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Basic RF Transmission Concepts

Basic RF Transmission Concepts. Ant. Ant. Feedline. Feedline. Radio Systems . This presentation concentrates on the propagation portion. Transmitter. Information. Modulator. Amplifier. Filter. RF Propagation. Receiver. Information. Demodulator. Pre-Amplifier. Filter.

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Basic RF Transmission Concepts

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  1. Basic RF Transmission Concepts

  2. Ant Ant Feedline Feedline Radio Systems This presentation concentrates on the propagation portion Transmitter Information Modulator Amplifier Filter RF Propagation Receiver Information Demodulator Pre-Amplifier Filter

  3. As the wave propagates, the surface area increases The power flux density decreases proportional to 1/d2 Waves from an Isotropic source propagate spherically • 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

  4. Most real antennas do not radiate spherically The wavefront will be only a portion of a sphere Gain in this area Real antennas are non-isotropic • 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

  5. Pt = power of transmitter Gt = gain of transmitting antenna system Transmitted Power • Radiated power often referenced to power radiated by an ideal antenna • The isotropic radiator radiates power uniformly in all directions • Effective Isotropic Radiated Power calculated by: The exact same formulas andprinciples apply on the receiving side too! Gt = 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)

  6. 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

  7. Propagation Models • f = frequency • d = distance (m) • = wavelength (m) c = speed of light For Free Space (no buildings, trees, etc.) For Urban environments, use the Hata model • hb = base station antenna height (m) • hm = mobile antenna height (m) • a(hm) is an adjustment factor for the type of environment and the height of the mobile. • a(hm) = 0 for urban environments with a mobile height of 1.5m. • Note: Hata valid only with d in range 1000-20000, hb in range 30-200m

  8. Applying formulas to real systems A transmission system transmits a signal at 960MHz with a power of 100mW usinga 16cm dipole antenna system with a gain of 2.15dB over an isotropic antenna.At what distance can far-field metrics be used?  = 3.0*108 m/s / 960MHz = 0.3125 meters Fraunhofer distance = 2 D2/  = 2(0.16m)2/0.3125 = 0.16m What is the EIRP? Method 1: Convert power to dBm and add gainPower(dBm) = 10 log10 (100mW / 1mW) = 20dBmEIRP = 20dBm + 2.15dB = 22.15dBm Method 2: Convert gain to linear scale and multiplyGain(linear) = 102.15dB/10 = 1.64EIRP = 100mW x 1.64 = 164mW Checking work: 10 log10 (164mW/1mW) = 22.15dBm

  9. 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.What is the power received at a distance of 2km (assuming free-space transmission and an isotropic antenna at the receiver)? Loss(dB) = 20 log10(960MHz) + 20 log10(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 / 1098.0dB/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(hb)) – a(hm)+ 44.9-6.55(log(hb))(log(d)-3) =69.55 + 78.01 – 27.79 – 0 + (35.22)(0.30) = 130.34 dB  Received power = 22.15dBm – 130.34dB = -108.19dBm

  10. Gain Ant Ant Loss Feedline Feedline Gain Link Budget Analysis • A Link Budget analysis determines if there is enough power at the receiver to recover the information Transmitter Information Modulator Amplifier Filter RF Propagation Receiver Information Demodulator Pre-Amplifier Filter

  11. Ant Feedline 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 Transmitter Information Modulator Amplifier Filter RF Propagation

  12. Component Value Scale Power Amplifier 25 Watts 44 dBm Filter loss (0.3) dB Jumper loss (1) dB Feedline loss 150 ft. at 1dB/100 foot (1.5) dB Antenna gain 12 dBi Total 53 dBm Calculating EIRP All values are example values

  13. Ant Receiver Information Demodulator Pre-Amplifier Filter Feedline Receiver System Components • 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

  14. 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

  15. 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 = SNRin/SNRout (positive because amplifiers always generate noise) • May be expressed linearly or in dB

  16. Receiver Sensitivity Calculation • The smaller the sensitivity, the better the receiver • Sensitivity (W) = kTB * NF(linear) * minimum SNR required (linear) • Sensitivity (dBm) =10log10(kTB*1000) + NF(dB) + minimum SNR required (dB)

  17. 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 (W) = (8.006 x 10-16 W )(1.318)(31.62) = 3.33 x 10-14 W • Sensitivity (dBm) = -121dBm + 1.2dB + 15dB = -104.8dBm • Sensitivity decreases when: • Bandwidth increases • Temperature increases • Amplifier introduces more noise

  18. 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.

  19. Ant Ant Feedline Feedline Link Budget Analysis EIRP Transmitter Information Modulator Amplifier Filter RF Propagation Prop Loss Receiver RSS Information Demodulator Pre-Amplifier Filter Sensitivity

  20. 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

  21. Forward (Base to Mobile) Amplifier power 45dBm Filter loss (2dB) Feedline loss (3dB) TX Antenna gain 10dBi Path loss X Fade Margin (5dB) Vehicle Penetration (12dB) RX Antenna gain 3dBi Feedline loss (3dB) Signal into mobile’s LNA has strength 33dBm – path loss If Mobile Sensitivity is -100dBm Maximum Path loss = 133dB Forward/Reverse Link Budget Example • Reverse (Mobile to Base) • Amplifier power 28dBm • Filter loss (1dB) • Feedline loss (3dB) • TX Antenna gain 3dBi • Fade Margin (5dB) • Vehicle Penetration (12dB) • Path Loss X • RX Antenna gain 10dBi • Feedline loss (3dB) • Signal into base’s LNA has strength 17dBm – path loss • If Base Sensitivity is -105dBm • Maximum Path loss = 122dB Unbalanced – Forward path can tolerate 11dB more loss (distance) than reverse

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