WO2012047146A1

WO2012047146A1 - Triggering of sounding reference signals

Info

Publication number: WO2012047146A1
Application number: PCT/SE2011/050712
Authority: WO
Inventors: Kristina Jersenius; Richard Abrahamsson; Robert Baldemair; David Astely
Original assignee: Telefonaktiebolaget L M Ericsson (Publ)
Priority date: 2010-10-04
Filing date: 2011-06-09
Publication date: 2012-04-12

Abstract

Embodiments relate to a method in a base station (110) for triggering a user equipment UE (120) to transmit aperiodic sounding reference signal SRS to the base station, and to such a base station. The method comprises: using existing code points of a resource allocation field comprised in an uplink grant to indicate to the UE whether or not to transmit aperiodic SRS (40); and transmitting to the UE the uplink grant (42). Further embodiments relate to a method in the UE for transmitting aperiodic SRS to the base station when triggered by the base station, and to such a UE. The method comprises: receiving from the base station an uplink grant; interpreting existing code points of the resource allocation field as an indication of whether or not to transmit aperiodic SRS; and transmitting aperiodic SRS if existing code points is interpreted as aperiodic SRS is to be transmitted.

Description

TRIGGERING OF SOUNDING REFERENCE SIGNALS

TECHNICAL FIELD

Embodiments herein relate to a method in a base station, a base station, a method in a user equipment and a user equipment. In particular, embodiments relate to triggering and transmission of aperiodic sounding reference signals.

BACKGROUND

Communication devices such as mobile stations are also known as e.g. mobile terminals, wireless terminals and/or User Equipments (UEs). Mobile stations are enabled to communicate wirelessly in a cellular communications network or wireless

communication system, sometimes also referred to as a cellular radio system. The communication may be performed e.g. between two mobile stations, between a mobile station and a regular telephone and/or between a mobile station and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.

Mobile stations may further be referred to as mobile telephones, cellular telephones, or laptops with wireless capability, just to mention some further examples. The mobile stations in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another mobile station or a server.

The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. "eNB", "eNodeB", "NodeB", "B node", or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the mobile stations within range of the base stations.

In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.

In the context of this disclosure, the expression DownLink (DL) is used for the transmission path from the base station to the mobile station. The expression UpLink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.

In LTE Release 8 and 9, users, herein also referred to as User Equipments (UEs), can be configured to transmit sounding reference signals (SRS) periodically over the entire or parts of the uplink (UL) bandwidth (3GPP TS 36.213 v.9.2.0, Physical layer procedures, http://www.3gpp.org/ftp/Specs/archive/ 36_series/36.213/36213-920.zip). SRS can provide a base station, e.g. an eNodeB, with information about channel quality. This information can be utilized for many purposes such as, for example, for uplink channel dependent scheduling, uplink link adaptation and for downlink beam forming in case of reciprocal channels as in Time Division Duplex (TDD). The sounding reference signals are transmitted on the last symbol in the subframe. Several user equipments can transmit sounding in the same subframe. The multiplexing of users can be performed by frequency domain multiplexing and by code division multiplexing.

In LTE Release 10, which is currently being standardized, the user equipment can, in addition to being able to transmit periodic sounding reference signals, transmit Sounding Reference Signals (SRS) aperiodically. Aperiodic sounding means that the user transmits SRS upon request from the eNodeB. The standards provide for requesting the aperiodic sounding reference signal transmission using the uplink grant (Final report of 3 GPP TSG RAN WG1 #60bis v.l .0.0, http://www.3gpp.org/ftp/tsg_ran/

WGl_RLl/TSGRl_60b/Report/Rl-102601.zip).

The uplink grant is used to schedule the user for transmission and includes information such as resource allocation and modulation and coding scheme (3GPP TS 36.212 V9.2.0, "Multiplexing and Channel Coding", http://www.3gpp.org/ftp/Specs/ archive/36_series/36.212/36212-920.zip). The number of bits allocated to describe the number of consecutive resource blocks and the frequency position of these resource blocks ("Multiplexing and Channel Coding", http://www.3gpp.org/ftp/Specs/

archive/36_series/36.212/36212-920.zip) are: [log₂ (N^ (N^ + l) / 2)] bits, where

represents the number of resource blocks over the system bandwidth.

One possibility of using the uplink grant for triggering aperiodic SRS is to let the user always transmit SRS as a response to receiving uplink grant. This, however, results in an unnecessary large amount of SRS signaling with unnecessary overhead causing interference. SUMMARY

An object of embodiments herein is to provide a way of improving the performance in a communications network.

According to a first aspect of embodiments herein, the object is achieved by a method in a base station for triggering a user equipment to transmit an aperiodic sounding reference signal to the base station. The base station and the user equipment are comprised in a communications network. The base station uses one or more existing code points of a resource allocation field comprised in an uplink grant to indicate to the user equipment whether or not to transmit the aperiodic sounding reference signal. Further, the base station transmits the uplink grant to the user equipment.

According to a second aspect of embodiments herein, the object is achieved by a method in a user equipment for transmitting an aperiodic sounding reference signal to a base station when triggered by the base station. The base station and the user equipment are comprised in a communications network. The user equipment receives from the base station an uplink grant. It interprets one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic sounding reference signal. Further, the user equipment transmits the aperiodic sounding reference signal to the base station if the one or more existing code points is interpreted as the aperiodic sounding reference signal is to be transmitted. According to a third aspect of embodiments herein, the object is achieved by a base station for triggering a user equipment to transmit an aperiodic sounding reference signal to the base station. The base station and the user equipment are comprised in a communications network. The base station comprises a transmitter configured to transmit to the user equipment an uplink grant, wherein the uplink grant comprises a resource allocation field comprising one or more existing code points configured to indicate whether or not to transmit the aperiodic sounding reference signal. The base station comprises further a receiver configured to receive a transmitted aperiodic sounding reference signal.

According to a fourth aspect of embodiments herein, the object is achieved by a user equipment for transmitting an aperiodic sounding reference signal to a base station when triggered by the base station. The base station and the user equipment are comprised in a communications network. The user equipment comprises a receiver configured to receive from the base station an uplink grant, and a processor configured to interpret one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic sounding reference signal. Further, the user equipment comprises a transmitter configured to transmit the aperiodic sounding reference signal.

Since one or more existing code points of a resource allocation field comprised in the uplink grant is used as an indication whether or not to transmit aperiodic SRS, no additional bits need to be added to the uplink grant, and no unnecessary SRS signaling need to occur. This results in an improved performance in the communications network, since the SRS will be transmitted only aperiodically when requested by the base station.

An advantage of embodiments herein is that unnecessary overhead is avoided since no additional bits need to be added to the uplink grant. A further advantage of embodiments herein is that interference is avoided since no unnecessary SRS transmissions need to occur. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described in more detail with reference to the following drawings, in which:

Figure 1 is a schematic block diagram illustrating embodiments in a communications network;

Figure 2 is a flowchart depicting embodiments of a method in a base station;

Figure 3 is a flowchart depicting embodiments of a method in a user equipment;

Figure 4 is a flowchart depicting embodiments of a method in a user equipment;

Figure 5 is a schematic block diagram illustrating embodiments of a base station; and Figure 6 is a schematic block diagram illustrating embodiments of a user equipment.

DETAILED DESCRIPTION

Embodiments will be exemplified in the following non-limiting description.

As part of developing embodiments herein, a problem will first be identified and described. As mentioned above, an alternative to the possibility of using the uplink grant for triggering aperiodic SRS is to let the user always transmit SRS as a response to receiving uplink grant. Another approach might be to provide for indicating in the uplink grant whether or not to send aperiodic SRS. One solution for such an indication is to include one bit in the grant indicating if SRS would be to be transmitted or not. This could, however, potentially cause extra overhead. Furthermore, current uplink grant message formats have very limited or no room for additional explicit bits; to the extent such unused bits are available, other 3GPP Release 10 features compete for the same available bits.

This causes problems with extra overhead, unnecessary SRS transmission and/or interference when triggering aperiodic SRS.

In LTE, the uplink utilizes Single-Carrier Frequency Division Multiple Access (SC-FDMA). A block of the time domain signal is pre-coded by an Fast Fourier Transform (FFT) and the resulting frequency domain block is input on consecutive subcarrier of an inverse FFT (IFFT) to produce the time domain signal corresponding to one SC-FDMA symbol. To simplify the implementation at the user equipment side, not all FFT sizes are permitted. The sizes are limited to 2^¾ · 3^as · 5^as < number of resource blocks, where a₂,a₃,a₅ is a set of non-negative integers and is the number of resource blocks over the system bandwidth (3GPP TS 36.211 v.9.2.0, Physical Channels and Modulation, http://www.3gpp.org/ftp/Specs/archive/36_series/36.211/ 36211 -920.zip)

Due to this restriction, there are unused bit combinations in the resource allocation field which is designed to handle all kinds of resource allocations without any restrictions on the number of resource blocks.

In exemplifying embodiments, the already existing code points of the resource allocation field comprised in an uplink grant may be utilized to indicate to a user equipment whether or not to transmit the aperiodic SRS to a base station. Figure 1 schematically illustrates a communications network 100 comprising a base station 110 serving a cell 115 and a user equipment 120 located within the cell 1 15.

The base station 110 may be an eNB, eNodeB, or a Home Node B, a Home eNode B, a GSM/EDGE radio base station or any other network unit capable to serve a user equipment in a cellular communications network.

The user equipment 120 is configured to communicate within the cellular communications network 100 via the base station 1 10 over a radio link 125 when the user equipment 120 is present in the cell 1 15 served by the base station 110. The user equipment 120 may be e.g. a mobile terminal or a wireless terminal, a mobile phone, a computer such as e.g. a laptop, a tablet pc such as e.g. an iPad, a Personal Digital

Assistant (PDA), or any other radio network unit capable to communicate over a radio link in a cellular communications network.

A number of other user equipments, not shown, may also be located within the cell

1 15. Embodiments of a method in the base station 110 for triggering a user equipment 120 to transmit an aperiodic SRS to the base station 1 10 will now be described with reference to the flowchart depicted in Figure 2. As mentioned above, the base station 110 and the user equipment 120 are comprised in the communications network 100. The method comprises the following actions, which actions may as well be carried out in another suitable order than described below.

Action 201

The base station 110 uses existing code points of a resource allocation field in an uplink grant to indicate to a user equipment 120 whether or not to transmit an aperiodic SRS to the base station.

The base station 110 may by means of a look-up table determine which one or more existing code points to use in order to indicate to the user equipment 120 whether or not to transmit the aperiodic SRS to the base station 1 10.

In some embodiments, the look-up table is created in advance by the base station or by a third party, such as a standardization authority, according to any of the methods described in more detail below.

Action 202

The base station 1 10 transmits the uplink grant to the user equipment 120. The uplink grant is transmitted over the radio link 125.

Embodiments of a method in the user equipment 120 for transmitting an aperiodic sounding reference signal to a base station 110 when triggered by the base station 110 will now be described with reference to the flowchart depicted in Figure 3. As mentioned above, the base station 110 and the user equipment 120 are comprised in the communications network 100. The method comprises the following actions, which actions may as well be carried out in another suitable order than described below. Action 301

The user equipment 120 receives an uplink grant from the base station 1 10. The uplink grant is received over the radio link 125. Action 302

The user equipment 120 interprets one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic SRS.

The user equipment 120 may by means of a look-up table determine whether the one or more existing code points is to be interpreted as an indication to transmit the aperiodic SRS to the base station 110 or not.

As preciously mentioned, in embodiments, the look-up table is created in advance by the base station or by a third party such as a standardization authority, according to any of the methods described in more detail below.

Action 303

The user equipment 120 transmits the aperiodic SRS to the base station 110 if the one or more existing code points is interpreted as the aperiodic SRS is to be transmitted. The user equipment 120 transmits the SRS to the base station 110 over the radio link 125.

The one or more existing code points may indicate a number of resource blocks which are not permitted for transmission, thereby indicating to the user equipment 120 to transmit the aperiodic SRS, or according to some embodiments, indicating to the user equipment 120 not to transmit the aperiodic SRS.

Further, the one or more existing code points may indicate a number of resource blocks which are permitted for transmission, thereby indicating to the user equipment 120 to transmit the aperiodic SRS, or according to some embodiments, indicating to the user equipment 120 not to transmit the aperiodic SRS.

Thus the code points of the resource allocation field may be utilized not only to indicate which resource blocks to use but to also indicate if aperiodic SRS is to be triggered or not. In order to provide extra flexibility, the one or more existing code points may comprise previously unused bit combinations which can also be used to indicate one of the valid resource allocations and if aperiodic SRS is to be transmitted or not.

Interpretation of each code point in terms of resource allocation and in terms of whether or not to transmit aperiodic SRS may be performed in a fixed manner or in a configurable way. The interpretation by the user equipment 120 may be performed by, for example, using a look-up table with constant or configurable values. This is illustrated in FIG. 4 which will be described below. Embodiments of a method in the user equipment 120 for transmitting an aperiodic sounding reference signal to a base station 110 when triggered by the base station 110 will now be described in further detail with reference to the flowchart depicted in Figure 4. As mentioned above, the base station 1 10 and the user equipment 120 are comprised in the communications network 100. The method comprises the following actions, which actions may as well be carried out in another suitable order than described below.

Action 401

By means of e.g. a look-up table, the user equipment 120 maps the one or more code points comprised in the uplink grant received from the base station 1 10 to resource allocation and to aperiodic SRS triggering. This action corresponds to Action 302.

As preciously mentioned, in embodiments, the look-up table may be created in advance by the base station or by a third party such as a standardization authority, according to any of the methods described in more detail below.

Action 402

The user equipment 120 determines based on the mapping whether or not aperiodic SRS is to be transmitted to the base station 110. This action also corresponds to Action 302. Action 403

If aperiodic SRS is not to be transmitted, the user equipment 120 transmits data on indicated resources. The indicated resources are determined by the mapping of the one or more code points in the uplink grant to resource allocation.

Action 404

If aperiodic SRS is to be transmitted, the user equipment 120 transmits data on indicated resources and transmits aperiodic SRS. This action corresponds to Action 303. Embodiments of a look-up table and embodiments of methods for creating a lookup table will now be described in more detail. As previously mentioned, the look-up table may be created by the base station or by a third party.

In some embodiments, the look-up table is created in advance by the base station 1 10 and transmitted from the base station 1 10 to the user equipment 120 at handover when the user equipment 120 enters a new cell. The base station 1 10 may be configured to reconfigure the look-up table and to send a new or updated look-up table to a user equipment 120 already existing in the cell.

In some other embodiments, the look-up table is created by a third party, such as a standardization authority. The look-up table may be implemented in the base station 1 10 and/or the user equipment 120 since it has been standardized that the look-up table should be used. The implementation may be performed during manufacture of the base station 1 10 or the user equipment 120 but may be performed at another suitable point of time.

According to a first category of exemplifying embodiments, the one or more existing code points of the resource allocation field comprise one or more unused bit combinations of the resource allocation field. These one or more unused bit combinations are bit combinations not permitted for transmission.

By the wording "resource blocks or bit combinations not permitted for transmission" or similar is meant one or more resource blocks or bit combinations which by a standardization authority or the like is not permitted to be used for data transmission. In some embodiments, the "resource blocks or bit combinations not permitted for transmission" refers to resource blocks or bit combinations corresponding to resource allocation sizes that are not permitted for transmission in order to simplify the FFT implementation.

When used herein, the wording "resource blocks or bit combinations permitted for transmission" or similar, should be understood as one or more resource blocks or bit combinations which by a standardization authority or the like is permitted to be used for data transmission. In some embodiments, the "resource blocks or bit combinations permitted for transmission" refers to resource blocks or bit combinations corresponding to resource allocation sizes that are permitted for transmission in order to simplify the FFT implementation.

The unused bit combinations of the resource allocation field may be used to indicate that aperiodic SRS is to be triggered. The number of resource blocks to be used for the data transmission may be determined or specified from a look-up table where the unused bit combinations may be mapped to a certain resource allocation.

The mapping of the unused bit combinations to a certain resource allocation may be performed by the base station or by a third party, and it may take place as follows: Let K denote the number of unused bit combinations for a certain system bandwidth and, without loss of generality, enumerate these unused combinations from 0 to K-\ in the order in which they occur when considering the bit combinations in sequence according to their natural number representation. Each of these K values may correspond to a message. In this description, message should be understood as any bit combination comprising information relating to aperiodic SRS signaling and to resource block allocation indication. The message does not have to comprise a header and a payload but it may do so. The messages may be grouped in a look-up table as illustrated in Table 1 for example.

Message number Meaning

0 Transmit aperiodic SRS and transmit data on Resource Block (RB) #0

1 Transmit aperiodic SRS and transmit data on RB #1

2 Transmit aperiodic SRS and transmit data on RB #2

K-l Transmit aperiodic SRS and transmit data on RB #0-1 10 Table 1 - Example of how the unused messages in the uplink grant for resource allocation on Physical Uplink Shared Channel (PUSCH) may be used for requesting aperiodic SRS According to second category of exemplifying embodiments, the one or more existing code points of the resource allocation field comprises one or more used bit combinations of the resource allocation field. Here one or more used bit combinations are bit combinations permitted for transmission.

According to these second exemplifying embodiments, a combination of the used code points, i.e. bit combinations, of the resource allocation field may be used to indicate that aperiodic SRS is to be triggered.

In this case: Let K denote the number of used bit combinations for a certain system bandwidth and, without loss of generality, enumerate these used combinations from 0 to K-\ in the order in which they occur when considering the bit combinations in sequence according to their natural number representation. This procedure may be performed in advance by the base station 110 or a third party. The enumerated used bit combinations may be stored in a look-up table by the base station 1 10 or the third party.

Let every x^th used bit combination in the look-up table mean that aperiodic SRS is to be transmitted, wherein x can be a constant or a fixed number or x can be a

configurable number. In such an embodiment, the base station 110 is configured to interpret every x^th used bit combination in the look-up table as an indication that aperiodic SRS is to be transmitted. If the base station 1 10 wants to trigger aperiodic SRS, the base station 1 10 may use one of every x^th used bit combination in an uplink grant sent to the user equipment 120. Further, in such an embodiment, the user equipment 120 is configured to interpret every x^th used bit combination in the look-up table as an indication that aperiodic SRS is to be transmitted.

An alternative possibility is to link, i.e. map, the aperiodic SRS transmission to certain resource allocation sizes. For example, a resource allocation size that is a factor of y may indicate that aperiodic SRS is to be transmitted, wherein y may be a fixed or a constant number or y may be a configurable number. According to third category of exemplifying embodiments, the size of the system bandwidth may determine whether to utilize the first exemplifying embodiment or the second exemplifying embodiment. That is, one, i.e. the base station 1 10 and/or the user equipment 120, may switch between the first and second exemplifying embodiments (as described above) based on the size of the system bandwidth.

For example, the second exemplifying embodiments may be used for a system bandwidth of a size smaller than z resource blocks and the first exemplifying

embodiments may be used for a system bandwidth of a size equal to or larger than z resource blocks. The value of z may be, for example, fifty i.e. 50. resource blocks. As understood by those skilled in the art, the number of resource blocks is depending on the system bandwidth and the number of resource blocks could thus be in the range of 6-110 resource blocks for LTE Release 8, 9 and 10. However, as understood by those skilled in the art, the number of resource blocks may increase as the system bandwidth increases.

It may be noted that for contiguous resource allocations, i.e. allocation of consecutive resource blocks, the following set of resource allocation lengths are supported: {1, 2, 3, 4, 5, 6, 8, 9, 10, 12, 15, 16, 18, 20, 24, 25, 27, 30, 32, 36, 40, 45, 48, 50, 54, 60, 64, 72, 75, 80, 81, 90, 96, 100, 108} .

From the decoded value for the length in terms of contiguously allocated resource blocks, Lcrb, the user equipment 120 may determine whether or not to transmit aperiodic SRS and the actual bandwidth in terms of contiguous resource blocks on which PUSCH is to be transmitted.

While exemplifying embodiments have been described in the context of contiguous uplink resource allocation, it should be noted that LTE Release 10 also specifies non-contiguous uplink resource allocations, i.e. allocation of non-consecutive resource blocks. The difference is that the FFT pre-coded signal is not mapped contiguously but in multiple clusters to the input subcarriers of the subsequent IFFT.

The principles outlined above for triggering aperiodic SRS transmission also apply for non-contiguous uplink resource allocations. In the non-contiguous resource allocation, which is typically one bit more than the contiguous resource allocation for the same system bandwidth, not all available code points are used. The first exemplifying embodiments are applicable to the case of non-contiguous uplink resource allocations.

The first variant of the second exemplifying embodiments (i.e. every x^th resource allocation) are also applicable.

The second variant of the second exemplifying embodiments, wherein the resource allocation contains factor y, may be generalized to cover non-contiguous uplink resource allocations. The resource allocation size may either be the bandwidth of one cluster, i.e. the bandwidth of this cluster contains factor y, or the total allocated bandwidth, i.e. the summed cluster bandwidth contains factor y. While both alternatives are possible, in some embodiments herein, the total allocated bandwidth should be factorable by y.

The third exemplifying embodiments which provides for switching between the first and second embodiments also apply to the case of the non-contiguous uplink resource allocations.

Exemplifying embodiments as described above provide several advantages. In general, aperiodic SRS triggering may be indicated without additional overhead.

The first category of exemplifying embodiments may be applicable for large bandwidth sizes where there are as many unused bit combinations as used bit combinations. In such cases, it is possible to trigger aperiodic SRS independent of the resource allocation. For small bandwidth sizes, there are not so many unused bit combinations and the eNodeB, i.e. the base station 110, may then be forced to use certain resource allocations in order to trigger SRS, as has been described with reference to the second category of exemplifying embodiments. The third category of exemplifying embodiments has the advantage that they work well for both large and small bandwidth sizes.

To perform the method actions in the base station 1 10 described above for triggering aperiodic SRS, the base station 110 comprises the following arrangement depicted in Figure 5. As mentioned above the base station 110 and the user equipment 120 are comprised in the communications network 100.

The base station 110 comprises a transmitter 112 configured to transmit an uplink grant to the user equipment 120. The uplink grant comprises a resource allocation field comprising one or more existing code points configured to indicate whether or not to transmit the aperiodic SRS.

The base station 110 further comprises a receiver 114 configured to receive a transmitted aperiodic SRS.

The base station 110 further comprises a processor 116 configured to determine which one or more existing code points to use in the uplink grant by interpreting each existing code point in terms of resource allocation and as an indication of whether or not to transmit the aperiodic SRS.

The processor 116 may be configured to perform the interpretation by means of a look-up table with constant or configurable values relating the resource allocation to whether or not the aperiodic sounding reference signal is to be transmitted.

In some embodiments, the processor 116 is configured to use a method according to the first exemplifying embodiments described above. Further, in some embodiments, the processor 116 is configured to use a method according to the second exemplifying embodiments described above. Furthermore, in some embodiments, the processor 1 16 may be configured to determine based on the size of a system bandwidth whether to use a method according to the first exemplifying embodiments or the second exemplifying embodiments described above.

The base station 1 10 may be configured to create the one or more of the look-up tables as described above, e.g. by means of the processor 116. In embodiments, the one or more look-up tables are created by a third party, such as a standardization authority, and implemented in the base station at a suitable point of time, e.g. during manufacture of the base station.

To perform the method actions in the user equipment 120 described above for transmitting aperiodic SRS to a base station 1 10 when triggered by the base station 1 10, the user equipment 120 comprises the following arrangement depicted in Figure 6. As mentioned above the base station 110 and the user equipment 120 are comprised in a communications network 100. The user equipment 120 comprises a receiver 122 configured to receive an uplink grant from the base station 1 10. The user equipment 120 further comprises a processor 124 configured to interpret one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic sounding reference signal. The user equipment 120 further comprises a transmitter 126 configured to transmit the aperiodic sounding reference signal.

In some embodiments, the processor 124 is configured to interpret each existing code point in terms of resource allocation and as an indication of whether or not to transmit the aperiodic sounding reference signal. The interpretation may be performed in a fixed manner or in a configurable way, by means of a look-up table with constant or configurable values. The constant or configurable values relate to the resource allocation and to whether or not the aperiodic sounding reference signal is to be transmitted.

In some embodiments, the processor 124 is configured to use the method according to the first exemplifying embodiment described above. Further, in

embodiments, the processor 124 is configured to use the method according to the second exemplifying embodiments described above. Furthermore, in some embodiments, the processor 124 may be configured to determine based on the size of a system bandwidth whether to use a method according to the first exemplifying embodiments or the second exemplifying embodiments described above.

The user equipment 120 may be configured to receive the one or more of the look-up tables described above from the base station 110 during for example hand-over. The one or more look-up tables may be created by a third party, such as a standardization authority, and implemented in the user equipment at a suitable point of time, e.g. during manufacture of the user equipment. When using the word "comprise" or "comprising" it shall be interpreted as non- limiting, in the meaning of "consist at least of.

When using the word "action"/" actions" it shall be interpreted broadly and not to imply that the actions have to be carried out in the order mentioned. Instead, the actions may be carried out in any suitable order other than the order mentioned. Further, some action/actions may be optional.

The embodiments herein are not limited to the above described examples. Various alternatives, modifications and equivalents may be used. Therefore, the above examples should not be taken as limiting the scope of the invention, which is defined by the appending claims.

Claims

1. A method in a base station (110) for triggering a user equipment (120) to transmit an aperiodic sounding reference signal to the base station (110), the method comprising: - using one or more existing code points of a resource allocation field comprised in an uplink grant to indicate to the user equipment (120) whether or not to transmit the aperiodic sounding reference signal (40); and

- transmitting to the user equipment (120) the uplink grant (42).

2. The method according to claim 1 , wherein the one or more existing code points indicates a number of resource blocks which are not permitted for transmission, thereby indicating to the user equipment (120) to transmit the aperiodic sounding reference signal.

3. The method according to claim 1 , wherein the one or more existing code points indicates a number of resource blocks which are not permitted for transmission, thereby indicating to the user equipment (120) not to transmit the aperiodic sounding reference signal.

4. The method according to claim 1, wherein the one or more existing code points indicates a number of resource blocks which are permitted for transmission, thereby indicating to the user equipment (120) to transmit the aperiodic sounding reference signal.

5. The method according to claim 1 , wherein the one or more existing code points indicates a number of resource blocks which are permitted for transmission, thereby indicating to the user equipment (120) not to transmit the aperiodic sounding reference signal.

6. The method according to any of claim 1-5, wherein the base station (1 10) determines which one or more existing code points to use in the uplink grant by interpreting each existing code point in terms of resource allocation and as an indication of whether or not the user equipment (120) is to transmit the aperiodic sounding reference signal, and wherein the base station (110) performs the interpretation by means of a look-up table with constant or configurable values relating the resource allocation to whether or not the aperiodic sounding reference signal is to be transmitted.

7. The method according to claim 6, wherein the one or more existing code points of the resource allocation field comprises one or more unused bit combinations of the resource allocation field, and wherein the one or more unused bit combinations are bit

combinations not permitted for transmission.

8. The method according to claim 7, wherein the look-up table comprises each of the one or more unused bit combinations mapped to a certain resource allocation, whereby the number of resource blocks to be used for a data transmission can be determined.

9. The method according to claim 8, wherein the one or more unused bit combinations are mapped to a certain resource allocation by enumerating the one or more unused bit combinations from 0 to K- 1 in the order in which they occur when considering the bit combinations in sequence according to their natural number representation, wherein K denotes the number of unused bit combinations for a certain system bandwidth and wherein each K-value corresponds to an indication indicating whether or not to transmit the aperiodic sounding reference signal.

10. The method according to claim 6, wherein the one or more existing code points of the resource allocation field comprises one or more used bit combinations of the resource allocation field, and wherein the one or more used bit combinations are bit combinations permitted for transmission.

11. The method according to claim 10, wherein the look-up table comprises the one or more used bit combinations enumerated from 0 to K- 1 in the order in which they occur when considering the used bit combinations in sequence according to their natural representation, and wherein K denotes the number of used bit combinations for a certain system bandwidth.

12. The method according to claim 11 , wherein the base station (1 10) interprets every x^th used bit combination in the look-up table as an indication that the aperiodic sounding reference signal is to be transmitted, and wherein x is a fixed number or a configurable number.

13. The method according to claim 11 , wherein the aperiodic sounding reference signal transmission is mapped to a certain resource allocation size; wherein the base station

(1 10) uses the certain resource allocation size corresponding to a factor of y as an indication to the user equipment (120) that the aperiodic sounding reference signal is to be transmitted, and wherein y is a fixed number or a configurable number.

14. The method according to claim 1 -6, wherein the base station (110) determines based on the size of a system bandwidth whether to use a method according to any of claim 7-9 or a method according to any of claim 10-13.

15. The method according to claim 14, wherein the base station (1 10) uses the method according to any of claim 7-9 when the system bandwidth is smaller than z resource blocks, and the method according to any of claim 10-13 when the system bandwidth is equal to or larger than z resource blocks.

16. The method according to claim 15, wherein z is an integer between 6 and 1 10, for example 50.

17. A method in a user equipment (120) for transmitting an aperiodic sounding reference signal to a base station (1 10) when triggered by the base station (1 10), the method comprising:

- receiving from the base station (1 10) an uplink grant (50); - interpreting one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic sounding reference signal (52); and

- transmitting the aperiodic sounding reference signal if the one or more existing code points is interpreted as indicating that the aperiodic sounding reference signal is to be transmitted (54).

18. The method according to claim 17, wherein the one or more existing code points indicates a number of resource blocks which are not permitted for transmission thereby indicating to the user equipment (120) to transmit the aperiodic sounding reference signal.

19. The method according to claim 17, wherein the one or more existing code points indicates a number of resource blocks which are not permitted for transmission thereby indicating to the user equipment (120) not to transmit the aperiodic sounding reference signal.

20. The method according to claim 17, wherein the one or more existing code points indicates a number of resource blocks which are permitted for transmission thereby indicating to the user equipment (120) to transmit the aperiodic sounding reference signal.

21. The method according to claim 17, wherein the one or more existing code points indicates a number of resource blocks which are permitted for transmission thereby indicating to the user equipment (120) not to transmit the aperiodic sounding reference signal.

22. The method according to any of claim 17-21 , wherein the user equipment (120) interprets each existing code point in terms of resource allocation and as an indication of whether or not to transmit the aperiodic sounding reference signal; and wherein the user equipment (120) performs the interpretation by means of a look-up table with constant or configurable values relating the resource allocation to whether or not the aperiodic sounding reference signal is to be transmitted.

23. The method according to claim 22, wherein the one or more existing code points of the resource allocation field comprises one or more unused bit combinations of the resource allocation field, and wherein the one or more unused bit combinations are bit combinations not permitted for transmission.

24. The method according to claim 23, wherein the look-up table comprises each of the one or more unused bit combinations mapped to a certain resource allocation, whereby the number of resource blocks to be used for a data transmission can be determined.

25. The method according to claim 24, wherein the one or more unused bit combinations are mapped to a certain resource allocation by enumerating the one or more unused bit combinations from 0 to K- 1 in the order in which they occur when considering the bit combinations in sequence according to their natural number representation, wherein K denotes the number of unused bit combinations for a certain system bandwidth and wherein each K-value corresponds to an indication indicating whether or not to transmit the aperiodic sounding reference signal.

26. The method according to claim any of claim 22, wherein the one or more existing code points of the resource allocation field comprises one or more used bit combinations of the resource allocation field, and wherein the one or more used bit combinations are bit combinations permitted for transmission.

27. The method according to claim 26, wherein the look-up table comprises the one or more used bit combinations enumerated from 0 to K- 1 in the order in which they occur when considering the used bit combinations in sequence according to their natural representation, and wherein K denotes the number of used bit combinations for a certain system bandwidth.

28. The method according to claim 27, wherein the user equipment (120) interprets every x^th used bit combination in the look-up table as an indication that the aperiodic sounding reference signal is to be transmitted, and wherein x is a fixed number or a configurable number.

29. The method according to claim 27, wherein the aperiodic sounding reference signal transmission is mapped to a certain resource allocation size; wherein the user equipment (120) interprets the certain resource allocation size corresponding to a factor of y as an indication that the aperiodic sounding reference signal is to be transmitted, and wherein y is a fixed number or a configurable number.

30. The method according to claim 22, wherein the user equipment (120) determines based on the size of a system bandwidth whether to use a method according to any of claim 23-25 or a method according to any of claim 26-29.

31. The method according to claim 30, wherein the user equipment (120) uses the method according to any of claim 23-25 when the system bandwidth is smaller than z resource blocks, and the method according to any of claim 26-29 when the system bandwidth is equal to or larger than z resource blocks.

32. The method according to claim 31 , wherein z is an integer between 6 and 110, for example 50.

33. A base station (110) for triggering a user equipment (120) to transmit an aperiodic sounding reference signal to the base station (110), the base station (110) comprising:

- a transmitter (1 12) configured to transmit to the user equipment (120) an uplink grant; said uplink grant comprising a resource allocation field comprising one or more existing code points configured to indicate whether or not to transmit the aperiodic sounding reference signal; and

- a receiver (1 14) configured to receive a transmitted aperiodic sounding reference signal.

34. The base station (1 10) according to claim 33, further comprising a processor (1 16) configured to determine which one or more existing code points to use in the uplink grant by interpreting each existing code point in terms of resource allocation and as an indication of whether or not to transmit the aperiodic sounding reference signal, and wherein the processor (1 16) is configured to perform the interpretation by means of a look-up table with constant or configurable values relating the resource allocation to whether or not the aperiodic sounding reference signal is to be transmitted.

35. The base station (110) according to claim 34, wherein the processor (116) is configured to use a method according to claim 7-9.

36. The base station (110) according to claim 35, wherein the processor (116) is configured to use a method according to claim 10-13.

37. The base station (110) according to claim 35, wherein the processor (116) is configured to determine based on the size of a system bandwidth whether to use a method according to any of claim 7-9 or a method according to any of claim 10-13.

38. A user equipment (120) for transmitting an aperiodic sounding reference signal to a base station (110) when triggered by the base station (1 10), the user equipment (120) comprising:

- a receiver (122) configured to receive from the base station (1 10) an uplink grant;

- a processor (124) configured to interpret one or more existing code points of a resource allocation field comprised in the uplink grant as an indication of whether or not to transmit the aperiodic sounding reference signal; and

- a transmitter (126) configured to transmit the aperiodic sounding reference signal.

39. The user equipment (120) according to claim 48, wherein the processor (124) is configured to interpret each existing code point in terms of resource allocation and as an indication of whether or not to transmit the aperiodic sounding reference signal, and wherein the interpretation is performed in a fixed manner or in a configurable way by means of a look-up table with constant or configurable values relating to the resource allocation and to whether or not the aperiodic sounding reference signal is to be transmitted.

40. The user equipment (120) according to claim 39, wherein the processor (124) is configured to use the method according to claim 23-25.

41. The user equipment (120) according to claim 39, wherein the processor (124) is configured to use the method according to claim 26-29.

42. The user equipment (120) according to claim 39, wherein the processor (124) is configured to determine based on the size of a system bandwidth whether to use a method according to any of claim 23-25 or a method according to any of claim 26-29.