Strongest-users-only detectors for randomly spread CDMA

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

The reverse link performance of direct sequence code division multiple access (DS-CDMA) system in wireless communication over a frequency selective multipath fading channel has been investigated in this paper. The channel has been modeled as Nakagami-m distributed frequency selective multipath fading channel. A conventional correlator type receiver has been considered for the analysis. Noise is taken as additive white Gaussian noise (AWGN) with two sided power spectral density N/sub 0//2 . To model multiple access interference (MAI), standard Gaussian approximation (SGA) has been considered in this paper. To mitigate the near-far problem, a power control scheme, but imperfect has been assumed in this paper. The bit error rate (BER) expression for the system has been derived and simulated by varying number of users, number of fading path, processing gain, fading parameter, SNR per bit E/sub b//N/sub 0/ and variation in the amplitude of the received signal. The analysis shows that as the number of simultaneous active user, the number of fading path and the variation in the power of the received signal increases, the BER of the system increases indicating the performance degradation. As the value of the processing gain, the ratio E/sub b//N/sub 0/ and the fading parameter increases, the BER performance of the system improves.

Criteria for selecting spreading codes for a downlink transmission in a direct sequence code division multiple access (DS-CDMA) system are reviewed. For the most general case of a non-code inversion keying system (CIK) it turns out that the codes must have good partial aperiodic cross and autocorrelation properties as far as the optimisation of the RAKE receiver performance is concerned. Codes generally used such as the Gold or Kasami codes have good cross and auto correlation properties on their whole period but do not have any particular partial correlation properties. In order to solve this limitation, the author proposes a method for building a set of so-called extended structured codes. Each long code is obtained by concatenation of cyclically different short codes of period the processing gain, the short codes of the initial set presenting good correlation properties. This construction is an extension of the structured codes proposed by T. O'Farrell (1991) that overcomes some of their drawbacks such as a short period and a small number of long codes in the constructed set. Simulation results show that the 'extended structured codes' bring a large improvement with respect to Gold and m-sequences.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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>

The conventional minimum mean squared error (MMSE) receiver which actively suppresses multiuser interference (MUI), outperforms the RAKE receiver for direct sequence code division multiple access (DS-CDMA) in multipath slow fading channels. However the performance gap between the conventional MMSE receiver and the theoretical bound is still large. We therefore propose an extended MMSE receiver which spans three symbol intervals the desired symbol interval as well as the previous and the next symbol interval. The extended MMSE receiver outperforms the conventional MMSE receiver especially for large system load and large number of propagation paths in the channel. Its computational complexity is approximately three times the complexity of the conventional MMSE receiver but still independent of the number of active users and the number of paths in the channel. As a consequence the extended MMSE receiver allows a low-complexity low-power implementation in the mobile user-terminal for communication over multipath slow fading channels in the forward link.

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.

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.

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.

Rapid converging adaptive equalization algorithms for interference suppression in direct sequence code division multiple access (DS/CDMA) systems are presented. These adaptive algorithms, based on an orthogonal transformation, do not require a priori knowledge on interfering signals parameters such as spreading sequences and relative signal power levels. The convergent rate of these algorithms is independent of the eigen-value spread of the input data correlation matrix. The performance of these proposed rapid converging adaptive equalizers is investigated both in the absence and presence of frequency offsets among transmitters. >

The performance of a multicarrier direct sequence code division multiple access (MC-DS-CDMA) system in fading environment has been extensively analysed in the literatures. The analysis that is available in the literatures did not take into account the impact of jamming and channel estimation errors on the performance of the system. In this study, closed-form expressions of the average bit error probability (BEP) of the MC-DS-CDMA system with channel estimation errors in the presence of partial band, broadband and multitone jamming are derived in a Rayleigh fading channel. The analysis shows that the magnitude of the normalised correlation between the actual and the estimated complex fading channel gain, A, greatly affects the performance of the MC-DS-CDMA system. For large signal-to-noise ratio, the average BEP with A=1 and without multiple access interference (MAI) tends to zero. When A≠1, the BEP of the system exhibits an error floor even without MAI. This error floor increases as the number of users increases. It is also shown that increasing the number of the system subcarriers enhances the system performance against jamming.

Chaotic sequences have good correlation properties and they can be used as address sequences in Spread Spectrum Communication. Chaotic functions are highly sensitive to initial condition and exhibit non-linear behavior. In Chaotic spread spectrum communication systems, different user may be assigned different sequences generated with different initial conditions. Set of binary sequences with good pair wise cross- correlation property finds applications in Direct Sequence- Code Division Multiple Access (DS-CDMA) systems. Security in Direct Sequence-Code Division Multiple Access systems is relatively limited. To enhance security in Direct Sequence- Code Division Multiple Access systems, chaotic sequences are generated. In this paper we analyze the generation of chaotic binary sequences and study of auto correlation and cross correlation properties.

Blind detection of a desired user's signal in a code-hopping (CH) direct sequence code-division multiple access (CDMA) system is considered. In CH CDMA systems each user switches between a predetermined set of short-code sequences. A user signal in such systems may be treated as the superposition of several virtual short-code signals. A code-constrained inverse filter criterion (IFC)-based blind detector for short-code CDMA signals in asynchronous multipath channels to detect a desired user's signal was recently presented by Tugnait and Li (2001). A novel approach combining the code-constrained IFC and a penalty function is proposed to simultaneously extract all virtual users associated with a given CH user. Global minima of the proposed cost function are analyzed. An extension of an existing subspace-based approach is also investigated. An illustrative simulation example is provided where the proposed algorithm is compared with a clairvoyant matched filter receiver, two linear minimum mean-square error (MMSE) receivers with channels known, and the subspace-based approach.

In the last decade, increasing research investigations on spread spectrum communications have addressed direct sequence code division multiple access (DS-CDMA) systems, where all users transmit on the same band at the same time and are distingnished only by means of a code signature of spreading sequence. A DS-CDMA system has well known desirable features: universal frequency reuse, fast and accurate power control, robustness to multipath channel, graceful degradation, and ease of cellular planning, etc. These advantages result from the assignment of spreading sequences to every potential user of the system.