The spread spectrum technique spreads the carrier containing the information over a very wide bandwidth, typically 1.25 MHz, using pseudo-noise generation techniques as described in Chapter 12. The transmitter uses what is in effect a digital key to spread the bandwidth and the receiver is equipped with an identical key for despreading. A number of users with different keys can occupy the same band at the same time. The system operates well in poor signal-to-noise or high interference environments.
A continuous wave (CW) transmission concentrates all the radiated energy on a single frequency (Figure 8.12). Amplitude modulation and narrow band FM widen the radiated bandwidth, reducing the energy at the carrier frequency and per kHz. Wide band FM carries the process a stage further until with spread spectrum the band width is increased to the extent that the signal almost disappears into the noise floor.
Spreading of the bandwidth is achieved by multiplying the digitally modulated signal by a spreading code at a much higher bit rate (100–1000 times the signal bit rate). This is done by combining the signal with the output of a random code generator running at 2 or 3 orders of magnitude faster than the binary signal rate. Figure 8.13 is a block diagram of a spread spectrum system. A clock running at the spreading rate Rc is used to drive both the spreading generator and, after frequency division, the data encoder. The carrier is first BPSK modulated by the encoded data and then in a balanced modulator (the spreading correlator) by the high rate code from the spreading generator. The resultant transmitted bits are referred to as chips to distinguish them from data bits. In the receiver, the clock pulses Rc are recovered and used to drive both the despreading generator and decoder.
CWCarrier generator Spreading correlator
RF power amplifierRc Code clock
generator Spreading code
generator
De-spreading
correlator Demodulator Receiver RF stages
Spreading code
generator Clock recovery
Many users can be accommodated by allocating each a unique spreading code. It is common to use a pseudo-noise (PN) generator to multiply the bit rate and then to modulate the carrier with either FSK or PSK.
Although spread spectrum is a digital system, in quality of signal there are similarities with analogue:Analogue
Signal/noise ratio Intelligibility, signal/ noise + distortion
Digital
Energy per bit, Eb/No Bit error rate