Yoedy Moegiharto

This work investigates the implementation of hybrid AF and DF relay protocols in D2D multi-relay wireless communication systems. To obtain system performance we propose four scenarios for using the AF and DF relay protocols and three S – R and R – D distance conditions that have an impact on system performance in terms of throughput values. From the simulation results it is concluded that the R – D distance will produce the highest SNR value which produces the best system throughput value compared to . The simulation results also prove that applying the AF relay protocol to all relays produces better SNR values ​​and better throughput values ​​than applying the DF relay protocol. Simulation results with the impact of hardware impairment (HI) levels show that system performance in the first scenario is actually worse than other scenarios when the κ value is above 0.1.

Modern wireless communication systems use orthogonal frequency division multiplexing (OFDM), a multi-carrier modulation method that resists multipath channels and provides bandwidth efficiency. OFDM is generally used with a Multiple-Input Multiple-Output (MIMO) system to boost diversity gain and channel capacity. MIMO-OFDM has several advantages, but its high PAPR value is a drawback. A non-linear high-power amplifier (HPA) can distort signals with high PAPR values. This issue can be resolved by employing predistortion, which compensates for nonlinear HPA. In addition to PD, channel coding can be used to improve the quality of systems with high PAPR values by adding redundant bits to the bits to be sent. In this paper, we report the experimental evaluations of the joint channel coding and pre-distortion (PD) technique on a 2x2 MIMO OFDM system using USRP hardware. The experiments are conducted in two scenarios: line-of-sight (LOS) and nonline-of-sight (NLOS) scenarios. The channel coding used in this scenario is convolutional code with code rates of 1/2, 2/3, and 3/4. From the results of the experiment, it can be seen that the system that uses PD combined with the convolution code produces better performance in the LOS and NLOS scenarios compared to the system without PD. In the LOS scenario, the use of PD can improve the SNR value of code rates 1/2, 2/3, and 3/4 by approximately 58.74%, 75.97%, and 96.20%. In the NLOS scenario, the use of PD can improve the SNR value of code rates 1/2, 2/3, and 3/4 by about 60.71%, 73.59%, and 71.84%. The measurement of the LOS scenario gives a better SNR value than the NLOS scenario, with a maximum SNR value of 30.86 dB, while the maximum SNR value of the NLOS scenario is 30.23 dB. This happened because the LOS scenario suffered minimal multipath fading compared to the NLOS scenario

Orthogonal Frequency Division Multiplexing (OFDM) is a transmission technique that uses several subcarrier frequencies (multicarrier) that are perpendicular to each other (orthogonal). This OFDM modulation technique effectively eliminates Intersymbol Interference (ISI) on a channel caused by the effects of multipath fading. To overcome this weakness, OFDM uses a guard interval (cyclic prefix) inserted in its transmission. This paper analyses the effect of different cyclic prefix lengths on OFDM performance on multipath channels using Matlab. Here, the cyclic prefix length as the main parameter is varied for the different number of subscribers. Meanwhile, Bit Error Rate (BER) and Signal Noise Ratio (SNR) values obtained at the receiver are used to see the system's performance. Based on our simulation results, the best value of BER is obtained using a cyclic prefix length of 1/4 with a Fast Fourier Transform (FFT) size of 512.

A high PAPR value is one of important weaknesses in OFDM systems. Several reduction techniques are applied to reduce PAPR including partial transmit sequence (PTS) as well as clipping and filtering (CF). Adaptive iterative clipping and filtering is a development of iterative clipping and filtering techniques. In this paper, a combination of Partial transmits sequence and adaptive iterative clipping filtering (PTS-AICF) techniques on multi-antenna OFDM transmitters was carried out. The simulation results showed that combined technique application had a better performance than non-combined technique (PTS), either for two or four antennas, and also for different sub block numbers. Performance was also influenced by iterations number on AICF section, the more iterations were used, the better the reduction technique performance was because it produced smaller PAPR 0 value.

Multicarrier modulation technique as Orthogonal Frequency Division Multiplexing (OFDM) is able to vanishIntersymbol Interferences (ISI) caused by multipathchannel. Merging with multi- antenna transmission techniques such asMIMO become MIMO- OFDM system is able to improve (data rate transmission)the transmission of data and capacity of the system. Pilot channel estimation technique with the algorithm Linear Minimum Mean Square Error (LMMSE) with modulation m-PSK is applied in this study. The results obtained in the form of graphs BER against EbNo which shows that the system performance with LMMSEestimator has a different gain for QPSKmodulation better about 0.75 dB, for 8 PSKmodulation is about 1.5 dB and for 16 PSKmodulation around 1 dB compare to LS estimator. Number of the largest antenna both in the transmitter and receiver produce the best system performance. Observation for QPSKmodulation shows a scheme transmitter- receiver antenna 4x2 having gain approximately 9 dB better than the number of the lowest transmitter antenna(1x2). For scheme 4x4 shows gain around 8,5 dB than the number of the lowest receiver antenna.Keywords:MIMO, OFDM,LS, LMMSE, MPSK.