A Multi-Service Oriented Multiple-Access Scheme for Next-Generation Mobile Networks

One of the key requirements for fifth-generation (5G) cellular networks is their ability to handle densely connected devices with different quality of service (QoS) requirements. In this article, we present multi-service oriented multiple access (MOMA), an integrated access scheme for massive connections with diverse QoS profiles and/or traffic patterns originating from both handheld devices and machine-to-machine (M2M) transmissions. MOMA is based on a) stablishing separate classes of users based on relevant criteria that go beyond the simple handheld/M2M split, b) class dependent hierarchical spreading of the data signal and c) a mix of multiuser and single-user detection schemes at the receiver. Practical implementations of the MOMA principle are provided for base stations (BSs) that are equipped with a large number of antenna elements. Finally, it is shown that such a massive-multiple-input-multiple-output (MIMO) scenario enables the achievement of all the benefits of MOMA even with a simple receiver structure that allows to concentrate the receiver complexity where effectively needed.Comment: 6 pages, 3 figures, accepted to the European Conference on Networks and Communications (EuCNC 2016

Multi-rate access schemes and successive interference cancellation for wireless multimedia MC-CDMA communications

To catch up with the fast changes of the information challenges, providing multimedia services has become a very important requirement for future wireless communications. A proper system, capable of supporting multi-rate transmissions as well as handling high quality of service (QoS) requirements in hostile wireless communication environments, should be sought. Multi-carrier CDMA (MC-CDMA), a combination of multi-carrier modulation (MCM) and direct-sequence CDMA (DS-CDMA), appears to be one of the most elegant solutions. In this dissertation, four multi-rate access schemes, termed uncoded fixed spreading length (UFSL), coded fixed spreading length (CFSL), multi-code fixed spreading length (MFSL) and variable spreading length (VSL), are constructed for MC-CDMA. Due to different sub-carrier assignment strategies, they present different properties in spectral utilization efficiency (SUE), rate matching capability, receiver structure and bit-error-rate (BER) performance in correlated Rayleigh fading channels. With these schemes, different information traffic such as voice, video and higher rate data can be transmitted seanilessly through one MC-CDMA infrastructure. The performance of the multi-rate MC-CDMA is mainly limited by multiple access interference (MAI). For example, in the MFSL MC-CDMA systems, the interference is not only presented among different users, but also among different symbols of the same user transmitted in parallel on different spreading codes. To mitigate this problem, a nonlinear zero-forcing successive interference cancellation (ZF-SIC) receiver and a minimum mean square error SIC (MMSE-SIC) receiver are applied in the MFSL MC-CDMA systems. It is well known that SIC is sensitive to the receive power distribution. By providing channel state information (CSI) at the receiver and reliable feedback of power distribution from the receiver to the transmitter, SIC can be integrated with power distribution control (PDC), which improves the system capacity significantly. In this dissertation, the PDC algorithms, under both a short-term power constraint (STPC) and a long-term power constraint (LTPC) are investigated for two different SIC receivers. For the ZF-SIC receiver, the PDC under the equal BER criterion, which ensures the same performance after SIC for all parallel transmit symbols, is first considered. It is found that for a multi-code system, such equal BER PDC is only suboptimal from the viewpoint of minimizing each user\u27s BER, hence, an optimal PDC algorithm is proposed, which significantly outperforms the equal BER PDC, particularly under the STPC and highly-loaded systems. For the MMSE-SIC, the PDC under the equal BER criterion is derived, which cancels interference very effectively, resulting in a performance of a fully-loaded system close to the single user bound (SUB). In comparison to the nonlinear matched-filter SIC (MF-SIC) with the equal BER PDC, studied extensively in the literature, the ZF-SIC and MMSE-SIC with the proposed PDC algorithms present remarkable performance advantage. Finally, the effect of channel estimation errors (CEE) on the performance of the MMSE-SIC with the equal BER PDC is analyzed. A method of second-order approximation is used to estimate the mean excess MSE (MEMSE) of the parallel transmit symbols, under a given decision order. The approximation accuracy is confirmed by simulation results. Furthermore, it is also interesting to find out that the MMSE-SIC with the equal BER PDC presents significant robustness to CEE

Design and performance of CDMA codes for multiuser communications

Walsh and Gold sequences are fixed power codes and are widely used in multiuser CDMA communications. Their popularity is due to the ease of implementation. Availability of these code sets is limited because of their generating kernels. Emerging radio applications like sensor networks or multiple service types in mobile and peer-to-peer communications networks might benefit from flexibilities in code lengths and possible allocation methodologies provided by large set of code libraries. Walsh codes are linear phase and zero mean with unique number of zero crossings for each sequence within the set. DC sequence is part of the Walsh code set. Although these features are quite beneficial for source coding applications, they are not essential for spread spectrum communications. By relaxing these unnecessary constraints, new sets of orthogonal binary user codes (Walsh-like) for different lengths are obtained with comparable BER performance to standard code sets in all channel conditions. Although fixed power codes are easier to implement, mathematically speaking, varying power codes offer lower inter- and intra-code correlations. With recent advances in RF power amplifier design, it might be possible to implement multiple level orthogonal spread spectrum codes for an efficient direct sequence CDMA system. A number of multiple level integer codes have been generated by brute force search method for different lengths to highlight possible BER performance improvement over binary codes. An analytical design method has been developed for multiple level (variable power) spread spectrum codes using Karhunen-Loeve Transform (KLT) technique. Eigen decomposition technique is used to generate spread spectrum basis functions that are jointly spread in time and frequency domains for a given covariance matrix or power spectral density function. Since this is a closed form solution for orthogonal code set design, many options are possible for different code lengths. Design examples and performance simulations showed that spread spectrum KLT codes outperform or closely match with the standard codes employed in present CDMA systems. Hybrid (Kronecker) codes are generated by taking Kronecker product of two spreading code families in a two-stage orthogonal transmultiplexer structure and are judiciously allocated to users such that their inter-code correlations are minimized. It is shown that, BER performance of hybrid codes with a code selection and allocation algorithm is better than the performance of standard Walsh or Gold code sets for asynchronous CDMA communications. A redundant spreading code technique is proposed utilizing multiple stage orthogonal transmultiplexer structure where each user has its own pre-multiplexer. Each data bit is redundantly spread in the pre-multiplexer stage of a user with odd number of redundancy, and at the receiver, majority logic decision is employed on the detected redundant bits to obtain overall performance improvement. Simulation results showed that redundant spreading method improves BER performance significantly at low SNR channel conditions

UMTS Network Model for Interference Analysis - Optimization of Spreading Codes Order

This article deals with mathematical modeling of UMTS network. The presented model is designed for interference analysis in this network. The paper presents the set of simulations based on an idea of a specific order of Walsh codes. In the first part the mathematical model is presented. This model is designed for Matlab and is based on 3GPP specifications. The second part describes the method of specific ordering of Walsh spreading codes. The specific order can decrease the interference level in the radio network of UMTS which is caused by multipath propagation. The simulation results are presented, too

Application of advanced on-board processing concepts to future satellite communications systems: Bibliography

Abstracts are presented of a literature survey of reports concerning the application of signal processing concepts. Approximately 300 references are included

On rate capacity and signature sequence adaptation in downlink of MC-CDMA system

This dissertation addresses two topics in the MC-CDMA system: rate capacity and adaptation of users\u27 signature sequences. Both of them are studied for the downlink communication scenario with multi-code scheme. The purpose of studying rate capacity is to understand the potential of applying MC-CDMA technique for high speed wireless data communications. It is shown that, to maintain high speed data transmission with multi-code scheme, each mobile should cooperatively decode its desired user\u27s encoded data symbols which are spread with different signature sequences simultaneously. Higher data rate can be achieved by implementing dirty paper coding (DPC) to cooperatively encode all users\u27 data symbols at the base station. However, the complexity of realizing DPC is prohibitively high. Moreover, it is found that the resource allocation policy has profound impact on the rate capacity that can be maintained in the system. Nevertheless, the widely adopted proportional resource allocation policy is only suitable for the communication scenario in which the disparity of users\u27 channel qualities is small. When the difference between users\u27 channel qualities is large, one must resort to non-proportional assignment of power and signature sequences. Both centralized and distributed schemes are proposed to adapt users\u27 signature sequences in the downlink of MC-CDMA system. With the former, the base station collects complete channel state information and iteratively adapts all users\u27 signature sequences to optimize an overall system performance objective function, e.g. the weighted total mean square error (WTMSE). Since the proposed centralized scheme is designed such that each iteration of signature sequence adaptation decreases the WTMSE which is lower bounded, the convergence of the proposed centralized scheme is guaranteed. With the distributed signature sequence adaptation, each user\u27s signature sequences are independently adapted to optimize the associated user\u27s individual performance objective function with no regard to the performance of other users in the system. Two distributed adaptation schemes are developed. In one scheme, each user adapts its signature sequences under a pre-assigned power constraint which remains unchanged during the process of adaptation. In the other scheme, pricing methodology is applied so that the transmission power at the base station is properly distributed among users when users\u27 signature sequences are adapted. The stability issue of these distributed adaptation schemes is analyzed using game theory frame work. It is proven that there always exists a set of signature sequences at which no user can unilaterally adapt its signature sequences to further improve its individual performance, given the signature sequences chosen by other users in the system

A General Framework for Analyzing, Characterizing, and Implementing Spectrally Modulated, Spectrally Encoded Signals

Fourth generation (4G) communications will support many capabilities while providing universal, high speed access. One potential enabler for these capabilities is software defined radio (SDR). When controlled by cognitive radio (CR) principles, the required waveform diversity is achieved via a synergistic union called CR-based SDR. Research is rapidly progressing in SDR hardware and software venues, but current CR-based SDR research lacks the theoretical foundation and analytic framework to permit efficient implementation. This limitation is addressed here by introducing a general framework for analyzing, characterizing, and implementing spectrally modulated, spectrally encoded (SMSE) signals within CR-based SDR architectures. Given orthogonal frequency division multiplexing (OFDM) is a 4G candidate signal, OFDM-based signals are collectively classified as SMSE since modulation and encoding are spectrally applied. The proposed framework provides analytic commonality and unification of SMSE signals. Applicability is first shown for candidate 4G signals, and resultant analytic expressions agree with published results. Implementability is then demonstrated in multiple coexistence scenarios via modeling and simulation to reinforce practical utility