RF Propagation No. 2 Seattle Pacific University Radio Systems InformationModulatorAmplifier Ant Feedline Transmitter InformationDemodulatorPre-Amplifier Ant Feedline Receiver Filter RF Propagation This presentation concentrates on the propagation portion
RF Propagation No. 3 Seattle Pacific University Waves from an Isotropic source propagate spherically As the wave propagates, the surface area increases The power flux density decreases proportional to 1/d 2 At great enough distances from the source, a portion of the surface appears as a plane The wave may be modeled as a plane wave The classic picture of an EM wave is a single ray out of the spherical wave
RF Propagation No. 4 Seattle Pacific University Real antennas are non-isotropic Most real antennas do not radiate spherically The wavefront will be only a portion of a sphere The surface area of the wave is reduced Power density is increased! The increase in power density is expressed as Antenna Gain dB increase in power along “best” axis dBi = gain over isotropic antenna dBd = gain over dipole antenna Gain in this area
RF Propagation No. 5 Seattle Pacific University Transmitted Power Radiated power often referenced to power radiated by an ideal antenna P t = power of transmitter G t = gain of transmitting antenna system The isotropic radiator radiates power uniformly in all directions Effective Isotropic Radiated Power calculated by: G t = 0dB = 1 for isotropic antenna This formula assumes power and gain is expressed linearly. Alternatively, you can express power and gain in decibels and add them: EIRP = P(dB) + G(dB) The exact same formulas and principles apply on the receiving side too!
RF Propagation No. 6 Seattle Pacific University Propagation Models Large-scale (Far Field) propagation model Gives power where random environmental effects have been averaged together Waves appear to be plane waves Far field applies at distances greater than the Fraunhofer distance: D = largest physical dimension of antenna = wavelength Small-scale (Near Field) model applies for shorter distances Power changes rapidly from one area/time to the next
RF Propagation No. 7 Seattle Pacific University Propagation Models For Free Space (no buildings, trees, etc.) f = frequency d = distance (m) = wavelength (m) c = speed of light h b = base station antenna height (m) h m = mobile antenna height (m) a(h m ) is an adjustment factor for the type of environment and the height of the mobile. a(h m ) = 0 for urban environments with a mobile height of 1.5m. Note: Hata valid only with d in range 1000-20000, h b in range 30-200m For Urban environments, use the Hata model
RF Propagation No. 8 Seattle Pacific University Calculation of Received Signal Strength 1.Confirm that far-field metrics can be used: Use Fraunhofer distance 2.Calculate EIRP = Transmit Power * Antenna Gain 3.Calculate propagation loss (free space or Hata) 4.Received signal strength (RSS) = EIRP – propagation loss
RF Propagation No. 9 Seattle Pacific University Applying formulas to real systems A transmission system transmits a signal at 960MHz with a power of 100mW using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna. 1. Confirm that far-field metrics can be used. = 3.0*10 8 m/s / 960MHz = 0.3125 meters Fraunhofer distance = 2 D 2 / = 2(0.16m) 2 /0.3125 = 0.16m 2. Calculate EIRP. Method 1: Convert power to dBm and add gain Power(dBm) = 10 log 10 (100mW / 1mW) = 20dBm EIRP = 20dBm + 2.15dB = 22.15dBm Method 2: Convert gain to linear scale and multiply Gain(linear) = 10 2.15dB/10 = 1.64 EIRP = 100mW x 1.64 = 164mW Checking work: 10 log 10 (164mW/1mW) = 22.15dBm
RF Propagation No. 10 Seattle Pacific University Applying formulas to real systems A transmission system transmits a signal at 960MHz with a power of 100mW using a 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna. 3. Calculate propagation loss (use free space in this example). Loss(dB) = 20 log 10 (960MHz) + 20 log 10 (2000m) – 147.56dB = 179.6dB + 66.0dB – 147.56dB = 98.0dB Received power(dBm) = EIRP(dB) – loss = 22.15dBm – 98.0dB = -75.85dBm Received power(W) = EIRP(W)/loss(linear) = 164mW / 10 98.0dB/10 = 2.6 x 10 -8 mW = 2.6 x 10 -11 W Checking work: 10 -75.85dBm/10 = 2.6x 10 -8 mW What is the power received at a distance of 2km (use Hata model with base height 30 m, mobile height 1.5 m, urban env.)? Loss(dB) = 69.55+26.16(log(f)-6) – 13.82(log(h b )) – a(h m )+ [44.9-6.55(log(h b )](log(d)-3) =69.55 + 78.01 – 20.41 – 0 + (35.22)(0.30) = 137.7 dB Received power = 22.15dBm – 137.7dB = -115.55dBm 4. Calculate RSS.
RF Propagation No. 11 Seattle Pacific University Link Budget Analysis InformationModulatorAmplifier Ant Feedline Transmitter InformationDemodulatorPre-Amplifier Ant Feedline Receiver Filter RF Propagation Gain Loss A Link Budget analysis determines if there is enough power at the receiver to recover the information
RF Propagation No. 12 Seattle Pacific University Transmit Power Components Begin with the power output of the transmit amplifier Subtract (in dB) losses due to passive components in the transmit chain after the amplifier Filter loss Feedline loss Jumpers loss Etc. Add antenna gain dBi Result is EIRP InformationModulatorAmplifier Ant Feedline Transmitter Filter RF Propagation
No. 13 Seattle Pacific University Calculating EIRP dBi12Antenna gain dB(1.5)150 ft. at 1dB/100 footFeedline loss dB(1)Jumper loss dB(0.3)Filter loss dBm4425 WattsPower Amplifier ScaleValueComponent dBm53Total All values are example values
RF Propagation No. 14 Seattle Pacific University Receiver System Components InformationDemodulatorPre-Amplifier Ant Feedline Receiver Filter The Receiver has several gains/losses Specific losses due to known environment around the receiver Vehicle/building penetration loss Receiver antenna gain Feedline loss Filter loss These gains/losses are added to the received signal strength The result must be greater than the receiver’s sensitivity
RF Propagation No. 15 Seattle Pacific University Receiver Sensitivity Sensitivity describes the weakest signal power level that the receiver is able to detect and decode Sensitivity is dependent on the lowest signal-to-noise ratio at which the signal can be recovered Different modulation and coding schemes have different minimum SNRs Range: <0 dB to 60 dB Sensitivity is determined by adding the required SNR to the noise present at the receiver Noise Sources Thermal noise Noise introduced by the receiver’s pre-amplifier
RF Propagation No. 16 Seattle Pacific University Receiver Noise Sources Thermal noise N = kTB (Watts) k=1.3803 x 10 -23 J/K T = temperature in Kelvin B=receiver bandwidth Thermal noise is usually very small for reasonable bandwidths Noise introduced by the receiver pre-amplifier Noise Factor = SNR in /SNR out (positive because amplifiers always generate noise) May be expressed linearly or in dB
RF Propagation No. 17 Seattle Pacific University Receiver Sensitivity Calculation The smaller the sensitivity, the better the receiver Sensitivity (W) = kTB * NF(linear) * minimum SNR required (linear) Sensitivity (dBm) = 10log 10 (kTB*1000) + NF(dB) + minimum SNR required (dB)
RF Propagation No. 18 Seattle Pacific University Sensitivity Example Example parameters Signal with 200KHz bandwidth at 290K NF for amplifier is 1.2dB or 1.318 (linear) Modulation scheme requires SNR of 15dB or 31.62 (linear) Sensitivity = Thermal Noise + NF + Required SNR Thermal Noise = kTB = (1.3803 x 10 -23 J/K) (290K)(200KHz) = 8.006 x 10 -16 W = -151dBW or -121dBm Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dB Sensitivity (W) = (8.006 x 10 -16 W )(1.318)(31.62) = 3.33 x 10 -14 W Sensitivity decreases when: Bandwidth increases Temperature increases Amplifier introduces more noise
RF Propagation No. 19 Seattle Pacific University RSS and Receiver Sensitivity Transmit/propagate chain produces a received signal has some RSS (Received Signal Strength) EIRP minus path loss For example 50dBm EIRP – 130 dBm = -80dBm Receiver chain adds/subtracts to this For example, +5dBi antenna gain, 3dB feedline/filter loss -78dBm signal into receiver’s amplifier This must be greater than the sensitivity of the receiver If the receiver has sensitivity of -78dBm or lower, the signal is successfully received.
RF Propagation No. 20 Seattle Pacific University Link Budget Analysis InformationModulatorAmplifier Ant Feedline Transmitter InformationDemodulatorPre-Amplifier Ant Feedline Receiver Filter RF Propagation EIRP Prop Loss RSS Sensitivity
RF Propagation No. 21 Seattle Pacific University Link Budgets A Link Budget determines what maximum path loss a system can tolerate Includes all factors for EIRP, path loss, fade margin, and receiver sensitivity For two-way radio systems, there are two link budgets Base to mobile (Forward) Mobile to base (Reverse) The system link budget is limited by the smaller of these two (usually reverse) Otherwise, mobiles on the margin would have only one-way capability The power of the more powerful direction (usually forward) is reduced so there is no surplus Saves power and reduces interference with neighbors
RF Propagation No. 22 Seattle Pacific University Forward/Reverse Link Budget Example Forward (Tower to Mobile) Amplifier power45dBm Filter loss-2dB Feedline loss-3dB TX Antenna gain+10dB Path lossX Vehicle Penetration-12dB RX Antenna gain+3dB Feedline loss-3dB RSS at mobile = 38dBm – X (path loss) If Mobile Sensitivity is -100dBm Maximum Path loss = 138dB Reverse (Mobile to Tower) Amplifier power28dBm Filter loss-1dB Feedline loss-3dB TX Antenna gain+3dB Vehicle Penetration-12dB Path LossX RX Antenna gain+10dB Feedline loss-3dB RSS at Tower = 22dBm – X (path loss) If Tower Sensitivity is -105dBm Maximum Path loss = 127dB Unbalanced – Forward path can tolerate 11dB more loss (distance) than reverse. Reduce Tower transmit power by 11dB.