Timothy A. Thomas

Bio: Timothy A. Thomas is an academic researcher from Motorola. The author has contributed to research in topics: Communication channel & Orthogonal frequency-division multiplexing. The author has an hindex of 35, co-authored 97 publications receiving 5193 citations. Previous affiliations of Timothy A. Thomas include Nokia Networks & Nokia.

Method and apparatus for choosing a modulation and coding rate in a multi-user, mimo communication system

Abstract: A method and apparatus for choosing a modulation and coding rate in a MU-MIMO communication system is provided herein. During operation, a node will determine the MCR to feed back to the base even though the mobile does not know which of the possible interferers (if any) will be using the same time/frequency resources as the mobile. This takes place via the mobile node calculating best antenna weights (codebook choice) for each group of subcarriers that can be potentially used by the mobile. Transmit weights v for each interferer are then determined and the weights are utilized to determine a best modulation and coding rate for the mobile.

Method and apparatus for finite impulse response cyclic-shift diversity

Abstract: A generalized form of cyclic shift diversity is described for use in an OFDM system with multiple transmit antennas. Multiple cyclic shifts are performed for each transmit antenna and the shifted signals are scaled and summed to form a time-domain data stream for each transmit antenna. A cyclic extension is added to each data stream prior to transmission.

OFDM channel estimation in the presence of interference

Abstract: We develop a frequency-domain channel estimation algorithm for single-user orthogonal frequency division multiplex (OFDM) wireless systems in the presence of interference. The received measurement is correlated in space with a covariance matrix dependent on frequency. Hence, the commonly used least-squares algorithm is suboptimal. On the other hand, accurate estimation of the spatial covariance matrix in such a model using the multivariate analysis of variance (MANOVA) method would impose significant computational overhead, since it would require a large number of pilot symbols. To overcome these problems, we propose to model the covariance matrix using an a-priori known set of frequency-dependent functions of joint (global) parameters, resulting in a structured covariance matrix. We estimate the interference covariance parameters using a residual method of moments (RMM) estimator and the mean (user channel) parameters by maximum likelihood (ML) estimation. Since our RMM estimates are invariant to the mean, this approach yields simple non-iterative estimates of the covariance parameters while having optimal statistical efficiency. Therefore, our algorithm outperforms the least-squares method in accuracy, and requires a smaller number of pilots than the MANOVA method and thus has smaller overhead. Numerical results illustrate the applicability of the proposed algorithm.

Pilot signal transmission method and apparatus

Abstract: A pilot (or reference) transmission scheme is utilized where different transmitters are assigned pilot sequences with possibly different cyclic time shifts. A pilot signal is transmitted concurrently by the transmitters in a plurality of pilot blocks, and a receiver processes the plurality of received pilot blocks to recover a channel estimate for at least one of the transmitters while suppressing the interference due to the pilot signals from the other transmitters.

OFDM channel estimation in the presence of asynchronous interference

Abstract: We have recently proposed a frequency-domain channel estimation algorithm for a single-user orthogonal frequency division multiplexing (OFDM) wireless system in the presence of synchronous interference. However, the interference cyclic prefix does not usually align with the desired user's cyclic prefix (i.e., the interferer is asynchronous) and thus a different approach is required. For an asynchronous interferer in a rich multipath environment, the received frequency-domain measurement is correlated in space with a full-rank covariance matrix on each subcarrier. Therefore, the synchronous algorithms may give poor detection performance since they assume reduced rank interference and use a smaller number of parameters. We overcome these problems by employing an appropriately structured model with a properly defined number of covariance parameters. We estimate the interference covariance parameters using a residual method of moments (RMM) estimator and the mean (i.e., the desired user's channel) parameters by maximum likelihood (ML) estimation. Since the RMM estimates are invariant to the mean, we obtain simple non-iterative estimates of the covariance parameters while having optimal statistical efficiency. Numerical results illustrate the applicability of the proposed algorithm.

Internet of Things: A Survey on Enabling Technologies, Protocols, and Applications

Abstract: This paper provides an overview of the Internet of Things (IoT) with emphasis on enabling technologies, protocols, and application issues. The IoT is enabled by the latest developments in RFID, smart sensors, communication technologies, and Internet protocols. The basic premise is to have smart sensors collaborate directly without human involvement to deliver a new class of applications. The current revolution in Internet, mobile, and machine-to-machine (M2M) technologies can be seen as the first phase of the IoT. In the coming years, the IoT is expected to bridge diverse technologies to enable new applications by connecting physical objects together in support of intelligent decision making. This paper starts by providing a horizontal overview of the IoT. Then, we give an overview of some technical details that pertain to the IoT enabling technologies, protocols, and applications. Compared to other survey papers in the field, our objective is to provide a more thorough summary of the most relevant protocols and application issues to enable researchers and application developers to get up to speed quickly on how the different protocols fit together to deliver desired functionalities without having to go through RFCs and the standards specifications. We also provide an overview of some of the key IoT challenges presented in the recent literature and provide a summary of related research work. Moreover, we explore the relation between the IoT and other emerging technologies including big data analytics and cloud and fog computing. We also present the need for better horizontal integration among IoT services. Finally, we present detailed service use-cases to illustrate how the different protocols presented in the paper fit together to deliver desired IoT services.

Spatially Sparse Precoding in Millimeter Wave MIMO Systems

Abstract: Millimeter wave (mmWave) signals experience orders-of-magnitude more pathloss than the microwave signals currently used in most wireless applications and all cellular systems. MmWave systems must therefore leverage large antenna arrays, made possible by the decrease in wavelength, to combat pathloss with beamforming gain. Beamforming with multiple data streams, known as precoding, can be used to further improve mmWave spectral efficiency. Both beamforming and precoding are done digitally at baseband in traditional multi-antenna systems. The high cost and power consumption of mixed-signal devices in mmWave systems, however, make analog processing in the RF domain more attractive. This hardware limitation restricts the feasible set of precoders and combiners that can be applied by practical mmWave transceivers. In this paper, we consider transmit precoding and receiver combining in mmWave systems with large antenna arrays. We exploit the spatial structure of mmWave channels to formulate the precoding/combining problem as a sparse reconstruction problem. Using the principle of basis pursuit, we develop algorithms that accurately approximate optimal unconstrained precoders and combiners such that they can be implemented in low-cost RF hardware. We present numerical results on the performance of the proposed algorithms and show that they allow mmWave systems to approach their unconstrained performance limits, even when transceiver hardware constraints are considered.

Millimeter-Wave Cellular Wireless Networks: Potentials and Challenges

Abstract: Millimeter-wave (mmW) frequencies between 30 and 300 GHz are a new frontier for cellular communication that offers the promise of orders of magnitude greater bandwidths combined with further gains via beamforming and spatial multiplexing from multielement antenna arrays. This paper surveys measurements and capacity studies to assess this technology with a focus on small cell deployments in urban environments. The conclusions are extremely encouraging; measurements in New York City at 28 and 73 GHz demonstrate that, even in an urban canyon environment, significant non-line-of-sight (NLOS) outdoor, street-level coverage is possible up to approximately 200 m from a potential low-power microcell or picocell base station. In addition, based on statistical channel models from these measurements, it is shown that mmW systems can offer more than an order of magnitude increase in capacity over current state-of-the-art 4G cellular networks at current cell densities. Cellular systems, however, will need to be significantly redesigned to fully achieve these gains. Specifically, the requirement of highly directional and adaptive transmissions, directional isolation between links, and significant possibilities of outage have strong implications on multiple access, channel structure, synchronization, and receiver design. To address these challenges, the paper discusses how various technologies including adaptive beamforming, multihop relaying, heterogeneous network architectures, and carrier aggregation can be leveraged in the mmW context.

MIMO Broadcast Channels With Finite-Rate Feedback

Abstract: Multiple transmit antennas in a downlink channel can provide tremendous capacity (i.e., multiplexing) gains, even when receivers have only single antennas. However, receiver and transmitter channel state information is generally required. In this correspondence, a system where each receiver has perfect channel knowledge, but the transmitter only receives quantized information regarding the channel instantiation is analyzed. The well-known zero-forcing transmission technique is considered, and simple expressions for the throughput degradation due to finite-rate feedback are derived. A key finding is that the feedback rate per mobile must be increased linearly with the signal-to-noise ratio (SNR) (in decibels) in order to achieve the full multiplexing gain. This is in sharp contrast to point-to-point multiple-input multiple-output (MIMO) systems, in which it is not necessary to increase the feedback rate as a function of the SNR

5G : A tutorial overview of standards, trials, challenges, deployment, and practice

Abstract: There is considerable pressure to define the key requirements of 5G, develop 5G standards, and perform technology trials as quickly as possible. Normally, these activities are best done in series but there is a desire to complete these tasks in parallel so that commercial deployments of 5G can begin by 2020. 5G will not be an incremental improvement over its predecessors; it aims to be a revolutionary leap forward in terms of data rates, latency, massive connectivity, network reliability, and energy efficiency. These capabilities are targeted at realizing high-speed connectivity, the Internet of Things, augmented virtual reality, the tactile internet, and so on. The requirements of 5G are expected to be met by new spectrum in the microwave bands (3.3-4.2 GHz), and utilizing large bandwidths available in mm-wave bands, increasing spatial degrees of freedom via large antenna arrays and 3-D MIMO, network densification, and new waveforms that provide scalability and flexibility to meet the varying demands of 5G services. Unlike the one size fits all 4G core networks, the 5G core network must be flexible and adaptable and is expected to simultaneously provide optimized support for the diverse 5G use case categories. In this paper, we provide an overview of 5G research, standardization trials, and deployment challenges. Due to the enormous scope of 5G systems, it is necessary to provide some direction in a tutorial article, and in this overview, the focus is largely user centric, rather than device centric. In addition to surveying the state of play in the area, we identify leading technologies, evaluating their strengths and weaknesses, and outline the key challenges ahead, with research test beds delivering promising performance but pre-commercial trials lagging behind the desired 5G targets.