Technical Field

The present invention is related to methods for detecting a downlink control structure for carrier aggregation in a communications network.

Background Art

[0002]

Long Term Evolution (LTE) is a mobile network technology standard based on a 3 GPP standard. It is a set of enhancements to the Universal Mobile Telecommunications Systems (UIvlTS) and is designed to increase data rates for mobile wireless users, improve user throughput and make more efficient use of the radio ftrequency spectrum. LTE-Advanced is currently being standardized by the 3GPP as an enhancement of LTE.

[0003]
Fig. 1 shows the configuration of a heterogeneous LTE-Advanced or LTE Rel-10 mobile communication network 10. In the system 10, base stations 12,14, also known as evolved Node Bs (eNode B), support communication for multiple User Equipments (UEs) 16,18, for example mobile phones, laptops, personal digital assistants. The base stations 12,14 are fixed and each provide communication coverage for a particxdar geographical area. Base station 12 is a femto cell, which connects to the service provider's network via broadband, and provides coverage over component carriers CC#0 and CC#l. Base station 14 is a macro ceil which provides radio coverage over component carriers CC#0 and CC#l over different distances for each component carrier.

[0004]
In the downlink channel, from the base stations 12, 14 to the UEs 16, 18, the LTE standard uses Orlhogonal Frequency Division Multiplexing (OFDM). OFDM is a digital multi-carrier modulation method that uses "a large number of closely spaced orthogonal sub-carriers to carry data. Orthogonal Frequency Division Multiple Access (OFDMA) is employed as a multiplexing scheme in the LTE downlink. In OFDMA, individual UEs are allocated sub-carriers for a predetermined amount of time. This allows simultaneous data transmission from several users.

[0005]
The downlink chamiel supports physical channels, which convey information from higher layers in the LTE stack. Two physical downlink channels are the Physical Downlink Shared Channel (PDSCH), which, is used for data transmission and the Physical Downlink Control Channel (PDCCH), which is used for transmitting control information. Schediiling of downlink data reception (in PDSCH) or uplink data transmission in the Physical Uplink Shared Channel (PUSCH) to the UE is typically performed through downlink control signaling using the PDCCH.

Summary of Invention Technical Problem

[0006]
A major feature to be introduced for LTE-Advanced is carrier aggregation. Component carriers (CCs) that are contiguous or non-contigiious in frequency may be aggregated. AUE may be configured to aggregate a different number of CCs of possibly different bandwidths in the uplink (UL) and downlink (pL). Carrier aggregation is UE specific, each UE in the same cell may have a different configuration of carrier aggregation.

[0007]
Once a UE is configured with carrier aggregation, the UE is capable of simultaneously receiving or transmitting on all the CCs that are aggregated. Thus, the UE may be scheduled over multiple CCs simultaneously. The scheduling of downlink assignments and uplink grants for each CC may be via an additional carrier indicator field of 0-3 bits in DCI fonnat(s) for a single CC. In case of 0 bits, there is no carrier indicator.

[0008]
An example illustrating the carrier aggregation of 5 CCs and Hie corresponding carrier indicator index to CC index mapping for a PDCCH with the carrier indicator field in CC #2 is shown in Fig. 2 and 3, respectively.

[0009]
The use of a carrier indicator in the PDCCH is not without cost. The disadvantages of having a PDCCH carrier iodicator include:

increased complexity in PDCCH scheduling as the scheduling may.have,to be. performed jointly over multiple CCs, Increased payload size of up to 3 bits for DCI formats if tbe carrier indicator is sspiidtly signaled.

Potential increased numbei of blind decoding attempts per CC if UE is expected to blindly detect whether the non-2ero-bit carrier indicator field exists in a DCI format and if the CCs can have different bandwidth size.

[0010]
From the UE's point of view, the increase in blind decodinig attempts for a CC is undesirable due to the increased PDCCH processing latency and increased power consumption especially if the UE is required to perform the extra blind detections all the time but the benefit of PDCCH with configurable linkage is only limited to certain scenarios.

[0011]
It would therefore be desirable to provide a method for detecting carrier aggregation that minimizes the number of PDCCH blind decoding attempts required to be performed by the UE for each CC.

[0012]
The above discussion of badcground art is included to explain the context of the present invention. It is not to be taken as an admission that any of the documents or other material referred to was pubhshed, known or part of the common general knowledge at the priority date of any one of the claims of this specification.

Solution to Problem

[0013]
According to one aspect, the present invention provides a method for detecting a downlink control structure for carrier aggregation in a communications network in which data transmission is scheduled by a physical downlink control channel (PDCCH), the method including the steps of, at a UE: receiving higher layer signaling enabling carrier aggregation for the UE, and reading the PDCCHs of component carriers (CCs), -wdierein the downlink control information (DCI) in the PDCCHs of each CC is read according to one of aplvirality of predefined formats derived fi-om the higher layer signaling.

[0014]
The higher layer signaling allows the carrier aggregation to be turned on or off, and allows the number of blind decoding attempts as well as the payload sizes of DCI formats to be kept to minimum when cross-carrier control is not needed (depending on deployment scenario or network operator's prcfcmce). The higher-layer signaling is transmitted only to UEs with carrier aggregaaon c^aabiliry. Tbs defeult setting assumed by both the eNodeB and the UE before the higher layer signaling is sent is no cross-carrier control, i.e. all DCI formats are with zero-bit carrier indicator fields.

[0015]
As the PDCCH of each CC is read in accordance with a predetermined DCI format signaled to the UE, the UE power and latency budget for PDCCH processing per CC may be reduced.

[0016]
The higher layer signaling may indicate that a CC is a host CC that is able to transmit the PDCCHs of cHent CCs, ihe predefined format for the DCI in the PDCCHs for the host CC having a non-zero-bit carrier indication field.

[0017] Thus, PDCCHs containing carrier indicators are only transmitted on a subset of CCs (say K, where K=l M and M is the total number of CCs aggregated for the UE), called the host CCs.

[0018]
The higher layer signaling may indicate that a CC is a client CC that does not transmit the PDCCHs of other CCs, the predefined format for the DCI m the PDCCHs for the client CC having a zero-bit carrier indication field.

[0019]
The PDCCHs of client CCs can be transmitted on a host CC. It is possible for a CC to be a host CC as well as a client CC at the same time. In this case, the CC can transmit Lie PDCCHs of other CCs as well as having its own PDCCHs transmitted on other CCs.

[0020]
The higher layer signaling may indicate that the UE is not required to detect PDCCHs on a client CC. Instead, the PDCCHs for the client CC are transmitted in the host CC. The method may then fiirther include the step of selectively reading &e PDCCHs of CCs so that the UE does not detect PDCCHs on that client CC.

[0021]
A client CC can thus be configured such that all PDCCHs (with zero-bit carrier indicator) for a UE are not transmitted on the CC. Hence, the UE is not required to detect any PDCCH on-the-client CC. However, such configuration cannot be applied if the.client CC is also a host CC at the same time.

[0022]
Tbis configuration is beneficial for iieterogsacots -xr^orks deployment where the interference level of the client CCs can be so high that control channels cannot be reliably transmitted. As PDCCH detection is not required for the client CCs, power saving can be achieved at the UE.

[0023]
However, for other deployment scenarios such as homogenous networks where frequency diversity gain may be more important, the diversity gain can be harnessed by the UE also detecting PDCCHs on the client CCs.

[0024]
The higher layer signaling may not indicate thai a CC is a cHent CC or host CC, in this case, the CC may be taken to be a normal CC which is used to transmit all of its own PDCCHs and only its own PDCCHs, the predefined format for the DCI in the PDCCHs for the normal CC havmg a zero-bit carrier indication field.

[0025]
Thus, using higher layer signaling, an eNodeB can configure a CC to be one or more of the following types:

Host CC: The CC which can be used for transmission of the PDCCHs of client CC(s) and its own PDCCHs.

Client CC: The CC of which its PDCCHs can be transmitted on a host CC. The client CC can also be used to transmit its own PDCCHs if configured to do so.

Nomial CC: The CC which is used to -transmii all of is own PDCCHs and only its own PDCCHs (same as in LTE Rel-8).

[0026]
All PDCCHs transmitted on the host CCs always contain carrier indicators with non-zero bits, even for the PDCCHs that correspond to the host CCs. The actual number of bits for the carrier indicator field can be a fimction of the actual number of carriers aggregated for the UE (i.e. ceil(log2 M).). The PDCCHs transmitted on the chent CCs or the normal CCs do not contain carrier indicators with non-zero bits.

[0027]
The higher layer signaling may be used to configure CCs as host CCs, client CCs and normal CCs in a semi-static manner according to need or change in the radio channel characteristics in the network. For example,.for a heterogeneous network with.imcoordinated deployment of femto cells, the interference characteristics of each CC may change several times in a day.

[0028]
The higher-layer signaling may be UE-specific, as some UEs may not have carrier aggregation capability. Furthermore, for a heterogeneous network, the interference characteristics for each CC experienced by different UEs may be different As shown in Fig. 1, UE 16 and UE 18 clearly experience different radio characteristics for CC #0 and CC #1.

[0029]
The host CC may have the same CC bandwidth as a client CC fen- which they are transmitting PDCCHs. In this case, a CC which is a part of carrier aggregation has to have the same bandwidth size with at least one other CC within the carrier aggregation for it to be eligible as a candidate CC for a host or a client CC.

Advantageous Effects of Invention

[0030]
The advantage of the invention is that the number of PDCCH blind decoding attempts that the UE has to perform for the host CC does not double due to two different payioad sizes for the same DCI format as a result of the difference in CC bandwidth. In conjunction with using the predefined formats as described above, the number of blind decoding attempts required to be performed by the UE may be kept the same as that needed for a normal CC.

Brief Description of Drawings

[0031] [Fig. 1]

Fig. 1 is a diagram illustrating a configuration of a heterogeneous LTE-Advaaced or LIE Rel-10 mobile communication networL [Fig-S]

Fig. 2 is a schematic diagram of carrier aggregation of 5 CCs. [Fig.3]

Fig. 3 is a table providing examples of 3-bit carrier indicator fields. [Fig. 4A]

Fig. 4A is schematic diagrams of PDCCH-PDSCH linkage scenarios for two CCs. [Fig. 4B]

Fig. 4B is schematip diagrams pf PDCCH-PDSCH linkage scenarios for two CCs, [Fig.4C]

Fig. 4C is schematic diagrams of PDCCH-PDSCH linkage scenarios for two CCs.
[Fig. 5]

Fig. 5 is a flowcliart showing a method according to an embodiment of the invention.

Description of Embodiments

[0032]

Fig. 4A, Fig. 4B and Fig. 4C show possible PDCCH-PDSCH Imkage scenarios for an example of two CCs. In Fig.4A, the PDCCHs 20,22 are in the same CC 24,26 as the PDSCHs 28,30 that they schedule. In Fig. 4B, the PDCCHs 32,34 are ia a different component carrier 36,38 to the PDSCHs 42,40 that they schedule. In Fig. 4C, the PDCCHs 44,46 are both in a single component carrier 48, although the PDCCHs 44,46 schedule PDSCHs 50, 52 in component carriers 48, 54.

[0033]

According to the control structure used in the present invention, CCs 24 and 26 are normal CCs, CCs 36, 38 are both host and client CCs, CC 48 is a host CC and CC 54 is a clientt CC.

[0034]
"With reference to Fig. 5, according to an embodiment of the invention, the PDCCHs of CCs are read according to one of a plurality of predefined formats derived fix)m higher layer signaling between the eNodeB (for example base station 12) and UE (for example UE 16).

[0035]
At step 70 it is determined if the UE is required to detect DCI formats with non-2Bro-bir carrier indicator fields. The de&ult setting assumed by botii the eNodeB and UE is no carrier aggregation. Thus if no higher layer signaling has been received, at step 72, the UE assumes that the CC is a normal CC, and reads PDCCHs of the CC according to the predefined format of the DCI in the PDCCHs having a zero-bit cairira: indication field.

[0036]
However, if the UE receives higher layer signaling firom the eNodeB, such as a carrier_ind_config signal to switch on carrier aggregation, then at step 74, the UE determines if the downlink CC is a host CC. If the higher layer signaling fi-om the eNode B, such as a control_cc_config signal, indicates that a CC is a host CC, the UE reads the PDCCHs of the CC according to the predefined format of the DCI having a non-zero-bit carrier indication field at step 76.. The physical signal.processing and.procedure of PDCCHs detection can be.the same as the Rel-8 processing and procedure as specified in TS 36.211 and TS 36.213.

[0037]
If the higher layer signaling fiom the eNodeB, such as the control_cc_config signal, instead indicates that the CC is a client CC, the UE detennines that the downlink CC is a client CC at step 78. The UE tihen determines whether the UE is required to detect PDCCHs for that client CC at step 80 via higher layer signaling firom the eNodeB, such as a client_cc_config signal.

[0038]
If the UE is required to detect PDCCHs, at step 82, the UE reads the PDCCHs of the CC according to the predefined format of the DCI having a zero-bit carrier indication field. The physical signal processing and procedure of PDCCHs detection can be the same as the Rel-8 processing and procedure as specified in TS 36.211 and TS 36.213.

[0039]
If the UE is not required to detect PDCCHs on the client CC, then at step 84, the UE does not attempt to detect PDCCHs.

[0040]
If no control_cc_config signal is received by the UE (i.e, the higher layer signaling does not indicate that the CC is a host or cKent), the UE assumes that the downlink CC is a normal CC at step 86. The UE reads PDCCHs of the normal CC according to the predefined format of the DCI in the PDCCHs having a zero-bit carrier iadication field. The physical signal processing and procedure of PDCCHs detection can be the same as the Rel-8 processing and procedure as specified in TS 36.211 and TS 36.213.

[0041]
The UE can therefore detect the downlink control structure for carrier aggregation m a manner which minimizes the number of PDCCH blind decoding attempts reqmred to be performed by the UE to tihe same as tihe LTE Rel-8 requiremait (a maviTrmm of 44 times).

[0042]
It is to be understood that various alterations, additions and/or modifications may be made to the parts previously described without departing firom the ambit of the present invention, and that, in the light of the above teachings, the present invention may be implemented in software, firmware and/or hardware in a variety of manners as would be understood by the skilled person.

[0043]