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.

Quasi-static antenna array processing for rapidly time-varying channels

Abstract: This paper explores the use of quasi-static frequency-domain antenna combining weights for multi-user (e.g., SDMA) or multi-stream (e.g., MIMO) communication systems operating in rapidly time-varying frequency-selective channels. By implementing combining weights that are constant across a time slot but are updated from slot-to-slot (i.e., "quasi-static"), great computational complexity savings can be realized compared to calculating new weights at each data block within a time slot. The quasi-static weights are computed by first modeling, the time-varying channel for each user as the superposition of multiple time-invariant channels, called Doppler channels. The weights are then calculated based on all users' Doppler channels. These new weights work by using some of the degrees of freedom of the antenna array to suppress time variations as well as multiple access interference. Despite being fixed across a time slot, these weights can equalize and suppress SDMA interference even when the channel varies significantly over the slot. Simulation results show the effectiveness of these weights for equalization and interference suppression.

Error-resistant differential feedback of long-term covariance matrix in closed-loop MIMO

Abstract: One method of obtaining channel information for closed-loop SU-MIMO or MU-MIMO is for the mobile to feed back a covariance matrix based on the measured downlink channel However for MU-MIMO to be able to operate properly (ie, have low crosstalk), the covariance matrix must be fed back with fairly high precision which would result in a very high level of overhead This paper describes two new methods for feeding back accurate covariance matrix estimates from a mobile to a base station with low overhead, while being very robust to transmission errors System-level results show that one of the proposed feedback methods has performance very close to an unquantized covariance matrix when the feedback bit error rate is less than 10% When the feedback bit error rate is 20% one of the proposed feedback methods has better performance than an existing single-shot covariance matrix quantization method operating with no feedback errors while additionally requiring less feedback overhead than the existing method (16 bits versus 28 bits)

Receiver in wireless communication systems with hierarchical pilot structure

Abstract: A wireless terminal comprises a receiver and a controller coupled to the receiver. The controller is configured to control the receiver to perform steps comprising receiving, from a base station, a time-frequency resource block comprising a first dedicated pilot signal, a second dedicated pilot signal, an allocation control channel that includes control information, a data channel that includes data. The controller is configured to control the receiver to receive, from the base station, signaling indicating the number of spatial streams the base station is using to transmit the control information, to determine, based on the signaled number of spatial streams, the format of the at least one of the first dedicated pilot signal and the second dedicated pilot signal, to decode the allocation control channel based on the first dedicated pilot signal, and to decode the data channel based on the allocation control channel and the second dedicated pilot signal.

Vertical beam design

Abstract: A method includes transmitting reference signals to user equipment from an active antenna array system including a multiple of antennas arrayed in at least a vertical direction. The active antenna array system is configured to perform the transmitting of the reference signals using multiple vertical beams having vertical patterns designed to provide desired vertical patterns after codebook feedback from a codebook is applied on transmissions from logical ports of the antenna array. The method includes receiving codebook feedback from the user equipment in response to transmitting the reference signals. The method includes transmitting information to the user equipment at least by applying selected entries of the codebook corresponding to the codebook feedback to the logical ports of the antenna array, wherein application of the selected entries causes the active antenna array system to perform transmitting information using multiple beams having the desired vertical patterns. Apparatus and program products are disclosed.

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.