9 The EM spectrum

Prof. Bhushan Trivedi

Introduction

We have mentioned about the EM spectrum in the previous module. In this module, we will elaborate on the types of waves which are part of EM spectrum and used for data communication. The lowest range used for data communication, as seen in the previous module, is the radio waves, the highest usable range is the visible light. We will be discussing these types of waves, their characteristics and where are they used for transmission. We will also brief about spread spectrum method which uses the typical frequency for transmission in a typical way. Both, the introduction to EM spectrum as well as the spread spectrum acts as a prerequisite to the next module, the wireless physical layer. We will begin with the radio waves.

Radio waves

The radio waves are at the lowest part of the spectrum. The frequency range of the radio wave is 104 to 107 Hz. The frequency, thus, is less and the corresponding wavelength is more, i.e. the waves are long.

Following famous equation describes a well-known principle of physics.

l  f = c

The f is the frequency of the wave, c is the velocity of the wave in the vacuum (which is same as the speed of light) and l is the wavelength of the wave in the vacuum. This equation clearly indicates that the multiplication of both wavelength and frequency being a constant, increasing value of one will decrease the value of the other1.

Such waves are easy to produce and transmitted across comparatively long distance. They are able to pass through obstacles and are omnidirectional so sender and receiver do not need to be aligned carefully. Thus the old fashioned radio can be placed anywhere which suits the listener. Even when we use a radio in a moving car, the orientation of the car does not matter in the quality of the transmission. Unlike that, the higher range transmission used in TV sets, the antenna (a dish antenna or a typical TV antenna with multiple pipes of varying length) must exactly point to the TV tower or satellite in the right direction, otherwise, the transmission cannot be achieved. Another desirable property of the radio wave is that they can pass through obstacles. Thus the old fashioned radio was able to work even indoors. The TV antenna cannot work indoors, on the contrary, as it works on microwaves and not radio waves.

1 If the wave is traveling some other media, for example, air, the speed is reduced to almost 2/3rd of the speed of light. Even in that case, the frequency value does not change. The wavelength is reduced to match the decreased value of C.

Radio waves have quite a few subcategories and each has different physical properties. One common property is that a radio wave travels a long distance. Most radio stations are heard at long distances due to this reason. The downside of this characteristic is that it is possible for multiple radio waves to merge into each other and thus garble. When radio signals travel longer than expected, a radio set sometimes produces mixed output due to this reason. This is the reason FCC in US and DoT in India are established for regulating the use of radio spectrum. All radio stations can only use frequency slot dedicated to them and no other.

Our interest is to use the radio frequency for data communication. Unfortunately, there are a few reasons which make radio spectrum not very suitable for data communication. First, a law of physics. At lower frequencies, data encoding per frequency is much lesser than higher frequencies. At 1 MHz, a radio wave is capable of carrying 1 Mb of data. Higher frequency microwaves, for example, can carry 4 Mb to 5Mb on the contrary. Second reason is that radio spectrum is quite crowded. Find out the frequencies which are not used by radio stations and use them is not an easy job.

The properties that we generally attached to radio waves are actually frequency dependent. Lower range radio waves pass through thicker obstacles and travel in all directions but they have very high attenuation. When a signal is traveling in any direction, the signal tends to spread in all directions surrounding that signal, and drastically lose power. The power loss is in proportion to the square of the distance traveled. That means if the wave travels double the distance, lose power 4 times the original. This power loss is also known as path loss. As the signals travel in all directions, the path loss is huge. At higher frequencies, the radio waves move from omnidirectional travel to single directional travel and at higher and higher frequencies, they tend to travel in straight line. Higher the frequency, more they stop passing through obstacles and bounce off them. Path loss still occurs but to the lesser extent as the wave does not travel in all directions and lose power in all directions.

There are many devices which work in that range and that is why the radio waves always face interference. Moreover, there are other devices like electric motors, and scooters which produce (as a side effect of their work), some radio waves. When somebody starts the electric motor in the vicinity and if your radio voice is distorted, now you know why that happens.

Attenuation in the air is quite analogous to attenuation in media. The power of the signal drops with respect to distance it travels. The attenuation, also, is a property of a typical frequency of the radio waves.

The radio spectrum is divided into multiple sub-bands. It begins with VLF (very low frequency), LF (Low Frequency), MF (medium frequency), HF (High Frequency), VHF (Very high frequency), UHF (ultra-high frequency), SHF (super high frequency), EHS (extremely high frequency), THF (tremendously high frequency).

Last few names may surprise you. This is a problem of premature naming. After naming HF as high frequency (which actually is quite low), when higher level frequencies started being used, naming them in the as funny manner as above was devised. Also, the ranges UHF to THF are considered under microwave by most others (above is a specification from ITU) and we will not mention them further as a radio range.

The LF, MF and VF waves travel along the earth surface. They follow the curvature of the earth and thus can go a long way without needing any towers (which are needed for higher frequency waves). The downside is that their power drastically drops with distance as mentioned before. The advantage is that such waves can move anywhere without reflection. Navigation signals, AM radio stations, submarine communication etc. use this range.

HF and VHF, on the contrary, travel in straight lines and curvature of the earth comes in between any transmission. We need tall towers to overcome that problem and make sure the signals travel as long as they can.

Again, to double the distance we need to increase the size of the towers (on both sides, a sender’s as well as receiver’s) four-fold.

HF and VHF waves have one more advantage. There is an atmospheric layer consisting of charged particles around the earth, known as ionosphere at about 100 km above the earth surface. The HF and VHF waves tend to bounce off from the ionosphere and thus can travel a much longer distance. Most of the long range radio stations (for example, BBC) use this range to transmit their signals, so are heard very far, even other continents at times. There are ham radio enthusiasts who use radio transmission to communicate to other like-minded people over the world. These people built their own radio sets and use them over these frequency bands. In a case of some calamities in past, these ham radio operators have helped to signal messages for relief and directions for saving people. The other operator who uses this range is military. Aircraft to aircraft communication also use this range2. Maritime mobile communication also is benefited by this range. The figure 10.1 showcases both ground waves and waves which bounce off from the ionosphere.

Figure 10.2 shows the position of each of the types of waves. We can see that the radio waves are at the lowest frequency range. The next in that range is the microwave and that is the theme of next section.

Microwave transmission

The microwave range is about 100 MHz to 1011 Hz. In the beginning, the higher end of the microwave was not used but now some part of it, most notably, one range at around 10GHz and another at around 50GHz is started being used. The higher range of frequencies of the microwave are highly directional; i.e. they travel in very straight line so much so that even if a sender and receiver move a millimeter sideways they lose the sync. Both sender and receiver need to be aligned precisely for communication. This requirement is called line of sight (LOS) requirement. Sometimes special mechanism for defocusing the beam is needed to make sure the communication is not disturbed even when sender and receiver are little misaligned. Unlike radio waves, they only travel in the single direction and thus experience much lesser path loss. That means microwaves can travel much longer distance than radio waves.

The difference between the radio and microwave range does not come abruptly. The waves gradually become more focused as they go shorter in length. In the lower range, if the communicating parties are little misaligned, they can still communicate with a lower data rate but at the higher frequencies, special defocusing is needed. On the other hand, such a focused transmission allows the sender to use a technology called MIMO (Multiple InputMultiple Output). MIMO is a method to increase the capacity of the carrier to be multiplied by pairs of antennas used for transmission. In MIMO, there are multiple antennas sending and receiving (like multiple wires are used together for sending and receiving), and thus multiply the effective bandwidth by a number of such pairs of antennas used. Multiple transmitters are lined up in a single row to multiple receivers without having any interference from each other. Thus if one such pair of antenna gives a 10 Mb and we have 10 such pairs of antennas lined up, we get an effective bandwidth of 100Mb without using additional frequency band. Latest Wi-Fi solutions have increased the capacity of the transmission using many techniques, one of them is MIMO.

2  aircraft to aircraft communication can use radio, microwave as well as visible light

The antennas used by the microwave is the parabolic antenna that we mentioned in the previous module. It helps focus the beam precisely at the center of the antenna and travel much longer. These parabolic antennas must be aligned precisely for clear communication. Unlike HF and VHF, microwaves do not bounce off the ionosphere and we need towers at sender and receiver for communication. However, the microwave is an attractive solution when no wiring is possible between sender and receiver. Microwave is the only solution when the terrain in between the sender and the receiver requires right-of-way to dig trench for wired communication (for example, between two high-rise building in the city where the connecting line passes through resident colony), or impossible to lay cables (for example, between islands, or peaks of hills etc.). The sheer dimension of the obstacle prevents a wired solution. The Microwave can bypass these obstacles and thus can be really handy. It is relatively inexpensive as one just need to raise sender and receiver towers. The mobile phone companies save a lot when whey use high-rise building rooftops to raise their towers for communications. They avoid wiring the passage containing the busy government owned roads and other buildings. They use repeaters atop rooftops of tall buildings at the distance of 50 to 60 km to further their coverage. The microwave communication is not affected by natural disasters like flood or earthquake, as long as the sender, receiver, and repeaters are intact and aligned, the transmission remains unaffected.

The microwaves do not pass through the obstacles like radio waves and bounce off the obstacles. Even when traveling in straight lines, some divergence is introduced by atmospheric conditions3. Due to typical atmospheric conditions, a typical range of microwaves refracted from atmospheric layers and thus reach to the receiver little later than the direct wave. This little delay is good enough to make them out of phase with the direct wave. If they reach the receiver in an out of phase with the original wave, they will cancel out the original wave and the receiver does not get any signal. This is known as multipath fading and happen quite often. Different atmospheric conditions affect different frequency bands and generally, there is no alternative for the sender to use some other frequency to transmit. It is a natural phenomenon and there is no way to prevent it. As a

3  It has a similar effect which the visual light has when the mirage is created.

practical solution, sender usually keeps a spare frequency. When one of the transmitting range experience multipath fading, they use the spare frequency band instead and transfer the communicating going on that frequency to the reserved band.

Another problem with the high-end microwave is that they are so short that they are absorbed by rain and vegetation. The reason for dish television programs stops working while there is a heavy rain is due to this reason. The waves coming from the satellite are absorbed by rain drops. This is also true for cases where the trees come in between.

However, microwaves are quite useful for many applications like mobile phone communication, long range communication by other mobile devices like satellite phones, long range point to point communication and communication satellite to dish antenna communication and also between ground stations of TV channels to satellites. Due to all these usages (especially use of this spectrum by mobile companies), they are highly in demand and invite arbitration from the government. The government, from time to time, auction these bands. Unfortunately, these frequency auctions are in the news for all wrong reasons, the world over and not only in India. International bodies like ITU (International Telecommunication Union) are working in the direction. The advantage of such international agreement is huge. It is always a good idea to have consistent frequency allocation across countries as devices working in one country (for example mobile phones and TV sets) also work in other countries as they use the same frequency spectrum. One can mass produce the device in a country where such manufacturing is cheaper. Government bodies like DoT (Department of Telecommunication) in India and FCC (Federal Communications Commission) in the US are controlling the auction process. Instead of technological reasons, decisions of such bodies are controlled by strong lobbies who has acquired or would like to acquire specific frequency band and do not want to release. That means, there is a little consensus across countries to adopt the suggestions by bodies like ITU for a typical spectrum. The politically influenced parties do not allow any such agreement between countries. One such example is of 3G. It took almost 10 years to arrive and lose the advantage it would have otherwise. ITU recommended it but nobody except China reserved the specified bandwidth on the specified spectrum. Originally designed to be 2 Mb, it has to be reduced to 384 kb and even that is not actually possible to be delivered in most cases.

The next range that is used is called infrared which has the size of wave reduced to millimeters in size and thus are also known as millimeter waves.

Infrared transmission

The infrared is used for short range transmission, for example, the remote controls of our TV sets and other electronic devices use infrared transmission. The devices which one need to generate such rays can be built easily so quite cheap, it is not hazardous to human health (microwaves are) so one can use them in household operations and remote controlled toys etc., and contains a huge bandwidth so can carry a lot of data. It is highly directional so one needs to align sender with the receiver and it cannot pass obstacles. Both of these properties anyone would have realized if ever stood between a TV set and a remote.

The infrared waves are very focused so much so that if the sender and receiver are not properly aligned, the communication cannot happen. Older mobile phones used to have infrared ports. When one uses such ports, both devices must be kept exactly opposite to each other.

The infrared cannot work outdoors as well. A portion of the sunlight contains infrared waves apart from visible light and many other waves. We cannot ‘see’ anything4 other than the visible light as our eyes are equipped to see only a small part of the spectrum5. If we use an infrared device outdoors, the signals will collide with the infrared signals present in the sunlight and result in garbage. Governments do not impose any licensing on the usage of infrared waves.

The confinement to indoors and the inability of pass through obstacles can actually be boon for devices which demand security. For example, one cannot remotely operate our TV from outside our house and play a prank. A communication will not be ‘heard’ in the adjacent rooms. However, infrared use is limited to connecting wireless devices like mice and printers in past. That job is taken over by the Bluetooth in recent years.

The unlicensed bands (ISM bands)

Looking at the issues of licensing and politics associated with, one may wonder if we can forgo licensing and start using the spectrum in some collaborative fashion. There are indeed some ranges known as “Industrial, Scientific and Medical” bands which are free for anybody to use primarily set aside to be used in industrial equipment, apparatus used for the scientific purpose and medical devices. However, they have used for non-ISM purposes also. We are interested in learning about its usage in data communication here. Out of a few ISM ranges, there are three different ranges used worldwide for non-ISM usage. The exact ranges available are different from place to place and there is no worldwide consensus except for 2.4 GHz range. Looking at the use of such bands, many prefer to call these range as “unlicensed” and not ISM bands. Calling them ISM may be considered misnomer in future. Let us concentrate on three ranges that are primarily used for such non-ISM purpose in the following.

4That is why we cannot see the rays travelling from our remote to the TV set

5   In true sense, hearing to a small fraction of the spectrum and can only be able to see a small fraction of the spectrum is a blessing. It would be a very noisy atmosphere otherwise for us.

The first range is about 915 MHz6 which is used by 802.15.4 networks (refer to module 31-33 for the description of these networks) and ZigBee networks. That range is also used by cordless telephones. The second range is 2.4 GHz to 2.48 GHz, popularly known as middle ISM range. Unlike the first range, this range is available worldwide in most countries. The Wi-Fi network’s two version 802.11b and 802.11g works in this range. Bluetooth also works in this range. Some of the 802.15.4 and ZigBee devices also use this band. Many remote controlled devices, toys, garage door openers, microwave ovens, etc. also use this band. The third range which exists at 5 GHz, is basically a collection of three different ranges. One is between 5.25 GHz to 5.35 GHz (a 100 MHz band), another is between 5.47 to 5.7 GHz which is 255 MHz wide, and last is 5.725 GHz to 5.825 GHz which is also 100 MHz wide. These ranges are known as Unlicensed National Information Infrastructure (U-NIII). The range is also known as Broadband Radio Access Network (BARN). One typical version of Wi-Fi, the 802.11a, uses this range. HyperLAN which is a wireless LAN used in Europe also uses this range.

In India, only the middle range is available and other two ranges are licensed. The unlicensed ranges have other restrictions by authorities, for example, a cap on the power to be used. It is quite obvious. When a range is free for all to use if one uses that with high powers, others get so much interference that the range becomes effectively useless for them. That is why there is always a power limit specified for devices working in these unlicensed bands.

Out of three bands available, the lower bands have less bandwidth compared to higher end bands. However, they better for some reasons cited as follows.

1. They consume less power as compared to higher end band to cover the same distance. That means the devices used in this range need less power and battery runs longer.

2. For the same power, the range they cover is more than double the range covered by high-frequency bands

3.  High-frequency bands suffer from absorption by rain and vegetation, these bands do not have any such problem.

The downside is that there are many devices which use this range. The range is highly occupied by many radio devices and other long range communicating devices. There is every chance that the transmission experience some interference.

Optical light, FSO, and Li-Fi

The final band that is used for transmission is the visible light that we are used to. Light is also a wave with frequency and well suited for transmission except for the case that there is a lot of interference due to the existence of our normal light. The term Optical Wireless Communication is used to describe unguided visible, infrared and UV light for carrying the data signal. FSO or free space optics is a way of transmitting the optical light in free space. FSO is generally used to provide a terrestrial point to point link. The very light pulses which work in FO cable can work without the confinement of cable. The idea about data transmission using optical light has emerged from that idea. Thus we will be using the very light that we use for our day to day life for data communication purpose without using any physical media. It is quite similar to use the torch to signal a message.

6  In some parts of the world a range starts from 433.05 to 434.79 MHz is used instead.

FSO can be used wherever laying an FO cable is inappropriate, or impossible, for example, when we want two aircraft to communicate. FSO is demonstrated to work even with distances of 40,000 of kilometers between spacecrafts. In other experiments, it was demonstrated to work at 1Gbps speed for about 2 kilometers. Though the FSO is the quite attractive proposition from that ground, has a few limitations as well. The atmosphere is the biggest issue. Everything including sunlight, fog, vegetation, rain, lightning, dust can hinder the propagation of light, and thus, data communication. We have already commented on the problem of dispersion.

In recent years, an interesting indoor application has emerged called Li-Fi, which is basically a Wi-Fi for home. The difference is, instead of using the 2.4GHz spectrum, it uses visible light, infrared and near UV spectrum for data communication. The sender uses LED and receiver uses a photodiode, like an FO cable.

The LED switches off and on while transmitting the light pulses, too fast for the human eye to notice. The light is dimmed below human visibility to make sure humans do not get disturbed by the transmission. Li-Fi is a boon for cases like aircraft cabins, nuclear plants, hospitals where any form of radiation is unwelcome. Li-Fi is capable of achieving much better data speeds than Wi-Fi. Researchers are able to generate speeds of 224 Gb in a closed space. Even when the senders and receivers are not aligned, the light bouncing off the walls can provide about 70 Gb of speed. The light cannot pass through walls which make this network much more secure than Wi-Fi.

Using the spectrum

In most cases, the devices which use the spectrum focus on a very small part of it. For example, a radio station generally picks up a very small band a few kb wide. A GSM mobile phone uses a 200 MHz band. Such bands are known as the narrow band and most transmission works in that fashion.

The benefit for such narrow band transmission is clear. It uses very less bandwidth and thus many such transmissions can be packed in the available range.

Some transmissions, favor a wide band transmission, i.e. use a much wider range than needed. This method seems wasteful but it is not in true sense. One typical method, known as DSSS or direct sequence spread spectrum was used in Wi-Fi transmission. Another method, known as CDMA or Code Division Multiple Access, is one popular method of mobile communication. Yet another version, known as FHSS (Frequency Hopping Spread Spectrum) is also used in Bluetooth and other cases. All these methods use more bandwidth than they actually need.

The FHSS utilizes the large bandwidth in a typical way. The transmission which uses FHSS moves from one frequency to another from time to time. The sender and receiver both have the clear idea about the sequence in which the frequency changes and the time it spends in a given frequency band. An intruder fails to jam signals from such transmission for a long period as he might use trial and error to capture a transmission but cannot hold on to it for a longer period.

The DSSS uses a larger band to avoid errors related to big signals (higher frequency harmonics will cut off if the transmission uses a big signal. It uses multiple smaller signals. This mechanism can use the spectrum more efficiently and is more immune to other transmission related errors.

CDMA allows multiple users to use the same frequency and thus can entertain a much larger number of users in a given band than a narrow band transmission might allow. So CDMA was a popular choice in 3G mobile phones.

Another mechanism which uses a wider band is called UWB or Ultra-Wide Band. It spreads the radio energy over a very wide frequency band, with very low power spectral density. Usually, the range is many GHz and the data rate is also around some GHz speed. However, it is spread over a much larger band. That means for any narrow frequency part chosen out of this wide band, the power used is very less and a number of bits it covers are also less. The high bandwidth allows very high data rate for a short distance or reasonably good data rate for a longer distance. The advantage of UWB is its capacity to withstand interference, pass through obstacles and even share the bandwidth with another narrow band transmission. For a narrow band transmission, the low powered UWB transmission going on in that very range seems like little noise and nothing else. The UWB signal is said to underlay the narrow band signal in that case.

UWB actually is designed to transmit using such a wide range that it also covers many bands which are commercially licensed to transmit narrow band. The proponents of it claim that the interference the UWB devices generate is less than the normal noise that the narrow band transmission is expected to encounter during normal transmission. Thus UWB does not cause significant interference with other licensed users.

There are a few advantages of this wide band. The multipath fading problem does not affect UWB and low power utilization improves the battery life of the devices. UWB is well suited for sensor devices with constrained power. Normally UWB operates in 3 GHz to the 10GHz range. Vehicular networks use UWB in the range of 22Gb to 29Gb frequency range. PANs also use UWB and get about 1Gbps speed.

References

1. Computer Networks by Bhushan Trivedi, Oxford University Press
2. Data Communication and Networking, Bhushan Trivedi, Oxford University Press