TECHNICAL FIELD

The disclosure relates to wireless communications, and in particular, to assignment of short Physical Downlink Control Channel (sPDCCH) candidates for short Transmission Time Interval (TTI) communications.

BACKGROUND

Latency Reduction with Short TTI Operation

Packet data latency is one of the performance metrics that vendors, operators and also end-users (via speed test applications) regularly measure. Latency measurements are done in all phases of a radio access network system lifetime, when verifying a new software release or system component, when deploying a system and when the system is in commercial operation.

Shorter latency than previous generations of Third Generation Partnership Project (3GPP, a standardization organization) Radio Access Technologies (RATs) was one performance metric that guided the design of Long Term Evolution (LTE). LTE is also recognized by the end-users to be a system that provides faster access to internet and lower data latencies than previous generations of mobile radio technologies.

Packet data latency is important not only for the perceived responsiveness of the communication system, but packet data latency is also a parameter that indirectly influences the throughput of the system. For example, Hypertext Transfer Protocol (HTTP)/Transmission Control Protocol (TCP) is one common application and transport layer protocol suite used on the internet. The typical size of HTTP based transactions over the internet are in the range of a few 10's of Kbyte up to 1 Mbyte. In this size range, the TCP slow start period is a significant part of the total transport period of the packet stream. During TCP slow start, the performance is latency limited. Hence, improved latency can be showed to improve the average throughput, for this type of TCP based data transactions.

Radio resource efficiency could be positively impacted by latency reductions. Lower packet data latency could increase the number of transmissions possible within a certain delay bound, hence higher Block Error Rate (BLER) targets could be used for the data transmissions, thereby freeing up radio resources to potentially improve the capacity of the communication system.

One area of packet latency reduction relates to the reduction of transport time of data and control signaling, within the context of the length of a transmission time interval (TTI). In Long Term Evolution (LTE) release 8 (Rel-8), a TTI corresponds to one subframe (SF) of length 1 millisecond. One such 1 ms TTI is constructed by using 14 Orthogonal Frequency Division Multiplexing (OFDM) or single-carrier frequency-division multiple access (SC-FDMA) symbols in the case of normal cyclic prefix and 12 OFDM or SC-FDMA symbols in the case of extended cyclic prefix.

Work in 3GPP is ongoing on standardizing “short TTI” or “sTTI” operation, where scheduling and transmission can be performed on a faster timescale. Therefore, the legacy LTE subframe is subdivided into several sTTI. Supported lengths for sTTI of 2, 4 and 7 OFDM symbols are currently discussed. Data transmission in downlink (DL), i.e., from the network node such as a base station to the wireless device, may happen per sTTI via the short Physical Downlink Shared Channel (sPDSCH), which may include a control region short Physical Downlink Control Channel (sPDCCH). In the uplink (UL), i.e., from the wireless device to the network node, data is transmitted per sTTI via a short Physical Uplink Shared Channel (sPUSCH), and control information can be transmitted via a short Physical Uplink Control Channel (sPUCCH).

Scheduling sTTI

Different alternatives are possible to schedule a sTTI in the UL or the DL to a wireless device. In one alternative, individual wireless devices receive information about sPDCCH candidates for short TTI via RRC configuration, configuring wireless devices where to look for the control channel for short TTI, i.e., sPDCCH. The downlink control information (DCI) for sTTI is included directly in the sPDCCH. In another alternative, the DCI for sTTI is split into two parts, a slow DCI is sent in the Physical Downlink Control Channel (PDCCH) and a fast DCI is sent in the sPDCCH. The slow grant can contain the frequency allocation for a DL and an UL short TTI band to be used for short TTI operation. The slow grant can also contain refinement about sPDCCH candidate locations.

Control Signaling in LTE

3GPP LTE technology is a mobile broadband wireless communication technology in which transmissions from base stations (also referred to as network nodes) to mobile stations (also referred to as wireless devices) are sent using orthogonal frequency division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in the frequency domain. The basic unit of transmission in LTE is a resource block (RB) which in its most common configuration consists of 12 subcarriers and 7 OFDM symbols (one slot) in the case of normal cyclic prefix. In the case of extended cyclic prefix, a RB consists of 6 OFDM symbols in the time domain. A common term is also a physical resource block (PRB) to indicate the RB in the physical resource. Two PRBs in the same subframe that use the same 12 subcarriers are denoted a PRB pair. This is the minimum resource unit that can be scheduled in LTE.

A unit of one subcarrier and 1 OFDM symbol is referred to as a resource element (RE) as illustrated inFIG. 1. Thus, a PRB consists of 84 REs. An LTE radio subframe is composed of multiple resource blocks in frequency with the number of PRBs determining the bandwidth of the system and two slots in time as illustrated inFIG. 2.

In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms.

Messages transmitted over the radio link to wireless devices can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system as well as proper operation of each wireless device within the system. Control messages can include commands to control functions such as the transmitted power from a wireless device, signaling of RBs within which the data is to be received by the wireless device or transmitted from the wireless device and so on.

In Rel-8, the first one to four OFDM symbols, depending on the configuration, in a subframe are reserved to contain such control information, as illustrated inFIG. 2. Furthermore, in 3GPP release 11 (Rel-11), an enhanced control channel (EPDCCH) was introduced, in which PRB pairs are reserved to exclusively contain EPDCCH transmissions, although excluding from the PRB pair the one to four first symbols that may contain control information to wireless devices of releases earlier than Rel-11, as illustrated inFIG. 3, where a downlink subframe is illustrated having 10 RB pairs and configuration of three EPDCCH regions (Regions 1-3) of size 1 PRB pair each. The remaining PRB pairs can be used for PDSCH transmissions.

Hence, the EPDCCH is frequency multiplexed with PDSCH transmissions, contrary to PDCCH which is time multiplexed with PDSCH transmissions. The resource allocation (RA) for PDSCH transmissions exists in several RA types, depending on the downlink control information (DCI) format. Some RA types have a minimum scheduling granularity of a resource block group (RBG). An RBG is a set of adjacent (in frequency) resource blocks and when scheduling the wireless device, the wireless device is allocated resources in terms of RBGs and not individual RBs.

When a wireless device is scheduled in the downlink from an EPDCCH, the wireless device assumes that the PRB pairs carrying the DL assignment are excluded from the resource allocation, i.e., rate matching applies. For example, if a wireless device is scheduled PDSCH in a certain RBG of size 3 adjacent PRB pairs, and one of these PRB pairs contain the DL assignment, the wireless device may assume that the PDSCH is only transmitted in the two remaining PRB pairs in this RBG. Note also that multiplexing of PDSCH and any EPDCCH transmission within a PRB pair is not supported in Rel-11.

The PDCCHs and EPDCCHs are transmitted over radio resources that are shared between several wireless devices. Each PDCCH consists of smaller parts, known as control channel elements (CCE), to enable link adaptation (by controlling the number of CCEs a PDCCH is utilizing). For PDCCH, a wireless device monitors 4 aggregation levels of CCEs, namely, 1, 2, 4, and 8, for wireless device-specific search space and 2 aggregation levels of CCEs, namely, 4 and 8, for common search space.

In 3GPP Technical Specification (TS) 36.213, Section 9.1.1, a search space Sk(L)at aggregation level L∈{1,2,4,8} is defined by a set of PDCCH candidates. For each serving cell on which PDCCH is monitored, the CCEs corresponding to PDCCH candidate m of the search space Sk(L)are given by
L{(Yk+m′)mod└NCCE,k/L┘}+i
where i=0, . . . , L−1. For the common search space m′=m. For the PDCCH wireless device specific search space, if the wireless device is configured with carrier indicator field then m′=m+M(L)*nCI, where nCIis the carrier indicator field value, otherwise m′=m, when m=0, . . . , M(L)−1. M(L)is the number of PDCCH candidates to monitor in the given search space. Each CCE contains 36 QPSK modulation symbols.

The value of M(L)is specified by Table 9.1.1-1 in 3GPP TS 36.213, as shown below in Table 1. For the common search spaces, Ykis set to 0 for L={4, 8}. For the wireless device-specific search space Sk(L)at aggregation level L, Ykis defined by Yk=(A*YK−1)mod D. YK−1=nRNTI≠0, A=39827, D=65537 and k=└ns/2┘, nsis the slot number within a radio subframe. The RNTI value used for nRNTIis defined in subclauses 7 and 8 in TS 36.213.

TABLE 1M(L)vs. Aggregation Level L for PDCCHNumber ofPDCCHSearch space Sk(L)candidatesTypeAggregation level LSize [in CCEs]M(L)UE-specific16621264828162Common41648162

PDCCH Processing

After channel coding, scrambling, modulation and interleaving of the control information, the modulated symbols are mapped to the resource elements in the control region. To multiplex multiple PDCCH onto the control region, control channel elements (CCE) has been defined, where each CCE maps to 36 resource elements. One PDCCH can, depending on the information payload size and the required level of channel coding protection, consist of 1, 2, 4 or 8 CCEs, and the number is denoted as the CCE aggregation level (AL). By choosing the aggregation level, link-adaptation of the PDCCH is obtained. In total, there are NCCECCEs available for all the PDCCH to be transmitted in the subframe, and the number NCCEvaries from subframe to subframe depending on the number of control symbols n and the number of antenna ports configured.

As NCCEvaries from subframe to subframe, the wireless device needs to blindly determine the position and the number of CCEs used for its PDCCH which can be a computationally intensive decoding task. Therefore, some restrictions in the number of possible blind decoding a wireless device needs to go through have been introduced. For instance, the CCEs are numbered, and CCE aggregation levels of size K can only start on CCE numbers evenly divisible by K, as illustrated inFIG. 4.

The set of candidate control channels formed by CCEs where a wireless device needs to blindly decode and search for a valid PDCCH are called search spaces. This is the set of CCEs of an aggregation level (AL) that a terminal should monitor for scheduling assignments or other control information, as illustrated in the example ofFIG. 5. InFIG. 5, the search space that a certain wireless device is to monitor is indicated by the darker hatching while the common search space of NCCE=15 CCEs is indicated by the lighter hatching. In each subframe and on each AL, a wireless device will attempt to decode all the PDCCHs that can be formed from the CCEs in its search space. If the CRC checks, then the content of the PDCCH is assumed to be valid for the terminal and it further processes the received information. Often two or more terminals have overlapping search spaces and the network has to select one of them for scheduling of the control channel. When this happens, the non-scheduled wireless device is “blocked”. The search spaces vary pseudo-randomly from subframe to subframe to minimize this blocking probability.

A search space is further divided to a common and a wireless device specific part. In the common search space, the PDCCH containing information to all or a group of wireless devices is transmitted (paging, system information, etc.). If carrier aggregation is used, a wireless device will find the common search space present on the primary component carrier (PCC) only. The common search space is restricted to aggregation levels 4 and 8 to give sufficient channel code protection for all terminals in the cell (since it is a broadcast channel, link adaptation cannot be used). The m8 and m4 first PDCCH (with lowest CCE number) in an AL of 8 or 4 respectively belong to the common search space. For efficient use of the CCEs in the system, the remaining search space is wireless device specific at each aggregation level.

A CCE consists of 36 Quadrature Phase Shift Keying (QPSK) modulated symbols that map to the 36 REs unique for this CCE. To maximize the diversity and interference randomization, interleaving of all the CCEs is used before a cell specific cyclic shift and mapping to REs, the steps of this process are in illustrated in the flow diagram ofFIG. 6. For example, a network node structures all PDCCH into CCE (Block S100). The network node scrambles and modulates (Block S102). The network node optionally performs layer mapping and transmit diversity (Block S104). The network node performs quadruplex based interleaving (Block S106). The network node performs a cyclic shift based on the Cell ID (Block S108). The network node performs mapping to the resource element group (REG) (Block S110).

Note that in most cases some CCEs are empty due to the PDCCH location restriction to wireless device search spaces and aggregation levels. The empty CCEs are included in the interleaving process and mapping to RE as any other PDCCH to maintain the search space structure. Empty CCE are set to zero power and this power can instead be used by non-empty CCEs to further enhance the PDCCH transmission.

Furthermore, to enable the use of 4 antenna TX diversity, a group of 4 adjacent QPSK symbols in a CCE is mapped to 4 adjacent RE, denoted a RE group (REG). Hence, the CCE interleaving is quadruplex (group of 4) based and mapping process has a granularity of 1 REG and one CCE corresponds to 9 REGs (=36 RE).

There will also be a general collection of REG that remains after the set of size NCCECCEs has been determined (although the remaining REGs are fewer than 36 RE) since the number of REGs available for PDCCH in the system bandwidth is in general not an even multiple of 9 REGs. These remaining REGs are in LTE unused by the communication system.

EPDCCH Details

Similar as for PDCCH, the EPDCCH is transmitted over radio resources shared by multiple wireless devices and enhanced CCE (eCCE) is introduced as the equivalent to CCE for PDCCH. An eCCE has also a fixed number of RE but the number of RE available for EPDCCH mapping is generally fewer than this fixed number because many RE are occupied by other signals such as CRS and CSI-RS. Code chain rate matching is applied whenever a RE belonging to a eCCE contains other colliding signals such as the CRS, CSI-RS, legacy control region or in case of Time Division Duplex (TDD), the Guard Period (GP) and Uplink Pilot Time Slot (UpPTS) 3GPP TS 36.211.

Consider the example inFIGS. 7aand 7bthat illustrate the difference between CCE (a) and eCCE (b), where (a) illustrates the PDCCH mapping, which avoids the CRS so that a CCE consist of Tavail=36 available RE. In (b), there is shown how the eCCE consists of 36 REs nominally, but the number of available REs is less in case there are colliding signals, hence Tavail≤36 RE for EPDCCH. Since the colliding signals is subframe dependent, the value of Tavailbecomes subframe dependent as well and could even be different for different eCCEs if the collisions impact on the eCCEs unevenly.

When the number of eCCE per PRB pair is 2 the nominal number of RE per eCCE is not 36 but instead 72 or 64 for normal and extended CP length respectively.

In 3GPP Rel-11, the EPDCCH supports wireless device specific search space whereas the common search space remains to be monitored in the PDCCH in the same subframe. In future 3GPP TS releases, the common search space may be introduced also for EPDCCH transmission. It is specified that the wireless device monitors eCCE aggregation levels 1, 2, 4, 8, 16 and 32 with restrictions shown.

In distributed transmissions, an EPDCCH is mapped to resource elements in up to D PRB pairs, where D=2, 4 or 8 (the value of D=16 is also being considered in 3GPP). In this manner, frequency diversity be achieved for the EPDCCH message.FIG. 8illustrates a schematic example in which downlink subframe showing 4 parts belonging to an EPDCCH is mapped to multiple of the enhanced control regions known as PRB pairs, to achieve distributed transmission and frequency diversity or subband precoding.

In localized transmission, an EPDCCH is mapped to one PRB pair only, if the space allows (which is always possible for aggregation level one and two and for normal subframes and normal CP length also for level four). In case the aggregation level of the EPDCCH is too large, a second PRB pair is used as well, and so on, using more PRB pairs, until all eCCEs belonging to the EPDCCH has been mapped.FIG. 9is an illustration of localized transmission in which the downlink subframe showing the 4 eCCEs belonging to an EPDCCH is mapped to one of the enhanced control regions, to achieve localized transmission.

As an example, in normal subframe and with normal CP length and with nEPDCCH≥104, localized transmission is using aggregation levels (1,2,4,8) and they are mapped to (1,1,1,2) PRB pairs respectively.

To facilitate the mapping of eCCEs to physical resources each PRB pair is divided into 16 enhanced resource element groups (eREGs) and each eCCE is split into 4 or 8 eREGs for normal and extended cyclic prefix, respectively. An EPDCCH is consequently mapped to a multiple of four or eight eREGs depending on the aggregation level.

These eREG belonging to an ePDCCH resides in either a single PRB pair (as is typical for localized transmission) or a multiple of PRB pairs (as is typical for distributed transmission).

The exact division of a PRB pair into eREG.

Assignment of EPDCCH candidates

A wireless device is configured by higher layers with one or two EPDCCH-PRB-sets for EPDCCH monitoring, as described in 3GPP TS 36.213. Each EPDCCH-PRB-set consists of a set of ECCEs numbered from 0 to NECCE,p,k−1, where NECCE,p,kis the number of ECCEs in EPDCCH-PRB-set p of subframe k.

The wireless device monitors a set of EPDCCH candidates, that is, attempting to decode each of the possible EPDCCHs, at different aggregation levels, within the set. The set of EPDCCH candidates to monitor are defined in terms of EPDCCH wireless device-specific search spaces.

The ECCEs corresponding to an EPDCCH candidate m of the wireless device-specific search space are given by the following formulation:

L⁢{(Yp,k+⌊m·NECCE,p,kL·Mp(L)⌋+b)⁢mod⁢⌊NECCE,p,k/L⌋}+i
where Yp,krefers to wireless device Radio Network Temporary Identifier (RNTI) based offset, L is aggregation level, i=0, . . . , L−1, b is equal to the carrier indicator field value (if the wireless device is configured with it, otherwise b=0), m=0,1, . . . , Mp(L)−1, and Mp(L)is the number of EPDCCH candidates to monitor at aggregation level L in EPDCCH-PRB-set p.

sPDCCH for sTTI

In order to quickly schedule low latency data on the short TTIs, a new short PDCCH (sPDCCH) needs to be defined. Since the short TTI operation is desired to co-exist with legacy TTI operation, the sPDCCH should be placed in-band within PDSCH, still leaving resources for legacy data.

Legacy control channels PDCCH and EPDCCH use Cell Specific Reference Signal (CRS) and Demodulation Reference Signal (DMRS) demodulation, respectively. For operation in both these environments, an sPDCCH should support both CRS and DMRS, and to maintain efficiently, resources not used by sPDCCH should be used by sPDSCH (short PDSCH).

To facilitate the definition of the sPDCCH mapping to resource elements special entities are defined: sREG and sCCE. This follows the methodology used so far in the LTE specifications, i.e., 3GPP TSs, for defining PDCCH and ePDCCH, as described above. The definition of the same mapping can also be done without using these terms or by using equivalent terms.

sREG configuration

The candidate lengths for sPDCCH in time domain are 1 or 2 Orthogonal frequency-division multiplexing (OFDM) symbols for sTTI operation. The REs of a PRB in a given OFDM symbol of the sTTI can build one or more sREG. The number of REs in a sREG may also be variable in order to provide allocation flexibility and to support good frequency diversity.

The sREG configuration for sPDCCH is defined as the complete number of REs in a PRB within 1 OFDM symbol (i.e., 12 REs per sREG in 1 OFDM symbol). These sREG configurations are depicted inFIG. 10where sREG configuration is based on 12 REs within 1 OFDM for 1 OFDM symbol sPDCCH (left), 2 OFDM symbol sPDCCH (center) and 2 OFDM symbol sPDCCH (right), considering 1 OFDM symbol sPDCCH, 2 OFDM symbol sPDCCH and 3 OFDM symbol sPDCCH. Each index, i.e. {0, 1, 2}, represents an sREG group.

sCCE Configuration

The number of sREG required to build up a sCCE for a given sPDCCH can vary as well as their placement scheme along the frequency resources used for sTTI operation. One option is to define a sCCE to be composed ideally by 36 REs like an eCCE or a CCE. For that, and based onFIG. 10, an sCCE is composed by three sREG, i.e. 1 sCCE=3 sREG.

For DMRS-based sPDCCH, a further option to consider in order to increase the number of REs available within 2 OFDM symbols SPDCCH is that an sCCE is defined to be composed by 48 REs instead of 36 REs, i.e. 1 sCCE=4 sREG. The 12 additional REs help compensating the DMRS overhead compared to CRS based sPDCCH.

In order to support good frequency diversity, or a more localized placement, localized and distributed placement schemes of sREG building up the same sCCE are defined:Localized scheme: sREGs building the same sCCE can be localized in frequency domain to allow for a sPDCCH resource allocation confined in a limited frequency band. This facilitates the use of beamforming for DMRS based sPDCCH.Distributed scheme: A distributed sREG location can be used to allow frequency diversity gains. In this case, multiple UEs may have the sREG of their sPDCCH mapped to the same PRB on different REs. Distributing over a wide frequency range also more easily makes the sPDCCH fit into one single OFDM symbol. For wireless devices with DMRS based demodulation, wireless device-specific beamforming is not recommended with distributed sCCE locations.

These schemes, which are described below for building sCCE based on 1 OFDM symbol sPDCCH, 2 OFDM symbol sPDCCH and 3 OFDM symbol sPDCCH, can be used for CRS and DMRS transmissions.

Likewise, this takes into account the following considerations:CRS and DMRS wireless devices can coexist on the same sTTI, since sPDCCH design is the same.If both CRS and DMRS users are given DCI in the same PRB, CRS wireless devices need to be indicated with this. Then the wireless devices know that some REs are not used for sCCE. Otherwise, CRS and DMRS users have to be sent DCI in different PRBs.

Configuration of PRBs that can be Used for sPDCCH

At least one set of PRBs that can be used for sPDCCH is configured per wireless device. It has been recommended to support the configuration of several sets of PRBs used for sPDCCH in order to configure one set of PRBs following the localized sPDCCH mapping and another set with the distributed mapping. The wireless device would monitor both sets and the network node could select the most favorable configuration/PRB set for a given sTTI and wireless device.

The set of PRBs assigned for the sPDCCH, which includes PRBs (not necessarily consecutive) from the available sTTI band, may be configured via RRC signaling. However, the set may include a potential resource allocation refinement in the slow DCI transmitted in PDCCH, e.g., a reduced set of PRBs or a specific set in case that several sPDCCH sets were defined.

The set of PRBs may be configured independently, e.g., as a PRB bitmap. The set can also be configured based on groups of PRBs. One example of already defined group of PRBs in LTE is called RBG and can be used as basis in the proposed sPDCCH mapping. Then all PRBs within the same PRB group, e.g. RBG, are jointly used.

The PRBs or groups of PRBs included in the configured PRB set may be ordered according to a sequence signaled to the wireless device before mapping the sPDCCH to them.

sPDCCH Configuration

1 OFDM symbol sPDCCH is defined for CRS based transmissions due to the advantage of early decoding for 2 OFDM symbol sTTI and slot TTI. 2 OFDM symbol sPDCCH can also be configured for both 2 OFDM symbol sTTI and slot TTI as an alternative to allow a small sTTI band, i.e., to limit the number of frequency resources used for sTTI operation.

For DMRS based transmissions with 2 OFDM symbol sTTI, assuming a design based on DMRS pairs in time domain as in legacy LTE, a 2 OFDM symbol sPDCCH is defined, since wireless devices need anyway to wait for the end of sTTI for channel estimation. In that case, DMRS is thus not shared between sPDCCH and sPDSCH in a given PRB of the sTTI. This gives more freedom for applying beamforming for sPDCCH. Furthermore, for some sTTI in a subframe, the TTI length is 3 symbols instead of 2 symbols. To allow beamforming flexibility, a 3 symbol sPDCCH can be considered for the 3-symbol long sTTI.

For DMRS with 1-slot sTTI, a 2 symbols sPDCCH is suitable. One DMRS pair for 1-slot TTI is preferred to be able to do channel estimation for sPDCCH and early sPDCCH decoding. Likewise, 3 OFDM symbol sPDCCH is also suitable for 1-slot TTI for those cases with only few REs available within the first 2 symbols in the slot due to reference signals and other kind of overhead.

Thereby, considering the presence of potential reference signals in a sTTI such as DMRS, CRS or CSI-RS, those REs occupied by these signals within a PRB are not used for a given sREG.

Consider the example where sPDCCH spans only the first OFDM symbol of a 2 symbol sTTI and that an sCCE is composed of 36 REs like an ECCE or a CCE. In this case, 3 PRBs are needed to build a sCCE (i.e., 3 sREG). These 3 PRBs can be distributed over the sPDCCH-PRB-set or can be localized as three consecutive PRBs. InFIG. 11, an example of distributed and localized configurations are depicted for 4 sCCEs composed of 3 sREGs each with a los-sPDCCH-PRB-set, and 1 OFDM symbol sPDCCH (the unused PRBs shown inFIG. 9can be further assigned for building other sCCE as well as the possibility to be used for sPDSCH allocation).

SUMMARY

There may be a need to define sPDCCH transmission characteristics and to decode sPDCCH, respectively, in an efficient, easy and flexible manner.

Some embodiments advantageously provide a method and system for assignment of short Physical Downlink Control Channel (sPDCCH) candidates for short Transmission Time Interval (sTTI).

In one embodiment, a method, i.e., first method, for the assignment of sPDCCH candidates of the wireless device-specific search space for sTTI operation is provided. Furthermore, a method to reduce the number of blind decodes to be performed by a wireless device is provided.

In one or more embodiments, a method, i.e., a second method, is provided for assigning sPDCCH candidates at different aggregation levels for a wireless device within a configured search space region. This method can be based on the specified definition for EPDCCH or the one for PDCCH. This method also allows sharing the same search space region among different wireless devices. Furthermore, the second method is defined in order to limit the total number of blind decodes to be performed by the wireless devices per each sTTI. Based on the second method, a wireless device can skip determining again the sPDCCH candidates found by the first method. This leads to facilitate wireless device's implementation and capabilities. Besides, the proposed methods provide full flexibility to the network node for the transmission of sPDCCH. This is achieved by leaving the freedom to the network node to make the wireless device search space of different wireless devices overlap or not overlap.

According to one aspect of the disclosure, a network node for configuration of a downlink control channel for a short transmission time interval, sTTI, in a communication network is provided. The network node includes processing circuitry configured to assign a search space region to be monitored by a first wireless device in the communication network where the search space region includes a plurality of downlink control channel candidates associated with a plurality of aggregation levels. Each of the plurality of downlink control channel candidates have at least one corresponding short Control Channel Element, sCCE. The processing circuitry is further configured to transmit, to the first wireless device, a bitmap indicating at least one of the plurality of downlink control channel candidates for at least one aggregation level to monitor.

According to one embodiment of this aspect, the bitmap includes a plurality of groups of bits where each group of bits corresponding to a respective one of the plurality of aggregation levels. According to one embodiment of this aspect, each group of bits indicates sCCEs for an aggregation level to monitor. According to one embodiment of this aspect, the plurality of aggregation levels include at least aggregation levels 1, 2 and 4. According to one or more embodiments, the bitmap includes a plurality of bits where each bit corresponds to a respective one of the plurality of aggregation levels.

According to one embodiment of this aspect, the bitmap indicates to monitor a subset of configured sCCEs for at least one of the plurality of aggregation levels. According to one embodiment of this aspect, the bitmap is transmitted via Radio Resource Control, RRC, signaling. According to one embodiment of this aspect, each downlink control channel candidate is a short Physical Downlink Control Channel, sPDCCH, candidate. According to one embodiment of this aspect, the bitmap corresponds to a set of physical resource block, PRB, pairs to which the search space is mapped where the set of PRB pairs enables localized or distributed mapping of the search space to the set of PRB pairs.

According to one embodiment of this aspect, the plurality of sCCE are consecutive sCCEs. According to one embodiment of this aspect, the processing circuitry is further configured to assign another search space region to be monitored by a second wireless device in the communication network, and transmit, to the second wireless device, another bitmap indicating at least one of the plurality of downlink control channel candidates for at least one aggregation level to monitor. According to one embodiment of this aspect, the at least one of the downlink control channel candidates for the second wireless device to monitor partially overlaps with the at least one of the downlink control channel candidates for the first wireless device to monitor. According to one or more embodiments, the bitmap is a bitmap field.

According to another aspect of the disclosure, a method for a network node for configuration of a downlink control channel for a short transmission time interval, sTTI, in a communication network is provided. A search space region to be monitored by a first wireless device in the communication network is assigned. The search space region includes a plurality of downlink control channel candidates associated with a plurality of aggregation levels where each of the plurality of downlink control channel candidates having at least one corresponding short Control Channel Element, sCCE. A bitmap indicating at least one of the plurality of downlink control channel candidates for at least one aggregation level to monitor is transmitted to the first wireless device.

According to one embodiment of this aspect, the bitmap includes a plurality of groups of bits, each group of bits corresponding to a respective one of the plurality of aggregation levels. According to one embodiment of this aspect, each group of bits indicates sCCEs for an aggregation level to monitor. According to one embodiment of this aspect, the plurality of aggregation levels include at least aggregation levels 1, 2 and 4. According to one or more embodiments, the bitmap includes a plurality of bits where each bit corresponds to a respective one of the plurality of aggregation levels.

According to one embodiment of this aspect, the bitmap indicates to monitor a subset of configured sCCEs for at least one of the plurality of aggregation levels. According to one embodiment of this aspect, the bitmap is transmitted via Radio Resource Control, RRC, signaling. According to one embodiment of this aspect, each downlink control channel candidate is a short Physical Downlink Control Channel, sPDCCH, candidate. According to one embodiment of this aspect, the bitmap corresponds to a set of physical resource block, PRB, pairs to which the search space is mapped where the set of PRB pairs enabling localized or distributed mapping of the search space to the set of PRB pairs.

According to one embodiment of this aspect, the plurality of sCCE are consecutive sCCEs. According to one embodiment of this aspect, another search space region to be monitored by a second wireless device in the communication network is assigned. Another bitmap indicating at least one of the plurality of downlink control channel candidates for at least one aggregation level to monitor is transmitted to the second wireless device. According to one embodiment of this aspect, the at least one of the downlink control channel candidates for the second wireless device to monitor partially overlaps with the at least one of the downlink control channel candidates for the first wireless device to monitor. According to one or more embodiment, the bitmap is a bitmap field.

According to another aspect of the disclosure, a wireless device for configuration of a downlink control channel for a short transmission time interval, sTTI, in a communication network is provided. The wireless device includes processing circuitry configured to receive a bitmap corresponds to an assigned search space region to be monitored by the wireless device in the communication network where the search space region including a plurality of downlink control channel candidates associated with a plurality of aggregation levels where each of the plurality of downlink control channel candidates having at least one corresponding short Control Channel Element, sCCE. The processing circuitry is further configured to monitor the search space region based on the bitmap, the bitmap indicating at least one of a plurality of downlink control channel candidates for at least one aggregation level to monitor.

According to one embodiment of this aspect, the bitmap includes a plurality of groups of bits, each group of bits corresponding to a respective one of the plurality of aggregation levels. According to one embodiment of this aspect, each group of bits indicates sCCEs for an aggregation level to monitor. According to one embodiment of this aspect, the plurality of aggregation levels include at least aggregation levels 1, 2 and 4. According to one embodiment of this aspect, the bitmap indicates to monitor a subset of configured sCCEs for at least one of the plurality of aggregation levels. According to one or more embodiments, the bitmap includes a plurality of bits where each bit corresponds to a respective one of the plurality of aggregation levels.

According to one embodiment of this aspect, the bitmap is transmitted via Radio Resource Control, RRC, signaling. According to one embodiment of this aspect, each downlink control channel candidate is a short Physical Downlink Control Channel, sPDCCH, candidate. According to one embodiment of this aspect, the bitmap corresponds to a set of physical resource block, PRB, pairs to which the search space is mapped, the set of PRB pairs enabling localized or distributed mapping of the search space to the set of PRB pairs. According to one embodiment of this aspect, the plurality of sCCE are consecutive sCCEs. According to one or more embodiments, the bitmap is a bitmap field.

According to another aspect of the disclosure, a method for a wireless device for configuration of a downlink control channel for a short transmission time interval, sTTI, in a communication network is provided. A bitmap corresponding to an assigned search space region to be monitored by the wireless device in the communication network is received where the search space region including a plurality of downlink control channel candidates associated with a plurality of aggregation levels. Each of the plurality of downlink control channel candidates have at least one corresponding short Control Channel Element, sCCE. The search space region is monitored based on the bitmap where the bitmap indicating at least one of a plurality of downlink control channel candidates for at least one aggregation level to monitor.

According to one embodiment of this aspect, the bitmap includes a plurality of groups of bits, each group of bits corresponding to a respective one of the plurality of aggregation levels. According to one embodiment of this aspect, each group of bits indicates sCCEs for an aggregation level to monitor. According to one embodiment of this aspect, the plurality of aggregation levels include at least aggregation levels 1, 2 and 4. According to one or more embodiments, the bitmap includes a plurality of bits where each bit corresponds to a respective one of the plurality of aggregation levels.

According to one embodiment of this aspect, the bitmap indicates to monitor a subset of configured sCCEs for at least one of the plurality of aggregation levels. According to one embodiment of this aspect, the bitmap is transmitted via Radio Resource Control, RRC, signaling. According to one embodiment of this aspect, each downlink control channel candidate is a short Physical Downlink Control Channel, sPDCCH, candidate. According to one embodiment of this aspect, the bitmap corresponds to a set of physical resource block, PRB, pairs to which the search space is mapped, the set of PRB pairs enabling localized or distributed mapping of the search space to the set of PRB pairs. According to one embodiment of this aspect, the plurality of sCCE are consecutive sCCEs. According to one or more embodiments, the bitmap is a bitmap field.

