This paper presents outdoor field experimental results in the 28-GHz frequency band of a 5G radio access system where base stations (BSs) are installed on buildings to deploy a 5G trial area in the Tokyo Odaiba waterfront area. This is an early field trial assessment of key 5G features such as BS and mobile station (MS) beam tracking, being deployed in dense urban areas. In the trial area, a variety of key 5G technologies and conditions are evaluated such as BS and MS massive multiple-input multiple-output (MIMO) antennas, beamforming and tracking, use of the high 28-GHz frequency, the wide frequency bandwidth of 400 MHz, intra-baseband unit (BBU) hand over (HO), and a coverage area in a realistic urban deployment with four different trial courses. Experimental results show that throughput exceeding 1 Gbps is achieved in the experiment area, and approximately 200 Mbps is achieved at 500 m away from the BS. We confirm that beam tracking and intra-BBU HO operate effectively while compensating for high path loss in the 28-GHz band to achieve a coverage area of 500 m. On the other hand, line of sight (LoS) and Non-LoS conditions are critical to 5G performance in the 28-GHz band, and we find that 5G connections may drop behind trees, buildings, and/or under a footbridge. The trial results show that the downlink throughput exceeds 50 Mbps in 85%, 50%, 70%, and 33% of the four different trial courses, respectively.

This paper presents outdoor experimental results with a 4 layer single-user (SU)-multiple-input multiple-output (MIMO) 5G cellular radio access system. Beam tracking with narrow beams are evaluated in the 15-GHz frequency band in order to investigate the effects of user mobility. In addition, distributed MIMO beamforming functionality is implemented to extend coverage using multiple transmission points (TPs) supporting 2 layers each. The experimental result in an outdoor open space environment shows that the peak throughput is 12.1 Gbps on a public road at an average speed of approximately 10 km/h. The result also shows that more than 8 Gbps is achieved at an average speed of approximately 40 km/h. Furthermore, it is observed that throughput is remarkably degraded when the difference in the received power between the best and second best transmission point is larger than 15 dB and only 2 layer spatial multiplexing is supported from one of the transmission points.

This paper presents indoor and outdoor experimental results on the achievable throughput for downlink 8-by-16 multi user (MU)-multiple-input multiple-output (MIMO) using 2 sets of user equipment (UE) and carrier aggregation with 8 component carriers in various measurement areas for 5G radio access. In the presence of four centralized antenna units, the throughput performance is evaluated using the implemented 5G testbed for MU-MIMO operation. The experimental results show that the peak system throughput of 18.6 Gbps and 25.9 Gbps are achieved in indoor and outdoor environments. Furthermore, we confirm that to avoid interference a vertical angle exceeding 18 deg. or a horizontal angle exceeding 10.7 deg. is required between UEs to achieve the throughput of 12 Gbps for a UE when employing MU-MIMO in an outdoor open space.

This paper presents indoor and outdoor experimental results on 4-by-8 single-user (SU)-multiple-input multiple-output (MIMO) multiplexing based on 730.5 MHz bandwidth transmission when applying carrier aggregation (CA) with 8 component carriers (CCs) in a 15 GHz frequency band in the downlink of 5G cellular radio access. Beam tracking with massive MIMO is implemented in a 5G testbed to support user mobility with a narrow beam. In addition, channel state information (CSI) acquisition functionality is implemented for the scheduler to select appropriate beams. Experimental results in an indoor multi-path rich environment show the gain up to 34 % with CSI acquisition compared to the case of without CSI acquisition in a line-of-sight (LoS) environment with high rank MIMO transmission. The results also show the gain up to 55 % with CSI acquisition compared to the case of without CSI acquisition in LoS conditions due to reflections by buildings.

This paper presents outdoor field experimental results that clarify 4-by-8 multiple-input multiple-output (MIMO) in the downlink throughput performance when applying distributed MIMO in the 28 GHz frequency band of a 5G cellular radio access system. A variety of transmission point (TP) deployments with narrow antenna beam forming is evaluated in order to investigate the downlink and uplink throughput performance and appropriate TP deployment for distributed MIMO. The experimental results show that the throughput is limited to approximately 7 Gbps in the downlink and 1 Gbps in the uplink in a parking area with localized MIMO. A distributed MIMO throughput gain is achieved with various TP deployments for both the downlink and uplink using two radio units. The outdoor parking area is an open space and multipaths are not anticipated, so a distinct advantage from distributed MIMO is expected. The distributed MIMO gain of 60% in the downlink and from 50 to 200% in the uplink is achieved at the cumulative distribution function of 80%. As a result of implementing distributed MIMO, throughput exceeding 10 Gbps in the downlink and 1.9 Gpbs in the uplink is achieved in the 28-GHz frequency band.

This paper presents indoor and outdoor field experimental results that clarify 4-by-8 multiple-input multiple-output (MIMO) downlink throughput performance applying distributed MIMO with a variety of transmission point (TP) deployments and narrow antenna beam tracking in the 15 GHz frequency band of a 5G cellular radio access system in order to investigate the appropriate TP deployment for distributed MIMO. Experimental results show that a throughput gain from distributed MIMO is achieved in various TP deployments in the lobby of an office building and an outdoor parking area using two radio units (RUs). In the lobby, a deployment where TP #1 covers the entire lobby and TP #2 covers areas around TP #1 that are non-line-of-sight exhibits a significant advantage over when distributed MIMO is not used yielding a throughput gain of 11 % over most of the area. In the outdoor parking area, a distinct advantage when using distributed MIMO is expected, and a gain in throughput of 124 % is achieved when TPs face the same direction and have a large separation between them over when distributed MIMO is not used. However, a large difference in the mobility reference signal received power (MRSRP) is also confirmed, especially in front of the TPs due to the large difference in the distance between the mobile station and the TPs. We find that if the second TP has a signal power below 14 dB of the first TP's signal, it does not contribute to the MIMO transmission. In order to compensate for the difference in MRSRP, four RUs are installed in the parking area, and the distributed MIMO gain of 77 % is achieved.

This paper presents measurement results from different deployments with a 5G test-bed on a 15-GHz frequency band. The beam gain with a grid-of-beam solution applied to an 8x8 element antenna array is compared with a reference wide-beam power equivalent antenna. The beamforming gain in outdoor environment is found to be large, it is in the range of 10-13 dB in line-of-sight (LoS) and 7-12 dB in non-LoS. In a reflective and rich indoor environment, the gain is as expected lower but still substantial, 5-11 dB. The potential of a hybrid analog-digital solution is also assessed as an upper bound with perfect phase coherent combination of several beams. To reach an average beamforming gain of 14 dB it is sufficient with the combination of three beams in outdoor LoS, while eight beams are required in the indoor scenario.

This paper presents indoor experimental results showing the achievement of 14.5 Gbps throughput performance based on 730.5 MHz bandwidth transmission when applying carrier aggregation (CA) with 8 component carriers (CCs) and 4-by-8 single-user multiple-input multiple-output (MIMO) multiplexing in the 15 GHz frequency band in the downlink of 5G cellular radio access. Beam tracking with massive MIMO is implemented in a 5G testbed to support user mobility with a narrow beam. Experimental results in an indoor multi-path rich environment show that the peak throughput is 14.5 Gbps in a line-of-sight (LoS) environment with high mobility-reference signal received power (MRSRP) and low antenna correlation. The results also show that 12.5 Gbps is achieved behind a wall in non-LOS conditions due to ceiling reflections.

We propose a trial concept that utilizes advanced multi-antenna solutions such as beamforming and spatial multiplexing with a massive number of antenna elements for use in a previously established trial system for the fifth generation (5G) mobile broadband communications system. Furthermore, we present experimental results showing that multi-user aggregated data rates exceeding 20 Gbps are possible in a real field environment.

5G is the next step in the evolution of mobile communication and a key component of the future networked society. With capabilities such as massive system capacity, higher data rates, very low latency and ultra-high reliability, 5G will provide significantly enhanced mobile-broadband experience but also support a wide range of new wireless applications and use cases. In this paper, a trial system for 5G mobile broadband operating at 15 GHz is outlined, together with initial experimental results showing that data rates of many Gbit/s are possible.

This paper presents outdoor field experimental results showing the achievement of 3-Gbps throughput performance based on 400-MHz bandwidth transmission when applying carrier aggregation with 4 component carriers and 4-by-4 single-user multiple-in multiple-out multiplexing in the 15-GHz frequency band in the downlink of 5G cellular radio access. A new radio interface with time division duplexing and radio access based on orthogonal frequency-division multiplexing is implemented in a 5G testbed to support ultra-high speed transmission with low latency. Experimental results in an outdoor open-space parking area show that the peak throughput is 2.8 Gbps in front of a base station (BS) antenna with a high reference signal received power (RSRP) although rank 2 is selected due to the high antenna correlation. The results also show that the average throughput of 2 Gbps is achieved 120 m from the BS antenna. In a courtyard enclosed by building walls, 3.6 Gbps is achieved in an outdoor-to-outdoor environment with a high RSRP and in an outdoor-to-indoor environment where the RSRP is lower due to the penetration loss of glass windows, but a multipath rich environment contributes to actualizing a low antenna correlation.

5G is the next step in the evolution of mobile communication and a key component of the future networked society. With capabilities such as massive system capacity, higher data rates, very low latency and ultra-high reliability, 5G will provide significantly enhanced mobile-broadband experience but also support a wide range of new wireless applications and use cases. In this paper, a trial system for 5G mobile broadband operating at 15 GHz is outlined, together with initial experimental results showing that multi-Gbps data rates are possible.

This paper presents the link-level performance of downlink non-orthogonal multiple access (NOMA) with successive interference cancellation (SIC) receiver, considering the influence of error vector magnitude (EVM). In our evaluations, for the case of a 1-by-2 transmission and the number of NOMA multiplexed users is two, we clarify the performance of NOMA for cell-center user equipment (UE) with codeword level SIC (CWIC) receiver and the performance of the cell-edge UE, which does not apply SIC. Based on link-level performance evaluations, we show that the performance of CWIC receiver is almost the same as that of ideal SIC when the allocated transmit power ratio to cell-center UE is less than 0.35, the modulation and coding scheme (MCS) of cell-center UE is 16QAM (R = 0.51) or 64QAM (R = 0.47), and the MCS of cell-edge UE is QPSK (R = 0.51). Furthermore, we also show that, when EVM value is 4 % and the allocated power ratio to cell-center UE is between 0.2 and 0.37, the level of degradation due to the influence of EVM is below 0.6 dB, which is almost the same level of degradation as that of orthogonal frequency division multiple access (OFDMA) for the EVM value required by 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). On the other hand, we show that the influence of EVM is marginal for cell-edge UE compared to cell-center UE. However, when the cell-center UE uses a higher order modulation such as 64QAM, we show that the required SNR of cell-edge UE increases due to the increase in inter-user interference. Based on these results, we clarify that it is important for the base station (BS) to take into account the target BLER and influence of EVM when selecting the MCS and allocated transmit power ratio of NOMA UEs.

Non-orthogonal multiple access (NOMA) is one of the promising radio access techniques for further cellular enhancements toward 5G. Compared to orthogonal multiple access (OMA) such as orthogonal frequency-division multiple access (OFDMA), large performance gains were confirmed via system-level simulations. However, NOMA link-level simulations and the design of the receiver remain of great importance to validate NOMA performance gains. In this paper, we evaluate downlink NOMA link-level performance with multiple receiver designs and propose a novel NOMA transmitter and receiver design, where the signals of multi-users are jointly modulated at transmitter side and detected at receiver side. The predominant advantage of the proposed scheme is that at receiver side interference cancellation to the interference signal is not needed, thus low complexity is achieved. The performances of codeword-level SIC, symbol-level SIC and the proposed receiver are evaluated and compared with ideal SIC. Simulation results show that compared with ideal SIC, downlink NOMA link-level performance depends on actual receiver design and the difference in the power ratio split between the cell edge user and cell center user. In particular, it is shown that codeword-level SIC and the proposed receiver can both provide a good performance even when the power ratio difference between the cell center user and cell edge user is small and with real channel estimation.

Non-orthogonal multiple access (NOMA) is a promising radio access technique for LTE release 13 and beyond. In this paper, we focus on the investigation of the system performance for uplink NOMA with an advanced successive interference cancellation (SIC) receiver applied in the base station side. In NOMA, more than one user can be multiplexed simultaneously in the same frequency bandwidth, which demands multi-user scheduling. In our study, we propose an enhanced proportional fair (PF) based scheduling scheme for non-orthogonal multiplexed users with contiguous resource allocation to retain the SC-FDM property. In order to reduce the scheduling complexity for non-orthogonal multiple access, a greedy consecutive resource allocation method is adopted. Moreover, since the interference condition becomes more complicated in NOMA, we consider the application of fractional frequency reuse (FFR) to NOMA to further enhance the performance of cell-edge users. In simulation results, the system performance of NOMA by using the proposed PF-based scheduling algorithm is evaluated. The results show that NOMA significantly enhances the uplink system performance compared to the conventional orthogonal multiple access.

This paper investigates inter-cell interference mitigation of non-orthogonal multiple access (NOMA) in the cellular downlink. A novel adaptive power and frequency resource allocation algorithm in the NOMA system is proposed for performance enhancement. The objective of the algorithm is to improve both the average cell throughput and cell-edge throughput through adaptive resource allocation in the power and frequency domains. The proposed algorithm improves the cell-edge throughput by providing better protection against inter-cell interference in the power domain while increasing the average cell throughput by providing more resources in the frequency domain. Numerical results show that the proposed algorithm yields significant improvements in the cell average throughput and cell-edge throughput by reducing the interference for cell-edge users and expanding the bandwidth for cell interior users.

Non-orthogonal multiple access (NOMA) utilizing power domain and advanced receiver is a promising technique for future radio access. Multi-antenna techniques have been deeply studied during the past decade to boost system performance by virtue of spatial multiplexing and/or spatial diversity. It is of great interest to study how to efficiently and effectively combine the two techniques together for further system performance improvement. This paper investigates on the combination of NOMA with opportunistic beamforming (OBF), which has been proposed as a practical multi-antenna transmission scheme with good performance and very limited feedback. The key design issues of NOMA with OBF are investigated including precoding, transmission power assignment, user feedback and scheduling. Furthermore, a method to improve the system performance with OBF is proposed by utilizing historical information. The performance of NOMA with OBF is investigated by system-level simulations with very practical assumptions and parameters in order to provide insights into the suitability of combining OBF with NOMA for future radio access. Simulation results show the proposed method is effective and NOMA with OBF is a promising scheme for future wireless communication systems.

In order to satisfy the increasing traffic requirement, dynamic Time Division Duplexing (TDD) Downlink/Uplink (DL/UL) time configuration is introduced to provide a more flexible service and improve the system performance. In a dynamic TDD system, the DL/UL time configuration can be changed based on the traffic load. Since Hybrid Automatic Repeat Request (HARQ) timing may become misaligned in dynamic TDD system, HARQ timing and feedback signalling design need to be considered for dynamic TDD system. In this paper, HARQ timing issue in LTE system is investigated and novel HARQ feedback signalling design schemes for dynamic TDD system are proposed. Performance evaluation and numerical analysis are provided to show the effectiveness of the proposed schemes. By using the proposed schemes, the feedback signalling overhead can be reduced and spectrum efficiency can be improved.

Coordinated Multi-Point (CoMP) transmission is a key technology for LTE-Advanced (LTE-A) to increase the downlink cell-edge throughput. Generally speaking, multi-cell channel state information (CSI) is needed at multiple coordinated eNBs to achieve the performance gain promised by CoMP in downlink. Sounding reference signal (SRS) could be an efficient tool to enable multi-cell CSI acquisition for time division duplex (TDD) LTE-A (TD-LTE-A) systems because of the inherent channel reciprocity in TDD systems. In this study, we investigate multi-cell SRS coordination schemes for TD-LTE-A downlink CoMP. We introduce a CoMP-specific sounding zone that facilitates multi-cell SRS coordination and then propose and compare three coordinated SRS resource allocation schemes within the CoMP-specific sounding zone, i.e., cyclic shift (CS) coordination, comb coordination, and orthogonal cover code (OCC) for the SRS. Computer simulations show the effectiveness of multi-cell SRS coordination and its necessity is revealed by the sum-rate analysis of TD-LTE-A downlink CoMP.

In Long Term Evolution (LTE) networks, two operators might deploy their networks based on frequency division multiplexing (FDD) and time division multiplexing (TDD) in the same geographical area utilizing adjacent frequency bands. In this kind of an environment, the mutual interference, also called adjacent channel interference (ACI), caused by the coexisting adjacent channels of those networks will result in capacity degradation in both systems. In this paper, we investigate the coexistence problem between two macrocells in LTE networks and propose a power allocation-based solution comprising two methods for the base station (BS)-to-BS ACI scenario. Compared to the guard band (GB)-based solution, the proposed methods offer higher spectrum efficiency and even better performance when the GB is equal to or narrower than 10 MHz.

Coexistence research for LTE systems is indispensable to ensure that the time division duplex (TDD) Long Term evolution (LTE) system can coexist with the frequency division duplex (FDD) LTE system operating at the adjacent frequency band within the same geographical area. Previous studies on this topic only focus on the throughput loss of the victim system from the data channel viewpoint. In this paper, the performance of control channels (CCHs) in the typical coexistence scenario is investigated, since the performance of communication systems is largely dependent on the quality of CCHs. The CCHs' interferences are analyzed in detail, including the coexistence issue for CCHs, Co-Channel Interference (CCI) analysis, and Adjacent Channel Interference (ACI) on CCHs. The simulation results indicate that the ACI generated by the Base Station (BS) of collocated LTE system degrades the performance of the uplink (UL) CCHs of the victim LTE system significantly.

In this paper, we investigate the feasibility of applying interference alignment (IA) technology to multi-cell coordinated beamforming (CB) in a future release of the Long Term Evolution (LTE) standard. Specifically, we establish a link level model for a two-cell system according to the current LTE- Advanced (LTE-A) specifications, and evaluate the performance of an IA based CB scheme. The link-level performance of the IA based CB scheme is compared to that of conventional coordination schemes to identify the potential gain. We establish different simulation configurations to identify the factors that restrict the practical deployment of the IA based CB scheme and the potential requirements for extra standardization support.

In wireless communications, when multiple radio access networks operate in adjacent frequency bands, there is the potential for adjacent channel interference (ACI). In this paper, we investigate the ACI problem in coexisting frequency division duplex and time division duplex systems. Inserting a guard band (GB) is a straightforward way to avoid the performance degradation due to ACI; however, this method sacrifices spectrum efficiency. To overcome this shortcoming, we propose a novel frequency spectrum allocation scheme to mitigate the ACI. The proposed scheme effectively avoids the ACI from base stations to relay nodes. We verify the effectiveness in a 3G Long Term Evolution-Advanced network coexistence scenario for the 2.6-GHz band. Numerical results show that the proposed scheme significantly improves the performance by avoiding the interference even compared to the GB scheme, and greatly improves the efficiency in the spectrum resource utilization.

This paper investigates low complexity coordinated beamforming (CB) schemes that adopt interference alignment (IA). The majority of existing IA CB schemes rely on iterative solutions that have drawbacks such as sensitivity to initialization conditions, high computational complexity, and infeasible system requirements. Low-complexity non-iterative solutions only exist for specific scenarios. In this paper, we first propose an IA scheme for a new scenario. Secondly, we discuss low complexity diversity enhancement methods which can be applied on top of any IA method to form the final IA CB scheme. The proposed scheme can achieve high multiplexing gain via IA while improving the capacity of the conventional IA schemes at the low signal-to-noise-ratio (SNR) regime.

This paper considers the problem of adjacent channel interference between two different operators using different duplexing methods. Focusing on the interference between two colocated base stations, which existing guard-band based methods still cannot solve efficiently, we propose a scheme that adjusts the downlink power of the aggressor system to maximize the downlink transmission capacity while keeping the interference introduced to the victim system in a tolerable range. In addition to the power allocation scheme, a downlink user allocation scheme is proposed to improve further the downlink capacity. Simulations on 2.6-GHz systems show that the proposed schemes enable coexistence of two co-located base stations, and that the frequency spectrum is utilized more efficiently.

In relay systems, the quality of the received signal at a mobile user is quite different from that at a relay node making it a good application scenario for hierarchical modulation. In conventional multi-user transmission schemes for relay systems where hierarchical modulation is applied, high level power is always assigned to the same mobile user, at both the source and relay node. The performance of one mobile user is always superior to that of the other; therefore, the performance of mobile users is unbalanced. In this paper, we propose a novel multiuser transmission scheme that implements different hierarchical modulation signal in a relay node to achieve balanced bit error rate performance among the mobile users. The strategy we employ is that when high power is allocated to a mobile user at a base station, low power is allocated to the hierarchical modulation signal of the same mobile user at the relay node. Hence, the strengths of the signals at different mobile users, which are a combination of the signals from the base station and relay node, are comparable. Then mobile users can achieve excellent performance simultaneously, and a balanced performance can be obtained by using proposed scheme. Simulation results show effectiveness of proposed scheme.

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.

This paper proposes a novel frame structure for Orthogonal Frequency Division Multiplexing (OFDM) based broadband wireless access and its application. The proposed frame structure, which is robust against inter-symbol interference (ISI) caused by a very large delay spread of multi-paths, is applicable to ultra-large-cell environments where the cell radius is comparable to that for broadcast services without changing key parameters of the OFDM frame such as the guard interval duration and symbol duration, which are optimized for cellular networks. Simulation results show that the proposed method has an advantage compared to the conventional OFDM frame structure when the delay spread is extremely large, i.e., excess delay exceeds the guard interval duration, and the ISI is the dominant factor in degrading the performance.

We propose a low complexity synchronization method for the 3rd Generation Partnership Project (3GPP) long term evolution (LTE) mobile communication systems. A low complexity detection window, synchronization state machine, and two-stage detection are used to lower the computational complexity. The positions of the detection window and received samples are recorded, and the position and length of the detection window are controlled by the state machine. The positions are compared to decide if the received sample is useful for synchronization signal detection. Such techniques avoid unnecessary computations especially when implementing a receiver on a software defined radio (SDR) platform with general-purpose processors. We analyze the computational complexity of the proposed method and evaluate its performance based on simulations and using a Universal Software Radio Peripheral (USRP)-based SDR platform. The evaluation shows that the proposed method provides reliable and real-time synchronization signals on the considered SDR platform.

This paper investigates the optimization of precoders for uplink multi-user wireless communication systems, where the transmitters and receiver are equipped with multiple antennas. It is assumed that the centralized receiver has perfect knowledge of each uplink channel and designs the precoders for all users. The precoders are quantized to the vectors of a codebook, and the precoder vector index (PVI) is sent to each user through a limited channel. Since interference exists among users, the traditional single-user optimization-based precoding scheme exhibits poor performance. To improve the multi-user uplink sum-rate performance, a scheme incorporating an iterative multiple user PVI selection algorithm is proposed that iteratively projects one user channel into the null space of the precoded channels of other users, and obtains the PVI from the projected space. Simulation results show significant performance improvement compared to the conventional scheme. We also show that the proposed scheme achieves performance near to that for an exhaustive search, and outperforms the scheme that directly quantizes the precoders calculated from un-quantized joint optimization algorithms.

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.

Cross subframe scheduling is introduced to improve spectrum efficiency in co-channel heterogeneous network (Het-Net) scenarios. In practical protocol development, the design of control channel signaling, for instance, how to determine the subframe carrying control channel, plays an important role for efficient resource usage. This paper proposes a novel method to determine the subframe carrying the downlink Hybrid Automatic Repeat-request (HARQ) feedback channel in HetNets. In the proposed method, we use a threshold-based iterative algorithm and distribute the HARQ information to all available downlink subframes to reduce the blocking probability. Simulation results show that the proposed method has a large advantage, e.g., more than 50 percent improvement in some cases, from the viewpoints of the Physical Hybrid-ARQ Indicator Channel (PHICH) blocking probability and control channel performance.

3GPP Long Term Evolution (LTE) is the next generation network technology for beyond 3G. In the LTE system deployment, there may not be much separation between the frequency bands assigned to operators. Therefore, adjacent channel interference analysis between two coexisting LTE systems is a potentially important research field. In this paper, we investigate the coexistence problem between a macrocell and small cells, i.e., microcells and picocells, in LTE networks. We focus on analyzing the adjacent channel interference (ACI) between two coexisting systems with different duplex modes, i.e., frequency division multiplexing and time division multiplexing, deployed in adjacent frequency bands. Main contribution of this paper is to present the coexistence results of two systems by evaluating the macrocell performance degradation caused by ACI when it operates with microcell/picocell on adjacent channel in some typical interference scenarios. Based on comparison and analysis of simulation results, we find that in some scenarios two LTE systems could not coexist well under current LTE radio frequency requirements. We also present some suggestions and mitigation technologies to avoid large throughput loss due to ACI.

We investigate the problem of two-cell multi-user co-channel transmission, wherein both inter-cell interference and intra-cell inter-user interference should be well managed. Interference alignment (IA) is theoretically proved to be a promising technology for managing co-channel interference. It can be used to mitigate inter-cell interference. When multiple users are served by a single cell, advanced multi- user multiple-input multiple output (MU-MIMO) transmission can be used to manage the intra-cell inter-user interference. In this paper, we present an efficient two-cell coordinated linear beamforming method where IA is combined with MU-MIMO to achieve high performance multi-cell cooperative transmission. The effectiveness of the proposed method is verified by numerical evaluations.

Heterogeneous network (HetNet) is considered to be a promising approach to improve spectrum efficiency per area in a Long Term Evolution-Advanced (LTE-A) system and beyond, which allows multiple cells to provide service in an overlaid area. To guarantee stability of the HetNet, one challenge is how to handle inter-cell interference between different cells especially for a control channel that does not support link adaptation or retransmission. In this paper, a novel method is proposed that improves performance of downlink Hybrid Automatic Repeat-request (HARQ) feedback channel in HetNet. In the proposed method, legacy control channel transmission is shifted to another subframe with a lower interference level. Simulation results demonstrate that the downlink HARQ feedback channel performance can be significantly improved in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) systems.

Multi-cell cooperative processing (MCP) is widely recognized as a key technology for next generation cellular networks due to its promising potential of increasing the spectrum utilization efficiency. The performance of novel MCP schemes in terms of e.g., capacity, throughput, are mostly evaluated numerically but not analytically. Although the latter approach offers more insights into the true nature, the high complexity of coordinated multi-cell systems usually makes analytical derivation intractable. In this paper, we make a comprehensive study of the rate region achieved by different downlink MCP schemes with per-cell power constraints. We derive in closed form the corresponding rate region boundaries based on simple but representative Wyner type model. The obtained analytical results provide a useful reference of the performance and complexity tradeoff of those schemes.

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.

An efficient multipath-based user selection (MPUS) algorithm for multiuser beamforming in multiple-input multiple-output (MIMO) distributed antenna systems (DAS) is presented in this paper. Motivated by the idea behind the M algorithm of MIMO detection, in this paper, we generate a scheduling tree having M nodes. From each of the M retained nodes, the M "best" users having the largest branch metrics are extended, and of the M2contenders that result, only the M best are retained before advancing to the next level. Meanwhile, to limit the search range, candidate groups are firstly generated by QR decomposition. In the last stage, the best scheduling path having the largest sum rate is selected as the output. The sum rate performance evaluation of the proposed algorithm is given by computer simulation and its complexity is evaluated by measuring the total number of floating-point operations (flops).

Coherent space-time codes perform not so excellent in high moving speed case due to the inaccurate channel estimation. However, differential space-time code is appealing in this case because no channel estimation is needed. While in low and middle signal-to-noise ratio regions differential space-time codes are inferior to the coherent ones. To achieve an excellent performance in a various moving speed environments, we develop an adaptive transmission scheme in this paper where coherent space-time code is applied in low and middle moving speed cases, while differential space-time code is used in high moving speed case. Because the proposed scheme adopts the space-time codes wisely according to the moving speed, it outperforms the scheme which purely utilizes coherent space-time code or differential space-time code and thus is suitable for various moving speed environments. Moreover, two differential space-time codes based on differential Alamouti code are proposed in this paper with the merits of low decoding complexity, suitable for general number of transmit antennas and no code rate loss. The effectiveness of the proposed scheme and the proposed differential space-time codes is supported by numerical simulations.

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. This paper focuses on enhancements of downlink control channel resource allocation for CA system in order to improve the performance of control information transmission in cross-carrier scheduling. A novel metric, named impact factor, is proposed to represent the impact of control information location selection in the search space of one UE (User Equipment) to other UEs. On the basis of the proposed metric, an enhanced control information resource allocation algorithm is proposed to assign control region resource to multiple CCs (Component Carrier) and multiple UEs with low mutual impacts among each other. Furthermore, a novel control information placement method is proposed to solve the problem of active CC set mismatch between eNodeB and UE. Through this novel control information placement after CC activation, data transmission interruptions in cross carrier scheduling can be effectively reduced. Simulation results show the proposed method can effectively improve the control information transmission performance compared with existing methods.

A user authentication system using "unclear images" as pass-images has been proposed, in which only legitimate users can understand their meaning by viewing the original images corresponding to these unclear pass-images, which are meaningless to unauthorized users. Hence, it is difficult for attackers to memorize them, even though they may have observed authentication trials by legitimate users. This paper reports another advantage of the user-authentication system using unclear images: their adaptation enables decoy images to be automatically generated, which are displayed along with the pass-images in the authentication window. Here, we explore various methods of automatically generating decoy images.

A user authentication system using "unclear images" as pass-images has been proposed, in which only legitimate users can understand their meaning by viewing the original images corresponding to the unclear pass-images. These unclear images are meaningless to unauthorized users. Hence, it is difficult for attackers to memorize these unclear pass-images, even though they may have observed legitimate users' authentication trials. However, as long as the same pass-images are always used, it would not be completely impossible for unauthorized users to memorize unclear-images. Therefore, this paper discusses enhancement to the authentication system by permitting users to memorize a number of pass-images and using a different subset of pass-images at all authentication trials. To mitigate the burden imposed by memorizing additional pass-images in the enhanced system, we modified it by extracting a certain number of images from a video sequence. The pass-images were obtained by making the extracted images unclear. As only legitimate users were allowed to view the video sequence in their registration phase, only they could understand what the unclear pass-images meant. Thus, the unclear pass-images become easy for legitimate users to memorize. We verified the effectiveness of the enhanced system through comparative experiments.

This paper proposes an adaptive radio parameter control scheme that utilizes the maximum number of retransmissions in hybrid automatic repeat request (ARQ) in addition to the data modulation and channel coding scheme (MCS) according to quality of service (QoS) requirements and propagation conditions such as the delay spread in the forward link of orthogonal frequency and code division multiplexing (OFCDM) broadband wireless access. We elucidate that for non-real time (NRT) class data transmission, it is appropriate to employ three MCSs, QPSK with the coding rate of R = 1/2, 16QAM with R = 1/2 and 3/4. Application of a much higher MCS set. 16QAM with R = 5/6 and 64QAM with R = 3/4, in addition to the three MCSs improves the throughput under much higher average received E/sub s//N/sub 0/ conditions in a short-delay spread environment. Meanwhile, for real time (RT) data transmission assuming the number of retransmissions of M = 2, it is appropriate to use five MCSs, QPSK with R = 1/3, 1/2 and 16QAM with R = 1/3, 1/2, and 3/4. Thus, we elucidate that the appropriate MCS associated with the maximum number of retransmissions, i.e., major radio parameters should be controlled according to the QoS requirements and the propagation channel conditions. Furthermore, the proposed adaptive radio parameter control scheme that considers the QoS requirements is unified by controlling the threshold value for selecting the appropriate MCS associated with the maximum number of retransmissions in the hybrid ARQ.

In wideband direct sequence code division multiple access (W-CDMA), employing an adaptive antenna array is a very promising technique to reduce severe multiple access interference (MAI) from high rate users. A four-antenna pilot symbol-assisted coherent adaptive antenna array diversity (PSA-CAAAD) receiver comprising an adaptive antenna array based on a minimum mean squared error (MMSE) criterion and a RAKE combiner is implemented in preliminary laboratory and field experiments. There are two important design concepts of the PSA-CAAAD receiver. The first is that the adaptive antenna array forms an antenna beam for each resolved propagation path and tracks only slow changes in the directions of arrival (DOAs) and average powers of the desired and interfering user signals. The second is that the RAKE combiner tracks the instantaneous changes in channel conditions and coherently combines the signals of the desired user propagating along the resolved paths to maximize the instantaneous signal-to-interference plus background noise power ratio (SINR). This paper presents, both by laboratory and field experiments, the effectiveness of PSA-CAAAD receiver as a powerful means to reduce severe MAI from high rate users, and that it is more effective than using a space diversity receiver with the same number of antennas in the W-CDMA reverse link.