Ciphered BCH Codes for PAPR Reduction in the OFDM in Underwater Acoustic Channels

We propose an effective, low complexity and multifaceted scheme for peak-to-average power ratio (PAPR) reduction in the orthogonal frequency division multiplexing (OFDM) system for underwater acoustic (UWA) channels. In UWA OFDM systems, PAPR reduction is a challenging task due to low bandwidth availability along with computational and power limitations. The proposed scheme takes advantage of XOR ciphering and generates ciphered Bose–Chaudhuri–Hocquenghem (BCH) codes that have low PAPR. This scheme is based upon an algorithm that computes several keys offline, such that when the BCH codes are XOR-ciphered with these keys, it lowers the PAPR of BCH-encoded signals. The subsequent low PAPR modified BCH codes produced using the chosen keys are used in transmission. This technique is ideal for UWA systems as it does not require additional computational power at the transceiver during live transmission. The advantage of the proposed scheme is threefold. First, it reduces the PAPR; second, since it uses BCH codes, the bit error rate (BER) of the system improves; and third, a level of encryption is introduced via XOR ciphering, enabling secure communication. Simulations were performed in a realistic UWA channel, and the results demonstrated that the proposed scheme could indeed achieve all three objectives with minimum computational power.

CPM-OFDM Performance over Underwater Acoustic Channels

We propose and evaluate the performance of a continuous phase modulation based orthogonal frequency division multiplexing (CPM-OFDM) transceiver for underwater acoustic communication (UAC). In the proposed technique, the mapper in traditional OFDM is replaced by CPM while a realistic model of underwater channel is employed. Bit error rate (BER) as well as peak to average power ratio (PAPR) performance of the proposed scheme is evaluated using Monte-Carlo simulations. The error performance observed clearly establishes the superiority of CPM-OFDM over traditional OFDM schemes. Specifically, a value of 7/16 or 9/16 for the modulation index gives the best error performance. Furthermore, the error performance of the proposed scheme is within acceptable values up to a transmitter–receiver distance of 1.5 km. Additionally, the PAPR performance of the proposed scheme suggests that like other OFDM schemes, a PAPR reduction scheme is mandatory for acceptable PAPR performance of CPM-OFDM.