Transmit power is limited by EMS regulations and, in the case of portable devices, battery life. Linear modulation translates the spectrum of the data up to the carrier frequency. Nonlinear modulation uses a nonlinear process to modulate the carrier, producing a broadband transmission much greater than the data bandwidth. The receiver retrieves the data by multiplying the incoming signal by the identical PN sequence. With both methods, the transmitter and receiver PN sequences must be synchronized in order for the receiver to extract the data from the sequence.
Synchronization is the greatest challenge of a spread-spectrum communication system.
A common receiver synchronization strategy is to acquire synchronization when a transmitter or receiver is first turned on, then continuously track synchronization while the devices are communicating.