Joseph Farkas

Distributed scheduler design for multiuser detection enabled wireless mobile ad-hoc networks

There is a need for military and commercial wireless radio networks that can operate in dynamic environments while supporting high spectral efficiency with throughput guarantees and low latency. This is particularly challenging in wireless mobile ad-hoc networks (MANET). Multiuser detection (MUD) technology promises to address these needs. But most research in MUD technology to date has focused on the physical layer (PHY) challenges with little attention being paid to design of efficient MUD scheduler in medium access control layer (MAC). Our research described in this paper presents a distributed scheduler that addresses many challenging issues associated with a wireless MANET such as dynamic allocation of resources, handling of hidden and exposed nodes, QoS, and scalability. In particular, our research shows that the exposed node problem in MUD enabled radio systems is different from that in conventional interference avoidance systems. We provide guidelines to resolve this problem. Some simulation results are presented. The scheduler design is used in the DARPA Interference Multiple Access (DEVIA) communications program.

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.

Multiuser detection enabled medium access control in mobile ad hoc networks

Increasing spectral efficiency has been a constant challenge in wireless communications. Many military and commercial applications require that wireless networks operate in dynamic environments and provide high data rates. Multiuser detection (MUD) has been demonstrated to increase spectral efficiency by increasing spectrum reuse. Most MUD research to date has focused on the physical layer (PHY) technology. Our research has focused on design of an efficient wireless media access controller (MAC) for MUD enabled mobile ad-hoc networks (MANET). Beyond MUD, other issues addressed in this design include overhead efficiency, optimization of dynamic resource allocation, and support for dense topologies, mobility, scalability, and Quality of Service (QoS). The MAC design is used in the DARPA Interference Multiple Access (DEVIA) communications program. In this paper, a frame structure and architecture of the MAC design are presented. Technical challenges are discussed and motivating factors behind the design are highlighted. The MAC described in this paper has been prototyped and demonstrated in laboratory environment and field trial. Some test results are presented.

Real time implementation of a multiuser detection enabled ad-hoc network

In contrast to the interference avoidance paradigm that conventional wireless communication systems follow, the DARPA Interference Multiple Access (DIMA) Communications program intentionally structures multiple-access interference to enable high-capacity, low-latency, spread spectrum communications. The systempsilas adaptive multi-user detection (MUD) receiver algorithms enable the transmission of multiple data streams in the same time and frequency channel without need for centralized control of synchronization or power levels. This paper focuses on the DIMA system level implementation of the prototype radio network, along with the results from both non-real-time testing and real-time performance characterization efforts. Extensive simulations and non-real-time over-the-air testing helped to drive the design of a real-time MUD receiver implemented on a single FPGA. The Media Access Control Layer, implemented on a General Purpose Processor (GPP), facilitates intentional interference through a defined frame structure that also enables distributed synchronization and scheduling. During the real-time performance characterization period, the DIMA prototype network demonstrated 3.5 times the spectral efficiency of comparable 802.11 ad-hoc systems at low signal to noise ratios (SNRs). DIMA demonstrated up to 11x reductions in latency jitter in the same comparison. These gains can be leveraged in future military applications to provide increased capabilities and improved performance to the war fighter.

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.

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.

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.