Investigation of the dynamic properties of multicarrier modulation schemes

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Dr. Kollár Zsolt
Department of Broadband Infocommunications and Electromagnetic Theory

Orthogonal frequency division multiplexing (OFDM) is a commonly used modulation scheme for high speed digital communication. This modulation scheme is applied in many high rate wired and wireless communication systems such as Digital Video Broadcasting (DVB), Digital Audio Broadcasting (DAB), Wireless Local Area Network (WLAN) and in Asymmetric digital Subscriber Line (ADSL) as well.

The high peak-to-average power ratio (PAPR) of the modulated signal is a major drawback, which may negatively affect the overall system performance if amplifiers with limited linear range are applied. The distortion caused by these high peaks, which bring the amplifiers to its nonlinear range, may increase the bit error rate at the reciever side and the amount of the transmitted power to other channels.

To eliminate this problem, one can apply PAPR reduction algorithms. Multiple methods were developed for this task, but only two of them are part of the DVB-T2 standard. These are the tone reservation (TR) and the active constellation extension (ACE) techniques. In this paper I investigate two well-known TR realisation, namely the gradient based and the kernel based method, and a novel, third variant, the enlipping based tone reservation is also introduced. The maximal amount of PAPR reduction can be determined with the aid of linear programming, but due to its complexity in practical life rather the above mentioned, more heuristic methods were adopted.

These methods were compared through various aspects. I investigated the efficiency as well as the degree of PAPR reduction in correspondence of the number of iterations of these techniques, compared with the results of the LP simulation. The convergence of these TR realisations were also investigated. Finding the optimal value of the clipping ratio (CR) was also a significant task during the simulation, because this parameter occurs in all three methods, moreover the amount of PAPR reduction is highly dependant from its value. I introduced a new method based on an adaptive clipping trajectory as well, where defining the CR for each iteration results in better PAPR values, than using a constant clipping factor.

The performance of the three previously described techniques were compared through simulations in MATLAB, where due to the modularity of the system I created, the new techniques could be evaluated. After the comparison of the three techniques, I proposed implementation parameters, which helps to achieve the best PAPR reduction performance. Finally, I also introduce a measurement enviroment, where the results of the simulations using a software defined radio (SDR) were evaluated in real life scenario as well.


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