WO2017084703A1

WO2017084703A1 - Fractional pilot reuse in multi-user mimo

Info

Publication number: WO2017084703A1
Application number: PCT/EP2015/076916
Authority: WO
Inventors: Jesus ARNAU YANEZ; Italo ATZENI
Original assignee: Huawei Technologies Co., Ltd.
Priority date: 2015-11-18
Filing date: 2015-11-18
Publication date: 2017-05-26
Also published as: CN108292979A

Abstract

A controller for assigning one or more pilot sequences to one or more user terminals of at least one base station of a communication network, the controller comprising a load and balance sub-controller, LBSC, configured to receive an operating condition information of the base station and determine, based on the received operating condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled; and a pilot assignment sub-controller, PASC, configured to receive one or more channel quality indicators, CQIs, of the user terminals to be scheduled, and assign, based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

Description

FRACTIONAL PILOT REUSE IN MULTI-USER MIMO

TECHNICAL FIELD

The present invention relates to a controller for assigning one or more pilot sequences, a base station, a system of base stations and a method for assigning pilot sequences to one or more user terminals. The present invention also relates to a computer-readable storage medium storing program code, the program code comprising instructions for carrying out such a method.

BACKGROUND

Massive multiple-input multiple-output (MIMO) refers to a type of cellular network based on multiuser MIMO in which a number of antennas at the base station (BS) is much larger than a number of served user terminals (UTs). Under certain propagation conditions, such a system renders quasi-orthogonal channels among user terminals and very simple linear processing is shown to be optimal in terms of throughput.

In this context, a problem that appears when obtaining channel state information when time- division duplex (TDD) is adopted is that the number of available pilot sequences is finite and limited by the channel behavior, as their duration cannot span larger than the coherence interval of the channel to be estimated. In consequence, user terminals in different cells might have to reuse the same pilot sequences, thus resulting in corrupted channel estimates at each BS: this is known as the pilot contamination effect and represents a major impairment affecting massive MIMO communications.

The problem of pilot contamination has been intensively investigated in the literature in the last few years. For instance, a low-rate coordination phase between different BSs; a subspace projection to improve the channel estimation accuracy; and exploited coordination, comparing the amount of training needed with respect to the uncoordinated case and its implications, have been suggested. More recently, the idea of pilot reuse was introduced. Several publications exploit the concept of reusing subsets of pilots to reduce the pilot contamination effect while limiting the length of the pilot sequences. Such reuse can take place in different forms, namely:

1. Pilot subsets can be reused across groups of user terminals with similar geographical position or with similar second-order statistics within the cell.

2. Pilots subsets can be reused across cells and, in particular:

(a) across clusters of cooperating BSs,

(b) across adjacent cells according to a reuse factor, and

(c) across cell centers.

The reuse of orthogonal pilot subsets across adjacent cells borrows from the concept of frequency reuse in cellular systems: given an integer β≥ 1 representing the number of orthogo nal pilot subsets (or, equivalently, the ratio between the number of available sequences and the number of scheduled user terminals per cell), for each cell, only a fraction l/β of the interfering cells reuse the same pilot subset.

On the other hand, the reuse of orthogonal pilot subsets across cell centers was proposed and is termed as fractional pilot reuse. This idea is borrowed from the concept of fractional frequency reuse: user terminals that are closer to the cell center reuse the same pilot subset across all cells, whereas user terminals that are closer to the cell edges are allocated pilot subsets that are orthogonal to that of the cell centers. This is achieved by dividing the pilot book into β + 1 pilot subsets, where i) the β + 1-th subset is assigned to all cell centers, and ii) each of the remaining β subsets is assigned to adjacent cell edges according to a specific pattern. In addition, frequency bands can be also reused in the cell edges so that, although the same pilot sequences are reused in the cell edges of adjacent cells, there is no pilot contamination for the cell edges.

Fractional pilot reuse allows greatly improving the spectral efficiency; however, it can present the following disadvantages:

1. It requires exhaustive-search optimization of metrics that depend on path- loss parameters.

2. The number of user terminals to be served and the pilot reuse parameters can be optimized only for a particular cell and by means of coordination with the other BSs. SUMMARY OF THE INVENTION

The objective of the present invention is to provide a controller, a base station, a system and a method for assigning one or more pilot sequences to one or more user terminals, wherein the controller, the base station, the system and the method overcome one or more of the above- mentioned problems.

A first aspect of the invention provides a controller for assigning one or more pilot sequences to one or more user terminals of at least one base station of a communication network, the controller comprising:

a load and balance sub-controller, LBSC, configured to receive an operating condition information of the base station and determine, based on the received operating condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled; and

a pilot assignment sub-controller, PASC, configured to receive one or more channel quality indicators, CQIs, of the user terminals to be scheduled, and assign, based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

This has the advantage that the controller can assign the one or more pilot sequences without having to exchange information with neighboring cells. Thus, an overall network complexity is reduced, while still ensuring good pilot reuse.

Embodiments of the first aspect combine choosing the optimal number of user terminals to be served and reusing pilot sequences across cell groups. This can be achieved for example by incorporating the controller of the first aspect at the base station.

Preferably, center user terminals are terminals for which the controller has determined that they are at the center of a cell, far from an edge of the cell, and/or unlikely to experience severe interference from neighboring cells. In particular, a center user terminal can be defined as a user terminal for which the controller has determined that it is not near a neighboring cell. Preferably, the remaining user terminals of the user terminals to be scheduled are edge user terminals. The controller can be configured to determine whether a user terminal is a center user terminal or an edge user terminal based on channel quality information of the user terminal that it receives from the base station. Preferably edge user terminals can be defined based on a signal strength of an associated base station. For example, if a signal is lower than a pre- defied threshold, the user terminal is defined as an edge user terminal. Additionally or alternatively, edge user terminals can be defined based on a position or distance. For example, if a distance to an associated base station is larger than a pre-defined threshold the user terminal is defined as an edge user terminal. Preferably, a controller according to the first aspect can interact with one or more base stations associated to it independently from other controllers. Embodiments of the controller of the first aspect only consider standardized measures available locally at each base station (as, e.g., the estimated signal strength from each user terminal). Furthermore, the controller can be configured to exploit multiple types of information that might be available.

In a first implementation of the controller according to the first aspect, the operating condition information includes a number of active antennas, a global pilot reuse factor and/or a minimum user terminal data rate of the base station. Preferably, the operating condition information includes at least the number of active antennas and the global pilot reuse factor.

The controller of the first implementation has the advantage that the controller can make use of relevant information available at the base station and appropriately determine the number of user terminals to be scheduled and the center fraction. In a second implementation of the controller according to the first aspect, the LBSC comprises a look-up table that maps the operating condition information, in particular the number of active antennas, the global pilot reuse factor and/or the minimum user terminal data rate to the center fraction. This has the advantage that the controller can with low computational effort determine the optimum center fraction and number of user terminals to be scheduled. In a third implementation of the controller according to the first aspect, a CQI comprises a geographic position indicator, a received signal strength, a modulation and coding scheme, and/or an estimate of a spatial covariance matrix. This has the advantage that the controller can use information that are available at the base station, quickly obtain an estimate of which user terminals are closer to the base station and which are further away. Thus, the controller can efficiently and accurately assign user terminals as center user terminals or edge user terminals. In a fourth implementation of the controller according to the first aspect, the PASC comprises a scoring unit which is configured to assign a score value to one or more user CQIs of a user terminal, wherein in particular assigning the score value comprises a step of determining a measure of a distance of the user terminal to the base station. This has the advantage that complex information that is available about a user terminal can be reduced to a score function that is related to distance of the user terminal to the base station. For example, a scalar score value can be assigned to a complex information structure. Thus, further processing of the information is simplified. In a fifth implementation of the controller according to the first aspect, the PASC is configured to sort the user terminals to be scheduled in an order of an increasing or a decreasing score value, and wherein in particular the PASC is configured to determine a first group of user terminals with higher score values as center user terminals and a second group of user terminals with lower score functions as edge user terminals.

This has the advantage that the controller can efficiently and accurately determine which terminals are closest to the base station and thus should be assigned as center user terminals and which user terminal should be assigned as edge user terminals. In a sixth implementation of the controller according to the first aspect, the controller is configured to set a received minimum user terminal data rate to a predetermined threshold data rate if it is smaller than the predetermined threshold data rate. This has the advantage that it is ensured that each of the user terminals reaches at least a certain minimum data rate, which may be required for basic connectivity.

In a seventh implementation of the controller according to the first aspect, the PASC is con- figured to assign the one or more pilot sequences without considering information from neighboring base stations and/or neighboring controllers. In other words, the controller can be configured to assign pilot sequences independent of neighboring base stations and/or controllers. The controller can be configured to assign pilot sequences to a plurality of base stations but do so independently of another plurality of (neighboring) base stations.

This has the advantage that signaling between neighboring controllers and/or neighboring base stations is avoided, thus reducing a complexity of the communication network.

In an eighth implementation of the controller according to the first aspect, the controller is configured to assign pilot sequences to a plurality of base stations, wherein in particular the controller is configured to sequentially assign the pilot sequences to the plurality of base stations.

This has the advantage that the same controller can be used for a plurality of base stations.

In a ninth implementation of the controller according to the first aspect, the one or more center pilot sequences are orthogonal to the one or more edge pilot sequences.

The center pilot sequences being orthogonal to the edge pilot sequences has the advantage that the center pilot sequences can be reused in all the cell centers, whereas the edge pilot sequences can be reused in all cell edges or only in non-adjacent cell edges according to a reuse factor.

A second aspect of the invention refers to a base station of a communication network, the base station comprising a controller according to one of the previous claims.

This has the advantage that no separate controllers for assigning the pilot sequences are necessary. Thus, a complexity of the network can be reduced. A third aspect of the invention refers to a system of base stations that are connected to one or more controllers according to one of implementations of the first aspect of the invention, wherein each of the one or more controllers is configured to assign pilot sequences independently of the remaining one or more controllers.

This has the advantage the signaling between neighboring controllers and/or neighboring base stations is avoided, thus reducing a complexity of the system.

A fourth aspect of the invention refers to a method for assigning pilot sequences to one or more user terminals of at least one base station of a communication network, the method comprising:

receiving an operating condition information of the base station and determining, based on the received operation condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be sched- uled,

receiving one or more channel quality indicators, CQIs, for the user terminals to be scheduled, and

assigning, based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

The method according to the fourth aspect of the invention can be performed by the controller according to the first aspect of the invention. Further features or implementations of the method according to the fourth aspect of the invention can perform the functionality of the controller according to the first aspect of the invention and its different implementation forms.

A fifth aspect of the invention refers to a computer-readable storage medium storing program code, the program code comprising instructions for carrying out the method of the fourth aspect or one of the implementations of the fourth aspect.

BRIEF DESCRIPTION OF THE DRAWINGS

To illustrate the technical features of embodiments of the present invention more clearly, the accompanying drawings provided for describing the embodiments are introduced briefly in the following. The accompanying drawings in the following description merely show some embodiments of the present invention. Modifications of these embodiments are possible without departing from the scope of the present invention as defined in the claims.

FIG. 1 is a simplified block diagram illustrating a controller in accordance with an embodiment of the present invention,

FIG. 2 is a simplified block diagram illustrating a base station in accordance with a further embodiment of the present invention,

FIG. 3 is a simplified block diagram illustrating a system for a base station in accordance with an embodiment of the present invention,

FIG. 4 is a flow chart of a method for assigning pilot sequences in accordance with a further embodiment of the present invention,

FIG. 5 is a sequence diagram illustrating exchange of information between a base station and a controller in accordance with a further embodiment of the present invention,

FIG. 6 is a simplified block diagram illustrating a controller in accordance with a further embodiment of the present invention, and

FIG. 7 is a flow chart of a method for assigning pilot sequences in accordance with a further embodiment of the present invention.

Detailed Description of the Embodiments

FIG. 1 shows a controller 100 for assigning one or more pilot sequences to one or more user terminals of at least one base station of a communication network. The controller can be integrated into the base station or it can be external to the base station, but connected to it via a wired or wireless connection. The controller can be connected to a plurality of base stations. In embodiments, the controller can be implemented as a unit which can be added to an existing base station, e.g., as a kind of plugin-module for a base station. The controller comprises a load and balance sub-controller, LBSC, 110, and a pilot assignment sub-controller, PASC, 120. Optionally, the LBSC 110 and the PASC 120 can be implemented in the same unit, for example they can be implemented on the same processor.

The LBSC 110 is configured to receive an operating condition information of the base station and determine, based on the received operating condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled. The center fraction determined by the LBSC 110 can be e.g. a floating variable. In other embodiments of the invention, the center fraction is expressed in different ways, e.g. as two integers indication the fraction (such as 3/8) and/or a number that indicates the number of center terminals (which in conjunction with the number of user terminals to be scheduled also defines a fraction). The center fraction can also be another number or information from which the part of the user terminals that are to be scheduled as center user terminals can be directly derived.

The PASC 120 is configured to receive one or more channel quality indicators, CQIs, of the user terminals to be scheduled, and assign, based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

The LBSC 110 can be configured to transmit the center fraction to the PASC 120 and the PASC 120 can be configured to receive the center fraction from the LBSC 110. The PASC can comprise a sequence storage unit which stores a plurality of center pilot sequences and a plurality of edge pilot sequences.

FIG. 2 shows a base station 200 of a communication network, the base station comprising a controller, e.g. the controller 100 shown in FIG. 1. Optionally, as indicated with dashed lines in FIG. 2, the base station 200 comprises a transmission unit 210, which is configured to transmit signals to one or more user terminals 220A, 220B and receive signals from the one or more user terminals 220 A, 220B. In particular, the transmission unit 210 can be configured to transmit a center pilot sequence to a center user terminal 220 A that is located close to a center of a cell of the base station 200. Also, the transmission unit 210 can be configured to transmit an edge pilot sequence to an edge user terminal 220B that is located further away from a center of the cell of the base sta- tion 200 and nearer to a cell boundary of the cell of the base station.

FIG. 3 shows a system 300 of base stations 31 OA, 310B that are connected to a first controller 100A and a second controller 100B. The controllers 100A, 100B can each be configured for example as the controller 100 illustrated in FIG. 1. Preferably, the controllers 100A, 100B are configured to operate independently, i.e., the first controller 100A is configured to assign pilot sequences to the base station 310A independent of any information exchange with the second controller 100B. Preferably, the controller 100 A is not configured to exchange any information related to a generation of pilot sequences with the second controller 100B. The second controller 100B can be configured in the same way as the first controller 100A.

FIG. 4 shows a method 400 for assigning pilot sequences to one or more user terminals of at least one base station of a communication network. The method 400 can be carried out e.g. by a controller, in particular by the controller 100 shown in FIG. 1. In a first step 410, an operating condition information of the base station is received and determined based on the received operation condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled.

In a second step 420, one or more channel quality indicators, CQIs, for the user terminals to be scheduled are received.

In a third step 430, based on the received CQIs and the center fraction, one or more center pilot sequences are assigned to one or more center user terminals and one or more edge pilot sequences are assigned to one or more edge user terminals. The third step 430 can comprise a sub-step of determining for a plurality of user terminals whether each of the plurality of user terminals is a center user terminal or an edge user terminal.

A center user terminal can be a user terminal for which the controller has determined that it is at a center of a cell, far from a boundary of the cell, and/or unlikely to experience interference from neighboring cells. It is worth pointing out that a center user terminal is not necessarily located in the middle of a cell. In embodiments, a center user terminal is a user terminal for which the controller has determined that it is not near a neighboring cell. Similarly, an edge user terminal can be a user terminal for which the controller has determined that it is near a neighboring cell and/or likely to experience severe interference from neighboring cells.

It is understood that the controller can be implemented in hardware and/or software. For ex- ample, in embodiments a processor can be configured to carry out the steps of the method 400 shown in FIG. 4.

Consider a plurality of cells, each comprising a base station and of a plurality of user terminals to be potentially served. In a preferred embodiment, each cell has its own independent controller. In another embodiment, one controller serves the cells in a sequential fashion.

For clarity of presentation, we focus hereafter on the preferred embodiment. The controller engages in a two-step information exchange with its associated base station. This can for example be performed as illustrated by FIG. 5 and as described below in detail.

In a first step 510, the base station describes its operating conditions to the controller, which will respond with an optimal number of user terminals to be served. In FIG. 5, the controller is referred to as CLORE (controller for joint loading and pilot reuse). The operating conditions can include:

i. a number of active antennas N,

ii. a global pilot reuse factor /?, and

iii. an indicative minimum rate per user terminal R₀. In an embodiment of the controller, if R₀ is not sent, the controller assumes it to be 0.

In a second step 520, the controller sends the optimal number of user terminals to serve to the base station. In a third step 530, the base station responds with a channel quality indicator (CQI) per user terminal. The CQI can comprise:

i. a geographic position (i.e., GPS coordinates) of the user terminal,

ii. a received signal strength (RSS) of the user terminal,

iii. a modulation and coding scheme (MCS) in use by the user terminal, and/or iv. one or more estimated spatial covariance matrices of the user terminal.

In a fourth step 540, the controller sends a pilot allocation pattern to the base station, specifying the pilot sequence corresponding to each user terminal.

FIG. 6 illustrates a controller 600 in accordance with a further embodiment of the present invention. The controller 600 comprises a Load and balance sub-controller (LBSC), indicated with reference number 610, and a Pilot assignment sub-controller (PASC), indicated with reference number 620. These sub-controllers are responsible for the decisions taken by the con- trailer and for fulfilling the information exchange protocol with the base station. In FIG. 6, these sub-controllers and the information they exchange are illustrated in more detail.

The LBSC 610 is responsible for determining the number of user terminals that should be served and how many of them will be allocated to the center set for improved spectral efficiency (center fraction). Preferably, the LBSC 610 is configured to increase the number of served user terminals with the number of active base station antennas.

In one embodiment, the LBSC 610 is configured to perform a simple search on a look-up table (LUT) like the one in Table 1 shown below. The table can be obtained e.g. from detailed computer simulations.

In another embodiment, the LBSC 610 is not provided with the tentative minimum rate R₀, and is configured to stop after performing the above-described operation.

In yet another embodiment, the LBSC 610 is provided with a non-zero value for R₀. It is configured to then compare it with the internally-kept variable R_th; if it is higher, a correction will be applied to the output values, decreasing them by a fixed pre-defined amount. The PASC 620 is responsible for determining the pilot sequence assigned to each user terminal, depending on the fraction of user terminals to be allocated to the cell center and on a set of CQIs (one per user terminal).

The process is described by FIG. 7. In a first step 710, CQI values are received for a number (K_opt) of user terminals.

In a second step 720, a function / is applied to each of the received CQI values. The function / maps the domain of the CQI values into an ordered one-dimensional domain. The particular description of / depends on the type of CQI. i. In one embodiment, the CQI are the GPS coordinates of the user terminal; function / would then map them into the inverse distances to the base station. ii. In another embodiment, the CQI are the RSSI; function / would then be the identity function.

In a third step 730, the outputs of function / are sorted. For clarity of presentation, and coherence with the items above, we will hereafter assume decreasing ordering.

In a fourth step 740, the

user terminals with higher values of /(CQ/)are assigned the pilot sequences from the β + 1 subset (center subset), and the rest are assigned to the corresponding edge subset.

Table 1

As illustrated in FIG. 7, by means of a two-step exchange, a controller can provide a base station with information about i) the number of user terminals to be served, and ii) the pilot se- quences corresponding to each of them. The pilot allocation can be changed dynamically depending on the number of active antennas at the BS, and also on some estimated rate requirement. This prevents user terminals from certain groups from being excessively penalized. The controller can govern the channel estimation process in a distributed, computationally efficient way, with no required cooperation among cells and minimal input of data.

Relevant points of some embodiments of the presented invention include:

• a controller that exploits information available at the base station (CQI, MCS, GPS coordinates of the user terminals) to reduce the impact of pilot contamination in TDD- based cellular systems, with no coordination among cells; and

• a two-part method and/or apparatus for a controller that efficiently implements fractional pilot reuse in multi-cell systems without cell coordination or information exchange.

The foregoing descriptions are only implementation manners of the present invention, the scope of the present invention is not limited to this. Any variations or replacements can be easily made through person skilled in the art. Therefore, the protection scope of the present invention should be subject to the protection scope of the attached claims.

Claims

A controller (100; 100A, 100B; 600) for assigning one or more pilot sequences to one or more user terminals (220A, 220B) of at least one base station (200; 31 OA, 310B) of a communication network, the controller comprising:

a load and balance sub-controller, LBSC (110; 610), configured to receive an operating condition information of the base station and determine, based on the received operating condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled; and

a pilot assignment sub-controller, PASC (120; 620), configured to receive one or more channel quality indicators, CQIs, of the user terminals to be scheduled, and assign, based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

The controller (100; 100A; 100B; 600) of claim 1, wherein the operating condition information includes a number of active antennas, a global pilot reuse factor and/or a minimum user terminal data rate of the base station.

The controller (100; 100A; 100B; 600) of claim 2, wherein the LBSC (110; 610) comprises a look-up table that maps the operating condition information, in particular the number of active antennas, the global pilot reuse factor and/or the minimum user terminal data rate to the center fraction.

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein a CQI comprises a geographic position indicator, a received signal strength, a modulation and coding scheme, and/or an estimate of a spatial covariance matrix.

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein the PASC (120; 620) comprises a scoring unit which is configured to assign a score value to one or more user CQIs of a user terminal, wherein in particular assigning the score value comprises a step of determining a measure of a distance of the user terminal (220A, 220B) to the base station (200; 310A, 310B).

The controller (100; 100A; 100B; 600) of claim 5, wherein the PASC (120; 620) is configured to sort the user terminals (220A, 220B) to be scheduled in an order of an increasing or a decreasing score value, and wherein in particular the PASC is configured to determine a first group of user terminals with higher score values as center user terminals (220A) and a second group of user terminals with lower score functions as edge user terminals (220B).

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein the controller is configured to set a received minimum user terminal data rate to a predetermined threshold data rate if it is smaller than the predetermined threshold data rate.

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein the PASC (120; 620) is configured to assign the one or more pilot sequences without considering information from neighboring base stations and/or neighboring controllers.

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein the controller is configured to assign pilot sequences to a plurality of base stations (31 OA, 310B), wherein in particular the controller is configured to sequentially assign the pilot sequences to the plurality of base stations.

The controller (100; 100A; 100B; 600) of one of the previous claims, wherein the one or more center pilot sequences are orthogonal to the one or more edge pilot sequences.

A base station (200) of a communication network, the base station comprising a controller according to one of the previous claims.

A system (300) of base stations (31 OA, 310B) that are connected to one or more controllers (100; 100A, 100B; 600) according to one of claims 1 to 10, wherein each of the one or more controllers is configured to assign pilot sequences independently of the remaining one or more controllers.

13. A method (400) for assigning pilot sequences to one or more user terminals (220A,

220B) of at least one base station (200; 31 OA, 310B) of a communication network, the method comprising:

receiving (410) an operating condition information of the base station and determining, based on the received operation condition information, a number of user terminals to be scheduled by the base station and a center fraction of the user terminals to be scheduled,

receiving (420) one or more channel quality indicators, CQIs, for the user terminals to be scheduled, and,

assigning (430; 740), based on the received CQIs and the center fraction, one or more center pilot sequences to one or more center user terminals and one or more edge pilot sequences to one or more edge user terminals.

14. A computer-readable storage medium storing program code, the program code comprising instructions for carrying out the method of claim 13.