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Coherent multichannel systems have unique properties that enable beam forming, direction finding and improved reception in a dispersive channel. Interest in diversity antenna systems, phased array antennas and multiple-input multiple-output (MIMO) systems continues to grow. These coherent multichannel systems offer capabilities and performance that are unavailable with other technologies.

We shall look at two applications of coherent diversity systems, MIMO communication systems and direction finding. These applications are critical to the aerospace and defense communities since recent developments in military tactics require these technologies.

Aerospace and defense radio applications often demand that radio systems are placed aboard platforms that have high motion dynamics such as a high-performance jet. The antenna-shadowing problem aboard these high dynamic vehicles can dictate many antennas just to provide ‘visibility’ around airframes or other vehicle obstructions. Antenna coverage of 360° could take six or more antennas to achieve. One benefit of using many narrow beam antennas to cover an omnidirectional view is the increased antenna gain. The challenge of selecting the appropriate antenna can be difficult due to the rapid signal fading characteristics as the vehicle maneuvers. High-speed switching or combining signals of a MIMO system is one approach to providing seamless communications.

In recent years, the tremendous advances in digital signal processing (DSP) now enable separation of the transmit datastreams using inverse channel property estimation. This allows each receiver to take in multiple signals and separate the datastreams by applying the appropriate channel modeling. Usually these systems use multipath-resistant modulations such as orthogonal frequency-division multiplexing (OFDM). Modern MIMO systems also offer some advantages in data security, since they can take advantage of spatially diverse transmitter sources.

Multichannel coherent systems are also useful for determining the location of a signal emitter. Signal emitters are sometimes referred to as targets. Using multichannel and coherent systems it is possible to find the direction from which the signal is originating.

There are several methods for determining the location of a signal emitter using different types of direction-finding (DF) radio equipment. Let's briefly review the different approaches to locating a signal emitter and the types of multichannel DF equipment needed for each approach.

There are three basic approaches to locating a signal emitter from an airborne platform using only passive receivers with many variations on these basic techniques. Azimuth angles, elevation angles and altitude can all be used to determine the direction and distance an emitter lies from the aircraft. Another approach is to measure the angle and distance as the aircraft passes by the signal emitter. The distances and angle measurements can be used to triangulate the position of the emitter. Often, this approach uses many triangles averaged together in sophisticated Kalman filters to multilaterate a more precise location. Doppler shifts and accurate timing can further aid multilateration systems in determining precise measurement fixes. Finally, the signal time of arrival (ToA) at several points or time difference of arrival (TDoA) delay can be measured to determine the origin of the emitter. Precision ToA systems are based on the intersection of hyperbola formed by constant delay differences. All these methods rely on either measuring the angle of arrival (AoA) of the signal or precision timing measurements. Multichannel and coherent radio systems are ideal for these applications.

Using a single high gain antenna (Figure 3), it is possible to determine the AoA of a signal and convert it to the cardinal direction of arrival (DoA) for location on a map. However, the scanning highly directive antenna, though simple in concept and receiver design, has several significant disadvantages. Mechanically scanned antennas are difficult to rotate 360° quickly and do not provide rapid location of emitters. They can also be mechanically challenging to adapt to airborne and other platforms. To provide high directivity and gain, it is necessary to use large aperture antennas that are physically large, further complicating rapid scanning. Finally, since the antennas scan slowly, their ability to capture short pulses or bursts of signals is limited. Mechanically scanned antennas have a low probability of intercept (LPI) of pulsed signals. These single aperture antenna drawbacks make the multichannel and coherent systems for AoA measurement attractive alternatives.

A simple approach to determining AoA is to use several antennas pointing in different directions and measure the received signal strength from each antenna. In Figure 4, a six-channel system is used to determine the angle of arrival of the signal. By comparing the signal strengths between horn antennas, it is possible to further improve the accuracy of the angle measurement. Deriving AoA from amplitude measurements does require multichannel receivers. Fortunately, the receivers do not have to be coherent. The horn antennas tend to be small in size, low cost and the measurement speed can be extremely fast. The primary disadvantage of using amplitude measurements to determine AoA or location is their low angular resolution.

The phase interferometer (Figure 5) is another approach to measuring AoA, and has characteristics nearly opposite that of the antenna amplitude comparison method. Interferometers require expensive coherent receivers and can provide high angular resolution on the direction of arrival of the signal. Unlike the amplitude measurement approach, interferometers are often large in size.

The phase interferometer compares the phase shift observed between antennas created by the wave front arriving from different directions. This phase shift, f, is related to the AoA of the signal. Unfortunately, since the phase shift becomes ambiguous after 360° it is often necessary to have many antennas and coherent receivers to provide a complete 360° of directional coverage.

Interferometer systems can become complex with multiple coherent channels being driven from entirely phase coherent LO synthesizers. LO phase noise is also an important consideration for multichannel coherent systems like interferometers. Phase noise on the LOs can reduce the precision of the interferometer.

One challenge to building multichannel phase coherent systems is phase matching the different coherent signals. Multichannel phase coherent systems depend on precise phase relationships between antennas. At microwave frequencies, even small differences between cable lengths, amplifier devices and filters can create delays or phase shifts that destroy the desired relationships. Phase-matching systems that are sensitive to less than one degree of phase shift can be exceedingly difficult. Phase stability of components, non-linear AM/PM effects and group delay variations can make phase matching a test concern for the multichannel designer.

Another approach to determining the location radio signals are emanating from is to use a time difference of arrival (TDoA) system. These systems are particularly useful in locating radar pulse emitters or communications signal bursts from time-division duplex (TDD) transceivers. They also work well over larger areas.

TDoA systems contain multiple receivers that work together to measure the time difference of arrival of a signal pulse between receivers. The constant time difference of arrival between two points is represented by a hyperbola. The locus of points, which always equals a fixed difference in distance or delay at the speed of light is a hyperbola with the receiver antennas at the foci.

Unfortunately, since the hyperbola represents all the places the signal emitter might be, it requires at least one more receiver to create an intersecting hyperbola to determine the exact location of the emitter. The accuracy of the system is variable and dependent on the locations of the receivers relative to each other and the emitter. TDoA systems also require precision delay measurement for each receiver channel, which requires accurate timing reference clocks. These clocks may use GPS for synchronization with one another but this introduces an ambiguity of up to 40 ns in timing. For truly accurate direction-finding measurements using the TDoA method, each receiver platform is synchronized by a point-to-point millimeter wave or laser link.

Since pulse timing can be carefully controlled, a signal emitter can be artificially located at a wide range of simulated positions. This enables testing of a complex TDoA system without having to actually fly the receivers around to validate correct operation. Using either Agilent's Signal Studio for pulse building software or third-party software such as Synergent Technologies' Six Pack DF software, complex multi-emitter wave front signals can be created.

The most basic problem all these multi-antenna aperture systems share, is that they require more than one source to stimulate them for test and more than a single-port analyzer to measure some of their performance attributes. The multiple-source inputs and multiple-measurement output ports often require coherency and phase control between measurement ports. Single-port sources and signal analyzers often cannot stimulate or measure key system parameters. Further complicating matters, unless the test and measurement equipment is correctly built, tying reference clock inputs together will not assure the proper signal coherency and phase control.

Multichannel coherent systems create added test challenges for the development engineer and production test department. Beyond the necessity of requiring multiple phase coherent stimulus ports and measurement ports, coherent test requires unique signals and measurements. Coherent sources with adjustable phase offsets and sophisticated software are needed to simulate complex wave fronts arriving at multiple antennas. Cross- channel measurements such as correlation, coherence, cross spectrum, frequency response and impulse response are also needed for diagnosing many multichannel designs.

Agilent has recognized these issues for the coherent multichannel system test and created the synthesizer distribution solution to provide coherent signal control. This solution allows up to eight signal sources to be tied together and provide complete phase and amplitude control of the RF carrier and modulation.

The distribution network enables one of the sources to serve as a master reference signal for up to seven slave signal sources. It also provides a means to coherently drive the arbitrary waveform generators, so they are phase related and triggered at the same time. This not only allows for phase control of the modulation, but also accurate pulse timing control (Figure 8).

With trigger timing accuracies of 500 ps, pulses can be transmitted at the precise time to provide exceptional time delay accuracy. This enables testing of very-high-resolution TDoA systems for positional accuracy. Each source is also equipped with a wide-range output attenuator that allows independent control of signal amplitude. Agilent offers both signal source solutions and dual-channel vector signal analyzers that are designed to solve these problems for the coherent multichannel system designer.

John Hansen is a senior product manager for Agilent Technologies' Electronic Measurements Group. He is responsible for the launch of new high-frequency microwave signal generator products. He has more than 20 years of experience in system engineering and new product development within the wireless, microelectronics and defense industries. Hansen received his BS in Engineering from UCLA, an MSEE (communications systems) from USC, and his MBA from San Diego State University. He is a registered Professional Engineer in the State of California.

John Barfuss is the Aerospace Defense program manager for Agilent's RF and microwave (wireless) divisions. He joined Agilent in 1999 as a marketing engineer in Santa Rosa, CA. While at Agilent, he has worked extensively with Agilent's spectrum analyzers, vector signal analyzers, and signal generators providing product training and market development activities. Prior to joining Agilent, Barfuss was a test engineer for 3Com Corporation. He graduated from the University of Utah with his MBA in 2000 and his BSEE in 1997.