Wireless Communication: Spectrum, Transmission, and Fading.pptx

Overview of Wireless Spectrum, Transmission Impairments, and Channel Correction

1. Wireless Communication: Spectrum, Transmission, and Fading Overview of Wireless Spectrum, Transmission Impairments, and Channel Correction

2. 1. Importance of Wireless Spectrum The wireless spectrum is the range of frequencies used to transmit data without cables. Choosing the right frequency range is crucial for a technology’s performance. Lower frequencies travel longer distances. Higher frequencies penetrate buildings and walls more effectively. The spectrum is shared by many users and must be regulated to avoid interference.

3. 2. Spectrum Regulation Wireless frequencies are managed by regulatory bodies to ensure fair use. In the United States , this is done by the Federal Communications Commission (FCC) . Other countries have similar agencies. The FCC determines which frequencies are: Government-exclusive Non-government exclusive Shared government/non-government

4. Cont.. Licenses are often required and may be purchased through auctions . Allocated services include: Aeronautical, Amateur, Broadcasting Maritime, Meteorological, Mobile Satellite and Space communications

5. 3. Signal Characteristics that Differentiate Wireless Transmissions a. Carrier Frequency Each wireless signal is shifted to a higher carrier frequency . Example: A 22 MHz Wi-Fi signal centered at 2.412 GHz occupies 2.401–2.423 GHz . b. Signal Power Determines how far a signal can travel. Signals with limited power allow frequency reuse in distant areas

6. Cont.. c. Multiple Access Schemes Methods that allow multiple users to share the same spectrum: TDMA (Time Division Multiple Access): Users share by time slots. FDMA (Frequency Division Multiple Access): Users share by separate frequencies. CDMA (Code Division Multiple Access): Users share the same time and frequency but use different codes.

7. 4. Dynamic Spectrum Access and Cognitive Radio Traditional allocation often leaves parts of the spectrum underutilized. Dynamic Spectrum Access (DSA): Allows secondary users to use frequencies temporarily when primary users are inactive. Cognitive Radio: A technology that scans the spectrum, detects unused frequencies, and automatically switches channels.

8. 5.Unlicensed ISM Bands Some frequency ranges can be used without a license if devices: Stay within power limits Use spread spectrum techniques Common ISM (Industrial, Scientific, and Medical) bands: 915 MHz 2.45 GHz 5.8 GHz 57–64 GHz Technologies using these bands: Wi-Fi (IEEE 802.11) Bluetooth (IEEE 802.15)

9. 6.Propagation Modes & Transmission Propagation Modes Ground Wave : Travels along Earth's surface; effective < 3 MHz (e.g., AM radio). Sky Wave : Reflected by ionosphere; 3–30 MHz; covers thousands of km (e.g., shortwave radio). Line-of-Sight (LOS) : Frequencies > 30 MHz (Wi-Fi, LTE, microwave); requires mostly clear path.

10. Cont.. Propagation Mechanisms Free-space propagation : No obstacles, signal weakens with distance. Transmission : Signal passes through medium and bends (refraction). Reflection : Bounces off large surfaces (walls, ground). Diffraction : Bends around sharp edges. Scattering : Bounces off small objects (trees, signs).

11. 7. Transmission Impairments Wireless signals face several impairments as they travel: a. Free-Space Path Loss Signal power decreases as distance increases. Caused by natural spreading of the wave in open space. Calculated using: b. Attenuation Gradual loss of signal strength due to medium resistance or obstacles. Measured in decibels (dB) .  

12. Cont.. c. Distortion Occurs when different frequency components of a signal arrive at different times, altering the original signal shape. d. Noise Unwanted random energy that interferes with the signal. Sources: electronic components, atmosphere, nearby devices. e. Signal-to-Noise Ratio (SNR) Indicates the quality of a signal. Higher SNR = better communication reliability.  

13. 8. Fading Fading is the variation in received signal strength over time or space. Caused by: Multipath propagation (signal reflections and scattering) Movement of transmitter, receiver, or obstacles Types: Slow fading: Due to large obstacles or terrain. Fast fading: Due to rapid movement or multiple reflection paths.

14. 9 . Correction and Compensation Techniques a. Equalization Adjusts the receiver to counteract distortion caused by multipath propagation. b. Diversity Uses multiple antennas or frequencies to reduce fading effects. c. Error Control Coding Adds redundant bits to detect and correct transmission errors. d. Adaptive Modulation Dynamically changes modulation type based on channel quality (e.g., from QPSK to BPSK).