Ham Radio Digital Modes
The ITU uses Emission Designators to define a “mode” as demonstrated in the Modulation chapter. These designators include the bandwidth, modulation type and information being sent.
This system works well to describe the physical characteristics of the modulation, but digital modes for HAM radio create some ambiguity because the type of information sent could be text, image or even the audio of a CW session. As an example, an FM data transmission of 20K0F3D could transmit spoken audio (like FM 20K0F3E or 2K5J3E) or a CW signal (like 150H0A1A).
These designators don’t help identify the type of data supported by a particular mode, the speed that data can be sent, if it’s error-corrected, or how well it might perform in hostile band conditions.
HAM radio digital modes have more characteristics that define them and there are often many variations on a single mode that are optimized for different conditions. We’ll need to look at the specifics of these unique characteristics to be able to determine which HAM radio digital modes offer the best combination of features for any given application.
Symbols, Baud, Bits and Bandwidth
The basic performance measure of a digital mode in HAM radio is the data rate. This can be measured a number of ways and is often confused. Each change of state on a transmission medium defines a symbol and the symbol rate is also known as baud. (While commonly used, “baud rate” is redundant because “baud” is already defined as the rate of symbols/second.)
Modulating a carrier increases the frequency range, or bandwidth, it occupies. The FCC currently limits digital modes by symbol rate on the various bands as an indirect (but easily measurable) means of controlling bandwidth.
The bit rate is the product of the symbol rate and the number of bits encoded in each symbol. In a simple two-state system like an RS-232 interface, the bit rate will be the same as baud. More complex waveforms can represent more than two states with a single symbol so the bit rate will be higher than the baud.
For each additional bit encoded in a symbol, the number of states of the carrier doubles. This makes each state less distinct from the others, which in turn makes it more difficult for the receiver to detect each symbol correctly in the presence of noise.
A V.34 modem may transmit symbols at a baud rate of 3420 baud and each symbol can represent up to 10 discrete states or bits, resulting in a gross (or raw) bit rate of 3420 baud × 10 or 34,200 bits per second (bit/s). Framing bits and other overhead reduce the net bit rate to 33,800 bit/s. Bits per second is abbreviated here as bit/s for clarity but is also often seen as bps.
Bits per second is useful when looking at the protocol but is less helpful determining how long it takes to transmit a specific size file because the number of bits consumed by overhead is often unknown. A more useful measure for calculating transmission times is bytes per second or Bps (note the capitalization).
Although there are only eight bits per byte, with the addition of start and stop bits, the difference between bps and Bps is often tenfold. Since the net bit rate takes the fixed overhead into account, Bytes per second can be calculated as bpsnet/8. Higher data rates can be expressed with their metric multipliers as shown in Table 16.1. Digital modes constantly balance the relationship between symbol rate, bit rate, bandwidth and the effect of noise.
The Shannon-Hartley theorem demonstrates the maximum channel capacity in the presence of Gaussian white noise and was discussed in the Modulation chapter in the Channel Capacity section. This theorem describes how an increased symbol rate will require an increase in bandwidth and how a reduced signal-to-noise ratio (SNR) will reduce the potential throughput of the channel.