| Radio |
A diverse and robust collection of technologies and engineering techniques used to enable the communication of signals representing information via the electromagnetic spectrum, a property of the physical universe. The general process used is (a) the creation of a carrier wave, via an oscillator, at a given frequency and with a given bandwidth, (b) the modulation of this wave with the information to be communicated, (c) additional coding of the signal in an effort to at least partially compensate for the uncertain nature of radio communications and technical artifacts associated with electromagnetic waves, (d) amplification of the signal, and (e) applying the signal to a transmit antenna, at which point the signal enters the radio channel, sometimes (if archaically) called the "ether." Radio waves naturally fade in amplitude (signal strength) as they propagate across the radio channel, both with the square of distance and due to interactions with objects, including other radio signals, in the environment. Assuming sufficient signal strength at the receiving antenna, the signal is (a) detected, (b) amplified, and (c) decoded and demodulated, and the information contained in the signal is thus recovered.
In reality, a vast number of options and alternatives confront contemporary radio designers. Very-large-scale integration (VLSI) has, however, has enabled device manufacturers to easily and cost-effectively take advantage of standards-based wireless systems, and thus has been largely responsible for the proliferation of wireless as the medium of choice in much of networking today. |
| Amplifier |
Amplifiers are electronic circuits that boost the power (increase the amplitude) of a signal fed to them. While many different types of amplifiers are broadly applied across many electronic applications, two important amplifiers used in wireless include the power amplifier (PA), which is used to boost the signal sent to a transmit antenna, and the low-noise amplifier (LNA), used to boost the typically very weak signal appearing at a receiving antenna. |
| Analog (and Digital) |
The "real world" is the domain of continuous signals often described as analog, typically sine waves at a given frequency. Radio waves are an example of an analog value. Modern communications systems, however, are based on digital technology, where the goal is to encode and communicate the ones and zeros of the digital world via analog signals. Digital is used because communications systems need only be concerned with the successful transmission of two discrete values rather than the fidelity of continuous waves, and because digital processing is far more efficient, convenient, cost-effective and adaptable than analog circuity. Note, however that in transmission, digital signals are in fact represented as analog values. Analog and digital are therefore always tied together in communications systems, with circuitry known as analog-to-digital and digital-to-analog converters used to translate the representation of information between these two domains. |
| Antenna |
An antenna in a wireless communications system is analogous to the tires on a car or any other vehicle -- the antenna is the only portion of the radio machine that actually touches the medium upon which radio waves propagate. Antennas cover a broad range of designs and applications, from simple wire dipoles (sometimes called "whip" or "rabbit ears" antennas) to designs based on fractals and other complex mathematics to so-called "smart" antennas with active (powered) electronic components. In general, antenna designs are optimized for a specific range of frequencies, with lower frequencies usually requiring physically larger antennas, and are either "omnidirectional", transmitting towards and receiving from all directions simultaneously, or "directional", optimizing both transmission and reception across a limited number of degrees of arc. Antennas can also be designed to provide a certain amount of gain when transmitting and receiving, acting like passive amplifiers, and can be ganged together on the receiving end using a technique known as "antenna diversity" to at least partially compensate for fading. See also beamforming and MIMO. |
| Band |
A specific and contiguous range frequencies utilized for a given transmission, this is most often determined by spectrum regulation. A band is often subdivided by a given frequency range into channels that are statically or dynamically assigned and individually (and distinctly and simultaneously) utilized for a given service or application. |
| Bandwidth |
The range and thus amount of spectrum utilized in a given transmission, with the nominal frequency being the center of this range. See also spread-spectrum and orthogonal frequency-division multiplexing. |
| Baseband |
That portion of a radio that performs processing other than that required for functions directly related to radio frequencies. Increasingly, this processing is performed by digital signal processing components. In an extreme case, an analog-to-digital converter might directly convert an over-the-air waveform into a digital stream suitable for baseband processing. Many implementations, however, often utilize significant analog processing for reasons of cost, design requirements such as physical space allotted and environmental concerns, and the experience level and preferences of the radio design engineers involved. |
| Beamforming |
A set of techniques that utilize multiple transmit antennas and related signal processing to combine independent transmissions so as to increase reliability and/or to bias the transmitted radio wave in a particular direction (typically known as "beamsteering"). While two or more antennas can be used in this manner, implementations with larger numbers of antenna elements are often described by the term "phased array." |
| Broadband |
An imprecise term typically used to describe the improvement in one or more dimensions of performance (throughput, range, reliability, etc.) that accrues from the utilization of more bandwidth than would otherwise be required in a given application. Broadband is also commonly used, again imprecisely, as a descriptive term for comparatively higher performance in general. With respect to radio, a more appropriate term to use in this case is "wideband," implying the utilization of more spectrum than would otherwise be required again in the service of the above goals. See also spread-spectrum. |
| Capacity |
Capacity refers to the upper bound of performance for a given channel. Capacity is by definition an imprecise term given that the behavior and thus the capacity of any radio channel is variable under normal operating conditions. Capacity can therefore vary, but the term is often used to describe the ability of a shared channel to carry the maximum information at any given moment in time, as opposed to maximum throughput for any given transmitter. Given that channel capacity in many operational systems can easily provision far more throughput at any given moment than is otherwise required for a given communication, the ability to utilize the "excess" spectrum is often desirable if not essential. Capacity therefore refers to an upper bound on the ability of any given channel to support simultaneous, distinct, and diverse communications and is usually more important than throughput in the specification, design and operation of most implementations. |
| Cellular |
Cellular architectures were developed in the late 1940s as a means to deal with both capacity and fading challenges. The core idea is to enable the connection between a client and the wireless network infrastructure to be "handed off" between individual radio base stations called cells both as clients move and also in some cases for load balancing. This function is often called "roaming". While not always economically or logistically practical, in principle cells can be deployed over any geographic scale and are widely used in both wide-area and local-area (see Wi-Fi) implementations. The placement of cells can be challenging, and such considerations as local zoning rules and the availability and provisioning of power and backhaul (interconnect) facilities come into consideration. While an individual cell can potentially cover a very large geographic area, subject to transmit power regulations, local terrain, etc., the emphasis today is on "small cells," an imprecise term." While more handoff activity may result as the coverage area of a given cell decreases, the ability to more rapidly reuse the frequencies allocated within a given cellular installation can yield a dramatic increase in overall system capacity, and thus an improvement in per-connection throughput and latency. |
| Channel |
A channel is a specific range of frequencies within a band. Bands are often subdivided into channels in order to allow multiple, distinct communications to occur simultaneously. Channel as defined here is not to be confused with radio channel when used to mean the physical elements of the universe that support radio communications. The bandwidth of a given channel may vary depending upon the radio system being used. For example, the IEEE 802.11ac wireless-LAN standard specifies the availability of 20-, 40-, 80-, and 160-MHz channels. But the decision as to what bandwidth to use under a given set of conditions is left to the implementer and/or end-user of a given product. Wider channels potentially enable higher throughput at the expense of the potential for greater interference and a reduction in the total number of channels available. Specified channels may also overlap under some circumstances. This overlap is not necessarily a problem if the distance between transmitters is sufficient to avoid damaging and potentially mutual interference, and thus may be utilized to compensate for interference in some cases. |
| Channel Adaptation |
Channel adaptation is the ability of a radio system (transmitter and receiver) to vary elements of a given communication connection, including modulation, coding, number of MIMO streams, and transmit power to improve the capacity of a given radio channel under dynamically varying operating conditions. While often described in applicable technical standards like 802.11, the actual real-time behavior of a given product is at the discretion of the vendor and a given implementation, usually via firmware, of a specific channel-adaptation strategy. Given that channel conditions can vary widely from moment to moment, different channel-adaptation schemes also vary widely in their ability to improve reliability, throughput and capacity, and sometimes, from moment to moment, these schemes can even act counter and detrimental to these goals. |
| Coding |
As the loss of information in a wireless channel is a possibility due to the fundamental uncertainty associated with wireless communications, information theory dictates the use of coding schemes that send a "code" representing information rather than the information itself. Codes provide a degree of resilience that counter, at least to some degree, the possibility of errors in communication that is always present. Codes usually result in more bits being transmitted than are otherwise required to represent transmitted information and are also often designed to equalize the number of positive and negative charges placed on the transmission medium. Codes are often described in the form Db/Cb, where Db is the number of data bits that the code represents, and Cb is the number of channel bits transmitted. For example, 8b/10b indicates a code where 10 bits are sent to represent eight. This notation is often also expressed as a rate, such as 1/2, 2/3, etc. Larger ratios here can serve to improve reliability at the expense of capacity. Finally, codes can also be used for automatic error detection and correction of many errors that occur during transmission, improving overall reliability and thus capacity as well. |
| Congestion |
Congestion is a phenomenon that occurs when the capacity of a channel is oversubscribed, leading to the requirement for queuing that delays access to the channel, at least for some traffic. Techniques designed to address congestion include prioritization schemes such as quality of service (QoS) and class of service (CoS); data compression; and the addition of more capacity via the utilization of additional bands or channels, or the denser deployment of base stations (typically using small cells) or Wi-Fi access points. |
| Decibel (dB) |
A measure of relative signal strength on a logarithmic scale expressed as a ratio relative to a particular measurement standard. For example, with respect to milliwatts (frequently used to specify transmit power) the term is dBm, with 0 dBm usually defined as 1 milliwatt and 30 dBm as 1 watt. Since the scale is logarithmic, power doubles with every additional 3 dB. Antenna gain is also expressed in dB, but with many variants depending upon the type of antenna being referenced. The most common measurement here is dBi, relative to an idealized hypothetical uniform antenna referred to as "isotropic" -- hence the "i." |
| Doppler Shift |
Doppler Shift is an apparent, relativistic change in frequency often observed when one or both ends of a given connection are in motion. In the domain of acoustics, this is the familiar apparent change in pitch of a siren on an emergency vehicle -- the pitch appears higher as the vehicle approaches and lower as it moves away, when in reality the pitch being generated is constant. A similar effect occurs with radio waves as one or both ends of a connection move, and circuitry within a receiver must compensate for this variance over a specified range. A related problem can occur due to variance across the thermal operating range of a given radio system. |
| Electromagnetic Spectrum |
The electromagnetic spectrum, sometimes simply called "spectrum," is a property of the physical universe, and enables the propagation of electromagnetic waves through space or the air. The spectrum itself ranges from direct current (DC), where signals are not vibrating at all, to, in theory, infinitely fast vibrations. The spectrum is divided into bands that are often grouped together based on the physical characteristics of wave propagation at a given frequency, such a microwave or millimeter-wave bands. As there is only one electromagnetic spectrum, making the best use of a given band in a given location (due to fading, radio waves at a given frequency can be re-used over distance) is paramount to regulatory authorities. Some bands are reserved for particular functions, such as the receive-only radio astronomy bands, while others are licensed, at least in one physical location, for exclusive use by the licensee. Some bands are made available on an unlicensed basis for uses such as Wi-Fi and Bluetooth, provided that manufacturers of the radios here certify compliance with appropriate regulations. |
| Error Detection and Correction |
Because of the fundamental uncertainty associated with wireless communications of any form, additional overhead in the form of forward error correction codes may be added to a given transmitted signal. This information can be used by the receiver to detect and even under some circumstances correct errors that can occur in transmission. Not all errors are recoverable, and the option of retransmission is therefore always included in modern radio-protocol implementations. It is possible that successful communications simply cannot occur under certain circumstances due to fading, jamming (intentional interference) or other issues, no matter what error detection and correction facilities might be implemented. |
| Fading |
Fading is a property of transmitted signals that causes them to become weaker. The fading of radio signals occurs at a constant, natural rate, and is phenomenon is known as "flat fading," with signals losing power with the square of the distance between transmitter and receiver. Other types include "shadow fading" caused by to the presence of physical objects between a given transmitter and receiver, multipath or "Rician fading" caused by echoes and reflections of a given signal interfering with itself, and "Rayleigh fading" caused by scattering of the signal. Fading is frequency-selective, with given frequencies in a given relationship between a given transmitter and receiver exhibiting more or less fading relative to each other than might be realized, given all other radio elements held constant, at other frequencies. |
| Frequency |
Frequency is the speed at which a given electromagnetic wave is vibrating. "Wavelength" expresses the same value as a distance of a single 360-degree wave cycle, rather than as a time-based value. |
| Gain |
Gain is an increase in the power of a signal, expressed in dB. Most often provided by an amplifier, gain can also be realized via the clever design of passive antennas, on both the transmit and receive sides of a connection. The corresponding reduction in signal strength is called "loss." |
| Generation (cellular communications) |
The letter "G "is used as shorthand for generation when discussing major advances in the technologies used to implement wide-area wireless communications, often simply called cellular. 1G was analog-only, 2G digital but narrowband, 3G broadband, 4G broadband with higher performance and based on a single technology (LTE), and 5G will have even higher performance and be based entirely on the IP stack. |
| Interference |
Interference is a general term for signals that conflict with a given signal that is intentionally being transmitted. The net effect of interference is the same as that resulting from fading – a given signal in effect becomes weaker, even to the point where the intended receiver can no longer detect the signal. Except in very rare cases, interference in the unlicensed bands is unintentional, resulting from other signals legitimately allowed to use a given channel. Interference, however, can also be intentional, and in this case is known as jamming. Most jamming is illegal under local regulations and is reserved for electronic warfare applications. The most common countermeasures taken in the presence of existing or potential interference involve the use of multiple-access protocols, multiplexing or spread-spectrum techniques, channel adaptation, and switching to a different channel. Boosting transmit power is also sometimes an option, although always subject to regulation. While rare, the potential that a given channel could become so oversubscribed as to become unusable is always present. And, finally, multipath can cause self-jamming in some cases. |
| Line of Sight (LoS) |
Line of sight refers to a clear, unobstructed physical path between a given transmitter and receiver. For many applications, LoS is essential, especially at very high frequencies which only propagate linearly and not very well (if at all) through obstructions. When LoS is not feasible, it may be possible to use additional radios to relay around the obstruction, with the use of mesh techniques (see Wireless Network Topology) an increasingly popular solution. |
| Link Budget (or Link Margin) |
The amount of loss in signal strength (see gain), usually called "path loss," that is permissible before a transmitted signal becomes too weak to be detected and demodulated. The link budget is a function of transmit power, receiver sensitivity, the quality of the radio channel (which can vary from moment to moment), antenna gain, the physical path between endpoints, and many other factors, so it is common to express link budget/margin as a worst-case value. |
| Microwave |
Microwaves are the portion of the electromagnetic spectrum from 1 GHz. to 30 GHz. The frequency range of 1 GHz. to about 6 GHz. has become the most popular home for consumer and organizational communications and is thus now heavily subscribed for both licensed and unlicensed activities, with power management and spectral reuse essential for managing capacity. |
| Millimeter Wave |
Millimeter waves are the portion of the electromagnetic spectrum from 30 GHz. To 300 GHz. These are much more directional than microwaves, but, thanks to advances in electronics, are increasingly being applied (up to around 90 GHz., anyway) in fixed and an increasing number of mobile applications as well. Once requiring exotic and expensive electronic technologies, millimeter waves can now be generated with inexpensive electronic components, increasing their availability, reliability and utilization. |
| MIMO |
An acronym for multiple input, multiple output, and representing a set of techniques that involve the use of multiple independent transmissions and corresponding processing at the receiving end to improve performance and reliability. MIMO is perhaps the most complex and counterintuitive technology ever developed for radio communications and was viewed skeptically for many years because the mathematics involved seemed to violate the laws of physics and other accepted electronic laws. The key to MIMO's success, however, is three-dimensional processing, extending the frequency and time typically used to describe radio to include a third spatial dimension. MIMO is thus sometimes referred to as "spatial multiplexing." MIMO involves the coding of a given signal to be transmitted into multiple, simultaneous signals, each assigned to its own transmit antenna. These are referred to as "streams," and beamforming may also be applied here. Multiple receiving antennas, with the number of these typically being equal to or greater than the number of transmitting antennas, are utilized on the receiving end, along with complex processing. Multipath, usually a challenge for radio designers, is essential in MIMO systems, adding to their counterintuitive nature. While a potentially very large number of streams can be applied to a given implementation, yielding potentially quite remarkable throughput, the physical and cost constraints of real-world products generally limit the number of streams to between two and four, but still with a remarkable improvement over non-MIMO implementations. MIMO is widely applied in wireless LANs and increasingly in wireless WANs, and is expected to be a key element in 5G implementations. |
| Modulation (and Demodulation) |
Modulation is a set of techniques used to modify a carrier wave so as to encode information for transmission. A corresponding demodulation function is required at the receiving end, and the amalgam "modem" is often used to describe the pair. Modulation can be achieved via modifications to the amplitude, frequency, or phase of the carrier wave, creating "symbols" that encode one or more channel bits. Modulation based on the combination of two or more of the above are possible; for example, the very popular quadrature amplitude modulation (QAM) uses a combination of amplitude and phase modulation. There are literally hundreds of different modulation techniques in use. The effectiveness of any given modulation scheme is evaluated via gains in efficiency and/or reliability and is often expressed as bits per Hz. More aggressive modulation schemes with more bits per Hz. can be more efficient but are also more subject to errors in transmission. |
| Multiple Access |
Multiple access techniques are widely applied in communications where multiple unrelated data streams must share a single communications channel. Common multiple-access techniques include those based on allocating the channel (or a portion of it) using fixed or variable time slots (time-division multiple access, or TDMA), a portion of available spectrum (frequency division multiple access, or FDMA), or, using a technique called Code-Division Multiple Access (CDMA), by encoding each stream so that all streams can coexist simultaneously without meaningfully interfering with one another. Most contemporary multiple access schemes are adaptive and often self-optimizing. See also multiplexing. |
| Multiplexing |
Multiplexing is a technique that enables multiple distinct data streams to coexist simultaneously (or apparently simultaneously) in a given data channel. Multiplexing is closely related to multiple access, and the two terms are often used interchangeably. The primary difference, however, is that multiple access refers to multiple independent data streams, while multiplexing can be used for a single data stream. For example, orthogonal frequency division multiplexing (OFDM) is a technique that divides a single data stream into multiple, simultaneous and independent data streams called "tones," each consuming a small portion of the total available bandwidth. It is orthogonal in the sense that these individual streams do not interfere with one another. OFDM has largely replaced CDMA, even as the two perform largely the same function in different ways. OFDM can also be used for multiple access by allocating a subset of the total number of tones to a given stream, allowing multiple streams to be processed simultaneously, albeit with only a portion of the total bandwidth available to each connection. In this case, OFDM is known as orthogonal frequency-division multiple access (OFDMA). |
| Multipath |
Multipath refers to the echoes and reflections of radio waves as they interact with objects in the environment – in other words, a given signal might reach its destination by multiple simultaneous paths. Multipath has historically served as a source of fading or self-jamming and is therefore normally destructive, but MIMO techniques actually depend upon multipath and cannot operate optimally without some degree of multipath present in the environment. Indoor settings are typically rich in multipath, as are urban environments, and the corresponding pervasiveness of multipath has been a key driver in the adoption of MIMO-based solutions. |
| Multi-User MIMO (MU-MIMO) |
A feature of 802.11ac Wave 2 products, MU-MIMO is a technique that enables multiple stations to each receive an independent data stream during a single transmission cycle of a given Wi-Fi access point. It can thus be thought of as a multiple-access technique. MU-MIMO can also be applied in the case of multiple stations transmitting to a single base station (or access point), and it is expected to be included in the upcoming IEEE 802.11ax standard. |
| Noise |
Noise is a term often used to describe any signal present in a channel other than that intentionally being transmitted, and as such can also be used to mean interference. More properly, however, noise is an artifact of (a) the universe itself, given sources such as the cosmic background radiation and other natural sources like the Sun, and (b) noise resulting from the internal function of electronic components like transistors and capacitors. The term signal-to-noise ratio (SNR) is used to describe the relationship of the strength of the desired signal to that of any noise present in the channel, expressed in decibels, with a larger number being desirable. |
| Propagation |
Propagation is the manner in which radio waves move from transmitter to receiver. Considered here are environmental factors, the physical characteristics of specific frequencies, fading and many other elements. Radio-wave propagation is usually non-linear and impossible to predict in any given case. |
| Range |
In general, a term used to describe the distance between endpoints in a given wireless communication. Given the many artifacts associated with radio communications (noise, interference, fading, etc.), the probability that a given radio communication will be successful declines with greater distance, and increases the likelihood that channel adaptation techniques will be utilized in an attempt to compensate – although almost always with at least some compromise to throughput. It is therefore more appropriate to discuss radio performance in terms of rate given a specific range ("rate-vs.-range"), rather than either of these elements alone. In general, it is desirable to keep the range as short as possible so as to maximize throughput and reliability; this strategy has led to the development of small cells (see cell) and, similarly, the dense deployment of Wi-Fi access points. |
| Received Signal-Strength Indication (RSSI) |
RSSI is a method for communicating the quality (as strength) of a received signal to the receiver. RSSI is often visible to end-users as a bar graph on cell phones and an OS function supporting wireless LANs. A low RSSI indicates either excessive distance between transmitter and receiver or fading most often due to environmental challenges. RSSI is imprecise, with each product vendor free to interpret signal strength as they may so desire. RSSI is also often expressed in dBm. |
| Spectrum Regulation |
Spectrum (or spectral) regulation is a function of governmental or quasi-governmental regulatory authorities that assign permissible activities and related parameters, such as maximum allowable transmit power, to specific bands. This function is performed by the Federal Communications Commission (FCC) in the United States. Regulation has technical, political and often economic elements, but regulators usually attempt to match the propagation characteristics of any given band to one or more specific and appropriate applications. |
| Spread-Spectrum |
Spread spectrum is a set of techniques that utilize more radio spectrum than would otherwise be required by a narrowband signal in the interest of improving reliability, especially in the unlicensed bands. In other words, spectral efficiency is traded off against an improvement in reliability. Since interference and fading tend to be frequency-dependent, a spread-spectrum technique known as "frequency hopping" (FHSS) moves the center frequency of a narrowband signal about through a given channel in a pattern known to both transmitter and receiver in order to minimize any impacts from that fading. Direct-sequence spread spectrum (DSSS) is very similar to code-division multiple access (CDMA; see Multiple Access), utilizing multi-bit and often orthogonal codes to represent a single data bit, but without the requirement for multiple access. A number of other techniques including time-hopping and ultra-wideband also fall under this term. The use of spread-spectrum techniques is mandated by the FCC for unlicensed use in certain bands. |
| Throughput |
Throughput is a measure of the speed at which information is sent through a particular channel. The term, however, is imprecise without additional context. For example, Wi-Fi vendors frequently note throughput in their advertising materials that refers only to the maximum theoretical channel speeds, which, due the requirement for coding and the possibility of error resulting from radio artifacts of many forms, are completely unrealistic. Consequently, terms such as "goodput" -- in this case meaning Layer-7 throughput net of retransmissions and other real-world degradations to throughput -- are also sometimes used, especially in testing and performance-evaluation exercises. Throughput should be measured only at Layer-3 or above in order to provide a common basis of comparison for the performance of any communications system. Throughput is sometimes expressed as bits/Hz., although this is more precisely a measure of efficiency. |
| Uncertainty |
Uncertainty results from the various artifacts intrinsic to radio communications, including fading, interference and the inability to precisely determine the path that a given signal will take from transmitter to receiver and allowing for the possibility that no such path may be available at any given moment in time and for reasons that are not always obvious. Radio designers therefore need to apply techniques to compensate for this uncertainty. These include both radio-specific (PHY-layer) techniques and network protocols that can detect, correct, and/or compensate for this uncertainty. |
| Wi-Fi |
Wi-Fi has become the generic term used to described wireless local-area networks ("WLANs"). More properly, Wi-Fi describes a set of specifications produced by the Wi-Fi Alliance, an industry trade association, that enable interoperability and define other features for WLANs based on IEEE 802.11 standards and additional developments within the Alliance. Wi-Fi does not stand for any other term and is not an acronym. It is, however, no longer used as a trademark. |
| Wireless Network Topology |
Topology refers to the logical and/or geometric orientation of a given network implementation. In wireless, three key topologies come into play. point-to-point (PtoP or P2P) means that every node in a given network must be able to communicate directly with every other node. The logistics here can become quite complex, so P2P is limited to single connections between two nodes or for connections among a small number of nodes within close physical proximity to one another. Point-to-multipoint (PtoMP or P2MP) is a "star" configuration where all traffic between nodes in the network (and beyond, via bridging) must go through a single central point. Cellular networks are P2MP with handoff of client traffic to other cells via backhaul facilities. Finally, "mesh" techniques enable the construction of arbitrarily large and complex configurations by enabling traffic to flow through intermediate nodes, which act like switches forwarding traffic not intended for the relaying node itself. Many possible implementations and configurations are possible here, and both infrastructure and client nodes can, in theory, serve as relay points. |
