Increasing the talk-time of mobile radios with efficient linear transmitter architectures

In order to meet the linearity specification of standards such as GSM EDGE and TETRA the power amplifier (PA) linearisation methods in common usage today result in low DC to RF efficiency (e.g. Cartesian loop and adaptive predistortion). The transmitter places a heavy demand on the battery and therefore utilising more efficient transmitters can increase talk-time or reduce the battery size. For GSM EDGE and TETRA modulation it is shown that to obtain a transmitter efficiency (of >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 compile and transmit a burst or packet. 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 effect is largest at high 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 & restoration (EER), predistortion methods, and synthesis techniques (e.g. LINC & 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