Showing papers by "Thomas L. Marzetta published in 2011"

Energy and Spectral Efficiency of Very Large Multiuser MIMO Systems

Abstract: A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. Lower capacity bounds for maximum-ratio combining (MRC), zero-forcing (ZF) and minimum mean-square error (MMSE) detection are derived. A MRC receiver normally performs worse than ZF and MMSE. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option. The tradeoff between the energy efficiency (as measured in bits/J) and spectral efficiency (as measured in bits/channel use/terminal) is quantified. It is shown that the use of moderately large antenna arrays can improve the spectral and energy efficiency with orders of magnitude compared to a single-antenna system.

Pilot Contamination and Precoding in Multi-Cell TDD Systems

Abstract: This paper considers a multi-cell multiple antenna system with precoding used at the base stations for downlink transmission. Channel state information (CSI) is essential for precoding at the base stations. An effective technique for obtaining this CSI is time-division duplex (TDD) operation where uplink training in conjunction with reciprocity simultaneously provides the base stations with downlink as well as uplink channel estimates. This paper mathematically characterizes the impact that uplink training has on the performance of such multi-cell multiple antenna systems. When non-orthogonal training sequences are used for uplink training, the paper shows that the precoding matrix used by the base station in one cell becomes corrupted by the channel between that base station and the users in other cells in an undesirable manner. This paper analyzes this fundamental problem of pilot contamination in multi-cell systems. Furthermore, it develops a new multi-cell MMSE-based precoding method that mitigates this problem. In addition to being linear, this precoding method has a simple closed-form expression that results from an intuitive optimization. Numerical results show significant performance gains compared to certain popular single-cell precoding methods.

Analysis of the pilot contamination effect in very large multicell multiuser MIMO systems for physical channel models

Abstract: We consider multicell multiuser MIMO systems with a very large number of antennas at the base station. We assume that the channel is estimated by using uplink training sequences, and we consider a physical channel model where the angular domain is separated into a finite number of directions. We analyze the so-called pilot contamination effect discovered in previous work, and show that this effect persists under the finite-dimensional channel model that we consider. We further derive closed-form bounds on the achievable rate of uplink data transmission with maximum-ratio combining, for a finite and an infinite number of base station antennas.

System and method of wireless communication using large-scale antenna networks

Abstract: A communication method uses a distributed cell-free network of spatially independent service antennas. According to the method, pilot sequences are allocated to a user population of access terminals by an allocation procedure that imposes local relative orthogonality of pilot sequences. Channel coefficients for access terminals are determined by measuring allocated pilot sequences as received by each of the service antennas. In embodiments of the invention, the determination of channel coefficients is performed independently and the resulting channel coefficients are locally stored at the respective service antennas. At each service antenna, a processor independently uses locally stored channel coefficients to precode forward link signals or to at least partially decode reverse link signals.

Method and apparatus for interference cancellation for antenna arrays

Abstract: Embodiments of the claimed subject matter provide methods and apparatuses for interference cancellation. One embodiment of a method includes estimating, for an antenna in an antenna array including a plurality of antennas, interference parameters using analog signals received at the antenna on each of a plurality of subcarriers. Each interference parameter is associated with one of a plurality of symbols transmitted to one of a plurality of users on one of the plurality of subcarriers. This embodiment also includes canceling interference from analog signals received by the antenna on the plurality of subcarriers using the estimated interference parameters.

Uplink power efficiency of multiuser MIMO with very large antenna arrays

Abstract: A multiplicity of autonomous terminals simultaneously transmits data streams to a compact array of antennas. The array uses imperfect channel-state information derived from transmitted pilots to extract the individual data streams. The power radiated by the terminals can be made inversely proportional to the square-root of the number of base station antennas with no reduction in performance. In contrast if perfect channel-state information were available the power could be made inversely proportional to the number of antennas. A maximum-ratio combining receiver normally performs worse than a zero-forcing receiver. However as power levels are reduced, the cross-talk introduced by the inferior maximum-ratio receiver eventually falls below the noise level and this simple receiver becomes a viable option.

A Random Matrix-Theoretic Approach to Handling Singular Covariance Estimates

Abstract: In many practical situations we would like to estimate the covariance matrix of a set of variables from an insufficient amount of data. More specifically, if we have a set of N independent, identically distributed measurements of an M dimensional random vector the maximum likelihood estimate is the sample covariance matrix. Here we consider the case where N <; M such that this estimate is singular (noninvertible) and therefore fundamentally bad. We present a radically new approach to deal with this situation based on the idea of dimensionality reduction through an ensemble of isotropically random unitary matrices. We obtain two estimates cov and invcov which are estimates for the covariance matrix and the inverse covariance matrix respectively. Both estimates retain the original eigenvectors while altering the eigenvalues. We have a closed form analytical expression for cov and invcov in terms of the eigenvector/eigenvalue decomposition of the sample covariance. We motivate the use of invcov through applications to linear estimation, supervised learning, and high-resolution spectral estimation. We also compare the performance of these estimators with other more conventional methods.

Large-scale antenna method and apparatus of wireless communication with suppression of intercell interference

Abstract: Methods are provided for mitigating interference due to pilot contamination in a cellular network in which there is reuse of pilot signals. In embodiments, forward-link signals are precoded, using knowledge of slow-fading coefficients, to mitigate the interference. In embodiments, interference in reverse-link signals destined for a given base station is mitigated by linearly combining reverse-link signals destined for the given base station and for other base stations of the network, using knowledge of slow-fading coefficients.

MIMO System Having A Plurality Of Service Antennas For Data Transmission And Reception And Method Thereof

Abstract: Embodiments provide a MIMO system having a plurality of service antennas and method for data transmission and reception. The system includes a plurality of service antennas, where each service antenna is configured to simultaneously serve a plurality of terminals and independently receive a pilot sequence from the plurality of terminals. The system further includes a plurality of channel estimation units configured to independently generate an antenna-specific channel estimate based on the received pilot sequence and a plurality of pre-coding units configured to independently generate a coded signal to be transmitted to the plurality of terminals via a respective service antenna based on a set of data symbols and the antenna-specific channel estimate.

Method of accessing a wireless network and a wireless device

Abstract: In one embodiment, this method includes sending, from a terminal, a random access request; and receiving, at the terminal, system information from a base station via a unicast transmission in response to the random access request. The system information includes an identifier, uplink bandwidth information and downlink bandwidth information. The identifier identifies the base station, the uplink bandwidth information indicates a bandwidth to use for uplink transmission, and the downlink bandwidth information indicates a bandwidth used for downlink transmission.

Multiple antenna method for reducing inter-cell interference in multi-user wireless systems

Abstract: A method is provided to reduce inter-cell interference in mobile wireless systems, and particularly in TDD wireless systems. In an embodiment, a base station receives a pilot signal from at least one of the mobile terminals that it serves at a plurality of base station antennas, which include both main antennas and auxiliary antennas. In response, each of the base station antennas provides an output that is processed to obtain a set of precoding weights for a transmission from the main antennas. The processing includes nulling at least one interfering signal using the outputs from at least the auxiliary antennas.

Power Allocation in a Wireless System with Base Stations Having Antenna Arrays

Abstract: Provided is a method and apparatus for allocating transmit power in a wireless network. The method includes determining powers at which a base station transmits downlink signals to each active user equipment associated with the base station such that each of the downlink signals is associated with a different active user and the base station is permitted to transmit downlink signals with different powers to the associated active user equipments. The method further includes transmitting the associated downlink signals, by the base station, to the active user equipments at the determined powers.

MIMO system having a plurality of service antennas for data transmission thereof

Abstract: Embodiments provide a MIMO system having a plurality of service antennas and method for data transmission and reception. The system includes a plurality of service antennas, where each service antenna is configured to simultaneously serve a plurality of terminals and independently receive a pilot sequence from the plurality of terminals. The system further includes a plurality of channel estimation units configured to independently generate an antenna-specific channel estimate based on the received pilot sequence and a plurality of pre-coding units configured to independently generate a coded signal to be transmitted to the plurality of terminals via a respective service antenna based on a set of data symbols and the antenna-specific channel estimate.

Mimo transmission system with decentralized channel estimation and precoding

Abstract: It is provided a MIMO system having a plurality of service antennas and method for data transmission and reception. The system includes a plurality of service antennas, where each service antenna is configured to simultaneously serve a plurality of terminals and independently receive a pilot sequence from the plurality of terminals. The system further includes a plurality of channel estimation units configured to independently generate an antenna - specific channel estimate based on the received pilot sequence and a plurality of pre-coding units configured to independently generate a coded signal to be transmitted to the plurality of terminals via a respective service antenna based on a set of data symbols and the antenna- specific channel estimate.