Copyright ©2019 Thomas Schwengler. A significantly updated and completed 2019 Edition is available.
Many textbooks begin with a good historical overview of wireless communications and excellent insights (see for further reading informative introductory chapters in refs. [3, 6, 8]). We do not chose to expand much on these historical aspects or radio communications, still a few statements might be interesting to recall on the major innovations in wireless technology.
Early smoke signals and carrier pigeons may of course be considered as a form of wireless communications, but offer little modern interest. Early coding schemes, however, are more interesting: British scientist Robert Hooke invented large mobile panels coding the letters of the alphabet (1684). nd more elaborate schemes appear in the late 18th century, including the noteworthy optical telegraph invented by French physicist Claude Chappe (1791), transmitting coded words (rather than letters) over long distances; large signaling towers were developed in the following years into a large network over major cities in France and surrounding countries. These precursors to radio communications already emphasize some valuable points: 1) coding information is used for efficiency, 2) transmission works well in line of sight, and may suffer from outages due to obstructions, or even weather. In a sense these systems are rather reminiscent of current fixed wireless systems such as microwave, millimeter-wave, or infrared links.
True radio communications of course rely on electromagnetic waves, and build on the work of Maxwell and the experiments of Hertz. The first use of radio to transmit coded information was proposed by Tesla in the 1880’s, and the first radio communication systems were described in his papers around 1891. Nearly simultaneously, Marconi patented the telegraph and demonstrated to the world the usefulness of mobile communications with ships crossing the English Channel. These facts already emphasize the importance of some aspects of radio communications: 1) certain radio frequencies overcome line-of-sight obstructions and weather impediments, 2) mobility is the main application, 3) patent protection is paramount.
The next major advances in radio systems were developed during and after world war two, and benefitted from significant research around radar and remote sensing. Subsequently, different applications flourished: TV broadcasting in the 1940’s probably has the merit of introducing the first standardization of communications technology, leading to major television standards, (NTSC Color Standard in 1953, and recently ATSC Digital Standard in 2009). Standards have become very important in all aspects of wireless communications, and will be analyzed in more details further.
Cellular systems were devised by AT&T Bell Labs in the seventies. Continued improvements in standards and products provide increasing spectral efficiencies, lower prices, and wider consumer acceptance. Amazing growth occurred in the wireless industry during the 1980’s and 1990’s, which led to almost ubiquitous service availability and cheap service plans; some irrational exuberance in the industry also caused failures and bankruptcies around 2000 (such as excessive spectrum bidding, expensive satellite services, or some early broadband wireless initiatives).
Current events and trends in the wireless industry are in constant evolution and their descriptions quickly become obsolete; still the following milestones are noteworthy:
The above points show a fast-paced change of landscape. The best sources of information for these data points are periodicals, publications, and websites that specialize in the trade. When searching them, always pay attention to publication dates, and beware not to use obsolete data; take very good care to search for a trustworthy and recent fact source.
“Prediction is very difficult, especially about the future.” (Attributed to Niels Bohr, Danish physicist, 1885–1962.) Nevertheless, most predictions show a continuing growing trend. The growth is based on continuing increase in data use in smart phones, tablets, as well as the proliferation of applications including in new areas such as the Internet of Things. We will investigate new standards and technologies that will lead us to better understand some of these wireless trends.
Spectrum is a very important notion for any wireless system: it refers to the range of frequencies used by the system’s electromagnetic waves; when several services use the same spectrum in the same location interferences occur that may be harmful to these services. Governments therefore step in and set rules and regulations for spectrum coordination.
Spectrum is a valuable resource for many applications such as radio communications (terrestrial and satellite), radar, remote sensing. Some applications require very quiet portions of spectrum (such as deep-space observations), others may share or reuse spectrum fairly aggressively. In all cases rules and regulations are in place for each spectrum bands, which will be reviewed further.
Different bands of spectrum benefit from very different properties. Lower spectrum generally propagates better through the atmosphere and through obstacles; it has been traditionally used for radio and TV broadcasting, and is excellent for mobile applications. Higher spectrum is more abundant and therefore used for higher throughput applications, but is much more attenuated by atmospheric particles and weather variations, and needs near line-of-sight conditions.
A wide range of spectrum is used in modern communications: megahertz (MHz) radio frequencies, gigahertz (GHz) microwaves and millimeter waves, and terahertz (THz) infrared lasers for free-space optics. Accurate study of radio waves at any given frequencies can be performed by studying electromagnetic fields properties: propagation, scattering, etc. Maxwell’s equations are used to determine wave characteristics in complex situations, but in many practical situation full-wave modeling is too complex and time consuming; instead the industry has been relying on simple approximations, rules, and models. We will see some of these models in chapter 3, but we start here with some high-level properties of radio frequencies.
Communication systems use diverse frequencies either in a wired mode (with a wave guide from transmitter to receiver such as coaxial cable, copper wires, or fiber), or in a wireless mode, which is our focus here. To summarize:
The earth atmosphere is a complex gaseous mixture and has various effects on radio propagation; the dominant effect is usually attenuation, though depolarization also occurs. Most wireless cellular system use frequencies below 6 GHz for good coverage, and are therefore not significantly affected by atmospheric variations. Weather conditions do have an impact on wireless cellular networks, but their effects are typically not dominant, and are usually not part of initial system design.
Above 6 GHz, atmospheric effects may be significant and must be taken into account in the design of a wireless system. This is especially important for the backhaul, which may rely on wireless links up to several tens of miles. (See §5.4.3).
Atmospheric absorption usually dominated by absorption by water vapor molecules or oxygen molecules; these molecules have specific frequencies of resonance that create some peaks of absorption around these frequencies. Water vapor has an absorption peak around 24 GHz and its multiples, and oxygen has it main peak at 60 GHz.
In addition to gaseous absorption, hydrometeors are also a severe cause of absorption. Heavy rain with large drop size has an especially severe impact. At 30 GHz for instance heavy 50-100 mm/hr precipitation causes 10-20dB/km attenuation, and will usually cause radio outages. This may be the case for LMDS (28 GHz) or point-to-point microwave links (6, 11, 18, 23 GHz). Refer to chapter 5 on backhaul for more details.
Communications require bidirectional exchange of information; in wireless communication, the two communications links are often referred to as uplink and downlink, or as forward and reverse link. The terms downlink and forward link refer to the transmission from a base station to a customer device like a mobile phone; uplink or reverse link refer to the transmission from the customer to the base station.
A fundamental aspect of a spectrum band is how it is used to provide both uplink and downlink communications; which is called duplexing. The two common duplexing schemes used for wireless communications are: time division and frequency division. Frequency Division Duplexing (FDD) uses two different bands of spectrum for uplink and downlink, these bands are said to be paired; one portion of the spectrum is reserved for uplink operations, the other for downlink. Time Division Duplexing (TDD) uses the same band for uplink and downlink, and uses different time slots to separate uplink from downlink traffic; the spectrum band is usually continuous, and is said to be unpaired; unlicensed spectrum usually uses TDD.
Each duplexing scheme has advantages in some situations. FDD schemes tend to be slightly more spectrum efficient for symmetric traffic; they are typically preferred for long-range links such as private line and for voice communications (typically based on symmetrical standards: DS0, DS1, etc). FDD schemes allow simultaneous uplink and downlink transmission in paired bands of spectrum with enough separation to avoid interference.
TDD schemes are technically slightly less spectrum efficient since some quiet times are required between uplink and downlink traffic (at least greater than round-trip delay between the base and the furthest device); but they offer the great advantage of allowing dynamic changes in the amount of time (and therefore bandwidth) dedicated for uplink versus downlink, which may actually be a more spectral efficient solution for asymmetric or bursty data services. TDD schemes are therefore useful for small cells and LAN data applications.
FDD vs. TDD preferences sometimes give rise to endless arguments: one argument is that adaptive antenna systems (smart antennas) and MIMO systems require fast channel estimation and are better suited for TDD schemes (although efficient FDD adaptive system certainly have been demonstrated). Other considerations of cost of equipment are sometimes important to choose between TDD and FDD: FDD devices are typically more expensive to manufacture since frequency diplexers must be used to transmit and receive at the same time. To alleviate that cost, some cheaper devices use hybrid frequency division duplexing (H-FDD) in which they use paired spectrum like an FDD device (different transmit and receive bands), but are not capable of transmitting and receiving at the same time (like a TDD device). Although H-FDD may allow for cheaper devices, the drawback is a very inefficient use of the spectrum.
Practically duplexing choices are often determined by local spectrum rules and regulations: when regulators make spectrum available (e.g. for an auction), they announce if it is paired or unpaired, practically forcing the use of FDD or TDD (see for instance figure 1.9). Consequently most standards have TDD and FDD profiles to adapt to multiple regulatory environments, though sometimes not with the same time to market and equipment availability.
In the US, spectrum is regulated by several government institutions: the National Telecommunications and Information Administration (NTIA) Office of Spectrum Management (OSM) for government and military use, the Federal Communications Commission (FCC) for commercial use.1 Some bands are also opened for license exempt use, and have contributed to the widespread of wireless LAN technologies like Wi-Fi.
In the US, the Code of Federal Regulations (CFR) is the set of rules and regulations for federal administrative law governing anything from agriculture to public health, including telecommunications. The FCC rules around telecommunications (including spectrum) are under title 47 of that code, and are referred to as CFR title 47 [10]. Title 47 has multiple parts governing many spectrum bands and their different applications: amateur radios, satellite, microwave, cellular, etc. In particular part 101 deals with specific radio applications and is covered in chapter 5. Part 15 deals with unlicensed transmissions, from spurious emissions (§1.4) to wireless communications in unlicensed bands detailed next.
The main bands for unlicensed (or license exempt) use in the United States are listed below. These bands are governed by different FCC rules; device power and EIRP are limited in order to alleviate interference concerns, and equipment must be approved by the FCC (mainly to verify power levels and emission masks).
The amount available varies by geographic area (and presence of broadcasters). Databases exist for white-space devices to determine which frequencies they can transmit on. See for instance the following URL’s:
Initial rules made channels 2-51 available for fixed service (except 3, 4, 37) up to 1W power output, 4 W EIRP as long as no TV signal is in the channel or adjacent channels. Antennas should be mounted outdoors (above 10m and below 30m). Channels 21-51 (except 37) are for personal/portable service, including mobile. Devices that use spectrum scanning and an Internet database can operate at 100 mW when no adjacent TV broadcast. Portable devices that only scan the are limited to 50 mW EIRP on all channels and 40 mW EIRP in an adjacent TV channel.
Subsequent actions restricted the amount of spectrum available for unlicensed use, and instead is planning on auctioning some of the precious spectrum (see 600 MHz section, §1.3.3). [11]
The two new bands proposed in 2013: UNII-2B and UNII-4 are not quite available for use (as of mid 2016), and are likely to have rules similar to UNII-2 and 3 [12].
These various UNII bands are actually allocated on a primary basis to various services, either federal or commercial, such as aeronautical or maritime radionavigation, fixed satellite service, earth exploration, space research, which is why their license-exempt use has restrictions to avoid interferences.
The 60 GHz band was recently extended from 57-64 GHz to now include 64-71 GHz (under part CFR 47, part 15.255) as part of an effort to provide more 5G spectrum [14]. Power levels are limited to 43 dBm indoor, and 82 dBm outdoor (minus 2 dB for every dB that the antenna gain is less than 51 dBi).
In summary the unlicensed spectrum landscape now contains: the busy but well established 900 MHz and 2.4 GHz ISM bands and a rich 775 MHz of unlicensed U-NII bands. This spectrum combines with powerful and ubiquitous wireless standards (like Wi-Fi 802.11n/ac/ax) to offer very powerful wireless solutions.
Different spectrum bands are made available for commercial use at different times. The spectrum bands listed in this section are interesting for US activities, they have a very different history and different rules. Many of the detailed band plans are available at www.fcc.gov under auctions.
A new band plan was proposed by FCC to transition the old 6 MHz analog TV channels to 5.5 MHz channels. In 2006 broadband radio service (BRS) operators began filing their plans to initiate transitions of the 2.5 GHz band in various basic trading areas (BTAs) under the new rules adopted by the FCC in 2004.
Power is regulated in several categories: Category A limited to 1 W (30 dBm) EIRP maximum in 10 MHz, Category B allows 10 W (47 dBm) EIRP in 10 MHz. End-user devices can transmit up to 23 dBm EIRP in 10 MHz – see the new part 96 for CFR 47 [10].
In addition existing non-exclusive licenses in the 3.65 GHz band are grandfathered until April 2020, and is otherwise reserved for GAA use. [13] Power in that grandfathered band is limited to 25 W EIRP maximum in 25 MHz (or 1 W in 1 MHz to keep same power spectral density), 1 W EIRP per 25 MHz maximum for mobile stations. (Also 150 km exclusion buffer zones were created around existing fixed satellite stations).
Rules around CBRS are still subject to change, and an auction (for PAL licenses) is expected in 2019.
That band is the area of focus of 802.11p, and may be an opportunity for public safety applications. Automobile industry has had most of the activity so far around smart cars and collision avoidance.
More recently, bands above 24 GHz have been identified for mobile use as part of 5G. [14] Mobile rights are granted to existing license holders, license areas subdivided by county. FSS earth stations continue to share the band. LMDS and 39 GHz were regulated in part 101, and are now augmented with new mobile flexibility and moving under a new part 30 of 47 CFR [10] under the new denomination of Upper Microwave Flexible Use Service (UMFUS). Power is limited to 75 dBm per 100 MHz EIRP at the base station (average power of the sum of all antenna elements), 43 dBm EIRP for mobile, and 55 dBm for fixed transportable stations (such as in homes or cars).
The new 5G millimeter wave mobile bands are:
All the above licenses are auctioned in different areas: some are very large (Regional Economic Areas cover the continental US in only 6 service areas), while others are small (like the 416 Partial Economic Areas covering the US and territories). The various sizes are by design to allow large providers to bid on contiguous areas, while also letting small providers focus in local markets. The many FCC areas are detailed and mapped on the FCC website at www.fcc.gov/oet/maps/areas.
The above sections show a fairly complex spectrum landscape in the US, with different bands coming from different use at different times; this overview shows the following points:
A final note: in some cases pre-existing systems may be operational at a given frequency, and bidders have to move these systems to another band before utilizing the spectrum they bid on. That requirement is called spectrum clearing, it can be time consuming and costly.
Different spectrum bands have been identified by regulators for very different applications: some require high power (e.g. for long-haul communications), others on the contrary require very low power levels (e.g. for radars, or deep-space observations). Consequently regulations also have to deal with how much power is allowed in these bands, and how much can overflow into neighboring bands.
It may be important here to consider other electrical devices: in general they all emit electromagnetic radiations, wanted or not, sometimes guided (as in a DSL or cable modem), sometimes for wireless applications over an antenna. Consequently, the FCC has a minimum allowed emission level in all bands. Part 15 (of CFR title 47 [10]) deals with these spurious emissions of any device in any band; it deals mostly with fairly low power devices (typically less than 1mW devices). The unlicensed bands identified in §1.3.2 are also covered by part 15, and allow for reasonably high power levels, all other bands are strictly limited. Power limitations are set in terms of electric field strength at a certain distance from the device: typically at 3 meters 200 μV/m below 960 MHz, and 500 μV/m above. Outside of unlicensed bands, spurious or unwanted EIRP radiated are limited to less than -40 dBm, which is very low... (Note that there are however some provisions that “periodic” signals or “intermittent control” signals can be higher.)
To determine if a system radiates within the limits allowed by part 15, refer for instances to details in FCC document OET bulletin no. 63 [16], p. 29, which gives a commonly used approximation to relate radiated power (P in W), antenna gain (G relative to an isotropic source) and electric field strength (E in V/m) at a distance D:
 | (1.1) |
For the purpose of assessing limitations at a distance of 3 meters, the accepted relation is simply EIRP = PG = 0.3 E2 (where EIRP is in W, E in V/m). The relation can simply be converted to dB’s by: EIRP (dBm) = 24.77 + 20log(E(V/m)).
Other levels of emission important for good cohabitation of services are out of band emission levels. Technical specifications as well as local regulators address these my mandating a spectrum mask for adjacent bands.
The complex US spectrum landscape is due in part to the fact that multiple government and commercial entities compete for a scarce and precious resource. It can easily be pictured how difficult such coordination might be on a worldwide basis. Such coordination attempts occur regularly at the World Radiocommunication Conference (WRC), a conference organized by the ITU where governments delegations and standards bodies provide contributions, studies, and discussions to attempt to harmonize worldwide use of spectrum. Progress and coordination is typically very slow to achieve at the WRC, as different countries have very different priorities and needs. Some levels of harmonization have been successful, which have allowed for instance global roaming services in 3G cellular bands. WRC 2015 also started identifying 5G bands, including in the millimeter band mentioned above, and some bands above 95 GHz that will be discussed further at WRC 2019.
In addition, different governments have different incentives to use spectrum efficiently. In the US, typical rules associated with a spectrum band are fairly technology agnostic: the FCC tends to be standard agnostic in order to allow all technologies and promote innovation; in other countries, governments sometimes mandate the use of certain standards in certain bands in order to enforce some minimal spectrum efficiency. (In the late 90’s for instance, IMT-2000 provided a family of 3G standards for third generation cellular systems; the European Union mandated that these standards could be used in newly created bands, whereas the US did not have any such requirements).
Also, throughput this class and your professional career, please proof-read ; if you don’t have the patience to re-read your work, do not expect anyone else to.
Copyright ©2019 Thomas Schwengler. A significantly updated and completed 2019 Edition is available.
