Optical Communication system with multimode fibers at low-frequency passbands have been studied for signal transmission. In previous studies, it was found that the most commonly known bandwidth is called 3-dB modal bandwidth and is limited between 200 to 500 MHz·km. There have been several studies of low-frequency passbands which used subcarrier signals in order to increase the bandwidth for the optical communication system. Some of these possible passbands are affected by nulls of frequency response of MMF. To overcome this problem, they must be selected carefully using the methods discovered in recent studies. It was found that to achieve the proper Bit-Error-Rate (BER), the optical communication system required more optical power. In this paper, the optical communication system with MMF at low- frequency passbands is modelled by applying a 4-Amplitude Shift Keying (4-ASK) with linear block encoding. Using the subcarrier multiplexing (SCM) technique with 3 low-frequency passbands and 3-dB modal bandwidth as channels for transmitting a high data rate signal, from the simulation result, it is found that using (31, 26) linear block encoding, the data rate of 546 Mbps can be obtained with a BER lower than 10-8 and a 6-dB coding gain is achieved. The total transmitted data rate is higher by approximately 2.73 times than the rate sent by the 3-dB modal bandwidth.
Multimode fibers are normally known to have a channel for carrying a signal mainly by their 3-dB modal bandwidth ranging between 200 to 500 MHz-km, depending on the material and structure of the fiber. To use only this 3-dB modal bandwidth, a higher data rate signal cannot be successfully transmitted. Alternatively, it has been shown that the response of the multimode fibers at low-frequency region, defining as the frequency next to the 3-dB modal band, contains many passbands. Additionally, these low-frequency passbands have been shown to be predictable in terms of their peak frequencies; thus, suitable subcarrier frequencies can be obtained and used in SCM system. In this paper, the formula from the previous work for determining the peak frequency of all 6 low-frequency passbands is applied. These 6 passbands and the 3-dB modal band of the multimode fiber are used to convey a high data rate signal. The signal is separated into 7 subcarrier signals and transmitted over these 7 channels using SCM system. The performance of the received signal in terms of the bit-error-rate (BER) is determined and shown. Some modification and adjustment are done in order to improve the performance of the system. It is found that for a multimode fiber with a 200-MHz 3-dB modal bandwidth, a 500-Mbps data rate signal can be successfully transmitted with a BER of lower than 10-6 . The data rate transmitted over a multimode fiber can be increased 2.5 times comparing to the 3-dB modal bandwidth, without any coding technique applied.