Turbo equalizer

In digital communications, a turbo equalizer is a type of receiver used to receive a message corrupted by a communication channel with intersymbol interference (ISI).

It approaches the performance of a maximum a posteriori (MAP) receiver via iterative message passing between a soft-in soft-out (SISO) equalizer and a SISO decoder.

[1] It is related to turbo codes in that a turbo equalizer may be considered a type of iterative decoder if the channel is viewed as a non-redundant convolutional code.

Turbo codes were invented by Claude Berrou in 1990–1991.

In 1993, turbo codes were introduced publicly via a paper listing authors Berrou, Glavieux, and Thitimajshima.

[2] In 1995 a novel extension of the turbo principle was applied to an equalizer by Douillard, Jézéquel, and Berrou.

[4] This discovery made turbo equalization computationally efficient enough to be applied to a wide range of applications.

[5] Before discussing turbo equalizers, it is necessary to understand the basic receiver in the context of a communication system.

Encoding adds redundancy by mapping the information bits

The main reason for doing this is to insulate the information bits from bursty noise.

At the receiver, the operations performed by the transmitter are reversed to recover

The down-converter mixes the signal back down to baseband.

The A/D converter then samples the analog signal, making it digital.

were transmitted through the digital baseband equivalent of the channel plus noise.

The equalizer attempts to unravel the ISI in the received signal to recover the transmitted symbols.

If the equalizer makes soft decisions, it outputs information relating to the probability of the bit being a 0 or a 1.

The block diagram of a communication system employing a turbo equalizer is shown below.

Due to the structure of the code, the decoder not only estimates the information bits

, are fed into the equalizer as a priori symbol probabilities.

The equalizer uses this a priori information as well as the input signal

to estimate extrinsic probability information about the transmitted symbols.

The turbo equalizer repeats this iterative process until a stopping criterion is reached.

In practical turbo equalization implementations, an additional issue need to be considered.

Firstly, in order to improve the reliability of the CSI, it is desirable to include the channel estimation block also into the turbo equalization loop, and parse soft or hard decision directed channel estimation within each turbo equalization iteration.

[6][7] Secondly, incorporating the presence of CSI uncertainty into the turbo equalizer design leads to a more robust approach with significant performance gains in practical scenarios.