1. Orthogonal frequency duplex multiple access, omnidirectional frequency division multiple access, orthogonal frequency divider multiple access, and orthogonal frequency division multiple access are the acronyms for OFDMA.
Answer: d Explanation: The well-known orthogonal frequency-division multiplexing (OFDM) digital modulation scheme has a multi-user variant called orthogonal frequency-division multiple access (OFDMA). By allocating specific users different subsets of subcarriers, OFDMA achieves multiple access.
2. To ensure symbol time is an integer number, prevent multipath and ISI, maintain orthogonality, and make OFDMA scalable are all reasons why a cyclic prefix is needed in an OFDMA.
Clarification: A crucial component in ensuring the dependability of the OFDM signal is the use of cyclic prefix. For the purpose of shielding the OFDM signals from intersymbol interference, the cyclic prefix functions as a buffer region or guard interval.
3. A) The cell’s identity B) The antenna configuration C) The center of the OFDM channel D) The format of the data channel
Clarification: An OFDM symbol’s subcarriers do not all carry useful data. The DC subcarrier in OFDM and OFDMA PHY layers is the subcarrier whose frequency is the same as the transmitting station’s RF center frequency.
4. A) FFT, B) IFFT, C) RF combining, D) Channel mapping is the processing step that combines multiple OFDM subcarriers into a single signal for transmission.
Answer: b Explanation: For transmission, IFFT combines various OFDM subcarriers into a single signal. From the perspective of OFDM, these transforms are crucial because they can be thought of as the process of mapping digitally modulated input data (data symbols) onto orthogonal subcarriers.
5. A) Orthogonality b) Orthodoxy c) Octagonality d) Originality Which characteristic of the OFDMA system allows the use of adjacent subcarriers without interference?
Clarification: Orthogonality of sub-carriers simply denotes the absence of any correlation. In the OFDMA system, orthogonality permits the use of adjacent subcarriers without interference.
6. What is the relationship in OFDMA between the symbol time t and the subcarrier spacing f? a) f=t b) f=1/2t c) f=1/t d) no relation
Answer: c Explanation: In OFDMA, the subcarrier spacing f and symbol time t have the relationship f=1/t. They are inversely proportional.
In applications like digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G mobile communications, OFDM has grown to be a well-liked wideband digital communication scheme.
Answer: b Clarification: ODMA does not use complex equalizers. Each detected sub-carrier (each Fourier coefficient) in an OFDM symbol only needs to be multiplied by a fixed complex number, or a rarely altered value, by the equalizer.
9. The effects of __________ are lessened when Orthogonal Frequency Division Multiplexing (OFDM) is divided into subbands: a) noise b) collision c) interference d) signals absence
Answer: c Explanation: By incorporating numerous smaller-bandwidth channels, OFDM uses the same bandwidth to deliver roughly the same data rate as a single carrier modulation. The original rate is achieved by combining the lower-bandwidth channels, each of which has a lower rate.
10. Common data rates of IEEE 802. 11 OFDM have the following data rates: 18 Mbps, 200 Mbps, 50 Mbps, and 54 Mbps.
Answer: a Clarification: The IEEE 802. A modulation that splits a high-speed serial information signal into numerous lower-speed sub signals is described by the 11a standard. Common data rates of IEEE 802. 11 OFDM is 18 Mbps.
How to Answer Behavioral Interview Questions Sample Answers
The first step in designing an OFDM receiver is to establish the number of subcarriers, the number of symbols, and the length of the cyclic prefix. Making a pilot subcarrier matrix, which is used to predict the channel response, is the next step. The channel response is then estimated for each data subcarrier after the data subcarriers are mapped to the pilot subcarriers. Finally, the data is demodulated and decoded.
In an OFDM system, the Cyclic Prefix is used to minimize Inter Symbol Interference (ISI). When the symbols in a digital communication system are not exactly aligned, ISI can happen. Numerous factors, including frequency-selective fading and multipath propagation, can cause this. When ISI happens, the received signal may become distorted and challenging to decode. Before being transmitted, the Cyclic Prefix is added to the OFDM symbol to guarantee that the symbols are correctly aligned when they are received.
A crucial component of OFDM is dynamic resource allocation, which describes the system’s capacity to adjust to changing circumstances and utilize resources as effectively as possible. This includes adjusting the size of the subcarriers or the number of carriers being used in accordance with the current needs.
The process of figuring out a signal’s carrier frequency offset is known as carrier frequency offset estimation. The carrier frequency offset in Orthogonal Frequency Division Multiplexing (OFDM) systems can result in Inter-Symbol Interference (ISI), so it’s important to keep in mind. The carrier frequency offset can be calculated using a variety of techniques, such as the use of a pilot signal or a known data sequence.
Using FDM, multiple signals can be multiplexed onto a single carrier by splitting the carrier’s frequency spectrum into a number of smaller bands, each of which is then used to carry a different signal. Using orthogonal frequencies in addition to the similar technique of OFDM reduces crosstalk between the various signals even more.
6) What are the advantages of using OFDM? Answer: Spectrum efficiency, also known as bandwidth efficiency, is the first advantage. That phrase actually refers to the fact that more data can be transmitted faster in the presence of noise within a given bandwidth. Bits per second per Hertz, or bps/Hz, is a unit of measurement for spectral efficiency. Different modulation techniques will result in drastically different maximum data rates for a given chunk of spectral space for a given bit error rate (BER) and noise level. Amplitude shift keying (ASK) and frequency shift keying (FSK) are only decent but straightforward digital modulation techniques. BPSK and QPSK are much better. While QAM is excellent, it is more susceptible to noise and weak signal levels. Code division multiple access (CDMA) methods are even better. However, none can match OFDM when it comes to extracting the most data possible from a given channel. The channel capacity C, measured in bits per second (bps), is defined as C = B * log2(1 + S/N), where B is the channel’s bandwidth in hertz and S/N is the power signal-to-noise ratio. The holy grail of wireless technology is spectral efficiency due to the scarcity or high cost of spectrum.
5) How is OFDM implemented in the real world? Answer: Digital signal processing (DSP) is used to implement OFDM. The IFFT and FFT math functions can be programmed on any fast PC, but they are typically implemented using a DSP IC, an appropriately programmed FPGA, or some hardwired digital logic. Even complex math routines like FFT are relatively simple to implement with today’s lightning-fast chips. In a nutshell, we can fit everything on a single chip.
9) What is OFMDA? Ans: The A stands for access. It means that OFDM is not only a fantastic modulation technique but can also give numerous users access to a single channel or bandwidth. Numerous access techniques like frequency-division multiplexing (FDM) and time-division multiplexing (TDM) are probably already familiar to you. Each digital signal that needs to be transmitted is digitally coded before being transmitted using the same spectrum by the widely used cellular technology known as CDMA. They simply appear to one another as low-level noise because of their random nature. The receiver can later separate each individual signal thanks to the digital coding. Multiple users can share a single bandwidth thanks to OFDMA, which offers essentially the same advantages.
1) What is OFDM, and why is it better than older, more traditional single-carrier modulation methods? Orthogonal frequency division multiplexing, or OFDM, is a broadband multicarrier modulation technique that operates in the UHF and microwave spectrum and provides superior performance and advantages over older, more traditional single-carrier modulation methods.
8) What are the drawbacks of OFDM? Ans: OFDM is not perfect, just like anything else. It is very complex, making it more expensive to implement. However, modern semiconductor technology makes it pretty easy. OFDM is also sensitive to carrier frequency variations. OFDM systems transmit pilot carriers along with the subcarriers for synchronization at the receiver to get around this issue. An OFDM signal’s high peak to average power ratio is another drawback. As a result, the complex OFDM signal requires linear amplification. As a result, the RF power amplifiers will be less effective and use more energy.
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1. Popular Interview Question in Wireless Communication Dr. Varun Kumar ************************************************************************************************** 1. What do you mean favourable propagation? Ans It refers the mutual orthogonality among vector value wireless channel that can maximize the total system throughput. Ex- Let H is the channel matrix and P is the transmitted power and σ2 is noise variance. Hence, channel capacity is C = log2
2. det I + P σ2 HHH
3. Let H = h11 h12 h21 h22 # ⇒ HH = h∗ 11 h∗ 21 h∗ 12 h∗ 22 # C = log2 P σ2 n (1 + |h2 11| + |h12|2 )(1 + |h2 21| + |h22|2 ) − (h11h∗ 21 + h12h∗ 22)(h21h∗ 11 + h22h∗ 12) o Favourable propagation refers: (h11h∗ 21 + h12h∗ 22)(h21h∗ 11 + h22h∗ 12) → 0 C = Cmax = log2 P σ2 n (1 + |h2 11| + |h12|2 )(1 + |h2 21| + |h22|2 ) o 2. What are the challenges of TDD ? Ans The challenges of TDD are as follow • It is relatively more complex compare to FDD. How?? • It works on larger power compare to FDD. Why?? • The UL and DL channel matrix doesn’t remain same for UL/DL same carrier frequency. The major reason is hardware mismatch across BS and UE side both. 3. Can we achieve the channel reciprocity using frequency division duplexing ? Ans For reciprocal channel, the UL and DL carrier frequency should be same. For practical consideration, if the UL/DL carrier frequencies are nearly same the associated hardware circuitry will respond same. In this way, we can say reciprocity can be achieved through FDD. But if UL and DL signals are not nearly equal then, channel reciprocity can’t be achieved using FDD. 4. What is coherent and non-coherent detection ? Ans Message signal is detected, when received signal is passed through product modulator then after through LPF. Ex- Let received signal is m(t) cos 2πfct. Hence, there will be two case arise. 1
4. Case 1: When local oscillator (LO) produces the same carrier, ie cos 2πfct then the resultant signal can be expressed as m(t) cos 2πfct × cos 2πfct = m(t) 2 + m(t) cos 4πfct 2 LPF −−−−−−→ m(t) 2 This detection is called as coherent detection. Case 2: When local oscillator (LO) produces the same carrier, ie cos 2π(fc + ∆fc)t then the resultant signal can be expressed as m(t) cos 2πfct × cos 2π(fc + ∆fc)t = m(t) cos 2π∆fct 2 + m(t) cos 2π(2fc + ∆fc)t 2 LPF −−−−−− → m(t) cos 2π∆fct 2 This detection is called as non-coherent detection. 5. Why FDD is not suitable for massive MIMO communication ? Ans For coherent detection, the coherence time Tc can be expressed as follow Tc = Tch + TUL(Data) + TDL(Data) + Toth Tch → Time elapsed in channel estimation TUL(Data) → Time allocation for UL data transmission TDL → Time allocation for DL data transmission Toth → Guard period etc Let a base station (BS) has M antenna and K UEs connected to the respective BS. The channel estimation time for TDD and FDD scheme can be expressed as Tch = (2M + K)τ → FDD = Kτ → TDD If channel estimation time is more, then it will not support the highly mobile user or when operating carrier frequency is very high. 6. What is TDD and FDD ? Ans TDD refers to time division duplexing, whereas FDD refers to frequency division multiplexing. The basic difference has been described through below table. 7. What are practical difficulty with reciprocal channel ? Ans Channel reciprocity: When Tx − Rx position are interchanged then the transfer charac- teristics of channel should remain same. This process or act referred as channel reciprocity. Practical difficulties with BS-UE based wireless network. • Man-made noise levels: The antenna of the BS is at more height, so that it does not affected by obstacles in its vicinity. On the other hand, UE antenna is at most mounted a few meters above ground level. The man-made noise level is likely to be substantially higher at the mobile antenna than at the BS antenna. 2
5. TDD FDD MAC layer complexity is less, because it operates on a single frequency. MAC layer complexity is relatively high. Latency is more, hence it is not suitable for large distance communication. No major issues of latency in FDD. Spectrum efficiency is high, because in bi- directional communication most of the DL channel utilized. Spectrum efficiency is less in unbalanced traffic scenario. Equipment cost is less, because no diplexer is used due to single carrier frequency. Equipment cost is high. Signal processing complexity is more. Signal processing complexity is less. • Effect of antenna diversity: Multipath scatters mostly occur in the immediate vicinity of the mobile antenna. The base station receives more or less a transversal electromagnetic wave, whereas the mobile station receives a superposition of a set of reflected waves from random angles. Two antennas at the mobile terminal are likely to receive uncorrelated signal powers if their separation is more than a wave length. At the base station site, however, all reflections arrive from almost identical directions. Therefore, diversity at the base station requires much larger separation of the antennae to ensure uncorrelated received signal powers at the two antennas. For the same reason, antenna directivity has different effects at the mobile and the base station. • Correlation of shadow fading of desired signal and interfering signals: In a cellular network, shadow fading of the wanted signal received by the mobile station is likely to be correlated with the shadow fading of the interference caused by other base stations, or, in a spread-spectrum network, with the shadowing of simultaneously transmitted signals from the same base station. In contrast to this, at the base station, shadow fading of the wanted signal presumably is mostly statistically independent from shadow fading of the interference. However, experimental results for correlation of shadow attenuation are scarce. • Full-duplex channels: In full-duplex operation, multipath fading of inbound and outbound channel, which operate at widely different frequencies, may be uncorrelated. This will particularly be the case if the delay spread is large. • Multiplexing and multiple access In a practical multi-user system with intermittent transmissions, inbound messages are sent via a multiple-access channel, whereas in outbound channel, signals destined for different users can be multiplexed. In the latter case, the receiver in a mobile station can maintain carrier and bit synchronisation to the continuous incoming bit stream from the base station, whereas the receiver in the base station has to acquire synchronisation for each user slot. Moreover, in packet-switched data networks, the inbound channel has to accept randomly occurring transmissions by the terminals in the service area. Random-access protocols are required to organise the data traffic flow in the inbound channel, and access conflicts (’contention’) may occur. In cellular networks with large traffic loads per base station, spread-spectrum modulation can be exploited in the downlink to combat multipath fading, whereas in the uplink, the signal powers from the various mobile subscribers may differ too much to effectively apply spread- spectrum multiple access unless sophisticated adaptive power control techniques are employed. • industrial design From a practical point of view, the downlink and the uplink will be designed under entirely different (cost) constraints, such as power consumption, size, weight and other ergonomic aspects, energy radiated into the human body, and consumer cost aspects. • Data traffic patterns In packet data networks applied for traffic and transportation, the characteristics 3
6. of the data traffic flows are known to differ for the uplink and the downlink. For instance, (outbound) messages from a fleet management centre to the vehicles are likely to be of a more routine type, of a more uniform length and occur in a more regular pattern than messages in the opposite (inbound) direction. 8. Can massive MIMO nullify the effect of inter-user interference ? Ans Beamforming is the essential feature of massive MIMO. In same time-frequency slot all UEs observed the vector valued channel. Due to large number of links, it is supposed that all wireless link are mutually orthogonal. This orthogonality implies the nullification of inter- user interference. 9. Difference between Rayleigh, Racian and Nakagami fading. Ans Fading refers the fluctuation of signal, when it travel into a wireless media. (a) Rayleigh fading: When transmitter and receiver are not in the LOS, then observed fluctuated channel can be modeled as a Rayleigh distributed random variable. f(r) = r σ2 e− r2 2σ2 (b) Racian fading: When transmitter and receiver are in the LOS, then observed fluctuated channel can be modeled as a Racian distributed random variable. (c) Nakagami fading: When transmitter and receiver are in the LOS, then observed fluc- tuated channel can be modeled as a Racian distributed random variable. 10. Difference between time flat and time selective, frequency-flat and frequency- selective, and doubly selective channel. Ans The received signal has two components, ie channel coefficient and transmitted data symbol. Mathematically, it can be expressed as y(t) = N X i=1 h(t − τi)x(t − ti) ∀ i = 0, 1, 2, …N Case 1: When y(t) = hx(t − t0) (a) h(t) = h → Time flat (b) x(t − ti) = x(t − t0) → Single delay element → Frequency flat Case 2: When y(t) = h0x(t − t0) + h1x(t − t0) + … + hN x(t − t0) (a) h(t − τ0) = h0, …., h(t − τN ) = hN → Time selective (b) x(t − ti) = x(t − t0) → Single delay element → Frequency flat Case 3: When y(t) = h0x(t − t0) + h1x(t − t1) + … + hN x(t − tN ) (a) h(t − τ0) = h0, …., h(t − τN ) = hN → Time selective (b) x(t − ti) = x(t − t0) → Single delay element → Frequency flat 4
7. 11. What is coherence time and coherence bandwidth ? Ans Coherence time and coherence bandwidth are the statistical measures for a communication system. We can understand the mathematical relation of the received wireless signal y = hx + n (1) h → Channel coefficient, that is also a time varying quantity. x → Transmitted symbol/data/information n → Additive noise Coherence time refers a time, where channel is supposed to highly correlated. Let h(t) and h(t + τ) are channel coefficient for two time instant. For 70% correlation, the value of coherence time can be expressed as Tc = 9 16πfd , fd = vfc c → Doppler frequency v → Velocity of UE, fc → Carrier frequency, c → Speed of light Coherence bandwidth: From (1), coherence bandwidth is associated with x. Supportable bandwidth, so that channel became flat. 12. What is equalizer and how it improves the signal quality ? Ans Equalization is the reversal of distortion incurred by a signal transmitted through a channel. X(f) =⇒ H(f) Channel | {z } =⇒ H(f)X(f) =⇒ 1 H(f) Equalizer | {z } =⇒ X(f) Equalizer is a method for combating with multi-path fading. Ex- let a current received symbol is y(k) that depends on current and one unit delayed input signal, i.e, x(k), and x(k − 1). Mathematically, it can be expressed as y(k) = h0x(k) + h1x(k − 1) + n(k) y(k + 1) = h0x(k + 1) + h1x(k) + n(k + 1) y(k + 2) = h0x(k + 2) + h1x(k + 1) + n(k + 2) Let current input is a three tapped output symbol, i.e x̂(k) = f y(k), y(k + 1), y(k + 2) Above expression can also be written in the matrix form y(k + 2) y(k + 1) y(k) = h0 h1 0 0 0 h0 h1 0 0 0 h0 h1 x(k + 2) x(k + 1) x(k) x(k − 1) + n(k + 2) n(k + 1) n(k) 5
8. Let x̂(k) = c0y(k+2)+c1y(k+1)+c2y(k), C̄ = [c2 c1 c0]T , X̄ = [x(k+2), x(k+1), x(k), x(k− 1)]T and Ȳ = [y(k + 2), y(k + 1), y(k)]T . The aim of equalizer is to find out the vector C̄, such that C̄T H → [0 0 1 0]. Let 12 = [0 0 1 0] Here, x̂(k) = C̄T Ȳ = C̄T HX̄ + N̄ Applying the least square based equalization, the value of C̄ is C̄ = HT H −1 HT 12 (2) 13. What is the significance of auto-correlation ? Ans It refers the degree of similarity between signal itself. It is widely applicable in wireless communication, when we work on a random process. The best example of auto-correlation that gives some practical sense is coherence time, finding the energy and power of the signal. Rxx(τ) = Z ∞ −∞ x(t)x(t + τ)dt ⇒ Rxx(0) = Z ∞ −∞ |x(t)|2 dt = Eg P = lim T→∞ 1 T Z T/2 −T/2 |x(t)|2 dt = 14. What is the difference between large and small-scale effect? Ans When signal propagate into a wireless media then two types of effect is observed. (a) Large scale effect: When separation is of the order of 100m or more. Ex- i. Propagative loss • Free space • Indoor • Outdoor ii. Propagative mode • Reflection • Refraction • Diffraction iii. Shadowing (b) Small scale effect: When separation is of the order of 10 to 20λ. Ex- Multi-path fading • Rayleigh fading • Ricean fading • Flat fading • Frequency selective fading • Fast fading 6
9. • Slow fading • Time dispersion • Random FM 15. Difference between FDM, OFDM, and OFDMA. Ans Multiplexing is process when resources are shared over a common channel. (a) FDM:(Frequency division multiplexing) It refers a process where frequency re- sources are shared over a common channel. In FDM, Guard band is required and the spectral efficiency of FDM is low. (b) OFDM (Orthogonal frequency division multiplexing): It is a special kind of FDM, where two sub-carrier are orthogonal to each other and no need of guard band. Hence, it maximize the overall spectrum efficiency. (c) OFDMA(Orthogonal frequency division multiplexing access): It is also a mul- tiple access technique. It can be understood as the combination of TDMA+OFDM. 16. What do you mean by interleaver in communication system? Ans Interleaving in communication is a process by which the ordering sequence of the encoded bits is changed so that the burst of errors gets distributed across various codewords present in the data rather than being bound to a single codeword. 17. What is the difference between multiplexing gain and diversity gain. Ans ⇒ Multiplexing gain refers the number of bits/symbol transmission from the transmitter node at a time through a common channel. ⇒ Diversity gain refers the link reliability, when signal is coming from multiple link. Higher the number of link greater be the diversity gain. Capacity can also be increased by diversity combining. In MIMO system, let Nt and Nr be the total number of transmit and receive antenna. Mathematically capacity expression can be written as as R = min{Nt, Nr} | {z } Multiplexing log2 1 + P σ2 (|h1|2 + |h2|2 + … + |hN |2 ) | {z } Diversity 18. What is water-filling algorithm ? 7
10. Ans It is a method for power allocation across each spatial direction. This algorithm is used in the MIMO communication network. Let a MIMO network of size N × N having spatial gain α1, α2,….., αN and allocated power across each spatial directions are P1, P2,….,PN , respectively. During communication, let the noise variance is σ2 . Hence, the observed data rate under such a scenario can be expressed as R = N X i=1 log2 1 + Piαi σ2 In communication system the total transmit power (PT ) is a fixed quantity. Hence, P1 + P2 + …… + PN ≤ PT It is a optimization problem and solving these problem (minimize the power and maximize the data rate), we get Pi = max n1 λ − σ2 αi + , 0 o ∀ α1 ≥ α2…… ≥ αN Here, 1 λ is the reference allocated power. From above relation, it is clear that as per water filling algorithm maximum power is allocated for the highest spatial gain. 19. What do you mean by transmit diversity and receive diversity ? Ans Diversity is a technique that provides the robustness in communication system. Multiple diversity techniques are as follow (a) Time diversity (b) Frequency diversity (c) Antenna diversity or Space diversity (d) Macro diversity 20. What are the functions of precoder and decoder ? Ans (a) Precoder: It is a device that send or transmit the signal for fulfilling the following objective. ⇒ Mitigate the interuser interference ⇒ Cancellation of inter base station interference ⇒ Nullify the effect of pilot contamination (b) Decoder It is a device that send or transmit the signal for fulfilling the following objec- tive. 21. What is the usage of circular convolution and correlation in communication sys- tem ? 8
11. Ans Circular convolution is the convolution of two periodic functions that have the same period. For example, the DTFT of the product of two discrete sequences is the periodic convolution of the DTFTs of the individual sequences. And each DTFT is a periodic summation of a continuous Fourier transform function. Although DTFTs are usually continuous functions of frequency, the concepts of periodic and circular convolution are also directly applicable to discrete sequences of data. 22. What is the usage of I-FFT block in OFDM transmitter ? Ans There are two major advantage of I-FFT block in OFDM transmitter. (1) It reduces the hardware requirement like filter bank across the receiver unit. (2) Sample based sub-carrier information can be coherent decoded by only the IFFT opera- tion. Because, each adjacent samples are orthogonal to each other. In OFDM communication system, let total bandwidth and number of allocated sub-carriers are B and N respectively. Bandwidth allocation across each sub-carrier is 4f = B N Total time scale integration = 1 4f = N B . Let us see the mathematical operation of IFFT xn = 1 N N−1 X k=0 X(k)ej2π kn N From the mathematics of the IFFT, nth sub-carrier for integer value of k = 0, 1, …N − 1, all are orthogonal among each other. 23. What do you mean by channel correlation and its drawback on communication system ? Ans Channel correlation: Correlation is a statistical measure between the similarity between two random variable. Let h(t) is the time-varying channel and h(t − τ) is the delayed version of channel. The auto-correlation can be expressed as Rh(τ) = lim T→∞ 1 T Z T/2 −T/2 h(t)h(t + τ)dt If a channel is highly correlated then it is supposed to less time varying. In another words, if auto-correlation has Gaussian PDF then larger variance shows the high correlation. It can be easily observed from above figure. Impact on communication system: (a) A mobile user experience correlated wireless channel and if channel gain is poor then the mobile user experience poor SNR for a very time. 9
12. (b) In a MIMO wireless network, if all links are highly correlated and at any instant the channel gain is very poor or in deep fade then the diversity gain can’t be reach to its theoretical expectation. (c) Channel correlation also helps for estimating the coherence time, that provides a time resource to the wireless network for doing the channel estimation, uplink/downlink data transmission and other signal processing activity. 24. Difference between random variable and random process. Ans (a) Random variable: A fundamental quantity, whose outcome is not certain called as random variable. Ex- temperature, pressure, attenuation, velocity, etc. Note: For converting a random quantity into its deterministic equivalent, we need prob- ability density function (PDF), mean, variance and higher order moment. (b) Random process: Time dependent random variable is called as random process. Ex- i Variation of temperature in day, week or year is an example of random process. ii Wireless channel coefficient, etc As per the types of random process, it has been classified into several categories. – Stationary process (strictly stationary) – Wide-sense stationary (WSS) – Cyclo-stationary – Non-stationary 25. What are the differences among stationary, non-stationary, wide sense stationary and cyclo-stationary processes ? Ans Fundamental differences are as follow (a) Strictly stationary process: A random process, whose outcome remain same for all time or a time invariant. In other words, variance of this time dependent random variable is equal to zero. In real world, strictly stationary process is not found. (b) Wide-sense stationary process: It is also a stationary process but not in strict sense. Time dependent random variable is classified in terms of mean and auto-correlation. 10
13. Higher order moment is not required for analyzing this process. Ex- Wireless channel coefficient. (c) Cyclo-stationary process: It is also a weak stationary process. A cyclostationary process is a signal having statistical properties vary cyclically with time. Ex- 21 July is considered to be hot and 20 December to be cold day of the year. (d) Non-stationary process: A random process whose outcome is un-certain is called as non-stationary process. For hardware making, no hardware can be made for non- stationary process. 26. What is the significance of linear phase ? Ans Linear phase assure the distortionless wireless communication. Let y and x are output and input respectively. If y = kx(t − τ) Now taking the Fourier transform Y (jΩ) = kX(jΩ)e−jΩτ = kX(jΩ)e−θ(f) Here, θ(f) = 2πfτ → Linear phase. Note: It means constant gain/attenuation and fixed delay in input signal has linear phase and it is main condition for distortion free communication. 27. What do you mean by linear and non-linear distortion ? What are the cause for arising these type of distortion ? Ans For general consideration, the channel is supposed to be linear time invariant. Signal dis- tortion can happen either due to magnitude distortion or phase distortion. This type of distortion is called as linear distortion. Linear distortion causes time domain spreading of a signal Let a memoryless non linear channel where input g and output y are related by some non-linear equation y = f(g) and y(t) = a0 + a1g(t) + a2g2 (t) + a3g3 (t) + ….+ If we take the spectrum of the above signal then output signal has larger bandwidth compare to input one. Non-linear distortion causes spectral broadening. 28. What do you mean by vector space and signal space ? Ans Signals that are defined for only a finite number of time instant (say N) can be written as vectors (of dimension N). Consider g(t) is defined over a time interval [a, b]. Let we pick N point uniformly on the time interval [a, b] such that t1 = a, t2 = a + , ….., tN = a + (N − 1) = b = b − a N − 1 11
14. Then we can write a signal vector g as an N− dimensional vector. g = [g(t1), g(t2), …., g(tN )] 29. What is the significance of negative frequency ? Ans Negative frequency does not describes the rate of variation of sine or cosine wave. It describes the direction of rotation of a unit length exponential sinusoid and its rate of revolution. 30. What do you mean by group delay and phase delay ? Ans The output envelope is the same as the input envelope delayed by tg = − 1 2π dθh(f) df called as group delay. If output carrier is the same as the input carrier delayed by tp = − θh(f) 2πf is called as phase delay. 31. What is uniform and non-uniform quantization ? Ans Quantization is process, where amplitude of signal is divided into several chunks. If this division is done in a uniform fashion then it is called as uniform quantization otherwise it is a non-uniform quantization. 32. What do you mean by source coding, channel coding, and line coding ? Ans Fundamental differences between all coding schemes are as follows (a) Source coding: It is process to reduce the redundancy so that bandwidth requirement became less in communication system. (b) Channel coding: It is process by which we add some redundancy with the information bits so that the error detection and correction could be easily possible. Due to redundant bit addition the BER reduces with greater extent in the communication system. (c) Line coding: It is process by which information bits are converted into electrical pulses that is used for following purposes. – Bandwidth efficiency – Power efficiency – Reducing the BER – Maintaining the transparency for long bit stream. Most popular line coding are like, RZ, NRZ, polar, manchester, etc. 33. What is the difference between regenerative repeaters and amplifier ? Ans The basic difference between regenerative repeaters and amplifier are as follow. 12
15. (a) Repeater: It is a device that correct and reshape the incoming signal. (b) Amplifier: It only amplify the amplitude level of incoming signal. 34. What is the Nyquist’s criterion for zero ISI ? Ans For zero ISI, the pulse shape is chosen in such a way that has non-zero amplitude at its center (say t=0) and zero amplitude at t = ±nTb (n = 1, 2, …), where Tb is the separation between successive transmitted pulses thus p(t) = 1 t = 0 = 0 t = ±nTb Tb = 1 Rb Now transmission of Rb bits/s requires a theoretical minimum bandwidth Rb/2 Hz. It would be nice if a pulse satisfying Nyquist’s criterion had this minimum bandwidth Rb/2 Hz. 35. What is the difference between independent, un-correlated, and orthogonal ? Ans The basic difference between independent, uncorrelated and orthogonal are as follows (a) Independent: If X and Y are two random variables such that the conditional den- sity is equal to the marginal density then these two random variable will be called as independent. Mathematically it can be expressed as P(Y/X) = P(Y ) → Independent, P(Y/X) 6= P(Y ) Dependent (b) Un-correlated: If X and Y are two random variables said to be uncorrelated, when cross-covariance is equal to zero. cov(X, Y ) = E[X, Y ] − E[X]E[Y ] = 0 (c) Orthogonal: If X and Y are the random variables of two disjoint sets then it is said to be orthogonal. Mathematically, it can be expressed as E[X, Y ] = 0 36. What is central limit theorem ? Ans Let X1, X2, ….., XN are the independent and identically distributed random variable. If all random variables having same mean and variance then the sum of all random variables converges to a Gaussian distributed random variable for N → ∞, Z = X1 + X2 + ….. + XN N , then Z ∼ N(µz, σ2 z ) 37. What are the important properties of Gaussian random process ? Ans Important properties of Gaussian random process are as follows 13
16. (i) Addition of two Gaussian distributed rv is also a Gaussian. Let X1 ∼ N(µX1 , ΣX1X1 ) and X2 ∼ N(µX2 , ΣX2X2 ) are two Gaussian distributed rv. Z = X1 + X2 ∼ N(µX1 + µX2 , ΣX1X1 ΣX2X2 ) (ii) Normalization is also a Gaussian. Z = Z y p(y, µ, Σ)dy = 1 → Gaussian distribution (iii) Marginalization is also a Gaussian distribution. p(X1) = Z ∞ 0 p(X1, X2, µ, Σ)dX2 → Gaussian distribution (iv) Conditioning: The conditional distribution of X1 on X2 p(X1/X2) = p(X1, X2, µ, Σ) R X1 p(X1, X2, µ, Σ)dX1 → Gaussian distribution X1/X2 ∼ N(µX1 + ΣX1X2 Σ−1 X2X2 (X2 − µX2 ), ΣX1X1 − ΣX1X2 Σ−1 X2X2 ΣX2X1 )) 38. What do you mean by colored channel noise ? Ans If the noise power spectral density is variable across different frequency components that causes frequency distortion in the communication system. Such type of noise is called as colored noise. 39. Difference between multiplexing and multiple access ? Ans The basic differences are as follows (i) Multiplexing: It is a method by which we combine multiple signal and it is transmitted over a single channel. Ex-TDM and FDM (ii) Multiple access: In multiple access method, wireless resources are shared among the multiple users. Ex-TDMA, FDMA, CDMA→ Wireless resources are time, frequency and code. 40. What is near-far problem in CDMA ? Ans Let the PN sequence length in CDMA system is N = 256. Let two users are U0 and U1, which are d0 and d1 distance far from the base station. Let d0 = √ Nd1, it means user U1 is nearer to BS. In such a scenario, nearer user produces larger interference to the farther user SINR. The resultant SINR can be expressed as SINR = P|h|2 d2 0 P|h|2 Nd2 1 + σ2 N = P|h|2 Nd2 1 P|h|2 Nd2 1 + σ2 N 41. What do you mean by a memory-less channel and channel having memory ? 14
17. Ans The basic difference is as follow (a) Memory-less channel: If channel is assumed to be static or output current symbol depends on the instantaneous channel coefficient not on the past coefficients is called as memory-less channel. (b) Channel having memory: If channel is assumed to be dynamic or output current symbol depends on the instantaneous channel coefficient as well as the past coefficients is called as channel with memory. 42. What is channel hardening ? Ans One of the main impairments in wireless communications is small-scale channel fading that are caused by microscopic changes in the propagation environments. The fluctuations make the channel unreliable. The diversity is a key to combating small-scale fading Suppose the probability of a bad channel gain realization is p. If we have M antennas with independent channel gains, then the risk that all of them are bad is pM . For example, with p=0.1, there is a 10% risk of getting a bad channel in a single-antenna system and a 0.000001% risk in an 8-antenna system. This shows that just a few antennas can be sufficient to greatly improve reliability. Channel hardening means that a fading channel behaves as if it was a non-fading channel. The randomness is still there but its impact on the communication is negligible. 43. Explain channel capacity in terms of mutual information. Ans Let X and Y are random variable across input and output side. Mathematical expression for mutual information can be expressed as I(X, Y ) = H(X) − H(X/Y ) = H(Y ) − H(Y/X) (3) H(X), H(Y ) and H(X/Y ), H(Y/X) are the marginal and conditional entropy. Mathe- matical expression for entropy is H(X) = m X i=1 p(xi) log2 1 p(xi) (4) We have I(X, Y ) = m X i=1 p(xi) log2 1 p(xi) − m X i=1 n X j=1 p(xi, yj) log2 1 p(xi/yj) (5) Also n X j=1 p(xi, yj) = p(xi) (6) 15
18. Hence, from (5) I(X, Y ) = m X i=1 n X j=1 p(xi, yj) log2 1 p(xi) − m X i=1 n X j=1 p(xi, yj) log2 1 p(xi/yj) = m X i=1 n X j=1 p(xi, yj) log2 p(xi/yj) p(xi) = m X i=1 n X j=1 p(xi, yj) log2 p(xi, yj) p(xi)p(yj) = m X i=1 n X j=1 p(xi, yj) log2 p(yj/xi) p(yj) = m X i=1 n X j=1 p(xi, yj) log2 p(yj/xi) Pm i=1 p(xi, yj) = m X i=1 n X j=1 p(xi, yj) log2 p(yj/xi) Pm i=1 p(xi)p(yj/xi) (7) I(X, Y ) is the average amount of information received per symbol transmitted.From (7), it is clear that I(X, Y ) is the function of transmitted symbol probability p(xi) and the channel matrix. The maximum channel capacity can be expressed as Cs = max{I(X, Y )} (8) 44. What is the difference between MLE and MAP ? Ans Let y = ax + n, where n ∼ N(0, σ2 ) x̂MLE(y) = arg max x fY (y/x) = 1 √ 2πσ e− (y−ax)2 2σ2 Measure y = ȳ = ax̂MLE Note: There is no requirement of the distribution of x. 1 Maximum apriori xapriori = arg max x fX(x) 2 Maximum aposteriori probability (MAP) x̂MAP = arg max x fX(x/y) = fY (y/x)fX(x) fY (y) = fY (y/x)fX(x) R X fY (y/x)fX(x)dx ⇒ If xapriori is uniformly distributed then x̂MLE = x̂MAP 16
19. 45. What is full duplex communication ? Ans There are five wireless resources for communication system, which are as follows (i) Power (ii) Time (iii) Frequency (iv) Space (v) Code In earlier technology, resource allocation for bi-directional communication is done in the share basis. In this practice, one resource is always in a scarce mode. Ex- TDMA (time resource is shared among all user). In full duplex communication, all resources are allocated to all wireless devices or no resource sharing is done among other wireless devices. In such case, there will be a chance of huge interference. 46. What is 5G-NR ? Ans In earlier communication standard, sub 6-GHz band was being utilized for the communication perspective. Due to explosive growth in ICT industry, nearly 2GHz available bandwidth is sufficient for fulfilling the demand. Therefore ICT researcher proposed the new radio band for the communication perspective that was beyond the sub-6GHz band. mm-Wave is more suitable for fixed wireless communication. This new band for communication is called as 5G-NR. 47. If transmitter and receiver units are in static mode then can diversity be achieved with greater extent ? Ans If transmitter and receiver are in static mode then the effect of multipath can also be observed. This multi-path effect may increase or decrease the effective SNR. 48. What is the major role of L2 layer in data communication ? Ans This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer. 49. What is the application of network layer ? Ans The network layer provides the means of transferring variable-length network packets from a source to a destination host via one or more networks. Within the service layering semantics of the OSI network architecture, the network layer responds to service requests from the transport layer and issues service requests to the data link layer. 17
20. 50. What is URLLC in 5G ? Ans It is a one of the major specification of 5G communication. URLLC stands for ultra reliable low latency communication. Latency is a major performance degradation factor in the wire- less communication. It limits the overall data rate or system capacity. In 5G communication, link reliability and low latency is one of the application area. For overcoming this prob- lem, network densification and suitable interference cancellation techniques are the possible solution. 18
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FAQ
What is the principle of OFDM?
Working Principle of OFDM The sub-streams into which the main signal is divided must be orthogonal to one another for OFDM to function. Orthogonal signals are signals that are perpendicular to each other. The fact that orthogonal signals do not interfere with one another is a key characteristic of them.
What problem does OFDM solve?
The main benefit of OFDM over single-carrier schemes is its ability to handle difficult channel conditions without the use of complicated equalization filters, such as attenuation of high frequencies in a long copper wire, narrowband interference, and frequency-selective fading due to multipath.
Which modulation is used in OFDM?
In order to achieve high data rates and spectral efficiency, another popular modulation technique is orthogonal frequency division multiplexing (OFDM). Due to the use of multiple carriers rather than just one, it is known as a multicarrier modulation method.
What is OFDM and its purpose?
Orthogonal Frequency Division Multiplexing, or OFDM, is a type of signal modulation that spreads out a high data rate modulating stream onto numerous narrowband close-spaced subcarriers that are slowly modulated, making it less susceptible to frequency selective fading.
