Direct sequence spread spectrum modulation occupies more bandwidth than conventional communication schemes by spreading the signal before transmission with a pseudorandom noise (PN) sequence. It provides benefits like secure communication, rejection of interference, and multipath rejection. The two main types are direct sequence, which spreads the signal by multiplying it with the PN sequence before transmission, and frequency hopping, which rapidly switches the carrier frequency. PN sequences are generated using feedback shift registers and have important properties like maximal-length sequences that repeat after a specific period. Processing gain is a measure of the system's immunity to interference and is calculated as the ratio of the spread bandwidth to the original signal bandwidth.