Speech Processing (EC-803 (C)) - Unit 5 Short Notes

UNIT 5: WIRELESS NETWORKS & COMMUNICATION SYSTEMS

(Compiled from RGPV EC-803(C) Past Papers: 2022–2025)

1.0 INTRODUCTION TO WIRELESS NETWORKS

1.1 Wireless Medium Characteristics

• Broadcast nature: Signals propagate in all directions, enabling one-to-many communication but causing security vulnerabilities and uncontrolled interference.

• Limited bandwidth: Spectrum is a scarce, regulated resource. Frequency reuse in cellular systems mitigates this.

• Signal propagation issues:

  • Path loss: Power decay with distance ($$\displaystyle \propto d^{-n} $$, $n$ = path loss exponent).

  • Multipath: Reflected signals cause constructive/destructive interference → fading.

  • Doppler shift: $$\displaystyle f_d = \frac{v}{\lambda} \cos\theta $$, where $v$ = mobile speed, $\lambda$ = wavelength.

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Exam Focus: Distinguish between large-scale fading (path loss) and small-scale fading (multipath).

2.0 CELLULAR NETWORK EVOLUTION & ARCHITECTURES

2.1 GSM to UMTS Evolution

2.2 UMTS Network Architecture

graph LR

    UE --> NodeB;

    NodeB --> RNC;

    RNC --> MSC[Circuit-Switched CN];

    RNC --> SGSN[PS Core SGSN];

    SGSN --> GGSN[GGSN];

    MSC --> VLR;

    MSC --> HLR;

• Core Network (CN):

  • MSC (Mobile Switching Centre): Circuit-switched calls.

  • VLR (Visitor Location Register): Temporary subscriber data.

  • HLR (Home Location Register): Permanent subscriber data.

  • SGSN (Serving GPRS Support Node): PS mobility, data routing.

  • GGSN (Gateway GPRS Support Node): External network gateway.

• UTRAN (UMTS Terrestrial RAN):

  • Node B: Base station (physical layer).

  • RNC (Radio Network Controller): MAC/RLC scheduling, handover control.

  • Interfaces: Iub (NodeB-RNC), Iur (RNC-RNC), Iu (RNC-CN).

2.3 3GPP Standards & Role

• 3GPP (3rd Generation Partnership Project): Defines global specs for GSM, UMTS, LTE, 5G.

• LTE Objectives:

  • High spectral efficiency (downlink: 5× HSDPA).

  • Low latency (<10 ms).

  • Simplified architecture (all-IP, flat core).

• Key Releases:

  • Rel-8: First LTE specs (2008).

  • Rel-10: LTE-Advanced (carrier aggregation, MIMO).

2.4 E-UTRAN (LTE) Architecture

• Components:

  • eNodeB (evolved Node B):

    • MAC scheduling (dynamic, QoS-aware).

    • RLC (ARQ), PDCP (header compression, encryption).

    • Handover decisions (X2-based).

  • Interfaces:

    • X2: eNodeB-to-eNodeB (handover coordination).

    • S1: eNodeB to MME/S-GW (control/user plane split).

• Mobility Management:

  • X2-based handover: Direct eNodeB communication → low latency.

  • S1-based handover: Via MME → used when X2 unavailable.

• Resource Allocation:

  • Dynamic scheduling: eNodeB assigns RB (Resource Block) per TTI (1 ms).

  • QoS: QoS Class Identifier (QCI) maps to ARQ, scheduling priority.

2.5 IEEE 802.16 (WiMAX) vs. Fixed Standard

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Exam Focus: Mobile WiMAX introduces scalable OFDMA (variable FFT size: 128–2048) to support diverse channel bandwidths.

3.0 WIRELESS LOCAL AREA NETWORKS (WLANs)

3.1 IEEE 802.11 Protocol Architecture

• Physical Layer Variants:

  • FHSS/DSSS (802.11 legacy): Spread spectrum, low rate (1–2 Mbps).

  • OFDM (802.11a/g/n/ac): Multi-carrier, high rate (up to Gbps).

• MAC Sublayer:

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

  • PCF (Point Coordination Function): Central polling (optional).

  • HCF (Hybrid Coordination Function): 802.11e QoS (EDCA, HCCA).

• MAC Management: Scanning → Authentication → Association.

3.2 MAC Layer Functions in Detail

• CSMA/CA with RTS/CTS:

  1. DIFS idle → transmit if frame < RTS threshold.

  2. Else, send RTS → CTS from receiver → DATA → ACK.

• Frame Format:

  • MAC header: Duration/ID, Addr1–4, Sequence Control.

  • Frame Body: Payload (≤2304 bytes).

  • FCS: CRC-32.

• Power Management:

  • PS-Poll: Station wakes, polls AP for buffered frames.

  • U-APSD (802.11e): Uplink/downlink access during TXOP.

3.3 Medium Access Challenges

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Exam Focus: NAV (Network Allocation Vector) in MAC header implements virtual carrier sensing to mitigate hidden/exposed terminals.

3.4 HIPERLAN vs. IEEE 802.11

3.5 Interface between 802.11 and Bluetooth

• Coexistence Mechanisms:

  • AFH (Adaptive Frequency Hopping): Bluetooth avoids 802.11 channels.

  • Time-division: Schedule transmissions in non-overlapping slots.

  • Packet scheduling: Prioritize based on traffic type.

4.0 MIMO & OFDM SYSTEMS

4.1 SISO vs. MIMO Systems

• SISO (Single-Input Single-Output):

  • Capacity: $$\displaystyle C = B \log_2(1 + \text{SNR}) $$ (Shannon).

• MIMO (Multi-Input Multi-Output):

  • Spatial diversity: Alamouti code → $$\displaystyle d_{\text{min}} $$ improvement.

  • Spatial multiplexing: V-BLAST → capacity scales with $$\displaystyle \min(M_t, M_r) $$.

  • Beamforming: Directional transmission → coverage gain.

  • Capacity: $$\displaystyle C = \sum_{i=1}^{\min(M_t,M_r)} \log_2(1 + \lambda_i \text{SNR}) $$, $$\displaystyle \lambda_i $$ = eigenvalues.

4.2 MIMO Configurations & Applications

• Alamouti Coding (2×1 or 2×2):

  • Transmission matrix:

$$\mathbf{X} = \begin{bmatrix} s_1 & -s_2^* \\ s_2 & s_1^* \end{bmatrix}$$

• Full diversity, no channel state info (CSI) at transmitter.

• V-BLAST (Vertical Bell Labs Layered Space-Time):

  • Layered spatial multiplexing with successive interference cancellation.

• LTE/LTE-A:

  • Transmit Diversity (2/4 antennas).

  • Spatial Multiplexing (up to 8 layers in LTE-A).

4.3 OFDM Fundamentals

• Orthogonality: Subcarriers spaced by $$\displaystyle \Delta f = 1/T_s $$, where $$\displaystyle T_s $$ = symbol duration.

$$\int_0^{T_s} e^{j2\pi (f_m - f_n)t} dt = 0 \quad (m \neq n)$$

• Transmitter Block Diagram:

  
  Serial Bits → QAM Mapper → Serial/Parallel → IFFT → CP Add → DAC → RF → Antenna
  
  

• Cyclic Prefix (CP):

  • Length $$\displaystyle T_{cp} \geq \tau_{\text{max}} $$ (max delay spread).

  • Converts linear convolution to circular → simple frequency domain equalization.

4.4 OFDM-MIMO Integration

• Addresses frequency selectivity: OFDM splits wideband channel into flat-fading subcarriers.

• Mitigates ISI: CP absorbs multipath delay spread.

• Channel Estimation:

  • Pilot subcarriers (comb-type/block-type).

  • MIMO-OFDM requires per-antenna channel estimation.

• Equalization:

  • ZF (Zero-Forcing): $$\displaystyle \mathbf{H}^{-1} $$ per subcarrier.

  • MMSE: $$\displaystyle \mathbf{H}^H(\mathbf{H}\mathbf{H}^H + \sigma^2\mathbf{I})^{-1} $$.

[!TIP]

Exam Focus: In MIMO-OFDM, per-subcarrier MIMO processing is key; each subcarrier sees a MIMO flat-fading channel.

5.0 WIRELESS SENSOR NETWORKS (WSNs)

5.1 WSN Architecture & Node Components

[ Sensing Unit ] → [ Processing Unit (MCU) ] → [ Communication Unit (RF) ]

         ↑

[ Power Unit (Battery/Energy Harvesting) ]

• Network Topologies:

  • Star: Single-hop to sink.

  • Mesh: Multi-hop, peer-to-peer.

  • Tree: Hierarchical, cluster-based.

5.2 WSN vs. Traditional Wired Networks

5.3 Routing Protocols in WSNs

[!TIP]

Exam Focus: LEACH (Low-Energy Adaptive Clustering Hierarchy) is a hierarchical, cluster-based protocol that rotates cluster heads to balance energy.

5.4 Topology Management

• Importance:

  • Energy efficiency: Sleep scheduling reduces idle listening.

  • Robustness: Redundancy via multi-path.

  • Coverage: Ensure area monitoring (deployment planning).

• Techniques:

  • Clustering (LEACH, PEGASIS): Data aggregation at CHs.

  • Sleep scheduling: TDMA-based wake/sleep cycles.

  • Topology control: Power adjustment to maintain connectivity.

5.5 Security in WSNs

[!TIP]

Exam Focus: TinySec is a lightweight link-layer security protocol for WSNs using CBC mode with a 64-bit IV.

5.6 Underwater WSNs (UWSNs)

• Architecture:

  • Sensor nodes (anchored/mobile) → Surface gateway (buoy) → Onshore base station → Control center.

  • Acoustic modems for communication (RF/optical limited).

• Applications:

  • Oceanography (temperature, salinity), disaster prevention (tsunami detection), surveillance (pipeline monitoring).

• Challenges:

  • High propagation delay: 1500 m/s vs. 3×10⁸ m/s (RF) → latency ~1–5 s/km.

  • Limited bandwidth: ~10–100 kHz (acoustic).

  • Node mobility: Water currents cause drift → topology changes.

  • Energy: Battery replacement difficult → energy harvesting (vibration, salinity).

6.0 MOBILITY & TRANSPORT LAYER PROTOCOLS

6.1 Mobility Management

• Mobile IP:

  • Home Agent (HA): Maintains binding of home address → care-of address.

  • Foreign Agent (FA): Provides care-of address (CoA) in visited network.

  • Tunneling: Encapsulates packets to HA → FA → mobile node.

  • Triangular Routing: CN → HA → MN (suboptimal).

• Optimizations:

  • Route Optimization: CN learns MN’s CoA → direct tunneling.

  • MIPv6: Uses IPv6 addressing, no FA needed (MN acquires CoA via Router Advertisements).

6.2 TCP over Wireless/Mobile Networks

• Problems:

  • High packet loss: Due to bit errors (not congestion) → TCP misinterprets as congestion → $\text{cwnd}$ reduction.

  • Variable latency: Handovers cause RTT spikes → spurious timeouts.

  • Frequent disconnections: Connection breaks → slow restart.

• Traditional TCP Variants:

  • Tahoe: Timeout → $$\displaystyle \text{cwnd}=1 $$, no fast recovery.

  • Reno: Fast retransmit + fast recovery (halve $\text{cwnd}$).

  • New-Reno: Partial ACK for multiple losses.

  • Vegas: Congestion avoidance based on RTT increase (proactive).

6.3 TCP Variants for Wireless Environments

6.4 Congestion Window Management

• Phases:

  1. Slow Start: $$\displaystyle \text{cwnd} \leftarrow 1 $$, increase by 1 per ACK → exponential.

  2. Congestion Avoidance: $$\displaystyle \text{cwnd} \leftarrow \text{cwnd} + 1/\text{cwnd} $$ per ACK → linear.

  3. Fast Retransmit: 3 duplicate ACKs → $$\displaystyle \text{cwnd} \leftarrow \text{cwnd}/2 $$.

  4. Fast Recovery: Reno: $$\displaystyle \text{cwnd} \leftarrow \text{cwnd}/2 + 3 $$, New-Reno: partial ACK handling.

• Wireless Consequences:

  • Spurious timeout: $\text{cwnd}$ drops to 1 → throughput collapse.

  • Throughput: $$\displaystyle \approx \frac{\text{MSS}}{\text{RTT} \sqrt{p}} $$ (Padhye model), where $p$ = loss probability.

7.0 INTERNET OF THINGS (IoT)

7.1 IoT Architecture

• Three-Layer:

  • Perception: Sensors/actuators (data acquisition).

  • Network: Gateways, communication protocols (LPWAN, WPAN).

  • Application: Data analytics, user interfaces.

• Five-Layer (extended):

  • Perception → Transport (data forwarding) → Processing (cloud/edge) → Application → Business (management).

7.2 IoT Design Principles & Capabilities

• Principles:

  • Scalability: Millions of devices.

  • Interoperability: Standard protocols (MQTT, CoAP).

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

  • Energy efficiency: Duty cycling, low-power radios.

• Capabilities:

  • Ubiquitous sensing, real-time analytics, autonomous control.

7.3 Emerging IoT Standards

8.0 WIRELESS PERSONAL AREA NETWORKS (WPANs)

8.1 Bluetooth Technology

• Piconet:

  • Master (1) ↔ Slaves (≤7 active, 255 parked).

  • Frequency hopping: 79 channels (1 MHz spacing), 1600 hops/s.

  • Coverage: ~10 m (Class 2).

• Scatternet:

  • Interconnected piconets via bridge nodes (time-sharing).

  • Scalability limit: Complexity in synchronization, reduced throughput.

8.2 IEEE 802.15 WPAN Family

8.3 Zigbee Protocol Stack

Application (Profiles: Home, Industrial)

    ↓

Network (NWK): Mesh routing (AODV), star/cluster-tree

    ↓

MAC (CSMA/CA, GTS for guaranteed slots)

    ↓

PHY (O-QPSK, DSSS, 2.4 GHz/915 MHz/868 MHz)

• Mesh networking: Multi-hop, self-healing.

• Low power: Sleep modes, beacon-enabled mode.

9.0 SPECIALIZED WIRELESS TECHNOLOGIES

9.1 Wireless ATM

• Concepts:

  • Fixed-length 53-byte cells (header + payload).

  • QoS guarantees via VC/VP switching, traffic contracts.

• Architecture:

  • W-ATM switch → Base station → Mobile terminal.

  • Radio link protocol (RLP) for error control.

• Challenges:

  • Mobility support: VC re-routing during handover.

  • Resource reservation: CAC (Call Admission Control) in fading channels.

9.2 GPS & GAGAN

• GPS-Aided GEO Augmented Navigation (GAGAN):

  • SBAS (Satellite-Based Augmentation System) for aviation.

  • Purpose: Improve accuracy (from ~10 m to <3 m), integrity monitoring (alert if GPS faulty).

  • Architecture: GEO satellites → ground reference stations → correction signals.

9.3 IPv4 vs. IPv6 Addressing

10.0 COMPARATIVE ANALYSIS & INTEGRATED TOPICS

10.1 TCP vs. UDP in Wireless Contexts

10.2 Wireless ATM vs. Traditional IP-Based Networks

10.3 Integration Challenges in Heterogeneous Networks

• Vertical Handoff:

  • Decision metrics: RSS, bandwidth, cost, user preference.

  • Seamless handoff: Maintain session continuity (Mobile IP, SIP).

• Unified Mobility Management:

  • IP-based: Single address across networks (Mobile IPv6).

  • Cross-layer signaling: Link-layer events → network-layer decisions.

• Interworking:

  • Cellular-WLAN: Offloading (3GPP ANDSF).

  • IoT-WPAN: Gateway protocol translation (MQTT ↔ CoAP).

BOXED KEY FORMULAS & RESULTS

1. OFDM Subcarrier Orthogonality:

$$\Delta f = \frac{1}{T_s} \quad \text{(subcarrier spacing)}$$

2. MIMO Capacity (i.i.d. Rayleigh fading):

$$C = \mathbb{E} \left[ \sum_{i=1}^{\min(M_t,M_r)} \log_2(1 + \text{SNR} \cdot \lambda_i) \right]$$

3. Path Loss Model:

$$PL(d) = PL(d_0) + 10n \log_{10}\left(\frac{d}{d_0}\right) + X_\sigma$$

where $n$ = path loss exponent, $$\displaystyle X_\sigma $$ = shadowing (log-normal).

4. Doppler Shift:

$$f_d = \frac{v f_c}{c} \cos\theta$$

5. TCP Throughput (Padhye Model):

$$\text{Throughput} \approx \frac{\text{MSS}}{\text{RTT} \sqrt{\frac{2bp}{1+2b p}}}$$

where $b$ = duplicate ACKs, $p$ = loss probability.

PAST PAPER QUESTION MAPPING

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Exam Strategy: For 7m questions, structure answer as:

1. Definition (1m)

2. Key components/mechanisms (4m)

3. Advantages/challenges (2m)

For 3–4m short notes: Definition + 2–3 bullet points only.