Brandon Hombs

Interference multiple access wireless network demonstration enabled by real-time multiuser detection

In recent outdoor, over-the-air tests and simulations, DARPA demonstrated greater than 3X improvement in aggregate wireless network throughput and latency compared to the 802.11b wireless local area network. This achievement is based on allowing multiple users to simultaneously use the same channel (time, frequency, code, etc.) without requiring a centralized infrastructure for coordination and spectrum preplanning. DARPA has developed new multi-user detection (MUD) algorithms that make it possible to exploit multi-access interference to increase network capacity. A key program goal is to apply these algorithms to next-generation warfighter communications systems for substantial capacity, latency and scalability improvements.

Prototype implementation of a visual communication system employing video imagery

Information transmission through the use of video imagining systems has the potential to build upon the success of imaging communication systems, such as QR Codes and 2D Barcodes. Video communication systems add a temporal dimension to image-based information transmission systems, greatly expanding the amount of data that can be transferred. Such a system allows for secure transfer of data through the use of a line-of-sight (LOS) visual channel, making it attractive for several applications, including in-store purchases, banking, and stealth military missions. The nature of the LOS visual channel also reduces interference from other applications, making it a very attractive option for implementing short range communications systems from a cell phone, computer, tablet, or other device. This paper investigates how a video imaging system can be used for information transmission. A prototype system has been designed and implemented as a proof of concept, and the performance of the prototype is evaluated.

Power aware scheduling and power control techniques for multiuser detection enabled wireless mobile ad-hoc networks

Multiuser Detection (MUD) based receivers theoretically require no power control (PC) as they have the ability to separate signals regardless of their relative power levels as long as these signals achieve a suitable SNR. In practice, receiver designs have finite dynamic range. In this paper, power aware scheduling (PAS) and power control (PC) algorithms are investigated to address the finite MUD dynamic range and performance results are shown. The final PAS algorithm and motivating factors behind the design selections made on the DARPA Interference Multiple Access (DIMA) program are highlighted as well as different approaches involving both scheduling and PC. The techniques selected on the DIMA program are currently operating as part of the DEVIA mobile real-time experiments.

System design of tactical communications with Solar Blind Ultraviolet Non Line-of-Sight systems

Recent advances in component technology now make it possible to exploit of the ultraviolet (UV) spectrum for short range, medium bandwidth, low probability of intercept (LPI) communications. BAE Systems will report on analysis of hardware and phenomenology for the near term application of Solar Blind Ultraviolet Non Line-of-Sight Communications (SB UV NLOS Comm). Communications in the presence of Radio Frequency (RF) interference from Counter-IED jamming is a formidable challenge for our war-fighters on the battlefield of today and the future. Here we discuss approaches to exploit the Solar Blind (SB) Ultraviolet (UV) spectrum to provide a short range (∼500m), medium bandwidth (28.8 kbps), Non-Line-of-Sight (NLOS), networked communications system as an alternative to traditional RF communications systems.

Multiple-access capacity gains using multiuser detection under uniform linear power spacing

This paper reviews the driving factors that affect performance gains in wireless networks when receivers are enabled by multiuser detection (MUD). Assuming the many-to-one, Gaussian multiple-access channel, we compare the capacity of a jointly optimal MUD receiver with the single user match filter using TDMA, FDMA and orthogonal CDMA access schemes. Comparisons are made under both equal power and unequal power conditions. Unequal power cases are constrained to uniform linear power spacing where the received powers are equally spaced over a given power spread. Motivation for this analysis is to better understand what multiple-access schemes will realize the highest potential improvements by incorporating MUD technology and under what system constraints those gains are maximized.

Increasing User Capacity by Interference Avoidance and Intentional Asynchrony in CDMA Systems

This correspondence examines the number of users that can be supported at a given signal-to-interference ratio in asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems. It is shown that for many chip waveforms the user capacity for asynchronous systems is larger than for optimally designed synchronous systems. The degree to which the user capacity improves by exploiting asynchrony is characterized by defining the effective dimension of the chip waveform. Finally, simulation results are shown to verify the analysis.

Exploiting asynchrony in DS-CDMA systems through interference avoidance

User capacity in synchronous CDMA systems has been well characterized and is fixed by the processing gain and target SIR required by each user. Asynchronous systems have not been as well studied. This paper examines the user capacity of asynchronous CDMA systems. It is shown that the user capacity can be increased over synchronous systems by allowing the users to be chip asynchronous and adapting their spreading codes to decrease interference. The degree to which the system experiences a gain by asynchrony is characterized by the effective dimension of the chip waveform. Simulation results are provided to verify that the user capacity is improved by allowing asynchrony when interference avoidance algorithms are employed.

Impact of power spread constraints on a multiuser detection enabled ad hoc network

Multiuser detection (MUD) in a mobile ad-hoc network can significantly improve throughput by exploiting the natural power spreads (PS) between users. Scheduling algorithms can take advantage of the network geometry to enable this increase in throughput by reusing space to allow multiple transmissions at the same time when there is no penalty to the individual links. We show through simulation that MUD power scheduling algorithms can significantly increase network throughput as a function of the number of simultaneous transmissions and MUD algorithm capabilities. Without the need for spreading, a receiver is able to decode multiple transmitted signals when there is a power spread between the signals. Thus with a priori knowledge of scheduling information and received powers, a MUD power scheduling algorithm can efficiently allocate transmission times to increase network throughput at virtually no cost to the individual links. In this paper we motivate the usefulness of MUD without spreading in an ad-hoc network. Next we present the range of acceptable power spreads (both upper and lower limits) that lead to these gains. Additionally, using random node locations we show the likelihood of finding scenarios where scheduling leads to this low cost MUD network gain and show the corresponding network throughput gain.

Application of multiuser detection to the common data link waveform

The growing use of small form factor unmanned aerial systems (UAS) in theater presents severe co-channel interference issues with todaypsilas radio technology. This paper addresses this concern by describing multiuser detection receiver technology and its application to the Common Data Link (CDL) ISR waveform protocol. By equipping CDL radios with MUD receiver technology developed under the DARPA Interference Multiple Access Communications Program, we show a significant reduction in spectrum pre planning requirements and increases in spectral efficiency. This technique enables multiple UASs to occupy the same channel at the same time without centralized coordination or power control.