Wireless media access control for highly mobile information servers

A DS/CDMA (direct-sequence code division multiple access) system using BPSK (binary phase shift keying) modulation is simulated to determine the bit error probabilities for various numbers of users with a processing gain close to that of a cellular network. The results are compared to results obtained using the Gaussian approximation equations. The results obtained are to the feasibility of a DS/CDMA cellular network. It is concluded that DS/CDMA cellular networks appear to offer higher subscriber capacities than existing analog or TDMA (time division multiple access) formats. In addition, DS/CDMA offers additional advantages such as inherent privacy through the use of spreading codes and the ability to combat problems such as multipath fading and narrowband noise. >

The future wireless mobile communication systems will be required to support high-speed transmission rate and high quality of service. Direct sequence code division multiple access (DS-CDMA) is an important scheme for high-rate wireless communication. The capacity of DS-CDMA can be impaired by two problems; near-far effect and multiple-access interference (MAI). The use of conventional-matched filter detector for multiple users in DS-CDMA fails to combat any of these problems. The performance degradation caused by MAI can be overcome using multiuser detection (MUD). The use of maximum likelihood (ML) sequence estimation detector provides excellent results, but involves high computational complexity. In this paper, we propose a new meta-heuristic approach for MUD using honeybees mating optimization (HBMO) algorithm to detect the user bits based on the ML decision rule for DS-CDMA systems in additive white-Gaussian noise and flat Rayleigh fading channels. In order to improve the solutions generated by the HBMO, a second meta-heuristic method simulated annealing is used. By computer simulations, the bit error rate performance and the complexity curves show that the proposed HBMO-SA MUD is capable of outperforming the other conventional detectors and genetic algorithm detector.

In most direct sequence code division multiple access (DS-CDMA) systems, the modulation scheme used is a simple one. In this paper, employing a combined modulation scheme in DS-CDMA is proposed. By combining two or more modulation schemes, the resultant symbol energy can be increased, and thus a better symbol error rate performance can be achieved. For the combined modulation scheme, the symbol error rate is determined by the less powerful one of the basic modulation schemes. So a proper selection of modulation combination is essential for power efficiency. In a DS-CDMA system, unlike a narrow-band system, the bandwidth efficiency is closely related to its power efficiency. Thus a combined modulation concept may result in a bandwidth efficient DS-CDMA system. In this paper, we present two DS-CDMA schemes by combining MFSK and coherent MPSK and differential BPSK. These two schemes are respectively denoted as (1) MFSK-MPSK/DS-CDMA, and (2) MFSK-DPSK/DS-CDMA. The proposed schemes have better performance than the conventional CDMA schemes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

We introduce a novel merger of direct sequence code division multiple access (DS-CDMA) and smart antenna arrays. With regard to the DS-CDMA scheme, we employ carrier interferometry DS-CDMA (CI/DS-CDMA), a novel implementation of DS-CDMA where chips are decomposable into N narrowband frequency components. With regard to the antenna array, we deploy the oscillating-beam smart array. Here, applying proper time-varying phases to the array elements, we create small movement (oscillation) in the antenna array's pattern, while steering the antenna pattern main lobe to the position of the intended user. The oscillating antenna pattern creates a time-varying channel with a controllable coherence time. This, in turn, provides transmit diversity in the form of a time diversity gain at the mobile receiver side. At the receiver, three stages of combining are available: combining time components of the received signal within symbol duration Ts (each experiencing a different fade) to enhance performance via time diversity; combining frequency components which make up the CI/DS-CDMA chip to enhance the performance via frequency diversity; and combining across chips to eliminate the interfering users on the system. Merging CI/DS-CDMA with the oscillating-beam smart antenna at the base station, we achieve very high capacity via the merger of SDMA (available through directionality of the antenna array) and code division multiple access (inherent in CI/DS-CDMA), and very high performance via the construction of receivers that exploit both transmit diversity and frequency diversity. We present the performance gains of the proposed merger.

This paper proposes a new packet rate scheduling scheme for a non-real time data services over the uplink of a burst switching based direct sequence code division multiple access (DS/CDMA) system to support integrated voice/data services. We consider the most general form of optimization problem formulation to determine the optimal number of transmission time groups along with their data rates, which minimizes the average packet transmission delay. An ordered packet length based groupwise transmission (OLGT) scheme is proposed as a simple heuristic solution to this problem, and we present some analytical results for performance comparison with other possible schemes.

This paper proposes a new packet rate scheduling scheme for a non-real time data service over the uplink of a burst switching-based direct sequence code division multiple access (DS/CDMA) system to support the integrated voice/data service. We consider the most general form of optimization problem formulation to determine the optimal number of transmission-time groups along with their data rates, which minimize the average packet transmission delay. An ordered packet length-based group-wise transmission (OLGT) scheme is proposed as a simple heuristic solution approach to this problem and present some analytical results for performance comparison with other possible schemes.

We propose a multistage joint adaptive array (AA) and spatial interference cancellation (IC) structure for a direct sequence code division multiple access (DS/CDMA) system. The structure utilizes both space and time processing to improve the performance of the base station receiver under the influence of multiple access interference (MAI) and in the absence of power control. The AA comprises a bank of temporal matched filters followed by a beamformer. IC is then used to remove the detected strong user from the received array input. This stage can be repeated for recover and remove more users. Computer simulation shows that, in the absence of power control, the proposed algorithm efficiently detects the weak user information at MAI levels of 30 dB where the conventional space-time structure fails.

The nonlinear Multiuser Detection (MUD) in Direct Sequence Code Division Multiple Access (DS/CDMA) system can be viewed as a two-class classification task. A new classification method called Probabilistic Tangent Subspace (PTS) is introduced to be used as an MUD. Due to the mobility of communicator, wireless communication channels are in fact time variant. The uncertainties of the time-varying channel’s coefficients cause the uncertainties of the Multiuser Interference (MUI). On the other hand, the probabilistic tangent subspace method is designed to encode the pattern variations. Therefore, we are motivated to adopt this method to develop a classifier as a multiuser detector for time-varying channels. Simulation results show that this MUD performs better than that based on Support Vector Machine (SVM) for Rayleigh fading channel in DS/CDMA system.KeywordsSupport Vector MachineMinimum Mean Square ErrorCode Division Multiple AccessRayleigh Fading ChannelMultiuser DetectionThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

The evaluation of a hybrid acquisition performance has been considered for the pilot signal in the direct sequence code division multiple access (DS/CDMA) forward link. The hybrid acquisition is introduced by the combination of two schemes, parallel and serial acquisitions. The mean acquisition time of the proposed scheme is derived by considering both case 1 (the correct code-phase offsets are included in one subset) and case 2 (the correct code-phase offsets exist at the boundary of two subsets), which are caused by the distribution of correct code-phase offsets in the subset. Detection, false alarm, and miss probabilities are derived for the cases of multiple correct code-phase offsets and multipath Rayleigh fading channel. Results are provided for the acquisition performance with respect to design parameters such as postdetection integration length in search and verification modes, subset size, and number of I/Q noncoherent correlators. Also, comparison between the hybrid acquisition and the fully parallel acquisition is provided in terms of the minimum mean acquisition time.

It is difficult to assign independent radio frequency bands to wideband DS-CDMA (direct sequence code division multiple access) systems because of lack of radio frequency resources, so that CDMA overlay technique, which means co-existence of wideband and narrowband signals in the same frequency band, has been considered to improve spectral efficiency. In the overlay situation, however, signals from existing narrowband systems are not necessarily transmitted with low power level enough not to give damage to DS-CDMA signals, so that an efficient rejection technique for high power narrowband interferers is required for successful CDMA overlay. We propose a high power narrowband interference rejection method which connects several narrowband interference cancellers with simple construction in series and show its rejection performance obtained from computer simulation.

Direct sequence code division multiple access (DS-CDMA) techniques have received a great deal of attention for current and future communication systems. One of the major considerations in implementing a DS-CDMA system is the necessity for accurate power control to ensure adequate quality-of-service and capacity. This paper investigates the power control errors that occur in a real microcellular environment for a variety of architectures. Furthermore, the mitigation of the effect of power control errors with varying spreading bandwidth is quantified.

Performance analysis of multicell multiuser MIMO MC DS/CDMA system with MMSE user-ordered SIC technique

In this paper an efficient direct sequence code division multiple access (DS-CDMA) transmission protocol is proposed for centralized packet radio networks. This protocol reduces the central station complexity while maintaining the throughput at an acceptable level. First the authors propose a partial DS-CDMA protocol which reduces the number of receivers required at the central station. The central station complexity reduction is usually obtained at the price of throughput reduction. Then to mitigate the problem of throughput reduction, a new protocol which uses DS-CDMA in conjunction with polling is proposed to increase the throughput while maintaining the complexity reduction of the central station.

Spatial diversity is an attractive technology for coping with the fading channels encountered in mobile communications. In this paper, the bit error rate (BER) is analyzed theoretically for diversity reception with a RAKE receiver in a Nakagami fading environment using either selection or maximal ratio combining. A coherent binary phase-shift keying (CBPSK) direct sequence code division multiple access (DS/CDMA) system is considered. An arbitrary branch correlation is also considered for any diversity order in the case of identical fading severity on the branches.

The multiuser detection (MUD) performance analysis in a synchronous direct sequence code division multiple access (DS-CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.