MXPA00011124A - Method for improving signal reception in a mobile communication unit by using pilot symbols - Google Patents

Abstract

A mobile unit for communicating with a transmitter which transmits signals in data bursts, such data bursts having a plurality of symbols therein including data symbols and a plurality of predetermined pilot symbols. The symbols have a discrete number of possible values. The unit further includes a receiver adapted to receive a data burst of the transmitted signals, memory with the predetermined pilot symbols and a plurality of signals filters (interpolators), a comparator, a processor, and an output. The comparator compares the pilot symbols in a received data burst with the predetermined pilot symbols in the memoryto determine correction factors for the pilot symbols. The processor uses the pilot symbol correction factors and the plurality of signal filters to derive a set of data symbol correction factors for each of the signal filters. The processor also adjusts discrete possible values of the data symbols by the data symbol correction factors and compares those adjusted values with the received data symbols to determine a cumulative error value among adjusted data symbols for each signal filter. The processor uses the data symbols derived by the signal filter having the lowest cumulative error value to demodulate a received data burst.

Description

METHOD FOR IMPROVING SIGNAL RECEPTION IN A UNIT OF MOBILE COMMUNICATION BY USING PILOT SYMBOLS with RELATED APPLICATION Priority is claimed for the provisional patent application of the U.S.A. Serial No. 60 / 085,769, titled "Mobile Unit for Pilot Symbol Assisted Wireless System and Method of Improving Performance Thereof", (Mobile Unit for Wireless System Assisted by Pilot Symbol and Method to Improve Performance), by Ayman Mostafa and Rajaram Ramesh, presented May 15, 1998. BACKGROUND OF THE INVENTION TECHNICAL FIELD The present invention is directed to communication systems, and more particularly to wireless systems assisted by pilot symbols. PREVIOUS TECHNIQUE Signals for wireless systems are subject to varying conditions that can degrade the signal received by the mobile units that use the system. ft For example, a mobile unit can receive the signal from multiple directions (for example, directly from the base unit, and be reflected from very different objects in the ground) with the variable signal sources that are potentially out of phase and from this way they tend to cancel each other to some degree, reducing signal strength. This signal fading, generally known as Rayleigh fading, occurs spatially over the system area, with specific areas that potentially have significant fading that can cause the mobile unit to lose the signal altogether. Furthermore, mobile units are subjected to the Doppler effect as they move in the system. As is known in the art, whenever there is relative movement between the signal source / transmitter and the receiver / receiver of signal, there is a Doppler shift of the frequency components of the received signal. In this way, when the recipient is in a vehicle moving at a speed v, the maximum Doppler shift frequency fd (which occurs when the vehicle moves to or directly away from the signal source) is: fd = v /? Component waves arriving from the front of the vehicle experience a positive Doppler shift (that is, the frequency increases) while those arriving behind the vehicle have a negative displacement © (that is, the frequency decreases). In this way, if a vehicle travels 60 m / hr, at 900 MHz _ (? "0.33 m) the maximum Doppler shift (when the vehicle travels directly to or away from the signal source) is: fd = [60,000 m / hr / 3600 sec / hr] /0.33 m = 50 Hz Of course, a proportional change in frequency, or velocity, will produce a proportional change in fd This frequency shift results in maximum signal strength that is at the shifted frequency instead of the assigned frequency, with signal strength that is significantly less than the Designated frequency (as perceived by the mobile unit in motion) that is demodulated by the mobile unit If the mobile unit also happens to pass through an area in the system subject to significant Rayleigh fading, a significant loss in signal strength may In any case, the net result of these and other factors is that the signal that is transmitted by the transmitter (for example, a cell tower) will be distorted at the same time reaches the receiver (for example cell phone). On a cell phone, for example, this can result in objectionable distortion to the ear, or even a lost signal. In order to take into account this distortion, channel estimates have been used to determine the Signal distortion in known pilot symbols in data bursts and correction factors in other symbols in data bursts, have been interpolated based on the channel estimates in the pilot symbols. As an example, bursts of data have been transmitted in the IS-136 system with 162 symbols, each symbol comprising two bits. In a proposed extension of the IS-136 system, the data bursts of 162 symbols at known, predetermined sites P_ in the data bursts are predetermined, known pilot symbols SPl (where i = 1 to n, n is the number of pilot symbols used). In the proposed extension of the IS-136 system, each symbol contains three bits. As also described below, the correction factors (ie, channel estimates) derived from the pilot symbols can be used to estimate the most probable value for each data symbol in a data burst. That is, the channel estimates derived from the pilot symbols can be interpolated to determine the correction factors in the other symbols (ie data symbols) in the data burst when using a selected interpolator or filter to work better under the conditions that are more likely to be found in the communication unit. In order to provide acceptable performance, this interpolator or The filter essentially needs to be designed to handle the highest possible vehicle speed. For example, an interpolator or filter designed to accept a Doppler effect that is at 60 kph will not provide acceptable performance 'for a vehicle traveling 70 kph to or away from the cell tower). Unfortunately, this has inevitably resulted in the required use of an interpolator or filter that causes degradation in the estimation of symbols that are received under conditions other than those parameters in which the interpolator or filter is designed (eg degradation at low speed of the vehicle) . This degradation can even get worse in areas where very high vehicle speeds have to be anticipated since the interpolator must be designed based on very high anticipated speeds that often will not be found. The present invention is directed to overcome one or more of the previously established problems. SUMMARY OF THE INVENTION In one aspect of the present invention, a mobile communication unit that is subjected to conditions that degrade the reception of a signal is provided. The unit includes a receiver adapted to receive a signal having multiple symbols including predetermined ß-pilot symbols, a processor adapted to demodulate received symbols is based on an interpolator that uses the error in the received pilot symbols and an output adapted to receive demodulated symbols using the interpolator that is best adapted to correctly demodulate selected symbols received under the conditions in which the communication unit is submitted when the symbols that are demodulated, are received. The processor chooses, that interpolator of at least two possible interpolators, which is better adapted to correctly decode the received symbols under the conditions in which the communication unit is subjected when the symbols that are demodulated are received. In a preferred form of this aspect of the present invention, the received signals include encoding for error detection and a decoder for error detection decodes the signal using the two interpolators at least with the processor choosing that interpolator that * detects the decoder that It has the minimum errors as the best adapted interpolator.

In another preferred form of this aspect of the present invention, the select ones of the received symbols comprise less than half of the received symbols and in a preferred form, those selected from the symbols received are from more than one burst of data in the signal. Still in a preferred form of this aspect of the present invention, the mobile communication unit also includes a memory that maintains at least two interpolators. In an alternative, the processor of this preferred form demodulates the selected ones of the symbols received using at least two of the two interpolators and selects that interpolator that has the minimum cumulative error in the demodulated symbols of possible discrete values of the symbols as the best adapted to demodulate the received symbols correctly. In alternative alternative of those preferred form of this aspect of the present invention, the communication unit includes an estimator to determine the conditions in which the communication unit is subjected when the symbols that need to be demodulated are received, and the memory also stores information regarding the conditions under which each of the two interpolators is at least better adapted to demodulate the received symbols correctly. In a preferred form of this other alternative, the estimator is an algorithm for the Doppler shift of the unit, and the information in the memory is the range of Doppler shifts in which each of the two interpolators is at least adapt better to demodulate correctly received symbols. In another preferred form of this aspect of the present invention, the mobile communications unit includes memory storage of a first algorithm to derive any of a plurality of interpolators with. base in conditions! selected in which the communication unit is subjected, when the symbols that are demodulated are received, and also includes a detector to determine the selected conditions in which the communication unit is subjected when the symbols that are demodulated are received, with the processor that uses the selected conditions determined by the detector to derive an interpolator from the algorithm. In a preferred form, a selected condition determined by the detector is the Doppler shift and the first algorithm stored by the memory derives interpolators based on Doppler shift. In another preferred form, the detector is a second algorithm for determining the Doppler shift and the first algorithm stored by the memory derives interpolators based on Doppler shift. In another aspect of the present invention, a mobile unit is provided to communicate with a transmitter that sends signals in bursts of data, these ^ ^ bursts of data have a plurality of symbols, including data symbols and a plurality of predetermined pilot symbols. The symbols have a discrete number of possible values. The unit further includes a receiver adapted to receive a burst of data from the transmitted signals, in memory with the predetermined pilot symbols and a plurality of signal filters, a comparator, a processor and an output. The comparator is adapted to compare the pilot symbols in a burst of received data with the predetermined pilot symbols in the memory to determine channel estimates at the sites of the pilot symbols. The processor is adapted to use the channel estimates of pilot symbols and the plurality of signal filters, to derive a set of data symbol correction factors for each of the signal filters. The processor is adapted to adjust the discrete possible values of the data symbols by the data symbol correction factors and compares the adjusted data symbols with selected data symbols received to determine a cumulative error value among the data symbols received. Select] 3fer each signal filter. The output is adapted to receive symbols demodulated by the processor using the interpolator that has the lowest cumulative error value.

In a preferred form of this aspect of the present invention, the processor is adapted to measure an error such as the difference between a select received data symbol and the nearest discrete adjusted possible value of the data symbols and the cumulative error value. for each signal filter it is the sum of the squares of the absolute values of the error in each selected received data symbol. In another preferred form of this aspect of the present invention, the cumulative error value is determined from selected data symbols received from more than one data burst. In still another aspect of the present invention, a mobile unit is provided to communicate with a transmitter that sends signals. The unit includes a receiver adapted to receive the transmitted signals, memory with a plurality of signal filters, the signal filters are chosen to correct selected Doppler shifts, an estimator for the Doppler shift of the unit and a demodulator that responds to the estimator, to demodulate the transmitted signal as it is received by the receiver using the select of the signal filters that better corrects the estimated Doppler effect.

In a preferred form of this aspect of the present invention, the estimator is an algorithm for estimating the Doppler shift of the unit. In another preferred form of this aspect of the present invention, the estimator is a processor that for each signal filter compares select symbols in the received signal with discrete possible values of the symbols adjusted by each filter, to determine an error figure for each signal filter, the estimated Doppler shift falls within a better Doppler shift range corrected by the signal filter that has the determined figure smaller. In still another aspect of the present invention, there is provided a method for improving the signal reception of a mobile communication unit, including the steps of a) transmitting a signal from a base station with data symbols having discrete possible values and symbols pilot with predetermined values at predetermined sites in the signal, b) receiving the transmitted signal in the mobile communication unit, c) deriving correction factors at the predetermined sites of the pilot symbols in the signal by comparing the symbols as received by the mobile communication unit with the predetermined values of the pilot symbols, d) use a plurality of interpolators, to interpolate corrections for data symbols, based on the correction factors of stage c), e) correct the data symbols using the interpolated corrections of a step d) and compare the received data symbols with the discrete possible values adjusted for the data symbols, to generate a cumulative error value for each interpolator, f) select a set of data symbols that are the closest possible values to the corrected data symbols of the interpolator, with the lowest cumulative error value; and g) output the received signal using the selected data symbols. In a preferred form of this aspect of the present invention, two interpolators are used in step d), an interpolator is specially adapted to interpolate corrections for a communications unit, which travels at high speeds and the other interpolator is specially adapted to interpolate Corrections for a communications unit that travels at low speeds. An objective of the invention is to provide mobile communication units, such as cell phones that provide maximum reliability and the highest signal quality. Another object of the invention is to provide mobile communication units that can demodulate high quality signals using current transmission standards BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic illustration of a _ burst of data in a transmitted signal. Figure 2 is a schematic illustration of a communication system including a mobile communications unit embodying the present invention.

Figure 3 is a schematic illustration of a mobile communications unit, incorporating an alternate embodiment of the present invention, and Figure i is a schematic illustration of a mobile communications unit, incorporating another alternate embodiment of the present invention. THE PREFERRED MODE The preferred embodiments of the present invention are illustrated in the Figures. In the data burst 10 illustrated in Figure 1, five pilot symbols Spl, Sp2, Sp3, Sp_, Sp5 are provided in positions P1; P2, P3, P. and P5 Of course, more or less than five pilot symbols can also be used in a data burst. In each pilot position P_, the pilot symbols Spl can be one or more symbols. As an example, in the system extensions _ 1 IS-136, the number of symbols in the positions P_, P2, P3, P, and P5 are 1 .3, 3, 3, and 3 respectively. As illustrated in Figure 2, the transmitter of the base station 1 transmits signals 16 including data bursts 10 to receivers (e.g., a mobile communication unit 20 such as a cellular telephone) at a selected frequency within a width of band suitable for signals between% transmitter 1 and mobile communication unit 20. In the prior art, the pilot symbols Sp? (where i is 1 to 5 in the example of Figure 1) are used to correct variations in the channel conditions on the burst of data 10. That is, known pilot symbols Spl are provided at known positions within the burst of 10. The communications unit 20 compares the received symbol Rpl received at those known positions P_ with the known symbols Spl and determines an appropriate factor Cp? for each position P_ to correct that signal degradation. For example, if the symbol at P_ is known to have been transmitted as Sp ?, and the communications unit currently receives R_ to P_ Cpl it can be determined during synchronization such that:? | Rp_ - Cpl * Spl | 2 is minimized. Cpl determined for the selected positions Px on the data burst are then used to determine Cpl for all symbol positions on the data burst 10 by & convenient interpolation. Simple linear interpolation is illustrated schematically in Fig. 1. However, interpolation to determine appropriate correction factors C_ for all data symbols S_ between the pilot symbols Sp, is preferably done by use of an appropriate interpolator or Weiner filter as is known in the art, and, as explained in "Designs for Pilot-Symbol-Assisted Burst-Mode Communications with Fading and * Frequency Uncertai? ty" (Designs for Burst-Mode Communications Assisted by Pilot Symbol with Fade and Uncertainty of Frequency), by Wen-yi Kuo and Michael P. Fitz, International Journal of Wireless Information Networks, (International Journal of Wireless Information Networks) Vol. 1, No. 4, 1994, p. 239-252, the entire description of which, here incorporated by reference. . The C_ interpolated at each symbol position on the burst of * 9ths 10, then used to demodulate the symbols in the burst 10. For example, in the symbol 20 of the data burst, a C20 factor is interpolated as discussed above . The appropriate symbol is then determined by calculating which signal S2? 3 results in the minimum error factor or "metric" m20 at position 20 according to the following:, m20 = | R20 - C20 * S2? 3 | 2, where: R20 is the symbol received at position 20; and 'S2? 3 are the possible hypothetical symbols at position 20 (where for example there are four possible discrete values for symbols [ie four possible waveforms, J is 1 to 4], when the modulation is from QPSK [Encryption with Phase Displacement of Square] and there are eight possible symbols [ie, J is 1 to 8], when the modulation is 8-PSK [Encryption with Phase Displacement 8]). a According to a prior art channel estimation form, whatever the discrete J number of symbols S20: gives the lowest m20, it is used as the most probable symbol for position 20, this symbol is used to demodulate the data burst In the preferred embodiment, the present invention also uses Cpl as determined in the P_ positions of the pilot symbols and how they are interpolated between them (C_ in 'each Symbol position). However, it will be understood that the present invention can be used with different methods to determine Cpl at the positions of _ 17 pilot symbol P_ and interpolate C_ for the data symbols, and shall not be limited to the methods for determining Cpl, C_ described herein for purposes of illustration. Stated more simply, according to the preferred embodiment of the present invention. The communication unit 20 includes a receiver 24 and memory 28 that stores not only the information regarding the pilot symbols Spl but also stores multiple filters or interpolators 30, 32, such as Wiener filters. The mobile communication unit 10 also includes a convenient processor 40 for interpolating C_, based on the symbols Rp? received in the pilot positions Pa using each of the Wiener filters or other interpolators 30, 32 stored in the unit memory 28 as described further below. The bearer 44, which may be part * of the processor 40, compares the results of the processed data bursts 10 (as described below), to select the symbols to demodulate in order to provide the best signal quality in the unit 20. The processor 40 also uses these correction factors Ca to estimate the most probable value (of the discrete possible values) for each symbol and also generates an error or "metric" factor m_ such as previously described for each symbol S_ in data burst 10. These error factors are summed to generate a cumulative error value for each filter according to the following: Mr = SmFl, where? = l MF is the cumulative error value for filter F; and mFl is the error factor in the symbol S_ for the filter F, where there are -JJ symbols, where according to the previous discussion: = | R_ - C._ * S_j | , where R_ is the symbol received from position i; and S_j are the hypothetical possible symbols in position i, with J possible discrete values for these symbols. * Comparator 44 compares these cumulative errors to essentially determine which cumulative error value M. is the lowest. The filter 30 or 32 having the lowest cumulative error value MF, is used to determine the appropriate symbols S_ in the data burst 10, and those symbols are demodulated by a demodulator 50 so as to provide a high quality output signal at the output 54, such as a horn in the communication unit 20 (the demodulator 50 can be separated as illustrated in the Figures, or it can be part of the processor 0 , with Fl processor 40 performing demodulation, as indicated by dotted lines 40 in Figures 2 to 4). The calculation of the cumulative metric can be performed at the sites of all the data symbols in the slot or preferably only a subset of data positions in the data slot or burst (for example at approximately 20 data points). In an altered mode, the receiver accumulates the cumulative error through multiple slots and chooses the interpolator with base ^ n this accumulated value. Data symbols are typically protected by some combination of coding with error correction and coding with error detection. In an alternate mode, the demodulated symbols obtained using the multiple interpolators are passed to the decoder for error detection and the symbols with the minimum errors detected (if it is a slot or are multiple slots) lead to the selection of the interpolation filter. As another alternate morbidity shown in Figure 3, instead of each filter 30, 32 being tested for a cumulative error value according to the preferred embodiment previously described, the filters 30, 32 can be stored with information 30a, 32a, that a filter 30 is better suited for use under certain conditions of the communication unit (e.g. low vehicle speeds) and the other filter 32 is better adjusted for use with other conditions (eg high vehicle speeds), and unit 20 also includes a convenient detector 60 ar * determine those conditions. Specifically, filters that better adapt for different ranges of Doppler shift can be stored and the detector 60 can be of a type that detects or estimates Doppler shift in the unit 20 '. Ways to estimate Doppler shift as may be employed with the detector 60 of this embodiment are illustrated in US Patents. Us , - 4, 723, 303 and 5,016,017 and the Taiwanese patent 59862, the complete discussions of which here? they are incorporated comfortably by reference. Another alternative to the embodiment of Figure 3 is illustrated in Figure 4, wherein a convenient algorithm 64 for developing an appropriate filter 30 is stored in the memory 28 of the unit 20", based on selected variables indicative of the condition of unit 20"and that can be determined or estimated, such as Doppler shift. These algorithms are known by those skilled in the art to develop for example Wiener filters, and exemplary algorithms are described in "Designs ßor Pilot-Symbol -Assisted Burst-Mode Communications with Fading and Frequency Uncertainty", (Designs for Burst Mode Communications Assisted by Fading Symbol and Frequency Uncertainty) by Wen -yi Kuo and Michael P. Fitz, International Journal of Wireless Information Networks, Vol. 1, No. 4, 1994, p. 239-252, the entire description of which is here fully incorporated by reference. While other algorithms may require significant memory and processing speeds, these requirements shift somewhat by eliminating the need to store multiple filters and also eliminate the need to process multiple signal times (since with this alternative, a single filter can be generated). and used based on the conditions of the unit, as opposed to testing / comparing the signal against multiple filters as occurs with the preferred embodiment described above). Of course, as processing speeds and memory costs * increase with technology, any disadvantages resulting from these requirements can be effectively eliminated. Still other aspects, objects and advantages of the present invention can be obtained from a study of the specification, the drawings and the appended claims.

It should be understood, however, that the present invention can be used in alternate forms where less than all objects and advantages of the present invention and preferred embodiment will be obtained as described above. *

Claims (8)

1. CLAIMS 1. A mobile communication unit subjected to conditions that degrade the reception of a signal, characterized in that it comprises: a receiver adapted to receive a signal having multiple symbols having a select number of discrete possible values; a memory for storing at least two interpolators; a processor to selectively demodulate the received symbols, using the two interpolators as a minimum, the processor is based to demodulate all received symbols, based on a select one of the two interpolators at least, the selected one of the interpolators has the minimum cumulative error in the demodulated select ones of the received symbols, from the discrete possible values; and an output adapted to receive symbols demodulated by the processor St using the selected ones of the interpolators.
2. 2. The mobile communications unit according to claim 1, characterized in that the selected ones of the received symbols comprise less than half of the received symbols.
3. 3. The mobile communications unit according to claim 2, characterized in that the selected ones of the received symbols are more than one burst of data in the signal.
4. 4. A mobile unit for communicating with a transmitter that sends signals in bursts of data having a plurality of symbols, including data symbols with a discrete number of possible values, characterized in that it comprises: a receiver adapted to receive a burst of signal data transmitted; a memory for storing a plurality of signal filters; a comparator adapted to compare the pilot symbols in a received data burst, with the predetermined pilot symbols in the memory to determine channel estimates for the pilot symbols; a processor adapted to use the pilot symbol channel estimates and the plurality of signal filters to derive a set of data symbol correction factors for each of the signal filters, the processor is also adapted to adjust the possible values discrete of the data symbols by the data symbol correction factors and comparing the adjusted data symbols with selected received data symbols to determine a cumulative error value among the data symbols of selected received by each signal filter, the processor is adapted to measure an error as the difference between a select received data symbol and the most closely set possible value of the data symbols, the cumulative error value for each signal filter in the area of the squares of the absolute values of the error in each selected received data symbol and an output adapted to receive symbols demodulated by the processor using the signal filter having the lowest cumulative error value i *.
5. The mobile unit according to claim 4, characterized in that the cumulative error value is determined from selected data symbols received from more than one data burst.
6. 6. A mobile unit for communicating with a transmitter that sends signals, characterized in that it comprises: a receiver adapted to receive the transmitted signals; memory with a plurality of signal filters, the signal filters are chosen to correct selected Doppler shifts; an estimator for the Doppler shift of the unit; a demodulator that responds to the estimator, to demodulate the transmitted signal as it is received by the receiver, using the select of the signal filters that better corrects the estimated Doppler shift; and an output adapter for receiving the transmitted signal as it is processed by the demodulator.
7. 7. The mobile unit according to claim 15, characterized in that the estimator comprises an algorithm for estimating the Doppler shift of the unit.
8. 8. A mobile communication unit subjected to conditions that degrade the reception of a signal, characterized in that it comprises: a receiver adapted to receive a signal having multiple symbols, including predetermined pilot symbols; a memory that maintains at least two interpolators and the Doppler shift range in which each of at least two interpolators is better adapted to demodulate the received symbols correctly; an algorithm that estimates Doppler shift to which the communication unit is subjected; a processor adapted to demodulate all the received symbols, based on a select one of the at least two interpolators, using errors in the received pilot symbols, the selected of the interpolators is better adapted to correctly demodulate the received symbols for the Doppler shift to which it is submitted to the communication unit, when the symbols that are demodulated are received, as determined by the algorithm; and an output adapted to receive symbols demodulated by the processor using the select of the interpolators. SUMMARY OF THE INVENTION a A mobile unit for communicating with the transmitter, which sends signals in bursts of data, the data bursts have a plurality of symbols including data symbols and a plurality of predetermined pilot symbols. The symbols have a discrete number of possible values. The unit further includes a receiver adapted to receive a burst of data from the transmitted signals, memory with the predetermined pilot symbols and a plurality of signal filters a (interpolators), a comparator, a processor and an output. The comparator compares the pilot symbols in a received data burst with the predetermined pilot symbols in the memory to determine correction factors for the pilot symbols. The processor uses the pilot symbol correction factors and the plurality of signal filters to derive a set of data symbol correction factors for each of the signal filters. The processor also adjusts values to possible discrete data symbols by the data symbol correction factors and compares these adjusted values with the received data symbols to determine a cumulative error value between data symbols adjusted for each signal filter. . The processor uses the data symbols derived by the signal filter that has the lowest cumulative error value to demodulate a burst of received data. to