Dispersion Signatures, a Statistically Based, Dynamic, Digital Microwave Radio System Measurement Technique

Recent studies have shown that trellis-coded modulation can achieve coding gains in the residual intersymbol interference environment that exists after adaptive equalization on terrestrial digital microwave radio (DMR) systems. Simulated system signatures for 140 Mb/s DMR systems incorporating trellis-coded 512-QAM modulation are presented and the results compared to the system signatures of equivalent uncoded 256-QAM systems. At a threshold bit-error rate (BER) of 10/sup -3/, the signature of the uncoded system can be degraded by the addition of coding. However, at a threshold BER of 10/sup -4/, it is found that trellis coding improves the system signature. Conditional probabilities of outage and the percentage of error-free seconds are also calculated for the coded and uncoded systems. >

The authors discuss radar interference and the effect on digital microwave radio systems. Bit interleaving is proposed as a way of canceling radar interference. In particular, 3-GHz maritime radar interference to 5- or 6-GHz (16-QAM) (quadrature amplitude modulation) digital microwave radio systems is described. The magnetron oscillator performance is discussed. The construction of a bit interleaving circuit and experimental results are described, showing the positive effect of bit interleaving used as a radar interference canceler. Bit interleaving equipment being designed for a digital microwave system with 16 QAM is described. >

A reduction of complexity in full-digital implementations of the adaptive equalizing function for digital microwave radio (DMR) is attained using a modified intersymbol interference canceller instead of a conventional equalizer. The reduction of complexity is mostly due to the fact that preliminary decisions are coded in a relatively small number of bits and additional cancelling stages are used. For equalizers typically employed in 140 Mbit/s DMR systems, the complexity reduction can exceed 50%. A numerical method of calculating error rates was extended and used to obtain signatures for modified cancellers and conventional equalizers, which are presented. >

Two block-coded modulation (BCM) families particularly suited for high-capacity digital microwave radio systems are presented. The first family is based on one-step partitioning, and the second family is based on two-step partitioning of the signal constellation. The alphabet expansion is a decreasing function of the block length, and the constellation is constructed in such a way as to minimize both the average and peak signal powers. Using short block lengths, specific modulation schemes are described that transmit 4, 6, and 8 information bits per symbol. The asymptotic coding gain is only on the order of 2 dB in the first family, and of 3 dB in the second family, but their detection simplicity makes the presented BCM schemes particularly attractive for high-speed applications where trellis-coded modulation (TCM) decoders may be difficult to implement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The concept, design, and evaluation of a cross-polarization canceler for use on digital dual-polarization microwave radio systems is presented. A cross-polarization canceler has been developed and tested that minimizes the effects of cross-polarization interference. The canceler uses a transversal structure along with a powerful decision-directed control algorithm. Extensive laboratory and field tests are discussed. The results demonstrate that with this cross-polarization canceler, a high-performance dual-polarization system can be realized. The system mitigates the cross-polarization leakage sufficiently that performance is comparable to single-polarization systems. >

This paper presents a novel technique for power amplifier linearization in digital microwave radio systems. The proposed technique is based on the use of a predistortion circuit, whose AM/AM and AM/PM responses are separately implemented as polynomial approximations of the respective responses of the ideal linearizer. The proposed scheme is shown to attain superior performance in comparison with other well-known predistortion structures, such as those based on the cancellation of third or fifth order distortion, with no substantial aggravation in implementation complexity.

In microwave digital radio transmission, waveform distortion due to frequency selective fading becomes a dominant factor of degradation. To evaluate the degradation of different systems in a unified way, the waveform factor is defined first. With the aid of concept “waveform factor,” the eye aperture is easily obtained. We also evaluate the waveform factors for various modulation schemes. Next, on the basis of the relation between propagation parameters and waveform factor in two‐ray fading, the estimation formula of the outage probability for various digital radio systems in line‐of‐sight propagation is derived. The proposed factor is determined uniquely by system parameters, such as modulation scheme, system impulse response, and symbol rate. With the aid of this factor, digital radio system performance during multipath fading is easily evaluated.

A series of algorithms that significantly extend the range of control for decision-directed structures are presented. Experimental results show that increased control ranges of approximately 20 dB can be obtained for 64-QAM systems. The algorithms can eliminate digital radio outage due to hysteresis during dispersive multipath fading and allow for enhanced control of cross-pol interference cancelers in dual-polarized digital radio systems. The performance of 64-QAM digital radios utilizing these techniques verifies that the substantial improvements predicted by theory are realizable. Examples of performance improvements with thermal noise, FM interference, and multipath fading are given to demonstrate the power of the algorithms. Examples taken from standard production AT&T DR6-30-135 digital radio show that timing and carrier synchronization are easily maintained for 64 QAM systems at signal-to-noise ratios approaching 0 dB. The results can be applied to other communications systems and may eliminate data training sequences in many applications such as voice band modems. >

Digital radio performance degradation due to interference is significant in the latest 64 QAM technology and must be considered during the path design of a digital radio facility. A route engineering tool, based on a model that considers radio interference to be time-varying has been developed for designing rugged digital radio paths in a radio interference environment. The long-term statistics of interference dynamics during multipath fading is modeled as a lognormal distribution with a standard deviation dependent on terrain roughness. Analysis of multipath fading data obtained from a distance experiment has shown a linear relationship between the standard deviation of the interference and the fade depth of the desired signal. When this interference dynamics model is used to calculate the expected outage in a 64 QAM digital radio system equipped with space diversity (at a junction station with average terrain roughness, a free space C/l of 60 dB, and a standard deviation of 4 dB), a 200% increase in outage is predicted over the outage predicted by the constant interference model. >

New 40-GHs band digital radio equipment is described. In the equipment we adopted a new circuit configuration consisting of a single IMPATT diode oscillator which functions as both transmitter frequency converter and receiver load oscillator simultaneously. The principal system design factors, a unique IMPATT diode oscillator mount configuration and test results are described. The compact radio equipment is designed so that it ensures excellent cost performance for communication systems in local trunk service, even in short hop applications resulting from rainfall attenuation to the new band.

Modern wireless communication equipment, point-to-point and point-to-multipoint microwave digital radios, radar equipment, microwave sources require high stability, low power consumption, and very low phase noise frequency sources in UHF band. Existing TCXO solutions can achieve -150 dBc/Hz phase noise floor at the carrier frequency of 100 MHz. The goal of this work was to create a 500 MHz TCXO with Stratum-3 stability performance (0.28 ppm over temperature, 4.6 ppm over all conditions), which can reach -155 dBc/Hz noise floor, -120 dBc/Hz at 1 KHz offset from the carrier, while providing +7 dBm of output power. The goal was accomplished by integrating in a small SMD package high performance, low frequency, Stratum-3 digitally compensated reference TCXO, low noise off-the-shelf phase locked loop (PLL) IC, and ultra low noise 500 MHz VCXO. The key solutions for this development besides solid digitally compensated TCXO design were optimization of the PLL circuit, and design of a VCXO. The VCXO is based on a 3rd overtone 100 MHz AT-cut crystal resonator with relatively high Q, passive band-pass filter, tuned on the fifth harmonic of the 100 MHz VCXO, and a free-running L-C oscillator, which is injection locked to the above mentioned fifth harmonic of the VCXO. The resulting device performance met all the stated goals and even somewhat exceeded the phase noise expectations. The sub-harmonic suppression was better than 50 dBc

As radio communication has progressed, reduction of intersystem interference has become a problem. In a conventional interference canceller, the interference signal as a source of cancellation is obtained at the receiving station and the interference component in the desired signal is canceled based on this interference signal. However, the interference signal for cancellation may not be obtained at the receiving station. As a remedy, an extraction and reinjection‐type interference canceller has already been proposed.In this paper, a pre‐interference canceller is proposed in which, if the transmitting station of the desired signal and that of the interference signal are identical, the interference signal is mixed with the desired signal from the transmitting station beforehand and the interference signal leaking into the desired signal is cancelled at the receiving station.First, the applicable range and the fundamental configuration of the present canceller are presented. Subsequently, the method for determining the amplitude and phase of the interference signal injected into the desired signal of the transmitting station and the method for evaluation of the cancellation effect are presented theoretically.Next, an experiment is carried out with the desired signal of 16 QAM or 256 QAM and the interference signal of 256 QAM. Interference improvement by about 20 dB is obtained for both the 16 and 256 QAM signals. Also, the response speed of control for the fading is 60 ms. In addition to the validity of the theoretical results, effectiveness of the proposed canceller is shown.

An efficient data predistortion technique is presented to compensate for transmitter high-power amplifier (HPA) nonlinearities in digital microwave radio systems employing quadrature amplitude modulation (QAM) signal formats. The proposed technique makes use of a specific transmitter pulse shaping that leads to discrete signal levels at time instants spaced by half a symbol interval. The predistortion circuit is designed so as to cancel nonlinear distortion at the HPA output at these instants. With respect to conventional data predistortion, which can only compensate for warping of the signal constellation, this technique also compensates for nonlinear intersymbol interference (ISI). It is shown that, using the 64 and 256 QAM signal formats, the presented technique leads to a considerably more efficient utilization of the available HPA power. >

The need to predict the performance of digital microwave radio systems is introduced and the approach adopted by BT in the estimation of radio link fading for the outage prediction model is described. An examination of the limitations of fading laws given in the CCIR is made, and the need to separate composite fading into flat and selective components is identified. A simple method to perform this split is proposed and results from several links are presented. The certification of several radio performance prediction models submitted to the European Telecommunication Standards Institute is discussed. >

The design and implementation of a relatively complete simulator for a high-speed digital microwave radio system is underway. This simulator is used to model a number of channel conditions as well as several radio imperfections such as modulator imbalance, carrier leakage and phase noise. It is already possible to predict the transmitted frequency spectrum and the BER for a white Gaussian noise channel. A complete simulator will then permit one to conveniently compare different design alternatives. >