Redundancy

Protects digital data by introducing redundancy in the transmitted data

Error detection codes: can identify certain types of errors.

Error correction codes: can fix certain types of errors.

Block codes provide Forward Error Correction (FEC) for blocks of data

(n, k) code: n bits are transmitted for k information bits.

Simplest example: parity codes.

Many different codes exist: Hamming, cyclic, Reed-Solomon, …

Convolutional codes provide protection for a continuous stream of bits

Coding gain is n/k.

Turbo codes: convolutional code with channel estimation.

Multiple Users Can Share the Spectrum

So Why Don’t we Always Send a High Bandwidth Signal?

Channels have a limit on the type of signals it can carry.

• Good transmission of signals only in certain frequency range.

• Signals outside of that range get distorted, e.g. attenuated.

Distortion can make it hard for receiver to extract the information.

• It is beneficial to match the signal to the channel.

• Limits the throughput of the channel.

Spread Spectrum

Spread transmission over a wider bandwidth.

• Don’t put all your eggs in one basket!

Good for military: jamming and interception becomes harder.

Also useful to minimize impact of a “bad” frequency in regular environments.

What can be gained from this apparent waste of spectrum?

• Immunity from various kinds of noise and multipath distortion.

  • Including jamming.

• Can be used for hiding/encrypting signals.

  • Only receiver who knows SS code can retrieve signal.

• Several users can independently share the same higher bandwidth with very little interference (later).

  • Code division multiple access (CDMA).

Concept

Input fed into channel encoder.

• Produces narrow bandwidth analog signal around central frequency.

Signal modulated using sequence of digits.

• Spreading code/sequence.

• Typically generated by pseudonoise/pseudorandom number generator.

  • Not actually random.

  • If algorithm good, results pass reasonable tests of randomness.

  • Need to know algorithm and seed to predict sequence.

Increases bandwidth significantly.

• Spreads spectrum.

Receiver uses same sequence to demodulate signal.

Demodulated signal fed into channel decoder.