Test apparatus and program

Provided is a measurement apparatus that measures power of an orthogonal frequency division multiplexing modulated signal (OFDM modulated signal) output from a transmitting device, comprising an output control section that causes the transmitting device to output the OFDM modulated signal having a prescribed waveform that repeats in each of a plurality of repetition periods; and a power measuring section that measures, over a measurement period that corresponds to an integer multiple of the repetition period, the power of the OFDM modulated signal output by the transmitting device. The output control section may cause the transmitting device to output the OFDM modulated signal having, as the repetition period, a period for which the arrangement of a pilot signal is the same in a direction of sub-carriers and a direction of OFDM symbols.

BACKGROUND

1. Technical Field

The present invention relates to a measurement apparatus and a program. In particular, the present invention relates to a measurement apparatus and a program for measuring power of an orthogonal frequency division multiplexing modulated signal (OFDM modulated signal) output from a transmitting device.

2. Related Art

Orthogonal frequency division multiplexing (OFDM) is used, for example, for wireless LAN (IEEE 802.16e) according to the communication standard. An OFDM communication apparatus that adopts OFDM modulation transmits and receives orthogonal frequency division multiplexing modulated signals (referred to hereinafter as “OFDM modulated signals”). During transmission, the OFDM communication apparatus generates OFDM modulated signals by performing an IFFT (Inverse Fast Fourier Transform) on the data to be transmitted. During reception, the OFDM communication apparatus performs an FFT (Fast Fourier Transform) on the received OFDM modulated signal and extracts the data modulated by each sub-carrier.Patent Document 1: Japanese Patent Application Publication No. 2000-206160

An apparatus is known that measures power of a transmission signal from a communication apparatus, as shown in Patent Document 1, for example. When measuring the transmission power of the OFDM communication apparatus to measure a characteristic of the OFDM communication apparatus, however, it is necessary to sample the OFDM modulated signal over a long time and calculate the average power.

SUMMARY

Therefore, it is an object of an aspect of the innovations herein to provide a measurement apparatus and a program, which are capable of overcoming the above drawbacks accompanying the related art. The above and other objects can be achieved by combinations described in the independent claims. The dependent claims define further advantageous and exemplary combinations of the innovations herein.

According to a first aspect related to the innovations herein, one exemplary measurement apparatus may include a measurement apparatus that measures power of an orthogonal frequency division multiplexing modulated signal (OFDM modulated signal) output from a transmitting device, comprising an output control section that causes the transmitting device to output the OFDM modulated signal having a prescribed waveform that repeats in each of a plurality of repetition periods; and a power measuring section that measures, over a measurement period that corresponds to an integer multiple of the repetition period, the power of the OFDM modulated signal output by the transmitting device.

According to a second aspect related to the innovations herein, one exemplary program may include a program that causes an information processing apparatus to function as a measurement apparatus that measures power of an orthogonal frequency division multiplexing modulated signal (OFDM modulated signal) output from a transmitting device, the program causing the information processing apparatus to function as an output control section that causes the transmitting device to output the OFDM modulated signal having a prescribed waveform that repeats in each of a plurality of repetition periods; and a power measuring section that measures, over a measurement period that corresponds to an integer multiple of the repetition period, the power of the OFDM modulated signal output by the transmitting device.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1shows a configuration of a test apparatus10according to an embodiment of the present invention, along with a transmission apparatus under test100. The transmission apparatus under test100is an example of a transmitting device according to the present invention. The transmission apparatus under test100uses OFDM modulation, and outputs a transmission signal obtained by orthogonally modulating an OFDM modulated signal with a carrier signal having a prescribed frequency.

The test apparatus10is an example of a measurement apparatus according to the present invention. The test apparatus10measures the power of the OFDM modulated signal output from the transmission apparatus under test100and judges the acceptability of the transmission apparatus under test100based on the measurement result.

The test apparatus10is provided with an output control section12, an input section14, a power measuring section16, and a comparing section18. The output control section12outputs, from the transmission apparatus under test100, a transmission signal obtained by modulating, with a carrier signal, an OFDM modulated signal having a prescribed waveform that repeats in each of a plurality of repetition periods. The OFDM modulated signal may be a signal as set forth in IEEE 802.16e, for example. The output control section12may include a signal generating section30. The signal generating section30may supply the transmission apparatus under test100with transmission data, and the transmission apparatus under test100may output a transmission signal obtained by modulating the transmission data.

The input section14receives the OFDM modulated signal output from the transmission apparatus under test100having a measurement period corresponding to an integer multiple of the repetition period. The input section14may include a down-converter32, an AD converting section34, an orthogonal demodulating section36, and a signal storing section38. The down-converter32may down-convert the carrier frequency of the transmission signal output from the transmission apparatus under test100. The down-converter32outputs an IF signal obtained as a result of the down-conversion on the transmission signal.

The AD converting section34samples the IF signal output from the down-converter32and digitizes the sampling result. The AD converting section34may instead sample and digitize the transmission signal output directly from the transmission apparatus under test100without passing through the down-converter32. The orthogonal demodulating section36orthogonally demodulates the IF signal digitized by the AD converting section34and outputs a baseband signal, i.e. an I-signal and a Q-signal.

The signal storing section38sequentially stores the baseband signal output from the orthogonal demodulating section36. The signal storing section38may sequentially store each sample of the baseband signal output from the orthogonal demodulating section36at sequential addresses. Here, the signal storing section38stores samples of the baseband signal for each measurement period. For example, the orthogonal demodulating section36may store a baseband signal having the number of samples contained in a single measurement period, which is an integer multiple of the repetition period.

The power measuring section16measures the power over the measurement period of the OFDM modulated signal output by the transmission apparatus under test100. For example, the power measuring section16may calculate the power of the baseband signal having a number of samples contained in the measurement period stored in the signal storing section38. More specifically, the power measuring section16calculates, for each sample n, a value (In2+Qn2) obtained as the sum of the square of the real component (In) and the square of the imaginary component (Qn), and may calculate the power to be the summation (Σ(In2+Qn2)) of this calculated value at each sample within the measurement period. Furthermore, the power measuring section16may calculate, as the power, an average value (Σ(In2+Qn2)/N) by dividing the summation value (Σ(In2+Qn2)) by the number of samples N in the measurement period.

The comparing section18judges the acceptability of the transmission apparatus under test100by comparing (i) an expected value for the power of the OFDM modulated signal to be output from the transmission apparatus under test100and (ii) the power measured by the power measuring section16. The test apparatus10described above can measure the power of the OFDM modulated signal output from the transmission apparatus under test100in a short time. Therefore, the test apparatus10can quickly test the transmission apparatus under test100.

FIG. 2shows examples of the repetition period of the OFDM modulated signal output from the transmission apparatus under test100and the measurement period of the power used by the power measuring section16. The output control section12causes the transmission apparatus under test100to output the OFDM modulated signal in which the same waveform repeats in each repetition period, which is an integer multiple of the OFDM symbol. As shown by “A” ofFIG. 2, for example, the output control section12may cause the transmission apparatus under test100to output an OFDM modulated signal in which the same waveform repeats for each set of three OFDM symbols.

The multipath effect can be ignored when measuring the power, and so the output control section12may cause the transmission apparatus under test100to output an OFDM modulated signal that does not include a guard interval. In this way, the test apparatus10can measure the power in a short time.

The power measuring section16measures the power over a measurement period, which is an integer multiple of the repetition period, in the OFDM modulated signal output from the transmission apparatus under test100. As shown by “B” ofFIG. 2, the power measuring section16may measure the power over a measurement period that is an integer multiple of the repetition period, which in this case is three OFDM symbols.

Here, the OFDM modulated signal output from the transmission apparatus under test100has the same waveform for each repetition period, and so the power of the extracted signal is the same no matter where in the OFDM modulated signal the signal having the length of the measurement period is extracted from. Accordingly, the power measuring section16may extract the signal having the length of the measurement period from any position, and may measure the power of this extracted signal. In other words, the power measuring section16can extract an OFDM modulated signal with a length of the measurement period from a position that is not bound by the boundaries of the OFDM symbols, and can measure the power of the extracted signal.

Therefore, the power measuring section16can measure the power of the OFDM modulated signal without performing a symbol synchronization process for the OFDM modulated signal. Accordingly, the test apparatus10can have a simple configuration that can measure the power of the OFDM modulated signal output from the transmission apparatus under test100.

FIG. 3shows exemplary arrangements of a data signal and a pilot signal of the OFDM modulated signal output from the transmission apparatus under test100by the output control section12according to the present embodiment. The output control section12causes the transmission apparatus under test100to output an OFDM modulated signal obtained by performing an IFFT on a signal sequence wherein a period is repeated in which the signal component arrangement is the same in the sub-carrier direction and in the OFDM symbol direction.

The sub-carriers of the OFDM modulated signal include a pilot signal and a data signal as signal components. The pilot signal has a predetermined amplitude and phase according to the technical specifications or the like. The pilot signal is modulated with a predetermined sub-carrier of a predetermined OFDM symbol. In the example ofFIG. 3, the pilot signal is output at sub-carriers numbered1,4,7,10, etc in each odd-numbered OFDM symbol.

The data signal is a signal component other than the pilot signal, and may have an amplitude and phase corresponding to a sign designated by a user. The data signal may be obtained by orthogonal phase modulation such as BPSK and QPSK or by orthogonal amplitude modulation such as 16 QAM or 64 QAM.

Here, the output control section12causes the transmission apparatus under test100to output an OFDM modulated signal in which the repetition period is set such that the arrangement of the pilot signal is the same in the sub-carrier direction and in the OFDM symbol direction. If the repetition period is set to be three consecutive OFDM symbols, as in the example ofFIG. 3, the arrangement of the pilot signal is the same within each repetition period. Accordingly, in the example ofFIG. 3, the output control section12may cause the transmission apparatus under test100to output an OFDM modulated signal in which three OFDM symbols are set as one repetition period. The output control section12causes the transmission apparatus under test100to output an OFDM modulated signal in which the arrangement of the sign of each data signal is the same for each period in which the arrangement of the pilot signal is the same, i.e. each period of three OFDM symbols in the example ofFIG. 3.

The arrangement of the pilot signal is determined by the technical specifications and cannot be changed by the user. However, even if a pilot signal that is unchangeable by the user is included, the output control section12described above can cause the transmission apparatus under test100to output an OFDM modulated signal in which the prescribed waveform repeats for each repetition period.

FIG. 4shows examples of OFDM symbols and a sampling frequency of the OFDM symbols. If fOFDM, e.g. 11.2 MHz, represents the sampling frequency of the OFDM symbols and M, e.g. 1024, represents the number of FFT points of the OFDM modulated signal, in order to sample an OFDM modulated signal with a length of n, e.g. 3, OFDM symbols, the AD converting section34must sample the OFDM modulated signal within a period T expressed below in Expression 1.
T=(1/(fOFDM))×M×n(seconds)  Expression 1:

Furthermore, if fCrepresents the sampling frequency of the AD converting section34and m represents the number of samples taken by the AD converting section34for sampling a period that is n OFDM symbols long, Expression 2 can be formed.
(1/(fOFDM))×M×n=(1/fC)×mExpression 2:

Expression 2 can be transformed into Expression 3 shown below.
fC=(fOFDM/M)×(m/n)  Expression 3:

The AD converting section34may set the sampling frequency fCto be a value obtained by multiplying (i) an arbitrary number L and (ii) a value obtained by dividing the sampling frequency fOFDMof the OFDM symbols by the number M of FFT points. The signal storing section38then stores the OFDM modulated signal sampled with this sampling frequency fCas a number of samples (n×L=m) obtained by multiplying the number n of OFDM symbols to be sampled by the arbitrary number L. In this way, the input section14can begin sampling from any point and can accurately sample an OFDM modulated signal having the length of the measurement period.

Furthermore, the AD converting section34may sample the OFDM modulated signal using a sampling clock fCthat is synchronized with the sampling frequency fOFDMof the OFDM modulated signal output from the transmission apparatus under test100. As a result, the input section14can more accurately sample a measurement period of the OFDM modulated signal output from the transmission apparatus under test100.

The sampling frequency fOFDMof the OFDM symbols and the number M of FFT points is determined in advance according to the technical specifications of the OFDM modulated signal output by the transmission apparatus under test100being measured. Furthermore, n corresponds to the number of OFDM symbols in a repetition period. In other words, n is predetermined according to the arrangement of the pilot signal in the direction of the sub-carriers and the direction of the OFDM symbols, which is predetermined by the technical specifications. Accordingly, the adjustable parameters in Expression 3 are fCand m. Furthermore, the range of the sampling frequency fCfor the AD converting section34is desirably within a certain range from the limit of the oscillator, for example.

Therefore, the input section14adjusts the parameter m such that the sampling frequency fCis within the desirable range. The signal storing section38may store the OFDM modulated signal having a number of samples corresponding to the adjusted value of m. As a result, even if the sampling frequency fCof the AD converting section34is within a prescribed range, the input section14can accurately sample the OFDM modulated signal having a length of the measurement period.

FIG. 5shows a Cartesian coordinate system in which are plotted signal points of a data signal that is QPSK modulated and a data signal that is 16 QAM modulated. The output control section12can cause a data signal that is orthogonally phase modulated to be included in the OFDM modulated signal and can cause a data signal that is orthogonally amplitude modulated to be included in the OFDM modulated signal.

The power of an OFDM modulated signal that includes an orthogonally phase modulated data signal is ideally constant. The power of an OFDM modulated signal that includes an orthogonally amplitude modulated data signal changes depending on the included sign.

Here, based on the first quadrant of the Cartesian plane inFIG. 5, it is understood that the average value of the power of the four 16 QAM signal points in the first quadrant matches the power of the one QPSK signal point in the first quadrant. Accordingly, when the OFDM modulated signal includes a 16 QAM modulated data signal, the output control section12can express the plurality of signal points acquired from the 16 QAM modulation as an average to match the power of the OFDM modulated signal that includes the QPSK modulated data signal.

Therefore, when the transmission apparatus under test100outputs an OFDM modulated signal obtained by orthogonally amplitude modulating the data signal, the output control section12causes the power over the repetition period of the OFDM modulated signal being output to be substantially equal to the power over the repetition period of the OFDM modulated signal obtained by orthogonally phase modulating the data signal. The comparing section18compares (i) the power measured by the power measuring section16to (ii) the expected power that is the same when the data signal is orthogonally phase modulated and when the data signal is orthogonally amplitude modulated. In this way, the comparing section18can use the same expected power regardless of the type of modulation. Therefore, the comparing section18can easily compare the measured power to the expected power.

Furthermore, the output control section12may cause the transmission apparatus under test100to output an OFDM modulated signal in which the difference in occurrence rates of the signs of the data signal in the repetition period is within a prescribed range. For example, the output control section12may cause the transmission apparatus under test100to output an OFDM modulated signal in which the difference between the minimum value and the maximum value of the occurrence rate of each type of signal in each repetition period, e.g. the occurrence rates of 00, 01, 10, and 11 for QPSK, is within a predetermined range. Therefore, the output control section12can prevent the transmission apparatus under test100from outputting an OFDM modulated signal in which the amplitude and the phase are skewed for prescribed symbols. In other words, the test apparatus10can measure averaged characteristics of the transmission apparatus under test100.

FIG. 6shows a configuration of the test apparatus10according to an embodiment of the present invention, along with a carrier modulation apparatus under test200. The test apparatus10of the present embodiment has substantially the same function and configuration as the test apparatus10described inFIG. 1and components that are the same as those shown inFIG. 1are given the same reference numerals. Therefore, the following description includes only differing points.

The test apparatus10of the present embodiment tests the carrier modulation apparatus under test200, which is an example of a transmitting device according to the present invention. The carrier modulation apparatus under test200may be an orthogonal modulator provided at an output stage of a transmission apparatus.

In the present embodiment, the output control section12further includes a transmitting section40. The signal generating section30supplies the transmitting section40with transmission data. The transmitting section40performs an IFFT on the transmission data to generate an OFDM modulated signal of a baseband signal or an IF signal, and supplies the resulting signal to the carrier modulation apparatus under test200. The transmitting section40causes the carrier modulation apparatus under test200to output a transmission signal obtained by modulating the OFDM modulated signal of the baseband signal or the IF signal with a carrier signal. The test apparatus10of the present embodiment can test the characteristics of an orthogonal modulator provided at an output stage of a transmission apparatus.

FIG. 7shows an example of a hardware configuration of a computer1900according to the present embodiment. The computer1900according to the present embodiment is provided with a CPU peripheral including a CPU2000, a RAM2020, a graphic controller2075, and a displaying apparatus2080, all of which are connected to each other by a host controller2082; an input/output section including a communication interface2030, a hard disk drive2040, and a CD-ROM drive2060, all of which are connected to the host controller2082by an input/output controller2084; and a legacy input/output section including a ROM2010, a flexible disk drive2050, and an input/output chip2070, all of which are connected to the input/output controller2084.

The host controller2082is connected to the RAM2020and is also connected to the CPU2000and graphic controller2075accessing the RAM2020at a high transfer rate. The CPU2000operates to control each section based on programs stored in the ROM2010and the RAM2020. The graphic controller2075acquires image data generated by the CPU2000or the like on a frame buffer disposed inside the RAM2020and displays the image data in the displaying apparatus2080. Instead, the graphic controller2075may internally include the frame buffer storing the image data generated by the CPU2000or the like.

The input/output controller2084connects the communication interface2030serving as a relatively high speed input/output apparatus, the hard disk drive2040, and the CD-ROM drive2060to the host controller2082. The communication interface2030communicates with other apparatuses via a network. The hard disk drive2040stores the programs and data used by the CPU2000housed in the computer1900. The CD-ROM drive2060reads the programs and data from a CD-ROM2095and provides the read information to the hard disk drive2040via the RAM2020.

Furthermore, the input/output controller2084is connected to the ROM2010, and is also connected to the flexible disk drive2050and the input/output chip2070serving as a relatively high speed input/output apparatus. The ROM2010stores a boot program performed when the computer1900starts up, a program relying on the hardware of the computer1900, and the like. The flexible disk drive2050reads programs or data from a flexible disk2090and supplies the read information to the hard disk drive2040via the RAM2020. The input/output chip2070connects the flexible disk drive2050to each of the input/output apparatuses via, for example, a parallel port, a serial port, a keyboard port, a mouse port, or the like.

The programs provided to the hard disk drive2040via the RAM2020are stored in a storage medium, such as the flexible disk2090, the CD-ROM2095, or an IC card, and provided by a user. The programs are read from storage medium, installed in the hard disk drive2040inside the computer1900via the RAM2020, and performed by the CPU2000.

The programs installed in the computer1900to make the computer1900function as the test apparatus10are provided with an output control module, an input control module, a power measuring module, and a comparing module. These programs and modules prompt the CPU2000or the like to make the computer1900function as the output control section12, the input section14, the power measuring section16, and the comparing section18, respectively.

The programs and modules shown above may also be stored in an external storage medium. The flexible disk2090, the CD-ROM2095, an optical storage medium such as a DVD or CD, a magneto-optical storage medium, a tape medium, a semiconductor memory such as an IC card, or the like can be used as the storage medium. Furthermore, a storage apparatus such as a hard disk or RAM that is provided with a server system connected to the Internet or a specialized communication network may be used to provide the programs to the computer1900via the network.