11-07-2012, 12:36 PM
SMART ANTENNA TECHNOLOGY
SMART ANTENNA TECHNOLOGY.pdf (Size: 280.36 KB / Downloads: 66)
In mobile communication systems, capacity and performance are usually limited by two major impairments. They are multipath and co-channel interference [5]. Multipath is a condition which arises when a transmitted signal undergoes reflection from various obstacles in the propagation environment. This gives rise to multiple signals arriving from different directions. Since the multipath signals follow different paths, they have different phases when they are arrive at the receiver. The result is degradation in signal quality when they are combined at the receiver due to the phase mismatch. Co-channel interference is the interference between two signals that operate at the same frequency. In cellular communication the interference is usually caused by a signal from a different cell occupying the same frequency band.
Types of Smart Antenna Systems
There are basically two approaches [3], [4], [5], [7], [14], [15] to implement antennas that dynamically change their antenna pattern to mitigate interference and multipath affects while increasing coverage and range. They are
• Switched beam
• Adaptive Arrays
The Switched beam approach is simpler compared to the fully adaptive approach. It provides a considerable increase in network capacity when compared to traditional omnidirectional antenna systems or sector-based systems. In this approach, an antenna array generates overlapping beams that cover the surrounding area as shown in figure 4.1. When an incoming signal is detected, the base station determines the beam that is best aligned in the signal-of-interest direction and then switches to that beam to communicate with the user.
Switched Beam Systems
This type of adaptive technique actually does not steer or scan the beam in the direction of the desired signal. Switched beam employs an antenna array which radiates several overlapping fixed beams covering a designated angular area. It subdivides the sector into many narrow beams. Each beam can be treated as an individual sector serving an individual user or a group of users. Consider a traditional cellular area shown below in figure 4.3 that is divided into three sectors with 120° angular width, with each sector served by six directional narrow beams. The spatially separated directional beams leads to increase in the possible reuse of a frequency channel by reducing potential interference and also increases the range.
Butler Matrix Arrays
In this approach a Butler Matrix [5], [15] is used to provide the necessary phase shift for a linear antenna array. Abutler matrix can producebeams looking in different directions with an N-element array. A butler matrix requires an ( 90° hybrids interconnected by rows of fixed phase shifters to form the beam pattern. When a signal impinges upon the input port of the Butler Matrix, it produces a different inter-element phase shifts between the output ports. The set of different inter-element phase shifts is given by:
Blass Arrays
The Blass matrix uses directional couplers and transmission lines to provide the necessary phase shift for the arrays in order to produce multiple beams. Figure 4.7 shows an 8-element array fed by a Blass Matrix. Each node is the direction coupler to cross-connect the transmission lines. Port 0 provides equal delays to all elements and hence produces a broad side beam, whereas other ports provide progressive time delays between elements and hence produces beams at different angles. Therefore, when you send signal into the different inputs, you will get different steering angles. The Blass Matrix, is simple but has a low performance because its loss is attributed to the resistive terminations.
SMART ANTENNA TECHNOLOGY.pdf (Size: 280.36 KB / Downloads: 66)
In mobile communication systems, capacity and performance are usually limited by two major impairments. They are multipath and co-channel interference [5]. Multipath is a condition which arises when a transmitted signal undergoes reflection from various obstacles in the propagation environment. This gives rise to multiple signals arriving from different directions. Since the multipath signals follow different paths, they have different phases when they are arrive at the receiver. The result is degradation in signal quality when they are combined at the receiver due to the phase mismatch. Co-channel interference is the interference between two signals that operate at the same frequency. In cellular communication the interference is usually caused by a signal from a different cell occupying the same frequency band.
Types of Smart Antenna Systems
There are basically two approaches [3], [4], [5], [7], [14], [15] to implement antennas that dynamically change their antenna pattern to mitigate interference and multipath affects while increasing coverage and range. They are
• Switched beam
• Adaptive Arrays
The Switched beam approach is simpler compared to the fully adaptive approach. It provides a considerable increase in network capacity when compared to traditional omnidirectional antenna systems or sector-based systems. In this approach, an antenna array generates overlapping beams that cover the surrounding area as shown in figure 4.1. When an incoming signal is detected, the base station determines the beam that is best aligned in the signal-of-interest direction and then switches to that beam to communicate with the user.
Switched Beam Systems
This type of adaptive technique actually does not steer or scan the beam in the direction of the desired signal. Switched beam employs an antenna array which radiates several overlapping fixed beams covering a designated angular area. It subdivides the sector into many narrow beams. Each beam can be treated as an individual sector serving an individual user or a group of users. Consider a traditional cellular area shown below in figure 4.3 that is divided into three sectors with 120° angular width, with each sector served by six directional narrow beams. The spatially separated directional beams leads to increase in the possible reuse of a frequency channel by reducing potential interference and also increases the range.
Butler Matrix Arrays
In this approach a Butler Matrix [5], [15] is used to provide the necessary phase shift for a linear antenna array. Abutler matrix can producebeams looking in different directions with an N-element array. A butler matrix requires an ( 90° hybrids interconnected by rows of fixed phase shifters to form the beam pattern. When a signal impinges upon the input port of the Butler Matrix, it produces a different inter-element phase shifts between the output ports. The set of different inter-element phase shifts is given by:
Blass Arrays
The Blass matrix uses directional couplers and transmission lines to provide the necessary phase shift for the arrays in order to produce multiple beams. Figure 4.7 shows an 8-element array fed by a Blass Matrix. Each node is the direction coupler to cross-connect the transmission lines. Port 0 provides equal delays to all elements and hence produces a broad side beam, whereas other ports provide progressive time delays between elements and hence produces beams at different angles. Therefore, when you send signal into the different inputs, you will get different steering angles. The Blass Matrix, is simple but has a low performance because its loss is attributed to the resistive terminations.