An adaptive digital filtering algorithm that can compensate for both frequency-selective multipath and interference on constant envelope modulated signals is presented. The method exploits the fact that multipath reception and various interference sources generate incidental amplitude modulation on the received signal. A class of so-called constant modulus performance functions is developed which sense this AM term but are insensitive to the angle modulation. Simple adaptive algorithms for finite-impulse-response (FIR) digital filters are developed which employ a gradient search of the performance function. One of the resulting algorithms is simulated for the example of an FM signal degraded by specular multipath propagation. Substantial improvements in noise power ratio (NPR) are observed (e.g., 25 dB) with moderately rapid convergence time. These results are then extended to include tonal interference on a FM signal and intersymbol interference on a QPSK data signal.

A new approach to multipath correction of constant modulus signals

This paper presents three extensions of the constant modulus algorithm (CMA) introduced in an earlier paper as a means of correcting degradations in constant enyelope waveforms. As originally formulated, the CMA employs an FIR filter with complex coefficients and accepts complex (quadrature) input data. In this paper, first a real input, real coefficient version of the algorithm is shown to perform arbitrarily close to the fully complex version. Secondly, the algorithm is extended for the enhancement of signals having a nonconstant but known envelope, as might arise in data signals with pulse shaping. Lastly, a multichannel version of CMA, wherein several observations are linearly combined, is presented for joint adaptation of multiple filters. This approach can be used, for example, as a means of spatial or polarization "beamsteering" to reject additive interferers and compensate for channel-induced polarization rotation.

New processing techniques based on the constant modulus adaptive algorithm

This paper describes a radically new approach to the simulation of multipath fading in the civil land mobile radio environment. The simulation package described takes into account vehicle movement through a particular environment, signal shadowing and coherent waves. Statistical results obtained with the simulation are compared with those obtained by v.h.f. field trials for three areas within the City of Bath which exhibit differing fading characteristics, and are shown to be in excellent agreement.

Computer simulation of multipath fading in the land mobile radio environment

A method for co-channel interference suppression and signal separation was proposed in a previous work. It uses the amplitude variation of the composite signal to estimate the parasitic phase modulation impinged on the strong desired signal by the weak interference signal. This estimate is then used to cancel out the distortion of the composite signal, revealing the desired signal. In the cancellation process, amplitude estimates for both signals were obtained from measurements. Errors in estimating these magnitudes limit the quality of cancellation and signal separation performance. An adaptive method is proposed which improves these estimates and, hence, results in a better cancellation of interference. A criterion for performance measure was chosen to be the variance of the envelope of the weak signal estimate and is shown to be directly related to the cancellation quality. A random search algorithm was used to vary the signal magnitude estimates to achieve better estimates. Numerical simulation experiments were performed to prove the concept, including Gaussian noise. >

Adaptive co-channel interference cancellation and signal separation method

An interface for connecting a station using a time division multiplex format to a time division multi-access system comprising (a) a first channel for converting time division multiplexed signals (112) into bursts (119), (b) a duplex channel for re-constituting bursts (117) into time division multiplex frames (111) and (C) TDMA circuitry for controlling (a) and (b) to operate in conformity with the TDMA. The first channel is preferably adapted to remove unused slots from the multiplex and to collect the used slots over several frames into a single burst. The duplex channel restores the frame format by replacing slots removed by the transmitter.

Tdma/tdm interfacing

We discuss a possible method for interference control in radio systems whose desired signal is analog FM. The method utilizes the fact that the desired signal has a nearly constant envelope; with interference present, the composite envelope becomes noisy. A controlling processor measures this additional envelope noise, and iteratively adjusts the complex combining gain of an auxiliary signal to minimize the interference. The expected residual interference remaining after minimization is proportional to the reciprocal of the square of the rms envelope fluctuation of the desired signal. This result was confirmed in a laboratory simulation of an intersecting radio route case using both large and small index FM. Beginning with signal-to-interference ratios of 30 to 40 dB, the controller increased these numbers to about 60 dB, the expected theoretical limit for a signal with 0.1 percent rms envelope fluctuation.

Evaluation of a Method for Interference Control in Analog FM Systems

Using simulated Rayleigh-fading FM signals, the baseband performance of a diversity system that uses a separate pilot is studied. The effect of various RF/audio bandwidth expansion ratios is included. These results show an approach to a constant asymptotic baseband SNR for large expansion ratios, with diversity increasing this asymptote by 5 dB in the case of moderate to large interference and/or RF noise levels (9 to 15 dB below the carrier), to over 10 dB in the case of lower impairment levels (21 to 27 dB below the signal level). Assuming the usual hex-cell radio coverage plan, and a propagation law of the form r^{-n}, n = 3 or 4, bandwidth expansion ratios of about three to six seem to yield the most efficient RF spectrum utilization in many practical situations. The number of required channel subsets (cells) is strongly influenced by the propagation law ranging from about 19 for n = 3 , to about 7 for n = 4 .

Baseband Performance of a Pilot Diversity System with Simulated Rayleigh Fading Signals and Co-Channel Interference

We describe exploratory work designed to demonstrate the feasibility of directly interfacing terrestrial digital carrier systems to digital satellite systems. Designed to accept up to 30 T1 carrier systems on the terrestrial side, the interface is designed as a common-control, bus-oriented time-division switch using a common time-slot interchanger for simultaneous switching of up- and down-links. The output bit rate is about 50 Mbits/s. The controller is implemented as two Motorola M6800 microcomputer chips and associated memory. The switch can communicate with a variety of terrestrial originating switches, including those using either register-sender or cut-through operation. By proper software, a simultaneous mix of such originating switches can be handled, and either delay-start dial, wink-start or dialtone start outpulse control can be used in either direction of call setup (i.e., to or from a terrestrial switch connected to the satellite earth station).

A Microprocessor Controlled Digital Interface for Digital Satellite Systems

Using simulated Rayleigh-fading FM signals, the baseband performance of a diversity system that uses a separate pilot is studied. The effect of various RF/audio bandwidth expansion ratios is included. These results show an approach to a constant asymptotic baseband SNR for large expansion ratios, with diversity increasing this asymptote by 5 dB in the case of moderate to large interference and/or RF noise levels (9 to 15 dB below the carrier), to over 10 dB in the case of lower impairment levels (21 to 27 dB below the signal level). Assuming the usual hex-cell radio coverage plan, and a propagation law of the form r-n, n = 3 or 4, bandwidth expansion ratios of about three to six seem to yield the most efficient RF spectrum utilization in many practical situations. The number of required channel subsets (cells) is strongly influenced by the propagation law ranging from about 19 for n = 3, to about 7 for n = 4.

http://ieeexplore.ieee.org/iel5/25/34015/01622177.pdf

W. Aranguren

Papers

Evaluation of a Method for Interference Control in Analog FM Systems

Baseband Performance of a Pilot Diversity System with Simulated Rayleigh Fading Signals and Co-Channel Interference

A Microprocessor Controlled Digital Interface for Digital Satellite Systems

Baseband performance of a pilot diversity system with simulated Rayleigh fading signals and co-channel interference