WO2014005322A1

WO2014005322A1 - Search spaces for wireless communications

Info

Publication number: WO2014005322A1
Application number: PCT/CN2012/078284
Authority: WO
Inventors: Peter Skov; Chunye Wang; Frank Frederiksen; Bernhard Raaf; Lars Lindh; Pekka Janis; Honglei Miao; Klaus Hugl
Original assignee: Nokia Siemens Networks Oy; Nokia Corporation
Priority date: 2012-07-06
Filing date: 2012-07-06
Publication date: 2014-01-09

Abstract

Methods and apparatuses for providing adaptive search space definitions for wireless communications are provided where a code rate available for at least one selected downlink control information format is determined based on a determined control channel element size and an evaluation is performed whether said code rate is acceptable for transmission of control information. When said code rate is acceptable, at least one search space candidate is defined based on said control channel element size and code rate. When said code rate is unacceptable, iterative evaluation is provided whether a code rate determined on a higher aggregation level is acceptable.

Description

SEARCH SPACES FOR WIRELESS COMMUNICATIONS

This disclosure relates to search spaces for wireless communications, and more particularly to defining of search space candidates for communication of control information.

A communication system can be seen as a facility that enables communication sessions between two or more entities such as fixed or mobile communication devices, machine-type terminals, base stations, servers and/or other communication nodes. A communication system and compatible communicating entities typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. In a wireless communication system at least a part of communications between stations occurs over a wireless link. Examples of wireless systems include public land mobile networks (PLMN) such as cellular networks, satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN). A wireless system can be divided into cells or other radio coverage or service areas.

A user can access the communication system by means of an appropriate communication device. A communication device of a user is often referred to as user equipment (UE) or terminal. A communication device is provided with an appropriate signal receiving and transmitting arrangement for enabling communications with other parties. In a wireless system a communication device provides a transceiver station that can communicate with another communication node such as e.g. a base station and/or another user equipment.

An example of communication systems is an architecture that is being standardized by the 3rd Generation Partnership Project (3GPP). This system is often referred to as the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access technology. A further development of the LTE is often referred to as LTE-Advanced. The various development stages of the 3GPP LTE specifications are referred to as releases.

Various control channels are possible. A physical downlink control channel (PDCCH) is used to carry scheduling assignments and other control information. The channel is transmitted on one or several control channel elements (CCEs). In LTE blind decoding a device, for example a user equipment, checks predefined PDCCH locations, PDCCH aggregation levels, and downlink control information (DCI) formats and acts on messages satisfying predefined criteria. Carrying out such a 'blind decoding' of all the possible combinations would require the device to make many PDCCH decoding attempts in every subframe, and thus LTE specifications define that each device has only a limited set of control channel element (CCE) locations where a PDCCH may be placed. The set of CCE locations where a device may find its PDCCH can be considered as its 'search space'. A search space can be of a different size for each control channel format. Separate dedicated, user specific and common search spaces can also be defined. In such case a dedicated search space can be configured for each device individually (i.e. user specific) while all devices can be informed of the extent and location of a common search space. Such common search space is typically located in a fixed location such that it is possible to address all user devices within a cell using only one signalling message on the PDCCH. Each space can comprise 2, 4 or 6 PDCCH candidates whose data length depends on the PDCCH format where each PDCCH is transmitted on 1 , 2, 4 or 8 CCE(s). The common and device specific search spaces can overlap with each other. The size of search space is determined by the number of PDCCH candidates and the size of CCE aggregation level.

A new control channel known as the enhanced physical downlink control channel (ePDCCH) is to be specified by 3GPP. The ePDCCH is multiplexed with physical downlink shared channel (PDSCH) using frequency division multiplexing (FDM) such that certain physical resource blocks (PRBs) are used for ePDCCH and others for PDSCH instead of multiplexing the ePDCCH inside one PRB. An ePDCCH can be based on a fixed number of control channel elements (CCEs) per a physical resource block (PRB) where the size of each CCE adapts to the number of resource elements (REs) available in the PRB. The adaptation of the CCE sizes can depend on the presence of legacy signals such as common reference signal (CRS), PDCCH, physical control format indicator channel (PCFICH), physical hybrid automatic repeat request (ARQ) indicator channel (PHICH), and channel state information reference signal (CSl-RS). This is in contrast to the case of channels such as the PDCCH where a fixed CCE size of 36 resource elements is provided.

The inventors have found that a problem may arise when using variable sized CCEs in how to achieve a proper selection of code rates for transmitting a physical downlink control channel. This may also occur e.g. where Downlink Control Information (DCI) element sizes can vary but the search space size and control channel element size is fixed. For example, DCI element sizes can vary from about 30 bits to 70 bits depending on DCI type and system bandwidth. A varying control channel element size with potentially smaller enhanced CCE (eCCE) size for a ePDCCH compared to the CCE size for legacy PDCCH size can enhance this problem. For example, if there are four aggregation levels available (as is the case e.g. in 3GPP Release 8) the range of code rates may be acceptable even with varying DCI message sizes. However, if varying, potentially smaller CCE sizes are used in association with a channel such as the ePDCCH the available code rates may become too limited for certain aggregation levels to offer decent system performance.

It is noted that the above discussed issues are not limited to any particular communication environment.

Embodiments of the invention aim to address one or several of the above issues.

In accordance with an embodiment there is provided a method for adaptive search space definition for wireless communications, comprising selecting at least one downlink control information format, determining a control channel element size, determining a code rate available for the at least one downlink control information format based on the control channel element size, evaluating whether said code rate is acceptable for transmission of control information, and when said code rate is acceptable, defining at least one search space candidate based on said control channel element size and code rate, and when said code rate is unacceptable, evaluating whether a code rate determined on a higher aggregation level is acceptable.

In accordance with an embodiment there is provided a method for adaptive search space definition for wireless communications, comprising determining at least one potential downlink control information format, determining a control channel element size, determining a code rate available for the at least one downlink control information format based on the control channel element size, evaluating whether said code rate is acceptable for communication of control information, and when said code rate is acceptable, defining at least one search space candidate based on said control channel element size and code rate, and when said code rate is unacceptable, evaluating whether a code rate determined on a higher aggregation level is acceptable.

In accordance with an embodiment there is provided an apparatus for controlling wireless communications, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to determine a code rate available for at least one selected downlink control information format based on a determined control channel element size, evaluate whether said code rate, is acceptable for transmission of control information, and when said code rate is acceptable, define at least one search space candidate based on said control channel element size and code rate, and when said code rate is unacceptable, evaluate whether a code rate determined on a higher aggregation level is acceptable.

In accordance with an embodiment there is provided an apparatus for controlling wireless communications, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to determine at least one potential downlink control information format, determine a control channel element size, determine a code rate available for the at least one downlink control information format based on the control channel element size, evaluate whether said code rate is acceptable for communication of control information, and when said code rate is acceptable, define at least one search space candidate based on said control channel element size and code rate, and when said code rate is unacceptable, evaluate whether a code rate determined on a higher aggregation level is acceptable.

In accordance with a more detailed embodiment the at least one potential downlink contra! information format is used for blind decoding at a mobile device.

When said code rate is found acceptable for an aggregation, all candidates for higher aggregation levels may be included in the definition of search space candidates.

The evaluating may comprise comparing the determined code rate to a predefined maximum value. The predefined maximum value may be communicated on higher layer signalling and/or preconfigured in a device based on a standard specification.

An iterative algorithm may progress from an aggregation level to a next available aggregation level until a maximum supported aggregation level is reached or an acceptable code rate is determined.

The evaluation may be performed per each transmission time interval or per each control channel element.

The determining of control channel element size may comprise at least one of using a reference control channel element as the basis for evaluation, the reference control channel element comprising a predefined complete control channel element counted from a predefined subcarrier, using the smallest or largest control channel element as the basis for evaluation, and using an average of control channel elements in the region of a control channel assigned to a receiving device as the basis for evaluation.

A downlink control information format specific search spaces may be provided.

A selection between a higher and lower order modulation schemes may be provided. The control channel element may comprise an enhanced control channel element to be transmitted on an enhanced physical downlink control channel.

Nodes implementing the embodiments may be provided. The can includes a base station control apparatus and a mobile device.

A computer program comprising program code means adapted to perform the claimed method may also be provided.

Various other aspects and further embodiments are also described in the following detailed description and in the attached claims.

The invention will now be described in detail, by way of example only, with reference to the following examples and accompanying drawings, in which:

Figure 1 shows a schematic diagram of a network according to some embodiments;

Figure 2 shows a schematic diagram of a mobile communication device according to some embodiments;

Figure 3 shows a schematic diagram of a control apparatus according to some embodiments;

Figure 4 shows a flow chart according to a certain embodiment for operation at a network element; and.

Figure 5 shows a flow chart according to a certain embodiment for operation at a mobile device.

In the following certain exemplifying embodiments are explained with reference to a wireless or mobile communication system serving mobile communication devices. Before explaining in detail the exemplifying embodiments, certain general principles of a wireless communication system and mobile communication devices are briefly explained with reference to Figures 1 to 3 to assist in understanding the technology underlying the described examples.

In a wireless communication system mobile communication devices or user equipments (UE) 12, 13 are provided wireless access via at least one base station or similar wireless transmitting and/or receiving node or point. In the figure 1 example an access system or radio service area of a cellular system 10 provided by a base station 16 are shown. Each mobile communication device and station may have one or more radio channels open at the same time and may send signals to and/or receive signals from more than one source. It is noted that the radio service area border is schematically shown for illustration purposes only in Figure 1. it shall be understood that the size and/or shape of a radio service area may vary considerably from that of Figure 1.

Base stations are typically controlled by at least one appropriate controller apparatus so as to enable operation thereof and management of mobile communication devices in communication with the base stations. In Figure 1 control apparatus 18 is shown to control the station 16. The control apparatus can be interconnected with other control entities. The control apparatus is typically provided with memory capacity and at least one data processor. The control apparatus and functions may be distributed between a plurality of control units.

A non-limiting example of the recent developments in communication system architectures is the long-term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) that is being standardized by the 3rd Generation Partnership Project (3GPP). As explained above, further development of the LTE is referred to as LTE-Advanced. Non-limiting examples of LTE access nodes are macro level base stations known as NodeB (NB) and enhanced NodeB (eNB) in the vocabulary of the 3GPP specifications. Home eNBs (HeNB), pico eNodeBs (pico-eNB), femto nodes, and radio remote heads (RRH) connected to an eNB. The LTE employs a mobile architecture known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Base stations of such systems (evolved or enhanced Node Bs) provide E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical layer protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the user devices. Of these Radio Resource Control (RRC) protocol is used to configure and control the radio resource between eNodeBs and user equipment. For example, RRC is used to configure the RLC/MAC and PHY layer at a user equipment and eNodeB. Other examples of radio access system include those provided by base stations of systems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave Access).

A possible mobile communication device will now be described in more detail in reference to Figure 2 showing a schematic, partially sectioned view of a communication device 20. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) such as a mobile phone or what is known as a 'smart phone', a portable computer provided with a wireless interface card or other wireless interface facility, personal data assistant (PDA) provided with wireless communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email), text message, multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non-limiting examples of these services include two-way or multi-way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. User may also be provided with broadcast or multicast data. Non-limiting examples of the content include downloads, television and radio programs, videos, advertisements, various alerts and other information. The mobile device may receive signals over an air interface 27 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 2 transceiver apparatus is designated schematically by block 26. The transceiver apparatus 26 may be provided for example by means of a radio part and associated antenna arrangement. The antenna arrangement may be arranged internally or externally to the mobile device.

A mobile device is also typically provided with at least one data processing entity 21 , at least one memory 22 and other possible components 23 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 24. The control apparatus of a user equipment can be configured to process information in association with search space defining and to monitor for control information from a base station accordingly.

Figure 3 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling a station of an access system. In some embodiments base stations comprise a separate control apparatus. In other embodiments the control apparatus can be another network element. The control apparatus 30 can be arranged to provide control on communications in the service area of the system. The control apparatus 30 can be configured to provide control functions in association with scheduling and adaptive search space definition by means of the data processing facility in accordance with certain embodiments described below. For this purpose the control apparatus comprises at least one memory 31 , at least one data processing unit 32, 33 and an input/output interface 34. Via the interface the control apparatus can be coupled to a receiver and a transmitter of the base station. The control apparatus can be configured to execute an appropriate software code to provide the control functions. It shall be appreciated that similar components can be provided in a control apparatus provided elsewhere in the system.

Figure 4 shows a flowchart for operation at a scheduling control apparatus for an adaptive definition of search space candidates for wireless communications. A network element can select a downlink control information (DCI) element format at 40. A control channel element size is determined at 41. The control channel element size can be determined, for example, based on the amount of available resource elements taking into account the varying amount of legacy signals. A code rate available for the selected DCI element based on a control channel element size and aggregation level is then determined at 42. An evaluation is provided at 43 whether said code rate is acceptable for transmission of control information. The evaluating may comprise comparing the determined code rate to a predefined maximum value. If the result is positive i.e. said code rate is acceptable, at least one search space candidate is defined at 44 based on said determined control channel element size and selected DCI format.

The process is iterative such that if it is determined at 43 that said code rate is not acceptable, the process progresses to a higher aggregation level at 45 and a further evaluating is made at 43 whether a code rate determined in the higher aggregation level is acceptable. When progressing to higher level the code rate is recalculated before testing its acceptability.

If the highest possible aggregation level is not reached in an iteration round, all higher supported aggregation levels can be automatically included in the search space at 43 as search space candidates. Thus, if an acceptable code rate is achieved for a lower aggregation level thus can be used for higher aggregation level automatically as well.

For example, the method may iteratively progress from aggregation level 1 to aggregation level 2 if the evaluation shows a non-acceptable code rate at 42 for aggregation level 1 , and from aggregation level 2 to aggregation level 4 at if a subsequent evaluation shows a non-acceptable code rate for aggregation level 2. If the further evaluation provides a positive result, at least one search space candidate can be defined based on the evaluation on the higher aggregation level. The process may continue and progress from aggregation level 4 to aggregation level 8 if the relevant evaluation shows a non-acceptable code rate for aggregation level 4, and so on until either an acceptable code rate is determined or a maximum supported aggregation level is reached.

A network element such as an eNB can be configured to select a DCI format to be transmitted. In accordance with an embodiment the eNB is configured to iteratively define a finite number of search space candidates of varying aggregation levels.

In accordance with a possibility a network element is configured to identify a minimum aggregation level and the relevant search space candidates. However, the network element may also be configured to select a lowest acceptable aggregation level for transmission whilst the aggregation level actually selected being not being the identified lowest level, for example for operational reasons. No single best candidate may be found but a list of potential ones which are selected by excluding those whose code rate would be too high.

A minimal aggregation level can be defined based on the effective enhanced CCE (eCCE) size. Calculation of the code rate of the ePDCCH can be based on the available eCCE size. In the calculations the effect of legacy signals such as CRS, PDCCH, CSI-RS, and so on can be removed.

In accordance with an embodiment according to Figure 5 definition of search space(s) can also be performed at a mobile device receiving the control information. Instead of the DCI selection step 40 of Figure 4, the mobile device can be adapted to consider different potential DCI formats separately to define code rate restricted search spaces for each potential DCI format, it shall be appreciated that the format definition shall be understood as covering definition of the DC! element size. More particularly, a mobile device can determine at 50 at least one potential downlink control information format for blind decoding. A control channel element size is determined at 51 and available code rate for the at least one downlink control information format is determined at 52 based on the control channel element size. At 53 it is evaluated whether said code rate is acceptable for communication of control information. When the code rate is found acceptable, at least one search space candidate is defined based on said control channel element size and code rate at 54. When said code rate is found unacceptable, the iteration progress to next level at 55 to evaluate whether a code rate determined on a higher aggregation level is acceptable. As with the network element, it is possible to have an operation where, If the highest possible aggregation level is not reached in an iteration round, all higher supported aggregation levels can be automatically included in the search space as potential search space candidates. Thus, if an acceptable code rate is achieved for a lower aggregation level thus can be used for higher aggregation level automatically as well.

For each potential DCI size the mobile device can evaluate if it should try to blind decode a search space candidate or not. If the code rate is not acceptable, for example exceed a predefined maximum, the mobile device will not include the search space candidate in the code-rate restricted search space and therefore will not even try blind decoding for that specific DCI format / size and initial search space candidate. This can reduce the total amount of blind decoding performed by the mobile device.

In accordance with a more detailed embodiment an evolved physical downlink control channel (ePDCCH) is used for scheduling a physical downlink shared channel (PDSCH). ePDCCH is a channel that has been designed to improve control channel performance. ePDCCH may be in particular useful in connection with capacity and/or performance enhancing features such as coordinated multipoint (CoMP), DL MIMO, heterogeneous networks (HetNet) and carrier aggregation, including use of extension carriers. For example, ePDCCH may be used to provide support for increased control channel capacity, support for frequency-domain interference control and interference coordination (ICIC), improved spatial reuse of control channel resources, support for beamforming and/or diversity, support for operation on new carrier types and in Multicast Broadcast Single Frequency Network (MBSFN) subframes, capability to coexist on the same carrier as legacy user equipment, ability to be scheduled frequency-selectively, ability to mitigate inter-cell interference and so on.

Below certain examples for dynamically building of code-rate restricted search space based on an enhanced CCE (eCCE) size available in a current transmission time interval (TTI) are explained in greater detail. In accordance with an embodiment a search space is adapted so that an eNB can always have a set of reasonable code rates available for transmitting a certain DCI. This may be provided by calculating the effective code rate of a single CCE allocation. A maximum acceptable value for the code rate can defined. For example, it can be determined whether the calculated code rate is at most ¾. If the calculated code rate is determined to satisfy the predefined criteria, the selected single CCE can be taken as a starting point for defining search space candidate(s) and allocations with the possibility of adding higher aggregation levels. The adapted search space by removing candidates with a too high code rate can be used to decrease the amount of unnecessary blind decoding at a mobile device. The maximum value can be provided at the relevant device in various manners. According to a possibility the value is fixed in the relevant standard specifications and the device is configured to use the standardized value. The parameter may also be communicated by higher layers signalling between the devices, for example an eNB and user equipment. In the platter option the value of the parameter can be varies and optimised e.g. to further reduce the maximum code rate.

In accordance with an embodiment the eCCEs are designed to be of or almost equal size. If there are eCCEs of different sizes the code rate of each individual allocation may need to be generated and evaluated.

For example, aggregation levels 2, 4, 8, 16 and so on could be added. In case the single CCE allocation does not give a useful code rate, code rate with aggregation level 2 can be checked and a search built from this aggregation level and upwards. If aggregation level 2 code rate is not acceptable either then the process can go on to aggregation level 4 and so on until a proper search space candidate is found. If a search space candidate of a certain aggregation level is found, all search space candidates of higher aggregation levels can be automatically made possible as well.

CCE sizes may change from transmission time interval (TTI) to TTI. The size may change depending for example on the Physical Control Format Indicator Channel (PCFICH) value, the starting symbol for ePDCCH transmission, presence of channei state information reference signal (CSl-RS), special sub frame lengths such as used for Downlink Pilot Timeslot (DwPTS) in time division duplex (TDD) frame structure, MBSFN sub frames, channel state information reference signal (CSl-RS) presence with zero and non-zero power, and so on. As the CCE sizes may change from transmission time interval (TTI) to TTI then the search space candidates based on the selected downlink control channel message (i.e. DCI) needs to be potentially rebuilt every TTI. Comparing to PDCCH design where the region size is changing dynamically the re-evaluation of search space is not expected to add substantial additional complexity. In accordance with a possibility a terminal device can simply select among a few predefined search space candidates for each potential DCI size depending on the actual CCE size and is therefore not required to try to blindly decode a DC! from all search space candidates resulting in insufficient code rate.

In accordance with a scenario eCCE size may vary between the different candidate eCCEs in one TT1. A rule can be defined for selection of an eCCE size to be used for code rate calculations. For example, a maximum code rate a user equipment can assume may be used. The smallest or largest eCCE may be used. One possible solution is to define a reference eCCE which could be the first complete candidate eCCE when counting from subcarrier 0. A yet other solution is to use the average CCE size of all the CCEs in a ePDCCH region assigned to a user equipment. It is also possible to have the evaluation for each potential eCCE in the search space. These are oniy examples and other options are also available. Regardless of the manner how the size is determined, each party ends to the communications should base the determination on the same principles.

Search spaces constructed in this way in general can include fewer aggregation levels and/or smaller amount of candidates for a specific aggregation level for the different DCI sizes than what was agreed for release 8 because the ones included are insured to be usable for the transmission.

DCI format specific search spaces may be created for each subframe depending on the effective size of the individual eCCE candidates. There can be different search spaces for uplink and downlink DCIs, as they vary in size. This can be implemented relatively easily since search for different DCI sizes at a user equipment is done independently and the eNB selecting only a single DCI format for a user equipment in a TTI. !t is expected that the ePDCCH region would be the same for different DCIs.

A selection between different modulation schemes may be used as a part of the space definition procedure. For example, selection between a higher order modulation scheme, such as 16 Quadrature Amplitude Modulation (16 QAM) and a lower order modulation scheme, such as Quadrature Phase Shift Keying (QPSK), can be a part of a search space generation process. Moving to a higher order scheme such as 16QAM divides the code rate by a factor 2. If 16 QAM is supported the smallest supported CCE size and/or aggregation level to consider can be reduced to half. An advantage of the embodiment is creation of an ePDCCH solution which adapts well to different configurations of legacy signals. Moreover reduced blind decoding at a mobile device may be expected as all code rates are useful.

The required data processing apparatus and functions of a control apparatus for the determinations and control of scheduling of transmission in subframes at a communication device, a base station and any other node or element may be provided by means of one or more data processors. The described functions may be provided by separate processors or by an integrated processor. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs), application specific integrated circuits (ASIC), gate level circuits and processors based on multi core processor architecture, as non limiting examples. The data processing may be distributed across several data processing modules. A data processor may be provided by means of, for example, at least one chip. Appropriate memory capacity can also be provided in the relevant devices. The memory or memories may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory.

An appropriately adapted computer program code product or products may be used for implementing the embodiments, when loaded or otherwise provided on an appropriate data processing apparatus, for example for selection of a CCE, determination of available (maximum) code rates, adaptive evaluation of acceptability of the code rates and for any related operations. The program code product for providing the operation may be stored on, provided and embodied by means of an appropriate carrier medium. An appropriate computer program can be embodied on a computer readable record medium. A possibility is to download the program code product via a data network. In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Embodiments of the inventions may thus be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

It is noted that whilst embodiments have been described in relation to LTE-Advanced, similar principles can be applied to any other communication system. Therefore, although certain embodiments were described above by way of example with reference to certain exemplifying architectures for wireless networks, technologies and standards, embodiments may be applied to any other suitable forms of communication systems than those illustrated and described herein.

The foregoing description has provided by way of exemplary and non- limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. For example, a combination of one or more of any of the other embodiments previously discussed can be provided. All such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims.

Claims

What is claimed is:

1. A method for adaptive search space definition for wireless communications, comprising

selecting at least one downlink control information format,

determining a control channel element size,

determining a code rate available for the at least one downlink control information format based on the control channel element size,

evaluating whether said code rate is acceptable for transmission of control information, and

when said code rate is acceptable, defining at least one search space candidate based on said control channel element size and code rate, and

when said code rate is unacceptable, evaluating whether a code rate determined on a higher aggregation level is acceptable.

2. A method for adaptive search space definition for wireless communications, comprising

determining at least one potential downlink control information format, determining a control channel element size,

evaluating whether said code rate is acceptable for communication of control information, and

3. A method as claimed in claim 2, comprising using the at least one potential downlink control information format for blind decoding at a mobile device.

4. A method as claimed in any preceding claim, wherein, when said code rate is acceptable for an aggregation, the method comprises including all candidates for higher aggregation levels in the definition of search space candidates.

5. A method as claimed in any preceding claim, wherein the evaluating comprises comparing the determined code rate to a predefined maximum value.

6. A method as claimed in claim 5, wherein the predefined maximum value is communicated on higher layer signalling and/or preconfigured in a device based on a standard specification.

7. A method as claimed in claim 5 or 6, wherein the predefined maximum value is ¾.

8. A method as claimed in any preceding claim, comprising progressing from aggregation level 1 to aggregation level 2 if the evaluation shows a non- acceptable code rate for aggregation level 1 , from aggregation level 2 to aggregation level 4 if a subsequent evaluation shows a non-acceptable code rate for aggregation level 2, and iteratively to a next available aggregation level until a maximum supported aggregation level is reached or an acceptable code rate is determined.

9. A method as claimed in any preceding claim, comprising performing the evaluation per each transmission time interval or per each control channel element.

10. A method as claimed in any preceding claim, wherein the determining of control channel element size comprises at least one of using a reference control channel element as the basis for evaluation, the reference control channel element comprising a predefined complete control channel element counted from a predefined subcarrier, using the smallest or largest control channel element as the basis for evaluation, and using an average of control channel elements in the region of a control channel assigned to a receiving device as the basis for evaluation.

11. A method as claimed in any preceding claim, comprising defining downlink control information format specific search spaces.

12. A method as claimed in any preceding claim, comprising selecting between a higher and lower order modulation schemes.

13. A method as claimed in claim 11 , comprising reducing the smallest acceptable size of the control channel element or aggregation level by selecting a higher order modulation scheme.

14. A method as claimed in any preceding claim, wherein the control channel element comprises an enhanced control channel element to be transmitted on an enhanced physical downlink control channel.

15. An apparatus for controlling wireless communications, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to

determine a code rate available for at least one selected downlink control information format based on a determined control channel element size,

evaluate whether said code rate is acceptable for transmission of control information, and

when said code rate is acceptable, define at least one search space candidate based on said control channel element size and code rate, and

when said code rate is unacceptable, evaluate whether a code rate determined on a higher aggregation level is acceptable.

16. An apparatus for controlling wireless communications, the apparatus comprising at least one processor, and at least one memory including computer program code, wherein the at least one memory and the computer program code are configured, with the at least one processor, to

determine at least one potential downlink control information format, determine a control channel element size,

determine a code rate available for the at least one downlink control information format based on the control channel element size,

evaluate whether said code rate is acceptable for communication of control information, and

when said code rate is acceptable, define at least one search space candidate based on said control channel element size and code rate, and when said code rate is unacceptable, evaluate whether a code rate determined on a higher aggregation level is acceptable.

17. apparatus according to claim 15 or 16, configured to compare the determined code rate to a predefined maximum value.

18. An apparatus according to any of claims 15 to 17, configured to iteratively progress from aggregation level 1 to aggregation level 2 if the evaluation shows a non-acceptabie code rate for aggregation level 1 , from aggregation level 2 to aggregation level 4 if a subsequent evaluation shows a non-acceptable code rate for aggregation level 2, and to a next available aggregation level until a maximum supported aggregation level is reached or an acceptable code rate is determined.

19. An apparatus according to any of claims 15 to 18, configured to determine a reference control channel element based on a predefined complete control channel element counted from a predefined subcarrier, based on the smallest or largest control channel element, and/or based on an average of control channel elements in the region of a control channel assigned to a receiving device.

20. An apparatus according to any of claims 15 to 19, configured to select between a higher and lower order modulation schemes.

21. An apparatus according to any of claims 15 to 20, wherein the control channel element comprises an enhanced control channel element to be transmitted on an enhanced physical downlink control channel.

22. An apparatus according to claim 15 or any claim dependent on claim 15, configured to cause selection of the downlink control information format at a network element.

23. An enhanced NodeB comprising the apparatus as claimed in claim 22.

24. An apparatus according to claim 16 or any claim dependent on claim 16, configured to define potential search space candidates for blind detection by a mobile device.

25. A mobile device comprising the apparatus as claimed in claim 24.

26. A node for a communication system comprising the apparatus as claimed in any of claims 5 to 22 or 24.

27. A computer program comprising code means adapted to perform the steps of any of claims 1 to 13 when the program is run on a processor.