(SEM VII) THEORY EXAMINATION 2022-23 WIRELESS AND MOBILE COMMUNICATION

SECTION A (2 Marks Each)

(a) General Model of Wireless Communication System

A wireless communication system consists of information source → transmitter → channel → receiver → destination. The channel introduces noise, fading, and interference.

(b) Condition for Zero ISI

Zero Inter-Symbol Interference occurs when the overall impulse response is zero at all sampling instants except the desired one, satisfying the Nyquist criterion.

(c) Selection Diversity

Selection diversity selects the branch with the highest signal-to-noise ratio (SNR) among multiple received signals to improve performance.

(d) PDF of Rician Fading Channel

The probability density function of Rician fading is
f(r) = (r/σ²) exp [−(r² + s²)/(2σ²)] I₀(rs/σ²)
where s is LOS component and σ is variance.

(e) Efficiency of ALOHA

Pure ALOHA: Maximum efficiency = 18.4% (1/2e)

Slotted ALOHA: Maximum efficiency = 36.8% (1/e)

(f) Pooling

Pooling is the sharing of resources like channels or bandwidth among users to improve utilization efficiency.

(g) LTE

LTE (Long Term Evolution) is a 4G wireless standard providing high data rates, low latency, and packet-switched architecture.

(h) GPRS

GPRS (General Packet Radio Service) is a packet-based data service for GSM networks supporting internet access.

(i) Wireless Ad-hoc Network

It is a self-configuring network without fixed infrastructure where nodes communicate directly or via multi-hop routing.

(j) Li-Fi Communication

Li-Fi uses visible light for data transmission with LEDs, offering high speed and security.

SECTION B (10 Marks Each – Attempt Any Three)

(a) Nakagami Fading Channel

Nakagami fading is a statistical model that describes multipath fading. Its PDF is flexible and can model Rayleigh and Rician fading. The shape parameter (m) controls severity of fading, while Ω represents average power. It is widely used in wireless channel modeling.

(b) Direct Sequence Spread Spectrum (DSSS)

In DSSS, the data signal is multiplied by a high-rate pseudo-noise (PN) sequence, spreading the signal bandwidth. At the receiver, despreading recovers the original data. It provides interference resistance, security, and multipath immunity.

(c) Transversal Filters

A transversal filter is a finite impulse response (FIR) filter with delay elements, multipliers, and adders. Its response is generated by summing weighted delayed versions of the input signal. It is widely used in adaptive equalizers.

(d) Mobile Satellite Communication

Mobile satellite communication provides connectivity to mobile users via satellites. The system consists of space segment (satellite), ground segment (earth stations), and user segment. It supports navigation, maritime, aeronautical, and remote communication.

(e) Bluetooth

Bluetooth is a short-range wireless technology operating at 2.4 GHz ISM band.
Architecture includes master-slave configuration (piconet).
Applications include wireless headsets, IoT devices, keyboards, and file transfer.

SECTION C (10 Marks Each)

Q3

(a) Strategies to Increase Capacity of Wireless Systems

Capacity can be increased using cell splitting, sectoring, smart antennas, power control, frequency reuse, MIMO systems, and adaptive modulation. These techniques improve spectral efficiency.

(b) Handoff Strategies and Ping-Pong Effect

Handoff transfers an active call from one base station to another. Strategies include hard handoff, soft handoff, signal-strength-based, and mobile-assisted handoff.
Ping-pong occurs when frequent handoffs happen between cells due to signal fluctuation.

Q4

(a) Diversity Combining Techniques

Techniques include selection combining, maximal ratio combining (MRC), and equal gain combining (EGC).
Advantages: reduced fading, improved SNR, and better reliability.

(b) Vocoders

Vocoders compress speech for transmission. Types include channel vocoder, LPC vocoder, and CELP vocoder. They analyze speech parameters and reconstruct speech at the receiver.

Q5

(a) Need of Equalization and Decision Feedback Equalizer

Equalization compensates ISI caused by multipath.
A Decision Feedback Equalizer (DFE) uses previous decisions to cancel post-cursor ISI, improving performance in wireless channels.

(b) RAKE Receiver

A RAKE receiver combines multiple delayed signal paths using correlators. Each finger processes a multipath component.
Main advantage: efficient utilization of multipath energy in CDMA systems.

Q6

(a) IMT-2000

IMT-2000 defines 3G mobile standards offering data rates up to 2 Mbps, global roaming, multimedia support, and enhanced voice quality.

(b) Wireless Local Loop (WLL)

WLL replaces copper lines with wireless links for last-mile connectivity. It provides voice and data services with lower installation cost.

Q7

(a) Wi-MAX

Wi-MAX (IEEE 802.16) provides broadband wireless access over long distances.
Challenges include interference, mobility management, high deployment cost, and spectrum issues.

(b) Next Generation Networks (NGN)

NGN is an IP-based network integrating voice, data, and multimedia services.
Key characteristics include packet switching, QoS support, mobility, and convergence.