Wednesday, September 23, 2009

Phase Shift Keying, Bob, No. 70

Tonight I am continuing with my series on Spread Spectrum Radio. I am going to discuss Phase Shift Keying (PSK), which is the form of modulation used by Direct Sequence Spread Spectrum (DSSS) Radio.

Spread Spectrum Radio Review
Let's have a short review of spread spectrum. The two main kinds in use in ham radio are Frequency Hopping and Direct Sequence. Frequency hopping is relatively easy to understand. Your carrier, rather than being on a fixed frequency, jumps from one frequency to another. As long as the equipment receiving makes the same jumps you will be able to transmit the signal and prevent interference on any one frequency from causing a problem. This can be used for analog or digital transmission but is most commonly used for digital. Note that you must have a pattern of frequency changes that is known to both parties in order to allow the receiver to follow the transmission.

In the other method, Direct Sequence, you only transmit digital information. In fact the name "Direct Sequence" comes from the 11 digit sequence of ones and zeros that is used to modulate the carrier. If your data bit is a zero then you send the sequence normally. If your data bit is a one then you send a one where the sequence has a zero and send a zero where the sequence has a one. For example 101 would turn into 010. I mentioned that the carrier is modulated by this sequence but I didn't say how. It is modulated using Phase Shift Keying.

Phase Shift Keying (PSK)
So, what exactly is phase shift keying? It is a way of changing the phase of a carrier to transmit a digital signal. In essence what you are doing with PSK is phase hopping. In the simplest example called Binary Phase Shift Keying (BPSK) you hop 180 degrees out of phase to transmit a certain piece of digital data. So the signal is either in it's "normal" phase or in it's 180 degree out phase which is to say, inverted. Note that the frequency of the carrier isn't changed but when you change the phase and modulate the carrier you create side-band emissions. The faster you modulate the signal, the wider the side-bands. In addition to BPSK there is Quadrature PSK (QPSK) where the phase can be one of four values, 0, 90, 180 or -90. There is also 8PSK using 8 phase angles and so on. The general term is Multiple Phase Shift Keying (MPSK).

Differential Phase Shift Keying (DPSK)
Basic BPSK has the signal at normal 0 phase to represent a digital zero and at 180 degrees out to represent a digital 1. A commonly used variation on this is called Differential Binary Phase Shift Keying (DBPSK) and instead of a 1 being 180 degrees out of phase, a 1 always changes the phase while a 0 always keeps the phase the same. This way you don't need a reference signal to know which is 0 degrees and which is 180 degrees.

If you want a very narrow signal you are limited to a slow digital transfer rate. This is the case with PSK-31. The 31 comes from the 31.25 bits per second data rate which generates 31.25Hz sidebands. This is pretty much the opposite of spread spectrum. The whole point of PSK-31 was to use such a small slice of the band that you could actually fit many PSK-31 channels in the space of a typical SSB voice bandwidth.

PSK for Spread Spectrum
With spread spectrum you want a very wide bandwidth; by definition, one that is much wider than necessary to convey the information. By encoding each bit of data with the 11 bit sequence used in DSSS you now modulate the signal 11 times for each bit transferred. So for a 1 Mbit/sec transmission, you modulate at 11 MHz. This achieves the spread spectrum you are looking for. This modulation is encoded using DBPSK and a suppressed carrier.

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