5G challenges : Waveform design and D2D communication

Abstract

Beşinci nesil (5G) iletişim sistemlerinin, farklı uygulamalarla ilgili çok çeşitli gereksinimleri karşılaması beklenmektedir. Esneklik, düşük gecikme, düşük güç tüketimi en önemli olanlardır. OFDM'nin dezavantajlarını ortadan kaldıran ortogonal frekans bölmeli çoğullama (OFDM) dalga formundan başka bir dalga formu kullanmak, gereksinimlerin karşılanmasına yardımcı olabilir.Cihazdan Aygıta (D2D) iletişim, gelecek nesil iletişim sistemlerinde önemli bir rol oynar. D2D iletişimini etkinleştirmek, gecikmeyi azaltır ve hücresel ağlarda bir hücrenin kapsamını genişletir. Ek olarak, D2D'nin hücresel kullanıcıları desteklemesi, yüksek spektral verimden faydalanır. Ancak, hücresel haberleşmelere müdahale oluşturur.Kablosuz ağlarda, doğru ve düşük karmaşıklık yerelleştirme yöntemlerine olan ihtiyaç artmaktadır. Her ne kadar sinyallerin 'varış zamanı (TDOA) ve varış açısı (AOA)' na dayanan birçok konumlandırma yöntemi önerilmişse de, bu yöntemler çoklu baz istasyonları (BS) ve yüksek karmaşıklık gerektiren hesaplamalar gerektirir.

Fifth generation (5G) of communication systems is expected to satisfy a wide variety of requirements with regards to different applications. Flexibility, low latency, low power consumption are the most important ones. Using another waveform other than orthogonal frequency division multiplexing (OFDM) waveform which avoids the disadvantages of OFDM can help to satisfy the requirements. In the literature, there are multiple research works about proposing new waveform. One of the most important thing which needs to be satisfied by the 5G waveform is the flexibility. The first part of the thesis is focused on the proposing two flexible waveform. The first proposed waveform is DFT-s- ZW OFDM which has similar transceiver to ZT DFT-s-OFDM. In DFT-s-ZW OFDM instead on zeros in ZT DFT-s-OFDM in time domain we create those zeros at the tail of output of IFFT. With this approach the flexibility of ZT DFT-s-ODM in terms of guard interval length is kept but the problem of ISI is solved.Device-to-Device (D2D) communication plays an important role in the next generation of communication systems. Enabling D2D communication decreases latency and expands the coverage of a cell in cellular networks. Additionally, D2D underlaying cellular users benefits from high spectral efficiency. However, it creates interference to cellular communications. In the second part of this thesis, we propose a genetic algorithm-based method to minimize the interference and maximize the spectral efficiency. One of the advantages of genetic algorithm is that it escapes from local maximums and evolves toward global maximum by searching different parts of search space simultaneously. Since D2D underlay cellular network degrades the signal to interference plus noise ratio (SINR), a minimum SINR is considered for cellular users. Numerical evaluations demonstrate the superior performance of the proposed technique in terms of spectral efficiency and interference mitigation.In wireless networks, the need for accurate and low complexity localization methods are growing. Although many positioning methods based on signals' time difference of arrival (TDOA) and angle of arrival (AOA) have been proposed, these methods require multiple base stations (BS) and calculations with high complexity. Furthermore, the distance between a target and the BSs are usually larger than the distance between different BSs, which causes geometric dilution of precision (GDP) problem. In third part of this thesis, to circumvent these issues, we propose a novel and linear method for positioning by only one BS. Our method uses both AOA and TDOA of incoming signals and called positioning using one BS (PuOB). In this method, we take measurements from one BS at different time instances instead of taking measurements from multiple BSs simultaneously. The ability to estimate the mobile transmitter position accurately by using one BS is the highlighted advantage of the PuOB over the conventional methods. The positioning accuracy of PuOB for different BS numbers are presented. According to simulation results, PuOB outperforms TDOA and AOA methods using three and two BSs, respectively.