Some less common wireless communications techniques are effective under certain circumstances.
Block diagram of a dual-diversity radio receiver system.
A dual-diversity receiver can reduce the fading that occurs in radio reception at high frequencies (approximately 3 to 30 MHz) when signals are propagated by means of the ionosphere. Two receivers are used. Both are tuned to the same signal, but they employ separate antennas, spaced several wavelengths apart. The outputs of the receiver detectors are fed into a common audio amplifier, as shown in above figure.
Dual-diversity tuning is critical, and the equipment is expensive. In some installations, three or more antennas and receivers are employed. This provides superior immunity to fading, but it compounds the tuning difficulty and further increases the expense.
Digital signals require less bandwidth than analog signals to convey a given amount of information per unit time. Synchronized communications refers to a specialized digital mode, in which the transmitter and receiver operate from a common time standard to optimize the amount of data that can be sent in a communications channel or band.
In synchronized digital communications, also called coherent communications, the receiver and transmitter operate in lock-step. The receiver evaluates each transmitted binary digit, or bit, for a block of time lasting for the specified duration of a single bit. This makes it possible to use a receiving filter having extremely narrow bandwidth. The synchronization requires the use of an external frequency/time standard. The broadcasts of standard time-and-frequency radio stations such as WWV or WWVH can be used for this purpose. Frequency dividers are employed to obtain the necessary synchronizing frequencies. A tone or pulse is generated in the receiver output for a particular bit if, but only if, the average signal voltage exceeds a certain value over the duration of that bit. False signals, such as can be caused by filter ringing, sferics, or ignition noise, are generally ignored, because they rarely produce sufficient average bit voltage.
Experiments with synchronized communications have shown that the improvement in S/N ratio, compared with nonsynchronized systems, is several decibels at low to moderate data speeds. Further improvement can be obtained by the use of DSP.
How DSP Can Improve Reception
Digital signal processing can clean up a signal, improving reception.
In DSP with analog modes such as SSB or SSTV, the signals are first changed into digital form by A/D conversion. Then the digital data is cleaned up so the pulse timing and amplitude adhere strictly to the protocol (standards) for the type of digital data being used. Finally, the digital signal is changed back to the original voice or video by D/A conversion. Digital signal processing can extend the range of a wireless communications circuit, because it allows reception under worse conditions than would be possible without it. Digital signal processing also improves the quality of marginal signals, so that the receiving equipment or operator makes fewer errors. In circuits that use only digital modes, A/D and D/A conversion are irrelevant, but DSP can still be used to clean up the signal. This improves the accuracy of the system, and also makes it possible to copy data over and over many times (that is, to produce multigeneration duplicates).
The DSP circuit minimizes noise and interference in a received digital signal as shown in above figure. A hypothetical signal before DSP is shown at the top; the signal after processing is shown at the bottom. If the incoming signal is above a certain level for an interval of time, the DSP output is high (also called logic 1). If the level is below the critical point for a time interval, then the output is low (also called logic 0).
Signals in a communications channel or band can be intertwined, or multiplexed, in various ways. The most common methods are frequency division multiplexing (FDM) and time division multiplexing (TDM). In FDM, the channel is broken down into subchannels. The carrier frequencies of the signals are spaced so they do not overlap. Each signal is independent of the others. In TDM, signals are broken into segments by time, and then the segments are transferred in a rotating sequence. The receiver must be synchronized with the transmitter by means of a time standard such as WWV. Multiplexing requires an encoder that combines or intertwines the signals in the transmitter, and a decoder that separates or untangles the signals in the receiver.
In spread-spectrum communications, the main carrier frequency is rapidly varied independently of signal modulation, and the receiver is programmed to follow. As a result, the probability of catastrophic interference, in which one strong interfering signal can obliterate the desired signal, is near zero. It is difficult for unauthorized people to eavesdrop on a spread-spectrum communications link unless they gain access to the sequencing code, also known as the frequency spreading function. Such a function can be complex, and can be kept secret. If the transmitting and receiving operator do not divulge the function to anyone, and if they do not tell anyone about the existence of their contact, then no one else on the band will know the contact is taking place.
During a spread-spectrum contact between a given transmitter and receiver, the operating frequency can fluctuate over a range of kilohertz, megahertz, or tens of megahertz. As a band becomes occupied with an increasing number of spread-spectrum signals, the overall noise level in the band appears to increase. Therefore, there is a practical limit to the number of spread-spectrum contacts that a band can handle. This limit is roughly the same as it would be if all the signals were constant in frequency, and had their own discrete channels.
A common method of generating spread spectrum is frequency hopping. The transmitter has a list of channels that it follows in a certain order. The receiver must be programmed with this same list, in the same order, and must be synchronized with the transmitter. The dwell time is the interval at which the frequency changes occur, which is the same as the length of time that the signal remains on any given frequency. The dwell time should be short enough so that a signal will not be noticed, and not cause interference, on any frequency. There are numerous dwell frequencies, so the signal energy is diluted to the extent that, if someone tunes to any particular frequency in the sequence, the signal is not noticeable.
Another way to get spread spectrum, called frequency sweeping, is to frequency-modulate the main transmitted carrier with a waveform that guides it up and down over the assigned band. This FM is independent of signal intelligence. A receiver can intercept the signal if, but only if, its tuning varies according to the same waveform, over the same band, at the same frequency, and in the same phase as that of the transmitter.