Pulse or digital communications – Receivers – Interference or noise reduction
Reexamination Certificate
1998-06-09
2002-04-30
Corrielus, Jean (Department: 2631)
Pulse or digital communications
Receivers
Interference or noise reduction
C455S063300
Reexamination Certificate
active
06381290
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention is directed toward communication systems, and more particularly toward pilot symbol assisted wireless systems.
2. Background Art
Signals for wireless systems are subjected to varying conditions which can degrade the signal received by the mobile units using the system.
For example, a mobile unit can receive the signal from multiple directions (e.g., directly from the base unit, and reflected off of many different ground objects), with the varying signal sources potentially being out of phase and thereby tending to cancel each other out to some degree, reducing signal strength. Such signal fading, generally known as Rayleigh fading, occurs spatially over the area of the system, with specific areas potentially having significant fading which could cause the mobile unit to lose the signal entirely.
Still further, mobile units are subject to the Doppler effect as they move about the system. As is known in the art, whenever relative motion exists between the signal source/transmitter and signal recipient/receiver, there is a Doppler shift of the frequency components of the received signal. Thus, where the recipient is in a vehicle moving at a speed v, the maximum Doppler shift frequency f
d
(occurring when the vehicle is moving directly at or directly away from the signal source) is:
f
d
=v
/&lgr;
Component waves arriving from ahead of the vehicle experience a positive Doppler shift (i.e., the frequency increases) while those arriving from behind the vehicle have a negative shift (i.e., the frequency decreases). Thus, if a vehicle is traveling 60 km/hr, at 900 MHz (&lgr;~0.33 m) the maximum Doppler shift (when the vehicle is traveling directly toward or away from the signal source) is:
f
d
=[60,000 m/hr/3600 sec/hr]/0.33 m=50 Hz
Of course, a proportional change in frequency, or speed, would produce a proportional change in f
d
. This shift in frequency results in the maximum signal strength being at the shifted frequency rather than the assigned frequency, with the signal strength being significantly less at the assigned frequency (as perceived by the moving mobile unit) which is demodulated by the mobile unit. If the mobile unit happens to also pass through an area in the system subjected to significant Rayleigh fading, a significant loss in signal strength can accordingly result.
In any event, the net result of these and other factors is that the signal which is transmitted by the transmitter (e.g., a cell tower) will be distorted by the time it reaches the receiver (e.g., cellular telephone). In a cellular telephone, for example, this can result in distortion objectionable to the ear, or even a lost signal.
In order to account for this distortion, channel estimates have been used to determine the signal distortion at known pilot symbols in the data bursts and correction factors at other symbols in the data bursts have been interpolated based on the channel estimates at the pilot symbols. As an example, data bursts 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 predetermined, known locations P
i
in the data bursts are predetermined, known pilot symbols S
Pi
(where i =1 to n, n being the number of pilot symbols used). In the proposed extension of the IS-136 System, each symbol contains three bits.
As also described further below, the correction factors (i.e., channel estimates) derived from the pilot symbols can be used to estimate the most likely value for each data symbol in a data burst. That is, the channel estimates derived from the pilot symbols may be interpolated to determine the correction factors at the other symbols (i.e., data symbols) in the data burst by using an interpolator or filter selected to best work under the conditions most likely to be encountered by the communication unit. In order to provide acceptable performance, such interpolator or filter essentially needs to be designed so as to handle the highest possible vehicle speed. For example, an interpolator or filter designed to accommodate a Doppler effect encountered at 60 kph will not provide acceptable performance for a vehicle traveling at 70 kph toward or away from the cell tower). Unfortunately, this has unavoidably resulted in required use of an interpolator or filter which causes degradation in the estimation of symbols which are received under conditions other than those parameters upon which the interpolator or filter is designed (e.g., degradation at a low vehicle speed). This degradation can be even worse in areas where very high vehicle speeds must be anticipated since the interpolator must be designed based on very high anticipated speeds which oftentimes will not be encountered.
The present invention is directed toward overcoming one or more of the problems set forth above.
SUMMARY OF THE INVENTION
In one aspect of the present invention, a mobile communication unit which is subjected to conditions which degrade the receipt of a signal is provided. The unit includes a receiver adapted to receive a signal having multiple symbols therein including predetermined pilot symbols, a processor adapted to demodulate received symbols based on an interpolator using the error in the received pilot symbols, and an output adapted to receive symbols demodulated using the interpolator which is best adapted to correctly demodulate selected ones of the received symbols under the conditions to which the communication unit is subjected when the symbols being demodulated are received. The processor selects that interpolator from at least two possible interpolators which is best adapted to correctly demodulate the received symbols under the conditions to which the communication unit is subjected when the symbols being demodulated are received.
In a preferred form of this aspect of the present invention, the received signals include error detection coding and an error detection decoder decodes the signal using the at least two interpolators with the processor selecting that interpolator which the decoder detects as having the least errors as the best adapted interpolator.
In another preferred form of this aspect of the present invention, the selected ones of the received symbols comprise less than half of the received symbols and, in one preferred form, the selected ones of the received symbols are from more than one data burst in the signal.
In still another preferred form of this aspect of the present invention, the mobile communication unit also includes memory maintaining at least two interpolators. In one alternate, the processor of this preferred form demodulates the selected ones of the received symbols using all of the at least two interpolators and selects that interpolator which has the least cumulative error in the demodulated symbols from discrete possible values of the symbols as the one best adapted to correctly demodulate the received symbols. In another alternate of this preferred form of this aspect of the present invention, the communication unit includes an estimator for determining the conditions to 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 at least two interpolators is best adapted to correctly demodulate the received symbols. In a preferred form of this other alternate, the estimator is an algorithm estimating the Doppler shift of the unit, and the information in the memory is the range of Doppler shifts at which each of the at least two interpolators is best adapted to correctly demodulate the received symbols.
In another preferred form of this aspect of the present invention, the mobile communication unit includes memory storing a first algorithm for deriving any of a plurality of interpolators based on selected conditions to which the communication unit is subjected when symbols being demodulated are received, and
Mostafa Ayman
Ramesh Rajaram
Coats & Bennett P.L.L.C.
Corrielus Jean
Ericsson Inc.
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