Continuous Phase Modulation (CPM) schemes are advantageous for low-power radios. The constant envelope transmit
signal is more efficient for both linear and non-linear amplifier architectures. A standard, coherent CPM receiver can
take advantage of modulation memory and is more complex than a coherent Phase Shift Keyed receiver. But the CPM
signal can be demodulated non-coherently and still take advantage of the trellis structure inherent in the modulation.
Prior analyses of several different non-coherent CPM schemes have been provided with many providing coherent or near
coherent performance. In this paper we will discuss a new, reduced complexity decoder that improves upon the noncoherent
performance. In addition, this new algorithm generates soft decision metrics that allow the addition of a
forward error correction scheme (an outer code) with coherent equivalent performance gains.
Continuous Phase Modulation (CPM) schemes are advantageous for low-power radios. The constant envelope transmit
signal is more efficient for both linear and non-linear amplifier architectures. A standard, coherent CPM receiver can
take advantage of modulation memory and is more complex than a coherent Phase Shift Keyed receiver. But the CPM
signal can be demodulated non-coherently and still take advantage of the trellis structure inherent in the modulation.
With this complexity reduction, the CPM receiver is comparable in performance to a Phase Shift Keyed radio with the
power utilization of a Frequency Shift-Keyed design. In this paper, we discuss two methods for increasing the
modulation memory of the CPM signal. In the first method, the distribution of the transmitted symbol across multiple
phase pulses is investigated and the bit error rate analyzed. In the next method we address the addition of convolutioncodes.
In both cases the effects of the CPM memory to quasi-coherent demodulation is analyzed and discussed. The
differences in complexity will be analyzed and the overall performance enhancements of several different modulation
schemes will be illustrated.
1
KEYWORDS: Receivers, Modulation, Demodulation, Phase shift keying, Doppler effect, Global system for mobile communications, Distortion, Monte Carlo methods, Frequency shift keying, Standards development
Constant Envelope, Continuous Phase Modulation (CPM) is highly desirable for low-power, battery-operated systems as
well as for small-profile vehicular and aircraft systems where large amplifiers won't fit. In the past, CPM was noted for
increased demodulator complexity (over simple PSK or FSK receivers) but with modern computational power it is
possible to continue to improve the power efficiency of CPM modulation at the receiver. In the specific case of
multipath, there are several known methods (Rake Receiver is one example) to resolve and correct for inter-symbol
interference and phase distortion. This paper develops a standard CPM demodulation and compares the optimal coherent
performance with a partially coherent receiver. Several methods are developed to compensate and correct for ISI due to
various types of multipath and the power efficiency is compared to the original, coherent demodulation.
KEYWORDS: Modulation, Receivers, Phase shift keying, Global system for mobile communications, Doppler effect, Monte Carlo methods, Error analysis, Performance modeling, Fourier transforms, Binary data
Constant Envelope, Spread Spectrum Modulation is highly desirable for low-power, battery-operated systems. It has
been demonstrated that Hybrid CPM is a constant envelope modulation with similar frequency diversity properties to the
standard spread-spectrum m-PSK DSSS and spread-MSK modulation schemes while retaining a superior emissions
profile. This paper continues the analysis of the novel constant envelope spread spectrum modulation technique with an
analysis of the commonly utilized rake receiver signal processing. Initially, a simple channel model is developed to
illustrate and compare the convergence of the channel estimate over a fixed, non-time-varying channel. A more complex,
wireless channel model is then developed and a new corresponding method for channel estimation created. A Monte-
Carlo simulated bit error rate performance of Hybrid CPM is then generated to evaluate the overall performance of the
Hybrid CPM modulation scheme.
Hybrid Continuous Phase Modulation (HCPM) is a variant of Continuous Phase Modulation that is optimized for low-power,
wireless communications applications. Hybrid CPM uses either additional Amplitude or Phase pulses to create a
higher order modulation without the increase in trellis complexity that normally accompanies higher order CPM
modulation schemes. For standard CPM, an increase in the modulation order results in the increase of the symbol
alphabet and a corresponding increase in the transmit bandwidth and exponential increase in the complexity of the
decoder trellis structure. Hybrid CPM however achieves higher order modulation by adding parallel branches to the base
CPM trellis structure thus reducing the receiver complexity. The novelty of this paper is the application of the techniques
devised to reduce the demodulator complexity of standard modulation types like PSK and CPM and apply them to
Hybrid CPM using a method that would not result in the loss of performance or require any sort of compromise. As an
example, this paper provides an analysis of the power and spectral efficiency of the two hybrid CPM waveforms and
gives specific examples of the application of reduced state techniques. Both set-partitioning and reduced-state sequence
estimation with decision feedback techniques are analyzed and compared. The results will demonstrate that reduced-state
sequence estimation can be coupled with Hybrid CPM demodulation without any loss in bit error rate performance.
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