Resumen de Performance and Spectral Efficiency of OFDM systems on urban radio channels
The increasing in the demand of mobile data, generally Internet access, at rates near 45% per year, but not exclusively, and the decreasing of the gigabyte price are pushing the telecommunication industry to improve the spectral efficiency of the networks. The work on this PhD Thesis is involved in this path. Many simulations were done to know the details of the performance and the spectral efficiency of fourth generation (4G) systems. These simulations have been done using both 4G standards, Worldwide Interoperability for Microwave Access (WIMAX) and Long Term Evolution (LTE), with some modifications exploring the alternatives that should increase the spectral efficiency at reasonable distances and with an acceptable bit error rate (BER). Multiple input multiple output (MIMO) and Code Division Multiple Access (CDMA) schemes, added to the basic standard system, have been tested as possible solutions with its advantages and disadvantages. Two techniques of channel estimation and pre-equalization are proposed to use, along this PhD Thesis, to reduce the BER reaching a reasonable value at low signal to noise ratio (SNR), chosen after evaluation of some candidates. Also many urban channels models were used in the simulations because they are present where the high demand is required: Modified Stanford University Interim channels as SUI4 or SUI6 and actual radio channels indoor. The additive white Gaussian noise (AWGN) channel is also used for standard comparisons. Another topic to be improved is related to radio wave propagation model. A high density building urban environment is modeled based in COST 231 WI model, upgrading in the following items: the loss dependence on the angle between the propagation direction and the street axis"; takes into account the street crossings when predicting the signal attenuation along the block;" and the dependence of propagation loss with the terrain height at the mobile and at the base station locations. This model gives the loss as a function of the distance, so it is useful for radio network planning purposes but not for system technology performance as SUI channels models. As the presence of trees also affects the radio wave propagation, many vegetal species are modeled with a measurements-derived attenuation factor, which depends on their geometrical arrangement. Thus, the use of vegetal barriers to diminish interference in Wi-Fi networks inside buildings or to isolate networks from outdoors is investigated, as a way to take advantage of the attenuation induced by such obstacles. Due to the scarcity of radio spectrum, the frequency reuse within a network deployment has been analyzed with and without inter-cell interference coordination (ICIC), using the channel sizes proposed by service providers and the minimum SNR needed for QPSK, 16 and 64 QAM modulations. A relation between the distances of two mobiles to their serving base stations when the frequency is reused is presented. The total capacity of a sector with multi-modulation scheme is derived. The efforts were put mainly at the 64 QAM, due to its high throughput capacity. As a collateral product, a simulation platform was developed in Simulink-Matlab© to make more tests of this type of systems, making easy the implementation of new ideas for the physical layer of orthogonal frequency division multiplexing (OFDM) systems. It brings the possibility to compare performances, basically BER vs. SNR, between these systems, over these or other channels.
