Scrambler (randomizer)
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In telecommunications, a scrambler (often erroneously referred to as a randomizer) is a device that manipulates a data stream before transmitting. The manipulations are reversed by a descrambler at the receiving side. Scrambling is widely used in satellite, radio relay communications and PSTN modems. A scrambler is usually placed just before a FEC coder, similarly, a descrambler is placed after a FEC decoder.
It has nothing to do with encrypting, as the intent is not to render the message unintelligeable, but to give the transmitted data useful engineering properties.
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[edit] Purposes of scrambling
There are two main reasons why scrambling is used:
- It facilitates the work of a timing recovery circuit, an automatic gain control and other adaptive circuits of the receiver (eliminating long sequences consisting of '0' or '1' only).
- It eliminates the dependence of a signal's power spectrum upon the actual transmitted data, making it more dispersed to meet maximum power spectral density requirements (because if the power is concentrated in a narrow frequency band, it can interfere with adjacent channels due to the cross modulation and the intermodulation caused by non-linearities of the receiving tract).
[edit] Types of scramblers
- Additive (synchronous) scramblers
- Multiplicative (self-synchronizing) scramblers
[edit] Additive (synchronous) scramblers
Additive scramblers (they are also referred to as synchronous) transform the input data stream by applying a pseudo-random binary sequence (PRBS) (by modulo-two addition). Sometimes a pre-calculated PRBS stored in the Read-only memory is used, but more often it is generated by a linear feedback shift register (LFSR).
In order to assure a synchronous operation of the transmitting and receiving LFSR (that is, scrambler and descrambler), a sync-word must be used.
A sync-word is a pattern that is placed in the data stream through equal intervals (that is, in each frame). A receiver searches for a few sync-words in adjacent frames and hence determines the place when its LFSR must be reloaded with a pre-defined initial state.
The additive descrambler is just the same device as the additive scrambler.
Additive scrambler/descrambler is defined by the polynomial of its LFSR (for the scrambler on the picture above, it is 1 + x14 + x15) and its initial state.
[edit] Multiplicative (self-synchronizing) scramblers
Multiplicative scramblers are called so because they perform a multiplication of the input signal by the scrambler's transfer function in Z-space. They are discrete linear time-invariant systems.
Unlike additive scramblers, multiplicative scramblers don't need the frame synchronization, that's why they are also called self-synchronizing.
A multiplicative scrambler is recursive and a multiplicative descrambler is non-recursive.
Unfortunately, multiplicative scramblers have significant drawbacks: they are not as good randomizers as additive scramblers are, and they lead to error multiplication (i.e. an error at the descrambler's input will result into three or more errors at its output).
Multiplicative scrambler/descrambler is defined similarly by a polynomial (for the scrambler on the picture it is 1 + x18 + x23), which is also a transfer function of the descrambler.
