The ENCAP-4N (ENCoded Amplitude and Phase-4N), related with a vast class of OQPSK (Offset Quadri-PhaseShift) Keying) type signals, can be advantageous when it is important to achieve good tradeoffs between spectral efficiency and envelope fluctuations when a nonlinear amplification is employed. This paper presents a simplified representation of FQPSK (Feher-Patented Quadrature-Phase Shift Keying), showing also the potential improvement in power efficiency, without sacrifice of spectral compactness. Usually, the coding rule for ENCAP-4N format is based on the complex envelope decomposition in pulses with bit duration T, directly related with the 4N generating functions set. In this paper, it is proposed a new interpretation based in this signal format that results in a coding rule, based on a complex envelope description as function of pulses with duration 2T. The signal, in accordance with the foregoing coding rule, can be viewed as a trellis coded modulation with 22N+2 states and pulses shapes.

Highly Efficient Encoded OQPSK Signals: Emission and Reception Design Aspects

Offset quaternary PSK-type (OQPSK) digital modulation schemes are strongly recommended for radio transmission within land mobile and/or satellite communication systems requiring good power/bandwidth tradeoffs and low-cost, power-efficient implementations. The ENCAP-OQPSK design method (encoded-amplitude-and-phase) previously proposed by the authors was shown to provide a wide class of OQPSK-type schemes of practical interest; a flexible, digital transmitter implementation can be directly based on the encoding rules of this design method. The paper deals with the transmission modelling of radio links where ENCAP-OQPSK modulators are employed. In spite of the apparent complexity of the encoding rules, it was possible to define an appropriate signal model, both accurate and user-friendly. In fact, the nonlinearly modulated ENCAP-OQPSK signals can always be exactly represented as a sum of linear OQPSK-type components. Similar representations, also exact, can be adopted for any signal obtained by filtering and/or nonlinear amplification of the signals. This modelling approach simplifies spectral computations and performance evaluations, and is quite accurate, in general, when the sum of the two main linear components is used to approximate the transmitted signals.

Transmission modelling of ENCAP-OQPSK radio links

The use of encoded-amplitude-and-phase offset quaternary phase-shift keying (ENCAP-OQPSK) signal encoding techniques are considered for power/bandwidth efficient transmission systems. It is shown that a coding gain is achievable. Connections with trellis-coded offset quadrature amplitude modulation (OQAM) schemes previously proposed by the authors are established.

Highly power/bandwidth efficient digital transmission with selected ENCAP-OQPSK modulation schemes

A class of encoding schemes for generating offset-QPSK-type signals is introduced. Each scheme supposes the specification of 4N functions directly related to the eye-diagrams of the in-phase and quadrature signal components. The digital implementation of the encoding rules, by table look-up techniques, is straightforward, and leads to a flexible and cost-effective transmitter that allows high spectral efficiencies to be achieved without any selective linear filtering. The main features of the proposed modulation format are described, namely its connections with well known modulation formats. It is shown that the new signal design method provides a unified representation of all OQPSK-type (MSK-type) signals recommended so far (IJF-OQPSK, TFM, GMSK, XPSK, 12PM3X, etc.), and allows the generation of a large variety of new signals. The conditions under which a linear filter and a nonlinear amplifier preserve the proposed format are discussed. It is also pointed out that, in most practical situations, the encoding rules can be modified so as to allow the transmission of a nonlinearly amplified signal of the proposed class, having a variable envelope.

A. Gusmao

Papers

Generalised ENCAP-4N digital modulation technique