Communications and data handling system improves signal to noise ratio when transmission channel is perturbed by noise. Telemetry system consists of airborne source, Gaussian additive noise channel, and ground receiver unit. Advantages of system are given

Miller, W.

Muller, R.

Taylor, T.

Yagelowich, J.

English

NASA Technical Reports Server

August 1971	 Brief 71-10324 
NASA TECH BRIEF 
Goddard Space Flight 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. 
Self-Synchronizing, Bi-Orthogonal Coded PCM Telemetry System 
A communications and data handling system em-
ploys a self-synchronizing, bi-orthogonal binary code 
that improves the signal-to-noise ratio when the trans-
mission channel is perturbed by noise. The coded 
Modifying I	 I 
Code 
Generator Noise 
S L+NI
Bi-Orthogonal 
Code 
Generator
The ground receiver unit consists of a digit con- 
ditioner, a storage unit, a set of cross-correlators, 
a maximum likelihood detector, and a word syn- 
chronizer. The digit conditioner input consists of 
Digit 
Digit	 Clock Word 
Conditioner	 ?	 • Synchronizer 
Analog I	 I Word	 Maximum Clock	 Correlation 
I I Storage I	 _______ Level 
Serial 
Data	 I	 I	 Delayed (Uncoded)	 I	 Data	 2N
rrelators	 Likelihood Detector 
itt ••• ti 
	
I	 I	 Stored Codes 
Communications i Source	 Channet —'-	 Receiver 
telemetry system for long distance communication 
consists of an airborne source, a gaussian additive-
noisr, channel, and a ground receiver unit (see fig.). 
The airborne unit receives a random binary signal 
from the aircraft data handling subsystem and trans-
forms it into a bi-orthogonal code. This code is 
modified by a self-synchronizing code, and the result 
is a binary code which needs only a single com-
munication channel without additional bandwidth 
n word	 hr	 information fnr	  evnni7Itirn i
Detected 
Word 
the coded source digits which have been perturbed by 
channel noise. The output is a stream of periodic 
pulses, which are synchronous with the incoming 
source digits and channel noise for a digit clock 
period. 
Correlating the received signal with stored rep- 
licas of the code set is accomplished by first storing 
the analog level from the digit conditioner for one 
word period. This stored information is then cor- 
•	 ... .. .. . .....	 ..• . 	
related with the stored code replicas (one correlator 
(continued overleaf) 
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is used for each possible source code). The gaussian 
noise perturbs a transmitted code, and the noisy 
signal is cross-correlated with all members of the 
code set. The output of each correlator is normally 
distributed about the noise-free correlation values 
once synchronization is obtained. The maximum 
likelihood detector selects the correlator with the 
maximum correlation level, thereby providing an 
optimum decision for each word-period at word 
synchronization time. 
Word synchronizing is obtained by utilizing the 
self-synchronization property of the source code 
set and the digit clock which generates word pulses. 
Coherence between the derived word pulses and the 
code synchronizing time is obtained by using the 
self-synchronizing property of the code set. In the 
out-of-phase position, the maximum correlation 
level at the correlator outputs can vary from maximum 
to minimum values in the absence of noise. In this 
class of code sets, the set with the lowest average 
correlation in the out-of-phase position has the best 
synchronizing property, since coherence is achieved 
by synchronizing on the maximum word-period 
correlation value occurring at the word synchroniz-
ing time. A word error-detector is incorporated to 
provide a quantitative measure of overall system 
performance. Word errors are detected by comparing
a known source signal, before coding, with the 
detected signal at the output of the maximum like-
lihood detector. 
The advantages achieved by this telemetry system 
as compared to an uncoded system include a 40% 
farther communications capability, with given prob-
ability of bit error and with the same transmitter 
power. 
Note: 
The following documentation may be obtained 
from:
National Technical Information Service 
Springfield, Virginia 22151 
Single document price $3.00 
(or microfiche $0.95) 
Reference: 
NASA-TR-R-292 (N68-36333), Self-
Synchronizing Bi-Orthogonal Coded 
PCM Telemetry System 
Patent status: 
No patent action is contemplated by NASA. 
Source: W. Miller, R. Muller,

T. Taylor, and J. Yagelowich

Goddad Space Flight Center

(G SC- 1 1237)
. 
Brief 71-10324	 Category 02


Self-synchronizing, bi-orthogonal coded PCM telemetry system

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