Basak Can

Efficient Cooperative Diversity Schemes and Radio Resource Allocation for IEEE 802.16j

This paper studies various cooperative diversity schemes for orthogonal frequency division multiple access (OFDMA)- time division duplex (TDD) based two-hop cellular networks in low mobility scenarios, where the instantaneous channel state information is available at the base station. A user scheduling and radio resource allocation technique is developed in order to efficiently integrate various cooperative diversity schemes for the emerging IEEE 802.16j based systems. The analysis of the system with this scheduler shows that a simple cooperative diversity scheme which dynamically selects the best scheme between conventional relaying and direct transmission is promising in terms of throughput and implementation complexity. The conventional relaying refers to the scheme where the destination relies solely on the signals received through the relay.

Implementation Issues for OFDM-Based Multihop Cellular Networks

In this article we present various issues that need careful design for the successful implementation of OFDMA-based multihop cellular networks which need incorporation of relay terminals. The first issue we present is synchronization. We show that it is not a problematic issue for infrastructure-based relaying, where the relay is deployed by a system operator at strategic points in the cell. Second, we focus on the advantage of adaptive relaying and provide a frame structure to enable adaptive relaying in a cellular network operating according to the IEEE 802.16e standard. The third issue we present is related to hardware implementation aspects. Hardware performance and resource usage analysis will show that cooperative diversity schemes increase hardware resource usage and power/energy consumption at mobile terminals. The last issue we present is within the context of link layer ARQ, where we propose a novel retransmission method, named local retransmission-ARQ (LR-ARQ), which is designed to take advantage of the multihop nature of the cellular network. Practically, we show that LR-ARQ improves performance with respect to its single-hop counterpart in terms of cell latency, goodput, and throughput.

Link adaptation and selection method for OFDM based wireless relay networks

We propose a link adaptation and selection method for the links constituting an orthogonal frequency division multiplexing (OFDM) based wireless relay network. The proposed link adaptation and selection method selects the forwarding, modulation, and channel coding schemes providing the highest end-to- end throughput and decides whether to use the relay or not. The link adaptation and selection is done for each sub-channel based on instantaneous signal to interference plus noise ratio (SINR) conditions in the source-to-destination, source-to-relay and relay- to-destination links. The considered forwarding schemes are amplify and forward (AF) and simple adaptive decode and forward (DF). Efficient adaptive modulation and coding decision rules are provided for various relaying schemes. The proposed end-to-end link adaptation and selection method ensures that the end-to-end throughput is always larger than or equal to that of transmissions without relay and non-adaptive relayed transmissions. Our evaluations show that over the region where relaying improves the end- to-end throughput, the DF scheme provides significant throughput gain over the AF scheme provided that the error propagation is avoided via error detection techniques. We provide a frame structure to enable the proposed link adaptation and selection method for orthogonal frequency division multiple access (OFDMA)-time division duplex relay networks based on the IEEE 802.16e standard.

Traffic Analysis and Video Quality Evaluation of Multiple Description Coded Video Services for Fourth Generation Wireless IP Networks

For the performance evaluation of future wireless communication systems, such as the fourth generation wireless networks, traffic traces of realistic services are needed. Multiple description coding (MDC) is gaining a lot of interest lately and is a viable solution to provide robust video services over single or multi hop wireless networks and MDC introduces more flexibility for network coding. Furthermore it has the capability to support heterogeneous terminals as they are accepted to be used in 4G wireless networks. By means of twelve well known video sequences, in different video formats, we generate the frame size traces and evaluate the multiple description coding characteristics. In addition to that we highlight the expected overhead due to the underlying RTP/UDP/IP protocol suite. As an objective quality measurement at the application layer, we investigate the video quality in dependency of lost and error-prone descriptors. This allows researcher to convert the network losses of their network models directly into video quality values. This step makes the work unique as single layer coded video streams would always need further postprocessing to retrieve the video quality.

Hardware aspects of fixed relay station design for OFDM(A) based wireless relay networks

Hardware aspects of infrastructure based (i.e., fixed) relay station design with focus on digital baseband processing in OFDM(A) based systems are investigated. A novel architecture for the digital baseband processor for fixed relay stations is proposed to minimize the relay complexity and the implementation cost. Two methods of signal forwarding -amplify and forward (AF) and -decode and forward (DF) are developed and implemented on FPGA, exploiting parallel processing and a pipelined system architecture. In contrast to common heuristic conclusions existing in the literature, we conclude that the AF based implementation introduces significant hardware complexity resulting from high memory usage. On the other hand, the DF implementation requires higher clocking frequencies, making it more power-consuming as compared to AF-based relaying.

MAC-PDU Size Optimization for OFDMA Modulated Wireless Relay Networks

Hop adaptive MAC-PDU (Protocol Data Unit) size optimization is proposed for Orthogonal Frequency Division Multiple Access (OFDMA) based wireless relay networks. Such optimization improves the end-to- end goodput via MAC-PDU size optimization. We provide design and evaluation for two hop cellular networks with fixed relays where the Base Station (BS) to Relay Station (RS) links are potentially robust and are Line of Sight (LOS). The proposal reduces the total overhead transmitted in the end-to-end path via transmitting longer length packets in the potentially robust BS-to-RS links. The link level simulations show that, the reductions in overhead that consists of header and tailer is 25% to 44%. The system level investigations in a multi-user environment with a total of 3 relays in the cell show an average end-to-end goodput gain of 13%.

Novel Fractional Spur Relocation in All Digital Phase Locked Loops

We present a new model that estimates the locations of the fractional spurs relative to the center carrier. Based on this model, we present a novel technique to mitigate the effect of fractional spurs generated in All-Digital Phase Locked Loops. Instead of using power-consuming spur cancellation algorithms, the proposed scheme enables to move the fractional spurs to locations more preferable by the modulation fidelity. This new location of the spurs is decided based on the analytical model of the new scheme. Modulation fidelity is defined in terms of phase noise spectrum which has direct effect on mask and Error Vector Magnitude (EVM) for the transmitter (TX) and Bit Error Rate (BER) and sensitivity for the receiver (RX). Simulation results with this new scheme show that the fractional spurs are estimated correctly and are moved out of the band of the desired modulation, hence improving performance.