User-to-User Clustering, Channel Estimation & Cross-Link Interference Mitigation for Dynamic TDD Systems

In next-generation wireless communication systems, a dynamic time division duplex (TDD) system is promising to support simultaneous uplink (UL) and downlink (DL) transmissions. We herein consider a fixed number of feedback bits per DL user, where each DL user reports its quantized channel direction information (CDI) of the interference channel to a serving DL base station via its dedicated feedback channel. Based on the quantized CDI feedback, we design the UL and DL transmit zero-forcing beamforming (ZFBF) to mitigate the interference signals caused by the coexistence of the UL and DL transmissions. To maximize the DL data rate, the challenge of interference cancellation lies in how many feedback bits are adaptively allocated for the transmission of the quantized CDI of each channel link. This paper proposes an adaptive feedback bits allocation scheme which minimizes the mean rate loss between perfect channel knowledge and limited feedback system. We also propose the UL transmit power allocation scheme under the interference power constraint from the UL user to the DL user when the number of feedback bits for the quantized CDI is given. To jointly solve the feedback bits allocation and the UL transmit power allocation problems, we use an iterative scheme to maximize the DL data rate while satisfying the constraint of interference power. Finally, we investigate how many feedback bits per DL user are required to maintain a constant mean rate loss. The simulation results show that the proposed adaptive feedback bits allocation scheme significantly increases the DL data rate compared with the conventional schemes.

This paper investigates the passive beamforming and deployment design for an intelligent reflecting surface (IRS) aided full-duplex (FD) wireless system, where an FD access point (AP) communicates with an uplink (UL) user and a downlink (DL) user simultaneously over the same time-frequency dimension with the help of IRS. Under this setup, we consider three deployment cases: 1) two distributed IRSs placed near the UL user and DL user, respectively; 2) one centralized IRS placed near the DL user; 3) one centralized IRS placed near the UL user. In each case, we aim to minimize the weighted sum transmit power consumption of the AP and UL user by jointly optimizing their transmit power and the passive reflection coefficients at the IRS (or IRSs), subject to the UL and DL users' rate constraints and the uni-modulus constraints on the IRS reflection coefficients. First, we analyze the minimum transmit power required in the IRS-aided FD system under each deployment scheme, and compare it with that of the corresponding half-duplex (HD) system. We show that the FD system outperforms its HD counterpart for all IRS deployment schemes, while the distributed deployment further outperforms the other two centralized deployment schemes. Next, we transform the challenging power minimization problem into an equivalent but more tractable form and propose an efficient algorithm to solve it based on the block coordinate descent (BCD) method. Finally, numerical results are presented to validate our analysis as well as the efficacy of the proposed passive beamforming design.

In a full-duplex (FD) cellular network, a base station transmits data to the downlink (DL) user and receives data from uplink (UL) users at the same time; thereby the interference from UL users to DL users occurs. One of the possible solutions to reduce this interuser interference in the FD cellular network is user pairing, which pairs a DL user with a UL user so that they use the same radio resource at the same time. In this paper, we consider a user pairing problem to minimize outage probability and formulate it as a nonconvex optimization problem. As a solution, we design a low-complexity user pairing algorithm, which first controls the UL transmit power to minimize the interuser interference and then allows the DL user having a worse signal quality to choose first its UL user giving less interference to minimize the outage probability. Then, we perform theoretical outage analysis of the FD cellular network on the basis of stochastic geometry and analyze the performance of the user pairing algorithm. Results show that the proposed user pairing significantly decreases the interuser interference and thus improves the DL outage performance while satisfying the requirement of UL signal-to-interference-plus-noise ratio, compared to the conventional HD mode and a random pairing. We also reveal that there is a fundamental tradeoff between the DL outage and UL outage according to the user pairing strategy (e.g., throughput maximization or outage minimization) in the FD cellular network.

In a full duplexing (FD) wireless cellular network, a base station operates in FD mode, while the downlink (DL) and uplink (UL) users operate in half duplexing (HD) mode. Thus, the downlink and uplink transmissions occur simultaneously so that interuser interference from a UL to a DL user occurs. In an FD network, the main challenge to minimize the interuser interference is user pairing, which determines a pair of DL and UL users who use the same radio resource simultaneously. We formulate a nonconvex optimization problem for user pairing to maximize the cell throughput. Then, we propose a heuristic user pairing algorithm with low complexity. This algorithm is designed such that the DL user having a better signal quality has higher priority to choose its paired UL user for throughput maximization. Thereafter, we conduct theoretical performance analysis of the FD cellular system based on stochastic geometry and analyze the impact of the user paring algorithm on the performance of the FD cellular system. Results show that the FD system that uses the proposed user pairing algorithm effectively reduces the interuser interference and approaches optimal performance. It also considerably outperforms the FD system using a random user pairing and almost doubles the conventional HD system in terms of cell throughput.

In this paper, we propose a user selection algorithm for full-duplex cellular system. In full-duplex cellular system, besides self-interference (SI), the user-equipment (UE)-UE interference at downlink (DL) users is induced by adjacent uplink (UL) users thereby degrading the performance severely. Thus, we alleviate the UE-UE interference and improve the achievable rate of UL users at the same time by means of our proposed user selection algorithm. Specifically, we first adopt semiorthogonal user selection (SUS) algorithm [13] with ZF precoder for DL users and then we select UL users considering the trade-off between increasing the rate of UL users and decreasing the UE-UE interference. Moreover, we also propose a mode selection based on our proposed algorithm. As a result, we provide the improved achievable sum-rate of full-duplex system as guaranteeing better achievable rate for both DL and UL users, compared with those of users in half-duplex system. In addition, we analyze the complexity of the proposed user selection algorithm along with analysis of full-search algorithm. In the end, we observe that the numerical results show the improved achievable sum-rate of the proposed user selection algorithm.

In this paper, we present a comparative study of the performance of a dynamic time division duplex (DTDD) enabled cell free massive multiple-input multiple-output (CF-mMIMO) with a full-duplex (FD)CF-mMIMO system. Both DTDD and FD enable a CF-system to serve uplink (UL) and downlink (DL) user equipments (UEs) simultaneously over the same time-frequency resources. However, interference from other access points (APs) affects the performance of both of these duplexing strategies. Additionally, the intra-AP self-interference suppression level has a significant impact on the sum UL-DL spectral efficiency (SE) of an FD CF-system. On the other hand, in DTDD, concurrent UL/DL reception/transmission is facilitated by judiciously apportioning the UL and DL time slots across half-duplex APs based on the localized UL/DL traffic load. Due to this, DTDD obviates the need for intra-AP interference cancelation. Our numerical experiments reveal that the achievable sum SE with DTDD can match, and even outperform, an FD system with similar antenna density. Thus, DTDD is a potential duplexing scheme that can be incorporated in the next generation wireless systems to serve concurrent UL-DL traffic load without the need for implementing complex hardware and algorithms for self-interference cancelation as in FD systems.

We consider a cellular system with a full-duplex (FD) base station (BS) serving multiple uplink (UL) and downlink (DL) users simultaneously, where all the nodes are equipped with multiple antennas. The self-interference at the BS and the co-channel interference (CCI) caused by the UL users on the DL users are both taken into account. We address the proportional fairness (PF) issue of this system, which is important for networks with asymmetric topology and/or asymmetric traffic demands. The sum of the logarithm of the achievable rate of UL and DL users is maximized subject to power constraints at the BS and UL users. We develop a gradient projection (GP) method to solve this non-convex optimization problem, and demonstrate that the proposed algorithm provides a good balance between maximizing the sum-rate and maintaining fairness among users.

This paper analyzes the spectral/energy efficiency (SE/EE) of a full-duplex (FD) cell-free (CF) massive multiple-input multiple-output (mMIMO) over Rician fading channels. Due to the FD radios, the access points (APs) suffer from self-interference (SI) and inter-AP interference (IAI) while the downlink (DL) users’ signals are corrupted by the uplink (UL) users’ transmissions. We consider the case, where low-resolution analog-to-digital converters (ADCs) are utilized at the APs and DL users, which introduces the quantization noise (QN). The combined effects of Rician κ-factor, residual SI/IAI, UL-to-DL interference, multi-user interference, and QN on the UL/DL SEs are characterized. The UL SE is degraded by the increase in DL power, whereas the growth in UL power deteriorates the DL SE. The effects of the residual SI/IAI and UL-to-DL interference are worsened by the low-resolution ADCs. We optimize the UL/DL transmit powers to maximize the overall sum SE of the network. It is observed that the proposed power allocation algorithm brings substantial sum SE gain. A trade-off analysis between the EE and SE, as a function of the ADCs’ resolution, shows that the entire envelope of the operating region of FD CF mMIMO is enhanced in Rician channels.

To enable ultrareliable and low-latency communications (URLLCs) in the Internet of Things (IoT), a sparse-code multiple-access (SCMA)-enhanced full-duplex (FD) scheme (FD-SCMA) is proposed in this article. FD-SCMA can support short-packet transmissions of several SCMA users in the uplink (UL) and downlink (DL) simultaneously by an FD next generation node B (gNB). First, the gNB and UL users can generate and superpose signals according to the preconfigured SCMA codebooks, and simultaneously transmit the signals via occupied subcarriers in a joint SCMA pattern. The receivers at the gNB and DL users can demodulate and decode the signals with multiuser detection (MUD). With the imperfect self-interference suppression (SIS) of FD considered, the effective signal-to-noise ratio (SNR) of FD-SCMA at the gNB and DL users is formulated. The error probability of FD-SCMA in the UL and DL is also derived under a given transmission latency constraint of short-packet transmissions. In the stationary flat-fading channel, it is proved that FD-SCMA can achieve better reliability than the existing FD and SCMA schemes. In the time-invariant frequency-selective fading channel, the upper bounds for error probability of the UL and DL users in FD-SCMA are derived, respectively. Through the theoretical calculation and Monte Carlo simulation, it is verified that the superiority of FD-SCMA in supporting ultrareliable and low-latency short-packet transmissions in IoT.

Dynamic allocation of subframes to uplink (UL) or downlink (DL) in time division duplex (TDD), termed ‘Dynamic TDD,’ has been studied by the 3rd Generation Partnership Project (3GPP) since the Long Term Evolution (LTE) Release 11 timeframe. At the same time, 3GPP is also standardizing macrocell-assisted small cell heterogeneous architectures for inclusion in LTE Release 12 as a solution offering high data rate to user terminals (UEs) along with high system capacity through spatial reuse of spectrum. In this paper, we focus on a particular small cell architecture proposed by DOCOMO, known as the Phantom Cell architecture, which provides the option to support dynamic TDD. For an arbitrarily-located UE in a small cell network, we apply results from stochastic geometry to derive expressions for the distribution of DL signal to interference plus noise ratio (SINR) at an arbitrary UE and the distribution of UL SINR at its serving base station (BS). The analytical results are verified by system level simulations. These results can be used to study aspects of system design for small cells, and the sensitivity of SINR to the extent of synchronization across small cells employing dynamic TDD. In order to deal with the severe inter-cell interference (ICI) problem in dynamic TDD, we further propose a frequency domain interference coordination technique. Finally, system level simulations based on more realistic system models and assumptions are conducted to evaluate the performance of dynamic TDD systems and the proposed interference coordination technique.

As the promising technologies for providing higher network throughput, full-duplex (FD) communications and non-orthogonal multiple access (NOMA) have attracted wide attention in recent years. In this study, the authors propose a user scheduling algorithm for the full-duplex non-orthogonal multiple access system (FD-NOMA) where an FD base station equipped with a pair of reconfigurable antennas serves multiple half-duplex downlink (DL) and uplink (UL) users simultaneously. Firstly, they formulate a user scheduling problem to maximise the sum rate of the FD-NOMA system. Then, they eliminate the inter-user interference from caused by the UL users to the DL users through the blind interference alignment based on reconfigurable antennas and decouple the user scheduling problem into a DL user scheduling problem and a UL user scheduling problem. Next, for the DL and UL user scheduling subproblems, they pair the users aligned with the same directions based on the channel correlation metric. Finally, they employ the exhausting search and iterative power allocation algorithm to solve the subproblems. Simulation results show that the proposed FD-NOMA system can achieve up to 233 of gain in the average sum rate compared with the existing baseline schemes.

This paper proposes a downlink (DL)/uplink (UL) transmission power control (TPC) scheme for dynamic time division duplex (TDD) based small cells under multi-cell environment. In the dynamic TDD, an eNB (evolved node B) selects an adequate UL-DL configuration according to the ratio of DL to UL data bits in each cell. However, in this case, eNB- eNB and user equipment (UE)-UE interferences could be additional interference since the transmission directions can be different among cells. Especially, eNB-eNB interference significantly degrades the UL transmission performances. Therefore, we investigate the DL TPC which is applied to subframes which can be different directions among cells and the benefits to decrease eNB-eNB interference by this scheme. We also investigate the different UL TPC parameters that are applied according to the subframe types in order to alleviate the impact of eNB-eNB interference. Computer simulation confirms that the proposed DL/UL TPC scheme can achieve 21.6 % gain at maximum for UL throughput without significant DL throughput degradation.

In this paper, we study user association in a full-duplex (FD) cellular network where the base station (BS) serves a set of near and far users via non-orthogonal multiple access (NOMA) technique. In order to reduce received interference at the NOMA downlink (DL) user caused by the NOMA uplink (UL) user we propose guard zone based user association scheme. In particular, we propose DL user associate based on the nearest near and nearest far scheme. In order to schedule the UL users, we assume a guard around the selected DL user with a specific radius. Finally, UL near and far users are randomly selected from those UL users located outside the guard zone, to upload their data packet to the BS. We derive closed-form expressions for the DL and UL coverage probability of the proposed scheme. Numerical results show that the DL coverage probability of the proposed scheme achieves up to 43% gain over the conventional NNNF scheme.

In-band full-duplex (FD) systems have been widely studied because they can double the spectral efficiency (SE) compared with conventional half-duplex (HD) systems, theoretically. However, inherent interference caused by both self-interference (SI) and co-channel interference (CCI) makes FD systems to achieve theoretical SE hard. In this paper, we propose a CCI estimation and cancellation (CCI-EC) protocol for the FD systems to overcome performance degradation due to CCI. We suggest to use a coherence channel block for two phases: CCI channel is estimated at a downlink (DL) user through uplink (UL) pilot signals in the first phase, then a base station (BS) and a UL user transmit their data simultaneously in the second phase, in which a DL user cancels interfering signals from a UL user by utilizing the estimated CCI channel information. We analyze the achievable SE of FD systems with CCI-EC and formulate a resource allocation problem to maximize it, where resource includes pilot and data transmission time, and transmit power of a BS and a UL user. A closed-form optimal power allocation for UL pilot and data transmissions and a suboptimal power allocation for DL data transmission are provided. The simulation results show that FD systems with the proposed CCI-EC achieve near-optimal achievable SE and outperform the conventional HD systems and FD systems without CCI-EC in terms of the achievable SE. Especially, the SE gain achieved by the proposed CCI-EC protocol in FD systems is remarkable under severe interference environment.

Ultra-reliable low-latency communication (URLLC) is one of the most important components in the fifth generation (5G) cellular networks for realizing mission-critical applications. In this paper, we jointly optimize the transceiver design and decoding error probability (DEP) of a full-duplex (FD) URLLC system, where the base station (BS) operates in FD mode, while the uplink (UL) and downlink (DL) users work in half-duplex (HD) mode. Accordingly, an optimization problem is formulated to maximize the achievable total (UL plus DL) rate for an FD URLLC system under finite blocklength, subject to the end-to-end (E2E) reliability constraint from the UL user to each DL user and the total transmission power constraint at the UL user and at the BS. We analyze the problem structure and convexify the problem by approximating the channel dispersion in scenarios of high and mid-to-high signal-to-interference plus noise ratio (SINR) regimes, respectively. Next, efficient iterative algorithms are proposed to find the near-optimal power allocation for the UL user and transceiver weights for the BS. Furthermore, closed-form expressions of the transceiver weights are derived, and the convergence of the proposed algorithms is proved. Simulation examples demonstrate the impact of the code blocklength, number of DL users, transmitter/receiver distortion and DEP threshold on the system performance.