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Abstract-This paper addresses the carrier-phase estimation problem under the low SNR conditions often encountered in turbo and LDPC-coded applications. In [1] a decision-directed carrier synchronization (DDCS) circuit that uses soft information from an iterative LDPC decoder was presented for BPSK and QPSK constellations with constant phase offsets. In this work, we present a method that is able to handle arbitrary constellations with a random-walk phase noise process. Loop SNR equations are derived and the performance for different constellations is shown. An extension of the DDCS algorithm is described that uses a search and tracking method based on measuring the number of satisfied LDPC constraint-node equations equations at the decoder and is able to track arbitrary carrier phase.

Abstract-In traditional receiver architectures, symbol acquisition and tracking are performed using phase lock techniques that are independent of the channel-code decoding process. In [1] feedback from the constraint-node side of a bi-partite graph is used to estimate symbol frequency and timing offset in a baseband pilotless transmission. In [2] soft information feedback from an LDPC decoder is used to recover carrier phase information under the assumption of perfect symbol timing. In this paper we address the problem of joint carrier-phase and symbol timing recovery. The proposed system is able to perform within 0.3 [dB] of the code performance with perfect knowledge of carrier phase and symbol timing.

Abstract-Timing recovery and channel decoding are traditionally performed independently. However, we show here that the information generated during the iterative decoding of LowDensity Parity-Check (LDPC) coded data can be fed back to the timing recovery circuit to enable accurate estimation of frequency and phase errors without the need for any pilot symbols. We describe a method capable of handling large offsets with complexity that grows linearly with offset size. Combining the LDPC constraint node observations with a properly calibrated phase locked loop allows successful tracking of a constant time delay, a frequency offset and a random phase walk.

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ABSTRACT

This paper presents results on the implementation of pilotless carrier synchronization algorithms at low SNRs using joint decoding and decision-directed tracking. A software test bed was designed to simulate the effects of decision-directed carrier synchronization (DDCS) techniques. These techniques are compared to non-decision directed algorithms used in phase locked loops (PLLs) or Costas loops. In previous work by the authors, results for direct M-ARY modulation constellations, with no code spreading were introduced. This paper focuses on the application of the proposed family of decision directed algorithms to direct sequence spread spectrum (DSSS) wave forms, typical of GPS signals. The current algorithm can utilize feedback from turbo codes in addition to the prior support of LDPC codes.

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