A Linear Multi-User Detector for STBC MC-CDMA Systems based on the Adaptive Implementation of the Minimum-Conditional Bit-Error-Rate Criterion and on Genetic Algorithm-assisted MMSE Channel Estimation

The implementation of efficient baseband receivers characterized by affordable computational load is a crucial point in the development of transmission systems exploiting diversity in different domains. In this paper, we are proposing a linear multi-user detector for MIMO MC-CDMA systems with Alamouti’s Space-Time Block Coding, inspired by the concept of Minimum Conditional Bit-Error-Rate (MCBER) and relying on Genetic-Algorithm (GA)-assisted MMSE channel estimation. The MCBER combiner has been implemented in adaptive way by using Least-Mean-Square (LMS) optimization. Firstly, we shall analyze the proposed adaptive MCBER MUD receiver with ideal knowledge of Channel Status Information (CSI). Afterwards, we shall consider the complete receiver structure, encompassing also the non-ideal GA-assisted channel estimation. Simulation results evidenced that the proposed MCBER receiver always outperforms state-of-the-art receiver schemes based on EGC and MMSE criterion exploiting the same degree of channel knowledge (i.e. ideal or estimated CSI)

High Capacity CDMA and Collaborative Techniques

The thesis investigates new approaches to increase the user capacity and improve the error performance of Code Division Multiple Access (CDMA) by employing adaptive interference cancellation and collaborative spreading and space diversity techniques. Collaborative Coding Multiple Access (CCMA) is also investigated as a separate technique and combined with CDMA. The advantages and shortcomings of CDMA and CCMA are analysed and new techniques for both the uplink and downlink are proposed and evaluated. Multiple access interference (MAI) problem in the uplink of CDMA is investigated first. The practical issues of multiuser detection (MUD) techniques are reviewed and a novel blind adaptive approach to interference cancellation (IC) is proposed. It exploits the constant modulus (CM) property of digital signals to blindly suppress interference during the despreading process and obtain amplitude estimation with minimum mean squared error for use in cancellation stages. Two new blind adaptive receiver designs employing successive and parallel interference cancellation architectures using the CM algorithm (CMA) referred to as ‘CMA-SIC’ and ‘BA-PIC’, respectively, are presented. These techniques have shown to offer near single user performance for large number of users. It is shown to increase the user capacity by approximately two fold compared with conventional IC receivers. The spectral efficiency analysis of the techniques based on output signal-to interference-and-noise ratio (SINR) also shows significant gain in data rate. Furthermore, an effective and low complexity blind adaptive subcarrier combining (BASC) technique using a simple gradient descent based algorithm is proposed for Multicarrier-CDMA. It suppresses MAI without any knowledge of channel amplitudes and allows large number of users compared with equal gain and maximum ratio combining techniques normally used in practice. New user collaborative schemes are proposed and analysed theoretically and by simulations in different channel conditions to achieve spatial diversity for uplink of CCMA and CDMA. First, a simple transmitter diversity and its equivalent user collaborative diversity techniques for CCMA are designed and analysed. Next, a new user collaborative scheme with successive interference cancellation for uplink of CDMA referred to as collaborative SIC (C-SIC) is investigated to reduce MAI and achieve improved diversity. To further improve the performance of C-SIC under high system loading conditions, Collaborative Blind Adaptive SIC (C-BASIC) scheme is proposed. It is shown to minimize the residual MAI, leading to improved user capacity and a more robust system. It is known that collaborative diversity schemes incur loss in throughput due to the need of orthogonal time/frequency slots for relaying source’s data. To address this problem, finally a novel near-unity-rate scheme also referred to as bandwidth efficient collaborative diversity (BECD) is proposed and evaluated for CDMA. Under this scheme, pairs of users share a single spreading sequence to exchange and forward their data employing a simple superposition or space-time encoding methods. At the receiver collaborative joint detection is performed to separate each paired users’ data. It is shown that the scheme can achieve full diversity gain at no extra bandwidth as inter-user channel SNR becomes high. A novel approach of ‘User Collaboration’ is introduced to increase the user capacity of CDMA for both the downlink and uplink. First, collaborative group spreading technique for the downlink of overloaded CDMA system is introduced. It allows the sharing of the same single spreading sequence for more than one user belonging to the same group. This technique is referred to as Collaborative Spreading CDMA downlink (CS-CDMA-DL). In this technique T-user collaborative coding is used for each group to form a composite codeword signal of the users and then a single orthogonal sequence is used for the group. At each user’s receiver, decoding of composite codeword is carried out to extract the user’s own information while maintaining a high SINR performance. To improve the bit error performance of CS-CDMA-DL in Rayleigh fading conditions, Collaborative Space-time Spreading (C-STS) technique is proposed by combining the collaborative coding multiple access and space-time coding principles. A new scheme for uplink of CDMA using the ‘User Collaboration’ approach, referred to as CS-CDMA-UL is presented next. When users’ channels are independent (uncorrelated), significantly higher user capacity can be achieved by grouping multiple users to share the same spreading sequence and performing MUD on per group basis followed by a low complexity ML decoding at the receiver. This approach has shown to support much higher number of users than the available sequences while also maintaining the low receiver complexity. For improved performance under highly correlated channel conditions, T-user collaborative coding is also investigated within the CS-CDMA-UL system

Spatial diversity in MIMO communication systems with distributed or co-located antennas

The use of multiple antennas in wireless communication systems has gained much attention during the last decade. It was shown that such multiple-input multiple-output (MIMO) systems offer huge advantages over single-antenna systems. Typically, quite restrictive assumptions are made concerning the spacing of the individual antenna elements. On the one hand, it is typically assumed that the antenna elements at transmitter and receiver are co-located, i.e., they belong to some sort of antenna array. On the other hand, it is often assumed that the antenna spacings are sufficiently large, so as to justify the assumption of independent fading. In this thesis, the above assumptions are relaxed. In the first part, it is shown that MIMO systems with distributed antennas and MIMO systems with co-located antennas can be treated in a single, unifying framework. In the second part this fact is utilized, in order to develop appropriate transmit power allocation strategies for co-located and distributed MIMO systems. Finally, the third part focuses on specific synchronization problems that are of interest for distributed MIMO systems

Space-time coding for CDMA-based wireless communication systems

Thesis (Master)--Izmir Institute of Technology, Electronics and Communication Engineering, Izmir, 2002Includes bibliographical references (leaves: 72-75)Text in English; Abstract: Turkish and Englishx, 75 leavesMultiple transmit antennas giving rise to diversity (transmit diversity) have been shown to increase downlink (base station to the mobile) capacity in cellular systems.The third generation partnership project (3GPP) for WCDMA has chosen space time transmit diversity (STTD) as the open loop transmit diversity technique for two transmit antennas.On the other hand, the CDMA 2000 has chosen space time spreading (STS) and orthogonal transmit diversity (OTD) as the open loop transmit diversity.In addition to all the standardization aspects, proposed contributions such as space time coding assisted double spread rake receiver (STC-DS-RR) are exist.In this thesis, open loop transmit diversity techniques of 3GPP, CDMA 2000 and existing contributions are investigated.Their performances are compared as a means of biterror- rate (BER) versus signal-to-noise ratio (SNR)

Improved Spatial Modulation Techniques for Wireless Communications

Transmission and reception methods with multiple antennas have been demonstrated to be very useful in providing high data rates and improving reliability in wireless communications. In particular, spatial modulation (SM) has recently emerged as an attractive transmission method for multiple-antennas systems due to its better energy efficiency and lower system complexity. This thesis is concerned with developing transmission techniques to improve the spectral efficiency of SM where antenna/subcarrier index involves in conveying information bits. In the first part of the thesis, new transmission techniques are developed for SM over frequency-flat fading channels. The first proposed scheme is based on a high-rate space-time block code instead of using the classical Alamouti STBC, which helps to increase the spectral efficiency and achieve a transmit diversity order of two. A simplified maximum likelihood detection is also developed for this proposed scheme. Analysis of coding gains and simulation results demonstrate that the proposed scheme outperforms previously-proposed SM schemes at high data transmission rates. Then, a new space-shift keying (SSK) modulation scheme is proposed which requires a smaller number of transmit antennas than that required in the bi-space shift keying (BiSSK). Such a proposed SSK-based scheme is obtained by multiplexing two in-phase and quadrature generalized SSK streams and optimizing the carrier signals transmitted by the activated antennas. Performance of the proposed scheme is compared with other SSK-based schemes via minimum Euclidean distance analysis and computer simulation. The third scheme proposed in this part is an improved version of quadrature SM (QSM). The main feature of this proposed scheme is to send a second constellation symbol over the in-phase and quadrature antenna dimensions. A significant performance advantage of the proposed scheme is realized at the cost of a slight increase in the number of radio-frequency (RF) chains. Performance comparisons with the most recent SM schemes confirm the advantage of the proposed scheme. The last contribution of the first part is an optimal constellation design for QSM to minimize the average probability of error. It is shown that, the error performance of QSM not only depends on the Euclidean distances between the amplitude phase modulation (APM) symbols and the energies of APM symbols, but also on the in-phase and quadrature components of the QSM symbols. The analysis of the union bound of the average error probability reveals that at a very large number of transmit antennas, the optimal constellations for QSM converge to a quadrature phase shift keying (QPSK) constellation. Simulation results demonstrate the performance superiority of the obtained constellations over other modulation schemes. In the second part of the thesis, the applications of SM in frequency-selective fading channels are studied. First, a new transmission scheme that employs SM for each group of subcarriers in orthogonal frequency-division multiplexing (OFDM) transmission is investigated. Specifically, OFDM symbols in each group are passed through a precoder to maximize the diversity and coding gains, while SM is applied in each group to convey more information bits by antenna indices. Performance analysis and simulation results are carried out to demonstrate the superiority of the proposed scheme over a previously-proposed combination of SM and OFDM. Next, the performance of OFDM based on index modulation and a flexible version of OFDM, knows as OFDM with multiple constellations, is compared for both case of "no precoding'' and "with precoding'' of data symbols. It is shown that the precoded OFDM with multiple constellations outperforms precoded-IM based OFDM systems over frequency-selective fading channels. The last part of the thesis investigates a multiuser downlink transmission system based on in-phase and quadrature space-shift keying modulation and precoding to reduce the minimum number of transmit antennas while keeping the complexity of the receiver low. In addition to the maximum likelihood (ML) detection, the low complexity zero forcing (ZF) receiver is also studied. Theoretical upper bounds for the error probabilities of both ML and ZF receivers are obtained and corroborated with simulation results

Large-System Analysis of Correlated MIMO Multiple Access Channels with Arbitrary Signaling in the Presence of Interference

Presence of multiple antennas on both sides of a communication channel promises significant improvements in system throughput and power efficiency. In effect, a new clas s of large multiple-input multiple-output (MIMO) communication systems has recently emerged and attracted both scientific and industrial attention. To analyze these systems in realistic scenarios, one has to include such aspects as co-channel interference, multiple access and spatial correlation. In this paper, we study the properties of correlated MIMO multiple-access channels in the presence of external interference. Using the replica method from statistical physics, we derive the ergodic sum-rate of the communication for arbitrary signal constellations when the numbers of antennas at both ends of the channel grow large. Based on these asymptotic expressions, we also address the problem of sum-rate maximization using statistical channel information and linear precoding. The numerical results demonstrate that when the interfering terminals use discrete constellations, the resulting interference becomes easier to handle compared to Gaussian signals. Thus, it may be possible to accommodate more interfering transmitter-receiver pairs within the same area as compare d to the case of Gaussian signals. In addition, we demonstrate numerically for the Gaussian and QPSK signaling schemes that it is possible to design precoder matrices that significantly improve the achievable rates at low-to-mid range of signal-to-noise ratios when compared to isotropic precoding

Antenna Design for 5G and Beyond

With the rapid evolution of the wireless communications, fifth-generation (5G) communication has received much attention from both academia and industry, with many reported efforts and research outputs and significant improvements in different aspects, such as data rate speed and resolution, mobility, latency, etc. In some countries, the commercialization of 5G communication has already started as well as initial research of beyond technologies such as 6G.MIMO technology with multiple antennas is a promising technology to obtain the requirements of 5G/6G communications. It can significantly enhance the system capacity and resist multipath fading, and has become a hot spot in the field of wireless communications. This technology is a key component and probably the most established to truly reach the promised transfer data rates of future communication systems. In MIMO systems, multiple antennas are deployed at both the transmitter and receiver sides. The greater number of antennas can make the system more resistant to intentional jamming and interference. Massive MIMO with an especially high number of antennas can reduce energy consumption by targeting signals to individual users utilizing beamforming.Apart from sub-6 GHz frequency bands, 5G/6G devices are also expected to cover millimeter-wave (mmWave) and terahertz (THz) spectra. However, moving to higher bands will bring new challenges and will certainly require careful consideration of the antenna design for smart devices. Compact antennas arranged as conformal, planar, and linear arrays can be employed at different portions of base stations and user equipment to form phased arrays with high gain and directional radiation beams. The objective of this Special Issue is to cover all aspects of antenna designs used in existing or future wireless communication systems. The aim is to highlight recent advances, current trends, and possible future developments of 5G/6G antennas

Ultra-Reliable Short-Packet Communications: Fundamental Limits and Enabling Technologies

The paradigm shift from 4G to 5G communications, anticipated to enable ultra-reliable low-latency communications (URLLC), will enforce a radical change in the design of wireless communication systems. Unlike in 4G systems, where the main objective is to provide a large transmission rate, in URLLC, as implied by its name, the objective is to enable transmissions with low latency and, simultaneously, very high reliability. Since low latency implies the use of short data packets, the tension between blocklength and reliability is studied in URLLC.Several key enablers for URLLC communications have been designated in the literature. Of special importance are diversity-enabling technologies such as multiantenna systems and feedback protocols. Furthermore, it is not only important to introduce additional diversity by means of the above examples, one must also guarantee that thescarce number of channel uses are used in an optimal way. Therefore, it is imperative to develop design guidelines for how to enable reliable detection of incoming data, how to acquire channel-state information, and how to construct efficient short-packet channel codes. The development of such guidelines is at the heart of this thesis. This thesis focuses on the fundamental performance of URLLC-enabling technologies. Specifically, we provide converse (upper) bounds and achievability (lower) bounds on the maximum coding rate, based on finite-blocklength information theory, for systems that employ the key enablers outlined above. With focus on the wireless channel, modeled via a block-fading assumption, we are able to provide answers to questions like: howto optimally utilize spatial and frequency diversity, how far from optimal short-packet channel codes perform, how multiantenna systems should be designed to serve a given number of users, and how to design feedback schemes when the feedback link is noisy. In particular, this thesis is comprised out of four papers. In Paper A, we study the short-packet performance over the Rician block-fading channel. In particular, we present achievability bounds for pilot-assisted transmission with several different decoders that allow us to quantify the impact, on the achievable performance, of imposed pilots and mismatched decoding. Furthermore, we design short-packet channel codes that perform within 1 dB of our achievability bounds. Paper B studies multiuser massive multiple-input multiple-output systems with short packets. We provide an achievability bound on the average error probability over quasistatic spatially correlated Rayleigh-fading channels. The bound applies to arbitrary multiuser settings, pilot-assisted transmission, and mismatched decoding. This makes it suitable to assess the performance in the uplink/downlink for arbitrary linear signal processing. We show that several lessons learned from infinite-blocklength analyses carry over to the finite-blocklength regime. Furthermore, for the multicell setting with randomly placed users, pilot contamination should be avoided at all cost and minimum mean-squared error signal processing should be used to comply with the stringent requirements of URLLC.In Paper C, we consider sporadic transmissions where the task of the receiver is to both detect and decode an incoming packet. Two novel achievability bounds, and a novel converse bound are presented for joint detection-decoding strategies. It is shown that errors associated with detection deteriorates performance significantly for very short packet sizes. Numerical results also indicate that separate detection-decoding strategies are strictly suboptimal over block-fading channels.Finally, in Paper D, variable-length codes with noisy stop-feedback are studied via a novel achievability bound on the average service time and the average error probability. We use the bound to shed light on the resource allocation problem between the forward and the feedback channel. For URLLC applications, it is shown that enough resources must be assigned to the feedback link such that a NACK-to-ACK error becomes rarer than the target error probability. Furthermore, we illustrate that the variable-length stop-feedback scheme outperforms state-of-the-art fixed-length no-feedback bounds even when the stop-feedback bit is noisy