Wireless technologies
There are different types of applications for wireless information transfer like speech, sound, video, computer data, alarm, remote control, position location etc. All these applications have their own requirements in terms of range, bandwidth, reliability and mobility. In the last decade wireless technologies have taken a great leap forward and governments have opened up a multitude of frequency bands to allow these technologies to penetrate many aspects four life. This description gives an overview of the major technologies, frequencies and the applications that use them. The description is for European applications. In the different continents there are similar bands and applications, usually at slightly different frequencies and with different transmitting power regulations.

This is just an overview: many details are left out and some figures are just an estimate.

This is an old analog technology that uses two channels in the 40 MHz range. It was primarily used for cordless telephones and baby monitors. Because there are only two channels available these devices are usually plagued by noisy signals and interference from other users. Some CT0 applications are still available.

These are standards for wireless telephones at different frequencies. CT2 was a precursor to the DECT standard. None of these are in use anymore.

ISM stands for Industrial, Scientific and Medical use but these bands are actually used mostly for consumer applications like remote controls, walky talkies, baby monitors and other audio and video technologies. There are three frequency ranges allocated for this application. These bands are also called ‘license-free bands’ because any application can use them (within certain technical constraints) without any license.

The frequency bands for ISM use are:

433 MHz
There are several analog audio channels and some narrow band data channels in this range. The audio channels are mostly used for walky talkies and some baby monitors. The data channels are mostly used for small remote controls like car keys and garage door openers. The range can be up to a few hundred meters.

868 MHz
This band contains a small mix of narrowband and wideband channels. The narrowband channels are used for remote controls and data monitoring. The wideband channels can be used for analog audio and digital (hifi) audio like wireless microphones. The range can be up to 50 meters or even a few hundred meters, depending on the application.

2.45 GHz
Although this is an ISM band, it is also shared with other technologies like WLAN and Zigbee. There are more channels in this band that allow more sophisticated communication schemes like Spread Spectrum and Frequency Hopping. These schemes are designed to minimize interference between different users. Applications are (higher speed) data transfer and audio and video communication but the range is smaller than 868 MHz, up to about 30 meters.


GSM is perhaps the most widely spread radio speech technology these days. It uses an extensive network of base stations that route the traffic from one telephone to another. In Europe there are two frequency ranges around 900 MHz and 1800 Mhz. To allow many calls at the same time it uses a digital speech technology with different frequency channels and time slots. A carefully coordinated geographic allocation of the channels avoids interference between base stations and callers that are using the same channel. The actual range between a base station and a telephone is a few kilometers but the interconnection between base stations and national networks results in a range around the world.

Dual Band, Tri Band and Quad Band
While in Europe there are two frequency bands in use for GSM (900 and 1800 MHz), other continents can use different bands at 850 MHz and 1900 MHz. A Dual Band GSM phone can work only in Europe . A Tri-Band phone can also be used in the US and for truly global use we need a Quad-Band GSM phone.

GPRS is a data technology that uses the GSM infrastructure. Since GSM telephones convert the speech into a digital signal being transferred to another telephone, it is a small step to allow the same network to transfer other data like computer data. GPRS allows higher data transfer rates (up to 114 kbps) by combining several timeslots into one data channel.

The GPRS network is connected to the Internet and this allows any GPRS device to communicate to any fixed or mobile internet site or server.

EDGE (Enhanced Data GSM Environment) is another technique similar to GPRS that uses the GSM infrastructure to deliver up to 384 kbps data transfer. This technology is mostly used in the US.

This system seems similar to GPRS in that it also uses a widespread network of base stations but the technology is much more sophisticated than GSM and GPRS. While those systems allow only one telephone or base station to send a signal at any one channel and time, UMTS allows multiple signals to be present at the same channel and time. The stations can separate these signals by scrambling and descrambling with a unique code for each station. Thus, by re-using the same channels this technology allows much more data from the same users to be transferred. This is especially suited to transfer video signal streams and large amounts of computer data, up to 2 Mbps. UMTS uses frequency bands between 1900 and 2200 MHz.

This is a position location technology that uses an infrastructure of satellites of the US military. While it was originally intended to be used on the battle fields around the world, its use was released for the public in the late 90s. Each satellite transmits a signal at around 1550 MHz that is precisely synchronized with all the other satellites. A receiver on the ground receives the signals from several satellites and it carefully compares the time of arrival of these signals. By using a table of positions of these satellites it can then calculate the position on the ground, or even in the air, with a precision of a few meters. To get a 2D (ground) fix it is necessary to receive at least 3 satellites and for a 3D (air) fix it is necessary to receive 4 satellites. A receiver usually receives more than 5 satellites, these extra signals are used to improve the reliability and accuracy of the location fix.
For a car-navigation system the position is continuously fed into a map-database that shows where the receiver is located on a road-map. A small computer tracks this position and the map and with the road-map and the end-destination it can tell the user which road to take and where to turn. Additional traffic information can also take into account the traffic jams and calculate alternative routes to avoid these jams.

Up until recent it was necessary to have a clear view of the sky (satellites) to get a location fix. Recent receiver technology has made it possible to locate a position in heavily developed inner cities and to a certain extent even indoors. Some additional technologies have made it possible to get even higher sensitivity and faster fixes (Aided-GPS), or centimeter-accuracy (Differential-GPS) but these technologies require the aid of additional ground-based services.

The DECT standard was developed for cordless telephones with a much better quality than the CT0 telephones. It was originally named ‘Digital European Communication Technology’ but was later renamed ‘Digital Enhanced Communication Technology’ to give it a more global character. It uses a frequency band around 1890 MHz with 10 channels and 24 timeslots. For 2 way communication this gives a maximum of 120 communication channels. With a decent 200 milliwatts of power it allows for a robust speech connection in and around the house up to 50 meters away, while outdoors a range of 300 meters is easily obtainable. Some DECT devices can reach over a kilometer away.

The DECT standard is for telephone speech signals and gives a clear and low-noise quality. With its digital (encrypted) modulation it is basically free of interference and protects against eaves dropping. This makes it also very useful for quality baby monitors and some other speech applications.

When we are using several devices like computers, audio players, headphones etc, we always get tangled in a knot of wires. The idea of Bluetooth was to make cables redundant and transport all signals over the air at a low cost. The Bluetooth standard was originally conceived by Ericsson and was named after a Danish king who strived for communication between people.

Bluetooth transmits at 2.45 GHz and is intended for medium speed data communication between two or more devices. Although it is capable of setting up small networks it is usually used to connect two devices, like a headset to a GSM telephone or a palmtop to a GPRS telephone. Usually the communication range of Bluetooth is up to 10 or 20 meters.

Wireless LAN is used as an extension of the wired computer networks in a company or at home. It allows laptops and palmtops to smoothly integrate into a fixed network. These days, there are also WLAN telephones that connect to a network to allow Voice Over IP (Voip) telephone calls.

WLAN uses 2.45 GHz and 5 GHz and the range is up to about 30 meters . The data rate is up to 54 Mbps.

More and more public areas are getting equipped with a WLAN access point that allows travelers or guests to access internet with their mobile phones and computers while they are away from home or work.

This is an emerging standard for home automation (domotica). Although there are already many applications that use the ISM bands for low speed remote control and monitoring (light switches, thermostats etc), ZigBee is intended to improve on this and make a standard protocol so that different (brand) devices can communicate with each other, much like Bluetooth. Although there is a ZigBee standard for one channel in the 868 MHz band, most applications are focussing on the 10 available channels in the 2.45 GHz band.

ZigBee focuses on applications with very low datarate (just a few light switches a day, a thermostat that switches a few times per hour) and allows for devices with such a low power consumption that a battery can last for many years. You might have a battery powered light switch you can stick to the wall at any place and that connects to your home ZigBee network. This switch can then control a number of ZigBee lights somewhere in the room. The range is about 10 to 20 meters.

This system is designed for high speed networks over fairly long distances up to 10 or even 50 km . It can use frequency bands between 2 and 11 GHz but these are highly dependent on the countries. It can be used as a wideband, wide-area internet network and provide public internet access.

The Ultra Wide Band standard is an older technology originally used for military purposes. In the late 90’s an investigation has started for commercial use of UWB. Only recently UWB is allowed in Europe . It uses an extremely wide bandwidth with an extremely low power so that it is hardly detectable by conventional receivers. Because of the wide bandwidth it allows high datarate signals like digital video streams and high speed computer data to be transferred. A special receiver can detect the undetectable and convert the signal back to the high speed data stream.

UWB uses several bands in the 3 – 10 GHz range and its range is up to about 10 meters.

Multiple users
Usually in radio systems there are many users that all want to use the same frequency band. For this purpose there are several schemes that allow several users in a band to communicate without interference. All users of a particular scheme have to follow the same standard so that they don’t interfere with each other.

Frequency Division Multiple Access (FDMA)

This is a basic sharing system. Each user uses a separate sub-band or channel in a certain frequency band. This is comparable with the tuning dial on an FM radio. Each station uses a different channel and you can tune to a different station by selecting its channel.

Time Division Multiple Access (TDMA)

It is also possible for several users to use the same channel as long as they don’t transmit at the same time. Each channel is then divided into several ‘time slots’ and each user transmits in a time slot that is assigned to him. You can compare this with a group of people all taking turns to talk. Usually this process is controlled by a central coordinator that synchronizes all the users so that they only transmit in their allocated time slot.

Code Division Multiple Access (CDMA)

Normally it is not possible for different users to transmit at the same frequency at the same time. This results in interference and none of the information from the users can be discerned from each other. However, CDMA is a technique where each user has a different code and uses that code to encrypt its information. The (central) receiver knows the code of each user and it can use that to separate the information from the other users.

To summarize these techniques:
FDMA is like having several baskets and each user throws its information in a different basket.
TDMA is like having one basket. The users take turns to throw their information in the basket and the receiver takes this out before the next user throws in new information.
CDMA is also like having one basket but this time each user paints its information with a unique color and they all throw it in the basket at the same time. The receiver takes all this out and separates the information according to color.

It is possible to combine these schemes to achieve a very high density of information to be transferred between the users and the controllers (base stations).

Frequency Hopping and Spread Spectrum

The above techniques are meant to be standardized and followed by all users of a certain frequency band. Some bands however allow different groups with different schemes to be used at the same time (eg. WLAN and ZigBee). To avoid structural interference between the schemes they employ Frequency Hopping or Spread Spectrum.

With Frequency Hopping the users use a certain channel for a short period of time and then ‘hop’ to the next channel, usually in a quasi random order. If one channel is disturbed then the information lost is minimal and the information transfer picks up at the next channel.
With Spread Spectrum parts of the information are transferred on different channels, again minimizing the risk that all information is corrupted by interference from other users.

There are several different techniques that combine frequency hopping and spread spectrum and also other schemes.

Communication protocols
For AM and FM radio signals it is easy to pick up the information. You just tune to the station and enjoy the music. When the information is digital it is another matter. You need to know when a certain packet of information starts and when it ends. You also need to know when to send something back, and in which format.

This is called a protocol, a set of agreements of how and when to send and receive information.
Data (information) is usually sent in packets of a certain length. This data can also be encrypted with a certain algorithm. This packet of data is then wrapped in a message that contains a message header that indicates what kind of data it is, it may indicate from whom it originates and to whom it is addressed. A trailer may be added that contains error correction code so that errors can be detected and possibly also corrected.

There are many different protocols for different kinds of communications. This can be protocols for wired communication (TCP/IP, UDP, SIP etc) or for wireless communications (BlueTooth, GSM, WLAN etc). Many of these protocols are standardized by the IEEE organization and are numbered according to the committee that design them and decide on the final version.
A widely used protocol is the 802.11 family of specifications. The plain .11 specification was originally designed to work on the 2.45 GHz (license free) ISM band and provide network capabilities at 2 megabits per second data transfer speed. This protocol was later improved upon and with each improved version a new letter was added, first the 802.11b and later the 802.11g specification that is nowadays used widely in WLANs and that provides a maximum of 54 megabits per second.

These 802 protocols define the basic structure of communication. These are then wrapped into an application like WLAN or ZigBee that provides a useful service for the users.

Other protocols were drawn up by other committees: the GSM and DECT protocols were developed by the European ETSI organization.

Radio Properties
Radio signals are Electro-Magnetic vibrations, they propagate through vacuum and (non-conductive) materials. They are essentially the same as light waves and X-rays but at a much lower frequency. Radio waves are not to be confused with acoustic waves or sonar, which are mechanical vibrations of air or water or another physical material.

Radio signals are described in terms of Frequency (or wavelength), Radiated Power, Bandwidth, Modulation and Propagation.

Frequency and wavelength
Since radio signals are electro-magnetic vibrations they vibrate at a certain frequency. We use the unit Hertz to express how many times per second this signal vibrates.
A 1 Hertz signal vibrates 1 time per second. Although it is possible to generate such a signal, it would be extremely slow and not very useful.

Useful radio signals vibrate at a much higher pace. The spectrum for radio signals ranges from tenths of thousands of Hertz up to many billions of Hertz. We use terms of KiloHertz (1kHz = 1000 Hertz), MegaHertz (1MHz = 1,000,000 Hertz) and GigaHertz (1GHz = 1,000,000,000 Hertz).
Radio waves propagate at the speed of light (since they are a kind of low frequency light waves). The wavelength is expressed as the distance a radio wave travels when it goes through one cycle. As the frequency increases, the cycles are faster and therefore the distance (wavelength) it travels in that cycle gets shorter. Light travels at a speed of 300 million meters per seconds. So a 1 Hertz radio signal has a wavelength of 300 million meters or 300,000 kilometers . A 300 MHz signal vibrates 300 million times per second, so in one cycle it travels just 1 meter.

A broad division of frequency ranges dates from many years ago but is still useful:

Low Frequency (LF): 0.3 – 3 MHz; 1000 – 100 meters
In this range we can find the old AM radio stations and some other special services.

High Frequency (HF): 3 – 30 MHz; 100 – 10 meters
This is also known as Short Wave radio and it is used for world wide (national) radio broadcast services (BBC, VOA etc) and other global radio services.

Very High Frequency (VHF): 30 – 300 MHz; 10 – 1 meter
This is the domain of FM radio broadcast, some TV broadcast and aircraft communications.

Ultra High Frequency (UHF): 300 – 3000 MHz (3 GHz); 1 meter – 10 cm
UHF is used for many of the common communication technologies: GSM, GPRS, UMTS, GPS, DECT, WLAN etc.

Super High Frequency (SHF): 3 – 30 GHz; 10 – 1 cm
The SHF range is used for satellite communication, (police) radars WLAN and UWB. As technology advances we will see more commercial use of these frequencies.

Extremely High Frequency (EHF): 30 – 300 GHz; 1 cm – 1 mm
This range is at the edge of our current technology. Some commercial (radar) applications exist and more and more research focuses on this frequency range.

Radio transmitters emit radio signals with a certain amount of power, expressed in Watts . Generally, the more power a transmitter uses, the longer the distance that can be bridged.

While broadcast stations use many kilowatts to reach many people over great distances, the modern communication techniques generally use milliwatts to bridge distances up to a few hundred meters.

GSM and GPRS phones need to reach the base stations up to a few kilometers away and therefore they use a fairly high power of up to 2 Watts. This power is regulated: if the phone is near a base station then the power is reduced in order to save batteries and to reduce the use of the radio spectrum.
DECT phones and Wireless LAN use a power of a few hundred milliwatts to bridge distances of up to 300 meters. This is comparable to the power of a penlight.

Other applications like Zigbee and Bluetooth use less power, up to 10 milliwatts, to reach distances of up to 50 meters.

Generally, if more information needs to be sent at higher speed (like WLAN), more power is needed to bridge a certain distance.

When a transmitter transfers a certain amount of information to a receiver, it occupies not only a certain frequency but it also uses a ‘band’ of frequencies around that frequency. This is often called a ‘channel’.
The occupied bandwidth depends on the amount of information per second that is being transferred. Voice signals use a few kilohertz and high speed WLAN use several megahertz of bandwidth.

A receiver receives the signal at the operating channel and does its best to reject all other signals at other frequencies. We call this the receiver bandwidth: the bandwidth of the signals that are passed through to the rest of the receiver for reconstructing the original information.
The receiver bandwidth must be as wide as the bandwidth of the transmitted signal in order not to lose any of the transmitted information.

If the receiver has a large bandwidth to receive high speed information from the transmitter, it will also receive more noise than a receiver with a narrow bandwidth for low speed information. When the transmitter is far away from the receiver so that the signal strength is quite low, a wideband signal will more easily ‘drown’ into the noise because the receiver also picks up more noise. This means that for a wideband signal more power is needed to stay above the noise level of the receiver than a narrowband signal.

A radio transmitter contains a circuit that generates a ‘carrier’ signal at the operating frequency. The information that needs to be transmitted is superimposed on this carrier and there are several different ways to to this. This is called ‘modulation’. The modulation signal can be (analog) voice, music, TV signals or digital signals like computer data or digitized voice etc.

Amplitude Modulation (AM)
We can vary the transmitted power with the information we want to transmit. This is called Amplitude Modulation. The earliest radio systems used Amplitude Modulation because it is easiest to generate but at high power it becomes harder to maintain the information without distortion in the output amplifiers.

The old Morse Code is a form of (digital) AM because it turns the transmitter (power) on and off in a certain pattern.

The receiver can reconstruct the information again by following the received signal power and passing on this signal to the rest of the system.

Frequency Modulation (FM)
When we keep a constant output power we can still vary the frequency of the carrier signal with the information. This is called Frequency Modulation. Although this is a little more difficult to generate, it is easier to amplify this signal to high power. FM signals occupy more bandwidth than AM but in the overall system it is easier to maintain the integrity of the information and is therefore more suited for HiFi audio and computer data.

An FM receiver reconstructs the information by following the frequency of the received signal.

Phase Modulation (PM)
Beside Amplitude and Frequency, there is also the Phase of the signal. This can be thought of as the timing of the signal and we can very this timing with the information. This is called Phase Modulation. FM and PM are quite similar but PM is easier to generate and results in a more efficient information system.
When we use a digital signal (1 or 0) for information this results in a signal where the phase has two states. This is called Phase Shift Keying (PSK). For higher speed information the digital 1’s and 0’s can be combined in 1, 0.75, 0.5 and 0 values and used for modulation. This results in ‘Quadruple PSK’ or QPSK. Even more states can be used to increase the bandwidth efficiency of the system.

Most of the time the goal of a radio system is to increase the throughput of information without increasing the bandwidth. Usually a form of compression is used in the transmitter to reduce the amount of information to transmit. In the receiver an inverse-compression, or expansion, is applied to reconstruct the original information again. This is comparable with ‘zipping’ a file to send it in an e-mail and ‘unzipping’ it again by the recipient.
MPEG, JPEG and MP3 are all forms of compression to reduce the amount of information to be transferred.

Radio waves travel through vacuum and many kinds of materials, including air. Light (a radio wave) travels in straight lines only: you can not look around a corner. But light can be reflected by a mirror and this makes it possible to see around corners. Radio waves at lower (radio) frequencies have different propagation properties and much of that has to do with the wavelength.

Low Frequency (LF) waves follow the surface of the earth and if you use enough power (many kilowatts or even megawatts) you can reach the other side of the world. This was done in the earlier radio days when the technology only allowed LF radio to be used.
When people started to explore higher frequencies (HF) they discovered that at those frequencies the radio waves would leave the earth’s surface and travel through the air and atmosphere. Reflections in the upper atmosphere made it possible to reach other continents without the need for enormous amounts of power. These atmospheric reflections are highly dependent on the weather and other factors and that makes the HF signals less suited for reliable worldwide radio communication. But under exceptional atmospheric conditions the author managed to have a two way voice communication with the other side of the world with a transmitting power of less than 1 Watt!
At even higher frequencies (UHF and up) radio starts to travel in straight lines but can still travel through building materials like bricks and concrete, though with increasing attenuation. This allows for in-house communication at the usual ISM, GSM, DECT and WLAN frequencies.
Going even higher in frequency (5GHz, 10GHz and up) the signals get more and more absorbed by building materials and these are mostly useful for ‘line of sight’ communication where transmitter and receiver can ‘see’ each other directly.

At UHF frequencies and higher a significant part of the signal is reflected against walls and furniture. In a building this results in a multitude of waves under different angles. At the receiver these waves can amplify each other if the peaks arrive at the same phase (time) or they can extinguish each other if they arrive at opposite phase. At, for example, the DECT frequency of 1.9 GHz the wavelength is about 16 centimetres . It is possible for the receiver to receive two waves in phase and get maximum signal strength. However, if the receiver is moved half a wavelength it could find itself at a position where those two waves are at opposite phase and almost completely extinguish each other.

This is of course very undesirable for a portable telephone so these receivers use ‘Antenna Diversity’. They have two antennas separated by about half a wavelength and continually monitor the signal strength at the two antennas. The software in the phone then decides which antenna to use for best reception. This technique greatly improves the stability of the received signal.

Antennas are a very important part of a radio system. The equipment generates a transmitting signal with several components and detects a received signal with other components. These components are connected to each other with wires or conductive structures like a Printed Circuit board. But we are interested in wireless communication and we need at least two antennas to transfer the electrical signal from the transmitter to an electromagnetic (radio) signal in free space and at the receiver we need to pick it up again and transfer it to the receiver.

Antennas are reciprocal: a transmitting antenna works exactly the same as a receiving antenna, with the same properties. When we design an antenna with good transmitting properties it will work equally well as a receiving antenna. However, the design goals can be different: an antenna of a GSM base station can be a fairly large structure but the antenna of a GSM phone must be small enough to fit in a pocket-sized ‘designer’ cabinet.
Generally, antennas must have a size of a half wavelength of the radio signal. This means that it is very impractical to make a portable antenna for the Low Frequency and High Frequency range: those antennas need to be several meters long. At the UHF range however the wavelengths range from 70cm to 12cm and therefore the antennas come into the range between 35cm and 6cm, a much more practical range for portable equipment.
To reduce the size even more we can trade in radiation and reception efficiency for a reduction in size and with special materials we can reduce the size even more. This can result in useful, but not ideal, antennas that can be used inside a GSM phone.

Generally, the more we reduce the antenna from its ideal half-wavelength size, the less efficient the antenna will perform.
Since most of present day communication equipment is two way (transmitting and receiving) the efficiency of an antenna works also both ways. If we make an antenna more efficient we can gain both receiving sensitivity and reduce the amount of transmitting power, which has a positive effect on battery life time.
It is therefore very important to consider antenna requirements and design at a very early stage in the design of a wireless product. A lot of performance and range can be gained with a careful antenna design.
Portable antennas come in many shapes and sizes. The most straightforward antennas are the (extendable) stub antennas that were used on earlier mobile phones. More modern designs use internal antennas that have a particular shape to tune them to the specific frequency used. Antennas may also consist of a certain copper pattern printed on the circuit board together with the rest of the electronics. By using certain ceramic materials antennas may be made much smaller than a quarter wavelength and these are useful for very small devices like Bluetooth applications but these antennas tend to be relatively expensive. Ceramic antennas are also used for GPS reception.