WO2009118704A1

WO2009118704A1 - Method for communicating in a telecommunication network and primary station therefor

Info

Publication number: WO2009118704A1
Application number: PCT/IB2009/051262
Authority: WO
Inventors: Ludovicus M.G.M. Tolhuizen; Timothy J. Moulsley; Matthew P.J. Baker
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-03-26
Filing date: 2009-03-26
Publication date: 2009-10-01

Abstract

The present invention relates to a method for communicating between a primary station and a plurality of secondary stations, comprising the steps (a) at the primary station, signaling to each secondary station an indication of an allocated search space comprising at least two resource sets consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be allocated to the considered secondary station, (b) at each secondary station, receiving the control signal and searching in the corresponding allocated search space for a control message addressed to the considered secondary station, wherein at least one search space consists of non-contiguous resource sets.

Description

METHOD FOR COMMUNICATING IN A TELECOMMUNICATION NETWORK

AND PRIMARY STATION THEREFOR

FIELD OF THE INVENTION The present invention relates to a method for communicating between a primary station and a plurality of secondary stations.

This invention is, for example, relevant for telecommunication systems like a mobile telecommunication system. More specifically, this invention is relevant for the UMTS.

BACKGROUND OF THE INVENTION

In a conventional UMTS system, a PDCCH (Physical Downlink Control Channel) message can use 1 , 2, 4 or 8 Channel Control Elements (CCEs or resource elements) - referred to as CCE aggregation levels 1, 2, 4 or 8. A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads possible for that aggregation level. Search spaces are defined per aggregation level; a secondary station in such a system thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3,4,5,6,7,8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1,2,..8 and 9,10,...,16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs.

In the known embodiments, CCEs within a search space are contiguous, and resource sets of aggregated CCEs are spaced a fixed number of CCEs apart. With contiguous CCEs within the search spaces blocking may occur - i.e. it is impossible to schedule a UE at a lower aggregation level because all the CCEs in its search space are already used. For example, it may be impossible to schedule a UE at an aggregation level 1 because of another UE being scheduled for an aggregation level 8.

Let us assume that the primary station (or Base station eNodeB) sends a PDCCH message for a certain UE at aggregation level eight in CCEs 1,2,...,8. If the search space for aggregation level 1 for another UE consists of the six consecutive CCEs 1,2,3,4,5,6 (or 2,3,..,7, or 3,...,8), it is impossible to send a message at aggregation level 1 to this UE. That is to say, a PDCCH message sent at aggregation level eight may block transmission of a PDCCH message at aggregation level 1 to certain UEs. SUMMARY OF THE INVENTION

It is an object of the invention to propose a method of communicating in a system reducing the risk of blocking allocations of elements of the search space.

It is another object of the invention to propose a method enabling the schedule different aggregation levels which avoids the blocking of a secondary station schedule blocking.

To this end, according to a first aspect of the invention, a method is proposed for communicating between a primary station and a plurality of secondary stations, comprising the steps (a) at the primary station, signaling to each secondary station an indication of an allocated search space comprising at least two resource sets consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be allocated to the considered secondary station, (b) at each secondary station, receiving the control signal and searching in the corresponding allocated search space for a control message addressed to the considered secondary station, wherein at least one search space consists of non-contiguous resource sets. As a consequence, this blocking effect is reduced if search spaces for for example 1- CCEs are designed in such a way that they contain just a few CCEs in common with all possible 8-CCEs. As 8-CCEs consist of eight contiguous CCEs, it is advantageous that the search spaces for 1-CCEs have as few CCEs as possible aligned with the sets of eight consecutive CCEs. This is also the case for all aggregation levels: the search spaces are as few as possible aligned with the higher aggregation levels CCEs.

The present invention also relates to a primary station comprising means for communicating to a plurality of secondary stations, further comprising means for signaling to each secondary station an indication of a search space configuration comprising at least two resource sets consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be allocated to the considered secondary station, means for transmitting a control signal to the secondary stations wherein at least one search space consists of non-contiguous resource sets. In accordance with another aspect of the invention, it is proposed a secondary station comprising means for communicating to a primary station, further comprising means for determining a plurality of search spaces to search, each search space comprising at least two resource sets, each consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be used to transmit a message to the secondary station, means for receiving a control signal from the primary stations wherein at least one search space consists of non-contiguous resource sets. These and other aspects of the invention will be apparent from and will be elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail, by way of example, with reference to the accompanying drawings, wherein:

Fig. 1 is a block diagram of a system in accordance with the invention comprising a primary station and at least a secondary station.

Fig. 2 is a time chart representing the allocated search spaces in accordance an embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a method for communicating in a network, like a cellular network. For instance, the network may be a UMTS network as depicted on Figl.

Referring to Fig. 1, a radio communication system in accordance with the invention comprises a primary station (BS) 100 and a plurality of secondary stations (MS) 110. The primary station 100 comprises a microcontroller (μC) 102, transceiver means (Tx/Rx) 104 connected to antenna means 106, power control means (PC) 107 for altering the transmitted power level, and connection means 108 for connection to the PSTN or other suitable network. Each MS 110 comprises a microcontroller (μC) 112, transceiver means (Tx/Rx) 114 connected to antenna means 116, and power control means (PC) 118 for altering the transmitted power level. Communication from primary station 100 to mobile station 110 takes place on a downlink channel, while communication from secondary station 110 to primary station 100 takes place on an uplink channel. One of the downlink control channels received by the secondary stations is the

PDDCH, where each secondary station has to blindly decode a plurality of sets of CCEs to find which set was allocated to it as set out in the preamble of the description.

In accordance with a first embodiment of the invention, results of various simulations carried out by the inventors are described. With these simulations, it is assumed that 48 CCEs are available. This corresponds to the illustrative exemplary first embodiment of the invention. Various sets of 48 search spaces for the 1-CCEs have been considered; to each user to which a 1-CCE is to be sent, one of these 48 search spaces is assigned at random (the choice corresponds to the outcome of a hash function of that UE that we model as being uniform over the numbers 1,2, ..,48). Each search space consists of six CCEs in this example.

The following sets of search spaces have been considered:

S_l : all search spaces contiguous - i.e. of the form {i,i+l,i+2,i+3,i+4,i+5} with 0< i < 47 where i is the CCE index, and all elements modulo 48.

S_5: all search spaces of the form {i,i+5,i+10,i+15,i+20,i+25} with 0< i < 47, and all elements modulo 48.

S_7: all search spaces of the form {i,i+7,i+14,i+21,i+28,i+35} with 0< i < 47, and all elements modulo 48.

S_d: all search spaces of the form {i,i+l,i+3,i+7,i+12,i+22} with 0< i < 47, and all elements modulo 48. S d is designed so that all search spaces overlap in just 1 CCE. So, for example, the search space of S_5 corresponding to i=25 consists of the CCEs indexed by 25,30,35,40,45,2 (as 50 modulo 48 equals 2).

Fig 2 illustrates the use of a pattern enabling the number of resource elements in common to be minimized, in accordance with the first embodiment, compared with the prior art. On Figure 2, a set of available resources 200 are depicted.

In a conventional system, if only sets of 1-CCEs and 8-CCEs are considered, the search space for one secondary station or UE for 8-CCE messages (2 positions 208 are constructed from contiguous groups of CCEs) is depicted on Figure 2. The positions 201 of 1-CCE messages (6 contiguous positions ) are such that it is likely that all possible positions are blocked if another UE is receiving an 8-CCE message.

In accordance with the first embodiment of the invention, the set of available resources 300 comprises search space for one UE for 8-CCE messages 308, as on Figure 2 where 2 positions are constructed from contiguous groups of CCEs. Regarding the search space for a UE for 1-CCE messages, 6 non-contiguous positions 301 are represented. These positions are non contiguous, so that they reduce overlap with higher aggregation- level search space and therefore increase likelihood that a position can be found to send a small message.

In simulations, it has been assumed that several CCEs are in use for UEs at aggregation level 2,4 or 8, and see how many of N users with a 1-CCE can be assigned a CCE from their respective search spaces. As a performance measure, the "blocking percentage" was estimated, the average percentage of these N users to which no CCE can be assigned. Several different schedulers were employed to show their influence on the relative ranking of the alternative search spaces:

The "greedy" scheduler considers UEs in turn, and assigns them at random to a not-yet- assigned CCE, whenever such a CCE exists. Assignments of CCEs to UEs is never undone. The "weighted greedy scheduler" considers CCEs in turn. At any iteration step, a CCE is assigned that is in the search space of as few UEs as possible that have not yet been assigned a CCE; the CCE is assigned to a UE that has not yet been assigned a CCE, has this CCE in its search space, and has the smallest number of not yet assigned CCEs in its search space among all such users. The "optimal scheduler" finds the largest possible assignment of channels to UEs (see [2]).

1-CCEs only

First, consider an example where all N UEs are to be assigned a 1-CCE. Even though no CCEs are allocated at higher aggregation levels, the fact that all CCEs in a search space are contiguous means that there is a likelihood of a high degree of overlap between the search spaces of different UEs.

The following tables show the percentage of blocked users, for different total numbers of users, N.

Greedy scheduler Weighted greedy scheduler

Optimal scheduler Next consider the situation that CCEs 1,2,...,8 are in use for an 8-CCE PDCCH message. There are N other UEs, to all of which a 1-CCE is to be transmitted. The tables depict the average percentage of these N UEs to which no CCE can be assigned.

Greedy scheduler Weighted greedy scheduler

Optimal scheduler

Next, consider the following situation: CCEs 1,2, ..,8 are in use for an 8-CCE. Furthermore, four consecutive CCEs are in use for a 4-CCE (starting at a random position of the form 4i+l). Finally, there are N other UEs, to all of which a 1-CCE message is to be transmitted. The tables show the average percentage of these N UEs to which no channel can be assigned.

N 24 28 32 34 36

S 1 14. 1 16.2 18. 7 20 .1 21 .6

S 5 4.5 7.5 11. 1 13 .1 15 .3

S 7 4.5 7.5 11. 1 13 .2 15 .3

S d 3.8 6.2 9.3 11 .2 13 .3

Greedy scheduler Weighted greedy scheduler

Optimal scheduler Next, consider the following situation: Three UEs have a 2-CCE message; the corresponding CCEs are placed in random positions. Furthermore, there are N other UEs, to all of which a 1-CCE message is to be transmitted. The tables show the average percentage of these N UEs to which no CCE can be assigned.

Greedy scheduler Weighted greedy scheduler

Optimal scheduler

The results above show that in all cases the assumption of contiguous CCEs at the higher aggregation levels (2, 4, 8) results in non-negligible blocking probabilities for single

CCE messages regardless of the scheduling algorithm employed.

Moreover, the blocking probability can be reduced very significantly by spacing out the CCEs in the aggregation level 1 search space in accordance with the invention.

Regarding the exact spacing of the CCEs in the aggregation level 1 search space, the results show that a spacing which is not a multiple of 2 (e.g. 5 or 7) gives a significant reduction in the blocking probability. However, the best results are obtained by a more random spacing, such as that denoted "S_d"in the analysis above (which was designed so that all search spaces overlap in just 1 CCE).

In an embodiment of the invention, a fixed non-contiguous pattern of CCEs is defined (for example predetermined in specifications for the system in question) for the aggregation level

1 search space (while retaining the contiguous property for aggregation levels 2, 4 and 8).

One embodiment of such a pattern is {i,i+l,i+3,i+7,i+12,i+22}, which works well for all total numbers of CCEs greater than about 42. Other embodiments are possible for smaller total numbers of CCEs. The invention may be applicable to mobile telecommunication systems like UMTS LTE and UMTS LTE-Advanced, but also in some variants to any communication system having allocation of resources to be done dynamically or at least semi persistently.

In the present specification and claims the word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. Further, the word "comprising" does not exclude the presence of other elements or steps than those listed.

The inclusion of reference signs in parentheses in the claims is intended to aid understanding and is not intended to be limiting.

From reading the present disclosure, other modifications will be apparent to persons skilled in the art. Such modifications may involve other features which are already known in the art of radio communication.

Claims

1. A method for communicating between a primary station and a plurality of secondary stations, comprising the steps (a) each secondary station is configured to search at least one of a plurality of search spaces, each search space comprising at least two resource sets, each consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be used to transmit a message to the considered secondary station, (b) at each secondary station, searching in the configured at least one search space for a control message from the primary station addressed to the considered secondary station, and receiving the control message, wherein at least one of the plurality of search spaces consists of non-contiguous resource sets.

2. The method of claim 1, wherein if one of the search spaces comprises resource sets of a number of resource elements being greater than a first threshold number of resource elements (CCEs), the resource sets of the considered search space are contiguous.

3. The method of claim 1 or claim 2, wherein if one of the search spaces comprises resource sets of a number of resource elements being less than a second threshold number of resource elements (CCEs), the resource sets of the considered search space are not contiguous.

4. The method of claim 3, wherein if a search space comprises resource sets of one resource element, the resource sets of the considered search space are not contiguous.

5. The method of claim 4, wherein the starting positions of the resource sets are distributed in accordance with one of the following patterns: {i, i + l, i + 3, i + 7, i + 12, i + 22} or {i, i + 4, i + 11, i + 13, i + 14, i + 19}, with 0 < / < total number of resource elements minus one, and all elements modulo the total number of resource elements.

6. The method of claim 3, wherein the resource sets of at least two of the noncontiguous search spaces are distributed in accordance with a pattern that minimizes the number of resource elements being common to a plurality of the non-contiguous search spaces comprising resource sets with the same number of resource elements (CCEs).

7. The method of claim 6, wherein the pattern is chosen so that the number of resource elements being common to a plurality of search spaces is minimized.

8. A primary station comprising means for communicating to a plurality of secondary stations, further comprising means for signaling to each secondary station an indication of a search space configuration comprising at least two resource sets consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be allocated to the considered secondary station, means for transmitting a control signal to the secondary stations wherein at least one search space consists of non-contiguous resource sets.

9. A secondary station comprising means for communicating to a primary station, further comprising means for determining a plurality of search spaces to search, each search space comprising at least two resource sets, each consisting of at least one resource element (CCE) of a control signal, where at least one resource set might be used to transmit a message to the secondary station, means for receiving a control signal from the primary stations wherein at least one search space consists of non-contiguous resource sets.

10. The secondary station of claim 9, wherein at least one further search space consists of contiguous resource sets.