1,720,976 research outputs found
Exact BER Performance of Asynchronous MC-DS-CDMA over Fading Channels
No full textIn this contribution an accurate average Bit Error Rate (BER) formula is derived for MC-DS-CDMA in the context of asynchronous transmissions and random spreading sequences. We consider a flat Nakagami-m fading channel for each subcarrier. Our analysis is based on the Characteristic Function (CF) and does not rely on any assumption concerning the statistical behavior of the interference. We develop a new closed-form expression for the conditional CF of the inter-carrier interference and provide a procedure for calculating the exact BER expressed in the form of a single numerical integration. The accuracy of the Standard Gaussian Approximation (SGA) technique is also evaluated. Link-level results confirm the accuracy of the SGA for most practical conditions
Exact BER performance of asynchronous MC-DS-CDMA over Nakagami-m fading channels
Full text linkIn this contribution an accurate average bit error rate (BER) formula is derived for Nakagami-faded MC-DSCDMA in the context of asynchronous transmissions and random spreading sequences. Our analysis is based on the Characteristic Function (CF) and does not rely on any assumption concerning the statistical behavior of the interference. We develop a new closed-form expression for the conditional CF of the inter-carrier interference and provide a procedure for calculating the exact BER expressed in the form of a single numerical integration. The accuracy of the standard Gaussian approximation (SGA) technique is also evaluated. Link-level results confirm the accuracy of the SGA for most practical conditions
Throughput and Efficiency Analysis of Network-coded HARQ for Two-way Wireless Systems
Full text linkDue to the explosive increase of wireless subscribers, it becomes more crucial to efficiently use the limited spectrum resources. To improve spectral efficiency, this study explores network-coded (NCed) automatic repeat request (ARQ) in two-way wireless systems. NCed-ARQ has been proved effective in multi-cast and broadcast channels by utilizing the side-information overheard during retransmissions. However, most wireless networks are two-way where each node transmits and receives. In two-way wireless systems, more side-information can be used to enhance the throughput performance with network coding, which has not received much attention. In this study, we propose a new NCed-ARQ scheme with reverse-link assistance to fully utilize the side-information generated in two-way wireless systems. We consider two typical two-way wireless system models. One is multi-access broadcast channel with one base-station and multiple users. In the second one, a relay is added as a low-cost solution for increasing the capacity. We derive the expressions of throughputs for both models. Our simulation results show that the proposed scheme achieves higher throughputs in both systems, especially for low/moderate SNRs. Additionally, NCed-ARQ requires more acknowledgments from users. The effect of the extra feedback, especially on the uplink of a two-way system, remains unclear. We also study the extra feedback required for NCed-ARQ and the resulting packet efficiency. With simulation, we reveal the trade-off in the throughput when considering the extra feedback
Optimization of limited feedback in two-way communications
No full textWireless communications are becoming more and more popular nowadays. Cell phones, wifi and other wireless communication methods seem to be in every corner of people\u27s daily life. The use of the wireless communications is becoming more diverse than before. Instead of simply talking on a phone, video chatting is more popular. Sharing a text message is not as well-liked as sharing a photo or a video. Cloud computing and remote storage are also developed to take over the place of local disk, etc. All of these needs require a highly increased data rate. However, the low utilization rate of transmitting packet gives a limit of increasing the data rate. To solve this problem, a two-way communication model is set up in the thesis to optimize the data rate with limited feedback. This model is under the condition that two nodes wishing to exchange information over a frequency division duplex, wireless additive white Gaussian noise with Rayleigh fading channel. The tradeoff between bits spent on learning channel, feedback and data must be carefully incorporated into the overall performance. In order to analyze the tradeoffs, a generalized framework is presented. Each packet contains three parts: a) training for estimating channel, b) feedback and c) data, where the feedback will contain ARQ (for requesting a re-transmission when a packet got lost) only or both ARQ and CSI (for transmitting channel state information). Under those conditions, an expression for the overall expected throughput in each direction is provided, which varies with the different allocating resources in a packet, channel conditions, SNR, transmitter protocols and receiver feedback protocols
Full duplex communication by means of backscattering modulation
No full textThe demand of high speed data is increasing day by day and hence wireless communication is getting popular. Nowadays we are using data through wireless communication for different aspect such as voice data, internet using, IP television and video conference. Due to high volume of downstream data the network is becoming more asymmetric. To accommodate this high volume of data researchers are working on different aspect. One of the key strategies is to make the wireless communication full duplex which till now is working as half duplex. Theoretically, making a wireless communication full duplex will double the channel capacity and hence it will open a new horizon of wireless communication system. In order to make the communication system full duplex we face a strong problem of self interference which basically contaminates the whole data signal. So we work to and an efficient way to get rid of the self interference and implement a successful full duplex system. In this thesis, we propose a design for full-duplex wireless communication system, based on backscatter modulation (BM) that is able to efficiently accommodate high downlink traffic. We consider two nodes that wish to exchange data over wireless additive white Gaussian noise with Rayleigh fading channel. Within this framework the end user receives self-interference free signal. The specific objectives of our research are to investigate a model of full-duplex wireless communication system by means of reflected power based on backscatter modulation (BM) where electromagnetic waves are modulated, amplified and reflected by the same antenna that receives them. Also to analyze the performance for using modulation impedance for guarantying sufficient power for the demodulator while exhibiting maximal difference in the backscattered field. In order to do so we design a framework to find out the optimized value of the modulation load for different scheme
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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Interference in Air to Ground CDMA Cellular Systems
Full text linkFor air-to-ground cellular systems with no frequency reuse, we provide an analysis of the inter-cell interference
under 3D hexagonal cell planning and a line of sight channel model with no shadowing assumptions. Based on
this model, we derive approximate bounds for the inter to intra-cell interference ratio for the air-to-ground link.
In addition, we provide upper bounds on the interference and the outage probability for the ground-to-air link.
Simulation results demonstrate that our approximations are extremely tight.Engineering and Applied SciencesAuthor's Origina
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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