A Study on Wireless Technologies to Improve Communication Performance by Utilizing Multi-frequency Bands

2.1 Use cases

Examples of leveraging ultra-broadband data are considered below and depicted in Fig. 1.

Fig. 1. Example of utilizing ultra-broadband data.

In these use cases, log data and database information saved on disk must be exchanged between the cloud database and the user anywhere and anytime through wireless networks. This requires disk-to-disk (D2D) communication, which until now has mainly been used to back up data on hard disks, between multiple devices and instruments that are far from each other through wireless networks. We set the target wireless transmission speed for this new D2D communication to 100 Gbit/s, which is 10 times faster than Wi-Fi^*1.

2.2 60-GHz unlicensed band

We have adopted the 60-GHz unlicensed band as the frequency band for wireless transmission speeds of 100 Gbit/s. This unlicensed band has the advantage of easily forming ultra-high-speed wireless access links because the wireless access points can be placed freely, and the wireless function can be installed on any device without a license. The characteristics of the 60-GHz band are described below.

The large bandwidth makes it possible to achieve the maximum transmission speed of 11.4 Gbit/s with a single stream [1]. For reference, the transmission speed of Wi-Fi is less than 1 Gbit/s [2]. By applying multiple-input multiple-output (MIMO) techniques, a transmission speed of 100 Gbit/s can be expected.

While the short communication range results in smaller communication coverage than that of the microwave band, the high antenna directivity enables interference signals from other stations to be reduced by both propagation loss and antenna directivity. Therefore, a stable wireless transmission speed of 100 Gbit/s is expected regardless of the other stations.

To achieve a transmission speed of 100 Gbit/s in the 60-GHz band, more than ten streams must be simultaneously transmitted via space division multiplexing because the maximum transmission speed with a single stream is 11.4 Gbit/s. We consider short range MIMO to be one of the most promising approaches to meet the requirements. The system image is shown in Fig. 2.

Fig. 2. System image: using short range MIMO techniques.

The MIMO technique is often used in multipath-rich environments, and with space division multiplexing, the number of streams can equal the number of multipath waves. However, general MIMO techniques cannot support more than ten streams because it is difficult to ensure more than ten multipath waves regardless of the propagation environment.

The short range MIMO technique is used in very close proximity environments in which the direct propagation path lengths between transmitter and receiver antennas are different from each other. The multiple streams can be simultaneously transmitted on direct paths by utilizing different phases on the propagation paths. When the linear antenna array is adopted as the MIMO antenna configuration, the carrier-to-interference ratio (CIR)^*2 degrades as the number of multiple streams increases. Moreover, the CIR strongly depends on the relative positioning of the transmitter and receiver, so the CIR significantly degrades when the positioning is not optimum.

To solve these problems, we are studying the use of two-dimensional antenna arrays and multiple MIMO configurations, the optimal design of those configurations, and adaptive selection of the MIMO configuration depending on the number of multiple streams and the positioning relationship at the time of use.

2.3 Standard specifications in 60-GHz band

The standard specifications in the 60-GHz band are introduced in this section. The key standards are listed in Table 1. IEEE802.15.3e is the next-generation standard specified by the Institute of Electrical and Electronics Engineers (IEEE). It has higher transmission speeds than TransferJet^*3 [3], and its target use is close proximity communication. IEEE802.11ad offers a higher transfer speed than Wi-Fi [2], and IEEE802.11ay is the next-generation standard with higher transmission speeds than IEEE802.11ad [4]. The standard IEEE802.11ad was completed in 2013, but IEEE802.15.3e and IEEE802.11ay are still in progress. All standards adopt the channel bandwidth of 2.16 GHz. Channel bonding^*4 and multiple stream transmission (with a MIMO function) will be standardized in IEEE802.15.3e and IEEE802.11ay. NTT Network Innovation Laboratories is active in contributing to IEEE802.15.3e and has proposed the MIMO function to achieve a transmission speed of 100 Gbit/s and a faster link setup of 0.2 seconds.

3.1 Advanced ITS

The development of an advanced intelligent transport system (ITS) is continuing in Japan, and industry, government, and academia are promoting the development of autonomous vehicle driving technologies for 2030 [6]. Any autonomous vehicle driving system requires cooperative technologies such as predicting events using vehicle-to-vehicle/roadside wireless communications and artificial intelligence, as well as self-driving technologies such as collision avoidance using on- and off-board vehicle sensors. One cooperative ITS technology that has already been launched is the safe driving support service called ITS Connect, which uses the 760-MHz band in Japan.

In North America and Europe, a similar DSRC (Dedicated Short Range Communications) service will be introduced in the 5.9-GHz band in the next several years. Both of these systems adopt IEEE 802.11p wireless local area network (LAN) standards [1] for vehicle-to-vehicle/roadside communications.

Meanwhile, the development and standardization of cellular communication systems called LTE (Long Term Evolution) V2X [7] are also being discussed. Since LTE V2X can utilize the existing LTE cellular infrastructure, additional investment is minimal.

As shown in the case of the autonomous driving systems, a variety of wireless communication technologies in both licensed and unlicensed bands is being studied. Of course, existing wireless LANs are also promising radio access technologies (RATs) for these systems. The characteristics or performance of wireless communications depends on the radio environments such as the frequency band, the RAT adopted, network infrastructure availability, terminal device position, and radio channel conditions such as whether there is interference. Here, it is assumed that access point 1 with RAT-a running on band A and access point 2 with RAT-b running on band B are available (Fig. 3). Even if RAT-a has good performance (such as transmission speed or latency performance) as a RAT, its actual communication performance (such as throughput) will fall dramatically due to the presence of many terminals, heavy interference, or obstacles in the channel. In contrast, RAT-b offers lower ultimate performance but is more consistent. This suggests that establishing more reliable radio connections requires the optimal RAT to be selected from several candidates according to the multiband radio environments.

Fig. 3. Multiband utilization approach for high-reliability and low-latency communication.

3.2 Multiband propagation evaluation technologies

NTT Network Innovation Laboratories is promoting the study of multiband propagation evaluation technologies and applied wireless access technologies. An overview is shown in Fig. 4. First, the radio propagation characteristics for each band (channel) are evaluated by simulating radio intensity and arrival time of multipath waves. For this, we consider 3D data of buildings and objects around the access points and terminal devices (Fig. 4(a)). Then the propagation results are combined to permit network simulations of different RATs for each band. These can evaluate overall system parameters such as processing latency and packet loss characteristics, taking into consideration the connection density of devices and other interference effects (Fig. 4(b)). These evaluation results make it possible to design multiband wireless systems that make maximum use of multiple bands and the diversity in available RATs.