On the design of a flexible waveform and low ici symbol boundary alignment

Abstract

Cellular systems of fifth generation (5G) radio access technology is expected to support a wide variety of service requirements in different applications. High reliability, flexibility, spectral efficiency, and low power consumption are some of the service requirements. In order to support these requirements, symbol boundary alignment design and the waveform selection play important roles. The current symbol boundary alignment, along with orthogonal frequency division multiplexing (OFDM) waveform, has some disadvantages, such as non-flexible guard interval (e.g., hard coded cyclic pre x (CP)), and severe intercarrier interference (ICI) in high speed communications like in unmanned aerial vehicles (UAV). In the literature, multiple di erent waveforms are proposed to be used instead of the OFDM in 5G. Although they try to prevent from the drawbacks of OFDM, they create other problems such as high complexity. Among the available waveform candidates, zero tail (ZT) DFT-spread (s) OFDM has flexible GI and low power consumption along with a low complex transceiver. However, unlike its name, ZT DFT-s OFDM contains non-zero samples at its tail causing intersymbol interference (ISI) in multipath channels. Additionally, ZT DFT-s OFDM does not solve the ICI problem of high speed communications. Regarding to these issues,this thesis presents two separate solutions as follows. First, an improved version of ZT DFT-s OFDM, called DFT-s zero word (ZW) OFDM, is proposed to reduce the ISI power. In DFT-s ZW OFDM, we utilize redundant subcarriers concept, like in unique word (UW) OFDM, to nullify the tail of ZT DFT-s OFDM. The achieved waveform bene ts from high mitigation in the ISI power compared to ZT DFT-s OFDM. Although DFT-s ZW OFDM has a superior performance in multipath channels, it consumes slightly more power than ZT DFT-s OFDM. Therefore, a hybrid waveform, constructed by ZT DFT-s OFDM and DFT-s ZW OFDM, is designed which provides a high flexibility in order to control the symbol power and bit error rate (BER) performance of the system. The hybrid waveform utilizes the similarity between the transceivers of these two waveforms to deploy them in one resource block (RB) for a user. Thus, it can control the symbol power, reliability, and even peak to average power ration (PAPR) of the system by tuning the dedicated subcarriers to each waveform with respect to the channel characteristics. The second part of this thesis focuses on ICI reduction in current LTE numerologies and presents a novel symbol boundary alignment called \Low ICI Symbol boundary alignment numerology (LICIS)'. LICIS utilizes large subcarrierspacing to reduce the ICI power (e.g. around 5 dB ICI power reduction with subcarrier-spacing of 30 kHz in high speed UAV communications). Moreover, LICIS is based on the same reference clock as LTE which guarantees its compatibility with the current LTE numerology. Additionally, this approach places only one guard-interval (GI) at the end of a sequence of OFDM symbols and creates a sub-slot. It leads to less overhead and preserves the spectral e ciency. Furthermore, a pre-FFT multipath channel equalizer is considered for preventing the intersymbol interference (ISI) between the OFDM symbols occurring within the sub-slot. However, only one additional FFT and IFFT operations are required for the equalizer which creates an acceptable complexity increment compared to the complexity of other available solutions. Numerical and analytical evaluations show the superior performance of the proposed technique in terms of reliability and spectral effciency.Beşinci nesil (5G) radyo erişim teknolojisinin hücresel sistemleri çok çeşitli hizmet gereksinimlerini farklı uygulamaları desteklemesi bekleniyor. Yüksek güvenilirlik, esneklik, spektral verimlilik, ve düşük güç tüketimi bazı hizmet gereksinimleridir. Bu gereksinimleri desteklemek için sembol sınır hizalama tasarımı ve dalga formu seçimi önemli rol oynamaktadır. Bu konularda bu tez şu şekilde iki ayrı çözüm sunmaktadır: Birincisi, ZT DFT-s OFDM'nın geliştirilmiş bir versiyonu olan DFT-s ZW OFDM, ISI gücünü azaltmak için önerildi. DFT-s ZW OFDM'de, ZT DFT-s OFDM kuyruğunu geçersiz kılıyoruz. Bu tezin ikinci bölümünde, mevcut LTE numerolojilerinde ICI azalması üzerinde duruluyor ve "Düşük ICI Sembol Sınır Hizalama (LICIS)" numerolojisi olarak adlandırılan yeni bir sembol sınır hizalanması sunuluyor.