To support the rapid increase of mobile traffic, the LTE-based air interface is expected to be employed in the unlicensed spectrum known as “Licensed-Assisted Access (LAA).” The LAA terminal, which employs an LTE-based air interface, suffers from interference from WiFi access points as well as the LAA base station. The interference rejection combining (IRC) receiver, which employs a linear minimum mean square error (MMSE) filter, can suppress this interference from WiFi access points in addition to that of the LAA base station. The IRC receiver is effective, since it requires no knowledge of the interference, which is generally difficult to obtain for different systems. In this paper, we use a link-level simulation to evaluate the performance of the IRC receiver in suppressing the interference from WiFi access points, and show that the IRC receiver can effectively cancel the interference from WiFi systems as well as LTE systems, although we observed a slight performance degradation due to the covariance matrix estimation error caused by the WiFi interference fluctuation in the frequency-domain.

This paper describes the broadband radio access techniques for Universal Mobile Terrestrial Systems (UMTS)/Wideband Code Division Multiple Access (W-CDMA), High-Speed Downlink Packet Access (HSDPA)/High-Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and LTE-Advanced. Major technical pillars are almost identical regardless of the radio access systems of the respective generations. However, the key techniques that provide distinct performance improvements have changed according to the system requirements in each generation. Hence, in this paper, we focus on the key techniques associated with the system requirements. We also describe the requirements, radio access technology candidates, and challenges toward the future 5G systems.

Many research efforts are being focused upon the design of dynamic Inter-Cell Interference Coordination (ICIC) schemes for macrocell/picocell heterogeneous networks employing Cell Range Expansion (CRE). In order to protect the expanded Pico User Equipments (ePUEs) located in the CRE region from severe Macro Base Station (MBS) interference in downlink, the conventional methods reduce the transmit power of the MBS in the Almost Blank Subframes (ABSs), where ePUEs can be scheduled. However, this severely limits the amount of usable resources/power for the MBS as compared to Resource Block (RB)-based dynamic allocation. Instead, we propose a self-organized RB-based dynamic resource allocation method. Based on the proposed partial Channel State Information (CSI) sharing, the MBS obtains ePUEs' CSI and predicts their RB allocation. Then, the MBS reduces its transmit power in RBs where the ePUEs' allocation probability is estimated to be high. The simulation results show that the proposed scheme achieves excellent macrocell/picocell performance trade-offs, even when taking into account the overhead increase due to the partial CSI sharing.

Currently, many operators worldwide are deploying Long Term Evolution (LTE) to provide much faster access with lower latency and higher efficiency than its predecessors 3G and 3.5G. Meanwhile, the service rollout of LTE-Advanced, which is an evolution of LTE and a “true 4G” mobile broadband, is being underway to further enhance LTE performance. However, the anticipated challenges of the next decade (2020s) are so tremendous and diverse that there is a vastly increased need for a new generation mobile communications system with even further enhanced capabilities and new functionalities, namely a fifth generation (5G) system. Envisioning the development of a 5G system by 2020, at DOCOMO we started studies on future radio access as early as 2010, just after the launch of LTE service. The aim at that time was to anticipate the future user needs and the requirements of 10 years later (2020s) in order to identify the right concept and radio access technologies for the next generation system. The identified 5G concept consists of an efficient integration of existing spectrum bands for current cellular mobile and future new spectrum bands including higher frequency bands, e.g., millimeter wave, with a set of spectrum specific and spectrum agnostic technologies. Since a few years ago, we have been conducting several proof-of-concept activities and investigations on our 5G concept and its key technologies, including the development of a 5G real-time simulator, experimental trials of a wide range of frequency bands and technologies and channel measurements for higher frequency bands. In this paper, we introduce an overview of our views on the requirements, concept and promising technologies for 5G radio access, in addition to our ongoing activities for paving the way toward the realization of 5G by 2020.

This paper investigates an interference mitigation technique for dynamic time division duplex (TDD) based frequency-separated small cell networks in future long term evolution advanced (LTE-A) based wireless access systems. In dynamic TDD, cross-link interference, i.e. evolved node B (eNB)-eNB interference and user equipment (UE)-UE interference, also occur, and eNB-eNB interference in particular significantly degrades the uplink (UL) transmission performance. In order to alleviate the impacts of eNB-eNB interference and to obtain high traffic adaptation gain, we investigate a transmit power control (TPC) based interference mitigation (IM) scheme. In TPC-IM, time-domain subframes are divided into two subframe sets according to whether the cross-link interference can occur or not, and different TPC parameters are applied depending on the type of subframe. To improve of UL signal to interference plus noise power ratio (SINR) in the subframe set with the potential to occur eNB-eNB interference, there are two approaches of UL power boosting and downlink (DL) power reduction. We investigate the adequate combination of these two approaches to avoid an impact of DL performance degradation and increase of UE power consumption. Moreover, we further investigate a combined scheme of the TPC-IM and a cell clustering interference mitigation (CCIM) to avoid the significantly strong cross-link interference from the neighbouring cells. Computer simulation confirms that the proposed TPC-IM scheme can achieve 4.4% and 26.2% gain in the average DL and UL throughputs, respectively, compared to the case without any IM schemes on dynamic TDD. Moreover, when the CCIM is applied to the TPC-IM scheme, 11.6% and 40.3% gain can be achieved in the average DL and UL throughputs, respectively.

The performance in terms of the user separation of multi-user multiple-input multiple-output (MU-MIMO) depends on not only the spatial correlation but also the location of the mobile stations (MSs). In order to take into account the performance in terms of the user separation, we need to consider the granularity of the beam and null width of the precoded antenna pattern in addition to the spatial correlation to determine the base station (BS) antenna configuration. In this paper, we propose Smart Vertical MIMO (SV-MIMO) as the best antenna configuration that achieves both spatial correlation and granularity of the beam and null width of the precoded antenna pattern. We evaluate SV-MIMO in a field experiment using a downlink 4-by-2 MU-MIMO configuration focusing on the dependency of the location of the MSs in Yokosuka, Japan. The majority of the measurement course is under line-of-sight (LOS) conditions in a single cell environment. The MSs are almost uniformly set 30 to 60 degrees in azimuth and 12 to 30 degrees in elevation and the distance from the BS antennas is approximately 150m at maximum. We also evaluate the performance of 4-by-2 MU-MIMO using the conventional type of horizontal array antenna and show the difference. The field experimental results show that throughput of greater than 1Gbps is achieved at the Cumulative Distribution Function (CDF) of 14% by employing SV-MIMO for Rank-4 MU-MIMO. The throughput of SV-MIMO is 30% higher than that for the horizontal array antenna configuration at the CDF of 50%.

In LTE (Long Term Evolution)-Advanced system, a transparent MU-MIMO (Multi-User Multiple-Input Multiple Output) scheduling is basically considered, so the performance degradation in channel estimation may occur due to the unpredictable interference from co-scheduled layers. In order to detect and mitigate the interference, traditional binary hypothesis testing based interference detection method and iterative channel estimation method can be applied. However, there are two major problems. First, the binary hypothesis testing based interference detection is not suitable solution for LTE-Advanced system which has four dynamically changing interference hypotheses. Second, the conventional iterative operation does not guarantee sufficient performance gain with limited iteration time due to the estimation error in initial estimation stage. To overcome these problems, we introduce an enhanced iterative channel estimation method which considers simple matrix operation-based partial interference estimation. Based on the outcomes of the partial interference estimation, we can not only detect interference layers individually, but also partially eliminate the interference in initial channel estimation stage. Consequently, the proposed method can effectively mitigate the interference adaptively to the dynamically changing interference condition.

A D2D (Device-to-Device) communication system needs to cope with inter-cell interference and other types of interferences between cellular network and D2D links. As a result, macro user equipments, particularly those located near a cell edge, will suffer from serious link performance degradation. We propose a novel interference avoidance mechanism assisted by the SRN (Shared Relay Node) in this letter. The SRN not only performs data re-transmission as a typical type-II relay, but has several newly defined features to avoid interference between cellular network and D2D links. The superb performance by the proposed scheme is evaluated through extensive system level simulations.

This paper presents indoor experimental results on 4-by-2 multi-user (MU)-MIMO transmission with carrier aggregation (90-MHz bandwidth) achieving real-time 1-Gbps data transmission using adaptive modulation and coding (AMC) in the LTE-Advanced downlink employing OFDMA radio access. In the experiments, eigenvalue decomposition (EVD)-based channel state information (CSI) feedback based on subband unit for MU-MIMO operation and inter-user interference whitening filter applied before maximum likelihood detection (MLD) are employed to achieve such a high data rate with realistic numbers of transmitter and receiver antennas. The indoor experiments are conducted in a conference room under line-of-sight conditions with multiple reflected waves where one mobile station (MS) travels at walking speed and the other MS is static. The experimental results show that the total throughput for the 2 MSs is greater than 1Gbps at the average received signal-to-interference plus noise power ratio (SINR) of approximately 25 and 17dB for the first and second streams of each MS, respectively, when the moving speed is up to approximately 1km/h. The results also show that a centralized transmitter antenna arrangement is more effective in order to achieve a high data rate such as 1Gbps compared to a distributed antenna arrangement for the measurement environment.

In this letter, we propose an Adaptive Modulation and Coding (AMC) scheme with relay protocols, such as Amplify-and-Forward (AF), Decode-and-Forward (DF) and De-Modulate-and-Forward (DMF). We perform simulations based on 3GPP Long Term Evolution-Advanced (LTE-A) parameters to compare the performance of an adaptive Modulation and Coding Scheme (MCS) using relay protocols of AF, DF, and DMF with non-adaptive MCS, with the same relay protocols. We analyze the performance of the proposed scheme and observe how the proposed AMC scheme with DMF performs at various Signal to Noise Ratio (SNR) regions. The simulation results have shown that the performance of the proposed AMC scheme with relay protocols of DMF is much better at lower and a higher SNR regions and also provides higher average throughput.

LTE-Advanced supports asymmetric carrier aggregation (CA) to achieve flexible bandwidth allocation by applying different numbers of component carriers (CCs) between the downlink and uplink. This paper experimentally clarifies the achievable downlink throughput performance when uplink control information (UCI) feedback mechanism using the physical uplink shared channel (PUSCH), which enables minimization of the UCI overhead while maintaining the required reception quality, is applied in asymmetric CA. The laboratory experimental results show that the stable reception quality control of the channel quality information (CQI) with the target block error rate (BLER) of 10-1 to 10-2 is achieved irrespective of the average received signal-to-noise power ratio (SNR) when the control offset parameter of approximately 1.25 is used. We also show that the achievable downlink throughput when the CQI error is considered is almost the same as that in no CQI error case. Furthermore, based on the experimental results in a real field environment, a suburban area of Yokosuka city in Japan, we confirm stable adaptive modulation and coding (AMC) operation including target BLER control of the CQI on the PUSCH in asymmetric CA. The field experimental results also show that when CA with 5 CCs (90-MHz bandwidth) and 2-by-2 rank-2 multiple-output multiple-input (MIMO) multiplexing are employed in the downlink, the peak throughput of approximately 640Mbps is achieved even considering the CQI error.

In this study, we investigate the use of scheduling algorithms to support coordinated multipoint (CoMP) operation for Long Term Evolution (LTE)-Advanced systems studied in the 3rd Generation Partnership Project (3GPP). CoMP, which improves cooperative transmission among network nodes (transmission points: TPs) and reduces or eliminates interTP interference, enabling performance improvements in cell edge throughputs. Although scheduling algorithms in LTE systems have been extensively investigated from the single cell operation perspective, those extension to CoMP where each user equipment (UE) has multiple channel state information (CSI) feedbacks require further consideration on proportional fairness (PF) metric calculation while maintaining PF criteria. To this end, we propose to apply a scaling factor in accordance with the number of CSI feedbacks demanded for the UE. To evaluate the benefits of this scaling factor, multicell system-level simulations that take account of channel estimation errors are performed, and the results confirmed that our improved algorithm enables fairness to be maintained.

This work investigates the cell range expansion (CRE) possible with time-domain multiplexing inter-cell interference coordination (TDM ICIC) in heterogeneous cellular networks (HCN). CRE is proposed to enable a user to connect to a picocell even when it is not the cell with the strongest received power. However, the users in the expanded region suffer severe interference from the macrocells. To alleviate the cross-tier interference, TDM ICIC is proposed to improve the SIR of pico users. In contrast to previous studies on CRE with TDM ICIC, which rely mostly on simulations, we give theoretical analysis results for different types of users in HCN with CRE and TDM ICIC under the Poisson Point Process (PPP) model, especially for the users in the expanded region of picocells. We analyze the outage probability and average ergodic rate based on the connect probability and statistical distance we obtain in advance. Furthermore, we analyze the optimal ratio of almost blank subframes (ABS) and bias factor of picocells in terms of the network fairness, which is useful in the parameter design of a two-tier HCN.

Bandwidth expansion in Long Term Evolution (LTE)-Advanced is supported via carrier aggregation (CA), which aggregates multiple component carriers (CCs) to accomplish very high data rate communications. Heterogeneous networks (HetNets), which set pico-base stations in macrocells are also a key feature of LTE-Advanced to achieve substantial gains in coverage and capacity compared to macro-only cells. When CA is applied in HetNets, transmission on all CCs may not always be the best solution due to the extremely high levels of inter-cell interference experienced by HetNets. Activated CCs that are used for transmission should be selected depending on inter-cell interference conditions and the traffic offered in the cells. This paper presents a scheme to select CCs through centralized control assuming a centralized baseband unit (C-BBU) configuration. A C-BBU involves pooling tens or hundreds of baseband resources where one baseband resource can be connected to any CC installed in remote radio heads (RRHs) via optical fibers. Fewer baseband resources can be prepared in a C-BBU than those of CCs in RRHs to reduce the cost of equipment. Our proposed scheme selects the activated CCs by considering the user equipment (UE) assigned to CCs under the criterion of maximizing the proportional fairness (PF) utility function. Convex optimization using the Karush-Kuhn-Tucker (KKT) conditions is applied to solve the resource allocation ratio that enables user throughput to be estimated. We present results from system level simulations of the downlink to demonstrate that the proposed algorithm to select CCs can outperform the conventional one that selects activated CCs based on the received signal strength. We also demonstrate that our proposed algorithm to select CCs can provide a good balance in traffic load between CCs and achieve better user throughput with fewer baseband resources.

The interference rejection combining (IRC) receiver effectively improves the cell-edge user throughput by suppressing interference from the surrounding cells. The work item (WI) for the specification of the IRC receiver is now ongoing for Release 11 Long-Term Evolution (LTE)-Advanced. Furthermore, heterogeneous networks where low power nodes such as picocells are overlaid onto macrocells are important to further improve the system throughput per unit area. In heterogeneous networks, to achieve an offloading gain from macrocells to picocells, cell range expansion (CRE) is applied. Additionally, inter-cell interference coordination (ICIC) is applied to reduce the severe inter-cell interference imposed from the macrocells onto the sets of user equipment (UEs) connected to picocells. In such cases, the interference statistics are completely different from traditional well-planned macrocell deployments, which have been investigated for the IRC receiver. This paper clarifies the effect of the IRC receiver in a heterogeneous network employing CRE and ICIC. Simulation results show that when both CRE and ICIC are applied, the effect of the IRC receiver becomes small due to a reduction in the severe inter-cell interference from ICIC. However, we clarify that the user throughput gain at the cumulative distribution function of 5% from the IRC receiver exceeding 10% is achieved compared to the conventional minimum mean square error (MMSE) receiver in a heterogeneous network regardless of the usage of ICIC. Furthermore, in heterogeneous networks employing CRE and ICIC, we clarify that an average user throughput gain exceeding 5% is achieved from the IRC receiver and the improvement in the average user throughput is high especially for the UEs connected to picocells compared to UEs connected to macrocells.

This paper evaluates the downlink performance of an LTE-Advanced (LTE-A) heterogeneous network that uses carrier aggregation (CA) between macro and small cells. The concept of utilizing the CA functionalities in LTE-A is effective in increasing the network capacity in a congested area through raising of the base station density using small cells overlaid onto an existing macro cell network. This concept is also effective in maintaining the mobility performance of user equipment (UE) because handover operation is not applied when entering/leaving a small cell, but component carrier addition/removal is only performed through CA while maintaining the connection to a macro cell. In order to present comprehensive performance evaluations in an LTE-A heterogeneous network with CA, this paper evaluates various performance criteria, e.g., downlink cell throughput and downlink user throughput. According to the obtained simulation results, the total downlink cell throughput achieved in an LTE-A heterogeneous network deployment with CA (four small cells overlaid onto a macro cell sector) exhibits a 3.9-fold improvement compared to a conventional-macro-cell-only network deployment using two frequency bands.

In Long-Term Evolution (LTE)-Advanced, heterogeneous networks are important to further improve the system throughput per unit area. In heterogeneous network deployment, low power nodes such as picocells are overlaid onto macrocells. In the downlink, the combined usage of inter-cell interference coordination (ICIC), which is a technique that reduces the severe interference from macrocells by reducing the transmission power or stopping the transmission from the macrocells, and cell range expansion (CRE), which is a technique that expands the cell radius of picocells by biasing the received signal power, is very effective in improving the system and cell-edge user throughput. In this paper, we consider two types of ICIC. The first one reduces the transmission power from the macrocells (referred to as reduced power ICIC) and the second one stops the transmission from the macrocells (referred to as zero power ICIC). This paper investigates the impact of the reduction in the transmission power when using reduced power ICIC and the restriction on the modulation scheme caused by the reduction in the transmission power when using reduced power ICIC on the user throughput performance with the CRE offset value as a parameter. In addition, the throughput performance when applying reduced power ICIC is compared to that when applying zero power ICIC. Simulation results show that the user throughput with reduced power ICIC is not sensitive to the protected subframe ratio compared to that with zero power ICIC even if the modulation scheme is restricted to only QPSK in the protected subframes. This indicates that reduced power ICIC is more robust than zero power ICIC for non-optimum protected subframe ratios.

In order to support the increasing amount of mobile data traffic, Third Generation Partnership Project (3GPP) is actively discusses cell range expansion (CRE) and time domain multiplexing – inter-cell interference coordination (TDM-ICIC). They have shown to be attractive techniques for heterogeneous network (HetNet) deployment where pico base stations (BSs) overlay macro BSs. There are two control schemes of the TDM-ICIC. One, named ZP-scheme, stops radio resource assignments for data traffic in predetermined radio resources in the time domain (subframes). The other, named RP-scheme, maintains the resource assignment whereas it reduces the transmission power at macro BSs at predetermined subframes. In this paper, we clarify the effective ranges of both ZP-scheme and RP-scheme by conducting the system level simulations. Moreover, the appropriate power reduction value at predetermined subframes is also clarified from the difference in the effective range of various power reduction values. The comprehensive evaluation results show that both ZP-scheme and RP-scheme are not effective when the CRE bias value is 0 dB or less. If the CRE bias value is larger than 0 dB, they are effective when the ratio of predetermined subframes in all subframes is set to appropriate values. These values depend on the CRE bias value and power reduction in the predetermined subframes. The effective range is expanded when the power reduction in the predetermined subframes changes with the CRE bias value. Therefore, the effective range of RP-scheme is larger than that of ZP-scheme by setting an appropriate power reduction in the predetermined subframes.

Heterogeneous networks (HetNets) can provide higher capacity and user throughput than homogeneous networks in Long Term Evolution (LTE)-Advanced systems. However, because of increased interference from neighboring cells and the characteristics of the embedded small cells, handover performance is impacted adversely, especially when the user equipment (UE) moves at medium or high speeds. In this paper, to improve mobility performance, we propose two schemes, i.e., 1) using wideband signal-to-interference noise ratio (SINR) as the handover metric and 2) emergency attaching. The schemes can enhance mobility performance since handovers are performed based on the quality of the radio link. Importantly, the two schemes compliment rather than contradict each other. System-level simulations show that both the individual proposed schemes and the joint schemes can improve mobility performance significantly.

In Long-Term Evolution (LTE)-Advanced, heterogeneous networks where femtocells and picocells are overlaid onto macrocells are being extensively discussed in addition to traditional well-planned macrocell deployment to improve further the system throughput. In heterogeneous networks, cell range expansion (CRE), which is a technique for expanding the cell radius of picocells by biasing the handover criteria, e.g., the downlink received signal power, is applied so that the UEs will more frequently select the picocells. This paper investigates a fractional transmission power control (TPC) method suitable for the heterogeneous networks that employ CRE in the LTE-Advanced uplink and evaluates the cell-edge user throughput and cell throughput performance. Simulation results (2-8 picocells and 25 (30) UEs are located within one macrocell with a uniform (cluster) distribution, the difference in transmission power between the macro and picocells is 16 dB, and the Typical Urban and Pedestrian-A channel models are employed) show that almost the same cell-edge user throughput is obtained by setting an appropriate difference in the target received signal power between the macro and picocells according to the CRE offset value.