Clark Robertson

Performance analysis and simulation of cyclic code-shift keying

Cyclic code-shift keying (CCSK) is the baseband symbol modulation scheme used by Joint Tactical Information Distribution System (JTIDS), the communication terminal of Link-16. Since CCSK is non-orthogonal, an analytic evaluation of its performance in terms of probability of symbol error is nontrivial. In this paper, an analytic upper bound on the probability of symbol error of CCSK is derived for the 32-chip CCSK sequence chosen for JTIDS. The probability of symbol error obtained with the analytic method is compared with that obtained by Monte Carlo simulation for additive white Gaussian noise. The results show that the analytic method yields a tight upper bound. In addition to the 32-chip CCSK sequence chosen for JTIDS, a new 32-chip CCSK sequence with a smaller maximum off-peak cross-correlation is obtained and evaluated both analytically and by Monte Carlo simulation. The results obtained for the new CCSK sequence compare favorably with the sequence chosen for JTIDS.

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.

Co-channel interference reduction on the forward channel of a wideband CDMA cellular system

Wideband code-division multiple access (CDMA) systems are interference-limited, and so must utilize some form of interference reduction in order to maintain an acceptable quality of service and capacity. In this paper, co-channel interference for several different CDMA architectures is evaluated. For wideband CDMA systems such as W-CDMA and cdma2000 with carrier stealing, co-channel interference is significantly reduced by the implementation of either microzoning or sectoring. The disadvantage of microzoning is that intra-cell interference is no longer ideally zero on the forward channel, as it is with sectoring and omnidirectional architectures. For wideband CDMA systems such as cdma2000 without carrier stealing, co-channel interference is reduced by both microzoning and sectoring architectures even more than in the case of W-CDMA and cdma2000 with carrier stealing. In this case, since forward channel intra-cell interference remains ideally zero, the significant reduction of co-channel interference by microzoning makes microzoning clearly superior to omnidirectional architectures.

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.

Reed Solomon coded M-ary hyper phase-shift keying

Non-binary forward error correction (FEC) coding in conjunction with M-ary hyper phase-shift keying (MHPSK) is considered in order to improve the robustness of a satellite communications uplink. MHPSK is a spectrally efficient modulation technique that uses four orthonormal basis functions to increase the distance between different symbols in the signal space. Spectral efficiency and probability of bit error are two key figures of merit used to evaluate digital modulation techniques. The use of four orthonormal basis functions provides an advantage over traditional modulation techniques such as M-ary phase-shift keying (MPSK) and M-ary quadrature amplitude keying (MQAM) that only possess two degrees of freedom. MHPSK offers an improvement in bit error performance over other spectrally efficient modulation techniques for the same average energy per bit-to-noise power spectral density ratio and similar spectral efficiency. As a result, MHPSK offers a novel way to improve both throughput and reduce power requirements using easy to generate waveforms. In this paper, Reed Solomon coded symbols are assumed to be transmitted with MHPSK. MHPSK, MPSK, and MQAM are compared in terms of probability of bit error and bandwidth efficiency, where the number of bits per coded symbol are typically designed to match the number of bits per channel symbol.

Detection of frequency-hopped waveforms embedded in interference waveforms

Military communication systems do not necessarily operate within FCC frequency bands. Hence, they may be subject to interference from other waveforms using the same frequency band. In this paper we first investigate a technique to estimate the spectrum of competing signals utilizing the same bandwidth as a desired frequency-hopped waveform. Next, we show that the desired frequency-hopped waveform can be recovered from the composite received signal by dividing the composite signal spectrum by an estimate of the interference spectrum. Since the interference estimate is imperfect, spectral division is significantly better than spectral subtraction of the interference spectrum from the composite spectrum for the detection of frequency-hopped waveforms

An improved link-16/JTIDS receiver in pulsed-noise interference

Link-16 provides presumably secure and jam-resistant tactical information for land, sea, and air platforms. Its communication terminal, Joint Tactical Information Distribution System (JTIDS), is a hybrid direct-sequence/frequency-hopping spread spectrum system and features Reed-Solomon (RS) codes for channel coding, cyclic code-shift keying (CCSK) for 32-ary baseband symbol modulation, and minimum-shift keying (MSK) for waveform modulation. In this paper, a noise-normalization combining MSK chip demodulator and an errors-and-erasures RS decoder (EED) are proposed in the JTIDS receiver to replace the original MSK chip demodulator and errors-only RS decoder in order to enhance the anti-jam capability of JTIDS. The symbol error rate (SER) performances of the proposed JTIDS receiver are investigated in pulsed-noise interference (PNI) by a combination of analysis and simulation assuming perfect frequency de-hopping, sequence and chip synchronization, and de-scrambling. Given various fraction of time the jammer is on, the SER performances obtained with the proposed JTIDS receiver are compared to those obtained with the original JTIDS receiver. The results show that the proposed JTIDS receiver not only significantly outperforms the original system as the fraction of time the jammer is on is large, but completely eliminates the effect caused by PNI as the fraction of time the jammer is on is small.

Performance analysis and simulations of 32-ary cyclic code-shift keying

Cyclic code-shift keying (CCSK) is the baseband 32-ary symbol modulation scheme used by the Joint Tactical Information Distribution System (JTIDS), the communication terminal for Link-16. CCSK is not orthogonal and an analytic expression for the probability of symbol error for CCSK has thus far been elusive. In this paper, an analytic upper bound on the probability of symbol error of CCSK is derived for the 32-chip CCSK starting sequence chosen for JTIDS. The analytically obtained probability of symbol error is compared with two different Monte Carlo simulations for additive white Gaussian noise. The results of both simulations match the analytic results very well and show that the analytic method yields a tight upper bound. A new 32-chip CCSK starting sequence which has a smaller maximum off-peak cross-correlation value than the current JTIDS starting sequence is proposed and evaluated both analytically and by simulation. The results obtained for the new CCSK starting sequence compare favorably with the CCSK starting sequence chosen for JTIDS. Published in 2010 by John Wiley & Sons, Ltd. Cyclic code shift keying is used in important military communications systems designed for jamming resistance. This paper provides the most accurate analysis and simulation of CCSK performance to date in addition to a proposed improvement over the CCSK employed in the widely used Link-16 tactical communications system. (This article is a U.S. Government work and is in the public domain in the U.S.A.)

M-ary hyper phase-shift keying with Reed Solomon encoding and soft decision reliability information

Reed Solomon (RS) forward error correction (FEC) coding in conjunction with M-ary hyper phase-shift keying (MHPSK) and soft decision decoding is considered in order to improve the robustness of a high spectral efficiency, non-linear satellite communications link. In this paper, a system that utilizes RS encoding of the information symbols which are then transmitted with MHPSK is evaluated in terms of probability of bit error and spectral efficiency. Using standard RS hard decision decoding, the receiver either correctly decodes the received block or returns a decoding failure. In the event of a decoding failure, soft decision reliability information is used to identify received code symbols with a low probability of being correctly received and to generate new code symbol estimates that are used in the traditional RS decoding algorithm. Because the majority of decoding failures are caused when the total number of code symbol errors exceeds the error correction capability t of the RS code by only a few symbols, only a few code symbols must be corrected in order to successfully decode the received block. The performance of this system is compared to a two-subcarrier OFDM system with either 8-PSK or 8-QAM on each subcarrier and single carrier 8-PSK where the data bits are encoded with the Digital Video Broadcast (DVB) standard rate 0.83 low density parity check (LDPC) code. The MHPSK system with RS encoding and soft decision decoding, the two-subcarrier OFDM system with either 8-QAM or 8-PSK on each subcarrier with LDPC encoding, and single carrier 8-PSK with LDPC encoding are compared in terms of probability of bit error, peak-to-average power ratio, amplifier backoff, and spectral efficiency for very long block lengths.