WO2013068135A1

WO2013068135A1 - Radio base station, user equipment and methods for transmitting and receiving data in a radio communications system

Info

Publication number: WO2013068135A1
Application number: PCT/EP2012/064567
Authority: WO
Inventors: Thomas Chapman; Terence Edwin Dodgson; Matthias HESSE
Original assignee: Nokia Siemens Networks Oy
Priority date: 2011-11-08
Filing date: 2012-07-25
Publication date: 2013-05-16

Abstract

The present invention refers to transmitting and receiving data in a radio communications system. In particular, the present invention refers to methods, a radio base station and a user equipment for transmitting and receiving data in a radio communications system from a radio base station to a user equipment. The radio base station comprises a number of transmit antennas. Data is transmitted via a pilot channel and via a data channel. The method comprises the step of applying, depending on a condition of the user equipment, a virtual antenna mapping to the pilot channel and to the data channel, or to the pilot channel only. According to some embodiments of the present invention,the condition of the user equipment is further being defined as the user equipment comprising information on the virtual antenna mapping.

Description

DESCRIPTION

Radio base station, user equipment and methods for transmitting and receiving data in a radio communications system

FIELD OF THE INVENTION

The present invention relates to transmitting and receiving data in a radio communications system. Particularly, the present invention refers to methods for the transmitting and receiving data; a base station configured to perform the transmitting; and a user equipment configured to perform the receiving .

BACKGROUND OF THE INVENTION

For efficient operation of a radio base station, or Node B, in a radio communications system, in particular in a High Speed Downlink Packet Access (HSDPA) system, it is important that all available transmit (Tx) antennas transmit with equal power .

A current Node B can be of type lTx (Single Input Multiple Output, or SIMO) , 2Tx (2x2 Multiple Input Multiple Output, or 2x2 MIMO) , or 4Tx MIMO, i.e. comprising one, two, or four transmit antennas for transmitting data. However, legacy MIMO user equipments (UEs) are only aware of two transmit antennas at the base station, and legacy non-MIMO UEs only of one transmit antenna.

To facilitate equal power transmission from all four transmit antennas at the radio base station, so-called virtual antenna mapping (VAM) , or power balancing, is needed, which is achieved by selecting appropriate precoding vectors. Prior to transmission, the signals to be applied to the antennas of a MIMO system are subjected to a further precoding based on the selected precoding vectors. In case of a 2x2 MIMO system, the signal that was originally intended for the first antenna only is transmitted over both antennas and similarly the signal that was intended for the second antenna only is transmitted over both antennas. This allows for the signals intended for the first or second antennas originally to be transmitted with unequal power, whilst the actual transmitted signal is the same at both of the physical antennas. An antenna from which a signal was originally intended to be transmitted prior to applying the VAM precoding is referred to as a "virtual" antenna. An antenna from which a signal is actually transmitted is referred to as a "physical" antenna. Virtual antenna mapping means the process of mapping a signal for a "virtual" antenna to one or more physical transmit antennas, based on a set of precoding vectors comprised in a precoding matrix, or virtual antenna mapping matrix. The virtual antenna mapping matrix is a fixed matrix which normally never changes.

VAM allows the Node B to appear to a UE as if it has fewer Tx antennas than it actually has. By applying VAM, unbalanced power at virtual antenna ports is transformed into balanced power at physical antenna ports.

The precoding applied for TxAA is user and time specific and is referred to as "TxAA precoding". The second precoding applied for VAM is fixed for all users and time and is referred to as "VAM precoding".

VAM is transparent to terminals, which are not aware of the fixed precoding applied at the base station. Since the pilot channels are also subject to VAM precoding, the terminal views the signal from the "virtual" antennas as if it would have been transmitted from one real antenna. VAM does not cause performance degradation for single antenna or dual stream MIMO users. For single stream 2x2 MIMO users, a restriction in the selection of precoding vectors for TxAA has to be applied. However, deriving VAM precoding vectors for a 4Tx system that do not interact with Transmit Antenna Array (TxAA) precoding vectors (for two or four antenna users) is difficult, and in a real system, it is likely that it would not be possible to derive such vectors without a precoding loss caused by interaction between the VAM and TxAA precoding.

Furthermore, the VAM precoding must be transparent to existing SIMO and MIMO terminals but does not need to be transparent to new 4x2 and 4x1 UEs which can perfectly profit from 4Tx antennas used in 3GPP 4Tx MIMO (also called 4 branch MIMO) . Thus, there is still a need for an improved method for transmitting and receiving data in a radio communications system.

SUMMARY OF THE INVENTION

Object of the present invention is improving of transmitting and receiving data in a radio communications system.

This object is achieved by a method for transmitting data in a radio communications system from a radio base station to a user equipment, the radio base station comprising a number of transmit antennas, wherein data is transmitted via a pilot channel and via a data channel. The method comprises the step of applying, depending on a condition of the user equipment, a virtual antenna mapping to the pilot channel and to the data channel, or to the pilot channel only.

According to some embodiments of the present invention, the condition of the user equipment is further being defined as the user equipment comprising information on the virtual antenna mapping.

According to further embodiments of the present invention, the number of transmit antennas is four. According to further embodiments of the present invention, the method further comprises the step of selecting, prior to said applying, a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas

The object of the present invention is also achieved by a radio base station for transmitting data in a radio

communications system to a user equipment, the radio base station comprising a number of transmit antennas; data transmission means configured for transmitting data via a pilot channel and via a data channel; and virtual antenna mapping means configured for applying, depending on a condition of the user equipment, a virtual antenna mapping to the pilot channel and to the data channel or to the pilot channel only.

According to some embodiments of the present invention, the radio base station further comprises selecting means for selecting a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas. The object of the present invention is also achieved by a user equipment for receiving data in a radio communications system from a radio base station. The user equipment

comprises data channel receiving means configured for receiving a data channel; and pilot channel receiving means configured for receiving a pilot channel, the pilot channel receiving means comprising information on a virtual antenna mapping applied to the pilot channel by said radio base station . According to some embodiments of the present invention, said information comprises a virtual antenna mapping matrix used for said virtual antenna mapping. As an advantage of the invention, there can be a mixture of UEs in a radio communications system expecting a different number of transmit antennas in which UE-transparent VAM is applied for UEs receiving from less than the total number of radio base station transmit antennas, whilst UEs that can receive from all of the radio base station transmit antennas are aware of the VAM precoding and where VAM precoding is not applied to the HS-PDSCH for such UEs. This allows for a more efficient data transmission and therefore for an improved method for transmitting and receiving data in a radio communications system.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be more clearly understood from the following description of the preferred embodiments of the invention read in conjunction with the attached drawings, in which : Fig. 1 shows a radio telecommunications system, a radio base station and a user equipment according to the invention;

Fig. 2 shows an implementation of the present invention according to some embodiments of the present invention;

Fig. 3 shows an implementation of the present invention according to some embodiments of the present invention;

Fig. 4 shows an implementation of the present invention according to some embodiments of the present invention;

Fig. 5 shows an implementation of the present invention according to some embodiments of the present invention; Fig. 6 shows an implementation of the present invention according to some embodiments of the present invention; and

Fig. 7 shows an implementation of the present invention according to some embodiments of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, preferred embodiments of the invention will be described, wherein the term "pilot channel" refers to a Common Pilot Indicator Channel (CPICH) , and in particular to a primary Common Pilot Indicator CHannel (P-CPICH) and to secondary common pilot indicator channels (S-CPICH) . The term "data channel" refers to a High-Speed Downlink Shared Channel (HS-DSCH) as used in a HSDPA system. However, it is apparent to the person skilled in the art that the invention is not limited to a HSDPA system, but can be applied to any radio telecommunications system using data channels and pilot channels .

Fig. 1 shows components of a radio telecommunications system according to the invention.

In particular, Fig. la shows a radio cell in a radio telecommunications system. The radio cell is spanned by a radio base station 11 which serves a user equipment 12.

Fig. lb shows the radio base station 11 and the user equipment 12 in more detail: The radio base station 11 comprises a number of transmit antennas 111 and the user equipment comprises a receive antenna 121. The radio base station 11 furthermore comprises data transmission means 112 configured for transmitting data via a pilot channel and via a data channel; and virtual antenna mapping means 113 configured for applying, depending on a condition of the user equipment 12, a virtual antenna mapping to the pilot channel and to the data channel or to the pilot channel only.

According to some embodiments of the invention, the radio base station 11 further comprises selecting means 114 for selecting a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas 111. The user equipment 12 further comprises data channel receiving means 122 configured for receiving a data channel; and pilot channel receiving means 123 configured for receiving a pilot channel. The pilot channel receiving means comprises information 1231 on a virtual antenna mapping applied to the pilot channel by said radio base station 11.

Fig. 2 shows an implementation of the present invention according to some embodiments of the present invention. In particular, Fig. 2a shows a method for transmitting data in a radio communications system from a radio base station to a user equipment, the radio base station comprising a number of transmit antennas. Data is transmitted via a pilot channel and via a data channel. The method comprises the step of applying 22, depending on a condition of the user equipment, a virtual antenna mapping to the pilot channel and to the data channel, or to the pilot channel only.

According to some embodiments of the invention, the condition of the user equipment 12 is further being defined as the user equipment 12 comprising information 1231 on the virtual antenna mapping.

In the following, preferred embodiments of the invention will be described with a radio base station comprising four transmit antennas and a user equipment comprising any number of receive antenna. An example for such a UE is a UE comprising four receive antennas. Such a UE is also referred to as 4x4 UE . Other examples of UEs comprise one or two receive antennas, i.e. 4x1 or 4x2 UEs.

4x4 UEs, or more precisely UEs capable of 4Tx MIMO, will be new to the market and thus it is not harmful to introduce a VAM scheme that is not transparent to 4x4 UEs. However, it is essential that the scheme is transparent to SIMO and 2x2 MIMO UEs, i.e. UEs with only one or two receive antennas configured for receiving data transmitted by one or two transmit antennas at the radio base station. Fig. 2b shows the method as shown in Fig. 2a, the method further comprising the step of selecting 21, prior to said applying 22, a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas 111.

The method of Fig. 2 is shown in more detail in Fig. 3. Each precoding vector relates to a "virtual" antenna. A pilot signal, or pilot, is transmitted via a pilot channel from each virtual antenna and precoded with the relevant VAM vector .

As shown in Fig. 3, for SIMO UEs, the HS-DSCH is mapped to the first virtual antenna. The first VAM vector then maps the signal on this first virtual antenna to all four physical radio base station transmit antennas using a fixed precoding vector VI. Since the P-CPICH is precoded with the same weights as the HS-DSCH, the UE is not aware that the VAM precoding has occurred. Furthermore, since the first virtual antenna signal is mapped to all four physical radio base station transmit antennas, the power on each of the physical antennas is equal.

As further shown in Fig. 3, for 2Tx MIMO UEs, the HS-DSCH is precoded to two virtual antennas using the two-weight TxAA precoding codebook Wl . Each of the two virtual antennas is then mapped to four physical radio base station transmit antennas using two VAM precoding vectors. From the UE perspective, since the P-CPICH is precoded with the first VAM precoding vector VI and the S-CPICH with the second VAM precoding vector V2, the VAM is transparent. Furthermore, since the TxAA precoding is still applied, from the UE perspective TxAA and MIMO operate as in the 3GPP standard specification. Since the signal intended for both virtual antennas is transmitted from all four physical radio base station transmit antennas, power balancing between the physical antennas is achieved. As further shown in Fig. 3, according to the present invention, 4x4 MIMO UEs are aware of the VAM precoding, as the pilot channel receiving means 123 comprised in the UE 12 comprises information 1231 on the applied virtual antenna mapping.

In particular, according to some embodiments of the invention the information 1231 comprises a virtual antenna mapping matrix used for said virtual antenna mapping.

Thus, 4x4 MIMO UEs can combine the four pilot signals in order to estimate the channel impulse response for each physical antenna. The HS-DSCH for four antenna UEs is mapped directly to the physical antennas using TxAA precoding; no VAM precoding is applied to the HS-DSCH. Power balancing is maintained as the HS-DSCH is mapped to all four antennas. Based on the channel impulse responses from each of the four physical antennas, the UE can combine the antenna channel impulse responses to correctly demodulate the HS-DSCH whenever TxAA precoding is applied.

According to further embodiments of the present invention, the VAM precoding vectors applied to the pilots and HS-DSCH for SIMO and 2x2 MIMO UEs is a subset of the TxAA precoding codebook for 4x4 MIMO UEs.

The above description considers the situation of 4x4 MIMO being introduced to the existing specifications in a manner that is backwards compatible to 2x2 MIMO and SIMO UEs and includes VAM.

However the concept could be extended such that new 2x2 MIMO UEs would be aware of the VAM precoding. This would have the advantage of avoiding the need for precoding weight restriction, which occurs when VAM is applied currently.

Fig. 4 shows the concept of the invention when applied to a 2x2 system. Summarizing, according to some embodiments of the present invention, an antenna mapping system for SIMO and MIMO schemes is proposed where there can be a mixture of UEs expecting a different number of transmit antennas in which transparent (to the terminals) VAM is applied for UEs receiving from less than the total number of radio base station transmit antennas, whilst UEs that can receive from all of the radio base station transmit antennas are aware of the VAM precoding and where VAM precoding is not applied to the HS-PDSCH for such UEs.

According to further embodiments of the present invention, the VAM precoding vectors are a subset of the highest antenna order TxAA precoding vectors.

In the following, further details on some embodiments of the present invention are given.

Fig. 5 shows a transmission to a SIMO UE . The pilot, control channels and HS-PDSCH are mapped to the first virtual antenna for this UE . The first VAM precoding vector VI maps the signal on the first virtual antenna to the four physical transmit antennas. As an advantage, since the P-CPICH is mapped to the four antennas with the same precoding vector as all of the other channels, the UE will make its channel estimation on P-CPICH and will not be aware that the received signal is a combination of signals from four antennas rather than just a signal from a single antenna.

Fig. 6 shows a transmission to a 2x2 MIMO UE using dual stream. The P-CPICH is transmitted via the first virtual antenna. The first virtual antenna is mapped to the four physical antennas using the first VAM precoding vector VI . The S-CPICH is mapped to the second virtual antenna. The second virtual antenna is mapped to the 4 physical antennas using VAM precoding vector V2. The UE makes channel estimates on the P-CPICH and the S-CPICH. As an advantage, in each case, since the UE receives the combined signal from four antennas, the UE is unaware of the VAM precoding. However, since the VAM precoding is different for the P-CPICH and the S-CPICH, the UE will experience different channel impulse responses from the P-CPICH and S- CPICH and hence view them as coming from separate transmit antennas . The HS-PDSCH is mapped to the virtual antennas 1 and 2 using TxAA precoding weights W±,_j (i,j = l,2), as defined in the HSDPA D-TxAA 3GPP standard specification. The TxAA precoding is indicated by the UE in exactly the same manner as without VAM. The UE can use the channel estimates from the P-CPICH and S-CPICH to demodulate the HS-PDSCH.

As a further advantage, from a UE perspective, therefore HS- PDSCH demodulation is exactly the same as if there would be no VAM.

Fig. 7 shows a transmission to a 4x4 MIMO UE using two streams. It should be noted that there is no reason why the UE could not be transmitting four streams, but two stream transmission is depicted for clarity of the diagram. The UE must make channel estimates for each of the four pilots, P- CPICH, S-CPICH, CPICH3 and CPICH4. In this case, the UE is aware of the VAM precoding vectors VI to V4 that have been applied to the pilots. By adding and subtracting the channel estimates from each of the pilots, the UE can get channel estimates from each of the four physical antennas. Then, the UE can select an optimal TxAA precoding vectors Hi from its TxAA vector codebook.

The four HS-PDSCH channels are precoded using the selected vectors from the TxAA codebook Hi. The UE must combine its channel estimates relating to each of the physical antennas in order to get channel estimates. Thus, unlike SIMO or 2x2 MIMO UEs, 4x4 MIMO UEs must be aware of the applied precoding. The present invention allows for this, as the pilot channel receiving means comprised in the UE comprises information on the applied virtual antenna mapping.

A main advantage of the invention is that for UEs that receive from all base station antennas, the HS-DSCHs are mapped directly to the transmit antennas using TxAA pre- coding, without applying VAM. As VAM is costly and time consuming, not applying VAM to the data channels allows for a more efficient data transmission.

The pilot channels, however, are always pre-coded with VAM, because pilots 1 and 2 have to be VAM pre-coded due to legacy reasons, and mixed channel estimation (from pilots with VAM and pilots without VAM) would be very complicated.

While embodiments and applications of this invention have been shown and described above, it should be apparent to those skilled in the art, that many more modifications (than mentioned above) are possible without departing from the inventive concept described herein. The invention, therefore, is not restricted except in the spirit of the appending claims. Therefore, it is intended that the foregoing detailed description should be regarded as illustrative rather than limiting .

LIST OF ABBREVIATIONS:

HSPA High-speed packet access

HSDPA High Speed Downlink Packet Access

MIMO Multiple Input Multiple Output SIMO Single Input Multiple Output

Tx antenna transmit antenna

TxAA transmit antenna array

UE User equipment

VAM Virtual antenna mapping

LIST OF REFERENCES:

11 Radio base station

111 number of transmit antennas

112 data transmission means

113 virtual antenna mapping means

114 selecting means

12 User equipment

122 data channel receiving means

123 pilot channel receiving means

1231 information on the virtual antenna mapping

21 selecting a number of virtual antenna precoding vectors

22 applying a virtual antenna mapping

Claims

1. Method for transmitting data in a radio communications system from a radio base station (11) to a user equipment (12), the radio base station (11) comprising a number of transmit antennas (111), wherein data is transmitted via a pilot channel and via a data channel,

the method comprising the step of

- applying (22), depending on a condition of the user

equipment (12), a virtual antenna mapping to the pilot channel and to the data channel or to the pilot channel only.

2. Method according to claim 1, the condition of the user equipment (12) further being defined as:

- the user equipment (12) comprising information (1231) on the virtual antenna mapping.

3. Method according to claim 1 or 2, wherein the number of transmit antennas (111) is four.

4. Method according to any preceding claim, the method further comprising the step of

- selecting (21), prior to said applying (22), a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas (111) .

5. Radio base station (11) for transmitting data in a radio communications system to a user equipment (12), the radio base station (11) comprising:

- a number of transmit antennas (111);

- data transmission means (112) configured for transmitting data via a pilot channel and via a data channel; and - virtual antenna mapping means (113) configured for

applying, depending on a condition of the user equipment (12), a virtual antenna mapping to the pilot channel and to the data channel or to the pilot channel only.

6. Radio base station according to claim 5, the condition of the user equipment (12) further being defined as:

7. Radio base station according to claim 5 or 6, wherein the number of transmit antennas (111) is four.

8. Radio base station according any of claims 5 to 7, the radio base station (11) further comprising:

- selecting means (114) for selecting a number of virtual antenna precoding vectors, wherein the number of virtual antenna precoding vectors is equal to the number of transmit antennas (111) .

9. User equipment (12) for receiving data in a radio

communications system from a radio base station (11), the user equipment comprising (12) :

- data channel receiving means (122) configured for

receiving a data channel; and

- pilot channel receiving means (123) configured for

receiving a pilot channel, the pilot channel receiving means comprising information (1231) on a virtual antenna mapping applied to the pilot channel by said radio base station (11) .

10. User equipment (12) according to claim 9, wherein said information (1231) comprises a virtual antenna mapping matrix used for said virtual antenna mapping.