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In a soft decision Viterbi decoder, the log-likelihood ratio (LLR) metric is used to improve the performance of the error correction. However, this cannot be directly applied in frequency shift keying (FSK) wireless transmission system which uses a frequency discriminator for FSK de-modulation -an inherently non-linear circuit. The LLR for a linear system cannot be used for the FSK wireless transmission system. It is necessary to calculate LLR which considers the non-linear characteristic of the frequency discriminator. In this paper, we derive the modified LLR which considers the non-linear characteristic of the frequency discriminator. Simulation shows the effectiveness of the derived LLR for 4-ary FSK wireless transmission system.

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Direct conversion receiver is a well-known receiver architecture that enables small, low power and cheap radio. However, I/Q imbalance caused by non-orthogonality between in-phase component and quadrature-phase component from the imperfections of quadrature demodulator is a major problem in this architecture. In addition, carrier frequency offset (CFO) occurs as well. In this paper, we present a register transfer level (RTL) design of joint CFO and I/Q imbalance compensator. We verify that our system can compensate CFO from -500 Hz to 500 Hz when sampling rate is 9.6 kbps under the effect of I/Q imbalance.

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