Abstract:
A message format and associated method for estimating the frequency and timing offset of a reference burst of a random access channel in a time-division multiple access communications system. A signal structure in which the continuous wave segment is split into two parts separated by some portion of the content of the random access channel burst is used to provide more precise frequency estimation. Iteration of separate frequency and timing estimation procedures is used to refine both estimates, as the individual estimation processes are more accurate when there is less error in the other parameter. Multiple hypothesis testing is used, in which more than one initial frequency estimate is carried through the iteration process, with thresholding to identify the best frequency offset. Reliable rejection of false ambiguities is achieved because the degradation in timing estimation, due to large frequency offsets, can be detected with thresholding.

Description:
This application claims benefit of Prov. No. 60/044,753 filed Apr. 21, 1997. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates in general to time-division multiple-access communications and in particular to frequency and timing estimation for burst acquisition of random access channels. 
     BACKGROUND OF THE INVENTION 
     Several communication techniques are well suited for applications where a number of independent users need to share a common band of frequencies without the benefit of an external synchronizing mechanism, such as in time-division multiple access (TDMA) cellular radio and satellite systems. TDMA is a digital radio frequency signaling technique, in which the transmission of a message occupies the communication channel for only a fraction of the total time (i.e., a time slot  22 ), in a periodic fashion, thus allowing other users access to the same spectrum on a time-shared basis. 
     In order to achieve this sharing of the spectrum, TDMA systems rely on tight synchronization of time bases, between transmitter and receiver, to enable communication. To synchronize the stations, each time slot  22  begins with a reference burst. 
     In an ideal system, the receiver knows that a burst always arrives at the beginning of a slot at a known radio frequency (RF) and prepares to detect it at the beginning of each slot. However, due to a multitude of factors such as propagation delay differences among users at different locations, Doppler shift, and oscillator drift, the offset of the arrival time relative to the expected time and the actual radio frequency of the random access channel burst have a wide range of uncertainty. Therefore, a large detection window in both time and frequency must be processed to look for the burst. Current transmissions begin each data frame (i.e., a complete period comprising one time slot from each transmitter) with a preamble containing a synchronization pattern. The receiving system attempts to detect this pattern, by looking for spectral energy in certain frequency bins (i e., a series of adjacent frequency ranges), and then estimates the frequency and timing using interpolation. 
     However, these systems are often inaccurate, and extraneous signals (e.g., noise) can cause the receiver to fail to detect the random access channel burst when present, erroneously detect a burst when none is present, or successfully detect the burst but fail to estimate the correct timing or frequency offset from the expected timing and/or frequency offset, thereby losing data. What is needed, therefore, is an improved method of estimating the frequency and timing offsets of a random access reference burst. 
     SUMMARY OF THE INVENTION 
     The present invention relates to communication systems that use a reference burst to synchronize a receiver with a transmitter. The present invention describes a message format and associated method to improve the accuracy of estimating the frequency and timing offset, during initial burst acquisition of a random access channel, in a time-division multiple access communications system. 
     In accordance with one aspect of the present invention, a novel signal structure is shown in which a continuous wave (CW) segment used primarily for frequency estimation is separated into at least two parts separated by some portion of the content of the random access channel burst to provide more precise frequency estimation. Increased precision in frequency estimation is achieved as a result of having longer periods of time over which to measure the time rate of change of the phase of the CW signal. 
     In accordance with another aspect of the present invention, iteration of separate frequency and timing estimation procedures is used to refine both estimates, as the individual estimation processes are more accurate when there is less error in the other estimate. This method detects the presence of random access channel bursts by correlating the frequency of the continuous wave segment of an incoming data packet with the expected frequency of the burst. This is accomplished with a plurality of filters near the expected frequency, configured to alert the system when a preset threshold is exceeded. The center frequency of the detection filter provides the receiver with a coarse estimate of the unknown frequency offset. A coarse timing estimate is obtained by continuing to monitor the detection filter after the alert and determining the sample point of the peak filter response. The frequency estimation is refined by interpolating the filter response from the detection filter and its neighbors at the acquired sampling time. The timing estimation is refined by correlating the frequency corrected received data against a template representing both the continuous wave and unique word segments. Because the accuracy of the fine timing estimation depends on the residual frequency error and vice versa, the fine timing and fine frequency estimations are performed iteratively. 
     In accordance with yet another aspect of the present invention, multiple hypothesis testing is used, in which more than one initial frequency estimate is carried through the iteration process, with thresholding to identify the best frequency offset. Reliable rejection of false ambiguities is accommodated since the degradation in timing estimation due to large frequency offsets can be easily detected with thresholding. 
     The present invention provides significant increases in the precision of timing and frequency offset estimations, when acquiring random access channels using a reference burst, by separating the continuous wave portion of the message, iterating between frequency and timing estimates, and carrying multiple hypotheses through the iteration process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become more apparent from a detailed consideration of the invention when taken in conjunction with the drawings in which: 
     FIG. 1 is a block diagram of a typical time-division multiple access communication system which relies on the use of a continuous wave reference burst to synchronize communication and is capable of utilizing the present invention; 
     FIG. 2 is a block diagram showing more detail of the detection circuit of FIG. 1 according to the preferred embodiment of the present invention; 
     FIG. 3 is a message diagram showing more detail of the traditional random access channel message format, with contiguous continuous wave preamble, of FIG. 1; 
     FIG. 4 is a message diagram of a random access channel format with a split continuous wave segment according to the preferred embodiment of the present invention; 
     FIG. 5 is an ideal plot of the spectra for the contiguous and split random access channel formats of FIG.  3  and FIG. 4; 
     FIG. 6 is a simulation plot showing click noise spikes and improved frequency estimates of the split random access channel format of FIG. 4; 
     FIG. 7 illustrates the time correlation of unique word with frequency effect as a parameter; and 
     FIG. 8 illustrates a flow chart, which represents a program that can be implemented by the detection hardware of FIG.  2 . 
    
    
     DETAILED DESCRIPTION 
     By the way of example only, the message format and method of the present invention is used to accurately estimate the frequency and timing of an initial burst of a TDMA signal in order to synchronize a transmitter with a receiver. It should be understood, however, that any communication system requiring frequency or timing estimation may alternatively employ the techniques shown herein. Such systems might include other coded communications techniques not traditionally referenced as TDMA. For example, a cordless telephone handset and base having independent clocks could employ the present invention. Generally, however, the techniques of the present invention are best used by frequency divided or time slotted messaging systems employing a transmitter and receiver pair that can benefit from an accurate frequency and timing estimate. 
     Generally, the present invention estimates the frequency and timing of an incoming burst by detecting the burst using coarse filtering, estimating the frequency as the center of the coarse bin in which the burst was detected, estimating the timing by the time of peak coarse detection, and refining the estimates through iteration and interpolation. Gross errors are eliminated by carrying multiple hypotheses and correlating each against a unique word. This iterative multi-hypothesis technique, combined with the unique split continuous wave message format, achieve the improvements in accuracy of frequency and timing estimation of the present invention. 
     Illustrated in FIG. 1 is a typical time-division multiple access communication system capable of implementing the present invention. One or more transmitters  10  sends a message  12  to one or more receivers  14  (e.g., antenna, satellite dish, cable-type decoder, fiber optic detector, electrical circuit, etc.) through a communication channel  16  (e.g., hardwire connection, fiber optic cable, satellite relay, etc.). A portion of the message  12  and a continuous wave preamble  18  are combined into packets  20  and transmitted during a time slot  22  in a frame  24 . The receiver  14  knows approximately what time and at what frequency to recover the message  12  if sent; however, for random access messages, it is not known a priori when a user will request access, so timing and frequency offsets can be significantly larger than would be the case for ongoing continuous communication between the transmitter and receiver. In addition, there is no common clock between the transmitter  10  and the receiver  14 , therefore the receiver  14  must acquire timing and frequency synchronization directly from the incoming data packets  20 . The continuous wave  18  and unique word (UW) (shown in FIGS. 3 and 4) portions of the random access message  20  are reference signals that are used by a detection circuit  26  to perform timing and frequency synchronization. 
     Illustrated in FIG. 2 is a block diagram of the detection circuit  26  according to the preferred embodiment of the present invention. The packets  20  of FIG.1 are captured by the receiver  14  and sent through a filter  30  to eliminate out of band signals. The filtered signal is down converted at a block  32  and fed to an analog to digital converter  34  before going to a burst detector  36 . Well known burst detectors  36  include programmed digital signal processors (DSPs) and/or application specific integrated circuits (ASICs). The burst detector  36  is responsible for determining when a reference burst has occurred and estimating from the sampled burst the frequency, timing, and phase offsets required by the receiving system&#39;s demodulator. The accuracy of these estimates is important to the system&#39;s ability to communicate and is the subject of the present invention. Subsequently, the signal is recovered by a demodulator  37 . 
     Illustrated in FIG. 3 is a traditional reference burst message format with all the symbols of the continuous wave segment  18 , used for frequency estimation, coming contiguously as a preamble at the beginning of the message. The difference between the expected frequency and the received frequency, required to decode the message, can be estimated as the time rate of change in phase shift over the duration of the continuous wave segment  18 . The longer the expected continuous wave segment  18 , the more accurately the frequency offset can be estimated. However, increasing the size of the continuous wave segment  18  leaves less time available to transmit message packets. 
     Illustrated in FIG. 4 is a non-traditional reference burst message format  20 ′, according to the preferred embodiment of the present invention. The continuous wave segment  18  is split, half coming at the beginning and the other half at the end of the message data packet  20 ′. Increased precision in frequency estimation is achieved by the split CW signal format as a result of observing the time rate of change of phase over a longer observation window, even though the total duration of the CW signal itself does not change. The random access channel symbols consist of information symbols and unique words (UW) symbols which are used for the timing estimation. The UW symbols are separated into a few portions instead of grouping them together to achieve better channel estimation. 
     FIG. 5 illustrates the spectra for both the contiguous continuous wave format of FIG.  3  and the split continuous wave format, according to the preferred embodiment of the present invention, of FIG.  4 . The mainlobe peak  50  occurs at the receive frequency to be estimated. The width of the mainlobe  50  is smaller for the split continuous wave spectra  52  than it is for the contiguous continuous wave spectra  54 , resulting in better resolution for the frequency estimation, because the variance is reduced. However, there are additional lobes  56  in the vicinity of the mainlobe  50  for the split continuous wave spectra  52 . In a noisy environment, it is possible that one of these near lobes  56  will have a higher amplitude than the true mainlobe  50 . In order to retain the advantages of the increased resolution and reduced variance and eliminate the potential that the estimator would be confused and pick the wrong lobe, resulting in a biased frequency estimate (click noise spike  60  of FIG.  6 ), a more sophisticated algorithm is used to estimate the frequency and timing of the incoming signal, as illustrated in FIG.  8 . 
     FIG. 6 is a simulation plot showing the improvement in frequency estimates and the click noise spikes  60  caused by ambiguities in the split continuous wave spectrum  52  when the incorrect peak is selected. After the click noise spikes  60  are rejected, according to a method illustrated in FIG. 7, the remaining RMS error  62  is much smaller for the split continuous wave spectra  52  than it is for the contiguous continuous wave spectra  54 . 
     FIG. 7 shows the simulated time correlation value at correct epoch versus frequency error for the burst format in FIG.  4 . From this figure we see that the time correlation will work well if the frequency error is within 40 Hz. To use the multi-hypothesis testing (MHT) processing, there is about 10dB difference of correlation between the two adjacent frequency correction versions. 
     FIG. 8 illustrates a program  70  which may be implemented by the detection circuit  26  of FIG.  2 . Specifically, the burst detector  36  (e.g., a DSP) may use the program  70  to accurately estimate the frequency and timing offsets of a detected burst. Step-wise refinement is used because the initial frequency and timing uncertainties are too large for the demodulator to work effectively. 
     The program  70  detects the presence of a random access channel burst at a block  72  using the continuous wave portion  18  of the data packet  20 . A frequency domain correlation is performed using a plurality of coarse Discrete Fourier Transform filters covering an initial range of frequencies. The filtering function is preferably implemented using a digital signal processor as is well known to persons of ordinary skill in the art. However, the filtering function could also be implemented using discrete filters as is well known. Preferably, the initial range of frequencies encompasses the eventual outcome of the frequency estimation. The filter magnitudes are thresholded using well known constant false alarm rate (CFAR) techniques. An alert occurs whenever a filter magnitude exceeds the CFAR threshold, indicating the potential presence of a reference burst. At a block  74 , the center frequency of the detection filter provides the receiver with a coarse estimate of the unknown frequency offset. At a block  76  a coarse timing estimate is obtained by continuing to monitor the detection filter after the alert and determining the sample point of the peak filter response. 
     At block  78 , the program  70  performs a fine frequency estimation by interpolating the filter response from the detection filter and its neighbors at the acquired sampling time. Known interpolation methods include zero-padded Discrete Fourier Transforms, fractional Discrete Fourier Transforms, and quadratic minimum mean squared error fits. At a block  80 , fine timing estimation is performed by correlating the frequency corrected received data against a template representing both the continuous wave and any unique word or known Barker sequence. Because Barker sequences have excellent cross correlation properties, the response is a narrow peak, allowing precise estimation of the correct sampling time. Since the accuracy of the fine timing estimation  80  depends on the residual frequency error and vice versa, the fine timing  80  and fine frequency  78  estimations may be done iteratively, as decided by a block  82 , to improve performance further. Multiple hypothesis testing (MHT) is used to eliminate the click noise spikes  60  by performing the fine timing estimation step  80  for the maximum amplitude peak and both of its nearest neighbors. Each of the incorrect peaks will result in an uncompensated frequency offset so large that the correlation against the random access channel&#39;s unique word will be poor. In the unlikely event that the true peak is not among the three candidates, all three unique word correlations will be poor and the burst detection will be rejected, because the best correlation does not meet a minimum acceptable threshold or is significantly stronger than the other two correlation magnitudes. Of course, the multiple hypothesis approach could be generalized to N peak candidates to correct all errors. 
     When the block  82  determines the correlation is of sufficient magnitude, the phase can be estimated and passed along to the demodulator at a block  84 . The phase is estimated by taking the known portions of the frequency corrected receive data, namely the continuous wave and unique word segments, along with the fine timing estimate and performing minimum mean squared error techniques. 
     Certain modifications of the present invention have been discussed above. Other modifications will occur to those skilled in the art of the present invention. For example, the ratio and mixture of the continuous wave portion and the data portion of the reference burst could be varied and optimized in numerous ways. For instance, an arrangement comprising half of the continuous wave symbols, followed by half of the random access channel symbols, followed by the other half of the continuous wave symbols, followed by the other half of the random access channel symbols, achieves most of the benefits of increased timing and frequency estimation, but requires less processing complexity. 
     The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.