Automation and process engineers increasingly need an understanding of radio basics to properly harness the many wireless automation products now coming on the market. Fortunately, new generations of wireless products are easier to use, configure and install. However, it still pays enormous dividends to understand something of the strengths and limitations of radio frequency wireless technology.
In early RF systems, a transmitter was connected to a sensor that transmitted information to a simple receiver. Early systems had a limited bandwidth – the range of frequencies they broadcast across and the amount of data transmitted in that band.
Additionally, sometimes the receiver would not receive the signal, because signals can degrade, or attenuate, as they reflect off or pass through objects such as walls or filing cabinets. Because reliability was poor, those who tried these systems were generally disappointed and developed a jaded view of RF systems.
Because higher frequency components generally operate across a wider RF band, they can transmit more data than their low frequency counterparts. A drawback is that higher frequencies degrade – that is, they are absorbed – more readily as they pass through objects, which decreases range. To counter this effect, designers add amplifiers to the transmitter and design more sensitive receivers, increasing cost. However, practical limits to the use of brute force power are soon reached.
Modulation is the way data is impressed onto a radio wave. Today, many radio systems use a technique called spread spectrum to modulate data across the bandwidth of the radio. Spread spectrum allows multiple users to share the same frequency channel at the same time.
Two different spread spectrum technologies are commonly used:
Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS). Some hybrid schemes use both DSSS and FHSS.
Another modulation scheme commonly used in ISM bands – those used for industry, science and medicine – is Orthogonal Frequency Division Multiplexing (OFDM). Two newer spectrum sharing modulation techniques employ ultra wide band (UWB) and chirp modulation, both of which are gaining popularity.
Frequencies & Modulation
Most designs presently use some type of spread spectrum technology. For technologies such as WiFi that must transfer data rapidly, the IEEE has standardised DSSS technology. Because no such standards currently exist in meaningful form for industrial sensing systems, it almost inevitably means that wireless sensing products will use a proprietary communication protocol. The two most popular spread spectrum designs are Direct Sequence (DS) and Frequency Hopping (FH). DS and FH systems use this bandwidth very differently.
In a Direct Sequence system, the data to be transmitted, or input signal, is applied to a circuit, called a spreader, which uses software to apply a unique coding sequence to the data. The system spreads the data randomly across the bandwidth and outputs it to the air at a very low power level and at a particular time.
At essentially the same time, the other end of the radio link listens for low levels of signal and applies the same coding sequence to the raw data. If the data seem meaningful – correlates over a number of samples – the receiver processes it to the output. If the information is not meaningful, the receiver assumes it is noise and discards the information. The unique coding sequence is the key element, because it enables many different DS systems to use the same frequency range without interfering with each other. It is also the method by which the incredibly weak signals received from GPS satellites are recovered from the noise and turned into accurate positioning information. GPS provides the perfect example of the powerful data recovery possible with DS systems: all 24 satellites transmit their individual data in the same frequency band, the received signals being typically 10 times weaker than the ambient background noise level.
In the Frequency Hopping system, the bandwidth is used differently. In these systems, the bandwidth is broken up into multiple smaller frequency bands or channels. The system breaks data to be transmitted into smaller chunks. The transmitter then broadcasts these chunks across the various channels in accordance with a unique pattern, known as the hop code pattern. The receiver is synchronised with the transmitter and listens to the unique channels in the order of the hop code pattern. In this fashion, the data is reassembled and recovered to the system output.
When a modulation scheme is combined with the rules and regulations governing the use of a frequency band, the resultant method that transmitters and receivers use to talk to each other is referred to as the communications protocol.
Environment & Propagation
In a perfect world, with no interference, great radio designers, and a perfectly benign atmosphere, DS and FH systems work equally well. But the world is not perfect and interference does exist. Once we move outside of the lab and into the auto plants, steel mills, refineries, and cereal manufacturing facilities of the world, environmental factors – such as radio wave absorption and reflection – can significantly affect radio performance. The probability of reception at a given frequency on a particular channel at a particular moment varies. That’s why a cell phone connection can be dropped, even when both parties on the call are standing still. A dropped signal is referred to as radio link loss. One way to address the problem is to vary, or hop, the frequency according to a predefined pattern.
With frequency hopping, if interference on one frequency prevents a data packet from being received, the transmitter moves to the next frequency in the pattern and resends the data packet.
An alternative solution is to intentionally build multiple paths between the transmitter and receiver so that if one path fails, another will take over. The error correction algorithms of packet data transmission – the ubiquitous Ethernet TCP/IP network layer provides the perfect example – can work with any and all these varieties of wireless transmission systems to ensure totally accurate data transfer.
Interferers are objects that block radio signals or generate electrical interference. Interferers reflect, refract, or absorb radio signals, resulting in signal loss. Common interferers include objects, walls, ceilings, and other radios. As with the problem of radio link loss, frequency hopping can help stabilise the path.
Direct Sequence Spread Spectrum Modulation
Direct Sequence modulation spreads a low level signal over a specific range of frequencies simultaneously. Digital processing by oversampling can recover extremely weak signals in the presence of severe electrical noise in a direct trade-off with transmission channel data rate. GPS provides the perfect example of DSSS in action. it also offers a route to very high data rate, short range systems.
Frequency Hopping Spread Spectrum Modulation
Frequency Hopping modulation transmits a data signal over a number of different carrier frequencies at different times, following a specific pattern of frequency switching.The switching of RF carriers may be combined with the data packet error correction capability of TCP Ethernet to auto-optimise a wireless network link by taking out the ‘hops’ which always produce a packet resend.
The antenna is an important component of a wireless network. The proper antenna can optimise the range and reliability of a radio network while the wrong antenna causes otherwise good wireless devices to apparently misfunction.
Every antenna has specific characteristics that determine the range and radiation pattern of the radio signal. In sensor networking systems there are two basic types of antennas: omnidirectional – usually a single vertical structure – and multiple element directional types. Within these two broad categories there are many different designs, but all antennas shape the RF signal into a signal radiation pattern that increases the transmission or listening range of the radio.
As the name implies, an omnidirectional antenna transmits and receives radio signals equally in all directions. Because of the physics associated with radio, an omnidirectional antenna’s effective signal pattern looks like a doughnut with the antenna located in the centre of the hole. Antennas used on cell phones or handheld transceivers are good examples of omnidirectional antennas.
Vertical Omnidirectional aerial radiation viewed Side-On
In a wireless network, omnidirectional antennas are best suited to an environment where the associated wireless devices are at the centre of a star topology network. For long range point-to-point communications, omnidirectional antennas would not be the best option.
The most common type encountered is the Yagi antenna, named after its inventor Prof Hidetsugu Yagi. This type of antenna, which comprises a number of metal elements all set in the same physical plane, focuses most of the energy of the transmitter or receiver in a single direction. Yagi antennas are ideally suited for long range, line of sight communications. In sensor networks, Yagis are often used in outdoor applications like tank level monitoring back to a central point. If there is no line of sight, no antenna will perform well, the Yagi type included.
Yagi Antenna Radiation: The length of the lobes relate to link distance limit Gain
The shaping of the radio signal by an antenna is what is referred to as antenna gain, a measure of its ability to focus a radio signal in a particular direction and radiation pattern. The higher the gain of an antenna, the more focused the signal.
Adding gain to a radio system does not amplify the signal; the gain focuses the signal. Adding gain to a system usually minimises wasted energy sent vertically and instead focuses that energy into the horizontal plane.
Antenna gain is measured in decibels. A decibel is a Log10 logarithmic ratio between a specific value and a base value of the same unit of measure. With respect to radio power, dBm is a ratio of power relative to one milliwatt, where 1mW equals 0dBm.
A small change measured in dBm is equivalent to a numerically much larger change in power. For every reduction of 10dBm the power reduces ten times. Power levels below 1mW are quoted as negative dB values. Thus a power level of 100µW would be written as –20dBm. Thus a system’s power is halved with every 3dB reduction.
Experimentation indicates that for every 6dB increase in gain, the radio signal range doubles. Therefore, if a radio system with a unity gain antenna (0dB gain) transmits two miles, a 6dB antenna on the same radio transmits the signal four miles.
Specifications for most antennas refer to the gain in either dBd (dB gain of a standard dipole source) or dBi (dB gain of an isotropic source). While it isn’t important to understand the precise difference in these decibel ratings, a good rule is:
dBi = dBd – 2.15.
In other words, performance described in dBd usually provides a more realistic measure.
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