Frequency-hopped Spread-spectrum Multiple Access Research Articles

Concatenated Codes with Two Inner Coding Schemes in Frequency-Hopping Spread Spectrum Multiple-Access Channels

In this paper, we analyze the performance of a concatenated code with two different inner decoding schemes. One is the error- detecting inner coding, and the other is the error-detecting-and-correcting inner coding scheme. We compare the performance of the two decoding schemes for finite and infinite block length cases when the concatenated code is applied to slow frequency-hopping spread-spectrum multiple access (FH-SSMA) communication systems. Our results show that the error-detecting-and-correcting inner decoding scheme outperforms the error-detecting inner decoding scheme for finite block lengths, while there exists a performance trade-off for infinite block length case.

Cochannel interference cancelation for multiple access systems

Many properties of frequency-hopped spread-spectrum multiple access (FH/SSMA), such as, robust-ness against frequency selective fading and jamming make it a preferred multiple access communication scheme. But cochannel interference (CCI) can seriously harm the system's performance. On the basis of the analysis of the CCI pattern of frequency hopped multiple access signal, a new multiuser detection algorithm for canceling the CCI in frequency hopped multiple access systems is proposed. In the proposed scheme, the CCI is canceled by first detecting the frequency hopping slot, and then classifying and regenerating the candidates of the interference pattern from the time-frequency matrix. Both the theoretical analysis and computer simulation results show that compared to the conventional detector and Mabuchi's detector, this new multiuser detector has lower complexity, better convergence stability, and is suitable for both additive white Gaussian noise (AWGN) channels and Rayleigh fading channels.

Soft-Decision Decoding in Asynchronous FH/SSMA Networks Using MFSK Modulation

Soft-decision decoding techniques are applied to asynchronous frequency-hop/spread-spectrum multiple-access (FH/SSMA) networks, where M-ary frequency shift keying (MFSK) is employed to transmit one modulated symbol per hop. Coding schemes using soft-decision decoded binary convolutional codes or turbo codes are considered, both with or without bit-interleaving. Performances of several soft metrics are examined for each coding scheme. It is shown that when multiple access interference is the main source of errors, the product metric offers the best performance among the soft metrics considered for all coding schemes. Furthermore, the application of soft-decision decoded convolutional codes or turbo codes without bit-interleaving is shown to allow for a much larger number of simultaneously transmitting users than hard-decision decoded Reed-Solomon codes. Finally, it is observed that when soft-decision decoding techniques are employed, synchronous networks attain better performance than asynchronous networks.

Performance of FH/MFSK systems with readable erasures

We propose a readable-erasure channel for frequency-hopping spread-spectrum multiple access systems with MFSK modulation scheme (FH/MFSK). In contrast to the conventional erasure-only channel having M + 1 output symbols, the proposed readable-erasure channel has 2 M output symbols. Half of the symbols are related to hit-free transmissions, and the rest of the symbols, referred to as weak symbols, are related to transmissions that were hit. The essence of the readable erasure is in the information hidden inside it. One may explore such information by using channel coding. Our analysis shows that in conjunction with channel coding, the readable-erasure channel may enjoy performance advantage over the erasure-only channel. By taking the spectrum efficiency into consideration, we point out that using RS codes over large channel alphabet size in FH/MFSK systems is inefficient. Thus, for FH/MFSK systems, we suggest BCH codes over GF(4) for use with the readable-erasure channel.

Logical and diversity combining in FH/SSMA packet radio networks

We propose the logical and diversity combining in slotted frequency-hopped spread spectrum multiple access (FH/SSMA) packet radio networks. The proposed diversity scheme employs the symbol-by-symbol logical AND operation between the currently received packet and the previously combined packet. Our results show that the proposed low-complexity diversity combining scheme provides a significant performance improvement over existing diversity combining schemes.

Improved cochannel interference cancellation for MFSK/FH-SSMA systems

There has been much interest in the application of frequency hopping-spread spectrum multiple access (FH-SSMA) to wireless systems because of its frequency diversity and resistance to the near-far problem. In FH-SSMA systems, cochannel interference (CCI) is the dominant factor limiting the user capacity. Various techniques used to resolve the CCI problem have been extensively investigated. We propose several multiuser detection schemes based on canceling CCI to increase user capacity in a multilevel frequency shift keying (MFSK)/FH-SSMA system. The performance of systems with the proposed interference cancellers over a Rayleigh fading channel are analyzed and compared with those of the conventional and other interference cancellers. Our results show that the proposed schemes have the best performance in both nonfading and fading channels.

An optimum control strategy of channel input traffic in FH/SSMA systems with central controllers

An optimum channel input-traffic control (OCIC) scheme is proposed and investigated for slotted frequency-hopped spread spectrum multiple access communication systems. The conditional throughput is obtained, the state transition probability is derived, the steady-state throughput is obtained, and the mean packet delay is calculated. It is shown that the performance of the OCIC scheme is better than that of other schemes. It is also shown that the smaller the number of frequency slots is, the more the OCIC scheme outperforms the other schemes.

FH‐SSMA with Band‐efficient modulations over cellular fading channels

AbstractWe present frequency hopped spread‐spectrum multiple‐access (FH‐SSMA) employing bandwidth‐efficient modulations that are phase‐continuous during each hop. The particular cases of minimum‐shift keying (MSK), and Gaussian minimum‐shift keying (GMSK) are studied in detail. Noncoherent sequence estimation on a hop‐by‐hop basis is considered. System performance has been evaluated in selective Rayleigh fading with additive white Gaussian noise (AWGN) and the presence of multiple‐access cellular interference. This system performance is better than the performance of FH‐SSMA with noncoherent binary FSK modulation. The effects of power control, different fading covariance functions and different power bandwidth definitions are investigated.

Influence of guard times on the performance of slotted frequency-hopped spread spectrum multiple access systems

A number of authors have examined the problem of determining the codeword error probability, P/sub e/, in an asynchronous frequency-hopped spread spectrum multiple access (FH-SSMA) communication system. In this letter, we present a method for determining P/sub e/ in a time-slotted system which incorporates a guard time and study the effect of the guard time on P/sub e/ and some other performance measures.

Performance of FHSS multiple-access networks using MFSK modulation

The main concern of this paper is to estimate the symbol error probabilities of synchronous and asynchronous frequency-hop spread-spectrum multiple-access (FHSS-MA) networks through semi-analytic Monte Carlo simulations. We concentrate on systems transmitting one M-ary (M/spl ges/2) FSK modulated symbol per hop with noncoherent demodulation. The usual practice when analyzing the performance of such networks is to upper-bound the probability of symbol error when a hop is hit by K/sup '//spl ges/1 interfering users with (M-1)/M or 1. Previous work on the derivation of accurate approximations to this probability for the case when M=2 has indicated that the aforementioned bound not only gives excessively pessimistic results but may also lead to wrong tradeoff decisions. Using the simulated values for the error probabilities, we show that a similar argument holds for the cases when M>2 as well. Also, by employing a normalized throughput measure that takes into account the bandwidth and time expansion associated with the modulation order M, we find that there exists an optimum value of M that achieves the maximum possible throughput for the cases when binary and M-ary error correcting codes are employed. Throughput results are also given for the case when the signals from the active users in the network suffer from independent Rayleigh fading.

Code diversity transmission in a slow-frequency-hopped spread spectrum multiple-access communication system

At every hop in a conventional frequency-hopped spread spectrum multiple-access (FH-SSMA) system, each transmitter sends its data using a single frequency bin. In this paper, a scheme in which several frequency bins are simultaneously used by a transmitter is studied. It is found that such a code diversity scheme can yield a significant reduction in symbol error rate over that of a conventional FH-SSMA system. A priority system can also be implemented by assigning different diversity degrees to the transmitters.

A robust rate-adaptive hybrid ARQ scheme for frequency-hopped spread-spectrum multiple-access communication systems

This paper considers the application of rate-adaptive coding (RAC) to a spread spectrum multiple access (SSMA) communication system. Specifically, RAC using a variable rate Reed-Solomon (RS) code with a single decoder is applied to frequency-hopped SSMA. We show that this combination can accommodate a larger number of users compared to that with conventional fixed-rate coding. This increase is a result of a reduction in the channel interference from other users. The penalty for this improvement in most cases is a slight increase in the delay (composed of propagation and decoding delay). The throughput and the undetected error probability for a M-ary symmetric channel are analyzed, and performance results are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Multiuser detection scheme based on canceling cochannel interference for MFSK/FH-SSMA system

The paper proposes and investigates a multiuser detection scheme baseEEon canceling cochannel interference (CCI) to improve spectral efficiency or to increase user capacity in an MFSK (multilevel frequency shift keying)/FH-SSMA (frequency hopping-spread spectrum multiple access) system. In the MFSK/FH-SSMA system, an address code is employed as a hopping sequence to hop the carrier frequency in MFSK. In the proposed scheme, it is assumed that the address codes of all users in MFSK/FH-SSMA are known. Then, candidates of the transmitted vector which are regenerated from the time-frequency matrices decoded by all the users' address codes are added with logical OR operation to produce candidates of the received matrix. The candidates of the received matrix are utilized in order to estimate a pattern of all users' data symbols which has the most number of coincident entries with the received matrix. Its BER (bit error rate) performance is evaluated by theoretical analysis and computer simulation in order to show the improvement of user capacity. Moreover, the authors investigate a hybrid scheme combining a multiuser detection scheme and the decoding scheme of an error-correcting code for the coded MFSK/FH-SSMA system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Adaptive retransmission control for slotted frequency-hopped spread-spectrum multiple access systems

The authors propose a simple adaptive retransmission control scheme for slotted frequency-hopped spread-spectrum multiple access systems. The scheme changes the retransmission probability adaptively according to the system state such that channel throughput can be maximised. It is shown that a performance quite close to the maximum possible throughput can be achieved.

Frequency hopped spread spectrum multiple access over cellular dispersive fading channels

The authors present a general method based on the concept of circularly symmetric signals, for the evaluation of the average probability of error of uncoded asynchronous noncoherent frequency hopped spread spectrum multiple access (FH-SSMA) communications over cellular dispersive fading channels. This method provides an accurate approximation and a tight upper bound to the symbol error probability of orthogonal MFSK over nonselective and selective fading channels, respectively. Nonselective fading is considered for the two extreme cases: a system with no interleaving and a channel with fading which is slow relative to the hopping rate; and a system which is fully interleaved. The authors consider a general class of Gaussian wide-sense-stationary uncorrelated scattering (GWSSUS) fading channels. Several conclusions are drawn from the application of this method, namely the influence of power control on nonselective fading. This is compared with the selective fading case, for which a different situation occurs. The Gaussian approximation method is also presented and compared.

Spread spectrum and coding for multiple access communications

Discusses the use of spread-spectrum modulation and error correction coding for multiple access communications such as mobile communications. The emphasis is on the connection between, and interdependence of, these two techniques. This connection is illustrated through three examples. The authors consider frequency-hopped (FH) spread-spectrum multiple access communications employing <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary modulation and error correction coding. A mismatch between, for example, <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -ary Reed-Solomon (RS) codes and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary modulation, which is most likely to occur in practice, can drastically reduce network capacity. As a remedy, a matched <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</i> -ary BCH code is shown to outperform an RS code with a comparable code rate and decoding complexity when the number of users in the system is large. Frequency hopping is a nonlinear multiple access system. For a linear multiple access system such as a cellular, direct-sequence, code division multiple access (CDMA) system, it is pointed out that the capacity is linearly inversely proportional to the signal-to-noise ratio ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> ) required at the receiver to maintain a given error performance. Therefore the key to increasing the CDMA capacity is to increase the coding gain to reduce the required <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">b</sub> / <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> . The authors demonstrate a connection between spread spectrum and coding by considering the analysis of the multiple access interference in an FH system.

Multiple hops/symbol fast frequency-hopping spread spectrum multiple access with coding for indoor radio

The paper is concerned with the bit error rate (BER) performance of multiple hops/ symbol fast-frequency-hopped spread-spectrum multiple-access (FFH-SSMA) systems operating through an indoor radio multipath Rayleigh fading channel. M-ary frequency-shift-keying (MFSK) modulation with noncoherent square-law envelope demodulation and Reed-Solomon (RS) coding is considered. Results obtained in this study show that an FFH-SSMA system is not sensitive to the changes in the number of paths. Employing RS coding with an increased number M of MFSK signals yields a significant improvement in BER performance. Assuming a given system bandwidth, using a single hop per symbol is the best solution when the multiaccess interference can be large. Using several hops per symbol is more advantageous when the multiaccess interference is small.