Abstract:
The present embodiments are directed to systems and methods for detecting random access channel requests, while excluding false random access signals using search windowing and distance-based peak suppression techniques. The present embodiments additionally include further techniques for suppression of fake random access signals, including amplitude thresholds and preamble-based signal exclusion. Beneficially, the present embodiments significantly reduce the false alarm rate, while maintaining a low hardware complexity requirements. In some embodiments, worst-case false alarm rates can be reduced from as much as 20% down to nearly 0.1%.

Description:
TECHNICAL FIELD 
       [0001]    This invention relates to data communications and more particularly relates to false alarm reduction with search window and peak suppression. 
       BACKGROUND OF THE INVENTION 
       [0002]      FIG. 1  illustrates cellular network  100  of the prior art. Each cell  102   a - c  in cellular network  100  is generally defined by an area in which base stations  104   a - c  are able to communicate with User Equipment (UE)  106   a - d . Examples of UEs  106   a - d  include telephones, smartphones, Personal Data Assistants (PDAs), tablet computers, cellular data devices for use with laptop computers, and the like. Generally, each cell  102   a - c  includes one corresponding base station  104   a - c.    
         [0003]    Any number of UEs  106   a - d  may be found in cells  102   a - c , depending on the use habits of the users of cellular network  100 . For example, cell  102   b  includes two UEs  106   c - d . In such an embodiment, both UEs  106   c - d  may communicate with base station  104   b  at the same time. Depending on the protocol used by base station  104   b,  UEs  106   c - d  may communicate simultaneously, or substantially simultaneously. Alternatively, UEs  106   c - d  may communicate with base station  104   b  within a time slot. Additionally, UEs  106   a - d  may move between cells as the user travels from one area to another. As shown, UE  106   a  may move from cell  102   a  to cell  102   c.  In this sort of case, cell  102   a  includes two UEs  106   a - b  initially, but once UE  102   a  moves to cell  102   c,  cell  102   a  only includes UE  106   b.  Thus cellular networks  100  are generally dynamic in nature, and changes in the topology of cellular network  100  may be random, based upon the user&#39;s habits. 
         [0004]      FIG. 2  illustrates an example of a topology for any of cells  102   a - c . In addition to base station  104 , cells  102  may include antenna  202  coupled to base station  104 . Antenna  202  receives random access signals from UEs  106  operated by user  204  in a multipath environment and mobile user  206  over Random Access Channels (RACHs) operated by base station  104 . The RACH allows UEs  106  to gain initial access to cellular network  100  and facilitates uplink synchronization. 
         [0005]      FIG. 3  illustrates RACH detection circuit  300  according to the prior art. RACH detection circuit  300  is often included in base station  104 . RACH detection circuit  300  includes CP removal module  302  for removing the Cyclic Prefix (CP) from received symbols. RACH detection circuit  300  also includes downsampling/resampling module  304  for reducing a sample rate to a frequency that is suitable for use by correlator  306 . The reduced sample rate simplifies operations of the correlator  306 , particularly in FFT module  308 . Correlator  306  includes Fast Fourier Transform (FFT) module  308  configured to transform the downsampled symbol into frequency domain, correlator  306  also includes subcarrier demapping module  310  and multiplier  312 . Multiplier  312  multiplies the demapped subcarriers with a conjugate of a root sequence in the frequency domain. The multiplication result is then converted back to time domain by Inverse Discrete Fourier Transform (IDFT) module  314 . In prior systems  300 , signature detection and timing offset estimation module  316  detects a random access signal from UE  106  and determines the timing offset of the detected random access signal. 
         [0006]    Unfortunately, as described below, RACH detection circuit  300  generates a large number of false alarms. A false alarm is an event that is a result of a RACH detection circuit detecting energy that is outside of the boundary of a signature sent by UE  106   c.  For example, when a UE  106   c  is very close to a base station  104   b,  the timing offset may be very low. In such an embodiment, power leakage from a random access signal sent by UE  106   c  to base station  104  may fall outside of detection interval for a first signature and fall within a signal detection interval of a second signature. Thus, the power leakage may appear to be a second random access signal from a second UE  106   d.  Such situations may trigger false alarm events. 
         [0007]    Additionally, noise, interference, frequency offset and Doppler shifts may all contribute to false detections. For example, in a second situation, UE  106   a  may be far from base station  104   c.  Thus, the time delay may be large. If UE  106   a  is in a multipath environment, for example, then the multipath reflections may cross into a detection interval for a second signature. Because of the time shift, RACH detection circuit  300  may experience a false alarm because random access signals may appear to be from two separate UEs  106  to RACH detection circuit  300 . 
         [0008]    In cellular systems, UEs  106   a - d  send random access signals to base stations  104   a - c  to gain initial network access to cell network  100 . Ideally base station  104   a - c  would detect the random access signals with high detection accuracy while maintaining a low false alarm rate. Currently, two primary methods are used by base stations for detection of random access signals. The first is a full frequency-domain method for achieving a high level of accuracy. Unfortunately, most common RACH detection circuits  300  are not able to perform full frequency-domain analysis because of the complexity of the FFT and high system resources requirements associated with such methods. The second method for detection includes down-sampling to significantly reduce hardware complexity while achieving a minimum level of acceptable performance. Unfortunately, the minimum level of acceptable performance is still not particularly accurate, and there is much room for improvement. Both of these approaches detect fake random access signals, thus causing a high false alarm rate under some circumstances. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The present embodiments are directed to systems and methods for detecting random access channel requests, while minimizing false alarm events using search windowing and distance-based peak suppression techniques. The present embodiments additionally include further techniques for suppression peaks which may cause false alarm events, including amplitude thresholds. Beneficially, the present embodiments significantly reduce the false alarm rate, while maintaining a low hardware complexity requirement. In some embodiments, worst-case false alarm rates can be reduced from as much as 20% down to nearly 0.1%. 
         [0010]    In one embodiment, search windowing may be applied to the correlator output to reduce the false alarm rate on a physical random access channel signal. Additionally, a Peak Suppression Algorithm (PSA) may be applied to the correlator output to remove extra preambles which may cause false alarm events in various channel conditions. Beneficially, search windowing may remove power leakage in short time offset cases. Further, the PSA may use a distance threshold to remove extra peaks due to long time offset and multipath. In one embodiment, the first peak detected may be kept. In another embodiment, the strongest peak detected may be kept. 
         [0011]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0013]      FIG. 1  is a schematic diagram illustrating a cellular network of the prior art. 
           [0014]      FIG. 2  is a schematic diagram illustrating a cellular network environment. 
           [0015]      FIG. 3  is a schematic block diagram illustrating a random access channel circuit of the prior art. 
           [0016]      FIG. 4  is a graphical diagram illustrating an ideal output of the correlator of  FIG. 3  where all preambles are sent with zero time offset in perfect channel conditions. 
           [0017]      FIG. 5  is a graphical diagram illustrating the concept of RACH signature detection and methods for peak selection. 
           [0018]      FIG. 6  is a graphical diagram illustrating examples of network conditions that may lead to false alarms. 
           [0019]      FIG. 7  is a schematic block diagram illustrating one embodiment of a random access signal detection circuit. 
           [0020]      FIG. 8  is a schematic block diagram illustrating another embodiment of a random access signal detection circuit. 
           [0021]      FIG. 9  is a graphical diagram illustrating a method for calculating a margin size in response to an oversampling rate. 
           [0022]      FIG. 10  is a graphical diagram illustrating a method for detecting a random access channel signal with a search window and a PSA. 
           [0023]      FIG. 11  is a graphical representation of one embodiment of a method for excluding superfluous preambles. 
           [0024]      FIG. 12  is a graphical diagram illustrating a method for peak selection through identification of highest amplitude. 
           [0025]      FIG. 13  is a graphical diagram illustrating a method for peak selection though a race condition. 
           [0026]      FIG. 14  is a schematic flowchart diagram illustrating one embodiment of a method for peak selection in a random access channel system. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]      FIG. 4  is a graphical diagram illustrating an ideal output of correlator  306  where all preambles are sent with zero time offset in perfect channel conditions. RACH is used for communication between UEs  106   a - d  and base stations  104   a - c  in several ways. First, a RACH signal allows UEs  106   a - d  to gain access to cellular network  100  and perform uplink synchronization tasks. Several RACH preamble signatures may be used per cell. The preamble signatures are often constructed from cyclic shifts of pseudo-random sequences, for example Zadoff-Chu (ZC) sequences. ZC sequences have Constant Amplitude Zero Autocorrelation (CAZAC) properties, and good cross-correlation properties as compared with other sequences. Thus,  FIG. 4  illustrates how preamble signatures may look under ideal operating circumstances. 
         [0028]    Unfortunately, the real world almost never operates in ideal conditions. Therefore, the preamble signatures received by base stations  104   a - c  may look more like those illustrated in  FIG. 5 . Highest peak  502  is typically accompanied by a plurality of multipath peaks  504 . In one embodiment, only the highest peak  502  in each signature is selected, and others  504  discarded. In addition, there is often some time offset  506  between the zero time offset point for the preamble signature and the actual arrival time. These non-ideal properties can be caused by several issues as illustrated above in  FIG. 2 . For example, base station  104  may receive multiple multipath peaks  504  because in addition to the actual signal sent by user  204 , base station  104  also receives reflected versions of the true signal which are reflected from, e.g., building  208 . Time offset is caused by the round trip delay between the base station and the UE. 
         [0029]    One method for RACH signature detection involves picking highest peak  502  in each zero correlation zone (ZCZ)  508  defined by each cyclic shift as illustrated further above in  FIG. 4 . In addition, an amplitude threshold  510  may be applied on low peaks  504  to eliminate any peaks (not shown) that are below a minimum amplitude level as defined by amplitude threshold  510 . In such a method, the output of correlator  316  may include preamble indexes and estimates of time offset  506  for each ZCZ in which a peak was detected. Amplitude threshold  510  may be adjusted to balance missed detection and false alarm rates. 
         [0030]      FIG. 6  is a graphical diagram illustrating examples of network conditions that may lead to false alarm events. A high false alarm rate can significantly overload the base stations  104   a - c  and block real preambles from other UEs  106   a - d . False detection of preambles may be caused by a variety of issues, such as power leakage due to oversampling when UE  106  is close to base station  104 . Other issues that may cause false alarms include large time offsets due to round-trip delay, and large channel delay spread.  FIG. 6  shows how these types of factors may contribute to false detections. For example, a highest peak  502  may exhibit a large time offset  506  due to round-trip delay, or the like. Several additional peaks  504  may be received by base stations  104   a - c  due to a multipath environment causing reflections of the RACH signature. In one embodiment, extra preamble  602  may be a multipath  504  of the highest peak  502  that has spilled over into the next ZCZ  508 . Without the peak suppression techniques described according to the present embodiments, random access channel circuit  300  may identify extra preamble  602  as a signal received from a UE  106  on the new ZCZ  508 , when in fact, it has received no such signal. This situation is one example of a false alarm event caused by extra preamble  602 . In various embodiments, either a false detection or a false negative may occur. 
         [0031]      FIG. 7  illustrates a further embodiment of random access detection circuit  300 . An embodiment of correlator  306  may provide an output to windower module  702  for window-based signal filtering. Windower module  702  may pass the windowed output to signature detection module  704 , which may remove extra preambles  602  caused power leakage and short time delay as described further with reference to  FIGS. 9-10 . The peak suppression module  706  may then suppress other extra preambles  602  which may be caused by long large time offset due to round-trip delay, or the like. The functional operations of these various modules are described below with respect to  FIGS. 8-14 . 
         [0032]      FIG. 8  is a schematic block diagram illustrating another embodiment of random access signal detection circuit  300 . In this embodiment, the random access detection circuit  300  includes frequency shift module  802 . Frequency shift module  802  may perform a frequency shift operation on received samples to avoid noise propagation due to decimation in later stages of circuit  300 . Once frequency shift module  802  performs the frequency shift operation on the samples, it then passes the frequency shifted samples to low pass filtering and decimation module  804 . In one embodiment, the low pass filter may be centered at zero frequency. The filter coefficients and downsampling rate of low pass filter and decimation module  804  may be variable, and may be determined in response to the PRACH format chosen. CP removal, FFT &amp; normalization module  806  may remove Cyclic Prefix (CP), perform a Fast Fourier Transform (FFT) on the received signal to convert the signal into frequency domain for processing and normalize the signal. The frequency domain signals are then sent to module  808  so that the PRACH subcarriers may be extracted from the frequency domain signals. The PRACH subcarriers are then multiplied with conjugates of ZC sequences in module  810 . To assist in further processing, the sequences may be zero-padded and inverse Fourier transformed by module  812 . Equal gain combining module  814  further conditions the signals and the search window is applied by module  704 . Peak search and threshold computation module  816  may identify peaks in each signature detection interval that are within set threshold guidelines. Signature detection and timing offset estimation module  818  may then detect a signature and determine the timing offsets of each peak in the signatures. Additionally, peak suppression module  708  may suppress any extra peaks  602  that are identified through use of timing offset thresholds. 
         [0033]      FIG. 9  is a graphical diagram illustrating a method for calculating a margin size in response to an oversampling rate and a representation of how oversampling affects power leakage. Here the oversampling rate is the ratio between the FFT size and the number of PRACH subcarriers. Power leakage can be represented as the sinc squared waveform shown in  FIG. 9  as opposed to perfect sharp peaks in  FIG. 2 . In short time offset cases, i.e. UEs close to base stations, the wider peak leaks power to the signature detection interval on the left causing an extra detected preamble. In addition, side peaks  504   a - b  may become large enough that they are identified as additional extra peaks  602  as shown in  FIG. 10 . In such an embodiment, the performance of the system may be degraded. To combat system performance degradation, a window margin should be larger than twice the oversampling rate, which is the distance between the main peak and the first side peak. Additionally, the window size should correspond to the size of the cell  102   a - b.    
         [0034]      FIG. 10  is a graphical diagram illustrating a method for detecting a random access channel signal with a search window and a PSA. In the depicted embodiment, the main peak  502  has a short time offset. First side peak  504  may be generated by the power leakage from oversampling and may fall to the right of highest peak  502 . Additionally, when UE  106  is close to base station, power leakage may spill over into an adjacent ZCZ  508  on the left of highest peak  502 , resulting in an extra peak  602 . In such instances, extra peak  602  to the left of the highest peak  502  is ignored, because it does not fall within search window  1002   a.  Thus, in such an embodiment, only peaks  502 ,  504  considered for signature determination, and peak  504  may also be discarded because it is lower than highest peak  502 . Beneficially, such an embodiment is effective when UEs  106  are close to base station  104 . Only the highest peak  502  in each window  1002   a - c  is selected. 
         [0035]      FIG. 11  is a graphical representation of one embodiment of a method for suppressing superfluous preambles. This embodiment of a method is effective for situations where UEs  106   a - d  are located far from base stations  104   a - c  and in where large delay spread is exhibited. In such situations, random access detection circuit  300  may receive multiple random access channel signals. In one embodiment, a time difference threshold  1102  may be used to determine whether the received peaks in adjacent ZCZs  508  are from the same UE  106 . For example, the random access detection circuit  300  may receive three high-amplitude peaks  502   a ,  502   b,  and  602 . In order to determine which of the three peaks are real, and which are to be suppressed in order to avoid false alarm events, a time difference threshold  1102  is used. For example, time difference  1104  between peak  502   a  and  502   b  is relatively large, so those peaks would be determined to be from different UEs  106 . On the other hand, peaks  502   b  and  602  are received relatively close in time, so they are determined to be from the same UE  106  because they are within time difference threshold  1102 . Thus, peak  602  is an extra detected preamble and is therefore suppressed by peak suppression module  708 . The time difference threshold may be tuned to provide various degrees of accuracy. In one embodiment, however, the time difference threshold is larger than the predetermined channel delay spread, so that sufficient accuracy may be achieved. 
         [0036]      FIG. 12  is a graphical diagram illustrating a method for peak selection through identification of a higher amplitude. If it is determined that two of the peaks  502 ,  602  received by the random access detection circuit  300  are from the same UE  106 , then the peak suppression module  708  may select the peak with the lowest amplitude for suppression and allow the higher amplitude peak to pass through. For example, as shown in  FIG. 12 , it is determined that two peaks  502 ,  602  were received from the same UE  106 . In order to determine which peak is suppressed, the peak suppression module  708  may measure the amplitude of both peaks  502 ,  602  and suppress the peak with the lowest amplitude. The peak suppression may be accomplished through various signal processing methods. In the depicted embodiment, the lower amplitude signal  602  may be suppressed, even though it is inside of the search window  1002 . Thus, the search windowing may be irrelevant for the purposes of peak suppression in one embodiment, so it can be disabled. 
         [0037]      FIG. 13  is a graphical diagram illustrating an alternative embodiment of a method for peak selection, which is based on race conditions. In this embodiment, the same two peaks in  FIG. 12  may be received by random access detection circuit  300 , but the first peak received may be determined to be the true peak  502 , and the second peak received is deemed to be the extra preamble  602 . Since the initial step of windowing eliminates extra preambles caused by power leakage to the left, this embodiment of the method is based on the premise that extra peaks due to large time offsets and channel delay spread always slight on the right. Thus, the first peak received  502  is deemed to be true, and any subsequent peak  602  is deemed to be fake. One of ordinary skill in the art will recognize situations in which the method of  FIG. 12  is more suitable for peak suppression, and other situations where  FIG. 13  is more suitable for peak suppression. In one embodiment, both options may be provided to a user as configuration settings of the random access detection circuit  300 . 
         [0038]      FIG. 14  is a schematic flowchart diagram illustrating one embodiment of method  1400  for peak selection in a random access channel system. In one embodiment, windower module  704  may apply a windowing filter to identify peaks that fall outside of a predetermined window of timing offset at block  1402 . In one embodiment, the timing offset being referenced to a zero-offset point of a random access signature slot. Signature detection module  706  may then identify a primary peak for each signature slot in which peaks appear within the predetermined window, as shown in block  1404 . Peak suppression module  704  may then determine whether a first primary peak has a common source with a second primary peak as shown at block  1406 , and suppress at least one of the first primary peak and the second primary peak in response to a determination that the first primary peak and the second primary peak have a common source as shown at block  1408 . 
         [0039]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.