Adaptive amplify-and-forward relaying for cellular downlink

This paper proposes interference-based decode and forward scheme that utilizes relay stations (RSs). In Long-Term-Evolution (LTE-) Advanced, heterogeneous networks in which femto- and picocells are overlaid onto macrocells are extensively discussed. However, interference between macro- and pico(femto)cells arises due to their different transmit power levels. Unlike conventional cooperative transmission schemes, the RS decodes interference in the first transmit timing period and forwards it to the user equipment (UE) in the second period. Moreover, cooperative transmission can be achieved without stopping the transmission from the base station (BS) to UE when forwarding the interference from the RS to the UE by utilizing the fact that signal-to-noise power ratio (SNR) between the RS and UE is much greater than that between the BS and UE. The basic performance of the proposed method is shown based on computer simulation. Moreover, the interference temperature and shadowing effect are measured when considering the coexistence between macro- and femtocells, and the performance of the proposed method is verified using measured shadowing effect.

This paper addresses channel state information (CSI) estimation in a wireless relay network, which consists of a base station (BS), a relay station (RS), and a mobile station (MS). We propose a method for the BS to obtain the instantaneous CSIs of the three links (BS-RS, BS-MS, and RS-MS links) with minimized overhead. The CSI availability plays an important role for the BS to adapt its transmission strategies to the given channel conditions. Our proposed method exploits the fact that the link condition between fixed BS and fixed RS tends to be stable, and the frequent CSI update is not necessary for this link. In order for the BS to obtain the CSI of the distant link (i.e. RS-MS link), RS amplifies a pilot signal received from MS with a pre-defined amplification factor, and forwards it to BS. This enables the BS to obtain the CSI of the RS-MS link based on the received pilot signal and pre-knowledge on the CSI of BS-RS link. We show by computer simulation that our proposed method can reduce the required overhead for CSI estimation with little degradation on the accuracy of the obtained CSI.

This paper investigates an adaptive frequency and transmission power partitioning method between cell-interior and cell-edge users in the downlink orthogonal frequency division multiple access (OFDMA) with fractional frequency reuse (FFR). In the conventional FFR method, the frequency and transmission power partitioning between cell-interior and cell-edge users is fixed and this results in limited system-level performance in terms of the average and cell-edge user throughput. This paper presents enhancements to a previously proposed partitioning algorithm to improve the system-level performance. The computer simulation results show that the enhanced method increases both the average and cell-edge user throughput compared to the conventional FFR and the previous method.

The link performance between base station (BS) and user equipment (UE) in long term evolution-advanced (LTE-A) system is already quite close to the Shannon limits and from a pure link-budget perspective, the highest data rates supported by LTE-A require a relatively high signal-to-noise ratio. In later releases of LTE-A, various tools such as relaying, enhancing the support for low-power BSs and improving the link budget are included. This paper studies the relay stations (RSs) for LTE-A downlink physical layer. Multi-input multi-output amplify-and-forward RS is used to enhance the received signal of UEs at cell edge. This paper proposes RSs with orthogonal space–time block codes (OSTBC) transmission for LTE-A system. The received signals for different UEs scenarios are mathematically derived and analyzed. The symbol error rate performance of proposed LTE-A system is illustrated for the scenarios of UEs. Results show that the proposed LTE-A system based RSs with OSTBC transmission improve link performance and achieve higher diversity gains.

For conventional multiple-input and multiple-output (MIMO) selective decode-and-forward (S-DF) relaying network, without global channel knowledge, base station (BS) normally allocates its transmission power for the channel links between itself and its served users. In this case, if the received signal power at relay station (RS) is larger enough, RS will help BS decode and forward the messages. However, such strategy may lose system optimality if BS has the location information of RS. Motivated by this, in this paper, a RS-oriented BS power allocation algorithm for MIMO S-DF relaying is proposed to enhance the system performance. Specifically, BS firstly allocates its transmission power to help as much as data streams to be correctly decoded at RS. Then, if there is still some power left, BS will allocate the residual power for its direct channel links to the users to further increase the system performance. Computer simulations are carried out to examine the proposed scheme in terms of sumrate and bit-error-rate. It is shown that the proposed scheme outperforms the conventional schemes, especially when the RS is placed in the middle of BS and the users.

Time-domain inter-cell interference coordination, so called eICIC, has been introduced in Long Term Evolution-Advanced (LTE-Advanced) system as a throughput improvement method for heterogeneous network, where a lot of low power nodes (LPNs) are located in the coverage area of high power base station (Marco-BS). eICIC enables Macro-BSs to configure almost blank subframe (ABS), in which Marco-BSs reduce their transmit power or mute data transmission so as to reduce interference to the users served by LPNs. However, the improvement of the overall throughput is affected by the ABS setting. This paper proposes a novel user throughput estimation method for LTE-Advanced system where ABS and proportional fairness scheduler are applied. Numerical results show that the proposed method accurately estimates the user throughput for arbitrary ABS ratio with the estimation error of 1-7% for cell edge user throughput and around 1% for average user throughput, respectively. As a result, using the proposed method, Macro-BSs can decide proper ABS ratio to maximize cell edge or average user throughput.

In this contribution, a maximum-likelihood-based approach is proposed for estimating the attitudes of sectoring base station (BS) antennas based on the power of the received signals observed by multiple user equipments (UEs) in a cellular network. The proposed method calculates the likelihood of the antenna attitude by taking into account both the large-scale fading statistics, i.e. path loss, shadowing, and the small-scale fading attributed to multi-path propagation. The performance of the method is evaluated by simulations in both urban and rural scenarios, where a random-propagation-graph method is applied to generating realistic channel impulse responses by using directional antenna patterns. Results show that by using the proposed method, accurate estimates of the antenna attitude can be obtained when the observations from more UEs are available and the line-of-sight (LoS) condition dominates the channels between the BS and the UEs.

Traditional cellular network requires that a user equipment (UE) should associate to the same base station (BS) in both the downlink (DL) and the uplink (UL). Based on dual connectivity (DC) introduced in LTE-Advanced R12, DL/UL decouple access scheme has been proposed, which is especially suitable for heterogeneous networks (HetNets). This paper is the pioneer to take the DL/UL decouple access scheme into consideration and develop a novel resource allocation algorithm in a two-tier HetNet to improve the total system throughput in the UL and ease the load imbalance between macro base stations (MBSs) and pico base stations (PBSs). A model is formulated as a nonlinear integer programming, and the proposed algorithm is a sub-optimal algorithm based on the graph theory. First, an undirected and weighted interference graph is obtained. Next, the users are grouped to let users with large mutual interferences to be assigned to different clusters. Then, the users in different clusters are allocated to different resource blocks (RBs) by using the Hungarian algorithm. Simulation results show that the proposed algorithm can provide great promotions for both the total system throughput and the average cell edge user throughput and successfully ease the load imbalance between MBSs and PBSs.

In Japan, the 5th generation mobile communication system (5G) became commercially available in 2020. The millimeter wave bands such as 28GHz is being used to achieve the peak rate of 10 [Gbps] or higher targeted for 5G. In the late 2020s, low latency and high-capacity data transmission over both the up and down links will become important. This is because 5G will be utilized in the late 2020s, and telemedicine and teleoperation using 4K/8K and other high-definition video transmission will become widespread. In this study, we propose a relaying method that converts frequency multiplexing into spatial multiplexing during relaying, with the goal of achieving low latency and high capacity relaying communications. The user equipment, base stations, and relay stations have different conditions in terms of transmission power and number of antennas. Therefore, the proposed method achieves high capacity by frequency multiplexing in the link where the number of antennas is limited. In addition, the proposed method uses spatial multiplexing to achieve high capacity while suppressing the increase in resource usage in the link where multiple antennas are available. The 39 GHz band, which has more frequency resources than the existing 5G bands, is used for the evaluation in the link of frequency multiplexing. Then, the 28 GHz band, which is used commercially in 5G, is used for the evaluation in the link of spatial multiplexing. For low latency relaying, analog circuits are used during the relaying process to convert between frequency-multiplexed and space-multiplexed signals without modulation and demodulation, while maintaining the number of multiplexes. In this paper, simulation evaluations show that the proposed method improves the communication distance where the throughput exceeds 4 [Gbps] to 6.5 times that of 39 GHz band 5G communications without relaying, and to 1.3 times that of RF repeaters in conventional relaying methods that use the 39 GHz band both before and after relaying, indicating that the uplink communication distance can be extended.

This paper considers the two-hop relay broadcast channel (BC) in a wireless cellular network where the downlink transmission from the base station (BS) to mobile users is assisted by a fixed relay station (RS). Assuming both the BS and RS equipped with multi-antennas, we consider joint optimization of beamforming weights and power allocations at the BS and RS so as to minimize their weighted sum-power consumption under the minimum signal-to-interference-noise-ratio (SINR) constraints at user terminals. We extend the two well-known criteria for design of linear precoding in the conventional MIMO-BC without relay, namely, the SINR balancing and the channel-inversion, to the MIMO relay BC. First, an iterative SINR-balancing algorithm is proposed that in each iteration optimizes transmit parameters at one station (BS or RS) with those at the other fixed. Secondly, a joint BS and RS channel-inversion algorithm is proposed together with a novel technique termed eigenmode switching at the RS. Simulation results show that optimized joint beamforming and power control are able to achieve substantial power savings for the MIMO relay BC.

This paper studies the two-hop relay broadcast channel (BC) for a relay-assisted wireless cellular network where the multiuser independent downlink signals from the base station (BS) are first transmitted to a fixed relay station (RS), and then forwarded by the RS to multiple mobile users. Assuming both the BS and RS are equipped with multiantennas, we study the joint optimization of linear beamforming and power control at the BS and RS so as to minimize their weighted sum-power consumption under the user minimum signal-to-interference-noise-ratio (SINR) - quality-of-service (QoS) - constraints. We apply two well-known criteria in the literature, namely, the ldquoSINR balancingrdquo and the ldquochannel-inversion,rdquo for the design of linear precoding in the traditional nonrelay-assisted multiantenna BC to the relay-assisted multiantenna BC. First, a convergence-ensured iterative SINR-balancing algorithm is proposed to successively in turn optimize the transmit parameters at one station (BS or RS) with those at the other station being fixed. Second, a joint BS and RS channel-inversion algorithm is proposed together with a novel technique, termed ldquoeigenmode switching,rdquo at the RS to reduce the power penalty of the channel inversion. Simulation results show that the proposed joint beamforming and power control schemes provide substantial power savings to achieve the assigned user QoS constraints.

In a cellular communication system, deploying a relay station (RS) is an effective alternative to installing a new base station (BS). A dual-hop network enhances the throughput of mobile stations (MSs) located in shadow areas or at cell edges by installing RSs between BSs and MSs. Because additional radio resources should be allocated to the wireless link between BS and RS, a frame to be transmitted from BS is divided into an access zone (AZ) and a relay zone (RZ). BS and RS communicate with each other through the RZ, and they communicate with their registered MSs through an AZ. However, if too many MSs are registered with a certain BS or RS, MS overloading may cause performance degradation. To prevent such performance degradation, it is very important to find the proper positions for RSs to be deployed. In this paper, we propose a method for finding the sub-optimal RS deployment location for the purpose of load-balancing and throughput enhancement. The advantage of the proposed method is the efficiency in find the sub-optimal location of RSs and its reliable tradeoff between load-balancing throughput enhancement. Since the proposed scheme finds the proper position by adjusting the distance and angle of RSs, its computational complexity lower than other global optimization approach or learning-based approach. In addition, the proposed scheme is constituted with the two stages of load-balancing and throughput enhancement. These procedures result in the appropriate tradeoff between load-balancing and throughput enhancement. The simulation results support these advancements of the proposed scheme.

Heterogeneous networks (HetNets) are expected to handle increasing demands for mobile network capacity. This paper presents an adaptive control cell range expansion (CRE) technique that improves the cell edge user throughput in HetNets, in which user equipment (UE) can automatically choose an optimal cell selection offset (CSO). The aspects and algorithm of the proposed CRE are described by comparing them with the conventional method operated by a fixed CSO. Computer simulation results such as average and cell edge user throughput are provided as parameters of the number of UEs. We confirmed that the proposed CRE can improve cell edge user throughput, while maintaining the average user throughput performance. The proposed CRE will play an important role in future mobile communication systems such as LTE-Advanced and beyond.

A K-tier heterogeneous downlink millimeter wave (mmWave) cellular network with user-centric small cell deployments is studied in this paper. In particular, we consider a heterogeneous network model with user equipments (UEs) being deployed according to a Poisson Cluster Process (PCP), i.e., Thomas cluster process, where the UEs are clustered around the base stations (BSs) and the distances between UEs and the BS are modeled as Gaussian distributed. In addition, distinguishing features of mmWave communications including directional beamforming and a sophisticated path loss model incorporating both line-of-sight (LOS) and non-line-of-sight (NLOS) transmissions, are taken into account. In this paper, the complementary cumulative distribution function (CCDF) and probability density function (PDF) of the path loss are provided. Also, using tools from stochastic geometry, we derive a general expression of the signal-to-interference-plus-noise ratio (SINR) coverage probability. Our results demonstrate that coverage probability can be improved by decreasing the size of UE clusters around BSs, and interference has noticeable influence on the coverage performance of our model.

Performance enhancement of repacking and borrowing mechanisms for IEEE 802.16j multihop resource scheduling