WO2005083916A1

WO2005083916A1 - Simulating data transfer between transmitter and receiver

Info

Publication number: WO2005083916A1
Application number: PCT/FI2005/050050
Authority: WO
Inventors: Aarne MÄMMELÄ; Markku Kiviranta; Tommi JÄMSÄ; Jyrki Patrikainen; Janne Kolu
Original assignee: Elektrobit Testing Oy
Priority date: 2004-02-26
Filing date: 2005-02-25
Publication date: 2005-09-09
Also published as: FI20040306A0; FI20040306A

Abstract

The invention relates to a method and a simulator for simulating a radio channel between a transmitter (100) and a receiver (102). The simulating device comprises means (308) for digitally simulating a radio channel between the transmitter and the receiver. The device also comprises means (304, 316) for digitally simulating interferences and distortions caused by analog parts of the transmitter and the receiver between transmitters and receivers.

Description

SIMULATING DATA TRANSFER BETWEEN TRANSMITTER AND RECEIVER

FIELD [0001] The invention relates to a method and an apparatus for implementing the method of simulating data transfer between a transmitter and a receiver. The invention particularly relates to digital simulation of data transfer.

BACKGROUND [0002] The design and testing of data transfer systems, such as radio systems, for example, is a multistage and complex process. Simplified, the working phases can be divided into three stages: analyses, simulations and prototypes. At the analysis stage, mathematical models are created for the system being designed, allowing the desired system and its properties to be described and adjusted. Models are capable of giving accurate results, but in order for mathematical equations to remain sufficiently simple, numerous simplifications have to be made when describing the operation of the physical world. [0003] Building and testing prototypes in practice under field conditions gives valuable information about the actual circumstances, but is extremely expensive and time-consuming. Tests performed under real conditions are difficult, since for instance radio system tests performed outside are influenced by the weather and time of year, for example, which change continuously. Even measurements made at the same location produce different results at different times. In addition, a test performed in one environment is not valid in another fully similar environment. Usually it is not either possible to test the worst possible situation under real conditions. [0004] Simulation plays a significant role in all stages of the design and testing of telecommunication systems. Simulation enables the testing of the performance and properties of systems with the desired accuracy and in the desired environment substantially more easily than is possible by means of analysis or actual testing. For example, a device simulating a radio channel can be very well used to simulate the desired type of radio channel between two radio devices in such a manner that the radio devices operate at their natural transfer rates, such as in an actual usage situation. [0005] Different alternatives for simulating data transfer between a transmitter and a receiver in a telecommunication system utilizing a radio- frequency data transfer channel exist in different steps of system and equip- ment design. In the example of Figure 1A, data transfer between a transmitter 100 and a receiver 102 in a radio channel is simulated at radio frequency in a radio channel simulator 104. Both the transmitter 100 and the receiver 102 comprise baseband parts 106, 108, a converter 110, 112, and radio frequency parts 114, 116. This enables simulation of the operation of existing devices in conditions simulating actual conditions of use. In the example of Figure 1 B, data transfer between the transmitter 100 and the receiver 102 in a radio channel is simulated at an analog baseband. In this case, the transmitter and the receiver do not comprise radio frequency parts 114, 116. In the example of Figure 1 C, data transfer between the transmitter 100 and the receiver 102 in a radio channel is simulated at a digital baseband. In this case, the transmitter and the receiver only comprise the digital baseband parts 106, 108. The arrangement of Figure 1C is advantageous at early stages of the design, when the devices are not yet completely designed. However, a drawback in the arrangement is that the operation of the radio frequency parts of the transmitter and the receiver is excluded from the simulation. [0006] In a prior art solution, interferences and distortions caused by the analog parts of a transmitter and a receiver are simulated by software using various computing software, such as Matlab, for example. However, software does not enable real-time simulation, and the long simulation time required by the complex computing presents a problem. Literature also knows a method in which radio frequency parts are simulated by using analog components. The drawback of this method is the structure of the simulator. When analog components are used, the simulator has to be assembled separately for each testing, and its conversion for the different simulations is laborious, since changing simulation parameters requires a change in the structure of the simulator. Such a method has therefore not become common.

BRIEF DESCRIPTION [0007] An object of the invention is to provide a method, and equipment for implementing the method, for advantageous and versatile testing of data transfer between a transmitter and a receiver. This is achieved with a simulation device comprising means for digital simulation of a radio channel between a transmitter and a receiver, and means for digital simulation of interferences and distortions caused by analog parts of the transmitter and the receiver between transmitters and receivers, all said means being under com- mon control. [0008] The invention also relates to a method for digital simulation of a radio channel between a transmitter and a receiver in a simulation device. The method of the invention comprises digital simulation of interferences and distortions caused by analog parts of the transmitter and the receiver between transmitters and receivers. [0009] The solution of the invention brings forth a plurality of advantages. The solution enables more flexible and faster simulation than previously of data transfer between a transmitter and a receiver. In the solution, the multi- path propagation and fading of a radio channel between a transmitter and a receiver are simulated. In the solution, the analog parts of the transmitter and the receiver are also taken into account in the simulation. This results in the simulation results becoming more realistic and reliable than previously. The solution allows the long simulation times performed with software implementations to be essentially shortened. The solution enables real-time simulation. Changing the configuration of the simulator is easy, changes can be made by software, and various simulations can be performed without extensive preparations.

LIST OF FIGURES [0010] In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawings, in which Figures 1A to 1 C, described above, show different simulation alternatives, Figures 2A to 2C illustrate examples of a transmitter, a channel and a receiver, Figure 3 illustrates an example of a simulator according to an embodiment, Figure 4 illustrates an example of an implementation according to an embodiment for simulating phase noise, Figure 5 illustrates an example of an implementation according to an embodiment for simulating l/Q imbalance and DC offset, and Figures 6A to 6C illustrate an example of an implementation according to an embodiment for simulating a non-linear power amplifier. DESCRIPTION OF EMBODIMENTS [0011] Figure 2A illustrates an example of the analog parts of a transmitter. As input is a digital signal 200, 201 from baseband parts of the transmitter. The I branch 200 of the signal is applied to a D/A converter 202, which converts the signal into an analog form. The converted signal is applied via a low-pass filter 204 to a mixer 206, wherein the signal is mixed with a local oscillator signal (I FL_O) obtained from a generator 208. The Q branch 201 of the signal is applied to a D/A converter 203, which converts the signal into an analog form. The converted signal is applied via a low-pass filter 205 to a mixer 207, wherein the signal is mixed with a local oscillator signal (IF_Lo) obtained from the generator 208 via a 90° phase shift 209. The I and Q signals are combined in an adder 210, yielding an intermediate frequency signal fip. [0012] The intermediate signal is applied via a filter 211 to an amplifier 212, which performs gain control. The purpose is to control signal strength to the desired level. The amplified signal is applied to an RF mixer 214, wherein the signal is mixed with a local oscillator signal (RF_Lo) 216 obtained from a generator 217. This yields a radio-frequency signal f_RF, which is applied via a filter 218 to a power amplifier 220, and further to an RF filter 222, after which the signal is ready to be applied to antenna parts. [0013] Typical errors occurring in the D/A converter 202 include a quantization error, an offset error, non-linearity and a gain error, and non- monotonicity. Typical errors of the adder 210 include l/Q imbalance and DC offset. In connection with power adjustment, adjustment errors may occur in the amplifier 212. Errors in the generator 217 of the local oscillator signal RFL_O include frequency offset, phase offset, and phase noise, for example, which are shown in the output of the RF mixer 214. Non-linearity may occur in the power amplifier 220. [0014] Figure 2B illustrates an example of a channel model of a radio channel between a transmitter and a receiver. A transmitted signal propagates in the channel, wherein fading and multipath propagation 224 exist. In addition, noise 226 is summed up in the signal, and possibly interfering signals 228 originating from other radio devices. [0015] Figure 2C illustrates an example of the analog parts of a receiver. As input is an analog signal 230 received with the antenna parts of the receiver. The signal 230 is first applied to an RF filter 232, from where the signal is further applied to a low-noise amplifier (LNA) 234. The amplified signal is mixed with a signal of a local oscillator signal RF_Lo obtained from a generator 238 from radio frequency to intermediate frequency in a mixer 236. The intermediate-frequency signal is applied via a filter 240 to gain control to an AGC amplifier 242. In some cases, the amplifier may also be replaced with a iimiter. [0016] The output signal of the amplifier is divided into I and Q branches. In the I branch, the output signal of the amplifier is mixed to baseband with an IF_Lo signal obtained from a generator 246 in a mixer 244. The baseband signal is filtered in a low-pass filter 248 and converted into digital form in an A/D converter 250. In the Q branch, the output signal of the amplifier is mixed to baseband in a mixer 245 with an IF_Lo signal obtained via a 90° phase shift from the generator 246. The baseband signal is filtered in a low- pass filter 249 and converted into digital form in an A/D converter 251. [0017] Typical errors occurring in an analog part of a receiver include for instance a noise factor of the LNA amplifier 234, a frequency offset seen 236 at the output of the RF mixer, a phase offset, and phase noise, l/Q imbalance of the mixer 244, and DC offset, and a quantization error, an offset error, non-linearity, and a gain error of the A/D converter 250. [0018] Let it be mentioned that the aforementioned transmitter, receiver and typical errors are only examples, and variations in the configurations of the devices and the errors occurring are possible, as is apparent to a person skilled in the art. [0019] Figure 3 illustrates an example of a simulator 300 according to an embodiment. As input 302, a baseband digital signal of the transmitter arrives at the simulator. The simulator comprises a transmitter simulator 304 that simulates interferences and distortions, which are caused to the signal by the radio-frequency and possible intermediate-frequency analog parts of the transmitter and added to the signal in an alternative. The thus processed signal 306 is further applied to a channel simulator 308 that simulates changes caused to the signal by the radio-frequency channel between the transmitter and the receiver. The channel simulator may model multipath propagation, for example. From the output of the channel simulator 308, the signal 310 is applied to a noise simulator 312 that adds white Gaussian noise to the signal. The output signal 314 of the noise simulator is applied to a receiver simulator 316 that simulates interferences and distortions, which are caused to the signal by the radio-frequency and possible intermediate-frequency analog parts of the receiver and which interferences and distortions are added to the signal in an alternative. The thus obtained signal 318 is at the output of the simulator, and it can be applied to the baseband parts of the receiver. The simulator may further comprise a control unit 320, by means of which simulation parameters may be applied to the simulator and which controls the simulation. The control unit may also serve as the user interface of the simulator and enable the connection of other devices, such as a computer, for example, to the simulator, if desired. [0020] The analog parts of the transmitter and receiver may cause a plurality of interferences and distortions to the signal to be transferred. These include for instance non-linear amplifiers, l/Q imbalance and DC offset of the mixers, frequency and phase offsets, and phase noise in oscillators, synthesizers and mixers. [0021] Let us study an example of an implementation of the effect of phase noise in a simulator by means of Figure 4. In the implementation, noise is generated and added to the angle component of the signal. As input is a signal 400, which is divided into I and Q branches in a manner known to a person skilled in the art. The implementation comprises a noise generator 402, which generates white Gaussian noise. The noise is applied to a digital filter 404. The filtered noise is applied as a phase input to a converter 406, to whose amplitude input a desired scalar value 408 is applied. The desired spectrum of the phase noise is determined by means of a digital filter. The converter performs the conversion from polar coordinates into rectangular coordinates, and thus converted, complex multiplication 410 can be performed, whereby the effect of phase noise can be added to the signal. [0022] Let us study an example of an implementation of the effect of l/Q imbalance and DC offset in a simulator by means of Figure 5. When the signal is shifted from one frequency to another in the mixers, errors may be created in the phase of the signal and in the amplitudes of the I and Q branches. Other components besides mixers may also affect the balance of the I and Q branches. [0023] In the implementation, the signal is divided into I and Q branches, each of which are separately subjected to a given amplitude and phase error. The obtained signals are combined into a complex signal, to which an estimate of the DC offset is also added. The result is a signal 500 divided into I and Q branches in a manner known to a person skilled in the art. Both branches comprise a multiplier 502, 504, wherein a real amplitude error 506, 508 is added to the signals of the branches. The thus obtained signals are applied as an amplitude input to converters 510, 512, to whose phase input an estimated phase error 514, 516 is applied. The obtained I and Q signals are combined in a complex adder 518 and applied to a DC offset biock, wherein complex estimates of the DC offset 520, 522 are added to the I and Q branches in an adder 524. [0024] Let us study an example of an implementation of the effect of a non-linear power amplifier in a simulator by means of Figures 6A to 6C. Output power and phase shift relative to input power are variables typically descriptive of a power amplifier. Figures 6A and 6B illustrate an example of the behaviour of power and phase in a saturated amplifier as a function of the input power. The power curve of Figure 6A is generally called AM/AM conversion, and the phase curve of Figure 6B AM/PM conversion. These show the effect of non-linearity on the complex envelope of a signal in polar coordinates. The non-linearity of each amplifier is usually found out by empiric measurements. [0025] In the implementation of Figure 6C, the input is a signal 600 divided into I and Q branches in a manner known to a person skilled in the art. The signal is multiplied by a real coefficient 602, descriptive of input amplification, in a complex multiplier 604. The multiplied signal is applied to a converter 606 that converts the signal into the form of polar coordinates, i.e. into an amplitude and phase signal 608, 610. The amplitude signal 608 is subjected to AM/PM conversion in a converter 612 by using the conversion data of the desired amplifier. The result of the conversion is summed to the phase signal 610 in an adder 614. The amplitude signal 608 is also subjected to AM/AM conversion in a converter 616 by using the conversion data of the desired amplifier. The output of the converter 616 is applied as amplitude data to a converter 618, which receives the output of the adder 614 as phase data. In the converter 618, conversion is performed from polar coordinates into rectangular coordinates. The output signal of the converter, the signal being composed of the I and Q branches, is applied to a complex multiplier 620, wherein the signal is multiplied by a real coefficient 622 descriptive of the output amplification of the amplifier. [0026] The implementations of Figures 4, 5 and 6C can be employed for instance in the blocks 304 and 316 of the simulator device of Figure 3, which simulate the interferences and distortions generated by the analog parts of the transmitter and the receiver. [0027] In an embodiment, the simulation of the interferences and distortions generated by the analog parts of the transmitter and the receiver is implemented in the simulation device by means of FPGA (Field Programmable Gate Array) circuits. These are integrated circuits which enable the implementation of digital applications. FPGA circuits are composed of logics blocks and I/O blocks between them, and the desired implementation can be programmed therein. [0028] The desired functions may also be implemented with other digital circuits, such as for instance ASIC circuits (Application-Specific Integrated Circuit) or a real-time signal processing processor. [0029] Although the invention is described above with reference to the example in accordance with the accompanying drawings, it will be appreciated that the invention is not to be so limited, but it may be modified in a variety of ways within the scope of the appended claims.

Claims

CLAIMS 1. A simulation device comprising means (308) for digital simulation of a radio channel between a transmitter and a receiver, characterized in that the device comprises means (304, 316) for digital simulation of interferences and distortions caused by analog parts of the transmitter and the receiver between transmitters and receivers, all said means (304, 308, 316) being under common control.

2. A simulation device as claimed in claim 1 , characterized in that the means for simulating the interferences and distortions caused by the analog parts of the transmitter and the receiver are implemented by means of FPGA (Field Programmable Gate Array) circuits.

3. A simulation device as claimed in claim 1, characterized in that the means for simulating the interferences and distortions caused by the analog parts of the transmitter and the receiver are implemented by means of ASIC circuits.

4. A simulation device as claimed in claim 1 , characterized in that the means for simulating the interferences and distortions caused by the analog parts of the transmitter and the receiver are implemented by means of a signal processor.

5. A simulation device as claimed in claim 1 , characterized in that the simulation device comprises a control unit (320) for controlling the operation of the means.

6. A simulation device as claimed in claim 1 , characterized in that the simulation device comprises a control unit (320) for setting operational parameters of the means.

7. A simulation device as claimed in claim 1 , characterized in that the analog parts comprise one or more of the group: amplifier, Iimiter, oscillator, mixer, A/D or D/A converter.

8. A simulation device as claimed in claim 1, characterized in that the simulation device is configured to simulate multipath propagation and fading of a radio channel.

9. A method for digital simulation of a radio channel between a transmitter (100) and a receiver (102) in a simulation device (104), characte ized by digital simulation of interferences and distortions caused by analog parts of the transmitter and the receiver between transmitters and receivers.

10. A method as claimed in claim 9, characterized by setting operational parameters of the simulations of the analog parts by software.

11. A method as claimed in claim 9, characterized by the analog parts to be simulated comprising one or more of the group: amplifier, Iimiter, oscillator, mixer, A/D or D/A converter.