933 research outputs found

    Massive MIMO for Next Generation Wireless Systems

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    Multi-user Multiple-Input Multiple-Output (MIMO) offers big advantages over conventional point-to-point MIMO: it works with cheap single-antenna terminals, a rich scattering environment is not required, and resource allocation is simplified because every active terminal utilizes all of the time-frequency bins. However, multi-user MIMO, as originally envisioned with roughly equal numbers of service-antennas and terminals and frequency division duplex operation, is not a scalable technology. Massive MIMO (also known as "Large-Scale Antenna Systems", "Very Large MIMO", "Hyper MIMO", "Full-Dimension MIMO" & "ARGOS") makes a clean break with current practice through the use of a large excess of service-antennas over active terminals and time division duplex operation. Extra antennas help by focusing energy into ever-smaller regions of space to bring huge improvements in throughput and radiated energy efficiency. Other benefits of massive MIMO include the extensive use of inexpensive low-power components, reduced latency, simplification of the media access control (MAC) layer, and robustness to intentional jamming. The anticipated throughput depend on the propagation environment providing asymptotically orthogonal channels to the terminals, but so far experiments have not disclosed any limitations in this regard. While massive MIMO renders many traditional research problems irrelevant, it uncovers entirely new problems that urgently need attention: the challenge of making many low-cost low-precision components that work effectively together, acquisition and synchronization for newly-joined terminals, the exploitation of extra degrees of freedom provided by the excess of service-antennas, reducing internal power consumption to achieve total energy efficiency reductions, and finding new deployment scenarios. This paper presents an overview of the massive MIMO concept and contemporary research.Comment: Final manuscript, to appear in IEEE Communications Magazin

    Analog Feedback Communication System with Receive Diversity and MRC

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    This study investigates the analog feedback communication system (AFCS) architecture considering Rayleigh fading channel model and receive diversity maximum ratio combining (MRC). This architecture employs a power-efficient transmitting unit, an estimator at the receiver side and iterative algorithm that minimizes the mean square error (MSE). Using the feedback channel, the estimated received sample is fed back to the transmitter. The performance of AFCS with the Rayleigh fading channel model is evaluated using MSE optimization. The investigation revealed that when compared to a single input single output AFCS system, the diversity-enabled AFCS system achieves negligible MSE in fewer iterations. MSE of order 10 −3 is achieved by 6 receive antennas with MRC in only 4 iterations at 0dB channel signal-to-noise ratio (SNR), compared to a single input single output AFCS system that requires more than 10 iterations to achieve the same order of MSE

    Timing synchronization for cooperative wireless communications

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    In this work the effect of perfect and imperfect synchronization on the performance of single-link and cooperative communication is investigated. A feedforward non- data-aided near maximum likelihood (NDA-NML) timing estimator which is effective for an additive white Gaussian noise (AWGN) channel and also for a flat-fading channel, is developed. The Cramer Rao bound (CRB) and modified Cramer Rao bound (MCRB) for the estimator for a single-link transmission over an AWGN channel is derived. A closed form expression for the probability distribution of the timing estimator is also derived. The bit-error-rate (BER) degradation of the NDA-NML timing estimator with raised cosine pulse shaping for static timing errors over an AWGN channel is characterized. A closed form expression is derived for the conditional bit error probability (BEP) with static timing errors of binary phase shift keying modulation over a Rayleigh fading channel using rectangular pulse shaping. The NDA-NML timing estimator is applied to a cooperative communication system with a source, a relay and a destination. A CRB for the estimator for asymptotically low signal-to-noise-ratio case is derived. The timing complexity of the NDA-NML estimator is derived and compared with a feedforward correlation based data-aided maximum likelihood (DA-ML) estimator. The BER performance of this system operating with a detect-and-forward relaying is studied, where the symbol timings are estimated independently for each channel. A feedforward data and channel aided maximum likelihood (DCA-ML) symbol timing estimator for cooperative communication operating over flat fading channels is then developed. For more severe fading the DCA-ML estimator performs better than the NDA- NML estimator and the DA-ML estimator. The performance gains of the DCA-ML estimator over that of the DA-ML estimator become more significant in cooperative transmission than in single-link node-to-node transmission. The NDA-NML symbol timing estimator is applied to three-node cooperative communication in fast flat-fading conditions with various signal constellations. It is found that timing errors have significant effect on performance in fast flat-fading channels. The lower complexity NDA-NML estimator performs well for larger signal constellations in fast fading, when compared to DA-ML estimator. The application of cooperative techniques for saving transmit power is discussed along with the related performance analysis with timing synchronization errors. It is found that power allocations at the source and relay nodes for transmissions, and the related timing errors at the relay and the destination nodes, have considerable effect on the BER performance for power constrained cooperative communication. The performance of multi-node multi-relay decode-and-forward cooperative com- munication system, of various architectures, operating under different fading con- ditions, with timing synchronization and various combining methods, is presented. Switch-and-stay combining and switch-and-examine combining are proposed for multi-node cooperative communication. Apart from the proposed two combining methods equal gain combining, maximal ratio combining and selection combining are also used. It is demonstrated that synchronization error has significant effect on performance in cooperative communication with a range of system architectures, and it is also demonstrated that performance degradation due to synchronization error increases with increasing diversity. It is demonstrated that decode-and- forward relaying strategy with timing synchronization, using a very simple coding scheme, performs better than detect-and-forward relaying with timing synchronization. Analytical expressions are derived for BEP with static and dynamic timing synchronization errors over Rayleigh fading channels using rectangular pulse shaping for amplify-and-forward and detect-and-forward cooperative communications. Moment generating function (MGF) based approach is utilized to find the analytical expressions. It is found that timing synchronization errors have an antagonistic effect on the BEP performance of cooperative communication. With the relay intelligence of knowing whether symbols are detected correctly or not, detect- and-forward cooperative communication performs better than the low complexity amplify-and-forward cooperative communication

    Cooperative diversity for the cellular uplink: Sharing strategies, performance analysis, and receiver design

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    In this thesis, we propose data sharing schemes for the cooperative diversity in a cellular uplink to exploit diversity and enhance throughput performance of the system. Particularly, we consider new two and three-or-more user decode and forward (DF) protocols using space time block codes. We discuss two-user and three-user amplify and forward (AF) protocols and evaluate the performance of the above mentioned data sharing protocols in terms of the bit error rate and the throughput in an asynchronous code division multiple access (CDMA) cellular uplink. We develop a linear receiver for joint space-time decoding and multiuser detection that provides full diversity and near maximum-likelihood performance.;We also focus on a practical situation where inter-user channel is noisy and cooperating users can not successfully estimate other user\u27s data. We further design our system model such that, users decide not to forward anything in case of symbol errors. Channel estimation plays an important role here, since cooperating users make random estimation errors and the base station can not have the knowledge of the errors or the inter-user channels. We consider a training-based approach for channel estimation. We provide an information outage probability analysis for the proposed multi-user sharing schemes. (Abstract shortened by UMI.)

    Investigation of Channel Adaptation and Interference for Multiantenna OFDM

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