Patent Application
20080139137
A multiple-input-multiple-output (MIMO) network comprises a base transceiver station (BTS) with multiple antennas and multiple mobile stations (MS), of which at least one has multiple antennas. Utilizing a beamforming technique can enhance the performance of a MIMO network.
In a MIMO network deploying BTS equipped with multiple antennas, the BTS computes beamforming weighting vectors for an MS using signals transmitted from the MS. The BTS sends messages to the MS via beamformed signals generated with the beamforming weighting vectors. The signals sent from the multiple antennas on the BTS are weighted based on phase and magnitude and are coherently combined at the receiving MS.
Given that there are M antennas on the BTS and N antennas on one of the MSs, there will be an M×N MIMO channel between the BTS and the MS. By applying L beamforming weighting vectors to the antennas on the BTS, an L×N MIMO channel is created between the BTS and the MS. The quality of the beamforming weighting vectors is crucial to the performance of the L×N MIMO channel.
Several methods utilizing signals transmitted from the MS antennas have been developed to compute beamforming weighting vectors for the BTS. When applied to the multiple antennas on the BTS, these beamforming weighting vectors facilitates the increasing of the signal strength.
An often-used method for computing beamforming weighting vectors is to acquire the primary eigenvector of a covariance eigenvalue problem that describes the communication channel. Using this method, signals sent from the target antenna are regarded as desired signals while those sent from other antennas are regarded as interference signals.
According to the method described above, an MS equipped with multiple antennas must transmit signals from each antenna individually. A BTS detects signals transmitted from each antenna individually and separates interference signals from desired signals.
As a result, the transmitter of the MS must switch among multiple antennas and transmit signals from one antenna at a time so that the BTS can receive signals from all MS antennas. This requirement increases the complexity of MS design and communication protocol significantly. As such what is desired is a method and system for creating MIMO channel with beamforming using signals transmitted from single transmit antenna on an MS.
The present invention discloses a method for generating a beamformed multiple-input-multiple-output (MIMO) channel. The method comprises receiving by a first wireless station a first plurality of signals transmitted from a first antenna on a second wireless station, deriving by the first wireless station a second plurality of signals corresponding to a second antenna on the second wireless station from the first plurality of signals, computing a first and second beamforming weighting vectors, using the first and second plurality of signals, creating a beamformed MIMO channel between the first and second wireless stations using the first and second beamforming weighting vectors, and allocating a predetermined transmitting power to signals beamformed by the first and second beamforming weighting vectors.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. It should be noted that the features illustrated in the drawings are not necessarily drawn to scale.
The following detailed description of the invention refers to the accompanying drawings. The description includes exemplary embodiments, not excluding other embodiments, and changes may be made to the embodiments described without departing from the spirit and scope of the invention. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims.
The present invention discloses a method for creating a multiple-input-multiple-output (MIMO) channel with beamforming in a MIMO network. The beamforming weighting vectors are computed by partially nulling out interference signals for a mobile station (MS) equipped with multiple antennas. Transmission power of each logical antenna created by applying the beamforming weighting vectors to a plurality of antennas on a base transceiver station (BTS) is determined in accordance with a predetermined power distribution method.
By applying the method disclosed in the present invention, the M×N MIMO network forms an L×N virtual MIMO channel.
The method 200 begins with step 210 where the M antennas on the first wireless station receive signals transmitted from a first antenna i on the second wireless station. A vector of signals transmitted from the antenna i on the second wireless station to the M antennas on the first wireless station is denoted as Si, where Si=(Si1, Si2, . . . , Si(M-1), SiM). The Sij represents signals transmitted from the antenna i on the second wireless station to an antenna j on the first wireless station, where j=1 . . . M.
In step 220, the first wireless station generates derivative receiving signals, denoted as Sk, using receiving signals transmitted from the antenna i on the second wireless station. The vector Sk of derivative receiving signals is considered as signals transmitted from an antenna k, where k=(1,N) and k≠i, on the second wireless station. The details of the generating of derivative receiving signals are described in
In step 230, the first wireless station calculates a beamforming weighting vector for each antenna on the second wireless station with all St, where t=(1,N). A beamforming weighting vector for an antenna t on the second wireless station, where t=(1,N) is represented by Wt=(Wt1, Wt2, . . . Wt(M-1), WtM), where Norm(Wt)=1. One having skills in the art would recognize that the Norm(.) represents a vector norm.
When the first wireless station computes a beamforming weighting vector Wt for the antenna t, signals transmitted from the antenna t on the second wireless station to the first wireless station are regarded as desired signals. By contrast, signals transmitted from one or more remaining antennas on the second wireless station to the first wireless station are regarded as interference signals.
The beamforming weighting vector Wt for the antenna t on the second wireless station is the primary eigenvector of the following matrix: (αt*Ri+σn2*I)−1Rs*Wt=λ*Wt (1), where Ri is a covariance matrix calculated from interference signals; an is the standard deviation of channel noise; Rs is a covariance matrix calculated from desired signals; I is the identity matrix; λ is the maximum eigenvalue; and πt is a scaling factor for partially nulling out interference signals, where 0<at<1.
The scaling factor αt in equation 1 defines the degree of partial nulling of interference signals. The larger πt is, the less correlated the signals in the beamformed MIMO channels are and the smaller the beamformed gain is. The scaling factor αt can be changed dynamically according to operating conditions.
In step 240, a beamformed MIMO channel is created between the first and the second wireless stations by applying the beamforming weighting vectors to the M antennas on the first wireless station. The beamforming weighting vectors are normalized to find a balanced distribution of transmitting power.
Power is distributed according to the following formulas. Let P denote the total transmitting power. The power allocated to the signals beamformed with the beamforming weighting vector Wt is Pt=AtP, where t=(1,N−1); P is the total transmitting power; and 0≦At≦=1. The power allocated to the signal beamforme with the last beamforming weighting vector is equal to
The method disclosed in the present invention creates a plurality of beamformed signals that have a certain level of de-correlation. Nulling out all interference signals de-correlates signals on the beamformed MIMO channel completely, which makes the MIMO signal detection trivial for the receiver of the wireless station. However, applying such beamforming weighting vectors could reduce the gain of signal strength, and the level of reduction is proportional to the degree of nulling of interference signals.
In step 320, a covariance matrix of derivative receiving signals Sk is computed, where k=(1,N) and k≠1. There are two ways to generate a covariance matrix of derivative receiving signals Sk. The first one is to use the last set of receiving signals, denoted as a vector Si,l, to generate a vector of derivative receiving signals Sk. The vector Sk is generated according to the following equation: Sk=αk×Si,lβk×V, where αk and βk are numbers between 0 and 1; V is a randomly generated vector; and Si,l is the vector representing the last set of receiving signals. A covariance matrix Rk of the derivative receiving signals is computed according to the following equation: Rk=(Sk)HSk, where ( . . . )H is a Hermitian operator.
The second way to generate a covariance matrix of derivative receiving signals Sk is to use all receiving signals Si,w, where w=1 . . . l, to generate a covariance matrix of derivative receiving signals. A covariance matrix Rk of derivative receiving signals k is computed according to the following equation:
where Si,w is a vector of signals transmitted from an antenna i on the second wireless station to the M antennas on the first wireless station at time instance w; ( . . . )H is a Hermitian transpose operator; and V is a randomly generated vector. Coefficients ak,w and b are predetermined numbers between 0 and 1. The coefficients change dynamically according to predetermined channel conditions.
The above illustration provides many different embodiments or embodiments for implementing different features of the invention. Specific embodiments of components and processes are described to help clarify the invention. These are, of course, merely embodiments and are not intended to limit the invention from that described in the claims.
Although the invention is illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention, as set forth in the following claims.
The present application claims the benefit of U.S. Provisional Application Ser. 60/873,721, which was filed on Dec. 9, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60873721 | Dec 2006 | US |
