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Channel-independent equalizer device
| Details |
Inventors: Dent, Paul W.;
Assignee: Ericsson Inc. (Research Triangle Park, NC)
Primary Examiner: Tse; Young T.
Assistant Examiner:
Attorney, Agent or Firm: Burns, Doane, Swecker & Mathis
A demodulator for demodulating a signal modulated with digital information symbols so as to extract the information symbols is disclosed. A receiver receives a signal over a communications channel. Samplers and digitizers then produce a sequence of numerical sample values representative of the received signal. Memories are provided each having a number of state memories each associated with a hypothesized symbol string. A controller selectively retrieves values from the memories and controls the timing of operations thereupon. A metric computer computes candidate metrics using a hypothesis of a next of the information symbols to be demodulated made by the controller, one of the numerical sample values, path metric values, B-matrices, and U-vectors and the candidate metrics associated state number selected by the controller from the memories. A best predecessor computer determines the best of the candidate metrics to be selected to be written back into the memory means along with a successor B-matrix, U-vector and path history. The successor B-matrices, U-vectors and path history are then updated using corresponding values associated with the best predecessor and one of numerical sample values. |
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DETAILED DESCRIPTION OF THE DISCLOSURE The present invention is primarily intended for use in cellular communication systems, although it will be understood by those skilled in the art that the present invention can be used in other various communication applications.
A sequence of transmitted symbols is denoted by s1,s2,s3 .
.
.
These symbols can take on binary values, such as .
+-.
1, quaternary values such as .
+-.
1/.
+-.
j, or higher order modulation values.
Complex received samples r1,r2,r3 .
.
.
taken one symbol time apart are assumed to depend linearly on the transmitted symbols through a set of channel echo coefficients c1,c2,c3 .
.
.
cL according to the equation: ##EQU1## This equation can be abbreviated as Rn=Sn.
multidot.
C where suffix n is the first n received samples included in R and likewise the matrix S has n rows and L columns.
The receiver's task is to find the sequence Sn that best explains the received waveform Rn.
In addition, the channel coefficients may not be known by any other means other than by observing the received signal.
Some constraints have to be placed on how rapidly the channel can vary.
For example, the channel cannot be permitted to totally change between one symbol and the next, otherwise for any hypothesized symbol sequence, a set of varying channel coefficients could be found which would explain the received waveform.
Therefore, the channel must be assumed to vary at a rate which is slower than the symbol rate.
A solution for the static channel case will now be described below.
The errors between the expected waveform for a symbol sequence Sn and the received samples Rn are: En=Sn.
multidot.
C-Rn The sumsquare error En'.
multidot.
En=C'Sn'SnC-C'Sn'Rn-Rn'SnC+Rn'Rn where "'" is a conjugate transpose.
For a given sequence Sn, this sumsquare error may be minimized with respect to C by differentiating with respect to each C value and setting equal to 0.
The set of simultaneous equations Sn'SnC=Sn'Rn results, which can be rewritten as C=(Sn'Sn).
sup.
-1 Sn'Rn When this value for C is substituted in the sumsquare error equation, several terms cancel out leaving: En'En=Rn'Rn-Rn'Sn(Sn'Sn)
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Cell Phones 
