Patent Publication Number: US-9407484-B2

Title: Synchronization sequence with tone notching in OFDM systems

Description:
The present invention relates to a system, apparatus and method for designing a synchronization sequence when a subset of sub-carriers is turned off. 
     Detect and avoid (DAA) technology allows ultra-wideband (UWB) systems to coexist peacefully with current and future wireless technologies sharing the same spectrum. DAA techniques mitigate interference by searching for broadband wireless signals and automatically switching UWB devices to another frequency to prevent any conflict. 
     In current UWB multi-band orthogonal frequency division multiplexing (MB-OFDM) systems, each device needs to have the capability to detect existing primary narrow-band devices and the bandwidth of these systems and avoid transmission in that frequency region. These so-called detect-and-avoid (DAA) capabilities have been designed as a format of tone notching to operate during the payload transmission as well as channel estimation preamble transmission. However, design of the synchronization sequence with DAA ability without too much change made to the current sync sequence is needed to add switching flexibility between different modes as well as taking care of receivers. 
     Thus, there is a need in the art for a mechanism that efficiently addresses this problem while at the same time having minimal impact on existing sync sequences. The synchronization sequence of UWB MB-OFDM systems should not only have good correlation property but also satisfy the emission mask requirement issued by FCC. If some tones in the frequency domain for the sync sequence are required to be zero (notched to zero in the spectrum of the sync sequence), the present invention avoids a redesign of the sync sequence in the frequency domain. Usually the receiver doesn&#39;t know whether certain tones have been notched or not at the transmitter, and the present invention maintains the good correlation properties of an existing sync sequence. 
     When the OFDM system requires a sync sequence taking into consideration DAA, the sync sequence designer can notch the corresponding sub-carriers to zero in the spectrum of a current sync sequence. In a preferred first embodiment, the new sync sequence is the inverse-FFT version of the notched spectrum according to a first formula of the present invention. When symmetric notching is needed, in a second preferred embodiment of the present invention, a second formula is provided to generate a real sync sequence. The receiver needn&#39;t be aware of the notching at the transmitter. 
    
    
     
         FIG. 1  illustrates the spectrum of an original sync sequence; 
         FIG. 2  illustrates the spectrum of  FIG. 1  notched using one-sided notching and symmetric notching; 
         FIG. 3  illustrates the correlation results for synchronization at the receiver under CM 1  channel; 
         FIG. 4  illustrates the correlation results for synchronization at the receiver under CM 2  channel; 
         FIG. 5  illustrates an apparatus diagram for generating sync sequences with tone notching from the original sequence, according to the present invention; and 
         FIG. 6  illustrates an OFDM system to switch between DAA and non-DAA mode by using a generated sync sequence, according to the present invention. 
     
    
    
     The present invention can be readily understood by one skilled in the art in view of the following detailed description with reference to the accompanying drawings presented only for illustrative purposes. In the drawings, the same reference symbols denote identical components in order to eliminate redundant description thereof. 
     In the present invention, a transform domain approach is provided for generating sync sequences with notched tones whose cross-correlation with the non-notched sequences is maintained as good as the auto-correlation of the notched sequences. Although the sync sequences have similar correlation properties, the approach of the present invention for constructing the desired sequence with DAA ability is elementary and simple. Basically, the present invention transforms the correlation requirements into frequency domain identities and can be used to generate a number of sequences with different tone notching requirements in OFDM systems. 
     Suppose x(0), x(1), x(2), . . . , x(N−1) is a time-domain sync sequence, its frequency domain response is X(0), X(1), X(N−1), where 
     
       
         
           
             
               
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     If the system requires a subset A of sub-carriers to be turned off, redesign of the sync sequence is not necessary, in a first preferred embodiment. In a first preferred embodiment, the present invention notches spectrum X(k) to be zero in subset A and then obtains its time-domain version. Therefore, the resultant time-domain sequence is 
     
       
         
           
             
               
                 
                   
                     
                       
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     The transmitter uses x′(n) as a new sync sequence, while the receiver doesn&#39;t need to know this information and still uses the original sync sequence x(n) to correlate with the received signal. 
     This technique, of a first preferred embodiment, guarantees that a receiver finds a peak because in the frequency domain the energy captured by correlation is only reduced by a small amount in subset A of sub-carriers. On the other hand, this notching method can be viewed from the perspective that the channel gets deep-faded from sub-carrier subset A and as a result the original sync sequence still works for this kind of channel. 
     A symmetric notching is needed whenever the resultant sync sequence is required to be real. For certain transmission modes with frequency domain spreading (put one symbol on sub-carrier k and put its conjugate on sub-carrier N−k, except k=0), the time-domain signal is real for payload transmission. As is well known, the real part of the signal is passed into I-branch of the transmitter chain and the imaginary part of the signal is passed into the Q-branch of the transmitter chain. In this case, only the I-branch is needed to reduce the transmitter complexity in this low-rate mode. To be consistent with the transmission of data part, this symmetric notching method is needed in the sync sequence. It is well known that AGC (automatic gain control) is at the front end of a demodulator to eliminate chain gain variations, which could affect the amplitude detector. Therefore, the notching must be consistent from the aspect of AGC to the subsequent notching pattern in the channel estimation sequence and the payload part so that the amplitude of received signals for the sync sequence can be maintained at the same level as that of received signals for the notched channel estimation sequence and the notched payload part. In a second preferred embodiment, the mathematical formula for the new sync sequence for the transmitter is 
     
       
         
           
             
               
                 
                   
                     
                       
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     A sync sequence with good correlation properties is desired in many communication systems. Oftentimes, it is desirable to have a family of sequences whose autocorrelation function has a single peak at zero delay. Since the sync sequence with tone notching of the present invention is different from the sync sequence stored at the receiver, the cross-correlation function between them has been checked to determine that it has a single peak at zero delay. 
     Some simulation results are presented below for CM 1  and CM 2  channel models generally used in UWB systems. Synchronization is performed by running cross-correlation between the received signals for notched sync sequence x′(n), (x″(n) and original sequence x(n). 
       FIG. 1  illustrates the spectrum of the original sync sequence, which is fat to satisfy the emission mask. 
       FIG. 2  illustrates that according to the present invention the spectrum of  FIG. 1  is notched to zero from sub-carrier  32  to  39  for one-sided notch and is notched to zero from sub-carrier  89  to  96  for symmetric notching as well. 
       FIG. 3  illustrates the correlation results for synchronization at the receiver under CM 1  channel. The peak value for one-sided notching and symmetric notching, according to the first and second embodiments of the present invention, respectively are found as 1008.8  302  and 940.4 compared to 1041.8  303  for synchronization without notching  301 . 
       FIG. 4  illustrates the correlation results for synchronization at the receiver under CM 2  channel. The peak value for one-sided notching and symmetric notching, according to the first and second embodiment of the present invention, respectively are found as 1322.9  402  and 1228.9  403  compared to 1427.5 for synchronization without notching  401 . 
       FIG. 5  illustrates a sync generator apparatus  500  for generating sync sequences with tone notching from the original sync sequence, according to the first and second embodiments of the present invention. The sync generator apparatus  500  comprises a first memory  501  for storing the time-domain sequence x(N), an FFT module  502  operably connected to the first memory  501  for access to the time-domain sequence x(N) stored therein. The FFT module  502  outputs the frequency domain sequence X(k) and stores it in a second memory  503  to which it is also operably connected. A sync generator module  504  is operably connected to the first  501  and second  503  memory for respective access to the time-domain sequence x(N) and the frequency domain sequence X(k) respectively stored therein and generates therefrom sequences x′(n)  505  and x″(n)  506  as a new sync sequences using formulas (1) and (2) above. A transmitter (not shown) would then transmit the generated sequence whenever DAA is needed. 
       FIG. 6  illustrates an OFDM system  600  switching between DAA and non-DAA modes by using a generated sync sequence, without any change to the receiver structure. At  601  it is determined at the transmitter whether or not DAA is needed. If not, no new sync sequence is generated and transmission takes place using the original sync sequence at  602 . If DAA is needed then, in accordance with either a first or a second embodiment, at  603  a sync sequence is generated using formula (1) or (2) above and the generated sync sequence is transmitted. No change is required at the receiver at step  604 . 
     It can be seen from the simulations graphically presented in  FIGS. 3 and 4  that the peak is reduced by such a small amount such that replacing the original sync sequence with a generated sync sequence according to a first or a second embodiment does not affect the synchronization process at the receiver at all. 
     One of the immediate applications is the next generation Multi-Band OFDM UWB systems. 
     While, embodiments of the present invention have been illustrated and described, one skilled in the art will understand that changes may be made in these embodiments without departing from the principles and spirit of the present invention, the scope of which is defined in the appended claims and their equivalents.