Intermediate Electronics & Communication interview questions

150 questions with answers (ECE-151 to ECE-300). Try each one aloud before reading the answer.

Signals & Systems

ECE-151 · What is the difference between Fourier series and Fourier transform?

Fourier series represents a periodic signal as a discrete sum of harmonics at multiples of f₀. The Fourier transform represents an aperiodic signal with a continuous spectrum. A periodic signal's transform is a train of impulses at the harmonics.

ECE-152 · How is the Laplace transform related to the Fourier transform?

The Laplace transform X(s) = ∫x(t)e^(−st)dt uses s = σ + jω. Setting s = jω gives the Fourier transform, valid if the ROC includes the jω axis.

ECE-153 · State Parseval's theorem.

Energy is the same in time and frequency domains: ∫|x(t)|²dt = ∫|X(f)|²df. It lets signal energy or power be computed from the spectrum.

ECE-154 · What is the convolution property of the Fourier transform?

Convolution in time equals multiplication in frequency: x(t) * h(t) ↔ X(f)·H(f). The dual holds too: multiplication in time is convolution in frequency, which explains modulation and windowing.

ECE-155 · What is the time-shifting property of the Fourier transform?

x(t − t₀) ↔ X(f)·e^(−j2πft₀). A delay leaves the magnitude spectrum unchanged and adds a phase that is linear in frequency.

ECE-156 · What is the Fourier transform of a rectangular pulse?

A pulse of amplitude A and width T has transform A·T·sinc(fT), where sinc(x) = sin(πx)/(πx). The first nulls are at ±1/T, so narrower pulses have wider spectra.

ECE-157 · How are the unit impulse and unit step related?

The unit step is the integral of the impulse, u(t) = ∫δ(τ)dτ from −∞ to t, and the impulse is the derivative of the step. In discrete time, δ[n] = u[n] − u[n − 1].

ECE-158 · What does the Wiener–Khinchin theorem state?

For a wide-sense stationary random process, the power spectral density is the Fourier transform of its autocorrelation function: S(f) = F{R(τ)}.

ECE-159 · How do pole locations determine stability of a continuous-time LTI system?

A causal LTI system is stable if all poles of H(s) lie in the left half of the s-plane. Poles on the jω axis give sustained oscillation, and right-half-plane poles give growing responses.

ECE-160 · What is the Hilbert transform used for in communication?

It shifts every frequency component by −90°. It is used to build the analytic signal, generate SSB by the phase-shift method and extract envelope and instantaneous phase.

ECE-161 · What is the condition for distortionless transmission?

The channel must have H(f) = K·e^(−j2πft_d) over the signal bandwidth: constant magnitude and phase linear in frequency. The output is then a scaled, delayed copy of the input.

ECE-162 · What is group delay?

τg = −dφ(ω)/dω, the delay experienced by the envelope of a narrowband signal. Constant group delay across the band means no envelope distortion.

ECE-163 · What is the Nyquist rate of x(t) = cos(200πt) + sin(600πt)?

The components are at 100 Hz and 300 Hz, so f_max = 300 Hz. The Nyquist rate is 2 × 300 = 600 samples/s.

Analog Communication

ECE-164 · Why is FM less affected by noise than AM?

Most noise and interference appear as amplitude variations. An FM receiver's limiter removes amplitude variations before demodulation, and the information is carried in frequency, which noise disturbs less.

ECE-165 · What is a superheterodyne receiver?

It mixes the incoming RF signal with a tunable local oscillator to convert it to a fixed intermediate frequency (IF). Most filtering and gain happen at the IF, giving good selectivity and sensitivity across the tuning range.

ECE-166 · An AM carrier of 100 W is modulated with μ = 1. What are total power and efficiency?

Pt = Pc(1 + μ²/2) = 100 × 1.5 = 150 W. Sideband power is 50 W, so transmission efficiency is 50/150 ≈ 33.3%, the maximum for single-tone AM.

ECE-167 · What is Carson's rule? Apply it to broadcast FM.

FM bandwidth ≈ 2(Δf + fm). With Δf = 75 kHz and fm = 15 kHz, B ≈ 2(75 + 15) = 180 kHz, which fits within the 200 kHz channel spacing.

ECE-168 · What is the image frequency in a superheterodyne receiver?

The frequency that also mixes to the IF: f_image = f_signal + 2·f_IF for a high-side LO. For 1000 kHz with a 455 kHz IF, the image is 1910 kHz, and it must be rejected before the mixer.

ECE-169 · What is the difference between narrowband and wideband FM?

NBFM has modulation index β ≪ 1 and bandwidth about 2fm, similar to AM. WBFM has large β, bandwidth roughly 2(Δf + fm), and a much larger SNR improvement.

ECE-170 · Why does coherent detection of DSB-SC need phase synchronization?

If the receiver's carrier has phase error φ, the output is scaled by cos φ. At φ = 90° the output vanishes, and a slowly varying φ causes fading of the recovered message.

ECE-171 · What is a Costas loop?

A carrier-recovery circuit with in-phase and quadrature arms whose product drives a VCO. It locks to the suppressed carrier of DSB-SC or BPSK signals.

ECE-172 · What is the threshold effect in FM?

When the input carrier-to-noise ratio falls below a threshold (about 10 dB for a conventional discriminator), output SNR drops sharply and clicks appear. FM's noise advantage only holds above this threshold.

ECE-173 · How can an SSB signal be generated?

By the filter method (generate DSB-SC and filter out one sideband), the phase-shift method (combine the message and its Hilbert transform with quadrature carriers) or the Weaver method.

ECE-174 · What does AGC do in a receiver?

Automatic gain control adjusts receiver gain according to signal strength so the output level stays nearly constant despite fading or distance changes. It also prevents overload by strong signals.

ECE-175 · How do AM, DSB-SC, SSB and FM compare in figure of merit?

Figure of merit is output SNR divided by baseband reference SNR. It is 1/3 for AM with μ = 1 and envelope detection, 1 for DSB-SC and SSB, and 3β²/2 for FM above threshold.

ECE-176 · What is the capture effect in FM?

When two FM signals share a frequency, the receiver locks onto the stronger one and largely suppresses the weaker, even if the power difference is only a few dB.

Digital Communication

ECE-177 · What is the bandwidth of a BPSK signal?

With rectangular pulses, the null-to-null bandwidth is 2Rb. With ideal Nyquist pulse shaping the minimum bandwidth is Rb, and with raised-cosine roll-off α it is Rb(1 + α).

ECE-178 · What is Nyquist's criterion for zero ISI?

The overall pulse must have zero crossings at every other symbol instant: p(nTs) = 0 for n ≠ 0. The minimum bandwidth that achieves this is Rs/2, using an ideal sinc pulse.

ECE-179 · What is a raised-cosine filter?

A practical Nyquist pulse-shaping filter with roll-off factor α between 0 and 1. Baseband bandwidth is B = (Rs/2)(1 + α); larger α eases filtering and timing but uses more bandwidth.

ECE-180 · Why is QPSK more bandwidth-efficient than BPSK?

QPSK sends 2 bits per symbol on two orthogonal carriers, so it doubles the bit rate in the same bandwidth. Its BER versus Eb/N0 is the same as BPSK.

ECE-181 · How are Eb/N0 and SNR related?

SNR = (Eb/N0)·(Rb/B). Eb/N0 normalizes energy per bit, allowing fair comparison of modulation schemes with different bit rates and bandwidths.

ECE-182 · What is the BER of BPSK in AWGN?

Pb = Q(√(2Eb/N0)). A BER of 10⁻⁵ needs Eb/N0 of about 9.6 dB.

ECE-183 · What is the difference between coherent and non-coherent detection?

Coherent detection needs a phase-synchronized carrier reference and gives better BER. Non-coherent detection (for example envelope detection of FSK) needs no phase reference, making receivers simpler at a cost of 1–3 dB.

ECE-184 · What is DPSK?

Differential PSK encodes data in the phase change between successive symbols. The receiver compares each symbol with the previous one, so no absolute carrier phase is needed, at a small Eb/N0 penalty compared with coherent BPSK.

ECE-185 · What are MSK and GMSK?

MSK is continuous-phase FSK with modulation index 0.5, giving constant envelope and compact spectrum. GMSK adds a Gaussian pre-filter to further reduce side lobes and is used in GSM with BT = 0.3.

ECE-186 · What are slope overload and granular noise in delta modulation?

Slope overload occurs when the signal changes faster than the step size Δ per sample can track. Granular noise occurs when the signal is nearly flat and the output hunts by ±Δ. Adaptive delta modulation varies Δ to reduce both.

ECE-187 · What is the bit rate of standard telephone PCM?

Voice sampled at 8 kHz with 8 bits per sample gives 64 kbps (DS0). A T1 line multiplexes 24 such channels plus framing for 1.544 Mbps; E1 carries 32 timeslots at 2.048 Mbps.

ECE-188 · What is companding in PCM?

Compressing the signal before uniform quantization and expanding after, to give roughly constant SQNR for weak and strong signals. North America and Japan use μ-law (μ = 255); Europe uses A-law (A = 87.6).

ECE-189 · What is the purpose of an equalizer?

It compensates for channel frequency distortion to reduce ISI. Zero-forcing equalizers invert the channel but amplify noise; MMSE equalizers balance ISI removal against noise enhancement.

Information Theory & Coding

ECE-190 · State Shannon's channel capacity theorem.

For a band-limited AWGN channel, C = B·log₂(1 + SNR) bits/s. Error-free transmission is possible at any rate below C with suitable coding, and impossible above it.

ECE-191 · What is the Shannon limit for Eb/N0?

As bandwidth goes to infinity, the minimum Eb/N0 for reliable communication approaches ln 2 ≈ 0.693, or −1.59 dB. No coding scheme can work below it.

ECE-192 · What is the capacity of a 3 kHz channel with 30 dB SNR?

SNR = 1000, so C = 3000 × log₂(1001) ≈ 3000 × 9.97 ≈ 29.9 kbps.

ECE-193 · What is the maximum entropy of a source with M symbols?

H_max = log₂M bits/symbol, reached when all symbols are equally likely. A binary source has maximum entropy of 1 bit when p = 0.5.

ECE-194 · What is mutual information?

I(X;Y) = H(X) − H(X|Y), the reduction in uncertainty about the input X from observing the output Y. Channel capacity is the maximum mutual information over all input distributions.

ECE-195 · What is the capacity of a binary symmetric channel?

C = 1 − H(p) bits per use, where H(p) = −p·log₂p − (1−p)·log₂(1−p). For p = 0.5 the capacity is zero; for p = 0 or 1 it is 1 bit.

ECE-196 · What are the properties of the Hamming (7,4) code?

It encodes 4 data bits with 3 parity bits, has dmin = 3 and corrects any single-bit error, or detects up to two errors if used only for detection. Its code rate is 4/7.

ECE-197 · How many errors can a code with minimum distance dmin detect and correct?

It can detect up to dmin − 1 errors and correct up to t = ⌊(dmin − 1)/2⌋ errors.

ECE-198 · What are convolutional codes and how are they decoded?

Each output depends on current and previous input bits held in a shift register of constraint length K. They are decoded optimally (maximum likelihood) by the Viterbi algorithm on a trellis.

ECE-199 · What are the generator and parity-check matrices of a linear block code?

The generator G (k × n) produces codewords c = mG. The parity-check matrix H satisfies GHᵀ = 0. The syndrome s = rHᵀ is zero for valid codewords and identifies correctable errors.

ECE-200 · Why is interleaving used?

It reorders coded bits before transmission so that a burst of channel errors is spread into isolated errors after de-interleaving, which random-error-correcting codes can handle.

ECE-201 · What is the source coding theorem?

The average codeword length L of any uniquely decodable code satisfies L ≥ H, the source entropy. Coding efficiency is η = H/L, and long block codes can approach H arbitrarily closely.

EM Theory & Transmission Lines

ECE-202 · What is VSWR?

Voltage standing wave ratio, the ratio of maximum to minimum voltage on a line: VSWR = (1 + |Γ|)/(1 − |Γ|). VSWR = 1 means a perfect match with no reflection.

ECE-203 · What is return loss?

RL = −20·log₁₀|Γ| dB, the ratio of incident to reflected power. Higher is better; a VSWR of 2 corresponds to |Γ| = 1/3 and a return loss of about 9.5 dB.

ECE-204 · What is the input impedance of a lossless transmission line?

Zin = Z₀·(ZL + jZ₀·tan βl)/(Z₀ + jZL·tan βl), where β = 2π/λ and l is the line length. It repeats every λ/2.

ECE-205 · How do short-circuited λ/4 and λ/2 lines behave?

A shorted λ/4 line looks like an open circuit at its input, and an open λ/4 line looks like a short. A λ/2 line of any termination repeats its load impedance.

ECE-206 · What is a Smith chart?

A polar plot of the reflection coefficient with overlaid circles of constant normalized resistance and reactance. It is used to find input impedance, VSWR and matching networks graphically.

ECE-207 · What is the propagation constant of a transmission line?

γ = α + jβ = √((R + jωL)(G + jωC)). α is attenuation in Np/m and β is phase constant in rad/m.

ECE-208 · What is the velocity factor of a cable?

The ratio of signal speed on the line to c: VF = 1/√εr for a TEM line. Solid polyethylene coax (εr ≈ 2.25) has VF ≈ 0.66.

ECE-209 · A 100 Ω load is connected to a 50 Ω line. Find Γ and VSWR.

Γ = (100 − 50)/(100 + 50) = 1/3. VSWR = (1 + 1/3)/(1 − 1/3) = 2. Reflected power is |Γ|² ≈ 11%.

ECE-210 · What are the boundary conditions at a dielectric interface?

Tangential E and tangential H are continuous (with no surface current). Normal D is continuous if there is no surface charge, and normal B is always continuous.

ECE-211 · What is displacement current?

Jd = ∂D/∂t, the term Maxwell added to Ampère's law. It allows current to "flow" through a capacitor's dielectric and is essential for EM wave propagation.

ECE-212 · What is the Brewster angle?

The angle of incidence at which a parallel-polarized (TM) wave is fully transmitted with no reflection: θB = tan⁻¹(n₂/n₁). Reflected light at this angle is purely perpendicular-polarized.

ECE-213 · What is stub matching?

Placing a short- or open-circuited line section (stub) in shunt or series at a chosen distance from the load to cancel the reactive part of the impedance. Single-stub tuning can match any non-zero load at one frequency.

ECE-214 · When is a material considered a good conductor for EM waves?

When σ ≫ ωε, so conduction current dominates displacement current. Fields then decay rapidly with depth, and the wave's attenuation and phase constants both equal 1/δ.

Antennas & Wave Propagation

ECE-215 · State the Friis transmission equation.

Pr = Pt·Gt·Gr·(λ/(4πR))² for antennas in each other's far field. In dB: Pr = Pt + Gt + Gr − FSPL.

ECE-216 · What is free-space path loss in dB?

FSPL(dB) = 20·log₁₀(d_km) + 20·log₁₀(f_MHz) + 32.44. Doubling distance or frequency adds 6 dB of loss between isotropic antennas.

ECE-217 · What is the far-field distance of an antenna?

The Fraunhofer distance R ≥ 2D²/λ, where D is the largest antenna dimension. A 1 m dish at 10 GHz (λ = 3 cm) has a far field beyond about 67 m.

ECE-218 · What is the effective aperture of an antenna?

The area that captures power from an incident wave: Ae = G·λ²/(4π). Received power is Pr = S·Ae, where S is the incident power density.

ECE-219 · What is an antenna array factor?

The pattern produced by an array of identical isotropic elements, determined by spacing, number, amplitude and phase of the elements. Total pattern = element pattern × array factor.

ECE-220 · What is the difference between broadside and endfire arrays?

In a broadside array, elements are fed in phase and maximum radiation is perpendicular to the array axis. In an endfire array, a progressive phase shift steers the maximum along the axis.

ECE-221 · What is a Yagi-Uda antenna?

A directional antenna with one driven dipole, a slightly longer reflector behind it and several shorter directors in front. It gives moderate gain and is widely used for TV and VHF/UHF links.

ECE-222 · What is the gain of a parabolic dish antenna?

G = η·(πD/λ)², where D is diameter and η is aperture efficiency (typically 0.55–0.7). A 1 m dish at 10 GHz with η = 0.6 gives about 38 dBi.

ECE-223 · What is the first Fresnel zone and why does clearance matter?

An ellipsoid around the line-of-sight path with radius r₁ = √(λ·d₁·d₂/(d₁ + d₂)). Keeping at least 60% of it free of obstacles keeps the link close to free-space loss.

ECE-224 · What are critical frequency and maximum usable frequency?

Critical frequency fc is the highest frequency reflected back at vertical incidence by an ionospheric layer. For oblique incidence, MUF = fc/cos θi (the secant law).

ECE-225 · What is the radiation resistance of a short dipole?

For a short (Hertzian) dipole of length l ≪ λ with uniform current, Rr = 80π²(l/λ)² Ω. With the triangular current of a real short dipole, it is one quarter of this.

ECE-226 · What is polarization mismatch loss?

Received power is scaled by cos²ψ, where ψ is the angle between the incoming field and the antenna's polarization. Crossed linear polarizations ideally give total loss, and linear to circular gives 3 dB.

Microwave Engineering

ECE-227 · What is the TE₁₀ cutoff frequency of a WR-90 waveguide?

WR-90 has a = 22.86 mm, so fc = c/(2a) = 3 × 10⁸/(0.04572) ≈ 6.56 GHz. Its recommended X-band operating range is about 8.2–12.4 GHz.

ECE-228 · What is the guide wavelength in a waveguide?

λg = λ₀/√(1 − (fc/f)²). It is longer than the free-space wavelength and goes to infinity as f approaches cutoff.

ECE-229 · Why are waveguides not used at low frequencies?

The broad dimension must exceed λ/2 for TE₁₀ propagation, so at 100 MHz a guide would need to be over 1.5 m wide. Coax is far more practical at low frequencies.

ECE-230 · What are the standard IEEE radar band letters?

L 1–2 GHz, S 2–4 GHz, C 4–8 GHz, X 8–12 GHz, Ku 12–18 GHz, K 18–27 GHz and Ka 27–40 GHz.

ECE-231 · What is a magic tee?

A four-port waveguide hybrid combining E-plane and H-plane tees. A signal into the H-arm splits in phase to the collinear arms; into the E-arm it splits 180° out of phase. The E and H arms are isolated.

ECE-232 · What are the properties of the S-matrix of a lossless reciprocal network?

Reciprocity makes it symmetric (Sij = Sji). Losslessness makes it unitary ([S]ᴴ[S] = I), so the squared magnitudes in each column sum to 1.

ECE-233 · What is a Gunn diode?

A two-terminal device, typically n-type GaAs, that shows negative differential resistance from the transferred-electron effect, where electrons move to a lower-mobility valley at high fields. It is used as a microwave oscillator.

ECE-234 · What is an IMPATT diode?

An impact-ionization avalanche transit-time diode that combines avalanche multiplication and transit-time delay to give negative resistance. It generates high microwave power but is noisy.

ECE-235 · What is a travelling-wave tube (TWT)?

A vacuum amplifier in which an electron beam interacts continuously with an RF wave slowed by a helix to nearly the beam velocity. It offers very wide bandwidth and high gain, and is used in satellite transponders.

ECE-236 · What is the difference between microstrip and stripline?

Microstrip has a conductor on top of a dielectric over a ground plane; it is easy to fabricate but quasi-TEM and radiates somewhat. Stripline sits between two ground planes in a uniform dielectric, giving pure TEM and better shielding.

ECE-237 · What is the Q factor of a resonator?

Q = ω × (energy stored)/(average power lost), approximately f₀/Δf₃dB. Higher Q means narrower bandwidth and lower loss. Unloaded Q excludes external loading; loaded Q includes it.

ECE-238 · What are coupling, directivity and isolation in a directional coupler?

Coupling C = 10·log(P₁/P₃), directivity D = 10·log(P₃/P₄) and isolation I = 10·log(P₁/P₄), with port 1 input, 3 coupled and 4 isolated. In dB, I = C + D.

Digital Signal Processing

ECE-239 · What is the bilinear transform and frequency warping?

It maps s to z by s = (2/T)(1 − z⁻¹)/(1 + z⁻¹), mapping the whole jω axis onto the unit circle without aliasing. Frequencies warp nonlinearly as Ω = (2/T)·tan(ω/2), so critical frequencies are pre-warped.

ECE-240 · What is impulse invariance and its limitation?

It designs a digital filter by sampling the analog impulse response, h[n] = T·ha(nT). It preserves time response but aliases the frequency response, so it is unsuitable for high-pass or band-stop filters.

ECE-241 · What is the condition for a linear-phase FIR filter?

The impulse response must be symmetric, h[n] = h[N−1−n], or antisymmetric, h[n] = −h[N−1−n]. The group delay is then constant at (N − 1)/2 samples.

ECE-242 · What is the stability condition for a causal discrete-time system?

All poles of H(z) must lie strictly inside the unit circle, |z| < 1. FIR filters have poles only at the origin, so they are always stable.

ECE-243 · What does zero-padding do before a DFT?

It samples the DTFT more densely, giving a smoother-looking spectrum. It does not improve true frequency resolution, which depends only on the actual data length.

ECE-244 · What is the frequency resolution of an N-point DFT?

Δf = fs/N, the spacing between bins. With fs = 48 kHz and N = 1024, Δf ≈ 46.9 Hz.

ECE-245 · What are the overlap-add and overlap-save methods?

Block methods for FFT-based linear convolution of a long signal with an FIR filter. Overlap-add sums overlapping output tails; overlap-save overlaps input blocks and discards wrapped-around output samples.

ECE-246 · What is spectral leakage?

When a non-integer number of cycles is captured in the DFT window, a single tone's energy spreads into neighbouring bins. Windowing reduces the leakage at the cost of a wider main lobe.

ECE-247 · What is the Gibbs phenomenon?

Truncating a Fourier series or ideal filter response causes about 9% overshoot and ripple near discontinuities that does not shrink as more terms are added. Smooth windows reduce it in FIR design.

ECE-248 · How many complex multiplications does a 1024-point FFT save?

Direct DFT needs N² ≈ 1,048,576. Radix-2 FFT needs (N/2)·log₂N = 512 × 10 = 5120, about 200 times fewer.

ECE-249 · What is the difference between Direct Form I and Direct Form II IIR structures?

Direct Form I implements the zeros then the poles with separate delay lines. Direct Form II shares one delay line, using the minimum number of delays (canonic), but has larger internal signal swing.

ECE-250 · What are finite word-length effects in digital filters?

Coefficient quantization shifts poles and zeros, round-off adds noise, overflow can cause large errors, and limit cycles create small oscillations in IIR filters. Cascaded second-order sections reduce sensitivity.

ECE-251 · Why must you filter before decimating?

Downsampling by M folds all content above fs/(2M) into the new baseband. A low-pass filter with cutoff π/M must precede downsampling to prevent aliasing.

Wireless & Cellular

ECE-252 · What is the difference between TDMA, FDMA and CDMA?

They share a channel among users by different time slots (TDMA), different frequency bands (FDMA) or unique spreading codes over the same time and frequency (CDMA).

ECE-253 · How are cluster size and co-channel reuse ratio related?

Valid hexagonal cluster sizes are N = i² + ij + j² (1, 3, 4, 7, 12…). The reuse ratio Q = D/R = √(3N), so N = 7 gives Q ≈ 4.58.

ECE-254 · What are delay spread and coherence bandwidth?

Delay spread στ is the spread of multipath arrival times. Coherence bandwidth is inversely proportional to it, roughly Bc ≈ 1/(5στ) for 0.5 correlation. If signal bandwidth exceeds Bc, fading is frequency-selective.

ECE-255 · What is coherence time and how does it define fast and slow fading?

Coherence time Tc ≈ 0.423/fd, the duration over which the channel is roughly constant. If the symbol period exceeds Tc, fading is fast; if much shorter, it is slow.

ECE-256 · What is the difference between Rayleigh and Rician fading?

Rayleigh fading models many scattered paths with no dominant line-of-sight. Rician fading includes a dominant LOS component; its K-factor is the ratio of LOS to scattered power.

ECE-257 · What diversity techniques are used against fading?

Space (multiple antennas), frequency, time (coding plus interleaving), polarization and multipath (RAKE) diversity. Signals are combined by selection, equal-gain or maximal-ratio combining (MRC), which is optimal.

ECE-258 · What are cell splitting and sectoring?

Cell splitting divides congested cells into smaller cells with lower power, increasing reuse. Sectoring uses directional antennas (for example three 120° sectors) to reduce co-channel interference and boost capacity.

ECE-259 · What are the key air-interface features of LTE?

OFDMA in the downlink, SC-FDMA in the uplink, MIMO, channel bandwidths from 1.4 to 20 MHz, 15 kHz subcarrier spacing, adaptive modulation up to 64-QAM or higher, and an all-IP core (EPC).

ECE-260 · Why does LTE use SC-FDMA in the uplink?

SC-FDMA (DFT-spread OFDM) has a lower peak-to-average power ratio than OFDMA. This lets the handset's power amplifier run more efficiently, saving battery while keeping OFDM-like equalization.

ECE-261 · What is the path loss exponent?

In PL ∝ dⁿ, n = 2 in free space, about 2.7–3.5 in urban cellular areas and 3–5 in shadowed urban areas. Higher n means power falls faster with distance.

ECE-262 · What is log-normal shadowing?

Large-scale variation of received power around the mean path loss caused by obstacles like buildings and terrain. In dB it follows a Gaussian distribution, with standard deviation typically 4–12 dB.

ECE-263 · What is the difference between hard and soft handoff?

In hard handoff the old link is broken before the new one is made (GSM, LTE). In soft handoff the mobile connects to several base stations at once before dropping the old one (CDMA, WCDMA).

ECE-264 · What is the near-far problem in CDMA?

A nearby user's strong signal can drown out a distant user's weak signal at the base station, because all share the same band. Fast closed-loop power control equalizes received powers.

Optical Communication

ECE-265 · Find NA and acceptance angle for n₁ = 1.48 and n₂ = 1.46.

NA = √(1.48² − 1.46²) = √(2.1904 − 2.1316) = √0.0588 ≈ 0.243. Acceptance angle θa = sin⁻¹(0.243) ≈ 14°.

ECE-266 · What is the V-number of a fiber?

V = (2πa/λ)·NA, where a is core radius. A step-index fiber is single-mode when V < 2.405.

ECE-267 · How many modes does a multimode step-index fiber support?

Approximately M ≈ V²/2 for large V. A fiber with V = 30 supports about 450 modes.

ECE-268 · What are the types of dispersion in optical fiber?

Modal (intermodal) dispersion in multimode fiber; chromatic dispersion, made of material and waveguide dispersion; and polarization-mode dispersion caused by birefringence.

ECE-269 · Why is 1550 nm preferred for long-haul links?

Silica fiber has its lowest loss there (about 0.2 dB/km), and erbium-doped fiber amplifiers operate in this band. Dispersion can be managed with dispersion-shifted fiber or compensation.

ECE-270 · What is an EDFA?

An erbium-doped fiber amplifier, pumped by a laser at 980 nm or 1480 nm, that directly amplifies optical signals in the C-band near 1550 nm. It amplifies many WDM channels at once without O-E-O conversion.

ECE-271 · How does a laser diode compare with an LED as an optical source?

A laser diode gives coherent, narrow-linewidth, high-power output with high modulation bandwidth, suited to long single-mode links. An LED is cheaper and more robust but has broad spectrum, lower power and lower speed.

ECE-272 · How does an APD compare with a PIN photodiode?

An APD has internal avalanche gain, giving better sensitivity, but needs high reverse bias, is temperature-sensitive and adds excess noise. A PIN is simpler, cheaper and lower-noise but has no gain.

ECE-273 · What is photodetector responsivity?

R = Ip/Pin = η·q·λ/(h·c) ≈ η·λ(µm)/1.24 A/W. A PIN with η = 0.8 at 1.55 µm has R ≈ 1.0 A/W.

ECE-274 · Calculate the maximum span from this power budget: Pt = 0 dBm, sensitivity = −30 dBm, fiber loss 0.25 dB/km, connectors 2 dB, margin 3 dB.

Available budget is 30 dB. Subtract 2 dB connector loss and 3 dB margin to get 25 dB for fiber. Maximum length = 25/0.25 = 100 km.

ECE-275 · What is the difference between CWDM and DWDM?

CWDM uses wide 20 nm channel spacing with cheap uncooled lasers and few channels. DWDM uses dense spacing such as 100 GHz (about 0.8 nm) or 50 GHz on the ITU grid, giving many channels for long-haul amplified systems.

ECE-276 · What is Rayleigh scattering in fiber?

Scattering from microscopic density fluctuations frozen into the glass. It varies as 1/λ⁴, so it dominates loss at shorter wavelengths and sets the fundamental loss floor.

Data Communication & Networking

ECE-277 · What is the TCP three-way handshake?

The client sends SYN, the server replies SYN-ACK, and the client sends ACK. This synchronizes initial sequence numbers and establishes the connection.

ECE-278 · What is the difference between Go-Back-N and Selective Repeat ARQ?

Go-Back-N retransmits the erroneous frame and all frames sent after it, with a receiver window of 1. Selective Repeat retransmits only the lost frames but needs buffering, and its window must not exceed half the sequence space.

ECE-279 · What is the efficiency of stop-and-wait ARQ?

η = 1/(1 + 2a), where a = propagation delay/transmission time. Long or fast links have large a and very low efficiency, so sliding-window protocols are used.

ECE-280 · What is the difference between CSMA/CD and CSMA/CA?

CSMA/CD (classic Ethernet) detects collisions while transmitting and backs off. CSMA/CA (Wi-Fi) tries to avoid collisions with random backoff and acknowledgements, because a radio cannot reliably detect collisions while transmitting.

ECE-281 · How many usable hosts does a /26 subnet provide?

A /26 leaves 6 host bits, so 2⁶ = 64 addresses. Removing the network and broadcast addresses leaves 62 usable hosts.

ECE-282 · What is ARP?

Address Resolution Protocol maps an IPv4 address to a MAC address on a local network. A host broadcasts a request, and the owner replies with its MAC address.

ECE-283 · What are the maximum throughputs of pure and slotted ALOHA?

Pure ALOHA peaks at 1/(2e) ≈ 18.4% at load G = 0.5. Slotted ALOHA halves the vulnerable period and peaks at 1/e ≈ 36.8% at G = 1.

ECE-284 · What are the main differences between IPv4 and IPv6?

IPv4 has 32-bit addresses; IPv6 has 128-bit addresses. IPv6 also has a simpler fixed header, built-in autoconfiguration, no router fragmentation and no broadcast (it uses multicast).

ECE-285 · What is the difference between distance-vector and link-state routing?

Distance-vector (such as RIP) shares routing tables with neighbours and can converge slowly (count-to-infinity). Link-state (such as OSPF) floods topology information so every router computes shortest paths with Dijkstra's algorithm.

ECE-286 · How does TCP congestion control work?

TCP starts with slow start (exponential window growth), then congestion avoidance (additive increase). On loss it cuts the window multiplicatively (AIMD). Fast retransmit and fast recovery handle isolated losses.

ECE-287 · What is NAT?

Network Address Translation rewrites private IP addresses (and often ports) to a public address at a router. It lets many devices share one public IPv4 address.

ECE-288 · What is the bandwidth-delay product?

Link capacity × round-trip time, the amount of data in flight needed to keep the pipe full. A 100 Mbps link with 50 ms RTT needs a window of about 625 kB.

RF Circuits

ECE-289 · What is the Friis formula for cascaded noise factor?

F_total = F₁ + (F₂ − 1)/G₁ + (F₃ − 1)/(G₁G₂) + …, using linear values. The first stage dominates when its gain is high.

ECE-290 · Why is the LNA placed first in a receiver chain?

By the Friis formula, the first stage's noise adds directly while later stages' noise is divided by preceding gain. A low-NF, high-gain LNA therefore sets the system noise figure.

ECE-291 · How are noise figure and noise temperature related?

Te = T₀(F − 1), with T₀ = 290 K. NF = 3 dB (F = 2) corresponds to Te = 290 K, and NF = 1 dB to about 75 K.

ECE-292 · What is IIP3?

The input third-order intercept point is the extrapolated input power at which the third-order intermodulation products (2f₁ − f₂, 2f₂ − f₁) would equal the fundamental. IM3 rises 3 dB per 1 dB of input, so higher IIP3 means better linearity.

ECE-293 · What is the thermal noise floor?

N = kTB, which at 290 K is −174 dBm/Hz. For a 1 MHz bandwidth, the noise floor is −174 + 60 = −114 dBm.

ECE-294 · What is mixer conversion gain or loss?

The ratio of IF output power to RF input power. Passive diode mixers have conversion loss (about 6–7 dB for a double-balanced design), while active mixers such as Gilbert cells provide gain.

ECE-295 · What is phase noise?

Random fluctuation of an oscillator's phase, seen as noise skirts around the carrier and specified in dBc/Hz at an offset. It causes reciprocal mixing in receivers and constellation spreading in digital links.

ECE-296 · How do PA classes A, B, AB and C compare?

Class A conducts 360° with maximum efficiency 50% and best linearity. Class B conducts 180°, up to 78.5%. Class AB is between them. Class C conducts under 180°, is highly efficient but nonlinear, suiting constant-envelope signals.

ECE-297 · How is receiver sensitivity calculated?

Sensitivity (dBm) = −174 + 10·log₁₀(B) + NF + SNR_min. With B = 200 kHz, NF = 8 dB and SNR_min = 9 dB, sensitivity ≈ −174 + 53 + 8 + 9 = −104 dBm.

ECE-298 · What is an L-section matching network?

Two reactive elements, one series and one shunt, that match a load to a source at one frequency. Its Q is fixed by the resistance ratio, Q = √(R_high/R_low − 1).

ECE-299 · What are the trade-offs of a direct-conversion receiver versus superheterodyne?

Direct conversion mixes straight to baseband, removing image filters and IF stages for easy integration. It suffers DC offsets, LO leakage and self-mixing, flicker noise and I/Q imbalance.

ECE-300 · What is spurious-free dynamic range (SFDR)?

The range between the noise floor and the input level at which third-order products rise to the noise floor: SFDR = (2/3)(IIP3 − N_floor) in dB. It defines the usable input range for weak and strong signals together.

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