In a discrete multitone receiver, a time-domain equalizer (TEQ) reduces intersymbol interference (ISI) by shortening the effective duration of the channel impulse response. Current TEQ design methods such as minimum mean-squared error (MMSE), maximum shortening SNR (MSSNR), and maximum geometric SNR (MGSNR) do not directly maximize bit rate. In this paper, we develop two TEQ design methods to maximize bit rate. First, we partition an equalized multicarrier channel into its equivalent signal, noise, and ISI paths to develop a new subchannel SNR definition. Then, we derive a nonlinear function of TEQ taps that measures bit rate, which the proposed maximum bit rate (MBR) method optimizes. We also propose a minimum-ISI method that generalizes the MSSNR method by weighting the ISI in the frequency domain to obtain higher performance. The minimum-ISI method is amenable to real-time implementation on a fixed-point digital signal processor. Based on simulations using eight different carrier-serving-area loop channels, 1) the proposed methods yield higher bit rates than MMSE, MGSNR, and MSSNR methods; 2) the proposed methods give three-tap TEQs with higher bit rates than 17-tap MMSE, MGSNR, and MSSNR TEQs; 3) the proposed MBR method achieves the channel capacity (as computed by the matched filter bound using the proposed subchannel SNR model) with a five-tap TEQ; and 4) the proposed minimum-ISI method achieves the bit rate of the optimal MBR method.
In this paper, we present a new class of blind cyclic-based estimators for carrier frequency offset and symbol-timing error estimation of orthogonal frequency-division multiplexing (OFDM) systems. The proposed approach exploits the properties of the cyclic prefix subset to reveal the synchronization parameters in the likelihood function of the received vector. In this paper, a new likelihood function for the joint timing and frequency-offset estimation is derived, which globally characterizes the estimation problem. The resulting probabilistic measure is used to develop three classes of unbiased estimators, namely, maximum-likelihood, minimum variance unbiased, and moment estimator. In comparison to the previously proposed methods, the proposed estimators in this study are computationally and statistically efficient, which makes the estimators more attractive for real-time applications. Performance of estimators is assessed by simulation for an OFDM system.
A noise-shaped direct digital IF to RF (DIF2RF) DAC with embedded upconverter mixer is presented. The digital IF signal is noise shaped by a bandpass 6-1 modulator with 1-bit IF output followed by a semidigital finite impulse response (FIR) filter. The current mode FIR filter combines scaled values of the local oscillator (LO) signal for performing reconstruction filtering and upconversion in a single module. The DIF2RF design modulates the digital IF signal with a digital LO signal. This topology eliminates the transconductance nonlinearity of conventional mixers and is inherently linear due to single-bit digital IF input. The presented architecture reduces clock jitter sensitivity of 1-bit DACs by masking IF clock transitions with LO signals. A prototype of the DIF2RF DAC is designed and fabricated in a five-layer metal 0.25-m digital CMOS process. The architecture can be used in low-power software-defined digital-IF transmitters. The DIF2RF DAC consumes 49 mA from a 2.5-V supply, achieving 64.7-dBc third-order intermodulation at 1.03 GHz with a spurious-free dynamic range of 72 dB in a 15-MHz bandwidth.
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