This paper discusses a number of tone-aided modulation techniques which have been studied as part of the Mobile Satellite Experiment (MSAT-X) Program. In all instances tone(s) are inserted into data-free portions of the transmit spectrum and used by the receiver to sense the amplitude and frequency/phase distortions introduced by the channel. The receiver then uses this information in a feedforward manner to lessen the effect of the distortions on the data detection performance. Particular techniques discussed are the Tone Calibration Technique (TCT), the Dual Tone Calibrated Technique (DTCT), Transparent Tone-In-Band (TTIB), and Dual-Tone Single Sideband (DTSSB)

Rafferty, William

Saulnier, Gary J.

English

NASA Technical Reports Server

N88-25728
PILOT-AIDED MODULATION FOR NARROW-BAND SATELLITE COMMUNICATIONS
DR. GARY J. SAULNIER, Electrical, Computer and Systems Engineering,
Rensselaer Polytechnic Institute, United States; DR. WILLIAM RAFFERTY,
Mobile Satellite Program, Jet Propulsion Laboratory, United States.
RENSSELAER POLYTECHNIC INSTITUTE
Troy, New York, 12180-3590
United States
ABSTPJ%CT
This paper discusses a number of tone-aided
modulation techniques which have been studied as part
of the Mobile Satellite Experiment (MSAT-X) Program.
In all instances tone(s) are inserted into data-free
portions of the transmit spectrum and used by the
receiver to sense the amplitude and frequency/phase
distortions introduced by the channel. The receiver
then uses this information in a feedforward manner to
lessen the effect of the distortions on the data
detection performance. Particular techniques
discussed are the Tone Calibration Technique (TCT),
the Dual Tone Calibrated Technique (DTCT), Transparent
Tone-In-Band (TTIB) and Dual-Tone Single Sideband
(DTSSB).
INTRODUCTION
The goal of the Mobile Satellite Experiment (MSAT-X) is to develop
concepts and technologies which enable the creation of a mobile
satellite service. To accommodate a large number of users, it has been
decided that each channel will be 5 kHz wide. In addition, these
channels must support a data rate of 4800 bps in order to allow
telephone quality digital voice communication using vocoders of
reasonable complexity. A major challenge, then, is to develop a
modulation format which can provide adequate detection performance for
operation of the digital voice system while maintaining adequately low
signal powers. It has been determined that bit error rates under 10 -3
at a signal-to-noise ratio of I0 dB are needed for this system.
The mobile satellite system will involve communications from
moving vehicles directly to and from the satellite. Consequently, the
system must contend with the presence of reflected multipath signals as
well as Doppler frequency shifts. Due to the narrow bandwidth of the
signals and the desired operation at L-Band, these effects can become
quite severe and some type of compensation must be applied in the
receiver. Highly directional antennas will help reduce some of the
multipath effects but some reflected components are expected in
conjunction with the desired line-of-sight component. A modem for octa-
Phase Shift Keying (8-PSK) using differential detection with a Doppler
correction technique has been developed as part of the MSAT-X program.
This system is discussed in (Simon, 1988).
An alternate method for mitigating the effects of multipath fading
and Doppler frequency shifts is the use of tone aided modulation. A
number of tone aided modulation formats have been investigated in
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https://ntrs.nasa.gov/search.jsp?R=19880016344 2020-03-20T06:52:17+00:00Z
conjunction with the MSAT-X program. The common feature of all these
formats is that one or more tones are inserted into a portion of the
transmit spectrum which does not contain any of the transmit data
signal. These tones may reside at the edges of the data spectrum or in
a frequency null that has been deliberately created within the data
spectrum. In any case, the one or more tones are separated from the
data signal at the receiver and used to determine the channel- and
equipment-induced amplitude and phase/frequency distortions. It is
assumed in this system that, due to the narrow channel bandwidth,
these channel effects will be nearly uniform over the channel and,
therefore, the distortion introduced in the tone(s) is the same as that
introduced in the data signal.
The recovered pilot tone(s) contains the necessary information to
reduce the effects of the channel distortions on the data signal. The
envelope of the recovered pilot tone(s) provides information about the
signal amplitude distortion. In addition, the frequency/phase
modulation of the tone(s) provides an indication of the frequency and
phase distortions. The basic structure of the receiver involves the
recovery of amplitude and frequency/phase distortions from the pilot
tone(s) and the subsequent feedforward processing of this information to
compensate for the distortion effects in the data signal. Figure 1
shows the general receiver structure. The bandpass filter on the input
performs the channel select function. As can be seen in the figure, the
data signal is delayed in a path parallel to the pilot processor in
order to maintain time alignment with the recovered distortion
information obtained from the tone(s). This feedforward structure is
superior to feedback techniques for this application because it allows
the instantaneous "tracking" of fluctuations. In other words, the
information used to correct the data signal at a particular time is
obtained from the tone signal at that same time.
This paper discusses several tone-aided systems and provides
information on the structure of the various receivers. Important
considerations in the evaluation of the viability of a particular tone-
aided format are the complexity of the modulator/demodulator structure,
and the amount of extra bandwidth and power required for the insertion
of the tone(s). The receiver structures presented are all bandpass
(i.e. "IF") implementations, though baseband implementations are
probably better suited to implementation using digital signal
processors. The bandpass implementations are presented here because
they more clearly illustrate the functions performed in each receiver
type.
TONE CALIBRATED TECHNIQUE (TCT)
The TCT system (Davarian, 1984) employs Manchester encoding to
create a null in the center of the data signal spectrum. A single pilot
tone that is in phase coherence with the carrier is inserted into this
null. Figure 2 shows the resulting transmit spectrum.
The TCT receiver, shown in Figure 3, consists of the pilot
bandpass filter (PBPF) for the recovery of the pilot tone, the
calibration system, a coherent demodulator and a Manchester decoder.
The calibration system processes the pilot tone in two paths. The upper
path squares the pilot tone and then lowpass filters the result tD
obtain the square of the pilot amplitude modulation. The lower path
simply delays the pilot tone by an amount equal to the delay of the
330
upper path. Finally, the delayed pilot tone is divided by the square of
the amplitude modulation. The net result is that, if the pilot at the
input to the calibration system has amplitude modulation A(t), the pilot
at the output of the calibration system has amplitude modulation I/A(t).
The pilot processor output is fed to the data demodulator where it is
used to coherently demodulate the data signal. Assuming that the data
signal also has amplitude modulation A(t), the amplitude modulation
terms vanish in the mixing process. Finally a Manchester decoder
recovers the data.
The TCT system has been analyzed and implemented for binary PSK
signalling. Although BPSK doe8 not provide the required 4800 bps
throughput in the 5 kHz channel, the results indicate that the major
contribution of the pilot tone is the removal of the irreducible error
floor associated with coherent receivers in the presence of Rician
fading (Rafferty, 1987).
Despite this important performance advantage, the drawbacks of the
TCT system reduce its value for mobile satellite applications. First,
the use of Manchester coding to create a null for the pilot has the
undesirable by-product of doubling the signalling rate and, therefore,
the transmit bandwidth. As a consequence, the number of signalling
states must be doubled to maintain the same throughput as a non-tone
aided PSK system, causing a loss in detection efficiency. Second, the
width of the spectral null proved to be too narrow for proper operation.
A sufficiently wide null must be used because the pilot tone must be
recovered using a filter which will pass the faded pilot tone when
subjected to the Doppler frequency shifts. Using a pilot recovery
filter with a minimum bandwidth necessary to accommodate the expected
Doppler spread, it was found that unacceptable amounts of data signal
energy passed through the filter and appeared in the recovered pilot
tone. This signal energy resulted in pilot tone amplitude and phase
modulation that was not channel-induced and, as a result, degraded
system performance. Later studies used high pass filters in the
transmitter to increase the width of the null,_resulting in the
introduction of inter-symbol interference (Rafferty, 1985).
DUAL TONE CALIBRATED TECHNIQUE (DTCT)
The problems encountered with the TCT system centered upon the
creation of the spectral null for the pilot tone and the related
bandwidth expansion. As a solution to this problem the DTCT system
(Simon, 1986), which uses two pilot tones, was derived. This system
places one tone on each side of the data signal as shown in Figure 4.
The two tones are used in the receiver to create a coherent reference at
the carrier frequency which, as in the TCT system, is used to demodulate
the data.
Figure 5 is a block diagram of an implementation of the DTCT
system. The input signal is bandlimited to set the channel bandwidth.
The pilot tones are recovered using two pilot bandpass filters, one at
frequency _0+_p and a second at frequency _0-_p. The outputs from the
pilot recovery filters are mixed together to produce a tone at frequency
2_0, i.e. twice the carrier frequency of the data signal. This signal
is then fed to a calibration system similar to the TCT calibration
system which outputs the 2_ 0 signal with amplitude modulation i/A(t).
A tone at frequency _0 is produced using a frequency divider and the
result is used to coherently demodulate the data signal.
331
The main drawbacks of the DTCT system are the extra bandwidth
required for two tones, the increased processing noise in the pilot
recovery system due to the use of two pilots (each recovered pilot
signal contains noise) and the placement of the tones at the band edge
where they are more likely to be subject to distortions. Frequency
uncertainty combined with non-ideal amplitude and phase characteristics
of the receiver front end filters will create some phase distortion in
the pilot tones which, in turn, will affect the efficiency of the
coherent demodulation process.
TRANSPARENT TONE-IN-BAND (TTIB)
The TTIB system (McGeehan, 1984) in many ways combines the best
attributes of the TCT and DTCT systems. Like the TCT system, the TTIB
system utilizes a single pilot tone placed at the center of the transmit
signal where the distortions due to filtering are expected to be
minimal. However, the spectral null for the tone is created by the
somewhat unusual approach of splitting the data spectrum in a single-
sideband fashion and frequency shifting the upper half upwards by _T
and the lower half downward by the same amount. The frequency shift
value, _T, is set to accommodate the expected Doppler spread and allow
for non-zero transition bands of the sideband separation filters.
Figure 6 shows the TTIB spectrum. Like the DTCT system, the data
bandwidth is not expanded by the insertion of the tone, however, the
TTIB data signal appears as two sections.
Figure 7 is a block diagram of a TTIB receiver. The two main
functions performed by the receiver are the pilot processing, which
removes the amplitude and frequency/phase distortions, and the
demodulator which which uses a phase-locked loop (PLL) to obtain an
estimate of _T and, then, recombines the data sidebands.
The pilot processor splits the input signal into two paths, one
path uses a notch filter to remove the pilot tone from the data
sidebands and a second mixes the input signal with a subcarrier at _s
and then uses a bandpass filter to isolate the pilot tone, now at
frequency _s+_T. The pilot calibration system then inverts the
amplitude modulation on the pilot. Finally, the data sidebands are
mixed with the output of the pilot amplitude processor, resulting in the
data sidebands on the subcarrier _s and some higher-order mixer
products.
The output of the pilot processor is the data signal centered at
frequency _s and containing the 2_ T spectral null. At this point, the
amplitude and frequency uncertainty due to Doppler or fading has been
removed through the pilot processing. The next portion of the receiver
is a PLL which is a unique implementation of a Costas-type tracking loop
which recombines the two sidebands by recovering the frequency shift
term, _T- Note that the pilot processing effectively removes any
significant frequency uncertainty at the input to the PLL.
Consequently, a first order PLL is sufficient. Therefore the loop
filter in Figure 7 is simply a gain block. The upper and lower
sidebands of the pilot processor output are isolated by bandpass filters
on the input to the PLL. These sidebands are then mixed to baseband
using a locally generated references at _T+_s and _T-_s, respectively.
The error signal is generated by mixing these two baseband signals
together. The error signal controls the voltage controlled oscillator
(VCO) which generates the estimate of _T" Note that the PLL tracks _T
332
which is a stable frequency value and is not affected by the channel
impairments.
The main disadvantages of the TTIB system are the complexity of
the receiver and the non-ideal operation of the PLL which recovers _T"
It has been found that, in processing the TTIB signal, this PLL contains
a data dependent noise term in the error signal. This unwanted noise
term is due to the single sideband nature of the signals and is
proportional to the Hilbert Transform of the data signal (Saulnier,
1988). These data dependent terms, however, do not prevent the PLL from
acquiring or maintaining lock, but may degrade its performance.
Currently, experiments are being conducted to characterize the loop
performance in AWGN and multipath fading. In particular the degradation
in performance due to the undesired terms in the PLL error signal is
being evaluated.
DUAL-TONE SINGLE SIDEBAND (DTSSB)
The DTSSB system transmits only one sideband of the data signal
and uses two pilot tones, both on the same side of the data sideband, to
provide frequency and amplitude compensation as well as a coherent
reference for demodulation. Like the DTCT and TTIB systems, this
approach does not expand the bandwidth of the data signal. Unlike the
TTIB system, the DTSSB system does not require a PLL for data recovery
and therefore, will not introduce the data dependent noise. However,
the use of two pilots does introduce additional noise in the pilot
processing and, consequently, it is not clear whether the DTSSB system
provides a performance advantage over the TTIB system.
Figure 8 shows the DTSSB spectrum. One pilot tone is placed a
frequency _T from the carrier of the single sideband. The second tone
is placed _T from the other tone, i.e. 2_ T from the carrier. The
receiver recovers both tones and uses the difference frequency to
determine _T and, then, uses _T to regenerate a coherent carrier
reference. The net result is that the single sideband is coherently
demodulated.
Figure 9 is a block diagram of the DTSSB receiver. Two bandpass
filters, one centered at frequency _0 and the other at _0+_T , are used
to isolate the two pilots. One pilot is processed by a calibration
system to replace its envelope with its inverse, i.e. A(t) is replaced
by 1/A(t). The amplitude-processed pilot is then mixed with the other
recovered pilot to produce a frequency term at frequency _0" This _0
term is then mixed with the calibration system output to produce a tone
at the data sideband carrier frequency, _0-_T. Finally, coherent
demodulation is performed to recover the data.
The main concerns with the DTSSB system are the asymmetry of the
transmit signal and the distortion of the phase relationship between the
pilots by the receiver front end filtering. Any error in this phase
relationship will degrade the coherent demodulation process. Also, the
use of two pilots introduces additional noise into the demodulation
process as in the DTCT system. An advantage of the DTSSB system is the
efficient use of the bandwidth by the transmission of only a single data
sideband.
DISCUSSION AND CONCLUSIONS
This paper presents four pilot-aided modulation schemes and
discusses some of the properties of each. The TCT system uses a very
333
simple receiver but suffers from the excessive bandwidth expansion
caused by the use of Manchester encoding. The DTCTsystem uses two
pilots and is more bandwidth efficient than the TCTsystem. The
placement of the tones at the edges of the channel make this technique
susceptible to degradation from non-ideal receiver filtering and, in
particular, non-uniform group delay distortion. The TTIB system is more
bandwidth efficient than the DTCTsystem because it requires only one
pilot tone. The TTIB receiver,however, is muchmore complex than those
for the other techniques due to the need for the PLL. Currently, it has
not been determined if the presence of data dependent terms in the PLL
error signal will seriously degrade performance. Finally, the DTSSB
system provides excellent bandwidth efficiency and uses a simple
receiver structure. However, the asymmetry of the signal spectrum and
the additional noise due to the use of two pilots may degrade the
performance of this system.
REFERENCES
Davarian, F. 1984. High performance communication in mobile
channels. IEEE 34th Vehicular Technoloqy Conference.
McGeehan, J., Bateman, A. 1984. Phase-locked transparent tone-in-band
(TTIB) : a new spectrum configuration particularly suited to the
transmission of data over SSB radio network. IEEE Trans. on Comm.
COM-32: 81-87.
Rafferty, W., Anderson, J.B., Saulnier, G.J., Holm, J.R. 1987.
Laboratory measurements and a theoretical analysis of the TCT
fading channel radio system. IEEE Trans. on Comm. COM-35: 172-
180.
Rafferty, W., Bechtel, L.K., Lay, N.E. 1985. An additional study and
implementation of tone calibrated technique of modulation. Final
report on JPL Contract 957190, Jet Propulsion Laboratory, Pasadena,
CA.
Saulnier, G.J., Hladik, S., Kolb, S., Varghese, A. 1988. A study of
transparent tone-in-band modulation for binary PSK signalling.
(JPL internal interim report on JPL Contract 958050), Jet Propulsion
Laboratory, Pasadena, CA.
Simon, M.K. 1986. Dual pilot tone calibration technique [DTCT]. IEEE
Trans. Vehicular Tech. VT-35: 63-70.
Simon, M.K., Divsalar, D. 1988. Doppler corrected
differential detection of MPSK. To be published in IEEE
Trans. on Comm.
334
PILOT
P_X_SSe_
DETECTION
CIRCUITS
OATA
-- OUT
Fig. 1 General tone-aided receiver
PILOT
_0
Fig. 2 TCT spectrum
Fig. 3 TCT receiver structure
PILOT PILOT
(_0 (_p _0 _0• ÷ C_p
Fig. 4 DTCT spectrum
CALIBRATION t_
SYSTEM
DATAOU
Fig. 5 DTCT receiver structure
335
PILOT
LOWER UPPER
¢_0" coT °_0 m0 _¢OT
Fig. 6 TTIB spectru_
PHASE .LOCKED LOOP
DATA
-- - OUt
_'×
PILOT pROCESSOR
_ 0 o
IF IN 8PF NOTCH X
'°i×
$UBCoAsRRIE R
Fig. 7 TTIB receiver structure
PILOTS
o_0- coT m 0 0)0"co T
Fig. 8 DTSSB spectrum
.0I_EL/W
Fig. 9 DTSSB receiver structure
336


Pilot-aided modulation for narrow-band satellite communications

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