Wenjaun Yu

In this paper, we theoretically investigate the performance of non-orthogonal and orthogonal spectrum access protocols (more generically known as NOMA) in supporting ultra-reliable low-latency communications (URLLC). The theory of effective capacity (EC) is adopted as a suitable delayguaranteed capacity metric to flexibly represent the users? delay requirements. Then, the total EC difference between a downlink user-paired NOMA network and a downlink orthogonal multiple access (OMA) network is analytically studied. Exact closed-form expressions and the approximated closed-forms at high signal-tonoise ratios (SNRs) are derived for both networks and validated through simulation results. It is shown that for a user pair in which two users with the most distinct channel conditions are paired together, NOMA still achieves higher total EC (compared to OMA) in high SNR regime as the user group size becomes larger, although the EC performance of both NOMA and OMA reduces with the increase in group size. It is expected that the derived analytical framework can serve as a useful reference and practical guideline for designing favourable orthogonal and nonorthogonal spectrum access schemes in supporting low-latency services.

In this paper we perform subcarrier and power allocation for the downlink transmission in a cognitive radio network by exploiting the location information of mobile secondary users. A mixed integer nonlinear problem is formulated which maximizes the aggregate capacity of the secondary network, subject to the constrained interference at the primary user and the maximum transmit power limit of the secondary base station. To solve the formulated problem, an optimal subcarrier allocation is first obtained based on a practical assumption, followed by an optimal power allocation achieved using standard optimization methods. The proposed solution can facilitate to reduce latency and provide high speed communication of big data for mobile secondary users since speed, direction and location are all taken into account for effective allocation of resources. Simulation results verify our design intentions and confirm the efficiency of the proposed resource allocation strategy.

This paper addresses the problem of opportunistic spectrum access in support of mission-critical ultra-reliable and low latency communications (URLLC). Considering the ability of supporting short packet transmissions in URLLC scenarios, a new capacity metric in finite blocklength regime is introduced as the traditional performance metrics such as ergodic capacity and outage capacity are no longer applicable. We focus on an opportunistic spectrum access system in which the secondary user (SU) opportunistically occupies the frequency resources of the primary user (PU) and transmits reliable short packets to its destination. An achievable rate maximization problem is then formulated for the SU in supporting URLLC services, subject to a probabilistic received-power constraint at the PU receiver and imperfect channel knowledge of the SU-PU link. To tackle this problem, an optimal power allocation policy is proposed. Closedform expressions are then derived for the maximum achievable rate in finite blocklength regime, the approximate transmission rate at high signal-to-noise ratios (SNRs) and the optimal average power. Numerical results validate the accuracy of the proposed closed-form expressions and further reveal the impact of channel estimation error, block error probability, finite blocklength and received-power constraint.

A device-to-device (D2D) ultra reliable low latency communications (URLLC) network is investigated in this paper. Specifically, a D2D transmitter opportunistically accesses the radio resource provided by a cellular network and directly transmits short packets to its destination. A novel performance metric is adopted for finite block-length code. We quantify the maximum achievable rate for the D2D network, subject to a probabilistic interference power constraint based on imperfect channel state information (CSI). First, we perform a convexity analysis which reveals that the finite block-length rate for the D2D pair in short-packet transmission is not always concave. To address this issue, we propose two effective resource allocation schemes using the successive convex approximation (SCA)-based iterative algorithm. To gain more insights, we exploit the mono- tonicity of the average finite block-length rate. By capitalizing on this property, an optimal power control policy is proposed, followed by closed-form expressions and approximations for the optimal average power and the maximum achievable average rate in the finite block-length regime. Numerical results are provided to confirm the effectiveness of the proposed resource allocation schemes and validate the accuracy of the derived theoretical results.

In this paper, we investigate the throughput performance of single-packet and multi-packet hybrid-automatic repeat request (HARQ) with blanking for downlink non-orthogonal multiple access (NOMA) systems. While conventional singlepacket HARQ achieves high throughput at the expense of high latency, multi-packet HARQ, where several data packets are sent in the same channel block, can achieve high throughput with low latency. Previous works have shown that multi-packet HARQ outperforms single-packet HARQ in orthogonal multiple access (OMA) systems, especially in the moderate to high signal-tonoise ratio regime. This work amalgamates multi-packet HARQ with NOMA to achieve higher throughput than the conventional single-packet HARQ and OMA, which has been adopted in the legacy mobile networks. We conduct theoretical analysis for the throughput per user and also investigate the optimization of the power and rate allocations of the packets, in order to maximize the weighted-sum throughput. It is demonstrated that the gain of multi-packet HARQ over the single-packet HARQ in NOMA systems is reduced compared to that obtained in OMA systems due to inter-user interference. It is also shown that NOMAHARQ cannot achieve any throughput gain with respect to OMAHARQ when the error propagation rate of the NOMA detector is above a certain threshold.