Recently (almost a month back), I happened to ask a simple question to an interview candidate. The candidate apparently worked, among other things, on the design of scrambler and descrambler blocks in 3G/LTE terminal development .Naturally, I expected him to know in and out of the block and of course the scrambling idea itself. To my innocence, the answer came: “Scrambling adds protection to the information source”. That was a bit of foot in the mouth reply, which I seldom expected from such a candidate profile. Anyway, come to think of it I shouldn’t overly blame the candidate because many a time, engineers overlook the primary goal of placing a block or unit in the overall design, even when they do a glory traverse with it. I personally think this is ignorance on part of an individual engineer if he or she doesn’t spend time to figure out the reasoning behind every single unit in design, especially the one he or she works on, if not to an expert level, the intuitive idea gathering is paramount.
Anyway, let us get some facts right with scramblers. Scramblers are there in almost all the standardized data communication systems. It is some kind of randomize acting upon the information bit stream (Well, one can also think of it operating on non-binary symbol sequence or packets, but let us consider binary for the time being, for simplicity.). The input stream upon the act of a scrambler changes the patter of the original bit stream. Naturally, at the receiver should be doing a reverse operation (descrambler) to make sense of the source information. One can argue that, a receiver not having the right descrambler (descrambler is very well based on the scrambler algorithm) cannot retrieve the intended information, adding protection/security on the data stream is NOT the reason for the existence a scrambler/descrambler unit in a communication system. The why? Let us see why.
A typical receiver will have several algorithmic stages such as synchronization, equalization, decoding etc ( These are broad classifications. One can go into finer details. For the discussion on scrambling we can suffice with this much). All these stages are usually designed with the assumption that the source stream is i.i.d (continuous stream of identically independent ‘0’s and ‘1’s). For example, a long stint of ‘0’s or ‘1’s can degrade the timing/clock synchronizer performance or may even result in loss of synchronization.
The other important argument on the scrambler is the spectral density (spectral mask as defined in standard documents) requirements, usually imposed by the standard specification and (country specific) regulation requirements. Any periodicity (short or long), can result in spectral spikes, which in turn may result in co-channel and adjacent channel interference due to various nonlinear blocks present in the transceiver chain.
So, scrambler’s job is to make an uniform i.i.d stream so that, the spectrum stays as close to white. Similarly, the receivers do not misbehave because of arbitrary source distribution.