1. Field of the Invention
The present invention relates to communications systems and methods. More specifically, the present invention relates to systems and methods for improving the performance of cellular telephone systems.
2. Description of the Related Art
Cellular telephone systems are characterized by a number of base stations, each of which is equipped with a transceiver. The transceiver is conventionally connected to an antenna arrangement that provides a coverage area or “cell”. The conventional antenna arrangement typically includes three antennas, each of which radiate energy over a 120° arc to provide the 360° coverage required for the cell.
Smart antennas are arrays of antenna elements, each of which receive a signal to be transmitted with a predetermined phase offset and relative gain. The net effect of the array is to direct a (transmit or receive) beam in a predetermined direction. The beam is steered by controlling the phase and gain relationships of the signals that excite the elements of the array. Thus, smart antennas direct a beam to each individual mobile unit (or multiple mobile units) as opposed to radiating energy to all mobile units within a predetermined coverage area (e.g., 120°) as conventional antennas typically do. Smart antennas increase system capacity by decreasing the width of the beam directed at each mobile unit and thereby decreasing interference between mobile units. Such reductions in interference result in increases in signal-to-interference and signal-to-noise ratios that improved performance and/or capacity. In power controlled systems, directing narrow beam signals at each mobile unit also results in a reduction in the transmit power required to provide a given level of performance.
While smart antennas effectively improve the capacity of a system, such systems require a method for determining where to direct the beam. In the reverse link (i.e., the signal from the mobile unit to the base station), the angle of arrival of energy transmitted by the mobile unit may be used to calculate the direction in which the beam should be directed. Unfortunately, current techniques for calculating angle of arrival information require complex computations and furthermore are subject to measurement error due to noise and interference introduced by the channel. In addition, systems that perform angle of arrival computations works best in environments where energy is received from the mobile unit via a “line of sight”. Unfortunately, in some environments (e.g., urban environments) signals transmitted from mobile units often reflect off buildings and other structures and are therefore received by base stations as a multipath signal.
For a CDMA based system, an optimal solution (from a mobile unit capacity perspective) for determining how to direct the beams of a smart antenna is achieved by maximizing the signal-to-noise-plus-interference ratio. Typical methods, such as the “optimal Wiener solution”, are relatively complex, costly and result in potential time delays within the system.
The process 100 includes detecting the mobile unit's request for access to the system (STEP 110) and generation of a pilot signal in response to the request (STEP 120). A received signal vector is sampled (STEP 130) and used to generate an equation of the beamformer output (STEP 140). An error function is generated between the pilot signal and the beamformer output (STEP 150). Next, the error function is minimized using the Wiener-Hopf equation or the optimum Wiener solution (STEP 160). Finally, the optimized weights are applied to the beamformer (STEP 170). In accordance with this process, eigenvalues must be calculated and other operations involving linear algebra must be performed. These calculations and operations result in numerous processor operations.
Hence, a need remains in the art for an efficient method and apparatus for increasing system capacity for cellular telephone systems without the need for complex computation. In addition, there is a need for a system that is robust in environments in which multipath signals are often received by base stations from mobile units and in environments where a significant amount of noise and interference is added by the channel.
The need for an efficient method and apparatus for increasing system capacity for cellular telephone systems without the need for complex computation and that is robust in environments in which multipath signals are often received is satisfied by the teachings of the present disclosure. The inventive method and apparatus disclosed herein includes both a mobile unit and a base station. The mobile unit includes a system for generating position information and a transceiver for transmitting the position information. In the preferred embodiment of the disclosed method and apparatus, the transceiver is a preferably implemented as a CDMA (Code Division Multiple Access) transceiver. The system for generating position information preferably includes a receiver for receiving signals from Global Positioning System (GPS) satellites.
The base station receives position information from a remote unit and responds by transmitting a forward link signal in a narrow beam in the direction of the position indicated by the received position information. The direction in which the forward link signal is transmitted may also be determined by taking into account terrain data that is available to the base station. In the illustrative embodiment, the mechanism for directing the beam is a smart antenna system including an antenna array and a beamforming network.
It should be understood that throughout the present description, like reference numbers are to refer to like elements.
As discussed more fully below, in one embodiment of the disclosed method and apparatus, GPS data is received at the base station 20. Assistance data is derived from the received GPS data. The assistance data is transmitted to the mobile unit 30. The mobile unit 30 uses the assistance data to shorten the amount of time required to acquire GPS satellites. Position location data is transmitted by the array 40 to the base station 20.
GPS signals are received by the GPS antenna 120. These signals are coupled to the GPS signal processor 110. The GPS signal processor 110 generates position location data from the received GPS signals. The GPS signal processor is coupled to the system processor 100. The system processor 100 provides position data to the smart antenna processor 50.
Each receiver 52 is connected to all of the beamformers 54 and a spatial processing unit 60. Each beamformer 54 includes a set of complex multipliers 56 and a summing circuit 58. The beamformers 54 each accept the baseband signals from the receivers 52. Each complex multiplier 56 multiplies the received baseband signal by a complex weight provided by the spatial processing unit 60. The beam is formed by summing the complex-multiplied samples with an adder 58 in each beamformer 54. Each beamformer 54 performs this operation for one beam. Due to the fact that the signal from one particular mobile unit 30 may arrive at the base station 20 over several distinct paths, there are typically multiple beams per mobile unit 30. In addition, there are typically many mobile units 30.
The summed signals are supplied to the rake receiver 70. The rake receiver 70 accepts the outputs of the beamformers 54. Since there may be multiple beams associated with one mobile unit 30, the rake receiver 70 delays and combines signals received in beams that are directed at the same mobile unit 30. This delaying and combining operation is performed in an optimal fashion to ensure that energy that is transmitted from a mobile over an indirect path is combined with energy from other indirect paths as well as energy transmitted over the direct path between the mobile unit 30 and the base station 20. This delaying and combining operation takes place under the control of the spatial processing unit. Accordingly, the spatial processing unit 60 is not only responsible for determining the characteristics of the beams to be formed, but also for determining which beams are to be combined in the rake receiver. The spatial processing unit 60 implements an advantageous beamforming algorithm in accordance with the present teachings as discussed more fully below.
In many cases, a “near optimal” solution can achieve satisfactory results. Such a near optimal solution requires far less complexity, cost and and amount of processing then solutions that require eigenvalues to be calculated and that require linear algebra to be performed. One such near optimal solution is illustrated in
Next, the number and direction of the beams is calculated (STEP 605). One method for calculating the number and direction of the beams to be used relies on information supplied by a multipath database 62 (see
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Thus, the present invention has been described herein with reference to a particular embodiment for a particular application. Those having ordinary skill in the art and access to the present teachings will recognize additional modifications, applications and embodiments within the scope thereof.
It is therefore intended by the appended claims to cover any and all such applications, modifications and embodiments within the scope of the present invention. While the disclosed method and apparatus is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is defined by the claims appended to this disclosure. Those having ordinary skill in the art and access to the presently disclosed method and apparatus will recognize additional modifications, applications, and embodiments within the scope of the claimed invention. Furthermore, those skilled in the art will note that there may be additional fields in which the present invention would be of significant utility.
Accordingly,
This application claims priority to U.S. Provisional Application No. 60/249,870, filed on Nov. 16, 2000. This application is a continuation of U.S. patent application Ser. No. 09/989,875, filed on Nov. 20, 2001, which is a continuation of U.S. patent application Ser. No. 09/998,860, filed on Nov. 15, 2001
Number | Date | Country | |
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Parent | 09989875 | Nov 2001 | US |
Child | 11055920 | Feb 2005 | US |
Parent | 09998860 | Nov 2001 | US |
Child | 09989875 | Nov 2001 | US |