This paper presents a VLSI design of a Tomlinson-Harashima (TH) precoder for multi-user MIMO (MU-MIMO) systems. The TH precoder consists of LQ decomposition (LQD), interference cancellation (IC), and weight coefficient multiplication (WCM) units. The LQ decomposition unit is based on an application specific instruction-set processor (ASIP) architecture with floating-point arithmetic for high accuracy operations. In the IC and WCM units with fixed-point arithmetic, the proposed architecture uses an arrayed pipeline structure to shorten a circuit critical path delay. The implementation result shows that the proposed architecture reduces circuit area and power consumption by 11% and 15%, respectively.

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.

In wide-area wireless access systems such as satellite communications systems and stratospheric platform systems, electric power supplies for radio communications are realized using solar photovoltaic cells and/or fuel cells. However, the on-board weight limits restrict the number of cells that can be equipped. In addition, the transmission power of such systems is limited taking account of issues and regulations on sharing the same frequency band with other systems. Hence, both the frequency band and electric power is limited, which are crucial radio resources for those systems. Although radio channel allocation methods taking account of the frequency constraint only or the power constraint only have been proposed, radio channel allocation methods taking account of both constraints simultaneously have been insufficiently studied. This paper proposes a radio channel allocation method that provides global optimum allocation results by utilizing the linear programming method. The proposed method has features such that the method first allocates radio channels in proportion to the traffic demand distributed over the service coverage area and then maximizes the total radio channels allocated to systems. Numerical results are presented for a stratospheric platform system that covers an area of Japan, as an example, to demonstrate that the proposed method optimally allocates radio channels taking account of both constraints while efficiently allocating excess resources. In addition, whether a system reaches either the frequency or power limit can be estimated, by investigating the radio channel allocation results. Furthermore, enhanced linear programming models based on a method aiming at practical use of the radio channel allocation results in operation are also introduced. The enhanced model is demonstrated to work effectively to avoid unbalanced radio channel allocations over geographical areas. The proposed method and linear programming models are useful not only for making pre-plans but also for determining the amount of necessary frequency and power resources in designing systems.

Cell Range Expansion (CRE) is a promising technique for the enhancement of traffic offload to pico cells. CRE is realized by adjusting the trigger timing of handover (HO) toward/from pico cells. However, inappropriate setting of trigger timing results in HO failures or Ping-Pong HOs. Both the HO failures and the Ping-Pong HOs degrade the continuity of user data services. Therefore, when CRE is applied, both the HO failures and the Ping-Pong HOs should be kept suppressed in order to guarantee the continuity of services for users. However, in the conventional studies, the application of CRE is discussed without consideration of HO performance. This paper clarifies the application range of CRE from the perspective of HO performance by taking the HO failure rates and the Ping-Pong HO rates as HO performance measures. As an example, we reveal that there is an appropriate CRE bias values which keep both the HO failure rate and Ping-Pong HO rate less than 1%. Such an appropriate CRE bias value range is smaller than the one without consideration of HO performance, which is reported in the conventional studies. The authors also observed that Ping-Pong HO occurs due to the short staying time of users at pico cells in high velocity environment. The rate of such Ping-Pong HOs becomes more than about 1% when the user velocity is more than 60 km/h. Therefore, it is more difficult in high velocity environment than that in low velocity environment to find appropriate CRE bias values.

FDMA/TDMA non-geostationary earth orbit satellite systems (Non-GEOS) generally require a pre-planned pool of radio resource, i.e., frequency and time slot plan (FTSP), for each gateway earth station (GES) prior to the real-time channel assignment by the multiple GES's sharing the resources harmoniously. The time-variant nature of those systems implies that a dynamic FTSP planning method is crucial to the operation to cope with the time-variant traffic demand and the inter-beam interference condition. This paper proposes and compares three algorithms (Serial-numbering, DP-based, and Greedy algorithms) mixed with two strategies (concentrated- and spread-types) for the resource allocation. The numerical evaluation demonstrates that Greedy algorithm with the spread-type strategy is very effective and promising with feasible calculation time for the FTSP generation.

In conventional wireless cellular networks, cell coverage is static and fixed, and each user equipment (UE) is connected to one or a few local base stations (BS). However, the users' distribution in the network area commonly fluctuates during a day. When there are congeries of users in some areas, conventional networks waste idle network resources in sparse areas. To address this issue, we propose a novel approach for cooperative cluster formation to dynamically transfer idle network resources from sparse cells to crowded cells or hotspots. In our proposed scheme, BS coverage is directed to hotspots by dynamically changing the antennas' beam angles, and forming large optimal cooperative clusters around hotspots. In this study, a cluster is a group of BSs that cooperatively perform joint transmission (JT) to several UEs. In this paper, a mathematical framework for calculation of the system rate of a cooperative cluster is developed. Next, the set of BSs for each cluster and the antennas' beam angles of each BS are optimized so that the system rate of the network is maximized. The trend of performance variation versus cluster size is studied and its limitations are determined. Numerical results using 3GPP specifications show that the proposed scheme attains several times higher capacity than conventional systems.

Electronic mail (email) is one of the most popular services using Internet Protocol (IP). Emails are now sent to and from portable phones in addition to laptop computers. The volume of emails is increasing because of the recent addition of attached electronic files. In addition, short message delivery services (SMS) that send email header information and the essence of email contents are also commonly used. Global short message delivery services via satellites are considered attractive for meeting the demand of increased short message delivery services. To establish a communications link for indoor subscribers, it is preferable to concentrate high transmission power into a spot-beam even using a non-geostationary satellite orbit (non-GSO) satellite system with low Earth orbits. The idea can be realized by a kind of TDMA scheme where only a single spot-beam per satellite is fired at a timeslot in an efficiently scheduled manner. This paper proposes a global short message delivery scheme and a scheduling algorithm that enables each satellite to concentrate its power into a spot-beam illuminating the addressed area in a prescribed timeslot, so that a subscriber in the area can efficiently receive a message addressed to the subscriber. Since the algorithm must guarantee that the spot-beam assignment causes no interference with other spot-beams from adjacent satellites, numerical evaluations are given for a 6-hour period MEO system, as an example, to demonstrate the efficacy and performance.

In recent years, heterogeneous cellular network (HetNet) topology has been attracting much attention. HetNet, which is a network topology with low power base stations installed inside the cell range of conventional macrocells, can realize network capacity enhancement through the effects of macrocell offloading and cell shrinkage. Due to the heterogeneity nature of HetNet, network designers should carefully consider about the interference management, resource allocation, user association and cell range expansion. These issues have been well studied in recent literatures. However, one of the important problems which has not been well investigated in conventional works is the base station (BS) deployment problem in HetNet. This paper investigates the optimal pico base station deployment in heterogeneous cellular networks especially with the existence of hotspots. In this paper, pico BS locations are optimized together with other network parameters including spectrum splitting ratio and signal-to-interference-noise ratio (SINR) bias for cell range expansion to maximize the total system rate, by considering two spectrum allocation strategies, i.e. spectrum overlapping and spectrum splitting. Numerical results show that the optimized pico BS locations can improve the system rate, the average user rate and outage user rate in HetNet with hotspots.

5G service has been launched in various countries, and research for the beyond 5G is already underway actively around the world. In beyond 5G, it is expected to expand the various capabilities of communication technologies to cover further wide use cases from 5G. As a candidate elemental technology, cell free massive MIMO has been widely researched and shown its potential to enhance the capabilities from various aspects. However, for deploying this technology in reality, there are still many technical issues such as a cost of distributing antenna and installing fronthaul, and also the scalability aspects. This paper surveys research trends of cell free massive MIMO, especially focusing on the deployment challenges with an introduction to our specific related research activities including some numerical examples.

This work studies the benefits of heterogeneous cellular networks with overlapping picocells in a large macrocell. We consider three different strategies for resource allocation and cell association. The first model employs a spectrum overlapping strategy with an SINR-based cell association. The second model avoids the interference between macrocell and picocell through a spectrum splitting strategy. Furthermore, picocell range expansion is also considered in this strategy to enable a load balancing between the macrocell and picocells. The last model is a hybrid one, called as fractional spectrum splitting strategy, where spectrum splitting strategy is only applied at the picocell-edge, while the picocell-inner reuses the spectrum of the macrocell. We constructs resource allocation optimization problem for these strategies to maximize the system rate. Our results show that in terms of system rate, all the three strategies outperform the performance of macrocell-only case, which shows the benefit of heterogeneous networks. Moreover, fractional spectrum splitting strategy provides highest system rate at the expense of outage user rate degradation due to inter-macro-pico interference. Spectrum overlapping model provides the second highest system rate gain and also improves outage user rate owing to full spectrum reuse and the benefit of macro diversity, while spectrum splitting model achieves a moderate system rate gain.

We have seen a rapid increase in mobile data traffic in cellular networks, especially in densely populated areas called “hotspots.” In order to deal with this trend, heterogeneous networks (HetNet) are attracting much attention as a method of effectively accommodating such traffic increases using the Long Term Evolution (LTE)-Advanced system in the 3rd Generation Partnership Project (3GPP). This paper first presents an overview of HetNet, where various wireless nodes can be deployed over the coverage area formed by macro base stations (BSs). Next, various evaluation results are provided for HetNet, where pico BSs (“Pico-BSs”) are deployed over the coverage area of macro BSs (“Macro-BSs”). Then, this paper presents a comprehensive analysis, not only of the effect of overlaying Pico-BSs but also a detailed analyses of the techniques called “cell range expansion (CRE)” and “enhanced inter-cell interference coordination (eICIC)” for facilitating the offloading of user terminals (UEs) from Macro-BSs to Pico-BSs and mitigating interference, respectively, for both downlink and uplink. Noteworthy outcomes found through the comprehensive study are that CRE provides throughput improvements for uplinks, especially for UE connected to Pico-BSs. In addition, this paper demonstrates that CRE contributes to improving downlink throughput especially for low traffic loads. The outcome regarding eICIC is that eICIC provides improvements in total throughput, in spite of the fact that eICIC causes unfairness between UE connected to the Pico-BSs and those with Macro-BSs.

Single Carrier — Frequency Domain Multiple Access (SC-FDMA) is a multiple access technique employed in LTE uplink transmission. SC-FDMA can improve system throughput by frequency selective scheduling (FSS). In cellular systems using SC-FDMA in the uplink, interference arising from user equipments (UEs) in neighboring cells degrades the system throughput, especially the throughput of cell-edge UEs. In order to overcome this drawback, many papers have considered fractional frequency reuse (FFR) techniques and analyzed their effectiveness. However, these studies have come to different conclusions regarding the effectiveness of FFR because the throughput gain of FFR depends on the frequency reuse design and evaluation conditions. Previous papers have focused on the frequency reuse design. Few papers have examined the conditions where FFR is effective, and only the UE traffic conditions have been evaluated. This paper reveals other conditions where FFR is effective by demonstrating the throughput gain of FFR. In order to analyze the throughput gain of FFR, we focus on the throughput relationship between FFR and FSS. System level simulation results demonstrate that FFR is effective when the following conditions are met: (i) the number of UEs is small and (ii) the multipath delay spread is large or close to 0.

This paper proposes an open loop beamforming scheme for downlink multiuser multiple-input multiple-output (MIMO) transmissions in frequency division duplex (FDD). The proposed scheme uses the uplink direction of arrival (DOA) estimation, and then generates the beamforming weight such that the interference caused by the overlapping beams is removed by applying the dirty-paper coding (DPC) principle. The simulation results show that the proposed scheme provides the gain of 32.3% at minimum in terms of the spectral efficiency at the CDF of 50% compared to the conventional DOA based beamforming scheme. In addition, it is shown that the proposed scheme has superior performance to closed loop scheme with the limited feedback information.

This paper proposes a multiple-input multiple-output (MIMO) precoding scheme for the down link of single user (SU) cooperative base station (BS) systems. The proposed precoding scheme mitigates the performance degradation caused by large inter-BS path-loss imbalance and large intra-BS antenna correlation by controlling two parameters. The proposed precoding scheme multiplexes the multiple layers by adjusting the amplitude of each layer, and then decreases the occurrence probability of the small absolute value of the log likelihood ratio (LLR), and so reduces the bit error rate (BER). Link level simulation results show that the proposed precoding scheme decreases the required signal-to-noise ratio (SNR) of BER = 0.001 by 5.5 dB, 2.2 dB, and 0.7 dB in the case of QPSK and coding rate 1/1, 3/4, and 1/2 respectively. The proposed precoding scheme is also evaluated in terms of spectrum efficiency using rank adaptation and adaptive modulation, showing that it improves the spectrum efficiency when the SNR per a receiver antenna is higher than 4 dB.

Coordinated Multi-point (CoMP) transmission has long been known for its ability to improve cell edge throughput. However, in a CoMP cellular network, fixed CoMP clustering results in cluster edges where system performance degrades due to non-coordinated clusters. To solve this problem, conventional studies proposed dynamic clustering schemes. However, such schemes require a complex backhaul topology and are infeasible with current network technologies. In this paper, small power base stations (BSs) are introduced instead of dynamic clustering to solve the cluster edge problem in CoMP cellular networks. This new cell topology is called the diamond cellular network since the resultant cell structure looks like a diamond pattern. In our novel cell topology, we derive the optimal locations of small power base stations and the optimal resource allocation between the CoMP base station and small power base stations to maximize the proportional fair utility function. By using the proposed architecture, in the case of perfect user scheduling, a more than 150% improvement in 5% outage throughput is achieved, and in the case of successive proportional fair user scheduling, nearly 100% improvement of 5% outage throughput is achieved compared with conventional single cell networks.