Uplink Code Division Multiple Access Systems Research Articles

Reduced-Rank Adaptive Filtering Using Localized Processing for CDMA Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> An integrated beamforming (spatial processing) and multiuser-detection (temporal processing) scheme is an effective approach to increase system capacity but is also impractical due to the high associated computational costs. To overcome this problem, researchers have developed reduced-rank approaches. Adding to this class of algorithms, this paper introduces a joint-domain adaptive algorithm, which processes spatial and temporal data within a localized region after transformation to a “beamspace.” This new joint-domain-localized (JDL) adaptive algorithm is developed for single-cell uplink code-division multiple-access (CDMA) systems with a receive-antenna array. Given a rank constraint on the JDL transformation, this paper develops the optimal choice of transformation matrix and justifies a simpler suboptimal choice. The JDL algorithm is shown to have relatively low computation load. We also introduce the JDL-Z algorithm that combines JDL processing with zero forcing for multicell uplink CDMA systems. At the cost of higher computational complexity, this new scheme provides better performance and a faster convergence rate than the JDL algorithm. However, the key contribution is a framework wherein adaptive processing can be followed by yet another stage of adaptive processing. Simulations are used to illustrate the efficacy of the two algorithms. </para>

Signal Design to Optimize Trade-Off between Bandwidth Efficiency and Power Efficiency in Uplink CDMA Systems

In this paper, we compare two signal designs for uplink quasi-synchronous code division multiple access (CDMA) channels in order to optimize the trade-off between bandwidth efficiency and power efficiency. The design we call band-limited DS/CDMA design, is based on the time-domain assignment of Gold sequences, just as in the ordinary DS/CDMA, but with band-constrained cyclic chip interpolation functions, which is unlike the ordinary DS/CDMA. The other design, MC/CDMA design, is based on frequency-domain assignment of the sequences, as in the ordinary MC/CDMA. In both designs, we assume insertion of guard intervals at the transmitter and frequency-domain processing in reception. Assuming quasi-synchronous arrival of CDMA signals at the CDMA base station and FFT in the effective symbol interval, the intersymbol interference is evaded in both designs. First we identified the signal parameters that optimize bandwidth efficiency in each of the band-limited DS design and MC design. Second, we clarified the signal parameters that optimize the power efficiency as functions of frequency efficiency in each of the two designs. Finally, we derived and compared the trade-off between the bandwidth efficiency and power efficiency of band-limited DS and MC designs. We found a superiority of band-limited DS design over MC design with respect to the optimized trade-off.

On pilot-based multipath channel estimation for uplink CDMA systems: an overloaded case

The paper considers a coding scheme for multipath channel estimation in uplink code-division multiple-access systems where each user transmits an individual pilot signal (sequence) to estimate its multipath channel coefficients. Assuming a common radio channel model with a uniform power delay profile, we derive lower bounds on the maximum mean square error for two types of linear channel estimators: an inverse filter and a linear minimum mean square error (MMSE) estimator. In contrast to previous work, the main focus here is on overloaded systems where the total number of multipath channel coefficients of all users is greater than processing gain. We show that the inverse filter bound is attained if and only if each pilot sequence is a perfect root-of-unity sequence. Interestingly, the conventional matched filter achieves the same lower bound if pilot sequences form a complementary periodic sequence set. In case of the MMSE estimator, the lower bound is either met or not depending on some system parameters. We provide a necessary and sufficient condition for achieving the bound when pilot sequences are arbitrary vectors on the unit sphere. This paper gives insight into the performance limits of practical systems.