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The rapid growth in the scale of the intelligent connected vehicles (ICVs) brings about the need for novel random access schemes that realize collision avoidance and reliable communication. The coded random access (CRA) scheme utilizes successive interference cancellation (SIC) across slots to enable the extraction of multiple users' packets from the collision slots and proves to be an efficient solution to collision resolution. In this paper, we propose a random backoff-based CRA scheme to achieve a tradeoff between the access success probability and access delay for CRA. Specifically, we partition the traditional CRA frame into subframes, so that the user access delay is reduced to the same scale of the subframe length. In addition, the backoff operation among subframes is introduced so that access failures originated from the subframe length limit can be reduced with the retransmission in subsequent subframes. In particular, we consider practical Nakagami-m fading channel in the CRA performance analysis and take into account the capture effect in the iterative interference cancellation process. Simulation results show that backoff-based CRA can significantly reduce the average access delay without severe negative effect on the access success probability for moderate system load compared with traditional CRA. Comparisons with the state-of-the-art non-orthogonal random access (NORA) scheme proposed for 5G indicates that backoff-based CRA provides a feasible solution for random access of large-scale ICVs.

This research uses repetition codes based on Coded Random Access (CRA) to support Internet of Things (IoT) to give priority to emergency commutations in super-dense networks. Degree distribution for emergency group and general group are obtained with extrinsic information transfer (EXIT) analysis to achieve small error performance shown by the very small gap between emergency group curve and general group curve. This research also evaluates performance by observing throughput and packet-loss rate (PLR) parameters from every groups. Offered traffic in PLR for emergency group user is G= 0,7 packet/slot without fading and G= 0,65 packet/slot with fading, while for public group is G=0,699 packet/slot without fading and G=0,42 packet/slot with fading. Peak throughput for emergency group is G= 0,737 packet/slot without fading and G= 0,729 packet/slot with fading. Peak Throughput for public group is G= 0,699 packet/slot without fading and G=0,685 packet/slot with fading. Throughput values of emergency group are higher than those of the general group, indicating successful process of giving priority for emergency group.

This paper focuses on new communication paradigms arising in massive multiple-input-multiple-output systems where the antenna array at the base station is of extremely large dimension (xMaMIMO). Due to the extreme dimension of the array, xMaMIMO is characterized by spatial non-stationary field properties along the array; this calls for a multi-antenna transceiver design that is adapted to the array dimension but also its non-stationary properties. We address implementation aspects of xMaMIMO, with computational efficiency as our primary objective. To reduce the computational burden of centralized schemes, we distribute the processing into smaller, disjoint subarrays. Then, we consider several low-complexity data detection algorithms as candidates for uplink communication in crowded xMaMIMO systems. Drawing inspiration from coded random access, one of the main contributions of the paper is the design of low complexity scheme that exploits the non-stationary nature of xMaMIMO systems and where the data processing is decentralized. We evaluate the bit-error-rate performance of the transceivers in crowded xMaMIMO scenarios. The results confirm their practical potential., Accepted in IEEE Global Communications Conference (Globecom) 2018 Workshops- 5GNR

Recent vehicular communications problems entail various aspects, such as rapidly changing topologies and large number of users. We consider massive uncoordinated multiway relay networks (mu-mRN) that can serve a massive number of users expecting to fully exchange information among them via a common multiway relay. Relay vehicular networks system is one of potential applications of the mu-mRN. In this paper, we are interested in improving normalized throughput T of the network using multiuser detection (MUD) capability of K > 1 based on the graph-based random access to flexibly adapt the topology changes. First, we present a network capacity bound of the mu-mRN with general K to investigate the theoretical limit of the networks. Then, we search for many optimal and practical degree distributions for each theoretical bound. Second, we aim to improve the normalized throughput by 10× from the maximum normalized throughput of conventional systems. To achieve the goal, we propose the mu-mRN applying doubly irregular coded slotted ALOHA. We also propose an optimal practical encoding of the mu-mRN to closely approach the target with finite number of users.

We consider multiway multirelay (MWMR) systems composed of massive number of users exchanging information among themselves via two relays. To avoid high complexity scheduling involving massive number of users, we consider a novel uncoordinated transmission schemes for MWMR systems. The relays keep forwarding (broadcasting) the received packet to all users even though it is erroneous. When both links between users and two relays are erroneous, this situation is regarded as massive and dynamic CEO problems in the uncoordinated networks. Joint decoding that exploits correlation of information received from two relays is proposed for final decoding process to reduce the power consumption of all users. The results show that the proposed techniques outperform the traditional systems in terms of bit-error-rate (BER) and throughput performances.

Supervisor:Tadashi Matsumoto, 情報科学研究科, 修士

Densely deployed wireless network is one of the most important solutions for spectrum crunch expected by 2020 with a huge economic impact. This paper proposes decoding schemes for high dense multiway multirelaying (hd-MWMR) systems comprising two multiway relays to serve huge number of users. Due to the nature of huge number, instead of using perfect scheduling, we consider coded random access schemes, where all users transmit their messages without coordination. Although the transmission is uncoordinated, the network structure still has a structure that provide an advantage. The theoretical bound for hd-MWMR systems exploiting two multiway relays is derived. To achieve the bound, we propose simple coding and decoding schemes based on iterative interference cancellation over a sparse graph involving two multiway relays. The results confirm that the second relay helps on the improvement of throughput performances, which is highly required for future wireless networks.

We study coded multichannel random access schemes for ultra-reliable low-latency uplink transmissions. We concentrate on non-orthogonal access in the frequency domain, where users transmit over multiple orthogonal subchannels and inter-user collisions limit the available diversity. Two different models for contention-based random access over Rayleigh fading resources are investigated. First, a collision model is considered, in which the packet is replicated onto $K$ available resources, $K' \leq K$ of which are received without collision, and treated as diversity branches by a maximum-ratio combining (MRC) receiver. The resulting diversity degree $K'$ depends on the arrival process and coding strategy. In the second model, the slots subject to collisions are also used for MRC, such that the number of diversity branches $K$ is constant, but the resulting combined signal is affected by multiple access interference. In both models, the performance of random and deterministic repetition coding is compared. The results show that the deterministic coding approach can lead to a significantly superior performance when the arrival rate of the intermittent URLLC transmissions is low., 2019 IEEE 20th International Workshop on Signal Processing Advances in Wireless Communications (SPAWC) - Special Session on Signal Processing for NOMA Communication Systems