Patent Publication Number: US-8116419-B2

Title: Methods and apparatuses for estimating time delay and frequency offset in single frequency networks

Description:
BACKGROUND OF THE INVENTION 
     In a conventional wireless digital transmission system, there is often a need to detect or estimate time and frequency offsets in a received signal relative to a transmitted signal. In a single frequency network (SFN), for example, detected time and frequency offsets are used to synchronize time and frequency of transmitted and received signals. As is well-known, a SFN is a broadcast network in which several transmitters simultaneously transmit the same signal over the same frequency channel. One type of conventional SFN is known as a hybrid satellite and terrestrial SFN. An example hybrid SFN is defined in the Digital Video Broadcasting-Satellite services to Handhelds (DVB-SH) standard “Framing Structure, Channel Coding and Modulation for Satellite Services to Handheld devices (SH) below 3 GHz,”  DVB Document A 111  Rev.  1, July 2007. 
     A DVB-SH SFN is a hybrid satellite and terrestrial SFN in which signals are simultaneously transmitted over the same frequency channel via both satellite and terrestrial communications links. 
     In a conventional DVB-SH SFN, a signal from a satellite has a varying time delay and frequency offset with respect to a terrestrially repeated signal due to the relative motion of the satellite in an inclined orbit. Because of this varying time delay and frequency offset, time and frequency synchronization is necessary to ensure proper reception of signals by receivers in the DVB-SH SFN. 
     SUMMARY OF INVENTION 
     Example embodiments provide methods and apparatuses for estimating time delay and frequency offset between transmitted and received signals in a wireless network. The estimated time delay and frequency offset are used to compensate for time delay and frequency offset between signals received concurrently or simultaneously over satellite and terrestrial connections. 
     In one embodiment, time delay and frequency offset are estimated using a particular correlation algorithm, and the actual time delay and frequency offset are compensated based on the estimated time delay and frequency offset. The same correlation is used to provide estimates for both time delay and frequency offset. The algorithm is independent of waveform and applies to Orthogonal Frequency Division Multiplexed (OFDM), time-division multiplexed (TDM) waveforms, as well as other waveforms. 
     Furthermore, the correlation contains adjustable parameters that may be used to make trade-off between accuracy and complexity, and improve the reliability of detection according to channel conditions. 
     In one embodiment of the method, an uplink signal carrying at least one block of transmitted samples is transmitted, and a distorted copy of the uplink signal is received as a downlink signal. A plurality of blocks of received samples are generated based on the received downlink signal, and a time delay and frequency offset between the uplink and downlink signals are estimated based on a correlation between the block of transmitted samples and at least one of the plurality of blocks of received samples. The actual time delay and frequency offset between subsequent transmitted and received signals are compensated based on the estimated time delay and frequency offset. 
     In one embodiment, the apparatus for time delay and frequency offset compensation in a hybrid single frequency network includes a transmitter, receiver and detector. The transmitter transmits an uplink signal carrying at least one block of transmitted samples. The receiver receives a downlink signal, which is a distorted copy of the transmitted signal and carries a plurality of blocks of received samples. The detector estimates a time delay and frequency offset between the uplink and downlink signals based on a correlation between the at least one block of transmitted samples and at least one of the plurality of blocks of received samples. The plurality of blocks of received samples are generated based on the received downlink signal. The apparatus further includes a modulator for compensating for time delay and frequency offset between subsequent transmitted and received signals based on the estimated time delay and frequency offset. 
    
    
     
       BRIEF SUMMARY OF THE DRAWINGS 
       The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings, wherein like elements are represented by like reference numerals, which are given by way of illustration only and thus are not limiting of the present invention and wherein: 
         FIG. 1  illustrates an example of a portion of a hybrid single frequency network; and 
         FIG. 2  is a flow chart illustrating a method for time delay and frequency offset compensation according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Various example embodiments of the present invention will now be described more fully with reference to the accompanying drawings in which some example embodiments of the invention are shown. 
     Detailed illustrative embodiments of the present invention are disclosed herein. However, specific structural and functional details disclosed herein are merely representative for purposes of describing example embodiments of the present invention. This invention may, however, may be embodied in many alternate forms and should not be construed as limited to only the embodiments set forth herein. 
     It will be understood that, although the terms first, second, etc. may be used herein to describe various elements, these elements should not be limited by these terms. These terms are only used to distinguish one element from another. For example, a first element could be termed a second element, and, similarly, a second element could be termed a first element, without departing from the scope of example embodiments of the present invention. As used herein, the term “and/or,” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that when an element is referred to as being “connected,” or “coupled,” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected,” or “directly coupled,” to another element, there are no intervening elements present. Other words used to describe the relationship between elements should be interpreted in a like fashion (e.g., “between,” versus “directly between,” “adjacent,” versus “directly adjacent,” etc.). 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of example embodiments of the invention. As used herein, the singular forms “a,” “an,” and “the,” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises,” “comprising,” “includes,” and/or “including,” when used herein, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     It should also be noted that in some alternative implementations, the functions/acts noted may occur out of the order noted in the figures. For example, two figures shown in succession may in fact be executed substantially concurrently or may sometimes be executed in the reverse order, depending upon the functionality/acts involved. 
     Specific details are provided in the following description to provide a thorough understanding of example embodiments. However, it will be understood by one of ordinary skill in the art that example embodiments may be practiced without these specific details. For example, systems may be shown in block diagrams in order not to obscure the example embodiments in unnecessary detail. In other instances, well-known processes, structures and techniques may be shown without unnecessary detail in order to avoid obscuring example embodiments. 
     Also, it is noted that example embodiments may be described as a process depicted as a flowchart, a flow diagram, a data flow diagram, a structure diagram, or a block diagram. Although a flowchart may describe the operations as a sequential process, many of the operations may be performed in parallel, concurrently or simultaneously. In addition, the order of the operations may be re-arranged. A process may be terminated when its operations are completed, but may also have additional steps not included in the figure. A process may correspond to a method, a function, a procedure, a subroutine, a subprogram, etc. When a process corresponds to a function, its termination may correspond to a return of the function to the calling function or the main function. 
     Moreover, as disclosed herein, the term “buffer” may represent one or more devices for storing data, including random access memory (RAM), magnetic RAM, core memory, and/or other machine readable mediums for storing information. The term “storage medium” may represent one or more devices for storing data, including read only memory (ROM), random access memory (RAM), magnetic RAM, core memory, magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “computer-readable medium” may include, but is not limited to, portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data. 
     Furthermore, example embodiments may be implemented by hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine or computer readable medium such as a storage medium. A processor(s) may perform the necessary tasks. 
     A code segment may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     As used herein, the term “receiver” may be considered synonymous to, and may hereafter be occasionally referred to, as a client, mobile, mobile unit, mobile station, mobile user, user equipment (UE), subscriber, user, remote station, access terminal, receiver, etc., and may describe a remote user of wireless resources in a wireless communication network. 
     As described herein, x(t) is referred to as an uplinked or transmitted version of a signal, whereas y(t) is referred to as the downlink or received version of the transmitted signal. The received signal y(t) is a distorted copy of the transmitted signal x(t), but carries the same information. The distortion may be Gaussian noise, frequency offset, time delay, etc. 
       FIG. 1  illustrates a portion of hybrid satellite and terrestrial single frequency network (hybrid SFN). As shown in  FIG. 1 , the terrestrial repeated version of the signal travels from the broadcast head end (BHE) to terrestrial repeating antenna  110  via satellite  104 , or some other transmission means. The signal then travels from repeating antenna  110  to receiver  108  via a wireless link. The same signal is also received at the receiver  108  via satellite  106 , without being repeated by the terrestrial repeating antenna  110 . 
     Due to the relative motion of satellite  106  in an inclined orbit with respect to the receiver  108  on Earth, the signal y(t) received via the satellite  106  has a varying time delay and frequency offset with respect to a terrestrially repeated version of the same signal. 
     To achieve time and frequency synchronization in such a hybrid SFN, the BHE  102  includes a modulator  1020  for adjusting the time and frequency of subsequent uplink signals so that the downlink satellite signal y(t) arriving at the BHE  102  has a fixed time delay and a fixed center frequency—as if the satellite  106  is stationary with respect to the location of the BHE  102 . 
       FIG. 2  is a flow chart illustrating a method of compensating for time delay and frequency offset according to an example embodiment. The method shown in  FIG. 2  may be performed iteratively at the BHE  102  shown in  FIG. 1 . For the sake of brevity, only a single iteration will be described in detail. 
     Referring to  FIGS. 1 and 2 , at step S 202  the modulator  1020  converts multimedia content (e.g., voice, video, pictures, etc.) from a service and network head end (not shown) into digital samples x n  to be transmitted. The manner in which the modulator  1020  generates the samples x n  via sampling is well known in the art, and thus, a detailed discussion will be omitted for the sake of brevity. Consecutive samples x n  are grouped into blocks or frames of samples. Each block or frame of samples includes N samples, where N is an integer (e.g., 1000, 2000, etc.). As discussed herein, the samples x n  may be referred to as “transmitted samples.” 
     The modulator  1020  outputs the generated blocks of samples to a transmitter processing unit  1104  and a reference frame buffer  1032 . 
     At step S 204 , the consecutive blocks of samples from the modulator  1020  are stored in the reference frame buffer  1032  on a per block basis. For the sake of clarity, example embodiments will be discussed with respect to a single block of samples being stored in the reference frame buffer  1032 . However, it will be understood that one or more blocks of samples may be stored at the reference frame buffer  1032 . The stored block(s) of samples are indexed by an index b, which is an integer. 
     At step S 206 , the transmitter processing unit  1104  sequentially converts each block of samples x n  into an analog uplink signal x(t) suitable for transmission on a wireless uplink channel. Although shown as successive steps, the storing step S 204  and the processing step S 206  in  FIG. 2  may be performed concurrently or simultaneously. In this case, the modulator  1020  outputs the block of samples to the reference frame buffer  1032  and the transmitter processing unit  1104  in parallel. 
     At step S 208 , the uplink signal x(t) is broadcast on the uplink channel. 
     Upon receipt, the satellite  106  broadcasts the signal (now y(t)) on a downlink channel. The broadcast downlink signal is received at the BHE  102  as well as receiver  108 . As noted above, the received signal y(t) is a distorted copy of x(t). 
     Still referring to  FIGS. 1 and 2 , at step S 209  the receiver processing unit  1102  processes the received signal y(t) to recover (or generate) digital samples y n  (referred to as “received samples”) carried by the received signal y(t). The sample rate used in generating the transmitted samples x n  and recovering the received samples is assumed to be the same for the sake of clarity. However, the example embodiments may be easily adapted for different sample rates by those skilled in the art. 
     As is the case with the transmitted samples x n , consecutively received samples y n  are grouped into blocks or frames of samples, each block or frame of samples also including N samples. The consecutive blocks of recovered samples are stored in a feed back capture buffer  1106  on a per block basis. The consecutive blocks of samples are also indexed using index k, where k=0, ±1, ±2, . . . , K. The index k associated with each block of received samples represents a location of a block of received samples within the plurality of blocks of received samples. 
     To ensure that at least one recovered block of samples y n  stored in the feed back capture buffer  1106  corresponds to the block of transmitted samples stored in the reference frame buffer  1032 , the BHE  102  begins storing received samples y n  in the feed back capture buffer  1106  a given period of time after filling the reference frame buffer  1032 . That is, after the reference frame buffer  1032  has reached its capacity. 
     The reference frame buffer  1032  may have the capacity to store 1 or 2 blocks of samples. The size of feed back capture buffer  1106  may vary, but typically is large enough to hold a plurality of blocks of received samples (e.g., about 10 milliseconds of received samples). 
     The interval of time that the BHE  102  waits before storing received samples may be equal to the round trip delay (RTD) of the signal traveling from the modulator  1020  to the receiver processing unit  1102 ; namely, between transmission of signal x(t) and reception of signal y(t) carrying corresponding information at the BHE  102 . 
     In one example, if T represents the sample duration, the nominal RTD between transmission and reception of corresponding signals is expressed in terms of the number of samples, and is denoted by D. That is, D·T is the nominal RTD for the transmitted signal to travel from the modulator  1020  to the receiver  1102  via the satellite  106 . 
     After the feed back capture buffer  1106  is full—has reached its capacity—the blocks of received samples y n  are output to the detector  1030  on a per block basis. The reference frame buffer  1032  also outputs the transmitted block of samples x n  to the detector  1030 . 
     At step S 212 , the detector  1030  estimates a time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)} between the transmission and reception of corresponding signals based on the at least one block of transmitted samples from the reference frame buffer  1032  and at least one of the plurality of blocks of received samples from the feed back capture buffer  1106 . An example process for estimating the time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)} will be described in more detail below. The estimated time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)} are output to the modulator  1020 . 
     At step S 214 , the modulator  1020  compensates for the actual time delay Δt and frequency offset Δf between transmission and reception of corresponding signals based on the estimated time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)}. The manner in which the modulator  1020  compensates for the time delay and frequency offset is well-known in the art and thus a detailed discussion will be omitted. 
     An example method for estimating time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)} will now be described. As noted above, the method may be performed at the detector  1030  in  FIG. 1 . The method will be described, for the sake of clarity, with regard to an example situation in which the only distortion in the received signal y(t) are actual time delay Δt, frequency offset Δf and Gaussian noise. In this example, the received signal y(t) is represented by Equation (1) shown below.
 
 y ( t )= √{square root over (P)}x ( t−Δt )· e   2πΔft +ω( t )  (1)
 
In Equation (1), P is the power of the received signal y(t) relative to the transmission power of the transmitted signal x(t), and ω(t) is the Gaussian noise. The actual time delay Δt represents the round trip delay (RTD) of the signal traveling from the modulator  1020  to the receiver  1102  via the satellite  106 . The actual frequency offset Δf is a result of the Doppler effect due to the motion of the satellite  106 .
 
     Assuming that the time delay Δt is an integer multiple of sample duration T, each received sample y n  is given by Equation (2) shown below.
 
 y   n   =√{square root over (P)}x   n-M   ·e   2πΔfnT +ω n   (2)
 
In the above equation, M is an additional delay with respect to the nominal delay D, expressed as a number of samples. The additional delay M is related to the time delay Δt and given by Equation (3) shown below.
 
                   M   =         Δ   ⁢           ⁢   t     T     -   D             (   3   )               
In Equation (3), M represents the instantaneous variation of the time offset with respect to the nominal offset D.
 
     Referring back to  FIG. 1 , in estimating time delay and frequency offset, the detector  1030  calculates a correlation C k  between the stored block of transmitted samples x n  and each stored block of recovered samples y n . As discussed above, each block of transmitted samples and each block of recovered samples includes the same number of samples—namely N samples. The number N may be determined based on empirical data at a network controller. 
     The detector  1030  calculates the correlation C k  between the block of transmitted samples and each corresponding block of recovered samples according to Equation (4) shown below. 
                       C   k     =       ∑     n   =   0       N   -   1       ⁢       y     n   +   k       ·       (     x   n     )     *     ·       (       y     n   +   k   +   q       ·       (     x     n   +   q       )     *       )     *           ⁣           (   4   )               
In Equation (4), notation ( )* represents complex conjugate, and q is a parameter that indicates the distance between the samples represented by y n+k  and x n  and respective samples y n+k+q  and x n+q . According to example embodiments, parameter q determines the accuracy of the frequency offset estimate. The larger q becomes, the more accurate the estimate becomes. The value of q may be determined experimentally for a given accuracy requirement. Typically, q may be on the order of between about 10N to about 100N. For a given block of transmitted samples, a correlation is computed for each block of received samples, which are indexed by k=0, ±1, ±2, . . . , K.
 
     According to example embodiments, a single correlation C k  given by Equation (4) is used to estimate both time delay and frequency offset between the transmitted and received signals. The estimate of the time delay Δ{tilde over (t)} is obtained by maximizing the amplitude of correlation C k  over index k=0, ±1, ±2, . . . , ±K. That is, the time delay is estimated by identifying the index k associated with the maximum correlation value C k . As discussed herein, the maximum correlation value is referred to as C k     max    and the index k associated with the maximum correlation C k     max    is referred to as k max . In this example, k max  represents a location of the block of received samples associated with the maximum correlation within the plurality of blocks of received samples. 
     In one example, identification of the maximum correlation C k     max    may be regarded as searching within a given or desired search window [−K, K], for some K&gt;0 as represented by Equation (5) shown below.
 
| C   k     max   |=max {| C   k   |, −K≦k≦K}   (5)
 
The estimated time delay Δ{tilde over (t)} is then calculated based on the index k max  associated with the maximum correlation value C k     max    as shown below in Equation (6).
 
Δ {tilde over (t)} =( D+k   max ) T   (6)
 
As noted above, D is the nominal delay and T is the sample duration. Stated another way, the estimated time delay Δ{tilde over (t)} may be calculated as a function of the index k max , the nominal delay D and the sample duration T.
 
     According to example embodiments, the estimated time delay Δ{tilde over (t)} given by Equation (6) is valid when the condition given by Equation (7) is met.
 
( D−K ) T≦Δt ≦( D+K ) T   (7)
 
Consequently, in choosing the search window [−K, K], the values of D and K are chosen such that condition (7) is satisfied. The search window [−K, K] may be selected automatically or by a human network operator based on empirical data.
 
     The frequency offset is also estimated based on the maximum correlation value C k     max   . In more detail, the frequency offset is estimated based on the phase of the maximum correlation value C k     max   ; that is, the correlation value C k  evaluated at the index k max . 
     The estimated frequency offset Δ{tilde over (f)} between the transmitted signal x(t) and the received signal y(t) is given by Equation (8) shown below. 
                     Δ   ⁢           ⁢     f   ~       =         -   1       2   ⁢           ⁢   π   ⁢           ⁢   qT       ⁢     arg   (     C     k   max       )               (   8   )               
As noted above, q is a parameter indicating a distance between pairs of samples and T is the sample duration used in generating the samples. The value arg(C k     max   ) is the phase of the correlation C k  evaluated at k max . Because computation of the phase of a complex number is well known in the art, only a brief discussion will be provided. In one example, arg(C k     max   ) may be computed according to Equation (9) shown below:
 
     
       
         
           
             
               
                 
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     In Equation (9), Im(C k     max   ) is the imaginary part of complex number C k     max   , and Re(C k     max   ) is the real part of the complex number C k     max   . 
     According to example embodiments, the estimated frequency offset Δ{tilde over (f)} is valid for frequency offsets within the range given by the following inequality: 
     
       
         
           
             
               
                 
                   
                     
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     The inequality (10) places a condition on the maximum range of the frequency offset for which the algorithm is able to detect/estimate. This range is referred to as the range of frequency detection. As shown in inequality (10), the range of frequency detection is a function of parameter q. There is a trade-off between the size of the range of frequency detection and the accuracy of estimate. 
     According to example embodiments, the estimated time delay Δ{tilde over (t)} and frequency offset Δ{tilde over (f)} are used in modulator  1020  to adjust the time and frequency of subsequently transmitted signals. The modulator  1020  is designed to compensate for the time delay and frequency offsets such that Δ{tilde over (t)}=D·T and Δ{tilde over (f)}=0 in the steady state. 
     Because the manner in which the estimated time delay and frequency offsets are utilized by the modulator  1020  to compensate for actual time delay and frequency offsets is well-known, a detailed discussion will be omitted for the sake of brevity. 
     After compensating, a new set of samples is captured in the reference frame buffer  1032  and the feedback capture buffer  1106 , and another iteration of processing begins. 
     The above discussed methods for estimating time delay and frequency offsets and compensating for actual time delay and frequency offsets between transmitted and received signals may be performed iteratively. Iterations between the modulator  1020  and the detector  1030  form a closed loop. 
     An example application in which the frequency offset for an OFDM waveform is estimated will now be described. However, example embodiments are applicable to other waveforms. In this example, N is defined as the length of a fast Fourier transform (FFT) symbol. If F s  is the frequency spacing between two OFDM sub-carriers, the sub-carrier frequency spacing is given by the following expression: 
                     F   s     =     1   NT             (   11   )               
In this example, parameter q is normalized with respect to the FFT symbol size N, and the normalized parameter (referred to as Q) is given by Equation (11).
 
                   Q   =     q   N             (   12   )               
In Equation (12), Q represents the number of FFT symbols between a pair of received symbols in the correlation given by Equation (4). As was the case with parameter q, Q in equation (12) is a parameter that determines the accuracy of the frequency offset estimate. The larger Q becomes, the more accurate the estimate becomes. The value of Q may be determined experimentally for a given accuracy requirement. Typically, Q may be on the order of between about 10 to about 100.
 
     Substituting Equation (12) into Equation (8), the estimated frequency offset Δ{tilde over (f)} in an OFDM system is given by Equation (13). 
                     Δ   ⁢           ⁢     f   ~       =         -   1       2   ⁢           ⁢   π   ⁢           ⁢   Q       ⁢       arg   ⁡     (     C     k   max       )       ·     F   s                 (   13   )               
In terms of the normalized parameters, the range of frequency detection in this example is given by Equation (14).
 
     
       
         
           
             
               
                 
                   
                     
                       
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     Example embodiments may be implemented in a broadcast head end in a DVB-SH single frequency network. The correlations used for detecting the time and frequency offsets in a single frequency network provide more reliable and accurate estimates of time and frequency offsets. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the invention, and all such modifications are intended to be included within the scope of the invention.