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Keeping a Finger on the Pulse of Spectrum Management

February 2001
By Christian B. Sheehy

Radio wave technology provides clarity without added interference.

An experimental radio technology may provide a more efficient means of alleviating bandwidth congestion in wireless communications. Operating at lower power than most radio devices, time-modulated ultrawideband technology fuses communications, radar and tracking capabilities into one piece of hardware that can deliver improved performance while remaining compatible with most legacy and commercial off-the-shelf systems.

The prospect of greater data transmission speed and security with less accepted or projected interference has increased interest in ultrawideband signaling. Time Domain, Huntsville, Alabama, has developed PulsON technology, a technique that could meet many of the connectivity and spatial requirements for conducting reliable radio communications in the next decade. By operating in a lower spectrum band, the technology offers enhanced signal intelligibility while minimizing clutter in crowded portions of the radio wave environment. Low-power independent pulses from a chipset effectively eliminate the destructive signal cancellation associated with higher-powered continuous sine wave transmissions.

The U.S. Army experimented with the technology at last summer’s roving sands field training exercise held at Fort Bliss, Texas, as part of the Army Space and Missile Defense Battle Laboratory’s demonstration of a prototype next-generation missile defense tactical operations center. Tests revealed that at a center frequency of 2 gigahertz, the PulsON chipset propagates signals using standard local area network (LAN) raw bit rates of between 1.25 and 1.54 megabits per second. In all cases, external interference was not noted as received or produced by the PulsON demonstration system. In future applications, the technology could deliver enhanced communications capabilities in air and missile defense operations and, when used with Ethernet LANs, support home or office personal computer use.

PulsON technology uses a silicon-germanium chipset to control the emission of 10 million coded pulses per second across multiple gigahertz of spectrum to within 5 picoseconds of predicted arrival time. The pulses are randomly spaced and sent through a sequence of noise-encoded hops that identify them to the receiving unit. Data being transmitted is relative to each pulse position and corresponds to the precisely timed spacing of each pulse. The use of time dithered pulse position modulation (TDPPM) instead of continuous sine waves eliminates potential signal cancellation because pulses, not continuous waves, are the transmission method. Correspondingly, TDPPM allows the correlating receiver to recognize a pulse as a zero if it arrives early or a one if it arrives late.

“A fundamental difference between the Time Domain approach and regular analog sine wave systems is in the lack of multiplicity of self-canceling waveforms. In the case of continuous sine waves, the threat of massive signal corruption is inherent because the transmission is on one pathway,” Kevin Davis, vice president director of government defense sector, Time Domain, states. Interference in waveform can cause a rippling effect, leading to spiked distortions in the message. Signals are often canceled out completely by reflected waves in reverse phase within a given section of the sine wave. These distortions do not occur with time-modulated signaling because each pulse train is time hopped and correlated only to the expected sequence of pulses, he adds.

Based on precisely controlled pulse positioning, PulsON technology incorporates an intrinsic position location capability for tracking the exact coordinates of intrasystem links within a common wireless network. Pseudo-random hopping of the time slots that each pulse occupies enables the easy identification of sending and receiving systems as linked units. Over the course of every 1-second emission, the pulses hop to longer time slots, further refining the characteristics of the transmission. Based on a digital architecture built into each chipset, the system channels the pulses into correlating receptors without disturbance from outside interference. With the proper channelization of all the pulses, the probability of enough signal corruption occurring to distort a message beyond recognition is relatively low.

Potential uses of time-modulated ultrawideband technology currently are being explored for areas of high signal interference such as in urban, mountainous or densely forested terrain. High-power analog transmissions can be disrupted by the external noise of other radio signals in the area. Frequencies higher than 3 gigahertz often lose portions of their signals to attenuation spikes caused by a reverse rippling effect on sine waves. To avoid this problem, PulsON technology uses pulse emissions of between 50 microwatts and 1 milliwatt that transmit in the “grass or garbage” layer of the radio frequency spectrum. At this low power level, Time Domain’s signal propagation frequency of around 2 gigahertz is below the destructive signal cancellation that occurs at higher power levels. “Using coherent pulse integration, time-modulated ultrawideband signals can tolerate most in-band frequency domain users while maintaining good channelization at high data rates,” Davis remarks.

In environments of high transmission noise such as urban areas, the coding of data is essential so that only intended personnel hear particular signals. PulsON technology offers an inherent encoding feature that distinguishes individual time-hopping pulses from external signals, enabling the reception of precise information by linked receivers only. In the time it takes a pulse to reach a receiver unit and return to the sender, an algorithm, applied automatically, determines the position of both units. In situations where a second or third radio link is involved, a three-dimensional geospatial picture of a moving target’s location can be achieved using a point-of-reference ping to collect the desired coordinate information. “The PulsON architecture provides a built-in referential capability, allowing the construction of geospatial relationships between linked radio units,” Davis remarks. “In potential future military applications, the system could be used to keep patrolling units in better coordination with regard to their own position.”

According to Cory Youmans, project director for the U.S. Army Simulation, Training and Instrumentation Command in Orlando, Florida, precise position location is becoming an area of greater importance to today’s military as rules of engagement change to accommodate longer range defense capabilities. “Enabling systems to conduct pinpoint targeting without having to rely on linked units to perform routines like referential triangular locating is a high priority,” he says.

In the communications arena, simultaneous voice transmission limitations as well as over-the-horizon air-to-ground radar systems are driving the development and implementation of ultrawideband technology. Systems that supplement global positioning system (GPS)-based equipment and are usable indoors for tracking and range finding will be invaluable assets on the new urban battlefield, he adds.

Unlike differential GPS or acoustic sensing, time-modulated ultrawideband enables the pinpoint tracking of users down to a few centimeters of actual location within complex urban or natural environments. The short duration of each pulse, around 500 picoseconds, allows a sending unit to collect highly accurate range information through the numerous pulses emitted. With specially designed antenna arrays, the rapid transmission of millions of pulses can result in a high-resolution radar image of an intended target.

Large, high-powered antenna systems used in military tactical operations take into account the probability that signals will encounter extraneous interference. “Increasing the power of a transmission decreases the likelihood that the signal will succumb to attenuation by outside disturbances,” Davis explains. “The problem arises when too many signals of similar strength are propagated in a shared spectrum space. A way around this is to use very low power in the noise bands and transmit pulses intermittently with enough redundancy to get the signal from the sender to receiver.” With PulsON technology, signals not only are tolerant of greater external interference, but also are less prone to interfere with other signals because the pulse waves contain less energy. High-energy waves tend to weaken other transmissions by drawing energy from them, he explains.

Transportability and adaptability are other key factors in the system’s applicability to military operations. As a lightweight chipset-based technology, PulsON could be integrated with existing legacy systems before being built into an independent format. Substituting large, high frequency military antenna systems that require 10 to 100 megahertz with a more robust architecture that uses less power is one of Time Domain’s future objectives. Davis explains that a combination of communications and position location and tracking capabilities within a single 2-inch antenna would enable increased interoperability while reducing the complexities of bulky, multiple-part systems. “These capabilities, coupled with inherent functions like low probability of detection and fusion of sensory, tracking and communications [signals] in high noise urban environments, will provide the warfighter with assets that simply don’t exist today,” he says.

The technology also addresses the issue of data security. Because a majority of radio wave systems operate in typical frequency ranges, traffic often overlaps and users hear extraneous signals. Encryption normally is used to hide data being transmitted, but in many cases it is not feasible to upgrade systems to include this functionality. PulsON systems have an inherent means of maintaining information security without compromising scalability if added encryption is necessary. “Aside from the fact that the very environment that our pulses are propagated in lends itself to covertness, a major reason signals remain untranslatable by outside systems is the time hopping that each pulse experiences as it is channeled to its correlating receiver,” Davis explains. “Since the pulses are encoded for recognition and are randomly hopped, they are only translatable by a linked receiver unit.” In highly confidential military communications, both the technology’s architecture and modularity support the need for maximum data security.

Before the military can use time-modulated ultrawideband technology, it must be approved by the Federal Communications Commission (FCC) for commercial use. A new product line for civilian markets would be developed first, which could lead to the dissemination of the commercial products into the military services.

“Once the technology finds a home in the armed forces, scalable applications enabling fused net-centric command and control, intelligence, surveillance and reconnaissance capabilities may gradually replace old legacy systems that cannot support a seamless transition to newer technologies,” Davis remarks. Lower cost commercial products will allow the military to streamline acquisition of the technology to transition it from training to mission planning to combat operational systems, he adds.

To date, a large number of products are compatible with PulsON emissions of 1 to 3 gigahertz. “Many European nations that now use multiple radio systems to interoperate across national borders will be able to use one ultrawideband system without interfering with their existing communications infrastructures,” Davis adds. Further demonstration and application of Time Domain’s time-modulated PulsON technology will continue as hardware production increases.