THEORY EXAMINATION (SEM–VI) 2016-17 WIRELESS COMMUNICATION

WIRELESS COMMUNICATION (EEC028)

B.Tech – Semester VI
 

Time: 3 Hours | Max. Marks: 100

SECTION – A (10 × 2 = 20 marks)

(Attempt all questions)

(a) Explain the term FDMA.

FDMA (Frequency Division Multiple Access) is a multiple access technique in which the total available bandwidth is divided into different frequency bands, and each user is assigned a unique frequency band.

(b) Define the term wireless.

Wireless communication refers to the transmission of information between two or more points without the use of physical conductors, using electromagnetic waves.

(c) Explain the term Brewster Angle.

Brewster Angle is the angle of incidence at which reflected waves are completely polarized parallel to the surface, resulting in zero reflection of one polarization.

(d) Explain the role of an Equalizer.

An equalizer is used in wireless receivers to reduce intersymbol interference (ISI) caused by multipath propagation by compensating channel distortion.

(e) Explain Coherence Bandwidth and Coherence Time.

Coherence Bandwidth is the frequency range over which the channel response is constant.
Coherence Time is the time duration over which the channel characteristics remain unchanged.

(f) List techniques used to improve coverage and capacity in cellular systems.

Techniques include cell splitting, sectoring, microcell zoning, and frequency reuse.

(g) Find the Fraunhofer distance.

Given:
Maximum antenna dimension (D) = 1 m
Frequency (f) = 900 MHz

Wavelength (λ) = c/f = 3×10⁸ / 9×10⁸ = 0.333 m

Fraunhofer distance:
df = 2D² / λ = 2 × 1² / 0.333 = 6 m

(h) Explain the term Doppler Shift.

Doppler Shift is the change in frequency of a received signal caused by relative motion between transmitter and receiver.

(i) Explain Reflection and Scattering.

Reflection occurs when a radio wave strikes a smooth surface larger than the wavelength.
Scattering occurs when the wave hits rough surfaces or objects smaller than the wavelength.

(j) Explain Fast Fading and Slow Fading.

Fast fading occurs due to rapid changes in signal amplitude caused by multipath effects.
Slow fading occurs due to shadowing from large obstacles over longer distances.

SECTION – B (5 × 10 = 50 marks)

(Attempt any five questions)

2(a) Practical link budget using Free Space Propagation Model

Given:                                                          Transmitter power = 50 W
Distance = 10 km                                        Frequency = 900 MHz
Gt = 1, Gr = 2                                              Free space path loss:
Pr = Pt Gt Gr (λ / 4πd)²                                λ = 0.333 m

Received Power ≈ –80 dBm

Power Flux Density:
S = Pt / (4πd²)

2(b) Factors influencing small scale fading and Doppler shift calculation

Factors include multipath propagation, relative speed, bandwidth, and antenna characteristics.

Given:
fc = 900 MHz                                               Velocity = 70 km/hr = 19.44 m/s

Doppler shift:
fd = v / λ = 19.44 / 0.333 ≈ 58.3 Hz

(i) Toward transmitter:
fr = fc + fd = 900 MHz + 58.3 Hz

(ii) Away from transmitter:
fr = fc − fd = 900 MHz − 58.3 Hz

2(c) Types of small scale fading

Small scale fading types include flat fading and frequency selective fading.

Flat fading occurs when signal bandwidth is less than coherence bandwidth.
Frequency selective fading occurs when signal bandwidth exceeds coherence bandwidth.

2(d) Vocoders and Linear Predictive Coding

Vocoders are speech coders that compress speech signals.

Types include LPC, CELP, and Channel vocoders.

Linear Predictive Coders model speech using linear prediction coefficients to represent vocal tract characteristics.

2(e) Diversity techniques and space diversity

Diversity techniques improve signal reliability by transmitting the same signal over different paths.

Types include time, frequency, and space diversity.

Space diversity uses multiple antennas separated by distance to reduce fading.

2(f) FH-SS system and performance analysis

Frequency Hopping Spread Spectrum changes carrier frequency periodically according to a hopping sequence.

It improves resistance to interference, jamming, and multipath fading.

2(g) Short notes

(i) Channel Planning:
Allocates frequencies efficiently to reduce interference.

(ii) Adjacent Channel Interference:
Occurs due to imperfect filtering causing signal overlap between neighboring channels.

SECTION – C (2 × 15 = 30 marks)

(Attempt any two questions)

3. Linear and Non-linear Equalization Techniques

Linear equalizers use FIR or IIR filters to minimize ISI.

Non-linear equalizers include Decision Feedback Equalizers (DFE), which use past decisions to cancel interference.

Non-linear equalizers perform better in severe ISI conditions.

4. Multiple Access Techniques and TDMA

Multiple Access techniques include FDMA, TDMA, and CDMA.

Narrowband systems have bandwidth less than coherence bandwidth, while wideband systems exceed it.

TDMA:
Users share the same frequency but transmit in different time slots.

Efficiency = Useful time / Total frame time

Number of channels depends on slots per frame.

5. Explain the following

(i) Handoff Strategies:
Process of transferring a call from one cell to another without call drop.

(ii) Channel Assignment Strategies:
Includes fixed, dynamic, and hybrid assignment.

(iii) Frequency Reuse:
Reuse of same frequencies in different cells to increase capacity.