| The fifth generation (5G) wireless communication systems are expected to be deployed beyond 2020, aiming to face the various scenarios and demands in future wireless communication, provide overall network coverage and high-speed data transmission for the whole society. However, with the limited spectrum, the future 5G wireless communication systems are facing great challenges in order to meet the demand for ex-plosive growth of wireless communication data rate and higher quality of network coverage. On one hand, promising technologies should be implemented to increase the spectrum efficiency. For example, heteroge-neous networks (HetNet) with dense deployed low power nodes such as pico cells, relay, femto cells, along with traditional macro cells can improve the system capacity and spectrum efficiency. However, the Hetnet with dense deployment of small cells also brings the challenge of more severe and more complex inter-cell interference (ICI). On the other hand, new spectrum such as visible light are expected to be more explored. In recent years, indoor visible light communications (VLC) have been widely researched all over the world. In this thesis, efficient modulation technologies in VLC as well as the inter-cell interference suppression for HetNet have been studied.Firstly, the thesis gives the overview of the VLC system, including the system structure, transmitting and receiving devices, as well as the visible light channel and modulation methods in VLC. Two popular OFDM schemes often used in VLC systems, DCO-OFDM and ACO-OFDM are also discussed. Bit error rate (BER) of these two OFDM schemes are simulated and compared in AWGN channel.Additionally, the conventional radio frequency (RF) OFDM systems are briefly investigated for its high peak-to-average-power ratio (PAPR) problem. Statistic models of the time domain OFDM signal and PAPR are built and confirmed by simulation of PAPR CCDF curves. The thesis has also briefly introduced the PAPR reduction technologies for RF OFDM systems. Similar with RF OFDM, DCO-OFDM and ACO-OFDM also have the problem of high PAPR. So the PAPR of both optical OFDM schemes are discussed and simulated. Moreover, the thesis elaborates the PAPR reduction schemes applied in VLC OFDM systems including Clipping, PTS, and SLM.In addition, we propose a PAPR reduction technique for DCO-OFDM system via Semi-Definite Re-laxation (SDR). We solve the tone injection problem by semi-deifinite relaxation and convex optimization. Simulation results verify that a significant PAPR reduction as large as 5 dB has been achieved, which con-tributes to a noticeable BER performance gain considering the limited linear range in the VLC transmitter. Furthermore, we have provided an optimized scaling and clipping scheme specially for DCO-OFDM systems. Concerning the limited linear range of the VLC transmitter, we investigated an optimized scaling factor for DCO-OFDM time domain signals allowing certain level of clipping noise. Simulation results demonstrate that the scaling and clipping schemes can achieve considerable PAPR reduction and BER improvement with very simple calculation.Finally, the thesis mainly focuses on the inter-cell interference in LTE-Advanced HetNet. Small Cell Networks (SCN) can increase the system capacity and improve the cell coverage. However, the dense de-ployment of small cells inevitably brings the problem of inter-cell interference. HetNet with various kinds of small cells is introduced and several inter-cell interference suppression technologies have been described briefly. Considering the synchronization problem in SCN, a simple but efficient prefix based approach is pre-sented for the SCN to alleviate the performance degradation due to synchronization errors. Simulation results show that the proposed method achieves a noticeable performance gain in terms of both outage probability and average data rate. |
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