Increasing talk-time with efficient linear PA's

Abstract

In order to meet the TETRA linearity specification the power amplifier (PA) linearisation methods in common usage today usually have low efficiencies (e.g. Cartesian loop and adaptive predistortion). The market drivers for lightweight terminals and long talk-time are directly related to transmitter efficiency. Techniques that make the radio more power efficient allow smaller (lower capacity) batteries or a longer useable time between battery charge-cycles. The transmitter places a heavy demand on the battery and therefore utilising more efficient transmitters can increase talk-time or reduce the battery size. Firstly it is shown that to obtain a high transmitter efficiency (>50%) the PA needs to be biased as class-C. The effect power amplifier efficiency has on talk-time is demonstrated for a handportable with fixed power overhead owing to the digital processing required to transmit a burst. It is shown that the PA efficiency for a transmitter operating backed off (under power control), has a marginal impact on the talk-time and that the PA efficiency effects on talk-time are largest at higher output powers. This is put in context by a review of the various linearisation schemes such as Cartesian loop, polar loop, 2nd harmonic injection, envelope elimination and restoration (EER), predistortion methods, and synthesis techniques (e.g. LINC and CALLUM). The review also discusses which methods are not suitable for handportable design, and documents the strengths and weaknesses of the remaining methods. The paper shows that to increase the power efficiency and (thus talk-time) hybrid solutions to power amplifier linearisation are required. Finally, existing hybrid solutions and their shortfalls are discussed. (22 pages)