- Recent Topics(from April 2017)
- Wireless Evolution
Recent Topics(from April 2017)
Distributed MIMO Network
Toward small-cell networks
The design of the mobile communications network is based on the cellular concept  to effectively utilize the limited available bandwidth and accordingly the area SE (ASE) (bps/Hz/km2). The cellular concept is that the service area is divided into small areas called cells, each of which is covered by a base station (BS). The spatially separated BSs reuse the same frequency as long as the CCI is kept below a predetermined allowable level. Improving the ASE is one important challenge for mobile communications networks. Another challenge for 5G is improving the EE (bits/Joule) since increasing the data rates leads to increasing the transmit signal power (this becomes a serious problem in particular for battery operated mobile terminals).
Distributed MIMO network
One promising approach for simultaneous improvement of the SE and the EE is a small-cell structured network . Reducing the BS coverage area (i.e., small-cell) enables the same frequency to be reused more densely in the same service area and accordingly the ASE can improve. The short distance communications allow the transmit signal power significantly lowered, thereby the EE improves. However, frequent handover may happen in order to enable the continuous communication while a user is traveling or even walking and may increase the control signal traffic.
There are two main approaches to avoid frequent handover  (Fig. 1). One is to introduce co-located MIMO network or massive MIMO network . The other is to deploy a number of distributed antennas over a macro-cell covered by a BS . Here, we call this network the distributed MIMO network (or a distributed antenna network [6-8]). The handover problem can be replaced with the selection of appropriate distributed antennas for the former network and with a selection of beams for the latter network within the same BS. Possible advantage of using distributed MIMO over co-located MIMO is its capability of alleviating problem raised by the shadowing loss and path loss.
User-centric virtual small-cell
Figure 2 illustrates the conceptual structure of distributed MIMO network. Each distributed antenna and macro-cell BS (MBS) are connected by optical fiber link. MBS performs radio signal processing for signal transmission and reception, whilst the distributed antennas near a user equipment (UE) form a user-centric small-cell and perform cooperative signal transmission using several antennas near a target user terminal. This is considered as an extension of CoMP of LTE-advanced .
Distributed MIMO cooperative transmission
The broadband channel is severely frequency-selective (Fig. 3). Therefore, it is necessary to introduce some powerful equalization technique, e.g., FDE used in LTE/LTE-A to tackle the frequency-selectivity of the channel. FDE requires the channel estimation to acquire the channel state information (CSI). In the time division duplex (TDD) transmission scheme, the same frequency is used for both uplink (UEàMBS) and downlink (MBSàUE) transmissions (Fig. 4). Therefore, in TDD, the CSI estimate of uplink can be reused for transmit equalization in the downlink transmission. Because of this reason, FDE for both uplink and downlink transmissions can be done at MBS. Figure 5 illustrates a simplified transmitter/receiver structure of distributed MIMO cooperative transmission for downlink. OFDM and SC block signal transmissions are considered for downlink and uplink, respectively. We have been developing single-user MIMO diversity, multi-user MIMO multiplexing, blind selective mapping (SLM) for reducing signal waveform peak-to-average ratio (PAPR), MIMO channel estimation, inter-cell interference coordination (ICIC), etc.
Recent achievement on distributed MIMO cooperative transmission is summarized in .
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- F. Adachi, K. Takeda, T. Yamamoto, R. Matsukawa, and S. Kumagai, “Recent advances in single-carrier distributed antenna network,” Wireless Commun. and Mobile Computing, Volume 11, Issue 12, pp. 1551–1563, Dec. 2011, DOI: 10.1002/wcm.1212.
- F. Adachi, W. Peng, T. Obara, T.Yamamoto, R.Matsukawa and M.Nakada, “Distributed antenna network for gigabit wireless access,” International Journal of Electronics and Communications (AEUE), Vol. 66, Issue 6, pp.605-612, 2012.
- M. Sawahashi, Y. Kishiyama, A. Morimoto, D. Nishikawa, and M. Tanno, “Coordinated multipoint transmission/reception techniques for LTE-advanced [coordinated and distributed MIMO],” IEEE Wireless Commun., Vol. 17, Issue 3, pp.26-34, June 2010.
- F. Adachi, A. Boonkajay, Y. Seki, T. Saito, S. Kumagai, and H. Miyazaki, “Cooperative Distributed Antenna Transmission for 5G Mobile Communications Network,” IEICE Trans., Vol.E100-B, No.8, pp.-, Aug. 2017. DOI:10.1587/transcom.2016FGP0019.
From the 1st generation to the 3rd generation
Ultimate goal of our desire is to communicate with anyone, anytime, from anywhere to exchange any type of information, beyond time and space. To realize this, wireless technology plays an important role. Now, we have started to enjoy Internet communications using the 2nd generation mobile phones developed in early 1990’s. Browsing text-based WWW sites is possible from mobile phones. However, transfer rate over wireless channel is quite slow, e.g., about 10kbps, which is far slower than with fixed communications. The 3rd generation mobile communications systems known as International Mobile Telecommunication Systems (IMT)-2000 have been standardized by the International Telecommunication Union (ITU). Wideband direct sequence code division multiple access (W-CDMA) technology is used as the wireless access. Its data transfer capability is far faster than the 2nd generation systems and up to 2Mbps transfer rates will be provided with the same quality as the fixed networks , . W-CDMA systems was put into service in Japan in 2001 and now, the services are expanding throughout Japan. W-CDMA services will soon appear European countries.
4th generation mobile wireless systems
Now, it seems that a mobile phone is not just for voice conversation, but is evolving into a multimedia communication tool that enables various types of electronic communications for private as well as business use . Internet communications, mobile wireless communications, and optical communications will soon become the information infrastructure of our society. However, information volume exchanged over the Internet will grow because of image information becoming popular and popular. This suggested that IMT-2000’s data transfer capability of maximum 2Mbps became insufficient. A 100M-1Gbps-class wireless technology was then desired. This technology was a key to realize the 4th generation mobile wireless systems that was said to appear in around 2010 years , .
The 4th generation (4G or called long-term evolution (LTE)-Advanced)) networks started in Japan in 2015 [7, 8, 9]. In 4G, high-quality video communications and close-to-1Gbps broadband data services have been becoming more popular. The cellular networks reuse the same frequency to utilize the limited available bandwidth efficiently. However, this frequency reuse introduces the co-channel interference (CCI), from adjacent cells, that limits the transmission quality. For macro-cell edge users, the received signal power reduces due to large path loss while strong CCI is received (i.e., the received signal-to-interference plus noise power ratio (SINR) degrades significantly) and therefore, we will not be able to achieve the required transmission quality. To solve this problem, 4G networks have adopted the coordinated multi-point (CoMP) transmission technique .
Evolution into the 5th generation
In the 5th generation (5G) networks, we expect much broader data services (>1Gbps/user). Recently, the development of 5G networks achieving higher spectrum efficiency (SE) and energy efficiency (EE) than 4G networks is on-going worldwide . In Japan, the research and development project for realizing the 5G mobile communications networks started in Sept. 2015 . One promising approach is a distributed antenna small-cell network [13, 14, 15] that deploys a number of distributed antennas over a traditional macro-cell area to exploit the spatial-domain more efficiently. For efficient signal transmission using distributed antennas, we have been studying distributed MIMO cooperative transmission for spatial diversity and multi-user spatial multiplexing. The distributed MIMO cooperative transmission can be viewed as an advanced version of CoMP in 4G networks.
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- F. Adachi, K. Takeda, T. Obara, T. Yamamoto, and H. Matsuda, “Recent advances in single-carrier frequency-domain equalization and distributed antenna network,” IEICE Trans. Fundamentals, Vol. E93-A, No. 11, pp. 2201-2211, Nov. 2010.
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