In orthogonal frequency-division multiplexing (OFDM)-based systems, such as 5 G, precise channel estimation (CE) is crucial for reliable connectivity and high data rates. However, in 5G, the sub-band-specific precoding structure of pilot signals, known as demodulation reference signals (DMRS), limits the number of pilot samples per sub-band, which can lead to significant CE errors. This paper provides an in-depth analysis of the influence of CE errors on pilot and data subcarriers in OFDM systems. We rigorously demonstrate that CE errors on data subcarriers can be modeled as independent additive noise, validating assumptions made in previous works but not formally proven. Additionally, we analyze how CE errors on pilot subcarriers introduce bias in noise variance estimation and derive closed-form expressions for this bias under both perfect and imperfect MMSE CE conditions. We propose practical bias removal schemes tailored for residual power-based noise variance estimation. Numerical simulations confirm the accuracy of our theoretical findings and show that the proposed noise variance estimation method significantly reduces the performance gap between narrow-band and wide-band CE.

In simultaneous transmitting and receiving systems, passive RF components can cause non-linear distortion products that lead to critical self-interference (SI) in the received signal. Adaptive digital filters are an effective approach to cancel this SI. However, when the source of SI involves multiple transmitters (TXs) with different frequency responses, the number of taps required for estimation increases significantly, leading to highly complex estimation methods. In this paper, we propose a computationally efficient digital cancellation solution that is shown to have substantially lower complexity compared to reference methods, while efficiently canceling intermodulation distortion (IMD) under a comprehensive modeling of the interference induced by two unrelated transmit signals. The simulation results demonstrate that the proposed method enables the cancellation of up to 12 dB of signal interference, while simultaneously reducing complexity by 90% compared to conventional full-complexity solutions.

Recently, LTE Release 12 and IEEE 802.11ac adopted 256-quadrature amplitude modulation (QAM) as a tool to accommodate high demand of data throughput in wireless systems. Furthermore, currently developing IEEE 802.11ax considers 1024-QAM to further increase the Wi-Fi throughput. In this paper, we propose low complexity schemes in soft detection of high order QAM symbols in multiple input multiple output (MIMO) channels with or w/o prior information (from decoder output). We focus on 256-QAM with two transmitted layer MIMO with the understanding that the scheme can be generalized to higher order modulation and higher rank MIMO channels. We propose an initial candidate reduction (ICR) scheme with 128 initial candidates (refer to the scheme as ICR-128) to reduce the number of Euclidean distance (ED) calculations by half in comparison with soft maximum likelihood (ML) detection and as a result reduce the total hardware size by almost 50%. For initial candidate set selection we rely on linear minimum mean square error (MMSE) detection. The main advantage of the proposed scheme is in initial candidate set selection where we only need to detect I and Q sign of the target layer which simplifies MMSE detection. We further investigate the use of prior information in initial candidate set selection with MMSE soft interference cancellation (MMSE-SIC) which uses prior information in case of iterative detection and decoding (IDD) and/or hybrid automatic re- transmission request (HARQ) re-transmissions. To avoid computational complexity of MMSE-SIC, we propose a low complexity initial detection with prior information.

We study memoryless channels with synchronization errors as defined by a stochastic channel matrix allowing for symbol drop-outs or symbol insertions with particular emphasis on the binary and non-binary deletion channels. We offer a different look at these channels by considering equivalent models by fragmenting the input sequence where different subsequences travel through different channels. The resulting output symbols are combined appropriately to come up with an equivalent input–output representation of the original channel which allows for derivation of new upper bounds on the channel capacity. We consider both random and deterministic types of fragmentation processes applied to binary and nonbinary deletion channels. With two specific applications of this idea, a random fragmentation applied to a binary deletion channel and a deterministic fragmentation process applied to a nonbinary deletion channel, we prove certain inequality relations among the capacities of the original channels and those of the introduced subchannels. The resulting inequalities prove useful in deriving tighter capacity upper bounds for: 1) independent identically distributed (i.i.d.) deletion channels when the deletion probability exceeds 0.65 and 2) nonbinary deletion channels. Some extensions of these results, for instance, to the case of deletion/substitution channels are also explored.

A relay communication system with two amplify and forward (AF) relays under flat fading channel conditions is considered where the signals received from the relays are not necessarily time aligned. We propose a new time-reversal (TR)-based scheme providing an Alamouti code structure which needs a smaller overhead in transmitting every pair of data blocks in comparison with the existing schemes and, as a result, increases the transmission rate significantly (as much as 20%) in exchange for a small performance loss. The scheme is particularly useful when the delay between the two relay signals is large, e.g., in typical underwater acoustic (UWA) channels.

We develop several lower bounds on the capacity of binary input symmetric output channels with synchronization errors, which also suffer from other types of impairments such as substitutions, erasures, additive white Gaussian noise (AWGN), etc. More precisely, we show that if a channel suffering from synchronization errors as well as other type of impairments can be decomposed into a cascade of two component channels where the first one is another channel with synchronization errors and the second one is a memoryless channel (with no synchronization errors), a lower bound on the capacity of the original channel in terms of the capacity of the component synchronization error channel can be derived. A primary application of our results is that we can employ any lower bound derived on the capacity of the component synchronization error channel to find lower bounds on the capacity of the (original) noisy channel with synchronization errors. We apply the general ideas to several specific classes of channels such as synchronization error channels with erasures and substitutions, with symmetric q-ary outputs and with AWGN explicitly, and obtain easy-to-compute bounds. We illustrate that, with our approach, it is possible to derive tighter capacity lower bounds compared to the currently available bounds in the literature for certain classes of channels, e.g., deletion/substitution channels and deletion/AWGN channels (for certain signal-to-noise ratio (SNR) ranges).

In cooperative UWA systems, due to the low speed of sound, a node can experience significant time delays among the signals received from geographically separated nodes. One way to combat the asynchronism issues is to employ orthogonal frequency division multiplexing (OFDM)-based transmissions at the source node by preceding every OFDM block with an extremely long cyclic prefix (CP) which reduces the transmission rates dramatically. One may increase the OFDM block length accordingly to compensate for the rate loss which also degrades the performance due to the significantly time-varying nature of UWA channels. In this paper, we develop a new OFDM-based scheme to combat the asynchronism problem in cooperative UWA systems without adding a long CP (in the order of the long relative delays) at the transmitter. By adding a much more manageable (short) CP at the source, we obtain a delay diversity structure at the destination for effective processing and exploitation of spatial diversity by utilizing a low complexity Viterbi decoder at the destination, e.g., for a binary phase shift keying (BPSK) modulated system, we need a two-state Viterbi decoder. We provide pairwise error probability (PEP) analysis of the system for both time-invariant and block fading channels showing that the system achieves full spatial diversity. We find through extensive simulations that the proposed scheme offers a significantly improved error rate performance for time-varying channels (typical in UWA communications) compared to the existing approaches.

We derive an upper bound on the capacity of non-binary deletion channels. Although binary deletion channels have received significant attention over the years, and many upper and lower bounds on their capacity have been derived, such studies for the non-binary case are largely missing. The state of the art is the following: as a trivial upper bound, capacity of an erasure channel with the same input alphabet as the deletion channel can be used, and as a lower bound the results by Diggavi and Grossglauser in [1] are available. In this paper, we derive the first non-trivial non-binary deletion channel capacity upper bound and reduce the gap with the existing achievable rates. To derive the results we first prove an inequality between the capacity of a 2K-ary deletion channel with deletion probability d, denoted by C2K(d), and the capacity of the binary deletion channel with the same deletion probability, C2(d), that is, C2K(d) ≤ C2(d)+(1-d) log(K). Then by employing some existing upper bounds on the capacity of the binary deletion channel, we obtain upper bounds on the capacity of the 2K-ary deletion channel. We illustrate via examples the use of the new bounds and discuss their asymptotic behavior as d → 0.

We develop several analytical lower bounds on the capacity of binary insertion and deletion channels by considering independent uniformly distributed (i.u.d.) inputs and computing lower bounds on the mutual information between the input and output sequences. For the deletion channel, we consider two different models: i.i.d. deletion–substitution channel and i.i.d. deletion channel with additive white Gaussian noise (AWGN). These two models are considered to incorporate effects of the channel noise along with the synchronization errors. For the insertion channel case, we consider Gallager's model in which the transmitted bits are replaced with two random bits and uniform over the four possibilities independently of any other insertion events. The general approach taken is similar in all cases, however the specific computations differ. Furthermore, the approach yields a useful lower bound on the capacity for a wide range of deletion probabilities of the deletion channels, while it provides a beneficial bound only for small insertion probabilities (less than 0.25) of the insertion model adopted. We emphasize the importance of these results by noting that: 1) our results are the first analytical bounds on the capacity of deletion-AWGN channels, 2) the results developed are the best available analytical lower bounds on the deletion–substitution case, 3) for the Gallager insertion channel model, the new lower bound improves the existing results for small insertion probabilities.

In this paper, we develop several lower bounds on the capacity of binary input symmetric q-ary output channels with synchronization errors, e.g., substitution/erasure channels with synchronization errors. More precisely, we show that if a channel with synchronization errors can be decomposed into a cascade of two independent channels where only the first one suffers from synchronization errors, a lower bound on its capacity related to the capacity of the one with only synchronization errors can be given. We present several examples with the new approach and demonstrate that for certain channels, e.g., deletion/substitution channel, it is possible to derive tighter capacity lower bounds than the existing ones.

We develop several analytical lower bounds on the capacity of deletion channels by considering independent uniformly distributed (i.u.d.) inputs and computing lower bounds on the mutual information rate between the input and output sequences. We consider the usual independent identically distributed (i.i.d.) binary deletion channel, i.i.d. deletion/substitution channel and i.i.d. deletion channel with additive white Gaussian noise (AWGN). We emphasize the importance of these results by noting that 1) our results are the first analytical bounds on the capacity of deletion-AWGN channels, 2) the results developed are the best available analytical lower bounds on the deletion/substitution case, 3) for the deletion only channel, our results compete well with the best available lower bounds for small deletion probabilities and they explicitly obtain the first order terms in the recently derived capacity expansions.

We consider achievable rates over binary input insertion channels for small values of insertion probabilities by computing bounds on the mutual information rate of insertion channels for independent uniformly distributed (i.u.d.) input sequences. We consider two scenarios: random insertions, where transmitted bits are replaced with a certain probability by two random bits independent of any other insertions, and sticky channels where bits are duplicated with a certain probability again independent of the other insertion errors. Derived lower bounds improve the existing results available in the literature as demonstrated by specific examples.