Multi-antenna transmission method and system and respective mobile device

Abstract

Multi-antenna transmission method for providing downlink radio coverage from a radio base station to a mobile terminal in a mobile communications network by means of a transmission system, comprising: - at least one multi-radiator transmitting antenna directed towards at least one mobile device, in particular a smart antenna or an array antenna, - at least one database that is associated with at least one coverage cell of the mobile communications network and contains information about the position of the mobile device; - Method means that interact with the at least one multi-radiator transmission antenna and the at least one database, wherein the method means are configured to connect a transmission direction with the position of the mobile device, wherein the transmission direction is suitable for reaching the mobile device, comprise the steps: a) Detecting the position of the mobile device; b) Storing the position in at least one database; c) Determining at least one transmission direction of at least one transmission beam from the at least one multiple-radiator transmission antenna to the mobile device, depending on the position stored in the at least one database, wherein at least one of the transmission parameters, which must be determined in order to transmit in the transmission direction in the at least one multiple-radiator transmission antenna, is linked to the stored position of the mobile device, and d) Comparing reception quality information signaled by the mobile device with the reception quality expected for the position achieved by the mobile device and stored in the database.

Description

[0001] The present invention relates to a method and a system for multiple antenna transmission, in particular used for providing cellular downlink coverage in a mobile communications network, and to a mobile terminal device thereof.

[0002] As is well known, cellular coverage of mobile telecommunications networks has developed rapidly since its introduction to the mass market in the 1980s.

[0003] Initially, these networks primarily supported voice services, but nowadays the demand for data services is increasingly important and more widespread. Technological development has thus been spurred by the constant demand for performance improvements in network transmission speed and capacity.

[0004] Standardization is now progressing at a speed that was unusual for the telecommunications sector during the last decade, but is now being incentivized by the increasing frequency range requirements of various new data applications, leading to the introduction of new technologies that enable better performance.

[0005] The increasing demand for frequency bands, however, clashes with the limited radio resources. In fact, the radio frequencies used for modern telecommunications services are limited and rigidly allocated to a few operators who cannot use frequencies for which they do not have a license. Therefore, it is generally not possible to increase frequency allocation to cope with the continuous increase in traffic.

[0006] The technical solutions that can be adopted to increase network capacity given a specific available radio frequency range can be classified into two main categories: - Techniques that allow for greater frequency reuse, typically by increasing the number of cells (which therefore become smaller and smaller, so much so that they are also known as "pico-cellular" coverings); - Techniques that employ more efficient transmission and modulation methods that allow the transmission of larger amounts of data to the available radio frequency range.

[0007] However, the number of cells and transmission efficiency are both subject to physical and economic limitations.

[0008] The limitations of pico-cellular coverage arise from interference between cells, which increases as the cells are located closer together; furthermore, the increasing number of radio base stations leads to significantly higher transmission costs due to the connections that must be provided between the radio base station and the transport network access points. Theoretically, the higher the number of cells in a cellular network, the greater the need for a complex transmission distribution infrastructure to connect the radio base stations, causing the cellular network to tend to resemble a fixed-line transmission network.

[0009] Network management costs also increase due to the number of radio base stations.

[0010] Nevertheless, economic limits depend on considerations of economic advantages and are therefore not absolute theoretical limits.

[0011] In contrast, technical limitations characterize every given technology in an absolute way.

[0012] The achievable transmission efficiency limits are determined by the fact that extreme modulations degrade the signal-to-noise ratio (SNR) until it becomes unsustainable for ensuring the accurate operation of the system.

[0013] Therefore, intensive studies are being conducted to optimize transmission technologies that focus on utilizing the largest portion of the available radio frequency range.

[0014] To provide an answer to the problems described above, MIMO techniques and techniques based on the use of "vector antennas" (also known as "smart antennas") or "multi-antenna systems" or "multi-radiator systems" have been developed over time.

[0015] In particular, MIMO techniques express a concept first proposed by Arogyaswami Paulraj and Thomas Kailath in 1994: these are transmission / receive techniques based on the use of multiple antenna transmitters and receivers. In a typical embodiment, MIMO transmission generates "N" signals on the same carrier frequency, but these signals are spatially separated. Spatial separation is achieved by radiating signals from multiple antennas positioned at different locations and by receiving the signals using a receiver system composed of multiple antennas, also spatially separated. This is based on the hypothesis that the different propagation paths between the transmitting and receiving antennas have different transmission functions due to the multiple propagation paths.

[0016] The fact that a signal generated by a transmission antenna propagates to a receiving antenna through a certain number of multiple propagation paths is generally known as "multipath".

[0017] This "multipath" propagation condition is typically found in cellular coverage in urban environments. Due to the multipath effect, the signal arriving at a receiving antenna is the sum of a certain number of signals, one for each different path the original signal can take to reach the receiving point. The different signals differ primarily in the phase at which the carrier frequency arrives at the receiving point, due to the varying lengths of their respective propagation paths.

[0018] It is therefore clear that cases can occur in which carrier frequencies balance each other in a constructive way, or cases in which carrier frequencies combine in a destructive way. Such differences in the way carrier frequencies combine depend on the exact point at which reception occurs, and the effects of this can be very different (or even opposite) at distances comparable to the carrier frequency wavelength, i.e., distances on the order of centimeters for current mobile networks.

[0019] The phenomenon described above, according to which the same signal can be strong at one point and very weak at another point very close by, is known as "multipath fading" or "fast fading".

[0020] Referring back to the application of MIMO techniques, it can be observed that different signals originating at different points are characterized by different "multipaths" that end in different "multipath fades".

[0021] A typical way to handle "multipath fading" is to equip the receiver with at least two antennas arranged at a distance of approximately one quarter of the carrier frequency wavelength, so that when the signal to be received is minimal at the receiving point where one antenna is located, it will not also be minimal at the receiving point occupied by the other antenna at the same time.

[0022] In the case of MIMO transmission, the different signals transmitted on the same carrier frequency have an effect on each other, but of course they can be distinguished in reception by the different propagation paths they take.

[0023] In MIMO transmissions, the different signals transmitted from different points appear to have different strengths (different “fadings”) at the different receiving elements of the receiver: in fact, in transmitting systems that implement MIMO techniques, the receivers, not just the transmitters, must also be multi-antenna systems.

[0024] A MIMO system therefore uses "multipath fading" to distinguish between different signals transmitted on the same carrier frequency but from different points.

[0025] In the case of MIMO transmission, a signal can be extracted from each element of the receiving antenna system at the receiver. This signal is the sum of all transmitted signals, each transformed according to a different transfer function. Therefore, to reconstruct the originally transmitted signals, it is necessary to solve a system consisting of as many equations as there are elements in the receiving antenna system.

[0026] It is clear that in a propagation environment not characterized by a satisfactory and substantial "multipath", the various transmission functions would all be very similar and almost identical to each other, making the system impossible to solve.

[0027] From a physical point of view, it can be stated that the more similar the transfer functions are to each other, the more the different signals will interfere with each other, with each being a disturbance to the others and contributing to making them undetectable.

[0028] To simplify the aforementioned system and enable the differentiation between various signals transmitted on the same carrier frequency, MIMO techniques also utilize an additional device called "precoding." Precoding takes into account that each signal is transmitted not only from a different antenna but also with controlled phase shift and amplitude. The goal of precoding is to simplify the system of equations to be solved during reception. In the most extreme and optimal case, a propagation situation is defined where the system to be solved during reception is characterized by a quasi-diagonal matrix. This matrix corresponds to a case where each receiving antenna receives only one distinct signal of significant strength, while the other signals received by each antenna are very weak.

[0029] In more sophisticated applications, MIMO techniques use an additional device according to which each individual signal is transmitted through more than one radiator and the various components (i.e. replicas of the same signal radiated by different radiators) which also have different and adjustable phases and amplitudes.

[0030] From a physical point of view, as will be better explained below, the fact is that each individual transmitted signal is radiated by different radiators according to the introduction of a directional effect into the transmissions of the individual signals.

[0031] It should be noted, however, that the “pre-coding” parameters must be continuously updated, insofar as the “multipath fading” conditions change very rapidly: in fact, it is sufficient for the receiver (which, it must be remembered, is usually a mobile device) to move only a few centimeters towards the system to vary significantly.

[0032] “Precoding” must therefore be based on highly sophisticated algorithms for updating the “precoding” parameters, and such algorithms must continuously use feedback information that communicates from the receiver to the transmitter.

[0033] Precoding efficiency is very important to determine the performance of the system; however, these algorithms can generally be very computationally intensive; for these reasons, much research is currently being conducted that focuses on optimizing this aspect of MIMO technology.

[0034] In summary, a MIMO system owes its efficiency to the simple fact that multiple information flows can be transmitted on the same carrier frequency, at the expense of generating many disturbances, which can be handled by exploiting "multipath fading" and "precoding" techniques.

[0035] It is clear that multipath fading is essential for the accurate operation of MIMO systems; thus, the technology is applied to propagation situations where multiple paths are much more uniform, without the usual "direct" component of the signal, i.e., in complex coverage situations. Generally, given the attenuation suffered by the signal due to multiple reflections, it is preferable for the transmission source to send signals at the highest possible mains current. This makes it less problematic to establish down-path MIMO transmissions, where the transmitter can be powered by the mains power lines, and then up-path transmissions, where the mains power source is limited to the capacity of the mobile station's battery, hereinafter also referred to as the mobile (radio) terminal or, more simply, terminal.

[0036] In short, it can be said that MIMO techniques represent a promising solution to the problem of more efficient use of radio frequency range, and that the algorithms used for updating the "pre-coding" parameters constitute a key factor in achieving the best performance from MIMO techniques applied to mobile communication systems.

[0037] Another well-known technique on which the present invention is based is that which uses "vector antennas", better known as "smart antennas" or "array antennas".

[0038] The so-called "vector antennas" are antennas that consist of several radiating elements (radiators), just like the antennas used for MIMO transmissions.

[0039] By appropriately supplying the different radiators with energy, it is possible to achieve directional transmission, whereby the emphasis can be adjusted electronically without the need for physical movements of the antenna system.

[0040] “Vector antennas” can also be used for reception, and by methods similar to those used for transmission (i.e., signals picked up by phase shifting through each element), it is possible to increase the signal gain of the overall system in some reception directions. However, the present invention focuses on downstream coverage and therefore on transmission systems.

[0041] In a transmission system using a "vector antenna," each individual element radiates the same signal transmitted by the other antenna elements, but with a correspondingly aligned carrier frequency phase shift. Due to the different starting points of the various transmissions, these components recombine with a phase shift that varies depending on the propagation direction. This occurs because the original phase shift group is combined with the phase shift determined by the geometry of the transmission system, which varies as a function of the transmission direction. It is therefore possible to tune the entire transmission in such a way that there are transmission directions in which the interference is maximally constructive and other transmission directions in which the interference is maximally destructive.The system, viewed as a whole, creates an adjustable directional antenna that, within certain angles, requires no mechanical movements, but only adjustments (to be carried out using software) of the initial phase shifts.

[0042] The effect of shaping a transmission lobe in a specific direction is known as "beamforming".

[0043] It must be pointed out that the directive capacity of such antenna systems is strictly related to the number of antenna elements, because antennas made up of a large number of elements are more directional (and therefore directional antennas are larger).

[0044] From what has been mentioned above, it is evident that there is an analogy between "precoding" and "beamforming": in both cases, a signal is transmitted from several different points, with a different phase being applied to each component. Despite this physical analogy, the two processes, i.e., "beamforming" and "precoding," are in practice two different methods and are considered distinct techniques because they pursue different goals.

[0045] The first method, "beamforming," is generally implemented with a large number of radiators to achieve relatively narrow transmission beams. For good directional performance, as mentioned earlier, it is necessary to use a number of elements on the order of 10, or more conveniently, several tens. Furthermore, the phase shift calculation can be performed very easily in an open loop, where the geometry of the "vector antenna" is known as a function of the propagation directions in which maximum radiation is desired.

[0046] The transmission efficiency of a "vector antenna" is not affected by the fact that an antenna made up of several elements is used for reception.

[0047] The second method, "precoding", can also be implemented with just a few radiators, even with only two antennas (this is the most common case).

[0048] In the "precoding" method, phase shift calculation is generally provided in a closed loop to optimize the separation of two or more signals at a multi-antenna receiver, which therefore must provide the transformer with continuous (or very frequent) feedback.

[0049] It is clear that "precoding" techniques require a number of calculations that increase rapidly with the number of antennas (i.e., with the number of elements that make up the antenna).

[0050] MIMO techniques have already been used in some mobile communication systems, and they will be used to a greater extent in currently standardized systems: however, due to the complexity of the design, it can be foreseen that their use will be limited to applications with a reduced number of antennas: this means that two or four transmitting elements and the same number of receiving elements will be used for the most part.

[0051] It should be mentioned that a reduced number corresponds to a reduced frequency of reuse, and therefore represents a limitation of the efficiency of the system as a whole.

[0052] Vector antenna technology, although already mature in many applications, has not yet been used in cellular coverage systems. The main reason for this is that the position of the mobile station within the cell could not be determined with the necessary accuracy, nor are any technology standards available that allow the use of sufficiently accurate information about the location of the end device. Furthermore, the radio paths involved in transmissions over cellular coverage networks are usually reflected paths, and therefore it is almost never practical to transmit in the direction of the transmitter-receiver link while transmitting towards an end device.

[0053] These and other problems of variable nature have prevented the discovery of a usable method for determining the pointing direction of antennas in mobile networks, effectively preventing the use of "vector antennas" for cellular coverage applications.

[0054] The industry is currently deeply involved in trying to combine MIMO techniques with "beamforming", which is typical for "vector antennas", but all the proposed solutions have not yet been tested as being satisfactorily efficient and easy to implement.

[0055] Indeed, as mentioned previously, if the parameters required to determine the beamforming (i.e., the phase shifts used by the components of the signal transmitted by multiple radiating elements to transmit it in a directed manner) are calculated in a closed loop using continuous feedback from the terminal device, the computational complexity is such that it is necessary to limit the size of the antennas (i.e., the number of radiating elements). This also limits the potential frequency reuse and directivity transfer.

[0056] Conversely, if one were to follow the path of determining the "beamforming" by calculating the necessary phase shift parameters in an open loop, this would lead to the problem of having to correctly choose the optimal direction of guidance in a complex propagation environment characterized by many reflections.

[0057] This latter problem can be addressed by several types of implicit feedback from the terminal device. The term "implicit feedback" refers to the fact that the terminal device does not explicitly transmit a signal indicating how it is being received, but nevertheless receives such information. It is indeed possible that there is also a directional receiver at the transmission point, capable of "listening" to the signals coming from the mobile terminal device. Therefore, in addition to acting as a receiver, the mobile terminal device must also transmit towards the base station, and the optimal propagation direction is determined by selecting the one in which the signal strength is highest.

[0058] This method was recently established at an experimental level and described, for example, in “A Proposal of DPC Beamforming for Open Loop Multiuser MIMO Transmissions” - Tomoko Matsumoto, Noriaki Miyazaki and Satoshi Konishi - KDDI R&D Laboratories, Inc. - IEEE ICC 2010 proceedings.

[0059] Although defining the procedural activities necessary for implementing the technology mentioned therein remains rather complex, it could yield significant advantages: in fact, it is possible to implement higher-order antennas and achieve very interesting levels of spectrum utilization efficiency.

[0060] However, such a technique has a structural limit: in fact, it requires that the propagation channels be symmetrical, in which the transmission direction is determined depending on the quality of reception in the same direction.

[0061] This assumption is not generally true: in fact, it is possible (and advantageous) that the coverage of next-generation networks is characterized by the use of various radio stations positioned at different points to support upstream radio transmission (UL = from the terminal device to the base station) and downstream transmission (DL = from the base station to the terminal device).

[0062] US 7,181,244 B2 describes a multi-antenna transmission system for providing downlink coverage from a radio base station to a mobile device in a cellular communications network. The transmission system includes a multi-radiating transmission antenna directed toward the mobile device and a database associated with a coverage cell of the cellular communications network, containing information about the mobile device's location.

[0063] DE 10 2009 017 426 A1 describes a method for determining the position of a mobile device and a base station in a mobile network.

[0064] It is therefore an object of the present invention to eliminate the aforementioned problems of the prior art by providing a method and a system for multiple antenna transmission, which are used in particular for providing cellular downlink coverage in a mobile communications network, which allow simultaneous execution of MIMO techniques and directional transmissions typical of a “vector antenna”, which can also be composed of many elements (and are therefore highly directional).

[0065] A further object of the present invention is to provide a method and a system for multi-antenna transmission, which is used in particular for providing cellular downlink coverage in a mobile communications network, wherein the calculation and optimization of the parameters of the “precoding matrix” (i.e., determining parameters that need to be updated regularly to ensure efficient signal transmission) are significantly simpler than the methods known in the design, even in the case of high MIMO orders.

[0066] A further object of the present invention is to provide a method and a system for multiple antenna transmission, which is used in particular for providing cellular downlink coverage in a mobile communications network, which allows a substantial reduction in the signal-to-noise ratio (SNR).

[0067] A further objective of the present invention is to provide a method and a system for multiple antenna transmission, which is used in particular for providing cellular downlink coverage in a mobile communications network, which allows the limitations of symmetrical coverage to be overcome, and thus further simplifies the process required for determining the beamforming.

[0068] These and other tasks and advantages of the invention, which will become clear from the following description, are achieved by a multiple-antenna transmission method as introduced in claim 1.

[0069] In addition, these and other tasks and advantages of the invention are achieved by a multi-antenna transmission system as introduced in claim 8.

[0070] These and other tasks and advantages of the invention are also achieved by a mobile terminal as introduced in claim 9.

[0071] Preferred embodiments and non-obvious variants of the present invention are specified in dependent claims.

[0072] It is immediately clear that what is described herein can be subject to countless variations and modifications (e.g. in form, dimensions, arrangements and parts having equivalent functions) without deviating from the scope of protection of the invention as introduced in the attached claims.

[0073] The present invention will be described in more detail with regard to a few preferred embodiments thereof, which will only be provided by means of non-limiting examples.

[0074] The method and system for multiple antenna transmission according to the present invention relates generally to mobile communications, in particular to a transmission system used to provide so-called downlink coverage (from a network station to a user's mobile station), i.e., radio coverage is necessary to establish communications from a network to mobile devices. In particular, as will be described in more detail below, the method and system for multiple antenna transmission according to the present invention makes particular use of "vector antenna" technology (which, as previously mentioned, is a special technique for generating directional antennas), which is combined with MIMO (Multiple Input Multiple Output) technology.

[0075] The method and system according to the present invention are based on the availability, which will be ensured in future networks, of information about the position of the devices integrated into the mobile communication networks, in particular the receiving devices represented by the mobile terminals. The LTE / LCS (Long Term Evolution / Localization Services) standard, for example, requires that the terminal's position be constantly communicated to the network via the system's control channels. This position can be communicated to the network, for example, by the mobile terminal via its acquisition means (GNSS tracking device, triple or multiple lateration systems, etc.), or it can be detected by the network using some method, usually in cooperation with the mobile terminal (e.g., triple or multiple lateration with various base stations).

[0076] In the future, mobile devices may be required to communicate their own position at specific intervals (for database updates, for example), or when changing cell sites, or generally whenever the mobile network considers this position as necessary. Additionally, or as an alternative, position transmission can occur via an explicit request from the base station, for example, on a radio signal channel, via a specific request message, or when signal messages or data packets are sent to the transmission system, such as the first time it comes into contact with it.

[0077] The LTE case is mentioned here as a symbolic example, in which the LTE standard will probably become the most widely distributed standard of future mobile networks; however, it can be predicted that location information, due to its great importance in relation to the development of a large number of services, will be present in all future mobile communication systems, and as this technology develops, such information will become more and more available (most likely initially only in environments with GPS or Galileo coverage or in other satellite positioning systems, but availability will increasingly increase in indoor environments as well), and it will become increasingly accurate over time.

[0078] If the position of the mobile terminal is known, a radio base station equipped with a "vector" antenna can send the transmission directly in the most suitable direction to allow the mobile terminal to best receive the transmission directed towards it in that position.

[0079] In an open environment between a station and a mobile device, the direction is that determined by the station-device connection line, whereas in complex propagation environments, the system according to the present invention requires that the optimal propagation direction is known in advance for each "location" of the mobile device.

[0080] The system of the present invention therefore advantageously comprises at least one database connected to at least one coverage cell of the mobile communication network, which includes the propagation directions to be connected to each "location" or position occupied by the receiver of the mobile communication device, which can be converted into the cell position. Nevertheless, this invention is also applicable to only one or more areas of one or more mobile communication cells, in which the implementation of the present invention makes it possible to solve technical problems that cannot be solved in any other suitable way by known techniques or in a more advantageous way than by using known techniques.

[0081] Naturally, the positions where a mobile device can be located can be defined by the accuracy ensured by its placement, which, for example, can be on the order of a few meters. However, it must be noted that not all positions necessarily need to be defined with the same accuracy; in fact, a single radiation pattern can reach quite distant locations. For example, a mobile device located within a room, even a very large room with only one window, will be reached exactly by a transmission radiation directed towards the window of that hypothetical room, regardless of the precise point where the device is located within the room.

[0082] Furthermore, as will be discussed in more detail later, the multi-antenna transmission method for downlink radio coverage (from the radio base station to the terminal device) according to the present invention in a mobile communication network by means of a transmission system such as the one described above comprises the following steps: a) Determining the position of the mobile device; b) Saving the position in the database; c) Determining at least one transmission direction of at least one transmission beam from at least one of the multiple radiator transmission antennas depending on the position stored in the database, preferably by a MIMO transmission technique.

[0083] The database is created during network installation or by activating suitable machine learning algorithms while the system is already operational. In the latter case, the database is initially empty or partially empty (i.e., it contains no data associated with propagation directions and corresponding transmission parameters), and therefore the optimal direction must be determined using very expensive methods. However, these methods only need to be activated once for each location or area that shares the same optimal direction. A slight decrease in efficiency during this phase is therefore acceptable, although the required utilization of radio sources to establish the connection is noticeably slow.

[0084] The use of the aforementioned machine learning algorithms can be based on many techniques, and they can be optimized to be faster and more efficient. In any case, the technical feasibility of such algorithms can undoubtedly be ensured by using very simple modalities, for example, by using directional transmission that scans a wide angle: in this case, the mobile device can begin to communicate the reception quality associated with each direction, and the algorithm will only need to select the best direction and correlate it with the device's position in the database.

[0085] It must be noted that the previously used principle for describing a possible machine learning algorithm has been mentioned here for the sake of feasibility, but many other improvements could certainly be made to reduce its complexity and execution time. In any case, the complexity of the algorithm briefly described above is similar to that accepted in the previously mentioned technique by Tomoko Matsumoto et al., which, however, has the disadvantage that it must be executed with every connection and not just in an initial phase for populating a database.

[0086] Algorithms like the ones mentioned above are not only suitable for initially populating the database; they can also be used regularly to update the database data. In fact, the optimal transmission direction can change over time depending on variations in the propagation environment, such as a new building.

[0087] The criteria used to update the database can also be subject to improvement and optimization. The calculation (or recalculation) of the best propagation direction associated with a given position can occur under various circumstances, such as when the data is unavailable (initially, or when the operator decides to delete the data because the coverage area has undergone significant changes in propagation characteristics), when the terminal device requests it because the signal was not received with adequate quality, or periodically when the network is idle and frequencies are used for the purpose of improving or refining the level of detail and / or accuracy of the database, or based on another criterion that can be defined by the user as desired.

[0088] By using the terminal device localization information, it is therefore possible to combine MIMO technology with "smart antennas" technology, which allows for directed transmissions: the method according to the present invention therefore comprises suitable method means that cooperate with the transmission antennas and the database to determine the transmission direction of the at least one transmission beam depending on the position, preferably by means of a transmission technology of the MIMO type, wherein the method means are therefore prepared to associate the transmission direction with the position of the mobile terminal device in such a way that the transmission direction is suitable, or even optimal, to reach the mobile terminal device receiver.

[0089] The invention will now be described in more detail with regard to a specific case, wherein the radiating antenna is composed of “M” radiating elements and “N” is the desired reuse factor.

[0090] In general, if the multi-antenna transmission system is composed of "M" radiating elements (antennas) and one wants to transmit a number "N" of signals on the same carrier frequency, each of the "N" signals will be transmitted "M" times because there are "M" available antennas, and the entire transmission will require "N" x "M" (complex) parameters, each of which is related to the phase (and amplitude) at which each individual component of the respective signal will be transmitted by each individual radiating element.

[0091] Maintaining the constant updating of these parameters makes the matrix of the “N” x “M” size (with respect to the phases of the transmitted signals) particularly challenging because “N” and “M” increase, and the investigations for optimal combinations, which are those that make it easier to find a solution to the problems to be solved during reception, can be a rather complex topic if it has to be done in real time.

[0092] As mentioned previously, the known techniques are usually applicable in the presence of low orders, i.e., when "N" and "M" are small. Therefore, the technical feasibility of such solutions is limited by the size of the matrix "N" x "M".

[0093] In the present invention, as well as in the prior art document by “Tomoko Matsumoto et al.”, which, however, is not generally applicable and exhibits a certain level of complexity, the main focus is on the physical meaning of the “N” x “M” parameters of the previously described matrix (hereinafter referred to as the “precoding” matrix), which aims to simplify the calculation of the “precoding” matrix.

[0094] In fact, the invention utilizes "vector antenna" technology, which, in its most interesting implementations, is combined with MIMO technology. According to the teaching of the present invention, the calculation and optimization of the parameters of the "pre-coding" matrix is ​​facilitated by the use of specific localization information. This facilitation consists of using a database that associates each position occupied by a mobile device, the information being necessary to configure a "vector antenna" so that it can be transmitted in the most suitable direction to reach the mobile devices in question.

[0095] In this way, when the radio base station is shown that a particular mobile device is in a position already stored in the database or in a position considered satisfactorily close to it, the procedural means will use the direction of reference associated with the point of transmission towards the mobile device, without having to perform any closed-loop parameter calculations, and therefore drastically reduce the time required to establish the transmission if the parameters for a given direction of reference are known.As an alternative or in addition to the information about the best transmission direction, one or more parameters necessary for configuring the optimal indication for the multi-antenna system can be stored directly in the database, thus further reducing the time that elapses between receiving the signal from the terminal and transmitting the subsequent response signal, because this eliminates the need to perform such calculations or significantly reduces their complexity.

[0096] It should be noted that the “M” parameters, which determine the transmission phase of the components of each individual signal at the “M” antenna elements, can be expressed as “M-1” values ​​of the relative phase shift between the varying components, and therefore the first “M-1” parameters determine (for the reasons explained above) which are the direction of maximum radiation, while another Mter parameter determines the phase of the carrier frequency of the signal in the direction of maximum radiation.

[0097] According to such a model, it can be concluded that the principal value of an electric field associated with a given signal with a relatively large area (determined by the radiation amplitude of the beam) is a function of the “M-1” parameters, where the phase of the carrier frequency in the direction of maximum radiation is a function of a single parameter, and therefore only one parameter influences the “fast fading”, i.e., the field dishomogeneity at short distances comparable to the wavelength of the carrier frequency.

[0098] As long as it is not necessary to continuously update the parameters that determine the direction of maximum radiation, it is nevertheless necessary to continuously optimize the parameters that determine the phase of the carrier frequency in the direction of maximum radiation. Consequently, only the latter parameters need to be updated using known methods for determining the "precoding" parameters via closed-loop calculations. Known MIMO techniques are based on the use of multiple antenna systems at the receiver, which are used to distinguish between the usable signal and the interfering signals present in the reception area.

[0099] This modeling explains why the best selection of the first “M-1” parameter during a call is quite stable in that it does not vary with small movements of the mobile device, while only the optimization of the last parameter requires faster and more constant updates.

[0100] Additionally: the first “M-1” parameters, which are a function of the mobile device’s position with an approximation on the order of meters, can theoretically be calculated in an open loop by starting from the device’s position (with the approximation), while only the last parameter needs to be updated with regular feedback from the device as in classic transmissions that use MIMO techniques.

[0101] At this point, it is clear that, from a conceptual standpoint, it is necessary to link each transmission system to a database that, in turn, connects the receiver position with the direction of transmission to be determined. Since the direction of transmission is known, it is relatively easy to calculate the parameters to be determined in the "vector antenna" in order to transmit in that direction. From a practical standpoint, it is therefore clear that such calculations, although simple, can be avoided by directly storing the transmission parameters to be determined in the "vector antenna" in the database in order to transmit in the desired direction.

[0102] With this measure, most of the parameters of the "pre-coding" matrix ("M"-1 x "N") can simply be calculated in an open loop, or, if they have been pre-calculated and are already available in the database, they do not need to be calculated at all.

[0103] It should be pointed out that searching the database, which is obviously very large, is a simple process because the initial search key used to recapture the data consists of the position of the mobile device and is therefore, by its very nature, an ordered and orderable key.

[0104] It is obvious that MIMO techniques are generally applicable in complex propagation environments (so much so that they cannot operate in the absence of multipath propagation); it is also obvious that, in any case, such techniques are needed in environments characterized by high traffic: it is therefore realistic to propose the application of machine learning algorithms, where the optimal propagation direction is initially determined using more complex approximations (requiring longer computation times), and then each transmission side includes a mapping of itself, where each approximated position corresponds to a predetermined optimal (previously learned) propagation direction.

[0105] The transmission system and method according to the present invention are therefore preferably implemented in a cellular DL coverage infrastructure for mobile communication networks, wherein a "vector antenna" is used to determine a suitable "beamforming", together with the MIMO technique with "precoding" (in particular the MU-MIMO technique), and wherein the algorithm for determining the optimal "precoding matrix" is substantially simplified, wherein the "M-1" x "N" parameters are determined in an open loop by utilizing the localization information that can be made available by applying various localization techniques (including developmental ones), and only the "N" parameters are continuously updated by performing continuous calculations enclosed in a feedback procedure, which is typical for MIMO techniques.

[0106] It is clear that the practical implementation of the teaching of the present invention is tied to the ability to determine the position of the terminal in the downward track cell.

[0107] As a result, all techniques for localizing a mobile device represent additional prior art that is useful for the invention: it is obvious that there are high expectations for future progressive improvements of such localization techniques, from which the performance of the invention can also benefit.

[0108] As mentioned earlier, one direct approach is to assign the task of determining its own position and communicating this position to the network, particularly the database, to a mobile device appropriately equipped with data acquisition capabilities. This can be achieved through suitable communication methods and procedures, some of which have already been addressed at the standardization level. The mobile device can determine its own position based on, for example, localization by the Galileo system (if it is operational) or by another satellite navigation system, such as GPS. This can be combined with other algorithms if the device is not within line of sight of the satellite (i.e., located in "deep, inland" environments), or through specially developed algorithms based on triangulation with terrestrial networks.

[0109] Additionally, it is conceivable that the method according to the present invention activates transfer methods to move a mobile device from one radio channel to another during a transmission. Indeed, considering that the present invention allows the disclosure of all positions of all devices received by a cell and the corresponding transmission directions to be used for each mobile device, it then becomes possible to organize the transmissions in such a way that the same carrier frequency is used for transmissions in directions as far as possible, including communications with devices located in unreachable positions.For example, if a cell transmits on two carrier frequencies and receives ten mobile devices, the network will be used to organize the transmissions in such a way that the transmission directions in the two carrier frequencies are switched: it is clear that in order to support this type of optimization, one must follow the movements of the device as well as the activations and deactivations of the connections, so that handoffs become necessary not only when the mobile device moves, but also when the context of the other active connections changes.

[0110] Other modalities by which a mobile device can be located with satisfactory accuracy with a view to an advantageous application of the teachings of the present invention will certainly be developed when a solution to the localization problem is sought (which is a separate problem), e.g. by using the “fingerprint” technique (based on the development of environmental mappings in such detail, point by point, that the presence of electromagnetic fields exhibits particular spectral characteristics) or by adopting “ad-hoc” infrastructures, or by methods of collaborative localization.

[0111] Another alternative solution is to have the network estimate the mobile device's location. This solution is certainly interesting, but requires the availability of sufficient information for this purpose, and in particular, it requires further development.The main advantage brought about by the present invention is that it makes it possible to achieve optimal separation in the receiving system between the usable signal and the interfering signals (in that the latter become significantly weaker), very quickly and with a substantial simplification of the calculations to be carried out for transmission: these advantages allow the number of transmissions on carrier frequencies at the same frequency to be increased, while maintaining the same tolerable SNR, and facilitate the creation of complex systems (transmission antennas with many elements and control of many signals on the same carrier frequency); the ultimate and concrete result is that the overall performance of the system increases when the same available frequency range is used.

[0112] Another advantage is that while MIMO techniques work very well in the presence of complex “multipaths”, it is possible to eliminate this limitation by applying the teaching of the present invention because, when the mobile receiver is within line of sight of the transmitter, the simple application of sufficiently directed “beamforming” is sufficient for the receiver to distinguish and isolate the usable signal directed at it.

[0113] Naturally, the present invention also relates to a receiving device, which in particular belongs to the previously described multi-antenna transmission system for downstream radio coverage, and which is suitable for implementing the method according to the present invention.

[0114] In particular, the mobile device according to the present invention can include a means of communication that is suitable for interacting with the database to transmit feedback information about its own operation to the system, e.g., reception quality, and that the system and the method can verify that the transmitted reception quality for the position occupied by the device is as expected and stored in the database; alternatively, it is possible to command an update of the parameters stored in the database. The mobile device can also send information about its own current position, as determined by any available means, to the transmission system.

[0115] The invention therefore allows for the use of an asymmetrical coverage scheme, which is macrocellular in the downstream segment and picocellular in the upstream segment. In its preferred application, the invention exhibits its most significant advantages in downstream coverage, which can be useful in avoiding extreme reductions in the size of the downstream cells. Regarding upstream coverage, it is advantageous to increase performance by increasing the number of receiving stations.

[0116] The invention significantly contributes to the use of "vector antenna" technology, thereby eliminating and controlling the reasons that have previously hindered its use. Such technology generally offers advantages for any cellular coverage system by drastically reducing inter-cell interference caused by frequency reuse. This has a positive impact on cellular planning, with positive effects also on the overall performance of the network.

[0117] It is obvious that the main advantages of the present invention are achieved with the version that adopts the MU-MIMO techniques, whereby the different signals transmitted on the same frequency are determined for different users, who are generally arranged at different points.

[0118] It must be emphasized that the application of the teaching of the present invention, in addition to adding up the advantages obtained from the application of MIMO techniques and those obtained from the use of "vector antennas", also significantly improves the performance of MIMO reception by combining these two technologies, in that the received signal is characterized by a much better SNR than that which corresponds to the same MIMO transmission carried out with non-directional antennas.

[0119] The extent of the SNR improvement depends on how the different signals intended for various users are dispersed in different directions: it is clear that the greatest benefits are achieved when the signals are transmitted on the same carrier frequency in directions as widely as possible. This latter effect can be optimized by defining new cellular coverage typologies and new channel allocation algorithms. For example, handover procedures can be implemented to optimize channel allocation to handle the movements of mobile stations, or when connections are established or triggered by other neighboring users.

[0120] It should also be noted how the set of signals to be transmitted on the same carrier frequency can be spatially separated according to MIMO technology (i.e. transmitted from different virtual physical points, thus with different “multipath fading” of the carrier frequency at different receiving points) or (most importantly) radiated in different directions.

[0121] The MIMO receiver can therefore make use of a further aid that is useful for distinguishing between the different signals; in fact, the signal directed towards the receiver will generally be stronger than the others, which are transmitted at the same frequency but in different directions.

[0122] The use of vector antennas, such as MIMO multiple antennas, also opens up a range of possibilities for technological optimizations and innovations for the efficient physical implementation of such multiple antenna systems. In fact, the diversity of elements can be exploited to achieve the spatial separation that is useful for MIMO techniques and to control beamforming. It must be emphasized that a vector antenna, which is already a multiple antenna system, can in principle be treated like a MIMO transmitter.

[0123] It should also be noted that the MIMO receiver can theoretically be used in this MIMO transmission mode combined with the "vector antenna" technology without requiring any hardware modifications; in fact, as previously stated, the efficiency of a "vector antenna" does not impose any requirements such as the characteristics of the receiver.

[0124] The preferred embodiments of the invention described herein may, of course, be subject to further modifications and variations without departing from the inventive idea. In particular, it will be obvious to those skilled in the art that the present invention may be subject to many variations and modifications which are functionally equivalent to those described herein and which fall within the scope of protection of the invention, as set out in the attached claims.

Claims

[1] Multi-antenna transmission method for providing downlink radio coverage from a radio base station to a mobile terminal in a mobile communications network by means of a transmission system, comprising: - at least one multi-radiator transmitting antenna directed towards at least one mobile device, in particular a smart antenna or an array antenna, - at least one database that is associated with at least one coverage cell of the mobile communications network and contains information about the position of the mobile device; - Method means that interact with the at least one multi-radiator transmission antenna and the at least one database, wherein the method means are configured to connect a transmission direction with the position of the mobile device, wherein the transmission direction is suitable for reaching the mobile device, comprise the steps: a) Detecting the position of the mobile device; b) Storing the position in at least one database; c) Determining at least one transmission direction of at least one transmission beam from the at least one multiple-radiator transmission antenna to the mobile device, depending on the position stored in the at least one database, wherein at least one of the transmission parameters, which must be determined in order to transmit in the transmission direction in the at least one multiple-radiator transmission antenna, is linked to the stored position of the mobile device, and d) Comparing reception quality information signaled by the mobile device with the reception quality expected for the position achieved by the mobile device and stored in the database. [2] Transmission method according to claim 1, characterized by, that at least one transmission parameter includes the phase of the carrier frequency of the transmission beam in the transmission direction, which is determined by a MIMO-type transmission technique. [3] Transmission method according to claim 1, characterized by , that step a) includes the substep of determining the position of the mobile device by a method executed by network elements. [4] Transmission method according to claim 1, characterized by that steps b) and / or c) are performed using machine learning algorithms. [5] Transmission method according to claim 1, characterized by , that it includes the possibility of activating hand-over procedures to move transmissions towards the mobile device from one radio channel to another, such that the transmission directions are changed in different carrier frequencies. [6] Transmission method according to claim 1, characterized by , that it includes the step of updating the database if the reception quality information signaled by the mobile device does not match the information stored in the database. [7] Transmission method according to claim 1, characterized by , that step a) includes the step of receiving information about the current position of one of the mobile devices from the mobile device. [8] Multi-antenna transmission system for downlink radio coverage from a radio base station to a mobile terminal in a mobile communications network by means of a transmission system comprising means for implementing the method according to any one of claims 1 to 7. [9] Mobile terminal device suitable for receiving transmissions from a multi-antenna transmission system for downlink radio coverage from a radio base station to a mobile terminal device in a mobile communications network according to claim 8 and configured to implement the transmission method according to any one of claims 1 to 7. [10] Mobile terminal according to claim 9, characterized by , that it includes procedural means set up to interact with the database for transmission feedback information to the system about its own operation. [11] Mobile terminal according to claim 10, characterized by , that it can send information about its own current position to the transmission system if at least one of the following events occurs: a) Receiving a command from the transmission system; b) Transmission of a connection request to a radio service channel in the direction of the transmission system; c) Expiration of a current time interval; d) Detecting a mobile phone cell change.