CIMTechniques has been providing wireless monitoring systems for over 15 years. During that time we’ve learned a lot about what to do to make wireless sensors work reliably. The main thing to remember is the fact that we are operating in the same two unlicensed bands (900 MHz or 2.4 GHz) along with millions of other wireless devices like Wi-Fi, wireless pagers, RFID, wireless instruments, etc. This means that the potential for interference is great and ever increasing as the wireless revolution explodes. Maintaining reliable communications in this mess is the challenge we all face implementing wireless systems.
Most of our competitors provide wireless units with output power levels of 1 mW. A few provide outputs of up to 20 mW, but rarely over that. In addition, most of these units operate on a single preset frequency. Consequently, if another station is operating on that frequency, units with low power levels will be drowned out and their communications will be interrupted. To combat this problem, we provide wireless devices with output power levels which can be adjusted to 40 or 158 mW to effectively punch through almost any interference. We also offer the ability to add external antennas that will increase the effective radiated power by a factor of three.
Another way to combat interference from another wireless device is to simply switch frequencies to a clear channel. This could be done manually whenever a new wireless device is in the area. Of course, this is impractical. A better way is to automatically switch using a technique called Frequency Hopping Spread Spectrum (FHSS). With this proven technology, our wireless sensors constantly change frequencies to find a clear channel within their operating bands. This ability, along with the higher output power levels, costs more and therefore our wireless components are a little more expensive than the less capable units from our competitors. (See the section on Spread Spectrum Technology in the paragraphs below.)
In the past, data security isn’t an important consideration in a wireless monitoring system. In today’s environment, that may not be case. How about the possibility of an unscrupulous person deciding to monitor the temperature levels from the blood bank refrigerators in hopes of detecting an abnormal condition on which to base a lawsuit? That sounds pretty farfetched, but it is a possibility. Also consider the situation where a disgruntled employee or even a terrorist wanted to disrupt hospital operations by falsifying measurement data and creating a huge number of alerts that would have to be responded to.
To prevent the above from happening, all of CIMScan’s wireless devices use AES-128 Encryption to provide protection from eavesdropping or the falsifying of measurement information.
The range of a wireless device operating indoors depends on 1) the construction of the walls between the transmitter and the receiver, 2) the power levels produced by the transmitters, and 3) the types of antennas used. Our CQ series of wireless sensors offers the highest power level of any competitive device and high gain antennas are also available to increase the range.
Our competitors may say that range isn’t important, “just add repeaters.” Repeaters are generally not particularly expensive. They do, however, require line power for operation. Installing a new power receptacle in an area where one doesn’t already exist will be very expensive.
Freedom of Movement
While freedom of movement is one of the great advantages of wireless sensors, we have occasionally heard “I don’t want wireless because when I moved my refrigerator just a few feet from where it was, I lost my sensor.” This is invariably due to the physical environment (metal objects which cause signal attenuation and reflections). The CQ sensor’s high power levels and the use of Spread Spectrum technology all but eliminates this objection. (See “multipath fading” in the Spread Spectrum section below.)
Faster Update Rates and More Available Sensor Types.
All battery-powered wireless sensors from any manufacturer (including CIMTechniques) normally operate at a very slow update rate (typically once every 5 to 15 minutes) to extend battery life. Our line-powered CQ-05 wireless bridge can provide updates every 25 seconds if necessary and has an interface with up to 8 inexpensive CT series SensorBus sensors. The following is a list of some of the available sensor types.
- Differential Pressure
- CO2 Level
- O2 Level
- Airborne Particles
- Voltage/4-20 ma Current
- LN2 Level
- Door Ajar
- Line Power
- Flow Rate
- Tank Level
The CQ series of wireless sensors utilize standard “AA” alkaline batteries available just about anywhere at low cost. Our competitors normally use expensive Lithium batteries which often must be purchased from the supplier of the sensor.
A Closer Look at Spread Spectrum Technology
A radio channel can be very hostile, corrupted by noise, path loss and interfering transmissions from other radios. Even in an interference-free environment, radio performance faces serious degradation from a phenomenon known as multipath fading. Multipath fading results when two or more reflected rays of the transmitted signal arrive at the receiving antenna with opposing phases, thereby partially or completely canceling the signal. This problem is particularly prevalent in indoor installations. In the frequency domain, a multipath fade can be described as a frequency-selective notch that shifts in location and intensity over time as reflections change due to motion of the radio or objects within its range. At any given time, multipath fades will typically occupy 1% – 2% of the band. From a probabilistic viewpoint, a conventional radio system faces a 1% – 2% chance of signal impairment at any given time due to multipath fading.
Spread spectrum reduces the vulnerability of a radio system to both multipath fading and jammers by distributing the transmitted signal over a larger region of the frequency band than would otherwise be necessary to send the information. This allows the signal to be reconstructed even though part of it may be lost or corrupted in transmission.
The two primary approaches to spread spectrum are direct sequence spread spectrum (DSSS) and frequency hopping spread spectrum (FHSS), either of which can generally be adapted to a given application. Direct sequence spread spectrum is produced by multiplying the transmitted data stream by a much faster, noise-like repeating pattern. The ratio by which this modulating pattern exceeds the bit rate of the base-band data is called the processing gain, and is equal to the amount of rejection the system affords against narrow-band interference from multipath and jammers. Transmitting the data signal as usual, but varying the carrier frequency rapidly according to a pseudo-random pattern over a broad range of channels produces a frequency hopping spectrum system.
One disadvantage of direct sequence systems is that due to design issues related to broadband transmitters and receivers, they generally employ only a minimal amount of spreading, often no more than the minimum required by the regulating agencies. For this reason, the ability of DSSS systems to overcome fading and in-band jammers is relatively weak. By contrast, FHSS systems are capable of hopping throughout the entire band, statistically reducing the chances that a transmission will be affected by fading or interference. This means that a FHSS system will degrade gracefully as the band gets noisier, while a DSSS system may exhibit uneven coverage or work well until a certain point and then give out completely.