03-08-2012, 01:37 PM
APPLICATION OF SMART ANTENNAS TO MOBILE COMMUNICATIONS
SYSTEMS
Smart or adaptive antenna arrays can improve the performance of wireless
communication systems. In this chapter, basic terms such as coverage and capacity are
defined. An overview of strategies for achieving coverage, capacity, and other
improvements is presented, and relevant literature is discussed. Multipath mitigation and
direction finding applications of arrays are briefly discussed, and potential paths of
evolution for future wireless systems are presented. Requirements and implementation
issues for smart antennas are also considered.
Smart antennas are most often realized with either switched-beam or fully
adaptive array antennas. An array consists of two or more antennas (the elements of the
array) spatially arranged and electrically interconnected to produce a directional radiation
pattern. In a phased array the phases of the exciting currents in each element antenna of
the array are adjusted to change the pattern of the array, typically to scan a pattern
maximum or null to a desired direction. Although the amplitudes of the currents can also
be varied, the phase adjustment is responsible for beam steering [4.1].
Strategies for Coverage and Capacity Improvement
Adaptive antennas can increase the coverage area and/or the capacity of a wireless
communication system. The coverage, or coverage area, is simply the area in which
communication between a mobile and the base station is possible. The capacity is a
measure of the number of users a system can support in a given area.
Three strategies that employ smart antennas are considered in this section. Range
extension is a means of increasing coverage, while the interference reduction/rejection
and spatial division multiple access (SDMA) approaches seek to increase the capacity of
a system.
Range extension
In sparsely populated areas, extending coverage is often more important than
increasing capacity. In such areas, the gain provided by adaptive antennas can extend the
range of a cell to cover a larger area and more users than would be possible with
omnidirectional or sector antennas. This approach is shown in Fig. 4-1.
Interference reduction and rejection
In populated areas, increasing capacity is of prime importance. Two related
strategies for increasing capacity are interference reduction on the downlink and
interference rejection on the uplink. To reduce interference, directional beams are steered
toward the mobiles. Interference to co-channel mobiles occurs only if they are within the
narrow beamwidth of the directional beam. This reduces the probability of co-channel
interference compared with a system using omnidirectional base station antennas.
Interference can be rejected using directional beams and/or by forming nulls in the base
station receive antenna pattern in the direction of interfering co-channel users.
Interference reduction and rejection can allow N
c (which is dictated by co-channel
interference) to be reduced, increasing the capacity of the system.
Spatial division multiple access
Adaptive antennas also allow a base station to communicate with two or more
mobiles on the same frequency using space division multiple access (SDMA). In spatial
division multiple access (SDMA), multiple mobiles can communicate with a single base
station on the same frequency. By using highly directional beams and/or forming nulls in
the directions of all but one of the mobiles on a frequency, the base station creates
multiple channels using the same frequency, but separated in space. This approach is
shown in Fig. 4-3.
Figure 4-3. Spatial division multiple access (SDMA) using adaptive antennas
If SDMA can be achieved, the spectral efficiency can be increased dramatically.
SYSTEMS
Smart or adaptive antenna arrays can improve the performance of wireless
communication systems. In this chapter, basic terms such as coverage and capacity are
defined. An overview of strategies for achieving coverage, capacity, and other
improvements is presented, and relevant literature is discussed. Multipath mitigation and
direction finding applications of arrays are briefly discussed, and potential paths of
evolution for future wireless systems are presented. Requirements and implementation
issues for smart antennas are also considered.
Smart antennas are most often realized with either switched-beam or fully
adaptive array antennas. An array consists of two or more antennas (the elements of the
array) spatially arranged and electrically interconnected to produce a directional radiation
pattern. In a phased array the phases of the exciting currents in each element antenna of
the array are adjusted to change the pattern of the array, typically to scan a pattern
maximum or null to a desired direction. Although the amplitudes of the currents can also
be varied, the phase adjustment is responsible for beam steering [4.1].
Strategies for Coverage and Capacity Improvement
Adaptive antennas can increase the coverage area and/or the capacity of a wireless
communication system. The coverage, or coverage area, is simply the area in which
communication between a mobile and the base station is possible. The capacity is a
measure of the number of users a system can support in a given area.
Three strategies that employ smart antennas are considered in this section. Range
extension is a means of increasing coverage, while the interference reduction/rejection
and spatial division multiple access (SDMA) approaches seek to increase the capacity of
a system.
Range extension
In sparsely populated areas, extending coverage is often more important than
increasing capacity. In such areas, the gain provided by adaptive antennas can extend the
range of a cell to cover a larger area and more users than would be possible with
omnidirectional or sector antennas. This approach is shown in Fig. 4-1.
Interference reduction and rejection
In populated areas, increasing capacity is of prime importance. Two related
strategies for increasing capacity are interference reduction on the downlink and
interference rejection on the uplink. To reduce interference, directional beams are steered
toward the mobiles. Interference to co-channel mobiles occurs only if they are within the
narrow beamwidth of the directional beam. This reduces the probability of co-channel
interference compared with a system using omnidirectional base station antennas.
Interference can be rejected using directional beams and/or by forming nulls in the base
station receive antenna pattern in the direction of interfering co-channel users.
Interference reduction and rejection can allow N
c (which is dictated by co-channel
interference) to be reduced, increasing the capacity of the system.
Spatial division multiple access
Adaptive antennas also allow a base station to communicate with two or more
mobiles on the same frequency using space division multiple access (SDMA). In spatial
division multiple access (SDMA), multiple mobiles can communicate with a single base
station on the same frequency. By using highly directional beams and/or forming nulls in
the directions of all but one of the mobiles on a frequency, the base station creates
multiple channels using the same frequency, but separated in space. This approach is
shown in Fig. 4-3.
Figure 4-3. Spatial division multiple access (SDMA) using adaptive antennas
If SDMA can be achieved, the spectral efficiency can be increased dramatically.