Multicast is an effective technology to save network bandwidth while transmitting data packets in WiFi networks. However, it lacks of a mechanism to recover lost packets. When interference or congestion occurs in wireless networks, the receiver cannot receive complete data. In this Thesis, we propose a multicast retransmission mechanism which integrates NACK in WiFi networks. Also, the sender can dynamically adjust retransmission parameters according to the network status reported from the receiver. To demonstrate our proposed mechanism, three parts are implemented on the Linux platform. First, the RTP header is added in each packet and the buffer is maintained in both the sender and the receiver before the sender can transmit data. Second, a module is installed in the wireless card driver to examine the CRC errors caused by interference. Then the sender can be notified by NACK such that the sender may know the reasons of packet loss and the numbers of dropped packets. Third, the sender utilizes RTCP to calculate RTT and derive RTO. The retransmission time is calculated based on RTO, interferences, and congestion. Finally, our measurement results show that the sender can effectively use multicast retransmission according to various background traffics. Besides, the number of retransmitted packets can be significantly reduced.

Transmission Control Protocol (TCP) designed to deliver seamless and reliable end-to-end data transfer across unreliable networks works impeccably well in wired environment. In fact, TCP carries the around 90% of Internet traffic, so performance of Internet is largely based on the performance of TCP. However, end-to-end throughput in TCP degrades notably when operated in wireless networks. In wireless networks, due to high bit error rate and changing level of congestion, retransmission timeouts for packets lost in transmission is unavoidable. TCP misinterprets these random packet losses, due to the unpredictable nature of wireless environment, and the subsequent packet reordering as congestion and invokes congestion control by triggering fast retransmission and fast recovery, leading to underutilization of the network resources and affecting TCP performance critically. This thesis reviews existing approaches, details two proposed systems for better handling in networks with random loss and delay. Evaluation of the proposed systems is conducted using OPNET simulator by comparing against standard TCP variants and with varying number of hops.

Transmission Control Protocol (TCP) designed to deliver seamless and reliable end-to-end data transfer across unreliable networks works impeccably well in wired environment. In fact, TCP carries the around 90% of Internet traffic, so performance of Internet is largely based on the performance of TCP. However, end-to-end throughput in TCP degrades notably when operated in wireless networks. In wireless networks, due to high bit error rate and changing level of congestion, retransmission timeouts for packets lost in transmission is unavoidable. TCP misinterprets these random packet losses, due to the unpredictable nature of wireless environment, and the subsequent packet reordering as congestion and invokes congestion control by triggering fast retransmission and fast recovery, leading to underutilization of the network resources and affecting TCP performance critically. This thesis reviews existing approaches, details two proposed systems for better handling in networks with random loss and delay. Evaluation of the proposed systems is conducted using OPNET simulator by comparing against standard TCP variants and with varying number of hops.

Transmission Control Protocol (TCP) designed to deliver seamless and reliable end-to-end data transfer across unreliable networks works impeccably well in wired environment. In fact, TCP carries the around 90% of Internet traffic, so performance of Internet is largely based on the performance of TCP. However, end-to-end throughput in TCP degrades notably when operated in wireless networks. In wireless networks, due to high bit error rate and changing level of congestion, retransmission timeouts for packets lost in transmission is unavoidable. TCP misinterprets these random packet losses, due to the unpredictable nature of wireless environment, and the subsequent packet reordering as congestion and invokes congestion control by triggering fast retransmission and fast recovery, leading to underutilization of the network resources and affecting TCP performance critically. This thesis reviews existing approaches, details two proposed systems for better handling in networks with random loss and delay. Evaluation of the proposed systems is conducted using OPNET simulator by comparing against standard TCP variants and with varying number of hops.

Les demandes de haut débit, faible latence et grande fiabilité augmentent dans les nouvelles générations de systèmes de radiocommunications. Par conséquent, on propose de combiner la transmission non orthogonale avec les retransmissions HARQ afin de combattre les fluctuations de canal de transmission à haut débit. Dans la première partie de la thèse, on propose des protocoles de retransmissions HARQ avec l'aide d'un relai afin d'améliorer le débit et la fiabilité du système. Une version renforcée du protocole HARQ qui prend en compte le délai de retour est proposée dans la seconde partie de la thèse. / HARQ has become an important research field in the wireless digital communications area during the last years. In this thesis, we improve the HARQ mechanisms in terms of throughput and/or latency which are the bottleneck of next generation wireless communication systems. More precisely, we improve the time-slotted HARQ systems by mimicking NOMA, which means using superposed packets in a single-user context. In the first part of the thesis, we propose HARQ protocols using the help of a relay to improve the transmission rate and reliability. An enhanced HARQ protocol adapted to delayed feedback is proposed in the second part. In this new multi-layer HARQ protocol, additional redundant packets are sent preemptively before receiving the acknowledgement, and in superposition to other HARQ processes.

The research accomplished in this dissertation is focused on the design of effective solutions to the problem that error codes occur in the ZigBee-based meeting attendance registration system. In this work, several different check algorithms are compared, and the powerful error-detecting Cyclic Redundancy Check (CRC) algorithm is studied. In view of the features of the meeting attendance registration system, we implement the check module of CRC-8. This work also considers the data reliability. We assume use retransmission mechanism to ensure the validity and completeness of transmission data. Finally, the potential technical improvement and future work are presented.

In an LTE (Long-Term Evolution) network, HARQ (Hybrid Automatic Repeat reQuest) is used to reduce the error probability of retransmitted packets. However, HARQ cannot guarantee delay constraints for real-time traffic when RBs (Resource Blocks) are allocated improperly. To avoid the retransmitted real-time packets exceeding their delay constraints, we propose an HARQ scheduling scheme based on RTT (Round-Trip-Time) estimation. In this scheme, traffic are classified into real-time and non-real-time queues in which real-time queue are further classified into four sub-queues according to their retransmission times; i.e., the first transmission queue, the first retransmission queue, the second retransmission queue, and the third retransmission queue. For the four real-time queues, we estimate RTT and compute the number of RBs required satisfying the delay constraints. To prevent from starvation of non-real-time traffic, after allocating the RBs for real-time traffic, the remaining RBs are allocated for non-real-time traffic according to their MBR (Minimum Bit Rate). To analyze the proposed scheduling scheme, we build a mathematical model to derive the successful probability of retransmitted packets and the expected value of packet retransmission times. Finally, we compute average packet delay, average packet loss rate, and the throughput for both real-time and none-real-time traffic by varying packet error probability and the delay constraints of real-time traffic.

Driven by the ambition for ubiquitous networking, wireless networks had gained substantial technical advances in recent years. Using radio signals in air as data links, wireless networks can get rid of the tangling of wired cables. However, due to the inherent limitations of wireless channels and legacy protocol design, users of wireless networks today still suffer from the problems on low bandwidth and high error rates. The seven-layer Open System Interconnection (OSI) model was originally designed with wired network environments in mind. Following the seven-layer OSI model, each layer is responsible for handling specific tasks without communicating with each other. Due to the relative stability of wired channels, the strictly-layered approach works well in wired network environments. However, its adequacy is a controversy in wireless environments, since wireless networks have completely different characteristics from its wired counterparts. In wireless environments, channel conditions are highly time-varying and are affected by many factors. External interference or signal degradation may lead to severe packet loss. Even signal-to-noise ratios are fine, transmissions may still fail due to collisions when contention-based MAC protocols are adopted. Conventional protocols developed with wired network environments in mind cannot appropriately response to the characteristics of wireless channels and may make wrong reactions. For these reasons, a flexible framework to capture the rapid change conditions of wireless channels and respond to them immediately is necessary. In this dissertation, we design a cross-layer framework with the consideration of wireless network characteristics. By the coordination between the involved layers, the cross-layer framework can adapt to wireless channel conditions and significantly improve QoS in wireless networks. In order to reduce collision probabilities in wireless networks, we propose a novel protocol named Wait-and-Transmit, which effectively alleviates contentions in wireless networks. By reducing collision probabilities of wireless networks, transmission delays can be shortened and throughputs can be significantly improved. Aiming at the transmission paths containing at least one wireless link, a flexible and efficient cross-layer transmission scheme is also present in this dissertation, which separates the rapid change conditions such as collision probabilities from the relatively stable conditions and well responds to these changes. The proposed approaches significantly improve the performance of wireless networks. We believe that these approaches can contribute to the development of wireless networking.

No description available.

Multiple-input multiple-output (MIMO) communication systems employ multiple antennas at the transmitter and the receiver. Multiple antennas provide capacity gain and/or robust performance over single antenna communications. Traditional automatic-repeat-request (ARQ) techniques developed for single-input single-output (SISO) communication systems have to be modified in order to be employed in MIMO communication systems. In this dissertation, we propose and analysis some ARQ techniques for MIMO communication systems. The basic retransmission protocols of ARQ, stop-and-wait (SW-ARQ), go-back-$N$ (GBN-ARQ), and selective repeat (SR-ARQ), designed for SISO communication systems are generalized for parallel multichannel communication systems. The generalized ARQ protocols seek to improve the channel utilization of multiple parallel channels with different transmission rates and different packet error rates. The generalized ARQ protocols are shown to improve the transmission delay as well. A type-I hybrid-ARQ error control is used to illustrate the throughput gain of employing ARQ error control into MIMO communication systems. With the channel information known at both the transmitter and the receiver, the MIMO channel is converted into a set of parallel independent subchannels. The performance of the type-I hybrid-ARQ error control is presented. Simulation results show the throughput gain of using an ARQ scheme in MIMO communication systems. When the channel state information is unknown to the transmitter, error control codes that span both space and time, so-called space-time coding, are explored in order to obtained spatial diversity. As a consequence, the coding scheme used for ARQ error control has to be designed in order to consider coding across both space and time. In this dissertation, we design a set of retransmission codes for a type-II hybrid-ARQ scheme employing the multidimensional space-time trellis code as the forward error control code. A concept of sup-optimal partitioning of the (super-)constellation is proposed. The hybrid-ARQ error control scheme, consisting of the optimal code for each transmission, outperforms the hybrid-ARQ error control scheme, consisting of the same code for all transmissions.