英文report怎么写 Phased Array Antennas For Mobile Earth
3.3 Mobile satellite earth station network performance
A phased array antenna with its ability to direct its main beam, can improve the performance of a mobile satellite communications network, it improves the resilience against co-channel interference and also multipath fading, thus giving a better QOS, it provides a reduced BER and outage probability. The ability to form many beams can be used to service many network users in simultaneously, giving improved spectral efficiency; the ability to adapt beam shapes to match traffic conditions can reduce the handoff rate, which increases efficiency.
An array with beam forming capability in wanted directions and nulls in others is able to mitigate the delayed signals. It can in transmitting mode focus the transmitted energy in the wanted direction, which helps to reduce multipath reflections, which then reduces delay spread. In the receiving mode, the antenna array achieves compensation in multipath fading by the use of diversity combining.
3.4 Co-channel Interference
An antenna array also has the ability to provide spatial filtering, to reduce the co-channel interferences.
In the transmit mode, it can focus the radiated wave by forming a highly directional beam in a smaller area where the receiver is expected to be. This means there is less interference from other directions, where the beam is not pointing. Co-channel interference in transmit mode can also be reduced by forming beams with nulls in the directions of other receivers. This system reduces transmitted energy in the direction of co-channel receivers and requires receiver position information. The reduction of co-channel interference in the receive mode is a major feature of phased array antenna. It does not require prior information about co-channel interferences.
Chapter 4 Phased array antenna application
4.1 Radar systems
Radar was first was first used in the 1930's at the start of the 2nd World War. At
this time the frequencies used where VHF in the 100 MHz to 200 MHz region,
The physical sizes of the antennas were tens of centi-metres. As Radar technology improved, it became possible to use microwave frequencies and several radar systems were allocated to use S band or 3 GHz band.
As a result of this higher gain antennas could be used with dimensions, similar to lower gain antennas at VHF. Higher gain antennas resulted in a narrower beam widths was attainable. Therefore, by rotating the antenna to potential targets, the angle of objects or targets could be discovered within a beam width of the antenna.
The rotation was mechanical whereby an electric motor would rotate the antenna structure. The antenna structure could also be rotated in elevation thus allowing a two-dimensional resolution of the target to be found. Signal processing would allow the range to be found.
At the present time Radar systems, are not rotated mechanically they have being replaced by phased array antenna, where the beam direction is controlled electronically.
And depending on the array configuration, if a planar or spherical array is used it enables a targets to be located in both azimuth and elevation.
The control of the beam in azimuth and elevation allows targets to be
detected and identified, the Radar system can then switch to tracking mode
to follow the movement of the target. By using multiple beam-formers, many targets can
be detected and tracked at the same time.
