Method of calibrating power for MIMO-OFDM transmitter

Abstract

Provided is a method of calibrating a power for a multiple input and multiple output-orthogonal frequency division multiplexing transmitter having a plurality of antennas via a measurement equipment, the method including: receiving a cyclic delay diversity (CDD) signal simultaneously output from the transmitter to obtain a starting point of a frame of a corresponding signal; performing a fast Fourier transform on a sample proceeded from the starting point by an extent of a maximum CDD delay; calculating a channel coefficient; calculating a channel impulse response and a power of the channel impulse response; converting CDD delay values of each antenna into values in units of samples by using a sampling rate in the channel impulse response; mapping a peak point position of the channel impulse response to each antenna using CDD delay sample values; and simultaneously performing a power calibration of each antenna based on each peak point power.

Claims

1. A method of calibrating power for a multiple input and multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) transmitter having a plurality of antennas via a measurement equipment coupled to the MIMO-OFDM transmitter, the method comprising: receiving, via the measurement equipment, a cyclic delay diversity (CDD) signal which is cyclically delayed for a predetermined time period and simultaneously output from the MIMO-OFDM transmitter to obtain a starting point of a frame of a corresponding signal; performing, via the measurement equipment, a fast Fourier transform (FFT) on a sample preceded by a maximum CDD delay from the starting point of the frame; calculating, via the measurement equipment, a channel coefficient H(n) by using an FFT calculated training symbol Y(n) and an ideal training symbol X(n), wherein n=0, 1, 2, . . . ; calculating, via the measurement equipment, a channel impulse response h(k) by performing an inverse fast Fourier transform (IFFT) on the channel coefficient H(n) and calculating a power p(k) of the channel impulse response, wherein k=0, 1, 2, . . . ; converting, via the measurement equipment, CDD delay values of each antenna into CDD delay sample values by using a sampling rate in the channel impulse response; mapping, via the measurement equipment, a peak point position of the channel impulse response to each antenna by using the CDD delay sample values; and simultaneously performing, via the measurement equipment, a power calibration of each antenna based on each peak point power of the plurality of antennas.

2. The method of claim 1, wherein the channel coefficient H(n) is obtained by $H\left(n\right)=\frac{Y\left(n\right)}{X\left(n\right)},n=0,1,2,\mathrm{.Math.},\mathrm{FFT}\mathrm{size}-1.$

3. The method of claim 1, wherein the channel impulse response h(k) and the power p(k) thereof are obtained by
h(k)=IFFT{H(n)}, k=0,1,2, . . . ,FFT size1 and
P(k)=real(h(k))*real(h(k))+imag(h(k))*imag(h(k)), k=0,1,2, . . . ,FFT size1.

4. The method of claim 2, wherein the channel impulse response h(k) and the power p(k) thereof are obtained by
h(k)=IFFT{H(n)}, k=0,1,2, . . . ,FFT size1 and
P(k)=real(h(k))*real(h(k))+imag(h(k))*imag(h(k)), k=0,1,2, . . . ,FFT size1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a block diagram illustrating a system for calibrating power for a MIMO-OFDM transmitter according to the present invention.

(2) FIG. 2 is a view illustrating a CDD principle applied to a conventional MIMO-OFDM transmitter.

(3) FIG. 3 is a block diagram illustrating units constituting the MIMO-OFDM to which CDD is applied.

(4) FIG. 4 is a flowchart illustrating a method of calibrating power for a MIMO-OFDM according to the present invention.

(5) FIG. 5 is a timing chart illustrating an FFT start position for power calibration according to the method of calibrating power for a MIMO-OFDM transmitter of the present invention.

(6) FIG. 6 is an exemplary power graph illustrating a channel impulse response transmitted for four transmission antennas according to the method of calibrating power for a MIMO-OFDM transmitter of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) Hereinafter, a method of calibrating power for a MIMO-OFDM transmitter according to a preferable embodiment of the present invention will be described in detail with reference to accompanying drawings.

(8) FIG. 1 is a block diagram illustrating a system for calibrating power for a MIMO-OFDM transmitter according to the present invention. As shown in FIG. 1, for example, the system for calibrating power for a MIMO-OFDM transmitter according to the present invention may substantially include user equipment, a macro base station or a device under test (DUT) 300 supporting MIMO, measuring equipment 100 such as a spectrum analyzer or a signal analyzer, and a signal distributor 200 for connecting the measuring equipment 100 and the DUT 300.

(9) In the system described above, MIMO transmission antenna ports Ant-1, Ant-2, . . . , Ant-n of the DUT 300 are wire connected to input ports of the signal distributor 200, respectively. An output port of the signal distributor 200 is wire connected to an RF input port of the measuring equipment 100. In this case, the DUT 300 transmits delayed CDD signals through the transmission antenna ports Ant-1, Ant-2, . . . , Ant-n at the same time, and the measuring equipment 100 performs power calibration after simultaneously receiving the signals transmitted from the DUT 300 through the RF input port.

(10) FIG. 2 is a view illustrating a CDD principle applied to a conventional MIMO-OFDM transmitter. As shown in FIG. 2, the CCD scheme is a scheme of increasing a diversity gain by assigning predetermined delay time periods to antennas, respectively. Since the cyclic delay in time domain causes a phase shift between subcarriers in frequency domain, the correlation between the subcarriers is reduced so that the diversity gain may be obtained in a simple manner, so the CDD scheme has been applied to many OFDM systems.

(11) FIG. 3 is a block diagram illustrating units constituting the MIMO-OFDM to which CDD is applied. As shown in FIG. 3, the MIMO-OFDM transmitter to which CDD is applied includes a forward error correlation (FEC) encoder 310, a modulator 320, an IFFT calculator 330, a plurality of delays 340, a plurality of cyclic prefix (CP) inserters 350 and a plurality of antennas 360.

(12) In the configuration described above, the encoder 310 encodes transmitting data at a coding rate to output coded symbols. The modulator 320 modulates the data from the encoder 310 according to a give modulation scheme and outputs the modulated symbols. The IFFT calculator 330 performs an IFFT operation on the data from the modulator 320 to output sample data. Next, the delays 340 cyclically delay the sample data for a predetermined time period, respectively. Each of the CP insertors 350 inserts a cyclic prefix into the sample data from the corresponding delay 340 and transmits the CP-inserted sample data through the corresponding antenna 360.

(13) FIG. 4 is a flowchart illustrating a method of calibrating power for a MIMO-OFDM according to the present invention. As shown in FIG. 4, according to the method of calibrating power for a MIMO-OFDM transmitter, in the system shown in FIG. 1, the measuring equipment 100 receives a MIMO signal from the DUT 300 to obtain a starting point of a frame of a corresponding signal in step S10. Next, in step S20, FFT is performed on the sample preceded from the starting point of the frame obtained by extent of the maximum CDD delay in step S10. FIG. 5 is a timing chart illustrating an FFT start position for power calibration according to the method of calibrating power for a MIMO-OFDM transmitter of the present invention. Meanwhile, values specified in the standard are utilized as the CDD delay values of each transmission antenna.

(14) In step S30, a channel coefficient H(n) is calculated by using the FFT calculated training symbol Y(n) and an ideal training symbol X(n) as following Equation 1.

(15) $\begin{array}{cc}H\left(n\right)=\frac{Y\left(n\right)}{X\left(n\right)},n=0,1,2,\mathrm{.Math.},\mathrm{FFT}\mathrm{size}-1& \left[\mathrm{Equation}1\right]\end{array}$

(16) Next, in step S40, a channel impulse response h(k) is calculated by IFFT calculating the channel coefficient H(n) calculated in step S30 as following Equation 2 and then, power p(k) of the channel impulse response (h) is calculated as following Equation 3.
h(k)=IFFT{H(n)}, k=0,1,2, . . . ,FFT size1[Equation 2]
P(k)=real(h(k))*real(h(k))+imag(h(k))*imag(h(k)), k=0,1,2, . . . ,FFT size1[Equation 3]

(17) FIG. 6 is an exemplary power graph illustrating a channel impulse response transmitted for four transmission antennas according to the method of calibrating power for a MIMO-OFDM transmitter of the present invention.

(18) Meanwhile, since the MIMO-OFDM signal received by the measuring equipment 100 is the sum of several signals obtained by applying CDD to the same signal, that is, delaying the same signal, the power P of the channel impulse response h has the forms shown in FIG. 6. In this case, the sampling resolution is 1/fs (where fs is a sampling rate). In step S50, by applying it and using the sampling rate fs, the CDD delay values of each transmission antenna are converted into values in units of samples as following Equation 4.

(19) $\begin{array}{cc}\mathrm{CDD}\mathrm{delay}\mathrm{sample}\mathrm{of}\mathrm{antenna}=\frac{\mathrm{CDD}\mathrm{delay}\left(\sec \right)\mathrm{of}\mathrm{antenna}}{\mathrm{fs}\left(\mathrm{Hz}\right)}& \left[\mathrm{Equation}4\right]\end{array}$

(20) Meanwhile, since FFT is performed on the sample preceded by from the starting point by extent of the maximum CDD delay in step S20 after the frame position is detected, the peak position of CDD delay 0 is the rightmost peak point (Peak 1) in FIG. 6. Thus, in step S60, peak point positions (Peak 2, Peak 3 and Peak 4 in FIG. 6) of channel impulse responses are mapped to the transmission antennas by using the CDD delay sample values of each transmission antenna obtained in step S50.

(21) In this case, since the power in frequency domain is equal to that in time domain, as a result, the powers of each peak point positions Peak 1 to Peak 4 of the channel impulse responses h in FIG. 6 are equal to the powers of MIMO-OFDM symbols of the transmission antennas of the DUT 300. Thus, in step S70, based on the powers of each transmission antennas of the DUT 300, the power calibration for the MIMO-OFDM transmitter may be performed at each peak point position of the channel impulse responses h, so that the time taken to perform the power calibration may be effectively reduced.

(22) The method of calibrating power for a MIMO-OFDM described with reference to accompanying drawings in this disclosure is for an illustrative purpose only, and the present invention is not limited thereto. Thus, it should be understood that numerous other modifications and embodiments can be devised by those skilled in the art within the spirit and scope of the present invention and they will fall within the scope of the present invention.