2.4. Microwave Link Design
Channel
The air channel has attenuation due to absorption by various gases, and multi-path scattering from various objects/structures in the transmission path or terrain. The former is predictable, while the latter is usually not.
Transmitter
Major Tx performance metrics include output power transmitted, which determines the range of transmission, and spurious emissions.
Friis’ Radio-Link Formula
The power density (Poynting vector) S radiated by an isotropic antenna with directivity expressed by a gain factor Gt at a distance R away is simply:
Sisotropic,actual = GtPt/(4πR2) (W/m2), where Pt in Watts is transmitted power.
The power received by the Rx, Pr, is determined by Sactual and the receiving antenna area know as the effective aperture area A = Grλ2/(4π) in m2, where Gr is the directivity of the receiver and λ is the operating wavelength:
Pr = ASactual = GrGtλ2Pt/(4πR)2 (W), this is Friis’ formula.
To account for inevitable loss due to atmospheric absorption, the above equation must be appended by including an attenuation-loss factor LA in dB:
Pr(dBm) = Pt + Gr + Gt – 20log(4πR/λ) – LA
Receiver
Rx performance includes two important parameters: selectivity and sensitivity.
Selectivity
Rx selectivity is determined by the bandwidth which is dominated by the IF filter bandwidth. Selectivity can be measured by the ability of an Rx to block a potential interference frequency in an adjacent channel to the operating frequency fRF:
Selectivity = 10log|G(fRF + foffset)/G(fRF)| < 0
Sensitivity
Rx sensitivity is limited by the presence of noise, which comes from internal and external sources. Internal noise is produced within the Rx by its circuitry, while external noise is picked up by the antenna. The combined noise sets a threshold for the weakest signal that can be reliably detected. The minimum detectable signal (MDS) is the minimum received power Pr which meets a specified BER at the Rx output. MDS is expressible in either Watts or Volts.
Link Budget
The link budget includes all aspects discussed above to describe the performance of an Rx-Tx communication link. We redefine signal-strength loss due to free-space propagation over the distance R as path loss, L0(dB) = 20log(4πR/λ).
In addition, the link budget must include another type of loss representing antenna line losses Lt & Lr – representing losses in the feedlines leading to the transmitting and receiving antennas, respectively.
In practice, communication systems are designed so that the received power Pr exceeds the minimum power level Pr,min required for ensuring a specified BER performance. Pr,min is often expressed in terms of the SNR or CNR (carrier-to-noise ratio). The excess of received power forms a link margin LM:
LM(dB) = Pr – Pr,min.
Typically, LM ranges from 3dB to 20dB.
Rx Noise
The total Rx noise is defined as a noise floor established by external noise and an excess noise contributes by internal circuitry of Rx itself.
Thermal Noise
Noise Figure (NF)
NF is the most important noise metric for a circuit/system. The ratio SNRi/SNRo is defined as the noise factor F, or noise figure NF(dB) = 10logF. Note F is always specified at standard temperature. For a noisy circuit block with a power gain G and contributing excess noise Ne at output,
So = GSi, No = GNi + Ne,
F = SiNo/(SoNi) = 1 + Ne/(GNi).
NF of a Cascade
Note that Ftot is dominated by F1, and progressively less by subsequent stages. In a multi-stage system such as an Rx, on should strive to minimize the noise of the first stage. For a passive filter or attenuator with G<1 and matched impedance between input and output, F = 1/G and NF = -G (dB)
