Takayuki Sotoyama

A study on beam tilt angle of base station antennas for Base Station Cooperation systems

Base Station Cooperation (BSC) is one of the key technologies for next generation cellular systems. The BSC can improve channel capacity at the cell-edge unlike conventional single cell transmission. In this paper, we will study about beam tilt angle of base station antennas for the BSC systems. In conventional systems, beam tilt angle is designed to maximize the power of its own cell while minimizing the power from adjacent cells to reduce Inter-Cell Interference (ICI). Unfortunately, performance at the cell-edge cannot be improved since Signal-to-Interference Ratio (SIR) is unchanged regardless of the beam tilt angle. However, since the problem of ICI is removed in the BSC system, beam tilt angle is required to be designed differently from the conventional way. In this paper, we will derive optimal beam tilt angle for the BSC systems based on these considerations. For that purpose, an objective function to maximize average channel capacity of cooperative cell is defined, and the optimal beam tilt angle is derived by solving the problem. By numerical analysis, it is shown that the optimal beam tilt angle for the BSC system is smaller than that of the single cell transmission. By using the optimal base station antennas, channel capacity of the BSC system is greatly improved, especially at the cell-edge.

Theoretical analysis of base station cooperation MIMO channel by using eigenvalue theory of wishart matrix

Multiple-Input Multiple-Output (MIMO) technology is indispensable for future wireless communication systems. Capacity theorem of the MIMO system has been developed by I. E. Telatar [1]. In the case of ideal MIMO system, channel capacity increases drastically due to multiplexing and diversity gains. When applying MIMO technology to cellular network, there is a problem of performance degradation at the cell-edge due to inter-cell interference (ICI). Base station cooperation (BSC) MIMO has been proposed in order to solve the cell-edge problem [2, 3]. In BSC-MIMO, different base stations cooperatively transmit signals to users at the same channel to control the ICI and to achieve cooperative multiplexing and diversity gains. In this paper, we will derive theoretical channel capacity of BSC-MIMO by using eigenvalue analysis of the cooperative MIMO channel.

AM-AM characteristics of low noise block converters

The low noise block converter is one of the key nonlinear devices for spectrum sharing between IMT-A and FSS in the 3GHz band. We measure the AM-AM characteristics of several off-the-shelf LNBs and compare the characteristics against those of conventional nonlinear device models. The comparison shows that the conventional models work well on the whole, however their accuracy has room for improvement. This paper also proposes novel AM-AM models based on Rapps and Salehs AM-AM model.

AM-PM characteristics of low noise block converters

Low noise block converter is one of the key nonlinear devices for spectrum sharing between IMT-A and FSS on 3GHz band based on the result of World Radiocommunication Conference 07. We measured AM-PM characteristics of several off-the-shelf Low Noise Block converters (LNBs) and compare the characteristics with conventional nonlinear device models. On measuring AM-PM characteristics of LNBs, LNBs have frequency conversion function and their local oscillator may not be stable in a long period for the measurement. Then we propose a method for AM-PM measurement which eases required frequency tuning accuracy of the measurement system. The results show that conventional models fit well on the whole, however there are rooms to be improved in their accuracy and usability. Then, in this paper, we also propose a novel AM-PM model based on the concept of the Rapps AM-AM model.

A study on applying nonlinear models for spectrum shape estimation

In this study, it is investigated the validity of applying nonlinear device model to estimate distorted spectrum shapes of the OFDM signals for spectrum sharing studies. We used an LNA and two LNBs, which are off-the-shelf devices and the spectrum shape of OFDM signal distorted by the nonlinear devices, are evaluated by the means of model-based estimation and indoor experiment. As the results, the spectrum shapes obtained by the two evaluation show good agreement, thus the model-based estimation is considered to be an effective method on the spectrum sharing studies.

Capacity achievable by spectrum sharing with adaptive transmit power control: Based on field measurements

We conduct field measurements to assess the effectiveness of the spectrum sharing method using adaptive transmit power control [2]. This investigation assumes a fixed satellite service as the victim and an IMT-Advanced system as the interferer. We evaluate the capacities achieved by adaptive transmit power control in a comparison with fixed-level transmit power control. The results show that adaptive transmit power control attains higher capacities, especially in severe sharing conditions such as the victim receiver operates near a interfering transmitter. We also discuss the impact of introducing interference criteria based on desired signal level at the victim receiver instead of the long-standing noise-based criteria.