Propagation of Mobile Communication with Tree Obstacle using OFDM-QAM at 10 GHz

This research focused on mobile communication system on the road. The communication frequency used was 10 GHz. The tree was modeled as communication's obstacle for every node. The communication itself was modeled with single diffraction using the single knife-edge model. The communication transmission used Orthogonal Frequency Division Multiplexing. This paper used modulations that consisted of 16 QAM and 64 QAM. This paper utilized the variation of modulation and transmitter power. As a result, the SNR was decreased when transmitter power was increased, the value of BER 64 QAM was lower than BER 16 QAM, and the percentage of the coverage area for communication around the tree was around 96%.


B. Propagation
Noise value for communication systems could be seen in equation (1). At Equation (1), the noise figure (F) value was 7 dB, the T0 parameter was 290 O K, the Boltzman constant was 3 x 10 -23 [18], and the amount of 200 MHz bandwidth's channel was 8. Path loss value could be seen in equation (2).
Transmitter power that was used consisted of 0.1 watts and 1 watt. L parameter was a loss, GT parameter was transmitter gain (dB), GR parameter was receiver gain (dB), d parameter was communication distance, and λ parameter was wavelength (m). The SNR value could be seen in equation (3).
The SNR value was obtained from S value as a signal (dB), and N parameter of noise. The transmission communication of this research was used orthogonal frequency division multiplexing (OFDM), which transmits multiple data symbols continuously used subcarriers orthogonal. The transmission system diagram that used OFDM could be seen in Fig.2. That figure showed OFDM systems such as transmitter and receiver. The transmitter block was used consisted of send data, QAM modulation, serial to parallel, IFFT, cyclic prefix, parallel to serial, Digital Analog Converter (DAC), and transmitter [19]. The receiver block was used consisted of the receiver, Analog Digital Converter (ADC), remove a cyclic prefix, serial to parallel, FFT, parallel to serial, QAM demodulation, and receive data.
Send data block was a bit sequence that will be sent. QAM principle was an information data sequence that was sent and converted to parallel form, formerly bit rate of R, M parameter was a parallel line amount that was the same with subcarrier amount. The S/P block that used for changing serial data bit became a parallel data bit. Generate subcarrier at OFDM system used technique of inverse fast furrier transform (IFFT) subcarriers and orthogonal for every duration at OFDM symbol. The Guard interval was used by cyclic prefix (CP). The distance between subcarrier (∆f) and time duration from the OFDM symbol was 1/∆f + cyclic prefix, functioning as the orthogonal guard between subcarrier.
The cyclic prefix used for extending the OFDM symbol, such as multiplication at the last sample part of OFDM symbol, becomes addition at the first sample part. At equation (4), Tsymbol parameter was symbol period of Tsample. Tsample parameter was period for part of data, and Tcp parameter was the addition part of data that is taken from the data.
The data (dn,k) that was placed at scale group of N and n block modulation that became waveform of exponential complex ϕk(t), can be seen at equation (5) until equation (7), The dn,k parameter was a symbol that transmitted along interval time n using k subcarrier. Td parameter was symbol duration. N parameter was the amount OFDM subcarrier. fk parameter was frequency subcarrier k, and f0 parameter became the lower value [19]. P/S block at the receiver side was blocked that used for change of parallel data bit become serial bit data. CP addition block at transmitter side constitutes cyclic prefix block, where at that block carried was add bit of CP. P/S block at the transmitter side constitute block that used to change parallel data bit become serial data bit. Generated subcarrier at OFDM systems used fast furrier transform (FFT) technique. At Fig.2 IFFT block that used for accomplishing modulation block at data constellation of single carrier orthogonal. Inverse Fast Fourier Transform (IFFT) can decrease of necessities RF divider so that block more efficient for divider at subcarriers. IFFT accomplished transformation at parallel data symbol from the frequency domain become the time domain. S/P block at the receiver side constitutes block that used for change of serial data bit become parallel data bit. Block for CP remove at the receiver side was processed where data bit was removed process at cyclic prefix bit. That FFT block was used for demodulation at data constellation from single carrier orthogonal. IFFT input from constellation N showed number at FFT node. That constellation has taken from QAM modulation. Demodulation process based on orthogonal at subcarrier ϕk(τ), could be seen in equation (8) [19].
Demodulator was implemented to digital with orthogonal relation at subcarrier. IFFT was used for modulation at OFDM signal, and FFT was used for demodulation at OFDM signal. Equation (9) was implemented for FFT block [20].
C. Atmosphere Atmospheric attenuation was influenced by oxygen and water vapor. Equation (10) consisted of γa parameter that was attenuation atmospheric specification (dB/km), d parameter was LOS (line of sight) distance (km). Equation (11) showed γa value, that consist of γw and γo [11]. Where γw parameter was water vapor attenuation, although γo parameter was oxygen attenuation.      For some data that used parameter consisted of the transmitter power of 1 watt and 16 QAM, the SNR value of 33 dBW was obtained for MS location at 14 meters. With the same condition, the SNR value of 7 dBW was obtained for MS location at 290 meters. However, if 64 QAM was used with the same condition, the SNR value of 43 dBW was obtained for MS location at 14 meters, and the SNR value of 17 dBW was obtained for MS location at 290 meters.
For some data that used parameter consisted of the transmitter power of 0.1 watts and 16 QAM, the BER value of 0 was obtained for MS location at 14 meters. With the same condition, the BER value of 0,017882 dBW was obtained for MS location at 290 meters. However, if 16 QAM and the transmitter power of 1 watt were used with the same condition, the BER value of 0 was obtained for MS location at 14 meters, and the BER value of 0 was also obtained for MS location at 290 meters.
For some data that used parameter consisted of the transmitter power of 0.1 watts and 64 QAM, the BER value of 0 was obtained for MS location at 14 meters. With the same condition, the BER value of 0,0833 was obtained for MS location at 290 meters. However, if 64 QAM and the transmitter power of 1 watt were used with the same condition, the BER value of 0 was obtained for MS location at 14 meters, and the BER value of 0,000188 was obtained for MS location at 290 meters.

V. CONCLUSION
The result showed some data that consist of SNR, BER, and percentage of coverage area. The SNR value was increasing when the transmitter power was increased. The BER value was increasing when the mobile station was moved far away from the base