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.[1]
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).[2]
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.
The radio 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.[3] 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.[4]
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.[5]
The frequency is the number of oscillations per unit time.[6]
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.
Spectrum
Radio spectrum is the section of electromagnetic spectrum that comprises radio waves with frequencies ranging from 3 kHz to 300 GHz.[7] 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.
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.
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.
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.
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.[8]
There are various types of analog modulations: amplitude modulation, frequency modulation, phase modulation etc.
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.[9]
There are various reasons why you need to use modulation:
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.[10]
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 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.
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:
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:
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:
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. 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:
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:
Frequency Stability, Frequency Drift and Phase Instability
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[11].
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.[12]
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.[13]
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.
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 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.[14] 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.
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:
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:
Fading
Fading is a a phenomenon cause by the constructive and destructive interference of two or more copies of the same signal that arrive at the receiver at different times.
Fading is likely to lead to poor communication performance because it causes loss of signal power without reducing the power of noise.
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.
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:
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.
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, the 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.[15]
Antenna Parameters
Gain
The gain is the ratio of the power gain in a particular direction relative to that of an ideal isotropic antenna.
Isotropic Antenna
The isotropic antenna is an ideal, theoretical antenna that radiates the same amount of power in all directions, in a 3D space.. This theoretical type of antenna is used as a reference for measuring the gain.
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.
Antenna Types
For Coverage
The antennas used for ensuring coverage are of two types:
Omni antennas radiate power uniformly in all directions. The power drops with the elevation angle above or below the plane, reaching zero on the antenna’s vertical axis.
Sector antennas radiate power on a particular sector of a circle. The antenna radiates a horizontal beam, shaped as a fan. These antennas have different designs, covering either 60°, 90° and 120° portions of a circle’s circumference.
Note: All things being equal – antenna size, power level, operating frequencies – a 120° sector antenna will have 5 dB more gain than an omni antenna, yet the coverage area will be about the same in both antenna types.
For more information about various antenna types and their radiation patterns, please access the following Cisco article: “Antenna Patterns and Their Meaning”.
For Backhaul
Backhaul antennas are used for increasing the capacity and the quality of service in a wireless network.
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:
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. PAs can be distinguished between themselves by three characteristics:
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.
A duplexer has a number of characteristics:
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.
Tower Mounted Booster
A tower mounted booster (TMB) is a type of equipment used when deploying a network, meant to extend the signal coverage. A TMB will increase the downlink signal with its RF power amplifier and will boost the uplink signal with its low-noise amplifier.
Tower mounted boosters are installed right under the antenna to increase both the receiver sensitivity and the transmit power and prevent cable losses.
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[16]
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 communications field.
A basic SDR implementation is comprised of a computer, equipped with a type of analog-to-digital converter, as a sound card, and an RF front end.[17] Ultimately, this system produces a radio able to tune into different frequencies and receive and transmit a multitude of radio protocols via its software alone.
Through SDR, some of a typical radio’s functions are implemented in easy to modify programmable processing technologies, such as: FPGAs, DSPs, SoCs etc.
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.
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.
Coverage
In telecom, the coverage of a basestation is represented by the geographical area where the station can communicate.
Coverage depends on: the antenna type, the amplifier power levels, the radio propagation and the receiver sensitivity. To be more precise, the coverage level is influenced by: the terrain type, buildings, technology, radio frequency and perhaps most importantly for two-way telecommunications the sensitivity and transmit efficiency of the consumer equipment. Some frequencies provide better regional coverage, while other frequencies penetrate better through obstacles, such as buildings in cities.
Important Note: Coverage is highly dependent on the mobile station’s ability to “see” the antenna, as well as on the antenna’s ability to reach the mobile station.
The ability of a mobile phone to connect to a basestation depends on the strength of the signal. The signal can be boosted by higher power transmissions, more efficient antennas, taller antenna masts etc.
The coverage also depends on the choice of antenna type, be it omni or sector. Some of the factors influencing this choice are the location of the basestation, the terrain, the environment etc.
To measure the coverage, network planners use simple models to roughly calculate what can be expected in a specific environment, or they use more accurate tools that take into account the exact type of environment where the network is to be deployed.
Coverage Maps
Coverage maps to indicate to users the station’s intended service area. The maps may also separately denote supplementary service areas where good reception may be obtained but other stations may be stronger, or where reception may variable but the service may still be usable. Often coverage maps show general coverage for large regions and therefore any boundary indicated should not be interpreted as a rigid limit. The biggest cause of uncertainty for a coverage map is the quality (mainly sensitivity) of receiving apparatus used. A coverage map may be produced to indicate the area in which a certain signal strength is delivered. Even if it is 100% accurate (which it never is), a major factor on whether a signal is receivable depends very much on whether the receiving apparatus is sensitive enough to use a signal of that level.
Coverage in Network Planning
The coverage plan is mostly dependent on geographical and environmental factors: the type of the terrain or if the area is urban, suburban or rural.
The “location probability” is a concept that needs to be determined in order to obtain the best coverage quality in the respective area. Location probability refers to the probability of having a field of strength at a higher level then the sensitivity of the target area. This prediction is typically performed through radio propagation models.
There are standard models used depending on general characteristics of the designated network area, or if need be, network planners can design their own propagation models. Using standard propagation models leads to lowering costs and saving time, yet they are also less accurate then the personalized propagation models.
To obtain coverage areas predictions, network planners need to have used propagation models, performed drive tests and established correction factors.
For a very efficient coverage of the GSM signal, planners can use both omnidirectional and sector antennas. Ensuring the coverage for a particular area implies mounting more antennas, and each antenna creates a radio cell.
The distance between the antennas must be thoroughly thought out. If the antennas are too close to one another, they will overlap, and the network deployment will be very costly. At the same time, if they are installed too far away from one another, there will be areas without GSM coverage.
If the planner chooses to use omni antennas, the following formula needs to be used for calculating the coverage area:
2.6km × r2
For calculating the distance between the antennas, the planner will use the following formula:
1.73 × r
If the planner opts for sector antennas, the coverage area will be calculated with the formula:
1.59km × r2
The distance between the antennas will be calculated with the formula:
1.5 × r
Keep in mind that these formulas are only representative for two sector antennas.
Note: To obtain the total number of basestations to be deployed, divide the total coverage to the basestation coverage area.
Link Budget
After finishing the design, a network planner needs to calculate the link budget. This ultimately leads to a network design that functions correctly according to all requirements at a reasonable cost. 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.
The 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.
Radio frequency propagation models
Hata Model
The Hata model is probably the most efficient radio frequency propagation model for predicting mobile communications in suburban areas. These suburban areas have distances between buildings that are larger than the buildings themselves. It predicts the total path loss experienced along a link of terrestrial cellular radio transmissions.
However, this model is not recommended for scenarios where the antennas are below the rooftop levels.
Requirements
The carrier frequency for this model must be from 150 to 1500 MHz. The basestation antenna height must be from 30 to 200 meters. The distance between the basestation and the mobile station must be from 1 to 20 km.
COST231 Model
COST231 is a radio propagation model that extends the urban Hata model to cover more complex frequencies. It is mostly suited for urban areas.
This model works best if the antenna is at the rooftop level or just below it.
Requirements
The carrier frequency for this model must be from 1500 to 2000 MHz. The basestation antenna height must be from 30 to 200 meters. The distance between the basestation and the mobile station must be from 1 to 20 km.
Walfisch-Ikegami Model
The Walfisch-Ikegami radio propagation model works best in urban environments with buildings in the vertical plane between the transmitter and the receiver.
This propagation model is most efficient when the antenna is above the roof height. It takes into account various factors, such as the density and height and width of the buildings, the width of the streets or the direction of the streets in relation to the direct trajectory of the antenna and the mobile station.
Note: Each site has a particular capacity. To obtain the total number of basestations, after having performed the capacity estimation, divide the total number of subscribers by the capacity of each site.
Our solutions
Requirements
The carrier frequency for this model must be from 800 to 2000 MHz The basestation antenna height must be from 4 to 30 meters. The distance between the basestation and the mobile station must be from 0.02 to 5 km.
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