MICROWAVE RADIO SYSTEMS GAIN PENTel.Com Engr. Josephine Bagay, Ece faculty
SYSTEM GAIN G s is the difference between the nominal output power of a transmitter (P t ) and the minimum input power to a receiver (C min ) necessary to achieve satisfactory performance; Must be greater than or equal to the sum of all gains and losses incurred by a signal as it propagates from a transmitter to a receiver In essence, system gain represents the net loss of a radio system, which is used to predict the reliability of a system for a given set of system parameters.
SYSTEM GAIN Ironically, system gain is actually a loss. Losses are much higher than the gains. Therefore, the net system gain always equates to a negative db value (i.e., a loss) Because system gain is defined as a net loss individual losses are represented with positive db individual gains are represented with negative db
Mathematically, system gain in its simplest form is G s = P t - C min where G s = system gain (db) P t = transmitter output power (dbm or dbw) C min = minimum receiver input power necessary to achieve a given reliability and quality objective
G s = P t - C min losses gains P t - C min FM (db) + L p(db) + L f(db) + Lb (db) - A t(db) - A r(db) Gains: A t = transmit antenna gain relative to an isotropic radiator (db) A r = receive antenna gain relative to an isotropic radiator (db) Losses FM = fade margin for a given reliability objective (db) L p = free-space path loss in (db) L f = transmission line loss in (db) L f = total coupling or branching loss in (db)
TABLE 13-3 System Gain Parameters
FADE MARGINS (LINK MARGIN)
FADING The reduction in receive signal level; Reduction in signal strength at the input to a receiver; It applies to propagation variables in the physical radio path that affect changes in the path loss between transmit and receive antennas
WHAT IS FADE MARGIN? Under interference-free conditions, the fade margin is defined as the difference between the received signal level under normal wave propagation conditions (fadefree time) and the receiver s threshold level at a given bit-error level Considers the non-ideal and less predictable characteristics of radiowave propagation, such as multipath propagation and terrain sensitivity; these characteristics cause temporary, abnormal atmospheric conditions
TYPES OF FADE MARGIN Thermal Fade Margin (TFM) Also called Flat Fade Margin Interference Fade Margin (IFM) Dispersive Fade Margin (DFM) Composite Fade Margin (CFM)
THERMAL OR FLAT FADE MARGIN The difference between the unfaded received signal level (RSL) and receiver s static or dynaminc threshold, as measured with back-to-back radios, at a given BER. Thermal Fade Margin is the only fade margin that needs to be considered on analog LOS links since interference affects unfaded baseband noise. Mathematically: TFL = Recieved Signal Level Reciever Threshold
INTERFERENCE FADE MARGIN Defines the digital link s vulnerability to cochannel and adjacent channel interferrence and is provided by the frequency search company based on the manufacturer s threshold-to-interference (T/I) curves and the interference ambiance. Based on congestion of systems within the path using the same band of frequencies. Taken from graphs from a specific location and varies over time.
DISPERSIVE FADE MARGIN Dependent on the type of equipment and modulation used. These are gains in the equipment which are factored in because of technical improvements on the system and how they improve the information signal itself It is determined by the type of modulation, the effectiveness of equalization employed in the receive path, and the multipath signal s delay time.
DISPERSIVE FADE MARGIN DFM is calculated based on the W-curves using computation DFM = 17.6 log 10 (S w /158.4) where
COMPOSITE FADE MARGIN This is the total of all fade margins Mathematically: CFM = TFM + DFM + IFM In decibels CFM = -10 log ( 10 -TFM/10 + 10 -IFM/10 10 -DFM/10 ) where TFM,IFM,DFM in decibell (db)
RECEIVER THRESHOLD (RECEIVER SENSITIVITY)
WHAT IS RECEIVER THRESHOLD? Receiver threshold means the lowest signal your receiver will pick up and still operate. When nearing threshold, radio will sound noisy with static, TV will show snow and your cell phone will show only one bar or drop out
RECEIVER THRESHOLD The receiver threshold is the minimum signal required for the demodulator to work at a specific error rate. Two thresholds are normally defined, one at a BER of 10^ 6 and the other at a BER of 10^ 3. The reason for this is the original cutoff for audio applications was 10^ 3, whereas it is generally considered data requires at least 10^ 6 for an acceptable throughput rate.
RECEIVER THRESHOLD Explaining the value 10^ 3,or the loss of frame synchronization point (2 ^10 5 for SDH/SONET), is the correct threshold to use from a performance objective perspective as it is related to the severely eroded second ratio (SESR) but the industry tends to use 10^ 6 due to the data concerns.
RECEIVER THRESHOLD The receiver threshold is dependent on the minimum S/N required at the receiver input, the noise figure of the receiver s front-end, and the background thermal noise (Pn) Pn = ktb where k - Boltzmann s constant (1.38 10^ 23) T - temperature in Kelvin B - bandwidth of the receiver.
RECEIVER THRESHOLD In general, the receiver threshold considered depends both on the required output performance at base- band, and on the type of interference For linear modulation, such as AM and SSB, and any Gaussian interference, the relation between the SNR at the detector output and the (RF) C/Iratio is linear
RECEIVER THRESHOLD In non-linear modulation, such as phase modulation (PM) or frequency modulation (FM), the post-detection signal-to-noise ratio can be greatly enhanced as compared to baseband transmission or compared to linear modulation Typically, for FM signals, the threshold is in the range of 3 to 10 db. This threshold fundamentally limits the noise immunity of various types of non-linear modulation techniques
Table: Typical receiver thresholds below which a voice baseband signal becomes almost unintelligible. Source: Gosling FM 25 khz 3.. 8 db FM 12.5 khz 6.. 15 db SSB 5 khz heavy companding moderate companding no companding 8.. 11 db 11.. 15 db 15.. 20 db
CARRIER-TO-NOISE VS SIGNAL-TO-NOISE RATIO
CARRIER-TO-NOISE RATIO In communications, the carrier-to-noise ratio, often written CNR or C/N, is a measure of the received carrier strength relative to the strength of the received noise. High C/N ratios provide better quality of reception, and generally higher communications accuracy and reliability, than low C/N ratios. Carrier to noise ratio is the ratio of the carrier signal power to the noise power in some specified channel, usually expressed in decibels (db). For the analog channels the noise is assumed flat and the result of thermal and amplifier noises.
CARRIER-TO-NOISE RATIO (MATHEMATICAL DEFINITION) Engineers specify the C/N ratio in decibels (db) between the power in the carrier of the desired signal and the total received noise power. If the incoming carrier strength in microwatts is P c and the noise level, also in microwatts, is P n, then the carrier-to-noise ratio, C/N, in decibels is given by the formula: C/N = 10 log 10 (P c /P n )
CARRIER-TO-NOISE RATIO The C/N ratio is measured in a manner similar to the way the signal-to noise ratio (S/N) is measured, and both specifications give an indication of the quality of a communications channel. However, the S/N ratio specification is more meaningful in practical situations. The C/N ratio is commonly used in satellite communications systems to point or align the receiving dish; the best dish alignment is indicated by the maximum C/N ratio.
CARRIER-TO-NOISE RATIO Graphical representaion of C/N ratio (http://www.cisco.com/application/pdf/en/us/guest/products/ps2209/c1244/cdccont_0900aecd800fc94c.pdf)
SIGNAL-TO-NOISE RATIO In analog and digital communications, signal-to-noise ratio, often written S/N or SNR, is a measure of signal strength relative to background noise. The ratio is usually measured in decibels (db). Signal-to-noise ratio, or SNR, is a measurement that describes how much noise is in the output of a device, in relation to the signal level. SNR is actually two level measurements, followed by a simple calculation. First, we measure the output level of the device under test with no input signal. Then we apply a signal to the device and take another level measurement. Then we divide.
SIGNAL-TO-NOISE RATIO (MATHEMATICAL APPROACH) If the incoming signal strength in microvolts is V s, and the noise level, also in microvolts, is V n, then the signal-to-noise ratio, S/N, in decibels is given by the formula: S/N = 20 log 10 (V s /V n )
SIGNAL-TO-NOISE RATIO Graphical representation of SNR propagation (http://www.cisco.com/en/us/prod/collateral/video/ps8806/ps5684/ps2209/prod_white_paper0900aecd805738f5.htm l)
CNR VS. SNR RECAP CNR is a predetection measurement performed on RF signals. Raw carrier power to raw noise power in the RF transport path only say, a coaxial cable distribution network or a standalone device such as a converter or headend hetrodyne processor; Ideal for characterizing network impairments
CNR VS. SNR RECAP SNR is a pre modulation or post-detection measurement performed on baseband signals. Includes noise in original signal, transmitter or modulator, transport path, and reciever and demodulator Ideal for characterizing end-to-end performance the overall signal quality seen by the end user
NOISE FACTOR AND NOISE FIGURE
WHAT IS NOISE FACTOR? Simply a ratio of input signal-to-noise ratio to output signal-to-noise ratio Any unwanted input Limits systems ability to process weak signals Sources: 1. Random noise in resistors and transistors 2. Mixer noise 3. Undesired cross-coupling noise 4. Power supply noise Dynamic range capability of detecting weak signals in presence of large-amplitude signals
NOISE FACTOR IEEE Standards: The noise factor, at a specified input frequency, is defined as the ratio of (1) the total noise power per unit bandwidth available at the output port when noise temperature of the input termination is standard (290 K) to (2) that portion of (1) engendered at the input frequency by the input termination. F = available output noise power available output noise due to source
NOISE FACTOR noisiness of the signal measure = signal-to-noise ratio (frequency dependant) SNR = S( f N( f ) ) = average signal power average noise power
NOISE FACTOR The noise factor F of a system is defined as: F = (SNR in )/(SNR out ) where SNR in = input signal-to-noise power ratio SNR out = output signal-to-noise power ratio
WHAT IS NOISE FIGURE? Indicates how much the signal-to-noise ratio deteriorates as a waveform propagates from the input of a circuit It is a measure of the degradation of SNR due to the SNRi noise added F = 1 SNRo Implies that SNR gets worse as we process the signal Na Spot noise factor F =1+ kt The answer is the bandwidth
NOISE FIGURE The noise figure NF is defined as: The noise figure is the factor, given in db
NOISE FIGURE IN TEMPERATURE(K)
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