WO2011103912A1

WO2011103912A1 - Transmitting in non-beamforming mode with a beamforming antenna array

Info

Publication number: WO2011103912A1
Application number: PCT/EP2010/052253
Authority: WO
Inventors: Christian Mahr
Original assignee: Nokia Siemens Networks Oy
Priority date: 2010-02-23
Filing date: 2010-02-23
Publication date: 2011-09-01
Also published as: EP2540005A1

Abstract

An apparatus (10) comprises a beamforming antenna array which includes n antennas (14₁ to 14_n) having k antenna elements (15₁₁ to 15_1k,..., 15_n1 to 15_nk), n being an integer greater than one and k being an integer greater than zero. The apparatus (10) further comprises n transmitters (13₁ to 13_n) provided in correspondence with the antennas (14₁ to 14_n), each of the n transmitters having an average transmit power of 1/n of the total transmit power, and a processor (11) which splits a signal to be radiated in a non-beamforming mode into n adjacent frequency ranges, thereby obtaining n split signals, and applies the n split signals to the n transmitters (13₁ to 13_n) such that power is distributed equally to all (n x k) antenna elements (15₁₁ to 15_1k,..., 15_n1 to 15_nk).

Description

DESCRIPTION

TITLE

Transmitting in Non-Beamforming Mode with a Beamforming Antenna Array

The present invention relates to an apparatus and a method of transmitting in a non-beamforming mode with a beamforming antenna array.

Recent radio technology uses multi-element antennas plus mul^¬ tiple transmitters and receivers to create fixed or variable beams to transmit to multiple individual user equipments in different directions at the same time. This technique to nar^¬ row the radiation to an individual user equipment (UE) is called "beamforming". By doing so, an individual transmission covers only a certain subsector of a whole cell of a cellular network.

Individual transmitters for n antenna elements usually have 1/n-th of the power normally required for this cell in a non- beamforming case.

When using beamforming (BF) in a cell, the "beamforming mode" cannot be applied to all of the transmissions within this cell. Several transmissions such as broadcasting/paging channels and transmissions to non-BF capable user equipments need to be addressed to the whole cell, not only to a subsector. Even if the user equipment is capable of the beamforming mode, it might be not advisable in certain propagation situa^¬ tions to use beamforming, but better radiate to the whole cell instead. When radiating a signal to the whole cell with a horizontal antenna array, this can be done by supplying the signal to only one of the antenna elements. The problem in this situa^¬ tion is that one of the transmitters needs to be dimensioned with much more power than the other ones. This inequality re^¬ sults in significant design issues, as transmitters for BF underlie extreme requirements for phase and amplitude align^¬ ment, so in practice identical designs are highly advisable. The present invention aims at solving the above problems and provides an apparatus and a method in which an antenna array can be used both in beamforming mode and non-beamforming mode without restrictions. The invention may also be implemented by a computer program product.

According to the invention, an overall equal power distribu^¬ tion of all antenna elements of a beamforming antenna array can be achieved. In the following, embodiments of the invention will be de^¬ scribed by referring to the accompanying drawings, in which:

Fig. 1 shows a schematic block diagram illustrating a structure of an apparatus according to an embodiment of the inven- tion; and

Fig. 2 shows a diagram illustrating an example of a beamforming antenna array with cross-polarized antennas used for transmission in a non-beamforming mode.

As shown in Fig. 1, an apparatus 10 comprises a processor 11, a memory 12, first to n-th transmitters 13i to 13_n, and first to n-th antennas 14i to 14_n. Each antenna 14i to 14_n may have one or more antenna elements 15n to 15i_k, 15_ni to 15_nk. The processor 11 is connected to the memory and the transmitters 13i to 13_n which are provided for each of the antennas 14i to 14_n. transmitter 13i is coupled to an antenna 14i, and a transmitter 13_n is coupled to an antenna 14_n, for example. The transmitters 13i to 13_n transmit signals by means of the antennas or antenna elements with which they are coupled. The antennas 14i to 14_n form an antenna array for radiating sig^¬ nals. The apparatus 10 may be part of a base station of a mo^¬ bile communications network, or part of a user equipment.

The terms "connected, " "coupled, " or any variant thereof, mean any connection or coupling, either direct or indirect, between two or more elements, and may encompass the presence of one or more intermediate elements between two elements that are "connected" or "coupled" together. The coupling or connection between the elements can be physical, logical, or a combination thereof. As employed herein two elements may be considered to be "connected" or "coupled" together by the use of one or more wires, cables and printed electrical connec^¬ tions, as well as by the use of electromagnetic energy, such as electromagnetic energy having wavelengths in the radio frequency region, the microwave region and the optical (both visible and invisible) region, as non-limiting examples.

The memory 12 may store program instructions that, when exe^¬ cuted by the processor 11, enable the apparatus 10 to operate in accordance with the exemplary embodiments of this inven^¬ tion, as detailed below. Inherent in the processor 11 is a clock to enable synchronism among the various apparatus for transmissions and receptions within the appropriate time in^¬ tervals and slots required, as the scheduling grants and the granted resources/subframes are time dependent.

In general, the exemplary embodiments of this invention may be implemented by computer software stored in the memory 12 and executable by the processor 11 of the apparatus 10, or by hardware, or by a combination of software and/or firmware and hardware in any or all of the devices shown. The memory 12 may be of any type suitable to the local tech^¬ nical 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 processor 11 may be of any type suitable to the local techni^¬ cal environment, and may include one or more of general pur^¬ pose computers, special purpose computers, microprocessors, digital signal processors (DSPs) and processors based on a multi-core processor architecture, as non-limiting examples.

Embodiments of the invention may be practiced in various com^¬ ponents such as integrated circuit modules. The design of in- tegrated circuits is by and large a highly automated process. Complex and powerful software tools are available for con^¬ verting a logic level design into a semiconductor circuit de^¬ sign ready to be etched and formed on a semiconductor sub^¬ strate .

Programs, such as those provided by Synopsys, Inc. of Moun^¬ tain View, California and Cadence Design, of San Jose, Cali^¬ fornia automatically route conductors and locate components on a semiconductor chip using well established rules of de- sign as well as libraries of pre-stored design modules. Once the design for a semiconductor circuit has been completed, the resultant design, in a standardized electronic format (e.g., Opus, GDSII, or the like) may be transmitted to a semiconductor fabrication facility or "fab" for fabrication.

According to an embodiment of the invention, the processor 11 splits a signal to be radiated from the apparatus 10 in a non-beamforming mode into n adjacent frequency ranges, whereby n split signals are obtained. The n split signals are applied by the processor 11 to the n transmitters 13i to 13_n such that power is distributed equally to all (n x k) antenna elements 15n to 15i_k, 15_ni to 15_nk. n is an integer greater than one and k is an integer greater than zero. The transmit^¬ ters 13i to 13_n are provided in correspondence with the an^¬ tennas 14i to 14_n, and each of the transmitters 13i to 13_n has an average transmit power of 1/n of the total transmit power.

For example, each antenna 14i to 14_n has one antenna element 15ii, 15_ni (i.e. k=l) . Signals to be radiated in non-BF mode are split into adjacent frequency ranges so that the power of non-BF transmissions is equally distributed to all n antenna elements.

In an LTE (long term evolution) system, n equidistant frequency sub-bands can be used. The split signals are achieved in OFDM (orthogonal frequency division multiplex) transmis^¬ sions by splitting the signal to be radiated in the non- beamforming mode into n groups of subcarriers and then assign the resulting n sub-bands to the n antenna elements. In the case of a cross-polarized antenna array used in MIMO (multi-input, multi-output) mode, antenna elements can be used in an arrangement to compensate antenna pattern inaccu^¬ racies of individual elements by using - in the same fre^¬ quency band - such elements of the other polarisation layer which are located opposite to each other within the array. Those will complement each other for the overall radiation pattern in this frequency band to approximate the full sector coverage better. For example, referring to Fig. 2, a 4+4 cross-polarized an^¬ tenna array 20 is assumed, with four antennas in a lambda/2 distance to cover a 120-degree sector. Each of the four an^¬ tennas comprises two antenna elements. That is, a first an^¬ tenna comprises elements 15n and 15i₂, a second antenna com- prises elements 15₂i and 1522_/ a third antenna comprises ele^¬ ments 15₃₁ and 15₃₂, and a fourth antenna comprises elements 15₄₁ and 15₄₂ - It is to be noted that Fig. 2 is a schematic drawing and does not show vertical replication of antenna elements . In the BF mode, all four antennas receive the same signal with specific phase shifts to create user specific beams, ac^¬ cording to an applied BF algorithm. For the sake of simplic^¬ ity it is assumed that all four antennas receive the same power from BF mode signals.

In the non-BF mode, a signal to be radiated by the apparatus 10 is split into four adjacent frequency ranges. The fre^¬ quency split can easily be done in particular for OFDMA (Orthogonal Frequency-Division Multiple Access) or SC-DMA (Sin- gle-Carrier Frequency-Division Multiple Access) based systems, which employ an IFFT (Inverse Fast Fourier Transforma^¬ tion) at the end of the baseband symbol layer processing prior to cyclic prefix insertion. To ease UE reception and eNB implementation, the frequency split may be aligned with PRB (subband group) boundaries. The input signal to the IFFT constitutes the baseband frequency domain representation of the transmitted signal which allows for a straightforward splitting into the desired adjacent frequency ranges. This makes the present invention very appropriate for application in LTE or LTE-Advanced . Application to other systems which are based on code division multiplex, time division multi^¬ plex, frequency division multiplex and combinations thereof may be considered, even though these systems per se may not employ such a final inverse Fourier Transformation, like the IFFT in OFDMA and SC-FDMA based systems. But one may obtain the required split into separate signals related to adjacent frequency bands for transmission over different antennas in such a system by appropriate filtering in time domain or by an additional Fourier Transformation through FFT or DFT, separation into sets of adjacent frequency bands and inverse Fourier Transformation through IFFT or IDFT and transmission of the obtained time-domain signals associated with the vari^¬ ous frequency bands via different antennas.

In the example according Fig. 2 for a system employing OFDMA the F=4 frequency ranges (f=0,l... F-l) are preferably equidis^¬ tant within the system bandwidth which may comprise L subcar- riers . Split signal s_f(t) (t=0,... L-l) constitutes the time domain representation after cyclic prefix insertion of the frequency band S in the subcarrier range 1= [Lf/F; L ( f+1 ) /F -1] according to

before cyclic prefix insertion. Each of the four split signals is applied to two transmitters of two opposite antenna elements for the same frequency range. That is, the two opposite antenna elements are ele^¬ ments of orthogonal polarisation layers and are located oppo^¬ site to each other within the array. As shown in Fig. 2, a first split signal (f=0) of a first 5MHz band is applied to transmitters of the antenna elements 15n and 15₄₂- A second split signal (f=l) of a second 5MHz band is applied to trans^¬ mitters of the antenna elements 15₂i and 1532 - A third split signal (f=2) of a third 5MHz band is applied to transmitters of the antenna elements 15₃i and 15₂2 · A fourth split signal

(f=3) of a fourth 5MHz band is applied to transmitters of the antenna elements 15₄i and 15₂i.

As a result of this split, the sum of beam-formed and non- beam-formed signals is equal for all 8 antenna elements.

Thus, the transmitters can be identical and need not to be over-dimensioned . For example, also referring to Fig. 1, the antenna elements 15ii and 15i₂ belong to antenna 14i, the antenna elements 15₂i and 1522 belong to antenna 14₂, the antenna elements 15₃i and 15₃₂ belong to antenna 14₃, and the antenna elements 15₄i and 15₄₂ belong to antenna 14₄ (i.e. n=4, k=2) . Transmitters 13i, 132, 13₃ and 13₄ are provided for the antennas 14i, 14₂, 14₃, 14₄, respectively. The processor 11 may form the split sig^¬ nals and apply the split signals to the respective transmit^¬ ters as described above, e.g. by processing a program stored in the memory 12.

According to an embodiment of the invention, a beamforming antenna array comprises n antennas 14i to 14_n having k an^¬ tenna elements 15n to 15i_k, 15_ni to 15_nk, n being an inte- ger greater than one and k being an integer greater than zero, n transmitters 13i to 13_n are provided in correspon^¬ dence with the antennas 14i to 14_n, each of the n transmit^¬ ters having an average transmit power of 1/n. The processor 11 splits the signal to be radiated in a non-beamforming mode into n adjacent frequency ranges, thereby obtaining n split signals. The processor applies each of the n split signals to k transmitters of k antenna elements for the same frequency range, wherein the k antenna elements are elements of or^¬ thogonal polarisation layers and are located opposite to each other within the array. In the example shown in Fig. 2, n=4 and k=2.

Alternatively, transmitters may be provided for each of the antenna elements 15n to 15i_k, 15_ni to 15_nk. Then an average transmit power of each transmitter is 1/ (n x k) .

With the above embodiments, transmit amplifiers can be de^¬ signed identically and for minimum desired power level. As the power radiated in non-BF mode covers only 1/nth of a bandwidth compared to a beam-formed signal, the average-to- max ratio (Crest-factor) for the transmitters is improved ac^¬ cording to the ratio of power used in non-BF mode to BF mode.

Moreover, since all antenna elements are partially used, a better overall coverage can be achieved in case of asymme^¬ tries in the antenna pattern of a single antenna element. As a radio channel is frequency selective anyway, the use of a different antenna at a different frequency can be compensated automatically within normal channel estimation mechanisms. Thus, no additional procedure is needed on a receiving side.

According to an embodiment of the invention, an apparatus, e.g. the apparatus 10 of Fig. 1, comprises a beamforming an^¬ tenna array comprising n antennas having k antenna elements, n being an integer greater than one and k being an integer greater than zero, n transmitting means provided in correspondence with the antennas, each of the n transmitting means having an average transmit power of 1/n, and processing means for splitting a signal to be radiated in a non-beamforming mode into n adjacent frequency ranges, thereby obtaining n split signals, and applying the n split signals to the n transmitting means such that power is distributed equally to all (n x k) antenna elements. The n adjacent frequency ranges may be n equidistant fre^¬ quency sub-bands . k may be equal to one and the processing means may apply the f=n split signals to the n transmitting means, respectively, for radiation by the n antenna elements. k may be greater than one and the processing means may apply each of the f split signals to k transmitters. The processing means may comprise the processor 11 and the transmitting means may comprise the transmitters 14i to 14_n. It is to be understood that the above description is illus^¬ trative of the invention and is not to be construed as limit^¬ ing the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the ap^¬ pended claims.

Claims

CLAIMS :

1. An apparatus comprising:

a beamforming antenna array comprising n antennas having k antenna elements, n being an integer greater than one and k being an integer greater than zero;

n transmitters provided in correspondence with the an^¬ tennas, each of the n transmitters having an average transmit power of 1/n of the total transmit power; and

a processor configured to split a signal to be radiated in a non-beamforming mode into n adjacent frequency ranges, thereby obtaining n split signals, and apply the n split sig^¬ nals to the n transmitters such that power is distributed equally to all (n x k) antenna elements.

2. The apparatus of claim 1, wherein the n adjacent frequency ranges are n equidistant frequency sub-bands.

3. The apparatus of claim 1 or 2, wherein k is equal to one and the processor is configured to apply the n split signals to the n transmitters, respectively, for radiation by the n antenna elements.

4. The apparatus of claim 1 or 2, wherein the processor is configured to apply each of the n split signals to k trans^¬ mitters of k antenna elements for the same frequency range, wherein the k antenna elements are elements of orthogonal po^¬ larisation layers and are located opposite to each other within the array.

5. A method comprising:

splitting a signal to be radiated in a non-beamforming mode into n adjacent frequency ranges, thereby obtaining n split signals; and

applying the n split signals to n transmitters provided in correspondence with antennas of a beamforming antenna ar- ray comprising n antennas having k antenna elements, n being an integer greater than one and k being an integer greater than zero, each of the n transmitters having an average transmit power of 1/n of the total transmit power,

wherein the n split signals are applied to the n trans^¬ mitters such that power is distributed equally to all (n x k) antenna elements.

6. The method of claim 5, wherein the n adjacent frequency ranges are n equidistant frequency sub-bands.

7. The method of claim 5 or 6, wherein k is equal to one and the n split signals are applied to the n transmitters, re^¬ spectively, for radiation by the n antenna elements.

8. The method of claim 5 or 6, wherein each of the n split signals is applied to k transmitters of k antenna elements for the same frequency range, wherein the k antenna elements are elements of orthogonal polarisation layers and are lo- cated opposite to each other within the array.

9. A computer program product including a program for a processing device, comprising software code portions for perform^¬ ing the method of any one of claims 5 to 8 when the program is run on the processing device.

10. The computer program product according to claim 9, wherein the computer program product comprises a computer- readable medium on which the software code portions are stored.

11. The computer program product according to claim 9, wherein the program is directly loadable into an internal memory of the processing device.