Smart Antenna, previously known as Adaptive Array (AAA), was initially developed for radar, sonar, and military applications. It is primarily used for spatial filtering and signal localization. Phased array radar is a basic form of an adaptive antenna array. In the field of mobile communications, researchers have given this technology a more appealing name: "smart antenna," also referred to as "intelligent antenna" in English.
The basic structure of a smart antenna involves multiple antenna elements, each followed by a weighting device—typically a complex number that adjusts both amplitude and phase. In phased array systems, only the phase is usually adjustable. These weighted signals are then combined through an adder. This configuration allows spatial domain processing. However, when the system supports both spatial and time-domain processing, the structure becomes more complex. Each antenna element is connected to a delay tap weighting network, similar to a time-domain FIR filter. The term "adaptive" or "intelligent" refers to the ability of these weights to change dynamically based on the environment.
When used as a receiving antenna, the structure remains relatively straightforward. However, for transmission, the weighting network is placed before the antenna, and no combiner is needed.
The working principle of a smart antenna relies on the phase differences between signals arriving at different antenna elements. If the incoming signal is a plane wave, these phase differences depend on the wavelength, angle of arrival, and the positions of the antennas. By adjusting the weights, the output gain can be optimized for specific directions, creating a directional pattern with a main lobe and side lobes. This allows the antenna to focus on the desired signal while suppressing interference.
To maximize the useful signal and suppress interference, the main lobe is typically aligned with the direction of the desired signal, while the nulls are directed toward interfering sources. However, real-world environments are complex, with multiple interferences and limited antenna elements. Thus, the goal is often to maximize the signal-to-noise ratio (SINR) rather than achieving perfect alignment.
Smart antennas are particularly useful in mobile communication systems, where channel conditions are harsh due to multipath fading, inter-symbol interference (ISI), and co-channel interference (CCI). They help improve signal quality and system capacity by leveraging spatial information, which is not fully utilized by traditional techniques like equalization or RAKE receivers.
One key advantage of smart antennas is their ability to enhance coverage and reduce the number of required base stations, saving costs. In later stages of system development, they enable smaller cell sizes and higher frequency reuse, allowing more users to be supported simultaneously. Smart antennas also help mitigate multipath effects, especially in CDMA systems, by separating angularly distinct paths.
Research into smart antennas focuses mainly on base station applications, as mobile devices face limitations in size and power. Downlink beamforming is more challenging because the system must estimate the downlink channel characteristics without direct feedback. While TDD systems allow for channel estimation using uplink data, FDD systems face greater difficulties due to the lack of correlation between uplink and downlink channels.
Uplink reception with smart antennas is more mature, with two main approaches: fully adaptive algorithms and pre-beam switching. Fully adaptive methods use algorithms like MMSE or LMS to adjust weights in real-time, while beam switching selects from pre-defined patterns, offering simpler but less optimal performance.
Despite challenges, smart antenna technology remains a promising area of research, especially for improving downlink performance in future mobile networks. The core of this research lies in developing efficient adaptive algorithms and optimizing antenna placement for better spatial resolution.
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