Download Wireless Communication Challenges and Techniques and more Schemes and Mind Maps Wireless Communication Systems in PDF only on Docsity!
EC2401 WIRELESS COMMUNICATION
16 MARK IMP QUESTION WITH KEY POINTS
PART-B
UNIT-I
- Explain in detail the different types of services in wireless communication? ( Marks) Ans. Explanation of the following services with diagram Types of Services 1. Broadcast 2. Paging 3. Cellular Telephony 4. Trunking Radio 5. Cordless Telephony 6. Wireless Local Area Networks 7. Personal Area Networks 8. Fixed Wireless Access 9. Ad hoc Networks and Sensor Networks 10. Satellite Cellular Communications
- Discuss briefly about the requirements of services for a wireless system. (8 marks) Ans. Explanation of following requirement in detail
- Data Rate
- Range and Number of Users
- Mobility
- Energy Consumption
- Use of Spectrum
- Direction of Transmission
- Service Quality
- Compare and contrast wired and wireless communication (8 marks) Ans-
Wired communications Wireless communications The communication takes place over a more or less stable medium like copper wires or optical fibers. The properties of the medium are well defined and time-invariant.
Due to user mobility as well as multipath propagation, the transmission medium varies strongly with time.
Increasing the transmission capacity can be achieved by using a different frequency on an existing cable,and/or by stringing new cables.
Increasing the transmit capacity must be achieved bymore sophisticated transceiver concepts and smaller cell sizes (in cellular systems), as the amount of available spectrum is limited. The range over which communications can be performed without repeater stations is mostly
The range that can be covered is limited both by the transmission medium
limited by attenuation by the medium (and thus noise); for optical fibers, the distortion of transmitted pulses can also limit the speed of data transmission.
(attenuation, fading, and signal distortion) and by the requirements of spectral efficiency (cell size).
The delay in the transmission process is also constant, determined by the length of the cable and the group delay of possible repeater amplifiers.
The delay of the transmission depends partly on the distance between base station and Mobile Station (MS), and is thus time- variant. Interference and crosstalk from other users either do not happen or the properties of the interference are stationary.
Interference and crosstalk from other users are inherent in the principle of cellular communications. Due to the mobility of the users, they also are time-variant The Bit Error Rate (BER) decreases strongly (approximately exponentially) with increasing Signal-to-Noise Ratio (SNR). This means that a relatively small increase in transmit power can greatly decrease the error rate.
For simple systems, the average BER decreases only slowly (linearly) with increasing average SNR. Increasing the transmit power usually does not lead to a significant reduction in BER. However, more sophisticated signal processing helps. Due to the well-behaved transmission medium, the quality of wired transmission is generally high.
Due to the difficult medium, transmission quality is generally low unless special measures are used. Jamming and interception of dedicated links with wired transmission is almost impossible without consent by the network operator.
Jamming a wireless link is straightforward, unless special measures are taken. Interception of the on-air signal is possible. Encryption is therefore necessary to prevent unauthorized use of the information. Establishing a link is location based. In other words, a link is established from one outlet to another, independent of which person is connected to the outlet.
Establishing a connection is based on the (mobile) equipment, usually associated with a specific person. The connection is not associated with a fixed location. Power is either provided through the communications network itself (e.g., for traditional landline telephones), or from traditional power mains (e.g., fax). In neither case is energy consumption a major concern for the designer of the device.
MSs use rechargeable or one-way batteries. Energy efficiency is thus a major concern.
- Describe the technical challenges to wireless communication Ans- Explain each of the following in details multipath propagation: i.e., the fact that a transmit signal can reach the receiver via different paths (e.g., reflections from different houses or mountains); spectrum limitations; energy limitations; user mobility.
- Explain Inter symbol Interference is caused and how it is eliminated
Ans-
- Adaptive modulation and coding : employs the knowledge at the TX of the transmission channel, and chooses the modulation format and coding rate that are “just right” for the current link situation. This approach makes better use of available power, and, among other effects, reduces interference
- Reduction of cell radius : this is an effective, but very expensive, way of increasing capacity, as a new BS has to be built for each additional cell. For FDMA systems, it also means that the frequency planning for a large area has to be redone.8 Furthermore, smaller cells also require more handovers for moving users, which is complicated, and reduces spectral efficiency due to the large amount of signaling information that has to be sent during a handover.
- Use of sector cells : a hexagonal (or similarly shaped) cell can be divided into several (typically three) sectors. Each sector is served by one sector antenna. Thus, the number of cells has tripled, as has the number of BS antennas. However, the number of BS locations has remained the same, because the three antennas are at the same
- Use of an overlay structure : an overlay structure combines cells with different size and different traffic density. Therefore, some locations may be served by several BSs simultaneously.
- Multiple antennas : these can be used to enhance capacity via different scenarios: (a) diversity increases the quality of the received signal, which can be exploited to increase capacity – e.g., by use of higher order modulation formats, or reduction of the reuse distance; (b) multiple-input multiple-output systems increase the capacity of each link; (c) space division multiple access allows several users in the same frequency channel in the same cell to be served.
- Fractional loading : this system uses a small reuse distance, but uses only a small percentage of the available timeslots in each cell. This leads to approximately the same average capacity as the “conventional” scheme with large reuse distance and full loading of each cell.
- Partial frequency reuse : in this scheme, the available spectrum is divided into N + 1 subbands. One subband is used in all the cell centers, while the other subbands are used at the cell edges, employing a conventional frequency reuse (with cluster size N ). The “cell edges” must be large enough so that interference from one cell center to another is sufficiently weak.
- With neat diagrams explain TDMA & FDMA Ans- Frequency Division Multiple Access (FDMA), where different frequencies are assigned to different users. Explain the concept using diagram The advantages are The transmitter (TX) and receiver (RX) require little digital signal processing ( Temporal ) synchronization is simple. the disadvantages are Frequency synchronization and stability are difficult Sensitivity to fading : Sensitivity to random Frequency Modulation (FM) : Intermodulation Time Division Multiple Access (TDMA), where different timeslots are assigned to different users. Explain the concept using diagram and explain its ad, disadvantages. o Users occupy a larger bandwidth o Temporal guard intervals are required Each timeslot might require a new synchronization and channel estimation, as transmission is o not continuous
o For interference-limited systems
- Explain about noise and interference limited system
Ans a. Noise limited system When MS moves further away from the BS, the received signal power decreases, and at a certain distance, the SNR does not achieve the required threshold for reliable communications. Therefore, the range of the system is noise limited; equivalently, we can call it signal power limited. Depending on the interpretation, it is too much noise or too little signal power that leads to bad link quality. Explain the following types of noise Thermal noise Man made noise Receiver noise And link budget b. Interference-Limited Systems The interference is so strong that it completely dominates the performance, so that the noise can be neglected.
- Write short notes channel assignment. (8 Marks)
Ans- Fixed channel assignment Dynamic channel assignment
- Explain the different hand off strategies used in wireless communication (8 Marks) Ans- Concept of Handoff Different types of handoff
UNIT-II
- With neat diagrams explain the Free Space Propagation Model? (8 marks) Derivation of the Friis’ law :
- Derive the equation of the Path loss using Two-Ray Model with neat diagrams Derivation of the following Equation, the power received at a distance d from a transmitter is given by,
For derivation refer ( Rappaport Book)
Scattering
- Explain the wideband channel models
- Frequency selective model
- Time selective models
- Gaussian models
- WSSUS model
- Non stationary channels
- Explain different types of wide band and narrow band models
For a narrowband channel, the impulse response is a delta function with a time-varying attenuation, so that for slowly time-varying channels: h(t, τ ) = α(t)δ(τ )
UNIT-III
- Explain in detail the generation & detection of MSK technique? (16 Marks)
Principle of MSK MSK transmitter block diagram &explanation MSK receiver block diagram &explanation
- Explain in detail the generation & detection of QPSK technique? (16 Marks)
Quadrature-Phase Shift Keying A Quadrature-Phase Shift Keying (QPSK)-modulated signal is a PAM where the signal carries bit per symbol interval on both the in-phase and quadrature-phase component. When interpreting QPSK as a PAM , the band pass signal reads S BP (t ) = E B /T B[ p 1D (t ) cos ( 2 πf c t) − p 2D (t ) sin ( 2 πf c t) ] Explain the following for different types of QPSK and its comparison Circuit diagram Waveform Constellation diagram
- Explain in detail the generation & detection of GMSK modulation? (16 Marks)
Principle of GMSK GMSK transmitter block diagram &explanation GMSK receiver block diagram &explanation
- Explain the performance of digital modulation in slow flat fading channel.(16 Marks)
Explanation & comparison of digital modulation Performance results for ASK, FSK,QPSK,MSK
- Explain error probability of coherent receivers
The modulation formats can be any of the form and explain error probability for the following signal formats
- Binary Phase Shift Keying (BPSK) signals are antipodal signals.
- Binary Frequency Shift Keying (BFSK), and Binary Pulse Position Modulation (BPPM), areorthogonal signals.
- Quadrature-Phase Shift Keying (QPSK), π/ 4-DQPSK (Differential Quadrature-Phase Shift Keying), and Offset Quadrature-Phase Shift Keying (OQPSK) are bi-orthogonal signals
6. Explain Error Probability in Flat-Fading Channels
- Explain Error Probability in Delay- and Frequency-Dispersive Fading Channels
UNIT-IV
- Explain the different methods of microdiversity in detail ((16 Marks)
Micro diversity can be used to combat small-scale fading, which are therefore called “microdiversity.” The five most common methods are as follows:
- Spatial diversity : several antenna elements separated in space.
- Temporal diversity : transmission of the transmit signal at different times.
- Frequency diversity : transmission of the signal on different frequencies.
- Angular diversity : multiple antennas (with or without spatial separation) with different antenna patterns.
- Polarization diversity : multiple antennas with different polarizations (e.g., vertical and horizontal).
Explain each types of diversity using diagrams (refer book Molisch)
2 .Explain several signal combining techniques in detail ((16 Marks)
Combining Signals - How to use diversity signals in a way that improves the total quality of the signal that is to be detected. In general, we can distinguish two ways of exploiting signals from the multiple diversity branches:
- Selection diversity , where the “best” signal copy is selected and processed (demodulated and decoded), while all other copies are discarded. There are different criteria for what constitutes the “best” signal.
- Combining diversity , where all copies of the signal are combined (before or after the demodulator), and the combined signal is decoded. Again, there are different algorithms for combination of the signals. Combining diversity leads to better performance, as all available information is exploited. On the downside, it requires a more complex RX than selection diversity. 1. Selection Diversity Received-Signal-Strength-Indication-Driven Diversity Bit-Error-Rate-Driven Diversity _Switched Diversity
- Combining Diversity Principle_ Selection diversity wastes signal energy by discarding (N r − 1 ) copies of the received
signal. This drawback is avoided by combining diversity, which exploits all available signal
copies. Each signal copy is multiplied by a (complex) weight and then added up.
Maximum Ratio Combining Equal Gain Combining Optimum Combining Hybrid Selection – Maximum Ratio Combining
- Explain the performance of diversity reception in Fading Channels a. Error Probability in Flat-Fading Channels
- Explain in detail the CDMA multiple access technique. (16 Marks) CDMA working principle Advantages & comparison with other techniques.
- Explain the principles of Frequency Hopping spread spectrum technique and direct sequence spread spectrum technique. (16) The principles of Frequency Hopping spread spectrum technique The basic thought underlying FH is to change the carrier frequency of a narrowband transmission system so that transmission is done in one frequency band only for a short while. The ratio between the bandwidth over which the carrier frequency is hopped and the narrowband transmission bandwidth is the spreading factor. Basic Principle behind the Direct Sequence-Spread Spectrum The DS-SS spreads the signal by multiplying the transmit signal by a second signal that has a very large bandwidth. The bandwidth of this total signal is approximately the same as the bandwidth of the wideband spreading signal. The ratio of the bandwidth of the new signal to that of the original signal is again known as the spreading factor. As the bandwidth of the spread signal is large, and the transmit power stays constant, the power-spectral density of the transmitted signal is very small – depending on the spreading factor and the BS–MS distance, it can lie below the noise power-spectral density. This is important in military applications, because unauthorized listeners cannot determine whether a signal is being transmitted. Authorized listeners, on the other hand, can invert the spreading operation and thus recover the narrowband signal (whose power-spectral density lies considerably above the noise power).
- Explain the principles of Code Division Multiple Access and compared with TDMA Each user is assigned a different spreading code, which determines the wideband signal that is
multiplied by the information symbols. Thus, many users can transmit simultaneously in a
wide band
- Explain about Cellular Code-Division-Multiple-Access Systems and power control Multiple Access In a TDMA/FDMA system, the answer is the limited number of available timeslots/frequencies. Users can occupy those slots, and not interfere with each other. But when all possible timeslots have been assigned to users, there are no longer free resources available, and no further users can be accepted into the system. In a CDMA system, this mechanism is subtly different. Different users are distinguished by different spreading codes; however, as user separation is not perfect, each user in the cell contributes interference to all other users. Thus, as the number of users increases, the interference for each user increases as well. Consequently, transmission quality decreases gradually ( graceful degradation ), until users find the quality too bad to place (or continue) calls. Consequently, CDMA puts a soft limit on the number of users, not a hard limit like TDMA. Therefore, the number of users in a system depends critically on the Signal-to- Interference-and-Noise Ratio (SINR) required by the receiver.
It also implies that any increase in SINR at the receiver, or reduction in the required SINR, can be immediately translated into higher capacity.
Power control Distinguish between power control for the uplink and that for the downlink:
- Power control in the uplink : for the uplink, power control is vital for the proper operation of
CDMA. Power control is done by a closed control loop: the MS first sends with a certain power, the BS then tells the MS whether the power was too high or too low, and the MS adjusts its power accordingly. An open control loop (where the MS adjusts its transmit power based on its own channel estimate) cannot be used to compensate for small-scale fading in a Frequency Domain However, an open loop can be used in conjunction with a closed loop. The open loop compensates for large-scale variations in the channel (path loss and shadowing), which are approximately the same at uplink and downlink frequencies. The closed loop is then used to compensate for small-scale variations.
- Power control in the downlink : for the downlink, power control is not necessary for CDMA
to function: all signals from the BS arrive at one MS with the same power (the channel is the same for all signals). However, it can be advantageous to still use power control in order to keep the total transmit power low. Decreasing the transmit power for all users within a cell by the same amount leaves unchanged the ratio of desired signal power to intra-cell interference – i.e., interference from signals destined for other users in the cell.
- Effects of Multipath Propagation on Code Division Block diagram of RAKE receiver Working principle
- Explain the operations of orthogonal frequency division multiplexing and define and list the benefits of cyclic prefix cyclic prefix. (16 Marks)
Orthogonal Frequency Division Multiplexing (OFDM) is a modulation scheme that is especially suited for high-data-rate transmission in delay-dispersive environments. It converts a high-rate data stream into a number of low-rate streams that are transmitted over parallel, narrowband channels that can be easily equalized. Principle of Orthogonal Frequency Division Multiplexing OFDM splits a high-rate data stream into N parallel streams, which are then transmitted by modulating N distinct carriers (henceforth called subcarriers or tones ). Symbol duration on each subcarrier thus becomes larger by a factor of N. In order for the receiver to be able to separate signals carried by different subcarriers, they have to be orthogonal. Implementation of Transceivers OFDM can be interpreted in two ways: one is an “analog” –Explain using diagram Splitting original data stream into N parallel data streams, each of which has a lower data rate. We furthermore have a number of local oscillators (LOs) available, each of which oscillates at a frequency fn = nW/N , where n = 0 , 1 ,.. .,N − 1. Each of the parallel data streams then modulates one of the carriers.
IS-95 specifies two possible speech coder rates: 13.3 or 8.6 kbit/s. In both cases, coding increases the data rate to 28.8 kbit/s. The signal is then spread by a factor of 64, resulting in a chip rate of 1.2288 Mchip/s. theoretically; each cell can sustain 64 speech users. In practice, this numberis reduced to 12–18, due to imperfect power control, non orthogonality of spreading codes, etc. The downlink signals generated by one Base Station (BS) for different users are spread by different Walsh–Hadamard sequences (see Section 18.2.6), and thus orthogonal to each other. This puts an upper limit of 64 channels on each carrier. In the uplink, different users are separated by spreading codes that are not strictly orthogonal. Furthermore, interference from other cells reduces signal quality at the BS and Mobile Station (MS). Explain Spreading and Modulatio in the uplink and downlink
- Discuss about 3G standards – WCDMA/UMTS for wireless network.
Explain the following
Physical-Layer Overview Network Structure Data Rates and Service Classes Air Interface
