Yogi Y. Krikorian

Communication systems modeling techniques

Modem simulation and modeling is frequently used to design and understand communications systems from both a theoretical and practical point of view. This can result in very significant cost savings, speed up the design process and add another dimension into the reliability of the system design. Nevertheless. the simulation of advanced communications systems is a non-trivial task and the errors introduced due to the very nature of the digital simulation process must he properly modeled, where possible avoided, and their impact on the simulation results must be properly ascertained. The purpose of this paper is to discuss, demonstrate, and give examples of the many important considerations and potential pitfalls in the modeling and simulation of communications systems. For example, it is sometimes possible and expeditious to model the communications systems at a baseband frequency resulting in orders of magnitude reduction in sampling rate and thereby speeding up the simulation process and reducing requirements on the computing resources. This is demonstrated in the paper by simulating a P C W M Communication system. The paper also discusses other important DSP concepts including zone aliasing when simulating nonlinear devices, spectral analysis techniques, and frequency binning effects.

Simulation and modeling of amplifier nonlinearities for multicarrier wireless communication systems

Amplifier nonlinearities impact the performance of multicarrier frequency division multiple access (FDMA) wireless communication systems. It is important to accurately model limiters and high power amplifiers to predict the bit error rate performance of multicarrier communication systems. Modern simulation tools can be used to accurately determine intermodulation distortion and its effect on the bit error rate in a computationally timely manner. This paper presents simulation results in a parameterized form when the input to the amplifier is comprised of a number of 8-PSK modulated signals with raised cosine filter shaping. One of the important parameter in these simulated results is the amplifier power back off level, which is varied over a range of about 6 dB. The simulation results include the probability of bit error and the intermodulation distortion both within the signal band and outside the signal band. Bit error rate curves, eye diagrams and constellation figures are used in presenting the results.

Communication system performance - detailed modeling of a power amplifier with two modulated input signals

The performance of multicarrier frequency division multiple access (FDMA) wireless communication systems is impacted by amplifier nonlinearities. It is vital to precisely model limiters and high power amplifiers (HPAs) to determine the spectrum distortion and other types of degradation associated with multicarrier communication systems. Contemporary simulation tools can be employed to precisely ascertain intermodulation (IM) distortion and its effect and impact on both in-band and out-of-band IM performance in a computationally timely manner. This paper illustrates analysis and simulation results in a parameterized form when the amplifier input consists of two 8-PSK modulated signals with raised cosine filter shaping. One of the significant parameters in these analyses and simulated results is the HPA operational back-off (OBO) power level, which is studied over a range of about 6 dB. The simulation illustrates the intermodulation distortion both within the signal band and outside the signal band. A novel approach is used to represent an HPA in terms of a power series expansion, which converges very rapidly and gives significant insight and provides a useful tool in predicting the spectral content of the HPA output. The results contained in this submission were generated in whole, or in part, through work supporting the MILSATCOM joint program office (MJPO). The authors are very appreciative of the support provided by the SMC/MC program office of the Space and Missile Center in this effort

Telemetry, tracking, and commanding (TT&C) link considerations for a LEO Sat

Telemetry, tracking, and command are very important functions necessary for the proper operation of a satellite. It becomes a critical issue for the LEO-Sat program, however, when the command and control is provided from a small ground station with limited capabilities (low transmit EIRP and receive G/T), and when the satellite is in a contingency mode of operation, such as tumbling (antennas off pointed). Previous link analyses have shown that the satellite link would be adequate when commanding from large remote tracking stations (RTS) with a larger antenna (45- or 60-foot dish). This paper deals with command and control of this new LEO-Sat in its early orbit, using a small remote tracking station (RTS), with a 33-foot antenna. Our analyses have proven that the uplink and downlink can be closed even with a small RTS station with a 33-ft antenna. The details of these analyses are given in this paper. The communication link performances, both during normal operations as well as in contingency tumbling mode of operations, are presented here. Both the SNR (signal to noise ratio) and the threshold techniques were shown side-by-side, in the link analyses, for normal and anomalous tumbling cases, with similar results. The conclusion in Section 5 summarizes the analyses. Aerospace personnel and the satellite contractor came to a similar conclusion: that the uplink and downlink (to and from the satellite) using small RTS stations with a 33-ft antenna have adequate link margin. In normal mode, worst-case LEO-Sat and nominal small RTS station parameters were used in the analysis. In contingency mode or tumbling mode, the worst worst-case parameters for LEO-Sat and for a small RTS station were used. A summary of the results of the analyses performed for the uplink and downlink communication paths are given in this paper

Analog-to-Digital Converter Loading Analysis Considerations for Satellite Communications Systems

In contemporary communication satellite systems, uplink radio frequency (RF) signals are amplified, downconverted to intermediate frequency (IF) and/or baseband, and after appropriate filtering, are input to an analog-to-digital converter (ADC). The ADC digital output is signal processed for a variety of purposes, such as signal channelization and switching. In these systems, a foremost realization problem is the ADC, which must operate to satisfy the sampling theorem, which necessitates a sampling rate at least twice the received signal bandwidth. When the signal consists of numerous multiplexed signals, a critical matter in ADC performance is the degree of signal clipping, which arises when the instantaneous ADC input signal magnitude surpasses the maximum range of the ADC. Since at least some clipping is often present, the total ADC noise output consists of clipping plus quantization noise. A figure of merit for the ADC is the signal-to-noise ratio (SNR) of the ADC, which is defined as the ratio of input signal average power to the ADC output average noise power. The SNR is determined by, among other things, the ADC load factor, which is the ratio of the ADC input signal average power, and the ADC maximum peak power output. This paper describes analysis and simulation results on SNR versus the ADC load factor when the input signal is composed of many digitally modulated carriers. A nine-signal 8-ary phase shift key (8-PSK) modulated carrier case is considered with each signal band limited. It is important to note that for this particular ADC input, it is shown that the probability density function (PDF) is Gaussian-like. This is significant since this means that the SNR versus ADC load factor curve for the nine 8-PSK signal case will have nearly identical characteristics to that when the ADC input is white Gaussian noise. Additionally, this paper discusses what occurs when the ADC is strongly driven into the clipping region. This discussion is enhanced by comparing the ADC to a limiter in this highly distorted region. Analysis and simulation results are provided to describe ADC performance characteristics in this highly distorted region.

Modeling, simulation, and analysis of analog-to-digital converters for wireless communication [broadband satellite systems]

In modern satellite systems the uplink RF wideband signal after downconversion to IF is input to an ADC, which is a major limitation as it needs to operate at a rate at least 2/spl times/ the received signal bandwidth. In the case of a wideband signal comprised of many multiplexed signals, the spectral distribution of quantization noise is also of significant interest. Another very important consideration is the signal clipping effect occurring whenever the instantaneous input signal amplitude exceeds the maximum linear range of the quantizer. Since clipping cannot be avoided in most practical situations, the signal power to the quantization plus clipping noise power ratio is of utmost interest. Such a ratio is in general a function of the quantizer load factor that in turn is the ratio of the input signal average power to the maximum peak power at the quantizer output. This paper presents simulation results on the quantizer analysis when the input signal is comprised of a specified number of digitally modulated carriers. The results are obtained in terms of the probability of symbol error and the probability of bit error as a function of quantizer load factor.

PCM/FM performance enhancement using Reed Solomon channel coding

This paper presents analytical and detailed computer simulation results of the performance enhancement of pulse code modulation/frequency modulation (PCM/FM) links using Reed Solomon (RS) channel coding. PCM/FM is a commonly deployed technique for a wide variety of telemetry and other links. Nevertheless, the required SNR to provide acceptable bit error rate (BER) can be considerable. Thus, it is desired to provide channel coding to reduce the SNR requirement. The simulation of PCM/FM with and without RS coding requires the application of a variety of signal processing techniques discussed in the paper. RS channel coding is considered in this paper and is often used as a forward-error-correction strategy for digital links. In communication links, random and burst errors can result from a variety of interferences. This results in high symbol rate errors, some of which can be corrected by a RS code. RS codes are powerful burst-error correcting codes. An analytical model is developed for this paper, and detailed simulations are performed for PCM/FM with and without RS coding. The FM demodulator for small SNR input creates click noise, while for very large signals Gaussian type noise is anticipated. The implications of these types of noise and how it relates to BER are discussed in the paper. Detailed BER versus SNR curves are produced from both analysis and simulation. The results of the analysis and the simulation indicate that coding can substantially enhance link closure.

A dynamic deep space communication link analysis tool for the deep space network (DSN)

A dynamic deep space communication link analysis tool is described in this paper. This tool, developed by The Aerospace Corporation, provides the capability to analyze coverage and data throughput for communication links between a spacecraft and the Jet Propulsion Laboratorys (JPL) Deep Space Network (DSN). The tool determines the link margin and data throughput over time during the trajectory of a spacecraft as monitored by the DSN. The analysis takes into account several dynamic effects in calculating the link budget, including transmit and receive antenna gains and space loss. This tool consists of a database derived from JPLs DSMS Telecommunications Link Design Handbook, from which relevant link budget parameters are extracted for all of the 70-m and 34-m antennas. Some of the relevant DSN link budget parameters include receive and transmit gain, as well as system noise temperature. Since these parameters vary, depending on operating conditions, the database takes into account various conditions, such as cumulative weather distribution, month of operation, elevation angle, particular DSN antenna being used, and uplink and downlink frequency. By utilizing the corresponding DSN link budget parameters and the spacecrafts trajectory and link budget parameters, one is able to determine the amount of coverage and data throughput versus time

Modeling and simulation of amplifier nonlinearities for single 8-ary PSK modulated signal input

The performance of band limited M-ary PSK communication systems is impacted by amplifier nonlinearities. It is critical to precisely model high-power amplifiers (HPAs) to ascertain the bit error rate (BER) performance of M-ary PSK communication systems, which are band limited. State-of-the-art simulation techniques can be employed to exactly ascertain intersymbol interference (ISI) and its degradation on the BER in a computationally efficient approach. Simulation results are demonstrated in this paper, in a parameterized form with the HPA back-off level varied over a range of several dB. The HPA input is composed of a single 8-ary PSK modulated signal employing raised cosine filter shaping to band limit the signal structure. The effect of the shaping factor used in raised cosine filtering is determined in detail. The results for the simulation encompass the spectral distortion outside the signal band and the probability of bit error. Eye diagrams, constellation figures, cross correlation plots, and timing diagrams are employed in illustrating the results

Dynamic proximity communication link analysis tool for orbiting satellites and ground assets on Mars

A dynamic proximity link analysis tool developed by The Aerospace Corporation is described in this paper. Given appropriate link budget parameters, the tool can return the data throughput for proximity links from ground assets to communications satellites. In order to determine the data throughput, the tool calculates the link margin as a function of time and determines the variable data rate(s) achievable as a function of time. Given any time period, the tool is then able to compute the total data throughput based on the data rate computations. Given a spacecrafts trajectory, the location of the ground asset, antenna gain patterns, and standard link budget parameters (i.e., transmitter power, carrier frequency, etc.), the tool can compute link closure duration, variable data rate(s) achievable, and the data throughput. As a demonstration of the tools capability, several examples of proximity links between a Mars ground asset and an orbiting relay satellite will be shown