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Bai Y.,Hainan University | Chen L.,DOCOMO Beijing Communications Laboratories Co.
Eurasip Journal on Wireless Communications and Networking | Year: 2013

When deploying two-tier LTE cellular networks with coexistence of macrocells and underlaid femtocells, the strategies of spectrum arrangement need to be investigated to efficiently utilize the scarce spectrum resource. Co-channel spectrum usage and dedicated-channel spectrum usage are two traditional strategies of spectrum arrangement. Nevertheless, there are pros and cons of them from the perspective of achievable network capacity in two-tier LTE network as they result in different amounts of available spectrum at each tier and disparate cross-tier interferences. To improve overall spectrum utilization, we propose a novel approach of spectrum arrangement, which is called hybrid spectrum arrangement, to take advantage of their merits. In our proposal, an underlaid femtocell can select its spectrum usage mode according to a criterion aiming to benefit both macrocell and femtocell in terms of achieved capacities. Consequently, the femtocells embedded in macrocell are self-organized as inner and outer femtocells, which operate in dedicated-channel spectrum usage and co-channel spectrum usage, respectively. Then, we examine distinct characteristics of cross-tier interferences in the context of hybrid spectrum arrangement and present corresponding schemes to mitigate the residual significant interferences. Analysis and system level simulation are given to validate the effectiveness of our proposed methods for two-tier LTE cellular network. © 2013 Bai and Chen.

Bo H.,Beijing University of Posts and Telecommunications | Hui T.,Beijing University of Posts and Telecommunications | Lan C.,DoCoMo Beijing Communications Laboratories Co. | Jianchi Z.,DoCoMo Beijing Communications Laboratories Co.
IEEE Communications Letters | Year: 2010

The Long Term Evolution (LTE) system adopts Discontinuous Reception (DRX) in the link level for power saving to extend the battery life of the User Equipment (UE). A novel real-time DRX-aware scheduling method is proposed for delay-sensitive services in wireless downlink systems. In the scheme, DRX parameters are introduced into the scheduling determinants, so as to reduce packet loss caused by the sleeping process during DRX. Simulation results show that, significant performance gain in terms of packet loss rate and power consumption, can be achieved by the proposed scheme compared with traditional schemes. © 2010 IEEE.

Song Y.,DOCOMO Beijing Communications Laboratories Co. | Yun X.,DOCOMO Beijing Communications Laboratories Co. | Nagata S.,NTT DoCoMo | Chen L.,DOCOMO Beijing Communications Laboratories Co.
2013 IEEE International Conference on Communications Workshops, ICC 2013 | Year: 2013

This paper investigates and analyzes three typical elevation beamforming scenarios which are most likely to be applied in future LTE-Advanced systems: vertical sectorization with same carrier frequency, vertical sectorization with different carrier frequency based on carrier aggregation, and user-specific elevation beamforming. Preliminary evaluation using WINNERII/WINNER+ based 3D MIMO channel modeling is carried out to compare these scenarios with the conventional system with single downtilting. It is shown that the latter two elevation beamforming scenarios can achieve good performance. © 2013 IEEE.

Benjebbovu A.,NTT DoCoMo | Li A.,DOCOMO Beijing Communications Laboratories Co. | Saito Y.,NTT DoCoMo | Kishiyama Y.,NTT DoCoMo | And 2 more authors.
2013 IEEE Globecom Workshops, GC Wkshps 2013 | Year: 2013

This paper investigates the system-level performance of downlink non-orthogonal multiple access (NOMA) with power-domain user multiplexing at the transmitter side and successive interference canceller (SIC) on the receiver side. The goal is to clarify the performance gains of NOMA for future LTE (Long-Term Evolution) enhancements, taking into account design aspects related to the LTE radio interface such as, frequency-domain scheduling with adaptive modulation and coding (AMC), and NOMA specific functionalities such as error propagation of SIC receiver, multi-user pairing and transmit power allocation. In particular, a pre-defined user grouping and fixed per-group power allocation are proposed to reduce the overhead associated with power allocation signalling. Based on computer simulations, we show that for both wideband and subband scheduling and both low and high mobility scenarios, NOMA can still provide a hefty portion of its expected gains even with error propagation, and also when the proposed simplified user grouping and power allocation are used. © 2013 IEEE.

Liu L.,DOCOMO Beijing Communications Laboratories Co. | Mu Q.,DOCOMO Beijing Communications Laboratories Co. | Takeda K.,NTT DoCoMo | Chen L.,DOCOMO Beijing Communications Laboratories Co.
IEEE Wireless Communications and Networking Conference, WCNC | Year: 2013

This paper focuses on investigating the search space design of Enhanced Physical Downlink Control CHannel (EPD-CCH), which is introduced in Long Term Evolution-Advanced (LTE-A) system to increase the capacity of the downlink control channel. Since EPDCCH is frequency-division-multiplexed with downlink data channel, resource utilization efficiency is a very important issue in addition to blocking probability. This paper proposes the innovative methods of locating the search spaces of EPDCCH with different transmission schemes, i.e. distributed transmission and localized transmission, considering the UE complexity, the capacity of control channel and the impacts on Physical Downlink Shared CHannel (PDSCH). By simulation results, it is proven that the proposed methods could get higher resource utilization and lower blocking probability while maintain the same UE complexity in blind decoding. © 2013 IEEE.

Na C.,DOCOMO Beijing Communications Laboratories Co. | Hou X.,DOCOMO Beijing Communications Laboratories Co. | Harada A.,DOCOMO Beijing Communications Laboratories Co.
IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC | Year: 2011

Multi-user multiple input multiple output (MU-MIMO) systems can achieve high spectral efficiency for wireless communication. The design of optimal transmit precoding for max-sum-rate (MSR) MU-MIMO downlink transmission is a non-trivial problem. Recent research approached this problem by exploiting the relationship between max-sum-rate and the minimum-mean-squared-error (MMSE) and proposed several iterative schemes. Based on these research results, we propose enhanced iterative MSR algorithms for linear MU-MIMO precoding which improve the sum rate performance and reduce the computational complexity. The effectiveness of the proposed method is verified by numerical simulations. © 2011 IEEE.

Li A.,DOCOMO Beijing Communications Laboratories Co. | Harada A.,DOCOMO Beijing Communications Laboratories Co. | Kayama H.,DOCOMO Beijing Communications Laboratories Co.
IEICE Transactions on Fundamentals of Electronics, Communications and Computer Sciences | Year: 2014

Multiple access (MA) technology is of most importance for beyond long term evolution (LTE) system. Non-orthogonal multiple access (NOMA) utilizing power domain and advanced receiver has been considered as a candidate MA technology recently. In this paper, power assignment method, which plays a key role in performance of NOMA, is investigated. The power assignment on the basis of maximizing geometric mean user throughput requires exhaustive search and thus has an unacceptable computational complexity for practical systems. To solve this problem, a novel power assignment method is proposed by exploiting tree search and characteristic of serial interference cancellation (SIC) receiver. The proposed method achieves the same performance as the exhaustive search while greatly reduces the computational complexity. On the basis of the proposed power assignment method, the performance of NOMA is investigated by link-level and system-level simulations in order to provide insight into suitability of using NOMA for future MA. Simulation results verify effectiveness of the proposed power assignment method and show NOMA is a very promising MA technology for beyond LTE system Copyright © 2014 The Institute of Electronics.

Hou X.,DOCOMO Beijing Communications Laboratories Co. | Harada A.,DOCOMO Beijing Communications Laboratories Co. | Suda H.,DOCOMO Beijing Communications Laboratories Co.
IEEE International Symposium on Personal, Indoor and Mobile Radio Communications, PIMRC | Year: 2012

As the 3GPP long term evolution (LTE) system actively evolves, advanced multi-user multiple-input multiple-output (MU-MIMO) schemes have attracted increasing interest in the engineering field. We have proposed a QR decomposition (QRD) based MU-MIMO scheme for the evolving LTE time division duplex (TDD) system to achieve high spectrum efficiency, assuming ideal channel reciprocity. In order to investigate the promising performance of TDD MU-MIMO in practice, we implement the proposed QRD-based TDD MU-MIMO scheme together with an actual over-the-air (OTA) antenna calibration scheme into our FPGA-based hardware testbed for the evolving LTE TDD system to examine its performance under the constraint of practical channel reciprocity. Measurement results show that channel reciprocity indeed could be achieved with the OTA antenna calibration and the QRD-based MU-MIMO scheme can effectively make use of this channel reciprocity to achieve high spectrum efficiency data transmission for both downlink and uplink. Moreover, the influence of antenna calibration on the TDD MU-MIMO performance is further investigated and several considerations related to antenna calibration are addressed. © 2012 IEEE.

Wang W.,DOCOMO Beijing Communications Laboratories Co. | Harada A.,DOCOMO Beijing Communications Laboratories Co. | Kayama H.,DOCOMO Beijing Communications Laboratories Co.
IEEE Transactions on Wireless Communications | Year: 2013

This paper proposes new limited-feedback Channel State Information (CSI) calculation schemes for Zero Forcing (ZF)-precoded downlink Multi-User Multiple-Input Multiple-Output (MU-MIMO) systems. It is a common understanding that the feedback quantized by the codebook limits the performance of MU-MIMO. In this paper, through a quasi-ZF or a quasi-Minimum Mean-Squared Error (MMSE) weight, the channel matrix is transferred to one of the codebook vectors, based on which, the CSI is calculated. Thus, the quantization error is minimized. Meanwhile, the selection for the codebook vector guarantees the maximizing of the estimated Signal to Interference plus Noise Ratio (SINR). We verify that the proposed scheme obtains accurate feedback information, and the predicted weight can be the same as the optimal linear decoder as if the receiver knew all the precoder information that is fed-forward from the BS, as long as the number of antennas for each receiver equals that for the transmitter, and equals that for the total transmit data streams. Compared to the commonly used Precoding Matrix Index (PMI) based method, which uses rank-one single user (SU)-MIMO feedback, simulation results show that the proposed schemes achieve higher sum capacities in different scenarios. Moreover, since the weight can be directly used as the decoder, the feed-forward overhead is reduced. © 2002-2012 IEEE.

Kayama H.,DOCOMO Beijing Communications Laboratories Co. | Jiang H.,DOCOMO Beijing Communications Laboratories Co.
Conference Record - Asilomar Conference on Signals, Systems and Computers | Year: 2015

The paradigm shift toward smartphones caused distinct traffic expansion in mobile networks. Indeed, the total amount of traffic in Japan is glowing nearly 1.8-fold in yearly basis during the past few years. To cope with this problem, many mobile operators are now speeding up their deployment of LTE/LTE-A. In this paper, current situation of LTE deployment in Japan, including area deployment and spectrum assignment are introduced. Then, the technical trend for further enhancement of LTE-A is presented based on the current standardization activities in 3GPP Rel.11 and Rel.12. Finally, the requirements and some expected technologies for the next generation mobile system, namely 5G or Future Radio Access (FRA), will be presented. Here, small cell deployment and its interwork with macro-cell is regarded as one of promising ways for increasing channel capacity while maintaining mobility support. As to radio access technologies, advanced interference mitigation and non-orthogonal multiple access (NOMA) are likely to be key issues for FRA. Initial evaluations of Massive-MIMO performance are also presented for clarifying the feasibility in the assumed scenario. © 2014 IEEE.

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