D. Richard Brown

Receiver-coordinated distributed transmit beamforming with kinematic tracking

A distributed transmit beamforming technique is described for a scenario with two or more transmit nodes and one intended receiver. The protocol includes a measurement epoch, feedback from the intended receiver to the transmit nodes, and a beamforming epoch. The intended receiver tracks the clock and kinematic parameters of the independent transmit nodes and coordinates the transmit nodes by feeding back state predictions which are then used as phase corrections to facilitate passband phase and frequency alignment at the receiver. A three-state dynamic model is developed to describe the stochastic kinematics and clock evolution of each transmit node relative to the frame of the receiver/coordinator. Steady-state analysis techniques are used to analytically predict the tracking performance as well as the beamforming gain as a function of the system parameters. Numerical results show that near-ideal beamforming performance can be achieved if the period between successive observations at the receiver/coordinator is sufficiently small.

Retrodirective distributed transmit beamforming with two-way source synchronization

Distributed transmit beamforming has recently been proposed as a technique in which several single-antenna sources cooperate to form a virtual antenna array and simultaneously transmit with phase-aligned carriers such that the passband signals coherently combine at an intended destination. The power gains of distributed transmit beamforming can provide increased range, rate, energy efficiency, and/or security, as well as reduce interference. Distributed transmit beamforming, however, typically requires precise synchronization between the sources with timing errors on the order of picoseconds. In this paper, a new two-way synchronization protocol is developed to facilitate precise source synchronization and retrodirective distributed transmit beamforming. The two-way synchronization protocol is developed under the assumption that all processing at each source node is performed with local observations in local time. An analysis of the statistical properties of the phase and frequency estimation errors in the two-way synchronization protocol and the resulting power gain of a distributed transmit beamformer using this protocol is provided. Numerical examples are also presented characterizing the performance of distributed transmit beamforming in a system using two-way source synchronization. The numerical results demonstrate that near-ideal beamforming performance can be achieved with low synchronization overhead.

Receiver-coordinated distributed transmit nullforming with channel state uncertainty

A distributed coherent transmission scheme in which two or more transmit nodes form a beam toward an intended receiver while directing nulls at a number of other “protected” receivers is considered. Unlike pure distributed beamforming, where the ith transmit coefficient depends only on the ith transmit nodes channel to the intended receiver, the transmit coefficients of a distributed nullformer each depend on the channel responses from all of the transmit nodes to all of the protected nodes. The requirement for each transmit node to know all of the channels in the system makes distributed transmit nullforming challenging to implement in the presence of channel time variations. This paper describes a receiver-coordinated distributed transmission protocol, in the context of a state-space dynamic channel model, in which the receive nodes feed back periodic channel measurements to the transmit cluster. The transmit nodes use this feedback to generate optimal channel predictions and then calculate a time-varying transmit vector that minimizes the average total power at the protected receivers while satisfying an average power constraint at the intended receiver during distributed transmission. We demonstrate via analysis and numerical simulation the efficacy of the technique even with low channel measurement overhead, infrequent update intervals, and significant feedback latency.

Quantized Distributed Reception for MIMO Wireless Systems Using Spatial Multiplexing

We study a quantized distributed reception scenario in which a transmitter equipped with multiple antennas sends multiple streams via spatial multiplexing to a large number of geographically separated single antenna receive nodes. This approach is applicable to scenarios such as those enabled by the Internet of Things (IoT) which holds much commercial potential and could facilitate distributed multiple-input multiple-output (MIMO) communication in future systems. The receive nodes quantize their received signals and forward the quantized received signals to a receive fusion center. With global channel knowledge and forwarded quantized information from the receive nodes, the fusion center attempts to decode the transmitted symbols. We assume the transmit vector consists of arbitrary constellation points, and each receive node quantizes its received signal with one bit for each of the real and imaginary parts of the signal to minimize the transmission overhead between the receive nodes and the fusion center. Fusing this data is a nontrivial problem because the receive nodes cannot decode the transmitted symbols before quantization. We develop an optimal maximum likelihood (ML) receiver and a low-complexity zero-forcing (ZF)-type receiver at the fusion center. Despite its suboptimality, the ZF-type receiver is simple to implement and shows comparable performance with the ML receiver in the low signal-to-noise ratio (SNR) regime but experiences an error rate floor at high SNR. It is shown that this error floor can be overcome by increasing the number of receive nodes.

Receiver-coordinated zero-forcing distributed transmit nullforming

A coherent cooperative communication system is proposed in which a distributed array of transmit nodes forms a beam at a desired receiver while simultaneously steering nulls at several protected receivers. Coherent transmission is achieved through a receiver-coordinated protocol where the receivers in the system use state-space channel tracking and provide feedback to the transmit cluster to facilitate distributed transmission. Analytical estimates for the performance degradation in the nulls due to channel estimation errors are verified by simulations. Numerical results demonstrate that the technique is effective even with low channel measurement overhead, infrequent measurement intervals, and feedback latency.

Implementation and demonstration of receiver-coordinated distributed transmit beamforming across an ad-hoc radio network

Distributed transmit beamforming using an ad-hoc network of 10 RF transmitters was demonstrated using radio nodes developed from off-the-shelf components and modules. A time-slotted protocol allowed carrier phases from each transmitter to be measured at a receiver and fed back to the transmitters where Kalman filters were used to predict the offset phases and frequencies. Offsets were digitally compensated for during beamforming intervals. Beamforming gain within 0.1dB of ideal was demonstrated across 1 km at 910MHz. This is the first report (to our knowledge) of a successful outdoor RF distributed transmit beamforming experiment using independent clocks at this scale.

An experimental study of acoustic distributed beamforming using round-trip carrier synchronization

This paper describes the development of an acoustic distributed beamforming system and presents experimental results for two-source and three-source acoustic distributed beam-forming using the time-slotted round-trip carrier synchronization protocol. Each source node in the system was built using commercial off-the-shelf parts including a Texas Instruments floating-point digital signal processor, microphone, speaker, audio amplifier, and battery. The source node functionality, including phase locked loops and the logic associated with the time-slotted round-trip carrier synchronization protocol, was realized through real-time software independently running on each source nodes C6713 digital signal processor. Experimental results for two-source and three-source realizations of the acoustic distributed beamforming system in a room with multipath channels are presented. The two-source and three-source experimental results show mean power gains of approximately 97.7% and 90.7%, respectively, of an ideal beamformer.

On throughput efficiency of geographic opportunistic routing in multihop wireless networks

Geographic opportunistic routing (GOR) is a new routing concept in multihop wireless networks. In stead of picking one node to forward a packet to, GOR forwards a packet to a set of candidate nodes and one node is selected dynamically as the actual forwarder based on the instantaneous wireless channel condition and node position and availability at the time of transmission. GOR takes advantages of the spatial diversity and broadcast nature of wireless communications and is an efficient mechanism to combat the unreliable links. The existing GOR schemes typically involve as many as available next-hop neighbors into the local opportunistic forwarding, and give the nodes closer to the destination higher relay priorities. In this paper, we focus on realizing GORs potential in maximizing throughput. We start with an insightful analysis of various factors and their impact on the throughput of GOR, and propose a local metric named expected one-hop throughput (EOT) to balance the tradeoff between the benefit (i.e., packet advancement and transmission reliability) and the cost (i.e., medium time delay). We identify an upper bound of EOT and proof its concavity. Based on the EOT, we also propose a local candidate selection and prioritization algorithm. Simulation results validate our analysis and show that the metric EOT leads to both higher one-hop and path throughput than the corresponding pure GOR and geographic routing.

SINR, power efficiency, and theoretical system capacity of parallel interference cancellation

This paper analytically derives exact expressions for the SINR of the two-stage linear parallel interference cancellation (LPIC) and two-stage hard-decision parallel interference cancellation (HPIC) multiuser detectors in a synchronous, nonorthogonal, binary, CDMA communication system with deterministic short spreading sequences. We consider approximations to the SINR expressions that are justified in typical operating scenarios to obtain a more intuitive understanding of the SINR performance of the HPIC detector. We consider the case where a specific SINR requirement is given for each user in the system and derive expressions for the set of transmit powers necessary to meet this requirement when two-stage LPIC or HPIC detection is used. We also derive expressions for a measure of the theoretical system capacity using LPIC and HPIC detection, defined as the maximum number of users possible in a system with finite available transmit power. Numerical results are presented that compare the HPIC and LPIC detectors to the hard-decision successive interference cancellation (SIC) detector and matched filter (MF) detector. Our results suggest that HPIC detection may offer the best SINR and power efficiency performance when the number of users in the system is low to moderate and that SIC detection may offer superior performance when the number of users in the system is large.

Precise timestamp-free network synchronization

This paper describes an approach to master/slave network synchronization based on bidirectional message exchanges without the use of timestamps. Rather than the usual approach of exchanging digital timestamps through a dedicated synchronization protocol, an approach is described in which synchronization information is conveyed implicitly at the physical layer through the timing of the master node¿s responses to the slave nodes. This approach can reduce overhead and allow the embedding of synchronization functions in existing network traffic. A timestamp-free synchronization protocol is described and its performance is quantified in the presence of delay estimation error and stochastic local oscillator dynamics. A filtering framework is also developed to allow each slave node to accurately infer and correct local clock drifts from multiple noisy clock offset estimates. Based on fundamental delay estimation bounds for narrowband signals, numerical results show that synchronization among the slave nodes can be achieved quickly and that the resulting steady-state accuracy can be sufficient to support distributed transmission techniques requiring carrier phase alignment, e.g. distributed beamforming.