Abstract—We investigate turbo equalization, or iterative equalization and decoding, as a receiver technology for systems where data is protected by an error-correcting code, shuffled by an interleaver, and mapped onto a signal constellation for transmission over a frequency-selective channel with unknown time-varying channel impulse response. The focus is the concept of soft iterative channel estimation, which is to improve the channel estimate over the iterations by using soft information fed back from the decoder from the previous iteration to generate “extended training sequences ” between the actual transmitted training sequences. Index Terms—Channel estimation, high-frequency communications, iterative channel estimation, turbo equalization.

Abstract. In 2005, Germany introduced a new Amateur Radio Ordinance prohibiting encrypted radio traffic at home. Crypto-bans can be circumvented using steganography. However, present steganographic methods are not eligible because the embedded message will not survive the usual distortions in a radio transmission. Robust as current watermarking methods are, they leave clearly detectable traces and have a smaller capacity. This paper presents measures that improve the robustness of steganographic communication with respect to non-intentional, random channel errors and validates their effectiveness by simulation. For the scenario of a radio communication, we determine practicable parameters for least detectability under six different short wave conditions. The resulting method embeds messages with a length of up to 118 bytes in a narrow-band Slow Scan Television connection in Martin-M1 mode. 1 Requirements for Robust Steganography Steganography is the art and science of invisible communication. Its aim is the transmission of information embedded invisibly into carrier data. Secure watermarking methods embed short messages protected against modifying attackers (robustness, watermarking security) while the existence of steganographically embedded information cannot be proven by a third party (indiscernibility, steganographic security). The existence of steganographic methods is one of the main arguments against a crypto-ban, since steganography facilitates the confidential exchange of information like cryptography, but goes unnoticed and consequently cannot be effectively persecuted. Nevertheless, Germany expanded the regulation that international amateur communications should be “in plain language ” [1] to domestic ones in its Amateur Radio Ordinance in 1998. The new German Amateur Radio Ordinance from 2005 [2] explicitly prohibits encrypted amateur communications