Frame synchronization for PSAM in AWGN and Rayleigh fading channels

Pilot Symbol Assisted Modulation (PSAM) is a good method to compensate for the channel fading effect in wireless mobile communications. In PSAM, known pilot symbols are periodically inserted into the transmitted data symbol stream and the receiver uses these symbols to derive amplitude and phase reference. One aspect of this procedure, which has not received much attention yet, is the frame synchronization, i.e. the method used by the receiver to locate the time position of the pilot symbols. In this study, two novel non-coherent frame synchronization methods are introduced in which only the magnitude of received signal is used to obtain the timing of the pilot symbol. The methods are evaluated for both AWGN and frequency non-selective slow Rayleigh fading channels. One synchronization technique is derived by standard maximum likelihood (ML) estimation formulation, and the other is obtained by using maximum a Posteriori probability (MAP) with a threshold test. Signal processing in the receiver uses simplifying approximations that rely on relatively high signal-to-noise ratio (SNR) as consistent with the reception of 16-QAM. Computer simulation has been used to test the acquisition time performance and the probability of false acquisition. Several lengths and patterns of pilot symbol sequences were tested where every 10th symbol was a pilot symbol and all other symbols were randomly selected data symbols. When compared with the other published synchronizers, results from this study show better performance in both AWGN and fading channels. Significantly better performance is observed in the presence of receiver frequency offsets

Pilot Pouring in Superimposed Training for Channel Estimation in CB-FMT

Cyclic block filtered multi-tone (CB-FMT) is a waveform that can be efficiently synthesized through a filter-bank in the frequency domain. Although the main principles have been already established, channel estimation has not been addressed yet. This is because of assuming that the existing techniques based on pilot symbol assisted modulation (PSAM), implemented in OFDM-like schemes, can be reused. However, PSAM leads to an undesirable loss of data-rate. In this paper, an alternative method inspired by the superimposed training (ST) concept, namely pilot pouring ST (PPST), is proposed. In PPST, pilots are superimposed over data taking advantage of the particular spectral characteristics of CB-FMT. Exploiting the sub-channel spectrum, the pilot symbols are poured in those resources unused for data transmission. This spectral shaping of pilots is also exploited at the receiver to carry out channel estimation, by enhancing those channel estimates that exhibit a low data interference contribution. Furthermore, a frequency domain resource mapping strategy for the data and poured pilot symbols is proposed to enable an accurate estimation in strongly frequency-selective channels. The parameters of the proposed scheme are optimized to minimize the channel estimation mean squared error (MSE). Finally, several numerical results illustrate the performance advantages of the proposed technique as compared to other alternatives

Parallel RAM from Cyclic Circuits

Known simulations of random access machines (RAMs) or parallel RAMs (PRAMs) by Boolean circuits incur significant polynomial blowup, due to the need to repeatedly simulate accesses to a large main memory. Consider two modifications to Boolean circuits: (1) remove the restriction that circuit graphs are acyclic and (2) enhance AND gates such that they output zero eagerly. If an AND gate has a zero input, it 'short circuits' and outputs zero without waiting for its second input. We call this the cyclic circuit model. Note, circuits in this model remain combinational, as they do not allow wire values to change over time. We simulate a bounded-word-size PRAM via a cyclic circuit, and the blowup from the simulation is only polylogarithmic. Consider a PRAM program $P$ that on a length $n$ input uses an arbitrary number of processors to manipulate words of size $\Theta(\log n)$ bits and then halts within $W(n)$ work. We construct a size-$O(W(n)\cdot \log^4 n)$ cyclic circuit that simulates $P$. Suppose that on a particular input, $P$ halts in time $T$; our circuit computes the same output within $T \cdot O(\log^3 n)$ gate delay. This implies theoretical feasibility of powerful parallel machines. Cyclic circuits can be implemented in hardware, and our circuit achieves performance within polylog factors of PRAM. Our simulated PRAM synchronizes processors by simply leveraging logical dependencies between wires

Channel estimation with TCH codes for machine-type communications

TCH codes possess several properties that allow us to use them efficiently in various applications. One of these applications is channel estimation and, in this dissertation, it is studied the performance of TCH codes to estimate the channel in an Orthogonal Frequency Division Multiplexing system, regarding Machine-Type Communications. Bit error rate performance results were obtained by executing simulations that allowed the evaluation of the impact of using two different pilot techniques, such as data multiplexed and implicit pilots, different pilot power levels and different modulations, QPSK and 64-QAM. Pilots based on TCH codes are also compared with other conventional pilots. Results show that TCH codes have a very positive and reliable performance. Joint timing synchronization and channel estimation is also performed using different sparse based approaches, such as Orthogonal Matching Pursuit, L1- regularized and Iterative Reweighted L1. TCH codes are compared against different sequence types, namely Zadoff-Chu sequences and pseudorandom codewords, and variations in the pilot size, the channel length and the observation window size are executed in order to understand their effects. Results ultimately illustrate that TCH codes can be effectively used in joint channel estimation and synchronization, managing to withstand worst simulation conditions better than its counterparts. It is also proven that compressed sensing can successfully be utilized in joint synchronization and channel estimation, an area where its use has not been very explored.Os códigos TCH possuem várias propriedades que nos permitem usá-los eficientemente em diversas aplicações. Uma delas é a estimação de canal e nesta dissertação é estudado o desempenho dos códigos TCH em estimação de canal num sistema OFDM, tendo em conta as comunicações Machine-Type. Resultados que ilustram a taxa de erro de bit foram obtidos através de simulações que permitem avaliar o impacto de usar diferentes técnicas de pilotos, nomeadamente multiplexados e implícitos, diferentes valores de potência para os pilotos e diferentes modulações, QPSK e 64-QAM. Também é feita a comparação entre os pilotos TCH e pilotos convencionais. Os resultados mostram que os pilotos TCH tem um desempenho muito positivo e confiável, dentro dos parâmetros testados. Também é efetuado o estudo de sincronização e estimação de canal conjunta usando métodos esparsos como o OMP, o L1-regularized e o Iterative Reweighted L1. Os códigos TCH são comparados com outros tipos de sequências, tais como as sequências Zadoff-Chu e os códigos pseudo-aleatórios. São consideradas variações no tamanho dos pilotos, no comprimento do canal e no tamanho da janela de observação para perceber quais são os seus efeitos no desempenho. Os resultados demonstram que os códigos TCH podem ser utilizados com sucesso em estimação de canal e sincronização conjunta e conseguem aguentar condições adversas de simulação melhor que os outros pilotos utilizados. Também é provado que compressed sensing pode ser utilizado com sucesso em sincronização e estimação conjunta, que é uma área onde o seu uso ainda não foi explorado aprofundadamente

Phase-domain Injected Training for Channel Estimation in Constant Envelope OFDM

Constant envelope orthogonal frequency division multiplexing (CE-OFDM) is a multi-carrier waveform with 0 dB peak-to-average power ratio (PAPR). This property enables the exploitation of multi-carrier waveforms with non-linear power amplifiers, avoiding the undesirable clipping effects. However, the existing channel estimation techniques designed for OFDM cannot be reused, since the use of a phase modulator makes CE-OFDM a non-linear waveform. Previous works assumed that the channel estimation process relies on the transmission of preambles, and the data symbols are equalized using a frequency domain equalizer (FDE). To avoid the overhead induced by preambles, a phase-domain injected training (PIT) is proposed, where the pilot sequence is embedded in the phase dimension of the data symbols. This novel approach does not waste time and/or frequency resources as in preamble-based schemes. Moreover, it does not require additional power for the training. The received symbols are averaged with a dual procedure, and owing to the particular structure of CE-OFDM, the channel estimates are recovered. Also, the analytical expression of the channel estimation mean squared error (MSE) is derived. Finally, several numerical results illustrate the performance of the proposal, showing that the MSE, bit error rate (BER) and achievable rate are improved, as compared to the existing works.This work was supported by the Spanish National Project IRENE-EARTH under Grant PID2020-115323RBC33/AEI/10.13039/501100011033. The work of Andrea M. Tonello was supported in part by the Chair of Excellence Program of the Universidad Carlos III de Madrid.Publicad

Symbol level decoding of Reed-Solomon codes with improved reliability information over fading channels

A thesis submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Doctor of Philosophy in the School of Electrical and Information Engineering, 2016Reliable and e cient data transmission have been the subject of current research, most especially in realistic channels such as the Rayleigh fading channels. The focus of every new technique is to improve the transmission reliability and to increase the transmission capacity of the communication links for more information to be transmitted. Modulation schemes such as M-ary Quadrature Amplitude Modulation (M-QAM) and Orthogonal Frequency Division Multiplexing (OFDM) were developed to increase the transmission capacity of communication links without additional bandwidth expansion, and to reduce the design complexity of communication systems. On the contrary, due to the varying nature of communication channels, the message transmission reliability is subjected to a couple of factors. These factors include the channel estimation techniques and Forward Error Correction schemes (FEC) used in improving the message reliability. Innumerable channel estimation techniques have been proposed independently, and in combination with di erent FEC schemes in order to improve the message reliability. The emphasis have been to improve the channel estimation performance, bandwidth and power consumption, and the implementation time complexity of the estimation techniques. Of particular interest, FEC schemes such as Reed-Solomon (RS) codes, Turbo codes, Low Density Parity Check (LDPC) codes, Hamming codes, and Permutation codes, are proposed to improve the message transmission reliability of communication links. Turbo and LDPC codes have been used extensively to combat the varying nature of communication channels, most especially in joint iterative channel estimation and decoding receiver structures. In this thesis, attention is focused on using RS codes to improve the message reliability of a communication link because RS codes have good capability of correcting random and burst errors, and are useful in di erent wireless applications. This study concentrates on symbol level soft decision decoding of RS codes. In this regards, a novel symbol level iterative soft decision decoder for RS codes based on parity-check equations is developed. This Parity-check matrix Transformation Algorithm (PTA) is based on the soft reliability information derived from the channel output in order to perform syndrome checks in an iterative process. Performance analysis verify that this developed PTA outperforms the conventional RS hard decision decoding algorithms and the symbol level Koetter and Vardy (KV ) RS soft decision decoding algorithm. In addition, this thesis develops an improved Distance Metric (DM) method of deriving reliability information over Rayleigh fading channels for combined demodulation with symbol level RS soft decision decoding algorithms. The newly proposed DM method incorporates the channel state information in deriving the soft reliability information over Rayleigh fading channels. Analysis verify that this developed metric enhances the performance of symbol level RS soft decision decoders in comparison with the conventional method. Although, in this thesis, the performance of the developed DM method of deriving soft reliability information over Rayleigh fading channels is only veri ed for symbol level RS soft decision decoders, it is applicable to any symbol level soft decision decoding FEC scheme. Besides, the performance of the all FEC decoding schemes plummet as a result of the Rayleigh fading channels. This engender the development of joint iterative channel estimation and decoding receiver structures in order to improve the message reliability, most especially with Turbo and LDPC codes as the FEC schemes. As such, this thesis develops the rst joint iterative channel estimation and Reed- Solomon decoding receiver structure. Essentially, the joint iterative channel estimation and RS decoding receiver is developed based on the existing symbol level soft decision KV algorithm. Consequently, the joint iterative channel estimation and RS decoding receiver is extended to the developed RS parity-check matrix transformation algorithm. The PTA provides design ease and exibility, and lesser computational time complexity in an iterative receiver structure in comparison with the KV algorithm. Generally, the ndings of this thesis are relevant in improving the message transmission reliability of a communication link with RS codes. For instance, it is pertinent to numerous data transmission technologies such as Digital Audio Broadcasting (DAB), Digital Video Broadcasting (DVB), Digital Subscriber Line (DSL), WiMAX, and long distance satellite communications. Equally, the developed, less computationally intensive, and performance e cient symbol level decoding algorithm for RS codes can be use in consumer technologies like compact disc and digital versatile disc.GS201

Channel Estimation in Coded Modulation Systems

With the outstanding performance of coded modulation techniques in fading channels, much research efforts have been carried out on the design of communication systems able to operate at low signal-to-noise ratios (SNRs). From this perspective, the so-called iterative decoding principle has been applied to many signal processing tasks at the receiver: demodulation, detection, decoding, synchronization and channel estimation. Nevertheless, at low SNRs, conventional channel estimators do not perform satisfactorily. This thesis is mainly concerned with channel estimation issues in coded modulation systems where different diversity techniques are exploited to combat fading in single or multiple antenna systems. First, for single antenna systems in fast time-varying fading channels, the thesis focuses on designing a training sequence by exploiting signal space diversity (SSD). Motivated by the power/bandwidth efficiency of the SSD technique, the proposed training sequence inserts pilot bits into the coded bits prior to constellation mapping and signal rotation. This scheme spreads the training sequence during a transmitted codeword and helps the estimator to track fast variation of the channel gains. A comprehensive comparison between the proposed training scheme and the conventional training scheme is then carried out, which reveals several interesting conclusions with respect to both error performance of the system and mean square error of the channel estimator. For multiple antenna systems, different schemes are examined in this thesis for the estimation of block-fading channels. For typical coded modulation systems with multiple antennas, the first scheme makes a distinction between the iteration in the channel estimation and the iteration in the decoding. Then, the estimator begins iteration when the soft output of the decoder at the decoding iteration meets some specified reliability conditions. This scheme guarantees the convergence of the iterative receiver with iterative channel estimator. To accelerate the convergence process and decrease the complexity of successive iterations, in the second scheme, the channel estimator estimates channel state information (CSI) at each iteration with a combination of the training sequence and soft information. For coded modulation systems with precoding technique, in which a precoder is used after the modulator, the training sequence and data symbols are combined using a linear precoder to decrease the required training overhead. The power allocation and the placement of the training sequence to be precoded are obtained based on a lower bound on the mean square error of the channel estimation. It is demonstrated that considerable performance improvement is possible when the training symbols are embedded within data symbols with an equi-spaced pattern. In the last scheme, a joint precoder and training sequence is developed to maximize the achievable coding gain and diversity order under imperfect CSI. In particular, both the asymptotic performance behavior of the system with the precoded training scheme under imperfect CSI and the mean square error of the channel estimation are derived to obtain achievable diversity order and coding gain. Simulation results demonstrate that the joint optimized scheme outperforms the existing training schemes for systems with given precoders in terms of error rate and the amount of training overhead

Study and implementation of a low complexity receiver using TCH codes

The use of coding in telecommunications systems reveals to be a technique with an essential contribution to the improvement of the recovery of transmitted signals. Depending on the circumstances to which a signal is subjected at transmission, by recurring to coding, it is possible to attenuate the unfavorable effects that result from this process, obtaining a signal with superior quality in comparison with a scenario where the presence of coding is absent. This study aims to test the feasibility of a high-rate wireless communications system using TCH codes being applied to an OFDM signal, subjected to noise components introduced by a wireless AWGN channel, considering a free path propagation model. Due to their correlation properties, the use of TCH codes reveals to be adequate since they allow the same codewords to be used to realize both error correction and channel estimation, mitigating the channels effects, leading to the realization of a receiver with lower complexity. With the intent of performing a qualitative analysis to this system, a simple simulation is executed in MATLAB where an OFDM signal is generated, being therefore applied various TCH codes and, through channel estimation, obtain the BER for their respective code lengths and, consequently, coding gains. The results were obtained for the modulation indexes of 16, 64 and 256-QAM. These demonstrate that the implementation of TCH codes is a viable option to reduce the rate of recovered errors, enabling the reception of a signal with better reliability, especially for higher code lengths and modulation indexes.O uso de codificação em sistemas de telecomunicações revela-se uma técnica com um contributo essencial na melhoria da recuperação de sinais transmitidos. Dependendo das circunstâncias às quais um sinal é submetido aquando a sua transmissão, com recurso à codificação, é possível atenuar os efeitos adversos resultantes deste processo, obtendo uma qualidade no sinal recebido superior face a um cenário com ausência da mesma. Este estudo tem como objetivo testar a exequibilidade de um sistema de comunicações sem fios para alto débito usando códigos TCH e aplicando-os a um sinal OFDM, sujeito a componentes de ruído introduzidos por um canal sem fios AWGN, considerando um modelo de propagação em espaço livre. Devido às propriedades de correlação destes códigos, a sua utilização revela-se adequada pois permite que as mesmas palavras de código sejam utilizadas para efetuar correção de erros, minimizando os efeitos do canal, possibilitando o desenvolvimento de um recetor com menor complexidade. Visando efetuar uma análise qualitativa do sistema, é realizada uma simulação simples em MATLAB onde é gerado um sinal OFDM ao qual são aplicados diversos códigos TCH, realizando estimação de canal com a finalidade de obter o BER para os diferentes comprimentos dos códigos e, consequentemente, ganhos de codificação relativos aos mesmos. Os resultados obtidos foram realizados para os índices de modulação 16, 64 e 256-QAM. Estes demonstram que o uso destes códigos é uma opção viável para reduzir os erros detetados, permitindo recuperar o sinal com maior fiabilidade, especialmente para comprimentos de código e índices de modulação elevados

Performance of LDPC decoder with accurate llr metric in LDPC-coded pilot-assisted OFDM system

Master'sMASTER OF ENGINEERIN