Method and transmitter for transmit beamforming in a wireless communication system

1. A phased array transmitter comprising:

a first splitter configured to split a first intermediate frequency (IF) signal into a plurality of first input IF signals;

a plurality of transmitting branches each configured to up-convert a respective one of the plurality of input IF signals to a respective radio frequency (RF) signal with a desired phase shift, wherein each transmitting branch is connected to an antenna element configured to transmit the respective RF signal;

wherein each transmitting branch of the plurality of transmitting branches comprises an IF phase shifter configured to phase shift the first input IF signal with a first IF phase shift to obtain a first phase shifted IF signal, a local oscillator (LO) phase shifter configured to phase shift an LO signal with an LO phase shift to obtain a phase shifted LO signal, a mixer configured to mix the first phase shifted IF signal with the phased shifted LO signal to obtain the RF signal, wherein the combined IF phase shift and LO phase shift is the desired RF phase shift of each transmitting branch;

a second splitter configured to split a second IF signal into a plurality of second input IF signals; wherein each transmitting branch is further configured to up-convert a second input IF signal to an RF signal with a second desired phase shift and wherein each transmitting branch is further comprising a second IF phase shifter configured to phase shift the second IF signal with a second IF phase shift to obtain a second phase shifted IF signal, wherein the combined second IF phase shift and LO phase shift is the second desired RF phase shift of each transmitting branch;

a combiner configured to add the second phased shifted IF signal to the phase shifted IF signal before mixing with the phase shifted LO signal; and

wherein the mixer is further configured to mix the combined phase shifted IF signal with the phased shifted LO signal to obtain the RF signal.

2. The phased array transmitter according to claim 1 , wherein each transmitting branch is further comprising a band-pass filter configured to suppress unwanted spurious signals.

3. The phased array transmitter according to claim 1 , wherein each transmitting branch is further comprising a power amplifier configured to amplify the RF signal.

4. The phased array transmitter according to claim 1 , wherein at least one of the first and second IF phase shifters in each transmitting branch is tunable.

5. The phased array transmitter according to claim 1 , wherein the first and second IF phase shifter of each transmitting branch is configured to provide a continuously variable phase shift; and

wherein the LO phase shifter of each transmitting branch is configured to provide a continuously variable phase shift.

6. The phased array transmitter according to claim 1 , wherein the first and second IF phase shifter of each transmitting branch is configured to provide a discrete set of variable phase shifts; and

wherein the LO phase shifter of each transmitting branch is configured to provide a discrete set of variable phase shifts.

7. The phased array transmitter according to claim 1 , wherein each of the first and second phase shifted IF signals comprises a digital domain phase shifted IF signal;

wherein each transmit branch is further configured to convert the digital domain phase shifted IF signal to the analog domain; and

wherein the mixer is configured to mix the analog domain phase shifted IF signal with the phased shifted LO signal to obtain the RF signal.

8. The phased array transmitter according to claim 1 , wherein for each transmitting branch the LO phase shifts are chosen randomly in order to suppress unwanted spurious signals.

9. A method for beamforming in a phased array transmitter, the method comprising:

splitting a first intermediate frequency (IF) signal into a plurality of first input IF signals and feeding each of the plurality of first input IF signals to a corresponding transmitting branch;

phase-shifting, for each transmitting branch, the first input IF signal with a first IF phase shift to obtain a first phase shifted IF signal, and phase-shifting a local oscillator (LO) signal with an LO phase shift to obtain a phase shifted LO signal, wherein the choice of the first IF phase shift and the LO phase shift is determined such that the combined IF and LO phase shift is the desired RF phase shift of each transmitting branch;

up-converting, for each transmitting branch, the first input IF signal to a radio frequency (RF) signal with a desired RF phase shift by mixing the first phase shifted IF signal and the phased shifted LO signal to obtain the RF signal with the desired RF phase shift; and

transmitting, for each transmitting branch, the RF signal by feeding the RF signal to an antenna element;

wherein splitting further comprises splitting a second IF signal into a plurality of second input IF signals and feeding each of the plurality of second input IF signals to a corresponding transmitting branch;

wherein phase-shifting, for each transmitting branch, further comprises phase-shifting the second input IF signal with a second IF phase shift to obtain a second phase shifted IF signal that is added to the phase shifted IF signal to obtain a combined phase shifted IF signal, wherein the choice of the second IF phase shift is determined such that the combined IF and LO phase shift of each transmitting branch is a second desired RF phase shift; and

wherein up-converting, for each transmitting branch, comprises mixing the combined phase shifted IF signal and the phased shifted LO signal to obtain the RF signal with the desired RF phase shift.

10. The method in accordance with claim 9 , wherein the method further comprises:

filtering the RF signal with a band-pass filter configured to suppress unwanted spurious frequency components.

11. The method in accordance with claim 9 , wherein the method further comprises:

amplifying the RF signal before transmitting.

12. The method in accordance with claim 9 , wherein at least one of the first and second IF phase shifter and the LO phase shifter in each transmitting branch is tunable.

13. The method according to claim 9 , wherein the first and second IF phase shift comprises a continuously variable phase shift; and

wherein the LO phase shift comprises a continuously variable phase shift.

14. The method according to claim 9 , wherein the first and second IF phase shift comprises a discrete set of variable phase shifts; and

wherein the LO phase shift comprises a discrete set of variable phase shifts.

15. The method according to claim 9 , wherein each first and second phase shifted IF signal comprises a digital domain phase shifted IF signal;

wherein phase-shifting, for each transmitting branch, further comprises converting the digital domain phase shifted IF signal to the analog domain; and

wherein up-converting, for each transmitting branch, further comprises mixing the analog domain phase shifted IF signal with the phased shifted LO signal to obtain the RF signal.

16. The method according to claim 9 , wherein for each transmitting branch the LO phase shifts are chosen randomly in order to suppress unwanted spurious signals.

17. A computer readable program product comprised on a non-transitory computer-readable medium, the computer readable program product comprising executable instructions that when executed by a processor of a phased array transmitter causes the processor to operate to:

split a first intermediate frequency (IF) signal into a plurality of first input IF signals and feeding each of the plurality of first input IF signals to a corresponding transmitting branch of the phased array transmitter;

phase-shift for each transmitting branch, the first input IF signal with a first IF phase shift to obtain a first phase shifted IF signal, and phase-shifting the a local oscillator (LO) signal with an LO phase shift to obtain a phase shifted LO signal, wherein the choice of the first IF phase shift and the LO phase shift is determined such that the combined IF and LO phase shift is the desired RF phase shift of each transmitting branch;

up-convert, for each transmitting branch, the first input IF signal to a radio frequency (RF) signal with a desired RF phase shift by mixing the first phase shifted IF signal and the phased shifted LO signal to obtain the RF signal with the desired RF phase shift; and

transmit, for each transmitting branch, the RF signal by feeding the RF signal to an antenna element;

split a second IF signal into a plurality of second input IF signals and feeding each of the plurality of second input IF signals to a corresponding transmitting branch;

for each transmitting branch, phase-shift the second input IF signal with a second IF phase shift to obtain a second phase shifted IF signal that is added to the phase shifted IF signal to obtain a combined phase shifted IF signal, wherein the choice of the second IF phase shift is determined such that the combined IF and LO phase shift of each transmitting branch is a second desired RF phase shift; and

mix the combined phase shifted IF signal and the phased shifted LO signal to obtain the RF signal with the desired RF phase shift.

18. The computer readable program product according to claim 17 , wherein for each transmitting branch the LO phase shifts are chosen randomly in order to suppress unwanted spurious signals.