[Show abstract][Hide abstract] ABSTRACT: In this paper, families of rate-compatible protograph-based LDPC codes that are suitable for incrementalredundancy hybrid ARQ applications are constructed. A systematic technique to construct low-rate base codes from a higher rate code is presented. The base codes are designed to be robust against erasures while having a good performance on error channels. A progressive node puncturing algorithm is devised to construct a family of higher rate codes from the base code. The performance of this puncturing algorithm is compared to other puncturing schemes. Using the techniques in this paper, one can construct a rate-compatible family of codes with rates ranging from 0.1 to 0.9 that are within 1 dB from the channel capacity and have good error floors.
Preview · Article · Sep 2009 · IEEE Journal on Selected Areas in Communications
[Show abstract][Hide abstract] ABSTRACT: Abstract—This paper,considers the application of cooperative base-station (BS) transmission,schemes,to the downlink,of multi- cell networks. Based on a simplified Wyner-type network,model with users clustered at the cell-edges, closed form sum rate expressions,for non-fading channels,are derived for dirty-paper coding (DPC), linear zero-forcing (ZF) precoding, and co-phasing with,reuse. By extending,the,model,to include,cell-interior users, the capacity region for various transmission strategies is determined,for the rate pairs achievable,by the two,classes of users. In addition,to the upper,bound,of DPC across the whole,network,and,the simple,approach,of having,adjacent BSs alternate between serving cell-edge and cell-interior users, we,consider,several hybrid,approaches,to serve,cell-interior users in each cell but cell-edge users in alternating cells. These hybrid,approaches,allow the cooperation,to be localized among adjacent,BSs based,on,either DPC or superposition,coding (SPC). The resulting capacity,regions,show,the tradeoff,for improving,performance,based,on techniques,that have differing levels of BS cooperation,and processing complexity,and,differing requirements,for channel,state information,at the transmitter.
Full-text · Article · Jan 2008 · EURASIP Journal on Wireless Communications and Networking
[Show abstract][Hide abstract] ABSTRACT: In this paper, we study the potential benefit of base-station (BS) cooperation for downlink transmission in a modified Wyner-type multicell model. Besides the Dirty-Paper-Coding (DPC) precoder, we also analyze several linear precoding schemes, including co-phasing, zero-forcing (ZF) and MMSE precoders. For the nonfading case, analytical sum rate expression is obtained for each scheme. In networks of a large number N of cells, a high signal-to-noise-ratio (SNR) asymptotic gap is shown between the sum rate performances of DPC and ZF precoders. Moreover, the MMSE precoder sum rate expression in large networks indicates different behaviors of MMSE precoder in different SNR regimes: in the SNR ≫ N<sup>2</sup> regime, it coincides with the ZF precoder, while, in the SNR ≪ N<sup>2</sup> regime, it coincides with the co-phasing precoder. For the Rayleigh fading case, Monte-Carlo simulations demonstrate the effectiveness of linear precoding schemes with the proposed user selection criterion.
[Show abstract][Hide abstract] ABSTRACT: This paper addresses the network level aspects of incorporating interference cancellation into the CDMA2000 1xEV-DO Revision A reverse link. We illustrate how a physical layer analysis of interference cancellation can be applied to an extensive network simulation environment that models inter-cell interference, hybrid-ARQ, multipath fading channels and the MAC-layer dynamics of power control, rate allocation, and rise- over-thermal control. An investigation of reverse link pilot channel performance and rise-over-thermal distribution shows that interference cancellation can be added to the base station processing without modifying the overall network operation or system stability. Network simulation results demonstrate how interference cancellation increases the data rate of each user to significantly improve the throughput achieved with both 2 and 4 receiver antennas per sector.
[Show abstract][Hide abstract] ABSTRACT: We describe ongoing research and product development focused on the theory, system design, and capacity gains from incorporating reverse link interference cancellation into commercial CDMA basestations. The work considers receiver architecture design tradeoffs, link cancellation efficiency in multipath fading channels, and the multi-cell network operation and performance
[Show abstract][Hide abstract] ABSTRACT: A general framework is provided to characterize the link level performance of CDMA systems with interference cancellation. This closed-form residual power analysis accounts for the impact of channel estimation errors due to SNR, channel variation, chip asynchronism, and filter mismatch. Simulations further quantify the link level cancellation performance on more realistic sub-chip multipath channels. This work demonstrates that properly designed channel estimation and signal reconstruction techniques achieve high cancellation efficiency over a variety of multipath fading channels.
[Show abstract][Hide abstract] ABSTRACT: This paper investigates the design of commercially viable CDMA basestation receivers that incorporate interference cancellation. Results are given for cdma2000 1xEV-DO but also apply to WCDMA HSUPA. The effect of receiver memory size is determined for canceling packets transmitted with hybrid-ARQ. Performance-complexity tradeoffs are presented for implementing iterative and successive interference cancellation of asynchronous user transmissions. Multipath channel estimation techniques of varying complexity are compared based on sector throughput. Practical interference cancellation designs with moderate complexity are shown to achieve a large fraction of the gains of more elaborate techniques.
[Show abstract][Hide abstract] ABSTRACT: In this paper, we design families of rate-compatible structured LDPC codes suitable for hybrid ARQ applications with high throughput. We devise a systematic technique of low complexity for the design of structured low-rate LDPC codes from higher rate ones. These codes have a good performance on the AWGN channel and are robust against erasures and puncturing. The codes designed here are protograph-based codes and have fast encoding and decoding structures. These low rate codes are used as the parent codes of rate-compatible families. Then, we propose a number of algorithms for puncturing the codes in a rate compatible manner to construct codes of higher rates. The two most promising ones are the random puncturing search technique and progressive node puncturing. We show that using the techniques in this paper one could construct a high throughput rate compatible family with codes whose rates are in the range from 0.1 to 0.9 and which are within 1 dB from the channel capacity and have good error floors
[Show abstract][Hide abstract] ABSTRACT: We design serial concatenated multi-input multi-output systems based on low-density parity-check (LDPC) codes. We employ a receiver structure combining the demapper/detector and the decoder in an iterative fashion. We consider the a posteriori probability (APP) demapper, as well as a suboptimal demapper incorporating interference cancellation with linear filtering. Extrinsic information transfer (EXIT) chart analysis is applied to study the convergence behavior of the proposed schemes. We show that EXIT charts match very well with the simulated decoding trajectories, and they help explain the impact of different mappings and different demappers. It is observed that if the APP demapper transfer characteristics are almost flat, the LDPC codes optimized for binary-input channels are good enough to achieve performance close to the channel capacity. We also present a simple code-optimization method based on EXIT chart analysis, and we design a rate-1/2 LDPC code that achieves very low bit-error rates within 0.15 dB of the capacity of a two-input two-output Rayleigh fading channel with 4-pulse amplitude modulation. We next propose to use a space-time block code as an inner code of our serial concatenated coding scheme. By means of a simple example scheme, using an Alamouti inner code, we demonstrate that the design/optimization of the outer code (e.g., LDPC code) is greatly simplified.
Preview · Article · May 2005 · IEEE Transactions on Communications
[Show abstract][Hide abstract] ABSTRACT: For some channels, channel side information at the transmitter (CSIT) may not improve channel capacity. For example, for binary input and symmetric output memoryless channels, uniform input distribution is capacity achieving with or without CSIT. However, we show that CSIT can help reduce the complexity of codes if the code is designed using channel knowledge and provide for lower bounds on complexity. A code optimization problem for minimizing complexity for a given rate loss is also posed.
[Show abstract][Hide abstract] ABSTRACT: We design multilevel coding (MLC) and bit-interleaved coded modulation (BICM) schemes based on low-density parity-check (LDPC) codes. The analysis and optimization of the LDPC component codes for the MLC and BICM schemes are complicated because, in general, the equivalent binary-input component channels are not necessarily symmetric. To overcome this obstacle, we deploy two different approaches: one based on independent and identically distributed (i.i.d.) channel adapters and the other based on coset codes. By incorporating i.i.d. channel adapters, we can force the symmetry of each binary-input component channel. By considering coset codes, we extend the concentration theorem based on previous work by Richardson et al. ( see ibid., vol.47, p.599-618, Feb. 2001) and Kavcˇic´ et al.(see ibid., vol.49, p.1636-52, July 2003) We also discuss the relation between the systems based on the two approaches and show that they indeed have the same expected decoder behavior. Next, we jointly optimize the code rates and degree distribution pairs of the LDPC component codes for the MLC scheme. The optimized irregular LDPC codes at each level of MLC with multistage decoding (MSD) are able to perform well at signal-to-noise ratios (SNR) very close to the capacity of the additive white Gaussian noise (AWGN) channel. We also show that the optimized BICM scheme can approach the parallel independent decoding (PID) capacity as closely as does the MLC/PID scheme. Simulations with very large codeword length verify the accuracy of the analytical results. Finally, we compare the simulated performance of these coded modulation schemes at finite codeword lengths, and consider the results from the perspective of a random coding exponent analysis.
Preview · Article · Oct 2003 · IEEE Transactions on Information Theory
[Show abstract][Hide abstract] ABSTRACT: A numerical method has been presented to determine the noise
thresholds of low density parity-check (LDPC) codes that employ the
message passing decoding algorithm on the additive white Gaussian noise
(AWGN) channel. In this paper, we apply the technique to the
uncorrelated flat Rayleigh fading channel. Using a nonlinear code
optimization technique, we optimize irregular LDPC codes for such a
channel. The thresholds of the optimized irregular LDPC codes are very
close to the Shannon limit for this channel. For example, at rate
one-half, the optimized irregular LDPC code has a threshold only 0.07 dB
away from the capacity of the channel. Furthermore, we compare simulated
performance of the optimized irregular LDPC codes and turbo codes on a
land mobile channel, and the results indicate that at a block size of
3072, irregular LDPC codes can outperform turbo codes over a wide range
of mobile speeds
Preview · Article · Jun 2001 · IEEE Journal on Selected Areas in Communications
[Show abstract][Hide abstract] ABSTRACT: We design multilevel coding (MLC) schemes with low-density
parity-check (LDPC) codes as component codes at each level. We develop a
method to analyze the performance of an LDPC code at any level as the
codeword length goes to infinity, even if the equivalent binary-input
component channels are not symmetric. By joint optimization of code
rates and degree distributions, the optimized irregular LDPC codes at
each level are capable of achieving reliable transmission at
signal-to-noise ratios (SNR) very close to the capacity of the additive
white Gaussian noise (AWGN) channel as the codeword length tends to
infinity. Simulation results show that the optimized LDPC codes also
perform very well at moderate codeword lengths
[Show abstract][Hide abstract] ABSTRACT: We design low-density parity-check (LDPC) codes for bandwidth
efficient modulation using a multilevel coding (MLC) technique. We
develop a method to analyze the asymptotic performance of the LDPC codes
using message-passing decoding at each level of the MLC scheme as the
codeword length goes to infinity. Simulation of very large block size
LDPC codes verifies the accuracy of the analytical results. We jointly
optimize the code rates and code parameters of the LDPC codes at each
level of the MLC scheme, and the asymptotic performance of the optimized
irregular LDPC codes is very close to the channel capacity of the
additive white Gaussian noise (AWGN) channel
[Show abstract][Hide abstract] ABSTRACT: The potential capacity gains of coordinated processing in the reverse link of a cellular system are evaluated. Three different matched filter-based receiver schemes, successive intra-cell interference cancellation, successive intra-and inter-cell interference cancellation, and centralized successive interference cancellation, are analyzed considering different path loss exponents and various limiting factors such as non-detectable paths and imperfect interference cancellation. Performance of the three schemes is compared to that achievable by the single user matched filter receiver and by the optimum joint decoding scheme. Capacity gains ranging from 41% up to 183% over the classical single user matched filter receiver are highlighted.
[Show abstract][Hide abstract] ABSTRACT: channel 2 for different rates. Simulation Results: As a comparison parameter, we consider complexity by introducing the sparseness parameter as the ratio of the number of ones in the parity check matrix to the number of variable nodes N, i.e., average variable node degree. We take the two-channel case where the SNR (Eb/No) of the first channel is fixed at 0.391 dB and A1 ,A 2 are set at 0.4 and 0.6 respectively. Simulation results as in Figures 2 and 3 indicate that the joint coding with CSIT gives slightly higher thresholds than the other cases and also shows a significant decrease in the sparseness parameter. Figure 4 presents simulation results showing the better performance of our joint design owing to larger block lengths. In this simulation, the component channel block lengths are 2000 and 3000. The noise standard deviation of the first channel is fixed at 0.4212.