Proposed π/4-CQPSK with increased capacity in digital cellular systems

Truly coherent π/4-quadrature phase shift keying (π/4-CQPSK) is shown to increase the capacity of digital cellular systems over coherently demodulated and differentially encoded π/4-QPSK (π/4-DEQPSK). The authors study the degradation of DEQPSK for different fading rates and operating bit error rates (BERs). Frequency-reuse systems and power-limited systems are studied. It is shown that the degradation of DEQPSK in fading channels is not as high as in linear static channels. However, for any practical fading rate and operating BER, the degradation is as high as 2 dB. It is also shown that the degradation is independent of the fading rate. For the practical operating BER, the degradation is also constant. Thus, an unambiguous phase estimation scheme should be used in fading channels if coherent detection is essential. Unambiguous phase estimation can be achieved by utilizing the preamble sequence or inserting pilot symbols. It is also shown that the power efficiency is also improved by 2 dB when truly coherent demodulation is employed in a power-limited system