4,399 research outputs found
Channel Estimation for Two-Way Relay Networks in the Presence of Synchronization Errors
This paper investigates pilot-aided channel estimation for two-way relay
networks (TWRNs) in the presence of synchronization errors between the two
sources. The unpredictable synchronization error leads to time domain offset
and signal arriving order (SAO) ambiguity when two signals sent from two
sources are superimposed at the relay. A two-step channel estimation algorithm
is first proposed, in which the linear minimum mean-square-error (LMMSE)
estimator is used to obtain initial channel estimates based on pilot symbols
and a linear minimum error probability (LMEP) estimator is then developed to
update these estimates. Optimal training sequences and power allocation at the
relay are designed to further improve the performance for LMMSE based initial
channel estimation. To tackle the SAO ambiguity problem, the generalized
likelihood ratio testing (GLRT) method is applied and an upper bound on the SAO
detection error probability is derived. By using the SAO information, a scaled
LMEP estimation algorithm is proposed to compensate the performance degradation
caused by SAO detection error. Simulation results show that the proposed
estimation algorithms can effectively mitigate the negative effects caused by
asynchronous transmissions in TWRNs, thus significantly outperforming the
existing channel estimation algorithms.Comment: 14 pages, 9 figure
Synchronisation of the superimposed training method for channel estimation in the presence of DC-offset
The superimposed training method estimates the channel
from the induced first-order cyclostationary statistics
exhibited by the received signal. In this paper,
using vector space decomposition, we show that the
information needed for training sequence synchronisation,
and for DC-offset estimation, can be extracted
from the first-order cyclostationary statistics as well.
Necessary and sufficient conditions for channel computation
and equalisation are derived, when training
sequence synchronisation and DC-offset removal are
required. The computational burden of the practical
implementation of the method presented here is much
lighter than for existing algorithms. At the same time,
simulation results show that the performance, in terms
of the MSE of the channel estimates and BER, is not
diminishedwhen compared to these existing algorithms
Short Packet Structure for Ultra-Reliable Machine-type Communication: Tradeoff between Detection and Decoding
Machine-type communication requires rethinking of the structure of short
packets due to the coding limitations and the significant role of the control
information. In ultra-reliable low-latency communication (URLLC), it is crucial
to optimally use the limited degrees of freedom (DoFs) to send data and control
information. We consider a URLLC model for short packet transmission with
acknowledgement (ACK). We compare the detection/decoding performance of two
short packet structures: (1) time-multiplexed detection sequence and data; and
(2) structure in which both packet detection and data decoding use all DoFs.
Specifically, as an instance of the second structure we use superimposed
sequences for detection and data. We derive the probabilities of false alarm
and misdetection for an AWGN channel and numerically minimize the packet error
probability (PER), showing that for delay-constrained data and ACK exchange,
there is a tradeoff between the resources spent for detection and decoding. We
show that the optimal PER for the superimposed structure is achieved for higher
detection overhead. For this reason, the PER is also higher than in the
preamble case. However, the superimposed structure is advantageous due to its
flexibility to achieve optimal operation without the need to use multiple
codebooks.Comment: Accepted at ICASSP 2018, special session on "Signal Processing for
Machine-Type Communications
Coded DS-CDMA Systems with Iterative Channel Estimation and no Pilot Symbols
In this paper, we describe direct-sequence code-division multiple-access
(DS-CDMA) systems with quadriphase-shift keying in which channel estimation,
coherent demodulation, and decoding are iteratively performed without the use
of any training or pilot symbols. An expectation-maximization
channel-estimation algorithm for the fading amplitude, phase, and the
interference power spectral density (PSD) due to the combined interference and
thermal noise is proposed for DS-CDMA systems with irregular repeat-accumulate
codes. After initial estimates of the fading amplitude, phase, and interference
PSD are obtained from the received symbols, subsequent values of these
parameters are iteratively updated by using the soft feedback from the channel
decoder. The updated estimates are combined with the received symbols and
iteratively passed to the decoder. The elimination of pilot symbols simplifies
the system design and allows either an enhanced information throughput, an
improved bit error rate, or greater spectral efficiency. The interference-PSD
estimation enables DS-CDMA systems to significantly suppress interference.Comment: To appear, IEEE Transactions on Wireless Communication
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
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