Speech Processing (EC-803 (C)) - Unit 1 Short Notes

I. Foundations of Wireless Communication

Characteristics of the Wireless Medium:

• Mobility & Ubiquity: Users can move while maintaining connectivity.

• Broadcast Nature: Signals propagate in all directions, leading to interference and security vulnerabilities.

• Limited & Shared Bandwidth: Spectrum is scarce and regulated; multiple users share channels.

• High Path Loss & Attenuation: Signal strength decreases with distance and obstacles.

• Multipath Propagation: Signals arrive via multiple paths causing constructive/destructive interference.

• Doppler Shift: Frequency change due to relative motion between source and receiver.

• Noise & Interference: From other wireless systems, natural sources, and electronic devices.

Propagation Effects:

• Multipath Fading:

  • Caused by reflection, diffraction, scattering.

  • Results in flat fading (frequency-selective if bandwidth > coherence bandwidth).

  • Deep fades occur when multipath components cancel each other.

• Doppler Shift:

  • $$\displaystyle f_d = \frac{v}{\lambda} = \frac{v f_c}{c} $$, where $v$ = relative velocity, $\lambda$ = wavelength, $$\displaystyle f_c $$ = carrier frequency, $c$ = speed of light.

  • Causes time-selective fading; limits coherence time.

Comparison with Wired Networks:

[!TIP] Exam often asks to contrast wired vs. wireless focusing on reliability, security, and mobility.

II. Cellular Mobile Communication Systems

Evolution (GSM → UMTS → LTE):

Role of 3GPP: Standards body developing GSM → UMTS → LTE specifications; ensures global interoperability and evolution.

UMTS Architecture

Key Components & Interactions:

• UE (User Equipment): Mobile device.

• Node B: Base station (analogous to BTS in GSM); handles radio transmission/reception.

• RNC (Radio Network Controller): Manages radio resources, handovers, connects Node B to core network.

• Core Network:

  • Circuit-Switched: MSC (call control), VLR (visitor location), HLR (home location).

  • Packet-Switched: SGSN (serving GPRS support node), GGSN (gateway to external IP networks).

• Interfaces:

  • Iub: Between Node B and RNC (control/user plane).

  • Iu-CS/Iu-PS: Between RNC and core network (circuit/packet).

E-UTRAN (Evolved UTRAN) Architecture for LTE

Components & Roles:

• eNodeB (evolved Node B): All radio resource management, scheduling, handover decisions. Flat architecture—no RNC.

• EPC (Evolved Packet Core):

  • MME (Mobility Management Entity): Signaling, authentication, bearer management.

  • S-GW (Serving Gateway): Data routing/forwarding, mobility anchor.

  • P-GW (PDN Gateway): Connectivity to external networks, IP address allocation, policy enforcement.

• Interfaces:

  • X2: Between eNodeBs for handover coordination.

  • S1: Between eNodeB and EPC (S1-MME for control, S1-U for user data).

Mobility Management in Cellular Networks:

• Handover Types:

  • Hard Handover: Break-before-make (GSM).

  • Soft Handover: Make-before-break (CDMA).

  • Softer Handover: Within same Node B (UMTS).

• Location Management: Tracking area updates, paging.

• Key Procedures: Authentication, security setup, bearer establishment.

[!TIP] E-UTRAN’s flat architecture reduces latency; eNodeB handles all radio tasks unlike UMTS’s RNC.

III. Wireless Local Area Networks (WLAN)

IEEE 802.11 Protocol Architecture

• Physical Layer (PHY): Defines modulation (DSSS, OFDM), frequency bands (2.4 GHz, 5 GHz), data rates.

• MAC Sublayer:

  • DCF (Distributed Coordination Function): CSMA/CA, mandatory, contention-based.

  • PCF (Point Coordination Function): Optional, contention-free, polling by AP.

  • Management/Control: Association, authentication, beacon frames.

MAC Layer Functions

• Frame Format: MAC header, payload, FCS.

• Addressing: 4 addresses (To DS, From DS bits indicate which are used).

• Medium Access:

  • DCF: CSMA/CA with DIFS for data, SIFS for ACK/CTS.

  • PCF: AP polls stations in contention-free period.

• Fragmentation & Reassembly: To combat interference.

• Power Management: Stations sleep to conserve power.

Medium Access Challenges:

• Hidden Terminal Problem:

  • Stations A and C cannot sense each other but both transmit to B → collision at B.

  • Mitigation: RTS/CTS exchange (optional).

• Exposed Terminal Problem:

  • Station B transmitting to A; station C wants to transmit to D but senses B’s transmission → unnecessarily defers.

  • Mitigation: RTS/CTS with appropriate thresholds, directional antennas, or protocol modifications (e.g., 802.11e).

[!TIP] Exposed terminal reduces capacity; RTS/CTS helps but adds overhead. Hidden terminal is more common exam question.

Comparison: IEEE 802.11 vs. HIPERLAN

IV. Wireless Personal Area Networks (WPAN)

Bluetooth Technology

• Piconet: Up to 8 devices (1 master, 7 slaves). Master controls clock and hopping sequence.

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

• Topology: Ad-hoc, frequency-hopping spread spectrum (FHSS) in 2.4 GHz (79 channels, 1 MHz spacing).

• Scalability: Limited by piconet size and interference in scatternets.

• Coverage: ~10 m (Class 2), up to 100 m (Class 1).

• Device Interaction: Master polls slaves; slaves respond only when polled.

IEEE 802.15 WPAN Standards:

• 802.15.1: Bluetooth (legacy).

• 802.15.2: Coexistence with WLAN.

• 802.15.3: High-rate WPAN (up to 55 Mbps).

• 802.15.4: Low-rate WPAN (Zigbee, 250 kbps).

• 802.15.6: Body Area Networks (BAN).

Zigbee Technology:

• Based on IEEE 802.15.4 (PHY/MAC) + Zigbee network layer.

• Low-power, low-data-rate (up to 250 kbps), long battery life.

• Topologies: Star, tree, mesh.

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

• Security: AES-128 encryption.

[!TIP] Bluetooth piconet max 8 devices; Zigbee supports larger networks via mesh routing.

V. Wireless Sensor Networks (WSN)

WSN Architecture and Components

• Sensor Node: Sensing unit, processor, memory, transceiver, power source.

• Sink/Base Station: Collects data from nodes, connects to external network.

• Gateway: Interfaces WSN with Internet.

• Management Center: Controls network.

Types and Applications:

• Terrestrial: Land monitoring (agriculture, environment).

• Underground: Soil monitoring, mining.

• Underwater (UWSN): Oceanographic, pollution monitoring; uses acoustic waves (high attenuation, low bandwidth).

• Mobile: Vehicles, robots; dynamic topology.

• Body Area Network (BAN): Healthcare, biometrics; IEEE 802.15.6.

Topology Management

• Importance: Energy efficiency (sleep scheduling), network longevity, robustness to node failures.

• Techniques: Clustering (LEACH), sleep/wake cycles, topology control (power adjustment).

Routing Protocols

[!TIP] Proactive suits dense, stable networks; reactive suits sparse, dynamic networks.

Security in WSN

• Challenges: Resource constraints (energy, computation), deployment in hostile areas, lack of physical security.

• Techniques:

  • Confidentiality: Symmetric encryption (AES-128).

  • Integrity: Message Authentication Codes (MACs).

  • Authenticity: Pre-shared keys, public-key (Ecc for low overhead).

  • Key Management: Distributed key generation, key predistribution.

Coverage and Node Placement

• Coverage: Ensures area is monitored; depends on sensing range and node density.

• Placement Strategies:

  • Deterministic: Planned deployment for optimal coverage.

  • Random: Dropped from aircraft; requires redundancy.

• Metrics: Coverage ratio, connectivity, hole detection.

Sensor Node Technologies

• Processors: Low-power microcontrollers (ARM Cortex-M, AVR).

• Transceivers: IEEE 802.15.4 (Zigbee), Bluetooth Low Energy (BLE).

• Sensors: Temperature, humidity, accelerometer, acoustic.

• Power: Batteries (Li-ion), energy harvesting (solar, vibration).

VI. Advanced Transmission and Antenna Techniques

SISO vs. MIMO Systems

Advantages of MIMO:

1. Spatial Multiplexing: Multiple data streams → higher data rates.

2. Diversity: Multiple paths combat fading → reliability.

3. Beamforming: Directed transmission → extended coverage, reduced interference.

OFDM (Orthogonal Frequency Division Multiplexing)

Principle: High-rate data stream split into many low-rate subcarriers that are orthogonal ($$\displaystyle \Delta f = 1/T_{sym} $$), eliminating ICI.

• Transmitter:

  1. Serial-to-parallel conversion.

  2. Modulation (QPSK, QAM) on each subcarrier.

  3. IFFT to convert to time domain.

  4. Cyclic Prefix (CP) insertion to combat ISI.

  5. Parallel-to-serial, RF upconversion.

• Receiver:

  1. RF downconversion, CP removal.

  2. FFT to frequency domain.

  3. Equalization (1-tap per subcarrier due to orthogonality).

  4. Demodulation, parallel-to-serial.

Key Formula: Subcarrier spacing $$\displaystyle \Delta f = \frac{1}{T_{sym}} $$, where $$\displaystyle T_{sym} $$ = symbol duration including CP.

OFDM-MIMO

• Combines MIMO spatial multiplexing with OFDM’s robustness to ISI.

• Each MIMO layer uses OFDM subcarriers.

• Addresses Channel Variability: Frequency diversity across subcarriers; time diversity via coding.

• Addresses ISI: CP converts linear convolution to circular, enabling simple FFT-based equalization.

• Challenges: High peak-to-average power ratio (PAPR), channel estimation overhead.

[!TIP] OFDM-MIMO is core of LTE/5G; CP length chosen to exceed channel delay spread.

VII. Transport Layer Protocols for Wireless Networks

Traditional TCP (Reno) and Limitations:

• Assumes packet loss = congestion → reduces congestion window aggressively.

• Problems in wireless: High bit error rates, variable latency, handovers cause non-congestion losses → throughput collapse.

TCP Variants:

Mobile TCP (M-TCP) Adaptations:

• Split Connection: Break end-to-end connection at base station; local retransmissions.

• Selective Retransmission: Avoids unnecessary window reduction.

• Frozen Timers: During handovers, timers paused to avoid spurious timeouts.

• Explicit Notification: Base station notifies sender of wireless segment loss.

Congestion Window Management:

• Slow Start: $$\displaystyle cwnd \leftarrow cwnd + 1 $$ per ACK → exponential growth until threshold $ssthresh$.

• Congestion Avoidance: $$\displaystyle cwnd \leftarrow cwnd + 1/cwnd $$ per ACK → linear growth.

• Loss Event: $$\displaystyle ssthresh \leftarrow cwnd/2 $$, $$\displaystyle cwnd \leftarrow 1 $$ (Tahoe) or $$\displaystyle cwnd \leftarrow ssthresh $$ (Reno).

[!TIP] Vegas uses RTT variation $$\displaystyle \Delta = (cwnd - \frac{RTT_{min}}{RTT} \cdot cwnd) $$ to adjust $cwnd$.

VIII. Mobility Management Protocols

Mobile IP

Operation (MN away from home network):

1. Agent Discovery: MN learns home agent (HA) and foreign agent (FA) via advertisements.

2. Registration: MN registers Care-of Address (CoA) (FA’s address or co-located) with HA.

3. Data Forwarding:

   • CN sends packet to MN’s home address.

   • HA tunnels packet (encapsulation) to MN’s CoA.

   • FA decapsulates and delivers to MN.

4. Return Route: Packets from MN to CN may go via HA (triangle routing) or directly (route optimization).

Optimizations:

• Reverse Tunneling: MN’s packets tunneled back through HA to avoid ingress filtering (when CN expects packets from MN’s home address).

• Route Optimization: CN learns MN’s CoA and sends directly, reducing latency.

General Mobility Management Concepts:

• Location Management: Track MN’s current point of attachment (paging, updates).

• Handover Management: Seamless transfer during movement (layer 2 vs. layer 3 handover).

[!TIP] Mobile IP’s triangle routing inefficiency addressed by route optimization; reverse tunneling for security.

IX. Internet of Things (IoT)

IoT Architecture

• Perception Layer: Sensors/actuators collect data; RFID, barcodes.

• Network Layer: Wired/wireless networks (WSN, WPAN, cellular); gateways for protocol translation.

• Application Layer: Data processing, cloud services, user interfaces (smart homes, healthcare).

Design Principles:

• Scalability: Support billions of devices.

• Interoperability: Standard protocols (MQTT, CoAP, HTTP).

• Security: End-to-end encryption, authentication.

• Energy Efficiency: Low-power operation, duty cycling.

• Data-Centric: Focus on data collection/analysis.

Required Capabilities:

• Unique identification (IP, EPC).

• Sensing/actuation.

• Communication (short/long-range).

• Data processing (edge/cloud).

Emerging IoT Standards for Networking Engineers:

• LPWAN: LoRaWAN, NB-IoT, Sigfox (long-range, low-power).

• WPAN: Bluetooth 5, Zigbee 3.0, Thread.

• Edge Computing: MEC (Multi-access Edge Computing).

• Frameworks: OneM2M, AWS IoT, Azure IoT.

Case Study: Sensor Body Area Network (BAN) Implementation

• Architecture: Wearable sensors (ECG, temperature) → BAN coordinator (e.g., smartphone) → healthcare server via WBAN (IEEE 802.15.6).

• Applications: Remote patient monitoring, fitness tracking.

• Challenges: Power constraints, interference, security of health data.

• Implementation: Use BLE for low-power communication; AES encryption; gateway aggregates data to cloud.

X. Specialized and Emerging Wireless Technologies

Wireless ATM (WATM)

• Goal: Extend ATM’s QoS guarantees to wireless.

• Architecture:

  • Radio Access Point (RAP): Connects mobile to WATM switch.

  • WATM Switch: Handles mobility, QoS, cell relay.

  • Fixed ATM Backbone: High-speed core.

• High-Speed Data Transmission: Uses small fixed-size ATM cells (53 bytes) for low delay and predictable performance.

• Research Challenges:

  • Wireless channel errors → cell loss; need error correction.

  • Mobility management during handover (cell loss, delay).

  • Limited bandwidth vs. wired ATM.

  • Power constraints for mobiles.

WiMAX Overview:

• IEEE 802.16: Broadband wireless access (fixed/mobile).

• PHY: OFDMA (downlink), SC-FDMA (uplink).

• MAC: Connection-oriented, supports QoS (UGS, rtPS, nrtPS, BE).

• Coverage: Up to 50 km; data rates up to 1 Gbps.

• Role: Last-mile broadband, alternative to DSL/cable.

GAGAN (GPS-Aided GEO Augmented Navigation):

• Purpose: Satellite-based augmentation system (SBAS) for improved GPS accuracy, integrity, and availability in aviation.

• Architecture: Geostationary satellites relay correction signals from ground stations.

• Benefits: Vertical guidance (LPV approaches), reduces flight delays, enhances safety.

IPv4 vs. IPv6 Addressing:

IEEE 802.15 WPAN (Detailed):

• 802.15.1: Bluetooth (up to 3 Mbps, FHSS).

• 802.15.2: Coexistence with 802.11 (interference mitigation).

• 802.15.3: High-rate (55 Mbps), QoS support for multimedia.

• 802.15.4: Low-rate (250 kbps), basis for Zigbee; star/mesh topologies.

• 802.15.6: Body Area Networks (operates in ISM/medical bands); ultra-low-power.

[!TIP] GAGAN is Indian SBAS; WiMAX uses OFDMA; IPv6 eliminates NAT.