Abstract:
Methods and apparatus are provided for processing a signal received on a channel. The intersymbol interference on the channel is initially evaluated and a detection method is selected from a plurality of available detection methods based on the intersymbol interference evaluation. For example, the plurality of available detection methods may include a differential detection technique and an equalization-based technique. The equalization technique may be, for example, a decision feedback equalization technique. The intersymbol interference evaluation may comprise, for example, a comparison of the signal to noise ratio at an output of a differential detector for at least a portion of a frame to a predefined threshold.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates generally to digital communication receivers, and more particularly, to techniques for addressing Intersymbol Interference in such digital communication receivers.  
       BACKGROUND OF THE INVENTION  
       [0002]     Differential detection techniques are used in many receivers, such as Personal Handy Phone System (PHS) digital receiver systems. The performance of such differential detection techniques, however, is limited by Intersymbol Interference (ISI) from the operating environment. For wireless channels that exhibit strong ISI, equalization based techniques, such as Decision Feedback Equalization (DFE) techniques, are used instead to improve system performance. For channels where there is only mild ISI, however, equalization is unnecessarily computationally intensive.  
         [0003]     In fact, for those channels where the ISI is mild, the performance of the equalizer based receiver is not improved by the equalization technique (relative to a differential detection technique) and the performance might even be degraded. In addition, an equalization based receiver is generally more computationally intensive, usually requiring more MIPS (million instructions per second) for the more complex signal processing algorithms, and hence consumes more power (thereby shortening the battery life of the receiver).  
         [0004]     While the above-described equalization and differential detection methods each perform in a satisfactory manner under appropriate conditions, a need exists for a detection method that demonstrates improved performance in any communication environment and is more resilient to ISI without unnecessarily consuming additional power.  
       SUMMARY OF THE INVENTION  
       [0005]     Generally, methods and apparatus are provided for processing a signal received on a channel. The intersymbol interference on the channel is initially evaluated and a detection method is selected from a plurality of available detection methods based on the intersymbol interference evaluation. For example, the plurality of available detection methods may include a differential detection technique and an equalization-based technique. The equalization technique may be, for example, a decision feedback equalization technique. The intersymbol interference evaluation may comprise, for example, a comparison of the signal to noise ratio at an output of a differential detector for at least a portion of a frame to a predefined threshold.  
         [0006]     A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]      FIG. 1  is a schematic block diagram of a receiver incorporating features of the present invention;  
         [0008]      FIG. 2  is a schematic block diagram illustrating the differential detection subsystem of  FIG. 1  in further detail;  
         [0009]      FIG. 3  is a schematic block diagram illustrating the equalization subsystem of  FIG. 1  in further detail; and  
         [0010]      FIG. 4  is a flow chart describing an exemplary implementation of the methods of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0011]     The present invention recognizes that a differential detection based receiver can work very well for the channels where the ISI is mild, while an equalization based receiver system can attain high performance for the channels with serious ISI. According to one aspect of the present invention, a receiver selectively applies either the differential detection or equalization technique, based on ISI conditions. In this manner, the appropriate detection technique is applied based on channel conditions, without unnecessarily consuming battery power.  
         [0012]     Generally, the present invention characterizes the operating environment into at least two categories based on an assessment of ISI. A first operating environment category is characterized by a mild ISI environment, where the differential detection based receiver performs well, thereby consuming less power. A second operating environment category is characterized by a more severe ISI environment, where the equalizer based receiver would provide improved performance (and warrants the increased battery consumption). The differential detection based receiver or the equalization based receiver is then selected based upon the operating environment that has been identified. The disclosed approach enables a receiver to take advantage of both the differential detection and equalization approaches, as needed, and hence can significantly improve and overcome the issue of jitter in PHS receiver system performance while reducing power consumption.  
         [0013]     As previously indicated, the present invention first identifies the operating environment, based on the associated ISI. Thereafter, the differential detection based receiver or the equalization based receiver is enabled based upon the operating environment assessment.  FIG. 1  is a schematic block diagram of a receiver  100  incorporating features of the present invention. As shown in  FIG. 1 , the received RF signal is first demodulated into I and Q baseband components by a demodulator  110 . These I and Q signals are then each sampled by respective Analog to Digital Converters (ADC)  120 -I,  120 -Q with a sampling rate equal to twice the symbol rate. The resulting digitized baseband signals I k , Q k  are fed to a carrier detection measurement procedure  130  and an ISI measurement procedure  140 , in a well-known manner.  
         [0014]     For a more detailed discussion of the demodulation, sampling and carrier detection aspects of a conventional receiver, see, for example, Theodore Rappaport, Wireless Communications: Principles and Practice (2001), incorporated by reference herein.  
         [0015]     As shown in  FIG. 1 , the exemplary receiver  100  includes a switch  145  that selectively enables a differential detection based receiver subsystem  200 , discussed further below in conjunction with  FIG. 2 , to further process the signals; or enables an equalization based receiver subsystem  300 , discussed further below in conjunction with  FIG. 3 , for further processing. As previously indicated, the switch  145  enables the appropriate detection method  200 ,  300 , based on the ISI measurement performed at stage  140 , relative to a specified threshold. Following the selected detection method  200 ,  300 , de-encryption, de-scrambling, CRC check and bit packing are performed at stage  150 .  
         [0016]     In one exemplary implementation, the ISI is evaluated at stage  140  by applying the differential detector  200  on some portion of the frame, such as the preamble and a unique word having predefined values. The signal to noise ratio (SNR) at the output of this detector  200  can be compared to a threshold to provide a reliable indicator of whether the differential detector  200  is adequate for detection. An SNR above the threshold indicates a low level of ISI, for which equalization is not required, while a SNR below the threshold indicates a high level of ISI for which equalization will give a significant improvement in performance, justifying the extra signal processing power.  
         [0017]      FIG. 2  is a schematic block diagram illustrating the differential detection subsystem  200  of  FIG. 1  in further detail. As shown in  FIG. 2 , the input signals are further over sampled by a factor of 4 via interpolation at stage  210  and the resulting samples are then applied to a synchronizer  220  where the optimal symbol rate sampling phase is found (in order to synchronize the received frame). The signals are then down sampled at stage  230  back to the symbol rate at this optimal phase. Thereafter, differential detection and de-mapping are performed at state  240  to generate the bit streams which will be further processed with de-encryption (if necessary), de-scrambling, CRC and bit packing (stage  150 ,  FIG. 1 ) to get final results.  
         [0018]     For a more detailed discussion of suitable differential detection techniques, see, for example, Theodore Rappaport, Wireless Communications: Principles and Practice, Ch. 6 (2001), or U.S. patent application Ser. No. ______, filed contemporaneously herewith, entitled “Method and Apparatus for Compensation of Doppler Induced Carrier Frequency Offset in a Digital Receiver System,” incorporated by reference herein.  
         [0019]     It is noted that the synchronizer  220  is a correlation calculator for each phase of the symbol, plus a trace-back check to find the real optimal phase information. The reason for use of the trace-back check is that although the point of maximal value of the correlation among all phases is an effective estimate of the optimal phase, this estimate can have error due to the impairments of the operating environment. In order to get the refined estimate of the optimal phase, the trace-back check is carried out on the neighbors of the maximal point.  
         [0020]      FIG. 3  is a schematic block diagram illustrating the equalization subsystem  300  of  FIG. 1  in further detail. The equalization subsystem  300  may be implemented as a Decision Feedback Equalizer or a linear feedforward equalizer.  
         [0021]     Generally, as shown in  FIG. 3 , the digitized baseband signals I k , Q k  are fed to a feed-forward FIR  310 . The N complex coefficients of the feed-forward FIR are initialized by a central-tap algorithm which is based on the information provided by the carrier detector  130  of  FIG. 1 , while the M complex coefficients of a feed-back FIR  330  are set to values of zero. The learning algorithm for the exemplary DFE  300  is least mean square (LMS)-based using a block-multiple-epoch adaptive update approach.  
         [0022]     It is noted that for rapid training on a short ideal reference sequence for PHS, it is important to do the multiple updates in a block, rather than on a symbol by symbol basis. The ideal reference training epoch is comprised of the corresponding preamble (PR) and unique word (UW) which are specified in the PHS STD-28 standard (Version 3.3). It is further noted that even though PR is not normally used for training, being a periodic sequence, a short portion of PR can be used effectively to increase the training in the case of a very short training sequence, as found in PHS. After the ideal reference training procedure is completed, the DFE  300  enters the data-directed training phase and the output of the slicer  320  is fed to the de-mapping module  340  to get the bit streams which will be processed by the following modules of de-encryption, de-scrambling, CRC check and bit packing (stage  150 ,  FIG. 1 ).  
         [0023]      FIG. 4  is a flow chart describing an exemplary implementation of the methods of the present invention. As shown in  FIG. 4 , the process  400  is initiated during step  410  upon receipt of a frame, n. Thereafter, the received RF signal r(t) is transformed during step  420  into baseband I(t) and Q(t) signals by the demodulator  110 . The I and Q signals are then each sampled by respective Analog to Digital Converters (ADC)  120 -I,  120 -Q during step  430  to obtain digitized baseband signals I k , Q k .  
         [0024]     Carrier detection is performed at step  440  and a test is performed during step  450  to determine if the carrier frequency has appeared. If it is determined during step  450  that the carrier frequency has not appeared, then program control returns to step  440 .  
         [0025]     Once it is determined during step  450  that the carrier frequency has appeared, then the ISI is measured during step  455  (at stage  140  of  FIG. 1 ). A test is performed during step  460  to determine if the measured ISI is greater than a predefined threshold. If it is determined during step  460  that the measured ISI is greater than a predefined threshold, then the equalization based subsystem  300  is selected for detection during step  465 . If, however, it is determined during step  460  that the measured ISI is not greater than the predefined threshold, then the differential detection based subsystem  200  is selected for detection during step  470 .  
         [0026]     Following detection during step  465  or  470 , de-encryption, de-scrambling, CRC check and bit packing are performed during step  480  (using stage  150 ,  FIG. 1 ), before the frame is incremented during step  490 , for processing of the next frame.  
         [0027]     In this manner, the present invention improves performance and reduced power consumption of a PHS receiver system, or another receiver, by efficiently selecting an appropriate detection method based on channel conditions. In addition, the present invention eliminates the jitter that occurs with current PHS handset receiver implementations.  
         [0028]     It is to be understood that the embodiments and variations shown and described herein are merely illustrative of the principles of this invention and that various modifications may be implemented by those skilled in the art without departing from the scope and spirit of the invention.