Adam R. Margetts

Full-Duplex Bidirectional MIMO: Achievable Rates Under Limited Dynamic Range

In this paper we consider the problem of full-duplex bidirectional communication between a pair of modems, each with multiple transmit and receive antennas. The principal difficulty in implementing such a system is that, due to the close proximity of each modems transmit antennas to its receive antennas, each modems outgoing signal can exceed the dynamic range of its input circuitry, making it difficult—if not impossible—to recover the desired incoming signal. To address these challenges, we consider systems that use pilot-aided channel estimates to perform transmit beamforming, receive beamforming, and interference cancellation. Modeling transmitter/receiver dynamic-range limitations explicitly, we derive tight upper and lower bounds on the achievable sum-rate, and propose a transmission scheme based on maximization of the lower bound, which requires us to (numerically) solve a nonconvex optimization problem. In addition, we derive an analytic approximation to the achievable sum-rate, and show, numerically, that it is quite accurate.

Adaptive chip-rate equalization of downlink multirate wideband CDMA

We consider a downlink direct sequence-code division multiple access (DS-CDMA) system in which multirate user signals are transmitted via synchronous orthogonal short codes overlaid with a common scrambling sequence. The transmitted signal is subjected to significant time- and frequency-selective multipath fading. In response to this scenario, a novel two-mode receiver is proposed that accomplishes chip-rate adaptive equalization aided by filtering and/or cancellation of multiaccess interference (MAI). In the acquisition mode, a code-multiplexed pilot is used to adapt the equalizer from cold start or loss-of-lock. The use of MAI filtering results in a third-order least mean squares (LMS) algorithm, which has significant advantages over standard (i.e., first-order) LMS in nonstationary environments. In the tracking mode, decision-direction facilitates MAI-cancellation in the equalizer update, which enhances performance. The receiver monitors pilot decision quality as a means of switching between the two modes. The performance of the adaptive receiver is studied through analysis and simulation.

Direct Space-Time GF(q) LDPC Modulation

Wireless communication using multiple-input multiple-output (MIMO) systems improves throughput and enhances reliability for a given total transmit power. In this paper, potential performance gains are investigated via a direct Galois field [GF(q)] low-density parity-check (LDPC) space-time coding and modulation scheme. The field order q is chosen such that the number of bits per GF(q) LDPC symbol matches the number of bits per space-time symbol. The result is an elegant coding and decoding scheme that leverages the powerful LDPC iterative decoding technique. Results for 2, 3, and 4 transmitter systems with relatively short block lengths of around 2000 bits are provided, demonstrating frame-error- rate performances as close as 0.5 dB to the probability of outage upper bound.

Networked airborne communications using adaptive multi-beam directional links

Advances in digital arrays provide new techniques for increasing throughput in airborne adaptive directional networks. By adaptive directional linking, we mean systems that can dynamically change both the transmit and receive spatial patterns used for a link on a packet-by-packet basis. Using several arrays at each node, and several beams per array, these systems are able to both focus transmit energy in favorable directions, and reject interference at reception. Our problem space overlaps considerably with Multiple Input Multiple Output (MIMO) systems and distributed MIMO, but differs in several important ways. First and foremost, our links are largely line of sight, and as such, typically have a maximum rank of one and little time variation; hence, point-to-point multi-stream and diversity techniques have little benefit. We do however consider multiple transmissions at each transmitting array face, not to a common receiver, but to several distributed receiving nodes. As we will show, the primary driver of network performance becomes geometry and array technology rather than channel phenomenology. We explore the utility of various spatial processing strategies both on the receive and transmit side of the networked links, and the gains associated with multiple simultaneous transmissions.

Loss characterization of distributed space-time transmit beamforming with embedded channel probing

Distributed space-time transmit beamforming approaches potentially improve the throughput and robustness of wireless communications links. In this paper, we propose a distributed, space-time, transmit beamformer formulation that performs simultaneous data transmission and channel probing in the presence of interference. The difficulty is that the data signal corrupts the whitened channel estimate and leads to a loss in signal-to-interference and noise ratio. We examine this loss relative to the optimal solution and propose a diagonal loading technique to reduce the loss.

Scale-lag diversity reception in mobile wideband channels

We consider the effect of mobility on a wideband direct sequence spread spectrum (DSSS) communication system, and study a scale-lag Rake receiver capable of leveraging the diversity that results from mobility. A wideband signal has a large bandwidth-to-center frequency ratio, such that the typical narrowband Doppler spread assumptions do not apply to mobile channels. Instead, we assume a more general temporal scaling phenomenon, i.e., a dilation of the transmitted signals time support. Such analysis applies, for example, to ultra-wideband (UWB) radio frequency channels and underwater wideband acoustic channels.

Optimal training and data power allocation for distributed transmit beamforming

Performing distributed transmit beamforming across multiple cooperative radios can focus energy at a receiver to significantly improve the power, rate, and range tradeoff. In frequency division duplexed systems, the transmit beamformer is estimated at the receiver from a training signal and fed back to the transmitters. A caveat is that the training signal is not focused and as transmitters are added training becomes a large fraction of the total transmit power. We determine the optimal allocation of power between training and data that minimizes the outage probability. Results depend on the target spectral efficiency, coherence time, SNR per receiver, and number of transmitters and receivers.

Full-duplex MIMO relaying: Achievable rates under limited dynamic range

In this paper we consider the problem of full-duplex multiple-input multiple-output (MIMO) relaying between multi-antenna source and destination nodes. The principal difficulty in implementing such a system is that, due to the limited attenuation between the relays transmit and receive antenna arrays, the relays outgoing signal may overwhelm its limited-dynamic-range input circuitry, making it difficult-if not impossible-to recover the desired incoming signal. While explicitly modeling transmitter/receiver dynamic-range limitations and channel estimation error, we derive tight upper and lower bounds on the end-to-end achievable rate of decode-and-forward-based full-duplex MIMO relay systems, and propose a transmission scheme based on maximization of the lower bound. The maximization requires us to (numerically) solve a nonconvex optimization problem, for which we detail a novel approach based on bisection search and gradient projection. To gain insights into system design tradeoffs, we also derive an analytic approximation to the achievable rate and numerically demonstrate its accuracy. We then study the behavior of the achievable rate as a function of signal-to-noise ratio, interference-to-noise ratio, transmitter/receiver dynamic range, number of antennas, and training length, using optimized half-duplex signaling as a baseline.

Informed MIMO implementation of distributed transmit beamforming for range extension

Radios are ubiquitous today as embedded air interfaces to smartphones, electronic wearables, sensors, and autonomous systems. In many instances these radios are in close proximity to one another and share a common goal of relaying a message to a distant terminal. Examples include sensor networks, a swarm of UAVs, search and rescue teams, emergency response teams, police and military squads, a group of people beyond cell coverage attempting to send text messages, etc. Currently, the radio resources of the group offer no help when a single member attempts to contact the base. In this paper, we describe the implementation of a new approach to leverage group radio resources to gain a square-law growth in receive power at the base - while simultaneously suppressing a moderate amount of incidental interference at the base radio.

Reducing the fractional rank of interference with space-time-frequency adaptive beamforming

When wideband interference is subject to multi-path, the rank of the interference component sensed at a receive-antenna array increases. In a MIMO communications system, this makes it more challenging to suppress the interference and close the link. In this paper we investigate the use of adaptive filters to suppress interference, by quantifying the impact of multiple time taps and frequency “taps” on the rank of the interference. We show that as the number of taps increases, the relative rank of the interference decreases asymptotically to one over the number of receive antennas. We verify this with measurements of a broadcast interference source.