Performance analysis of digital modulation schemes used in mobile radio

Abstract

Mobile communication is becoming an important field where people need to communicate while moving to follow rapid changes occurring in the world. The mobile communication channel is considered as a multipath fading channel where frequency selective fading can occur. Also, in the mobile environment, the available bandwith is limited and usually non-linear power amplifiers are used. Under all these severe conditions, many digital modulation schemes have been investigated to be used for mobile radio. The Gaussian minimum shift keying (GMSK) is the modulation scheme used in the GSM system which is installed in Europe while the modulation scheme used in North America is the n/4-shifted-DQPSK. The QAM system is being proposed for mobile communication because of its spectral efficiency but it suffers from employing non-constant envelope signals which creates problems when nonlinearity is present. The aforementioned schemes are simulated in this thesis under different channel conditions and their bit error rate (BER) performance is evaluated. The effect of non-linear amplification to the signals on the QAM systems is also investigated. Linear equalization is employed to alleviate the effect of the frequency-selective fading channel. Simulation results showed that both the GMSK & n/4-shifted-DQPSK have comparable BER performance. However, this BER increases in the case of QAM. To reduce this BER and make it comparable with the other two systems, the transmission rate has to be reduced giving only 20% bit rate advantage to the QAM system. A new transmitter structure for the GMSK system is proposed. It is more spectrally efficient compared to the existing transmitter. This reduces the interference between adjacent channels which is an important property especially in the mobile radio.