The paper proposes a new desicion method of a Log Likelihood Ratio (LLR) under cochannel interference. A Maximum A posteriori Propability (MAP) decoding, which is mainly used to turbo codes, requires the LLR derived from the amplitude distribution of the received signal. Since, under cochannel interference, the received signal is composed of the interference signal in addition to the desired signal and thermal noise, the amplitude distribution of the received signal doesn't necessarily follow the Gaussian distribution. In the proposed scheme, the amplitude distribution of the received signal is derived by taking the amplitude distribution of the interference signal into account. By means of the computer simulations, the BER performance is evaluated, comparing with that of the scheme on the assumption that the amplitude distribution of the interference signal is characterized by the Gaussian distribution.
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