Telecommunications – Transmitter and receiver at separate stations – Plural transmitters or receivers
Reexamination Certificate
1998-04-15
2001-01-23
Hunter, Daniel S. (Department: 2749)
Telecommunications
Transmitter and receiver at separate stations
Plural transmitters or receivers
C455S060000, C342S373000
Reexamination Certificate
active
06178333
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the simulcasting of signals from a multibeam antenna system and, more particularly, to systems and methods for delaying signals simulcast from various of the multibeams to avoid destructive nulls.
BACKGROUND OF THE INVENTION
It is often desirable to provide a signal simultaneously in multiple beams of a multibeam antenna system. For example, a cellular communication system may provide communications between a base transceiver station (BTS), having an antenna system associated therewith, and a plurality of mobile units operating within a predefined area, or “cell,” defined by the antenna system's radiation pattern. Often such cells, although providing communications in a full 360° about the BTS, are broken down into three 120° sectors in order to provide more capacity and less interference over that of an omni cell 360° system. Additionally, such a sectorized cell achieves extended range as compared to an omni cell 360° system due to the greater signal gain at the sector antennas resulting from their more focused coverage.
Further advantage may be realized by providing multiple narrow beams at the BTS rather than the three 120° sectors. For example, twelve 30° narrow antenna beams may be utilized to provide the same 360° communication coverage within the cell as the 360° omni cell configuration and its 120° sectorized cell replacement. Such a multiple narrow beam arrangement is desirable because, as with the 120° sector system described above, the multiple beams provide a greater signal gain resulting from their greater focused coverage. A further advantage of the multiple narrow beams is the flexibility offered in synthesizing any desired sector size. Combining adjacent narrow beams provides a wider composite beam, with a beam width roughly equal to the sum of the individual beams widths. Accordingly, synthesized sectors may be formed of any size from a full 360°, by simulcasting a signal on each of the narrow beams, to as small as the narrow beams themselves, by providing the signal only within one narrow beam.
However, it should be appreciated that there is a potential for phase nulling associated with simulcasting of identical signals within multiple beams. For example, it may be necessary to synthesize a 120° sector pattern comprised of four 30° beams. Conceptually the 120° sector may be synthesized by simply simulcasting the desired signal (CDMA waveform or AMPS signaling channel) over the four contiguous 30° narrow beams making up the desired 120° radiation pattern. However, phase differences between the signals radiated by the four constituent beams can cause signal cancellation. This cancellation leads to undesirable shaping, i.e. “holes,” in the composite radiation pattern. For example if the antennas creating the narrow beams are separated by several wavelengths, deep nulls occur in the resulting antenna pattern, giving it a “rippled” appearance azimuthally. This pattern is not desirable for a BTS as it implies that there are “holes” in the coverage corresponding to the nulls in the pattern.
This phase nulling problem created by simulcasting is an artifact of the multibeam approach and is potentially a problem regardless of the type of signal transmitted, i.e., phase nulling is not unique to digital systems such as with respect to dynamic beam mapping in code division multiple access (CDMA) systems, but also exists with respect to signalling in analogue systems such as the advanced mobile phone system (AMPS) and narrowband advanced mobile phone system (NAMPS). Therefore, it is desirable to identify a solution that addresses both CDMA and AMPS/NAMPS. However, due to dynamic beam mapping desirable in CDMA, there may be some unique aspects associated with CDMA.
A need therefore exists in the art for systems and methods by which signals may be provided to a multibeam antenna system for simulcasting over multiple ones of the antenna beams without producing undesirable nulls.
A further need exists in the art for the systems and methods to avoid nulls in the simulcasting over multiple antenna beams of digital signals as well as analogue signals.
A still further need exists in the art for the systems and methods avoiding nulls to be adapted so as not to adversely affect forward link performance.
SUMMARY OF THE INVENTION
These and other objects, features and technical advantages are achieved by a system and method which introduces delays in ones of the simulcast signals in order to provide simulcast signals which do not destructively combine and produce an undesired composite radiation pattern. Time delay elements can smooth the composite antenna beams substantially when the antennas are separated by a significant distance. Accordingly, a preferred embodiment of the present invention uses a switch matrix adapted to selectively provide non-delayed or delayed versions of a signal to be simulcast to each antenna beam of the synthesized sector.
In order to provide the delays, the preferred embodiment includes delay elements associated with each signal of the sectors to be synthesized. Preferably, a switch selection is utilized to provide delay
on-delayed signals for each beam of the synthesized sector. Accordingly, a sector signal may be delayed by an associated delay element before being provided to a particular antenna beam of the antenna beams utilized to synthesize a sector. Therefore, a technical advantage of the present invention is that destructive combining of the wideband signals, whether digital or analogue, of multiple antenna beams used in synthesizing a desired sector radiation pattern is avoided.
The use of delays results in the addition of delay boundaries, as between the antenna beams having differing amounts of delay (or no delay) introduced into the simulcast signal. Such delay boundaries may create, for example, regions in which all demodulator fingers of a mobile unit's CDMA Rake receiver are utilized. Furthermore, the forward link performance may also be adversely affected, depending upon the relative strengths of the paths available relative to the amount of unrecovered energy acting as interference. Accordingly, it is desirable to introduce as few delay boundaries in the synthesized sector as possible while providing sufficient difference between the signals of the antenna beams in order to avoid destructive nulls. By intelligently selecting the antenna beams for which delayed versions of a signal are simulcast, the present invention operates so as not to adversely affect forward link performance.
Often ones of the multiple antenna beams of a multibeam antenna system are generated with a common phase center, i.e., a single antenna panel, such as a phased array, provides multiple antenna beams, often each associated with a different input of a beam forming matrix, from the same radiation elements. Because the same radiation elements are energized in various phase relationships to form the beams, each of these antenna beams has a common phase center. Assuming these antenna beams are otherwise calibrated to be in phase with each other, i.e., the respective signals experience substantially the same path lengths, signals simulcast over these beams having a common phase center should not destructively combine. Accordingly, the preferred embodiment of the present invention identifies antenna beams having a common phase center and does not introduce a delay as between these signals of a synthesized sector. Therefore, a technical advantage of the present invention is that introduction of unnecessary delay boundaries in the synthesized sector is avoided.
However, often multiple antenna beam sources must be utilized in order to provide the desired geographical coverage. For example, multiple ones of the above described phased array panels may be utilized to provide coverage in a full 360°. Synthesized sectors including antenna beams from multiple ones of these panels will generally not enjoy common phase centers due to the physical separation of the multiple panels. Therefore, in order to av
Elson J. Todd
Feuerstein Martin J.
Fulbright & Jaworski L.L.P.
Hunter Daniel S.
Metawave Communications Corporation
Tran Pablo
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