PATENT DOCUMENT

Publication Number: US-8279978-B2
Application Number: US-201113210251-A
Country: US
Kind Code: B2

Title: Pilot signal in an FDMA communication system

Abstract:
A method for receiving a pilot symbol in a receiver is disclosed. In one embodiment, the method includes removing a cyclic prefix from a received sequence to produce a modified sequence, transforming the modified sequence to a first frequency domain sequence according to a first transform, demapping a plurality of distributed subcarriers in the transformed modified sequence to extract a plurality of received symbols, deriving an intermediate channel estimate for each of the plurality of received symbols, and interpolating a final channel estimate based on the plurality of derived intermediate channel estimates. In one exemplary embodiment, the received symbols have one or more predefined characteristics such as a constant amplitude, and zero autocorrelation (CAZAC sequence).

Claims:
1. A method for receiving a pilot symbol in a single carrier frequency division multiple access (SC-FDMA) receiver comprising:
 removing a cyclic prefix from a received sequence to produce a modified sequence; 
 transforming the modified sequence to a first frequency domain sequence according to a first transform; 
 demapping a plurality of distributed subcarriers in the transformed modified sequence to extract a plurality of received pilot symbols; 
 deriving an intermediate channel estimate for each of the plurality of received pilot symbols, based on one or more characteristics of each of the plurality of pilot symbols; and 
 interpolating a final channel estimate based on the plurality of derived intermediate channel estimates. 
 
     
     
       2. The method of  claim 1 , wherein the first transform comprises a discrete Fourier transform. 
     
     
       3. The method of  claim 1 , wherein the deriving the intermediate channel estimate is based on at least one of the one or more predefined characteristics of the received plurality of pilot symbols. 
     
     
       4. The method of  claim 3 , wherein the one or more predefined characteristics comprises a constant magnitude property. 
     
     
       5. The method of  claim 3 , wherein the one or more predefined characteristics comprises a zero circular autocorrelation property. 
     
     
       6. The method of  claim 3 , wherein the one or more predefined characteristics comprises a flat frequency domain response property. 
     
     
       7. The method of  claim 3 , wherein the one or more predefined characteristics comprises a low and constant magnitude result when circularly cross-correlated with another sequence. 
     
     
       8. The method of  claim 1 , wherein the plurality of received pilot symbols comprises a Zadoff-Chu sequence. 
     
     
       9. The method of  claim 1 , further comprising demapping a second plurality of subcarriers to extract a datastream. 
     
     
       10. The method of  claim 1 , wherein the plurality of distributed subcarriers in the transformed modified sequence are distributed substantially evenly throughout the transformed modified sequence. 
     
     
       11. The method of  claim 1 , wherein the distribution of the distributed subcarriers in the transformed modified sequence changes between successive receptions of the received sequence. 
     
     
       12. A single carrier frequency division multiple access (SC-FDMA) receiver, comprising:
 a wireless interface comprising:
 first apparatus configured to remove a cyclic prefix from a received sequence to produce a modified sequence; 
 second apparatus configured to transform a received sequence to a first frequency domain sequence; 
 third apparatus configured to demap a plurality of distributed subcarriers in the first frequency domain sequence to extract a plurality of received pilot symbols; 
 fourth apparatus configured to derive an intermediate channel estimate for each of the plurality of received pilot symbols, based on one or more characteristics of each of the plurality of pilot symbols; and 
 fifth apparatus configured to interpolate a final channel estimate based on the plurality of derived intermediate channel estimates. 
 
 
     
     
       13. The SC-FDMA receiver of  claim 12 , wherein the second apparatus is configured to execute a discrete inverse Fourier Transform. 
     
     
       14. The SC-FDMA receiver of  claim 12 , wherein the third apparatus is additionally configured to demap a second plurality of distributed subcarriers in the first frequency domain sequence to extract a plurality of received data symbols. 
     
     
       15. The SC-FDMA receiver of  claim 14 , wherein the cyclic prefix is present within the received sequence in order to ensure orthogonality. 
     
     
       16. The SC-FDMA receiver of  claim 12 , additionally comprising apparatus configured to perform serial to parallel conversion in communication with the first and second apparatus. 
     
     
       17. The SC-FDMA receiver of  claim 11 , wherein the plurality of distributed subcarriers is distributed substantially evenly throughout the first frequency domain sequence. 
     
     
       18. The SC-FDMA receiver of  claim 17 , wherein the distribution of the plurality of distributed subcarriers changes between successive receptions of the received sequence. 
     
     
       19. A method for extracting one or more pilot symbols within an Orthogonal Frequency Division Multiplexing (OFDM) system, comprising:
 receiving a sequence comprising a plurality of pilot symbols and data symbols interspersed in the frequency domain; 
 extracting the plurality of pilot symbols and data symbols from the received sequence; 
 estimating an intermediate channel response for each of the extracted pilot symbols; and 
 interpolating an overall channel response based at least in part on the estimated intermediate channel response. 
 
     
     
       20. The method of  claim 19 , wherein the pilot symbols comprise a Zadoff-Chu sequence. 
     
     
       21. The method of  claim 19 , where the overall channel response is applied to the extracted data symbols. 
     
     
       22. Wireless receiver apparatus configured to extract one or more pilot symbols from a wireless Orthogonal Frequency Division Multiplexing (OFDM) signal, comprising:
 first apparatus configured to receive a sequence comprising a plurality of pilot symbols and data symbols interspersed in the frequency domain; 
 second apparatus configured to extract the plurality of pilot symbols and data symbols from the received sequence; 
 third apparatus configured to estimate an intermediate channel response for each of the extracted pilot symbols; and 
 fourth apparatus configured to interpolate an overall channel response based at least in part on the estimated intermediate channel response. 
 
     
     
       23. The receiver apparatus of  claim 22 , wherein the pilot symbols comprise a Zadoff-Chu sequence. 
     
     
       24. The receiver apparatus of  claim 22 , where the overall channel response is applied to the extracted data symbols.

Description:
PRIORITY AND RELATED APPLICATIONS 
     This application is a divisional of co-owned U.S. patent application Ser. No. 12/806,944 of the same title filed Aug. 24, 2010 (now issued as U.S. Pat. No. 8,000,228), which is a divisional of and claims priority to U.S. patent application Ser. No. 11/334,606 of the same title filed Jan. 18, 2006 (now issued as U.S. Pat. No. 7,808,886), each of the foregoing incorporated herein by reference in its entirety. This application is also related to co-owned and co-pending U.S. patent application Ser. No. 13/210,240 of the same title filed contemporaneously herewith on Aug. 15, 2011. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to communication systems using a carrier comprising multiple sub-carriers, and more specifically to techniques and apparatus for generating and using a pilot signal in a multi-carrier communication system. 
     BACKGROUND OF THE INVENTION 
     Multicarrier modulation systems divide the transmitted bitstream into many different substreams and send these over many different subchannels. Typically the subchannels are orthogonal under ideal propagation conditions. The data rate on each of the subchannels is much less than the total data rate, and the corresponding subchannel bandwidth is much less than the total system bandwidth. The number of substreams is chosen to ensure that each subchannel has a bandwidth less than the coherence bandwidth of the channel, so the subchannels experience relatively flat fading. This makes the inter symbol interference (ISI) on each subchannel small. 
     In more complex systems, which are commonly called orthogonal frequency division multiplexing (OFDM) systems (or multi-carrier or discrete multi-tone modulation systems), data is distributed over a large number of carriers that are spaced apart at precise frequencies. The frequency spacing provides the “orthogonality,” which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other through inter symbol interference (ISI), it becomes very hard for the receiver to extract the originally transmitted data. 
     In an OFDM system, data must be coherently demodulated. Therefore, it is necessary to know the amplitude and phase of the channel in the receiver. A pilot signal is transmitted with the data so that the receiver can determine the amplitude and phase of the channel. The pilot signal also allows the receiver to measure the transfer characteristics of the channel between the transmitter and receiver through a process known as “channel estimation.” 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying figures, wherein like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to further illustrate various embodiments and to explain various principles and advantages, all in accordance with the present invention. 
         FIG. 1  depicts, in a simplified and representative form, a high-level block diagram of portions of a single carrier frequency division multiple access (SC-FDMA) transmitter for use in a data communications system in accordance with one or more embodiments; 
         FIG. 2  shows, in a representative form, a high-level block diagram of portions of an SC-FDMA receiver used to receive data transmitted by the transmitter of  FIG. 1  in accordance with one or more embodiments; 
         FIG. 3  depicts a more detailed high-level representative block diagram of portions of the SC-SDMA transmitter of  FIG. 1  in accordance with one or more embodiments; 
         FIG. 4  depicts a more detailed high-level representative block diagram of portions of the SC-SDMA receiver of  FIG. 2  in accordance with one or more embodiments; 
         FIG. 5  is a high-level flowchart of processes executed by the SC-SDMA transmitter of  FIGS. 1 and 3  in accordance with one or more embodiments; 
         FIGS. 6 and 7  show, in representative form, distributions of pilot signal information to sets of subcarriers in accordance with one or more embodiments; 
         FIG. 8  is a high-level flowchart of processes executed by the SC-SDMA receiver of  FIGS. 2 and 4  in accordance with one or more embodiments; and 
         FIG. 9  shows an alternative high-level representative block diagram of portions of the SC-SDMA receiver of  FIG. 2  in accordance with one or more embodiments. 
     
    
    
     DETAILED DESCRIPTION 
     In overview, the present disclosure concerns a pilot signal to be used for estimating the transfer characteristics of a communication channel in a communication system. More particularly various inventive concepts and principles embodied in methods and apparatus may be used for generating a pilot signal for transmitting with a data signal in a communications system, and for demodulating the pilot signal in a receiver in the communication system. 
     While the pilot signal (generator/demodulator) of particular interest may vary widely, one embodiment may advantageously be used in a wireless cellular communications system having a transmitter and receiver using a single carrier frequency division multiple access (SC-FDMA) modulation scheme. However, the inventive concepts and principles taught herein may be advantageously applied to other broadband communications systems having multiplexed communication links transmitted in other media. 
     The instant disclosure is provided to further explain in an enabling fashion the best modes, at the time of the application, of making and using various embodiments in accordance with the present invention. The disclosure is further offered to enhance an understanding and appreciation for the inventive principles and advantages thereof, rather than to limit in any manner the invention. The invention is defined solely by the appended claims, including any amendments made during the pendency of this application, and all equivalents of those claims as issued. 
     It is further understood that the use of relational terms, if any, such as first and second, top and bottom, and the like, are used solely to distinguish one entity or action from another without necessarily requiring or implying any actual such relationship or order between such entities or actions. 
     Much of the inventive functionality and many of the inventive principles are best implemented with, or in, integrated circuits (ICs), including possibly application specific ICs, or ICs with integrated processing controlled by embedded software or firmware. It is expected that one of ordinary skill—notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations—when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. Therefore, in the interest of brevity and minimization of any risk of obscuring the principles and concepts according to the present invention, further discussion of such software and ICs, if any, will be limited to the essentials with respect to the principles and concepts of the various embodiments. 
     Referring to  FIG. 1 , a high-level diagram of portions of a single carrier frequency division multiple access (SC-FDMA) transmitter for use in a data communications system in accordance with one or more embodiments will be briefly discussed and described. In  FIG. 1 , SC-FDMA transmitter  100  includes data source  102 , which generates a data sequence  104  that is traffic data or user data generated by, for example, an application running in a subscriber unit of a cellular communications system, or perhaps by the transmission of streaming media, or by transferring a file, or other similar processes that transfer data. 
     Because the transmitted signal will be coherently demodulated in a receiver, it is necessary to know the amplitude and phase of the channel in the receiver. A pilot signal is transmitted with the data so that the receiver can determine the amplitude and phase of the channel. The pilot signal also allows the receiver to measure the transfer characteristics of the channel between the transmitter and receiver through a process known as “channel estimation.” 
     As shown in  FIG. 1 , pilot sequence generator  106  generates a pilot sequence  108  that can be alternately and periodically transmitted with traffic data  104  from data source  102 . The pilot sequence  108  comprises a known data sequence having known characteristics. The characteristics of pilot sequence  108  are: (1) a constant magnitude; (2) zero circular autocorrelation; (3) a flat frequency domain response; and (4) a circular cross-correlation between two sequences that is low and has a constant magnitude, provided that the sequence length is a prime number. 
     One sequence that has these properties is known as a Zadoff-Chu, Frank-Zadoff, and Milewski sequence, which is also known as a constant-amplitude zero-autocorrelation (CAZAC) sequence. The CAZAC sequence is defined as follows: Let L be any positive integer, and let k be any number which is relatively prime with L. Then the n-th entry of the k-th Zadoff-Chu CAZAC sequence is given as follows: 
     
       
         
           
             
               
                 
                   
                     
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     where n ranges from 0 to L−1. 
     The CAZAC sequence is a special subset of polyphase Chirp sequences, such as the Generalized Chirp Like (GCL) sequences. Note that the “constant-amplitude zero-autocorrelation” property of the CAZAC sequence is preserved as the sequence is transformed to and from the time and frequency domains. Thus, the CAZAC sequence has benefits for the radio frequency power amplifier because transmission using SC-FDMA techniques provides a signal with a low peak-to-average power ratio in the time domain. Additionally, the CAZAC sequence aids channel estimation because the signal has a constant amplitude in the frequency domain. 
     Switch controller  110  controls switch  112 , which alternates between selecting data sequence  104  and pilot sequence  108 . In one embodiment, switch controller  110  allows 6 symbols of data sequence  104  for every 2 pilot sequences  108 . Other ratios of data and pilot sequences may be used. 
     After being selected by a switch  112 , pilot sequence  108  is coupled to serial-to-parallel converter  114 . Serial-to-parallel converter  114  receives a serial stream of data and converts it to a parallel data output. If pilot sequence  108  is received, serial-to-parallel converter  114  converts it to an M-point parallel CAZAC sequence  116 , where M is the number of complex valued samples in the sequence. 
     The M-point parallel output of serial-to-parallel converter  114  is coupled to the input of single-carrier frequency division multiple access (SC-FDMA) modulator  118 . SC-FDMA modulator  118  is a multi-carrier modulation block that uses digital mathematical techniques to generate the orthogonal carriers required for OFDM transmission. The output of SC-FDMA modulator  118  is a digital baseband representation of an OFDM modulated pilot symbol  120 . A more detailed view of the digital processing system of data transmitter  100  is shown in  FIG. 3 , discussed below. 
     The OFDM modulated pilot symbol  120  is coupled to transmitter block  122 . Within transmitter block  122 , the digital signal is converted to an analog signal by a digital-to-analog (D/A) converter. The analog signal is then coupled to an upconverter, and a power amplifier, to produce amplified radio frequency signal  124 . Radio frequency signal  124  is coupled to antenna  126  for transmitting transmitted radio signal  128 . Transmitted radio signal  128  is affected by the environment according to the characteristics of the channel, and thus signal  128  becomes a “received” signal at the receiver. 
     Referring to  FIG. 2 , there is depicted a representative block diagram of a receiver that illustrates functional blocks for receiving, demodulating, and using a pilot signal in accordance with one or more embodiments. As shown, receiver  200  includes antenna  202  for receiving received signal  204 . Antenna  202  is coupled to downconverter and analog-to-digital (A/D) converter  206 , which removes the radio frequency carrier from received signal  204  and converts the analog signal to a serial stream of digital samples  208 . 
     Digital samples  208  from downconverter and A/D converter  206  are coupled to synchronization controller  210  and OFDM multi-carrier demodulator  212 . Synchronization controller  210  monitors digital samples  208  to detect a pilot signal using pilot signal detector  214 . The purpose of synchronization controller  210  is to recover a reference signal (e.g., a pilot signal), which is a known signal that provides symbol timing or synchronization information, and which also allows channel estimation and the calculation of channel equalization coefficients. Such synchronization and channel estimation information  216  is coupled to OFDM multi-carrier demodulator  212  so that demodulator  212  may be precisely synchronized in order to demodulate the OFDM signal represented by digital samples  208 . OFDM multi-carrier demodulator  212  provides, at its output, traffic data or user data  218 , which is data carried in received signal  204 . Functions similar to those within OFDM multi-carrier demodulator  212  may be used within synchronization controller  210  to receive and demodulate pilot symbols. 
     With reference now to  FIG. 3 , there is depicted a more detailed representative diagram of portions  300  of transmitter  100  (see  FIG. 1 ) for generating a pilot symbol in accordance with one or more embodiments. As illustrated in  FIG. 3 , a serial pilot sequence  304  is generated in short CAZAC sequence generator  302 . In one embodiment, the sequence is M complex samples in length, wherein M is equal to, for example, 37. 
     The CAZAC sequence generated by CAZAC sequence generator  302  is referred to as “short” sequence because the length of the sequence is shorter than the number of available subcarriers. For example, if there are  301  available subcarriers, a long CAZAC sequence would occupy all of them, and a short CAZAC sequence may only occupy, for example, 37 of them. As noted above, the “constant-amplitude zero-autocorrelation” property of the CAZAC sequence is preserved in both the time and frequency domains. 
     The output of short CAZAC sequence generator  302  is coupled to serial-to-parallel converter  306 , which converts serial pilot sequence  304  into an M-point parallel CAZAC sequence  308 . M-point parallel CAZAC sequence  308  is coupled to an input of M-point discrete Fourier transformer (DFT)  310 . M-point DFT  310  analyzes the frequency components of the M-points of serial pilot sequence  304 , and converts serial pilot sequence  304  into the frequency domain to output an M-point parallel transform sequence  312 . 
     Note that in some embodiments, M-point parallel transform sequence  312  may be recalled from transform memory  314 . Thus, rather than generating sequences and calculating discrete Fourier transforms (i.e. real-time synthesis), the system and process may recall a pre-calculated M-point parallel transform sequence  312  from a data storage device using a table look up technique. 
     M-point parallel transform sequence  312  is coupled to the input of distributed subcarrier mapper  316 , which is used to distribute the M-point parallel transform sequence  312  to a set of M subcarriers among N subcarriers to form an N-point frequency-domain sequence  318 , where N is greater than M and wherein the M subcarriers are evenly spaced apart among the N subcarriers. N is the number of subcarriers used in the SC-FDMA transmitted signal. In one embodiment, N is equal to 512, and N maybe larger or smaller in alternative embodiments. Outputs  318  of distributed subcarrier mapper  316  that are not mapped to one of the M-point parallel transform sequence  312  values are set to a zero value. 
     Referring again to the output of distrubuted subcarrier mapper  316 , N-point frequency-domain sequence.  318  is coupled. to N-point. inverse fast Fourier transformer (IFFT)  320 . N-point IFFT  320  converts N-point frequency-domain sequence  318  into N-point time-domain sequence  322 . The inverse fast Fourier transform is an efficient mathematical algorithm for reversing a fast Fourier transform. N-point time-domain sequence  322  is coupled to an input of parallel-to-serial converter  324 , which converts the parallel data into serial sequence  326 . 
     Serial sequence  326  is coupled to an input of cyclic prefix adder  328 . Cyclic prefix adder  328  copies a predetermined number of complex samples from the end of serial sequence  326  and places those samples at the beginning of serial sequence  326 . In one embodiment, the number of samples copied is  32 . The purpose of adding a cyclic prefix is to ensure orthogonality, which prevents one subcarrier from interfering with another (which is called intercarrier interference, or ICI). The output of cyclic prefix headers  328  is pilot symbol  330 . 
     Note that the functional blocks within dashed box  332  may be referred to as an SC-FDMA modulator, such as SC-FDMA modulator  118  in FIG,  1 . Also note that in other embodiments, the order of functional blocks parallel-to-serial converter  324  and cyclic prefix adder  328  may be reversed so that the cyclic prefix is added before converting the data to a serial stream. 
     Turning now to  FIG. 4 , there is depicted a more detailed high-level representative block diagram of portions  400  of the SC-SDMA receiver  200  of  FIG. 2  in accordance with one or more embodiments. As illustrated, received waveform  402 , which is a baseband digital stream similar to digital samples  208  in  FIG. 2 , is coupled to the input of cyclic prefix remover  404 . Cyclic prefix remover  404  removes the cyclic prefix from the beginning of the digital samples to produce modified pilot waveform  406 . 
     Modified pilot waveform  406  is coupled to an input of serial-two-parallel converter  408 . The output of serial-to-parallel converter  408  is N-point parallel modified pilot symbol  410 , which is coupled to an input of N-point fast Fourier transformer (FFT)  412 . 
     N-point FFT  412  produces received transformed pilot symbol  414  at its output, as it converts the pilot signal from the time domain to the frequency domain. 
     Received transformed pilot symbol  414  is coupled to an input of distributed subcarrier de-mapper  416 . Distributed subcarrier de-mapper  416  de-maps the M distributed subcarriers in the received transformed pilot symbol to produce an M-point received signal  418 . Distributed subcarrier demapper  416  uses carrier mapping information  420  to perform the demapping function. 
     Carrier mapping information  420  describes the selected set of M subcarriers (Le., the locations of the subcarriers containing pilot information within the N received subcarriers). Carrier mapping information  420  is known in the receiver before receiving received waveform  402 . Such carrier mapping information can be agreed upon according to a standard describing the data communication interface, or it can be transmitted in a control message the to the receiver before it is needed. 
     M-point received signal  418  is coupled to the input of M-point parallel multiplier  422 . M-point parallel multiplier  422  multiplies received signal  418  by M-point channel estimate multiplier  424  to produce an M-point intermediate channel estimate  426 . M-point channel estimate multiplier or sequence  424  is, in one or more embodiments, a Fourier transformed time-reversed conjugate sequence derived from serial pilot sequence  302  (see  FIG. 3 ). The net effect of the transformation of, and multiplication by, the pilot sequence is that the frequency domain equivalent of a time domain circular correlation with the pilot sequence is performed 
     Following M-point parallel multiplier  422 , M-point intermediate channel estimate  426  is coupled to two dimensional (2D) interpolator  428 . The output of 2D interpolator  428  is final channel estimate  430 . The final channel estimate gives the channel estimate at the location of each assigned data carrier. 
     Referring now to  FIG. 5 , there is depicted a high-level flowchart  500  of exemplary processes executed by portions of a transmitter, such as transmitter  100 , which is shown in the system of  FIGS. 1 and 3 , or other similar apparatus, in accordance with one or more embodiments. As illustrated, the process begins at block  502 , and thereafter passes to block  504  wherein the process generates a short M-point constant-amplitude zero autocorrelation (CAZAC) serial pilot sequence, wherein M is an integer representing the number of complex samples in the sequence. M is also referred to as a short sequence because M is significantly less than a number of carriers used when mapping transmitted data symbols on subcarriers of the SC-FDMA modulator. This is important because it allows the pilot symbols of different users to be orthogonal in the frequency domain. Furthermore, it allows the same CAZAC sequence to be reused by multiple users at different frequency offsets. 
     As shown in  FIG. 3 , this process is implemented by short CAZAC sequence generator  304 , which generates pilot sequence  302 . This process may be implemented by recalling complex samples of sequence  302  from data memory  314  (see  FIG. 3 ). In other embodiments, short CAZAC sequence  302  may be generated using specially designed logic circuits, or by executing specially programmed software code (e.g., microcode in a microchip). 
     As discussed above, the CAZAC sequence has a constant magnitude, zero circular autocorrelation, flat frequency-domain response, and low cross-correlation between two sequences. In other embodiments, the pilot sequence may be implemented with a GCL sequence having similar characteristics. 
     The CAZAC pilot symbol has the properties of a CDMA signal in that it is able to average interference because of its superior correlation properties. In addition, the signal has the benefits of frequency domain orthogonality. This means that the same CAZAC sequence can be reused on different frequency sets of M subcarriers by different users. Thus, the same CAZAC sequence can be used to estimate different user&#39;s channels. 
     Next, the process converts the M-point CAZAC pilot sequence to an M-point parallel pilot sequence, as illustrated at block  506 . This process may be implemented by a serial-to-parallel converter, such as serial-to-parallel converter  114  in  FIG. 1 . 
     After converting the pilot sequence to parallel data, the process performs an M-point discrete Fourier transform to produce an M-point parallel transform sequence, as illustrated at block  508 . This process may be implemented in the M-point discrete Fourier transform block  304  in  FIG. 3 . The M-point parallel transform sequence is a frequency-domain representation of the time-domain modulation of the pilot sequence. 
     After transforming to the frequency-domain, the process distributes the M-point parallel transform sequence to a selected set of M subcarriers among N subcarriers, as depicted at block  510 . In this distribution process, N is the number of subcarriers transmitted in the OFDM signal, and N is greater than M. The M selected subcarriers are evenly spaced apart, as represented in  FIGS. 6 and 7 . In one embodiment, this distribution process is implemented in distributed subcarrier mapper  308  shown in  FIG. 3 . Note that the N subcarriers that have not been selected in the set of M subcarriers will have their values set to zero. 
     As shown in  FIGS. 6 and 7 , different sets of M subcarriers may be selected among the N subcarriers while still maintaining even carrier spacing in the group of N subcarriers. In  FIG. 6  for example, a first subcarrier  602 , labeled SC 0 , is selected, and every fourth subcarrier  604  is selected among a group of sixteen subcarriers  600 , SC 0 -SC 15 . These selected subcarriers  602 ,  604  may represent a first group or set of selected subcarriers having a carrier spacing  606 . Similarly,  FIG. 7  shows the selection of a second set of subcarriers  702 ,  704 , beginning with the second subcarrier  702 , SC 1 , and every fourth subcarrier  704  thereafter. In  FIG. 7 , the carrier spacing  606  remains at four carriers apart. In practice, N will probably be much larger than sixteen. In one embodiment, N is equal to 512, and M is equal to 37. Different sets of subcarriers may be selected in alternate frames of pilots symbols. Information specifying which set of selected subcarriers is used should be known in advance in the receiver for proper demodulation. Such information may be known by communicating the selected set in advance through a control message, or by agreeing upon a known sequence of sets beginning at a selected time. 
     Next, after distribution, the process performs an N-point inverse fast Fourier transform (IFFT) to convert the N-point frequency-domain sequence to an N-point time-domain sequence, as illustrated at block  512 . This process is implemented using N-point IFFT block  312  in  FIG. 3 . The IFFT converts the mapped set of M-points in the parallel transform sequence, and the zero values for the non-selected subcarriers, to an N-point distrubuted mode time-domain sequence. 
     Next, the process converts the N-point time-domain sequence to a serial sequence, as illustrated at block  514 . Then, at block  516 , the process adds a cyclic prefix to the serial sequence. The purpose of adding the cyclic prefixes to reduce inter symbol interference. This process is implemented by copying a number of complex samples from the end of the serial sequence to the beginning of the serial sequence, as is known in the art of OFDM modulation. In one embodiment, the number of complex samples copied is equal to 32. 
     Finally, the process of generating a pilot symbol in an OFDM transmitter ends at block  518 . It should be apparent that while the process of generating a pilot signal ends at block  518 , the process depicted may be repeated as necessary to provide multiple pilot symbols as dictated by the requirements of the particular system in which the process is used. 
     Referring now to  FIG. 8 , there is depicted a high-level flowchart  800  of exemplary processes executed by portions of a receiver, such as receiver  400 , which is shown in the system of  FIGS. 2 and 4 , or other similar apparatus, in accordance with one or more embodiments. As illustrated, the process begins at block  802 , and thereafter passes to block  804  wherein the process receives the OFDM signal. This process of receiving the OFDM signal includes downconverting the received signal and converting the down converted analog waveform to digital samples. As shown in  FIG. 2 , receiving the OFDM signal is implemented in downconverter and A/D converter  206 , which produces received waveform  402  shown in  FIG. 4 . Note that the received signal  204  is equal to the transmitted signal  126  multiplied by the transfer function of the channel. 
     Next, the process removes the cyclic prefix from the received pilot waveform to produce a modified pilot waveform, as depicted at block  806 . Removing the cyclic prefix is implemented in the cyclic prefix remover block  404  shown in  FIG. 4 . After removing the cyclic prefix, the process converts the serial modified pilot waveform into an N-point parallel modified pilot symbol, as illustrated at block  808 . 
     Next, the process performs an N-point fast Fourier transform on the N-point parallel modified pilot symbol to produce a received transformed pilot symbol, as depicted at block  810 , This process converts a signal in the time-domain to a signal in the frequency-domain. 
     After the fast Fourier transform, the process de-maps M distrubuted subcarriers in the received transformed pilot symbol to produce an M-point received signal as illustrated at block  812 . This process may be implemented using distrubuted subcarrier de-mapper  416  in  FIG. 4 . 
     Next, the process multiplies the M-Point received pilot symbol by an M-point channel estimate multiplier to produce an M-point intermediate channel estimate, as depicted at block  814 . The M-point channel estimate multiplier is derived from the transmitted pilot symbol, and is a Fourier transformed time-reversed conjugate sequence of the transmitted pilot symbol. 
     After multiplying, the process performs a two dimensional interpolation to produce a final channel estimate, as illustrated at block  816 . In one embodiment, the two dimensional interpolation is performed using 2D interpolator  428  in  FIG. 4 . 
     Finally, the process of receiving and using a pilot symbol in an OFDM receiver ends at block  818 . It should be apparent that while the processing of the pilot signal ends at block  818 , the process depicted may be repeated as necessary to receive and process multiple pilot symbols as dictated by the requirements of the particular system in which the process is used. 
     With reference now to  FIG. 9 , there is depicted high-level representative block diagram of an alternate embodiment of portions  900  of the SC-SDMA receiver  200  of  FIG. 2  in accordance with one or more embodiments. The functional blocks comprising receiver  900  are similar to those of receiver  200 , except for the addition of the following functional blocks: M-point inverse discrete Fourier transform (IDFT)  902 , interference mitigation  904 , and M-point DFT  906 . The purpose of the IDFT is to convert the pilot signal into the time domain where additional processing and optimization can be performed to mitigate interference and multipath. This produces a refined time-domain channel estimate. The refined time-domain channel estimate must then be transformed back into the frequency domain for interpolation. 
     The above described functions and structures can be implemented in one or more integrated circuits. For-example, many or all of the functions can be implemented in the signal processing circuitry that is suggested by the block diagrams shown in  FIGS. 1-4  and  9 . 
     The processes, apparatus, and systems, discussed above, and the inventive principles thereof are intended to produce an improved and more efficient pilot symbol in an SC-FDMA transmitter and receiver system by combining CDMA and FDMA pilot signals. By using a generalized chirp like (GCL) sequence—such as the CAZAC sequence—for the pilot signal, the peak to average ratio of the transmitted signal can be lowered, and the characteristics of the channel may be estimated more accurately because the pilot signal has a constant amplitude in the frequency-domain, which is better suited for channel estimation. When a CAZAC sequence is inserted in an FDMA manner for the pilot channel, the receiver may use interference averaging techniques to receive the pilot signal without sacrificing the link benefits of FDMA pilots These significant improvements can be made with relatively low cost and minimal added complexity. 
     While the embodiments discussed above primarily relate to transmitting a radio frequency signal in a wireless communications system, this system for generating a pilot symbol, and processes therein, may be used in other data transmission applications, such as transmitting data via a wireline media, such as a wideband coaxial cable, twisted-pair telephone wire, or the like. 
     This disclosure is intended to explain how to fashion and use various embodiments in accordance with the invention, rather than to limit the true, intended, and fair scope and spirit thereof. The foregoing description is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications or variations are possible in light of the above teachings. The embodiment(s) were chosen and described to provide the best illustration of the principles of the invention and its practical application, and to enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims, as may be amended during the pendency of this application for patent, and all equivalents thereof, when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.

Metadata:
Filing Date: 20110815
Publication Date: 20121002
Grant Date: 20121002
Priority Date: 20060118
Inventors: MCCOY JAMES W.
Assignee: APPLE INC
CPC Classifications: [{"code": "H04J1/12", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/26134", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L27/2613", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/26134", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L5/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0224", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04J1/04", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04B7/208", "inventive": true, "first": true, "tree": "[]"}, {"code": "H04L5/0048", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2613", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2672", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/26134", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L25/0224", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2672", "inventive": true, "first": false, "tree": "[]"}, {"code": "H04L27/2613", "inventive": true, "first": false, "tree": "[]"}]
Family ID: 38263067