Showing papers on "Continuous phase modulation published in 2001"

Serially concatenated continuous phase modulation with iterative decoding

Abstract: Serially concatenated and interleaved continuous phase modulation (CPM) with iterative decoding is investigated. An a posteriori probability (APP) algorithm for CPM is developed based on the classic APP algorithm for channel codes. The system is analyzed through upper bounds on the average bit error probability. For coded and interleaved minimum shift keying, the weight spectrum is computed, resulting in a transfer function bound. This is cumbersome for a general CPM system; instead, only the most significant error events contributing to the weight spectrum are identified. Simulations show that, firstly, these events give a satisfactory view of system performance when equal outer codes are used, and secondly, that remarkably good performance can be obtained for some simple systems. Finally, power spectral densities and bandwidths are computed, allowing for a bandwidth/performanee comparison of different combinations.

Performance of trellis-coded CPM with iterative demodulation and decoding

Abstract: Interleaved trellis-coded systems with full response continuous-phase modulation (CPM) are considered. Upper bounds on the bit-error rate performance are derived for coherent detection on the additive white Gaussian noise and flat Rayleigh fading channels by considering the trellis code, interleaver, and CPM modulator as a serially concatenated convolutional code. A coherent receiver that performs iterative demodulation and decoding is shown to provide good bit error performance. Finally, a noncoherent iterative receiver is proposed and is shown to perform close to the coherent iterative receiver.

Cyclic spectral analysis of continuous-phase modulated signals

Abstract: Continuous-phase modulated (CPM) signals play a prominent role in modern communication systems due to their desirable constant-modulus property and the ability to control their power and bandwidth efficiencies. Popular CPM signals include the classical minimum-shift keyed (MSK) signal, the LREC family of signals also known as continuous-phase frequency-shift-keyed (CPFSK) signals, and Gaussian MSK, which is used in state-of-the-art GSM and PCS mobile communication systems. CPM signals, like virtually all man-made communication signals, are known to exhibit cyclostationarity, which implies that their probabilistic parameters, such as mean, second moment, and higher order cumulants, are almost-periodic functions of time. A novel representation of CPM signals as a sum of PAM signals is presented for both integer and noninteger modulation index cases. Then, the Nth-order cyclostationarity properties of binary CPM signals are derived in terms of Nth-order temporal and spectral moment and cumulant functions. Moreover, the case of M-ary CPM signals is briefly addressed. The results are illustrated with simulations involving MSK, LREC, and GMSK signals.

Synchronization method and apparatus for modems based on jointly iterative turbo demodulation and decoding

Abstract: A bandwidth efficient advanced modulation waveform modem using concatenated iterative turbo coding and continuous phase modulation is disclosed A demodulator in the modem has a turbo decoder and a decision feedback carrier and time tracking algorithm to track a carrier and adjust timing The decision feedback carrier and time tracking algorithm may use an APP decoder as a decision device to provide symbol decisions at a high error rate and low latency for a coded input data stream A symbol phase estimator produces a symbol phase error estimate from the symbol decisions An erasure decision function decides which symbol decisions are correct and which symbol decisions are erasures A carrier tracking function receives the symbol phase error estimates when the symbol decisions are correct and receives erasure inputs when the symbol decisions are erasures to maintain carrier tracking

Design of carrier recovery algorithm for high-order QAM with large frequency acquisition range

Abstract: We propose a polarity decision carrier recovery algorithm that is useful for carrier acquisition in high order-QAM (quadrature amplitude modulation). The PD (phase detector) output and its variance characteristics are mathematically derived and the simulation results are presented. We design a carrier recovery loop with this polarity decision PD and ATC (automatic transfer-mode controller), which is proposed to detect a proper time for mode change and to yield fine phase tracking error in the steady state. While the conventional DD (decision directed) algorithm can only acquire frequency offsets of less than /spl plusmn/10 kHz, the proposed carrier recovery algorithm can acquire up to /spl plusmn/200 kHz without AFC (automatic frequency control). We show the RMS (root mean square) phase error performance using the proposed carrier recovery PLL (phase locked loop).

Optical to microwave converter using direct modulation phase shift keying

Abstract: A point-to-point microwave radio link that operates in a Frequency Division Duplex (FDD) mode using direct digital modulation with a Continuous Phase Shift Keyed (PSK) scheme. The transmit signal is generated by a circuit that uses a Voltage-Control Oscillator (VCO) operating in a microwave radio band to obtain the modulated signal. The VCO output is fed to a phase modulator to obtain the modulated signal. The output of the VCO is then frequency multiplied by the predetermined factor to produce the modulated microwave output signal at the desired band.

A GMSK modulator using a /spl Delta//spl Sigma/ frequency discriminator-based synthesizer

Abstract: This paper describes a new transmitter architecture suitable for wideband GMSK modulation. The technique uses direct modulation of /spl Delta//spl Sigma/ frequency discriminator (/spl Delta//spl Sigma/FD)-based synthesizer to produce the modulated RF signal without any up-conversion. Digital equalization is used to extend the modulation data rate far beyond the synthesizer closed-loop BW. A prototype 1.9-GHz GSM transmitter was constructed consisting of a /spl Delta//spl Sigma/FD-based synthesizer and a digital transmit filter. The synthesizer consists of an 0.8-/spl mu/m BiCMOS /spl Delta//spl Sigma/FD chip, a digital signal processor FPGA, and an off-chip D/A converter, filter, and VCO. Measured results, using 271-kbit/s GSM modulation, demonstrate data rates well in excess of the 30-kHz synthesizer closed-loop BW are possible with digital equalization. Without modulation, the synthesizer exhibits a -76-dBc spurious noise level and a close-in phase noise of -74 dBc/Hz.

Carrier phase estimation based on single-axis constant modulus cost criterion and Bussgang criteria

Abstract: Carrier phase recovery employs a single-axis blind cost criterion from the Bussgang class of functions, and its stochastic gradient, to generate a carrier phase error used to adjust a received and demodulated signal to near baseband. For one implementation, the estimate is derived in accordance with a Single-Axis Constant Modulus (SA-CM) criterion and its stochastic gradient via a SA-CM algorithm (SA-CMA). The carrier phase error is then used to adjust the carrier frequency and phase of the received and demodulated signal toward the frequency and phase of the carrier used to modulate the transmitted symbols, driving the carrier phase error to zero. The values used for the phase recovery may be either i) an IIR filtered signal, ii) a processed signal (e.g., decisions for the signal symbols), or iii) an equalized and processed signal.

Turbo coded continuous phase modulation

Abstract: Continuous phase modulation (CPM) provides an attractive option for spectrally efficient communications systems. Turbo-coding combined with CPM has the potential to result in a spectrally efficient scheme that is also energy efficient in that it yields low error rates at low signal-to-noise ratios. In this paper we consider the concatenation of a turbo encoder and a CPM modulator which can be decoded and demodulated iteratively. For demodulation of the CPM signal we consider a design based on the Walsh signal space. Multiple iterations of the turbo decoder, feedback of a priori CPM symbol probabilities from the turbo-decoder to the CPM demodulator and multiple iterations of this turbo-decoder/CPM demodulator operation enhances the performance of the system.

Suppressed cycle based carrier modulation using amplitude modulation

Abstract: A sinusoidal RF carrier is modulated for the transmission of digital binary data streams through the amplitude suppression of carrier wavelets. These wavelets are defined between zero crossover positions representing zero energy locations. This modulation is accomplished when the carrier is slightly amplitude modulated with a modulation signal that is equal in frequency to the carrier itself and the modulation always begins or ends upon the exact zero voltage crossing point of the RF cycle phase. The modulation is applied as a slight shift of the amplitude of any single cycle, each cycle representing a single bit of data. A single cycle of RF will either represent a “1” or “0” depending upon the amplitude of the cycle, relative to other adjacent cycles in the same carrier.

Space-time code design with CPM transmission

Abstract: Design criterion for space-time codes with continuous phase modulation (CPM) in a quasistatic fading channel is given. Space-time codes are proposed and simulation results demonstrate their good performance.

Phase-locked loop initialization via curve-fitting

Abstract: A method and apparatus for accurately estimating the carrier frequency offset and the carrier phase offset of a digitally modulated signal using a signal processing algorithm to initialize the state variables of a Phase-Locked Loop (PLL) is disclosed. A sequence of phase values is estimated from a received sequence of symbols and the angular effect due to the modulation format is removed from the sequence of phase values. A curve-fit algorithm based in one embodiment on the RLS algorithm is then applied to a sequence of unwrapped phase values to estimate the carrier frequency offset and the carrier phase offset.

Reduced state adaptive SISO algorithms for serially concatenated CPM over frequency-selective fading channels

Abstract: Iterative detection (ID) has proven to be a near-optimal solution for concatenated finite state machines (FSMs) with interleavers over an additive white Gaussian noise (AWGN) channel. When perfect channel state information (CSI) is not available at the receiver, an adaptive ID (AID) scheme is required to deal with the unknown, and possibly time-varying, parameters. The basic building block for ID or AID is the soft-input soft-output (SISO) or adaptive SISO (A-SISO) module. The complexity of SISOs and A-SISOs increases exponentially with constraint length. Reduced state SISO (RS-SISO) algorithms have been applied to reduce the complexity of the A-SISO module. We show that serially concatenated CPM (SCCPM) with AID has turbo-like performance over fading ISI channels and also RS-A-SISO systems have large iteration gains. Various design options for RS-A-SISO algorithms are evaluated and performance and complexity are compared.

On forward-adaptive versus forward/backward-adaptive SISO algorithms for Rayleigh fading channels

Abstract: Two types of soft-input soft-output (SISO) algorithms have previously been proposed for iterative decoding of concatenated codes applied over fading channels; forward-adaptive algorithms exploit the dependency between the present symbol interval and past symbol intervals, while forward/backward-adaptive algorithms take both the past and the future into account. Numerical results presented for interleaved serially concatenated continuous phase modulation (CPM) over the Rayleigh fading channel indicate that the performance of forward-adaptive algorithms approaches the performance of forward/backward-adaptive algorithms when a long channel-memory depth is exploited.

Parameter estimation of binary CPM signals

Abstract: Estimation of frequency and symbol timing in continuous phase modulated (CPM) signals is investigated. Several well-known statistical approaches, classically applied to the sensor array problem, are used to derive non-data-aided (NDA) algorithms under a unifying general framework (estimation-directed). A new cost function is proposed which is shown to provide a good compromise between additive and pattern noise cancellation, when the additive noise power is unknown.

ML estimation of symbol timing and carrier phase for CPM in Walsh signal space

Abstract: An algorithm to estimate jointly the symbol timing and carrier phase for continuous-phase modulation (CPM) is described. It is designed to optimize the steady-state performance of the CPM synchronizer and exhibits excellent performance, very close to the optimum maximum-likelihood estimate. The algorithm is simple and practical and may be employed with any CPM scheme. It is based on a signal space decomposition of the CPM signal in the Walsh signal space.

Modulator and method of modulating optical carrier with clock signal before or after the carrier is modulated with data pulse

Abstract: In an optical transmitter which modulates a continuous-wave optical carrier with an electrical data pulse, the carrier is modulated with a clock signal having ½ n (where n is an integer equal to or greater than unity) of the frequency of a fundamental sinusoidal component of the data pulse so that the modulated carrier contains at least a center frequency spectral component. On the modulated optical carrier a filtering operation is performed so that the filtered carrier contains a carrier frequency component and first sideband components. Before or after the clock modulation is performed, the data pulse is used to perform data modulation on the optical carrier.

System implications in designing a 60 GHz WLAN RF front end

Abstract: In this paper, we seek to evaluate the performance of a 60 GHz WLAN system taking into account RF circuitry imperfections and hardware requirements for various modulation techniques. A model of an RF frontend is developed, including physical imperfections of the circuitry such as power amplifier (PA) nonlinearity and voltage controlled oscillator VCO phase noise. Given the RF front-end model, several modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) and Continuous Phase Modulation (CPM) are considered. The evaluation of the system performance in terms of bit error rate allows a better understanding of physical circuitry limitations, and optimal modulation parameters as well as circuit design recommendations can be derived.

Optimized quantization for diffractive phase elements by use of uneven phase levels

Abstract: Many applications of diffractive phase elements involve the calculation of a continuous phase profile, which is subsequently quantized for fabrication. The quantization process maps the continuous range of phase values to a limited number of discrete steps. We present a new scheme with unevenly spaced levels for the design of diffractive elements and apply it to the design of intracavity mode-selecting elements. We show that this modified quantization can produce significantly better results than are possible with a regular or even the bias-phase-optimized quantization scheme that we reported here earlier. In principle this process can be employed to a greater or lesser extent in any quantization process, allowing the fabrication of diffractive elements with much improved performance.

A method of non-data-aided carrier recovery with modulation identification

Abstract: A non-data aided carrier recovery technique using modulation format identification is proposed. This technique can also be interpreted as a modulation identification method that is robust against static phase and frequency offsets. The performance of the proposed technique is studied and analytical expressions derived for the mean acquisition time to detect lock in the cases of M-PSK, M=2,4,8, and 16-QAM modulation, with respect to frequency offset and signal-to-noise ratio. The results are verified with Monte Carlo simulations. The main advantage of the proposed method lies in its simpler implementation and faster lock detection, when compared to conventional methods.

Convergence analysis of SCCPM with iterative decoding

Abstract: A convergence analysis of serially concatenated continuous phase modulation (SCCPM) with iterative decoding is presented, based on extrinsic information density evolution. Earlier conclusions from union bounds to maximum likelihood decoding that could not be verified by simulations can now be explained. This includes the effect of more complex outer codes and of inner CPM systems with input weight 1 error events. Due to their trivial feed-forward polynomial, all CPM systems provide good initial decoder convergence. Still the role of the CPM state complexity is not well explained. Finally, bit error rate performance per iteration is estimated from the diagrams, showing good coherence with simulation results.

Maximizing minimum Euclidean distance of spectrally constrained partial response CPM

Abstract: Optimized phase pulses, which maximizes the minimum Euclidean distance of spectrally constrained partial response CPM with different pulse lengths and different alphabet sizes, are numerically found. The channel is specified by a narrow spectrum mask, which puts hard requirements on both the width of the spectral main lobe and the size of the spectral side lobes, and AWGN. The phase pulse is parameterized by equidistant sample points, which are connected by cubic spline interpolation and the modulation index is also treated as a parameter. A gain of up to 0.9 dB is obtained compared to GMSK. In addition, some of the new phase pulses provide an advantage for reduced state sequence detection.

Acquisition of a continuous-phase modulation waveform

Abstract: Acquisition of a waveform such as a continuous-phase modulation (CPM) waveform is described. In one embodiment of the invention, the invention is directed to a method and apparatus for acquiring a waveform as defined by MIL-STD-188-181B including the preamble of such a waveform at a performance level defined by the standard. The present invention provides solutions to at least four primary issues presented in acquiring a CPM waveform such as the MIL-STD-188-181B compliant waveform. These primary problems include searching for the preamble, determination of the symbol rate, determination of an initial carrier frequency error (Doppler), determination of an initial carrier phase, and determination of the start-of-message to establish an absolute time marker within the waveform.

Soft output diversity combining for CPM signals over space-time correlated Rayleigh fading channels

Abstract: Soft output detection algorithms are useful in systems where soft-decision metrics are important, e.g., systems with interleaved coded modulation. Receive diversity techniques can provide significant performance improvement for fading channels encountered in mobile communication systems. A soft-output receive diversity combining algorithm for the demodulation of continuous phase modulated (CPM) signals over space-time correlated frequency flat Rayleigh fading channels in the absence of perfect channel state information (CSI) is developed. The derived soft output algorithm is used in the iterative detection of interleaved TC-CPM. The performance of the algorithm is extensively characterized by means of Monte-Carlo simulations.

Reduced complexity iterative demodulation and decoding of serial concatenated continuous phase modulation

Abstract: We present a structure for reduced complexity iterative demodulation and decoding of partial response continuous phase modulation (CPM) when serially concatenated with a convolutional code. The proposed receiver uses a single front-end filter and a 2 or 4 state trellis for decoding the inner CPM code. This can yield significant savings, as the complexity of the optimal demodulator increases exponentially with the length of the CPM frequency pulse. Simulations show that for GMSK with BT=1/6, the performance penalty for simplified iterative demodulation and decoding is less than 0.25 dB, compared to the receiver which performs optimal demodulation (which requires 64 filters and a 64 state inner trellis).

Noncoherent coded continuous phase modulation

Abstract: Continuous phase modulation (CPM) systems for noncoherent coded transmission are proposed and analyzed. Specifically, the application of a feedback-free modulator is regarded. For demodulation, a receiver structure which requires only two or three linear filters is considered. For decoding, noncoherent sequence estimation (NSE) with Viterbi decoding and per-survivor processing is applied. Noteworthy, in our approach the problems of low-complexity filtering and reduced-state decoding can be treated separately. Since we give a recursive formula for the phase reference symbol necessary for NSE metric calculation, computational effort is further decreased. Overall, in terms of complexity, the proposed noncoherent receiver provides significant advantages over approaches in the literature. The high performance of the novel noncoherent CPM system is confirmed by simulation results.

Demodulation of CPFSK and GMSK Signals Using Digital Signal Processing DPLL with Sequence Estimator

Abstract: SUMMARY Phase locked loops (PLL's) are well known as a threshold extension demodulator for analogue FM signals. This capability may lead to the low bit error rate demodulation for digital FM signals. A PLL has also its native frequency tracking ability and is suited to the demodulation of the signals having large Doppler shifts, for example signals from Low Earth Orbit (LEO) satellites. In this paper, we study the demodulation scheme of Continuous Phase FSK (CPFSK) and Gaussian filtered MSK (GMSK) signals using a Digital Signal Processing type Digital PLL (DSP DPLL). First we propose a DSP DPLL completely equivalent to an Analog PLL (APLL). Next we adopt the sequence estimation scheme to compensate the Inter-Symbol Interference (ISI) associated with the finite loop bandwidth of the DSP DPLL. Through computer simulations it is clarified that the proposed DSP DPLL with sequence estimator can achieve better BER performance compared with the conventional Limiter Discriminator (LD) detection on the AWGN channel. We have also shown that the DSP DPLL with sequence estimator has excellent BER characteristics on Rician fading channels having actual large Doppler shifts.

Adjacent channel interference susceptibility of serial concatenated continuous phase modulation

Abstract: Iterative decoding of a serially concatenated outer convolutional code and an inner continuous phase modulation (CPM) separated by a random interleaver is examined. This scheme, which is known to produce a high coding gain, is examined for various CPMs such as MSK, LRC, GMSK and TFM. CPM signals are constant envelope signals with very high bandwidth efficiency, providing good immunity to adjacent channel interference (ACI). The susceptibility of the serial concatenated system to ACI is examined. Simulations show that this configuration can tolerate much stronger ACI than previous CPM schemes, including those detected with the Viterbi algorithm.

Null-pilot symbol assisted fast automatic frequency control (AFC) system for coherent demodulation of continuous phase modulation (CPM) signals and method for implementing same

Abstract: A null-pilot symbol assisted fast automatic frequency control (AFC) system for coherent demodulation of carrier phase modulation (CPM) includes ( 209 ) a pilot clock driven phase differentiator ( 252,253,255 ) for operating once every pilot clock cycle to determine the difference between the phase at a current pilot symbol location and the phase at a previous pilot symbol location. A frequency offset selector ( 256 ) is then used for choosing the most likely frequency offset from amongst a set of all frequency offsets that give rise to the same phase difference.