Spread Spectrum System Using Noise Band Shift Keying

A new spread spectrum technique using band-limited Gaussian noise for transmission and incoherent detection at the receiver has been discussed. The proposed system called noise band shift keying (NBSK) may be considered as a modified form of the frequency-shift keying (FSK) system. In the NBSK system nonoverlapping bandlimited noise is transmitted instead of pure tones as in the FSK system and the center frequency of the noise is chosen on the basis of the binary data signal. For this system, an expression for the probability of error has been derived and it has been shown that its performance under nonfading channel conditions is identical to that of the FSK system with frequency diversity under Rayleigh fading conditions. Compared to the conventional spread spectrum systems, the proposed method is considerably more simple to implement and is attractive where adequate bandwidth is available.     

Design of binary bandlimited signals imbedded in colored Gaussian noise and intersymbol interference

This paper considers the design of binary bandlimited receiver signals as well as the design of the receiver filter for signal immunity to intersymbol interference and additive colored Gaussian noise. The design problem is treated in the frequency domain. The maximum probability of error is minimized by choosing signals with zero intersymbol interference. When transmission is below the Nyquist rate, bandlimited signals with gradual cutoff are designed. Other waveforms, with intersymbol interference but possessing no sequences of unbounded amplitude, are also investigated. Examples are given demonstrating the application of the theory.     

An optimal signal design for band-limited asynchronous DS-CDMAcommunications

An optimal signal design for band-limited, asynchronous, direct-sequence code-division multiple-access (DS-CDMA) communications with aperiodic random spreading sequences and a conventional matched filter receiver is considered in an additive white Gaussian noise (AWGN) channel. With bandwidth defined in the strict sense, two optimization problems are solved under finite bandwidth and zero interchip interference constraints. First, a chip waveform optimization is performed given the system bandwidth, the data symbol transmission rate, and the processing gain. A technique to characterize a band-limited chip waveform with a finite number of parameters is developed, and it is used to derive optimum chip waveforms which minimize the effect of multiple-access interference (MAI) for any energy and delay profile of users. Next, a joint optimization of the processing gain and the chip waveform is performed, given the system bandwidth and the data symbol transmission rate. A sufficient condition for a system to have lower average probability of bit error for any energy profile is found, and it is used to derive some design strategies. It is shown that the flat spectrum pulse with the processing gain leading to zero excess bandwidth results in the minimum average probability of bit error. Design examples and numerical results are also provided     

新的扩频通信调制方法

针对传统扩频调制方法频谱效率低的问题,提出了一种新的扩频通信方法——时移位置调制方法,该方法利用伪随机序列良好的自互相关特性,通过扩频调制码元之间的相对位置加载信息,建立了信息传输速率与扩频码或扩频调制带宽之间的联系。理论分析和仿真结果表明,这种调制方法可以大大提高扩频系统的频带利用率,并可有效对抗宽带、窄带以及扩频干扰,为扩频通信技术的研究提供了一种新方法。     

Multitone spread spectrum multiple access communications system ina multipath Rician fading channel

The paper proposes a new multiple access communications system based on the combination of multitone transmission and direct sequence spectrum spreading (DS/SS). Multitone transmission is proposed because the associated larger symbol duration is favorable to combat the effects of multipath propagation. The multiple access capability of the system is achieved by means of DS/SS which also helps to combat the presence of several paths. On the other hand, DS/SS is also attractive to decrease the multiple access interference. The performance of this system is investigated for a multipath, slowly Rician fading and frequency selective channel, and a suboptimal receiver based on matched filters. The multiple access interference is also taken into account, A model for the computation of the bit error probability is derived for QPSK modulation of the orthogonal tones by the spread symbols. The system performance is investigated for selection diversity and maximal ratio combining (RAKE reception). Then the influence of the number of tones is investigated for given bandwidth, bit rate and transmission power. The positive effect of a multitone spread spectrum transmission is demonstrated     

Rejection of frequency sweeping signal in DS spread spectrumsystems using complex adaptive filters

It is already known that the performance of narrowband interference adaptive filtering in DS spread spectrum systems is additionally degraded in the presence of a frequency hopped jamming signal compared to the case with CW interference. Even so, the adaptive filter can be rather efficient because during the hopping interval, while the jamming signal parameters are constant, the filter adjusts its weights; and if the hopping rate is not too high, there are time periods when the interfering signal is suppressed. In order to have a better insight into the performance of these systems, we analyze the DS spread spectrum receiver behavior, when the jamming signal frequency is continuously changing (chirp signal). We find that this is a reasonable step for a jammer that is trying to be more efficient. In such a case, a two-sided filter structure shows worse overall performance than a prediction error structure. As the jamming frequency sweeping rate or bandwidth is increased, the system performance is degraded. The analysis provides an insight into how much the system performance will be degraded as a function of the interfering (frequency sweeping) signal parameters     

On the optimality of direct sequence for arbitrary interferencerejection

Communication over a waveform channel corrupted by additive white Gaussian noise, and by an unknown and arbitrary interfering signal of bounded power is considered. For this channel, the authors derive an upper bound to the worst case error probability of direct-sequence spread spectrum communication with a correlation receiver, and also a lower bound applicable to any binary signaling technique and any receiver. By comparing these two bounds, it is shown that, if a small error probability is required, then no other binary signaling scheme or receiver can substantially improve upon the performance of direct-sequence with a correlation receiver for the same power and bandwidth     

Short-time Fourier transform receiver for nonstationaryinterference excision in direct sequence spread spectrum communications

A new adaptive excision approach for nonstationary interference excision in direct sequence spread spectrum (DS/SS) communications is introduced. The proposed excision approach is based on the attractive localization properties of the impulse responses of the multiple pole filters. These impulse responses have Gaussian-like shapes and decrease in bandwidth with higher pole multiplicities. When used as data windows, they field a large class of computationally efficient short-time Fourier transforms (STFTs). Localization measures can be applied to determine the optimum window that maximally concentrates the interference in the time-frequency (t.-f.) domain. Interference mitigation is then achieved by applying a binary excision mask to the corresponding STFT for each data bit. We show that the proposed interference excision method permits both data-dependent windowing and time-varying filtering and leads to improved BER performance of the DS/SS system. The paper also derives the general optimum receiver implementing the STFT-based interference excision system     

直扩码分多址个人通信系统在衰落信道中的多用户信号检测

本文在ＤＳ－ＣＤＭＡ ＰＣＮ系统抗干扰性能的分析中，首次提出了用户归一化扩频信号等效左半部分和等效右半部分的分析模型，并在此基础上有效地结合传统扩频通信ＲＡＫＥ接收机的抗多径性能和多用户信号检测器的抗多址干扰特性，得到了一种能同时消除同频干扰和对抗多径多用户信号检测新方法。     

Performance of a fast frequency-hopped noncoherent MFSK receiverwith nonideal adaptive gain control

An error probability analysis is performed for an orthogonal noncoherent M-ary frequency-shift keying (MFSK) communication system employing fast frequency-hopped (FFH) spread spectrum with diversity. The signal is assumed to be transmitted through a frequency-nonselective slowly fading channel with partial-band noise interference. The partial-band interference is modeled as a Gaussian process. Both the information signal and the partial-band noise interference signal are assumed to be affected by channel fading; it is assumed that the two fading processes are independent and that channel fading need not necessarily affect the information signal and the interference signal in the same way. Each diversity reception is assumed to fade independently according to a Rician process. Adaptive gain control is employed to minimize partial-band interference effects, and the effect of inaccurate noise measurement on the ability of the adaptive gain control receiver to reject partial-band interference is examined. The effect of thermal noise is included in the analysis     

Maximum Likelihood Sequence Estimation of Binary Sequences Transmitted Over Bandlimited Nonlinear Channels

The problem of designing and evaluating the performance of a maximum likelihood sequence receiver for binary PSK transmission over bandlimited nonlinear channels is considered in this paper. The effects of intersymbol interference followed by AM/AM and AM/PM conversions are taken into account while optimizing the performance in the presence of white Gaussian noise. A new representation for the output of a bandpass nonlinearity is given when the input consists of a carrier signal modulated by a sum of binary overlapping pulses. The structure of a maximum likelihood sequence receiver for a bandlimited nonlinear channel is derived using this representation. The receiver uses a modified Viterbi algorithm to determine the most likely sequence of data symbols transmitted. An upperbound on the probability of symbol error for this receiver is obtained. Numerical results illustrating the applicability of the present work to optimizing the performance of a digital satellite communications link are also presented.     

On the detection of bandlimited direct-sequence spread-spectrumsignals transmitted via fading multipath channels

Recent investigations suggest bandlimited direct-sequence spread-spectrum transmission for use in CDMA mobile radio systems. In this paper, transmission systems with signal bandwidths not exceeding the chip rate are considered. First, the optimum noncoherent single-user receiver and various suboptimum approximations thereof are derived. These receivers utilize the fact that the received signals can be sampled at chip rate. This results in receiver concepts, which are well suited for fully digital implementations. Then, it is shown that the main result on the structure of the optimum single-user receiver is directly applicable to optimum noncoherent multiuser joint detection and interference cancellation. In this paper, noncoherent detection is considered. By this, we mean that only the channel statistics and not the channel itself is known to the receiver. It is shown that due to this less restrictive assumption, more powerful channel coding and interleaving schemes can be employed     

P-order metric UWB receiver structures with superior performance

The generalized Gaussian probability density function is shown to better approximate the probability density function of the multiple access interference in ultra-wide bandwidth systems than the Gaussian approximation and the Laplacian density approximation. Two ultra-wide bandwidth receiver structures based on this new approximation using a p-order metric receiver decision statistic are investigated for the detection of time-hopping ultra-wide bandwidth wireless signals in multiple access interference channels. The first receiver outperforms both the conventional matched filter ultra-wide bandwidth receiver and the soft-limiting ultra-wide bandwidth receiver when only multiple access interference is present in UWB channels. The second new receiver with adaptive limiting threshold outperforms the conventional matched filter ultra-wide bandwidth receiver, the soft-limiting ultra-wide bandwidth receiver, and the adaptive threshold soft limiting ultra-wide bandwidth receiver in all multiple access interference-plus-noise environments. In multipath channels, a new Rake receiver based on the p-order metric receiver is proposed for signal detection. Mathematical analysis and numerical results show that this new Rake receiver can achieve larger signal-to-interference-plus-noise ratio than the standard matched filter Rake receiver when multipath components are resolvable in UWB channels.     

Analysis and optimization of DS-CDMA systems with time-limited partial response chip waveforms

Conventional direct sequence code division multiple access systems (DS-CDMA) using offset quadrature phase shift key (OQPSK) usually employ a strictly bandlimited partial response square-root raised cosine pulse as the chip waveform. They have the disadvantage of large envelope fluctuation that will incur performance degradation due to the intermodulation and bandwidth enlargement caused by post nonlinear processing. To improve the performance of DS-CDMA systems, the chip waveform and receiver should be properly selected. This paper presents a systematic performance analysis of a matched filter receiver and zero-forcing filter (ZF) receiver for DS-CDMA using a time-limited partial response chip waveform. Nevertheless the systematic performance analysis is applicable to bandlimited chip pulse as well. For the zero-forcing filters, we propose to select the frequency responses that satisfy the first Nyquist criterion. With this class of filters, we can choose the roll-off factor to minimize the total power of multiple access interference and noise power. The zero-forcing filter with proper choice of roll-off factor, referred to as optimum ZF, yields a performance better than the matched filter counterpart. The bit error rate (BER) performance of the optimum ZF with superposed quadrature amplitude modulation signal as the time pulse waveform is evaluated. It is shown that the optimum ZF provides better BER performance than conventional OQPSK and minimum shift keying, and its envelope uniformity is much better than that of OQPSK.     

Narrow‐band interference suppression in wavelet packets based multicarrier multicode CDMA overlay system

The wavelet packets based multicarrier (MC) multicode (MCD) code‐division multiple‐access (CDMA) transceiver consists of the MCD part, which ensures the transmission for high speed and flexible data rate; the MC part contributing to robustness to frequency‐selective fading and flexibility for handling multiple data rates; and wavelet packets (WPs) modulation technique, which contributes to the mitigation of the interference problems. As WPs have lower sidelobes compared with sinusoidal carriers, this system is very effective in reducing the problem of inter‐carrier interference. Of course, like any CDMA system, the system can suppress a given amount of interference. This paper considers an interference suppression scheme which will enhance the performance of the system. The receiver employs suppression filters to mitigate the effect of narrow‐band jammer interference. The framework for the system and the performance evaluation are presented in terms of the bit error rate and the outage probability over a Nakagami fading channel. Also, we investigate how the performance is influenced by various parameters, such as the number of taps of the suppression filter and the ratio of narrow‐band interference bandwidth to the spread‐spectrum bandwidth. Finally, the performance of the system is compared with the performance of sinusoidal based MC/MCD‐CDMA system denoted Sin‐MC/MCD‐CDMA. Copyright © 2012 John Wiley & Sons, Ltd.     

Adaptive interference suppression for CDMA overlay systems

It has been proposed that CDMA systems can be assigned to spectral bands which are presently occupied by narrowband users to further increase spectral capacity. Such CDMA overlay systems could provide new options for efficient utilization of the spectrum with minimal disruption to existing narrowband users, especially if adaptive interference suppression techniques are utilized in the spread spectrum receiver. Previous studies have defined the SNR improvement ratio which can be achieved for tone interferers and for narrowband interferers for which the center frequency of the interference is at the carrier frequency of the CDMA signal. In this paper the bit-error-rate (BER) performance of the mobile-to-base link of a CDMA system for a single narrowband user which occupies a significant portion of the CDMA bandwidth is evaluated. It is shown that the narrowband model used in previous studies does not apply in this case, especially for the large, effective, bandwidths which are characteristic of the interferers in the overlay system. The dependence of the BER on the filter order, the bandwidth of the interference, and its center frequency relative to the CDMA carrier frequency are defined. Additionally the increase in BER for a digital implementation of the adaptive suppression filter relative to the optimal Wiener filter is characterized with respect to the adaptive time constant and the quantization errors due to finite wordlength. It is shown that these implementation errors can be made negligible compared to the errors which are characteristic of the optimal Wiener filter. Analytic results are validated by simulation for typical system parameters     

Digital Transmission Performance on Fading Dispersive Diversity Channels

The reception and detection of a single digit under known channel conditions are investigated. The probability of error for an optimum one-shot receiver instantaneously matched to the channel state is averaged over an ensemble of dispersive diversity channels. The average probability of error as a function of energy to noise ratio is found to be solely dependent on the ratio of rms dispersion width to data symbol width. For these dispersive channels an implicit diversity effect is qualitatively explained in terms of eigenvalues that depend on the ensemble statistic. The one-shot receiver performance provides a bound for practical receivers. In a comparison with a decision feedback equalizer, it is shown that on moderately dispersive channels the equalizer nearly achieves optimum one-shot performance. Since an adaptive version of this equalizer exists, this means data transmission on slowly fading channels is possible at rates above the natural rate suggested by the channel dispersion spread without bandwidth expansion and with small intersymbol interference penalty. The use of one-shot receiver performance curves can also be used as estimates of equalizer performance in situations where computation of the latter is impractical.     

Space-Time Optimum Combining for CDMA Overlay for Personal Communications Systems

The requirement to suppress narrowband interferences in CDMA communication stems from the overlay concept, i.e., coexistence of different types of signals in the same frequency band. This paper describes the performance analysis of a direct-sequence (DS) CDMA personal communication system sharing a common spectrum with narrowband microwave radio links in the 1.85 to 1.99 GHz band. The coexistence of these two systems within the same frequency band will improve the overall spectrum efficiency, but will also cause interference to both systems. In this paper it is shown that joint spatial and temporal optimum combining provides an efficient means of improving the performance of the DS-CDMA system through cancellation of the narrowband signal and the co-channel interferences. The proposed space-time architecture provides degrees of freedom for both diversity and interference cancellation. It is shown that the joint space-time receiver is robust with respect to the narrowband interference signal bandwidth and its carrier frequency offset from the DS-CDMA carrier frequency.     

Code tracking performance analysis of GNSS signal in the presence of CW interference

Continuous wave (CW) interference can cause severe performance degradation of global navigation satellite system (GNSS) receiver. In analysing the code tracking performance of pilot channel under CW interference, the signal spectrum consists of discrete spectral lines and the interference term of the correlator output shows the characteristics of direct current or sine. Thus the traditional assumptions that the interference obeys the Gaussian distribution and that the spectrum of the signal is continuous no longer hold. Considering this fact, this paper presents the analytic expressions of the code tracking error bound for the early-minus-late power (EMLP) discriminator and the dot-product (DP) discriminator. The derived expressions can be used to assess the susceptibility of code tracking performance under CW interference for the pilot channel of GNSS signals. Then the GPS L5 signal is taken as an example of the application of the derived mathematic expressions. The effects of finite front-end bandwidth, correlator spacing, and discrete spectrum of pseudo-random noise (PRN) code on performance are discussed. It is shown that the code tracking performance can be improved by choosing proper receiver parameters and some of the L5 codes are more vulnerable to the interference compared with the average level.