Digital Image Processing (EC-803 (B)) - Unit 1 Short Notes

Unit 1: Fundamentals of Wireless Networks and Standards

1. Introduction to Wireless Communication

Definition and Scope

Wireless networks enable communication without physical cables, using electromagnetic waves. Scope includes:

• Cellular networks (2G/3G/4G/5G)

• Wireless Local Area Networks (WLAN) – e.g., Wi-Fi (IEEE 802.11)

• Wireless Personal Area Networks (WPAN) – e.g., Bluetooth, Zigbee

• Wireless Sensor Networks (WSN) – distributed sensing

• Satellite and navigation systems – e.g., GPS, GAGAN

Characteristics of Wireless Medium

• Mobility support: Users can move while maintaining connectivity.

• Shared medium: Multiple devices share limited spectrum; requires medium access control.

• Limited bandwidth & high error rates: Spectrum scarcity and propagation impairments cause losses.

• Susceptibility to interference, noise, and fading: Signal degrades due to obstacles, multipath, and environmental factors.

Major Challenges

• Multipath propagation: Signals arrive via multiple paths, causing intersymbol interference (ISI).

• Doppler effect: Frequency shift due to relative motion, degrading signal in high-mobility scenarios.

• Hidden terminal problem: Node A cannot sense node B’s transmission to a common receiver, causing collisions at receiver.

• Exposed terminal problem: Node defers transmission though it wouldn’t cause collision, reducing spatial reuse.

• Security vulnerabilities: Eavesdropping, spoofing, and denial-of-service attacks due to open medium.

[!TIP]

Common Pitfall: Hidden vs. exposed terminal – hidden causes collisions; exposed causes unnecessary deferral. Both are mitigated by RTS/CTS in 802.11.

Wireless Sensor Networks (WSN)

• Architecture:

  • Sensor nodes: Sense physical phenomena, process data, communicate.

  • Sink/base station: Aggregates data from nodes.

  • Gateway: Connects to external networks (e.g., Internet).

  • Control center: Monitors and manages network.

• Types:

  • Terrestrial (ground-based), Underwater (UWSN), Aerial (drones).

  • Static vs. mobile nodes.

• Applications: Environmental monitoring, health care, smart agriculture, military surveillance.

• Challenges:

  • Energy constraints: Battery-powered nodes require low-power operation.

  • Deployment: Often unattended in harsh/difficult terrains.

  • Scalability: Hundreds to thousands of nodes.

  • Security: Data confidentiality, integrity, authenticity in resource-constrained devices.

• Security Techniques:

  • Cryptography: Symmetric (AES) for efficiency; asymmetric for key exchange.

  • Authentication: Pre-shared keys, lightweight protocols.

  • Intrusion detection: Monitor for malicious nodes.

2. Evolution of Mobile Telecommunication Systems

Generational Advancements (1G to 5G)

GSM to UMTS Evolution

• Data rates: From kbps (GSM with GPRS/EDGE) to Mbps (UMTS).

• Access technology: TDMA (GSM) → CDMA (UMTS) for better capacity and spectral efficiency.

• Enhanced services: Video calling, mobile Internet, improved coverage via hierarchical cell structures.

3GPP (3rd Generation Partnership Project)

• Role: Standardizes cellular technologies (GSM, UMTS, LTE, 5G NR).

• Key objectives for LTE/LTE-A:

  • High spectral efficiency.

  • Low latency (<10 ms).

  • Simplified architecture (all-IP).

  • Backward compatibility.

  • Support for flexible bandwidth (1.4–20 MHz).

E-UTRAN (Evolved UTRAN) Architecture for LTE

• Components:

  • eNodeB (eNB): Base station; handles radio resource management, scheduling, and MAC layer.

  • MME (Mobility Management Entity): Signalling for mobility, authentication.

  • S-GW (Serving Gateway): Data routing/forwarding within LTE network.

  • P-GW (Packet Data Network Gateway): Interface to external networks (e.g., Internet); IP address allocation, policy enforcement.

• Functions:

  • Mobility management: Handovers between eNBs.

  • Resource allocation: Dynamic scheduling via eNB.

  • Control plane: Signalling via MME.

  • User plane: Data via S-GW/P-GW.

[!TIP]

Exam Focus: E-UTRAN is flatter than UTRAN (no RNC); eNB handles more functions.

Wireless ATM (Asynchronous Transfer Mode)

• Concept: Combines ATM’s QoS with wireless access for high-speed data.

• Architecture:

  • Wireless ATM switch: Connects base stations to backbone.

  • Base stations: Manage radio resources, perform cell handoffs.

  • Mobile terminals: Use ATM-like cells over wireless link.

• Challenges:

  • High bit error rates over wireless.

  • Mobility management (handoffs with QoS guarantees).

  • Limited bandwidth and power constraints.

3. Wireless Local Area Networks (WLAN)

IEEE 802.11 Standards Family

Protocol Architecture

• Physical Layer (PHY):

  • Modulation: DSSS (b), OFDM (a/g/n/ac/ax).

  • Spread spectrum: FHSS (not used), DSSS.

  • OFDM: Divides channel into orthogonal subcarriers; robust to ISI.

• MAC Layer:

  • CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance): Listen before talk; uses DIFS and random backoff.

  • Frame formats: Control (RTS/CTS), data, management frames.

  • Two access methods:

    • DCF (Distributed Coordination Function): Contention-based (default).

    • PCF (Point Coordination Function): Contention-free (optional, AP-centric).

MAC Layer Functions

1. Medium access control: DCF/PCF, NAV (Network Allocation Vector) for virtual carrier sense.

2. Association & authentication: Station joins BSS; authentication (e.g., WPA2/3).

3. Security: Encryption (WEP, TKIP, CCMP/AES).

4. Power management:

   • PSM (Power Save Mode): Station sleeps, wakes for buffered frames.

   • APSM (Automatic Power Save Delivery): AP buffers frames for sleeping stations.

5. Fragmentation & reassembly: Split large frames to combat errors.

HIPERLAN (High-Performance Radio LAN)

• Comparison with IEEE 802.11:

  | Feature | HIPERLAN/2 | IEEE 802.11 (e.g., 802.11n) | |------------------|--------------------------|-----------------------------| | Data rate | Up to 54 Mbps | Up to 600 Mbps (n) | | Range | ~30 m indoor | ~100 m indoor | | QoS support | Yes (via MAC) | Limited (EDCA) | | Architecture | Centralized (AP) | Infrastructure/ad-hoc |

• HIPERLAN/2 architecture:

  • Central controller (AP): Manages resources, QoS.

  • Mobile terminals: Communicate via AP.

  • Uses OFDM at PHY; connection-oriented MAC with time-division multiplexing.

[!TIP]

Common Pitfall: 802.11 uses CSMA/CA (collision avoidance), not CSMA/CD (wired Ethernet).

4. Wireless Personal Area Networks (WPAN)

Bluetooth Technology

• Piconet topology:

  • Master-slave: One master (coordinates), up to 7 active slaves.

  • Frequency hopping: 79 channels (1 MHz each) in 2.4 GHz ISM band.

• Scatternet: Multiple piconets interconnected; a device can be master in one and slave in another.

• Device interaction:

  • Active mode: Participates in traffic.

  • Parked mode: Synchronized but not active (saves power).

• Comparison:

  | Feature | Piconet | Scatternet | |---------------|-----------------------------|-----------------------------| | Topology | Star (1 master, ≤7 slaves) | Multiple piconets overlapped| | Scalability | Limited to 8 active devices | Higher (via inter-piconet links) | | Coverage | ~10 m | Extended via relaying | | Device roles | Fixed master/slave | Dynamic (master/slave per piconet) |

IEEE 802.15 Standards

• WPAN overview: Low-power, short-range networks.

  • 802.15.1: Bluetooth (high-rate).

  • 802.15.4: Low-rate WPAN (LR-WPAN); basis for Zigbee.

• Zigbee:

  • Features: Low-power, low-data-rate (~250 kbps), mesh networking.

  • Topology: Star, tree, mesh.

  • Applications: Home automation, industrial control, sensor networks.

  • Protocol stack: PHY/MAC (802.15.4), network/security (Zigbee alliance).

Interoperability between WLAN and WPAN

• Interface mechanisms:

  • Coexistence: Both operate in 2.4 GHz; use adaptive frequency hopping (Bluetooth) and channel selection (Wi-Fi) to avoid interference.

  • Gateway functions: Device with both radios (e.g., smartphone) bridges traffic.

• Use cases:

  • Smartphone connects to Wi-Fi for Internet, Bluetooth for peripherals (headset, smartwatch).

  • IoT gateway aggregates Zigbee sensors and forwards via Wi-Fi.

5. Introduction to Key Enabling Technologies

Orthogonal Frequency Division Multiplexing (OFDM)

• Principle: Divides wideband channel into many narrowband orthogonal subcarriers; each carries low-rate data.

  • Orthogonality: Subcarrier spacing Δf = 1/T, where T is symbol duration.

$$\boxed{\Delta f = \frac{1}{T}}$$

Ensures no inter-carrier interference (ICI).

• Block diagram:

  DiagramCANVAS: OFDM transmitter: serial data → serial-to-parallel → QAM mapping → IFFT → parallel-to-serial → add CP → RF. Receiver: RF → remove CP → FFT → demapping → parallel-to-serial → serial data.

• Advantages:

  • Robust to ISI via cyclic prefix (CP).

  • Efficient spectrum usage (overlapping subcarriers).

  • Simple equalization (one-tap per subcarrier).

  • Used in 802.11a/g/n/ac/ax, LTE, 5G.

Multiple Input Multiple Output (MIMO)

• SISO vs. MIMO:

  • SISO: Single antenna at Tx/Rx.

  • MIMO: Multiple antennas at both ends (Nt × Nr).

• Advantages of MIMO:

  1. Spatial diversity: Multiple paths improve reliability (e.g., Alamouti coding).

  2. Spatial multiplexing: Parallel streams increase data rate (gain = min(Nt, Nr)).

  3. Beamforming: Focuses energy toward receiver, extending range and SNR.

• Applications:

  • LTE: 2×2, 4×4 MIMO for high throughput.

  • Wi-Fi (802.11n/ac/ax): MU-MIMO serves multiple users simultaneously.

OFDM-MIMO Combination

• How it addresses challenges:

  • Frequency-selective fading: OFDM converts wideband channel into flat-fading subcarriers; MIMO exploits spatial diversity per subcarrier.

  • High mobility: OFDM reduces ISI; MIMO diversity combats fast fading.

  • Inter-symbol interference: Cyclic prefix in OFDM absorbs multipath delay spread.

• Benefits:

  • High spectral efficiency (bits/s/Hz).

  • Robustness in diverse channel conditions.

  • Enables high data rates in LTE-A and Wi-Fi 6 (802.11ax).

[!TIP]

Exam Focus: MIMO multiplexing gain increases throughput linearly with antennas; diversity gain improves reliability without rate increase.