, which can interfere with other communication channels.

Real-world Scenario: In a satellite communication system, if the amplifier experiences clipping, the transmitted signal could interfere with adjacent frequency bands, causing crosstalk and data corruption.

• Compression: In a compressed signal, the gain decreases for higher input levels, resulting in a "squashed" output. This can happen in real amplifiers when they approach their power limits, and it leads to less effective transmission of strong signals.

Real-world Scenario: In mobile phone transmitters, compression can occur when the phone is far from a base station and tries to transmit at full power. The resulting compression distorts the transmitted signal, making it harder for the base station to decode it accurately.

Additional Types of Nonlinearity:

Harmonic Distortion

Nonlinearities can generate harmonics, which are multiples of the input signal frequency. These harmonics can interfere with the desired signal or with adjacent frequency channels.

Example: In an audio amplifier, harmonic distortion could make a musical note sound harsh and unnatural. In RF amplifiers, harmonics can interfere with nearby communication channels, leading to cross-channel interference.

• Input Signal for Harmonic Distortion (Top Chart - Blue): This is the original input signal, a simple sine wave.
• Output with Harmonic Distortion (Second Chart - Red): The output contains the fundamental frequency (same as the input) along with second and third harmonics. These additional harmonics cause distortion in the output waveform, altering its shape. Harmonics are multiples of the input frequency and can cause interference with other signals in the system.

Real-world Impact: Harmonic distortion can cause adjacent channel interference in communication systems, making it difficult to accurately recover signals.

Intermodulation Distortion (IMD)

When two or more signals pass through a nonlinear amplifier, they can interact to create new signals at frequencies that are sums and differences of the original frequencies. These new frequencies are called intermodulation products and can fall within the operating frequency band, causing interference.

Example: In a cell tower, multiple signals from different users may be transmitted through the same amplifier. Nonlinearity in the amplifier can cause intermodulation products, leading to interference and reduced call quality.

• Input Signals for Intermodulation Distortion (Third Chart - Green & Orange): Two sine waves at different frequencies are input into a nonlinear amplifier.
• Output with Intermodulation Distortion (Fourth Chart - Purple): The nonlinear amplifier generates new frequencies that are sums and differences of the input frequencies. These are called intermodulation products, which can fall within the operating bandwidth and cause interference with the original signals.

Real-world Impact: Intermodulation distortion is particularly problematic in wireless communication systems where multiple signals are transmitted through a single amplifier. The intermodulation products can interfere with nearby communication channels, reducing overall system performance.

Effects of Nonlinearity on Communication Systems

Nonlinear amplifiers can cause severe performance degradation in wireless communication systems:

• Spectral Regrowth: Nonlinear distortion causes the signal's spectrum to spread beyond its assigned bandwidth, a phenomenon called spectral regrowth. This can cause interference with neighbouring channels in the frequency spectrum.

Example: In a cellular network, spectral regrowth from a distorted signal could interfere with adjacent channels, reducing overall network capacity and performance.

• Error Vector Magnitude (EVM): EVM is a measure of the difference between the ideal signal and the actual transmitted signal. Nonlinearity increases the EVM, leading to higher bit error rates (BER) and degraded system performance.

Example: In 4G/5G systems, nonlinearity can cause a high EVM, leading to corrupted data transmission and necessitating retransmissions, which reduces data throughput.

Compensating for Nonlinearity: Digital Pre-Distortion (DPD)

To combat the effects of nonlinearity, techniques such as Digital Pre-Distortion (DPD) are often employed, especially in modern high-power amplifiers:

• How DPD Works: DPD involves applying an inverse distortion to the signal before it enters the amplifier. The goal is for the amplifier’s nonlinearity to cancel out this pre-distortion, resulting in a clean, linear output signal.

Example: In 4G and 5G communication systems, DPD is applied to signals before they are fed into high-power amplifiers. This ensures that even when the amplifier is operating near its saturation point, the output signal remains clean and linear.

Example to show all in one Trends

• Linearity in Amplifiers:

Blue: Input is a clean sine wave; Green: Output is an amplified but undistorted sine wave (linear amplification).

• Non-Linearity in Amplifiers (Clipping):

Blue: Input is a sine wave; Red: Output is clipped, with flat peaks showing distortion (non-linear amplification).

• Digital Pre-Distortion (DPD):

Orange: Pre-distorted input signal; Purple: Output is a clean sine wave after DPD cancels out distortion.

• Harmonic Distortion:

Blue: Input is a clean sine wave; Red: Output contains additional harmonics, resulting in distortion.

• Intermodulation Distortion (IMD):

Green and Orange: Two sine waves as inputs; Purple: Output contains intermodulation products (sum and difference frequencies causing distortion).

https://www.techedgewireless.com/post/basic-understanding-of-linearity-nonlinearity-in-amplifiers-dpd-harmonic-and-intermodulation-dis

references :

• Linearity in Amplifiers
• Non-Linearity in Amplifiers
• Digital Pre-Distortion (DPD)
• Harmonic Distortion
• Intermodulation Distortion (IMD)

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