WO2010139516A2

WO2010139516A2 - Method and apparatus for antenna virtualization for multimode diversity transmission

Info

Publication number: WO2010139516A2
Application number: PCT/EP2010/056093
Authority: WO
Inventors: Hannu Tapio Hakkinen; Kari Petri Juhana Sipila
Original assignee: Nokia Siemens Networks Oy
Priority date: 2009-06-02
Filing date: 2010-05-05
Publication date: 2010-12-09
Also published as: WO2010139516A3; GB0909466D0

Abstract

The present invention refers to methods and an apparatus for transmitting data in a radio communications system, wherein for a multiple downlink transmission of a signal of a Node B, the Node B comprising at least a first antenna and a second antenna, precoding weights are applied for mapping the signal to the first antenna and to the second antenna, with a first precoding weight formula (I) for mapping the signal to the first antenna, and a second precoding weight w ₂ for mapping the signal to the second antenna, with w ₂ being the first precode or the fourth precode out of a set of four alternative precodes, the set of four alternative precodes being defined as formula (II).

Description

Method and apparatus for antenna virtualization for multimode diversity transmission

The present invention relates to a method and apparatus for antenna virtualization for multimode diversity transmission.

BACKROUND OF INVENTION

WCDMA/HSPA+ evolution support multiple downlink transmission modes starting from basic single antenna transmission up to 2 antenna MIMO transmission. The HSPA+ network therefore has to support a mixture of UEs with different capabilities. Typically, geographic area is covered by a low number of WCDMA carriers.

The rollout of MIMO feature for low penetration of MIMO- capable UEs doesn't justify allocation of the whole carrier only for MIMO mode operation. UEs with 1-antenna receiver configuration prefer 1-TX transmission in downlink, while the

UEs are implemented with equalizer receivers. Simulations have shown that system capacity is higher when using 1-TX transmission compared to using 2-TX diversity transmission.

Transmitting a combination of 1-TX and 2-TX transmission in standard manner would cause unbalanced total transmission power in the two TX paths of the Node B. Thus, the typical Node B configuration can't use its full transmit power capabilities under such unbalanced power condition. Conventional way of mapping 1-TX transmission into single physical TX path can use only partially power capability of the Node B with multiple TX hardware paths. This unbalanced power setup can be avoided only in specific large multi- carrier configurations.

SUMMARY OF INVENTION

The present invention aims to avoid the above mentioned problem.

In the basic configuration the Node B has two physical paths for downlink transmission. The two transmitters feed the two diversity antennas, which should provide low correlation. In the following it is assumed that both hardware transmitters have equal maximum power capability.

In the signal processing of the Node B each antenna port

(which is according to the 3GPP standard) is virtualized by mapping the signal to both of the two physical transmission paths with balanced power split. The mapping of the two antenna ports is orthogonal. I.e., the mapping shall not increase correlation between virtual antenna ports in comparison to conventional physical antenna based mapping. The correlation properties are maintained in statistical terms, while the mapping modifies the actual time-variable correlation case-by-case in random radio channel.

Every physical channel is transmitted via virtual antenna ports according to standard definition. Thus payload channels and related reference channels are processed through the equal mapping. Both 1-TX and 2-TX transmission modes are supported. Simultaneously the low correlation between the two virtualized transmission channels is maintained. Further, the specific virtual antenna mapping and scheduling functions can be applied to maintain power balance in case of 3GPP MIMO single stream transmit mode. That will be described in the implementation section.

In addition, a small opposite frequency offset between the two physical transmission paths may be produced. The frequency offset causes slow rotation of the phase difference between the two physical paths. This additional function modifies the interference impact between the two virtual transmissions in time domain. That protects the reception of the 1-TX transmission against potential severe semi-permanent interference due to transmission in the other virtual antenna port.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are shown in the drawing in which: Fig. 1 shows a MIMO cell with single stream MIMO transmission according to 3GPP, using S-CPICH as a phase ref for Ant2

Fig. 2 shows a MIMO cell with single stream MIMO transmission, using virtual antennas

Fig. 3 shows a MIMO cell with a non-MIMO transmission using virtual antennas

Fig. 4 shows a MIMO cell with a dual stream MIMO transmission according to 3GPP, using S-CPICH as a phase ref for Ant2 Fig. 5 shows a MIMO cell with a dual stream MIMO transmission, using virtual antennas

Fig. 6 shows a non-MIMO cell with a virtual single antenna in a non-MIMO transmission

Fig. 7 shows a MIMO cell with a single stream MIMO transmission, using preferred virtual antenna mapping

DETAILLED DESCRIPTION OF THE INVENTION

The antenna virtualization can be implemented e.g. in the baseband signal processing. The mapping is basically a cell based process. However, implementation may be alternatively implemented in equivalent manner on physical channel basis.

The most simple orthogonal mapping function would be summing and subtraction of the two virtual antenna ports to the two physical TX transmit paths, respectively. That would fulfil both basic power balance and orthogonality conditions.

However, the above sum-subtract mapping doesn't match properly with the precoding applied in the 3GPP HSPA/MIMO. The sequential processing with precoding and virtual antenna mapping impacts on the power balance in the physical antenna paths. Four alternative precodes defined by the 3GPP cause a different power split between the physical transmit paths. The basic sum-subtract mapping causes unbalanced power split in single stream mode. Dual stream MIMO mode is effectively balanced, while orthogonal precodes are applied per stream. Also the basic 1-TX transmission will be balanced, while it applies only virtual antenna mapping but no precoding.

The 3GPP defines alternative precoding weights for single stream as follows:

[1 + 7 1-7 -1 + 7 -I-7"

W_n

One applicable orthogonal mapping from virtual antennas, [S₁S₂ to physical antennas, Ja₁,^) is as follows:

a_n = s, /v2 - 1 +7

2 ^Λ In single stream case virtual antenna signals are as follows:

where di denotes the data pattern of the single stream

thus the following signal pairs {α_1;α₂/ ^are possible in the physical antennas, respectively:

^ J , respectively

The 1st and 4th precodes provide balanced power split also with single stream transmission, while the 2nd and 3rd precodes steer all power into one out of the two physical transmission paths. Also the 2nd and 3rd precodes provides balanced power split in dual stream MIMO mode, while the dual stream mode applies two orthogonal precodes. The power balance is achieved when equal power is applied for both data streams .

The Node B may choose at least from the following alternative or complementing actions to manage power balance in case of single stream transmission:

1. The UEs with power balanced precodes are scheduled preferably first.

2. The Node B should schedule both orthogonal unbalanced precodes with code multiplexing within TTI.

3. Postpone scheduling for an UE to later TTI to fulfil the power balancing condition, like in alternative 2.

4. The Node B may omit the precode feedback from the UE and apply a balanced precode instead.

There are several mapping alternatives, which behave in equivalent manner in MIMO single stream mode.

The frequency offset between the two transmission paths may be implemented by synthesizer configuration in the upconverters, if those should support such high frequency resolution. Alternatively, the frequency offset can be implemented as phase rotation in the processing of complex base band signal. However, the phase rotation resolution must be smooth enough not to confuse channel estimation in the UE receiver .

No change is required for the reception algorithms of the UE, while the payload channels and related reference channels are processed in equal manner. The frequency offset should be small enough to be handled as equivalent to Doppler phenomena of radio channel. Further the precode feedback reported by the UE can follow slow enough phase changes of the modified transmission.

From UE receiver performance point of view, it is preferable to schedule 1-TX and MIMO transmissions separate in time domain. Thus the UEs with 1-RX receiver and MIMO capable UEs with 2-TX reception would operate under most suitable conditions .

In HSPA+, MIMO requires transmission of continuous S-CPICH in the second virtual antenna port. Thus S-CPICH overlaps also with the 1-TX transmissions when those occupy the same carrier. 1-RX receiving UE may see a combination of applicable 1-TX transmission and interfering S-CPICH transmission. The two signals propagate through two low- correlating radio channels. The frequency offset or equivalent phase rotation function modifies interference conditions in time domain. Thus the average interference shouldn't stay at high level. The frequency offset between the two transmitter paths may modify phase relation the UE seen between the two virtual antennas. Thus the UE reports modified precode indicator towards Node B. The precode variation is favourable behaviour in the special scheduling described above for single stream power balancing.

Claims

Cl aims

1. Method for transmitting data in a radio communications system, wherein for a multiple downlink transmission of a signal of a Node B, the Node B comprising at least a first antenna and a second antenna, precoding weights are applied for mapping the signal to the first antenna and to the second antenna, with a first precoding weight

for mapping the signal to the first antenna, and a second precoding weight W₂ for mapping the signal to the second antenna, with W₂ being the first precode or the fourth precode out of a set of four alternative precodes, the set of four alternative precodes being defined as [1+₇ 1-₇ -1+₇ -I-₇]

2. Method according to claim 1, wherein the mapping is defined as

with virtual antennas Js₁S₂J being mapped to physical antennas

{a_λ,a₂} , the physical antennas {a_λ,a₂} corresponding to the first and second antenna.

3. Method according to any preceding claim, wherein said multiple downlink transmission is done in 3GPP MIMO single stream transmit mode, providing balanced transmission power split with regard to the first antenna and the second antenna.

4. Method according to any preceding claim, wherein

- said Node B further comprises upconverters, two physical transmission paths for said multiple downlink transmission are related to the first and second antenna, and

- a small opposite frequency offset between the two physical transmission paths is produced by synthesizer configuration in the upconverters, the frequency offset causing slow rotation of a phase difference between the two physical transmission paths.

5. Method according to any of claims 1 to 3, wherein

- two physical transmission paths for said multiple downlink transmission are related to the first and second antenna, and

- a small opposite frequency offset between the two physical transmission paths is produced by a phase rotation in processing of the signal, with the signal being a complex base band signal, the frequency offset causing slow rotation of a phase difference between the two physical transmission paths.

6. Method according to any preceding claim, wherein transmissions using the least two antennas, so-called MIMO transmissions, and single antenna transmissions using only one antenna are scheduled separately in time domain.

7. Apparatus for transmitting data in a radio communications system, the apparatus comprising:

- at least a first antenna and a second antenna;

- means configured for a multiple downlink transmission of a signal; and means configured for applying precoding weights for mapping the signal to the first antenna and to the second antenna, with a first precoding weight

for mapping the signal to the first antenna, and a second precoding weight W₂ for mapping the signal to the second antenna, with W₂ being the first precode or the fourth precode out of a set of four alternative precodes, the set of four alternative precodes being defined as

[1+₇ 1-₇ -1+₇ -I-₇]