Engineering & Technologies, 2016, 9(2), 197-203 ~ ~ ~ УДК 621.397.22 Effect of Changing the Symbol Rate of QAM Modem on the Performance of 32kb/s ADPCM System Muhanned AL-Rawi*a b and Muaayed AL-Rawib a AL-Mustansiryia University Iraq Received 28.05.2015, received in revised form 16.09.2015, accepted 06.02.2016 This paper presents three models of Quadrature Amplitude Modulation (QAM) modem operating at data rate of 16.8kb/s to be transmitted over 32kb/s Adaptive Differential Pulse Code Modulation (ADPCM) channel. <...> These modems operate at symbol rates of 2400, 2800, and 3360 baud with associated number of bits per symbol of 7, 6, and 5 respectively. <...> The simulation results show that the performance of ADPCM degrades as the symbol rate decreases or number of bits per symbol increases. <...> Also, the performance with circular constellation is better than rectangular one. <...> Citation: AL-Rawi M., AL-Rawi M. Effect of changing the symbol rate of qam modem on the performance of 32kb/s ADPCM system, J. Sib. <...> All rights reserved * Corresponding author E-mail address: muhrawi@yahoo.com, muaayed@yahoo.com University of Ibb Yemen # 197 # Muhanned AL-Rawi and Muaayed AL-Rawi. <...> As one of these coding methods is international standard 32 kb/s Adaptive Differential Pulse Code Modulation (ADPCM) [1]. <...> A recommended defi nition of ADPCM algorithm was published by International Telephone & Telegraph Consultative Committee [CCITT, the new name is International Telecommunication Union (ITU)] as Recommendation G.721 [1]. <...> It was recognized at study group XVIII meeting [1] that voiceband data performance at 9.6 kb/s would not be acceptable with standard 32 kb/s ADPCM because ADPCM adds severe nonlinear distortion to the voiceband data signal with speed greater than 4.8 kb/s. <...> Thus, the interest of many research workers has been directed towards the ADPCM codec capable of providing better performance for speech and voiceband data signal at speed greater than 4.8 kb/s. # 198 # Muhanned AL-Rawi and Muaayed AL-Rawi. <...> Effect of Changing the Symbol Rate of QAM Modem on the Performance… Exhaustive work had been done to accommodate high speed voiceband data signal either by modifying <...>