Multicode Code-division Multiple-access System Research Articles

Algorithm for reducing PAPR of Multicode CDMA using Reed–Muller codes

A new algorithm for reducing peak-to-average power ratio (PAPR) of multicode code division multiple access (MC-CDMA) systems under binary PSK modulation is presented. The algorithm is based on Reed-Muller (RM) codes that are modified by finding a suitable binary sequence that minimises the PAPR of the MC-CDMA. Three types of RM codes are investigated: namely, zero order, first order and second order. Simulation studies conducted show that the proposed algorithm achieves transmitter power savings of up to 6 dB. Zero-order and first-order codes exhibit 0 dB PAPR, while several others maintain a constant and low PAPR as compared with the original MC-CDMA systems.

Sum-Rate Optimal Multicode CDMA Systems: An Equivalence Result

In this paper, the sum rate of a multicode code-division multiple-access (CDMA) system with asymmetric-power users is maximized, given a processing gain and the power profile of users. Unlike the sum-rate maximization for a single-code CDMA system, the optimization requires the joint optimal distribution of each user's power to its multiple data streams as well as the optimal design of signature sequences. The crucial step is to establish an equivalence of the multicode CDMA system to restricted frequency-division multiple-access (FDMA) and time-division multiple-access (TDMA) systems. The CDMA system has upper limits on the numbers of multicodes of users, whereas the FDMA system has upper limits on the bandwidths, and the TDMA system has upper limits on the duty cycles of users. The equivalence facilitates the complete characterization of the maximum sum rate of the multicode CDMA system and also provides new insights into single- and multicode CDMA systems in terms of the parameters of the equivalent FDMA and TDMA systems.

Effect of jamming and diversity on the performance of a wavelet packet based multicarrier multicode CDMA communication system

In this paper, the performance of multicarrier (MC), multicode (MCD), code division multiple access (CDMA) system employing wavelet packets (WPs) as subcarriers for modulation in the presence of narrowband interference is investigated. The performance analysis include the effects of diversity techniques such as selection, equal gain and maximal ratio combining. The system performance is evaluated by means of bit error rate (BER) and outage probability. The effects of system parameters such as channel intensity profile, diversity order and fading parameter on the system performances were investigated by means of analytical methods and numerical results. Also, the effect of jamming power is investigated. The performance of the system is compared to that of MC/MCD-CDMA based on sinusoidal carrier. Results reveal considerable performance improvement of the system that employ diversity techniques over the one without diversity and of our proposed system over the other system.

BER Analysis of Multi-Code Multi-Carrier CDMA Systems in Multipath Fading Channel

This paper proposes a Multi-Code Multi-Carrier Code Division Multiple Access (CDMA) scheme for next generation wireless communication systems. This system retains the advantages of Multi-Carrier CDMA in combating multipath and rejecting interference, and provides variable and adaptive data rates through the use of Multi-Code scheme. The rate adaptation algorithm proposed computes the user data rate as a function of the instantaneous channel condition and application dependent data rates. Exploiting the channel information improves the effective data rate and overall capacity of the system. The performance improvement of the proposed system to the Multi-Code CDMA system is shown through simulations. Walsh-Hadamard, Gold and Kasami codes are explored as possible choices for the multiple codes in the system.

A Hybrid Receiver Scheme for Multiuser Multicode CDMA Systems in Multipath Fading Channels

In this paper, we present a system model for multiuser multicode code-division multiple access (CDMA) in a multipath fading channel and propose a hybrid receiver scheme for the system. Conventional receivers for CDMA systems utilized interference cancellation (IC) schemes, with the RAKE receiver as an initial data estimator. However, the RAKE receiver generates inaccurate data estimates due the interference that is caused by the cross correlation between the spreading codes. This inaccurate data estimate leads to significant error propagation to the subsequent stages and thus degrades the bit error rate (BER) performance. Our proposed scheme overcomes the problem by using a more accurate initial data estimator. It consists of an equalizer for obtaining initial data estimation, followed by several stages of parallel IC (PIC). Computer simulation results show that the BER curves of our proposed scheme do not encounter an error floor under all channel conditions; in contrast, the BER curves of the conventional RAKE-PIC receiver are constrained by an irreducible error floor. In addition, in an uplink four-path channel and at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , our simulation results show that a one-stage PIC achieves a 2.5-dB gain over the first stage equalization receiver.

Improving the capacity of multi-code multimedia CDMA system

A novel method for improving the capacity of multi-code multimedia Code Division Multiple Access (CDMA) systems is proposed. By factitiously improving the transmission bit rate of the call, the number of the orthogonal codes used by a user is increased which leads to the decreasing of the interference. Simulation results shows that the proposed scheme results in better throughput than traditional multi-code CDMA systems.

Linear MMSE space-time equalizer for MIMO multicode CDMA systems

We propose an enhanced chip-level linear space-time (ST) equalizer for multiple-input multiple-output (MIMO) multicode code-division multiple-access (CDMA) systems. In the MIMO multicode CDMA systems, the reuse of the same spreading codes in different transmit antennas significantly degrades the equalization performance if the ST equalizer uses a minimum mean-squared error (MMSE) weighting vector that minimizes the mean-squared error of the equalizer output chip sequence. As the CDMA despreader concatenated to an ST equalizer distorts interstream interference components differently from multipath interference and background noise components, the chip-level MMSE weighting vector usually steers in suboptimal directions in the signal space. Therefore, we propose a new MMSE weighting vector that takes the despreading effect into account in this paper. Simulation results show a substantial performance improvement through the new weighting vector.

A Software Definable Architecture for Adaptive Space Diversity at Handsets in MC-CDMA Systems

Software-Defined Radio (SDR) represents a major paradigm shift in the design of radios, allowing a large fraction of the functionality to be implemented through programmable signal processing devices, enabling the radio to change its operating parameters to accommodate new air interface, features and capabilities. However, the actual realization of innovative and software-reconfigurable receiver diversity at mobile handsets in intermediate frequency band to provide wide-ranging benefits, including more effective filtered result, less cost of the mixed channel access, improved capacity, better link reliability, and reduced power consumption, has been slowed down largely due to an absence of effective architecture reducing the complexity of adaptive combining algorithms. This paper proposes a novel reconfigurable architecture for adaptive space diversity at handsets in MC-CDMA (multicode code-division multiple-access) systems. The key to which is the development of a valid and effective alternative to the time-consuming multiplication operation and despreading acquisition. A software definable algorithm can become a multiplier-free architecture if it can restrict the weight factors to power-of-two values and repetitive gradient search procedure to contain shift operations and predicate functions. The results of numerical simulation and experimentation confirm the expectation that the constrained approach should perform comparably to, but not better than the traditional diversity algorithm. That is, the feasibility of SDR depends on its trading some performance for reduced computational complexity, improved area efficiency and less power consumption.

Analytical Study of QAM With Interference Cancellation for High-Speed Multicode CDMA

In this paper, we consider the multilevel quadrature amplitude modulation (M-QAM) for forward link of multicode code-division multiple-access (CDMA) systems with interference cancellation to support high data rate service, and provide an analytical bit error rate (BER) performance of the system. In the current third-generation (3G) wide-band CDMA systems, in addition to multicode transmission, M-QAM is employed for high-speed downlink packet access (HSPDA) due to its high spectral efficiency. In frequency-selective fading channels, multipath interference seriously degrades the system performance, therefore interference cancellation technique is employed at the receiver. In this paper, an analytical closed-form BER of multicode CDMA systems with QAM is presented, where two important factors (channel estimation error and additive interference) on system performance are considered. Numerical results show that the system is very sensitive to additive multipath interference. With the help of interference cancellation technique, M-QAM can be adaptively employed in high signal-to-noise ratio cases to increase system throughput, within 5 MHz bandwidth. Meanwhile, simulation results are shown to illustrate the accuracy of the analytical method. Moreover, it is found that extra pilot power should be invested for more accurate channel estimation and consequently better BER performance when using the interference cancellation technique.

Channel adaptive fair queueing for scheduling integrated voice and data services in multicode CDMA systems

Code Division Multiple Access (CDMA) systems continue to be critical building blocks of future high performance wireless and mobile computing systems. While CDMA systems are very mature for voice services, their potentials in delivering high quality data services (e.g. multimedia messaging services) are yet to be investigated. One of the most crucial components in an advanced wideband CDMA system is the judicious allocation of band width resources to both voice and high data rate services so as to maximize utilization while satisfying the respective quality of service requirements. Specifically, in a multicode CDMA system, the goal is to intelligently allocate codes to the users' requests. While previous work in the literature has addressed this problem from a capacity point of view, the fairness aspect, which is also important from the users' point of view, is largely ignored. In this paper, we propose a new code allocation approach that is channel adaptive and can guarantee fairness with respect to the users' channel conditions. Simulation results show that our approach is more effective than the proportional fair approach.

Improvement of power efficiency of HPA by the PAPR reduction and predistortion

In this paper, we propose the PAPR (peak to average power ratio) reduction methods of SLM (selected mapping), and PTS (partial transmit sequence) to decrease the nonlinear distortion and to improve the power efficiency of the nonlinear HPA (high power amplifier) in the multi-code CDMA (code division multiple access) system. The phase rotation factors transmitted as side information in PTS and SLM methods play an important role in the BER performance. So, we newly derive the theoretical BER equation when the effects of side information are considered in the multi-code CDMA communication system. Simulation results show that the PTS reduces the PAPR by about 2 dB while SLM can reduce by about 3 dB, compared with the original MC-CDMA when QPSK is used for 32 branches. Also, we find the BER performance of multi-code CDMA system with and without predistorter in front of the HPA. When predistorter is used, 3 dB IBO (input backoff) is required in SLM method, and 5 dB IBO in PTS method to closely match with the linear amplifier. Without PAPR reduction method, more than 7 dB is required. Finally, we show that the simulation results are very close to the theoretical results.

Channel assignment with QoS guarantees for a multiclass multicode CDMA system

In a wireless system that supports multimedia services, each traffic requires different quality of service (QoS) at both communication on radio links and connection admission. We initially derive the uplink capacity satisfying the QoS constraint on radio links in a multiclass multicode code-division multiple-access (CDMA) system. Based on the derived capacity, the number of channel elements, which is one of the system resources, is determined. Then, we define the QoS parameters associated with connection processes. To guarantee the defined QoS at the connection level, under given channel elements, we propose a channel-assignment scheme with dynamic priority adjustment (DPA). The proposed scheme gives multipriority to different traffic classes. Real-time classes can preempt non-real-time classes with restricted preemptive priority. Such restriction is regulated by preemption-free code channels and a buffer threshold for non-real-time classes. Among real-time classes, different priorities are assigned to each traffic class by code reservation parameters. These multipriority parameters are dynamically adjusted in order to guarantee different QoS requirements. We analyze the DPA scheme by the matrix-geometric method, and evaluate the performance of each traffic class. The results show that the proposed scheme flexibly guarantees QoS depending on traffic loading condition and achieves high channel utilization.

Capacities of spectrally overlaid single-code and multicode CDMA systems

This paper is concerned with the capacities of spectrally overlaid narrowband and wideband systems using two code-division multiple-access (CDMA) techniques - single-code and multicode. The capacity is defined here as the maximum achievable total throughput in the narrowband and wideband subsystems. User signal power in the overlay systems affects the performance of the systems. The signal power is characterized as the relative power level of wideband and narrowband users according to the spreading bandwidth, spectral overlay ratio, and the number of spreading codes assigned to a user in single-cell and multiple-cell environments. The capacities of the overlay systems are then analyzed. The results of the analysis show that the single-code technique yields better performance than the multicode scheme in terms of the capacities of the overlay systems. Furthermore, the use of a multicode technique for wideband subsystems and a single-code scheme for narrowband subsystems is desirable for enhancing the performance of CDMA overlay systems.

MCI cancellation for multicode wideband CDMA systems

Multicode code division multiple access (CDMA) is a new transmission scheme for flexible and high-speed data communications. The basic idea of multicode CDMA, is to assign multiple channelization codes to any given user. If these channel codes are orthogonal, the self-interference among them can be eliminated in an additive white Gaussian noise (AWGN) channel. However, in a multipath environment, these intrauser signals from different delay paths no longer maintain orthogonality and, thus, cause interference, i.e., multicode interference (MCI), to each other. In high-speed data networks, where the number of users is much less than in the voice networks, the MCI may represent a large portion of the total interference and has a great impact on the system performance. A complex spreading multicode wideband CDMA receiver with RAKE structure and multistage MCI cancellation is studied. By pilot aided channel estimation, the MCI associated with the reference user is regenerated and subtracted from the received signal by a cancellation factor of /spl lambda/. A complete and consolidated theoretical analysis is presented to show that the system performance is significantly improved by the MCI cancellation. The optimal cancellation factor in the kth stage is approximated by /spl lambda//sub opt//sup (k)//spl les/1-2P/sub e//sup (k-1)/, where P/sub e//sup (k-1)/ the error probability of the (k-1)th stage. The optimal value of each stage can be chosen from 0.5 to 0.85 for a wide range of signal to noise ratios.

Traffic management in a multicode CDMA system supporting soft handoffs

A traffic management scheme is proposed in a multicode code-division multiple-access system supporting soft handoff that uses guard channels and a queue for real-time traffic. Preemptive queue control gives priority to queued handoff calls. Handoff traffic is derived as a function of the new call arrival rate, the size of the soft handoff region, mobile speed, the new call blocking probability, and the handoff failure probability. System performance with K types of calls is analyzed by introducing a concept of effective channel. The effects of the number of guard channels, the number of effective channels, system capacity, and other factors are numerically investigated. The effectiveness of the proposed queue control scheme is also observed in terms of handoff processing delay.

Capacity estimation for a multicode CDMA system with SIR-based power control

Capacity estimation in code division multiple access (CDMA) systems is an important issue which is closely related to power control. Strength-based power control has been assumed in most analyses in which other cell interference was considered as a known and fixed variable. However, in signal-to-interference ratio (SIR)-based power-control systems, power control and other cell interference are closely related to each other and capacity can be obtained by considering this relationship. This study derives the reverse-link capacity of an SIR-based power-controlled multicode CDMA system supporting heterogeneous CBR and on-off traffic in a multiple cell environment. Mean and variance statistics of total other cell interference, and the effects of traffic and propagation parameters on system capacity are investigated.