JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III Year SPREAD SPECTRUM COMMUNICATIONS Previous Question Paper (November/December - 2020 - R16)

Pseudo-Noise (PN) codes are generated using Linear Feedback Shift Registers (LFSR), which consist of a sequence of flip-flops connected in series with a feedback path. The feedback is typically implemented using XOR gates connected to specific taps of the shift register.

The generation process follows these steps:

• The shift register is initialized with a non-zero seed value, as an all-zero state would result in a permanent zero output.
• At each clock pulse, the contents of the registers are shifted one position to the right.
• The feedback bit is calculated by performing an XOR operation on the outputs of the selected tap positions.
• This feedback bit is fed back into the input of the first flip-flop.
• The output of the last flip-flop represents the PN code sequence.

For a register of length n, the maximum length of the sequence generated is 2ⁿ - 1, known as an m-sequence. The properties of these codes include a balance property (number of 1s and 0s differ by at most one), a run property (distribution of runs of consecutive 1s and 0s), and a correlation property (low cross-correlation and impulse-like auto-correlation).

Frequency Hopped Spread Spectrum (FHSS) is a technique where the carrier frequency of the transmitted signal is changed periodically according to a pseudo-random sequence. The system model consists of a transmitter and a receiver.

Transmitter: The incoming data is modulated using techniques like FSK or PSK. The resulting signal is then mixed with a frequency synthesizer output. The frequency synthesizer is controlled by a PN code generator, which selects a specific frequency from a set of available channels at each hop interval. The frequency-hopped signal is then transmitted over the channel.

Receiver: The received signal is mixed with the output of a local frequency synthesizer that is synchronized with the transmitter's PN code generator. This process, known as de-hopping, brings the signal back to a fixed intermediate frequency or baseband. The signal is then demodulated to recover the original data.

The block diagram involves the following components: Data Source, Modulator, Frequency Synthesizer, PN Code Generator, Mixer, and Power Amplifier at the transmitter, and corresponding de-hopping and demodulation blocks at the receiver. The key to successful operation is the precise synchronization between the transmitter and receiver PN code generators to ensure the local synthesizer tracks the hopping pattern of the incoming signal.