The Bhagvad Gita Thus, designing an UWB communication system requires the understanding of how excess bandwidth and very low transmitted powers can be used jointly to provide a reliable radio link. UWB offers systems transceiver potential for very simple implementations.
Comparison between UWB and traditional narrow-band systems highlights the following features: Large bandwidth enables very fine time-space resolution for accurate lo- tion of the UWB nodes and for distributing network time stamps. Very short pulses are effectively counter-fighting the channel effect in very dense multipath environments. Data rate number of pulses transmitted per bit can be traded with power emission control and distance coverage.
Very low power density leads to low probability of signal detection and adds security for all the layers of the communication stack. Very low power density is obtained through radio regulation emission masks; UWB systems are suitable for coexistence with already deployed narrow-band systems.
Smolik,Joseph E. The deployment of wireless communications over the last decade has been phenomenal. With over 28, new cellular subscribers a day, the public's desire Personal Communications Systems is keeping this frenzy alive.
Author : Ke-Lin Du,M. Uniquely, a detailed introduction to the properties, design, and selection of RF subsystems and antennas is provided, giving readers a clear overview of the whole wireless system.
While the plain signal is fully masked by the jammer and is not clearly seen at the filter output, the spread spectrum one, time-compressed by the matched filter, is observable distinctively. In Figure 3. The plots of the second row give the power spectra of two random realizations of different barrage jammers, the same fixed average jammer power being distributed over the signal bandwidth. Because of this the average level of the spectrum in column b is about 50 times lower than that in column a.
The third row shows example observed waveforms, where the intensity of the jammer is approximately the same for both signals, which are well hidden under the jammer. As for the lower plots, they again confirm explicitly the superiority of a spread spectrum signal in resistance to a barrage jammer.
In closing, note once again that this section is in no way aimed to answer questions on what sort of jammer is most dangerous in a concrete scenario and what the system should 1 Signal Signal 1 0 —1 0. The idea was just to demonstrate the principal advantages of spread spectrum in countering a jammer. An interested reader may consult numerous specific works on the issue and confirm that whatever sophisticated systems and strategies are investigated, the general tendency is always the same: spread spectrum raises the jamming immunity potential.
The other reason we will discuss in this section. In the confrontation of electronic systems effective jamming may be organized only after detecting the presence of an adversary system on the air and estimation of its parameters, such as carrier frequency and bandwidth. This entails a very popular scenario of the confrontation of two systems, when the first call it intended tries to operate as covertly as possible and escape unintended detection of its signal, while the 83 Merits of spread spectrum second interceptor or eavesdropper is on the alert constantly, doing all in its power to discover an active state of the first.
From the perspective of the intended system, let us explore how the spread spectrum can help in its conflict with an interceptor. They may be rather sophisticated and hard to analyse see [6,9] and their bibliographies. We are again pursuing the goal of getting the general idea of why a spread spectrum appears to be a good option in this case. Let us assume that the intended system uses a signal with some non-trivial modulation law, details of which are not known to the interceptor, depriving the latter of the chance to use a matched filter or a correlator for signal detection.
It is natural to believe, then, that the eavesdropper has no other choice but to treat the intercepted signal as random and base its detection on just the presence or absence of some extra energy in the suspicious frequency band.
Thus, an energy detector, also called a radiometer, which is optimal for detecting a band-limited noise signal against the AWGN background, is accepted as the operational instrument at the intercepting side.
A bandpass filter, whose bandwidth Wi spans the whole signal spectrum or only part of it, filters the observation to remove any off-band noise. In practice, an interceptor may not know beforehand the frequency zone and time interval occupied by a signal. In such circumstances he tries various combinations of these parameters, implementing the whole procedure with the aid of either scanning the time—frequency area or a bank of parallel channels, each analysing its specific time—frequency zone.
In any case the performance of the interceptor receiver will depend radically on the performance of the energy detector tuned to the true signal time— frequency zone. First of all, an output voltage ud of the square-law detector equals the input instant power.
Strictly speaking, with the signal advent the filtered observation may differ from a Gaussian process, making applicability of the result just obtained doubtful. This detail, however, has no importance to the case in question, since the intended system does its best to hide its signal under the thermal noise and we have every reason to assume that a signal has a negligible effect on the variance of instant power and thus on the variance of the detector response.
To make a constant component at the detector output distinctive enough against the random fluctuations the latter should be smoothed as a result of integration.
This is possible only if fluctuations of the detector response around ud are sufficiently fast and change their polarity many times during the period T to compensate each other and produce an averaging effect. In other words, the number of statistically independent samples ns of the detector response within T should be large enough. Although practically integration may be implemented as continuous, its result is rather close to that of just summation of ns independent samples [6,9], which is even a more practicable technique, especially in digital circuitry.
These PDFs are subject to the chi-square law, which is a bit bulky and not quite transparent enough for a physical treatment. However, we may again exploit the fact that a signal is weak and its reliable detection requires a large number of integrated samples ns. Making use of 3. Certainly, q2 should be maintained large enough otherwise the intended system will not be able to do its main job.
It is quite clear, then, that the intended system has the only way to prevent detection of its signal by a potential interceptor: use a spread spectrum signal with as large a processing gain WT as possible. Coming back to Figure 3. Widening the spectrum of the signal of a constant energy 87 Merits of spread spectrum and duration reduces the signal power spectrum density, hiding it under the background spectrum of the natural thermal noise.
Example 3. Consider the system transmitting sporadically and rather infrequently one of 64 messages using orthogonal signals. To provide error probability no worse than 10 3 it needs to use SNR around 7 dB per one bit, or 15 dB per 6-bit message see Figure 2. SNR 3. Finishing this section, note that the discussed advantage of spread spectrum is widely utilized today not only by the military or special services.
The fact that a spread spectrum signal may be practically unnoticeable for the equipment that monitors the state of the radio air has serious implications for licensing policy. In particular, the range of commercial systems that may actively operate on the air without applying for a licence becomes broader, and in some regions special spectral zones are currently allocated for such licence-free use.
As a result, the interceptor cannot process the signal in the manner used by the intended receiver matched filtering. Of course, if the signal structure is not complicated enough and the interceptor is aware that it was chosen from only a few alternatives he may try them all. Appropriate equipment for doing so may be a bank of parallel matched filters or a single filter several filters reconfigured to fit the candidate signal structures serially in time, if the signal is known to be received for an adequate duration.
Therefore, another aspect of the strategy of the intended system in its conflict with an interceptor consists in making a signal structure practically unbreakable. A similar task is characteristic of military or commercial systems that do not tend to make the fact of their operation a mystery, e. The satellite-based global navigation system GPS is a convincing example of this kind.
It has two positioning channels see Section The signal transmitted over the second channel allows super-high precision of positioning, and the US government, which runs the system, does not permit unconditional access 88 Spread Spectrum and CDMA to this channel.
In order to protect it from unauthorized use some special measures are undertaken concerning the signal modulation. In disciplines dealing with information security, the extent of data protection is measured by a number of competitive equiprobable keys, which an enemy cryptanalyst eavesdropper should try to crack the ciphertext, i.
In application to the signal structure, each of those keys is just a modulation law, which is typically repeated with some period T. Suppose that a signal is built of chips see the example in Section 2. If the bandwidth allocated to the system is W then the total signal space has dimension measured as WT ignoring bandpass doubling; see Sections 2.
It is clear, then, that M WT is the total number of possible modulation laws, i. Being hidden under the thermal noise, this signal cannot be retrieved by symbol-wise reception and only knowledge of its fine structure permits it to be cleared with highest efficiency off the AWGN.
To prevent an unauthorized interceptor from accessing the P-code, a secret binary key W-code is modulo 2 added to it, masking the structure of the resulting Y-code. For this reason the Y-code is believed to be unbreakable and no reports have emerged in nearly 10 years of its history on any successful cryptanalytical attack on it. We conclude the section with another declaration on the advantages of spread spectrum: this technology is very conducive to cryptographic protection of a signal structure.
EMC implies friendly co-existence of different systems on the air despite each of them receiving not only its proper signal but also the signals of the other systems. Certainly, it is impossible to root out entirely mutual disturbance when several systems are operating simultaneously within a relatively small area. Any active system, i. There are two parties playing the EMC game. The motivation for this is not only ethical Merits of spread spectrum 89 but is also enforced by strict international and domestic regulations, compliance with which is carefully monitored by the services authorized to impose relevant sanctions.
Among the traditional ways of providing EMC are stringent frequency allocation controlled by national and world institutions, employing antennas with high directivity, careful design of RF circuitry etc.
Here we briefly show why spread spectrum technology may also be included in this list. From the point of view of the emanating system, the following logic is justified. As long as it is possible to make the emitted signal almost imperceptible for a special monitoring receiver see Section 3. The issue is only in the choice of processing gain guaranteeing that the signal power spectral density appears to be sufficiently low compared to the natural noise spectrum intensity at the input of an outside receiver.
In a real design estimates like this have to be coordinated with distance so that some circle around the emanating system exists outside of which the signal of the latter is practically harmless for other systems [16].
From the position of a susceptible system, any outside signal at its receiver output may be treated as a narrowband or broadband jammer and all the reasoning behind the benefit of spread spectrum in anti-jamming see Section 3. Therefore we see that the spread spectrum philosophy fits well with the issue of EMC. First of all, a key parameter affecting performance in any reception problem is signal intensity or SNR.
Certainly, signal energy and power in all preceding formulas expressing error probability, variance of estimate etc. Hence, it is important to be able to predict signal intensity at some point in space remote from the transmitting antenna, allowing for effects accompanying electromagnetic wave propagation. The issue of wave propagation is quite complicated and hard to analyse theoretically.
There are a great variety of factors causing both deterministic and random attenuation of a signal reaching the receiver input. Due to them the received signal is corrupted not only by additive noise AWGN but also by multiplicative interference, whose name stems from the fact that it changes signal intensity, or putting it another way, scales signal amplitude.
Let D denote the distance between the transmitter and the receiver. If the transmitting antenna is directional it emanates in the receiver direction power which is Gt higher than that of the omnidirectional one, and Gt is called a transmit antenna power gain. In this case the received power becomes Gt times higher as well. The free-space model may be directly applied to communication links whose environment is well described as an open space, e. The propagation medium of terrestrial systems is much less favourable and in its influence on the signal intensity two main components are typically categorized: shadowing and multipath fading.
Due to them the signal intensity drops with distance much faster than equation 3. Of course, the irregular nature of terrestrial patterns makes attempts at creating some universal theoretical model of shadowing impossible or worthless. A great deal of field testing has been carried out to collect knowledge about the general character of the dependence between the received power and the length of the propagation path and a number of empirical models have been proposed [17—19].
The received power predicted by this model gives only a very rough reference point, being the result of averaging over the different positions of the receiver with the same distance D from the transmitter. Attenuation of power caused by shadowing has a static character and, even when the receiver is in motion, usually changes in time comparatively slowly due to the large scale of landscape components tens or hundreds of metres. For this reason shadowing is also often referred to as large-scale or long-term fading.
As a matter of fact, the transmitted signal can reach the receiving antenna travelling by many paths. The LOS may appear as one of them or be utterly obstructed, all the other paths emerging as a result of the transmitted wave being reflected by various objects. To better understand the phenomenon, consider first one simple scenario, which may take place in mobile communications, TV broadcasting or elsewhere.
Both reflectors are oriented so that they emit the secondary wave towards each other. This is exactly the phenomenon called multipath fading. Since the space distance between adjacent peaks of Pr is comparable with a wavelength, for the system operating in metre or decimetre band the time cycles of changing Pr at the moving receiver input will be rather short typically split seconds.
The plot of Pr in dependence on time in Figure 3. As is seen, even with rather small speed of movement, changes of the received power are very rapid. This explains why multipath fading is also called short-term fading or small-scale fading.
In practice, the number of multipath signals L received simultaneously may be very large and as a result the interference pattern becomes more complicated.
The phasor diagram in Figure 3. The chaotic character of the distribution of reflectors or scatterers in the receiver environment makes the interference pattern unpredictable and its statistical description most appropriate.
The received power is normalized to the average one. The irregular character of the power change is fairly explicit as well as the presence of deep drops of the received signal intensity.
Therefore, numerous multipath signals obeying these conditions produce a bandpass Gaussian process at the receiver input. If a dominating deterministic component like the LOS one is not present among them the resulting Gaussian process will be a zero-mean one. But the envelope of such a process has Rayleigh distribution see Section 3.
A plot of PDF 3. The deep falls in signal intensity inherent in it are not as a rule neutralized by sporadic rises of Ar when multipath signals arrive nearly in phase. As a result, the overall effect of Rayleigh fading on the system performance appears to be pretty destructive, as the analysis below corroborates. The first of the attributes means that the interference pattern remains stable during many symbols and the current reference phase may be retrieved from the received signal by averaging over an appropriate time interval.
In other words, signal randomness does not exclude the BPSK from the available options. As a result, successive BPSK symbols do not overlap with each other, i. ISI see Section 2. The only sort of corruption which the signal undergoes due to multipath propagation in such a case is Rayleigh amplitude fluctuations described by 3.
Plots simulated in Matlab show a slow flat fading a as opposed to the fast flat one b for the case of the bell-shaped symbol pulses. The second of the fading types is characterized by a rapid change of the interference pattern in time so that distortions of successive symbols are practically independent. Equation 2. Actual amplitude Ar is random and fluctuates from one receiving session to another according to the Rayleigh PDF 3.
It is natural, then, to characterize the performance of data transmission by the value of Pe Ar averaged over all Ar. The physical explanation of a quite detrimental fading effect is rather straightforward. Sporadic sharp drops in signal intensity due to the multipath interference are rather likely in the Rayleigh channel. Elementary integration of PDF 3. Then, as is seen from Figure 3. Since the share of such sessions is 0. This effect cannot in any way be compensated by possible favourable sessions with high SNR, since their contribution to the total error probability is never negative.
The consequence of multipath propagation may potentially be even more dramatic when the fading is frequency selective. This happens if the delay spread covers several transmitted bits so that at the channel output the previous bits overlap with the current one. To counter this ISI, special filters equalizers are used, which rectify the channel transfer function nonuniformity. On the other hand, frequency selectivity when used properly is a good resource for countering fading by arranging the multipath diversity discussed in Section 3.
Thanks to this, despite every individual branch remaining liable to Rayleigh or other fading, the probability that the interference patterns in all of them are simultaneously poor is defined by the multiplication rule and thus diminishes radically. Take the figures of the example at the end of the previous section and suppose that two identical independent branches are somehow organized.
With a larger number of branches this diversity gain becomes more and more substantial. Branches operate in parallel, as though they secured each other, mitigating fading impairment. In other words, we know that the poor performance resulting from multipath fading is entirely due to the deep drops of SNR occurring from time to time.
Such joint processing is called combining. What, then, is the best linear processing producing the maximum possible resultant SNR? Then the resultant power SNR q2r is just the ratio between the magnitude of the deterministic component of this sum and the variance of its noise component. The latter is simply the sum of branch noise variances weighted by jwi j2 , since branches are independent. Such weights, as is readily seen, realize joined matched filtering of the responses of the diversity branches.
Technically it is possible only when accurate values of all signal amplitudes and phases are known. Then signals can be summed coherently with an appropriate amplitude weighting. This combining technique is often referred to in the literature as the maximal ratio technique [5,18].
In practice, some other combining schemes find application, too, because maximal ratio processing is rather demanding as to the extra arrangements necessary to measure SNR and phase in a diversity branch some special pilot signal may appear necessary etc.
Alternative combining modes are equal-weight combining and selection of a maximum SNR branch. The first approaches the maximal ratio mode in effectiveness if all the diversity branches have nearly equal SNR. The gain of the second is close to that of the optimal scheme if one of the diversity branches dominates over the rest in value of SNR. Consider now traditional ways of organizing independent diversity branches.
Space diversity Frequency diversity Time diversity Polarization diversity Multipath diversity. Space diversity implies creating several independent propagation paths at the expense of involving multiple antennas, which explains the other popular name for this technique: antenna diversity.
Duplicating antennas may be used at the receiving side as well as at the transmitting side. Being spaced from each other by a distance of 7—10 wavelengths or more, they provide practical independence of parallel interference patterns at the receiver input.
When used at the receiver Figure 3. In this case diversity signals are separated automatically since different antennas receive them.
Being matched-filtered individually, they may be further combined as described above. Transmitting antenna diversity transmit diversity is not that straightforward. First, as is seen from Figure 3. Second, the receiver antenna receives the mixture of signals emitted by all transmitting antennas.
Therefore, some measures should be taken to provide an opportunity for separation and individual processing of those signals by the receiver before combining. These factors make this sort of diversity a sophisticated optimization problem, solving which is the subject of a special branch of communication theory called space—time coding see Section Reflector Transmitter Receiver Figure 3.
The idea of frequency diversity is based on the concept of the channel coherence bandwidth. This notion determines the frequency range within which fading is considered as flat, i. On the other hand, the harmonics with frequency space beyond the coherence bandwidth may be treated as independently distorted by the channel.
As was already underlined in the previous section, the frequency range of the flat fading depends inversely on the delay spread, so the wider the range of dispersing signals in time, the shorter the coherence bandwidth.
Evidently, transmitting the same signal simultaneously at nd carriers whose frequencies are offset by coherence bandwidth or more creates nd diversity branches. We may say that frequency diversity puts frequency selectivity of fading to good use. When one of these phase differences leads to attenuation of the resultant signal the other may appear less destructive.
With many parallel propagation paths present statistical interpretation comes into force, and frequency difference exceeding the channel coherence bandwidth provides the independence of diversity branches in this scheme. An appropriate choice of frequencies in this diversity scheme provides separation of branches at the receiver with the help of bandpass filtering. Even when the receiver does not move, the multipath profile may be unstable due to the motion of the transmitter or surrounding reflectors.
Thereby Doppler scattering of the received signal arises, and the greater is its spread, the smaller is the coherence time of the channel, i.
Consider again the time—frequency duality: the correlation range in the frequency domain coherence bandwidth is inverse to the spread in time delay spread , while in the time domain coherence time it is inverse to the frequency shift Doppler spread.
Since at the time moments spaced apart by coherence time or more, fading patterns may be treated as independent, retransmission of nd replicas of the same information at appropriate time intervals creates nd diversity branches.
With a slight modification this principle is universally used in telecommunications in the form of interleaving. Polarization diversity, which exploits the difference of multipath profiles of waves with different polarization, has not yet found wide application. As for the last item in the list above, it possesses quite an important role in our context and will be discussed separately in the next section.
Typically this technique is used when the signal spectrum is narrow enough compared to the coherence bandwidth to make the fading flat. The other version of frequency diversity is multipath diversity, exploiting signals with spectrum deliberately extended beyond the coherence bandwidth. Thereby fading becomes frequency selective, allowing in principle time resolution of multipath signals. Thus, the multipath diversity scheme is based on the fact that the signals propagating along the different paths reach the receiver with different time delays.
Suppose that the resultant received signal with the complex envelope 3. Then, taking into account filter linearity and the connection between signal ACF and the matched filter response see Section 2. It is obvious that in such a situation i all multipath signals after the matched filter will not overlap. Since they are fully resolved in time and do not interfere with each other, we may treat them as signals of the independent diversity branches and process according to one of the combining algorithms described above.
If, for instance, their time positions, amplitudes and initial phases are known say, preliminarily measured using a separate pilot channel , maximal ratio combining is the best choice.
As the discussion of Sections 2. This, however, means transmission of high peak power, which cannot be afforded in numerous cases.
Much more attractive is the use of special signals featuring time-compression in the matched filter, i. These signals can only be found among spread spectrum ones, so we may add to the list of the merits of spread spectrum one more advantage: feasibility of organizing the multipath diversity scheme. Multipath diversity is unique in the sense that it radically changes the attitude towards multipath effects, which at first sight are taken as implicitly harmful.
As the discussion above exhibits, reflection of waves has a fruitful side, too. Actually, any contributing reflector directs to the receiver part of the emitted energy which would otherwise be entirely lost. When these reflected signals may be separated from each other time-resolved , this energy is utilized, improving the system performance against that in their absence.
The channel itself, in effect, creates diversity branches in this scheme, the only problem being adequate signal design allowing the multipath replicas to be resolved. Consider the illustration of the diversity scheme applied to digital communications given by Figure 3.
For the sake of transparency, only baseband equivalents complex envelopes of all signals are shown. The left column a corresponds to the transmission of one bit by a plain rectangular pulse, zero bit content being sent by positive polarity upper plot. The second plot of the column shows the resultant signal at the channel output, ignoring noise.
Although ISI exhibits itself by distortion of the initial part of a bit pulse, the major portion of the pulse undergoes flat fading. The lowest plot demonstrates five superimposed realizations of the matched filter response to the resultant signal, in the presence of noise.
The destructive effect of multipath propagation is clearly seen: reliable decisions on the transmitted bits are hardly possible at all. The upper plot shows three such pulses manipulated by the same bit pattern as earlier. In the second plot a resultant noiseless signal at the channel output is given. The lowest plot demonstrates the matched filter response to the resultant signal corrupted with white noise of the same intensity as in the previous case.
The three distinctive peaks per one transmitted bit are all available to retrieve the transmitted data with high confidence. Assuming the channel model is known beforehand, the samples may be taken at the accurate moments of the maximums of each noiseless multipath component at the matched filter output.
As is seen, a properly chosen spread spectrum bit pulse provides resolution of all multipath components at the filter output with no mutual interference. The three samples may then be combined optimally, i.
An equivalent realization of combining is shown in Figure 3. The SNR gain is apparent in Figure 3. Since then it has been widely known by the nickname RAKE, because the peaks at the matched filter output Figures 3. Numerous implementations of the RAKE algorithm are around. One of them uses nd parallel correlators instead of a matched filter, nd being the number of fingers, i.
This structure is most practical when delays of multipath signals are estimated precisely and may be considered as known.
The advantage of such a structure compared to the matched filter one is that for a complicated spread spectrum modulation, implementation of the correlator is often much more feasible than that of the matched filter, since, unlike the latter, the former computes only a single sample of the correlation see Section 2. Where the barrage jammer is considered neglect AWGN. A system needs the highest possible ratio of useful signal power to total interference power, including AWGN and a jammer.
A narrowband jammer is present. Which of the two strategies is better: ignoring the jammer or band-elimination filtering, if: a Jammer power equals AWGN power within the signal bandwidth and jammer bandwidth is half of the signal bandwidth? Spectra of the signal and narrowband jammer are given in Figure 3. What is the most harmful central frequency of the jammer when a receiver ignores the jammer and when it employs band-elimination filtering?
Signal f1 f 0 f 2 Jammer f Figure 3. In some system power SIR at the matched filter output degrades times as compared to power SNR, while the system can preserve its operation capability with only two-times degrading SIR. What should be changed in the signal, SNR remaining constant, if a Only plain signals are allowed?
In some system a band-elimination filter neutralizes the narrowband jammer. Due to this, the matched filter SNR degrades by 3 dB. If so, what should the signal processing gain be? A system can operate with SNR no smaller than 10 dB.
Due to a barrage jammer SNR drops to 3 dB. How could the signal parameters be changed to neutralize the jammer, if: a b c d Only plain signal of the fixed energy is allowed? Only plain signal of the same peak power can be used? Peak power and energy of the signal are fixed with no other constraints?
Peak power of the signal is fixed and its bandwidth can be increased only 10 times? Find the processing gain in cases c and d. In conflict with a barrage jammer transmitter, a system increases signal duration by 4 times with simultaneous halving of signal power and widening of the bandwidth by 50 times. The jammer transmitter is capable of increasing its power no more than 13 dB.
Who will be the winner in this game? A signal occupies two separate sub-bands of identical width W0. Total signal energy is distributed between them in the proportion In the receiver two matched filters process both sub-band parts and their outputs are combined optimally to maximize a resultant SNR. There is a barrage jammer transmitter. What is the most harmful distribution of its total power between the signal sub-bands? Matched filter SNR for an intended receiver equals 14 dB.
A system designer wants the SNR of the interceptor radiometer to be no greater than 10 dB per one transmitted bit duration. What processing gain per one bit would be satisfactory? What should its minimal bandwidth be? Two spread spectrum BPSK data transmission systems are compared. The first employs binary modulation to widen the bandwidth, providing processing gain of per data bit. The second operates with a ternary modulation resulting in processing gain of 50 per data bit.
Which of them has better immunity to cracking a modulation law, the data rate being the same? Two spread spectrum BPSK data transmission systems operating at the same data rate are compared. For the second system these parameters are 16 and 4 dB, respectively.
Which of them is more immune to breaking a modulation law? A system designer takes care of the EMC of a developed system. The maximal SNR provided by the new system over the entire zone covered by the old ones is 20 dB. Any old system operates with a satisfactory quality if the extra power spectrum density does not exceed 10 dB compared to the background AWGN spectrum. What should the minimum processing gain of the new system be? There are two spread spectrum systems occupying the same total bandwidth and operating inside the same geographical area.
The maximal SNR over all the intersection of their operational zones are 20 and 17 dB, respectively. For their compatibility, the extra power spectrum density due to the emission of the other system should be at least 7 dB below the natural AWGN level.
Find the minimal processing gain of each of the systems. The transmitter antenna has gain of 5 dB, while the receiving antenna is omnidirectional. Find the necessary transmitted power for a free-space propagation model, if the system coverage zone should have a radius no smaller than 30 km. How will this power increase for the conditions typical of mobile telephone with the distance-attenuation exponent 3. A system survives if the received voltage SNR drops no more than 4 times below the average predicted level.
Find the probability of system failure due to long-term fading with the standard deviation of the decibel power content equal to 9 dB. There are two propagation paths: LOS and one through a reflector located 3 km away from the LOS and equidistant from both receiver and transmitter. What binary transmission mode is advisable in this case? Binary data are transmitted over a channel with lognormal long-term and Rayleigh short-term fading. Due to the long-term fading, the signal power fluctuates around Merits of spread spectrum 3.
Is bit error probability of 10 3 achievable in this channel without error-control coding? There are two Rayleigh diversity branches with identical average signal energies.
Using average power SNR at the combiner output as a criterion, compare the energy gains of two combining techniques: the maximal ratio one and selection of a maximum SNR branch. A signal of a system occupies bandwidth of 60 kHz. The delay spread of the channel is 20 ms. The total bandwidth is no greater than kHz.
How many frequency diversity branches may be arranged? There are four propagation paths with lengths 5 km, 5. Estimate the approximate signal bandwidth and minimal processing gain necessary for arranging a 4-finger RAKE receiver.
The minimum difference of lengths of paths in a channel is m. The delay spread of the channel is within 10 ms. The system transmits data using QPSK at the rate 20 kbps. What is the maximal available number of RAKE fingers? Find the necessary bandwidth and processing gain of the signal for arranging the RAKE receiver.
A RAKE receiver splits the resultant signal at the output of a Rayleigh channel into nd non-fading signals of equal power using maximal ratio combining. In the light of the answer how can the energy gain of the RAKE technique be explained? Murali Mohan. Ba Dong. Rahul Bhardwaj. Rachel Wheeler.
Julian Duran. Suraj Pratap. Perez Wa Mugoha. Shriram Swaminathan. Amandeep Sappal. Gopi Chowdary. Yingying Chok. Digital Communication via Multipath Fading Channel. More From faizazohra. Lucas Ramero.
Pravat Karki. Monson H. Rajeev Ranjan. Pranata Agriawan. Nurul Hafiza Mohd Jani.
0コメント