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BPSK
milstar: Missions not requiring a residual carrier and having modest data rates (20 ks/s - 200 ks/s) should consider BPSK/NRZ modulation first. ######################## nonreturn-to-zero (NRZ) to binary phase-shift keying It provides a good compromise between spectrum efficiency and simplicity of design. While data imbalance does not result in system losses as in the case of PCM/PM/NRZ modulation, the statistics of each application should be reviewed. Agencies employing a DTTL architecture in their symbol synchronizers, must ensure a sufficient transition density to acquire and maintain synchronization. Manchester encoding prior to BPSK modulation can ensure sufficient transitions. As with PCM/PM/Bi-N modulation, there is a 100% penalty in spectrum efficiency over the NRZ equivalent https://deepspace.jpl.nasa.gov/files/phase3.pdf https://deepspace.jpl.nasa.gov/dsndocs/810-005/208/208B.pdf
milstar: Fast fading is the case when any or both of the transmitting or receiving ############## nd is moving with some relative speed to the other. As the signal is transmitted rough multipath characteristic, the movement in the surrounding objects of the transitter/receiver also causes fast fading https://core.ac.uk/download/pdf/61799926.pdf The inter symbol interference is independent of the signal to noise ratio, because as the power of the signal is increased the ISI also gets increased The error floor starts to dominate for values of fdTb > 0.001, where Tb is the time interval for one bit t fd Dopller shift
milstar: https://trs.jpl.nasa.gov/bitstream/handle/2014/43454/11-1691_A1b.pdf?sequence=1
milstar: https://pdfs.semanticscholar.org/929b/74ef14d842ca8be7d63fe88282c7f4d9e54d.pdf The BER performance for BPSK in the average channel is still the best among the three, and a/4 DQPSK performs better than FSK. Specifically, at BER = 0.01, BPSK requires an E, /No = 12.1 dB, whereas for the same error performance, a/4 DQPSK requires 15.2 dB, and FSK requires 18.3 dB. The theoretical BER performance of BPSK in a Rayleigh fading channel is also shown in the figure for comparison The theoretical E, /No required for BPSK in a Rayleigh fading channel is 34.1 dB.
milstar: s. For example, in a narrowband 30-kHz channel (such as that used in the North American TDMA cellular standard IS-136) with a Doppler spread of 100 Hz, the coherence time Tc is roughly 80 symbols and in this case the channel can be estimated with minimal overhead expended in the pilot https://web.stanford.edu/~dntse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter3.pdf
milstar: While the coherent approach to signal detection based on the separation of the detection problem into explicit channel estimation and signal detection is most commonly deployed in digital transmission systems, the noncoherent approach appears to be more natural, since the receiver is usually primarily interested in the transmitted information, but not in information about the current state of the channel. Furthermore, noncoherent detection schemes are more robust in rapidly varying transmission scenarios than their coherent counterparts, which rely on the accuracy of the externally obtained channel estimates [3]. They are therefore particularly apt for 1. discontinuous transmission, where coherent transmission would require a relatively large portion of pilot symbols for accurate channel estimation, 2. systems with low cost, high frequency components, where e.g. strong fluctuations of phase and frequency of local oscillators may occur, and 3. systems with time–variant interferences https://pdfs.semanticscholar.org/f9ab/7951ae721b3937032e998d2814810d86e03a.pdf
milstar: Values of JvT ranging from 0.001 to 0.1 are understood to mean very slow to very fast fading respectively. Figures 1.2 and 1.3 show simulated fading envelopes u(t) for values of JvT of 0.01 and 0.1 respectively. The typical behaviour of the amplitude of the Rayleigh fading process is an oscillatory motion with sudden rapid deep fades occurring at almost regular intervals. The depth of the fades can easily be as much as 20 dB and these are the cause of most error events in a communication system. https://core.ac.uk/download/pdf/35466351.pdf
milstar: Conventional differential detection is known to perform poorly on the Rayleigh fading channel due to the fluctuation of the channel state with time. The worles by Ho and Fung [32] and Divsalar and Simon introduce the method of multiple symbol differential detection (MSDD), a technique where a decision is made on a sequence of symbols, rather than on a per symbol basis. MSDD vastly improves performance over conventional differential detection even when decoding over just a few symbols) by exploiting knowledge of the correlation of the fading process. MSDD works best when the channel fading is highly correlated - a contradicting requirement to diversity. We circumvent this problem by interleaving blocks of symbols over which the MSDD is computed, and constructing a multi-level code over these blocks of symbols. The resulting multi-level codes achieve high coding gains and can perform very well in relatively fast fading environments, that is, values of iDT as high as 0.1. ############ https://core.ac.uk/download/pdf/35466351.pdf
milstar: CDMA outperforms FDMA and TDMA as regards to combating fading, capacity and frequency https://www.ee.iitb.ac.in/~comlab/seminar/ashwini1.pdf • Low mobility Users move at walking speeds (3 km/hr, Rayleigh). • High mobility Users move at 30 km/hr, Rayleigh. https://web.stanford.edu/~dntse/Chapters_PDF/Fundamentals_Wireless_Communication_chapter6.pdf
milstar: . When one examines the numerical results in [1], one observes that as the ratio of data rate (R = 1/T) to loop bandwidth (BL) increases, the optimum fractional allocation of power to the carrier, i.e., m2 4 = Pc/Pt, diminishes. In fact, defining this ratio by δ 4 = R/BL = 1/BLT, then for values of δ on the order of a few hundred (which is typical of most system designs), the fraction of total power allocated to the carrier that yields the minimum average error probability is on the order of m2 = 0.1 or less over a large range of total signal-tonoise ratio (SNR), Rt = PtT /N0. This trend suggests the possibility of using a suppressed-carrier system, i.e., m2 = 0, which itself requires replacing the PLL with a loop capable of tracking a fully suppressed carrier, e.g., a Costas loop https://pdfs.semanticscholar.org/26cc/24d7be6fdf82b87fc22316371b5e590d536c.pdf
milstar: . Indeed, since as mentioned above, the Costas loop requires a larger loop SNR than does the PLL to yield a given tracking accuracy, the same is true in terms of the loop SNR required to maintain lock (herein referred to as the threshold value of loop SNR.) Thus, depending on the threshold SNR values decided upon for the two loops (to be discussed later on), there will exist a region of system parameters where one would be forced to employ a residual rather than a suppressed-carrier system, since in this region the loop SNR of the latter is below its threshold value whereas the loop SNR of the former is still above its threshold value. The purpose of this article is to define these regions, which will then clearly spell out for the system designer when to choose the suppressed-carrier option over the residual carrier one or vice versa. In the next section, we present the theoretical background necessary to establish these regions.
milstar: https://ieeexplore.ieee.org/document/950321 When the order of diversity L increases, we can notice a great improvement of the performance but this comes at the expense of a more complicated system and a slower transmission rate (for a fixed transmission bandwidth).
milstar: http://www.ijsrp.org/research_paper_jun2012/ijsrp-June-2012-100.pdf
milstar: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=483276 https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=1292540
milstar: Direct modulation schemes are inherently more bandwidth efficient than those employing subcarriers. This is due, in part, to the way that the ITU defined Occupied Bandwidth to be that span of frequencies, covered by the modulated signal, which excludes only the lower 0.5% and the upper 0.5% of the transmitted power. Thus, large frequency gaps between the RF carrier and the subcarrier are included in the Occupied Bandwidth calculation despite the fact that there is no significant modulation sideband energy in large portions of these frequency gaps. https://deepspace.jpl.nasa.gov/files/phase1.pdf
milstar: to analyze the performance of a pure phase coherent slow frequency hopped (SFH) receiver with 1 bit/hop in the presence of AWGN and partial band interference and compared the BER performance with non-coherent FSK systems. When the phase distortion in the channels was not excessive, improved performance compared to a coherent system was observed. The importance of fast frequency hopping (FFH) in mitigating follower jamming was emphasized due to its inherent frequency diversity. Moreover, the frequency hops occurring during each symbol could be combined to achieve a reliable decision state. Considering the scenario when a smart receiver is able to track the hopped frequencies, a hybrid system was proposed in which each hopped frequency is spread with a PN sequence. FFH was found to combat both frequency selective fading (because of frequency diversity) and non-selective fading (when hopped frequencies are combined properly). Under severe fading conditions, follower jamming was found to be less effective against a hybrid spread spectrum system. https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes
milstar: In general, coherent systems (with slow frequency hopping) provided better performance against PBN, Ricean fading, multiple access interference and AWGN. Under severe Rayleigh fading, coherent reception became difficult. Muammar [40] studied the effects of frequency selective Rayleigh fading and log normal shadowing on a DS/FH system with differential phase shift keying (DPSK) modulation. Error probabilities were examined for a Rayleigh fading channel with and without the effects of log normal shadowing. System degradation with log normal shadowing was much smaller than that caused by Rayleigh fading. Byun et al. [42] analyzed a hybrid DS/SFH system subject to a Nakagami fading channel. The bit error probability over a Nakagami fading channel was calculated as a function of the number of jamming tones used by the jammer. Various combinations of number of hopping frequencies and spreading code sequences that satisfied an equal bandwidth constraint were employed. For a low jamming to signal power ratio (JSR) of about 10dB, a pure DSSS system was found to achieve a lower BER than a hybrid DS/SFH system. However, for higher JSR values (20 or 30dB), the hybrid DS/SFH system exhibited superior performance. Also, the worst case performance of a hybrid DS/SFH system was found to be almost equal to the nominal performance of a pure DSSS system. https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes
milstar: . The proposed transceiver is a slow frequency hopping (SFH) system, meaning the hop rate is slower than the symbol rate. It should be noted, though, that our system still requires a high hop rate. There are many fundamental differences in the transceiver design and implementation between a very low hop rate SFH-TDMA system, which hops to a new frequency every frame (i.e., hop rate 50 Hz), and the proposed SFH-CDMA transceiver. It hops every eight symbols, which results in a hop rate of 20 kHz at a symbol rate of 80 kHz. https://pdfs.semanticscholar.org/c2f9/e8284cbaa43eff5ee97c9fe1d89d38e636bb.pdf
milstar: Figure 5.2 : Performance of a Single User System in an AWGN and Rayleigh Fading Environment for Selected SFs. page 79 The BER performance curves for a varying number of users in a Rayleigh fading environment are presented in Figure 5.3 for SF=64. It is clear that the performance degrades gradually as the number of users is increased from 10 to 30. Even in the presence of 30 users, the system is able to provide an acceptable voice BER of 3 10− at a Eb No / value of just 10 dB. Thus the system can accommodate many users even without error correction coding. It can be inferred from Figures 5.2 and 5.3 that a lower spreading factor than 64 will provide a BER greater than 3 10− at 10 dB, while a higher SF than 64 will reduce the BER and thus many more users than 30 will be accommodated at 10 dB for a BER of 10−3 . https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes
milstar: 5.2 Slow Frequency Hopping: Performance Results For slow frequency hopping systems, 3 sets of simulations were carried out. First, the SFH system was simulated in a Rayleigh fading environment for a single user using 64 hopping frequencies. Following the equal bandwidth constraint, this is equivalent to a SF of 64 in DSSS system. BER performance of a SFH system with 64 hopping frequencies is compared with DSSS system of SF=64 in Figure 5.5. It is evident that the performance of the DSSS system under Rayleigh fading is far better than the SFH system for the same processing gain in a single user system. A second set of SFH system performance results are based on multi-user interference analysis. All the users use two frequencies from a total of 64 frequencies used by the desired user. The SFH system was simulated for a hopping rate equal to 8 bits/hop and 64 hopping frequencies with a varying number of users. Performance results are shown in Figure 5.6 https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes
milstar: Performance of the FFH system for different numbers of users in a Rayleigh fading environment is plotted in Figure 5.11. A hopping rate of 8 hops/bit and 64 hopping frequencies were used. As with Rayleigh fading performance, FFH multiuser performance is better than SFH but inferior to DSSS performance. A second difference between SFH and FFH performance curves is that FFH performance Figure 5.10 : Comparison of SFH, FFH and DSSS Systems under Rayleigh Fading https://trace.tennessee.edu/cgi/viewcontent.cgi?referer=https://www.google.de/&httpsredir=1&article=3781&context=utk_gradthes
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