Theoretical analysis of M-ary/SSMA systems utilizing multicarrier techniques

This paper presents a theoretical analysis of the performance of a filter bank-based multicarrier spread spectrum (FB-MC-SS) system. In FB-MC-SS, each data symbol is spread across multiple subcarriers, but - contrary to some other systems - there is no spreading in time. The results of this system are then compared with those of the well-known direct sequence spread spectrum (DS-SS) system with a rake receiver for its best performance. We compare the two systems when the channel noise is white. We prove that as the processing gains of the two systems tend to infinity both systems approach the same performance. However, numerical simulations show that in practice, where processing gain is limited, FB-MC-SS outperforms DS-SS.

In this report we consider adaptive multistage interference cancellation within asynchronous spread spectrum multiple access (SSMA) communication systems using spreading sequences of Markov chains. We show that for synchronous systems, the empirical mean of the multiple access interference (MAI) at the second stage of interference cancellation satisfies the large deviations principle (LDP) with the convex, good rate function. By refinement of the large deviations analysis, we obtain exact asymptotics of bit error probabilities in such systems. We also discuss evaluations of bit error probabilities in asynchronous systems based on the LDP.

This paper investigates the performance of a hybrid system based on the combination of slow frequency hopping (SFH) with multitone direct sequence (DS) spreading as originally proposed by Vandendorpe (see International Zurich Seminar on Digital Communications, 1994). The coherent receiver structure considered in the paper is optimum for AWGN only but its performance is analyzed for a multipath slowly Rician fading channel, in the presence of it multiple access interference. Selection diversity and Rake reception are considered. >

The performance of a hybrid direct sequence/frequency hopped (DS/FH) spread spectrum multiple access communication system is evaluated. The modulation considered is quadrature phase shift keying modulation. A mobile radio channel is investigated in the presence of interference and noise. Both deterministic and random signature sequences and memoryless random frequency-hopping patterns are considered. The maximum number of simultaneous users (capacity) at a desired level of bit error probability (10/sup -3/ - 10/sup -2/) and the spectrum efficiency are considered as performance measures. Simulation results for both synchronous and asynchronous hybrid systems are presented.

Hybrid spread-spectrum (HSS) systems have recently received considerable interest in commercial, Smart Grid, and military communication systems because they accommodate high data rates with high link integrity, even in the presence of significant multipath effects and interfering signals. A highly useful form of this modulation scheme is the specific code-related combination of standard direct-sequence spread spectrum (DSSS) with fast frequency-hopping (FFH) spread spectrum, denoted hybrid DS/FFH, wherein multiple frequency hops occur within a single data-bit time. In this paper we perform a simulation-based study of the DS/FFH performance as compared to the existing standard DSSS and FHSS wireless networks. The performance metrics are biterror probability and multiple-access capability. The parameter space of DS/FFH, including the DS spreading rate, frequency hopping rate, carrier frequencies, and numbers of users, is explored to show its performance under frequency-selective Rayleigh fading environments and multiuser interference. Direct digital synthesizers to achieve fast hopping speeds are also considered in our study.

This paper presents a comparison of direct sequence spread spectrum (DS-SS) and filter bank multi carrier spread spectrum (FB-MC-SS) systems when applied for communications over HF channels. Such systems by their spread spectrum nature are, clearly, broadband. On the other hand, noting that the presence of narrowband interferers are inevitable in HF channels, interference suppression performance has to be given a special consideration when such systems are designed. In this paper DSSS and FB-MC-SS are compared using measured HF interference data. FB-MC-SS is shown to have very good performance while using a low complexity and robust maximum ratio combining receiver. The partitioned structure of the FB-MC-SS waveform enables this implementation. Among the DS-SS receivers in the literature, we find that only the highly complex minimum mean square error (MMSE) receiver can provide a comparable performance to its FB-MC-SS counterpart. Considering the receiver complexity, it becomes obvious that FB-MC-SS is the right choice for further developments of HF spread spectrum modems in the future.

A direct sequence spread spectrum multiple access communication system based on differential /spl pi/4-shifted QPSK modulation and convolutional coding is analyzed. The channel model is frequency selective fading typical of mobile digital radio systems. An efficient decoding scheme is proposed to relieve performance losses at the receiver due to user interference. Sequential decoding is a valid alternative to Viterbi decoding, and particularly the backtracking algorithm when long constraints convolutional codes are imposed by system performance. Purely breadth first sequential decoding algorithms such as the M-algorithm present inherent parallelism (adapt to a parallel HW realization) but are, as pointed out by Anderson [1992], less efficient in terms of average number of paths searched with respect to the backtracking algorithm, while representing a search performed by the Viterbi algorithm. The authors propose a technique which is a modified version of a Fano-like algorithm in which the forward operations are performed by means of a breadth first algorithm: this modification presents many advantages. First of all, backtracking events are limited. In this way the decoder proceeds faster and with a lower delay. Second: the searches are made in parallel, allowing a smaller buffering and hence a simpler VLSI realization with respect to the traditional Fano decoder. An analysis is performed of the uncoded detection scheme and the results of the simulations of the encoded scheme are compared. Large coding gains seem possible when employing an adapt interleaving and an efficient power control system. >

The authors investigate the performance of hybrid direct sequence (DS) slow frequency hopped (SFH) spread spectrum multiple access communication systems over a Rayleigh multipath fading channel for indoor wireless communications. Systems with binary, M-ary frequency shift keying, as well as differential phase shift keying modulation and noncoherent demodulation, are studied. Their average error probabilities are analyzed for both deterministic and random frequency hopping pattern signature sequences. Numerical results show that the performance of the hybrid system degrades considerably over the multipath fading channel as compared to the nonfading channel. The system's performance degrades gracefully as the number of simultaneous users increases. >

In the presence of narrowband interference, non-Wiener effects have been observed for the normalized least-mean-square (NLMS) algorithm and the affine projection algorithm (APA). The convergence performance of the affine projection algorithm (APA) is analyzed in the presence of strong narrowband interference. Firstly, the relationship between the APA and the NLMS algorithm is derived, with the help of decorrelation of the input signal. Then, the statistical properties of the decorrelated input signals are derived, from which we can see that the convergence performance of the APA is almost not affected by the strong narrowband interference. The computer simulation results support the theoretical analysis.

Proposes a neural network for a multiuser detector in a parallel combinatory/spread spectrum (PC/SS) multiple access communication system. In the PC/SS system, M orthogonal sequences are assigned for one user, and q out of M (q/spl les/M) sequences are used for transmitting information. The combination of sequences can transmit more information than the conventional CDMA systems. In general, all of the users' signals overlap in time and frequency and cause mutual interference. Multiuser detection is a vital technique for combating mutual interference and near-far problems. The paper proposes a neural system for the multiuser detector in the PC/SS, which can output three values, +1, -1 and 0. It is compared with the conventional detector, and its performance evaluated.

The use of filter banks for implementing multicarrier spread spectrum systems leads to a class of effective waveforms that are highly resilient to partial-band interferers. Such waveforms can be also designed to keep the peak-to-average power ratio (PAPR) of the resulting signal at a minimum level. The use of multiple spreading gain vectors (known as multicodes), on the other hand, is an effective method for increasing the data rate in spread spectrum systems, in general. This paper presents a detailed analysis of a class of filter bank multicarrier spread spectrum (FBMC-SS) waveforms and demonstrates an effective receiver implementation of them when multicodes are applied. Application of the developed multicode waveform for communications over high-frequency (HF) skywave channels is also explored, and the benefits that it provides are studied both numerically, through computer simulations, and experimentally, by examining the receivers performance over a variety of skywave links.

The frequency offset estimation processing scheme for the multi-carrier direct sequence spread spectrum (MC-DSSS) transceiver is proposed. The design principle of the proposed frequency offset estimation processing for MC-DSSS transceiver was derived. The coarse frequency offset can be preliminarily measured by transmitting a single pulse to save the required estimation processing time. The simulation results demonstrate that the resolution of the proposed frequency offset estimation processing scheme improves with the increasing order of the QAM symbol signal. When the receiver signal-to-noise ratio (SNR) exceeds 4.2 dB, the frequency estimation accuracy of the frequency offset estimation processing using 64-QAM symbol signal is better than that using 16-QAM symbol signal.

In this paper two fundamental aspects for wireless data transmission are addressed, namely the interrelation between multicarrier spread spec- trum transmission and multilevel coding and the view of downlink trans- mission as the broadcast channel from information theory. In the first part it will be shown that multicarrier spread spectrum systems can be interpreted as a multilevel code with repetition codes in all levels and different mappings onto the modulation alphabet. This point of view might provide new insights into both, multicarrier spread spec- trum systems as well as multilevel coding. In the second part we will describe the information theory point of view for the situation when data are transmitted from one sender to many receivers. This is called the broadcast channel. The opposite case when data are transmitted from many senders to one receiver is called the multiple access chan- nel. It is known that the broadcast channel has a larger capacity than the multiple access channel which might be exploited in the downlink transmission .W ith the help of elementary examples these relations will be illustrated.

Power line communications (PLC) are gaining popularity in smart grid applications. However, current PLC systems with high complexity and sensitiveness to impulse noise and carrier frequency error remain difficult to employ for robust communications over highly unpredictable harsh power line channel. This paper presents a low complexity and high flexibility multicarrier (MC) spread spectrum (SS) system by frequency division multiple access (FDMA) approach to cope with hostile environment with multipath frequency-selective fading. With the number of subcarriers given by the length of sampled data block, experimental results demonstrate that the developed system can effectively combat the channel effects by enabling differentially coherent detection between the subcarriers with variable channel spacing, leading to robust communications.

Tone interference suppression in DS-SS systems with modified DFT