Excerpt

From the previous chapters, it is known that any linear analog signal is accompanied by an interfering signal. Interference is composed of noise, which is a random process, and distortions. Distortions result from the nonlinear (NL) response of a network and can be divided into two categories: distortions created by the electronic circuit and those created by the optics. Nonlinear distortions (NLDs) may result in undesired tones, which may interfere with the desired signal detection process, thus reducing the system's dynamic range. NLD is a stochastic random process as well. There are two definitions for that purpose, signal-to-noise ratio (SNR) or carrier-to-noise ratio (CNR), in analog transport that measure the ratios of the signal-to-noise level, and signal-to-noise and distortion (SINAD). In digital modulation schemes, CNR is replaced by Eb∕N0, which is the bit energy per noise. However, in both cases, noise and distortions may reduce the signal quality of service by affecting the bit error rate (BER) in digital transport, CNR, and dynamic range. To overcome this problem, linearization techniques are used. Linearization can be done in the optics and in the electronic circuit that drives the optics.

In wideband systems such as community access television (CATV) transport, the goals are to reduce both second-order distortions and third-order distortions, since due to the enormous bandwidth (BW) of 50 to 870 MHz, both composite second order (CSO) and composite triple beat (CTB) distortions fall within the channel's receiver band. Thus, these linearizers should be wideband. In narrowband systems such as cellular and radio over fiber (ROF), second-order distortions are out of band and are filtered by the transmitter and receiver. However, third-order distortions are in band and pass through the system's intermediate frequency (IF) as a legal signal, or in direct conversion, zero IF creates dc offsets due to IP2 and IP3. Therefore, when designing a narrow-band system, high IP3 is a major consideration, while in wideband systems, both IP2 and IP3 are important. In other words, linearization circuits improve the system's IP2 and IP3, thus the system's dynamic range is improved.

The transmitter's linearization circuits are divided into two categories of predistortion circuits.

© 2008 Society of Photo-Optical Instrumentation Engineers

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