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Author

Sudhir Srinivasa

Bio: Sudhir Srinivasa is an academic researcher from University of California, Irvine. The author has contributed to research in topics: Communication channel & Cognitive radio. The author has an hindex of 27, co-authored 61 publications receiving 5757 citations.


Papers
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Journal ArticleDOI
24 Apr 2009
TL;DR: This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.
Abstract: Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowledge about the activity, channels, codebooks, and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay, or interweave the cognitive radios' signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.

2,516 citations

Journal ArticleDOI
TL;DR: The trade-off between regulation and autonomy inherent in the design and performance of cognitive networks is examined through a simple example, which shows that the optimal amount of licensing is equal to the duty cycle of the traffic arrivals.
Abstract: Cognitive radios are promising solutions to the problem of overcrowded spectrum. In this article we explore the throughput potential of cognitive communication. Different interpretations of cognitive radio that underlay, overlay, and interweave the transmissions of the cognitive user with those of licensed users are described. Considering opportunistic communication as a baseline, we investigate the throughput improvements offered by the overlay methods. Channel selection techniques for opportunistic access such as frequency hopping, frequency tracking, and frequency coding are presented. The trade-off between regulation and autonomy inherent in the design and performance of cognitive networks is examined through a simple example, which shows that the optimal amount of licensing is equal to the duty cycle of the traffic arrivals

379 citations

Proceedings ArticleDOI
01 Oct 2006
TL;DR: This work summarizes different cognitive radio techniques that underlay, overlay and interweave the transmissions of the cognitive user with those of the licensed users and investigates the inherent tradeoff between the primary detection and the cognitive link capacity.
Abstract: Cognitive radios are promising solutions to the problem of overcrowded and inefficient licensed spectrum. In this work we explore the throughput potential of cognitive communication. We summarize different cognitive radio techniques that underlay, overlay and interweave the transmissions of the cognitive user with those of the licensed users. Recently proposed models for cognitive radios based on the overlay technique are described. For the interweave technique, we present a `two switch' cognitive radio model and develop inner and outer bounds on the secondary radio capacity. Using the two switch model, we investigate the inherent tradeoff between the sensitivity of primary detection and the cognitive link capacity. With numerical results, we compare the throughputs achieved by the secondary user in the different models.

373 citations

Patent
12 Apr 2010
TL;DR: In this article, a first field and a second field are generated to be included in a preamble of the data unit to determine the duration of a data unit and modulated using a modulation scheme specified for a field corresponding to the first field.
Abstract: In a method for generating a data unit conforming to a first communication protocol, a first field and a second field to be included in a preamble of the data unit are generated. The first field includes a first set of one or more information bits that indicate a duration of the data unit and is formatted such that the first field allows a receiver device that conforms to a second communication protocol to determine the duration of the data unit. The second field includes a second set of one or more information bits that indicate to a receiver device that conforms to the first communication protocol that the data unit conforms to the first communication protocol. The first field and the second field are modulated using a modulation scheme specified for a field corresponding to the first field and the second field, respectively, by the second communication protocol.

338 citations

Journal ArticleDOI
TL;DR: In this paper, the capacity of opportunistic communication in the presence of dynamic and distributed spectral activity is investigated, where the time varying spectral holes sensed by a cognitive transmitter are correlated but not identical to those sensed by the cognitive receiver.
Abstract: We investigate the capacity of opportunistic communication in the presence of dynamic and distributed spectral activity, ie, when the time varying spectral holes sensed by the cognitive transmitter are correlated but not identical to those sensed by the cognitive receiver We develop a two switch model that captures the localized spectral activity estimates at the transmitter and receiver The information theoretic framework of communication with side information is employed to characterize the capacity of the cognitive link with both causal and non-causal side information at the transmitter and/or the receiver These capacity results are used to determine the benefits of any feedforward and feedback information We find that cognitive radio capacity is robust to the uncertainties arising out of distributed and dynamic spectral environments, even when the communication occurs in bursts of only 3-5 symbols The capacity depends strongly on the correlation of the local spectral environment at the cognitive transmitter and receiver

238 citations


Cited by
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Journal ArticleDOI
24 Apr 2009
TL;DR: This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.
Abstract: Cognitive radios hold tremendous promise for increasing spectral efficiency in wireless systems. This paper surveys the fundamental capacity limits and associated transmission techniques for different wireless network design paradigms based on this promising technology. These paradigms are unified by the definition of a cognitive radio as an intelligent wireless communication device that exploits side information about its environment to improve spectrum utilization. This side information typically comprises knowledge about the activity, channels, codebooks, and/or messages of other nodes with which the cognitive node shares the spectrum. Based on the nature of the available side information as well as a priori rules about spectrum usage, cognitive radio systems seek to underlay, overlay, or interweave the cognitive radios' signals with the transmissions of noncognitive nodes. We provide a comprehensive summary of the known capacity characterizations in terms of upper and lower bounds for each of these three approaches. The increase in system degrees of freedom obtained through cognitive radios is also illuminated. This information-theoretic survey provides guidelines for the spectral efficiency gains possible through cognitive radios, as well as practical design ideas to mitigate the coexistence challenges in today's crowded spectrum.

2,516 citations

Journal ArticleDOI
TL;DR: A key finding is that the feedback rate per mobile must be increased linearly with the signal-to-noise ratio (SNR) (in decibels) in order to achieve the full multiplexing gain.
Abstract: Multiple transmit antennas in a downlink channel can provide tremendous capacity (i.e., multiplexing) gains, even when receivers have only single antennas. However, receiver and transmitter channel state information is generally required. In this correspondence, a system where each receiver has perfect channel knowledge, but the transmitter only receives quantized information regarding the channel instantiation is analyzed. The well-known zero-forcing transmission technique is considered, and simple expressions for the throughput degradation due to finite-rate feedback are derived. A key finding is that the feedback rate per mobile must be increased linearly with the signal-to-noise ratio (SNR) (in decibels) in order to achieve the full multiplexing gain. This is in sharp contrast to point-to-point multiple-input multiple-output (MIMO) systems, in which it is not necessary to increase the feedback rate as a function of the SNR

1,717 citations

Journal ArticleDOI
TL;DR: This tutorial provides a broad look at the field of limited feedback wireless communications, and reviews work in systems using various combinations of single antenna, multiple antenna, narrowband, broadband, single-user, and multiuser technology.
Abstract: It is now well known that employing channel adaptive signaling in wireless communication systems can yield large improvements in almost any performance metric. Unfortunately, many kinds of channel adaptive techniques have been deemed impractical in the past because of the problem of obtaining channel knowledge at the transmitter. The transmitter in many systems (such as those using frequency division duplexing) can not leverage techniques such as training to obtain channel state information. Over the last few years, research has repeatedly shown that allowing the receiver to send a small number of information bits about the channel conditions to the transmitter can allow near optimal channel adaptation. These practical systems, which are commonly referred to as limited or finite-rate feedback systems, supply benefits nearly identical to unrealizable perfect transmitter channel knowledge systems when they are judiciously designed. In this tutorial, we provide a broad look at the field of limited feedback wireless communications. We review work in systems using various combinations of single antenna, multiple antenna, narrowband, broadband, single-user, and multiuser technology. We also provide a synopsis of the role of limited feedback in the standardization of next generation wireless systems.

1,605 citations

Journal ArticleDOI
TL;DR: Both analytical and numerical results are provided to demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the performance gain of F- NOMA over conventional MA can be further enlarged by selecting users whose channel conditions are more distinctive.
Abstract: Nonorthogonal multiple access (NOMA) represents a paradigm shift from conventional orthogonal multiple-access (MA) concepts and has been recognized as one of the key enabling technologies for fifth-generation mobile networks. In this paper, the impact of user pairing on the performance of two NOMA systems, i.e., NOMA with fixed power allocation (F-NOMA) and cognitive-radio-inspired NOMA (CR-NOMA), is characterized. For F-NOMA, both analytical and numerical results are provided to demonstrate that F-NOMA can offer a larger sum rate than orthogonal MA, and the performance gain of F-NOMA over conventional MA can be further enlarged by selecting users whose channel conditions are more distinctive. For CR-NOMA, the quality of service (QoS) for users with poorer channel conditions can be guaranteed since the transmit power allocated to other users is constrained following the concept of cognitive radio networks. Because of this constraint, CR-NOMA exhibits a different behavior compared with F-NOMA. For example, for the user with the best channel condition, CR-NOMA prefers to pair it with the user with the second best channel condition, whereas the user with the worst channel condition is preferred by F-NOMA.

1,391 citations

Journal ArticleDOI
01 Dec 2017
TL;DR: This work provides a comprehensive overview of the state of the art in power-domain multiplexing-aided NOMA, with a focus on the theoretical N OMA principles, multiple-antenna- aided NomA design, and on the interplay between NOMa and cooperative transmission.
Abstract: Driven by the rapid escalation of the wireless capacity requirements imposed by advanced multimedia applications (e.g., ultrahigh-definition video, virtual reality, etc.), as well as the dramatically increasing demand for user access required for the Internet of Things (IoT), the fifth-generation (5G) networks face challenges in terms of supporting large-scale heterogeneous data traffic. Nonorthogonal multiple access (NOMA), which has been recently proposed for the third-generation partnership projects long-term evolution advanced (3GPP-LTE-A), constitutes a promising technology of addressing the aforementioned challenges in 5G networks by accommodating several users within the same orthogonal resource block. By doing so, significant bandwidth efficiency enhancement can be attained over conventional orthogonal multiple-access (OMA) techniques. This motivated numerous researchers to dedicate substantial research contributions to this field. In this context, we provide a comprehensive overview of the state of the art in power-domain multiplexing-aided NOMA, with a focus on the theoretical NOMA principles, multiple-antenna-aided NOMA design, on the interplay between NOMA and cooperative transmission, on the resource control of NOMA, on the coexistence of NOMA with other emerging potential 5G techniques and on the comparison with other NOMA variants. We highlight the main advantages of power-domain multiplexing NOMA compared to other existing NOMA techniques. We summarize the challenges of existing research contributions of NOMA and provide potential solutions. Finally, we offer some design guidelines for NOMA systems and identify promising research opportunities for the future.

1,008 citations