Wireless Access Technologies to Provide Various Services in Future Networks

2.1 Technologies for linking unlicensed radio band systems

Various wireless standards for radio access in unlicensed radio bands have been developed for different uses, so it is envisioned that users will install APs as they see fit according to their target applications. To manage these APs across the network in an optimal manner, our aim is to achieve radio resource management through an unlicensed-band platform and software-based white-box radio base stations. Additionally, as part of the 5G comprehensive demonstration test launched in 2017 by the Ministry of Internal Affairs and Communications of Japan, some of these technologies will be used in the deployment of high-density wireless LAN and in control tests of wireless LAN linked to 5G cellular [2].

2.2 High-capacity wireless relay technology

We aim to promote the deployment of small-cell base stations and APs in massive quantities and are therefore studying high-capacity wireless relay technology in order to replace some relay lines with wireless networks in locations where the laying of optical fiber is difficult (Fig. 2). For this technology, we are assessing the use of high-frequency bands such as the millimeter-wave band capable of comparatively high-capacity transmission, but there is also a need for technology that can simplify installation requirements and achieve high efficiency. To solve these issues, the NTT laboratories are developing spatial multiplexing technology in the millimeter-wave band capable of high-capacity communications, antenna high-gain technology based on large-scale arrays to compensate for propagation loss, and point-to-multi-point technology for simultaneously accommodating multiple stations.

Fig. 2. High-capacity wireless relay technology.

2.3 Next-generation advanced wireless LAN technology

We are developing next-generation wireless LAN technology to provide unlicensed-band radio with sophisticated functions. Up to now, wireless LAN standards have been progressing as part of efforts to increase throughput in one-to-one communication and improve the area coverage rate. Recently, however, throughput degradation in dense environments, for example, congested train stations, has brought to light the problem of how to improve communication quality in such environments. To solve this problem, the NTT laboratories have developed distributed smart antenna technology that performs high-speed switching among multiple antennas in units of packets at a wireless LAN AP and an algorithm for performing optimal control of the radio resources (frequency channel, transmission output, etc.) (Fig. 3). We tested this technology in an actual stadium and found it to be effective in reducing signal interference between APs and in improving transmission speeds even for densely installed APs [3].

Fig. 3. Cooperative wireless LAN technology with distributed smart antenna system.

At the same time, we have been promoting the drafting of related standards and have participated in formulating the IEEE (Institute of Electrical and Electronics Engineers) 802.11ax standard for achieving high transmission efficiency in dense environments since the task group was established for this purpose in May 2014 [4].

2.4 Multi-frequency signal propagation modeling technology

Technology for modeling the characteristics of wireless signal propagation is essential to all wireless communication systems, including 5G/5G+. The NTT laboratories have established a new method of frequency allocation through a study on ongoing coexistence with current wireless systems plus a new system design and communication method through an evaluation of propagation characteristics. We have been reflecting these new methods in cell design and area design according to the propagation conditions of individual locations.

More recently, we have been conducting propagation measurements and modeling using multiple frequency bands in the 800 MHz–66 GHz range in diverse environments, including urban and congested environments that are expected to become key targets of 5G/5G+ (Fig. 4). We are working to standardize these results as part of standardization efforts at ITU-R (International Telecommunication Union - Radiocommunication Sector) and 3GPP (3rd Generation Partnership Project) with the aim of promulgating new methods and specifications for frequency allocation.

Fig. 4. Investigation of radio propagation models for 5G+.

3.1 Terrestrial wireless systems for mountainous areas and isolated islands

The terrestrial wireless systems for mountainous areas and isolated islands consist of a fixed microwave communications system and a subscriber radio system (Fig. 5). The fixed microwave communications system [5] is a wireless system that provides transmission links to areas where optical transmission paths are difficult to construct for topographical and/or economic reasons. The subscriber radio system, meanwhile, is a wireless system that provides telephone lines, for example, to extremely rural areas, mountain huts, or other remote facilities where the laying of metal or optical cables is difficult.

Fig. 5. Terrestrial radio systems.

The NTT laboratories are expanding the applicable areas for such wireless systems through technology that achieves long-distance and high-quality transmission while increasing transmission capacity. The goal here is to bring uniformity and standardization to multiple terrestrial wireless systems and to reduce equipment costs.

3.2 Terrestrial wireless systems for disaster recovery

There are four NTT laboratories products that make up these terrestrial wireless systems for disaster recovery: a business radio system, a relay radio system, a wireless access system, and a cell and radio frequency planning tool. The business radio system [6] is a wireless system for establishing communications within a company to disseminate current disaster conditions, instructions on recovery tasks, or other necessary information to personnel in the field. The relay radio system [7] is a portable wireless system capable of high-capacity wireless communications for quickly restoring services when relay transmission paths are disrupted during a disaster. The wireless access system [8] provides special public telephone and Internet services to customers at evacuation centers and other locations at the time of a disaster. Finally, the cell and radio frequency planning tool [9] is used to select optimal buildings for installing base stations in terrestrial wireless systems for disaster recovery.

These four products have smaller and lighter configurations and more advanced functions than past systems. The objective is to downsize the antennas in the business radio system and wireless access system to contribute to even greater portability.

3.3 Satellite communications systems

The satellite communications systems being researched and developed at the NTT laboratories mainly consist of a satellite communications system for disaster recovery and a satellite communications system for isolated islands (Fig. 6). The system for disaster recovery provides special public telephone and Internet services to customers at evacuation centers in the event of a disaster. The system for isolated islands provides a communications infrastructure for areas where it is difficult to lay submarine optical cable.

Fig. 6. Satellite communications systems.

We have developed a satellite communications modem unit, or common unit (COM-U) [10] that can be applied to base stations in both types of satellite communications systems. COM-U enables frequency placement to be set as desired and therefore improves spectrum utilization efficiency compared with existing satellite communications modem units. Furthermore, although current satellite communications systems combine multiple pieces of equipment, the equipment is consolidated and mounted on a single rack, thereby eliminating the need for wiring and improving maintainability and operability.