Another approach adopted by researchers to find the BER of the DS/SSMA scheme is to ignore MAI completely. Most of these techniques are basically an extension of pre- viously studied inter-symbol interference (ISI) techniques. Some of these techniques are moment space method, characteristic function method, moments method, and the approximate Fourier series method. It has been reported in the literature that these techniques are supe- rior to the central limit theorem-based techniques in approximating the BER but involve higher computational costs. A study of the signal-to-noise ratio (SNR) of Rician fading channels at the correlator receiver’s output was done by . The BER performance of the DS-CDMA sys- tem in frequency non-selectiveRayleighfading channels for deterministic sequences using the SGA approach was evaluated in . The characteristic function (CF) tech- nique was utilized to assess the performance of the spread spectrum multiple access (SSMA) scheme in an AWGN environment in . The CF method used to evaluate the performance of the DS-SSMA scheme in multipath fad- ing channels with multipath intersymbol interference was applied in  without taking into account the MAI effect. An approximate Fourier series technique was utilized in [11, 12] to evaluate the BER performance in selective and non-selectiveRayleighfading environments. System degradation caused by an imperfect chip and phase syn- chronization were also assessed in this technique.
The BER performance of DS-CDMA system in a Rayleighfading channel has been investigated in this re- search. So far, most of the researches on DS-CDMA mobile communication systems were done considering the mobile wireless channel either AWGN channel or Rayleigh distributed flat or frequencyselectivefading chan- nel. The objectives of the current research work were: 1) To study a Direct Sequence Code Division Multiple Access (DS-CDMA) system in a Rayleighfading channel, 2) To carry out the analysis for carrier to multiple access interference ratio and the compression for bit error rate (BER) of a DS-CDMA system in a Rayleigh fad- ing channel, 3) To carry out numerical computations to evaluate the carrier to interference ratio (CIR) and to find the BER as a function of system parameters, 4) To carry out computer simulations to evaluate the signal waveforms and the amount of signal distortions due to fading and 5) To find the optimum system parameters in presence of fading.
In this paper, we present a way to obtain the analytical BER of MC-CDMA uplink with MRC in channels with corre- lated Rayleigh, Rician, or Nakagami fading paths. We first achieved a simplified signal model for the MRC output and established its time-frequency equivalence, which states that combining the subcarriers using MRC has exactly the same eﬀect as combining the channel paths using MRC. This prin- ciple is exploited to achieve very simplified BER formulas based on (CIR) information of the MC-CDMA system under study. A new closed-form BER formula is derived for chan- nels with correlated Nakagami fading paths using the tech- niques of Cholesky decomposition and gamma pdf approx- imation. The BER formula is exact if all the channel paths have identical m l /Ω l ratio; otherwise, it is approximate, but
Channel estimation in frequencyselective has different approach then compared with flat fading environment. A comparative study using Minimum Mean Squared Error (MMSE) and Least square (LS) estimator in frequencyselectivefading environment has been presented in . The channel estimation based on comb type pilot ar- rangement is studied using different algorithms by bahai et al. . A novel channel estimation scheme for OF- DMA uplink packet transmissions over doubly selective channels was suggested in . The proposed method uses irregular sampling techniques in order to allow flexible resource allocation and pilot arrangement. In flat fading environment, estimation of the channel using trained sequence of data has been studied and implemented in . He presented the channel estimation in flat fading environment using some trained data. Channel phase was estimated during each coherence time. Then pilot data of some required percentage of data length (referred as training percentage in simulation) is inserted into the source data. It is used to estimate the random phase shift of the fading channel and train the decision to adjust the received signal with phase recover. The results obtained showed the great variation in BER for with and without estimation curves. It is clear from literature reviewed that phase estimation using training symbol can be imple- mented in flat fading environment to improve the per- formance of system.
be seen that the increase in the number of multipath components from (Fig. 7) to (Fig. 8) increases the threshold SNR by only 1 dB or less. We also see that results in Figs. 7 and 8 are very close to that in Fig. 6. Thus, despite that the channel model used for derivation of the ML frequency estimator was based on the assumptions of uncorrelated path amplitudes and path delays being multiples of the sampling period, the simu- lation results in Figs. 7 and 8 show that this estimator demon- strates approximately the same performance in a more realistic channel with correlated path amplitudes and fractional path de- lays. Fig. 9 shows the dependence of the frequency error on the estimated frequency for SNR dB. It can be seen that, in a wide frequency range, the accuracy is close to the CRLB in a single-path channel.
Up until now we have looked at wireless systems with one transmitter and one receiver. For the remainder of the thesis we study networks with more than one transmitter-receiver pair. This introduces interference into the system as users compete for access to the shared wireless medium. The receivers then have to decode signals perturbed not only by fading and noise but also by additive interference from other users. The essential question we ask here is, how can we ensure reliable communications without jeopardizing on the total rate of the system? For example, we do not wish to have the transmitter-receiver pairs communicate one at a time. While this avoids the interference problem and is reliable, it gives a low rate per user. Are there cleverer schemes? Here we will investigate ways that other nodes cooperate with communication between source and destination, ensuring a higher rate while maintaining reliable communication. Again we study this in the high SNR regime, where we can measure reliability by diversity, the exponent of the error probability due primarily to fading. From now we also suppose all communication nodes have a single antenna.
can be optimized according to specific design requirements tailored to the communication environments encountered, in order to achieve the optimum performance in terms of . For example, for a given total system bandwidth, can be optimized, in order to minimize the multiuser interference, since it has an influence on both the overlap of the modulated subcarrier signals and on the processing gain. In this context a clear trade-off exists between the overlap and the processing gain. On the one hand, if is low—for example —in the context of multitone DS-CDMA, then a subcarrier signal will overlap with a high number of subcarrier signals of both the same user and with those of the interfering users. On the other hand, given a total bandwidth and a low value of , a high spreading gain can be maintained, which leads to the reduction of the multiuser interference. By contrast, if is high—for example, —which means that there exists no spectral overlap between the main lobes of the subcarrier signals, then the modulated subcarrier signals benefit from a low interference inflicted by the other subcarrier signals of both the reference and the interfering users. However, in this case, the spreading gain of each subcarrier signal is low, which leads to an increased multiuser interference. The influence of the subcarrier spacing on both the spreading gain and on the spectral overlap of the subcarrier signals highlights that there exists an optimum spacing which minimizes the multiuser interference inflicted upon each of the subcarrier signals. Another example in the context of optimizing is that the subcarrier spacing can be adjusted for matching the re- ceiver’s requirements. For example, assuming that the receiver employs a three-finger RAKE receiver, a specific spacing can be selected such that the number of resolvable paths becomes three in the propagation environment encountered. In this case, the highest possible diversity gain is achieved, since the receiver can combine all the energy scattered over the multipath components. The adjustment of the subcar- rier-spacing parameter can be implemented as follows. If there exists a low-delay feedback channel between the receiver and the transmitter—which is the case in the existing wireless systems, such as in the 3G systems—the parameter as well as the corresponding chip-rate of the spreading sequences can be reconfigured according to the near-instantaneous channel quality. However, if the near-instantaneous channel quality cannot be readily estimated, the subcarrier-spacing parameter can be designed according to the typical channel character- istics encountered—such as indoor, outdoor, urban, rural, etc., environments.
short filters are preferred. In this context, a novel short filter allowing for near perfect reconstruction and having the same size as one OFDM symbol is proposed. Using frequency-spread (FS) implementation for the FBMC/OQAM receiver, analytical analysis and simulation results show that the proposed filter exhibits better robustness to several types of channel impairments when compared to state-of the-art prototype filters and OFDM modulation. In addition, FS-based hardware architecture of the filtering stage is proposed, showing lower complexity than the classical polyphase network-based implementation. Proposed-In this proposed the impact of nonlinear distortion induced by High Power Amplifiers (HPA) for medium and high power signals is analyzed for Filter Bank Multi-Carrier (FBMC) systems using Offset Quadrature Amplitude Modulation (OQAM). A closed form expression for Bit Error Rate (BER) expression is derived and analyzed for MMSE equalizer models for FBMC/OQAM system with nonlinear HPA in frequencyselectiveRayleigh channel by varying constellation size in OQAM modulation and Input Back-Off. The performance is compared for the models with 64 sub channels and input back-off for 6 and 8 decibels with 4*4 MIMO system. In lieu of validating the obtained simulation results, theoretical results have also been compared.
Abstract In recent years, there has been considerable interest in the use of CDMA in mobile communications. Bit error rate is one of the most important parameters in the evaluation of CDMAsystems. In this paper, we develop a technique to find an accurate approximation to the probability of bit error for asynchronous direct–sequence code division multiple–access (DS/CDMA) systems by modeling the multiple access interference (MAI) as an improved Gaussian process. The channel is modeled as a multipath flat Rayleighfading channel. Numerical results are obtained by using the improved Gaussian approximation (IGA) and compared to the standard Gaussian approximation (SGA).
proposed in . The study of advanced receivers also leads us to a better characterization of the MIMO-CDMA link. Such characterization of a wireless link, usually known as channel quality indicator (CQI), is very important from the overall system evaluation perspective, both in terms of link adaptation and link-to-system mapping . In SISO sys- tems, the CQI of a wireless link is usually reported to the base station (BS) in the form of the instantaneous SNR seen at the mobile station (MS). At the BS, the scheduler performs the link adaptation by comparing this CQI with some preset threshold to determine the proper modulation and coding scheme (MCS) for this MS. The CQI is also used in gener- ating the so-called short-term frame error rate (FER) ver- sus SNR curves, which provides a simple abstraction of the link for the purpose of system-level simulations. In SISO sys- tems, the mappings of CQI to both MCS and FER, denoted as MCS(CQI) and FER(CQI), are single-dimensional map- pings. For MIMO systems, if an SE MIMO scheme is used, the CQI of each coded stream can still be represented by a single SNR and hence, the single-dimensional mapping of both MCS(CQI) and FER(CQI), just as in the SISO case. However, for JE MIMO schemes, various portions of a packet see diﬀerent SNRs, and hence the mapping is potentially a complicated multidimensional problem.
Then the prediction algorithm needs to be improved in order to compensate for the effect of power adjustment to the correlation of input samples into the predictor. Predictive algorithm aims at predicting the future sample values using the correlation property of the past sample values. In the predictive power control algorithm, however, the correlation of past sample values is destroyed by power control updating factors. In this study, the number of CDMA users is assumed to be large and therefore using the central limit theorem the multiple access interference is Gaussian distributed. As a result the SIR measured at the base station corresponds with the received signal strength which reflects the channel gain. However, since the received signal level or SIR has been updated by power control updating, in this paper the past power-controlled fading factor or the past SIR values, γ(i – D – v), v = 0, 1, …, V – 1 need to be compensated for by the same factors that were given by power control updating. The restored SIR values as input samples to the predictor can be expressed as
Abstract: This paper presents Trellis coded modulation with 8-PSK for Space communication. TCM is more suitable for space communication compared to other existing modulation techniques because of its bandwidth efficiency and better performance over fading channel. In TCM system higher coding gain can be achieved without expanding the bandwidth or increasing the average energy of the signal set. This includes concatenated system in which outer code is BCH and inner code TCM. The proposed concatenated system provides bandwidth efficiency compared to concatenated RS-TCM system. The simulated results show that significant coding gain by the proposed system over Additive White Gaussian Noise (AWGN) and Rayleighfading channel.
The two main problems in the multiuser environment are Multiple Access Interference (MAI) and multipath fading. In the reverse link (from mobile to base station), the detector faces a challenging problem since user signals are asynchronous and the link suers from the "near-far" problem, where weaker users are poorly detected. The simplest receiver structure for the multiuser detection problem is the conventional (or single-user) detector. However, its performance is generally poor for bandwidth ef- cient systems and weaker users suer greatly from the near-far problem, necessitating the use of strict power control to ensure that signals from the dierent users arrive at the base station with close to equal power levels. The Maximum-likelihood detector, on the other hand, oers optimum performance  at the expense of high complexity which becomes prohibitive even for systems with a moderate number of users. Several suboptimum detectors that attempt to oer a better performance/complexity ratio have therefore been proposed, e.g. [2, 3, 4, 5]. However, most of the research eort reported so far has considered uncoded systems over the Additive White Gaussian Noise (AWGN) channel with a few exceptions (e.g. [6, 7, 8]).
Furthermore, the data detection of the proposed DSTBC can be decoupled in frequency domain for a low-complexity receiver. The main idea in our design is that each relay con- veys a distinct column of the conventional STBC developed for colocated antennas. We prove that the data rate of the proposed DSTBC is higher than that of repetition code which has the maximum data rate of 1/3 for a two-relay system. The design of DSTBC in our paper is inspired by the idea of linear dispersion codes where each column of the code matrix is a linear summation of transmit data and their conjugations . In this paper, we analyze the PEP with a mixed Rayleigh and Rician frequency-selectivefading channel model. The analysis shows that the LOS component e ﬀ ectively improves the error rate performance at the destination. Simulation results over diﬀerent channel parameters and diﬀerent n- factors were provided to verify the theoretical analysis.
considered. Here, the way pick up segment has been viewed as Rayleigh appropriation. The final observation of our simulation is that the BER performance of DS-CDMA system is highly influenced by this path gain component and noise variance. This work can be further extended considering path gain component and other parameters in multicarrier direct sequence CDMA (MC-DS-CDMA) and orthogonal frequency division multiplexing (OFDM). Mr. G. A. Bhalerao and Prof.R.G.Zope in their paper,” BER Improvement of DS CDMA with rake receiver using Multipath fading channel”, had studied the performance of DS CDMA system for various users.
simulation results show that the DF and the 2S provide significant performance gain over the linear decorrelator, and of course, the conventional detector. In particular, we prove that the BER of the ideal DF approaches the single user bound. To evaluate the theoretical performance of multiuser detectors under channel mismatch conditions, we model a flat fading channel as the second order AR process, and explore the use of Kalman filtering for channel estimation under the assumption that decisions at input to the Kalman filter are correct. We find that the error floors are determined by physical parameters of the fading channel. Performance comparison results do not agree with the perfect channel estimation case: the 2S and the DF do not always outperform the decorrelator. The performance gain of these two more complex detectors is observed only in the lower SNR region, and is not as significant as in the perfect channel estimation case. In the high SNR region, the residual interference of the these two detectors caused by channel mismatch becomes large enough to degrade their performance whereas the decorrelator tends to the single user bound in this case.
We also note that, from an implementational point of view, the available hardware may favor one system over the other. For instance, the problem of peak-to-average ratio in MC-CDMA system may make implementing DS-CDMA sys- tem easier. Similarly, the di ﬃ culty in implementing RAKE receivers in high data rate systems may make implementing MC-CDMA system easier. We showed in Propositions 1, 2, and 3 that all the systems can perform as well provided that the su ﬃ cient conditions are met. We again emphasize that
In this paper, we applied MSDD to OFDM over time- and frequency-selectiveRayleighfading channels under the as- sumption that the guard time is longer than the delay spread, thus causing no e ﬀ ective ISI. Optimal decision metrics of TD-MSDD and FD-MSDD have been derived based on the exact covariance matrix conditioned on transmitted infor- mation symbol sequence. The theoretical BER performance of MSDD for OFDM has been analyzed, and based on these analytical results, we have shown that when simple receiver structure is preferable, both TD-MSDD and the suboptimal FD-MSDD provide a good performance because of their robustness against the time- and frequency-selective nature of the channel. Thus, as opposed to need of careful selec- tion between TD-DD and FD-DD according to the channel statistics, the di ﬀ erence in BER performance between TD- MSDD and the suboptimal FD-MSDD is negligible. Further- more, it has been shown that if the enhancement of compu- tational complexity at the receiver is acceptable, the optimal FD-MSDD may be a very eﬀective strategy due to its robust- ness against the ICI over such channels.