Achievable DoF-delay trade-offs for the K-user MIMO interference channel with delayed CSIT

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.The degrees of freedom (DoFs) of the K-user multiple-input multiple-output (MIMO) interference channel are studied when perfect, but delayed channel state information is available at the transmitter side (delayed CSIT). Recent works have proposed schemes improving the DoF knowledge of the interference channel, but at the cost of developing transmission involving many channel uses (long delay), thus increasing the complexity at both transmitter and receiver side. This paper proposes three linear precoding strategies, limited to at most three phases, based on the concept of interference alignment, and built upon three main ingredients: delayed CSIT precoding, user scheduling, and redundancy transmission. In this respect, the interference alignment is realized by exploiting delayed CSIT to align the interference at the non-intended receivers along the space-time domain. Moreover, a new framework is proposed where the number of transmitted symbols and duration of the phases is obtained as the solution of a maximization problem, and enabling the introduction of complexity constraints, which allows deriving the achievable DoF as a function of the transmission delay, i.e., the achievable DoF-delay trade-off. Finally, the latter part of this paper settles that the assumption of time-varying channels common along all the literature on delayed CSIT is indeed unnecessary.Peer ReviewedPostprint (author's final draft

On the superiority of improper Gaussian signaling in wireless interference MIMO scenarios

©2016 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Recent results have elucidated the benefits of using improper Gaussian signaling (IGS) as compared to conventional proper Gaussian signaling (PGS) in terms of achievable rate for interference-limited conditions. This paper exploits majorization theory tools to formally quantify the gains of IGS along with widely linear transceivers for MIMO systems in interferencelimited scenarios. The MIMO point-to-point channel with interference (P2P-I) is analyzed, assuming that received interference can be either proper or improper, and we demonstrate that the use of the optimal IGS when received interference is improper strictly outperforms (in terms of achievable rate and mean square error) the use of the optimal PGS when interference is proper. Then, these results are extended to two practical situations. First, the MIMO Z-interference channel (Z-IC) is investigated, where a trade-off arises: with IGS we could increase the achievable rate of the interfered user while gracefully degrading the rate of the non-interfered user. Second, these concepts are applied to a two-tier heterogeneous cellular network (HCN) where macrocells and smallcells coexist and multiple MIMO Z-IC appear.Peer ReviewedPostprint (author's final draft

Performance analysis of feedback-free collision resolution NDMA protocol

To support communications of a large number of deployed devices while guaranteeing limited signaling load, low energy consumption, and high reliability, future cellular systems require efficient random access protocols. However, how to address the collision resolution at the receiver is still the main bottleneck of these protocols. The network-assisted diversity multiple access (NDMA) protocol solves the issue and attains the highest potential throughput at the cost of keeping devices active to acquire feedback and repeating transmissions until successful decoding. In contrast, another potential approach is the feedback-free NDMA (FF-NDMA) protocol, in which devices do repeat packets in a pre-defined number of consecutive time slots without waiting for feedback associated with repetitions. Here, we investigate the FF-NDMA protocol from a cellular network perspective in order to elucidate under what circumstances this scheme is more energy efficient than NDMA. We characterize analytically the FF-NDMA protocol along with the multipacket reception model and a finite Markov chain. Analytic expressions for throughput, delay, capture probability, energy, and energy efficiency are derived. Then, clues for system design are established according to the different trade-offs studied. Simulation results show that FF-NDMA is more energy efficient than classical NDMA and HARQ-NDMA at low signal-to-noise ratio (SNR) and at medium SNR when the load increases.Peer ReviewedPostprint (published version

Joint user scheduling, precoder design, and transmit direction selection in MIMO TDD small cell networks

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.New short-length single-direction frame structures are proposed for 5G time division duplex (TDD) systems, where the transmit direction [i.e., either downlink (DL) or uplink (UL)] can be independently chosen at each cell in every frame. Accordingly, high flexibility is provided to match the per-cell DL/UL traffic asymmetries and full exploitation of dynamic TDD is allowed. As a downside, interference management becomes crucial. In this regard, this paper proposes a procedure for dynamic TDD in dense multiple-input multiple-output small cell networks, where the transmit direction selected per small cell (SC) is dynamically optimized together with the user scheduling and transmit precoding. We focus on the maximization of a general utility function that takes into account the DL/UL traffic asymmetries of each user and the interference conditions in the network. Although the problem is non-convex, it is decomposed thanks to the interference-cost concept and then efficiently solved in parallel. Simulation results show gains in DL and UL average rates for different traffic asymmetries and SC/user densities as compared to existing dynamic TDD schemes thanks to the proposed joint optimization. The gains become more significant when there is high interference and limited number of antennas.Peer ReviewedPostprint (author's final draft

Signal-timing offset compensation in dense TDD OFDM-based networks

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Orthogonal Frequency Division Multiplexing (OFDM)-based networks rely on time synchronization to obtain their best performance. Time synchronization with neighboring nodes can be satisfied by increasing the cyclic prefix (CP) length (at the expenses of spectral efficiency reduction) We show that by optimizing the transmit pre-compensation and receive post-compensation we can meet the time synchronization constraints and keep the CP at its minimum value. This concept is applied to paired-bands Frequency Division Duplexing (FDD) systems which tend to show inefficient occupancy of the uplink (UL)-band due to the traffic asymmetry. We consider the possibility of deploying multiple Time Division Duplexing (TDD) small eNBs (SeNBs) in the unused UL spectrum. In this scenario, time synchronization with macro eNB (MeNB) and neighboring SeNBs becomes essential. Two algorithms are proposed in order to ensure orthogonality of OFDM transmissions network-wide.Peer ReviewedPostprint (author's final draft

Hierarchical beamforming in random access channels

Managing a massive number of terminals in a contention-based multiple access is challenging due to its intrinsic limited efficiency. For example, in the random access channel considered in LTE-A and 5G NR, Base Station (BS) is just aware of the collided and non-collided preambles. Several time-based protocols have been investigated to redistribute the overload under high terminal activity, thus avoiding the congestion. In this work, we explore the use of the spatial domain by means of a hierarchical codebook-based beamforming, where the BS selects the appropriate beams as a function of the number of non-collided and collided preambles. Since the activity and placement of terminals may be dynamic over time, the sequential selection of parameters can benefit from a reinforcement learning (RL) framework. We propose an algorithm that can exploit both domains, temporal and spatial, with the goal of reducing collisions and enhancing transmission delay. Our approach is able to efficiently learn whenever there is a non-homogeneous spatial distribution of terminals and adapt the spatial beams accordingly.The work of A.Agustin was supported by the Spanish Government through the Statistical Learning and Inference for Large Dimensional Communication Systems (ARISTIDES, RTI2018-099722-B-100) Project. The work of J.Vidal and M.Cabrera-Bean was supported by the project ROUTE56 (Agencia Estatal de Investigación, PID2019-104945GB-I00/AEI/10.13039/501100011033), and in part by the Grant 2017 SGR 578 (AGAUR, Generalitat de Catalunya).Peer ReviewedPostprint (author's final draft

Linear transceiver design in nonregenerative relays with channel state information

This paper deals with the design of nonregenerative relaying transceivers in cooperative systems where channel state information (CSI) is available at the relay station. The conventional nonregenerative approach is the amplify and forward (A&F) approach, where the signal received at the relay is simply amplified and retransmitted. In this paper, we propose an alternative linear transceiver design for nonregenerative relaying (including pure relaying and the cooperative transmission cases), making proper use of CSI at the relay station. Specifically, we design the optimum linear filtering performed on the data to be forwarded at the relay. As optimization criteria, we have considered the maximization of mutual information (that provides an information rate for which reliable communication is possible) for a given available transmission power at the relay station. Three different levels of CSI can be considered at the relay station: only first hop channel information (between the source and relay); first hop channel and second hop channel (between relay and destination) information, or a third situation where the relay may have complete cooperative channel information including all the links: first and second hop channels and also the direct channel between source and destination. Despite the latter being a more unrealistic situation, since it requires the destination to inform the relay station about the direct channel, it is useful as an upper benchmark. In this paper, we consider the last two cases relating to CSI.We compare the performance so obtained with the performance for the conventional A&F approach, and also with the performance of regenerative relays and direct noncooperative transmission for two particular cases: narrowband multiple-input multiple-output transceivers and wideband single input single output orthogonal frequency division multiplex transmissions.Peer Reviewe

Hybrid turbo FEC/ARQ systems and distributed space-time coding for cooperative transmission

Cooperative transmission can be seen as a "virtual" MIMO system, where the multiple transmit antennas are in fact implemented distributed by the antennas both at the source and the relay terminal. Depending on the system design, diversity/multiplexing gains are achievable. This design involves the definition of the type of retransmission (incremental redundancy, repetition coding), the design of the distributed space-time codes, the error correcting scheme, the operation of the relay (decode&forward or amplify&forward) and the number of antennas at each terminal. Proposed schemes are evaluated in different conditions in combination with forward error correcting codes (FEC), both for linear and near-optimum (sphere decoder) receivers, for its possible implementation in downlink high speed packet services of cellular networks. Results show the benefits of coded cooperation over direct transmission in terms of increased throughput. It is shown that multiplexing gains are observed even if the mobile station features a single antenna, provided that cell wide reuse of the relay radio resource is possible

Receive antenna selection and hybrid precoding for receive spatial modulation in massive MIMO systems

©2018 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Recently, a receive spatial modulation (RSM) for massive multiple-input-multiple-output operating in millimeter wave (mmWave) was introduced with the purpose of simplifying user terminal circuit by employing only one radio-frequency chain and attaining high spectral efficiency by exploiting the receive spatial dimension. However, when RSM is applied in a mmWave channel, it demands a challenging receive antenna selection (RAS) procedure. On the other hand, the power consumption at the transmitter side is high when a full digital (FD) precoder is envisioned. We consider the joint problem of RAS and precoder designs based low complexity hybrid architecture. For the sake of simplicity, we divide this problem into two subproblems. First, we design the RAS assuming FD precoder, and then, we design the hybrid precoder. We propose two novel and efficient RAS methods. First, we formulate the RAS as non-convex optimization problem. Then, we convert it into a convex optimization problem by introducing novel lower bounds and relaxing non-convex constraints. Second, we provide sequential algorithms that approach the optimal selection where we (add/remove) one (good/poor) antenna per iteration. We propose novel zero forcing hybrid precoder based convex optimization that maximizes the received power. We prove that the proposed precoder is optimal when the channel is highly spatially sparse. The proposed designs have been compared with the best known methods in terms of average mutual information and energy efficiency showing significant improvements.Peer ReviewedPostprint (author's final draft

Receive spatial modulation for massive MIMO systems

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.In this paper, we consider the downlink of a massive multiple-input-multiple-output (MIMO) single user transmission system operating in the millimeter wave outdoor narrowband channel environment. We propose a novel receive spatial modulation architecture aimed to reduce the power consumption at the user terminal, while attaining a significant spectral efficiency and low bit error rate. The energy consumption reduction is obtained through the use of analog devices (amplitude detector), which reduces the number of radio frequency chains and analog to- digital-converters (ADCs). The base station transmits spatial and modulation symbols per channel use. We show that the optimal spatial symbol detector is a threshold detector that can be implemented by using one bit ADC. We derive closed form expressions for the detection threshold at different signal-to noise-ratio (SNR) regions. We derive expressions for the average bit error probability in the presence and absence of the threshold estimation error showing that a small number of pilot symbols is needed. A performance comparison is done between the proposed system and fully digital MIMO showing that a suitable constellation selection can reduce the performance gap.Peer ReviewedPostprint (author's final draft