1,721,043 research outputs found
Optimized Prunable Single Cycle Interleavers for Turbo Codes
No full textThis paper is aimed at the problem of designing optimized interleavers for parallel concatenated convolutional codes (PCCC) that satisfy several requirements simultaneously: 1) designing interleavers tailored to the constituent codes of the PCCC; 2) improving the distance spectra of the resulting turbo codes which dominate their asymptotic performance; 3) constructing optimized interleavers recursively so that they are implicitly prunable; and 4) completely avoiding short permutation cycles in order to reduce the risk of having strong correlations between the extrinsic information during iterative decoding. To this end, we present two theorems that lead to a modification of a previously developed iterative interleaver growth algorithm (IGA) that can be used to design optimized variable-length interleavers, whereby at every length the optimized permutation implemented by the interleaver is a single-cycle permutation. Two more modifications of the IGA are presented to improve the performance of the optimized interleavers at a reduced complexity. The optimization is achieved via constrained minimization of a cost function closely related to the asymptotic bit-error rate or frame-error rate of the code
Time-frequency density for wavelets
No full textThe authors demonstrate that given a function orthogonal with respect to shifts of integer multiples of a constant A, its bandwidth is related to the shift parameter A. This result is used to establish an upper bound on the density of the independent states in the time-frequency plane for scaling functions and wavelets
Clock synchronisation without self-noise using wavelets
No full textThe authors present a symbol synchronisation scheme using wavelets that completely eliminates pattern-dependent jitter [I]. This is achieved by obtaining the optimal prefilter using a particular linear combination of the wavelet and its translates
On Design of Optimized Low-Density Parity-Check Codes Starting From Random Constructions
No full textIn this paper we present a novel two step design technique for Low Density Parity Check (LDPC) codes, which, among the others, have been exploited for performance enhancement of the second generation of Digital Video Broadcasting- Satellite (DVB-S2). In the first step we develop an efficient algorithm for construction of quasi-random LDPC codes via minimization of a cost function related to the distribution of the length of cycles in the Tanner graph of the code. The cost function aims at constructing high girth bipartite graphs with reduced number of cycles of low length. In the second optimization step we aim at improving the asymptotic performance of the code via edge perturbation. The design philosophy is to avoid asymptotically weak LDPCs that have low minimum distance values and could potentially perform badly under iterative soft decoding at moderate to high Signal to Noise Ratio (SNR) values. Subsequently, we present sample results of our LDPC design strategy, present their simulated performance over an AWGN channel and make comparisons to some of the construction methods presented in the literatur
Iterative Joint Channel Decoding of Correlated Sources Employing Serially Concatenated Convolutional Codes
No full textThis correspondence looks at the problem of joint decoding of serially concatenated convolutional codes (SCCCs) used for channel coding of multiple correlated sources.W e assume a simple model whereby two correlated sources transmit SCCC encoded data to a single destination receiver. W e do not assume the existence of, nor do we use channel side information at the receiver.In particular, we present a novel iterative joint channel decoding algorithm for correlated sources by using the empirical cross-correlation measurements at successive decoding iterations to provide extrinsic information to the outer codes of the SCCC configuration. Two levels of soft metric iterative decoding are used at the receiver: 1) iterative maximum a posteriori probability (MAP) decoding is used for efficient decoding of individual SCCC codes (local iterations) and 2) iterative extrinsic information feedback generated from the estimates of the empirical cross correlation in partial decoding steps is used to pass soft information to the outer decoders of the global joint SCCC decoder (global iterations).W e provide analytical results followed by simulation studies confirming the robustness of the cross-correlation estimates to channel-induced errors, justifying the use of such estimates in iterative decoding.Experimental results suggest that relatively few global iterations (two to five) during which multiple local iterations are conducted are sufficient to reap significant gains using this approach specially when the sources are highly correlated
Permutation Fixed Points With Application to Estimation of Minimum Distance of Turbo Codes
No full textIn this paper we present a systematic technique for obtaining all the input sequences that are mapped by a given permutation either to themselves or to shifted versions of themselves (generically called permutation fixed points). Such sequences or their subsets, represent the primary candidates for examination in connection with obtaining estimates of the minimum distance of parallel concatenated codes, specially for interleaver lengths for which the determination of the actual minimum distance may be very difficult. Subsequently, we present a new class of permutations that nearly achieve the lower bound on the number of possible fixed points associated with a given permutation of prime length . Preliminary experimental evidence suggests that certain permutations of this class lead to turbo codes with large minimum distances for short interleaver lengths
Modulator quality figures
No full textEVM (Error Vector Magnitude) and BER (Bit Error Rate) are compared using simulation for some realistic modulators, in order to understand whether EVM, which is easily measured through a VSA (Vector Signal Analyzer), can be considered a sufficient parameter to assess the quality of a modulator. The system loss at a given BER is actually the most important figure for the system design, and it is shown that in general the loss cannot be predicted from EVM alone. However, VSA outputs can be easily used to measure BER through the same semi-analytic techniques commonly adopted in simulation, and thus it is possible to directly estimate the loss. The semi-analytic techniques however cannot be used in presence of nonlinear blocks after the receiver such as channel decoders. However, at least in the considered cases, the loss measured with semi-analytic technique at the output of the demodulator is equal to that found at the output of the decoder. Hence, even if BER at the decoder output cannot be estimated, the loss can be predicte
Linear subcodes of turbo codes with improved distance spectra
No full textAbstract—In this correspondence, we present a technique for generation of linear subcodes of a given turbo code with better distance spectrum than the original mother turbo code, via an iterative process of trace-bit injection which minimallyr educes code rate, followed byselecti ve puncturing that allows recoveryof the rate loss incurred during the trace-bit injection. The technique allows for asymptotic performance improvement of any linear turbo code. In effect, we trim the distance spectrum of a turbo code via elimination of the lowdistance and/or high multiplicitycodew ords from the output space of the code. To this end, we perform a greedyminimization of a cost function closelyr elated to the asymptotic bit error probability (or frame error probability) of the code. This improves the performance of the code everywhere, but its main impact is a reduction in the error floor of the turbo code which is important for delayconstrained applications employing short interleavers
Shaping the Power Spectrum of TCM Codes Using Wavelet Packet Modulation
No full textMany TCM codes do not shape the power spectrum. It is desirable to shape the power spectrum of the coded modulation to compensate for the linear filtering effect of the channel, in order to maximise the data transmission rate. A technique is proposed whereby several independent TCM codes are used to encode different subbands of the available transmission bandwidth such that they can all be decoded efficiently in a single decode
Iterative joint channel decoding of correlated sources
No full textIn this article we exploit the potential correlation existing between multiple information sources to achieve additional coding gains from the channel codes used for data protection. We do not assume the existence of, nor do we use channel sideinformation at the receiver. Instead, empirical estimates of the cross-correlation are used in partial decoding steps in an iterative joint soft decoding paradigm. Experimental results suggest that relatively few iterations (2 to 4) are sufficient to reap significant gains using this approach specially when the sources are highly correlated. Finally, we provide analytical performance bounds of the proposed technique showing a close match with the simulation results at sufficiently high SNR
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