Soft output metrics generation for symbol detectors

#1. Field of the Invention

#The present invention is directed to a method for generating and calculating reliability information with the use of a symbol detector used in a receiver for a digital communication system.

#2. Description of the Related Art

#FIG. 1 is a simplified block diagram of a communication system with a receiver having a Soft Output Detector;

#FIG. 2 shows Gray mapping constellations for different groupings of bits into symbols;

#FIG. 3 is a trellis chart for a 4-DPSK modulation;

#The present invention provides a method for calculating and generating reliability information used to detect symbols transmitted over communication channels. The reliability information is calculated using the samples received by the receiver and a priori information. Multiplicative distortion introduced by the channel on the transmitted symbols is defined as the channel state information (CSI). It will be shown herein that in a communication system for which the CSI is known, the generation of the reliability information by an SOD reduces to a symbol by symbol calculation. Thus, for a communication system in which symbol sequences x, are generated by a constant envelope modulator and are transmitted and propagated through a channel having characteristic h and noise n and where such transmitted symbols are received by a receiver having Na receiving devices, the reliability information for the ith bit at the mth time interval is expressed in the following equations: Ω m , i ⁡ ( b ) ≅ max x m ∈ X ⁡ ( d m , i ) ⁢ { ∑ a = 0 N a - 1 ⁢ ⁢ ( Re ⁢ { y a , m ⁢ h a , m * ⁢ x m * } σ a 2 ) + ∑ l = 0 N - 1 ⁢ ⁢ ( ln ⁢ ⁢ P ⁡ ( d m , l ) ) } ( i ) Ω m , i ⁡ ( b ) = ln ⁢ ∑ x m ∈ X ⁡ ( d m , i ) ⁢ ( e ∑ a = 0 N a - 1 ⁢ ⁢ ( Re ⁢ { y a , m ⁢ h a , m * ⁢ x m * } σ a 2 ) + ∑ l = 0 N - 1 ⁢ ⁢ ( ln ⁢ ⁢ P ⁡ ( d m , l ) ) ) ( ii ) Ω m , i ⁡ ( b ) ≅ max x _ m - 1 m ∈ X _ m - 1 m ⁡ ( d m , i ) ⁢ { ∑ a = 0 N a - 1 ⁢ ⁢ ∑ k = m - 1 m ⁢ ⁢ ( Re ⁢ { y a , k ⁢ h a , k * ⁢ x k * } σ a 2 ) + ∑ l = 0 N - 1 ⁢ ⁢ ( ln ⁢ ⁢ P ⁡ ( d m , l ) ) } ( iii ) Ω m , i ⁡ ( b ) = ln ⁢ ∑ x _ ⁢ m - 1 m ∈ X _ m - 1 m ⁡ ( d m , i ) ⁢ ⁢ ( e ∑ a = 0 N a - 1 ⁢ ⁢ ∑ k = m - 1 m ⁢ ( Re ⁢ { y a , m ⁢ h a , m * ⁢ x m * } σ a 2 ) + ∑ l = 0 N - 1 ⁢ ⁢ ( ln ⁢ ⁢ P ⁡ ( d m , l ) ) ) ( iv )

#where a indicates a particular antenna and m indicates a particular signaling interval.

#Equations (i) and (ii) are the algorithms for generating reliability information for a communication system in which the transmitted symbols xi are generated with the use of a memoryless modulator (e.g., PSK modulator). A memoryless modulator, such as a PSK modulator, generates symbol xm at time m where the generation of symbol xm does not depend on any previously generated symbol. In contrast, a modulation scheme such as a DPSK scheme introduces memory in the generation of a symbol. In particular, the transmitted symbol xm is based on at least the previously generated symbol xm−1. Equations (iii) and (iv) are the algorithms for generating reliability information for a communication system with a modulation scheme that introduces memory to the transmitted symbols. The algorithms are interpreted as transformations from the space of observations Y to the space of probabilities P. The additive noise sequence n is modeled as a sequence of complex variables that follow Gaussian statistics with zero mean and variance 2σa2, and the modulation scheme is a constant envelope scheme.