Nobuhiko Miki

Performance of Advanced Receiver Employing Interference Rejection Combining to Suppress Inter-Cell Interference in LTE-Advanced Downlink

The interference rejection combining (IRC) receiver is effective in improving the cell-edge user throughput because it suppresses inter-cell interference. The IRC receiver is typically based on the minimum mean square error (MMSE) criteria, which requires channel estimation and covariance matrix estimation including the inter-cell interference with high accuracy. The paper investigates the gain from the IRC receiver taking into account the estimation of the interference signal, i.e., the covariance matrix, in terms of the downlink user throughput performance in a multi-cell environment. For the estimation of the covariance matrix, two estimation schemes are considered one based on data signals and the other based on the demodulation reference signal (DM-RS). In the evaluation, to assess the actual gains of the two schemes, the inter-cell interference signals from the surrounding 56 cells are actually generated in the same way as the desired signals including reference signals, and the channel propagation from all of the cells is explicitly taken into account considering pathloss, shadowing, and multipath fading. The simulation results when the inter-site distance is 500 m and the numbers of transmitter and receiver antennas are 2 and 2, respectively, show that the IRC receiver employing the covariance matrix comprising the interference and noise component estimation improves the cell-edge user throughput (defined as the 5% value in the cumulative distribution function) by approximately 22% compared to the simplified MMSE receiver that approximates the inter-cell interference as AWGN, while the IRC receiver employing the full covariance matrix estimation degrades the average user throughput due to less accurate channel and covariance matrices.

Performance evaluation of inter-cell interference coordination and cell range expansion in heterogeneous networks for LTE-Advanced downlink

In Long-Term Evolution (LTE)-Advanced, heterogeneous networks where femtocells and picocells are overlaid onto macrocells are extensively discussed in addition to traditional well-planned macrocell deployment to improve further the system throughput. In heterogeneous network deployment, combined usage of inter-cell interference coordination (ICIC) and cell range expansion (CRE) is very effective in improving the system and cell-edge throughput. In this combined usage, the fraction of the sets of user equipment (UEs) connected to the picocells, which are controlled through CRE, and that connected to macrocells affect the gain from the ICIC. Therefore, this paper evaluates the performance of ICIC and CRE in picocell deployments in the LTE-Advanced downlink. Simulation results (4 picocells and 30 UEs are located within 1 macrocell) assuming a full buffer model show that when the CRE offset value is set between 8 to 20 dB, almost the same user throughput performance is obtained by allocating the appropriate resources to protect UEs that connect to the picocells.

Evolved UTRA - physical layer overview

This paper presents the physical layer features and structures, along with the NTT DoCoMos proposals on solutions for the Evolved UTRA. After completion of study item investigations, outlines for physical channel structures were determined. The proposals for the orthogonal reference signal structures, multiplexing of the synchronization signal and broadcast channel appropriate to scalable multiple transmission bandwidths, the hierarchical synchronization signal structure, the downlink Layer 1 (L1) /Layer 2 (L2) control channel structure, and block-wise resource block-level distributed transmission for the shared data channel in the downlink are described. Other proposals such as purpose grouping for the random access channel in the uplink, the uplink L1/L2 control channel structure, and a link adaptation scheme for the shared data channel are also presented.

Investigation on Mobility Management for Carrier Aggregation in LTE-Advanced

In Long-Term Evolution-Advanced (LTE-A), which is currently in the process of standardization in the 3rd generation partnership project (3GPP), carrier aggregation (CA) was introduced as a main feature for bandwidth extension while maintaining backward compatibility with LTE Release 8 (Rel. 8). In the CA mode of operation, since two or more component carriers (CCs), each of which is compatible with LTE Rel. 8, are aggregated, mobility management for CCs such as inter/intra-frequency handover, CC addition, and CC removal is needed to provide sufficient coverage and better overall signal quality. Therefore, the signaling overhead for Radio Resource Control (RRC) reconfiguration for the mobility management of CCs in LTE-A is expected to be larger than that in LTE Rel. 8. In addition, CA allows aggregation of cells with different types of coverage, and therefore, the signaling overhead might be dependent on the coverage of each CC assumed in a CA deployment scenario. This paper presents evaluation results on the CC control overhead with several CC management policies in some CA deployment scenarios, and shows that the increase in the control overhead is not significant even in a CA deployment scenario with overlaid picocells.

Layered OFDMA Radio Access for IMT-Advanced

This paper proposes layered orthogonal frequency division multiple access (OFDMA) radio access for LTE (long-term evolution)-advanced to achieve higher-level system requirements than those in Release 8 LTE called Evolved UMTS terrestrial radio access (UTRA) and UMTS terrestrial radio access network (UTRAN). Layered OFDMA comprises layered transmission bandwidth assignment according to the required data rate, a layered control signaling structure, and support for layered environments in which an adaptive multi-access scheme with hybrid single-carrier and multicarrier based radio access is applied. layered OFDMA radio access will support all functionalities provided in Release 8 LTE and its enhancement. Key radio access techniques such as fast inter-cell radio resource management that take advantage of remote radio equipment (RRE) aiming at inter-cell orthogonality, multi-antenna transmissions with more antennas, and coverage enhancing techniques are used to achieve a high level of capacity and cell-edge spectrum efficiency.

Investigation on Interference Rejection Combining Receiver for Space–Frequency Block Code Transmit Diversity in LTE-Advanced Downlink

The interference rejection combining (IRC) receiver, which can strictly suppress intercell interference based on the minimum mean square error (MMSE) criteria, is effective in improving cell-edge user throughput. When assuming the Long Term Evolution (LTE) or LTE-Advanced downlink and open-loop transmit diversity employing the space-frequency block code (SFBC) using Alamouti coding, the IRC receiver must detect the Alamouti coded signals and suppress the interference signals using a couple of received signals in the frequency domain at the same time. To achieve this, the IRC receiver weight matrix, which consists of the channel matrix of the serving cell and the statistics of the covariance matrix, including the interference and thermal noise components, must be extended in the frequency domain, i.e., due to the effect of Alamouti coding, in addition to the spatial domain. These extended matrices can be estimated using the downlink reference signals (RSs) from the serving cell. However, some elements, including the effect of Alamouti coding in the extended covariance matrix, cannot be estimated using a practical estimation scheme that subtracts the replica symbols of the serving cell generated by the estimated channel matrix and the known RS sequence from the received RSs of the serving cell. This is because the RSs in LTE/LTE-Advanced are not transmitted using two adjacent subcarriers. This paper investigates the statistics of these unknown elements and proposes appropriate values, specifically inserting zero values, for these elements assuming the LTE/LTE-Advanced downlink. The results of simulations show that the IRC receiver using the proposed scheme, which has two receiver antenna branches, suppresses the intercell interference and improves the throughput by more than 10% compared with that for the conventional maximal ratio combining (MRC) receiver when a cell-edge environment is assumed.

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.

Multipath interference canceller using soft-decision replica combined with hybrid ARQ in W-CDMA forward link

This paper proposes a multipath interference canceller (MPIC) that employs soft-decision replica generation combined with hybrid ARQ (HARQ) to achieve a peak throughput of more than 10 Mbps in a multipath fading environment in the W-CDMA forward link. In the proposed scheme, more accurate multipath interference (MPI) replica generation is possible by decreasing the influence of data-decoding errors associated with the reliability of the originally transmitted and retransmitted packets. Simulation results elucidate that by applying the soft-decision replica, a four-stage MPIC improves the peak throughput by approximately 300 kbps and 1 Mbps with 64QAM data modulation in an average equal level 2- and 3-path Rayleigh fading channel compared with the MPIC using the hard-decision replica, respectively. In addition, by effectively combining the originally transmitted packet with the identical retransmitted one in soft-decision MPI replica generation, the throughput performance is further improved by approximately 300 kbps in a 3-path environment. Consequently, we clarify that the maximum peak throughput using the proposed MPIC achieves approximately 11.8 and 10.4 Mbps with 64QAM data modulation in a 2- and 3-path Rayleigh fading channel, respectively, within a 5 MHz bandwidth.

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 evaluation on effect of hybrid ARQ with packet combining in forward link for VSF-OFCDM broadband wireless access

This paper experimentally investigates the throughput performance employing hybrid automatic repeat request (ARQ) with a packet combining algorithm, i.e.. incremental redundancy and Chase combining, from the viewpoints of the achievable throughput satisfying the required residual packet error rate (PKR), coupled with real-time adaptive modulation and channel coding (AMC) operation, for variable spreading factor-orthogonal frequency and code division multiplexing (VSF-OFCDM) broadband wireless access in a forward-link multipath fading channel. Since the implemented base station (BS) and mobile station (MS) transceivers can achieve a high peak throughput of greater than 100 Mbps, the time diversity effect by packet combining or code combining in hybrid ARQ is thoroughly examined associated with the frequency diversity effect over a broadband channel and antenna diversity reception. The experimental results demonstrate that even in a six-path Rayleigh fading channel, incremental redundancy is superior to Chase combining under high Doppler frequency conditions such as f/sub D/ = 140 Hz, especially when the number of maximum retransmissions is small, because the larger coding gain after code combining is effective in compensating for packet errors due to inappropriate modulation and coding scheme (MCS) selection caused by the tracking error of the AMC loop for the fast fading variations. Furthermore, it is elucidated that the throughput values of 7 Mbps and 10 Mbps (achieved at the average received signal energy per symbol-to-background noise power spectrum density ratio (E/sub s/M/sub 0/) of approximately -5 dB) satisfying the required PER of below 10/sup -4/ are achieved when the maximum number of retransmissions is approximately 3 and 7, respectively, in the six-path fading channel with f/sub D/ = 20 Hz.