Patent Publication Number: US-2009225877-A1

Title: Method and system for characterization of filter transfer functions in ofdm systems

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS/INCORPORATION BY REFERENCE 
     This application makes reference to, claims priority to, and claims the benefit of U.S. Provisional Application Ser. No. 61/033,489, filed on Mar. 4, 2008 and U.S. application Ser. No. 61/092,944, filed on Aug. 29, 2008. 
     This application also makes reference to U.S. application Ser. No. ______ (Attorney Docket No. 19437US03), which is filed on even date herewith. 
     Each of the above referenced applications is hereby incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Certain embodiments of the invention relate to wireless communication systems. More specifically, certain embodiments of the invention relate to a method and system for characterization of filter transfer functions in OFDM systems. 
     BACKGROUND OF THE INVENTION 
     Mobile communications have changed the way people communicate and mobile phones have been transformed from a luxury item to an essential part of every day life. The use of mobile phones is today dictated by social situations, rather than hampered by location or technology. While voice connections fulfill the basic need to communicate, and mobile voice connections continue to filter even further into the fabric of every day life, the mobile Internet is the next step in the mobile communication revolution. The mobile Internet is poised to become a common source of everyday information, and easy, versatile mobile access to this data will be taken for granted. 
     Third (3G) and fourth generation (4G) cellular networks have been specifically designed to fulfill these future demands of the mobile Internet. As these services grow in popularity and usage, factors such as cost efficient optimization of network capacity and quality of service (QoS) will become even more essential to cellular operators than it is today. These factors may be achieved with careful network planning and operation, improvements in transmission methods, and advances in receiver techniques. To this end, carriers need technologies that will allow them to increase downlink capacity. 
     In order to meet these demands, communication systems may become increasingly complex and increasingly miniaturized. It may hence be important to strive for solutions that may reduce, for example, the system complexity while offering high performance. 
     Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings. 
     BRIEF SUMMARY OF THE INVENTION 
     A system and/or method for characterization of filter transfer functions in OFDM systems substantially as shown in and/or described in connection with at least one of the figures, as set forth more completely in the claims. 
     These and other advantages, aspects and novel features of the present invention, as well as details of an illustrated embodiment thereof, will be more fully understood from the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an exemplary wireless communication system, which may be utilized for characterization of filter transfer functions, in accordance with an embodiment of the invention. 
         FIG. 2A  is a diagram of an exemplary analog OFDM receiver front end, which may be utilized for characterization of filter transfer functions, in accordance with an embodiment of the invention. 
         FIG. 2B  is a diagram of an exemplary filter impulse response measurement setup for OFDM systems, in accordance with an embodiment of the invention. 
         FIG. 3  is a diagram of an exemplary double-sided frequency response with and without I/Q mismatch, in accordance with various embodiments of the invention. 
         FIG. 4  is a diagram of an exemplary double-sided phase response of an I branch filter and a Q branch filter mismatch, in accordance with an embodiment of the invention. 
         FIG. 5  is a flow chart illustrating a transfer function characterization, in accordance with various embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Certain embodiments of the invention may be found in a method and system for characterization of filter transfer functions in Orthogonal Frequency Division Multiplexing (OFDM) systems. Aspects of a method and system for characterization of filter transfer functions in OFDM systems may comprise receiving at a filter, a calibration signal which is generated from conversion of a digital input signal comprising N samples to an analog signal, wherein the digital input signal comprises one (1) full scale sample and N- 1  zero samples and N is an integer. In response to receiving the calibration signal, the filter may generate an output analog signal, wherein the output analog signal may be converted to an output digital signal, and a transfer function of the filter may be determined via a Fast Fourier transformation of the output digital signal. 
     The OFDM system may be compliant with a wireless standard, wherein the wireless standard may comprise UMTS EUTRA (LTE), WiMAX (IEEE 802.16), and/or WLAN (IEEE 802.11). A transfer function of an in-phase branch filter and/or a quadrature branch filter may be measured. The filter may be an in-phase branch filter or a quadrature branch filter. The transfer function may comprise a magnitude and/or phase response, wherein the magnitude and/or phase response mismatch may be a function of frequency. A number of the samples N may be chosen arbitrarily. The calibration signal may approximate an impulse signal. The Fast Fourier transformation may be performed with an arbitrary number of coefficients. 
       FIG. 1  is a diagram illustrating an exemplary wireless communication system, which may be utilized for characterization of filter transfer functions, in accordance with an embodiment of the invention. Referring to  FIG. 1 , there is shown an access point  112   b , a computer  110   a , a router  130 , the Internet  132  and a web server  134 . The computer or host device  110   a  may comprise a wireless radio  111   a , a host processor  111   c , and a host memory  111   d . There is also shown a wireless connection between the wireless radio  111   a  and the access point  112   b.    
     The access point  112   b  may comprise suitable logic, circuitry and/or code that may be enabled to transmit and receive radio frequency (RF) signals for data communications, for example with the wireless radio  111   a . The access point  12   b  may also be enabled to communicate via a wired network, for example, with the router  130 . The wireless radio  11   a  may comprise suitable logic, circuitry and/or code that may enable communications over radio frequency waves with one or more other radio communication devices. The wireless radio  111   a  and the access point  112   b  may be compliant with one or more communication standards, for example, GSM, UMTS EUTRA (LTE), CDMA2000, Bluetooth, WiMAX (IEEE 802.16), and/or IEEE 802.11 Wireless LAN. 
     The host processor  111   c  may comprise suitable logic, circuitry and/or code that may be enabled to generate and process data. The host memory  111   d  may comprise suitable logic, circuitry and/or code that may be enabled to store and retrieve data for various system components and functions of the computer  110   a.    
     The router  130  may comprise suitable logic, circuitry and/or code that may be enabled to communicate with communication devices that may be communicatively coupled to it, for example the access point  112   b  and/or one or more communication devices that may be communicatively coupled to the Internet  132 . 
     The Internet  132  may comprise suitable logic, circuitry and/or code that may be enabled to interconnect and exchange data between a plurality of communication devices. The web server  134  may comprise suitable logic, circuitry and/or code that may be enabled to communicate with communication devices that may be communicatively coupled to it via, for example the Internet  132 . 
     Various computing and communication devices comprising hardware and software may be enabled to communicate using one or more wireless communication standards and/or protocols. For example, a user of the computer or host device  110   a  may access the Internet  132  in order to consume streaming content from the Web server  134 . Accordingly, the user may establish a wireless connection between the computer  110   a  and the access point  112   b . Once this connection is established, the streaming content from the Web server  134  may be received via the router  130 , the access point  112   b , and the wireless connection, and consumed by the computer or host device  110   a.    
     In many communication devices, the in-phase (I) channel and the quadrature (Q) channel may be processed separately. Because of component variation, and/or slight mismatch due to fixed hardware that may be operated on multiple communication protocols and/or frequencies, there may be instances when the I-channel and Q-channel processing chains may not be identical. This mismatch may affect communication performance. Various embodiments of the invention may be operable to compensate for mismatch between the I-channel and Q-channel, in accordance with various embodiments of the invention. In this regard, a transfer function mismatch between an in-phase processing branch, and/or a quadrature processing branch of an OFDM receiver may be determined. To determine transfer function mismatch, the transfer functions may be measured. In this regard, a calibration signal may be received at a filter. The calibration signal may be generated from conversion of a digital input signal comprising N samples to an analog signal, wherein the digital input signal comprises one (1) full scale sample and N- 1  zero samples and N is an integer. In accordance with various embodiments of the invention, in some instances, the N- 1  zero samples may be generated by grounding the filter input. In response to receiving the calibration signal, the filter may generate an output analog signal, wherein the output analog signal may be converted to an output digital signal, and a transfer function of the filter may be determined via a Fast Fourier transformation of the output digital signal. 
       FIG. 2A  is a diagram of an exemplary analog OFDM receiver front end, which may be utilized for characterization of filter transfer functions, in accordance with an embodiment of the invention. Referring to  FIG. 2A , there is shown an antenna  240 , amplifiers  242  and  244 , multipliers or mixers  202   a  and  204   a , a local oscillator  246 , a phase shifting block  248 , and in-phase (I) branch filter  210   a , and a quadrature (Q) branch filter  212   a.    
     The antenna  240  may comprise suitable logic, circuitry and/or code that may be enabled to convert electromagnetic radio-frequency waves to electrical signal. The amplifier  242  may comprise suitable logic, circuitry and/or code that may be enabled to amplify and/or filter an input signal. The amplifier  244  may be substantially similar to amplifier  242 . 
     The multiplier  202   a  may comprise suitable logic, circuitry and/or code that may be enabled to generate an output signal that may be proportional to the product of a plurality of input signals. The multiplier  204   a  may be substantially similar to the multiplier  202   a . The local oscillator  246  may comprise suitable logic, circuitry and/or code that may be enabled to generate a alternating current (AC) and/or voltage signal. This AC signal may, for example, be a sinusoidal signal. 
     The phase shifting block  248  may comprise suitable logic, circuitry and/or code that may be enabled to generate an output signal that may be a phase shifted version of the phase shifting block  248  input signal. The I-branch filter  210   a  may comprise suitable logic, circuitry and/or code that may be enabled to attenuate certain frequency components of an input signal and/or a phase of an input signal. The Q-branch filter  212   a  may be substantially similar to the I-branch filter  212   a.    
     In most instances, OFDM wireless communication systems may employ complex valued signals that may be processed as two separate, real-valued signal branches/paths, I branch and the Q branch, as illustrated in  FIG. 2A . One signal branch may process the in-phase (I) signal component, and the other signal branch may process the quadrature (Q) signal component. Signal processing elements in the analog front-end of an OFDM receiver may comprise one or more amplifiers  242  and  244 , mixers/multipliers  202   a  and  204   a , and filters  210   a  and  212   a . The processing elements in the analog front-end of the receiver may appear in pairs for processing of signals along the I and Q branches, respectively, as illustrated in  FIG. 2A . The pairs of signal processing elements may have similar, matching characteristics, for example gain, bandwidth, phase, and/or magnitude response, in accordance with various embodiments of the invention, and depending on specific components used. However, component imperfections and manufacturing tolerances in discrete components and/or integrated circuits may lead to mismatches in, for example, I-branch filter  210   a  and Q-branch filter  212   a  that may have signal transfer characteristics that may not perfectly match. Such transfer function mismatches may lead to differences in signal processing between the I and Q branches of the receiver, which in turn may degrade receiver performance, for example bit error rate performance. 
     In some instances, mismatches that may exist between I and Q branch elements, for example I-branch filter  210   a  and Q-branch filter  212   a , or amplifiers  242  and  244 , may be compensated for by using equalizing techniques, which may significantly reduce mismatches. However, for effective equalization, the sources of mismatch may be characterized, to appropriately adjust the equalizer. In accordance with various embodiments of the invention, the I and Q branch filter  210   a  and  212   a  impulse transfer functions may be characterized. For example,  FIG. 2A  may illustrate an exemplary, simplified block diagram of an analogue RF front-end for an OFDM receiver. In accordance with an embodiment of the invention, it may be desirable to determine the frequency response of the I branch filter  210   a , and the Q branch filter  212   a . The I and/or Q branch characterization may be used to assist equalizing the I and/or Q branch mismatches. 
       FIG. 2B  is a diagram of an exemplary filter impulse response measurement setup for OFDM systems, in accordance with an embodiment of the invention. Referring to  FIG. 2B , there is shown amplifiers  242   b  and  244   b , multipliers  202  and  204 , a digital-to-analog converter  206 , selectors or multiplexers  208   a , and  208   b , a local oscillator  246   b , a phase shifting block  248   b , an I-branch filter  210  and a Q-branch filter  212 , analog-to-digital converters  214  and  218 , and FFT blocks  216  and  220 . There is also shown a calibration input signal, a normal/calibration mode selection signal, a real FFT in-phase output Re(I), an imaginary FFT in-phase output Im(I), a real FFT quadrature output Re(Q), and an imaginary FFT quadrature output Im(Q). 
     The amplifiers  242   b  and  244   b , the multipliers  202  and  204 , the phase shifting block  248   b , the local oscillator  246   b , the I-branch filter  210 , and the Q-branch filter  212  may be substantially similar to the amplifiers  242  and  244 , the multipliers or mixers  202   a  and  204   a , the phase shifting block  248 , the local oscillator  246 , the I-branch filter  210   a , and the Q-branch filter  212   a , respectively, as described with respect to  FIG. 2A . 
     The digital-to-analog (D/A) converter  206  may comprise suitable logic, circuitry and/or code that may be enabled to convert a digital input signal into an analog output signal. The analog-to-digital (A/D) converter  214  and  218  may comprise suitable logic, circuitry and/or code that may be enabled to convert an analog input signal into a digital representation output signal. The selector or multiplexer  208  may comprise suitable logic, circuitry and/or code that may be enabled to switch a plurality of input signals through to one or more outputs. The FFT block  216  and  220  may comprise suitable logic, circuitry and/or code that may be enabled to compute an FFT of an input signal. 
     In accordance with various embodiments of the invention,  FIG. 2B  may illustrate an analog front-end section of an OFDM receiver, where the outputs from the quadrature demodulators via multipliers  202  and  204  may be bypassed, and a digital-to-analog (D/A) converter  206  output may be switched to the I branch and/or Q branch analog filter inputs by means of the selectors or multiplexers  208   a , and  208   b . Because the analog front-end components, amplifiers  242   b  and  244   b , multipliers or mixers  202  and  204 , local oscillator  246   b , and phase shifting block  248   b  may not be active during the transfer function characterization phase, they may be depicted in dashed lines. For characterization of the transfer functions of the I-branch filter  210  and/or the Q-branch filter  212 , the calibration signal may be switched to the outputs of the multiplexer  208  via the selectors  208   a  and  208   b , which may be controlled by the normal/calibration mode selection signal, for example. The outputs of the selectors or multiplexers  208   a  and  208   b  may be communicatively coupled to the input of the I-branch filter  210  and the Q-branch filter  212 , respectively. 
     For example, a series of N samples may be sent to the D/A converter  206  input, of which the first sample may be a full-scale (with respect to D 2 A  206  input/output dynamic range) sample, the remaining N- 1  samples may be zero, obtained either from the D/A  206  converter output, or by grounding the filter input. Such an input sequence to the D/A converter  206  may generate an output signal that may approximate a unit impulse function. An impulse function communicatively coupled to an I branch filter  210  and/or a Q branch filter  212  may be used to measure a transfer function of a filter. 
     For example, K samples may be taken at the I branch filter  210  output, for example, the first sample of which may coincide with the time when the first input sample was sent to the filter  210  input. A Fast Fourier Transform (FFT) of a set of output samples from the filter  210  may be computed via the Analog-to-Digital (A/D) converter  214 , and the FFT block  216 , generating the required transfer function of the filter  210  in the frequency-domain. Similarly, a transfer function may be determined for the Q-branch filter  212  by sending similar signal samples via the multiplexer to the filter  212 , and by computing the filter transfer function via the A/D converter  218  and the FFT block  220 . The Fast Fourier Transform in FFT block  216  and  220  may be performed with an arbitrary number of coefficients. 
     In accordance with various embodiments of the invention, the functional blocks that may be required to perform the transfer function characterization of the I branch filter  210  and/or the Q branch filter  212  may comprise elements of an OFDM transceiver, in particular the FFT blocks  216  and  220 . Hence, the number or additional components to determine the transfer filter characteristics may be limited. 
       FIG. 3  is a diagram of an exemplary double-sided frequency response with and without I/Q mismatch, in accordance with various embodiments of the invention. There is shown an input signal  302 , a non-ideal frequency response  304 , and a near-ideal frequency response  306 . The horizontal axis may illustrate OFDM sub-carriers (frequency) relative to DC level, and the vertical axis may illustrate magnitude. The plot in  FIG. 3  may illustrate an exemplary magnitude transfer function that may be measured, in accordance with various embodiments of the invention. 
     It may be observed that the peak magnitudes of the near-ideal response  306  may approximately coincide with the input signal  302  magnitude, whereas the non-ideal response  304  magnitude peaks may be lower and/or higher than the input signal  302  magnitude. 
       FIG. 4  is a diagram of an exemplary double-sided phase response of an I branch filter and a Q branch filter mismatch, in accordance with an embodiment of the invention. There is shown an I-branch filter phase response  402  and a Q-branch filter phase response  404 . As illustrated in  FIG. 4 , the phase response may be similar but not precisely the same. Even small mismatches may in some circumstances deteriorate system performance. Thus, in accordance with various embodiments of the invention, a transfer function of the I branch filter and/or the Q branch filter may be characterized, for example, as described with respect to  FIG. 2B . 
       FIG. 5  is a flow chart illustrating a transfer function characterization, in accordance with various embodiments of the invention. After initialization in step  502 , selectors or multiplexers, for example selectors or multiplexers  208   a  and  208   b , may switch a calibration signal branch to its outputs in step  504 . The outputs of the selectors or multiplexers  208   a  and  208   b  may be communicatively coupled to the input of an I branch filter  210  and/or to a Q branch filter  212 , respectively. In step  506 , for example, a digital impulse response calibration signal, comprising one full scale sample and N- 1  zero samples, may be communicatively coupled to a D/A converter  206 . The D/A converter  206  may be operable to generate an analog output signal, which may approximate a unit impulse function. The output signal of the D/A converter  206  may be communicated to the I branch filter  210  and/or the Q branch filter  212  via the multiplexers  208   a  and  208   b . In step  508 , the output of the I branch filter and/or Q branch filter in response to the calibration signal may be sampled, for example in the A/D converters  214  and  218 . The sampled impulse response may, in step  510 , be converted to the frequency domain by generating an FFT, for example in FFT blocks  216  and  220 , of the samples generated in the A/D  214  and  218 . 
     In accordance with an embodiment of the invention, a method and system for characterization of filter transfer functions in OFDM may comprise receiving at a filter, for example I branch filter  210 , a calibration signal which is generated from conversion of a digital input signal comprising N samples to an analog signal in D/A converter  206 . The filter may be an in-phase branch filter  210  or a quadrature branch filter  212 . The calibration signal may approximate an impulse signal. The digital input signal may comprise one (1) full scale sample and N- 1  zero samples and N is an integer. In response to receiving the calibration signal, the filter  210 , for example, may generate an output analog signal, wherein the output analog signal may be converted to an output digital signal in the A/D converter  214 , and a transfer function of the filter may be determined via a Fast Fourier transformation in the FFT block  216 , for example, of the output digital signal. The Fast Fourier transformation may be performed with an arbitrary number of coefficients 
     The OFDM system may be compliant with a wireless standard, wherein the wireless standard may comprise UMTS EUTRA (LTE), WiMAX(IEEE 802.16), and/or WLAN (IEEE 802.11). A transfer function of an in-phase branch filter  210  and/or a quadrature branch filter  212  may be measured. The transfer function may comprise a magnitude and/or phase response, wherein the magnitude and/or phase response mismatch may be a function of frequency, as illustrated in  FIG. 3  and  FIG. 4 . A number of the samples N may be chosen arbitrarily. 
     Another embodiment of the invention may provide a machine-readable and/or computer-readable storage and/or medium, having stored thereon, a machine code and/or a computer program having at least one code section executable by a machine and/or a computer, thereby causing the machine and/or computer to perform the steps as described herein for a method and system for characterization of filter transfer functions in OFDM. 
     Accordingly, the present invention may be realized in hardware, software, or a combination of hardware and software. The present invention may be realized in a centralized fashion in at least one computer system, or in a distributed fashion where different elements are spread across several interconnected computer systems. Any kind of computer system or other apparatus adapted for carrying out the methods described herein is suited. A typical combination of hardware and software may be a general-purpose computer system with a computer program that, when being loaded and executed, controls the computer system such that it carries out the methods described herein. 
     The present invention may also be embedded in a computer program product, which comprises all the features enabling the implementation of the methods described herein, and which when loaded in a computer system is able to carry out these methods. Computer program in the present context means any expression, in any language, code or notation, of a set of instructions intended to cause a system having an information processing capability to perform a particular function either directly or after either or both of the following: a) conversion to another language, code or notation; b) reproduction in a different material form. 
     While the present invention has been described with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the present invention without departing from its scope. Therefore, it is intended that the present invention not be limited to the particular embodiment disclosed, but that the present invention will include all embodiments falling within the scope of the appended claims.