Nonlinear Transponder Research Articles

RPAS COMMUNICATION CHANNELS BASED ON WCDMA 3GPP STANDARD

First built models of Remotely Piloted Air System (RPAS) communication channels based on Wideband Code Division Multiple Access (WCDMA) 3GPP Standard were designed. Base Station (BS) transmission within the Radio Line of Sight (RLoS) and through the satellite using Beyond Radio Line of Sight (BRLoS) was considered. The dependencies of the Bit Error Rate (BER) on the signal-noise ratio for different RPAS velocities and WCDMA сhannel models were obtained. The dependences of the BER on the signal-noise ratio for different levels of satellite transponder nonlinearity were studied. The dependence of the BER on the BS antenna diameter in case of the transponder nonlinearity was analysed. The dependencies for satellite channel characteristics, first obtained taking into account the motion of the RPAS, make it possible to predict the behavior of the communication system in critical conditions.

RPAS Satellite Communication Channel Based on IEEE 802.11b Standard

Abstract Original models of RPAS communication channels based on IEEE 802.11b Standard, including both Base Station transmission within the Radio Line of Sight, and through the satellite using Beyond Radio Line of Sight, were built. Dependencies of the Bit Error Rate on the Signal-Noise Ratio for different payload data rates were obtained. Transponder nonlinearity and Base Station antenna diameter impact were analysed.

INFLUENCE OF TRANSMITTER NONLINEARITIES ON DATA TRANSMISSION FROM REMOTELY PILOTED AIR SYSTEM

Purpose: 1) to develop a model of a communication channel of an unmanned aerial vehicle UAV-Ground with adaptive modulation and orthogonal frequency division of channels; 2) to calculate the channel parameters with different types of fading for different levels of satellite transponder nonlinearity. Method: MATLAB Simulink software was used to simulate the channel operation. Results: Based on the IEEE 802.16d standard, a realistic model of the communication channel of the unmanned aerial vehicle UAV-Ground Station was developed, which is used to estimate the channel parameters. The channel model with adaptive modulation consists of a source of information, a transmitter, a downlink channel with fading, and a terrestrial station receiver. Dependences of the signal-to-noise ratio of the terrestrial receiver on the signal-to-noise ratio in the channel down for different types of nonlinearity of the unmanned aerial transmitter, various modulations (BPSK, QPSK, 16QAM, 64QAM), various types of fading are obtained. The signal constellations of the received signals for different Doppler frequency shifts are compared. Conclusion: The developed model allows determining the conditions under which the channel is open for a given type of a modulation and a data rate. The proposed approach can be considered as a method for estimating the parameters of the satellite communication channel of an unmanned aerial vehicle with fading.

RPAS ADS-B AND TRAJECTORY CONTROL DATA TRANSMISSION VIA SATELLITE

Purpose: to develop a model of the satellite communication channel for an remotely piloted air system with adaptive modulation and orthogonal frequency division of channels; 2) to calculate the channel parameters with Rayleigh fading and various types of satellite transponder nonlinearity; 3) analyze the effect of fading and the type of nonlinearity on the parameters of the satellite communication channel. Method: MATLAB Simulink software was used to simulate the channel operation. Results: For the first time, based on the IEEE 802.16d standard, a realistic model of the satellite communication channel of an unmanned aerial vehicle was developed, which is used to estimate the channel parameters. The created model takes into account the Rayleigh fading in the downlink and the nonlinearity of the satellite transponder amplifier. Dependences of the signal-to-noise ratio in the terrestrial receiver on the signal-to-noise ratio in the downlink for various types of modulation (BPSK, QPSK, 16QAM, 64QAM) and data transmission rates are obtained. The nonlinearity of satellite amplifiers was analyzed on the basis of a linear model, a cubic polynomial model, a hyperbolic tangential model, the Gorbani model, and the Rapp model. The results for the cubic polynomial model and the hyperbolic tangential model are similar to the linear model, but differ significantly from the Gorbani model and the Rapp model. For the Gorbani and Rapp models, very low values of the signal-to-noise ratio in the receiver are observed. Conclusion: The proposed approach can be considered as a method of estimating the parameters of the satellite communication channel of an unmanned aerial vehicle with fading. It is shown how the type of modulation varies depending on the level of the signal-to-noise ratio and the type of fading. The developed model allows to predict the operation of the channel with Rayleigh fading and can be useful for the design of communication systems.

Intermodulation Distortion in Multicarrier Satellite Systems: Analysis and Turbo Volterra Equalization

The urgent objective of transmitting high data rates over satellite, coupled with the challenge to maximize satellite mass efficiency, has necessitated that multiple carriers share the same transponder high-power amplifier (HPA). This paper presents analytical framework that characterizes the resulting intermodulation distortion (IMD) by utilizing Volterra series representation to account for the memory within the carrier itself and those associated with other carriers. Also provided is analytical evaluation of nonlinear IMD which involves computing statistical averages of higher-order products of Volterra series containing complex-valued symbols from multiple carriers. Using this theoretical characterization, novel algorithms are developed to overcome IMD in highly distortion-limited environments by employing the powerful Turbo equalization method with linear minimum mean-squared error criterion. Further, the solution is adaptive so compensation does not require prior knowledge of the HPA characteristics and can be rapidly responsive to variations in the environment. Through extensive simulations, it is shown that the proposed multicarrier analysis and Turbo Volterra techniques can be used to substantially remove IMD resulting from operating the nonlinear transponder HPA, shared by multiple carriers, near saturation. By applying more iterations of joint equalization and decoding, the solution can approach the ideal performance when feeding back correct decisions.

Analysis and compensation for nonlinear interference of two high-order modulation carriers over satellite link

We introduce analytical characterization of the nonlinear interference that results when passing more than one high-order modulation carrier through the same nonlinear transponder high-power amplifier (HPA). A Volterra filter is proposed which is novel in its implementation of this analytical characterization and modeling of intersymbol interference (ISI) and adjacent channel interference (ACI). Our focus is on adaptive algorithms with pilot-based training so that the solutions are completely blind to unknown transponder HPA characteristics, and can rapidly respond to varying operating back-off level. Furthermore, two families of adaptive solutions are provided to compensate for nonlinear ISI and ACI. The first set performs adaptive channel inversion and then applies equalization. The second set of solutions performs adaptive channel identification and then applies cancellation. The effectiveness of the proposed analysis and techniques is demonstrated via extensive simulations for high-order QAM and APSK modulations. Also included is the coded performance with selected LDPC codes designed for the DVB-S2 standard. Finally, computational complexity is assessed and performance impact is quantified when complexity is reduced by decreasing the number of Volterra coefficients.

AUTOMATIC CLASSIFICATION OF MPSK SIGNALS IN PRESENCE OF SATELLITE CHANNEL IMPAIRMENTS

ABSTRACTSatellite communications have received an increasing attention in the last three decades because of their potential service to rural areas that cannot economically be incorporated into terrestrial systems. For satellite communications, M-ary Phase Shift Keying (MPSK) schemes are widely used because they offer a good trade-off between power and bandwidth. in this paper an algorithm for automatic real time classification of MPSK with M= 2, 4, and 8 in presence of satellite channel impairments is introduced. A combination of digital signal processing and pattern recognition methods is used for solving the recognition problem. The developed algorithm has been tested by computer simulations and has proven to be reliable and robust against the expected channel impairments (interference) such as noise, fading, shadowing, and transponder non-linearity. It is found that all digital modulation types of interest have been correctly classified with a success rate > 94 % at signal-to-interference ratio, SIR, of 10 dB.

A performance comparison of orthogonal code division multiple-access techniques for mobile satellite communications

In recent years, code division multiple-access (CDMA) techniques have received a great deal of attention for mobile terrestrial/satellite communication systems. Primarily considered for the noteworthy features of low power flux density emission and robustness to interference and multipath, CDMA is known to bear reduced bandwidth and power efficiency when compared to traditional TDMA and FDMA due to the intrinsic cochannel self-noise. Early attempts to increase the capacity of CDMA-based systems for commercial applications relied on voice activation and frequency reuse. More recently, practical solutions to implement (synchronous) orthogonal CDMA signaling are being developed independently in Europe and in the USA. This paper is focused on the comparative performance analysis of those two orthogonal CDMA schemes in the operating renditions of a mobile satellite communications system. In particular, the two CDMA systems are compared in the presence of that and frequency-selective multipath fading and a typical satellite transponder nonlinearity. Most numerical results are derived through a time-domain system simulation that confirms and integrates the theoretical findings.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effect of Satellite Transponder Nonlinearity on Uplink Thermal Noise*

The satellite transponder nonlinearity causes the uplink thermal noise to be suppressed relative to the carriers. It also causes the uplink thermal noise to interact with the carriers to generate intermodulation products. For the single carrier access where-one FM-TV carrier, one FM-FDM carrier or one TDMA-PSK carrier operating at or very close to saturation, the net effect is an improvement in the carrier to uplink thermal noise ratio (C/N)u of, about 2.2 to 3.5 dB. When there are two or more carriers accessing the transponder at the same time, the net effect is reversed. For a typical dual TV operation, it results in a degradation in (C/N)u of 0.5 to 0.7 dB when the total carrier input power level is 1 dB IPBO and is negligible when the total carrier input power level is 3 dB IPBO or lower; and for a typical multiple SCPC carrier access, due to the backoff operation, it is negligible.

Offset frequency allocation to reduce intermodulation noise

AbstractIn SCPC satellite systems intermodulation (IM) noise is generated due to transponder nonlinearity. It is known that this noise can be reduced by proper signal frequency allocation. In the conventional method the signal frequencies are allocated at integral multiples of a basic interval, that is, the minimum signal frequency separation. This paper is concerned with offset frequency allocation in which the signal frequencies are not necessarily integral multiples of a basic interval. Specifically, the IM noise power spectrum is analyzed for 2‐phase PSK signals and studies are made for the IM power reduction characteristics after passing the receiving filter, specific signal frequency allocations with reduced IM noise and IM noise improvement over equal‐interval frequency allocation. Also, starting with the Babcock allocation, offset frequency allocations are derived and their IM characteristics and frequency bandwidth are evaluated.

Error Probability Variation Due to the Nonlinearity of the TDMA/PSK Multicarrier System Caused by On-Off Bursting

Representing a satellite transponder as a nonlinear, memoryless, band-limited device, the error probability variation due to on-off transitions of the TDMA signal envelopes in TDMA/PSK multicarrier operation is analyzed. The effects of AM/AM and AM/PM conversion are also taken into account. The results of this study are applied to <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4\phiPSK</tex> and <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8\phiPSK</tex> systems, for which the parameters describing the transponder nonlinearity have been taken from the real characteristics obtained by measurements. The numerical calculations show that on-off transitions of the TDMA signal envelopes could cause significant variation of the error probability.

Analytical comparisons of CSSB and TDMA/DSI satellite transmission and techniques

At the same time that Time-Division Multiple Access (TDMA) with Digital Speech Interpolation (DSI) is gaining wide acceptance, being planned for almost every modern satellite communication network as the basic transmission technique, a variation of the classical single sideband using companders (CSSB) has recently been proposed, accompanied by the claim of higher transponder capacities. The higher capacity of CSSB is supported under a variety of assumptions (lower speech levels, increased transponder linearity, benign interference environment) that hinders the comparison of CSSB with other well-established satellite transmission techniques such as FDM/FM or TDM/D51. The purpose of this paper is to "normalize" CSSB to a common set of assumptions that permits its capacity to be calculated on the same bases that apply to other satellite transmission methods, and in this manner to identify the ranges of ground and space segment parameters that would render CSSB superior to existing alternatives. In particular, CSSB is compared with TDMA/DSI, using analytical approaches that permit the simultaneous consideration of the principal factors involved in the capacity of both systems. These factors are • speech and compander characteristics • ground segment parameters, G/T, and antenna sidelobe radiation • space segment parameters, in particular EIRP, the transponder nonlinearities, and orbital spacing. These factors have been quantified for a typical 36-MHz C-band transponder with EIRP--34 dBW. The results show that CSSB's claims of higher capacity are realizable only under idealized transponder characteristics or with larger ground segment antenna sizes. In addition, CSSB's performance degrades faster than TDMA/DSI under fading conditions, suggesting that margins must be subsequently improved for this technique.

MSK and offset QPSK signal transmissions through nonlinear satellite channels in the presence of intersymbol interference

An expression is derived for the bit error probability of minimum shift keying (MSK) and offset quaternary phase shift keying (OQPSK) signals which have been transmitted through nonlinear satellite channels after bandlimiting. The transponder nonlinearity considered in the paper is of the bandpass type, which exhibits AM/AM and AM/PM conversion effects. The effects of up-link and down-link thermal noise are also taken into consideration in the analysis. The resulting expression for the bit error probability is an infinite series containing double integrals. In order to illustrate the usefulness of the method, the error rates of MSK and OQPSK signals are computed for a Butterworth bandlimiting filter hard limiting transponder channel model. The error rates of MSK and OQPSK signals are compared with the error rate of binary PSK signals for this communication system model.

Effects of satellite transponder nonlinearities on spread spectrum satellite communication systems

AbstractCorrelation analysis using the autocorrelation function is applied to assess the effects of satellite transponder non‐linearities on multiple access SS (spread‐spectrum) signals when SS techniques are applied to satellite communication systems. Both AM/PM conversion and AM/AM conversion nonlinearities are employed in our model (parameters k and L). The model can represent various kinds of nonlinearities by choosing the pair of parameters (k, L) and can discuss the effects of AM/AM and AM/PM conversions on SS signals synthetically and individually. The same analysis is then applied to SCPC (single channel per career) signals for comparison. It is shown that AM/PM conversion degrades the output DUR's of both SS and SCPC systems as severely as does AM/AM conversion. The difference between the characteristics of under linear and nonlinear systems is as large for SS signals as for SCPC signals. Direct sequence SS signals are assumed and the DU (Desire/Undesire) ratio at baseband signal demodulation is adopted as an evaluation criterion.

Spectral Properties and Band-Limiting Effects of Time-Compressed TV Signals in a Time-Compression Multiplexing System

Time-compression multiplexing (TCM) has recently been proposed for application in multiple TV transmissions through satellites. It is advantageous over frequency-division multiplexing because of its relative immunity to nonlinear transponder effects. Here we study two important and fundamental aspects of TCM — the spectral properties and band-limiting effects on the time-compressed signal. We derive the output spectrum of a time-compressed signal and show that if the original input signal is assumed to be: (i) band-limited to B Hz, (ii) segmented into T-second intervals before time compression by a factor of α(α ≥ 1), and (iii) 1/T ≪ B, then essentially all the spectral power in the output time-compressed signal is contained in the bandwidth |f| ≥ αB Hz. This result is applicable to the TV case. Numerical examples on various types of spectra are also presented. Using the TV example, we further demonstrate that the ripples created by low-pass filtering the time-compressed signal up to αB Hz are small, and interburst interference due to these ripples can be kept negligible with a small guard time (about 2 percent of the burst duration) between different signal bursts. We also provide a brief discussion on some interesting spectral properties of time-compressed signals in spectrum-expansion applications.

Erratum: E.I.R.P. allocation for a nonlinear satellite channel with multiple transmit beams and carriers

Sharing of communication-satellite capacity among several earth terminals with different traffic requirements is considered, assuming multicarrier f.d.m.a. operation through one nonlinear transponder connected to a multibeam transmit antenna. Analytic results are given for the strategy which maximises spare useful E.I.R.P. and for the resulting upper bound on the capacity when all E.I.R.P. resources have been consumed to overcome system noise.

Input Power Assignment of Multicarrier Systems from Given Output Power Levels

A problem associated with signal power control during multicarrier transmission in the presence of an amplifier nonlinearity in a satellite communications channel is analyzed. The problem is to calculate input carrier power levels wheii output carrier power levels are given. The Fletcher-Powell optimization technique is used for a small number of transmitted carriers, and an example shows the effect of weak signal suppression in the presence of large signal(s) because of nonlinear effects. This technique is applied to three types of transponder nonlinearities to illustrate realistic cases. The Gaussian assumption is used for a large number of carriers. Cases in which one or two input signals are much larger than the other signals are also treated.

The Evaluation of Satellite Link Availability

The need to predict overall link availability in a satellite communications system has motivated efforts to devise theoretical and empirical models to predict fade statistics for line of site propagation between an earth station and a satellite. Investigators have largely concentrated on measuring and analyzing the key propagation phenomena, especially attenuation and depolarization due to rainfall, and deriving simple parametric models which relate rain drop geometry, frequency, and available local meteorological data to probability distributions of attenuation and depolarization. However, in allocating margins for carrier-to-noise ratios, the designers of operational satellite systems still rely mainly on operational experience, and where limited experience is available margins tend to be conservative. While this approach may be the most practical with limited experimental and theoretical information, the sufficient variability of earth station to earth station link behavior suggests that without more detailed knowledge of the underlying mechanisms, system inefficiencies and unexpected problems may arise. Furthermore, an accurate link model is essential in the proper assessment of new systems, especially those systems using the 14/12 GHz, or higher, allocated satellite frequency bands. This paper derives the long term probability distribution of C/N over a complete satellite link that includes the effects of signal attenuation due to transmitting earth station pointing error, up-path propagation, geostationary satellite position error, down-path propagation, and receiving earth station pointing error. Non-tracking and tracking antennas subject to wind loading are considered, and the effects of the non-linear Satellite power transfer characteristic and the correlation of up- and down-link pointing losses due to satellite position error are accounted for. Results presented for 6/4 GHz links show that the satellite non-linear transponder, the proportion of noise in the up- and down-links, and satellite station keeping all affect the net fade distribution. It is therefore demonstrated that system parameters as well as uppath and down-path propagation statistics determine fade margin requirements for a satellite link.

E.I.R.P. allocation for a nonlinear satellite channel with multiple transmit beams and carriers

Sharing of communication-satellite capacity among several earth terminals with different traffic requirements is considered, assuming multicarrier f.d.m.a. operation through one nonlinear transponder connected to a multibeam transmit antenna. Analytic results are given for the strategy which maximises spare useful E.I.R.P. and for the resulting upper bound on the capacity when all E.I.R.P. resources have been consumed to overcome system noise.

The Effects of Transponder Nonlinearity on Binary CPSK Signal Transmission

An analytical approach is developed to investigate the performance of coherent phase-shift keying (CPSK) systems transmitted through a nonlinear satellite transponder. A novel quadrature model of the satellite nonlinearity is introduced, and is found to yield an excellent fit to the actual traveling wave tube (TWT) characteristics as commonly used in a satellite transponder, with the advantage of requiring the choosing of only a few parameter values. Based on this quadrature model an expression for the bit error rate is derived in the form of an infinite series where the leading term corresponds to the error-rate for a linear channel and the remaining terms are correction terms accounting for nonlinear effects. The performance of a binary PSK system is computed and presented as a function of the up-link and down-link carrier-to-noise-power ratios (CNR) and a bandwidth spread factor.