Scott Enserink

Towards simultaneous radar and spectral sensing

DARPAs Shared Spectrum Access for Radar and Communications (SSPARC) program seeks to improve radar and communications capabilities through spectrum sharing. Spectrum sensing is an integral component of this vision. For example, a spectrally-agile military radar system could adapt the frequency bands it uses according to the sensed spectrum. Currently, radar transmission and spectrum sensing must be multiplexed in time. That is to say, sensing the use of a particular frequency band is only possible during time slices where the radar is inactive in that band. In this paper, we describe how emerging Simultaneous Transmit and Receive (STAR) technology could be evolved to support simultaneous radar and spectrum sensing. We review the state-of-the-art in STAR technology and identify the key challenges associated with achieving contemporaneous radar operation and spectrum sensing on the same platform.

Estimation of Constrained Capacity and Outage Probability in Rayleigh Channels

Ergodic capacity and outage probability are two important performance measures of communication systems in fading channels. The goal of this work is to develop a simple method for tightly estimating the ergodic capacity and outage probability of a Rayleigh channel with and without antenna combining for constellation constrained capacity. Researchers have determined methods for calculating the ergodic Shannon capacity for Rayleigh channels for single and multiple receive antenna systems. However, in practical communications systems, the input signal is constrained to a discrete signalling set such as finite-size quadrature amplitude modulation constellations. Under these conditions the ergodic constellation constrained capacity is a more accurate measure. Relatively loose bounds for the ergodic constellation constrained capacity of a Rayleigh channel have been exposited in the literature for a single antenna system. This paper details a method that provides a uniform expression for accurately estimating the ergodic capacity, both Shannon and constellation constrained, of Rayleigh channels with and without antenna combining. Expressions are derived using this method for both the noise limited and interference limited cases. These expressions facilitate straightforward computation of outage probability as well. To date, no other method for calculating the ergodic constellation constrained capacity of an interference limited Rayleigh channel or estimating the outage probabilities for the constellation constrained capacity of a noise limited or interference limited Rayleigh channel has been published.

Estimation of Constrained Capacity and Outage Probability in Lognormal Channels

The driving force behind this work is the desire to obtain one method for estimating the ergodic capacity of lognormal (LN) channels for both Shannon capacity and constrained capacity. In recent years, researchers have determined methods for calculating the ergodic Shannon capacity for LN channels. However, in practical communication systems, the input signal is constrained to a discrete signaling set such as finite-size quadrature amplitude modulation constellations. At a high SNR, the Shannon capacity greatly overestimates the capacity of these practical systems, particularly for low-order constellations. For this reason, a method is needed to evaluate the capacity and outage probability for LN channels when the signal set is constrained to a finite alphabet. The main contribution of this paper is the introduction of a simple but accurate method for calculating both the ergodic Shannon capacity and the ergodic constrained capacity of practical signals for LN channels. This method also facilitates straightforward computation of outage probability and outage capacity. Prior to this work, the ergodic constrained capacity, outage probability, and outage capacity of practical signals for LN channels had not been dealt with in the literature.

Constrained Capacities of DVB-S2 Constellations in Log-Normal Channels at Ka Band

Researchers have investigated the Shannon capacity of satellite links that have log-normal rain-fading channels in the Ka band (20-30 GHz). This paper uses information theoretic tools to expand on the previous work by computing the constrained capacity of log-normally distributed rain-fading channels in which practical constellations, such as those found in the DVB-S2 standard are used, as opposed to the Gaussian distribution assumed for the input signal in Shannon capacity calculations. Two types of rain environments are considered: a tropical environment and a fairly dry European environment. The results of this approach will help the satellite planner to determine realistic upper limits for the throughput of satellite links that use adaptive coded modulation to mitigate the effects of rain fades.

Mitigation of scintillation using antenna receive diversity for Ka band satellite signals

The desire for higher data rates and the need for available bandwidth has pushed satellite communications into the Ka band (20-30 GHz). At these higher carrier frequencies the effects of scintillation due to turbulence-induced index of refraction irregularities in the troposphere is known to become more prevalent, particularly on slant paths. Mitigation of scintillation can be very important for certain links such as those operating at a low-margin along a low elevation path in a dry climate, where scintillation, rather than rain, is the key link factor. We will look at mitigating the deleterious scintillation effects on a satellite signal in the Ka band through the use of two receive antennas. This paper quantifies the possible benefits of such a mitigation technique through the use of a multiple phase screen (MPS) simulation model. We will show that in moderate to strong turbulence cases there can be significant diversity gains achieved by combining two receive antennas, and that even in mild turbulence there is often a diversity gain of 0.5 dB, and that MPS simulations are a useful tool for quantifying scintillation effects on Ka-band satellite signals.

A critique of HF NVIS channel models

A wideband HF NVIS measurement campaign in Southern California collected 7 terabytes of channel sounding across 2-12 MHz from 72 days of data collection between October 2014 and February 2015. This massive dataset demonstrates that the traditional HF NVIS channel models are ill-suited to describe the observed HF NVIS propagation phenomena. In particular, it is observed that the HF NVIS channel is often non-stationary over a few minutes; with taps rapidly shifting in delay, exhibiting abrupt changes in phase characteristics, and periodic variations in magnitude-all having significant implications on modem design, e.g. block length, preamble length and repetition rate, interleaving size, and modulation type. To this end, this study summarizes the empirical findings of the delay and Doppler characteristics, as well as the non-traditional fading modes, of wideband HF NVIS channels.

A model for dual polarized HF MIMO communications

The ionosphere has two modes of propagation, each with a different polarization. Theory and measurements show that the paths associated with these two polarization modes have independent fades. Transmitting and receiving on two co-located orthogonal linear polarized antennas 1) makes the link more robust to fading, 2) removes possibility of polarization loss, 3) can nearly double the data throughput by effectively establishing two links, and 4) is a form of multiple input multiple output communications (MIMO). This paper introduces a simple and flexible model for a dual polarized (DP) HF MIMO channel, illustrates how to develop specific instantiations of the model based on channel measurements, including ionosondes, and presents some performance comparisons between single input single output HF communications and DP HF MIMO. A contribution of this paper is the introduction of this flexible DP HF MIMO channel model that will allow researchers to develop instantiations of HF DP channel models from readily available ionosonde data and determine the performance benefits of HF DP MIMO under a variety of ionospheric propagation conditions and antenna orientations.

Joint Analog and Digital Interference Cancellation

The mitigation of in-band interference from co-located emitters is an increasingly important problem in commercial and military wireless communications systems design. In this work, we present a co-located interference mitigation approach that is targeted for scenarios in which the interfering signal is received at an extremely high power and the delay between the received (over-the-air) and reference (wired) copies of the interferer is not negligible. Such scenarios are common in military applications (e.g., simultaneous jamming and communications). The proposed Joint Analog and Digital Interference Cancellation (JADIC) system is self-configuring and can provide over 100 dB of suppression of wideband and narrowband jammers.

On the Calculation of Constrained Capacity and Outage Probability of Broadband Satellite Communication Links

Outage probability and outage ergodic constrained capacity are important performance metrics of broadband satellite communication networks operating above 10 GHz. The objective of this letter is to propose a simple method, using piecewise linear fitting, for the calculation of these figures of merit in rain fading channels. Rain fading affecting the signal amplitude is described by log-lognormal distribution. The results are focused on well-accepted DVB-S2 satellite communication system. Finally, a lower bound for the ergodic constrained capacity of log-lognormal channels is given. The numerical results highlight the accuracy of the proposed method.

Use of antenna receive diversity to mitigate scintillation on Ka band satellite links

According to the MPS simulation of Ka band satellite signals, used in conjunction with a typical cloud-present turbulence profile, there can be a substantial diversity benefit from using multiple antennas for links with elevation angles less than 20 degrees. It should be noted that this analysis method can also be applied to horizontal paths (i.e. terrestrial links), in which case the Cn 2 value remains approximately constant over the entire path. In a broader sense, this paper has shown the utility of using MPS simulations to model the effects of the troposphere on Ka band signals. This is of particular benefit in the strong scattering regime where the mathematics are intractable.