LU100757B1

LU100757B1 - Method for controlling the transmission of signals of a multibeam broadband satellite

Info

Publication number: LU100757B1
Application number: LU100757A
Authority: LU
Inventors: Targarona Eva Lagunas; Symeon Chatzinotas; Vahid Joroughi; Stefano Andrenacci
Original assignee: Univ Luxembourg
Priority date: 2018-04-04
Filing date: 2018-04-04
Publication date: 2019-10-07
Also published as: WO2019193085A1

Abstract

The invention provides a method of providing a beam illumination schedule for controlling the transmission of signals of a multibeam broadband satellite. The assumptions underlying the method provide a realistic setting and consider frequency reuse among beams, co-channel interference and preceding for mitigating the latter. The method allows for flexible resource allocation in terms of allocated capacity and satellite power among multiple beams, while at the same tame taking into account heterogeneous traffic demands among the beams.

Description

METHOD FOR CONTROLLING THE TRANSMISSION OF SIGNALS OF A MULTIBEAM BROADBAND SATELLITE

Technical field

The invention lies in the field of satellite communication systems, and relates in particular to a beam hopping method for a multibeam broadband satellite, wherein aggressive frequency reuse is employed among several beams.

Background of the invention

Communication satellites have evolved to provide broadband data transfer from a transmitting ground station via the satellite to a geographical area on the ground, which is defined by the area covered by the satellite’s transmission beam. Multibeam architectures have been proposed, in which information is simultaneously transmitted to a plurality of spot beams on the ground.

In known approaches, all of the satellite’s beams are used to transmit data at the same time, while adjacent beams transmit at different frequencies and/or using different polarizations in order to reduce the co-channel interference. This approach fails to cope with heterogeneous data traffic demands in each ground spot, as the satellite resources are equally distributed over the beams, so that the per-beam offered capacity is fixed and the same for all beams.

It has also been proposed to use beam hopping techniques, in which a first subset of beams, serving a first set of ground spots, is active at a given time and transmitting at a given frequency. Subsequently, the first subset of beams is not transmitting, and a second subset of beams, preferably serving a second set of ground spots, transmits at the same frequency. This approach allows to flexibly allocate scarce on-board resources over the service coverage.

However, none of the known solutions are able to provide flexibility in allocating the satellite’s resources per beam in an aggressive frequency reuse scenario among the beams, implying high cochannel interference levels.

Technical problem to be solved

It is an objective to present a method and device, which overcome at least some of the disadvantages of the prior art.

Summary of the invention

According to a first aspect of the invention, a method for determining a beam illumination schedule for a broadband satellite transmitting signals using multiple beams is provided. At least a group of beams of the multiple beams use the same transmission frequency resources. Each beam has an associated coverage area. The method comprises the steps of: a) providing, at a control unit, a capacity demand for each beam in said group of beams; b) determining, using scheduling means, a beam illumination schedule defining a sequence of K subsets of said group of beams to be used by the satellite for transmitting signals, wherein the sequence allocates an average capacity for each beam, and wherein the schedule is selected so that the difference between the allocated average capacity and the capacity demand for each beam is minimized, subject to achievable rate indications for each beam.

The achievable rate indication for a given beam in a given subset depends on the precoding overhead applied at a data transmitter for mitigating the co-channel interference induced within said beam’s coverage area by signal transmissions carried on other beams in said subset.

The control unit and scheduling means may preferably comprise data processor such as a central processing unit, CPU, operatively coupled via a data bus or a data communication channel to a memory element and a persistent data storage element. The data processor may preferably be configured by a computer software program loaded into said memory element so as to perform the steps in accordance with aspects of the invention.

Preferably, the achievable rate indication of a beam within a subset in which said beam is not used for transmitting signals, may be zero.

The method may preferably comprise the preliminary step of providing, at the control unit, for each subset of beams in the group of beams, said achievable rate indication for each beam in the subset.

Preferably, the method may further comprise the step of transmitting, using data transmission means, information describing the beam illumination schedule to said satellite, so that the satellite is able to transmit signals in accordance therewith.

Preferably, the beam illumination schedule may be repeatedly used by said broadband satellite.

Preferably, the beam illumination schedule may have a duration of twelve hours, split into K=4 four time slots of preferably the same duration.

The data transmission means may preferably comprise a networking interface device.

Said group of beams may preferably comprise all the beams of said satellite.

It may further be preferred that the method further comprises the step of feeding precoded data to the satellite using data transmission means, wherein the precoding takes into account the cochannel interference induced by the beams that are scheduled to be used for transmitting signals in accordance with said beam illumination schedule.

Preferably, the capacity demands for each beam may be approximated values of capacity requests received at the control unit for each beam’s coverage area.

The provision of the achievable rate indications for the beams in a given subset of the group of beams may further preferably comprise the following steps: instructing the satellite to transmit data that has been precoded using an initial precoding matrix, using said subset of beams at an initial available capacity; receiving, at the control unit, an indication of the respective transmission channel states from data receivers located in the coverage areas associated with said beams; adjusting the values in said precoding matrix to reflect said transmission channel states, in order to mitigate co-channel interference arising from data transmissions on said beams; for each beam in said subset, updating said initial available capacity by taking he precoding overhead induced by said adjusted precoding matrix into account.

The beam illumination schedule may preferably be stored in a memory element of said control unit.

According to another aspect of the invention, a device for determining a beam illumination schedule for a broadband satellite transmitting signals using multiple beams is provided. At least a group of beams among said multiple beams use the same transmission frequency resources, each beam having an associated coverage area. The device comprises a control unit having computing means and a memory element, wherein said memory element is configured for storing: a capacity demand for each beam in said group of beams; for each subset of beams in the group of beams, an achievable rate indication for each beam in the subset, the achievable rate indication for a given beam depending on the precoding overhead applied at a data transmitter for mitigating the co-channel interference induced within said beam’s coverage area by data transmissions on other beams in said subset.

The computing means are configured for: determining a beam illumination schedule defining a sequence of K subsets of said group of beams to be used by the satellite for transmitting signals, wherein the sequence allocates an average capacity for each beam, and wherein the schedule is selected so that the difference between the allocated average capacity and the capacity demand for each beam is minimized, subject to achievable rate indications for each beam.

The computing means may preferably comprise a data processor, such as a central processing unit, which is operatively connected by means of a data bus to said memory element. The memory element may comprise a Random Access Memory and/or a persistent memory device, such as a Hard Disk Drive or Solid State Disk.

The device may preferably further comprise data receiving means for receiving said capacity demands, and data transmission means for transmitting information describing the beam illumination schedule to said satellite.

Preferably, the control unit may further be configured to perform the method in accordance to aspects of the invention.

According to a further aspect of the invention, a computer program is provided. The computer program comprises computer readable code means, which when run on a computer, causes the computer to carry out the method according to aspects of the invention.

According to a final aspect of the invention, a computer program product is provided. The computer program product comprises a computer-readable medium on which the computer program according to the previous aspect of the invention is stored.

Embodiments of the present invention allow for flexibly allocating the data transmission capacity available among a plurality of beams of a multibeam broadband satellite, in order to address possibly heterogeneous traffic demands per beam. In accordance with embodiments of the invention, multiple beams reuse the same transmission frequency. While this results in co-channel interference among signals transmitted on different beams, the effect thereof is mitigated using precoding at the data transmitter that feeds the satellite link. The beam illumination schedule resulting from embodiments of the invention presents the advantage that it explicitly accounts for the precoding overhead. Therefore, while providing a resource allocation that is capable of addressing heterogeneous traffic demands per beam, the invention provides at the same time realistically achievable transmission rates that take into account co-channel interference. By using embodiments of the invention, the onboard resources of a broadband satellite, i.e., power and bandwidth, may be used more efficiently as compared to previously known solutions.

Brief description of the drawings

Several embodiments of the present invention are illustrated by way of figures, which do not limit the scope of the invention, wherein: figure 1 provides an illustration of a multibeam broadband satellite using four different subsets of beams during a time period of duration Γ; figure 2 provides a flow diagram showing the main method steps in accordance with a preferred embodiment of the invention; figure 3 provides a schematic illustration of the main components in a satellite communication architecture for putting into practice the method in accordance with a preferred embodiment of the invention; figure 4 provides an exemplary look-up table indicating achievable rates for subsets of beams of a multibeam broadband satellite.

Detailed description

This section describes aspects of the invention in further detail based on preferred embodiments and on the figures.

The capability to flexibly allocate scarce on-board resources, i.e., power and bandwidth, of a multibeam broadband satellite over its service coverage is becoming an important feature for broadband multibeam satellites. Beam hopping has been proposed as a promising technological enabler to provide a very high level of flexibility to manage irregular and time variant traffic requests in the satellite coverage area. However, using known solutions in certain scenarios, like the high throughput full frequency reuse scenario, the performance of beam bopping is heavily degraded by the self-interference generated by the system. This is in particular the case when neighbouring co-channel beams are activated at the same time. Such co-channel interference can be mitigated in the satellite forward link by adopting precoding techniques.

The proposed method provides a way to design an optimal beam illumination pattern so as to satisfy specific capacity needs within the satellite coverage areas in the presence of co-channel interference.

In accordance with embodiments of the invention, a broadband multibeam satellite communication system employing beam hopping and interference mitigation techniques is considered. At any given time, only a subset of the satellite beams is illuminated, i.e., activated for actual signal transmission. On the one hand, this procedure mitigates the co-channel interference while an aggressive frequency reuse scheme is employed among the beams, and on the other hand, this allows to flexibly allocate the available satellite resources. The set of illuminated beams changes in each time-slot based on a time-space transmission pattern that is periodically repeated. Figure 1 illustrates a non-limiting example in which an overall illumination period of duration T is split into K=A equal time slots, and in which at each timeslot a subset of seven beams of the multibeam satellite is illuminated. By modulating the period and duration that each of the beams is illuminated, different offered or allocated capacity values can be achieved in different beams. While any durations are possible, T may for example being equal to twelve hours.

The aim of the proposed method is to design the beam illumination pattern so as to satisfy specific capacity needs within the satellite coverage areas in the presence of co-channel interference taking into account users’ traffic demand when employing aggressive frequency reuse. In doing so, the use of the satellite’s resources is optimized and a flexible service is offered to different beams according to the corresponding inhomogeneous traffic demands. In particular, the focus of the invention is the forward satellite link (i.e., the link from gateway/data transmitter to user terminal), where precoding is applied as an enabler to mitigate co-channel interference.

Embodiments of the invention concern a broadband multibeam satellite system where each beam has an associated area of coverage and uses the same frequency. Each beam “ό” has an associated capacity demand, rd(b), measured in bits per second, which is known and given to the satellite operator. All capacity demands are grouped into a single column vector as follows:

where B denotes the total number of beams of the satellite.

The goal of the method is to design the appropriate beam illumination schedule, i.e., the set of beams to be activated and the duration of such set to be active in order to offer an average capacity as close as possible to the requested demand.

Figure 2 provides a flow diagram showing the main steps defining the method in accordance with the invention. In a first step a), said capacity demands for each beam using the same frequency on the broadband satellite are provided at a control unit that is configured for determining the sought beam illumination schedule. The determination of that beam illumination schedule, which provides a sequence of subsets of beams to be used by the satellite, is performed in a subsequent step b). Method step b) takes into account the provided capacity demands, as well as a set of achievable rate indications for each beam, wherein the achievable rate indication for a given beam in a given subset of beams depends on the precoding overhead applied at a data transmitter for mitigating the co-channel interference induced within said beam’s coverage area by signal transmissions carried on other beams in said subset.

Figure 3 provides an illustration for a first embodiment in accordance with the invention. A broadband satellite 10 in orbit around the earth has the capability of illuminating a plurality of spots 20 on the ground using multiple beams. The satellite may therefore transmit data signals to said ground spots. In this example, three beams 30, 32 and 34 use the same frequency resource, while the ground spots shown in dashed lines are supposedly served on different transmission frequencies. The signals carried in said beams 30, 32 and 34 are therefore prone to undergo cochannel interference.

Within the ground segment of the satellite communication infrastructure, a control unit 100 having scheduling means 110 is provided with a capacity demand rd (30), rd (32), rd (34) for each beam in the group of three beams. This corresponds to step a) as shown in Figure 1. The capacity demands may be exact values corresponding to actual capacity requests received from users, or they may represent average values stemming from a number of users, or approximate values. The use of approximate values allows the scheduling means to possibly reuse a previously computed and stored beam illumination schedule if the received capacity requests are similar to those used in a previous scheduling run.

The scheduling means are for example implemented using a data processor configured using a computer software program implementing the method as described here below. The scheduling means determine a beam illumination schedule which defines a sequence of K subsets S, of active beams within said group of three beams. The sequence of active beam subsets is to be used by the satellite 10 for transmitting signals. There is a total of 23 such subsets to select from for each of the K available scheduling slots, as each one of the three beams may be either active or inactive. Each subset is further defined by achievable rate indications 5,(30), 5,(32), 5,(34) for each beam, wherein an inactive beam in a given subset has an achievable rate indication equal to zero. The achievable rate indication 5, for a given active beam in a given subset 5, depends on the precoding overhead applied at a data transmitter for mitigating the co-channel interference induced within said beam’s coverage area by signal transmissions carried on other active beams in said subset 5,. The beam illumination schedule A is determined so as to minimize the difference between the average capacity allocated by the schedule and the capacity demand rd for each beam, subject to achievable rate indications 5/(30), 5,(32), 5,(34) for each beam. This corresponds to step b) as shown in Figure 1.

The so-determined beam illumination schedule A is preferably transmitted using data transmission means 120 to the satellite. The satellite is configured to apply and repeat the received beam illumination schedule for a predetermined time duration. During that time duration, the transmission means 120 feed the satellite link with precoded data for distribution over the beams as provided in the beam illumination scheduled, and wherein the precoding reflects the precoding overhead that was used to determine said beam illumination schedule. A second embodiment of the invention provides a detailed computation example, using values that may not reflect a real-life scenario, for the sake of providing a clear description of the embodiment. A satellite with three beams is considered, each beam using the same transmission frequency, and in accordance with step a) as shown in Figure 1, the capacity demands for these three beams in this example are given as:

Rrf=[4.75 6.25 5.00f (1)

This demand has to be ensured (on average) during a time period [0,T]. The time period is dived into K subsequent time slots of equal duration T/K. It should be noted that the system may further be extended to time slots of unequal durations for providing further flexibility while remaining within the scope of the invention.

For three beams, there are 23=8 possible illumination snapshots, corresponding to different subsets of active beams within the group of three beams. These subsets Si to Ss are illustrated in the lookup table provided in Figure 4. The look-up table provides the composition of each subset together with the corresponding achievable capacity or rate for each active beam within the subsets. For the sake of clarity of the used notation, vector S2 describes a subset of beams in which only the third beam is selected to be active, and provides an achievable rate of 10 data units per second. The first two beams that are not used in this subset, so that their corresponding achievable rate indications are reflected as being 0. It should be noted that a given beam achieves different transmission rates depending on whether one or more neighbouring beams are also used for transmission, or not.

Since every beam is served using the same frequency resource (aggressive frequency reuse among beams), co-channel interference takes place and linear precoding is applied at the data transmitter to compensate such effect. Therefore, the indication of the achievable rate of each snapshot illumination or subset St is strongly linked with the selected active beams and the resulting precoding design. Conventional MMSE precoding is assumed without the invention being limited thereto, as described for example in G. Gallinaro, et al., “Perspectives Of Adopting Interference Mitigation Techniques In The Context Of Broadband Multimedia Satellite Systems” in Proc. 23rd AIAA Int. Commun. Sat. Syst. Conf. (ICSSC 2005), Sep. 2005, and L. Cottatellucci et al., “Interference mitigation techniques for broadband satellite system” in Proc. 24th AIAA Int. Comm. Sat. Syst. Conf. (ICSSC 2006), Jun. 2006.

Precoding is a technique which exploits multi-antenna diversity by weighting (tuning the phases and amplitude) the transmitted signals so that the effect of interference at the receiver side is minimized. Precoding therefore needs the Channel State Information, CSI, of each destination and the signal to be transmitted. The precoding matrix represents the phases and amplitudes of such weights, see e.g., A. Wiesel, et al., “Zero-Forcing Precoding and Generalized Inverses,” IEEE Trans. Signal Processing, vol. 56, no. 9, pp. 4409-4418, Sept. 2008.

The look-up table akin to Figure 4 is assumed to be available to a control unit of the satellite operator’s ground segment infrastructure. The control unit implements the computations described here below. In a preferred embodiment, an initial version of this look-up table is obtained once the satellite is put in orbit and before the full operational phase. Therefore, a training phase is preferably considered, during which the satellite measures the achievable rate of the different snapshot illuminations. Once the satellite starts to operate normally, the look-up table is preferably periodically updated, based on feedback obtained from the data receivers, which are able to provide updated information on the state of the satellite link.

Assuming the availability of such look-up table, the question is which snapshot illumination to use in each k-th time slot, such that the offered or allocated capacity is matched with the demanded capacity on average.

Let the average offered capacity vector be defined as,

where r0(b) denotes the average offered capacity at beam In particular,

Where A denotes the set of selected snapshot illuminations or subsets of beams (with cardinality K). Therefore, the problem reduces to find the set A such that the difference between Ro and Ro are minimized, i.e.:

The problem is reformulated using: o A new optimization variable A of dimensions KxP, P being the number of possible different subsets/snapshot illuminations, which is a binary matrix such that the [A]k,P, element indicates:

o A new matrix variable grouping all the possible achievable capacity S of dimension PxB. Following the example, the matrix S is provided in Table I.

Given the new variables, the problem can be reformulated as,

Where lx denotes the column-vector of size “X” with all elements equal to 1.

Table I: An example of eight possible subsets of three beams, together with the an indication of the achievable rate for each of the three beams in each row.

The problem in (2) corresponds to a Mixed-integer convex programming (MICP). Despite its inherent NP-Hardness, it can be solved to optimality with proper convex optimization tools. Here, we solve problem (2) with the convex optimization program CVX, Μ. Grant and S. Boyd, "CVX: Matlab Software for Disciplined Convex Programming’, available at http://stanford.edu/boyd/cvx. For the example at hand, this provides the following solution for matrix A, which provides an exact match on the demanded vs allocated rate capacity per beam:

This corresponds to step b) as shown in Figure 1. The matrix Λ is interpreted as a beam pattern illumination schedule as outlined in Table II here below, which illustrates the optimal beam pattern illumination schedule A matching the demand in (1) according to the look-up table in Figure 4 for Æ=4.

TABLE II: illumination schedule - “1” indicates that the corresponding subset of beams is active during the corresponding time slot.

It should be noted that there is no unique solution in terms of A since the permutation of the order on how the illumination snapshots are executed does not affect the final average allocated capacity. In this case, the set of snapshot illuminations {2,7,8,7} {2,7,7,8), {2,8,7,7}, {8,7,2,7), etc, gives the same (optimal) result.

Therefore, the proposed method achieves the best possible illumination pattern solution and thus, provides the satellite operator with a flexible and efficient allocation of satellite payload resources over the service coverage area.

In all embodiments, the determined beam illumination schedule is preferably stored in a memory element for later use. The memory element may preferably structured as a look-up table or a database record. If the control unit receives the same or similar capacity demands given the same or similar available rate indications for each beam using the same transmitting frequency resources at a future instant in time, the scheduling means will be able to search and retrieve the previously stored beam illumination schedule from the memory element without computing it anew.

It should be noted that features described for a specific embodiment may be combined with the features of other embodiments, unless the contrary is explicitly mentioned. Based on the description and figures that have been provided, a person with ordinary skills in the art will be able to construct a computer program for implementing the described method steps without undue burden.

It should be understood that the detailed description of specific preferred embodiments is given by way of illustration only, since various changes and modifications within the scope of the invention will be apparent to the person skilled in the art. The scope of protection is defined by the following set of claims.

Claims

A method for determining a beam illumination program for a broadband satellite transmitting signals with multiple beams (20), wherein at least one beam group (30, 32, 34) utilizes the same frequency resources of transmission, each beam having an associated coverage area, the method comprising the steps of: a) providing, at a control unit (100), a capacity request r <j (30), r <i (32) ), rd (34) for each beam (30, 32, 34) in said group of beams; b) determining, using scheduling means (110), a beam illumination program A defining a sequence of K subsets Si of said group of beams for use by the satellite for transmitting signals, the sequence assigning an average capacity at each beam (30, 32, 34), and in which the program is selected so as to minimize the difference between the average capacity allocated and the capacity demand rd for each beam, subject to the achievable flow indications s , (30), Si (32), Sj (34) for each beam; wherein the achievable flow indication Si for a given beam in a given subset Si depends on the precoding overhead applied at a data transmitter to attenuate the co-channel interference induced in said coverage area by transmissions of signals carried on other beams in said subset Si.

2. Method according to claim 1, comprising the preliminary step of providing, at the level of the control unit (100), for each subset S; beams in the group of beams (30, 32, 34), said rate indication achievable Si (30), Si (32), s; (34) for each beam in the subset.

The method of any one of claims 1 or 2, wherein the method further comprises the step of transmitting, using data transmission means, information describing the beam illumination program at satellite so that the satellite can transmit signals in accordance with it.

The method of any one of claims 1 to 3, wherein said group of beams comprises all the beams of said satellite.

The method of any one of claims 1 to 4, wherein the method further comprises the step of providing precoded data to the satellite using data transmission means, the precoding taking into account the co-channel interference. induced by the beams programmed to be used to transmit signals in accordance with said beam illumination program.

The method of any one of claims 1 to 4, wherein said capacity demands for each beam are approximate values of capacity requests received at the control unit for the coverage area of each beam.

The method of any one of claims 1 to 6, wherein providing the achievable bit rate indications for the beams in a given subset Si of the beam group (30,32,34) further comprises the following steps : Charging the satellite to transmit data that has been precoded using an initial precoding matrix, using said subset Si of beams at an initial available capacity; receiving, at the control unit, an indication of the respective states of the transmission channels from the data receivers in the coverage areas associated with said beams; adjusting the values in said precoding matrix to reflect said transmission channel states to mitigate co-channel interference resulting from transmissions of data on said beams; for each beam of said subset, updating said available initial capacity by taking into account the precoding overhead induced by said adjusted precoding matrix.

The method of any one of claims 1 to 7, wherein said beam illumination program is stored in a memory element of said control unit.

9. A device for determining a beam illumination program for a broadband satellite (10) transmitting signals using multiple beams (20) of which at least one beam group (30, 32, 34) uses the same beam resources. transmission frequency, each beam having an associated coverage area, the device comprising a control unit having computing means and a memory element, wherein said memory element is configured to store: a capacity request rd (30) , rd (32), rd (34) for each beam in said group of beams; for each subset of beams S, in the bundle group, a feasible rate indication for each bundle in the subset, the achievable bit rate indication for a given bundle depending on the precoding overhead applied at a transmitter of data for attenuating co-channel interference induced within said beam coverage area by data transmissions over other beams in said subset; and wherein the calculating means is configured to: determine a beam illumination program A defining a sequence of K subsets Si of said group of beams to be used by the satellite to transmit signals, wherein the sequence assigns a average capacity for each beam (30,32, 34), and in which the program is chosen so that the difference between the average capacity allocated and the capacity demand rd for each beam is minimized, subject to the achievable flow indications Si (30), Si (32), Si (34) for each beam.

The apparatus of claim 9, further comprising data receiving means for receiving said capacity requests, and data transmission means for transmitting information describing the beam illumination program to said satellite.

Apparatus according to claim 9, wherein the control unit is further configured to perform the method of any one of claims 2 to 8.

A computer program comprising computer readable code means, which, when run on a computer, causes the computer to perform the method of any one of claims 1 to 8.

A computer program product comprising a computer readable medium on which the computer program of claim 12 is stored.