A novel approach to modeling of OQPSK-type digital transmissionover nonlinear radio channels

Offset quaternary PSK-type (OQPSK) digital modulation schemes are often recommended for radio transmission systems employing grossly nonlinear amplifiers, and have found widespread use in land mobile and/or satellite communications. Most of the recommended OQPSK-type schemes-also called MSK-type schemes-jointly exhibit the following features: a constant envelope or, at least, a low envelope fluctuation; a compact spectrum after a power-efficient, saturated amplifier, and a high detection efficiency obtained with simple, low-cost receivers. We introduce a novel approach to computer-aided modeling of OQPSK-type digital transmission over nonlinear radio channels. By taking into account the specific nature of OQPSK-type modulations, when using the Volterra approach as proposed by Benedetto et al. (1987), we conclude that two signal representations (“parallel” and “serial”) can be achieved which are similar to the conventional Laurent (1986) representation of binary CPM signals with h=1/2. With the proposed approach, the OQPSK-type signals have an invariant structure along the transmission path (which is supposed to include, at least, one nonlinearity) in the sense that they are all made of several linear components of the same type. Some examples are given, including signal representation and power spectrum results. The proposed unified signal representations can be very useful for design and performance evaluation of radio communication systems (mobile and/or satellite applications). They combine flexibility, accuracy, and simplicity, and allow high computational efficiencies     

Per‐antenna power minimization in symbol‐level precoding for the multibeam satellite downlink

This paper addresses the problem of multiuser interference in the forward downlink channel of a multibeam satellite system. A symbol‐level precoding scheme is considered, to exploit the multiuser interference and transform it into useful power at the receiver side, through a joint utilization of the data information and the channel state information. In this context, a per‐antenna power minimization scheme is proposed, under quality‐of‐service constraints, for multilevel modulation schemes. The consideration of the power limitations individually for each transmitting radio frequency chain is a central aspect of this work, and it allows to deal with systems using separate per‐antenna amplifiers. Moreover, this feature is also particularly relevant for systems suffering nonlinear effects of the channel. This is the case of satellite systems, where the nonlinear amplifiers should be properly driven to reduce the detrimental saturation effect. In the proposed scheme, the transmitted signals are designed to reduce the power peaks, while guaranteeing some specific target signal‐to‐noise ratios at the receivers. Numerical results are presented to show the effectiveness of the proposed scheme, which is compared both with the state of the art in symbol‐level precoding and with the conventional minimum mean square error precoding approach.     

A novel correlation adaptive receiver structure for high speed transmissions in ultra wide band systems with realistic channel estimation

Impulse radio ultra wide band (UWB) communications require robust receivers; typically Rake receivers are required to capture a large number of resolvable paths, (even hundred of paths), so large number of correlators are needed; otherwise, adaptive receivers use complex filters and channel estimation algorithms. Therefore, traditional Impulse radio receivers demand non-practical implementation structures. In this paper we propose a novel correlation-adaptive receiver structure with low complexity for indoor high speed ultra wide band systems. This novel structure combines correlation characteristics from Rake receivers with recursive filters from adaptive receivers. The receiver includes a low complexity recursive channel estimation filter capable of estimating hundreds of channel impulse responses, and a single filter-correlation filter used for coherent bit demodulation. Furthermore, we derive by simulations the bit error rate for high density multipath environments for several impulse radio modulations like TH-PPM, DS-BPSK and TH-BPSK and we compare the performance of the proposed structure with typical Rake receivers.     

The implementation of channel diversity in mobile software radio receivers

A novel channel diversity concept is proposed and demonstrated, which avoids receiving signal deterioration due to multipath fading in mobile receivers. The system is based on coherent superposition of the signals received from several transmitters supplying the same information at different frequencies. Based on a software radio architecture this concept may increase the quality of mobile reception in modern car receivers considerably. Compared with multiantenna-receivers which overcome the multipath fading problem by simultaneously receiving the same program with several antennas, the proposed solution is advantageous, since it requires only a single antenna.     

A hierarchical modulation for upgrading digital broadcast systems

A hierarchical modulation scheme is proposed to upgrade an existing digital broadcast system, such as satellite TV, or satellite radio, by adding more data in its transmission. The hierarchical modulation consists of a basic constellation, which is the same as in the original system, and a secondary constellation, which carries the additional data for the upgraded system. The upgraded system with the hierarchical modulation is backward compatible in the sense that receivers that have been deployed in the original system can continue receiving data in the basic constellation. New receivers can be designed to receive data carried in the secondary constellation, as well as those in the basic constellation. Analysis will be performed to show the tradeoff between bit rate of the data in secondary constellation and the penalty to the performance of receiving the basic constellation.     

The probability of multiple correct packet receptions indirect-sequence spread-spectrum networks

We provide methods to evaluate the probabilities P(l, m-l|K), l=0, 1, ..., m and m⩽K of exactly l correct packet receptions in a group of m receivers, given that K packets are transmitted simultaneously from users employing direct-sequence spread spectrum (DS/SS) signalling schemes. This quantity is useful for the design and performance evaluation of protocols for admission control and dynamic code allocation in multiple-access spread spectrum packet radio networks intended for terrestrial or satellite applications. The evaluations are carried out for DS/SS networks employing BPSK modulation with coherent demodulation and convolutional codes with Viterbi decoding. Systems with geographically dispersed receivers and systems with colocated receivers are considered. Approximations based on the independent receiver operation assumption (IROA) and the Gaussian multivariate distribution are developed, and their accuracy is checked against the exact expressions derived for synchronous systems. The joint first error event approximation (JFEEA) is also developed for coded systems and compared to the IROA     

Parametric macromodels of differential drivers and receivers

This paper addresses the modeling of differential drivers and receivers for the analog simulation of high-speed interconnection systems. The proposed models are based on mathematical expressions, whose parameters can be estimated from the transient responses of the modeled devices. The advantages of this macromodeling approach are: improved accuracy with respect to models based on simplified equivalent circuits of devices; improved numerical efficiency with respect to detailed transistor-level models of devices; hiding of the internal structure of devices; straightforward circuit interpretation; or implementations in analog mixed-signal simulators. The proposed methodology is demonstrated on example devices and is applied to the prediction of transient waveforms and eye diagrams of a typical low-voltage differential signaling (LVDS) data link.     

MC-BOC:应用于导航通信一体化系统的新型调制技术

针对导航通信一体化系统面临的在有限带宽内实现高速率信号传输和高精度测量的难题,结合正交频分调制与恒包络调制思想,提出了多载波二进制偏移载波（multi-carrier binary offset carrier,MC-BOC）调制,其能够有效利用频谱资源,提升信号的接收性能,并具备恒包络特性.分析北斗无线电测向卫星业务（radio determination satellite service,RDSS）采用该调制后的性能,结果表明:该调制方式具备优良的通信性能、抗干扰能力、测距性能以及抗多径能力.本文提出的MC-BOC调制技术为导航通信一体化系统信号体制设计和优化提供了一种可行的参考方案,可以用于北斗RDSS和站间时间同步/数据传输网络的信号设计.     

A model for channel sharing in land mobile radio dispatch services

A modeling approach to the problem of channel sharing in land mobile radio dispatch services, which is based on real life traffic data, is presented. In order to apply the GI/G/1 queueing model, the first step consists in identifying a message arrival process for each user by means of observed available data. It is then shown how to use simple approximation formulas to evaluate the expected waiting time, when two or more independent users must share a common channel. GPSS simulations were performed to verify that the proposed approach is suitable to practical applications.     

Land mobile GNSS availability and multipath evaluation tool

Applications of global navigation satellite system (GNSS) in land transportation systems are already extensively deployed and will certainly continue to grow especially in the framework of intelligent transport systems. However, one of the best-known drawbacks of such a system is the lack of satellite visibility in dense urban areas as well as in some specific embedded railway environments. This restricts considerably GNSS use for extended safety related applications. In this paper, a new tool is proposed to predict the availability of a satellite constellation from the point of view of the land transportation user. Knowing the trajectory of a land vehicle, the tool predicts the number of satellites that will be received and produces a safety criterion able to qualify the GNSS localization result. A first version of the tool, already in operation, merges an image processing approach providing the knowledge of the land environment, and the output of a satellite tracking program predicting satellite positions in the sky. This allows us to determine, using a simple optical approach, the number of satellites received in line-of-sight or blocked, with regard to the nearby environment of the receiving antenna. Results obtained in railway as well as in road environments show that satellite signals received by multipath are often used by GNSS receivers in the localization process. Thus, propagation characteristics of the satellite signals in an urban canyon configuration were characterized to determine when a signal received by reflected ray is used by the receiver or not. A criterion related to the satellite elevation is defined to improve the overall performance of the predictive tool. Comparisons with real measurements are commented on. Both simulations and measurements are very similar.     

Application of convolutional error correcting codes in ultrawideband M-ary PPM signaling

The interference issues related to ultrawideband (UWB) radio pose tight restrictions on the maximum data rate of UWB radio telecommunication systems. A possible solution is to reduce the required signal to interference ratio (SIR) that gives satisfactory performance to the UWB system. In this letter, we propose coded M-ary UWB radio communication systems. Two classes of convolutional codes, namely, low-rate superorthogonal codes and high-rate punctured codes are considered for this purpose. Simulation results on the bit error rate of the proposed system indicates that the system is capable to work in lower SIR's and therefore supports higher data transmission rates in a real interference environment compared to the previously proposed UWB communication systems.     

Robust Linear Receivers for Multiaccess Space–Time Block-Coded MIMO Systems: A Probabilistically Constrained Approach

Traditional multiuser receiver algorithms developed for multiple-input–multiple-output (MIMO) wireless systems are based on the assumption that the channel state information (CSI) is precisely known at the receiver. However, in practical situations, the exact CSI may be unavailable because of channel estimation errors and/or outdated training. In this paper, we address the problem of robustness of multiuser MIMO receivers against imperfect CSI and propose a new linear technique that guarantees the robustness against CSI errors with a certain selected probability. The proposed receivers are formulated as probabilistically constrained stochastic optimization problems. Provided that the CSI mismatch is Gaussian, each of these problems is shown to be convex and to have a unique solution. The fact that the CSI mismatch is Gaussian also enables to convert the original stochastic problems to a more tractable deterministic form and to solve them using the second-order cone programming approach. Numerical simulations illustrate an improved robustness of the proposed receivers against CSI errors and validate their better flexibility as compared with the robust multiuser MIMO receivers based on the worst case designs.     

Performance of FH SS radio networks with interference modeled as amixture of Gaussian and alpha-stable noise

We consider the performance of frequency-hopping spread spectrum (FH SS) radio networks in a Poisson field of interfering terminals using the same modulation and power. The problem is relevant to wireless random-access communication systems where little information about transmitters requires stochastic modeling of their positions. Assuming that the signal strength is attenuated over distance r on average as 1/r ^m, we show that the interference in the network could be modeled as a mixture of Gaussian and α-stable noise. Based on this modeling, we derive expressions for the probability of error (P_e ) for systems with M-ary frequency shift keying (FSK) which use conventional envelope detectors. Because conventional envelope detectors are optimum only in Gaussian noise and are suboptimum in the noise considered, we also investigate noncoherent detectors which offer improved performance. We examine receivers with limiting nonlinearities and detectors which are optimal in Cauchy noise. Numerical calculations and Monte Carlo simulations are provided to confirm the accuracy of the analysis presented. The results obtained are useful in the performance evaluation of multiple-access radio networks in environments varying from urban settings to office buildings with deterministic and stochastic propagation laws such as lognormal shadowing and Rayleigh fading     

A New and Exact Analysis of DS-CDMA Cellular Radio System Based on the Physical Interpretation of Nakagami Wideband Channel

In this paper we present the performance of two uplink/downlink DS-CDMA receivers in a Nakagami wideband channel. Two detection methods were considered; namely, coherent and differential detection RAKE receivers based on maximal ratio combining technique. We consider a new physical interpretation of the Nakagami channel and the instantaneous power of the multiple access interferers. In comparison with the previous researches in the literature, which are based on Gaussian approximation, this approach results in a more accurate solution for the problem. We analyzed and derived closed form formulae for probability of bit error for DS-CDMA cellular radio system in the Nakagami wideband channel. We presented a set of numerical results for coherent downlink receivers.     

Neural network modeling and identification of nonlinear channelswith memory: algorithms, applications, and analytic models

This paper proposes a neural network (NN) approach for modeling nonlinear channels with memory. Two main examples are given: (1) modeling digital satellite channels and (2) modeling solid-state power amplifiers (SSPAs). NN models provide good generalization performance (in terms of output signal-to-error ratio). NN modeling of digital satellite channels allows the characterization of each channel component. Neural net models represent the SSPA as a system composed of a linear complex filter followed by a nonlinear memoryless neural net followed by a linear complex filter. If the new algorithms are to be used in real systems, it is important that the algorithm designer understands their learning behavior and performance capabilities. Some simplified neural net models are analyzed in support of the simulation results. The analysis provides some theoretical basis for the usefulness of NNs for modeling satellite channels and amplifiers. The analysis of the simplified adaptive models explains the simulation results qualitatively but not quantitatively. The analysis proceeds in several steps and involves several novel ideas to avoid solving the more difficult general nonlinear problem     

Satellite radio interface and radio resource management strategy for the delivery of multicast/broadcast services via an integrated satellite-terrestrial system

A variety of hybrid systems combining third-generation mobile communication networks with broadcast systems have been proposed for the delivery of multimedia broadcast multicast services (MBMS) to mobile users. The article discusses one of these alternatives, which involves the use of a geostationary satellite component for MBMS delivery. In particular, it proposes a radio access scheme for the satellite component of the system that features maximum commonalities with the standardized T-UMTS WCDMA-based interface. The ultimate advantages of this approach are more efficient delivery of MBMS as far as the mobile network operator is concerned. The required adaptations at the interface layers are described, and the radio resource management strategy that fulfills the particular requirements of the satellite system is presented.     

Control scheme for micro/macro/megacell-overlaying cellular systemincluding an LEO satellite system

A flexible radio resource management technique is proposed for multilayered cellular systems that consist of megacells using LEO satellites, and macrocell and microcell overlaid terrestrial cellular systems. In the proposed system, the radio spectra for microcell and macrocell terrestrial systems are adaptively and autonomously shared according to the traffic conditions of each microcell. Moreover, the radio resource for a satellite system is also dynamically shared by a temporarily unused satellite band. Computer simulation confirms that the proposed system can achieve high system capacity by reusing the radio resource allocated for the satellite band     

WAW Location-based Service Positioning

Coordinated positioning operations require the user to be aware, not only, of his own position but also of the positions of the other elements on his team. Its deployment can be supported in WAW (??Where Are We???) location-based services, where the position of each user is made available to the rest of the group. This paper presents a method for the estimation of a user??s position based on measurements from a group of Global Positioning System (GPS) receivers. None of the GPS receiver positions, used in the estimation process, is considered to be known. The proposed approach leads to an improvement on the accuracy of the position estimates, when compared with those computed on an autonomous way, and has a direct application in the development of enhanced services in WAW location-based services. The positioning estimator takes advantage from the fact that, in this type of location-based service, GPS measurements are available from several receivers. Instead of estimating the user??s position based only on the measurements of his own GPS receiver, the proposed approach uses the measurements from all the GPS receivers in the group. The paper presents details of the positioning model and estimator used to achieve the accuracy improvements. Simulation results are presented based on real GPS satellite ephemeris, collected at the University campus.     

导航卫星阵列天线的快速现场系统校准算法

阵列天线接收处理全球卫星导航信号时,由于天线和射频前端的非理想性,会在不同方向卫星信号中引入不同的载波相位偏差,从而破坏卫星导航接收机距离观测量准确性。针对此问题,提出了一种易实现的卫星导航阵列天线的快速现场系统校准算法。校准过程分为通道校准和天线校准：通过开机和周期进行的通道校准实现阵列天线接收射频前端的通道响应测量;通过与接收处理不同卫星基带数据的数字接收机配合,实现阵列天线的现场阵列流形矢量校准。仿真验证了所提校准算法的正确性和有效性。