precoding

In recent years, the need for reliable transmission of digital data is increasing rapidly And extend high-speed data networks and wireless networks in public and private business were caused It would be considered.With OFDM multi-carrier modulation and consists of several sub-carrier is modulated have to be perpendicular to each other. The disadvantages of multi-carrier modulation, such as OFDM, greater than Peak-to-Average Power Ratio PAPR at the transmitter output signal is PAPR large signal, causes the amplifier with a wider range of linear functions necessary. That is effective and if the amplifier can be used with low linear operation area, causing distortion in band and out of band signal will be sent radiation. In this paper, both Precoding techniques and Companding μ-law used to reduce PAPR Further studies using different methods and number of sub-carrier modulation is different. A significant decrease in the values of PAPP using these techniques have been. And efficacy compared with each other.

Massive MIMO system is one of the advanced and attractive technologies for fifth generation of wireless networks. In such systems, base stations employ a very large number of antennas (Hundreds or even thousands of antennas). As a result, in these systems, capacity, spectral efficiency and energy efficiency increases in comparison with conventional MIMO systems. In the channel estimation, interference, which is produced by reuse of the same pilot sequences by different users of neighboring cells, is the most important factor in the performance limits. This phenomenon is referred to as pilot contamination. In this paper, pilot contamination is described and then the research works presented by the researchers on this topic has summarized. We categorize the different proposed mitigation techniques for pilot contamination using the following taxonomy: protocol-based methods, precoding methods, AOA-based methods, blind methods and smart pilot assignment methods. Finally, we propose a method based on smart pilot allocation, to reduce pilot contamination. Simulation results show that both uplink rate of user’s increases based on the proposed method and the accuracy of channel estimation improved greatly than exciting methods. Then we propose a method based on pilot precoding and protocol based allocation to reducing pilot contamination effects. In addition, we study a range of challenges and future research topics.