Abstract:
A system for implementing a transmit uplink channel in MIMO RDN architecture is provided herein. The system includes a multiple-input-multiple-output (MIMO) transmitting system that includes a MIMO baseband module having N branches; and a radio distribution network (RDN) connected to the MIMO receiving system. The RDN includes at least one beamformer, wherein each of the beamformers feeds K transmit antennas, so that a total number of transmit antennas in the system is M=N*K. At least some of the beamformers include passive splitters/combiners configured to split the signal coming from the transmitter into multiple signals which are weighted individually going to each transmit antenna. The baseband module is configured to repeatedly apply a “blind scanning” process to at least some of the transmit antennas, one at a time, so that performance of the tested transmit beam can be graded.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit from U.S. provisional patent applications: 61/658,014 filed on Jun. 11, 2012; 61/658,009 filed on Jun. 11, 2012; 61/665,592 filed on Jun. 28, 2012; 61/665,597 filed on Jun. 28, 2012; and 61/665,638 filed on Jun. 28, 2012, all of which are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of radio frequency (RF) multiple-input-multiple-output (MIMO) systems and in particular to systems and methods for enhanced performance of RF MIMO systems using RF beamforming and/or digital signal processing. 
     BACKGROUND 
     Prior to setting forth a short discussion of the related art, it may be helpful to set forth definitions of certain terms that will be used hereinafter. 
     The term “MIMO” as used herein, is defined as the use of multiple antennas at both the transmitter and receiver to improve communication performance, where more than one radio, supported by digital signal processing are used. MIMO offers significant increases in data throughput and link range without additional bandwidth or increased transmit power. It achieves this goal by spreading the transmit power over or collecting the received signal from the antennas to achieve spatial multiplexing that improves the spectral efficiency (more bits per second per Hz of bandwidth) or to achieve a diversity gain that improves the link reliability (reduced fading), or increased antenna directivity. 
     The term “beamforming” sometimes referred to as “spatial filtering” as used herein, is a signal processing technique used in antenna arrays for directional signal transmission or reception. This is achieved by combining elements in the array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. 
     The term “beamformer” as used herein refers to RF circuitry that implements transmit or receive beamforming and usually includes a splitter/combiner and may further include switches, controllable phase shifters, and in some cases controllable amplifiers and/or controllable attenuators. 
     The term “Transmit Radio Distribution Network” or “Tx RDN” or simply “RDN” as used herein is defined as a group of beamformers as set forth above. 
     The term “hybrid MIMO RDN” as used herein is defined as a MIMO system that employ two or more antennas per channel (N is the number of channels and M is the total number of antennas and M&gt;N). This architecture employs a beamformer for each channel so that the radio circuit is connected to a splitter going to two or more antennas where phase and amplitude of each antenna signal could be adjusted individually. 
     In hybrid MIMO RDN transmitting systems, when the phases of the transmitted signals from each antenna are properly adjusted with respect to one another, the individual signals may be combined and result in an improved SINR for the receiving system. 
       FIG. 1  is a block diagram illustrating a standard 3G-4G cellular communication system  10  having a base station (BTS)  20  equipped with P antennas,  22 - 1  to  22 -P. Additionally, a user equipment (UE)  11  in a MIMO configuration is shown. UE  11  includes an N channels MIMO baseband module  12  connected to N receivers  14 - 1  to  14 -N and N transmitters  16 - 1  to  16 -N in a duplex configuration which couples receivers and transmitters of the same channel to a corresponding antenna  18 - 1  to  18 -N. 
     In operation, and in accordance with known 3G-4G standards, UE  11  transmits via antennas  18 - 1  to  18 -N to base station  20  over an uplink channel (UL) both data and pilot signals. The signal properties of the pilot signals are known to base station  20  and once received by base station antennas  22 - 1  to  22 -P a channel estimation operation is applied. Consequently, the phase of each and every transmitting antenna  18 - 1  to  18 -N can be derived for the MIMO demodulation. This channel estimation information is then used to derive the best pre-coding weight and feed back to UE  11  in the form of a codebook over the downlink channel (DL). The code book includes phases that are to be applied to the respective transmit antenna so that the multiple signal streams can be de-correlated, and separated cleanly. In addition, the SINR of each signal stream at the base station receiver has to be maintained at certain levels to guarantee the quality of service. In 3G (CDMA/UMTS/HSPA) network, base station feeds back power control bit to maintain the SINR. In addition, the data rate for each data stream could change per TTI (2 ms or 10 ms). Both data rate and power control (i.e., TX power) can be used as the metrics of beamformer performance. In 4G (LTE), Modulation and Coding Scheme (MCS) is fed back to UE to change the transmit data rate for maintaining the service quality. 
     While the aforementioned mechanism is straightforward to apply in standard MIMO, it would be challenging to achieve in MIMO RDN architecture where there are more transmit antennas (M) than pilot signals (N) in accordance with the 3G-4G standards. Additionally, it would be impossible to determine, when a plurality of UE transmit signals are combined how the phase shift of each of the transmit antennas contributes to the combined pilot signal associated with transmit antennas with a common beamformer. It would be therefore advantageous to find a solution for using existing feedback to UE from the BTS, in order to better adjust the phases of the UE transmit antennas in MIMO RDN configuration. 
     SUMMARY 
     According to one aspect of the present invention, a system is provided for implementing RDN architecture for transmitting the uplink channel in MIMO. The system includes a multiple-input-multiple-output (MIMO) transmitting system comprising a MIMO baseband module having N branches; and a radio distribution network (RDN) connected to the MIMO transmitting system, the RDN comprising at least one beamformer, wherein each of the beamformers feeds or provides input to K transmit antennas, so that a total number of transmit antennas in the system is M=N*K, wherein each of the beamformers includes a passive splitter configured to split the signal coming from the transmitter into multiple signals which are weighted individually going or being supplied to each transmit antenna, wherein the baseband module is configured to perform blind scan process which uses MCS as a quality indicator. The blind process may be performed by manipulating antennas; initially, when none of the beamformers are tuned, a single antenna is chosen for each un-tuned beamformer; once all are tuned, iterative tuning is performed in a round robin sequence (or some other sequence); during such iterations, while one beamformer undergoes tuning the others freeze the weights of their set of antennas. 
     According to some embodiments of the present invention, a Mobility Monitoring Indicator (MMI) can be used to help determining the number of active antennas for a beamformer. MMI can be used in the system to help to optimize the beamformers. 
     These additional, and/or other aspects and/or advantages of the present invention are set forth in the detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and in order to show how it may be implemented, references are made, purely by way of example, to the accompanying drawings in which like numerals designate corresponding elements or sections. In the accompanying drawings: 
         FIG. 1  is a high level block diagram illustrating a system according to the prior art; 
         FIG. 2  is a high level block diagrams illustrating several aspects of a system according to embodiments of the present invention; 
         FIG. 3  is a high level flowchart illustrating aspects of the method according to some embodiments of the present invention; 
         FIG. 4  is a more detailed flowchart illustrating one aspect according to some embodiments of the present invention; and 
         FIG. 5  is a more detailed flowchart illustrating another aspect according to some embodiments of the present invention. 
     
    
    
     The drawings together with the following detailed description make the embodiments of the invention apparent to those skilled in the art. 
     DETAILED DESCRIPTION 
     With specific reference now to the drawings in detail, it is stressed that the particulars shown are for the purpose of example and solely for discussing the preferred embodiments of the present invention, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention. The description taken with the drawings makes apparent to those skilled in the art how the several forms of the invention may be embodied in practice. 
     Before explaining the embodiments of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following descriptions or illustrated in the drawings. The invention is applicable to other embodiments and may be practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
       FIG. 2  is a high level block diagrams illustrating several aspects of a system according to embodiments of the present invention. A cellular communication system  100  includes a UE  102  having a MIMO RDN architecture and including a MIMO baseband module  110  connected to N receivers  112 - 1  to  112 -N connected to respective receive antennas  113 - 1  to  113 -N. MIMO baseband module  110  is further connected to transmitters  114 - 1  to  114 -N. Each of transmitters  114 - 1  to  114 -N is further connected to a respective beamformer  120 - 1  to  120 -N which is in turn connected to K transmit antennas  122 - 1 - 1  to  122 - 1 -K through  122 -N- 1  to  122 -N-K totaling M antennas altogether so that in this example M=N*K. 
     A base station  130  is equipped with P antennas  130 - 1  to  130 -P which communicate with both receive and transmit antennas  113 - 1  to  113 -N and  122 - 1 - 1  to  122 - 1 -K through  122 -N- 1  to  122 -N-K. As noted above, there is a challenge, due to the MIMO RDN architecture, to carry out efficient MIMO transmit scheme that is based on pilot signals that were originally designed to support a smaller number of antennas. 
     According to some embodiments of the present invention, MIMO baseband module  110  may be configured to determine or calculate the weights of the transmitting antennas for each beamformer based on quality indicators derived from error rate measured at a receiving side associated with the transmit signals such as base station  130 . 
     In accordance with some embodiments of the present invention, the aforementioned challenge may be addressed by implementing a “blind scanning” approach to each of transmit antennas  122 - 1 - 1  to  122 -N-K separately. In accordance with the “blind scanning” approach, the scanning process is carried out in order to systematically test and adjust each and every one of the transmit antennas  122 - 1 - 1  to  122 -N-K. This is achieved by transmitting the signals through one transmit antenna per beamformer at a time while all other M-K antennas in all other N- 1  beamformers are operating with all of their assigned antennas as described below. 
     Another challenge due to the aforementioned “blind scanning” process is that the scanning sequence may take relatively long time to accomplish and the fading channel rate may change during a specified training sequence. It is therefore important to determine the circumstances in which it is advisable to use a subset L of the K antennas. 
     In accordance with some embodiments of the present invention, one or more beamformers  120 - 1  to  120 -N may go or operate under an optimization process, in which antenna beam configurations of the beamformer are checked using a blind scanning process, in which the quality indicators are used to determine a desirable antenna configuration. 
     In accordance with some embodiments of the present invention, MIMO baseband module  110  may be configured to use Mobility Monitoring Indicators (MMI) to determine a subset L of K antennas in at least one of beamformers  120 - 1  to  120 -N, for participating in beamforming, wherein the L antennas are selected using a grading mechanism. 
     In accordance with some embodiments of the present invention, the RDN (which accounts to all beamformers  120 - 1  to  120 -N) is configured to apply an optimization process, using the quality indicators for at least some of the antenna configurations forming the beam, for at least some of beamformers  120 - 1  to  120 -N, sequentially. 
     In accordance with some embodiments of the present invention, one or more beamformers  120 - 1  to  120 -N may be configured to apply a blind scanning process for their tested antennas, in a sequential manner, and wherein the quality indicators are used to determine the desirable weights for the beamformer, wherein the beamformer is further configured to maintain the determined weights until a next optimization process for the beamformer. 
     In accordance with some embodiments of the present invention, MIMO baseband module  110  may be configured to repeatedly apply a “look through” process to all of the transmit antennas one at a time, so that a single transmit antenna in a specified beamformer is operative at a time, wherein antennas at other beamformers are further operative with all their antennas. 
     In accordance with some embodiments of the present invention, the aforementioned grading mechanism may include for example grading antennas using MCS; picking a best graded antenna as an anchor; pairing the anchor with each of the other antennas, one by one; registering MCS for two or more relative phases; and selecting the best paired antennas and the anchor, as the L antennas. 
     In accordance with some embodiments of the present invention, MIMO baseband module  110  may be further configured to apply a fine tuning process, to the selected L antennas. 
       FIG. 3  is a high level flowchart illustrating the training/testing algorithm  300  according to some embodiments of the present invention. The antenna testing algorithm  300  starts by setting up or configuring a revisit timer  310 . Then, a preferred number of L antennas out of the participating antennas is determined or gauged via MMI  320 . The testing sequence then goes on to the step of selecting the L specific antennas that constitutes the best subset  330 . Then, in an iterative or repetitive process, the beamformers are being tuned one by one  340  until the revisit time has expired  350 , after which the antenna testing sequence starts over with a new revisit time. 
       FIG. 4  is a flowchart  400  illustrating in further details aforementioned step  330  of the antenna selecting sub process. An initial operation includes picking beamformers one by one for antenna grading using “look through”, while maintaining the other beamformers in a normal operation  410 . Then, the picked beamformers antennas are graded one by one, using MCS measurements (e.g., via antenna&#39;s bypass switches)  420 . Then the best graded antenna is selected as an anchor  430 . The selected anchor is then paired with each of the other antennas one by one, each time measuring MCS in both zero and 180° phase difference  440 . Then, each of the pairs are graded by using a best phase difference (by comparing pair&#39;s performance per the MCS)  450 . Then, in an iterative step, the selection process repeats anchor selection, best paired antenna selection, the next best paired antenna etc., till L antennas are chosen  460 . 
       FIG. 5  is a flowchart  500  illustrating in further details the aforementioned fine tuning sub process  340 . The sub tuning starts by keeping the best graded antenna (anchor) with the same setting  510 . The fine tuning then goes on to modifying the next best antenna phase at 90° clockwise  520 . Then if the data rate of the +90° is not larger than the original data rate  530 , the antenna phase is modified to −90° (being 180° counter clockwise). Then, if the data rate of −90° is not larger than the original data rate  550 , the original antenna phase is kept  560 . Then, in any case that the phase modification contributed to the increase of the data rate, the modified phase is selected and as long as all L antennas are not all fine tuned yet  570 , the next best antenna&#39;s phase is modified by +90° and the aforementioned process is repeated till all L antennas are finely tuned. 
     Several example guidelines for determining the reduced number of transmit antennas to which the training sequence should be applied are provided below. The suggested quality indicator herein is mobility monitoring indicator (MMI). 
     For high Mobility situations i.e. Vehicular (e.g. V&gt;10 km/h), only a small number of antennas (e.g. 2) will be selected as a fixed number; as a default, it will be the ones with best radiation patterns (i.e. subset is not selected per dynamic grade of performance, but per factory based antennas grading). 
     For Medium Mobility situations (e.g. 3 km/h&lt;V&lt;10 km/h), an optimizing number of participating antennas process is used; this process is a combination of MMI and trial and error method; the specific antennas to be participating in the subset is selected via best radiation pattern (and not per dynamic performance evaluation). 
     For Low Mobility situations (e.g. V&lt;3 km/h), an optimizing number of participating antennas process is used; this process is a combination of MMI and trial and error method; the specific antennas to be participating in the subset is selected via antennas&#39; path-loss estimation. 
     As will be appreciated by one skilled in the art, aspects of the present invention may be embodied as a system, method or an apparatus. Accordingly, aspects of the present invention may take the form of an entirely hardware embodiment, an entirely software embodiment (including firmware, resident software, micro-code, etc.) or an embodiment combining software and hardware aspects that may all generally be referred to herein as a “circuit”, “module” or “system.” 
     In various embodiments, computational modules may be implemented by e.g., processors (e.g., a general purpose computer processor or central processing unit executing software), or DSPs, or other circuitry. The baseband modem may be implanted, for example, as a DSP. A beamforming matrix can be calculated and implemented for example by software running on general purpose processor. Beamformers, gain controllers, switches, combiners, and phase shifters may be implemented, for example using RF circuitries. 
     The flowchart and block diagrams herein illustrate the architecture, functionality, and operation of possible implementations of systems and methods according to various embodiments of the present invention. In this regard, each block in the flowchart or block diagrams may represent a module, segment, or portion of code, which comprises one or more executable instructions for implementing the specified logical function(s). It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved. It will also be noted that each block of the block diagrams and/or flowchart illustration, and combinations of blocks in the block diagrams and/or flowchart illustration, can be implemented by special purpose hardware-based systems that perform the specified functions or acts, or combinations of special purpose hardware and computer instructions. 
     In the above description, an embodiment is an example or implementation of the inventions. The various appearances of “one embodiment”, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
     Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
     Reference in the specification to “some embodiments”, “an embodiment”, “one embodiment” or “other embodiments” means that a particular feature, structure, or characteristic described in connection with the embodiments is included in at least some embodiments, but not necessarily all embodiments, of the inventions. 
     It is to be understood that the phraseology and terminology employed herein is not to be construed as limiting and are for descriptive purpose only. 
     The principles and uses of the teachings of the present invention may be better understood with reference to the accompanying description, figures and examples. 
     It is to be understood that the details set forth herein do not construe a limitation to an application of the invention. 
     Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
     It is to be understood that the terms “including”, “comprising”, “consisting” and grammatical variants thereof do not preclude the addition of one or more components, features, steps, or integers or groups thereof and that the terms are to be construed as specifying components, features, steps or integers. 
     If the specification or claims refer to “an additional” element, that does not preclude there being more than one of the additional element. 
     It is to be understood that where the claims or specification refer to “a” or “an” element, such reference is not be construed that there is only one of that element. 
     It is to be understood that where the specification states that a component, feature, structure, or characteristic “may”, “might”, “can” or “could” be included, that particular component, feature, structure, or characteristic is not required to be included. 
     Where applicable, although state diagrams, flow diagrams or both may be used to describe embodiments, the invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
     The term “method” may refer to manners, means, techniques and procedures for accomplishing a given task including, but not limited to, those manners, means, techniques and procedures either known to, or readily developed from known manners, means, techniques and procedures by practitioners of the art to which the invention belongs. 
     The descriptions, examples, methods and materials presented in the claims and the specification are not to be construed as limiting but rather as illustrative only. 
     Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
     The present invention may be implemented in the testing or practice with methods and materials equivalent or similar to those described herein. 
     While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.