WO2016119901A1

WO2016119901A1 - Method and apparatus for virtual beamforming

Info

Publication number: WO2016119901A1
Application number: PCT/EP2015/052007
Authority: WO
Inventors: Wolfgang Zirwas
Original assignee: Nokia Solutions And Networks Management International Gmbh
Priority date: 2015-01-30
Filing date: 2015-01-30
Publication date: 2016-08-04

Abstract

There is provided a method comprising causing a data symbol to be transmitted N times at a time interval of nT_shiftto a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment.

Description

Title

METHOD AND APPARATUS FOR VIRTUAL BEAMFORMING

Field of the invention

The present application relates to a method, apparatus and system and in particular but not exclusively, virtual

beamforming of user data.

Background

A communication system can be seen as a facility that enables communication sessions between two or more entities such as user terminals, base stations and/or other nodes by providing carriers between the various entities involved in the

communications path. A communication system can be provided for example by means of a communication network and one or more compatible communication devices. The communications may comprise, for example, communication of data for carrying communications such as voice, electronic mail (email) , text message, multimedia and/or content data and so on. Non- limiting examples of services provided include two-way or multi-way calls, data communication or multimedia services and access to a data network system, such as the Internet.

In a wireless communication system at least a part of

communications between at least two stations occurs over a wireless link. Examples of wireless systems include public land mobile networks (PLMN) , satellite based communication systems and different wireless local networks, for example wireless local area networks (WLAN) . The wireless systems can typically be divided into cells, and are therefore often referred to as cellular systems. A user can access the communication system by means of an appropriate communication device or terminal. A communication device of a user is often referred to as user equipment (UE) . A communication device is provided with an appropriate signal receiving and transmitting apparatus for enabling

communications, for example enabling access to a

communication network or communications directly with other users. The communication device may access a carrier provided by a station, for example a base station of a cell, and transmit and/or receive communications on the carrier.

The communication system and associated devices typically operate in accordance with a given standard or specification which sets out what the various entities associated with the system are permitted to do and how that should be achieved. Communication protocols and/or parameters which shall be used for the connection are also typically defined. Summary

In a first aspect there is provided a method comprising causing a data symbol to be transmitted N times at a time interval of nT_shift to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment .

Tghift may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The method may comprise causing said data symbol to be transmitted using grid of beams beamforming. The user equipment may comprise a plurality of physical antennas, and the effective number of antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment.

The method may comprise causing a plurality of multi-coded streams per beam to be transmitted. The streams may be rotationally coded.

The method may comprise performing spatial multiplexing for the user equipment. The method may comprise causing the data symbol to be

transmitted in a resource block, said block having a time duration of T_shif_t.

The method may comprise causing the data symbol to be

transmitted from a multiple input multiple output

transmitter .

In a second aspect, there is provided a method, said method comprising receiving a data symbol N times at a time interval of nTghift at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment . Shift may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The user equipment may comprise a plurality of physical antennas, and the effective number of overall antennas user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment. The method may comprise receiving a plurality of multi-coded streams per beam.

The streams may be rotationally coded. The method may comprise receiving the data symbol in a resource block, said block having a time duration of T_shi_ft

The method may comprise receiving the data symbol from a multiple input multiple output transmitter.

In a third aspect, there is provided an apparatus said apparatus comprising means for causing a data symbol to be transmitted N times at a time interval of nT shift to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment. Shif_t may be dependent on at least one user equipment

mobility and intended beamforming pattern. The apparatus may comprise means for causing said data symbol to be transmitted using grid of beams beamforming.

The user equipment may comprise a plurality of physical antennas, and the effective number of antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment . The apparatus may comprise means for causing a plurality of multi-coded streams per beam to be transmitted.

The streams may be rotationally coded.

The apparatus may comprise means for performing spatial multiplexing for the user equipment.

The apparatus may comprise means for causing the data symbol to be transmitted in a resource block, said block having a time duration of T_shift .

The apparatus may comprise means for causing the data symbol to be transmitted from a multiple input multiple output transmitter.

In a fourth aspect there is provided an apparatus, said apparatus comprising means for receiving a data symbol N times at a time interval of nT_shif_t at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment. Shif_t may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The user equipment may comprise a plurality of physical antennas, and the effective number of overall antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment.

The apparatus may comprise means for receiving a plurality of multi-coded streams per beam. The streams may be rotationally coded.

The apparatus may comprise means for receiving the data symbol in a resource block, said block having a time duration

Of Tghiff

The apparatus may comprise means for receiving the data symbol from a multiple input multiple output transmitter.

In a fifth aspect, there is provided an apparatus, said apparatus comprising at least one processor and at least one memory including a computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:

cause a data symbol to be transmitted N times at a time interval of nT_shift to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment .

Tghift may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The apparatus may be configured to cause said data symbol to be transmitted using grid of beams beamforming.

The user equipment may comprise a plurality of physical antennas, and the effective number of antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment . The apparatus may be configured to cause a plurality of multi-coded streams per beam to be transmitted.

The streams may be rotationally coded.

The apparatus may be configured to perform spatial

multiplexing for the user equipment.

The apparatus may be configured to cause the data symbol to be transmitted in a resource block, said block having a time duration of T_shift .

The apparatus may be configured to cause the data symbol to be transmitted from a multiple input multiple output

transmitter.

In a sixth aspect, there is provided an apparatus, said apparatus comprising at least one processor and at least one memory including a computer program code, the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:

receive a data symbol N times at a time interval of nT shift cLt a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment. Shif_t may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The user equipment may comprise a plurality of physical antennas, and the effective number of overall antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment. The apparatus may be configured to receive a plurality of multi-coded streams per beam. The streams may be rotationally coded.

The apparatus may be configured to receive the data symbol in a resource block, said block having a time duration of T_shi_ft The apparatus may be configured to receive the data symbol from a multiple input multiple output transmitter.

In a seventh aspect there is provided a computer program embodied on a non-transitory computer-readable storage medium, the computer program comprising program code for controlling a process to execute a process, the process comprising: causing a data symbol to be transmitted N times at a time interval of nT_shif_t to a user equipment to form a transmission beam, wherein N is the number of virtual

antennas at the user equipment. Shif_t may be dependent on at least one user equipment

mobility and intended beamforming pattern. The process may comprise causing said data symbol to be transmitted using grid of beams beamforming.

The user equipment may comprise a plurality of physical antennas, and the effective number of antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment . The process may comprise causing a plurality of multi-coded streams per beam to be transmitted.

The streams may be rotationally coded.

The process may comprise performing spatial multiplexing for the user equipment.

The process may comprise causing the data symbol to be transmitted in a resource block, said block having a time duration of T_shift .

The process may comprise causing the data symbol to be transmitted from a multiple input multiple output

transmitter.

In an eighth aspect there is provided a computer program embodied on a non-transitory computer-readable storage medium, the computer program comprising program code for controlling a process to execute a process, the process comprising: receiving a data symbol N times at a time

interval of nT_shif_t at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment. Shif_t may be dependent on at least one user equipment

mobility and intended beamforming pattern.

The user equipment may comprise a plurality of physical antennas, and the effective number of overall antennas at the user equipment may be determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment. The process may comprise receiving a plurality of multi-coded streams per beam. The streams may be rotationally coded.

The process may comprise receiving the data symbol in a resource block, said block having a time duration of T_shi_ft The process may comprise receiving the data symbol from a multiple input multiple output transmitter.

In a ninth aspect there is provided a computer program product for a computer, comprising software code portions for performing the steps of any one of the first and second aspects when said product is run on the computer.

In the above, many different embodiments have been described. It should be appreciated that further embodiments may be provided by the combination of any two or more of the

embodiments described above.

Description of figures

Embodiments will now be described, by way of example only, with reference to the accompanying Figures in which: Figure 1 shows a schematic diagram of an example communication system comprising a base station and a plurality of communication devices; Figure 2 shows a schematic diagram, of an example mobile communication device;

Figure 3 shows a schematic diagram of a communication system with an eNB and a UE moving along a straight line;

Figure 4 shows a schematic diagram of an example communication system with an eNB and a UE moving along a straight line; Figure 5 shows simulation results of the normalised means square error over different prediction horizons;

Figure 6 shows a simplified representation of virtual beamforming;

Figure 7 shows CIR (amplitude) of SISO (single input ingle output) channel (top) and beamformed channel (bottom)

Figure 8a shows a flowchart of an example method of virtual beamforming for data transmission] ;

Figure 8b shows a flowchart of an example method of virtual beamforming for data transmission; Figure 9 shows a simplified representation of virtual beamforming on user data; Figure 10 shows a simplified representation of blockwise data transmission;

Figure 11 shows a simplified representation of virtual beamforming on user data to a UE with a number of physical antennas ;

Figure 12 shows minimum crosstalk over 20MHz frequency bandwidth

Figure 13 shows CDF code crosstalk between code 1 and code 2 for 16 virtual antenna elements; Figure 14 shows an example control apparatus;

Figure 15 shows a schematic diagram of an example apparatus;

Figure 16 shows a schematic diagram of an example apparatus.

Detailed description

Before explaining in detail the examples, certain general principles of a wireless communication system and mobile communication devices are briefly explained with reference to Figures 1 to 2 to assist in understanding the technology underlying the described examples.

In a wireless communication system 100, such as that shown in figure 1, mobile communication devices or user equipment (UE) 102, 104, 105 are provided wireless access via at least one base station or similar wireless transmitting and/or

receiving node or access point. Base stations are typically controlled by at least one appropriate controller apparatus, so as to enable operation thereof and management of mobile communication devices in communication with the base

stations. The controller apparatus may be located in a radio access network (e.g. wireless communication system 100) or in a core network (not shown) and may be implemented as one central apparatus or its functionality may be distributed over several apparatus. The controller apparatus may be part of the base station and/or provided by a separate entity such as a Radio Network Controller. In Figure 1 control apparatus 108 and 109 are shown to control the respective macro level base stations 106 and 107. The control apparatus of a base station can be interconnected with other control entities. The control apparatus is typically provided with memory capacity and at least one data processor. The control apparatus and functions may be distributed between a

plurality of control units. In some systems, the control apparatus may additionally or alternatively be provided in a radio network controller.

LTE systems may however be considered to have a so-called "flat" architecture, without the provision of RNCs; rather the (e)NB is in communication with a system architecture evolution gateway (SAE-GW) and a mobility management entity (MME) , which entities may also be pooled meaning that a plurality of these nodes may serve a plurality (set) of (e)NBs. Each UE is served by only one MME and/or S-GW at a time and the (e)NB keeps track of current association. SAE-GW is a "high-level" user plane core network element in LTE, which may consist of the S-GW and the P-GW (serving gateway and packet data network gateway, respectively) . The

functionalities of the S-GW and P-GW are separated and they are not required to be co-located. In Figure 1 base stations 106 and 107 are shown as connected to a wider communications network 113 via gateway 112. A further gateway function may be provided to connect to another network.

The smaller base stations 116, 118 and 120 may also be connected to the network 113, for example by a separate gateway function and/or via the controllers of the macro level stations. The base stations 116, 118 and 120 may be pico or femto level base stations or the like. In the

example, stations 116 and 118 are connected via a gateway 111 whilst station 120 connects via the controller apparatus 108. In some embodiments, the smaller stations may not be

provided.

A possible mobile communication device will now be described in more detail with reference to Figure 2 showing a

schematic, partially sectioned view of a communication device 200. Such a communication device is often referred to as user equipment (UE) or terminal. An appropriate mobile

communication device may be provided by any device capable of sending and receiving radio signals. Non-limiting examples include a mobile station (MS) or mobile device such as a mobile phone or what is known as a 'smart phone', a computer provided with a wireless interface card or other wireless interface facility (e.g., USB dongle) , personal data

assistant (PDA) or a tablet provided with wireless

communication capabilities, or any combinations of these or the like. A mobile communication device may provide, for example, communication of data for carrying communications such as voice, electronic mail (email) , text message,

multimedia and so on. Users may thus be offered and provided numerous services via their communication devices. Non- limiting examples of these services include two-way or multi- way calls, data communication or multimedia services or simply an access to a data communications network system, such as the Internet. Users may also be provided broadcast or multicast data. Non-limiting examples of the content include downloads, television and radio programs, videos,

advertisements, various alerts and other information. The mobile device 200 may receive signals over an air or radio interface 207 via appropriate apparatus for receiving and may transmit signals via appropriate apparatus for transmitting radio signals. In Figure 2 transceiver apparatus is designated schematically by block 206. The transceiver apparatus 206 may be provided for example by means of a radio part and associated antenna arrangement. The antenna

arrangement may be arranged internally or externally to the mobile device. A wireless communication device can be provided with a Multiple Input / Multiple Output (MIMO) antenna system. MIMO systems use multiple antennas at the transmitter and receiver along with digital signal processing to improve link quality and capacity. Although not shown in Figures 1 and 2, multiple antennas can be provided, for example at base stations and mobile stations, and the transceiver apparatus 206 of Figure 2 can provide a plurality of antenna ports. A station may comprise an array of multiple antennas. Signalling and muting patterns can be associated with transmitter antenna numbers or receiver port numbers of MIMO arrangements.

A mobile device is typically provided with at least one data processing entity 201, at least one memory 202 and other possible components 203 for use in software and hardware aided execution of tasks it is designed to perform, including control of access to and communications with access systems and other communication devices. The data processing, storage and other relevant control apparatus can be provided on an appropriate circuit board and/or in chipsets. This feature is denoted by reference 204. The user may control the operation of the mobile device by means of a suitable user interface such as key pad 205, voice commands, touch sensitive screen or pad, combinations thereof or the like. A display 208, a speaker and a microphone can be also provided. Furthermore, a mobile communication device may comprise appropriate

connectors (either wired or wireless) to other devices and/or for connecting external accessories, for example hands-free equipment, thereto.

The communication devices 102, 104, 105 may access the communication system based on various access techniques, such as code division multiple access (CDMA) , or wideband CDMA (WCDMA) . Other non-limiting examples comprise time division multiple access (TDMA) , frequency division multiple access (FDMA) and various schemes thereof such as the interleaved frequency division multiple access (IFDMA), single carrier frequency division multiple access (SC-FDMA) and orthogonal frequency division multiple access (OFDMA) , space division multiple access (SDMA) and so on.

An example of wireless communication systems are

architectures standardized by the 3rd Generation Partnership Project (3GPP) . A latest 3GPP based development is often referred to as the long term evolution (LTE) of the Universal Mobile Telecommunications System (UMTS) radio-access

technology. The various development stages of the 3GPP specifications are referred to as releases. More recent developments of the LTE are often referred to as LTE Advanced (LTE-A) . The LTE employs a mobile architecture known as the Evolved Universal Terrestrial Radio Access Network (E-UTRAN) . Base stations of such systems are known as evolved or

enhanced Node Bs (eNBs) and provide E-UTRAN features such as user plane Radio Link Control/Medium Access Control/Physical layer protocol (RLC/MAC/PHY) and control plane Radio Resource Control (RRC) protocol terminations towards the communication devices. Other examples of radio access system include those provided by base stations of systems that are based on technologies such as wireless local area network (WLAN) and/or WiMax (Worldwide Interoperability for Microwave

Access) . A base station can provide coverage for an entire cell or similar radio service area.

The following is relevant for the future evolution of mobile radio systems beyond LTE Advanced and 5G systems. Massive MIMO and coordinated multi point (CoMP) are expected to be one of the ingredients of a 5G system and results promise large gains, but also indicate that accurate channel

knowledge will be key to success. Virtual beamforming for channel estimation and prediction has been proposed and latest simulation results indicate that channel prediction quality may improve as a result.

Figure 3 depicts a scenario used in an analysis, in which one eNB at about 32m height is provided and one UE is moved along a straight line over 51 locations in steps of 1cm are

provided. This scenario is investigated twice i) by ray tracing and ii) by measurements. For the measurements only one Tx antenna is available, while for raytracing additionally a 16 element uniform linear array (ULA) can be used .

Figure 4 further illustrates a basic intended concept being investigated, the so called grid of beam (GoB) concept, which uses a 16 (or more) antenna element ULA for forming a set of fixed beams .

As the UE moves along a straight line over several locations, the UE measures and stores for each location the channel state information (CSI) . By combining these measurement with suitable beamforming weights, so-called virtual beamforming is possible. The difference to conventional beamforming is that the UEs do the beamforming with only one physically available antenna element that is moved to different

locations. For each location the channel measurement is stored and combined afterwards by an according postcoder filter. The motivation for virtual - instead of real - beamforming is the expectation that UEs may have - even for 5G - a limited number of antenna elements due to, for example space limitations and costs for RF frontends

Figure 5 shows some simulation results of the normalized mean square error (NMSE) over different prediction horizons with and without virtual beamforming and with and without the GoB precoding. Where possible the ray tracing results are

verified by the measurements, which is important as pure artificial channels do not include moving objects, diffuse scatterers etc.

It can be seen from figure 5 that virtual beamforming

together with Tx-sided beamforming by the GoB concept has the potential to reduce the channel prediction error (NMSE) by some 10 to 20dB. This may boost system level performance for example, for massive MIMO and/or joint transmission CoMP schemes, which may play a major role for 5G. The prediction gains are mainly a result of reduced number of multipath components of the effective channel components with

corresponding smoother channel evolution and easier

trackability .

Figure 6 shows a simplified visualisation of virtual

beamforming, where the UE forms narrow beams by according beamforming weights W_UE . Using the shaded beam only multi path components from this direction will contribute to the effective beam. As a result the channel evolution may become smoother, as shown in Figure 7 by comparison of a single input single output (SISO) versus the virtually beamformed radio channel. Figure 7 shows the comparison CIR (amplitude) of SISO channel (top) versus virtually beamformed channel (bottom) . Virtual beamforming gains may be applicable not only for channel estimation and prediction, but also for user data transmission. Gains may include, amongst others, according signal to noise ratio (SNR) gains, suppression of

interference, reduction of impact of moving objects as well as diffuse scatterers etc.

Direct application of virtual beamforming to user data transmission may be desirable. Virtual beamforming requires re-transmission of the same reference or data symbol for N time instances, where N is the number of virtual antenna elements. For data transmission, the UE data rate will go down by a factor of N in case of N virtual antenna elements. For example 32 antenna elements means a reduction by a factor of 32, which may not be compensated by in higher spectral efficiency or overall capacity.

Figure 8a shows a flow chart of an example method of virtual beamforming for user data transmission. The method comprises causing a data symbol to be transmitted N times at a time interval of nT_shi_ft to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment .

Figure 8b shows a flowchart of a method of virtual

beamforming for user data transmission. The method comprises receiving a data symbol N times at a time interval of nT_shif_t at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment .

The data symbol may be caused to be transmitted (or received) from a multiple input multiple output device or a single antenna devices, such as, for example, from small cell devices or, in case of CoMP from many single antenna eNBs.

The basis for the method is the retransmission of the same data signal over several time instants suitably aligned with the user movement. This is illustrated in Figure 9 and Figure 10 where a particular data symbol is transmitted N times with a certain time delay of n times Tshift, which may depend on the UE mobility and the intended beamforming pattern. In an embodiment, the beam may be caused to be transmitted using beamforming, for example GOB beamforming. In the example shown in figure 9, the shaded beam at eNB retransmits the same data symbol N times and with a time delay of n x Tshift. N is number of virtual antenna elements and Tshift is adapted to mobile speed and intended spatial distance between virtual antenna elements. For a continuous data transmission the virtual beamforming may be done blockwise and the size of the blocks may be adapted so that one block has exactly the time duration of Tshift. For low load conditions this mode may be useful, but resource usage may be inefficient. For example a 32 element array will have a l/32th lower symbol rate compared to a conventional transmission. Note, this symbol rate ignores potentially higher data rates due to better interference suppression and better SNR due to virtual beamforming. One way of keeping the virtual beamforming gains together with higher resource usage would be to serve more users simultaneously by more multi user (MU) MIMO spatial

multiplexing. This is possible as long there are enough degree of freedoms at the eNB side and enough active users in the cell and relies on the fact that virtual beamforming improves the rank - or more correctly the condition - of the channel matrix due to the higher number of Virtual' Rx antenna elements. Extreme MU-MIMO as indicated above cannot be assumed possible in all scenarios.

Figure 11 illustrates an embodiment in which a UE has a plurality of physical antennas. In this example, a UE has for example, 4 Rx antennas per UE . The number of antennas may be any suitable number of antennas. Targeting, for example an antenna array with overall 16 antenna elements, the number of virtual antenna elements reduces to four. For transmission, a data symbol is thus transmitted four times with four time shifts and at each time instance it is being received by four spatially separated physical antennas. For a regular grid this may require a further adaptation of the transmit timing to the mobile speed and the physical antenna distances to get a regular uniform linear array. An ULA with irregular grid might be, but the UE beamformer should know the resulting physical locations of the real and the virtual antennas so that it can form narrow beams. In one embodiment, multiple stream transmission per UE is provided. Similar as for MU-MIMO this is possible as long as there are sufficient number of Tx antennas available.

Significant gains in the channel rank due to strong virtual beamforming may support such multi stream transmission per UE . The main benefit over MU-MIMO is that multiple stream transmission per UE is also possible with a limited number of UEs. For full resource usage the above described case, for example, in four parallel streams should be transmitted per UE.

For an overall cell wide - or in case of CoMP multicell - precoder, the number of data streams may become large. For example with future massive MIMO, ten or more UEs might be served, i.e. in case of four spatial streams per UE this would result overall in 40 data streams simultaneously on air per cell. It may be desirable to apply the CDMA principle, i.e. to transmit two or more data streams simultaneously with different codes. Any code with low inter code interference and simultaneously suitable beamforming characteristics, such as, for example rotationary codes, may be a suitable code. In one embodiment, unitary rotational codes of the form C=exp(- jkR2n/N); [k=1...4; R is the number of rotations with a typical value of 4] provide, as one possible example, very low code crosstalk together with moderate effects on the beamforming patterns .

The UE may estimate the CSI for all virtual antenna elements based on the regularly transmitted CSI reference symbols (RSs) , but the eNB knows only the effective virtually

beamformed channel based on according reporting and channel prediction. Therefore the eNB may not be able to do any adaptive precoding for reduction of the inter code crosstalk, which should therefore be sufficiently low. Inter code crosstalk varies between very low and very high values for different virtual beams and different PRBs . The solution has been to select suitable beams per PRB with lowest inter code crosstalk at UE side. Fortunately the crosstalk is also relative slowly varying for certain beams and certain PRBs so that an according semi static adaptation is possible.

Code design may be important as mentioned above as

beamforming and inter-code crosstalk should be optimized simultaneously. Rotational codes may lead to smooth effective channel evolution and simultaneously to code crosstalk below -20dB.

As an advanced solution beams or PRBs experiencing very strong crosstalk might be used as well in combination with a corresponding successive interference canceller (SIC)

receiver. In Figure 12 the crosstalk between three codes is shown for a measured radio channel for a 16 element ULA from which 4 are physical antennas. As can be seen for two codes the achieved minimum crosstalk between code 1 and 2 per PRB in a 20MHz bandwidth is most of the time below -20dB (see also corresponding cumulative distribution function (CDF) in Figure 13, which shows CDF crosstalk between code 1 and 2 for 16 virtual antenna elements, optimally selected for the best fitted beam) .

Virtual beamforming may be used to counteract rotations of the user device. For example rotational sensors can provide according information about the user device movement and the virtual beam is then rotated into the opposite direction. The virtual beam may be rotated step wise, for example, over angles of few to several tens of degrees.

A single step solution providing virtual beamforming gains for user data transmission together with full resource usage is complex. In one embodiment, a combination of the above described solutions, i.e. a plurality of physical antennas with a plurality of virtual antennas leads to an effectively larger virtual antenna array, e.g. for four physical antennas and four virtual antennas an effective 16 element virtual array may be realised. The benefit of large UE antenna arrays with a single to few physical UE antennas may be achieved. Per UE two spatial streams may be transmitted with two rotational codes each. That way, the same resource usage as for a system with 16 physical antennas may be achieved. Using only two codes a low inter code crosstalk can be maintained while the number of spatial streams on air may is still limited, i.e. only increased by factor less than two.

The overall benefits may be, amongst others, a large

prediction horizon for the channel estimation, strong

beamforming gains for the user data transmission, suppressing of inter cell interference and - per se almost unpredictable diffuse scatterers - as well as of channel variations of moving objects, SNR gains at least from the physical antenna elements . The method may achieve high virtual beamforming gains for user data transmission in combination with effective resource usage in terms of number of transmitted symbols per resource element.

High number of physical as well as virtual antennas may allow for strong beamforming gains, which reduces the number of relevant multi path components (MPC) per channel component. That way prediction horizon for channel prediction increases significantly. By transmitting user data over the same predicted virtual beams as used for channel

estimation/prediction a simple and robust solution may be achieved, avoiding complex reconstruction of single antenna CSI from many virtual beams.

Combining virtual and physical antennas may lead to very narrow Rx beams . The according SNR gain depends on the relation of physical to virtual antennas. Assuming a 4 ULA with 16 overall antenna elements after virtual beamforming the SNR gain would be 10. *logl0 (16) =12dB for a single stream transmission. If two codes plus two streams per UE are used for full resource utilization 6dB would be lost and the residual SNR gain will be 6dB. But the UE keeps full IF cancelation capability and in combination with the GoB concept further 10 dB SNR gains can be expected leading to overall 16dB coverage gains on system level (beside higher CSI predictability as explained above) . Beamforming at UE side may be powerful - and even more powerful than on eNB side -as the angle of arrival (AoA) spread is typically very large, i.e. UEs receive in non-line- of-sight (NLOS) conditions multi path components from almost all directions from close by reflectors. In such scenarios narrow beams provide significant MPC reduction.

Originally virtual beamforming was proposed for improvement of channel estimation and prediction. Directly reusing the virtual beams not only for channel estimation, but also for user data transmission is of vital importance for a robust system design and the success of massive MIMO and JT CoMP. The overall concept may be harmonized and standardized between eNB and UE . Amongst others, selection of transmission modes like number of beams per UE, and number of codes per UE, potentially adaptive to certain set of physical resource blocks (PRBs) to minimize the code crosstalk, may be

harmonised. Furthermore the block size, the time intervals for virtual beamforming may be harmonised. The number of physical antennas and/or information about their relative location may be useful to adapt the virtual beamforming scheme accordingly. Reporting of sensor data like rotations and accelerators may be helpful as well.

Selection of used beams per UE may include criteria like beam directions with no or only weak MPCs of moving objects as well as very low diffuse scattering, as these effects may be relatively difficult to predoct. Note this is assumed to be one of the main reasons for significantly shorter prediction horizons in real world radio channels compared to artificial ones. Minimizing these effects should close the gap between channel prediction for real world and artificial radio channels.

It should be noted that there are different options to combine physical and virtual antennas depending on antenna element distance speed and intended beamforming patterns. Instead of the interlaced solution according to Figure 11 one could as one example also shift the physical antennas as a whole sequentially. This may be suitable if the antenna spacing is small and the mobile speed is large.

It should be understood that each block of the flowchart of Figures8 and any combination thereof may be implemented by various means or their combinations, such as hardware, software, firmware, one or more processors and/or circuitry.

Embodiments described above by means of figures 1 to 13 may be implemented on a control apparatus as shown in figure 14 or on a mobile device such as that of figure 2. Figure 14 shows an example of a control apparatus for a communication system, for example to be coupled to and/or for controlling a station of an access system, such as a base station or (e) node B, or a server or host. In some embodiments, base stations comprise a separate apparatus unit or module. In other embodiments, the control apparatus can be another network element such as a radio network controller or a spectrum controller. In some embodiments, each base station may have such a control apparatus as well as a control apparatus being provided in a radio network controller. The control apparatus 300 can be arranged to provide control on communications in the service area of the system. The control apparatus 300 comprises at least one memory 301, at least one data processing unit 302, 303 and an input/output interface 304. Via the interface the control apparatus can be coupled to a receiver and a transmitter of the base station. The receiver and/or the transmitter may be implemented as a radio front end or a remote radio head. For example the control apparatus 300 can be configured to execute an appropriate software code to provide the control functions. Control functions may include at least causing a data symbol to be transmitted N times at a time interval of nT_shif_t to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment.

Alternatively, or in addition, control functions may comprise receiving a data symbol N times at a time interval of nT_shif_t at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment.

An example of an apparatus 1500 is shown in figure 15 and comprises means 1510 for causing a data symbol to be transmitted N times at a time interval of nT shift to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment.

An example of an apparatus 1600 is shown in figure 16 and comprises means 1610 for receiving a data symbol N times at a time interval of nT_shif_t at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment.

It is also noted herein that while the above describes example embodiments, there are several variations and modifications which may be made to the disclosed solution without departing from the scope of the present invention.

In general, the various embodiments may be implemented in hardware or special purpose circuits, software, logic or any combination thereof. Some aspects of the invention may be implemented in hardware, while other aspects may be implemented in firmware or software which may be executed by a controller, microprocessor or other computing device, although the invention is not limited thereto. While various aspects of the invention may be illustrated and described as block diagrams, flow charts, or using some other pictorial representation, it is well understood that these blocks, apparatus, systems, techniques or methods described herein may be implemented in, as non-limiting examples, hardware, software, firmware, special purpose circuits or logic, general purpose hardware or controller or other computing devices, or some combination thereof.

Embodiments as described above by means of figures 1 to 13 may be implemented by computer software executable by a data processor, at least one data processing unit or process of a device, such as a base station, e.g. eNB, or a UE, in, e.g., the processor entity, or by hardware, or by a combination of software and hardware. Computer software or program, also called program product, including software routines, applets and/or macros, may be stored in any apparatus-readable data storage medium or distribution medium and they include program instructions to perform particular tasks. An apparatus-readable data storage medium or distribution medium may be a non-transitory medium. A computer program product may comprise one or more computer-executable components which, when the program is run, are configured to carry out embodiments. The one or more computer-executable components may be at least one software code or portions of it.

Further in this regard it should be noted that any blocks of the logic flow as in the Figures may represent program steps, or interconnected logic circuits, blocks and functions, or a combination of program steps and logic circuits, blocks and functions. The software may be stored on such physical media as memory chips, or memory blocks implemented within the processor, magnetic media such as hard disk or floppy disks, and optical media such as for example DVD and the data variants thereof, CD. The physical media is a non-transitory media .

The memory may be of any type suitable to the local technical environment and may be implemented using any suitable data storage technology, such as semiconductor-based memory devices, magnetic memory devices and systems, optical memory devices and systems, fixed memory and removable memory. The data processors may be of any type suitable to the local technical environment, and may include one or more of general purpose computers, special purpose computers, microprocessors, digital signal processors (DSPs) , application specific integrated circuits (ASIC) , FPGA, gate level circuits and processors based on multi-core processor architecture, as non-limiting examples.

Embodiments described above in relation to figures 1 to 8 may be practiced in various components such as integrated circuit modules. The design of integrated circuits is by and large a highly automated process. Complex and powerful software tools are available for converting a logic level design into a semiconductor circuit design ready to be etched and formed on a semiconductor substrate.

The foregoing description has provided by way of non-limiting examples a full and informative description of the exemplary embodiment of this invention. However, various modifications and adaptations may become apparent to those skilled in the relevant arts in view of the foregoing description, when read in conjunction with the accompanying drawings and the appended claims. However, all such and similar modifications of the teachings of this invention will still fall within the scope of this invention as defined in the appended claims. Indeed there is a further embodiment comprising a combination of one or more embodiments with any of the other embodiments previously discussed.

Claims

1. A method comprising:

causing a data symbol to be transmitted N times at a time interval of nT_shif_t to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment.

2. A method according to claim 1, wherein T_shi_ft is

dependent on at least one user equipment mobility and intended beamforming pattern.

3. A method according to any preceding claim, comprising causing said data symbol to be transmitted using grid of beams beamforming.

4. A method according to any preceding claim, wherein the user equipment has a plurality of physical antennas, and the effective number of antennas at the user equipment is determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment.

5. A method according to any preceding claim, comprising causing a plurality of multi-coded streams per beam to be transmitted .

6. A method according to claim 5, wherein the streams are rotationally coded.

7. A method according to any preceding claim, comprising performing spatial multiplexing for the user equipment.

8. A method according to any preceding claim, comprising causing the data symbol to be transmitted in a resource block, said block having a time duration of T_shift.

9. A method according to any preceding claim, causing the data symbol to be transmitted from a multiple input multiple output transmitter.

10. A method comprising: receiving a data symbol N times at a time interval of nT_shift at a user equipment in the form of a transmission beam, wherein N is the number of virtual antennas at the user equipment.

11. A method according to claim 10, wherein Tshift IS

dependent on at least one user equipment mobility and

intended beamforming pattern.

12. A method according to any one of claims 10 or 11, wherein the user equipment has a plurality of physical antennas, and the effective number of overall antennas at the user equipment is determined by the number of virtual antennas N multiplied by the number of physical antennas at the user equipment.

13. A method according to any one of claims 10 to 12, comprising receiving a plurality of multi-coded streams per beam.

14. A method according to claim 13, wherein the streams are rotationally coded.

15. A method according to any one of claims 10 to 14, comprising receiving the data symbol in a resource block, said block having a time duration of T_shift-

16. A method according to any one of claims 10 to 15, comprising receiving the data symbol from a multiple input multiple output transmitter.

17. An apparatus comprising means for carrying out the method according to any one of claims 1 to 16.

18. A computer program product for a computer, comprising software code portions for performing the steps of any one of claims 1 to 16 when said product is run on the computer.

19. An apparatus, said apparatus comprising:

at least one processor;

and at least one memory including computer program code; the at least one memory and the computer program code configured to, with the at least one processor, cause the apparatus at least to:

cause a data symbol to be transmitted N times at a time interval of nT_shift to a user equipment to form a transmission beam, wherein N is the number of virtual antennas at the user equipment 20. An apparatus, said apparatus comprising:

at least one processor;

receive a data symbol N times at a time interval of nTghift at a user equipment in the form of a transmission beam, wherein N i s the number of virtual antennas at the user equipment .