Date of Award
Doctor of Philosophy (PhD)
Despite the unprecedented success and proliferation of wireless communication, sustainable reliability and stability among wireless users are still considered important issues in the underlying link protocols. Existing link-layer protocols, like ARQ  or HARQ [57,67] approaches are designed to achieve this goal by discarding a corrupted packet at the receiver and performing one or more retransmissions until the packet is successfully decoded or a maximum number of retransmission attempts is reached. These strategies suffer from degradation of throughput and overall system instability since packets need to be en/decode in every hop, leading to high burden for relay nodes especially when the traffic load is high. On the other hand, due to the broadcast nature of wireless communication, when a relay transmits a packet to a specific receiver, it could become interference to other receivers. Thus, rather than activating all the relays simultaneously, we can only schedule a subset of relays in each time slot such that the interference among the links will not cause some transmissions to fail. Accordingly, in this dissertation, we mainly address the following two problems: 1) Relay selection: given a route (i.e., a sequence of relays), how to select the relays to en/decode packets to minimize the latency to reach the destination? 2) Link scheduling: how to schedule relays such that the interference among the relays will not cause transmission failure and the throughput is maximized? Relay Selection Problem. To solve the relay selection problem, we propose a Code Embedded Distributed Adaptive and Reliable (CEDAR) link-layer framework that targets low latency. CEDAR is the first theoretical framework for selecting en/decoding relays to minimize packet latency in wireless communication networks. It employs a theoretically-sound framework for embedding channel codes in each packet and performs the error correcting process in selected intermediate nodes in packet's route. To identify the intermediate relay nodes for en/decoding to minimize average packet latency, we mathematically analyze the average packet delay, using Finite State Markovian Channel model and priority queuing model, and then formalize the problem as a non-linear integer programming problem. To solve this problem, we design a scalable and distributed scheme which has very low complexity. The experimental results demonstrate that CEDAR is superior to the schemes using hop-by-hop decoding and destination-decoding in terms of both packet delay and throughput. In addition, the simulation results show that CEDAR can achieve the optimal performance in most cases. Link Scheduling Problem. As for the link scheduling problem, we formulate a new problem called Fading-Resistant Link Scheduling (Fadin-R-LS) problem, which aims to maximize the throughput (the sum data rate) for all the links in a single time slot. The problem is different from the existing link scheduling problems by incorporating the Rayleigh-fading model to describe the interference. This model extends the deterministic interference model based on the Signal-to-Interference Ratio (SIR) using stochastic propagation to address fading effects in wireless networks. Based on the geometric structure of Fadin-R-LS, we then propose three centralized schemes for Fadin-R-LS, with O(g(L)), O(g(L)), and O(1) performance guarantee for packet latency, where g(L) is the number of length magnitudes of link set L. Furthermore, we propose a completely distributed approach based on game theory, which has O(g(L)^2\alpha) performance guarantee. Furthermore, we incorporate a cooperative communication (CC) technique, e.g., maximum ratio combining (MRC), into our system to further improve the throughput, in which receivers are allowed to combine messages from different senders to combat transmission errors. In particular, we formulate two problems named cooperative link scheduling problem (CLS) and one-shot cooperative link scheduling problem (OCLS). The first problem aims to find a schedule of links that uses the minimum number of time slots to inform all the receivers. The second problem aims to find a set of links that can inform the maximum number of receivers in one time slot. We prove both problems to be NP-hard. As a solution, we propose an algorithm for both CLS and OCLS with g(K) approximation ratio, where g(K) is so called the diversity of key links. In addition, we propose a greedy algorithm with O(1) approximation ratio for OCLS when the number of links for each receiver is upper bounded by a constant. In addition, we consider a special case for the link scheduling problem, where there is a group of vehicles forming a platoon and each vehicle in the platoon needs to communicate with the leader vehicle to get the leader vehicle's velocity and location. By leveraging a typical feature of a platoon, we devise a link scheduling algorithm, called the Fast and Lightweight Autonomous link scheduling algorithm (FLA), in which each vehicle determines its own time slot simply based on its distance to the leader vehicle. Finally, we conduct a simulation on Matlab to evaluate the performance of our proposed methods. The experimental results demonstrate the superior performance of our link scheduling methods over the previous methods.
Qiu, Chenxi, "A Time-Efficient Strategy For Relay Selection and Link Scheduling In Wireless Communication Networks" (2015). All Dissertations. 1588.