Shinobu Nanba

A new IQ data compression scheme for front-haul link in Centralized RAN

In the Centralized-RAN (C-RAN), some baseband units (BBUs) are centralized in the same location, which is typically less than 20 km away from remote radio heads (RRHs) locally distributed. Each RRH is connected to BBUs via front-haul link by optical fibers which convey the digital IQ data by CPRI protocol. This paper proposes a new IQ data compression scheme with 1/2 compression ratio, which can be easily implemented by using both IQ bit width reduction and a common lossless audio compression scheme. Through performance evaluation, it is verified that the proposed method meets the requirements from an implementation point of view. By applying the proposed scheme to the front-haul link, the installation cost of optical fibers can be reduced by half.

MIMO-OFDM Throughput Performances on MIMO Antenna Configurations Using LTE-Based Testbed with 100 MHz Bandwidth

This paper presents throughput performance of 22 multiple-input multiple-output (MIMO)-orthogonal frequency division multiplexing (OFDM) using a testbed in line with the Long Term Evolution (LTE) numerology. Then, this paper clarifies the relation among the throughput, modulation schemes and antenna polarization. The field experiments clearly demonstrate that a measured throughput exceeding 200 Mbps can be obtained in a coverage area more than 50% and a throughput of 456.0 Mbps is achieved for a limited number of users even when a maximum Doppler frequency is higher than 125 Hz. In addition, it is found that the single polarization antenna configuration is suitable for 16QAM while the dual polarization antenna configuration is suitable for 64QAM. It is concluded from analysis of the eigenvalue ratio and signal to noise power ratio that the aforementioned trade-off is caused by sensitivity to the eigenvalue ratio depending on the modulation scheme and distribution of the eigenvalue ratio depending on the MIMO antenna configuration.

Cell association method for multiband heterogeneous networks

In traditional heterogeneous cellular networks (Het-Net), base stations (BS) basically associate users based on received signal power. However, in the case of multiband HetNet, in which macro BSs and smallcell BSs use different frequency bands, this conventional cell association method is not effective because there is no interference between macro BSs and smallcell BSs. Additionally, there are huge differences in coverage and available bandwidth. These differences cause inefficient association which causes the imbalance between achievable rate and traffic demand. In order to overcome this problem, we propose a novel cell association method based on the combinatorial optimization. The proposed method considers achievable rates, traffic demands and the number of users belonging to each BSs simultaneously. Numerical simulation results show that the proposed association method achieves system rate gain twice as high as the conventional one.

MIMO capacity estimation based on single and dual-polarization MIMO channel measurements

Multiple-Input Multiple-Output (MIMO) wireless system is one of the most promising technologies for providing a large transmission capacity without expanding the frequency band in cellular mobile systems. This paper focuses on 2×2 MIMO capacity estimation through propagation measurements at 2GHz band. The authors conducted the measurement campaign in two regions in Fujimino (Saitama prefecture) and Yokohama (Kanagawa prefecture) Japan. The former is a typical residential area and the latter is an urban area. The paper compares MIMO spatial multiplexing efficiency in terms of the eigenvalue ratio between the single and orthogonal dual-polarization antenna configurations. The eigenvalue ratio is characterized by two parameters. One is cross-polarization discrimination (XPD) and the other is the normalized received signal strength indicator (RSSI), which represents the difference from the RSSI when free space propagation is assumed. It is verified that the MIMO capacity of the dual-polarization can be estimated from just the SNR because the dependency of the eigenvalue ratio on the propagation environment is small. In contrast, that of the single-polarization needs to consider both the eigenvalue ratio and the SNR because the eigenvalue ratios change significantly due to the propagation environment.

Optimum traffic distribution algorithm for multiple-satellite systems under power constraints

In satellite communications systems, in addition to frequency resources, total satellite transmitting power, (i.e., satellite power) is a crucial radio resource, due to the limited capacity of onboard electric generators. To increase system capacity, satellite power constraints have to be taken into account, as well as the frequency constraints. As one approach to this issue, we propose a traffic distribution algorithm using linear programming, which maximizes the accommodated traffic in a multiple-satellite system under satellite power constraints. The algorithms are applicable to all types of satellite systems with multiple satellites (geosynchronous Earth orbit, medium Earth orbit, low Earth orbit, or a combination of any) to increase system capacity with respect to the total amount of traffic accommodated by the systems. Finally, this paper evaluates the performance of the proposed algorithm by distributing traffic demand on the earths surface to cells irradiated by spot-beams of a satellite and allocating frequency resources to the cells. By using the algorithm that permits power constraints, the ratio of accommodated traffic in a system was improved 12% compared with the case when no power constraints were used.

2 × 2 MIMO channel characteristics under received power imbalance through indoor propagation measurements

Multiple-Input Multiple-Output (MIMO) wireless systems are one of the most promising technologies for providing large transmission capacity without expanding the frequency band in cellular mobile systems. This paper discusses 2 × 2 MIMO channel characteristics with different antenna spacing for indoor line-of-site (LOS) environments through propagation measurements considering received power imbalance. The measurements were conducted for three different transmitter antenna spacing (one, two, and five wavelengths), which is set above 2.7 meters from the floor to a downward direction in the corridor. In this situation, the received power imbalance is caused by differences in the antenna pattern gain in the direction of a receiver point between two transmitter antennas. The received power imbalance increases as antenna spacing increases. Finally, we verified that channel capacity decreases when antenna spacing increases. Careful attention to the received power imbalance caused by transmitter antennas should be required when providing MIMO technologies in indoor environments using antennas with different positions in the ceiling.

Advanced load balancing in LTE/LTE-A cellular network

Due to the nature of mobile communications, traffic load is not equal throughout the cellular network. While much traffic is concentrated on a specific cell (i.e., called a hotspot cell), the traffic load on adjacent cells is often low. To efficiently accommodate the imbalance in traffic, load balancing (LB) between base stations has attracted attention in recent years. This paper proposes an advanced LB method for LTE/LTE-A cellular networks which utilizes exiting mechanisms such as handover parameter control (HPC) and cell coverage control (CCC). Since the two mechanisms have advantages and disadvantages, the proposed method selects the appropriate LB method according to the situation. The paper presents overall procedures and a detailed selection algorithm to maximize the offloading effect on the hotspot cell with avoidance of interference on surrounding cells. In addition, through a simulation, we confirmed that 1) the proposed method could achieve higher offloading effect than the native HPC by 20% of cell capacity and that 2) it could avoid interfering with the surrounding cells compared to CCC when the traffic load on the hotspot cell is relatively low.

Cell discovery in 5G HetNets using location-based cell selection

Future heterogeneous networks (HetNets) are going to consist of cellular macro cell base stations (macro-BSs) and various types of small cell base stations (SC-BS), each operating in microwave and/or millimeter bands. Cell discovery of mm-wave SC-BSs is a challenging issue because of the limited communication range. In this paper, a location-based cell selection scheme that takes into account the effects of inaccuracy of the location estimation is introduced. The proposed scheme selects the SC-BSs according to the probability that they can reach the UE. This probability is dynamically calculated and updated during the search process to assure that at any step the SC-BS with highest detection probability is selected and assigned. The simulation results show a 30% reduction in the latency of the proposed cell discovery compared to the conventional scheme. Furthermore, even more than 70% reduction in latency is possible if the proposed algorithm is extended to simultaneously select three SC-BSs with maximum joint probability of detection.

2GHz band MIMO propagation measurement of dual polarized antennas in a residential area

Multiple Input Multiple Output (MIMO) wireless system is the most promising technology for next generation cellular mobile systems due to its high-speed data transmission capability. This paper focuses on the MIMO propagation characteristics in 2GHz bands. From the viewpoint of cell coverage planning of OFDMA-MIMO systems, it is important to know the key parameters required to estimate MIMO capacity. This paper shows the difference in the propagation characteristics of dual and single polarized MIMO by conducting experimental measurements. It emerged that the ratios of the eigenvalues of the channel matrix, which represent the channel capacity, depend on cross-polarization discrimination (XPD). It was confirmed that there is a positive correlation between XPD and the eigenvalue ratio for dual polarization and a negative correlation for single polarization respectively.

An experimental study on switching threshold for adaptive modulation in broadband FWA systems

A number of wireless systems have recently adopted an adaptive modulation scheme to improve frequency efficiency. Unlike mobile systems, FWA (fixed wireless access) systems including IEEE 802.16 systems used for MAN (metropolitan area network) are allowed to use quasi-millimeter and millimeter bands, but those are vulnerable to rain attenuation. Adaptive modulation scheme is effective in those systems to improve the availability of wireless links. A 26 GHz-band broadband FWA system with a five-level (1024 QAM to BPSK) adaptive modulation scheme was proposed and developed by the authors for field experiments. This paper introduces field experimental results concerning switching thresholds for adaptive modulation controls using the developed FWA system. The numerical results reveal that conventional ideas, in which identical switching thresholds in terms of BER (bit error rate) apply to all modulation methods and switching thresholds maximizing Layer-2 throughput, can neither obtain the maximum TCP (transmission control protocol) throughput nor satisfy the upper limit of the UDP (user datagram protocol) packet loss rate