Electronic Predistortion by Polar Coordinate Transformation Using the CORDIC Algorithm

In this paper, we propose a polar coordinate transformation in real time using the COordinate Rotation DIgital Computer (CORDIC) algorithm in order to realize an electronic predistorting transmitter with a practical circuit implementation. The proposal enables a dual-drive Mach-Zehnder modulator (DDMZM) to use vector in-phase/quadrature-phase (I/Q) modulation to generate predistorted complex electronic fields. The basic theory of coordinate transformation and the CORDIC algorithm are explained in detail. The expected dispersion compensation performance for 40-Gb/s differential quadrature phase-shift keying (DQPSK) is evaluated by numerical simulations. The required number of iterations of the CORDIC algorithm and its impact on signal quality are also clarified. Simulation shows that only seven iterations are required to achieve signal quality sufficient for predistortion transmission. A practical size of 6.9-M gates is estimated to be sufficient for the large scale integration (LSI) implementation for 40-Gb/s DQPSK systems.