In the earlier
chapters, we presented the principles of HF communications, and gave you
some insight into where the technology of HF communications has been and
where it is now. Today, and for the future, HF radio fills two roles. First,
it is the primary medium for long-haul communications, where there is a
need for a mobile or quickly deployable system to support emer-gency or
military operations. Second, it is a highly cost-effective
alternative and backup to other communications media, such as telephone and satellite systems. In either capacity, HF has to support a variety of traffic, including voice, data, and images. Advances in digital signal processing (DSP) technology will lead to continued improvements in HF systems and equipment. In particular, we expect to see advances in the following areas:
Higher-speed devices allow more precise and frequent link-quality analysis, enabling better and faster frequency selection. Also, higher ALE system data rates allow faster transmission of channel-quality information.
Adaptive channel equalization improvements will allow increases in channel bit rates to up to 9600 bps in a 3-kHz channel, giving HF communication an economical advantage over other long-haul communications media. Also, for certain less restricted applications allowing greater than 3-kHz band-width, transmission of 64 kbps can be achieved over HF. Advances in DSP devices improve adaptive filtering, which in turn combats unintentional interference and jamming. Modem capabilities will expand so that waveforms will be optimized for use, not only with HF, but for other frequencies in the next gener-ation of multi-band radios.
in HF system performance and computer-based technology provide networks
that achieve highly reliable levels of communications through automatic
message routing and adap-tive signaling techniques. Network design includes
ways to deter-mine periodically the link quality between each pair of its
stations at each of their assigned frequencies, and send this information
to all nodes so that they route messages automatically. Thus, if station
A transmits a message to B, a routing algorithm detemines
if direct point-to-point communication is possible or whether the message from A to B must be routed through other stations. The ability to transfer information over a network enables simultaneous transfer of several messages or speeds up the transfer of long messages. For example, multiple radios in a station simultaneously send messages to several destinations
over several channels. Also, a long message can be divided so that portions of it can be sent in parallel. If channel bandwidths increase beyond the current 3-kHz restriction (which requires international agreement), improvements in real-time channel equalization techniques will allow data transmission rates considerably higher than the current rate of 9600 bps.
HF radio is becoming an increasingly important element in multi-media
networks that incorporate landline, VHF, and UHF. Recent and projected improvements in HF communications technology mean that constraints on the passing of information through networks that include an HF leg will be significantly reduced.
continue to move toward multi-band designs, ranging from MF through UHF
in a single radio. Digital circuits will continue to replace analog circuits,
resulting in lower costs and more versatile and reliable designs. Digital
processing circuitry will handle higher and higher
frequencies, as higher speed analog-to-digital converters and other DSP circuits become available. The versatility, made possible by “going digital,” allows radios to be quickly reprogrammed for broadband modes of operation, resulting in new levels of performance, such as higher data rates and improved frequency-hopping capabilities.