Spread spectrum Processing gain looks like something for nothing. Is it?
No, something for nothing violates the second law of thermodynamics. Let’s look at processing gain. When we transmit a signal, we require a certain amount of bandwidth to pass our data stream. This is sometimes known a the information bandwidth. The amount of bandwidth is related to the bit speed and method of modulation. Normally it is only several kHz. A spread spectrum transmission takes the data stream and combines it with a very wide band signal (direct sequence) or combines it with a signal that is sequencing over many frequencies spaced over a wide bandwidth (frequency hopping). The term for this effect is spreading loss … The receiver must have a wide bandwidth to pass the signal to the detector. After the detector, the wideband signal is processed into an equivalent of the original narrow band data stream. Processing gain refers to the gain of this processor. This gain should be equal to the loss encountered during the spreading process.
