Abstract:
A method and apparatus for improving the transmission of an information signal having message information. A received information signal is formed from an information signal transmitted through an information channel. The received information signal has signal noise. The message information is removed from the received information signal to provide a symbol sequence that is then applied to a first filter to provide a first filtered signal. The power of the first filtered signal is determined and applied to a second filter to provide a second filtered signal representative of the signal noise of the received information signal.

Description:
BACKGROUND OF THE INVENTION 
     I. Field of the Invention 
     This invention relates to the field of communications systems and, in particular, to the transmission of message signals in a communications system. 
     II. Description of the Prior Art 
     In mobile radio communication systems, signals containing message information are transmitted for reception by a plurality of receivers. The signals are transmitted by way of communication channels wherein fading can occur. The fading in the communications channels can cause interference with the received signal and can degrade the received signal, thereby causing message information in the signal to be lost. Furthermore, other transmitting sources of varying power level usually exist and create noise at a receiver. Examples of other noise sources include signals from the same transmitter, signals from other transmitters or signals from different devices such as electric motors, televisions or compact disk players. 
     Successful recovery of transmitted information is a function of the ratio of the power of the received signal containing the information to the power of the received noise. An indication of the amount of noise that occurs during reception can therefore significantly improve the recovery of information from a received signal. For example, a Turbo decoder which relies on an accurate knowledge of the noise power at the receiver, can be used to improve recovery of information from a received signal. An indication of the amount of noise that occurs during reception can also be used to control the transmit power of the signal, so as to maintain the received signal to noise power at an appropriate level. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for improving the transmission of an information signal having message information. A received information signal is formed from an information signal transmitted through an information channel. The received information signal has signal noise. The message information is removed from the received information signal to provide a symbol sequence that is then applied to a first filter to provide a first filtered signal. The power of the first filtered signal is determined and applied to a second filter to provide a second filtered signal representative of the signal noise of the received information signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features, objects, and advantages of the present invention will become more apparent form the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify corresponding elements throughout and wherein: 
     FIG. 1 shows a graphical representation of the spectral density of a Doppler spectrum; 
     FIG. 2 shows a graphical representation of the spectral density of known symbols within a fading communications channel; 
     FIG. 3 shows a block diagram representation of the noise estimator system of the present invention; 
     FIG. 4 shows a block diagram representation of one preferred embodiment of a high pass filter suitable for use in the noise estimator system of FIG. 3; 
     FIG. 5 shows a block diagram representation of a further preferred embodiment of a high pass filter suitable for use in the noise estimator system of FIG. 3; and 
     FIG. 6 shows a block diagram representation of a further preferred embodiment of a high pass filter suitable for use in the noise estimator system of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to FIG. 1, there is shown graphical representation  10  of the spectral density of a Doppler spectrum. The spectral density of graphical representation  10  is well known to those skilled in the art. The Doppler spectrum of graphical representation  10  has a Doppler frequency of f d . 
     When a coherently modulated signal carrying information is transmitted through an information channel and received in the absence of fading conditions, it is possible for the receiver to demodulate the signal into a sequence of samples s i  of the form: 
     
       
           s   i   =Aa   i   +n   i   (1) 
       
     
     wherein A is a constant or relatively slow varying complex number, n i  is a real or complex noise term of constant or slow varying variance σ 2 , and a i  is a real or complex term that contains the transmitted information. For example, if the transmitted signal were BPSK modulated it would be possible to demodulate the received signal to obtain the sampled signal shown in equation (1) with a i =+1 or −1, depending on the transmitted information. a i  is usually referred to in the art as a point of the constellation associated with the modulation scheme. 
     If the transmitted information (a i ) is already known to the receiver or can be determined using a decoding process, then it is a simple task to remove the information from s i  of equation (1) by a rotation combined with a scaling and obtain equation (2) below: 
     
       
           s   i   ′=A+n   i   (2) 
       
     
     In the theoretical case wherein there is no fading within the transmission channel, the spectrum of s i ′ is a centered Dirac corresponding to f d  equal to zero, surrounded by a constant spectrum of noise due to the term n i  However, in most practical cases there is fading in transmission channels carrying a signal with message information. When fading is present in this manner the value of A varies with time and has the band limited power spectral density illustrated by graphical representation  10 . The time varying value of A can be represented as A i  and equation (2) can therefore be represented as s i   ′=A   i+n   i . Furthermore, note that the power σ 2  of n i  also varies with time, but usually relatively slowly. The spectrum of s i ′ resulting from the time varying A i  is depicted in representation  20  of FIG.  2 . 
     In order to obtain an estimate of the level of interference σ 2  of a received signal, the method of the present invention uses a priori knowledge of the message information (a i ) present in the received signal to derive equation (2) from equation (1). The a priori knowledge can be obtained using a pilot signal or a training sequence. Additionally, it can be obtained by decoding the received signal or by any other technique known by those skilled in the art. Using the a priori knowledge of the information in the received signal, the information (a i ) is removed from the received signal. This provides a signal (s i ′) that is substantially without any information content and therefore representative of the fading conditions and noise of the channel through which the received signal transmitted. In a particular embodiment of the present invention discussed below, it will be shown that complete knowledge of the message information (a i ) is not required as equation (3) can be derived from equation (1) with limited a priori knowledge. 
     Referring now to FIG. 2, there is shown graphical representation  20  of a typical spectral density of the sequence s i ′ within a fading transmission channel. Regardless of the amount of fading in the transmission channel carrying the received signal, a low pass band limited spectrum  24  is present within sequence s i ′ as shown in graphical representation  20 . Thus, spectrum  26  of graphical representation  20  corresponds to A i  and spectrum  24  corresponds to the noise n i . Therefore, the spectrum of sequence s i ′ can be characterized as a noise floor combined with a low pass band limited Doppler spectrum. 
     Referring now to FIG. 3, there is shown a block diagram representation of noise estimator system  50  of the present invention. Noise estimator system  50  receives the sequence s i ′ and provides an estimate of the noise power σ 2  (also called variance) that is present in the channel that transmits the sequence s i . The input sequence s i ′ of noise estimator system  50  is first received by high pass filter and sampler  52 . High pass filter and sampler  52  eliminates the effect of A i  within the sequence s i ′ and preferably re-samples the sequence at a lower rate. 
     The samples at the output of high pass filter and sampler  52  are designated as the signal u i . The average power of the signal u i  is determined by obtaining its norm raised to the power N within norm operator block  54 , with N being any real number different from zero. In addition to being a norm to the power N operator, block  54  can be any other type of operator that removes the sign of a symbol and provides a value that is directly related to the power of its input signal. The output of operator block  54  is applied to low pass filter and sampler  56 . The samples provided at the output of low pass filter and sampler  56  are representative of the variance (σ 2 ) of the noise (n i ) of the received signal. 
     In the preferred embodiment of noise estimator system  50 , high pass filter and sampler  52  can be realized as a finite impulse response filter with taps [1, −1]. This is a straightforward way to implement the required function because it requires only the subtraction of two consecutive symbols of the sequence s i ′. 
     Referring now to FIG. 4, there is shown a preferred embodiment of high pass filter and sampler  52  within noise estimator system  50 . The input sequence s i ′ of estimator system  50  is received by low pass filter  62  within filter and sampler  52 . The filtered output of low pass filter  62  is applied to difference device  64 . Difference device  64  computes the difference between the filtered and unfiltered values of the sequence s i . The result of the computation performed by difference device  64  is therefore the desired signal u i . When this embodiment is used the low pass filter can be non-causal. In this case, it is necessary to delay the unfiltered symbols s i ′ before computing the difference. The delay operation can be performed by delay block  74  located between the input sequence s i ′ and difference device  64 . 
     Referring now to FIG. 5, there is shown another preferred embodiment of high pass filter and sampler  52  within noise estimator system  50 . If the input sequence s i ′ of noise estimator system  50  is a stream of pilot symbols, it is possible to use channel estimation filter  72  as a low pass filter and thereby eliminate the need for low pass filter  62 . In this case the desired output of noise estimator system  50  is the difference between the symbols s i ′ and the output of channel estimation filter  72 . When this embodiment is used the channel estimation filter can be non-causal. In this case, it is necessary to delay the unfiltered symbols s i ′ before computing the difference. The delay operation can be performed by delay block  74  located between the input sequence s i ′ and difference device  64 . The channel estimation filter will usually also be used to perform coherent demodulation of the transmitted data. Additionally, in this embodiment it may be desirable to follow difference operator  64  by an additional high pass filter in order to remove any bias introduced by an incorrect channel estimation filter  72  in a particular situation. 
     Both embodiments depicted in FIGS. 4 and 5 can be directly preceded or followed by a down sampler which will reduce the necessary computation rate. 
     In a further alternative embodiment, the signal s i  of equation (1) can be transformed with even less a priori knowledge into a signal s″ shown in equation (3) below: 
     
       
           S   i   ″=B   i   +n   i   (3) 
       
     
     where B is an unknown and possibly time varying complex number of known phase α. The knowledge of α can be derived from a channel estimation filter or any other means known in the art. It is then possible to replace high pass filter  52  of FIG. 3 by quadrature operator  80  of FIG. 6, which returns a measure of the component of s i ″ that is not co-linear (perpendicular) with the complex vector e j*α , where j is the imaginary number defined by sqrt(−1). It will be understood by those skilled in the art that quadrature operator  80  can be implemented by simply projecting s i ″ onto vector e −j*α and returning the imaginary part of the resulting projection. Quadrature operator  80  may also be implemented by any other way known by those skilled in the art. 
     When practicing the present invention in a code division multiple access (CDMA) mobile radio communication environment, the different elements of all of the embodiments can be located anywhere between the finger level of the receiver and the output of the RAKE combiner. For example, the high pass filter and the norm or norm to the N operator can be implemented on a per finger basis and the output of all of the fingers can be combined before insertion into a common low pass filter. 
     The previous description of the preferred embodiments is provided to enable a person skilled in the art to make and use the present invention. The various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments without the use of the inventive faculty. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed.