1,720,961 research outputs found
Block Diagonalization and User Selection for MIMO Broadcast Channels with Limited Feedback
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On Achievable Rate of User Selection for MIMO Broadcast Channels With Limited Feedback
No full textWe consider the block diagonalization (BD) and user selection based on limited feedback in multiple antenna broadcast channels. With limited feedback, due to the imperfect channel state information at the transmitter (CSIT), BD cannot completely eliminate multiuser interference, and the throughput is correspondingly lower than that achieved with perfect CSIT. Nevertheless, the achievable multiuser diversity gain can be the same, such that limited-feedback-based BD can achieve an optimal throughput growth as the number of users increases. To show this, we first propose a channel quality indicator (CQI) for user selection. The CQI is designed to accurately estimate an achievable rate of each user and is given by an expected rate, where the expectation is solely taken over precoding matrices which cannot be known at the feedback stage. With the proper CQI, user selection can benefit from a large number of users in the system. As a result, we show that the BD can achieve an asymptotically optimal growth in throughput with the proposed CQI, based solely on a finite-rate feedback of channel information.11sciescopu
On Achievable Multiplexing Gain of BD in MIMO Broadcast Channels With Limited Feedback
No full textTo maintain a specific degree of multiplexing gain with limited feedback in multiple-input/multiple-output broadcast channels, the number of feedback bits has been increased linearly with the signal-to-noise ratio (SNR) measured in decibel, and conditions for the slope of this increase have been derived in previous studies. However, previous studies mostly focused on the chordal distance (ChD) for channel quantization, although it is not an optimally designed distance measure. Thus, the possibility exists that the same degree of multiplexing gain can be obtained by using fewer feedback bits than the existing method requires. In this paper, we propose an optimal distance measure that can maximize the multiplexing gain achieved by limited-feedback-based block diagonalization over broadcast channels. Then, we provide a sufficient condition for the number of feedback bits to achieve a specific degree of multiplexing gain with the proposed method. The sufficient number of feedback bits is given by a linear function of the SNR (decibel) as in previous studies, but the slope of the linear increase can be much less than that obtained in previous studies. As a consequence, the proposed method achieves higher multiplexing gain and corresponding throughput than the existing methods with the same number of feedback bits.11Nsciescopu
Time-Domain Differential Feedback for Massive MISO-OFDM Systems in Correlated Channels
No full textMassive multiple-input multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) is a promising technology for next-generation wireless communications. However, when channel state information (CSI) at the transmitter is obtained using channel feedback, the benefits of this system are severely limited by the tradeoff between downlink capacity and feedback overhead. To solve this problem, we propose a time-domain differential feedback scheme for massive multiple-input single-output (MISO) OFDM systems. The proposed scheme exploits channel correlations in time, frequency, and space domains simultaneously by considering differential channel impulse response (CIR). To simplify the codebook design, and to reduce codeword-search complexity, we partition and then quantize the differential CIR using a number of subcodebooks. Because the total feedback bits are shared by the subcodebooks, we further optimize the bit allocation for them to minimize the total quantization error. For this, we analyze the quantization error of the proposed scheme and then use the analysis results for the bit-allocation optimization. In simulations, the proposed scheme efficiently exploits all correlations and achieves significant spectral-efficiency gain compared to conventional feedback schemes.11sciescopu
Rate Adaptation and Power Allocation for Cognitive Radio Networks with HARQ-Based Primary System
No full textIn this paper, we propose a protocol for the coexistence of primary and secondary systems over block-fading channels. In the protocol, the primary system employs a hybrid automatic repeat request (HARQ). When the primary system retransmits the data signal, the secondary system serves as a relay for the retransmission of the primary system and simultaneously transmits its data signal. To efficiently accomplish the protocol, we analyze the average throughput of the primary and secondary systems by using the long-term average throughput (LAT). We formulate an optimization problem to maximize the LAT of the secondary system. The constraint of the optimization problem is that the LAT of the primary system with secondary system is not less than that of the primary system alone. Through the optimization problem, we obtain the closed-form solutions of the transmission rate of the secondary system and the fraction of the transmit power for relaying the primary system's data signal and transmitting the secondary system's data signal. Numerical results show that the primary system does not lose the average throughput, and rather achieves an additional throughput gain by adjusting the fraction of the transmit power of the secondary system.
MSE-Based Transceiver Design for Decentralized Transmit Power Minimization of Multi-Cell MIMO Cognitive Radio Systems
No full textThis paper considers multiple-input multiple-output (MIMO) cognitive radio (CR) networks in which multiple primary and secondary systems coexist. We propose a transceiver design algorithm to minimize the total transmit power of secondary systems subject to the signal to interference plus noise ratio (SINR) constraint for each secondary user and the interference constraint for each primary user. Due to the complexity of backhaul among secondary systems, perfect knowledge about the channel state information (CSI) may not be available in practice. To overcome this problem, we decompose the transmit power minimization problem into cascade problems of decentralized beamformer design with the interference constraint and power allocation with the SINR constraint. For beamformer design, we formulate the total mean squared error of data symbol minimization problem, to enhance the feasible rate and only use the local CSI via dual decomposition. The power allocation problem is established as linear programming. Simulation results show that, compared to the conventional centralized algorithm, the proposed decentralized algorithm achieves much higher feasible rate with slightly increased transmit power
Maximization of Long-Term Average Throughput for Cooperative Secondary System With HARQ-Based Primary System in Cognitive Radio Network
No full textWe consider a cognitive radio network in which a secondary system (SS) coexists with a hybrid-automatic-repeat-request (HARQ)-based primary system (PS) over block-fading channels. We propose a coexistence scheme in which the SS acts as a relay for the PS and gains spectrum access opportunities as a reward for the cooperation. The proposed scheme aims at achieving nontrivial throughput gain for the SS without affecting the performance of the PS. To this end, we first analyze the long-term average throughputs (LATs) of the PS and SS by applying total probability theorem and a mixture-of-Gamma approximation. Then, we optimize the transmit power ratio and the target rate of the SS to maximize its LAT. In simulations, the proposed scheme provides significant LAT gain for the SS while maintaining the LAT of the PS.
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Variations on the Author
Get PDF“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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