Abstract:
A method for use in a receiver in an orthogonal frequency division multiplexing-based data transmission system of detecting frame synchronization with respect to a signal received from a transmitter in the system comprises the following steps. First, the received signal is searched at a first predetermined sub-carrier frequency and at least a second predetermined sub-carrier frequency for a previously inserted data pattern. Then a frame boundary in the received signal is identified as a position where the data pattern is detected at both the first predetermined sub-carrier frequency and the second predetermined sub-carrier frequency.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to digital audio broadcasting systems and, more particularly, to frame synchronization techniques in digital audio broadcasting systems. 
     BACKGROUND OF THE INVENTION 
     The amplitude modulation In-Band-On-Channel (AM IBOC) Digital Audio Broadcasting (DAB) system, similar to the hybrid frequency modulation (FM) IBOC system, uses an orthogonal frequency division multiplexing (OFDM) scheme. In the OFDM scheme, a number of sub-carriers are modulated by a digital signal and then multiplexed together such that, in the time domain, adjacent symbols do not interfere with each other. To further minimize the inter-symbol interference, a guard period is added at the transmitter between each adjacent symbol-pair. Using a cyclic prefix or suffix in this guard period, one can determine OFDM frame boundaries. A combination of 32-QAM (quadrature amplitude modulation) and BPSK (binary phase shift keying) has been proposed as an approach for this system. 
     By way of an example of the use of a cyclic prefix or suffix, suppose that the useful symbol period contains 512 samples denoted as x 0 , x 1 , . . . , x 511 . A signal {y n } may be constructed in the following manner:          y   n     =     {               x     n   +   498       ,     0   ≤   n   &lt;   14                                  x     n   -   14       ,     14   ≤   n   &lt;   526                                x     n   -   526       ,     526   ≤   n   &lt;   540                                      
     In other words, the first 14 samples of {y n } are equal in amplitude and phase to the last 14 samples of the useful symbol period. They are said to constitute the cyclic prefix. Similarly, the last 14 samples of {y n } are the same as the first 14 samples of the useful symbol period. They are said to constitute the cyclic suffix. 
     However, in the AM IBOC DAB system, since each symbol is passed through a pulse shaper for additional signal conditioning, the standard OFDM frame synchronization procedure based on the correlation property of the cyclic prefix does not work. The cause of this problem is as follows. Since the output of a filter is given by the convolution of the input signal with the impulse response of the filter, when the input is passed through a pulse shaping filter, the samples in the guard period are no longer equal to the corresponding samples in the useful symbol period. 
     There is another, more difficult problem with the AM IBOC system. To support a higher data rate of the digital signal, digital data is transmitted over frequencies not only outside of the analog host bandwidth, but also in the bandwidth occupied by the host. The host bandwidth is 10 kHz (kilohertz). The digital signal bandwidth is 20 kHz. The signal in the outer digital lobes is about 5 dB (decibels) higher than the signal under the host. To minimize the interference from the digital signal to the analog AM, the level of the digital signal in the ±5 kHz frequency range must be about 20 dB below the analog signal. Also, to be able to recover the digital signal in the ±5 kHz in the presence of the stronger analog signal, it is necessary for the digital and analog signals to be in quadrature with respect to each other. Since the latter requirement should be met for any arbitrary data pattern, BPSK should be used for the OFDM sub-carriers under the analog host. 
     Therefore, there is a need for methods and apparatus for performing frame synchronization in a DAB system, such as the AM IBOC DAB system, which do not rely on the use of the cyclic prefix or suffix, and which eliminate or at least reduce the effects associated with the shortcomings of the prior art as discussed above and which otherwise exist in the art. 
     SUMMARY OF THE INVENTION 
     The present invention provides a robust OFDM frame synchronization procedure that does not depend on the use of the cyclic prefix or suffix. The procedure is based on sending, in each symbol period, a known data pattern, for example, 1, over two OFDM sub-carriers, for example, k and −k. In one exemplary embodiment, 1≦|k|≦36. It is to be appreciated that k may be selected arbitrarily, however, k is selected such that the sub-carrier frequency is within the host spectrum and computations become simpler. Also, more than two sub-carriers can be used. That is, the known pattern may be inserted into more than two sub-carriers and transmitted. 
     In any case, in a preferred embodiment, the two sub-carriers are modulated using BPSK. To obtain frame synchronization, we search the demodulated signal for the known data pattern by taking the Fast Fourier Transform (FFT) of the received signal at sub-carriers k and −k. Synchronization is achieved when the same pattern is detected in both sub-carriers. Once synchronization is achieved, a phase-locked loop is used to track the OFDM frames. If synchronization is lost as indicated by the phase-locked loop, we return to the search mode. 
     Advantageously, the procedure works in the presence of a wide range of additive white Gaussian noise (AWGN) and interference from the analog host. For example, synchronization may be achieved when the SNR (signal-to-noise ratio) is 23 dB or more and host interference is 20 dB above the digital signal. 
     Although the procedure is particularly well-suited for sub-carriers in the above range, it can be used with any sub-carrier over the entire ±10 kHz range and other data patterns. Similarly, the frame synchronization algorithm of the invention works for higher analog AM signal levels as well. 
    
    
     These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating an exemplary IBOC AM receiver for use in accordance with the present invention; 
     FIG. 2 is a flow diagram illustrating a frame synchronization procedure according to an embodiment of the present invention; 
     FIG. 3 is a block diagram illustrating a phase locked loop circuit according to an embodiment of the present invention; 
     FIG. 4 is a graphical representation illustrating the time to achieving OFDM frame synchronization according to the present invention; 
     FIG. 5 is a graphical representation illustrating the probability of correct synchronization for different values of a signal-to-noise ratio according to the present invention; 
     FIG. 6 is a graphical representation illustrating the probability distribution for achieving OFDM frame synchronization according to the present invention; and 
     FIG. 7 is a block diagram illustrating an exemplary transmitter for use in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description will illustrate the invention using an exemplary AM IBOC DAB system. It should be understood, however, that the invention is not limited to use with this particular DAB system. The invention is instead more generally applicable to any DAB system in which it is desirable to provide a frame synchronization procedure in which a cyclic prefix is not necessarily relied upon. 
     Referring initially to FIG. 1, a block diagram illustrating an exemplary IBOC AM receiver is shown. The receiver  100  includes: an antenna  102 ; a radio frequency (RF) amplifier, down converter and filter stage  104 ; analog-to-digital (A/D) converters  106  (in-phase or I component of baseband signal) and  107  (quadrature or Q component of baseband signal), a Fast Fourier Transform (FFT) and symbol decoder stage  108 ; an OFDM frame synchronization and timing stage  110 ; a clock generator  112 ; a forward error correction and de-interleaver stage  114 ; and an audio decoder  116 . A description of the overall operation of the elements of the receiver  100  will be given below. 
     First, it is to be noted that the frame synchronization procedure implemented by the receiver is based on sending from a corresponding transmitter, in each symbol period, a known data pattern over two OFDM sub-carriers, for example, k and −k. In one embodiment, for example, the data pattern may simply be a value of 1. However, any suitable data pattern may be used. Also, in one exemplary embodiment, 1≦|k|≧36. The two sub-carriers are modulated using BPSK. The insertion of the data pattern, in each symbol period, in accordance with the two OFDM sub-carriers k and −k may be accomplished in a conventional manner. Given the inventive teachings described herein, one of ordinary skill in the art will appreciate various manners of implementing the data pattern insertion and sub-carrier modulation operations in a suitable OFDM transmitter. Nonetheless, an example of a transmitter for use with the invention is illustrated in FIG.  7  and will be described following the description of the receiver. 
     We turn now to the description of the overall operation of the elements of the receiver  100  depicted in FIG.  1 . The RF signal received at antenna  102  is amplified, down-converted and filtered in block  104  to obtain the in-phase (I) and quadrature (Q) components of a baseband signal. These I and Q components are sampled by A/D converters  106  and  107 , respectively, and fed into decoder  108  and also into the OFDM frame synchronization and timing circuit  110 . The latter uses a frame synchronization algorithm, in accordance with the present invention, to generate a frame sync output that points to the sample number identifying the start of an OFDM frame. This frame sync output is used in block  108  to remove the samples of the guard period. The remaining samples of the useful symbol period are converted into the frequency domain by FFT. The output of the FFT engine is demodulated to recover the digital data that was used at the transmitter to modulate the sub-carriers. The output of block  108  is applied to the input of block  114  which performs error correction, if necessary, and de-interleaving. The resulting output is fed into the audio decoder. The output of the OFDM frame synchronization and timing circuit  110  is also used in timing circuit  112  to provide a synchronized time base for use at different points in the receiver. 
     It is to be appreciated that the frame synchronization methodology of the invention may be employed in the OFDM frame synchronization unit  110  of the receiver  100 . A detailed description of an embodiment of the frame synchronization methodology of the invention will now follow. 
     Referring now to FIG. 2, a flow diagram illustrating a frame synchronization procedure  200  according to an exemplary embodiment of the present invention is shown. In step  202 , the method includes obtaining samples of both the in-phase (I) and quadrature (Q) components of the baseband signal. Such samples of the I and Q components of the received baseband signal are taken in the A/D converters  106  and  107  (FIG.  1 ), respectively. The frame synchronization unit  110  (FIG. 1) reads the samples from the A/D converters. Preferably, NN samples x 0 , . . . ,x NN-1  of the baseband I and Q components are read where NN may be, e.g.,540. That is, 540 I samples and 540 Q samples may be read, such that N=512, as explained below, and NN=540. 
     In step  204 , the samples are saved in a FIFO associated with the frame synchronization unit  110 . Preferably, the FIFO has the capacity to store samples associated with the latest 17 OFDM frames and is thus referred to as a 17-deep FIFO. 
     Next, in step  206 , the received baseband signal, stored as samples in the 17-deep FIFO, is averaged on a sample-by-sample basis. That is, the I component samples are averaged and the Q component samples are averaged. The averaged samples are then stored in another FIFO, preferable a 2-deep FIFO, called rx_buf. Also, an index i is set equal to 0. 
     In step  208 , starting from i, where i is initially set to zero, the method includes taking complex samples x 1 , . . . , x i+N−1  of rx_buf and high-pass filtering the I components of this signal using a high pass filter associated with the frame synchronization unit  110  having a cutoff frequency of preferably 4400 Hz. In other words, staring from i, we pass the real part of N samples (N=512) of rx_buf through a high pass filter with a cutoff frequency of 4400 Hz. 
     In step  210 , we take the filtered I components and unfiltered Q components of N samples from rx_buf and compute the complex FFT at sub-carrier k using the following relation:          F        (   k   )       =       ∑     n   =   0       N   -   1                         x   n                   -   j2Π                     kn   /   N         .                                
     Recall that OFDM sub-carrier k is modulated by the desired digital signal that is being used for OFDM frame synchronization. 
     Next, we examine the sign of the imaginary part of F(k), or Im(F(k)). If the sign of Im(F(k)) is negative, we increment i by 1. If i&lt;NN, return to step  208 , else, we go to step  212 . If the sign of Im(F(k)) is positive, we compute Im(F(k))/ |Re(F(k))|, where Re(F(k)) is the real part of F(k), and save it in position i of an N-element array, for example, rx_mod  1 . We increment i by 1. If i&lt;NN, return to step  208 , else, we go to step  212 . 
     The same sub-steps of step  210  described above are repeated for sub-carrier −k. In other words, we take the FFT of the same samples as above, but now with k replaced by −k. The method then includes examining the sign of the imaginary part of F(−k), or Im(F(−k)). If Im(F(−k)) is negative, increment i by 1. If i&lt;NN, return to step  208 , else, we go to step  212 . If the sign of Im(F(−k)) is positive, we compute Im(F(−k))/ |Re(F(−k))|, where Re(F(−k)) is the real part of F(−k), save it in position i of an N-element array, e.g., rx_mod  2 . We increment i by 1. If i&lt;NN, return to step  208 , else, we go to step  212 . Notice that OFDM sub-carrier −k is modulated by the same desired digital signal that is being used for OFDM frame synchronization. 
     When i=N, the method includes searching array rx_mod  1  and determining index kk 1  where the array has the maximum value. Similarly, we determine kk 2  such that rx_mod  2  is a maximum value at this index. This is accomplished in step  212 . If kk 1 =kk 2 , this sample number is taken to be the frame boundary, in step  214 . This frame boundary is indicated by sample number p n . If kk 1 ≠kk 2 , the next NN samples of the received data are read into the 17-deep FIFO (steps  202  and  204 ) and the entire procedure is repeated. 
     In step  216 , the sample p n , indicative of the frame boundary, is applied to the input of a phase-locked loop associated with the frame synchronization and timing stage  110  (FIG.  1 ), as shown in FIG.  3 . It is to be appreciated that a purpose of this loop is to generate a steady sync pulse q n , in the presence of channel impairments and receiver timing that is in synchronism with the transmitter. 
     Referring now to FIG. 3, a block diagram illustrating a phase locked loop circuit  300  according to an embodiment of the present invention is shown. The phase-locked loop  300  works in the following way. The estimated frame boundary, p n , which is actually a pointer, is compared in a comparator  302  with the output q n  of a block  312 , and the difference is saved in a 36-deep FIFO  303 . Depending on this difference, the loop gain is adjusted in each symbol period. The differences in the pointer values saved in the FIFO are averaged over the latest 36 frames and passed through filter  304 , integrated in modulo integrator  306 , and amplified in block  310 , whose gain is adjusted dynamically. The output of the amplifier ±m is rounded off to the nearest integer, and added algebraically in block  312  to its past output q n−1  generated via delay unit  314 , to generate the current loop output q n . For fast acquisition, the initial value q 0  of q n  is set to p 0 . Further, a voltage controlled oscillator (VCO)  308  is provided which is phase-locked to the transmitter, and provides timing to various points in the receiver, e.g., A/D converter, de-interleaver, symbol decoder, etc. 
     In an experiment to evaluate a frame synchronization procedure of the invention, the following performance parameters were measured: (i) the time it takes to achieve synchronization for different values of SNR in the presence of an analog host; (ii) the probability of correct synchronization as a functions of SNR when the analog host is present; and (iii) the probability distribution function of the exact synchronization. 
     FIG. 4 shows the time it takes to achieve OFDM frame synchronization in the presence of an AM host as a function of SNR. This time is given in terms of the number of OFDM frames. The experiment was conducted in the presence of varying amounts of noise with the AM host signal strength set to 20 dB above the digital signal. The noise is white Gaussian. The SNR was measured by averaging the signal and noise power over the entire ±20 kHz bandwidth of the digital signal. A digital one value was transmitted in all OFDM frames over sub-carriers k=1 and −1, using BPSK satisfying the conditions that were discussed above in the background section. For SNR of 25 dB or more, synchronization is achieved within about 12 frames, and takes longer with lower values of SNR. 
     FIG. 5 shows the probability of correct synchronization for different values of SNR. The AM host signal level was held constant at 20 dB above the digital signal. It should be emphasized here that perfect synchronization is achieved for SNR of 23 dB or more. 
     Synchronization is not perfect for smaller values of SNR. If the SNR is, for example, 20 dB, perfect synchronization is achieved with a probability of about 0.83. In other words, at this SNR, an occasional error would be made in synchronization. Nevertheless, in most cases, synchronization would be correct within 1 or 2 samples. This is shown in the probability distribution function of FIG.  6 . Here, the probability that correct synchronization is achieved within ±n frames is plotted as a function of n. For FIG. 6, the SNR is 10 dB and the host signal strength is 20 dB above the digital signal. Notice that even with 10 dB SNR, the synchronization is off by no more than 2 samples. It is to be appreciated that the frame synchronization methodology of the invention works for higher analog AM signal levels as well. 
     Referring now to FIG. 7, a block diagram illustrating an exemplary transmitter for use in accordance with the present invention is shown. As mentioned, the frame synchronization procedure implemented by the receiver is preferably based on sending from a corresponding transmitter, in each symbol period, a known data pattern over two OFDM sub-carriers, for example, k and −k. The two sub-carriers are modulated using BPSK. FIG. 7 illustrates one exemplary OFDM transmitter for implementing the data pattern insertion and sub-carrier modulation operations. 
     As shown in FIG. 7, audio signal to be transmitted is input to a perceptual audio coder (PAC)  702  where it is digitally encoded. The output of the PAC is then block coded in a block coder  704 . Then, in multiplexer  706 , the encoded audio data is multiplexed with synchronization and other control data. It is to be appreciated that this is where the synchronization data pattern used by the receiver, according to the invention, is preferably inserted. The output of the multiplexer is encoded in accordance with an error-correcting convolutional code in a channel coder  708 . To mitigate the effects of Rayleigh fades and clustered errors that are characteristic of this channel, the data bits of the channel coder output are re-ordered in an interleaver  710  according to some rules, and mapped into symbols, in accordance with a symbol map  712 , which then modulate a number of carriers of an OFDM system including, for example, k and −k. The output of the map  712  is subjected to an Inverse Fast Fourier Transform (IFFT) in block  714 . It is to be appreciated that the symbol map block and the IFFT block comprise the OFDM transmission system. Then, as is well known in the art, blocks  716  through  734  comprise IF and RF sections with antenna  736 . 
     It should be noted that the elements of the receiver  100  (FIG. 1) and, in particular, the frame synchronization and timing stage  110 , (and the elements of the transmitter  700  (FIG.  7 )) may be implemented using a central processing unit, microprocessor, application-specific integrated circuit, digital signal processor or other data processing device in a computer or an audio receiver (transmitter). The central processing unit, microprocessor, application-specific integrated circuit, digital signal processor, or other data processing device may also have memory associated therewith for storing data and results associated with each element&#39;s function when necessary. The invention may be utilized in conjunction with numerous types of audio processing or transmission systems. 
     Although illustrative embodiments of the present invention have been described herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be affected therein by one skilled in the art without departing from the scope or spirit of the invention.