WO2016082887A1

WO2016082887A1 - An ofdm transmitter for subcarrier based beamforming

Info

Publication number: WO2016082887A1
Application number: PCT/EP2014/075946
Authority: WO
Inventors: Thomas Olsson; Leif Wilhelmsson
Original assignee: Telefonaktiebolaget Lm Ericsson (Publ)
Priority date: 2014-11-28
Filing date: 2014-11-28
Publication date: 2016-06-02

Abstract

In a beamforming transmitter (27) in a wireless orthogonal frequency division multiplexing system, at least two modified sets (x_a(n), x_b(n)) of frequency domain data signals are provided, each one comprising a subset of a first set (x(n)) of frequency domain data signals corresponding to a number of sub-carriers, while information content is removed from frequency domain data signals not belonging to said subset. An inverse fast Fourier transform operation is performed on each modified set individually (2a, 2b), and for each modified set, a plurality of phase shifted signals is provided with the phase shifts of each modified set being determined according to a beam direction defined for that modified set. For each one of the plurality of antennas phase shifted signals from the modified sets are combined, and the plurality of combined signals are provided to the plurality of antennas (6.0, 6.1, 6.k-1) forming the antenna array.

Description

AN OFDM TRANSMITTER FOR SUBCARRIER BASED BEAMFORMING

Technical Field

The invention relates to a method of transmitting radio frequency signals from a beamforming transmitter in a wireless orthogonal frequency division multiplexing system and to a beamforming transmitter for transmitting such signals. Background

The modulation format Orthogonal Frequency Division Multiplexing, OFDM, is and will be used for many recent and future wireless and telecommunications standards, especially where high data rates are needed. In OFDM a high data rate stream is divided and placed on a number of slowly modulated and closely spaced narrowband subcarriers that are orthogonal to each other, so that they can be received without interference from each other. In other words, the data to be transmitted is spread across the subcarriers, so that each subcarrier carries a part of the payload. Further, many future radio access technologies, including technologies using OFDM, for communication e.g. between a base station and a user equipment are expected to heavily rely on beamforming techniques, especially when radio frequencies in the Extremely High Frequency (EHF) band are utilized, i.e. frequencies in the range from 30 to 300 GHz. Radio waves in this band have wavelengths from ten to one millimeter, and thus the band is also called millimeter band or millimeter wave, abbreviated as the mmW frequency band.

Beamforming is a signal processing technique used e.g. in antenna arrays for directional signal transmission or reception. This is achieved by combining antenna elements in a phased array in such a way that signals at particular angles experience constructive interference while others experience destructive interference. Beamforming can be used at both the transmitting and receiving ends in order to achieve spatial selectivity. To change the directionality of the array in a transmitter, a beamformer controls the phase and relative amplitude of the signal at each antenna in order to create a pattern of constructive and destructive interference in the wave front. Thus, beamforming is becoming an increasingly important feature for wireless OFDM systems.

One obstacle, however, is that the implementation complexity of an OFDM transmitter is increased when beamforming is included.

A solution of relatively low complexity to beamforming in an OFDM transmitter is to do the beamforming in time domain by phase-shifting (delaying) the signal to each antenna. Since time domain beamforming is usually implemented in analog domain, the term analog beamforming is also widely used. A drawback with this solution is that the beamforming is not frequency selective, i.e. the beamformer controls the phase shifts so that the beam transmitted from the antenna is directed in one particular direction, which is the same for all subcarrier frequencies. In some situations, it may be desirable to assign some subcarriers for transmission to one receiver located in one direc- tion and other subcarriers for transmission in one or more other directions.

A completely frequency selective solution can be obtained by performing the phase-shifting in frequency domain using frequency domain beamforming, where the signal is phase-shifted in frequency domain by applying digital beamforming weights in a digital precoder or codebook. Since frequency domain beamforming is usually implemented in digital domain, the term digital beamforming is also widely used. The difference in phase shift between different antenna elements gives possibility to steer the beam in different directions, and the phase shift may be different for different parts of the OFDM spectrum. Although this is a very flexible solution regarding frequency selectivity, the main problem with frequency domain beamforming is that the hardware complexity is considerably increased compared to the time domain beamforming, because the frequency domain beamforming solution requires one Inverse Fast Fourier Transform and one digital-to-analog converter for each antenna stream, while a single Inverse Fast Fourier Transform is common for all antenna streams in time domain beamforming. In WO2013/169055 a Multiple Input Multiple Output, MIMO, base station for performing hybrid beamforming is shown, which is a combination of analog beamforming and digital beamforming, i.e. the base station comprises an analog beamformer as well as a digital beamformer and a controller is configured to control both beamformers. In this way Multiple Input Multiple Out- put capacity can be increased. However, due to the use of an analog as well as a digital beamformer the hardware complexity is still considerable.

Summary

Therefore, it is an object of embodiments of the invention to provide a method of transmitting radio frequency signals from a beamforming transmitter, which can achieve frequency selectivity with a hardware complexity that is considerably lower than the complexity of a frequency domain beamforming transmitter, and which can thus also be implemented at a lower cost than a frequency domain beamforming transmitter.

According to embodiments of the invention the object is achieved in a method of transmitting radio frequency signals from a beamforming transmitter in a wireless orthogonal frequency division multiplexing system, the method comprising the steps of providing a first set of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system; performing an inverse fast Fourier transform operation on said frequency domain data signals to generate a time-domain signal; providing from said time-domain signal a plurality of phase shifted signals; and providing each one of said plurality of phase shifted signals to one of a plurality of antennas forming an antenna array. The object is achieved when the method further comprises the steps of providing at least two modified sets of frequency domain data signals, each modified set comprising a subset of said first set of frequency domain data signals, while information content is removed from frequency domain data signals not belonging to said subset; performing an inverse fast Fourier transform operation on each modified set of frequency domain data signals individually to generate a time-domain signal for each modified set; providing for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set; combining phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and providing the plurality of combined signals to the plurality of antennas forming the antenna array.

When the set of frequency domain input signals or samples corresponding to the number of OFDM subcarriers is split into two parts or modified sets that are IFFT processed and phase shifted individually before they are combined and provided to the plurality of antenna elements, the complexity in the digital domain of a frequency selective beamforming transmitter can be reduced to be approximately proportional to the number of different beam directions for different subcarriers, which is considerably less than the complexity for the complete digital and frequency selective solution in frequency domain beam- forming, where the hardware complexity, and thus also the cost, is approximately proportional to the number of antenna elements.

In one embodiment, the method further comprises the steps of converting the time-domain signal for each modified set in a digital-to-analog converter be- fore the step of providing the phase shifted signals; and up-converting each one of the plurality of combined signals to a radio frequency signal before the step of providing the signals to the plurality of antennas forming the antenna array. Performing the conversion from the digital to the analog domain before applying the different phase shifts results in a reduced number of digital-to- analog converters, since there is only needed one for each beam direction.

In another embodiment, the method further comprises the steps of converting the time-domain signal for each modified set in a digital-to-analog converter and up-converting it to a radio frequency signal before the step of providing the phase shifted signals. When the digital-to-analog conversion as well as the up-conversion are both performed before applying the different phase shifts, the number of digital-to-analog converters and the number of mixers for up-conversion can both be reduced to one for each beam direction.

In still another embodiment, the method further comprises the steps of converting each one of the plurality of combined signals in a digital-to-analog converter and up-converting them to radio frequency signals before the step of providing the signals to the plurality of antennas forming the antenna array. Applying the different phase shifts and combining in the digital domain before converting to the analog domain requires a digital-to-analog converter for each antenna element, but on the other hand, the phase shifts may be implemented simpler in the digital domain.

In one embodiment, the method further comprises the step of providing the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data signals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system. This is a simple way of splitting the subcarriers for different beam directions.

In another embodiment, the method further comprises the step of providing the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data signals corresponding to interleaved subcarriers in said wireless orthogonal frequency division multiplexing system. Distributing the subcarriers for the different beam directions over the subcarrier bandwidth is advantageous in case fading should occur in a part of the bandwidth.

When the method further comprises the step of removing information content from frequency domain data signals not belonging to the subset of each modified set by setting these frequency domain data signals to zero, a very simple implementation of providing the modified sets can be achieved.

As mentioned, embodiments of the invention further relate to a beamforming transmitter for transmitting radio frequency signals in a wireless orthogonal frequency division multiplexing system, the transmitter comprising an inverse fast Fourier transform unit configured to perform an inverse fast Fourier transform operation on a first set of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system to generate a time-domain signal; and a beam- former configured to provide a plurality of phase shifted signals from said time-domain signal, and to provide each one of said plurality of phase shifted signals to one of a plurality of antennas forming an antenna array. The transmitter is further configured to provide at least two modified sets of fre- quency domain data signals, each modified set comprising a subset of said first set of frequency domain data signals, while information content is removed from frequency domain data signals not belonging to said subset; perform an inverse fast Fourier transform operation on each modified set of frequency domain data signals individually to generate a time-domain signal for each modified set; provide for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set; combine phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and provide the plurality of combined signals to the plurality of antennas forming the antenna array.

When the set of frequency domain input signals or samples corresponding to the number of OFDM subcarriers is split into two parts or modified sets that are IFFT processed and phase shifted individually before they are combined and provided to the plurality of antenna elements, the complexity in the digital domain of a frequency selective beamforming transmitter can be reduced to be approximately proportional to the number of different beam directions for different subcarriers, which is considerably less than the complexity for the complete digital and frequency selective solution in frequency domain beam- forming, where the hardware complexity, and thus also the cost, is approximately proportional to the number of antenna elements. In one embodiment, the beamforming transmitter further comprises a digital- to-analog converter configured to convert the time-domain signal for each modified set to be provided at the input of the beamformer; and a mixer unit configured to up-convert each one of the plurality of combined signals at the outputs of the beamformer to radio frequency signals to be provided to the plurality of antennas forming the antenna array. When the transmitter is configured to perform the conversion from the digital to the analog domain before applying the different phase shifts, this results in a reduced number of digital- to-analog converters, since there is only needed one for each beam direction. In another embodiment, the beamforming transmitter further comprises a digital-to-analog converter configured to convert the time-domain signal for each modified set and a mixer unit configured to up-convert it to a radio frequency signal to be provided at the input of the beamformer. When the digital-to-analog conversion as well as the up-conversion are both performed before applying the different phase shifts, the number of digital-to-analog converters and the number of mixers for up-conversion can both be reduced to one for each beam direction.

In still another embodiment, the beamforming transmitter further comprises a digital-to-analog converter configured to convert each one of the plurality of combined signals and a mixer unit configured to up-convert them to radio frequency signals to be provided to the plurality of antennas forming the antenna array. Applying the different phase shifts and combining in the digital domain before converting to the analog domain requires a digital-to-analog converter for each antenna element, but on the other hand, the phase shifts may be implemented simpler in the digital domain. In one embodiment, the beamforming transmitter is further configured to provide the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data signals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system. This is a simple way of splitting the subcarriers for different beam directions.

In another embodiment, the beamforming transmitter is further configured to provide the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data signals corresponding to interleaved subcarriers in said wireless orthogonal frequency division multiplexing system. Distributing the subcarriers for the different beam directions over the subcarrier bandwidth is advantageous in case fading should occur in a part of the bandwidth.

When the beamforming transmitter is further configured to remove information content from frequency domain data signals not belonging to the subset of each modified set by setting these frequency domain data signals to zero, a very simple implementation of providing the modified sets can be achieved.

Brief Description of the Drawings

Embodiments of the invention will now be described more fully below with reference to the drawings, in which Figure 1 shows a block diagram of an example of an OFDM transmitter;

Figure 2 shows a block diagram of a first example of a beamforming OFDM transmitter; Figure 3 shows a block diagram of a second example of a beamforming OFDM transmitter; Figure 4 shows a block diagram of a third example of a beamforming OFDM transmitter;

Figure 5 shows a block diagram of a fourth example of a beamforming OFDM transmitter;

Figure 6 shows a block diagram of an example of a digital beamforming OFDM transmitter; Figure 7 shows a block diagram of a first embodiment of a frequency selective beamforming OFDM transmitter;

Figure 8 illustrates the splitting of a frequency domain input signal into two parts in a splitter in the transmitter of Figure 7;

Figure 9 shows a block diagram of a second embodiment of a frequency selective beamforming OFDM transmitter;

Figure 10 shows a block diagram of a third embodiment of a frequency selec- tive beamforming OFDM transmitter;

Figure 11 shows a block diagram of a fourth embodiment of a frequency selective beamforming OFDM transmitter; Figure 12 shows a block diagram of an embodiment of a frequency selective beamforming OFDM transmitter with three beam directions;

Figure 13 illustrates the splitting of a frequency domain input signal into three parts in a splitter in the transmitter of Figure 12;

Figure 14 illustrates an alternative way of splitting a frequency domain input signal into two parts in a splitter in the transmitter of Figure 7; and Figure 15 shows a flow chart of a method of transmitting radio frequency signals from a beamforming transmitter.

Detailed Description

Figure 1 shows a block diagram of an example of an OFDM (Orthogonal Frequency Division Multiplexing) transmitter 1 . A frequency domain input signal x(n) is OFDM modulated in an IFFT (Inverse Fast Fourier Transform) 2. Although not shown, the IFFT 2 includes a serial-to-parallel converter at its input and a parallel-to-serial converter at its output. The OFDM modulated signal is then converted from the digital domain to the analog domain in a digital-to-analog converter 3. In a mixer 4 the analog signal is then up converted to a radio frequency signal. The mixer 4 is clocked by clock signals from a not shown local clock generator. The radio frequency signal is amplified in a power amplifier 5 and transmitted from an antenna 6.

Beamforming can be implemented in an OFDM transmitter by using an antenna array with a plurality of antennas or antenna elements, e.g. arranged in a linear array or a planar array. This is illustrated in the transmitter 21 shown in Figure 2 having the antenna elements 6.0, 6.1 , 6.k-1 . Directionality of the antenna array is achieved by individually controlling the phase of the signal supplied to each antenna such that a pattern of constructive and destructive interference is created in the transmitted wave front. Thus in Figure 2, the output signal from the digital-to-analog converter 3 is supplied to a plurality of phase shifters 7.0, 7.1 , 7.k-1 in which individual phase shifts cpo, cpi , cp_k-i are applied to the signal. Then in each channel the phase shifted signal is up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 . The individual phase shifts cp₀, cp-i , cp_k-i can be determined in different ways that are well known in the art, and which will therefore not be described in further detail here. Instead of phase shifting the input signals to the mixers 4.0, 4.1 , 4.k-1 in the individual paths to the antenna elements as shown in Figure 2, the phase shifts cpo, φ-ι , cpk-i can also be introduced by controlling the phases of the local oscillator signals supplied to the mixers that up convert the signals to radio frequency signals. This is illustrated in the transmitter 22 shown in Figure 3, where the mixers 8.0, 8.1 , 8.k-1 are clocked by local oscillator signals 9.0, 9.1 , 9.k-1 .

Another possibility, which is illustrated in the transmitter 23 shown in Figure

4, is to introduce the individual phase shifts directly at the output of the IFFT 2. Thus in Figure 4, the output signal from the IFFT 2 is supplied to a plurality of phase shifters 10.0, 10.1 , 10.k-1 in which individual phase shifts cpo, φ-ι , c k-i are applied to the signal. Then in each channel the phase shifted signal is converted from the digital domain to the analog domain in a digital-to- analog converter 3.0, 3.1 , 3.k-1 , up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 ,

5. k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 . It is noted that although the phase shifts are here applied to the signal in the digital domain, i.e. before the digital-to-analog converters, it is considered as time domain beamforming, because the beamforming is performed after the IFFT 2, i.e. in time domain, which is in contrast to frequency domain beamforming, where the beamforming is performed before the IFFT. Frequency domain beamforming will be described later. The individual phase shifts can also be applied after the analog signal has been up converted to a radio frequency signal in the mixer 4. This is illustrated in the transmitter 24 shown in Figure 5, where the radio frequency output signal from the mixer 4 is supplied to a plurality of phase shifters 1 1 .0, 1 1 .1 , 1 1 .k-1 in which individual phase shifts (po, φ-ι , ... , Ψ are applied to the signal. Then in each channel the phase shifted signal is amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 . As mentioned above, time domain beamforming as it is illustrated in Figures 2 to 5 is not frequency selective, i.e. the beamformer controls the phase shifts so that the beam transmitted from the antenna elements is directed in one particular direction, which is the same for all subcarrier frequencies. In some situations, it may be desirable to assign some subcarriers for transmission in one direction and other subcarriers for transmission in one or more other directions.

A completely frequency selective solution can be obtained by performing the phase-shifting using frequency domain beamforming, where the signal is phase-shifted in frequency domain before the IFFT by applying digital beam- forming weights in a digital precoder or codebook. An example of frequency domain beamforming is illustrated in the transmitter 25 shown in Figure 6, where the frequency domain input signal is supplied to a port expansion and beamforming unit 12 including a digital precoder, in which the signal is phase-shifted in frequency domain by applying digital beamforming weights for phase shift. The difference in phase shift between different antenna elements gives possibility to steer the beam in different directions, and the phase shift may be different for different parts of the OFDM spectrum. From the port expansion and beamforming unit 12 the phase shifted signals for each antenna element are supplied to the IFFTs 2.0, 2.1 , 2.k-1 , in which they are OFDM modulated. Each phase shifted and OFDM modulated signal is then converted from the digital domain to the analog domain in a digital-to- analog converter 3.0, 3.1 , 3.k-1 , up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 ,

5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1.

As it can be seen from Figure 6, this solution has a high hardware complex- ity, and thus a high cost, in the digital domain, because an IFFT as well as a digital-to-analog converter is required in each antenna stream in addition to the port expansion and beamforming unit. Thus although the frequency do- main beamforrning is the most flexible regarding frequency selectivity, its high hardware complexity and cost may prevent its use in many applications.

One embodiment of a solution that provides frequency selective beamform- ing in an OFDM transmitter with a hardware complexity close to the time domain beamformers as shown in Figures 2 to 5 is illustrated in the transmitter 26 shown in Figure 7. By splitting the signal in the frequency domain and then OFDM modulating it in a separate IFFT for each part, i.e. for each beam direction, the output from each IFFT can be used for all antenna elements, as it will be described below. The embodiment shown in Figure 7 is a transmitter with two different beam directions. However, as it will also be described below, this can easily be expanded to more beam directions. The transmitter 26 of Figure 7 corresponds to the beamforrning transmitter 23 of Figure 4 modified so that the frequency domain input signal is split and OFDM modulated in separate IFFTs for different frequency dependent beam directions.

In the embodiment of Figure 7, the frequency domain input signal x(n) is split into two different frequency-selective parts x_a(n) and x_b(n) in a first stage consisting of a splitter 13, so that part x_a(n) includes subcarriers intended for one beam and part x_b(n) includes subcarriers intended for another beam. For each part, so-called zero-padding is performed for all subcarriers that should not be included in that beam. Each one of the two parts is then supplied to a separate IFFT, i.e. one of the IFFTs 2a and 2b. Figure 8 shows an example of the zero-padding in the splitter 13, where the frequency domain signal x(n) is split into x_a(n) and x_b(n). The part x_a(n) contains all frequency domain samples of x(n) from 0 to N_b-1 , and the part x_b(n) contains all frequency domain samples from N_b to N-1. All other samples of x_a(n) and x_b(n) are set to 0. The number N of frequency domain samples of the frequency domain input signal x(n) corresponds to the number of subcarriers. Thus Xa(n) is equal to x(n) up to subcarrier N_b-1 and set to zero from subcarrier N_b, and x_b(n) is set to zero up to subcarrier N_b-1 and equal to x(n) from subcarrier N_b up to subcarrier N-1. After the splitter 13, the two frequency selective parts x_a(n) and Xb(n) are processed individually in the IFFTs 2a and 2b. Each one of the output signals from the IFFTs 2a and 2b is supplied to a plurality of phase shifters 10. a.0, 10.a.1 , 10.a.k-1 and 10.b.0, 10.b.1 , 10.b.k-1 , respectively, in which individual phase shifts cp_a,₀, φ_3> , ... , cp_a,_k-i and (p_b,o_> (Pb,i , · · · , pb,k-i are applied to the signals. The phase shifts applied to each one of the IFFT output signals are determined according to the corresponding intended beam direction. Thus, the dual phase shift is done once per antenna element. In each chan- nel, i.e. in the path for each antenna element, the two phase shifted signals are summarized in an adder 14.0, 14.1 , 14.k-1 to one time-domain signal containing the signal divided into two different frequency selective phase shifts. The remaining part of the circuit may be identical to that of Figure 4. Thus in each channel the phase shifted signal is converted from the digital domain to the analog domain in a digital-to-analog converter 3.0, 3.1 , 3.k- 1 , up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 . This gives a similar effect as a frequency domain beamformer with two different phase shifts, one up to subcarrier N_b-i and one from subcarrier N_b. Since the post-IFFT phase shift is less computational intensive than the IFFT, this gives a potential reduction in complexity from N_antennas ^x 0(nxlog2(n)) + O(n) for frequency domain beamforming to 2*0(nxlog2(n)) + N_antennas^x2^xO(n) for the proposed solution, where N_antennas is the number of antenna elements, n is the number of samples in each IFFT frame, and O(n) is n complex multiplications.

In the embodiment described above and illustrated in Figure 7, the idea of splitting the frequency domain input signal into two (or more) different parts is used in a beamforming transmitter of the type shown in Figure 4. However, the same idea can of course be used in the beamforming transmitters illustrated in Figures 2, 3 and 5 as well. Thus, the beamforming transmitter 27 shown in Figure 9 is a modified version of the beamforming transmitter 21 shown in Figure 2, where the individual phase shifts are applied to the analog output of the digital-to-analog con- verter. The OFDM modulated outputs of the two IFFTs 2a and 2b are therefore converted from the digital domain to the analog domain in the two corresponding digital-to-analog converters 3a and 3b, before each one is supplied to a plurality of phase shifters 7.a.0, 7.a.1 , 7.a.k-1 and 7.b.0, 7.b.1 , 7.b.k-1 , respectively, in which individual phase shifts φ₃,ο, <p_a,i . · · · , <Pa,k-i and ⁽pb,o, ⁽Pb,i _> · · · _< ⁽Pb,k-i are applied to the signals. In each channel, i.e. in the path for each antenna element, the two phase shifted signals are summarized in an adder 15.0, 15.1 , 15.k-1 to one time-domain signal containing the signal divided into two different frequency selective phase shifts. The remaining part of the circuit may be identical to that of Figure 2. Thus in each channel the phase shifted signal is up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 .

Compared to the beamforming transmitter 26 shown in Figure 7, in the beam- forming transmitter 27 shown in Figure 9 the phase shifters have been moved into the analog domain after the digital-to-analog conversion with the result that the number of digital-to-analog converters can be reduced, which has heavy impact on hardware complexity. Thus, the beamforming transmitter 27 can be implemented at a lower hardware cost. Compared to Figure 7, the digital complexity in Figure 9 is reduced by N_antennas^xO(nx2) from removing digital phase shifters, and the analog complexity is decreased by (N_anten- nas - 2) DAC instances and increased by 2x N_antennas analog phase shifters.

Thus, the embodiment shown in Figure 9 has the ability of frequency selec- tive beamforming in two different directions while having a complexity close to the time domain beamformer shown in Figure 2. Similarly to Figure 3, instead of phase shifting the two paths of input signals to the mixers 4.0, 4.1 , 4.k-1 as shown in Figure 9, the phase shifts φ₃,ο, ⁽p_a,i , q>a,k-i and c b.o, ⁽pb,i , · · · , <Pb,k-i can also be introduced by controlling the phases of the local oscillator signals supplied to mixers 8.a.0, 8.a.1 , 8.a.k- 1 and 8.b.0, 8.b.1 , 8.b.k-1 , respectively, that up convert the signals to radio frequency signals. This is illustrated in the transmitter 28 shown in Figure

10, where the mixers 8.a.0, 8.a.1 , 8.a.k-1 and 8.b.0, 8.b.1 , 8.b.k-1 are clocked by local oscillator signals 9.a.0, 9.a.1 , 9.a.k-1 and 9.b.0, 9.b.1 , 9.b.k-1 . In each channel, i.e. in the path for each antenna element, the two phase shifted and up converted signals are then summarized in an adder 16.0, 16.1 , 16.k-1 to one signal representing the two different frequency selective phase shifts. The remaining part of the circuit may be identical to that of Figure 3. Thus in each channel the radio frequency signal is amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corre- sponding antenna element 6.0, 6.1 , 6.k-1 .

As mentioned, the idea of splitting the frequency domain input signal into two (or more) different parts can also be used in the beamforming transmitter 24 illustrated in Figure 5, where the individual phase shifts are applied after the analog signal has been up converted to a radio frequency signal. This is illustrated with the transmitter 29 in Figure 1 1 . Similarly to the previous embodiments, the frequency domain input signal x(n) is first split into two different frequency-selective parts x_a(n) and x_b(n) in the splitter 13, so that part x_a(n) includes subcarriers intended for one beam and part x_b(n) includes subcarri- ers intended for another beam. The two frequency selective parts x_a(n) and x_b(n) are processed individually in the IFFTs 2a and 2b, and the OFDM modulated outputs are then converted from the digital domain to the analog domain in two corresponding digital-to-analog converters 3a and 3b and up converted to a radio frequency signal in mixers 4a and 4b.

Each one of the radio frequency signals from the mixers 4a and 4b is supplied to a plurality of phase shifters 1 1 .a.0, 1 1 . a.1 , 1 1 .a.k- and 1 1 .b.0,

1 1 . b.1 , 1 1 .b.k-1 , respectively, in which individual phase shifts cp_a,o. <Pa,-i , c a,k-i and ⁽p_b,o, <Pt_>,i, ·■■, <Pt_>,k-i are applied to the signals. Thus, the dual phase shift is done once per antenna element. In each channel, i.e. in the path for each antenna element, the two phase shifted signals are summarized in an adder 17.0, 17.1 , 17.K-1 to one radio frequency signal contain- ing the signal divided into two different frequency selective phase shifts. Then in each channel the combined phase shifted signal is amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , . 6.k-1 . As mentioned, the embodiments shown in Figures 7, 9, 10 and 1 1 are beam- forming transmitters with two different frequency dependent beam directions. However, the embodiments can easily be expanded to more beam directions with only a slight increase in hardware complexity. Thus as an example, Figure 12 illustrates an embodiment of a transmitter 30 with three different fre- quency dependent beam directions. The transmitter 30 is similar to the transmitter 27 shown in Figure 9 except for the number of frequency dependent beam directions. Thus, the frequency domain input signal x(n) is first split into three different frequency-selective parts x_a(n), x_b(n) and x_c(n) in the splitter 18, so that the three parts includes subcarriers intended for three different beam directions.

Figure 13 shows an example of the zero-padding in the splitter 18, where the frequency domain signal x(n) is split into three parts x_a(n), Xb(n) and x_c(n). The part x_a(n) contains all frequency domain samples of x(n) from 0 to N_b-1 , the part x_b(n) contains all frequency domain samples from N_b to N_c-1 and the part x_c(n) contains all frequency domain samples from N_c to N-1. All other samples of x_a(n), x_b(n) and x_c(n) are set to 0. The number N of frequency domain samples of the frequency domain input signal x(n) corresponds to the number of subcarriers.

After the splitter 18, the three frequency selective parts x_a(n), x_b(n) and x_c(n) are processed individually in the IFFTs 2a, 3b and 2c. The OFDM modulated outputs of the three IFFTs 2a, 2b and 2c are then converted from the digital domain to the analog domain in the three corresponding digital-to-analog converters 3a, 3b and 3c, before each one is supplied to a plurality of phase shifters 7.a.0, 7.a.1 , 7.a.k-1 , 7.b.0, 7.b.1 , .... 7.b.k-1 and 7.C.0, 7.C.1 , 7.c.k-1 , respectively, in which individual phase shifts φ₃,ο. <Pa,i,■·· _> p_a,k-i. Ψ_>.ο_> c b,i , 9b,k-i and φ₀,ο, cp_c,i. _{· · ·} , cp_c,k-i are applied to the signals. In each channel, i.e. in the path for each antenna element, the three phase shifted signals are summarized in an adder 19.0, 19.1 , 19.k-1 to one time-domain signal containing the signal divided into three different frequency selective phase shifts. The remaining part of the circuit may be identical to that of Figure 9. Thus in each channel the phase shifted signal is up converted to a radio frequency signal in a mixer 4.0, 4.1 , 4.k-1 and amplified in a power amplifier 5.0, 5.1 , 5.k-1 , before it is transmitted from the corresponding antenna element 6.0, 6.1 , 6.k-1 . Similarly, embodiments with four of more frequency dependent beam directions may be implemented, but of course, the hardware complexity increases with the number of beam directions.

In the embodiments described above, the splitters 13 and 18 are configured to provide the parts or subsets x_a(n), x_b(n) and x_c(n) such that each part comprises signals corresponding to a number of adjacent subcarriers, as it was shown in Figures 8 and 13. However, the subcarriers of each part may also be selected in other ways. An example of this is shown in Figure 14, where each part x_a(n) and x_b(n) comprises signals corresponding to a number of interleaved subcarriers. The subcarriers of each set may also be selected in other ways or combinations thereof.

Although embodiments of frequency selective beamforming transmitters with low complexity as described above can be used in many different applications, there are certain applications in wireless communications where they are particular attractive. A few will be mentioned below.

As an example, such transmitters may be used together with Orthogonal Frequency Division Multiple Access (OFDMA). In OFDMA, different users are allocated different sub-carriers, which means that the signal carries data in- tended for more than one receiver. As the two or more receivers are typically not located very close to one another, it is typically not feasible to use time domain beamforming. With the low complexity beamforming described above, it is feasible to beamform different sets of sub-carriers to different us- ers.

In another application, the frequency selective beamforming is used for the case where the link is just between two devices, but where the frequency selective beamforming is used as a low complexity means to achieve spatial diversity. Specifically, if the full bandwidth is beamformed in one direction, and this direction becomes obstructed, the link may become very bad. For this reason, frequency selective beamforming can be introduced as a means to avoid that the entire bandwidth becomes useless if one path suddenly becomes obstructed.

Figure 15 shows a flow chart 100 illustrating a method of transmitting radio frequency signals from a beamforming transmitter. In step 101 , at least two modified sets of frequency domain data signals are provided by splitting a frequency domain input signal x(n) into e.g. two different frequency selective parts x_a(n) and x (n) as illustrated in Figures 8 and 14. An example where the frequency domain input signal x(n) is split into three different frequency selective parts x_a(n), x_b(n) and x_c(n) is illustrated in Figure 13. Each part contains frequency domain samples of the frequency domain input signal x(n) corresponding to subcarriers intended for one beam direction, while the in- formation content is removed from samples corresponding to subcarriers intended for other beam directions, e.g. by setting the samples to zero.

In step 102, the two or more parts, e.g. x_a(n) and Xb(n), are OFDM modulated individually in a separate IFFT for each part. For each part or modified set a plurality of phase shifted signals corresponding to the number of antenna elements used by the beamforming transmitter are provided in step 103 by applying a plurality of phase shifts determined according to the corresponding beam direction for that part. For each antenna element, the phase shifted signals for each part or modified set are combined or summarized in an adder in step 104 and provided to the plurality of antenna elements in step 105.

In other words, a method of transmitting radio frequency signals from a beamforming transmitter in a wireless orthogonal frequency division multiplexing system is provided that comprises the steps of providing a first set of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system; performing an inverse fast Fourier transform operation on said frequency domain data signals to generate a time-domain signal; providing from said time- domain signal a plurality of phase shifted signals; and providing each one of said plurality of phase shifted signals to one of a plurality of antennas forming an antenna array. The object is achieved when the method further comprises the steps of providing at least two modified sets of frequency domain data signals, each modified set comprising a subset of said first set of frequency domain data signals, while information content is removed from frequency domain data signals not belonging to said subset; performing an inverse fast Fourier transform operation on each modified set of frequency domain data signals individually to generate a time-domain signal for each modified set; providing for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set; combining phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and providing the plurality of combined sig- nals to the plurality of antennas forming the antenna array.

When the set of frequency domain input signals or samples corresponding to the number of OFDM subcarriers is split into two parts or modified sets that are IFFT processed and phase shifted individually before they are combined and provided to the plurality of antenna elements, the complexity in the digital domain of a frequency selective beamforming transmitter can be reduced to be approximately proportional to the number of different beam directions for different subcarriers, which is considerably less than the complexity for the complete digital and frequency selective solution in frequency domain beam- forming, where the hardware complexity, and thus also the cost, is approximately proportional to the number of antenna elements. The method may further comprise the steps of converting the time-domain signal for each modified set in a digital-to-analog converter before the step of providing the phase shifted signals; and up-converting each one of the plurality of combined signals to a radio frequency signal before the step of providing the signals to the plurality of antennas forming the antenna array. Per- forming the conversion from the digital to the analog domain before applying the different phase shifts results in a reduced number of digital-to-analog converters, since there is only needed one for each beam direction.

In one embodiment, the method further comprises the step of providing the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data signals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system. This is a simple way of splitting the subcarriers for different beam directions. In another embodiment, the method further comprises the step of providing the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data signals corresponding to interleaved subcarriers in said wireless orthogonal frequency division multiplexing system. Distributing the subcarriers for the different beam directions over the subcarrier bandwidth is advantageous in case fading should occur in a part of the bandwidth.

A beamforming transmitter for transmitting radio frequency signals in a wireless orthogonal frequency division multiplexing system comprises an inverse fast Fourier transform unit configured to perform an inverse fast Fourier transform operation on a first set of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system to generate a time-domain signal; and a beam- former configured to provide a plurality of phase shifted signals from said time-domain signal, and to provide each one of said plurality of phase shifted signals to one of a plurality of antennas forming an antenna array. The transmitter is further configured to provide at least two modified sets of frequency domain data signals, each modified set comprising a subset of said first set of frequency domain data signals, while information content is re- moved from frequency domain data signals not belonging to said subset; perform an inverse fast Fourier transform operation on each modified set of frequency domain data signals individually to generate a time-domain signal for each modified set; provide for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set; combine phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and provide the plurality of combined signals to the plurality of antennas forming the antenna array.

In one embodiment, the beamforming transmitter further comprises a digital- to-analog converter configured to convert the time-domain signal for each modified set to be provided at the input of the beamformer; and a mixer unit configured to up-convert each one of the plurality of combined signals at the outputs of the beamformer to radio frequency signals to be provided to the plurality of antennas forming the antenna array. When the transmitter is configured to perform the conversion from the digital to the analog domain before applying the different phase shifts, this results in a reduced number of digital- to-analog converters, since there is only needed one for each beam direction.

In another embodiment, the beamforming transmitter further comprises a digital-to-analog converter configured to convert the time-domain signal for each modified set and a mixer unit configured to up-convert it to a radio frequency signal to be provided at the input of the beamformer. When the digital-to-analog conversion as well as the up-conversion are both performed before applying the different phase shifts, the number of digital-to-analog converters and the number of mixers for up-conversion can both be reduced to one for each beam direction.

In still another embodiment, the beamforming transmitter further comprises a digital-to-analog converter configured to convert each one of the plurality of combined signals and a mixer unit configured to up-convert them to radio frequency signals to be provided to the plurality of antennas forming the antenna array. Applying the different phase shifts and combining in the digital domain before converting to the analog domain requires a digital-to-analog converter for each antenna element, but on the other hand, the phase shifts may be implemented simpler in the digital domain.

In one embodiment, the beamforming transmitter is further configured to provide the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data signals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system. This is a simple way of splitting the subcarriers for different beam directions. In another embodiment, the beamforming transmitter is further configured to provide the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data signals corresponding to interleaved subcarriers in said wireless orthogonal frequency division multiplexing system. Distributing the subcarriers for the different beam directions over the subcarrier bandwidth is advantageous in case fading should occur in a part of the bandwidth.

When the beamforming transmitter is further configured to remove information content from frequency domain data signals not belonging to the subset of each modified set by setting these frequency domain data signals to zero, a very simple implementation of providing the modified sets can be achieved. Although various embodiments of the present invention have been described and shown, the invention is not restricted thereto, but may also be embodied in other ways within the scope of the subject-matter defined in the following claims.

Claims

C l a i m s :

1. A method of transmitting radio frequency signals from a beamforming transmitter (26; 27; 28; 29; 30) in a wireless orthogonal frequency division multiplexing system, the method comprising the steps of:

• providing a first set (x(n)) of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system;

• performing an inverse fast Fourier transform operation on said frequency domain data signals to generate a time-domain signal;

• providing from said time-domain signal a plurality of phase shifted signals; and

• providing each one of said plurality of phase shifted signals to one of a plurality of antennas (6.0, 6.1 , 6.k-1 ) forming an antenna array, c h a r a c t e r i z e d in that the method further comprises the steps of:

• providing (101 ) at least two modified sets (x_a(n), Xb(n); x_a(n), Xb(n), x_c(n)) of frequency domain data signals, each modified set comprising a subset of said first set (x(n)) of frequency domain data signals, while information content is removed from frequency domain data signals not belonging to said subset;

• performing (102) an inverse fast Fourier transform operation on each modified set (x_a(n), x_b(n); x_a(n), x_b(n), x_c(n)) of frequency domain data signals individually to generate a time-domain signal for each modified set;

• providing (103) for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set;

• combining (104) phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and • providing (105) the plurality of combined signals to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

2. A method according to claim 1 , wherein the method further comprises the steps of:

• converting the time-domain signal for each modified set in a digital-to- analog converter (3a, 3b) before the step of providing the phase shifted signals; and

• up-converting each one of the plurality of combined signals to a radio frequency signal before the step of providing the signals to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

3. A method according to claim 1 , wherein the method further comprises the steps of:

· converting the time-domain signal for each modified set in a digital-to- analog converter (3a, 3b) and up-converting it to a radio frequency signal before the step of providing the phase shifted signals.

4. A method according to claim 1 , wherein the method further comprises the steps of:

• converting each one of the plurality of combined signals in a digital-to- analog converter (3.0, 3.1 , 3.k-1 ) and up-converting them to radio frequency signals before the step of providing the signals to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

5. A method according to any one of claims 1 to 4, wherein the method further comprises the step of:

• providing the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data sig- nals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system.

6. A method according to any one of claims 1 to 4, wherein the method further comprises the step of:

• providing the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data sig- nals corresponding to interleaved subcarriers in said wireless orthogonal frequency division multiplexing system.

7. A method according to any one of claims 1 to 6, wherein the method further comprises the step of:

· removing information content from frequency domain data signals not belonging to the subset of each modified set by setting these frequency domain data signals to zero.

8. A beamforming transmitter (26; 27; 28; 29; 30) for transmitting radio fre- quency signals in a wireless orthogonal frequency division multiplexing system, the transmitter comprising:

• an inverse fast Fourier transform unit (2a; 2b) configured to perform an inverse fast Fourier transform operation on a first set (x(n)) of frequency domain data signals corresponding to a number of subcarriers in said wireless orthogonal frequency division multiplexing system to generate a time-domain signal; and

• a beamformer configured to provide a plurality of phase shifted signals from said time-domain signal, and to provide each one of said plurality of phase shifted signals to one of a plurality of antennas (6.0, 6.1 , 6.k- 1 ) forming an antenna array,

c h a r a c t e r i z e d in that the transmitter is further configured to:

• provide at least two modified sets (x_a(n), Xb(n); x_a(n), x_b(n), x_c(n)) of frequency domain data signals, each modified set comprising a subset of said first set (x(n)) of frequency domain data signals, while information content is removed from frequency domain data signals not belonging to said subset; • perform an inverse fast Fourier transform operation (2a, 2b) on each modified set (x_a(n), Xb(n); x_a(n), Xb(n), x_c(n)) of frequency domain data signals individually to generate a time-domain signal for each modified set;

· provide for each modified set a plurality of phase shifted signals from the corresponding time-domain signal, the phase shifts of each modified set being determined according to a beam direction defined for that modified set;

• combine phase shifted signals from said at least two modified sets for each one of the plurality of antennas; and

• provide the plurality of combined signals to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

9. A beamforming transmitter according to claim 8, wherein the transmitter further comprises:

• a digital-to-analog converter (3a, 3b) configured to convert the time- domain signal for each modified set to be provided at the input of the beamformer; and

• a mixer unit (4.0, 4.1 , 4.k-1 ) configured to up-convert each one of the plurality of combined signals at the outputs of the beamformer to radio frequency signals to be provided to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

10. A beamforming transmitter according to claim 8, wherein the transmitter further comprises:

• a digital-to-analog converter (3a, 3b) configured to convert the time- domain signal for each modified set and a mixer unit (4a, 4b) configured to up-convert it to a radio frequency signal to be provided at the input of the beamformer.

11. A beamforming transmitter according to claim 8, wherein the transmitter further comprises: • a digital-to-analog converter (3.0, 3.1 , 3.k-1 ) configured to convert each one of the plurality of combined signals and a mixer unit (4.0, 4.1 , 4.k-1 ) configured to up-convert them to radio frequency signals to be provided to the plurality of antennas (6.0, 6.1 , 6.k-1 ) forming the antenna array.

12. A beamforming transmitter according to any one of claims 8 to 11 , wherein the transmitter is further configured to provide the modified sets of frequency domain data signals such that the subset of each modified set comprises frequency domain data signals corresponding to a number of adjacent subcarriers in said wireless orthogonal frequency division multiplexing system.

13. A beamforming transmitter according to any one of claims 8 to 11 , wherein the transmitter is further configured to provide the modified sets of frequency domain data signals such that the subsets of the modified set comprise frequency domain data signals corresponding to interleaved sub- carriers in said wireless orthogonal frequency division multiplexing system.

14. A beamforming transmitter according to any one of claims 8 to 13, wherein the transmitter is further configured to remove information content from frequency domain data signals not belonging to the subset of each modified set by setting these frequency domain data signals to zero.