Patent Publication Number: US-8116804-B2

Title: Method and apparatus for providing biasing criteria for binary decisions for use in wireless communications to enhance protection

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/133,019, filed Jun. 4, 2008, which is a continuation of U.S. patent application Ser. No. 10/369,836, filed Feb. 19, 2003, now U.S. Pat. No. 7,428,425, issued on Sep. 23, 2008, which claims the benefit of U.S. Provisional Application Ser. No. 60/357,947, filed Feb. 19, 2002, all of which are incorporated by reference herein as if fully set forth. 
    
    
     FIELD OF INVENTION 
     This application is related to wireless communications. More particularly the present invention relates to providing biased binary decisions for high speed downlink packets (HSDP). 
     BACKGROUND 
     In HSDP Access (HSDPA), as well as many other communication systems, there are applications where a binary decision is required but wherein the penalty of error is unequal as between the two decisions. 
     For example, HSDPA uses an acknowledge/not acknowledge (ACK/NACK) signal where the user equipment (UE) indicates whether or not a transmitted block of data has been successfully decoded. It has been recognized that it is more injurious to system performance for a NACK message to be incorrectly interpreted as an ACK than for an ACK message to be incorrectly decoded as a NACK. In the latter case, the error would result in a transmission block being unnecessarily retransmitted; which amounts to only a small loss in efficiency. In the former case, the transmitting side would assume success for the previously transmitted block, and would not resend it. This is a catastrophic failure, causing serious system disruption. 
     Several obvious techniques have been recognized to bias the answer in favor of identifying the condition with the NACK. For example, in normal coherent demodulation of binary phase shift keying (BPSK), the output signal is often normalized, e.g., an ideal signal representing 1 is +1.0, while an ideal signal representing 0 would be −1.0 and, in a typical channel, which has impairments, interference and noise, the normalized output may take on any value therebetween. 
     In an unbiased decision, if an output z&gt;0 then the process declares 1, and, if the output z&lt;0 then the process declares the output=0. In a biased decision, if the output z&gt;X then declare 1, otherwise declare 0. X is identified as the threshold value and is selected based on the analysis. If it is desired to favor the output 1, then X will be negative; e.g., a small negative number; e.g., minus 0.1, etc. 
     Employing the above process, normalizing the output and deriving the optimum threshold can be delicate and complex to implement and is subject to degradations due to tolerances in the implementation. 
     SUMMARY 
     A method and apparatus for providing biasing criteria for binary decisions for use in wireless communications to enhance protection is disclosed. 
     In accordance with the present invention a criterion is utilized which makes the decision much simpler and more reliable in application, which criteria is significantly more reliable than the simple X threshold. 
     In accordance with this criteria, for time division duplex (TDD), the criteria is based on the measured signal to interference ratio (SIR), which is derived from the channel estimation process. It can be shown that virtually all instances of error occur when the SIR value is low. 
     Correlation between failures, or near failures of the radio modulation (RM) decoder and errors in the other bits of the transmission is high; therefore, the criterion of the present invention is highly reliable and can be easily implemented. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic representation of apparatus for practicing the novel methods of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a routine utilized to practice the technique of the present invention. In the example shown, which is a downlink example, a device such as a base station (BS 10 ), based on a communication from UE 12 , provided at step S 1 , utilizes this communication to obtain channel estimation, at step S 2 . 
     The BS 10  derives a signal to interference ratio (SIR), from the channel estimation, at step S 3  using well known techniques. At step S 4  BS 10  compares the SIR obtained at step S 3  with a given threshold. 
     The threshold may be determined by employing any one of a variety of different techniques. One technique which may be used is to determine, through a combination of analysis, aided by simulation, and actual testing, with a test set-up. Test signals plus noise are input to the test set-up and output signals are examined. The output signals are measured and the correctness of the binary decision is also determined. This test is repeated over an entire range of useful signals, including very low signals. For each amplitude level, the number of instances of correct decisions and incorrect decisions are stored. Generally, strong signals are associated with very high frequency of correct decisions and weak signals are associated with high instances of incorrect decisions. The threshold is preferably greater than the level of noise taken alone. 
     Since the objective is to avoid one particular type of error; e.g. probability of declaring an ACK, given that a NACK was sent, the threshold is selected i.e., biased, so that, when signals are above the threshold, there is an acceptably low frequency of errors and when signals fall below this threshold there is an unacceptable frequency of occurrence of errors. Since the objective is to avoid false declarations of ACK, any decision that occurs with a signal below the threshold is declared as a NACK. More generally, when the signal is below the threshold the binary decision is thus biased to be the choice that causes the least catastrophic result, even if in error. This technique may be performed either off-line or periodically by the BS 10  (or UE 12 ), as shown by step S 11 . Also, if desired other techniques may be used to obtain a threshold value. 
     If the SIR is less than the threshold, BS 10 , at step S 5 , transmits a NACK signal to UE 12 . UE 12 , at step S 6  receives and detects the NACK signal and, at step S 7  retransmits the temporarily stored communication received by BS 10  at step S 8 . 
     Returning step S 4 , assuming that the SIR is greater than the threshold value, the routine branches to Step S 9  to transmit an ACK signal. The UE 12  at step S 10  receives the ACK signal and clears the communication temporarily stored at step S 1 . 
     It should be understood that the routine shown in  FIG. 1  is equally applicable to uplink wherein the functions performed by the BS 10  and UE 12  are reversed, and it is the BS 10  that transmits and temporarily stores a communication at step S 1 . The UE 12  obtains channel estimation at step S 2 , derives the SIR, at step S 3 , compares the SIR derived with a given threshold, at step S 4 , and transmits a NACK to the BS at step S 5 . BS 10 , receiving the NACK and retransmits the temporarily stored communication at steps S 6  and S 7  respectively, the retransmitted communication being received by the UE 12  at step S 8 . In the event that the SIR is greater than the threshold setting, the UE transmits an ACK condition at S 9 , which is received by the BS 10  at step S 10  whereby the ACK signal causes the BS 10  to clear the temporarily stored communication. 
     It should be understood that the technique of the present invention may also be used simultaneously in both uplink and downlink communications between a UE and a BS.