RFSpreading vs. Scrambling: Understanding the Differences
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This page describes the difference between spreading and scrambling. Both techniques are used to spread energy over a wider frequency band. This is beneficial because it helps prevent unauthorized access to the signal. The spread signal power is typically below the noise power level.
A user who knows the spreading code (a PN code or channelization code) can “despread” the spread signal. Similarly, a user who knows the scrambling code can “descramble” the scrambled signal. As illustrated in the figures below, both spreading and scrambling utilize XOR circuits.
Spreading
Image alt: Spreading
In the spreading process, the binary data stream is XORed with a high bit rate PN code (channelization code or spreading code). The resulting sequence has the same bit rate as the PN code. This effectively spreads the data across a larger bandwidth, reducing the power spectral density and pushing it below the noise floor.
At the receiver, a user with the correct PN code (identical to the one used at the transmitter) can despread the signal, recovering the original data.
Scrambling
Image alt: Scrambling
In scrambling, the binary data is input to a scrambler, which typically consists of a shift register and XOR gates. The primary purpose of scrambling is to remove long strings of consecutive ‘1’s or ‘0’s and randomize the data. This randomization lowers the energy within a specific channel and helps mitigate the effects of adjacent channel interference.
Key Differences
| Feature | Spreading | Scrambling |
|---|---|---|
| Primary Goal | Spread signal over a wider bandwidth | Randomize data to reduce peak power and interference |
| Code Used | PN code (Spreading or Channelization Code) | Pseudo-random sequence generated by a shift register |
| Implementation | XOR with a high bit rate code | Shift register and XOR gates |
| Application | CDMA, secure communication | Communication systems to reduce interference |
