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Antenna Systems For Radar Applications Information Technology Essay

Paper Type: Free Essay Subject: Information Technology
Wordcount: 5398 words Published: 1st Jan 2015

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The project will examine a variety of beam forming techniques which can be used in order to make radar electronic beam steering feasible. Commonly used mechanical rotating antennas for a 360 degrees views coverage are difficult to operate and expensive to implement. Thus, electronic beam forming is an attractive solution. This report is mostly interested in radar applications performing in 24 GHz frequencies, which can be used by car industries, in order to avoid obstacles on the road, for example, or security radars, covering 360 degrees views.

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Radar

2.1. Radar Definition

Radar means “radio detection and ranging”, determining thus the original and still significant application of radar. The main reason for using radar is to estimate certain characteristics, such as the position, motion and presence of the specific surroundings in which the user is interested. Radar is actually a sensor which transmits electromagnetic energy into the surroundings and detects energy which is reflected by objects. If a directive antenna transmits electromagnetic energy through a narrow beam it is easy to predict the bearing of an object because of the energy reflected of it. The time needed for the transmission and reception of the energy represents the distance between the radar and the object. [21]

Kinds of Radar

There is a great variety of radars. Some radars provide navigation aid and safety on small boats and their size might be less than 15cm. Others are widely used by the police in order to measure the speed of the vehicles. Moreover, there are some radars so large as to cover many kilometers of land, long arrays of antennas and they all work together in order to supervise the flight of astronomical bodies or space vehicles. In addition, there are many radars at airport, with a more common size and rotating antennas. Finally, there are several radars, more complex, for mobile use. [21]

Radars can be classified in many categories. As much as the waveforms are concerned, radars can be classified in 2 categories, they can be either Continuous Wave (CW) or Pulsed Radars (PR). CW radars use different antennas for transmission and receiving, and they emit electromagnetic energy continuously. Unmodulated CW radars precisely determine the target radial velocity and angular position, while information about the target range have to use some form of modulation in order to be gathered. In order to search and track target velocity, primarily unmodulated CW radars are used. Pulse Radars (PR) use a series of pulsed waveforms, mostly with modulation and can be separated based on the Pulse Repetition Frequency (PRF) in 3 categories, high, medium and low PRF radars. CW and PR radars are both able to determine target range and velocity by using different forms of modulation. [23]

Continuous Wave Radar (CW)

CW radar sets continuously transmit a high-frequency signal. Then, the received signal is permanently processed. In such a system, two problems have to be solved:

avoid a direct connection between the transmitted and received energy (feedback connection),

conduct the received echoes into a time system capable of doing run time measurements.

A feedback connection can be prevented by:

spatial separation  between the transmitting and receiving antenna

frequency dependent separation by the Doppler-frequency during the measurement of speeds. [4]

Frequency Modulated Continuous Wave Radar (FMCW)

CW radars are not capable of measuring distance, because the timing mark necessary lacks, preventing thus the system to time precisely the transmit and receive cycle and exchange the measured round-trip-time into range. This problem can be solved by using phase or frequency shifting techniques. As far as the frequency shifting method is concerned, a signal is used, which continuously changes in frequency around a specific reference, in order to identify stationary objects and measure the range. In order to achieve an up-and-down or a sawtooth-like alternation in frequency, Frequency-Modulated Continuous Wave radars (FMCW) are used, changing the frequency in a linear fashion. By constantly changing the frequency, there will be a difference between the frequency of the echo signal and the one transmitted. Thus, the difference transmitters frequency shift will be relative to round trip time and so the range of the target too. The frequencies can be examined, when a reflection is received, and by comparing the received echo with the actual step of transmitted frequency, a range calculation like using pulses can be done. Consequently, the range of the stationary objective is given by comparing the transmitted and received frequencies. It is difficult to make a broadcaster able to send out random frequencies cleanly, and as an alternative, this frequency-modulated continuous-wave radar, use an easily changeable “ramp” of frequencies up and down. If the frequency modification is linearly over a broad area, by making a comparison among frequencies within this region, the distance can be easily determined. It is possible to measure only the complete value of the difference and thus, the results with increasing frequency modification signify a decreasing frequency change at a static scenario. [4]

Characteristics e of FMCW radar:

measuring the distance is potential by comparing the definite frequency of the received signal to a given reference (regularly direct the transmitted signal)

the time required for transmitting a signal as longer than the duration of the measurement of the installed maximum range of the radar [4]

By selecting the appropriate frequency deviation per time unit, the radar resolution can be different, and choosing the frequency shift duration, the maximum range can be varied. For instance, if the linear frequency of radar increases over 1 ms duration, the time-limited maximum range might be 150 km. If the maximum frequency deviation is 65 MHz, then stay about 433 Hz per meter for the filter for analysis. It is important that the amount of frequency modulation is considerably greater than the estimated Doppler shift otherwise, the outcome will be affected. The most common and easy way to modulate the wave is by linearly increasing the frequency. In this way, the transmitted frequency will change at a continuous rate. If a single antenna is used, a ferrite circulator has to separate the transmit and receive. However, using to different antennas, one for transmission and one for reception, is easier and cheaper to perform. On a ordinary substrate transmitting and receiving antenna are placed exactly above each other as an antenna array. The direction of the linear polarization is rotated against each other by 180 degrees. An extra shielding plate reduced a direct “cross talk” (i.e. a direct coupling of both antennas) usually. From this direct coupling, arises a signal, which is suppressed due to the same frequency, since the measurement is performed to as a frequency difference between transmit and receive signal. [4]

Radar Beamforming

In order to create a beam with the appropriate and desired characteristics, radar beamforming, which combines signals from a set of sources, is essential. As much as an RF antenna system is concerned, each source may be a single array element or a subarray. A steerable beam is able to control the combination process electronically. Moreover, it can be replicated so as to create various independent beams, limited by hardware difficulty, complication and losses. [22]

3.1. Analog Beamforming

A feed system is a network used in order to connect the antenna input to its radiators. The main reason for using such a system is to transmit power to the elements or collect signal from them. (transmit mode, or receive mode). While being on transmit or receiving mode, the required phase and amplitude excitations needed for the radiation performance must be maintained. The feed network is able to scan the beam, select among different antenna beam shapes and communicate with active sectors, by containing several switches and other devices, appropriate to execute such processes. Moreover, the feed network may contain amplifiers and other active devices. There are also many new developments, such as Switch matrix systems, Butler matrix feed systems and Vector transfer matrix systems, but the most significant are the RF lens feed systems. [1]

3.1.1. RF Lens

One of the biggest problems when using a transmission line feed network is that amount of losses. Therefore, systems which are based on RF/optical principles are preferred. There is a large variety of RF Lens and many RF/optical feed systems also incorporate different types of beam scanning functions. RF refractive lenses are similar to their classical optical counterparts, which function by using the refraction amongst different materials. When using constrained lenses, the waves are forced to follow some specific paths, like in a geodesic lens. Another type of lens is the bootlace lens which in which the signals between the input surface and the output surface are routed on transmission lines. Occasionally, a conformal array feed uses different combinations of lens types, or lenses and matrices. Small array antenna elements are used by an RF lens as input/output probes that couple to the lens region. These probes exist in an array environment which is characterized by reflections and mutual coupling and the associated design problems. In particular in circular lens designs, there can also be standing waves caused by reflections from the opposite side of the lens. Another problem is the variation of the element phase center with frequency. [1]

Rotman Lens

A Rotman lens is a parallel-plate structure used as the beam forming network (BFN) for a linear array of radiating antenna elements. It is easy to form a beam forming network suitable for use with a planar array, by stacking numerous lenses. Rotman lenses are preferred because of the advantages that they offer, such as ease of manufacture, light weight, low cost, monolithic construction and availability of many beams at the same time. Rotman lens is capable of extremely wide-band operation, because it is a true time-delay device which produces frequency-independent beam steering. Because of these characteristic, Rotman lens is a possible candidate for use in multi-beam satellite-based applications. [2]

The electrical area that a Rotman lens occupies is very large (usually hundreds of square wavelengths) and because of this, an entirely precise analysis is not possible. The planar circuit approximation applies to structures which are electrically thin in one dimension, like parallel-plate lenses. The effort required for their analysis is reduced to that of solving a (line) integral equation for the relationship between the RF voltage and current at the periphery of the structure. [2]

The R-2R Lens

The R-2R lens feed (Figure 1) has feed ports on the perimeter of a parallel-plate lens with radius R, in order to illuminate the output ports on the opposite side of the lens. These output ports are linked to the element ports on the 2R radius circular array with cables of equal length. The number of feeding ports is half the number of element ports. This type of arrangement allows all feed points to be ideally focused, resulting in a plane-phase front. In order to scan the antenna beam at angle Φ, the feed point has to be moved an angle 2Φ. One illumination taper can be achieved, by combining three to four adjacent feed ports, resulting in lowered sidelobes. [1]

Figure1: The R-2R lens feed system [1]

It is essential to add several switches on the lens ports, in order to scan the beam. One has to be allowed to use numerous beam ports at the same time in order to achieve a multiple beam generation. This problem could be solved by using half the lens for beam ports and connect the other half to a 90° arc array. R-2R lens are considered to be a special case of the Rotman lens, which is typically used for linear array feeds. Furthermore, for circular arcs up to 90°, the Rotman lens can be used. Actually, the curvature does not have to be circular, as the design in general, curvature of lens input and output lines, cable lengths, and so on can be optimized together with the array shape. It is possible to achieve ideal focusing in the Rotman lens only for three beam directions. [1]

The R-kR Lens

The R-kR lens feed system has as much ports on the lens as there are radiators on the circular array. In order to cover 360° views, the lens ports have to be used more than once, both as feeding points and for connecting to the radiating elements. In order to achieve this, switches are used, circulators (Figure 2), or two lenses at the same time. The radiators placed on radius R are connected by cables of the same length to the ports of the circular lens with radius kR. When k is about 1.9, a planar phase front for rays within a sector of about 120° is obtained. This shows that the lens is nearly two times the size of the circular array, thus, it can’t fit inside the circular array if it is not filled with a dielectric with permittivity more than 4.

If broadband radiators are used, the R-kR lens-fed circular array can be very broadband. The bandwidth could be limited by using switches or circulators. The phase center of the radiators is a design parameter of critical importance and must be located on the design radius R. [1]

Figure 2: The R-kR lens, here with circulators. [1]

In order to limit the focusing performance, several types of element have a phase center which is able to change position with frequency. [1]

Mode-Controlled Lenses

A radial transmission line which forms a circular parallel-plate lens is possible to act like a circular array feed. If it is excited by several probes placed close to the center, the modes generated will direct the energy toward a part of the lens periphery.

Therefore, by controlling the modes using phase shifters or a hybrid network connected to the input probes commutates the excitation. Then it is easy to connect these pick-up probes to the radiating elements, via additional phase shifters if needed. [1]

Luneburg lenses

In order to achieve wide angle scanning, the Luneburg lens, is the appropriate and desired device. As far as land mobile operations are concerned, an antenna able to scan in a two-dimensional (2D) plane is required, particularly if the scan angle is large. The Luneburg Lenses are used in order to provide single or multiple mechanically scanned beams, at microwave frequencies. Nevertheless, because of the advent of phased arrays the lenses are now usually used for radar applications as a wide angle passive reflector.

This is why nowadays there are appropriate lens configurations which can be established by permitting the inclusion of controllable dielectric material into a Luneburg Lens so as to make the lens suitable for electronic scanning at 24 Ghz. [1]

Digital Beamforming

When performing beamforming in the digital area, it is called digital beamforming. The realization can demand huge volumes of digital information to be processed at extremely high rates, but current improvements in processing hardware have made Digital Beamforming a useful alternative to RF combining in many ways. Moreover, it has allowed the formation of systems which were not practical with legacy technologies. Below are presented the benefits of Digital Beamforming. [22]

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Simplicity of hardware

If the RF and analog hardware becomes a ”minimal” device, collecting data, it would be an ideal case. Then, all the difficult and complicated process of the signal is done in firmware, which is a more flexible and gainful way of processing comparing to RF ”plumbing”. In addition, it is possible that the overall size of the system, as long as its weight, will be reduced a lot, and this is particularly significant in airborne systems. [22

Replication

Digital beamforming is the best option when many independent beams are needed. By using digital beamforming, it is easy to form each beam completely digitally, without any analog or RF hardware further required. The quantity of beams like these is then partially limited by power, speed and synchronization of the processing elements, which become even more cost-effective and flexible each year. [22]

Scanning and Tracking

It is not possible to steer electronically each beam (e.g., to track a moving source). However, by using the precisely same stream of digital samples from each antenna element, it is potential to turn each independent beam to a different source. Thus, it is easy to reduce extremely difficult receiver scenarios to firmware buildings blocks which are now usual. [22]

Flexibility

These digital systems can be adapted without any difficulty to varying requirements, such as multipath combination, application bandwidth, tracking requirements or interference rejection. A SMOP (Simple Matter of Programming) is able to perform numerous adaptations. [22]

Radar

An array antenna which is a low Cost Transmit/Receive one provides agile beams to track multiple targets at the same time. [22]

Anything that can be done by using an analog beam forming can easily be done digitally too. Choosing to do everything digitally might lead to several difficulties because of the extreme requirements on data transmission, storage, and signal processing. However, nowadays such problems are easily solved because of the rapid growth of computer power, either software or hardware. When using an analog reception beam forming, the element signals are combined with weights determined by feed networks and/or phase and amplitude controlled receiver modules. In digital computer, it is possible to do the same operations on the element signals just by converting analog signals to digital ones. Thus, the formation of many receive beams can take place at the same time, without feed losses, which are common when using analog systems. Moreover, the element modules in the digital systems have low noise amplifiers (LNA) preceding the analog-to-digital conversion. A ”lossless beam forming” is created as the LNAs set the signal-to noise ratio, so that it is not affected by transmission losses. The advantages of a digital beam forming in this case are not so obvious. After the transmission of the beam, it is not possible to change the beam shape or to perform any other signal processing. Nevertheless, digital synthesis of the transmitted waveform on the element level combined with DBF on reception can offer remarkable system capabilities in terms of, for example, LPI (low probability of intercept) radar with jamming resistance. A wide transmission beam illuminating the area of interest and multiple, narrow, digitally formed receive beams has also been suggested for LPI systems-“ubiquitous radar” and “OLPI radar” (Omnidirectional LPI). There are many aspects which can best be performed digitally, such as the need for amplitude and phase control, polarization control, switching of the active sector, compensating for element patterns in the beam steering algorithms and calibration. A DBF antenna system has a combination of numerous subsystems and components. Receiver channel imbalance, , A/D converter offset errors, amplitude and phase errors and frequency dependent errors are some of the possible imperfections in these subsystems and component which can influence the performance of the overall system. The type and requirements of each processing used influence the importance of such imperfections. Usually, array calibration and special error correction schemes are included in the antenna system design. [1]

3.3. Beamforming Transmitter Architectures

Several beamforming transmitter architectures exist, suitable for integrated circuit implementation as well as many well-known topologies for separate implementations of phased array transmitters. The goal is topologies appropriate for performance in consumer products at 24 GHz. Electrical beamforming is achievable if the phase of the signal to each antenna element in the array is separately set. Moreover, a larger number of patterns can be achieved and the sidelobe level can be reduced compared to uniform power distribution if the power to each antenna element is set individually. [3]

3.3.1. Baseband Phase Shifting

In the baseband phase shifting architecture the phases and amplitudes of the signals are created in the digital baseband. The phase control is very accurate, but the architecture demands an entire signal path between the baseband and the antenna for each element (Figure 3). Also, the architecture can be called digital array, because the beamforming is being performed in the digital domain. Such an architecture lead in a large hardware cost and power spending because there are many signal paths, but also in big flexibility. As a result, this architecture is perhaps very complex for radar at 24 GHz. In order to transmit individual information in various directions, in MIMO systems (multiple input multiple output), the flexibility of the architecture with parallel paths is available too. [3]

Figure 3: Transmitter architecture for baseband phase shifting [3]

3.3.2 Local Oscillator Phase Shifting

Phase shifting can occur in the LO path as well (Figure 4) Moreover, it is likely to use phase shifters in the signal path, at IF or RF. Whether performing the phase shift at LO or RF or place them at different places, the same amount of hardware is achievable. If they are placed in the LO path, amplitude variation among dissimilar phase settings is less significant if the mixers are driven hard. In this way, amplitude variation in the LO path will not influence the signal path a lot. Thus, it is easier to implement the phase shift in the LO path. [3]

Figure 4: Transmitter architecture for phase shifting in the local oscillator path, polar modulation [3]

3.3.3. Offset Local Oscillator Phase Shifting

If the power amplifier and local oscillator are used at the same frequency, injection pulling is possible to occur. It might not be easy to accomplish a sufficient isolation so as to avoid the corruption of the oscillator signal by the PA. To moderate this on an architectural level, offset LO phase shifting may be used as shown in Figure 5. Beamforming transmitters have applications like radar (24 GHz and 77 GHz) and WLAN (60 GHz) which are placed at high frequencies. It is valuable to use the lowest frequencies possible on the chip, and multiply the frequency close to the PA. A reduced VCO frequency makes allows a wider tuning range, and the increasing MOS varactor quality factor. [3]

Figure 5: Offset local oscillator phase shifting for beamforming transmitter [3]

3.3.4. Ring Oscillator Based Phase Shifting

A ring oscillator which has a tunable phase shift among the oscillating elements is used in such architecture (Figure 6). The tuned oscillators in the ring are separately detuned from their center frequency. The LC-loads is capable of sustaining up to +-90 degrees phase shift. It is important that the phase shift around the ring is constantly equal to 360 degrees, or a multiple thereof. The phase shift among consecutive elements is zero degrees if each oscillating element is non-inverting, and no excess phase shift is introduced in the loop. By putting an excess phase shift of Kδ degrees it will have as a result a phase shift of δ degrees in each of the equal K oscillators in the loop. [3]

Figure 6: Transmitter architecture for variable phase ring oscillator in a phase locked loop [3]

3.3.5. Radio Frequency Phase Shifting

The phase shifting which is the most hardware efficient, including numeral building blocks, is to carry it out just before the power amplifier. The power amplifiers are the only circuit components that have to be duplicated (Figure 7). The disadvantage is that the phase shifting is being performed at the highest frequency and signal level in the system. When an envelope modulation scheme is used, the linearity of the phase shifters may be a problem while noise is not as significant when the power level is high. It might be useful to implement the phase shifters at the highest frequency.

If transmission lines are used as separate phase shifters, they become shorter with frequency. This is an ordinary architecture in radar systems. Several fixed phase shifts are in that case implemented and switches controlled by selection logic determine the phase shift. Certainly, the transmission lines are linear and thus, these phase shifters can easily be used in envelope modulated systems. Moreover, the delay is stable over a wide bandwidth.

A set of fixed phase shifts is then implemented and switches controlled by a selection logic choses the phase shift. Of course the transmission lines are linear so these phase shifters can very well be used in envelope modulated systems. Another advantage is that the delay is constant over a wide bandwidth. [3]

Figure 7: Transmitter architecture for phase shifting in the radio frequency path. [3]

Applications for 24GHz Radar Sensors

Modular 24 GHz Radar Sensor for Digital Beamforming on Transmit and Receive

In order to increase the angular resolution, numerous switched transmitters are preferred, as they need less hardware effort. The FMCW radar sensor is the best solution, providing up to eight transmitters, switchable ones, and eight receiving channels which provide parallel receiving, and they all allow digital beadforming. An innovative switching technique via switchable amplifiers is preferred. [5]

Four Channel 24-GHz FMCW Radar Sensor with Two-Dimensional Target Localization Capabilities

Results on the angular measurements are improved when using an FMCW radar sensor, compared to standard beamforming methods, as far as the target localization is concerned. Furthermore, the determination of other characteristics required will be allowed, such as the range or velocity. [6]

.

24-GHz Automotive Radar Transmitter with Digital Beam Steering in 130-nm CMOS (Complementary metal-oxide-semiconductor)

Many Pas are connected to different antenna elements so as to control the steering of the beam. The output phases of the PAs are controlled separately through 360 degrees by binary weighting of quadrature phases. The circuit has 18 PAs,and each one of them delivers 0 dBm to the antenna, ensuring an output power of 13 dBm. The antenna array, which is constituted of 18 elements, will be 11 cm at 24 GHz and will have 12 dB directivity and a half power beam width of 5 degrees. [7]

Design and Performance of a 24-GHz Switch-Antenna Array FMCW Radar System

One transmitter, one transmitting antenna, four receiving antennas, one receiving channel and an SP4T switch (single-pole four-throw) are the elements which compose a 24-GHz FMCW radar system. In order to increase the inter-connection loss and create a compact whole size, radio-frequency (RF) and intermediate-frequency (IF) circuits are integrated in the antennas. The receiving antennas are sporadically switched to the receiving channel. Beamforming methods are used in order to evaluate the performance of such a developed system, by estimating the angle, velocity and range. [8]

Imaging Radar Sensor Front-End with a Large Transmit Array

Automotive applications need medium range imaging radars, such as the 24 GHz imaging radar front-end. In this radar, a large switched transmit antenna array is combined with a coherent FM-CW architecture. It permits two dimensional electronic scanning in range and cross range with excellent crass range resolution over a wide angle of new using very low EIRP. The advantage of using such radar is that it requires just a small number of active millimeter wave components. [9]

Novel Photonic Rotman-Lens Design for Radar Phased Array Antennas

A new microwave photonic implementation of a Rotman-lens is proposed in this project, providing superior performance and functionality. The scanning unit presented is an optical element, where photo-detectors attached to the transmitting/receiving antennas are the interfaces, doing conversions among the RF signals and their particular optical waves. Actually, the optical module is a photonic Rotman lens, designed like its RF complement. Despite the advance of practicing the solution in a photonic module, the recommended photonic Rotman lens superior design is able to realise a linear phase profile with a varied slope at the output of the lens for any potential spot at the input to the lens. This is contrary to what is presently accessible with the usual RF Rotman lens, where output phase front linearity is achieved for a small quantity of input spots. A better performance is achieved by increasing the curves of the photonic input and output surfaces of the lens, having an off-centered elliptical profile, and not the typically used spherical curvatures. [10]

Virtual Antenna Beamforming (VAB) for Radar Systems by Using Orthogonal Coding Waveforms

An original way of creating virtual transmitting and receiving radar antenna beams at the same time is to use orthogonal coding waveforms from the antenna elements and deal out digitally their echoes at the receiver. Many virtual transmitting-receiving radar antenna beams can be produced at the same time by using the same quantity of beamforming filters with no any increase on the transmitted power or antenna gain or resolution loss. Both virtually formed antenna beams and common phased arrays of equal size are able to achieve the same antenna gains and spatial resolutions. Since the antenna radiation pattern can be completed almost isotropic, the original system has low probability of intercept (LPI) property. While the transmitting and receiving beams are both virtually implemented through digital filtering, expensive radiation phase shift used in phased arrays is unnecessary for beam scanning in this actual system. [11]

Compact Two-Layer Rotman Lens-Fed Microstrip Antenna Array at 24 GHz

A new way of realizing a compact Rotman lens-fed antenna array is presented in this paper. The lens-fed antenna has the construction of two layers, which is an original option of reducing the Rotman lens size. This is performed at 24 GHz approaching automotive sensing radar.

The lens has a metal layer on the top, a dielectric, a regular ground, a dielectric, and a metal layer on the bottom, in sequential order. The antennas are put on the top layer, while the layout of the lens body is positioned on the bottom layer. They are both connected electrically via slot transitions. This structure, composed of two layers, offers many advantages, because it reduces the entire size of the lens, as well as the total loss of the delay lines, as the lines can be as short and straight as possible. This two-layer Rotman lens-fed antenna array is evaluated in terms of scattering parameters and beam patterns. [12]

Cylindrical arrays with electronic beam scanning

In order to provide a continuously 360 degrees scan by the directional pattern of a cylindrical array using electronic means, there are several methods proposed. It is important that the circular aperture distribution related to the far-field directional pattern is subjected to rotation comparative to the fixed array. With the intention of synthesizing appropriate forms of directional pattern, there are various techniques describing the independent control of the amplitude and phase of the aperture distributio

 

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