Interference as a Source of Green Signal Power in Cognitive Relay Assisted Co-Existing MIMO Wireless Transmissions
TL;DR: The results show that by exploiting free interference power, the secondary downlink can access the primary resources without deteriorating the primary users' performance.
Abstract: This paper investigates the potential of exploiting interference as a source of green signal energy in parallel transmissions of co-existing radio systems assisted by a cognitive relay. Assuming a cognitive radio setup, the purpose of the relay is to allow the resources of a primary downlink to be efficiently utilised by a secondary system. While conventionally the relay aims to completely remove the interference, we investigate a strategy of making use of interference energy when the cross-interference between the two systems is mutually constructive. In this way, the interference that already exists in the communication medium provides a source of green signal energy that mutually enhances the received signal power of primary and secondary users without the need to raise the transmitted power. In this direction two adaptive linear precoding techniques are proposed for the cognitive relay and compared to conventional precoding. The effect of green interference on the received signal to noise ratio (SNR) of the primary and secondary users is studied through theoretical analysis and used to predict the resulting error probability and outage performance. The results show that by exploiting free interference power, the secondary downlink can access the primary resources without deteriorating the primary users' performance.
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TL;DR: A maximum ratio transmission (MRT) based algorithm that jointly exploits DI and CSI to glean the benefits from constructive multiuser interference and novel constructive interference precoding techniques that tackle the transmit power minimization with individual SINR constraints at each user's receivers are proposed.
Abstract: This paper investigates the problem of interference among the simultaneous multiuser transmissions in the downlink of multiple-antenna systems Using symbol-level precoding, a new approach to exploit the multiuser interference is discussed The concept of exploiting the interference between spatial multiuser transmissions by jointly utilizing data information (DI) and channel state information (CSI), in order to design symbol-level precoders, is proposed To this end, the interference between data streams is transformed under certain conditions into useful signal that can improve the signal to interference noise ratio (SINR) of the downlink transmissions We propose a maximum ratio transmission (MRT) based algorithm that jointly exploits DI and CSI to glean the benefits from constructive multiuser interference Subsequently, a relation between the constructive interference downlink transmission and physical layer multicasting is established In this context, novel constructive interference precoding techniques that tackle the transmit power minimization (min-power) with individual SINR constraints at each user's receivers is proposed Furthermore, fairness through maximizing the weighted minimum SINR (max-min SINR) of the users is addressed by finding the link between the min power and max min SINR problems Moreover, heuristic precoding techniques are proposed to tackle the weighted sum rate problem Finally, extensive numerical results show that the proposed schemes outperform other state of the art techniques
204 citations
TL;DR: The theoretical analysis and simulations show the somewhat surprising result that for a given number of receivers the improved transmit diversity dominates the performance of practical linear precoders.
Abstract: We explore the performance of multiple input multiple output (MIMO) transmitters in correlated channels where increasing numbers of antenna elements are fitted in a fixed physical space. As well investigated in the literature, two main effects emerge in such a design: transmit spatial correlation and mutual antenna coupling. In contrast to the literature however, here we investigate the combined effect of reducing the distance between the antenna elements with increasing the number of elements in a fixed transmitter space. In other words, towards the implementation of large-scale MIMO transmitters in limited physical spaces, we investigate the joint effect of two contradicting phenomena: the reduction of spatial diversity due to reducing the separation between antennas and the increase in transmit diversity by increasing the number of elements. Within this context, we analytically approximate the performance of two distinct linear precoding designs. The theoretical analysis and simulations show the somewhat surprising result that for a given number of receivers the improved transmit diversity dominates the performance of practical linear precoders. Consequently, important benefits in the system sum rate can be gleaned by fitting more antenna elements in a fixed space by employing separations smaller than the wavelength of the transmit frequency.
203 citations
Cites background from "Interference as a Source of Green S..."
...While (4) contradicts this, it is an affordable and common simplification [15],[45][48] to attain a closed form expression of the received SNR....
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TL;DR: The definition of constructive interference (CI) is presented and the corresponding mathematical characterization is formulated for popular modulation types, based on which optimization-based precoding techniques are discussed.
Abstract: Interference is traditionally viewed as a performance limiting factor in wireless communication systems, which is to be minimized or mitigated. Nevertheless, a recent line of work has shown that by manipulating the interfering signals such that they add up constructively at the receiver side, known interference can be made beneficial and further improve the system performance in a variety of wireless scenarios, achieved by symbol-level precoding (SLP). This paper aims to provide a tutorial on interference exploitation techniques from the perspective of precoding design in a multi-antenna wireless communication system, by beginning with the classification of constructive interference (CI) and destructive interference (DI). The definition for CI is presented and the corresponding mathematical characterization is formulated for popular modulation types, based on which optimization-based precoding techniques are discussed. In addition, the extension of CI precoding to other application scenarios as well as for hardware efficiency is also described. Proof-of-concept testbeds are demonstrated for the potential practical implementation of CI precoding, and finally a list of open problems and practical challenges are presented to inspire and motivate further research directions in this area.
177 citations
Cites background from "Interference as a Source of Green S..."
...Compared with [131] and [132] that adopt the ‘strict phase-...
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...Early works include [131] and [132] based on selective CI precoding and correlation-rotation CI precoding, where a parallel transmission system aided with a cognitive relay is considered in [131] while an overlay CR network is investigated in [132]....
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...improved performance, which is however more computationally expensive than [131] and [132] since it needs to solve an optimization problem on a symbol level....
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...CI-based precoding has shown to be effective in CR networks [120], [131]–[136]....
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TL;DR: A unified view and classification of precoding techniques with respect to two main axes is presented: 1) the switching rate of the precoding weights, leading to the classes of block-level and symbol-level precoding and 2) the number of users that each stream is addressed to, hence unicast, multicast, and broadcast precoding.
Abstract: Precoding has been conventionally considered as an effective means of mitigating or exploiting the interference in the multiantenna downlink channel, where multiple users are simultaneously served with independent information over the same channel resources. The early works in this area were focused on transmitting an individual information stream to each user by constructing weighted linear combinations of symbol blocks (codewords). However, more recent works have moved beyond this traditional view by: 1) transmitting distinct data streams to groups of users and 2) applying precoding on a symbol-per-symbol basis. In this context, the current survey presents a unified view and classification of precoding techniques with respect to two main axes: 1) the switching rate of the precoding weights, leading to the classes of block-level and symbol-level precoding and 2) the number of users that each stream is addressed to, hence unicast, multicast, and broadcast precoding. Furthermore, the classified techniques are compared through representative numerical results to demonstrate their relative performance and uncover fundamental insights. Finally, a list of open theoretical problems and practical challenges are presented to inspire further research in this area 1 . 1 The concepts of precoding and beamforming are used interchangeably throughout this paper.
119 citations
Cites background from "Interference as a Source of Green S..."
...The applications of symbol-level precoding span different research areas in wireless communications: underlay cognitive radio system [99], [100], [102], [212], coor-...
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TL;DR: Scenarios where the interfering signal enhances the desired signal's useful power on an instantaneous basis and provides an unexplored source of additional signal power are discussed.
Abstract: Interference in wireless communications is traditionally treated as a cause of performance degradation. Whilst from a statistical viewpoint this is entirely justified, this article discusses scenarios where the interfering signal enhances the desired signal's useful power on an instantaneous basis and provides an unexplored source of additional signal power. The potential of harvesting this energy, which naturally exists in the communication system, is shown to be readily available in downlink systems where interference can be predicted. The main concept discussed is that instead of using knowledge of the interference to cancel, eliminate or avoid it, it is potentially more fruitful to use this knowledge to manipulate and make use of the interference towards the system's advantage. A significant source of useful signal power, which with conventional transmission techniques is left unexploited, can be used to improve the radio systems' performance and achieve reliable power-efficient communications where performance benefits are yielded without the need to increase the average transmitted power.
110 citations
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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
"Interference as a Source of Green S..." refers background in this paper
...Cognitive radio (CR) networks have recently gained attention as a promising concept towards using the licensed frequencies more efficiently [1]....
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...100734 obtains access to the licensed spectrum provided that the interference level caused to the primary network remains below some predefined interference temperature [1]....
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2,299 citations
TL;DR: A simple encoding algorithm is introduced that achieves near-capacity at sum rates of tens of bits/channel use and regularization is introduced to improve the condition of the inverse and maximize the signal-to-interference-plus-noise ratio at the receivers.
Abstract: Recent theoretical results describing the sum capacity when using multiple antennas to communicate with multiple users in a known rich scattering environment have not yet been followed with practical transmission schemes that achieve this capacity. We introduce a simple encoding algorithm that achieves near-capacity at sum rates of tens of bits/channel use. The algorithm is a variation on channel inversion that regularizes the inverse and uses a "sphere encoder" to perturb the data to reduce the power of the transmitted signal. This work is comprised of two parts. In this first part, we show that while the sum capacity grows linearly with the minimum of the number of antennas and users, the sum rate of channel inversion does not. This poor performance is due to the large spread in the singular values of the channel matrix. We introduce regularization to improve the condition of the inverse and maximize the signal-to-interference-plus-noise ratio at the receivers. Regularization enables linear growth and works especially well at low signal-to-noise ratios (SNRs), but as we show in the second part, an additional step is needed to achieve near-capacity performance at all SNRs.
1,796 citations
"Interference as a Source of Green S..." refers background in this paper
...The two cases of precoding investigated here for the BSs are maximum ratio combining (MRC) [14] and zero forcing (ZF) [12] given by...
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...Assuming that the number of antennas at the relay base station is equal to the total number of receive antennas of the primary and secondary systems, the cross-interference can be completely removed by adapting the conventional channel inversion linear precoding [12] to the cognitive relay scenario....
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TL;DR: In this article, Matrix Variate Distributions (MVD) are used to measure the degree of similarity between distributions in a set of distributions, i.e., matrix variance distributions.
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