Adaptive CDI-CQI Feedback Bit Partitioning for Quantized MISO-SDMA in Downlink HetNets

ABSTRACTIn this paper, several multi‐user scheduling algorithms with limited channel feedback are proposed for multi‐antenna downland channels. With little channel quality information (CQI) and channel direction information (CDI) at multi‐antenna base station, multi‐user scheduling algorithms are carefully designed to maximise the potential system throughput while reducing the feedback load. We first propose a scheduling algorithm that requires a quantised CDI and a threshold CQI. Then we propose a scheduling that needs limited feedback of a threshold CDI and a perfect CQI. Both scheduling algorithms are studied in terms of the achievable throughput and the feedback load. Analytical results show that the proposed scheduling with limited channel feedback has a small performance degradation compared with the scheduling with unlimited channel feedback when the thresholds for user scheduling are judiciously designed. Moreover, under a per user total bit rate constraint of the feedback channel, we propose a scheduling with the quantisation of both CQI and CDI. It is demonstrated that allocating 1 bit to the CQI with an optimal threshold and the remaining bits to the CDI achieves the near optimal sum‐rate performance. Copyright © 2011 John Wiley & Sons, Ltd.

Multi-antenna transmission, or MIMO, is a major enabling technique for broadband cellular networks. The current implementation, however, is mainly for the point-to-point link, and its potential for Space-Division Multiple Access (SDMA) has not been fully exploited. In this paper, we will analytically evaluate the performance of SDMA in multicell networks based on a spatial random network model, where both the base stations (BSs) and users are modeled as two independent Poisson point processes. The main difficulty is the evaluation of the interference distribution, for which we propose a novel BS grouping approach that leads to a closed-form expression for the network area spectral efficiency. We find that the number of active users (U) served with SDMA is critical, as it affects the spatial multiplexing gain, the aggregated interference, and the diversity gain for each user. The optimal value of U can be selected based on our analytical result, with which SDMA is shown to outperform both the single-user beamforming and full-SDMA for which U is the same as the number of BS antennas. In particular, it is shown that the performance gain of SDMA is higher when the BS density is relatively small compared to the user density, but the optimal value of U is almost the same for different scenarios, which is close to half of the BS antenna number.

In this paper, we find the optimum lengths of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">UL</sub> , T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DL</sub> and B i.e. the uplink (UL) and downlink (DL) pilots and feedback bits respectively maximizing the sum of the lower bounds of the UL and the DL rates. With the aid of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">UL</sub> symbols, the Base Station (BS) estimates the UL channel of each user. T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DL</sub> symbols are means of estimating the downlink channel, which after being normalized and quantized with B bits, is made available at the BS. Estimated Channel State Information (CSI) and quantized Channel Direction Information (CDI), which may be received erroneously, are thus available at the BS for computing the receive and the transmit Zero-forcing (ZF) filters, respectively. The use of ZF enables the derivation of analytical DL and UL mean squared error (MSE) expressions that give a lower bound on the DL and UL rates, respectively, needed for our optimization.

In this paper schemes for feedback reduction in MIMO broadcast channels are considered for a multi-user system. We consider a case where the number of single antenna users K >> M where M is the size of antenna array at the base station (BS). We consider zero-forcing beamforming system with semi-orthogonal user selection algorithm in which channel state information is splitted into channel quality information (CQI) and channel direction information (CDI). In such a user-dense multi-user system feedback channel is inundated if all the users are feedbacking even in the case where only a limited feedback is employed. In order to reduce the feedback load we propose threshold based schemes in which threshold is applied on CQI or CDI. We show that by employing proposed feedback reduction schemes feedback load can be significantly reduced without sacrificing much of the sum-capacity of the system.

This letter proposes a robust joint linear precoding scheme based on the minimum mean squared error (MMSE) criterion for amplify-and-forward (AF) multiple-input multiple-output (MIMO) relay broadcast systems with limited feedback, where only the quantized channel direction information (CDI) of the forward channel is available for the base station (BS) and the relay station (RS). The proposed scheme employs an iterative algorithm which alternately optimizes the BS and RS precoders to jointly minimize the expected MSE conditioned on the quantized CDI.

Space division multiple access (SDMA) is a multiple access scheme for mobile communications that improves system capacity. We developed the SDMA base station (BS) and carried out several tests. We describe the configuration of the SDMA BS and the results of the tests. The result of the indoor measurement indicates that the SDMA BS has sufficient interference suppression performance. The results of the field test indicate that the traffic capacity is several times greater than existing systems and interference emitted from the SDMA BS is decreased. Therefore, it is confirmed that the SDMA BS achieves substantial improvements in spectrum efficiency without any degradation.

Since flying base stations (FlyBSs) are energy constrained, it is convenient for them to act as transparent relays with minimal communication control and management functionalities. The challenge when using the transparent relays is the inability to measure the relaying channel quality between the relay and user equipment (UE). This channel quality information is required for communication-related functions, such as the UE association, however, this information is not available to the network. In this letter, we show that it is possible to determine the UEs’ association based only on the information commonly available to the network, i.e., the quality of the cellular channels between conventional static base stations (SBSs) and the UEs. Our proposed association scheme is implemented through deep neural networks, which capitalize on the mutual relation between the unknown relaying channel from any UE to the FlyBS and the known cellular channels from this UE to multiple surrounding SBSs. We demonstrate that our proposed framework yields a sum capacity that is close to the capacity reached by solving the association via exhaustive search.

This article dives into the fundamentals of dense and ultradense small cell wireless networks, discussing the reasons why dense and ultradense small cell networks are fundamentally different from sparse ones, and why the well‐known linear capacity scaling law with the base station (BS) density in the latter does not apply to the former. In more detail, we review the impact of the following factors on ultradense networks (UDNs): (i) closed‐access operations and line‐of‐sight conditions, (ii) the near‐field effect, (iii) the antenna height difference between small cell BSs and user equipment (UE), and (iv) the surplus of idle‐mode‐enabled small cell BSs with respect to UEs. Combining all these network characteristics and features, we present a more realistic capacity scaling law for UDNs, which indicates (i) the existence of an optimum BS density to maximize the area spectral efficiency (ASE) for a given finite UE density and (ii) the existence of an optimum density of UEs that can be simultaneously scheduled across the network to maximize the ASE for a given finite BS density.

Joint linear beamforming and scheduling are performed in a system where limited feedback is present at the transmitter side. The feedback conveyed by each user to the base station consists of channel direction information (CDI) based on a predetermined codebook and a scalar metric with channel quality information (CQI) used to perform user scheduling. In this paper, we present a design framework for scalar feedback in MIMO broadcast channels with limited feedback. An approximation on the sum rate is provided for the proposed family of metrics, which is validated through simulations. For a given number of active users and average SNR conditions, the base station is able to update certain transmission parameters in order to maximize the sum-rate function. On the other hand, the proposed sum-rate function provides a means of simple comparison between transmission schemes and scalar feedback techniques. Particularly, the sum rate of SDMA and time division multiple access (TDMA) is compared in the following extreme regimes: large number of users, high SNR, and low SNR. Simulations are provided to illustrate the performance of various scalar feedback techniques based on the proposed design framework.

The second part of the study of beamforming issues, based on positioning in ultra-dense millimeter wave radio access networks, is devoted to the formalization and software implementation of a complex simulation model of the functioning of a set of directional radio links. Each directional radio link between a base station (gNodeB – gNB), equipped with an antenna array, and a user equipment (UE), operating in omnidirectional mode, is formed according to the location of the UE, known at the gNB. The set of gNB→UE directional radio links, simultaneously operating in a common frequency range, is studied as a set of traffic beams, that implement space division multiple access (SDMA). Spatial multiplexing is implemented through three-dimensional beamforming at the gNB and makes it possible to compensate for propagation losses and high levels of interference. In the first part of the study, it was shown that the problem of practical implementation of SDMA in ultra-dense radio access networks is a significant (tens of dB) spread in the signal to interference plus noise ratio (SINR), depending on the arrangement of two devices. The purpose of this study is to establish the dependence of SINR on 1) the beamwidth of the gNB sector in the direction of the user equipment in the radio link of the signal of interest (SOI); 2) uncertainty of the UE location; 3) interference from radio links of signal not of interest (SNOI): a) within its sector, b) other sectors of its cell and c) other cells in the network. The simulation model developed and implemented in software in this work for the first time made it possible to establish the interdependence of the UE positioning error factors and the required width of the traffic beam for its service. In particular, it was found, that as the positioning error decreases from 10 to 1 m, the required beam width in the horizontal and vertical planes narrows to 3 °, which makes it possible to increase the SINR to 25 dB. A simultaneous transmission multiplexing study showed that for 64 spatially multiplexed UEs, as the cell size increases from 20 to 300 m, the SINR increases by approximately 30 dB, subject to a beamwidth constraint of 3°. Unlike similar studies, in this model, the contribution from interference from simultaneously operating traffic beams within its sector, other sectors of its cell and other cells in the network is shown separately for the first time, which allows to differentiate the origin of interference and use scientifically based beamwidth control for their compensation.

This paper studies the impact of the base station (BS) idle mode capability (IMC) on the network performance of multi-tier and dense heterogeneous cellular networks (HCNs). Different from most existing works that investigated network scenarios with an infinite number of user equipments (UEs), we consider a more practical setup with a finite number of UEs in our analysis. More specifically, we derive the probability of which BS tier a typical UE should associate to and the expression of the activated BS density in each tier. Based on such results, analytical expressions for the coverage probability and the area spectral efficiency (ASE) in each tier are also obtained. The impact of the IMC on the performance of all BS tiers is shown to be significant. In particular, there will be a surplus of BSs when the BS density in each tier exceeds the UE density, and the overall coverage probability as well as the ASE continuously increase when the BS IMC is applied. Such finding is distinctively different from that in existing work. Thus, our result sheds new light on the design and deployment of the future 5G HCNs.

Time-varying channel is an important characteristic of a mobile communication system, which will cause random fluctuations of the quality of received signal at the user equipment (UE). Link adaptation is one of the key technologies to deal with the variant channel condition. It maximizes the data transmission rate over a limited bandwidth. In this paper, we design and implement an end-to-end link-adaptive system based on the 5G New Radio (NR) standard, adopting software defined radio (SDR) equipment as the baseband processing module. In the designed system, the selection of channel quality information (CQI) and the link-adaptive module are implemented at the base station (BS). The design and implementation of the physical layer according to 5G NR standard, including system parameters, synchronization scheme and duplex mode, are provided. Further, the over-the-air (OTA) results validate the feasibility of the system design, and show that link adaptation module can significantly improve the quality of the received signal.

This paper is dedicated to better understanding of the achievable sum rate in the single carrier FDMA (SC-FDMA) system. We investigate the achievable sum rate in a two-user SC-FDMA system, where a mix of SIMO (single input multiple output) and SDMA (space division multiple access) transmission mode is assumed for the wideband communication. The problem is formulated which appears to be non-convex. Relation to the conventional waterfilling solution is pointed out for single mode (SIMO or SDMA) transmission. We show that a mix of SIMO and SDMA transmission mode in the wideband is not favorable for any pair of the user equipments. A better strategy would be to switch to SIMO mode or SDMA mode adaptively. We also propose a simple algorithm to find the group of users for SDMA mode based on the wideband average spatial correlation. Performance improvement is shown by combining adaptive SIMO mode and SDMA mode selection with our proposed user grouping algorithm.

With the diversification of mobile services, mobile traffic exhibits new characteristics which is different from traditional voice traffic. In this paper, we investigate the optimal base station (BS) density in cellular networks by taking into account the self- similar characteristics of mobile traffic using stochastic geometry approach. Based on the traffic load distribution satisfying self-similar characteristics, traffic coverage probability is defined and its relation with BS density is derived. An optimization problem with the aim of minimizing the BS density is formulated under the constraint of traffic coverage probability. Using binary search algorithm, the optimal BS density and the bounds (both upper and lower bound) for simplified analysis is obtained. The simulation results confirm the accuracy of theoretical analysis and verify the impact of the self-similar characteristics on the optimal BS density.

The base station (BS) density influences the coverage performance of cellular communication system. In massive multiple input multiple output (MIMO) system, each BS is equipped with large scale antenna array. The stochastic geometric model is employed because of its analytical tractability. This paper demonstrates that BS density has to be below a certain bound in order to satisfy the coverage requirement in massive MIMO system. Closed-form expression for the BS density bounded by maximum outage probability is derived using Lambert W function. Moreover, the expression for the upper bound of successful area spectral efficiency (ASE) is also derived. Our study claims that increasing BS density will lead to the deterioration of coverage performance in massive MIMO system.