CP-free OFDM for Future Wireless Communication Systems
Bo, Sun (2019)
Sähkötekniikan DI-ohjelma - Degree Programme in Electrical Engineering
Informaatioteknologian ja viestinnän tiedekunta - Faculty of Information Technology and Communication Sciences
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Orthogonal frequency division multiplexing (OFDM) is a multicarrier transmission scheme and has been widely used in communication systems due to the advantages like simpler equalization, flexible signal arrangement and so on. Also, wireless communication can provide high data speed in cellular systems, which makes internet connection via mobile and other user equipment the primary approach in daily life. The existing long term evolution (LTE) and fourth-generation (4G) systems use OFDM as a downlink scheme and provide very high peak data speed. However, the development of user equipment (UEs) operating with high data rates and the continuously growing number of users creates needs for more features and high capacities for wireless communications. For meeting the needs of the latest and future applications, fifth-generation (5G) technologies are under intensive research and development, while the first phase of 5G is actively deployed by the operators. Those technologies aim to reduce the latency, reduce power consumption, improve the throughput and support more users. In addition, the usage of 5G technologies can be categorized into three branches: enhanced mobile broadband (eMBB), ultra reliable low latency communication (URLLC) and massive machine type communication (mMTC). Because of the high spectral efficiency and other advantages of OFDM, it is still seen as a suitable modulation technique for 5G wireless communication applications. The most common OFDM scheme is CP-OFDM, which can cancel inter symbol interference (ISI) and ease the equalization process. However, the use of cyclic prefix (CP) introduces overheads which reduce the spectral efficiency and increase power consumption and latency. To meet the requirements of low latency and spectral efficiency in 5G New Radio (5G NR) applications, avoiding the usage of CP would be an interesting possibility if it could be done without severe drawbacks. This thesis provides one novel CP-free method called modified CP-free OFDM which can be used in 5G single input single output (SISO) communication. Comparing with existing CP-free designs, it does not require complex equalization algorithms or extra procedures for receivers’ detection process. Also, it can achieve similar performance with conventional CP-OFDM with minor modifications on the receiver side with respect to basic CP-OFDM receiver. In the experiment part of the thesis, the modified CP-free OFDM was tested under TDD SISO communication scenarios with line-of-sight (LOS) and non-line-of-sight (NLOS) channel models, considering both, normal and pre-equalized configurations. Simulations are used to evaluate different variants of the scheme regarding the three main performance aspects: bit-error-rate (BER), peak-to-average power ratio (PAPR), and power spectral density (PSD) of the transmitted signal.