Author: | Zheng, Yijie |

Title: | Partial transmit sequence based reduction of OFDM peak-to-average power ratio using simulated annealing |

Degree: | M.Sc. |

Year: | 2010 |

Subject: | Hong Kong Polytechnic University -- Dissertations Orthogonal frequency division multiplexing. Signal processing -- Digital techniques Sequences (Mathematics) |

Department: | Department of Electronic and Information Engineering |

Pages: | vii, 84 leaves : ill. ; 31 cm. |

Abstract: | Orthogonal frequency-division multiplexing (OFDM) is a modulation for many current and future wireless communication systems. OFDM wins its reputation for saving bandwidth, avoiding frequency selective fading and improving transmission efficiency. This technique has already been applied to video broadcasting, wireless local area networks and many other regions. OFDM adopts multi-carrier system, which divides one original carrier into several independent subcarriers, and these subcarriers are ensured to be orthogonal with each other. As a proverb says: "One coin has two sides" . Though the technique obtains these wonderful advantages, one side-effect can not be neglected, which is the production of high peak power. People use peak-te-average power ratio (PAPR) to express the extent of this situation. Negative effect of signal power value comes out in the time domain after adding all the subcarriers together. Assume the number of subcarriers is N, in a most unfortunate situation, which means all subcarriers are in the same phases in an instant time, then the combined signal will have a highest signal power value which will be N times larger than the average. In this condition, a tough problem will happen in the part of power amplifier (PA). Generally, PA has a saturation limit, it works in a linear state normally when signal value is smaller than the limit, but if the signal value becomes larger than the limit, the operation of PA won't be well and signals will be distorted and finally the communication leads to a failure. This is the most important reason that we care about high PAPR value. PTS is an efficient approach for mitigating PAPR, it uses a certain number of scramble sequences to combine with the input data and try to change the waveform of the signals. Here scramble sequences can be phase factors, which are in the range of [0,2pi]. Optimal PTS uses a full (exhaustive) enumeration method to calculate all the possible results, then selects one group of the phase factors which can minimize PAPR as the ideal answer. Though PTS actually provides a useful solution, huge calculation task turns to be time-consuming, if the number of subcarriers increases largely, this problem is thus highlighted. In order to improve searching efficiency, a heuristic technique which is called "Simulated Annealing(SA)" is applied. SA is a scientific algorithm which combines PTS as a combinatorial problem. In the process of searching the smallest PAPR, this algorithm tries to compare a new produced solution with the current solution, monitor the changed state and select the better one, i.e. do a downhill movement to accept smaller value. What's more, it allows some uphill action when a predefined relation is met. With this step running on and on, simulation results show that this measure really saves large calculation tasks and approximates the PTS method, which eventually proves it is a great improvement to achieve good PAPR reduction. |

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