Detector operates in conjunction with radio frequency portion of receiver to detect digital signals transmitted in known modulation formats. Signal is constructed by assigning known and distinct modulation waveforms to sequence of message symbols. It reconstructs transmitted digital sequence with minimum probability that any reconstructed digit will be in error

Painter, J. H.

English

NASA Technical Reports Server

September 1974	 B74-10137 
*	 &
NASA TECH BRIEF 
•	 Langley Research Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Anti-Multipath Digital Signal Detector 
A digital signal detector was developed which operates 
in conjunction with the radio frequency portion of a 
receiver to detect digital signals transmitted in known 
modulation formats. The transmitted signal is con-
structed by assigning known and distinct modulation 
waveforms to a sequence of message symbols, or 
digits. The basic digital message, which the detector is 
to reconstruct, consists of a sequence of digits, each 
lasting for a fixed time and each picked from an 
alphabet of arbitrary, but fixed, size. It reconstructs the 
transmitted digital sequence with a minimum probability 
that any reconstructed digit will be in error. 
The detector does not operate on the received signal 
continuously in time. Rather, operation is on samples 
of the received signal, taken at time intervals spaced 
uniformly within the basic digit interval. The number 
of data samples taken during detection of one message 
digit Is a design variable, restricted only by the speed
of the detector digital logic. Each data sample is encoded 
into a digital format, suitable to the detector logic, by 
the analog-to-digital converter. The detector operates 
on the samples recursively, requiring storage of no more 
than one data sample at any given time. 
Internally, the detector produces estimates of the 
perturbations of the received signal which were caused 
by the multipath. The estimates are used to improve 
detection' of the transmitted digital symbol. It is a 
multipath canceling device which makes maximum use 
of digital computation, which markedly reduces physical 
size and complexity over analog, nonsampled.data 
versions, or nonrecursive digital versions. 
Digital data from an analog-to-digital converter is 
routed to paralleled inputs of Kalman filters, as shown 
in the IllUstration. This data may be In vector form, 
resulting from in-phase and quadrature demodulation 
of the signal carrier. Innovations processes and their 
Data
Input 
Vector
Kalman 
Filters 
First 
- - - - —i'a,iaticesance  
Innovations 
Processes,
I	 Probability 
I	 Computers 1---------
First
Probabilities 
_...................1/1
Second 
_
Second
Last	 I	 Last 
eL 
1. Multiplicative Process Statistics 
2. Additive Noise Statistics 
3. Digital Signal Waveforms 
4. Doppler and Timing References 
Digital Signal Detector
Output 
Decision 
Comparator
(continued overleaf) 
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variances are routed from each filter to a corresponding 
probability computer. There are as many filter-probability 
computer combinations as there are digits in the trans-
mitted alphabet (two for a binary signal, three for a 
ternary signal, etc.). 
The first filter produces (possibly vector) estimates 
of the multipath perturbations of the received signal, 
under the assumption that the modulated signal cor-
responding to the first digit in the alphabet was 
transmitted. Each other filter produces multipath esti-
mates under the differing assumptions that the other 
digits were transmitted. From these estimates and the 
corresponding computed error variances, the innovations 
processes and their variances are formed, as a byproduct 
of the filtering process. These latter quantities are 
routed to each corresponding probability computer. 
The first computer calculates the probability that the 
first digit in the alphabet was transmitted. Each other 
computer calculates the alternative probabilities that the 
other digits in the alphabet were transmitted. These 
calculations are made each time a data sample is 
processed. The probability numbers are updated each 
time a data sample is processed. At the end of the time 
period allotted to the transmitted message digit after 
the last data sample has been processed, the updated 
probability numbers are compared in amplitude in the 
comparator. The message digit which yields the highest 
probability is produced at the output. 
At the beginning of the next message digit period, 
the estimates in the filters which were operating under
the "wrong" assumptions during the previous period 
are reset to equal the last estimates of the "correct" 
filter, as determined by the decision on which digit 
was transmitted. In this manner, the detector proceeds 
to reconstruct each digit in the received message 
sequence. 
Note: 
Requests for further information, including com-
putational algorithmic descriptions of the operations 
performed during each message period by any one of 
the filter-probability computer branches, may be directed 
to:
Technology Utilization Officer 
Langley Research Center 
Mail Stop 139-A 
Hampton, Virginia 23665 
Reference: B74-101 37 
Patent status: 
Inquiries concerning rights for the commercial use of 
this invention should be addressed to: 
Patent Counsel 
Langley Research Center 
Mail Stop 456 
Hampton, Virginia 23665 
Source: John H. Painter
Langley Research Center

(LAR-1 1379)
. 
B74-10137	 CategoiyO2


Anti-multipath digital signal detector

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