Lecture 3: Transmission Impairments & Link Budget

Wireless Transmission Impairments

Factors that degrade signal quality during propagation.

1. Attenuation

Reduction in signal strength over distance.

• Free-Space Path Loss (FSPL): Loss of power in an unobstructed LOS path.$$L_p=20{\log }_{10}(d)+20{\log }_{10}(f)+32.44$$(where $d$ is in km and $f$ is in MHz).

2. Distortion & Dispersion

• Distortion: Higher frequencies attenuate more than lower ones, changing the signal shape.
• Dispersion: Signal spreads in time/frequency; different components travel at different speeds.

3. Noise Categories

• Thermal Noise: Due to electron agitation (unavoidable).
• Intermodulation: Sum/difference frequencies from sharing a medium.
• Crosstalk: Coupling between signal paths.
• Impulse Noise: Short, high-amplitude spikes (external interference).

4. Multipath Propagation

Signals reach the receiver via multiple paths due to:

• Reflection: Large surface relative to $\lambda$.
• Diffraction: Edges of impenetrable bodies.
• Scattering: Small objects relative to $\lambda$.
• Result: Fading (variation in power over time/distance).

Link Budget Analysis

Quantifying link performance by summing all gains and subtracting all losses.

Link Margin

The "safety buffer" between received power and receiver sensitivity.

• If Margin $>0\text{ dB}$, the link is feasible.

Example: 5 km Link Feasibility

System Parameters:

• Distance: $5\text{ km}$
• Frequency: $2.4\text{ GHz}$ ($⟹\text{FSPL}=114\text{ dB}$)
• AP: $P_t=20\text{ dBm}$, $G=10\text{ dBi}$, $P_{\text{sens}}=-89\text{ dBm}$, $L_{\text{cable}}=2\text{ dB}$.
• Client: $P_t=15\text{ dBm}$, $G=14\text{ dBi}$, $P_{\text{sens}}=-82\text{ dBm}$, $L_{\text{cable}}=2\text{ dB}$.

Part A: AP $\to$ Client

1. Transmit Power (AP): $+20\text{ dBm}$
2. Antenna Gain (AP): $+10\text{ dBi}$
3. Cable Loss (AP): $-2\text{ dB}$
4. FSPL (5 km): $-114\text{ dB}$
5. Antenna Gain (Client): $+14\text{ dBi}$
6. Cable Loss (Client): $-2\text{ dB}$

Received Signal ($P_r$): $-74\text{ dBm}$
Client Sensitivity: $-82\text{ dBm}$
Link Margin: $-74-(-82)=\mathbf{8}\text{ dB}$ (FEASIBLE)

Part B: Client $\to$ AP

1. Transmit Power (Client): $+15\text{ dBm}$
2. Antenna Gain (Client): $+14\text{ dBi}$
3. Cable Loss (Client): $-2\text{ dB}$
4. FSPL (5 km): $-114\text{ dB}$
5. Antenna Gain (AP): $+10\text{ dBi}$
6. Cable Loss (AP): $-2\text{ dB}$

Received Signal ($P_r$): $-79\text{ dBm}$
AP Sensitivity: $-89\text{ dBm}$
Link Margin: $-79-(-89)=\mathbf{10}\text{ dB}$ (FEASIBLE)

continue:./l4.md
before:./l2.md