Transmission Characteristics of an OFDM signal for Power Line Communication System with High Bit Rate

Abstract

In this report, we measured what influence the characteristics of the electric power line with various forms gave to the transmission characteristic of OFDM (Orthogonal Frequency Division Multiplexing) signal through PLC (power line communication system) modem based on the OFDM method.We classified the electric power line transmission line with various forms in a real environment into basic elements as shown in Fig. 1, which are important elements influencing the power line transmission characteristics. In Fig. 1, (a) is no branch, (b) an outlet type branch and (c) a switch type branch that is used in a power line for lamp. Next, differential permeation characteristics and group delays for the basic elements were measured. When the outlet was short in case (b), the differential permeation attenuation increased extremely in low-frequency band. Moreover, in case (c) attenuation like the resonance appeared in the low-frequency band at the branch length of 160cm.We measured PHY rate (Physical rate) for each basic element as shown in Fig. 1, which is connected with the PLC modem. The measurement system of PHY rate is shown in Fig. 2 and set in G-TEM (Giga-Hertz Transverse Electromagnetic) cell in order to suppress power supply coupling between sending and receiving modems. Normally, PLC modem is connected to AC power line, but in this case, the measurement system connected to DC power line as shown in Fig. 2 is used to suppress the power supply coupling. The communication link between sending and receiving PCs through the modem was established and measured PHY rate. The frequency band of the modem is from 4MHz to 34MHz.At this time, the transmission characteristics of the electric power line are simulated from measured data. We composed the OFDM sending and receiving system on the computer, and simulated the PHY rate. The OFDM system consists of general FFT-base. The composition of the measured transmission characteristics for the electric power line is simulated as follows. First, wideband and uniform signal attenuation is imitated in the ATT (attenuator) block. Next, we imitated the attenuation and the group delay characteristics according to each basic element by a digital filter block. In addition, the thermal noise of the equipment is imitated by AWGN (Additive White Gaussian Noise) block. Table 1 shows the results of PHY rate corresponding to each basic element as well as their termination conditions, and the calculated values agree well with the measured values. By comparing with measured and calculated values, it is revealed that PHY rate of PLC modem is most affected in the case of the power line transmission characteristics having broad band and high level attenuation and group delay variation, and is not affected in the case of that having narrow band attenuation and group delay variation. Transmission characteristics measurement system using PLC modem

Transmission characteristics simulation on power line communications

“…There have been many investigations of the transmission characteristics of PLC, including evaluations based on field data and analytical models for transmission lines [5][6][7][8][9][10][11][12][13][14][15]. Investigations in the past have either generated analytical models of transmission lines [5][6][7][8][9][10][11][12][13], or have used measured results [14,15].…”