In this work, we investigate the issue of semantics in non real-time communication scenarios, that is, without stringent information delivery constraints. While semantics has been widely used in the context of communications, the definition of the semantic metrics has been limited to the real-time applications. We thus revise the basic concepts of the timeliness and the accuracy and propose their formal definitions in the case of non real-time applications characterized by a possibility of non-causal signal reconstruction at the destination. In particular, we link the concept of accuracy to the canonical first-order auto-regressive model of the signal, while the timeliness is defined by the time difference to reference samples of the signal. We compare the proposed semantic metrics in the case of different buffer management strategies and using numerical examples we show that the optimality of the transmission strategy changes when the real-time or non real-time applications are considered.

This paper investigates the incremental-redundancy hybrid-automatic repeat request (IR-HARQ) transmission over independent block-fading channels in the presence of an eavesdropper, where the secrecy of the transmission is ensured via introduction of dummy messages. Since the encoder only knows the statistics of the channel state, the secrecy and the reliability are defined in a probabilistic framework. Unlike previous works on this subject, we design a coding strategy tailored to IR-HARQ by splitting the dummy-message rate over several rate parameters. These additional degrees of freedom improve the match between the dummy-message rates and the realizations of the eavesdropper channels. We evaluate the performance in terms of secrecy outage probability, connection outage probability, and throughput and compare it with the benchmark paper by Tang et al. Numerical examples illustrate that, comparing to existing alternatives, splitting of the dummy-message rate provides higher throughput and lower expected duration/average delay.

In this paper, we investigate a coding strategy devised to increase the throughput in hybrid ARQ (HARQ) transmission over a block fading channel. In our approach, the transmitter jointly encodes a variable number of bits for the each round of HARQ. The parameters (rates) of this joint coding can vary and may be based on the negative acknowledgment provided by the receiver or, on the past (outdated) information about the channel states. These new degrees of freedom allow us to improve the match between the codebook and the channel states experienced by the receiver. The results indicate that gains obtained using the proposed cross-packet coding strategy are particularly notable for the large values of the throughput. In this region, the conventional HARQ fails to offer throughput improvement even if the number of transmission rounds is increased. We implement the proposed cross-packet HARQ using turbo codes, where we show that the theoretically predicted throughput gains materialize in practice; the implementation challenges are also discussed.

In this paper, we propose a coding strategy designed to enhance the throughput of hybrid automatic repeat request (HARQ) transmissions over independent identically distributed block-fading channels with the channel state information unknown at the transmitter. We use a joint packet coding where the same channel block is logically shared among many packets. To reduce the encoding and decoding complexity, we use a two-layer coding where, first, packets are punctured, mixed, and then passed to the conventional channel encoder. We show how to optimize the puncturing rates on the basis of the empirical error-rate curves. We also discuss how the parameters of the practical turbo-codes may be modified to take advantage of the proposed HARQ scheme. Finally, simple and pragmatic rate adaptation strategies are developed. In numerical examples, our scheme is compared with the conventional incremental redundancy HARQ (IR-HARQ), which remains a strategy of choice in the region of small throughput; however, our scheme shows a notable gain of $1-2~\textrm {dB}$ in the region of high throughput, where IR-HARQ fails to provide any improvement.

In this work, we propose and optimize a new coding strategy designed to enhance the throughput of hybrid ARQ (HARQ) transmissions over i.i.d. block-fading channels where the channel state information (CSI) is unknown at the transmitter. Unlike in the conventional approach, where many blocks are exclusively assigned to carry the HARQ rounds of a single packet, we use a joint coding and the same channel block is logically shared among many packets. To reduce the complexity, we use a two-layer coding, where, first, packets are mixed on the binary level, and the resulting mixture is then passed for the conventional channel encoder. This allows us to decode packets on a block-by-block basis. Knowing the decoder error rate curves, the challenge is to find the optimal packets mixing that maximizes the total throughput. We show how to solve this problem using a dynamic programming (DP) approach, which we next apply to the off-the-shelf turbo-codes. We also discuss how the parameters of the practical turbo-encoder may be modified to take advantage of the proposed HARQ scheme. In numerical examples, our scheme is compared to the conventional incremental redundancy HARQ (IR-HARQ), and it yields a notable gain of 1 - 2dB in the region of high throughput, where IR-HARQ fails to provide any improvement.

To ensure reliable communications in randomly varying and error-prone channels, wireless systems use adaptive modulation and coding (AMC) as well as hybrid ARQ (HARQ). In order to elucidate their compatibility and interaction, we compare the throughput provided by AMC, HARQ, and their combination (AMC-HARQ) under two operational conditions: in slow- and fast block-fading channels. Considering both incremental redundancy HARQ and repetition redundancy HARQ, we optimize the rate-decision regions for AMC/HARQ and compare them in terms of attainable throughput. Under a fairly general model of the channel variation and the decoding functions, we conclude that: 1) adding HARQ on top of AMC may be counterproductive in the high average signal-to-noise ratio regime for fast fading channels and 2) HARQ is useful for slow fading channels, but it provides moderate throughput gains. We provide explanations for these results which allow us to propose paths to improve AMC-HARQ systems.

In this work, we propose and investigate a new coding strategy devised to increase the throughput of hybrid ARQ (HARQ) transmission over block fading channel. In our proposition, the transmitter jointly encodes a variable number of bits for each round of HARQ. The parameters (rates) of this joint coding can vary and may be based on the negative acknowledgment (NACK) signals provided by the receiver or, on the past (outdated) information about the channel states. The results indicate that significant gains can be obtained using the proposed coding strategy especially where the conventional HARQ fails to offer throughput improvement even if the number of transmission rounds is increased.

Modern wireless systems deal with the adverse and unpredictable channel conditions using two main transmission schemes considered as complimentary: adaptive modulation and coding (AMC) and hybrid ARQ (HARQ). In this work we use the effective capacity as the performance measure to evaluate different design options. We thus show how to calculate this performance measure under independent, identically distributed (i.i.d.) block-fading channel model considering AMC, HARQ and more importantly, a combination of thereof. Numerical results indicate that AMC alone outperforms the other schemes in the region of high average SNR; we provide an explanation for this observation and discuss possible paths for improvement.

In this work, various channel coding schemes that can be used in hybrid automatic repeat request (HARQ) transmission protocols are investigated from an energy efficiency point of view. Conventional HARQ, where only one bit is used to inform the transmitter about the decoding success or failure, is compared to adaptive HARQ where the transmitter adapts either the length or the transmit power of the codewords using outdated channel state information (i.e., experienced by the receiver during the past transmissions). Describing the problems within a Markov decision process framework, we find optimal adaptation policies for both persistent (unlimited number of transmission) and truncated HARQ protocols. Numerical examples obtained in a Rayleigh block fading channel show that, in terms of energy efficiency, the adaptation of the codewords length provides notable gains over power adaptation and conventional HARQ.

In this paper, we focus on the analysis of the end-to-end outage probability of hybrid automatic repeat request (HARQ) in a multi-hop, block-fading channel. First, we use a Markov chain to model the multi-hop system with truncated HARQ type-I (HARQ-I) and derive closed-form expressions for the outage probability and throughput; we also obtain lower and upper bounds on the energy efficiency of the system. Then, we extend the analysis to cover HARQ with packet combining where we provide a general formulation of the outage probability; in special cases, closed-form expressions for outage probability and throughput are obtained. Finally, for a multi-hop system with opportunistic routing, we present recursive formulas to obtain outage probability and throughput. The expressions we derived allow us to draw conclusions about the outage diversity of different multi-hop protocols. In particular, we show that the diversity order is improved for opportunistic multi-hop system and is linearly increasing with the number of allowed transmissions for each packet. Numerical results illustrate how the analytical expression can be used to compare various multi-hop HARQ transmission strategies.

In this work, we consider incremental redundancy (IR) hybrid automatic repeat request (HARQ), where the transmission rounds are carried over independent block-fading channels. We analyze the multipacket HARQ, where the transmitter allows two different packets to share the same channel block; this stands in contrast with the conventional HARQ, where each packet occupies the entire block. We then optimally assign resources within the block to each packet and in each transmission round. We study superposition coding and time-sharing encoding strategies, and we optimize their parameters to maximize the throughput. Besides the conventional, one-bit (ACK/NACK) signaling we also consider a multibit feedback. We formulate our problem as a Markov decision process (MDP), where the decisions concerning the encoding strategies and their parameters are taken using additional information obtained from the receiver via feedback channel. In the case of the one-bit (ACK/NACK) signaling, the partial state information Markov decision process (PSI-MDP) framework is used to obtain the optimal policies. Numerical examples obtained in a Rayleigh-fading channel indicate that, the proposed multipacket HARQ outperforms the conventional one, by more than 5 dB for high-spectral efficiencies.

In this work, we analyze hybrid ARQ (HARQ) transmission over the independent block fading channel. We consider two scenarios with respect to the message sent by the receiver via the feedback channel: i) “conventional”, one-bit feedback used to inform the transmitter about the decoding success/failure (ACK/NACK), and ii) the multi-bit feedback message, where on top of ACK/NACK, the transmitter is provided with additional information about the state of the receiver. To minimize the outage probability under long-term average and peak power constraints, we cast the problems into the dynamic programming (DP) framework and solve them for Nakagami-m fading channels. An approximate, closed-form solution for the high signal-to-noise ratio (SNR) regime is proposed using geometric programming (GP). The obtained results quantify the advantage of the multi-bit feedback over the conventional one-bit approach, and show that the power optimization can provide significant gains over conventional power-constant HARQ transmissions even in the presence of peak-power constraints.

End-to-end outage probability and end-to-end throughput are analyzed in a multi-hop channel with truncated automatic repeat request type-I (ARQ-I). The system is modelled as a Markov chain and the closed-form expressions for the outage and throughput are derived for the cases of variable and equal distances between the nodes. The limits on the performance in high SNR and large number of transmissions are derived. We show that the diversity order is linearly increasing with the number of allowed transmission for each packet.

In this work we consider incremental redundancy hybrid ARQ (HARQ) transmission of data packets over independent block-fading channels. In HARQ, the transmitter, upon request (NACK), resends the "NACKed" packet in the next available block. Conventionally, the original and retransmitted packets occupy one block, each. In our approach, we allow the transmitter to share the resources between the new and the retransmitted packets. To implement such a multi-packet transmission we consider the superposition coding and the timesharing modulation as possible encoding schemes for simultaneous transmission of both packets. Targeting the maximization of the throughput, we cast the problem into the framework of Markov decision process (MDP), where the decisions about the optimal transmission strategy are taken using information about the state of the receiver in the previous transmissions. Numerical examples obtained in a Rayleigh block-fading channel show that proposed transmission strategy provides notable gains over a conventional HARQ.

In this work, we analyze hybrid ARQ (HARQ) protocols used over independent block fading channels. We assume that the transmitter is unaware of the channel state information (CSI) at the moment of transmission but knows the statistics of the channel. We consider two scenarios with respect to the feedback received by the transmitter: (i) “conventional”, one-bit feedback carrying the message about the decoding success/failure (ACK/NACK), and (ii) the multi-bit feedback scheme when, on top of ACK/NACK, the receiver feeds back information about the state of the decoder, which is equivalent to the CSI experienced by the receiver in the past (unsuccessful) transmissions. The feedback can then be used to allocate power across the HARQ transmission attempts (in the case of one-bit feedback) or adapt the power for each re-transmission as a function of multi-bit feedback. The objective in both cases is the minimization of the outage probability under peak and long term average power constraints. We solve the optimization problems using the dynamic programming framework. The obtained results quantify the advantage of the multi-bit feedback over the conventional approach and indicate that power optimization can provide significant gains over the conventional constant-power HARQ transmission even in the presence of peak-power constraints.

We analyze HARQ transmission over a fading channel with the help of a relay node. In the absence of the channel state information (CSI) at the transmitters, we establish the problem of maximizing the overall throughput of HARQ transmission by optimizing the variable transmission rate. We present a closed form of the throughput with respect to the transmission rates and use the well-known Dynamic Programming technique along with the necessary simplifications on the problem to find the optimal set of transmission rates for both the Source node and the Relay node in source-relay-destination scenario. We show that such an approach yields a significant increase in throughput compared to fixed-rate HARQ.

We analyze the average throughput and the outage probability for relay-based incremental redundancy HARQ transmission over block fading channel. We focus on the effects of having an error-free multi-bit feedback channel instead of the single-bit ACK/NACK feedback used in conventional HARQ. This multi-bit feedback message provides the cooperating nodes with outdated channel state information (CSI) so that they can adapt their transmission rate. We discuss a network with M relay nodes and assume adaptive transmission rate for all the cooperating nodes. We describe the adaptation problem as a Markov Decision Process (MDP) and employ the Dynamic Programming (DP) for optimization. The numerical results obtained in the case of one relay transmission, indicate that significant throughput gains can be obtained when compared to non-adaptive, i.e, fixed-rate HARQ. Moreover, we study the performance limits of the system model by finding an upper bound of the throughput for the cooperative HARQ channel which is applicable to both adaptive and non-adaptive models. Finally, we analyze the discretization issues and conclude that only a small number of feedback bits is required by the adaptive system to outperform the conventional single-bit HARQ.

In this paper we analyze the maximum achievable throughput of Hybrid ARQ (HARQ) protocol for transmission over cooperative block-fading channels. It is assumed that the instantaneous channel state information (CSI) is not available at the encoders and only its outdated version is available via the error-free feedback channel. The outdated CSI is then used to adapt the transmission rate of all cooperating nodes. Considering source-relay-destination scenario, we use Dynamic Programming (DP) to find the optimal rate adaptation policy for both the Source node and the Relay node. Such an approach yields a significant increase in throughput compared to non-adaptive fixed-rate HARQ. Moreover, the maximum achievable throughput is shown for the cooperative HARQ channel which is applicable to both adaptive and non-adaptive models.

In this work we consider the front-end receivers for flat fading MIMO transmission, whose essential feature is the calculation of the log-likelihood ratios (LLRs) for the transmitted bits. When the number of transmit antennas and the modulation size grow, the exact calculation of the LLRs is most often unfeasible due to the necessity to enumerate the symbols affecting the output. In this work, to avoid the burden of explicit enumeration, instead of calculating the likelihoods exactly, we explore the possibilities offered by the saddlepoint approximation of the likelihood functions. The resulting fixed-complexity approach significantly outperforms the well known linear-filter based minimum mean square error detection.

This paper is dedicated to the study of HARQ protocols under a secrecy constraint. An encoder sends information to a legitimate decoder while keeping it secret from the eavesdropper. Our objective is to provide a coding scheme that satisfies both reliability and confidentiality conditions. This problem has been investigated in the literature using a coding scheme that involves a unique secrecy parameter. The uniqueness of this parameter is sub-optimal for the throughput criteria and we propose a new coding scheme that introduces additional degrees of freedom. Our code involves Secrecy Adaptation and Rate Adaptation and we called it SARA-code. The first contribution is to prove that the SARA-code has small error probability and small information leakage rate. The second contribution is to show, over a numerical example, that the SARA-code improves the secrecy throughput.

In this work, we consider the front-end receivers for flat fading MIMO transmission, whose essential feature is the calculation of the log-likelihood ratios (LLRs) for the transmitted bits. When the number of transmit antennas and the modulation size grow, the exact calculation of the LLRs becomes unfeasible due to the necessity to enumerate the symbols affecting the output. In this work, to avoid the burden of explicit enumeration, instead of calculating the likelihoods exactly, we explore the possibilities offered by the saddlepoint approximation of the likelihood functions. The resulting fixed-complexity approach significantly outperforms the well known linear-filter based minimum mean square error detection. A complexity analysis identifies the main computational bottleneck of the proposed detection algorithm.

This paper considers incremental redundancy hybrid ARQ (HARQ) transmission over independent block-fading channels. The transmitter, having no knowledge of the instantaneous channel state information (CSI) can or - allocate the transmission rate knowing the statistics of the channel, or - adapt the transmissions rates using the outdated CSI, i.e., the one experienced by the receiver in the past transmissions that resulted in a packet decoding failure. Aiming at throughput maximization problems under constraint on the outage probability, we show how to optimize the rate-adaptation and rate-allocation policies using dynamic programming framework. Numerical examples obtained in a Rayleigh-fading channel show that rate adaptation provides notable gains over a rate allocation and non-adaptive HARQ, and, for high SNR, only a few transmissions are necessary to approach closely the ergodic capacity.

In this work we analyze the problem of linear correction of the reliability metrics (L-values) in bit-interleaved coded modulation (BICM) receivers. We want to find the correction factors that minimize the probability of error of a maximum likelihood decoder that uses the corrected L-values. To this end, we use the efficient approximation of the pairwise error probability in the domain of the cumulant generating functions (CGF) of the L-values and conclude that the optimal correction factors are equal to the twice of the saddlepoint of the CGF. We provide a simple numerical example of transmission in the presence of interference where we demonstrate a notable improvement attainable with the proposed method. The proposed method is compared with the one based on the maximization of generalized mutual information.

In this work we analyze the problem of correction of the reliability metrics (L-values) in bit-interleaved coded modulation (BICM) receivers. First, we propose a method for finding the linear correction factors that minimize the probability of error of a maximum likelihood decoder that uses the corrected L-values. To this end, we use the efficient approximation of the pairwise error probability in the domain of the cumulant generating functions (CGF) of the L-values and conclude that the optimal correction factors are equal to the twice of value of the saddlepoint of the CGF. Next, a simple extension of the proposed method to the non-linear correction is presented. We provide numerical examples demonstrating improvements attainable with the proposed method comparing it to the competitive solutions.

In this paper, we propose a new BICM design which considers hierarchical (nonequally spaced) constellations, a bit-level multiplexer, and multiple interleavers. It is shown that this new scheme outperform previous BICM designs because it exploits the temporal structure of the coded sequence. An analytical bound on the bit error rate (BER) of the proposed BICM scheme in terms of the constellation parameters and the multiplexing is developed for Nakagami-m fading channels. This bound is used to design a BICM transceiver with improved BER performance. Numerical results show that the gains compared to conventional BICM designs depend on the fading parameter and reach 2 dB for a target BER of 10-7and m=5.

We analyze the throughout achievable in transmission over block-fading channels when the instantaneous channel state information (CSI) is not available at the transmitter. Assuming operation with incremental redundancy hybrid ARQ (HARQ), we propose to adapt the transmissions rates using the outdated CSI, i.e., the one experienced by the receiver in the past transmissions that resulted in a packet decoding failure. We show that, even if the CSI is fully outdated, i.e., in independently block-fading channel, the adaptation provides notable gains over a non-adaptive HARQ and for high SNR, a few transmissions are necessary to approach closely the ergodic capacity.

Recent results have shown that the performance of bit-interleaved coded modulation (BICM) using convolutional codes in nonfading channels can be greatly improved if the bit-level interleaver takes a trivial form (BICM-T), i.e., if it does not interleave the bits at all. The reported gains reach a few decibels and are obtained using a less complex BICM system. In this paper, we give a formal explanation for these results and show that BICM-T is in fact the combination of a TCM transmitter and a BICM receiver. Analytical bounds that predict the performance of BICM-T are developed and a new type of distance spectrum for the convolutional code is introduced.

Recent results have shown that the performance of bit-interleaved coded modulation (BICM) using convolutional codes in nonfading channels can be significantly improved when the interleaver takes a trivial form (BICM-T), i.e., when it does not interleave the bits at all. In this paper, we give a formal explanation for these results and show that BICM-T is, in fact, the combination of a TCM transmitter and a BICM receiver. To predict the performance of BICM-T, a new type of distance spectrum for convolutional codes is introduced, analytical bounds based on this spectrum are developed, and asymptotic approximations are presented. It is shown that the free Hamming distance of the code is not the relevant optimization criterion for BICM-T. Asymptotically optimal convolutional codes for different constraint lengths are tabulated and BICM-T is shown to offer asymptotic gains of about 2 dB over traditional BICM designs based on random interleavers. The asymptotic gains over uncoded transmission are found to be the same as those obtained by Ungerboeck's one-dimensional trellis-coded modulation (1D-TCM), and therefore, in nonfading channels, BICM-T is shown to be as good as 1D-TCM.

We propose a simple relay-based adaptive cooperation scheme to improve the spectral efficiency of "Detect-and-Forward" (DetF) half-duplex relaying in fading channels. In a new common framework, we show that the proposed scheme offers considerable gains-in terms of the achievable information rates-compared to conventional DetF relaying schemes for both orthogonal and non-orthogonal source/relay transmissions. The analysis leads on to a general adaptive cooperation strategy based on the maximization of information rates at the destination which needs to observe only the average signal-to-noise ratios of the links.

Bit-interleaved coded modulation (BICM) transceivers often use equally spaced constellations and a random interleaver. In this paper, we propose a new BICM design, which considers hierarchical (nonequally spaced) constellations, a bit-level multiplexer, and multiple interleavers. It is shown that this new scheme increases the degrees of freedom that can be exploited in order to improve its performance. Analytical bounds on the bit error rate (BER) of the system in terms of the constellation parameters and the multiplexing rules are developed for the additive white Gaussian Noise (AWGN) and Nakagami-m fading channels. These bounds are then used to design the BICM transceiver. Numerical results show that, compared to conventional BICM designs, and for a target BER of 10^{-6}, gains up to 3 dB in the AWGN channel are obtained. For fading channels, the gains depend on the fading parameter, and reach 2 dB for a target BER of 10^{-7} and m=5.

In this paper, we analyze the impact of carrier frequency offset (CFO) on the performance of orthogonal frequency division multiplexing (OFDM) transmission employing space-frequency coding over relay channels. The challenge in such systems lies in the difficulty of canceling the interference resulting from the different CFOs that correspond to the relays involved in the transmission. We first analyze the CFO correction schemes and examine their impact on the achievable information rates. Further, we analyze the interference cancellation (IC) technique based on the so-called turbo-principle, that is, which jointly detects and decodes the received data. The increase of the rates achievable thanks to IC is assessed via parametric description of the iterative process. We provide examples that demonstrate the efficacy of the proposed scheme and numerical results are contrasted with theoretical performance limits.

We analyze the throughput achievable by a truncated Hybrid ARQ protocol (HARQ) using incremental redundancy (IR) when transmitting over a block-fading channel whose state is unknown at the transmit end. We allow the transmission rates to vary, optimize them, and show that such a variable-rate HARQ-IR provides gains with respect to a fixed-rate transmission in terms of increased throughput and decreased average number of transmissions.

In this paper, we analyze the coded performance of a cooperative system with multiple parallel relays using "Detect-and-Forward'' (DetF) strategy where each relay demodulates the overheard signal and forwards the detected binary words. The proposed method is based on the probabilistic characterization of the reliability metrics given under the form of L-values. First, we derive analytical expressions of the probability density functions (PDFs) of the L- values in the elementary two-hop DetF relay channel with different source-relay channel state information assumptions. Then, we apply the obtained expressions to derive tight approximations for the end-to-end coded bit error rate (BER) of a general cooperative scheme with multiple parallel relays. Simulation results demonstrate the accuracy of our derivations for different cooperation configurations and conditions.

To improve the spectral efficiency of Detect-and-Forward (DetF) half-duplex relaying in fading channels, we propose a cooperation scheme where the relay uses a modulation whose order is higher than the one at the source. In a new common framework, we show that the proposed scheme offers considerable gains-in terms of achievable information rates-compared to the conventional DetF relaying schemes for both orthogonal and non-orthogonal source/relay cooperation. This allows us to propose an adaptive cooperation scheme based on the maximization of the information rate at the destination which needs to observe only the average signal-to-noise ratios of direct and relaying links.

In this paper, the rate achievable in Multiple-Input-Multiple-Output (MIMO) systems with iterative receivers based on linear front-end (FE) processing is investigated. First, the communication with Gaussian input signal is assumed and the Extrinsic Information Transfer (EXIT) chart is applied to evaluate the achievable sum rate. Then, the method for deriving the EXIT chart for a more practical case has been introduced. As a specific model, which fits the real turbo receivers better, communication with large size uniform constellations is discussed where the information being exchanged between the receiver's iterative block are Log-Likelihood-Ratios (LLRs) of the transmitted bits. It is shown that, in this situation, the iterative process does not improve the performance from achievable sum rate point of view in both high and low SNR regimes. However, the iterative process is shown to help in the medium SNR range, which is the range of interest.

In this paper we study bit-interleaved coded modulation (BICM) transmission using non-uniform (NU) quadrature amplitude modulation (QAM) constellations. For such a NUQAM-BICM transmission, we develop closed-form approximations for the probability density function of the L-values, and we use them to predict the coded bit error rate (BER)performance of the system in the AWGN channel. We then numerically optimize NU-QAM-BICM based on convolutional codes. Compared to uniform QAM-based BICM transmission, and for a target BER of 10-7, we reach gains up to 1 dB. When the design is applied to turbo codes a decrease in the error floor can be obtained.

In this paper we formally analyze the interleaver and code design for QAM-based BICM transmissions using the binary reflected Gray code. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds the optimum design of interleaver and code is found, and numerical results for representative configurations are presented. When the new design is used, the improvements may reach 2 dB, and they are obtained without any increase on the transceiver's complexity. We also introduce the concept of generalized optimum distance spectrum convolutional codes, which are the optimum codes for QAM-based BICM transmissions.

In this paper, we analyze the coded performance of a cooperative system with multiple parallel relays using “Detect-and-Forward” (DetF) strategy where each relay demodulates the overheard signal and forwards the detected binary words. The proposed method is based on the probabilistic characterization of the reliability metrics given under the form of L-values. First, we derive analytical expressions of the probability density functions (PDFs) of the L-values in the elementary two-hop DetF relay channel with different source-relay channel state information assumptions. Then, we apply the obtained expressions to calculate the theoretically achievable rates and compare them with the practical throughput of a simulated turbo-coded transmission. Next, we derive tight approximations for the end-to-end coded bit error rate (BER) of a general cooperative scheme with multiple parallel relays. Simulation results demonstrate the accuracy of our derivations for different cooperation configurations and conditions.

In this paper, we propose a Detect-and-Forward (DetF) relaying scheme for coded communications over fading channels based on the transmission of partial channel state information from the relay to the destination. In order to assist the detection at the destination, the relay transmits a quantized source-relay signal-to-noise ratio and the quantization criterion is based on the minimization of the estimated symbols transition probabilities at the relay. We show that the overhead necessary to communicate the quantized information between the relay and the destination is negligible for practical transmission rates and fading speeds. The efficiency of the proposed scheme is demonstrated through constrained capacity analysis along with throughput simulations based on practical coding schemes.

In this paper we present a general methodology for the interleaver and code design for QAM-based BICM transmissions. We develop analytical bounds on the bit error rate and we use them to predict the performance of BICM when unequal error protection (UEP) is introduced by the constellation labeling. Based on these bounds, the optimum design of interleaver and code is presented. The improvements obtained reached 2 dB for the analyzed cases, and are obtained without complexity increase. Although previous works noted the influence of the interleaver design and the UEP, to the best of our knowledge, this paper is the first to analyze formally this problem for BICM transmissions.

In this paper the issue of improving the performance of iterative decoders based on sub-optimal calculation of the messages exchanged during iterations (L-values) is addressed. It is well known in the literature that a simple-yet very effective-way to improve the performance of suboptimal iterative decoders is based on applying a scaling factor to the L-values. In this paper, starting with a theoretical model based on the so-called consistency condition of a random variable, we propose a methodology for correcting the L-values that relies only on the distribution of the soft information exchanged in the iterative process. This methodology gives a clear explanation of why the well-known linear scaling factor provides a very good performance. Additionally, the proposed methodology allows us to avoid the exhaustive search required otherwise. Numerical simulations show that for turbo codes the scaling factors found closely follow the optimum values, which translates to a close-to-optimal BER performance. Moreover, for LDPC codes, the proposed methodology produces a better BER performance compared with the known method in the literature.

In this paper we develop closed form approximations for the probability density function (PDF) of the reliability metrics (L-values) in bit-interleaved coded modulation (BICM). The expressions are valid for M2-ary quadrature amplitude modulations (M2-QAM) with binary reflected Gray mapping when the metrics are calculated using the so-called max-log approximation. Based on the developed expressions, we also propose two simple Gaussian mixture approximations that are analytically tractable. We apply our developments to efficiently calculate the BICM capacity, and to develop bounds on the coded bit-error rate when a convolutional code is used. The coded performance of a hybrid automatic repeat request (HARQ) based on constellation rearrangement is also evaluated.

In this letter we derive closed-form expressions for the probability density functions (PDFs) of the bits' reliability metrics (L-values) in bit-interleaved coded modulation (BICM) transmission over fully-interleaved fading channels. The expressions are valid for the relevant case of quadrature amplitude modulation (QAM) with Gray mapping when the metrics are calculated via the so-called max-log approximation. Using the developed expressions, the performance of coded BICM transmissions is efficiently evaluated, i.e., without resorting to otherwise required two-dimensional numerical integration. The BICM capacity for different fading channels and constellation sizes is also evaluated.

In this paper we consider hybrid automatic repeat request transmission based on binary phase shift keying modulation. Our objective is to improve the performance of the retransmissions, keeping at the same time the complexity and the performance of the first transmission unaltered. We conclude that the so-called mapping rearrangement (MR) may considerably improve the performance if multi-dimensional modulation is applied. We evaluate the theoretical limits of bit-interleaved coded modulation with MR, verify the functioning of the practical coding scheme, and propose to improve the performance via iterative detection-decoding (BICM-ID) which brings considerable gains. In particular, for high coding rates, a 4 dB SNR gain over transmission without MR may be achieved.

In this paper, we present space shift keying (SSK) as a new modulation scheme, which is based on spatial modulation (SM) concepts. Fading is exploited for multiple-input multiple-output(MIMO) channels to provide better performance over conventional amplitude/phase modulation (APM) techniques. In SSK, it is the antenna index used during transmission that relays information, rather than the transmitted symbols themselves. This absence of symbol information eliminates the transceiver elements necessary for APM transmission and detection (such as coherent detectors). As well, the simplicity involved in modulation reduces the detection complexity compared to that of SM, while achieving almost identical performance gains. Throughout the paper, we illustrate SSK's strength by studying its interaction with the fading channel. We obtain tight upper bounds on bit error probability, and discuss SSK's performance under some non-ideal channel conditions (estimation error and spatial correlation). Analytical and simulation results show performance gains over APM systems (3 dB at a bit error rate of 10-5), making SSK an interesting candidate for future wireless applications. We then extend SSK concepts to incorporate channel coding, where in particular, we consider a bit interleaved coded modulation (BICM) system using iterative decoding for both convolutional and turbo codes. Capacity results are derived, and improvements over APM are illustrated (up to 1 bits/s/Hz), with performance gains of up to 5 dB.

In this paper we consider hybrid automatic repeat request transmission based on binary phase shift keying modulation. Our objective is to improve the performance of the retransmissions, keeping at the same time the complexity and the performance of the first transmission unaltered. We conclude that the so-called mapping rearrangement (MR) may considerably improve the performance if multidimensional modulation is applied. We evaluate the theoretical limits of bit-interleaved coded modulation with MR, verify the functioning of the practical coding scheme, and propose to improve the performance via iterative detection-decoding (BICM-ID) which brings considerable gains. In particular, for high coding rates, a 4 dB SNR gain over transmission without MR may be achieved.

In this work, we propose and analyze coded modulation (CM) schemes suitable for mapping rearrangement (MaRe) in Hybrid ARQ (HARQ). Two known MaRe schemes for 16-ary quadrature amplitude modulation (16-QAM) are first studied. We demonstrate that the performance of the studied CM receiver is intrinsically limited by the MaRe design and we propose iterative detection to improve the performance. Further, we propose to change the design of the mapping to take into account the property of the CM receiver and we obtain well appreciable gains over other detection schemes and known mapping designs.

A fundamental component of spatial modulation (SM), termed generalized space shift keying (GSSK), is presented. GSSK modulation inherently exploits fading in wireless communication to provide better performance over conventional amplitude/phase modulation (APM) techniques. In GSSK, only the antenna indices, and not the symbols themselves (as in the case of SM and APM), relay information. We exploit GSSKpsilas degrees of freedom to achieve better performance, which is done by formulating its constellation in an optimal manner. To support our results, we also derive upper bounds on GSSKpsilas bit error probability, where the source of GSSKpsilas strength is made clear. Analytical and simulation results show performance gains (1.5-3 dB) over popular multiple antenna APM systems (including Bell Laboratories layered space time (BLAST) and maximum ratio combining (MRC) schemes), making GSSK an excellent candidate for future wireless applications.

In the popular bit-interleaved coded modulation (BICM) the output of the channel encoder and the input of the modulator are separated by a bit-level interleaver. From the decoder's point of view, the modulator, the transmission channel, and the demodulator (calculating bits' reliability metrics) become a memoryless BICM channel with binary inputs and real outputs. In unfaded channels, the BICM channel's outputs (reliability metrics) are known to be Gaussian for binary- or quaternary phase shift keying but their probability density function (PDF) is not known for higher-order modulation. We fill this gap by presenting an algorithmic method to calculate closed-form expressions for the PDF of reliability metrics in BICM with arbitrary modulation and bits-to-symbol mapping when the so-called max-log approximation is applied. Such probabilistic description of BICM channel is useful to analyze, from an information- theoretic point of view, any BICM constellation/mapping design.

Abstract In this paper, we propose a method for evaluation of the bit error rate (BER) for packet combining based on constellation rearrangement (CoRe). Such mapping diversity scheme, adopted in the high speed downlink packet access (HSDPA), uses Gray‐mapped constellations and is based on suboptimal accumulation of the reliability metrics generated in each of the transmissions. We present an exact model for the logarithmic likelihood ratios (LLR) obtained by means of the so‐called max‐log approximation, and we show that their conditional probability density functions (pdf) are piecewise Gaussian. We then present the derivation of the uncoded BER and illustrate it with simulation results that confirm our formulation. Finally, we propose simplifications which significantly reduce the complexity of the evaluation method and provide results with a very good accuracy; an extension to transmissions over faded channel is also presented. Copyright © 2007 John Wiley & Sons, Ltd.

Abstract Hybrid ARQ with packet combining for high‐order modulations (such as 16‐QAM) may be significantly enhanced if the bits‐to‐symbols mappings are appropriately changed throughout the transmissions. In this paper, we analyze the relationship between such mapping diversity and channel coding. We calculate the capacity of the popular bit‐interleaved‐coded modulation (BICM) to draw qualitative and approximate quantitative conclusions that are valid for strong codes approaching the capacity limits. We conclude that the choice/design of the appropriate mapping depends on the targeted spectral efficiency and we demonstrate that certain forms of mapping diversity may be counterproductive. We also show that iterative demapping may be successfully applied to significantly reduce (by more than 1 dB) the gap between the BICM and coded modulations (CM) capacities. The analysis is illustrated with results obtained when the mapping diversity is combined with practical turbo codes. Copyright © 2007 John Wiley & Sons, Ltd.

Abstract We present closed‐form expressions for the bit error rate (BER) of rectangular quadrature amplitude modulation (QAM) in Nakagami‐mfading channels. The presented formulas, which are valid for arbitrary bits‐to‐symbols mapping, thus may be used when non‐Gray mapping is employed, are particularly useful in the low signal‐to‐noise ratio (SNR) range and/or for small values of the parameterm. The advantage of the proposed expressions over the known bounding techniques is illustrated through numerical simulations. Copyright © 2006 John Wiley & Sons, Ltd.

In this paper we develop formulas for the probability density function (PDF) of the reliability metrics in bit- interleaved coded modulation (BICM) for arbitrary M2-ary quadrature amplitude modulation (M2-QAM) with Gray mapping. The advantage of this approach over an entirely general one proposed previously, is that the resulting formulas are obtained without resorting to any algorithmic steps. We also propose Gaussian mixture approximations that are analytically tractable, and we analyze their accuracy when evaluating uncoded bit error rate (BER) and BICM capacity.

Hybrid ARQ with packet combining for high-order modulations (such as 16-QAM) may be significantly enhanced if the bits-to-symbols mappings are appropriately changed throughout the transmissions. In this paper, we analyze the relationship between such mapping rearrangement (or mapping diversity) and channel coding. We calculate the capacity of the popular bit-interleaved coded modulation (BICM) to draw qualitative and approximate quantitative conclusions that are valid for strong codes approaching the capacity limits. We conclude that the choice/design of the appropriate mapping depends on the targeted spectral efficiency and we demonstrate that certain forms of mapping rearrangement may be counterproductive. We also show that iterative demapping may be successfully applied to significantly reduce the gap between the BICM and coded modulations capacities. The analysis presented is illustrated with the results obtained when the mapping rearrangement is combined with practical turbo codes.

In bit interleaved coded modulation (BICM), the probability density function (PDF) of the reliability metrics (L- values) fully defines the BICM transmission and may be used to analyze it from an information-theoretic point of view. In this paper, the closed-form expressions for the PDF of the L-values in Rayleigh fading channels for 16-QAM with Gray mapping are derived and contrasted with histograms of the L-values obtained via numerical simulations. The obtained closed-forms show that the PDF is either a sum of exponentials or is bounded by such a sum.

In this paper we address the issue of probabilistic modelling of the extrinsic L-values, used as reliability metrics in the context of bit interleaved coded modulation with iterative demapping (BICM-ID). Starting with a simple piece-wise linear model of the L-values obtained via the max-log approximation, we derive the expressions for the cumulative distribution functions of the L-values, that differentiated produce the desired forms of the probability density functions for Gray-mapped 16-QAM. To illustrate the usefulness of our analytical expressions we applied them to efficiently compute the so-called EXIT functions of the demapper for different values of SNR. The proposed analytical expressions are also compared to thehistograms of the L-values obtained through time-consuming simulations.

We present a numerically efficient method to evaluate uncoded bit-error rate for arbitrary two-dimensional constellation and nonuniform signaling in fading channels. Compared to the bounding techniques, treating the problem at the same level of generality, the proposed method is shown to be more accurate (by construction) and numerically more efficient: for moderate constellation size, the complexity reduction may reach two orders of magnitude. The operation of the proposed method is illustrated with numerical examples obtained in Nakagami-m fading.

In this paper, we propose a method to evaluate analytically the bit error rate (BER) for packet combining with Gray mapping and constellation rearrangement (CoRe) between retransmissions - a mapping diversity scheme adopted in the high speed downlink packet access (HSDPA) standard. We first develop the model for the logarithmic likelihood ratios (LLR) and show that their conditional probability density functions (pdf) are piecewise Gaussian. We then present the derivation of the BER and illustrate it with simulation results that validate our formulation.

An analytical method to predict accurately the performance of iterative demapping in bit-interleaved coded modulation (BICM-ID) is presented. It is based on the analytical evaluation of the bit error rate at the output of the demapper and, in comparison with previously known methods (e.g., those based on EXIT charts), considerably reduces the need for simulations. The results obtained with the proposed method are compared with those obtained by EXIT analysis and through simulations.

Bit-interleaved coded modulation (BICM) is a coded-modulation scheme where the output of the channel encoder and the input of the modulator are separated by a bit- level interleaver. From the decoder's point of view, the modulator, the transmission channel, and the demodulator (calculating bits' reliability metrics) become a memoryless BICM channel with binary inputs and real outputs. The BICM channel's outputs (reliability metrics) are known to be Gaussian for binary-or quaternary phase shift keying but no expression for their probability density function (PDF) is known for higher-order modulation. We fill this gap presenting closed-form expressions for PDF of reliability metrics in BICM based on M-PSK with gray mapping. Such probabilistic description of BICM channel is necessary to calculate the channel's information-theoretic parameters such as capacity, or cut-off rate but it is also a tool required to analyze the performance of coded transmission.

In the popular bit-interleaved coded modulation (BICM) the output of the channel encoder and the input of the modulator are separated by a bit-level interleaver. From the decoder's point of view, the modulator, the transmission channel, and the demodulator (calculating bits' reliability metrics) become a memoryless BICM channel with binary inputs and real outputs. The BICM channel's outputs (reliability metrics) are known to be Gaussian for binary- or quaternary phase shift keying but no expression for their probability density function (PDF) is known for higher-order modulation. We fill this gap presenting an algorithmic method to calculate closed-form expressions for the PDF of reliability metrics in BICM with arbitrary modulation and bits-to-symbol mapping. Such probabilistic description of BICM channel provides a useful tool to analyze, from an information- theoretic point of view, any BICM constellation/mapping design.

We present a numerically efficient method to evaluate exactly uncoded bit error rate for transmission in fading channels. The presented method deals with arbitrary two-dimensional constellation, arbitrary bits-to-symbols mapping, and with non-uniform signaling. Compared to the bounding techniques, treating the problem at the same level of generality, the proposed method offers not only accuracy but also is numerically much more efficient: for moderate constellation size, the complexity reduction may reach two orders of magnitude. The operation of the proposed method is illustrated with numerical examples considering non-uniform signaling in transmission over Nakagami-mfading channel.

The use of generalized logarithmic likelihood ratio (GLLR), as a metric for soft-input decoding, was shown to provide gains over the conventional logarithmic likelihood ratio (LLR). However, computational complexity of the GLLR is greater than in the case of LLR, so the choice of GLLRs must be justified by a notable performance improvement. In this paper, we investigate the judiciousness of the adoption of GLLRs or LLRs for soft decoding analyzing the performance of both metrics in two different scenarios. Metrics expressions are derived and channel estimation errors are considered in analysis and simulation. Numerical results show that GLLRs outperform LLRs but, in general, the achieved improvement is relatively small. Therefore, application of GLLR metrics requires cautious analysis.

Adaptive modulation and coding (AMC) consists in adjusting the transmission parameters according to the channel state which affects the performance of the receiver. Practical implementation of the AMC is commonly done in scalar channels, where the signal to noise ratio characterizes sufficiently well the quality of the transmission. In this paper, we propose to use the AMC in matrix (MIMO) channels with the receiver employing the iterative (turbo) processing. Such solutions were not yet proposed in the literature due to the lack of appropriate performance evaluation tools. In the paper we employ the recently proposed method based on the so-called EXIT analysis, which was shown to predict accurately the performance of the turbo receivers. The numerical results obtained in the space-time bit-interleaved coded modulation MIMO transmission indicate that the throughput obtained exceeds considerably a non-adaptive transmission, and approaches the optimal water-filling solution

Exact evaluation of the bit- and symbol-error rates in a 2-D constellation has closed solutions for particular modulations and/or bits-to-symbol mapping. To solve this fundamental problem of digital communications, we propose a general method which yields the exact results for arbitrary modulation symbols' set, arbitrary bits-to-symbol mapping, and deals with the case of nonuniform signaling. These three conditions define any digital transmission using memoryless modulation, so the proposed method is a general tool solving all problems tackled in the literature, under constraints imposed on one or more of the parameters defining the modulation or signaling type. Such an evaluation tool is of practical importance during the design of the modulation. Our analysis and numerical simulations show the advantages offered by the new method when compared with the bounding techniques.

Exact evaluation of bit- and symbol-error rates in a 2-D constellation is a fundamental problem of digital communications, which only for particular modulations and/or bits-to-symbol mapping has closed-form solutions. Here, we propose a general, numerically efficient algorithmic method, which yields the exact results for arbitrary modulation symbols' set and arbitrary bits-to-symbol mapping, and which deals with the case of nonuniform signaling. These three conditions define any digital transmission using memoryless modulation, so the proposed method is a general tool solving all problems tackled in the literature under constraints imposed on one or more of the parameters defining the modulation or signaling type. Such an evaluation tool is of practical importance during the design of the modulation. Our analysis and numerical simulations show the advantages offered by the new method when compared with the bounding techniques

The exact closed-form expression for the bit-error rate (BER) of rectangular quadrature amplitude modulation (QAM) is given. The presented formula is independent of the bit mapping and it is thus particularly useful in the design and analysis of modulation schemes employing non-Gray mapping. Compared with the so-called expurgated bound and the union bound, our expression is shown to accurately predict the BER in the low signal-to-noise ratio range where the bounding techniques fail.

Exact evaluation of the bit- and symbol error rates in two dimensional constellation is a fundamental problem of digital communications which has closed-form solutions for particular modulations and/or bits-to-symbol mapping. In this paper we propose a general method which yields the exact results for arbitrary modulation constellation, arbitrary bits-to-symbol mapping and deals with the case of non-uniform signalling. These three conditions define any digital transmission using memoryless modulation thus, our method is a general tool solving all problems tackled in the literature under constraints imposed on one or more of the parameters defining the modulation or signalling type. Such evaluation tool is of practical importance during the design of the modulation. Through numerical simulations we illustrate the advantages offered by the new method when compared to the bounding technique.

In this paper we propose two algorithms for adaptation of linear and successive interference cancellation (SIC) MIMO receivers based on the MMSE criterion. The algorithms are compared to the so-called fast V-BLAST algorithm in terms of implementation simplicity and the required number of arithmetic operation. We conclude that both proposed algorithm offer advantages over the algorithm fast V-BLAST. When compared to the latter, the first proposed algorithm has much simpler implementation but the same arithmetic complexity, while the second proposed algorithm lowers by 33% the required number of arithmetic operations.

In this letter, an efficient algorithm to determine the optimal delay in linear finite impulse response equalizers based on the minimum mean-square-error criterion is proposed. The algorithm uses the Levinson-Durbin (L-D) recursion as a starting point to find the values of the mean-square error for equalizers with all nontrivial delays. Despite the exhaustive search approach, the complexity of the proposed algorithm is only doubled when compared to the calculation of the equalizer with one prescribed delay. Such increase in complexity may be fully justified in practice because it yields globally optimal equalizer's design.

Turbo receivers have gained significant importance in recent years, combating efficiently the effect of interference-limited propagation channels through the iterative exchange of information between the front-end receiver and the channel decoder. Such iterative process is difficult to describe in a closed form so the performance evaluation is usually done by means of extensive numerical simulations. We propose an analytical method allowing for performance evaluation of turbo receivers which may be used for system-level analysis or link adaptation. The proposed approach, based on the so-called EXIT analysis, allows for an accurate prediction of the receiver's coded bit- or block-error rates for each iteration as a function of the channel state. We illustrate the paper applying the proposed method to MIMO systems.

MIMO systems promise high spectral efficiency attainable if appropriate signal processing algorithms are used at the receiver. The paper compares several turbo-processing algorithms which may be used in narrow-band systems. The algorithms are studied in the common simulation setup for two transmitter classes defined as horizontal and vertical encoding. The coded BER and BLER are compared to their respective lower-bounds using numerical simulations.

Space-time processing is often used to eliminate intersymbol interference (ISI) and multiple access interference (MAI) which are main sources of transmission errors in communication systems. This paper carries out an analysis of applicability of the decision feedback equalizer (DFE) for multi-user detection in multiple antennas commercial TDMA standards: IS-136 and GSM/EDGE. The principal feature of the studied algorithm is a feedback connection among all detected users. The algorithm is implemented using indirect reduced-order adaptation and bootstrap processing. Its advantages and drawbacks are shown through simulations.

Turbo equalization (TEQ) is an iterative technique which lowers the transmission error rates allowing the equalizer and the decoder to exchange information. The objective of this work is to answer the question if turbo equalization may offer significant improvement and at what cost, in the EDGE systems. The simplified SISO MAP equalizer and decoder are used and various schemes of frame and block data processing are analyzed. Conclusions are drawn from simulations based on definition of the TCH/F43.2 EDGE channel.