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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.
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