TechVani: Wireless and Mobile Communication Important Questions with Answers (Units 1-3)

Wireless and Mobile Communication exam prep. The questions come from the course question bank for the B.Tech Wireless and Mobile Communication syllabus, which covers Units 1 to 3, grouped by topic with the bank's own 1-mark / 5-mark / 10-mark tags. The three course assignments repeat the same question areas, so they are covered by the blocks below. General study notes, not an official university syllabus or paper prediction.

Unit 1: Cellular System Design - Frequency Reuse, Handoff, Interference and Trunking

1 mark: What is a cellular system?

A mobile communication system that divides a service area into small regions called cells, each served by its own low-power base station, so frequencies can be reused across the area.

1 mark: Define a cell in mobile communication.

A cell is the small geographic area covered by one base station; cells are drawn as hexagons for analysis because hexagons tile a plane without gaps and approximate a circular coverage area.

1 mark: What is frequency reuse?

Using the same set of radio channels in cells that are far enough apart that interference stays acceptable - the idea that lets a limited spectrum serve unlimited users.

1 mark: What is the frequency reuse factor?

The fraction of total channels available in each cell, equal to 1/N where N is the cluster size; every channel is reused once per cluster.

1 mark: Define cluster size.

The number of cells N in a cluster that together use the full channel set exactly once; valid values follow N = i² + ij + j² (1, 3, 4, 7, 9, 12, ...).

1 mark: What is a frequency reuse pattern?

The fixed arrangement showing which cells in each cluster use which channel groups, repeated across the whole service area.

1 mark: What is channel assignment?

The strategy by which the available radio channels are allocated to cells and to individual calls - fixed, dynamic or hybrid.

1 mark: Name the types of channel assignment strategies.

Fixed Channel Assignment (FCA), Dynamic Channel Assignment (DCA), and Hybrid Channel Assignment (HCA).

1 mark: What is handoff (handover)?

The process of transferring an ongoing call from one base station or channel to another as the mobile moves, without dropping the call.

1 mark: What is co-channel interference?

Interference between cells that use the same set of channels; it is the main capacity limiter in cellular systems and is controlled by cluster size.

1 mark: What is adjacent channel interference?

Interference from signals on neighbouring channels leaking into the wanted channel because filters are imperfect and nearby transmitters are strong.

1 mark: What is the signal-to-interference ratio (S/I)?

The ratio of wanted signal power to total interference power at a receiver; a minimum S/I (for example 18 dB in AMPS) is required for acceptable call quality.

1 mark: What is trunking?

Sharing a limited pool of channels among many users on demand, exploiting the fact that not everyone calls at once, so fewer channels serve more users.

1 mark: Define Grade of Service (GoS).

A measure of how easily users get a channel during the busiest hour, usually expressed as the probability that a call is blocked or delayed beyond a limit.

1 mark: What is the blocking probability?

The probability that a new call finds all channels busy and is denied service, given by the Erlang B formula for blocked-calls-cleared systems.

1 mark: What is cell splitting?

Replacing a congested cell with several smaller cells, each with its own lower-power base station, to increase capacity in high-traffic areas.

1 mark: What is sectoring?

Dividing a cell into sectors (typically 3 or 6) served by directional antennas, so co-channel interference reaches fewer receivers and capacity improves.

1 mark: What is an omnidirectional antenna?

An antenna that radiates equally in all horizontal directions, covering a full 360-degree cell.

1 mark: What is a directional antenna?

An antenna that concentrates radiation in one direction, used to cover a sector or to aim signal away from interferers.

1 mark: State one advantage of frequency reuse.

It multiplies system capacity: the same spectrum serves many times more users because channels are reused in distant cells.

5 marks: Explain the cellular concept with a neat diagram.

The cellular concept replaces one high-power transmitter covering a whole city with many low-power base stations, each covering a small hexagonal cell. Channels are divided into groups and assigned to cells so that cells using the same group (co-channel cells) sit far apart. (Diagram: honeycomb of hexagons with a marked cluster of 7 cells and arrows showing reuse distance.) Benefits: capacity grows by adding cells, power stays low so handsets are small, and spectrum is reused. Costs: more infrastructure, and the need for handoff and interference management.

5 marks: Describe the principle of frequency reuse.

Radio coverage decays with distance, so beyond a reuse distance D the same channel can serve another cell with tolerable interference. Cells are grouped into clusters of N cells; the total channels are split into N groups, one per cell in a cluster, and the pattern repeats. The co-channel reuse ratio q = D/R = √(3N) links geometry to interference: larger N means farther reuse, less interference, but fewer channels per cell. Capacity per cell = total channels / N.

5 marks: Explain the importance of cluster size in cellular systems.

Cluster size N fixes both interference and capacity: S/I improves as N grows because co-channel cells move apart (q = √(3N)), but channels per cell shrink as 1/N.
Small N (3, 4): high capacity, higher interference - used where demand is heavy and links are robust.
Large N (7, 12): cleaner links, lower capacity - used where coverage quality matters more.
Only certain N tile the plane: N = i² + ij + j².

5 marks: Compare Fixed Channel Assignment (FCA), Dynamic Channel Assignment (DCA), and Hybrid Channel Assignment (HCA).

Point	FCA	DCA	HCA
Allocation	Permanent channel set per cell	Channels assigned per call from a common pool	Fixed set plus a borrowable dynamic pool
Advantage	Simple, predictable	Adapts to traffic hotspots, higher trunking efficiency	Balance of stability and flexibility
Disadvantage	Wastes channels in low-traffic cells, blocks in busy ones	Complex, needs central control, more signalling	Borrowing rules must avoid interference
Suits	Uniform traffic	Varying traffic	Mostly-uniform traffic with peaks

5 marks: Explain different handoff strategies used in cellular communication.

Hard handoff: break-before-make; the old link drops an instant before the new one connects (FDMA/TDMA systems).
Soft handoff: make-before-break; the mobile talks to two base stations during the transition (CDMA).
Mobile-controlled (MCHO): the handset measures and initiates (DECT).
Network-controlled (NCHO): base stations measure, the network decides (first-generation systems).
Mobile-assisted (MAHO): the handset measures and reports, the network decides (GSM).
Priority schemes: queuing handoff calls and reserving guard channels so ongoing calls are dropped less often than new calls are blocked.

5 marks: Distinguish between hard handoff and soft handoff.

Point	Hard handoff	Soft handoff
Connection	Break before make	Make before break
Interruption	Brief gap possible	None
Channels	Different frequency usually	Same frequency (CDMA)
Complexity	Low	High - two links maintained
Extra benefit	-	Macro-diversity gain at cell edges

5 marks: Explain co-channel interference and its effects on system performance.

Co-channel interference comes from cells reusing the same channels. Unlike noise, it cannot be fought with more transmit power - raising power raises interference equally. It sets the floor on call quality: S/I must stay above the receiver threshold. Effects: reduced capacity (cluster size must grow), dropped or noisy calls at cell edges, and a hard limit on how small clusters can be. Control: increase reuse distance, use sectoring, and manage power carefully.

5 marks: Discuss adjacent channel interference and methods to reduce it.

Adjacent channel interference (ACI) leaks from neighbouring channels: imperfect receiver filters let a strong signal on the next channel bleed into the wanted one, worst when a nearby mobile transmits beside a weak far one (the near-far effect). Reduction: sharp filtering, guard bands, careful channel assignment within a cell (never adjacent channels in the same cell), power control so all mobiles arrive at the base station at similar strength, and good frequency planning.

5 marks: Explain trunking and its significance in mobile communication.

Trunking pools channels and assigns one per call on demand. Because users call randomly and briefly, a pool of C channels serves far more than C users. Significance: it makes cellular economics work - trunking efficiency rises with pool size, so one pool of 50 channels carries much more than ten pools of 5. The trade-off is blocking: at busy hour some calls fail, and that probability is the Grade of Service.

5 marks: Define Grade of Service (GoS). Explain the factors affecting GoS.

GoS quantifies service accessibility at busy hour, commonly as blocking probability (Erlang B) or the probability of waiting beyond a limit (Erlang C). Factors: number of trunked channels (more channels, better GoS), offered traffic in Erlangs (more traffic, worse GoS), call duration and arrival pattern, and the queueing discipline (blocked-cleared vs blocked-delayed). Operators pick a target, for example 2% blocking, and size channels to meet it.

5 marks: Explain the concept of Erlang in trunking analysis.

One Erlang = one channel occupied continuously for one hour; traffic intensity A = λH (call rate × mean holding time).
A user making 2-minute calls twice per busy hour offers 2×2/60 ≈ 0.067 Erlang.
Erlang B gives blocking for C channels at A Erlangs; Erlang C gives queueing delay probability.
Example: A = 10 Erlangs on 14 channels blocks far less than the same 10 on 11 channels - efficiency and GoS trade off through these tables.

5 marks: Describe the cell splitting technique with its advantages and disadvantages.

Cell splitting subdivides busy cells into smaller ones (radius halved), each with a new lower-power base station; transmit power drops by 12 dB when radius halves (path loss exponent 4), keeping the reuse geometry intact. Advantages: capacity grows where demand is, without new spectrum; the reuse pattern is preserved. Disadvantages: more base stations and cost, more handoffs, and careful power/channel re-planning so new microcells do not disturb neighbours.

5 marks: Explain sectoring and how it improves system capacity.

Sectoring replaces the cell's omnidirectional antenna with directional ones covering 120° (3 sectors) or 60° (6 sectors). A co-channel interferer now illuminates only one sector, so the number of effective interferers drops by the sectoring factor and S/I improves by roughly 5 dB (3-sector) or more (6-sector). Better S/I allows a smaller cluster size N, and smaller N means more channels per cell - that is the capacity gain. Cost: more antennas and channels split per sector (trunking efficiency falls), and more intra-cell handoffs.

5 marks: Compare cell splitting and sectoring.

Point	Cell splitting	Sectoring
Mechanism	New smaller cells, new base stations	Same cell, directional antennas
Capacity gain from	More cells per area	Lower interference allowing smaller clusters
Infrastructure cost	High (new sites)	Low (antenna change)
Handoffs	Increase	Increase (between sectors)
Trunking efficiency	Preserved per channel pool	Drops (channels split per sector)

5 marks: Explain various methods of improving coverage and capacity in cellular systems.

Cell splitting - capacity where demand grows.
Sectoring - interference down, cluster size down.
Repeaters and range extenders - coverage into holes and buildings.
Microcell zones / distributed antennas - one cell served by several antennas, combining splitting's capacity with fewer handoffs.
Power control and careful frequency planning - keeps interference at the designed floor.

10 marks: Explain the cellular concept in detail. Discuss frequency reuse and cluster size with suitable diagrams.

Combine the 5-mark answers: hexagonal geometry and why hexagons; cluster of N cells using the full channel set; reuse distance D = R√(3N) with a marked 7-cell cluster diagram; reuse factor 1/N; capacity C = M×(total channels/N) for M clusters; worked example - 33 MHz system, 50 kHz channels: how N = 7 vs N = 4 changes channels per cell and S/I.

10 marks: Explain various channel assignment strategies (Fixed, Dynamic, and Hybrid) with their advantages and disadvantages.

Extend the comparison table with mechanisms: FCA pre-splits channels by the reuse plan (with borrowing variants), DCA keeps all channels in a central pool and assigns on demand with interference checks, HCA fixes a base set and borrows from the pool under lockout rules. Add signalling cost, MSC involvement, and when each wins: FCA under uniform traffic, DCA under spiky traffic, HCA in practice.

10 marks: Describe handoff strategies in detail. Explain the handoff process, types, and decision criteria.

Process: monitor signal strength of serving and neighbour cells, trigger when serving drops below threshold while a neighbour exceeds it by a margin (hysteresis), select target, re-route. Types: hard vs soft; intra-cell, inter-cell, inter-MSC; control styles MCHO/NCHO/MAHO. Decision criteria: received signal strength, threshold, hysteresis margin, dwell timer, speed of mobile, and traffic on candidate cells; handoff prioritisation with guard channels and queuing reduces forced call drops.

10 marks: Explain co-channel interference and adjacent channel interference. Discuss methods to reduce interference in cellular systems.

Define both, their sources (reuse distance vs filter leakage/near-far), and why co-channel interference cannot be solved with power. Mitigation toolbox: raise q (larger N), sectoring, power control, microcells, discontinuous transmission, good site engineering, tight filters, guard bands, no adjacent channels in one cell, and frequency planning that keeps strong interferers far. Include the S/I ≈ (√(3N))ⁿ/i₀ relation for the first tier.

10 marks: Explain trunking theory and Grade of Service (GoS). Discuss Erlang concepts and blocking probability.

Trunking principle and efficiency; Erlang definition and A = λH; Erlang B model (blocked calls cleared) with the recursion and a worked table lookup (for example 2% GoS: 5 channels ≈ 2.0 Erlangs, 10 channels ≈ 5.1 Erlangs); Erlang C for queued systems; how operators size a cell's channels from forecast busy-hour traffic; why large pools are more efficient per channel.

10 marks: Discuss techniques for improving coverage and capacity in cellular systems. Explain cell splitting and sectoring with neat diagrams.

Coverage vs capacity distinction first. Capacity: cell splitting (radius halved, power -12 dB, diagram of one cell becoming four/seven), sectoring (120°/60° antenna patterns, S/I improvement, smaller N), microcell zones. Coverage: repeaters, leaky feeders in tunnels, distributed antennas. Trade-offs of each: cost, handoff load, trunking efficiency.

10 marks: Derive the relationship between co-channel reuse ratio, cluster size, and system capacity. Explain the impact of cluster size on interference.

Geometry: hexagon centres separated by D = R√(3N) for a valid cluster, so q = D/R = √(3N). With first-tier interferers i₀ = 6 and path loss exponent n, S/I ≈ qⁿ/6. Capacity: channels per cell = S/N for total channels S, so system capacity with M clusters = M×S. Impact: doubling N roughly halves channels per cell but raises S/I strongly; the design picks the smallest N that still meets the S/I threshold.

10 marks: Compare co-channel interference and adjacent channel interference. Explain their causes, effects, and mitigation techniques.

Point	Co-channel	Adjacent channel
Source	Cells reusing same channel	Leakage from neighbouring channels
Worst case	Cell edge	Near-far: strong near mobile beside weak far one
Controlled by	Cluster size, sectoring, power control	Filters, guard bands, channel planning, power control
Effect	Capacity limit, call quality floor	Local quality loss, cross-talk

10 marks: Explain frequency reuse planning in cellular networks. Discuss its impact on capacity, interference, and spectrum efficiency.

Planning steps: choose N from the S/I requirement, lay out the reuse pattern, assign channel groups, then refine with real terrain and traffic. Impact chain: N sets q, q sets S/I, S/I sets quality; N sets channels per cell, which with cell count sets capacity; spectrum efficiency (Erlangs/MHz/km²) rises as cells shrink and N tightens, but interference management gets harder - planning is the balance point.

10 marks: Explain the complete system design fundamentals of a cellular network, including frequency reuse, channel assignment, handoff, interference, trunking, Grade of Service, cell splitting, and sectoring.

Capstone answer linking the unit: geometry and reuse (hexagons, N, q) set the interference floor; channel assignment (FCA/DCA/HCA) distributes capacity; handoff strategies keep calls alive across cells; trunking and GoS size each cell's channels from busy-hour Erlangs; and when demand outgrows the design, cell splitting and sectoring raise capacity without new spectrum. Present as one design narrative with the key relations q = √(3N), S/I ≈ qⁿ/6, A = λH, and Erlang B sizing.

Unit 2: Mobile Radio Propagation - Path Loss, Reflection, Diffraction and Scattering

1 mark: What is mobile radio propagation?

The study of how radio waves travel from transmitter to receiver in a mobile environment, including how terrain, buildings and motion weaken and distort the signal.

1 mark: Define large-scale path loss.

The average signal power loss over long transmitter-receiver distances, caused by distance and large obstacles; it predicts mean received strength, ignoring rapid local fluctuations.

1 mark: What is path loss?

The reduction in signal power density as the wave spreads and is absorbed along its path, expressed in dB as the difference between transmitted and received power.

1 mark: State the Free Space Propagation Model.

A model for a clear, unobstructed line-of-sight link where received power falls with the square of distance, given by the Friis equation.

1 mark: What is free space path loss (FSPL)?

FSPL = (4πd/λ)² = (4πdf/c)²; in dB, FSPL = 20 log₁₀(d) + 20 log₁₀(f) + 32.44 for d in km and f in MHz.

1 mark: What is the Friis transmission equation?

Pr = Pt Gt Gr λ² / ((4π)² d² L): received power from transmitted power, antenna gains, wavelength, distance and system loss L.

1 mark: Define reflection in radio wave propagation.

The bouncing of a radio wave off a surface much larger than its wavelength - buildings, walls, the ground - sending energy in a new direction.

1 mark: What is a dielectric material?

An electrical insulator (like glass, brick or dry ground) that a radio wave can partly penetrate and partly reflect from, characterised by its permittivity.

1 mark: What is ground reflection?

Reflection of the transmitted wave from the earth's surface, creating a second path to the receiver that adds to or cancels the direct wave.

1 mark: What is the Two-Ray Ground Reflection Model?

A model combining the direct line-of-sight ray and one ground-reflected ray; it predicts received power falling as d⁻ beyond a break point, faster than free space.

1 mark: Define diffraction.

The bending of radio waves around sharp edges of obstacles, letting signal reach shadowed areas behind buildings and hills where no line of sight exists.

1 mark: What is the Fresnel zone?

One of the ellipsoidal regions between transmitter and receiver within which path lengths differ by multiples of half a wavelength; obstructions in these zones weaken the link.

1 mark: What is the first Fresnel zone?

The innermost ellipsoid where the path length exceeds the direct path by at most λ/2; keeping it mostly clear is required for near-free-space propagation. Its max radius is r = √(λd₁d₂/(d₁+d₂)).

1 mark: Define scattering.

The spreading of a wave in many directions when it hits objects comparable to or smaller than the wavelength - lamp posts, foliage, rough surfaces, rain.

1 mark: Name the three basic propagation mechanisms.

Reflection, diffraction, and scattering.

1 mark: What is multipath propagation?

The arrival of the same signal at the receiver over several paths (direct, reflected, diffracted, scattered) with different delays and phases, causing fading and distortion.

1 mark: What is an outdoor propagation model?

A model predicting path loss over outdoor terrain - examples: Okumura, Hata, COST-231, and Longley-Rice - tuned by environment type.

1 mark: What is an indoor propagation model?

A model for propagation inside buildings, accounting for walls, floors and furniture - for example partition-loss models and the ITU indoor model.

1 mark: Define line-of-sight (LOS) propagation.

Propagation where a clear direct path exists between transmitter and receiver, the strongest and most predictable component of the received signal.

1 mark: Mention one factor affecting radio wave propagation.

Obstacles such as buildings and terrain (also: frequency, antenna heights, weather, and surface roughness).

5 marks: Explain the concept of mobile radio propagation.

Mobile radio propagation describes how the transmitted wave reaches a moving receiver through a cluttered environment. The signal spreads (path loss), bounces (reflection), bends around edges (diffraction), and breaks up on rough surfaces (scattering), so the receiver sees many delayed copies adding with different phases. Engineers split the result into large-scale behaviour (average loss over distance, used for coverage planning) and small-scale behaviour (rapid fading over metres, used for link design).

5 marks: Describe the characteristics of large-scale path loss.

Grows with distance as dⁿ, where n ranges from 2 (free space) to 4 or more (urban clutter).
Varies with environment: open, suburban, urban each get different exponents and constants.
Depends on frequency: higher frequencies lose more in free space and penetrate walls worse.
Predicts the average over many wavelengths; slow shadowing variations sit on top of it.

5 marks: Explain the Free Space Propagation Model with the Friis transmission equation.

The free space model assumes a clear LOS path with no obstacles or reflections. Friis: Pr = Pt Gt Gr λ²/((4π)²d²L). Consequences: received power falls as 1/d² (6 dB per distance doubling), improves with antenna gains, and shrinks at higher frequencies for fixed gain. It is the reference model - real models describe how much worse than free space the environment is.

5 marks: Discuss the assumptions and limitations of the Free Space Propagation Model.

Assumptions: unobstructed LOS, no reflections or obstacles, antennas in far field, isotropic reference unless gains specified.
Limitations: cities break every assumption - ground reflection, building blockage and multipath make real loss much worse; the model is valid only for satellite links, microwave LOS hops, and as a first-order reference at short distances.

5 marks: Explain the mechanism of reflection in radio wave propagation.

When a wave meets a boundary between two media larger than its wavelength, part of its energy reflects. The reflection coefficient depends on the materials' permittivities, polarisation and incidence angle; a perfectly conducting surface reflects with a 180° phase shift. Reflected rays arrive delayed and phase-shifted at the receiver, combining with the direct ray to produce the interference patterns behind multipath fading.

5 marks: Describe reflection from dielectric surfaces.

Real ground and walls are dielectrics, not perfect conductors: the wave partly reflects and partly penetrates, with coefficients computed from Fresnel equations using the material's permittivity and conductivity. Reflection strength falls for rough surfaces and for horizontal polarisation at grazing angles (Brewster effect). In prediction models, dielectric ground reflection is exactly what the two-ray model captures for flat earth.

5 marks: Explain the Ground Reflection (Two-Ray) Model with a neat diagram.

(Diagram: transmitter at height ht, receiver at hr, direct ray plus a ray reflecting off the ground between them.) The two rays travel different distances and the reflected one flips phase, so they interfere. Near the transmitter the field oscillates; beyond the break distance 4πhthr/λ the phases nearly cancel, giving Pr ≈ Pt Gt Gr ht²hr²/d⁴ - power falls as d⁴ (12 dB per doubling) and becomes independent of frequency.

5 marks: Discuss diffraction and its importance in mobile communication.

Diffraction lets waves bend around sharp edges (the Huygens principle: every point on a wavefront re-radiates). Importance: it is why mobiles work behind buildings and hills at all - shadowed areas receive diffracted, weaker copies rather than nothing. Modellers compute the extra loss from knife-edge diffraction theory using the Fresnel-Kirchhoff parameter v = h√(2(d₁+d₂)/(λd₁d₂)), and multiple edges compound the loss.

5 marks: Explain Fresnel zone geometry and its significance.

Fresnel zones are concentric ellipsoids with transmitter and receiver at the foci; the nth zone boundary marks paths exactly nλ/2 longer than the direct path. Adjacent zones tend to cancel, so keeping the first zone largely clear preserves free-space-like propagation. Rule of thumb: an obstruction is tolerable if it leaves about 60% of the first Fresnel zone clear; otherwise knife-edge diffraction loss must be added. This drives antenna height choices on point-to-point links.

5 marks: Describe the scattering mechanism in wireless communication.

Scattering occurs when the wave hits objects comparable to or smaller than the wavelength or surfaces rough on that scale: foliage, street signs, rain, rough walls. Energy sprays in many directions. Effects: additional diffuse multipath components (feeding fading), extra loss for the main path, and - usefully - energy into shadowed regions; radar cross-section and surface roughness criteria (Rayleigh criterion) decide when a surface acts rough.

5 marks: Compare reflection, diffraction, and scattering.

Mechanism	Object scale vs λ	Result	Typical source
Reflection	Much larger	One strong redirected ray	Ground, building faces, walls
Diffraction	Sharp edge, larger	Bending into shadow zones	Building corners, hilltops
Scattering	Comparable or smaller	Many weak diffuse rays	Foliage, lamp posts, rough surfaces

5 marks: Explain common outdoor propagation models used in cellular communication.

Okumura: empirical curves from Tokyo measurements; base median loss plus correction factors for terrain and antenna heights.
Hata: closed-form equations fitted to Okumura, for 150-1500 MHz, with urban/suburban/open formulas and mobile-height corrections.
COST-231 Hata: extends Hata to 1500-2000 MHz for PCS/3G planning.
Longley-Rice: point-to-point model handling irregular terrain with diffraction and scatter modes.

5 marks: Explain the Indoor Propagation Model and its applications.

Indoor models add partition losses to distance loss: path loss = free-space loss + Σ(wall losses) + floor attenuation factors, with same-floor distance exponents of 2-6 depending on building type (ITU and Keenan-Motley style models). Applications: planning Wi-Fi access points and indoor small cells, estimating how many floors one base station can serve, and deciding between outdoor penetration vs dedicated indoor systems.

5 marks: Compare indoor and outdoor propagation models.

Point	Outdoor	Indoor
Scale	Hundreds of m to km	Metres to tens of m
Dominant effects	Terrain, buildings, antenna heights	Walls, floors, furniture, people
Typical models	Okumura, Hata, COST-231, Longley-Rice	Partition-loss, ITU indoor, Keenan-Motley
Inputs	Frequency, heights, environment class	Floor plan, partition materials

5 marks: Discuss the factors affecting large-scale path loss in mobile radio systems.

Distance and the environment's path-loss exponent.
Carrier frequency (free-space loss rises with f).
Antenna heights at both ends (taller antennas, less loss).
Terrain and clutter: buildings, vegetation, hills.
Construction materials for penetration scenarios.
Weather and atmospheric conditions at higher bands.

10 marks: Explain large-scale path loss in detail. Derive the Free Space Propagation Model using the Friis transmission equation.

Start from an isotropic radiator: power density at distance d is Pt/(4πd²). An antenna with gain Gt concentrates it to PtGt/(4πd²). The receiving antenna's effective aperture Ae = Grλ²/(4π) collects Pr = PtGtGrλ²/((4π)²d²) - the Friis equation, with system loss L in the denominator for real hardware. Then discuss large-scale path loss: dⁿ generalisation, log-normal shadowing about the mean, and the standard form PL(dB) = PL(d₀) + 10n log₁₀(d/d₀) + Xσ.

10 marks: Discuss the three basic radio propagation mechanisms - reflection, diffraction, and scattering - with neat diagrams.

One diagram per mechanism (flat-surface bounce; knife-edge bending with the shadow region lit; rough surface spray). For each: physical condition on object size vs wavelength, governing quantity (Fresnel reflection coefficients; knife-edge parameter v and its loss curve; Rayleigh roughness criterion), and its role in the received multipath profile of a mobile link.

10 marks: Explain reflection from dielectric surfaces. Discuss the factors affecting reflected waves.

Dielectric reflection via Fresnel coefficients for both polarisations; partial penetration and absorption; grounding parameters εr and σ of real materials. Factors: incidence angle (grazing reflects strongly), polarisation, surface roughness vs wavelength, material permittivity and conductivity, and frequency. Connect to practice: ground and building reflections create the second ray of the two-ray model and the strong echoes in city multipath.

10 marks: Explain the Ground Reflection (Two-Ray) Model with a neat diagram. Compare it with the Free Space Propagation Model.

Point	Free space	Two-ray
Rays	Direct only	Direct + ground-reflected
Far-field decay	d² (6 dB per doubling)	d⁴ (12 dB per doubling)
Frequency dependence	Yes (λ² in Pr)	Cancels out beyond break point
Antenna heights	Not in formula	Central: ht²hr²
Use	Satellite, LOS hops, reference	Macrocell land-mobile links

10 marks: Describe diffraction in mobile communication. Explain Fresnel zone geometry and its significance in wireless propagation.

Diffraction via Huygens secondary wavelets; knife-edge model with v parameter and the standard loss curve (0 dB at v ≤ -0.8, rising to >15 dB deep in shadow); multiple-edge extensions (Bullington, Epstein-Peterson). Fresnel zones: definition, first-zone radius r₁ = √(λd₁d₂/(d₁+d₂)), the 60% clearance rule, and how zone blockage converts directly into added path loss on real links.

10 marks: Explain scattering in detail. Discuss its causes, effects, and applications in mobile communication.

Causes: objects at or below wavelength scale - foliage, street furniture, rough facades, precipitation - plus the Rayleigh criterion for when a surface counts as rough. Effects: diffuse multipath feeding fast fading and delay spread, extra loss of the specular component, and delivered energy into non-LOS spots. Applications: urban microcell coverage benefits from scattered energy, radar cross-section models, and diffuse components in ray-tracing simulators.

10 marks: Discuss various outdoor propagation models used in wireless communication. Compare their characteristics and applications.

Compare Okumura (empirical, most general, curve-based), Hata (parametric, fast, 150-1500 MHz), COST-231 Hata (to 2 GHz), Longley-Rice (irregular terrain, point-to-point), and the two-ray model (flat-earth theoretical baseline). Columns: frequency range, inputs, accuracy, best use - rural planning vs urban macros vs microwave hops.

10 marks: Explain indoor propagation models in detail. Discuss the factors affecting indoor radio wave propagation.

Models: log-distance with partition losses (Keenan-Motley: PL = FSPL + kw·Wf + kf·Ff), ITU site-general indoor model with floor-penetration factors, attenuation-factor models. Factors: wall and floor materials (concrete vs plasterboard vs glass), number of floors, furniture density, human traffic, corridor waveguiding, and frequency. Close with design use: access-point placement and capacity planning indoors.

10 marks: Compare free-space propagation, ground reflection, diffraction, and scattering in terms of propagation characteristics, advantages, and limitations.

Mechanism/model	Characteristic	Advantage	Limitation
Free space	LOS only, d²	Simple, exact for clear links	Unrealistic in clutter
Two-ray	Direct + ground, d⁴	Matches macrocells, shows heights' role	Flat-earth only
Diffraction	Bends into shadows	Explains non-LOS coverage	High loss, hard for many edges
Scattering	Diffuse spray	Energy everywhere, feeds diversity	Unpredictable, deep fades

10 marks: Explain mobile radio propagation comprehensively, covering large-scale path loss, Free Space Propagation Model, reflection, diffraction, scattering, outdoor propagation models, and indoor propagation models with suitable diagrams.

Capstone structure: the three mechanisms produce multipath; multipath plus distance gives the two description scales - large-scale (log-distance dⁿ + shadowing) and small-scale (fading, covered in Unit 3). Free space (Friis) is the ideal; two-ray corrects it for earth; empirical outdoor models (Okumura/Hata/COST-231/Longley-Rice) handle terrain classes; indoor partition models handle buildings. Diagram set: Friis geometry, two-ray, knife-edge, scattering spray. Conclude with the engineering chain: pick model by environment, predict loss, size the link budget.

Unit 3: Small-Scale Fading - Multipath, Doppler and Coherence Parameters

1 mark: What is small-scale fading?

Rapid fluctuation of signal strength, phase and delay over short distances (half a wavelength) or short times, caused by multipath components adding with changing phases.

1 mark: Define multipath propagation.

The arrival of one transmitted signal over multiple reflected, diffracted and scattered paths, each with its own delay, amplitude and phase.

1 mark: What is fading in wireless communication?

The variation of received signal strength over time, distance or frequency due to changing interference between multipath components.

1 mark: What is Doppler shift?

The change in received frequency caused by relative motion between transmitter and receiver; a moving mobile shifts every arriving path's frequency.

1 mark: State the Doppler shift formula.

fd = (v/λ) cos θ, where v is the mobile speed, λ the wavelength, and θ the angle between the direction of motion and the arriving wave.

1 mark: Define Doppler spread.

The range of Doppler shifts across all arriving paths, BD = 2fm where fm = v/λ is the maximum shift; it measures how fast the channel changes.

1 mark: What is coherence time?

The time span over which the channel stays essentially constant, inversely related to Doppler spread: Tc ≈ 0.423/fm (common approximation).

1 mark: Define coherence bandwidth.

The frequency span over which the channel response stays essentially flat, inversely related to delay spread: Bc ≈ 1/(5σ𝜏) for 50% correlation (common approximation).

1 mark: What is delay spread?

The spread in arrival times among significant multipath components, from the first to the last meaningful echo.

1 mark: Define RMS delay spread.

The standard deviation σ𝜏 of the power-delay profile - a single number summarising how dispersed the echoes are; the key parameter behind coherence bandwidth.

1 mark: What is flat fading?

Fading where the signal bandwidth is smaller than the coherence bandwidth, so all frequencies fade together and the signal keeps its shape, only its strength varies.

1 mark: What is frequency-selective fading?

Fading where the signal bandwidth exceeds the coherence bandwidth, so different frequencies fade differently and the signal distorts (intersymbol interference).

1 mark: Define slow fading.

Fading where the channel changes slowly relative to the symbol duration (coherence time much longer than the symbol time), so the channel is constant over many symbols.

1 mark: What is fast fading?

Fading where the channel changes within a symbol duration (coherence time shorter than the symbol time), distorting the pulse shape.

1 mark: What are the factors influencing small-scale fading?

Multipath propagation, the speed of the mobile, the speed of surrounding objects, and the transmission bandwidth of the signal.

1 mark: What is constructive interference?

Addition of multipath components in phase, producing a received signal stronger than any single path.

1 mark: What is destructive interference?

Addition of multipath components out of phase, partially or fully cancelling the signal - the cause of deep fades.

1 mark: Define multipath time delay spread.

The time between the first and last significant multipath arrivals; large delay spreads smear symbols into their neighbours.

1 mark: What is a mobile radio channel?

The time-varying transmission path between base station and mobile, characterised by path loss, shadowing, multipath delays and Doppler shifts.

1 mark: State one effect of Doppler shift in mobile communication.

It spreads the signal in frequency and makes the channel time-varying, causing fast fading at high speeds (also: irreducible error floors and lost synchronisation if untracked).

5 marks: Explain the concept of small-scale multipath propagation.

In a city, the receiver sees not one wave but dozens: a direct ray plus reflections off buildings, diffractions over rooftops and scatter from street furniture. Each copy arrives with its own delay, amplitude and phase. Because a half-wavelength at 900 MHz is only ~17 cm, moving even that far changes the phases completely, swinging the sum from constructive to destructive - deep fades within metres. This rapid, multipath-driven variation is small-scale fading, distinct from the slow average loss of Unit 2.

5 marks: Discuss the factors influencing small-scale fading.

Multipath propagation: more paths with larger delay differences mean deeper and more selective fades.
Speed of the mobile: faster motion means faster phase change, i.e. larger Doppler spread and quicker fades.
Speed of surrounding objects: moving traffic and swaying foliage shift the environment even for a still user.
Transmission bandwidth: wideband signals resolve paths separately, turning flat fades into frequency-selective distortion.

5 marks: Explain Doppler shift and derive its expression.

A mobile moving toward a wavefront meets crests more often; away, less often. If the mobile covers distance vt in time t and the wave arrives at angle θ to the motion, the extra path length is vt cos θ, so the phase changes by 2πvt cos θ/λ. Frequency is phase rate, giving fd = (v/λ) cos θ. Maximum shift fm = v/λ occurs when moving straight into the wave. Each path has its own angle, hence its own shift - the basis of Doppler spread.

5 marks: Describe Doppler spread and its significance in wireless communication.

Paths arrive from all directions, so their Doppler shifts span roughly -fm to +fm; the occupied width is the Doppler spread, classically with a U-shaped (Jakes) spectrum for uniform arrival angles. Significance: Doppler spread measures channel time-variation - it sets how often channel estimates must be refreshed, limits symbol rates before fast fading bites, causes adjacent-channel spill if uncompensated, and through coherence time tells designers how long a channel estimate stays valid (for example at 100 km/h and 2 GHz, fm ≈ 185 Hz, Tc ≈ 2.3 ms).

5 marks: Explain coherence time and its relationship with Doppler spread.

Coherence time Tc is the duration over which two received signals remain strongly correlated - how long the channel "holds still". Larger Doppler spread means faster change, so Tc ∩ 1/Doppler spread: common rules are Tc ≈ 0.423/fm (50% correlation) or the geometric mean Tc ≈ √(9/(16πfm²)) = 0.423/fm. A channel with Tc far above the symbol period is slow fading; below it, fast fading.

5 marks: Define coherence bandwidth. Explain its importance in wireless communication.

Coherence bandwidth Bc is the frequency range over which the channel's gain stays correlated - a statistical maximum gap between two frequencies that fade alike. It is set by delay spread: Bc ≈ 1/(5σ𝜏) for 50% correlation (or 1/(50σ𝜏) for 90%). Importance: compare Bc with the signal bandwidth. Signal narrower than Bc: flat fading, an equaliser may not be needed. Signal wider: frequency-selective fading, so equalisation, OFDM or RAKE combining becomes necessary. This one comparison drives receiver design.

5 marks: Explain multipath time delay spread and its effects.

Delay spread measures echo dispersion: the last significant reflection arrives τmax after the first, and σ𝜏 summarises the profile. Effects: echoes of one symbol spill into the next (intersymbol interference) when the symbol period approaches the delay spread; the channel's frequency response develops ripples spaced ~1/σ𝜏 apart; and the link gets an irreducible error floor no extra power can fix - only equalisation, diversity or OFDM helps.

5 marks: Differentiate between flat fading and frequency-selective fading.

Point	Flat fading	Frequency-selective fading
Condition	Signal bandwidth < coherence bandwidth (symbol period > delay spread)	Signal bandwidth > coherence bandwidth (symbol period < delay spread)
Frequency response	Roughly constant across the signal	Varies across the signal; deep notches
Time-domain effect	Strength varies, shape kept	Pulses smear, ISI
Fix	Power margin, diversity	Equalisation, OFDM, RAKE

5 marks: Explain the conditions under which flat fading occurs.

The signal's bandwidth is smaller than the channel's coherence bandwidth (Bs < Bc).
Equivalently, the symbol period exceeds the RMS delay spread by a comfortable factor.
Typical settings: narrowband channels, small cells, open areas with short echo delays.
Result: all frequency components rise and fall together - gain varies, waveform survives.

5 marks: Explain the conditions that cause frequency-selective fading.

Signal bandwidth exceeds coherence bandwidth (Bs > Bc), i.e. symbol period shorter than the delay spread.
Strong distant echoes: hilly terrain, dense urban cores with large σ𝜏.
Wideband or high-rate transmission, which resolves individual paths.
Result: different frequencies experience different fades - notches in the spectrum and intersymbol interference in time.

5 marks: Distinguish between slow fading and fast fading.

Point	Slow fading	Fast fading
Condition	Coherence time >> symbol period	Coherence time < symbol period
Doppler	Low spread (slow motion, low frequency)	High spread (fast motion, high frequency)
Channel during one symbol	Essentially constant	Changes within the symbol
Main harm	Long deep fades (bursts)	Pulse distortion, sync loss
Fix	Interleaving, coding, diversity	Faster tracking, robust design

5 marks: Explain the relationship between coherence bandwidth and delay spread.

They are Fourier-linked reciprocals: the power-delay profile and the frequency correlation function are a transform pair, so a wide delay spread means a narrow coherence bandwidth. Working rules: Bc ≈ 1/(5σ𝜏) for 50% correlation, Bc ≈ 1/(50σ𝜏) for 90%. Example: σ𝜏 = 1 μs gives Bc ≈ 200 kHz - a 1 MHz signal through that channel is frequency-selective; the same signal where σ𝜏 = 0.1 μs (Bc ≈ 2 MHz) sees flat fading.

5 marks: Compare Doppler spread and coherence bandwidth.

Point	Doppler spread	Coherence bandwidth
Describes	Time variation of the channel	Frequency variation of the channel
Caused by	Motion (mobile or scatterers)	Multipath delay differences
Dual parameter	Coherence time (Tc ∩ 1/BD)	Delay spread (σ𝜏 ∩ 1/Bc)
Failure it predicts	Fast fading	Frequency-selective fading

5 marks: Discuss the effects of multipath fading on communication quality.

Deep short-term fades causing burst errors and dropped frames.
Intersymbol interference when delay spread approaches the symbol period, giving an irreducible error floor.
Doppler-induced frequency spread disturbing demodulation and synchronisation.
Random frequency response notches destroying parts of wideband signals.
Net: higher required margins, worse throughput, and the need for equalisation, diversity, interleaving and error-control coding.

5 marks: Explain the classification of small-scale fading with suitable examples.

Two independent axes. By delay spread vs signal: flat fading (GSM-era narrowband in a suburb) vs frequency-selective fading (a 20 MHz Wi-Fi channel in a long-echo warehouse). By Doppler spread vs symbol rate: slow fading (pedestrian at 900 MHz) vs fast fading (a train user at high carrier frequency). Every real link lands in one cell of this 2×2 grid, and the cell decides the receiver design.

10 marks: Explain small-scale multipath propagation in detail. Discuss the factors influencing small-scale fading with suitable diagrams.

Expand the 5-mark core: impulse-response model h(t) = Σ ak δ(t - τk) with time-varying amplitudes, delays and phases; the power-delay profile diagram; the four factors (multipath richness, mobile speed, scatterer speed, signal bandwidth) with one numeric illustration each; and how the receiver sees a walking interference pattern in space - fades every ~λ/2.

10 marks: Explain Doppler shift, Doppler spread, and coherence time. Discuss their significance in mobile communication.

Derivation of fd = (v/λ)cosθ, the fan of shifts over arrival angles forming the Jakes U-spectrum, BD = 2fm, and Tc ≈ 0.423/fm. Significance chain: speed and carrier frequency set fm; fm sets how fast the channel decorrelates; that sets pilot spacing, channel-estimation refresh, and whether the link is slow or fast fading. Worked example: 60 km/h at 900 MHz - fm = 50 Hz, Tc ≈ 8.5 ms, so GSM bursts (0.577 ms) see an almost frozen channel.

10 marks: Explain coherence bandwidth and delay spread. Discuss their relationship and importance in wireless communication.

Define both, show the transform relationship (frequency correlation vs power-delay profile), give the 50%/90% rules Bc ≈ 1/(5σ𝜏) and 1/(50σ𝜏), and tabulate typical σ𝜏 (indoor ~50 ns, urban ~1-3 μs, hilly ~10+ μs) against which systems stay flat or go selective. Importance: the Bs-vs-Bc test decides flat vs selective and therefore whether the receiver needs an equaliser, OFDM or RAKE.

10 marks: Discuss different types of small-scale fading based on multipath time delay spread with neat diagrams.

The delay-spread axis of the classification: flat fading (narrowband, Bs<Bc, undistorted pulses, gain ripple only) vs frequency-selective fading (Bs>Bc, smeared pulses with ISI, notched spectrum). Diagrams: power-delay profiles with overlaid symbol periods for both cases, and frequency responses showing flat vs rippled channels. Conditions, effects and remedies for each.

10 marks: Explain flat fading and frequency-selective fading. Compare their characteristics, causes, and effects.

Definitions, the Bs/Bc and Ts/σ𝜏 conditions, causes (bandwidth choice, environment echo length), characteristics (uniform gain variation vs spectral notches; preserved vs smeared pulses), and effects (burst fades fixable by margin and diversity vs ISI floors needing equalisation/OFDM/RAKE). Close with the 2×2 classification showing these two on the delay-spread axis.

10 marks: Explain slow fading and fast fading. Compare their characteristics and applications in wireless communication.

Definitions via Tc vs Ts; causes (low vs high Doppler spread); characteristics (constant-over-bursts vs within-symbol change); effects (long error bursts vs pulse distortion); remedies (interleaving and ARQ-friendly coding vs dense pilots and short symbols). "Applications" angle: which designs assume which regime - GSM assumes slow fading per burst; high-mobility systems insert more pilots precisely because they expect fast fading.

10 marks: Describe the effects of multipath propagation on mobile communication systems. Explain methods to reduce multipath fading.

Effects: deep fades, ISI and error floors, Doppler spread, random FM, sync and equaliser stress. Countermeasures: diversity (space, frequency, time, polarisation) and combining (selection, MRC); equalisation for ISI; OFDM turning a selective channel into many flat ones; RAKE receivers in CDMA harvesting echoes; channel coding with interleaving to spread burst errors; power control; and adaptive modulation that follows the channel.

10 marks: Compare flat fading, frequency-selective fading, slow fading, and fast fading in terms of causes, characteristics, and effects.

Type	Caused by	Condition	Characteristic	Main effect
Flat	Short delay spread	Bs < Bc	Uniform gain ripple	Burst fades
Frequency-selective	Long delay spread	Bs > Bc	Spectral notches	ISI, error floor
Slow	Low Doppler	Tc >> Ts	Stable per burst	Long fades
Fast	High Doppler	Tc < Ts	Changes within symbol	Distortion, sync loss

10 marks: Explain the relationship among Doppler shift, Doppler spread, coherence bandwidth, coherence time, and delay spread. Illustrate with suitable examples.

The two duality pairs: motion turns into Doppler shift per path (fd = (v/λ)cosθ), the angle fan widens it into Doppler spread (BD = 2fm), and its time dual is coherence time (Tc ≈ 0.423/fm). Echoes spread arrival times into delay spread (σ𝜏), whose frequency dual is coherence bandwidth (Bc ≈ 1/(5σ𝜏)). Worked pair: highway user at 2 GHz (fm ≈ 185 Hz, Tc ≈ 2.3 ms) in an urban cell (σ𝜏 ≈ 2 μs, Bc ≈ 100 kHz) - a 1 MHz signal there is both time- and frequency-selective; a pedestrian indoor user (fm ≈ 5 Hz, σ𝜏 ≈ 50 ns) faces slow, flat fading.

10 marks: Explain the complete concept of small-scale fading in mobile communication, including multipath propagation, Doppler effects, coherence bandwidth, coherence time, delay spread, flat fading, frequency-selective fading, and slow fading, with suitable diagrams.

Capstone narrative: multipath creates the echo set; motion Doppler-shifts each echo; the statistics of echoes (σ𝜏) and motion (fm) define the two coherence measures; comparing signal bandwidth to Bc and symbol time to Tc places any link on the 2×2 fading grid; and each cell of the grid prescribes its cures (margin/diversity for flat, equalisation/OFDM for selective, interleaving for slow, dense tracking for fast). Include power-delay profile, Jakes spectrum and classification diagrams.

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