In the upcoming B5G and 6G, orbital angular momentum (OAM) can be used as the new dimension for data transmission between the base-station (BS) and the self-backhauled small cell (SBSC) station. However, the OAM beam is not suitable for the long distance transmission due to its inverted cone shape. Moreover, the commonly used uniform circular array (UCA) communication system with fixed radius can only obtain limited channel capacity. In order to obtain higher channel capacity, this paper proposes an optimized transmission system combining UCA and non-twisted parabolic antenna (PA). The optimization of UCA deployment radius enables the combined system to obtain the maximum channel capacity for the transmission at a specific distance. Compared with the traditional OAM system and the multiple-input multiple-output (MIMO) system, the channel capacity can be increased nearly up to two and four times respectively without increasing the bit error rate (BER) and complexity significantly.