AEiE0901 Telecommunication and wireless communication¶
Core ideas and timeline view¶
- Telecommunication means transmission of information over distance by electrical, electromagnetic, or optical means.
- Wireless communication removes the guided medium and uses radio propagation.
- A compact historical progression is wired telegraph -> telephone and circuit switching -> radio -> digital PCM and electronic switching -> cellular generations -> packet/all-IP networks.
- Generational language in mobile systems usually means 1G analog voice, 2G digital voice, 3G data services, 4G packet broadband, and 5G low-latency high-capacity service.
- Future trends typically include all-IP core networks, software-defined control, IoT-scale connectivity, virtualization, and tighter spectrum reuse.
Guided and unguided media¶
| Medium | Class | Strength | Common limit |
|---|---|---|---|
| Twisted pair | Guided | Cheap, simple, telecom access lines | Lower bandwidth, more interference than fiber |
| Coaxial cable | Guided | Better shielding, moderate bandwidth | Bulkier than twisted pair |
| Optical fiber | Guided | Very high bandwidth, low attenuation | Higher installation precision |
| Radio | Unguided | Mobility, broadcast capability | Fading and interference |
| Microwave | Unguided | Point-to-point high capacity | Line-of-sight dependence |
| Satellite | Unguided | Very wide coverage | Large propagation delay |
Free-space propagation model¶
- In ideal free space, received power falls with the square of distance.
- Friis transmission formula:
\[
P_r = P_t G_t G_r \left(\frac{\lambda}{4\pi d}\right)^2
\]
- \(P_r\): received power.
- \(P_t\): transmitted power.
- \(G_t, G_r\): antenna gains.
- \(\lambda\): wavelength.
- \(d\): separation distance.
In decibel form, free-space path loss grows with distance and frequency:
\[
FSPL(dB) = 32.44 + 20\log_{10}(f_{MHz}) + 20\log_{10}(d_{km})
\]
Reflection, diffraction, and scattering¶
| Phenomenon | Cause | Recognition cue |
|---|---|---|
| Reflection | Wave strikes surface larger than wavelength | Multipath from buildings or ground |
| Diffraction | Wave bends around obstacle edges | Non-line-of-sight reach behind obstruction |
| Scattering | Wave hits rough or small objects | Many weak random components |
- These three effects create multipath propagation.
- Multipath can cause constructive or destructive interference at the receiver.
Channel assignment and handover¶
- Channel assignment allocates radio resources to cells or users.
- Fixed channel assignment uses predetermined channels per cell.
- Dynamic channel assignment allocates channels according to current demand.
- Handover transfers an active connection from one channel, sector, or base station to another.
| Handover type | Cue |
|---|---|
| Hard handover | Break before make |
| Soft handover | Make before break |
- Hard handover is common in many cellular systems.
- Soft handover reduces drop probability when simultaneous links are possible.
Small-scale multipath propagation and fading¶
- Small-scale fading is rapid fluctuation of amplitude, phase, or delay over short time, distance, or frequency changes.
- It is caused by multipath and motion.
- Delay spread leads to time dispersion and possible intersymbol interference (ISI).
- Doppler spread reflects channel variation with motion.
Rayleigh fading model¶
- Rayleigh fading applies when there is no dominant line-of-sight component.
- The received envelope is Rayleigh distributed when many scattered components add randomly.
- Power may fluctuate deeply even when average distance changes little.
- A line-of-sight dominant path suggests Rician rather than Rayleigh fading.
Recognition cues for telecommunication and wireless communication¶
- "No dominant LOS component" indicates Rayleigh fading.
- "LOS plus scattered components" indicates Rician fading.
- "Signal bends around obstacle" indicates diffraction.
- "Rapid local fluctuation" indicates small-scale fading, not large-scale path loss.
Common traps in telecommunication and wireless communication¶
- Increasing distance may reduce average received power, but it does not by itself eliminate fading.
- Diversity across space, frequency, time, or polarization is the canonical fading countermeasure.
- Path loss and fading are related but distinct concepts.
- Shadowing is large-scale variation, while multipath fading is small-scale variation.
One-step examples for telecommunication and wireless communication¶
- Doubling distance in free space reduces received power by a factor of 4, or about 6 dB.
- If a channel has strong direct LOS, do not call it Rayleigh without qualification.
- If a call switches cells without user action, that event is handover.
Telecommunication-and-wireless revision box¶
Guided versus unguided media is basic classification. Free-space path loss is a smooth average model; fading is local fluctuation. Rayleigh fading requires no dominant LOS path. Reflection, diffraction, and scattering are the multipath causes.