WO2009040747A2

WO2009040747A2 - Change of pilot symbol pattern

Info

Publication number: WO2009040747A2
Application number: PCT/IB2008/053887
Authority: WO
Inventors: Matthew P.J. Baker; Timothy J. Moulsley; Filippo Tosato
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2007-09-28
Filing date: 2008-09-24
Publication date: 2009-04-02
Also published as: TW200931903A; WO2009040747A3

Abstract

A pilot symbol pattern comprises a set of pilot symbol locationsτ in a time- frequency reference frame, wherein pilot symbols are transmitted in said pilot symbol locations in a radio transmission (41 ). A current pilot symbol pattern is used in a radio transmission from a network entity (31 ) to a terminal (32) in a radio communication network. Then, a change request (43) of pilot symbol pattern is sent from the terminal to the network entity; and a next pilot symbol pattern is used in said radio transmission.

Description

CHANGE OF PILOT SYMBOL PATTERN

Field of the invention

The invention relates generally to radio communication networks, and more particularly, it relates to such networks whereinpilot symbols are used, like a UMTS LTE network ('Universal Mobile Telecommunication System Long Term Evolution') for instance.

Background of the invention

Such a radio communication network comprises network entities and mobile terminals which are able to exchange information via radio channels. In order to allow radio transmission between network entities and mobile terminals, it is possible to use an access scheme like OFDMA (Orthogonal Frequency Division Multiple Access') or like SC-FDMA ('Single Carrier Frequency Division Multiple Access') for instance. An access scheme of this type is a digital multi-carrier modulation scheme, which uses a number of closely-spaced orthogonal sub-carriers. A signal is transmitted in successive signal frames comprising symbols at respective locations along an axis of time and an axis of frequency in a time- frequency reference frame. A received signal can differ from the corresponding transmitted signal due to several effects linked to the radio transmission medium, such as thermal noise or multi path fading caused by the interaction of the signal with reflections or echoes of itself, or interference from other signals passing through the same medium. All these effects cause distortions of the transmitted signal. In order to be able to retrieve the transmitted signal from the received signal, it can be useful to use pilot symbols in the radio transmission.

A pilot symbol is one or more predetermined symbols that may be transmitted from the transmitter to the receiver. The transmitter and the receiver are able to know locations of pilot symbols in a transmitted signal frame and, on the basis of them, it is possible to control, to synchronize or to supervise the radio transmission, to carry out measurements on radio channels or to estimate radio channels.

In a mobile radio communication system like an UMTS LTE network, a base station determines a pattern of pilot symbols associated to a mobile terminal for using in its signal transmission to this terminal. In these conditions, the pilot symbol pattern is set at the network level.

Such a characteristic can be effective and relevant when, at the base station level, it is possible to predict an optimal usage of pilot symbols. This is the case, for instance, in the context of usage of Multimedia Broadcast Multicast Service (MBMS) wherein a mobile terminal receives the signals from multiple cells as if they were transmitted from a single cell in Single- Frequency Network (SFN) mode. Indeed, in this context, it may be reliably predicted that a large delay-spread in the combined radio channel may result, and then, that closely spaced pilot symbols in the frequency domain are required in order to make the mobile terminal able to estimate the radio channel unambiguously. Here, consequently, the network may configure a pilot symbol pattern with appropriately placed pilot symbols without needing additional information. In these conditions, the network can be an appropriate level to set the pilot symbol pattern.

Figures 1 and 2 illustrate two different pilot symbols patterns respectively used in two different radio transmission types.

More precisely, Figure 1 illustrates a pilot symbol pattern 11 which is set at the base station level when the handled radio transmission corresponds to a non Multimedia Broadcast Single Frequency Network transmission. Figure 2 illustrates a pilot symbol pattern 21 which is set at the base station level effectively to handle a radio transmission corresponding to a Multimedia Broadcast Single Frequency Network transmission.

These pilot symbol patterns are illustrated in a time-frequency reference frame according to which the horizontal axis is the time axis t and the vertical axis is the frequency axis f.

The pilot symbol pattern 11 comprises more pilot symbols on the time axis and on the frequency axis than the pilot symbol pattern 21 .

Thus, in some conditions, it can be effective to set the pilot symbol pattern at the network level on the basis of the type of the radio transmission used in the considered network.

However, in some other conditions of transmission, the network, and more particularly the base station, may not have sufficient information to determine an effective and relevant pilot symbol pattern for transmission to a mobile terminal. Typical feedback from the mobile terminal, such as channel quality indicators (CQI) or Hybrid ARQ (HARQ) acknowledgements do not provide sufficient information for the pilot symbol pattern to be adapted appropriately.

However, it is important to use a relevant pilot symbol pattern. Indeed, on the one hand, if the used pilot symbol pattern comprises not enough pilot symbols, the quality level of transmission can be insufficient. On the other hand, if the used pilot symbol pattern comprises more pilot symbols than are required, the resources available for transmitting useful data symbols are reduced.

Summary of the invention

A first aspect of the present invention is a method for adapting a pilot symbol pattern used in a radio transmission through a radio channel from a network entity to a terminal in a radio communication network; the pilot symbol pattern comprising a set of pilot symbol locations in a time- frequency reference frame, wherein pilot symbols are transmitted in said pilot symbol locations in the radio transmission; and a current pilot symbol pattern being used in the radio transmission, this method comprising the following steps at the network entity :

/a/ receiving a change request (43) of pilot symbol pattern from said terminal; and in response to said change request,

IbI using a different pilot symbol pattern in said radio transmission. In these conditions, a change of pilot symbol pattern used in a radio transmission from the base station to the terminal is triggered by the terminal itself, and the terminal has relevant information to recommend a change of pilot symbol pattern. Consequently, such a characteristic allows the used pilot symbol pattern in such a radio transmission to be adapted correctly.

Indeed, classically, a terminal in a radio communication network has information about the quality of a transmission from the base station, since it is adapted to determine at least some of the following indications:

- indication relative to Doppler spread;

- indication relative to channel impulse response; and

- indication relative to the reasons of decoding failures which occur on the terminal side at the signal reception.

For instance, the terminal is able to determine whether decoding failure occurs because of poor data reception (insufficient data symbol energy) or because of poor channel estimation (insufficient pilot symbol energy or insufficient pilot symbol density). Consequently, the terminal can detect if the radio transmission can be carried out with a better quality level if the pilot symbol pattern is changed.

Thus, according to one embodiment of the present invention, the pilot symbol pattern used for a radio transmission between the base station and the terminal can be adapted in an effective, reliable and relevant way. One embodiment of the present invention can be applied to a mobile radio communication network, and in this case the terminal is a mobile terminal. In such a mobile radio communication network, the radio transmission may comprise a continuous signal in the time domain occupying substantially the same bandwidth, or a sequence of signals of limited duration having different bandwidths and occupying different parts of the radio spectrum. But, no limitation is attached to this aspect regarding the present invention. Indeed, it can be applied to a non mobile radio communication network.

In one embodiment, the different pilot symbol pattern is determined at the terminal. In this case, the network entity can handle a plurality of pilot symbol patterns to be used for the radio transmission, and the change request indicates which pilot symbol pattern among the plurality of pilot symbol patterns is required.

Alternatively, the different pilot symbol pattern can be determined at the network entity based on information received from the terminal.

In this case, the change request on which the network entity bases the determination of the next pilot symbol pattern can be either explicit or implicit. Implicit information received from terminal and used to determine the different pilot symbol pattern at the network entity can comprise an indication of the reason for a failed decoding of a data packet.

Alternatively or in a complementary way, the change request may be explicit.

In this case, the network entity can use for instance the following feedback indications: - indication relative to Doppler spread; and/or

- indication relative to channel impulse response; and/or Such information comprised in the change request can indicate, for instance, if more or less pilot symbols are required in the next pilot symbol pattern. Thus, the network entity is able to modify the current pilot symbol pattern in order to obtain a different pilot symbol pattern which is better adapted to the radio transmission. Such information comprised in the change request can indicate time and/or frequency characteristics relative to the radio transmission. Thus, knowing such characteristics relative to the radio transmission, the network entity is itself able to adapt correctly the pilot symbol pattern. In one embodiment, an initial pilot symbol pattern is used for the radio transmission initially, the initial pilot symbol pattern being known by the terminal and the network entity. This can be predetermined as a default pilot symbol pattern which is always available for use by the terminal(s). In some such embodments, the different pilot symbol patterns only insert additional pilot symbols relative to the default pilot symbol pattern, and do not remove pilot symbols contained in the default pilot symbol pattern. In such a case, the default pilot symbol pattern represents the minimum density of pilot symbols.

A second aspect of the present invention is a network entity being appropriate to adapt a pilot symbol pattern for a radio transmission to a terminal in a radio communication network; the pilot symbol pattern indicating, in a time-frequency reference frame, pilot symbols transmitted in a signal frame in the radio transmission; a current pilot symbol pattern being used in the radio transmission; the network entity comprising means adapted to use a next pilot symbol pattern in said radio transmission after reception of a change request of pilot symbol pattern sent from said terminal.

In one embodiment, the network entity further comprises means adapted to determine the different pilot symbol pattern based on information received from the terminal. This information received from the terminal can correspond to feedback indication relative to a quality level of the radio transmission.

In alternative or complementary way, this information received from the terminal can correspond to information comprised in the change request.

A third aspect of the present invention is a terminal being appropriate to adapt a pilot symbol pattern for a radio transmission received from a network entity in a radio communication network; the pilot symbol pattern indicating, in a time-frequency reference frame, pilot symbols transmitted in a signal frame in the radio transmission; a current pilot symbol pattern being used in the radio transmission; the terminal comprising means adapted to send a change request of pilot symbol pattern to the network entity and to use a different pilot symbol pattern in the radio transmission.

A fourth aspect of the present invention is a radio communication system comprising at least a network entity according to the second aspect of the present invention and a terminal according to the third aspect of the present invention.

A fifth aspect of the present invention is a computer program product comprising instructions for implementing a sending change request of pilot symbol pattern according to method according to the first aspect of the present invention when loaded and run on computer means of a terminal according to the third aspect of the present invention.

A sixth aspect of the present invention is a computer program product comprising instructions for implementing a method according to the first aspect of the present invention when loaded and run on computer means of a network entity according to the second aspect of the present invention.

Brief description of the drawings

Further features and advantages of the present invention will become more apparent from the description below. The latter is given purely by way of illustration and should be read in conjunction with the appended drawings, of which : figure 1 and 2 illustrate two different pilot symbols pattern respectively associated to two different transmission types which are used according to the art ; - figure 3 illustrates a radio communication network according to one embodiment of the present invention; figure 4 illustrates the main steps of a method for adapting a pilot symbol pattern according to one embodiment of the present invention; and figure 5 illustrates the main steps of a method for adapting a pilot symbol pattern according to another embodiment of the present invention.

Description of the preferred embodiments

Figure 3 illustrates a radio communication network according to one embodiment of the present invention.

Such a network comprises at least one network entity 31 and terminals 32-35, which can be mobile terminals. Such a radio communication network can be based on any transmission protocol including the transmission of pilot symbols. For instance, this network can be an UMTS LTE network.

Hereafter, by way of example, the considered network is an UMTS network and the network entity corresponds to a base station.

According to one embodiment of the present invention, the terminal is in charge of requesting a change of pilot symbol pattern, and the determination of the new pilot symbol pattern to be used can be determined either at the terminal or at the base station level.

Figure 4 illustrates the main steps of a method for adapting a pilot symbol pattern according to one embodiment of the present invention wherein the determination of the new pilot symbol pattern to be used for the considered radio transmission with the terminal is carried out on the terminal side.

The base station 31 performs a radio transmission 41 towards the terminal 32 by using a current pilot symbol pattern. At the beginning of the radio transmission from the base station to the terminal, meaning on the downlink, the current pilot symbol pattern can be an initial pilot symbol pattern which is known by the terminal and by the base station beforehand. During the radio transmission, the terminal detects that this current pilot symbol pattern is not adapted to this radio transmission from the base station 31. Consequently, it decides to request the base station to change the pilot symbol pattern for this radio transmission. In this embodiment, the terminal determines itself the next pilot symbol pattern to be used and indicates it to the base station by sending a message 43 comprising a change request.

To carry out such an embodiment, beforehand, on one hand, the base station can know a list of several different pilot symbol patterns and different indexes corresponding respectively to these different pilot symbol patterns, and on the other hand, the terminal can know the list of these indexes. Thus, advantageously, the terminal is able to indicate to the base station which pilot symbol pattern is required for the radio transmission by sending in the change request message 43 the corresponding index. This has the advantage of limiting the signalling overhead for sending the message 43. Upon reception of the change request message 43, the base station is able to retrieve the required pilot symbol pattern and, then, to use it for the radio transmission towards the terminal 32.

Alternatively, the determination of a next pilot symbol pattern to be used by the base station 31 for the radio transmission towards the terminal can be carried out at the base station itself. Figure 4 illustrates this case.

Thus, in these conditions, as in Figure 3, a radio transmission 41 from the base station 31 to the terminal 32 is currently carried out, on the basis of a current pilot symbol pattern, which can be, at the beginning of the radio transmission, an initial pilot symbol pattern which is known beforehand on both sides, on the terminal side and on the base station side.

Then, the terminal 32 detects that this current pilot symbol pattern seems to be no more adapted to this radio transmission. Consequently, it decides to request a change of pilot symbol pattern by sending a change request message 43 to the base station. In this alternative, the next pilot symbol pattern is determined on the base station side, on the basis of information comprised in this change request message 43. Thus, at step 52, upon reception of the change request message 43, the base station 31 exploits information comprised in this message 43 to determine the next pilot symbol pattern to be used in this radio transmission. Such information comprised in the change request message 43 can be an indication on the fact that more or less pilot symbols are required in the next pilot symbol pattern. It is possible to specify that more or less pilot symbols are required on the time axis and/or on the frequency axis.

This change request message 43 can also indicate time and/or frequency characteristics relative to the radio channel used for this radio transmission. Such characteristics may for example comprise one or more of a Doppler spread, a channel impulse response, a coherence time and a coherence bandwidth.

No limitation is attached to such information comprised in the change request message 43.

Thus, advantageously, in the alternative wherein the base station in charge of determining the next pilot symbol pattern, the change request message 43 sent by the terminal contains information specifying for instance if more or less pilot symbols are required in the next pilot symbol pattern, or indicating time and/or frequency characteristics relative to the used radio channel as described above.

In a general way, as the terminal is in charge of requesting a change of pilot symbol pattern, it is also in charge of detecting the need to carry out such a change. No limitation is attached to the way of such detection on the terminal side. Thus, the terminal can take into account any information useful to make it able to decide that a change of pilot symbol pattern is needed to enhance the considered radio transmission.

For instance, the terminal can take into account one of the following indications: - relative to Doppler spread; or

- indication relative to channel impulse response; or - indication relative to decoding failures which occur on the terminal side at the signal reception.

Thus, thanks to the fact that this is the terminal which takes the decision to request a change of pilot symbol pattern, the current pilot symbol pattern is closely adapted to the radio transmission which is considered.

Indeed, the terminal has more relevant information to carry out such a decision than a network entity in a radio communication network.

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 invention has been largely described in relation to pilot symbol patterns transmitted by a network entity to a mobile terminal, but it will be understood that the invention applies equally to opposite or other allocations of roles to nodes.

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 and which may be used instead of or in addition to features already described herein.

Claims

1. Method for adapting a pilot symbol pattern used in a radio transmission through a radio channel (41 ) from a network entity (31 ) to a terminal (32) in a radio communication network; said pilot symbol pattern comprising a set of pilot symbol locations in a time- frequency reference frame, wherein pilot symbols are transmitted in said pilot symbol locations in said radio transmission; a current pilot symbol pattern being used in said radio transmission; said method comprising the following steps at the network entity :

/a/ receiving a change request (43) of pilot symbol pattern from said terminal; and, in response to said change request,

IbI using a different pilot symbol pattern in said radio transmission.

2. Method for adapting a pilot symbol pattern according to claim 1 , wherein the different pilot symbol pattern is determined at the terminal.

3. Method for adapting a pilot symbol pattern according to claim 2, wherein the change request comprises an indication of a pilot symbol pattern selected from a plurality of predetermined pilot symbol patterns.

4. Method for adapting a pilot symbol pattern according to claim 1 , wherein the different pilot symbol pattern is determined at the network entity.

5. Method for adapting a pilot symbol pattern according to claim 4, wherein the different pilot symbol pattern is selected from a plurality of predetermined pilot symbol patterns.

6. Method for adapting a pilot symbol pattern according to claim 4, wherein the change request comprises an indication of the reason for a failed decoding of a data packet, said reason comprising at least one of insufficient data symbol energy or insufficient pilot symbol energy or insufficient pilot symbol density.

7. Method for adapting a pilot symbol pattern according to claim 4, wherein the change request comprises an indication of whether a higher or lower density of pilot symbols is required in time and/or frequency domains.

8. Method for adapting a pilot symbol pattern according to claim 4, wherein the change request comprises time and/or frequency characteristics of the radio transmission (41 ) as received at the terminal.

9. Method for adapting a pilot symbol pattern according to claim 8, wherein the change request comprises an indication that the degree of variation of the radio channel in the frequency domain, as estimated at the terminal, has increased.

10. Method for adapting a pilot symbol pattern according to claim 9, wherein the different pilot symbol pattern comprises a higher density of pilot symbol locations in the frequency domain.

11. Method for adapting a pilot symbol pattern according to claim 8 or 9, wherein the change request comprises an indication that the degree variation of the radio channel in the time domain, as estimated at the terminal, has increased.

12. Method for adapting a pilot symbol pattern according to claim 11 , wherein the different pilot symbol pattern comprises a higher density of pilot symbol locations in the time domain.

13. Method for adapting a pilot symbol pattern according to claim 2 or claim 5, wherein a predetermined default pilot symbol pattern is used for the radio transmission until a different pilot symbol pattern is used in response to receiving a change request , said default pilot symbol pattern comprising a set of pilot symbol locations which is a subset of the set of pilot symbol locations in at least one of the plurality of the predetermined pilot symbol patterns.

14. Network entity (31 ) being appropriate to adapt a pilot symbol pattern for a radio transmission through a radio channel (41 ) to a terminal (32) in a radio communication network; said pilot symbol pattern comprising a set of pilot symbol locations in a time- frequency reference frame, wherein pilot symbols are transmitted in a signal frame in said radio transmission; a current pilot symbol pattern being used in said radio transmission; said network entity comprising means adapted to use a different pilot symbol pattern in said radio transmission after reception of a change request of pilot symbol pattern sent from said terminal.

15. Network entity (31 ) according to claim 14 further comprising means adapted to determine the different pilot symbol pattern based on information received from the terminal (32).

16. Network entity (31 ) according to claim 15, wherein information received from the terminal corresponds to feedback indication relative to a quality level of the radio transmission (41 ).

17. Network entity (31 ) according to claim 11 , wherein information received from the terminal (32) corresponds to information comprised in the change request (43).

18. Terminal (32) being appropriate to adapt a pilot symbol pattern for a radio transmission through a radio channel (41 ) received from a network entity (31 ) in a radio communication network; said pilot symbol pattern comprising a set of pilot symbol locations in a time- frequency reference frame, wherein pilot symbols are transmitted in said pilot symbol locations in said radio transmission; a current pilot symbol pattern being used in said radio transmission; said terminal comprising means adapted to send a change request of pilot symbol pattern to said network entity and to use a different pilot symbol pattern in said radio transmission.

19. Radio communication system comprising at least a network entity (31 ) according to any one of claims 10 to 13 and a terminal (32) according to claim 14.

20. A computer program product comprising instructions for implementing a sending change request (43) of pilot symbol pattern according to method as claimed in claim 1 when loaded and run on computer means of a terminal (32) according to claim 18.

21. A computer program product comprising instructions for implementing a method according to claim 1 when loaded and run on computer means of a network entity (31 ) according to claim 14.