Smart antennas and smart antenna technology using an adaptive antenna array are being introduced increasingly with the development of other technologies including the software defined radio, cognitive radio, MIMO and many others.
Smart antenna technology or adaptive antenna array technology enables the performance of the antenna to be altered to provide the performance that may be required to undertake performance under specific or changing conditions.
The smart antennas include signal processing capability that can perform tasks such as analysis of the direction of arrival of a signal and then the smart antenna can adapt the antenna itself using beam-forming techniques to achieve better reception, or transmission. In addition to this, the overall antenna will use some form of adaptive antenna array scheme to enable the antenna to perform is beam formation and signal direction detection.
There are two basic types of smart antennas. As shown in Fig.1, the first type is the phased array or multibeam antenna, which consists of either a number of fixed beams with one beam turned on towards the desired signal or a single beam (formed by phase adjustment only) that is steered toward the desired signal. The other type is the adaptive antenna array as shown in Fig. 2, which is an array of multiple antenna elements, with the received signals weighted and combined to maximize the desired signal to interference plus noise power ratio. This essentially puts a main beam in the direction of the desired signal and nulls in the direction of the interference.
A smart antenna is therefore a phased or adaptive array that adjusts to the environment. That is, for the adaptive array, the beam pattern changes as the desired user and the interference move; and for the phased array the beam is steered or different beams are selected as the desired user moves.
Smart antenna functions
While the main purposes of standard antennas are to effectively transmit and receive radio signals, there are two additional functions that smart antennas or adaptive antennas need to fulfil:
- Direction of arrival estimation: In order for the smart antenna to be able provide the required functionality and optimisation of the transmission and reception, they need to be able to detect the direction of arrival of the required incoming signal. The information received by the antenna array is passed to the signal processor within the antenna and this provides the required analysis.
- Beam steering: With the direction of arrival of the required and any interfering signals analysed, the control circuitry within the antenna is able to optimise the directional beam pattern of the adaptive antenna array to provide the required performance.
Smart Antenna Advantages
Smart antennas can help systems meet these requirements in the following manner: First, both phased and adaptive arrays provide increased power by providing higher gain for the desired signal. Phased arrays use narrow pencil beams, particularly with a large number of antenna elements at higher frequencies, to provide higher gain (power) in the direction of the desired signal. Adaptive arrays place a main beam in the direction of the desired signal for an M-fold power gain with M antenna elements.
In terms of interference suppression, phased arrays reduce the probability of interference with the narrower beam, and adaptive arrays adjust the beam pattern to suppress interference. For multipath mitigation, smart antennas can provide diversity, of which there are three basic types: spatial, polarization, and angle (or pattern) diversity. These appear in more detail below.
Use of Smart Antennas
.Phased arrays are mainly being studied for point-to-point wireless systems, e.g., for wireless local loops. They are also being considered for macrocellular base stations. For example, in Europe there is work on using 8-element phased arrays on GSM base stations. In Japan, there is work on using very large phased arrays on satellites, as well as on satellite terminals such as on car tops.
Adaptive arrays are being studied for indoor systems, i.e., systems with wide angular spread where the received signals arrive via widely separated paths where a phased array may not be useful in achieving gain. Also they are being studied in microcells and in some cellular base stations. For example, currently in the TDMA system ANSI-136 adaptive antenna algorithms have been widely deployed commercially in the United States. Also adaptive arrays are being considered on cellular terminals where local scattering causes wide angular spread.
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