Since the introduction of fifth-generation new radio (5G-NR) in Third Generation Partnership Project (3GPP) Release 15, swift progress has been made to evolve 5G with 3GPP Release 18 emerging. A critical aspect is the design of massive multiple-input multiple-output (MIMO) technology. In this line, this paper makes several important contributions: We provide a comprehensive overview of the evolution of standardized massive MIMO features from 3GPP Release 15 to 17 for both time/frequency-division duplex operation across frequency range in (FR)-1 and FR 2-1. We analyze the progress on channel state information (CSI) frameworks, beam management frameworks and present enhancements for uplink CSI. We shed light on emerging 3GPP Release 18 problems requiring imminent attention. These include advanced codebook design and sounding reference signal design for coherent joint transmission (CJT) with multiple transmission/reception points (multi TRPs). We discuss advancements in uplink demodulation reference signal design, enhancements for mobility to provide accurate CSI estimates, and unified transmission configuration indicator framework tailored for FR 2–1 bands. For each concept, we provide system level simulation results to highlight their performance benefits. Via field trials in an outdoor environment at Shanghai Jiaotong University, we demonstrate the gains of multi-TRP CJT relative to single TRP at 3.7 GHz.

In this paper, we consider a distributed multi-input-multi-output (D-MIMO) system wherein multiple radio remote heads (RRHs) distributed in a cell are connected with a single baseband unit. To enable coherent joint transmission from multiple RRHs in the D-MIMO system, we propose several channel state information (CSI) codebooks as candidates for enhancements in the context of 3rd Generation Partnership Project (3GPP) 5G New Radio (NR) standardization. The proposed codebooks are developed based on the 5G Release-16 Type-II CSI codebook framework. In addition, we propose dynamic RRH selection (DRS) methods that are able to obtain performance gain and reduce the amount of feedback by sending the CSI only for the selected RRHs having dominant channel qualities. System-level simulation (SLS) results under realistic scenarios are provided to validate the potential of the proposed CSI codebooks and DRS methods.

High-resolution Type II CSI feedback is a key differentiator between 4G (LTE) and 5G (NR). The large feedback overhead of Type II CSI reporting, however, is a bottleneck for UE implementations in real deployments. In this paper, a space-frequency compression based Type II CSI overhead reduction has been proposed. The system-level simulation results are provided to show that the performance similar to Type II CSI is achievable with significantly reduced CSI overhead.

To support downlink full-dimension (FD-MIMO) in FDD scenarios, an accurate yet reasonably efficient channel-state-information (CSI) feedback scheme is required to harness the potential beamforming gain offered by the use of a large number of antenna elements in a 2D active antenna array. In this paper, a reduced-spaced channel quantization and feedback scheme which exploits slow variation of the angle-of-departure (AoD) distribution is proposed. By perceiving the instantaneous spatial channel to be a linear combination of slowly varying subset of matrices, it is demonstrated that the proposed scheme offers up to 62.5% dimensionality reduction with negligible throughput loss in 16-antenna FD-MIMO system. When used together with vector quantization, the proposed scheme offers substantial gain over that supported by 3GPP LTE.

Multi-cell coordination is a technique for interference mitigation in cellular systems. For joint-transmission (JT), the received signal power is boosted by diversity combining of signals from multiple transmission points. This however results in reduced frequency-reuse ratio and cell-splitting order, which may subsequently degrade the network-level throughput. In order to strike a balance between the diversity combining gain and frequency-reuse ratio, in this paper we propose a dynamic scheduling algorithm that adaptively schedules users in single-cell or multi-cell transmission mode to maximize the system sum throughput. Furthermore, basestation association for each user is meticulously addressed to avoid negatively impacting the frequency-reuse gain. The proposed algorithm is evaluated with system-level evaluation tools fully compliant with 3GPP LTE specification, thereby providing a realistic assessment on the gain of multi-cell coordination in terms of system capacity and coverage.

In orthogonal frequency division-multiple access (OFDMA) systems, the available downlink frequency is divided into subbands which are shared among users. The transmitter determines the users that are scheduled on each subband, and the data rate for each user. To enable these transmitter choices, each user feeds back a channel quality indicator (CQI), giving a compressed version of its observed signal-to-interference-noise ratio (SINR) on all subbands. In this paper, we classify existing CQI compression schemes into symmetric and asymmetric schemes. Symmetric schemes aim to accurately represent the CQI on all subbands, and are shown to be suitable when the receiver cannot accurately predict the subbands on which it will be scheduled. This happens, for instance, when the receiver is fast-moving, or when a round- robin scheduler is employed, or during low load conditions. Asymmetric schemes, on the other hand, greedily achieve accuracy only in high-SINR subbands, and are shown to perform well in high-load cellular networks with proportional fair scheduling and low-speed users. We propose one new scheme in each category. Simulation results for a 3GPP LTE cellular network show that our proposed schemes achieve similar or better performance than the current best-M feedback scheme used in LTE, with 30% less overhead.

In this paper, we investigate several possibilities to increase the efficiency of hybrid automatic retransmission request (HARQ) for multi-input multi-output (MIMO) systems. At the receiver side, two HARQ combining schemes, namely pre-combining and post-combining, are discussed and analyzed. We show that pre-combining is superior to post-combining. In addition, we propose a transmission technique, termed the basis hopping, which improves the HARQ diversity gain especially in slow fading channels. We demonstrate that the use of pre-combining in conjunction with the proposed basis hopping technique provides dramatic gain over a simplistic application of HARQ scheme for MIMO.

We assume a CDMA downlink multiuser scenario, where the base station is equipped with multiple transmit antennas, and each user has multiple receive antennas. For this setup we propose a new, computationally effective MIMO multiuser detector for frequency selective channels. We exploit the fact that the interference mainly comes from the spatial domain, while the residual interference is due to temporal dispersion and multiuser interference. This motivates us to suppress the residual interference in a linear fashion, while combating the spatial interference iteratively. This approach is an efficient performance-complexity tradeoff between fully iterative and linear detectors.

In this paper, a capacity analysis of multi-input multi-out (MIMO) channels with multipath assuming different types of sub-optimal detectors is presented. The results are instrumental for evaluating the potential of multipath MIMO channels when sub-optimal detectors are utilized. It is demonstrated that for some types of detectors, an increase in the number of paths may result in some performance loss, especially at low transmit power. In addition, it is shown that a type of iterative detector can attain near-capacity performance when highly reliable decision feedback is used.

In this paper, we study the performance of space-time block coding, also known as space-time transmit diversity (STTD) scheme for high data rate applications with 4 transmit antennas. The scheme consists of two STTD units at the transmitter and an interference-resistant detector at the receiver, termed the double-STTD (DSTTD) scheme. We compare the performance of DSTTD to general multi-input multi-output (GMIMO) signaling for a given data rate and under different channel correlation profiles. It is demonstrated that DSTTD outperforms GMIMO in all cases. In addition, additional performance gain can be attained for DSTTD in correlated channels by selectively ordering the transmit antennas to minimize the average interference terms.

Optimal multi-antenna wide-band signaling schemes are derived for multipath channels assuming perfect channel state information at the transmitter. The scheme that minimizes the bit-error probability in the single-user case is a rank-one space-time beamformer which focuses the signal transmission in the direction of the most dominant channel mode. Several suboptimal variations are discussed for multiuser applications. The optimal signaling scheme given channel statistics at the transmitter is also derived. The optimal scheme in this case is a full-rank space-time beamformer that transmits on all channel modes. Analysis and simulation results are used to compare the schemes proposed in this paper. Finally, we discuss the optimal signaling scheme when a delayed version of the channel state is available at the transmitter. It is shown that in this case the optimal scheme is a rank-1 beamformer when the channel variations are sufficiently slow and is a full rank beamformer in a sufficiently fast fading channel.

We propose a decentralized space-time multiuser detection scheme based on the notion of canonical space-time coordinates (CSTCs) for representing the received signal. The CSTC representation provides a natural framework for decentralized multi-access interference (MAI) suppression in lower dimensional subspaces that results in complexity reduction relative to existing chip rate filtering schemes. The framework is based on a partitioning of the signal space into active and inactive CSTCs. The active CSTCs contain the signal of the desired user, facilitate maximal diversity exploitation and minimal complexity interference suppression. The inactive CSTCs only contain MAI and can be included progressively to attain a desired level of MAI suppression at the cost of increased complexity. We develop CSTC-based linear coherent multiuser detectors using the linearly constrained minimum variance (LCMV) criterion. We characterize the set of inactive coordinates and analyze the performance of the LCMV receiver as a function of the number of inactive CSTCs. Channel estimation and detector sensitivity to channel estimation errors are discussed. We demonstrate that the low-complexity adaptive receivers designed via the CSTC framework are more robust to channel estimation errors than existing chip-domain filtering schemes.

An orthogonal decomposition of a general wideband space-time multipath channel is derived assuming antenna arrays at both the transmitter and receiver. This decomposition provides a framework for efficiently managing the available space-time channel dimensions using channel state information at the transmitter and receiver. Signaling and receiver designs for high-throughput applications are derived using this decomposition. For a fixed-throughput system, we investigate a power allocation scheme that minimizes the effective bit-error rate. In addition, a strategy to maximize the average throughput is discussed.

We describe a technique to control the trade-off between the error floor and the waterfall performance of parallel concatenated trellis-coded. modulation (PCTCM), which is also known as turbo TCM, via the choice of coded-bits-to-signal mapping for a fixed component code. We demonstrate that different choice of mapping leads to different error floor and waterfall performance. One mapping may have the lowest error floor but result in poor waterfall performance, while another mapping may achieve the opposite. We then present the time-varying mixed mapping scheme for PCTCM systems. The proposed mixed mapping scheme enables us to generate a set of turbo trellis codes from two choices of coded-bits-to-signal mapping with various waterfall performance and error floor.

In this paper, a bit-error-rate (BER) analysis for closed-loop transmit diversity in a time-selective Rayleigh fading channel containing feedback delay is presented. In the absence of feedback delay, closed-loop transmit diversity always outperforms open-loop transmit for a given transmitted signal energy. This is no longer true in the presence of feedback delay. We derive closed-form expressions of the average BER for this case assuming QPSK and BPSK signaling. The results of the analysis are instrumental for comparing closed-loop with open-loop schemes under given operating conditions. In particular, we demonstrate that, for a given transmitted energy and number of transmit antennas, open-loop outperforms closed-loop at sufficiently fast channel fading. We also show that, for a given transmitted signal energy and fading rate, closed-loop outperforms open-loop for sufficiently large numbers of transmit antennas while the total average transmitted signal energy is kept constant. For some special cases, closed-form expressions for the fading rate at which the performance of open-loop is equal to closed-loop are obtained.

A spread-spectrum spatio-temporal signaling scheme based on electric field polarization is proposed. The information symbol is transmitted via two perpendicular transverse electric field components, each with a unique signature sequence. At the receiver, all three electric field components are measured. It is demonstrated that polarization signaling provides another degree of freedom for increasing spatial diversity to improve the performance of the wireless link.

The time scale over which a channel is observed determines whether a deterministic or statistical characterization is appropriate. We show that the channel estimation algorithm of Tsatsanis and Xu (see Proc. 31st Conf. Inform. Sci. Systems (CISS) John Hopkins Univ., Baltitmore, MD, p.724-29, 1997) and the acquisition algorithm of Sayeed and Aazhang (see Proc. 91st Conf. Inform. Sci. Systems (CISS) Princeton, NJ, 1998) share a common MMSE formulation. They differ only in the assumption of the channel characterization. From this observation we develop a new blind MMSE interference resistant algorithm for both timing acquisition and channel estimation that is applicable to both deterministic and statistical channel models. Simulations illustrate the effectiveness of the new method.

A canonical space-time characterization of mobile wireless channels is introduced in terms of a fixed basis that is independent of the true channel parameters The basis captures the essential degrees of freedom in the received signal using discrete multipath delays, Doppler shifts, and directions of arrival (DOA). The canonical representation provides a robust representation of the propagation dynamics and eliminates the need for estimating delay, Doppler and DOA parameters of different multipaths, Furthermore, it furnishes a natural framework for designing low-complexity space-time receivers. Single-user receivers based on the canonical channel representation are developed and analyzed, It is demonstrated that the resulting canonical space-time receivers deliver near-optimal performance at substantially reduced complexity compared to existing designs.

We propose a canonical space-time receiver structure for wireless communications based on front-end processing with respect to a fixed basis that is independent of the true channel parameters. The basis is dictated by a canonical characterization of channel propagation dynamics in terms of discrete multipath delays, Doppler shifts, and directions of arrival that capture the essential degrees of freedom in the received signal. In addition to dramatically reducing the number of channel parameters to be estimated, performance analysis demonstrates that canonical space-time receivers can deliver optimal performance at substantially reduced complexity compared to existing designs.

A design framework for space-time multiuser detectors is introduced based on the notion of canonical space-time coordinates (CSTC). The CSTC system employs a fixed set of basis signals which eliminates the need for estimating direction of arrivals (DOAs) and delays of the channel, thereby allowing significant reduction in receiver complexity. The proposed framework is based on a hybrid structure for multiuser detection which combines the power of centralized processing and the flexibility of decentralized reception. In the first stage, a subset of users whose spreading codes are assumed known are suppressed in a centralized fashion. The remaining interference is suppressed via the second stage in an adaptive decentralized mode. The hybrid scheme offers a flexible design framework by offering lower complexity compared to fully centralized receivers and improved adaptive performance compared to fully decentralized schemes.