Understanding BPSK Modulation

All communications systems are a trade-off between bandwidth, S/N (signal to noise ratio) and channel capacity. For example, the channel capacity (for transmitting data) will be higher with an increase in channel bandwidth and/or better signal-to-noise ratio. These tradeoffs can be illustrated using Shannon's Law:

Typically, amateur radio contacts tend to have a much poorer S/N than would be acceptable for reliable communications in commercial systems.

Until relatively recently, CW was the mode of choice for weak signal communication. This was largely due to its narrow bandwidth and the amazing ability of the human ear (and brain) to pick out and decode weak signals transmitted in Morse code. Over the past 20 years more advanced digital ‘machine decoded’ modes have become available which have performance comparable to CW. These use less bandwidth than older machine-decoded systems such as RTTY, which uses FSK (frequency shift keying).

One of the most popular of these digital modes is BPSK (Binary Phase Shift Keying). BPSK changes the phase of the carrier by 180 degrees to determine whether a '1' or a '0' is being transmitted. PSK uses a single carrier and therefore uses less bandwidth than FSK systems such as RTTY, which use two frequencies (separated by 170 Hz typically) to transmit the two binary digits. With basic BPSK the 'hard' switching of the phase at these transitions causes an increase in the signal bandwidth. A basic BPSK signal is shown below.

(Dunlop and Smith, 1989)

In PSK31 the amplitude of the carrier is significantly reduced at the phase transition, thus reducing the required bandwidth. PSK31 is a form of BPSK modulation developed by Peter Martinez (G3PLX) in the late 1990s. The waveform and modulation envelope is shown below. 

(Browne and Stevens, 2016)

PSK31, as its name suggests, has a bit rate of 31.25 bit/s and a bandwidth of only 32 Hz.The attenuation of the carrier at the phase transition points means that the PSK31 signal has amplitude modulation as well as phase modulation. This has practical implications as, unlike traditional RTTY, the radio transmitter must be linear in order to prevent distortion of the waveform, and the corresponding increase in bandwidth. This means that the transmitter drive level needs to be adjusted below the threshold of the ALC (Automatic Level Control). An unfortunate consequence of this is that, without the ALC, the transmit power will vary slightly depending on where within the transmitter audio passband the signal is set.

A 'waterfall' display showing mostly PSK63 signals. The excessive width of the PSK63 signal on the centre right is probably due to an overdriven transmitter.

Unlike RTTY, the transmitted characters will not all be of equal length, i.e. the same number of bits. More common letters in the English language such as 'e' are given shorter codes as they will occur more frequently in typical messages. These are known as "varicodes". In fact, this concept is no different to Morse code, where, for example, an 'e' is a single dot and a 'q' is dash-dash-dot-dash. For this reason, when transmitting messages using PSK31, it is unwise to type everything in upper case (capital) letters, since capitals have longer codes and will take longer to transmit.

PSK63 is similar to PSK31 but is faster (and requires more bandwidth). Other bitrates are available but are less popular.

Other Digital Modes

Since PSK31 was developed, numerous other digital modes have become available, many of them developed for specific applications. For example, QRSS (very slow Morse) and WOLF (Weak Signal Operation on Low Frequency) are used on the VLF bands, where receive bandwidths of less than 1Hz are common! Recently ‘FT8’ has become very popular, especially for DX (long distance) communications on the HF bands. FT8 works very well with weak signals but is only suitable for exchanging basic information, rather than ‘rag-chewing’ (chatting).

References

1. Dunlop, J and Smith, D (1989). Telecommunication Engineering - Second Edition. Chapman & Hall, London.
2. Browne, M and Stevens, M (2016). Radio Communication Handbook. RSGB, Bedford.

Related posts:
Portable PSK Operation with UBITX
Review of DroidPSK Android App


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