Modulation and channel effects in digital communication

Abstract: This thesis investigates three possible methods to increase the performance of digital communication systems, with focus on wireless systems, by accounting for some of the channel effects that may occur. The modulation scheme plays an important role in the impact of different channel effects on system performance and this work considers both a single-carrier system and orthogonal frequency-division multiplexing (OFDM). The first work investigates effects of the channel estimation errors resulting from blind channel estimation. The performance of a communication system in terms of throughput may be increased by using blind channel estimation instead of non-blind. This will allow more useful information to be sent through the system, but the channel estimation will be less reliable. The effects of the channel estimation errors on the performance of separation in a multiple-input multiple-output (MIMO) system are investigated for a specific blind channel estimation method. This work quantifies the expected performance reduction, in terms of cross-channel power, due to channel estimation errors. The second and third work consider the OFDM framework, which enables simple equalization and has been adopted in several standards. However, OFDM is sensitive to frequency-selective fading and introduces a large peak-to-average power ratio (PAPR) of the transmitted signal. These problems can be alleviated by pre-multiplying the OFDM block by a spreading matrix, e.g. the Walsh-Hadamard matrix. It is shown that spreading by the Walsh-Hadamard matrix reduces the PAPR of the transmitted signal and increases the frequency diversity. Surprisingly, with a joint implementation of the spreading and the OFDM modulation, the spread OFDM system requires less computations than the conventional OFDM system. An alternative to PAPR reduction is to allow clipping of the signal in the transmitter. Clipping will however introduce losses due to the clipping distortion of the signal. In the thesis, receiver methods to mitigate such clipping losses are investigated. It is shown that for an OFDM system with low-density parity-check (LDPC) coding, the cost of completely ignoring the clipping effects in the receiver is minimal.

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