Asymptotic Behavior of Error Exponents in the Wideband Regime

In this paper, we complement Verd\'{u}'s work on spectral efficiency in the wideband regime by investigating the fundamental tradeoff between rate and bandwidth when a constraint is imposed on the error exponent. Specifically, we consider both AWGN and Rayleigh-fading channels. For the AWGN channel model, the optimal values of $R_z(0)$ and $\dot{R_z}(0)$ are calculated, where $R_z(1/B)$ is the maximum rate at which information can be transmitted over a channel with bandwidth $B/2$ when the error-exponent is constrained to be greater than or equal to $z.$ Based on this calculation, we say that a sequence of input distributions is near optimal if both $R_z(0)$ and $\dot{R_z}(0)$ are achieved. We show that QPSK, a widely-used signaling scheme, is near-optimal within a large class of input distributions for the AWGN channel. Similar results are also established for a fading channel where full CSI is available at the receiver.Comment: 59 pages, 6 figure

Coding in 802.11 WLANs

Forward error correction (FEC) coding is widely used in communication systems to correct transmis- sion errors. In IEEE 802.11a/g transmitters, convolutional codes are used for FEC at the physical (PHY) layer. As is typical in wireless systems, only a limited choice of pre-speci¯ed coding rates is supported. These are implemented in hardware and thus di±cult to change, and the coding rates are selected with point to point operation in mind. This thesis is concerned with using FEC coding in 802.11 WLANs in more interesting ways that are better aligned with application requirements. For example, coding to support multicast tra±c rather than simple point to point tra±c; coding that is cognisant of the multiuser nature of the wireless channel; and coding which takes account of delay requirements as well as losses. We consider layering additional coding on top of the existing 802.11 PHY layer coding, and investigate the tradeo® between higher layer coding and PHY layer modulation and FEC coding as well as MAC layer scheduling. Firstly we consider the joint multicast performance of higher-layer fountain coding concatenated with 802.11a/g OFDM PHY modulation/coding. A study on the optimal choice of PHY rates with and without fountain coding is carried out for standard 802.11 WLANs. We ¯nd that, in contrast to studies in cellular networks, in 802.11a/g WLANs the PHY rate that optimizes uncoded multicast performance is also close to optimal for fountain-coded multicast tra±c. This indicates that in 802.11a/g WLANs cross-layer rate control for higher-layer fountain coding concatenated with physical layer modulation and FEC would bring few bene¯ts. Secondly, using experimental measurements taken in an outdoor environment, we model the chan- nel provided by outdoor 802.11 links as a hybrid binary symmetric/packet erasure channel. This hybrid channel o®ers capacity increases of more than 100% compared to a conventional packet erasure channel (PEC) over a wide range of RSSIs. Based upon the established channel model, we further consider the potential performance gains of adopting a binary symmetric channel (BSC) paradigm for multi-destination aggregations in 802.11 WLANs. We consider two BSC-based higher-layer coding approaches, i.e. superposition coding and a simpler time-sharing coding, for multi-destination aggre- gated packets. The performance results for both unicast and multicast tra±c, taking account of MAC layer overheads, demonstrate that increases in network throughput of more than 100% are possible over a wide range of channel conditions, and that the simpler time-sharing approach yields most of these gains and have minor loss of performance. Finally, we consider the proportional fair allocation of high-layer coding rates and airtimes in 802.11 WLANs, taking link losses and delay constraints into account. We ¯nd that a layered approach of separating MAC scheduling and higher-layer coding rate selection is optimal. The proportional fair coding rate and airtime allocation (i) assigns equal total airtime (i.e. airtime including both successful and failed transmissions) to every station in a WLAN, (ii) the station airtimes sum to unity (ensuring operation at the rate region boundary), and (iii) the optimal coding rate is selected to maximise goodput (treating packets decoded after the delay deadline as losses)

Performance Analysis for the RIS and the AmBC Systems at the Short Blocklength Regime

This thesis investigates and analyzes the performances of the RIS and AmBC systems, and three main research works are involved. The first research work of this thesis is to present the decoding error probability bounds for the optimal code in an RIS system within the short blocklength regime and given a code rate and signal-to-noise ratio (SNR). The approach uses sphere-packing techniques to derive the main results, with the Wald sequential t-test lemma and the Riemann sum serving as the primary tools for obtaining the closed-form expressions for both the upper and lower bounds. The numerical results are demonstrated to illustrate the performance of the findings. The last two research works focus on examining the maximal achievable rate for a given maximum error probability and blocklength in a system that employs reconfigurable intelligent surface (RIS) or ambient backscatter communication (AmBC) to aid a multiple-input and multiple-output (MIMO) communication system. The findings of these research include finite blocklength and finite alphabet constraints channel coding achievability and converse bounds, which are established through the use of the Berry-Esseen theorem, the Mellin transform, and the closed-form expression of the mutual information and the unconditional variance. The numerical analysis indicates that the maximum achievable rate is reached rapidly as the blocklength increases. Additionally, the channel variance accurately reflects the deviation from the maximum achievable rate due to the finite blocklength

BER Performance Improvement in UWA Communication via Spatial Diversity

In present era while wireless communication has become an integral part of our life, the advancements in underwater communications (UWA) is still seem farfetched. Underwater communication is typically essential because of its ability to collect information from remote undersea locations. It don’t use radio signals for signal transmission as they can propagate over extremely short distance because of degradation in signal strength due to salinity of water, rather it uses acoustic waves. The underwater acoustic channel has many characteristics which makes receivers very difficult to be realized. Some of the characteristics are frequency dependent propagation loss, severe Doppler spread multipath, low speed of sound. Due to motion of transmitter and receiver the time variability and multipath makes underwater channel very difficult to be estimated. There are various channel estimation techniques to find out channel impulse response but in this thesis we have considered a flat slow fading channel modeled by Nakagami-m distribution. Noise in underwater communication channel is frequency dependent in nature as for a particular range of frequency of operation one among the various noise sources will be dominant. Here they don’t necessarily follow Gaussian statistics rather follows Generalized Gaussian statistics with decaying power spectral density. The flexible parametric form of this statistics makes it useful to fit any source of underwater noise source. In this thesis we have gone through two step approach. In the first step, we have considered transmission of information in presence of noise only and designed a suboptimal maximum likelihood detector. We have compared the performance of this proposed detector with the conventional Gaussian detector where decision is taken based on a single threshold value and the threshold value is calculated by using various techniques. Here it is being observed that the ML detector outperforms the Gaussian detectors and the performance can be improved further by exploiting the multipath components. In the second step we have considered channel along with noise and have designed a ML detector where we have considered the receiver is supplied with two copies of the same transmitted signal and have gone through a two-dimensional analysis. Again we compared the performance with conventional maximal ratio combiner where we can observe the ML detector performance is better. Further we have incorporated selection combining along with these detectors and compared the performance. Simulation results shows that the proposed detector always outperforms the existing detectors in terms of error performance

Physical-Layer Security Enhancement in Wireless Communication Systems

Without any doubt, wireless infrastructures and services have fundamental impacts on every aspect of our lives. Despite of their popularities, wireless communications are vulnerable to various attacks due to the open nature of radio propagation. In fact, communication security in wireless networks is becoming more critical than ever. As a solution, conventional cryptographic techniques are deployed on upper layers of network protocols. Along with direct attacks from lower layer, wireless security challenges come with the rapid evolution of sophisticated decipher techniques. Conventional security mechanisms are not necessarily effective against potential attacks from the open wireless environment anymore. As an alternative, physical-layer(PHY) security, utilizing unique features from lower layer, becomes a new research focus for many wireless communication systems. In this thesis, three mechanisms for PHY security enhancement are investigated. Beginning with a discussion on the security vulnerability in highly standardized infrastructures, the thesis proposed a time domain scrambling scheme of orthogonal frequency division multiplexing (OFDM) system to improve the PHY security. The method relies on secretly scrambling each OFDM symbol in time domain, resulting in constellation transformation in frequency domain, to hide transmission features. As a complement to existing secrecy capacity maximization based optimal cooperative jamming systems, a security strategy based on the compromised secrecy region (CSR) minimization in cooperative jamming is then proposed when instantaneous channel state information(CSI) is not available. The optimal parameters of the jammer are derived to minimize the CSR which exhibits high secrecy outage probability. At last, security enhancement of OFDM system in cooperative networks is also investigated. The function selection strategies of cooperative nodes are studied. Our approach is capable of enhancing the security of broadband communications by selecting the proper function of each cooperative node. Numerical results demonstrate the feasibility of three proposed physical layer security mechanisms by examining the communication reliability, achievable CSR and secrecy capacity respectively

Performance analysis of MIMO-OFDM systems using complex Gaussian quadratic forms

En este trabajo se proponen aportaciones originales para el análisis de prestaciones en sistemas multiantena con múltiples portadoras, mediante el desarrollo de nuevas técnicas matemáticas para el cálculo de probabilidades de error. Así, ha sido posible analizar el efecto de no idealidades (estimación de canal imperfecta, offset de continua, desbalanceo I/Q…) en las prestaciones de sistemas de comunicaciones móviles e inalámbricas

Signal mapping designs for bit-interleaved coded modulation with iterative decoding (BICM-ID)

Bit-interleaved coded modulation with iterative decoding (BICM-ID)is a spectral efficient coded modulation technique to improve the performance of digital communication systems. It has been widely known that for fixed signal constellation, interleaver and error control code, signal mapping plays an important role in determining the error performance of a BICM-ID system. This thesis concentrates on signal mapping designs for BICM-ID systems. To this end, the distance criteria to find the best mapping in terms of the asymptotic performance are first analytically derived for different channel models. Such criteria are then used to find good mappings for various two-dimensional 8-ary constellations. The usefulness of the proposed mappings of 8-ary constellations is verified by both the error floor bound and simulation results. Moreover, new mappings are also proposed for BICM-ID systems employing the quadrature phase shift keying (QPSK) constellation. The new mappings are obtained by considering many QPSK symbols over a multiple symbol interval, which essentially creates hypercube constellations. Analytical and simulation results show that the use of the proposed mappings together with very simple convolutional codes can offer significant coding gains over the conventional BICM-ID systems for all the channel models considered. Such coding gains are achieved without any bandwidth nor power expansion and with a very small increase in the system complexity

An Analysis of Pilot Symbol Assisted Modulation for Rayleigh Fading Channels

Abstract-Proposals have appeared recently for the use of pilot symbols to mitigate the effects of rapid fading in mobile communications. Unlike the more familiar pilot tone systems, pilot symbol assisted modulation (PSAM) does not affect the transmitted pulse shape or the peak-to-average power ratio, and implementation is straightforward. This paper puts PSAM on a solid analytical basis, a feature missing from previous work. It presents closed form expressions for the BER in BPSK and QPSK, for a tight upper bound on SER in 16QAM, and for the optimized receiver coefficients. The error rates obtained are lower than for differential detection for any combination of SNR and Doppler spread, and the performance is within 1 dB of a perfect reference system under slow fading conditions, and within 3 dB when the Doppler spread is 5% of the symbol rate