Kei Sakaguchi

Millimeter-Wave Evolution for 5G Cellular Networks

Triggered by the explosion of mobile traffic, 5G (5th Generation) cellular network requires evolution to increase the system rate 1000 times higher than the current systems in 10 years. Motivated by this common problem, there are several studies to integrate mm-wave access into current cellular networks as multi-band heterogeneous networks to exploit the ultra-wideband aspect of the mm-wave band. The authors of this paper have proposed comprehensive architecture of cellular networks with mm-wave access, where mm-wave small cell basestations and a conventional macro basestation are connected to Centralized-RAN (C-RAN) to effectively operate the system by enabling power efficient seamless handover as well as centralized resource control including dynamic cell structuring to match the limited coverage of mm-wave access with high traffic user locations via user-plane/control-plane splitting. In this paper, to prove the effectiveness of the proposed 5G cellular networks with mm-wave access, system level simulation is conducted by introducing an expected future traffic model, a measurement based mm-wave propagation model, and a centralized cell association algorithm by exploiting the C-RAN architecture. The numerical results show the effectiveness of the proposed network to realize 1000 times higher system rate than the current network in 10 years which is not achieved by the small cells using commonly considered 3.5 GHz band. Furthermore, the paper also gives latest status of mm-wave devices and regulations to show the feasibility of using mm-wave in the 5G systems.

Enabling 5G backhaul and access with millimeter-waves

This paper presents the approach of extending cellular networks with millimeter-wave backhaul and access links. Introducing a logical split between control and user plane will permit full coverage while seamlessly achieving very high data rates in the vicinity of mm-wave small cells.

Cloud cooperated heterogeneous cellular networks

This paper introduces a concept of cloud cooperated heterogeneous cellular network (C-HetNet) where small power pico base stations (BSs) overlaid on a macrocell are connected to cloud radio access network (C-RAN) to operate cooperatively with the macro BS. Since the macro BS assists operation of the pico BSs, it is easy to deploy multi-band HetNet where e.g. macro BS is operated at 2GHz band and pico BSs at 3GHz or 60GHz band to expand operation bandwidth. Moreover, cell structure of pico BSs are controlled dynamically to track hotspot users via beam steering and cooperative transmission among pico BSs. In this paper, several numerical simulations show effectiveness of the proposed architecture, where 1,000 times system rate than that of current single band cellular network is achieved by using the proposed architecture.

Millimeter wave beamforming based on WiFi fingerprinting in indoor environment

Millimeter Wave (mm-w), especially the 60 GHz band, has been receiving much attention as a key enabler for the 5G cellular networks. Beamforming (BF) is tremendously used with mm-w transmissions to enhance the link quality and overcome the channel impairments. The current mm-w BF mechanism, proposed by the IEEE 802.11ad standard, is mainly based on exhaustive searching the best transmit (TX) and receive (RX) antenna beams. This BF mechanism requires a very high setup time, which makes it difficult to coordinate a multiple number of mm-w Access Points (APs) in mobile channel conditions as a 5G requirement. In this paper, we propose a mm-w BF mechanism, which enables a mm-w AP to estimate the best beam to communicate with a User Equipment (UE) using statistical learning. In this scheme, the fingerprints of the UE WiFi signal and mm-w best beam identification (ID) are collected in an offline phase on a grid of arbitrary learning points (LPs) in target environments. Therefore, by just comparing the current UE WiFi signal with the pre-stored UE WiFi fingerprints, the mm-w AP can immediately estimate the best beam to communicate with the UE at its current position. The proposed mm-w BF can estimate the best beam, using a very small setup time, with a comparable performance to the exhaustive search BF.

Outdoor millimeter-wave access for heterogeneous networks — Path loss and system performance

The millimeter-wave frequency bands, especially the license free band at 60 GHz, is a candidate for future broadband access links. Path loss measurements have been performed in a typical small cell access scenario. Path loss model parameters were derived for these results, including large and small scale effects. Using this model a system level analysis was performed to evaluate the performance of a heterogeneous network using millimeter-wave access links.

Indoor MIMO channel measurements for evaluation of effectiveness of array antenna configurations

Recent research has been done on the influence of the array antenna configuration on a multi-input multi-output (MIMO) communication system. In this paper, we present the results from a measurement experiment conducted in an indoor propagation environment, one chosen to represent the typical office environment. The IST-METRA channel model was used for data analysis. In the experiment, a variety of array configurations were used and their effects on the channel capacities were investigated. Based on the results, the channel model, which considers the spatial correlation at both ends of the link separately, is found to be fairly accurate. A criterion to evaluate both the correlation and the mean effective gains (MEG) of the antenna elements was proposed and supported by measurement data.

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.

Performance Analysis of MIMO-OFDM Systems using Indoor Wideband MIMO Channel Measurement Data

In this paper, throughput performance of MIMO-OFDM system in real residential home environment was evaluated by using wideband MIMO channel measurement data. Computer simulations were carried out to compare the performance of MIMO-OFDM systems with various detection methods, i.e. MMSE, VBLAST, QRM-MLD and SVD-MIMO, in addition to that of the SISO-OFDM system. The results showed that the SVD-MIMO transmission can provide the highest average and outage throughput under the assumption of perfect CSI feedback. On the other hand, QRM-MLD yielded the best performance among the systems without CSI available at the transmitter. It was also found that the outage throughput performance of MMSE and VBLAST degrades severely due to the existence of spatial correlation in the real home environment

Delayed offloading zone associations using cloud cooperated heterogeneous networks

Delayed offloading of mobile user delay tolerant traffic, through other networks such as WiFi networks or millimeter wave (mm-w) gates, is a promising solution for dealing with the ever-growing problem of wireless cellular network capacity. In this paper, we investigate the problem of how we can optimally associate a user with delayed traffic to a nearby offloading zone, i.e., to which nearby offloading zone a user with delayed traffic should be associated. Towards that, we propose an adaptive scheme for offloading zone association, including a weighted proportional fairness (WPF) online algorithm for optimal zone selection. The proposed scheme is a network-based, which maximizes the total system offloading and the users offloading experiences (OFEs). To efficiently implement the proposed scheme, we also suggest a cloud cooperated heterogeneous network (CC-HetNet) to hold the delayed offloading process in a cloud cooperated manner. In this CC-HetNet, the offloading zones and the Macro Base station (BS) are linked to the Centralized Radio Access Network (C-RAN) using high speed backhaul links. The proposed scheme of offloading zone association will be implemented in the C-RAN as an enhanced Access Network Discovery and Selection Function for delayed offloading networks.

Development of high resolution MIMO channel sounder for the advanced modeling of wireless channels

This paper describes the development of the multi-input multi-output (MIMO) channel sounder which can jointly estimate the direction of departure (DOD), the direction of arrival (DOA) and the delay time of arrival (DTOA), for the advanced modeling of wireless channels. The algorithm and the hardware architecture are described, as well as the experimental results using the prototype.