A novel soft demapper for ?/2-quadrature phase shift keying

In recent years, there has been enormous interest in millimeter (mm) wave communication systems such as IEEE 802.11ad and IEEE 802.15.3c operating in higher and unused frequency bands. To make system more resilient in time varying, multipath fading channels, a modulation scheme called π/2-quadrature phase shift keying (QPSK) along with a forward error control (FEC) scheme has been proposed in these systems. The performance of the FEC decoder is improved when soft information in the form of log likelihood ratio (LLR) is given to it In this paper, we propose a method for soft demapping of π/2-QPSK symbols. This method is also applicable to other schemes such as differential QPSK (DQPSK) and offset QPSK (OPSK). The complexity of the proposed method is very less and requires one adder and one multiplier. We demonstrate the bit error rate (BER) performance of the proposed method using simulations and compare it with that of other methods.