Antenna Downlink Research Articles

Multicell MISO Downlink Weighted Sum-Rate Maximization: A Distributed Approach

We develop an easy to implement distributed method for weighted sum-rate maximization (WSRMax) problem in a multicell multiple antenna downlink system. Unlike the recently proposed minimum weighted mean-squared error based algorithms, where at each iteration all mobile terminals needs to estimate the covariance matrices of their received signals, compute and feedback over the air certain parameters to the base stations (BS), our algorithm operates without any user terminal assistance. It requires only BS to BS signalling via reliable backhaul links (e.g., fiber, microwave links) and all required computation is performed at the BSs. The algorithm is based on primal decomposition and subgradient methods, where the original nonconvex problem is split into a master problem and a number of subproblems (one for each BS). A novel sequential convex approximation strategy is proposed to address the nonconvex master problem. In the case of subproblems, we adopt an existing iterative approach based on second-order cone programming and geometric programming. The subproblems are coordinated to find a (possibly suboptimal) solution to the master problem. Subproblems can be solved by BSs in a fully asynchronous manner, though the coordination between subproblems should be synchronous. Numerical results are provided to see the behavior of the algorithm under different degrees of BS coordination. They show that the proposed algorithm yields a good tradeoff between the implementation-level simplicity and the performance.

Multi User Diversity Evaluation in MIMO HSDPA Downlink Channels

A multiple transmit antenna, single receive antenna (per receiver) downlink channel with limited channel feedback is considered. Given a constraint on the total system-wide channel feedback, the following question is considered: is it preferable to get low-rate feedback from a large number of receivers or to receive high-rate/high-quality feedback from a smaller number of (randomly selected) receivers. Acquiring feedback from many users allows multi-user diversity to be exploited, while highrate feedback allows for very precise selection of beamforming directions. It is shown that systems in which a limited number of users feedback high-rate channel information significantly outperform low-rate/many user systems. The marginal benefit of channel feedback is very significant up to the point where the CSI is essentially perfect.

The Radio Observatory on the Lunar Surface for Solar studies

The Radio Observatory on the Lunar Surface for Solar studies (ROLSS) is a concept for a near-side low radio frequency imaging interferometric array designed to study particle acceleration at the Sun and in the inner heliosphere. The prime science mission is to image the radio emission generated by Type II and III solar radio burst processes with the aim of determining the sites at and mechanisms by which the radiating particles are accelerated. Specific questions to be addressed include the following: (1) Isolating the sites of electron acceleration responsible for Type II and III solar radio bursts during coronal mass ejections (CMEs); and (2) Determining if and the mechanism(s) by which multiple, successive CMEs produce unusually efficient particle acceleration and intense radio emission. Secondary science goals include constraining the density of the lunar ionosphere by searching for a low radio frequency cutoff to solar radio emission and constraining the low energy electron population in astrophysical sources. Key design requirements on ROLSS include the operational frequency and angular resolution. The electron densities in the solar corona and inner heliosphere are such that the relevant emission occurs at frequencies below 10 MHz. Second, resolving the potential sites of particle acceleration requires an instrument with an angular resolution of at least 2°, equivalent to a linear array size of approximately 1000 m. Operations would consist of data acquisition during the lunar day, with regular data downlinks. No operations would occur during lunar night. ROLSS is envisioned as an interferometric array, because a single aperture would be impractically large. The major components of the ROLSS array are 3 antenna arms arranged in a Y shape, with a central electronics package (CEP) located at the center. The Y configuration for the antenna arms both allows for the formation of reasonably high dynamic range images on short time scales as well as relatively easy deployment. Each antenna arm is a linear strip of polyimide film (e.g., Kapton™) on which 16 science antennas are located by depositing a conductor (e.g., silver). The antenna arms can be rolled for transport, with deployment consisting of unrolling the rolls. Each science antenna is a single polarization dipole. The arms also contain transmission lines for carrying the radio signals from the science antennas to the CEP. The CEP itself houses the receivers for the science antennas, the command and data handling hardware, and, mounted externally, the downlink antenna. We have conducted two experiments relevant to the ROLSS concept. First, we deployed a proof-of-concept science antenna. Comparison of the impedance of the antenna feed points with simulations showed a high level of agreement, lending credence to the antenna concept. Second, we exposed a sample of space-qualified polyimide film, with a silver coating on one side, to temperature cycling and UV exposure designed to replicate a year on the lunar surface. No degradation of the polyimide film’s material or electric properties was found. Both of these tests support the notion of using polyimide-film based antennas. The prime science mission favors an equatorial site, and a site on the limb could simplify certain aspects of the instrument design. A site on the lunar near side is sufficient for meeting the science goals. While the site should be of relatively low relief topography, the entire site does not have to be flat as the fraction of the area occupied by the antenna arms is relatively small (∼0.3%). Further, the antenna arms do not have to lay flat as deviations of ±1 m are still small relative to the observational wavelengths. Deployment could be accomplished either with astronauts, completely robotically, or via a combination of crewed and robotic means. Future work for the ROLSS concept includes more exhaustive testing of the radio frequency (RF) and environmental suitability of polyimide film-based science antennas, ultra-low power electronics in order to minimize the amount of power storage needed, batteries with a larger temperature range for both survival and operation, and rovers (robotic, crewed, or both) for deployment. The ROLSS array could also serve as the precursor to a larger array on the far side of the Moon for astrophysical and cosmological studies.

The effects of channel estimation errors on a nonlinear precoder for multiple antenna downlink channels

The effects of channel estimation errors on the performance of a system employing Tomlinson-Harashima precoding and the QR decomposition, operating over multiple antenna frequency-flat fading channels with decentralized receivers, is evaluated. The QR decomposition of the channel matrix is employed to arrive at an equivalent channel where successive interference cancellation at the transmitter can be used to remove the effect of the multiuser interference. However, in the case of imperfect channel estimation, it is not possible to remove the effect of the multiuser interference due to a mismatch between the precoder and the channel. Consequently, it is necessary to include the estimation error in deriving the probability of symbol error. We also provide simulations to corroborate the analytical results.

Throughput versus Fairness: Channel-Aware Scheduling in Multiple Antenna Downlink

Channel aware and opportunistic scheduling algorithms exploit the channel knowledge and fading to increase the average throughput. Alternatively, each user could be served equally in order to maximize fairness. Obviously, there is a tradeoff between average throughput and fairness in the system. In this paper, we study four representative schedulers, namely the maximum throughput scheduler (MTS), the proportional fair scheduler (PFS), the (relative) opportunistic round robin scheduler (ORS), and the round robin scheduler (RRS) for a space-time coded multiple antenna downlink system. The system applies TDMA based scheduling and exploits the multiple antennas in terms of spatial diversity. We show that the average sum rate performance and the average worst-case delay depend strongly on the user distribution within the cell. MTS gains from asymmetrical distributed users whereas the other three schedulers suffer. On the other hand, the average fairness of MTS and PFS decreases with asymmetrical user distribution. The key contribution of this paper is to put these tradeoffs and observations on a solid theoretical basis. Both the PFS and the ORS provide a reasonable performance in terms of throughput and fairness. However, PFS outperforms ORS for symmetrical user distributions, whereas ORS outperforms PFS for asymmetrical user distribution.

Multi-Antenna Downlink Channels with Limited Feedback and User Selection

We analyze the sum-rate performance of a multi- antenna downlink system carrying more users than transmit antennas, with partial channel knowledge at the transmitter due to finite rate feedback. In order to exploit multiuser diversity, we show that the transmitter must have, in addition to directional information, information regarding the quality of each channel. Such information should reflect both the channel magnitude and the quantization error. Expressions for the SINR distribution and the sum-rate are derived, and tradeoffs between the number of feedback bits, the number of users, and the SNR are observed. In particular, for a target performance, having more users reduces feedback load.

Antenna combining for the MIMO downlink channel

A multiple antenna downlink channel where limited channel feedback is available to the transmitter is considered. In a vector downlink channel (single antenna at each receiver), the transmit antenna array can be used to transmit separate data streams to multiple receivers only if the transmitter has very accurate channel knowledge, i.e., if there is high-rate channel feedback from each receiver. In this work it is shown that channel feedback requirements can be significantly reduced if each receiver has a small number of antennas and appropriately combines its antenna outputs. A combining method that minimizes channel quantization error at each receiver, and thereby minimizes multi-user interference, is proposed and analyzed. This technique is shown to outperform traditional techniques such as maximum-ratio combining because minimization of interference power is more critical than maximization of signal power in the multiple antenna downlink. Analysis is provided to quantify the feedback savings, and the technique is seen to work well with user selection and is also robust to receiver estimation error.

Blind Signal Reception and Channel Estimation in Downlink MC-CDMA Communication System with Transmit Diversity

In this paper, we exploit transmit diversity in downlink multicarrier CDMA (MC-CDMA) system to provide a solution to a high data rate, high capacity, yet reliable and robust to fading channel network. Our first emphasis is on designing a blind (without knowing the undesired users' signatures and channel parameters) MC-CDMA receiver in the mobile station that can suppress the multiple access interference (MAI) and combine the desired signal stemmed from all the transmitting antennas. Two blind signal reception algorithms are proposed and the performances in terms of signal-to-interference-and-noise-ratio (SINR) are comprehensively analyzed. One is based on the decorrelating RAKE (DRAKE) scheme proposed in [IEEE Trans. Communs. 47(7), 1036---1045, 1999] and the other is designed to meet the minimum output energy (MOE) criterion [IEEE Trans. on Information Theory 41(4), 944---996, 1995]. The MOE receiver, though optimum in ideal situation, is semi-blind that the vector channel impulse responses (VCIR) should be explicitly known. While the proposed modified DRAKE receiver is completely blind that it does not require any information of VCIR. Furthermore, we demonstrate that in practical scenario, the modified DRAKE scheme substantially outperforms the MOE receiver. The second part of this paper is to develop an efficient and blind (without requiring training sequences) algorithm to estimate the VCIR of downlink antenna array MC-CDMA system. Simulation results show that both the channel estimation algorithm and the blind reception schemes are reliable and near-far resistant.

Rate Scheduling in Multiple Antenna Downlink Wireless Systems

We consider scheduling strategies for multiantenna and multibeam cellular wireless systems for high-speed packet data services on the downlink. We establish a fundamental connection between the stability region of the queuing system and the set of feasible transmission rates, which provides the basis for the scheduling algorithm proposed in this paper. Transmission using adaptive steerable beams and fixed sector beams are considered and average delay versus throughput results are obtained through simulations for the proposed scheduling scheme in each case. While in single antenna systems multiuser diversity gains are achieved by the scheduling algorithms that transmit to a single user in each scheduling interval, our results show that with multiple antennas, transmitting to a carefully chosen subset of users has superior performance. The multiantenna scheduling problem is closely related to the problem of coordinated scheduling for transmission through multiple base stations, where a user can receive signals from several base stations simultaneously. We consider the special case when three single-antenna base stations are allowed to cooperate and transmit to the users in the triangular region between the base stations and propose scheduling strategies that demonstrate significant gains.

Performance loss due to imperfect parameter estimation in FDD downlink antenna array techniques

This paper examines, through simulations, the performance loss in various frequency division duplex (FDD) downlink antenna array techniques for digital mobile communication systems, when various parameters are estimated from pilot signals. Performance loss from noisy channel estimates decreases with increasing pilot signal power, while loss from channel coefficient quantisation decreases with increasing number of quantisation bits. Also, noisy channel correlation matrices lead to small loss when only the principal eigenvector is used for transmission and to much larger loss in techniques based on all eigenvectors. Finally, narrow beams are affected more than wide ones by noisy selection or feedback of the highest signal-to-noise ratio (SNR) transmit beam. Copyright © 2004 AEI.

Impact of optical baffle on antenna pattern

One of the major concerns of antenna design for spacecraft applications is the effect of surrounding structures which can reflect and diffract the antenna's radiated energy and cause degradation in the antenna directivity, beam shape, and sidelobe levels. A case in point is the NASA Earth Observing System (EOS) spacecraft which has a large number of instruments on board. Among these is the Multiple Imaging Spectral Radiometer (MISR). The downlink antenna system (DAS) for the spacecraft is a very broad beam antenna and as such irradiates much of the spacecraft leading to the possibility of distortion of its pattern. The MISR, on the other hand, must be protected from optical glint resulting from the structures onboard the spacecraft and to this end the instrument has been fitted with a baffle. The optimal position for this baffle (from the point of view of the MISR instrument) places it in the near-field of the downlink antenna. The baffle material must be chosen so as to minimize the effect on the antenna pattern while adequately protecting the instrument. In an effort to select an optimum material, several optically black materials were considered, all of which were RF transmissive in varying degrees. The RF reflection coefficient of samples of these materials was found to be less than 0.03 as measured in a waveguide. The results were then used in an analytical assessment of the antenna pattern perturbation. Details of the modeling are described.

Precise orbit determination of high-earth elliptical orbiters using differenced Doppler and ranging measurements

Recent advances in NASA's Deep Space Network station calibration methods have led to renewed interest in the use of differenced radio metric data types for interplanetary navigation, particularly differenced Doppler and range. An orbit determination error analysis is described which compares the performance of these differenced data types when used in concert with conventional two-way Doppler for precise navigation for high-earth orbiters. Three highly elliptical orbits were investigated, with apogee heights on the order of 20000, 70000, and 156000 km. The analysis assumes that each orbiter's downlink antennas has no ground footprint limitation in the near apogee regime. Results indicate that the most significant navigational accuracy improvements are seen for the lowest altitude orbit by using differenced Doppler measurements in conjunction with two-way Doppler. The results for the two higher-altitude orbits, although less dramatic, suggest accuracy improvements can also be achieved when differenced range measurements are combined with two-way Doppler.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Single shaped reflector antennas for broadcasting satellites

For future direct broadcasting satellites in Japan, precisely contoured beam antennas will be required for onboard antennas. Single shaped-reflector antennas are suitable for that purpose, since they do not need complex feed networks. However, in a previous study, discontinuities in the reflector surface were observed. The problem of the discontinuity was circumvented in this work and the shaped reflectors were successfully designed to produce contoured beams for covering the Japanese islands for the downlink and feederlink antennas. The downlink antenna was fabricated, and the radiation pattern was verified by measurement. The radiation pattern meets the radio regulations imposed on the onboard antenna, such as sidelobe and cross-polarization characteristics.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Antenna‐beam allocation for a satellite‐fed millimeter‐wave personal communication system

AbstractThe satellite‐fed millimeter‐wave personal communication system that is described requires a satellite with a multi‐beam antenna in order to guarantee the required link availability in point‐to‐point communications between numerous widely distributed small Earth stations. The usual method of antenna beam allocation requires high beam power in the downlink, giving rise to realizability problems due to limitations on the output power of the millimeter‐wave transmitter. This paper proposes an approach to optimizing the allocation of downlink antenna beams in which the beams are assigned to service areas that are chosen with reference to population distribution, rainfall attenuation in the millimeter‐wave region, and constraints on beam power. A computer simulation based on nationwide rainfall attenuation and population distribution data indicates that the proposed approach is more effective than with the usual beam allocation method. The equipment configuration of the satellite is described.