Frank Kragh

Performance analysis of a JTIDS/Link-16-type waveform transmitted over Nakagami fading channels with pulsed-noise interference

The Joint Tactical Information Distribution System (JTIDS) is the communication terminal of Link-16. JTIDS is a hybrid direct sequence/frequency-hopping spread spectrum system and features Reed-Solomon codes for channel coding, cyclic code-shift keying for 32-ary symbol modulation, minimum-shift keying for chip modulation, symbol interleaving, chip sequence scrambling and random jittering for transmission security, and a double-pulse structure for diversity. Assuming that coherent chip demodulation is practical, we investigate the probability of symbol error of a JTIDS/Link-16-type waveform for both the single- and the double-pulse structure transmitted over a slow, flat Nakagami fading channel in the presence of pulsed-noise interference (PNI) in this paper. In general, the results show that the double-pulse structure always outperforms the single-pulse structure, whether the PNI is present or not and whether the channel is fading or not. Furthermore, barrage noise interference has the most effect in degrading performance when signal-to-interference ratio (SIR) is small. When SIR is large, PNI with a smaller fraction of time that interference is on causes the greatest degradation.

Detection of Frequency-Hopped Waveforms Embedded in Interference Waveforms with Noise

Bandwidth usage has become more complex such that it is not uncommon that multiple signals of appreciable power may be present within the same bandwidth. The presence of multiple signals in addition to additive white Gaussian (AWGN) increases the difficulty of detecting frequency-hopped (FH) waveforms. This paper investigates the performance of an exponential-averaging based FH detection method in the presence of interfering signals and AWGN. The detection method provides an estimate of the noise plus inference spectrum using exponential averaging and then generates an estimate of the desired signal spectrum by combining the estimated noise plus interference spectrum with the composite (desired signal plus interference plus noise) spectrum. Finally, this paper evaluates the detectors performance as a function of the exponential coefficient, the combining method (division or subtraction), signal-to-AWGN ratio (SNR), and signal-to- interference ratio (SIR).

Performance analysis of a Link-16 compatible waveform using errors-and-erasures decoding when corrupted by pulse-noise interference

The Link-16 is the tactical data link utilized by the Joint Tactical Information Distribution System (JTIDS). The JTIDS system is important due to its wide use by U.S. armed forces, NATO, and other allied militaries. Link-16 is a hybrid frequency-hopped/direct sequence spread spectrum system that utilizes minimum-shift keying (MSK) to modulate the chips, cyclical code-shift keying (CCSK) to modulate the 32-chip symbols, and a (31, 15) Reed Solomon (RS) code with hard decision decoding (HDD) for forward error correction (FEC). This paper analyzes an alternative waveform compatible with the existing Link-16 which uses orthogonal modulation such as Walsh codes vice CCSK and errors-and-erasures decoding (EED) vice hard decision decoding. Both of these modifications are suggested for enhanced bit error rate (BER) performance. Orthogonal modulation for Link-16 with HDD has been explored before [1][2]. [2] shows that the proposed alternative waveform outperforms the Link-16 waveform slightly when HDD is used. This paper reveals potential further improvement through the use of EED. Currently, the Link-16 waveform is received noncoherently at the chip level, but in this paper the performance of the alternative Link-16-compatible waveform is evaluated for coherent as well as for noncoherent demodulation in order to ascertain the performance possible if coherent demodulation becomes practical. The performance of the alternative waveform for the relatively benign case where additive white Gaussian noise is the only noise present as well as when pulse-noise interference is present is investigated for both coherent and noncoherent demodulation.

A Bandwidth Efficient Constant Envelope Modulation

This paper presents an innovative, bandwidth efficient, constant envelope modulation technique that was originally developed by Resnikoff and Tigerman. This modulation technique applies the properties of phase realizability, phase shift orthogonality, and compact phase support to devise a bandwidth efficient modulation waveform that is also resistant to intersymbol interference. This paper defines the general complex envelope waveform and develops the general phaseform design process. An example phaseform is provided with accompanying shift orthogonality test results. In addition, this paper demonstrates power spectral density and bandwidth performance using the phaseform provided and M-ary symbols. The bandwidth results compare well to traditional continuous phase modulation and favorably to M-ary phase shift keying.

Performance of a JTIDS-type waveform with errors-and-erasures decoding in pulsed-noise interference

The Joint Tactical Information Distribution System (JTIDS) is the Link-16 communication terminal. JTIDS is a hybrid direct sequence/frequency-hopping spread spectrum system and features Reed-Solomon (RS) codes for channel coding. In this paper, the performance with an errors-and-erasures decoder (EED) in the JTIDS receiver is evaluated by a combination of analysis and simulation assuming perfect frequency dehopping, chip sequence synchronization, chip synchronization, and chip descrambling. Furthermore, maximum-likelihood chip detection is assumed rather than maximum-likelihood chip-sequence detection since the former represents a more practical assumption for a JTIDS-type signal. The probability of symbol error of a JTIDS-type waveform is evaluated for both the single- and the double-pulse structure in both additive white Gaussian noise and pulsed-noise interference. The results obtained with EED are compared to those obtained with errors-only RS decoding. In all cases considered, EED outperforms errors-only RS decoding in terms of probability of symbol error.

Maximum Likelihood Sequence Detection of a Bit-stuffed Data Source

Bit-stuffing is used in various communications protocols, typically as a way to remove ambiguity in the detection of control flags or as a method to aid clock recovery. Typically a demodulator makes hard symbol-by-symbol decisions without incorporating the knowledge that the data source is bit-stuffed. A demodulator making symbol-by-symbol decisions is clearly not optimum for a source that has been bit-stuffed. In this paper we investigate an optimum approach. We develop the maximum-likelihood sequence detector (MLSD) for a bit-stuffed data source. We start by modeling the transmitted signal as a Markov process. We then proceed to calculate approximate performance bounds for the MLSD. We then develop a trellis receiver structure and perform simulations using the Viterbi decoder algorithm.

Implementing a Software Defined Radio using the Maestro 49-tile processor

The Maestro 49-tile Radiation-Hard-by-Design chip was developed to demonstrate the application of space-qualified, multicore hardware. We have investigated the implementation of a single precision floating-point pipeline FFT to be used as part of a Software Defined Radio (SDR) application. The details of the software architecture that can adapt to the use of different numbers of tiles and the performance of the N-point FFTs for N = 128, 512, 1024, and 2048 are described. The maximum throughput achieved for a 2048-point FFT is 27 million samples per second when 20 of the 49 available tiles are used for separate FFT blocks, one tile is used for input data distribution, and one tile is used for output data collection. We also report on the performance of the SDR based upon the FFT experiments.

Performance analysis of the LINK-16/JTIDS waveform with concatenated coding in both AWGN and pulsed-noise interference

Link-16 is a tactical data link widely used in the United States Armed Forces and North Atlantic Treaty Organization. The communication terminal of Link-16 is called the Joint Tactical Information Distribution System (JTIDS) and features Reed-Solomon (RS) coding, symbol interleaving, cyclic code-shift keying for M-ary symbol modulation, minimum-shift keying for chip modulation and combined frequency-hopping and direct sequence spread spectrum. In this paper, we investigate the performance of a Link-16/JTIDS-type waveform in both additive white Gaussian noise (AWGN) and pulsed-noise interference (PNI) when an alternative error correction coding scheme for the physical layer waveform is employed. The waveform considered uses a concatenated code consisting of a rate r = 4 / 5 convolu-tional code as an outer code and a (31, k ) RS code as an inner code. Currently, the JTIDS waveform is received noncoherently at the chip level. In this paper the performance of the alternative, JTIDS-compatible waveform is evaluated for both coherent and noncoherent demodulation. The performance of the alternative waveform for the case where AWGN is the only noise present as well as when PNI is also present is investigated. The performance obtained using the alternative error correction coding scheme is compared to that of the existing JTIDS waveform when the same assumptions are made for both waveforms.

Blind signal separation via independent component analysis

The “Infomax” method of Independent Component Analysis (ICA) is applied to the problem of separating received signals that overlap in time and frequency, but are otherwise unrelated. The Infomax method separates unknown non-Gaussian signals from a number of signal mixtures by maximizing the entropy of a transformed set of signal mixtures. This work specifically focuses on mixtures of simple communications signals. The Infomax method, as implemented, is found to be successful and efficient for small numbers of signals.

A Bandwidth Efficient Constant Envelope Modulation with Reed-Solomon Coding

This paper presents Reed-Solomon (RS) coding applied to an innovative, constant envelope modulation technique that was originally developed by Resnikoff and Tigerman [1]. The general waveform and phaseform are presented as-well-as probability of bit error and symbol error expressions for uncoded binary and uncoded M-ary modulation, respectively. Bit error performance analysis is presented for the case of RS coded waveforms when the modulation symbol size (M) is less than the RS coded symbol size (Q). In addition, upper and lower performance bounds are derived for RS coded waveforms for M less than Q. Performance analysis curves are presented for the M = 8 and Q = 64 case. Finally, simulation results are also presented for the M = 8 and Q = 64 case that strongly agree with analysis.