A smart antenna consists of several antenna elements, whose signal are processed adaptively in order to exploit the spatial domain of the mobile radio channel . Usually the signals received at the different antenna elements are multiplied with complex weights W and then summed up the weights are chosen adaptively not the antenna itself, but the whole antenna system including the signal processing is called "adaptive".
The smart antenna technology can significantly improves wireless system performance and economics for a range of potential users. It enable operators of PC's cellular and wireless local loop networks to realize significant increase in signal quality, network capacity and coverage. In truth it is not the antenna that is smart but the antenna system are smart.
Types of Smart Antenna Systems:
Terms commonly heard today that embrace various aspects of a smart antenna system technology include intelligent antennas, phased array, SDMA, spatial processing, digital beam forming, adaptive antenna systems, and others. Smart antenna systems are customarily categorized, however, as either switched beam or adaptive array systems.The following are distinctions between the two major categories of smart antennas regarding the choices in transmit strategy:
• Switched beam- a finite number of fixed, predefined patterns or combining strategies (sectors).
• Adaptive array - an infinite number of patterns (scenario-based ) that are adjusting in real time.
Smart Antennas for TDMA:
In a convetional time division multiple access (TDMA) or frequency division multiple access (FDMA) cellular system, carrier frequencies that are used in one cell cannot be reused in the neighboring cells, because the resulting co-channel interference would be too strong. Rather, those frequencies are reused at a greater distance. The distance (related to cell radius) between two base stations which use the same carrier frequency is named reuse distance D/R. The number of cells that have to use different carrier frequencies is called cluster size N or reuse factor. Typically, a signal-to-noise-and-interferenfe ratio (SNIR) of lODb is required for each user, resulting in a cluster size of 3 or more (N.=3) for sector cells.Smart Antennas for TDMA (2): The increase in capacity can now be accomplished in different ways. One possibility is so-called spatial filtering for interference reduction (SFIR). Thereby, we can put base stations with the same carrier frequencies closer
It is assumed here that a smart antenna is only employed at the base station and not at the handset or subscriber unit. Such remote radio terminals transmit using omnidriectional antennas, leaving it to the base station to selectively separate the desired signals from interference selectively. Typically the received signal from the spatially distributed antenna elements is multiplied by a weight, a complex adjustment of an amplitude and a phase. These signals are combined to yield the array output. An adaptive algorithm controls the weights according to predefined objectives. For a switched beam system, this may be primarily maximum gain; for and adaptive array system, other factors may receive equal consideration. These dynamic calculations enable the system to change its radiation pattern for optimized signal reception.
Phase cancellation:
When waves of two multipath signals are rotated to exactly 180° out of phases, the signals will cancel each other. While this sounds severe, it is rarely sustained on any given call (and most air interface standards are quite resilient to phase cancellation). In other words, a call can be maintained for a certain period of time while there is no signal, although with very poor quality. The effect is of more concern when the control channel signal is cancelled out, resulting in a black hole, a service area in which call set-ups will occasionally fail.
Smart Antenna Systems Work:
Traditional switched beam and adaptive array systems enable a base station to customize the beams they generated for each remote user effectively by means of internal feedback control.
Generally speaking, each approach forms a main lobe toward individual users and attempts to reject interference or noise from outside of the main lobe.
Listening to the Cell (UplinkProcessing):
It is assumed that a smart antenna is only employed at the base station and not at the handset or subscriber unit. Such remote radio terminals transmit using omni driectional antennas, leaving it to the base station to selectively separate the desired signals from interference selectively. Typically the received signal from the spatially distributed antenna elements is multiplied by a weight, a complex adjustment of an amplitude and a phase. These signals are combined to yield the array output. An adaptive algorithm controls the weights according to predefined objectives. For a switched beam system, this may be primarily maximum gain; for and adaptive array system, other factors may receive equal consideration. These dynamic calculations enable the system to change its radiation pattern for optimized signal reception.
Speaking to the Users Downlink Process
The task of transmitting in a spatially selective manner is the major basis for differentiating between switched beam and adaptive array systems. The type of downlink processing used depends on whether the communication system uses time division duplex (TDD). Which transmits and receives on the same frequency (e.g., PHS and DECT) or frequency division duplex (FDD), which uses separate frequency for transmit and receiving. In most FDD systems, the uplink and downlink fading and other propagation characteristics may be considered independent, whereas in TDD systems uplink and downlink channels can be considered reciprocal. Hence, in TDD systems uplink channel information may be used to achieve spatially selective transmission. In FDD systems, the uplink channel information cannot be used directly and other types of downlink processing must be considered.
What Makes Them So Smart:A simple antenna works for a simple RF environment. Their smartness reside in their digital signal-processing facilities. In adaptive antenna systems, this fundamental signal-processing capability is augmented by advanced techniques that are applied to control operation in the presence of complicated combinations of operating conditions.
Features and Benefits:
Signal gain:
Inputs from multiple antennas are combined to optimize available power required establish given level of coverage
better range/coverage:
Focusing the energy sent out into the cell increases base station range and coverage lower power requirements also enable a greater battery life and smaller/lighter handset size.
Interference rejection:
Antenna pattern can be generated toward co-channel interference sources improving the signal to interference ratio of the received signals. Increased capacity: Precise control of signals quality and mitigation of interference combined to frequency reuse, reduce distance, improving capacity. Spatial diversity: Composite information from the array is used to minimize fading, other undesirable effects of multi path propagation.
Multipath rejection:
It can reduce the effective delay spread of the channel allowing higher bit rates to be supported without the use of an equalizer.
Power efficiency:
It combines the inputs to multiple elements to optimize available processing gain in the downlink(toward user).
Reduced expense:
Lower amplifier costs, power consumption, and higher reliability will result.
Multipath:
Multipath is a condition where the transmitted radio signal is reflected
Smart Antennas for TDMA:
In a convetional time division multiple access (TDMA) or frequency division multiple access (FDMA) cellular system, carrier frequencies that are used in one cell cannot be reused in the neighboring cells, because the resulting co-channel interference would be too strong. Rather, those frequencies are reused at a greater distance. The distance (related to cell radius) between two base stations which use the same carrier frequency is named reuse distance D/R. The number of cells that have to use different carrier frequencies is called cluster size N or reuse factor. Typically, a signal-to-noise-and-interferenfe ratio (SNIR) of lODb is required for each user, resulting in a cluster size of 3 or more (N.=3) for sector cells.
Summarizing, we have shown how smart antennas reduce fading and suppress interference. This in turn allows the increase in capacity of existing or future mobile communications networks. The used algorithms for weight determination can be divided into spatial reference, temporal reference and blind algorithms. The first use knowledge about the geometry of the antenna array, the second use a training sequence and the last employ knowledge about the structural and statistical properties of the transmitted signal. Future developments will include MIMO systems, which promise huge transmission capacities.Finally let us note that for the design and simulation of any smart antenna system, knowledge of the spatially resolved mobile radio channel is essential.
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