Jihwan P. Choi

Optimum power and beam allocation based on traffic demands and channel conditions over satellite downlinks

For data services over satellite networks, the efficient management of satellite downlink communication resources is crucial for economic competitiveness of the medium. To support a broad spectrum of users with small terminals at high rates, narrow transmit spotbeams on the satellite will be used. Since satellite transmitter resources are expensive and there can be many spotbeam-coverage cells within the satellite service area, it is attractive to use some form of agile scanning beam system and to time-share these precious resources. An optimized design of the multibeam antenna pattern and scheduling can further improve the efficiency of transmission and power management. In this paper, the advantage of parallel multibeams in terms of spectral efficiency and power gain is shown, and the issue of multibeam power allocation based on traffic demands and channel conditions over satellite downlinks with power and delay constraints is addressed. The study indicates that the use of a parallel multibeam scheme with optimum power allocation can achieve a substantial power gain and reasonable proportional fairness. By coupling power allocation with multibeam scheduling when the number of active beams is smaller than the number of cells, the authors show that a modest number of active parallel beams are sufficient to cover many cells efficiently.

Predicting and adapting satellite channels with weather-induced impairments

Efficiency improvements using predictive and adaptive methods over satellite channels with weather-induced impairments are presented. Scintillation and rain attenuation are the two dominant factors for signal fading over satellite-Earth paths at operating frequencies over 10 GHz. We develop statistical and spectral analyses of these processes, and obtain simple linear predictors for received signal attenuation using autoregressive (AR) models. For adaptation, we propose changing signal transmission power, modulation symbol size, and/or code rate as the state of the channel changes. In particular, we introduce a continuous power control and discrete rate control strategy. Quantitative analyses of power consumption and channel capacity indicate that there can be a substantial gain in performance with such adaptive schemes.

Generalized co-phasing for multiple transmit and receive antennas

We propose and study a class of transmit beamforming techniques for systems with multiple transmit and multiple receive antennas with a per-antenna transmit power constraint. The per-antenna transmit power constraint is more realistic than the widely used total (across all transmit antennas) power constraint, since in practice each transmit antenna is driven by a separate power amplifier with a maximum power rating. Under the per-antenna power constraint, from an implementation perspective, it becomes desirable to vary only the phases (as opposed to both power and phase variation) of the signals departing from the transmit antennas. We name this class of techniques generalized co-phasing and formulate an optimization problem to calculate the transmit antenna phases. Furthermore, we propose five heuristic algorithms to solve the optimization problem. All the proposed algorithms except one are optimal for the case of two transmit antennas and an arbitrary number of receive antennas. For an arbitrary number of transmit and receive antennas, simulations indicate that the proposed algorithms perform very close to the optimal solution calculated through an exhaustive search of all possible transmit phases.

Satellite multibeam allocation and congestion control with delay constraints

A data satellite network would have to support a broad spectrum of bursty unscheduled users with different delay constraints over time-varying atmospheric satellite channels. Since onboard resources are expensive and a broadband satellite will have many spotbeam-coverage cells within its service area, it is desirable to use agile scanning beam systems, adaptive to service requirements and channel conditions, and time-share a small number of active downlink beams. In addition, a congestion control mechanism of incoming traffic into the satellite service queues is required to provide a reasonable queueing delay within delay deadlines. In this paper, a jointly optimized scheme of multibeam allocation and congestion control based on incoming traffic and channel conditions over fading satellite downlinks with beam-sharing and delay constraints is developed. With the assumptions of quasistatic channel conditions and predictable incoming traffic rates (at least in the short term), an analytical solution for joint beam allocation and congestion control is considered. Numerical examples will be given to show that the joint scheme outperforms uniform allocation by serving, with fairness, more accepted traffic (e.g., a factor of 2 in the case of linearly distributed traffic across cells) and/or a smaller queueing delay.

Joint Maximum Likelihood Estimation of Channel and Preamble Sequence for WiMAX Systems

This paper examines the detection problem of the preamble sequence index in the WiMAX system. The mobile station receiver knows all the possible preamble sequences and should estimate which preamble sequence has been transmitted from the base station. Since the preamble in the orthogonal frequency division multiplexing (OFDM) transmission is usually the first received symbol, the channel is unknown to the receiver, which makes the problem of preamble sequence estimation complicated. In this paper, this problem is addressed by developing the joint maximum likelihood (ML) estimator of the preamble sequence and the channel. A simple decoupled estimator and a minimum mean square error (MMSE) estimator are also presented as benchmarks for the joint ML estimator. Then it is shown how the joint ML estimator can be used for the segment detection. Since the joint ML estimator can be computationally complex in its general form, low-complexity algorithms are developed depending on the type of pilot subcarrier locations for general OFDM systems including WiMAX. The simulation results show that the joint ML estimator detects the preamble sequence index very well in the absence of the channel knowledge.

Near-Field Magnetic Induction MIMO Communication Using Heterogeneous Multipole Loop Antenna Array for Higher Data Rate Transmission

In this paper, we propose a novel method realizing multi-input-multi-output (MIMO) transmission in near-field magnetic induction (NFMI) communication for increasing channel capacity where limited capacity is a major bottleneck problem in magnetic communication systems. Since conventional magnetic communication systems using the same antenna patterns cannot easily implement MIMO transmission due to strong crosstalk between transmitters, we propose heterogeneous antenna arrays with multipole antennas, enabling crosstalk cancellation. We confirm the crosstalk cancellation of the proposed antenna array through numerical simulations and experiments, and verify the capacity enhancement of the proposed NFMI communication system over conventional NFMI communication schemes.

Adaptive communications over fading satellite channels

Effectiveness of prediction and adaptation of fading satellite channels induced by bad weather conditions is presented in terms of power consumption and channel capacity improvement. We suggest adaptive schemes that vary transmission rate, code rate, transmission power or all of these, incorporated with prediction functions for channel states. For prediction, we develop simple linear predictors for received signal attenuation using autoregressive (AR) models. For adaptation, we introduce a continuous power control and discrete rate control strategy. The quantitative analyses of excess average power and channel capacity indicate that the adaptive schemes can achieve substantial performance gains.

Optimum multibeam satellite downlink power allocation based on traffic demands

For multimedia services over satellite networks, the efficient management of satellite downlink communications will be crucial due to the high cost of satellite transmission to many users with different quality of service (QoS) requirements and limited amount of onboard resources. An optimized design of the agile antenna multibeam pattern can improve the efficiency of transmission and power management. In this paper, we show the advantage of parallel multibeams in terms of spectral efficiency and power gain, and address the issue of multibeam power allocation based on traffic demands over satellite downlinks with power restriction. The analysis indicates that the use of a parallel multibeam scheme with optimized power splits can achieve a substantial power gain and reasonable proportional fairness. We also give a downlink multibeam scheduling solution when the number of beams is smaller than the number of cells, and show that a modest number of parallel beams are sufficient to obtain large power gains without need for many transmitters.

Joint Maximum Likelihood Estimation of Channel and Preamble Sequence for OFDM Systems

This paper examines the detection problem of the preamble sequence index in the orthogonal frequency division multiplexing (OFDM) system. The receiver knows all the possible preamble sequences and should estimate which preamble sequence has been transmitted. Preamble sequence detection is straightforward with the knowledge of channel conditions. However, since the preamble OFDM symbol is usually the first symbol that is received, the channel is unknown to the receiver, which makes this problem complicated. In this paper, the joint maximum likelihood (ML) estimator of the preamble sequence and the channel is derived and compared with a simple decoupled estimator. The estimators are applied to IEEE 802.16e systems, and it is shown by simulation that the joint ML estimator detects the preamble sequence index very well in the absence of the channel knowledge.

Joint phased array antenna gain patterning and scheduling for satcom transmission

The economic viability of data satellite networks requires efficient utilization of precious resources. The phased array antenna can provide flexibility of beam size, shape and transmission rates. The optimum design of antenna gain pattern and scheduling is critical for supporting a broad spectrum of unscheduled bursty users with small receiver antennas at high data rates. In this paper, we describe the optimum joint solution for phased array antenna gain patterning and scheduling. We examine two limiting cases: (i) when active users are located far enough apart so that they can be served simultaneously with very small interference and (ii) when active users are closer than one beamwidth. In the first case, the optimum scheme is to provide the narrowest spotbeams for non-interfering users. In the second case when co-channel interference is significant between close-in users, the optimum pattern of each signal depends on the distances between users and their signal-to-noise ratios (SNR). and the optimum, solution can be one of three possibilities; (i) complete cancellation of interference. (ii) optimum suppression of interference, and (iii) sequential service of close-in users. Due to its flexibility for power allocation, the phased array antenna can provide better performance than the multiple beam antenna especially when there are a small number of very demanding users