Adaptive Antenna Arrays
In most satellite personal communication systems (SatPCS), interference and multipath fading remain a problem for reliable reception of signals. Interference sources include another user in the same coverage area, terrestrial base stations, and other satellites that are operating in the same frequency band. Multipath fading is a result of cancellation between signals transmitted from a satellite which reach a receiver via direct paths and reflected paths. Multipath can be caused by scattering surfaces such as the ground, buildings, and vehicles. Figure illustrates the signals interfering with each other, resulting in the degradation of reception performance.
The antennas used for transmit and receive in most handheld terminals are ideally isotropic and circularly polarized. They are normally designed with a single low gain antenna element so that users are not required to point the antenna at the satellite. This is especially true in the case of a LEO system, where the satellite moves across the sky in a period of 8 to 15 minutes. Therefore, having the user point the antenna at the satellite is a difficult task.
However, these single element units are particularly vulnerable to interference and multipath effects because they receive signals from all directions. One approach to alleviate this problem is to use an adaptive antenna array mounted on the handheld unit. The adaptive array, as will be discussed, will have the ability to reject interference and track the satellite automatically as it moves across the sky.
SatPCS User in the presence of interference and multipath
In a satellite personal communication (SatPCS) system, the channels contain noise which may arise from electrical disturbances (e.g. lightning) or from man made sources, such as highvoltage transmission lines, the circuit switching systems of a nearby computer, or the ignition system of a car [Cou93]. In addition, multiple copies of the Signal of Interest (SOI) might arrive at the receiver at different times and with different attenuation characteristics. These multipath elements may introduce intersymbol interference (ISI), which will eventually result in significant amplitude variation in the signal envelope and can also cause co-channel interference.
As mentioned in Chapter 3, several SatPCSs proposed the use of frequency reuse in order to maximize the system capacity. As a result, for a given coverage area, there will be several cells that use the same set of frequencies. The cells which reuse the same frequency are defined as co-channel cells. The interference between signals from these cells is defined as co-channel interference [Rap 95].
Consequently, ISI and co-channel interference are only two of the many factors that will affect system capacity, range of operation and carrier-to-noise ratio. Moreover, if the multipath elements vary in time and have different attenuation characteristics, the result will be different levels of fading experienced at the channel output. Hence, multipath fading, ISI and co-channel interference are key elements that will degrade the communication link and may cause significant link failure in a wireless mobile environment.
Numerous techniques have been used to combat interference and multipath fading; diversity and spread spectrum are just some of the widely used methods. Diversity is a commonly used technique in mobile radio systems to combat multipath signal fading. The basic principle of diversity can be explained as follows: several replicas of the same information carrying signal are received over multiple channels with comparable strengths and exhibit independent fading. As a result of this, there is a good likelihood that at least one or more of these signals will not be in a fade at any given instant of time, thus making it possible to deliver adequate signal level to the receiver by combining or switching between the channel outputs.
Without diversity techniques, the transmitter will have to deliver a much higher power level to protect the link during the short intervals when the channel experiences a deep fade. However, in a satellite personal communication system, the power available is severely limited by the electrical power source on board the satellite. The more power required, the larger and heavier the satellite. Moreover, for a given satellite, low Carrier-to-Noise ratio is usually combated by narrowing the receiver bandwidth to reduce the effect of the noise level, N. This will result in lowering the data rate, and possibly more delays or longer time for the signals to arrive at the destination. In any case, diversity is very effective in reducing transmitter power. Diversity itself has numerous variations. These include polarization diversity, selection diversity, frequency/time diversity and space diversity. Adaptive arrays fall into the category of space diversity.
Space diversity has traditionally been the most common form of diversity in terrestrial mobile radio base stations. It does not require additional frequency spectrum and is easy to implement. Space diversity is exploited by spacing antennas (at the receiver side) apart so as to obtain sufficient decorrelation between the signals from each antenna. The key for obtaining uncorrelated fading of antenna outputs is adequate spacing of the antennas. The main goal of this technique is to minimize the effects of fading by the simultaneous use of two or more antennas that are spaced a number of wavelengths apart, which is a function of the frequency of operation.
Space diversity exploits the common polarization and spatial separation of two or more receiving antennas. Their signals may be combined at intermediate frequency (IF) or audio frequency (AF) stages to produce a single output signal. This technique makes use of the fact that fading is often a very localized phenomenon. The spacing of the antennas is usually 2 or 3 wavelengths for high-frequency (HF) systems.
Spacing between array elements is an important factor in designing antenna arrays. If the elements are spaced more than half a wavelength (l/2) apart, grating lobes can appear in the antenna pattern (applies to a linear, equally spaced array) which are generally undesirable.
Similarly, for every null formed, a grating null may appear for element spacings that are greater than one half a wavelength (l/2) [Com78][Stu81][Lib95]. The problem occurs because interference nulled by the array at one angle causes additional nulls (grating nulls) to appear at other angles. If the desired signals fall in a grating null, this will result in a low SINR
Therefore, it is generally advisable to maintain a small spacing (0.5l - 0.8l) between the array elements.
Antenna Arrays for Suppression of Interference and Multipath
When several antennas are arranged in space and interconnected to produce a directional pattern, an antenna array is formed [Stu81]. An antenna array can mimic the performance of a single large antenna and can often obtain the same level of performance. Large antennas produce narrow beams and must be mechanically steered to maintain maximum signal strength, while the antenna array can electronically scan the main beam of the antenna by phasing the elements in the array instead of mechanically moving the antenna.
Adaptive antenna arrays differ from conventional arrays in the sense that they are adaptive. An adaptive array is actually a phased array antenna that is capable of adjusting the phasing of elements automatically, thus controlling its own pattern. These antennas are use extensively in radar and communication systems that are subject to interference and jamming.
They adjust their pattern automatically to the signal environment to reduce the level of interference. This is accomplished by optimizing the signal-to-interference-plus-noise ratio (SINR) at the array output. As a result, adaptive arrays are extremely effective in protecting radar and communication systems from interference and jamming. The are several advantages of using an adaptive antenna array as opposed to a conventional array. Adaptive arrays are capable of sensing the presence of interference noise sources and are able to suppress the interference while simultaneously enhancing desired signal reception without prior knowledge of the signal/interference environment. Adaptive arrays can also be designed to complement other interference suppression techniques so that the actual suppression achieved is greater than that obtained solely through conventional means, for instance, by the use of spread spectrum techniques or the use of a highly directive antenna. In general, a null in an antenna pattern can be made very deep, even in a low gain array. A directional beam can only have high gain if the antenna is large (multiple wavelengths in diameter). Essentially, interference rejection works best on nulls. An adaptive array also offers enhanced reliability compared to a conventional array. When an element in a conventional array fails, the sidelobe structure of the array pattern may be significantly degraded as the sidelobes increase. However, in the case of an adaptive array, the response of the remaining operational elements in the array can be automatically adjusted until the array sidelobes are reduced to anacceptable level. Hence, adaptive arrays fail gracefully compared to conventional arrays, and they increase reliability results.
The operation of an adaptive array can most easily be visualized by considering the response in terms of the array pattern. Interference signal suppression is obtained by appropriately steering the beam pattern nulls and reducing sidelobe levels in the direction of interfering sources, while desired signal reception is maintained by steering the main beam.
Since an adaptive array is capable of forming deep nulls over a narrow angle region, very good interference suppression can be realized. As a result, interference suppression by null steering is a principal advantage of adaptive arrays.
Adaptive nulling is currently considered to be the principal benefit of adaptive techniques employed by adaptive array systems, and automatic cancellation of sidelobe jamming provides a valuable electronic-countermeasure (ECCM) capability for radar and communication systems
Array Classification
There are numerous ways of classifying antenna arrays. One of the most common ways is by geometrical configuration. The most elementary configuration is the linear array which can have equally or unequally spaced elements. A linear array is classified as having a single row of elements along a straight line. A planar array has several (or many) rows of elements in a plane.
In this thesis, we will limit the discussion to linear receiving arrays for convenience, but because of the reciprocity principle, the results obtained apply equally well to transmitting arrays.
The radiation pattern of an array is determined by the type of individual elements used, their orientations and position in space, and the amplitude and phase of the received signal
[Stu81]. In order to simplify our discussion, we will assume that each element of the array is an isotropic point source. The resulting phase pattern is known as the array factor. The array may also be used for transmission by reciprocity, but we will only look at the case of receiving arrays.
Albany Zambrano CI. 18089080.
CAF
No hay comentarios:
Publicar un comentario