29-10-2012, 10:59 AM
Bandwidth-Efficient Digital Modulation with Application to Deep-Space Communications
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Introduction
The United States Budget Reconciliation Act of 1993 mandates reallocation
of a minimum of 200 MHz of spectrum below 5 GHz for licensing to nonfederal
users. One of the objectives is to promote and encourage novel spectrum-inspired
technology developments and wireless applications. Many user organizations
and communications companies have been developing advanced modulation techniques
in order to more efficiently use the spectrum.
In 1998, the international Space Frequency Coordination Group (SFCG)
adopted a spectral mask that precludes the use of a number of classical modulation
schemes for missions launched after 2002. The SFCG has recommended
several advanced modulations that potentially could reduce spectrum congestion.
No one technique solves every intended application. Many trade-offs must
be made in selecting a particular technique, the trade-offs being defined by the
communications environment, data integrity requirements, data latency requirements,
user access, traffic loading, and other constraints. These new modulation
techniques have been known in theory for many years, but have become feasible
only because of recent advances in digital signal processing and microprocessor
technologies.
The Need for Constant Envelope
Digital communication systems operate in the presence of path loss and
atmospheric-induced fading. In order to maintain sufficient received power at
the destination, it is required that a device for generating adequate transmitter
output power based on fixed- but-limited available power be employed, examples
of which are traveling-wave tube amplifiers (TWTAs) and solid-state power
amplifiers (SSPAs) operated in full- saturation mode to maximize conversion
efficiency. Unfortunately, this requirement introduces amplitude modulationamplitude
modulation (AM-AM) and amplitude modulation-phase modulation
(AM-PM) conversions into the transmitted signal. Because of this, modulations
that transmit information via their amplitude, e.g., quadrature amplitude modulation
(QAM), and therefore need a linear amplifying characteristic, are not
suitable for use on channels operated in the above maximum transmitter power
efficiency requirement.1 Another consideration regarding radio frequency (RF)
amplifier devices that operate in a nonlinear mode at or near saturation is the
spectral spreading that they reintroduce due to the nonlinearity subsequent to
bandlimiting the modulation prior to amplification. Because of the need for the
transmitted power spectrum to fall under a specified mask imposed by regulating
agencies such as the FCC or International Telecommunications Union (ITU),
the modulation must be designed to keep this spectral spreading to a minimum.
This constraint necessitates limiting the amount of instantaneous amplitude fluctuation
in the transmitted waveform in addition to imposing the requirement for
constant envelope.
Differentially Encoded QPSK and Offset (Staggered)
QPSK
In an actual coherent communication system transmitting M-PSK modulation,
means must be provided at the receiver for establishing the local demodulation
carrier reference signal. This means is traditionally accomplished with the
aid of a suppressed carrier-tracking loop [1, Chap. 2]. Such a loop for M-PSK
modulation exhibits an M-fold phase ambiguity in that it can lock with equal
probability at the transmitted carrier phase plus any of the M information phase
values. Hence, the carrier phase used for demodulation can take on any of these
same M phase values, namely, θc + βi = θc + 2iπ/M, i = 0, 1, 2, · · ·,M − 1.
Coherent detection cannot be successful unless this M-fold phase ambiguity is
resolved.
One means for resolving this ambiguity is to employ differential phase encoding
(most often simply called differential encoding) at the transmitter and
differential phase decoding (most often simply called differential decoding) at
the receiver following coherent detection. That is, the information phase to be
communicated is modulated on the carrier as the difference between two adjacent
transmitted phases, and the receiver takes the difference of two adjacent phase
decisions to arrive at the decision on the information phase.4 In mathematical
terms, if Δθn were the information phase to be communicated in the nth transmission
interval, the transmitter would first form θn = θn−1 +Δθn modulo 2π
(the differential encoder) and then modulate θn on the carrier.5 At the receiver,
successive decisions on θn−1 and θn would be made and then differenced modulo
2π (the differential decoder) to give the decision on Δθn. Since the decision on
the true information phase is obtained from the difference of two adjacent phase
decisions, a performance penalty is associated with the inclusion of differential
encoding/decoding in the system.
Continuous Phase Modulation
Continuing with our discussion of strictly constant envelope modulations, we
now turn our attention to the class of schemes referred to as continuous phase
frequency modulation (CPFM) or more simply continuous phase modulation
(CPM). The properties and performance (bandwidth/power) characteristics of
this class of modulations are sufficiently voluminous to fill a textbook of their
own [15]. Thus, for the sake of brevity, we shall only investigate certain special
cases of CPM that have gained popularity in the literature and have also been
put to practice.