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We investigate in this paper uplink transmission schemes for 5G Ultra-Reliable Low Latency Communications (URLLC) traffic. The URLLC class of services has been defined for applications requiring extremely stringent latency and reliability. We show that, in systems with episodic traffic and many users compared with the number of transmission resources, randomly transmitting multiple copies of a packet allows to meet the URLLC requirements. We first develop an analytical model for the packet loss rate for a contention based multiple transmission scheme where replica positions are assigned randomly and show that it allows to meet the performance requirements of URLLC for small enough user activity rate. We then propose an advanced replication scheme where positions for the different replicas are pre-allocated to users so that users have pairwise-distinct positions. We show that this advanced scheme achieves very high reliability with low resource consumption.

International audience; This paper studies the problem of feedback allocation and scheduling for a multichannel downlink cellular network under limited and delayed feedback. We propose two efficient algorithms that select the link states that should be reported to the base station (BS). A novelty here is that these feedback allocation algorithms are performed at the users' side to take advantage of their local channel state information knowledge in order to achieve higher gains. The first algorithm is suitable for a continuous-time contention scheme and requires only one feedback per channel, whereas the second one is adapted for a discrete-time contention scheme and adopts a threshold-based concept. For this second algorithm, we study some implementation aspects related to the feedback period and investigate the tradeoff between knowing at the BS a small number of accurate link states and a larger but outdated number of link states. We show that these algorithms, combined with the Max-Weight scheduling, achieve good stability performance compared with the ideal system.

This work proposes a new joint link adaptation and HARQ (Hybrid Automatic Repeat Request) scheme for URLLC (Ultra-Reliable Low-Latency Communication) services. We consider the case where the transmitter knows only the average SNR (Signal-to-Noise Ratio) and not the instantaneous one. In the proposed scheme, the optimal maximum number of allowable HARQ transmissions and the optimal MCS (Modulation and Coding Scheme) level are determined for each packet to maximize the spectral efficiency. Our adopted approach exploits the channel diversity and increases the flexibility of the scheduling mechanism. Simulation results show that the proposed retransmission policy and link adaptation scheme increases the system performance in terms of spectral efficiency, while satisfying the latency and reliability constraints.

This paper characterizes the performance of interference alignment (IA) technique taking into account the dynamic traffic pattern and the probing/feedback cost. We consider a time-division duplex (TDD) system where transmitters acquire their channel state information (CSI) by decoding the pilot sequences sent by the receivers. Since global CSI knowledge is required for IA, the transmitters have also to exchange their estimated CSIs over a backhaul of limited capacity (i.e. imperfect case). Under this setting, we characterize in this paper the stability region of the system under both the imperfect and perfect (i.e. unlimited backhaul) cases, then we examine the gap between these two resulting regions. Further, under each case, we provide a centralized probing algorithm (policy) that achieves the max stability region. These stability regions and scheduling policies are given for the symmetric system where all the path loss coefficients are equal to each other, as well as for the general system. For the symmetric system, we compare the stability region of IA with the one achieved by a time division multiple access (TDMA) system where each transmitter applies a simple singular value decomposition technique (SVD). We then propose a scheduling policy that consists in switching between these two techniques, leading the system, under some conditions, to achieve a bigger stability region. Under the general system, the adopted scheduling policy is of a high computational complexity for moderate number of pairs, consequently we propose an approximate policy that has a reduced complexity but that achieves only a fraction of the system stability region. A characterization of this fraction is provided., 66 pages, 13 figures, 1 table, submitted to IEEE Transactions on Information Theory

International audience; This work studies the problems of feedback allocation and scheduling for a multichannel downlink cellular network under limited and delayed feedback. We consider a realistic scenario where a fixed and small number ¯ F of link states can be reported to the base-station (BS) per time-slot. We study the trade-off between knowing at the BS a small number of accurate link states (i.e. that can be reported within one time-slot) and a larger but outdated number of link states (i.e. number of link states > ¯ F that requires more than one slot to be reported). We propose an efficient algorithm that selects the link states that should be reported to the base-station. A novelty here is that this feedback allocation algorithm is performed at the users side. We show that this algorithm combined with the Max-Weight scheduling achieves at least a fraction η of the stability region achieved under the ideal system (i.e. with full and perfect feedback at no cost). We then provide numerical results that show the best aforementioned trade-off under various system setups.

This paper considers a MIMO interference system where interference alignment (IA) technique is adopted to manage the problem of interference. We consider a time division duplex (TDD) system where each transmitter estimates its channel state information (CSI) by probing the receivers. In addition, the transmitters share their local CSI estimate between each other using a backhaul links of limited capacity. A quantization over the backhaul is therefore required to reduce the amount of information to exchange. We study in this paper the impact of this quantization on the system performance and determine the optimal number of transmitter-receiver pairs that maximizes the system throughput.