Multi-user Multi-carrier Code Division Multiple Access Research Articles

Demonstration of high-speed multi-user multi-carrier CDMA visible light communication

We experimentally demonstrated a high-speed multi-user multi-carrier code-division multiple access (MC-CDMA) visible light communication (VLC) system. By employing a commercially available red light emitting diode (LED) and an avalanche photo diode (APD), we achieved a 16-user VLC system enabled by MC-CDMA, pre- and post-equalization, with an overall bit rate of 750Mb/s over 1.5m free-space transmission. The measured bit error ratio (BER) of each user is below the 7% pre-forward-error-correction (pre-FEC) threshold of 3.8×10−3.

Partial Equalization for MC–CDMA Systems in Non-Ideally Estimated Correlated Fading

Multicarrier code-division multiple access (MC-CDMA) can support high data rates in next-generation multiuser wireless communication systems. Partial equalization (PE) is a low-complexity technique for combining the signals of subcarriers to improve the achievable performance of MC-CDMA systems in terms of their bit error probability (BEP) and bit error outage (BEO) in comparison with maximal ratio combining, orthogonality restoring combining, and equal-gain combining techniques. We analyze the performance of the multiuser MC-CDMA downlink and derive the optimal PE parameter expression, which minimizes the BEP. Realistic imperfect channel estimation and frequency-domain (FD) block-fading channels are considered. More explicitly, the analytical expression of the optimum PE parameter is derived as a function of the number of subcarriers, number of active users (i.e., the system load), mean signal-to-noise ratio (SNR), and variance of the channel-estimation errors for the aforementioned FD block-fading channel. We show that the choice of the optimal PE technique significantly increases the achievable system load for the given target BEP and BEO.

Efficient Detection Algorithm for a Multiple-Input and Multiple-Output MultiuserMulti-Carrier Code Division Multiple Access in Time-Varying Channels

Problem statement: Maximum Likelihood (ML) decoding has been applied for the uplink of a Multi-Carrier Code Division Multiple Access (MC-CDMA) system based on Orthogonal Frequency Division Multiplexing (OFDM). Multiple-Input Multiple-Output (MIMO) channel from k users, which moves at vehicular speed, to the Base-Station (BS) is time-varying. For time-varying channels, Sphere Decoding (SD) was introduced to perform ML decoding. Whereas, computational complexity of SD (due to a QR-factorization for each symbol), is nevertheless high. Modified SD had been proposed to achieve near optimum solutions that called Subspace-Sphere Decoder (SPSD). Approach: Proposed algorithm was based on subspace and orthogonal projection with very small dimensionality as robustness scheme in an iterative Multi-User Detection (MUD) and Parallel Interference Cancelation (PIC) method. Results: This approach had been achieved intense reduction of computational complexity for time-varying channel via one and more than one order of magnitude at channel estimation and multiuser detection respectively. Furthermore, SPSD was robustness to channel estimation error (about 3.8 dB) as compared to the representative counterparts in literature. Conclusion: Effectiveness of proposed method was demonstrated by simulations.

Adaptive channel and power allocation of downlink multi-user MC-CDMA systems

The Multi-Carrier Code Division Multiple Access (MC-CDMA) is becoming a very attractive multiple access technique for high-rate data transmission in the future wireless communication systems. This paper is focused on the joint channel and power allocation in the downlink transmission of multi-user MC-CDMA systems and considers the throughput maximization problem as a mixed integer optimization problem. For simple analysis, the problem is divided into two less complex sub-problems: power allocation and channel allocation, which can be solved by a suboptimal Adaptive Power Allocation (APA) algorithm and an optimal Adaptive Channel Allocation (ACA) algorithm, respectively. By combining APA and ACA algorithms, an adaptive channel and power allocation scheme is proposed. The numerical results show that the proposed APA algorithm is more suitable for MC-CDMA systems than the conventional equal power allocation algorithm, and the proposed channel and power allocation scheme can significantly improve the system throughput performance.

Time-variant channel estimation using discrete prolate spheroidal sequences

We propose and analyze a low-complexity channel estimator for a multiuser multicarrier code division multiple access (MC-CDMA) downlink in a time-variant frequency-selective channel. MC-CDMA is based on orthogonal frequency division multiplexing (OFDM). The time-variant channel is estimated individually for every flat-fading subcarrier, assuming small intercarrier interference. The temporal variation of every subcarrier over the duration of a data block is upper bounded by the Doppler bandwidth determined by the maximum velocity of the users. Slepian showed that time-limited snapshots of bandlimited sequences span a low-dimensional subspace. This subspace is also spanned by discrete prolate spheroidal (DPS) sequences. We expand the time-variant subcarrier coefficients in terms of orthogonal DPS sequences we call Slepian basis expansion. This enables a time-variant channel description that avoids the frequency leakage effect of the Fourier basis expansion. The square bias of the Slepian basis expansion per subcarrier is three magnitudes smaller than the square bias of the Fourier basis expansion. We show simulation results for a fully loaded MC-CDMA downlink with classic linear minimum mean square error multiuser detection. The users are moving with 19.4 m/s. Using the Slepian basis expansion channel estimator and a pilot ratio of only 2%, we achieve a bit error rate performance as with perfect channel knowledge.

Bit Error Rate Analysis for MC-CDMA Systems in Nakagami-"Equation missing" Fading Channels

Multicarrier code division multiple access (MC-CDMA) is a promising technique that combines orthogonal frequency division multiplexing (OFDM) with CDMA. In this paper, based on an alternative expression for the Q-function, characteristic function and Gaussian approximation, we present a new practical technique for determining the bit error rate (BER) of multiuser MC-CDMA systems in frequency-selective Nakagami-m fading channels. The results are applicable to systems employing coherent demodulation with maximal ratio combining (MRC) or equal gain combining (EGC). The analysis assumes that different subcarriers experience independent fading channels, which are not necessarily identically distributed. The final average BER is expressed in the form of a single finite range integral and an integrand composed of tabulated functions which can be easily computed numerically. The accuracy of the proposed approach is demonstrated with computer simulations.