James K. Cavers

An analysis of pilot symbol assisted modulation for Rayleigh fading channels (mobile radio)

The author presents pilot-symbol-assisted modulation (PSAM) on a solid analytical basis, a feature missing from previous work. Closed-form expressions are presented for the bit error rate (BER) in binary-phase-shift-keying (BPSK) and in quadrature-phase-shift-keying (QPSK), for a tight upper bound on the symbol error rate in 16 quadrature-amplitude-modulation (16-QAM), and for the optimized receiver coefficients. The error rates obtained are lower than for differential detection for any combination of signal-to-noise ratio (SNR) and Doppler spread, and the performance is within 1 dB of a perfect reference system under slow-fading conditions and within 3 dB when the Doppler spread is 5% of the symbol rate. >

Amplifier linearization using a digital predistorter with fast adaptation and low memory requirements

A method is described for linearizing a power amplifier by predistorting its input. It is particularly well suited to baseband implementation with digital signal processor hardware. In comparison with the most powerful previously published predistorter, it requires four orders of magnitude less memory, reduces convergence time by over three orders of magnitude, eliminates reconvergence time following a channel switch, and eliminates the need for a phase shifter in the feedback path. The predistorter structure is described. Its ability to suppress intermodulation products using only a small table is demonstrated. The effect of predistorter nonidealities (especially limited table size) on the power amplifiers output are analyzed. A fast adaptation algorithm is introduced. >

Adaptive compensation for imbalance and offset losses in direct conversion transceivers

The current interest in linear modulation and multilevel signals has resulted in an emphasis on DSP implementations to achieve precision signal manipulation. However, most transceivers, and direct conversion designs in particular, rely on analog implementations of the quadrature modulator and demodulator, thereby sacrificing much of the precision gained through DSP. The present paper focuses on the three principal impairments of analog quadrature modulators and demodulators: gain imbalance, phase imbalance, and DC offset. The paper contains three main contributions. First is an analysis and quantitative assessment of the losses-primarily a degraded BER and out-of-band power in the transmitted signal-due to imbalances and offsets. The second contribution is an adaptive compensation technique for the quadrature modulator at the transmitter, and the third is a compensation technique for the quadrature demodulator at the receiver. Both compensation methods converge quickly and present only a modest computational load. >

Analysis of the error performance of trellis-coded modulations in Rayleigh-fading channels

This work presents an exact expression for the pairwise error event probability of trellis-coded modulation (TCM) transmitted over Rayleigh-fading channels. It includes phase shift keying (PSK) and multilevel quadrature amplitude modulation (QAM) codes, as well as coherent and partially coherent (e.g. differential, pilot tone, etc.) detection. Due to the form of the exact pairwise error event probabilities, this calculation technique cannot be used with the transfer function technique to obtain an upper (union) bound on the overall bit error probability. For this reason, the authors estimate the bit error probability by considering only a small number of short error events. Through simulations, they found that the estimation is usually very accurate at high signal-to-noise ratios but not as accurate at lower signal-to-noise ratios. They study several coded modulation schemes this way. Among the results are the fact that TCM provides significant improvement in the error floor when detected differentially, and an asymmetry in the pairwise error event probability for 16 QAM. >

The effect of quadrature modulator and demodulator errors on adaptive digital predistorters for amplifier linearization

With the increasing importance of spectral efficiency in mobile communications, the power amplifier linearity has become a critical design issue for nonconstant envelope modulation. Imperfections in quadrature modulators and demodulators-gain and phase imbalance and DC offset-have a crippling effect on amplifier linearization circuits, a fact that has previously been noted experimentally. This paper is the first analysis of these effects on an adaptive predistorter. The primary result is an expression that provides an explicit tradeoff among intermodulation power, accuracy of the quadrature modulator and demodulator, and speed of adaptation. Another useful result is a simple and easily measured error figure for quadrature modulators and demodulators.

Adaptation behavior of a feedforward amplifier linearizer

Feedforward linearization has advantages in bandwidth and generality over other linearization methods. However, it is based on the subtraction of nearly equal quantities, so its major parameters must adapt to changes in environmental or operating conditions. This paper is the first published analysis of adaptation behavior in feedforward amplifier linearizers. As such, it presents an analytical framework and several new results, including convergence time and coefficient jitter, a bias effect that leads to extreme accuracy requirements in one coefficient, the effect of delay mismatch, and the mitigating effects of a filter inserted in one adaptation path.

A fast method for adaptation of quadrature modulators and demodulators in amplifier linearization circuits

Imperfections in analog quadrature modulators (QMs) and demodulators-gain and phase imbalance and DC offset-have a devastating effect on amplifier linearization circuits. Correction circuits have a simple structure, but their parameters must be adjusted adaptively. This paper introduces two new methods for such an adaptation. The first is based on a training sequence consisting of a tone at one or more amplitudes, and the second needs no training sequence at all. They are faster and more robust than previously published methods.

Optimum table spacing in predistorting amplifier linearizers

Radio frequency (RF) power amplifiers require linearization to reduce the intermodulation (IM) power if the signal does not have a constant envelope. Digital predistortion (PD) is one of the more successful methods. However, implementations usually employ uniform spacing of the PD lookup table entries, since the question of optimum nonuniform spacing has been unresolved. This paper provides the solution: a way to compute the IM for different spacing methods, a comparison of several table spacing schemes, a derivation of the optimum table spacing method, and a demonstration that the IM of optimum spacing is only a few decibels lower than that of equal spacing by amplitude when variations in operating point are considered.

Simulation and analysis of an adaptive predistorter utilizing a complex spectral convolution

An adaptive predistorter for linearizing a power amplifier in a mobile transmitter is studied, and the analytical, simulation, and measured results are presented. This predistorter does not have the problems of loop delay or phase shifting in its feedback path. The feedback is used only periodically to update the predistorter parameters so that it adapts to changes in the amplifier characteristics. An adaptation method for predistorters of the polynomial type is described. Its complexity is significantly lower than that of the previously described methods and its convergence speed, though low, is more than sufficient to track amplifier drift. Analytical verification that the measurement of out-of-band power is sufficient to drive the adaptation, a complex convolution method for measuring the out-of-band power that requires no additional local oscillator or phase reference, and a demonstration of the system performance utilizing a 16-QAM signal in a 25-kHz bandwidth, centered at 850 MHz, are provided. >

A linearizing predistorter with fast adaptation

An adaptive predistorter particularly suited to baseband implementation with digital signal-processing hardware is presented and analyzed. It is applicable to any modulation format and any AM/FM and FM/PM characteristic. With a table of well under 100 complex words, it reduces intermodulation products in a typical Class AB amplifier to levels suitable for the mobile environment. It adapts to a new amplifier in a few milliseconds, and its reconvergence time following a channel switch is virtually zero. Since it allows linearization of the power amplifier over 95% to 98% of the saturated output power, it provides power efficiency as well. In comparison with the most powerful predistorter previously reported, it requires four orders of magnitude less memory, reduces convergence time by over three orders of magnitude, eliminates reconvergence time following a channel switch, and eliminates the need for a phase shifter in the feedback path.<<ETX>>