METHOD AND BEAM STEPPING APPARATUS FOR A SATELLITE CELLULAR COMMUNICATION SYSTEM
FIELD OF THE INVENTION
This invention relates in general to the field of cellular communication system, in particular to satellite communications and more particularly to phased array antennas for cellular communications.
BACKGROUND OF THE INVENTION
One problem which cellular communications systems address is the handing-off of communications between cells. Relative movement between subscriber units and cells causes the subscriber units and the communication links directed thereto to move between cells. In order to permit continuous communications in an ongoing call, the system must "hand-off' the communication when the end user crosses a cell boundary. If a communication is not handed off to a new cell upon leaving an old cell, the communication will eventually be lost because the strength of signals over which communications take place would diminish to a point where the system's radio equipment cannot receive the subscriber unit's transmissions, or vice versa.
A satellite cellular communication system with nongeosynchronous orbiting satellites must address the motion of the satellite antenna beam coverage areas relative to the subscriber units. The satellite beams sweep over the earth's service at such a high rate of speed that a given subscriber will move through a number of beams in the course of a particular call. In many cases, a subscriber unit may also be serviced by two or more different satellites during a call.
Typically, a conventional cellular communication system deals with the satellite motion by "handing-off" subscriber units from beam to beam or satellite to satellite when the subscriber unit detects that better service is available in a new antenna beam. Whenever one of these hand-off s occurs, the subscriber unit is assigned a new communication channel in the new beam. Each of these hand-offs requires the exchange of signaling data between the satellite and the subscriber unit. If these signaling messages are not received in a timely manner, the call may be dropped when the current beam no longer provides enough signal strength to maintain the link.
The hand-off rate increases rapidly as the number of beams is increased to provide more antenna gain, better link performance or a smaller channel reuse distance. In some satellite systems, there are many antenna beams per satellite. As a result, subscriber units may be hand-off from beam to beam as often as once per minute and between satellites about once in five minutes. When the number of beams is increased or more satellites are used, the beam to beam hand-off interval may be reduced to 15 seconds or less. The additional processing and bandwidth required to support this increased hand-off rate, along with the loss in call completion reliability creates an undesirable situation, especially with systems using smaller antenna beams.
A second problem with the channel to channel hand-off is that channels must be available to receive the hand-off. If no free channel exists in the requested antenna beam, the hand-off cannot be completed and the call may be dropped. As a result, a pool of available channels must be available for hand-offs.
Thus what is needed are a method and apparatus that reduces the amount of processing and bandwidth required
for handing-off communications between cells. What is also needed are a method and apparatus that reduces the hand-off rate and reduces the risk of dropped calls. What is also needed are a method and apparatus that allows a subscriber 5 unit to use the same communication channel in subsequent cells. What is also needed are a cellular communication system that does not require a pool of channels to be maintained for hand-offs.
10 BRIEF DESCRIPTION OF THE DRAWINGS
The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description and claims when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and:
FIG. 1 illustrates a highly simplified diagram of a satellite-based communication system with which the 20 present invention may be practiced;
FIG. 2 illustrates a simplified diagram of a portion of a cellular pattern formed on the surface of the Earth in accordance with a preferred embodiment of the present invention;
25 FIG. 3 illustrates a simplified block diagram of a satellite radio communication station in accordance with a preferred embodiment of the present invention;
FIG. 4 illustrates a simplified block diagram of a system control station and an earth terminal in accordance with a 30 preferred embodiment of the present invention;
FIG. 5 illustrates a simplified block diagram of a dynamic beamformer in accordance with a preferred embodiment with the present invention; 35 FIG. 6 is a simplified schematic of a portion of a phaseshift matrix in accordance with the present invention;
FIGS. 7, 8 and 9 illustrate satellite antenna beam projections at different times during a time interval in accordance with a preferred embodiment of the present invention; 40 FIG. 10 illustrates satellite antenna beam projections near the beginning of a time interval in accordance with a preferred embodiment of the present invention; and
FIG. 11 is a simplified flow chart of a beam stepping procedure suitable for use in a preferred embodiment of the 45 present invention.
The exemplification set out herein illustrates a preferred embodiment of the invention in one form thereof, and such exemplification is not intended to be construed as limiting in any manner.
DETAILED DESCRIPTION OF THE DRAWINGS
The present invention provides, among other things, a method and apparatus that reduces the amount of processing
55 and bandwidth required for handing-off communications between cells. The present invention also provides a method and apparatus that reduces the hand-off rate and reduces the risk of dropped calls. The present invention also provides a method and apparatus that allows a subscriber unit to use the
60 same communication channel in subsequent cells. The present invention also provides a cellular communication system that does not require a pool of channels to be maintained for hand-offs.
In one preferred embodiment, these advantages of the
65 present invention are accomplished through a method of stepping antenna beams in a satellite cellular communication system. Each of the beams are stepped back to compensate