Radio Concepts

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This page describes concepts and notions relevant to better understanding GSM (2.5G), its specifications, and how YateBTS was designed to work as a software defined radio.

Radio Waves Related Concepts

Radio waves

The wave, as an element in physics, is an oscillation transferring energy or information, which travels through space and matter. The most common examples of waves are: light, sound or water waves.

There are two main types of waves: mechanical waves and electromagnetic waves. While mechanical waves transmit energy in a material medium, electromagnetic waves travel though space itself. Electromagnetic waves consist of periodic oscillations of electrical and magnetic fields generated by charged particles. These types of waves vary in wavelength and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. Its respective frequency range is what determines a wave's type.

Radio waves are a type of electromagnetic radiation with frequencies ranging from 300 GHz to as low as 3 Hz, and wavelengths ranging from 1 millimeter (0.039 inches) to 100 kilometers (62 miles).

Similar to all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are made by lightning or by astronomical objects. Artificially generated radio waves are used in a large number of applications: such as fixed and mobile radio communication, broadcasting, radar, communications satellites, computer networks etc.

Radio waves frequencies for 2.5G networks are further explained in the GSM Concepts section.

Phase, Amplitude, Wavelength and Frequency

In typical representations, the wave has a sinusoidal movement. Below you can see a diagram illustrating the phase and the amplitude of a wave in the time domain.

File:wave.png

The phase is the distance between the point of origin of any given wave and its first zero crossing.

Phase can also refer to the difference between two waves which are at the same frequency and referenced to the same point in time. If the two waves have no difference, they are in phase. However, if they they have the same frequency and different phases, they are out of phase with each other. This phase difference can be expressed from 0º to 360º (degrees), or from 0 to 2π (radians).

The amplitude (or height) is the distance between the middle line of a wave and either its crest or through.

Any give wave will have a wavelength which is defined by the distance between two consecutive locations, such as crests or zero crossings, that are in phase with each other.

The frequency is the number of oscillations per unit time.

The wavelength is inversely proportional to the frequency, i.e. the higher the frequency of a wave, the shorter its wavelength and viceversa.

Radio frequencies range from around 3kHz to 300 GHz. See below an illustration of high and low radio frequencies.

Frequency.png

Spectrum

Radio spectrum is the section of electromagnetic spectrum that comprises radio waves with frequencies ranging from 3 kHz to 300 GHz. Essentially, there are two main domains of representing radio waves: the time domain and the frequency domain.

A time domain illustration of a particular signal shows the how it changes over time. As seen in the diagram below, there are two waves in the same frequency, represented in the time domain. However, the second wave (in blue) leads by 90º, thus causing the two waves to be out of phase with each other.

Waves timedomain.png

In the next diagram, you can see two waves with different frequencies and, in purple, a third, which is the sum of the first two. The frequency of a wave is correlated to its number of oscillations: the higher the frequency, the more oscillations the wave will have.

Summing waves.png

The frequency domain representation typically shows how much signal each frequency band (from a range of frequencies) carries. A spectrum of more frequencies is the frequency domain representation of the signal.

The illustration below represents the two waves with different frequencies from the previous diagram, in the frequency domain. The image, therefore, shows the blue wave on the higher frequency scale. The red wave, with a higher amplitude, has more power.

Waves frequencydomain.png

To view the signal in the frequency domain, one must use a tool called spectrum analyzer. This device can either analyze the entire signal, or short segment of it. The diagram below illustrates the spectrum of a non-trivial signal (be it data, audio etc.), as seen in the spectrum analyzer.

Spectrum analyzer.png

Simple Radio Modulation and Tuning

In electronics and telecommunications, modulation is the process of mixing the low frequency baseband signal with a modulating (radio frequency) carrier signal for the purpose of transmitting information.

In radio communications, the bandwidth is the range of higher and lower frequencies in a continuous set of frequencies. Its main characteristic is that it can hold the same amount of information irrespective of where it is located on the frequency spectrum.

There are various reasons why you need to use modulation:

  • to transmit more simultaneous signals over the same channel of communications at different frequencies
  • to reduce the baseband signal's wavelength, by shifting it to a higher frequency and, thus, facilitating the transmission with a much smaller antenna

For example, AM or amplitude modulation is a technique that modulates the amplitude of the carrier signal in proportion to the message signal. In AM radio communications, the radio signal is transmitted in a continuous wave with its amplitude modulated by an audio waveform before transmission.

In the example below, you can see the graphical representation of a basic principle of modulation. First, the baseband signal, s(t), is modulated by multiplying it with a radio frequency carrier signal, 2πfct, obtaining a modulated signal, s(t)sin2πfct. Next, the the modulated baseband signal, containing the same information, is shifted on a much higher frequency. Another important aspect is the presence of the negative frequency, a mirror image of the spectrum on the positive side.

Note: When multiplying real valued signals, the positive and the negative frequencies are mirrors of the same spectrum. For this reason, we only analyze and take into account the spectrum on the positive side.

Radio modulation positiveandnegative freq.png

Fundamental digital modulation methods

In digital modulation, a discrete signal modulates an analog carrier signal. Digital modulation methods can be considered as digital-to-analog conversion, and the corresponding demodulation or detection as analog-to-digital conversion.

Below are the primary digital modulation techniques:

  • PSK (phase-shift keying): a finite number of phases are used.
  • FSK (frequency-shift keying): a finite number of frequencies are used.
  • ASK (amplitude-shift keying): a finite number of amplitudes are used.
  • QAM (quadrature amplitude modulation): a finite number of at least two phases and at least two amplitudes are used.

Note: GSM uses GMSK (Guassian minimum-shift keying), a form of FSK.

Legal Aspects

Radio spectrum is a public national resource and almost all national government have strict legislation that allows them to regulate it.

Spectrum Management

At a national level there are coordinating organizations that perform spectrum management through a series of measures:

  • elaborate and adopt legislation regulating the national radio spectrum
  • plan, allocate and designate the entire nation radio spectrum
  • manage, license and coordinate radio spectrum
  • monitor and control the use of the national radio spectrum

The RF spectrum is a reusable national resource and is a property of each national state. Is a natural monopoly and, as such, there is typically one regulator per RF band.

The International Telecommunication Union (ITU) is the international coordinating organization managing the shared use of radio spectrum.

ITU Regions

ITU has defined three regions for managing the global radio spectrum. These regions are:

  • 1. Europe, Africa, the Middle East west of the Persian Gulf including Iraq, the former Soviet Union and Mongolia
  • 2. the Americas, Greenland and a few Eastern Pacific Islands
  • 3. most of non-former-Soviet-Union Asia, east of and including Iran, and most of Oceania

Please refer to this link for a world map of all the regions.

GSM Standardized Bands

There are four globally standardized GSM bands, as you can see in the table below:

System Band Uplink (MHz) Downlink (MHz) Region
GSM 850 850 824 – 849 869 – 894 North America, the Caribbean and Latin America
E-GSM 900 900 880 – 915 925 – 960 Europe, the Middle East, Africa and Asia-Pacific
GSM 1800 1800 1,710 – 1 ,785 1,805 – 1,880 Europe, the Middle East, Africa and Asia-Pacific
GSM 1900 1900 1,850 – 1,909 1,930 – 1,989 North America, the Caribbean and Latin America

Note: E-GSM or Extended GSM-900 Band and includes the Standard GSM-900 band.

Radio Performance Concepts

Sensitivity

Sensitivity is a concept that refers to the minimum usable signal level at the receiver. In the case of the GSM handset, the sensitivity level is around -100 dBm, while for the BTS this level reaches approximately -106 dBm. This difference appears due to the higher quality electronics used in the basestation.

Signal-to-Noise Ratio (SNR)

Signal-to-noise ratio is a method that calculates the ratio of the desired signal power to the background noise power and is expressed in decibels.

Any ratio higher than 1:1, or greater than 1 dB, means that there is more signal than noise.

Receiver Error Rates and Error Correction Coding

Bit Error Rates

In digital transmissions, bit errors represent the number of alerted bits entering a data stream through a communication channel. The bits can be distorted by noise, interference or bit synchronization errors.

The bit error rate is the ratio between the bit errors and the total number of transferred bits during a particular time.

A receiver's bit error rate can be affected by noise, interference or multipath fading. To improve it, one must use a more robust modulation scheme that would lead to transmitting a stronger signal, or to apply error correction coding schemes.

There are two types of bit error rates:

  • transmission bit error rate – the number of incorrect bits divided by the total number of transmitted bits
  • information bit error rate – the number of decoded bits that remain incorrect after applying error correction schemes divided by the total number of decoded bits

Usually, the transmission bit error rate is greater than the information bit error rate.

Error Correction Coding

Error correction coding is a method of controlling errors during data transmissions over noisy channels of communication, using redundant message encoding. The main purpose of error correction coding is to allow the receiver to spot a number of errors during the message transmission and to correct them without retransmission and, as such, without a reverse channel for the retransmission.

Thermal Noise

Thermal noise is the electronic noise produced by the natural motion of the electrons in a receiver's atoms. It largely affects the receiver's quality. If your receiver is made out of atoms, then it will automatically produce thermal noise.

This phenomenon is directly proportional to resistance and temperature, therefore, the lower the temperature, the lower the thermal noise. Nevertheless, a change of 20-30°C in temperature doesn't make any difference on the decibel scale.

The natural thermal noise power of a GSM radio channel is of -120 dBm. However, this power mostly depends on the channel's bandwidth.

Note: A difference between the natural thermal noise power of a GSM radio channel of -120dBm and the sensitivity of a BTS of -106 dBm can be explained in two ways:

  • the unavoidable differences in the receiver
  • the signal to noise ratio requires for the signal to be decoded correctly

Frequency and Phase Stability

Frequency stability represents, as its name suggests, the stability in frequency over time, or the measure to which the signal can produce a stable frequency for an certain amount of time[1].

Frequency Drift

Frequency drift is a nonlinear phenomenon that causes unwanted progressive changes in the frequency in time.

Frequency drift can cause interference when, for example, a radio station switches to an adjacent channel. This phenomenon appears when the radio components are old or have flaws or during thermal changes.

Phase Instability

Phase instability, or phase noise, refers to the speedy, short-term variations caused by a signal's random frequency variations. In a spectrum analyzer, these fluctuations would appear as a noise spectrum on either side of the signal.

It is common phenomenon and it is widely believed that all signals experience a certain amount of phase noise. When the phase noise is too strong, it affects the signal quality and may increase the bit error rate in radio systems using phase modulation.

Radio Propagation Concepts

Power

The concept of power, in physics, refers to the an amount of energy consumed per unit time. There are various units of measuring power: the joule per second, the watt, the horsepower etc.

In radio, microwave and fiber optic networks the unit of measure of power is the Decibel-milliwatt (dBm). The dBm is an electrical power unit in decibels (dB) referenced to 1 milliwatt (mW).

Here is the formula for representing the power in dBm:

P(dBm) = 10 · log10(P(mW) / 1mW)

The transmit power (Tx) is the energy transmitted through a specific bandwidth, generated by the radio into the Radio Frequency (RF). Tx power is typically measured in dBm or W.

The receive power (Rx) is the energy of the receive signal and is also measured in dBm.

Below you have a conversion table with a few examples.

dBm Linear
0 dBm 1 mW
30 dBm 1 W
-30 dBm 1 μW

Path Loss

Path loss, as a concept in radio communications, refers to the phenomenon of power density decrease of an electromagnetic wave, as it propagates through space. Path loss is a determining factor in analyzing the link budget (accounting of all gains and losses from the transmitter to the receiver through a particular medium)of a telecom system.

Path loss can be caused by various factors: refraction, diffraction, free-space loss, reflection, aperture-medium coupling loss or absorption. Other variables in determining the path loss are the environment, the type of terrain, the medium of propagation, the distance between the emitter and the receiver or the type and mounting of the antennas.

Path losses occur during the natural expansion of radio waves in the free space, when the signal is obstructed by an impenetrable obstacle or when the signal's medium of transmission is not transparent to electromagnetic waves.

Multipath is an effect related to path loss and is caused by a signal transported from the transmitter to the receiver through multiple different paths. Thus, the signal arriving at the receiver is variable, depending on the distribution of intensity, and the propagation time, and the bandwidth of the transmitted signal.

Small scale fading is another phenomenon caused by rapid changes in the radio signal amplitude in a short time frame or on a short distance.

Prediction Models

Free space path loss, also expressed as 1/r2 is an elementary model to be considered when designing a radio communications system. It is the standard free space loss caused by the expanding wavefront area, as the wave travels through free space.

Other widely used path loss prediction models are: Hata, Cost231 or Walfisch-Ikegami. They are based on measured and averaged losses through various classes of radio links.

Path loss.png

Link Margin

Link margin is the result of the difference between the receiver's sensitivity and the the actual received power, and is measured in dB.

It is a system performance parameter.

The link margin value indicates three distinct outcomes:

  • link margin equals 0 – the system is on the edge of proper functioning
  • link margin is greater than 0 – the system can tolerate a certain amount of attenuation of the RF power
  • link margin is less than 0 – the system is not set up properly and cannot transfer the information; a receiver with better sensitivity is needed

Link Budget

Link budget takes into account all the gains and losses from the transmitter to the receiver from: path losses, antennas, antenna feeders, power levels and receiver sensitivity. It is a key element in designing a mobile network and ultimately leads to a network design that functions correctly according to all requirements at a reasonable cost.

Link budget can be calculated with the below formula:

RP = TP + G − L 

where: RP – received power (dBm)

TP – transmitted power (dBm)

G – gains (dB)

L – losses (dB)

The factors that determine link budget can vary in time, as path loss does, and, in such cases, the worst case scenario will be taken into account. The link budget calculation essentially leads to determining the cell size by accounting for the maximum allowed path loss without affecting the communication quality.

Range

In mobile telecommunications, the range is the usable distance determining the reach of the radio wave propagation.

Range.png

To find out what the range is in a mathematical manner, the equation below may be used:

PR = PT + G − LP

where: PR – received power

PT – transmitted power

G – the combined antenna gains at Tx and Rx

LP – path loss.

The range is defined as the maximum distance at which the received power (PR) is greater than the sensitivity, which can be symbolized as PS , in both uplink and downlink. Path loss (LP) increases with distance, and is symmetric in uplink and downlink, but since the transmitted power (PT) and the received power (PR) are different, the link itself may not be symmetric. Therefore, the range of a basestation is determined as the distance that allows a maximum path loss value without losing connectivity.

The formula used above is also a simplified version of calculating link budget.

The range is variable and various factors influence it:

  • the radio masts – higher radio masts increase the range
  • the space – open and flat spaces vs. urban spaces, with high buildings, forests, mountains etc.
  • the antennas used – a sector antennas have greater range than omni antennas
  • the frequency band – low-band (850/900) systems have better range than high-band (1800/1900) systems

Propagation Delay

In wireless communications, propagation delay refers to the amount of time a signal's first bit travels from the transmitter to the receiver. It is dependent on the propagation medium, but also on the type of electromagnetic signal.

It can be calculated as the ratio between the distance and the signal's propagation speed (D/S). In wireless communications, the speed equals the speed of light.

In the case of fiber or copper wires, the speed is in the range of 2 * 108 meters/second.

Interference

Interference is the phenomenon that disrupts a signal as it travels on a channel from the transmitter to the receiver. The disturbance may interrupt, obstruct, degrade or limit the effective reception of signals. These effects can range from a simple degradation of data to a total loss of data.

There are several types of interference, such as:

  • Co-channel interference – caused by two transmitters using the same frequency
  • Unintentional interference (EMI) – caused by a source that radiates power in the same range as other equipments
  • Adjacent-channel interference (ACI) – caused by the external power of a signal from an adjacent channel: inadequate filtering, poor frequency control etc.

Interference.png

Fading

In radio communications, signals travel through different types of matter such as concrete, wood, air or fog, which can limit the range of the signals. This phenomenon is called attenuation. Fading deviates the attenuation affecting a signal as it travels through a propagation media. Fading is likely to lead to poor communication performance because it causes loss of signal power without reducing the power of noise. It is essentially caused by constructive and destructive interference.

Inphase outofphase.png

Understanding the effects of interference and fading is an important aspect in GSM networks because the environment or area type (urban, rural) has a big impact on the design of the network. Depending on building's layouts and population density, operators need to deploy more cells in cities, while the same cell can cover a significantly larger rural area.

A fading cause can also be multipath propagation or signals that interfere with the signal's propagation. The latter leads to an effect that is sometimes called shadow fading.

Fading2.png

Multipath Propagation

Multipath is a propagation phenomenon that causes the transmitted signal to be sent on two or more paths to the receiver. The most frequent causes of multipath propagation are:

  • refraction
  • reflection from water sources or objects such as mountains and buildings
  • atmospheric ducting
  • ionospheric reflection

Therefore, multipath propagation causes the reflected radio waves to interfere with the direct line of sight radio waves, resembling a typical echo effect. This is a common phenomenon and mobile networks are designed to minimize the damaging effects of reflections.

Multipath.png

Radio Electronics

Antennas

The antenna is a key component of any equipment that uses radio or is an electrical device that emits and/or receives radio waves. It is typically used with a radio transmitter or a radio receiver. A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing.

In transmission, the radio transmitter supplies an oscillating radio frequency electric current to the antenna's terminals, and the antenna radiates the energy from the current as radio waves.

In reception, an antenna intercepts some of the power of an electromagnetic wave in order to produce a tiny voltage at its terminals, that is applied to a receiver to be amplified.

Antenna gain

Antenna gain a performance indicator describing how well an antenna converts the power received into radio waves directed towards a specific target.

Gain

Gain is the ratio of the power gain in a particular direction to the power gain of an ideal lossless isotropic antenna.

Radiation Pattern

Radiation pattern is the representation of the antenna's radiation distribution, usually in a spherical graph.

Front to back ratio

Front to back radio describes the ratio between the peak gain in the front of the antenna to the 180° gain behind the antenna.

Beamwidth

Beamwidth is the defined angle between the half power (-3 dB) points to the main lobe of radiation in a circular pattern; measured in degrees.

You can see a typical 3D radiation pattern, azimuth plane pattern and elevation plane pattern for a Yagi-Uda antenna below.

File:yagi.png

Amplifiers

An amplifier is an electronic device meant to increase a signal's power.

There are two main types of amplifiers that are essential in radio communications:

  • RF power amplifiers
  • Low-noise amplifier

The RF power amplifier (PA) has the role of boosting the power of a high frequency, high power signal.

Power amplifiers are used to increase voice and data signals that are sent or received through an antenna. To increase their efficiency, they must accomplish a number of conditions: to have a high output power compression, optimal head dissipation, good gain and good return loss on the input and output.

A low-noise amplifier (LNA) has the role of increasing very weak signals received by an antenna. It is typically installed very close to the antenna to be more efficient.

Duplexers

The duplexer is an electronic device meant to allow both the receiver and the transmitter to use the same antenna.

It does so by isolating the receiver from the transmitter.

Duplexers have two main roles: to eliminate any sideband noise coming from the transmitter on the receiving frequency and to attenuate the transmitter carrier, thus preventing the receiver to overload.

Software-Defined Radio

Software-defined radio is the "radio in which some or all the physical layer functions are software defined", according to a definition of the Wireless Innovation Forum in collaboration with Institute of Electrical and Electronic Engineers P1900.1 group.

To be more exact, it is the type of radio that implements as software components that are usually found as hardware: filters, modulators, demodulators, mixers etc. The implementation is done on a computer or an embedded system.

SDR was initially employed in the military, but it has become the most used technology in the radio communication field.

Radio technology exists in a variety of common use objects such as: mobile phones, TVs, cars, computers etc. Before SDR it was difficult and expensive to bring modifications to traditional radio hardware. In the SDR era, modifications are done by simply performing software upgrading to wireless devices.

Through SDR, some of a typical radio's functions are implemented in easy to modify programmable processing technologies, such as: FPGAs, DSPs, SoCs etc.

Basic Radio Design

Below is a simplified basic radio design diagram containing a digital radio transceiver, a control processor, a power amplifier, a low-noise amplifier, a duplexer and an antenna.

Basic radio design.png

Notes

  1. Time and Frequency from A to Z. "Frequency Stability"; [cited 2014 Jan 23]. National Institute of Standards and Technology, Boulder, CO, USA. Available from: http://tf.nist.gov/general/enc-f.htm#frequencydrift

Some of the material here is used under the CC-BY or CC-BY-SA license. The original source document is at: