(SEM VIII) THEORY EXAMINATION 2017-18 WIRELESS MOBILE COMMUNICATION

SECTION A (Brief Answers)

Hard vs Soft Handoff
In hard handoff, the connection with the current base station is broken before establishing a new connection, so it is called “break before make.” In soft handoff, the mobile device connects to multiple base stations simultaneously during transition, known as “make before break,” which provides smoother communication.

Channel Assignment
Channel assignment is the process of allocating radio channels to users in a cellular system. It can be fixed channel assignment, dynamic channel assignment, or hybrid assignment.

Applications of Spread Spectrum
Spread spectrum modulation is used in military communication, GPS systems, CDMA mobile systems, and wireless LANs due to its resistance to interference and jamming.

Diversity
Diversity is a technique used to reduce fading effects by receiving the same signal through multiple independent paths, such as time, frequency, or space diversity.

Number of Duplex Channels
Total spectrum = 20 MHz.
Each simplex channel bandwidth = 25 kHz.
Number of simplex channels = 20,000 kHz / 25 kHz = 800.
Since duplex requires two simplex channels,
Number of duplex channels = 800 / 2 = 400 duplex channels.

OFDMA
OFDMA (Orthogonal Frequency Division Multiple Access) divides the available bandwidth into multiple orthogonal subcarriers and assigns subsets to users. It is used in 4G systems like LTE for high data rates and spectral efficiency.

IS-95 Frequency Range
IS-95 operates typically around 824–849 MHz for reverse link and 869–894 MHz for forward link.

Services Offered by GSM
GSM provides voice calls, SMS, MMS, data services, call forwarding, call waiting, and roaming facilities.

IMT-2000
IMT-2000 is a global standard for 3G mobile communication. It enables multimedia services, high data rates, and node-to-node communication across networks.

WiMax
WiMax (Worldwide Interoperability for Microwave Access) is a wireless broadband technology based on IEEE 802.16 standard, providing long-range high-speed internet access.

SECTION B (Medium Length Answers)

Coverage and Capacity in Cellular Systems
Coverage refers to the geographic area served by a cellular network, while capacity refers to the number of users the system can support simultaneously. Capacity can be increased by cell splitting, sectoring, microcell zoning, and reducing cluster size. Cell splitting increases number of cells, while sectoring reduces interference and improves frequency reuse.

Direct Sequence Spread Spectrum (DS-SS)
In DS-SS using BPSK, the data signal is multiplied by a high-rate pseudo-noise (PN) sequence before transmission. This spreads the signal bandwidth. At the receiver, the same PN sequence is used to despread the signal and recover original data. It improves security and interference resistance.

ALOHA Protocol
ALOHA is a random access protocol used in wireless communication. In Pure ALOHA, data can be transmitted anytime, which leads to higher collision probability. In Slotted ALOHA, time is divided into slots and transmission occurs at slot boundaries, improving efficiency.

IMT-2000 Network Structure
IMT-2000 consists of mobile stations, base stations, radio network controller, core network, and gateway systems. It supports multimedia services and seamless roaming.

4G Technology and Comparison
4G provides high-speed internet, low latency, and IP-based services. Compared to 1G (analog voice), 2G (digital voice and SMS), and 3G (multimedia and moderate data speed), 4G offers broadband-level speed and improved quality of service.

SECTION C (10-Mark Style Answers)

Small Scale Fading and Types
Small scale fading occurs due to multipath propagation. It is classified as flat fading and frequency selective fading. In flat fading, entire signal bandwidth experiences uniform fading. In frequency selective fading, different frequency components fade differently due to time delay spread.

Cluster Size Formula Verification
Cluster size in cellular systems is given by
N = i² + j² + ij
where i and j are integers representing cell movement pattern. This formula determines frequency reuse pattern and co-channel interference control.

PN Sequence and Properties
PN sequence is a pseudo-random binary sequence generated using shift registers. It has properties like balance property, run property, and autocorrelation property, making it suitable for spread spectrum systems.

Selection Diversity Improvement
Selection diversity chooses the branch with strongest signal among multiple branches. Probability of correct reception improves as number of branches increases. For M branches, outage probability reduces significantly compared to single branch.

RAKE Receiver
A RAKE receiver is used in CDMA systems to combine multipath signals. It has multiple correlators called fingers that align with different delayed signal paths. An m-branch RAKE receiver uses m fingers to combine signals and improve reception quality.

Signal Processing in GSM and Channel Calculation
In GSM, speech is digitized, encoded, modulated, transmitted, received, demodulated, and decoded to reproduce speech.
Given total bandwidth = 60 MHz.
1 MHz for control → remaining 59 MHz for voice.
Each duplex channel uses 2 × 50 kHz = 100 kHz.
Voice channels = 59,000 kHz / 100 kHz = 590 channels.
With reuse factor 9, channels per cell = 590 / 9 ≈ 65 voice channels per cell.

CDMA Working
CDMA assigns unique spreading codes to users. All users share same frequency band but are separated by codes. GSM uses TDMA/FDMA while CDMA uses spread spectrum technique, offering higher capacity and security.

Mobile Ad-Hoc Networks (MANET)
MANET is a self-configuring network without fixed infrastructure. Nodes act as routers and communicate directly. It is useful in military and emergency applications.

Next Generation Network (NGN)
NGN is an IP-based network integrating voice, data, and multimedia services. It supports mobility, security, and flexible service delivery.