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In digital systems such as fiber optical communications the ratio between probability of errors of type 1 → 0 and 0 → 1 can be large. Practically, one can assume that only one type of errors can occur. These errors are called asymmetric. Unidirectional errors differ from asymmetric type of errors, here both 1 → 0 and 0 → 1 type of errors are possible, but in any submitted codeword all the errors are of the same type.We consider q-ary unidirectional channels with feedback and give bounds for the capacity error function. It turns out that the bounds depend on the parity of the alphabet q. Furthermore we show that for feedback the capacity error function for the binary asymmetric channel is different from the symmetric channel. This is in contrast to the behavior of that function without feedback.

Cryptographic algorithms rely on the secrecy of their corresponding keys. On embedded systems with standard CMOS chips, where secure permanent memory such as flash is not available as a key storage, the secret key can be derived from Physical Unclonable Functions (PUFs) that make use of minuscule manufacturing variations of, for instance, SRAM cells. Since PUFs are affected by environmental changes, the reliable reproduction of the PUF key requires error correction. For silicon PUFs with binary output, errors occur in the form of bitflips within the PUFs response. Modelling the channel as a Binary Symmetric Channel (BSC) with fixed crossover probability p is only a first-order approximation of the real behavior of the PUF response. We propose a more realistic channel model, refered to as the Varying Binary Symmetric Channel (VBSC), which takes into account that the reliability of different PUF response bits may not be equal. We investigate its channel capacity for various scenarios which differ in the channel state information (CSI) present at encoder and decoder. We compare the capacity results for the VBSC for the different CSI cases with reference to the distribution of the bitflip probability according a work by Maes et al.