Kazuaki Takeda

Non-orthogonal transmission technology in LTE evolution

Non-orthogonal transmission, although not entirely new to the wireless industry, is gaining more attention due to its promised throughput gain and unique capability to support a large number of simultaneous transmissions within limited resources. In this article, several key techniques for non-orthogonal transmission are discussed. The downlink technique is featured by MUST, which is being specified in 3GPP for mobile broadband services. In the uplink, grantfree schemes such as multi-user shared access and sparse code multiple access, are promising in supporting massive machine-type communication services. The multi-antenna aspect is also addressed in the context of MUST, showing that MIMO technology and non-orthogonal transmission can be used jointly to provide combined gain.

Search Space Design for Cross-Carrier Scheduling in Carrier Aggregation of LTE-Advanced System

CA(Carrier aggregation) is one of the most important techniques for LTE-Advanced system because of its capability to support wide bandwidth while achieving backward compatibility. In order to improve performance of control information transmission, cross-carrier scheduling is to be supported, i.e. control information on one CC (Component Carrier) can assign radio resource on another CC. Design of search space, in which control information is mapped, for different CCs is one of the most essential issues of cross-carrier scheduling. This paper presents two novel methods of search space design. In the first method, UE (User Equipment)-specific offset is introduced among search spaces of different CCs. Due to the UE-specific offsets, search spaces of different UEs are staggered with each other and probability that search space of one UE is wholly overlapped with that of another UE can be greatly reduced. In the second method, a novel randomization scheme is proposed to generate independent hashing functions for search spaces of different CCs. Because of ideal randomization of the proposed method, search space overlapping of different UEs is reduced. Simulation results show the proposed methods can effectively reduce blocking probability compared with existing methods.

Effects of Wideband Scheduling and Radio Resource Assignment in OFDMA Radio Access for LTE-Advanced Downlink

This paper presents the effects of applying channeldependent dynamic scheduling and transport block (TB) assignment that take advantage of a wider transmission bandwidth than that for a component carrier (CC) in OFDMA radio access for the LTE-Advanced downlink. At the 3GPPRAN WG1 meeting, independent resource block (RB) assignment of one TB within one CC was adopted. However, gains in dynamic scheduling and TB assignment over a transmission bandwidth wider than that for a CC have not yet been sufficiently investigated, and these are crucial in obtaining target insight which is to be achieved by alternative techniques according to the adopted CC based TB assignment. Hence, we investigate the cell throughput and celledge user throughput gains for the three types of channeldependent dynamic scheduling and TB assignment: TB assignment and dynamic scheduling within one CC, TB assignment within one CC and dynamic scheduling over multiple CCs, and TB assignment and dynamic scheduling over multiple CCs. Then, based on system-level simulation results, we present the comprehensive insights assuming the current working assumption forLTE-Advanced.

Experimental evaluations on carrier aggregation and multi-user MIMO associated with EVD-based CSI feedback for LTE-Advanced downlink

This paper presents laboratory experimental results on 4-by-2 multi-user (MU)-MIMO multiplexing with two mobile stations (MSs) in the LTE-Advanced downlink in combination with carrier aggregation (CA) using 5 component carriers (CCs). Extended channel state information (CSI) feedback functionality based on eigenvalue decomposition (EVD) is implemented into real-time experimental equipment based on the LTE-Advanced radio interface to support MU-MIMO operations applying minimum mean square error (MMSE)-based precoding. Laboratory experiments are conducted assuming a multi-path fading environment based on different antenna configurations. The experimental results show that 4-by-2 MU-MIMO with two data streams per MS achieves a peak throughput of greater than 950 Mbps, i.e., nearly 1 Gbps, in a correlated antenna configuration scenario when the maximum Doppler frequency is 10 Hz and the propagation channel model with the root mean squared delay spread is 0.3 μsec. The results also show that, high throughput performance such as greater than 750 Mbps for 4-by-2 MU-MIMO with two data streams per MS is acheived even in a non-correlated antenna configuration scenario although the environment is limited to low mobility and a small delay spread.

Investigation on Optimum Radio Parameter Design in Layered OFDMA for LTE-Advanced

This paper investigates the optimum radio parameters, i.e., numerology, in layered orthogonal frequency division multiple access (OFDMA) for long-term evolution (LTE)- Advanced. Similar to the layered OFDMA concept, it was agreed at the 3GPP RAN WG1 meeting that the entire transmission bandwidth in LTE-advanced should comprise multiple frequency blocks called component carriers to achieve a wider system bandwidth than that for the current LTE (Rel-8 LTE). We investigate the optimum radio parameters in the frequency domain including the need for an inter-component carrier guard band (GB). In the simulation evaluations, we set the parameters for the receiver filter assuming the worst case that satisfies the minimum requirement specified in Rel-8 LTE. Simulation results show that the loss in the average received signal energy per symbol-to-noise power spectrum density ratio (Es/No) satisfying the required block error rate compared to the case without an inter-component carrier GB is negligible irrespective of the modulation scheme and coding rate. As a result, we conclude that the inter-component carrier GB is unnecessary and can be minimized to 19 subcarriers in the downlink while retaining the same number of subcarriers within a component carrier as that for Rel-8 LTE.

Investigation of Two-Dimensional Orthogonal Sequence Mapping to Multi-Layer Reference Signal for LTE-Advanced Downlink

In the LTE-Advanced downlink, to satisfy high-level performance requirements, advanced multi-antenna transmission techniques such as higher-order single-user (SU)-MIMO, multi-user (MU)-MIMO, and coordinated multi-point transmission/reception (CoMP) are key. At the 3GPP RAN WG1 meeting, the demodulation reference signal (DM-RS) was defined to support channel estimation and data demodulation for up to eight transmission layers. A hybrid code division multiplexing (CDM) and frequency division multiplexing (FDM) scheme was adopted as a DM-RS multiplexing scheme. The time-domain orthogonal cover code (OCC) is used for CDM since time domain orthogonality among OCCs is relatively robust against channel variation. However, in a medium-to-high mobility environment, orthogonality distortion occurs among OCCs, which results in performance degradation. In this paper, we propose a two-dimensional (2D) OCC that achieves two-dimensional orthogonality in the time and frequency domains to improve the performance of CDM-based DM-RSs while reducing the peak transmit power of each OFDM symbol where CDM-based DM-RS are mapped. Simulation results show that the 2D OCC is effective in improving the block error rate performance especially in a medium-to-high mobility environment. Furthermore, it is shown that the proposed 2D OCC effectively reduces the peak transmit power compared to the time-domain OCC.

Search Space Design in Enhanced Physical Downlink Control Channel for LTE-Advanced

In Long Term Evolution- Advanced (LTE-A) Release 11, an enhanced physical downlink control channel (ePDCCH)was introduced to increase the capacity of the physical downlink control channel (PDCCH), in order to support Multiple User-Multiple Input Multiple Output (MU-MIMO) and interference mitigation in heterogeneous networks. Design of the search space, in which control information is mapped, for the ePDCCH is an essential issue. Conventional search space designs suffer from low resource utilization efficiency and a high blocking probability. This paper presents a novel search space design. The proposed design introduces proper resource division in conjunction with a search space split. Simulation results show that the proposed design effectively improves the resource utilization efficiency and reduces the blocking probability.

Time Domain Inter-Cell Interference Coordination for Dense Small Cell Deployments

In order to improve further the user experience in high-traffic areas, dense small cell deployments in a new and high frequency band, e.g., 3.5 GHz, have been identified as a very interesting topic in Long Term Evolution-Advanced (LTE-Advanced) Release 12. The interference among small cells becomes a challenge instead of the cross layer interference for macro and small cell co-channel deployments. The conventional time domain inter-cell interference coordination (ICIC) method employing almost blank subframes (ABSs) is no longer suitable. A new and effective semi-static time domain ICIC method is proposed. The small cells that need to perform ICIC are carefully selected, and different ABS ratios and ABS patterns are well designed considering user distribution and traffic load for those small cells. Evaluation results show that the proposed method achieves significant performance gain under different traffic loads, even when considering more realistic cell-specific reference signal (CRS) interference. The proposed method is also quite robust against backhaul latencies and suitable for non-ideal backhaul networks.

Search space design of enhanced physical downlink control channel for long term evolution advanced system

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.

Investigation on Rate Matching and Soft Buffer Splitting for LTE-Advanced Carrier Aggregation

In the LTE-Advanced downlink, carrier aggregation (CA) employing multiple component carriers (CCs) is an essential technique to achieve a target peak data rate of 1 Gbps. 3GPP RAN WG1 meeting decided to adopt one transport block (TB) per CC, where the TB is the unit of channel coding and hybrid automatic repeat request (HARQ). When CA is applied to an LTE-Advanced UE, the soft buffer which is used for HARQ packet combining must be equally split according to the number of CCs, where the soft buffer size is one of the factors to in defining the LTE-Advanced UE categories. When soft buffer splitting is applied to UEs in a lower UE category, the soft buffer size per CC may not be sufficient and the performance of the HARQ packet combining is not ensured. In this paper, we propose a rate-matching scheme for CA and a new receiver suited to this rate matching scheme. Using the proposed scheme, degradation in the HARQ throughput performance due to the soft buffer splitting can be avoided and the same rate-matching scheme can be applied irrespective of the number of CCs. Simulations results show that the proposed scheme always provides better performance than that for equal splitting rate-matching scheme that reuses Rel-8 LTE rate-matching scheme.