Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-09-30
2001-04-17
Olms, Douglas W. (Department: 2732)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S328000
Reexamination Certificate
active
06219345
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates in general to time domain multiple access (TDMA) mobile communication systems.
(2) Description of Related Art
One standard for mobile communication is the global system for mobile communication (GSM) which is a widely used form of TDMA digital wireless telephony technology. Being a mobile system, power consumption is of primary concern since batteries are heavy and expensive. Further, if power drain is high, use of the mobile telephone will be limited by battery life. Clearly, it is desirable to minimize power consumption in mobile communication applications.
One way of limiting power drain is by minimizing the amount of time the receiver section is powered especially since transmissions in TDMA based systems are received in spaced bursts of information. As will be appreciated, timely application of power to the receiver section of the mobile unit is highly dependent on remaining closely synchronized with the transmission rate of the base station. Such synchronization is even more critical as the mobile unit roams from one base station to another. Accordingly, GSM systems rely on extracting the timing information required for synchronization with the base station during an active call from the burst. Mobile needs to obtain this information with accuracy of ⅛ th of a bit to ensure synchronization with the base station.
Traditionally, a software implemented finite impulse response (FIR) filter is used to locate the position of such signal. Typically, the FIR filter will have a large number of taps. e.g., ninety-six or more taps in order to ensure the necessary accuracy in recovering the timing information with desired accuracy. However, it will be appreciated that software filtering is computationally demanding in terms of compute time, system resources (for example, each tap coefficient must be stored together with intermediate calculated values) and power drain.
By way of example, in a ninety-six tap FIR filter, the filtering process will perform 96 multiply and accumulate operations on every sample. And since the signal must be filtered in real time, the processor must be fast enough to fully execute the filter process. It being well known that power consumption increases as the operating rate of the processor increases since power is proportional to switching activity.
Although it is possible to improve timing resolution by over-sampling the burst signal and rapidly processing the signal within the GSM frame rate, it will be appreciated that processors operating at a clock rate sufficient to process will have high power consumption.
SUMMARY OF THE INVENTION
To overcome the limitations in the related art described above, and to overcome other limitations that will become apparent upon reading and understanding this specification, the present invention discloses an improved system and method for increasing the timing accuracy of a time domain multiple access (TDMA) mobile communication system. In TDMA systems correct operation depends on proper timing of the transmission and reception of signal bursts. Accordingly, transmissions between a base station and a plurality of mobile units must compensate for transmission delay so as to avoid collision of bursts. The mobile in GSM is required to obtain the timing with a ⅛ th bit period accuracy. Since sampling and processing at such a high rate introduces excessive power consumption and cost, the next best thing is to obtain such accuracy via interpolation. The most common method of interpolation for obtaining high accuracy is filtering. To increase timing accuracy, the present invention employs a resource efficient parabolic interpolator that performs local curve fitting to refine the location of an embedded peak after digitization of the burst signal. In the context of the present invention, we can use a local interpolation due to the fact that a coarse peak location is known and the fact that a TDMA system is band-limited.
With the present invention, the burst signal is digitized, stored and an embedded peak in a mid-amble of the burst is located. Specifically, the largest sample obtained from a correlation between received mid-amble with a copy of mid-amble residing in the receiver is selected together with two adjacent samples. The largest sample is designated as a coarse approximation of the embedded peak. The first of the two adjacent samples is merely the immediately preceding sample and the second is the immediately following sample.
A second order polynomial is then fitted to these three samples and the sample values substituted in to a single expression for evaluation to determine the time offset of a refined peak relative to the coarse peak. Since the present invention minimizes the number of processing steps necessary to locate the refined peak, a relatively slow processor may be used.
Advantageously, compute time and power consumption are significantly reduced since the plurality of multiply and accumulate operations necessary to locate the refined peak using a FIR filter are replaced by a single divide and a relatively insignificant number of addition and multiply steps. Further, system resources are minimized since there is no need to store filter coefficients or intermediate calculations.
Once the time offset of the refined peak relative to the coarse peak is known, the offset may be used to position the peak in one of a plurality of bins defined between the coarse peak and each of the adjacent samples. In this manner, the refined peak is more precisely located and system-level timing of the mobile unit more accurately synchronized with the base station.
These and other advantages of the present invention not specifically described above will become clear within the detailed discussion herein.
REFERENCES:
patent: 4602375 (1986-07-01), Inukai
patent: 5276706 (1994-01-01), Critchlow
patent: 5553064 (1996-09-01), Paff et al.
patent: 5898684 (1999-04-01), Currivan
Clark Ricke W.
Komaili Jaleh
Conexant Systems Inc.
Howrey Simon Arnold & White , LLP
Olms Douglas W.
Pizarro Ricardo M.
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