364 research outputs found
Trellis-Based Equalization for Sparse ISI Channels Revisited
Sparse intersymbol-interference (ISI) channels are encountered in a variety
of high-data-rate communication systems. Such channels have a large channel
memory length, but only a small number of significant channel coefficients. In
this paper, trellis-based equalization of sparse ISI channels is revisited. Due
to the large channel memory length, the complexity of maximum-likelihood
detection, e.g., by means of the Viterbi algorithm (VA), is normally
prohibitive. In the first part of the paper, a unified framework based on
factor graphs is presented for complexity reduction without loss of optimality.
In this new context, two known reduced-complexity algorithms for sparse ISI
channels are recapitulated: The multi-trellis VA (M-VA) and the
parallel-trellis VA (P-VA). It is shown that the M-VA, although claimed, does
not lead to a reduced computational complexity. The P-VA, on the other hand,
leads to a significant complexity reduction, but can only be applied for a
certain class of sparse channels. In the second part of the paper, a unified
approach is investigated to tackle general sparse channels: It is shown that
the use of a linear filter at the receiver renders the application of standard
reduced-state trellis-based equalizer algorithms feasible, without significant
loss of optimality. Numerical results verify the efficiency of the proposed
receiver structure.Comment: To be presented at the 2005 IEEE Int. Symp. Inform. Theory (ISIT
2005), September 4-9, 2005, Adelaide, Australi
Experimental Assessment of Different Receiver Structures for Underwater Acoustic Communications over Multipath Channels
Underwater communication channels are often complicated, and in particular multipath propagation may cause intersymbol interference (ISI). This paper addresses how to remove ISI, and evaluates the performance of three different receiver structures and their implementations. Using real data collected in a high-frequency (10–14 kHz) field experiment, the receiver structures are evaluated by off-line data processing. The three structures are multichannel decision feedback equalizer (DFE), passive time reversal receiver (passive-phase conjugation (PPC) with a single channel DFE), and the joint PPC with multichannel DFE. In sparse channels, dominant arrivals represent the channel information, and the matching pursuit (MP) algorithm which exploits the channel sparseness has been investigated for PPC processing. In the assessment, it is found that: (1) it is advantageous to obtain spatial gain using the adaptive multichannel combining scheme; and (2) the MP algorithm improves the performance of communications using PPC processing
Interference Cancellation for UWA Random Access Data Packet Transmission
In underwater acoustic (UWA) random access communication networks with
multiple users and data packet transmissions, the packet collisions are the
main cause of the network performance degradation. The aim of this paper is to
investigate interference cancellation (IC) techniques capable of resolving such
collisions in a low-complexity modem with single-carrier modulation and single
transducer. More specifically, in this modem, the IC is used at multiple stages
of the receiver. Firstly, the IC is performed for cancelling the multipath
interference to improve the equalization performance in comparison with the
linear equalization and Rake combining. Secondly, the IC removes the
interference from collided data packets within extracted signal segments after
identifying the collisions. Finally, the IC is applied to the received baseband
signal to improve the data packet detection. The modem performance is
investigated in a lake experiment with intensive multipath channels. The
experimental results demonstrate high detection performance of the proposed
modem design and show that the proposed IC techniques can significantly improve
the throughput of random access UWA networks.Comment: 13 pages, 13 figure
A low-cost time-hopping impulse radio system for high data rate transmission
We present an efficient, low-cost implementation of time-hopping impulse
radio that fulfills the spectral mask mandated by the FCC and is suitable for
high-data-rate, short-range communications. Key features are: (i) all-baseband
implementation that obviates the need for passband components, (ii) symbol-rate
(not chip rate) sampling, A/D conversion, and digital signal processing, (iii)
fast acquisition due to novel search algorithms, (iv) spectral shaping that can
be adapted to accommodate different spectrum regulations and interference
environments. Computer simulations show that this system can provide 110Mbit/s
at 7-10m distance, as well as higher data rates at shorter distances under FCC
emissions limits. Due to the spreading concept of time-hopping impulse radio,
the system can sustain multiple simultaneous users, and can suppress narrowband
interference effectively.Comment: To appear in EURASIP Journal on Applied Signal Processing (Special
Issue on UWB - State of the Art
Pilot-Aided Equalization with a Constrained Noise-Estimation Filter
In this paper we focus on a single carrier pilotassisted
transmission scheme where one pilot symbol is periodically
inserted in the transmitted sequence on a time-division
multiplexing basis. A new equalization scheme, where the knowledge
of pilot symbols is exploited by the equalizer to generate
an estimate of the noise affecting the symbol to be detected,
is introduced and analyzed. The criterion used to compute the
equalizer coefficients is the minimization of the mean-square
error (MSE). The main new result of our analysis is that the
optimal pilot aided equalizer (PAE) can be decomposed as the
cascade of an unconstrained minimum MSE (MMSE) linear
equalizer (LE) and a data-aided noise estimation filter. This
result completes and extends the noise-predictive view of decision
feedback equalization to general data-aided equalization. The
PAE is compared here to the MMSE-LE and to the MSE decision
feedback equalizer on two frequency selective wireless channels
Sparsity Enhanced Decision Feedback Equalization
For single-carrier systems with frequency domain equalization, decision
feedback equalization (DFE) performs better than linear equalization and has
much lower computational complexity than sequence maximum likelihood detection.
The main challenge in DFE is the feedback symbol selection rule. In this paper,
we give a theoretical framework for a simple, sparsity based thresholding
algorithm. We feed back multiple symbols in each iteration, so the algorithm
converges fast and has a low computational cost. We show how the initial
solution can be obtained via convex relaxation instead of linear equalization,
and illustrate the impact that the choice of the initial solution has on the
bit error rate performance of our algorithm. The algorithm is applicable in
several existing wireless communication systems (SC-FDMA, MC-CDMA, MIMO-OFDM).
Numerical results illustrate significant performance improvement in terms of
bit error rate compared to the MMSE solution
A Study on Efficient Receiver Design for UWA Communication System
Underwater Acoustic Channels are fast varying channel according to environmental conditions and exhibit strong random fluctuations in amplitude as well as phase due to reflection, refraction, and diffraction. Due to these highly space, time and frequency dependent channel characteristics, it is very difficult to establish reliable and long-range underwater acoustic communication. In this project, channel modeling has been done showing the different channel characteristics of underwater and their dependencies on frequency, temperature, pressure, salinity etc. Also, it has been shown through some theoretical and practical results that the nakagami fading is the best suitable generalized fading to be used in underwater. In this research work various techniques such as equalization, pilot based OFDM and LDPC Coding has also been done to mitigate the channel fading effect and to improve the performance. An adaptive equalizer has been implemented through three different algorithms LMS, NLMS and RLS for linear as well as non-linear channels to mitigate ISI and, their convergence characteristics along with bit error rate performance has been compared. Two types of pilot insertion, block and Comb type has also been done while implementing OFDM. Block type pilot based OFDM is suitable for slow fading and comb type pilot based OFDM is suitable for a fast fading channel. As in underwater, both types of fading exist, hence, lattice type pilot based OFDM is the best suitable for underwater acoustic communication. LDPC channel coding through which almost Shannon capacity performance can be achieved; has also been implemented taking nakagami channel fading. Bit error rate performance has been compared for different LDPC decoding techniques and for different code rate
Implementation of an Underwater Digital Acoustic Telemetry Reciever
This thesis presents the design and software implementation of an underwater acoustic modem receiver. Communication links in underwater environments face several undesired effects. These include multipath signal reflections, intersymbol interference, and channel fading. This receiver design uses a combination of time and spatial diversity inputs combined with an adaptive feedback equalizer to counteract those effects. The design is based on three modules. A front-end module demodulates and Doppler-compensates the incoming data. A channel combiner module receives data from one or more front ends for spatial diversity and combines repeated transmissions for time diversity. The data from each input channel is time aligned and stored in a \u27job\u27 structure. The channel combiner also calculates tap sizes and locations for the feedback equalizer. Completed \u27job\u27 structures from the channel combiner are then sent to an equalizer module. The modules are implemented in C language code written and compiled for Analog Devices SHARC digital signal processors. The hardware consists of several processors that are interconnected via link ports. This allows each module to run on a separate processor. It also allows for multiple instances of certain modules to be run simultaneously to provide real-time operation
Interference Suppression in WCDMA with Adaptive Thresholding based Decision Feedback Equaliser
WCDMA is considered as one of the 3G wireless standards by 3GPP. Capacity calculation shows that WCDMA systems have more capacity compared to any other multiple access technique such as time division multiple access (TDMA) or frequency division multiple access (FDMA). So it is widely used. Rake receivers are used for the detection of transmitted data in case of WCDMA communication systems due to its resistance to multipath fading. But rake receiver treat multiuser interference (MUI) as AWGN and have limitation in overcoming the effect of multiple access interference (MAI) when the SNR is high. A de-correlating matched filter has been used in this thesis, which eliminates and improves system performance. But the given receiver works well only in the noise free environment. A DFE, compared to linear equaliser, gives better performance at severe ISI condition. The only problem in this equalisation technique is to select the number of symbols that are to be fed back. This thesis gives an idea on multiple symbol selection, based on sparity where an adaptive thresholding algorithm is used that computes the number of symbols to feedback. Simulated results show a significant performance improvement for Regularised Rake receiver along with thresholding in terms of BER compared to a rake receiver, de-correlating rake receiver and regularised rake receiver. The performance of the receiver in different channels is also analysed
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