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.
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.
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.