Bandwidth
This describes the amount of electromagnetic spectrum needed or allocated for a particular communications channel or group of channels. It is usually defined in units of frequency and is computed as the difference between an upper and lower band edge limit. Example: (1) US PCS is defined as 1850 to 1990 MHz, and therefore has an allocated bandwidth of 140 MHz. (2) Individual PCS voice circuits operate on much narrower channels within the PCS band, requiring only 30 kHz of bandwidth for each conversation. Beamwidth
Beamwidth is a measurement of the antenna's radiation pattern. It is defined as the -3dB points (relative to the direction with the highest gain ) where the intensity falls off by ½ power. It is measured in degrees, representing an angular measurement of how wide the pattern is dispersed. Because the RF radiation is 3-dimensional, it is usually measured in two angular directions, azimuth and elevation. Example: Parabolic antennas have very high gain over a very narrow beamwidth, usually less than 5 degrees in azimuth. Directional
Antenna
This type of antenna transmits or receives maximum power in a particular direction. This design usually has much higher gain in the desired direction at the expense of the radiation in other directions. These designs are physically adjusted so that it points to the corresponding receiving (or transmitting) radio. (Opposite: See Omni-Directional Antenna) Example: Antennas designed to communicate with geosynchronous satellites or television broadcasts are directional antennas because the satellites and broadcast towers are fixed in location and do not move. Directivity
Directivity is a measure of how focused an antenna radiation pattern is in a given direction. This is similar to gain , but directivity disregards efficiency (heat losses) Efficiency
Efficiency is a measure of how much of the electrical power supplied to an antenna element is converted to electromagnetic power. A 100% efficient antenna would theoretically convert all input power into radiated power, with no loss to resistive or dielectric elements. Gain
Antenna gain is measure of the directive property of the antenna, as well as how efficiently it transforms available input power into radiated power as compared to a theoretical antenna element. It is usually measured in units of dBi (decibels as referenced to an isotropic antenna element) or dBd (decibels as referenced to a dipole antenna element, where 0 dBd = 2.1 dBi). An isotropic antenna is a theoretical point source radiating equal power in all directions, resulting in a perfect spherical pattern. This ideal reference point is defined to be 0 dBi. New antenna designs are usually compared to this common reference level. Multi-Band
Communication standards are assigned to a range of frequencies that are referred to as a frequency band. Multi-band refers to a radio that is designed to communicate in more than one frequency band. Example: Tri-band GSM mobile phones, support the 900, 1800, and 1900 MHz frequency bands. Omni-Directional
Antenna
This type of antenna radiates maximum power uniformly in all directions. The isotropic antenna has a theoretically perfect omni-directional radiation pattern and is used as a reference for specifying antenna
gain. In practice, omni-directional antennas provide a uniform radiation pattern in one reference plane. (Opposite: See Directional Antenna) Example: Handset antennas are usually designed to be omni-directional because the position of the wireless service provider's tower is not normally known by the user. Pattern
An antenna pattern is related to the 3-dimensional shape of the antenna's radiation field. It is sometimes subjectively described by how it looks, but it is usually objectively measured in azimuthal and elevation plots. Plots of gain vs. direction show 2-D cuts in different planes of the 3-D total shape. Example: The antenna used in cell phones typically has an omni-directional pattern with nulls in the zdimension, which is sometimes described as a donut pattern. Polarization
Polarization is a parameter describing the position and direction of an antenna's electric field with reference to the earth's surface. Linear polarization: The electric field is either parallel to the ground (horizontal polarization) or the electric field is perpendicular to the ground (vertical polarization) Circular polarization: The wave spins as it travels, covering all angles. polarization describes the direction in which it is spinning
Right or left-handed
Example: Satellite signals are normally circularly polarized, having both vertical and horizontal components rotating around the z-direction of wave propagation. Return
Loss
Return loss is a measure of the difference between the power input to and the power reflected from a discontinuity in a transmission circuit. Return loss is often expressed as the ratio in decibels of the power incident on the antenna terminal to the power reflected from the terminal at a particular frequency or band of frequencies. (Also see Voltage Standing Wave Ratio) Example: SkyCross antennas are all designed to have a return loss of -10dB or less, meaning that at least 90% of the electrical energy generated by the radio is actually transferred into electromagnetic wave energy. Specific Absorption Rate (SAR)
This is a measure that estimates the amount of radio frequency power absorbed in a unit mass of body tissue over time. In the interest of ensuring public and user safety, the FCC and other regulatory bodies have developed safety standards for mobile phone radio frequency emissions. All cellular and PCS phones manufactured after August 1, 1996 must be tested for compliance against these FCC guidelines for safe exposure. Example: The limit for SAR in Australia, United States, and Canada is 1.6 milliwatts per gram. Voltage Standing Wave Ratio (VSWR
This parameter is another way to measure return loss. VSWR is a ratio of the maximum to minimum amplitude (or the voltage or current) of the corresponding field components appearing on a line that feeds an anten
First let us take the definition of link budget, what it is exactly? Link budget is only a mathematical calculation in which you take consideration of losses and gains when a signal is transmitted through any medium (either wirelessly or wired). It is necessary to calculate the losses (more importantly) so that the link that establishes between a transmitter and receiver doesn't lose its connection. Now there are several factors that are taken in to consideration when you are calculating the losses. Majorly there are two factors: which are Attenuation and Dispersion (Fiber optics). Random Attenuation or fading is not calculated in Link Budget because it is supposed to be considered in diversity issue which is a separate discussion. Now lets go a step further, I'll try to explain the link budget in GSM first and then in later technologies like 3G and WiMAX. Earlier i have mentioned that calculating path loss is a part of calculating link budget. Path loss, in wireless communications includes the losses when a signal propogates in air (free space) and the losses when a signal is reflected from any object. In GSM, we know that the architecture is based on cells. In which every cell could be sectorised (mostly it is , except micro-cells or pico-cells). When a cell is sectorised then it means there are different channels that are transmitted in each sector. Also, you know that when you are calculating the link budget then you have to decide whether the receiving end is at LOS (line of sight) or NLOS (non-line of sight). GSM is based on FDMA-TDMA technology, which means there are multiple frequencies which can interefere each other (again a different topic to minimize interference). If suppose you are at LOS then you can use a Free Space Path Loss Model to calculate the path-loss. It is FSL (db) = 20log(d) + 20log(f) - 147.55 where d is the distance between the transmitter and receiver, f is the signal frequency. This model is only used when both transmitter and receiver are at LOS, if both are at NLOS then it is better to use other models, such as Okumara-Hata Model which you can search on internet (sorry for not writing it over here as it is very long). One thing is most important here is that every Model has got its limitations. for example Hata model can only be used if the coverage frequency range is between 200Mhz and 1900Mhz. It means Hata model can be used in 2G and 3G path loss calculations but not in WiMAX. These models are not cent % correct but it roughly gives an idea of the path loss. We all know when the signal is transmitted into the air there are several factors that effects its transmission therefore don't think that if i said it that this model should be implemented to calculate the path loss then it is correct. Our basic object is to establish such a link in which signal strength overcomes the path losses (link budget). Other more important thing is that location of a receiver is very important. We should consider it when calculating the link budget because calculations are different for rural, urban , sub-urban and dense-urban areas. Hata Model is also defined differently for these areas. What i have explained earlier is only a preview, if you go further in detail then i recommend you to read on internet about using which model in what conditions. Few conditions i can briefly describe below : Weissberger's Model (alternate of ITU Vegetation Model) --- Use it when there are one or more trees in point to point communication. (NLOS), Frequency range is 230Mhz to 95Ghz (You can use it in WiMAX deployments).
Egli Model --- LOS propogation only, it is more suitable when you are travelling on irregular terrain, not sure about the frequency range of it. (You can use it in 2G/3G i guess). Similarly there are other models available, but it only gives an idea or rough estimate of path loss. These models are made by collecting data at different times and in different conditions therefore no one can say that this model is cent % correct. What i have written means that Link Budget can be the same and can be different for all these technologies. It only depends on the conditions which are effecting the signal transmission.
RF Terms and Definitions dB The dB convention is an abbreviation for decibels. It is a mathematical expression showing the relationship between two values. RF Power Level RF power level at either transmitter output or receiver input is expressed in Watts. It can also be expressed in dBm. The relation between dBm and Watts can be expressed as follows: PdBm = 10 x Log Pmw For example: 1 Watt = 1000 mW; PdBm = 10 x Log 1000 = 30 dBm 100 mW; PdBm = 10 x Log 100 = 20 dBm For link budget calculations, the dBm convention is more convenient than the Watts convention. Attenuation Attenuation (fading) of an RF signal is defined as follows:
Pin is the incident power level at the attenuator input Pout is the output power level at the attenuator output Attenuation is expressed in dB as follows:PdB = 10 x Log (Pout/Pin) For example: If, due to attenuation, half the power is lost (Pout/Pin = 2), attenuation in dB is 10 x Log (2) = 3dB Path Loss Path loss is the loss of power of an RF signal travelling (propagating) through space. It is expressed in dB. Path loss depends on: The distance between transmitting and receiving antennas. Line of sight clearance between the receiving and transmitting antennas. Antenna height.
Free Space Loss Attenuation of the electromagnetic wave while propagating through space. This attenuation is calculated using the following formula: Free space loss = 32.4 + 20xLog F(MHz) + 20xLog R(Km) F is the RF frequency expressed in MHz. R is the distance between the transmitting and receiving antennas. At 2.4 Ghz, this formula is: 100+20xLog R(Km) Antenna Characteristics Isotropic Antenna
A hypothetical, lossless antenna having equal radiation intensity in all directions. Used as a zero dB gain reference in directivity calculation (gain). Gain
Antenna gain is a measure of directivity. It is defined as the ratio of the radiation intensity in a given direction to the radiation intensity that would be obtained if the power accepted by the antenna was radiated equally in all directions (isotropically). Antenna gain is expressed in dBi. Radiation Pattern
The radiation pattern is a graphical representation in either polar or rectangular coordinates of the spatial energy distribution of an antenna. Side Lobes
The radiation lobes in any direction other than that of the main lobe. Omni-directional Antenna
This antenna radiates and receives equally in all directions in azimuth. The following diagram shows the radiation pattern of an omni-directional antenna with its side lobes in polar form.
Directional Antenna
This antenna radiates and receives most of the signal power in one direction. The following diagram shows the radiation pattern of a directional antenna with its side lobes in polar form:
Antenna Beamwidth
The directiveness of a directional antenna. Defined as the angle between two halfpower (-3 dB) points on either side of the main lobe of radiation. System Characteristics Receiver Sensitivity
The minimum RF signal power level required at the input of a receiver for certain performance (e.g. BER). EIRP (Effective Isotropic Radiated Power)
The antenna transmitted power. Equal to the transmitted output power minus cable loss plus the transmitting antenna gain. Pout Output power of transmitted in dBm Ct Transmitter cable attenuation in dB Gt Transmitting antenna gain in dBi Gr Receiving antenna gain in dBi Pl Path loss in dB Cr Receiver cable attenuation is dB Si Received power level at receiver input in dBm Ps Receiver sensitivity is dBm Si = Pout - Ct + Gt - Pl + Gr - Cr EIRP = Pout - Ct + Gt Example: Link Parameters: Frequency: 2.4 Ghz Pout = 4 dBm (2.5 mW) Tx and Rx cable length (Ct and Cr) = 10 m. cable type RG214 (0.6 dB/meter) Tx and Rx antenna gain (Gt and Gr) = 18 dBi Distance between sites = 3 Km Receiver sensitivity (Ps) = -84 dBm Link Budget Calculation EIRP = Pout - Ct + Gt = 16 dBm Pl = 32.4 + 20xLog F(MHz) + 20xLog R(Km) @ 110 dB Si = EIRP - Pl + Gr - Cr = -82 dBm In conclusion, the received signal power is above the sensitivity threshold, so the link should work.
The problem is that there is only a 2 dB difference between received signal power and sensitivity. Normally, a higher margin is desirable due to fluctuation in received power as a result of signal fading. Signal Fading Fading of the RF signal is caused by several factors: Multipath The transmitted signal arrives at the receiver from different directions, with different path lengths, attenuation and delays. The summed signal at the receiver may result in an attenuated signal. Bad Line of Sight An optical line of sight exists if an imaginary straight line can connect the antennas on either side of the link. Radio wave clear line of sight exists if a certain area around the optical line of sight (Fresnel zone) is clear of obstacles. A bad line of sight exists if the first Fresnel zone is obscured. Link Budget Calculations Weather conditions (Rain, wind, etc.) At high rain intensity (150 mm/hr), the fading of an RF signal at 2.4 Ghz may reach a maximum of 0.02 dB/Km. Wind may cause fading due to antenna motion. Interference Interference may be caused by another system on the same frequency range, external noise, or some other co-located system.
Antenna gain
Antenna gain is the ratio of surface power radiated by the antenna and the surface power radiated by a hypothetical isotropic antenna:
The surface power carried by an electromagnetic wave is:
The surface power radiated by an isotropic antenna feed with the same power is:
Substituting values for the case of a short dipole, final result is:
= 1,5 = 1,76 dBi dBi are just deibels. The i is just a reminder that the indicated gain is taken against an isotropic antenna.
