Power in composite signal sidebands is used to enhance signal-to-noise ratio in carrier tracking loop, thereby reducing radio loss and decreasing probability of receiver error. By adding quadrature channel to phase-lock-loop detector circuit of receiver, dc component can be fed back into carrier tracking loop

Lindsey, W. C.

Simon, M. K.

English

The characteristics of a data-aided carrier tracking loop are discussed. The loop signal to noise ratio is improved in a phase-locked loop used for tracking a carrier an angle modulated communications system by a quadrature channel added to the phase locked loop. A d.c. signal derived from the quadrature channel is added to the signal fed back to the voltage controlled oscillator in the otherwise conventional phase-locked loop

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Data-aided carrier tracking loops

Auust 1973	 B73-10356 
•	 NASA TECH BRIEF NASA Pasadena Office 	 & 
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. 
Data-Aided Carrier Tracking Loops 
fl
The problem: 
To improve the performance of phase-lock-loop 
telemetry receivers which must acquire and track 
very weak, noisy signals. 
To Subcarrier and 
r, 
Received 
Signal - 
T5 reciprocal of data rate	 S(t)=referencesubcarrier in subcarrier 
d(t) = estimate of output data	 tracking loop (square wave) 
The solution: 
Use the power in the composite signal sidebands 
to enhance the signal-to-noise ratio in the carrier 
tracking loop, thereby reducing the radio loss (noisy 
reference loss) and decreasing the probability of error 
of the receiver. 
How it's done: 
The subcarrier components of power in the received 
signal, being centered around the subcarrier fre-
quency, are rejected by conventional phase-lock-loop 
receivers and hence are not available for improving 
the carrier tracking performance. On the other hand, 
the signal-to-noise ratio in the carrier tracking loop 
can be considerably enhanced if the power in the 
composite signal sidebands can be recovered. Thus,
by adding a quadrature channel to the phase-lock-
loop detector circuit of a receiver, there can be fed 
back a dc component into the carrier tracking loop 
VCO that is proportional to the power in the mod-
ulated data subcarriers. 
The proposed modification is based on the principle 
of decision-directed feedback. For example, when the 
received signal is a carrier modulated by a single 
biphase-modulated data subcarrier, an estimate is 
formed of the biphase-modulated subcarrier term 
which, when fed back to the carrier tracking loop, 
can be used to recover the power in the sideband 
components for carrier tracking purposes. 
The standard phase-locked loop used in present-
day (deep space) telemetry receivers is depicted in 
the block diagram. The proposed modification makes 
use of a quadrature channel to insert into the VCO a 
dc component with power proportional to the biphase-
modulated data subcarrier. The shape of the equiv-
alent loop S-curve remains essentially unaffected by 
the addition of the quadrature channel; its amplitude, 
however, is increased in proportion to the power con-
tained in the signal's sidebands, the phase jitter in 
the subcarrier tracking loop, and the conditional 
probability of error in the data detector. In general, 
the conditional error probability depends both upon 
the subcarrier and the rf phase-errors, and hence the 
exact solution to the problem requires a two-dimen-
sional iteration. However, cursory examination of 
these two degradations indicates that they are small 
relative to the radio loss caused by the carrier-track-
ing-loop phase error, and hence essentially all of the 
sideband power can be recovered and used for im-
proving this principal source of degradation. 
(continued,-overleaf) 
This document was prepared under the sponsorship of the National 	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States 	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States 	 will be free from privately owned rights.
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For the simple case discussed, very little additional 
equipment and complexity need be incorporated into 
current receiver configurations. In situations where 
several data and/or sync-modulated subcarriers are 
phase modulated onto a main carrier, an n-dimen-
sional extension of this technique can be applied. The 
power in any of the subcarrier components can, in 
principle, be recovered as above. Also, cross modula-
tion losses can be recovered and used for carrier 
tracking purposes. For example, several decision-
directed subcarrier-data demodulators are employed 
in parallel. Each of these requires an input-delay 
element proportional to the reciprocal of the data in 
that channel. In order to form an estimate of the 
cross modulation component of interest, the outputs 
of the channels involved must be delayed by the 
complementary delay before multiplication. The delay 
in the carrier loop is then the sum of the delays cor-
responding to the reciprocals of the channel data ratio. 
References: 
1. Lindsey, W. C.: Optimal Design of One-Way and 
Two-Way Coherent Communication Links. IEEE 
Transactions on Communications Technology, Vol. 
COM-14, No. 4, August, 1966, p. 418.
2. Lindsey, W. C.: Determination of Modulation In- 
dexes and Design of Two-Channel Coherent Com-
munication Systems. IEEE Transactions on Com-
munication Technology, Vol. COM-15, No. 2, April 
1967, p. 229. 
3. Lindsey, W. C.: Design of Block-Coded Communi-
cation Systems. IEEE Transactions on Communi-
cation Technology, Vol. COM-15, No. 4 August 
1967, p. 524. 
Patent status: 
This invention has been patented by NASA (U.S. 
Patent No. 3,710,261). Inquiries concerning nonex-
clusive or exclusive license for its commercial devel-
opment should be addressed to: 
NASA Patent Counsel 
Mail Code 1 
NASA Pasadena Office 
4800 Oak Grove Drive 
Pasadena, California 91103 
Source: William C. Lindsey and Marvin K. Simon of 
-	 Caltech/JPL 
under contract to 
NASA Pasadena Office 
(NPO-11282)
. 
S 
B73-10356	 Category 01


Data-aided carrier tracking loops

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