Frequently Asked Questions

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Below are a few good online resources for news and information in the wireless industry.

Courtesy of Patrick Leary, BreezeCOM and Marlon Schafer, Oddessa Office Equipment.

Abbreviations and Acronyms
AP	Access Point
ARK	Automatic Retransmission Queing
ARS	Automatic Rate Switching
AU	Access Unit (same as AP)
BWA	Broadband Wireless Access
BSS	Basic Service Set
CDMA	Code Division Multiple Access
CIR	Committed Information Rate
CoS	Class of Service
CPE	Customer Premises Equipment
CSMA/CA	Carrier Sense Multiple Access/Collision Avoidance
DS (or DSSS)	Direct Sequence Spread Spectrum
EIRP	Effective Isotropic Radiated Power
EMI	Electromagnetic Interference
ESSID	Extended Service Set ID
FDD	Frequency Division Duplex
FH (or FHSS)	Frequency Hopping Spread Spectrum
GFSK	Gausian Frequency Shift Keying
IDU	Indoor Unit
IF	Intermediate Frequency
ISM	Industrial, Scientific, and Medical
LOS	Line Of Sight
MIR	Maximum Information Rate
NLOS	Near/Non Line of Site
ODU	Outdoor Unit
OFDM	Orthonagol Frequency Division Multiplexing
PAN	Personal Area Network
PtMP	Point-To-Multipoint
PtP	Point-to-Point
QAM	Quadrature Amplitude Modulation
QoS	Quality of Service
RFI	Radio Frequency Interference
RSSI	Receive(or receiver) Signal Strength Index/Indication
TDD	Time Division Duplex
TDMA	Time Division Multiple Access
UNII	Unlicensed-National Information Infrastructure
VOFDM	Vector Orthonagol Frequency Division Multiplexing
WEP	Wired Equivalent Privacy
WLL	Wireless Local Loop

Terminology
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	Loss of power, expressed in dB. 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).
Isotropic Antenna	A hypothetical, lossless antenna having equal radiation intensity in all directions. Used as a zero dB gain reference in directivity calculation (gain). The sun is often given as an example of an isotropic radiator.
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.
Directional Antenna	This antenna radiates and receives most of the signal power in one direction.
Antenna Beamwidth	The directiveness of a directional antenna. Defined as the angle between two half-power (-3 dB) points on either side of the main lobe of radiation.
Receiver Sensitivity	The minimum RF signal power level required at the input of a receiver for certain performance (e.g. > BER).
EIRP	The antenna transmitted power. Equal to the transmitted output power minus cable loss plus the transmitting antenna gain. EIRP = Pout - Ct + Gt 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 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.
The Line of Sight Concept	An optical line of sight exists if an imaginary straight line can be drawn connecting the antennas on either side of the link.
Clear Line of Sight	A clear line of sight exists when no physical objects obstruct viewing one antenna from the location of the other antenna. A radio wave clear line of sight exists if a defined area around the optical line of sight (Fresnel Zone) is clear of obstacles. Fresnel Zone (pronounced: fruh nell): The Fresnel zone is the area of a circle around the line of sight. The Fresnel Zone is defined as follows: R1 = ? square root of (lxD) R: radius of the first fresnel zone l: wavelength D: distance between sites.

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