(SEM-VII) THEORY EXAMINATION 2021-22 WIRELESS & MOBILE COMMUNICATION

WIRELESS & MOBILE COMMUNICATION – KEC 076

B.Tech (Semester VII) – Detailed Answers

SECTION A – Descriptive Answers (2 Marks Each)

a) Trade-off Between System Capacity and Co-Channel Interference

In cellular systems, increasing system capacity requires reducing the frequency reuse distance, which increases co-channel interference. Conversely, increasing reuse distance reduces interference but decreases capacity. Hence, a balance must be maintained between high capacity and acceptable interference levels to ensure satisfactory signal quality.

b) Channel Assignment Strategies

The channel assignment strategies in mobile radio systems are:        Fixed Channel Assignment (FCA)

Dynamic Channel Assignment (DCA)                                                   Hybrid Channel Assignment (HCA)

c) Multiple Access

Multiple access refers to techniques that allow multiple users to share the same communication medium simultaneously by dividing resources in frequency, time, code, or space.

d) Space Diversity Techniques

Types of space diversity techniques include:                     Transmit diversity

Receive diversity                                                                 MIMO (Multiple Input Multiple Output)

e) Persistent Methods in CSMA

CSMA persistence methods include:                                 1-persistent CSMA

Non-persistent CSMA                                                        p-persistent CSMA
These methods determine how a node behaves when the channel is busy.

f) Equalization

Equalization is a signal processing technique used at the receiver to reduce intersymbol interference (ISI) caused by multipath propagation and channel distortion.

g) Uplink and Downlink Frequency Band of GSM

In GSM-900:                                                                     Uplink: 890–915 MHz

Downlink: 935–960 MHz
Uplink is used for mobile-to-base transmission, while downlink is for base-to-mobile transmission.

h) Specifications of LEO, MEO and GEO

i) Advantages of NGN (Next Generation Networks)

NGN provides high data rates, multimedia services, IP-based architecture, reduced operational cost, scalability, and improved Quality of Service (QoS).

j) Wi-Fi vs WiMAX

SECTION B – Long Answers (10 Marks Each)

2(a) MAHO Technique and Queuing Concept in Handoff

Mobile Assisted Handoff (MAHO) allows the mobile station to measure signal strength of neighboring base stations and report to the network. The queuing concept prioritizes handoff calls over new calls to minimize call drops.                                         Types of handoff include:

Hard handoff                                                    Soft handoff

Softer handoff                                                   Horizontal and Vertical handoff

2(b) PN Sequence Generation Using 3-bit LFSR

A PN sequence is generated using a Linear Feedback Shift Register (LFSR) with XOR feedback. In a 3-bit LFSR, taps are selected based on a primitive polynomial. The sequence repeats after 2n−12^n - 12n−1 states, producing a pseudo-random pattern used in spread spectrum systems.

2(c) Pure ALOHA vs Slotted ALOHA

Slotted ALOHA improves efficiency by dividing time into slots.

2(d) UMTS Architecture and IMT-2000

UMTS architecture consists of:                                   User Equipment (UE)

UTRAN (Node-B, RNC)                                               Core Network

IMT-2000 is a global framework for 3G systems providing high data rates, global roaming, and multimedia services.

2(e) Wi-Fi and WiMAX Standards

Wi-Fi standards include IEEE 802.11a/b/g/n/ac, while WiMAX uses IEEE 802.16d and 802.16e. WiMAX supports large coverage and high-speed broadband access.

SECTION C – Very Long Answers (10 Marks Each)

3(a) SIR, Frequency Reuse Factor and Cluster Size

Given:                                                                           SIR = 15 dB → 31.6
Number of co-channel cells = 6

Using:                                                                           SIR=(D/R)ni\text{SIR} = \frac{(D/R)^n}{i}SIR=i(D/R)n​

For n = 4:

(D/R)4=6×31.6⇒D/R≈3.7(D/R)^4 = 6 \times 31.6 \Rightarrow D/R \approx 3.7(D/R)4=6×31.6⇒D/R≈3.7 N=(D/R)23≈4.6⇒N=7N = \frac{(D/R)^2}{3} \approx 4.6 \Rightarrow N = 7N=3(D/R)2​≈4.6⇒N=7

For n = 3:

(D/R)3=6×31.6⇒D/R≈5.7(D/R)^3 = 6 \times 31.6 \Rightarrow D/R \approx 5.7(D/R)3=6×31.6⇒D/R≈5.7 N≈11N \approx 11N≈11 

3(b) Frequency Reuse Concept

Frequency reuse divides the service area into cells, each using a subset of frequencies. Cells using the same frequencies are separated by reuse distance.
A 19-cell reuse pattern improves capacity while controlling interference. (Neat cellular diagram to be drawn in exam)

4(a) Vocoders and Properties of Speech Signal

Vocoders compress speech by modeling vocal tract parameters.

Types of vocoders:                                                        Channel vocoder

LPC vocoder                                                                    CELP vocoder

Speech signal properties include bandwidth limitation, voiced/unvoiced sounds, redundancy, and time-varying nature.

4(b) Frequency Hopped Multiple Access (FHMA)

FHMA rapidly changes carrier frequency according to hopping sequence. Types include:

Slow frequency hopping                                                  Fast frequency hopping
Proper hop timing diagrams illustrate frequency transitions.

5(a) Equalization Techniques

Equalization techniques include:                                      Linear equalizer

Decision Feedback Equalizer (DFE)                                   Adaptive equalizer

These reduce ISI and improve signal quality. (Block diagram to be drawn)

5(b) FDMA and TDMA

FDMA:    Divides spectrum into frequency bands assigned to users.

TDMA:    Divides time into slots assigned to users.

TDMA improves spectrum efficiency over FDMA.

6(a) GSM Architecture

GSM architecture includes:                                               Mobile Station

Base Station Subsystem (BTS, BSC)                                   Network Switching Subsystem (MSC, HLR, VLR)

Interface standards include Um, Abis, and A interfaces.

6(b) LTE Architecture and Mobile Satellite Communication

LTE uses flat IP architecture consisting of eNodeB, EPC, MME, SGW, and PGW.
Mobile satellite communication provides coverage in remote areas using LEO, MEO, and GEO satellites.

7(a) Short Notes

i) MANET
A Mobile Ad-hoc Network is a self-configuring, infrastructure-less wireless network.

ii) Bluetooth
Bluetooth is a short-range wireless technology for low-power device communication.

7(b) Short Notes

i) Li-Fi
Light Fidelity uses visible light for high-speed data transmission.

ii) Introduction to 4G and 5G
4G provides high-speed IP-based communication, while 5G offers ultra-low latency, massive connectivity, and very high data rates.