Generation of balanced quadrature phase shift keyed sequences through guided scrambling

Abstract

Balanced codes (also called DC-free codes) are widely used in binary communication systems to increase the likelihood of accurate symbol recovery with practical demodulators. Guided scrambling (GS) is recognised as a viable approach to efficiently generate DC-free binary sequences. In this study the authors extend the use of GS to generate balanced sequences of quadrature phase shift keyed (QPSK) symbols by using arithmetic from the ring of polynomials defined over the Galois field of four elements. In addition to ensuring adequate timing information and consistent decision thresholds to improve the performance of practical demodulators, balanced encoding of QPSK symbol sequences creates a null at DC in the spectrum of the equivalent complex baseband signal. This corresponds to a null at the centre frequency of the bandpass QPSK signal that enables the insertion of a pilot tone and avoidance of narrowband interference without filtering or distortion of the signal. The authors outline sufficient conditions for the generation of balanced GS QPSK sequences, and based upon these conditions the authors recommend scrambling polynomials and quotient selection criteria. The authors then present analytical and simulation results that confirm the generation of balanced sequences using this approach.

Convex optimization–based multi-user detection in underwater acoustic sensor networks

“…, C k (SF À 1) as the spreading code of the kth user. For this user, the input data sequence is modulated according to quadrature phase shift keying 33 (QPSK) and converted into Nc=SF substring after serial/parallel (S/P) conversion. The output of the Nc=SF substring can be expressed as fa k 0 (i), a k 1 (i), .…”

“…Given C k ( 0 ) , C k ( 1 ) , … , C k ( SF − 1 ) as the spreading code of the k th user. For this user, the input data sequence is modulated according to quadrature phase shift keying 33 (QPSK) and converted into Nc / SF substring after serial/parallel (S/P) conversion. The output of the Nc / SF substring can be expressed as { a 0 k ( i ) , a 1 k ( i ) , … , a ( Nc / SF ) − 1 k ( i ) } , where a ( Nc / SF ) − 1 k ( i ) represents the i th data on the Nc / SF − 1 line.…”

Guided Scrambling As a Peak to Average Power Ratio Reduction Technique for Digital Video Broadcasting-Terrestrial 2

“…Scrambling randomises a bit pattern but does not ensure a balanced line transmission. Guided Scrambling is a line coding technique which offers balanced, highly efficient transmission with a high transition density [6] [7].…”

Variable‐length balanced codes for quadrature phase shift keyed systems

“…Owing to the advantages they offer to practical demodulators, the use of balanced codes has recently been suggested in bandpass systems such as those using quadrature phase shift keyed (QPSK) signalling [3, 4]. The balanced nature of a QPSK signal also results in a null at the carrier frequency of the bandpass spectrum, which corresponds to a null at DC in the spectrum of the equivalent complex baseband signal.…”

“…Construction of fixed‐length balanced QPSK codes using table‐based approaches [3] and guided scrambling [4] has recently been reported. In this paper, we outline the construction of variable‐length DC‐free QPSK codes based on the technique described in [5].…”