Time Code Division Multiple Access Research Articles

Large device to device communication network including airborne drones for emergency scenarios

AbstractIn the case of emergency scenarios communication infrastructure might be destroyed, impairing communication abilities in the disaster area. In such situation device‐to‐device (D2D) communication networking plays an essential role to overcome the communication loss. To provide a reliable communication network, D2D devices are combined with airborne drones. The D2D devices provide a relay service while being able to transmit their own data. The airborne drones offer additional relay functionality and provide considerably reduced latency compared with D2D communication networks without any airborne drones. In this article, we will illustrate a large dynamic communication network (LDCN) and demonstrate its feasibility for even more than 64 000 D2D communication devices in the case of realistic channel conditions including path loss and fading. It will be shown that the illustrated LDCN is truly infrastructure‐less, that is, no infrastructure or backbone or base station functionality need to be reinstalled. Therefore, the airborne drones do not take over any coordination services. The illustrated LDCN has a simple packet switching architecture and since all communication is localized it is robust and fully flexible. In addition, no specific radio interface or preferred multiple access scheme such as code division multiple access or time division multiple access are required.

M-PAM Joint Optimal Waveform Design for Multiuser VLC Systems Over ISI Channels

Visible light communication is a candidate technique to provide high-speed data transmissions. This paper proposes a joint optimal waveform design algorithm for visible light communication systems using $M$ -ary pulse amplitude modulation to support multiple users. Transmitted waveforms and minimum mean squared error filters are jointly optimized to minimize the intersymbol and multiple access interferences. Based on different channel conditions, the designed waveforms and modulation constellation sizes can be adaptively adjusted to guarantee the highest bit rates. Channel uncertainty is also considered in this paper. Compared with optical code division multiple access and time division multiple access, the algorithm proposed can provide higher bit rates. In addition, the proposed algorithm can flexibly adjust the illumination level by changing the optimization constraints. An offline waveform design algorithm is then proposed to diminish the optimization computation time. A waveform lookup table can be established offline in advance, and the proper waveforms can be selected from the table based on the actual channel gains in real time. The performance of the offline algorithm can be estimated by using the channel uncertainty model.

3D Indoor Positioning of UAVs with Spread Spectrum Ultrasound and Time-of-Flight Cameras.

This work proposes the use of a hybrid acoustic and optical indoor positioning system for the accurate 3D positioning of Unmanned Aerial Vehicles (UAVs). The acoustic module of this system is based on a Time-Code Division Multiple Access (T-CDMA) scheme, where the sequential emission of five spread spectrum ultrasonic codes is performed to compute the horizontal vehicle position following a 2D multilateration procedure. The optical module is based on a Time-Of-Flight (TOF) camera that provides an initial estimation for the vehicle height. A recursive algorithm programmed on an external computer is then proposed to refine the estimated position. Experimental results show that the proposed system can increase the accuracy of a solely acoustic system by 70–80% in terms of positioning mean square error.

A robust adaptive array beamformer using particle swarm optimization for space–time code division multiple access systems

An adaptive beamformer often suffers from severe performance degradation when a mismatch exists in the steering vector of interest. In this study, a new approach to the robust adaptive beamforming technique is proposed based on minimum variance distortionless response (MVDR) for space–time systems. The proposed method combines particle swarm optimization (PSO) and Taylor series expansion to enable the use of direction-of-arrival (DOA) estimation. Taylor series expansion can be robust to pointer errors for the presumed steering vector. However, the Taylor approach is easily trapped at a local minimum, causing errors in DOA estimation. Therefore, a more accurate DOA estimate is necessary for MVDR beamforming. To resolve this problem, a DOA estimation approach featuring a high resolution and low computational load is presented in this paper. An initial DOA estimate is first determined using a PSO estimator. Next, the predominant DOA estimate is sent to the Taylor series expansion-based estimator to form an estimate. Several simulation results are provided to demonstrate the effectiveness of the proposed approach.

A Frequency Domain Multi-User Detector for TD-CDMA Systems

In this paper, a novel frequency domain multi-user detector is proposed for a time division-code division multiple access (TD-CDMA) up-link. Unlike conventional frequency domain detectors, the proposed detector first transforms the system matrix of TD-CDMA systems into a circulant matrix by cyclic truncation. It subsequently uses a new method to convert the circulant matrix into a frequency domain block diagonalized matrix through discrete Fourier transforms and permutations. Therefore, the proposed detector can utilize the channel frequency domain coherence to further decrease its computational complexity with a controlled performance loss. Moreover, a novel approach is proposed to calculate the frequency domain correlation matrix and matched filter. With the help of this novel approach, the proposed detector expresses significant complexity advantage over other frequency domain detectors for a real TD-CDMA system in a short-time-dispersive channel.

Downlink multiple access schemes for transmitted reference pulse cluster UWB systems

AbstractTransmitted reference pulse cluster (TRPC) signaling was recently proposed for low data rate ultra‐wideband communications. Compared with the conventional transmitted reference (TR), TRPC has a more compact and uniform spacing for the reference and data pulses and therefore solves the implementation problem posed by the long delay line requirement in the conventional TR. In this paper, downlink multiple access issue is further investigated for TRPC systems. According to the TRPC signaling structure, both code division multiple access (CDMA) and time division multiple access (TDMA) methods are discussed. Theoretical analysis and simulation results show that TDMA outperforms CDMA in both bit error rate performance and throughput. Copyright © 2010 John Wiley & Sons, Ltd.

A hybrid multirate MAC protocol providing trade‐off between throughput and fairness in future TD‐CDMA networks

AbstractIn this paper, a hybrid, frequency division duplex (FDD), time division–code division multiple access (TD‐CDMA), medium access control (MAC) protocol is proposed. Both multicode (MC) and orthogonal variable spreading factor (OVSF) transmission modes are used in order to utilise the system resources more efficiently. Moreover, a new rate‐scheduling algorithm is developed, aimed at providing a trade‐off between throughput maximisation and fairness, while satisfying QoS requirements. The above scheme is formulated as a constrained optimisation problem, which maximises the sum of normalised rates instead of throughput in order to achieve fairer rate allocations, while maintaining throughput at a high value. A new optimisation constraint is introduced in order to deal with the discrete set of available rates that the hybrid MC‐OVSF scheme supports. Copyright © 2009 John Wiley & Sons, Ltd.

Channel Assignment and Time Sharing for Multiple Access in Multicarrier Communication Systems

In this paper, we present a general 2-D multiple-access scheme, namely, code time division multiple access (CTDMA), on the downlink of multicarrier communication systems. CTDMA combines the idea of CDMA in the code domain and TDMA in the time domain. We consider suitable resource allocation strategies with our CTDMA scheme in the code domain and the time domain jointly. Under the total transmission-power minimization criterion, we propose a near optimal approach to allocate codes and transmission time to users appropriately, while the data rate of each user is guaranteed. To reduce the computational complexity, we also give some simplified suboptimal approaches for certain types of systems, namely, the one-to-one approach for the fully loaded system and the multiple-to-one approach for the nonfully loaded systems. To check the differences between our approaches and the optimal allocation strategy, we derive a pair of lower and upper bounds on the optimal resource allocation solution. Simulation results show that major improvement can be obtained by the near optimal approach, while the suboptimal approaches only cause minor performance degradation. We also show that our approaches will all converge to the lower bound as well as the optimal allocation solution, when the system size increases.

Performance of quasisynchronous scale time code division multiple access for AWGN channel

We analyze bit error rate performance of a recently proposed multiple access system called scale time code division multiple access (STCDMA) for quasisynchronous communication over an AWGN channel. STCDMA depends on code, time and scale orthogonality introduced by spreading sequences and wavelets. Wavelets are employed as an orthogonal set of symbols for signaling, and their orthogonality over scale and time is exploited. The channel is partitioned into different scales, and each scale into different time slots. Each user is assigned a specific scale, time slot, and spreading code. Information symbols of each user are encoded by the Haar wavelet in its scale and time slot, and then they are spread by its spreading code. Complex-valued Hadamard sequences are used as spreading sequences and conventional detector (i.e. matched filter) is used at the receiver. Results show that the performance of STCDMA gets much better than that of CDMA over the quasisynchronous AWGN channel as the number of scales increases.

Usability and evaluation of a deployed 4G network prototype

This article presents a field evaluation of an IP-based architecture for heterogeneous environments that has been developed under the aegis of the Moby Dick project, covering UMTS-like (universal mobile telecommunications system) TD-CDMA (time division-code division multiple access) wireless access technology, wireless and wired LANs. The architecture treats all transmission capabilities as basic physical and data-link layers, and replaces all higher-level tasks by IP-based strategies. The Moby Dick architecture incorporates mobile IPv6, fast handovers, AAA-control (authentication, authorisation, accounting), charging and quality of service (QoS) in an integrated framework. The architecture further allows for optimised control on the radio link layer resources. It has been implemented and tested by expert users, and evaluated by real users on field trials with multiple services available.

Transmitter-Based Inverse Filters for Reducing MAI and ISI in CDMA-TDD Downlink

Inverse filters (INVF) are introduced as a new transmitter-based precoding scheme for the code-division multiple access-time division duplex (TDD) downlink. Multiple access interference and intersymbol interference are significantly reduced while the mobile station's structure is reduced to a filter matched to the user's spreading code. The method assumes knowledge of the channel at the transmitter which is feasible under TDD. The new algorithm uses a minimum mean-squared error criterion with power constraint. When compared with existing schemes, INVF increase the system performance while maintaining a low computational cost. The theoretical analysis is supported with simulation results. Analytical bit-error rates for any precoding algorithm is also calculated.

Wide-Band TD-CDMA MAC With Minimum-Power Allocation and Rate- and BER-Scheduling for Wireless Multimedia Networks

A wide-band time-division-code-division multiple-access (TD-CDMA) medium access control (MAC) protocol is introduced in this paper. A new minimum-power allocation algorithm is developed to minimize the interference experienced by a code channel such that heterogeneous bit-error rate (BER) requirements of multimedia traffic are satisfied. Further, from analysis of the maximum capacity of a time slot, it is concluded that both rate and BER scheduling are necessary to reach a maximum capacity. Based on the new minimum-power allocation algorithm as well as on rate and BER scheduling concepts, a new scheduling scheme is proposed to serve packets with heterogeneous BER and quality of service (QoS) requirements in different time slots. To further enhance the performance of the MAC protocol, an effective connection admission control (CAC) algorithm is developed based on the new minimum-power allocation algorithm. Simulation results show that the new wide-band TD-CDMA MAC protocol satisfies the QoS requirements of multimedia traffic and achieves high overall system throughput.

Emerging Trends in Mobile Communication

With the advancement in the technology, wireless mobile communication has been witnessing unprecedented developments. The trend is to move from earlier narrow band standards to recent wide band standards suited for a wide variety of applications. Many schemes have evolved within different generations of wireless mobile digital communication. Code Division Multiple Access (CDMA) is evolving as one of the widely accepted major multiple access schemes in 3G mobile communication systems. Multi-carrier CDMA, Wide band CDMA (W-CDMA) and its hybrid associate Time Division CDMA (TD-CDMA) are coming up as key elements of standards such as IMT-2000. In this article, recent advances in this area of mobile communication and related issues have been discussed.

Admission control in TD‐CDMA networks

AbstractQuality of service (QoS) at the call or flow level in Time Division Code Division Multiple Access (TD‐CDMA) networks is an important topic that has not yet received much attention. In this paper, we propose a two‐tier admission and scheduling policy. The scheduling policy is comprised of two procedures, the User Prioritizer Procedure (UPP) and Packet Allocator Procedure (PAP). The UPP is used to determine the serving priority of users. The task of PAP is to allocate packets to a time slot without violating the outage probability by invoking the tier 1 and tier 2 admission control. The tier 1 admission keeps the probability of violating the Bit Error Rate (BER) limits for new call arrivals below a specified threshold. The tier 1 admission allocates bandwidth for newly arrived calls on the basis of the outage probability threshold. The tier 2 admission reserves bandwidth in all surrounding sectors for future handoff events, which minimizes the probability that the handoff call from one sector will cause an outage condition in the target sector. The performance of the proposed policy is evaluated through simulations in the presence of voice and data traffic types. We show that the proposed scheme can provide satisfactory quality for a call or data flow during its lifetime. Copyright © 2003 John Wiley & Sons, Ltd.

RS-CTDMA: a novel concept of multiple access design

RS-CDMA (rotated spectrum code division multiple access) is designed to yield a novel multiple access concept termed RS-CTDMA (rotated spectrum code time division multiple access) for mobile radio systems. The concept exploits the principle of CTDMA (code time division multiple access) to combine the advantages of the good user separation of TDMA and the spread spectrum of CDMA. Moreover, the concept allows multicode transmission, and thus high spectrum efficiency. Owing to its having the inherent features of both CDMA and TDMA, RS-CTDMA preserves maximum commonality with other air interfaces, and thus has the potential to be compatible with UMTS.

Comparative study of joint-detection techniques for TD-CDMA based mobile radio systems

Third-generation mobile radio systems use time division-code division multiple access (TD-CDMA) in their time division duplex (TDD) mode. Due to the time division multiple access (TDMA) component of TD-CDMA, joint (or multi-user) detection techniques can be implemented with a reasonable complexity. Therefore, joint-detection will already be implemented in the first phase of the system deployment to eliminate the intracell interference. In a TD-CDMA mobile radio system, joint-detection is performed by solving a least squares problem, where the system matrix has a block-Sylvester structure. We present and compare several techniques that reduce the computational complexity of the joint-detection task even further by exploiting this block-Sylvester structure and by incorporating different approximations. These techniques are based on the Cholesky factorization, the Levinson algorithm, the Schur algorithm, and on Fourier techniques, respectively. The focus of this paper is on Fourier techniques since they have the smallest computational complexity and achieve the same performance as the joint-detection algorithm that does not use any approximations. Similar to the well-known implementation of fast convolutions, the resulting Fourier-based joint-detection scheme also uses a sequence of fast Fourier transforms (FFTs) and overlapping. It is well suited for the implementation on parallel hardware architectures.

Joint detection with low computational complexity for hybrid TD-CDMA systems

In the hybrid multiple access technique time division-code division multiple access (TD-CDMA), both multiple-access interference and intersymbol interference (ISI) arise. In order to combat the overall interference, we propose four efficient joint detection schemes based on zero-forcing and minimum-mean square error criteria. By exploiting the Toeplitz-block structure of matrices and the asymptotic equivalence between finite-order Toeplitz matrices and circulant matrices, most computations can be carried out very efficiently through extensive use of discrete Fourier transform (DFT) and independent DFT (IDFT) transforms. Performance results based on the UMTS scenario are presented.

Perspectives of SAW Based Processor Implementation for Mobile Communications

The new approaches to the development of the mobile telecommunication systems' wireless interface based on the use of surface acoustic waves (SAW) are considered in the paper. The completed analysis has shown, that the proposed new architecture of the wireless interface with code-time division multiple access (CTDMA), based on SAW components, is potentially capable of approaching the UMTS/IMT2000 requirements for mobile telecommunication systems of 3-rd generation. The appropriate application in the system of 3rd generation of the CTDMA wireless interface, taking into account the features of SAW components and the possibility of an integration with the standards of the GSM-900/DCS-1800 system, can provide effective cellular telecommunications with the flexible adaptation to possible operating conditions.

Radio resource allocation in third generation mobile communication systems

Code-division multiple access has been widely accepted as the major multiple access scheme in third-generation mobile communication systems. Wide-band CDMA and its hybrid associate time-division CDMA are key elements of the IMT2000 framework of standards. Since the beginning of the 1990s there has been enormous research activity in analysis of the soft (i.e., interference limited) capacity of these CDMA-based systems. Optimal usage of the soft capacity to provide, maintain, and guarantee QoS for different service classes is now becoming a very important issue. Therefore, interest in radio resource allocation has recently. This article presents an overview of RRA schemes (primarily for CDMA-based systems) that are flexible, support traffic services with various QoS requirements, minimize call/session blocking and dropping probabilities, and have acceptable radio resource utilization.

Smart antennas for combined DOA and joint channel estimation in time-slotted CDMA mobile radio systems with joint detection

In cellular mobile radio systems, the directional inhomogeneity of the mobile radio channel can be exploited by smart antennas to increase the spectral efficiency. In this paper, a novel smart antenna concept applying receiver antenna diversity at the uplink receiver is investigated for a time-slotted code-division multiple-access (CDMA) mobile radio air interface termed time-division CDMA (TD-CDMA), which has been selected by the European Telecommunications Standards Institute (ETSI) in January 1998 to form part of the Universal Mobile Telecommunications System (UMTS) air interface standard. First, a combined direction-of-arrival (DOA) and joint channel estimation scheme is presented, which is based on DOA estimation using the Unitary ESPRIT algorithm and maximum likelihood estimation of the channel impulse responses associated with the estimated DOA's, which can also be used as an input for advanced mobile positioning schemes in UMTS. The performance of the combined DOA and joint channel estimation is compared with the conventional channel estimation through simulations in rural and urban propagation environments. Moreover, a novel joint data detection scheme is considered, which explicitly takes into account the signal DOA's and the associated channel impulse responses. The link level performance of a TD-CDMA mobile radio system using these novel schemes is evaluated by Monte Carlo simulations of data transmission, and average bit error rates (BER's) are determined for rural and urban propagation environments. The simulation results indicate that, depending on the propagation environment, the exploitation of the knowledge of the directional inhomogeneity of the mobile radio channel can lead to considerable system performance enhancements.