Abstract:
A synchronization system within a dual antenna receiver employs two timing recovery loops each coupled to a different antenna input. The timing error computed within each timing recovery loop is differentiated by a high pass filter and compared to a predefined threshold indicating convergence. Both sampling rates are then synchronized utilizing the timing error from whichever loop converges first. If synchronization lock for the selected loop is lost, the other loop is selected to provide timing error to both loops.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to timing recovery in wireless transmission systems and, more specifically, to timing recovery within synchronization loops for receivers coupled to two or more antennae. 
     BACKGROUND OF THE INVENTION 
     Current Advanced Television Systems Committee (ATSC) receivers employ single antenna systems to receive terrestrial digital television (DTV) signals. However, terrestrial wireless signals transmitted to or from a remote station may be reflected from terrain features, fixed or mobile objects such as buildings or vehicles, or discontinuities in the atmosphere. If the reflected signal is not sufficiently absorbed or attenuated, a plurality of different propagation paths are created between the transmitter and receiver, creating a situation referred to as multipath propagation. 
     Various problems associated with multipath propagation, typically referred to collectively as multipath fading, may dictate throughput and other performance criteria. One suggestion for minimizing the effects of multipath fading during wireless signal transmission, set forth in the Electronics Industry Association/Tele-communications Industry Association (EIA/TIA) proposed standard ISO-2000, employs space-time spreading (STS), in which identically coded data frames are transmitted on each of two (preferably orthogonal) channels utilizing physically displaced antennae. 
     In a single antenna system, a synchronization loop is required within the receiver to accommodate transmit and receive data rate variances; in a multiple antennae system, a synchronization loop is still required, but multiple inputs (from each antenna) are present. There is, therefore, a need in the art for a robust synchronization mechanism employing signals from two or more antennae. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in dual antenna receiver, a synchronization system employing two timing recovery loops each coupled to a different antenna input. The timing error computed within each timing recovery loop is differentiated by a high pass filter and compared to a predefined threshold indicating convergence. Both sampling rates are then synchronized utilizing the timing error from whichever loop converges first. If synchronization lock for the selected loop is lost, the other loop is selected to provide timing error to both loops. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
     Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation; the term “or” is inclusive, meaning and/or; the phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term “controller” means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, wherein like numbers designate like objects, and in which: 
     FIG. 1 depicts a multiple antennae receiver system including a differentiator-based synchronization loop for timing recovery according to one embodiment of the present invention; 
     FIG. 2 illustrates in greater detail a synchronization system employing differentiator-based antenna switching for timing recovery according to one embodiment of the present invention; and 
     FIG. 3 is a high level flow chart for a process of differentiator-based antenna switching for timing recovery according to one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 3, discussed below, and the various embodiment used to describe the principles of the present invention in this patent document are by way of illustration only and should not be construed in any way to limit the scope of the invention. Those skilled in the art will understand that the principles of the present invention may be implemented in any suitably arranged device. 
     FIG. 1 depicts a multiple antennae receiver system including a differentiator-based synchronization loop for timing recovery according to one embodiment of the present invention. Receiver system  100  includes an antenna array  101  having physically displaced antennae  101   a  and  101   b.  Rather than being spaced closely together (e.g., on the order of half a wavelength) for array gain, antennae  101   a  and  101   b  are spaced far enough apart to so that received signals fade (almost) independently. 
     Antenna array  101  is coupled to a receiver  102 , a digital television receiver in the exemplary embodiment, receiving separate inputs  104 ,  105  from antennae  101   a  and  101   b.  The present invention may also be employed for any receiver such as, for example, a broadband wireless Internet access receiver. Regardless of the embodiment, however, receiver  102  includes a synchronization system  103  employing differentiator-based antenna switching for timing recovery in accordance with the present invention, as described in further detail below. 
     Those skilled in the art will perceive that FIG. 1 does not explicitly depict every component within a receiver system. Only those portions of such a system that are unique to the present invention and/or required for an understanding of the structure and operation of the present invention are shown. 
     FIG. 2 illustrates in greater detail a synchronization system employing differentiator-based antenna switching for timing recovery according to one embodiment of the present invention. Synchronization system  103  includes two timing recovery loops  200  and  201 . Independent carrier recovery mechanisms are utilized since separate tuners are employed with antennae  101   a  and  101   b.    
     Each timing recovery loop  200  and  201  includes a sample rate converter (SRC)  202  and  203  coupled to inputs  104  and  105 , respectively, receiving wireless signals from one of antennae  101   a  and  101   b.  The output of sample rate converters  202  and  203  are passed to carrier recovery (CR) units  204  and  205 , respectively, the outputs of which are connected to square root raised cosine (SQRC) filters  206  and  207 . The outputs of square root raised cosine filters  206  and  207  are passed to timing recovery (TR) units  208  and  209 , which generate control signals for controlling sample rate converters  202  and  203  to complete timing recovery loops  200  and  201 . 
     Sample rate converters  202  and  203  require input from the timing recovery unit(s)  208  and/or  209  for proper operation, but the timing recovery loops  200  and  201  may fail in poor channel conditions so that a form of diversity is beneficial to the timing recovery algorithm. While signal amplitude and symbol arrival times may vary as a result of multipath propagation, inter-symbol timing is unlikely to be affected. Thus, the timing recovery error for either timing recovery loop  200  or  201  may be employed for the other timing recovery loop with, at most, the addition of some delay factor. 
     In the present invention, the timing recovery error from timing recovery units  208  and  209  are passed through high pass filters (HPF)  210  and  211 , which are differentiators which may be implemented by a simple one tap feedback filter. The outputs of high pass filters  210  and  211  provide a measure of how well the respective timing recovery loop  200  or  201  has converged. If a timing recovery loop has converged, the output of a differentiator operating on that output will be constant (that is—the differentiator output should tend towards zero for a well converged system). However, if the timing recovery loop has not converged, then the timing recovery error wanders over a large range. 
     In the present invention, a control unit  212  receives the differentiated outputs of high pass filters  210  and  211  and determines whether the magnitude of either output has dropped below a predefined threshold Δ. If the magnitude of either output drops below the threshold value Δ, the respective timing recovery loop  200  or  201  is selected as the “master” timing recovery loop and the error value produced by the corresponding timing recovery unit  208  or  209  is selected for controlling both sample rate converters  202  and  203 . Multiplexer  213 , under the control of control unit  212 , passes the output of the selected timing recovery unit  208  or  209  to both sample rate converters  202  and  203 . Sample rate converters  202  and  203  then utilize the received timing recovery error value to adjust processing of received wireless signals from inputs  104  and  105 . 
     In operation, the system  103  is initialized in “independent” mode, with each of the timing recovery loops  200  and  201  running independently on each antenna input  104  and  105  and multiplexer  213  passing the output of each timing recovery unit  208  and  209  to the sample rate converter  202 ,  203  within the corresponding timing recovery loop  200  and  201 , respectively. When the magnitude of the differentiated output of either high pass filter  210  or  211  drops below the threshold value Δ, the timing recovery error from that timing recovery loop  200  or  201  is selected to control both timing recovery loops  200  and  201 . If the timing recovery loop synchronization lock is lost, then the control unit switches to the other timing recovery loop as the master. Control unit  212  may optionally receive input from segment sync detect units  213  and  214  for this purpose. 
     Of course, numerous variations of the operational scheme described above may be alternatively employed. For example, the control unit  212  may periodically re-evaluate the outputs of high pass filters  210  and  211  after selecting one of the timing recovery loops  200  or  201  as the “master,” and either select the other timing recovery loop or continue to utilize the current timing recovery loop depending on the result. Moreover, if the timing recovery error of the current “master” timing recovery loop  200  or  201  rises above the threshold value Δ for any reason, the system  103  may be returned to “independent” mode with both timing recovery loops  202  and  203  running independently on the respective antenna input  104  and  105 . Similarly, when the timing recovery loop lock is lost for a current “master” timing recovery loop, the system may revert to independent mode rather than simply switching directly to the other timing recovery loop. 
     FIG. 3 is a high level flow chart for a process of differentiator-based antenna switching for timing recovery according to one embodiment of the present invention. The antenna switching process  300 , implemented by synchronization system  103  depicted in FIG. 2, begins with the two timing recovery loops being started and running independently on the inputs from the respective antennae (step  301 ). The timing errors for both loops are then separately computed (step  302 ) and differentiated, and the differentiated results are compared to a predefined threshold (step  303 ) to determine if the rate of change for either error has fallen below that predefined threshold, indicating convergence within the respective timing recovery loop. If not, the process returns for further calculation of timing error and differentiation. If so, however, the process selects the timing recovery loop having a differentiated timing error less than the threshold as a “master” timing recovery loop (step  304 ), employing the timing error from that loop for feedback within both timing recovery loops. 
     A determination of whether the loop lock has been lost is then made (step  305 ), and either the determination is repeated (step  305 ), or the other timing recovery loop is selected as the master (step  306 ). The process continues indefinitely until interrupted by an external process. 
     The present invention applies antenna diversity to timing recovery, specifically to antenna selection and/or switching for timing recovery. Differentiated timing recovery errors for both antennae are employed to chose which antenna input to employ in timing recovery and synchronization. The synchronization system thus depends only on the antenna which sees the best channel, rather than on the antenna which sees the worst channel or some combination of the best and worst channels. The probability of getting a timing recovery loop lock for an ATSC receiver utilizing timing recovery-based antenna switching in a dual antenna system is much higher than using some combination of both timing recovery loop errors. 
     Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions and alterations herein may be made without departing from the spirit and scope of the invention it its broadest form.